Dynamic top oil thermal model of oil-immersed power transformers with tap changer

The paper presents the improved method for on-line calculation of top oil temperature. Basic of the model is well-known lumped thermal circuit with nodes corresponding to each of phase windings and additional node for the core, oil and the tank. Improvement in respect to the standards is consideration of the influence of temperature dependent thermal conductivities (steady-state thermal characteristics) to the transient thermal characteristics. Change of the cooling mode (AN - natural air flow / AF - forced air flow) and change of the number of fans in operation is taken into account in the model. The finest content of the model, representing improvement in respect to the similar work published in [1], is the manner of the calculation of the losses, including the dependence on the load, tap changer position and average winding temperature.