High-voltage (HV) crosslinked polyethylene (XLPE) cables are subjected to electrothermal stress gradients during service, which often leads to nonuniform degradation within insulation systems, significantly affecting their endurance, resilience, and overall lifetime. This article presents a comprehensive study of dielectric mechanisms and health state estimation of XLPE insulation under nonuniform thermal aging, with a particular emphasis on 500 kV XLPE cables. First, the distinct effects of uniform versus nonuniform thermal aging mechanisms on XLPE systems are compared through selected five combinations in two groups. Furthermore, the mechanisms by which nonuniform thermal aging influences XLPE insulation systems are revealed using three dielectric analysis techniques. Additionally, this work innovatively introduces a quantitative framework for the health state estimation of XLPE cable insulation under nonuniform thermal aging, utilizing the previously developed equivalent circuit model that incorporates a fractional-order circuit module (FOCM). The aging features extracted from the FOCM are utilized to develop health state estimation models for XLPE insulation under different nonuniform thermal aging conditions. The performance and limitations of health estimation models are discussed using a stacked XLPE system. This study deepens the understanding of nonuniform thermal aging mechanisms and provides insights to support condition-based maintenance of HV cable insulation under complex aging conditions.

Epoxy (EP) resins find widespread application in power electronics (PE) applications characterized by multi-frequency electrical stresses. Transient thermal overloads are also not uncommon due to, for example, short circuits in the external circuit. This article investigates the influence of various temperatures on broadband dielectric properties of EP insulation, above and below the glass transition temperature ( T _g ). The underlying multi-timescale dynamic processes, conductivity and relaxation mechanisms are revealed based on multiple spectroscopy techniques. In particular, the dependence on the frequency and amplitude of loss peaks on temperatures is modeled considering the potential use of this model in the multi-physics design of PE applications. Results show that the high temperature above T _g substantially triggers noticeable low-frequency quasi-DC conductance behavior and multiple non-Debye relaxation processes in higher-frequency regions. Once the operating temperature exceeds the T _g , the low-frequency (0.1 to 100 Hz) real permittivity and loss factor will increase by more than dozens of times. The low-frequency quasi-DC conductivity will increase by about 6 magnitudes from 25 °C to 200 °C. As a result of the findings in this paper, the future insulation circuit modeling and reliable insulation design will consider the underlying multi-timescale physical mechanisms to support the multi-frequency applications.

With the rapid rise of new energy industry and its rapid development all over the world, there is a great market for submarine cable products. The world's first class AC 500kV cross-linked polyethylene (XLPE) insulated submarine cable will soon be put into operation in the “Zhoushan-Ningbo” interconnection project. Aging of insulation materials after operation will be an important factor restricting the safe operation of the 500kV XLPE cable. Taking into consideration that the frequency domain spectroscopy (FDS) is a nondestructive testing (NDT) method, in this paper, the changes of dielectric spectroscopy of the AC 500kV XLPE submarine cable material during 130°C thermal aging process were studied. The aged samples show increasing dielectric constant and dielectric loss. Besides, Iε‘ is defined to estimate the aging condition of XLPE samples. It can be clearly seen that the samples severely aged when the Iε’ exceeds 1. The Iε’ can be used to diagnostic the aging condition of the AC 500kV submarine cable insulation material. Besides, the ε‘rand tanδ in the lower frequency of the seriously aged sample increase significantly, and the inflection points of tanδ curves for the seriously aged sample move to higher frequency significantly.

Bio-based hydrocarbon liquids have excellent biodegradability, physicochemical properties and impulse insulating properties, and have become the focus of research in recent years. In this paper, using density functional theory (DFT) and first-principles calculations, the variations and differences in ionization energy, electron affinity, excitation processes, and molecular orbital characteristics of isoalkanes (the main component of bio-based insulating oil) and mineral oil molecules under varying electric field intensities were simulated and analyzed. Based on the key microscopic parameters of isoalkanes and mineral oil systems, their impulse insulation properties were comprehensively evaluated. The test results reveal that the ionization potential (IP), excitation energy, and band gap of C22-isoalkanes are the largest, followed by C18-isoalkanes, while those of mineral oil are the smallest. Hydrocarbon insulating liquids exhibit a clear “short board” effect in their ionization energy and excitation energy. Aromatic hydrocarbons with lower ionization energy, excitation energy, and energy gaps in mineral oil significantly influence its overall performance. Consequently, the pulse insulation performance of isoalkanes is similar to or even better than that of mineral oil in theory, which is basically consistent with the experimental results. The isoalkanes with shorter molecular chains are expected to have better pulse insulation performance. This work provides theoretical guidance for understanding the microscopic mechanism of the difference in impulse insulation properties between bio-based hydrocarbon liquids and mineral oils.