Non-destructive aging evaluation of 500-kV field-serviced silicone rubber composite insulators with photothermal radiometry

In this article, a non-destructive photothermal radiometry (PTR) method is proposed to quantitatively evaluate the aging degrees of field-servicing silicone rubber composite insulators via a thermal diffusivity ratio and degradation layer thickness. A sensitivity analysis is performed to investigate the sensitivity of the PTR amplitude and phase to the thermo-physical, optical, and structural parameters of the aged silicone rubber composite insulators consisting of a degradation layer and a substrate layer. It is found that the PTR amplitude and phase are highly sensitive to the thermal diffusivity, effective infrared absorption coefficient, and thickness of the degradation layer as well as the thermal diffusivity of the substrate layer but are much less sensitive to optical absorption coefficients of the degradation and substrate layers and the effective infrared absorption coefficient of the substrate layer. Thus, parameters with high sensitivity coefficients are set as free parameters and determined by multi-parameter fitting the frequency dependences of the PTR amplitude and phase measured directly from the surfaces of the field-serviced silicone rubber composite insulators to a two-layer PTR theoretical model, while in the multi-parameter fitting, the parameters with low sensitivity coefficients are set as “known” parameters whose values are either measured separately or taken from literature. By doing so, the thermal diffusivity ratio and degradation layer thickness are accurately determined in a non-destructive way. Experimental results demonstrate that the thermal diffusivity ratio and degradation layer thickness determined by the proposed non-destructive PTR method are in good agreement with that determined by a conventional destructive PTR method, thus, proving the feasibility of the non-destructive PTR method for quantitative evaluation of aging degrees of the silicone rubber composite insulators being serviced in the high-voltage power transmission systems.