I Introduction
Highly reliable three-phase on-line uninterrupted power supply systems (UPS) are formed by back-to-back connection, i.e. DC side coupling of a mains-side voltage source rectifier and a load-side voltage source inverter [1]. There, the battery energy storage is directly connected to the DC link or coupled via a DC/DC converter in order to allow a control of the charging or discharging current and a compensation of battery voltage changes. Furthermore, for safety reasons and voltage level adaption in general a 50/60Hz isolation transformer is employed on the input or output side which, however, constitutes a significant drawback concerning volume/weight, costs and efficiency.