Publication: American Institute Of Electrical Engineers
New York, NY, United States
LONG DISTANCE TRANSMISSION AT 10,000 VOLTS.
(THE POMONA PLANT.)
BY GEORGE HERBERT WINSLOW.
The Pomona plant was installed in the summer and fall of 1892 for the San Antonio Light and Power Company, of Pomona, Cal. It was increased in the following spring, and early last year the capacity of the plant was doubled by duplicating the entire equipment. At the present time, when the plant has been in regular operation for more than two years, and its complete success has established confidence in the successful outcome of many similar projects of greater magnitude, it seems fitting to present a careful description of the entire installation. The electric plant was installed under the personal direction of the writer, as electrical engineer, who presents many of his personal observations on its construction and operation.
The plant is used to transmit energy from a waterfall to substations at Pomona, 13 3/4 miles distant, and San Bernardino, 281 miles distant, from which points it is distributed for incandescent and arc lighting. It consists of a Pelton water power plant and a Westinghouse alternating current transmission plant in which generators supply currents to sets of raising and lowering transformers operating at 10,000 volts, and delivering current to the local circuits at 1,000 volts.
The water power for this plant is derived from the San Antonio creek, which is chiefly supplied by the melting snows and the rains on San Antonio Mountain. Side canyons, however, also furnish some water. The creek flows for several miles through a narrow valley at the upper end of the San Antonio canyon in a bed which it has washed for itself in the layer of boulders and gravel formed by the action of an immensely larger stream in past ages.
At the lower end of the valley, a sharp ridge extends eastward from the side of a neighboring mountain, from which it originally split off, and blocks up the valley except at a narrow place at which bed-rock is exposed, and through which the stream plunges suddenly downward at least 90 feet between precipitous walls of rock, forming the San Antonio Falls.
To utilize this fall, part of the water is diverted by a dam about 200 feet above the falls into a canal which conducts the water to a tunnel passing through the ridge. At the other end of this tunnel, the water enters a large pipe leading to the powerhouse, which is located 412 feet below the level of the outlet of the tunnel.
The pipe is of sheet steel, double-riveted throughout, and was delivered on the ground in sections having a length of 11 feet 6 inches. These sections consist of four sheets, each three feet long.
The diameter of the pipe up to within 450 feet of the powerhouse is 30", with the exception of the length which connects it to the sand-box at the top of the pipe, which length is considerably expanded, so as to allow the water to flow slower on entering, and thus to reduce the entrainment of air. Near the power-house a "reducer" is inserted in the pipe to reduce the diameter to 24", and this size is maintained from this point to the power-house. The pipe was designed to carry 2000 miner's inches of water (measured under a head of 6 inches), without unnecessary loss by friction. The capacity is equivalent to 50 cubic feet per second, or 1882 H.P. at 390 feet effective head, assuming
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Clark's insulation is used on all wires connected to the transformers and to the dynamo, and the terminal wires of the full bank, which must often be disconnected for testing, are further insulated by heavy glass tubes at points where they might come in contact with other wires. All other transformer-wires are supported upon double-petticoat glass insulators, and all dynamo wires upon porcelain knobs.
The switchboard is of narrow redwood boards, tongued, grooved and beaded, nailed on a framework of yellow pine, the latter supported on porcelain insulators to keep it dry. The switchboard outfit for one generator and one exciter consists of two 120 amp. fuse blocks, an A. C. field rheostat with a 25 amp. D. P. field switch with fuses, an exciter rheostat, one 150 amp. ammeter and a 200 amp. D. P. jaw-switch. From this switch the current passes to two 4-dynamo, marble switch-panels which are connected in multiple to the dynamo, and are each provided with two pairs of contact plugs. By means of these panels and of two 200 amp. dynamo-changing switches below them, any feeder can be operated from any dynamo which is connected to the switch panels. Between each panel and its switch is a pair of 65-ampere Wurts shunt-wire fuse-blocks, each provided with an extra fuse and shunt which can be connected by inserting a plug, should it be desired to double the fuses during the run on account of over-load or of weakness in the fuse. The remaining instruments on each feeder are a voltmeter, a No. 1 switchboard-converter and a 150 amp. type "E" compensator. When both feeders were run from one alternator, one voltmeter was connected to the generator and the other to the feeder, and in this way the amount of compensation could be watched.
