Modelling, Control and Optimisation of Low Capacitance Modular Multilevel Converters

With the growing wind energy market, the installed capacity of wind farms is on a steep incline. Looking at the near future, as the wind capacity further increases there is a need for high power converters for Wind Energy Conversion Systems (WECSs). Most of the power converter topologies employed for wind turbines are based on 2-level Back-to-Back (BTB) Voltage Source Converters (VSCs) with few industries offering 3-level neutral point clamped converters. Keeping a focus on future high power wind energy conversion systems, BTB Modular Multilevel Converters (MMCs) are investigated in this work. MMCs benefit from easy scalability, better power quality, higher efficiency, and low dv/dt stress on the generator/transformer as compared to the two-level and three-level converters. Although, application of MMCs to variable frequency WECSs is previously investigated in literature, the existing studies are outdated limiting to the popular HB-MMC (Half Bridge MMC) which suffers from increased SM capacitor size and don’t account for the recent research. Namely, (a) the recent advances in the control, operation, and design of the HB-MMC with low capacitance i.e., high SM capacitor voltage ripple more than the traditional design of ±10%, and (b) application of the Full Bridge SM (FBSM) and the hybrid (mix of HBSMs and FBSMs) SM based MMCs, both of which inherently benefit from reduced SM capacitance. Moreover, the required SM capacitance is inversely proportional to the ac side frequency of the MMC. With the generator frequencies of the DD-PMSGs used in WECSs typically in the range of 10-25Hz the SM capacitor size in the generator side MMC is relatively larger compared to the grid side MMC. Thus, for such systems with low ac side frequency reduction of the SM capacitor size is of prime importance as it directly defines the over-all system cost, size, and weight. Therefore, this thesis explores the modelling, control, and design of the potential topologies based on “Low Capacitance” (LC) HB-MMCs, Full Bridge MMCs (FB-MMCs) and Hybrid MMCs (Hy-MMCs) where the converter is forced to work with high SM capacitor voltage ripple, greatly benefitting from reduced SM capacitor size and thus over-all MMC size, cost, and weight. Novel modelling and control techniques are proposed to reduce the over-all SM capacitance requirements (i.e., converter size) which is dominant in MMC topologies especially for direct drive wind systems with low generator frequencies. At the same time attention is given to optimising and minimising the harmonic content of the MMCs in order to reduce the grid side filter requirements and also understand the design requirements for a filter-less grid connection. For this, accurate harmonic analytical models are developed based on which harmonic minimisation criteria are proposed. Lastly, the proposed modelling and optimal design techniques of the LC MMCs are used to design and optimise medium voltage mega-watt complete back-to-back MMC topologies of HB-MMC, FB-MMC and Hy-MMC for WECSs. The optimised MMC topologies are benchmarked against the popular standard (baseline) system of two-level voltage source converter topology