ABB RELION RET670, Applications Manual

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8.16.2 Application
SEMOD143248-4 v3
Normally, the generator operates synchronously with the power system, that is, all the
generators in the system have the same angular velocity and approximately the same
phase angle difference. If the phase angle between the generators gets too large the
stable operation of the system cannot be maintained. In such a case the generator loses
the synchronism (pole slip) to the external power system.
The situation with pole slip of a generator can be caused by different reasons.
A short circuit occurs in the external power grid, close to the generator. If the fault
clearance time is too long, the generator will accelerate so much, so the synchronism
cannot be maintained. The relative generator phase angle at a fault and pole slip,
relative to the external power system, is shown in figure
234 .
en06000313.vsd
IEC06000313 V1 EN-US
Figure 234: Relative generator phase angle at a fault and pole slip relative to the
external power system
The relative angle of the generator is shown for different fault duration at a three-phase
short circuit close to the generator. As the fault duration increases the angle swing
1MRK 504 163-UUS A
Section 8
Impedance protection
Transformer protection RET670 2.2 ANSI 467
Application manual