CHAPTER 1 INTRODUCTION

Transcription

1 1 CHAPTER 1 INTRDUCTIN 1.1 VERVIEW Power transmission at high voltages has gained a considerable importance recently. Glass and ceramics have been preferred for the manufacturing of insulators, bushings, cable terminations and surge arrestors for many years. Presently, polymeric insulators are increasingly used both in the distribution and transmission systems and steadily capture a wider share of the market because of their better dielectric properties, low weight, easy handling, vandal resistance, and cost effectiveness (Gorur et al 1999). With the advancement in power transmission capability, it has become more important to design and develop compact, cost effective and reliable insulating structures. utdoor insulation is simultaneously subjected to various stresses such as electrical, thermal and mechanical stresses which cause degradation or aging of the insulating material. The electrical and mechanical strengths of the insulator become diminishing and it results in material deteriorations. A great problem yet to be overcome is the tracking and erosion of outdoor polymer insulators. Tracking is a peculiar phenomenon which occurs on the surface of the insulator due to the creepage distance resulting from surface contamination. The researchers all over the world are making an effort to reduce the tracking and erosion effects in outdoor insulators through their research works.

2 2 After tracking occurs, the insulation property of the material is totally lost and there are no ways to improve the insulation property. The tracking phenomenon is investigated worldwide to improve the reliability and performance of insulating materials thereby solving the problems of surface degradation and adverse effects of tracking and erosion of the materials. The micro sized inorganic fillers are incorporated into the polymer materials so as to reduce the above mentioned problems and the results have been discussed by Meyer et al (2004a). The selection of appropriate filler is one of the most important aspects in the formulation of silicone composite and is based on the filler properties such as particle size, surface area, thermal and electrical conductivity. El-Hag et al (2006) reported that the micro filler with 30-65% filler content are required to get the desired electrical insulation properties for outdoor applications. The recent increase in demand for high quality outdoor insulating materials tends to focus on polymer nano-composite materials. Nano composites are in the range of nanometers in size, different with three orders of magnitude in length compared to micro composites. This would mean a difference of approximately nine orders in their number density. Therefore, the distance between the neighboring fillers are much smaller in nano composites than in micro composites. In terms of specific surface area, nano composites have high specific surface area of fillers (about three orders larger than micro composites). With this, the interaction of polymer s matrices with fillers is expected to be much more in nano composites and this will enhance the electrical and mechanical properties of the insulating materials. Research findings on nano-sized particle filled dielectrics show similar property improvements for considerably reduced filler additions compared to a higher amount of micro-sized fillers.

3 3 Considering this fact, in the present work, due emphasis is given to analyse and understand the tracking and erosion resistances of the micro and nano sized alumina (Al 2 3 ), nano sized silica ( 2 ) and nano sized aluminium hydroxide Al(H) 3 )filled silicone rubber materials by conducting experiments according to (International Electro Technical Commission) IEC under AC and DC Voltage with ammonium chloride as a contaminant. The characteristics variations in the fundamental and harmonic components of leakage current signals of the filled silicone rubber of both micro and nano sized alumina composite materials are investigated thoroughly. The effect of leakage current in initial stage, dry band formation, surface discharge and during severe is also analysed in micro and nano Al 2 3 filled silicone rubber (SIR). Moving average fundamental component of the initial and final stage of leakage current (LC) signals of the filled silicone rubber of nano sized silica is also investigated. To understand the electrical performance of the polymeric insulating materials under certain environmentally polluted conditions, the artificially aged nano sized Al(H) 3 and 2 filled silicone rubber have been analysed. The Thermo Gravimetry- Derivative Thermo Gravimetric (TG-DTG), Scanning Electron Microscope (SEM) with Energy Dispersive X-ray analysis (EDAX) and Fourier Transform Infra-red Spectroscopy (FTIR) were used to characterise the thermal, physical and chemical properties of the silicone rubber specimens. 1.2 VERHEAD LINE INSULATRS AND MATERIALS There are two main types of insulator materials: Inorganic materials and polymeric materials. The inorganic materials consist of ceramic and glass, whereas the polymeric material consists of composite insulators and resin insulators. These materials act as the dielectric of the insulator, when it is attached to the terminal or end fitting. Ceramic or porcelain is a very stable

4 4 material and thus very immune to degradation through electrical discharge activities, ultra violet (UV) radiation and other environmental factors. The basic components used to make the ceramic insulator are clay, fine sand, quartz and feldspar. Alumina and cristobalite are usually added as filler. Glazing is used to smooth the insulator surface, to improve hydrophobicity, and also to increase the mechanical strength. The cement that is used to attach the ceramic insulator shed to the metal cap and pin is believed to cause electrical puncture on the ceramic insulator in some cases (Gorur et al 1999). Ceramic is a brittle material, and cracks and breaks are common problems. However, it is not easily shattered. The glass insulators consist of a mixture of silica, limestone, dolomite, feldspar and soda ash. They are made by annealed glass. It is used for high voltages, which require heavier conductors and higher strength insulators. The problem with glass is that it is a brittle material and easily fractures due to mechanical stress on the surface. Ceramics are first used and then followed by glass. Ceramics and glass have similar general shapes. The higher the voltages, the bigger are the number of units attached. It means that the insulators are heavier for higher line voltages. This has made ceramics and glass insulators difficult to use with the increase in system voltage requiring longer insulators. Considering the polymeric insulators, it consists of composite and resin insulators. The composite insulators are composed from more than one insulator material whereas the resin insulators are made only one insulator material. Resin insulators are made from various types of heavily filled polymeric resins, including polyester, polyurethane and cycloaliphatic. The fillers are added in order to improve the tracking and erosion resistance of the polymers, since their formulations include a fair amount of carbon. However, the high amounts of filler make it more difficult to cast the insulator shape.

