ADVANTAGES OF THE CONTINUOUS AND ALTERNATING CURRENT.
Electrical transmissions over long distances in America have been mainly
carried out with alternating current. In Europe, on the other hand,
continuous current is widely used on long transmissions at high
voltages. So radical a difference in practice seems to indicate that
neither system is lacking in points of superiority.
A fundamental feature of long transmissions is the high voltage
necessary for economy in conductors, and this voltage is attained by
entirely different methods with continuous and alternating currents. In
dynamos of several hundred or more kilowatts capacity the pressure of
continuous current has not thus far been pushed above 4,000 volts,
because of the danger of sparking and flashing at the commutator. Where
10,000 or more volts are required on a transmission line with continuous
current a number of dynamos are connected in series so that the voltage
of each is added to that of the others. In this way the voltage of each
dynamo may be as low as is thought desirable without limiting the total
line voltage. There is no apparent limit to the number of
continuous-current dynamos that may be operated in series or to the
voltage that may be thus obtained. In the recently completed
transmission from St. Maurice to Lausanne, Switzerland, with continuous
current, ten dynamos are connected in series to secure the line voltage
of 23,000. When occasion requires twenty or thirty or more dynamos to be
operated in series, giving 50,000 or 75,000 volts on the line, machines
exactly like those in the transmission just named, may be used. No
matter how many of these dynamos are operated in series the electric
strain on the insulation of the windings of each dynamo remains
practically constant, because the iron frame of each dynamo is insulated
in a most substantial manner from the ground. The electric strain on the
insulation of the windings of each dynamo in the series is thus limited
to the voltage generated by that dynamo. There is no practical limit to
the thickness or strength of the insulation that may be interposed
between the frame of each dynamo and the ground, and hence no limit to
line voltage as far as dynamo insulation is concerned.
It is impracticable to operate alternating dynamos in series so as to
add their voltages, and the pressure available in transmission with
alternating current must be that of a single dynamo or must be obtained
by the use of transformers. The voltage of an alternating may be carried
much higher than that of a continuous-current dynamo of very large
capacity, and in many cases pressures of 13,200 volts are now supplied
to transmission lines by alternating dynamos. Just how high the voltage
of single alternating dynamos will be carried no one can say, but it
seems probable that the practical limit will prove to be much less than
the voltages now employed in some transmissions. As the voltage of
alternating dynamos is carried higher the thickness of insulation on
their armature coils and consequently the size or number of slots in
their armature cores and the size of these cores increase rapidly. The
dimensions and weight of an alternating dynamo per unit of its capacity
thus go up with the voltage, and at some undetermined point the cost of
the high-voltage dynamo is greater than that of a low-voltage dynamo of
equal capacity with raising transformers. To the voltage that may be
supplied by transformers there is no practical limit now in sight. Lines
have been in regular operation from one to several years on which
transformers supply 40,000 to 50,000 volts; some large transformers have
been built for commercial use at 60,000 volts, and other transformers
for experimental and testing purposes have been employed in a number of
cases for pressures of 100,000 volts and more.
Available voltages for continuous- and alternating-current transmissions
are thus on a practically equal footing as to their upper limit. The
amount of power that may be generated and delivered with either the
alternating- or continuous-current system of transmission is practically
unlimited. Single alternating dynamos may be had of 5,000 or even 8,000
kilowatts capacity if desired, but it is seldom that these very large
units are employed, because the capacity of a generating station should
be divided up among a number of machines. It is perhaps impracticable to
build single continuous-current dynamos with capacities equal to those
of the largest alternators, but as any number of the continuous-current
machines may be operated either in series or multiple, the power that
may be applied to a transmission circuit is unlimited.
