Axial internal forces in power transformer active part elements after short circuit

Abstract

The paper examines core type transformers in which windings clamping and magnetic system end yokes clamping is accomplished by means of the same yoke beams. With regard to forces of friction between the yoke beams and the yoke there were determined internal forces in the active part elements after short circuits. Based on numerical analysis, there was studied influence of axial short circuit forces upon the internal forces in windings. One of the reasons for reduction of winding clamping forces are the forces of static friction between the yokes and yoke beams. With the short circuit forces acting on the yoke beams decreasing, the static friction forces change direction, thus preventing the yoke beams from returning to the initial location before short circuit. The reduction of the winding clamping forces is in direct dependency from the windings rigidity and clamping structure compliance. These phenomena were studied on a set of 110 kV class core type transformers with the indicated type of the clamping structure. It was demonstrated that the forces of static friction between the yokes and the yoke beams have a significant influence upon the windings clamping forces. The obtained results are consistent with transformer testing and service data.