HVDC System Energization via Grid-forming Offshore Wind Turbines: Sensitivity Analysis and Technical Requirements

Renewable energy sources (RES), especially HVDC-connected wind power plants (WPP), have been put into focus to provide services for the onshore grid, such as black-start. To achieve black-start capability, new requirements are imposed on the controls, requiring grid-forming capability for the offshore WPP as well as for the onshore HVDC converter. Recent publications have shown the general feasibility of grid-forming WPPs for black-starting HVDC links, but focus on single aspects of the energization or rely on simplified wind farm, converter, or control models. Until now, important aspects such as the influence of converter or WPP design, as well as a derivation of the requirements posed on the different components for each step of the black-start have not yet been analysed. This paper fills these gaps through an in-depth analysis of the entire energization process, considering different startup methods with a close-to-real converter design. From the investigations, technical requirements for black-start capable HVDC-connected offshore WPPs are derived. The energization of the HVDC system was found to pose the highest requirements on active power availability of the WPP, independently of the chosen startup method. For the WPP energization, reactive power was found to be the main system design factor.