Control And Protection Strategy Of Modular Multilevel Converter

Modular Multilevel Converter (MMC) is a new type of voltage source converter(VSC)topology, compared to the traditional two-level or three-level VSC topologies, it has many advantages, for example:it can expand to any level for its modular structure; its output voltage is high quality and switching frequency is low. For these advantages, it is widely used in flexible high dc transmission system.This thesis firstly classifies the traditional converters and the research with aspect to MMC in home and abroad. The structure of MMC and its operational principles are analyzed, then the mathematical model of MMC is established. This thesis studies two kinds of carrier phase-shift sinusoidal pulse width modulation and conclud their own characteristics with simulation software.This thesis studies the control strategies of DC capacitor voltage, including pre-charging control and balancing control. The way of pre-charge have two schemes for MMC, and this thesis pay emphasis on self-excitation mode, and the process is divided into uncontrollable stage and controllable stage, The strategy accomplishes fleet pre-charging, limit the over-current and over-voltage when it starts charge with ac source from power grid. In order to suppress the capacitor voltage unbalanced, this thesis proposes a hierarchical voltage balancing control strategy. Circulating current’s mathematical expression is derived under ideal working conditions. The simulation results show that, by circuiting current suppressing controller, the frequency-doubled circulating current is limited and the distortion of arm current is reduced.Redundancy protection is adopted and the strategy is to make sure that the redundant sub-modules can be put into operation very quickly.This thesis studies the application of MMC in the field of flexible high voltage dc transmission. A decoupling control strategy is designed for achieving active power and reactive power control respectively.Finally, a platform is designed to test operating state of sub-module.