Silicone Housing for High Voltage Applications

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SiliconeHousingforHighVoltageApplicationsCONFERENCEPAPERJANUARY2003

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INTERNATIONAL COLLOQUIUM: Asset Management of Switching Equipment and New Trends in Switching Technologies

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SILICONE HOUSINGS FOR HIGH VOLTAGE APPLICATIONS

K. Sokolija University of Sarajevo

Faculty of electrical Engineering Bosnia and Herzegovina

R. Gorur

Arizona State University USA

ABSTRACT Although today porcelain still holds the majority of ma-terial consumption for high voltage apparatus, the obvi-ous importance of composite polymer insulators (CPIs) with silicone housings is evident from growing number of different high voltage apparatus equipped with this kind of insulators. Conservative power industry is being changed under the pressure of the development of the market focused more and more on safety and increased environmental concerns as well as on cost reduction and shorter delivery times. In this paper, the well known advantages of CPIs in the field of high voltage engineering for electrical apparatus (low weight, non-brittleness, explosion safety, hydro-phobicity etc.) as well as also known disadvantages (age-ing under the influence of multidimensional stresses) gained by in-service experiences for breakers, bushings, measuring transformers, surge arresters and cable ter-minations are reviewed. Key words: Silicone housing, Arresters, Bushings, Cir-cuit breakers, Cable Terminations, Measuring trans-formers 1. INTRODUCTION The initial purchase price and the past experience have been the predominant practice for transmission and dis-tribution insulator, as well as for station and apparatus (external) insulation selection, until now. Some users, for example, claim that they have no problems with por-celain insulators in contaminated locations as long as they are washed on regular basis. These same users are often reluctant to consider to use of CPIs even though

they can significantly reduce the need for such cleaning. By the same token, there are users who have installed CPIs extensively on their network and are spending a lot of resources (time and personnel) for monitoring the condition on these insulator in service. Perhaps this also has not been taken into consideration when calculating total overall cost !1". However, it is very important to emphasise that the ini-tial purchase price of a component is only a proportion of total cost over the lifetime (final lifetime cost) of that component. Israel Electric !2", for example, has calcu-lated that over the life time of a typical porcelain insula-tor, the maintenance costs will typically be at least three times higher then the purchase price. The same might be said about the cost of nondelivered power due to insu-lator failures, expect that here the multiple could be hundreds or even thousands of times insulators pur-chase price !3". So, the critical problem influencing the selection of a particular insulator (housing) from the various alternatives available is life expectancy, but not under ideal conditions than real life conditions !4". There exists widespread doubt about the expected life-time of CPIs due to premature ageing and this is proba-bly the main factor limiting their more extend use. However premature ageing is not the only factor to the soonerthanexpected enoflife of a CPI. It could be also any event which leads to a limitation in expected service life of conventional insulators, for example: ex-cessive pin corrosion on cap and pin discs, damage due to vandalism, earthquake causing systemwide black-out, change in environmental conditions requiring re-placement existing with another type insulator etc. On the other hand, light weight of composite polymer insu-lator (meaning less installation cost) and less purchase cost, including other cost savings, reduce their lifetime cost (life cycle cost) in such dramatic way that even

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their two times shorter life expectancy in relation to con-ventional insulators could be accepted !5". Accordingly, in order to achieve an optimal choice of insulator/housing type for the specific application an engineer should take into consideration the following factors: operating environment (reallife conditions), performance expectations and total expected costs over the lifetime (purchase, installation, maintenance, loss of revenues due to outages). At the same time suppliers of electrical equipment have to accept the fact that offering new technology at the same or higher final lifetime cost and along with some risk factor gives no motivation for the customer to buy. After a rather slow start during 1960s, CPIs for over-head lines since the early 1990s can be considered a mature product !6". The results of a recently published CIGRE survey !7" show that CPIs are now well acce-pted substitute for conventional porcelain or glass insu-lators used in transmission lines all over the world, showing the failure rate similar to that usually reported for conventional insulators (0,5 to 1,5 failed insulator per 10000 installed insulators per year). This result agree with the findings of a Japanese survey conducted in 1998 !8". At the same time, polymeric housing have now found the virtual replacement of porcelain for manufacturing distribution arresters and more increasing applications in cable accessories as well as in hollow core insulators for station and apparatus insulation. Although today conventional solutions, primary porce-lain, still holds the majority of material consumption for high voltage insulators, conservative power industry is being changed under the influence of the following ad-vantages of CPIs technology: (1) Light weight resulting in more economic design of

the towers or alternatively enabling to upgrade the voltage of existing system without changing the tower dimensions. The light weight also permits an increase in the clearance distance between the conductor to ground and increase in the phasetophase distance reducing electric and magnetic fields. The light weight obviates the need to use heavy cranes for handling and installation thus saving on labour cost.

(2) Much better performance than conventional insula-tors in outdoor service in the presence of heavy pol-lution !911", specially in the case of silicone rubber which have an intrinsic hydrophobicity that is recov-ered after being lost and also transferred to pollution layers.

(3) Higher mechanical strength to weight ratio which enables the construction of longer spans of towers.

(4) In distinction from conventional line post insulators where gunshots could cause drop conductor to ground !12", composite polymer insulators can withstand severe gunshot damage without immediate electrical or mechanical failure.

