Comparative study and non-linear simulations of harmonics in conventional E-core and hexa-core transformers

Abstract

The power system is very advanced and complex, it is considered to be the largest machine ever built in the history of humankind. Its uninterruptable and reliable functioning is the key for further progress of the modern society. The power system includes generation, transportation and consumption of electrical energy and its efficiency on all these stages is determined by losses in the system. Power loss during energy transportation is inversely proportional to the operating voltage. Therefore, the use of higher voltages is desirable. Moreover, with higher voltages it is possible to transport more energy. In order to raise the voltage, transformers are used on both transmission and distribution levels. There are a countless number of them operating around the world. Conventional three-phase transformers are mainly constructed using E-type magnetic cores where magnetic fluxes are generated in each core leg by phase currents flowing in the windings. The arrangement of the core and its non-linear magnetic properties lead to asymmetry and distortion of voltages and/or currents waveforms. In general, presence of harmonics is strongly undesirable due to additional energy losses and their impact on equipment connected to the transformer. One promising way to prevent generation of harmonics in the power system is to arrange the magnetic circuit of a transformer in a special manner that would, e.g., provide conditions for compensation or even cancellation of harmonics produced in different phases. Such non-conventional (hexa-core) transformers are manufactured today by the company Hexaformer AB (Västervick, Sweden). Preliminary tests have shown that these machines may have several advantages over traditional technology, in particular, reduced generation of harmonics and better performance in presence of them. The objective of this thesis work is to analyze sources and conditions for generation of harmonics in power distribution transformers and to compare a hexa-core transformer with a conventional E- core transformer. The study is performed by means of computer simulations and experimental studies of the behavior of the machines under different load conditions. The developed computer models are validated with measurements done on real transformers. The data obtained from both measurements and simulations are used to compare the performance of the machines. The results of the work will contribute to further development and better understanding of hexa-core technology.