Breakdown Characteristics of Transformer Liquids With Winding Conductor Segments Under Lightning Impulse

Historically, transformer insulation design is based on experience with mineral oil (MO). The basic insulation level (BIL) determines the lightning impulse withstand level (LIWL) for a specific design. These design values are not necessarily appliable to other insulating liquids than MO. There are few published studies on BIL using nonmineral-based liquids in design-relevant insulation geometries. The present study compares lightning impulse (LI) breakdown voltages (BDVs) of a synthetic ester (SE) liquid and a gas-to-liquid (GTL) oil with those of MO using winding conductors as electrode geometries. The field enhancement factors for the bare electrodes are below 4.1 in all cases indicating the quasi-uniform fields. The effects of liquid gap distance, paper wrapping, and impulse polarity are all considered. It is found that the LI BDVs of the alternative transformer liquids are comparable to those of the MO. The streamer propagation characteristics during the breakdown process are also captured. The results show that the breakdowns are led by the fast streamer under positive LI, and there is a streamer mode transition under negative LI. The BDVs are found to be practically the same for both polarities, and they barely differ among the three investigated transformer liquids. This is because the breakdowns are dominated by the streamer initiation under the present test electrode geometries and conditions.