Examination of Harmonics in Power Systems with Embedded Power Electronic Converters during Transients and Steady State

Rapid growth in the renewable energy sector and the upcoming growth of the electrical energy storage sector are resulting in an ever-growing number of grid-connected power converters. These converters inject harmonic currents into the grid that can lead to unacceptable levels of harmonic pollution during both transients and steady state. To study transient harmonics of power converters, the concept of generalized averaging has recently attracted considerable interest. Two distinct approaches have been employed in the literature to obtain dynamic harmonics. However, embedded in these approaches are restricting assumptions that have been overlooked by some researchers. This has resulted in several misconceptions and misapplications of models. This thesis determines implicit underlying assumptions and identifies appropriate and inappropriate areas of applications for these two approaches. A simple circuit is employed to clearly communicate ideas. To study steady state harmonics of power converters, few pragmatic tools presently exist for utilities to predict the impact of grid-connecting a new power converter or to assess the potential grid impact of one converter versus another. Converter data sheets contain insufficient information to make such determinations. It is generally known that the amount of harmonic injection depends not only on the background voltage harmonics, but also on the interactions that occur between the grid harmonic impedance and the converter. The thesis proposes new pragmatic techniques based on the frequency coupling matrix concept that permit accurate modeling and analysis of power converters, without requiring detailed information about the internal parameters of the converters. An online technique for identification of the frequency coupling matrix model of grid-connected converters is also proposed. Proposed techniques are verified through experimental measurements on a commercial, three phase, photovoltaic inverter.Ph.D