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values are in the case of a 5 A rating.
(*2) DEF: Instant or Definite time, DEP: Inverse time
Sensitive setting of DNOC1 and DNOC2 thresholds is restricted by the negative phase sequence
current normally present on the system. The negative phase sequence current is measured in the
relay continuously and displayed on the metering screen of the relay front panel along with the
maximum value. It is recommended to check the display at the commissioning stage and to set
DNOC1 and DNOC2 to 130 to 150% of the maximum value displayed.
The delay time setting TDNC1 and TDNC2 is added to the inherent delay of the measuring
elements DNOC1 and DNOC2. The minimum operating time of the DNOC elements is around
200ms.
2.1.2.6 Blocked Busbar Protection
GRD150 can be applied to provide busbar zone schemes. For a simple radial system where a
substation has only one power source, then non-directional overcurrent relays are suitable. In the
case of more complex systems where the substation has multiple incomers, or where power can
flow into the substation from the feeders, then the directional protection must be applied. Figure
2.1.49 shows one half of a two-incomer station. A directional overcurrent relay protects the
incomer, with non-directional overcurrent units on the feeders.
Feeder Trip
Non-directional
I> (IDMTL)
I> (Inst)
For a fault on an outgoing feeder, the non-directional feeder protection sends a hardwired blocking
signal to inhibit operation of both incomers, the signal being generated by the instantaneous phase
fault and earth fault alarm outputs. Meanwhile, the feeder is tripped by the OC1 and EF1 elements,
programmed with inverse time delays and set to grade with downstream protections.
The incomer protection is programmed for directional operation such that it will only trip for
faults on the busbar side of its CTs. Hence, although a fault on the HV side may be back-fed from
the busbars, the relay does not trip.
Directional
Delayed Back-up Trip
I> (IDMTL)
I>> (50ms)
I>>> (250ms)
High Speed Block
Trip Incomer
Feeder Trip
Non-directional
I> (IDMTL)
I> (Inst)
Figure 2.1.49
Blocked Busbar Protection Scheme
⎯
⎯
55
Trip Bus Section and Bus Coupler
Bus Section
Bus Coupler
6 F 2 S 0 8 4 2
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