Indonesian Journal of Electrical Engineering and Computer Science Vol. 21, No. 1, January 2021, pp. 1~9 ISSN: 2502-4752, DOI: 10.11591/ijeecs.v21.i1.pp1-9  1 Journal homepage: http://ijeecs.iaescore.com Power quality improvement using fuzzy logic controller based unified power flow controller Ahmed N. Alsammak, Hasan A. Mohammed Department of Electrical Engineering, Colege of Engineering, University of Mosul, Iraq Article Info ABSTRACT Article history: Received Mar 24, 2020 Revised Jun 15, 2020 Accepted Jul 8, 2020 The power quality of the electrical system is an important issue for industrial, commercial, and housing uses. An increasing request for high quality electrical power and an increasing number of distorting loads had led to increase the consideration of power quality by customers and utilities. The development and use of flexible alternating current transmission system (FACTs) controllers in power transmission systems had led to many applications of these controllers. A unified power flow controller (UPFC) is one of the FACTs elements which is used to control both active and reactive power flow of the transmission line. This paper tried to improve power quality using a fuzzy logic controller (FLC) based UPFC, where it used to control both active and reactive power flow, decreas the total harmonic distortion (THD), correct power factor, regulate line voltage and enhance transient stability. A comparison study of the performance between the system with a conventional PID controller and FLC has been done. The theoretical analysis has been proved by implementing the system using MATLAB/SIMULINK package. Keywords: FACTs Power quality SSSC UPFCSTATCOM Voltage stability This is an open access article under the CC BY-SA license. Corresponding Author: Hasan A. Mohammed Department of Electrical Engineering The University of Mosul, Mosul, Iraq Email: hasan82adnan@uomosul.edu.iq 1. INTRODUCTION Recently, the term power quality (PQ) takes more consideration by the electrical engineers. The power quality is a very important issue related to the voltage, current, and frequency of the power system. Many circuits such as magnetic circuits, non-linear loads, converters, and flexible alternating current transmission system (FACTs) devices represent source of harmonics and problems with adversely influence to the power quality. In electrical engineering the term power quality may take several meanings such as (voltage quality), (current quality), (service quality), (supply reliability), (quality of source). The power quality of the power system may be faced with many problems like voltage sag, voltage swell, harmonics distortion, absence or lack of VAR compensation units, voltage interruptions, and transient conditions. To improve the performance of a microgrid, it is required to manage the fluctuation of reactive power efficiently and this is known as reactive power compensation. The FACTs can mitigate electrical power problems in different conditions (steady-state, transient, and post transient steady-state) [1-4]. It can be classified into four types, the first type is shunt controllers such as static synchronous compensator (STATCOM), the second type is series controllers, for example, static synchronous series compensator (SSSC), the third type is a combination between the series and shunt controllers such as unified power flow controller (UPFC) and the final type is a combination between two series controllers such as interline power controller (IPFC) [5-9]. The SSSC is used to control power flow and to improve transient stability [10-12], while the STATCOM