In order to facilitate online monitoring of the aging state of high-voltage (HV) cables, a novel method based on impedance spectroscopy has been proposed. Firstly, the method uses the Debye model to restore the dielectric response of the insulating material of the mechanism. Secondly, based on the transmission line equation, a calculation model for the impedance spectroscopy of HV cables was established. Then, the characteristics of impedance spectroscopy under different overall aging and local deterioration states were analyzed. Further, simulation analysis of the influence of factors such as load factor, length, metal resistivity, and geometric parameters on impedance spectroscopy was carried out. Finally, diagnostic criteria were proposed, which are divided into two major categories: known cable parameters and unknown cable parameters. If the geometric parameters are known, the aging state can be determined using the resonance peak values and the resonance peak frequencies. If the geometric parameters are unknown, it is necessary to test impedance spectroscopy using the healthy condition of the cable as the baseline curve. By comparing the impedance spectroscopy of the cable during tests with the baseline curve, the aging development trend of the cable can also be determined.

The interfacial pressure between cable accessories and cable insulation plays a decisive role in the insulation characteristics of the accessories and the long-term reliable operation of the cable. Considering the actual rough contact interface between the cable body and the intermediate joint, a finite element simulation model of stress field-sound field coupling was constructed using points with Gaussian probability distribution. Based on this model, the differences in echo signals in the time domain and frequency domain at different positions of the cable accessories, as well as the differences in echo signals under different interfacial pressures at the same position, were analyzed. Subsequently, the functional relationship between interfacial pressure and nonlinear parameters was fitted. Finally, a nonlinear ultrasonic testing platform was established to measure the interfacial pressure at different positions of the cable intermediate joint, verifying the accuracy of the relationship between interfacial pressure and nonlinear parameters. The experimental and simulation results showed consistent patterns in the time domain and frequency domain of the echo signals, with the nonlinear parameters increasing as the interfacial pressure increased. The experimental results indicated that the detected nonlinear parameter value was around 7.46 × 10⁻³ (with an error range within 5.80%), corresponding to an interfacial pressure of 0.21 MPa; and the detected nonlinear parameter value was around 6.38 × 10⁻³ (with an error range within 7.60%), corresponding to an interfacial pressure of 0.16 MPa. The nonlinear parameters can characterize the interfacial pressure at different locations, verifying the validity of the simulation fitting function and providing a new approach for measuring the interfacial pressure of cable accessories.