Page 250 - Practical Power System and Protective Relays Commissioning

Protection Relays Chapter | 18 247

FIGURE 18.7.15 Rotor earth fault protection for generators.

18.7.12 FREQUENCY PROTECTION
Over-frequency protection is:
Caused by a sudden loss of load;
Can be considered as a backup protection for governor failure.
Underfrequency protection is caused by excessive overload and can cause
a rise in:
Overfluxing of the stator core;
Plant drives operate at lower speeds affecting generator output;
Mechanical resonant condition in turbines.
This protection initiates load shedding.

18.7.13 VOLTAGE PROTECTION
Overvoltage protection is a condition of overvoltage which occurs when the
prime mover overspeeds due to a sudden loss of load, and the voltage regula-
tor is defective.
This condition can cause failure of insulation and also causes
overexcitation.