Effects of 2D boron nitride (BN) nanoplates filler on the thermal, electrical, mechanical and dielectric properties of high temperature vulcanized silicone rubber for composite insulators

Condition assessment of naturally aged composite insulators, aiming to better understand the ageing processes involved, is essential for maintaining high reliability in power networks. In this study field-ageing effects on insulating materials (EPDM and VMQ/HCR) of two 150 kV composite suspension insulators are evaluated. The insulators had been in service for 17 years on a coastal transmission network, exposed to seaborne salt spray, windborne soil deposits and intense UV solar radiation. Several diagnostic techniques, including visual inspection, wettability classification, FTIR spectroscopy, SEM and EDX analysis were employed to assess the condition of the insulators. Field-ageing effects were not assessed as critical to dictate insulator replacement. For both insulators morphological and material degradation due to field-ageing was restricted to the upper surface layer of the polymeric housing. The resistance of housing material against tracking and erosion remained unaffected by field-ageing. Surface deterioration of EPDM was more intense than that of VMQ/HCR, despite the fact that the EPDM insulator had been in operation at a site with a lower pollution severity. The EPDM housing was classified as hydrophilic; however, after removing pollutant deposits its hydrophobicity was improved. The hydrophobic properties of the VMQ/HCR housing were only slightly reduced. Field-ageing effects on insulator housing are discussed based on material structural changes caused by ageing stresses.

Evaluation of field-ageing effects on insulating materials of composite suspension insulators

High voltage insulator detection and monitoring via drone-based aerial images is a cost-effective alternative in extreme winter conditions and complex terrains. The authors examine different surface conditions of the outdoor electrical insulator that generally occur under winter condition using image processing techniques and state-of-the-art classification methods. Two different types of classification approaches are compared: one method is based on neural networks (e.g. CNN, InceptionV3, MobileNet, VGG16, and ResNet50) and the other method is based on traditional machine learning classifiers (e.g. Bayes Net, Decision Tree, Lazy, Rules, and Meta classifiers). They are evaluated to discriminate the images of insulator surface exposed to freezing, wet, and snowing conditions. The results indicate that traditional machine learning methods with proper selection of features can show high classification accuracy. The classification of the insulator surfaces will assist in determining the insulator conditions, and take preventive measures for its protection.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/hve.2019.0079

High voltage outdoor insulator surface condition evaluation using aerial insulator images

Corona activity on the surface of silicone rubber has high impact on variation in hydrophobicity of the material, and on corona inception voltage (CIV) of water droplet sitting on the corona degraded surface. The CIV is high under negative DC as compared to positive DC/AC voltages. Increase in conductivity of water droplet reduces CIV. Surface charge accumulation studies indicate that the charge retention capability of the insulating material gets reduced drastically with silicone rubber damaged due to corona causing degradation of surface. Irrespective of the number of cycles of corona discharge activities carried out, the time constant of charge decay, after the removal of charge injection, remains the same. The water droplet initiated discharges under AC and DC voltages radiate UHF signal in the bandwidth 0.3–1.2 GHz. With single droplet sitting on top of the corona damaged silicone rubber, the amount of charge retained by the water droplet is low when the droplet is placed and charged immediately after corona discharge activity to surface of the insulating material. When the water droplet is placed after a certain time period, the charge accumulation characteristics of silicone rubber are regained. Leakage current measured during water droplet discharge studies through recurrent plot (RP) analysis reveals that, under AC voltage, it is possible to classify corona inception, arcing and bridging of water droplet between high voltage and ground electrode. The RP obtained for the current signals generated due to water droplet discharges under positive and negative DC voltages are different. The amount of damage caused to silicone rubber due to water droplet is high under negative DC voltage as compared to positive/AC voltages. While correlating the level of damage to the insulating material due to water droplet and the RQA parameters of RP, it is observed that higher the recurrence rate along with high determinism can cause severe damage to the insulating material. Structural analysis using ATR-FTIR and pyrolysis-GC/MS studies reveal significant functional group variation on surface of silicone rubber, especially when the number of cycles of corona ageing is more.

Understanding the influence of water droplet initiated discharges on damage caused to corona-aged silicone rubber

Composite insulators are the critical components of power transmission systems. 371 composite insulators with an operating duration between 3 and 22 years from the lines in high temperature and humidity regions were sampled. The material properties, mechanical properties, electrical properties and interface properties of these insulators were researched systematically. Moreover, 19 aging characteristic index were put forward to describe the aging conditions of insulators. We analyzed the correlation between the aging index and operating years, and corresponding aging mechanism was discussed in this paper. Meanwhile, according to the analysis results, we established lifespan prediction models based on physics and statistics respectively. The results showed: lifespan of the sampled insulators was determined by the thermal oxidation process of silicone rubber. Lifespan of the silicone rubber material was 14.6 years while the electrical lifespan of composite insulator was 18.9 years. With regard to the rest of the insulators in high-temperature and high-humidity regions, their remaining lifespan could be predicted in accordance with the mechanical load and Fourier translation infrared spectroscopy (FTIR) test results. In the end, we selected 20 insulators which had run for 14 years, and predicted their lifespan with the method mentioned in this article, the forecast error was 1 year.

Research on aging evaluation and remaining lifespan prediction of composite insulators in high temperature and humidity regions

During the field service of composite insulators, high temperature vulcanized (HTV) silicone rubber (SIR) material used for insulator’s sheath and sheds is gradually aging, and thus how to effectively evaluate its aging state has become an inevitable issue. In this paper, artificially corona-aged and naturally site-aged SIR materials are employed as samples, and their properties such as hydrophobicity, leakage current, trap density/energy level, surface microstructure and chemical composition are investigated for comparative study. With the increasing of corona aging intensity, the contact angle of SIR samples drops gradually and recovers more slowly. The hydrophobicity of site-aged insulators also declines, from HC1 for 2-year service to HC5 for 15-year service. The leakage current of corona-aged and site-aged samples both increase with aging, which indicates that aging induces the increase of surface conductivity of SIR material. The peak trap density of corona-aged and site-aged samples increases with the aging duration or service duration remarkably. The changes of these properties are attributed to the changes of micro-structures and compositions in the surface layer of SIR. The scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) analysis results reflect that a hardened inorganic silica-like (SiO2) layer with many polar chemical groups and distributed micro-pores is formed on the surface of SIR material after corona aging. Because of this silica-like layer, the hydrophobicity decreases while surface conductivity and trap density of SIR material increases. Besides the traditional properties like hydrophobicity and surface conductivity, the trap density is expected to be a novel parameter for effective evaluation of aging state of HTV silicone rubber material.

Aging characterization of high temperature vulcanized silicone rubber housing material used for outdoor insulation

At present, composite insulator is widely used in the power system, and its safe operation is seriously affected by its aging condition. In order to research the aging feature of composite insulator material, this paper analyzes the surface characteristics of composite insulator after accelerated aging process via corona discharge, including surface hydrophobic, trap parameters and chemical components based on X-ray Photoelectron Spectroscopy (XPS), etc. It is revealed that, with the enhancement of corona discharge along with applied voltage and time etc, the hydrophobic property continuously decreases, trap parameters regularly changes, and surface chemical results show that content of hydrophobic micro groups decrease than before. The XPS analysis can be used to well explain the aging mechanism. With the aggravation of aging condition of composite insulator, its surface trap density gradually increases, which could be considered as a novel aging characterization parameter of composite insulator.

Wei Song

Papers

Aging characterization of high temperature vulcanized silicone rubber housing material used for outdoor insulation

Analysis on surface characteristics of composite insulator materials after corona discharge