Aging characterization of 500-kV field-serviced silicone rubber composite insulators with self-normalized photothermal radiometry

•Self-normalized photothermal radiometry (PTR) is used to quantitatively characterize the aging degree of field-serviced composite insulators to eliminate the influence of the wide variation of the initial thermal diffusivities.•A thermal diffusivity ratio is proposed to characterize the aging degree of silicone rubber composite insulators with improved accuracy.•The thermal diffusivity ratio decreases and the aged layer thickness increases monotonically with the increasing field-service years, in consistent with hydrophobicity classification results. Aging of silicone rubber composite insulators widely used in high-voltage transmission lines is a dynamic process starting from surface and gradually forming an aged surface layer with reduced thermal diffusivity. Based on the demodulation of the modulated infrared radiation signal and the subsequent data analysis, self-normalized photothermal radiometry (PTR) is used to quantitatively characterize the aging degree of field-serviced composite insulators. The ratio of the thermal diffusivity of the aged layer normalized by the substrate thermal diffusivity is proposed to characterize the aging degree to eliminate the influence of the wide variation of the initial thermal diffusivities of fresh silicone rubber composite insulators which are normally not known. Experimental results show that the thermal diffusivity ratio decreases and the aged layer thickness increases monotonically with the increasing field-service years, in consistent with the measurements of hydrophobicity class. The thermal diffusivity ratio and thickness of the aged layer are expected to be used as numerical indices for understanding the aging mechanism and estimating the remaining lifetime of silicone rubber composite insulators in field service.