Composite insulators are considered mature products and a broad range of insulator types is nowadays available, including line insulators, hollow-core insulators and apparatus insulators, for applications ranging from distribution voltages to the highest transmission voltages. This development has created a need to elaborate diagnostic methods and tools for determining the quality of composite insulators. A review on recognized insulator faults and methods for their detection is provided.

Diagnostic Methods for Outdoor Polymeric Insulators

We present a simple method for fabricating the superhydrophobic coatings on composite silicone rubber used for electrical outdoor applications. The coating is characterized by contact angles as high as 170° and is mechanically durable in contact with the aqueous phase. We discuss the impact of mechanical durability of the surface texture on the anti-icing performance of the coating on the basis of the experimental data on freezing delay of sessile aqueous droplets. A set of complementary data obtained in laboratory and outdoor experiments on freezing delay time, variation of wettability and practical work of adhesion for supercooled aqueous sessile droplets, impacting behavior of droplets at low negative temperatures, as well as the results of snow and ice accumulation in outdoor experiments indicate the very prospective icephobic properties of the developed coating.

Reinforced Superhydrophobic Coating on Silicone Rubber for Longstanding Anti-Icing Performance in Severe Conditions.

Failure of insulators leads to failure of transmission system and causes heavy loss to the power sector. This has necessitated helicopter inspection on transmission line. Therefore, localizing insulators in collected aerial image is an essential step for automatic fault detection. This paper proposes a localization method of multiple insulators with different angles in complex aerial image based on Orientation Angle Detection and Binary Shape Prior Knowledge (OAD-BSPK). Detect possible orientation angles of insulators preliminarily. For each possible angle, reserve insulator by its binary shape prior knowledge. Traverse all the possible angles, and multiple insulators can be localized in aerial image finally. This paper uses a large number of real aerial captured images as experimental images. The results show that this method can localize multiple insulators with different orientation angles in complex aerial image, and has higher positioning precision, lower computational complexity and lower time consuming compared with existing methods. This method can meet the requirements of insulators’ real-time and accurate localization.

Localization of multiple insulators by orientation angle detection and binary shape prior knowledge

In recent years, a new kind of abnormal fracture phenomena of composite insulator occurred in high voltage transmission lines in several countries, which was totally different from brittle fracture and normal fracture. According to the general characteristic features of the fracture, this type of failure was named decay-like fracture of composite insulator by the author. In this paper, a decay-like fractured composite insulator was investigated and microscopic physic-chemical properties of decay-like fractured Fiber Reinforced Plastic (FRP) core rod were analyzed. Scanning Electron Microscope (SEM), Thermal Gravimetric Analysis (TGA) and Fourier Transform Infrared (FTIR) Spectroscopy methods were employed to explain the microscopic morphology characteristics and chemical component changes of decay-like fractured composite insulator, and to further explain the reasons for the name of decay-like fracture in terms of "crisp" and "decay-like". The main microscopic feature of decay-like fracture is the degradation and deterioration of the epoxy resin matrix in FRP rod. The mechanism of decay-like fracture of composite insulator could be summarized as follows. Due to the liquids penetration into the sheath-core interface, hydrolysis of silane coupling agent with bad quality leads to poor adhesion strength and interface voids, which results in distortion of electric field at the sheath-core interface. Under the combined action of damp condition and high electric field, discharge and current appear along the FRP rod surface. Epoxy resin matrix is eroded in the presence of discharge and current, and then is soften, melt, evaporated. Meanwhile, ion-exchange and hydrolysis process happen between glass fibers under the combined action of mechanical stress and nitric acid produced by discharge in the presence of moisture. Fiber-resin interface in FRP rod is separated owing to the erosion of epoxy resin. The degradation direction of the decay-like fracture is from outside to inside of FRP rod, and the interface between silicone rubber sheath and FRP rod is confirmed to be the initial deterioration area of the decay-like fracture. The criterion to distinguish the differences between the decay-like fracture, brittle fracture and normal fracture depends on whether the epoxy resin matrix is degraded and deteriorated or not.

