A single-phase UPS system with series-parallel power-line conditioning

This work proposes a control strategy applied to a single-phase uninterruptible power supply (UPS) system with series-parallel active power-line conditioning capabilities, allowing sinusoidal input current and sinusoidal output voltage, which results in an effective power factor correction. The UPS system is composed of two single-phase PWM converters to perform the active power line filter functions. The series converter acts as a series active power filter, becoming a sinusoidal current source in phase with the input voltage. The parallel converter acts as a parallel active power filter, becoming a sinusoidal voltage source in phase with the input voltage, which provides sinusoidal voltage with low total harmonic distortion (THD). The detection of the phase-angle is performed using a single-phase phase-locked loop (PLL) structure, which is based on instantaneous active power theory. The PLL system will be implemented into a two-phase stationary reference frame. Accordingly, a fictitious quadrature voltage needs to be generated, which must be orthogonal to the measured utility voltage. Both the control algorithm of the UPS system, which uses the synchronous reference frame (SRF) method, and the active power, which flows through the UPS converters, will be described and analytically studied. Mathematical analysis of the SRF-based algorithm is presented and simulation results are performed to validate the theoretical development and confirm the performance of the UPS system.