Research On The Control Strategy Of Three-phase PWM Rectifiers Under Unbalanced Grid Voltage

This paper focuses on the control strategies to improve the performance of the three-phase PWM rectifier under unbalanced grid voltage.The problems such as negative sequence current,voltage and current harmonic are discussed in this paper.The analytical solutions for negative sequence current and harmonic current elimination are obtaining positive-sequence or negative-sequence voltage by filters and producing a non-harmonic current command of the inner-loop.Also the solutions for dc voltage harmonics elimination are achieved by controlling a constant power flow from ac to dc side in spite of the unbalanced voltage conditions.In this paper,a few design methods of filters such as notch filter and low-pass filter (LPF) are analysed.Also the positive-sequence-filter(PSF) and the negative-sequence filter(NSF) are studied.Simulation models of the filters in the control system are as well given.Simulation results proves the filters are feasible.Current-loop and voltage-loop have been introduced to the unbalanced control system of PWM rectifier.With the control system design and simulation work,the first unbalanced control method achieves symmetrical and sinusoidal ac current,and the second control method achieves dc voltage harmonic elimination.Finally,the design of unbalanced experimental system including hardware and software is described in detail.And a laboratory prototype has been established to verify the discussions and analyses of the control method based on eliminating ac side negative sequence current.The experimental results verify the correctness of the proposed control strategy.