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Kuncoro
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Join Date: Aug 2008
Posts: 31

Power Transformer Probem

08/26/2008 1:28 AM

Dear all,

We have some problem about my transformer with capacity 28MVA.Voltage 115kV/13.8kV The result of DGA is very high, several taps of tap changer the deviation of the result based on TTR are more than 0.5%. Poor insulation at LV but PI results of HV-LV is good, HV-Grnd is good and LV-Grnd is bad. The result of core bushing insulation resistance is bad too equal to 42kΩ.

My question are

1. What is relationship between DGA and core bushing

2. What will be taken action by us regard to this condition.

Iwill look forward your immediately response

Kuncoro
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Anonymous Poster
#1

Re: Power Transformer Probem

08/26/2008 8:38 AM

Sounds like a short in the LV windings, but could be bushing etc.. I recommend doing a powerfactor test, using Doble or similar..

The transformer will need repairs either way..
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Kuncoro
Associate

Join Date: Aug 2008
Posts: 31
#3
In reply to #1

Re: Power Transformer Probem

08/26/2008 7:52 PM

Thank you for your prompt response

We have performed tan delta test by doble, SFRA test and those result is good.

Thanks,

Kuncoro
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Sparkstation
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#5
In reply to #3

Re: Power Transformer Probem

08/26/2008 9:52 PM

Hello again Kuncoro

That seems to prove the LV Winding is OK.

The LV to Earth resistance is either a faulty bushing, or in the tap-changer, if internally connected.

Dependent on Make/Model of tapchanger, some used phenolic-based insulation in the on-load tapchanger, and it may show tracking.

Advise your further progress, with

Kind Regards....

__________________
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Sparkstation
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#2

Re: Power Transformer Probem

08/26/2008 7:32 PM

Hello Kuncoro

"Guest" above is correct.

1. The high DGA analysis shows probable damaged insulation.

2. LV to Ground Insulation test being low, either the LV bushing/s has/have failed, or the LV winding is damaged.

The fault may not be established without removing the transformer from service, tank draining, disconnect LV Bushings, then redo the LV to Ground Insulation test.

If the LV winding to ground then tests OK, the problem should be in a bushing, and it is quick and easy to establish which bushing, replace it.

Bushings sometimes have hairline crack/s which show up much later, after transformer has been in service a year or more.

The 42kΩ test is extremely low.

From your description, the transformer should be removed from service and repaired as soon as possible, before further damage occurs.

Advise your progress here, with

Kind Regards....

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Kuncoro
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Join Date: Aug 2008
Posts: 31
#4
In reply to #2

Re: Power Transformer Probem

08/26/2008 9:49 PM

Thank you for your prompt response.

My question is If the LV winding to ground then tests OK why the problem should be in a bushing? What is the corelation of both cases?

Regards,

-kg
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Sparkstation
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#6
In reply to #4

Re: Power Transformer Probem

08/26/2008 9:54 PM

Hello Kuncoro

Your original post stated that Insulation test of: LV to Earth was 42kΩ, and that is far too low for safety.

Unless there is misunderstanding re that figure.

Kind Regards....

__________________
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Anonymous Poster
#7
In reply to #2

Re: Power Transformer Probem

09/02/2008 1:51 AM

Hi,

Tan delta to all bushing will give us an idea about hairline crack in the Bushing. Replace the bushing.

Also, Polaristion Index HV to N, LV - N, HV-LV will give the Insulation Strength of the Transformer. PI of 1.5 between any Winding shows transformer is in better condition.

Regards,

N.Suresh
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Kuncoro
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Join Date: Aug 2008
Posts: 31
#8
In reply to #7

Re: Power Transformer Probem

09/02/2008 6:27 PM

Hello Suresh,

No hairline crack in the bushing, tan delta testing shown a good result. PI shown a good result too. But only LV - Grnd shown 0.99. I was visual inspect inside the main tank but I didn't find electrical damage or mechanical damage. 2 measurements shown bad result dfr and insulation resistance LV to grnd.

Should you have any comment please advice

Thanks,

Kuncoro
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