Part 1: Transient Over-reach

Transient over-reach and under-reach may occur in a distance relay protecting a two (or multi) end transmission lines. For example, in the following network a two end transmission line is supplied from sources EA and EB.  A fault occurs in %k*100 of the line having resistance Rf.

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To find what is calculated in a distance protection, we should find the corresponding impedance value. Therefore, we have:

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It can be easily found that the fault resistance affects the calculated impedance of the relay and it considerably depends to local and remote infeed currents. The fault resistance seen by the distance protection can be increased compared to its real value.

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The apparent fault resistance can also get a phase shift, depending on the load conditions before the fault. There is a risk that zone 1 will trip for faults outside its border. For example, in the following figure, the relay will trip for a fault more than its predefined reach. In other words, the operation reach of the relay are virtually increased. This is why, we say distance protection relay is over-reached.

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To prevent relay mal-operation, Zone 1 in the numeric relays is tilted accordingly.

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At the end, we should notice that the transient overreach may occurred when we have the following conditions at the same time:

1) Transmission line is supplied from both local and remote ends

2) The fault has a considerable resistance value

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