Long Time Storage Of Transformers 1

[Kiribanda]

Dear members,

We have a situation where, an old 60/ 80 MVA, 230/ 13.8 kV oil immersed power transformer has to kept out of site in long time storage for at least 2-3 years due to some site conditions. Could anybody help me to find out an IEEE/ CIGRE technical paper, IEEE guide or a manufacturer’s general guide line or some other relevant document in this type of transformer preservation method (long time storage).

I had scanned the IEEE digital library and the GE web site. But no luck.

Thanks in advance!

Kiribanda

 

[busbar]

Power-factor {Doble} testing would probably give the most information for a single test. Then DGA?

 

[alehman]

The tests suggested by busbar should be conducted when the transformer is taken out of service and repeated when preparing to restore it to service. Results can be compared to determine if there has been significant change.

 

[electricpete]

I developed long-term storage strategy for 3 different generator stepup transformer spares.

I am pretty sure there is no IEEE guide but I did have OEM recommendations from Westinghouse, McGraw Edison and Schmitt. Sorry I don't have them anymore except a little bit in my head.

There are a lot of decisions to be made. One big question which may dictate some of the answers is: How fast does the transformer need to be available when needed. At any rate the PM strategy should be integrated with the strategy for transportation, restoration/testing and putting the unit back in service.

Store with oil or without oil? Without oil there should be a nitrogen bottle/regulator hooked up with pressure indication either alarmed or verified periodically. With oil you still need nitrogen blanket and also as I remember the recommendation from McGraw Edison and Schmitt was to run the pumps for 30 minutes every three months and draw an oil sample at the end of that run. I recall there was some discussion about stratification etc. It seems to me like a lot of work and as you know OEM's are notoriously conservative on this type of thing. Doble test once per two years. If you end up with parts missing for various reasons your options are limited. No bushings (borrowed one for use in another transformer in our case) or no oil => no doble test.

As I said you really need to think through the sequence that will be used when the transformer is to be placed in service. In almost all cases transformer needs to be drained for moving. In some cases bushings and radiators etc need to be removed for moving. Depending on what you have been through during storage and moving you may or may not require a long vacuum processing upon reinstallation. If you have time for complete vacuum processing upon reinstallation and you are resigned it will be necessary you might slack off a little on the PM.

Reinstallation testing will needless to say include leak checks, vacuum processing, checking out all the accessories... cooling fans cycle at proper simulated temperature etc. Again if you don't have time for long testing upon installation you may pay more attention to these items during storage.

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

 

[electricpete]

I should clarify - for our purposes (long term storage 10 years or more), none of the three manufacturers recommended storing without oil.

But it always seemed to me that we could avoid the cost/effort of the periodic running of pumps and sample if we stored it with nitrogen and did periodic dew-point tests. I'm not sure what's wrong with this idea but none of the OEM's supported it for these transformers (138kv and 345kv gsu's).

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

 

[prc]

As electricpete said the maintenance activities on the stored transformer depends on how fast you want to switch on the standby unit or spare unit after the storage period.
However it is not advisable or safe to store trfs or spare windings in gas filled condition .You should fill with oil at the earliest but in no case transformer shall be kept in gas filled condition for more than 6-9 months.The reason is air with moisture can seep inside through the gaskets etc .

I remember a case from Japan where a EHV trf failed soon after commissioning and the reason was analysed to be the long storage of trf in gas filled condition for two years.
When trf is kept in oil filled condition ,care shall be taken as moisture ingress will be more in such case than in a trf in energised condition.

To prevent or reduce the moisture ingress ,the following are some of the methods used

Air cell in conservator in addition to silical gel breather as back up.

Oil heater in the conservator

Use a small distribution trf to circulate 25- 50% of full load current in Hv winding with Lv shorted so that oil and winding will remain warm -Disadvantage load loss.

Keep the trf in energised condition with secondary open and cooler shut condition - Disadvantage:low Pf,loss, need for a HV breaker .But this is some times adopted where a single phase trf from a 3 phase bank is kept as idle standby .

Monitoring regime and frequency depends on the size/importance of trf ,time available for overhauling in case of a pull out etc and it has to be drawn out in individual cases . IR ,power factor and oil evaluation are the common tests called for.
I think oil pump operation was suggested only to get uniformity in oil mass before oil sampling and it may not help improvement in trf condition.Outside US ,most of the trfs are cooled with out oil forced cooling ie with out an pump.

 

[tinfoil]

prc:

I am a bit confused with your post.

Anytime I have heard of filling a trf with inert gas (for shipping or storage), a positive pressure is always maintained (with low-pressure alarms on the guage if storage is going ot be long enough to ensure continuity of +ve pressure).

How does air and mositure ingress through gaskets or whatever against a higher pressure? I must be misunderstanding what you are saying.

 

[prc]

We must understand that size and construction of a large transformer is not that of a GIS .Even with GIS ,with double o-rings etc ,%loss of SF-6 per annum with 5-6 bar gas is not insignificant.It is true that transformers are transported and stored with dry gas at a pressure of 0.1-0.2 bar pressure.But with long storage, chances of pressure drop /breathing through gaskets etc cannot be ruled out and a wet transformer is the most difficult one to handle.I donot know whether long storage of oil impregnated paper in gas will affect the insulation properties.But trf manufacturers always try to avoid such a situation and recommend to store under oil filled condition.