Transmission line insulators

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2017


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TRANSMISSION LINE INSULATOR

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SR.NO.

CONTENT PG NO.

1. Introduction 4

2. 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18

Definitions Tracking Treeing Erosion Chalking Crazing Cracking Hydrolysis Puncture Specified mechanical load (s.m.l) Tensile load Routine test load (r.t.l.) Cantilever load Compressive load Maximum working combined loads Working cantilever load (w.c. L.) Maximum design rating (mdr) Proof-test load Delamination

5 5 5 5 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6

3. 3.1

Dielectric properties of insulation Dielectric strength or breakdown voltage

6 6

4. 4.1 4.2 4.3 4.3.1 4.3.2 4.4 4.4.1 4.4.2 4.4.3 4.4.4 4.4.5 4.4.6 4.4.7 4.4.8 4.4.9

Insulating material Properties of insulating material Porcelain insulator Glass insulator Advantages of glass insulator Disadvantages of glass insulator Polymer insulator Core Housing Weathersheds End fitting Coupling zone Interface Characteristics Advantages of polymer insulator Disadvantages of polymer insulator

7 7 7 8 8 8 9 9 9 9 9 9 9 10 10 10

5. 5.1 5.2

Types of insulators based on construction Type a Type b

11 11 11






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6. 6.1 6.1.1 6.1.2 6.1.3 6.1.4 6.1.5 6.1.6 6.2 6.2.1 6.2.2 6.3 6.3.1 6.3.2 6.4 6.4.1 6.4.2 6.4.3 6.4.4 6.5

Types of insulators based on use Pin insulator Structure Petticoats Causes of insulator failures Designing consideration of pin insulator Dimensions for pin insulators Helically formed pin insulator types Post insulator Suspension insulator Advantages of suspension insulator Disadvantages of suspension insulator Strain insulator Ball & socket type Tongue & clevis type Stay/guy strain insulator Types of guy insulators Type of insulators for guy insulators Basic insulator level for guy insulators Mechanical strength for guy insulators Shackle insulator or spool insulator

11 11 11 12 12 13 13 13 14 15 15 15 16 16 17 17 17 17 18 18 18

7. Comparison between ceramic and polymer insulator

19

8. Clearance and creapage distance 20

9. Marking for insulator 20

10. 10.1 10.2 10.3 10.4 10.5 10.6

Causes of insulator failure Cracking of insulator Defective insulation material Porosity in the insulation materials Improper glazing on insulator surface Flash over across insulator Mechanical stresses on insulator

20 20 20 20 21 21 21

11. 11.1 11.2 11.2.1 11.2.2 11.2.3 11.3 11.4 11.5 11.6 11.7 11.8 11.8.1 11.8.2 11.8.3 11.8.4

Insulator design Basic design concepts Material selection Core Weathersheds End fittings Insulator design Pollution consideration Table-1 Pollution severity levels Relation between the pollution level and the specific creepage distance Application of the "specific creepage distance" concept Parameters characterizing the profile Influence of the position of insulators Influence of the diameter Determination of the creepage distance

21 21 21 21 22 22 22 23 23 24 24 24 24 24 24 25






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12. 12.1 12.2 12.3 12.4 12.5 12.6

Insulation consideration Waveform- lightening impulse Waveform- switching impulse Critical flashover voltage (cfo) Withstand voltage Basic (lightning) impulse insulation level (bil) Basic (switching) surge impulse insulation level (bsl)

26 26 26 27 27 27 27

13. 13.1 13.2

Consideration of interference and corona Generating processes Surface corona discharges around highly stressed electrodes

27 27 27

14. Consideration of capacitance effects

28

15. 15.1 15.1.1 15.1.2 15.1.3 15.2 15.2.1 15.2.2 15.2.3 15.2.4 15.2.5 15.2.6 15.2.7 15.2.8 15.2.9 15.2.1

Insulator selection Mechanical parameters Loading considerations-ice and wind Table-3: loading parameters Table-4: steps used in calculating the total load on a conductor Electrical parameters Cifo selection Table-5 recommended bil at various operating voltages Low frequency ling duration (60hz) selection Switching surge selection Table-6 typical switching surge levels Contamination performance Calculation of insulator leakage distance example: Grading ring selection Minimum corona extinction level Maximum riv

29 29 29 29 30 30 30 31 32 32 32 32 33 33 33 33

16. 16.1 16.2 16.3 16.4

Insulator characteristics Service conditions Physical characteristics Electrical characteristics Mechanical characteristics

34 34 34 34 34

17. 17.1 17.2 17.3 17.4 17.4.1 17.4.2 17.4.3 17.4.4 17.5 17.5.1 17.5.2 17.5.3 17.5.4 17.6 17.6.1 17.6.2

Insulator testing Type tests Acceptance tests Routine tests Flashover test Power frequency dry flashover test Power frequency wet flashover test or rain test Power frequency flashover voltage test Impulse frequency flashover voltage test Performance tests Temperature cycle test Puncture voltage test Porosity test Mechanical strength test Routine tests Proof load test Corrosion test

36 36 36 36 36 36 36 36 37 37 37 37 37 37 37 38 38






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INTRODUCTION

In the case of a power system joined together by overhead transmission lines, the transmission conductors are required to carry the current between the source and the receiver and the insulators are required to insulate the mechanical supports from the current carrying lines.

Traditionally, the line insulators were made out of ceramics and glass. These materials were readily available and had the advantage of being cheap, having excellent insulating and dielectric properties and ease of construction. On the other hand, however, these insulators had the disadvantage of being heavy, easily broken and suffering from disintegration when subjected to polluted and exposed atmospheric conditions.

Thus, the need was felt to develop new type of insulators and new type of insulating material that, while carrying all the advantages of the traditional insulators, overcame the disadvantages. In the nineteen thirties and forties, new organic insulators were developed as a possible replacement for the conventional organic insulators; however, these insulators suffered from the problems of vulnerability from exposure to atmosphere and thus were deemed to be unfit for outdoor applications.

Later in the nineteen fifties, epoxy resin insulators were developed; these were heavy, susceptible to degradation from exposure to ultraviolet radiations and thus were never fully integrated into the power systems.

Since the advent of nineteen eighties, silicone rubber has been widely used in manufacturing of line insulators because of its resistance to weather conditions and the hydrophobic characteristics of rubber, which are permanent and allow advancement in the capacity of voltage of pollution the insulator can withstand. As a result of the above stated properties, the use of the composite polymers has increased tremendously in power system applications.

Generally, a basic insulator comprises of a core that bears the mechanical stress, a housing to protect the core from the weather and end fitting to transfer the tensile load. A mixture of glass with epoxy resin is for the formation of matrix inside the core that imparts the mechanical strength to the structure. The end fittings that are used to transfer the mechanical tension of the line and the supports are made from aluminium, cast iron, forged steel, etc. The use of rubber housing is to introduce insulation and isolation and also shield the core from atmospheric elements.

As a result of the hydrophobic properties of the silicone rubber composite, the new generation insulators can be designed to be more compact and lighter.




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DEFINITIONS

TRACKING Tracking is an irreversible deterioration by the formation of paths starting and developing on the surface of

an insulating material. These paths are conductive even under dry condition. Tracking can occur on surface

in contact with air and also on the interfaces between different insulating materials.

TREEING Treeing is the formation of micro-channels within the material. The micro-channels can be either conducting

or non-conducting and can progress through the bulk of the material until electrical failure occurs.

EROSION Erosion is an irreversible and non-conducting deterioration of the surface of the insulator that occurs by loss

of material. This can be uniform, localized or tree-shaped.

