Condition assessment of epoxy-impregnated insulating materials

The residual lifetime of primary and secondary insulation materials is a critical parameter for many electrical machines, especially when in service. The analysis of epoxy impregnated materials with conventional techniques is a particularly difficult one, as this often requires removal and repair of at least part of the insulation. We have investigated methods based on reflectance spectroscopy that demonstrate the ability to predict the temperature to which carefully air-aged epoxy-impregnated glass mats have been subjected. The resulting calibrations also provide excellent predictions of the exposure time at a known temperature. Using a time-temperature superposition function based on first-order kinetics to rationalize the widely different temperatures and exposure times examined in our samples, we found that the correlation of this combined function with the reflectance spectra was much better than with temperature or exposure time on their own. The calibrations are constructed using multivariate statistical methods, and one of the outputs is a spectral regression coefficient that shows which chemical components are correlated with the cumulative ageing metrics, and so give insight into the strengths and weaknesses of the resin composition under conditions of use. As the analysis methods can be in-situ and non-destructive, this provides a means to determine the degree of thermally accumulated damage in the insulator.