Electrical hollow core insulator

The present invention refers to an electrical hollow core insulator, preferably in tubular form, for medium and high voltage applications, wherein the wall of said electrical insulator is made from a fiber reinforced organic polymer composite system comprising a hardened or cured electrically insulating matrix resin composition and a reinforcing fiber, said wall further comprising an outer layer and an inner layer, both layers together forming said wall as integral parts. Said two layers are reinforced with different kinds of fibers whereby said hardened or cured matrix resin composition has a glass transition temperature (Tg) within the range of 160 ⁰ C to 250 ⁰ C.

State of the Art

In the field of medium and high voltage applications glass fiber impregnated hollow core insulators are widely used. Due to the good mechanical performance of glass fibers the resulting insulators have high mechanical stiffness. In the usual production process there is typically applied a winding and impregnation process, where the glass fiber is soaked with an epoxy resin matrix. Thereby the uncured insulator body is obtained which is put into an oven to cure the resin matrix yielding the hardened electrical insulator body. Such conventional electrical insulators, however, can be used only up to temperatures of about 130 ⁰ C to 150 ⁰ C. At higher temperatures these electrical insulators loose the required stiffness. Therefore conventional electrical insulators typically are used up to about 100 ⁰ C.

Another drawback of the glass fiber reinforced insulators is their bad corrosion resistance, which is mainly due to the presence of acid sensitive oxides as for instance SiC> ₂ . In high voltage applications where there is typically used sulfur hexafluoride gas (SF ₆ -gas) as an arc-quenching insulating gas, corrosive by-products such as hydrogen fluoride (HF) attack the

glass fiber reinforcement. Under harsh service conditions harmful damages due to corrosion can occur which finally lead to the break-down of the insulator. Other oxide rich fibers, as for instance basalt fibers, have the same drawbacks.

To replace the reinforcing glass fiber by a corrosion resistant fiber such as polyester fiber or aramide fiber results in other disadvantages. Polyester has a poor mechanical strength. Aramide fiber for example has a high moisture uptake; the fiber is ex- pensive and its processing to the finished product is cost intensive. Therefore, the resulting insulator tube is for many applications not competitive.

Description of the Invention It has now been found that an electrical hollow core insulator, preferably in tubular form, does not have these disadvantages when the wall of the electrical insulator is made from a fiber reinforced organic polymer composite system comprising a hardened or cured matrix resin composition and a reinforcing fiber, said wall further comprising an outer layer and an inner layer, both layers together forming said wall as integral parts. According to the present invention said two layers are reinforced with different kinds of fibers whereby said hardened or cured matrix resin composition has a glass transition tempe- rature (Tg) within the range of 160 ⁰ C to 250 ⁰ C. Thereby an electrical hollow core insulator system is obtained which is suitable for use at high temperatures, high service pressures and under corrosive conditions. As is known, conventional impregnation resins compositions used for electrical insulators typically have a glass transition temperature (Tg) below 130 ⁰ C to 150°C.

According to the present invention, the inner layer is reinforced with a corrosion resistant fiber and forms a corrosion resistant insulating surface as the internal surface of the

inner insulator wall whilst the outer layer is formed by a conventional corrosion sensitive fiber reinforced polymer system. The hollow core insulator as described in the present invention can be manufactured by using conventional winding and curing techniques.

The present invention is defined in the claims. The present invention specifically refers to an electrical hollow core insulator, preferably in tubular form, wherein the wall of said electrical insulator is made from a fiber reinforced organic polymer composite system comprising a hardened or cured electrically insulating matrix resin composition and a reinforcing fiber, said wall further comprising an outer layer and an inner layer, both layers together forming said wall as integral parts, characterized in that

(i) the outer layer of the wall is reinforced with a corrosion sensitive fiber; (ii) the inner layer of the wall is reinforced with a corrosion resistant fiber, and (iϋ) the hardened or cured matrix resin composition has a glass transition temperature (Tg) within the range of 160 ⁰ C to

250 ⁰ C.

The present invention further refers to a method of producing said electrical hollow core insulator. The present invention further refers to the use of said electrical hollow core insulator in the production of electrical insulations in electrical articles, especially in the production of electrical components, preferably in the field of cylindrical insulators, preferably metal enclosed applications, preferably metal enclosed switch- gear applications as well as in the production of spacer insulators and related applications. The present invention further refers to the electrical articles comprising said electrical hollow core insulator.

