THE PHYSICAL LIMITS OF ELECTRIC-POWER TRANSMISSION.
Electrical energy may be transmitted around the world if the line
voltage is unlimited. This follows from the law that a given power may
be transmitted to any distance with constant efficiency and a fixed
weight of conductors, provided the voltage is increased directly with
the distance.
The physical limits of electric-power transmission are thus fixed by the
practicable voltage that may be employed. The effects of the voltage of
transmission must be met in the apparatus at generating and receiving
stations on the one hand, and along the line on the other. In both
situations experience is the main guide, and theory has little that is
reliable to offer as to the limit beyond which the voltage will prove
unworkable.
Electric generators are the points in a transmission system where the
limit of practical voltage is first reached. In almost all high-voltage
transmissions of the present day in the United States alternating
generators are employed. Very few if any continuous-current dynamos with
capacities in the hundreds of kilowatts and voltages above 4,000 have
been built in Europe, and probably none in the United States. Where a
transmission at high voltage is to be accomplished with continuous
current, two or more dynamos are usually joined in series at the
generating station, and a similar arrangement with motors is made at the
receiving station, so that the desired voltage is available at the line
though not present at any one machine.
Alternating dynamos that deliver current at about 6,000 volts have been
in regular use for some years, in capacities of hundreds of kilowatts
each, and may readily be had of several thousand kilowatts capacity. But
even 6,000 volts is not an economical pressure for transmissions over
fifteen to fifty miles, such as are now quite common; consequently in
such transmissions it has been the rule to employ alternators that
operate at less than 3,000 volts, and to raise this voltage to the
desired line pressure by step-up transformers at the generating station.
More recently, however, the voltage of alternating generators has been
pushed as high as 13,000 in the revolving-magnet type where all the
armature windings are stationary. This voltage makes it practicable to
dispense with the use of step-up transformers for transmissions up to or
even beyond 30 miles in some cases. This voltage of 13,000 in the
armature coils is attained only by constructions involving some
difficulty because of the relatively large amount of room necessary for
the insulating materials on coils that develop this pressure. The
tendency of this construction is to give alternators unusually large
dimensions per given capacity. It seems probable, moreover, that the
pressures developed in the armature coils of alternating generators must
reach their higher limits at a point much below the 50,000 and 60,000
volts in actual use on present transmission lines. In the longest
transmissions with alternating current there is, therefore, little
prospect that step-up transformers at the generating stations and
step-down transformers at receiving stations can be dispensed with. The
highest voltage that may be received or delivered at these stations is
simply the highest that it is practicable to develop by transformers and
to transmit by the line.
A very high degree of insulation is much more easy to attain in
transformers than in generator armatures, because the space that can be
readily made available for insulating materials is far greater in the
transformers, and further because their construction permits the
complete immersion of their coils in petroleum. This oil offers a much
greater resistance than air to the passage of electric sparks, which
tend to set up arcs between coils at very high voltages and thus destroy
the insulation. Danger to insulation from the effect known as creeping
between coils at widely different pressures is largely avoided by
immersion of the coils in oil. For several years groups of transformers
have been worked regularly at 40,000 to 60,000 volts, and in no instance
is there any indication that the upper limit of practicable voltage has
been reached. On the contrary, transformers have repeatedly been worked
experimentally up to and above 100,000 volts.
From all these facts, and others of similar import, it is fair to
conclude that the physical limit to the voltages that it is practicable
to obtain with transformers is much above the 50,000 or 60,000 volts now
in practical use on transmission systems. So far as present practice is
concerned, the limit to the use of high voltages must be sought beyond
the transformers and outside of generating and receiving stations. As
now constructed, the line is that part of the transmission system where
a physical limit to the use of higher voltages will first be reached.
