Photothermal Radiometric Evaluation Of Aging Status Of Silicone Rubber Composite Insulators

Silicone rubber composite insulators are widely used in high-voltage transmission lines due to their excellent hydrophobicity,light weight,robust structure,and pollution resistivity.However,long-term exposure to adverse environments inevitably causes the aging problem and leads to performance degradation,which may seriously threaten the safety of transmission power systems.Hence,it is of great importance to evaluate the aging status of the composite insulators and monitor the remaining lifetime.This dissertation focuses on the influence of aging on the thermophysical properties based on Photothermal Radiometry(PTR),and has developed a novel evaluation method for the degree of aging.The main aspects of this dissertation are as follows:Correlations between thermophysical properties and the aging status of silicone rubber composite insulators with surface contaminations have been investigated.By introducing a surface-absorption parameter,an effective PTR theoretical model was developed,and dependences of PTR amplitude and phase to the thermal diffusivity and the absorption coefficients were analyzed.After measuring the PTR signals of samples using an experimental system,the thermal diffusivity and the absorption coefficients were simultaneously extracted through multi-parameter fitting.Investigation on composite insulators with different aging degrees showed that the severer the aging is,the lower the thermal diffusivity will be,indicating that aging causes changes of thermophysical properties of composite insulators.Considering that the change of the thermal diffusivity is the key point in the evaluation process,the initial thermal diffusivities of different types of composite insulators were also determined and compared.By introducing the infrared absorption coefficient,the theoretical model was optimized.Initial thermal diffusivities of composite insulators with different voltage grades,different mechanical load grades,and from different manufacturers were determined.Only slight differences were found in the thermal diffusivities of composite insulators with different voltage grades,different mechanical load grades,while those from different manufacturers showed significant differences due to discrepant compositions.Therefore,the difference of the initial thermal diffusivities among composite insulators should also be considered so as to guarantee the rationality of the results when assessing the aging degree using PTR.To gain a deeper insight into the effect of structural changes on thermophysical properties,depth profiles of the thermal diffusivity were obtained and the change mechanism of the thermophysical properties of composite insulators after aging was analyzed.A two-layer PTR theoretical model which divides the composite insulator into a surface aged layer and a substrate layer was developed to study the influence of aging on the thermophysical properties.After measuring the parameters of a 500 k V aged composite insulator,it was found that the thermal diffusivity of the surface layer is significantly lower than that in the bulk,and the effective depth profile of thermal diffusivity approximates to a hyperbolic tangential profile.The results indicate that the aging process of the composite insulator starts from surface and gradually spreads into the bulk along the depth direction.It was inferred that the destruction of compact packing structures and bond breaking of polymer matrix are two main factors that cause the decrease of the thermal diffusivity.Based on the change mechanism of thermophysical properties,aging degree of five silicone rubber composite insulators with different service years were investigated.The thermal diffusivity and the thicknesses of the aged layer show high relevance to the aging degree.The composite insulators with lower thermal diffusivities and the thicker aged layers were aged more seriously.To cancel out the influence of the wide variation of the initial thermal diffusivities of composite insulators,a self-normalized method was proposed to obtain more accurate quantitative characterization results.In this method,the thermal diffusivity of the aged layer was normalized by the substrate thermal diffusivity,defined as the thermal diffusivity ratio.The value of the ratio and the aged layer thickness can be used as the two numerical indices to characterize the aging status of a composite insulator,which features a higher resolution and is more reliable than the hydrophobicity class criterion.As the abovementioned method for determining thermophysical parameters has physical damages to tested samples(polishing the surface aged material),a non-destructive assessment solution was explored.Based on the results of sensitivity analysis,the optical absorption coefficient,the infrared absorption coefficient of the substrate layer and the optical absorption coefficient of the aged layer can be fixed in advance,and subsequently the remaining parameters can be simply fitted.Results showed that the thermal diffusivities of both the aged and the substrate layer and the aged layer thickness can be obtained effectively,further verifying the feasiblility of developing a non-destructive aging characterization method for composite insulators.In conclusion,the effect of aging of silicone rubber composite insulators on the thermophysical properties was studied based on PTR,and a preliminary evaluation method has been developed.As a sensitive,simple,low-cost,noncontact,and non-destructive characterization method,PTR evaluation of the aging degree of composite insulators based on thermophysical property measurements is very promising for industrial application.