Transformer refurbishment - implications

[babyG]

Hi,

I am trying to determine the implications of carrying out transformer refurbishments on transformers (only on decent transformers - for those with extremely low DPs we dont touch them). I would like to know if there is a general rule of thumb on how much longer a transformer's life will be prolonged after carrying out a refurbishment...

I am currently assuming 7-10 years for transformers that are xpected to die within the next 10 years.

 

[dpc]

What do you consider "refurbishment"?

 

[electricpete]

I guess I haven't heard of it and would like to know what you mean as well?

Are we talking dry or oil-filled transfomrers?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[Zogzog]

You should get a waranty with your refirbishment

 

[SphincterBoy]

We bought a used transformer w/tap changer seven years ago.

We had our contract written such that if any of the critical tests (partial discharge, impulse, etc.) were poor, the dealer had to untank and rewind the core.

Also: Make certain YOU are present to witness any tests. I would not put it past some of the rebuild shops to fudge test results in order to get your money. I know of one utility which bought several used transformers, which failed within one year after delivery. My gut instinct tells me the transformers had bad or marginal test results, which were not passed on to the customer. I know none of this particular utility's engineers/technicians were present to witness the final tests.

The particular unit we bought was small: 10MVA The initial tests showed the insulation was bad, and the refurb shop (per the specs) untanked the core and re-wound it. We were there to witness the Doble and impulse tests, which failed, even after rewind. The shop had to untank and rewind the core several times and repair/reinforce the insulation in order to pass the impulse tests. Our cost: $60,000. New cost: $325,000.

We also spec'd new fans (Krenz), new bushings, new gaskets, new LTC contacts, new oil, and new wiring (fan power, instrument leads, etc.).

The refurb shop we chose was Sunbelt Transformer in Temple, Texas. They were very good to us, and the transformer has shown no problems since we received it.

 

[babyG]

sorry should've been more specific..

No major refurbs - like rewinding of core etc, dont do that.

Just oil reclamation, remove moisture etc, refurbish the ancillary equipment etc.

wrt testing, our contractors are pretty trustworthy - abb or some other but I try to be on site to hang around anyways.

 

[SphincterBoy]

OK. Again, insist on being present for ALL testing to keep the refurb shop honest. Try to place a re-wind clause in the purchase contract if the unit fails the power factor test. Make certain the shop has in-house impulse testing. If they don't........stay away.

 

[waross]

I would like to suggest that whatever part of the life of the transformer has already been used is gone and cannot be reclaimed. The percentage of the life used will be dependant on the history of the transformer. If moisture and poor oil condition is causing accelerated ageing, then oil reclamation/drying may stop the accelerated ageing and give you the full expected remaining life.
Comments Gentlemen?
respectfully

 

[electricpete]

For awhile in a former life I worked on transformers with a very large utility T&D department. There was no periodic refurbishment program.

There was substantial condition monitoring. If a problem was seen in Doble testing or oil analysis etc, then we might do something - change bushings, clean oil, internal inspection etc.

The one possible exception I believe was type U bushings. There was a program to proactively replace all type U bushings regardless of Doble tests.

Other than that, we didn't go fix anything unless it was broke.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[jmmee97]

BabyG
You may want to consider refilling with natural ester oils. There was a good panel discussion on these at the IEEE PES T&D conference last month. The ester oil will drive out moisture from the paper prolonging its life. The biggest drawback I heard was that units ran hotter. Also, results from tests must be interpreted differently for these oils. This could be an interesting topic to discuss in this forum.

 

[SphincterBoy]

babyG, by natural ester oil, do you mean Cooper Environtemp?

From my understanding Environtemp is derived from soybeans, and supposedly it does indeed improve insulation life, but since it is somewhat new, I'm a bit skeptical as to it's claims of prolonging insulation life.

Cooper also claimed it was also "fireproof", but I have never seen an actual independent test to prove this.

Thanks for the info. regarding the dissolved gas analysis figures being different and higher operating temps., as these were not mentioned by the Cooper sales reps.

If you really want to increase insulation life and are willing to pay for it........NOMEX for insulation.

 

[jmmee97]

SphincterBoy,
Yes, the oil is produced from soybeans. I believe you will see other companies produce it as it becomes more accepted. I work for a distribution coop and we have started specifying it for our three phase pad units (not substation units yet).

One of the presenters at the conference showed results from accelerated tests on paper insulation. The paper tested using ester oil did age significantly slower than one tested using mineral oil.

As far as "fireproof", the flashpoint of ester is also about twice as high as mineral oil, but not fireproof. I don't know if Cooper puts additives in the oil that would further up this number.

As I stated in my other post, since this was not a rewind the ester oil could be a viable option to get more life out of the unit. It was also stated that there had been no reports of failures using this oil. I'm not saying everyone should start specifying ester, but it is an alternative to petroleum based oil

 

[stevenal]

Why is the transformer expected to die in ten years? What do the tests show? The life of a transformer is the life of the insulation. If oil reclamation can fix what the testing shows, then reclaim. SD Myers says a properly maintained transformer's lifespan should be 50 to 75 years.

 

[Zogzog]

Also make sure your oil is tested to the new ASTM 1275B standard, some new oils have been found to contain corrosive sulfurs which can lead to sudden failures. Also you might want an independent testing firm to conduct your testing, you can find one at

http://www.netaworld.org

Scott Peterson
Training Manager
Power Plus Engineering

http://www.epowerplus.com