SELECTION OF TRANSMISSION CIRCUITS.
Maximum power, voltage, loss, and weight of conductors having been fixed
for a transmission line, the number of circuits that shall make up the
line, and the relations of these circuits to each other, remain to be
determined.
In practice wide differences exist as to the number and relations of
circuits on a single transmission line between two points. Cases
illustrating this fact are the 147-mile transmission from Electra
power-house to San Francisco and the 65-mile transmission between Cañon
Ferry, on the Missouri River and Butte, Mont. At the Electra plant the
generator capacity is 10,000 kilowatts, and the transmission to San
Francisco is carried out over a single pole line that carries one
circuit composed of three aluminum conductors, each with an area in
cross section of 471,000 circular mils. From the generators at Cañon
Ferry, which have an aggregate capacity of 7,500 kilowatts, a part of
the energy goes to Helena over a separate line, and the transmission to
Butte goes over two pole lines that are 40 feet apart. Each of these two
pole lines carries a single circuit composed of three copper conductors,
and each conductor has a cross section of 105,600 circular mils. The
difference in practice illustrated by these two plants is further
brought out by the fact that their voltages are not far apart, as the
Cañon Ferry and Butte line operates at 50,000, and the Electra and San
Francisco line at 60,000 volts.
Economy in the construction of a transmission line points strongly to
the use of a single circuit, because this means only one line of poles,
usually but one cross-arm for the power wires per pole, the least
possible number of pins and insulators, and the smallest amount of labor
for the erection of the conductors. In favor of a single circuit there
is also the argument of greatest mechanical strength in each conductor,
since the single circuit is to have the same weight as that of all the
circuits that may be adopted in its place. Where each conductor of the
single circuit would have a cross section of less than 83,690 circular
mils, if of copper, corresponding to a No. 1 B. & S. gauge wire, the
argument as to mechanical strength is of especial force, since two equal
circuits instead of one, in the case where one circuit of No. 1 wires
would have the required weight, reduce the size of each conductor to No.
4 wire, of 41,740 circular mils cross section, and this is the smallest
wire that it is practicable to use on long lines for mechanical reasons.
Opposed to these arguments for a single circuit are those based on the
supposed greater reliability of two or more circuits, their greater ease
of repair, their more effective means of regulation, and the influence
on inductance of a reduction in the size of conductors.
In spite of the consequent reduction in the size of each conductor, the
use of two or more separate circuits for the same transmission is
sometimes thought to increase its reliability, because in case of a
break or short-circuit on one of the circuits the other will still be
available. Breaks in transmission conductors are due either to
mechanical strains alone, as wind pressure, the falling of trees, or the
accumulation of ice, or else to an arc between the conductors that tends
to melt them at some point. As a smaller conductor breaks or melts more
readily than a large one, the use of two or more circuits instead of a
single circuit tends to increase troubles of this sort. It thus seems
that while two or more circuits give a greater chance of continued
operation after a break in a conductor actually occurs, the use of a
single circuit with larger conductors makes any break less probable.
When repairs must be made on a transmission line, as in replacing a
broken insulator or setting a pole in the place of one that has burned,
it is certainly convenient to have two or more circuits so that one may
be out of use while the repairs on it are made. It is practicable,
however, to make such repairs on any high-voltage circuit, even when it
is in use, provided the conductors are spaced so far apart that there is
no chance of making a contact or starting an arc between them. To get
such distance between conductors there should be only one circuit per
pole, and even then more room should be provided for that circuit than
is common in this type of construction. On each of the two pole lines
between Cañon Ferry and Butte there is a single circuit of three
conductors arranged in triangular form, two at the opposite ends of a
cross-arm and one at the top of the pole, and the distance from each
conductor of a circuit to either of the other two is 6.5 feet. This
distance between conductors is perhaps as great as that on any
transmission circuit now in use, but it seems too small to make repairs
on the circuit reasonably safe when it is in operation at a pressure of
50,000 volts. There seems to be no good reason why the distance between
the conductors of a single circuit to which a pole line is devoted might
not be increased to as much as ten feet, at the slightly greater
expense of longer cross-arms. With as much as ten feet between
conductors, and special tools with long wooden handles to grasp these
conductors, there should be no serious danger about the repair of even
60,000-volt lines when in operation. As the 60,000-volt line between
Electra and San Francisco consists of only one circuit, it seems that
repairs on it must be contemplated during operation.
