Study on the Lightning Strike Discharge Characteristics of Air Gap at Low Air Pressure Condition

The average altitude of Qinghai Tibet railway is 4000m. The correct and reasonable design of air gap of electrical insulation is the technical bottleneck to determine the clearance height of Qinghai Tibet railway tunnel and the design of overhead contact system insulation structure. However, it costs a lot to carry out the typical air gap true type test in the actual site with an average altitude of 4000m and above. Therefore, it is necessary to establish tunnel model by using the large-scale multi-functional artificial climate chamber of Chongqing University, to test and study the insulation characteristics of typical air gap under different applied voltages. According to the actual parameters of the tunnel, the stainless steel ceiling is built. Since the Golmud - Lhasa section of Qinghai Tibet railway is in single line operation, the vertical plane where the highest line of ceiling is located is taken as the measurement datum of catenary. The constant voltage rise and fall method and uniform voltage rise method recommended by IEC60507, GB / t4585, DL / t859 and other standards are used to test the breakdown characteristics of air gap. The 50% breakdown voltage under lightning impulse voltage is obtained by constant voltage rise and fall method, and the breakdown voltage under AC power supply is obtained by uniform voltage rise method. The altitude parameters are classified according to 500m, and the atmospheric parameters of 3500∼5500m are simulated. The altitude correction methods based on (d, P, h) and (d, $\delta$, h) are respectively used to fit the test data to obtain the lightning impulse discharge voltage correction formula of the air insulation gap. The test results show that the error between the calculation results and the test results is less than 5.0%, which means that the test and fitting meet the engineering requirements. The calculation results show that: (1) The lightning impulse voltage of the air gap at high altitude is a function of air pressure, temperature and absolute humidity. (2) Taking the gap distance of 1000m above sea level as the reference value (di), in order to maintain the same electrical strength, a correction factor should be added to check the gap distance (dH) at high altitude.