To integrate offshore wind power, facilitate long-distance power transmission and interconnect asynchronous AC grids, HVDC systems are a crucial element of the European power system. While the majority of existing HVDC systems are designed as point-to-point (PtP) links, current grid development plans foresee a development towards multi-terminal HVDC (MTDC) systems. To protect such systems against DC-side faults, dedicated DC protection systems, typically based on DC circuit breakers (DCCBs) and current-limiting reactors, are required. The design of this protection equipment governs the converters’ dynamic behavior during DC-side and AC-side fault scenarios. Therefore, an adaptation of established PtP converter designs may be required to ensure compliance with fault-ride-through requirements. Previous research indicates that this can be achieved by increasing the number of submodules. However, existing research has not quantified how different degrees of freedom in HVDC system design translate into varying converter upscaling requirements, such as the required number of submodules.

This paper investigates how HVDC design choices, as well as AC-side system strength and infeed conditions, affect the requirements for converter upscaling. Specifically, the considered parameters include (i) the protection philosophy, covering fully-selective (FS) strategies with DCCBs at each line end and partially-selective (PS) strategies with a reduced number of DCCBs, (ii) converter control design and overcurrent capabilities and (iii) the strength of the connected AC grids. Reduced test systems for bipolar MTDC systems with PS and FS protection are developed and implemented in an EMT simulation environment. For each parameter variation, the dynamic converter response to AC-side and DC-side faults is evaluated and mapped to the required converter hardware upscaling relative to a reference, close-to-reality PtP converter design. Additionally, sensitivity analyses on the selection of DCCB technology and current-limiting inductance are performed.

The results indicate that the converter control design – in particular, the response to DC-side faults – strongly affects the necessary converter upscaling. Control designs with the objective of maintaining a constant DC voltage during faults are identified as the worst case, requiring the largest increase in submodule count. FS protection requires greater upscaling than PS protection. AC grid strength significantly affects the dynamic converter performance. If weak grid conditions, such as connecting an offshore wind farm via HVDC, are considered, more submodules are needed to meet fault-ride-through requirements. Across all investigated impact factors, the sensitivity analyses highlight that faster DCCBs consistently reduce the required upscaling. Overall, the findings highlight the importance of considering key HVDC system parameters when designing converters for MTDC systems.