Thermal model of oil power transformers with a tap changer

The paper presents a new method for on-line temperature calculation taking into account change in the tap changer position. The method introduces accurate calculation of the losses, depending on the current, average temperature of the winding, and tap changer position (it affects both Joules (DC) losses and stray losses). The thermal model itself is the common three-body model (for a transformer with two windings per phase), with temperature dependent thermal conductances. Elements of the model describing steady-state characteristics are determined based on detailed construction and calculation of distribution of losses in the windings using the finite-elements method (FEM) and calculation of distribution of temperatures using the detailed thermal-hydraulic network model (THNM). Detailed calculation of the losses and temperatures enables the determination of the hot-spot factor (H = Q S); factor Q describes the nonuniform losses and factor S describes nonuniform cooling. The paper shows how these factors depend on tap changer position and the load.