1. Solubility of gases in transformer oil

Air and various gases in contact with the oil may become dissolved in it, sometimes in significant quantities. In a transformer equipped with expansion tank, the rate of gas absorption by the oil depends to a large degree on transformer design. The rate is low, but not negligibly so, considering transformer life time.

Aging rate:
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The effect of moisture content in the paper, W, on the rate of thermal aging. 1 is the aging rate of dried and oil saturated paper with residual moisture content of 0.3% (e.g. paper with moisture content of 1% ages 6 times faster than paper with 0.3% moisture content).

In transformers where oil contact gas (air in transformers with silica gel dryer or nitrogen in transformers with nitrogen layer), the oil is gradually saturated with gas. Different gases have different solubility in oil. The following is the solubility of gases in transformer oil at 25 °C and 736 mm. hg.

Gas Volume, %

Air 10,0

Pure nitrogen 8,56

Pure oxygen 15,92

Air nitrogen 6,98

Air oxygen 3,02

Carbon oxide 9,0

Carbon dioxide 120,0

There is a direct correlation between the quantity of solved gas and its stabilized pressure over the surface of the oil. Solubility is directly proportional to temperature. However, the above data may not be correct for all oils, due to varying surface tension at the border between gas and oil. Different solubility of various gases in oil may cause the composition of gases in the oil to be very different from the gases above the oil.

2. Reduction of dielectric strength of the oil containing solved gas.

Data regarding dielectric strength of the oil containing gas is scarce. It is considered, however, that the effect of solved gas below saturation value is relatively low.

Some data indicate that gases in oil decrease its dielectric strength by as much as 20 – 30% if no bubbles are formed. This phenomenon manifests itself near the state of saturation as a result of temperature change or due to other reasons, such as the influence of strong electric field or very high flow rate in cooler pipelines.

Regular methods of transformer oil drying ensure degassing to the point when the oil only contains trace amounts of gas. Its content is difficult to measure. However, this oil may absorb gas during transformer operation unless it is separated from the air by a protective membrane in the expansion tank or is covered with a layer of inert gas insoluble in the oil.

Besides, gas may be emitted from oil as it travels from areas of high static pressure to areas of low static pressure and from areas of high temperature to areas of low temperature.

Accelerated agitation of oil with pumps and cavitation may also cause gas emission.

Load fluctuations and changes of ambient pressure and temperature have a more pronounced effect, more so if the fluctuations are rapid. The gas initially forms micro-bubbles with flow upward. Similar process occurs when large transformers are powered down, causing rapid oil cooling. Therefore, average values are not enough when considering gas (and moisture) content in insulation only.