Model Predictive Direct Power Control of PWM Rectifiers Under Unbalanced Network Conditions

Abstract

Model predictive direct power control (MPDPC) has been proposed as an effective alternative to conventional direct power control for pulsewidth-modulation (PWM) rectifiers. However, the sampling frequency of MPDPC still has to be high to achieve satisfactory performance. Furthermore, the grid currents of MPDPC would become highly distorted under unbalanced network conditions. To cope with the problems above, this paper proposes an improved MPDPC for PWM rectifiers, which is able to operate under both balanced and unbalanced grid voltages. By using a new definition of instantaneous reactive power in the predefined cost function, the proposed MPDPC can obtain sinusoidal grid currents and eliminate twice grid-frequency oscillation in both active power and the new reactive power. Neither complicated positive/negative-sequence extraction of grid voltage/current nor power compensation is required. Depending on the desired performance, two variants of the improved MPDPC are proposed and comparatively studied. At the same sampling frequency, the first approach achieves relatively low switching frequency, whereas the second approach obtains lower power ripples. Both simulation and experimental results are presented to confirm the effectiveness of the proposed methods.