Small-signal modelling for in-depth modal analysis of an MTDC system

In this paper, the state-space model of a VSC-MTDC system is derived from individual components and a modified PI section model for cables is proposed which considers both core and screen conductors as well as their coupling. A modal analysis performed on the complete system reveals the influence of each component on the system modes and permits the identification of a dominant DC voltage mode. This mode has proved to be largely affected by the energy storage level of the DC grid, and to be predominantly influenced by the voltage-droop parameters of the converters, meaning that the DC voltage dynamic of the MTDC system can be imposed thanks to a judicious choice of the voltage-droop parameters.