Optimization of the Modular Multilevel Converters Performance using the Second Harmonic of the Module Current

At the design framework, in this Thesis s the modular multilevel converter (MMC) is analysed and proposed for connecting the power grids. In order to mathematically describe the converter behaviour, the linear time-varying model (LTV) is developed. The system is considered linear between two switching instants. Since the switching functions have a periodical nature, the steady state of the system is calculated. Using the (LTV) model, both balanced and imbalanced operations of the converter are analyzed. Since the calculation of the steady state requires a low computational effort, the proposed method showed a good alternative to the numerical iterative procedures. Applying the LTV method to the modular multilevel converter, the different operating points are determined depending from the converter parameters. Special attention Was paid on finding the optimal operating point concerning the module current. Since the presence of the second harmonic of the converter's arm current and its dependence from the converter parameters is determined, it is used to optimize the module conditions. The resonance points of the converter's arm current were also detected and presented.