High-temperature vulcanized silicone rubber (HTV-SR) employed for composite insulators is continuously subjected to a complex environment of alternating heat, corona discharge, humidity, etc. These stresses (especially alternating heat) complicate the aging mechanism of HTV-SR, which lacks systematic investigation. In this paper, a multi-factor aging platform considering temperature cycling, moisture, and corona discharge is established. Specifically, four temperature-cycling settings are employed, each of which lasts for 15 cycles. The surface morphology, hydrophobicity, and chemical, mechanical, and electrical properties of aged samples are methodically characterized. Experimental results show that the aging degree is correlated to the range of temperature cycling, which is attributed to diverse crosslink-degradation degrees with different temperature differences. Under a large temperature difference (70 °C), HTV-SR possesses a high crosslinking degree and a low degradation degree, making the material hard but easy to crack with alternating thermal stress. Then, severe defects and water condensation emerge on the HTV-SR surface, which promote the diffusion of corona products and water molecules into the material. The subsequent rise in crosslinking density caused by in-depth oxidation further exacerbates the aging of the material. Consequently, it brings about poor hydrophobicity, high interfacial polarization, and shallow trap energy levels in HTV-SR. This work provides a detailed analysis of the aging mechanism of HTV-SR in a simulated on-site environment. 

Mechanism of Accelerated Deterioration of High-Temperature Vulcanized Silicone Rubber under Multi-Factor Aging Tests Considering Temperature Cycling

On the Thermo-Oxidative Aging of Elastomers: A Comprehensive Review

Abstract Silicone rubber (SR) is widely used for cable insulation and operates in multi‐stress coupling conditions. Its dielectric properties and ageing performance are the essential factors affecting cable operation. In this study, the frequency domain spectroscopy of unaged SR and SR aged under various conditions is measured. The dielectric responses before and after ageing are analysed in depth by the conjoint analysis via multiple complex planes (permittivity, conductivity, and modulus). Typical conductivity and interfacial polarisation properties, as well as property changes resulting from thermal and tensile ageing, are examined. Experimental results show that tensile ageing promotes conductance and interfacial polarisation, while thermal ageing shows the opposite effects. Based on the multi‐core model and charge carrier hopping transport model, thermal and tensile ageing mechanisms on the conductance and interfacial polarisation behaviours of SR are demonstrated. Both behaviours exhibit positively correlated changing characteristics, suggesting that the two behaviours may originate from a common process. The carrier transport process, promoted by the tensile effect and slightly suppressed by the thermal effect, is the primary factor and considered an important indicator to evaluate the dielectric properties of SR insulation. 

Analysis of conductance and interfacial polarisation behaviours of silicone rubber based on frequency domain spectroscopy

Understanding the degradation characteristics and assessing the operating status are crucial for improving the service quality of distribution cables. Moisture ingress in cables is a common defect leading to cable failures and associated economic losses. However, conducting targeted detection and prevention is challenging due to the lack of knowledge on the development process and assessment methods. In this study, cables with various common defects were operated in a water immersion environment for a 90-cycle accelerated moisture degradation test. The influence of moisture was highlighted in addition to other setting factors, including electric fields, high temperatures, and typical defects. Additionally, the electrical performances of the test cables were periodically tested, and a combined weighting approach based on the subject-oriented analytic hierarchy process (AHP) and object-oriented entropy weight method (EWM) was proposed. Furthermore, a comprehensive depiction of degradation status was provided through an assessment method utilizing the improved radar chart and two parameters, degradation index DI and degradation rate DR, derived from curve area calculation. Consequently, the findings revealed varying degradation characteristics in cables subjected to moisture ingress via different pathways. The moisture ingress through the cable joint and damaged body had the most significant impact on insulation, exceeding that of other defects. The proposed methods and parameters could effectively assess the moisture ingress and degradation status of distribution cables, which can offer critical guidance for the detection and prevention of cable dampness issues.

Moisture Degradation Characteristics and Multi-Performance-Based Status Assessment Method of Distribution Cables

High magnetic field causes premature failure of insulation. In order to improve the breakdown strength of silicone rubber (SiR), different filler contents of ferroferric oxide (Fe 3 O 4 ) nanoparticles were added. Experimental results show that 1 wt% addition amount of ferromagnetic nanoparticles is beneficial to improve the AC breakdown strength of SiR by 5.40% in 12 T magnetic field due to their ability to regulate dielectric polarization and partial discharge (PD) behavior, resulting from the relative higher permittivity and permeability of Fe 3 O 4 . Magnetization of Fe 3 O 4 nanoparticles results in 3.33% decrease of DC breakdown strength with 2 wt% filler content in 12 T magnetic field. Fluorination treatment was carried out and proved to be able to improve the DC breakdown strength of SiR/Fe 3 O 4 nanocomposites by 8.84% in 12 T magnetic field with the treatment time being 5 minutes, resulting from the regulation of surface traps. Fluorination treatment weakened the magnetism of nanoparticles, weakening their regulation on insulation polarization and PD. Adding an appropriate amount of ferromagnetic nanoparticles before the fluorination of SiR/Fe 3 O 4 nanocomposites can improve the AC and DC breakdown strength by 0.64% and 8.81% respectively at the same time in high magnetic field.

