Research on single-phase grounding fault protection on the AC side of VSC-HVDC systems considering third harmonic modulation

To achieve simple and reliable single-phase grounding fault protection on the AC side of VSC-HVDC systems， this paper focuses on systems where the neutral point of the converter valve's AC side is impedance-grounded. The zero-sequence fundamental component on the AC side and the common-mode component on the DC side are analyzed， and analytical expressions for the zero-sequence fundamental voltage and current at both the faulted and non-faulted terminals during a metallic grounding fault are derived. For VSC-HVDC systems utilizing third harmonic injection modulation， the phase characteristics of the longitudinal third harmonic voltage induced by this modulation are examined， and expressions for the third harmonic voltage and current are derived. Based on this analysis， protection criteria using the zero-sequence fundamental voltage and third harmonic voltage at neutral point are proposed to detect grounding faults and determine their location， along with the corresponding setting calculation method. Simulation results under various operating conditions show that the protection mechanism correctly activates when a grounding fault occurs at the protection installation terminal and does not operate erroneously when faults occur at other terminals.