Research On Control Strategy Of Three-phase Current-source PWM Rectifier Under Unbalanced Power Grid

With the rapid development of power electronics technology,current-source PWM rectifier has been widely used in various fields with the advantages of adjustable output voltage and bidirectional power transmission.The control technology of three-phase current-source PWM rectifier under balanced power grid is relatively mature,but in practical application,it is often caused by various factors such as load change,parameter asymmetry and external interference,which often results in three-phase unbalance of AC network.Under unbalanced conditions,direct use of PWM rectifier control strategy under balanced power grid can easily lead to abnormal operation of equipment.Therefore,the control strategy of current-mode PWM rectifier under unbalanced grid conditions is studied in depth in this paper.The main aspects are as follows:Firstly,the topological structure and basic working principle of three-phase current-source PWM rectifier are analyzed.Based on the balanced and unbalanced grid conditions,the corresponding mathematical models in different coordinate systems are established,and the positive and negative sequence separation is carried out under unbalanced conditions.Because of the existence of LC filter,the mathematic model in d-q coordinate system has strong coupling characteristics.This paper will simplify the mathematic model and eliminate the coupling characteristics by orienting the capacitance voltage,which greatly reduces the difficulty of controller design.Secondly,since the current of three-phase CSR DC-side energy storage inductor circuit can not change abruptly,it is necessary to use ternary logic to realize PWM control signal.Therefore,this paper elaborates on the differences in sector division,binary-ternary logic conversion,and the generation of logical values and vector formation.The sinusoidal pulse width modulation technology and space vector modulation technology are compared.Although both of them respond quickly and flexibly,space vector pulse width modulation(SVPWM)strategy has higher voltage utilization,stronger harmonic elimination ability and easier digitization.Then,aiming at the problems of three-phase current-source PWM rectifier in unbalanced power grid,a control strategy of high frequency,low frequency,double frequency and three-band component independent control is designed.Active damping control of virtual resistance is used in AC side and state feedback control strategy is used in DC side.High-pass filter is used to reduce the influence of low-frequency signal on AC side,and LC low-frequency filter on DC side restrains the influence of high-frequency signal on AC side,thus realizing decoupling control of AC and DC side.Based on the instantaneous power theory,the instantaneous power flow of three-phase current source PWM rectifier is analyzed.On the basis of AC and DC decoupling control,the negative sequence current component is compensated.In addition,the influence of parameters design of AC/DC side controller on system performance is analyzed in detail,and the parameters are tuned according to standard transfer function to achieve the best control effect.Compared with traditional PID control,it not only simplifies the design difficulty of the controller,but also reduces the interaction between AC and DC sides of the system,and further improves the dynamic response performance of the system.At the same time,this paper studies and compares three methods of separation and extraction of positive and negative sequence components,and decides to adopt double synchronous coordinate system decoupling extraction method and phase-locked loop with fast response and strong anti-jamming ability.The rationality of the control strategy proposed in this paper are verified by the simulation of Matlab/Simulink.Finally,based on the above theory and simulation analysis,experiments are carried out on the experimental prototype of the three-phase current-mode PWM rectifier to ensure the correctness of the control algorithm.