Decay-like fracture mechanism of silicone rubber composite insulator

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

This paper investigates the liquid permeation performance of silicone rubber containing alumina tri-hydrate (ATH) that has been modified with N-[3-(trimethoxysilyl) propyl] aniline (Y-9669). From the change in weight during immersion in water, saline solution or nitric acid, and during subsequent drying in air as well as a second immersion test, it is revealed that the majority of the absorbed liquid exists in filler and interface for silicone rubber with original ATH. Surface modification of ATH filler can protect silicone rubber from liquid permeation. It is easier for liquid to permeate into silicone rubber than to escape from it due to the hydrophobicity of the silicone rubber matrix. A cycle of immersion and drying is found to have little impact on subsequent immersion in the case of water and saline solutions; however, damage inflicted on the matrix and filler by nitric acid means that the rate of permeation increases during subsequent immersion. The volume resistivity and surface resistivity of samples with unmodified ATH also decrease more appreciably than those with modified ATH after a cycle of immersion and air drying due to the amount of the residual absorbed liquid.

Effect of alumina tri-hydrate surface modification on liquid permeation and electrical performance of silicone rubber

The effects of liquids immersion and subsequent drying on the surface properties of filled high temperature vulcanized (HTV) silicone rubber have been reported in this series of paper. In this first paper, the surface hydrophobicity characterized by static contact angle and surface chemical properties characterized by a comprehensive analysis of Fourier transform infrared spectroscopy (FTIR), X-Ray diffraction (XRD) and thermal desorption gas chromatography mass spectrometry (TD-GC/MS) have been studied. The liquids employed in this study were deionized water, NaCl solution and nitric acid solution, which represented the actual liquids that composite insulators could encounter in practice use. As for the NaCl solution, the lower the concentration, the greater its impact on the hydrophobicity of the silicone rubber (the deionized water can be considered as the NaCl solution of concentration of 0 mol/L). As for the nitric acid solution, the higher the concentration, the greater its impact on the hydrophobicity of silicone rubber. During the permeation process of deionized water and NaCl solution into HTV silicone rubber, some Alumina Tri-hydrate (ATH) filler and low molecular weight (LMW) could be dissolved into the liquids. Nitric acid solution not only dissolved the ATH filler, but also interacted with the ATH filler and degraded the polymer matrix.

Effects of liquids immersion and drying on the surface properties of HTV silicone rubber: Part I-contact angle and surface chemical properties

Silicone rubber is widely used in the outdoor insulation for its hydrophobicity, at the same time, superhydrophobic surface with self-clean property is more ideal for outdoor insulating material. This paper reports an efficient way to fabricate the superhydrophobic silicone rubber whose contact angle reaches 151° and sliding angle reaches 5°. The long term stability of polymeric superhydrophobic insulating surfaces is examined under electrical and non-electrical stresses. The result shows that in the water aging process, the static contact angle of both SIR and superhydrophobic SIR can recover, but the dynamic contact angle shows a little recovery. In the corona aging process, both the static and dynamic contact angle of superhydrophobic SIR can almost restore to the original level, but the dynamic contact angle of normal SIR increases after restoration. The finding shows that micro-nano surface structure and the stability of the surface chemical composition are important for the anti-aging performance of the superhydrophobic SIR.

Aging and recovery of superhydrophobic silicone rubber under electrical and non-electrical stresses

In order to solve the problem of pollution flashover in the strain insulator-strings on transmission line in China, a large number of insulators with RTV silicone coatings are used for the transmission line. This paper summarize the experience of using RTV coatings for disc insulators on transmission lines in China, the main existing problems of RTV operation, such as poor adhesion, hydrophobicity decline, powder and product pollution, are pointed out. Some problems are related to the way of spraying on the tower in jobsite, because the detection methods are not effective and the quality of spraying could not be controlled, at the same time. So, the pre-coated RTV insulators have been widely used in UHV projects. Experimental studies show that pre-coated RTV insulators have a clear technological advantage in the quality of appearance, thickness uniformity, adhesion and so on. But it also founds that the combination of disc insulator with RTV coating leads to the sharp decline of steep-front impulse breakdown performance of insulator. Most of breakdown happened at the negative voltage polarity moment. Under the negative polarity voltage, the discharge channel tends to select the RTV coatings surface.

Application of pre-coated RTV insulators in UHV OHLs and steep-front impulse voltage test

In the second part of this review, the chemical properties of all known fracture modes of suspension composite insulators are summarized; a novel criterion for discerning fracture mode is proposed.

Xidong Liang

Papers

Effect of alumina tri-hydrate surface modification on liquid permeation and electrical performance of silicone rubber

Effects of liquids immersion and drying on the surface properties of HTV silicone rubber: Part I-contact angle and surface chemical properties

Aging and recovery of superhydrophobic silicone rubber under electrical and non-electrical stresses

Application of pre-coated RTV insulators in UHV OHLs and steep-front impulse voltage test

Comparative Investigation on Fracture of Suspension High Voltage Composite Insulators: A Review—Part II: Chemical Properties and Criteria System