5 5 The interest in very high voltage power transmissions has encouraged the industry to produce a lightweight insulator with better electrical and mechanical properties compared to ceramic and glass. General Electric introduced the non-ceramic non-glass insulator called composite insulator. Composite literally means something which is made of different parts. This type of insulator is called composite because it consists of three parts: (i) a core made of glass fiber, (ii) external weather sheds made of polymer, and (iii) metal fittings made of aluminium for attachment. Their designs are also significantly different from ceramic and glass types. The physical description, service environmental factors affecting and advantages of composite insulators (polymeric) are clearly discussed in the following sections Composite Insulator Development History The composite insulators were first developed in the 1960s and installed in the 1970s. The initial designs of the composite insulators had a rod coated with silicone rubber and shed that were separately mounted on them. These designs failed in short periods while they were put into the service. It was because of the slipping of sheds from rod or opening damage of rod shed interface. It was then followed by designs improved by mounting shed on the rod directly and then encapsulating it with silicone rubber. However, these designs also failed shortly. It was found that the mechanical failures were the main cause of the failures. With the advancement of modeling and fabrication techniques, new designs were introduced that have a rod covered with silicone rubber having sheds in it s own mould as one unit. This whole outer portion was called sheath. This modern structure proved to be very successful and is implemented till today. Also the composite insulators have not yet attained the same level of experience or

6 6 standardization as glass or porcelain, and their weaknesses were still being discovered Physical Description of Composite Insulators Figure 1.1 shows the physical description of composite insulator. Composite insulators have three main components namely, a fiberglass core, end fitting hardware connected to the core, and a protective housing with weather sheds for the core. The fiberglass rod is the most important part of a composite insulator because it is the mechanical load bearing part. The hardware allows the insulator to be attached to an overhead tower and to an energized conductor. The end attachment hardware is typically forged steel, ductile cast iron, malleable iron or aluminum and is selected for mechanical strength. The core by itself is inappropriate for use outdoors, since moisture and voltage cause electrical tracking on the core and result in mechanical failure. Therefore, housing is mounded onto the core, protecting the core from electrical tracking and the design of the housing provides the electrical strength under wet and polluted conditions. The materials for housing (sheds and sheath) are required to have excellent aging resistance under multiple environmental stresses. The possible materials of the weather sheds include epoxy resins, Ethylene-Propylene Diene Monomer (EPDM), Ethylene- Propylene Rubber (EPR) and licone Rubber (SIR). The SIR is preferred due to its characteristic hydrophobicity and its good performance in polluted environments.

7 7 Figure 1.1 Cross Section Diagram of Composite Insulators Common Failure Modes of Insulating Materials The insulating materials of various types of insulators are subjected to failure when they are used in outdoor insulation applications. Some of the modes are discussed below Mechanical failure Porcelain and Glass insulators are mostly affected in this type of failure due to shattering. Shattering is caused by impurities gathered up during the manufacturing process. These impurities cause stress concentrations which can result in the insulator shattering suddenly. The shattering or breaking of the insulators also takes place due to abrupt temperature changes or uneven heating due to power arcs Thermal runway The electrical conduction through glass insulators is mainly accomplished by ionic conduction. The ionic migration within the material can be aggravated by applying the voltage, depending on the resistivity and temperature of the insulating materials. The temperature of the insulating materials are increased when the electrical current flowing through them and

8 8 the resistivity of the materials are decreased. This decreased resistivity causes the increased electrical current flow and temperature. If this process can repeat itself, until the thermal capacity of the material reached, will lead to the failure of the insulating materials Electrical punctures These types of failures mostly take place due to the steep front electrical pulses, which are caused by the external processes of lightning strikes or switching rather than the normal system operating voltage. The porcelain and resin type insulators are mostly affected in this type of failures. 1.3 SERVICE ENVIRNMENTAL F PLYMERIC INSULATRS The polymeric insulators are very effectively used in outdoor services. The outdoor service environment consists of moisture in the form of rain, fog, dew, direct spray, pollutants from the sea and roads that are salted during winter months in cold climates, and chemicals from industry. The polymeric insulators are very much suitable to be used in the above environmental conditions. These insulators have also been used in coastal areas, industrial areas, desert areas and in regions with a high incidence of vandalism. The reduced number of metallic parts in this kind of insulator makes it suitable for saline areas and the corrosion of the hardware is a significant issue Advantages of Polymeric Insulators The polymeric insulators have many advantages when compared with the conventional insulators. Some of the important advantages are listed below:

9 9 90% weight reduction Reduced breakage Lower installation costs Improved resistance to vandalism Improved handling of shock loads Improved power frequency insulation Improved contamination performance Figure 1.2 shows the exact reasons for the polymeric insulators used as outdoor insulators when compared with conventional insulators. It is clear that the polymeric insulators are the right choice to be used in the outdoor insulators. Eventhough the polymeric insulators have many advantages, the housing material of the insulators are subjected to ultraviolet radiation, temperature extremes, overvoltage due to switching and lightning surges, and mechanical loads due to heavy wind flow and ice coating. In order to overcome the above effects, the silicone rubber (SIR) is used as an insulating material of the polymeric insulators. The reason for the SIR material used in the polymeric insulators is discussed in Section 1.5.

10 10 Figure 1.2 Reasons for the Polymeric Insulator used for utdoor Insulators (Elizabeth da lva, University of Manchester 2008) 1.4 FACTRS AFFECTING THE PLYMERIC INSULATRS Factors affecting the polymeric insulators are categorized by mechanical factors, environmental conditions, power system design and operation. The mechanical factor includes polymer base quality, core quality, and end fitting gap attachment method and damage during installation. The environmental condition includes Ultraviolet radiation (UV), wind, rain, fog, ozone, temperature, pressure, humidity, and organic or inorganic pollution. Power system design and operation includes electric field stress and leakage current. Due to the above factors, the polymeric insulator surface can be degraded. The degradation of insulator is called aging. Due to the above factors, the electric field stress and environmental pollution become very important factors to be investigated thoroughly.

11 Factors Affecting Polymeric Insulator Material Aging Aging of insulators describe the deterioration of the insulator material over time or it is the effect produced on it in field after a specified period of service. Aging of insulators is mainly concerned with deterioration of outer sheath/shed. Deterioration of insulator material is concerned with the breakdown of macromolecules causing reduction in molecular weight. This slow break down of the material is induced by the external factors. It starts at the insulator surface and then proceeds deeper into the material. This aging decreases the electrical and mechanical performance of the insulator. utdoor weathering is a natural phenomenon which ages the insulator materials to some extent. The following are some of the major factors responsible for the aging of insulating materials: Pollution It is a broad term used for any particle that may accumulate on the surface of the insulator. There are two types of pollution Pre- deposited pollution It accumulates over a long period of time. It can be either active, which means that it can form a conducting electrolyte when wetted. Sources for this type of pollution include salt, chemical water products, bird droppings and many more. Instantaneous pollution It occurs when the insulator surface becomes covered in a highly conductive pollutant. Sources are salt fog, acid fog, or bird streams.

12 Humidity and rain Humidity is one of the pollutions to form the conducting electrolyte on the insulator surface. nce it occurs, leakage currents can flow along the length of the insulators, leading to dry band formation and electrical leakage current discharges take place. This can damage the insulating materials of the insulators. The humidity is normally drawn from the surrounding air or from the rain and fog. Rain is not the only a source of humidity. The acid rain can increase the pollution layer s conductivity and cause the tracking and erosion of the surface of the insulator material. Erosion or degradation is irreversible and the non-conduction of the surface of the insulator occurs by a major loss of material (that is more than 1 mm). It significantly reduces the thickness of the polymer sheath that ingresses to the core rod. It is slower from material loss and normally does not lead to failure unless it is so severe that it reaches up to the rod. Tracking or carbonizing is caused by leakage current activity. This is an irreversible deterioration by the formation of paths starting and developing on the surface of insulators. These tracks have the appearance of carbon tracks which cannot be easily removed and are conductive even under dry conditions. Figure 1.3 shows the tracking and erosion effects that have taken place on the polymeric insulator surface. Figure 1.3 Tracking and Erosion Effects on the Polymeric Insulator Surface

13 Solar radiation It is the one of the major factors responsible for degradation of insulator materials. The energy from the sunlight destructive to insulator materials are between 320 and 270 nm. This destructive energy constitutes less than five percent of the total radiation reaching the surface of the planet. The absorption of this UV radiation results in mechanical and chemical degradation in the insulating materials of the insulators. The rate at which the degradation occurs depends on the intensity and wavelength of the radiation. These factors vary with season and elevation. The effects of solar radiation include: chalking, crazing or cracking on the surface. Also the UV-B photons can cause in some degradation insulator materials. Chalking is the appearance of a rough and whitish powdery surface giving the insulator a chalky appearance. The factors which are responsible for chalking are UV radiation. When a small quantity of rubber is removed from a surface because of these factors, the filler material is exposed on the surface. This filler material is a white powdery substance, giving the insulator a chalky appearance. It allows more accumulation of water and contamination on the surface of the insulator which leads to degradation of the surface. Crazing is the appearance of shallow cracks on the insulator surface. Depth of these micro fractures is less than 0.1 mm. The reason is UV radiation and electrical stress. Figure 1.4 illustrates the chalking and crazing effects on the polymeric insulator surface.

