An improved control method for unified power quality conditioner with unbalanced load

Abstract

This paper proposes an improved control method for Unified Power Quality Conditioner (UPQC) with unbalanced load. In UPQC, shunt APF is overburdened when it alone supplies total load reactive power. PAC method aims at effective utilization of series and shunt APFs by sharing reactive power burden between the two. In presence of unbalanced load, existing PAC methods can lead to circulation of reactive power between two APFs and thereby result in overloading of UPQC. Also, due to unbalanced compensating currents, DC link voltage contains second order oscillations, which deteriorates source current quality. In this work, a new PAC method is proposed, which avoids circulation of reactive power and unnecessary VA burden on UPQC. To suppress DC link voltage oscillations, proposed control employs a ‘mean block’ (moving average) at output of PI controller, which ensures balanced and harmonic free source currents. Also, a ‘percentage unbalance’ parameter has been proposed to quantify the unbalance in three phase quantities. Performance of proposed UPQC system is tested in the presence of non-linear, reactive and unbalanced loads. Dynamic performance of system is studied during grid disturbances such as voltage sag, swell, and change in load. Real time simulation carried out in Opal-RT validate the effectiveness of proposed method. The electrical circuit of UPQC is simulated with sub-microsecond time step on FPGA based computational engine of Opal-RT for better verification of the proposed contro