ABB Relion 670 Series Applications Manual page 119

1MRK 511 407-UUS Rev. J
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 52.
1.1
1.05
1
0.95
0
IEC10000108 V2 EN-US
Figure 52: Stable and unstable case. For the fault clearing time tcl = 200 ms, the generator
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.
Measurement of the magnitude, direction and rate-of-change of load impedance relative to a
generator's terminals provides a convenient and generally reliable means of detecting whether
pole-slipping is taking place. The out-of-step protection should protect a generator or motor (or
two weakly connected power systems) against pole-slipping with severe consequences for the
machines and stability of the power system. In particular it should:
Phasor measurement unit RES670
Application manual
unstable
stable
3-ph
fault
260 ms
← For 260 ms long 3-phase fault generator
200
1-st
ms
pole-slip
1 corresponds
← For fault clearing time 200 ms generator remains
to 50 or 60 Hz
stable and in synchronism. After oscillations around
the nominal speed, the rotational speed returns to
the nominal, corresponding to 50 or 60 Hz
500 1000
time in milliseconds →
remains in synchronism, for tcl = 260 ms, the generator loses step.
© Copyright 2017 Hitachi Power Grids. All rights reserved
← 3-rd pole-slip
← 2-nd pole-slip
loses synchronism. Generator operates in
asynchronous mode at speeds > nominal
← damped
oscillations
1500 2000 2500
IEC10000108-2-en.vsd
Impedance protection
3000
Section 7
113