| Modern electric trains almost exclusively use induction machines fed by voltage source inverters(VSI)for propulsion.To reduce external as well as internal disturbances,an LC-filter is often inserted between the DC voltage source and the inverter.However,interaction between the input filter and the inverter is well known to cause degradation of system stability and oscillations of DC-link voltage,which are especially pronounced in traction applications for large powers.This thesis focuses on suppressing DC-link oscillations of DC/AC converters and improving system stability.Main work is as follows:The mathematical model of induction motors and the basic principle of vector control are introduced,based on which a more accurate model of the inverter-motor drive system and an expression for the inverter input admittance are derived and validated.Furthermore,the mechanism of DC-link oscillations caused by negative impedance instability is thoroughly discussed using the Nyquist stability criterion.The inverter is commonly seen as a perfect constant power load as to simplify analysis in literature.This paper discusses the error caused by such approximation.Analysis shows that the inverter input admittance is not only determined by the varying dynamics of the drive,but also properties of the control strategies and the steady state conditions.How control parameters such as current loop bandwidth and the estimated DC-link voltage(used for calculating the modulation index),affect the inverter input admittance is thoroughly studied.A stabilization technique based on a ―virtual resistor‖ is proposed which is essentially to assure a phase shift of the inverter input admittance around the input filter resonance frequency region,so that the inverter appears more passive and the drive becomes more stable.The effectiveness of the proposal is verified both by PLECS simulation and experimental results based on a motor control platform. |
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