Insulating Covers. Insulators: EPBI: Light Weight Polymeric Post Insulator. EPBI: Light Weight Polymeric Stand Off Insulator

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1 Exit Catalogue Return to Contents Energy Division Section 6: Insulators & Insulating Covers. Insulators: EPBI: Light Weight Polymeric Post Insulator EPBI: Light Weight Polymeric Stand Off Insulator EPCI: Light Weight Polymeric Tension Insulator An Overview of Porcelain versus Polymeric Insulators Introduction to Silicon Elastomer Insulators Selection of Polymeric Insulators - Material Considerations Insulating Covers: BCAC: Bushing Connection Animal Cover - up to 35kV BCIC: Raysulate Bird Protection Cover - up to 24kV BISG: Bus Insulator Squirrel ( Possum ) Guard MVLC: Medium Voltage Line Cover - up to 24kV OLIC: Overhead Line Insulating Cover - up to 24kV OLIT: Overhead Line Insulating Tape - up to 24kV Creepage Extenders: HVCE: High Voltage Creepage Extenders - up to 66kV

2 R EPBI Insulator for outdoor equipment These lightweight polymeric insulators are ideal for outdoor equipment applications such as overhead line switches and fuse cut outs. They provide equipment manufacturers and utilities with the benefit of a very lightweight and shatter proof construction without the need to compromise on reliability or longterm performance. The insulators combine the advantages of a mechanically strong and lightweight polymeric core with the Raychem HV material used in the outer insulator profile. The core contains no glass fibres and is therefore proof against wicking problems. The outer high voltage material has proven non tracking, UV stable properties backed up by 20 years field experience of HV polymers in widely varying climatic conditions. Features Engineering polymer core with no glass fibres Proven HV materials, backed up by 20 years successful field performance throughout the world Proven interfacial seal system Stainless steel end caps Benefits Lightweight easy installation, easy erection and reduced transport costs for equipment Shatterproof breakages eliminated during delivery and erection Vandal resistant Long term reliability. High resistance to water ingress High corrosion resistance

3 EPBI Insulator for outdoor equipment Technical Specification Electrical Impulse +ve (kv) Impulse -ve (kv) Creepage (mm) Dry A.C 50Hz (kv) Wet A.C 50Hz (kv) C D1 D2 D3 S L Mechanical Max loadf (N) Max cantilever (Nm) Tensile (M16 pull out) (KN) Torque withstand (Nm)* Max M16 bolt torque (Nm) Dimensions Length L (mm) Diameter D1 (mm) Diameter D2 (mm) Diameter D3 (mm) M16 bolt depth S (mm) ø 6 mm pin spacing C (mm) * limited by end fittings + Intermittent loading using H.T bolts Further details: Ref UVR 5166 Ordering Information Description U.O.M. Weight (g) EPBI-0210/07-048/01 1 Insulator 1400 EPBI-0345/11-056/01** 1 Insulator 2400 **The EPBI-0345/11-056/01 will be delivered with M16 bolts which have to be removed before installation Vandal Resistance 12 bore shot gun; full choke; 10 yards range:- no immediate electrical or mechanical failure. Drop test: 5 m on to concrete: no mechanical failure. Further details including interfacial seal tests: Ref UVR 8150 Applications EPBI-0210/07-048/01 is recommended for applications up to 24kV (line voltage). EPBI-0345/11-056/01 is recommended for applications up to 36kV (line voltage). Both insulators have sufficient creepage to operate in heavily polluted environments up to and including Class 3 IEC 815. EPBI insulators are designed for applications with high intermittent loadings such as overhead line switches and fuse cut outs. Installation The end fittings are designed to be compatible with IEC 273 dimensions and require high tensile M16 bolts and 6 mm diameter anti-rotation pins. Bolts should be tightened to a maximum torque of 50Nm. Wedge technology products Electrical connectors Cable accessories Asset protection Surge arresters Insulators Fittings Associated toolings All the above information, including drawings, illustrations and graphic displays, reflects our present understanding and is to the best of our knowledge and belief correct and reliable. It does, however, under no circumstance constitute an assurance of any particular qualities. Such an assurance is only provided in the context of our product specifications. Our liability for this product is set forth in our standard terms and conditions of sale. AMP, ELCON, ELO TOUCHSYSTEMS, HTS, MACOM, MADISON, NETCONNECT, RAYCHEM, SIMEL are trademarks of Tyco International Ltd. Members of the Tyco Electronics Corporation: Raychem EPP /00 Tyco Electronics Raychem GmbH Energy Division Haidgraben Ottobrunn/Munich Germany Tel. (089) Fax (089) Tyco Electronics Corporation Energy Division 8000 Purfoy Road Fuquay-Varina, NC , USA Tel. (800) Fax (800) Tyco Electronics Corporation Energy Division c/o AMP Singapore Pte Ltd No. 26 Ang Mo Kio Industrial Park 2 Singapore Tel Fax

