Study On The Propagation Law Of Thermal Wave In Composite Insulator With Defects And The Method Of Nondestructive Testing

The interface defect of composite insulators is the main reason affecting the safe operation of composite insulators.The thermal wave nondestructive testing method has the advantages of fast detection speed and far-field measurement,which provides a new idea for the detection and identification of micro defects in composite insulators.Composite insulator interface defects are the main reason for the safe operation of composite insulators.Thermal wave nondestructive testing has the advantages of fast detection speed and far field measurement,which provides a new idea for the detection and identification of micro defects in composite insulators.Firstly,based on the related theory of heat transfer,the one-dimensional heat conduction model under the pulse excitation source and constant current excitation source is obtained.The advantages and disadvantages of the two excitation sources for the internal defect detection of composite insulator are compared and analyzed.High energy excitation can be achieved by the pulse excitation energy in a short time,the detection efficiency is high,and the lateral diffusion of heat flow is small,but the thickness of the sheath has a great influence on the detection result;the constant current excitation energy continuously increases the incident energy to the sample,and has the ability to identify deeper defects,but the energy incident time is longer and be more affected by the lateral diffusion of heat flow.The three-dimensional slab model,cylindrical model and insulator model were constructed by finite element simulation software.The effects of the geometric characteristics of the defect and the thickness of the sheath surface,the applied heat flux density,the excitation steepness and the surface contamination on the surface temperature of the defect were obtained.Compared with the pulsed heat source,the constant current heat source can significantly improve the temperature difference of the defect surface.Therefore,the constant current heat source has better effect on identifying the deeper defects,but the defect contour is more blurred.Under the constant current excitation,the semicircle of the insulator surface is obtained at a certain time during the cooling process.The temperature distribution on the circumference can realize the quantitative identification of the internal defects of the insulator.For samples with a sheath thickness of 4 mm or less,the pulse thermal wave method is preferred for measurement.For samples with a sheath thickness of 5 mm or more,the flow wave method is better used for measurement.A pulsed thermal wave non-destructive testing experimental platform was built to test the samples containing the artificial defects and the insulator samples.The experimental results and simulation results were compared.The pulsed thermal nondestructive testing method was not applicable to samples with a sheath thickness exceeding 4 mm.For solving the problem of the deviation which is between the amount of radiation on the surface of the sample defect and the amount of radiation on the reference surface is extremely small,a method for detecting the internal defect of the insulator according to the relative deviation of the rate of change of the radiation amount is proposed in the double logarithmic coordinate,which can identify the 4 mm sheath 1 mm interface air defect at thickness.A constant-current thermal wave nondestructive testing experimental platform was built to test the plate samples and insulator samples containing artificial defects.Firstly,the distribution of the surface radiation amount of the sample was obtained by software,and the surface was observed for abnormal temperature rise.If there is any suspected defect,the surface of the internal defect is analyzed,and then processed by double logarithmic coordinates to identify the possible minor defects under the surface.The method can identify the air interface defects at the interface of the composite insulator with the thickness of the sheath of 5 mm or more,and the detection accuracy can reach 1mm.