Numerical assessment of the electrical transformer heating test

Abstract

The active components of an electric transformer are the windings and the magnetic core. The losses in these components originate warming, if they are very large, accelerate the deterioration of winding insulation and finally the entire machine. Using a numerical method it is now possible to assess the heat removal process inside an electrical transformer driven by natural convection. This computational tool solves the thermally coupled Navier-Stokes equations. The final goal is the optimization of the design and the operation of these machines. The physical phenomena taking place in the interior of the machine and also the fluid flow with the energy sources coming from the magnetic core and windings and their losses towards the tank walls are
captured. Specifically, in this work a distribution transformer is considered and the prediction of the temperature as the most representative parameter is obtained. The results have been achieved by the Finite Element Method, which solves numerically a mathematical model based on the Navier-Stokes equations and the transport equation of thermal energy, in a coupled way using an incompressible assumption for the flow and using a flotation term incorporated into the momentum equation to allow the assumption of constant density (Boussinesq hypothesis). The high Prandtl’s number of the transformer oil force to use very refined meshes on the interfaces solid - fluid. It also highlights the heterogeneity of the windings, in which is necessary to study the temperature distribution with a significant anisotropy of the lower voltage winding. The numerical results have been correlated positively with experimental data of thermal test (IRAM 2018, 1995) over a distribution transformer (315 kV 0231 kVA 13.2/0.4-) built by Tadeo Czerweny SA company Results of the simulation are presented, not only for the fluid dynamics also for the thermal regime, concluding that they are consistent with the phenomena being observed in the physical essays. In this way the procedure seems to be a robust tool for the study of the durability of such a machine. In an attached appendix the procedures used for determining the thermal conductivities of the windings are shown.