Effects of electric field homogeneity and voltage polarity on prebreakdown and breakdown behaviors in ester liquid and mineral oil

Streamer propagation and breakdown behavior under lightning impulse voltage were studied in point-plane and sphere-sphere electrode geometries. In case of breakdown streamer velocity in semi-uniform field behaved similarly with esters under positive and negative polarity, whereas in mineral oil streamers were slower under positive polarity. This had not been observed in divergent field. Pre-breakdown processes were characterized by recording streamer propagation currents and light emissions. Understanding the breakdown behavior and pre-breakdown processes is essential for assessing the suitability of insulating fluids in high voltage transformers. It was shown that mechanisms explaining pre-breakdown processes in strongly divergent fields in mineral oil are not easily transferable to semi-uniform fields and ester liquids. Varying molecular structure and electric field distribution between the electrodes acting on the molecules must be taken into account to describe breakdown processes in realistic geometries.