110kV Transformer Bushing Failure

[GerH]

A bushing failed on a 50/60MVA 110/20kV ONAN/ONAF transformer during FAT yesterday. The failure occurred a few seconds into the high voltage 50Hz test after the transformer tank had been on pressure test for about 16 hours. The porcelain surrounding paper insulation on the inner side of the bushing fractured, spreading broken bits on to the top of the windings and down the tank. No visible arc damage. The bushing paper surface which was under the broken porcelain is ripped along one side - presumably this was where the electrical breakdown occurred. Very reputable manufacturer. I have not seen this type of failure before. Should I be concerned or is it just one of those things that we should agree to replace the bushing, clean out the debris, retest and move on. I have asked for Tan Delta measurements on the bushing. Any comments appreciated.

 

[dpc]

So, FAT = FACTORY acceptance test or FIELD acceptance test. Beware of TLAs.

I'd not be overly concerned, but make sure you have a spare bushing on site when you energize it. Also, not all bushings are created equal. When the transformer spec was not specific about bushing manufacturer and country of origin, I have come to regret it in some instances.

Also, check your specifications. The manufacturer may be on the hook for travel costs to send someone back for re-testing prior to shipment. Are bushings to be removed for shipping?

 

[prc]

The situation is not unusual. The manufacturer is the best person to analyse and give root cause analysis. See whether the same is convincing.
Pressure testing before dielectric testing seems unusual. Normally that is the last test. Hope no air was used for testing. Otherwise it can be dangerous.

 

[GerH]

Thanks. Happened In the Factory Test.

 

[crshears]

TLA's? The OP didn't mention anything about the transformer having tertiary lightning arresters...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]