Automatic Voltage Control

the process of maintaining voltage at the junction points of an electrical system within given limits to ensure technically feasible conditions of operation for users of electrical energy and of separate systems and to increase the economic efficiency of their operation.

Most users of electrical energy may tolerate long-term voltage deviations from the nominal value of not more than ±5 percent. An increase of the voltage above its nominal value leads to a shorter service life for electrical equipment, and a decrease lowers the productivity and economically efficient operation of the machinery and the capacity of the transmission lines; it may also interfere with the stable operation of synchronous machines and induction motors.

Automatic voltage control is needed because of variable operating conditions at the user’s plant and at electrical energy sources. Because an increase in load leads to an increase in current strength and consequently also to an increase in voltage losses in various sectors of the grid, the voltage at the machine may be out of permissible limits. Therefore, the buses of electric power stations and the secondary voltage buses of regional substations are usually regulated by reverse (balancing) means. In this case, the voltage is held above its nominal value with increased loads and lowered with decreased loads. This lessens the variation of voltage fluctuation for the user. However, such regulation does not generally exclude the need for automatic voltage control for each user.

Automatic voltage control at electric power stations is obtained by regulating the excitation of the synchronous generators. In substations automatic voltage control is obtained by regulation of the excitation of synchronous compensators, if such have been installed in these substations, by automatic change under load of the ratio of transformers, and by control of the power of static capacitor banks. At the users of electrical energy, automatic voltage regulation is applied by means of control of high-power synchronous motors and of the power of static capacitor banks. The problem of specific selection of regulating devices is usually solved on the basis of technical and economic analysis.