Mineral oil is traditionally used in liquid immersed transformers to act as a coolant, an information carrier, and as an electrical insulator. Emerging alternative transformer liquids provide advantages, such as improved fire safety and better biodegradability, of which transformer operators would like to utilize. In this paper, an experimental study is conducted to compare the thermal performance of a mineral hydrocarbon transformer oil, a gas-to-liquid hydrocarbon transformer oil, and a synthetic ester transformer liquid as coolants in a zigzag disc type winding model. Comparisons are made under liquid directed cooling modes and under liquid natural cooling modes. It was found that under both cooling modes, the mineral based transformer oil and the gas-to-liquid based transformer oil behaved almost with comparable liquid flow and temperature distributions. Under liquid directed cooling modes, the synthetic ester gave more uniform flow distribution and delayed the occurrence of liquid reverse flow compared to the other oils. Under liquid natural cooling modes and using the zigzag disc type winding model, synthetic ester due to its higher viscosity caused lower inlet flow rate to develop under the specific tested retrofilling conditions which resulted in less uniform oil flow distribution within the pass and higher hot spot temperature.
This study investigated the partial discharge (PD) behaviour of a mineral oil Gemini X, a synthetic ester Midel 7131 and a Gas-To-Liquid (GTL) based synthetic hydrocarbon liquid Diala S4 ZX-I, and examined how their PD behaviour can be affected by moisture. Measurements of PD inception voltage (PDIV), PD magnitude represented by maximum apparent charge and PD number represented by pulse repetition rate of dry and moist liquids were compared. Results indicate that the PD magnitude of Midel 7131 is higher than the other two liquids at the same voltage. At voltages higher than PDIV, those liquids exhibit rather different pulse repetition rates. Midel 7131 has ahigher pulse repetition rate than the other two liquids at the same voltage. As a result of increasing the relative moisture content of the liquid samples to 50%, it has been observed that the pulse repetition rates of both Gemini X and Diala S4 ZX-I increase, however, in contrast, that ofMidel 7131 decreases.
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