Analysis of Sheath Breakdown Induced by Air Gap at the Interface Between Hardware and Sheath of Silicone Rubber Composite Insulator on the Roof of High-Speed Trains

To cope with the environmental adaptability of roof insulators amid the expanding coverage of high-speed railways, it is crucial to assess the insulation performance of novel roof insulators before they are put into service. During the flashover test of a batch of novel roof insulators, it was observed that the outer sheath of the high-voltage (HV) hardware had experienced an abnormal breakdown. Based on this case, first, based on the results of the sheath incision inspection, it was inferred that the breakdown may have been caused by bonding issues between the sheath and the HV hardware. Due to its advantage in detecting thin defects, ultrasonic testing was employed to identify that the breakdown was caused by an air gap at the interface between the HV hardware and the sheath. Additionally, the influence of the air gap on the electric field distribution was analyzed through electrical field simulation. Second, various gap sizes were constructed to investigate the impact of gap size variations on the electric field parameters. To ascertain the functional relationship between gap size and electric field parameters, a nonlinear regression equation with variable coupling was then established to obtain the functional relationship between gap size and electric field parameters using regression analysis. Subsequently, in order to reduce the probability of breakdown, a grading ring can be installed on the HV side. The influence of the installation of the grading ring on the electric field and flow field was analyzed using electrostatic and fluid-structure coupling simulations. The evaluation criteria for optimizing the grading ring were selected, including $\overline {E^{2}}$ within the air gap, the maximum electric field strength ( $E_{\text {rmax}}$ ) on the surface of the grading ring, and the maximum stress value on the surface of the 1# shed. By comparing and analyzing the differences in these criteria using orthogonal experimental design, the dimensions of the grading ring were optimized. Finally, the feasibility and effectiveness of the grading ring were verified through power frequency flashover tests and lightning impulse tests.