Hawkins Electrical Guide v. 07 (of 10)
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Title: Hawkins Electrical Guide v. 07 (of 10)
Author: N. Hawkins
Release date: June 21, 2015 [eBook #49253]
Most recently updated: October 24, 2024
Language: English
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*** START OF THE PROJECT GUTENBERG EBOOK HAWKINS ELECTRICAL GUIDE V. 07 (OF 10) ***
THE THOUGHT IS IN THE QUESTION THE INFORMATION IS IN THE ANSWER
[Illustration:
HAWKINS
ELECTRICAL GUIDE
NUMBER
SEVEN
QUESTIONS
ANSWERS
&
ILLUSTRATIONS]
A PROGRESSIVE COURSE OF STUDY
FOR ENGINEERS, ELECTRICIANS, STUDENTS
AND THOSE DESIRING TO ACQUIRE A
WORKING KNOWLEDGE OF
ELECTRICITY AND ITS APPLICATIONS
A PRACTICAL TREATISE
by
HAWKINS AND STAFF
[Illustration: THEO. AUDEL & CO. 72 FIFTH AVE. NEW YORK.]
COPYRIGHTED, 1915,
BY
THEO. AUDEL & CO.,
NEW YORK.
Printed in the United States.
TABLE OF CONTENTS
GUIDE NO. 7.
~ALTERNATING CURRENT SYSTEMS~ 1,531 to 1,586
Advantages of the alternating current--~classification
of systems~--vector summation; examples--~forms of
circuit~: series, parallel, parallel series, series
parallel--~transformer systems~: individual transformers;
transformation at distribution centers--~single phase
system~; two wire transmission and three wire distribution;
objections to single phase systems; advantages--~monocyclic
system~--~two phase systems~: adaptation; ordinary voltages
used; two phase three wire system; two phase five wire
system--~three phase systems~: six wire; four wire; three
wire; connections: star, delta, star delta, delta star;
evolution of three wire system; pressure and current relations;
connection of transformers; open delta connection--~change
of frequency~--Schaghticoke-Schenectady transmission
line--~transformation of phases~: three to one, three to
two, two to six, and three to six phase--~Scott connection~
for transforming from three to two phase--three to two phase
with three star connected transformers--~economy of a.c.
systems~--~relative weights of copper required for polyphase
systems~--aermotor towers of Southern Power Co.--~choice of
voltage~--~usual transmission voltages~--~diagram of three
phase distribution~--~mixed current systems~; usual d.c.
pressure on traction lines; use of mixed systems.
~AUXILIARY APPARATUS~ 1,587 to 1,588
~Classification of auxiliary devices~: switching
devices, types--current or pressure limiting devices,
types--lightning protection devices, types--regulating devices,
types--synchronous condensers, types--indicating devices.
~SWITCHING DEVICES~ 1,589 to 1,612
Definition of a switch--behaviour of the current when the
circuit is broken--points on design--installation of single
throw and double throw switches--plug switches--~forms of
break~: open, enclosed, fuse, horn, oil--disconnecting
switches--pole top switches--~horn break switches~--motor
starting switch--~oil switches~; nature of an oil
break--~remote control oil switches~--~motor operated
switches~--rupturing capacity of oil switches--~float switches~.
~CURRENT AND PRESSURE LIMITING DEVICES~ 1,613 to 1,676
Necessity for these devices; steam analogy--~fuses~: advantages
and disadvantages; types: plug, cut out, expulsion, no arc,
magnetic blow out, quick break fuse, etc.--metal used--~current
limiting inductances~: construction, location--~circuit
breakers~: progressive breaking of the circuit; carbon
contacts--~automatic features~: overload trip, underload trip,
low voltage trip, auxiliary circuit trip--~relays~: adaptation;
classification: protective, regulative, communicative, a.c. and
d.c., circuit opening, circuit closing, primary, secondary,
overload, underload, over voltage, low voltage, reverse energy,
reverse phase, instantaneous, time limit, inverse time limit,
differential--~how to select relays~.
~LIGHTNING PROTECTION DEVICES~ 1,677 to 1,714
~Essential parts~: air gaps, resistances, inductances,
arc suppressing devices--requirements--~air gap
arresters~--~multi-gap arresters~; difference between spark
and arc; distribution of stress; sparking at the gaps; how
the arc is extinguished; effect of frequency; graded shunt
resistances; the cumulative or breaking back effect--arresters
for grounded ~Y~ and non-grounded neutral systems--multiplex
connection--~horn gap arresters~: operation; objection to
the horn gap--~electrolytic arresters~: critical voltage,
temporary and permanent; determination of number of cell;
putting cell in commission; nature of the film; horn gaps on
electrolytic arresters; charging of electrolytic arresters;
charging arresters for non-grounded circuits--~grounded and
non-grounded neutral circuits~--~ground connections~--~choke
coils~: principal objects; principal electrical conditions to
be avoided; why choke coils are made in the form of an hour
glass; cooling--~static interrupters~; how to connect condenser
and choke coil; effect of condenser.
~REGULATING DEVICES~ 1,715 to 1,762
~Regulation of alternators~--~a.c. feeder
regulation~--~application of induction type regulators~;
types: induction, and variable ratio transformer regulators;
operation of induction regulators; neutral position;
regulator capacity--~polyphase induction regulators~:
construction, operation; _automatic control_; why two relays
are used; difficulties encountered in operation of relays;
vibration or chattering of the contacts; poor contact
of primary relay--~variable ratio transformer voltage
regulators~: types: drum, and dial; dial type for high
voltage--~small feeder voltage regulators~: construction and
operation; adjustment--~automatic voltage regulators for
alternators~: method of regulation--~line drop compensators~:
essential parts; connections; construction and operation;
diagram of automatic voltage regulator using a line drop
compensator--~starting compensators~: necessity for;
construction and operation--~star delta switches~.
~SYNCHRONOUS CONDENSERS~ 1,763 to 1,776
Characteristics--effect of fully loaded and lightly
loaded induction motors on the power factor--synchronous
motor used as condenser--~effects of low lagging power
factors~; example--~cost of synchronous condenser vs. cost
of copper~--location of condenser--synchronous condenser
calculations and diagram for same.
~INDICATING DEVICES~ 1,777 to 1,838
Virtual value of an alternating current or pressure--the
word ~effective~ _erroneously_ used for ~virtual~: steam
engine analogy illustrating this error--classification of
instruments: electromagnetic or moving wire, hot wire,
induction, dynamometer--~electromagnetic or moving iron
instruments~: types: plunger, inclined coil, magnetic vane;
character of scale; objections and precautions--~inclined
coil instruments~--~magnetic vane instruments~--~hot wire
instruments~--~induction instruments~: types: shielded pole,
rotary field; operation of both types--~dynamometers~:
construction and operation; how arranged to measure
watts--~watthour meters~: types: commutator, induction,
Faraday disc; essential parts; object of the motor; object
of generator; objection to commutator meter--~principles
of induction watthour meters~: essential parts; strength
of rotating field; moving element; retarding element;
registering element; frame and bearings; friction compensator;
power factor adjustment; frequency adjustment--~Faraday
disc, or mercury motor ampere hour meter~: construction
and operation--~frequency indicators~: types: synchronous
motor, resonance, induction; synchronous motor as
frequency indicator--~resonance frequency indicators~:
adaptation--~induction frequency meter~: construction
and operation--~synchronism indicators~: types: lamp or
voltmeter, resonance or vibrating reed, rotating field--~power
factor indicators~: wattmeter type; disc, or rotating field
type--~ground detectors~.
CHAPTER LV
ALTERNATING CURRENT SYSTEMS
The facility with which alternating current can be transformed from
one voltage to another, thus permitting high pressure transmission of
electric energy to long distances through small wires, and low pressure
distribution for the operation of lighting systems and motors, gives a
far greater variety of systems of transmission and distribution than is
possible with direct current.
Furthermore, when the fact that two phase current can be readily
transformed into three phase current, and these converted into direct
current, and vice versa, by means of rotary converters and rectifiers,
is added to the advantages derived by the use of high tension
systems, it is apparent that the opportunity for elaboration becomes
almost unlimited. These conditions have naturally tended toward the
development of a great variety of systems, employing more or less
complicated circuits and apparatus, and although alternating current
practice is still much less definite than direct current work, certain
polyphase systems are now being generally accepted as representing the
highest standards of power generation, transmission and distribution.
A classification of the various alternating current systems, to be
comprehensive, should be made according to several points of view, as
follows:
1. With respect to the arrangement of the circuit, as
_a._ Series;
_b._ Parallel;
_c._ Series parallel;
_d._ Parallel series.
2. With respect to transformation, as
Transformer;
3. With respect to the mode of transmitting the energy, as
_a._ Constant pressure;
_b._ Constant current.
4. With respect to the kind of current, as
_a._ Single phase { two wire;
{ three wire;
_b._ Monocyclic
{ four wire;
_c._ Two phase { three wire;
{ five wire;
{ six wire;
{ three wire;
{ four wire;
_d._ Three phase { star connection;
{ delta connection;
{ star delta connection;
{ delta star connection;
_e._ Multi-phase { of more than
{ three phases;
5. With respect to transmission and distribution, as
_a._ Frequency changing;
_b._ Phase changing;
_c._ Converter;
_d._ Rectifier.
In order to comprehend the relative advantages of the various
alternating current systems, it is first necessary to understand the
principle of _vector summation_.
~Vector Summation.~--This is a simple geometrical process for
ascertaining the pressure at the free terminals of alternating current
circuits. The following laws should be carefully noted:
_1. If two alternating pressures which agree in phase are connected
together in series, the voltage at the free terminals of the circuit
will be equal to their arithmetical sum, as in the case of direct
currents._
[Illustration: FIG. 2,123.--_Vectors._ ~A vector is defined as~: _a
line, conceived to have both a fixed length and a fixed direction in
space, but no fixed position_. Thus A and B are lines, each having a
fixed length, but no fixed direction. ~By adding an arrow head~ the
direction is fixed and the line becomes a vector, as for example vector
C. The fixed length is usually taken to represent a definite force,
thus the fixed length of vector C is 4.7 which may be used to represent
4.7 lbs., 4.7 tons, etc., as may be arbitrarily assumed.]
When there is phase difference between the two alternating pressures,
connected in series, the following relation holds:
2. _The value of the terminal voltage will differ from their
arithmetical sum, depending on the amount of their phase difference._
When there is phase difference, the value of the resultant is
conveniently obtained as explained below.
~Ques. How are vector diagrams constructed for obtaining resultant
electric pressure?~
Ans. On the principle of the _parallelogram of forces_.
~Ques. What is understood by the parallelogram of forces?~
Ans. It is a graphical method of finding the resultant of two forces,
according to the following law: _If two forces acting on a point
be represented in direction and intensity by adjacent sides of a
parallelogram, their resultant will be represented by the diagonal of
the parallelogram which passes through the point._
[Illustration: FIG. 2,124.--Parallelogram of forces. OC is the
resultant of the two forces OA and OB. The length and direction of the
lines represent the intensity and direction of the respective forces,
the construction being explained in the accompanying text.]
Thus in fig. 2,124, let OA and OB represent the intensity
and direction of two forces acting at the point O, Draw AC
and BC, respectively parallel to OB and OA, completing the
parallelogram, then will OC, the diagonal from the point at
which the forces act, represent the intensity and direction of
the resultant, that is, of a force equivalent to the combined
action of the forces OA and OB, these forces being called the
_components_ of the force OC.
~Ques. Upon what does the magnitude of the resultant of two forces
depend?~
Ans. Upon the difference in directions in which they act, as shown in
figs 2,125 to 2,128.
~Ques. Is the parallelogram of forces applied when the difference in
direction or "phase difference" of two forces is 90 degrees?~
Ans. It is sometimes more conveniently done by calculation according to
the law of the right angle triangle.
[Illustration: FIGS. 2,125 to 2,128.--Parallelograms of forces
showing increase in magnitude of the resultant of two forces, as
their difference of direction, or electrically speaking, their _phase
difference_ is diminished. The diagrams show the growth of the
resultant of the two equal forces OA and OB as the phase difference is
reduced from 165° successively to 120, 60, and 15 degrees.]
According to this principle, if two alternating pressures
have a phase difference of 90 degrees they may be represented
in magnitude and direction by the two sides of a right angle
triangle as OA and OB in fig. 2,129; then will the hypotenuse
AB represent the magnitude and direction of the resultant
pressure. That is to say, the resultant pressure
AB = √((OA)² + (OB)²) (1)
EXAMPLE.--A two phase alternator is wound for 300 volts on one
phase and 200 volts on the other phase, the phase difference
being 90°. If one end of each winding were joined so as to
form a single winding around the armature, what would be the
resultant pressure?
By calculation, substituting the given values in equation (1),
Resultant pressure = √(300² + 200²) = √(130,000) = 360.6 volts.
This is easily done graphically as in fig. 2,129 by taking a
scale, say, 1" = 100 volts and laying off OA = 3" = 300 volts,
and at right angles OB = 2" = 200 volts, then by measurement
AB = 3.606" = 360.6 volts.
[Illustration: FIG. 2,129.--Method of obtaining the resultant of two
component pressures acting at right angles by solution of right angle
triangle. The equation of the right angle triangle is explained at
length in Guide No. 5, page 1,070.]
~Ques. When the two pressures are equal and the phase difference is
90°, is it necessary to use equation (1) to obtain the resultant?~
Ans. No. The resultant is obtained by simply multiplying one of the
pressures by 1.41.
This is evident from fig. 2,130. Here the two pressures OA
and OB are equal as indicated by the dotted arc. Since they
act at right angles, OB is drawn at 90° to OA. According
to the equation of the right angle triangle, the resultant
AB = √(1² + 1²) = √2̅ = 1.4142 which ordinarily is taken as
1.41.
_This value will always represent the ratio between the
magnitude of the resultant and the two component forces,
when the latter are equal, and have a phase difference of 90
degrees._
~Forms of Circuit.~--Alternating current systems of distribution may
be classed, with respect to the kind of circuit used, in a manner
similar to direct current systems, that is, they may be called series,
parallel, series parallel, or parallel series systems, as shown in
figs. 2,131 to 2,134.
[Illustration: FIG. 2,130.--Diagram for obtaining the resultant of two
equal component pressures acting at right angles.]
~Series Circuits.~--These are used in arc lighting, and series
incandescent lighting, a constant current being maintained; also for
constant current motors and generators supplying secondary circuits.
[Illustration: FIGS. 2,131 to 2,134.--Various forms of circuit. These
well known forms of circuit are used in both alternating and direct
current systems. The simple series circuit, fig. 2,131, is suitable
for constant current arc lighting. Fig. 2,132, shows the parallel
constant pressure circuit; this form of circuit is largely used but
is seldom connected direct to the alternator terminals, but to a step
down transformer, on account of the low pressure generally required.
Fig. 2,133 illustrates a parallel series circuit, and 2,134, a series
parallel circuit.]
Several forms of constant current alternator, analogous to the
Thompson-Houston and Brush series arc dynamos, have been introduced.
In the design of such alternators self-induction and armature reaction
are purposely exaggerated; so that the current does not increase very
much, even when the machine is short circuited. With this provision, no
regulating device is required.
[Illustration: FIG. 2,135.--Typical American overhead 6,600 volt single
phase interurban trolley line, Baltimore and Annapolis short line,
Annapolis, Md.]
An objectionable feature is that the voltage of a constant current
alternator will rise very high if the circuit be opened, because it is
then relieved of inductance drop and armature reaction.
To guard against a dangerous rise of voltage, a film cut out or
equivalent device is connected to the terminal of each machine so that
it will short circuit the latter if the voltage rise too high.
~Ques. What advantage have constant current alternators over constant
current dynamos?~
Ans. The high pressure current is delivered to the external circuit
without a commutator, hence there is no sparking difficulty.
The above relates to the revolving field type of alternator.
There are, however, alternators in which the armature revolves,
the current being delivered to the external circuit through
collector rings and brushes. This type of alternator, it should
be noted, is for moderate pressures, and moreover there is
no interruption to the flow of the current such as would be
occasioned by a tangential brush on a dynamo in passing from
one commutator segment to the next.
In the revolving field machine, though the armature current be
of very high pressure, the field current which passes through
the brushes and slip rings is of low pressure and accordingly
presents no transmission difficulties.
[Illustration: FIG. 2,136.--Diagram of ~parallel circuit~. _It is a
constant pressure circuit_ and is very widely used for lighting and
power. If each lamp takes say ½ ampere, the current flowing in the
circuit will vary with the number of lamps in operation; in the above
circuit with all lamps on, the current is ½ × 5 = 2½ amperes.]
~Ques. State a disadvantage.~
Ans. Some source of direct current for field excitation is required.
~Ques. In a constant current series system, upon what does the voltage
at the alternator depend?~
Ans. The number of devices connected in the circuit, the volts required
for each, and the line drop.
~Parallel Circuits.~--These are used for constant pressure operation.
Such arrangement provides a separate circuit for each unit making them
independent so that they may vary in size and each one can be started
or stopped without interfering with the others. Parallel circuits are
largely used for incandescent lighting, and since low pressure current
is commonly used on such circuits they are usually connected to step
down transformers, instead of direct to the alternators.
[Illustration: FIG. 2,137.--Diagram of ~parallel series circuit~,
_showing fall of pressure between units_. ~This system~ _is very rarely
used_; it has the disadvantage that if a lamp filament breaks, the
resistance of the circuit is altered and the strength of the current
changed. The voltmeter shows the fall of pressure along the line.
~It should be noted~ that, although the meter across AB is shown as
registering zero pressure, there is, strictly speaking, a slight
pressure across AB, in amount, being that required to overcome the
resistance of the conductor between A and B.]
~Parallel Series Circuits.~--Fig. 2,137 shows the arrangement of a
parallel series circuit and the pressure conditions in same. Such
a circuit consists of groups of two or more lamps or other devices
connected in parallel and these groups connected in series.
Such a circuit, when used for lighting, obviously has the disadvantage
that if a lamp filament breaks, the resistance of the group is
increased, thus reducing the current and decreasing the brilliancy of
the lamps. This arrangement accordingly does not admit of turning off
any of the lights.
~Series Parallel Circuits.~--The arrangement of circuits of this kind
is shown in fig. 2,134; they are used to economize in copper since by
joining groups of low pressure lamps in series they may be supplied by
current at correspondingly higher pressure.
Thus, if in fig. 2,134, 110 volt, ½ ampere lamps be used, the
pressure on the mains, that is, between any two points as A
and B would be 110 × 3 = 330 volts. Each group would require ½
ampere and the five groups ½ × 5 = 2½ amperes.
[Illustration: FIG. 2,138.--44,000 volt lines entering the Gastonia
sub-station of the Southern Power Co. The poles used are of the twin
circuit two arm type, built of structural steel, their height varying
from 45 to 80 feet, the latter weighing 9,000 pounds each. These poles
have their bases weighted with concrete.]
~Transformer Systems.~--Nearly all alternating current systems are
transformer systems, since the chief feature of alternating current
is the ease with which it may be transformed from one pressure to
another. Accordingly, considerable economy in copper may be effected by
transmitting the current at high pressure, especially if the distance
be great, and, by means of step down transformers, reducing the voltage
at points where the current is used or distributed.
Ordinarily and for lines of moderate length, current is sent out
direct from the alternator to the line and transformed by step down
transformers at the points of application.
With respect to the step down transformers, there are two arrangements:
1. Individual transformers;
2. One transformer for several customers.
[Illustration: FIG. 2,139.--Diagram of transformer system with
individual transformers. The efficiency is low, but such method of
distribution is necessary in sparsely settled or rural districts.]
Individual transformers, that is, a separate transformer for each
customer is necessary in rural districts where the intervening
distances are great as shown in fig. 2,139.
~Ques. What are the objections to this method of distribution?~
Ans. It requires the use of small transformers which are necessarily
less efficient and more expensive per kilowatt than large transformers.
The transformer must be built to carry, within its overload capacity,
all the lamps installed by the customer since all may be used
occasionally.
Usually, however, only a small part of the lamps are in use,
and those only for a small part of the day, so that the average
load on the transformer is a very small part of its capacity.
