Glass Insulator

6.4.2 Glass Insulators

The glass insulators usually are manufactured at high temperatures by mixing the different materials, including lime and quartz powder, and then it suddenly cools in the mold. This action (“Toughening” causes tightening of the glass). In this way, a glass insulator is obtained. Fig. 6.4 shows a sample of the glass insulator [8].

5.7.3 Nonceramic and Composite Insulators

The use of nonceramic and composite insulators in place of traditional porcelain and glass insulators for line insulation has become widespread in the last 20 years. Such insulators have several advantages over porcelain and glass insulators, such as lighter weight, easier handling, better resistance to damage from vandals, lower cost (in some countries), and superior contamination performance. Different material families have been used for the exposed part of the insulator (hereafter called the housing). High temperature vulcanized (HTV) silicone rubber, ethylene propylene rubber (EPR), cycloaliphatic epoxy, and EVA are among the materials proven suitable for outdoor use, with the first two varieties dominating for transmission voltages.

A nonceramic and composite insulator's performance under polluted conditions merits careful consideration. For porcelain or glass insulators, flashover resulting in a temporary outage is the end result of a contamination event. This does not usually cause any major permanent damage to the insulator string although burning of the glaze and/or fragmentation of the bells in contact with the power-follow fault current can occur. There is little risk of the insulator failing mechanically. It is also unlikely that the inorganic dielectric is degraded due to surface discharge activity, which could be long-lasting.

For nonceramic insulators, there is actually less risk than with the porcelain or glass insulators from the flashover event itself due to the elastic nature of the material. But it is from the surface discharge activity that the exposed insulation can be subjected to degradation, and this can be a major concern. In addition, the organic nature of the insulating materials can make them vulnerable to degradation from natural elements, such as heat, UV from sunlight, moisture, and chemicals. A permanent reduction of their desirable properties under service conditions can occur with time, referred to as aging. It is also important to note that some degradation modes may actually occur even in clean conditions, such as from exposure to corona activity. In fact, mechanical failure of nonceramic insulators from a mode of failure called brittle fracture has been experienced in relatively benign outdoor conditions. Users should be aware of all these possibilities.

Despite these concerns, it should be said that judicious selection and application of nonceramic insulators has resulted in improved reliability and lower installation costs for both transmission and distribution lines. Progress at all fronts, namely research, development, testing, manufacturing, and usage, has made this possible.

Just from geometrical considerations alone, nonceramic insulators should offer superior performance under contaminated conditions when compared to their porcelain and glass counterparts due to their smaller diameter. Additional improvement in contamination performance can be obtained by using materials that are hydrophobic, hence suppressing leakage current and discharge activity; they can remain in this state for a long time in service. Silicone rubber is one type of material that fits into this category. Within a particular material family, the leakage current suppression capability is dependent on the formulation, but in general, silicone polymers have better leakage current suppression capability than other outdoor insulating materials.

The typical practice is to use a leakage distance similar to porcelain for silicone rubber insulators; and for nonceramic insulators employing materials other than silicone rubber, the leakage distance is about 20 to 30% higher than the distance for porcelain insulators.

Contributed By: Isabel Diaz-Tang

14.1 Introduction

Silicone rubber (SiR), as a basic insulating material, has been widely used to coat porcelain and glass insulators. The high voltage direct current (HVDC) technology is considered as the most efficient and economical solution to the high voltage, large capacity, and long distance transmission and the power network interconnection [1]. Several±800 kV UHVDC transmission lines have been put into operation in China [2]. Due to better contamination resistance, temperature resistance, electrical insulation, and elasticity, SiR is extensively used in insulators and cable accessories for the HVDC transmission lines [3].

