Research On Modulation Strategies Of Cascaded Three-phase-bridge Grid-connected Inverter

Cascaded multi-level inverter has been one of the research forcus in the field ofpower electronics technology, which is widely used in the new energy power generationand grid systems, HVDC, large capacity motor drive and other occasions. At present,cascaded H-bridge (Cascaded H-Bridge, CHB) multi-level inverter based on a singleH-bridge structure is widely used due to its modulation strategy and control methodshave been very mature. In recent years, some scholars have proposed a cascadethree-phase bridge multi-level inverter based on a three-phase bridge structure.Compared with the cascaded H-bridge inverter, cascade three-phase bridge inverter ismore suitable for the occasion of three-phase power and new energy power generationand grid systems. In this paper, cascade three-phase bridge multi-level inverter based ontheir modulation strategy, control methods and grid technologies were studied.First, mathematical model of a cascade three-phase bridge multi-level inverter isestablished, their cascade relationship and control method is viewed from amathematical point, and the voltage, current and power and other power relations ofsecondary cascade structure are analyzed, while current and voltage space vector vectordescription of inverter with secondary cascade structure and the structure of thecoupling transformer were studied，and more levels expansions of the latter wereanalyzed. Then, based on carrier phase shift PWM (CPS-PWM) modulation strategy,the paper proposes a new modulation strategy—carrier phase shift distributed PWM(CPSD-PWM).By analyzing output line voltage harmonic mathematical expressions inthe AC side of the inverter under constitute thinking, mathematical models and differentmodulation strategy，studies show that the nature of the CPSD-PWM is putting amathematical expression math factorejm into the second harmonic component of theline voltage mathematical expressions.Then, the cascade three-phase bridge inverter and its grid control strategy based onLCL filter structure are studied, the conversion method of suppression-current inductorconnected with inverter module is given and grid inverter equivalent circuit model isestablished. Simulations verifiy grid control strategy of cascade three-phase bridgemulti-level inverter, and indicate the correctness of the new modulation strategy.Meanwhile, by comparing the simulation results of both, the results show a negativesequence component of the output current in the AC side of the module is one of the reasons affecting the DC bus voltage fluctuations.Finally, a cascade of three-phase bridge inverter experimental platform is built,three separate DC bus is constructed by boost module, CPSD-PWM modulationstrategies are completed by using DSP and FPGA digital controller, experimentalresults confirm the correctness of the new modulation strategy.In summary, the mathematical model, topological expansion, modulation strategiesand grid inverter control methods of cascaded three-phase bridge multi-level inverterare analysised and researched in this paper detailedly. The correctness of theoreticalanalysis is demonstrated by simulation and experiment.