International Journal of Computer Applications (0975 – 8887) Volume 5– No.7, August 2010 21 Enhancement of Power Quality by using Unified Power Quality Conditioner with PID and Fuzzy Logic Controller R V D Rama Rao Assoc. Professor Narasaraopeta Engineering College, Narasaraopet – 522 601, India Dr.Subhransu Sekhar Dash Professor SRM University, Chennai,India ABSTRACT In this paper unified power quality conditioner (UPQC) is being used as a universal active power conditioning device to mitigate both current as well as voltage harmonics at a distribution end of power system network. The performance of UPQC mainly depends upon how quickly and accurately compensation signals are derived. The steady state and dynamic operation of control circuit in different load current and/or utility voltages conditions is studied through simulation results. The presented method has acceptable dynamic response with a very simple configuration of control circuit. This paper presents performance validation of Current Source Inverter (CSI)-based UPQC using Fuzzy Logic Controller (FLC) and Results are compared with conventional PID Controller and improvements are observed by FLC. The FLC-based compensation scheme eliminates voltage and current magnitude of harmonics with good dynamic response. Extensive simulation results using Matlab/Simulink for RL load connected through an uncontrolled bridge rectifier validates the performance of FLC compensator. Keywords active power filter; Fuzzy Logic Controller, FLC; current source inverter, CSI; harmonics; power quality; unified power quality conditioner. 1. INTRODUCTION Improvement of Power Semiconductor Technology since 1970, made it possible using these devices in electric utility applications. One of the recent developed of these applications is unified power quality conditioner (UPQC). According to the basic idea of UPQC, it consists of back-to-back connection of two three-phase active filters (AFs) with a common dc link. The point of common coupling (PCC) could be highly distorted, also the switching ON/OFF of high rated load connected to PCC may result into voltage sags or swells on the PCC has been discussed [2]-[3]. There are several sensitive loads, such as computer or microprocessor based AC/DC drive controller, with good voltage profile requirement; can function improperly or sometime can lose valuable data or in certain cases get damaged due to these voltage sag and swell conditions. One of the effective approaches is to use a unified power quality conditioner (UPQC) at PCC to protect the sensitive loads. A UPQC is a combination of shunt and series APFs, sharing a common dc link has been well presented in [4]. It is a versatile device that can compensate almost all power quality problems such as voltage harmonics, voltage unbalance, voltage flickers, voltage sags & swells, current harmonics, current unbalance, reactive current, etc. Recently more attention is being paid on mitigation of voltage sags and swells using UPQC. The swells are not as common as sags, but the effects of a swell can be more destructive than sag. For example, the excessive over voltage during swell condition may cause breakdown of components or equipments. The common cause of voltage sag and swell is sudden change of line current flowing through the source impedance. The steady state analysis of UPQC during voltage sag and swells on the system well presented in [5]- [6]. The main objective is to maintain the load bus voltage to be sinusoidal and the major concern is the flow of active and reactive power during these conditions. It plays animportant role to select the KVA ratings of both shunt and series APFs. Among different new technical options available to improve power quality, unified power quality conditioner has found to be more promising for compensation of current as well as voltage harmonics simultaneously. As per available literature, use of UPQC for mitigation of voltage and current harmonics has been presented in [1]. It is commonly configured with two voltage source converters connected back to back through a DC link capacitor. However, voltage source topology of UPQC has a drawback of slow control of onverter output voltage and no short circuit protection [7]- [8]. In addition, when the active rectifier inside UPQC is used as a power factor corrector, DC bus voltage oscillations appears which makes the control of the series filter output voltage more difficult. Before mentioned problems are overcome by using current source inverter (CSI). CSI-based UPQC has a faster phase voltage control loop and inherent short circuit protection capability. It also minimizes the cost as in this case passive filter connection between UPQC and the load is not necessary. The only disadvantage of CSI-based UPQC is that its dc link inductor is bulky and heavy which leads to high dc link losses. It uses synchronously rotating frame to derive reference signals, which has increased time delay in filtering dc quantities. The concept of FLC is to utilize the qualitative knowledge of a system to design a practical controller [9]. For a process control system, a fuzzy control algorithm embeds the intuition and experience of an operator, designer and researcher. The control doesn‟t need accurate mathematical model of a plant, and therefore, it suits well to a process where the model is unknown or ill-defined and particularly to systems with uncertain or complex dynamics [10]. In this paper the application of fuzzy logic in control of shunt and series active power filters used a hystersis band comparator for control of three-phase active power filter. This paper presents a novel method for derivation of compensation signals in CSI-based UPQC using Fuzzy logic Controller.The performance of the system is verified by extensive simulation on Matlab/Simulink