REGULATION OF TRANSMITTED POWER.
Regulation of voltage at incandescent lamps is a serious problem in the
distribution of electrically transmitted energy. Good regulation should
not allow the pressure at incandescent lamps rated at 110 to 120 volts
to vary more than one volt above or below the normal.
Electric motor service is much less exacting as to constancy of voltage,
and the pressure at motor terminals may sometimes be varied as much as
ten per cent without material objection on the part of users. A mixed
service to these three classes of apparatus must often be provided where
transmitted energy is used, and the limitations as to variations at
incandescent lamps are thus the ones that must control the regulation of
pressure.
Transmission systems may be broadly divided into those that have no
sub-stations and must therefore do all regulation at the generating
plant, and those that do have one or more sub-stations so that
regulation of voltage may be carried out at both ends of the
transmission line.
[Illustration: FIG. 64.--Arc-lighting Switchboard at Central
Sub-station, Montreal.]
As a rule, a sub-station with an operator in attendance is highly
desirable between transmission and distribution lines, and this is the
plan generally followed at important centres of electrical supply, even
though the transmission is a short one. One example of this sort may be
noted at Springfield, Mass., where energy for electrical supply is
transmitted from two water-power plants on the Chicopee River only about
four and a half and six miles, respectively, from the sub-station in the
business centre of the city. The voltage of transmission for two-phase
current in this case is 6,000, and is reduced to about 2,400 volts at
the sub-station for the general distribution of light and power. A
similar instance may be seen at Concord, N. H., where electrical energy
at both 2,500 and 10,000 volts is delivered to a sub-station in the
business section from a water-power plant at Sewall’s Falls, on the
Merrimac River, four and one-half miles distant. From this sub-station
the current is distributed at about 2,500 volts for the supply of lamps
and motors. A sub-station was found desirable at Concord for purposes
of regulation before the voltage of transmission was raised above that
of distribution. Subsequently, when the load increased, the voltage of
10,000 was adopted on a part of the transmission circuit in order to
avoid an increase in the size of their conductors.
[Illustration: FIG. 65.--Area of Electrical Distribution at Montreal.]
[Illustration: FIG. 66.]
[Illustration: FIG. 67.]
[Illustration: FIG. 68.]
In some instances, however, transmission and distribution lines are
joined without the intervention of a sub-station, where regulation of
voltage can be accomplished, though this practice has little to
recommend it aside from the savings in first cost of installation and
subsequent cost of operation. These savings are more apparent than real
if fairly constant pressure is to be maintained at the lamps, because
what is gained by the omission of sub-stations will be offset, in part
at least, by additional outlays on the lines if good regulation is to be
maintained. This fact may be illustrated by reference to Figs. 66, 67,
and 68, in each of which _D_ represents a generating station and _A_,
_B_, and _C_ towns or cities where energy from the station is to be
distributed. In the case of each figure it is assumed that the distance
between the generating station and each of the cities or towns is such
that distributing lines with a loss of, say, not more than two per cent
in voltage at full load cannot be provided between the generating
station and each city or town because of the cost of conductors. This
being so, one or more centres of distribution must be located in each
town, and the transmission lines must join the distribution lines at
these centres either on poles or in sub-stations. If several of these
towns are in the same general direction from the generating plant so as
to be reached by the same transmission line, as _A_, _B_, and _C_ in
Fig. 66, this one line will be all that is necessary with a sub-station
in each town. Where sub-stations are not employed a separate
transmission circuit must be provided between the generating plant and
each town for reasons that will appear presently. The percentage of
voltage variation in a transmission line under changing loads will be
frequently from five to ten, and is thus far beyond the allowable
variations at incandescent lamps. To give good lighting service the
centre of distribution, where the transmission line joins the
distribution circuits, must be maintained at very nearly constant
voltage if no sub-station is located there. Regulation at a generating
station will compensate for the changing loss of pressure in a line
under varying loads so as to maintain a nearly constant voltage at any
one point thereon. No plan of station regulation, however, can maintain
constant voltages at several points on the same transmission line when
there is a varying load at each. The result is that even though the
several towns served are in the same general direction from the
generating station, as in Fig. 67, yet each town should have its
separate transmission line where no sub-stations in the towns are
provided. In the case illustrated by Fig. 68, where the towns served are
in very different directions from the generating station, there should
be a separate transmission line to each, regardless of whether there is
a sub-station or only a centre of distribution there.
Even in the case illustrated by Fig. 68, as in each of the others, there
is a large saving effected in the cost of distribution lines by the
employment of a sub-station at the point where these lines join the
transmission circuit, provided that the variation of pressure at lamp
terminals is to be kept within one volt either way from the standard.