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6 E L E C T R I C A L. P R O D U C T S. D I V I S I O N EPCI 24 kv Tension Insulator The high tensile strength of glass fibre has been combined with a Raychem HV shedded profile, to produce this rugged, lightweight tension insulator for overhead line applications up to 24 kv. The glass fibre core provides high mechanical strength with tensile values of greater than 70 kn. The Raychem insul ator profile utilises the same materials technology that has been employed for over 25 years in Raychem s high voltage terminations. Its proven track and erosion resistance and UV stability have given outstanding performance in the widest possible range of climatic and pollution conditions. The construction consists of Raychem s compact creepage design insulator profile which has alternating large and small diameter sheds to optimise the pollution flashover performance. It is sealed to a glass fibre rod with track resistant viscoelastic mastic. The mastic remains mobile at service temperatures and ensures that an effective moisture barrier is constantly maintained. The hot dip galvanised steel end fittings are crimped onto the glassfibre core providing high strength corrosion resistant fixing points. Features High strength glass fibre core Raychem s proven HV polymer technology with 25 years of successful field performance Proven interfacial seal system Hot dip galvanised steel end fittings Tested to IEC 1109 polymeric insulator specification Benefits Lightweight easy installation, easy erection and reduced transport costs Shatterproof breakages eliminated during delivery and erection Vandal resistant Long term reliability. High resistance to moisture ingress High corrosion resistance mastic

7 EPCI 24 kv Tension Insulator Technical Specification ø71 ø Dimensions in millimetres Electrical (tested according to IEC 383) Impulse withstand (positive) 158 kv Impulse withstand (negative) 190 kv Creepage (nominal) 600 mm Dry withstand A.C 50Hz 91 kv Wet withstand A.C 50Hz (horizontal) 75 kv (vertical) 60 kv Mechanical Specified Mechanical Load (S.M.L.) Routine Test Load (R.T.L.) Weight 70 kn 50 kn 1300 g Applications The EPCI-0380/06-016/EE insulator is recommended for vertical or horizontal applications at voltages up to 24 kv (system voltage). The insulator has sufficient creepage to operate in heavily polluted environments up to and including Class 3 IEC 815. Ordering Information Kit No. U.O.M. Weight (g) EPCI-0380/06-016/EE 1 PC Supplied as 3 pcs PCN: per box At Raychem we are committed to continuous quality improvement in every aspect of our business. Raychem Corporation Electrical Products Division Insulator Group 300 Constitution Drive Menlo Park, CA , U.S.A. Tel. (415) Fax (415) All of the above information, including illustrations is believed to be reliable. Users, however, should independently evaluate the suitability of each product for their application. Raychem makes no warranties as to the accuracy or completeness of the information and disclaims any liability regarding its use. Raychem s only obligations are those in the Standard Terms and Conditions of Sale for this product and in no case will Raychem be liable for any incidental, indirect, or consequential damages arising from the sale, resale, use or misuse of the product. Raychem Specifications are subject to change without notice. In addition, Raychem reserves the right to make changes in materials or processing, without notification to the Buyer, which do not affect compliance with any applicable specification. Raychem and RayBowl are trademarks of Raychem Corporation. Raychem Corporation Electrical Products Division 8000 Purfoy Rd. Fuquay-Varina, NC , U.S.A. Tel. (800) Fax (800) Raychem Ltd. Electrical Products Division Faraday Road Dorcan, Swindon Wiltshire SN3 5HH, U.K. Tel. (07193) Fax (07193) Raychem GmbH Electrical Products Division Haidgraben Ottobrunn Munich, Germany Tel. (089) Fax (089) Raychem EPP /97

8 E L E C T R I C A L. P R O D U C T S. D I V I S I O N An Overview of Porcelain and Polymer Electrical Insulation

9 Introduction Porcelain as an insulating material has over one century of service history, while polymer materials have three decades. Early generation polymer products did not provide the expected service life, and users still have concerns about polymer insulating material performance. The chemical stability of porcelain resists aging, however it also allows the surface to easily wet, which can lead to flashover in contaminated locations. Polymer outdoor insulation is organic in nature and it can age with exposure. In a worst case condition, it can result in possible surface damage leading to surface cracking and loss of mechanical integrity. It does, however, initially resist wetting. Adequate compound formulation, combined with appropriate insulator housing design, resist weathering and aging with the benefit of enhanced flashover performance. In this paper, consideration is given to the relative advantages of each material type. Flashover mechanisms and mitigation techniques are also discussed. Porcelain Insulation Experience The vast majority of the installed electrical power system insulation directly exposed to the environment is porcelain, with over 100 years of history. This material has proven itself to resist environmental aging, be self-supporting, and is used in a wide variety of applications. It is also inherently large, bulky and heavy, broken in handling, transit and by vandalism, and subject to flashover in contaminated environments. Despite the many advantages of porcelain, system reliability can still suffer. Advantages Stability The strong ionic bonding and close packing of the atoms which constitute ceramics, such as between silicon and oxygen in silica and silicates, yield structures which tend to be very stable and are not generally degraded by environmental stresses. This means that the ceramic housing should not be damaged by UV, surface electrical activity, humidity, etc. [1] Mechanical strength The rigid nature of the ceramic material imparts significant mechanical strength. Insulators can be fabricated for both tension and cantilever loads. The porcelain housings employed for cable terminations, bushings and surge arresters are self-supporting and do not require other materials or components for strength. Low Raw Material Costs The principal raw materials of porcelain, such as clays, feldspar and quartz, are relatively inexpensive and readily available. Processing and Lead-time The manufacturing process for porcelain involves many steps. For larger housings, long periods of waiting are required to reduce the extruded core water content, prior to shaping and firing. Lead-times tend to be long as a result. Limitations Breakage Ceramics are very brittle. This means that they are easily broken in handling, transit or installation. Vandalism is a primary contributor to in-service mechanical damage. It is common practice to include a breakage or loss factor when purchasing porcelain insulators for line construction, which is an added unit cost factor. Weight The very dense nature of the ceramics means that porcelain bodies are very heavy. As the voltage rating increases, there is a compounding effect. This not only makes for difficult handling which can require cranes, but it also means that expensive and large structural supports are necessary. The large size and weight of porcelain products usually dictates the least expensive and most time consuming means of transport, in order to minimize cost. Thus, shipment by sea or other less costly methods also extend delivery time. Hollow Core Housing Failure Mode Hollow core housings for insulators, bushings, instrument transformers, cable terminations and surge arresters can experience a violent failure mode as a result of an internal dielectric breakdown. When an internal power arc occurs, there is a rapid increase in the pressure. If the pressure cannot be relieved before the bursting strength of the housing is exceeded, the housing can shatter. When this occurs, pieces of porcelain are expelled with considerable force. This type of failure mode is well known