Electrical hollow core insulators are known. The wall of the electrical hollow core insulators according to the present invention is made from a fiber reinforced organic polymer composite system. The matrix resin composition of the fiber reinforced organic polymer composite system may be selected from polymers known in the art of being used in electrical insulator compositions, such as polyesters, preferably poly (methyl-meth- acrylate) , or poly (alkylacrylonitrile) , or duroplastic polymers, preferably polyurethanes or epoxy resin compositions or cyanate- esters. Preferred are epoxy resin compositions.

The inventive feature is that said fiber reinforced organic polymer composite system is based on a hardened or cured matrix resin composition having a glass transition temperature (Tg) within the range of 160 ⁰ C to 250 ⁰ C, preferably within the range of 170°C to 220 ⁰ C and preferably within the range of 170°C to 200 ⁰ C, and that said fiber reinforced organic polymer organic system is combined with the further features as defined and described herein.

The fiber reinforced electrical insulation system comprises the hardened or cured matrix resin composition which optionally may comprise additives selected from filler materials, wetting/dispersing agents, flexibilizers, plasticizers, antioxidants, light absorbers, silicones, pigments, flame retardants and other additives generally used in electrical applications.

If the matrix resin composition is an epoxy resin composition, said composition comprises the epoxy resin, the hardener, e.g. an anhydride hardener, and preferably a curing agent/accelerant as an additive for enhancing the polymerization of the epoxy resin with the hardener. These additives are known to the expert and are not critical for the present invention.

The total amount of reinforcing fiber and optional additives preferably are within the range of from 60% to 90% by weight and preferably within the range of from 60% to 85% by weight of the total weight of the fiber reinforced organic polymer composite system.

The wall of the electrical hollow core insulator system comprises an outer layer and an inner layer, both layers together form said wall as integral parts. The figure illustrates an example of such an electrical hollow core insulator wall.

The basic function of the inner layer 1 which is reinforced with a corrosion resistant fiber and which also forms the internal surface of the insulator wall is to protect the outer layer which is reinforced with a corrosion sensitive fiber. As corrosion sensitive fiber, known fibers as a glass fiber or a basalt fiber or a mixture of both can be used. The inner layer 1 generally has a thickness within the range of 0.5 mm to 5 mm, preferably within the range of 1 mm to 3 mm. The thickness of the outer layer is variable and depends on the size of the electrical insulator and on the physical loadings.

The outer layer 2 of the wall is reinforced with a glass fiber. Such glass fibers as used in glass fiber reinforced resins in electrical insulators are known in the art.

The inner layer 1 of the wall is reinforced with a corrosion resistant fiber. The corrosion resistant fiber is preferably selected from the group comprising polyesters, preferably poly- ethyleneterephthalate (PET) or polybutyleneterephthalate (PBT) ; aramides; fluoropolmyers as for example from the group of poly- tetrafluoroethylene (PTFE) (Teflon) , and tetrafluoroethylene/- ethylene-copolymer (ETFE), polyimides (PI) , polyphenylene- sulfides (PPS), polyestersulfones (PES), liquid crystal polymers (LCP) , polybenzimidazols (PBI) , and poly-ether-etherketones

(PEEK) ; or from inorganic fibers made from alumina. The corrosion resistant fiber is preferably selected from the group comprising polyesters, preferably polyethyleneterephthalate (PET) or polybutyleneterephthalate (PBT) ; aramides; polytetrafluoro- ethylene (PTFE) , tetrafluoroethylene/ethylene-copolymer (ETFE) , and inorganic fibers made from alumina. Most preferably, the corrosion resistant fiber is polyethyleneterephthalate (PET) or alumina) .

All the mentioned fibers are well known including also the additives used in the production of these fibers. Also the physical and mechanical properties of these fibers are well known so that it is no problem for the expert in the art to chose the optimum fiber with the optimum physical and mechanical properties for producing the outer layer 2 and the inner layer 1 of the fiber reinforced organic polymer composite system which forms the wall of the tubular electrical insulator according to the present invention .

Examples of commercially available fibers are:

Polyester fibers: Diolen® from Acordis Corp., DSP® Polyester and

ACE® Polyester from Honeywall Corp.;

Aramide fibers: Kevlar®, Nomex® from DuPont, Twaron® from

Acordis Corp. ; Polybenzimidazole fibers: PBI Gold®, from Celanese Acetate

Corp. ;

Polyphenylenssulfide fibers: Procon® from Toyobo Corp.;

Liquid crystal polymer fibers: Vectran® from Celanese Acetate

Corp. ; Polyimide fibers: Pentex® from Allied Signal Corp.;

PEEK fibers: Zyex®

Alumina fibers: Saffil® fibres from SAFFIL Corp.

