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2.1.2
Directional Overcurrent Protection
In a system including parallel feeder circuits, ring main circuits or sources at both line terminals,
the fault current at the relay location can flow in any direction. In such a case, directional control
elements should be added to overcurrent elements.
GRD150 has directional phase fault and earth fault overcurrent elements DOC1 to DOC4, DEF1
to DEF4, DSEF1 to DSEF4, DNOC1 and DNOC2 which can be enabled or disabled by scheme
switch setting. The directional characteristic can be selected to "Forward" or "Reverse" by scheme
switch setting. The DOC1, DOC2, DEF1, DEF2, DSEF1, DSEF2 and DNOC1 elements have
selective inverse time and definite time characteristics.
Note: DOC1 to DOC4, DEF1 to DEF4, DSEF1 to DSEF4, DNOC1 and DNOC2 elements that have
forward or reverse direction characteristic are discriminated with DOCF1 to DOCF4, DEFF1
to DEFF4, DSEFF1 to DSEFF4, DNOCF1, DNOCF2 or DOCR1 to DOCR4, DEFR1 to
DEFR4, DSEFR1 to DSEFR4, DNOCR1, DNOCR2 respectively.
Further, DOC1, DOC2, DEF1, DEF2, DSEF1, DSEF2 and DNOC1 elements that have inverse
time overcurrent characteristics are discriminated with DOC1I, DOC2I, DEF1I, DEF2I.
DSEF1I, DSEF2I and DNOC1I respectively.
2.1.2.1 Application of Directional Overcurrent Protection
Parallel Feeder Circuits
If non-directional protection were applied to the circuit shown in Figure 2.1.28, then a fault at F
would result in both feeders being tripped at points A and B, and total loss of supply to the load.
Directional relays can be applied to look back into the feeder, thereby ensuring that only the faulty
feeder is disconnected. The relays at A and B would normally be set to operate at 50% of the full
load current of the circuit, via their inverse time elements DOC1 and DEF1, with a directional
characteristic looking in the direction shown by the arrows.
The various overcurrent elements of GRD150 are independently programmable for directional
operation. Therefore, elements OC2 and EF2 could be set for non-directional operation to provide
time-delayed back-up protection for the load.
Figure 2.1.28
Ring Main Circuits
A ring main circuit is commonly protected by directional overcurrent relays, since current may
flow in either direction past the relaying points. The normal grading procedure is applied
separately in both the clockwise and anti-clockwise directions. Conventionally, two directional
relays would be required at each load connection point, one for each direction.
A simple system is illustrated in Figure 2.1.29 showing definite time grading, although inverse
time can also be applied. Non-directional relays are applied at the in-feeds to the ring. All other
F
GRD150
Non-directional
GRD150
Non-directional
Application of GRD150 to Parallel Feeders
⎯
⎯
38
6 F 2 S 0 8 4 2
A
Load
GRD150
Directional
B
GRD150
Directional
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