Off Line UPS for Computer Power Applications

Critical computer power applications require some form of UPS protection. Achieving very high reliability from the UPS while keeping equipment and operating cost to a minimum is the main goal of all UPS designers. This paper addresses a new inverter design which permits the solid-state UPS to remain in an off-line mode and only operate during a utility power failure. The paper traces the redevelopment of an existing UPS technique which has been made practical today due to changes in inverter design. The reality of a true off-line solid-state UPS is made possible by taking advantage of the flywheel effect inherent in all motor generator (MG) sets used as power stabilizers for large computer systems. All aspects of the inverter design are presented including performance characteristics and system operating modes. Reliability of the off-line approach is discussed in detail. Alternate UPS configurations are presented which reduce energy consumption without reducimg overall system reliability.