[Trade Journal]
Publication: American Institute Of Electrical Engineers
New York, NY, United States
p. 220-223, col. 1
APPENDIX III—TRANSMISSION LINE PROBLEMS IN THE WEST
BY P. M. DOWNING
The different power companies operating on the Pacific Coast have done a great deal in the way of pioneering in the transmission of power over long distances at high voltages. This was due, to a considerable extent, to the fact that there is an abundance of water available for the hydroelectric generation of power, and also to the fact that the cost of fuel is very high as compared with that further east.
The first long-distance transmission to be put into successful operation on this coast, if not the first in the United States, was that of the San Antonio Light & Power Company in southern California, at what is known as their Pomona plant. This transmission was for a distance of 30 miles (48.2 km.) at 10,000 volts.
Transformers were at that time not manufactured for this voltage, and it was necessary to connect in series ten 1000-volt transformers for stepping up. The low-tension windings were connected in multiple.
The first polyphase installation was made in 1893, at Mill Creek, and furnished power to the city of Redlands.
The success of these undertakings gave a great impetus to the hydroelectric industry, and inside the next five or six years, there were several different companies operating at voltages as high as 60,000.
The climatic conditions of the Pacific Coast, and particularly of California, are most favorable to the operation of high-voltage lines. Except in the higher mountain districts, there is practically no snow to contend with, and there are but very few places where lightning ever causes any trouble. Some operating companies have installed lightning arrester equipment, but it is not common practise to do so. The well-known horn gap arrester, one side of which is connected directly to ground without resistance, has in many instances been used to advantage, more as a voltage limiting device than as a lightning arrester. These have proved quite satisfactory for this work. Ground wires strung above the line wires have been used to some extent, but there is so little trouble from lightning that engineers have never been able to decide as to whether or not there is any advantage in using them.
Very seldom, if ever, is it possible to find where an insulator has been punctured due to lightning. This is, possibly, due to the fact that the high voltage dissipates itself over the surface of the insulators. A number of instances is known where lightning has gotten onto the line, burned off one or more wires between poles, and gone to ground without in any way damaging the insulators, and only slightly damaging the wood poles.
The greatest trouble on lines using the ordinary four-part, 14-in. (35.5 cm.) pin type insulators at voltages around 60,000 is that due to leakage over the insulator. The climate of this coast is peculiar in that there are two seasons, one being dry and lasting for six months, the other being wet. During the dry season, and particularly in the sections near the coast, dust and salt fog will accumulate on the insulators to such an extent that leakage will eventually burn off the pole, and sometimes the line wire, without damaging the insulator itself.
Different ways of overcoming this trouble have been tried, but nothing seems to improve the condition, except cleaning the insulator. Even this does not entirely overcome the trouble. After the first heavy rains come on in the early winter, the insulators are washed off, and little trouble need then be expected, until they have again become covered. The thing that seems to eliminate the trouble most is to tie the pins together electrically, but not ground them. The grounding has been tried, but with unsatisfactory results, because arcs will oftentimes be established between line- wire and ground, which will hold until the voltage drops sufficiently for them to be extinguished.
The regulation of voltages on a high-tension network is not always an easy problem. No one particular point can be selected at which voltage can be kept constant, but it is necessary to keep it as near constant as possible over the entire network. This is accomplished by the use of synchronous condensers located at the more important distributing centers. These condensers can be made automatic by varying the fields. Several such installations are now in satisfactory operation.
The first high-voltage oil switches used at 60,000 volts were put into operation on the transmission lines in this State. The same general type and design that was originally selected as being suitable for 40,000 volts is still being used. On the 110,000-volt lines the same general type, with slight modifications, is also being used. In this particular switch the circuit is opened at two or four points, depending on the voltage and load carried. The contacts are made by rotating blades moving in a horizontal plane. The separate oil containers for each leg of the circuit are insulated from each other and from the ground by means of properly designed porcelain bushings or insulators.
Another very successful design of switch for the same character of service is that using a pantagraph arrangement of links carrying a contact and operated by a vertical movement of one corner of the pantagraph, thus giving a break in a vertical plane. Both of these types of switch have for years been in successful operation under all conditions of load. They have a weakness common to all oil switches, in that when operated under a heavy short circuit they will at times throw oil out of the container.
Outdoor oil switches, either automatic or non-automatic, are being used very generally for branch lines, or for sectionalizing trunk lines. For economic reasons the automatic switches are very often operated by means of a solenoid placed directly in the main line. Such an arrangement does away with the use of current transformers, but has the objection that the connection from the movable plunger to 'the relay, or trip, must stand the entire voltage from line to ground.
As the sensitiveness of the trip is, to a considerable extent, dependent upon the weight of the moving parts, the importance of keeping this weight down to a minimum is apparent. Well-treated rods of sufficient length have been used to make the connection from the plunger to the trip, or relay, but they have not been entirely satisfactory, because sooner or later they cause trouble from burning.
Transformer designs have more than kept pace with the art of long-distance transmission at high voltages, and it is now possible to get satisfactory apparatus for any voltage that can be handled on the lines.
One thing, however, that some manufacturers have failed to give proper attention to is that of supporting the windings to prevent injury or distortion due to mechanical strains set up when short circuits occur. In units of large capacities and high voltages, the coils are generally built of strap copper, using one turn per layer, these turns being separated by one or more thicknesses of insulating material. Obviously, with such a construction, the turns will be easily displaced, unless they are well supported on both sides.
A number of failures have recently occurred where straight vertical separating strips have been used in assembling the coils, thus leaving the turns between these separating strips unsupported throughout their entire length, and allowing them to be displaced when short circuits came on. Shorter strips placed diagonally across the coils, or wave-shaped strips, which will allow a free circulation of oil, will entirely overcome this trouble.
