Breaker Failure Question 1

[nightfox1925]

We have traditionally followed the breaker failure scheme such that associated lockout relays will initiate breaker failure through the 50BF. The 50BF has current level detectors to supervise the breaker failure initiation to verify true breaker opening. The 50BF then sets off the 62BF timer which will perform actual breaker tripping.

There is a proposal to wire the controls such that the 62BF will be ahead of the 50BF. The 62BF will be initiated by protection trips (lockouts, relays) and then initiates the 50BF after time out. They say it will be faster since the 50BF reset time will not be part of the total breaker failure clearing time.

Anybody who made a scheme similar to this? I will appreciate any comments.

Secondly, what is a typical current setting for the 50BF?

Thank you...

 

[dpc]

I'm not following your proposed logic. Normally the timer starts when a trip signal is issued to the breaker. When the timer times out, if there is still current flowing as detected by the 50BF, the breaker is assumed to have failed to open and the breaker failure lockout is tripped, IIRC.

You might refer to the old Westinghouse Applied Protective Relaying book, or J. Lewis Blackburn's Protective Relaying text. I'm sure both have examples of traditional BF schemes.

The 50BF setting can be quite low, since the current should be zero after the breaker trips. If there is any current at all, it is generally not a good thing.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[nightfox1925]

Is there any standard xIn pickup setting for 50BF? Like, what are the basis for setting this.

I'm pretty much sure it isn't based on Fault Current.

 

[slavag]

Hi Nightfox
Isn't simple Q.
Setting of BFP, 52BF.

We used standard 0.2In for many years.
Today with newer 52Bf relays, we used something other.
BFP:
1. Fot the protection with high level of fault current is stewts about 0.8-1.2In
2. With low current with CB position only, but from time to time with additional criterion of current about 0.2-0.3In. Used for signals like to reverse power, buhholz trip, etc.
3. Timer used with two stages:
repeat trip and back-up trip.

Best Regards.
Slava

 

[nightfox1925]

Thanks slavag. What doyou mean by BFP?

What do you mean if you say "low current with CB position only"?

 

[slavag]

BFP-breaker failure protection or 52BF or CBFP.
For big generator, setting of reverse power about 0.5%, that menas a very low current, for samll, is about 2-4%.
Standard BFP current detector isn't sensitive in those cases. In case of elctro-mechanical trips of trafo's possible "zero" current too.
In the low current faults ( same for voltages or freq. faults) we used 52a contact as criterions of BFP operated.

 

[nightfox1925]

I also noticed in one of our client's application that a breaker failure is integrated as one of the elements of the multifunction transformer differential relay. The transformer are protected by two differential relays (SR745 and AREVA 634) with the SR 745 doing the breaker failure protection.

In the event that the SR 745 is disabled for testing, that part of the zone is no longer covered when the primary switch is open. The SR745 primary CT inputs extends beyond the switch into the two breakers of the upstream ring bus.

I believe a separate Breaker Failure relay should have been installed so maintenance and testing of the differential relays won't disable the BFP.

 

[cranky108]

The problem with using the breaker failure within a protective relay, is that the BF would not be available to the compainion relay for that relay failing. Unless you use the breaker failure function in each relay.
I.E. If a relay trips a breaker, it also starts the internal breaker failure. But only as long as both relays have that function.

Other wise use a seprate relay for BF.

We use typical BF values of 5 amps. But you can use lower, but above 0.5 amps.

On generators you may need a BF that uses a 52a instead of a 50.

 

[JMatt]

I vaguely remember a scheme like this implemented in a SEL relay.

The breaker failure timer would start the instant that an overcurrent trip condition was detected. Then, a breaker failure would be declared if that overcurrent pickup did not dropout within a specified amount of time AND a trip had been sent to the 86BF. If the 86BF is being tripped by the same relay, this is a way of initiating your breaker failure timer quicker, no waiting on the 86BF. Just make sure that your 50BF is set high enough so that it won't pickup for out-of-zone faults.

This was used in a situation where system stability was unusually precarious. Faults absolutely needed to be cleared quickly.