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COMPOSITE INSULATORS

CHAPTER 1

Introduction

Electrical insulators are used to prevent the loss of electric charge or current from conductors in electric power transmission lines. Electrical insulators are electrically insulating components in various electric circuits and electrical installations. Electrical insulators are used as a barrier layer used in a circuit, an insulating sheathing of a current-carrying conductor or a printed-circuit board for electronics. An electrical insulator is also an insulator as used in power engineering for routing current-carrying lines or keeping them apart. Power transmission and distribution systems include various insulating components that must maintain structural integrity to perform correctly in often extreme environmental and operational conditions.

The overhead line conductors should be supported on the pole or towers in such a way that current from the conductors do not flow to earth through supports i.e., line conductors must be properly insulated from supports. This is achieved by securing line conductors to supply with the help of insulators. The insulators provide necessary insulation between line conductors and supports and thus prevent any leakage current from conductors to earth.

In general, the insulators should have following desirable properties:

1. High mechanical strength in order to withstand conductor load, wind

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load etc.2. High electrical resistance of insulators material in order to avoid

leakage currents to earth.3. High relative permittivity of insulators material in order that dielectric

strength is high.4. The insulator material should be non –porous , free from impurities and

cracks otherwise the permittivity will be lowered.

5. High ratio of puncture strength to flashover.

The insulators have conventionally been made of ceramics or glass. These materials have outstanding insulating properties and weather resistance, but have the disadvantages of being heavy, easily fractured, and subject to degradation of their withstand voltage properties when polluted. There was therefore a desire to develop insulators of a new structure using new materials that would overcome these drawbacks.

The 1930s and '40s saw the appearance of the first insulators to replace inorganic materials with organic, but these suffered problems of weather resistance, and their characteristics were unsatisfactory for outdoor use. In the 1950s epoxy resin insulators were developed, but they were heavy, suffered from UV degradation and tracking, and were never put into actual service. By the mid-1970s a number of new insulating materials had been developed, and the concept of a composite structure was advanced, with an insulator housing made of ethylene propylene rubber (EPR), ethylene propylene diene methylene (EPDM) linkage, polytetrofluoro ethylene (PTFE), silicone rubber (SR) or the like, and a core of fiber reinforced plastic (FRP) to bear the tensile load. Since these materials were new, however, there were many technical difficulties that had to be remedied, such as adhesion between materials and penetration of moisture, and the end-fittings, which transmit the load, had to be improved. Since the 1980s, greater use has been made of silicone rubber due to its weather resistance, which is virtually permanent, and its hydrophobic properties, which allow improvement in the maximum, withstand voltage of pollution, and this had led to an explosive increase in the use of composite insulators. In 1980, Furukawa Electric was engaged in the development of inter-phase spacers to divvent galloping in power transmission lines, and at that time

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developed composite insulators that had the required light weight and flexibility. In 1991 the first composite insulators having silicone rubber housing were used as inter-phase spacers for 66-kV duty, and in 1994 their use was extended to 275-kV service with a unit 7 m in length the worlds largest. Thus as composite insulators have established a track record in phase spacer applications and their advantages have been recognized, greater consideration has been given to using them as suspension insulators with a view to cutting transportation costs, simplifying construction work and reducing the cost of insulators in order to lower the costs of laying and maintaining power transmission lines. Recently Furukawa Electric developed composite insulators for suspension and delivered, for the first time in Japan, 154-kV tension insulators and V-type suspension insulator strings. Subsequently they were also used on a trial basis as tension-suspension devices in 77-kV applications. Work is also under way on the development of composite insulators for 1500-V DC and 30-kV AC railway service.

. There are following types of composite insulators .

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Silicon rubber (SR) – Room temperature vulcanizing (RTV), High temperature vulcanizingand liquid rubber (LSR).1. Ethylene Propylene rubber (EPR) Ethylene Propylene Monomer (EPM), 2. Ethylene Propylene Di Monomer (EPDM).3. Epoxy Resin – Bisphenol, cycloaliphate4. Polyurethene5. Polyfluoro carbons – Teflon, PTFE

2. RESEARCH SIGNIFICATION:

The insulators have generally been made of ceramics or glass.These materials have outstanding insulating properties and weather resistance, but have the disadvantages of being heavy, easily fractured, and subject to degradation of their withstand voltage properties when polluted. There was therefore a desire to develop insulators of a new structure using new materials that would overcome these drawbacks. Non-ceramic insulators, also referred to as composite insulators, polymer or polymeric insulators are used in power transmission lines.

