Current Loop Constant Frequency Model Predictive Control of Three Phase PWM Converter under Unbalanced Grid Condition

The unbalanced grid condition is one of common fault in the power system, the performance of electronic equipment will reduce when the controller designer without considering the unbalanced grid input. The traditional controller design methods of three-phase pulse width modulation (PWM) converter, are based on the instantaneous power model and the double-closed loops proportional integral (PI) controller, where the positive- and negative-sequence current components are controlled separately. Thus, there are eight mutual influenced control parameters need to be tuned, it is a difficult task. In addition, the performance of PI controller will degrade when the working point changes in a large scale. In this paper, a constant frequency current model predictive control (MPC) strategy is proposed for the three-phase PWM converter under unbalanced grid condition. In this strategy, the positive- and negative-sequence predictive current values are calculated by the predictive model and the cost function of MPC is designed, the duration times of voltage vectors are produced directly by minimizing the cost function in each sector and the space vector pulse width modulation (SVPWM) is used, thus the constant switching frequency of system is guaranteed. The proposed method has less control parameters need to be tuned and can realize the accurately and quickly current adjustment under unbalanced grid condition, which is proved by the simulation results.