14 14 Figure 1.4 Chalking and Crazing Effects on the licone Rubber Insulator Surface Bird droppings and streams Nitrogen content is affluent in bird droppings. In the presence of water, this may lead to the formation of nitric acids which can damage the insulator surface material. Moreover, it is highly conductive in nature and it increases the leakage current discharges. Bird streams, which are long strings of bird excrement with high conductivity can lead to immediate flashover Damage through vandalism and animals Eventhough these factors do not actively cause aging, the damage caused by animals and /or vandalism can create areas on the surface of the insulator which deteriorates at a faster rate than the undamaged material and thus promote aging of the insulator. Birds, rodents and termites are usually responsible for animal damage to silicone rubber insulators. Figure 1.5 shows

15 15 the sources of damage and damaged silicone rubber insulators. The bird attacks eat part of the insulator and render it useless. Figure 1.5 Sources of Damage and Damaged Polymeric Insulator 1.5 HUSING MATERIAL FR PLYMERIC INSULATRS The behavior of a polymeric insulator is closely linked with the properties of the housing material, as its lifetime depends mainly on the persistency of the housing. As suggested by Ansorge et al (2010), the most important property of the housing material is its resistance against tracking and erosion followed by the hydrophobicity. In the world wide development of polymeric insulators, a lot of housing materials are tested, such as epoxy resin, ethylene propylene rubber (EPR) and silicone rubber. However, it turned out quite soon that for mechanical reasons the housing must be rubberlike. So the most common material to be used nowadays for polymeric insulators is silicone rubber. licone rubber is not a unique material, but consists basically of a base polymer, inorganic fillers and a cross linking agent. Additionally to their mechanical reinforcing function, fillers that improve the tracking and erosion property might be added. The erosion of the material in the tracking and erosion test was mainly due to the thermal impact of dry band arcing (Kumagai and Yoshimura 2003a). Figure 1.6 shows the benefits of silicone rubber while used as housing material in polymeric insulators for outdoor insulation applications.

16 16 Figure 1.6 Benefits of licone Rubber for utdoor Insulations SIR is the only housing material able to transfer its water-repellent property to a pollution layer on the surface. Therefore, leakage currents are suppressed and the risk of flashover is reduced. Moreover, polymeric insulators with silicone rubber housing do not require cleaning. It has proven its outstanding suitability for outdoor applications for more than 30 years even under severe environmental conditions. 1.6 SILICNE RUBBER CLASSIFICATIN FR HIGH VLTAGE INSULATIN The ASTM D standard denotes the main classes of silicone rubbers. The Q class represents the silicon and oxygen in the polymer chain. In MQ class, M preceding Q indicates that methyl is one of the substituent groups on the polymer chain. In VMQ class, MQ is preceded by V when vinyl side groups are present. Type VMQ is commonly used for insulators. The most common base polymer for the housing in outdoor insulation is VMQ, composed of organic methyl groups (-CH3), vinyl groups ( CH=CH2), and a

17 17 linear silicon-oxygen backbone. Various fillers such as fumed silica, alumina and aluminium hydroxide are added to the base material and mixed, thus forming a compound for injection molding of insulator housings. Figure 1.7 Structure of licone Rubber Molecule licone rubber consists of a polymer fused together with a filler material by a process called vulcanization. licone and oxygen are the backbone of the polymer, which is bonded in an alternating pattern to form either long molecular chains. The length of the chain and the organic groups attached to the silicon atoms will decide the viscosity of the compound. Figure1.7 shows the structure of a typical silicone rubber molecule. It illustrates a silicone oxygen (-) backbone with two methyl groups ( ) attached to the silicone. The use of silicone as a base molecule (in conjunction with oxygen) offers many advantages. The advantages are: 1.The silicone oxygen bond is very strong, which offers thermal stability for the final molecule over a wide range of temperatures. Also it offers resistance to weathering, corona discharge, and oxidation by ozone. 2. The resistance to natural attack, good dielectric properties, fire resistance and it being a relatively harmless substance.

18 CHEMICAL REACTINS F SILICNE RUBBER MATERIALS DURING DRY BAND ARCING The discharges induce changes in the insulation material through chemical reactions occurring at the locations of the electrical discharges. The breaking or dissociation of the bond between the molecules requires sufficient amount of energy. Accordingly, such processes usually occur in the insulation material at the location of the dry band arcing, due to frequency of high temperature observed areas. However, heat can also be transmitted along the surface and through the bulk of the material to regions adjacent to the locations of electrical discharge activity. This results in a gradual heating of these areas until the accumulated heat energy is large enough to cause gradual chemical changes in these regions as well. The chemical reaction taking place in the insulating material fully depends on the relative bond strengths of the molecules presents in the insulating materials. Bond dissociation energy is one of the measures of the bond strength which is the energy required to break the bonds for one mole of a given molecule in its gaseous form. When the heat energy is being supplied to the insulating material, those molecules showing the lowest bond strength are separated first. The - bond has a high bond energy, which offers the high thermal resistance of the silicone rubber materials. The -C and H-C bonds show weaker bond strengths. The chemical changes in silicone rubber thus usually occur in the methyl groups rather than in the silicone backbone. The types of elements present in the insulating material and the surrounding condition decide the type of chemical reaction that takes place. The chemical changes in a silicone rubber is generally caused by the following three processes 1. Scission and interchange of either bonds or chains. 2. Hydrolysis of siloxane bonds and hydrocarbon groups

