Abstract—A unified power quality controller (UPQC) using a fuzzy controller (FC) has been proposed. The FC replaces the conventional PI controller in this paper. The results obtained through the FC are good in terms of dynamic response because of the fact that the FC is based on linguistic variable set theory and does not require a mathematical model of the system. Moreover, the tedious method of tuning the PI controller is not required in case of FC. Simulations are carried out using MATLAB/Simulink to validate the theoretical findings. Index Terms—Fuzzy controller, harmonics, PI controller, reactive power, unified power quality controller. I. INTRODUCTION HERE has been a continuous rise of nonlinear loads over the years due to intensive use of power electronic control in industry as well as by domestic consumers of electrical energy. The utility supplying these nonlinear loads has to supply large vars. Moreover, the harmonics generated by the nonlinear loads pollute the utility. The basic requirements for compensation process involve precise and continuous var control with fast dynamic response and on-line elimination of load harmonics. To satisfy these criterion, the traditional methods of var compensation using switched capacitor and thyristor controlled inductor [1-3] coupled with passive filters are increasingly replaced by active power filters (APFs) [4-8]. The APFs are of two types; the shunt APF and the series APF. The shunt APFs are used to compensate current related problems, such as reactive power compensation, current harmonic filtering, load unbalance compensation, etc. The series APFs are used to compensate voltage related problems, such as voltage harmonics, voltage sag, voltage swell, voltage flicker, etc. The unified power quality conditioner (UPQC) aims at integrating both shunt and series APFs through a common DC link capacitor. The UPQC is similar in construction to a unified power flow controller (UPFC) [9]. The UPFC is employed in power transmission system, where as the UPQC is employed in a power distribution system. The primary objective of UPFC is to control the flow of power at R. Mahanty is with the Department of Electrical Engineering, Institute of Technology, Banaras Hindu University, Banaras Hindu University, Varanasi 221005, India (corresponding author phone: +91-542-2575388; e-mail: mahantyr@yahoo.co.in). Chirag Patel is with the Department of Electrical Engineering, Dr. S & S. S. Ganddhy College of Engineering & Technology, Surat 395001, India (e- mail: chirag6903@gmail.com). fundamental frequency. On the other hand the UPQC controls distortion due to harmonics and unbalance in voltage in addition to control of flow of power at the fundamental frequency. The schematic block diagram of UPQC is shown in Fig. 1. It consists of two voltage source inverters (VSIs) connected back-to-back, sharing a common DC link in between. One of the VSIs act as a shunt APF, where as the other as a series APF. The performance of UPQC mainly depends upon how quickly and accurately compensation signals are derived. Control schemes of UPQC based on PI controller has been widely reported [10-13]. The PI control based techniques are simple and reasonably effective. However, the tuning of the PI controller is a tedious job. Further, the control of UPFC based on the conventional PI control is prone to severe dynamic interaction between active and reactive power flows [10]. In this work, the conventional PI controller has been replaced by a fuzzy controller (FC). The FC has been used in APFs in place of conventional PI controller for improving the dynamic performance [14, 15]. The FC is basically nonlinear and adaptive in nature. The results obtained through FC are superior in the cases where the effects of parameter variation of controller are also taken into consideration. The FC is based on linguistic variable set theory and does not require a mathematical model. Generally, the input variables are error and rate of change of error. If the error is coarse, the FC provides coarse tuning to the output variable and if the error is fine, it provides fine tuning to the output variable. In the normal operation of UPQC, the control circuitry of shunt APF calculates the compensating current for the current harmonics and the reactive power compensation. In the conventional methods, the DC link capacitor voltage is sensed and is compared with a reference value. The error signal thus derived is processed in a controller. A suitable sinusoidal reference signal in-phase with the supply voltage is multiplied with the output of the PI controller to generate the reference current. Hysteresis band is normally (most often but not always) is imposed on top and bottom of this reference current. The width of the hysteresis band is so adjusted such that the supply current total harmonic distortion (THD) remains within the international standards. The function of the series APF in UPQC is to compensate the voltage. The control circuitry of the series APF calculates the reference voltage to be injected by the series APF by comparing the terminal voltage with a reference value of voltage. Fuzzy Logic Controlled Unified Power Quality Conditioner for Power Quality Improvement Chirag Patel and R. Mahanty T 16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, 2010 681 Department of Electrical Engineering, Univ. College of Engg., Osmania University, Hyderabad, A.P, INDIA.