Comparison of Grid-Forming Control Solutions for Offshore Wind Farms Connected with Diode Rectifier-Based High Voltage DC Transmission

For offshore wind power transmission systems based on diode rectifier-based DC transmission, the voltage and frequency support of the offshore AC grid based on the grid-forming control of offshore wind turbines has become a key issue. For the currently most studied Q/f droop-based grid-forming control (GFM) scheme and the newly emerged P/f droop-based GFM control scheme, a detailed analysis of the multi-machine power coupling characteristics and the stability aspects of the two schemes is carried out. The analysis of the control mechanism reveals that the Q/f droop-based GFM control scheme has coupling characteristics between active and reactive power control path, while the P/f droop-based GFM control scheme has a natural power decoupling characteristic. Detailed analysis based on the small signal model shows that the power control coupling characteristics of the Q/f droop-based GFM control scheme may lead to power oscillation problems between multiple machines. And the P/f droop-based GFM control scheme has a natural decoupling characteristic and has better stability robustness under different load conditions, making it more conducive to achieving the synchronous and stable operation of hundreds of wind turbines in offshore high-capacity wind farms.