Considerations on the Design of Composite Suspension Insulators

21, rue d'Artois, F-75008 Paris http://www.cigre.org 33-204 Session 2000 © CIGRÉ CONSIDERATIONS ON THE DESIGN OF COMPOSITE SUSPENSION INSULATORS BASED ON EXPERIENCE FROM NATURAL AGEING TESTING AND ELECTRIC FIELD CALCULATIONS by K. SOKOLIJA University of Sarajevo Bosnia and Herzegovina M. KAPETANOVIĆ ENERGOINVEST Bosnia and Herzegovina R. HARTINGS STRI Sweden M. HAJRO University of Sarajevo Bosnia and Herzegovina Summary: The results of natural ageing tests have shown that other design parameters than the creepage distance may be critical for the short and long term performance of composite insulators such as: the design of the triple junction, the form and direction of the moulding line and the distance between the bottom flange and the first shed. Based on finite elements method field calculation, the paper illustrates how the insulator design, in general, and above mentioned specific design items, in particular, may influence the behaviour of composite insulators, providing confirmation and explanation of the results of natural observations. The approach illustrated in the paper may assist in extending and improving the concepts and criteria used for an effective design of composite insulators. Keywords: Composite insulators, natural ageing, design, triple junction, moulding line, first shed position. 1 INTRODUCTION Unlike porcelain and glass these polymers have low surface free energy which makes the virgin surface (new and without exposure to the environment) of the polymers inherently hydrophobic (water repellent). Hydrophobic surfaces present a higher resistance to leakage current flow than porcelain or glass surfaces (hydrophilic surfaces). They also require higher current and commensurate energy dissipation to initiate the well-known phenomenon of dry band arcing which, ultimately, is responsible for material degradation in the form of tracking and erosion. The lower leakage current and consecutive lower probability of dry band formation require a higher applied voltage to cause flashover. In one word, due to the hydrophobic surface, the polymer materials typically offer much better contamination performance than porcelain and glass [2]. Service stresses, such as surface discharge activity on hydrophobic surface, UV exposure and chemical attack, cause the reduction or complete loss of hydrophobicity and dry band formation under the same process as porcelain or glass. It has been observed that, in case of silicone rubber, the surface, due to the diffusion of mobile low molecular polymer chains (LMW) from the bulk to the surface [3] and the rotation of surface hydrophilic groups away from the surface [4], recovers hydrophobicity when there is little or no dry band arcing [5]. The ability of the material to control leakage current, which represent the first defence line of the insulating device, varies significantly based on polymer material used, but also on its interaction with the product design. However, even housing materials that have a tendency to recover their lost hydrophobicity must be able to withstand dry band arcing without tracking or erosion – a secondary line of defence against contamination – induced flashover. Housing design can also influence leakage current during the periods of reduced or lost Polymer materials such as silicone elastomers, hydrocarbon elastomers and epoxy resins are being increasingly used instead of porcelain and glass for outdoor insulation applications such as line insulators, bushings, hollow core insulators, cable terminations, etc., mostly due to the following advantages [1]: • light weight = lower construction and transportation costs; • vandalism resistance = less gunshot damage; • high strength to weight ratio = longer spans/new tower design; • improved transmission line aesthetics; • unexplosive housing = improved safety for the staff in the station and for the installation equipment. there is still a possibility of manufacturing a bad insulator if the design is poor. Insulators according to Fig. However. When the requested number of sheds is positioned as designed. As we can see. polymer materials usually outperform porcelain and glass in contaminated environments. Housing material formulation and leakage current path design are two interdependent tools that manufacturers have available to solve the performance optimisation problem. in determining the life time of these insulators. Obviously. product design. H. it is quite normal to produce a bad insulator from bad materials as well as from a good material with a bad design. As we just said. the mould is opened and insulating body is taken out. 2 Three principal design of composite insulators .the "can". enabling the sheath to obtain chemical cross-linking to the rod surface. a good composite insulator could be obtained with a perfect combination of design and polymer material formulation. Kärner [7] – Fig. a primer is applied to the rod surface prior to extrusion.2b are produced in a single shot moulding process where FRP rod is positioned between two halves of a parted mould and the housing (including the sheds at the same time) material is injected into the mould. Due to heating. To transfer the "could" to Fig. and manufacturing process are interdependent and that manufacturer. including the triple junction point. 2 TYPICAL INSULATOR DESIGNS Fig. Finally. 