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

This paper presents a review discussing methods of determination the resistance to corona discharges and ozone of polymeric materials for applications in outdoor insulation. It also describes a part of international activities within CIGRE WG D1.01 aiming to develop suitable methodology for the testing, which includes designing the necessary test arrangement, defining test conditions and finally exemplifying results of the treatment by monitoring changes of electrical, mechanical and structural properties of some materials.

Evaluating resistance of polymeric materials for outdoor applications to corona and ozone

Outdoor insulators are often subject to corona discharges and the problem is becoming more prevalent with the increasing use of higher transmission voltage levels. For polymeric insulators, exposure to such discharges can alter the chemical structure of basic polymer and degrade surface properties. This paper investigates the effect of micro and/or nano fillers in silicone rubber composites in suppressing such damage. Four different types of samples are fabricated: pristine silicone rubber (PR), 30wt% micron–sized silica/silicone rubber (MC), 27.5wt% micron + 2.5wt% nano silica/silicone rubber (NMC), and 5wt% nano silica/silicone rubber (NC) composites. Samples are exposed to AC corona using a needle to ground-plane electrode setup. Experimental results are analyzed based on five different measurement methods: phase-resolved partial discharge (PD), hydrophobicity loss-recovery, Scanning Electron Microscopy (SEM), Surface roughness and Fourier Transform Infrared Spectroscopy (FTIR). Results indicate that NC shows a strong resistance to partial discharges and hydrophobicity loss. In the area below the needle tip, higher hydrophobicity loss and higher recovery are observed as compared to the vicinity region. Variations in surface roughness, appearance of crackles, voids, pits, surface splitting into blocky structures and damages to chemical structure of silicone rubber are appreciably retarded in NC as compared to PR, MC and NMC. Based on NMC results, it is found that addition of nano–sized silica can be an attractive approach to improve the corona resistance of micron–sized silica filled silicone rubber.

Performance of silicone rubber composites with SiO 2 micro/nano-filler under AC corona discharge

On the way of making efforts to standardize the selection of polymeric materials for high voltage outdoor environments, the resistance to corona and ozone has been recognized as one among twelve parameters to be evaluated within the OGRE Working Group D1.14. Our previous related work reported on the evaluation of the resistance of five commercially available polymeric materials to ac corona-ozone treatment, according to the test procedure recommended by this Working Group. With the same test arrangement in use, this report focuses on comparing the influences of ac and dc corona-ozone exposures on high temperature vulcanized (HTV) silicone rubbers. For this purpose, two types of HTV materials were used, each one characterized by different resistivity levels and tensile strength. For defining the treatment conditions, the ac and dc coronas were characterized by measuring the associated discharge current as well as the power released from the electrode. The intensity of ac corona discharge was found to be higher than that of dc corona, the latter being controlled by electrostatic charging of the treated polymeric surfaces, which also resulted in significant differences of the resulting ozone concentration. Experimental investigations after the treatments concentrated on evaluating dielectric properties (e.g. surface and volume resistivities, dielectric response (DR)), mechanical properties (e.g. tensile strength and elongation at break), as well as chemical and structural analyses (e.g. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR)). Results obtained from the measurements of dielectric properties indicated a strong sensitivity of surface resistivity to the treatments, though the impact of the dc corona remained less pronounced. On the other hand, the dielectric response and volume resistivity did not exhibit a high susceptibility to the exposure. No substantial differences were also found among the ac and dc corona treated samples as regards the mechanical properties. The XPS and FTIR analyses revealed proofs of surface oxidation for the samples treated by ac corona, while the samples treated by dc corona showed much less tendency for oxidation.

AC and DC zone-induced ageing of HTV silicone rubber

https://iopscience.iop.org/article/10.1088/2053-1591/ab3f0d/pdf

Aging of silicone rubber-based composite insulators under multi-stressed conditions: an overview

This paper presented the results of a study on the hydrophobicity loss of HTV silicone rubber under the different corona discharge conditions. The experimental results show that the speed and grade of hydrophobicity loss vary with the corona discharge time and the corona intensity. Moreover, it is noticed that the hydrophobicity would not become worse as long as the corona intensity keeps below a certain level.

Hydrophobicity Loss of HTV Silicone Rubber after Corona Discharge

The hydrophobicity recovery characteristics of HTV silicone rubber (SIR) at different corona deterioration stage have been investigated with static contact angle method. The surface microstructure variation with the corona deterioration processing, along with the chemical composition change at the end of corona discharge have been analyzed by scanning electron microscopy (SEM), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS), respectively. Experiment results show that the hydrophobicity recovery characteristics vary with corona deterioration processing and the hydrophobicity recovery rate goes through four periods, which are "increase-reduction-increase-reduction". Moreover, after corona deterioration a certain time, the hydrophobicity can't recovery back to the initial value, even if the storage time is long enough. On the basis of surface appearance variation of HTV SIR, as well as the SEM, FTIR and XPS analysis results, the hydrophobicity recovery courses are explained in details and it is confirmed that the changes of surface microstructure and chemical composition act as important roles influencing the recovery speed and grade of hydrophobicity of HTV SIR.

Study on hydrophobicity recovery characteristics and mechanism of HTV silicone rubber after corona deterioration

This paper has investigated the hydrophobicity recovery characteristics of HTV silicone rubber after different corona deterioration time using the static contact angle method. The experimental results show that the hydrophobicity of HTV silicone rubber can get some recovery after the corona deterioration, and the procedure of hydrophibicity recovery would be varied with the corona deterioration condition, moreover, the hydrophobicity would not recover back to the initial value after a certain condition of corona deterioration, even if the storage time is long enough.

Effect of Corona on the Hydrophobicity Recovery Characteristics of HTV Silicone Rubber

This paper has investigated the thermally stimulated current (TSC) characteristics of the deteriorated HTV silicone rubber under varied corona discharge duration, the hydrophobicity, scanning electron microscopy (SEM), and Fourier transform infrared (FTIR) of the corresponding test samples have also been measured. The experimental results show that the TSC characteristics of HTV silicone rubber gradually varied with the corona deterioration process, and it has relationship with the change of its hydrophobicity, SEM, and also FTIR. As a result, it could be suggested that the TSC characteristics might be used as an effective index for assessing the corona deterioration of HTV silicone rubber.

A method to detect the deterioration of HTV silicone rubber under corona discharge

This paper presents the thermally stimulated current (TSC) results obtained from an experiment which was carried out in order to identify the feasibility to evaluate the aging state of field HTV composite insulators by TSC technique. The test samples were prepared from four HTV composite insulators, which were randomly picked in-situ. The TSC experimental results show that the rough TSC shapes of four samples were similar despite different serving time, while the peak at high temperature area was only observed in one sample (insulator C). Further calculated results indicate that insulator C had the deepest trap level in our investigation. The performance difference between insulator C and the other three insulators can be obviously observed from the SEM morphologies, but is difficult to discern from their corresponding HC levels. Based on the above results, we can infer that although there are certain inner relations between the TSC characteristics of HTV composite insulators and their hydrophobicity, comparing to the later, the former is more sensitive to the aging performance under some conditions. Also the aging states of field HTV composite insulators have no direct relations with the servicing time in our investigation, when the TSC characteristics are taken into account.

Kun Yang

Papers

Hydrophobicity Loss of HTV Silicone Rubber after Corona Discharge

Study on hydrophobicity recovery characteristics and mechanism of HTV silicone rubber after corona deterioration

Effect of Corona on the Hydrophobicity Recovery Characteristics of HTV Silicone Rubber

A method to detect the deterioration of HTV silicone rubber under corona discharge

The TSC characteristics of HTV composite insulators