Control Strategy Of Modular Multilevel Converter Based On Common-mode And Differential-model Decomposition

The Modular Multilevel Converter(MMC)is a popular subject of modern research in power electronics.MMCs are distinguished by their modular nature that makes them suitable for a wide range of high power and high voltage applications.The increased utilization of HVDC power transmission solutions has resulted in MMC becoming a more common type of converter.Other applications include interfacing renewable energy power sources to the grid and motor drives.MMC uses numerous low-rated IGBTs to produce the desired voltage.MMC is made up of a series of IGBT half-bridge circuits with a capacitor across both devices.To improve the control performance of MMC,a novel control strategy based on common-mode and differential-mode decomposition are presented.This master dissertation will evaluate the operating principles of the MMC.A phase shift carrier modulation strategy is used to reduce the size and cost of the passive components,such as the arm inductances and the cell capacitors.Control objectives,as well as the evaluation and classification of the control strategy,has been proposed.Among the main control targets in a modular multilevel converter(MMC)are the control of the Common-mode current and Differential-mode current within the phase leg of topology.The Common-mode current increases the RMS(Root Mean Square)value of the arm current and results in an increase in the power loss and reduction of the system efficiency.Therefore,the suppression of common-mode current is necessary for modular multilevel converters(MMC),a common-mode current in each phase unit is caused by the difference between the submodule capacitors voltage in the upper and lower arm of the converter leg and also by the DC voltage ripples of the phase unit.For controlling such common-mode current,a closed-loop PI+R controller strategy is implemented.The role of this controller is to suppress the second-order harmonic current in common mode current,which increases the RMS value and harmonic in the output current.Due to the existence of these ripples in common-mode,the output voltage also encompasses a dc and fundamental frequency component,which is not appropriate for the load side.To minimize the adverse impact of such a problem on the output voltage and current waveforms,a dedicated PR controller is used in leg topology.The primary purpose of this controller is to prevent the development of a dc component in the ac side waveforms and to suppress fundamental oscillation in the output current.Additionally,to balance the voltages of the capacitor within the leg topology of a converter,the unbalance capacitor voltages are passed through a notch filter in order to balance and eliminate high fundamental frequency voltage ripple and extract the DC voltage component.Simulation work is done in MATLAB/SIMULINK.