TABLE OF CONTENTS Power distortions such as power interruptions, voltage sags and swells, voltage spikes, and voltage harmonics can cause severe impacts on sensitive loads in the electrical systems. Uninterruptible power supply (UPS) systems are used to provide uninterrupted, reliable, and high quality power for these sensitive loads. Applications of UPS systems include medical facilities, life supporting systems, data storage and computer systems, emergency equipment, telecommunications, industrial processing, and on-line management systems [1-3]. The UPS systems are especially required in places where power outages and fluctuations occur frequently. A UPS provides a backup power circuitry to supply vital systems when a power outage occurs. In situations where short time power fluctuations or disturbed voltage occur, a UPS provides constant power to keep the important loads running. During extended power failures, a UPS provides backup power to keep the systems running long enough so that they can be gracefully powered down.
Most of the UPS systems also suppress line transients and harmonic disturbances. Generally, an ideal UPS should be able to simultaneously deliver uninterrupted power and provide the necessary power conditioning for the particular power application. Therefore, an ideal UPS should have the following features: regulated sinusoidal output voltage with low total harmonic distortion (THD) independent from the changes in the input voltage or in the load, on-line operation that means zero transition time from normal to back-up mode and vice versa, low...
