Effect of Thermal Aging on Medium Frequency Breakdown of Polyester Films

This article discusses the effects of thermal stress on polyethylene terephthalate (PET) films focusing on its impact on the capability of the insulation to withstand voltage waveforms with an elevated frequency content. To properly mimic the degradation the material may face during its service life, thin (∼50 μm) PET films were subjected to accelerated thermal aging at 150, 165 and 180 °C. Aging effects (including oxidation) on the chemical structure of PET were investigated by means of Fourier Transform InfraRed spectroscopy. Dielectric spectroscopy was performed on samples to evaluate the dielectric losses at the different aging stages. To simulate the worst-case scenario during operation, high frequencies (up to 30 kHz) were selected and used for the AC breakdown tests, thus assessing the evolution of the PET dielectric strength with aging. Results showed that no direct correspondence between dielectric strength and dissipation factor (tanδ) values is present, suggesting that thermal runaway is not the only factor causing the dielectric breakdown.