[Trade Journal]
Publication: Transactions of the American Institute of Electrical Engineers
New York, NY, United States
vol. 12, p. 405-432, col. 1
A paper presented at the Twelfth General
Meeting of the American Institute of Electrical
Engineers, Niagara Falls, N. Y., June 27th
1895, President Duncan in the Chair.
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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.
PIPE-LINE.
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.
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| Fig. 1. — Pipe Line, Pomona Plant. Effective Head 390 Feet. |
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
[missing pages 407-417]
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.
SWITCHBOARD.
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.
LINES.
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.
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| Fig. 4. — Route of Lines. |
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.
Commercial lighting in Pomona was begun November 28th, 1892, and a telegraphic order was at once sent for another set of reducing transformers, to be used at San Bernardino, where the local circuits were already nearly completed. It was a condition of the franchise for the latter place that lighting should be begun before January 1st, and work was pushed rapidly on the transmission line, over 2S miles long, until it was finished. The order for transformers had, however, been given too late for them to be delivered in time, and it was decided to reduce the pressure at Pomona to 5,000 volts and to take to San Bernardino the half of the Pomona batik thus made available. In order to avoid the re-heating which would have been required had the transformers been dismounted for shipment, the latter were hauled to San Bernardino on wagons without springs, after the lids had been packed so that the oil could not splash out. The transformers readied their destination after midnight, and the next day, December 31st, they were connected to the transmission line for a few moments to see that the latter was all right, after which all the lights on the circuit were thrown on for the night.
When the new transformers arrived they were set up and heated in the mine way as the others. The heating was continued night and day until complete, without interrupting the regular lighting service. The change of both circuits to 10,000 volts was made February 16th, 1893.
SUB-STATIONS.
The sub-stations at Pomona and San Bernardino are small brick buildings, one story high, with flat tin roofs and numerous windows. The Pomona building contains two rooms connected by a door near the switchboard. The front room is used as the office of the company, and the other as a transformer room. The 10,000-volt circuit enters over the door, and is connected directly to the bank of lowering transformers, no switches or other appliances being used on the high tension lines either there or at San Bernardino. The switchboard appliances are a Stillwell regulator, two 100-amp. marble fuse-blocks with double plugs and fuses; a 100-amp. jaw-switch, 150-amp. ammeter, compensating voltmeter, compensator and converter.
The San Bernardino sub-station is longer than that at Pomona, and is not divided by a wall. The transformers are placed in a row along a side wall, and along part of the end wall next the switchboard. The transformers near the switchboard heat the most. This is probably due to the slight drop in wiring between them and the more distant converters. During the first year of operation a little new oil was put into the converters about every four months to make up for that evaporated, but none of the old oil was drawn off. The switchboard apparatus is like that at Pomona.
The Stillwell regulator has long been recognized as a valuable adjunct to the central station operating a number of feeders of different lengths from a single dynamo. Its utility is still greater in a system of long-distance transmission in which the transmission circuits are supplied from one dynamo, as was the case in this plant during its first year of operation, since it is not practicable to install such a system so as to operate with small line-loss, and therefore means must be provided to compensate for the large differences in the pressure at the ends of the lines. The use of regulators at the power-house was impossible when but one bank of raising transformers was used for the two circuits. Even when it became possible by the use of separate banks of transformers, it was still undesirable because the attendant at the powerhouse would often have had to work both the regulators at once to properly compensate for changes in load, and his attention would have been required by the regulators at exactly the time he should have been free to attend to the generators. A regulator was therefore placed at each sub-station, as already stated. These are each of 2000 lights capacity and have a range of 10 per cent. up, and 10 per cent. down. This variation of 10 per cent. (100 volts) is divided into 14 equal parts, so that each step corresponds to 7.1 volts. The diagram (Fig. 5) was taken one Sunday, while using only 5000 volts, and shows the number of changes required during the evening at each station. The line marked "off" is the base line, and the divisions above and below correspond to the steps on the dial and show to what an extent the regulators were used to increase and decrease the pressure. This diagram shows how impossible it would be to furnish lights without using regulators, even on Sunday when the load is much smaller than on other days. It also shows that on that day the pressure at Pomona was being mostly reduced while that at San Bernardino at times (as at the point marked "very bad light") could not be sufficiently in-creased. This indicates that the pressure of the lowering trans formers at one of the sub-stations should be changed so as to make the pressures at both inure nearly equal, and thus increase. the effective range of the regulators. In the present plant this is done by changing the number of transformers.
