DC Grid Protection Method Based on Phase Planes of Single-End Common-and Differential-Mode Components

This study develops a high-speed protection method for a modular multilevel converter (MMC)-based DC grids. The method utilizes the single-end derivative of common-mode (CM) and differential-mode (DM) currents to construct a protection phase-plane that divides different DC fault types into different regions. When the protection starts, the first sampling point that enters the fault regions of the protection phase-plane has the largest value. When five sampled data satisfy the corresponding fault conditions, the protection signal will be sent. This scheme ensures the rapidity of protection. CM and DM circuits of MMC, DC transmission line, and fault transition resistance under different fault types are derived following the principle of modulus decomposition. Finally, the synthesized equivalent circuits are obtained. On this basis, the expressions of CM and DM currents are derived to analyze the characteristics of fault currents on the protection phase-plane and provide the rules for distinguishing different fault types. A four-terminal MMC-based DC grid simulation model is established in PSCAD/EMTDC. The rapidness and reliability of the proposed protection method are then verified using different simulation cases. The proposed method is also validated on a scaled-down hardware setup in the laboratory.