CHALKING Chalking is a surface condition where in some particles of the filler become apparent during weathering,

forming a powdery surface.

CRAZING Crazing is the formation of surface micro- fractures of depths up to 0.1 mm.

CRACKING Cracking is any surface fracture of a depth greater than 0.1mm.

HYDROLYSIS Hydrolysis is a chemical process involving the reaction of a material with water in liquid or vapour form. It

can lead to electrical or mechanical degradation.

PUNCTURE Puncture can be characterized by a disruptive discharge occurring through a solid dielectric (e.g., shed,

housing, or core) causing permanent loss of dielectric strength.

SPECIFIED MECHANICAL LOAD (S.M.L) The S.M.L. is a load specified by the manufacturing, used for mechanical tests in this specification. It forms

the basis of the selection of composite insulators.

TENSILE LOAD Tensile load is the load applied in- line with the longitudinal axis of the insulator rod and away from the end

metal fitting.

ROUTINE TEST LOAD (R.T.L.) The R.T.L. is the load applied to assembled composite insulators during Routine Tests. It is equal to 50% of

the S.M.L.

CANTILEVER LOAD Cantilever load is a load applied at the conductor position on the insulator, perpendicular to the conductor,

and perpendicular to the rod of the insulator. This load is also called bending.






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COMPRESSIVE LOAD Compressive Load is applied in- line with the longitudinal axis of the insulator rod and towards the base

end.

MAXIMUM WORKING COMBINED LOADS The maximum working combined loads are the simultaneously applied cantilever and compression loads.

They produce a bending moment that should not exceed the bending moment induced by the working

cantilever load rating alone.

WORKING CANTILEVER LOAD (W.C. L.) Working cantilever load is a load that must not be exceeded in service.

MAXIMUM DESIGN RATING (MDR) The maximum mechanical load that the insulator is designed to withstand continuously for the life of the

insulator.

PROOF-TEST LOAD The routine mechanical load that is applied to an insulator at the time of its manufacture.

DELAMINATION Delamination is the loss of bonding of fibers to matrix.

DIELECTRIC PROPERTIES OF INSULATION

DIELECTRIC STRENGTH OR BREAKDOWN VOLTAGE The dielectric material has only some electrons in normal operating condition. When the electric

strength is increased beyond a particular value, it results in breakdown. That is, the insulating

properties are damaged and it finally becomes a conductor. The electrical field strength at the time

of breakdown is called breakdown voltage or dielectric strength.

DIELECTRIC MATERIAL

DIELECTRIC STRENGTH(KV/MM)

DIELECTRIC CONSTANT

Air 3 1 Oil 5-20 2-5 Mica 60-230 5-9




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INSULATING MATERIAL The main cause of failure of overhead line insulator, is flash over, occurs in between line and earth

during abnormal over voltage in the system. During this flash over, the huge heat produced by

arcing, causes puncher in insulator body. Viewing this phenomenon the materials used for electrical

insulator, has to possess some specific properties.

PROPERTIES OF INSULATING MATERIAL It must be mechanically strong enough to carry tension and weight of conductors. It must have very high dielectric strength to withstand the voltage stresses in High Voltage

system. It must possess high Insulation Resistance to prevent leakage current to the earth. The insulating material must be free from unwanted impurities. It should not be porous. There must not be any entrance on the surface of electrical insulator so that the moisture or

gases can enter in it. There physical as well as electrical properties must be less affected by changing temperature.

PORCELAIN INSULATOR

The porcelain is aluminium silicate. The aluminium silicate is mixed with plastic kaolin, feldspar and

quartz to obtain final hard and glazed porcelain insulator material. The surface of the insulator

should be glazed enough so that water should not be traced on it. Porcelain also should be free

from porosity since porosity is the main cause of deterioration of its dielectric property. It must also

be free from any impurity and air bubble inside the material which may affect the insulator

properties.




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PROPERTY VALUE(APPROXIMATE)

Dielectric Strength

60 KV / cm

Compressive Strength

70,000 Kg / cm2

Tensile Strength

500 Kg / cm2

GLASS INSULATOR Now a days glass insulator has become popular in transmission and distribution system. Annealed tough glass is used for insulating purpose. Glass insulator has numbers of advantages over conventional porcelain insulator.

ADVANTAGES OF GLASS INSULATOR It has very high dielectric strength compared to porcelain. Its resistivity is also very high. It has low coefficient of thermal expansion. It has higher tensile strength compared to porcelain insulator. As it is transparent in nature, it is not heated up in sunlight as porcelain. The impurities and air bubble can be easily detected inside the glass insulator body because

of its transparency. Glass has very long service life as because mechanical and electrical properties of glass do

not get affected by ageing. After all, glass is cheaper than porcelain.

DISADVANTAGES OF GLASS INSULATOR

Moisture can easily condense over glass surface and hence air dust will be deposited on the wet glass surface which will provide path to the leakage current of the system.

For higher voltage glass cannot be cast in irregular shapes since due to irregular cooling internal strains are caused.







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PROPERTY VALUE(APPROXIMATE)

Dielectric Straingth

140 KV / cm

Compressive Strength

16,000 Kg / cm2

Tensile Strength

35,000 Kg / cm2

POLYMER INSULATOR CORE The core is the internal insulating part of a composite insulator. It is intended to carry the mechanical load. It consists mainly of glass fibres positioned in a resin matrix so as to achieve maximum tensile strength.

HOUSING The housing is external to the core and protects it from the weather. It may be equipped with weather sheds. Some designs of composite insulators employ a sheath made of insulating material between the weathersheds and the core. This sheath is part of the housing.

WEATHERSHEDS Weathersheds are insulating parts, projecting from the housing or sheath, intended to increase the leakage distance and to provide an interrupted path for water drainage. Basic polymer shed materials used are silicone rubber, EPM, EPDM, CE, and polytetrafluorethylene (PTFE or Teflon). To obtain desired electrical and mechanical properties these basic material are combined with various fillers, including aluminium trihydrate.

END FITTING End fitting transmit the mechanical load to the core. They are usually made out of metal.

COUPLING ZONE The coupling zone is the part of the end fitting that transmits the load to the line, to the tower, or to another insulator. It does not include the interface between the core and the fitting.

INTERFACE An interface is the surface between different materials. Examples of interface in composite insulators are as follows: Glass fibre/impregnating resin Filler/polymer Core/housing Housing/weathersheds Housing/end fitting Core/end fittings






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CHARACTERISTICS whole moulding and whole injection crimped end fittings artistic looking small volume high bending and torsion strength good anti-explosive performance Light weight (65-80% less than ceramic insulator) Silicon rubber sheds provide perfect hydrophobic performance good resistance to ageing, tracking and erosion Stable behaviour at extreme climatic conditions Long term surface hydrophobicity Suitable for polluted environment, salty atmospheres etc. Resistance to breakage and vandalism, practically unbreakable Superior anti-tracking properties High mechanical strength Ease of installation (easier handling with lighter equipment and labour at the job site) Resistance to Seismic Shock Free of cleaning, economical maintenance and suitable for difficult maintenance area No need of zero value check easy and economical to transportation and installation Excellent anti-pollution performance, suitable for high polluted area. Free of cleaning, economical maintenance and suitable for difficult maintenance area. No need of zero value check. Reduce the purchasing quantities for spare parts.

ADVANTAGES OF POLYMER INSULATOR

It is very light weight compared to porcelain and glass insulator. As the composite insulator is flexible the chance of breakage becomes minimal. Because of lighter in weight and smaller in size, this insulator has lower installation cost. It has higher tensile strength compared to porcelain insulator. Its performance is better particularly in polluted areas. Due to lighter weight polymer insulator imposes lesser load to the supporting structure. Less cleaning is required due to hydrophobic nature of the insulator.