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2. DESIGN OF COMPOSITE INSULATORS

2.1 Structure of Composite Insulators

Typically a composite insulator comprises a core material, end-fitting, and a rubber insulating housing. The core is of FRP to distribute the tensile load. The reinforcing fibres used in FRP are glass (E or ECR) and epoxy resin is used for the matrix. The portions of the end-fitting that transmit tension to the cable and towers are of forged steel, malleable cast iron, aluminium, etc.For line insulators, a high degree of standardisation has been achieved for the end fittings, which enables the easy replacement of existing conventional insulators by composite solutions. The glass fibre reinforced resin rod is responsible for bearing the mechanical loads, which can be tension, bending or compression, or a combination of all three, depending on the application and load scenario. Materials for the housing are as manifold as the corresponding methods of manufacturing. However, there are performance trends as a result of the existing service experience, details. The rubber housing provides electrical insulation and protects the FRP from the elements. For this reason silicone rubber is adopted, which has superior electrical

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characteristics and weather resistance, for use in the housing. Figure 2.1 shows the structure of a composite insulator. Figure 2.1 Structure of composite insulator.

Fig. 1: Parts of composite insulators

END FITTING-

Typical end fitting configurations are shown in Fig. 2.

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Dimensions are in accordance with IEC 60120, IEC 60471 or IEC 61466, as well as equivalent ANSI. For distribution level up to 70 KN, cast steel end fittings are used.For force ratings above 70 KN, forged steel end fittings are applied. For special applications such as railway catenary line fittings, A high strength coquille aluminium is often used. The steel end fittings are hot dip galvanized. The thickness of the galvanizing follows the recommendations of IEC 60383.Enhanced thickness for heavily corrosive in situ conditions or DC applications can be provided on request. Details of the dimensions and their relation to force rating are shown in the insulator design catalogue.

ROD-

Fig 3..

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The glass fibre reinforced resin rod is an important component of a composite insulator. The rod is typically produced in a continuous pultrusion process. Different diameters are available depending on the application (Fig. 3). The content of fibres determines the specific intrinsic tensile and bending strength of the rods. The glass sizing is important for the bond to the resin matrix. The resin matrix itself must be “electrically graded”, characterized by low moisture absorption and by negligible change of electrical and mechanical properties. The resin elongation must be balanced with the glass elongation to prevent cracks and fractures when being subjected to loadings. Typically, Epoxy-based resins are used today. Fillers are used for different purposes, which gives the rod a transparent or opaque appearance. When the raw materials are carefully checked, process parameters are accurately selected and routine checks are determined using statistics, both types of rod offer excellent and reliable performance.

HOUSING-

The electrical purpose of an insulator is the insulation of the high voltage potential to ground or between two phases against an external flashover. Simplified, a flashover event can be caused by an overvoltage or by pollution. With the invention of polymeric insulators, many different materials have been tried and tested in respect to their outdoor service performance. The experience has shown that there exists a close interaction between sole material properties and the overall design of an insulator. A survey conducted by CIGRE Working Group B2.03 and published in the year 2000 has shown

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that the majority of composite insulator applications use Silicone Rubber as housing material (Fig. 6). EPDM and others play a less important role.

Fig. 6: Use of housing material for composite insulators > 100 kV

Meanwhile, the striking distance determines the behaviour during an overvoltage, the shape (geometry) of the insulator and wetting behaviour of the insulator surface become the dominating factors for the pollution performance.

