Evaluating Stability in Offshore Wind Farm Transmission Systems Using Hybrid DR-MMC Rectifier-Based HVDC

Recently, a hybrid rectifier combining a diode rectifier with a modular multilevel converter (DR-MMC) has been recognized as an advantageous solution for High Voltage Direct Current (HVDC) systems, notable for its enhanced power transmission capabilities and cost efficiency. The integration of this innovative rectifier type into HVDC systems, especially for offshore wind farms (OWF), necessitates a comprehensive examination of system stability. In this paper, a DC impedance model for the DR-MMC rectifier is proposed. This model incorporates the impact of the connected OWF at the AC terminal on the DR-MMC rectifier's DC impedance. Following this, the AC impedance of the HVDC transmission system as seen from the onshore converter station is developed. Stability analysis for the onshore station connected to a weak AC grid is conducted using the general Nyquist criterion. How the simplification of the DC terminal influences the stability of the onshore AC system is investigated. The study shows that oversimplification of the offshore rectifier introduces inaccuracies into the stability analysis result. Validation of the impedance model and the stability analysis result is achieved through time-domain simulations in Matlab/Simulink.