1 Interdependence matrix in composite insulators manufacturing hydrophobicity. has to solve the higher order optimisation equation. When a stable state of housing material is reached. the third condition has to be fulfilled – manufacturer's know–how.1. in order to offer a good insulator. Moreover. even if one starts with a good material.2a consist of a fibre reinforced polymer (FRP) rod (tube in case of hollow insulators) covered with a seamless sheath. An extrusion process used in manufacturing of cables applies the sheath. even with a good design. This paper discusses some design aspects. To produce a good insulator from a bad material. to Fig. is virtually impossible. but they must be adequately designed and manufactured to withstand such conditions without accelerated ageing (in dry and non–contaminated environments these insulators normally have a very long life). sheds and sheath are vulcanised together at elevated temperatures (HVT – high temperature vulcanisation). Therefore the key to longevity in polymeric (non-ceramic or composite) insulators is to ensure that leakage current is kept low.2 shows three principal designs of composite insulators. Insulators acc. Fig. one can produce a bad insulator starting with a good material and having a good design if there are poor manufacturing process and/or poor quality control. The sheds are moulded separately and pushed onto the sheath by means of a slippery vulcanising paste. the vulcanising process starts to crosslink the housing materials as well as the bond between housing and the rod surface. such as the moulding line and the optimal distance between the first shed and the metal flange. design weaknesses (lack of voltage stress relief. improper method of coupling the endfittings) as well as the quality control problems play a very important and probably a primary role. The bonding between fittings and housing is realised using metastable silicone rubber sealing. For the reason of bonding. poor sealing between materials and connecting hardware. we shall use our extension [6] of a matrix developed by Prof. To summarise our earlier discussion which shows that housing polymer formulation. All three designs are strongly related to the manufacturing process.4 Simplified model showing distortion of the electrical field caused by moulding lines (a) without moulding lines. The sheds themselves show the moulding line at the outer periphery of the sheds. erosion and splitting may occur at the moulding line – Fig.The design of composite insulators acc. The progress of the erosion process may in the end lead to core exposure resulting in a possible impact on the mechanical integrity of the insulator.3.198 Fig. These two aspects. The designs acc. This. to Fig. the material properties of the rubber of moulding line are different compared to the properties of the rubber on the rest of the insulator surface. to Fig. which is not easily performed in case of the design showed in Fig. may have an important influence on the erosion of the moulding line. 2b and 2c result in moulding lines on all sheds as well as on the insulator shank. Furthermore. a) Etanmax=1 b) Etanmax=1. Note: tangential components of the field are rotated by 90o for better visualisation [6].2b and 2c. The modules are mechanically sealed to the end fittings within an integral grading disk. The design and manufacturing method acc. towards the outside). a scrutinised technique is required (housing must not be damaged).10. The moulding lines represent a natural barrier where pollution and moisture can be easily accumulated.11] show general weakness of moulding lines running parallel with the electric field: at first the moulding lines change colour and increase surface roughness.2c uses modular weathershed housings including a number of the weathershed in a single module. The silicone compound is held in place by internal orings moulded into weathershed housing. combined with the distortion of the electric field (Fig. which could erode the rubber even below the surface of the shed or shank.4) under dry conditions by the dimensions of the moulding line. facilitates a fixed and narrow leakage current path. In order to remove the moulding lines (caused by excessive housing material movement between the two parts of the mould. 3 Two examples of erosion at the moulding line between sheds on EPDM insulators after 8 years in a test site in a clean. These moulding lines are arranged perpendicular to the main direction of the electrical field. The modules are then mechanically bonded to the adjacent module by an external polymer collar. These moulding lines are arranged parallel to the main direction of the electrical field. each having their specific technical and economic advantages and/or disadvantages. due to the manufacturing process.2a results in no moulding line along the insulator shank between the sheds. at the third stage the first chalking. 4 DESIGN OF THE TRIPLE JUNCTION The design of the interface between the metal end fittings and organic polymer of the housing is a very . to Fig. in turn. (b) with moulding lines. inland environment. 3 MOULDING LINE AND ITS POSITION Long term service experience [8] as well as laboratory experiments [9. The modules are assembled to the rod using a high dielectric strength silicone compound in the interface. in the second phase further blacking occurs in the surroundings of the moulding lines indicating that the material properties of the rubber of moulding line are different compared to the properties of the rubber on the rest of the insulator surface. Fig. For longer insulators. The electric field is calculated as a function of the distance between the lowest shed and the flange (Fig. This effect is due to the difference in permittivity of the rubber (about 4-5). perturbations on the metal flanges will increase the field drastically and corona discharges are expected to be directed towards the rubber. not only the creepage distance. The aim of our current research is to compare different designs from this point of view using finite element stress calculation and we expect the results to be published in our next paper. Therefore. this solution is not a realistic solution. possible perturbations on the flanges and the presence of materials of a higher permittivity than air (ε=1). In case of a design where the point of the highest field strength and the triple junction point are the same (Fig. the position of the lowest shed is not very important for the total creepage distance. the field is low when the distance is short or relatively large. Note: tangential components of electrical field are rotated by 90o for better