ELECTRICAL TRANSMISSION UNDER LAND AND WATER.
Energy transmitted over long distances must sometimes pass through
conductors that are underground or beneath water. In some other cases it
is a question of relative advantages merely, whether portions of a
transmission line go under water or overhead. Where the transmitted
energy must enter a sub-station in the heart of a large city, it not
infrequently must go by way of underground conductors without regard to
the voltage employed. In some cities the transmission lines may be
carried overhead, provided that their voltage is within some moderate
figure, but not otherwise. Here it becomes a question whether
transmission lines at high voltage shall be carried underground, or
whether transforming stations shall be established outside of the
restricted area and then low-pressure lines brought into the business
section overhead or underground, as desired. Where a transmission line
must cross a steam railway track it may be required to be underground,
whether the voltage is reduced or not. The distance across a body of
water in the path of a transmission line may be so great that a span is
impossible and a cable under the water therefore necessary. Such a cable
may work at the regular line voltage, or a transformer station may be
established on one side or on each side of the body of water. Even where
it is possible to span a body of water with a transmission line, the
cost of the span and of its supports may be so great that a submarine
cable is more desirable. A moderate increase in the length of a
transmission line in order to avoid the use of a submarine cable is
almost always advisable, but where rivers are in the path of the line it
is generally impossible to avoid crossing them either overhead or
underneath. Thus, St. Paul could only be reached with the 25,000-volt
line from the falls on Apple River by crossing the St. Croix River,
one-half mile wide, on the way. In order to carry out the 40,000-volt
transmission between Colgate and Oakland, the Carquinez Straits, which
intervened with nearly a mile of clear water, were crossed. Sometimes,
as in the former of the two cases just named, an existing bridge may be
utilized to support a transmission line, but more frequently the choice
lies between an overhead span from bank to bank of a river and a
submarine cable between the same points.
The prime advantage of an overhead line at high voltage is its
comparatively small first cost, which is only a fraction of that of an
underground or submarine cable in the great majority of instances. At
very high voltages, like 40,000 to 50,000 or more, the overhead line
must also be given first place on the score of reliability, since the
lasting qualities of underground and submarine cables at such pressures
is as yet an unknown quantity. On the other hand, at voltages in which
cable insulation has been shown by experience to be thoroughly
effective, underground or submarine cables may be more reliable than
overhead lines because of the greater freedom from mechanical
disturbances which these cables enjoy.
In the business portion of many cities a transmission line must go
underground, whether its voltage is high or low. Under these conditions,
it may be desired either to transmit energy to a sub-station for
distribution within the area where conductors must be underground, or to
transmit energy from a generating station there located to outside
points. If the transmitted energy is reduced in pressure before reaching
such a sub-station, a transforming station must be provided, and this
will allow the underground cables to operate at a moderate voltage. For
such a case the advantages as to insulation at the lower voltage should
be compared with the additional weight of conductors in the cable and
the cost of the transforming apparatus and station. If the voltage at
which current is delivered from the transforming station does not
correspond with the required voltage of distribution at the sub-station,
the necessary equipment of step-down transformers is doubled in capacity
by lowering the voltage of the transmitted energy where it passes from
the overhead line to the underground cables. Conditions of just this
sort exist at Buffalo in connection with the delivery of energy from the
power-stations at Niagara Falls. This transmission was first carried out
at 11,000 volts, and a terminal station was located at the Buffalo city
limits where the overhead lines joined underground cables that continued
the transmission at the same voltage to several sub-stations in
different parts of the city. Later the voltage of the overhead
transmission line was raised to 22,000, and it not being thought
advisable to subject the insulation of the underground cables to this
higher pressure, transformers were installed at the terminal station to
lower the line voltage to 11,000 for the underground cables. As the
sub-stations in this case also have transformers, there are two
kilowatts of capacity in step-down transformers for each kilowatt of
delivery capacity at the sub-stations.
[Illustration: FIG. 74.--Cable Terminal House for the 25,000-volt
Chambly Line at Montreal.]
The saving effected in capacity of transformers and in the weight of
cables by continuing the full transmission voltage right up to the
sub-stations whence distribution takes place furnishes a strong motive
to work underground cables at the pressure of the overhead transmission
line of which they form a continuation. Thus, at Hartford, the
10,000-volt overhead lines that bring energy from water-power stations
to the outskirts of the city connect directly in terminal houses there
with underground cables that complete the transmission to the
sub-station at the full line voltage. In Springfield, Mass., the
overhead transmission lines from water-power stations connect directly
with underground cables at a distance of nearly two miles from the
sub-station, and these cables are thus subject to the full line pressure
of 6,000 volts. The overhead line that brings energy at 25,000 volts
from Apple River falls to St. Paul terminates about three miles from the
sub-station there, and the transmission is completed by underground
cables that carry current at the 25,000-volt pressure.
