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 ,78) in the line protection IED.
IEC06000314 V1 EN
Figure 233: Undamped oscillations causing pole slip
Section 8 1MRK 506 369-UUS -
Impedance protection
454 Line distance protection REL670 2.2 ANSI
Application manual
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