Vacuuming of the power transformer

[yiefang]

Dear All,

We've a power transformer with capacity of ~ 400MVA. For the purpose of the repairing the transformer it cause the transformer were exposed with moisture for around 200 hours. Did anyone have a suggestion or opinion on the vacuuming process in order to remove the moisture inside the transformer. Due to exposed for a long period, we're plan to fill up the hot oil inside the main tank & carry out filtration for 24 hours & drain it back. After that will do vacuuming for a period of 36 hours. Due to our unit need to put in service ASAP, did anyone have a idea to remove the moisture for a tx which have been exposed for a long period. Need your help..tq

 

[rcw retired EE]

I'm not an expert, but here's my experience:

Pull and hold the vaccum on the tank before you fill with oil. That will help dry out the windings. Filling with oil not under vacuum will probalby result in the windings sucking the water out of the oil more than the hot oil dries the windings.

Winding insulation is very hygroscopic. It probably sucks up water better than the filter paper in your oil filters. Pulling a vaccum on the tank for 24 hours prior to filling will help dry the windings. Filling the transformer under vaccum will cause any mositure in the oil to evaporate as it enters the tank and get sucked out by the pumps.

You could also heat up the windings using a low voltage current from a welder to help drive out moisture.

 

[TurbineGen]

I completely agree with rcwilson. Put it on vacuum first. I would also cold trap the unit to see how much water you are pulling out. My suggestion would be to hold the vaccuum for as long as possible or until the cold trap doesn't show significant water collection (ie 5ccs or less per hour). Then after that go with your hot oil cleaning. You want 2 complete passes of your oil beofre your final fill.


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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[DTR2011]

The length of the vacuum is VERY dependent on the voltage level and the moisture content of the insulation system.

From an ABB Xfmr book I have handy, vacuum times

<= 69 kV = 12 hrs @ <0.13 kPa hours
>69<325 = 24 hrs
>=325kV 36 hrs

The old rule of thumb is to also add 1 hour for ever hour the unit (core and coils) are exposed.

I've always relied on a final dew point test to confirm that the unit is dry, prior to final fill.

I would guess that 400 MVA unit is a generator step up and time is money. I would strongly recommend contacting the manufacturer for their specific vacuum times. If the core and coils were exposed, I would always pull vacuum.

 

[prc]

First of all you should find out the extent of moisture in insulation.

Fill with dry air or nitrogen with a dew point of less than -55C at a positive pressure of 0.3kg/cm2.Check for leaking points and stop the same.Draw vacuum to a level of 1mm mercury.Fill tank with dry air.Keep for 24 hours.Check dew point inside tank. If it is less than -30C at ambient of 30 C ,you can say insulation is reasonably dry.If not, start the drying process as below.

Vacuum the tank and fill with filtered,dry oil.Do hot oil circulation at 70 C for 24 hours.Drain oil quickly and fill with dry air.Keep for 12 hours.Check dew point inside.Start vacuum and keep at 0.1 mm Hg for 24 hours.Fill with oil.Conduct HOC for 24 hours .

The above cycle may be repeated till you get appropriate dew point values.

Drying of cellulose requires heat and vacuum.Heating may be by low frequency current heating in winding,tank heating by induced current from a current carrying winding on the tank, hot oil circulation as explained above,Vapour phase heating etc.

Remember it is extremely difficult to remove moisture from oil impregnated paper.It takes long periods of drying.

 

[yiefang]

thank to all..

prc: Agreed with you..the purpose of the using hot oil circulation is for to release the moisture trap inside the winding..it will not release all the moisture..the vacuum process will not get out the moisture inside the winding..i am thinking after we energize the tx & put it in full load, the moisture will out..we need to monitor the water content closely..if the result is vary..maybe we need to do online filtration..

 

[rcw retired EE]

If you energize with wet insulation, you may have to buy a new trasnformer or at least a rewind.

 

[bacon4life]

After we heat up the windings with hot oil circulation, we also measure the winding temperature using the winding resistance. As the water boils off the winding will cool down and make the process of extracting water much less efficient. Thus we have some curves for various winding temperatures showing how much water per 12 hour period is acceptable, or whether it is time to reheat the core. We recently got a wet transformer that took 3 reheat cycles.

Online filters are extremely slow compared to vacuum drying. As long as you already have the equipment set up, take the time to get it all the way dry.

 

[prc]

Let us remember these facts- 99 % of total moisture inside a transformer will be in paper and only 1 % in oil.HOC will remove only this 1 % of water in oil.Dispersion of water from paper to oil is a very slow process and takes a long time. Heating insulation by HOC or other methods will speed up the movement of water from paper to oil as moisture saturation limit in oil goes up with temperature.Heating the winding by circulating current will have only limited effect,esp in large units as it heats up the thin conductor insulation only while major quantity of water will be in thick insulation parts away from conductor.
Dont try to energise transformer thinking that operating temperature will bring water from paper in to oil.Paper can carry water up to 8-10 % by its weight while oil can carry only up to 30-100 ppm depending on oil temperature.

 

[yiefang]

rcwilson: We don't energize the transformer in wet condition..what i mean is we do a hot circulation for a 24 hrs period..and drain it back..after that we'll vacuum it to 36 hrs period..than we'll do filling up oil tx in vacuum condition..after everything ok which moisture is below 10ppm and BDV is >60KV..we'll do the tx test..i.e FRA, IR, ratio, winding resistance..if everything in order..we'll proceed to energize..is the flow ok?

 

[prc]

yiefang, water ppm in oil may not give correct condition of insulation dryness.Soon after HOC, you will get very low ppm as all water from oil has been removed.But after a couple of days you will see ppm agin gone up as it take some time for moisture to migrate from paper to oil.Hence my above recommendation.

 

[DTR2011]

If anyone is interested, there are 2 products on the market that can measure the moisture content of the paper directly through electrical measurement.

OMICRON DIRANA and MEGGER PAX.

Depending on the %MC and temp, the measurement takes ~30-3 hr. Much more accurate than oil sample.