High Frequency Model of Electrified Railway Propulsion System for EMC Analysis

Abstract: A model of the electrified railway propulsion system working in a wide frequency range is studied in this thesis. The high frequency modeling is the first stage to study and predict the Electromagnetic compatibility (EMC) problems in the electrified railway propulsion system, which are safety and reliability issues of high concern. Modeling methods and models for the line converter, motor power supply module, and the traction motor are developed. These models can work individually or be combined together to simulate the main part of the electrified railway propulsion system. A two level pulse width modulation (PWM) controlled rectifier used in propulsion system is built in Simulink. It provides different values of DC voltage for DC-link according to design requirements. Time domain line current as well as frequency domain spectra is studied. Harmonic value based on variation in time is given to better understand the behavior of the rectifier. High value of harmonics is found during starting time. The steady-state harmonics are also demonstrated. For the motor power supply module modeling, multitransmission line theory is used to model the cable used to connect the inverter and motor. A decoupling method to solve N multiconductor transmission lines into N single transmission lines in PSpice is developed and verified. Through simulation, the oscillation and overvoltage in the cable according to different layouts and length of cable are studied. The electromagnetic (EM) noise is dependent on the length of cables. A fourth grounded cable with optimized layout is introduced to reduce the oscillation on the line. By using this method, for the cable layout considered, the magnitude of the oscillation is reduced significantly from 19.48 V at 5.245 MHz to 2.1 V at 4.609 MHz for the 14 meter cable configuration. In order to improve accuracy of the modeling of propulsion system, a method to represent a traction motor with high accuracy in a wide frequency range in PSpice is presented. This model is based on vector fitting and synthesis of rational function in PSpice by equivalent circuits. It provides a new concept to model the traction motor for EM investigation compared with conventional circuit models. The simulated data can approach the measured data with extremely small error when high order of circuit is used. Verification of this method was done on a 200 kW induction motor. The relative simplicity of this method makes it a good candidate for analysis of industrialized traction motors. Fast transients induced by reverse-recovery of diodes in pre-charger or short circuit in power supplies are also studied, and it is possible to include them in the proposed model for propulsion system to study their impacts on the system from the EMC point of view. The fast transients with high magnitude are hazardous to the safe operation of the train. A test setup is proposed to study the fast transients caused by output diodes in the pre-charger. In the test setup, one efficient method to eliminate the contact bounce in the IGBT control circuit is also proposed. The voltage responses of the diode may be two or more times higher than the reverse bias voltage when nonlinear behavior of diode happens. A snubber, which has much bigger capacitor than conventional ones, is applied to solve the problem. The effect and robust operation of this snubber is experimentally verified. The overshoot voltage with snubber is reduced to less than 1.2 kV, which is about 60% of that without snubber, when the reverse bias is 1 kV. Short circuit may also lead to fast transient in the traction system. As there are numerous DC-DC converters in the propulsion system, especially in the auxiliary system, the importance of equipping short circuit protection circuit in power supplies is obvious. A test setup is proposed to study the fast transient induced by short circuit. An effective circuit, which has been experimentally verified, is proposed to reduce the transient current when short circuit occurs. As shown in the example converter, the peak value of the transient current resulting from short circuit is reduced to one fifth of that without protection circuit.