On the morning of December 9, 2016, the 200 kV HVDC circuit breaker independently developed by China was put into operation in Zhoushan. The circuit breaker is the first DC circuit breaker put into engineering application, can disconnect the fault current within 3 milliseconds, the speed is 100 times faster than the average person blink an eye, will effectively make up for the relevant gaps in the field of DC transmission in China, and promote the qualitative change of DC transmission from "line" to "network".

"Dc power transmission has low losses and is especially suitable for long-distance transmission. If the DC lines are connected into a network, it will revolutionize the efficient use of energy, complement the current mature AC power grid, and promote the global interconnection of energy networks." Qiao Min, an engineer from the transportation and inspection Department of State Grid Zhejiang Electric Power Company, said that the current successful attempt in Zhoushan will make China take a big step forward in the field of DC transmission and lay the foundation for the development of higher-level DC power grids in the future.

At present, it is not possible to form large-scale DC power grid due to the limitation of high voltage DC breaking technology and equipment. As the Zhoushan multi-terminal flexible DC power transmission project put into operation in 2014, there is a converter station on five islands, Dinghai, Daishan, Qushan, Yangshan and Si Jiao, and each station is connected by submarine cables and overhead lines. The input and exit of a single site cannot be realized. If a site fails, other sites will stop along with it, and the system operation mode is not flexible.

With the new equipment put into use, Zhoushan soft and direct transmission project can not only realize the stable operation of sound DC subsystems and network reconstruction after faults, reduce the impact of fault current on the power grid, ensure the safety of system equipment, but also realize the rapid on-line back-off of the converter station and line, and quickly start the system after the fault is cleared.

Xu Jeng, deputy director of the soft direct transport inspection room of Zhoushan Power Supply Company of the State Grid, said: "Take the soft direct project as a heating system of a community, and each converter station is equivalent to the heating equipment of each building. In the past, a building had a heating problem, and the heating system of the entire community needed to be shut down for maintenance. Now, with the DC circuit breaker, a problem in one building will not cause other buildings to stop, and the community heating system will always operate reliably and continuously."

At the same time, the development of distributed clean energy such as wind power and solar energy is increasingly rapid, and the flexible DC transmission system is considered to be the most friendly system for long-distance distributed energy access. Taking offshore wind power as an example, the flexible DC transmission system equipped with DC circuit breakers can quickly and flexibly realize the input and withdrawal of a single long-distance offshore wind power, ensuring the efficient consumption of offshore wind energy and the reliable operation of the power network.