Dry Type Transformer Failure

OK. I'll start. I'm guessing a >+10% voltage excursion event on the primary winding.

_{Is there a prize for getting this right, BTW?}

Good morning.

How do you get the >10% voltage on the LV primary side?

My calculation: P=EIx1.73; E=400V, I=58A

Thus, input power about 40kVA, less than normal rating 50kVA.

I don't get a >+10% voltage on my windings. I don't know anything about your power source so any surge in voltage will be on your equipment.

It's funny how transients happen so damned fast!!!

From past experience dirt springs to mind. What sets of as a slight leakage soon builds up until flashover occurs giving you the burning you see now.

Probable cause of the failure?

Voltage spike damaging inter-turn windings of HV coil resulting in eventual (not necessarily instant) arc/overheating and damage to HV winding within a localised part of the transformer.

Could also be a transformer manufacturing defect (or a combination of the two).

Your records showed no sign of any overload so that is not the cause

What would have cause the overheating? ...Inadequate cooling most likely if the above is really true.

The failure is the result of a process culminating in an event. Step by step and your other trafo looks like it is on the way out too.

Look carefully at the environment. Do you trust the manufacturer's rating claims? Did some one omit a derating factor at the onset?

Have your checked the inrush (start-up) current for the equipment being powered. We recently have found that some of the equipment installed on our site has higher inrush currents than specified by the manufacturer of the equipment. If the inrush is higher than the transformer rating this could be what's caused your problem. With our problem the inrush was found to be approx 4 times the nominal rating of the transformer and at present we are lucky that it has not burnt out.

It sounds more of a lightning strike, than over load, and the taps on the primary side...

The most common cause of failure on dry type HV transformers are insulation break downs on the magnet wire and inter-winding insulation. Deterioration of conformal coatings triggered by the combined accumulation of dirt and moisture. Heating is the after effect when HV arcing starts to develop due to the deterioration of this insulation.

Failure may be due to following reasons:

You have mentioned,' Average Output usage ( single phase, DC ) : 24kV, 1.35 A. That means the output AC voltage is converted to DC voltage for some special application( not mentioned ). If IGBTs are used for DC conversion, high volatge pulses may hit the HV windings and in effect interturn failure of HV windings may be happened. Due to interturn failure , localised heating may be occured and ultimately insulation may be damaged and trafo may be failed.

If there are harmomics in load side and fed to trafo, loaclised heating and failure of insulation may occur.

If the condition of epoxy insulation are not good,i.e.,there are pores/ air pockets in epoxy insulations, partial discharge at high voltage may occur and inturn localised heating and failure of epoxy insulations.

If photographs are available and nature of load or application in details are availabe, analysis of failure will be easier.

Thanks,

Manindra.

This is a rectifier Transformer application the failure may be attributed to the high dv/dt causing the damage to the start of the winding.
The actual reasons of the failure can only be commented after seeing the failure and kind of insulation scheme used for the transformer.

1.The class of insulation on the conductors?
2. During the design what was the stress level on the insulation?
3. What are the harmonics level Consideration during the design?

4. Background of Application?

I have got these questions from our technical manager, if you answer those, we can help you more. You may write to *

Regards,

Sujit

DuPont Nomex Team

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My opinion is failure was caused by corona. (partial discharge)

The overheating was caused by a partial short where the corona ate through the insulation.

If when you disassemble the coil, the insulation is eaten away in other areas of the coil, this is a sure indication that there was corona in the coil.