Ques. What form of single phase system should be used where the transmission distance is considerable?
Ans. The current should be transmitted at high pressure, a step down transformer being placed at each distribution center to reduce the pressure to the proper voltage to suit the service requirements as shown in fig. 2,146.
Thus, if 110 volt lamps be used on the three wire circuit, the pressure between the two outer wires would be 220 volts. A transformation ratio of say 10:1 would give 2,220 volts for the primary circuit. The current required for the primary with this ratio being only .1 that used in the secondary, a considerable saving is effected in the cost of the transmission line as must be evident.
With the high pressure alternator only one transformation of the current is needed, as shown at the distribution end.
In place of the high pressure alternator, a low pressure alternator could be used in connection with a step up transformer as shown in fig. 2,147, but there would be an extra loss due to the additional transformer, rendering the system less efficient than the one shown in fig. 2,146. Such an arrangement as shown in the fig. 2,147 might be justified in the case of a station having a low pressure alternator already in use and it should be desired to transmit a portion of the energy a considerable distance.
Ques. How could the system shown in fig. 2,147 be made more efficient than that of fig. 2,146?
Ans. By using a high pressure alternator in order to considerably increase the transmission voltage.
Thus, a 2,200 volt alternator and 1:10 step up transformer would give a line pressure of 22,000 volts, which at the distribution end could be reduced, to 220 volts for the three wire circuit, using a 100:1 step down transformation.
Fig. 2,147.—Diagram illustrating how electricity can be economically transmitted a considerable distance with low pressure alternator already in use.