ENTRIES FOR ELECTRIC TRANSMISSION LINES.
The entrance of transmission lines into generating plants and
sub-stations presents special problems in construction and insulation.
One of these problems has to do with the mechanical security of each
conductor at the point where it passes through the side or roof of the
station. Conductors are sometimes attached to the station so that the
strain of the line is borne by the side wall where they enter and tends
to pull it out of line.
This practice has but little to commend it, aside from convenience, for
unless the conductors are rather small, or the wall of the station is
unusually heavy, the pull of the former is apt to bulge the latter in
the course of time. For any heavy line the end strain is ultimately most
suitably taken by an anchor securely fixed. As special insulators must
be used where a conductor is secured directly to such an anchor, it is
usually more convenient to set one or more heavy poles with double
cross-arms at the end of a line, and then to make these poles secure by
large struts, or by guys attached to anchors. Extra heavy cross-arms on
these end poles should be provided with iron pins for the line
insulators; two or more of the insulators mounted in this way within a
few feet of each other, for each wire, will stand up against the end
strain on almost any line.
Insulators that are to take the end strain of a line in this way should
allow attachment of the wire at the side, so that the force exerted by
each conductor tends to press the insulator against the side of its pin,
rather than to pull off the top of the insulator. The end strain of the
line having been taken on poles close to the station, the conductors may
be attached to insulators on the wall, the latter thus being subjected
to very little mechanical strain.
Overhead lines usually enter a station through one of its side walls,
but an entry may be made in the roof. It is desirable to have a side
entry on the gable end of a building rather than on a side below the
eaves where there will be much dripping of water. If an entry must be
made below the eaves, a shelter should be provided above the entry, and
the roof of this shelter should have a gutter that will carry water
away from the wires.
Entrance of each conductor into a station must be effected in such a way
that ample insulation of the circuit will be maintained, and in some
cases so that rain, snow, and wind will be excluded. The line voltage
and the climate where the station is located thus have an important
bearing on the form of entry that is suitable in any particular case.
The simplest form of entry for a high-voltage line is a clear opening,
usually circular in form, through the wall of the station for each wire.
Insulators for each wire should be provided both inside and outside of
the wall to hold the wire at the centre of this opening. Such insulators
are usually most conveniently supported by fixtures attached to both
sides of the wall, and insulators on the outside should of course be
kept in an upright position, unless completely protected from rain and
snow.
The diameter of the openings through the wall should be great enough to
prevent any visible discharge of current between the wire and wall under
the worst conditions of snow, rain, fog, or dust. Such an opening must,
therefore, increase in diameter with the voltage of the line. The larger
these openings for the line wires, the greater is the opportunity for
rain, snow, dust, and cold air to enter the station through them.
Openings may be so protected as to keep out snow and rain by means of
shelters on the outside of the wall on which they are placed, but such
shelters cannot keep out the cold air. If the openings for the entrance
of wires are located in the wall of a room that contains air-blast
transformers, the area of openings for circuits of very high voltage may
be no greater than is necessary to allow the escape of heated air from
the transformers.
The milder the climate, other factors being the same, the higher the
voltage of circuits which may enter a station through openings that are
free for the movement of air. With circuits of only moderate voltage,
say less than 15,000, it is quite practicable to admit wires to a
station through perfectly free openings, in the coldest parts of the
United States. With voltages of 20,000 to 60,000 it is often necessary,
in the colder parts of the country, to close the opening in the wall
through which each wire enters with a disc of insulating material.
In order to keep the current leakage over these discs within proper
limits, the diameters of the discs must increase with the voltage of the
circuit. This increase of disc diameter obviously lengthens the path of
leakage current over the disc surface. Where the openings in a wall for
the entrance of high-voltage circuits are closed by insulating discs
about the wires, these discs may make actual contact with bare wires,
or the wire at each entry may have some special insulation.
