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Help for the design of polymer composite insulators


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Questions about the insulator design programs

Program instructions ?
Program sources ?
Our guaranty (dimensions, electrical and mechanical data) ?
Composite polymer only ? Glass & porcelain ?
Suspension and tension (dead-end) only ? Distribution insulators <50 kV ? Other composite polymer transmission applications ?
RIV, corona or partial discharges. Why no values ?
Silicone, EPDM. How to choose ?
Leakage (creepage) distance. How to choose ?
ANSI electrical data are higher than IEC's. Why ?
ANSI mechanical data are limited to 25,000 lbs (=222 kN). The IEC ones reach 320 kN. Why ?
Limits of the program ?
Corona ring. Why ?




Program instructions ? electrodesign.info strives to let you use its program without any particular instruction according to following principles:
If you wish to move to another available language, just click the corresponding one. (Depending on the browser and its settings, the modification is immediate or you shoud validate it by clicking the "submit" button at the end of the table).
Just set your targeted values in the table. Either through a choice in a scroll-down list, or just a click in front the proposed alternate, or you are prompted to enter your own numbers. Depending on your computer settings, the period (USA, UK) or the comma (Europe) represents the decimal point. Just do as you are used to. If any problem, just avoid decimals. The program might also react to the way you mark the thousand-separator with a comma (USA), a period (Germany) or a space (France). If any problem, just avoid any marking: for example, twenty thousand should be typed 20000 (not 20,000, not 20.000 and not 20 000).
If you don't have any value to write in a cell, just let the "0". (The program unerstands "0" as the equivalent of "no specific requirement").
Click on the validation button in order to send your parameters to electrodesign.info. Our server will return the resulting choice of designs.
The results are shown down the screen, under the table: the insulator drawing with the dimensions of the metallic end-fittings, a second table with electrical data and overall dimensions. Comments, if any.
The first line lists your targeted values as you have entered them.
Usually, several designs are submited to your evaluation, as your requirements are not necessarely coordinated.
Each line corresponds to a different insulator design, and the pivot parameter is repeated in the first column. Colours should help you sort your requests and the corresponding designs.
If not satisfied by any of the proposed designs, you may adjust your targeted values in the table at top of the page. You validate the new request.
The 10 last requests are kept available, following your last attempt (if your computer is set to accept Cookies). The program avoids however to repeat identical lines, as far as possible. See also the application limits of the program.
See also the program guaranty.
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Program sources ? The electrical, mechanical and dimensional data calculated by electrodesign.info are issued from international standards. As these standards state rather conservative values, in some cases rather far from the market offers, the algorithms have been adjusted accordingly.
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Our guaranty (dimensions, electrical and mechanical data) ? Test values obtained in a given laboratory are spread due to physical reasons. Therefore, an electrical value is never an absolute statement, but is given with a certain probability. Further, the standards make allowance for variations between different laboratories. As a result :
The values have not necessarely been evaluated correctly by the manufacturer, depending on their experience.
The proposed values are more or less conservative.
In order to obtain a market, a manufacturer may be tempted to be optimist when stating electrical values.
If you wish to compare competing electrical offers, it is better to base the evaluation on arcing distances, which are objective, than on displayed electrical values. Different manufacturing process bring about some variations, as well as ranges which are more or less complete.
The ambitious target of electrodesign.info is to maintain the results within +/-5% of technical offers as issued in the market. It is our guaranty. See also the application limits of the program.
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Composite polymer only ? Glass & porcelain ? Pages about porcelain and glass are being prepared, because electrodesign.info's intent is not to limit itself to polymer composites, nor to insulators.
By the way, some words about vocabulary: in North-America, electrical ANSI standards consider that a ceramic insulator is either in porcelain or in toughened glass. In other countries, where the IEC electrical standards are more common, a ceramic insulator is porcelain, never toughened glass.
For the electrodesign.info site, a polymer insulator is just another name for a composite insulator.
Even if many people use "Silicone insulator" as a generic word for composite polymer insulators in either Silicone, Silicone alloy or EPDM, electrodesign.info tries to avoid the confusion.
The choice between insulation materials depends on many factors. If we limit ourselves to state only one obvious advantage for each material:
Technical strength: the unmatched compression strength of porcelain.
Maintenance: the ultra-fast localization of a damaged toughened glass insulator.
Pollution: the unique withstand to contamination of composite polymer insulators. The choice between materials is such a vast domain that electrodesign.info will tackle the subject at a later stage, if ever. One only comment: better a good quality insulator, than a poor one in another material which seems more appropriate.
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Suspension and tension (dead-end) only ? Distribution insulators <50 kV ? Other composite polymer transmission applications ? Other applications will follow:
Line Posts
Distribution 11-50 kV, suspension, dead-ends or rigid type
Station posts Back to the question list / Search by keywords