DISADVANTAGES OF POLYMER INSULATOR

Moisture may enter in the core if there is any unwanted gap between core and weather sheds. This may cause electrical failure of the insulator.

Over crimping in end fittings may result to cracks in the core which leads to mechanical failure of polymer insulator.

Subject to bird attack by Parrots, Cockatoos and Galahs. Not resilient to bushfire temperatures.






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Not recommended for location near surf beaches due to salt spray. Let us give a practical example where many difficulties were faced in maintaining a distribution network in Victoria Australia due to polymeric insulator. There were many Cockatoos, Galahs and Parrots in that area of Australia, who love to chew on polymeric strain insulators. Here, the 22 KV network has many of polymeric strain insulators installed and now after a few years of installing polymeric strain insulators, the authority is now replacing many of them back with Glass disc insulators. Another disadvantage is that they had post type polymeric insulators melt and bend in bush fire areas. They have a concrete pole and a steel cross arm that survives a bush fire; however the polymers in some cases fail. This would not be the case with glass or porcelain insulators. They have also had polymeric insulators fail in areas close to the ocean coastline where there are high salt levels in the air.

TYPES OF INSULATORS BASED ON CONSTRUCTION

TYPE A An insulator or an insulator unit in which the length of the shortest puncture path through solid insulating material is at least equal to half the length of the shortest flashover path through air outside the insulator.

TYPE B An insulator or an insulator unit in which the length of the shortest puncture path through solid insulating material is less than half the length of the shortest flashover path through air outside the insulator.

Type A insulator are of solid core type.

TYPES OF INSULATORS BASED ON USE PIN INSULATOR Pin Insulator is earliest developed overhead insulator, but still popularly used in power network up to 33 KV systems. Pin type insulator can be one part, two parts or three parts type, depending upon application voltage. In 11 KV system we generally use one part type insulator where whole pin insulator is one piece of properly shaped porcelain or glass. As the leakage path of insulator is through its surface, it is desirable to increase the vertical length of the insulator surface area for lengthening leakage path.

STRUCTURE As the name suggests, the pin type insulator is secured to the cross-arm on the pole with a pin. There is a groove on the upper end of the insulator for housing the conductor. The conductor passes through this groove and is bound by the annealed wire of the same material as the conductor.






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PETTICOATS In order to obtain lengthy leakage path, one, two or more rain sheds or petticoats are provided on the insulator body. In addition to that rain shed or petticoats on an insulator serve another purpose. These rain sheds or petticoats are so designed, that during raining the outer surface of the rain shed becomes wet but the inner surface remains dry and non-conductive. So there will be discontinuations of conducting path through the wet pin insulator surface. In higher voltage like 33KV and 66KV manufacturing of one part porcelain pin insulator becomes difficult. Because in higher voltage, the thickness of the insulator become more and a quite thick single piece porcelain insulator cannot manufactured practically. In this case we use multiple part pin insulator, where a number of properly designed porcelain shells are fixed together by Portland cement to form one complete insulator unit. For 33KV two parts and for 66KV three parts of pin insulators are used.

CAUSES OF INSULATOR FAILURES

Insulators are required to withstand both mechanical and electrical stresses. The latter type is

primarily due to line voltage and may cause the breakdown of the insulator. The electrical

breakdown of the insulator can occur either by flash-over or puncture. In flashover, an arc occurs

between the line conductor and insulator pin (i.e., earth) and the discharge jumps across the air

gaps, following shortest distance. Figure shows the arcing distance (i.e. a + b + c) for the insulator.




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In case of flash-over, the insulator will continue to act in its proper capacity unless extreme heat

produced by the arc destroys the insulator. In case of puncture, the discharge occurs from

conductor to pin through the body of the insulator. When such breakdown is involved, the insulator

is permanently destroyed due to excessive heat. In practice, sufficient thickness of porcelain is

provided in the insulator to avoid puncture by the line voltage. The ratio of puncture strength to

flashover voltage is known as safety factor.

DESIGNING CONSIDERATION OF PIN INSULATOR

The live conductor attached to the top of the pin insulator is at a potential and bottom of the

insulator fixed to supporting structure of earth potential. The insulator has to withstand the

potential stresses between conductor and earth. The shortest distance between conductor and

earth, surrounding the insulator body, along which electrical discharge may take place through air,

is known as flash over distance.

When insulator is wet, its outer surface becomes almost conducting. Hence the flash over distance of insulator is decreased. The design of an electrical insulator should be such that the decrease of flash over distance is minimum when the insulator is wet. That is why the upper most petticoat of a pin insulator has umbrella type designed so that it can protect the rest lower part of the insulator from rain. The upper surface of top most petticoat is inclined as less as possible to maintain maximum flash over voltage during raining.

To keep the inner side of the insulator dry, the rain sheds are made in order that these rain sheds should not disturb the voltage distribution they are so designed that their subsurface at right angle to the electromagnetic lines of force.

The pins shall of single piece obtained preferably by the process of forging. They shall not be made by joining, welding, shrink fitting or any other process using more than

one piece material. The pins shall be of good finish, free from flaws and other defects. The finish of the collar shall be such that sharp angle between the collar and the shank is

avoided. Aluminium ferrous pins, nuts and washers, except those made of stainless steel, shall be galvanized. The threads of nuts and taped hole when cut after galvanizing shall be well oiled or greased.

DIMENSIONS FOR PIN INSULATORS

Pins shall be of small steel head type S 165 P as per IS : 2486 (Part-II) having stalk length of 165 mm and shank length of 150 mm with minimum failing load of 16 KN.

HELICALLY FORMED PIN INSULATOR TIES

Helically formed ties used for holding the conductor on the pin insulator shall be made of aluminium alloy or aluminized steel or aluminium clad steel wires and shall conform to the requirements of IS : 12048. The ties shall be suitable for pin insulator dimensions of and conductor sizes specified.

Elastomer pad for insulator shall be used with the ties to avoid abrasion of the conductor coming into direct contact with the insulator.






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POST INSULATOR

Post insulator is more or less similar to pin insulator but former is suitable for higher voltage application. Post insulator has higher numbers of petticoats and has greater height. This type of insulator can be mounted on supporting structure horizontally as well as vertically. The insulator is made of one piece of porcelain but has fixing clamp arrangement in both top and bottom end.

PIN INSULATOR POST INSULATOR

It is generally used up to 33KV system It is suitable for lower voltage and also for higher voltage

It is single stag It can be single stag as well as multiple stags

Conductor is fixed on the top of the insulator by binding Conductor is fixed on the top of the insulator with help of connector clamp

Two insulators cannot be fixed together for higher voltage application

Two or more insulators can be fixed together one above other for higher voltage application

Metallic fixing arrangement provided only on bottom end of the insulator

Metallic fixing arrangement provided on both top and bottom ends of the insulator






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SUSPENSION INSULATOR

In higher voltage, beyond 33KV, it becomes uneconomical to use pin insulator because size, weight

of the insulator become more. Handling and replacing bigger size single unit insulator are quite

difficult task. For overcoming these difficulties, suspension insulator was developed. In suspension

insulator numbers of insulators are connected in series to form a string and the line conductor is

carried by the bottom most insulator. Each insulator of a suspension string is called disc insulator

because of their disc like shape.

ADVANTAGES OF SUSPENSION TYPE

Suspension type insulators are cheaper than pin type insulators for voltages beyond 33 kV. If anyone disc is damaged, the whole string does not become useless because the damaged disc

can be replaced.

