A new parameter for the prediction of deterioration in epoxy ground wall insulation

A new partial discharge parameter, dynamic stagnation voltage, which is independent of voltage and polarity, is found to be a useful diagnostic measure in predicting remaining life and assessing insulation health. Twelve stator bars were taped with nine half flap layers of resin-rich press-curable mica-tape. Six examples were electrically stressed and three samples were subjected to combined thermal and electrical stresses. It is shown that the original electrical strength of epoxy-mica insulation can be interpreted in terms of this parameter, I_t. For new insulation, I_t=40 kV is assumed to be normal for this geometry and formulation. Insulation with I_t=20 kV has a tree channel which extends to the middle of the ground insulation. Generally, large-area delamination appears, which is verified by tear-down analysis. I_t<15 kV appears to be the critical value for insulation failure, since it is estimated that less than 1/2 of the insulation layer remains undeteriorated