Another example of a high-voltage transmission carried out with a single
circuit is that between Shawinigan Falls and Montreal, a distance of
eighty-five miles. In this case the circuit is made up of three aluminum
conductors, each of which has an area in cross section of 183,750
circular mils, and these conductors are located five feet apart, one at
the top of each pole, and two at the ends of a cross-arm below. This
single circuit is in regular operation at 50,000 volts for the supply of
light and power in Montreal, and it is hard to see how repairs while
there is current on the line are to be avoided.
Inductance varies with the ratio between the diameter of the wires in
any circuit and the distance between these wires, but as inductance
simply raises the voltage that must be delivered by generators or
transformers, and does not represent a loss of energy, it may generally
be given but little weight in selecting the number of circuits, the
distance between conductors, and the size of each conductor. If two or
more circuits with smaller conductors have a combined resistance in
multiple equal to that of a single circuit with larger conductors, the
loss of voltage due to inductance may be greater on the single circuit
than the corresponding loss on the multiple circuits, but the advantages
due to the single circuit may more than compensate for the higher
pressure at generators or transformers. That such advantages have been
thought to exist in actual construction may be seen from the fact that
the 147-mile line from Electra power-house to San Francisco, and the
83-mile line from Shawinigan Falls to Montreal, are composed of one
circuit each. As inductance increases directly with the length of
circuits, these very long lines are especially subject to its influence,
yet it was thought that the advantages of a single circuit more than
offset its disadvantages in each case.
Where several sub-stations, widely separated, are to be supplied with
energy by the same transmission line, another argument exists for the
division of the line conductors into more than one circuit, so that
there may be an independent circuit to each sub-station. As the pressure
for local distribution lines must be regulated at each sub-station, it
is quite an advantage to have a separate transmission circuit between
each sub-station and the power plant, so that the voltage on each
circuit at the power-house may be adjusted as nearly as possible to the
requirements of its sub-station. An interesting illustration of this
practice may be noted in the design of transmission circuits for the
line between Spier Falls on the Hudson River and the cities of
Schenectady, Troy, and Albany, located between thirty and forty miles to
the south, which passes through Saratoga and Ballston on the way. When
this transmission line is completed, four three-phase circuits, one of
No. 0 and three of No. 000 copper wire, will run to the Saratoga
switch-house from the generating plant at the Falls, a distance of some
eight miles.
From this switch-house two circuits of No. 0 conductors go to the
Saratoga sub-station, a little more than one mile away, two circuits of
No. 000 wires run to the Watervliet sub-station, across the river from
Troy and thirty-five miles from the generating station, and one circuit
of No. 0 and one circuit of No. 000 wires are carried to Schenectady,
thirty miles from Spier Falls, passing through and supplying the
Ballston sub-station on the way. Other circuits connect the sub-station
at Watervliet with that at Schenectady and with the water-power station
at Mechanicsville. From the Watervliet sub-station secondary lines run
to sub-stations that control the local distribution of light and power
in Albany and Troy. This network of transmission circuits was made
desirable by the conditions of this case, which include the general
supply of light and power in three large and several smaller cities, the
operation of three large electric railway systems, and the delivery of
thousands of horse-power for the motors in a great manufacturing plant.
In not every transmission system with different and widely scattered
loads it is thought desirable to provide more than one main circuit.
Thus, the single circuit eighty-three miles long that transmits energy
from Shawinigan Falls to Montreal is designed to supply power also in
some smaller places on the way.
So again, the 147-mile circuit from Electra power-house to San Francisco
passes through a dozen or more smaller places, including Stockton, and
is tapped with side lines that run to Oakland and San José. In cases
like this, where very long lines run through large numbers of cities and
towns that sooner or later require service, it is obviously
impracticable to provide a separate circuit for each centre of local
distribution. It may well be in such a case that a single main
transmission circuit connected to a long line of sub-stations will
represent the best possible solution of the problem. At the power-house
end of such a circuit the voltage will naturally be regulated to suit
that sub-station where the load is the most important or exacting, and
each of the other sub-stations will be left to do all of the regulating
for its own load.
