Modulation Method And Output Analytical Solution For Hybrid Multilevel Inverter

In recent years, multilevel power conversion technology has been becoming hot point in power electronic research due to extensive outlook in high power application. Novel topologies and modulation strategies concerning multilevel converters are emerging continuously. As far as the width and depth are concerned, research on multilevel technique is advanced greatly. To begin with traditional multilevel topologies, this paper focuses on topologies and modulation methods of hybrid multilevel inverters. Besides, mathematic models illustrating modulation process are constructed to parse complicated output waveforms. Based on single-phase diode-clamp 5-level inverter, diode-clamp cascaded multilevel inverter is presented. The topology consists of multiple single-phase diode-clamp 5-level units in series to synthesize more levels output. The concepts of hybrid multilevel topology and hybrid modulation method are defined clearly in the paper. As followed a new hybrid diode-clamp cascaded multilevel inverter is introduced. The inverter is based on connection of several diode-clamp inverter modules with different dc bus voltage values. An example is discussed in detail that two modules are linked with dc bus voltage 2:1. Also a new hybrid modulation method feasible in the inverter is put forward. Experiment validation is carried out. An asymmetric 5-level inverter is presented as simplification to single-phase diode-clamp multilevel topology, which is composed of a half-bridge diode-clamp inverter and a conventional half-bridge inverter in share with a common split-capacitor power supply. It is analyzed in the paper that square-wave operation combines with subharmonic PWM. In terms of the modulation scheme above, the new configuration makes faster devices and high voltage devices operating in synergism possible. Comparison with four other single 5-level inverters is made in the paper. In order to verify the proposed approach experiment is developed. Output spectral analysis provides essential information for optimal design on whole inverter system. In general, computer simulation is responsible for the work. However, simulation results are susceptible to modulation parameters choice owing to switch waveforms error and DFT algorithm adopted. Considering simulation results accuracy modulation parameters have to be limited to certain extent. Thus simulation on spectral analysis is short in generality. Alternatively rigorous mathematical analysis on output spectra exactly overcomes the deficiency above. Based on the anatomy of hybrid modulation process in nature, 3-D mathematic models are built under three kinds of carrier dispositions frequently used. Further, by double FOURIER series expansion and intricate derivation, strict output analytic solution is acquired. The output analytical solution contains complete original information on output waveform spectra. Output spectral characteristics are summed up in the paper. Results from mathematical analytical solution are applicable not only to non overmodulation but also to overmodulation, and valid not only to synchronous modulation but also to asynchronous modulation. Further more, analytical solution educes an illuminative clue for spectral analysis on complicated output PWM waveforms based on different modulation strategies.