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 (78) 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 (78) 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.
8.16.3 Setting guidelines
GlobalBaseSel: Selects the global base value group used by the function to define IBase,
VBase and SBase as applicable.
Operation: With the parameter Operation the function can be set Enabled or DIsabled.
MeasureMode: The voltage and current used for the impedance measurement is set by the
parameter MeasureMode. The setting possibilities are: PosSeq, AB, BC, or CA. If all phase
voltages and phase currents are fed to the IED the PosSeq alternative is recommended
(default).
Further settings can be illustrated in figure
234.
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 455
Application manual
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