(5) Reduction in the maintenance costs such as of insu-lator washing which is often required for conven-tional insulators in heavily polluted conditions.

(6) Reduction in the purchase price (increased quantities of insulators produced and competition on the mar-ket), lower transport and installation cost, less main-tenance cost etc. !13".

In addition to the above listed advantages specially ad-dressed to line insulators or common to all composite polymer insulators, hollow core composite insulators or directly applied polymeric housing used as a housing for different apparatus offer the following advantages in comparison with porcelain counterparts: # greater resistance to seismic forces of earthquakes; # explosive proof housing in the case of internal arc. The major disadvantages of CPIs are: the limited longterm experience, uncertain lifetime and difficulties in detecting faulty insulators as well as in monitoring per-formance and deterioration of insulators in service. A unmindful factormanufactures marketed the CPIs as a product which did not needed to be handled carefully has led to the insulators being used as tools and, thus, being destroyed by unnecessary mechanical treatment. Today proper handling instructions are available !14". Ageing and the expected life of CPIs depend on numer-ous factors, many of which are associated with service environment conditions while others are related to oper-ating conditions. Although natural weathering has been shown to cause CPIs ageing, there is a great experience showing that their life has been more related to design weaknesses and quality control during manufacturing process. Great efforts have been made in order to obtain an optimal mix of good properties of materials used today for production of CPIs, and to resolve different problems related to insulator design weaknesses. How-ever, and due largely to the lack of standardisation, not all insulator designs have reached the same level of quality. Although laboratory accelerated tests were de-veloped to evaluate CPI designs, the only sure method of distinguishing good designs from poor ones is their behavior in actual service conditions. 2. SILICONE RUBBER AS HOUSING MATERIAL 2.1 Silicone rubber formulation Silicone rubber (SIR) was first produced in 1944 !15" and in addition to outdoor insulation has been used in insulation of special purpose cables which operate of high temperature ($ 150o C). Silicone rubbers used in the area of high voltage insulation are mainly based on polydimethylsiloxane (PDMS). The uncrosslinked poly-mer is either in a pasta form or in a more liquid state. The compound contains silane treated fillers of amor-phous silica for rheological control and alumina tri-hydrate (ATH) is added as a flameretardant because unfilled PDMS is too flammable. ATH also improves the dielectric strength and the tracking resistance. Typi-cal compounds also contain smaller proportions of sili-cone oil for process control. Chemicals used for cross


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linking (vulcanisation) are added also. The PDMS back-bone consists of alternating silica and oxygen atoms with two methyl groups attached to each Si atoms. It is important to recognise that all silicone rubbers are not alike. The mechanical and electrical properties of the basic PDMS could be improved by changing or-ganic groups attached to silica atoms or the type and concentration of additives and fillers used. Each manu-facturer has his own specific formula of SIR, so the properties may differ widely from one to the next when subjected to the stresses of service conditions. In high voltage engineering the SIRs are often classified according the curing system used: # high temperature vulcanisation (HTV): peroxide

induced free radicals # room temperature vulcanisation (RTV): condensa-

tion reactions # liquid silicone rubber (LSR): hydrosilylationreaction The HTV rubber is cured of high temperature (180oC). This type has the highest thermal stability and better tear resistance when compared to the other types. It is used for manufacturing all types of composite polymer insulator applications (distribution, transmission and sta-tion). Since HTV rubber is stiff, there is a need for high pressure in the mould and therefore more expensive tooling. The RTV type is cured of room temperature and is available as a onecomponent and twocomponent system (RT2). The onecomponent system is used as sprayable coatings to improve the pollution performance of conventional ceramic insulators. Curing takes place on the exposure of coating to air at room temperature. The twocomponent system, which vulcanises at about 60o C, is used in production of hollow core composite insulators. The LSR is material with lower pressure re-quirements in the mould being vulcanised at a tempera-ture of 150o C to 200o C. The thermal stability of the LSR is almost as good as that of the HTV. A number of manufacturers of longrod and hollow core composite polymer insulators choosing this material have found the best compromise between the performance required and manufacturing costs.

2.2 Hydrophobicity In distinction from glass and porcelain characterised by high value of surface free energy (determines the strength of adhesion of their solid surface and water) enabling for such materials to be easily wettable (water forms a continuous layer on their surfaces), organic materials have the lower surface free energy. On such, virgin sur-faces, water forms discrete droplets much more easily than a continuous film. For this property in the insulator industry today is used term hydrophobicity. The SIRs represent a family of polymers used for high voltage outdoor insulation showing the best ability to resist wa-ter film formation. However, different environmental stresses, like the deposition of pollution, the influence of water, sunlight, corona discharge and surface discharge

activity can cause a loss of hydrophobic property and conversion from hydrophobic to hydrophilic (wettable) state of all polymeric materials. As distinguished from all other polymers used, which becomes hydrophilic after a short period of exposure to the weather and where this state is definitive, SIRs have unique ability to recover and remain hydrophobic even after long term service. This kind of SIRs behaviour is considered to be one of the main advantage of using them in high voltage outdoor insulation. The hydropho-bic behaviour preventing polluted water films to be formed on the insulator surface contribute to a suppres-sion of leakage currents which generally leads to surface discharge activities destroying insulator surface. The wide differences in the recovery kinetics reported in many research papers indicate that phenomenon of re-covery is complex mixture of several mechanisms. Owen et al !16" summarised the plausible mechanisms for hydrophobicity recovery of SIR after exposure to corona or plasma: 1. Migration of low molar mass species from the bulk

