| Modular Multilevel Converter Based Multi-Terminal HVDC(MMC-MTDC)has the advantages of high reliability,high economy,high flexibility,and high redundancy.The field of long-distance transmission capacity has been widely used at home and abroad flexible HVDC engineering sector,it is one of the core technology of the future rapid development of flexible DC power grid.At present,there are many DC transmission projects in operation or under construction in China,however,there are still a number of crux matters that need to be settled urgently.Therefore,this thesis takes the MMC-MTDC system as the research object,focusing on systematically researching three types of problems: small signal modeling and stability analysis of the system,design of adaptive droop control strategies,and improvements to traditional droop control to improve voltage quality and improve power distribution characteristics,the main work is as follows:(1)This paper analyzes the working principle and control system of MMC-MTDC system,introduces the topology of MMC-MTDC system,the advantages and disadvantages of system level control strategy and applicable scenarios,and compares the dynamic characteristics of master-slave control,voltage margin control and droop control through N-1 operation simulation.(2)A detailed small-signal model of the half-bridge MMC considering the internal dynamic characteristics was established.By building a four-terminal MMC-type flexible DC transmission test system,the electromagnetic transient model and the small-signal model were compared under small disturbance conditions.The step response verifies the accuracy of the proposed modeling method.Finally,the stability of the system is studied by selecting typical parameters.(3)According to the inherent defects of traditional droop control,the relationship between the droop coefficient and the power allocation is discussed.For this reason,an improved adaptive droop control method based on the influence factor is designed.This method focuses on the influence of DC system voltage deviation on power distribution of converter station.By introducing voltage deviation influence factor and power deviation influence factor,and combining with the control strategy of constant common DC voltage square,the droop coefficient is adjusted adaptively.The simulation waveform verifies the effectiveness of the method.(4)In order to further realize the purpose of precise power distribution and voltage deviation-free control,the influence of line impedance on power distribution is analyzed,adaptive virtual power distribution coefficient and voltage correction are introduced,and the P-V droop control mode is improved to distribute power accurately and eliminate voltage deviations,improve the economy and reliability of system operation furtherly.This paper has 58 pictures,5 tables,and 84 references. |
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