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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 either direction. In such a case, directional control
should be added to overcurrent elements.
GRD140 provides directional control for phase fault and earth fault overcurrent elements OC1 to
OC4, EF1 to EF4, SEF1 to SEF4, NOC1 and NOC2 which can be enabled or disabled by scheme
switch setting. The directional characteristic can be selected to "Forward" or "Reverse" or "Non"
by scheme switch setting [∗∗∗-DIR]. The OC1, EF1 and SEF1 elements have selective inverse
time and definite time characteristics.
2.1.2.1 Application of Directional Overcurrent Protection
Parallel Feeder Circuits
If non-directional protection were applied to the circuit shown in Figure 2.1.9, 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 OC1 and EF1, with a directional
characteristic looking in the direction shown by the arrows.
The various overcurrent elements of GRD140 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 2.1.9
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 2.1.10 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
protections are directional relays. It can be seen that a fault at F is cleared by tripping at A in 1.0s
and at B in 0.4s.
Alternatively, since GRD140 provides multiple, independent bi-directional overcurrent stages, a
scheme could be implemented in which a single relay can perform the necessary protection
functions in both directions at each load connection point. Each GRD140 overcurrent element can
be programmed with different settings for forward and reverse direction, thus allowing correct
grading to be achieved in both the clockwise and anti-clockwise directions.
F
GRD140
Non-directional
GRD140
Non-directional
Application of GRD140 to Parallel Feeders
⎯
⎯
17
A
Load
GRD140
Directional
B
GRD140
Directional
6 F 2 S 0 7 5 8
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