2.2 Designing of Composite Insulators An important feature of the composite insulators developed here is that the design of the shed configuration is extremely free, owing to the use of silicone rubber for the housing. Based on past experience, IEC 60815 "Guide for the selection of insulators in respect of polluted conditions" was adopted. Electrical and mechanical characteristics were designed to satisfy the requirements set forth in IEC 61109 "Composite insulators for ac overhead

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lines with a nominal voltage greater than 1000 V definitions, test methods and acceptance criteria". With regard to pollution design, it has been suggested that because of the hydrophobic properties of silicone rubber, composite insulators can be designed more compactly than in the past, but because of the absence of adequate data it was decided in principle to provide as great or greater surface leakage distances. The design value for leakage distance was referenced to the value per unit electrical stress as determined in IEC 60815, adjusted upward or downward according to customer requirements. Tensile breakdown strength was determined by applying a safety factor to the long-term degradation in tensile breakdown strength. The rubber and FRP of the housing were required not only to have sufficient mechanical adhesion but to be chemically bonded, so as to divent penetration of water at the interface. And because in general a large number of interfaces may result in electrical weak points, Furukawa Electric has adopted a composite insulator design in which the sheds and the shank are molded as a unit, resulting in higher reliability. The end-fittings comprise three elements, and have the greatest effect on insulator reliability. Specifically the penetration of moisture at this point raises the danger of brittle fracturing of the FRP and the electrical field becomes stronger. For this reason the hardware is of field relaxing structure and the silicone rubber of the housing is extended to the end-fitting to form a hermetic seal. The end-fitting is connected to the FRP core by a comdivission method that maintains long-term mechanical characteristics.

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The design requirements for composite insulators for 154-kV service are set forth below:-

• Overall performance (1) To have satisfactory electrical characteristics in outdoor use, and to be free of degradation and cracking of the housing. (2) To be free of the penetration of moisture into the interfaces of the end-fitting during long-term outdoor use. (3) To possess long-term tensile withstand load characteristics. (4) To be free of voids and other defects in the core material. (5) To be non-igniting and non-flammable when exposed to flame for short periods.

• Electrical performance (insulator alone) (1)To have a power-frequency wet withstand voltage of 365 kV or greater. (2) To have a lightning impulse withstand voltage of 830 kV or greater. (3) To have a switching impulse withstand voltage of 625 kV or greater. (4) To have a withstand voltage of 161 kV or greater when polluted with an equivalent salt deposition density of 0.03 mg/cm2. (5) To have satisfactory arc withstand characteristics when exposed to a 25kA short-circuit current arc for 0.34 sec. (6) Not to produce a corona discharge when dry and under service voltage, and not to generate harmful noise (insulator string).

• Mechanical performance (insulator alone) (1) To have a tensile breakdown load of 120 KN or greater. (2) To have a bending breakdown stress of 294 MPa or greater. (3) To show no abnormality at any point after being subjected to a comdivissive load equivalent to a bending moment of 117 Nm for 1 min. (4) To show no insulator abnormality with respect to torsional force producing a twist in the cable of 180°. (5) To be for practical purposes free of harmful defects with respect to repetitive strain caused by oscillation of the cable. Table-1 shows the characteristics of an insulator designed to satisfy these specifications

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3. ELECTRICAL DESIGN CRITERIA 3.1 Dry Arcing Distance (Strike Distance):-It is the shortest distance through the surrounding medium between terminal electrodes. In the figure given below red line shows the dry arcing distance.

Figure 3.1 Dry arcing distance

Figure 3.2 Leakage distance

3.2 Leakage Distance The sum of the shortest distances measured along the insulating surfaces between the conductive parts, as arranged for dry flashover test. In the given figure the distance covered by red line shows the leakage- distance. The design engineer can find general guidance on what leakage distance is provided by a properly designed shed shape. These recommendations have been devised for porcelain and glass insulators but were not meant to be used for composite insulators.

ADVANTAGES

Due to many advantages the use of composite insulators has grown steadily The polymeric products are demonstrating their capabilities in diverse

environments and are now routinely used to prevent contamination flashover. The advantage of composite insulators over ceramic insulator is given below:

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1. Leakage current control and flashover resistance:-One of their major advantages is their low surface energy and thereby maintaining a good hydrophobic (non wetting) surface property in the presence of wet conditions such as fog, dew and rain .- Water on the surface of insulators stays in form of droplets and does not form continuous film. So the leakage current along the insulator surface is strongly suppressed. -The efficient suppression of leakage current means the risk of flashover is reduced compared to porcelain insulators. Power frequency insulation is improved due to low leakage current. The energy loss is 1/10th when compared to Porcelain Insulators and have higher di-electric strength.

