Stability Analysis of Series-Connected Hybrid Converter for Offshore Wind Farm HVdc Integration

The system stability and MMC capacity are important issues in the diode rectifier-modular multilevel converter (DR-MMC) series topology in the high voltage dc (HVDC) system for offshore wind power integration. This study considers the DR, MMC, wind turbine converter and ac accumulation system, and establishes a 53-order state-space small-signal model, 8 for the wind turbine converters, 22 for the filters, transformers and ac cables, 19 for the MMC, 1 for the DR, and 3 for the dc cables. The eigenvalue and participation factor analyses indicate that if the MMC has a small capacity (e.g., 20%), an oscillation may occur at 4.8 Hz due to the interaction between the phase-locked loop of the wind turbine and the weak strength of the ac grid formed by the MMC. The study provides guidance on selecting appropriate MMC capacity mix and control parameters for reducing the required MMC capacity from a small-signal stability perspective. Simulation results confirm the accuracy of the small-signal analysis and the selected parameter values.