An Inrush Current Mitigation Technique for the Line-Interactive Uninterruptible Power Supply Systems

Unexpected voltage sags or outages events often interrupt the operation of manufacturing processes, or even lead to equipment damages. Numerous critical loads rely on the uninterruptible power supply (UPS) system to uphold the power during these events and maintain the normal operation. As disturbances occur in the utility, the UPS should take over the load within 1-5 ms to prevent any interruptions. However, the start-up process of UPS can cause the significant inrush current phenomenon on the load transformer due to the 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 linkage offset compensation technique based on the synchronous reference frame (SRF) to regulate the flux linkage of the transformer. The proposed technique can be integrated with the existing voltage control and the 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.