The distribution from both sub-stations is effected in the usual manner at 1400 volts for incandescent lighting, the only point of interest being, that a considerable number of Helios are lamps are successfully used on the incandescent circuits.
OBSERVATIONS
While in use, the transformers in the sub-stations give forth a continuous hum which depends for its tone on the number of alternations. This is an excellent indicator fur the attendant, whose attention is instantly called to any change in the running conditions of the plant by the resulting change of tone. Its indications not only mark changes which are taking place and which can be detected on the voltmeter, but also give notice of coining changes before there is any other indication of them_ It is thus possible to foretell a coining drop in voltage in time to use the regulator and thus keep the voltmeter needle perfectly still, though the voltmeter is a very sensitive instrument, and the regulator is often moved four or five notches. The hunt often changes, however, without any corresponding movement of the voltmeter, but the sound is then somewhat different. At rare intervals the Switchboard lights will suddenly change slightly in candle-power before any change is noticeable on the voltmeter.
It is noticeable that the needle will often stand for a time perfectly still on the center, and, on a slight rise in the hum, will start gently rising, never more than three quarters of an inch, and then as the tone gradually becomes lower, slowly fall back to the center and stop without passing it. At other times the variation in hum is inure sudden and the needle will rise and oscillate above the center. Again, the needle will oscillate equally about the center during a regular rise and fall of hum, its' movement being apparently due to one impulse and not seeming to be modified by subsequent variations. There is no apparent change in candle-power of the lamps during the voltmeter changes noted. These notes were made while the plant was running at only 5,000 volts, but they were later confirmed when-using 10,000 volts. During dry weather there is considerable intermittent oscillation of the voltmeter-needle without there being any change in load or any other apparent cause, while in wet weather, the needle remains perfectly still for many minutes at a time, often for as much as half an hour. A possible ex-planation of this oscillation may be found in the presence of static charges on the line, due to atmospheric electricity. That the line is often heavily eharged from the air, is shown by a number of observations. One afternoon a painful shock was obtained on touching the line at the canyon end, drifting clouds and a strong wind being noticed in the valley. Again, while using the telephone a report was heard in it so sharp as to cause momentary deafness. Later, after a moderate wind had been blowing for sonic time, loud reports were noticed on the telephone at long intervals. As the wind became higher the reports came oftener, and the intervals between the reports became shorter. It was evident that there was a discharge from the lines through the telephone (which was on a metallic circuit) and that it depended on the rate at which the wind blew. In order to get the strongest effect the two wires were connected in the usual way to the raising and lowering transformers, and one side of the telephone connected to one wire. On connecting the other side of the telephone to ground a sharp report was heard, and on maintaining the connection there was a sound as of steam escaping at a distance, with intermittent and very faint crackling. If the ground contact was made slowly, there was a bright spark before the metals touched, and a loud report. If the fingers were interposed a smart shock was received. By making and breaking the ground connection rapidly, the line was prevented from accumulating a heavy charge, and no spark was visible, though a faint crack was heard. If a slight space was left between the telephone wire and the ground, a spark occurred at fairly regular intervals, and when the space was lessened the sparks became smaller and more frequent. When the wind lessened, the sparks and reports became almost imperceptible, but on the wind becoming strong and blustery a large spark was again obtained. When one line wire was disconnected from the transformers at Pomona the effect obtained from grounding that wire was less, owing to the reduction in capacity.
