THD Reduction using 12-Pulse Converter for Vector Controlled Induction Motor Drive

THD Reduction using 12-Pulse Converter for Vector Controlled Induction Motor Drive
Authors:C. SUBBA RAMI REDDY, G.VENKATA LAKSHMI, P.RRAJESWARI

Abstract: In this paper, a novel autotransformer with a reduced kilovolt-ampere rating is presented for harmonic current reduction in twelve-pulse ac-dc converter-fed vector-controlled induction motor drives (VCIMDs). Different transformer arrangements for 12-pulse-based rectification are also studied and a novel harmonic mitigator capable of suppressing fifth, seventh, and 11th (most dominant harmonics) in the supply current is presented. The design procedure for the proposed autotransformer is presented to show the flexibility in the design for making it a cost-effective replacement suitable for retrofit applications, where presently a six-pulse diode bridge rectifier is being used. The effect of load variation on VCIMD is also studied to demonstrate the effectiveness of the proposed harmonic mitigator. A set of power-quality indices on input ac mains and on a dc bus for a VCIMD fed from different 12-pulse ac-dc converters is given to compare their performance. 

Keywords: Autotransformer, Multipulse AC–DC Converter, Power-Quality Improvement, Vector-Controlled Induction Motor Drive (VCIMD).

INTRODUCTION 

      With the proliferation of power-electronic converters, the majority of dc drives are being replaced by variable frequency induction motor drives. These variable frequency induction motor drives are generally operated in vector control [1], as it is an elegant way of achieving highperformance control of induction motors in a way similar to the dc motor. These vector-controlled induction motor drives (VCIMDs) are fed by an uncontrolled ac–dc converter which results in injection of current harmonics into the supply system. These current harmonics, while propogating through the finite source impedance, result in voltage distortion at the point of common coupling, thereby affecting the nearby consumers. Various methods based on the principle of increasing the number of pulses in ac–dc converters have been reported in the literature to mitigate current harmonics [2]–[4]. These methods use two or more converters, where the harmonics generated by one converter are cancelled by another converter, by proper phase shift. The autotransformer-based configurations [2] provide the reduction in magnetics rating, as the transformer magnetic coupling transfers only a small portion of the total kilovoltampere of the induction motor drive. These autotransformerbased schemes considerably reduce the size and weight of the transformer. Autotransformer-based 12-pulse ac–dc converters have been reported [4] for reducing the total harmonic distortion (THD) of the ac mains current. To ensure equal power sharing between the diode bridges and to achieve good harmonic cancellation, this topology needs interphase transformers and impedance-matching inductors, resulting in increased complexity and cost. Moreover, the dc-link voltage is higher, making the scheme nonapplicable for retrofit applications.

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