Terahertz Imaging Method for Composite Insulator Defects Based on Edge Detection Algorithm

Composite insulator is prone to internal defects such as air gaps which seriously endanger the safety of power systems. To detect the internal defects in composite insulators, a terahertz (THz) imaging method based on edge detection algorithm was proposed in this article. First, air gap defect sample consisting of planar multilayers was imaged based on the envelope area of the THz reflection waveform. Then, Canny operator was applied to extract the edges of the defects. After that, the time interval of defect characteristic pulses was used to calculate the defect depth and then 3-D imaging of defects was, therefore, obtained. The experimental results showed that the imaging method based on envelope area was very effective in determining the edges of air gap defect and it had better performance compared with those using pulse amplitude or time interval as parameter for defect imaging. According to the results, the accuracy was quite satisfactory when the defect depth was above 0.339 mm. To verify the effectiveness of the proposed method, the developed THz imaging system was also applied to the rod of composite insulator. It was found that the relative error was smaller along the axial direction of the rod than the lateral direction.