| HVDC develops very rapidly,because of the impetus of the power electronic technology,especially with the advent of Modular Multilevel Converter(MMC).It provides a new train of thought to solve the problems of large power DC transmission,high voltage and other technical problems.Since it appears,the MMC shows great advantages and potentials,and becomes a researched focus in recent years in the field of electric power.But as a newly researched topic,there is a lot of theories having not been thoroughly studied,and demanding further studied,such as in the new topology structure presented,in the sub module pre-charge ways,in the capacitor voltage balancing strategy,in the bridge arm circulation inhibited technology and so on.Based on MMC as the researched object,the paper studies the segmental control strategies,and the main content is as follows:(1)The paper combs the structure characteristics of the present HVDC projects and the projects under construction.It compares the working mode and structural characteristics of four kinds MMC topology structure,which are the half bridge,full bridge,fault crossing and DCM sub module.And it summarizes the research status and progress of the fast pre-charge control,the MMC Sub-Modules voltage balancing control,and the bridge arm circulation inhibited technology.(2)The paper detailedly studies the working principle of half bridge Sub-Module topology in the MMC system.Though the SM is stacked orderly,the DC can convert into alternating current(AC).Comparing three kinds of carrier phase shift modulation,and one kind of NLM characteristics,the NLM has a superior technical characteristics in the high voltage and multilevel structures.(3)The SM pre-charge is divided into uncontrolled and controlled two processes.In the uncontrolled stage,the paper detailedly studies the specific dynamic process,conducts the equivalent circuit processing,makes the range of the current-limiting resistance,and calculates the resistance value's range of secondary excision resistance.Though the single resistance is graded resection to control current,it can achieve the purpose of capacitance's quick charging.In the controlled charging stage,it puts sub module capacitance into simple grouping control to reduce the bigger voltage difference value of power supplying side and the sub module side,when it is unlocked to charge.It proposes a fast pre-charge strategy of sub module capacitance.Finally the effectiveness of the proposed strategy is validated by setting up 11 level double MMC module.(4)The SM voltage fluctuation can affect the external output characteristics by the modeling analysis of the MMC system.The paper compares with the static and dynamic two types of SM voltage grouping technique characteristics,from the SM specific modulation ways,the complexity of time,and the equalizing.It combines with their advantages to improve,and raises an improved balancing strategy of SM capacitance voltage.At the same time,the MMC controller of inner and outer loop is designed.Finally,the effectiveness of the proposed improvement strategy is verified through the double ports MMC system.(5)Through the modeling analysis of the single port and the theory of instantaneous power,the paper verifies that the MMC transmission system must be double frequency circulation.The circulation is not to export,but to bring with adverse effects,such as loss of power and so on.Thus it needs to design the circulation inhibition.The paper designs two kinds of circulation suppressors.The first one is based on dq coordinate transform,the second one is a filter by means of resonance frequency to filter the double circulation.By21 level single port MMC system simulation,comparing with analyzing the inhibitory effect of two kinds of circulation suppressors,it obtains that the CCSC is better than the second harmonic filter inhibitory effect,and the CCSC can inhibit the majority of the double frequency circulation by the NLM,but can not filter out completely. |
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