Driven by the increasing scale of offshore wind farms and the growing capacity of cross-country interconnections, High Voltage Direct Current (HVDC) systems based on bipolar Modular Multilevel Converter (MMC) technology are expected to play a key role in future transmission networks. However, concerns about techno-economic feasibility while relying exclusively on point-to-point links have prompted the industry to tackle the challenges of transitioning to Multi-Terminal (MT) configurations.

In this context, the InterOPERA project was launched to enable future HVDC systems from different suppliers to operate together, paving the way for Europe's first real-life MT, Multi-Vendor (MV), multi-purpose HVDC projects. InterOPERA has already made significant contributions, including the development of common functional specifications and minimum interface requirements. In the coming months, a Real-Time (RT) demonstrator will be implemented to validate and refine the proposed methods and processes, ensuring their practical applicability. This work focuses on activities supporting the deployment of the RT demonstrator, particularly on HVDC grid design studies using generic models to provide inputs to the detailed subsystem specifications. Three study packages have been defined:

1. DC load flow (LF)-based contingency analysis ensuring that DC voltage regulation capabilities and continuous operating ranges align with system operation under the selected risk policy.

2. Dynamic studies quantifying temporary excursions of electrical quantities following contingencies, to ensure they remain within equipment limits and do not trigger unwanted protection operations.

3. Transient studies assessing the required withstand capabilities of network assets and grid-connected devices, considering the performance of available protection system.

A series of three papers presents key findings from this work. This first part focuses on steady-state studies, determining secure DC voltage operating ranges and preliminary droop gains to ensure compliance with the N-1 rule. The proposed scenario-based approach was introduced in a previous work and applied to two configurations of a three-terminal HVDC system—with and without offshore wind generation—considering a pole-wise and multi-slope droop control strategy, as well as the presence of DC circuit breakers. The challenges of primary DC voltage control in highly inductive DC grids along with the impact of neutral voltage shifts during asymmetrical operation on pole-to-ground voltages have been highlighted.

Building on these findings, this follow-up work extends the analysis to degraded operating modes, including permanent asset unavailability and changes in the HVDC system grounding location. Various strategies for restoring operational security following unit outages are also explored.