PERFORMANCE EVALUATION OF CONTROL STRATEGIES FOR PWM RECTIFIERS

Abstract

All uncontrolled and phase-controlled rectifier topologies in use today suffer from poor input power factor and high harmonic content in input supply current. These may cause major problems in the power distribution system, be it when transferring power via a DC link, or when supplying input to, a DC motor. Various methods of improving power factor and decreasing harmonic content are discussed, and converters using PWM control are chosen for study. The topologies utilising PWM control for three-phase rectifiers are discussed and compared, and the optimum choice — the two-level six-switch reversible rectifier — is identified. Various control strategies for the pulse-width modulation, divided into two categories, are taken up. Under converter switching control strategies, hysteresis control, constant switching frequency control and space vector modulation are simulated. Under converter current control strategies, voltage and virtual-flux oriented controls, and direct power and virtual-flux-based direct power control strategies are simulated using MATLAB SimuIink software. A comparative analysis of all the methods is performed to choose the most suitable control strategy for PWM rectifiers so as to achieve unity input power factor and very low harmonic distortion in input supply current. It is seen that virtual flux-based direct power control strategy (VF-DPC) gives minimum total harmonic distortion in the input supply current, while also having low overshoot and undershoot along with good dynamic performance.