Long-term performance of micaceous insulation

Micaceous synthetic resin insulations, as used for the stator windings of high-voltage rotating machines, were investigated using laboratory-scale insulation models and original Roebel bars. The investigations were performed to clarify the relationships between the various stresses and the remaining lifetime, to determine changes in the aging condition by means of diagnostic tests, and to compile possible correlations between several measured and derived quantities and the remaining lifetime. Pure thermal stressing at 140°C over a test period of up to at least 12 months was found to produce practically no change in the dielectric behavior of the tested mica-resin insulation. Combined stressing (160°C+9 kV/mm) over three months of aging influences the remaining lifetime only insignificantly. The insulating capability is greatly reduced only after a six-month period of stressing. Therefore, the lifetime does not diminish uniformly. Should the insulation have entered into the final phase of its lifetime, a rough estimate of the remaining lifetime can be made by means of appropriate diagnostic examinations, e.g. loss-tangent test and partial-discharge measurements as a function of the voltage