In these and similar cases the relative advantages of underground cables
at the full voltage of transmission and of overhead lines at a much
lower pressure, in the central portions of cities, must be compared. The
overhead lines at moderate voltage will no doubt cost less in almost
every case than underground cables of equal length and at the full
transmission voltage.
As an offset to the lower cost of overhead city lines at moderate
voltage, where they are permitted by local regulations, comes the
increase in weight of conductors due to the low pressure on the overhead
lines, and also the cost of additional transformer capacity, unless the
lines that complete the transmission operate at the voltage of
distribution. The 10,000-volt lines that transmit energy from Great
Falls to Portland, Me., terminate in two transformer houses that are
distant about 0.5 mile and 2.5 miles, respectively, from the sub-station
there. In these transformer houses the voltage is reduced to 2,500, and
the transmission is then continued at this pressure to the sub-station
whence distribution takes place without further transformation.
Where a river or other body of water must be crossed by a transmission
line, either of three plans may be followed. The overhead line may be
continued as such across the water, either by a single span or by two or
more spans supported by one or more piers built for that purpose in the
water. The overhead line may connect directly with a submarine cable,
this cable being thus exposed to the full voltage of the transmission.
As a third expedient, a submarine cable may be laid and connected with
step-down transformers on one bank and with step-up transformers on the
other bank of the river or other body of water to be crossed. The
overhead lines connecting with these transformers can obviously be
operated at any desired voltage, and this is also true of the cable.
Even though the distance across a body of water is not so great that a
transmission line can not be carried over it in a single span, the cost
of such a span may be large. A case in point is that of the Colgate and
Oakland line, where it crosses the Carquinez Straits by a span of 4,427
feet. These straits are about 3,200 feet wide where the transmission
line crosses, and overhead lines were required to be not less than 200
feet above high water so as not to impede navigation. In order to gain
in ground elevation and thus reduce the necessary height of towers, two
points 4,427 feet apart on opposite sides of the straits were selected
for their location. Under these circumstances two steel towers, one
sixty-five feet and the other 225 feet high, were required to support
the four steel cables that act as conductors across the straits. To take
the strain of these four cables, each with a clear span nearly three
times as great as that of the Brooklyn Bridge, eight anchors with housed
strain insulators were constructed, four on the land side of each tower.
On each of these anchors the strain is said to be 24,000 pounds. At each
end of the cables making this span is a switch-house where either of the
two three-phase transmission lines may be connected to any three of the
four steel cables, thus leaving one cable free for repairs. It is not
possible to state here the relative cost of these steel towers and
cables in comparison with that of submarine cables for the same work,
but at first glance the question appears to be an open one. The voltage
of 40,000, at which this transmission is carried out, is probably higher
than that on any submarine cable in use, but it is possible that a
suitable cable can be operated at this voltage. Whatever the limitations
of voltage as applied to submarine cables, it would, of course, have
been practicable to use step-up and step-down transformers at the
switch-houses and thus operate a submarine cable at any voltage desired.
In another case, on a transmission between Portsmouth and Dover, N. H.,
it was necessary to cross an arm of the sea on a line 4,811 feet long
with a three-phase circuit operating at 13,500 volts. It was decided to
avoid the use of either a great span or of raising and lowering
transformers at this crossing, and to complete the line through a
submarine cable operating at the full voltage of transmission. To this
end a brick terminal house six by eight feet inside, and with an
elevation of thirteen feet from the concrete floor to the tile roof, was
erected on each bank of the bay at the point where the submarine cable
came out of the water. The lead-covered cable pierced the foundation of
each of these terminal houses at a point four feet below the floor level
and rose thence on one wall to an elevation eleven feet above the floor
to a point where connection was made with the ends of the overhead
lines. From this connection on each of the three conductors a tap was
carried to a switch and series of lightning arresters. A single
lead-covered cable containing three conductors makes connection between
these two terminal houses. At each end of this cable the lead sheath
joins a terminal bell one foot long and 2.5 inches in outside diameter,
increasing to four inches at the end where the three conductors pass
out. This terminal bell is filled nearly to the flaring upper end with
an insulating compound.
In the instance just named it is possible that the cost of the submarine
cable was less than would have been the outlay for shore supports and a
span nearly a mile long across this body of water.
