Study On The Characteristics And Influencing Mechanism Of Switching Overvoltage For High-speed EMU

In recent years,with the rapid development of China’s high-speed railway,the operation safety issues of the EMU have attracted a wide range of scholar’s attention.The train will operate circuit breaker frequently in the process of neutral-section passing,pantograph rising and lowering,and short circuit ocuuring.Inductors and capacitors in the loop of traction power supply system are energy storing element.When the circuit breaker operates,the magnetic energy stored in the inductor will transfer to the capacitor in the form of an electrtic field at an instant,resulting in an switching overvoltage.The whole traction power supply loop is in working condition as the circuit breaker operating for high-speed EMU.The high voltage equipment on the roof of train,which has a direct or indirect electrical connection with the circuit breaker,will suffer the impact of switching overvoltage.Thereby,it can cause a series of problems,such as the insulation aging,insulation breakdown,even insulator flashover and so on.Not only that,the overvlotage coupled to the car-body can impact on the normal work of the control and communications electronic equipment,such as speed sensors,smoke alarms,that even leads to major security incidents.Based on the traction power supply loop of ’traction supply substation-catenary-EMU’ the generation mechanism of circuit breaker switching overvoltage is introduced.A simulation model of the switching overvoltage is established by software PSpice.Based on the simulation model,the overvoltage characteristics of the EMU are simulated and analyzed.When the circuit breaker is turned on,the simulation results show that the overvoltage on the pantograph is maximal.The overvoltage on the transformer HV side comes second.The switching overvoltage on carriage is minimal and gradually reduces from No.2 car to No.7 car.As circuit breaker closed,the simulation results show that the waveforms of over-voltage on pantograph and transformer HV side are all the same.The overvoltage on each carriage is smaller than the circuit breaker opened.Based on the simulation model,the influence mechanisms of the key electrical parameters on the switching overvoltage are analyzed.The results are as follows:When the phase of catenary voltage is 90° and 270°,the operating voltage on the pantograph is maximal.The maximum of switching voltage on pantograph increases with the increase of the short-circuit current,but as the short-circuit current reaches certain value,the voltage is no longer significantly increased.The overvoltage value on the traction transformer HV side decreases with the increase of the distributed capacitance value,and the overvoltage is no significant decrease when the distributed capacitance value reaches certain value.When circuit breaker mounted on the roof,the overvoltage on the high voltage side of transformer reduces by 10.68%than circuit breaker placed under the carriage.Therefore,it is recommended that the circuit breaker should be located on the roof.The switching overvoltage value on each carriage and excitation inductance value of transformer is inversely proportional relationship.The overvoltage on carriage decreases first and then increases with the increase of the unit capacitance value of the high voltage cable.When the capacitance selects the optimal value,the overvoltage for each car is the smallest.It is best to use parallel capacitors with the grounding resistors to restrain grounding return current and reduce switching overvoltage on carriage.