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This paper describes the active power flow behavior of a three-phase line-interactive uninterruptible power supply system with series-parallel active power-line conditioning capabilities. The control algorithm uses synchronous reference frame (SRF)-based controller, which allows an effective harmonic and reactive power compensation, generated from any configuration of non-linear load. The control of the active power flow in the dc-bus of the UPS is obtained through an additional voltage dc-bus controller. Under normal line conditions, the UPS system works with universal filtering capabilities, such as, compensating the input currents and the output voltages. Two pulse width modulation (PWM) converters are used to perform the series and parallel active power-line conditioning. The active power flow control of the UPS system is discussed taking into account battery charging and undesired conditions, such as, input voltage unbalances, total harmonic distortion of the load currents and displacement factor (cosφ1). The control algorithm using SRF method are described and analytically studied. The performance of the UPS system is evaluated in three-phase, three-wire system. Experimental results are presented to confirm the theoretical studies.