Research On Control Strategy Of DC Transformer Based On MMC

Compared with AC power grid,DC power grid has a series of advantages.Therefore,it has become a highlight to solve the stability problem of grid connection of new energy generation.DC-DC converter has become the key role in connecting different voltage levels of DC power grid.However,due to the limitation of voltage withstand level of power switching devices,traditional DC-DC converters can’t be used in high voltage and high power applications and the existing technology of high voltage DC converters is not mature enough,it still has a series of defects.Although some scholars have proposed the topology of DC-DC converter based MMC which can be used in high voltage and high power applications,the research on it is still relatively small and there is no practical demonstration project at present.Therefore,further study of the topology is needed.This paper focuses on the control strategy of DC-DC converter based on MMC.In view of the defects of the traditional control method,the modulation and voltage balancing control strategy of the converter are improved.(1)The topology and basic principle of unidirectional and bidirectional MMC DC-DC converters are analyzed,the modulation strategies adopted at present and their merits and faults are summarized,the voltage sorting control strategy is introduced.(2)Aiming at the problem that the voltage of arm inductor is not taken into account in the traditional nearest level modulation(NLM)of unidirectional MMC DC-DC converter,which leads to large current fluctuation in DC side,a current direct tracking modulation strategy is proposed.Under this control strategy,the voltage of arm inductor is taken into account,so the current fluctuation can be reduced without adding filters and the output voltage can be increased at the same time,thus avoiding the distortion of the output voltage caused by arm inductor.However,because the control period of the proposed strategy is smaller than NLM,the switching frequency of the sub-module(SM)will be higher under the traditional voltage balancing control.For this reason,this paper proposes an improved voltage balancing control strategy which is match with the proposed modulation strategy,so that the modulation strategy can improve the current waveform without significantly increasing the switching frequency.(3)In order to reduce the switching loss of power devices of bidirectional MMCDC-DC converter under the traditional control method and improve the efficiency of converter,a hierarchical control strategy for the converter is proposed,which separately controls the system and the valve level.Realizing the voltage stabilization,at the same time,the switching frequency of the SM is equal to the frequency of the modulation wave,which reduces the switching loss.However,because the proposed control strategy only has once voltage balancing control in each cycle,the fluctuation of the SM’s voltage will be relatively large.In order to make the fluctuation of the SM’s voltage within the allowable range,the SM’s capacitance value under the proposed control strategy is calculated.(4)The simulation models of unidirectional and bidirectional MMC DC-DC converters are built in Matlab/Simulink simulation platform and the effectiveness of the proposed control strategy is verified.The results show that the proposed unidirectional MMC DC-DC converter’s control strategy can effectively improve the input current waveform without filter and reduce the cost of the system;the proposed bidirectional MMC DC-DC converter’s control strategy can realize voltage stability conversion while reducing the switching loss.