Non-unit protection method for multi-terminal MMCHVDC systems based on fault severity analysis and coordinated relay operation

•This paper proposes a non-unit coordinated protection method for MMCHVDC grids, which rapidly response to severe faults and apply a 3-ms decision-making time window to deal with high-resistance faults.•Coordination between the protective relays at different points of the system is evaluated comprehensively.•Equipped with various fault detection criteria, the proposed protection method distinguishes internal faults at any location along the line up to 700 Ω.•The proposed protection method is able to interrupt fault currents under 6 ms and avoid converter blocking phenomenon.