Input Impedance Specification Design for Solid-State Transformers in Multimodal Grid Environments

The widespread adoption of solid-state transformers (SSTs) in medium-voltage grids faces limitations due to system instability caused by SST/multimodal grid impedance mismatch. Existing impedance specifications struggle to balance conservativeness, robustness, and grid adaptability. To overcome this, this article proposes a dual-margin constrained design method for SST input impedance specifications. First, the instability mechanisms involving SST interaction with inductive-, capacitive-, and resonant-characteristic grids are analyzed, establishing a unified frequency-matching stability criterion and the partitioned impedance concept. Subsequently, a dual-margin specification design methodology combining point margins (gain/phase margins) and area margin is proposed, robustness visualization and quantitative evaluation methods are elaborated based on geometric properties of the area margin and a constructed robustness index. Then the implementation approach and case studies are discussed to show the superiority of the proposed method. Finally, hardware-in-the-loop and downscaled prototype tests under severely deteriorated grid conditions (including 80% inductive and 66.7% capacitive parameter drift) validate the proposed method’s superiority in robustness and grid adaptability over existing approaches.