An improved series-parallel processing applied to a single-phase line-interactive UPS system for low power and low cost applications

This paper deals with a single-phase line-interactive uninterruptible power supply (UPS), for low power and low cost applications. Two push-pull PWM converters with reduced DC-bus voltage are used to perform an improved active series-parallel conditioning for a performance enhancement, as well as power processing reduction. The series converter is current controlled while the parallel converter is voltage controlled. Both the converters are controlled using sinusoidal references in phase with the utility voltage. The control references are obtained by using a simplified strategy based on a self-tuning filter (STF) used in conjunction with a phase-locked loop, allowing high voltage harmonic rejection. In standby mode, an effective universal conditioning is carried out, which is obtained by performing sinusoidal input current and sinusoidal output voltage. A theoretical study involving the power rate handled by the series and parallel converters is developed. Validation results are presented by means of 1kVA UPS system implementation, in which the UPS performance and the power processing reduction are evaluated.