The suspension arrangement provides greater flexibility to the line. The connection at the cross arm is such that insulator string is free to swing in any direction and can take up the position where mechanical stresses are minimum.

In case of increased demand on the transmission line, it is found more satisfactory to supply the greater demand by raising the line voltage than to provide another set of conductors. The additional insulation required for the raised voltage can be easily obtained in the suspension arrangement by adding the desired number of discs.

The suspension type insulators are generally used with steel towers. As the conductors run below the earthed cross-arm of the tower, therefore, this arrangement provides partial protection from lightning.

Each suspension disc is designed for normal voltage rating 11KV (Higher voltage rating 15KV), so by using different numbers of discs, a suspension string can be made suitable for any voltage level. For instance, if the working voltage is 66 kV, then six discs in series will be provided on the string.

Mechanical stresses on the suspension insulator are less since the line hanged on a flexible suspension string.

DISADVANTAGES OF SUSPENSION INSULATOR Suspension insulator string costlier than pin and post type insulator. Suspension string requires more height of supporting structure than that for pin or post insulator

to maintain same ground clearance of current conductor. The amplitude of free swing of conductors is larger in suspension insulator system, hence, more

spacing between conductors should be provided.






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STRAIN INSULATOR When suspension string is used to sustain extraordinary tensile load of conductor it is referred as string insulator. When there is a dead end or there is a sharp corner in transmission line, the line has to sustain a great tensile load of conductor or strain. A strain insulator must have considerable mechanical strength as well as the necessary electrical insulating properties. For low voltage lines (< 11 kV), shackle insulators are used as strain insulators. However, for high voltage transmission lines, strain insulator consists of an assembly of suspension insulators. The discs of strain insulators are used in the vertical plane. When the tension in lines is exceedingly high, at long river spans, two or more strings are used in parallel.

BALL & SOCKET TYPE

RATED SYSTEM VOLTAGE

NUMBER OF DISC INSULATOR USED IN STRAIN TYPE TENSION INSULATOR STRING

NUMBER OF DISC INSULATOR USED IN SUSPENSION INSULATOR STRING

33KV 3 3 66KV 5 4 132KV 9 8 220KV 15 14






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TONGUE & CLEVIS TYPE

STAY/GUY STRAIN INSULATOR

For low voltage lines, the stays are to be insulated from ground at a height. The insulator used in the stay wire is called as the stay insulator and is usually of porcelain and is so designed that in case of breakage of the insulator the guy-wire will not fall to the ground. The porcelain insulator shall be sound, free from defects, thoroughly verified and smoothly

glazed. The design of the insulator shall be such that the stresses to expansion and contraction in any

part of the insulator shall not lead to its deterioration. The glaze, unless otherwise specified, shall be brown in colour. The glaze shall cover the entire porcelain surface parts except those areas that serve as supports

during firing.

TYPES OF GUY INSULATORS

The standard guy strain insulators shall be designated as ‘A’ or ‘C’ as per IS: 5300. The recommended type of guy strain insulators for use on guy wires of overhead lines of 11 KV

voltage level is as follows: Power Line Voltage: 11KV Designation of Insulators: C Dry one minute Power Frequency withstand Voltage: 27 KV (rms) Wet one minute Power Frequency withstand Voltage: 13 KV (rms) Minimum Failing Load: 88(KN)

TYPE OF INSULATORS FOR GUY INSULATORS

The standard guy strain insulators shall be of designations ‘A‘or ‘C‘as per IS: 5300. The recommended type of guy strain insulators for use on guy wires of overhead lines of

different voltage levels are as follows :

LINE VOLTAGE DESIGNATION OF INSULATOR

415/240Volt A Type

11KV C Type

33KV C Type (2 No’s of Strings in Series).






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BASIC INSULATOR LEVEL FOR GUY INSULATORS

DESIGNATION OF INSULATOR

DRY ONE MIN POWER FREQUENCY WITHSTAND

WET ONE MIN POWER FREQUENCY WITHSTAND

A Type 18 KV (rms) 8 KV (rms)

C Type 27 KV (rms) 13 KV (rms)

MECHANICAL STRENGTH FOR GUY INSULATORS

DESIGNATION OF INSULATOR MINIMUM FAILING LOAD

A Type 44 KN

C Type 88 KN

SHACKLE INSULATOR OR SPOOL INSULATOR The shackle insulator or spool insulator is usually used in low voltage distribution network. It can be used both in horizontal and vertical position. The use of such insulator has decreased recently after increasing the using of underground cable for distribution purpose. The tapered hole of the spool insulator distributes the load more evenly and minimizes the possibility of breakage when heavily loaded. The conductor in the groove of shackle insulator is fixed with the help of soft binding wire.







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COMPARISON BETWEEN CERAMIC AND POLYMER INSULATOR

FACTOR CERAMIC POLYMER

Weight

Heavy, approx. wt. of 400 kv is about 135 kgs.

90% lighter than porcelain insulators, but offer an equal to better strength. Approx. wt. of 400 kv is less than 14 kgs

Packing & transport Risky & expensive Easy & economical Installation Risky, expensive and more labour

required Very easy & economical

Handling Difficult Easy Maintenance cost High Low Vandalism More susceptible Highly resistant Breakages & secondary damage

Highly fragile - 16 to 15% breakages are reported during transportation, storage and installation

Composite insulators are flexible and therefore, highly resistant to breakages.

Mechanical failure Reduction in mechanical strength

and separation due to pings getting eroded.

Single piece hence no such problem

Resistance to flashovers & punctures

Low High

Anti-tracking and erosion resistance

Very low - poor tracking resistance Excellent tracking resistance avoids erosion or tracking of the housing material.

Dielectric strength Lower than polymer Excellent insulation performance Contamination & pollution

Highly affected Not affected and has longer life

Hydrophobicity Non hydrophobic, porcelain surface forms water film on the surface making easy path leading to more flash overs

The hydrophobicity properties of silicon rubber provide excellent insulating behaviour and resists wetting by forming beads of water without the need of washing or greasing even in humid or polluted climates. Hence low failure rate combined with low overall operating and maintenance costs.

Self-cleaning quality No. - dirt, sand, salt & snow are easily attracted

Yes. Due to hydrophobicity recovery characteristic

Tensile strength Good Excellent due to crimping technology. Maintenance Needs maintenance like cleaning,

washing and greasing No maintenance is required

Design Design flexibility is limited. Requires larger and heavier towers for installation and more space.

Polymer insulator design allows for adaption to suit specific needs such as creepage distance. Results in space saving and lower cost

Manufacturing process Porcelain insulators require long manufacturing process leading to long delivery time. Manufacturing process causes pollution & health risk .

Pollution free, safe, short process time leading to short delivery periods

Safety Porcelain insulators are susceptible to explosion & breakages, due to high fragile properties , stone throwing etc.

Composite insulators provide very high level of safety, superior flexibility and strength. Not susceptible to explosion. No breakages due to stone throwing etc.






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CLEARANCE AND CREAPAGE DISTANCE

CREEPAGE DISTANCE FOR INSULATOR

HIGHEST SYSTEM VOLTAGE

MODERATELY POLLUTED

ATMOSPHERES TOTAL

HEAVILY POLLUTED ATMOSPHERES

KV MM TOTAL (MM) PROTECTED(MM) 3.6 75 130 7.2 130 230 12 230 320 24 430 560 36 580 840 420 72 1160 1700 850 123 1850 2800 1400 145 2250 3400 1700 245 3800 5600 2800 420 6480 9660 4830

MARKING FOR INSULATOR Name or trademark of manufacturer. Month and year of manufacture. Minimum failing load in KN. ISI certificate mark, if any. Markings on porcelain shall be printed and shall be supplied before firing.