At the plant or plants where the power transmitted by continuous current
is received, a number of motors must be connected in series to operate
at the high-line voltage. These motors may all be located in a single
room, may be connected to machinery in different parts of a building, or
may be in use at points miles apart. The vital requirement is that the
motors must be in series with each other so that the line voltage
divides between them. If simply mechanical power is wanted at the places
where the motors are located, they complete the transmission system and
no further electrical apparatus is required. Where, however, as at
Lausanne, the transmitted power is to be used in a system of general
electrical supply, the motors that receive the current at the line
voltage must drive dynamos that will deliver energy of the required
sorts. In the station at Lausanne four of the motors to which the
transmission line is connected each drives a 3,000-volt three-phase
alternator for the distribution of light and power. The fifth motor at
this station drives a 600-volt dynamo which delivers continuous current
to a street railway. A sixth motor in the same series drives a cement
factory some distance from the station. Neglecting minor changes in
capacity due to losses in the line and motors, this continuous-current
system must thus include three kilowatts in motors and dynamos for each
kilowatt delivered for general electrical distribution at the receiving
station. In a case in which only mechanical power is wanted at the
receiving station, the dynamos and motors concerned in the transmission
must have a combined capacity of two horse-power for each horse-power
delivered at the motor shaft. In contrast with these figures, the
electrical equipment in a transmission with alternating current for
mechanical power alone includes two kilowatts capacity in generators and
motors, besides two kilowatts capacity in transformers for each
corresponding unit of power delivered at the motor shaft unless
generators and motors operate at the full line voltage. If a general
electrical supply is to be operated by the alternating system of
transmission, either motors and dynamos or rotary converters must be
added to transformers where continuous current is required. An
alternating transmission may thus include as little as one kilowatt in
dynamos and one in transformers, or as much as two kilowatts capacity in
dynamos, two in transformers, and one in motors for each kilowatt
delivered to distribution lines at the receiving station.
Line construction from the continuous-current transmission is of the
most simple character apart from the necessity of high insulation. Only
two wires are necessary and they may be of any desired cross-section,
strung on a single pole line and need not be transposed. On these wires
the maximum voltage for which insulation must be provided is the nominal
voltage of the system. It is possible under these conditions to build a
single transmission line with two conductors of such size and strength
and at such a distance apart that a high degree of reliability is
attained against breaks in the wires or arcing between them. In a
transmission of power by two- or three-phase alternating current at
least three wires are necessary and six or more are often employed. If
six or more wires carrying current at the high voltages required by long
transmissions are mounted on a single line of poles, it is not
practicable to obtain such distances between the wires as are desirable.
The repair of one set of wires while the other set is in operation is a
dangerous task, and an arc originating between one set of the wires is
apt to be communicated to another set. For these reasons two pole lines
are frequently provided for a transmission with alternating current, and
three or more wires are then erected on each line. Compared with a
continuous-current transmission, one with alternating current often
requires more poles and is quite certain to require more cross-arms,
pins, insulators, and labor of erection. For a given effective voltage
of transmission it is harder to insulate an alternating- than a
continuous-current line. In the first place the maximum voltage of the
alternating line with even a true sine curve of pressure is 1.4 times
the nominal effective voltage, but the insulation must withstand the
maximum pressure. Then comes the matter of resonance, which may carry
the maximum voltage of an alternating circuit up to several times its
normal amount, if the period of electrical vibration for that particular
circuit should correspond to the frequency of the dynamos that operate
it. Even where the vibration period of a transmission circuit and the
frequency of its dynamos do not correspond, and good construction should
always be planned for this lack of agreement, resonance may and often
does increase the normal voltage of an alternating transmission by a
large percentage. The alternating system of transmission must work at
practically constant voltage whatever the state of its load, so that the
normal stress on the insulation is always at its maximum. In a
transmission with continuous current on the other hand, if the
prevailing practice of a constant current and varying pressure on the
line is followed, the insulation is subject to the highest voltage only
at times of maximum load on the system. Lightning is a very real and
pressing danger to machinery connected to long transmission lines, and
this danger is much harder to guard against in an alternating system
than in a system with continuous constant current. The large degree of
exemption from damage by lightning enjoyed by series arc dynamos is well
known, the magnet windings of such machines acting as an inductance that
tends to keep lightning out of them. Moreover, with any
continuous-current machines lightning arresters having large
self-induction may be connected in circuit and form a most effective
safeguard against lightning, but this plan is not practicable on
alternating lines.
