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Join Date: Jun 2009
Location: Singapore
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Require Help for Transformer Flashover Incident

08/28/2019 9:07 PM

We have had at our install base a dry transformer with the following specs :

1) Rating = 3MVA

Ratio : 22000/400V

Impedence = 8%

Connection : Dyn11

Design of Low voltage winding is expoxy cast resin while high voltage winding is VPI.

The transformer have been energized for 2 months with no load applied and having no problem.

Just recently, we loaded up the transformer to 2.5MVA for the first time for about 30-45 mins, and the transformer restricted earth fault protection trigger and trip the transformer.

After which, we realized that the bottom of one of the windings have been burnt and this has probably to cause the winding to be shorted to ground.

This is an extremely rare incident, to which we have no clue on what are the probable causes that could have cause this to happen.

My guess, is somehow, the heat generated during actual load application has somehow break down the insulation and cause the inter-turn fault between the windings and then somehow this puncture a hole through the epoxy and then short the windings to ground.

If this is the case, then are there any additional tests that we can conduct to capture this problem?

The transformer has gone through all the necessary IEC tests prior to energizing it

If anyone has encountered any similar cases , please share with me your findings.

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Guru

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#1

Re: Require help for Transformer Flashover Incident

08/28/2019 9:16 PM

It's hard to say without a more thorough forensic exam...is it possible the winding was damaged? Possibly a foreign substance was trapped in the coil...possibly rodents have eaten away the insulation....there are many possibilities...

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#2

Re: Require Help for Transformer Flashover Incident

08/29/2019 11:15 AM

Since you have energised the transformer for two months the effect of the transfo being wet when installed is unlikely, it should have dried out before your load application.

Some questions to ask {I count 8}....

Because you refer to Restricted Earth Fault operation, I guess you had an LV winding fault, is that correct? {1}

Did you do an insulation resistance check on LV windings before energising? What was result? {2}

Since this is a commonly used size/type of transfo, it seems likely it was factory tested off-load and short-circuited but never given a heat run. You should find out what tests were actually done and the results [If there was a heat run, the supplier can claim it was AOK when it left him, otherwise a manufacture defect is more possible]. {3}

A turn to earth fault is far more likely than turn-turn, for same insulation thickness. When you write "bottom" of winding was burnt, is this in the visual way or also in the electrical sense of being at the neutral [earthed] end of winding? {4}

Fault at earthed end is more possible due to mechanical defect/damage than electrical stress for a uniform insulation thickness. Also low end usually runs cooler.

Have you checked if there was any damage to packing of transfo at arrival, suggesting mechanical damage during transport? Or rough handling on site, anything bent or dented or scratched? {5}

Is there any sign of "foreign bodies" near the fault? {6}

Blockage of ventilation passages I would expect to have slow effect, not within minutes of load application. Do the installation instructions require any transit blocks or braces to be removed? {7}

You have not given the transfo losses, but usually the magnetising losses & temperature rise no-load would be much less than full load loss/temperature rise. Full load would cause more expansion of windings than no-load.

Is there any forced-cooling and is it working OK? {8}

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Guru

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#3

Re: Require Help for Transformer Flashover Incident

08/29/2019 11:38 PM

I'm guessing a void in the insulation, there may be more. Expansion of copper and/or iron during load cycle could have distorted the void to reduce clearance enough to fail. A void might be difficult to detect without sophisticated inspection equipment.

I suppose considering this fail was on the LV side, assumed based on your mention of epoxy, a conductive inclusion might be a more likely culprit than a void. There may be more.

As for testing, the transformer manufacturer should have some comment on their QA equipment, all I can think of is radiography, cm by cm, a bulk test for a single defect doesn't seem like it could be sensitive enough.

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Active Contributor

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#4

Re: Require Help for Transformer Flashover Incident

08/29/2019 11:42 PM

Hi... Do not want to know manufacturers name... But can you tell me from where was it made.. The continent would be fine.. Thanks.

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#5

Re: Require Help for Transformer Flashover Incident

08/30/2019 4:13 AM

I think the advice offered is good and covers the many possible causes of failure.

But...The problem is easily divided into three responsibilities.

First...Has the manufacturer carried out the full amount of tests necessary?..

Load tests

Pressure tests

Most importantly ..as these are epoxy mouldings..the discharge hv tests?

The manufacturer should test for discharge values and then compare them

with historic tests of exactly the same design and performance.

Every design and configuration will give different values, and a product

should only pass if that design with a particular characteristic has proved

with use to be reliable.

Regretably epoxy moulding process is not just simply avoiding creating

voids during the moulding process.

Also we used to first mould conductors in a slightly cushion moulding and once set, complete the final moulding process.

This was to allow for expansion of copper conductors when reaching running temperatures.

So expansion of copper on load may have caused your problem.(sorry I still like the word "problem" and then "answer")

Second... Damage during transport or handling and positioning on site...packing not correct or strong or robust enough.

One would think a solid moulding would be very be strong...it could be

argued that an oil filled transformer is more forgiving.

Ofcourse if moulding was repaired for transport damage..there could be another cause.?

Third...The electrical environment the transformer is connected to?

Type and characteristics of the transformer load....highly inductive or

capacitive..switching devices? etc.

To summarise.... the manufacturer( not agent) should visit site , and they or with others must also investigate the electrical environment.

I personally would not accept any epoxy moulded trasformer that has been repaired,

because all the above mentioned tests and comparisons need to be carried out.

As I was involved for many many years in manufacture and later for many years in investigating power distribution problems and failures...i would consider it a really big favour if and when the event has been sorted out that you tell me what the outcome was.

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#6
In reply to #5

Re: Require Help for Transformer Flashover Incident

08/30/2019 6:53 AM

<...the manufacturer( not agent) should visit site...>

Quite. GA

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