Generator Protection Part 3
A basic differential protection scheme used for the generators is shown in the Fig. 1. It is known that the differential relay operates when the phasor difference of two or more similar electrical quantities exceeds a predetermined value.
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| Fig. 1 |
Suppose the current I flows through the primary of C.T.s to the external fault. If the two C.T.s have same ratio, then no current will flow through the relay and it remains inoperative.
But now if an internal fault occurs at point X as shown in the Fig. 2, the current flows through the fault from both the sides. The primary currents are I1 and I2 while the secondary currents are i1 and i2. so the current flowing through the relay will be i1 + i2. Even some current flowing out of one side while a large current entering the other side will cause the differential current. Such a current is responsible to operate the relay.
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| Fig. 2 |
Thus the relay current is proportional to the phasor difference between the currents entering and leaving the protected circuit and if the differential current exceeds the pickup value, the relay operates.
This basic differential scheme has following disadvantages :
1. This C.T.s may saturates inaccurately with heavy external faults.
2. The C.T.s may saturate and cause unequal secondary currents and the difference of secondary currents may approach the pickup value to operate the relay unnecessarily.
These disadvantages are overcome in the percentage differential relay.
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