Executive Summary ─The use of distributed floating capacitors in modular multilevel converters (MMC) makes their modelling and understanding of the dynamics increasingly challenging. In recent years, a number of traditional modelling methods of power system components, namely detail switching, switching function, averaged and Dommel pre-solved integration methods have been extended to the MMC, with the latter evolves to Thevenin or Norton equivalent MMC model. The increased number of electrical nodes of the detail switching model, and its demand for the much small time-step lead to very slow simulation speed especially for full-scale modelling of MMC systems. Among the remaining MMC modelling methods, the Thevenin equivalent method is widely known and used. However, many have recognized the computational superiority of the switching function over the Thevenin equivalent, and as a result switching function based MMC models have started to displace the Thevenin equivalent in many offline and real-time platforms.
This report reviews the modular multilevel converter with a balanced view of power electronics device and system, focusing on the fundamental concepts and mathematical relationships that govern its operation. Moreover, this report provides a critical assessment of different modelling methods, with emphasis on the capability and suitability of each modelling for a wide range of system studies such as ac and dc network faults and interaction between ac and dc sides. Systematic development of the switching function, averaged and Thevenin equivalent MMC models and their validations through comprehensive comparative studies are presented. Detail comparisons and simulations presented in this report reveal that the averaged and switching function MMC models are able to reproduce identical results of the Thevenin equivalent MMC model (provided by the PSCAD Library) during normal steady-state operation, and ac and dc network faults with increased simulation speed. Also, the MMCs with a reduced number of submodules per arm are able to reproduce practically identical results of the full-scale MMCs with hundreds of submodules per arm. Therefore, it can be concluded that the developed average and switching function MMC models can be used in a wide range of studies, including the steady-state, and ac and dc fault studies.
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