The ever-increasing use of power electronic devices and other non-linear loads in the power system has raised concerns about damaging effects of voltage and current harmonics, both in terms of their magnitude and frequency. In the power grid, transformers are the key components which interconnect different parts of the system at different operating voltage levels. Harmonic contamination in the electricity networks can generate excessive losses and associated temperature rise in transformer. Any

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... transformer failures will cause major outage and threaten the power supply reliability. Thus, it is important to accurately determine the impacts of harmonics on transformer life. Current standard provides an empirical approach to de-rate transformer capacities when transformers are subjected to non-sinusoidal load currents. The method was proposed based on some assumptions. The eddy current loss in transformer winding was taken to be proportional with square of the RMS current as well as the harmonic order, and the other stray loss in transformer structure was assumed to vary with square of harmonic current and a harmonic exponent factor. Furthermore, the standard ignored the effect of voltage harmonics on the transformer iron loss and temperature rise. In other words, the core loss was not considered in the de-rating process. The thesis focuses on practical assessment of voltage harmonics on transformer losses and temperature rise, specifically in oil-filled distribution transformers. Results demonstrated significant increase of transformer core loss under voltage harmonic contamination. An equation to calculate the resistance corresponding to the eddy-current loss was developed from empirical assumptions about losses in oil-filled transformer. Heat-run tests under voltage harmonic pollution were carried out on full- iv sized transformers. The temperature rise at different locations within the transformer was monitored by optical fibre temperature sensors. The analysis of winding temperature rise revealed increase of tem [...]