Page 327 - Practical Power System and Protective Relays Commissioning

Protection Relays Settings Chapter | 19 321

5. Phase unbalance;
6. Out-of-step operation for synchronous motors;
7. Loss of excitation for synchronous motors.

19.8.2.2 Phase Fault Protection
A high set Over Current (OC) unit is used.
Differential protection is also used as it is not affected by the motor start-
ing current.
For the H set OC relay the setting should be more than the motor starting
current:
I set . I start .

19.8.2.3 Earth Fault Protection
An inverse or very inverse OC relay is connected to the residual CT circuit
(as shown in Fig. 19.8.1).

FIGURE 19.8.1 Earth fault protection for motors.

I set 5 0.25 I F min , where I F min 5 minimum fault current.

19.8.2.4 Locked Rotor Protection
We use the OC relay with characteristics higher than the starting current
of the motor and less than the locked rotor current (as shown in
Fig. 19.8.2), where t s 5 motor starting time; t R 5 locked rotor time.
Also, the OC (51) relay characteristic is used to detect the locked rotor
condition of the motor or impedance relay is used to detect this condition, as
shown below in Fig. 19.8.3.