With the variations of loads the loss of pressure in the distribution
lines will range from zero to its maximum amount and the connected lamps
will be subjected to the change of voltage represented by this total
loss, unless the distribution start from a sub-station where the loss in
distribution lines can be compensated for by regulation. To give good
service the distribution lines should be limited to a loss of one per
cent at full load if there is no sub-station where they join
transmission lines. With opportunity for regulation at a sub-station the
maximum loss in distribution lines may easily be doubled, thus reducing
their weight by one-half in comparison with that required where there is
no sub-station.
Another advantage of connecting transmission and distribution lines in a
sub-station, where regulation of voltage can be had, lies in the fact
that it is practically impossible to maintain an absolutely constant
pressure miles from a generating plant at the end of a transmission line
that is carrying a mixed and varying load. A result is that without the
intervention of regulation at a sub-station it is almost impossible to
give good lighting service over a long transmission line. Furthermore,
the labor of regulation at a generating station is much increased where
there are no sub-stations, because it must be much more frequent and
accurate. The absence of sub-stations from a transmission system thus
implies more transmission circuits, heavier distribution circuits, more
labor at the generating plant, and a poor quality of lighting service.
Where stationary motors form the great bulk of the load on a
transmission system, and good lighting service is of small importance,
it may be well to omit sub-stations at some centres of distribution.
This is a condition that sometimes exists in the Rocky Mountain region
where the main consumers of power along a transmission line may be mines
or works for the reduction of ores. An example of this sort exists in
the system of the Telluride Power Transmission Company, in Utah, which
extends from Provo Cañon, on the river of the same name, entirely around
Utah Lake by way of Mercur, Eureka, and Provo, and back to the
power-house in Provo Cañon, a continuous circuit of 105 miles.
The transmission voltage on this line is 40,000, and at intervals where
there are distributing points the voltage is reduced to about 5,000 by
transformers on poles, and without the aid of regulation at sub-stations
in some cases. The power thus transmitted is largely used in mines and
smelters for the operation of motors, but also for some commercial
lighting.
Regulation at generating stations of the voltage on transmission lines
may be accomplished by the same methods whether there are sub-stations
at centres of distribution or not. In any such regulation the aim is to
maintain a certain voltage at some particular point on the transmission
line, usually its end, where the distribution circuits are connected. If
more than one point of distribution exists on the same transmission
line, the regulation at the generating plant must be designed to
maintain the desired pressure at only one of these points, leaving
regulation at the others to be accomplished by local means. One method
of regulation consists in the overcompounding of each generator so that
the voltage at its terminals will rise at a certain rate as its load
increases. If a generator and transmission line are so designed that the
rise of voltage at the generator terminals just corresponds with the
loss of voltage on the line when the output of that generator alone
passes over it to some particular point, then the pressure at that point
may be held nearly constant for all loads if no energy is drawn from the
line elsewhere. These several conditions necessary to make regulation by
the compounding of generators effective can seldom be met in practice.
If a varying number of generators must work on the same transmission
line, or if varying loads must be supplied at different points along the
line, no compound winding of generators will suffice to maintain a
constant voltage at any point on the line that is distant from the
power-station. For these reasons the compound winding of generators is
of minor importance so far as the regulation of voltage on transmission
lines is concerned, and on large alternators is not generally attempted.
An example may be noted on the 3,750-kilowatt generators at Niagara
Falls, where the single magnet winding receives current from the
exciters only.
A much more effective and generally adopted method of regulation of
voltage at the generating plants of transmission systems is based on the
action of an attendant who varies the current in the magnet coils of
each generator so as to raise or lower its voltage as desired. The
regulation must be for some one point on the transmission line, and the
attendant at the generating plant may know the voltage at that point
either by means of a pair of pressure wires run back from that point to
a voltmeter at the generating plant, by a meter that indicates the
voltage at the point in question according to the current on the line,
or by telephone connection with a sub-station at the point where the
constant voltage is to be maintained. Pressure wires are a reliable
means of indicating in the generating station the voltage at a point of
distribution on the line, but the erection of these wires is quite an
expense in a long transmission, and in such cases they are only
occasionally used. Owing to inductive effects and to variable
power-factors the amperes indicated on a line carrying alternating
current are far from a certain guide as to the drop in voltage between
the generating station and the distant point. In long transmissions,
telephone communication between the generating plant and the
sub-stations is the most general way in which necessary changes to
maintain constant voltage at sub-stations are brought to the attention
of the attendant in the generating plant. Few, if any, extensive
transmission systems now operate without telephone connection between a
generating plant and all of its sub-stations, or between a single
sub-station and the several generating plants that may feed into it.
