| Multilevel Converters have been under research and development for more than three decades and have found successful industrial application.Modular multilevel converter(MMC)is considered to be one of the most promising multilevel topologies from the medium-to high-voltage power applications,there are still new topologies and control algorithms been proposed to improve their performance and efficiency in the past few years.Firstly,the circuit topology,pulse modulation algorithm and DC voltage-balancing control algorithm commonly used in multi-level converters are introduced,and the combination strategy of pulse modulation algorithm and DC voltage balance control algorithm for multi-level converters under different topologies is given.Equivalent mathematic models under various combination strategies are established,the voltage fluctuation characteristics and harmonic characteristics of output voltage of multi-level converter under different combination strategies are analyzed theoretically,and the causes of voltage fluctuation on DC side of multi-level converter and the relationship between voltage fluctuation on DC side and harmonic characteristics of output voltage are given.Secondly,the basic theory of PWM modulation technology--volt-second equivalence is introduced,and a mathematical model of volt-second equivalence is established to illustrate its working principle which means two trigger pulses with different duty cycles are loaded on two unequal voltages,and their average output voltage is equal in one switching cycle on the premise of equal trigger pulse area.In the way of adjusting duty cycle of trigger pulse,two modulation methods: modulation wave modulation and carrier modulation are proposed,and the specific relationship among modulation wave,carrier amplitude and duty cycle of trigger pulse is given.A novel control algorithm based on carrier amplitude modulation is proposed for DC-side voltage fluctuation of multilevel converters based on volt-second equivalence principle.The control algorithm adjusts the amplitude of the corresponding carrier according to the specific fluctuation of the DC side voltage of the multilevel converter,and then adjusts the duty cycle of the trigger pulse.Finally,the control of the output voltage of the multilevel converter is realized,and the harmonic characteristics of output voltage of multilevel converter have also been improved noticeably.Finally,the cascaded single-phase H-bridge converter and modular combined multilevel converter are taken as examples,based on MATLAB and related simulation software,two kinds of simulation models of multi-level converter under conventional algorithm and new algorithm are established,and the simulation results of multi-level converter under new and old modulation algorithm are compared and analyzed.And the results further verify the feasibility of carrier amplitude modulation algorithm applied in different types of multi-level converters. |
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