10 Figure 1 15 kv class highly protected creepage insulators with open shed polymer surge arresters and porcelain fuse cutouts supplying a pole transformer in Brazil in a severely contaminated coastal application. The highly protected creepage design of the insulator enhances pollution withstand. The intrinsic stress grading of the arrester provides some assistance against flashover, however, the fuse cutout having relatively little strike and creepage distance is highly susceptible to flashover from the fuse contacts to the grounded mounting bracket during wetting conditions. Figure 2 66 kv insulators energized at 33 kv in a West Australia harbor were frequently washed because of routine flashover from salt deposition from the sea, iron ore dust from the handling equipment for ore loading from rail cars to ships and contamination from an adjacent coal fired generating station. Peak leakage current pulse monitoring of insulators fitted with single creepage extenders showed a significant reduction in leakage current, with additional savings realized from reduced washing expense. within the industry, especially for distribution class surge arresters. Complex Geometry Porcelain housings tend to have relatively low creepage distance per unit length, mm/kv, because of the cost associated with producing weathersheds with significantly larger diameters than the main core section. The shaping process to create the more complex weathershed shapes, to increase creepage distance and/or improve the contamination performance, also adds cost. For medium-voltage use, porcelain housings tend to have a simple weathershed design, and extra creepage is often obtained by using a higher voltage class rated housing. An example is the use of a 35 kv class insulator on 15 or 25 kv class systems. Pole-type transformers may also be specified with higher voltage class bushings, while surge arresters may be assembled with higher voltage class housings with internal spacers to make up for the increased longitudinal length. All of these examples seek to improve the pollution performance, often defined in terms of the mm/kv creepage distance. Pollution Performance The stable chemical bonds of the ceramic material also mean that it has high surface free energy, a property which describes the strength of the surface adhesion of contaminants. [1] With a high surface free energy, porcelain is easily wetted. Water on the surface tends to form filaments which sheet or coat sections of the surface. Materials with such characteristics are known as hydrophilic. Hydrophilic surfaces tend not to perform well under polluted conditions as the water filament dissolves the conductive pollution, lowering the overall surface resistance of the insulation with a conductive electrolyte along a continuous path, which can initiate the flashover process. Polymer Insulation Experience Initial Products Early polymer products did not provide the expected service life, primarily because of inadequate UV and tracking resistance. As a consequence of initial product failures, many users continue to be concerned about the long term performance of polymer materials. More Recent History High performance polymer materials have been in use for about 30 years. [2] During that time, the use of polymer insulation has grown steadily, and polymerics are now becoming the outdoor insulation material of choice. Common applications include cable terminations, surge arresters, insulators, bus bar insulation, and bushings. Figure 1 shows a highly protected creepage insulator installed in a severely contaminated site. This device is a hybrid design utilizing a porcelain core for strength and environmental aging resistance about the terminals. The elastomer housing provides the creepage in a highly protected geometry as well as the weathering resistance. Insulation enhancement products, such as creepage extenders, are installed over the weathersheds of porcelain insulators to improve the performance of porcelain insulators in contaminated applications. [3] Figure 2 shows how polymer material can enhance porcelain insulator performance. Other retrofit techniques are in use which apply polymer material to enhance the surface properties of existing porcelain insulators to reduce flashover in contaminated locations. Not only are polymeric products demonstrating their capabilities in diverse environments, but polymeric devices and materials are routinely used today for contamination flashover of a large installed base of porcelain insulation. Advantages Polymeric insulating materials offer numerous advantages over porcelain. A partial listing includes: Light Weight The density of polymer materials is much lower than ceramic, which results in significant product weight reduction. The weight differences increase with voltage class rating. Polymer devices tend not to require cranes or other lifting devices for handling or installation. The reduced weight also permits the use of lighter and less costly structures and mounting arrangements. The smaller size and weight result in lower shipping costs than equivalent porcelain devices. Polymer insulators are advantageous to use in dense urban areas and offer advantages in narrow rights of way. Non-ceramic insulators offer high strength to weight ratio which permit less expensive structures and improved visual aesthetics. Polymer insulators also facilitate new compact transmission line design with reduced electromagnetic

