Characteristics and Mechanism of Line-Line Gap Breakdowns Induced by Wood Crib Fires

The 30 kVA/50 kV power frequency discharge system was used to trigger the trips (flashover processes) of Aluminum Conductor Steel Reinforced (ACSR) powerline segments (LGJ-400/35) in a 10.0 cm air gap under a wood crib fire (comprehensive fire source), and the characteristics and mechanisms of such line-line air gap (conductor gap) were discussed. The results show that under the wood crib fire, the breakdown voltage of conductor gap is obviously lower than that under pure air. Under the same fire source power, the breakdown voltages of different flame zones are ordered as: pure air (70.0 kV) > intermittent flame zone (top, 32.0 kV-52.4 kV) > continuous flame zone 2 (upper, 21.0 kV-28.0 kV) > continuous flame zone 1 (bottom, 14.4 kV-24.4 kV). In the same flame zone, the breakdown voltage decreases from 52.4 kV to 14.4 kV (25.1 %-79.4% reductions to those in pure air) with the increase of fire source powers (1-3 wood cribs, 121.87 kW-323.66 kW). For the single wood crib fire source, the maximum leakage currents (within the period before the conductor gap breakdown) in the flame intermittent zone, the flame continuous zones 1 and 2 are less than 1.00 mA and 2.54 mA, respectively. Many soot particles and ashes will be generated when wood crib and other ground surface vegetation fire sources burn. They will fill the gaps between power lines and be polarized by the in-service line inducing electric field. Then distorted electric field will be formed between the two polarization ends and the lines. The opposite movements and interactive convections of a large number of electrons, ions and smoke particles in fire plumes are driven to form a discharge streamer channel and cause line-line gap breakdown at proper moments.