19 19 3. xidation of hydrocarbon groups and crosslinking of siloxane bonds In the scission process, the heat energy liberated by the arcing process causes scissions in either the methyl groups or even silicone backbone and it creates free radicals of form,, or with the dot designating the atom as a free radical. These free radicals are molecular fragments with a reactive nature, due to the free radicals one or more unpaired electrons in the outer electron cell. Because of their relative nature, they usually exist only in a transitional state before combining with other molecules to form new substances. Figure 1.8 shows the formation of the free radical from the silicone rubber structure. heat from dry band arcing + heat from dry band arcing + long chain backbone Figure 1.8 Scission of licone Rubber Chains and Formation of Free Radicals(Heger 2009) After the formation of the free radical, interchange reactions can occur. This means that two neighbored chains combine after having been broken into smaller chains, with the shorter chains fusing with their counterparts in the neighboring chain according to the type of radicals formed. This process is shown in Figure 1.9

20 20 S + S heat from dry band arcing S S S + S after chain scission S Short interchanging polymer backbones S Figure 1.9 Interchanging Reactions between Two Split Polymer Chains(Heger 2009) The hydrolysis processes usually take place in the presence of water (H 2 ), like rain or fog, deposited on the material surfaces. If such a condition exists, hydrolysis of the silicone rubber structure will occur due to the water being split into H and H groups through electrical discharge processes. Hydrolysis reaction can be responsible for a larger mass loss due to random scission occurring in the polymer chains. The H and H groups formed during the disassociation of water combine with the free radicals in the scission process to form chains with silanol (H) as side and end groups with possible liberation of methane gas (CH 4 ) depending on the free radicals present. Figure 1.10 shows typical hydrolysis reactions in the presence of water, indicating the formation of gaseous (CH 4 ) compound. S S + H 2 heat from dry band arcing S H S + CH 4 S + S + H 2 heat from dry band arcing S H + H S Hydrolytic scission Figure 1.10 Hydrolytic Reactions in the Presence of Water(Heger 2009)

21 21 Finally, the oxidation process follows the hydrolysis of the siloxane bonds and hydrocarbon groups. The crosslinking process can link two parallel polymer chains, create a branching from one polymer chain to several others or form several shorter polymer chains. All these different processes are presented in Figure However, the hydrolysis reactions do not occur on materials in well ventilated surroundings due to the low water vapor content in the atmosphere at the material s surface. Instead, further crosslinking reactions occur between the silicone rubber chains through oxidation of the methyl groups, which cause the material to become brittle if exposed to extended periods of high temperatures. Due to the dry band arcing process, the chemical changes on the silicone rubber surface take place when the surface temperature is raised to a level higher than 400 ºC. nce tracking and erosion in silicone rubber materials have only been observed to occur if the hotspots of a temperature above are 400 ºC. H H H H 3 heat from dry band arcing Crosslinking between polymer chains + H 2 H H H + H heat from dry band arcing H+H + 2H 2 branch formation H + H H + H heat from dry band arcing + 2H 2 formation of short chains Figure 1.11 Possible Forms of Crosslinking between Polymer Chains(Heger 2009)

22 RLE F FILLERS IN SILICNE RUBBER INSULATIN Inorganic fillers are essential in the formulation of SIR outdoor insulation. Their inclusion improves tracking and erosion degradation resistance. The desirable and undesirable are the types of effects due to the fillers in silicone rubber. Some of the desirable effects of fillers are as follows: a) Improved thermal conductivity of the compound, so that the heat dissipation is considerably improving in the composites. b) Reduced organic material exposure to heat from dry band arcing, thus decreasing the weight loss of the compound subsequent to aging. Considering the undesirable effect, the fillers act as a diffusion barrier for the Low Molecular Weight (LMW) fluid and slow down the recovery process. If the filler content is increased then the recovery process will be faster. Thus the quantity and type of filler to be included in the formulation is a critical task. The micro fillers are used to improve the physical properties of silicone compositions through molecular bonding with the silicone polymer. The micro silica filled SIR has been studied extensively in outdoor insulation applications. 1.9 INTRDUCTIN T NAN FILLERS The SIR as the base material with the addition of micro fillers is used in the manufacturing of outdoor insulation. Many researchers have been done in relation to the use of micro fillers in SIR. The fillers used in these materials are micro-sized, with a particle size of 5-10 µm. Currently, the industries are using 30% to 65%, by weight (hereafter referred to as t % wt), of micro fillers to achieve the required electrical properties for outdoor insulation applications. The nano fillers of nano technology have been introduced to enhance the electrical and mechanical properties of the insulating materials. The great advantage of the nano sized fillers is that they have larger specific surface area compared to that of the micro sized fillers. The particle sizes between 1 and 100 nm are used in the field of electrical

