Quantitative Detection of Interfacial Air Gap in Insulation Equipment Based on Terahertz Wave Contrast Method

The nondestructive testing of internal defects in power grid equipment has always been a spotlight of research. Since terahertz wave has good application in the nondestructive detection fields, we apply terahertz wave to detect the interfacial air gap of insulation equipment. At present, terahertz wave is difficult to measure the thickness of the internal air gap. Hence, this paper presents a method for quantitative measurement of internal air gap based on the waveform database. In addition, the propagation characteristics of the terahertz wave in medium are analyzed. Furthermore, based on the value of $n(\omega)$ and $\kappa (\omega)$ , the calculation formula of the transmission and reflection coefficients is simplified. After obtaining the reflected wave of the single outer layer material, we quantitatively calculate the time-domain reflection wave of interface. According to the principle of terahertz detection based on photoconductive antenna, the detection of multiple reflection waves is analyzed. To verify the superposition characteristics of multiple waves, the terahertz reflection waves of samples with interfacial air gap are tested. By comparing the measured waves and the artificially calculated waves, we prove that the superposition of the waveform is approximately linear. Based on the good coincidence between artificially generated waveform and measured waveform, we propose a method of quantitatively diagnosing the interface air-gap defect. Indeed, the deviation between the test results and the actual air-gap size is small.