CAUSES OF INSULATOR FAILURE

CRACKING OF INSULATOR

The porcelain insulator mainly consists of three different materials. The main porcelain body, steel

fitting arrangement and cement to fix the steel part with porcelain. Due to changing climate

conditions, these different materials in the insulator expand and contract in different rate. These

unequal expansion and contraction of porcelain, steel and cement are the chief cause of cracking

of insulator.

DEFECTIVE INSULATION MATERIAL

If the insulation material used for insulator is defective anywhere, the insulator may have a high

chance of being puncture from that place.

POROSITY IN THE INSULATION MATERIALS

If the porcelain insulator is manufactured at low temperatures, it will make it porous, and due to

this reason it will absorb moisture from air thus its insulation will decrease and leakage current will

start to flow through the insulator which will lead to insulator failure.




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IMPROPER GLAZING ON INSULATOR SURFACE If the surface of porcelain insulator is not properly glazed, moisture can stick over it. This moisture

along with deposited dust on the insulator surface produces a conducting path. As a result the

flash over distance of the insulator is reduced. As the flash over distance is reduced, the chance of

failure of insulator due to flash over becomes more.

FLASH OVER ACROSS INSULATOR If flash over occurs, the insulator may be over heated which may ultimately results into shuttering

of it.

MECHANICAL STRESSES ON INSULATOR If an insulator has any weak portion due to manufacturing defect, it may break from that weak

portion when mechanical stress is applied on it by its conductor. These are the main causes of

insulator failure.

INSULATOR DESIGN

BASIC DESIGN CONCEPTS

The basic designs of polymer insulators evolve around three essential components. These are, a

core, a sheath or weathersheds and metal end fittings.

The end fittings are attached to the core in various ways to develop the required mechanical strength for the intended application. The core consists of axially aligned glass fibres bonded together by means of an organic resin. The unprotected core with end fittings by itself is not suitable for outdoor high voltage application, as moisture, ultraviolet rays, contamination, acid rain, ozone and voltage are conducive to the degradation of the core material and leading to electrical and mechanical failure. Hence, a protective sheath or weathersheds made from various polymer materials that have been compounded for outdoor electrical applications are applied over the core in various ways to protect the core and to provide maximum electrical insulation between the attachment ends. It is quite clear that with such a diversity of possible constructions, the performance of polymer insulators depends on the selection of materials and on the design and construction of the insulators.

MATERIAL SELECTION CORE The mechanical strength member of polymer insulator is a fiberglass rod. The rod, normally referred to as the core of the insulator, consists of between 70 and 75% by weight of axially aligned glass fibres bonded by an organic resin. The resin system can be either polyester or epoxy and the rod either cast or protruded. Today’s core is protruded in various diameters with electrical grade E-type glass fibres and polyester Resin. Two critical process parameters in the protrusion of fiberglass rod for insulators are pulling speed and temperature of the forming die. An axial crack develops when the outside of the rod cures more quickly than the centre of the rod. This occurs when either the die temperature is too high for the pulling speed or outside of the rod sets, shrinkage during curing of the bulk of the rod and produces an axial crack in the centre of the rod. Bonding of glass fibres to the polyester resin is






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affected by process parameters as well. In pulling glass fibre, the fibre is sized or treated chemically for protection against mechanical damage during handling. Optimum bonding requires compatibility of the chemical systems; otherwise, the rod may exhibit porosity and wick moisture by capillary. The rod may still be acceptable for mechanical applications but not for polymer insulators.

WEATHERSHEDS weathersheds for polymer insulators are manufactured from materials such as bisphenol or cycloaliphatic epoxy resins, thermoplastic rubber, and ethylene-propylene diene Monomer (EPDM) and silicone elastomers. These materials are compounded with various types of inorganic fillers such as silica and hydrated alumina with concentrations ranging from a few percent to 70% by weight. Today, the elastomeric materials of EPDM and silicone containing a minimum of 70% by weight of hydrated alumina that are in use by most of manufacturers are favoured for weathersheds with silicone rubber clearly showing the best performance over all other types. Failures of some first generation polymer insulators with epoxy resin weathersheds have been attributed to depolymerisation by hydrolysis. Depolymerisation refers to the destruction of the molecular structure of polymer materials. Hydrolysis is the result of a chemical reaction, which takes place between the ions of water and the free ends of polymer chemical chain, which causes depolymerisation to occur. Also, insulators made from epoxy resins contain locked-in mechanical stresses that develop during curing of the resin. This occurs when mixing or curing of the resin is uneven. Circumferential cracks between sheds sometimes develop during storage of the insulator because of the locked- in stresses. However, more often the cracks develop in service as the stresses are aggravated by low temperature and line tension. The cracks extend down to the core, thereby exposing the core to moisture. Elastomers are the best weathershed material, as they do not contain locked- in mechanical stresses from the curing process. Also, elastomers are preferred at low temperatures where impact resistance is important. Another problem that surfaced early in the experience of first generation designs were the effect of outdoor weathering on weathersheds. Weathering affects all polymer materials to some extent and being a natural phenomenon includes the effects of heat, humidity, rain, wind, contaminants in the atmosphere and ultraviolet rays of the sun. Under such conditions, the weathersheds of polymer insulators may be permanently changed, physically by roughening and cracking and chemically by the loss of soluble components and by the reactions of salts, acids and other impurities deposited on the surface. Surface become hydrophilic and moisture more easily penetrates into the volume of the weathersheds.

END FITTINGS The end fittings are cast from either aluminium or malleable iron in sand castings. Some aluminium end fittings are cast in a permanent mould. The strengths of these fittings are more consistent than the sand-cast ones. Forged and extruded aluminium end fittings have strengths that are also quite consistent. However, if swaging is the method of attaching the fittings to the core, both permanent mould and forged aluminium fittings may develop cracks in service.

INSULATOR DESIGN Elastomeric weathersheds in first generation insulators are individually moulded and glued to the core and to each other by an epoxy adhesive. In some designs, silicone gel or silicone caulking sealant is applied. In other designs, silicone gel or grease is used to fill the air space between the sheds and the core. Although epoxy glue provides some measure of protection against water entry, there is some uncertainty in the lifetime of such a seal. Insulators constructed from individual sheds are known to permit water reaching the core and have failed during hot line high-pressure water washing.






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Most of the insulators are moulded from either EPDM or silicone elastomers, in one-piece having an aerodynamic design, that is fully vulcanized to the core. The insulators are terminated with end fittings that are swaged onto the core sealing the insulator from moisture reaching the core. Some designs or end fittings of the insulators are equipped with power arc interceptors. These serve to protect against loss of seal during flashover. Aluminium end fittings of a unique design are swaged onto the core there by effectively sealing the sleeved ends from moisture.

POLLUTION CONSIDERATION

TABLE-1 POLLUTION CATEGORY

ENVIRONMENT DESCRIPTION MINIMUM SPECIFIC

NOMINAL LEAKAGE

(MAX. S.D.D.) MM/KV IN/KV I-light (0.06 mg/cm2)

Areas without industrial and with low density of

Houses equipped with heating plants. Areas with low density of industries or houses But subjected to frequent wind and / or

rainfall. Agricultural areas. Mountainous areas. All areas situated 16 km to 20 km from the sea And not exposed to wind directly from the

sea.

16 0.63

Ii-medium (0.20 mg/cm2)

areas with industries not producing particularly

Polluting smoke and / or with average density of

Houses equipped with heating plants. areas with high density of houses and / or but Subjected to frequent winds and / or rainfall. areas exposed to winds from the sea but not

too Close to the coast (at least several kilometers Distance).

