Modeling and Stability Analysis of the ISOP-Type Cascaded Hydrogen Converter

Abstract

The input-series output-parallel (ISOP) cascaded hydrogen converter is a key device for interfacing the medium-voltage DC bus with the electrolyzers in off-grid hydrogen production systems. However, existing ISOP cascaded hydrogen converters are unable to simultaneously meet the comprehensive performance requirements, including wide input voltage range, high reliability, broad output power regulation capability, and the ability to supply power to multiple electrolyzers concurrently. Moreover, the stability analysis based on the closed-loop two-port small-signal model (CTSM), as well as the impact of multiple controller gains on system stability, has not been thoroughly investigated. To address these issues, this article proposes a novel two-stage ISOP cascaded converter topology along with a stability analysis method. First, the CTSMs are developed for both the switched-capacitor submodules and the output-stage phase-shift full-bridge converter interfaced with three PEM electrolyzers. Based on these models, the closed-loop transfer functions are derived, and a set of new stability criteria grounded in the Nyquist Criterion are formulated. Furthermore, a systematic sensitivity analysis is performed to evaluate the influence of multiple controller gains on the proposed criteria, providing theoretical guidance for multiloop controller tuning in cascaded converters. The proposed stability evaluation method can be further extended to the stability assessment of the other two-stage cascaded converters. Finally, both simulation and experimental results are presented to validate the effectiveness of the proposed stability assessment approach.