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Anonymous Poster #1

Transformer Problem

03/09/2017 3:58 AM

We have a transformer of 3.6MVA, 33KV/415,4800 Amp, Volt S Delta/Star , DY-11, in running condition.

But 1.5 years ago it came in idle condition so we stop the 33KV input supply .

6 months ago we found leakage from LT Bushing. And the oil become drain out up to bushing level for 15-20 days

For maintenance of bushing we drain the oil up to the level from where we can replace the bushing & gourmet of the bushing safely.

After compliance of job we fill up the oil with filtration.

Checked

BDV = 60 KV in 2.5 mm

Insulation resistance LT- Earth= 0 M-Ohm (500 V Meager)

Insulation resistance HT- Earth= 0 M-Ohm (1000 V Meager)

Insulation resistance HT- LT= 0 M-Ohm (1000 V Meager)

Filter OIL for 4 circulations Oil Temperature 70 deg C

BDV = 60 KV in 2.5 mm

Insulation resistance LT- Earth= 0.5 M-Ohm (500 V Meager)

Insulation resistance HT- Earth= 10 M-Ohm (1000 V Meager)

Insulation resistance HT- LT= 100 G-Ohm (1000 V Meager)

Filter OIL for circulations 7 days Oil Temperature 75 deg C

BDV = 65 KV in 2.5 mm

Insulation resistance LT- Earth= 0.5 M-Ohm (500 V Meager)

Insulation resistance HT- Earth= Infinitive (1000 V Meager)

Insulation resistance HT- LT=Infinitive (1000 V Meager)

Applied 415 Volt Supply in 33KV side

Measured LT Voltage-5 Volts

Connect the LT Terminals in a salt-water solution to pass the current 80 amp in all three phases for one day.

Next day Insulation resistance remain unchanged

Increase the LT Current up to 600 amp and kept for 4 days

Result as follows

Insulation resistance LT- Earth= 1.5 M-Ohm (500 V Meager)

Insulation resistance HT- Earth= Infinitive (1000 V Meager)

Insulation resistance HT- LT=Infinitive (1000 V Meager)

Remove the LT supply from HT side, drain total oil, disconnect all LT bushing from winding check insulation resistance of all bushings found all are infinitive but one FRP strip used for support all 3 phases and neutral with transformer body is 0 to 10 M-OHM in difference of FRP strip.

Insulate the FRP strip

Result as follows

Insulation resistance LT- Earth= 150 M-Ohm (500 V Meager)

Insulation resistance HT- Earth= Infinitive (1000 V Meager)

Insulation resistance HT- LT=Infinitive (1000 V Meager)

Riffle the transformer oil after filtration

Insulation resistance LT- Earth= 10 M-Ohm (500 V Meager)

Insulation resistance HT- Earth= 100 (1000 V Meager)

Insulation resistance HT- LT=100 (1000 V Meager)

Please suggest me what farther I can do?

With Thanks & Regards

Amit Nandi

9826009245

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Power-User

Join Date: Jul 2016
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#1

Re: Transformer Problem

03/09/2017 10:12 AM

So the really good readings you took were with the tank empty, before putting oil back in.

And with the oil back in, now primary 100? ohms to earth, and 100? ohms to secondary, but not 100 ohms to earth from secondary? Either something wrong with one of your meggers, or unlikely case that you have some insulation fault sensitive to 500 volts of potential?

One thing to consider is it is possible to charge the insulation with your test equipment, it can take some time to discharge the insulation. Look up dielectric absorption test, usually used for rotating machines. Because your transformer has a large ratio and high secondary current rating, maybe higher than typical insulation amount to charge, just a guess.

I would have put the switch to it with your 1.5 megohm secondary reading, knowing the history. Good work isolating and finding the problems, by the way. Not sure what could have happened to the LV lead support (FRP Strip), and how did you insulate it more? How did you measure zero to 10 Megohms, any (bare?) lead to earth?

The bushing leak is what hurt, moisture entered deep into the cellulose insulation structure, and it is a large effort to get it out, as any effort to dry wet paper, let alone buried in oil.

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#2

Re: Transformer Problem

03/09/2017 12:06 PM

Shouldn't you be paying a Consultant to answer this question?...

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#3
In reply to #2

Re: Transformer Problem

03/09/2017 5:34 PM

...but CR4 is free...and worth every penny...

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#4

Re: Transformer Problem

03/09/2017 7:46 PM

Here is a guy you could hire to advise you.

http://www.impomag.com/article/2017/03/7-myths-electrical-transformer-reliability

I don't work with beasts like this, so this is the best I can do. It looks like there may be some gems to harvest from this article. It sounds like you were hot filtering the oil to help clean the paper so that sounds prudent. Mr. Ross will know much more than probably anyone on this forum. The company has a nice website with more information.

Good luck with your quest.

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#5

Re: Transformer Problem

03/10/2017 10:15 PM

You need to get some expert like ABB, Siemens Westinghouse to recheck your transformer since they are expert in these field. Even maybe you are able to solve these insulation problem but it is also risky to switch the transformer on as there may be other undetected problem inside the transformer.

Dont take the risk as it is not worth it.

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