Design of Online Ups System with Over Voltage , Under Voltage and Phase Out Protection

Uninterruptible power supplies (UPS) play an important role in interfacing critical loads such as computers, communication systems, medical/life support system and industrial controls to the utility power grid. They are designed to provide clean and continuous power to the load under essentially any normal or abnormal utility power condition. Among the various UPS topologies such as online UPS, offline UPS and line interactive UPS, online UPS are widely used. In this paper hardware implementation of single phase 50Hz, online uninterruptable power supply with over voltage, under voltage and phase out protection have been carried. Atmega32 microcontroller is the heart of the system and controls entire system. By programming the microcontroller using embedded C, SPWM pulses to drive H-bridge are generated. By alternating switching switches of two legs of H-bridge alternating 9V DC voltage is converted into 9V Ac voltage. Output of H-bridge is given to step up transformer to step up the voltage to 220V, 50Hz.The microcontroller is so programmed that at every instant it checks the voltage that is supplied to the load through sensors. At any instant if it detects that there is over voltage, under voltage or phase out, microcontroller acts to isolate the load from the power source by sending tripping signal to relay. Once relay isolates the load from the power source, microcontroller supplies the load through the charged battery. Battery supplies the load until the power supply voltage reaches normal value of voltage.