Since the core loss continues whether the transformer be loaded
or not, but is not paid for by the customer, the economy of the
arrangement is very low.
In the second case, where one large transformer may be placed
at a distribution center, to supply several customers, as in
fig. 2,140, the efficiency of the system is improved.
~Ques. Why is this arrangement more efficient than when individual
transformers are used?~
[Illustration: FIG. 2,140.--Diagram of transformer system with
one transformer located at a distribution center and supplying
several customers as A, B, and C. Such arrangement is considerably
more efficient than that shown in fig. 2,139, as explained in the
accompanying text.]
Ans. Less transformer capacity is required than with individual
transformers.
~Ques. Why is this?~
Ans. With several customers supplied from one transformer it is
extremely improbable that all the customers will burn all their lamps
at the same time. It is therefore unnecessary to install a transformer
capable of operating the full load, as is necessary with individual
transformers.
~Ques. Does the difference in transformer capacity represent all the
saving?~
Ans. No; one large transformer is more efficient than a number of small
transformers.
~Ques. Why?~
Ans. The core loss is less.
For instance, if four customers having 20 lamps each were
supplied from a single transformer, the average load would
be about 8 lamps, and at most not over 10 or 15 lamps, and a
transformer carrying 30 to 35 lamps at over load would probably
be sufficient. A 1,500 watt transformer would therefore be
larger than necessary. At 3 per cent. core loss, this gives a
constant loss of 45 watts, while the average load of 8 lamps
for 3 hours per day gives a useful output of 60 watts, or an
all year efficiency of nearly 60 per cent., while a 1,000 watt
transformer would give an all year efficiency of 67 per cent.
For long distance transmission lines, the voltage at the
alternator is increased by passing the current through a _step
up_ transformer, thus transmitting it at very high pressure,
and reducing the voltage at the points of distribution by step
down transformers as in fig. 2,141.
[Illustration: FIG. 2,141.--Diagram illustrating the use of _step up_
and _step down_ transformers on long distance transmission lines. The
saving in copper is considerable by employing extra high voltages on
lines of moderate or great length as indicated by the relative sizes of
wire.]
~Ques. In practice, would such a system as shown in fig. 2,141 be used?~
Ans. If the greatest economy in copper were aimed at, a three phase
system would be used.
The purpose of fig. 2,141 is to show the importance of the
transformer in giving a flexibility of voltage, by which the
cost of the line is reduced to a minimum.
~Ques. Does the saving indicated in fig. 2,141 represent a net gain?~
Ans. No. The reduction in cost of the transmission is partly offset by
the cost of the transformers as well as by transformer losses and the
higher insulation requirements.
[Illustration: FIG. 2,142.--Single and twin circuit poles (Southern
Power Co.). The twin circuit pole at the right is used for 11,000 volt
circuits, while the single circuit poles at the left carry 44,000 volt
conductors, being used on another division for 100,000 volt line.]
Every case of electric transmission presents its own problem,
and needs thorough engineering study to intelligently choose
the system best adapted for the particular case.
~Single Phase Systems.~--There are various arrangements for
transmission and distribution classed as single phase systems. Thus,
single phase current may be conveyed to the various receiving units
by the well known circuit arrangements known as series, parallel,
series parallel, parallel series, connections previously described and
illustrated in figs. 2,131 to 2,134.
Again single phase current may be transmitted by two wires and
distributed by three wires. This is done in several ways, the simplest
being shown in fig. 2,143.
[Illustration: FIG. 2,143.--Diagram illustrating single phase two wire
transmission and three wire distribution. The simplified three wire
arrangement at A, is not permissible except in cases of very little
_unbalancing_. Where the difference between loads on each side of the
neutral may be great some form of balancing as an auto-transformer or
equivalent should be used, as at B.]
~Ques. Under what conditions is the arrangement shown in fig. 2,143
desirable?~
Ans. This method of treating the neutral wire is only permissible where
there is very little unbalancing, that is, where the load is kept
practically the same on both sides of the neutral.
~Ques. What advantage is obtained by three wire distribution?~
Ans. The pressure at the alternator can be doubled, which means,
for a given number of lamps, that the current is reduced to half,
the permissible drop may be doubled, the resistance of the wires
quadrupled, and their cost reduced nearly 75 per cent.
[Illustration: FIG. 2,144.--100,000 volt "Milliken" towers with one
circuit strung (Southern Power Co.). These towers are mounted on metal
stubs sunk 6 feet in the ground. Where the angle of the line is over
15 degrees, however, these stubs are weighted with rock and concrete,
and where an angle of over 30 degrees occurs, two and sometimes three
towers are used for making the turn. The weight of the standard
"Milliken" tower is 3,080 lbs., and its height from the ground to peak
is 51 feet. The towers are spaced to average eight to a mile and a
strain tower weighing 4,250 lbs. is used every mile. For particularly
long spans a special heavy tower weighing 6,000 lbs. is used. The
circuits are transposed every 30 miles. Multiple disc insulators are
used, four discs being used to suspend each conductor from standard
towers and ten discs to each conductor on strain towers. The standard
span is 600 feet, sag 11 ft at 50° Fahr.]
~Ques. What modification of circuit A (fig. 2,143), should be made to
allow for unbalancing in the three wire circuit?~
Ans. An auto-transformer or "balance coil" as it is sometimes called
should be used as at B.
This is a very desirable method of balancing when the ratio of
transformation is not too large.
~Ques. For what service would the system shown in fig. 2,143 be
suitable?~
Ans. For short distance transmission, as for instance, in the case of
an isolated plant because of the low pressure at which the current is
generated.
The standard voltages of low pressure alternators are 400, 480,
and 600 volts.
[Illustration: FIG. 2,145.--View of a typical isolated plant. The
illustration represents an electric lighting plant on a farm showing
the lighting of the dwelling, barn, tool house and pump house. The
installation consists of a low voltage dynamo with gas engine drive and
storage battery together with the necessary auxiliary apparatus.]
~Ques. In practice are single phase alternators used as indicated in
fig. 2,143?~
Ans. Alternators are wound for one, two or three phases. Three phase
machines are more commonly supplied and in many cases it will pay to
install them in preference to single phase, even if they be operated
single phase temporarily.
For a given output, three phase machines are smaller than
single phase and the single phase load can usually be
approximately balanced between the three phases. Moreover, if
a three phase machine be installed, polyphase current will be
available in case it may be necessary to operate polyphase
motors at some future time.
Standard three phase alternators will carry about 70 per cent.
of their rated kilowatt output when operated single phase, with
the same temperature rise.
~Ques. How are three phase alternators used for single phase circuits?~
Ans. The single phase circuit is connected to any two of the three
phase terminal leads.
[Illustration: FIG. 2,146.--Diagram showing ~arrangement of single
phase system~ for two wire transmission and three wire distribution,
_where the transmission distance_ ~is considerable~. In order ~to
reduce the cost~ of the transmission line, _the current must be
transmitted at high pressure_; this necessitates the use of a step down
transformer at the distributing center as shown in the illustration.]
~Ques. What form of single phase system should be used where the
transmission distance is considerable?~
Ans. The current should be transmitted at high pressure, a step down
transformer being placed at each distribution center to reduce the
pressure to the proper voltage to suit the service requirements as
shown in fig. 2,146.
Thus, if 110 volt lamps be used on the three wire circuit, the
pressure between the two outer wires would be 220 volts. A
transformation ratio of say 10:1 would give 2,220 volts for the
primary circuit. The current required for the primary with this
ratio being only .1 that used in the secondary, a considerable
saving is effected in the cost of the transmission line as must
be evident.
With the high pressure alternator only one transformation of
the current is needed, as shown at the distribution end.
In place of the high pressure alternator, a low pressure
alternator could be used in connection with a step up
transformer as shown in fig. 2,147, but there would be an
extra loss due to the additional transformer, rendering the
system less efficient than the one shown in fig. 2,146. Such an
arrangement as shown in the fig. 2,147 might be justified in
the case of a station having a low pressure alternator already
in use and it should be desired to transmit a portion of the
energy a considerable distance.
~Ques. How could the system shown in fig. 2,147 be made more efficient
than that of fig. 2,146?~
Ans. By using a high pressure alternator in order to considerably
increase the transmission voltage.
Thus, a 2,200 volt alternator and 1:10 step up transformer
would give a line pressure of 22,000 volts, which at the
distribution end could be reduced, to 220 volts for the three
wire circuit, using a 100:1 step down transformation.
[Illustration: FIG. 2,147.--Diagram illustrating how electricity can
be economically transmitted a considerable distance with low pressure
alternator already in use.]
~Ques. Would this be the best arrangement?~
Ans. No.
~Ques. What system would be used in practice for maximum economy?~
Ans. Three phase four wire.
~Ques. What are the objections to single phase generation and
transmission?~
Ans. It does not permit of the use of synchronous converters,
self-starting synchronous motors, or induction motor starting under
load. It is poorly adapted to general power distribution, hence it is
open to grave objections of a commercial nature where there exists any
possibility of selling power or in any way utilizing it for general
converter and motor work.
[Illustration: FIG. 2,148.--Angle tower showing General Electric strain
insulators. The tower being subject to great torsional strains is
erected on a massive concrete foundation. The construction is similar
to the standard tower but of heavier material, and having the same
vertical dimensions but with bases 20 ft. square.]
~Ques. For what service is it desirable?~
Ans. For alternating current railway operation.
There are advantages of simplicity in the entire generating,
primary, and secondary distribution systems for single phase
roads. These advantages are so great that they justify
considerable expense, looked at from the railway point of view
only, the single phase system throughout may be considered as
offering the most advantage.
~Ques. What are the objectionable features of single phase alternators?~
Ans. This type of alternator has an unbalanced armature reaction which
is the cause of considerable flux variation in the field pole tips and
in fact throughout the field structure.
In order to minimize eddy currents, such alternators must accordingly
be built with thinner laminations and frequently poorer mechanical
construction, resulting in increased cost of the machine. The large
armature reaction results in a much poorer regulation than that
obtained with three phase alternators, and an increased amount of field
copper is required, also larger exciting units. These items augment the
cost so that the single phase machine is considerably more expensive
than the three phase, of the same output and heating.
[Illustration: FIG. 2,149.--Elementary alternator developing one volt
at frequencies of 60 and 25, ~showing the effect of reducing the
frequency~. Since for the same number of pole, the R.P.M. have to
be decreased to decrease the frequency, increased flux is required
to develop the same voltage. ~Hence in construction~, low frequency
machines require _larger magnets, increased number of turns in series
on the armature coils, larger exciting units_ as compared with machines
built for higher frequency.]
~Ques. What factor increases the difficulties of single phase
alternator construction?~
Ans. The difficulties appear to increase with a _decrease_ in frequency.
The adoption of any lower frequency than 25 cycles may result
in serious difficulties in construction for a complete line
of machine, especially those of the two or four pole turbine
driven type where the field flux is very large per pole.
~Monocyclic System.~--In this system, which is due to Steinmetz, the
alternator is of a special type. In construction, there is a ~main~
single phase winding an auxiliary or ~teaser~ winding connected to the
central point of the main winding in quadrature therewith.
The teaser coil generates a voltage equal to about 25 per cent. of that
of the main coil so that the pressure between the terminals of the main
coil and the free end of the teaser is the resultant of the pressure of
the two coils.
[Illustration: FIG. 2,150.--Diagram of monocyclic system, showing
lighting and power circuits.]
By various transformer connections it is possible to obtain a
practically correct three phase relationship so that polyphase motors
may be employed.
In this system, two wires leading from the ends of the single phase
winding in the alternator supply single phase current to the lighting
load, a third wire connected to the end of the teaser being run to
points where the polyphase motors are installed as shown in fig. 2,150.
The monocyclic system is described at length in the chapter on
alternators, Guide No. 5, pages, 1,156 to 1,159.
~Two Phase Systems.~--A two phase circuit is equivalent to two
single phase circuits. Either four or three wire may be employed in
transmitting two phase current, and even in the latter instance the
conditions are practically the same as for single phase transmission,
excepting the unequal current distribution in the three wires. Fig.
2,151 shows a two phase four wire system.
[Illustration: FIG. 2,151.--Diagram of two phase four wire system.
It is desirable for supplying current for lighting and power. The
arrangement here shown should be used only for lines of short or
moderate length, because of the low voltage. Motors should be connected
to a circuit separate from the lighting circuit to avoid drop on the
latter while starting a motor.]
~Ques. For what service is the system shown in fig. 2,151 desirable?~
Ans. It is adapted to supplying current for lighting and power at
moderate or short distances.
Either 110 or 220 volts are ordinarily used which is suitable
for incandescent lighting and for constant pressure arc lamps,
the lamps being connected singly or two in pairs.
~Ques. Where current for both power and light are obtained from the
same source how should the circuits be arranged?~
Ans. A separate circuit should be employed for each, in order to avoid
the objectionable drop and consequent dimming of the lights due to the
sudden rush of current during the starting of a motor.
[Illustration: FIG. 2,152.--Diagram of two phase three wire system. A
wire is connected to one end of each phase winding as at A and B, and a
third wire C, to the other end of both phases as shown.]
Disagreeable fluctuation of the lights are always met with
when motors are connected to a lighting circuit and the effect
is more marked with alternating current than with direct
current, because most types of alternating current motor
require a heavy current usually lagging considerably when
starting. This not only causes a large drop on the line, but
also reacts injuriously upon the regulation of transformers and
alternators, their voltage falling much more than with an equal
non-inductive load.
~Ques. What voltages are ordinarily used on two phase lines of more
than moderate length?~
Ans. For transmission distances of more than two or three miles,
pressures of from 1,000 to 2,000 volts or more are employed to
economize in copper. For long distance transmission of over fifty
miles, from 30,000 to 100,000 volts and over are used.
~Ques. For long distance transmission at 30,000 to 40,000 volts, what
additional apparatus is necessary?~
Ans. Step up and step down transformers.
[Illustration: FIG. 2,153.--Diagram illustrating two phase three wire
transmission. The third wire C is attached to the connector between one
end of phase A, and phase B windings.]
~Ques. Explain the method of transmitting two phase current with three
wires.~
Ans. The connections at the alternator are very simple as shown in fig.
2,152. One end of each phase winding is connected by the brushes _a_
and _b'_, to one of the circuit wires, that is to A and B respectively.
The other end of each phase winding is connected by a lead across
brushes _a'_ and _b_, to which the third wire C is joined.
The current and pressure conditions of this system are
represented diagrammatically in fig. 2,153. The letters
correspond to those in fig. 2,152, with which it should be
compared.
As shown in the figure each coil is carrying 100 amperes at
1,000 volts pressure. Since the phase difference between the
two coils is 90°, the voltage between A and B is √2̅ = 1.414
times that between either A or B and the common return wire C.
The current in C is √2̅ = 1.414 times that in either outside
wire A or B, as indicated.
~Ques. How should the load on the two phase three wire system be
distributed?~
Ans. The load on the two phases must be carefully balanced.
[Illustration: FIG. 2,154.--Diagram of two phase three wire system and
connections for motors and lighting circuits.]
~Ques. Why should the power factor be kept high?~
Ans. A high power factor should be maintained in order to keep the
voltage on the phases nearly the same at the receiving ends.
~Ques. How should single phase motors be connected and what precaution
should be taken?~
Ans. Single phase motors may be connected to either or both phases,
but in such cases, no load should be connected between the outer wires
otherwise the voltages on the different phases will be badly unbalanced.
Fig. 2,154 shows a two phase three wire system, with two
wire and three wire distribution circuits, illustrating the
connection for lighting and for one and two phase motors.
[Illustration: FIG. 2,155.--Diagram of two phase system with four wire
transmission and three wire distribution. In the three wire circuits
the relative pressures between conductors are as indicated; that is,
the pressure between the two outer wires A and B is 141 volts, when the
pressure between each outer wire and the central is 100 volts.]
~Ques. Describe another method of transmission and distribution with
two phase current.~
Ans. The current may be transmitted on a four wire circuit and
distributed on three wire circuit as in fig. 2,155.
The four wire transmission circuit is evidently equivalent to
two independent single phase circuits.
In changing from four to three wires, it is just as well to
connect the two outside wires A and B together (fig. 2,152), as
it is to connect _a´_ and _b_. It makes no difference which two
secondary wires are joined together, so long as the other wires
of each transformer are connected to the outside wires of the
secondary system.
~Ques. For what service is the two phase three wire system adapted?~
Ans. It is desirable for supplying current of minimum pressure to
apparatus in the vicinity of transformers. It is more frequently
used in connection with motors operating from the secondaries of the
transformers.
~Ques. How should the third or common return wire be proportioned?~
Ans. Since the current in the common return wire is 41.4 per
cent. higher than that in either of the other wires it must be of
correspondingly larger cross section, to keep the loss equal.
[Illustration: FIGS. 2,156 and 2,157.--Conventional diagrams
illustrating star and delta connected three phase alternator armatures.]
~Ques. What is the effect of an inductive load on the two phase three
wire system and why?~
Ans. It causes an unbalancing of both sides of the system even though
the energy load be equally divided. The self-induction pressure in one
side of the system is in phase with the virtual pressure in the other
side, thus distorting the current distribution in both circuits.
~Ques. Describe the two phase five wire system.~
Ans. A two phase circuit may be changed from four to five wires by
arranging the transformer connections as in fig. 2,158.
As shown, the secondaries of the transformers are joined in
series and leads brought out from the middle point of each
secondary winding and at the connection of the two windings,
giving five wires.
With 1,000 volts in the primary windings and a step down ratio
of 10:1, the pressure between A and C and C and E will be 100
volts and between the points and the connections B or D at the
middle of the secondary coils, 50 volts.
The pressure across the two outer wires A and E is, as in the
three wire system, √2̅ or 1.41 times that from either outer
wire to the middle wire C, that is 141 volts.
The pressure across the two wires connected to the middle of
the coils, that is, across B and D, is 50 × √2̅ = 70.5 volts.
[Illustration: Fig. 2,158.--Two phase four wire transmission and five
wire distribution system. The relative pressures between the various
conductors are indicated in the diagram.]
~Three Phase Systems.~--There are various ways of arranging the circuit
for three phase current giving numerous three phase systems.
1. With respect to the number of wires used they may be classified as
_a._ Six wire;
_b._ Four wire;
_c._ Three wire.
[Illustration: FIG. 2,159.--Line connections of three phase three
wire long distance transmission, and distribution system. The three
phase alternator A, is driven by the water wheel B, and furnishes
current at say 2,200 volts plus sufficient pressure to compensate
for line drop. With 1:10 step up transformers C, this would give a
transmission pressure of 22,000 volts plus line drop. _It is_ ~this
transformation~ _that secures the_ ~copper economy~ _of the system_.
At the distribution end are the step down transformers; one set
reducing the voltage down to 2,200 volts, and supplying current direct
to the synchronous motor, and through another set of other step down
transformers, as L and K, to lighting and power circuits at 220 volts.
Another set of step down transformers M reduce the pressure directly to
120 volts for power and lighting, the pressure being regulated by the
regulators G. Arc lamps with individual transformers further reducing
the pressure to 50 volts are connected to this circuit as shown.]
2. With respect to the connections, as
_a._ Star;
_b._ Delta;
_c._ Star delta;
_d._ Delta star.
The six wire system is shown in fig. 2,160. It is equivalent to three
independent single phase circuits. Such arrangement would only be used
in very rare instances.
[Illustration: FIG. 2,160.--Three phase six wire system. It is
equivalent to three independent single phase circuits and would be used
only in very rare cases.]
~Ques. How can three phase current be transmitted by three conductors?~
Ans. The arrangement shown in fig. 2,160 may be resolved into three
single circuits with a common or grounded return.
When the circuits are balanced the sum of the current being
zero no current will flow in the return conductor, and it
may be dispensed with, thus giving the ordinary star or ~Y~
connected three wire circuit, as shown in fig. 2,163. The
transformation from six to three wires being shown in figs.
2,161 to 2,163.
