A Utility Interactive PWM Sine-Wave Inverter Configured as a High Efficiency UPS

Conventional UPS equipment employs a tandem charger, battery, and inverter to isolate critical loads from utility disturbances and failures. Since two distinct stages of power conversion -- charger and inverter -- are required, this method is both inefficient and expensive. This paper describes a new circuit topology and operating method that uses a single four-quadrant PWM sine-wave inverter to both regulate the critical load and charge the standby battery. The complete power system consists of an inverter coupled to the utility through an inductor. The critical load is connected directly across the inverter output. This allows the inverter complete control of the load voltage and waveform. When the inverter is operated synchronously with the utility, power flow is regulated by varying the phase angle between the inverter and the utility. The phase angle is automatically adjusted using an 8-bit microprocessor to meet the load and battery charge current requirements. The microprocessor also performs housekeeping and display functions. Since the system requires only one power conversion stage, throughput efficiencies exceeding 90% have been achieved at the 3kW power level. Power densities also have been improved because of its "chargerless" construction. General principles of operation with phasor diagrams including actual system waveforms are presented as well as advantages and disadvantages of this approach with respect to conventional UPS equipment.