Optimum Voltage and Harmonic Control PWM Techniques for Three-Phase Static UPS Systems

Pulsewidth modulation (PWM) is frequently employed with static ac power supplies to control the amplitude and harmonic content of load voltage. However, published work on the subject of PWM techniques has mainly dealt with variable-speed ac drive applications. Consequently, PWM techniques compatible with constant frequency and constant (load) voltage applications, such as uninterruptible power supply (UPS) systems, have been largely ignored. Several known and novel PWM techniques suitable for three-phase static UPS systems are analyzed, and attempts are made to evaluate them. Optimum techniques are subsequently selected on the basis of the low-order harmonic attenuation obtained with each technique and the number of thyristor commutations (per cycle) required to implement each technique.