ABB REG670 Applications Manual page 270

Section 8
Impedance protection
264
IEC06000314 V1 EN
Figure 135: 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.
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
1MRK 502 071-UEN -
en06000314.vsd
Application manual