ABB REG670 Applications Manual page 283

1MRK 502 071-UEN -
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
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 144.
1.1
3-ph
fault
260 ms
1.05
200
1-st
ms
pole-slip
1
1 corresponds
to 50 or 60 Hz
0.95
0 500
IEC10000108 V2 EN
Figure 144: 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.
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-
unstable
stable
← 3-rd pole-slip
← 2-nd pole-slip
← For 260 ms long 3-phase fault generator
loses synchronism. Generator operates in
asynchronous mode at speeds > nominal
← damped
oscillations
← 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
1000 1500 2000
time in milliseconds →
Section 8
Impedance protection
2500 3000
IEC10000108-2-en.vsd
277