Investigation on Gassing Behavior of Various Insulation Liquids in Power Transformers

Under normal operating conditions hermetically sealed power transformers undergo load cycles resulting in pressure and temperature variation. With decreasing pressure, the dissolved gases diffuse out of the insulation liquid and accumulate in gas phase. This gas leakage may be diagnosed as a fault in transformer by monitoring systems. However, it should be differentiated from a fault decomposing the insulation material. It is known that the conventional insulating liquid exhibit to some extent a different degassing behavior in comparison to the new alternatives. In addition to the erroneous fault indication, the gassing properties as well as the solubility of gasses influence the accuracy of the dissolved gas analysis. By determining the concentration of key gases and specific ratios among them, the type of fault can be classified. The gassing behavior of various liquids depends on many factors, such as pressure, temperature and of course the type of the insulation liquid. Therefore, the specification of the gassing and degassing behavior of new alternative insulating fluids is of critical importance. In this study, two different test vessels were built to determine the different gassing characteristics under laboratory conditions. On the one hand a simple arrangement with glass vessels was constructed to represent a kind of gas trap and on the other hand a scaled model of the transformer tank with a long diffusion way and a small contact to the gas phase above the liquid. The tests were performed at different pressure and temperature values by using a vacuum oven. To analyze the influence of these parameters on the gassing behavior, the pressure was kept at three different values: 800 mbar, 900 mbar and 950 mbar. Furthermore, the measurements were performed at various temperatures namely, 40 °C and 80 °C. A synthetic ester, an uninhibited mineral oil and a gas-to-liquid (GtL) oil were investigated. Before and after each test a dissolved gas analysis was performed. Moreover, the time, in which gas bubbles appeared, were registered.