11 Figure 3 66 kv polymer terminations installed in England in parallel with existing porcelain terminations. Polymer terminations require less structural support because of their light weight. Polymer products are becoming more common installed adjacent to existing porcelain devices on existing systems. Figure 4 24 kv distribution class polymer surge arrester showing polymer material hydrophobicity with water beading on surface. Discontinuous electrolyte increases voltage required for flashover on polymer material surface compared to porcelain. field effects. Polymer distribution surge arresters may be hung directly off fuse cutouts, reducing installation costs and improving aesthetics. Porcelain and polymer 66 kv terminations, installed in parallel, are shown in Figure 3, as evidence of the concurrent use of these two technologies. The polymer termination requires cable support since it is not self-supporting. Complex Geometry As polymer insulating housings are typically molded, it is not difficult to fabricate parts on a cost effective basis, which have higher creepage distance per unit length than porcelain. Weathershed profiles can be made more complex without production or yield problems. Alternating diameter weathersheds (big-small) are now commonly supplied, which improves the AC wet flashover by avoiding bridging of all sheds simultaneously during heavy wetting conditions. Pollution Performance Polymer materials typically used for outdoor insulating applications have low surface free energy. [4] When new and without exposure to the environment, polymer materials resist wetting and are inherently hydrophobic. Retention of hydrophobic properties with exposure is a desirable attribute. Water on the surface of hydrophobic materials form water beads, so the conductive contamination dissolved within the water beads is discontinuous. This condition results in lower leakage current flow and probability of dry band formation, which in turn requires a higher impressed voltage to cause flashover. Figure 4 shows the hydrophobic nature of polymer materials, where water tends to bead rather than form filaments along the surface. In severe conditions, all materials lose their hydrophobicity. Table 1 Subjective Comparison of Porcelain and Polymer Material Properties. Relative rankings assigned. Property Porcelain Polymer Strength Size Weight Breakage Aging Resistance Creepage/unit length - + Pollution Flashover - + However, the reduced diameters and superior shed geometry of polymer products can still give inherently better pollution flashover performance than porcelain. Hollow Core Housing Failure Mode Hollow core polymer housings are likely to have a very different failure mode from porcelain. The physical properties of the polymer material means that it will not shatter like porcelain. With the initiation of an internal fault, the expected failure mode is a rupturing or bursting of the hollow structure with venting of the internal pressure, leading to an external flashover and dissipation of the fault energy outside of the housing. The actual failure mechanisms between hollow core porcelain and polymer housings may differ depending upon the product design and function, and users are cautioned against assuming all polymer products are inherently safe. Depending upon the specific design, function, failure initiation mechanism and available fault current, internal components can be expelled. However, the volume of available dense material, compared to a porcelain housing, is commonly less. Processing The manufacturing process for polymer products is inherently shorter than for porcelain. Molding times typically are of the order of minutes, so the lead-time can be considerably shorter than for porcelain devices. Limitations Weathering Degradation Polymer materials have different chemical bonds than porcelain (covalent versus ionic), and they can be aged and changed by the multiple stresses encountered in service. [5] With proper material development and product design, polymer insulating products can and do provide high performance with desired service life as reported in Ref. 2. A complex formulation and design optimization process must be undertaken in order to achieve desired performance and service life over a diverse range of service conditions. This requires that the materials scientist formulate the polymer material with the appropriate additives in the necessary concentrations using appropriate mixing methods. The product designer must consider the specific properties of the formulation in product design and application. The manufacturing process

12 conditions must be evaluated as well to insure that compound degradation does not take place during production. Performance and service life remain user concerns. Polymer material formulation is a complex optimization process, but one which is attainable by a knowledgeable materials scientist with extensive experimentation and testing. Formulations can vary widely with base polymer comprising 20-80% by weight of the material. Additives are used to extend and reinforce mechanical properties. Typical extending and reinforcing fillers include antioxidants, plasticizers, pigments, cure agents, catalysts, flame retardants, UV stabilizers, tracking and erosion resistors, processing aids and others depending upon the specific formulation. In many situations, it is the additives and fillers, both collectively and individually, whose performance determines the overall material performance. The structure of polymer materials, such as flexible bonds and long chain mobility, provide many of the inherent advantages. However, as all polymer material formulations are organic, continuous service stress can lead to deterioration of the surface properties and pollution withstand characteristics, unless reinforced with a proper additive package and a housing design which limits leakage current. It is the recognition of this fact, combined with materials science and product expertise which results in material formulation and product designs with proven high levels of performance for diverse applications. Service stresses, such as corona discharge, UV exposure or chemical attack, cause chemical reactions on the polymer material surface. One result is the formation of hydrophilic groups which allow the surface to wet, which permits increased leakage current flow. A material, which may experience reduction in hydrophobicity, does not necessarily continue to change, such as by tracking or erosion, during wetting conditions. It is an important part of the material scientist s task to ensure that loss of hydrophobicity does not cause a disastrous increase in the degradation rate of the material. base polymer material not only to reduce cost, but also to enhance performance and facilitate processing. The base polymer may constitute of the order of 20%-80%, by weight, of the end material. Thus, the actual formulation of compounds claiming the same base polymer will be different, which will have a direct effect on performance. In today s highly competitive market, suppliers try to gain advantage via cost reduction. Service experience of an older generation of material may not predict performance of a different formulation. Mechanical Strength Polymer insulation is typically neither rigid nor self-supporting. For applications such as cable terminations, the cable must be supported by some other means such as clamping of the jacket (oversheath) to a structure, and/or rigid connection to the phase conductor. Where intrinsic mechanical strength is required, ceramic cores, fiber reinforced tubings or layers may be utilized, which are covered by the polymer material for weathering resistance as illustrated by both the hybrid insulators and polymer surge arresters in Figure 1. Material Compatibility Polymer products can have more than one axial interface, depending upon the product function and specific product design. Use of different strength member components results in different interfacial properties between the polymer housing and other internal materials. Where multiple interfaces exist, the primers and adhesives/sealants selected are of great importance. The material and processing properties must be known and carefully analyzed in order to assure long-term, stable performance over the broad range of conditions encountered in service. The polymer formulation can suffer from stress corrosion or brittle fracture, from typical service stresses, if improperly formulated. Material developers must ensure that this aging mechanism is minimized. High Raw Material Costs Polymer raw material costs are much higher than porcelain raw materials. Fillers and additives are blended with the

