IEC06000314 V1 EN-US
Figure 231: 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.
1MRK 506 369-UEN B Section 8
Impedance protection
Line distance protection REL670 2.2 IEC 425
Application manual
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