to the surface. 2. Reorientation of polar groups at the surface into the

bulk. 3. Condensation of silanol groups at the surface. 4. External contamination of the surface. 5. Changes in surface roughness. 6. Loss of volatile oxygenrich species to the atmos-

phere. Insulation engineers have several weapons in their ar-moury, including leakage distance, shed shape, material periodic maintenance, etc. to make insulators work well. Hydrophobicity is only one of these but quite a power-ful one !17". As we have just emphasized, although hydrophobicity is important, it is not the endoflife criterion for SIRs, if the insulator housings are not designed reducing creep-age distance on account of this property. Namely, if a silicone housing has adequate creepage distance sheds shape and distance between them arranged in proper manner, provided with properly designed corona rings (if necessary) , made from properly formulated silicone rubber regarding the tracking and erosion resistance, the longevity and performance should not be limited by loss of surface hydrophobicity over time. Naturally, the longterm performance of a SIR insulator are dependent on the loss and recovery of its surface properties. The mechanisms involved are complex and are dependent on material composition, design and envi-ronmental service conditions. Hydrophobicity loss causes permanent material change, but this does not necessarily means a deterioration in performance !18, 19". An adequate period, during which the cause of hy-drophobicity loss is absent or significantly diminished in intensity, is required to recover lost hydrophobicity. Empirical studies of recovery time give varying results shown in Table 1.


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Table 1. Recovery time according to different empirical studies

Reference !20" !21" !22" !23" !24" Recovery

time 90 min 2-5 h

several hours

6-15 h 24 h

Percentage of recovery (%)

66 90

Laboratory studies, many of which are much more se-vere than operational conditions, have not reached an irreversible loss of hydrophobicity or endoflife condi-tions !25". The reservoir of low molar mass species available for recovery has been found to be very large !24". In addition, the transfer of hydrophobicity to pollu-tion or to the surface after hydrophobicity loss is known to use minimal amounts of material !26". Some of SIR insulators have been in service for 20 years and still have good properties and not show seri-ous tracking !27". Studies have shown that after 8 to 12 years, there is no reduction in quantity of low molar mass molecules !28, 29". The other forms of SIR insulator housing ageing like excessive carbonisation, erosion by acids, destruction of sheds by arcing etc. can be dealt with through the selec-tion of proven material formulations and good design !30". 2.3 Ageing In the case of composite polymer insulators the term aging, which is generally synonym for weakening or gradual degradation of important properties, is dominantly ad-dressed to the housing material. Ageing is arisen from the different stresses submitted by an insulator in service: electrical, mechanical and environmental. Environmental stresses include such factors as pollution, moisture (fog,

dew, humidity, rain), ultraviolet radiation, pressure, tem-perature, chemicals, etc. All above mentioned stresses have a synergistic effect on an insulator (Fig. 1).

Fig. 1 Illustration of the processes of polymeric materials ageing

Since ageing can be best manifest by visual changes oc-curring on the insulator housing, R. Gorur on the base of his own experience with composite polymer insulators has proposed an Ageing Chart !31" provide in Fig. 2.

Unacceptable Ageing Acceptable Ageing

Flashover (reusable insulator) Cosmetic changes Flashover (damaged insulator) Increased losses when wet Line drop (damaged rod) 10 8 6 4 2 0

Failure Rod/shed Sheath erosion Minor shed Hydrophobicity New damage away erosion, loss from terminals Crazing Localized Sheath Extensive Light Discolor, sheath tracking chalking, chalking Loss of erosion Aligatoring gloss

Fig 2. The Gorur Ageing Chart

As it can be seen, the various surface changes are classi-fied as acceptable ageing (not expected to cause failure) and unacceptable ageing (can cause failure). Service experience shows that most CPI failures occurred so far are not related to real ageing but rather to poor material formulation (Number 6 in Fig. 2), manufacturing proc-

ess weaknesses and inadequacies in quality control (Number 8 in Fig. 2). With todays state-of-art in CPI technology those problems should not occur except of locations with very severe environmental conditions where such events like those quoted as Number 7 and 9 in Fig. 2 can occur.

Decrease of mechani-cal strength

Decrease of electrical performance

Environmental effects Electrical effects

Heat

UV

Acid Wind, Rain, Salt,

Snow

Surface discharging,

Corona

Erosion Roughness

Chalking Tracking

Loss of hydro-phobicity

Loss of elasticity

Loss of low molar mass species

Depolimerization (Chain scission, Oxidation)