Thus, the generating plant at Spier Falls, on the Hudson River, will be
connected by telephone with sub-stations at Schenectady, Albany, Troy,
and some half-dozen smaller places. On the other hand, the single
sub-station in Manchester, N. H., that receives the energy from four
water-power plants has a direct telephone line to each.
Where two or more transmission lines from the same power-station are
operated from the same set of bus-bars the voltage at a distant point on
each line cannot be held constant by changes of pressure on these
bus-bars. One generator only may be connected to each transmission line
and be regulated for the loss on that line, but this loses the
advantages of multiple operation. Another plan is to connect a
regulator in each transmission line before it goes from the generating
plant. One type of regulator for this purpose consists of a transformer
with its secondary coil divided into a number of sections and the ends
of these sections brought out to a series of contact segments. The
primary coil of this transformer may be supplied with current from the
bus-bars and the secondary coil is then connected in series with the
line to be regulated, so that the secondary voltage is added to or
subtracted from that of the main circuit. A movable contact arm on the
segments to which the sections of the secondary coil are connected makes
it possible to vary the secondary voltage by changing the number of
these sections in circuit. In another transformer used for regulating
purposes the primary coil is connected to the bus-bars as before and the
movable secondary coil is put in series with the line to be regulated.
The regulation is accomplished in this case by changing the position of
the secondary relative to that of the primary coil and thus raising or
lowering the secondary voltage. Both of these regulators require hand
adjustment, and the attendant may employ the telephone, pressure wires,
or the compensating voltmeter above mentioned, to determine the voltage
at the centre of distribution. The voltage indicated by this so-called
“compensator” is that at the generating station minus a certain amount
which varies with the current flowing in the line to be regulated. The
voltmeter coil of the compensator is connected in series with the
secondary coils of two transformers, which coils work against each
other. One transformer has its secondary coil arranged to indicate the
full station voltage, and the other secondary coil is actuated by a
primary coil that carries the full current of the regulated line. By a
series of contacts the effect of this last-named coil can be varied to
correspond with the number of volts that are to be lost at full load
between the generating station and the point on the transmission line at
which the voltage is to be held constant. If there is no inductive drop
on the transmission line, or if this drop is of known and constant
amount, the compensator may give the actual voltage at the point for
which the regulation is designed.
Automatic regulators are used in some generating stations to maintain a
constant voltage either at the generating terminals or at some distant
distributing point on a line operated by a single generator. These
regulators may operate rheostats that are in series with the magnet
windings of the generators to be regulated, and raise or lower the
generator voltage by varying the exciting current in these windings.
These regulators are much more effective to maintain constant voltage at
generating stations than at the distributing end of long transmission
lines with variable power-factors. In spite of the compound winding of
generators, of automatic regulators for the exciting currents in their
magnet coils, and of regulating transformers in the transmission
circuits, hand-adjustment of rheostats in series with the magnet coils
of generators remains the most generally used at the generating stations
of long transmission systems. Automatic regulators at the ends of
transmission lines in sub-stations are now being introduced, and may
prove very desirable.
[Illustration: FIG. 69.--Motor-generators in Shawinigan Sub-station at
Montreal.]
The more exacting and final work of regulation in transmission systems
is usually done at the sub-stations. After a nearly constant voltage is
delivered at the high-pressure coils of step-down transformers in a
sub-station, there remains the varying losses in these transformers, in
motor-generators or converters, in distribution lines and in service
transformers, to be compensated for. In general, three or four sorts of
loads must be provided for, namely, arc or incandescent lamps for street
lighting on series circuits, usually of 4,000 to 10,000 volts. Arc and
incandescent lamps on constant-pressure circuits of 2,000 to 2,500 volts
for commercial lighting, direct-current stationary motors on
constant-pressure circuits of about 500 volts, and alternating motors
which may be served at either 2,500 or 500 volts according to their
sizes and locations. To these loads may be added that of street-car
motors of 500 volts, direct current. Both the stationary and the
street-car motors, but more especially the latter, by their changes of
load give rise to large and rapid fluctuations of voltage on the
distribution lines to which they are connected. The problem of
regulation with combined lamp and motor loads is not, therefore, so much
to maintain a nearly constant voltage at the motors as to protect the
lamps from the fluctuations of voltage which the motors set up.
[Illustration: FIG. 70.--One of the 1,065-kilowatt Motor-generators in
the Shawinigan Sub-station at Montreal.]
