Fault Current Estimation in Multi-Terminal HVdc Grids Considering MMC Control

DC faults are critical events for the multi-terminal HVdc protection systems, where knowing the critical fault time is fundamental in the protections design. Existing literature analyzes the critical fault time with simulations or with complex analysis, requiring large computational effort and they are not robust to parameters variations. Furthermore, standard approaches rely on simplified modular multilevel converter (MMC) models neglecting the influence of converter control loops dynamic. As result, the prediction of the node voltages and especially the dc currents become increasingly inaccurate as bigger is the distance from the fault. This paper introduces a fault current estimation method in MMC-based multi-terminal dc (MTdc) grids, which improves the current differential equation based methodologies including the dynamics of power and dc voltage controllers. The proposed method gives the possibility of analyzing the fault current under different grid conditions (e.g., fault resistance and limiting inductor value). Furthermore, this method is extended to bipolar MTdc systems in case of pole-to-pole and pole-to-ground faults, and it is validated by means of simulations in PSCAD environment and standard CIGRE MTdc grid.