Small-signal model analysis of droop-controlled modular multilevel converters with circulating current suppressing controller

This paper presents a small signal eigenvalue analysis applied to a droop-controlled HVDC terminal based on the Modular Multilevel Converter (MMC) topology. The applied linearised model is derived from previous modelling efforts recently proposed in the literature, which rely on the application of three Park transformations at different frequencies (ω, -2ω and 3ω) applied to associated variables defined within the MMC model. The investigated configuration is controlled under the well-known Circulating Current Suppression Controller (CCSC). The developed small-signal model is utilized to evaluate two different approaches for calculating the insertion index for modulation of the MMC, and to reveal potential stability problems in the system. It is demonstrated by participation factor analysis that the potentially unstable modes of the system under the investigated control strategy are linked to the uncontrolled zero-sequence component of the common-mode current resulting from the CCSC.