The factors that tend most directly to this limit are two: temporary
arcing between the several wires on a pole, and the less imposing but
constant passage of energy from one wire to another. On lines of very
high voltage arcing is occasionally set up by one of several causes. At
a point where one or more of the insulators on which the wires are
mounted become broken or defective, the current is apt to flow from one
wire to another along a wet cross-arm, until the wood grows carbonized
and an arc is formed that ends by burning up the cross-arm or even the
pole. Where lines are exposed to heavy sea fog, the salt is in some
cases deposited on the insulators and cross-arms to an extent that
starts an arc between the wires, and ends often in the destruction of
the cross-arm. In some instances the glass and porcelain insulators
supporting wires used with high voltages are punctured by sparks that
pass right through the material of the insulator to the pin on which it
is mounted, thus burning the pin and ultimately the cross-arm. This
trouble is easily met, however, by the adoption of a better grade of
porcelain or of an insulator with a greater thickness of glass or
porcelain between the wires and the supporting pin. Arcs between lines
at high voltages usually start by sparks that jump from the lower edges
of insulators, when they are wet or covered with salt deposit, to the
cross-arm. As the lower edges of insulators are only a few inches from
their cross-arms, the sparks find a path of comparatively low resistance
by passing from insulator to cross-arm and thence to the other insulator
and wire. The wood of a wet cross-arm is a far better conductor than the
air. Where wires are several feet or more apart, sparks probably never
jump directly through the air from one to the other. Large birds flying
close to such wires, however, have in some instances started momentary
arcs between them. The treatment of cross-arms with oil or paraffine
reduces the number of arcs that occur on a line of high voltage, but
does not do away with them.
As the voltages of long transmissions have gone up, the distance through
the air between wires and the distances between the lower wet edges of
insulators and the cross-arms have been much increased. Most of the
earlier transmission lines for high voltages were erected on insulators
spaced from one to two feet apart. In contrast with this practice, the
three wires of the transmission line in operation at 50,000 volts
between Cañon Ferry and Butte are arranged at the corners of a triangle
seventy-eight inches apart, one wire at the top of each pole and the
other two at opposite ends of the cross-arm. A voltage that would just
start an arc along a wet cross-arm between wires eighteen inches apart
would be quite powerless to do so over seventy-eight inches of
cross-arm, the lower wet edges of insulators being equidistant from
cross-arms in the two cases. To reach the cross-arm, the electric
current passes down over the wet or dirty outside surface of the
insulator to its lower edge. In the older types of insulators the lower
wet edge often came within two inches of the cross-arm. For the
50,000-volt line just mentioned the insulators (see illus.) are mounted
with their lower wet edges about eight inches above the cross-arms. At
its lower edge each insulator has a diameter of nine inches, and a small
glass sleeve extends several inches below this edge and close to the
wooden pin, to prevent sparking from the lower wet edge of the insulator
to the pin. These increased distances between wires in a direct line
through the air, and also the greater distances between the lower wet
edges of insulators and their pins and cross-arms, are proving fairly
effective to prevent serious arcing under good climatic conditions, for
the maximum pressures of 50,000 to 60,000 volts now in use. If these
voltages are to be greatly exceeded it is practically certain that the
distance between wires, and from the lower wet edges of insulators to
the wood of poles or cross-arms, must be still further increased to
avoid destructive arcing.
The nearest approach to an absolute physical limit of voltage with
present line construction is met in the constant current of energy
through the air from wire to wire of a circuit. A paper in vol. XV.,
Transactions American Institute Electrical Engineers, gives the tests
made at Telluride, Col., to determine the rates at which energy is lost
by passing through the air from one wire to another of the same circuit.
The tests at Telluride were made with two-wire circuits strung on a pole
line 11,720 feet in length, at first with iron wires of 0.165 inch
diameter and then with copper wires of 0.162 inch diameter. Measurements
were made of the energy escaping from wire to wire at different voltages
on the line, and also with the two wires at various distances apart. It
was found that the loss of energy over the surfaces of insulators was
very slight, and that the loss incident to the passage of energy
directly through the air is the main one to be considered. This leakage
through the air varies with the length of the line, as might be
expected. Tests were made with pairs of wires running the entire length
of the pole line and at distances of 15, 22, 35, and 52 inches apart
respectively. Losses with wires 22 or 35 inches apart were intermediate
to the losses when wires were 15 and 52 inches apart respectively.
Results given in the original paper for the pair of wires that were 15
inches apart and for the pair that were 52 inches apart are here reduced
to approximate watts per mile of two-wire line. At 40,000 volts the loss
between the two wires that were 15 inches apart was about 150 watts per
mile, and between the two wires that were 52 inches apart the loss was
84 watts per mile. The two wires 15 inches apart showed a leakage of
approximately 413 watts per mile when the voltage was up to 44,000, but
the wires 52 inches apart were subject to a leakage of only 94 watts per
mile at the same voltage. At 47,300 volts, the highest pressure recorded
for the two wires 15 inches apart, the leakage between them was about
1,215 watts per mile, while an equal voltage on the two wires 52 inches
apart caused a leakage of only 122 watts per mile, or one-tenth of that
between the wires that were 15 inches apart. When about 50,000 volts
were reached on the two wires 52 inches apart, the leakage between them
amounted to 140 watts per mile; but beyond this voltage the loss went up
rapidly, and was 225 watts per mile at about 54,600 volts. For higher
pressures the loss between these two wires still more rapidly increased,
and amounted to 1,368 watts per mile with about 59,300 volts, the
highest pressure recorded. With a loss of about 1,215 watts per mile
between the two wires 52 inches apart, the voltage on them was 58,800,
in contrast with the 47,300 volts producing the same leakage on the two
wires 15 inches apart.