Despite all the advantages, SiR suffers from long-term operation and environment effects. In HVDC transmission lines, corona discharge could occur even on well-designed insulators, which can inject charge into the insulator surface and significantly damage the insulators [4]. It is well known that charge injection is predominantly dependent on the initial external field distribution [5,6]. Under DC voltage, the charges are more likely to accumulate on the insulator surface due to the constant electrostatic field, compared to that under AC voltage. The charges may remain on it over a certain period determined by the efficiency of decay process. The existence of surface charges causes early insulation failure and plays an important role during the development of the surface flashover [7]. It also has been reported that with the same field conditions the accumulation of pollutants under DC voltage is 1.2–1.5 times of that under AC voltage [8]. It is necessary to investigate the performance of SiR insulators under DC voltage. When exposed to long-term humidity and severe contamination, the hydrophobicity of SiR can be lost for sustained periods, which will lead to the development of a conductive film on the surface [9]. Thus, dry band arcing may occur and a large amount of heat will be generated. As the thermal conductivity of SiR is very low, the heat is accumulated in the discharge area and cannot spread quickly, which will gradually cause the degradation of SiR and may further induce tracking and erosion [10]. Especially under DC voltage, with more contaminants, conductivity and leakage current are higher, which can result in more severe degradation of SiR. Inclined plane tests on polymeric high temperature vulcanized SiR insulators have shown that the tracking and erosion are more severe under positive DC than under AC voltage [11]. Besides, the electrical field inside the accessories is not as uniform as that inside the power cable for its complicated physical structure, and some defects brought during the manufacturing process, such as nonuniform electrical field and defects, may cause dielectric failure inside the accessories [12]. The electrical tree is initiated from the enhanced point of the electric field which may be caused by the void, impurities, or irregular shapes [13]. It is a serious threat to the insulation and it can even cause insulation breakdown.

In order to improve the physical, chemical, mechanical, and electrical properties of SiR, the nanocomposite has recently drawn considerable attention. Venkatesulu and Thomas have investigated good performance on tracking and erosion resistance of nanocomposites due to the interaction between the host dielectric and the nanoparticles [14,15]. Previous studies showed that nanoparticles have a great effect on surface charge behavior [7]. Fleming et al. have presented the space charge profile data of Low Density Polyethylene (LDPE) in which different nanoparticles were incorporated [16]. Kumara et al. have observed that the flashover voltage level varied linearly with the amount of deposited charge for both positive and negative charging [5,6]. Many researchers have also investigated the thermal problems and the resistance to tracking and erosion of SiR. The results have shown that the material degradation is a function of the leakage current magnitude and the time for which dry band arcing exists in a particular spot [17,18]. According to the results in the field, thermal depolymerization activated by electrical discharge is the main degradation factor for SiR insulators exposed to a coastal environment [19]. A correlation study has shown that the resistance to erosion of SiR composites, filled with alumina trihydrate (ATH) or silica, is strongly correlated to the thermal conductivity of the composite [20,21]. In the field of electrical treeing, Chen et al. investigated the tree initiation time of pure epoxy resins and found that nanoparticles were able to increase tree initiation time [22]. Tanaka et al. found that alumina nanofillers were effective in suppressing both tree initiation and propagation [23].

On the bases of all the previous research activities, this chapter discusses three typical SiR nanocomposites and their dielectric properties. In Section 14.2, nano-Boron Nitride (BN) particles are mixed into RTV SiR to obtain SiR/BN nanocomposites. The tracking and erosion processes of SiR/BN nanocomposites were studied by employing a standard inclined plane test, with an exception that the supplied voltage was DC. In Section 14.3, SiR/SiO₂ nanocomposites are studied and the research focuses on the effects of fluorination time and mass fraction of nanoparticles on surface charge and DC flashover characteristics of SiR/SiO₂ nanocomposites. In Section 14.4, AC voltage with the frequency of 50 Hz was applied on the SiR/SiO₂ nanocomposites to initiate electrical trees with the temperature ranging from −30°C to −90°C. Both the structure and the growth speed of trees were observed by a digital microscope system, and the treeing proportion was introduced to describe the electrical tree propagation characteristics.

3.2.4 Insulators and Fittings

15.7 CELLULOSE ACETATE

5.3.1.2 Insulator pollution accumulation rules

The force analysis of pollutants moving to the insulator surface shows that, the deposition of pollutants on the insulator surface is affected by gravity, electric field force, and wind power, of which the wind power is the major factor influencing insulator pollution accumulation.

Gravity mainly influences the pollutants with higher density, while it has little deposition influence on pollutants with lower density, including gaseous pollutants. The characteristics of the pollutant deposition on insulator equipment caused by gravity are: the scale of influence is smaller, and the deposition is focused around the pollution source. In addition, the deposition mainly influences the pollution accumulation on the surface of the insulator equipment.