13 Contamination Flashover Common Porcelain Insulator Housing Flashover Process Contamination flashover is a multi-step process, which may result during a number of conditions. [6,] The basic steps in the more common process for porcelain insulator housing flashover include: 1. Contamination build up wind drives dust and/or other conductive contaminants onto the surface of the insulation. 2. Surface Wetting High humidity, dew, mist or light rain wets the surface and dissolves the contamination, creating a conductive electrolyte, which is continuous or nearly continuous along the insulator length. When the electrolyte forms, the surface resistance of the housing falls and appreciable leakage current flows. 3. Ohmic heating the leakage current flowing through the electrolyte causes a decrease in resistance and corresponding increase in current, since the electrolyte has a negative temperature coefficient of resistance. There is accumulated energy dissipation (I 2 Rt) heating which forces water evaporation, ultimately leading to a runaway increase in drying rate. 4. Dry band formation the power dissipation per unit area is the product of the electrical stress and current density. The areas of the surface with the highest power dissipation dry first. Geometry plays a role, and the current density tends to concentrate in the regions with the smallest crosssectional area, which is where drying is accelerated. Drying increases power dissipation because of increasing resistivity, leading to an unstable condition where dry bands form. As dry bands are insulating, surface activity continues within the band region until the band grows to sufficient length to withstand the applied voltage with only intermittent activity. 5. Partial arcing and flashover flashover occurs if one of the dry band discharges extends across the remaining wetted surface of the housing. Discharges usually extinguish just before a voltage zero. If, however, the stress and leakage current are high enough, the discharges may expand along the entire housing length and initiate flashover. Visible surface activity does not mean that flashover will occur. Flashover can only occur when the electrical stress in the discharge is less than the stress in the wet film. Note that characteristics of a discharge arc are inverse ohmic the higher the current, the lower the stress. Note that the above steps must all take place sequentially for flashover to occur. If the surface is altered by washing, such as rain, then the electrolyte conductivity is decreased. If wetting is by dew formation, the rising sun will reduce the wetting conditions. In such cases, the chances of flashover will be reduced. Additional Steps For Polymer Flashover and Effects of Aging The analyses of laboratory data and literature surveys suggest several additional steps that occur in the flashover of a hydrophobic polymer insulator housing. 1. Contamination deposition same as for porcelain. 2. Wetting high humidity, fog, dew or light rain deposit moisture on the surface which forms droplets because of the hydrophobic properties. Due to gravity,droplets roll down sloped areas. Where gravity does not encourage droplet movement, discrete droplets remain. Salt and/or conductive pollution dissolves in the water droplets, increasing the liquid conductivity. 3. The residual dry surface pollution is slowly wetted by the droplet migration. This forms a high resistance conductive layer, and changes the leakage current from capacitive to resistive. 4. Ohmic heating same as for porcelain. 5. Electric field effect on hydrophobic surface the applied electric field causes closely spaced droplets to join together into a larger single drop, known as a filament. Flashover tends to take longer for a hydrophobic surface because of the time to form a conductive path with filaments. The local electric field has to be sufficiently high to form filaments as well. 6. Spot discharges on hydrophobic surface filaments reduce the distance between housing terminals, increasing the electrical field between adjacent filaments. When the stress is sufficient, surface discharge activity can occur. 7. Reduction in hydrophobicity Discharge consumes the thin polymer layer around the droplets and reduces the hydrophobicity by rotation or breaking of the polymer chains. Loss or reduction of surface hydrophobicity results in droplet dispersion and the formation of a continuous conductive layer in a high stress area, allowing elevated leakage current flow. 8. Dry bands form under the same process as porcelain. The resultant activity causes surface erosion whose rate depends upon the