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3. HOLLOW CORE INSULATORS Over the last decade the application of hollow core com-posite polymer insulators in the area of high voltage electrical apparatus has made a big progress. Today nearly each type of apparatus equipped with porcelain housing has its counterpart dressed in polymeric clothing. The experience on development of hollow core com-posite polymer insulators fulfilling the dual function of a supporting structure as well as an insulating housing for different kind of bushings, instrument transformers, cable terminations, surge arrester arresters, capacitors, and station posts or switchgear post insulators represent an important breakthrough in high voltage equipment technology. Many utilities which has experienced a number of explosive failures of porcelainhoused equip-ment, possibly rather due to failure of the equipment inside the housing and not to insulator itself, in order to minimise damage to surrounding equipment has been commenced to convert from porcelain to polymerhoused apparatus. Another two reasons for changeover towards polymerhoused equipment is related to im-proved electrical performance in polluted conditions and greater resistance to seismic forces of earthquakes. Overvoltages or internal defects can cause internal arc-ing in substations equipment leading to an abruptly in-crease of the pressure and temperature inside the insula-tor. In the case of porcelain, the insulator can explode and flying fragments constitute hazard to personnel and equipment nearby. A composite hollow insulator is made up of a core tube on which the sheds are directly moulded on or pushed on. The tube is made from rav-ings of glass fibres impregnated with epoxy resin (FRP), wet wound around a mandrel at a predetermination an-gle for optimum bending, tensile and torsion strength (technique known as filament winding). After winding, the tube is cured in an oven after which together with its flanges is machined for a tight fit and assembled. This construction of housing precludes the occurrence of such hazardous conditions. In the case of the housings made with properly chosen silicon rubber, thanks to its high level of hydrophobicity even during severe environmental conditions, a much lower leakage current level along the insulator, and re-duced risk for flashover occurrence will result. Thanks to the high mechanical strength of FRP tube, lower weight of housing (4050%) and high damping factors, the equipment using hollow core composite poly-mer insulators has an increased ability to withstand earth quakes in comparison with conventional equipment. Last but not the least advantage offered by composite polymer hollow core insulators is a massive potential for reducing delivery times due to faster production lead times. In spite of the fact that this technology is now over twenty years old, hollow core composite polymer insu-lators have so far succeeded in capturing less than ten percent of the total world market for apparatus and bushing insulators [32]. The main reasons behind their very slow acceptance are:

1) Porcelain, in spite of its inherent limitations and weaknesses (high weight, poor performance under high pollution, seismic forces and internal arc) is a most universal and economical material which gen-erally has very acceptable mechanical and electrical performance during its rather long life time.

2) Purchase price has been and chiefly still is the key problem, especially in today de-regulated and extremely cost-conscious power supply industry. From the other hand, composite polymer hollow core insulators are still high-priced than their porcelain counterpart.

3) The well known conservative attitude of power sup-ply industry (specially in Europe) towards technol-ogy innovations make it reluctant to changes even in case of very obvious advantages which they could procure.

4) The first IEC standard (IEC 61462) for these insula-tors was published only in the late 1990s more than 15 years after their introduction.

5) Neutral instead of promoting attitude of most manu-facturers of high voltage substation equipment who actually purchase most apparatus insulators. The problem is more acute in developing countries, thanks to close co-operation between the equipment manu-facturers and local low-cost porcelain insulator in-dustry.

A faster acceptance of hollow core composite polymer insulators can be expected when utilities change their philosophy and understand that all previously men-tioned advantages of new technology cannot be ob-tained using them only as remedial measures but as in-tegrated part of re-designed electrical equipment (de-signed purely for composite polymer insulators) and installations.

3.1 Surge arresters The housing of the surge arresters traditionally have been made of porcelain. However, in the 10 years pe-riod, between the late 1980s and the late 1990s, the distribution arresters market converted almost entirely away from porcelain. Executives in the arrester industry estimate that possibly more than 90 per cent is now ac-counted for those units made with polymeric housing !33". This unprecedented change over to polymers is really quite remarkable given that the utility industry is very conservative requiring a lot of inoperation experi-ence before adopting any new technology. A porcelainhoused arrester should normally be equipped with a sealing system (Fig. 3) which has three tasks to fulfil: # to deter the ingress of moisture; # to act as a fast operating pressure relief device in the

event of arrester overload (N.B.: There were and still there are, many designs of distribution arresters with porcelain housing not provided with a pressure relief device at all);

# to enable current transfer from the flange to the re-sistor column.


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Fig. 3 Sealing system of a porcelainhoused MO arrester

The sealing system is one of the most critical compo-nents of this type arrester since moisture can penetrate inside arrester housing through pressure relief device or through housing flange interface leading to the most frequently mentioned type of failure in arrester literature and by users. Studies show that one per cent of the total installed population of porcelainhoused arresters failed each year, and that 86 per cent of the failures were due to moisture ingress !34". If pressure relief device fails to operate, or not provided at all, arrester might experience an explosive shattering of the housing. A porcelainhoused arrester, containing an enclosed gas volume be-tween the metaloxide (MO) resistors and housing (Fig. 4), might explode also due to internal pressure increase in the case of an overload (very infrequent event which can not in principle be ruled out), if the enclosed gas volume is not quickly vented.

