Transformer breakdown

[Guardiano]

Hi,

Something amazing happened to me yesterday. I had to intervence on a motor control centre and the transformer LV side was isolated. The transformer is a 5 MVA 22 k V/ 400 V Dyn11. A few seconds after the 400 V switchgear was opened, the transformer tripped on Bucholz. After verification, we noted that the LV winding was shorted to ground. The insulation of the HV winding is OK as well as the insulation between LV and HV. A di-electric test confirmed that the transformer oil is OK. This transformer was loaded to around 70 % and operated trouble free since it was installed five years ago.
What went wrong?

Guardiano

 

[jghrist]

I had to intervence on a motor control centre and the transformer LV side was isolated.

I'm not sure what you were doing. Was the transformer de-energized when you were working on the switchgear? Was the switchgear de-energized? Was a breaker or disconnect opened to den-energize the switchgear a few seconds before the transformer trip? Please explain further the circumstances.

 

[unclebob]

Lifespan of a transformer: 1 cycle to 1 century.

 

[prc]

You may check whether you inadvertently caused a shorting of LV side.Since Buchholz operated,definitely a failure is to be expected. Internal inspection may show you shorting of any LV lead to earth( if you are lucky) or collapse of LV winding to core.