[Illustration: FIGS. 2,161 to 2,163.--Evolution of the three phase
three wire system. Fig. 2,161 is a conventional diagram of the three
phase six wire system shown in fig. 2,160. A wire is connected to both
ends of each phase winding, giving six conductors, or three independent
two wire circuits. In place of the wires running from A, B, and C,
they may be removed and each circuit provided with a _ground_ return
as shown in fig. 2,162. The sum of the three currents being zero, or
nearly zero, according to the degree of unbalancing, the ground return
may be eliminated and the ends A, B, and C of the three phase winding
connected, as in fig. 2,163, giving the so called _star point_.]
Fig. 2,166 is a view of an elementary three phase three wire
star connected alternator.
~Ques. What are the pressure and current relations of the star
connected three wire system?~
[Illustration: FIGS. 2,164 and 2,165.--Three phase ~four wire~ _star
connected_ alternator and conventional diagram showing pressure and
current relations.]
[Illustration: FIGS. 2,166 and 2,167.--Three phase ~star connected~
alternator, and conventional diagram showing pressure relations.]
Ans. These are shown in the diagram, fig. 2,166 and 2,167.
Assuming 100 amperes and 1,000 volts in each phase winding, the
pressure between any two conductors is equal to the pressure in
one winding multiplied by √3̅, that is 1,000 × 1.732 = 1,732
volts.
The current in each conductor is equal to the current in the
winding, or 100 amperes.
~Ques. Describe the delta connection.~
Ans. In the delta connection, the three phase coils are connected
together forming an endless winding, leads being brought out from these
points.
Fig. 2,168 shows a delta connected three phase alternator, the
pressure and current relation being given in fig. 2,169.
[Illustration: FIGS. 2,168 and 2,169.--Three phase ~delta connected~
alternator and conventional diagram showing pressure and current
relations.]
~Ques. What are the pressure and current relations of the delta
connected three wire system?~
Ans. They are as shown in fig. 2,169.
Assuming 100 amperes and 1,000 volts in each phase winding,
the pressure between any two conductors is the same as the
pressure in the winding, and the current in any conductor is
equal to the current in the winding multiplied by √3̅, that
is 100 × 1.732 = 173.2 amperes, that is, disregarding the
fraction, 173 amperes.
~Ques. What are the relative merits of the star and delta connections?~
Ans. The power output of each is the same, but the star connection
gives a higher line voltage, hence smaller conductors may be used.
[Illustration: FIG. 2,170.--~T~ connection of transformers in which
three phase current is transformed with two transformers. The
connections are clearly shown in the illustration. The voltage across
one transformer is only 86.6% of that across the other, so that if each
transformer be designed especially for its work one will have a rating
of .866 EI and the other EI. The combined rates will then be 1.866 as
compared with 1.732 EI for three single phase transformers connected
either star or delta.]
When it is remembered that the cost of copper conductors is
inversely as the square of the voltage, the advantage of the
~Y~ connected system can be seen at once.
Assuming that three transformers are used for a three phase
system of given voltage, each transformer, star connected,
would be wound for 1 ÷ √3̅ = 58% of the given voltage, and for
full current.
For delta connection, the winding of each transformer is for
58% of the current. Accordingly the turns required for star
connection are only 58% of those required for delta connection.
~Ques. What is the objection to the star connection for three phase
work?~
Ans. It requires the use of three transformers, and if anything happen
to one, the entire set is disabled.
~Ques. Does this defect exist with the delta connection?~
Ans. No.
One transformer may be cut out and the other two operated at
full capacity, that is at ⅔ the capacity of the three.
~Ques. Describe the ~T~ connection.~
Ans. In this method two transformers are used for transforming three
phase current. It consists in connecting one end of both windings of
one transformer to the middle point of like windings of the other
transformer as in fig. 2,170.
[Illustration: FIG. 2,171.--Open delta connection or method of
connecting two transformers in delta for three phase transformation. It
is used when one of the three single phase delta connected transformers
becomes disabled.]
~Ques. What is the open delta connection?~
Ans. It is a method of arranging the connections of a bank of three
delta connected transformers when one becomes disabled as in fig. 2,171.
~Change of Frequency.~--There are numerous instances where it is
desirable to change from one frequency to another, as for instance to
join two systems of different frequency which may supply the same or
adjacent territory, or, in the case of a low frequency installation,
in order to operate incandescent lights satisfactorily it would be
desirable to increase the frequency for such circuits. This is done
by motor generator sets, the motor taking its current from the low
frequency circuit.
Synchronous motors are generally used for such service as the frequency
is not disturbed by load changes; it also makes it possible to use the
set in the reverse order, that is, taking power from the high frequency
mains and delivering energy at low frequency.
[Illustration: FIG. 2,172.--Course of the Schaghticoke-Schenectady
transmission line of the Schenectady Power Co. This transmission line
carries practically the entire output of the Schaghticoke power house
to Schenectady, N. Y., a distance of approximately 21 miles. The line
consists of two separate three phase, 40 cycle, 32,000 volt circuits,
each of 6,000 kw. normal capacity. These circuits start from opposite
ends of the power house, and, after crossing the Hoosic River, are
transferred by means of two terminal towers, fig. 2,173, to a single
line of transmission towers. The two circuits are carried on these on
opposite ends of the cross arms, the three phases being superimposed.
The power house ends of the line are held by six short quadrangular
steel lattice work anchor poles with their bases firmly embedded in
concrete, the cables being dead ended by General Electric disc strain
insulators. This equipment, together with the lightning arrester horn
gaps and the heavy line outlet insulators mounted on the roof of the
power house, is shown in fig. 2,174. While each circuit carries only
6,000 kw. under normal conditions, either is capable of carrying the
entire output of the station; in this case, however, the line losses
are necessarily augmented. This feature prevents any interruption
of the service from the failure of one of the circuits. There are
altogether 197 transmission towers, comprising several distinct types.]
~Ques. In the parallel operations of frequency changing sets what is
necessary to secure equal division of the load?~
Ans. The relative angular position of the rotating elements of motor
and generator must be the same respectively in each set.
[Illustration: FIG. 2,173.--Beginning of Schaghticoke-Schenectady
transmission line; view showing transfer towers with power house in
background.]
~Ques. How is this obtained?~
Ans. Because of the mechanical difficulty of accurately locating the
parts, the equivalent result is secured by arranging the stationary
element in one of the two machines so that it can be given a small
angular shift.
~Transformation of Phases.~--In alternating current circuits it is
frequently desirable to change from one number of phases to another.
For instance, in the case of a converter, it is less expensive and more
efficient to use one built for six phases than for either two or three
phases.
[Illustration: FIG. 2,174.--View from roof of power house of the
Schaghticoke-Schenectady transmission line, showing anchor poles,
strain insulators, lightning arrester horn gap and line entrance
bushings.]
The numerous conditions met with necessitate various phase
transformations, as
1. Three phase to one phase;
2. Three phase to two phase;
3. Two phase to six phase;
4. Three phase to six phase.
These transformations are accomplished by the numerous arrangements and
combinations of the transformers.
[Illustration: FIG. 2,175.--Three phase to one phase transformation
with two transformers. The diagram shows the necessary connections and
the relative pressures obtained.]
~Three Phase to One Phase.~--This transformation may be accomplished
by the use of two transformers connected as in fig. 2,175 in which one
end of one primary winding is connected to the middle of the other
primary winding and the second end of the first primary winding at a
point giving 86.6 per cent. of that winding as shown. The two secondary
windings are joined in series.
~Three Phase to Two Phase.~--The three phase system is universally used
for long distance transmission, because it requires less copper than
either the single or two phase systems. For distribution, however,
the two phase system presents certain advantages, thus, it becomes
desirable at the distribution centers to change from three phase to two
phase. This may be done in several ways.
~Ques. Describe the Scott connection.~
[Illustration: FIG. 2,176.--The Scott connection for transforming from
three phase to two phase. In this method one of the primary wires B
of the .866 ratio transformer is connected to the middle of the other
primary as at C, the ends of which are connected to two of the three
phase wires. The other phase wire is connected at D, the point giving
the .866 ratio. The secondary wires are connected as shown.]
Ans. Two transformers are used, one having a 10:1 ratio, and the other,
a ½√3̅:1, that is, an 8.66:1 ratio. The connections are arranged as in
fig. 2,176.
It is customary to employ standard transformers having the
ratios 10:1, and 9:1.
~Ques. What names are given to the two transformers?~
Ans. The one having the 10:1 ratio is called the ~main~ transformer,
and the other with the 8.66:1 ratio, the ~teaser~ transformer.
In construction, the transformers may be made exactly alike so
that either may be used as main or teaser.
In order that the connections may be properly and conveniently
made, the primary windings should be provided with 50% and
86.6% taps.
[Illustration: FIG. 2,177.--Three phase to two phase transformation
with three star connected transformers. Two of the secondary windings
are tapped at points corresponding to 57.7% of full voltage; these two
windings are connected in series to form one secondary phase of voltage
equal to that obtained by the other full secondary winding.]
~Ques. Describe another way of transforming from three to two phases.~
Ans. The transformation may be made by three star connected
transformers, proportioning the windings as in fig. 2,177, from which
it will be seen that two of the secondary windings are tapped at points
corresponding to 57.7 per cent. of full voltage.
~Three Phase to Six Phase.~--This transformation is usually made for
use with rotary converters and may be accomplished in several ways. As
these methods have been illustrated in the chapter on Converters (page
1,462), it is unnecessary to again discuss them here. Fig. 2,178, below
shows the _diametrical_ connection for transforming three phase to six
phase.
[Illustration: FIG. 2,178.--Diagram of ~diametrical connection~,
_three phase to six phase_. ~It is obtained~ _by bringing both ends
of each secondary winding to opposite points on the rotary converter
winding_, utilizing the converter winding to give the six phases. This
transformation of phases may also be obtained with transformers having
two secondary windings.]
~Alternating Current Systems.~--The saving in the cost of transmission
obtained by using alternating instead of direct current is not due
to any difference in the characteristics of the currents themselves,
but to the fact that in the case of alternating current very high
pressures may be employed, thus permitting a given amount of energy to
be transmitted with a relatively small current.
In the case of direct current systems, commutator troubles limit the
transmission pressure to about 1,000 volts, whereas with alternating
current it may be commercially generated at pressures up to about
13,000 and by means of step up transformers, transmitted at 110,000
volts or more.
[Illustration: FIG. 2,179.--End of Schaghticoke-Schenectady
transmission line at Schenectady; view showing entrance bushings and
lightning arrester horn gaps.]
~Relative Weights of Copper Required by Polyphase Systems.~--A
comparison between the weights of copper required by the different
alternating current systems is rendered quite difficult by the fact
that the voltage ordinarily measured is not the maximum voltage, and as
the insulation has to withstand the strain of the maximum voltage, the
relative value of copper obtained by calculation depends upon the basis
of comparison adopted.
As a general rule, the highest voltage practicable is used for long
distance transmission, and a lower voltage for local distribution.
Furthermore, some polyphase systems give a multiplicity of voltages,
and the question arises as to which of these voltages shall be
considered the transmission voltage.
If the transmission voltage be taken to represent that of the
distribution circuit, and the polyphase system has as many independent
circuits as there are phases, the system would represent a group of
several single phase systems, and there would be no saving of copper.
Under these conditions, if the voltage at the distant end be taken as
the transmission voltage, and the copper required by a single phase
two wire system as shown in fig. 2,180, be taken as the basis of
comparison, the relative weights of copper required by the various
polyphase systems is given in figs. 2,181 to 2,188.
[Illustration: FIG. 2,180.--Single phase line, used as basis of
comparison in obtaining the relative weights of copper required by
polyphase systems, as indicated in figs. 2,181 to 2,188.]
In the case represented in fig. 2,180, if the total drop on the line
be 100 volts, the generated voltage must be 1,100 volts, and the
resistance of each line must be 50 ÷ 1,000 = .05 ohms. Calculated on
this basis, a two phase four wire system is equivalent to two single
phase systems and gives no economy of copper in power transmission over
the ordinary single phase two wire system. This is the case also with
any of the other two phase systems, except the two phase three wire
system.
[Illustration: FIGS. 2,181 to 2,188.--Circuit diagrams showing relative
copper economy of various alternating current systems.]
In this system two of the four wires of the four wire two phase
system are replaced by one of full cross section.
The amount of copper required, when compared with the single
phase system, will differ considerably according as the
comparison is based on the highest voltage permissible for any
given distribution, or on the minimum voltage for low pressure
service.
If E be the greatest voltage that can be used on account of
the insulation strain, or for any other reason, the pressure
between the other conductors of the two phase three wire system
must be reduced to E ÷ √2̅.
The weight of copper required under this condition is 145.7%
that of the single phase copper.
On the basis of minimum voltage, the relative amount of copper
required is 72.9% that of the single phase system.
[Illustration: FIG. 2,189.--Twin circuit "aermotor" towers carrying
44,000 volt conductors (Southern Power Co.). These towers vary in
height from 35 to 50 feet, and the circuits are transposed every 10
miles. The towers are assembled on the ground and erected by means of
gin poles. They are normally spaced 500 feet apart with a sag of 5
feet 8 inches. The minimum distance between towers is 300 feet and the
maximum 700 feet.]
Figs. 2,187 and 2,188 are two examples of three phase four wire
systems. The relative amount of copper required as compared
with the single phase system depends on the cross section of
the fourth wire. The arrangement shown in fig. 2,188, where
the fourth wire is only half size, is used only for secondary
distribution systems.
[Illustration: FIG. 2,190.--General Electric standard tower for high
tension three phase transmission line.]
[Illustration: FIG. 2,191.--General Electric transposition tower for
high tension three phase transmission line.]
~Choice of Voltage.~--In order to properly determine the voltage for
a transmission system there are a number of conditions which must be
considered in order that the economy of the entire installation shall
be a maximum.
The nature of the diversely various factors which affect the problem
makes a mathematical expression difficult and unsatisfactory.
~Ques. What is the relation between the cross sectional area of the
conductors and the voltage?~
Ans. For a given circuit, the cross sectional area of the conductors,
or weight varies inversely as the voltage.
[Illustration: FIG. 2,192.--General Electric standard tower under
construction.]
~Ques. Would the highest possible voltage then be used for a
transmission line?~
Ans. The most economical voltage depends on the length of the line and
the cost of apparatus.
For instance, alternators, transformers, insulation and circuit
control and lightning protection devices become expensive when
manufactured for very high pressures. Hence if a very high
pressure were used, it would involve that the transmission
distance be great enough so that the extra cost of the high
pressure apparatus would be offset by the saving in copper
effected by using the high pressure.
In the case of the longest lines, from about 100 miles up,
the saving in copper with the highest practicable voltage
is so great that the increase in other expenses is rendered
comparatively small.
In the shorter lines as those ranging in length from about
one mile to 50 or 75 miles, the most suitable voltage must be
determined in each individual case by a careful consideration
of all the conditions involved. No fixed rule can be
established for proper voltage based on the length, but the
following table will serve as a guide:
[Illustration: FIG. 2,193.--Line of the Schenectady Power Company
crossing the tracks of the Boston and Maine Railroad near Schaghticoke.]
Usual Transmission Voltages
+----------------------------------------+
| Length of line | Voltage |
| in miles | |
+------------------+---------------------+
| 1 | 500 to 1,000 |
| 1 to 2 | 1,000 to 2,300 |
| 2 to 3 | 2,300 to 6,600 |
| 3 to 10 | 6,600 to 13,200 |
| 10 to 15 | 13,200 to 22,000 |
| 15 to 20 | 22,000 to 44,000 |
| 20 to 40 | 44,000 to 66,000 |
| 40 to 60 | 66,000 to 88,000 |
| 60 to 100 | 88,000 to 110,000 |
+------------------+---------------------+
~Ques. What are the standard voltages for alternating current
transmission circuits?~
Ans. 6,600, 11,000, 22,000, 33,000, 44,000, 66,000, 88,000.
The amount of power to be transmitted determines, in a
measure, the limit of line voltage. If the most economical
voltage considered from the point of view of the line alone,
be somewhere in excess of 13,200, step up transformers must
be employed, since the highest voltage for which standard
alternators are manufactured is 13,200. In a given case, the
saving in conductor by using the higher voltage may be more
than offset by the increased cost of transformers, and the
question must be determined for each case.
[Illustration: FIG. 2,194.--View of a three phase, 2,300 volt, 60 cycle
line at Chazy, N. Y. The current is transmitted at the alternator
voltage 2¾ miles over the single circuit pole line. The poles are of
cedar with fir cross arms, and are fitted with pin insulators. They are
from 35 to 40 feet high and are spaced at an average of about 120 feet.
The conductors are bare copper wire No. 00 B. & S. The alternators
consist of one 50 kw., and one 100 kw. General Electric machines.]
~Ques. What are the standard transformer ratios?~
Ans. Multiples of 5 or 10.
[Illustration: FIGS. 2,195 to 2,197.--Diagram showing electric railway
system. Three phase current is generated at the main station where it
passes to ~step up~ transformers to increase the pressure a suitable
amount for economical transmission. At various points along the railway
line are _sub-stations_, where the three phase current is reduced in
pressure to 500 or 600 volts by ~step down~ transformers, and converted
into direct current by rotary converters. The relatively low pressure
direct current is then conveyed by "feeders" to the rails, this
resulting in a considerable saving in copper.]
~Mixed Current Systems.~--It is often desirable to transmit electrical
energy in the form of alternating current, and distribute it as direct
current or vice versa.
Such systems may be classed as mixed current systems. The usual
conversion is from alternating current to direct current because of
the saving in copper secured by the use of alternating current in
transmission, especially in the case of long distance lines. Such
conversion involves the use of a rotary converter, motor generator set,
or rectifier, according to the conditions of service.
[Illustration: FIG. 2,198.--Example of converter sub-station, showing
the Brooklyn Edison Co. Madison sub-station. The transformers are
seen on the left, the converter shown at the right is a Westinghouse
synchronous booster rotary converter, consisting of a standard rotary
converter in combination with a revolving armature alternator mounted
on the same shaft with the converter and having the same number of
poles. _The function of the machine is to_ ~convert~ and ~regulate~
_the pressure_. By varying the field excitation of the alternator, the
A. C. voltage impressed on the rotary converter proper can be increased
or decreased as desired. Thus, the D. C. voltage delivered by the
converter is varied accordingly. This type of converter is well adapted
for any application for which a relatively wide variation, either
automatic or non-automatic, in direct current voltage is necessary.
Also especially for serving incandescent lighting systems where
considerable voltage variation is required for the compensation of drop
in long feeders, for operation in parallel with storage batteries and
for electrolytic work where extreme variations in voltage are required
by changes in the resistance of the electrolytic cells.]
The suburban trolley forms a good example of a mixed system, in
which alternating current is generated at the _central station_ and
transmitted to _sub-stations_, where it is transformed to low pressure,
and converted into direct current for use on the line. Fig. 2,195 shows
the interior of a sub-station of this kind.
~Ques. What direct current pressure is usually employed on traction
lines?~
Ans. 500 volts.
~Ques. Mention another important service performed by a mixed system.~
Ans. If the generator furnish alternating current it must be converted
into direct current in order to charge storage batteries.
CHAPTER LVI
AUXILIARY APPARATUS
For the proper control of the alternating current in any of the
numerous systems described in the previous chapter, various devices,
which might be classed as "auxiliary apparatus," are required. These
may be grouped into several divisions, according to the nature of the
duty which they perform, as
1. Switching devices;
_a._ Ordinary switches;
_b._ Oil break switches;
_c._ Remote control switches.
2. Current or pressure limiting devices;
_a._ Fuses;
_b._ Reactances;
_c._ Circuit breakers;
_d._ Relays.
3. Lightning protection devices;
_a._ Air gap arresters;
_b._ Multi-gap arresters;
_c._ Horn gap arresters;
_d._ Electrolytic arresters;
_e._ Vacuum tube arresters;
_f._ Choke coils;
_g._ "Static" interrupters.
4. Regulating devices;
_a._ Induction voltage regulators;
_b._ Variable ratio transformer regulators { drum type;
{ dial type;
_c._ Compensation shunts;
_d._ Pole type regulators;
_e._ Small feeder voltage regulators;
_f._ Automatic voltage regulators;
_g._ Line drop compensators;
_h._ Starting compensators;
_i._ Star delta switches.