23 23 insulating materials (Tanaka 2005). The use of nano particles in the matrix of polymeric material can improve the mechanical and electrical properties of polymeric composites. The various studies have been reported that the performance of nano and micro sized particle filled silicone rubber and these studies are discussed in detail the literature review section LITERATURE REVIEW Gorur et al (1992) had presented experimentally, the aging produced by dry band arcing in silicone rubber material. Aging indicated by permanent changes in the silicone rubber surfaces was recognized using analytical techniques of FTIR, EDX, X-ray Diffraction and surface roughness measurement. In this work, they have reported that the permanent changes occurring in the material lead to progressive degradation in the long run even though there can be a complete recovery of surface hydrophobicity in short time. Guoxiang Xu et al (1996) had presented the climatic aging experiments conducted on silicone rubber and ethylene propylene diene monomer (EPDM) outdoor polymer insulators by using a programmable weather-ometer. The accelerated aging stresses comprised of ultraviolet radiations, elevated temperature, temperature cycling, thermal shock and high humidity. The effects of aging on the insulator surface conditions and electrical performance were examined through visual inspection and SEM studies, contact angle measurements, TGA and EDAX analysis. It was found that a significant degradation on the EPDM insulator surface was caused by aging. The work also reported that the silicone rubber have superior resistance to climatic aging stresses over the EPDM. Gorur et al (1997) had conducted the laboratory test to evaluate and compare the tracking and erosion resistance of High Voltage (HV) outdoor polymeric insulating materials. The test was based on combining some features of the ASTM D-2132 Dust and Fog test, and ASTM D-2303 Inclined Plane test. The experimental conditions

24 24 employed could be related to actual field conditions and hence the results obtained provided a more realistic evaluation of materials tracking and erosion performance. The developed test method was capable of ranking materials in a timely manner, and also demonstrates the wide variations in the tracking and erosion resistance among different material families and formulations within the same family. It was found that the magnitude and harmonic content of the leakage current and discharge duration were significantly different during the portion of the test when there was no visible degradation, compared to their values at the onset of visible degradation. Torbjorn et al (1998) had presented two silicone rubber based polymeric materials for outdoor insulation under field conditions for about nine years and also evaluated them in the laboratory. The field test was conducted in an environment of light coastal pollution and the materials were non -energized as well as energized with both HVAC and HVDC. The long term study focused on the hydrophobicity and the material changes of the insulator surfaces combined with leakage current performance of the insulators. The results showed that the surface aging of the two types of silicone rubbers was relatively moderate but the chemical changes appeared to be greater in the HVDC exposed insulators. The laboratory studies revealed that the different insulators had dissimilar leakage current under clean fog conditions and this indicates that the hydrophobicity was not a good indicator of the surface resistance of the insulator material. Homma et al (1998) had used the thermo gravimetric analysis to evaluate the surface degradation of polymer insulators used in outdoor high voltage applications. The TGA measurements of silicone samples with various amount of ATH filler was performed with two different atmospheres. It was found that the decomposition of siloxane matrix was affected by oxygen and promoted chemical reactions on the surface.

25 25 Xinsheng Wang and Noboru Yoshimura (1999) had studied the tracking resistance of high temperature vulcanized silicone rubber under various types of precipitation. It was evaluated by the incline-plane method, using artificial rain water as a contaminant. The influences of ion concentration, acidity, and conductivity of rainwater on the energized silicone rubber material were studied. Experimental results were reported that the degradation of the energized materials increased with ion concentration of the rainwater, the discharge current and increase in erosion of material. The conductive ions in rainwater could induce more tracking and discharge on the energized material surface, and acidic rain water could accelerate the electrolyte process in the materials and dissolve the inorganic filler in silicone rubber. Although the silicone rubber degrades under the process of accelerated aging using artificial rainwater, the concentration, acidity and conductivity of actual precipitation was insufficient to exert a significant degradation effect on silicone rubber. It was found that the silicone rubber can resist the erosion of even the most corrosive water and does not fail and in addition, has a strong resistance to various types of precipitation. Moreno and Gorur (1999) had experimentally investigated the performance of several polymer outdoor insulator formulations under alternating current (AC) and direct current (DC) stresses. The performance of the polymers was evaluated in terms of their charge accumulation and decay characteristics as a function of humidity, and their tracking and erosion resistance. They obtained results which showed that the significant differences exist in surface charging phenomena between AC and DC. It was also proved that the DC polarization processes led to hydrophobicity loss when the materials were subjected to pollution and DC electrical stresses. In addition, it was found that the substantial reduction in the tracking and erosion resistance of the polymeric materials with DC stresses compared to alternation current (AC) stresses. Furthermore, the poorer electrical