[Illustration: FIG. 76.--Connections at Watervliet Sub-station on Spier
Falls Line.]
The greater the total loss of voltage on a transmission line supplying
sub-stations that are scattered along much of its length, the larger
will be the fluctuations of voltage that must be compensated for at all
of the sub-stations save one, under changing loads, if only one circuit
is employed between the power-plant and these sub-stations. Suppose, for
example, that a transmission line 100 miles long is composed of a single
circuit, and supplies two sub-stations, one located 50 miles and the
other 100 miles from the power-plant. Assume at first that there is no
load whatever at the intermediate sub-station. If the single
transmission circuit operates with 50,000 volts at the power-plant, and
45,000 volts at the sub-station 100 miles away when there is a full load
there, corresponding to a loss of ten per cent, then the pressure at the
intermediate sub-station will be 47,500 volts. If, now, the load at the
sub-station 100 miles from the power-house drops to a point where the
entire line loss is only 1,000 volts, and the pressure at the generating
plant is lowered to 46,000 volts so as to maintain 45,000 volts at the
more distant sub-station, then the pressure at the intermediate
sub-station will be 45,500 volts, or 2,000 volts less than it was
before. If the loss on the entire line at full load were only five per
cent, making the voltage at the sub-station 100 miles away 47,500 when
that at the generating station is 50,000, then the pressure at the
intermediate sub-station will be 48,750 volts. Upon a reduction of the
loss on the entire length of line to one-fifth of its maximum amount, or
to 500 volts, the pressure at the generating station must be reduced to
48,000 volts, if that at the more distant sub-station is to be held
constant at 47,500. At the intermediate sub-station the pressure will
then be 47,750 volts, or 1,000 volts less than it was at full load. From
these two examples it may be seen that the extent of pressure variation
at the intermediate sub-station will depend directly on the maximum line
loss, if the regulation at the generating station is such as to maintain
a constant voltage at the sub-station 100 miles away.
[Illustration: FIG. 77.--Sections of Switch-house on New Hampshire
Traction System.]
All the foregoing has assumed no load to be connected at the
intermediate sub-station, and with a load there the fluctuations of
pressure will of course depend on its amount as well as on the load at
the more distant sub-station.
One of the strongest reasons for the use of two or more circuits in the
same transmission line arises from the rapid fluctuations of load where
large stationary motors or an electric railway system is operated. When
a transmission line must carry a load of stationary or railway motors,
it is a common practice to divide the line into at least two circuits,
and to devote one circuit exclusively to railway or motor work and
another to lighting, at any one time. In some cases this division of the
transmission system into two parts, one devoted to the lighting and the
other to the motor load, is carried out not only as to the sub-station
apparatus and the line, but also as to the transformers, generators,
water-wheels, and even the penstocks at the power-plant. It is possible
even to carry this division of the transmission system still further,
and to separate either the motor or the lighting load, or both, into
sections, and then to devote a distinct transmission circuit, group of
transformers, generator, and water-wheel to the operation of each
section. An example of the complete division of generating and
transmitting apparatus into independent units may be noted in the case
of the system that supplies light and power in Portland, Me., from a
generating plant on the Presumpscot River, thirteen miles away. At this
station four steel penstocks, each provided with a separate gate at the
forebay wall, bring water to as many pairs of wheels, and each pair of
wheels drives a direct-connected generator. Four three-phase circuits
connect the generating plant with the sub-station at Portland, and each
circuit between the generating plant and a transformer-house outside the
business section of the city is made up of No. 2 solid soft-drawn copper
wires.
Each of these four sets of apparatus, from head-gate to sub-station, is
usually operated independently of the others, and supplies either the
motor load or a part of the electric lighting. In this way changes in
the amount of one section of the load cause no fluctuation of the
voltage on the other sections. At Manchester, N. H., the sub-station
receives energy from four water-power plants, and is provided with two
sets of low-tension, 2,300-volt, three-phase bus-bars, one set of these
bus-bars being devoted to the operation of the local electric railway
system, and the other set to the supply of lamps and stationary motors.
