ABB REG316 4 Series Operating Instructions Manual page 430

ABB Switzerland Ltd
Two timers ensure correct operation of the scheme:
T1 prevents operation during transients, t1 = 0.1 s
·
T2 prevents "pumping" when the grounding resistor R
·
switched in and out of circuit, t2 = 1 s.
The protection operates with a maximum real power component
in the ground fault current of 12...20 A for a ground fault at the
generator terminals. The neutral voltage is then a maximum. The
ground fault current is proportional to the neutral voltage and is a
maximum for a ground fault at the generator terminals and a
minimum for a ground fault at the star-point.
An offset of the of the neutral of the three phase voltages is
caused:
a) in normal operation by:
· asymmetries of the phase-to-ground
· the presence of a third harmonic component
b) under abnormal operating conditions by
· switching transients
· internal and external ground faults
To avoid any risk of mal-operation, the setting of the ground fault
detector must be higher than any voltage offset which can occur
during normal operation.
Under abnormal conditions, the voltage offset can be increased
by the ferroresonance of the capacitance with the inductance of
the v.t's.
The danger of mal-operation of the ground fault detector due to
switching operations is minimised or even eliminated altogether
by adding the resistor R
this measure depends on how low the resistor R
resistor, however, increases the current, the power of the resistor
and the load on the v.t's or grounding transformer.
4-32
REG 316*4 1MRB520049-Uen / Rev. B
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