The glass fibers and the corrosion resistant fibers to be used in the present invention have preferably a fiber diameter as

used in roving cloths and fabrics made from said fibers. Generally the diameter is within the range of about 0.4-200 micron (μm) , preferably within the range of about 1-100 micron and preferably within the range of about 5-50 micron. The diameter generally is not critical. It is within the knowledge of the expert in the art to optimize the diameter if required.

The reinforcing fiber may be present in form of chopped fibers having an average length preferably within the range of 0.5 mm to 15 mm, preferably within the range of 1.0 mm to 8 mm. Preferred, however, is that the fibers are used as a continuous non-woven filament, or as a continuous woven filament, such as a yarn, fiber roving, fiber web, fleece, woven fabric such as a cloth, tape, mat or net. In this sense the fiber reinforced organic polymer composite system comprises the reinforcing fiber evenly distributed within the electrically insulating matrix resin composition whereby the surface of the fibers is substantially covered by the matrix resin composition.

The reinforcing glass fiber is preferably present in an amount within the range of 40% to 75% by weight of the total weight of the glass fiber reinforced organic polymer composite system, preferably within the range of 50% to 75% by weight, and preferably within the range of 60% to 75% by weight of the total weight of the glass fiber reinforced organic polymer composite system.

Alternatively to the before-mentioned reinforcing glass fiber, a basalt fiber or a mixture of glass and basalt fibers can be used as a corrosion sensitive fiber.

The contents of the reinforcing corrosion resistant fiber depends on the type of fiber used and whether this fiber is used as a chopped fiber or in form of a woven or non woven fiber. The content of the corrosion resistant fiber is generally within the

range of from 40% to 70 % by weight of the total weight of the corrosion resistant fiber reinforced organic polymer composite system and can easily be optimized by the expert in the art.

The fiber reinforced electrical insulation system of the present invention may optionally contain a mineral filler or a mixture of such filler materials with an average grain size distribution within the micro range or nano range grain size. The mineral filler, however, has preferably an average grain size distribu- tion within the range of 1 μm-500 μm (micron) , preferably within the range of 5 μm-100 μm. Preferably at least 70% of the particles, and preferably at least 80% of the particles have a particle size within the range indicated.

The mineral filler is preferably selected from conventional filler materials as are generally used as fillers in electrical insulations. Preferably said filler is selected from silica, quartz, known silicates, aluminium oxide, aluminium trihydrate [ATH], titanium oxide or dolomite [CaMg (CO ₃ ) ₂ ] , metal nitrides, such as silicon nitride, boron nitride and aluminium nitride or metal carbides, such as silicon carbide. The surface of the filler material may have been surface treated in a manner known per se, for example with a compound selected from the group comprising silanes and siloxanes, preferably for example with 3- glycidoxypropyltrimethoxysilane or 3-glycidoxypropyldimethoxy- methylsilane .

Polyesters such as poly (methyl-methacrylate) or poly(alkyl- acrylonitrile) or duroplastic polymers such as polyurethanes or epoxy resin compositions or cyanate esters are known and have been described in the literature. According to the present invention the hardened or cured matrix resin composition has a glass transition temperature (Tg) within the range of 160 ⁰ C to 250 ⁰ C. Such polymerizable resin compositions are described for example in: Faserverbund-Kunststoffe, Gottfried W. Ehrenstein,

2. Auflage, Carl Hanser Verlag, 2006, where also several epoxy resin compositions are described (such as in Table 3.7, page 66) which may be used according to the present invention. Further references are for example: Seachtling, Kunststoff Taschenbuch, Oberbach, Baur, Brinkkmann, Hanser Verlag, 2004; or Chemistry and Technology of Cyanate Ester Resins, I. Hamerton, Blackie (1994); or http : //www.matweb .

