I. Introduction
Since the 1990s, with the development of global economic integration, the research of marine power transmission and the importance of environmental protection, the technology of submarine cable has been vigorously developed at home and abroad, and the demand for submarine cables has been increasing [1]–[3]. Especially, with the continuous progress of HVDC technology and its advantages in large transmission capacity, long transmission distance and little influence on the environment, the cross-sea power grid interconnection with HVDC transmission technology has been gradually recognized by domestic and foreign electric power construction [1], [4]–[6]. In addition, the VSC- HVDC technology, which based on VSC technology, is developing rapidly and maturely. Compared with the traditional HVDC technology, it has monopole operation, independent control of active and reactive power, without AC side reactive power and other technical advantages [7]–[9].