REG 316*4 1MRB520049-Uen / Rev. B ABB Switzerland Ltd
4-31
4.2.5. Stator ground fault protection for generators in parallel
This is a discriminative ground fault scheme for generators with
ungrounded star-points. It covers 80 % of the winding and op-
erates on the basis of a directional zero-sequence component in
the various generator feeders. Since the capacitive component
of a ground fault current does not usually provide a sufficiently
reliable criterion for determining the feeder concerned, the
ground fault current is artificially increased by adding a real
power component. The latter is generated either by three single-
phase v.t’s or a three-phase v.t. Whichever is the case, the sec-
ondaries are connected as a broken delta and a resistor (R
e
) is
switched briefly into the delta after a ground fault has been de-
tected. The combined v.t. and grounding transformer are con-
nected to the continuously energised busbars. The number of
generator feeders can vary.
The protection scheme comprises two parts:
The first part is a non-discriminative ground fault detector on
each busbar comprising a:
· grounding transformer
· ferroresonance damping resistor R
p
· switched grounding resistor R
e
· zero-sequence voltage detector for switching in the resistor
· contactor
· interposing v.t.
The second part comprises the power function and either a core-
balance or three bushing c.t’s to measure the zero-sequence
current and discriminatively locate the ground fault after the real
power component has been added.
4.2.5.1. Principle of operation
Initially a ground fault is detected non-discriminately due to the
occurrence of a neutral voltage measured by a sensitively set
voltage function. The grounding resistor R
e
is switched in circuit
after a short delay (t1 = 0.1 s) to prevent operation during power
system transients. Only then is the ground fault current large
enough to enable the power functions on the generators to de-
cide whether the ground fault is on their generator feeder or not.
The delay for the power function is set to isolate the fault after
0.5 s. The grounding resistor R
e
is connected for 1.9 s. The re-
sistor R
e
is switched out of circuit again after a delay of 2 s initi-
ated by the voltage function.
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