Evidently, however, at even 52 inches between line wires the limit of
high voltage is not far away. When the voltage on the 52-inch line was
raised from 54,600 to 59,300, the leakage loss between the two wires
increased about 1,143 watts per mile. If the leakage increases at least
in like proportion, as seems probable, for still higher pressures, the
loss between the two wires would amount to 6,321 watts per mile with
80,000 volts on the line. On a line 200 miles long this loss by leakage
between the two wires would amount to 1,264,200 watts. Any such leakage
as this obviously sets an absolute, physical limit to the voltage, and
consequently the length of transmission.
Fortunately for the future delivery of energy at great distances from
its source, the means to avoid the limit just discussed are not
difficult. Other experiments have shown that with a given voltage and
distance between conductors the loss of energy from wire to wire
decreases rapidly as their diameters increase. The electrical resistance
of air, like that of any other substance, increases with the length of
the circuit through it. The leakage described is a flow of electrical
energy through the air from one wire to another of the same circuit. To
reduce this leakage it is simply necessary to give the path from wire to
wire through the air greater electrical resistance by increasing its
length, that is, by placing the wires at greater distances apart. The
fact demonstrated at Telluride, that with 47,300 volts on each line the
leakage per mile between the two wires 15 inches apart was ten times as
great as the leakage between the two wires 52 inches apart, is full of
meaning. Evidently, leakage through the air may be reduced to any
desired extent by suitable increase of distance between the wires of the
same circuit. But to carry this increase of distance between wires very
far involves radical changes in line construction. Thus far it has been
the uniform practice to carry the two or three wires of a transmission
circuit on a single line of poles, and in many cases several such
circuits are mounted on the same pole line. For the 65 mile transmission
into Butte, Mont., only the three wires of a single circuit are mounted
on one pole line, and this represents the best present practice. The
cross-arms on this line are each 8 feet long, and one is attached to
each pole. The poles are not less than 35 feet long and have 8-inch
tops. One wire is mounted at the top of each pole, and the other two
wires near the ends of the cross-arm, so that the three wires are
equidistant and 78 inches apart. By the use of still heavier poles the
length of cross-arms may be increased to 12 or 14 feet, for which their
section should be not less than 4 by 6 inches. Placing one wire at the
pole top, the 12-foot cross-arm would permit the three wires of a
circuit to be spaced about 10.5 feet apart. The cost of extra large
poles goes up rapidly and there are alternative constructions that seem
better suited to the case. Moreover, a few tens of thousands of volts
above present practice would bring us again to a point where even 10.5
feet between wires would not prevent a prohibitive leakage. Two poles
might be set 20 feet apart, with a cross-piece between them, extending
out 5 feet beyond each pole and having a total length of 30 feet. This
would permit three wires to be mounted along the cross-piece at points
about 14 feet apart.
If the present transmission pressures of 50,000 to 60,000 volts are to
be greatly exceeded, the line structure may involve the use of a
separate pole for each wire of a circuit, each wire to be mounted at the
top of its pole. This construction calls for three lines of poles to
carry the three wires of a three-phase transmission. Each of these poles
may be of only moderate dimensions, say 30 feet long with 6- or 7-inch
top. The cost of three of these poles will exceed by only a moderate
percentage that of a 35- or 40-foot pole with an 8- to 10-inch top, such
as would be necessary with 12-foot cross-arms. The distance between
these poles at right angles to the line may be anything desired, so that
leakage from wire to wire through the air will be reduced to a trifling
matter at any voltage. Extra long pins and insulators at the pole tops
will easily give a distance of two feet or more between the lower wet
edge of each insulator and the wood of pin or pole. Such line
construction would probably safely carry two or three times the maximum
voltage of present practice, and might force the physical limit of
electrical transmission back to the highest pressure at which
transformers could be operated. With not more than 60,000 volts on the
line the size of conductors is great enough in many cases to keep the
loss of energy between them within moderate limits when they are six
feet apart, but with a large increase of voltage the size of conductors
must go up or the distances between them must increase.