Fig. 4 Principal designs of porcelainhoused MO arrester

Apart from the design utilised first of all for high and extra high voltage levels, where the porcelain housing has been replaced with composite polymer hollow core insulator and where the arrester also should be provided with a pressure relief device, a number of polymeric designs offer completely different constructions of a distribution as well as a HV surge arrester: polymeric housing applied directly on the MO resistor column. As a result, the air or gas filled gap between the housing and MO resistors no longer exists, and with appropriate constructive realisation of the interface between the polymeric housing and the end flanges, a sealing system can be completely omitted. Nevertheless, the unit re-mains absolutely leaktight and completely insensitive to moisture, no matter what the weather. In those designs two general methods are today utilised to mechanically contain the resistor column: open de-sign and closed design. In the first case, the mechanical containment of the re-sistor column may consist of loops of glass fibre, a cage of glass fibre weave or glass fibre rods around the resistor column. A body of silicone rubber (SIR) or ethylene propylene diene monomer rubber (EPDM) is moulded onto the internal part, and finally a prefabri-cated polymeric housing is slipped over on the inner body or the housing is moulded directly onto it. Such a design lacks an enclosed gas volume. At a possible in-ternal short circuit, material will be evaporated by the arc and cause a pressure increase. Since the open design deliberately has been made weak for internal overpres-sure, the arc will quickly rip the housing open, and with almost no resistance, find its way outside. In the case of closed design a fibre glass cloth im-pregnated with uncured epoxy resin is wrapped over the blocks in a number of layers under high tension and than heated in an oven so that resin will cure. The closed design might also be realised using a separate tube in which the blocks are mounted. The unit closed in one or other manner is than prepared for injection step where polymeric housing is moulded directly on to it. If there is none direct opening which could enable pressure relief during an internal short circuit, the gas generated can not easily escape. The internal overpres-sure could rise to a high value before cracking the hous-ing resulting in ejection, at high velocity, of pieces from the blocks which could cause damage to neighbouring equipment. An alternative is to arrange the windings or tube in a special manner to obtain weaknesses that pro-vide pressure relief and commutation of the internal arc to the outside thus preventing an explosion. Manufacturers who utilise the wrapping design concept defend it emphasising that wrap is an open weave type which, even after curing, still allows some venting of internal gases. Nevertheless, the two alternative designs different regarding the blocks containment may now be reviewed in the light of recent changes in the IEC shortcircuit test which requires that no internal parts should be ejected a distance of greater than the height of the arrester in the event of failure. In distinction from porcelainhoused arrester where they found the failure rate of 1 per cent per year !34",

Top cover plate Clamping ring

Pressure relief diaphragm

Sealing ring

Supporting ring

Venting outlet

Compression spring

High-voltage terminal

Porcelain housing

MO resistor stack with supporting construction

Mounting bracket

Sealing ring

Flange with venting outlet

Compression spring

Pressure relief diaphragm

Earth terminal with

disconnector


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one industry source estimates the failure rate for poly-meric arresters to be only around 0,1 per cent !33". It is obvious that this difference is not only the result of shifting from porcelain to polymeric housing, and that all designs utilised, types of materials used for housing production or manufacturing processes applied can not achieve this relatively high rating. The failure rate is determined first of all by isokeraunic level in the region where the arrester is in service (a factor which is out of the control of the industry), but also by design and ma-terials used in its construction and manufacturing proc-ess itself. Today, there are four major categories of polymeric materials used for arresters housing construction: ethylen propylen dien monomer (EPDM), silicone rub-ber (SIR), blends comprised of EPDM and SIR and eth-ylene vinyl accetate (EVA). Statistics as to which material dominates total sales are almost impossible to obtain, however, it appears that silicone rubber is probably the material found on the majority of such units sold today !33". The main reason for such situation could be found in the fact that silicone rubber, thanks to its unique property to maintain its hydrophobicity dur-ing the entire life time of the arrester even under pol-luted conditions, has superior performance in polluted conditions due to reduced surface leakage currents which could cause premature ageing. Apart from that, silicone rubber showing higher bond energy !19" than other polymers utilised is less vulnerable to the effects of UV radiation and ozone. The above statements can be found conformed by numerous reports from service experience !911, 3545". Although polymeric HV arrester designs have not al-ready come to dominate the market like in the area of distribution arresters, the acceptance of polymeric HV arresters has actually been growing more quickly than the current statistic might indicate. Namely, industry sources currently estimate that during the past five years the proportion of the all HV arresters with polymeric housing has risen by 10 percentage points, being today between 25 and 35 per cent !46". The main reasons which has made many utilities reluc-tant to change away from traditional porcelain housed HV arresters is the look at polymeric arresters as a sim-ple oneforone replacement for traditional porcelainhoused units and meeting the same mechanical require-ments. Namely, the cantilever strength of most poly-meric arrester designs in not able to permit such simple replacement. However, the lighter weight of polymeric arresters, enabling them to be used more and more in protection of overhead lines, should be, and probably very soon be, the reason to change the philosophy of their installation at stations, such as being suspended. As well as in the case of distribution polymeric arrest-ers, a number of alternative designs and manufacturing processes have been used in the past decade toward em-ploying polymeric housing in HV arresters: tube design, open design and closed design. The classical type of tube design which has to be pro-vided with pressure relief device as well as newer one

