Investigations on Three-Phase Front-End AC-DC Converters for Power Quality Improvement

Abstract

The revolution in power electronics has opened an era for widespread use of power converters of different power rating from few Watts to Mega-Watts. Three-phase AC-DC converters are most widely used power converters as the distributed electric power is AC supply, while the applications based on DC supply as well as variable frequency AC supply, need conversion of AC supply into DC supply. Large current harmonics and poor power factor in the utility interface are common problems in three-phase AC-DC converters. These AC-DC converters are used invariably at the front-end in numerous applications which may or may not be electrically isolated from the AC supply system depending on the rating and nature of the load and also the prevalent ‘Standards’ requirement. The applications such as electrochemical, electrometallurgical and electrical heating process, high voltage direct current systems, adjustable speed drives, battery charging, aerospace and naval equipment’s, uninterrupted power supplies etc., use AC-DC conversion at the front-end. These processing industries and adjustable speed drives are the main applications wherein large amount of power is involved in AC-DC converters. These AC-DC converters are generally diode-based, thyristor-based or self-commutating device-based converters depending on applications, size and cost. The wide spread use of AC-DC converters for various applications have resulted in power pollution leading to failure of sensitive equipment’s, reduced efficiency, etc. This has led to the development of power quality standards and hence attracted attention of many researchers for improving the power quality at AC mains. This research work aims at classifying and investigating different three-phase AC-DC converters that employ diodes. The AC-DC converters are mainly classified on the basis of the circuit configuration used for power quality improvement. They are passive, active and hybrid AC-DC converters. The detailed investigation in each category is carried out based on the circuit configurations and control techniques employed. Also, it has led to the formation ofiv some new AC-DC converter configurations that are investigated for the power quality improvement capability. Firstly, this research work aims at employing multi-pulse techniques for mitigating the power quality problems at the AC mains in front-end AC-DC converters. The multi-pulse technique uses autoconfigured transformer for power quality improvement and hence it is termed as passive technique. The investigations on two types of multi-pulse AC-DC converters is carried out covering a wide range of applications that use three-phase AC supply at the front-end for converting it to DC power. The two type of multi-pulse AC-DC converters are multi-phase staggering autoconfigured transformer and asymmetric multiphase converter. The former uses delta and zig-zag autoconfigured transformer and the latter uses delta connected autotransformer for power quality improvement. The main feature of these multi-pulse AC-DC converters is its ability to reduce current harmonics distortion. In these multi-pulse AC-DC converters the number of pulses (voltage ripples at output of AC-DC converters in one cycle of AC supply voltage or steps in the current at input of AC mains) is increased by using phase staggering (or phase shifting), multi-phase (or phase multiplication) and hybrid of these techniques. Further, the unconventional pulse numbered AC-DC converters having pulse number of 20 is considered for the power quality improvement at AC mains. Secondly, this research work aims at utilizing active front-end AC-DC converter for power quality improvement. The active topologies namely Vienna rectifier and Delta switch rectifier are chosen as front-end AC-DC converters for power quality improvement. The voltage sensorless control technique is proposed for the active front-end AC-DC converters such that the computational complexity and sensing effort have been reduced. The system results in improved power quality parameters with less engineering effort. Finally, this research work also focuses on hybrid front-end AC-DC converter for power quality improvement. The hybrid AC-DC converter uses three-level boost converter as active modulation circuit and zig-zag transformer as passive current injection circuit. The current injection circuit along with the modulation circuit at the output stage increases thev DC-link voltage. Further, the utilised current injection circuit avoids resonance problem and also resulted in less rating. Furthermore, the modulation circuit results in reduced ripple current and device rating. For visualizing the different advantages of the three-phase front-end AC-DC converters, the design and simulation of these converters are carried out in MATLAB/Simulink. The main emphasis of these investigations has been on compactness of configurations, simplicity and reduction in rating of components to reduce the overall cost of these frontend AC-DC converters. A laboratory prototypes of these front-end AC-DC converters are developed to validate the design. A high speed digital processor namely field programmable gate array controller, that consists of EP4CE30F484 processor is used to implement a control scheme for active and hybrid front-end AC-DC converters. The steady-state and transient-state performance of the front-end AC-DC converters are verified by changing the resistive load. These front-end AC-DC configurations and techniques have resulted in improved power quality indices with overall reduced rating and reduction in number of components.