Research On Control Strategy Of Interface Converter In Hybrid AC/DC Power Systems Based On Modular Multilevel Converter

With the decrease and pollution in use of fossil fuels, distributed generation based on renewable energy will be an important part of the future power generation. Because of the intermission and the variety of renewable energy, the power of renewable energy injects into grid directly will affect the system stability. The hybrid grids with the coexistence of AC and DC grid will be better transfered and utilized of the electric energy which generated from renewable energy. Grids with different types are connected by converters. With its characteristics, the Modular Multilevel Converter is well suited for transfering energy in the hybrid grid with high voltage and power.This paper takes the modular multilevel converter of the hybrid grid as research object. Firstly, the topology of the modular multilevel converter and the components of circulating current are analyzed. This paper proposes a current control method of the modular multilevel converter based on robust H￥ control. By using this method, the circulating current tracks the reference, so that the size of each current component can be controlled, the main components of the circulation harmonics can be eliminated, and the system can run more stable.When AC side of the modular multilevel converter is a weak AC system, the power transmission between the grid will be unstable, and then the System will be unstable. This paper proposes a energy control method of the modular multilevel converter based on the virtual synchronous generators. By emulating of the speed and excitation characteristics of the synchronous generator in powe r system, the power frequency controller and the excitation controller are design ed. With the virtual synchronous generators control, the system can still stable in weak AC system.In order to predict the stability of the system, the output impedance of the converter is analysed. This paper proposes a small-signal analysis method of the output impedance of converter. And the output impedance of converter when usd PLL and the virtual synchronous generators control are analysed.Finaly, the proposed control strategies have been verified in Matlab and the experimental platform.