Impulse Discharge Characteristics And Voltage Correction Of Tunnel-Catenary Air Gap At High Altitude

With the introduction of the 13th five year plan for railway development,more and more electrified railways will be built in the high-altitude areas in the western part of China.Located in the first step of the motherland,the Qinghai-Tibet Plateau is crisscrossed with several mountains,which inevitably requires tunneling.However,the cost of tunneling in high altitude area is much higher than that in plain area,so it is of great economic value to determine the reasonable tunnel clearance distance.At present,the research on the discharge characteristics of air at high altitude is mostly based on the rod-plate gap,but the research on the discharge characteristics of catenary in tunnel is less.Therefore,it is of great engineering value and theoretical significance to carry out the simulated high altitude test with the tunnel-catenary gap structure which is more practical.In this paper,the tunnel-catenary gap is built in the artificial climate chamber of Chongqing University.By simulating the high altitude（low air pressure）conditions,the impulse voltage breakdown characteristics of 420mm,500mm and 600mm gaps at the altitude of 243m,1000m,2000m,3000m and 4000m are studied.The discharge characteristics of rod-plate gap and tunnel-catenary gap are compared.The relationship between 50%breakdown voltage and gap length,air pressure and humidity of the tunnel-catenary gap is discussed.The 50%breakdown voltage correction formula is proposed and the correction coefficient at different altitudes is obtained.The results are as follows:（1）With the decrease of air pressure,the lightning and switching impulse 50%breakdown voltage of tunnel-catenary gap will be reduced,and the 50%breakdown voltage is a power function with the relative air pressure.The corresponding air pressure characteristic index of positive polarity lightning breakdown voltage of different length gap is 0.847～0.877,the corresponding air pressure characteristic index of negative polarity lightning breakdown voltage is 0.883～0.954,and the corresponding air pressure characteristic index of positive polarity switching breakdown voltage is0.563-0.691,and the corresponding pressure characteristic index of negative polarity switching breakdown voltage is 0.684-1.001.The breakdown voltage of the gap is approximately linear with the gap length.（2）With the increase of absolute humidity,the breakdown time of lightning and switching impulse decreases,and the 50%breakdown voltage of lightning and switching impulse increases.Under normal pressure,for every 1g/m³ increase of absolute humidity,the positive polarity lightning impulse breakdown voltage increases by about 0.71%;the negative polarity lightning impulse breakdown voltage increases by about 0.30%;the positive polarity switching impulse breakdown voltage increases by about 0.081%;and the negative polarity switching impulse breakdown voltage increases by about 0.73%.（3）The relationship between the lightning and switching impulse U₅₀ of the tunnel-catenary gap and the gap length d,relative air pressure（P/P₀）and absolute humidity h can be expressed as U₅₀=a×d^b×（P/P₀+ch）^d.The coefficientsa、b、c、d are0.792,0.929,0.0081 and 0.923 when positive polarity lightning voltage is applied,4.65,0.745,0.00317 and 0.950 when negative polarity lightning voltage is applied,1.45,0.836,0.00125 and 0.630 when positive polarity switching voltage is applied,2.65,0.805,0.00813 and 0.946 when negative polarity switching voltage is applied.