ABB Relion 670 series, Applications Manual

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0 500 1000 1500 2000 2500 3000
0.95
1
1.05
1.1
time in milliseconds →
G
enera
tor
ro
ta
tio
na
l s
pee
d
in
per
u
ni
t
→
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 generator
loses synchronism. Generator operates in
asynchronous mode at speeds > nominal
IEC10000108-2-en.vsd
IEC10000108 V2 EN-US
Figure 256: 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-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.
7.17.3 Setting guidelines
GUID-CB86FCF6-8718-40BE-BDF2-028C24AB367D v6
The setting example for generator protection application shows how to calculate the most
important settings
ForwardR, ForwardX, ReverseR, and ReverseX.
1MRK 505 343-UEN B Section 7
Impedance protection
391
Application manual