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

Abstract

The paper studies core type transformers, in which radial and axial electromagnetic short circuit forces act upon winding coils. In the conductors, these forces cause stresses due to bending in the radial and axial directions, as well as tensile or compressive stresses. Due to plastic deformations originating in the winding conductors during winding fabrication, there was suggested the method for checking conductor strength in bending by the limiting state, under which the stress in every point of the dangerous section of the conductor equals to the proof stress of conductor material. There were obtained analytical expressions for the limiting bending moments of conductors of the round cross-section and rectangular cross-section without regard to its corner round-offs. For calculation of the limiting bending moments of conductors with the rectangular cross-section with regard to its corner round-offs there was obtained a numerically solvable system of equations. It was demonstrated that this solution is a generalised one and covers all the shapes of conductor cross-sections in use. By means of numerical analysis it was indicated that the round-offs of the rectangular cross-section have a substantial impact upon the limiting bending moments of conductors and must be taken into account during practical calculations. By the example of a set of transformers that had been tested for short circuit withstand capability it was demonstrated that the obtained results agree with test results.