Transformer Fails High Voltage Test

What is your transformer primary voltage?

How do you know, if your transformer coil is still healthy?

Have you do any transformer oil test and what is the last time you do the oil test?

What is your loading from this transformer (KW)?

Dont repeate HV test as it should not be regular test

I have experienced this before in some manufacturing works. The test should be done as it tests the insulation withstand. I believe that the transformer you have is a distribution unit & therefore it would be fair to say that it is relatively small in physical size.

What I have done in the past is had the transformer cover removed, Disconnect the HT bushing leads & then connect the test equipment through the top opening. When you do the test & you get to the breakdown you should see some bubbles rise. You then have a better chance of finding where the problem is. You may also see a carbon track on the windings or leads or in the oil. If you see this before you retest go looking for the source & if not obvious do the test to pin point the problem area.

Sometimes the problem is very easy to fix because it is a lead that has not much clearance. Other times it is inside the windings & that sadly is expense & time consuming to fix. The manufacturer must do that as they were the builders - if it is new it is under their quality if it is an old unit then you need to decide on the cost to repair.

Some say do not do the test again - sorry but any damage is already done & you need to find it or it will fail in service & that is very expensive.

Good Luck.

what is the IEC standard for Trafo testing?

From the looks of it you have open this transformer to some point, you stated that there is no carbon spots on any part, this is a clear indication that the failure is inside the windings. That could be a turn to turn fault or insulation failure between layers.

If the transformer is still under warranty, then it is the manufacturer's responsibility whatever needs to get done. If you realy need to know where the problem is and if it is out of warranty and it is not a welded cover, take the cover off then let someone look inside from a safe position while you apply the voltage if you has the ability to slowly increase the voltage to 34kv. you will see smoke and bubbles coming from the defective area.

Regards

Granville