Impedance Modeling and Analysis of MMC in Single-Star Configuration

This paper presents sequence impedance modeling and analysis of modular multilevel converters (MMC) in a single-star configuration for Static VAR Compensation (STATCOM) application. Prior work on impedance modeling of MMC has focused on the double-star configuration for high-voltage dc (HVDC) transmission applications. In the single-star configuration, each phase uses only a single arm of switching modules, avoiding the circulating current problem of the double-star configuration. The absence of circulating current also implies that the arm currents contain no low-order harmonics, provided that phase current control is performed properly. These features have important effects on the small-signal (impedance) characteristics of the converter which this paper tries to model. Sequence impedance models are developed by using the matrix method presented in [1], taking into account the different circuit and control configurations as well as operational features. A particular application for the developed models is STATCOM for damping of power system resonances involving renewable energy generation and/or HVDC transmission. Implications of single-star MMC impedance characteristics on STATCOM control design for such application are discussed.