These observations, which were made on the Pomona circuit during hot, dry and cloudless weather, show conclusively that the lines were heavily charged by the action of the wind. The wind no doubt blows electrified air and dust against the wires, the latter thereby accumulating a static charge with a rapidity which we have seen depended on the speed of the wind.
Some curious conditions met with in the operation of the plant are shown in the ampere curves in Fig. 6, which were taken during very wet weather. Taking the maximum load measurements at 7 P. M. for March 14th and 8th, we note that the total apparent energy delivered by the sub-stations is 73 per cent. and 75 per cent., respectively, of that delivered by the generator, while for a smaller load at 9 P. M. the respective percentages decrease to 47.6 per cent, and 48.6 per cent. in spite of the fact that the apparent energy delivered by the generator is in the latter case-only half as great as in the former. These results are due to changes in the angle of lag caused by changing the load.
In the Canon curve of March 8th, the load at 9.30 P. M. is seen to have increased considerably, although the sub-station loads were decreasing. This increase was due to the fact that the rain which had previously ceased to fall at Pomona, began again.
TESTS.
The electrical resistance of the Pomona circuit is 74.9 ohms and that of San Bernardino 156.4 ohms at about 90 degrees F.
The insulation resistance of the circuits varies from far beyond the limits of an 11-megohm bridge in dry, hot summer weather, to as low as 0.65 megohm during light rain. The latter value is that obtained with the two San Bernardino wires in series, and represents an insulation-resistance per mile of over 37 megohms. On a clear, sunny day the insulation-resistances of one Pomona wire and of one San Bernardino wire were respectively 11 megohms and 5-1/2 megohms, which are in the exact relation which would be expected.
The lines cannot be tested for grounds or for continuity with a magneto bell, as their capacity is such that the bell will ring-loudly when the wire is perfectly insulated and the circuit open. This capacity also prevents the use of the ordinary converter test for line leakage, as the lamp on the secondary willburn as though there were a ground even when the line is perfectly clear. By adding lamps, however, the light may be cut down until the last lamp added extinguishes them all. This would not occur if the light of the first lamp was due to a difference of potential on the primary caused by a ground, as then the added lamps would be come as bright as the first. The primary current is thus seen to be limited, and to be duc to the capacity of the line. If the proper number of lamps has been added, to just secure darkness when the line is known to be free from grounds, the converter may be used to show grounds on that line, since the presence of a ground will cause all the lamps to brighten. The line test should only be made after ascertaining with one lamp that the generator is not grounded, as otherwise the leakage current for the number of lamps might burn the insulation of the generator. A very good example of Prof. Fleming's "condenser effect" was met with in the summer of 1893, while the engineer was trying this converter test for measuring line leakage. The primary of a 1000-100-volt converter was connected between the ground and one terminal of the generator, the other terminal of the latter being connected to one wire of the Pomona circuit. The resulting secondary pressure was 30 volts, which made one lamp burn dimly. When ten lamps were turned on, the pressure dropped to 14 volts. When all lamps were turned off the pressure became ten times as great, or 140 volts. One generator brush was then raised, and the second wire of the circuit was connected to the first. On completing the circuit again there was a sudden flash on the armature, followed by a crackling noise and a brush discharge which lasted until the circuit was broken. The Pomona wires were then disconnected from the dynamo terminal, which was left free. With this arrangement faint sparks and a glow were noticed, as though the armature winding were grounded on the core, but a Wheatstone bridge test showed an insulation resistance of over 10 megohms. Since then the machine has been used regularly and has not broken down, so it is evident the spark must have resulted from a sudden increase of the potential of the armature winding above that of its core, which latter was permanently connected to earth.