Fig. 10.1: Hydrophobicity of an unpolluted Silicone Rubber surface

7.1 LIGHT WEIGHT The density of polymer materials is lower than other materials. It makes construction and erection easier and faster. The reduced weight permits the use of lighter and less costly structures and mounting arrangements. The light weight of the composite insulator strings also permits an increase in the clearance distance between the conductor to ground and an increase in the phase-to-phase distance in order to reduce the electric and magnetic fields which are becoming a growing concern to some members of the general public. . The light weight of the composite insulators also obviates the need to use heavy cranes for their handling and installation and this saves shipping

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cost. Composite insulators are 90% lighter than Porcelain Insulators, but offer an equal to better strength. Approximate weight of 400 KV insulator will be less than 15kgs.

7.2 COMPLEX GEOMETORY The polymers insulators are typically molded therefore it may have a higher creep age distance per unit length than porcelain. Weathershed profiles can be made more complex and alternating diameter weathersheds are supplied, which improve the a.c. wet flashover by avoiding bridging of all sheds simultaneously during heavy wetting conditions.7.3Resistance to breakages- Composite Insulators are flexible and therefore, highly resistant to breakages. While 10 to 15% breakages are reported during transportation, storage and installation in case with porcelain insulators.

Safety against Vandalism & shatter proof.- Composite Insulators have superior flexibility and strength which provides improved seismic performance and are highly resistant to breakage due to stone throwing, etc.- No shattering/explosion. And not susceptible to breakages during earthquakes.

Excellent Tracking Resistance avoids erosion or tracking of the housing material.

7.3 POLLUTION PERFORMANCE The hydrophobic properties on the composite insulator have a better electrical performance in contaminated condition. Water on the surface of hydrophobic materials forms water bead, so the conductive contamination dissolved within the water beads is discontinuous. This condition results in lower leakage current flow and the probability of dry band formation, which in turn requires a higher impressed voltage to cause flashover. The higher resistance of silicone rubber helps to limit the arcing and minimizes the flashover. Another advantage of the composite insulator is that it contributes to reduce the maintenance costs, such, no need washing, and no need for application of silicone coatings and reduce the inspections.

7.4 HOLLOW CORE HOUSING FAILURE MODE The physical properties of the polymer material mean that it will not shatter like porcelain. With the initiation of an internal fault, the expected failure mode is rupturing or bursting of the hollow structure with venting of the internal pressure, leading to an external flashover and dissipation of the fault

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energy outside of the housing.

7.5 PROCESSING The processing time for polymer insulator is shorter than for porcelain.

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Safety- Composite Insulators provide very high level of safety for apparatus in case of inner over pressure of external causes. While porcelain Insulators are susceptible to explosion in case of inner over pressure or external causes.Composite Insulators have many ecological advantages. Manufacturing process is pollution free. Composite Insulators are safe and not health risk.

7.7 EARTHQUAKE RESISTANT Equipment using hollow core composite insulators can withstand seismic acceleration stresses up to 1 g (whether it is 0.5g in case of ceramics insulators) without damage due their lower weight, high damping factor and high strength design characteristics.

7.8 ECONOMICAL BENEFITS 1. Lower costs of manufacturing, shipping, loading/unloading work and installation (due to lesser weight and dimensions 2. No breakage during transportation, handling, loading /unloading assembly works (even so must to be handling carefully 3. Possible application in hard-reach-areas Swampy areas arm highlands costs tam necessity at all) insulators cleaners 4. Low costs of repair and replacement of insulators (due to increased reliability and shock resistance as well as easier assembly) .

FACTOR AFFECTING THE PERFORMANCE OF COMPOSITE INSULATORS 5.1 MATERIAL AND MANUFACTURING METHOD Polymer base and compound quality. Formulation and design. Core quality and end fitting gap attachment method. Manufacturing method and quality control. Handling, storage and delivery damage. Damage during installation. 5.2 ENVIRONMENTAL CONDITINS Ultraviolet radiations. Wind and ozone. Temperature and pressure. Humidity, rain, fog and snow. Organic and inorganic pollutions (fertilizers, dust, acid, salt etc).