Preferred thermosetting resins used within the context of the present invention are epoxy resins derived from aromatic and/or cycloaliphatic compounds. Preferred are epoxy resin compositions, preferably cycloaliphatic epoxy resins compound although in many cases aromatic epoxy resins as well are preferred giving excellent results. Said epoxy resin compositions generally contain the epoxy resin, a hardener, a curing agent to accelerate the curing process, as well as further additives. These compounds are known per se. Preferred examples are glycidyl ethers derived from Bisphenol A or Bisphenol F as well as glycidyl ethers derived from Phenol-Novolak-resins or cresol-Novolak-resins . Cycloali- phatic epoxy resins that may be used are for example also hexa- hydro-o-phthalic acid-bis-glycidylester or hexahydro-m-phthalic acid-bis-glycidyl ester or hexahydro-p-phthalic acid-bis-glyci- dyl ester, or 3, 4-epoxycyclohexylmethyl- (3, 4-epoxy) cyclohexane carboxylate or a mixture of these compounds. Also aliphatic epoxy resins, for example 1, 4-butane-diol diglycidylether, may be used, optionally as a component in a mixture of different hardeners .

The hardened or cured matrix resin composition has a glass tran- sition temperature (Tg) within the range of 160 ⁰ C to 250 ⁰ C. This also means that the epoxy resin compositions containing the epoxy resin, the hardener, the curing agent as well as further additives has a glass transition temperature within the range of 160 ⁰ C to 250 ⁰ C after hardening. Such an elevated glass transition tempera- ture can be obtained by changing either the epoxy resin or the

hardener or both components within the epoxy resin composition. This is within the knowledge of the expert in the art.

Examples of epoxy resin compositions having a glass transition temperature (Tg) higher than 160 ⁰ C are: mixtures of aromatic glycidylether of bisphenol A (e.g. Araldite F) with methylnadic- anhydride (MNA) in about equimolar concentrations; mixtures of Epikote 828 (epoxy resin), Epikure 878 (hardener) and Epikure 201 (curing agent) in a ratio of about 100:90:1 parts by weight, the components being available from Hexion Corp.

Further examples are mixtures of tetraglycidyldiaminodiphenyl- methane (TGDM) with methyltetrahydrophthalic anhydride (MTHA) ; or mixtures of epoxy resin XB9721, hardener Aradur 917 or Aradur 918 and curing agent DY070, in a ratio of about 100:141:1 parts by weight, the components being available from Huntsman Corp.; or mixtures of epoxy resin CY5825, hardener HY5825, in a ratio of about 100:121 parts by weight, the components being available from Huntsman; or mixtures of cycloaliphatic 3, 4-epoxycyclo- hexylmethyl- (3, 4-epoxy) cyclohexane carboxylate with hexahydro- phthalic anhydride; or mixtures of epoxy resin CY179 and hardener HT907 or hardener HY1235 and curing agent DY070 in a ratio of 100:105:8.5 parts by weight, the components being available from Huntsman Corp.

For producing the fiber reinforced organic polymer composite system according to the present invention known methods can be used in an analogous manner as described in the literature, using the fibers, the optional additives and the monomers of the polymers as described herein above. This is within the knowledge of the expert in the art.

When chopped reinforcing fibers are used for the production of the fiber reinforced electrical insulation system, the reinfor- cing fiber is preferably incorporated into the monomeric star-

ting material of the respective polymer so as to be uniformly dispersed therein. The non-hardened composition thus obtained, e.g. the non-hardened epoxy resin composition, is for example processed using conventional vacuum casting and/or automated pressure gelation (APG) manufacturing processes. The dispersion is formed into the desired shape, optionally with the help of a molding tool, and then hardened out or cured, optionally using post-curing. In producing the tubular form of the present invention wherein the wall comprises two layers and wherein each layer is reinforced with a different type of fiber, it is preferred to perform the production of the wall in two steps. In a first step the inner layer 1 of the wall is produced being reinforced with corrosion resistant fibers and hardened out at least partially and in a second step the outer layer is added to this inner layer 1 in a conventional manner by repeating the same production steps but using glass fibers as reinforcing fiber. The obtained tubular shape may subsequently be post- cured.

