Effects of Electrothermal Aging on Surface Morphology and Dielectric Properties of Poly(Ethylene Terephthalate) in Laminated Busbars

Poly(ethylene terephthalate) (PET) used as an insulating medium of laminated busbars is subjected to combined stresses of the dc electric field and elevated temperature. The coupling effect induces the electrothermal aging of PET, which weakens the busbar performance and thus threatens the long-term reliability of power electronics equipment. In this article, the dc electrothermal aging tests of a 50- $\mu \text{m}$ PET film are carried out at varied aging temperatures of 60 °C, $80~^{\circ }\text{C}$ , and $100~^{\circ }\text{C}$ and a fixed electric field of 120 kV/mm. The surface becomes rougher with ongoing electrothermal aging. It is found that the surface roughness Sa of samples increases from 95 to 2100 nm and the PET surface energy drops from 46.7 to 43.9 mN/m. Moreover, dc breakdown strength and volume resistivity decrease from 807 kV/mm and $6.75\times 10^{{16}}\,\, \Omega \cdot \text{m}$ to 556 kV/mm and $3.97\times 10^{{16}}\,\,\Omega \cdot \text{m}$ , respectively. The aging performance of dc breakdown strength and volume resistivity is discussed based on trap characteristics, which are involved in the evolution of aggregation structure and molecular chain during electrothermal aging. The results of differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) reveal that in the initial aging stage, recrystallization leads to the increased deep trap depth and the crystalline region destruction, and the molecular chain scission results in the decreased deep trap depth in the late aging stage.