20 0.79

Iii- heavy (0.60 mg/cm2)

areas with high density of industries and Suburbs of large cities with high density of Heating plants producing pollution. areas close to the sea or in any case exposed

to Relatively strong winds from the sea.

25 0.98

IV-very Heavy (>0.60 mg/cm2)

Areas generally of moderate extent, subjected to

Conductive dusts and to industrial smoke Producing particularly thick conductive

deposits. Areas generally of moderate extent, very close

to The coast and exposed to sea-spray or to very Strong and polluting winds from the sea. Desert areas, characterized by no rain for long Periods, exposed to strong winds carrying

sand And salt, and subjected to regular

condensation.

31 1.22






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S.D.D. = Salt Deposit Density

POLLUTION SEVERITY LEVELS For the purposes of standardization, four levels of pollution are qualitatively defined, from light Pollution to very heavy pollution. Table-1 gives, for each level of pollution, an approximate Description of some typical corresponding environments. Other extreme environmental conditions Exist which merit further consideration, e.g. Snow and ice in heavy pollution, heavy rain, and arid Areas.

RELATION BETWEEN THE POLLUTION LEVEL AND THE SPECIFIC CREEPAGE DISTANCE For each level of pollution described in Table-1, the corresponding minimum nominal specific Creepage distance, in millimetres per kilovolt (phase-to phase) of the highest voltage for insulator is Also given.

APPLICATION OF THE "SPECIFIC CREEPAGE DISTANCE" CONCEPT In order to successfully apply the "specific creepage distance" concept, certain dimensional Parameters characterizing the insulator shall be taken into account. The important parameters, that have to be taken into account, are:

PARAMETERS CHARACTERIZING THE PROFILE The profile of an insulator is characterized by the following parameters: Minimum distance, c, between sheds Ratio s / p between spacing and shed overhang Ratio ld / d between creepage distance and clearance alternating sheds Inclination of sheds Parameters characterizing the entire insulator Creepage factor C.F. Profile factor P.F.

INFLUENCE OF THE POSITION OF INSULATORS There is normally some change in the pollution performance of insulators designed for use in The vertical position when they are used in an inclined or horizontal position. Generally the Change is for an improvement in performance, but in certain cases a reduction may result, due For example to the cascade effect of heavy rain.

INFLUENCE OF THE DIAMETER Various laboratory tests appear to indicate that the pollution performance of post insulators Decreased with increasing average diameter. The following values for kd are proposed, k being a factor to increase the creepage distance With

average diameter Dm in millimeters.

average diameter Dm < 300 mm: kd = 1 300 ≤Dm ≤500 mm: kd = 1.1 Dm > 500 mm: kd = 1.2 For a given profile, the average diameter Dm is given by:






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1. Regular sheds Dm = D 1 + D2 / 2 2. Alternating sheds Dm = D1 + D2 + 2D 1 / 2 Where: L is the total creepage distance of the insulator. D(l) is the value of the diameter at a creepage distance l, measured from one electrode.

DETERMINATION OF THE CREEPAGE DISTANCE The minimum nominal creepage distance of an insulator situated between phase and earth is Determined, according to the pollution level of the site, by the relation: Minimum nominal creepage distance = minimum specific creepage distance X highest System voltage phase-to-phase for the equipment X kd Where: KD is the correction factor due to diameter If insulators are to be used between phases (phase-spacers for instance), the creepage distance should Be multiplied by v3 (for a three-phase system).






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INSULATION CONSIDERATION There are basically three factors to consider when designing the insulators. The 50/60-Hz power voltage. Surge voltages caused by lightning. Surge voltage caused by switching. Surge voltages provide the most stringent test and the rationale for the standard impulse voltage wave Form; that is, if the insulator is properly insulated to withstand surges, it can usually accommodate the Highest expected 50/60 Hz voltage. Insulators are more tolerant of short duration overvoltage than Sustained values. For the purpose of impulse testing, a standard waveform is defined, as shown in Figure. The waveform is referred to as T1 X T2, where both values are conventionally given in Microseconds. The crest value of the waveform V. The value T1 is the rise time to crest, whereas the Value T2 is the fall time to 0.5 V. A convenient analytical representation of the pulse waveform is the double-exponential expression V(t) = V1[exp(-t / t2) – exp(-t / t1)] Where T2 = T2 / ln (2) = 1.443T2 T1 = T1 / 5 = 0.2T1 V1 = V exp(T1 / 1.443T2)

WAVEFORM- LIGHTENING IMPULSE

WAVEFORM- SWITCHING IMPULSE






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For a given well-defined voltage waveform, under specified test conditions, the following Terminology is defined:

CRITICAL FLASHOVER VOLTAGE (CFO) The crest (maximum) voltage for which the probability of Flashover is 0.50.

WITHSTAND VOLTAGE The crest voltage 3s below the CFO

BASIC (LIGHTNING) IMPULSE INSULATION LEVEL (BIL) The crest voltage for which the probability of Flashover is 0.16, using a 1.2/50 ms test pulse.

BASIC (SWITCHING) SURGE IMPULSE INSULATION LEVEL (BSL) The crest voltage for which the probability of flashover is 0.16, using a 250/2500 s test pulse.

CONSIDERATION OF INTERFERENCE AND CORONA GENERATING PROCESSES Interference with radio and television (ri and tvi) may arise when electrical discharges run on Insulators and inject high-frequency currents into associated conductors, which radiate Electromagnetic waves. The types of discharge which generate interference are: Micro sparks between Water drops or metal fittings, the latter especially in cases of corrosion. Discharges across dry bands on leaky surfaces.

SURFACE CORONA DISCHARGES AROUND HIGHLY STRESSED ELECTRODES Surface corona discharges are again relatively slow phenomena, incapable of heavy generation at Vhf, but principal sources at lower frequencies. Surface discharges may be prevented by hydrophobic treatment. This not only inhibits dry-band formation but also gives good voltage Grading, thus removing the over voltages, which cause other types of discharge. It might appear that the installation of a corona ring would smother capacitive over voltages, while avoiding the usual Power loss, which follow installation of a corona ring. Both the above-mentioned types of discharge are, to some extent, weather dependent. Water may Cause droplet discharges while suppressing contact discharges by virtue of its high conductivity and Permittivity.






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Dry bands are not a fair-weather phenomenon, the existence of which depends on the design of the insulator and the geometry of the insulators of fittings. Surface corona, if in fair Weather, for which reason it has long been the practice to specify higher than normal voltages for Corona inception on insulator.

CONSIDERATION OF CAPACITANCE EFFECTS The distribution of capacitance along an insulator, and their size, govern the electric stresses, which excite generation of interference, and the coupling of the generator to the radiating antenna. A composite insulators like a cylinder of dielectric having a relative permittivity about 6. The field Intensity falls away rapidly with increasing distance from the live terminal. The generating discharges occur at or near the live terminal, and the capacitance, which couples the high frequency Currents into the radiating circuit, i.e. the line and tower, is small. Composite insulators are thus significantly quieter as interfering sources than string of discs. In a string of discs, quite large capacitance – of the order of 30 pf – is connected in cascade through the fittings. The voltage distribution is governed purely by these and by the stray capacitance to line and ground, in dry conditions. In such a voltage-dividing circuit the partition is independent of frequency: identical distributions therefore exist for the power- frequency and for the radio-frequency Voltage. The units at the line end are more prone to surface corona than the rest. Because of the high unit capacitance the sources are closely coupled into the line, which presents load impedance Equal to one-half of the line’s surge impedance. It is common practice to relieve the line end Overvoltage by means of stress- grading fittings. Some of the devices which are used to minimize surface corona, in cases like these where ‘quiet’ Insulators are essential. Tests showed those gradients between 16 and 14 kv/cm is sufficient to break Down air in contact with insulator over gaps of a few centimetres.






