| Recently, flexible high voltage direct current (Flexible-HVDC) transmissionsystem and flexible ac transmission system (FACTS) have played a vital role in thedevelopment of large-scaled and grid-connected renewable energy resource and smartgrid construction. Modular multilevel convertor (MMC) is a significant milestone inhistory of Flexible-HVDC development, not only improving operation benefit ofFlexible-HVDC projects, but also greatly promoting the development ofFlexible-HVDC technology and its applications. In order to make breakthrough ofMMC-HVDC system for high-voltage high-power application, it is very necessary toinvestigate new efficient modulation technology and control scheme. The dynamicbehaviors of MMC and its additional control strategies are investigated, thesimulation model of11-levels two-ends MMC-HVDC system is then established inthe PSCAD/EMTDC software environment with its control strategy implemented.This paper is organized as follows:Firstly, two electromagnetic-transient-type simulation modeling methods forMMC are researched, including detailed IGBT-based model and the fast model basedon Thevenin equivalent circuit. The two-ends MMC-HVDC system based on abovetwo models are established in PSCAD/EMTDC to evaluate their accuracy andefficiency under steady-state and fault conditions. Secondly, The capacitor voltageripples are estimated with derivation of arm energy variation equation. The minimumenergy storage requirement and influence factors of MMC power transfer region arealso analyzed for the sizing of sub-module capacitors. The capacitor voltage balancealgorithm based on Max-Min function is proposed to significantly reduce switchingfrequency and enhanced computational efficiency. Based on the average model ofMMC, the mechanism of interaction between harmonics of arm voltage and armcurrent are analyzed, and the magnitude of the circulating current is calculated. Acirculating current suppression controller is designed to eliminate it. Finally, the validation of the above analytical results is carried out in the MMC-HVDCsimulation system. |
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