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 UPS´s overcurrent protection. 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 technique, 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 on estimated transformer flux. The estimation error of transformer flux could lower quality of UPS´s 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 (SRF). The flux observer-based inrush current mitigation technique can make the UPS system to track the variation of transformer flux fast and precisely. Therefore, it can benefits the voltage quality and reduces the required voltage for inrush current mitigation.