low O2/N2 ratio in transformer oil

[kmkssanju]

We have recently noticed that there is high concentration of N2 dissolved in oil in free breathing TF while having low O2/N2 ratio. Please seen following test results. Is it indication of severe oil oxidation or oil contaminate during sampling?

 

[argotier]

I'm no expert on DGA and its hard to find answers with only one DGA analysis, gas trends in time are very important to take into account too.

That being said, IEC 60599 states that a low O2/N2 ratio (less than 0.3) could be considered as an indication of O2 excessive consumption.

This could have two main reasons: oil oxidation as you said or paper ageing.

CO, CO2 readings increased at the same time O2 was decreasing? That could be an indication of paper ageing.

Also you might want to check the past load of the transformer to confirm or not the paper ageing and the oil's inhibitor content (if applies) as an indication of the possible oxidation of the oil.

Hope it helps.

 

[prc]

1) Recheck measured value of O2.Hope it is not N2 sealed unit or with oil pump (forced oil cooled)
2) Normally in free breathing tarnsformers, O2 content will be about half of N2 content.
3) If the reported values are correct, there is a possibility of increased heating (a) any paper insulated lead wire (say to tap changers) or local heating at any current carrying contact areas (say tap-changer contacts). Can wait till next planned DGA measurement or internal inspection after oil draining(not urgent)

 

[Kiribanda]

kmkassanju,
From the results I would say that, typical ratio for O2/N2 is 0.5 for in-service transformers.
Your oil indicates a ratio of 0.11,
If it is less than 0.3, it indicates,
1) The transformer is absorbing outside air through the conservator (not through the bladder)
or through possible leaks in sealed equipment.
2) Consuming too much O2 due to age & degradation of paper insulation

The O2 inhibitor mixed in oil "ABSORBS" the O2 in oil and keep the O2 content low in oil.
A time may come that the inhibitor is saturated and cannot "ABSORB" any more O2.
Therefore, my advise is,
(a) Verify the current inhibitor level in oil
(b) Verify the current acid number
by a lab test and compare with the spec for new oil or in-service oil.
Hope this helps.

 

[dga1960]

Hi First the hydrogen seems to be quite elevated comparing to other gases , What is the transformer age and size? Before any significant conclusion, I suggest to send an oil sample to a laboratory for DGA tests. The portable device used for these measurement may be less accurate for those gases. Or replace the cartridge for air. Also I guess that you not have additional oil results from a laboratory, if you you send the oil to an specialize lab, also test IFT and inhibitor content if the oil is inhibited. because oxygen may affect the cellulose is recommended to test furan and/or methanol in oil.