Uninterruptible Power Supply (UPS) systems have become a standard to protect electronic devices such as servers and host computers. Also, the energy supply of whole buildings is linked with large UPS systems to ensure a steady power flow. Two system configurations are widely used which differ in price and their ability to protect very sensitive load. This thesis illustrates an analytical examination of all existing systems and concludes with the finding of new configurations with increased efficiency and reduced costs. A dual converter is proposed as the heart of the new UPS system. This converter links the necessary two sources of the UPS through a common transformer. The transformer operates at a high frequency which is enabled due to the resonant switching technique used. The results of this paper were achieved using mathematical analysis, electrical and electro-magnetic simulation as well as by experiments carried out on the self designed circuit boards in the laboratory. These boards were built in a modular way to enable series testing and thereby optimise the dimensioning of the system.