5. Power factor regulating devices;
_a._ Condensers;
_b._ Synchronous condensers.
6. Indicating devices;
{ plunger type;
_a._ Moving iron instruments { inclined coil type;
{ magnetic vane type;
_b._ Hot wire instruments;
_c._ Induction instruments { shielded pole type;
{ repulsion type;
_d._ Dynamometers;
_e._ Instrument transformers;
{ commutator type;
_f._ Watthour meters { induction type;
{ Faraday disc type;
{ synchronous motor type;
_g._ Frequency indicators { resonance type;
{ induction type;
{ lamp type;
_h._ Synchronism indicators { voltmeter type;
{ resonance type;
{ rotating field type;
_i._ Power factor indicators { wattmeter type;
{ rotating field type;
_j._ Ground detectors;
_k._ Earth leakage cut outs;
_l._ Oscillographs.
CHAPTER LVII
SWITCHING DEVICES
A switch is _a piece of apparatus for making, breaking, or changing the
connections in an electric circuit_.
The particular form and construction of any switch is governed by the
electrical conditions under which it must operate.
_Since the electric current cannot be_ ~stopped instantly~ _when the
circuit in which it is flowing, is broken_, ~an arc is formed~ _as the
switch contacts separate_; this tends to burn the contacts, and to
short circuit, the severity of such action depending on the voltage and
the proximity of the switch terminals. Accordingly in switch design,
provision must be made to counteract these tendencies. Thus,
1. The contacts should separate along their entire length, rather than
at a point;
2. The terminals should be far enough apart and properly protected to
prevent short circuiting of the arcs;
3. The break should be quick;
4. The gap should be surrounded by the proper medium (air or oil) to
meet the requirements of the electrical conditions.
A great variety of switches have been introduced to suit the different
requirements. Knife switches are used for low pressure service, the
multiple break form being used where it is desired to reduce the arcing
distance.
~Ques. How should single throw switches be installed?~
Ans. They should open downward so gravity will keep them open.
[Illustration: FIGS. 2,199 and 2,200.--General Electric triple pole
solenoid operated, single throw remote control switch, and push button
switch for operating same. Switch is a self-contained unit with two
sets of contacts, main laminated copper brushes, and carbon auxiliary
contacts to take the arc on breaking the circuit. The main brushes are
so made that each lamination makes an end on contact with the switch
blade without any tendency to force the laminations apart. A wiping
effect, given to the contacts every time the switch is closed, keeps
the contact surfaces clean and insures good contact at all times.
The carbon auxiliary contacts are made of blocks of carbon fastened
without screws. ~In operation~, the switch is actuated by a double coil
solenoid, one coil for closing and one for opening, controlled by the
single pole double throw push button switch shown in fig. 2,200, which
is normally in the open position and remains closed only when held by
the operator. One of these switches is furnished with each control
switch and must always be used, as the solenoid coils are not intended
for continuous service. The power required to operate the remote
control switch is small, being approximately 1.6 amperes at 110 volts,
0.81 amperes at 220 volts direct current, and 10 amperes at 110 volts,
and 6 amperes at 220 volts alternating current 60 cycles. The main
switch can be closed and opened by hand, and the push button located at
any point.]
~Ques. How should double throw switches be installed?~
Ans. Horizontally.
[Illustration: FIGS. 2,201 and 2,202.--Palmer service switch and
fuse box, for either plug, cartridge or open link fuses. Fig. 2,201
illustrates the box in open position for the inspection of fuses, etc.
The cover is held open by a simple lock so that the switch cannot
fall closed by gravity, the box may be mounted so that the service
wires lead directly into a sealed terminal chamber from any direction,
and all current carrying parts made accessible by the opening of the
switch are dead. Fig. 2,202 illustrates the device with side of box
and cover cut away to show interior and the normally sealed cover
of terminal chamber removed. The switch contacts do not enter their
contact clips until the flanged cover of the box has closed the switch
opening, no current connections being made to line or load until the
box is completely closed, and in consequence there is no opportunity
to make improper connections to any live parts of switch, when conduit
connections are used to the service and meter wires.]
~Ques. What is a plug switch?~
Ans. A switch in which the current is ruptured in a tube enclosed at
one end, thereby confining the arc and limiting the supply of air.
They are used on high pressure circuits of from 10,000 to
20,000 volts, for transferring live circuits and for voltmeter
and synchronizing circuits where there is very little energy.
The usual current capacity is from 4 to 7½ amperes.
[Illustration: FIG. 2,203 and 2,204.--~Bus transfer plug switch.~ The
method of supporting the contact farthest from the panel consists
of a porcelain pillar of the same height as the receptacle, clamped
to a brass connecting or bus bar which in turn is fastened to the
receptacle.]
[Illustration: FIG. 2,205.--~Ammeter jack.~ This plug switch is
insulated for high pressure and ~consists of two parts~: _the ammeter
jack, and the ammeter jack plug, cable, and bushing_. The receptacle,
which is simple in construction, consists of a brass bushing well
insulated from the panel and protected on the front of the panel by a
porcelain bushing. ~On the end of this tube~ and insulated from it,
is a phosphor bronze spring which, when the plug is out, rests on the
brass tube and keeps the circuit closed. ~The plug~ consists of a
brass rod well insulated and set in a brass tube, both being fastened
in a handle which is stained black and polished. Inside the handle
is run a twin conductor cable, one side being soldered in the brass
tube and the other to the brass rod. The other end of the cable is run
through a bushing set in the panel and thence to the ammeter or current
transformer. Where it is desired to remove the plug and cable from the
board, or to plug both ends of the cable in different receptacles, a
plug instead of a bushing should be used. In this case a cable should
be provided with a plug on each end.]
~Forms of Break.~--On high pressure circuits there are several types of
switch: they are classified with respect to the break, that is to say,
according as the break takes place,
1. In open air;
2. In an enclosed air space;
3. Aided by a metal fuse;
4. Aided by a horn;
5. In oil.
[Illustration: FIG. 2,206.--Westinghouse fused starting switch for
squirrel cage motors. It is arranged for National Electric Code fuses
on one end only and has springs on the other end to open the switch
automatically if left closed at this end. The corresponding terminals
at both ends of the switch are connected in grooves in the back of the
slate base so that the wiring need be connected to one set of these
terminals only, thus decreasing the number of connections necessary,
as shown in fig. 2,207. In starting an induction motor, the switch is
thrown to the end that is not fused and held there until the motor is
up to running speed; then it is quickly thrown to the fused position,
thus protecting the circuit under running conditions.]
[Illustration: FIG. 2,207.--Diagram of connections of Westinghouse
fused starting switch for squirrel cage motors. The starting current of
induction motors is several times the normal running current and, when
the controlling switch is fused to carry the running load only, the
fuses are apt to blow when the motor is started. The fuses must be of a
capacity to prevent overloads under running conditions. These switches
are designed to meet this difficulty and are used without auto-starters
to control motors up to 5 horse power rating.]
~Ques. What is the objection to open air break?~
Ans. The relatively long gap required to extinguish the arc, limiting
this form of switch to low or moderate pressure circuits.
The open air arc may cause very high voltage oscillations when
the circuit contains inductance and capacity unless the break
occur at zero value.
[Illustration: FIG. 2,208.--Westinghouse single pole disconnecting
switch. Disconnecting switches are used primarily for isolating
apparatus from the circuit for purposes of inspection and repair; also
for sectionalizing feeders. They are not designed for opening under
load, and therefore no attempt should be made to open them with current
in the circuit. In connection with lightning arrester installations,
disconnecting switches are particularly useful, providing a simple
and effective means for isolating the arresters while cleaning and
inspecting. The switch is opened and closed with a hook on the end of a
wooden pole, which hook engages in a hole provided in the switch blade.
This type of disconnecting switch is intended for wall mounting. The
live parts are mounted on porcelain insulators carried on a cast iron
yoke or base, forming a simple and substantial construction.]
~Ques. What are disconnecting switches?~
Ans. Knife switches in series with other switches so that the apparatus
controlled by the latter may be repaired in safety by entirely
disconnecting it from the bus bars or live circuit.
Such switches are not intended to rupture the load current.
[Illustration: FIGS. 2,209 and 2,210.--Westinghouse disconnecting
switches for pressures over 3,300 volts.]
[Illustration: FIGS. 2,211 and 2,212.--Westinghouse selector type
disconnecting switch. Fig. 2,211, view showing both sides closed; fig.
2,212, view with one side open. The selector type of disconnecting
switch is a transfer switch which does not require the circuit to be
interrupted while making the change. It can also be used to connect two
independent circuits in parallel. In construction, it is in effect two
single throw, single pole disconnecting switches with the hinge jaws
connected together and mounted on the same insulator. The hinge jaw is
also provided with dummy jaws to hold either blade of the switch in
the open position. Except for these differences in the hinge jaws, the
construction is similar to the switch shown in fig. 2,209. It should
not be used to open the circuit when loaded.]
[Illustration: FIG. 2,213.--Hook stick for operating a disconnecting
switch.]
~Ques. What are the features of the enclosed air break?~
Ans. The switch is more compact than the open air break type, but
pressure oscillations are caused on opening the circuit the same as
with the open air break, and it is not desirable for heavy current.
[Illustration: FIG. 2,214.--Baum 35,000 volt, 200 ampere, double break
pole type switch. While designed for disconnecting purposes only, it
can break considerable amperage. The levers and couplings are fastened
with tape pins. The control shaft coupling is adjustable to any
angle, and the switch can be locked in the open or closed position. A
removable wooden handle is supplied and the switch can be handled in
any weather. The arms can be extended to hold fuse fittings, or dead
end insulators in the event of a heavy strain, but it is preferable
to have fuses on another structure as a precaution against coming
in contact with the energized portion of the switch, and it is also
preferable to take the strain of the line on a pole a few feet from the
switch, rather than on the switch structure, particularly in the larger
sizes. An insulating wood section in the control shaft separates the
control handle from the remainder of the switch. Discharging horns can
be fitted to this type of switch and when so equipped they have been
found capable of breaking considerable loads.]
~Ques. How is the fuse arranged in the metal fuse break type of switch?~
Ans. It is placed in a tube fitted with powdered carbonate of lime or
some other insulating powder.
[Illustration: FIG. 2,215.--Pacific swivel type blade for Baum pole
top switches. The ~twist type~ _of blade_, here shown, is especially
adapted to switches operating in freezing or sleety weather. ~It
will be seen~ that the first few degrees through which the rotating
insulator is moved _have the effect of twisting the blade between the
shoes of the contact_, which breaks any seal through freezing, or
corrosion.]
[Illustration: FIG. 2,216.--Pacific 22,000 volt, 100 ampere, ~pole
top switch~ equipped with fuse tubes; designed to meet the need for a
small group controlled disconnecting switch, having several features
making it suitable for use with service transformer installations and
line branches. ~The switch is made~ with clamped pipe arms permitting
adjustment. It is equipped with fuse tubes and fittings, but should
the fuses be not desired, the arm may be shortened. Provision is made
for fitting insulator pins to the top of the arms, when the switch is
mounted vertically, which will hold insulators at right angles to the
switch, making it possible to end a line on the top of these arms and
then drop down through the switch to the bank of transformers. _The
switch is so constructed that_ ~gravity~ _tends to hold it in either
the open or the closed position_. Provision can be made for locking.]
[Illustration: FIG. 2,217.--Horn break switch. ~In operation~, the
arc formed at break, will travel toward the extremities of the horns
because of the fact that a circuit will tend to move so as to embrace
the largest possible number of lines of force set up by it. Hence, the
arc that starts between the horns where they are near together rises
between them until it becomes so attenuated that it is extinguished.]
[Illustration: FIG. 2,218.--Westinghouse rear connected motor starting
switch, for pressures up to 600 volts. It is used for starting rotary
converters and direct current motors of large capacity having starting
torque small enough to permit cutting out the starting resistance in
few steps. The clips can be connected to any type of resistor, the
steps of which are successively short circuited as the switch closes;
the amount of resistance in the armature circuit is thus gradually
reduced. A pause should be made after each step of resistance is thrown
in to allow the motor speed to accelerate. If the starting switch do
not have to carry the full load current and can be short circuited by
another switch, a starting switch of smaller capacity equivalent to 50
per cent of running current of the machine can be used. The switch is
of the single pole, single throw, rear connected, four point, knife
blade type.]
~Ques. Describe its operation.~
Ans. The moving arm of the switch draws the fuse through the tube, thus
opening the circuit without much disturbance.
[Illustration: FIG. 2,219.--Baum disconnecting switch with horns and
auxiliary contacts (Pacific Mfg. Co.). This switch is for use on
systems operating at 100,000 volts or over. It has a spacing of five
feet between outer insulators, is equipped with auxiliary shoes that
break the circuit between the horns, diverting it from the current
carrying contacts so that they are not attacked by the arc.]
~Ques. What is the objection to the metal fuse switch?~
Ans. The powder is set flying by the explosion of the arc, which, as
it settles, gets into the bearings of any machine that may be in the
vicinity.
~Ques. What is a horn break switch?~
Ans. One provided with horn shaped extensions to the contacts, as shown
in fig. 2,219.
The arc formed on breaking the circuit, as it travels toward
the extremities of the horns, becomes attenuated and is finally
ruptured.
[Illustration: FIG. 2,220.--Kelman switching mechanism. The pantograph
arrangement of the contact blades gives a double horizontal break deep
down in the oil. This gives over the break a heavy head of oil which
immediately closes in around the thin blades as they leave the contacts
in opening, thus effectually extinguishing the arc. The opening spring
acts within the pantograph itself without any intervening mechanism,
and the light weight of the few moving parts enables the spring to
accelerate the blades rapidly, thus obtaining a quick break. The
contacts are of the return bend type, which makes a flexible contact,
to obtain alignment with the blades at all times. The pantograph and
contacts are supported on corrugated porcelain insulators on a hardwood
base or insulator board. The insulators are fitted with iron ends for
securing the different parts. At each end of the insulator board is an
upright or lifting board which serves to lift the switching mechanism
out of the tank. The leads are heavily insulated.]
~Ques. What are the objections to this type of switch?~
Ans. The considerable space required for the horns and arcs, and the
line surges caused by the arc.
Horn switches were used extensively for high pressure
alternating current circuits before the introduction of oil
switches.
[Illustration: FIG. 2,221.--Sectional view of Pacific weatherproof oil
switch for use in places exposed to the weather. All moving and contact
parts are supported from the cast iron top and are readily removable
for inspection or repair.]
~Oil Switches.~--The extensive use of high pressure currents and
alternating current motors and other devices introducing inductance
make it necessary to use switches radically different from the ordinary
air break types.
[Illustration: FIG. 2,222.--General Electric central station triple
pole single throw oil switch; view of switch in tank. This type is for
pressures up to 110,000 volts, being adapted for stations employing
open wiring, since the connections are made at the top of the switch
and its construction obviates the need for isolating it in a cell. One
tank with two breaks in series are used for each phase.]
The opening of circuits of considerable current value with inductive
loads is not possible with old style switches which were quite
adequate for the service for which they were designed. These circuits
are controlled with ease and certainty by the oil switch.
[Illustration: FIGS. 2,223 to 2,226.--Westinghouse indoor, two pole
double throw oil switch for pressures not over 6,600 volts. Fig. 2,223,
open position; fig. 2,225, closed position. This type of switch is
suited for a wide range of application, being made in both switchboard
and wall mounting styles; also for remote mechanical control by the
use of bell cranks and connecting rods. The wall mounting style is
adaptable to motor installations on account of the facility with which
it may be mounted on any support, convenient to the motor operator. The
lever and handle extend outward over the oil tank, so that the switch
may readily be mounted against a wall, post or any vertical support.
The characteristic features of this type of switch are: knife blade
contacts submerged in oil; live parts carried on a porcelain base
affording a permanent insulation between adjacent poles, and between
the frame and live parts; compactness and accessibility; enclosure of
all live metal parts; and low first cost. Each contact jaw has attached
to it an arcing piece which takes the final break, thus preventing any
burning of the jaws. These arcing pieces are inexpensive and readily
replaced when worn or burnt away. The contact making parts are enclosed
in a sheet metal oil tank which has an insulating lining. The leads are
brought out at the top. Connections to the outside circuit are made
inside the switch and a porcelain insulator is slipped over the joint,
thus providing a straight continuous connection from the line with
maximum insulation. On the 6,600 volt switch, insulation is obtained
by the use of porcelain bases for supporting the live parts. In the
3,300 volt switch specially treated wooden bases are used, suitable
barriers being provided between the poles where necessary to prevent
arcs communicating.]
~Ques. What is an oil switch?~
Ans. One in which the contact is broken under oil.
This type of switch is the one almost universally used on high
pressure alternating current circuits, because of the fact that
the oil tends to cause the current to break when at its zero
value, thus preventing the heavy arcing which would occur with
an air break switch, and the consequent surges in the line
which are so often the cause of breakdown of the insulation of
the system.
[Illustration: FIG. 2,227.--Kelman electric control unit for oil
switch. It consists of an iron frame which contains the opening and
closing coils and the bearings for the operating bell crank. A small
switch on the frame automatically opens the coil circuit at the end of
the stroke in either direction and operates signal lamps to indicate
the open or closed position. The automatic overload release opens the
switch by closing the opening coil circuit. This electrical operating
unit gives satisfactory service through a wide variation of voltage.
It requires a momentary expenditure of energy of from 1,500 to 4,000
watts, depending on the size.]
~Ques. What is the nature of an oil break?~
Ans. It is not a quick break.
Oscillograph records show that the effect of the oil is to
allow the arc to continue during several cycles and then to
break the current, usually at the zero point of the wave.
~Remote Control Oil Switches.~--It is desirable in the case of switches
on high pressure circuits to locate the parts which carry the high
pressure current at some distance from the switchboard in order that
they may be operated with safety.
With respect to the manner in which the switches are operated they may
be classed, as
1. Hand operated;
2. Power operated.
[Illustration: FIGS. 2,228 and 2,229.--Views showing mechanism of hand
operated remote control switches. Fig. 2,228, straight mechanism; fig.
2,229, angular mechanism.]
~Ques. What kind of power is used?~
Ans. Electricity is used in most cases; in some installations, switches
are operated by compressed air.
~Ques. For what pressures should remote control switches be used?~
Ans. For pressures above 1,100 volts.
~Ques. Describe the operating mechanism of a remote control, hand, and
electrically operated switch.~
Ans. For hand operation, the mechanism between the operating lever and
switch proper, consists simply of a system of links and bell cranks.
Various shapes of bell crank are used, to permit change in direction or
position of the force applied to operate the switch.
[Illustration: FIG. 2,230.--Pacific ~oil switch with solenoid control~,
designed for 60,000 and 70,000 volt installations; it is capable of
handling a 25,000 kw. generating station. The break is horizontal,
made by the rotation of a flat member edgewise through the oil. The
solenoid, at its extreme outer position, has a free start before
commencing to move the control parts of the switch. As it approaches
the extreme inner position, where the opening spring and the contacts
begin to offer the greatest resistance, the magnetic action is, of
course, most powerful, and the leverage by which it is applied moves
to an increasing radius, by means of rollers working in the curved
slots of the control shaft levers. These curved slots and rollers have
the additional advantage of making the opening action very free and
smooth. The tripping coil does not act on the latch directly, but gives
a hammer blow that is positive. The latch proper is a roller having a
powerful hold and easy release. Current can not be left on either the
closing or opening coils, as they are automatically cut out by the
movement of the switch.]
~Ques. Name two classes of electrically operated remote control switch.~
Ans. Those operated by solenoids, and those operated by motors.
The solenoid type are closed by the action of a plunger
solenoid, and opened either by another solenoid called a
"tripping coil" or by gravity. Some examples of remote control
are shown in the accompanying illustrations.
~Ques. What indicating devices are used with electrically operated
switches?~
Ans. Red and green lamps; ~red~ for _closed_ and ~green~ for _open_ as
shown in fig. 2,231.
[Illustration: FIG. 2,231.--Diagram of connections of motor operated
remote control switch. The motor which operates the switch is
controlled by a small lever generally mounted on the panel with the
instruments which are in the circuit controlled by the switch. The
standard pressure for operating the motors is 125 volts.]
