Short circuit tests to derive the buckling strength of Continuously Transposed Cable for power transformers under the influence of the paper insulation thickness

Continuously Transposed Cable (CTC) are stranded multiple conductors which often are utilized in power transformers. It is assumed that the paper insulation around CTCs enhances their mechanical strength and thereby also influences the short circuit withstand capability of power transformers. This paper presents testing results from short circuit tests with CTC's in order to evaluate their mechanical buckling strength depending on the insulation thickness. A special test setup is used to generate the necessary electromagnetic forces on the CTC under test. Therefore, two test windings around an iron core are connected in series with opposite current direction. During the short circuit tests the CTC deformation is recorded by a high-speed camera. The force acting on the conductors is evaluated from the current time history and a FEA simulation model for the test setup. Several short circuit tests are conducted for every CTC by increasing the testing current between the steps, until the inner CTC winding buckles. Results from factory new CTCs with varying paper insulation thicknesses but identical copper dimensions are proposed. The investigations show that the paper insulation enhances the CTC stiffness in linear elastic load situations and increases the critical load for the beginning of buckling.