Mechanism of Dissolved Gas in Oil 1

[PowerTrains]

I was trying to explain some things about DGA to someone and realized I couldn't. So I want to clarify a few aspects for my own understanding.

In the situation where gas accumulates in the Buchholz relay (not talking about trip) - is this "free/non-dissolved gas" appearing because the fault is generating it a rate the oil cannot absorb? Is it because the oil is saturated and cannot absorb any more? Is it because the fault itself is causing gas bubble formation?

What I am truly trying to understand is why certain faults show up in DGA and do not result in gas accumulation, while others (usually a small minority) do result in gas accumulation?

Thanks

 

[prc]

When the fault is incipient, and the gas generation rate is low, the oil will absorb the hydrocarbon gases generated. This is detected ( presence and quantity) by conducting a dissolved gas analysis. Get the gas from the oil sample by applying a vacuum and analysing it to find out what types of gases are there and their content in ppm. But when the gas generation rate is high, the oil will not be able to dissolve the gases; the bubbles will move up and collect at the tank top and then move to the Buchholz relay and give an alarm.

If the arc with the fault releases too much energy, it will vaporise the oil as the arc temperature may be above 2000C. The volume of gas generated is several fold more than liquid volume; it will create a pressure wave and the consequent oil surge in Buchholz pipe causes the relay to trip. In case the arc energy is too much ( from high fault current), the pressure wave will be intense and fast; it may cause bursting of tank, before moving to the tank top and to the relay. A tank rupture releases hot carbon gases to atmosphere. Oxygen in air start a fire with the hot, combustible gases released.

 

[RRaghunath]

Gas accumulation in Buchholz relay can result under two cases.
1)Oil is already saturated with gas and thus cannot hold any more gas that is evolving due to an incipient fault.
It can also be that the oil is saturated with air (due to open conservator and over a period of time etc.) and as the temperature of oil increases while loading, the air starts evolving out of the oil atht accumulates in the Buchholz relay.
2) There is an incipient fault in the transformer but the rate at which the gas evolves is much faster than that the oil can absorb, resulting in gas accumulation in the Buchholz relay. In this case, the oil may not yet be saturated with gas.

R Raghunath

 

[SilverfoxUK]

Instead of complicating things it might be better to think of a hermetically sealed transformer i.e. no conservator / Buchholtz.
All of the Tx internals including the tap changer are under oil. Our transformers have an easy life and get a dissolved gas analysis from an oil sample every few years. Gases have to come from a fault, a slight arc at the tap changer or from something degrading. The impurities in the oil will let you know about internal arc-ing, over heating damage, former starting to decay and general breakdown. Restoring the nitrogen blanket is a chew but puts it all right again.