Phasor Model of Modular Multilevel Converter With Circulating Current Suppression Control

This paper presents the model for the modular multilevel converter (MMC) in phasor format, which is convenient for power-flow and parameter studies. The model is derived in rotating the d- q coordinate frame, and the coordinate frame at double the fundamental frequency, in steady state. A substantial analytical basis is presented in order to facilitate direct mathematical manipulations of nonlinear terms in the rotating frame. An eighth-order model is first derived which includes circulating current representation. Later, the circulating current suppression controller (CCSC) is modelled and the magnitude of second harmonic control inputs is studied. The final model for MMC with CCSC is given in a simple and convenient form for power-flow studies. The accuracy of the proposed models is verified against a detailed dynamic MMC benchmark model in PSCAD. Finally, a comparison between the MMC model and two-level voltage-source converter model is given.