Effects of electrode shape on the gap breakdown characteristics in propane injection flames

High-voltage transmission wires, a key part of electrical transmission systems, can cause serious fires. There have been few studies of the effect of electrode shapes on gap breakdown around the transmission wires. This study analyzed the effect of electrode shapes on gap breakdown with ignition conditions in terms of the breakdown voltage, breakdown polarity and breakdown arc characteristics. The tests used a high voltage power frequency discharge system (type 30 kV·A/50 kV), propane injection as a simulated fire source and combinations of rod or plate electrodes. For gaps less than 8.0 cm with flames present, the breakdown waveform had a discharge induction stage, a gap breakdown point, an arc conduction stage and a breakdown vanishing point. The average breakdown voltages and the average breakdown field strengths of the various electrode combinations were measured by setting various gap sizes. The rod-rod electrode had the slowest increase of the breakdown voltage with increasing gap size while the rod (negative)-plate (positive) electrode had the fastest increase. The air gap breakdown had polarity characteristics due to the flame flow field and buoyancy. The different electrode shapes affected the probability of forming conductive channels with positive and negative periods. When the breakdown was during a positive period, the combination of negative rod and positive plate electrodes had the highest breakdown probability, while when the breakdown was during a negative period, the combination of positive rod and negative plate electrodes had the highest breakdown probability. The results show that the evolution of the breakdown arc can be divided into the conduction stage, the combustion stage and the extinction stage. A flame significantly increases the arc volume and the upward drift. The flame flow field and the buoyancy both change the electrode discharge position. This research is important for forest fire prevention and safe operation of high voltage power grids in mountainous regions.