STRAY GASSING phenomenon in power transformers oil

[Vyacheslav Karol]

Hello everyone,
I work at a power plant and we have an issue regarding the oil of a main transformer.
The transformer is 18/161kV, 400MVA.
About two months ago, oil was purified in a transformer and it was returned to work. Recently we have seen an increase in the concentration of hydrogen in the oil up to about 200 ppm and the concentration continues to rise. There is also evidence of an increase in CO and CO2. It should be noted that there is no increase in concentrations of other flammable gases.
According to the conclusions of a chemical laboratory, this is a "STRAY GASSING" phenomenon in which oil itself distributes hydrogen when it is heated.
My questions:
1. How long will the increase in hydrogen concentrations continue until it stabilizes?
2. Will a decrease in the oil temperature cause a decrease in the hydrogen concentration? If so, where is the distributed hydrogen going? The transformer is sealed.
3. I have heard about "inhibitor" oil supplement that may reduce STRAY GASSING phenomenon. Does anyone have any information on this? Is it recommended? What is the recommended compound?

Thank you.

 

[Kiribanda]

What is your CO2/CO ratio? If this ratio is greater than seven that means lot of heat has generated
due to heat generation inside. Do you like to share a copy of the test report with the forum?

 

[Vyacheslav Karol]

 

[stevenal]

Inhibitor helps prevent oxidation and loss of dielectric strength. I’m not sure how it would affect gassing. But gassing itself is not the problem, it’s an indicator of heating within the transformer.

 

[prc]

Some clarifications- what is meant by purification.Understands filtered the oil in a vacuum filter machine. Or was it a reclamation (or regeneration) process?Did you top up with some new oil? What is the current load? Any overloading?Please post one DGA result prior to filtering.
Currently DGA values are not in alarm zone.

 

[cunin]

Looking at the DGA Results I believe you have stray gassing of oil at <200 Deg C. due to partial discharging. There is a high level of CO2 from high level of paper decomposition. Further investigation is strongly recommended.

 

[dga1960]

Your questions are very good and is a real situation appearing in many transformers in past 10 years. The gas concentration especially of hydrogen is quite high and you should be concern. They are few possibilities, stray gas indeed but they appear usually before oil treatment not after. The gases concentration with low methane can be stray gas and not be PD, but is not something that is 100% solid. Still PD is a possibility.
You are correct that additives may induce stay gas, and those are usually some of passivators used to suppress sulfur corrosion.
In case of stray gas, hydrogen can reach few hundred up to 800PPM, but if is PD it is not recommended to wait until there.
Are some ways to find out if is stray gas, first please provide other oil test results especially TAN delta and IFT. To be 100% sure that is a stray gas phenomenon you can send the oil to a specialize laboratory to test for it according to the oil type, ASTM or IEC. If you can provide the brand name of oil it is possible to recognize it is an oil type that may produce stray gas.
I have to emphasis that stray gas is still a controversial issue and some experts consider 200C a low temperature transformer failure that can become a non-pleasant phenomena.

 

[Corne Dames]

Hi, stray gassing has a specific pattern. With extremely high Hydrogen and Methane or Ethane concentrations.

Please note the following data on the link attached.

I think you have a thermal problem in the core the CO and CO2 involvement is stating a paper problem of high temperatures > 700 deg C