Study on diagnosis of HV rotating electrical machine insulation based on structural changes due to ageing

The model bars of generator stator winding manufactured under resin poor process and laminates made using both resin rich and resin poor systems were subjected to accelerated electrical, thermal and combined stress ageing. The insulation curing status and behaviour on ageing was studied by thermo analytical techniques like Differential Scanning Calorimeter (DSC), Thermo gravimetric analysis (TGA) and Thermo Mechanical Analysis (TMA). Various dielectric properties like Dissipation factor, Capacitance, Partial Discharge, Polarization currents etc. were measured at regular intervals during ageing. The structural and chemical changes that could have occured during ageing were examined using techniques like Fourier Transform Infrared Spectrometer (FTIR), Scanning Electron Microscope (SEM), and X-ray energy spectral analysis. The glass transition temperatures of fresh (sound) and thermally aged samples determined by DSC and TMA indicated post-curing effect to some extent. From the TGA study it was found that there was no change in total weight loss of sound and aged specimen. The decomposition temperatures of the specimens though are not that distinct from the normal curves, the derivatives precisely show the peaks at which decomposition is occurring. FTIR spectra of aged specimen showed a new peak at 1720 cm
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resulting from carbonyl (C=O) group in resin rich laminate. SEM Pictures showed a significant difference in surface morphology on account of heterogeneity.