Abstract

Standard uninterruptible power supply (UPS) systems are connected in series between the ac mains and the critical load. A phase controlled rectifier feeds a battery supported dc bus and an inverter supplies the load. These systems require two conversion stages thus reducing operating efficiency. Moreover, input power factor is poor and large harmonic currents are injected into the ac mains. The proposed topology overcomes these drawbacks by placing the inverter-battery subsystem in shunt between the ac mains and the load. The battery is charged through the inverter dc bus. In normal operation, the ac mains feeds the load directly, and the inverter supplies only the amount of reactive power required to regulate the load voltage and to maintain the power factor very close to unity. When the ac mains fails, the solid state breakers, which connect ac mains to the load, open automatically; and the load is supplied by the battery, through the inverter. The proposed system has the following advantages: a near unity power factor, reduced line current harmonics and higher efficiency. Furthermore, unlike other proposed one conversion stage UPS systems, the proposed structure has the capability of automatic compensation for line or load unbalance. The thesis presents a complete analysis of both the steady state operations and the dynamic performances of the proposed system, with simulation and experimental verifications.