SEMINAR REPORT ON COMPOSITE INSULATORS

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.                                                                                      Overhead power transmission lines require both cables to conduct the electricity and insulators to isolate the cables from the steel towers by which they are supported. 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 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.

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 fibers 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, aluminum, etc. The rubber housing provides electrical insulation and protects the FRP from the elements. For this reason we at Furukawa Electric have adopted silicone rubber, which has superior electrical 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.

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 a.c. overhead 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 divvent 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 comdivssion method that maintains long-term mechanical characteristics.

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 comdivssive 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.      

Mechanical Characteristics Product number Overall length (mm) Number of sheds (lrg/sml) Sheds diameter (lrg/sml) (mm) Effective length (mm) Surface leakage distance (mm) Weight (kg)                           Tensile strength (kN) Bending breakdown strength (MPa) Electrical Characteristics  Power frequency withstand voltage (kV) Lightning impulse withstand voltage (kV) Switching impulse withstand voltage(kV)

H154-120-1880CC 1884 26/25 117/83 1640 5400 10 120 294 440 835 635

  

table1. Specification of composite insulators 

Table-1 Characteristics of insulator

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

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.

Figure 3.2 Leakage distance

4. LEAKAGE CURRENT CONTROL AND

FLASHOVER RESISTANCE

·         Due to chemical nature of polymer, the surface of insulator is hydrophobic (non wetting).

·         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.

The following curve shows the current comparison in ceramic and polymer insulators during the “salt fog” test.

Figure 4.1 Leakage current

5. 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).

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.

7. 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:-

 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 smaller size and weight result in lower shipping cost.

Table-2 Comparison of weight

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.

7.3 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 energy outside of the housing.

7.5 PROCESSING

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

7.6 NO HAZARD CONDITION

In the event of any fault the characteristics of a composite insulator exclude the occurrence of hazardous condition to personnel and surrounding equipment.

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)

8. USES OF COMPOSITE INSULATORS

The composite insulators are used at following places:-

·         Distribution and transmission insulators.

·          Surge arresters.

·         Line surge arresters.

·         Bushings.

·         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 1.5 m 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.

    

                                 

Figure 8.3 ZnO Surge arresters

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 SF₆ circuit breaker.

Figure 8.5 SF₆ circuit breaker

8.6 INTERPHASE SPACERS

Interphase  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 interphase 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 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 interphase spacers.

Figure 8.6 Interphase spacers

8.7 INSTRUMENT TRANSFORMER

For a few years now, hollow composite insulators have been finding use as housing for the outdoor versions of current and volt age transformers and this in locations which put particular demands on the mechanical strength and elasticity of the dev ice's cases such as when there is a risk of explosion or where high mechanical stresses like earthquake, vandalism or high short circuit forces are likely. Rare faults leading to a CT’s or VT’s explosion and hence causing considerable risk of injury or damage to persons and property have in Switzerland recently led to an increase in the application of Current Transformers and Voltage Transformers using composite technology. Figure 8.7 shows current transformers with hollow composite Insulators.

Figure 8.7 Current transformer (CT)

9. CONCLUSION

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 divvent galloping.                                                                                                They have as yet, however, been infrequently used as suspension insulators. 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.

10. REFERENCES

1.      Composite insulators- Hollow insulators for use in indoor and outdoor electrical equipment Definitions, test method and acceptance criteria. IEC 1462. Ed. 1, Draft November 1996.

2.      IEC 1109: 1992. “Composite insulators for ac overhead lines with nominal voltage greater than 1000V- Annexure C”.

3.      IEEE Std 987-2001^TM. (Revision of IEEE Std 987-1985^TM). IEEE Guide for Application of Composite insulators.

4.      IEEE/PES 2010 Transmission and Distribution Conference and Exposition New Orleans, Louisiana April 20, 2010.