International Research Journal of Education and Technology

H-bridge-based multilevel converters (e.g., cascaded H-bridge converters) benefit from modularity and scalability. However, they suffer from the complexity and costs associated with a large count of semiconductor switches, together with their drivers and peripheral circuits, as well as higher conduction losses as compared to half-bridge-based counterparts, as two switches in each sub module must simultaneously conduct to provide a current path. To reduce the number of active switch semiconductors and conduction losses, this paper proposes novel multilevel converters with symmetrical half-bridge sub modules. The symmetrical half-bridge sub module features a bipolar voltage output, a reduced switch count, and simplicity. Further, this paper proposes a sensor less voltage balance scheme that successfully gets rid of capacitor voltage mismatch problems through diodes and sub module parallelization. This scheme can greatly reduce capacitor voltage ripples, thereby allowing the saving of dc capacitances, particularly in the case of numerous sub modules. Finally, simulation and experimental results validate the superiority of the proposed multilevel converters and voltage balance scheme. In the first control layer, the voltage vector of cells is adjusted so that the active power is uniformly distributed among the cells. In the second control layer, the switching redundancies of FNPC are used to provide voltage balancing. The introduced balancing scheme not only provides voltage balance among capacitors, but also distributes the reactive power equally among the cells and increases the reliability in this way. The performance of the proposed control is validated through simulation in MATLAB/SIMULINK environment and experimental results based on a single-phase nine-level HC-FNPC-based STATCOM laboratory prototype

Keywords – Cascaded H-bridge (CHB), half-bridge, modular multilevel converter (MMC), multilevel converter, sensor less voltage balance. DC fault reverse blocking capability.