Research on Control Method of VSC Based DC Grid with Weak AC System Integrated

Scenario of Passive grid connecting two VSC-HVDC transmission systems has been investigated in this paper. A control method with hierarchy is proposed. VSCs have been grouped into a hierarchy of various categories. Distance which denotes the number of connections with strong grid of VSC stations is used to define the hierarchy used in this paper. Different control methods are used in different categories to regulate different variables. This method can be used in DC grids with many VSCs grouped into a hierarchy of various categories. This paper presents a multilayer DC system with a passive AC grid inside the DC grid. The control principle of VSCs connected with the passive grid is investigated. The AC voltage control and active power control is assigned to different VSC based on its power flow in the inside passive AC grid. Also a DC voltage control is integrated in the hierarchy control system to improve the dynamic instability.