Effect of AC-voltage harmonics on oil impregnated paper in transformer bushings

With the global trend to establish smart distributed-generation grids, the level of AC-voltage harmonics is significantly increasing due to power electronic converters interfacing renewable energy sources with the grid. Such harmonics will not only degrade the power quality and increase the losses in the electricity grids but will also have a severe impact on oil impregnated paper (OIP) in power transformer bushings. While much attention has been given to investigate the deterioration of OIP under thermal and electrical stresses, the impact of voltage harmonics on the OIP structure is still uncertain and calls for more investigations. Therefore, this paper is aimed at investigating the variations of the multilevel structure of an OIP aged under high levels of AC-voltage harmonics. In this context, degree of polymerization of the aged OIP is measured to reveal the changes of OIP secondary structure. Also, X-ray diffraction, deuteration treatment and Fourier transform infra-red spectroscopy are conducted to analyze the changes of the OIP aggregate structure during the aging process. Further experiments are performed to investigate the variation of the microscopic appearance, thermal behavior and dielectric properties, including scanning electronic microscopy, thermal gravity analysis, differential scanning calorimetry and frequency domain spectroscopy. This investigation on the variation of the multilevel structure of aged OIP not only promotes further understanding to the aging behavior and mechanism of OIP under AC-voltage harmonics, but also proposes crystallinity and accessibility as two new effective aging indicators for the OIP structure.