Abstract
This dissertation presents a new flux control scheme for a solid-state transfer switch (STS) system and an uninterruptible power supply (UPS) system to accomplish fast load transfer and to mitigate the inrush current. Conventional STS system based on thyristors has been widely used in medium-voltage applications to enhance the power quality and reliability. However, conventional STS system often requires more than a quarter of cycle to complete the load transfer and its line transfer action also causes a considerable inrush current. In this dissertation, an improved STS with forced commutated circuit is presented to greatly reduce the transfer time and provide a better voltage sag ride-through capability for the critical loads. Based on this forced commutation capability, moreover, a flux estimation scheme and a thyristor gating scheme are presented to suppress the inrush current during the load transition process when the combination of the STS system and the transformer is used to serve the critical loads. Laboratory test results and design considerations are presented to validate the performance of proposed STS system. The inrush current issues associated with the solution for the UPS system are also presented in the dissertation. When the UPS systems are used for the voltage sag ride-through, the inrush current phenomenon often exists in the load transition process from a deformed grid voltage to battery power. To mitigate the inrush current, a closed-loop flux compensator is proposed and integrated with the voltage and current controllers. The proposed flux compensator can track the transformer flux and corrects the flux deviation in real time without sacrificing any voltage quality, thus completely avoiding the inrush current. Furthermore, the proposed flux control design is also extended to alleviate the inrush current when multiple transformers are energized by the UPS system. Detail description of the design issues and investigation of flux estimation error are given.