Stability Assessment and Improvement of MTDC System Connected With Offshore Wind Farms

This paper focuses on the assessment and improvement of the DC network stability of multi-terminal HVDC (MTDC) systems based on Modular Multilevel Converters (MMCs). Therefore, the DC terminal small-signal impedance models for MMCs with different controllers and AC side connections, including onshore AC networks and offshore wind farms (OWFs), are developed in this study. These models are based on the harmonic state space (HSS) method, which accurately captures the internal multi-harmonic couplings of the MMC. Further, by utilizing the impedance models, the paper investigates the effects of different active power controllers and DC cable distances between OWFs, and different DC cable technologies including Cross-linked polyethylene (XLPE) and High-Temperature Superconducting (HTS) cables on the stability of the DC network. To address the negative damping observed in the DC impedance of the MMCs, an improved damping controller implemented with the MMC circulating current controller is proposed to counteract the destabilizing effects and enhance the stability of the DC network. The time-domain simulation results demonstrate the accuracy of the DC impedance models and confirm the effectiveness of the proposed measures for improving system stability.