As an emerging renewable energy technology, offshore wind harnesses superior and steadier wind resources, enabling the implementation of large-scale projects exceeding gigawatt capacities. This rapid expansion pushes offshore wind power plants (OWPPs) further from shore. High voltage direct current (HVDC) has emerged as a key technology for connecting OWPPs due to its high efficiency in transmitting electrical energy over long distances. As a result of the ongoing energy transition, HVDC-connected OWPPs are increasingly being operated with grid-forming (GFM) control strategies, replacing the previously employed grid-following (GFL) control approaches. This shift enables the HVDC-link and the OWPPs to contribute to system stability by providing ancillary services to the grid.

A sensitivity analysis is conducted to evaluate the impact of the GFM HVDC control on the dynamic behavior of GFM offshore wind turbines. A symmetrical monopole HVDC link with Modular Multilevel Converters (MMCs) connects the offshore wind farm. The GFM control concept is giving rise to a fundamentally different behavior of the renewable energy sources. A voltage-sensitive GFM control scheme replaces the ideally voltage-independent GFL control schemes. Therefore, the number of grid disturbances per year, affecting the lifetime of the components of the renewable energy sources, might rise.

First, the investigated system is introduced. Subsequently, the operational strategy for propagating onshore grid events to offshore wind turbines is outlined. The wind turbines are modeled using both an electrical and a mechanical model. The mechanical model incorporates a two-mass drivetrain model. However, the wind turbines' modeling is not within this paper's scope. All grid-side converters in the system have implemented a GFM control. The sensitivity analysis performed in this paper's investigation focuses on HVDC control. A variation in the GFM onshore HVDC converter station and the overall energy control is carried out. The overall energy control influences the amount of disturbances directly transferred to the offshore HVDC converter station. Different onshore grid events, e.g., voltage phase angle jumps and Rate of Change of Frequency (RoCoF) events, are used to emphasize the provided phase step or RoCoF power and the provided energy from the MMC's and the wind turbines.

The performed investigations show how a variation in the HVDC control can distribute the energy supplied to the grid disturbance between the HVDC and the energy stored in the rotational mass of the wind turbines. The steepness of the onshore grid event, transferred to the offshore grid, influences the mechanical system of the wind turbines. Consequently, a higher contribution of the HVDC to the onshore grid event reduces the steepness of the event seen by the offshore wind turbines.