Zoran RADAKOVIC, Stefan TENBOHLEN

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.

PDF

___

[1] Georgilakis PS. Spotlight on Modern Transformer Design. London, UK: Springer, 2009.
[2] Radakovic Z, Kalic D. Results of a novel algorithm for the calculation of the characteristic temperatures in power oil transformers. Electr Eng 1997; 80: 205-214.
[3] Schmidt N, Tenbohlen S, Skrzypek R, Dolata B. Assessment of overload capabilities of power transformers by thermal modelling. In: CIGRE SC A2 and D1 Joint Colloquium 2011; 1116 September 2011; Kyoto, Japan: PS1-O-28.
[4] Radakovic Z, Jacic D, Lukic J, Milosavljevic S. Loading of transformers in conditions of controlled cooling system. Int T Electr Energy 2014; 24: 203-214.
[5] Radakovic Z, Sorgic M. Basics of detailed thermal-hydraulic model for thermal design of oil power transformers. IEEE T Power Deliver 2010; 25: 790-802.
[6] IEC. IEC Publication 600354, Loading Guide for Oil-Immersed Power Transformers. Geneva, Switzerland: IEC, 1991.
[7] IEC. IEC Publication 60076-7, Loading Guide for Oil-Immersed Power Transformers. Geneva, Switzerland: IEC, 2005.
[8] IEEE. IEEE Standard C57.91-1995, IEEE Guide for Loading Mineral-Oil-Immersed Transformers. New York, NY, USA: IEEE, 1996.
[9] Radakovic Z, Feser K. A new method for the calculation of the hot-spot temperature in power transformers with ONAN cooling. IEEE T Power Deliver 2003; 18: 1284-1292.
[10] IEC. IEC Publication 60076-2 ed.3.0, Power Transformers - Part 2: Temperature Rise for Liquid-Immersed Transformers. Geneva, Switzerland: IEC, 2011.
[11] Radakovic Z, Radoman U, Kostic P. Decomposition of the hot-spot factor. IEEE T Power Deliver 2015; 30: 403-411.
[12] Radakovic Z. Numerical determination of characteristic temperatures in directly loaded power oil transformer. Eur T Electr Power 2003; 13: 47-54.