ABB RELION 670 Series Applications Manual page 111

1MRK 511 364-UUS A
1.1
1.05
1
0.95
0
IEC10000108 V2 EN-US
Figure 48: 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:
1. Remain stable for normal steady state load.
2. Distinguish between stable and unstable rotor swings.
3. Locate electrical centre of a swing.
4. Detect the first and the subsequent pole-slips.
5. Prevent stress on the circuit breaker.
6. Distinguish between generator and motor out-of-step conditions.
7. Provide information for post-disturbance analysis.
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.
← 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
105