26 26 performance of the materials was found due to higher magnitudes and longer durations of the dc voltage discharge current. Kumagai and Yoshimura (1999) had studied the single and multiple effects of UV, corona, thermal, water absorption and acid rain stresses on the tracking and erosion characteristics of room temperature vulcanized silicone rubber (RTV). The test results showed that corona stress and water absorption stress had decreased the tracking and erosion resistance of RTV, while thermal stress and UV stress improved it. Also, the RTV subjected to simultaneous multiple stresses was evaluated and the obtained results showed significant variations in the tracking and erosion resistance. The chemical and morphological analysis for assessing the aging level was carried out by SEM-EDX, ATR-FTIR and Differential Scanning Calorimetry (DSC). It was found that the DSC played an important role to detect boiling and combustion temperatures of byproducts affecting tracking and erosion. Li Xuguang et al (2000) had investigated the influence of alumina trihydrate (ATH) and magnesium hydroxide on the tracking and erosion resistance of silicon rubber. The research showed that the tracking and erosion resistance of silicon rubber was improved with the increase of the filler concentration of ATH and magnesium hydroxide, but adding excessive filler had decreased the performance of silicon rubber. The experiment results also showed that the tracking and erosion resistance of compound hydroxide filler was superior to single hydroxide filler. It was found that the effect of hydroxide filler level played an important role on vulcanization characteristics of silicon rubber. Also, the experimentation indicated that lack or over vulcanization that decreased the tracking and erosion resistance capability of silicon rubber. Sarathi and Uma Maheshwar Rao (2001) explored the tracking phenomena study with ethylene propylene diene monomer (EPDM) material under AC and DC. It was reported that the tracking time depended on the conductivity and flow rate of the contaminant. They also carried out the

27 27 physico-chemical analyses using the wide angle X-ray diffraction (WAXD), thermo-gravimetric differential thermal analysis (TG-DTA) and the differential scanning calorimetry (DSC) studies. Finally, they confirmed that the tracking process was a surface degradation process and the tracking time was different for AC. and DC voltages. Sarathi et al (2002) explored the tracking phenomenon for silicone rubber material under AC and DC voltages with ammonium chloride as the contaminant. They made a detailed discussion on the tracking phenomenon with leakage current measurement. The physicochemical analysis was also carried out using the thermo gravimetric differential thermal analysis and differential scanning calorimetry. They concluded that the tracing was due to surface degradation and the tracking time was different for AC and DC voltages. The tracking time depended on the conductivity and flow rate of the contaminant further reported. Meyer et al (2002) explored the work of thermal behavior of silicone rubber filled with alumina trihydrate (ATH) and silica at different filler concentrations. Three particle sizes of 1.5µm, 5µm, and 10µm were employed in sample preparation. The infra-red laser was used to heat the samples and they found that the concentration of the filler played a major role on improving the thermal conductivity when compared to the particle size of the filler. mranipour et al (2002) studied the effect of filler particle size and concentration on the tracking and erosion resistance of silicone rubber loaded with silica and alumina tri-hydrate as fillers. The samples with different filler particle size and concentrations were tested in an inclined plane tracking and erosion resistance test apparatus. They explored that the loss of material was linked with the power dissipated by the fundamental and harmonic components, due do dry band arcing. Also their results showed that the smaller particle sizes and higher percentages of fillers, up to 50%, improved the tracking and erosion resistance of silicone rubber. Meyer et al (2002) investigated the influence of particle size and concentration of fillers used in

28 28 silicone rubber compounds for outdoor insulation. The alumina tri-hydrate and silica with mean particle sizes of 5 and 10 pm were used as fillers with concentrations of 10, 30 and 50 % by weight in a two-part room temperature vulcanized silicone rubber base polymer. The obtained results from inclined plane tests showed that the samples with high concentrations of filler and/or smaller particle size have better tracking and erosion resistance than samples with lower concentration and/or larger particle size. Afendi et al (2003) described the design and development of a leakage current monitoring system associated with surface tracking and erosion resistance test set-up of IEC 587. LabVIEW software package was used to develop a measurement program for recording and analysing leakage current signals. Experimental works have shown the capability of the developed system in detecting the performance of insulating materials as well as identifying the characteristics of the surface discharges. El Hag et al (2004) had studied the physicochemical properties of silica filled silicone rubber nano composites. They have discussed the erosion resistance of silicone composites with silica fillers with laser based method, used as a source of heat to treat the filled and unfilled silicone rubber. The thermal and chemical bonding behaviors have been analyzed using the atomic force microscopy (AFM), thermo-gravimetric analysis (TGA) and infrared microscopy (FTIR). It was concluded that the physical and chemical properties significantly improved the nano filled SIR than the unfilled SIR. El.Hag et al (2004) had studied the influence of nano size and micro size fillers in silicone rubber using the inclined plane tracking and erosion test. The low frequency components of leakage current and eroded volume were used to evaluate the performance of the composites. They found that the fundamental component of leakage current did not correlate with the erosion and the third harmonic component of the leakage current showed good correlation to the erosion in terms of volume. It was reported that 10% weight

