This study proposes a simplified space vector pulse-width modulation (SVPWM) method for multilevel converters. To avoid unnecessary coordination transformations, firstly, a further derivation is conducted on the basis of the 60° coordinate transformation. Then, all the nearest three vectors (NTVs), duty cycles and optimal switching sequence within one switching period can be identified through a simple comparison of three-phase modulation waves. To further reduce the switching frequency, detailed steps to achieve the minimum number of transitions among different NTVs are constructed. Combined with the optimal switching sequence within one switching period, the minimum number of transitions within a fundamental period can be achieved easily. Finally, a thorough discussion is conducted to simplify the procedure further and the flowchart for the proposed SVPWM is also presented. Compared with the previous methods, the proposed method does not require any iterative calculation or coordinate transformation. To the best of the authors' knowledge, the calculation amount is reduced by almost seven times compared with the methods proposed. In addition, the switching frequency and total harmonic distortions remain at the same level without any deterioration. Finally, the simulation and experiment for modular multilevel converter are performed to verify the correctness and efficiency of the proposed method.
A fractional frequency transmission system (FFTS) is the most competitive choice for long distance transmission of offshore wind power, while the Hexverter, as a newly proposed direct AC/AC converter, is an attractive choice for its power conversion. This paper proposes a novel control scheme characterizing the global stability and strong robustness of the Hexverter in FFTS applications, which are based on the interconnection and damping assignment passivity-based control (IDA-PBC) methodology. Firstly, the frequency decoupled model of the Hexverter is studied and then a port-controlled Hamiltonian (PCH) model is built. On this basis, the IDA-PB control scheme of the Hexverter is designed. Considering the interference of system parameters and unmodeled dynamics, integrators are added to the IDA-PB controller to eliminate the steady-state error. In addition, the voltagebalancing control is applied in order to balance the capacitor DC voltages to obtain a better performance. Finally, the simulation results and experimental results are presented to verify the effectiveness and superiority of the IDA-PB controller.
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