Transformer 1

[Guardiano]

Hi,
We had a breakdown of a cast resin tranformer, rated @ 3500 k VA 11 k V/ 415 V. The breakdown occured on one phase and the LV winding was shorted to ground. This transformer was working at a very high temperature, as do use to operate cast resin types,<= 115 deg C. I am having an argument with my insurance company because they are willing to pay for the repair/replacement of the the damaged coil only. I have advised for a replacement because I am of opinion that the other two coils will sooner or later break down. A simple megger test, HV and LV, on the two remaining coils did not show anything wrong. I don't want to effect any destructive testing on the two remaining coils. Any advice? I have attached a picture of the damaged winding.
The transformer is a Schneider one.
Regards.
Guardiano

 

[racookpe1978]

All three are in the same casing right? All three subject to the same temperature increase?

 

[waross]

Is that a cast resin or a dry type?
The much smaller sizes of cast resin transformers that I am familiar with are cast en-block. The windings are not visible.
Link
This thread may be informative:
Cast resin transformers
thread248-402679
And lastly, is there any core damage?
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ewo00]

A winding resistance test may help.

 

[Guardiano]

Hi,
Waross, thanks for the post. I refer to this quote from ScottyUK, "Schneider seem to have missed partial discharge as a problem, or at least seem to think their transformers don't experience it. I agree with Gunnar that the 'Geafol' type are among the best of the breed. I'm not convinced that Schneider have PD completely fixed. Equally I'm not sure how many users actually look for PD to know that they have it. Being PD-free on commissioning day doesn't always equate to being PD-free after years of service, especially with organic insulation materials. "
Guardiano

 

[ScottyUK]

PD wouldn't account for an LV fault to earth, that sounds more like a manufacturing defect. PD inception voltage is usually around 10kV in air, depending on altitude, so at 415V it's simply not going to happen. Was the LV system solidly earthed?