Stability Analysis and Parameter Optimization for DRU Connected Grid-Forming Offshore Wind Turbines

The growing capacity of offshore wind power poses great challenge on the offshore integration and transmission system. The diode rectifier unit (DRU) based high voltage direct current (HVDC) transmission has been regarded as a promising solution to realize low-cost offshore wind power transmission. However, due to the uncontrollability of DRU, the wind turbines (WTs) need to maintain the voltage and frequency of offshore AC grid. The reactive power-based synchronization control is commonly applied in the offshore WTs connected to the DRU-HVDC. However, the P-V and Q-ω control principles of offshore WTs haven’t been clearly clarified. Besides, the large number of WTs and cascaded multiple control loops of each WT make it difficult to design control parameters for system stabilizing. To solve the problems, a new V/PF type bus is proposed to equivalent the DRU-HVDC to illustrate the control principles, the frequency-mode analysis (FMA) is used to analyze the system interactive stability based on a new reactive power self-balancing control developed by our team under different conditions. The key control parameters and the adjustment schemes for improving the system stability are obtained from the analysis of this paper. Finally, the simulation results in PSCAD/EMTDC are given to verify the analysis.