Just how far up the voltage of alternators can be pushed for practical purposes is uncertain, but it seems that the limit must be much below that for transformers where there is ample room for solid insulation and the coils can be immersed in oil. The use of generators at 10,000 volts and above tends to lower the volts per mile on transmission lines, because it seems better in some cases to increase the weight of line conductors rather than to add step-up transformers, as in the 42-mile transmission from Portsmouth to Pelham.
CHAPTER X.
TRANSFORMERS IN TRANSMISSION SYSTEMS.
Transformers are almost always necessary in long electric systems of transmission, because the line voltage is greater than that of generators, or at least that of distribution. As transformers at either generating or receiving stations represent an increase of investment without corresponding increase of working capacity, and also an additional loss in operation, it is desirable to avoid their use as far as is practicable. In short transmissions over distances of less than fifteen miles it is generally better to avoid the use of transformers at generating stations, and in some of these cases, where the transmission is only two or three miles, it is even more economical to omit transformers at the sub-stations.
Thus, where energy is to be transmitted two miles and then applied to large motors in a factory, or distributed at 2,500 volts, the cost of bare copper conductors for the three-phase transmission line will be only about $6 per kilowatt of line capacity at 2,500 volts, with copper at 15 cents per pound, and a loss of 5 per cent at full load. The average loss in such a line will probably be as small as that in one set of transformers and a line of higher voltage. Furthermore, the first cost of the 2,500-volt generators and line without transformers will be less than that of generators and line of higher voltage with step-down transformers at the sub-station.
As generators up to 13,500 volts are now regularly manufactured, it is quite common to omit step-up transformers at the main stations of rather short transmission systems. This practice was followed in the 13,500-volt transmission to Manchester, N. H., the 10,000-volt transmission to Lewiston, Me., and the 12,000-volt transmission to Salem, N. C.
In most transmission over distances of twenty-five miles or more, step-up transformers at generating stations as well as step-down transformers at sub-stations are employed. As yet the highest voltages that have been put into practical use on transmission lines (that is, 50,000 to 60,000) are much below the pressures that have been yielded by transformers in experimental work. These latter voltages have in a number of instances gone above 100,000. The numbers and capacities of transformers used at main stations vary much in their relation to the numbers and individual capacities of generators there. In some cases there are three times as many transformers as three-phase generators, and the capacity of each transformer is either equal to or somewhat greater than one-third of the capacity of each generator.
Fig. 53.—Transformers at Central Sub-station, Montreal.