Gas Trapped in Distribution Transformers

Distribution transformers are generally sealed tank with gas space these days as opposed to the previous generation of conservator type transformers.

The oil filling of tanks of these transformers is made in open air, usually with a simple hose in the transformer. This type of oil filling results in trapping air in the oil, the windings and the core assembly and structure. Trapped air consists of Nitrogen, Oxygen and a small quantity of Carbon Dioxide.

This does not happen in high voltage power transformers since they are filled under vacuum and they go through an extensive degassing process.

The trapped air in distribution transformers may vary greatly from one transformer to the other and with time. The trapped air will migrate at different speed to the gas space.

During the transformer test, discharges might happen between the winding and the tank due to poor workmanship or insulation failure. In these cases different hydro-carbon gases are generated and added to the gas content due to the spark in the oil.

After the required retrofit action on the failed insulation, the gas content of the oil is measured and analyzed by experts to verify the safe operation of the transformer under the tests and normal operation of the transformer.

Based on the above-mentioned oil filling procedure, one should not be concerned by Nitrogen and Oxygen in analysis of dissolved gas.

In the test result of the gas content of these transformers, five gases that are more of concern are: Hydrogen, Methane, Ethane,Ethylene and Acetylene. Also, the ratios that shall be taken into account more seriously are: CH4/H2, C2H2/C2H4, C2H2/CH4, C2H6/C2H2 and C2H4/C2H6.