Discharge Studies of Epoxy/Mica Turbine Generator Insulation

This paper describes frequency accelerated experiments employed in an investigation into discharge resistance of small samples of epoxy resin bonded mica materials used for turbine generator stator bar insulation. Surface discharge experiments have been performed in nitrogen and helium at NTP, and internal discharge experiments in dry air at NTP and hydrogen at 650C and 310 kN m-2. An acceleration factor of 24, nominal stress 4 kV mm-1, was used for the particular experimental arrangement without any apparent change in the progression of dielectric deterioration. Only one sample failed at the above stress, equivalent service lifetimes in excess of 25 years being recorded at which point experiments were terminated. All mica based materials to some extent delaminated forming natural cavities; within these cavities white dentritic deposits were formed which were determined to be the result of discharge action on mica. Puncture of mica flakes between natural cavities and tracking between mica flakes was also observed. In general the materials made from small mica flakelets appeared to withstand the discharge attack better than large flake materials although all materials had lifetimes of at least 25 years. Samples made from relatively narrow tapes indicated severe damage at tape junctions.