Abstract
Unexpected voltage sags or outages often interrupt the operation of manufacturing processes or even lead to equipment damage. Numerous critical loads rely on the uninterruptible power supply system to uphold the power during these events and maintain the normal operation. As disturbances occur in the utility, the uninterruptible power supply (UPS) should take over the load within 15 ms to prevent any interruptions. However, the startup process of the UPS can cause the significant inrush current phenomenon on the load transformer due to magnetic saturation. The inrush current could result in reduced line voltages and even trigger the UPS's overcurrent protection. To prevent the inrush current, this paper proposes a flux offset compensation technique based on the synchronous reference frame (SRF) to regulate the flux of the load transformer. The proposed technique can be integrated with the existing voltage control and the pulse-width modulation (PWM) of the UPS inverter without any additional sensors. The proposed technique can provide a very smooth and timely transition without the risk of inrush current for critical loads under power outages and voltage sags. © 2010 IEEE.