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5.3 POWER SYSTEM OPRATION AND DESIGN Electric field stress (continuous and transient). Control stress ring. Leakage distance. Proximity of other lines. Mechanical stress traction, compression, torsion and vibration. 6. PREDICTING SERVICE LIFE The service life of a composite insulator involves both electrical and mechanical aspects. Electrical aging involves damage from erosion or tracking due to the thermal or chemical effects of discharge occurring when the insulation material is polluted or wet, and may even result in flashover. Mechanical aging includes long-term drop in the strength of the core material or in the holding force of the end-fittings, as well as brittle fractures of the core material, and can on occasion result in breakage of the insulator string. A drop in core strength or holding force of end-fitting can be countered by adopting an appropriate safety factor and using a reliable method of comdivssion. Brittle fractures, on the other hand, occur mostly near the interface between the insulation material and the end-fitting, and provided this area has been properly manufactured, the probability of their occurrence will be lower than that of electrical aging. To estimate service life from the electrical aspect, actual-scale composite insulators were exposed to electrical stress, and were subjected to an exposure test under a natural environment. A test chamber simulating environmental stress was also constructed, and accelerated tests were carried out according to international standards (IEC 61109 Annex C). Further, by comparing leakage current waveform and cumulative charge, which may be characterized as electrical aging, evaluation of composite insulator service life was carried out. Furthermore, since in Japan, a drop in insulation performance due to rapid pollution during typhoons is a familiar phenomenon, an investigation was made based on the characteristics of leakage current obtained during a typhoon into the effect of rapid pollution on electrical aging in composite insulators.

8. USES OF COMPOSITE INSULATORS The composite insulators are used at following places:- -Distribution and transmission insulators. - Surge arresters. -Line surge arresters. - Bushings.

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- Circuit breakers. - Instrument transformers. - Capacitors. 8.1 DISTRIBUTION AND TRANSMISSION INSULATORS Environmental demands on high-volt age transmission lines have increaser constantly in recent years both in qualitative and quantitative respects. For example, today it is of prime importance when planning an overhead line to pay attention to the achievement of a pleasing and environmentally tolerable towel configuration. A large power company in Western Switzerland has reached this goal in an exemplary manner with its new 400 kV lines in this case the wide use of silicone composite insulators brought positive results (figure 8.1). The composite insulator with a connection length of 30 m can be manufactured in a single piece and is almost

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shorter than the previously used porcelain insulator strings, each with three long rod insulators type LG 85/22/1470. Shorter insulators allow the use of shorter cross arms without the risk of flashover due to reduced clearances to the tower as a result of the swinging of the conductors. This has the further effect of reducing the torsional loads in the cross towers. It requires 37% narrower way-leave, which translates into 50% lower right-of-way cost. Figure 8.1 Transmission and distribution insulators

8.2 OUTDOOR SUBSTATION INSULATORS Switchyards are the nerve centers of every power grid and so the users' expect and demand a correspondingly high degree an operational safety. It is therefore not surprising that with the growing faith in composite insulators - particularly due to the good experience made in their application in overhead lines world-wide - great interest has developed in recent years in their applies bon in outdoor substations. Today, if the customer so desires, it is possible to design complete substations in silicone composite technology. Figure 8.2 Outdoor substation with composite insulators In the above figure 8.2 „a shows voltage controlled bushings for ‟transformers, „b shows surge arrester, „c shows live-tank circuit breaker, ‟ ‟„d shows current transformer, „e shows voltage transformer, „f shows ‟ ‟ ‟voltage controlled bushing for power transformer, and „g shows cable ‟termination. 8.3 SURGE ARRESTERS For the obvious reason of the danger explosion due to overloading, surge arresters were one of the first electrical devil that were built with silicone insulator she The advances in ZnO technology in arrester design, which replaced the spark-gap arresters, eased the realization of porcelain-free arresters. Today, ZnO arresters are manufactured either by applying the silicone shed directly onto the active part, which is sometimes done for voltage levels up to 36kV, or by using a fiberglass reinforced, silicone coated composite tube as an insulating housing for the arrester, which is possible up to the highest system voltages. Figure 8.3 shows ZnO surge arresters.