The electric field force influences motor direction and speed. For example, in the alternating-current electric field the pollutants will cyclically move back and forth under the electric field power; however, in the direct-current electric field, the pollutants will move directionally. The stronger the charging capacity of pollutants and the smaller the density, the more obviously they are affected by the electric field force.

The distribution of the pollution deposition of each individual insulator in the insulator string on a transmission line can directly reflect the influence of the electric field force on the pollution accumulation on the insulator equipment. The statistical result shows: The natural pollution accumulation degree of each individual insulator in a suspension-type insulator string on a transmission line is random under different region and terrain conditions. However, as for every string of glass or porcelain insulators, the insulators near the conductor side are those with the largest pollution accumulation (generally expressed by the ESDD), and the next are insulators on the grounding side. The pollution deposition rules for composite insulators generally are that, the salt density value of sheds near the conductor side and grounding side is slightly high, while the salt density value of sheds near the conductor side is the highest. The distribution of ESDD on the whole composite insulator is approximately proportionate to the voltage distribution on the composite insulator.

The wind directly determines the direction and speed of pollutants movement to the insulator equipment, and it also mainly controls the pollution accumulation rules for the insulator equipment surface. With the same pollution source, both the existence and the amount of wind influence the pollutant deposition process on the insulator equipment surface more greatly, and also directly influence the difference of degree of pollution accumulation on the top and bottom surfaces of the insulator equipment.

In addition, the strength of wind power always affects the scope of influence of pollutants on the power system. For instance, when atmospheric diffusion and transmission capacity is relatively weak, most common industrial pollution sources only influence the pollution accumulation of insulator equipment within 10 km; however, when the atmospheric diffusion and transmission capacity is relatively strong, its influence scope is up to 20–30 km.

With the increase of numbers of composite insulators in a network, the pollution accumulation rules for composite insulators are under research. The composite insulator is of a simple shed type, and its shed has no edge, which contributes to reducing pollution accumulation. However, the shed’s sheath on a composite insulator easily absorbs dust because it is easily charged, when rubbed with atmospheric particulates, which is bad for reducing pollution accumulation.

9.5.1 Ceramic insulators

Insulators are made of ceramic materials which include porcelain and glass. Their initial use precedes the construction of power systems. They were first introduced as components in telegraph networks in the late 1800s.

There are a number of basic designs for ceramic insulators, examples were shown in Figs 9.6(a) to (c). Porcelain is used for the production of cap and pin suspension units, solid and hollow core posts, pin type, multi-cone and long rod insulators, and bushing housings. Glass, on the other hand, is used only for cap and pin suspension and multi-cone posts. Porcelain and glass insulators are well established, as might be expected based on their long history of use. Currently these types of insulators comprise by far the majority of in-service units. Continuous improvements in design and manufacturing processes have resulted in insulators, which are both reliable and long lasting. Porcelain units are coated with a glaze to impart strength to the surface. Today's glass insulators are predominantly manufactured from thermally toughened glass, which prevents crack formation. Both of the materials have inert surfaces, which show very good resistance to surface arcing, and both are extremely strong in compression.

The manufacturing process for electrical porcelain is complex and involves numerous steps. With glass insulators, the manufacturing process is less complex, but still requires tight control. Failures of porcelain and glass insulators can usually be traced back to the manufacture, material or application of the units. If adequate caution and control in these areas is not maintained, the likelihood of an inferior product increases. However, as previously mentioned, when well made, both porcelain and glass insulators are highly reliable. The majority of bushings and lightning arresters installed in today's substations are contained within porcelain housings. Porcelain housings are, in essence, hollow core post insulators.

Cohesion

Adhesive forces bond a coating onto metal, whereas cohesive forces bond a coating with itself. Cohesion tests are similar to adhesion tests. But the specimen size and dolly diameter are larger so that area of sample under the dolly is far greater than the specimen’s cross-sectional area. This arrangement ensures that the coating fails by cohesion rather than by adhesion loss at the steel-coating interface.¹⁶

13.7 Hybrid nanocoatings

The use of hybrid nanocoatings i.e., containing organic/organic, organic/inorganic, and inorganic/inorganic nanoparticle combinations can result in a combination having the advantages of both mixtures.

