Coupled magneto-mechanical finite element analysis of a power transformer in short circuit conditions

Abstract

External short circuit is one of the most demanding load conditions a transformer can be subjected to. Short circuit withstand capability of power transformers is therefore quintessentially important in order to ensure the proper functioning of a power transformer during its lifetime. Accurate calculation of the forces and stresses a transformer is subjected to during a short circuit is a prerequisite for better, optimized design of the active part. Main focus of this paper is the investigation into dynamic electromagnetic and mechanical behaviour of a transformer winding subject to an external short circuit. For purposes of this simulation, a single-phase 100 MVA autotransformer active part was modelled using ANSYS and NACS software. Particular areas of the winding were modelled to a greater degree of detail in order to observe the effects of Lorentz forces during a short circuit on individual conductors. A transient coupled magneto-mechanical simulation of the transformer under short circuit conditions was carried out. When subject to dynamic short circuit forces, the winding discs exhibited a profoundly resonant behaviour indicating a strong relationship between the natural frequency of the winding and the resulting stresses and displacements incurred during a short circuit. It has been shown that the position of the yoke changes the orientation and the distribution of the magnetic field vectors at the top and the bottom of the winding, causing a non-uniform distribution of forces along the top discs of the winding. This non-uniform distribution of forces along the circular shape of the winding conductor caused high stresses at the positions within the winding which were previously considered to be under lower stress when calculated using 3D static FEM and analytical methods.