A modular multilevel converter (MMC) is one of the perfect topologies for high power and medium-/high-voltage energy conversion systems. The MMC has attractive features such as modularity, voltage and current scalability, transformerless operation, fault blocking capability, reduced filter size, a reduced ripple of the output current, high efficiency, and low expense on redundancy. These features attracted industries over the past few years, notable research has been carried out on MMC topologies, their operation, and control. This study presents a review of MMC topologies and their mathematical models. Furthermore, their control schemes (classical as well as model predictive controls) and modulation techniques are discussed. Finally, MMC applications and their future challenges are highlighted.

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Multiinfeed HVDC systems are planned to meet the high demand of power in heavy load centers in developing countries. A multiinfeed HVDC system inverting into a weak AC network are of serious concern. The possible solution for better stability is using a VSC-HVDC as one of the infeed for the multiinfeed system. This paper presents the dynamic performance of hybrid multiinfeed HVDC system carried out on Real Time Digital Simulator(RTDS). VSC-HVDC link is modeled using small time step simulation of RTDS which can accurately represent these models. VSC controls are modeled in detail and improved for better performance of LCC-HVDC link if the electrical proximity between the inverters is reduced.

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