Switching function analysis of half- and full-bridge modular multi-level converters for HVDC applications

This study presents a theoretical analysis of half- and full-bridge modular multi-level converters for HVDC transmission applications. The constraints and the degrees of freedom in the choice of switching functions, and the harmonic content of the sum and difference of arm voltages and currents, are highlighted by the analysis. The authors prove that under balanced switching conditions, dc and even harmonic components are absent from the difference variables and odd harmonics are absent from the sum variables, whereas the pole voltages are free from harmonics. Using an energy function approach, it is shown that the sorting strategy for module voltage balancing is stable and practically independent of the dynamics of the rest of the system. The module voltages settle down to a band around the average value because of the finite switching frequency, for which an upper bound is provided.