Expanded Grid-Forming Solution and SGFM Control in HVDC System to Harvest Additional Energy from Offshore Wind Farms

This paper introduces a novel control feature integrated within Grid-forming control (GFM) for offshore AC voltage regulation and Synchronous Grid-forming (SGFM) control for the onshore

DC voltage controller. The SGFM converter, recognized for its responsiveness during large disturbances, requires an energy source to stabilize the DC voltage effectively. In the absence of such a source, a rapid DC voltage loop becomes essential to effectively counteract AC grid disturbances. This inherent limitation significantly restricts the SGFM's ability to support the

AC grid through active power contributions, such as phase jumps or Rate of Change of

Frequency (RoCoF).

To address these challenges, this paper explores the establishment of a coupling between offshore and onshore AC systems indirectly, enabling the use of energy stored in offshore Wind Turbines (WTs) operating in (S)GFM mode efficiently. The proposed communication-free solution innovatively employs DC voltage as a metric to generate a phase jump at the offshore

HVDC station, effectively inducing (S)GFM-controlled WTs to inject necessary active power to support the onshore AC grid robustly.

Given the limited stored energy in HVDC schemes, which are primarily transmission systems, rapid energy exchange with the DC circuit during initial responses is crucial, resulting in rapid changes in the DC voltage. Enhanced active power exchange at the Grid Access Point subsequently reduces the DC transmission voltage. This paper discusses how the reduction of

DC voltage below a certain threshold is utilized to calculate a phase angle adjustment at the offshore converter, dynamically extracting additional inertial power from offshore wind farms, when available. This approach is used in both bipole and monopole offshore HVDC transmission scheme projects and aligns with instantaneous reserve requirements as required by grid codes, ensuring robust support for the onshore AC system.

This paper explores the proposed technique aimed at optimizing the interaction between offshore and onshore systems. Through detailed analysis, including sensitivity test cases concerning measurement time delays and inertia constants, the paper investigates the effectiveness of these new functionalities in supporting the offshore AC system's contribution to the onshore grid. Furthermore, it examines the impact of current limitations and resynchronizing functions on system performance, emphasizing their significance in maintaining stability and compliance with grid code requirements.

Ultimately, this work provides a comprehensive evaluation of this advanced control mechanism, highlighting its potential to significantly enhance offshore wind transmission technology and support sustainable energy initiatives by efficiently harvesting additional energy from offshore wind farms.