distance schemes

[powercam]

hi people

can a single distance relay be used to provide permissive under-reach or over-reach protection? from what i understand there must be 2 same relays across either end of the line to use POTT or PUTT.
we have a 7sa512 and a REL521 on one end and then ONLY a REL 521 on the remote end, but the specification says that we must have POTT configured on both the relays. is this correct? can it be done?
thanks in advance

 

[Bung]

You will need a relay at each end for each scheme, if they are to be independent. They don't necessarily have to be the same, but usually they have built-in logic which makes it much easier if they are the same. You could use one relay at one end, and two at the other, but then you lose the advantages of duplicate schemes. Depending on how liberal you want to be with definitions of POTT etc, and the nature of the network, you don't even have to use distance relays - directional overcurrent will do.

Bung
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