REG 316*4 1MRB520049-Uen / Rev. B ABB Switzerland Ltd
4-33
4.2.5.2. Busbar ground faults
Should no ground fault be located on one of the generator feed-
ers, it has to be on the busbars or possibly an outgoing feeder.
In such a case, the voltage function operates and the alarm
“Busbar ground fault” is generated after the set delay of 2 s.
4.2.5.3. Ground fault protection during start-up
The power function can only detect a ground fault on the gen-
erator feeder when the circuit-breaker is closed. For the time that
the circuit-breaker is open, ground fault protection is afforded by
a sensitive voltage function which trips the excitation switch in
the event of a stator ground fault after a delay of 0.5 s. This
ground fault protection scheme is blocked once the circuit-
breaker closes.
4.2.5.4. Grounding transformer
The following arrangements can be used to increase the ground
fault current:
· 3 single-phase v.t’s with a maximum rating of 100 kVA for
10 s. This arrangement can be used at 6.66 A up to a rated
generator voltage of 12 kV, at 5 A up to 16 kV and at 4 A up
to 20 kV.
· 3 single-phase dry v.t’s can conduct 6.66 A at voltages higher
than 12 kV.
Apart from a higher overload rating, a 3 single-phase grounding
transformer also has the advantage of a negligible voltage drop.
Example for generators with a rated voltage of 10.5 kV and a
ground fault current of 20 A (6.66 A per phase):
Voltage transformers Short-time
sec. rating
U
1N
[V] U
2N
[V]
S [kVA] [A] 10 s
10500
3
167 70 240
4.2.5.5. Ferroresonance damping resistor R
p
For a ground fault in the busbar zone, the ground fault detector
issues an alarm without interrupting operation. The resistor R
p
must therefore be rated for continuous operation. It generally has
a rating of 1 or 2 A, which is permissible for most v.t’s, but has
To Next Page
Loading...