14 specific material formulation discharge-free and promotes aging. 9. Full or partial recovery of hydrophobicity may be possible if the material is discharge free for a sufficient period of time. Recovery ability will depend upon the specific material, formulation, housing design and service environment. 10. Repetition of the aging cycle causes further erosion of the surface, which is enhanced by chemical reaction and local temperature rise. Local hot spots can be of the order of 400 C during heavy discharge activity. Other aging, such as UV damage, can cause surface crazing which traps and holds contaminants which can promote leakage current flow during wetting. 11. Flashover ultimately occurs along the same process as porcelain. The surface becomes hydrophilic, wets out, dry bands form and the discharge propagates to bridge the housing terminals. Hydrophobic surfaces (see Figure 4) present a higher resistance to leakage current flow than hydrophilic surfaces and require higher leakage current and commensurate energy dissipation to initiate flashover. This is why polymer insulators have higher flashover voltages than conventional porcelain insulators. As with porcelain, all of the above steps must take place sequentially for flashover to occur. If the process is interrupted, such as a change in wetting conditions or with surface hydrophobicity recovery, flashover does not take place. Thus, visible activity does not always result in flashover. Flashover Mitigation Porcelain Users can utilize several countermeasures to reduce flashover with porcelain insulating housings. They are: 1. Creepage extenders polymer sheds (Figure 2) are installed directly over porcelain insulator weathersheds to increase the creepage distance. [3] 2. Extra-creepage housings housings with extra-creepage, more creepage than typically used for the specific system, will reduce flashover risk. 3. Washing insulators may be washed live or de-energized with high pressure water or with solid materials such as ground up walnut shells. This is a costly process which may need to be carried out on a regular schedule to be effective. In addition, the use of solid cleaning materials may abrade away the protective glaze of the porcelain, exposing the underlying substrate to the environment, which can then hold contamination. 4. Complex weathershed profiles protected creepage and fog-type weathershed profiles are available, at a premium cost, which have profiles which resist contamination deposition in the protected areas. Such shapes, however, do not lend themselves to live line washing. 5. Surface coatings - greases or polymer coatings may be applied to the porcelain housing to improve the pollution performance. Coatings have a finite life, are costly to install, and potentially very costly to reapply because of the need to clean and/or remove prior applications. Performance improvement and time necessary for recoating are highly dependent upon the quality of the coating application. Polymer Separate or retrofit flashover countermeasures for polymer are rarely used. The development of standard products for a wide range of applications and conditions requires careful attention to the material formulation and insulator housing design. One important contamination application consideration is if a polymer product can be washed, either intentionally or unintentionally, along with porcelain insulators. Polymer products which are not firmly secured to the underlying layer, such as insulators or arresters which rely on grease as the housing interfacial sealing system, may not be suitable for high pressure washing. Such products may fail prematurely from washing, and users need to consider the use of such products in their specific service environment, if the supplier does not recommend washing. [7] Polymer insulating housings need leakage current control. While some degradation over service is likely to occur, one defense mechanism against flashover is retention of leakage current control. If the leakage current is insufficient then the flashover mechanism cannot progress. This is a result of the interaction of the polymer material and product design.

15 Conclusions 1. Polymer insulating materials offer significant advantages over porcelain. 2. Polymer materials have been proven with over 30 years service history. 3. Polymer material formulation must resist service stress degradation by a complex optimization process of material formulation and housing design, which includes consideration of the manufacturing conditions. 4. All insulators exposed to contamination can experience surface activity which does not necessarily lead to contamination-induced flashover. 5. Products need to be evaluated on an individual basis. Because of differences in formulation, product design and manufacturing conditions, polymer materials are not identical nor are they generic. Users cannot predict the performance of one polymer product based upon experience with another, which will have a different formulation and/or design. 6. The competitive nature of the marketplace is driving many suppliers to cost reduction and material reformulation. Newer generation materials may be untested and past service experience may not indicate future performance of newer generation materials. 7. Porcelain insulator flashover experience can be improved with the use of polymeric materials. Coatings have a limited service life, whereas other solutions, such as creepage extenders, offer long-term performance. References [1] Insulators for High Voltage, Looms, J.S.T., (Peter Peregrinus Ltd., London, UK, 1990), p. 17. [2] Thornley, D. and Shockett, A., 25 Years Experience of Outdoor Polymeric Insulation, 1994 IEEE Transmission and Distribution Conference, Chicago, IL, USA, April 10-15, [3] Pack, G., Creepage Extender Improves Insulator Performance, Transmission & Distribution International, Vol. 3, No. 3, September 1992, pp [4] Looms, p. 19. [5] Ibid., p. 17. [6] Karady, G., Shah, M. and Brown, R., Flashover Mechanism Of Silicone Rubber Insulators Used For Outdoor Insulation - I IEEE Trans. on Power Delivery, Vol. 10, No. 1, October 1995, pp [7] IEEE Std. 957 Guide for Cleaning Insulators, At Raychem we are committed to continuous quality improvement in every aspect of our business. Raychem Corporation Electrical Products 300 Constitution Drive Menlo Park, CA 94025, U.S.A. Tel. (650) Fax (650) All above information, including drawings, illustrations and graphic displays, reflects our present understanding and is to the best of our knowledge and belief correct and reliable. It does, however, under no circumstance constitute an assurance of any particular qualities. Such an assurance is only provided in the context of our product specifications. Our liability for this product is set forth in our standard terms and condition of sale. Raychem is a trademark of Raychem Corporation. Raychem Corporation Electrical Products 8000 Purfoy Rd. Fuquay-Varina, NC , U.S.A. Tel. (800) Fax (800) Raychem Ltd. Electrical Products 438 Alexandra Road # Alexandra Point Singapore Tel Fax Raychem GmbH Electrical Products Haidgraben Ottobrunn Munich, Germany Tel. (089) Fax (089) Raychem EPP /97

16 E L E C T R I C A L. P R O D U C T S. D I V I S I O N An Introduction to Raychem s Silicone Elastomer Outdoor Insulation Material