For street-car motors using direct current at about 500 volts, the
sub-station equipment includes either step-down transformers and
converters or motor-generators with or without transformers. It is the
practice in some cases where both lighting and street-railway service
are drawn from the same transmission system to keep these two kinds of
service entirely separate, devoting independent generators and
transmission lines, as well as independent transformers and converters
or motor-generators, to the street-car work. This is done in the
transmission system centring at Manchester, N. H., in which each one of
the four water-power plants, as well as the sub-station, has a double
set of bus-bars on the switchboard; and from each water-power plant to
the sub-station there are two transmission circuits. In operation, one
set of generators, bus-bars, transmission circuits, and transformers
supply converters or motor-generators for the street-car motors; and
another set of generators, bus-bars, transmission circuits, and
transformers are devoted to lighting and stationary motors in this
system. Where street-car motors draw their energy from the same
generators and transmission lines that supply commercial incandescent
lamps, some means must be adopted to protect the lighting circuits from
the fluctuations of voltage set up by the varying street-car loads. One
way to accomplish this purpose is to operate the lighting circuits with
generators driven by synchronous motors in the sub-stations. These
generators may, of course, be of either direct or alternating type and
of any desired voltage. The synchronous motors driving these generators
take their current from the transmission line either with or without the
intervention of step-down transformers. By this use of synchronous
motors the lighting circuits escape fluctuations of voltage
corresponding to those on the transmission line, because synchronous
motors maintain constant speeds independently of the voltage of the
circuits to which they are connected. This plan was followed at Buffalo,
where the street-car system and the lighting service are operated with
energy from the Niagara Falls stations over the same transmission line.
In one of the sub-stations at Buffalo, both 2,200-volt, two-phase
alternators, and 150-volt continuous-current generators for lighting
service, are driven by synchronous motors connected to the Niagara
transmission line through transformers. At other sub-stations in
Buffalo, the 500-volt continuous current for street-car motors is
obtained from the same transmission system through transformers and
converters. Another solution of the problem of voltage regulation where
street-railway and commercial lighting service are to be drawn from the
same transmission line is found in the operation of 500-volt
continuous-current generators in the sub-stations by synchronous motors
fed from the line either directly or through transformers. This plan has
been adopted in the transmission system of the Boston Edison Company,
which extends to a number of cities and towns within a radius of
twenty-five miles. The sub-stations at Natick and Woburn in this system,
where there are street-railway as well as lighting loads, contain
500-volt continuous-current generators driven by synchronous motors
connected directly to the three-phase transmission lines. In a case like
this the synchronous motors maintain their speed irrespective of the
voltage on the line and thus tend to hold that voltage steady in spite
of the variable losses due to fluctuating loads.
Stationary motors should not as a rule be operated from the same
distribution lines that supply incandescent lamps, especially in sizes
above one horse-power, and this is the better practice. Motor circuits
of about 2,400 volts and two- or three-phase, alternating, or 500 volts,
alternating or direct current, may be supplied at a sub-station either
by transformers alone in the first case or by transformers and
converters in the second. In either case no especial provision is
usually necessary for the regulation of constant pressure on the motor
circuits.
[Illustration: FIG. 71.--1,100-kilowatt, 2,300-volt, Three-phase,
30-cycle, Synchronous Motor at Sub-station of Shawinigan Line in
Montreal.]
In some transmission systems the distribution circuits for stationary
motors are not fed by the same transmission lines that carry the
lighting load, but draw their energy from lines that do no other work.
This practice is certainly desirable, as it frees the lighting circuits
from all fluctuations of voltage due to line losses with changing motor
loads. Examples of this sort may be seen at Springfield, Mass., and
Portland and Lewiston, Me., in each of which the load of stationary
motors is operated over independent transmission as well as distribution
lines.
In transmission systems series arc and incandescent lamps for street
lighting are commonly operated either by direct-current arc dynamos or
by constant-current transformers or constant-pressure transformers with
automatic regulators at the sub-stations. The arc dynamos are driven by
either induction or synchronous motors supplied directly from the
transmission line or through transformers. As the arc dynamos regulate
automatically for constant current no further regulation is required. If
the series arc and incandescent lamps are to be supplied with
alternating current, the constant-current transformer or the
constant-current regulator come into use. This type of transformer and
regulator alike depend for their regulating effect on the movement of a
secondary coil on a transformer core in such a way that the current in
this coil, which is in series with the lamps, is held nearly constant.
Such constant-current transformers and regulators are usually supplied
from the transmission line through regular constant-pressure
transformers, and they hold their currents sufficiently constant for the
purposes of their use.
The main problem of regulating thus comes back to the 250- or
2,200-volt, constant-pressure circuits for incandescent lighting,
supplied from transmission lines through transformers or motor
generators or both at the sub-station. For this regulation one of the
most reliable instruments is the hand of a skilful attendant, guided by
voltmeters connected with pressure wires from minor centres of
distribution, and adjusting the regulating transformers above mentioned,
or other regulating devices.