Underground and submarine cables have been operated at voltages suitable
for transmission during periods sufficiently long to demonstrate their
general reliability. The Ferranti underground cables between Deptford
and London have regularly carried current at 11,000 volts since a date
prior to 1890. During about five years cables with an aggregate length
of sixteen miles have transmitted power from St. Anthony’s Falls to
Minneapolis. At Buffalo, some thirty miles of rubber-insulated cables
have been in use for underground work at 11,000 volts since 1897, and
eighteen miles of paper-insulated cable since the first part of 1901.
These examples are enough to show that transmission through underground
cables at 11,000 to 12,000 volts is entirely practicable. At Reading,
Pa., an underground cable one mile long has carried three-phase current
at 16,000 volts for the Oley Valley Railway since some time in 1902. The
cables in the transmission from Apple River to St. Paul, which carry
three-phase current at 25,000 volts, have a total length of three miles,
and have been in use since 1900. This voltage of 25,000 is probably the
highest in regular use on any underground or submarine cable conveying
energy for light or power. From the experience thus far gained there is
much reason to think that the voltages applied to underground cables may
be very materially increased before a prohibitive cost of insulation is
reached.
On submarine cables the voltage of 13,000 in the Portsmouth and Dover
transmission, above mentioned, is perhaps as great as any in use. It
does not appear, however, that any material difference exists, as to the
strain on its insulation at a given voltage, between a cable when laid
in an underground conduit and when laid under water. In either case the
entire stress of the voltage employed operates on the insulation between
the several conductors in the cable and between each conductor and the
metallic sheath. Underground conduits have little or no value as
insulators of high voltages, because it is practically impossible to
keep them water-tight and prevent absorption or condensation of moisture
therein. For these reasons a cable that gives good results at 25,000
volts in an underground conduit should be available for use at an equal
voltage under water. The standard structure of high-voltage cables for
either underground or submarine work includes a continuous metallic
sheath outside of each conductor or of each group of conductors that
goes to make up a circuit. As most transmissions are now carried out
with three-phase current, the three conductors corresponding to a
three-phase circuit are usually contained in a single cable and covered
by a single sheath. The cables used in transmission systems at
Portsmouth, Buffalo, and St. Paul are of this type. If single-phase or
two-phase current is transmitted, each cable should contain the two
conductors that go to make up a circuit. In work with alternating
currents the use of only one conductor per cable should be avoided
because of the loss of energy that results from the currents induced in
the metallic sheath of such a cable.
[Illustration: FIG. 75.--Cable Terminal House at Piscataqua River
Crossing.]
Where the two, three, or more conductors that form a complete circuit
for alternating current are included in a single metallic sheath, the
inductive effects of currents in the several conductors tend to
neutralize each other and the waste of energy in the sheath is in large
part avoided. To neutralize more completely the tendency to local
currents in their metal sheath, the several insulated conductors of an
alternating circuit are sometimes twisted together, after being
separately insulated, before the sheath is put on. Distribution of power
at Niagara Falls was at first carried out through single-conductor,
lead-covered cables with two-phase current at 2,200 volts. One objection
to this plan was the loss of energy by induced currents in the lead
coverings of the cables. It was later decided to adopt three-phase
distribution at 10,000 volts for points distant more than two miles from
the power-station. Each three-phase circuit for this purpose was made up
of three conductors separately insulated and then covered with a single
lead sheath, so as to avoid losses through induced currents in the
latter. Underground and submarine cables for operation at high voltages
are generally covered with a continuous lead sheath and sometimes with a
spiral layer of galvanized iron wire. For high-voltage work underground
the lead covering is generally preferred without iron wire, but in
submarine work coverings of both sorts are employed. The lead sheath of
a cable being continuous completely protects the insulation from contact
with gases or liquids. As ducts of either tile, wood, or iron form a
good mechanical protection for a cable, the rather small strength of a
lead sheath is not a serious objection in conduit work. Submarine
cables, on the other hand, depend on their own outer coverings for
mechanical protection, and may be exposed to forces that would rapidly
cut through a lead sheath. Cables for operation under water should
usually be covered, therefore, with a layer of galvanized iron wires
outside of the lead sheath. These wires are laid closely about the cable
in spiral form and are usually between 0.12 and 0.25 inch in diameter
each, depending on the size of the cable and its location.
Underground conduits cannot be relied on to exclude moisture and acids
of the soil from the cables which they contain, and either of these
agents may lead to destructive results. If cables insulated with rubber,
but without a protecting covering outside of it, are laid in underground
conduits, the rubber is apt to be rapidly destroyed by fluids and gases
that find their way into the conduit. If a plain lead-covered cable is
employed the acids of the soil attack it, and if stray electric currents
from an electric railway find the lead a convenient conductor it is
rapidly eaten away where they flow out of it. To avoid both of these
results the underground cable should have a lead sheath, and this sheath
may be protected by an outside layer of hemp or jute treated with
asphaltum.