Each set of these bus-bars is divided into a number of sections, and by
means of these sections different transmission circuits are devoted to
different portions of the lighting and motor loads. As three of the four
water-power plants are connected to the sub-station by two circuits
each, the division of loads in this case is often carried clear back to
the generators, one generator in a power-house being operated, for
instance, on railway work and another on a lighting load at the same
time. This plan has the obvious advantage that much of the regulation
for the several parts of the entire load may be done at the generators,
thus reducing the amount of regulation necessary at the sub-station, and
that fluctuating motor loads do not affect the lamps. In this case the
conductors of the several transmission circuits are all of moderate
size, and the division of the lines was evidently adopted for purposes
of regulation, rather than to reduce the amount of inductance. Thus the
line between Gregg’s Falls and the sub-station, a distance of six miles,
is made up of one three-phase circuit of No. 4 and one circuit of No. 6
bare copper wires. The fourteen-mile line between the plant at Garvin’s
Falls and the sub-station, the longest of the four transmissions, is
made up of two three-phase circuits, each composed of No. 0 bare copper
wires. In the case of the Gregg’s Falls plant the subdivision of the
line has gone further than that of the generating equipment, for the
station there contains only a single generator, the rating being 1,200
kilowatts, while two circuits run thence to the sub-station. Another
instance showing extensive subdivision of a line into separate circuits
may be noted in the seven-mile transmission from Montmorency Falls to
Quebec, Canada, where sixteen conductors, each No. 0 copper wire, make
up four two-phase circuits that connect a plant of 2,400 kilowatts
capacity with its sub-station.
Such multiplication of transmission circuits has some advantages from
the standpoint of regulation, but there are good reasons for limiting it
to rather short lines, where it is, in fact, almost exclusively found.
On very long lines the use of numerous circuits composed of rather small
conductors would obviously increase the constant expense of inspection
and repairs and add materially to uncertainty of the service. Very few,
if any, transmission lines of as much as twenty-five miles in length are
divided into more than two circuits, and in several instances lines of
superlative length have only a single circuit each. The greatest single
power transmission in the world, that between Niagara Falls and Buffalo,
is carried out with two pole lines, one of which is about twenty and the
other about twenty-three miles long. The longer pole line, which is also
the older, carries two three-phase circuits, each of which is made up of
three 350,000 circular mil copper conductors. The shorter pole line
carries a single three-phase circuit composed of aluminum conductors,
each of which has an area in cross section of 500,000 circular mils. In
electrical conductivity the aluminum circuit is intended to be equal to
each of the two that are composed of copper. According to the
description of the Niagara Falls and Buffalo transmission system in vol.
xviii., A. I. E. E., pages 518 to 527, each of these three circuits is
designed to transmit about 7,500 kilowatts, and the maximum power
transmitted up to August, 1901, was 15,600 kilowatts, or about the
calculated capacity of two of the circuits. According to the description
just mentioned, the transmission circuits used to supply energy for use
at Buffalo are regularly operated in parallel, and this is also true of
the generators and the step-down transformers, though the uses to which
this energy is applied include lighting, large stationary motors, and
the electric railway system. Apparatus in the generating station at
Niagara Falls and in the terminal-house near the city limits of Buffalo
is so arranged, however, that two of the 3,750 kilowatt generators and
eight step-up transformers at the power-house, together with one
transmission circuit and three step-down transformers in the
terminal-house at Buffalo, may be operated independently of all the
other apparatus.
As already pointed out, the use of separate circuits for each
sub-station, and for lighting and power loads at each sub-station in
very long transmission systems, is often impracticable. Even in
comparatively short transmissions the multiplication of circuits and the
use of rather small and mechanically weak conductors increased the first
cost of installation and the subsequent expense of inspection and
repairs. An objection to operation with a single circuit in a
transmission line that supplies widely separated sub-stations with
lighting, power, and railway loads is the consequent difficulty of
pressure regulation on the distribution lines at each sub-station. Such
a transmission line necessarily delivers energy at different and
fluctuating voltages at the several sub-stations, and these fluctuations
are of course reproduced on the secondary side of the step-down
transformers. Fortunately, however, the use of synchronous motor
generators, either in place of or in connection with static
transformers, goes far to solve the problem of pressure regulation for
distribution circuits supplied with energy from transmission lines. This
is due to the well-known fact that with constant frequency the speed of
rotation for a synchronous motor is constant without regard to
fluctuations in the applied voltage or changes in its load. With a
constant speed at the motor and its connected generator it is of course
easy to deliver current at constant voltage to the distribution lines.
