Study on ultrasonic detection system for defects inside silicone rubber insulation material

摘要

During the manufacture and the installation process, defects are easy to introduce in prefabricated cable accessories. The existing standards of cable accessories product require that there should be no defects such as bubbles and metal impurities in insulation, but no corresponding detection means are put forward. At present, the main defect detection methods are high voltage test and partial discharge test after installation. It is easy to misjudge because of the installation process, environmental noise and other reasons, and cannot locate the specific location of the defect. It is useful to find these defects through ultrasonic detection and ensure the safety of the cable accessories. In this paper, a set of ultrasonic automatic defect detection system for SR (silicone rubber) was developed. The relationships between the detection ability and the system parameters were investigated. The axial resolution and sensitivity of the ultrasonic wave in the SR Cable joint sample were measured using excitation signal with different pulse width. The relationships between the resolution (or the sensitivity) and pulse width were investigated. The attenuation coefficient and axial resolution were measured by ultrasonic probes with different central frequencies. The relationships between the attenuation coefficient (or the axial resolution) and the central frequency were also investigated. Finally, the negative pulse with pulse width of 200ns was selected as the excitation signal and 2.5MHz was selected as the central frequency of the probe. With the proper system parameters, the attenuation coefficient was 0.059dB/mm and the axial resolution was about 3mm in SR. Two different defect imaging methods were developed by analyzing the amplitude, phase and frequency spectrum of the ultrasonic echoes. The first imaging method, which applied to defects such as air gaps and impurities, constructed images using the amplitude of the flaw echo. The second imaging method, which applied to defects such as uneven distribution of material, constructed images using the average sound velocity. SR samples with different thicknesses and types of defect (air gaps, and metal impurities) were scanned by the system. The results showed that the detection system could accurately identify defects larger than 1 mm. The depth of defects could reach 50mm. The ultrasonic detection method could be applied to instruct the insulation diagnosis for SR Cable accessories.