Input power factor improvement of a three phase rectifier by harmonic current injection

Abstract

A scheme for improving the input current wave shape of three phase bridge rectifier is proposed and analyzed in this thesis. Generally, to convert line frequency from ac to dc, a diode bridge rectifier is used. To reduce the ripple in the dc output voltage, a large filter is used at the output of a single-phase rectifier and a small output filter is used in a three phase rectifier. The capacitor used as filter causes the current drawn by converters to become non-sinusoidal in single-phase rectifiers. In three phase rectifiers input current takes the shape of periodic rectangular wave which is nonsinusoidal and discontinuous in nature. Due to the presence of low order harmonics, the total harmonic distortion of the current in lines become high and the input power factor in many cases become poor. The parallel active filter (PAF) is one of the solutions for harmonic current mitigation and reactive power compensation of nonlinear loads like rectifiers. This thesis analyzes design and control of a PAF for a 3-phase 3-wire rectifier load. A fifth-order harmonic injection PWM concept is developed for the harmonic reduction of the input current of a three-phase diode rectifier. The approach employs a direct selection of harmonic current. The fifth-order harmonic has been selected to be modulated by Pulse Width Modulator to suppress the dominant (fifth-order) harmonic in the input currents. To achieve THD less than 10% requirement of IEEE 519-1992, PAF technique has been introduced. The total scheme has been simulated by PSpice. The simulation result shows three phase rectifier’s input current becomes sinusoidal with less than ten percent THD. The scheme requires smaller input filter than passive filter method of input current shaping and it maintains high efficiency and maintains good input power factor at the same time.