Small Signal Stability Investigation of the MMC-HVDC Grid

This paper presents a small-signal dynamic model for a modular multilevel converter (MMC) based HVDC grid, and then systematically i) identifies the system dominant oscillatory modes, ii) quantifies their propensity to instability phenomena, and iii) devises appropriate countermeasures. Based on the developed model, the eigen structure of an HVDC grid is characterized and the impact of the system parameters, configuration, AC system strength and operating point on the damping ratios of the oscillatory modes, and consequently the HVDC grid stability, are investigated. The model is also applied to systematically determine the sensitive parameters and optimize them to mitigate oscillatory modes and prevent instability. The eigen analysis results are verified based on time-domain simulation studies in the PSCAD/EMTDC platform.