~Ques. For what service are motor operated switches used?~
Ans. For exceptionally heavy work where the kilowatt rupturing capacity
is greater than that for which the other types are suitable.
[Illustration: FIG. 2,232.--General Electric motor operated three
phase oil switch. The operation of the oil switch is accomplished by
a small hand controlling switch, generally mounted on the panel, with
the instruments which are in the circuit controlled by the oil switch.
The standard pressure for the operating motor is 125 volts. The switch
has six breaks, each break being a separate tank. In addition to this
isolation of the breaks, each phase is enclosed in a fireproof brick
compartment, making it impossible for trouble in one phase to be
communicated to another. The cells are constructed of brick with top
and bottom slabs of slate. The capacities of such switches, range from
2,500 to 60,000 volts, and from 100 to 1,000 amperes.]
~Rupturing Capacity of Oil Switches.~--While an oil switch may be
designed for a given pressure and to carry a definite amount of
current, it should not be understood that the switch will necessarily
rupture the amount of normal energy equivalent to its volt ampere
rating.
[Illustration: FIGS. 2,233 to 2,235.--Diagrams showing connections for
General Electric single, double, and triple pole, solenoid operated
remote control switches. The operating coils are shown connected to
main switch circuit, but may be connected to an entirely separate
control circuit. Connections are the same for either alternating or
direct current.]
Oil switches are often used on systems with generator capacity of
many thousand kilowatts. It is therefore essential that the switches
shall be able to break not only their normal current, but also greatly
increased current that would flow if a short circuit or partial short
circuit occur.
[Illustration: FIG. 2,236.--Westinghouse three pole hand operated
remote control oil switch, adapted for the control of alternating
current circuits of small and moderate capacities, the pressures of
which do not exceed 25,000 volts. Each unit is installed in a separate
masonry compartment. The open position of contacts is maintained by
gravity. Up to and including the 600 ampere capacity, the contacts
are cone shaped with an arcing tip, as shown for capacities in excess
of 600 amperes, brush contacts are furnished with auxiliary arcing
contacts of the butt type. Each pole has two sets of contacts, thus
providing a double break in each line. With both types of contact, the
final break of the arc is taken and the main contacts protected by
auxiliary arcing contacts which are inexpensive and readily renewable.
The upper or stationary contacts are mounted on porcelain insulators
secured in the soapstone base. The lower or movable contacts are
carried by a wooden rod connected to and moved vertically by the
operating mechanism. The operating mechanism of the hand operated
breaker consists of a simple system of levers, bell cranks, and rods.
The necessary energy for making a positive contact is small owing to
the use of a toggle mechanism. The leads are brought out of the top
of the breaker through heavy porcelain insulators. On breakers above
3,500 volts, the connections to the line wires are made by means of
a union which can be tightened with a socket wrench fitting inside
the insulator. As the leads coming into the switch are necessarily
insulated wire or cable, this arrangement eliminates all exposed live
parts and is well adapted to making connections readily to bus bars
located above or in the rear of the circuit breakers.]
[Illustration: FIG. 2,237.--Cutler-Hammer ~enclosed float switch~,
designed for the automatic control of alternating current motors
operating pumps used to fill or empty tanks, sumps or other reservoirs.
~The switch is operated~ _by the rise and fall of a copper float which
is connected to the switch lever by a brass rod or copper chain_. As
the water level rises and falls, the float moves up and down. This
movement is transmitted to the switch lever and the switch (if the
movement be sufficient) is tripped to make or break the motor circuit.
~To insure the best operation~ it is necessary that the float rod be
provided with a guide so that the float will move up or down in a
vertical line, as shown. The minimum difference in water level at which
the switch will operate is approximately 10 to 12 inches. When the
float is placed in a closed tank, the minimum height inside from the
bottom of the tank to the top should be at least 6 inches greater than
the difference in water level to provide sufficient clearance for the
float. ~When this type switch is used as a tank switch~, the contacts
are closed when the water level is low, putting the motor, driving the
pump, in motion. When the water in the tank reaches a predetermined
high level the float arm opens the switch contacts, and the motor is
disconnected from the line. ~For sump pump purposes~, the contacts open
on low level and close on high level, the lever being reversed for this
purpose. Two pole, three pole and four pole switches of this type are
made, all arranged to completely disconnect single phase, two phase and
three phase motors from their circuits. When used with small motors
which may be thrown across the line to start, the switch may be used
without a self starter if desired.]
Under short circuit conditions alternators develop instantaneously many
times their normal load current, while the sustained short circuit
current is approximately two and a half to three times normal, or even
higher with turbine alternators. Hence, circuit breakers of the so
called instantaneous type must be capable of rupturing the circuit
when the current is at a maximum, whereas, non-automatic switches, or
circuit breakers with time limit relays will be required to interrupt
only the sustained short current circuit. The reason is evident, since
the delay in opening the switch allows the current to approach the
sustained short circuit conditions.
CHAPTER LVIII
CURRENT AND PRESSURE LIMITING DEVICES
In any electric installation there must be provided a number of
automatic devices to secure proper control. The great multiplicity
of devices designed for this purpose may be divided into two general
classes, as
1. Current limiting;
2. Pressure limiting.
Because of the heating effect of the current which increases in
proportion to the square of the strength of the current, it is
necessary to protect circuits with devices which do not allow the
current to exceed a predetermined value.
Accordingly fuses, circuit breakers, reactances, etc., are used, each
possessing certain characteristics, which render it suitable for
particular conditions of service.
For instance, just as in analogy, steam boilers must be
protected against abnormal pressures by safety valves, electric
circuits must be guarded against excessive voltages by pressure
limiting devices, otherwise much damage would occur, such as
the burning out of incandescent lamps, grounding of cables, etc.
The control of steam is simple as compared to the electric
current, the latter being the more difficult to manage because
of its peculiar behaviour in certain respects, especially in
the case of alternating current which necessitates numerous
devices of more or less delicate construction for safety both
to the apparatus and the operator.
~Fuses.~--A fuse is "an electrical safety valve", or more specifically,
_the actual wire or strip of metal in a cut out, which may be fused by
an excessive current_, that is to say, by a current which exceeds a
predetermined value. A fuse, thus serves to protect a circuit from any
harm resulting from an undue overload.
Fuses have been treated at such length in Guide No. 2, Chapter
XXV, that very little can be said here, without repetition.
[Illustration: FIG. 2,238.--Sectional view of Noark 250 volt, 400
ampere ~enclosed fuse~. The ~fusible element~ _is divided into strips_
A, B, C, and D. This parallel link construction results, upon the
operation of the fuse, in the formation of a number of small arcs, thus
facilitating the absorption of the metal vapor formed when the fuse
blows. ~The fusible strips~, of which there are two or four in number,
according to the ampere capacity of the fuse, are entirely surrounded
by a granular material which is chemically inactive with respect to the
fusible link and whose function is to absorb the metallic vapor formed
upon the blowing of the fuse. ~The contact blades~ T and L are made of
round edge copper, the round edges facilitating the insertion of the
fuses in the circuit terminals. R and S are the end ferrules, attached
to cover E, by the pin M.]
~Ques. What effect have the terminals on a fuse?~
Ans. The current at which a fuse melts may be greatly changed by the
size and shape of the terminals.
If near together and large, they may conduct considerable heat
from the fuse thus increasing the current required to blow the
fuse.
~Ques. What is the objection to large fuses?~
Ans. The discharge of molten metal when the fuse blows is a source of
danger.
~Ques. What should be used in place of large fuses?~
Ans. Circuit breakers.
~Ques. What are the objections to fuses in general?~
Ans. The uncertainty as to the current required to blow them; the
constant expansion and contraction is liable to loosen the terminal
screws when screws are used.
~Ques. What is the advantage of fuses?~
Ans. They form an inexpensive means of protecting small circuits.
[Illustration: FIG. 2,239.--Cross section through plug fuse. With this
type of fuse it is impossible to place any except the correct size of
plug in the socket.]
~Ques. Describe a plug fuse.~
Ans. It is constructed as shown in fig. 2,239, the fuse wire being
visible and stretching between the two metal portions of the plug.
~Ques. What is a cut out fuse?~
Ans. One similar to a simple fuse, but provided with clip contacts as
used for knife switch contacts.
The fuse wire is usually contained in a china or porcelain
tube, which also serves the purpose of a handle for withdrawing
the fuse.
~Ques. What is an expulsion fuse?~
Ans. One in which the fuse is placed in an enclosed chamber with a vent
hole.
In operation, when the fuse blows, the hot air and molten metal
are expelled through the vent.
~Ques. What is a no arc fuse?~
Ans. A cartridge type fuse, in which the space surrounding the fuse
wire is filled with powdered material.
[Illustration: FIG. 2,240.--Inside view of end ferrule of Noark
enclosed fuse. Two prongs O and V, which are a part of the knife blade
K, pass through the square holes in the ends of the ferrule R, and
are riveted to the anchor plate T. ~The object of this plate~ _is to
stiffen the structure and to increase the current carrying capacity
of the metal between the holes, also to permit of proper alignment
of the plates_. In each ferrule is placed a ~vent screen~, composed
of reticulate material, such as cheese cloth. The fuzz between the
threads of the cheese cloth prevents the escape of the granular
material through the vent holes A, but when the fuse operates, allows
free egress of the air, thereby permitting the vapor formed upon the
operation of the fusible element to quickly and freely pass through the
interstices of the filling material and become cooled, eliminating any
possibility of flame issuing from the ends of the tube.]
The object of the powdered material is to assist in
extinguishing the arc formed when the fuse blows.
~Ques. What is a magnetic blow out fuse?~
Ans. An enclosed fuse which is subject to the action of a magnetic
field produced by the current, the magnetic field tending to blow out
the arc when fusing occurs.
~Ques. What is a quick break fuse?~
Ans. One having a weight suspended from its center, or springs attached
to its ends so that the arc formed at fusing is quickly attenuated and
extinguished.
~Ques. What is the disadvantage of a fuse as compared to an oil switch
circuit breaker?~
Ans. When a fuse blows, the arc causes oscillations in the line, which
cause excessive rise of pressure under certain capacity conditions,
whereas this disturbance is reduced to a minimum with an oil switch.
[Illustration: FIG. 2,241.--~Quick break fuse.~ The fuse wire is
connected between the fixed terminal A and the movable arm B, and is
held under tension by the spring which exerts pressure on the movable
arm in a direction tending to separate A and B. ~In operation~, when
the fuse blows, the movable arm quickly moves to the position B´, thus
attenuating the arc and accelerating its extinguishment.]
~Ques. What metal is used for fuse wires?~
Ans. Various metals. Ordinary fuse wire is made of lead or an alloy of
lead and tin.
~Ques. What is the objection to aluminum?~
Ans. It becomes coated with oxide or sulphide, which acts as a tube
tending to retain the metal inside and prevent rupture.
~Ques. What is the objection to copper?~
Ans. Its high fusing point.
~Current Limiting Inductances.~--The great increase in capacity of
power stations, for supplying the demands of densely populated centers
and large manufacturing districts, together with the decrease in the
reactance of modern alternators and transformers due to improvement
in design to obtain better regulation, has presented a problem
in apparatus protection not contemplated in the earlier days of
alternating current distribution. This problem is entirely separate and
distinct from that of eliminating the tendency toward short circuit,
incident to the high voltages now common in transmission lines. It
accepts that all short circuits must occasionally occur and considers
only the protection of the connected apparatus against the mechanical
forces due to the magnetic stresses of such enormous currents.
[Illustration: FIG. 2,242.--~Notched end fuse.~ This is a simple form
of fuse ~consisting of~ _a strip of metal (or wire) fixed between
two end pieces to fit around the terminals_. This type is often
proportioned so that it is only possible to place the correct size of
fuse in the terminals. Sometimes, in place of the end pieces as shown,
the fuse metal is fixed between two clamping screws.]
~Ques. What means are employed to limit the value of a short circuit
current?~
Ans. A current limiting inductance coil (called a _reactance_) is
placed in series with the alternators or transformers.
[Illustration: FIG. 2,243.--General Electric current limiting
reactance; view showing details of construction. The core consists
of a hollow concrete cylinder, alloy anchor plates or sockets being
embedded in the core near the ends to receive the radial brass bolts.
An extension at each end of the core provides for clamping and bracing
the reactance in installation. The supports for the winding are made of
resin treated maple and are located upon the core by radial brass studs
screwed into the alloy sockets, and insulated by mica tubes. The nuts
by which the structure is tightened, rest upon heavy fibre washers.
Wooden barriers fitted and shellacked into the supports add to the
creepage surface between layers of the winding and between the winding
and the core. The supports of the layer next to the core are separated
from the core by strips of treated pressboard. The coil consists of
bare stranded cable in several layers, usually three in number. It is
wound into grooves in the treated wood supports, which are protected
from contact with the cable by heat shields of asbestos shellacked
into the grooves. The winding is usually in the form of two back turn
sections, thereby allowing the terminals of the coil to be brought out
at the ends of the outside layer. This assures accessibility and ease
of connection, and the removal of the leads from proximity to the core.
Two turns at each end of the winding are given extra spacing for the
purpose of additional insulation. The final turn at each end of the
coil is securely held in place by alloy clamps bolted to the supports.
The wood is protected from contact with the clamps by shields of
asbestos. The ends of the cable between the two sections are welded by
the oxyacetylene process.]
~Ques. What are its essential features of construction?~
Ans. It consists of bare stranded cable wound around a concrete core
and held in place by wooden supports as shown in fig. 2,243.
In order to avoid the prohibitive expense of high voltage
insulation, the reactance coil is designed for the low tension
circuit. This requirement prohibits the use of a magnetic core
which, if economically designed for normal operation, would
become saturated at higher densities, or, if designed large
enough to avoid saturation at short circuit conditions, would
become prohibitive in cost and dimensions.
The elimination of all magnetic material from the construction
of the concrete core reactance permits of no saturation, and
assures a straight line voltage characteristic at all current
loads.
[Illustration: FIG. 2,244.--Westinghouse ~magnetic blow out circuit
breaker~, designed for the protection of street railway and electric
locomotive equipments; it serves the combined purpose of fuse block
and canopy switch. ~The contact tips~ _are surrounded by a moulded
arc chute which confines and directs the arc until the magnetic blow
out extinguishes it_. The current carrying contacts consist of copper
strips separated by air spaces. An auxiliary contact or "arcing tip"
at the end of the switch lever takes the burning of the arc when the
breaker opens, and thus confines the burning to a very small piece
which can be easily removed and replaced at small cost. The hand
tripping lever and the resetting lever have insulated handles, so that
they can be safely handled, even in the dark.]
~Ques. Where is the proper location for a current limiting reactance?~
Ans. As near the alternator as possible.
~Ques. Why?~
Ans. To lessen the possibility of a short circuit occurring between the
reactance and the alternator.
~Ques. Beside limiting the current, what other service is performed by
the reactance?~
Ans. It protects the alternator from high frequency surges coming in
from the outside, and limits the current from other machines on the
same bus.
[Illustration: FIG. 2,245.--General Electric magnetic blow out circuit
breaker. This type may be used in air or water tight boxes and is
peculiarly adapted for service where the arc must be confined.]
~Circuit Breakers.~--The importance of circuit protective devices,
commonly called circuit breakers, is fully recognized. The duty
of a circuit breaker is to protect the apparatus in an electrical
circuit from undesirable effects arising from abnormal conditions,
by automatically breaking the circuit. Accordingly a circuit breaker
must comprise a switch in combination with electrical control devices
designed to act under abnormal conditions in the circuit.
A circuit breaker is a _device which_ ~automatically~ _opens the
circuit in event of abnormal conditions, in the circuit_.
[Illustration: FIG. 2,246.--Magnetic blow out circuit breaker. This is
a direct current breaker in which the final break occurs in a magnetic
field. ~It is a principle in electromagnetics~ _that a conductor
carrying a current in a magnetic field will tend to move in a direction
at right angles to the field_. The arc set up on breaking a circuit
constitutes a conductor, and in magnetic blow out circuit breakers, as
generally manufactured, there is an electromagnet, energized by the
current to be broken, which produces a field in the neighborhood of
the arc, with the result that the arc moves outward, and so becomes
attenuated and is finally extinguished. The form shown in the figure is
used on cars equipped with heavy motors. When so used, it is in many
cases mounted in a box with the handle H projecting at one end. A and K
are the terminals of the breaker and B is the tripping coil, which also
serves to set up the magnetic field necessary for blowing out the arc.
X is the armature of coil B and is pulled down against the action of
the spring S whenever the current exceeds that for which the breaker is
set. The tripping current is adjusted by means of nut T. The iron plate
P and a similar one back of it are magnetized by the current in coil
B, and as the break takes place between these two poles, the arc is
promptly extinguished by the field that exists there. ~In operation~,
A and K are the terminals, D D is a contact that is forced up against
F, F when the breaker is set. The current then takes the path A-B-F-D
D-F-K. ~When the breaker trips~, the contact piece D D flies down
and the tendency is for an arc to form between F, F; the magnetic
field blows the arc upwards, and whatever burning takes place is on
the contacts E, E, which are so constructed that they may be readily
renewed. ~To trip the breaker by hand~, the knob N is pressed.]
In the design of circuit breakers, there are several methods used to
effect the rupturing of the arc between contacts when opened on heavy
overload, such as:
1. Magnetic blow out; 2. Thermal break; 3. Carbon break.
In the magnetic blow out type, the arc is extinguished between
auxiliary contacts confined by a chute in which the arc is rapidly
blown out due to a powerful magnetic field from one or more
electromagnets. This type may be used in air or watertight boxes and is
peculiarly adapted for service where the arc must be confined.
[Illustration: FIG. 2,247.--~Thermal overload circuit breaker.~ ~In
construction~ two contact blocks are fixed rigidly to, but insulated
from, the switch arm. They are connected electrically by two parallel
strips of suitable metal, each fitted with a steel catch piece. ~When
the switch is closed~ the strips are sprung apart over a fixed catch,
and the full rated current does not release the catch. Overload causes
the strips to move apart, and the circuit breaker flies off under the
action of a spring.]
In a carbon break type, the arc is finally ruptured between carbon
break contacts. The breaking of the circuit is accomplished
progressively, that is to say, it is done in three stages, by several
sets of contact, known respectively as
1. The main contacts;
2. The intermediate contacts;
3. The carbon contacts.
_In operation_, as the circuit breaker acts to break the circuit, first
the ~main~ _contacts_, separate, then the ~intermediate~ _contacts_,
and finally the ~carbon~ _contacts_ between which the arc is ruptured.
~Ques. What is the object of the intermediate contacts?~
Ans. To prevent the forming of an arc on the main contacts.
[Illustration: FIG. 2,248.--Carbon break discs of Condit circuit
breaker. The two pairs of similar discs which slide past each other
are so arranged that these surfaces coincide at the instant the
intermediate contacts separate after which, as the contact arm opens
further, they gradually disengage.]
~Ques. What is the object of the carbon contacts?~
Ans. First to protect the intermediate contacts by providing a path
for the current after the intermediate contacts separate, and 2, to
"slow down" the current by means of the considerable resistance of the
carbon, thus reducing to a minimum the arc which is formed when the
carbon contacts separate.
~Ques. How is the automatic operation of a circuit breaker usually
accomplished?~
Ans. Usually through the medium of a solenoid, or electromagnet
energized by current from the circuit controlled by the breaker.
[Illustration: FIG. 2,249.--Mechanically connected insulated latches
used on Condit circuit breakers to produce inter-locking tripping.]
The essential features of construction and operation of a
circuit breaker is shown in the elementary diagrams, figs.
2,250 to 2,253. ~In construction~ as shown in fig. 2,250 it
consists essentially of three sets of contacts, a swinging
contact arm which is set in the closed position by the handle
operating through the toggle joint, the movement of which is
limited in the closing direction by the stop. The latter is
made adjustable by an eccentric pin or equivalent. Connected
to the toggle is the plunger of the solenoid whose winding is
energized by current from the circuit which the circuit breaker
is to control.
