Control, Modeling and Stability Analysis of DRU-based Grid-forming Converter*

Diode rectifier unit (DRU)-based high-voltage direct current (HVDC) transmission systems are effective in achieving the stable and economical operation of offshore wind-power generation. Considering the uncontrollable characteristics of DRUs, a grid-forming (GFM) strategy for wind-turbine converters is necessary to support offshore AC voltage and frequency. However, the active power-synchronization control in traditional GFM converters is unsuitable for DRU-based GFM converters. Thus, the stability issue for DRU-based HVDC systems involving DRU-based GFM and grid-following (GFL) converters has not yet been addressed. To solve these issues, this study begins with the characteristics of a DRU-based HVDC system and presents a control scheme for DRU-based GFM converters for power synchronization. Subsequently, the dq-frame impedance model of the DRU-based GFM converter is proposed for the stability analysis of the entire HVDC system. Finally, a simulation platform is built to verify the model accuracy and system stability.