This paper was produced for the 2019 NAFEMS World Congress in Quebec Canada

Resource Abstract

This study deals with the multiphysics simulation of a three-phase power transformer under short-circuit fault conditions. In order to do so, a coupling between the electromagnetic FEM software packages Altair-Flux and the structural analysis tool Altair OptiStruct is carried out to obtain the structural behavior over both, the transformer winding and its structural parts loaded by Laplace Forces. The process final output is the set of stresses and strains all over the transformer geometry by considering the forces over the coils were maximal.



Windings of power transformers are loaded by electromagnetic forces due to the currents they are carrying. During normal operation the stresses and strains caused by the forces on the windings themselves and on the transformer structural parts usually have a limited influence and are not an important risk for the device integrity.



In spite of this fact, it is inevitable for power transformers to suffer from short circuit and in rush currents that are several times higher than normal operation ones. These currents can become an important thread not only from an electric and electromagnetic point of view but also for the structural one, due to very important forces than can even lead to plastic strains in the transformer structural parts or the complete crash down of the structure.



For this reason, it is not surprising that electromagnetic forces and structural deformation in short-circuit failure conditions are a major concern for transformers designers and manufacturers. To know the device behavior in such critical conditions, physical tests are usually carried out, but these tests are expensive, complex, time-consuming and make the tested transformer useless. Moreover, for high voltage transformers test became unaffordable since they most likely will destroy a high prize device.



In such scenario, an accurate multiphysics simulation can fulfil a double aim: On one hand it lets us know the maximum forces and deformations expected during the test, allowing to anticipate the structural impact and to limit the potential damage the transformer may suffer. On the other hand, it is also possible to replace the physical test by the simulation, especially when they are impossible to carry out.



This fact explains the great industrial interest of this electromagnetic-structural coupling. However, even to obtain an accurate estimation of short-circuit currents is not an easy task, which becomes more and more challenging due to the influence of non-linear magnetic effects such as saturation in the magnetic cores or hysteresis cycles. Once these currents have been calculated, Laplace forces should be obtained and structural stresses and local displacements estimated, considering the important anisotropies in the mechanical properties of the windings and core.



On top of that, electromagnetic and structural analysis must be coupled to evaluate in a complete, accurate and holistic way the real risks of a short-circuit failure over the global transformer structure. In this paper, a multiphysics approach based in the coupling of two 3D FEM software is proposed.



Firstly, an electromagnetic analysis is performed in Altair Flux. A short-circuit failure is simulated through a transitory simulation and the maximum current flowing through the coils identified. The second step consists in computing the forces due to these maximal currents and exporting them into Altair OptiStruct, which will use them as inputs to calculate the stresses and strains all over the transformer.



Using the proposed method, a power transformer has been modelled and unforeseen structural weaknesses identified.