Non-isolated Modular Multilevel DC/DC Converter Operated In Quasi-square-wave Mode

DC grids is an attractive means to achieve power exchange among regions and enhance the reliability of the power grids.To construct the dc grids,especially the high voltage direct current(HVDC)grids,the high-voltage high-power dc/dc converter is one of the key equipments.This thesis mainly studies the characteristics and control methods of the quasi-square-wave operated non-isolated modular multilevel dc/dc converter(NI-MMDCC).In this thesis,the basic operating principle of the NI-MMDCC and the energy balance mechanism of the chain-links are presented.Since the terminal voltage of the chain-links can be controlled to be the desired waveform,the chain-links is regarded as the voltage sources,and the equivalent circuits of the NI-MMDCC including the dc equivalent and ac equivalent circuits,are obtained.Through the analysis of dc equivalent and ac equivalent circuit,it is pointed out that ac component of the current can be used to achieve the energy balance of the chain-links.The terminal voltage of the chain-links in the NI-MMDCC are set to be quasi-square-wave in this thesis.The characteristics of the QSW NI-MMDCC with phase-shift control method are analyzed,and the power transfer characteristics are studied.A closed-loop control strategy composed of an output current control and submodule capacitor voltage balance scheme is presented.The dominant advantage of the closed-loop control strategy is simple and easy to be implemented.When the voltage transfer ratio is not equal to 0.5,the root mean square(RMS)value of the currents in the QSW NI-MMDCC with phase-shift control method will be a bit large.To overcome this problem,the phase-shift control method is improved by adding control of the amplitude of the ac components of the chain-link terminal voltages.Thus,a phase-shift plus amplitude control method is proposed.In order to minimize the RMS value of the currents,the phase-shift plus amplitude control method is optimized,resulting in a higher conversion efficiency.The proposed phase-shift plus amplitude control method can be applied to not only the NI-MMDCC,but also the isolated modular multilevel dc/dc converter.A 1 kW NI-MMDCC prototype is built and tested in the lab.The experimental results verify the correctness of the theoretical analysis.