Research On Control Strategy Of Inverter Power Supply With Nonlinear Load

Electric locomotives have developed rapidly with the continuous increase of the coverage area of China’s electrified railways.The high power and low switching frequency auxiliary inverter,as an important part of the electric locomotive,can provide stable AC energy for the electric equipment of the train and ensure the normal operation of the train.The output voltage harmonics of the inverter will increase when the load of the auxiliary inverter includes non-linear loads such as the charger and so on,and affect the normal operation of other loads.This paper studies the improvement of the control strategy to improve the output waveform of the inverter with nonlinear loads.This paper studies the working principle of the inverter and establishes mathematical model of the three-phase inverter by using state space modeling.Then the traditional double closed-loop proportional integral(PI)control strategy is analyzed.The working principle and working mode of the nonlinear load are analyzed with taking the three-phase uncontrolled rectification load corresponding to the charger as the representative.The output voltage harmonics of traditional double closed-loop PI control strategy are relatively large when the inverter power supply with nonlinear loads.To this end,this paper introduces resonant controller to improve the control strategy.Due to the low switching frequency and control frequency,the impact of digital delay on system performance cannot be ignored.In order to achieve the purpose of accelerating the dynamic response of the system,a vector proportional integral(VPIc)controller with delay compensation is added to the current control loop and a proportional resonance(PR)controller is added to the voltage control loop to achieve the purpose of suppressing harmonics and improving the anti-interference performance of the system.The design method of the control parameters of the inverter system is studied with ensuring the stability of the system at low switching frequency.Considering the effect of discretization on the performance of the controller,various commonly used discretization methods are compared using frequency response analysis method.In this paper,a method based on a second-order generalized integrator is used to implements the equivalent functions of VPIc controllers and transposed Direct Type II method is used to implements the equivalent functions of PR controllers.Based on the above design method and control strategy implementation method,this paper builds a simulation model in Matlab / Simulink,and the performance of the two control strategies with non-linear inverter load,mixed load at half load and full load is compared.The resonance control strategy can effectively suppress the harmonic components in the inverter output voltage and improve the quality of the output power.This paper considers the impact of low switching frequency on the control system,the same switching frequency and design method as the high power operating condition are adopted.A small power experimental prototype of the three-phase inverter system is built and the top-level logic of the software is designed in this paper.The experiment results verified the effectiveness of the improved control strategy and the correctness of the theoretical analysis.