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INSULATOR SELECTION MECHANICAL PARAMETERS The process for selecting the strength rating of composite or ceramic insulators is identical. Based on strength alone, a composite insulator can directly replace insulators made of porcelain or glass. Composite insulator’s strength rating is as defined in the Table-2. Historically, when selecting an insulator’s strength rating a two to one safety factor has been applied because all materials have a time- load characteristic that reduces their residual strength over time. Most manufacturer rate composite insulators at a maximum working load equal to 50% of the Ultimate load rating. TABLE-2

LOADING CONSIDERATIONS-ICE AND WIND Establishing the everyday working load of an insulator requires that ice and wind conditions for the area of

application be considered. Ice and wind can add considerable load to the conductor and insulators, resulting

in greater loads. Table-3 lists parameters to use for calculating ice and wind loads for each region.

The following example determines the total loading of ice, wind and conductor weight. This calculation

does not include the weight of the insulators and other items that increase the load on the insulators.

Normally, such items should be added to the final load. A composite insulator’s weight is a small percentage

of the total weight; therefore, omitting it from the calculation will have minimal effect on the outcome.

TABLE-3: LOADING PARAMETERS

LOADING RADIAL ICE THICKNESS (IN)

HORIZONTAL WIND PRESSURE (LBS./SQ. FT.)

TEMPERATURE (DEGREE F)

CONSTANT (LBS./FT.)

Heavy

0.50 4 (40 mph) 0 0.30

Medium 0.25 4 (40 mph) +15 0.20

Light 0 9 (60 mph) +30 0.05

LOADING NON CERAMIC CERAMIC

SUSPENSION POST Maximum Short Term

Load (one minute) SML (Specified

Mechanical Load) UCL (Ultimate

Cantilever Load) M&E (Mechanical and

Electrical Strength) Maximum Temporary

Load (one week) 50% to 60% of the

SML Not Defined Not Defined

Maximum Working

Load (continuous) RTL (Routine Test

Load) 50% of the

SML

WCL (Working

Cantilever Load)

50% or UCL

TPL (Tension Proof)

50% of the M&E and

WCL 40% of the UCL






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TABLE-4: STEPS USED IN CALCULATING THE TOTAL LOAD ON A

CONDUCTOR

1. Select ice and wind Parameters

Determine maximum radial ice thickness and wind pressure From the local standard.

2. Calculate vertical load Conductor and ice weight between insulator span, including Spacers and other equipment the conductor supports.

3. Calculate horizontal load Horizontal wind pressure on the conductor, radial ice thickness, Spacers and other equipment the conductor supports.

4. Calculate combined load Vector sum of the horizontal and vertical loads.

5. Calculate total conductor load Combine load plus the loading constant from table 3.

ELECTRICAL PARAMETERS An insulator’s electrical parameters should be selected to maintain an economic balance between system

performance and the cost of over insulation. Increasing an insulator’s electrical parameters will provide

greater system protection at a higher price and, for long power lines the cost can be substantial. Therefore,

insulation requirements for power systems should be considered on an individual basis because of the

uniqueness of power system configurations, grounding techniques, and protection scheme.

CIFO SELECTION Generally, an acceptable flashover probability is incorporated into the design of a transmission line because

it’s not economically feasible to protect against all occurrences of lightning overvoltages, These

overvoltages result from shielding failures caused by direct strokes to the conductor, from back-flashover

caused by strokes to the tower and shield wire, and from induced voltages caused by strokes to nearby

grounded objects. When a flashover occurs, an insulator is not normally affected and will recover, unlike

transformer insulation that will be damaged. This ability allows an insulator’s flashover level to be selected

by a statistical process that gives it an expected flashover probability. The capability of an insulator to resist

flashover caused by lightning strokes relies on the air gap distance (Dry Arc) between its live and ground

end. Therefore, as the system insulation level increases, so must the insulator’s length.






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TABLE-5 RECOMMENDED BIL AT VARIOUS OPERATING VOLTAGES

VOLTAGE CLASS (KV)

BIL (KV)

REDUCED BIL (KV)

15 116 23 150 34.5 200 125 46 250 69 350 92 450 115 550 450/350 138 650 550/50 161 750 650/550 196 900 230 1650 900/825/750 287 1300 1175/1650/900 345 1550 1425/1300/1650 500 1800 1675/1550/1300

The equipment used in a power system comprises items having different breakdown or withstand Voltages and different voltage time characteristics. In order that all items of the system are adequately Protected there is a need to consider the situation as a whole and not items of plant in isolation; i.e. The insulation protection must be coordinated. To assist this process, standard insulation levels are recommended and these are summarized in Table-5. Reduced basic insulation impulse levels are used when considering switching surges and are also summarized in Table-5.

Statistical data has shown that the CIFO should be applied at three standard deviations (s) above the system BIL; this will provide an acceptable flashover probability. Since the standard deviation is equal to 3% the required CIFO can be computed directly from the system BIL.






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1. The BIL is usually expressed as a per unit of the peak (crest) value of the normal operating voltage to earth; e.g. for a maximum operating voltage of 362 kV. 1 p.u. = v2 X (362 / v3) = 300 kV so that a BIL of 2.7 p.u. = 816 kV Calculation of using formula: Using Typical System BIL CIFO = BIL/ 0.91

LOW FREQUENCY LING DURATION (60HZ) SELECTION Insulators must perform under two types of electrical stresses, i.e., those caused by temporary overvoltages and those caused by the steady state nominal system voltage. Typically the occurrence of an overvoltage is an uncommon event and limited to a relatively short time. This type of stress can be determined by a system analysis and then be accounted for when selecting the insulator requirements. For system voltages through 230 kV, the dry flashover value is typically five times the maximum line to ground power frequency voltage. This margin results in a wet flashover value of three to five times the line to ground power frequency voltage. Normally, experience with the area and design or a system study is required to determine the minimum insulation strength.

SWITCHING SURGE SELECTION A switching overvoltage is caused by a switching operation or a fault on the system that occurs during line reclosing, fault clearing, and switching of capacitive or inductive circuits. Surge developed by these operations is generally of short duration, highly damped, and has unpredictable amplitudes. Switching surge requirements depend on the transmission- line design and location, i.e.; clearances between live parts and ground, altitude, and system grounding. For system voltages below 300 kV the probability of a switching surge flashover is negligible and lightning surges are of more concern. Above 300 kV, grounding and shield design determine whether a switching surge or lightning surge has the greater influence. Often, because of improved grounding techniques, tower designs, and line shielding methods, switching surge levels are more representative of the system requirements. Table-6 lists typical switching surge overvoltage from the Transmission Line.

TABLE-6 TYPICAL SWITCHING SURGE LEVELS

*Per unit voltage, expressed in maximum phase to ground system voltage.

CONTAMINATION PERFORMANCE The long term electrical performance of a polymer insulator is contingent upon the weatherability of the polymer material and the level of electrical stress subjected to the material. If the material weathers poorly or the electrical stress is too high, the polymer could prematurely degrade and lead to a shortened service life.

