Shed Profile Design for Hollow Composite Insulators on HV Apparatus Anders Holmberg R&D Manager ABB Composites PiteĂĽ, Sweden
1. Introduction It is now well accepted that composite insulators with silicone housing offer superior pollution performance compared to porcelain and glass insulators and it is suggested that the creepage distance in coastal areas can be reduced at least one pollution level compared to IEC 60815-1 [1]. The new IEC 60815-3 also allows for this option. The service experience with composite apparatus insulators is very good [2] and with a few exceptions they do fulfill the expectation of maintenance free service. In extremely harsh conditions some insulators with silicone housing have been carefully washed with rinsed water (no high-pressure washing), as in the case of the ¹350 kV DC cable termination station in Oteranga bay, New Zealand. The use of silicone insulators do however still demonstrate a huge advantage over porcelain since the required cleaning frequency could be reduced from once per hour to 2-3 times annually [3]. The quality of a composite insulator is closely linked with the properties of the housing material. Of particular importance are the tracking and erosion performance as well as the hydrophobicity. The silicone rubber used for HV insulators normally consists of a polydimethyl-siloxane (PDMS) base polymer, inorganic fillers and a cross linking agent. The inorganic fillers consist of silica, which is added to improve the mechanical properties, and ATH which is added to improve erosion and fire resistance. At present there is no clear consensus on the optimum balance between the tracking and hydrophobicity properties. Minimum requirements are however defined in IEC61462 which refers further to the 1000 h salt fog test in IEC62217 for the tracking/erosion performance for polymer hollow core insulators. A recommendation for the minimum requirement on the silicone given in IEC TR 62039 is class 1A3.5 in the inclined plane tracking test. A recent publication [4] gives a review of relevant test methods to verify the hydrophobic properties. As for porcelain insulators the shed profile design is important also for the pollution and ageing performance of composite insulators. Design constraints on the shed profile for insulators with polymeric housing are given in IEC60815-3 for AC applications. Subject to these, and possibly additional constraints imposed by utilities or OEM’s, it is up to the insulator manufacturer, usually in cooperation with the OEMs, to optimize the design of the insulator, including the shed profile, for the intended purpose. The shed design of commercial insulators is influenced by numerous factors such as the silicone quality used, the available production technology for the material/product, creepage distance requirements and costs. The short and in particular the long term performance of the final insulator will depend on the specific quality of the material used, the shed profile design, the possible presence of weak spots in the overall design and production of the insulator, and the interaction with the apparatus it is part of.
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The purpose of the present paper is to discuss the influence of three interrelated design parameters: selection of production process, shed profile geometry and housing material selection on the pollution performance and ageing of composite insulators and to give some background to the design of ABB Composite insulators.
2. Production processes The two most common production processes for the housing of high voltage composite apparatus insulators are injection molding and helical extrusion, see Figure 1. Some HTV (High Temperature Vulcanizing) silicone grades can be injection molded at high pressure. The most common material selection for injection molding is however LSR (Liquid Silicone Rubber) which can be injected at a lower pressure and enable thereby use of less costly equipment and moulds. Extrusion is only used with HTV silicone.
Figure 1. Application of silicone housing. Left: injection. Right: extrusion
As mentioned in the introduction one of the key factors to get a good pollution performance for the final product is the absence of weak spots. In the case of injection molding a multipart tool is required which usually results in a more or less visible molding line on the finished part, Figure 2. The molding line may be caused by silicone flowing into small gaps between the tools or a slight mismatch between the tools. The molding line is usually trimmed in a separate operation after demolding. If too much of the molding lines remain on the final part they will act as a natural barrier where pollution and moisture can be accumulated. The result of this is a fixed and narrow leakage current path which together with the distortion of the electric field by the geometry of the molding line may accelerate erosion of the molding line [5]. For long parts the housing is normally injected and cured in sections creating a potentially weak point if not performed correctly.
Figure 2. Mould lines
An intrinsic feature of the patented extrusion process used by ABB Composites is that it does not leave any weak spots. The one piece extrusion die can not leave any mould lines. All tubes, independent of shape and length, are continuously extruded in one step. For each new
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lap of the shed profile the uncured silicone on the trunk is mixed together with the previous lap. After the finished extrusion no traces of the “joint” between individual laps can be found by microscopy or mechanical testing.
