Electric power station

“ Another advantage of connecting transmission and distribution lines in a sub-station, where regulation of voltage can be had, lies in the fact that it is practically impossible to maintain an absolutely constant pressure miles from a generating plant at the end of a transmission line that is carrying a mixed and varying load. A result is that without the intervention of regulation at a sub-station it is almost impossible to give good lighting service over a long transmission line. ”

“ Owing to the relatively small load factor (say 8 or 10%) of a station giving electric supply for lighting only, the object of every station engineer is to cultivate a demand for electric current for power during the day-time by encouraging the use of electric motors for lifts and other purposes, but above all to create a demand for traction purposes. Hence most urban stations now supply current not only for electric lighting but for running the town tramway system, and this traction load being chiefly a daylight load serves to keep the plant employed and remunerative. ”

“ The alternating generators in a station of equal capacity and voltage would require a large switchboard fitted with bus-bars, oil switches, and automatic circuit-breakers. Relative efficiencies for the continuous-current and the alternating-transmission systems vary with the kind of service required at receiving stations and with the extent to which transformers are used in the alternating system, other factors being constant. ”

“ As a partial offset to the low cost of the continuous-current generators and to the absence of transformers, there is the necessity for motor-generators in a sub-station when current for lighting as well as power is to be distributed. ”

“ Where a number of water-power stations feed into a single sub-station the requirement that each generating station have its capacity divided up among quite a number of dynamos may not exist, since one station may be entirely shut down for repairs and the load carried meantime by the other stations. A good illustration of this point may be seen at Manchester, where a single sub-station receives energy transmitted from four water-power plants. At one of these plants the entire capacity of 1,200 kilowatts is in a single generator. ”

“ For street-car motors using direct current at about 500 volts, the sub-station equipment includes either step-down transformers and converters or motor-generators with or without transformers. It is the practice in some cases where both lighting and street-railway service are drawn from the same transmission system to keep these two kinds of service entirely separate, devoting independent generators and transmission lines, as well as independent transformers and converters or motor-generators, to the street-car work. ”

“ At sub-stations it is desirable to have a group of transformers for each transmission circuit, and it may be necessary to subdivide the transformer capacity still further in order to keep transformers in operation at nearly full load, or to provide a group of transformers for each sort of service or for each distribution circuit. All of the transformers at a sub-station should have a total capacity at least equal to that of the generators whose energy they are to receive, minus the losses in step-up transformers and the line. ”

“ In the case of each figure it is assumed that the distance between the generating station and each of the cities or towns is such that distributing lines with a loss of, say, not more than two per cent in voltage at full load cannot be provided between the generating station and each city or town because of the cost of conductors. This being so, one or more centres of distribution must be located in each town, and the transmission lines must join the distribution lines at these centres either on poles or in sub-stations. ”

“ The fact that a circuit-breaker may be designed to open the line which it connects, whenever the direction from which the flow of energy takes place is reversed, is taken advantage of at some sub-stations to guard against a flow of energy from a sub-station back toward the generating station. By this means a flow of energy from a sub-station to a short circuit in one of the lines or cables connecting it with the generating plant is prevented. ”

“ The money invested in central station business now exceeds six billion dollars, and the annual output of electric current is sufficient to keep eight billion 16-candle-power carbon filament electric lights burning continuously night and day. All this has more than doubled in the last five years. Central stations are now furnishing about five times as much current for heating, cooking, and charging automobiles as they did five years ago. About one third of all the central stations depend on water power.
[206]
Fig. ”

“ Even a small water-driven electric station that can be operated by a single attendant at any one time must have two attendants if it is to deliver energy during the greater part or all of every twenty-four hours. But a single attendant can care for a water-power plant of 2,000 horse-power or more capacity, so that two plants of 750 horse-power each will require double the operating force of one plant of 1,500 horse-power. ”

“ This station contains four three-phase generators of 750 kilowatts each, and six transformers of 500 kilowatts each, these latter being connected in two sets of three each. The use of three transformers for each three-phase generator instead of three transformers for each two or three generators, tends to keep transformers fully loaded when in use, and therefore to increase their efficiency. On the other hand, efficiency increases a little with the size of transformers, and the first cost per unit capacity is apt to be less the greater the size of each. ”

“ An electric automobile garage, situated close to a public generating station and offering a larger and more regular demand, will of course obtain current much cheaper. And it is possible that arrangements may be made for supplying electricity to automobiles at a much lower rate even than that customary for general power demands. In the metropolitan borough of Marylebone, for instance, an electric garage may obtain current during the small hours of the night at 1/2d. per unit, which is half the standard rate for power purposes. ”

“ This is particularly true in cases where energy transmitted over a single circuit is distributed for both incandescent lamps and large electric motors, because the starting and operation of such motors causes large fluctuations of amperes and terminal voltage on the transmission circuits. To hold such fluctuations within limits which a sub-station can readily compensate for, it is necessary that the loss in the transmission line be moderate, say often within ten per cent of the total voltage delivered to it at maximum load. ”