Rubber, paper, and cotton are extensively used as insulation for
underground and submarine cables, but the three are not usually employed
together. As a rule, the insulation is applied separately to each
conductor, and then an additional layer of insulation may be located
about the group of conductors that go to make up the cable. Where rubber
insulation is employed, a lead sheath may or may not be added, but where
insulation depends on cotton or paper the outer covering of lead is
absolutely necessary to keep out moisture. The radial thickness of
insulation on each conductor and of that about the group of conductors
in a cable should vary according to the voltage of operation.
Cables employed between the generating and sub-stations of the Manhattan
Elevated Railway, to distribute three-phase current at 11,000 volts, are
of the three-conductor type, rubber insulated, lead covered, and laid in
tile conduits. Each cable contains three No. 000 stranded conductors,
and each conductor has its own insulation of rubber. Jute is laid on to
give the group of conductors an outer circular form, and outside of the
group a layer of insulation and then a lead sheath is placed. Outside
diameter of this cable is nearly three inches, and the weight nine
pounds per linear foot.
The 11,000-volt, three-phase current from Niagara Falls is distributed
from the terminal house to seven sub-stations in Buffalo through about
30 miles of rubber-insulated and 18 miles of paper-insulated,
three-conductor, lead-covered cables, all in tile conduits. In each
cable the three No. 000 stranded conductors are separately insulated and
then twisted into a rope with jute yarn laid in to give an even round
surface for the lead sheath to rest on. A part of the rubber-insulated
cables have each conductor covered with 9/32-inch of 30 per cent pure
rubber compound, and the remaining rubber cables have 8/32-inch
covering on each conductor of 40 per cent pure rubber compound. The
paper-insulated cable has 13/64-inch of paper around each conductor, and
also another 13/64-inch of paper covering about the group of three
conductors and next to the lead sheath. In outside diameter the
rubber-insulated cable is 2-3/8 inches, and of the paper-insulated cable
2-5/8 inches, the radial thickness of the lead sheath being 1/8-inch in
each case. It is reported that the cables insulated with 9/32-inch of
the mixture, said to be 30 per cent pure rubber, have proved to be more
reliable than the cables insulated with 8/32-inch of a mixture said to
be 40 per cent pure rubber. Vol. xviii., A. I. E. E., 136, 836.
The six miles of underground cables that carry three-phase, 25,000-volt
current in St. Paul are of the three-conductor type, lead covered, and
laid in a tile conduit. One of the two three-mile cables is insulated
with rubber and the other with paper. In the former cable each conductor
is separately insulated with a compound containing about 35 per cent of
pure rubber and having a radial thickness of 7/32-inch. The three
conductors after being insulated are laid up with jute to give a round
surface, tape being used to hold them together, and then a rubber cover
5/32-inch thick is placed about the group, after which comes the lead
sheath over all. In the three miles of paper-insulated cable each
conductor is separately covered with paper to a thickness of 9/32-inch,
then the three conductors are laid together with jute and taped, and
next a layer of paper 4/32-inch thick is put on over the group. Outside
of all comes the lead sheath, which has an outer coating of tin. The
paper insulation in these cables was saturated with a secret insulating
compound. The lead sheath on both the rubber and paper insulated cables
is 1/8-inch thick and the sheath of the former contains 3 per cent of
tin. Each of the three conductors in each cable consists of 7 copper
strands and has an area of 66,000 circular mils. Outside of the lead
sheath each of these cables has a diameter of about 2-1/4 inches. By the
manufacturer’s contract these cables were tested up to 40,000 volts
before shipment, and might be tested up to 30,000 volts in their
conduits during any time within five years from their purchase. In first
cost the cable with rubber insulation was said to be about 50 per cent
more expensive than the cable in which paper was used. Vol. xvii., A. I.
E. E., 650.
Underground cables in which the separate conductors are covered with
cotton braid treated with an insulating compound, and then the group of
conductors going to make up the cable enclosed in a lead sheath, are
extensively used in Austria and Germany. For cables that operate at
10,000 to 12,000 volts the radial thickness of cotton insulation on each
conductor is said to be within 3/16-inch, and these cables are tested up
to 25,000 volts by placing all of the cable except its ends in water,
and then connecting one end of the 25,000-volt circuit to the water and
the other end to the conductors of the cable.
A test on the paper-insulated cable at St. Paul showed its charging
current to be 1.1 amperes at 25,000 volts for each mile of its length.
