Thank you very much for your discusion. For our transformer we took another oil sample after one month and send to the laboratory in Slovenia.The rezults are:
The value of all gas are litle bit lower but they are stil to high. Does that mean we do not have any more fault (hight temperature source) in transformer.
And I am asking to you: This value of gas in Transformer oil, do they always will be present in oil if in transformer is no more fault.Do we must to change this oil (75 t)
A am sorry about my english but momental I work so hard to learn this beautiful language

Dear Gazi,
Best regards to lovely Pristina and Kossovo from Ankara, Turkey. I have been at the power station in Pristina and have seen those big transformers. Are you talking about them? If you send me your test results I can have a look at them. My email ibrahim.caglayan(at)vibratek.com.tr Gazi if you want you can write to me in turkish also.
Ibrahim

Gazi,
It is simple to obtain DGA results. But it is not simple finding the failure from the analysis of these results. It is my understunding that you are having dissolved gases levels well beyond tolerable limits except for CO. You're missing N2 and O2 values. I recommend the following:
1. Take more samples and send them for analysis (be careful, avoid contamination of samples: clean and rinse syringe with same oil, do not use syringe used in other transformers. Clean threads of sample point. Flush 1L of oil. Load syringe, eliminate bubbles form sample but not bubbles formed after sample. Avoid sunligh, send sample inmediately to Lab). If you feel uncomfortable taking samples, hire experienced people or Lab, but it is critical you take good samples.
2. As you receives the results send them to the manufacturer of the transformer or hire another manufatcurer of transformers for analysis of results of the DGA. They are the most experienced persons anlyzing failures. Chem Laba are only Chem Labs,they do not build transformers
3. Buy IEC 60599. You can do it on-line in WWW.IEC.CH and have it in PDF format quickly. REad it and make sure you understand it. Take your time. It is not so easy as it seems.
4. Once you understood IEC 60599 Check and challenge the analysis of your manufacturer. You should be convinced about the action plan. You'll probably have to take many samples and take the transformer out of service for internal inspection. Have patience it is not easy to determine the point of failure in these cases.

The main point is #2. Good manufacturers (and national or continental distribution companies also) have experienced people that have observed many patterns of failure in many transformers through their lives. But be advised, some times it takes weeks and outages until you find the failure. It requires patience, many samples and qualified people (and find them).
Catastrophic failure in Power transformers lead to elevated repair and operational costs and MTTR, so do not take the cheap option. Spend the money to hire the right individual or company to do the analysis

Regards
ClaudioR