Modern protection of distribution systems

Abstract

Overcurrent relays are the most common form of protection used to operate only under fault conditions. They should not be installed purely as a means of protecting systems against overloads. The relay settings that are selected are often a compromise in order to cope with both overload and overcurrent conditions. Overcurrent relays can be classified as definite current, definite time, and inverse time as shown in Figure 9.1(a-c). The time delay units can work in conjunction with the instantaneous units as shown in Figure 9.1(d).