A Pilot Protection for LCC-HVDC Hybrid System Considering Three-Phase Coupling

LCC-HVDC systems are widely used in China. However, commutation failure remains an inherent flaw in thyristor-based converter systems that has yet to be eliminated. Additionally, during the occurrence and restoration of AC line faults at the inverter side, the equivalent DC system exhibits nonlinear characteristics, leading to uncertain impacts on the adaptability of traditional protections in AC grids with inverter stations. The Tellegen's theorem, which is only related to the circuit topology and is not affected by branch nonlinearity, is proposed in this paper to be applied in AC systems with inverter stations. Considering the three-phase coupling of lines, a novel pilot protection method that is not influenced by the characteristics of inverter stations is proposed and tested in a PSCAD simulation system to validate its feasibility.