TABLE OF CONTENTS Differential protection functions when the vector difference of two or more similar electrical magnitudes exceeds a predetermined value. Almost any type of relay can function as differential protection - it is not so much the construction of the relay that is important but rather its method of connection in the circuit. The majority of the applications of differential relays are of the current-differential type, but they can also be of the voltage-differential type, operating on the same principle as the current relays; the difference lies in the fact that the operating signal is derived from a voltage across a shunt resistance.
A simple example of a differential arrangement is shown in Figure 8.1. The secondaries of the current transformers (CTs) are interconnected and the coil of an overcurrent relay is connected across these. Although the currents I 1 and I 2 may be different, provided that both sets of CTs have appropriate ratios and connections then, under normal load conditions or when there is a fault outside the protection zone of the element, the secondary currents will circulate between the two CTs and will not flow through the overcurrent relay. However, if a fault occurs in the section between the two CTs the fault current would flow towards the short-circuit from both sides and the sum of the secondary currents would flow through the differential relay. In all cases the current in the differential relay would be proportional to the vector difference between the currents that...
