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Protection Relays Settings Chapter | 19 285

Subchapter 19.2

Feeder Backup Overcurrent Protections

19.2.1 INTRODUCTION

Overcurrent protection here is used as backup protection, with a considerable
time delay, which means it works as a second line of defense to protect the
line if the main or primary protection fails to isolate the line for the fault
instantaneously.

19.2.2 WORKED EXAMPLE
Fig. 19.2.1 gives an example of line backup overcurrent protection.
Line AB 500 kV maximum load current 5 I L Max 5 1200 A
For OC relay at A (R):
I set 5 1.2 3 I L Max
5 1.2 3 1200 5 1.44 A
1000
PSM calculation:
I F MVA F
PSM 5 andI F 5
I S O3 3 V
I F 5 80 KA
I S 5 1200 A
80; 000
PSM 5 5 80
1000
Using a standard inverse curve of the OC relay we then have:

0:14 0:14
T R 5 5 5 1:528
PSM 0:0221 80 0:0221
where T R 5 relay operation time assuming that the relay will operates on
1 second for this fault then t.operation 5 TM.T R , where TM 5 time
multiplier.

FIGURE 19.2.1 Line backup overcurrent protection.