“ The capacity of the Butte sub-station is 7,600 horse-power.
The great electric water power system marked by the Santa Ana station at one end and the city of Los Angeles at the other, eighty-three miles distant, includes more than 160 miles of transmission lines, several hundred miles of distribution circuits, and supplies light and power in twelve cities and towns. Among the customers of this system are an electric railway, a number of irrigation plants, and a cement works. These [9] works contain motors that range from 10 to 200 horse-power each in capacity. ”

“ In practice the volts lost through inductance are compensated for by operating generators or transformers in the power-plant at a voltage that insures the delivery of energy in the receiving-station at the required pressure. Thus, in a certain case, it may be desirable to transmit energy with a maximum loss of ten per cent in the line at full load, due to the resistance of the conductors, when the effective voltage at the generator end of the line is 10,000, so that the pressure at the receiving-station will be 9,000 volts. ”

“ This plan has the obvious advantage that much of the regulation for the several parts of the entire load may be done at the generators, thus reducing the amount of regulation necessary at the sub-station, and that fluctuating [240] motor loads do not affect the lamps. In this case the conductors of the several transmission circuits are all of moderate size, and the division of the lines was evidently adopted for purposes of regulation, rather than to reduce the amount of inductance. ”

“ Even where the power is to be utilized at some point distant from each of several waterfalls, it may be convenient to combine the power of all at one of them before transmitting it to the place of use. This is actually done in the case of two electric stations located respectively at Indian Orchard and Birchem Bend on the Chicopee River, whose energy is delivered to the sub-station of the electrical supply system in Springfield, Mass. At the Indian Orchard station the head of water is 36 feet, and at Birchem Bend it is 14 feet, while the two stations are about 2 miles apart. ”

“ When they are worked from a power station the great advantage is gained that the same plant which drives them can be used for many other purposes, such as working machine tools and supplying current for lighting. For dock-side jib cranes the use of electric power is making rapid strides. For overhead travellers in workshops, and for most of the cranes which fall into our second class, electricity as a motive power has already displaced nearly every other method. ”

“ In the rare case where the power to be developed is so great that the number of generators necessary to give security and reliability to the service leaves each generator with a capacity [106]
[107] larger than is desirable for structural reasons, the number must be increased simply to reduce the size of each generator. Such a state of facts existed at Niagara Falls, where the first station contains ten dynamos of 3,750 kilowatts each, and the second station contains eleven units of like capacity. ”

“ Transformers at the Niagara Falls station raise the voltage from 2,200 to 22,000 for the transmission to Buffalo.
As transformers can be wound for any desired ratio of voltages in their primary and secondary coils, a generator pressure that will allow the most economical construction can be selected where step-up transformers are employed. In general it may be said that the greater the capacity of each generator, the higher should be its voltage and that of the primary coils of step-up transformers, for economical construction. ”

“ With a constant speed at the motor and its connected generator it is of course easy to deliver current at constant voltage to the distribution lines. This constancy of speed makes the synchronous motor generator a favorite in large transmission systems with both power and lighting loads. The satisfactory lighting service in Buffalo, operated with energy transmitted from Niagara Falls, seems to be due in some measure to the use of synchronous motor generators at the sub-station in Buffalo, whence lighting circuits are supplied. ”

“ In a modern water-power station that contributes to electrical supply in a large city the generator capacity is 1,200 kilowatts. All of the labor connected with the operation of this station during nearly twenty-four hours per day is done by two attendants working alternate shifts. ”

“ The second station with vertical units at Niagara Falls has a capacity of 41,250 kilowatts in eleven generators on vertical shafts, and its floor area amounts to 0.86 square foot per kilowatt; narrow impulse-wheels of large diameter tend to economy of floor space, as in Electra station, where the room containing wheels and generators has an area of only 0.83 square foot per unit of its 10,000 kilowatts capacity. At the Colgate plant, where the total rating of generators is 11,250 kilowatts, the floor area under wheels and generators is almost exactly one square foot per kilowatt. ”

“ Let the problem be to deliver electrical energy in a city distant 100 miles from the generating plant. Transformers with approximately twice the capacity corresponding to the maximum rate of transmission must be provided, because one set is required at the generating and another at the delivery station. The cost of these transformers will be approximately $7.50 per horse-power for any large capacity.
Reliability is of the utmost importance in a great power transmission, and this requires a pole line of the most substantial construction. ”

“ From these examples it may be seen that the prevailing type of electric water-power station, whether designed for horizontal or vertical wheels of either the pressure or impulse type, is wide enough for only a single row of generators and wheels, and has sufficient length to accommodate the required number of units. ”

“ Synchronous Converters at Campbell Avenue and Homer Street Station
13 Power Supply The power for the early elevated lines was derived from engine-driven direct-current power plants all of which have since become obsolete. All power, therefore, is supplied from the network of the Commonwealth Edison Co., which maintains an ample reserve to supply all needs. While a large percentage of the power now being purchased is generated at 25 cycles, the policy of the Power Co. on all new equipment is to specify 60 cycles. ”

“ Where the electric generators operate at the maximum voltage of the system, as at Garvin’s Falls and in the power-house of the Manhattan Elevated Railway, there may be said to be only one general plan of connections possible. That is, the generators must connect directly with the main bus-bars at the voltage of the system, and the feeders or transmission lines must also connect to these same bars. ”