New approach in back-to-back m-level diode-clamped multilevel converter modelling and direct current bus voltages balancing

Multilevel power converter structures have been introduced as the solution for high-power high-voltage applications and also for grid interface connection of renewable energy sources systems, where they have several advantages, namely low distortion voltages and currents and low switching losses resulting in higher efficiency. As a consequence of increasing interest on the multilevel converter applications, accurate models of these power converters are essential to be used in computer simulation studies. This study presents a new systematic modelling approach suitable to obtain global generalised models for back-to-back m-level diode-clamped multilevel converters connecting AC loads. Using the proposed converter modelling, an advantageous generalised method to balance the (m - 1) DC bus capacitors, based on the DC bus average power flow is also presented. The generalised proposed model is particularised to the back-to-back multilevel structure for m = 5. This back-to-back converter is studied working with bidirectional power flow, connecting an induction machine (in motor or self-excited generator mode) to the power grid as a flexible AC transmission system.