In the matter of switches, controlling apparatus, and switchboards, an
alternating transmission requires much more equipment than a system
using continuous, constant current. The ten dynamos in the generating
station at St. Maurice, with a capacity of 3,450 kilowatts at 23,000
volts, are each connected and disconnected with the transmission by a
switch in a small circular column of cast-iron that stands hardly breast
high. An amperemetre and voltmetre are mounted on each dynamo. The
alternating generators in a station of equal capacity and voltage would
require a large switchboard fitted with bus-bars, oil switches, and
automatic circuit-breakers. Relative efficiencies for the
continuous-current and the alternating-transmission systems vary with
the kind of service required at receiving stations and with the extent
to which transformers are used in the alternating system, other factors
being constant. For purposes of comparison the efficiency at full load
of both alternating- and continuous-current dynamos and motors, also of
rotary converters, may be fairly taken at 92 per cent, and the
efficiency of transformers at 96 per cent.
For the line an efficiency of 94 per cent may be assumed at full load,
this being the actual figure in one of the Swiss transmissions of 2,160
kilowatts at 14,400 volts to a distance of 32 miles. Where the
continuous current system must simply deliver mechanical power at the
receiving stations, its efficiency under full load amounts to 92 × .94 ×
.92 = 79.65 per cent from dynamo shaft to motor shaft. An alternating
system delivering mechanical power will have an efficiency of 92 × .94 ×
.96 × .92 = 76.46 per cent between dynamo shaft and motor shaft, if the
line voltage is generated in the armature coils of the dynamo and the
line loss is 6 per cent. If step-up transformers are employed to secure
the line voltage the efficiency of the alternating transmission
delivering mechanical power drops to the figure of 92 × .96 × .94 × .96
× .92 = 73.40 per cent. It thus appears that for the simple delivery of
mechanical power the continuous current transmission has an advantage
over the alternating of three to six per cent in efficiency, depending
on whether step-up transformers are employed.
When the receiving station must deliver a supply of either continuous or
alternating current for general distribution, the efficiency of the
continuous-current transmission amounts to 92 × .94 × .92 × .92 = 73.27
per cent. The alternating-transmission system in a case in which no
step-up transformers are employed will deliver alternating current of
the same frequency as that on the transmission line at any desired
pressure for general distribution at an efficiency of 92 × .94 × .96 =
83.02 per cent, if step-down transformers are used, but the efficiency
drops to 83.02 × .96 = 79.70 per cent. when step-up transformers are
introduced. If the alternating transmission uses no step-up transformers
and delivers either alternating or continuous current by means of motor
generators, its efficiency at full load is 83.02 × .92 × .92 = 70.26 per
cent, but with step-up transformers added the efficiency drops to 70.26
× .96 = 67.43 per cent. In a transmission where electrical energy must
be delivered for general distribution, the full-load efficiency of an
alternating system ranges either higher or lower than that of a
continuous-current system depending on whether the current from the
transmission line must be converted or not.
Line loss is the same whatever the load in a constant-current
transmission, so that line efficiency falls rather rapidly with the
load. On the other hand, at constant pressure the percentage of energy
loss on the line varies directly with the load, but the actual rate of
energy loss with the square of the load. On partial loads the line
efficiency is thus much higher with alternating than with continuous
constant current.
Efficiency of electrical machinery is generally low at partial loads, so
that in cases in which the number or capacity of alternating dynamos,
transformers, motors, or rotary converters for a transmission would be
greater per unit of delivered power than the corresponding number or
capacity of machines for a transmission by continuous current, the
latter would probably have the advantage in the combined efficiency of
machinery at partial loads. In this way the lower-line efficiency of one
system might offset the lower efficiency of machinery in the other.
Energy is usually very cheap at the generating station of a transmission
system. For this reason small differences in the efficiencies of
different systems should be given only moderate weight in comparison
with the items of first cost, reliability, and expense of operation.
In the matter of first cost at least the continuous-current system seems
to have a distinct advantage over the alternating. Without going into a
detailed estimate, it is instructive to consider the figures given by a
body of five engineers selected to report on the cost of continuous- and
alternating-current equipments for the St. Maurice and Lausanne
transmission. According to the report of these engineers, a three-phase
transmission system would have cost $140,000 more than the
continuous-current system actually installed, all other factors
remaining constant. It should be noted that the conditions of this
transmission are favorable to three-phase working and unfavorable to
continuous-current equipment, because all of the energy except that
going to the 400 horse-power motor at the cement mill must be delivered
at the receiving station for general distribution. Moreover, four out of
the five motors at Lausanne drive three-phase generators, and only one
drives a continuous-current dynamo for the electric railway, so that a
three-phase transmission would have required only one rotary converter.