Silicone, EPDM. How to choose ? Electrical, dimensional and mechanical data proposed by electrodesign.info are valid whatever the choice of polymer (Silicone or EPDM).
But the choice of material has an impact for the tearing strength, the time life, the cost and the withstand to contamination (see also the choice of leakage distance).
Variations between manufacturers, process, chemical formulas and quality have an impact that widely exceed the advantages or drawbacks coming from the material choice.
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RIV, corona or partial discharges. Why no values ? Radio influences (RIV), corona and partial discharges depend mainly on other metallic parts in the complete insulator string: clamp, conductor, proximity to the tower, arcing horns, arcing rings... The RIV data of the naked insulator have no practical importance for the RIV evaluation of the complete string.
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Leakage (creepage) distance. How to choose ? In order to ascertain the needed leakage distance, publication IEC 60815 is the main reference document, as well in the ANSI as in the IEC world.
Table I states the contamination level in relation to a description of the pollution in some typical environments :
I = light
II = medium
III = heavy
IV = very heavy Table II states the minimal leakage distance for each pollution level :
Level I : 16 mm/kV of highest system voltage phase to phase (it is the lowest leakage applied by electrodesign.info)
Level II : 20 mm/kV
Level III : 25 mm/kV
Level IV : 31 mm/kV Higher leakage distances (up to 50 mm/kV) are specified in some parts of the world.
If you are moving from porcelain/toughened glass to composite polymers (Silicone or EPDM): for the leakage distance, just apply same values as porcelain or glass, if they gave you satisfaction.
The hydrophobicity of Silicone gives a better electrical withstand when submitted to pollution. But in the critical period just after a contamination, the Silicone looses its hydrophobicity, the heavier the pollution, the longer the recovery period. During the recovery period, the leakage distance is left alone as a shield. There is therefore no reason to apply a shorter leakage distance when using Silicone.
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ANSI electrical data are higher than IEC's. Why ? The ANSI electrical values (impulse and low frequency 50/60 Hz) are higher than IEC values.
The ANSI practice is to define the electrical values of a suspension or tension (dead-end) insulator according to its flashover values.
IEC defines the insulator according to its withstand strength: no flashover is accepted. Therefore, even with slightly different test procedures, the IEC withstand is always lower than the ANSI flashover.
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ANSI mechanical data are limited to 25,000 lbs (=222 kN). The IEC ones reach 320 kN. Why ? It is a question of practice and standards:
IEC defines sockets, clevis and tongues up to 530 kN. It is uncommon with a strength higher than 320 kN, which is therefore the highest in the electrodesign.info program.
ANSI standards and their users are generally satisfied with a maximum of 50,000 lbs.
Whatever the standard, please observe the possibility to increase the mechanical strength by using double, triple strings, or even more. Back to the question list / Search by keywords




Limits of the program ?
Composite polymer insulators for suspension, jumper and tension (dead-ends) for overhead lines and transformer stations.
Networks with a voltage beween phases 50 to 550 kV, at frequency 50 or 60 Hz
Components:
A core made of a glassfiber rod, impregnated with epoxy.
Hotdip galvanized end-fittings, forged steel or cast iron, crimped at each end of the rod.
A housing in EPDM or Silicone, totally covering the glassfiber rod.
The tightness between the end fitting and the rubber housing is insured by bounding (under high temperature and high pressure), or by the addition of seals.
The interesting possibility to obtain higher electrical performances by covering one pr both end fittings with EPDM or Silicone has not been taken in account in this program. (This possibility is limited to some process and manufacturers, and is not applied at voltages above 145 kV)
If needed, one or two corona rings in aluminum for grading the electrical field, minimum diameter 10 inches (255 mm).
Mechanical strength between 70 and 320 kN.
Insulator overall length between 20 and 160 inches (500 - 4000 mm).
Leakage distance equal or greater than 0.63 inch/kV (16 mm/kV) of the highest system voltage between phases. Back to the question list / Search by keywords




Corona ring. Why ? The extremities of an insulator are more sensitive to the electical field because:
The intensity of the electrical field is higher, specially at the live end (= conductor side).
The interfaces are complex: further to the rod/housing interface, the metallic end-fittings add their own interfaces and seals with the rod and the housing. In order to avoid a premature ageing of the insulator life, manufacturers may supply a corona ring in order to grade the electrical field. Critical points, which may lead to the recommendation to install a corona ring:
Sytem voltage: manufacturers and standards require a corona ring above 200 kV. Some utilities start already at 170 kV (limit used by electrodesign.info) or even lower.
Severe contamination is often considered as a factor accelelerating the need of a ring.
All manufacturers require 2 rings above 380kV (limit used by electrodesign.info).
The ring diameter is at least 8 inches (200 mm), but larger (up to the double) at higher voltages, which gives a better grading effect. This diameter is only one of the dimensioning criteria of the ring, others are the apparent tore diameter, the surface of the aluminum etc...
The materials, the shapes (in particular the end fittings) and the manufacturing processes influence the ring need.
The tore of the ring is usually in aluminum for cost reasons, lightness, and strength to corrosion. In case of a flashover, part of the aluminum will volatilize, with the benefit of helping to localize the impacted insulator to be replaced.
The aluminum corona ring is usually fixed directly to the insulator end-fitting.
This program doesn't handle hotdip galvanized steel horns and rings, which are used in some of the countries which follow the IEC standards. Such devices protect against power arcs, but have also a field grading effect. When the energy produced by the power arc is high enough to hazard the mechanical fixation of the end-fitting to the impregnated epoxy rod, the steel ring is not fixed directly to the end-fitting, but to an accessory mounted in series with the insulator.
The cost of a ring is significative: easily 15% of the naked insulator.
For the need of a corona ring, the market doesn't quantify the limits between "unnecessary", "preferably" et "indispensable". Note: electrical values are slightly lower when there is a corona ring. The electrodesign.info program takes it in account.
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