TABLE OF CONTENTS Based on the relay operating characteristics, overcurrent relays can be classified into three groups: definite current or instantaneous, definite time, and inverse time. The characteristic curves of these three types are shown in Figure 5.1, which also illustrates the combination of an instantaneous relay with one having an inverse time characteristic.
This type of relay operates instantaneously when the current reaches a predetermined value. The setting is chosen so that, at the substation furthest away from the source, the relay will operate for a low current value and the relay operating currents are progressively increased at each substation, moving towards the source. Thus, the relay with the lower setting operates first and disconnects load at the point nearest to the fault. This type of protection has the drawback of having little selectivity at high values of short-circuit current. Another disadvantage is the difficulty of distinguishing between the fault current at one point or another when the impedance between these points is small in comparison to the impedance back to the source, leading to the possibility of poor discrimination.
Figure 5.2a illustrates the effect of the source impedance on the short-circuit level at a substation, and for a fault at point B down the line. From Figure 5.2b it can be appreciated that the fault currents at F 1 and F 2 are almost the same, and it is this that makes it...