In the side wall of the sub-station at Manchester, N. H., the entrance
of transmission lines from four water-power plants is provided for by
circular openings in slate slabs that are built into the brickwork. The
transmission circuits from three of the water-power plants operate at
10,000 to 12,000 volts, and the circuit from the fourth plant at about
6,000 volts. Circular openings in the slate slabs are each five inches
in diameter, and they are spaced twelve to fifteen inches between
centres. A single wire enters through each of these openings and is held
at the centre by insulators both inside and outside of the wall. Each
wire is bare where it passes through the slate slab, and the circular
openings are not closed in any way. The largest wires passing through
these five-inch circular openings in the slate slabs are of solid
copper, No. 0, of 0.325-inch diameter each.
Before passing through the opening in the slate slabs the wires of these
transmission circuits are tied to regular line insulators supported by
cross-arms secured to the outside of the brick wall by iron brackets.
The point of attachment of each wire to its insulator is about nine
inches below the centre of the circular hole by which it enters the
sub-station.
This Manchester sub-station is equipped with air-blast transformers from
which the hot air is discharged into the same room that the transmission
lines enter. Along one side of the sub-station there are twenty-seven of
these five-inch circular openings in the slate slabs for entrance of the
high-voltage lines, and on another side of the sub-station there are a
greater number of smaller openings for the distribution circuits. Were
it not for the air-blast transformers, all of these openings would
probably admit more air than would be desirable in a climate as cold as
that at Manchester.
Another example of openings in the walls of a station for the entrance
of transmission circuits, where there is free movement of the air
between the inside and outside of the building, is that of the
33,000-volt line between Santa Ana River and Los Angeles, Cal. In this
case a sewer pipe twelve inches in diameter is built into the wall of
the station for each wire of the line, so that there is a free opening
of this size from inside to outside.
Each wire of the 33,000-volt circuit enters the station through the
centre of one of these twelve-inch pipes, and is thus surrounded by six
inches of air on every side. As the temperature near Los Angeles seldom
or never goes down to zero, these large openings do not admit enough
air to be objectionable. Besides this mild climate, air-blast
transformers add to the favorable features in the stations having the
twelve-inch openings.
In another case, however, where the openings for the entrance of wires
of very high voltage allow free movement of air between the inside and
outside of the station, the climate is cold and the winter temperatures
go down to 30° or more below zero. This condition exists on the
25,000-volt line between Apple River Falls and St. Paul, where six No. 2
wires enter the generating station through plain circular openings in
the brick side wall of a small extension where the lightning arresters
are located. Air-blast transformers are located in the end of the
station next to this lightning-arrester house, but it is not certain
that the hot air from them escapes through the openings for the wires.
In another case where the climate is about as cold as that just named, a
gallery is built along one side of the exterior of the station at some
distance above the ground, and two openings are provided for each wire
of the high-tension line. One of these two openings is in the horizontal
floor of the gallery and allows the entrance of the wire from the
outside, and the other opening is in the side wall of the station
against which the gallery is built. The two openings for each wire being
thus at right angles to each other, and the opening to the outside air
being protected from the wind by its horizontal position, no more than a
permissible amount of cold air, it is said, finds its way into the
station.
In some cases with lines of moderate voltage, say 10,000 to 15,000, and
in probably the majority of cases with lines of 25,000 volts or more,
the entry for the high-tension wires is entirely closed. An example of
this practice may be seen at the various sub-stations of the New
Hampshire Traction Company, which are located along their 12,000-volt
line between Portsmouth and Pelham, in that State.
For the entry of each wire on these lines a sixteen-inch square opening
is made in the brick wall of the sub-station. On the outside of this
wall a box is built about a group of three or more of these openings
located side by side. The top or roof of this box is formed by a slab of
bluestone three inches thick, which is set into the wall and extends
twenty-six inches from the face of the wall, with a slight slope from
the horizontal.
The ends, the bottom, and the outer side of this box are formed by slabs
of slate one inch thick, so that the enclosed space has an area in
vertical cross section at right angles to this building 15.5 inches high
and twenty-two inches wide.
[Illustration: FIG. 89.--Cable Entering Building.]
