ZVS Boundary Analysis and Design Guideline of MV Grid-Compliant Solid-State Transformer for DC Fast Charger Applications

Abstract

A solid-state transformer comprising a cascaded H-bridge and a dual-active bridge converter is a promising solution for a megawatt medium voltage DC fast charger application. The new IEEE Std 1547.9-2022 comprehensively discusses extending the minimum reactive power capability to electric vehicle chargers. This paper analyzes the impact of operating the grid compliant single-phase solid-state transformer on the overall system. The impact of the DC-link voltage due to the single-phase implementation of the H-bridges on the dual-active bridge converter zero-voltage switching mode at light-load operation is highlighted. The system’s operational boundary is analyzed, which defines the reactive power capability limit while ensuring the dual-active bridge converter zero-voltage switching mode operation for the defined operating points. This zero-voltage switching mode boundary analysis is then used to develop a design guideline as part of the solid-state transformer design process. The proposed guideline allows a simultaneous design of a DC-link capacitor and dual active bridge inductance to ensure zero-voltage switching mode for the defined operating points. It leads to DC-link capacitance reduction that offers cost-and footprint-savings. The proposed concept is validated through simulations and experimental results. Further, a potential benefit analysis is provided to emphasize the effectiveness of the proposed concept. A supplementary video is included to showcase the system’s dynamic active and reactive power operation.