Design of protection functions and block strategies for inverter station of a hybrid cascaded UHVDC transmission system

This paper focuses on the inverter station of ultra-high voltage direct current （UHVDC） transmission system with a hybrid cascaded structure. Based on its primary topological structure， adhering to the principles of protection without dead zones and easy fault location， and considering the installation position of measurement transformers， a protection partition suggestion and configuration plan are proposed. According to the location and severity of faults in inverter station， hierarchical block strategies including line-commutated converter （LCC） block， single modular multilevel converter （MMC） block， overall MMC block， sequential pole block， and step pole block are proposed， aiming to quickly eliminate faults and reduce the outage range. Finally， a real-time closed-loop simulation system is established based on the actual control and protection devices of the Baihetan-Jiangsu hybrid cascaded UHVDC transmission project and a real-time digital simulation （RTDS） model established according to the complete set of engineering parameters. The feasibility and effectiveness of the proposed protection scheme and block strategies are verified through simulation.