Study on the aging characteristics of PET for SF6 gas-insulated transformer

SF6 gas insulated transformers (GITs) are extensively utilized in underground substations within densely populated areas of large- and medium-sized cities, praised for their excellent non-flammability and operational reliability. However, the aging characteristics of gas-solid composite insulation (SF6-PET) within GITs remain unclear, complicating the effective evaluation of their aging condition. In this study, we performed accelerated thermal aging experiments on type 6020 polyethylene terephthalate (PET) films under an SF6 gas environment at 150 degrees C. We investigated the aging characteristics of PET materials at various stages using differential scanning calorimetry, tensile tests, near-infrared spectroscopy (NIRS), and frequency domain dielectric spectroscopy. Our findings indicate that the crystallinity and fracture elongation of PET materials initially increased before decreasing as aging progressed, while tensile strength steadily declined. In addition, the absorption rate observed in NIRS diminished with aging. Utilizing linear discriminant analysis, we achieved dimension reduction of characteristic parameters across different wavelengths, facilitating the classification of PET materials at diverse aging times. The tan delta-f curves of PET films initially decreased and subsequently rose with aging. By applying the Havriliak-Negami (H-N) dielectric relaxation model across three distinct frequency bands of the dielectric spectrum, a significant correlation was discerned between the aging state of PET materials and the characteristic relaxation peaks, tau(1) and tau(2). Fitting the H-N relaxation model and Arrhenius equation allowed us to negate the impact of "temperature drift," yielding an interface polarization activation energy of E-a = 0.69 eV. This research on the aging performance of PET materials in an SF6 gas atmosphere provides a vital foundation for assessing the aging state of SF6-insulated GITs.