For the cable with rubber insulation the charging current per mile of
length was found to be about twice as great as the like current for the
paper-insulated cable. Each of the two overhead transmission lines
connected with these cables consisted of three solid copper wires with
an area of 66,000 circular mils each, and all three so mounted on the
poles as to form the corners of an equilateral triangle twenty-four
inches apart. The charging current of one of these three-wire, overhead
circuits was found to be about 0.103 ampere per mile, at 25,000 volts,
or a little less than one-tenth of the like current for the paper cable.
These tests were made with three-phase current of sixty cycles per
second.
Where overhead transmission lines join underground or submarine cables,
either with or without the intervention of transformers, lightning
arresters should be provided to intercept discharges of this sort that
come over the overhead wires. Lightning arresters were provided in the
terminal house at Buffalo, where the 22,000-volt overhead lines feed the
11,000-volt cables through transformers, also at the terminal house in
St. Paul, where the 25,000-volt overhead lines are electrically
connected to the underground cables. If an underground or submarine
cable connects two portions of an overhead line, as in the Portsmouth
and Dover transmission above mentioned, lightning arresters should be
provided at each end of the cable, as was done in that case. One
advantage of a high rather than a low voltage on underground cables,
where power is to be transmitted at any given rate, lies in the fact
that the amperes flowing at a fault in the cable determine the
destructive effect there, rather than the voltage of the transmission.
It is reported that a fault or short-circuit in one of the 11,000-volt
cables at Buffalo usually melts off but little lead at the sheath and
does not have enough explosive force to injure the cable or its duct.
Ozone seems to destroy the insulating properties of rubber very rapidly,
and as it is well known that the silent electric discharge from
conductors at high voltages develops ozone, care should be taken to
protect rubber insulation from its action. This is especially true at
the ends of cables where connections are made with switches or other
apparatus, and the rubber insulation is exposed. To protect the rubber
at such points it is the practice to solder a brass cable head or
terminal bell to the lead sheath near its end, this head having a
diameter perhaps twice as great as that of the sheath, and then to fill
the space about the cable conductors in this head with an insulating
compound. Heads of this sort were used on the 11,000-volt cables at
Buffalo as well as on the 13,500-volt cable in the Portsmouth and Dover
transmission.
As insulating materials, whether rubber, cotton, or paper, may be
impaired or destroyed by heat, it is necessary that the temperature of
underground cables under full load be kept within safe limits. Rubber
insulation can probably be raised to 125° or 150° Fahrenheit without
injury, and paper and cotton may go a little higher. For a given size
and make of leaded cable the rise of temperature in its conductors above
that of the surrounding air, for a given loss in watts per foot of the
cable, may be determined by computation or experiment. The next step is
to find out how much the temperature of the air in the conduits where
the cable is to be used will rise above the temperature of the earth in
which the conduits are laid, with the given watt loss per foot of cable.
On this point there are but little experimental data. Obviously, the
material of which ducts are made, the number of ducts grouped together
with cables operating at the same time, and the extent to which ducts
are ventilated must have an important bearing on this question. At
Niagara Falls some tests were made to show the rise of air temperature
in a section of thirty-six-duct conduit lying between two manholes about
140 feet apart. For the purpose of this test twenty-four of the
thirty-six ducts in the conduit had one No. 6 drawing-in wire passed
through each of them. These twenty-four wires were connected into three
groups of eight wires each, so that one group was all in ducts next to
the surrounding earth, another group was one-half in ducts next to the
earth and the other half in ducts separated from the earth by at least
one duct, while the third group of wires was entirely in ducts separated
from the earth by at least one duct. It was found that when enough
current was sent through these wires to represent a loss of 5.5 watts
per foot of ducts in which they were located, the rise of temperature in
the air of the ducts next to the earth was about 108° Fahrenheit above
that of the earth. For the ducts separated from the earth by at least
one other duct the rise in temperature of contained air was 144°
Fahrenheit above the earth. If the earth about the ducts reached 70° in
hot weather, the temperature of air in the inner ducts, with a loss of
5.5 watts per duct foot, would thus be 214°. This temperature is too
high for either rubber, cotton, or paper insulation, to say nothing of
the amount by which the temperature of the conductors and insulation of
a cable in operation must exceed that of the surrounding air in its
duct. The cables actually installed in the ducts just considered were
designed for a loss of 2.34 watts per foot. As the No. 6 wire used in
the test did not nearly fill each duct as a cable would do, it would be
very interesting to know how much ventilation took place while the test
was going on. Unfortunately, this point was not reported. Vol. xviii.,
A. I. E. E., 508.