On the damping and coupling impacts of DC fault-limiting reactors in multi-terminal DC grids

This paper investigates the impact of DC fault-limiting reactors (DCRs) on the dynamics of Modular Multilevel converter-based multi-terminal DC (MTDC) grids, with a particular focus on their damping and coupling characteristics. The first part examines the instability introduced by large DCR values using a simplified low-frequency model and a comprehensive small-signal analysis of a four-terminal meshed DC grid. A dedicated controller is proposed to stabilize the system and is compared with other mitigation measures, such as modifying the control mode and changing the power flow direction. The second part investigates the coupling effects of DCRs in DC grids, showing that the commonly assumed equivalent DC grid capacitance becomes invalid in the presence of DCRs. This leads to different conclusions regarding DC voltage support between stations, particularly during fast transient responses. All findings are validated through time-domain simulations of a four-terminal DC grid. • Analysis of the impact of DC fault-limiting reactors (DCRs) on MTDC grid stability using small-signal and time-domain studies. • Instability mechanisms linked to DCRs are studies with simplified MMC representations and validated with full-grid simulations. • Lead compensation active damping is proposed to reshape MMC impedance and stabilize systems with large DCRs. • DCRs are proven to impact inter-station DC voltage coupling, invalidating the equivalent capacitance assumption during fast transients.