3. Shed geometry IEC 60815-3 specifies permissible, for AC conditions, ranges for the following parameters: • Spacing/overhang 0.65 • Minimum shed distance 40 mm (for alternating sheds) • Creepage distance versus clearance 4.5 • Difference between overhang of the larger and smaller sheds 15 mm • The sheds upper inclination angle 5-25° No requirement is put on the sheds under inclination angle but it is mentioned that protected creepage distance is beneficial in areas with type B pollution. This is also discussed in some more detail in Ref. (6). In direct coastal and liquid industrial environments the protected creepage distance is of great importance to prevent the whole surface from being covered with liquid electrolytes. In published studies on the ageing performance of line insulators in fog conditions it has been observed that insulators with protected creepage distance maintain their initial surface hydrophobicity for a much longer time [7, 8]. In a study on the ageing performance in rain conditions [9] it was observed that insulators with a small intershed spacing and a small shed inclination are particularly susceptible to degradation of their properties under rain conditions. Water drops collecting on the sheds of such insulators are the cause of corona discharges and lead to the formation of water channels, the collection of water on the sheds edges, and the bridging of intershed spaces by cascades of water drops. Highly detrimental was the wetting of the undersides of the sheds by water bouncing off the surface of the sheds below. For DC conditions no standard is agreed upon yet but based on experience with HVDC systems with voltage up to 600 kV recommendations have been formulated for UHVDC shed profiles for vertically installed silicone rubber insulators with alternating profile [10, 11]: • Spacing/overhang 0.9 • Spacing 65 mm (for vertical position) • Difference between overhang of the larger and smaller sheds 20 mm • The sheds upper inclination angle > 10° • The sheds under inclination angle > 3° For insulators installed in a near horizontal position and insulators with smaller diameter the sheds can be different from the requirement. ABB Composites has selected to use an upper inclination angle of 15°, see Figure 3, which is larger than the minimum requirement for ac in IEC 61805-3 and suggested for DC in [11] for two reasons. A steeper angle makes water roll off outside the underlying sheds instead of creating long pendant drops which may trigger flashover. The angle is also selected to ensure that water flows off the sheds instead of along the helical shape that is a consequence of the extrusion process. The under inclination angle is selected to at least 8° to obtain a good balance between protected creepage distance, water roll off from the sheds underside and natural cleaning. For most products different versions of the alternating profile shown in Figure 3 is used. A uniform profile is also used for some products and a different shed inclination is possible to produce.
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Figure 3. Principal design of ABB Composite shed profile.
One of the advantages with the extrusion process used by ABB is that the part specific investment costs associated with each shed profile is low and a single extrusion die can be used for a range of tube diameters. Due to this ABB can offer a range of shed profiles (different spacing, overhang and creepage ratio) to optimize the performance for each environment, including a tailored DC profile. This is in contrast to injection technologies that for cost reasons may limit the profile range. Another feature of the ABB shed profile that is enabled by the extrusion process is the drip edge at the shed tip. This feature increases the tear strength of the shed and reduces the electrical field at the shed tip. To facilitate the same tip radius with a conventional shed design would significantly increase the material cost and reduce the creepage distance. Figure 4 shows an example of a bushing where the difference in electrical field between a shed tip radius of 1 mm and 2.3 mm is 12 % in clean conditions. This feature may be utilized to compact the design of the apparatus.
Figure 4. Electrical field around shed tips with different radius. Left R=1.0 mm, right R = 2.3 mm.
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4. Materials At present the prevailing housing material for HV hollow composite insulators is silicone rubber. The silicone qualities used are often divided and discussed in two general classes; Liquid Silicone Rubber and High Temperature Vulcanizing rubber. Such simple generalizations are however dangerous since there are many different formulations used for HV insulators. Each manufacturer has chosen or developed a formulation adapted to the manufacturing process, the technical requirements of the finished product and cost constraints. This means that materials belonging to the same general material class may exhibit significant differences in the type and amount of filler, surface treatment of filler, catalyst type and formulation of the base polymer which affect the performance and durability of the finished product. A good hydrophobicity is required of the material to control leakage current and avoid dry band arcing which may trigger flashover. However, under severe conditions causing corona discharges or wetting of the pollution layer the hydrophobicity can be temporarily lost. A significant increase in leakage currents and dry band arcing can then occur and erode materials with insufficient tracking/erosion resistance. High quality HTV silicone with a high ATH filler content is a proven outdoor insulation material for HV applications and the dominant material for line and post insulators. A good balance between tracking/erosion and hydrophobicity can be obtained with high quality raw materials. The optimum ATH level is still not defined but it is suggested that more than 40% by weight is required to get a significant improvement of the tracking/erosion performance in the inclined plane test [12]. According to [13] optimum outdoor performance is obtained with an ATH filler content of 55% to 60% by weight. Production of smaller diameter insulators with such housing materials, e.g. line and post insulators and surge arresters, is made with injection molding. For larger diameter hollow core insulators extrusion of complete insulators or press molding of individual sheds need to be used due to the high viscosity of the material. Injection molding of hollow core insulators is in practice limited to less filled HTV or LSR. ABB Composites have selected to use a HTV silicone with 50% ATH content to obtain a good balance between the hydrophobic properties and the tracking/erosion resistance. The material fulfils tracking class 1A4.5 with is higher than the minimum requirement in IEC TR 62039. The hydrophobicity transfer performance has been verified according to the new method proposed by CIGRE WG D1-14 [4], the transfer time after pollution to recover a hydrophobic surface was less than 24 h (the first data point). The pollution performance of complete insulators has also been verified in laboratory and field test. 2009-2010 a production line hollow core insulator was subject to a one year field test at 132 kVAC phase to phase the Koeberg Insulator Pollution Test Station (KIPTS), managed by ESKOM in South Africa. KIPTS is generally accepted as a severe coastal test station for pollution and ageing of outdoor insulators. The winter cycle is considered as a light to medium pollution test and the summer cycle as a heavy to very heavy pollution test. The acceptance criteria for the test are similar to the IEC 62217 1000 h salt fog test and allows for some material degradation without failing the test. The insulator passed however the full year test without any signs of tracking, erosion, puncture or cracks in the material, see Figure 5a. The tested insulator had an alternating shed profile with 55 mm spacing and 55/25 mm shed overhang. The specific creepage distance was 32 kV/mm. During the same year two similar insulators with injection molded LSR housing were tested in parallel. High quality LSR materials from two different suppliers were evaluated in these tests. These insulators also passed the test but with traces of
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erosion at the trunk close to the HV flange. The tested LSR insulators had an alternating shed profile with 60 mm spacing and 60/40 mm shed overhang, specific creepage distance 36 mm/kV. It is interesting to note that one of the LSR materials evaluated do meet the 1A4.5 criteria in the inclined plane tracking test and the other LSR material is very close to do so. This means that the HTV and LSR materials demonstrate a similar tracking/erosion performance in pure material tests but significant differences are observed in the ageing performance of the complete insulators during the field test. It is however not known if this difference in behavior should be attributed to the material or to the lack of protected creepage distance on the LSR insulators or a combination of these factors. The upper shed angle was 11° and under shed angle was 0° for the LSR insulators.