where there is no more an enclosed amount of air be-tween the tube and MO blocks enable arrester construc-tions using only one single module up to 220 kV, and 400 kV, respectively. In the case of an open or closed wraptype design one of the critical question in design is that the maximum possible length of an individual unit is limited (cca. 1 m) by mechanical strength of col-umn and by certain performance parameters. The open design needs fewer units or modules to build up a HV arrester than a comparable wraptype design, but more than would be necessary with a tube design. The tube design principle offers the inner structure which could be so mechanically strong that can endure the most se-vere earthquake intact. In the case of an arrester over-load with this construction a housing breakage will not occur and not even any of the inner parts will be ejected. It is also very important to emphasis upon that with multiple unit arrester, the pollution performance be-comes much more important since it can result in a nonlinear voltage distribution across the arrester. With a pollution layer and greater moisture formation on lower units, most of the voltage drop is at the top and therefore overstresses the upper most part of arrester. On the other hand, the designs which have an enclosed air gap between the housing and MO blocks are more liable to pollution, because leakage currents flowing across the housing will ionise the air within the gas. This, in turn, can pose a threat to the resistors requiring immediate remedial action. Apart from better shortcircuit capability with the in-creased safety for the other equipment and personnel nearby there are another reasons for transition away from porcelain and toward polymeric housing for surge arresters: # Better behaviour in polluted conditions # Low weight # Non brittle Better performance in polluted environments compared to porcelain could be achieved as a result of application of silicone rubber housing as well as the proper external housing design. The possible weight reduction compared to porcelain housedarresters could be seen from the Table 2.

Table 2. Polymer arresters weight advantage

Type Voltage

(kV) Porcelain Polymer

Weight reduction

(%)

Distribution* 15 69,0 lbs. 3,8 lbs. 36,7

Substation* 69 124,0 lbs.

28,0 lbs.

77,4

Substation* 138 280 kg 98,9 lbs.

64,7

Substation** 550 450 kg 275 kg 39,0

Transmission**

line 550 450 kg 150 kg 66,6

(*) according to !34"; (**) according to !47"


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For longer arresters for HV and EHV applications, the desired increase in the mechanical strength of the hous-ing is obtained by using additional stays of polymer material (Fig. 5) or by for example a series parallel ar-rangement utilising a variable number of plates (Fig. 6).

Fig. 5 Polymerhoused arrester for 550 kV system volt-age designed to meet extreme earthquake requirements

Fig. 6 132 kV arrester with the series parallel arrangement

3.2 Transformer bushings For power transformer manufacturers as well as for the final users the oilimpregnated paper (OIP) bushings with porcelain housing still represent the most attractive solution. On a world wide basis, at least three out of every four installations of a bushing on power trans-former involve an OIP design. In spite of their short lead times in comparison with conventional porcelain-housed bushings the application of composite polymer insulators in place of porcelain in the design of OIP bushings is only of order of 2 to 3 per cent [48].

The only alternative technology for OIP bushing design is resin impregnated paper (RIP) design incorporating silicone composite polymer insulators. RIP bushings technology offers a total dry design (free of oil) over-coming all drawbacks of OIP design: oil leak, risk of explosion due to lighting strikes or other factors, mois-ture ingress, operating temperature limits, the problems in case of connection to SF6 due to oil presence etc. The well known advantages of silicone housing in com-parison with porcelain (better pollution performance, greater resistance to seismic forces and faster manufac-turing lead times) are most pronounced and accepted by customers when on an RIP core. According to executive at some of the largest suppliers in the industry, RIP de-signs incorporating silicone housing today represent the leading edge in bushing technology [48]. In recent years some producers have introduced a new manufacturing process which eliminates quite expensive FRP tube. This process consist in moulding the silicone weather sheds directly on to the core of RIP bushing, like in case of new arrester and classical cable termina-tion design. According to the producers estimation in this manner the cost of entire RIP bushing could be re-duced as much as between 5 and 10 per cent [48]. Apart from that, this design eliminates various internal parts in a bushing construction making it less compli-cated. However, there are some concerns in this design regarding the issues like: moisture ingress, quality of bonding between the core and silicone and issue of me-chanical strength, especially at higher voltages. Without regard to the manufacturing process, the appli-cation of SIR insulators to the RIP bushings, will grow significantly, to somewhere between 20 and 25 per cent within five years, according to executives at some of the largest suppliers in the industry [48] thanks to: # reduced risk for fire, oil leakage from bushing elimi-

nated, no monitoring of pressure and oil level, any mounting angle possible;

# protection of personnel and equipment, easy han-dling, high earthquake withstand;

# superior electrical performance washing normally not requested.

3.3 Circuit breakers Silicone rubber composite bushings for SF6 dead tank power circuit breaker applications have been introduced since the early 1990s. Many of such breakers of one of the manufacturers (145 kV and 245 kV) are in service for more than 10 years in different locations subjected to marine pollution, altitude or tropical environment. According to the inspection reports their external insula-tion is still hydroscopic, and has no punctures, cracks or significant erosion !49". An another manufacturer, started 1990 with 242 kV dead tank breakers, has al-ready in service composite bushings at 72 kV to 800 kV dead tank breakers. Their field experience has been ex-cellent with some minor problems: shed handling, clea-ning questions, mould growth, rodent damage !50".


