The Influence and Design Consideration of UPS Output Virtual Resistance on Parallel-Connected UPS System

The important characteristics of parallel-connected uninterruptible power supply (UPS) systems are excellent power-sharing quality and stability. In this paper we use the output active and reactive power to adjust the output voltage amplitude and phase to achieve the load sharing. Because of the difference of output filter inductor´s resistance and distance of connection wire of each machine in the paralleled UPS system, the equivalent output impedance of each UPS inverter is different. These output impedance difference can cause circulation current and decrease the load-sharing quality. In order to solve this problem, virtual resistance is introduced to each inverter. In this paper the influence of the virtual resistance on paralleled UPS system and how to choose the virtual resistance is analyzed. Experimental studies using DSP controlled parallel-connected three-phase four-wire UPS system are presented to show the effectiveness of the proposed design.