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Figure 8.3 ZnO Surge arresters

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8.4 BUSHINGS Increasingly, the design of high-voltage bushings is being influenced by higher demands on operational safety, damage-risk minimization (to persons and property) and not least, by a greatly increased public environmental consciousness. The consideration of these factors led to a new conception these important components on the basis composite technology. By using superior materials, as well as having their manufacture well under control, it has been possible to satisfy the above-mentioned demands o the bushings. Fig.8.4 shows 420 kV and 22 kV transformer bushings and GIS Bushings for 123 kV in composite technology. Figure 8.4 Power transformer bushing and GIS bushing

8.5 CIRCUIT BREAKERS For the various reasons already mentioned above there is also art increase in the use of hot low composite insulators in high voltage circuit breakers, including their associated control capacitors, and also recently in high voltage load disconnecting switches. The possibility of fitting an optical fiber cable into the composite tube for the transmission of measuring and a control signal, particularly in circuit breakers, is regarded as an additional advantage. Figure 8.5 shows SF6circuit breaker. Figure 8.5 SF6 circuit breaker 8.6 INTERPHASE SPACERS Inter phase spacers are fitted mainly the points on overhead lines at which either for reasons of design or due to external influences, there is a danger that required distance between the conductors of two phases will not be maintained a situation which would lead to a short circuit and hence an interruption in service. As early as 1990, a CIGRE questionnaire brought to light that around the world, 32 power utilities had around 13000 interphase spacers in operation a practically all voltage levels. Some of them had been in active service for many years (up to 20 years at the time of the questionnaire). Almost a third of the inter phase spacers registered in the above report are installed in Switzerland. As in any industrialized country, it is becoming increasingly difficult to obtain rights of way for routes for new lines. A possible solution to reduce the seriousness of this problem is to increase the power transmission capacity of existing lines, such as by installing a second circuit. In the case in question the cross arms of the concrete poles were originally designed to guarantee the required air clearance between the conductors at mid spam for one 12 kV

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circuit. So appropriate spacer made of silicone composite insulator were designed, and installed between conductors approximately 40m interval in order to maintain the required conductor separation. This solution is only possible by using the silicone composite insulators, which is very light compared with porcelain insulators, and thus do not add sever bending stress on conductors during dynamic load (ice shedding). Figure 8.6 shows Silicone composite insulators as inter phase spacers.

DIAGNOSTIC METHODS:

2.1 VISUAL INSPECTION

Visual inspection is presently the most commonly used inspection technique. It can be employed remotely from a long distance as well as by close-up visual inspections. Binoculars or telescopes are used to perform remote visual inspections. Better efficiency may be obtained when the inspections are made as close as possible to the insulator, e.g. operating from a tower, from a helicopter (including mini-helicopters), or from a bucket truck. A number of practical guides for visual inspection are available from CIGRE, EPRI and STRI. The guides typically include detailed descriptions of different types of possible defects with carefully selected colour photographic examples, enabling field personnel to quickly locate the photograph(s) and definition(s) of interest with respect to insulator deterioration and/or damages. In particular, it is important to define the criticality of a damage/defect, enabling required actions to be selected. The TB 481 can also assist in this regard.A simpler defect classification is necessary regarding LLW, as only conductive or semi conductive defects are recognized to be critical. This assumption is valid for the enforced rule that LLW is only permitted under dry weather conditions. While visual inspection allows most of the outside defects to be detected, internal defects, which might lead to flash thunder, cannot be observed. Furthermore, visual inspection can generally provide rather qualitative information, which can be better quantified by the other diagnostic methods examined in the following sections. Visual line inspection needs both an experienced helicopter pilot and an experienced line inspector (Fig. 36). Obviously, the upper side of tension string insulators can be visually investigated, which is rather difficult from the ground.

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Fig. 36: Helicopter inspection

Field measurement:

Measuring with a field probe (Fig. 37) is an accurate, but time consuming method. Initially, this technique was developed for the in-service evaluation of cap and pin insulator strings. With the increased service time of composite insulators, the field probe has subsequently been modified to also evaluate the new insulator technology. In principle, measuring involves mapping of the electrical field along the insulator. If a defect is found, the electrical field will show an immediate change. The ambient humidity has a strong influence on the field recordings, which makes interpreting the results difficult at such times.

