ABB Relion 670 series, Applications Manual

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en06000314.vsd
IEC06000314 V1 EN-US
Figure 247: Undamped oscillations causing pole slip
The relative angle of the generator is shown a contingency in the power system, causing un-
damped oscillations. After a few periods of the oscillation the swing amplitude gets to large
and the stability cannot be maintained.
If the excitation of the generator gets too low there is a risk that the generator cannot
maintain synchronous operation. The generator will slip out of phase and operate as an
induction machine. Normally the under-excitation protection will detect this state and trip the
generator before the pole slip. For this fault the under-excitation protection and PSPPPAM
function will give mutual redundancy.
The operation of a generator having pole slip will give risk of damages to the generator block.
• 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.
• At asynchronous operation the generator will absorb a significant amount of reactive
power, thus risking overload of the windings.
PSPPPAM function shall detect out of step conditions and trip the generator as fast as
possible if the locus of the pole slip is inside the generator. If the centre of pole slip is outside
the generator, situated out 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 pole slip protection, to prevent further damages to the generator block.
Section 7 1MRK 505 343-UEN B
Impedance protection
380
Application manual