Oil filled transformer partial discharge analysis 1

[TurbineGen]

We have a small power distribution transformer that is showing relatively high levels of PD (600pC) and we are trying to determine the cause. The transformer is less than 3 months old. Last night we pressurized the unit to 10psi and the PD level dropped to 200pC. My thoughts are this is a line to ground clearance issue, but I am in no way and expert. I noticed this morning that the PD level is not stable but seems to randomly increase for several cycles before settling back down to more acceptable levels.

I remember a few years back there was an article about reading the frequency domain locations of the PD on each phase to determine the possible causes of the issue but for the life of me I cannot find it. Perhaps someone could point me in the right direction to an article on reading partial discharge plots to determine what could be the cause?

What should I be looking for in the frequency domain and how might I interpret the data?

As a side note, when in the testing phase during the transformer manufacturing, it had an impulse test failure due to CT wiring not being properly secured which was repaired before it was retested. I am not sure if this is connected or not.

Thank you for any help.

 

[prc]

First of all, what is the MVA and kV of this transformer. Are you doing an on line Pd monitoring. If so, what prompted you go for it?Is the conservator open type? Never pressurise an energised transformer as it can end in disaster. When you release pressure air bubbles can be generated inside transformer( as in case of carbonted water bottles)which will ionise and cause a breakdown.Did you conduct DGA? Any excessive Pd can result in H2 and sometimes acetylene. Frankly I dont think 600 pC is such a big issue.With site PD testing accoustic method or VHF is found more easy to locate the problem area.

 

[TurbineGen]

It is a 30 MVA unit. The voltage is 138/34.5KV. The test is acceptance testing as the unit has yet to be put in service. It is a mobile transformer that has Nomex insulation if that makes a difference. We are doing the PD test at twice voltage and 3X frequency and we are monitoring from the bushings. Our spec is 100pC or less. In most cases we see less than 80pC in our testing of this class of transformer. I will look into acoustic testing. Like I said, I'm no expert here.

What puzzles me is that the PD went down when we pressurized it. Admittedly, I have no idea why they tried this last night.

We are waiting for the DGA results. I will post back with the results.

Thank you!

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.