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The application of CPIs in the area of circuit breakers has started with dead tank design since in that case is possible to make a relatively simple substitutions of composite bushings in place of porcelain bushings. How-ever, not all of advantages of CPIs technology could be expected as long as the composite insulator just replaces the old porcelain design. So the new designs of this type of breakers enabling the use of smaller insulators with-out additional grading shields can be found today on the world market !51". Due to the fragility of porcelain insulators significant care should be used during the handling, transport, in-stallation and maintenance of the live tank circuit where large insulators are required. With a view to overcome these problems some circuit breaker manufacturers have been introduced a new design concept using hollow core composite polymer insulators designed to withstand high mechanical leads as well as the heat developed during current interruption. As distinction from dead tank breakers, for live tank breakers, where the interrupters are not housed in a metal enclosed earthed tank (dead tank) than in insu-lators mounted at the system potential, oneforone substitution of composite insulator in place of its porce-lain counterpart would be quite impractical for eco-nomic reason. In order to integrate composite insulators in this type of the breaker, a new re-designed apparatus should be developed (see for example Fig. 7).

Fig. 7 Comparison between two different circuit breaker

design concepts !32"

Although live tank breakers with silicone insulators are today available in the range of 72,5 to 800 kV !52", they have not yet been found a widespread application, and are still under tests in some networks. This area is still seen as more evolving rather than evolved !53". The main reason for such situation is that there are still per-formance issues which need to be resolved in order for composite insulators to be as reliable or, in some cases, as economical as porcelain: # Composite polymer insulator may become a source of

leaks, especially under high temperatures (the area where the flange is bonded to the composite tube).

# The need to extinguish the arc inside the composite tube results in very aggressive decomposition prod-ucts of SF6. These products do not present harm to porcelain but could be very dangerous for organic materials. Therefore, a special protective lining inside the tube should be applied (it can not be the same liner used in equipment such as instrument transformer, terminals or bushings, where any systematic internal arcing is not expected). There is still the problem re-garding the longterm behaviour of this type of liner which could become another source of leaks.

# The behaviour of composite polymer insulators under stresses during bending (near the mid point) and pres-sure (at the bottom) should be checked with special attention (this issue is now covered by IEC 61462).

In order to be sure of the quality and longterm per-formance of composite polymer insulators used some of the above mentioned issues although not yet covered by the IEC standards should be particularly considered. Since testing of live tank circuit breakers equipped with composite polymer insulators under seismic conditions has shown the security factor twice that of porcelain it can be expected that high percentage of this breakers would be intended for service in earthquake-sensitive areas (by some estimates, about half of all dead tank breakers delivered with composite insulators have been specifically destined for such areas !53"). Greater safety margin, including also other advantages over porcelain insulators (impact resistance and explosionproof, light-weight), which have to be paid by 1020 per cent higher price of breakers do not present yet big attraction to the other still price driven customers. Why? Why to pay more and have to accept the additional risk (surface ageing of polymeric housing and problems with perme-ability and leaks of SF6 gas) would be their laconic answer. They have taken out of the equation all the various life cycle costs (other than initial purchase price) which should in truth be considered when select-ing a product, would be the argument of a composite insulators promoter. He is in the right as well, but the life is going on.

3.4 Cable terminations Traditional porcelain housed terminations which have been used successfully for many decades, due to draw-backs of the material itself as well as the designs used, have some disadvantages: # heavy, inflexible and large designs; # shatter under excessive electrical and mechanical loads; # time consuming installations requiring specially-

skilled staff. In order to overcome these problems, more than 30 years back, polymeric-housed cable terminations were introduced, particularly in the range of medium voltage. Modern cable installations from the lowest to the high-est voltage ranges world-wide are equipped with this type terminations, available in two basic variants:

Conventional 550 kV breaker with porcelain insulator

New 550 kV breaker with composite insulators

Interrupter head wight 100 %

52%

Support column weight 100 %

23%

Weight of base 100 %

100%

Total weight 100 %

62%


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# Flexible: a external tube with built-in sheds is shrunk over the prepared cable end;

# Rigid: the porcelain insulator used in old termination design is replaced with self-supporting polymeric counter part composite hollow core insulator.

Several advantages which offer polymers as the material of choice over porcelain, such as, light weight (net weigh reduction in this area is on the order of 50 per cent), and ready availability (prefabricated and tested products), enabled the users to be provided with highly simplified techniques making polymeric-housed termi-nations more reliable and less expensive than porcelain terminations !54, 55". Service experience has been largely successful with only few failures experienced in extremely harsh locations !54". Apart from a higher price in comparison with ethylene propylene or ethylene-vinyl acetate housings many manufacturers are using silicone rubbers. This decision is not only based on greater capability of SIRs to with-stand the effects of pollution in combination with mois-ture and ultraviolet radiation, but also on their other useful properties: # Silicone rubber shows high gas permeability which

enables gas bubbles enclosed in the interface during assembling to penetrate into the bulk against radial pressure of the silicone housing, resulting in a high electric strength of stressed interfaces !56".

# Siliconesilicone and siliconestresscontrolcompound interfaces life characteristics show very low decrease rate at fairly high electric stresses (no electrical age-ing of the inner insulation) !57".

The imperfections at the interface of cross-linked poly-ethylene (XLPE) cable insulation and SIR stress cone used for stress control in the terminations installed in a network (Dutch Electricity Network) caused ten 150 kV terminations broke down (1993) within 1 hour after one to six years of service !58". Without regard to the fact that this situation will still require attention in the com-ing years, the user have not rejected the general concept of polymer terminations application. It is to emphasize a reasonable approach of this user, since the failures re-ported could be associated with poor cable preparation (cleanliness and contact of surfaces, design issue etc.) rather to some of the weaknesses in the termination it-self. Having many outstanding qualities applicable to outdoor terminations, composite polymer insulator tech-nology is becoming more and more attractive in this area also at higher voltages !59".

