International Journal of Scientific and Research Publications, Volume 4, Issue 7, July 2014 1 ISSN 2250-3153 www.ijsrp.org Improvement of Power Quality Using PWM Rectifiers Mahasweta Bhattacharya* * ECE, Future Institute of Engineering & Management, Kolkata Abstract- The paper presents the modeling, simulation and analysis of an AC-DC converter based PWM rectifier. It provides a suitable control algorithm for a pulse width modulation rectifier which reduces ripple from the DC output side as well as shapes the input current properly. The basic objective of a PWM rectifier is to regulate the DC output voltage and also ensure a sinusoidal input current and unity power factor operation. This is implemented by high speed IGBT switches connected in anti parallel mode across the rectifier diodes. The output voltage is controlled by switching these IGBTS and higher order ripples at the output can be easily eliminated with the help of passive filters. Lower order harmonics are eliminated using PWM technique. The control subsystem generates gating pulse to the universal bridge by passing the output voltage through a network consisting of comparator, discrete PI controller and discrete PWM generator. The output of this generator are the gating pulses to be applied to the universal bridge. By this control method, we have tried to reduce the input current harmonic distortion and bring the input current and voltage in same phase as well as make it sinusoidal. The control of modulation index(m) and phi has been shown in the closed loop system. This paper presents the state of the art in the field of regenerative rectifiers with reduced input harmonics and improved power factor. The influence of the discussed modulation methods on the line current distortion and the switching frequency has been examined. The simulation results of the presented techniques have been demonstrated and concluded for various load resistance. Index Terms- Improvement of Power Quality, PWM Rectifiers I. INTRODUCTION The problems that tend to occur due to harmonic distortions and power factor variations are known and are under discussion for quite some time now. Large number of solutions has also been proposed like static VAR compensators passive or active filters which would improve the quality of the power that is delivered to the mains. A switching power rectifier in the power system converts one level of electrical energy into another level of electrical energy. Converters in the AC to DC conversion field are the most widespread and the operation of a converter can be explained in terms of the input quantities, output quantities and the switching pattern used to obtain the preferred output. Types of semiconductor devices used in the rectifier are as follows: a. Uncontrolled rectifiers - Diodes as switches b. Phase-controlled rectifiers - SCR (silicon controlled rectifiers) c. Pulse-width modulation rectifiers - IGBTs (insulated gate bipolar transistors) or power MOSFETs (metal oxide field-effect transistors) The ability to control the system to obtain unity power factor operation of a boost rectifier is an important feature of the rectifier topology. The power factor (PF) is defined as the ratio of working power to apparent power. The power quality problems, such as large values of harmonics, poor power factor and high total harmonic distortion, are usually associated with operations of AC to DC converters. An increase in the current harmonics and a decrease in the displacement power factor in AC power lines produced by diode and thyristors are serious problems. Pulse width-modulation (PWM) rectifiers in distribution systems represents the best solution, in terms of performance and effectiveness, for elimination of harmonic distortion as well as power factor correction, balancing of loads, voltage regulation and flicker compensation. Sinusoidal PWM is a technique employed where the sinusoidal waveform or modulation signal is compared with a very high frequency triangle or carrier signal to obtain the switching pulses for the device. A method of identification of supply current will be developed by using MATLAB/Simulink for elimination of the harmonics of current and to obtain a sinusoidal current of the line. I.1 Problem Identification From the study in this paper, there are a large number of switching converter topologies and composite switching converters are possible. The power quality problems, such as large values of harmonics, poor power factor and high total harmonic distortion, are usually associated with operation of AC/DC converters. The other important problem is that the input current and voltage waveforms are not in phase due to the distortions. There have been many approaches to mitigate the harmonics and other problems in the rectifier system. I.2 Objective of paper