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 zig-zag 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 zig-zag disc type winding model, synthetic ester, due to its higher viscosity which caused lower inlet flow rate to develop under the specific tested retrofilling conditions, caused less uniform oil flow distribution within the pass and higher hot spot temperature.

Investigating shockwave characteristics of insulating liquids is helpful for understanding the streamer characteristics since shockwave phenomenon is closely related to the streamer propagation. In recent years, a Gas-to-Liquids (GTL) based transformer oil has been increasingly used in large power transformers. However, there is no study to quantitatively investigate the dynamic propagation characteristics of Mach-cone shaped shockwave in GTL oil. This paper presents an experimental study focusing on the spatial distribution of Mach-cone shaped shockwave in GTL oil obtained with a laser Schlieren system. The needle-plane electrode geometry was used with the applied voltage increased from 21.0 kV to 32.5 kV under positive lightning impulse. The Mach-cone shaped shockwaves were observed after the streamer initiation in GTL oil and then they dissipated quickly. The instantaneous streamer velocity deduced from Mach cone angle in GTL oil tends to stabilise towards 1.5 km/s.