Fig 16 a shows an arrangement of an over current relay connected to operate as a differential relay. A pair of identical current transfonners is fitted on either end of the section to be protected (alternator winding in this case). The secondaries of CT’s are connected in series in such a way that they carry the induced currents in the same direction. The operating coil of over current relay is connected across the CT secondary circuit. This differential relay compares the current at the two ends of the alternator winding.

Under normal operating conditions, suppose the alternator winding carries a normal current of 1000 A. Then the current in the two secondaries of CT’s are equal as in figure. These currents will merely circulate between the two CT’s and no current will flow through the differential relay. Therefore, the relay remains inoperative. If a ground fault occurs on the alternator winding as shown in fig 16 b. the two secondary currents will not be equal and the current flows through the operating coil of the relay, causing the relay to operate. The amount of current flow through the relay will depend upon the way the fault is being fed.

(i) If some current (500 A in this case) flows out of one side while a larger current (2000 A) enters the other side as shown in fig 16 b, then the difference of the CT secondary currents i.e. 10 – 25 = 75 A will flow through the relay.

(ii) If current flows to the fault from both sides as shown in fig 16 c, then sum of CT secondary currents i.e. 1O + 5 = 15 A will flow through the relay

Disadvantages

a) The impedance of the pi1ot cables generally causes a slight difference between

the currents at the two ends of the section to be protected. If the is very sensitive, then the small differential current flowing through the relay may cause it to operate even under no fault conditions.

b) Pilot cable capacitance causes incorrect operation of the relay when a large

current flows

c) Accurate matching of current transformers cannot be achieved due to pilot

circuit impedance

The above disadvantages are overcome to a great extent in biased beam relay.

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