Mal Operation of Buchholz Relays

Mercury switches are the most reliable you will get. There are two in a Buchholtz unit usually connected to two separate relays for "Gas" and "Surge", which is operating?

Why do you suspect the Buchholtz unit is faulty when it operates during severe stress on the transformer, look at the transformer first especially if it has a manual tap changer. I've had three instances of Buchholtz trips due to tap changer failure. there's been others but they were due to oil levels.

Buchholz relay though an old technology is still one of the most reliable protective device for the safety of transformer internal faults for oil filled transformers. Any heating causes gas formation and results in immediate tripping signal. Mercury switch is one that is one hundred percent fail safe device. You should always provide additional protective relays for line fault ( overloading and short circuit) for both HT and LT side of switching panels.

Not surprising at all! The Buch Holz Relay is doing its job. When the transformer is feeding a "high intensity" fault as mentioned by you, the BH Relay also sees it as a fault only and hence should and would react. Be happy that your BH Relays are intact. It is not maloperation but only genuine operation. Even with reed switches this would happen. Avoid faults. Or have a high speed extrenal fault detection system so that this would operate before the BH Relay operates.

Wheather Buchholz Relays were operated or maloperated to be assessed at first instance as conventional mercury float switch are more reliable ones.And it is not clear about your transformers are equipped with automatic OLTC or not?In general,transformers are trouble free if strict O&M practices are followed and higher sensitive relays are part of your protection system.Relay coordination must be enhanced in order to avoid faults transferred on transformers so that stresses to be kept away.It is appropriate to analyse every tripping before concluding whether it was maloperation or not.

Operation of a Buchholtz unit usually means something is in the process of being fried. You say there's been a number of "mal-operations", if I were you I'd start getting worried about the rest of the system. Don't go altering the one bit that seems to be working OK.

To be honest I have serious doubts about the integrity of the rest of your system if high fault levels seem to be the normal way of things.

In my case the BR trips, and have done so a number of times, when the circuit BEFORE our BR had a fault. The last was when nearby roadworks damaged a cable of the same municipal transformer that fed our plant. According to the municipal engineers it is the way the (their) transformers are wired and the surge is passed into the BR, which then does its job. Good for us but annoying if you have to wait up to 4 hours for the council to come and reset the BR. ( For info- the council is planning to change to new transformers, but they cannot find 525v secondary units here.)

You did not mention the rating of the transformer, grounding system, the type of fault occured etc.. Assuming the connection is Dyn11, solid neutral grounding, fault occured in LV ( Secondary side ) and if there was no tripping of instantaneous Earth fault relay,during occurance of high intensity fault ( like three phase dead fault ), you may check the neutral grounding resistance ( the resistance may increase in passage of time if proper maintenance is not carried out once in every year ). If there was simultaneuos tripping instantaneous overcurrent relay and Buchholz relay, in addition to the above checking tightness checking of transformer core bolts should be carried out.

I would like to add some more information here before further discussions, The buchholz relay mal operation (there was no real operation of relay i.e. no gas formation, no change in the DGA pattern noted) noted in few Power transformers of rating 10-25 MVA, 66 & 33KV class with solid neutral grounding. Analysis revealed that BR was operating before operation of relays responsible for clearance of the fault. One more information, wheels are not clamped to the foundation in all transformers having this problem. Is the BR operating due to vibration? What is the possible solution.

Take the wheels off and bolt it down!

Have you ever stood next to a transformer under fault conditions? I have and to this day I'm convinced it jumped off it's foundations.