Dual-Sequence Multi-Objective Hierarchical Control for Virtual Synchronous Generators Considering Current Constraints Under Asymmetric Voltage Sags

Asymmetric grid voltage sags induce double-frequency power oscillations and three-phase current imbalance in virtual synchronous generators (VSGs). Though conventional multi-objective optimization via active negative-sequence current injection can achieve current imbalance mitigation and power fluctuation suppression, it often compromises positive-sequence current control performance, making it difficult to ensure converter fault ride-through (FRT). To address this, this paper analyzes the positive- and negative-sequence control coupling mechanism and proposes a dual-sequence multi-objective hierarchical control strategy that prioritizes grid voltage support, active-power retention, output current balance, and power fluctuation suppression under asymmetric voltage sags. Firstly, the positive-sequence current reference is generated to maximize active-power output and grid voltage support with consideration of current constraints. Subsequently, based on the remaining current capacity, a feasible region for the optimization algorithm is established to perform negative-sequence current multi-objective optimization, thereby achieving decoupled hierarchical control of positive- and negative-sequence components. Simulation and hardware-in-the-loop (HIL) experimental results demonstrate that this proposed strategy effectively mitigates control coupling between positive and negative sequences. It simultaneously ensures grid reactive-power support, maintains active-power output, and optimizes negative-sequence current while strictly adhering to current constraints.