Sequence Impedance Modeling and Analysis of MMC in Single-Star Configuration

This paper presents sequence impedance modeling and analysis of modular multilevel converters (MMCs) in a single-star configuration for static VAR compensation (STATCOM) application. Prior work on impedance modeling of MMC is limited to the double-star configuration for high-voltage dc transmission applications. Compared to that, single-star configuration uses only a single arm of switching modules in each phase, eliminating the second-harmonic circulating current and the need for the associated circulating current control. On the other hand, the lack of connection to a stiff dc bus makes it essential to include module capacitor voltage control in the model. A special form of the multi-harmonic linearization method is applied to take advantage of the simpler harmonic spectra and to reduce the complexity of the resulting models. The developed models are verified by simulation and experiment, respectively, and used to understand the similarities and differences in the impedance characteristics of the two configurations as well as the effects on stability. A representative wind power plant is presented as an example to demonstrate the application of STATCOM impedance models on the system stability analysis.