Momen at al. (2011) developed a one-step technique for manufacturing room temperature vulcanizing (RTV) silicone rubber and stearic acid icephobic coatings with micro/nano structure roughness. The coating was sprayed onto glass substrates to replicate the icing process for outdoor glass insulators. The increase in stearic acid concentration from 0% to 43% contributed to increasing CA from 115 to 160 degrees while enhancing droplet rolling-off properties.

Li et al. (2012) developed an icephobic nanocoating for power line insulators. The combination of materials consisted of nanosilica and PDMS. The average contact angle for the coatings was approximately 161 degrees i.e., a superhydrophobic surface. Insulators commonly form icicles due to their orientation and location. The presence of this hybrid coating resulted in two main advantages: firstly the concentration of the icicles and their lengths were both reduced, and the flashover voltage increased in comparison to the insulators with a standard silicone rubber coating.

Yan et al. (2012) developed a PDMS-modified nanostructured silica coating for glass slides, resulting in ice shear strength values 2149 and 139.6 times lower than bare and RTV coated substrates. Li et al. (2016b) modified this coating to add reactivated hydroxyl and carbon coating for glass slides as well as insulators. The FC-100/146 insulators were dip coated and cured in two consecutive steps before testing in an environmental chamber at −6°C, 3 m/s wind speed, 12 kV AC voltage, and 90 L/h water flow rate. The peak leakage current value was greatly reduced for the coated insulator (0.32–0.45 mA) as compared to the uncoated insulator (0.35–0.48 mA). Additionally, 30 min of glaze ice accretion in the climate chamber yielded sparse ice formation on the coated cylinder characterized by discrete droplets and minimal accretion.

Nosonovsky and Sobolev (2013) prepared a hydrophobic and icephobic emulsion containing polymethyl-hydrogen siloxane oil, metakaolin/silica fume (MK/SF), and nanostructured polyvinyl alcohol (PVA) fibers for Portland cement mortar tiles. Apart from low roll-off angles, shear stress tests with ice columns indicated good icephobicity along with hydrophobicity.

Liao et al. (2015a) used magnetic agitation to mix silica nanoparticles with fluorosilicone resin, liquid epoxy resin, and ethyl acetate. The entire mixture was dip coated onto glass slides and insulators respectively. After 80 min of 100 μm water droplet spray at −5°C, 90% of the coated glass slide remained ice free. Similar tests on the insulator exhibited icicle formation on only isolated points.

Wang et al. (2016a) spray coated a mixture of modified silica nanoparticles, PMMA and THF, followed by drying for 2 h onto substrates. Even at −20°C, the droplet easily slid off the lubricated surface, thereby indicating a good icephobic property.

Liao et al. (2017) used the RF magnetron sputtering technique to develop a nanostructured ZnO–silica–PTFE sandwich coating with CA 167.2 degrees and CAH equal to 1 degree. After a 2 h glaze ice spray, 82% of the surface remained uncoated. These coatings were manufactured on glass slides to mimic glass insulators for power transmission systems.

Qian et al. (2017) used a TiO₂-modified PDMS icephobic coating with fluoro-additives on glass substrates for outdoor insulators and windows for buildings. An air spray gun was used to coat the substrates with the alumina nanoparticles and the resultant CAs measured were between 108.4 and 112.1 degree. In addition to weathering tests which indicated no change in the icephobicity despite 1000 h exposure to wet/dry cycles, ultraviolet (UV) exposure, and humidity between 50% and 100%, 20 μL water droplets slid straight off the coated surface at −20°C and a tilt angle of 30 degrees.

Wang et al. (2017) developed a mixture of silica nanoparticles, ultra-high molecular weight polyethylene (UHMWPE), and decahydronaphthalene to be poured on various substrates including copper, aluminum, titanium alloys, steel, and polyethylene terephthalate (PET) wallpaper. The coating was infused with kerosene, silicon oil, and perfluorinated fluid to develop a SLIPS surface. Durability was tested by 3 h bombardment of 16,200 water droplets at 2.42 m/s and dipping for 24 h in HCL (pH 2–6) or NaOH (pH 8–12). No impact was seen on hydrophobicity. To overcome the issue of humidity, the SLIPS surface showed good water droplet mobility even at −15°C.