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Blocked Busbar Protection
Non-directional overcurrent protection can be applied to provide a busbar zone scheme for a
simple radial system where a substation has only one source, as illustrated in Figure 2.1.27.
For a fault on an outgoing feeder F1, the feeder protection sends a hardwired blocking signal to
inhibit operation of the incomer, the signal OCHS, EFHS and SEFHS being generated by the
instantaneous phase fault, and earth fault pick-up outputs of OC1, EF1 and SEF1 allocated to any
of the binary output relays. Meanwhile, the feeder is tripped by the OC1, EF1 and SEF1 elements,
programmed with inverse time or definite time delays and set to grade with downstream
protections.
The incomer protection is programmed to trip via its instantaneous elements OC2, EF2 and SEF2
set with short definite time delay settings (minimum 50ms), thus providing rapid isolation for
faults in the busbar zone F2.
At the incomer, inverse time graded operation with elements OC1I, EF1I and SEF1I are available
with the scheme switch [MOC1], [MEF1] and [MSE1] setting, thus providing back-up protection
in the event of failure of the blocked scheme.
GRD150 integrated circuit breaker failure protection can be used to provide additional back-trips
from the feeder protection to the incomer, and from the incomer to the HV side of the power
transformer, in the event of the first trip failing to clear the earth fault.
In the case of more complex systems where the substation has two incomers, or where power can
flow into the substation from the feeders, then directional protection must be applied (refer to
Section 2.1.2 Directional Overcurrent Protection).
Feeder Trip
OC2/EF2/SEF2
OCHS/EFHS/
SEFHS
F
1
GRD150
Delayed Back-up Trip
OC1I/EF1I/SEF1I
OC2/ EF2/SEF2
Fast Trip
F
2
Feeder Trip
GRD150
OC2/EF2/SEF2
OCHS/EFHS/
SEFHS
Figure 2.1.27
Blocked Busbar Protection
⎯
⎯
37
High Speed Block to Incomer for Feeder Fault
Feeder Trip
GRD150
OC2/EF2/SEF2
OCHS/EFHS/
SEFHS
6 F 2 S 0 8 4 2
GRD150
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