Page 249 - Practical Power System and Protective Relays Commissioning

246 Practical Power System and Protective Relays Commissioning

FIGURE 18.7.14 Out-of-step relay characteristic for generators.

18.7.10 NEGATIVE-PHASE SEQUENCE CURRENT I 2
PROTECTION

Unbalanced loading of generators which generates negative-sequence cur-
rents in the stator which cause a controlled-rotating magnetic field which
cuts the rotor at twice the synchronous speed inducing double-frequency cur-
rent in the field system and rotor body, which leads to eddy currents causing
overheating, there are two ratings for I 2 for machines as follows:
I 2 R 5 continuous Negative Phase Sequence (NPS) rating;
2
I 2 t 5 short time NPS rating.

18.7.11 ROTOR EARTH FAULT PROTECTION

A single fault will not cause any problem but a second fault in the field
winding of the rotor could cause serious damage including:
Heating-conductor burning;
Flux distortion in the field leading to generator vibrations.
A DC injection method is used by injecting a DC voltage into field wind-
ings to detect the earth faults, as shown in Fig. 18.7.15.