Startup Control of Grid-Forming Offshore Wind Turbines Connected to the Diode-Rectifier-Based HVDC Link

The Diode-Rectifier (DR) based HVDC has been considered as an economical solution to connect remote offshore wind turbines (WTs) to the onshore power grid. However, the DR is a passive device and cannot support the startup of the WTs. Therefore, it is worth finding an appropriate and cost-effective startup strategy for the DR-connected WTs. This paper investigates to use of wind energy to startup those WTs to avoid the need for additional grid support or the energy storage system, which can reduce the overall cost of such transmission system. The first challenge for this strategy is that the output active power of WTs can be extremely low during the startup process, which puts the WT rotor at a high risk of overspeed. Another challenge is to prevent power surges between synchronizing WTs. To address those issues, the pitch control has been innovated for the DR-connected WTs. In addition, a seamless synchronization control of the DR-connected WTs is proposed, which does not need the phase-locked-loop and can facilitate the whole startup process of the DR-connected WTs. The feasibility of these proposed control strategies for the startup of the DR-connected WTs is verified by comprehensive simulation studies.