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Proposal IEC 815, Chapter 5 Parameters characterising the insulators

5.0

Insulator Characteristics An insulators total function is characterised by material and design and consists of two principal parts. Primarily, the core supplying distance between electrodes to prevent direct flashover, in combination with dielectric and mechanical support. Secondarily, the outside surface supplying distance to prevent flashover derived from leakage currents. This chapter concern the outside surface in outdoor environment i.e. pollution performance. Choice of surface material, supplementary surface attributes, dimension and design characterise the function. Restrictions include high demands on durability and limitations of manufacturing processes.

5.1

Materials From the beginning of high voltage insulator manufacturing, many different materials and material compositions have been tried. Of the basic materials used today, porcelain provides the most extensive experience from service and laboratory tests, considering range of designs, dimensions, geographic areas and time span. Therefor it is convenient to establish elementary references for porcelain and derive corresponding information for other materials.

5.1.1

Glass Glass is mostly used for conventional cap and pin insulators. Standard material is toughened alkali-lime silica glass providing a pre-compressed surface very similar to porcelain glaze. Specifications can be found in IEC 60672. .

5.1.2

Traditional Porcelain Traditional porcelain is used for conventional cap and pin insulators as well as hollow and solid insulators without demanding strength requirements. Material is siliceous or cristobalite porcelain. Specifications can be found in IEC 60672. Design is limited due to the moderate mechanical strength, in addition to the silica filler’s disposition to local structure and stress. Progress of manufacturing methods has been very influential on design over a long time period. A wide range of dimensions can be manufactured.

5.1.3

Contemporary Porcelain Contemporary porcelain is used for all types of insulators and is suitable to supply advanced functions. Material is aluminious porcelain of standard and high strength. Specification can be found in IEC 60672. Design is flexible due to the strengthened and homogenous material as well as the ability of body to make use of modern production technologies. A wide range of dimensions can be manufactured. Contemporary porcelain is appropriate for development of new technologies and redesign of traditional insulators, in order to achieve improved insulator performance.

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Porcelain with Semiconducting Glaze Traditional and contemporary porcelain can be provided with semiconducting glaze, also called resistive glaze. The possibilities of contemporary porcelain insulators together with the property of semiconductive glaze admit sophisticated combination of material and design. Material is standard glaze containing conducting additives, which allows a continuous flow of current, in the order of 1 mA. Semiconductive glaze adds three main advantages to an insulator. First, the insulator surface is provided with enough heat to keep dry in dew and fog. Second, the insulator receives a very linear voltage distribution. Third, modern technologies make it possible to calculate and shape the electrical field of the insulator surface, thus affecting the surrounding electrical field. Semiconducting glazed insulators shows a withstand voltage of approximate three times that of ordinary glazed insulators. Design and dimensions are the same as for the porcelain used, but with semiconducting glaze it is often possible to reduce conventional dimensions.

5.1.5

Semiconducting Surface on and of Other Materials Semiconducting surface made of other materials than glaze and applied on porcelain and other materials has appeared in research and development. As of today there is not sufficient experience to provide adequate recommendations.

5.1.6

Porcelain with Hydrophobic Coating Both traditional and contemporary porcelain can be coated with a polymeric layer creating a hydrophobic surface.

5.1.7

Non Ceramic Non ceramic materials for insulators are also called composites and polymers. They are used for many types of insulators but most frequently as the non-ceramic equivalent of the porcelain long rod. Composite insulators are not specified but are described in IEC 1109. “Composite insulators consists of an insulating core, bearing the mechanical load protected by a polymer housing, the load being transmitted to the core by metal fittings. Despite this common features the material used and the construction details employed by different manufacturers may be quite different”. The most common housing materials are EPDM and Silicone. Filler materials and additives are used to provide different features as durability, hydrofobicity and resistance against influence from fungus, pests and birds. As for traditional porcelain, design is depending on progress of manufacturing processes. There are different forming technologies with varying capabilities A wide range of dimensions can be manufactured.

5.1.8

Hybrids Hybrid insulators as known today, consist of a core of porcelain, and a housing of EPDM or Silicone. They are considered to be of great potential by combining the best features from different materials and technologies, but has yet not made any major impact. They are not common in service.

