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This paper presents an improved method to generate the compensation reference currents applied to a three-phase line-interactive uninterruptible power supply (UPS) system with series and parallel active power line conditioning capabilities. The reference currents are used to compensate the reactive power and to eliminate harmonic currents generated from the non-linear loads, in a three-phase, four-wire system. The control strategy is based on the synchronous reference frame (SRF) method and the implemented current SRF-based controller does not have the task of compensating negative and zero sequence components at fundamental frequency of the load currents, due to unbalanced loads or unbalance source voltage conditions. Thus, the efficiency of the three-phase UPS system is increased when it is feeding unbalanced single-phase non-linear loads because the PWM parallel converter compensates only the harmonic load currents. Three strategies, employed to generate the reference currents, are discussed, such as, the SRF conventional strategy, the SRF single-phase strategy and the improved SRF single-phase strategy. Besides, the active powers that flow through the parallel PWM converter using the three presented strategies are shown and a comparative analysis of them is made to validate the predicted theoretical results.
Date of Conference: 17-21 June 2007