Fig. 37: Field probe measurements

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UV/IR MEASUREMENT:

2.2 INFRARED THERMOGRAPHY (IR)A thermal emission is associated with local heating caused by a current flowing along a defective part of the insulator, characterized by relatively high conductivity in comparison to the intact insulating material.With IR, the temperature distribution along the insulator axis is measured by means of an infrared camera, searching for hot spots associated with possible local defects. Compact5 cameras with high sensitivities and excellent performance were developed which currently permit a fast and reliable inspection of the insulators. An example of defect detection by IR is shown in Fig. 5

. Fig. 5: Examples of clear IR detection of internal conductive defects (from left-to-right: in laboratory; at test station; in service)

Guidelines for IR inspection from a helicopter are available .The method is particularly sensitive to defects developing between the housing and the core, leading possibly to a flash under. In this case the fault current is passing through the defective zone causing a significant temperature increase. The phenomenon is particularly evident when the tracking affects large parts of

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the insulator or when a sufficient conductivity is present all along the insulator. This could be because of humidity ingress in the interface or because of wetting of the part of the insulator not yet damaged. On the contrary, temperature measurements by means of an infrared camera are not suitable to detect conductive or semi-conductive defects developing on only a small section of the insulator with the remaining part sound and characterized by high resistivity, especially if the measurements are made in a low humidity condition (with the insulator dry). In this case, corona may instead occur on the tip of the defect, with a very low current associated with it, thus leading to very limited temperature increase, hardly detectable in service. 2.3 ULTRA-VIOLET DETECTION (UV)

The possibility of localizing initial corona activity constitutes an interesting technical challenge, especially in daylight conditions, thus different techniques are available for day and night measurements. For daylight corona cameras, the diagnostic indicator considered is the emission generated by the defects in the UV-C range (i.e. with wavelength in the range 240-280 nm), a bandwidth in which the solar light is filtered by the atmosphere. Corona emission intensity is calculated using the number of pulses of light emission (named “blobs” - Fig. 6). A counter gives a number proportional to the quantity of “blobs” received by the sensor.

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Fig. 6: Example of “blob” counting by a daylight corona camera (245 kV tension insulators)

. Corona activities can be measured in daylight (Fig. 38) or hot spots can be detected. The images can be superimposed (Fig. 39), which simplifies the interpretation of the results.However, this interpretation requires experience too. For example, it is important that the various types of electrical discharges occurring on an insulator are differentiated. Dry band discharges have an UV radiation as well, but are mainly caused by pollution on the insulating surface. On the other hand, (dry) corona discharge is initiated at areas of high electric field stress caused by sharp or irregular points on metallic or insulating surfaces. Since the corona discharge is formed by the partial breakdown of air, it is important to record the prevailing weather conditions simultaneously with the measurement. It has been empirically proven that observations/ measurements are required during periods of both high and low humidity before any conclusions on the corona performance of a particular insulator or an insulator set can be drawn.

Fig. 38: Corona activity can be measured in Fig. 39: Superimposed image of IR and corona daylight measurement of the same bushing.

Measurement of temperature enhancement provides a great deal of information also in stations and for current-carrying contacts

In composite insulator evaluation, initial experiences show that the late ageing state can be better detected with this combined UR/IR-measurement. The matter is currently under investigation by CIGRE Working Group B2.21. A

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first approach involves the validation of a failure interpretation matrix (Fig. 40).

In general, IR thermography and UV measurements principally detect different physical properties (heat and enhancement of the electric field in the form of corona respectively), thus a combination of these two methods/cameras or use of a multi-camera would be the optimum solution for the remote inspection of composite insulators, especially when a certain failure tendency is known for the age or vintage of the insulators in question.

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COMPARISON BETWEEN COMPOSITE AN PORCELAIN INSULATORS

POLYMERIC COMPOSITE INSULATORS

PORCELAIN INSULATORS

Excellent Hydrophobicity-The improved pollution and hydrophobicity properties of Silicon Rubber Provide excellent insulating behavior without the need of washing or greasing even in humid and/or polluted climates including dense fog, heavy rain with high conductivity, sea spray, dense saline fog and industrial pollutions. Hence low failure rate combined with low overall operating and maintenance costs.

Hydrophilic Properties-Porcelain surface forms a water film on the surface due to its high surface tension (called hydrophilic). As such flashovers and outages in humid and/or polluted climates will be very high.

Lower Leakage CurrentResulting in improved power frequency insulation. 1/10th the energy loss when compared to Porcelain Insulators. Higher di-electric strength.

Higher leakage current

Light weight- 90% Lighter than Porcelain Insulators, but offer an equal to better strength.- Approximate weight of 400 KV insulator will be less than 15kgs.

