Flux Estimation Techniques for Inrush Current Mitigation of Line-Interactive UPS Systems

The transformer inrush current contains a large amount of harmonic components and high current magnitude which could result in reduced line voltages and even trigger the overcurrent protection of an uninterruptible power supply (UPS). To mitigate the inrush current fast and also satisfy the load voltage requirement, the estimation of transformer flux is required in the UPS control design. However, in most of the flux estimation techniques, the transformer flux is calculated by integrating the measured line voltages directly. Therefore, a major problem with the pure integrator is that it lacks a virtual damper for convergence and a large error in the estimated transformer flux. The estimation error of transformer flux could lower the quality of UPS output voltage and affect the performance of inrush current mitigation. To improve the accuracy of flux estimation, this paper develops a flux-observer-based inrush current mitigation technique based on the synchronous reference frame. The flux-observer-based inrush current mitigation technique can make the UPS system track the variation of transformer flux fast and precisely. Therefore, it can improve the voltage quality and reduces the required voltage for the inrush current mitigation.