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5.2

Design

5.2.1

Typical Insulator Designs

5.2.1.1

Overhead Insulators The archetypes of insulators, the Cap & Pin and the Long Rod, are still the dominating types. Cap & Pin, made of glass or traditional porcelain, early on became widely used because of relatively easy manufacturing. The design was determined by early manufacturing processes and material quality, and remains practicable the same today. The Long Rod can offer considerable longer distance between electrodes, but demands high quality of material for manufacturing and tensional stress in service. The original porcelain Long Rod was a forerunner of contemporary porcelain insulators with its sophisticated use of material and design. The non-ceramic Long Rod was inspired by the original Long Rod and the first non-ceramic insulator in common use. Offering the same good design, lightweight but not yet the same durability, it has become the third standard overhead insulator.

5.2.1.2.

Post Insulators The original post insulators were designed as cap and pin insulators as jollying was the only production method at this time. With contemporary porcelain it is possible to produce solid cores posts which are stronger stiffer and electrically better designed. As they have smaller diameters and shed design can be made simpler they also have better pollution performance. The very big sheds with long distance between them were an effect of the production method.

5.2.1.3.

Hollow insulators Early hollow insulators or bushings were also jollied and joined together by slip jointing. This gave big sheds and long distances between the sheds to get as big parts as possible to joint. It was also a very costly production method. Today’s method with turning of big blanks has quite other possibilities with shed design and the shape of the trunk

5.2.2

Profile Design

5.2.2.1

Purpose The principal purpose of insulator surface profile is to extend the distance for a leakage current travelling on the polluted surface. In order to avoid local flashover which can damage the insulator or lead to total flashover, there are different important factors.

5.2.2.2.

Creepage Distance and Form Factor Creepage distance is the established method to calculate distance necessary to avoid flashover. However, the correct way to determine the resistance for a surface current is to use the form factor or K-value, as described in IEC 507. Established recommendations for creepage distance is normally most accurate when the insulators average diameter is close to 300 mm. Because the form factor is a relative new concept in design of insulators and require use of specific software, the chart in figure 1 (will be provided later) is included.The relationship between average diameter, creepage distance and form factor for standard shed types is shown in the graph .

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5.2.2.3

Minimum distance c between sheds. C is the minimum distance between adjacent sheds of the same diameter, measured by drawing a perpendicular from the lowest point of the outer rib of the upper shed to the shed below of the same diameter. This distance is important in rainfall conditions to avoid bridging between two successive sheds. C is not applicable to pedestal-type post and pin-type insulators.

5.2.2.4

Ratio s/p between spacing and shed overhang The ratio s/p describes the limitation on providing arbitrarily too high a leakage distance by either overdimentioning the shed overhang p or by unjustifiably increasing the number of sheds. The ratio is important for selfcleaning properties of insulators. s is the vertical distance between two similar points of successive sheds (spacing) p is the maximum shed overhang

5.2.2.5

Ratio ld/d between creepage distance and clearance The ratio ld/d describes the use of the creepage distance in order to avoid local shortcircuiting. This ratio should be checked for the worst case on any section, for example, of the under side of an antifog insulator profile. d is the straight air distance measured between two points situated on the insulating part or between any point located on the insulating part and the other on a metal part. Ld is the part of the creepage path measured between the above two points.

5.2.2.6

Alternating sheds The difference (p1-p2) between two consecutive shed overhangs is important in rain conditions to avoid bridging between them.

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5.2.2.7 5.2.2.8

5.2.2.9 5.2.2.10

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Inclination of sheds The inclination of sheds is important for the selfcleaning properties Creepage factor The creepage factor C.F. is used to characterise the entire insulator. C. F = l/s where l is the total creepage distance and s is the arcing distance Profile factor Do we need that ? For what purpose? Position All given factors are given for insulators in vertical position. Generally the change is improving the performance, but in certain cases a reduction may result due to for example to the cascade effect of heavy rain Hydrophobicity A hydrophobic surface will never be completely wet and transport the leakage current as well as a wet one. For hydrophobicity the silicone rubber is outstanding but will unfortunately loose hydrophobicity with time and also in very bad atmosphere when it is always wet Hydrofobicity 6

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HC

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År RTV+SIR

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Pors+EPDM

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