Dielectric changes of PET and Glass-fiber based epoxy composites during thermal aging

The paper deals with the problem of thermal aging of PET and Glass-fibre based composites. These electro-insulating materials are used in high voltage engineering as parts of insulation systems of large rotating machines like turbo or hydro generators. These materials were subjected to accelerated thermal aging to provide more information about the aging effects and allow us to estimate the lifetime of the materials at operating temperatures. The laboratory method used to compare the dielectric properties (relative permittivity, loss factor, parallel capacitance, parallel resistance and impedance) during the accelerated thermal aging was the dielectric spectroscopy. Thermal aging of the samples was performed in the laboratory thermal oven and the changes of their physical and chemical properties were measured and evaluated at given exposure times. Different temperatures (170-194 °C) were chosen for the accelerating of the aging process. The aging time was determined for each temperature value. Specimens of tested material were flat plates 100×100 mm. The measurement of these specimens was carried out by means of a special electrode test setup at frequencies from 200 Hz to 2 MHz. For comparison of the aging process of the investigated materials, the trends of measured parameters were studied and described in dependence on exposure times, temperatures and frequencies applied during the measurement. The measurement results show that the properties of the measured insulation materials change depending on the internal structure, temperature and the time of exposition.