strange DGA test
strange DGA test
(OP)
A client has recently obtained an DGA test on a transformer (which has not been tested for many years). Ethylene is high (115ppm), hydrogen was 2ppm and acetylene was 0.
I understand ethylene indicates an overheating condition. Hydrogen also is an indicator of overheating, and as I understand, is formed starting at a lower temperature than ethylene. Another clue is the CO2/CO ratio is about 5, which I think indicates oxidizing cellulose. I plan to have it re-tested, but can anyone suggest a reason we would have ethylene but little hydrogen in the oil?
I understand ethylene indicates an overheating condition. Hydrogen also is an indicator of overheating, and as I understand, is formed starting at a lower temperature than ethylene. Another clue is the CO2/CO ratio is about 5, which I think indicates oxidizing cellulose. I plan to have it re-tested, but can anyone suggest a reason we would have ethylene but little hydrogen in the oil?






RE: strange DGA test
One thing to consider is gas distribution and movement in the insulation itself and in the gas space.
Hydrogen is a lightest gas and least soluble in oil (lowest Ostwald coefficient) and therefor equilibrium concentration forces more of it into gas space. Anoter aspect H2 moves quicker into where the other gases take longer to move into gas space. So some scenario's:
1 - If the transformer does a lot of breathing due to thermal cycling, the high H2 removal rate from gas space drags down the amount in the oil
2 - if the gas was recently produced the H2 may have moved into the space and C2H4 not yet.
Another thing I have seen is that if a transformer with high concentration of combustibles is drained, vacuum processed and refilled, I have seen that the heavier gases can come back, apparently coming out of what was left in the oil soaked insulation during processing, but H2 is more mobile and completely sucked out by the vacuum process and doesn't come back.
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RE: strange DGA test