3.5 Instrument transformers Since the failure rate of instrument transformers equipped with conventional porcelain housing has been found very low (0,3%), and of these only small per cent has failed explosively, it is conspicuously that safety factor for composite hollow core insulators as replacements to porcelain is not as strong as in the case of, for example, surge arresters. On the other hand, in order to reach re-

quired ratio of leakage distance to flashover distance (4 : 1, according to IEC 60815), which naturally can be easily realized using silicone rubber, porcelain manufac-turers introduced the practice of bonding sections of relatively cheap 145 kV porcelains together to make units for higher voltage levels. Therefore, in the field of oil immersed instrument transformers hollow core com-posite polymer insulators are not to be attractive since now. In the field of SF6 insulated instrument transformers hollow core composite insulators are only solution to be considered since the internal gas pressure used repre-sents too great a stored energy for a porcelain design. Today there are many such instrument transformer op-erating for more than 20 years. The fact that their pur-chase price is 23 times as much as porcelain oil-filled transformers makes them too expensive for general ap-plications. The results of the non-destructive tests re-garding durability and life time of a current transformer with silicone housing after 17 years in service did not show any significant ageing of hollow core insulators as well as the transformer itself !61". Many manufacturers have developed electronic measur-ing transformers where a slim support structure is quite favorable and composite polymer hollow core insulators represent the proper choice. 4. CONCLUSIONS 1) There are some opinions that the application of the

technology of composite polymer insulators has not yet reached the level where these products are suffi-ciently cheap or technically attractive to the cus-tomer and that their advantages should be used only in specialist applications, mainly where seismic con-siderations make porcelain unsuitable. Cheap or ex-pensive should not be the matter in question since only total life cycle costs are supposed to be deci-sion-making instruments. Regarding technical is-sues, the past experience with different polymeric materials and different designs has made a great in-fluence on the manufacturers to introduce better ma-terials, designs and manufacturing techniques, as well as on the users to learn more and more about new technology. Today, failures of composite poly-mer insulators (excluding those produced when this technology was still in its infancy) are only isolated cases which are most likely due to poor quality con-trol resulting in internal defects.

2) Hollow core composite polymer insulators are gen-erally more complex than polymeric long rods which have no medium inside and no internal gas pressure. In addition hollow core insulators should be de-signed for complex type of cantilever loads. In spite of their relatively slow acceptance the advantages offered by this type of insulators constrain the re-search laboratories at large manufacturers of high voltage substations equipment to start developing new apparatus designs built completely around composite polymer hollow core insulators.


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3) Traditionally, porcelainhoused distribution arrest-ers have tended to fail due to problems with sealing. The benefits of a leaktight design using polymers have been generally accepted leading to the almost wholesale changeover from porcelain to polymers as the housing material for distribution arresters. An-other factor is that at present polymerichoused ar-resters are cheaper than those made with porcelain housing, since most customer today focus solely on price and not on technical benefits. Porcelain still dominates as the housing material for HV arresters. Major among the reasons for such situation is the fact that in the case of HV arresters porcelainhoused arresters have been always designed to handle inter-nal fault without catastrophic consequences (without explosive shattering). Another factor which to data has restricted acceptance of polymerhoused HV ar-resters is a continued lack of confidence about the long term behaviour of polymeric materials under service conditions. The last but not the least factor influencing greatly the market is that the large seg-ment of market yet consider the price as the only important issue. The manufacturers response to such slow acceptance of this promising technology was to introduce new designs where the original design con-cept which employs a relatively costly hollow tube has been replaced by the housing moulded directly onto the internal parts. One manufacturer , in order to drive conversion to polymerhoused arresters, of-fers them today, in spite of their advantages, at prices somewhat below standard porcelainhoused arrest-ers. Finally, the acceptance of HV polymeric arrest-ers is still relatively slow because users are not yet aware that with polymeric arresters the whole sys-tem performance could be greatly improved (for ex-ample by application of transmission line arresters where porcelainhoused arresters can not be used) or that they could be, thanks to the relatively light weight, the answer to the growing requirements for seismic withstand performance of arresters.

4) The state-of-the-art in new transformer bushing tech-nology represent resin-impregnated paper (RIP) de-sign incorporating silicone composite polymer insu-lators.

5) In spite of the fact that there are still performance issues which need to be resolved in order for circuit breakers with silicone insulators, specially for live tank breakers, to be as reliable or as economical as the breakers with porcelain insulators, leading manu-facturers have their products on the market being ready if the market shifts in the directions of com-posite polymer insulators.

6) Cable terminations were on of the first apparatus equipped with polymeric housings. Utility service experience worldwide has been very positive. Only few termination failures reported by utilities could be addressed to poor cable preparation rather to any weakness in the termination itself.

7) Hollow core composite insulators are not used in the field of oil immersed instrument transformers, but in

the field of SF6 insulated instrument transformers these insulators are only solution to be considered, showing very good service experience. They are also a good solution for new generation of electronic measuring transformers.

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Documents

Silicone Housing for High Voltage Applications