HV CONNECTION TRANSFORMER FAILURE

[GENELEC]

Concerning a distribution power transformer (> 200 MVA) connected to the grid (220 kV), I am looking for similar failures in similar HV bushing connection transformers to try to compare with the type of failure we have had. And i'm not able to find information about transformers or failures in transformers with this type of connection.

The connection between the HV bushing and the HV cable is made in this way:

1) Oil to oil bushing
2) Link (short flexible cable between bushing and cable termination)
3) Cable termination
4) Oil Container (oil box?), independant for each HV phase, in which the previous elements are inmersed. Every oil container has an independant Buchholz protection and is breathing through the main conservator tank.

In the first energization of the transformer it was running properly for about two hours. A second connection activated temporarily one buchholz relay of the HV bushing containers(due to oil flow, not gas bubble). After checking buchholz and electrical protections, finding all them correct, the third energization generated an explosion in one of the HV bushing containers (one HV phase connection).

There was a ground failure between the end of the cable termination (cable supplier) and the oil container (transformer supplier). So we are trying to determine the potential cause and origin of the failure.

If you know an address or link where I can find connections and/or transformer failures of this type it would be helpful to me.

Thank you vey much in advance and kind regards.

 

[RRaghunath]

To me it seems a bad termination job that is responsible. Was hi-pot done on 220kV cables before first energisation!
Conducting tests on the surviving cables to check the quality of workmanship of the termination may help in the investigation of failure and should be included.

CIGRE conducts user survey on various electrical equipment from all over the world. May be you may find some useful info if you google "transformer failures". You may like to search "cable failures" as well.

 

[GamezBeCJ]

Hi GENELEC:

My opinion is:

1) I agree with rraghunath. It is more likely that the cable termination was the problem. If the oil container was the problem, it would mean that either the design clearances were wrong or the oil quality was very poor.

2) Also, I would have not energized after the bucholz trip, specially if it was due to oil flow. That means that an arcing falut was developing inside the container. You should have drained and inspected the container in question before another energization attempt.

I hope this helps.

 

[prc]

Normally with the type of construction described ,at the time of installation/commmissioning of unit the following test will be conducted.The cable connection and pot head assembly (is the cable PILC or XLPE ?)will be done .Then after disconnecting the link between trf bushing & pot head & after oil filling cable box ,Cable plus pot head will be tested by applying high voltage at the sending end.Normally this voltage will be more than trf test voltage and trf designer will be providing oil clearances to suit this voltage.If you had done such a test ,chances of pot head failure as informed can be eliminated.

 

[benlanz]

If you don't perform an off-line PD test on the cable termination to IEEE specs, there is no way to be confident that the termination was installed properly.

-Cheers

 

[electricpete]

ben - good advice.

If you or your company sells pd test services or equipment I think you should make that clear in your comments.

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[benlanz]

electricpete,

We do sell cable test equipment and services, but I make every effort not promote our company's business on forum sites. If anyone would like to look at my background, it is clearly listed on this site.

I do, however, give the advice on basis of IEEE 400 and its point documents (400.1 DC, 400.2 VLF, 400.3 PD).

IEEE 400
Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems
“If the cable system can be tested in the field to show that its partial discharge level is comparable with that obtained in the factory tests on the cable and accessories, it is the most convincing evidence that the cable system is in excellent condition”.

An off-line PD test happens to be the only test which fulfills this recommendation, thus my recommendation.

IEEE 48 (termination standard) requires that terminations be PD free to 1.3Uo (calibrated off-line test comparable to a factory test). If a newly built termination can be tested to 1.3Uo with no apparent PD, the cable owner can be confident that the stress relief is built properly. Of course the termination can still fail thermally if the spade connector is not crimped properly. This can be detected with PD after the materials blister due to heating during operation. A quicker way to detect this problem is to use an infared camera while the cable is under high current conditions.