DC Link Balancing Method in Back-to-Back UPS System with Multi-Level Converters

Paper presents a novel topology of a grid-connected back-up uninterruptible power supply (UPS) system with multilevel converters. The star-configuration three-phase "back-to-back" topology with a five-level diode-clamped rectifier (5LDCR) and a five-level diode-clamped inverter (5LDCI) supplies a three phase output load. The proposed multi-level current mode control techniques in both converters provide an effective solution to an imbalance problem with the capacitor voltages in a DC link network. The interfaced battery energy storage (BES), which constitutes an integral part of the entire UPS system, supplies the DC link capacitor voltages to the 5LDCI in a standby emergency mode because the proposed balancing method is constrained to the "back-to-back" operation only. Importantly, the UPS system is able to operate with an asymmetrical, a nonlinear and an impact loads in three phases and it provides the power factor correction on the grid side