Predictive Direct Power Control of Three Phase PWM Rectifier Under Unbalanced Grid

Abstract

In a three-phase pulse-width modulation(PWM) rectifier, due to the unbalanced grid voltage, the input current may be distorted seriously under the traditional predictive direct power control (PDPC) strategy. In addition, the whole system more complex and costly because of the grid voltage sensor. In order to improve the input current quality of the PWM rectifier and reduce the cost of system, the grid voltage sensor free algorithm based on a improved second-order generalized integrator (SOGI) is combined with the PDPC strategy. The improved SOGI is used to generate orthogonal signals with the same frequency to estimate the grid voltage and realize the grid voltage sensor free PDPC. Based on the extended pq theory, the reasons of the active and reactive power double frequency oscillation under the unbalanced grid are analyzed. On this basis, the AC side voltage equation of the rectifier to eliminate the power oscillation is derived. Then the switching state is determined by the space vector pulse width modulation (SVPWM). In the simulation platform, the proposed method is simulated under the balanced and unbalanced grids. The simulation results show that the proposed method reduces the influence of unbalanced grid on the input current waveform, eliminates the double frequency oscillation of the power, accurately estimates the grid voltage, and realizes the normal operation under the balanced and unbalanced grids. The effectiveness of the proposed method is verified by comparing the simulation results of the two cases.