The electrical current must be transmitted from the power plant to different points of distribution in an economical manner; that is, with very little loss of electricity, and at the same time in a way that will reduce the danger to life to a minimum. The problem is not serious when the generating plant is in the same or an adjacent building, as in the case of a private plant; but it is a serious problem in a central power plant that supplies electricity over a large area.
The current is usually transmitted, as noted above, through copper wire supported on steel poles or towers, or in underground conduits. The wire used underneath the ground must be insulated, while the wire used overhead may or may not be insulated. Overhead wires should be separated as far as possible so they will not swing together. Over long distances, such as 15 to 20 miles or more, the energy is transmitted as alternating current at from 11,000 to 22,000 volts. If the central station is near the center of distribution, the voltage is about 2200 volts, and is reduced by transformers before it reaches the consumer.
Alternating current is usually generated at a medium voltage and then raised by step-up transformers for transmission purposes. When the current of high voltage reaches the substation, it is reduced by means of step-down transformers. If necessary, the alternating current may be changed over to direct current by means of a rotary converter.
Electrical energy must be furnished to meet the maximum demand during any part of the day, even if this maximum demand continues only for a short time. To avoid the expense and large investment of an equipment big enough to supply such a maximum, storage batteries are utilized to store up current during the slack hours and distribute it during the rush hours of the evening when many lights are burning. In this way the equipment is kept evenly at work throughout the day.