Control System Design Of Three-phase PWM Rectifier For Chemical Production Under Unbalanced Power Grid

The chemical industry plays an important role in the Chinese national economy.Three-phase PWM rectifier,as the interface of chemical equipment emergency power supply or DC chemical load with the power grid,is an important link in the configuration of chemical industrial power equipment,and its control performance plays a vital role in chemical production.When the power grid is unbalanced,the AC current is distorted and the quality of output DC voltage is decreased,which affects the stable operation of chemical equipment.Therefore,it is of great practical significance to study its control system under the unbalanced power grid.Three methods are designed to detect the positive and negative sequence components of AC voltage and current based on the mathematical model of three-phase PWM rectifier under the unbalanced power grid.The components of AC voltage are used to calculate command signal of power and current to build the multi-objective current command model.The twice-frequency notch filter method is implemented in d-q coordinate system,but there is static error between its detected component and the actual value.The time-delay calculation method and reduced order resonator(ROR)method implemented in α-β coordinate system can solve this problem.However,the time-delay calculation method depending on accurate formula calculation.To solve this problem,ROR method with frequency selection characteristics is designed,which reduce the dependence on the system model and will not be interfered by harmonic signal with high precision.In the double closed-loop of three-phase PWM rectifier based on PI control,the coupling relationship leading to the complexity of structure,which can not eliminate the harmonic effect in AC current and zero static error in the output DC voltage.To solve those problems,this paper designs the unbalanced control strategy based on proportional resonance(PR)control in α-β coordinate system,which using ROR to detect the positive and negative sequence components of AC voltage to calculate the command signal of power and current to build the new current command model,reducing the total harmonic distortion(THD)of the AC current effectively,improving the control precision of output DC voltage.The performance of the designed control system is verified by the MATLAB simulation experiment.Firstly,the effects of three methods detecting positive and negative sequence components are analyzed.The simulation model of main circuit and control system of three-phase PWM rectifier under unbalanced power grid is built.The control performance of PI control system in d-q coordinate system and PR control system in α-β coordinate system,output DC voltage,AC current and instantaneous power are compared and analyzed.The results show that advantages of unbalanced control strategy based on PR control,which can reduce THD of AC current,realize no static error control of output DC voltage,reduce twice-frequency AC component of instantaneous reactive power,and eliminate secondary ripple of instantaneous active power.