Non-Unit Line Protection Method for Multi-Terminal Hvdc System Based on Variational Mode Decomposition and Energy Entropy

In order to enhance the robustness against lightning disturbance and high-impedance fault, a non-unit protection based on variational mode decomposition and energy entropy (VMDEE) is proposed for multi-terminal high-voltage dc (MT-HVDC) system. Firstly, the instantaneous frequency of line-mode voltage is used for protection start-up, which is also utilized to avoid protection triggering mistakes under serious external faults. It is noted that VMDEE of pole current and fault impedance are positively correlated. Therefore, the ability of high-impedance fault recognition is significantly improved via this protection method. Moreover, the energy distribution pattern of VMD result is utilized to differ lightning disturbance from line faults and normal state. A four-terminal model of Zhangbei HVDC grid is developed in PSCAD/EMTDC environment for case study and the fault data is processed in MATLAB platform. The feasibility and effeteness of the proposed non-unit protection is verified with simulation research that considers different potential influence