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

In this paper, the permeation properties of three types of liquids into HTV silicone rubber with different Alumina Tri-hydrate (ATH) contents had been investigated by weight gain experiments. The influence of differing exposure conditions on the diffusion into silicone rubber, in particular the effect of solution type, solution concentration, and test temperature were explored. Experimental results indicated that the liquids permeation into silicone rubber obeyed anomalous diffusion ways instead of the Fick diffusion model. Moreover, higher temperature would accelerate the permeation process, and silicone rubber with higher ATH content absorbed more liquids than that with lower ATH content. Furthermore, the material properties of silicone rubber before and after liquid permeation were examined using Fourier infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA) and scanning electron microscopy (SEM), respectively. The permeation mechanisms and process were discussed in depth by combining the weight gain experiment results and the material properties analyses.

Investigation on permeation properties of liquids into HTV silicone rubber materials

In recent years, a new type of composite insulator mechanical failure appeared in high voltage transmission lines, which is a type of severe accident for power system, but only few studies on it have been reported all over the world. In this paper, this type of failure was named Decay-like Fracture by summarizing and analyzing the general features, and the decay-like fracture of composite insulator was treated and studied as a new type of composite insulator mechanical failure. By analyzing some field failed composite insulators in China, main features of decay-like fracture were then summarized. The fracture spot of decay-like fractured insulator became crisp and looked like decayed wood. The mechanisms of the normal fracture and brittle fracture of composite insulator cannot explain decay-like fracture, and boron-free Fiber Reinforced Plastic (FRP) core which has been widely used to prevent brittle fracture could not prevent decay-like fracture. Decay-like fracture is not a pure mechanical failure. The discharging and surface micro current on the core surface would finally lead to mechanical fracture of composite insulator. But the process and mechanism of decay-like fracture still need further researches.

Failure analysis of decay-like fracture of composite insulator

Power girds in different countries face different types of threats and in China the threats severely affecting grid reliability were in the past large power outage accidents caused by pollution flashovers. First records of pollution flashover accidents in China can be traced back to 1950s in industrial areas in Northeast and East China. Thereafter, such accidents occurred occasionally, but after 1970s they started to expand and were recorded in most of the country's provinces. The very first large area outage accident caused by pollution flashover happened in early 1974 in Northeast China around the local largest cities Shenyang and Fushun. Between 1971 and 1994, 3542 pollution-caused line trip-outs (several trip-outs on one overhead line (OHL) happening the same day is counted in China as a single event) were recorded on 35–500 kV OHLs and 1768 substation trip-outs. A total of 44 large area outage accidents caused by pollution flashovers took place during the 30 year period before 2007, as shown in Figure 1, while the outages recorded between 1981 and 2001 accounted for 43% of the total number. Among them, outages of super large character occurred every 5–6 years, e.g., in 1990 in North China and Northeast China, in 1996–1997 in East China and Central China, 2001 in North China and Northeast China [1–3]. Here we refer to the large area outage accidents as flashover events on several main transmission overhead lines and substations, appearing during one or several consequent severe meteorological events and continuing for several days. They affect several big cities within one province and stretch over an area of less than 100 thousand km2. For super large area outage accident, we refer to the accidents stretching across several provinces over the area of 200–300 thousand km2 and resulting in hundreds of pollution flashover events.

Improving the outdoor insulation performance of Chinese EHV and UHV AC and DC overhead transmission lines

The brittle fracture (BF) surface of a 500 kV composite insulator was analyzed. It was operated for 6 years. Chemical compositions on the field failed BF surface were examined by X-ray fluorescence spectrum and FTIR spectroscopy. Test results showed that the chemical functional groups NO3 -, NO2 -, together with SO4 2-, were formed both on the BF surface of FRP rod and in the rust on end fitting near BF crack. Laboratory simulated BF by stress corrosion under nitric acid or sulfuric acid were all analyzed and compared with the field failed BF surface. Analysis of the field failed insulator reveals that the BF is influenced not only by nitric acid, which may be formed by electrical discharge, but also by sulfuric acid, which may come from acid rain

Xidong Liang

Papers

Investigation on permeation properties of liquids into HTV silicone rubber materials

Failure analysis of decay-like fracture of composite insulator

Decay-like fracture mechanism of silicone rubber composite insulator

Improving the outdoor insulation performance of Chinese EHV and UHV AC and DC overhead transmission lines

Analysis of the acid sources of a field brittle fractured composite insulator