Dielectric and physico-chemical properties of epoxy-mica insulation during thermoelectric aging

Three stator bars were subjected to 1142 hours of thermoelectric aging. At intervals during this time, the aging of the insulation was investigated by capacitance (C) and dissipation factor (DF) measurements and by the physicochemical techniques of infrared spectroscopy (FTIR) and Differential Scanning Calorimetry (DSC). Results indicated different degradation paths under discharge conditions, where the volume of voids within the insulation played a major role. Postcuring reactions were predominant during the first aging cycles. Such reactions were detected as reductions in capacitance and dissipation factor parameters, as a depletion of the chemical group epoxide at the absorption band 909 cm-1 as measured by FTIR, and as an increase of the glass transition temperature (Tg) of the epoxy resin. Since the postcuring reactions occur globally in the insulation, they were detected by both dielectric and physicochemical techniques. With the advance of the aging program the epoxy resin underwent structural changes, seen as modifications of functional groups CH2, CH3 and C=O. The air atmosphere where partial discharges occurred also promoted the formation of reactive species. When the air volume was high enough, the findings of FTIR measurements suggested a local degradation mechanism of mica involving cation exchange reactions between partial discharge byproducts and potassium layers in the mica.