Page 240 - Practical Power System and Protective Relays Commissioning

238 Practical Power System and Protective Relays Commissioning

FIGURE 18.7.2 Overall differential scheme for generator units.

18.7.3 STATOR EARTH FAULT PROTECTION (100%)
This scheme is applied for large machines only.
A low-frequency injection method is shown in Fig. 18.7.4:
Complete protection for stator windings;
High cost of injection equipment is a disadvantage.

Third harmonic neutral voltage scheme as shown in Fig. 18.7.5:
27 Third harmonic undervoltage relay;
59P terminal voltage check permits for trip if circuit breaker is open but
a terminal voltage is present;
Neutral overvoltage protection operates at a fundamental frequency
voltage of 50 Hz.
The trip logic is shown in Fig. 18.7.6.
As shown in Fig. 18.7.7, the overlapping of the operation of fundamental
frequency overvoltage relay-59 and the third harmonic undervoltage relay-27
will cover by 100% the protection for the stator.