Page 264 - Practical Power System and Protective Relays Commissioning

Protection Relays Chapter | 18 261

FIGURE 18.10.1 Balanced circulating current system, external fault—relay should not operate.

FIGURE 18.10.2 Balanced circulating current system, internal fault—relay will operate.

There is another differential relay scheme as balanced circulating current
scheme is known as voltage balance differential relay. Refer Figs. 18.10.3
and 18.10.4.
The voltage balance differential scheme is used in feeder protection, but
current balance differential scheme is used for generator, transformer, and
switchgear main protection.
The principal of bias is applied to circulating current scheme to insure
the stability of the differential relay under external (through) fault condition,
then the relay will be driven to operate by the operating quantity and break-
ing by the bias quantity.