ABB Relion 670 Series Product Manual page 33

Generator protection REG670
Version 2.2
laminated components that are not designed to carry
flux. This will cause eddy currents to flow. These eddy
currents can cause excessive heating and severe
damage to insulation and adjacent parts in a relatively
short time. The function has settable inverse operating
curves and independent alarm stages.
Voltage differential protection VDCPTOV
A voltage differential monitoring function is available. It
compares the voltages from two three phase sets of
voltage transformers and has one sensitive alarm step
and one trip step.
100% Stator earth fault protection, 3rd harmonic
based STEFPHIZ
Stator earth fault is a fault type having relatively high
fault rate. The generator systems normally have high
impedance earthing, that is, earthing via a neutral point
resistor. This resistor is normally dimensioned to give an
earth fault current in the range 3 – 15 A at a solid earth-
fault directly at the generator high voltage terminal. The
relatively small earth fault currents give much less
thermal and mechanical stress on the generator,
compared to the short circuit case, which is between
conductors of two phases. Anyhow, the earth faults in
the generator have to be detected and the generator
has to be tripped, even if longer fault time compared to
internal short circuits, can be allowed.
In normal non-faulted operation of the generating unit
the neutral point voltage is close to zero, and there is no
zero sequence current flow in the generator. When a
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phase-to-earth fault occurs the neutral point voltage will
increase and there will be a current flow through the
neutral point resistor.
To detect an earth fault on the windings of a generating
unit one may use a neutral point overvoltage protection,
a neutral point overcurrent protection, a zero sequence
overvoltage protection or a residual differential
protection. These protections are simple and have
served well during many years. However, at best these
simple schemes protect only 95% of the stator winding.
They leave 5% close to the neutral end unprotected.
Under unfavorable conditions the blind zone may extend
up to 20% from the neutral.
The 95% stator earth fault protection measures the
fundamental frequency voltage component in the
generator star point and it operates when the
fundamental frequency voltage exceeds the preset
value. By applying this principle approximately 95% of
the stator winding can be protected. In order to protect
the last 5% of the stator winding close to the neutral
end the 3rd harmonic voltage measurement can be
performed. In 100% Stator E/F 3rd harmonic protection
either the 3rd harmonic voltage differential principle,
the neutral point 3rd harmonic undervoltage principle or
the terminal side 3rd harmonic overvoltage principle can
be applied. However, differential principle is strongly
recommended. Combination of these two measuring
principles provides coverage for entire stator winding
against earth faults.
1MRK 502 074-BEN M
33