Research On Control Strategy Of Modular Multilevel Converter

With the continuous development of the power system，especially the rapidgrowth of the emerging energy supply channels and capacity in recent years, thereare highly increasing demands on high-voltage and high-power energy conversion.However，conventional voltage source converter can no longer be able to meet thenew requirements. Modular Multilevel Converter (MMC) provides a new way topower conversion. The control methods of MMC are researched in this paper,including the comparison of several modulation methods, the control of MMC sub-module and the suppression of inner circulating current. Also, the control of sub-module includes the choice of capacitor parameter, the pre-charging method ofcapacitor, the balancing control of capacitor voltage and the optimization ofswitching frequency.Firstly, two typical modulating methods, which are staircase wave modulationand Carrier Phase Shifting SPWM (CPS-SPWM), are discussed and compared onseveral factors, including the quality of output voltage and current, circulatingcurrent suppressing effect, capacitor balancing control and the switching frequency.Then the choice of capacitor parameter is analyzed based on the limitation ofvoltage fluctuation and adjusting time. Moreover, the control methods of MMC sub-module are researched, this part mainly includes three aspects:(1) Through the study of the MMC sub-module pre-charging procedure, thegeneric pattern of sub-module charging out of control is discussed. Then the three-phrase equivalent model of MMC non-control charging is built, and the impulsecurrent formula based on this model is proposed. This formula is proved to be truesince the result is very close to the simulation. Additionally, a control strategy forsub-module pre-charging is presented, which can take advantages of AC side tomake the MMC startup stably.(2) Compare the switching sequence rotation and sorting redundancy selectionmethods in the same simulation environment, and point out the difference betweenthese two methods in current situation.(3) Propose a switching frequency optimization method to effectively reducethe switching frequency of sub-module without impact on the quality of the outputvoltage.After analyzing the modulating strategy and the sub-module control method,the theory of inner circulating current is inferred from the perspective of energyfluctuations. Based on direct feedback control princ iple, a novel circulating currentsuppressor (NCCS) for MMC is designed to suppress the circulating current. Finally, the correctness and feasibility of the proposed control strategy are verified bysimulation and experiment results.