Had the transmission been concerned merely with the delivery of
mechanical power, as at the cement mill, the advantage of the
continuous- over the alternating-current system in the matter of first
cost would have been much greater than it was.
Long-distance transmission with three-phase current began at Frankfort,
in 1891, when 58 kilowatts were received over a 25,000-volt line from
Lauffen, 109 miles away. Shortly after this historic experiment,
three-phase transmission in the United States began on a commercial
scale, and plants of this sort have multiplied rapidly here. Meantime
very little has been done in America with continuous currents in long
transmissions. In Europe, the birthplace of the three-phase system, it
has failed to displace continuous current for transmission work. About a
score of these continuous-current transmissions are already at work
there. If the opinion of European engineers as to the lower cost of the
continuous-current system, all other factors being equal, is confirmed
by experience, this current will yet find important applications to long
transmissions in the United States.
Systems of transmission with continuous-current may operate at constant
voltage and variable current, at constant current and variable voltage,
or with variations of both volts and amperes to correspond with changes
of load. Dynamos of several thousand kilowatts capacity each can readily
be had at voltages of 500 to 600, but the attempt to construct dynamos
to deliver more than two or three hundred kilowatts each at several
thousand volts has encountered serious sparking at the commutator. Thus
far, dynamos that yield between 300 and 400 kilowatts each have been
made to give satisfactory results at pressures as high as 2,500 volts.
Another one of the Swiss transmissions takes place over a distance of
thirty-two miles at 14,400 volts, the capacity being 2,160 kilowatts. To
give this voltage and capacity, eight dynamos are connected in series at
the generating station, each dynamo having an output of 150 amperes at
1,800 volts, or 216 kilowatts.
Continuous-current motors are, of course, subject to the same
limitations as dynamos in the matter of capacity at high voltage, so
that a series of motors must be employed to receive the high-pressure
energy from the line. The number of these motors may just equal, or may
be less or greater than the number of dynamos, but the total working
voltage of all the motors in operation at one time must equal the total
voltage of the dynamos in operation at that time minus the volts of drop
in the line.
Each constant-current motor may have any desired capacity up to the
practicable maximum, but it must be designed for the current of the
system. The voltage at the terminals of each motor varies with its load,
being greatest when the motor is doing the most work. Constant speed is
usually attained at each motor by means of a variable resistance
connected across the terminals of the magnet coils. The amount of this
resistance is regulated by a centrifugal governor, driven by the motor
shaft. This governor also shifts the position of the brushes on the
commutator to prevent sparking as the current flowing through the magnet
coils is changed.
For a constant-current transmission the magnet and armature windings of
both dynamos and motors are usually connected in series with each other
and the line so that the same current passes through every element of
the circuit, except that each motor may have some current shunted out of
its magnet coil for the purpose of speed regulation.
In some cases, however, the magnet coils of the dynamos are connected in
multiple with each other and receive their current from a separate
dynamo designed for the purpose. With this separate excitation of the
magnet coils, the dynamo armatures are still connected in series with
each other and the line.
The total voltage at the generating station and on the line of a
constant-current system varies with the rate at which energy is
delivered, and has its maximum value only at times of full load. To
obtain this variation of voltage, it is the general practice to change
the speed of the dynamos by means of an automatic regulator which is
actuated by the line current. Any increase of the line current actuates
the regulator and reduces the speed of the dynamos, while a decrease of
the line current raises the dynamo speed. With a good regulator the
variations of the line current are only slight. Under this method of
regulation the dynamos in operation have a substantially constant
current in both armature and magnet coils at all times, so that there is
no reason to shift the position of the brushes on the commutator.
Generating stations of constant current transmission systems are
generally driven by water-power and the speed regulator operates to
change the amount of water admitted to each wheel. Each turbine wheel
usually drives a pair of dynamos, but one or any number of dynamos might
be driven by a single wheel. The two dynamos driven by a single wheel
are generally connected in series at all times, and are cut in or out of
the main circuit together. When the load on a constant-current
generating station is such that the voltage can be developed by less
than all the dynamos, one or more dynamos may be stopped and taken out
of the circuit.
To do this the dynamo or pair of dynamos to be put out of service may be
stopped, their magnet coils having first been short-circuited, and then
a switch across the connections of their armatures to the lines closed,
after which the connections of the armatures to the line are opened. By
a reverse process, any dynamo or pair of dynamos may be cut into the
operating circuit.
