Research On New Control Method Of HVDC-flexible Based On MMC

Modular multilevel converter (MMC) is a new type of multilevel voltage sourceconverter which was put forward only in recent years, and has great significance in thedevelopment of multilevel converter. Thanks to the modular design, comparing with2-level,3-level voltage source converter, flexible DC transmission based on MMC can achieve theexpansion of transmission system voltage and power level just by changing the number ofsub-modules. Additionally, for advantages of the modular structure which has the sameinternal parameters, having no dynamic and static voltage balancing problem, no need fortransformer and filter etc., making MMC-HVDC become a future trend in the field of highvoltage DC transmission construction. This paper studies new control methods ofMMC-HVDC system, mainly include the following contents:(1) This paper introduces the research background and significance; summarizes themain kinds of multilevel converter at present; presents the research status, technicalcharacteristics and application prospects of MMC; describes the main contents and innovationpoints of this paper.(2) The MMC topology is introduced in this paper; low frequency dynamic mathematicalmodels of MMC-HVDC based on abc, αβ, dq coordinate system are also established. Besides,the structure of MMC are further studied; Working principle of sub module and three phaseMMC converter is analyzed; What’s more, mathematical models of MMC converter isderived; MMC operating characteristics are discussed.(3) This paper also elaborates the converter valve control strategies of MMC-HVDCsystem; It focuses on working principles and implementation of carrier phase shift PWMmodulation and nearest level modulation; Besides, the scope of application and advantages ofthe two modulation strategy are compared; What’s more, the importance of capacitor voltagebalancing control of sub module is elaborated; and two common capacitor voltage balancingcontrol strategies are described; In addition, it puts forward a new improved voltage sortingoptimization strategy and builds MMC simulation models in MATLAB/SIMULINKsimulation platform, does simulation experiments upon hierarchical capacitor voltagebalancing control strategy and improved voltage sorting optimization strategy. Moreover, itverifies the effectiveness of CPS, NLM, and the capacitor voltage control strategy mentionedby experiments.(4) In this paper, the generation mechanism of AC components in circulating current isderived, proving that circulating current in MMC only contains even harmonics and the expression of main component of the negative sequence second harmonic circulating currentcomponent is calculated; On the basis of analyzing of the mechanism of circulating currentand basic principles of circulating current suppression, this paper verifies the basic principleof circulating current suppression by using the capacitor voltage balancing control strategysimulation experiments mentioned in the third chapter; Finally, it proposes and designs twonew kinds of circulating current suppression controller. It builds MMC-HVDC simulationmodel and then verifies the correctness of the new circulating current suppression controller.(5) The role of system-level control of MMC-HVDC transmission system is alsointroduced in this paper; Based on vector control strategies, important functional modules likesynchronous phase locked loop, inner current decoupling controller and outer powercontroller are discussed and designed one by one. This paper proposes a new non-phaselocked loop control strategy. System-level controllers under this strategy don’t need phaselooked loop, it effectively avoid the influence of sampling delay, computational complexityand current coupling. Simulation results upon this control stragety show that the policy iseffective and have a good control effect.