Transformer dissolved gases decreasing through time even without degassing 1

[chao_david]

Has anyone encountered this before?

This is a 333 kVA 7620/266 single phase transformer. What could be the reason for the decreasing trend? Not sure if its sampling procedure error because majority of the decreasing trend is consistent. I'm not a chemist but my guess that the gas ppm will greatly vary depending if the particular volume of oil you sampled has a huge chunk of gases. But the consistent decreasing trend is bothering me.

Electrical tests such as IR, TTR, Winding resistance, PI, excitation, power factor are ok.

 

[bacon4life]

Many service transformers have atmospheric air exchange, which leads to decreasing combustible gasses over time.

Typically gas concentration in the oil tank becomes evenly distributed within a period of hours. (Sampling procedures generally drain any oil trapped inside the sampling pipe)

The operating temperature of the transformer can impact the H2O concentration, but I am not sure if temperature impacts other gasses.

 

[prc]

Yes. Such a decrease in gases is not uncommon. Here there was a minor arcing somewhere inside- may be loose core earthing or any current carrying connection. The high level of acetylene indicates such a scenario. The arcing fused and might have disappeared. Hence, the fault is cleared, and there is no more gas generation. The gas level will gradually come down due to breathing out.