Refined control of an Unified Power Quality Conditioner under nonlinear and asymmetrical loads

In this paper an UPQC (Unified Power Quality Conditioner) with a FLFB (Four-Leg Full-Bridge) inverter used as a Shunt Active Filter is presented. The UPQC works in a three-phase four-wire electrical system in order to compensate for nonlinear and asymmetrical loads, and disturbances in the power supply voltage. For the generation of reference currents for the FLFB the instantaneous power theory in the dqz reference frame is used. The FLFB inverter is modulated using the 3D-SVM (3D-Space Vector Modulation) algorithm to generate the positive, negative and zero sequence voltages required. Finally, PI (Proportional-Integral) controllers are used to adjust the steady state error of the controlled signals in the shunt and series compensator. Simulation results show that when a nonlinear and asymmetrical load is compensated with the proposed UPQC topology and control scheme, unitary power factor, sinusoidal currents and neutral current mitigation can be achieved at the PCC (Point Common Coupling), all of this with constant rms voltage at the load side, independently of the PCC voltage disturbances. As an example, a simulation study is performed in this work obtaining a reduction from 29% to 1.9% in PCC current THD, a reduction from 52.8 (A rms) to 3.4 (A rms) in PCC neutral current, and a 1.3% variation in rms load voltage when a 10% sag occurred in PCC side.