The oil-transformers were tested before shipment with 20,000 volts between the line-coil and the core, and were then taken out of the oil and boxed. In order to expel any moisture which might have been absorbed by the insulation of the coils or have condensed on the cores during their long journey, the transformers were connected in two banks of ten each, the line-coils of each set being connected in series to the generator, which was run at a reduced speed, and the secondary coils each short-circuited on itself. The coils were thus gradually heated to a point somewhat above the boiling point of water, which at that elevation was about 201 deg. F. They were kept at this temperature for a short time and then paraffin oil of a special grade ("Diamond") was poured slowly into the boxes at the edges so that the coils would begin to absorb oil at their lower ends, and thus drive upward the air and volatile gases occluded by the insulation. The transformers were then again brought to their former temperature, which caused expansion and partial expulsion of the air remaining in the insulation. Some of the air would however collect under the insulation at the top of the coils, and had to be freed by mechanical agitation, produced by stirring the folds of insulation or by pounding on the boxes. The heat caused volatization of some of the lighter elements of the oil, these coming to the surface as bubbles, just as the air did at first, and the agitation was kept up at intervals until bubbles from this cause also were entirely eliminated.
The 20 transformers were then connected as they would be when in regular use, and the two terminals of the line-coils, which were to give 10,000 volts, were connected in series with one hundred 100-volt lamps, which were then brought to full candle power, showing that the transformers were all in good condition. A similar test was then made at Pomona at the end of the 14-mile transmission line running to that place, after which the transformers there were prepared for work in the same way as at the power-house, except that the grouping and initial voltage were changed.
There are two transmission lines, one 13 3/4 miles long, which supplies Pomona, and another 28 3/4 miles long, which supplies San Bernardino. (See Fig. 4.) Each line consists of two No. 7 B. & S. gauge, hard-drawn copper wires. The joints in the wire are made with McIntire connectors. To further improve the joint, the ends of the wires were bent back side by side and soldered together. After the Pomona line was completed and the first ten miles of the San Bernardino line was put up, the supply of connectors ran out, and the regular telegraph joint was substituted. The conductivity was assured by soldering as before.
The wires are supported upon large double-petticoat flint-glass insulators designed for this plant. These insulators are of perfectly clear flint-glass, which gives a better surface-insulation than is attainable with any other kind of glass.
It was at first proposed to use oil insulators. The reason they were not used was because the glass companies which had undertaken to furnish them, found on trial that they could not make them without considerable experimenting, which would have delayed the installation of the plant. This was no doubt fortunate, as the country through which the line passes is subjected to hot, dry winds which not only blow dust onto the insulators, but also inside them, and during the day the sun beats on the insulators until they become so hot that they nearly blister one's hands. If oil were used under these conditions it would soon evaporate and thicken, and become filled with dust. It would therefore seem undesirable to have used oil insulators in this case, or to use them in any other until an increased voltage makes them necessary, and the transmission of greater amounts of energy over the circuits justifies the additional expense necessary to keep the insulators in good condition.
The two circuits are carried on the same pole line for 72 miles. (See Fig. 4.) The inside pair of pins was used for the circuit to Pomona, and the outside pair for that to San Bernardino, until after the acceptance of the plant, but in anticipation of the installation of another generator, the Pomona circuit was changed to the right-hand pair of pins, and the San Bernardino circuit to the left-hand pair, to avoid the fluctuation in lights which would result from inductive interference between two independent circuits.