An Improved Control Strategy for Three-phase AC/DC/AC Converter in UPS Application

This paper presents an advanced control scheme using repetitive controller (RC) to improve performance of three-phase AC/DC/AC converter in UPS application. To guarantee a high quality output voltage of UPS, i.e., balanced and sinusoidal under various types of load, a proportional integral (PI) plus RC is employed in the inverter control scheme. Meanwhile, to achieve unity power factor operation for the front-end rectifier of the UPS, an enhanced current controller is developed with a proportional plus RC controller. The RC developed in this study has delay time of one sixth of fundamental period, so that it can improve dynamic response of UPS without degrading the steady-state control performance. In addition, the use of three-level converters for both AC/DC and DC/AC stage in UPS system help to reduce converter power losses and to increase overall efficiency without negative impact on system performance. The effectiveness of the suggested control method is verified by experimental results using a 20 kVA prototype built in laboratory.