Testing Method for Composite Insulators Interface Based on Nonlinear Ultrasonic

The interface is the weakness of composite insulators. As the interface bonding fails, the uniform distribution of electric field will be affected, which accelerates the aging of composite insulators and affects the safe operation of transmission lines. Because of the stress between the sheath and the rod, the air gap at the interface is generally less than 10 ţm and there is extrusion contact in some areas. Therefore, conventional nondestructive testing technology based on refraction and reflection principle cannot effectively identify the faulty insulators in the initial stage of bonding failure. Based on the constitutive model of the interface of composite insulators, the friction force of the interface after bonding failure introduces more nonlinearity in mechanical properties than hydrogen bond under well bonding condition. The numerical results show that under the influence of the nonlinear constitutive relation, the higher harmonic components in the spectrum will increase significantly. The experimental results of plate and insulator samples confirm the validity of numerical calculation. Based on the normalized nonlinearity coefficient, the interface state of composite insulators at the initial stage of bonding failure can be effectively identified.