Magnetic Balance Test

"...are we not back charging a probably faulty transformer?..." Yes you are, your first goal is to find out what the gas composition is in the Bucholz, find out whether the relay is properly calibrated, then find out why/how that gas was generated.

Don't re-energize the transformer until you know what caused the trip and what its condition is.

The engineer assigned to carry out tests on the transformer started by doing an LV MBT that too at rated voltage. Yes no turns ratio, no winding resistance. In India MBT is used by many utilities as a diagnostic test on transformers. However there are no clear guidelines to carry out the test in any of the standards. So how should one conduct an MBT in this case i.e how much voltage should be applied and on which side??

RAMconsult: Good answer

I think you are going to the worse case scenario first... are the windings failing!!

Most Buchloz relays have two trips, gas and emergency trip, the later working when the windings do fail, the large volume of gas produced, rushes to the highest point, the header tank, tripping BOTH alarms. So I'm guessing that you had a 'gas trip'. That in itself is nothing to be overly concerned with as air bubbles can stay trapped in the windings for a long time.

As previously mentioned, check the gas, see what it is. If inflammable then you follow either your own test procedures or those of the transformer manufacturer to locate the fault.

Your procedures seem to be lacking in safety, if you have injected 230volts on the secondary side, and a transformer works both ways FYI.. it will step UP voltages and well as stepping DOWN voltages. If there is a fault in the windings, and is detected now, you might have an opportunity to do something about it, rather than hope for the best.

I suggest you do the following

Test the gas - if found to be air.. no problem

If found to be combustible, then call someone to check out your transformer or you replace it now, to be inspected at a later date.

I was trying to understand if the fault could have been escalated due to carrying out an MBT at rated voltage before doing any of the other tests like winding resistance and Turns ratio. The transformer in this case had tripped twice on bucholz, once in service and second time again after test charging on no load. After tripping for the first time the testing engineer carried out a rated voltage MBT on the LT side and the variac he was using started vibrating and burnt out. Even after this he again carried out an MBT after the transformer tripped for the second time. This time the variac did not burn and the MBT results were ok. Later on he carried out a winding resistance and got 4ohms, 8ohms , 4 ohms on the HV side while LT side resistances were ok. The transformer was declared failed on the basis of these measurements. BDV was found to be normal . However the DGA showed high electric discharge region in the duval triangle. Moreover can an open HV winding cause high electric discharge in the transformer when the Oil is good??

as you have declared the transformer not fit for service, then the whole topic and further comments would not be constructive, however your comment..

Moreover can an open HV winding cause high electric discharge in the transformer when the Oil is good??

How do you know the oil is/was good? when did you test it last?

Any dielectric oil has a dielectric strength value, measure in Mv/m, and while you may not reach the voltage, you will certainly get the fault current on the HV side to earth to break down the oil, causing gas. Once you have tested the gas which really should have been your first step, and it tested positive for gas, the oil is contaminated, and you need to either look at the transformer and complete more tests or again test the oil.

I suggest that you contact your transformer supplier to get further instructions and test procedures before you get another problem transformer.

A sweep frequency resonance will reveal any winding problems