Research On The Control Strategies For PWM Rectifier Under Unbalanced Grid Conditions

Pulsewidth modulation(PWM)rectifier are widely used in alternating current electric drive,active power filter,new energy power generation system and so on,which has the characteristics of controllable power factor,small alternating current distortion rate and bi-directional power flow.In practical applications,unbalanced distribution of three-phase loads and other reasons will lead to the unbalanced grid.At this point,the control strategies for PWM rectifier will cause the grid-side current distortion and dc-side voltage fluctuation,which are designed under balanced grid conditions.In more serious conditions,the unbalanced grid can affect the normal operation of electrical equipments.For this reason,the control strategies for PWM rectifier under unbalanced grid conditions are deeply researched in this thesis.First of all,according to the Kirchhoff's laws and instantaneous power theory,the voltage and current mathematical models and the instantaneous power mathematical models are established.By contrast,mathematical models of three-phase PWM rectifier under balanced grid conditions are also applicable to unbalanced grid conditions.When the grid's state changes,there is no need to consider the possible problems caused by changes in the system model.Then,based on the mathematical models of three-phase PWM rectifier,the dual current control strategy and the sliding-mode-based direct power control strategy for PWM rectifier under unbalanced grid conditions are deeply researched in this thesis.For the existing problems of these above two kinds of control strategies,the deadbeat predictive direct power control strategy for PWM rectifier under unbalanced grid conditions is proposed in this thesis.In order to improve the system dynamic performance,the fully digital deadbeat controller is designed as the power inner controller,which can make Instantaneous powers quickly track the reference powers.To improve the performance of the controller,analysis of the time delay influence on the controller is conducted and corresponding compensation scheme is proposed.At the same time,three control objectives are seted in this thesis to control system,which namely suppressing negative sequence current,suppressing active power fluctuation and suppressing reactive power fluctuation.Different control targets can be achieved through corresponding power component compensation of three control targets by a new instantaneous power analysis method.This method simplifies the structure of control system and reduces the amount of computation,because there is no need to extract positive and negative sequence components.The effectiveness of the strategy is verified by the results of simulation.Finally,the experimental platform for PWM rectifier under unbalanced grid conditions is established and the experiments on the control strategy which is proposed in this thesis are carried out.Through the analysis of the experimental results,the effectiveness of the strategy is further verified.