I. INTRODUCTION
In recent years, intensive research has been performed on the design of uninterruptible power supply (UPS) systems to provide clean and uninterruptible power to equipment in critical applications such as personal computers, medical equipment, telecommunication systems, industrial and commercial controls, etc under essentially any normal or abnormal utility power conditions. In order to supply continued power to the load in the absence of utility power, energy storage systems such as batteries or flywheels are incorporated in the UPS for this purpose. The power conversion is mainly accomplished using power electronics due to the advanced development of fast-switching high-current power transistors.