visualisation [6].6. As shown in Fig. When optimizing an insulator design. which tends to concentrate the field into areas with a lower permittivity (air). but also other electric design features. it is also important to emphasis the mechanical aspect. In the case of the design from Fig. Three positions are included to visualize the geometry. However.5 Calculated field distribution for different designs currently in use. simply because of the fact that the design of this part has a decisive influence on the behaviour of the partial discharge staring from the triple junction point. meaning that the design itself. Such discharges are triggered by concentrations of the electric field.6). a partial discharge triggered at the triple junction point (dry band flashovered in vicinity of the lower end fitting) will have this point as a stable foot-point causing burning and erosion of the isolating material at the triple junction point. such as the polymer housing material (ε=4-5). as indicated in Fig. it seems that a short distance is a possible solution. It is therefore interesting to investigate the influence of the position of the lowest shed on the electric field distribution. Therefore. .sensitive part of the composite insulator design. secures the protection of the interface between the insulating housing and the metal end fitting. causing the discharge instability. metal. perturbations are not included in the calculations.5c. in practical applications. the foot-point of the partial discharge triggered at the triple junction point will be moved to the point of the highest electric field strength. It is therefore recommended to b) c) Fig. thus reducing the risk of degradation of the polymer material. 5 POSITION OF THE FIRST SHED a) Sustained corona discharge activity from the flanges towards the rubber housing material may lead to a degradation of the rubber.2a and 2c.5. From this calculation. In case of a rigid connection of the housing. and rod to each other (with very different modules of elasticity and different coefficients of thermal expansion). only the large distance between the flange and the lowest shed is a practical solution. Apart from the electrical aspect of different designs of triple junction discussed here. This alternative is an interesting design as it reduces the electric field and changes the direction of the field away from the rubber. For simplicity reasons. Another solution is to integrate the lowest shed with the flange. mechanical stresses will occur at the interfaces in case of temperature changes and mechanical at the interfaces of the materials involved [13].2b). such as expected corona discharge activity should be regarded. caused by the design of the flanges. the triple junction point is not the point of the highest field strength – Fig. Bologna: "High Voltage Insulators: The Back bone of Transmission and Distribution Networks". Vol. 3. New York. 40 mm 10 mm 1. ETZ. 1. 4. Gorur.4 1. Vol. 1995. The field intensity near the triple junction (housing.2 1 0. Brown: "Flashover Mechanism of Silicone Rubber Insulators used for Outdoor Insulation – I". depending on the quality of the moulding.. Plenum Publishing Corp. serious problems with the mechanical integrity of the insulator may arise. Mackevich and M. Shah: "Polymer Outdoor Insulating Materials.2 0 Etanmax (p. erosion and splitting of polymer housing. 1988. IEEE Trans. No. Wustenberg: "Erosion und Alterung von Freiluftisolatoren aus cycloaliphatischen Epoxid-Polyurethan – Giepharzen". air.6 0..prioritize electric field considerations rather than creepage distance requirements when designing the area around the flanges. Interview given to Insulator News & Market Report. Polymer Surface Dynamics. Vol. [5] R. 3. The design of the area close to the flanges and the distance to the nearest shed should be based on electric field considerations rather than on creepage distance requirements. Cherney. Vol. ed. part I: Comparison of Porcelain and Polymer Electrical Insulation". on Power Delivery. Auxel. on Power Delivery. Orbeck and D. [3] G. on Power Delivery. If this erosion reaches the core of the insulator. R.) dista (m ) nce m [7] H. May/June 1997. Hackman and T. F. Moulding lines running in 5 15 25 35 45 55 65 75 85 95 105 115 . No. World Congress on Insulator Fig. 1993. M. Shah and R. 3.6 Tangential component of field versus distance between the lowest shed and metal flange [6] 6 CONCLUSIONS 1. and metal) must be controlled (by design) in such a way that discharges anchoring at the interface between housing and metal is prevented – the design of end fittings and position of triple junction ought to provide for instability of the discharges burning from triple junction point. 1988. T. 1992. Vosloo and F. L. Kärner. [8] H.4 0. IEEE Electrical Insulation Magazine. Orbeck: "The Electrical Performance of Polymeric Insulating Materials under Accelerated Aging in a Fog Chamber". Williams: "Dynamic Wettability of Hydrophobic Polymers".8 0. Owen. 8. F. Vol. The moulding line is an important part of the principal insulator design. 11th International Symposium on High-Voltage Engineering (ISH 99). 13. Andrade.u. 91. Gentle. J. T. No. 10. 7 BIBLIOGRAPHY [1] J. Karady. no sheds 3. No. No. Hall: "History and Bibliography of Polymeric Insulators for Outdoor Applications". May/June 1995. parallel with the main field direction could bring about electrical field distortion causing chalking. [2] J. F. E. IEEE Trans. [4] M. 3. 2. D. London. [9] W. 3. F. UK: 23-27 August 1999. Sokolija and M. Ehrard and K. IEEE Trans. Research at Braunschweig – Studies Interfacial Phenomena in Composite Materials. Kapetanovic: "About Some Important Items of Composite Insulators Design". [6] K.. Hartings: "Standard and Reduced Salinity 10004 Salt Fog Tests on Silicone rubber Apparatus Insulators". Lorenzo Thione. April 1987. Associazione Laboratory di Prova e Organismi di Ceerticazione Indepedenti – ALPI. Canada 1997. Gutman and R. [13] M. for his valuable. 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