/pdf/effects-of-ferromagnetic-nanoparticles-and-fluorination-on-1fe91ef7.pdf

Effects of Ferromagnetic Nanoparticles and Fluorination on Breakdown of SiR/Fe3O4 Nanocomposites in High Magnetic Field

Silicone rubber (SR) is widely used as insulation for cable accessories and the aging behavior significantly affects the cable operation. This study aimed to investigate the aging performance and mechanism of SR subjected to thermal effect and different mechanical tensions for up to 1000 h. We measure the crosslink density, mechanical and electrical properties to evaluate property variations, and material characterization methods, such as Fourier transform infrared spectrum and X-ray photoelectron spectroscopy (XPS), are conducted to explore the aging mechanism. Based on comprehensive analyses, competitive effects and synergistic effects between thermal aging and tensile aging are demonstrated. Thermal aging increases the crosslink density and improves the dielectric property, while tensile aging exhibits the opposite effects. Nevertheless, the effects of tensile aging on mechanical properties, especially stress relaxation, are significantly promoted by thermal aging. These complex effects can be attributed to the destruction of physical crosslink and the orientation of molecular chains induced by tensile stress.

Aging Mechanism of Silicone Rubber Under Thermal–Tensile Coupling Effect

Silicone rubber (SR) is widely used as insulation for cable accessories and silicone grease severs as a lubricant during its installation. However, the absorption of silicone grease during long-term operation can lead to swollen SR and interfacial cavity defects, whose effects can be severe and unpredictable. This study aims to investigate the changes in dielectric and mechanical properties of SR caused by silicone grease absorption. We measure the dielectric properties by frequency domain dielectric spectroscopy (FDS) and dynamic mechanical analysis (DMA) is applied to evaluate the complex modulus and stress relaxation. The effects of molecular polarity and absorption content of silicone grease are specifically considered and evaluated. As a consequence, silicone grease absorption reduces storage modulus and loss modulus, and accelerates stress relaxation of SR insulation, probably leading to insufficient interfacial pressure which is considered a major concern for cable accessories. In addition, changes in dielectric properties are dependent on the absorption content and molecular polarity of silicone grease. The deterioration effect of fluorinated silicone grease is stronger than that of phenyl methyl silicone grease. These results can be attributed to the occupation of free volume and weakening of intermolecular forces in SR, which provides an understanding of deterioration mechanism of SR caused by silicone grease absorption.

Changes in Dielectric and Mechanical Properties of Silicone Rubber Caused by Silicone Grease Absorption

Moisture is one of the most serious causes of cable joint failures in the distribution network, which can invade cable joints through the composite interface structures. Enhancing the sealing performance of interface structures is an important initiative that should be focused on, but the relevant aspects are lacking in the design and selection of cable accessories. In this work, a detection and evaluation method applicable to the water tightness of composite interface structures in distribution cables is proposed, and the water ingress process at different temperatures and interfacial pressures is measured. Two indexes, namely the time constant of penetration Tp and the water resistance coefficient Kr, are defined and calculated to evaluate the water tightness of cable joints. Experimental results show that increasing temperature and decreasing interfacial pressure have accelerated effects on the water ingress process, resulting in a worse water tightness of cable joints. When the temperature increases from 25°C to 75°C, Tp decreases to a minimum of 16.7% and Kr decreases to 49.8%. As the interfacial pressure decreases from 0.14 MPa to 0.015 MPa, Tp decreases to a minimum of 7.22% and Kr decreases to 55.2%. Similar results are obtained for water immersion environment and humid environment with 95% relative humidity. The proposed method is effective and accurate for testing and evaluating the water tightness of cable joints and can provide important guidance for the design and selection of cable accessories.

Water Tightness Testing and Evaluation of Composite Interface Structures in Cable Joints

Silicone grease helps lubricate and seal the cable joint interface during cable installation. However, silicone grease causes silicone rubber to swell during cable operation, which is an important factor| causing silicone rubber deterioration and equipment failure. In this paper, experiments are designed to study the effect of silicone grease on the dielectric properties of silicone rubber in cable joints. First, the dielectric properties of silicone grease absorbing different silicone rubbers in a wide frequency range were measured by using a broadband dielectric spectrum. Then, the experimental results are analyzed from the microscopic point of view by molecular dynamics simulation. It is found that dimethyl silicone grease can improve the charging properties of silicone rubber. Fluorinated silicone grease and long-chain alkanes deteriorate the charging properties of silicone rubber, and fluorinated silicone grease has the most obvious effect on the deterioration of the properties of silicone rubber. As the viscosity of dimethyl silicone grease decreases, its improvement on the dielectric properties of silicone rubber is more obvious. Therefore, the influence of silicone grease on the dielectric properties of silicone rubber is not only related to the molecular polarity of the silicone grease itself, but also to the swelling properties of the silicone grease.

Effect of Silicone Grease on the Dielectric Property of Silicone Rubber for Cable Joint

Synergistic Aging of Silicone Rubber by Wet/Dry Heat and Silicone Grease

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