[Illustration: FIGS. 2,250 to 2,253.--Elementary diagrams illustrating
the operation of a carbon circuit breaker of the overload type, showing
the progressive opening of such device. Fig. 2,250, closed position;
fig. 2,251, main contacts open; fig. 2,252, intermediate contacts open;
fig. 2,253, carbon contacts open, circuit broken.]
~In operation~, the circuit is closed by hand by turning the
handle downward to the position shown in fig. 2,250, that is as
far as it will go.
Since the toggle has passed the center line the arm will be
held normally in this position because of the spring action of
the contacts. Now, if the current rise above a predetermined
limit, the pull exerted by the solenoid will overbalance the
tendency of the toggle to remain in the closed position, and
pull the two toggle links downward below the center line,
drawing the contact arm back and breaking the circuit.
[Illustration: FIG. 2,254.--I-T-E overload circuit breaker. ~In
operation~: the current from one side of the circuit enters the circuit
breaker at A, passing through the laminated bridge B to contact block
C, thence through coil D and terminal E to the motor. The coil D
surrounds a magnetic core, having pole pieces F and G and armature H.
The effect of the current in the coil is to energize the magnet, thus
tending to lift the armature against the force of gravitation. The
volume of current required to trip the circuit breaker is determined
by the position of the armature, which is subject to ready adjustment,
and is indicated on the calibration plate P. From the opposite side
of the line, the current enters at I, passing downward through the
laminated bridge member J, into terminal K, whence it passes out to the
motor. When the current passing through the circuit breaker attains
sufficient volume, the force generated by the magnetic coil overcomes
the weight of the armature H; and the latter is drawn upward toward
the pole pieces with constantly increasing force, until the insulated
projections L and M strike against the respective restraining latches
N and O, thereby releasing the two switch members, which at once open
in response to the force supplied by the spring of the contact members
and auxiliary springs provided for the purpose. Positiveness in opening
is further assured by the blow of the armature, which is added to the
other opening forces; hence, the heavier the overload, the more violent
the blow and the quicker the circuit breaker opens; or the greater the
current the more promptly it is interrupted. This is the ~I-T-E~ or
~I~nverse ~T~ime ~E~lement principle.]
[Illustration: FIG. 2,255.--Condit 600 volt, 1,200 ampere, single pole,
type K, circuit breaker with pull down handle.]
[Illustration: FIG. 2,256.--Condit 600 volt, 6,000 ampere, single pole,
switch board mounting, circuit breaker, with pull down handle.]
[Illustration: FIG. 2,257.--General Electric triple pole, overload,
circuit breaker, with two overload coils, capacity 300 amperes, 480
volts.]
The progressive action which takes place during this operation
is shown in figs. 2,250 to 2,253 in which the main contacts
separate first, then the intermediate, and finally the carbon
contacts as mentioned before.
~Ques. What name is given to this type of circuit breaker?~
Ans. It is called an overload circuit breaker.
[Illustration: FIG. 2,258.--Parts of General Electric 2,000 ampere 650
volt circuit breaker. A, cover for secondary contact bracket; B, spring
washer for Ea.; C, pin for links and G; D, spring for carbon support;
E, plate for F; F, carbon support; G, secondary contact bracket; H,
contact plate; I, screw for H; J, nut for K and W; K, contact stud,
upper; L, laminated brush, complete with support; M, leather buffer for
L; N, main link; O, pin for Na and La left hand and Cb and Na right and
left hand; P, screw for N and magnet frame shaft; Q, washer for N and
magnet frame shaft; R, screw for S and V; S, index plate; T, plate for
Gb; U, screw for T; V, magnet frame; W, contact stud, lower; X, pin for
Cb, Na and V; Y, washer for X and O; Z, calibrating screw with thumb
nut; Aa, armature with contact plate; Ba, catch lever complete with
catch Ca, button handle for Ba; Da, spring cotter for Ea; Ea, pin for
F and Fa; Fa, operating link for G; Ga, pin for D; Ha, carbon holder
with copper and carbon contacts; Ia, flexible connections for G and F;
Ja, screw for G and flexible connection plate; Ka, screw for Na and Ha;
La, copper secondary contact; Ma, screw for La; Na, secondary contact
lever; Oa, cross bar for Na; Pa, screw for L and M; Qa, secondary
toggle link (left hand); Ra, spring cotter for Wa and O; Sa, brush
lever; Ta, buffer for Cb and Sa; Ua, secondary toggle link (right
hand); Va, washer for Wa; Wa, pin for Cb, Qa, Ua and N; Xa, pin for Sa
and Cv; Ya, spring cotter for all pins, except Wa, catch lever pin and
buffer; Za, secondary contact link; Ab, washer for Fb; Bb, guard for
Fb; Cb, handle lever; Db, catch for Cb; Eb, screw for Db; Fb, handle
with stud; Gb, secondary connection.]
~Automatic Features.~--There are three methods of connecting the
winding of the solenoid, or _trip coil_ as it is called:
[Illustration: FIGS. 2,259 to 2,262.--Elementary diagrams illustrating
the various methods of electromagnetic control for circuit breakers.
Fig. 2,259, overload trip; fig. 2,260, underload trip, fig. 2,261, low
voltage trip; fig. 2,262, control from auxiliary circuit by means of a
"relay."]
1. In series with the main circuit;
2. In shunt with the main circuit;
3. In shunt with an auxiliary circuit.
[Illustration: FIG. 2,263.--Diagram of General Electric low voltage
trip with tripping switch _normally_ ~open~.]
The automatic controls arising from these connections give various
kinds of protection to the circuit and are known as
1. Overload trip;
2. Underload trip;
3. Low voltage trip;
4. Auxiliary circuit trip.
[Illustration: FIG. 2,264.--Diagram of General Electric low voltage
trip, with tripping switch _normally_ ~closed~.]
~Ques. What is the object of the overload trip?~
Ans. It is intended to open the circuit when the current exceeds a
predetermined value.
~Ques. What modifications are made in the mechanism shown in the
elementary diagrams?~
Ans. Sometimes a latch is used in place of the toggle and a magnet in
place of the solenoid as in figs. 2,265 and 2,266.
~Ques. Why is a magnet used in combination with a latch?~
Ans. Because with this arrangement very little movement is required to
trip the breaker, and for such conditions, a magnet is more efficient
than a solenoid.
[Illustration: FIGS. 2,265 and 2,266.--Circuit breaker with automatic
control mechanism consisting of ~magnet and latch~; views showing
breaker in open and closed positions, and essential features. The
toggle is used to obtain sufficient leverage to easily close switch
against the pressure of the brush contacts but not to lock switch,
this being done by the latch as shown, the latter closing by the
action of a spring, there being a roller R at the end which engages
the arm to reduce friction. ~In operation~, when the current exceeds
a predetermined limit the magnet attracts the latch and releases the
contact arm. The brush contacts which are exerting pressure against the
contact arm, rapidly push it away, and assisted by gravity, the arm
flies open to the position shown in fig. 2,266.]
~Ques. How does the latch arrangement work?~
Ans. When the proper current is reached, the magnet pulls open the
latch and the contact arm of the breaker moves by the force of gravity
or other means and opens the circuit.
~Ques. How does the underload trip operate?~
Ans. The same as the overload type except that they operate on a
diminution of current instead of an excess.
[Illustration: FIGS. 2,267 and 2,268.--Positions in circuit of
_current_ and _pressure_ coils of circuit breakers.]
~Ques. Describe the no voltage trip.~
Ans. The energy for the trip of this breaker is derived from a high
resistance or fine wire coil which is arranged to be placed directly
across the line, in operation, when the current flowing through the
circuit falls below a predetermined value, the energy of the coil is
insufficient to counteract the force of a spring, which then trips the
breaker.
[Illustration: FIG. 2,269.--Diagram of General Electric shunt trip
with coil connected beyond breaker and thrown out of circuit after
tripping.]
~Ques. Describe the auxiliary circuit trip.~
Ans. A pressure coil is used which is energized by current from an
auxiliary circuit. The coil is only _momentarily_ energized, by push
button, relay or other control, as distinguished from the preceding
types, in which the coil is _constantly_ energized.
[Illustration: FIG. 2,270.--Diagram of General Electric shunt trip with
auxiliary circuit opening switch to throw coil out of circuit after
tripping.]
[Illustration: FIG. 2,271.--General Electric shunt trip attachment.
The shunt trip attachment has been designed to provide for conditions
under which the low voltage attachment cannot be successfully applied.
It resembles the low voltage attachment in construction, but differs in
that it trips the circuit breaker when energized. The shunt trip should
be allowed to remain only momentarily in circuit; hence it should
be so connected that the opening of the circuit breaker immediately
disconnects it from the circuit. Whenever it is impossible to connect
the shunt trip in this manner, the circuit opening auxiliary switch
should be used in connection with it.]
[Illustration: FIG. 2,272.--General Electric low voltage attachment
for circuit breakers. This low voltage trip is designed to operate the
circuit breaker when the line voltage drops to approximately 50 per
cent or less of the normal voltage. It should be noted that the coil
is always in circuit, as is the case with the overload and underload
coils, and that it operates with the _releasing_ of its armature. It is
always necessary to use a fixed amount of resistance (depending upon
the voltage of the system) in series with the low voltage release.
The low voltage release performs the functions of a shunt trip coil
when used in conjunction with a push button, auxiliary switch or
speed limiting device, and is generally preferred to the shunt trip
attachment.]
[Illustration: FIG. 2,273.--General Electric ~circuit opening~
_auxiliary switch_. This switch opens an auxiliary circuit when the
circuit breaker opens, and is intended to be used in connection with
a shunt trip attachment to insure the immediate disconnection of the
shunt coil from the circuit. It may also be employed to serve other
purposes, such as tripping another circuit breaker having a low voltage
attachment, and permitting another circuit breaker to remain closed
only when the circuit breaker equipped with the auxiliary switch is
open.]
~Ques. What other name is given to the auxiliary circuit trip?~
Ans. It is sometimes called the shunt trip, though ill advisedly so.
[Illustration: FIG. 2,274.--General Electric ~circuit closing~
_auxiliary switch_. This switch closes when the circuit breaker opens,
and may be used to announce the automatic opening of the circuit
breaker through the means of an indicating lamp or an alarm bell. It
is often necessary to arrange one circuit breaker so that, in opening,
it will trip others. This may be accomplished by using a circuit
closing auxiliary switch in connection with a low voltage or shunt trip
attachment on the circuit breakers to be tripped. The construction of
this type of switch is such that it may be opened by hand after the
circuit breaker opens, but it is automatically reset when the circuit
breaker is closed.]
~Relays.~--Oil break switches and carbon break circuit breakers are
commonly used to open electrical circuits at some given overload and
on short circuit. To secure additional protection under a variety of
abnormal conditions or to provide for a certain predetermined operation
or sequence of operations, relays may be employed.
[Illustration: FIG. 2,275.--General Electric type C circuit breaker.
Specially adapted to motor driven machine tool applications. For use in
mills, machine shops, factories, foundries and office buildings. For
general motor work, automobile charging outfits, storage batteries,
rectifier sets, cranes, etc. List of parts: A, calibrating post; B,
laminated contact; C, secondary contact spring; D, contact blade; E,
cotter pin for G; F, toggle link; G, pin for D and F; H, stop for Aa;
I, hinge frame; J, operating lever; K, pin for I and J; L, toggle
link; M, connection; N, screw for M, O and P; O, nut for N and P;
P, terminal; Q, tripping coil; R, calibrating screw; S, laminated
contact; T, calibrating scale; U, calibrating spring; V, connection
post; W, knob; X, washer for Y; Y, handle; Z, buffer; Aa, armature; Ba,
laminated connection; Ca, connection; Da, base.]
A relay is defined as: _A device which_ ~opens~ _or_ ~closes~ _an_
~auxiliary circuit~ _under predetermined electrical conditions in the
main circuit_.
The object of a relay is generally to act as a sort of electrical
multiplier, that is to say, _it enables a comparatively weak current to
bring into operation a much stronger current_.
[Illustration: FIG. 2,276.--Diagram of connections of General Electric
shunt trip coil ~with~ and ~without~ circuit opening auxiliary switch.]
~Ques. For what service are relays largely used?~
Ans. They are employed in connection with high voltage switches
where the small amount of energy derived from an ordinary instrument
transformer is insufficient for tripping.
The connections between relays and circuit opening devices are
usually electrical. Combinations of this nature are extremely
flexible since they permit the use of a number of devices, each
having a different function, with a single circuit breaker or
oil switch as well as with two or more switches, to secure the
desired operation and protection.
~Selection.~--In all electrical installations protection of apparatus
is important, but in some large central stations this is secondary to
continuity of service.
To combine maximum protection without interruptions of service is
not always possible, but these requirements can be approximated very
closely by the use of reliable and simple controlling or protecting
devices if proper care be taken to select the relays suited to the
special conditions of the installation. To do this intelligently, a
knowledge of the various types of relay is necessary.
[Illustration: FIG. 3,073.--Diagram of connections of General Electric
low voltage release coil when used with speed limiting device on rotary
converter.]
There is a multiplicity of types and a classification to be
comprehensive, should, as in numerous other cases, be made from several
points of view. Accordingly relays may be classified:
1. With respect to the nature of the service performed, as
_a._ Protective;
_b._ Regulative;
_c._ Communicative.
2. With respect to the operating current, as
_a._ Alternating current;
_b._ Direct current.
3. With respect to the manner of performing their function, as
_a._ Circuit opening;
_b._ Circuit closing.
4. With respect to the operating current circuit, as
_a._ Primary;
_b._ Secondary.
5. With respect to the abnormal conditions which caused them to
operate, as
_a._ Overload;
_b._ Underload;
_c._ Over voltage;
_d._ Low voltage;
_e._ Reverse energy;
_f._ Reverse phase.
6. With respect to the time consumed in performing their function, as
_a._ Instantaneous (so called);
_b._ Definite time limit;
_c._ Inverse time limit.
7. With respect to the character of its action, as
_a._ Selective;
_b._ Differential.
8. With respect to whether it acts directly or indirectly on the
circuit breaker, as
_a._ Main;
_b._ Auxiliary.
[Illustration: FIG. 2,278.--General Electric overload and low voltage
type C circuit breaker for 600 volts or less. It has one overload, and
one low voltage coil as shown. Screens are provided between contacts.]
~Protective Relays.~--These are used to protect circuits from abnormal
conditions of voltage, or current, which would be undesirable or
dangerous to the circuit and apparatus contained therein.
~Ques. How do protective relays operate?~
Ans. They act in combination with automatic circuit breakers, operating
when their predetermined setting has been reached, energizing the trip
coil of the circuit breaker and opening the circuit.
Fig. 2,279 shows the principles of relay operation. When
the current or pressure in the main circuit reaches the
predetermined value at which the protective system should
operate, the relay magnet attracts the pivoted contact arm and
closes the auxiliary circuit; this permits current to flow from
the current source in that circuit and energize the trip coil
thus opening the main circuit.
[Illustration: FIG. 2,279.--Diagram illustrating the operation of a
~circuit closing relay~. When the predetermined abnormal condition is
reached in the main circuit, the relay closes the auxiliary circuit,
thus energizing the trip coil and opening the breaker.]
~Regulative Relays.~--This class of relay is used to control the
condition of a main circuit through control devices operated by a
secondary circuit.
~Ques. For what service are relays of this class employed?~
Ans. They are used as feeder circuit or generator regulators.
~Ques. How do they differ from protective relays?~
Ans. They have differentially arranged contacts, that is to say,
arranged for contact on either side of a central or normal position.
[Illustration: FIG. 2,280.--Diagram showing a railway synchronous
converter protected by a single pole overload circuit breaker with
low voltage release attachment and bell alarm switch. The low voltage
attachment trips the breaker on failure of direct current voltage also
when speed limit device closes. Internal troubles are taken care of by
the alternating current automatic devices (not shown).]
~Communicative Relays.~--These are used for signalling in a great
variety of ways for indicating the position of switching apparatus or
predetermining the condition of electric circuits.
~A. C. and D. C. Relays.~--As here used, the classification refers to
the kind of current used on the auxiliary circuit. In some cases direct
current is used to energize the trip gear of the circuit breaker or oil
switch, and in others, alternating current.
[Illustration: FIG. 2,281.--Diagram showing three phase motors
protected by triple pole overload circuit breakers, with two overload
coils, also one overload coil and low voltage release coil. The use
of the low voltage release allows the breaker to be tripped from a
distance by means of a short circuiting switch or push button.]
A. C. and D. C. relays are respectively known as _circuit opening_ and
_circuit closing_ relays, being later fully described.
~Circuit Opening Relays.~--The duty of a circuit opening relay is
_to open the_ ~auxiliary circuit~, _usually alternating current, nd
thereby cause the oil switch or circuit breaker to be opened by the use
of a trip coil in the secondary of a current transformer, or by low
voltage release coil_.
The trip coil of the breaker is generally shunted by the relay
contacts and when the moving contact of the relay disengages
from the stationary contact, the current from the transformer
which supplies the relay, flows through the trip coil thus
opening the breaker. These features of operation are shown in
fig. 2,282.
[Illustration: FIG. 2,282.--Diagram illustrating the operation of a
~circuit opening relay~. When the relay contacts are in the normal
closed position, as shown, the coil is short circuited. When the
predetermined abnormal condition is reached in the main circuit, the
relay contacts are opened with a quick break, sending the current
through the trip coil momentarily, and opening the breaker.]
~Ques. Where are circuit opening relays chiefly employed?~
Ans. In places where direct current is not available for energizing the
trip coil.
~Ques. What is the objection to alternating current trip coils?~
Ans. They have relatively high impedance and impose a heavy volt ampere
load on the transformers.
~Circuit Closing Relays.~--The duty of a circuit closing relay is to
close the auxiliary circuit at the time when the predetermined abnormal
condition is reached in the primary circuit. The closing of the
auxiliary circuit energizes the trip coil and opens the breaker.
[Illustration: FIGS. 2,283 to 2,291.--General Electric instantaneous
overload ~circuit opening~ relays, covers removed. Circuit opening
relays are used chiefly in those cases where direct current for the
tripping circuit is not available. Alternating current trip coils
have relatively high impedance and impose a heavy volt ampere load on
the current transformers. To reduce this load during normal operation
the circuit opening relay is frequently used and is usually necessary
where instruments and meters are to be operated on the same current
transformers as the trip coils if the greatest accuracy be required.
The relay contacts in the normal, closed position, short circuits
the trip coil. When the relay operates on overload or other abnormal
condition the contacts are opened with a quick break, sending the
current through the trip coil circuit momentarily and tripping the
switch. With circuit opening relays, the trip coils of the oil switch
must be set to trip somewhat lower than the setting of the relay. ~In
construction~ the relay consists of a solenoid with iron frame forming
the support for the relay; a central plunger or armature of special
construction which is picked up or released by the magnetic action of
the solenoid; a plunger rod which actuates the relay contacts, which
are mounted on an insulated base usually above the solenoid; a tube or
plate for the calibration marking and adjustment; covers of glass or
metal to keep out dust; terminal boards with points corresponding to
tagged leads from relay coils and external wiring diagrams. The relay
contacts are of two kinds, circuit opening, as shown above, and circuit
closing, as shown in figs. 2,292 to 2,300.]
~Ques. What kind of current is generally used for the auxiliary circuit
of a circuit closing relay?~
Ans. Direct current.
~Ques. At what pressure?~
Ans. From 125 to 250 volts.
~Ques. Where is this current usually obtained?~
Ans. From a storage battery, or from the exciter.
~Ques. For what current are the contacts ordinarily designed?~
Ans. About 10 amperes.
[Illustration: FIGS. 2,292 to 2,300.--General Electric alternating
current, instantaneous overload ~circuit closing~ relays, covers
removed. The function of a circuit closing relay is to close an
electrical circuit, usually direct current, through a trip coil on an
oil switch or circuit breaker, or it may short circuit a low voltage
release coil, and thereby open the oil switch or circuit breaker
on occurrence of the condition upon which the relay is designed to
operate. Direct current at 125 or 250 volts taken from exciter bus
bars or storage battery system is generally used for the tripping
circuit. Circuit closing contacts have a cone shaped central element
of carbon or metal which makes contact with flexible contact fingers
symmetrically arranged above the cone. These contacts will make and
break a circuit of 10 amperes at 125 volts without the use of auxiliary
circuit opening switches. Relays are made with two or three contacts
for connecting one side of a direct current circuit through one or
two separate circuits, or trip coils respectively, to the side of
opposite polarity. Usually only two contacts are required. Where two
or more trip coils are used, which may not be connected permanently in
parallel, the three contact relays are selected and in some cases four
contacts furnished.]
