Effect of electrical tree propagation on breakdown in mica insulations

The work presented has been carried out to obtain a better insight into electrical aging phenomena of mica insulation as used in high voltage rotating machines. Model bars with typical insulating materials were exposed to standard electrical aging and stimulated tree propagation at a new electrode arrangement. The results show that time to breakdown under electrical load is mainly determined by electrical treeing. Tree growth must be considered to start at the same instant voltage is being applied. Micrographs show that the tree propagates mainly along the interface binder resin-mica-carrier material. If the mica layer is damaged the tree can also penetrate the mica tape itself. Higher breakdown time values were measured at insulation materials with higher mica content and a higher number of thinner tape layers. To judge from the results, electrical tree growth manifests itself as the main process of electrical aging. Further research for advanced materials and improved application processes in generator bar manufacturing has therefore to be focused on electrical treeing.