Evaluation of insulation layer of silicon rubber cable joint with thermal ageing by ultrasonic detection

Cable joint insulation withstanding high temperature for a long period may induce unrecoverable damage, which may result in reduced insulation performance or even failure. Therefore, to assess and monitor the situation of insulation layer accurately is very important to the fabrication and operating reliability of cable joints. To simulate the thermal ageing of 220kV integrally prefabricated cable joint silicone rubber (SR) insulation layer, the accelerated thermal ageing tests of the cable joint SR insulation layer at 225° were carried out in this paper. An ultrasonic detection system for evaluation of insulation layer of aged cable joint was established. The dielectric characteristics, such as relative permittivity and breakdown strength, were measured by dielectric tests. Ultrasonic velocity characteristics and frequency domain characteristics were also investigated by ultrasonic detection. The results showed that, with increasing of ageing time, the dielectric performance (as destructive parameters) degraded gradually, and the ultrasonic velocity (as a non-destructive parameter of ultrasonic detection) decreased gradually. In addition, the DC component of ultrasonic spectrum of cable joint insulation layer decreased, but the other spectral peaks increased simultaneously. The correlation between ultrasonic velocity and dielectric performance can be quantified by fractional expression. Based on the amplitudes of the DC component and the main peak of ultrasonic spectrum, the of thermal ageing damage factor (TADF) was proposed and the correlation between TADF and dielectric performance was found to satisfy the Boltzmann distribution. The results confirmed that the ultrasonic velocity and the TADF were suitable to evaluate the thermal ageing state of SR insulation layer of cable joint. The ultrasonic detection method may offer a broad prospect for nondestructive evaluation and diagnosis of the ageing state of cable joint insulation layer.