ABB Relion 670 series, Technical Manual

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two equivalent generators connected to each other via an equivalent transmission line and the
phase difference between the equivalent generators is 180°.
Centre of Pole Slip
en07000003.vsd
Angle = 90° Angle = -90°
IEC07000003 V1 EN-US
Figure 194: The centre of pole slip
The centre of the pole slip can occur in the generator itself or somewhere in the power system.
When a pole slip occurs within the generator it is essential to trip the generator. If the centre of
pole slip occurs outside any generator the power system should be split into two different parts
that could have the ability to get stable operating conditions.
Pole slip protection (PSPPPAM ,78) function in the IED can be used both for generator protection
application as well as, line protection applications.
SEMOD143246-4 v4
The situation with pole slip of a generator can be caused by different reasons.
A short circuit may occur in the external power grid, close to the generator. If the fault clearing
time is too long, the generator will accelerate so much, that the synchronism cannot be
maintained.
Undamped oscillations occur in the power system, where generator groups at different locations,
oscillate against each other. If the connection between the generators is too weak the magnitude
of the oscillations will increase until the angular stability is lost.
The operation of a generator having pole slip will give risk of damages to the generator, shaft and
turbine.
• At each pole slip there will be significant torque impact on the generator-turbine shaft.
• In asynchronous operation there will be 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 stator/rotor iron.
The Pole slip protection (PSPPPAM ,78) function shall detect pole slip conditions and trip the
generator as fast as possible if the locus of the measured impedance is inside the generator-
transformer block. If the centre of pole slip is outside in the power grid, the first action should be
to split the network into two parts, after line protection action. If this fails there should be
operation of the generator PSPPPAM (78) in zone 2, to prevent further damages to the generator,
shaft and turbine.
Section 7 1MRK 502 066-UUS B
Impedance protection
368
Technical manual