| In the multi-level converter,the input voltage is distributed on each series switch,which reduces the voltage stress of each switch,making it be the preferred topology for high voltage applications.The modular structure of the modular multilevel converter(MMC)makes it theoretically possible to achieve any voltage level output,and its capacitor voltage equalization is relatively easy to implement.In addition,MMC has the advantages of high output waveform quality,easy redundancy control and low switching frequency.However,as the application of MMC continues to expand,there are still issues related to MMC control and modulation that require further research.The control mode and modulation technology will have an important impact on the switching loss of the MMC,the fluctuation range of the capacitor voltage and the harmonic content of the output current,and directly affects the performance and operational stability of the converter.The main work of this paper is as follows:(1)The comparative analysis and simulation verification of the traditional control methods in medium voltage occasions are carried out.Through the mathematical modeling and analysis of MMC,the design of the voltage equalization of the traditional control scheme is elaborated.The advantages and disadvantages of the two most commonly used modulation methods in MMC(recent level approximation modulation and phase shift carrier modulation)are compared.The specific data characterization is given by simulation,and the insufficient of existing modulation scheme of MMC under medium voltage is pointed out.(2)It is pointed out for the first time that the deadbeat current control under the nearest level modulation has hysteresis characteristics,and the related mathematical derivation is carried out,and the circulating current suppression strategy is designed.In this paper,combined with the bridge arm current error control principle,the switching process of the deadbeat current control under the nearest level modulation is analyzed in detail.It is pointed out that the MMC deadbeat current control has the hysteresis characteristic under the nearest level modulation.The hysteresis loop width of MMC deadbeat current control under the nearest level modulation is quantitatively analyzed,and the expression of hysteresis loop width and output current fluctuation range is obtained.It provides a theoretical basis for theapplication of the deadbeat current control method in medium voltage applications,and bridge arm current circulation suppression strategy is designed by using the hysteresis characteristic of the bridge arm current.(3)The sub-module switching frequency under the control of the deadbeat current is optimized.For the deadbeat current control under the nearest level modulation,the optimization effect of the half-bridge sub-module MMC under the optimization strategy of the two sub-modules switching frequency(sub-module optimization strategy based on maximum capacitance voltage deviation and virtual capacitor voltage)are compared.Based on the idea of virtual capacitor voltage optimization,the corresponding switching frequency optimization strategy of the full bridge sub-module is designed,which significantly reduces the average switching frequency of the MMC sub-module.(4)The deadbeat current control is proposed for DC melting ice,and the integrated control scheme of the full bridge sub-module MMC DC ice melting device is designed.The overmodulation capability of the MMC full-bridge sub-module is analyzed in detail,and the wide-range voltage output of the MMC rectifier is realized by using the negative-level output capability of the full-bridge sub-module.In this paper,the deadbeat current control is applied to the MMC DC ice-melting device,and a comprehensive control scheme including circulating current suppression and sub-module switching frequency optimization is designed. |
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