REL 511 Relay Didn't Issue a Tripping Command in Case of a Fault?

One possible reason was that the relay wasn't in the same line as the lightning current.

Show us the connections of LA,relay,CB etc in a single line diagram with distances between them as well as relay type & settings.

Considering tha lack of information I will have to assume that you have a REL 511 relay installed on a 220 kV Power line circuit, power line also protected with surge arrestor after the 220 kV line isolator.

In the case the lightning strike took place somewhere outside the 220 kV bay, along the OHL, the current didn't pass thru the current transformer so the distance relay didn't detect the discharge current. Since the surge arrestors are installed before the voltage transformer and all other equipment, the overvoltage was cut thanks to the surge arrestor.

Overall, in this situation the arrestor did his job of protecting the power line and the power station without causing interruptions.

The surge arrester and CVT are in the same location at the OHL exit gantry. Could it be that the relay didn't have enough voltage to establish correct directionality! What does the ABB application engineer say! Appreciate if you could share.

This could be the possible reason e.g there is a minimum threshold voltage required for an AREVA make distance protection relay to operate but, Iam not aware of this REL 511 ABB make relay. But, there is a concept of "memory polarisation" as well. Iam also looking forward to a reply from ABB engineer or someone with a similar experience.

I wud like to add here that the directional O/C relay (installed at the same circuit)operated for overcurrent in R-phase.

The event list, what does it say?

If the distance was blocked you can find out from the event list.

What was the effect of the relay not functioning?

Did you have any other signals on the relay?

No signal appeared on this relay. and there is no event logging as well. Relay at the other end of the circuit sensed the fault in zone-2 and issued the tripping command but here at this end, the fault was cleared by directional O/C relay. Owing to the time delay in operation of back-up relay, other healthy circuits also tripped which would have been saved had REL511 sensed the fault in zone-1 and tripped the breaker instaneously.