Analysis on the dynamic behaviours and interactions of VSC-MTDC grid

Different from the synchronisation stability for alternating current (AC) systems, the potential risk caused by the interaction between different terminals in voltage source converter-based multi-terminal direct current (VSC-MTDC) grids is emerging, which may impose negative influences on system operation and control. Focusing on this topic, a comprehensive modal analysis is presented to analyse and characterise the dynamic behaviours and interactions of VSC-MTDC grids. The state-space model of VSC-MTDC grids associated with AC systems is developed and participation factor analysis is utilised to identify the physical source of the oscillation modes. This study investigates the voltage fluctuations in VSC-MTDC grids induced by the oscillation modes with the application of extended participation factors. A Monte Carlo-based statistic method has been adopted for the calculations. The interactions between different terminals are identified using mode shape analysis, and quantitative calculation of eigenvalue sensitivities is presented to assess the effect of parameter variation on modal shift. Furthermore, an approach based on the singular value decomposition is proposed for the quantitative analysis of the interaction strength among different control loops. Simulations on a multi-terminal test system are carried out to verify the analytical results.