Geometry impact on streamer propagation in transformer insulation liquids

Lightning impulse voltage breakdown tests were made in needle/plane, needle/sphere and U-type electrode systems and compared between ester fluids and mineral oil. Results show that the geometry of electrode has significant impact on both breakdown voltage and the velocity of streamer. The more divergent the electric field, the lower the breakdown voltage. The lowest breakdown voltage was found in needle/plane geometry having the most divergent field. In needle/sphere geometry the breakdown voltage was twice as high. In the U-type geometry the breakdown voltage was further increased. At breakdown voltage, streamer propagation velocity in needle/plane geometry was about 2 km/s and in the U-type geometry, the streamer velocity was about 10 km/s. Less difference was found between ester fluids and mineral oil in the U-type geometry for both streamer velocity and the values of breakdown voltage at an oil gap length of 35 mm, which is closer to the field distribution in a real transformer. The impact of electrode geometry dominated over the difference between liquids, which made the ester fluids and mineral oil more similar in the more homogeneous electric field of the U-type electrode system.