The key technologies of VSC-MTDC and its application in China

The application of VSC-HVDC (Voltage Source Converter Based High Voltage Direct Current Transmission) is one of the proven solutions to the transmission problems of offshore wind power. As AC and DC networks become more complex, VSC-MTDC (Multi-terminal High Voltage Direct Current Transmission Based on VSC) is developed based on two-terminal HVDC to offer technology support for the accessing among multi-supplier center, multi-loads center and distributed renewable powers. This paper reviews the MTDC and its implementation in China, mainly discusses the topologies of DC network of VSC-MTDC system and the control strategies of VSC-MTDC which includes converter station control and coordinated control. In most cases, double closed-loop space vector control (DCSVC) strategy applied in the Zhoushan project, which includes five-terminals and has been put into operation, is adopted for the converter station control. But the method can not work under the unbalanced voltage situation, thus an improved double closed-loop space vector control strategy, which includes positive and negative sequence components, is proposed in this paper. As for coordinated control, master-slave control, DC voltage droop control and DC voltage margin control are presented. As for the part of implementation of MTDC in China, Zhoushan project is presented in detail. This paper provides good theoretical foundation for the grid integration and the transmission of large offshore wind farms.