REG 316*4 1MRB520049-Uen / Rev. B ABB Switzerland Ltd
4-39
4.2.6. 100% stator and rotor ground fault protection
Stator ground fault protection
The ground fault protection of the entire stator winding comprises
a 95 % scheme and a 100 % scheme (see Fig. 4.18). The zones
of the two schemes overlap in the stator windings. Ground faults
in the region of the generator terminals are detected primarily by
the 95 % stator ground fault scheme. Ground faults near the
star-point, on the other hand, can only be detected by the 100 %
stator ground fault scheme.
The functions required for the two schemes are
· a “Voltage” function for the 95 % stator ground fault protection
· the “Stator-EFP” for the star-point zone protection.
The 95 % scheme uses the generator voltage and detects a
ground fault on the basis of the displacement of the star-point
voltage it causes.
The 100 % star-point scheme injects a voltage to permanently
bias the star-point. The injection voltage has an impulse wave-
form with an amplitude of about 100 V and a frequency of 12.5 or
15 Hz. It is provided by an injection unit Type REX 010 and an
injection transformer unit REX 011. The scheme measures the
ground fault leakage resistance.
The sensitivities of the two schemes can be set in the case of the
· 95 % scheme by the pick-up voltage (typically 5 V)
· 100 % star-point scheme by settings for the ground fault re-
sistance (typically 5 k
W for alarm and 500 W for tripping).
The zone of the 100 % scheme depends on the maximum zero-
sequence current at fundamental frequency flowing at the star-
point. This occurs for a fault at the generator terminals. The low-
frequency injection voltage is switched off when the zero-
sequence current component at power system frequency ex-
ceeds 5 A. For a current of I
E max
= 20 A, the pick-up current of
5 A is reached for a ground fault at 25 % of the winding from the
star-point. It is of advantage to limit the ground fault current to
I
E max
£ 5 A so that the zone of the 100 % scheme extends over
the whole stator winding.
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