17 Introduction Electrical insulating polymeric formulations first introduced by Raychem 30 years ago have been proven to have excellent long-term physical and electrical properties. [1] These materials were originally designed for field application by heat-shrinking and are based on radiation-crosslinked, semicrystalline polyolefin co-polymers. When their excellent weatherability and moisture-sealing properties were demonstrated in cable accessories, the use of these materials was expanded to such applications as surge arresters, insulators, insulation enhancement and bushings, supplied by Raychem and others. Whereas polyolefin co-polymer based formulations have definite advantages, other developmental materials research has taken place within Raychem over many years in order to fully exploit the properties achievable with elastomeric-based insulation materials. At Raychem, materials development is recognized as a complex formulation process where the entire package is optimized considering each of the steps involved in producing the final product. The base polymer grade and various additives which are used, combined with compounding procedures, material processing, product design and assembly all contribute to the overall product performance. The industry perception that polymer materials are generic or somehow similar in performance only on the basis of the claimed based polymer is incorrect. Performance is based on the specific characteristics of a unique formulation, utilizing a specific design produced under a defined set of process conditions. With the extreme variations and differences that exist in each key performance area, it is difficult to understand how materials could be commonly grouped and viewed as equivalent. Products need to be individually evaluated by users. Consideration needs to be given to the reputation and experience of the supplier, the material qualification and product performance testing conducted, who controls and is responsible for the polymer material supply chain, and the level of detail and data that is supplied about the material and its field performance. The silicone-based formulations now in use by Raychem have been specially developed for electrical insulating applications. They have undergone many years of development and optimization to yield exceptional electrical and weathering performance properties, comparable to the Raychem polyolefin co-polymer materials. The purpose of this paper is to highlight the advantages of properly formulated silicone materials in outdoor insulating applications and to answer the question, Why is Raychem also using silicone-based elastomeric insulation? Background Raychem s development of polymer outdoor insulating materials was initially based on supplying product for use as an outdoor cable termination. As was reported in early Raychem publications, [2,3] semi-crystalline materials with excellent properties were developed along with pioneering work on testing, evaluation and lifetime prediction models. Heat-shrink was selected in the 1960 s as the field delivery system to provide the ability to install the product on all cable types in different core constructions and conductor crosssections. This was to simplify installation and improve reliability versus other technologies in use at the time. The initial material was an alloy of a silicone elastomer with a polyolefin, the latter required to provide the crystallinity needed for supply in a heat-shrinkable form. Early terminations containing this material are still performing well in service today. The compounding and processing of this polymer-blend material was a formidable task since controls must remain extraordinarily tight to make consistent product. In the 1970 s, [4] development work resulted in a polyolefin co-polymer formulation without the silicone elastomer, which was introduced in wide use to the market in the 1980 s after an extensive and rigorous test program. The revised material had enhanced mechanical properties, enabling it to be delivered with larger application ranges, furthering the inherent benefits of heat-shrink materials with improved erosion resistance. The revised material provided improved durability in highly contaminated applications. To complement our products containing the polyolefin co-polymer formulation, Raychem has developed siliconeelastomer outdoor insulating materials which maximize the inherent material characteristics through formulation expertise to deliver good erosion resistance, weatherability, and ultimately excellent product performance. These materials can be moulded using a unique flashless process in open weathershed geometry or in a patented highly protected creepage weathershed for severe contamination applications. For applications where the polymer material can be factory installed or for field installation where all of the benefits of heat-shrink may not be required, Raychem silicone elastomer products

18 can also be used on a cost-effective basis. No one polymer material is universally superior than any other. The mechanical properties of the polyolefin co-polymer material are inherently better and offer advantages where such characteristics are needed, whereas the polyolefin copolymer material cannot be as readily produced in more complex shapes as the silicone-elastomer material, nor utilized as a cold applied cable termination. Uncrosslinked Gum Cross-linked Example CH3 CH3 -( - Si -- O - ) n - -( - Si -- O - ) n - CH = CH2 CH2 vulcanization CH2 CH3 CH2 -( - Si -- O - ) m - -( - Si -- O - ) m - CH3 CH3 As a materials science company, Raychem continues to develop highperformance materials and products which best serve the ever-changing needs of our customers, on a costeffective basis. Not every complex problem is solved with the same solution. Raychem is uniquely positioned to utilize its considerable skill and talent to prove the best solution for every problem every time. What is Silicone? In the literature, many generic comments have been made about silicone without a true understanding of its chemistry or microstructure. The term silicone refers to a polymer composed of an inorganic siloxane backbone (Si-O, silicon-oxygen link). The most common silicone is Polydimethylsiloxane (PDMS), which has a backbone of silicon and oxygen, but also contains two methyl groups (CH3) for every one silicone: CH3 -( - Si -O -)n- CH3 The above unit is linked with similar units to form a chain which is the polymer (n can be in the thousands). Hydrocarbon side-groups other than methyl that are commonly seen along the polysiloxane chain are ethyl, phenyl, and vinyl groups. There may be other types of side-groups (such as fluorine to form fluorosilicone or hydrogen such as in mono-substituted silicones), but the Si-O backbone is key for it to be called a silicone. Thus, with the significant presence of carboncontaining groups, the concept that silicone is completely inorganic is not true. These chemical constituents and their placement along the siloxane chain will determine many of the material properties of a particular grade of silicone. The length and distribution of lengths of the siloxane polymers can also vary and additionally contribute to the overall properties of the choice of silicone used. To form a silicone elastomer one must bind the chains together in some way to create a network that has rubbery properties (i.e. it will snap back from a stretched position). Vinyl and other reactive groups are usually also present as side-groups and end-groups; these groups allow crosslinks to form during chemical reaction (many of these groups contain multiple carbon atoms). Polymers can be joined at their endpoints or along various sections of a chain during a reaction, forming a crosslink. The nature and type of crosslink depends on the reactive groups, crosslinking agents and inhibitors, and catalysts; one kind of crosslink is shown in Figure 1. [5] Before crosslinking, the material is termed a silicone gum. After crosslinking, it is called a silicone elastomer or silicone rubber. Silicone elastomers usually also contain reinforcing silica (glass or quartz) to strengthen the polymer since its mechanical properties are inherently weak when compared with other polymers. This silica has reactive groups which bind to the silicone polymer and influence the strength, hardness, toughness, and other physical properties of different silicone grades. Other fillers such as coarser grades of silica are added to lower costs; these types of fillers are termed non-reinforcing fillers. Still other fillers are included to influence other properties such as processing characteristics, mold shrinkage, thermal expansion, tracking and erosion resistance, weathering performance, and color. These types of fillers chosen must meet two main criteria: 1) long term stability under expected service conditions and 2) chemical inertness towards the other components in the formulation such that the silicone retains its functionality over time. [6] Silicone must be thought of then as a broad-term covering materials with a wide variety of achievable properties; some silicones are better for outdoor electrical insulating applications, whereas others are not. However, once the base silicone polymer is chosen, it must be properly formulated with the inclusion of additives to optimize Figure 1. Schematic of a typical vinyl type crosslink in silicones. Note the number of carbon atoms present. Silicone elastomer housing with no longitudinal flash High strength ceramic core Cemented metal base fitting Figure 2. Protected creepage, doublebell, hybrid insulator design, 15 kv class.