In the bottom of this box there is a circular opening for each wire, and
into this opening fits a heavy glass or porcelain bushing through which
the wire passes. After reaching the inside of the box the wire turns at
right angles and passes through the sixteen-inch square opening into the
sub-station. Beneath the box a special insulator is secured by an iron
bracket to the outside of the brick wall for each line wire, and this
insulator takes the strain of the wire before it is carried up through
the bushing in the bottom of the box. This form of entry is permissible
where the desire is to exclude cold air from the station, and where the
voltage is not high enough to cause serious leakage over the surface of
the bushing and the slate forming the bottom of the box. In all of the
cases above mentioned the wires used to enter the stations were the
regular line conductors and were bare.
Another type of entry in sub-stations is that employed on the extensive
transmission system between Spier Falls, Schenectady, and Albany, N. Y.
The maximum voltage on this system is 30,000, and the lines usually
enter each sub-station through the brick wall at one of its gable ends.
Outside of and about the entry of each circuit or group of circuits a
wooden shelter is built on the brick wall of the sub-station. Each
shelter has a slanting roof that starts from the brick wall at some
distance above the openings for the entrance of the line, and terminates
in a gutter. The front of each shelter is carried down three feet below
the centre of the openings in the brick wall, and the ends go still
lower. The front of each shelter is four feet in height, is four feet
from the face of the brick wall, and has a circular opening of 10-inch
diameter for each wire of the transmission line.
In line with each circular opening in the wooden shield there is an
opening of 15-inch diameter in the brick wall of the sub-station, and
into this opening in the brickwork fits a ring of wood with 15-inch
outside and 11-inch inside diameter. To this wooden ring a 15-inch disc
of hard fibre 1/8-inch thick is secured, and a porcelain tube 24 inches
long and of 2-inch inside diameter passes through a hole in the centre
of this disc. Within the wooden shield and in line with each circular
opening in it and with the corresponding porcelain tube through the
fibre disc a line insulator is secured. Within the sub-station and in
line with each tube there is also an insulator, and the two insulators
near opposite ends of each tube hold the line wire that passes through
it in position.
Each wire of the transmission lines, of which the largest is No. 000
solid of 0.410-inch diameter, terminates at one of the insulators within
the wooden shield, and is there connected to a special insulated wire
that passes through one of the porcelain tubes into the sub-station. A
copper trolley sleeve 12 inches long is used to make the soldered
connection between the bare line wire and the insulated conductor that
passes through the porcelain tube. Each of these entry cables, whatever
its size, is insulated first with a layer of rubber 9/32-inch thick,
then with varnished cambric wound on to a thickness of 9/32-inch, and
lastly with two layers of weather-proof braid outside of the cambric.
This form of closed entry for the transmission lines obviously excludes
snow, rain, cold air, and dust from the station. Whether the fibre discs
and wooden rings, together with the insulation on the entry cables, are
as desirable as a glass disc at the entry is another question.
Another instance where the entry for a high-tension line is closed with
the aid of combustible material is that of the 25,000-volt transmission
between the water-power plant at Chambly, on the Richelieu River, and
the sub-station in Montreal. The four three-phase circuits of this line
are made up of No. 00 wires of 0.365-inch diameter each, which enter the
power-station at Chambly and the terminal-house in Montreal bare, as
they are outside.
At each end of the line the wires are secured to insulators on a
horizontal arm with their centres twenty-two inches outside of an end
wall of the station or terminal building. The insulators are mounted
with their centres thirty inches apart, and a few inches above the tops
of these insulators a corresponding row of wooden bushings pass through
the wall with an outward slant.
At the Chambly end of the line each of these bushings is of oak, boiled
in stearin, four inches in diameter and twelve inches long. At the
Montreal end the wall bushings are of boxwood, and each is four inches
square and twelve inches long. Each of the wooden bushings carries a
glass tube, and is itself held in position by the concrete of the wall
in which it is located. Entrance to the station by each of the bare No.
00 wires is gained through one of these glass tubes, and cold air is
excluded.
