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 third part focuses on transient stresses at the connection points of various subsystems during DC faults, specifically Short-Circuit Currents (SCC) and Temporary Overvoltages (DC-TOV) throughout the fault separation process. Detailed EMT simulations examine pole-to-ground faults along DC cables and at DC switching station (DCSS) busbars, as well as single-phase faults at the transformer’s converter-side.

The impact of critical design parameters is analysed, including DC circuit breaker (DC CB) fault neutralization times, converter current capabilities, DC reactor sizing, and system grounding considerations. The challenges and opportunities of applying the InterOPERA functional framework, as proposed in Deliverable 2.1, are discussed. Finally, insulation coordination aspects are briefly addressed, with preliminary guidelines on defining the dedicated metallic return (DMR) insulation level and setting grid-side surge arrester protective levels.