The authors describe a parallel redundant uninterruptible power system (UPS) with instantaneous pulsewidth modulation (PWM) control using insulated gate bipolar transistors (IGBTs) in the inverter units. The main features of the control method are that instantaneous parallel control at high frequency is realized and the parallel control for compensating power imbalance between UPS units in parallel and synchronization control to the bypass are completely digital by using a digital signal processor. The redundant UPS takes up less space, has low audible noise and good control characteristics for steady and transient states.