Electric Transmission of Water Power

by Alton D. Adams

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.