CAUSE OF SWITCHING SURGE MAXIMUM PER UNIT* OVERVOLTAGE

Energize 200 mile line, no closing resistors 3.5 Fault initiation on unfaulted phase 2.1 Fault clearing 1.7 to 1.9 Energize line and transformer 1.2 to 1.8 First restrike on capacitor bank 3.0






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The weathering of the material is a natural event caused by many factors, some of which are; wind, rain, heat, cold, humidity, pollutants from factories and automobiles, and ultraviolet rays from the sun. A polymer material's ability to endure these conditions for an extended period is dependent upon its chemical composition. Hydrophobicity is the characteristic of silicone rubber that sets it apart from other commonly used insulator materials. Since silicone rubber does not readily wet-out, leakage current remains low and flashover are prevented. Even when the insulators are heavily contaminated, silicone has the unique ability to impart hydrophobicity to the contaminated layer due to the migration of low molecular weight silicone polymer molecules into the contaminate. Contamination flashover performance of silicone is far superior to other insulation materials, and because the contamination layer is hydrophobic, power washing of silicone insulators is not required. To determine pollution category, use Table-1.

CALCULATION OF INSULATOR LEAKAGE DISTANCE EXAMPLE: Pollution category II-Medium Required Leakage = (system kV line) X (5% voltage regulation) X (leakage/kV line)

GRADING RING SELECTION Corona discharges can form on the metallic ends of an insulator when the electric-field intensity exceeds the dielectric strength of the surrounding air. Several factors that affect the onset of corona are air pressure, humidity, end fitting material, and voltage level. If the end fitting has sharp edges or is irregular in shape, corona can result from a concentration of the electric field. Grading rings reshape the electric field, thus, reducing corona discharge on the insulator, high RIV levels, and power losses. If corona persists, it may damage the polymer material and lead to insulator failure. The following recommendations for corona extinction and RIV are given by proposed ANSI standard C29.12:

MINIMUM CORONA EXTINCTION LEVEL 115% of the nominal line to ground voltage.

MAXIMUM RIV 160 mV at 115% of nominal line to ground voltage. The elevation at which insulator is applied is an important consideration when specifying grading rings. Tests have shown that corona extinction and inception levels decrease with increasing altitude; approximately a 1% change for every 1600 feet above sea level. However, the effect is not as significant below 3000 feet since the bulk of corona tests occur near this level. Therefore, corona extinction levels must be adjusted for applications above 3000 feet. Grading rings are selected according to the corona extinction level. A variety of ring shapes and sizes are necessary to meet the requirements of the different voltage levels and insulator configurations. Generally, by increasing a corona rings diameter or thickness the inception level increases.






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INSULATOR CHARACTERISTICS SERVICE CONDITIONS i) Weather Degree ii) Pollution Category iii) Altitude iv) Insulator Configuration

PHYSICAL CHARACTERISTICS i) Section Length ii) Dry Arc Distance iii) Leakage Distance

ELECTRICAL CHARACTERISTICS i) Max. System Voltage ii) System BIL iii) Insulator CIFO iv) Min. Dry 60 Hz Flashover v) Min. Wet 50/60 Hz Flashover vi) RIV

MECHANICAL CHARACTERISTICS Specified Mechanical Load (S.M.L.) - for Suspension Insulator Ultimate Cantilever Load - for Line Post or Station Post Insulators






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INSULATOR TESTING

TYPE TESTS Tests carried out to prove conformity with the specification. These are intended to prove the general qualities and design of a given type of insulator.

ACCEPTANCE TESTS Tests carried out on samples taken from the lot for the purpose of acceptance of the lot.

ROUTINE TESTS Tests carried out on each insulator to check requirements which are likely to vary during production. According to the British Standard, the electrical insulator must undergo the following tests

1. Flashover tests of insulator 2. Performance tests 3. Routine tests

FLASHOVER TEST There are mainly four types of flashover test performed on insulators-

POWER FREQUENCY DRY FLASHOVER TEST

1. First the insulator to be tested is mounted in the manner in which it would be used practically. 2. Then terminals of variable power frequency voltage source are connected to the both electrodes

of the insulator. 3. Now the power frequency voltage is applied and gradually increased up to the specified value.

This specified value is below the minimum flashover voltage. 4. This voltage is maintained for one minute and observes that there should not be any flash-over or

puncture occurred. The insulator must be capable of sustaining the specified minimum voltage for one minute without

flash over.

POWER FREQUENCY WET FLASHOVER TEST OR RAIN TEST

1. In this test also the insulator to be tested is mounted in the manner in which it would be used practically.

2. Then terminals of variable power frequency voltage source are connected to the both electrodes of the insulator.

3. After that the insulator is sprayed with water at an angle of 45o in such a manner that its precipitation should not be more 5.08 mm per minute. The resistance of the water used for spraying must be between 9 kΩ 16 11 kΩ per cm3 at normal atmospheric pressure and temperature. In this way we create artificial raining condition.

4. Now the power frequency voltage is applied and gradually increased up to the specified value. 5. This voltage is maintained for either one minute or 30 second as specified and observe that there

should not be any flash-over or puncher occurred. The insulator must be capable of sustaining the specified minimum power frequency voltage for specified period without flash over in the said wet condition.

POWER FREQUENCY FLASHOVER VOLTAGE TEST

1. The insulator is kept in similar manner of previous test. 2. In this test the applied voltage is gradually increased in similar to that of previous tests. 3. But in that case the voltage when the surroundings air breaks down is noted.




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IMPULSE FREQUENCY FLASHOVER VOLTAGE TEST

The overhead outdoor insulator must be capable of sustaining high voltage surges caused by

lightning etc. So this must be tested against the high voltage surges.

1. The insulator is kept in similar manner of previous test. 2. Then several hundred thousand Hz very high impulse voltage generator is connected to the

insulator. 3. Such a voltage is applied to the insulator and the spark over voltage is noted. 4. The ratio of this noted voltage to the voltage reading collected from power frequency flash over

voltage test is known as impulse ratio of insulator.

This ratio should be approximately 1.4 for pin type insulator and 1.3 for suspension type insulators.

PERFORMANCE TESTS

TEMPERATURE CYCLE TEST

1. The insulator is first heated in water at 70oC for one hour. 2. Then this insulator immediately cooled in water at 7oC for another one hour. 3. This cycle is repeated for three times. 4. After completion of these three temperature cycles, the insulator is dried and the glazing of

insulator is thoroughly observed. After this test there should not be any damaged or deterioration in the glaze of the insulator surface.

PUNCTURE VOLTAGE TEST

1. The insulator is first suspended in insulating oil. 2. Then voltage of 1.3 times of flash over voltage is applied to the insulator.

A good insulator should not puncture under this condition.

POROSITY TEST

1. The insulator is first broken into pieces. 2. Then these broken pieces of insulator are immersed in a 0.5 % alcohol solution of fuchsine dye

under pressure of about 140.7 kg ⁄ cm2 for 24 hours. 3. After that the sample is removed and examined.

The presence of a slight porosity in the material is indicated by a deep penetration of the dye into

it.

MECHANICAL STRENGTH TEST

The insulator is applied by 2½ times the maximum working strength for about one minute. The

insulator must be capable of sustaining this much mechanical stress for one minute without any

damage in it.

ROUTINE TESTS Each of the insulators must undergo the following routine test before they are recommended for

using at site.







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PROOF LOAD TEST

In proof load test of insulator, a load of 20% in excess of specified maximum working load is applied

for about one minute to each of the insulator.

CORROSION TEST

1. The insulator with its galvanized or steel fittings is suspended into a copper sulphate solution for one minute.

2. Then the insulator is removed from the solution and wiped, cleaned. 3. Again it is suspended into the copper sulphate solution for one minute. 4. The process is repeated for four times.

Then it should be examined and there should not be any disposition of metal on it.






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Transmission line insulators

Engineering