DC protection coordination for multi-terminal HB-MMC-based HVDC grids

Fast progress of High Voltage Direct Current (HVDC) technology has been considerable over the past decades. Emerging Modular Multilevel Converter (MMC) provides power systems with promising solutions to integrate large-scale renewable energy sources, as well as multiple asynchronous power grids. Consequently, implementation of a multi-terminal HVDC transmission system has been easier using MMC converters. However, protection of multi-terminal MMC-based HVDC grids under DC fault conditions is a challenge for the industry. This study sheds light on concept of system-level DC protection coordination for multi-terminal HVDC grids. Although, there are different protection methods for HVDC transmission systems including multi-terminal ones, there are no well-defined protection coordination strategies for transmission links protection and belonging HVDC converters under DC fault conditions. The proposed protection coordination prevents unnecessary tripping of MMCs in the multi-area HVDC grids and supports the system with minimum effect on healthy parts. For the first time, different HVDC Converter Tripping (HCT) profiles are presented based on a fully selective protection strategy and their characteristics are discussed comprehensively. Simulation results demonstrate the effectiveness of the proposed strategy for multi-terminal MMC-based HVDC grids. The proposed coordination can be applied to DC grid code requirements of meshed HVDC grids in the future. This study presents a new protection coordination between DCCBs as DC link protection and HB-MMC converters in multi-terminal HVDC grids under DC fault conditions. DCCBs tripping coordination considering Fault-Ride-Through (FRT) scenario assigned to MMC converters is investigated in the study. The proposed coordination not only prevents unnecessary disconnection of the MMC, but also determines when it is needed to disconnect the MMC. In this regard, for the first time, a HCT profile is proposed based on DC voltage and its parameters are analyzed. Different HCT profiles are applied to MMCs of a multi-terminal HVDC grid based on the converters' positions to fault location. The study discusses a coordinated protection approach using the HCT profiles assigned to local and remote MMCs in HVDC grids considering different FRT scenarios.image