At the terminals of each dynamo in the series, while in operation, the
voltage is simply that developed in its armature, so that the insulation
between the several windings is subject to only a corresponding stress.
The entire voltage of the line, however, tends to force a current from
the coils of the dynamo at one end of the series into its frame, thence
to any substance on which that frame rests, and so on to the frame and
coils of the dynamo at the other end of the series. To protect the
insulation of the dynamo coils from the line voltage, thick blocks of
porcelain are placed beneath the dynamo frames, and the armature shafts
are connected to those of the turbines by insulating couplings.
Besides the switches, already mentioned, a voltmeter and ammeter should
be provided for each dynamo and also for the entire series of machines.
This completes the switchboard equipment, which is, therefore, very
simple. As the line loss of a constant-current system is the same
whatever the load that is being operated, this loss may be a large
percentage of the total output when the load is light. If, for
illustration, five per cent of the maximum voltage of the station is
required to force the constant current through the line, the percentage
of line loss will rise to ten when the station voltage is one-half the
maximum, and to twenty when the station is delivering only one-quarter
of its full capacity.
In view of this property of constant-current working, the line loss
should be made quite small in its ratio to the maximum load, as most
stations must work on partial loads much of the time. Five per cent of
maximum station voltage is a fair general figure for the line loss in a
constant-current transmission, but the circumstances of a particular
case may dictate a higher or a lower percentage.
On the 32-mile transmission, above named, the loss in the line is six
per cent of the station output at full load.
If a transmission with continuous current is to be carried out at
constant pressure the limitation as to the capacity and voltage of each
dynamo is about the same as with constant current. Probably more energy
is now transmitted by continuous current at constant pressure than by
any other method, the greater part being devoted to electric railway
work at 500 to 600 volts. Dynamos for about these voltages can readily
be had in capacities up to several thousand kilowatts each, but the
length of transmission that can be economically carried out at this
pressure is comparatively small. For each kilowatt delivered to a line
at 500 volts and to be transmitted to a distance of five miles at a ten
per cent loss in the line, the weight of copper conductors must be 372
pounds, costing $56.80 at 15 cents per pound. This sum is twice to four
times the cost of good continuous-current dynamos per kilowatt of
capacity. If the distance of transmission is ten miles and the voltage
and line loss remain as before, the weight of copper conductor must be
increased to 1,488 pounds per kilowatt delivered to the line, costing
$227.20.
Experience has shown that in sizes of not more than 400 kilowatts,
continuous-current dynamos may safely have a voltage of 2,000 each, and
any number of such dynamos may be operated in multiple, giving whatever
capacity is desired. At 2,000 volts and a loss of 10 per cent in the
line the weight of copper conductors per kilowatt would be 93 pounds,
costing $13.95, for each kilowatt delivered to the line on a 10-mile
transmission. With 2,000 volts on a 20-mile transmission the weight of
conductors per kilowatt would be the same as their weight on a 5-mile
transmission at 500 volts, the percentage of loss being equal in the two
cases. Large continuous-current motors of, say, 50 kilowatts or more can
be had for a pressure of 2,000 volts, so that any number of such motors
might be operated from a 2,000-volt, constant-pressure line entirely
independent of each other. From these figures it is evident that a
transmission of 10 miles may be carried out with continuous-current at
constant pressure from a single dynamo with good efficiency and a
moderate investment in conductors.
When the distance is such that much more than 2,000 volts are required
for the constant-pressure transmission, with continuous current, resort
must be had to the connection of dynamos and motors in series. Any
number of dynamos may be so connected as in the case of constant-current
work. The combined voltages of the series of motors connected to the
constant-pressure transmission line must equal the voltage of that
line, so that the number of motors in any one series must be constant.
If the voltage of transmission is so high that more than two or three
motors must be connected in each series, there comes the objection that
motors must be operated at light loads during much of the time.
Moreover, each series of motors must be mechanically connected to the
same work, as that of driving a single dynamo or other machine, because
if the loads on the motors of a series vary differently, these motors
will not operate at constant speed. Continuous-current transmission at
constant pressure with motors in series thus lacks the flexibility of
transmission at constant current where any motor may be started and
stopped without regard to the others in the series, the line voltage
being automatically regulated at the generating station according to the
number of motors in use at any time and to the work they are doing.
