Research On Modeling And Suppression Methods Of High Frequency Bearing Currents In Traction Motors

Due to the characteristics of the urban rail transit such as speedability,convenience,large volume and high transportation efficiency,it has become an important part of urban public transportation.At present,variable frequency traction systems are widely used in urban rail transit systems.The common mode voltage generated by the traction converter causes serious bearing electric corrosions,which increases the cost of motor repair and maintenance and threatens the safe operation of rail transit systems.It has certain theoretical and engineering significance to carry out modeling and analysis on the bearing current problems of subway traction motors,and to propose effective suppression strategies on it.This thesis carries out the modeling,parameter extraction,system simulation and suppression measures of the bearing currents of the subway traction motors in the variable frequency power supply system.The specific contents are as follows:Firstly,the research is carried out on the bearing current lumped parameter model and parameter extraction.The bearing current lumped parameter model of the traction motor is established,and the test scheme and the parameter extraction scheme on the motor stray capacitances,bearing oil film capacitances,common mode resistance and common mode inductance arw designed.A test on the traction motor bearing voltage proceeds.A simulation analysis model of the bearing current of the traction motor is built,and the accuracy of the model is verified by comparing the simulated and measured common mode current and bearing voltage.Secondly,the research is carried out on the high frequency bearing current model and the model parameter extraction.Aiming at the situation that the frequency characteristics of the lumped parameter model motor port impedance does not match the actual situation in the high frequency range,based on the motor impedance frequency spectrum test,a bearing current high frequency model is proposed and a parameter extraction scheme is designed.A simulation analysis model of the bearing current of the traction motor is built to verify the time domain waveforms of the common mode current and the bearing voltage.And compared with the lumped parameter model of the bearing current,the applicable occasions of the two models are analyzed.The applicable occasions of the two models are analyzed.The result shows that when the frequency is less than the first resonant frequency of the common mode impedance,the lumped parameter model can also meet the needs within the error tolerance range,but when the frequency is higher than the first resonant frequency of the common mode impedance,the high frequency model should be used.Then the bearing current of the subway traction system is modeled and simulated The equivalent circuit of the bearing current of the traction motor taking into account the grounding impedance is established,and the simulation model under the variable frequency power supply system of the subway traction motor is built.Taking asynchronous modulation as an example,the influence of oil film breakdown on common mode voltage,common mode current,bearing voltage,and bearing current are analyzed,and the characteristics of bearing current at low speed is analyzed.The analysis shows that the current flowing through the bearings is relatively large under oil film breakdown and low-speed operation,which seriously threatens the life of the bearings and the stable operation of the system.Considering the influence of the grounding system,the analysis shows that safety capacitors,casing protection resistance,and inductance between the bus bar and the rail have little effect on the bearing current.Finally,the methods of restraining the bearing current are researched based on the shielding principle.This thesis analyzes the principle of electrostatic shielding to suppress the Electric Mischarge Machining(EDM)bearing current and the rotor grounding bearing current,and designs two bearing current suppression schemes,namely the end shielding and slot shielding.The stray capacitances of the motor before and after the shielding measures are calculated by ANSYS finite element simulation software,and the effectiveness of the shielding to suppress the bearing current is verified by MATLAB simulation.Comparing the suppression effect of the two shielding measures and the weight of the shield,the results show that the end shielding scheme is better than the slot shielding scheme.