The effect of tape overlappings in winding insulations on tree growth and breakdown time

The propagation of electrical trees in bent mica tapes of tape overlappings is described. The investigations were made because tapes in winding insulations of high voltage rotating machines are regularly bent at those sections. Geometrical sizes of bent mica tapes were analysed in micrographs and its influence on time to breakdown was determined in treeing experiments in a needle-plane-arrangement. It was found that a bent tape, which is penetrated by the tree, can lead to significantly reduced time to breakdown values. The probability of penetrating a tape depends on the size of bending and the support material. The results can be used by manufacturers to choose mica tapes that assure a higher quality of the insulation. Furthermore, the results provide a basis of developing tapes with lower sensitivity to damages at bends.