19 performance properties. At Raychem, candidate formulations are continually developed and evaluated prior to product introduction in order to assure that the user will have a reliable and economic product in the long term. Raychem has found that silicone-based formulations have both advantages and disadvantages. Among the advantages that are exploited in outdoor insulation applications are hydrophobicity and hydrophobic recovery, weathering resistance (when adequately formulated), processability, and elastomeric mechanical properties. Limitations include high raw materials costs, softness, and relatively low mechanical strength. The following discussion will explore a variety of these properties with respect to the choice of silicone elastomers, using a Raychem hybrid insulator design as a proxy. Hybrid Insulator Design The hybrid insulator consists of a smooth-bore ceramic core and suitable end fittings, depending upon the design and application, with a molded weathershed housing. The housing may have conventional open weathersheds or a patented highly protected double bell creepage design. Figure 2 shows the schematic diagram of a protected creepage hybrid post insulator. The synergistic performance realized comes about from maximum exploitation of the advantages of each material. The ceramic core provides mechanical strength for cantilever or tension applications. Because of its inherent chemical stability, [7] ceramic can withstand weathering, chemical attack and surface activity without damage. However, the chemical nature also results in high surface energy. This permits the surface to wet out easily, which can lead to flashover if the creepage distance is not sufficient. To overcome this intrinsic limitation, the silicone-elastomer housing is installed over the ceramic core with a suitable, stable interfacial sealant to maintain dielectric strength. The very high leakage distance and high surface resistance of the elastomer limit leakage current and prevent the onset of the flashover mechanism. In the event of elastomer surface damage and loss of hydrophobicity, ceramic is exposed at each terminal where there is the highest electrical field and likely location for electrical activity. With this design, the elastomer will not experience additional damage for electrical activity about the terminals. The reduced volume of the ceramic core contributes to reduced weight and easier handling. Hydrophobicity Silicone s property of hydrophobic recovery has been much publicized in the literature as a key feature for its use in outdoor insulation applications. Silicone s low surface energy imparts very good hydrophobicity which results in low leakage current during wetting conditions where contamination is present. [8] On a hydrophobic surface, water drops bead up and do not wet the surface completely. This reduces the leakage current, which leads to higher flashover voltages. [9] The organic groups which pack around the siliconeoxygen backbone are actually responsible for silicone s hydrophobicity which is similar to many organic polyolefin co-polymers. In comparing the two Raychem materials, the siliconeelastomer has some advantage in power loss, due to its faster hydrophobic recovery. The hydrophobic recovery property of silicone is attributed to the flexibility of the Si-O linkage and the presence of very mobile free silicone chains. Experimental studies indicated that heavy pollution and simultaneous wetting produced surface arcing, which destroys surface hydrophobicity and increases leakage current. However, the surface recovers hydrophobicity after hours of a dry and arc free period. The recovery of surface hydrophobicity is due to the diffusion of mobile low molecular polymer chains (LMW) from the bulk to the surface [9] and rotation of surface hydrophilic groups away from the surface. [10] Surface hydrophobicity can be evaluated in the laboratory by determining the contact angle of a droplet of deionized water on a material s surface. Hydrophobic recovery is monitored by treating the surface with corona and measuring the resulting contact angles as a function of time after exposure. Figure 3 shows the result of a study where 1cm x 3 cm of various silicone slabs were exposed to corona for 10 seconds (using a Model ED-20 corona treater, Electro-Technic Products, Inc.). Average contact angles along the exposed regions were determined from advancing contact angle measurements made with a Rame Hart model 100 Goniometer. In this figure, four experimental formulations (A,B,C and D), which contain the same silicone elastomer but different filler types and levels, are compared with a fifth compound, a commercially available silicone (E). All of these silicone compounds were found to have excellent hydrophobicity and all recovered their hydrophobicity within 8 hours after arc treatment. The experimental formulations (A,B,C, and D)