Modeling and Control of Modular Multilevel Voltage Source Converters for HVDC Application with Generalized DC Bus

Abstract

Control methods of the Modular Multilevel Converter (MMC) are classified into indirect modulation based control and direct modulation based control. In this thesis, a modified modeling of an indirect modulated MMC is proposed for generalized DC bus of High Voltage DC (HVDC) transmission. Based on the proposed modeling, a comprehensive arm capacitor energy control strategy is derived, which is valid regardless of the characteristics of the DC bus. In addition, for the direct modulated MMC, mechanism and dynamics of the natural balancing of the arm capacitor energy are analyzed. It is proven mathematically in this thesis that arm capacitor energy of six arms of the MMC converges to be balanced inherently regardless of the characteristics of the DC bus while an MMC is direct modulated. A novel control strategy of the MMC based Voltage Source Converter (VSC)-HVDC transmission system is also proposed. Different from the conventional two-level converter based transmission system or the direct modulated MMC based transmission system, the instantaneous DC bus voltage of the MMC is fully decoupled from the energy stored in the DC capacitors of the converter by the proposed method. The transmission line voltage fluctuation is also suppressed during power flow variation by the proposed method. Validity of the conducted work in this thesis is verified by both computer simulations and experiments.