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.