With the capacity of new energy resources such as wind power and photovoltaic in Zhangbei region increasing rapidly

the reliable transmission and consumption capabilities of large-scale and high-proportion new energy resources are urgently improved. Considering the advantages of the VSC-HVDC transmission technology

Zhangbei project is designed as symmetrical bipolar system which adopts half-bridge modular multi-level converter（MMC） topology and overhead lines for power transmission. In this paper

a system scheme was presented

and some results were provided. Meanwhile

a novel modular cascaded hybrid DC breaker concept was proposed

and 500 kV VSC valve and DC breaker prototype were developed to meet the requirement of the Zhangbei four-terminal DC grid.