The out-of-step condition of a generator can be caused by different reasons. Sudden events
in an electrical power system such as large changes in load, fault occurrence or slow fault
clearance, can cause power oscillations, that are called power swings. 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
242.
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
Figure 242: 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.
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 469
Application manual
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