~Primary and Secondary Relays.~--Primary relays are sometimes called
series relays as they have the current coils connected directly in
series with the line, both on high and low tension circuits.
Secondary relays receive their current supply from the secondary
circuits of current transformers. Alternating current relays connected
to secondary of pressure transformers and relays with both current and
pressure windings are included in this class.
~Ques. What is the usual winding of the coils?~
Ans. The current coils are usually wound for 5 amperes and the pressure
coils for 110 volts.
[Illustration: FIG. 2,301.--Alternating current low voltage circuit
closing low voltage relay, for 600 volts or less. The contacts are
similar to those of the circuit closing overload type except that they
are inverted. As long as the pressure is normal the contact cone is
held above the contacts. When the pressure falls below one half normal,
the cone and plunger rod drop and close the contact. This relay does
not pick up its own plunger. The plunger rod is pushed up by hand after
the pressure circuit is established. Low voltage relays are generally
used in connection with a low voltage release or shunt trip coil on
an oil switch or a circuit breaker. They are used in connection with
motor booster sets to prevent a disastrous speed of the booster which
might result from the loss of alternating current power. They are also
sometimes used for indicating purposes.]
~Ques. What refinement is made in the design of relays and why?~
Ans. Care is exercised to reduce to a minimum the volt ampere load
imposed by the relay on the current transformer to permit the use of
un-stranded meters and relays upon the same transformer.
The use of circuit opening relays to cut out the trip coil of
an oil switch during normal operation, has been described,
and in the short time that the trip coil is in circuit, it
does not affect the accuracy of the instrument readings. This
practice, however, does not apply in the case of curve drawing
meters, voltage compensators or other devices which have in
themselves sufficient load for separate current transformers.
In this connection it should be noted that to obtain accurate
instrument and meter readings; the current transformers should
not be loaded beyond certain limits which depend upon the volt
ampere load and power factor of each of the connected devices.
[Illustration: FIG. 2,302.--Condit type K circuit breaker with shunt
trip and no voltage attachment. ~The shunt trip~ is usually applied
as an auxiliary to other types of trip. It consists of a fine wire
coil which is mounted as a self-contained part of the breaker and
which when energized, trips the circuit breaker. It is used to open
the breaker from some distant point, and the coil is arranged to be
connected across the line. The coils are so arranged that the circuit
breakers will operate on a voltage 25% above or 25% below normal. The
shunt trip coil is not intended to remain across the line and should
be only momentarily energized. ~The no voltage trip~, receives energy
from a high resistance or fine wire coil which is arranged to be placed
directly across the line, but in contradistinction to the shunt trip
type, in which the coil is momentarily energized to trip the breaker,
the no voltage coil is _constantly_ energized and a _decrease_ or
failure of pressure trips the breaker. It can be used as a remote
control device the same as the shunt trip. Its general use, however, is
to cause the circuit breaker to open when the voltage of the line fails
from any cause. Its use is recommended on all motor circuits, as it
affords an additional protection against accidents, for if the voltage
should fail, the breaker immediately opens, and before the machine can
start again the attendant must close the breaker. It will not work for
the protection of storage batteries or of motor generator sets charging
storage batteries, as, when the voltage of the generator fails, the
voltage of the battery still maintains its full value. The action of
the coil is independent of the direction of flow of current; it simply
allows the breaker to stay closed as long as the voltage is on the line
and opens the breaker when the voltage on the line ceases. No voltage
circuit breakers are normally so adjusted that they will not release
until the voltage approaches 50% of normal.]
So great is the variety of combination used and the variations of these
factors in their several combinations at different loads and settings,
that special consideration of each arrangement is advisable.
[Illustration: FIG. 2,303.--General Electric alternating current high
pressure series overload relays controlling 45,000 volt oil switches.
These relays are connected in series with the line. If current
transformers are to be used on the same circuit for other purposes, and
have sufficient capacity to supply energy for operating relay coils,
then secondary relays would be more economical, otherwise the series
relays are much less expensive. By means of a specially treated wooden
rod, the relay operates a tripping switch, closing a separate tripping
circuit, usually 125 or 250 volts direct current. Relays and switches
are for mounting on flat surfaces. Series relays are essentially the
same as secondary relays except in the coil winding and insulation.
The corrugated horizontal arms which carry the relays, as shown, are
insulated posts, insulating the relays from the ground. The wood rod
from each relay is connected directly to a tripping shaft on the oil
switch which buckles an auxiliary toggle, thereby opening the main
toggle and tripping the oil switch.]
~Overload Relays.~--Series relays are connected directly in series with
the line and are chiefly used with high pressure oil break switches
for overload protection. If current transformers are to be used on the
same circuits for other purposes, and have sufficient capacity to admit
of adding a relay coil, secondary relays would be more economical;
otherwise, the series relays are less expensive.
By means of a specially treated wooden rod, the relay operates a
tripping switch, closing a separate tripping circuit, usually 125 or
250 volts direct current. Series relays are essentially the same as
secondary relays except in the coil winding and insulation.
~Underload Relays.~--These are similar in construction to low voltage
relays but have current instead of pressure windings.
~Over Voltage Relays.~--These are usually of the circuit closing type
and are similar to secondary overload relays, but have pressure instead
of current windings.
~Low Voltage Relays.~--Relays of this class are in most cases used
[Illustration: FIG. 2,304.--Condit 600 volt, 1,500 ampere single pole
back connected type K circuit breaker, ~motor operated~. The mechanical
and electrical features of the circuit breakers are no different than
when hand operated, the only difference being that the motor is used
for the operating means. This motor is so arranged that even should it
over travel, due to an accident to the controlling circuit, it cannot
produce more than a predetermined strain on the circuit breaker. In
other words, after the motor has closed the circuit breaker, further
travel of the motor will not result in putting a strain on the
operating parts. Suitable motors are supplied for this service, the
type of motor varying in accordance with the character of the operating
current supplied. The advantage of this type of electrical operation
is that it puts very little strain on the switch mechanism, takes very
little operating current, allows the use of standard parts, and makes
an extremely substantial and flexible structure. Its disadvantage is
that it closes slowly, and it must not, therefore, be used in places
where quick closing is essential.] for the protection of motors in the
event of a temporary weakening or failure of the pressure. They are
also used in connection with a low voltage release or shunt trip coil
on an oil switch or a circuit breaker.
~Reverse Energy Relays.~--The chief object of this species of relay is
to protect the generator. When so used, the overload adjustment is set
at the maximum value to give overload protection only at the maximum
carrying capacity of the generator and a sensitive reverse protection
to prevent a return of energy from the line.
[Illustration: FIG. 2,305.--General Electric direct current solenoid
control relay. Solenoids for operating large switches, etc., frequently
require comparatively large operating currents in the "closing" coils.
This necessitates the use of relatively heavy leads between the control
switch and the solenoid and is the cause of severe arcing at the
control switch, especially with solenoids of high inductance. These
objectionable features can best be eliminated by the use of a suitable
control relay located near the solenoids. The control relay consists
of a solenoid plunger and switch, the latter insulated from the frame
of the relay. It operates satisfactorily on one-half the rated voltage
and requires only a very small operating current. The terminals of the
switch and the relay coils are independent. The relay can be wound for
operation on 125, 250, or 600 volt circuits.]
~Reverse Phase Relays.~--This type of relay is used chiefly to prevent
damage in case of reversal of leads in reconnecting wiring to two or
three phase motors.
~Time Element.~--It is often inconvenient that a circuit breaker should
be opened immediately on the occurrence of what may prove to be merely
a momentary overload, so that time lag attachments are frequently
provided, particularly with relays. These devices, which may form part
of the relay or may be quite distinct from it, retard its action until
the overload has lasted for a predetermined time--several seconds or
more.
[Illustration: FIG. 2,306.--Alternating current series reverse phase
single pole, circuit closing, two contact relay for 600 volts or less.
This type of relay is used chiefly to open motor circuits for elevators
to prevent damage in case of reversal of leads in reconnecting wiring
to two or three phase motors. The relay is provided with a dust proof
metal cover.]
~Ques. What should preferably govern the time lag?~
Ans. It should depend on the extent to which the overload is reduced as
the time elapses.
~Instantaneous Relays.~--The so called instantaneous relays operate
almost instantly on the occurrence of the abnormal condition that they
are to control.
There is of course a slight time element comparable with that
of an overload circuit breaker, but for practical purposes, the
operation may be considered as instantaneous.
[Illustration: FIG. 2,307.--Electric circuits of Condit type "A" relay.
The construction is described in fig. 2,309. As here shown, the relay
is not in operation, but should the current passing through the coil
be of sufficient value to cause the lower movable half of the magnetic
circuit to approach the upper stationary half of the circuit, _the
relay will be transformed from an ordinary_ ~electromagnet~ _into a_
~repulsion motor~. The contact will short circuit the brushes of the
armature and thus cause it to revolve, the speed of rotation being
dependent on the amount of current flowing to a predetermined point,
and thereafter the speed of rotation of the motor remains constant
irrespective of the current value. ~Time adjustment:~ This is obtained
by varying the distance through which the contact travels, provision
being made whereby adjustment can be made as close as .1 of a second.
Current adjustment: This is obtained by means of a calibrated spring.
Standard relays are calibrated at 6, 8, 10, and 12 amperes, the coils
being designed to carry five amperes continuously, with a temperature
rise not exceeding 86° Fahr. ~Power to operate relay:~ The relay
requires twenty volt amperes for its operation at full load; the
influence of this type of relay on the ratio and phase angle of current
transformers is small.]
~Time Limit Relays.~--Under this classification there are two
sub-divisions.
1. Definite time limit;
2. Inverse time limit.
[Illustration: FIG. 2,308.--Characteristic curves of Condit type A
selective relay. Curves 1, 2, 3, and 4 show the time variation of
this relay with different settings at the various current values. The
relay may be adjusted to trip the switch at any point represented
between curves 1 and 4. This relay is a combination of an _inverse time
limit_ relay and a _definite time limit_ relay. The combination of the
characteristics of the two types are seen in the curve, the first part
of which is ~inverse~, and the latter part ~definite~ from a point of
three or four times full load current. This combination of features
being desirable as, for instance, in transmission work, particularly
where it is necessary to use circuit breakers set selectively, as, due
to the inverse feature of the curve, the relays can be set so that on
a moderate overload, they will require the proper length of time to
operate, and at the same time will operate quickly enough on heavy
short circuits to prevent damage to the distribution system or its
apparatus. Due to the definite feature of the latter part of the curve,
the relays of the varying circuit breakers when once set to operate at
different time values will never operate simultaneously irrespective of
the value of the short circuit current, thus tending toward continuity
of service.]
~Ques. Describe the time mechanism of a definite time limit relay.~
Ans. It consists of an air dash pot, and an air diaphragm or equivalent
retarding device connected to the contact mechanism.
~Ques. How does it operate?~
Ans. In some designs, when the contacts are released, they descend
by gravity against the action of the retarding device thereby making
contact a definite interval after the occurrence of the abnormal
condition.
[Illustration: FIG. 2,309.--Condit type "A" ~selective relay~, designed
for use with circuit breakers where selective or discriminating
action is required. The circuits and connections of this relay are
illustrated in fig. 2,307, and its characteristics in fig. 2,308. ~In
construction~, the relay consists of _a special motor with a short
circuited armature and a split field_. ~Under normal conditions~, the
fields are separated from each other and the motor armature does not
revolve. The force tending to pull the two faces of the field together
is opposed by a spring, the compression of which determines the number
of amperes necessary to cause the relay to begin operation. The motor
structure performs the whole work and the motor itself un-meshes and
meshes the gears without the aid of any external device.]
~Ques. How does the inverse time limit type operate?~
Ans. The actuating and contact mechanism is attached directly to an
air bellows and in operation tends to compress the bellows against the
action of a specially constructed escape valve in the latter.
[Illustration: FIG. 2,310.--Condit type "B" ~time limit attachment~,
designed to give sufficient time to allow an induction motor to start
without opening the circuit breaker, and not have the circuit breaker
trip on the momentary rush of current. ~Its action is inverse~; that
is, _the greater the current the less time it takes to operate_ and
is so arranged that four to five times full load current or a short
circuit will trip the circuit breaker instantly. The time limit
attachment is applied directly to the armature which trips the circuit
breaker and is adapted for the so called primary trip. It consists
of an air vacuum dash pot with a graphite piston, the dash pot being
fastened to the stationary calibrating ring of the trip coil and the
moving outside cylinder is fastened to the armature of the circuit
breaker. When the current reaches a point where it overcomes the weight
of the armature and lifts the same, the magnetic force tending to raise
the armature is opposed by the vacuum created in the interior of the
cylinder. As the magnetic force continues the vacuum is overcome due to
the leakage of air past the plunger and the armature gradually moves up
until it reaches the point where it trips the circuit breaker. If at
any point of the armature travel, the current drop back to normal, the
armature immediately resets itself by means of a ball valve in the top
of the brass cylinder.]
~Ques. Why is the arrangement called _inverse_ time limit?~
Ans. Because the retardation varies inversely with the pressure on the
bellows, and therefore inversely with the magnitude of the abnormal
condition.
~Ques. What other device may be used to retard the operation?~
Ans. A damping magnet is sometimes used which acts on a disc or drum
and which may be adjustable.
[Illustration: FIGS. 2,311 and 2,312.--General Electric alternating
current ~low pressure~ series overload relays. Fig. 2,311,
instantaneous time limit relay; fig. 2,312, inverse time limit relay.
These relays have carbon contacts and will make or break a direct
current circuit of 10 amperes at 125 volts without auxiliary circuit
opening switch. They are used where several circuits are controlled
by one automatic oil break switch or one shunt trip, overload and
shunt trip or low voltage release carbon break circuit breaker. These
relays may be used for signal purposes; they are back connected, the
connections can be seen in the illustrations.]
~Ques. How is the inverse time element introduced by this arrangement?~
Ans. The retardation is due to eddy currents induced by moving the disc
or drum through the magnetic field. The reaction thus induced varies
inversely with the magnitude of the force with which the disc or
drum is urged through the field and hence inversely with the abnormal
condition.
~Ques. What are the ordinary limits of adjustment for inverse time
limit relays?~
Ans. From one-half second to 30 seconds, depending upon the time
setting and magnitude of the overload current.
[Illustration: FIGS. 2,313 to 2,321.--General Electric time limit
overload circuit opening relays with covers removed. The construction
of this relay is similar to that of the inverse time limit relay,
except that it has a compression spring interposed between the plunger
and diaphragm. The plunger compresses the spring and further motion
is prevented by a stop, making the relay practically independent of
the amount of the overload, only the stored energy of the spring, if
the overload continue, applies power, dependent on its own mechanical
strength, to the diaphragm. The time limit therefore becomes
practically a constant for any given setting under ordinary conditions
of overload or short circuit. If, however, the overload come on slowly
so that the spring is not fully compressed at once, the time limit will
vary slightly. If the scheme of selective operation make it necessary
to take care of a creeping load of this character, two relays may
be used and definite time limit positively secured. In this case,
an instantaneous circuit closing, overload relay would be used and
a definite time limit relay, provided with a direct current coil in
circuit with the closing contacts of the first relay. The time limit
relay would be of the circuit closing type and control a direct current
trip coil on the oil switch.]
A setting of from two to six seconds is ordinarily used,
depending upon the requirements. Where selective operation is
desired a minimum setting of two seconds is recommended.
~Differential Relays.~--In this type of relay there are two
electromagnets. In normal working these oppose and neutralize each
other. Should, however, either winding become stronger or weaker than
the other, the balance is upset, the magnet energized, and the relay
comes into operation.
[Illustration: FIG. 2,322.--Differential relay transformer and reverse
current circuit breaker discriminating device. A differential relay is
_one whose electromagnet has two windings_. ~In normal working~ _these
oppose and neutralize one another_. Should however, either winding
become stronger or weaker than the other, the balance is upset, the
magnet is energized, and the relay comes into operation. A modification
of such a relay for alternating current is here shown, from which it
will be seen that when the currents are as indicated, the circuit A has
the larger pressure induced in it, whereas, should the main current
reverse with reference to the shunt current, the circuit B would have
the larger induced pressure.]
A modification of such a relay for alternating current is shown in
fig. 2,322, from which it will be seen that when the currents are
as indicated, the circuit A has the larger pressure induced in it,
whereas, should the main current reverse with reference to the shunt
current, the circuit B would have the larger induced pressure.
[Illustration: FIG. 2,323.--Diagram of modern power house wiring and
busses showing location of relays.]
~[1]How to Select Relays.~--The following general information on
relays, together with reference to the one line diagram, fig. 2,323,
will be of interest and assistance in making a selection from the
various relays previously described to meet the requirements of modern
power house and sub-station layouts.
[1] NOTE.--As suggested by the General Electric Co.
~Single pole relays~ are used on single phase and on balanced
three phase circuits.
~Double pole relays~ are used on ungrounded three phase and on
quarter phase.
[Illustration: FIG. 2,324 to 2,329.--General Electric inverse time
limit overload circuit closing relays. In this type of relay its
mechanism is so designed that a delay or lapse of time in opening the
circuit breaker after a predetermined condition of the circuit has
been reached, depends on the flow of current, that is, if the current
be great, the time will be small, and if the current be of a moderate
value, the time will be correspondingly longer.]
~Triple pole relays~ are used on three phase grounded neutral
and interconnected quarter-phase.
~Circuit closing relays~ are recommended in all cases where a
constant source of direct current is available for operating
trip coils.
The conditions for which relays have been designed for power
circuits may perhaps be best described, by considering a
one line diagram from the generator end to the sub-station
auxiliary machines and feeders.
Considering first alternating current circuits, the
prevailing practice is to make the circuit breakers by
which the alternators are connected to the low tension bus
_non-automatic_, in order to insure minimum interruption of
alternator service. The chance of trouble in this part of the
circuit is remote, but should it occur, the station attendant
could generally open the circuit breaker before the machines
would be injured.
~Reverse current relays~ of instantaneous or time limit types
are often connected to the secondaries of current and of
pressure transformers to indicate by lamp or bell any trouble
that may occur in the generator circuit.
These relays operate with a low current reversal at full
pressure and conversely with a proportionally greater current
at voltages less than normal. At zero pressure, the relay would
act as an overload one, set for high overload. At zero current,
a voltage considerably in excess of normal would be required to
operate it.
[Illustration: FIG. 2,330.--Diagram showing two phase motor or feeder
circuit protected by double pole double coil, overload circuit breaker
(or two single pole breakers interlocked) with bell alarm switch.]
Specifications sometimes call for automatic generator circuit breakers:
in this case _definite time limit overload relays_ are used. They are
connected in the secondaries of current transformers and are designed
to give the same time delay for all trouble conditions; they allow the
defective circuit to be opened, if possible, at a point more remote
from the generator than the generator circuit breaker.
When the total generator capacity exceeds the rated rupturing capacity
of the circuit breakers, one or more sectionalizing circuit breakers
are placed in each bus.
If operating conditions admit, these devices are made
non-automatic and are left disconnected except in case of
emergency; but if it be necessary for them to be continually in
service, they may be made automatic by _means of instantaneous
overload relays_ connected to current transformers in the low
voltage bus; the relays being adjusted to trip the circuit
breaker under short circuit conditions, confining the trouble
to one section and preventing the circuit breakers rupturing
more than their rated capacity.
Installations with but _one bank of power transformers_, and without
high voltage bus, are provided with automatic circuit breakers operated
by an _inverse time limit relay_.
The relay is connected to the secondaries of current
transformers, which in turn are connected in the low voltage
side of the power transformer.
