Answer / venki

Distance protection relay is mostly used to detect the fault which occurred at a specified distance normally these are used in 66 kV and above Transmission levels. the total line is divided in to some zone for a specified distance and those zones are named as A,B,C and etc., if a fault is occurred it will indicate the zone where it is occurred.

Answer / singa

distance protection relays are used in transmission lines
in order to have the proper selectivity.
If a simple directional or non directional overcurrent
relays are used, then for a fault at remote SS, will cause
tripping if grading is not proper.
In addition, overcurrent relays settings are deponds on
SIR. Whereas distance protection relays are not deponds on
SIR but on the line impedence value.

Exact selectivity can be done by providing the different
zones viz, Zone-1,2,3,4

zone-1 will be instantaneous trip covers 80% of the line
where as zone-2 / 3 will covers some protion of adjacent
lines and trip after the time delay of say 200mS and 400mS
respectively.

Simply saying distance relays are under impedence relays
with directional feature.

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