29 29 of the nano size filler in silicone rubber had given a performance that was similar to that obtained with 50% by weight of micro size filler. Dengke et al (2004) had reported that the addition of a small amount (2 to 5 %wt) of inorganic nano fillers to polymers should be sufficient for mechanical and thermal stability and performance improvement. Although the hydrophobicity of all the composites decreased after corona aging and the hydrophobicity was recovered after a few hours. Meyer et al (2004) had investigated the effect of filler particle size and concentration on the tracking and erosion resistance of silicone rubber loaded with silica and alumina tri-hydrate as fillers. The filler particle size and concentrations were tested with the inclined plane tracking and erosion resistance test apparatus. It was found that the loss of material is linked with the power dissipated by the fundamental and harmonic components due to dry band arcing. They also reported that the higher percentages of fillers improved the tracking and erosion resistance of silicone rubber. Meyer et al (2004) had reported how alumina tri-hydrate and silica fillers improved the erosion resistance of silicone rubber during dry band arcing. It was found that the thermal conductivity of the composite material is dependent on the concentration, particle size, and bonding of the filler particles to the silicone matrix. It was also found that the ATH filler imported better erosion resistance than silica in silicone rubber. Rajini et al (2004) had investigated the surface tracking phenomena in different polymeric insulating materials such as SIR, Ethylene-Propylene Diene Monomer (EPDM), and High density polyethylene (HDPE). The tracking test was conducted as per IEC (587) standard under AC and DC voltages. It was found that the silicone rubber tracking performance was superior compared with other materials. Ratzke et al (2005) had studied how nano fillers and micro fillers in an HTV (High Temperature Vulcanizing) silicone elastomer affect the resistance to arcing. In this work the best dispersion was obtained for nano silica. n the

30 30 other hand, large agglomerates were found to be formed by nano alumina. The results of the arcing tests demonstrated longer test time duration with increased filler concentrations of silica and alumina. The authors found that the thermal conductivity increased in an approximately linear fashion with the filler concentration. Enhanced resistance to arcing with nano silica was achieved only at a high concentration of filler, approximately40% wt. It was also found that the strong interfacial bonding and small inter-filler spacing of the nano dielectrics restrict material degradation. Chernery et al (2005) had studied the inorganic fillers used in silicone rubber to enhance the properties of thermal conductivity, relative permittivity, and electrical conductivity for outdoor insulation application. They used barium titanate fillers to contribute for increasing the relative permittivity and the fillers with antimony doped zinc oxide contributed towards electrical conductivity and preventing the partial discharge and corona discharge on the surface of the insulations. They also used silica and aluminium trihydrate to improve the thermal conductivity of the composites and increased the resistance to erosion due to heat produced by dry band arcing. It was reported that the relative permittivity of the composites predicted with greater accuracy if the relative permittivity of the individual filler particles was known and the predicted thermal conductivity should be possible if the gas layer is thin between the filler and polymer matrix. Roy et al (2005) had studied the voltage endurance behavior of cross-linked polyethylene (XLPE) significantly improved with the inclusion of treated nano particles (aminosilane- treated nano silica, vinylsilane-treated nano silica). It was found that all the nano scale fillers had significantly improved breakdown strength and endurance over the base resin. Yong Zhu et al (2005) had explored experimentally to examine the suppression effect of alumina trihydrate (ATH) filler on the erosion of filled SIR exposed dry band arc discharge. In this study, one simulated electrolyte electrode system was

31 31 used to generate dry band arc discharge and eroded mass of SIR filled with 0-50% wt ATH was measured. The TG-DTA was applied to study the thermal characteristics of filled SIR and clarify the suppression mechanism of ATH on the heat erosion. The physicochemical analyses on the degraded specimens were carried out by using FTIR and SEM. They demonstrated that the erosion of filled SIR due to dry band discharge can be well suppressed with the increase of ATH filler content. Ehsani et al (2005) reported that the experimental investigation concerned with electrical and surface properties of a silicone modified polymer in comparison with silicone rubber ethylenepropylene-dine monomer and alloy of SIR-EPDM. The loss of hydrophobocity of polymeric materials induced by ultraviolet radiation (UV), saltspary and water salinity aging was examined in this work. The ATR-FTR was used to study the surface degradation of polymers occurring during UV aging. Tracking and erosion induced by high electrical stress reduced the lifetime of polymeric materials. The results of standard tests showed that the SIR suffered from deterioration of tracking resistance caused by the loss of hydrophobicity from the action of water salinity stresses. The silicone modified polymer showed good hydrobobicity behavior in environmental conditions and excellent tracking and erosion resistance compared to SIR, EPDM and alloy of SIR-EPDM. Chandrasekar et al (2006) investigated the tracking phenomenon in silicone rubber material under AC and DC voltage. The leakage current during the tracking studies was measured and analysed by using moving average technique. They reported that the tracking was more severe and the tracking time was less under DC voltage. They also concluded that the leakage current magnitude was high in thermally aged specimens compared to the virgin sample. They extended their research to measure the surface condition of the insulation materials with water aged specimen and the diffusion co efficient of the material was calculated. The reduction in contact