Quite a different type of closed entry for the wires of a transmission
line is in use on that between Shawinigan Falls and Montreal, which
operates at 50,000 volts. For the entry of each of the three aluminum
cables that make up this line, each cable being composed of seven No. 6
B. & S. gauge wires, a tile pipe of twenty-four-inches diameter was set
into the station wall. The end of each tile pipe is closed by a glass
plate, with a small hole at its centre, through which the cable passes.
As the cable is thus held twelve inches from the terra cotta pipe all
the way around, any leakage of current must pass over this length of
glass surface at each cable or through the air.
A heavy coating of frost sometimes collects on these plates, and this
increases the amount of current leakage over them. Surface leakage in a
case of this sort, of course, varies with the size of the glass plate,
and if a tile pipe is used the limit of size is soon reached.
There seems to be no good reason, however, why a glass plate of any
desired dimensions should not be set directly into the brick wall of a
station for each line wire and the tile pipes entirely omitted. This
plan is followed on the system of the Utah Light & Power Company, which
extends to Salt Lake City, Ogden, Provo, and a number of other points in
that State.
On the 40,000-volt line of that system an entry for each wire is
provided by setting two plates of glass into the brick wall, one plate
being flush with the inner surface and the other with the outer surface
of the wall.
In the centre of each plate there is a hole of about 2.5-inch diameter,
into which a glass or porcelain tube fits. The line wire enters the
station through this tube, and it does not appear that any shelter for
the glass plates is located outside of the building. An entry of this
type for the 40,000-volt line with glass plates in a brick wall at a
gable end of the Murphy mill is said to have given satisfactory results
during four years, though that wall faces the southwest, from which
direction most of the storms come. At this entry each glass plate is not
more than eighteen inches in diameter, and the wires are about four feet
apart. On a 16,000-volt line of the same company, a glass plate twelve
inches square with a three-quarter-inch hole at its centre, and the bare
wire passing through without a tube, has given results that were
entirely satisfactory.
Two quite different types of entry to stations are used on the
50,000-volt line between Cañon Ferry and Butte, Mont. One type, employed
at the side wall of a corrugated iron building, consists of a thick
bushing of paraffined wood carrying a glass tube two inches in diameter,
four feet long, with a side wall of five-eighths to three-quarter-inch,
through which the line conductor passes.
On the roof of the power-station at Cañon Ferry a vertical entry is made
with the 50,000-volt circuit. For this purpose each line wire is brought
to a dead end on three insulators carried by a timber fixture on the
roof. A vertical tap drops from each line wire and passes through the
roof and into the station. This roof is of wood, covered with tin
outside and lined with asbestos inside. Each tap is an insulated wire,
and elaborate methods are adopted in the way of further insulation, and
to prevent water from following the wire down through the roof.
Over the point of entrance sits a large block of paraffined wood with a
central hole, and down through this hole passes a long cylinder of paper
that extends some distance above the block. Into the top end of this
cylinder fits a wood bushing, and a length of the tap wire that has been
served with a thick layer of rubber is tightly enclosed by this bushing.
The rubber-covered portion of the tap wire also extends above the
bushing, and has taped to it a paper cone that comes down over the top
of the paper cylinder to keep out the water. On the outside of this
paper cylinder, at a lower point, a still larger paper cone is attached
to prevent water from following the cylinder down through the wooden
block. At the lower end of the paper cylinder, within the station, there
is another bushing of wood, and between this and the wooden bushing at
the top of the cylinder and inside of the paper cylinder there is a long
glass tube. Down through this tube and into the station the insulated
tap wire passes.
From the experience thus far gained with high-voltage lines, it seems
that their entrance into stations should always be at a side wall,
unless there is some imperative reason for coming down through the roof.
If climatic conditions permit, no form of entry can be more reliable
than a plain, ample opening through the wall with a large air-space
about each wire. If the opening must be closed, it had better be done
with one or more large plates of thick glass set directly into the
brickwork of the wall. Some additional insulation is obtained by placing
a long glass or porcelain tube over each wire where it passes through
the central hole in the glass plates. Each conductor should be bare at
the entry, as it is on the line. Some of the above examples of existing
practice in entries for transmission lines are taken from Vol. xxii., A.
I. E. E.