Stations with _more than one bank of power transformers_, a
high voltage bus, and high and low voltage circuit breakers,
may have both circuit breakers arranged to trip at the same
time or one after the other. As in the former case, they are
operated from the inverse time limit relay connected in the low
voltage side.
[Illustration: FIG. 2,331.--Condit 600 volt, 1,500 ampere, single pole
type K circuit breaker ~pneumatically operated~. It is the same as the
electrically operated circuit breaker, except that a pneumatic cylinder
mechanism is supplied in place of either the electromagnet or the
motor. This cylinder mechanism is so arranged that the air pressure is
only on the cylinder at the instant of operation. At all other times
the air pressure is shut off by means of a control valve. The kind of
remote control to be used depends on local conditions. In general, the
hand operated remote control device is preferable where conditions are
such that it can be used, and where it is necessary to use electrically
operated, the motor operated type is recommended if conditions be such
that slow closing is not objectionable.]
[Illustration: FIGS. 2,332 and 2,333.--Diagram showing two phase
four wire no voltage connections for I-T-E circuit breaker. The two
no voltage coils for two phase four wire circuits are connected
respectively to binding posts B, C and A, D on the face of the base. B
and D are connected to lower spring contacts 2 and 1 respectively, of
the small disconnecting switch. (In instruments supplied on individual
bases, these connections are made in the factory, let into channels
in back of base and covered with wax.) Each of the upper contacts _a_
and _b_ of the disconnecting switch is connected respectively through
resistance R2 and R1 to one main in each phase at _aa_ and _bb_. C
and A are respectively connected to the other main in each phase at
3 and 4. Thus each of the no voltage coils operates across one phase
independent of the other. The terminals 3, _aa_, _bb_ and 4, must,
in all cases, be so connected that they will be subject to the full
voltage of the circuit, irrespective of the position of the starting
switch.]
[Illustration: FIGS. 2,334 and 2,335.--Diagram showing two phase
three wire no voltage connections for I-T-E circuit breaker. The two
no voltage coils for two phase, three wire circuits are connected
respectively to binding posts B, C and A D on the face of the base,
and from A and C connections are made to lower contacts 2 and 1
respectively of the disconnecting switch. Binding posts B and D are
connected together on the back of the board. (In instruments supplied
on individual bases, these connections are made in the factory, let
into channels in back of base and covered with wax.) Each of the upper
contacts _a_ and _b_ is connected respectively through resistance R2
and R1 to one of the mains at _aa_ and _bb_ as shown. D is connected
through resistance R3 to the common wire of both phases at 3 B and D
being connected as aforesaid, thus forming a common connection for
both no voltage coils. Terminals _aa_ and _bb_ of the resistances must
be connected to the outside main across the two phases, terminal 3 to
the main common to both phases, the connections being so made that
these terminals will be subject to the full voltage of the circuit
irrespective of the position of the starting switch.]
[Illustration: FIG. 2,336 and 2,337.--Diagram showing three phase no
voltage connections for I-T-E circuit breaker. The no voltage coils
for three phase circuits are connected in Δ by means of binding posts
A, B, C and D on the face of the base, and from the A and B of the no
voltage coils, connections are made respectively to spring contacts
1 and 2 of the small disconnecting switch. Each of the contacts
_a_ and _b_ of the disconnecting switch is connected respectively
through resistance R2 and R1 to one of the mains at _aa_ and _bb_.
The terminal C is connected through resistance R3 on the back of
the board to the middle main as shown at point 3. The terminal D is
linked on the back of the board to terminal B to complete the [Greek:
D] connection. The terminals _aa_, _bb_ and 3 of the circuit breaker
must, in all cases, be so connected that they will be _subject to full
voltage of the circuit_ irrespective of the position of the starting
switch. Each no voltage coil is supplied with two terminal wires, one
covered with green and one with black insulation. In replacing these
coils particular care should be taken to see that the terminal wires
connected to any one binding post are of unlike color.]
In plants in which two or more banks of transformers are operated in
parallel between high and low voltage busses, it is desirable to have
for each transformer bank, an automatic circuit breaker equipment which
will act selectively and disconnect only the bank in which trouble
may occur. With a circuit breaker on each side of transformer bank,
selective action may be secured in two ways as follows:
1. By means of an instantaneous differential relay connected in the
secondaries of current transformers installed on both the high and low
voltage sides of each transformer bank.
The relay operates on a low current, reversal on either side of
the bank.
2. By means of one inverse time limit, secondary or series relay
installed on that side of the transformer bank which is opposite the
source of power, the relay being arranged to trip both the high and low
voltage circuit breakers.
_The first method_ has the disadvantage of high first cost
due to the high voltage current transformers required, but is
more positive than the second method and is independent of the
number of transformer banks in parallel.
_The second method_ is the less expensive of the two and
protects against overloads as well as short circuits in the
transformers, but it is less positive and introduces delay
in the disconnection of the transformer when trouble occurs.
Furthermore, it is not selective when less than three banks are
operating in parallel.
The automatic circuit breakers in the outgoing line may be operated
from inverse time limit relays connected in the secondaries of current
transformers; or in case transformers are not necessary for use with
instruments, series high voltage inverse time limit relays connected
directly in the line may be used.
Whether to select current transformers with relays insulated for low
voltage, or to choose series relays, is a question of first cost and
adaptability to service conditions. Below 33,000 volts, the commercial
advantages in favor of the series relay are slight, and since it
is somewhat difficult to design this device for the large current
capacities met with at the lower voltage, it is generally the practice
to use the relay with current transformer, because of its operating
advantage. This practice, however, is not entirely followed, since some
service conditions (described later) make the use of series relays very
desirable and practical.
[Illustration: FIGS. 2,338 and 2,339.--General Electric instantaneous
direct current reverse current or ~"discriminating" relays~. Fig.
2,238, for 500 amperes; fig. 2,339 for 2,000 amperes. These relays are
designed for mounting directly on circuit breaker studs. ~These relays
consist~ _of a horseshoe magnet with a shunt wound armature pivoted
between its poles_. ~The magnet~ is mounted on the current carrying
stud of the circuit breaker between the back of the panel and the
first contact or supporting nut, and is placed in a vertical position.
~The contacts~ are insulated from the magnet permitting the use of an
auxiliary circuit for the tripping device, independent of the circuit
controlled by the circuit breaker. The magnet is excited by the current
flowing through the stud, and the armature is connected across the
line in series with suitable resistance. Rotation of the armature in
the normal direction is prevented by a stop. ~Reversal of the current~
flowing through the stud _changes the direction in which the armature
tends to rotate_, causing it to move away from the stop and close the
circuit through an auxiliary trip coil and trip the circuit breaker.
~These relays are used~ to protect dynamos, storage batteries, or main
station busses from damage on reversal of current due to short circuit,
or from the grounding of machines or connection. Relay contacts must
not be used to open the shunt trip coil circuits. An auxiliary switch
should be provided for this purpose in all cases where the opening of
the circuit breaker does not disconnect the trip coil from the source
of supply.]
_Inverse time limit_ relays are satisfactory for one, or more than two
outgoing lines in parallel as they act selectively to disconnect the
defective line only, but installations with only two outgoing lines
in parallel have the same load conditions in both lines and selective
tripping of the circuit breakers in the defective line is obtained by
means of a selective relay acting instantaneously under short circuit
conditions only.
[Illustration: FIG. 2,340.--General Electric ~direct current, reverse
current relay,~ _used to protect dynamos, storage batteries, or main
station busses from damage on reversal of current due to short circuits
or from the grounding of machine or connections._ It is mounted on
vertical bus bars as in the case of cables, on the side wall, or other
flat surface, and the cables threaded through the frame. ~When used
to trip a circuit breaker,~ the breaker is provided with a shunt trip
connected across the circuit, the tripping circuit being closed through
the relay contacts on the occurrence of sufficient reverse current to
lift the relay armature. The relay is either instantaneous or time
limit as desired. ~In the time limit relay,~ the time interval is
obtained by the leather bellows shown in the illustration. The time
setting can be varied within certain limits by means of a valve on the
bellows outlet. ~The operation of the relay~ _depends on the relative
value and direction of magnetic flux set up by a pressure coil, shown
in the illustration, and the current in the vertical bars._ Under
normal conditions these fluxes are in the same direction and circulate
around a closed magnetic circuit. ~When the current in the bars
reverses,~ _the two fluxes oppose each other and force flux through
the normally open leg of the magnetic circuit._ When the reversal of
current is of predetermined value, the relay armature is lifted and the
purpose of the relay accomplished.]
The relay design and action is similar to the reverse current
relay previously mentioned, and is connected to the secondaries
of current transformers in each high voltage line and pressure
transformers in the low voltage bus.
In the sub-station, the conditions are the reverse of those in
the main station, the incoming lines becoming the source of
power.
If there be only one incoming line and no high voltage bus,
the line circuit breaker is generally non-automatic. With
one incoming line and high voltage bus, the circuits from the
service side of the bus are equipped with automatic circuit
breakers and relays. These relays and those used for other
arrangements of two or more incoming lines in parallel, as
well as high and low voltage circuit breakers, are of the same
design and are applied in the same manner as for the generating
station.
Regarding the relay equipments for auxiliary machines, the same
practice is recommended with the generator end of alternating
current motor generator sets as with the main generators, the
outgoing feeder circuit breakers being tripped from inverse
time limit or instantaneous relays.
[Illustration: FIG. 2,341.--General Electric ~direct current
differential relay~ for balancer set; instantaneous, 500 (or less) volt
type for mounting on panel. In many power plants direct current, three
wire, power service is furnished by "high voltage" two wire dynamos
operating in connection with balancer sets consisting of two "low
voltage" machines on a common shaft. With this combination of machine,
a short circuit or heavy overload on one side of the system will shift
the neutral considerably, and the lamps on the opposite side may "burn
out". To protect the lamps, a differential relay operating on 15 volts
unbalancing, is commonly used; it is connected to trip either the
dynamo's circuit breakers (or a circuit breaker connected in the bus
between the balancer set and the other dynamos).]
With several synchronous machines in parallel, the relays
are arranged to operate with the least time delay with which
it is possible to get selective action, in order to prevent
the machines being thrown out of step in event of trouble
conditions causing a decrease of voltage.
The various types of _induction motor_ and various conditions
under which they are employed, have brought about the
development of several types of relay to protect the motors and
the apparatus with which they are used.
It is desirable to disconnect a _large motor_ in case of voltage
failure, and with conditions requiring either a motor operated, or
a solenoid operated circuit breaker, a _low voltage relay_ is used
to close the tripping circuit whenever the voltage decreases to,
approximately, 50 per cent. below normal.
[Illustration: FIG. 2,342.--Condit ~time limit relay~, designed
primarily for use in connection with feeder circuits, where close
selection or discrimination of circuit breakers is not required.
~It may be used~ satisfactorily on lighting and power circuits and
also where there are sudden, momentary fluctuations of current. This
relay is used in connection with series transformers. The contact
arrangements are provided so that the relays may be used as circuit
closing or circuit opening relays. ~The delayed action~ is produced
by an air vacuum dash pot with a graphite piston. The piston of the
dash pot is connected to an arm arranged to be moved by the armature.
When the current reaches a point where it overcomes the weight of the
armature and lifts the same, the magnetic force tending to lift the
armature is opposed by the pull of the vacuum created in the interior
of the shell into which fits the graphite piston. As the magnetic pull
continues the vacuum is overcome due to the leakage of air past the
piston, and the armature gradually moves until it reaches a point where
it causes the circuit breaker to trip, either by closing the contacts
in the circuit closing type, or by opening the contacts in the circuit
opening type. If, at any portion of its travel, the current drop to
normal, the armature immediately resets. ~The time adjustment~ consists
of an arrangement whereby the distance through which the armature moves
before tripping the breaker, may be changed, thus altering the time of
tripping. ~The current adjustment~ is made _by changing the effective
turns of the actuating coil_, the travel of the armature and the force
exerted by it being the same for all current adjustment. The winding is
designed to carry 5 amperes continuously with a temperature rise not
exceeding 68° Fahr. standard calibration is provided so that the relay
will start to operate at 5, 6, 8 and 12 amperes.]
Up to 550 volts, these relays may be connected across the line,
but for higher voltages they are connected to secondaries
of pressure transformers. _Smaller motors_ with which hand
operated circuit breakers are used, are generally provided with
low voltage release attachments that perform the same function
as the relay.
Induction motors are sometimes subjected to _high voltage conditions_
and to protect them from injury, high or excess voltage relays are
employed to trip the automatic circuit breaker. These relays are of
similar design and wired in the same manner as the low voltage relays.
[Illustration: FIG. 2,343.--Characteristic curves of Condit time limit
relay as illustrated in fig. 2,342. Settings: curve A, 5 amperes; curve
B, 6 amperes; curve C, 8 amperes; curve D, 12 amperes.]
_Reverse phase relays_ have been developed for operating conditions
under which a _reversal of phase_ would cause trouble, as for example,
in the case of _elevator motors_.
These are so designed that any phase reversal that would
reverse an induction motor, would operate the relay and
disconnect the automatic circuit breaker.
The design is based on the principle of the induction motor,
and in the case of low voltage motors of limited capacity,
the relay may be connected in series in the motor leads. If
the voltage or capacity of the motor make this arrangement
inexpedient, the relay may be placed in the secondaries of
current or pressure transformers connected in the motor leads.
_Underload relays_ are often used to trip the automatic circuit breaker
that is placed in the primaries of _arc lighting circuits_ to prevent
an abnormal rise of secondary voltage in case of a break in the
secondary circuit.
[Illustration: FIG. 2,344.--Diagram showing _storage battery and
charging dynamo protected by_ ~double pole single coil underload
circuit breaker. In operation,~ _the circuit breaker disconnects the
battery when fully charged, and protects the dynamo from reverse
current_.]
The underload relay is similar in design to the low voltage
relay excepting that it acts on a decrease of current.
The problem of _protecting induction motors_, from injury, that may
result from running on single phase, or from an overload, and at the
same time permit the motor to be started with the necessarily high
starting current that may be greatly in excess of the overload current,
has caused the development of the _series relay_.
[Illustration: FIG. 2,345.--Diagram showing direct current motors
protected by overload circuit breakers with bell alarm switches: _a_,
double pole single coil breaker no switch required. Low voltage device
is on the starting rheostat; _b_, single pole breaker in series with
lever switch. Low voltage attachment on the breaker.]
[Illustration: FIG. 2,346.--Diagram showing two wire dynamo, protected
by a single pole overload circuit breaker with bell alarm switch.
Breakers must be on opposite side from the series field.]
[Illustration: FIG. 2,347.--Diagram showing dynamo protected by a
single pole overload circuit breaker with reverse current relay and
combined circuit opening and bell alarm switch.]
This device may be connected in series with the motor leads for
voltages up to 2,500; it is designed with an inverse time limit
device which may be adjusted to give the desired protection.
The field for relays is more extensive for alternating current than
for direct current power circuits, the latter being generally confined
to much smaller and simpler systems and areas of distribution, and
generally sufficient selective action can be obtained by the use of
fuses or circuit breakers arranged with instantaneous trip.
[Illustration: FIG. 2,348.--Diagram showing three wire dynamo protected
by double pole double coil overload circuit breaker (or two single pole
breakers with interlock) with bell alarm switches. Complete protection
is secured as breaker is connected between armature and series field.]
Operating conditions sometimes make it advisable for the generator
circuit breakers to open only after the auxiliary and feeder circuit
breakers have failed to isolate the trouble.
This is accomplished by using direct current _series inverse
time limit relays_ to trip the generator circuit breakers.
_Instantaneous reverse current relays_ are used to trip the machine
circuit breaker of battery charging sets, rotaries and motor generator
sets to prevent their running as a motor on the charging or direct
current end. These relays can act only in case of current reversal.
To prevent serious unbalancing of voltages in Edison three-wire systems
causing trouble, _differential balance relays_ are used to trip the
circuit breakers on a small percentage of unbalancing.
CHAPTER LIX
LIGHTNING PROTECTION DEVICES
Lightning protection devices, or lightning _arresters_, are devices
for providing a path by which lightning disturbances or other static
discharges may pass to the earth.
Lightning arresters, designed for the protection of transmission lines,
must perform this function with a minimum impairment of the insulation
of the lines.
In general the construction of lightning arresters comprise
1. Air gaps;
2. Resistances;
3. Inductances;
4. Arc suppressing devices.
~Ques. What are the causes of static charges?~
Ans. They may be caused by sandstorms in dry climates, or may be due to
grounds on the high pressure side of a system.
~Ques. What causes high frequency oscillations?~
Ans. They are usually due to lightning discharges in the vicinity of
the line.
~Ques. What are the requirements of lightning protection devices?~
Ans. They must prevent excessive pressure differences between line and
ground, line and line, and between conductor turns in the electrical
apparatus.
~Air Gap Arresters.~--method of relieving any abnormal pressure
condition is to connect a discharge air gap between some point on an
electric conductor and the ground. The resistance thus interposed
between the ground and the conductor is such that any voltage very
much in excess of the maximum normal will cause a discharge to ground,
whereas at other times the conductor is ungrounded because of the air
gap. This forms the principle of air gap arresters.
[Illustration: FIG. 2,349.--Non-arcing multi-gap arrester. Based on
the principle of employing for the terminals across which the arc is
formed, such metals as are least capable of maintaining an alternating
arc between them. This so called non-arcing property of certain metals
was discovered by Alexander Wurtz. The action is such that the "line
current" which follows the lightning discharge follows as an arc, but
is stopped at the end of one alternation because of the property of the
non-arcing metals to carry an arc in one direction, but requiring an
extremely high voltage to start a reverse arc. The non-arcing metals
ordinarily employed are alloys of zinc and copper. Plain multi-gap
arresters as here shown operate satisfactorily with the smaller
machines and on circuits of limited power, but for large machines
of close regulation, and therefore of very large momentary overload
capacity, especially when a number of such are operated in parallel,
such arresters were found insufficient, the line current following the
lightning discharge frequently was so enormous that the circuit did
not open at the end of the half wave, that is the arrester held the
arc and was destroyed. The introduction of synchronous motors made it
necessary that the arc should be extinguished immediately, otherwise
the synchronous motors and converters would drop out of step, and the
system would in this way be shut down. To insure the breaking of the
arc, resistance was introduced in the arrester, the modified device
being known as the low equivalent arrester as shown in fig. 2,350.]
The single gap while adequate for telegraph line protection, was found
insufficient for electric light and power circuits, because since the
current in such circuits is considerable and usually at high pressure
it would follow the lightning discharge across the gap. Thus the
problem arose to devise means for short circuiting the line current
resulting in various modifications of gap arrester.
~Multi-gap Arresters.~--The essential elements of an arrester of this
type are a number of cylinders spaced with a small air gap between them
and placed between the line to be protected and the ground, or between
line and line.
_In operation_, the multi-gap arrester discharges at a much lower
voltage than would a single gap having a length equal to the sum of the
small gaps. In explaining the action of multi-gaps, there are three
things to consider:
[Illustration: FIG. 2,350.--Low equivalent arrester. This is a
modification of the multi-gap arrester shown in fig. 2,349. About half
of the total number of gaps are shunted by a resistance, and another
resistance inserted between the cylinders and the earth. With this
arrangement the middle point is at ground pressure, and there are
between line and ground only one half of the total number of gaps.
This is sufficient to prevent a bridging of the gaps under normal
conditions.]
1. The transmission of the static stress along the line of the
cylinders; 2. The sparking at the gaps; 3. The action and duration of
the current which follows the spark, and the extinguishment of the arc.
~Ques. What is a spark?~
Ans. The conduction of electricity by air.
~Ques. What is an arc.~
Ans. The conduction of electricity by vapor of the electrode.
~Distribution of Static Stress.~--The cylinders of the
multi-gap arrester act like plates of condensers in series.
This condenser function is the essential feature of its
operation.
When a static stress is applied to a series of cylinders
between line and ground, the stress is immediately carried from
end to end.
If the top cylinder be positive it will attract a negative
charge on the face of the adjacent cylinder and repel an equal
positive charge to the opposite face and so on down the entire
row.
The second cylinder has a definite capacity relative to the
third cylinder and also to the ground; consequ
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