Review of Current control strategies in Modular Multilevel Converter
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- Institutt for elkraftteknikk 
With the rapid development of offshore wind farms, the demand for high power, high quality transmission system becomes more urgent. Modular Multilevel converter (MMC) based high voltage direct voltage (HVDC) technology provides a promising solution. In the past decade, intense research has been done on challenges related to the control of MMC, including the outer dynamic control and two inner dynamic controls: capacitor balancing and circulating current suppressing. Many algorithms have been proposed, but an overview of them still lacks. This paper provides a review of most of control strategies for MMC in the literature about all three control purposes with clear classification and detailed explanations. In addition, the traditional proportional-integral (PI) controller based cascade control and model predictive control for AC side current control are compared by simulation in Matlab/Simulink. Different control strategies for capacitor voltage balancing and circulating current eliminating are also tested and compared. Finally, an option of comprehensive control of HVDC system for offshore wind farm is proposed and verified by simulation.