An introduction to the electrical winding connections of transformers for the supply of multipulse diode rectifiers and for Phase Shifting applications

This thesis represents an introduction to the use of special transformers for the supply of multi pulse diode rectifiers (MPDR) used in industrial environments to limit the harmonic content. In fact the energy power quality issue has become more and more important in the last years and the standards to prevent undesired effects such as harmonics and voltage dips require strong constraints for electrical equipment. From this point of view rectifiers are very important among power converters, as they act as the interfaces between loads (e.g. electrical drives) and the grid, and are responsible for a significant amount of electric pollution. A basic review of the parameters concerning the power quality, such as the Total Harmonic Distortion Factor is given. By starting from the most popular rectifiers used in industrial plant, i.e. three phase bridges having 6 pulse per period of the supply frequency, their standard operation is reviewed and different arrangements are presented. Then, as a large number of pulses helps to improve their effect on the grid, converter with 12 and 18 pulses are briefly described. Examples are also introduced of rectifiers supplied by 5-phase voltage systems and the extension to a larger number of phases is shown. In particular, a procedure is described which allows the calculations of the number of turns for 3 different kinds of transformer connections (Delta/Polygon, Delta/Fork and Delta/Hexagon) required to generate a five-phase system of sinusoidal voltages to supply a ten pulse bridge rectifier. A few simple simulations are presented to show the effect of an increased pulse number on the harmonic content of the line currents of the three phase system; a simple model of Inter Phase Reactor was also introduced and the behavior of series and parallel connections of rectifiers is shown. Finally, this work also gives evidence to Phase Shifting Transformers which are used in power networks for regulation of the power flow when proper phase displacements have to be defined between voltages at different nodes of an electrical system. A review of the most used connections such as Star/Delta, Zigzag, Extended-Delta, Polygon and Scott is presented, by analyzing and comparing their main features.