Oxygen and CO in GSU with no sign of overheating

[PowerGenJim]

I read the post about oxygen in transformer oil. I am assisting a client with a GSU issue that sounds similar, BUT different.
They have a >800MVA GSU with conservator, in service since late 2011. Looking at tests from 2015 thru last month August 2017, the unit has high CO (Condtion4 level of 1261 last month, CO2 at 4390, O2 at 14,000 and Nitrogen at 81,000. Moisture is excellent and steady at 0.1%by dry wt. Inhibitor levels steady at .25%. Unit has a continuous on line monitor that samples every 4hrs and agrees with lab results. Hydrogen varies from 15 to 80, Methane 3-18, Ethylene 3-5, and no ethane or acetylene and based on that I don’t have low or high energy arcing or PD at the top of my list as a cause.
Furnans are either not detected or <10ppb.

High CO is supposed to mean overheating but furans are zilch. Staff EE swears unit has no operating history of overheating. I know localized overheating can be masked when measuring bulk temp, and staff thinks they have a bladder leak in the conservator. That could explain high O2 and nitrogen, and you need oxygen to make CO and CO2.

I have asked them to plan to inspect all fittings, flanges, couplings etc in addition to inspecting the bladder leak for source of O2 and Nitrogen. Temperature degraded paper can also be a source but then where did N2 come from? What I can’t figure out is if air IS leaking into the unit, then I should see:
1) high nitrogen (which is high), TRUE
2) high moisture, which is not but could still increasing by absorbtion into the paper which gives a false assurance until paper is saturated but now FALSE
3) low inhibitor level as it should be consumed but has not changed even with high O2, FALSE.

So with some conflicting points, I recommend two paths-one, assume there is an O2 leak and look at all connections, flanges, fittings and bladder. And two, assume overheating is taking place and plan for possibility of localized overheating and do thorough IR from all four sides, and watch for gas generation, furans, and CO to go up and assume problem might require internal inspection

Any ideas?

 

[bacon4life]

Does the transformer have a dehydrating breather on the conservator? This could explain the lack of moisture increase for a bladder leak.

 

[PowerGenJim]

Thanks bacon4life. Yes it does, but the plants insurance inspector has reported in each of last two annual reports that the oil cup on the breather had oil level well below the normal line. I can't comment on the color of the desiccant (where color changes with saturation) at this time but have asked the plant if anything has changed since refilling the oil cup.

 

[prc]

There is nothing to worry as apart from high CO, there are no other thermal hydrocarbons present.

Latest IEC on DGA clause 5.5 of 60599-2015 states "The formation of CO2 and CO from oil-impregnated paper insulation increases rapidly with temperature. High values of CO (e.g., 1 000 ppm) and CO2/CO ratios less than 3 are generally considered as an indication of probable paper involvement in a fault, with possible carbonization, in the presence of other fault gases.
However, in some recent transformers of the closed-type or open (free breathing) transformers operating at constant load (i.e., with low breathing), CO can accumulate in the oil, leading to ratio CO2/CO < 3, without any irregularities or faults if no other gases such as
H2 or hydrocarbons are formed [7]."

The high N2 content looks strange. Normally the maximum solubility of O2 &N2 in oil are 32,000 ppm & 64,000 ppm and the ratio of 1:2 is the standard.