Time to breakdown of high voltage winding insulations with respect to microscopic properties and manufacturing qualities

The main factors that influence time to breakdown of high voltage winding insulations are described in this work. Micrographs of industrially-made winding insulations with different raw materials, production techniques, types of taping and manufacturing qualities are analysed and imperfections in the microscopic structure of the insulations are shown. The effect of electrical tree inception and tree propagation on time to breakdown is described and it is identified why and where trees are initiated in the material. Electrical treeing is analysed depending on material properties and it is quantified which insulations lead to fast and which to slow tree propagation. The knowledge about the tree propagation has been used to develop a method that allows testing of winding insulations with far less effort. Similarities and differences to tests according to the standards are described. Properties of winding insulations from the production lines of different companies are identified in micrographs and their influence on time to breakdown is quantified with the new method. Due to a high practical relevance, the impregnation and the type of taping were thereby given special consideration for vacuum pressure impregnation (VPI) and resin rich insulations. Finally, the influences of electric field, temperature and mechanical vibrations on time to breakdown are described. The results of all tests show that the manufacturing quality plays a major role to wards time to breakdown (or lifetime) of winding insulations. To extend lifetime, more attention has to be paid to the manufacturing process. Concluding, characteristics for high quality insulations with a long potential lifetime are given in this work.