A study on the analysis of degradation mechanism for generator stator windings

The purpose of this study is to determine the aging mechanism of Mica-Epoxy composite material used for large generator stator winding in order to estimate remaining life of the generator. After degrading artificially the Mica-Epoxy composite material, the surface analysis is performed to analyze breakdown mechanism of insulation in air and hydrogen atmosphere: (i) In the case of air atmosphere, it is observed that an aging propagation from copper to slot electrode by partial discharge effect and the formation of cracks between layers is widely carbonized surface. (ii) In case of hydrogen atmosphere, the partial discharge effect is reduced by the hydrogen pressure (4kg/cm²). Potassium ions forming the mica of sheet layer is replaced by hydrogen ions, which leads to microcracks. It is confirmed that the sizes of crack by SEM analysis are 10~20 μm in diameter, 200~400 μm in length under air, and 1~5 μm in diameter, 10~50 μm in length under hydrogen atmosphere respectively. The breakdown mechanism of stator winding insulation materials which are composed of mica-epoxy is analyzed by the component of materials with EDS, SEM technique. We concluded that the potassium ions of mica components are replaced by H⁺, H₃O⁺ at boundary area of mica-epoxy and/or mica-mica. It is proposed that through these phenomena, the conductive layers of potassium enable creation of voids and cracks due to thermal, mechanical, electrical and environmental stresses