Large Parallel UPS Systems Utilizing PWM Technology

In the past large Uninterruptible Power Supply (UPS) applications requiring parallel operation of Pulse Width Modulated (PWM) inverters was not practical. New developments in PWM logic design have made load sharing of PWM inverters very precise. This paper addresses the critical factors of the inverter and system design which permit parallel operation and enhancement of system reliability. Taking full advantage of the PWM technique in control of an inverter is discussed. New methods of accomplishing parallel operation are discussed. Particular emphasis is given to improving the "fault tolerance" capabilities of a UPS system. The paper discusses in detail the design of a parallel-redundant UPS system which eliminates some of the "single point failure modes" present in most UPS system configurations. The logic design which permits precision sharing of load currents between parallel inverter; and, the methods of removing faulted inverters from the common bus are presented. Actual operating sequences are used to demonstrate the effectiveness of the system design and the improved fault tolerance characteristics.