| The modular multi-level converter(MMC) has many advantages, such as highly modular architecture, superior redundant design and control performance, strong ability working on imbalance conditions, good fault riding through performance and self-start ability, lower component stresses etc. it is particularly suitable to applications with high voltage and large power capacity. Therefore modular multi-level converter has broad application prospects in high voltage and large power capacity fields, such as flexible HVDC, power quality improving, medium power motor driving with high voltage etc. This paper mainly gives a deeply analysis and study on the aspects of nearest level control(NLC) method applying to modular multi-level converter, on the basis of existing theory and results of modular multi-level converters’ modulation strategy.Firstly, the principles of MMC were introduced in detail by means of equivalent circuit model. The expression of Equivalent AC output voltage of NLC method were derived by analyzing the MMC equivalent circuit model of AC and DC side, and the paper makes a depth analysis on it as follows:There exists a problem of NLC in existing literature related to NLC method applying to MMC that the analytic expression of total level number on equivalent AC output voltage waveform is not comprehensive. Based on the method of numerical analysis and computation, the analytic expression of total level number was analyzed and derived, it has relation to sampling frequency, number of sub-modules in arms, output voltage modulation ratio, and it was further verified by simulation. According to the analytic expression of total level number, the paper makes a further analysis and derives conclusions on selecting sampling frequency in actual project.In view of analytic expression of NLC output waveform Fourier analysis, the paper makes a further analysis and derivation on analytic expression of NLC output waveform Fourier analysis, improving analytic calculation accuracy of the fundamental wave and harmonics in NLC output waveform Fourier analysis. According to the negative symmetry properties of the NLC output waveform in fundamental cycle, the simplified analytic expression of NLC output waveform Fourier analysis is obtained. And both of them are verified by simulation method. In order to obtain the NLC output waveform harmonic characteristics, the paper makes a further analysis on those analytic expressions by means of numerical computation and derives relative conclusions on harmonic characteristics.There exist many problems such as poor quality of output voltage, large harmonic content, large voltage fluctuation of sub-module capacitor, when NLC method applied to MMC with not enough number of sub-modules. Therefore two improved methods are proposed: one is increasing rounding level value, the other is increasing the numbers of output waveform level. Both methods can increase the total number of the output waveform level, thereby effectively improving the quality of the output waveform, and reducing the total harmonic distortion factor. Both methods are conducted further analysis and validation by simulation.Finally, a single phase rectifier based on MMC is built up, with 8 sub-modules in one arm. Its main circuit design and hardware control system are introduced in this paper, as for the basic principles of MMC and related NLC research in this paper, they are verified by experiments. |