a
b
Figure 5. State of silicone housing after 1 year field testing at KIPTS. a) HTV rubber, SCD = 32 mm/kV. b) LSR rubber, SCD 35 mm/kV.
5. Summary Shed profile design of hollow core composite apparatus insulators does not only concern the geometry of the profile itself. The material selection has a strong influence on the manufacturing processes available. The manufacturing process selection will put more or less constraints on the available design window for the shed geometry and the costs to tailor profiles for different apparatuses and environments. The possible presence of weak spots in the final product may also have a large influence on the long term performance of the insulator. The more than 10 years service experience with the ABB Composite shed profile design realized through a seamless helical extrusion of high quality HTV silicone with a high ATH filler content is excellent. More than 60 000 composite hollow core HV apparatus insulators are in service in all types of climates without a single failure due to flashover or ageing. Applications includes both AC and DC voltage including the UHV level.
6. References 1. 2. 3.
I. Gutman, L. StenstrĂśm, D. Gustavsson, D. Windmar, W.L. Vosloo. Optimised use of HV composite apparatus insulators: field experience from coastal and inland test stations. CIGRE Session 2004, A3-104. CIGRE Brochure prepared by WG A3.21. Aspects for the application of composite insulators to HV apparatus. to be published in 2011 Cable termination station fights ongoing battle against pollution. INMR Q1 2011.
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4. 5. 6. 7. 8. 9. 10.
11.
12. 13.
CIGRE WG D1-14. Evaluation of Dynamic Hydrophobicity Properties of Polymeric Materials for Non Ceramic Outdoor Insulation: Retention and Transfer of Hydrophobicity. Technical brochure, REF. 442, 2010 Sokolija K, Kapetanovi M, Hartings R, Hajro M. Considerations on the design of composite suspension insulators based on experience from natural ageing testing and electric field calculations. CIGRE 33-204, Paris, France, 2000. CIGRE WG C4.303. Outdoor insulation in polluted conditions: guidelines for selection and dimensioning. Part 1: General principles and the AC case. Technical brochure, REF. 361, 2008 Gorur R.S., Cherney E.A., Hackam R. Polymer insulator profiles evaluated in a fog chamber. IEEE Transactions on Power Delivery 1990:5;1078-1085. El-Hag A.H., Jayaram S.H., Cherney E.A. Effect of insulator profile on aging performance of silicone rubber insulators in salt-fog. IEEE-Transactions on Dielectrics and Electrical Insulation 2007:14(2);352-359. W. Bretuj, J. Fleszynski, A. Tyman, K. Wieczork. Effect of silicone rubber insulator’s profiles on their ageing performance in rain conditions. XVth International symposium on high voltage engineering. Ljubjana, Slovenia, August 27-31, 2007. Wu D, Åström U, Flisberg G, Zehong L, Gao L, Ma W, Su Z. External insulation design of converter stations for Xiangjiaba-Shanghai ±800 kV UHVDC project. 2010 International conference on power system technology, Hangzhou, China 24-28 October, 2010. Ma W.M., Luo B, Su Z.Y., Dang Z.P., Guan Z.C., Liang X.D., Åström U, Wu D. Long E.Y., Sun H.G., Preliminary recommendations on the suitable shed profile for HVDC station insulators with silicone rubber housing. 2006 International conference on power system technology. Paper DC1-07, C1538, Chongquing, China 22-26 October, 2006. Kumagai S., Yoshimura N. Tracking and erosion of HTV silicone rubbe and suppression mechanism of ATH. IEEE Transactions on dielectrics and electrical insulation 2001:8(2);203-211. Seifert J.M., Stefanini D., Janssen H. HTV silicone composite insulators for HVDC applications – long term experiences with material and design for 500 kV and above. 16th International symposium on high voltage engineering. Johannesburg, South Africa 2009.
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