Heavy in weight - Inferior in strength.- Approximate weight of 400 KV insulator will be about 135 Kgs.

Resistance to breakages Highly fragile.

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- Composite Insulators are flexible and therefore, highly resistant to breakages.

- 10 to 15% breakages are reported during transportation, storage and installation.

Safety against Vandalism & shatter proof.- Composite Insulators have superior flexibility and strength which provides improved seismic performance and are highly resistant to breakage due to stone throwing, etc.- No shattering/explosion.

Susceptible to breakages- Due to very fragile properties, Porcelain Insulators are highly susceptible to breakages due to vandalism such as stone throwing etc.- Porcelain Insulators are susceptible to breakages during earthquakes

Excellent Tracking Resistance avoids erosion or tracking of the housing material.

Poor Tracking resistance.

Compact Design- Results in space saving (Right of way) and lower costs.

Bulky in Design- Requires Larger and heavier towers for installation and more space

AestheticsAesthetically more pleasing design and appearance

AestheticsAesthetically not a pleasing design.

Safety- Composite Insulators provide very high level of safety for apparatus in case of inner over pressure of external causes.

Safety- Porcelain Insulators are susceptible to explosion in case of inner over pressure or external causes.

Composite Insulators have short process time and therefore short delivery periods.

Porcelain Insulators have long manufacturing process requiring long deliveries.

Composite Insulators are cost comparative.

Porcelain Insulators are higher cost on account of excessive heating required in the process.

Composite Insulators have many ecological advantages.Manufacturing process is pollution

Process of manufacturing causes pollution and health risk.

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free.Composite Insulators are safe and not health risk.

Stable long term operating behavior as demonstrated over more than 30 years of outdoor exposure experience abroad against ` degradation and deterioration of insulating properties.

Porcelain Insulators degrade over period of service and provide reduced insulating properties

Design flexibility of Polymer Insulators allows for adaptation to suit specific needs such as changes in creepage distance independent of insulator length and aero dynamic profile.

Design flexibility is limited.

Polymer Insulators cannot be used as ladder during maintenance.

Porcelain Insulators are rigid and are therefore used by maintenance workers as ladder during maintenance.

CONCLUSION: Composite insulators are used in increased numbers for the insulation of HV transmission lines. They are manufactured from different materials by different manufacturing processes and show different design aspects. Thus, these composite insulators are not equal. Their innovative details become obvious only if particular service stresses act together in such a way that the particular design items are challenged. Composite insulators are light in weight and have demonstrated outstanding levels of pollution withstand voltage characteristics and impact resistance, and have been widely used as inter-phase spacers to prevent galloping. They have as yet, however, been infrequently used as suspension insulators. Since the mechanical stress is smaller, the body diameter can be thinner, and so the polymeric insulators can be made smaller and lighter since their pollution withstand voltage performance is better than that of porcelain insulators.It is expected that the market share of polymer insulators will continue to

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grow worldwide. The expected life time of polymer insulators still presents an unknown quantity and therefore is of concern to some power utilities, particularly for applications in heavily polluted and wet conditions. Substantial improvements can still be made to the formulations, the designs of the weather sheds, stress relief rings and to the metal end fittings. These will undoubtedly bring about further improvements in the electrical performance of composite polymer insulators, leading to their acceptance worldwide and a further reduction in their cost. Worldwide service experience with silicon rubber insulators is excellent. Especially, under heavy pollution conditions silicon insulators outperform any other type of insulator by far. So far there have no laboratory test been known to judge performance in service or life expectancy. Suitability of composite insulator design and materials can be proven by long-term service experience only. The composite insulators for suspension use that were developed in this work have been proven, in a series of performance tests, to be free of problems with regard to commercial service, and in 1997 were adopted for the first time in Japan for use as V-suspension and insulators for a 154-kV transmission line. To investigate long-term degradation due to the use of organic insulation material, outdoor loading exposure tests and indoor accelerated aging tests are continuing, and based on the additional results that will become available, work will continue to improve characteristics and rationalize production processes in an effort to reduce costs and improve reliability.Composite insulators offer economic advantages such as low cost compact lines, low transportation and installation costs and drastically reduced maintenance costs. Due to their superior performance under any service conditions and their technical and economic advantages composite insulators with silicon rubber housing will represent the absolute future insulation material.