Journal of Theoretical and Applied Information Technology 31 st August 2011. Vol. 30 No.2 © 2005 - 2011 JATIT & LLS. All rights reserved . ISSN: 1992-8645 www.jatit.org E-ISSN: 1817-3195 102 THREE PHASE CONTROL FOR PWM-SWITCHED AUTOTRANSFORMER VOLTAGE-SAG COMPENSATOR BASED ON PHASE ANGLE ANALYSIS MUHAMAD MANSOR 1 , NASRUDIN ABD. RAHIM 2 1 Dept. Of Electrical Power, College Of Engineering, Universiti Tenaga Nasional, Malaysia. 2 Umpedac, University Of Malaya, Malaysia Email: 1 muhamadm@uniten.edu.my ABSTRACT Enhancing compensating capability is essential in a voltage-sag compensator. It involves two main aspects, first, sag detection technique and second, voltage sag compensator. Many detection techniques have been introduced to measure and to detect voltage sag, such as RMS- Value Evaluation and Peak-Value Evaluation. Unfortunately, most of the techniques require a delay for sag to be detected then compensated, whereas immediate sag detection is vital to improvement of transient performance. Presented is a three-phase PWM-switched autotransformer voltage-sag compensator that is based on AC-AC converter and that uses the proposed three phase controller based on Phase Angle Analysis as the detection technique. The significant advantage of this detection technique is the detection time. It is capable of detecting and compensating voltage sag the moment sag occurs without delay. Its effectiveness and capability were verified via MATLAB/Simulink simulation. Keywords: Voltage Sag, Voltage Sag Detection, Voltage Sag Compensator, RMS Detection, Peak Detection. 1. INTRODUCTION Sag detection is important as it determines the dynamic performance of a voltage-sag compensator. Research on voltage sag detection has also grown up and it is an essential part of the voltage sag compensator. It is the phenomenon of RMS voltage rapidly declining from 90% to 10% rated voltage, typically for 0.5 to 30 cycles [1]. Voltage sags commonly are caused by lightning, accidental short circuits, loose connections, the starting of large motors (or air-conditioners), or abnormal use of AC mains [2]. Short periods of voltage sag may cause irreversible damage to sensitive equipment and cause significant economic losses, owing to interrupted industrial production [3]. Disturbances caused by voltage sags cause losses to not only production but also utility [4-5], increasing demand for clean power as use of microelectronic processors increases in various types of equipment such as computer terminals, programmable logic controllers, and diagnostic systems. These are susceptible to disturbances in their supply voltage, and the widespread application of nonlinear electronic devices in power apparatuses and systems makes waveform distortion more significant. One of the most popular topologies for voltage-sag compensators is the dynamic voltage restorer (DVR), which requires a voltage-source inverter (VSI) for line-injection of series voltage, an injection transformer, and a dc link. Its obvious disadvantage, however, is that is incapable of compensating deep and long-duration voltage sag. Increasing its capability requires more energy storage devices, increasing cost. Another consideration is environmental, as battery is the energy storage device. Also, voltage regulation of the dc link demands use of a separate ac-dc converter, which requires one more stage of power conversion, increasing size, cost, control complexity, and power losses [6]. Energy storage is unnecessary in AC-AC sag compensator, though an AC-AC converter is needed to convert dropped ac voltage to regulated ac voltage. The three-phase PWM-switched autotransformer voltage-sag compensator presented here uses an AC-AC converter and three-phase controller based on Phase Angle Analysis as the