Generalised protection strategy for HB‐MMC‐MTDC systems with RL‐FCL under DC faults

Abstract

Half‐bridge modular multi‐level converter‐based multi‐terminal HVDC (HB‐MMC‐MTDC) technology is increasingly adopted as a promising option in the power transmission and distribution fields. However, how to cut off DC fault currents safely and ensure the continuous operation of the healthy parts of the HB‐MMC‐MTDC system under DC faults presents a huge challenge. This study proposes a generalised protection strategy on the use of a new RL‐fault current limiter (FCL) branch and hybrid DC circuit breakers (CBs), to deal with the bipolar DC faults for a given HB‐MMC‐MTDC system. It contributes to cut off the DC fault current before triggering MMC blocking, without influencing the continuous operation of the healthy parts of MTDC system. The operation characteristics of hybrid DC CBs and the equivalent circuit for the DC fault current is introduced, and the mathematical model of the DC fault current with the new RL‐FCL is discussed in detail. Theoretical and data analyses are performed to obtain the feasible parameters of the RL‐FCL. Simulation tests of an HB‐MMC‐MTDC system with different topologies are performed in PSCAD/EMTDC to validate the effectiveness of the generalised protection strategy.