In the efficiency of its dynamos, motors and line, a constant-pressure
system of transmission is substantially equal to one with constant
current at full load. At partial loads the constant-pressure line has
the advantage because the loss of energy in it varies with the square of
the load. Thus at constant pressure the line loss in energy per hour at
half-load is only one-fourth as great as the loss at full load. On the
other hand, the energy loss in the constant-current line is the same at
all stages of load. Because of these facts it is good practice to allow,
say, a ten-per-cent loss in a constant-pressure line and only five per
cent in a constant-current line at full load.
In a generating station at 2,000 volts or more constant pressure, it is
desirable to have the magnet coils of the main dynamos connected in
multiple and separately excited by a small dynamo at constant pressure.
This plan is especially desirable when the armatures of several dynamos
are connected in series to obtain the line voltage. Separately excited
magnet coils make it easier to control the operation of the several
dynamos, coils of low-voltage are cheaper to make than coils of high
voltage, and the low voltage windings are less liable to burn out. If a
series of constant-pressure motors is in use at one point, it may be
cheaper and safer to excite its magnet coils from a special dynamo than
from the line.
In a transmission carried out with series-wound dynamos and motors, the
speed of the motors may be constant at all loads without any special
regulating mechanism. To attain this result it is necessary that all the
motors be coupled so as to form a single unit mechanically and that the
dynamos be driven at constant speed. A transmission system of this sort
may include a single dynamo and a single motor, or two or more dynamos,
and two or more motors may be used in series.
When the dynamos of such a system are driven at constant speed and a
variable load is applied to the single motor, or to the mechanically
connected motors, both the voltage of the system and the amperes flowing
in all its parts change together so that practically constant speed is
maintained at the motors, provided that the design of both the dynamos
and motors is suitable for the purpose. With the maximum load on the
motors the volts and amperes of the system have their greatest values,
and these values both decline with smaller loads. The chief disadvantage
of this system lies in the fact that where more than one motor is
employed all the motors must be mechanically joined together so as to
work on the same load.
Compared with the constant-current system, this combination of series
dynamos with mechanically connected series motors has the distinct
advantage that neither the dynamos nor motors require any sort of
regulators in order to maintain constant motor speed. It is only
necessary that the dynamos be driven at constant speed and that both the
dynamos and motors be designed for the transmission. In comparison with
a constant-pressure system, the one under consideration has the
advantage that neither its dynamos nor motors require magnet coils with
a high voltage at their terminals and composed of fine wire or separate
excitation by a special dynamo. These features of the system with series
dynamos and motors, the latter being joined as a mechanical unit, make
it cheaper to install and easier to operate than either of the other
two. This system is especially adapted for the delivery of mechanical
power in rather large units. The voltage available may be anything
desired, but is subject to the practical limitations that all the motors
must deliver their power as a mechanical unit, so that unless the power
is quite large the number of motors in the series and, therefore, the
voltage is limited.
An interesting illustration of the system of transmission just described
exists between a point on the River Suze, near Bienne, Switzerland, and
the Biberest paper mills. At the river a 400 horse-power turbine
water-wheel drives a pair of series-wound dynamos, each rated at 130
kilowatts and 3,300 volts. These dynamos are connected in series, giving
a total capacity of 260 kilowatts and a pressure of 6,600 volts. At the
Biberest mills are located two series-wound motors, mechanically coupled
and connected in series with each other and with the two-wire
transmission line, which extends from the two dynamos at the River Suze.
Each of these motors has a capacity and voltage equal to that of either
of the dynamos previously mentioned. The coupled motors operate at the
constant speed of 200 revolutions per minute at all loads and deliver
over 300 horse-power when doing maximum work. Between the generating
plant at the river and the Biberest mills the distance is about 19
miles, and the two line wires are each of copper, 275 mils, or a little
more than one-fourth inch in diameter. The dynamos and motors of this
system are mounted on thick porcelain blocks in order to protect the
insulation of their windings from the strain of the full-line voltage.
Either of the three systems of transmission by continuous-current that
have been considered requires a smaller total capacity of electrical
apparatus for a given rate of mechanical power delivery than any system
using alternating current except that where both the dynamos and motors
operate at line voltage.