Flexible Grid Connection Technique of Voltage-Source Inverter Under Unbalanced Grid Conditions Based on Direct Power Control

This paper proposes a flexible grid connection technique of a voltage-source inverter (VSI) based on a direct power control strategy under unbalanced grid conditions. Based on the mathematical model in the $\alpha\beta$ stationary reference frame of the VSI, the generalized power reference compensation expressions for adjustable current unbalance and power fluctuations are deduced. By analyzing the active and reactive power expressions, three different algorithms based on the different control parameters are designed to make a compromise among the current negative sequence, the active power ripples, and the reactive power ripples. To satisfy the grid code, the third-harmonic current component is removed in the proposed flexible control target. Furthermore, the control scheme incorporates multiple complex coefficient filters to separate the positive and negative fundamental components of grid voltage and current to calculate the power reference compensation. The proportional-integral-resonant controller is applied to track the instantaneous power errors. The experimental results verify that the proposed control strategy is adaptive to the flexible control targets.