Robust Dual-Sequence Current Control of a Single-Stage Three-Phase EV Charger Under Unbalanced Grid Conditions and Grid Impedance Uncertainty

This paper proposes a robust dual-sequence current control strategy for a single-stage three-phase bidirectional electric vehicle (EV) charger operating under unbalanced grid conditions with grid impedance uncertainty. A conventional current controller employing only positive-sequence current regulation is first considered as the baseline, which exhibits second-order DC-link voltage oscillations under unbalanced grid voltages. To mitigate this issue, the proposed method employs a dual-sequence current control framework in which an outer proportional-integral (PI) controller regulates the DC-link voltage while positive- and negative-sequence current references are generated to compensate for the second-order power ripple. A phase-locked loop (PLL) is utilized solely to obtain the grid angle from the decomposed positive-sequence grid voltage for reference frame synchronization, thereby ensuring accurate current regulation under unbalanced operating conditions. The inner current control loop is designed using a linear matrix inequality (LMI)-based robust state-feedback controller that explicitly accounts for grid impedance uncertainty, thereby guaranteeing closed-loop stability and improving current-tracking performance over a prescribed uncertainty range. Comparative studies are conducted between the conventional positive-sequence current controller and the proposed dual-sequence current control strategy under balanced and unbalanced grid conditions with varying grid impedance. Simulation results demonstrate that the proposed approach effectively suppresses second-order DC-link voltage oscillations while maintaining accurate grid current tracking and robust performance against grid impedance variations. Furthermore, the proposed controller achieves improved DC-link voltage regulation without compromising system stability, demonstrating its effectiveness and suitability for reliable grid-connected EV charging applications under practical operating conditions.