As a preferred embodiment the fiber reinforced organic polymer composite system comprises the reinforcing fiber as a non-woven or woven continuous fiber, preferably as a fiber roving or as a woven fabric in a wound fiber structure impregnated with the matrix resin composition. The production of this electrical hollow core insulator, preferably in tubular form, includes the following steps: in the first step a non-woven or woven continuous corrosion resistant fiber or fleece is shaped into the required form, preferably by winding around a mandrel, whereby said corrosion resistant fiber is impregnated with the monomeric starting material of the respective polymer prior or after winding, thereby yielding the non-cured or non-hardened inner layer of the wall. In the second step a non-woven or woven continuous glass fiber is directly added by continued winding onto the corrosion resistant fiber windings, whereby said glass fiber is impregnated with the monomeric starting material of the

respective polymer prior or after winding, thereby yielding the non-cured or non-hardened outer layer 2 of the wall. The obtained non-cured or non-hardened fiber reinforced organic polymer composite system is then hardened or cured and optio- nally post-cured, whereby the cured or hardened fiber reinforced organic polymer composite system, i.e. the electrical hollow core insulator comprising the different layers as defined herein above is obtained.

In this sense the present invention refers also to a method for producing an electrical hollow core insulator, preferably in tubular form, comprising a fiber-reinforced composite system as defined herein above, said process being characterized by the following steps: (i) shaping the non-woven or woven corrosion resistant continuous fiber into the required form, preferably by winding around a mandrel and impregnating said corrosion resistant fiber with the monomeric starting material of the respective polymer prior or after winding; (ii) directly adding a non-woven or woven continuous glass fiber by continued winding onto the corrosion resistant fiber windings and impregnating said glass fiber with the monomeric starting material of the respective polymer prior or after winding; (iii) hardening or curing, and optionally post-curing the obtained non-cured or non-hardened fiber reinforced organic polymer composite system. The cured or hardened fiber reinforced organic polymer composite system, i.e. the electrical hollow core insulator in tubular form, is thus obtained. Instead of winding techniques also pultrusion techniques can be applied.

Alternatively, the manufacturing process can also be performed with a basalt fiber or a mixture of glass and basalt fibers as a reinforcing corrosion sensitive fiber.

The fiber-reinforced composite system is useful for the pro- duction of electrical insulations and in the production of elec-

trical components especially in the field of cylindrical insulators, preferably metal enclosed applications, especially metal enclosed switchgear applications, such as in the production of pressurized gas-insulated switchgear stations (GIS) or in the production of generator circuit breakers (GCB) , life-tank breakers, dead-tank breaker and related applications. Further electrical components to be made with the insulation system according to the present invention comprise the impregnation of electrical coils and the production of spacer insulators and related applications.

Preferred uses also are in the production of electrical components such as transformers, bushings, insulators, switches, sensors, converters and cable end seals, high-voltage insulations for indoor and outdoor use, especially for outdoor insulators associated with high-voltage lines, as long-rod, composite and cap-type insulators, and also for base insulators in the medium-voltage sector, in the production of insulators associated with outdoor power switches, measuring transducers, lead-throughs, and overvoltage protectors, in switchgear construction, in power switches. The following example illustrates the invention.

Example 1 A hollow core electrical insulator is produced whereby a commercial equipment for wet filament winding is used. The used steel or aluminium mandrel to define the inner diameter of the insulator is chosen according the requirements in length and diameter. In this example, the length is 4 meter and the diameter 800 mm. The mandrel is pre-heated if necessary to improve resin flow and curing behavior.

The epoxy resin composition is prepared from the following components: the epoxy resin (Araldite F-type) , the anhydride hardener like MNA, and an amine based curing agent (catalyst) .

Epoxy and hardener are mixed in equimolar ratio and the curing agent is added by 0.2 to 1.2 phr. The components are preheated to around 60 ⁰ C, mixed together, degassed, and filled into the impregnation bath of the filament winding machine.

In a first step the inner layer of the insulator wall is formed using a protective fleece made from polyester (polyester fleece) . The fleece is impregnated with the epoxy resin composition and afterwards wound around a rotating mandrel up to a deposited layer thickness of approximately 1 mm, in particular 1 mm.

Immediately afterwards, the outer layer is formed by winding around the inner layer glass fiber rovings (technical product) . The rovings are impregnated with the same resin prior to the winding. The winding is carried out until the wall thickness has reached the required value Typically, the total thickness can amount between 5 mm and 55 mm, depending on the constrictive requirements .

The curing of the obtained fiber composite body is done in several steps. In a first step the resin is solidified by a heat source next to the mandrel until gelation is reached. Alternatively it can be cured gently in an oven at 80 ⁰ C while the mandrel is kept rotating. Afterwards, the composite is cured in a hot air oven to reach complete cure. In a first curing step it is cured at 140 ⁰ C for several hours, and subsequently at 160°C for another several hours. After the completion of cure, the composite tube can be removed from the mandrel and machined to the final geometry.