ABB REG670 Applications Manual page 269

1MRK 502 071-UEN -
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
IEC06000313 V1 EN
Figure 134: 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
amplitude increases. When the critical fault clearance time is reached the stability
cannot be maintained.
Un-damped oscillations occur in the power system, where generator groups at
different locations, oscillate against each other. If the connection between the
generators is too weak the amplitude of the oscillations will increase until the angular
stability is lost. At the moment of pole slip there will be a centre of this pole slip, which
is equivalent with distance protection impedance measurement of a three-phase. If this
point is situated in the generator itself, the generator should be tripped as fast as
possible. If the locus of the out of step centre is located in the power system outside the
generators the power system should, if possible, be split into two parts, and the
generators should be kept in service. This split can be made at predefined locations
(trip of predefined lines) after function from pole slip protection (PSPPPAM) in the
line protection IED.
Section 8
Impedance protection
en06000313.vsd
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