Research On Overvoltage Distribution And Protection Of High-voltage System Of EMUs

During the operation of raising and lowering pantograph and opening and closing circuit breaker of high-speed EMU,there will be electromagnetic transient process and operating overvoltage.Overvoltage will accelerate the aging of equipment insulation and even lead to insulation breakdown,which seriously threatens the safe operation of train traction power supply system.Due to the steep overvoltage waveform,the overvoltage amplitude and waveform are different at different positions when it is transmitted through the high-voltage cable of the EMU,resulting in different distribution of overvoltage on the transformer and other high-voltage equipment in each compartment of the EMU.Therefore,this dissertation studies the operation overvoltage distribution and its temporal and spatial evolution characteristics of EMU high-voltage system,focusing on the distribution and evolution of pantograph overvoltage and on-off circuit breaker overvoltage in high-voltage system,the electric field distribution in the side cabinet of EMU networking under overvoltage,and the influence of overvoltage protection device on overvoltage distribution characteristics.Firstly,the electromagnetic transient simulation model of the whole high-voltage system and the arc simulation model of the EMU is established by using PSCAD software.The distribution of the over-voltage of the pantograph and the opening and closing circuit breaker in the high-voltage system of the EMU is studied,and the influence of the arrester position on the over-voltage distribution characteristics is also studied.The results show that the distribution of overvoltage is mainly related to the steepness of overvoltage waveform and the length of cable.The waveform steepness of pantograph lowering overvoltage and circuit breaker opening overvoltage is small,so the overvoltage waveforms of cables are basically the same.In addition,in case of arc reignition during pantograph lowering,the overvoltage will increase with time and reach about 250 k V at 0.1s.The waveform steepness of pantograph raising overvoltage and circuit breaker closing overvoltage is large.For example,when raising the pantograph of vehicle 6,the overvoltage amplitude at the cable end of vehicle 2 is21.83 k V higher than that at the voltage transformer side;When the circuit breaker is closed,the overvoltage at the primary side of the transformer of 2 vehicles is 14.56 k V higher than that at the voltage transformer.The lightning arrester can protect against overvoltage.In case of pantograph lowering overvoltage and circuit breaker opening overvoltage,the protection effect of the lightning arrester has nothing to do with the distance.One lightning arrester can limit the overvoltage in the whole high-voltage system.Due to the low amplitude of pantograph raising overvoltage and circuit breaker closing overvoltage,other protective devices are required.Secondly,taking the overvoltage distribution obtained above as the input,the finite element simulation model of the side cabinet of EMU networking is built by using COMSOL Multiphysics software to simulate and study the electric field distribution in the side cabinet of EMU networking under the action of overvoltage.The results show that the highest field strength in the cabinet is near the top of the voltage transformer.When the pantograph raising overvoltage occurs,the highest field strength in the cabinet is 38 k V/cm.The maximum field strength is 58 k V/cm in case of pantograph dropping overvoltage.When installing lightning arrester,it can be limited to 38 k V/cm.In addition,the field strength at the bottom of lightning arrester will be distorted,up to23 k V/cm.In case of circuit breaker closing overvoltage,due to its low amplitude,the maximum field strength is 29.4k V/cm.Finally,several overvoltage protection schemes are simulated and analyzed.One is to set temporary resistance.This method can suppress pantograph raising overvoltage and circuit breaker closing overvoltage.The greater the resistance value,the better the suppression effect.Second,series inductance at the head end of the high-voltage cable.This method can only suppress the overvoltage at the voltage transformer,but it will prolong the overvoltage transient time,so it will not inhibit the overvoltage at the end of the cable.The third is the parallel resistance capacitance protector,which can reduce the overvoltage steepness and reduce the voltage difference caused by the cable.