This constancy of speed makes the synchronous motor generator a favorite
in large transmission systems with both power and lighting loads. The
satisfactory lighting service in Buffalo, operated with energy
transmitted from Niagara Falls, seems to be due in some measure to the
use of synchronous motor generators at the sub-station in Buffalo,
whence lighting circuits are supplied. As above stated, the three
circuits that make up the transmission line between Niagara Falls and
Buffalo are operated in multiple, and in the latter place there is a
large load of both railway and stationary motors. As the three circuits
are operated in multiple, they of course amount to only a single
circuit so far as fluctuations of voltage due to changes in these
several sorts of loads are concerned. According to vol. xviii., A. I. E.
E., pages 125 and following, the load on the transmission system at
Buffalo in 1901 was made up of about 7,000 horse-power in railway
motors, 4,000 horse-power in induction motors, and 4,000 horse-power
divided up between series arc lamps, constant pressure incandescent
lamps, and continuous current motors. The railway load is operated
through step-down transformers and rotary converters. The induction
motors are connected either to the 2,000-volt secondary circuits of the
step-down transformers or to service transformers supplied by these
circuits. On these railway and stationary motor loads there is of course
no necessity for close pressure regulation. Series arc lamps are
operated through step-down transformers and synchronous motors
direct-connected to constant continuous current dynamos. Continuous
current stationary motors draw power from the transmission lines through
step-down transformers and rotary converters, like the railway load. For
the 2,200 volt circuits that supply service transformers for commercial
arc and incandescent lighting the transmitted energy passes through
step-down transformers and synchronous motor-generators. These
motor-generators raise the frequency from twenty-five to sixty cycles
per second. Finally the continuous current three-wire system for
incandescent lighting at about 250 volts between outside wires is
operated through step-down transformers and synchronous motors
direct-connected to continuous current generators. For this last-named
service rotary converters were at first tried, but were found to be
impracticable because voltage fluctuations on the transmission line (due
largely to the railway and motor loads) were reproduced on the
continuous-current circuits by the rotary converters. Since the adoption
of motor-generators this fluctuation of the service voltage is no longer
present.
Another case in which synchronous motor-generators deliver power from a
transmission line that carries both a lighting and a motor load is that
of the Shawinigan sub-station in Montreal. At this sub-station the
85-mile transmission line from the generating plant at Shawinigan Falls
terminates. As already pointed out, this line is composed of a single
three-phase circuit of aluminum conductors, each of which has a cross
section of 183,750 circular mils. In the Montreal sub-station the
thirty-cycle, three-phase current from Shawinigan Falls is delivered to
transformers that lower the voltage to 2,300. The current then goes to
five synchronous motor-generators of 1,200 horse-power capacity each,
and is there converted to sixty-three cycles per second, two-phase, at
the same voltage. This converted current passes onto the distribution
lines of the local electrical supply system in Montreal, which also
draws energy from two other water-power plants, and is devoted to
lighting, stationary motors, or to the street railway work, as may be
required. Though separate local distribution circuits are devoted to
these several loads, the fluctuations in the stationary and railway
motor work necessarily react on the voltage of the transmission line and
transformers at the sub-station. By the use of the synchronous
motor-generators the lighting circuits are protected from these pressure
variations.
As the numbers of sub-stations at different points on long transmission
lines increase, and stationary motor and railway loads at each become
more common, it is to be expected that the use of synchronous
motor-generators for lighting service will be much more frequent than at
present. With such use there will disappear one of the reasons for the
multiplication of transmission circuits.
[Illustration: FIG. 78.--Transfer Switches at Saratoga Switch-house on
Spier Falls Line.]
