Bucholtz relay (ANSI 63) 3

[electrolitic]

Situation: Oil filled power transformer fed through a fused combination starter (fuse + MV contactor)
Question: In case of heavy fault ( 63 trip signal) is it correct to trip the MV contactor?
Thanks

 

[electricpete]

Our sudden pressure relays cause trip if 2/3 logic is met. I assume Buchholz is similar.

I’d think the device to be tripped must be capable of interrupting the available fault current (the same devices that are tripped by your 87 relay).


=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[LionelHutz]

You can open the contactor when the fault current is less than the rated breaking current of the contactor. You need to find another way to open the circuit or allow the fuses to clear the fault when the fault current is above the rated breaking current of the contactor.

It doesn't really matter what type of relay it is. However, considering a "heavy fault" would suggest an internal arc has formed it's quite likey the current will be above the rated contactor breaking current.

 

[dpc]

Generally, the fuses will have to clear any faults. The contactor will be used for switching only. If the ground fault current is limited, you may be able to use the contactor to clear ground faults.

But Buchholz relay operation implies possible major internal transformer fault so it would be unwise to use the contactor unless it is certain that the contactor has adequate interrupting rating. Which is not likely.

Best engineering solution is to replace the contactor and fuse with a circuit breaker.





David Castor

http://www.cvoes.com

 

[electrolitic]

Sirs
Buchholz(german inventor) relay is connected between oil tank and conservator and detects, in case of major faults, sudden surge of pressure.Here in my country, a fuse+MV contactor combination is frequently used as a cheap transformer feeder option(up to 1,0MVA).
Sometimes, contactor breaking capacity is eight or more times smaller than network short circuit capacity.
So, is there a possibility,even remote,of contactor be tripped under short circuit condition by Buchholz actuation?
Thanks.

 

[rbulsara]

Your question has been adequately answered. Either keep the cheap (inexpensive) system without the relay or replace the fuse-contactor with a breaker. Typically, Buchholtz relay is not justified for 1 MVA transformer.

Fault has to be cleared by a fuse or a breaker. Contactor are for normal load or overload interrupting duties, within its rating.

Rafiq Bulsara

http://www.srengineersct.com

 

[LionelHutz]

So, is there a possibility,even remote,of contactor be tripped under short circuit condition by Buchholz actuation?

We don't know. We didn't design the protection scheme. Is the protection scheme wired so the Buchholz relay trips the contactor?

All I can tell you is that you should not open the contactor when the network fault current is eight or more times higher than the contactor breaking capacity.

 

[electricuwe]

If Buchholtz relay (and ciruit breaker as posted by dpc) are justified depends not only on the rating, but also on the kind of transformer and the load supplied.

If the transfomer is 1 MVA standard distribution transformer where you have a spare available within a few hours the fuse/contactor option may be suitable.

If you have to protect a one-of-design rectifier transformer where you might have to wait 3 month for a replacement unit built new, the combination of Buchholtz and breaker may be advisable.

 

[electrolitic]

According to RSULBARA my question has been adequately answered.In fact, it was answered in 2002,partially, in this forum. There, is explained that oil conservator/Buchholz relay are widely applied in Europe and Asia for small,medium and large transformers.
Additionally, I have found fused combination starters applied to feed transformers up to 1,0MVA.
My question it was:Is there a short circuit condition where Buchholz actuation can be faster than fuse actuation?
Summarizing, how fast can be Buchholz?
If yes, I will not wire Buchholz trip signal to the contactor.I will wire it to the first circuit breaker located upstream.
That is it.
Thanks

 

[dpc]

Is there a short circuit condition where Buchholz actuation can be faster than fuse actuation?

Yes, definitely.

David Castor

http://www.cvoes.com

 

[ScottyUK]

For example, an inter-turn short can result in immense currents within the shorted turn, yet remain within rating seen from the main terminals of the transformer. The Buchholz could well be the first protection to detect the fault, although the PRD might be close behind. One transformer accident which I am far too familiar with basically had the shorted turn problem: the Buchholz and PRD both operated, although the unit was actually tripped by the transformer diff relay when an interposing CT was flung from the transformer top as it broke up.


----------------------------------

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