Experimental Validation of Improved Power Quality in Unidirectional Multipulse AC - DC Converters due to Higher Pulse Number

Deployment of ac-dc converters invariably entails injection of current harmonics into the ac utility; and voltage ripple and hence, harmonics on the dc side. Multipulse unidirectional ac-dc converters are essentially rectifiers that have a high pulse number. The correlation of pulse number of an ac-dc converter and its power quality parameters on both the ac and dc sides is well established, however, in this paper the mathematical expressions of the instantaneous line current and load voltage and hence, their harmonic constituents, have been derived from the first principles for the most fundamental topologies and have been extrapolated to determine those of the higher pulse ones. This is important in the design of current harmonic and voltage ripple filters. Also, the correlation of pulse number with harmonic current distortion is theoretically established and experimentally validated with data from the three-, six- and twelve-pulse rectifiers.

I. Introduction

Ac to dc conversion of electrical energy is not only necessary but important as well. The wide range of applications that involve ac to dc conversion at power levels from a few watts to several thousands of megawatts clearly indicate that rectification is required to be efficient and in conformity with present day stringent power quality standards. The contemporary power quality norms [1] have exacting stipulations pertaining to permissible limits of harmonic distortion, power factor, distortion factor, displacement factor etc. Interestingly these power quality indices are related to the pulse number of the unidirectional ac-dc converters i.e., rectifiers. Warming and increasing awareness about the other ill effects of proliferation of carbon dioxide including air, water and soil pollution has not only deterred continual use of fossil fuels but also prompted the use of renewable energy resources. Since the complete transformation to clean and green resourced generation involves large scale dismantling of the conventional generation systems with concurrent deployment of renewable energy based ones to meet the ever growing energy demand the co-existence of both the systems is inevitable during the transition period. Also geographical and terrain limitations may not favour a total adoption of green technology. Thus, electric supplies comprising a combination of fossil fuel based bulk generation utility and distributed generation network based one or more renewable energy systems is a reality.