Where several transmission circuits connect a generating plant with a
single sub-station, or with several sub-stations in the same general
direction, it is desirable to have switches so arranged that two or more
circuits may be combined as one, or so that any circuit that ordinarily
operates a certain load or sub-station may be devoted to another when
occasion requires. For this purpose transfer switches on each circuit
are necessary at generating plants, sub-stations, and often at
switch-houses. These transfer switches will ordinarily be of the knife
type, and intended for manual operation when the circuits to which they
are connected are not in use. As such switches are exposed to the full
voltage of transmission, the insulation of their conducting parts should
be very high. In the extensive transmission system between the
power-plants at Spier Falls and Mechanicsville and the sub-stations at
Troy, Albany, and Schenectady, N. Y., a transfer switch of highly
insulated construction has been much used. The two blades of this switch
move independently of each other, but both are mounted between the same
metal clips. Each blade is of two by one-quarter inch drawn copper rod,
and the clips supporting the two blades are mounted on top of a circular
metal cap four and three-quarter inches in outside diameter and two
inches high, that is cemented over the top of a large, double petticoat,
porcelain line insulator.
[Illustration: FIG. 79.--Cross Section of Schenectady Switch-house on
Spier Falls Line.]
Clips into which these copper blades are swung in closing the switch are
also mounted in caps carried by insulators in the way just described.
Each of these insulators is mounted on a large wooden pin, and these
pins are secured in timbers at the points where the switches are wanted.
This construction of switches gives ample insulation for the line
voltage of 30,000 in this system. By means of the transfer switches
just described, either of the transmission circuits leaving the Spier
Falls power-plant may be connected to any one of the ten generators and
ten groups of transformers there. At the Saratoga switch-house, any one
of the twelve conductors, making up the four three-phase circuits from
Spier Falls may be connected to any one of the eighteen conductors
making up the six three-phase circuits that go south to Saratoga,
Watervliet, and Schenectady sub-stations, in the way indicated by the
drawing. So again at the Watervliet sub-station, where energy at 26,500
volts is received from Spier Falls and energy at 10,800 volts from
Mechanicsville, any single conductor from either of these water-power
plants may be connected, either directly or through a transformer, with
either conductor running to the railway and lighting sub-stations about
Albany and Troy. Where several transmission circuits are employed, this
complete flexibility of connection evidently adds materially to the
convenience and reliability of operation.
CIRCUITS IN TRANSMISSION LINES.
+----------------------------+------+------+------+----------+--------+
| | | | | | Cycles |
| |Length|Number|Number| Circular | per |
| Location of Lines. | in | of | of | Mils | Second |
| |Miles.|Cir- | Pole | per | of |
| | |cuits.|Lines.| Wire. |Current.|
+----------------------------+------+------+------+----------+--------+
|Electra to San Francisco | 147 | 1 | 1 |[A]471,034| 60 |
|Colgate to Oakland, Cal. | 142 | 2 | 2 | 133,100| 60 |
| | | | |[A]211,000| |
|Santa Ana River to Los | | | | | |
|Angeles | 83 | 2 | 1 | 83,690| 60 |
|Shawinigan Falls to Montreal| 85 | 1 | 1 |[A]183,750| 30 |
|Cañon Ferry to Butte | 65 | 2 | 2 | 106,500| 60 |
|Welland Canal to Hamilton | 35 | 1 | 1 | 83,690| 60 |
|Welland Canal to Hamilton | 37 | 1 | 1 | 133,100| 60 |
|Spier Falls to Schenectady | 30 | 2 | 1 | 105,600| 40 |
| | | | | 167,800| |
|Spier Falls to Watervliet, | | | | | |
|N. Y. | 35 | 2 | 1 | 167,800| 40 |
|Ogden to Salt Lake City | 36 | 2 | 1 | 83,690| 60 |
|Apple River Falls to St. | | | | | |
|Paul | 27 | 2 | 1 | 66,370| 60 |
|Niagara Falls to Buffalo | 23 | 2 | 1 | 350,000| 25 |
|Niagara Falls to Buffalo | 20 | 1 | 1 |[A]500,000| 25 |
|Farmington River to Hartford| 11 | 1 | 1 |[A]364,420| 60 |
|Niagara Falls to Toronto | 75 | 2 | 1[B]| 190,000| 25 |
+----------------------------+------+------+------+----------+--------+
[A] Aluminum conductor.
[B] Steel towers.