Detection of Double-layer Air Gap Defects Based on Terahertz Imaging Method

Composite insulators are prone to internal defects and the detection can be more difficult when the defects are overlap in the radial direction. A THz imaging system including THz spectrometer and sample scanning system was designed and set up to detect the defects. The flat sample with double-layer air gap defects of different shapes inside were made to simulate the composite post insulator. We collected reflection waveforms at different locations in the scanning step of 0.5mm based on the THz imaging system. Deconvolution method was used to reduce the pulse width and increase signal-to-noise ratio of the waveform. The method was proven to be effective in making the pulses more distinguishable. Three characteristic parameters, including maximum absolute value, signal power and envelop area were used to imaging the defects. All three parameters were proven to be able to identify double-layer defects which have the depth of 0.40mm and 0.83mm, respectively. Signal power imaging and envelop area imaging have better performances and have different advantages in defect morphology identification and resolution capability.