Under Voltage Protection In Parallel Connected Switchgear

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The question is how to sense under voltage by an under voltage relay individually when both incomers are connected, because each under voltage relay will read the voltage if there is any one feeder is live, hence there will be a back feed during power outage in any one feeder. the undervoltage will act only when both incomer is in under voltage

Er, so what's the problem?

If there is a two-out-of-three interlock between the bus coupler and the two incoming feeder isolators, as is normal on many twin-fed panels, a back-feed cannot occur.

No, 2 incomers and 1 buscoupler in the panel is normally closed condition, ie it will act as a single system. the thing is the undervoltage relays provided in each incomer, but the relays will not loose its supply since it is connected parallel. the only option i think the reverse power relay, but its costly. please suggest if any idea.

<...please suggest if any idea...>

Er, how about a two-out-of-three interlock between the bus coupler and the two incoming feeder isolators?

<...reverse power relay, but its costly...>

Compare it to the cost of not doing it. Which one gives the better whole-life solution (rhetorical question - NNTR)?

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agree, thanks

Definitely directional relays.

Regarding the cost - look at the costs of not providing protection.

If you need both feeders, you have to decide which loads to cut - and how - when one feeder goes low.

A sudden unusual flow of reactive power out might be another indicator of an impending problem, where you could isolate based on that. These 3 solutions depend on your history and who owns the power passing through, if any.

Your utility will likely have some guidance on how to disconnect/sectionalize in an event.