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(OP)
For transmission lines what are the benefits of having AR functionality reside in Bay Control Unit?

I am used to relays(X & Y) doing the AR; however, of late seen some schemes where BCU does this based on the AR initiate/block signal from relay.

RE: Auto Reclose

If you have the two line relays you have all you need. More devices adds more failure points.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

RE: Auto Reclose

You can choose the lead reclosing breaker, to make it easy for your operators.

We don't do that. We make the operators reclose the breakers by SCADA.

RE: Auto Reclose

Auto reclose, breaker fail, trip circuit supervision etc. are auxiliary functions and some clients prefer to have them in a separate relay instead of including them in the main protection relays.
It is more of a matter of preference and may have a small advantage when the feeder has redundant main protections.

RE: Auto Reclose

For single breaker scheme having no BCU, autoreclose from line protection relay is compact and efficient solution. For two breaker schemes it gets complicated. E.g. for one-and-half breaker scheme it is better to have separate AR relay for tie CB.
In SCADA based stations where BCU is already installed for control functions, using inbuilt function of BCU for AR makes sense.

RE: Auto Reclose

There’s no problem with the line relays reclosing the middle breaker in a BAAH bay. Have been doing it on a regular basis for many years and have never had a problem with it. An extra relay is just extra complexity.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

RE: Auto Reclose

When main breaker AR is from Main-1 relay, middle breaker AR from main-2 relay can be used. Seems logical but slightly complicated.

RE: Auto Reclose

Line relays can close two breakers with no difficulty. Two breaker reclosing is standard today, but back when it was only single breaker reclosing it wasn’t difficult to add a time delayed follow breaker close.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

RE: Auto Reclose

Two breaker reclose in a single relay is less complicated. But many relays do not have function for two breaker AR, or may be additional license / firware needs to be taken from manufacturer.

On the other hand, adding time delay for follower breaker has its own issues. like reclaim time for last AR occured from other line in same diameter, what will happen if main breaker AR is out of service.

RE: Auto Reclose

That’s all part of the basic relay, available for free once you’ve bought the relay. Maybe your choice of relays needs reevaluation 😀

I’ll see your silver lining and raise you two black clouds. - Protection Operations

RE: Auto Reclose

(OP)
Thanks All, Just a question, using BCU for current checked CBF can lead to a problem, as BCUs are generally feed from measuring CTs which may saturate during the fault condition and not yield replica current required for Current check? what are your thoughts on this?

RE: Auto Reclose

Since breaker failure protection is also a standard feature of line and transformer relays there’s likewise no reason to complicate things with a separate breaker failure relay.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

RE: Auto Reclose

In case of BF protection current check is employed to confirm that the CB is still closed at the end of set BF time delay. So, the setting is typically 20% and the BF protection resets if the current falls below the threshold during time-out.
Hence, metering CT is no concern and in a way preferable as it can have better accuracy at such low currents.

RE: Auto Reclose

(OP)
Hi RRaghunath, how does the metering CT behave during an active fault and when breaker fails? The CT will go into deep saturation and not yield the 10 or 20% I required for check current. Also remember CBF delay time may complicate things further.
PS: the breaker will continue to feed the active fault, which can be high, when the breaker fails leading the metering CT to saturation.

RE: Auto Reclose

Prot123,

A metering CT behaves as it's excitation curve indicates. You can have metering CT's that operate the same as a protective CT if they have enough core iron. I ran into this in switchgear when someone specified only metering CT's instead of protective CT's.

RE: Auto Reclose

Question, as to why are you concerned about how a metering CT behaves in a fault? Are you trying to use a metering CT to measure a fault?

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