150 MVA POWER TRANSFORMER

[afzumannu]

Dear technical friends,

WE HAVE TRANSFORMER 132/66 KV 150 MVA WITH OFAF AND 75 MVA WITH ONAN. AND OUR TOTAL LOAD IS EXCEEDING ITS LIMIT AND GOING BEYOND ITS RATING 150 MVA. MY QUESTION IS HOW WE CAN INCREASE TRANSFORMER CAPACITY IN EASY WAY EITHER BY COOLING SYSTEM OR ANY OTHER METHOD? BECAUSE WE NEED 20 MW EXTRA LOAD, IT IS ALREADY CROSSING >140MW. THANX IN ADVANCE... IS IT POSSIBLE?

 

[bacon4life]

Unifin offer external transformer coolers and pumps though I have never had occasion to use them.

Writing in all caps is usually interpreted as shouting, so you may not get a whole lot of responses. The forum is very understanding if English is not your native language but expects native English speakers to use proper of grammar and punctuation.

 

[davidbeach]

I agree with what bacon4life said about all caps. Redo the question in normal typing and I'll even read the question. Life's too short to wade through all caps.

 

[electricpete]

External cooler and pump is a good option.

A quicker, easier option which is probably not as effective is to being in supplementary cooling fans in various locations to try to supplement/assist the existing transformer.

I have also heard mixed results from using water spray on existing coolers or radiators. It has the potential to dramatically increase the existing cooler or radiator efficiency assuming the environment is dry (evaporative cooling). It also has the potential to foul the coolers and decrease their heat transfer capability in the long term (demin water would be better to address this concern). I guess you should also pay attention to the spray to make sure it's not spraying towards areas where flashover can occur.

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(2B)+(2B)' ?

 

[electricpete]

This sentence was bad enough before the typo... let me fix the typo:
A quicker, easier option which is probably not as effective is to being in supplementary cooling fans in various locations to try to supplement/assist the existing transformer.
should've been
A quicker, easier option which is probably not as effective is to bring in supplementary cooling fans in various locations to try to supplement/assist the existing transformer.

=====================================
(2B)+(2B)' ?

 

[dpc]

The most important variable is the temperature of the windings. If you can measure that, then you are in a position to load the transformer based on the actual winding temp and not the MVA.

If you don't have some approximation or simulation of winding temp available, the next best option is the oil temperature, which is almost certainly monitored via at least a thermometer on a 150 MVA unit. The transformer MVA rating is based on an assumption regarding the ambient temperature. This is generally stated on the nameplate. For an ANSI transformer it is based on an AVERAGE daily max temp of 30 deg C. If the average is higher, the transformer must be de-rated. If the ambient is lower, there MAY be some additional capacity in the transformer while the ambient is low.

I agree with the other suggestions regarding external cooling - this may provide some help in increasing the rating.

If this is a long-term situation, you really need to think about getting another transformer.

David Castor

http://www.cvoes.com

 

[electricpete]

good point dpc. I assumed he was approaching some temperature he considered limiting and therefore brought the question to the forum...probably a bad assumption from re-reading the op.

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(2B)+(2B)' ?

 

[ScottyUK]

As you already have forced oil circulation, liquid cooling by heat exchanger and cooling tower will offer good thermal efficiency, especially if you already have a cooling water circuit for a turbine condenser. Not much use if you don't have a ready supply of water though!

ePete's suggestion of demin water is good for cleanliness but demin is aggressive to many materials such as steel, concrete, etc. That's why concrete near steam traps gets eaten away, and demin pipework is stainless steel.


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If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

You guys all seem to be ignoring saturation. When do you stop ignoring it, with BIG transformers?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

It shouldn't saturate at rated voltage. Too much load is just a thermal problem.

 

[dpc]

Another point is that any external cooling added to increase transformer capacity would almost certainly void your transformer warranty, if any still exists, unless the manufacturer signs off on it.






David Castor

http://www.cvoes.com

 

[bacon4life]

The IEEE loading guide C57.91 discusses a number of other factors to consider in addition to saturation of the tank, including gas evolution, reduced mechanical strength, pressure buildup in bushings, increased wear to tap changers, and auxiliary equipment such as current transformers.

David, I think Keith was referring to saturation of the tank and other iron parts do to increased leakage flux, not saturation of the core from over voltage. Seeing the tank heating from leakage flux during a transformer heat run test was very enlightening.

If your load is cyclic, the loading guide would also help you determine the equivalent constant load.

 

[gerbii]

Hello

As mentioned before, the problem is strictly thermal.
To be honest, it all depends on type of transformer (construction features) which affect temperature distribution in windings -> hot spots really can be a nightmare. 30% more losses - it's a serious problem.

On the other hand, I know a unit which have rated power around 120 MVA but can withstand 160 MVA for quite long time.. or even without OFOF (in ONAN mode) for short time (all stated in warranty). But it's new unit, fresh design.

So.. ~13 % increase of power.. it's not so easy, I wouldn't be doing that to be honest

"If an experiment works, something has gone wrong"

 

[desrod]

Ambient operating temperature, load cycle, PF and expected overloading duration are other parameters that should be considered.

Up north (in Canada) we accepted operating transformers @ 135% MVA in peak contingencies situations (duration 4 hours).

We even accepted a 10% overload on industrial transformers in winter (steady load).

Cap banks on 66 kV might give you some trade-in between MVA and MW

 

[xj25]

Just to clearify for not-very-transformer-experts thread readers like ItSmoked and me:

Initially I was thinking about saturation problems due to overload, just as ItSmoked stated.

Eventually after DavidBeach reply I remembered that saturation depends just in magnetizing current (that depends at first order with input voltage).

Input voltage gives you a flux (Faraday´s law).

Flux needs some magnetizing current depending on core characteristics(but not depending at first order in load current).

Load current just "passes by" with no effect in core saturation, but so in thermal load.

Regards