DC FRT Analysis and Disturbance Propagation of Grid-Forming HVDC Converters in Future MTDC Systems

The ongoing energy transition is driving a growing interest in multiterminal DC (MTDC) systems and grid-forming (GFM) control strategies. As an increasing share of renewable energy sources (RES) is connected to power systems through converter-based interfaces, conventional synchronous generators are progressively displaced, leading to reduced system inertia and weaker voltage references. In this context, MTDC systems equipped with advanced control and protection schemes are expected to play a key role in ensuring both transmission capability and system stability. This paper presents a system-level analysis of an MTDC network behaviour during DC fault ride-through (DC FRT) events under a partially selective DC protection philosophy, with a gradual introduction of GFM control for DC voltage regulation. The sizing and coordination of protection elements are detailed, and a partially selective protection scheme with three fault segmentation zones is designed according to DC FRT performance. The study is motivated by future bipolar MTDC grids with large-scale offshore wind power integration and the progressive deployment of GFM control in converters interfacing AC/DC systems. To focus on MTDC dynamics, offshore wind farms are modelled using simplified models sufficient for DC FRT investigations. EMT simulations are carried out by progressively increasing the number of GFM-controlled converter stations providing DC voltage regulation, allowing the analysis of operational performance, stability, and disturbance propagation. The results corroborate the presented analysis, highlighting key control coordination challenges and hardware limitations inherent to conventional half-bridge MMC-based MTDC systems under