Derating of power transformers under dc excitation

In this thesis, firstly, to time efficiently and sensitively analyze the direct current (DC) excitation conditions' effects on single-phase shell type and three-phase five-leg core type power transformers, their reluctance-based models are developed. In the developed models, unlike other reluctance-based transformer models in the literature, the frequency-dependent nature of the winding losses has been taken into consideration. Secondly, for pure sinusoidal and DC biased voltage cases, the validity of the developed models has been verified by using the results of the finite element method as a reference. Then, the effects of DC excitation on the considered transformer types are analyzed with using the developed models. It is seen from the analysis results that DC excitation can cause saturation in transformers, and depending on the saturation, their losses, reactive power demand, and harmonic distortion of the excitation current can increase significantly. In addition, three different maximum loading ratio (MLR) determination methods, which are based on the rated primary current effective value, rated total loss, and rated primary winding loss limitations, are comparatively analyzed for the derating of transformers under the DC excitation conditions. From the analysis, it is concluded that the rated primary winding loss-based approach is more reliable than the other two approaches for transformers to operate without overheating under DC excitation conditions. Finally, under DC excitation, the effects of design choices such as magnetic core material type and core cross-sectional area sizing on the improvement of the MLR are analyzed. KEYWORDS: Power transformers, maximum loading ratio, direct current excitation, geomagnetically induced currents, electric-magnetic equivalent circuit model, reluctancebased model.