Condition Monitoring For DC-link Capacitors In Traction Drive System

With the rapid development of rail transit facilities in recent years,taking high-speed trains and subways has become an important way for people to travel.In order to ensure the stable operation of trains,the reliability research of traction drive system has been paid much attention.As an important part of reliability research,condition monitoring methods can effectively monitor the aging degree of system components to avoid the system shutdown and other safety problems caused by the failure of components.In this paper,condition monitoring methods of dc-link capacitor is studied and it focuses on the effectiveness of aging degree monitoring and its practicability in traction drive system.In this paper,the types of capacitor used in traction drive system,aging mechanism and aging characteristic parameters of the capacitor is introduced.The equivalent circuit model of the capacitor is established which provides the theoretical basis for the simulation verification of the following capacitor condition monitoring methods.In this paper,the existing capacitor condition monitoring methods are summarized.The simulation model is established to verified the methods,and the methods are compard.This paper focuses on the harmonic analysis of dc-link capacitor current,and then proposes a new method of capacitor condition monitoring based on two different frequency current harmonics.Different topology,control mode and converter switching frequency will affect the harmonic component and harmonic amplitude of the dc-link current.In this paper,the dc-link current harmonic analysis is carried out for the threephase uncontrol rectifier and four-quadrant rectifier circuit topologies.The harmonics produced by the rectifier and inverter are studied respectively,and the influence of these harmonics on the dc-link is compared.The simulation results show that the current flowing into the dc-link and the current flowing out of the dc-link can be used to estimate the capacitance current harmonics at a specific frequency.This paper estimates the current harmonics of the dc-link under light load and heavy load respectively.Harmonic estimation can effectively avoid adding dc-link current sensors to calculate dc-link current hamonics.According to the calculated current harmonics,the condition monitoring method based on two frequency current harmonics is simulated and verified in two different circuit topologies,and the aging parameters are calculated in the time domain and frequency domain.According to the calculation results,the new proposed condition monitoring method has the advantages of high accuracy and simultaneous monitoring both the equivalent series resistance and capacitance values compared with the Fourier analysis method.Change the system operating condition to verify the feasibility of the state monitoring method based on two different frequency current harmonics.The simulation shows that this method is suitable for traction and braking operating conditions,and has high monitoring accuracy.The capacitor condition monitoring platform is built.The design of the experimental platform includes the design of the main circuit and the design of the simulated capacitor aging circuit.The main circuit design includes the design of topology and parameters.The current on the rectifier side is collected by adding the flowing into filter current sensor,and the current on the inverter side is obtained according to the motor sensor and the inverter switch sequence.Then the dc-link current can be rebuilt.By changing the connection mode of capacitor,the capacitor aging characteristic parameters can be changed,so as to simulate capacitor aging.Finally,based on the experimental platform,the new proposed capacitor condition monitoring method is validated.