Page 245 - Practical Power System and Protective Relays Commissioning

242 Practical Power System and Protective Relays Commissioning

18.7.5 OVERCURRENT PROTECTION
This protection is the only protection for small units. It provides protection
for the generator and power system, therefore a time delay is required for
coordination.
Refer to Fig. 18.7.9 for the OC protection.

FIGURE 18.7.9 OC protection for generators.

The variable response of a generator for change in excitation and voltage
regulation led to use of overcurrent relays with different characteristics based
on the generator terminal voltage value.
There are two types of OC protection applied to generators:
1. Voltage-controlled overcurrent protection
During normal overloads, system voltage is near normal and the relay
operates on long inverse time characteristics but under close-up fault condi-
tions the busbar voltage falls and the voltage relay will change the relay
characteristics to standard IDMT characteristics. Refer to Fig. 18.7.10 for
voltage control overcurrent relay.
2. Voltage-restrained overcurrent protection
In this type of relay, the relay characteristic is modified continuously
based on the generator terminal voltage. Refer to Fig. 18.7.11 for voltage-
restrained overcurrent protection.

18.7.6 BACKUP DISTANCE PROTECTION
This impedance relay is used to provide backup protection for faults on a
power system which is not cleared by the power system protection. This
protection operates to trip generators after a time delay, as shown in
Fig. 18.7.12.