for transformer 7.5MVA 22/6.6KV oil conservator type,it was installed since 2010 and filled with oil.didn't run since this time.

i have checked the function protection of transformer(oil and winding) and it was working well then we
have enrgized it last week and it was working well without any problem .is it necessary to check the oil properties of transformer?

Unless the transformer has been in a damp atmosphere, there is no obvious reason to test the oil. Self protection is the only other reason!

PI values should be checked. To be on the safer side oil properties should also to be checked.

It is advisable to take an oil sample and have it tested for dissolved gasses (DG). This allows you to build a history of the DG in the oil and track the changes over time and you can also compare the present DG analysis with the levels when the transformer was new to verify the oil is still nice and dry and no anomalies exist. For large power transformers, it is common practice take an oil sample and perform a DG analysis on a yearly basis.

By taking a DG test now, you are choosing to be proactive in the operation and maintenance of the transformer. In my opinion, there is a greater benefit to being proactive than reactive and it is a quality expected in engineering.

Do not be cheap, do all recommended tests .... Really, I do mo know why you would think otherwise.

Hi Amr
When testing the transformer oil, you need some initial reference to compare the results with. You have probably missed it, because this reference point should have been created when the transformer was just erected on site. The earlier you test the oil, the better your comparison will be.
Another thing: if there is moisture or sediment, or any other unwanted substances in the oil, the transformer will most probably work properly. at the beginning. But the armature will corrode, the windings will oxidate, the tank will corrode etc, so this transformer's end of life will come quicker than expected, and with a lot of fireworks.
Good luck.
Ilia

Moreover, I recommand to take the oil samples from top (for gases), from middle (for insulating characteristics) and from bottom (for sediments). Also, all results must be included in "transformer history file" for have and follow the trend.

You said the transformer has been idle since installation in 2010 for sure the silica gel must have saturated.So to be on a safer side and have a maintenance reference there is the the need to carry out all tests on the transformer oil

Remove that old oil and fill in new oil in this particular transformer after of course carrying out all necessary tests on it(the transformer) and use that oil in smaller equipment after verifying also that its strength to break down is ideal.we do not expect Load shading or unnecessary power cuts because of short cuts or assumption from men at work.

Transformers not used after conducting all tests and left for>six months need to conduct OIL DGA analysis,Insulation resistance between-Hv-E,HV-MV,MV-E and 10minutes PI R10/R1-with 5KV motoriized Megger (preferably BM 21/Dobble)with scale 1Tera Ohm.
The PI should be greater than 1.5 and if the DGA report of Oil is OK then the Trafo can be charged and this is as per many reputed OEMs guidelines and a basic Prudent Engineering Practice.

before energising the transformer oil should be tested and the result should be compared with the orginal value at the time of installation. Alsothe insulation resistance should be measured