Is Breaker Fail protection relevant!!

[RRaghunath]

Present day breakers (both outdoor and indoor) have compact operating mechanisms with less moving components and made with better materials. Hence, breaker stuck conditions are extremely rare.
Hence, do you think the protection schemes can do away with breaker fail protection!!

 

[PHovnanian]

Breakers sometimes fail for reasons other than stuck mechanisms. Failures in protection sensing (CTs, PTs) or relays are possible as well. Modern digital relays have much better health monitoring capability, so the probability of such passive faults going unreported followed by a line fault is smaller. But digital relays make implementing breaker failure schemes much less expensive. So in the end, you might as well just put it in.

 

[TurbineGen]

I have seen several breaker failures in both new and old units. I would not do away with your breaker failure protection.

Three years ago we had a generator loose it's speed signal and went to trip on overspeed. The wires had not been hooked up due to an incomplete controls upgrade. Luckily our 86BF (Breaker fail) relay picked up and dropped the unit offline.

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[rcw retired EE]

Until we learned how to properly set the relays, we had many false trips from breaker failure schemes and no breaker failures. But the consequnces of a breaker failing to open are significant enough to recommend the continued use of BFP (Breaker Failure Protection).

When a BFP actuates incorrectly, the affected servcie area is much larger than a single breaker trip. That is why a lot of care must be taken in selecting the correct logic and settings for a BFP system.

An old IEEE survey identified maintenance work as a major cause of a breaker's failure to operate. A control power switch, manual interlock or lifted wire left in the wrong position after maintenance prevented correct breaker operation. Modern breakers require less maintenance so probability of failures due to the historical causes should be less. But the remianing failures can still be catastrophic.

 

[slavag]

Hi Raghun.
It's a intresting Q.
We also thinks about it.
On this moment:
1. For the voltage level from 69kV, we use 50BF/BFP.
2. For the MV, where used reversed logic, we don't use 50BF/BFP, but if the a generator bus, we used 50BF/BFP

For the HV, we used BFP as part of BBP(87B).

So, is depend.
Best Regards.
Slava

 

[ScottyUK]

Modern relays make it a lot easier to implement a BF scheme, so there are a lot more of them out there than in the past. Whether they have all been designed and implemented correctly is a very different question, as rcwilson has already alluded to.


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If we learn from our mistakes I'm getting a great education!

 

[rhatcher]

Breaker stuck conditions can still exist even with modern breakers. This is seen on breakers that remain closed for long periods of time without being opened. In this case, lubrication can harden and corrosion can develop that causes the breaker mechanism to stick.

PHovnanian, TurbineGen, and rcwilson also offer valid reasons for failure to open (failure of CT's or relays, improper control upgrade, and improper maintenance procedures).

I would not do away with the failure to open protection since this is a good backup to all of the above conditions.

 

[burnt2x]

Hi.
I think it is best to have layered/backup protection. With newer relay designs incorporating multi-functions, one could invest on a separate backup scheme (BFP, etc.). The key is to never rely on a single protection scheme or protection lumped in one physical space. However, it's the protection engineer's call! Extra relaying could be very prohibitive in some cases, IMHO.

 

[m3ntosan]

BFP is critical for generator units stability, also power system transient stability studies are made taking into account BFP settings.

May you grow up to be righteous, may you grow up to be true...

 

[cranky108]

Question: If BFP is so important, what is the best method to apply it?

Most applications I have seen use over current as a requirment.
However if set to low it can be misoperated for a tech. mistake and normal load currents.
Also for generator protection, a trip can be issued for a very low current condition.

Would it be prudent to apply a more than one misoperation mode BFP?

 

[Ohmly]

At our organisation, if we removed BF protection our clearance times would be too long, so we need it.

The risks are false trips by technicians, and bigger outages as well.

Our solution is to include BF Trip isolations in the formal switching instructions, along with formal training and authorisation of staff to carry out these isolations.

We also use a different type of link for the BF Trip, just so it stands out from the others, and makes the technician think.

 

[slavag]

Hi
cranky108, you are right.
Newer relays have a few criterions for the BFP scheme.
High current criterion
Low current with CB position criterion
Mix of both of criterions, for example: if current less then 0.3In CB position criterion will operated, if more, CB postition dosen't taked in account.

Is a good practic for the transformer and generator protection, like to reverse power, frequency and voltage protection, mechanical protection of transfromer, etc.

Newer relays have also option of SF6 signal (CB fault) for the BFP accelaration.
Best Regards.
Slava

 

[rcw retired EE]

Slavag - Good point on using low SF6 gas pressure alarm to trigger breaker failure protection. To prevent more widespread outages, low SF6 should only acutate a BFP trip if a trip signal is present. Most breakers can safely carry load with low SF6, they cannot interrupt fault currents. Another way of looking at the logic is the low SF6 input should bypass all time delays and operate the BFP trip outputs directly when a trip command is received.

Also in BFP logic, do many of you use the manual trip switch as a BFP initiate command? Some of our clietns reason that the operator should be aware that the breaker failed to open, so their BFP logic is only initiated by fault protections. This is to avoid nuisance trips.

 

[odlanor]

TurbineGen (Electrical) 5 Mar 10 15:06
If generator loose it's speed signal how breaker failure protection was initiated?