In a non-recoverable situation, the power swings become so severe that the
synchronism is lost: this condition is called pole slipping.
Undamped oscillations occur in power systems, where generator groups at different
locations are not strongly electrically connected and can oscillate against each
other. If the connection between the generators is too weak the magnitude of the
oscillations may increase until the angular stability is lost. More often, a three-
phase short circuit (unsymmetrical faults are much less dangerous in this respect)
may occur in the external power grid, electrically close to the generator. If the fault
clearing time is too long, the generator accelerates so much, that the synchronism
cannot be maintained even if the power system is restored to the pre-fault
configuration, see Figure
240.
0 500 1000 1500 2000 2500 3000
0.95
1
1.05
1.1
time in milliseconds
→
Generator rotational speed in per uni
→
unstable
stable
260 ms
200
ms
3-ph
fault
←
For fault clearing time 200 ms generator remains
stable and in synchronism. After oscillations around
the nominal speed, the rotational speed returns to
the nominal, corresponding to 50 or 60 Hz
← damped
oscillations
3-rd pole-slip
1 corresponds
to 50 or 60 Hz
←
2-nd pole-slip
1-st
pole-slip
←
For 260 ms long 3-phase fault generato
loses synchronism. Generator operates in
asynchronous mode at speeds > nominal
IEC10000108-2-en.vsd
IEC10000108 V2 EN-US
Figure 240: Stable and unstable case. For the fault clearing time tcl = 200 ms,
the generator remains in synchronism, for tcl = 260 ms, the
generator loses step.
A generator out-of-step condition, with successive pole slips, can result in damages
to the generator, shaft and turbine.
• Stator windings are under high stress due to electrodynamic forces.
• The current levels during an out-of-step condition can be higher than those
during a three-phase fault and, therefore, there is significant torque impact on
the generator-turbine shaft.
• In asynchronous operation there is induction of currents in parts of the
generator normally not carrying current, thus resulting in increased heating.
The consequence can be damages on insulation and iron core of both rotor and
stator.
Section 8 1MRK 506 369-UEN B
Impedance protection
438 Line distance protection REL670 2.2 IEC
Application manual
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