Transformer preservation

[yiefang]

Hi..

we have a generator transformer 350MVA which not be used for a 3 years. The last Furan analysis carried out found out the DP value is around 350. The transformer winding is still under oil without keep nitrogen. The OLTC diverter switch were removed before and only covered by blanketing plate. I assume the transformer were exposed with the moisture. We have a problem and plan to use back this transformer. Is this possible to use bak this transformer, What test to be carried out in order to make good the transformer?

 

[prc]

Remember an idle transformrer under storage will absorb more moisture than a running transformer.First of all find out the wetness of insulation.Find out the moisture level in oil by Carl Fischer.Express the ppm as a % of the saturation moisture level corresponding to temperature of sampled oil.If this is more than10-20 %, it calls more a dry out by hot oil circulation cum vacuum cycles.Dry out the diverter switch of tap changer too.

After above, keep the trnasformer with conservator and silicagel breather.Maintain the breather regularly.Run the oil pumps once a month.

 

[protocolo]

Evacuate completely the oil, after that fill up dry N2 100% to 0,5 bar , after 24 hour carry out DEW POINT TEST.

 

[stevenal]

New paper is over 1000, end of life is at 200. Looks like your unit was getting fairly old when last tested, and has likely degraded since. This is a process that might be slowed by a dry-out, but never reversed. Might want to retest the oil, or perform electrical insulation testing prior to making the decision to put it into service.

 

[yiefang]

thanks for the answer. we already carried out the oil test and the result shows the ppm is 8.9 & dielectric strength >60KV. The IR shows acceptable value. The problem is, before this the winding only preserved with oil for around 90% portion only. is the winding still safe? Another thing, during vacuuming process, is necessary to do 1KV IR for every hour? Polarization index also need to do? Just heard from tx manufacturing, normally they don't do IR during vacuuming process due to it will cause insulation damage/broken.Is this true?

 

[stevenal]

I thought you said winding was under oil. If exposed, you certainly have less dielectric strength, and I would follow manufacturer recommendations. Depending on voltage used; yes it could cause damage. Did you do a recent furan test?

 

[protocolo]

I don´t advice that you could make process vacuum only to 10 % of active parts(pappers, isolaters), remenber that furanos test is for aging of papper, you need to makea good process vacuum to all active part,after that dew point test
finally fill up the oil under vacuum then Power factor test of insulating could be performed.

 

[prc]

yiefang,
1)Moisture of 8.9 ppm at what oil temperature? That is important.Taht value at 70 C temperature is good,but if it is at 5C, wetness is to be suspected.

2) Dont rely on PI for oil filled transformers.Probably winding tan delta in oil filled condition will give some indication.

3) Yes voltage shall not be applied under partial vacuum as it may cause insulation breakdown between winding parts.Taking IR during vacuum not necessary.

 

[yiefang]

To PRC:

Thanks for your answer. Just have a question, is necessary to carry out vacuuming until get below 3 torr & start counting for 36 hrs period. is there any calculation for a period of vacuum?

let say this is old transformer & not all gasket was replaced. What is the best vacuum value to apply?

 

[TurbineGen]

One way to decide how long to hold vacuum is a cold trap. Of course this assumes you have no leaks in the unit. If you are pulling vacuum on the unit, hooking up a cold trap is quite simple. You should continue to hold vacuum until the moisture absorbed by the trap is at an acceptable level for three hours, sampled every hour. We specify a dried unit to have less than 5 ml/hour in the trap.

Once you have done the cold trap, perform the dew point test. If that comes out acceptable, then I see no reason you couldn't refill the transformer, circulate with hot oil for one complete pass and energize it.

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

 

[stevenal]

I must say that my colleagues here are recommending time consuming and expensive processes for a transformer that may have already reached the end of its insulation life. Suggest a new furan test first to gauge whether life support is warranted.

 

[DTR2011]

OMICRON and Megger have products for determining the moisture content of paper insulation through direct electrical measurement. A technical presentation was given at 2009 Fall IEEE Transformer Committee meeting. The test takes about 3 hours (worst case).

I'd recommend you contact either and have a measurement made.

 

[prc]

yiefang-The extent of vacuum duration depends on wetness.If insulation is not wet, vacuum duration of 12-36 hours is sufficient.

As I mentioned earlier first find out the extent of wetness by simple moisture in oil measurement and find out the % moisture saturation.Based on that, decide the type of dry out and time for heating,vacuum,HOC etc.

It is true that you may run in to problem to get proper vacuum.Apply slight over pressure over conservator oil level,say 3 psi.Look for oil leaks and stop by tightening or mark up for gasket change.

Drain oil and start vacuum.Achieve the possible vacuum level ( A pump of more than 250 m3/hour will be required).Stop the pump.After one hour take the pressure reading inside the tank.Measure again after 30 minutes. The pressure rise be P mbar.Then leak rate is P/1800 X oil Volume in litres.This value,leak rate in mbarlitres/sec shall be less than 20.

 

[protocolo]

You could take the moisture copling a intermdiate filter like a device to capture the water between the transformer compartment and vacuum pump.