How imperfections in mica tape barriers influence tree growth and breakdown time

The propagation of electrical trees in bent sections of mica tapes is described. The investigations have been made because tapes in coil insulations of high voltage rotating machines cannot always be wound in a perfectly even manner. The tapes are bent at the coil edges, at places where tapes overlap or due to the tapingand manufacturing process. Possible tape bends were therefore analysed by taking micrographs of industrially made model bars. To investigate the influence on time to breakdown, treeing experiments in needle-plane specimens with barriers of bent and unbent mica tapes were made. The results show that the tree can penetrate the tape below a certain bending radius, which depends on the tape material. Tree growth through a bent tape can lead to a significantly shorter time to breakdown. The results can be used for manufacturers to specify the minimum allowable tape bending and therefore assure a higher quality of the mica insulation. The results provide also a basis for development of tape materials with a lower sensitivity to damage at regions of tape bending.