Protection Engineering Basics and Schemes Protection Engineering | Page 125

The contact of directional and overcurrent relay are in
series, at time of fault-forward (when current at
secondary circuit of CT is exceed from the
Pick up value (PSM)). The breaker trip coil energizes and
got trip.
If the relay construction is electromagnetic
In it there are two coils on stator, one is current coil and
voltage coil and there is rotating disc, Our reference here
is Voltage And current is
and like watt hour
meter, the torque in rotating disc is;
When fault is at point P1 (figure 1), the current is lagging
voltage by 90 degree and torque is maximum, means disc
rotate with maximum torque and
Energized relay tripped circuit breaker.
T=KVA cos (90-90) = KVA cos 0= KVA
pg. 125