| For DC transmission has no synchronization issues such as phase and frequency in AC transmission,it is convenient for interconnection between different systems.therefore,DC transmission technology has gradually become a research hotspot both at home and abroad,the DC-DC converter is an indispensable part of the interconnection of DC grids of different voltage levels.However,most of the DC-DC converters currently studied in China are low-power,which are difficult to apply in practice.Modular multilevel converter(MMC)has gained wide attention because of its advantages of easy capacity expansion and high output waveform quality.The isolated MMC type DC-DC converter is to add a high-frequency AC transformer between two MMCs.This structure not only retains the advantages of MMC,but also has functions such as isolating faults.Therefore,this paper mainly studies the DC-DC converter of this structure.The specific research contents are as follows:Firstly,the operating principle and topology of this isolated MMC DC-DC converter are analyzed,the parameter design of the MMC and the intermediate AC transformer on both sides of the converter is carried out.Then in order to select the most suitable modulation method for this converter,the five commonly used modulation strategies are studied,the principle level is analyzed at first,and then the switching frequency,switching loss,harmonic distribution and harmonics of each modulation strategy are simulated.The relationship between the wave distortion rate and the level number is comprehensively compared,and the applicable occasions of each modulation method are obtained.Finally,the modulation method suitable for the DC-DC converter is selected.Secondly,in order to solve the problems of high switching frequency and large loss in the traditional sorting and voltage equalization algorithm,a new sorting and voltage equalization algorithm is proposed,and simulations verify that the proposed algorithm can effectively ensure the waveform quality,reduce the switching frequency and cut the switching loss.At the same time,in order to ensure the safe operation ofthe converter and improve the quality of the waveform,the circulating current suppression strategy is studied,and the advantages and disadvantages of different circulating current suppression strategies are comprehensively compared to lay the foundation for the selection of the circulating current suppression strategy of the DC-DC converter.Thirdly,according to actual needs,the system control strategy of the DC-DC converter is designed from the perspective of separate control and overall control.For separate control,the traditional double closed-loop control is improved by introducing an optimized PI adjustment algorithm;for the overall Control,that is,according to the relationship between the phase angle difference of the MMC systems on both sides and the power flow,the overall power control is achieved by controlling the phase angle difference.The above two system control strategies are combined with the aforementioned strategy of modulation voltage equalization and circulating current suppression,and the effectiveness of the designed control scheme is verified by Matlab/Simulink. |
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