A similar test was made on the San Bernardino wires in the previous January during dry weather, the only change being that the single lamp was not turned off. With only one wire connected, one 16 C. P. lamp came nearly to candle-power. On connecting the second wire to the first (at San B.) the lamp burned at full candle-power. Ten lamps (the full load) were then put on the converter and burned dimly. As they were turned off one by one, the others brightened until the last lamp lighted up as before. This test did not cause any discharge on the armature, in spite of the fact that the circuit was twice as long and the capacity thus twice as great. The probable explanation of this is that the inductive resistance of the converter was not so great as in the other case, on account of the single lamp being left burning. It is also possible that the breakdown in the other test was assisted by the presence of copper dust on the winding.
A comparison between the ammeters used at Pomona and San Bernardino showed that for readings above 674 amperes the San Bernardino instrument read higher than the other. As there was no reason for considering one instrument more accurate than the other, and no way of telling which was right, the San Bernardino readings taken during the test were reduced to the average of simultaneous readings on the two instruments.
A short test of the Pomona circuit was made on April 11th, the results of which are given in Table I.
A preliminary efficiency test on the San Bernardino circuit was made April 13th, 1893, with the results shown in curve in Fig. 7. In this, as in all the other tests, a variable load was obtained by the use of a water resistance.
An efficiency test of the San Bernardino circuit lasting five hours, during which the generator was kept at full load or overload, was made April 14th, 1893. The voltage at San Bernardino during the tests was measured with a Weston portable voltmeter. Readings were taken every fifteen minutes, and the efficiencies and the corresponding voltages for different loads are plotted in Fig. 7. The most prominent feature of this efficiency curve is its irregularity. This is, however, satisfactorily explained by referring to the curve of generator voltage, on which it is seen that the high efficiencies correspond to high voltages, and vice-versa.
It will be noted that the apparent efficiencies shown by the preliminary test at San Bernardino agree very closely with the results of the long efficiency test there. On calculating the apparent efficiencies for different loads from data obtained by lab oratory tests, and comparing, them with the measured apparent efficiencies, it is found that they agree within three or four per cent. This very close agreement is exceedingly gratifying, particularly when we consider that the tests of the plant were made with ordinary commercial instruments, and that the laboratory tests were made about a year before the commercial tests.
An interesting and unique test was made May 2d, 1893, by connecting the Pomona line wires in series with the San Bernardino circuit, and transmitting about 100 H. P. to San Bernardino by way of Pomona. The length of the circuit was Si miles, and the distance of transmission 42-1/2 miles. This is the greatest distance yet covered by any transmission since the Frankfort experiments. The measurements are given in Table II. The apparent efficiencies are much lower than those indicated by calculations, as in the latter no account was taken of the capacity and inductance of the circuit. The voltage of the generator as measured is about 5 per cent. above that calculated, and the amperes measured, notably exceed the amperes calculated. These results are attributable to the introduction of the Pomona loop, which added both capacity and inductance.
In order to reduce the cost of operation, the San Antonio company first dispensed with the sub-station attendant at Pomona after 11 P.M. This they did sometime before the acceptance of the plant, after assuming the responsibility for any damage which might result. No trouble occurred and, after the acceptance (May 6th, 18930 the same plan was put in operation at San Bernardino. After working in this way for several months, the Pomona station was started in the afternoon and then locked up until the next morning. This arrangement was made possible by running the generator so that the pressure would be right at Pomona, the pressure at San Bernardino being kept right by the attendant with the Stillwell regulator.
In January, 1894, another 120 K. w. generator was installed with an equal capacity of oil transformers, and the Pomona and San Bernardino circuits are now each operated from separate generators with separate banks of transformers at 10,000 volts. Since this change the San Bernardino attendant has also been dispensed with, and the book-keeper starts up in the afternoon and then locks up the station for the night, as at Pomona. The voltage of the lamps on each circuit is regulated by the engineer at the power-house, the generator pressure necessary at different loads to keep the lamps at the proper brilliancy being automatically indicated by the compensating voltmeter on each circuit. Thus after the plant is started the engineer has sole charge. The following associate members were elected at the meeting of Council held at Niagara Falls, June 27th, 1895:
