Electrical insulation system

The present invention refers to an electrical insulation system being a flber-remforced composite system which is suitable to be used in gas-insulated switchgear applications, preferably in gas-insulated metal enclosed switchgear applications, said electrical insulation system having improved mechanical properties and method of producing said electrical insulation system.

State of the art Cylindrical insulating parts in gas-insulated switchgears, such as chamber insulators and operating rods, are commonly made from flber-remforced materials. The material used is preferably a cured epoxy resin system reinforced with polyethylene tere- phthalate (PET) fibers or with aramide (Kevlar®, Twaron®) fibers or combinations thereof. These fibers have severe limitations.

Polyethylene terephthalate fibers for example have comparatively weak mechanical properties. Aramide fibers have a low adhesion between fibers and the epoxy matrix as well as a high moisture uptake. Therefore, there is a need for an electrical insulation system with improved properties, especially for the use as electrical insulator to be used m gas-msulated electrical applications, for example in pressurized gas-insulated switchgear stations (GIS) .

It has now been found that a fiber-reinforced composite system comprising a thermoplastic or duroplastic polymer composition, preferably a duroplastic polymer composition, preferably a hardened epoxy resin composition, and a reinforcing fiber selected from polyethylene naphthalate (PEN) fibers, poly- butylene naphthalate (PBN) fibers, from fibers being a copoly- ester of bisphenol A and/or bisphenol F with naphthalene dicarbonic acid, from fibers being a copolyester of hydroxy-benzoic acid and hydroxy-naphthoic acid preferably a copolyester of p-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid, or from a mixture of these fibers, yields an electrical insulation system with improved mechanical properties, which is especially useful

as electrical insulator for the production of electrical gas- insulated switchgear applications, preferably gas-insulated metal enclosed electrical applications such as pressurized gas- msulated switchgear stations (GIS) .

Description of the invention

The present invention is defined in the claims. The present invention refers to an electrical insulation system which is suitable to be used in electrical gas-insulated switchgear applications, preferably in gas-insulated metal enclosed switchgear applications, said electrical insulation system comprising a solidified polymer composition, preferably a cured epoxy resin composition, and a reinforcing fiber, characterized m that said reinforcing fiber is selected from polyethylene naphthalate (PEN) fibers, polybutylene naphthalate (PBN) fibers, from fibers being a copolyester of bisphenol A and/or bisphenol F with naphthalene dicarbonic acid, from fibers being a copolyester of hydroxy-benzoic acid and hydroxy-naphthoic acid preferably a copolyester of p-hydroxybenzoic acid and 6-hydroxy- 2-naphthoic acid, or from a mixture of these fibers, wherein said fiber-reinforced composite system optionally may contain further additives.

The present invention further refers to a method of producing said electrical insulation system which is suitable to be used m said electrical gas-insulated switchgear applications, preferably gas-msulated metal enclosed electrical applications such as pressurized gas-insulated switchgear stations (GIS) . The present invention further refers to the use of said electrical insulation system m the production of said electrical gas- insulated switchgear applications, preferably gas-insulated metal enclosed electrical applications such as pressurized gas- insulated switchgear stations (GIS) or spacer insulators and related applications. The present invention further refers to said electrical gas-insulated switchgear applications, preferably gas-insulated metal enclosed electrical applications

comprising an electrical insulation system according to the present invention.

Polyethylene naphthalate (PEN) is an ester of ethylene glycol with naphthalene dicarbonic acid and substantially contains the following chemical units:

Polybutylene naphthalate (PBN) is an ester of butylene glycol with naphthalene dicarbonic acid and substantially contains the following chemical units:

The copolyester of bisphenol A and/or bisphenol F with naphthalene dicarbonic acid substantially contains the following chemical units:

wherein if made from bisphenol A: G is methyl; and if made from bisphenol F: G is hydrogen.

The copolyester of p-hydroxybenzoic acid and 6-hydroxy-2- naphthoic acid substantially contains the following chemical units :

Said 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 (μra) , 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. The use according to the present invention, however, is that the fibers preferably are used in a non-chopped form, i.e. as a dry continuous non-woven filament, or as a fiber roving or as a woven fabric resp. cloth. The dry-body is preferably either produced by dry winding of a fiber roving or by dry winding of a fabric. Also one or more of such fiber rovings or cloths may be arranged within the hardened resin system in any required order, preferably by dry winding of the roving or cloth (e.g. around a mandrel) , or m a parallel and/or rectangular order. The system may contain also a combination of chopped fibers and/or continuous fibers and/or one or more fiber rovings or cloths.

The reinforcing fiber selected from the fibers to be used according to the present invention and as specified herein above is preferably present in an amount within the range of 20% to 70% by weight of the total weight of the fiber-remforced composite system, preferably within the range of 30% to 60% by weight, and preferably within the range of 35% to 55% by weight of the total weight of the fiber-reinforced composite system.

The polymers as used in the present invention are preferably selected from the group comprising epoxy resin systems, poly- urethanes, polyesters, polyamides, polybutylene terephthalate and polydicyclopentadiene. The polymer preferably is a duro- plastic polymer selected from epoxy resin systems and poly- urethanes and most preferably is a cured epoxy resin system.

As optional additives the fiber-reinforced composite system may further comprise components selected from filler materials, wettmg/dispersmg agents, plasticizers, antioxidants, light absorbers, silicones, and from further additives generally used m electrical applications. The total amount of reinforcing fiber, filler and the optional further additives may be up to 80% by weight and preferably up to 70% by weight of the total weight of the fiber-reinforced composite system.

The fiber- reinforced composite system of the present invention may optionally contain a mineral filler material of a micro size or nano size grain size distribution or a mixture of such filler materials. The mineral filler, however, has preferably an average grain size distribution within the range of 1 μm-500 μm, preferably within the range of 5 μm-100 μm. Preferably at least 70% of the particles, preferably at least 80% of the particles, and preferably at least 90% 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 gas-insulated switchgear applications. Such filler materials e.g. are stable against degradation products of sulfur hexa- fluoπde (SF ₆ ) such as aluminum oxide and are known per se. Preferably the electrical isolation system according to the present invention does not contain a filler material.

Epoxy resin systems, polyesters, polyamides, polybutylene tere- phthalate, polyurethanes and polydicyclopentadiene have been described m the literature. When a special fiber reinforced composite system according to the present invention is made, said fiber component (s) together with the optional additives may be incorporated into the respective monomeric starting material of the polymer in an analogous manner as described in the literature generally for filler materials and other additives.

Subsequently the starting material is hardened or cured. This is within the knowledge of the expert in the art.

When chopped reinforcing fibers as defined herein above are used, the reinforcing fiber material is incorporated into the monomeric starting materials of the respective polymer by known methods so as to be uniformly dispersed therein. The non- hardened composition thus obtained, e.g. the non-hardened epoxy resin composition, can for example be processed using conventio- nal vacuum casting and/or automated pressure gelation (APG) manufacturing processes. The dispersion is formed into the desired shape using known methods, optionally with the help of a molding tool, and then hardened out or cured, optionally using post-curing.

In this sense the present invention also refers to a method of producing an electrical insulation system which is suitable to be used in electrical gas-insulated switchgear applications, preferably gas-insulated metal enclosed electrical applications, characterized in that a chopped reinforcing fiber selected from polyethylene naphthalate (PEN) fibers, polybutylene naphthalate (PBN) fibers, from fibers being a copolyester of bisphenol A and/or bisphenol F with naphthalene dicarbonic acid, from fibers being a copolyester of hydroxy-benzoic acid and hydroxy- naphthoic acid preferably a copolyester of p-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid, or from a mixture of these fibers, and optionally further additives as defined herein above, is incorporated into the monomeric starting material of the respective polymer as defined herein above, preferably an epoxy resin composition, so as to be uniformly dispersed therein, the dispersion is then formed into the desired shape, optionally with the help of a molding tool, and then hardened or cured and optionally post-cured.

The fiber-reinforced composite system according to the present invention, being suitable for the use as an electrical

insulation system, preferably contains the reinforcing fiber in the form of a continuous non-woven filament, or as a fiber roving or as a woven fabric or as a non-crimp fabric. The dry- body is preferably either produced by dry winding of a fiber roving or by dry winding of a woven fabric or by dry winding of a non-crimp fabric. Also one or more of such fiber rovings or fabrics may be arranged within the hardened resin system m any required order, preferably by dry winding of the roving or fabric (e.g. around a mandrel) , or in a parallel and/or rectangular order. The system may contain also a combination of chopped fibers and/or continuous fibers and/or one or more fiber rovings or fabrics.

The present invention refers therefore also to a process for producing an electrical insulation system which is suitable to be used m electrical gas-insulated switchgear applications, preferably gas-insulated metal enclosed electrical applications, said process being characterized by the following steps: (i) providing a mold for the product comprising at least one dry layer of reinforcing material, said reinforcing material comprising a continuous non-woven filament and/or at least one fiber roving and/or at least one woven fabric and/or at least one non-crimp fabric within the mold as defined herein above; (ii) casting an unhardened monomer composition, prefe- rably a monomeric epoxy resin mixture, into the mold so that the mold becomes filled and the at least one layer of reinforcing material becomes fully impregnated; (in) hardening and/or curing the monomer composition within the mold at the appropriate temperature for a time long enough to get hardened resp. cured; and (iv) optionally post-curing the fiber- reinforced composite system obtained.

The above mentioned at least one dry layer of dry reinforcing material comprising a continuous non-woven filament and/or at least one fiber roving and/or at least one woven fabric and/or at least one non-crimp fabric is also named "drybody" . To

produce the drybody, the fibers, rovings or fabrics, as mentioned, are wound around a cylinder (the so called mandrel) . Polyester fleece layers may be used to stabilize the drybody. Subsequently the mandrel is put into another cylinder (outer mould) and afterwards the fibers are impregnated.

Preferred thermosetting resins used within the context of the present invention are epoxy resins made from aromatic and/or cycloaliphatic compounds. These compounds are known per se. Epoxy resins are reactive glycidyl compounds containing at least two 1,2-epoxy groups per molecule. Preferably a mixture of poly- glycidyl compounds is used such as a mixture of diglycidyl- and triglycidyl compounds.

Epoxy compounds useful for the present invention comprise unsub- stituted glycidyl groups and/or glycidyl groups substituted with methyl groups. These glycidyl compounds preferably have a molecular weight between 200 and 1200, especially between 200 und 1000 and may be solid or liquid. The epoxy value (equiv./lOO g) is preferably at least three, preferably at least four and especially at about five, preferably about 4.9 to 5.1 or higher. Preferred are glycidyl compounds which have glycidyl ether- and/or glycidyl ester groups. Such a compound may also contain both kinds of glycidyl groups, e.g. 4-glycidyloxy-benzoic acid- glycidyl ester. Preferred are polyglycidyl esters with 1 to 4 glycidyl ester groups, especially diglycidyl ester and/or triglycidyl esters. Preferred glycidyl esters may be derived from aromatic, araliphatic, cycloaliphatic, heterocyclic, heterocyc- lic-aliphatic or heterocyclic-aromatic dicarbonic acids with 6 to 20, preferably 6 to 12 ring carbon atoms or from aliphatic dicarbonic acids with 2 to 10 carbon atoms. Preferred are for

example optionally substituted epoxy resins of formula (IV) :

D = O SO2 CO CH2 C(CH3)2 C(CF3)2 n = zero or 1

or formula (V) :

C ^H 3 C H ₃

(V )

^=^ C H ₃ C H ₃

Examples are glycidyl ethers derived from Bisphenol A or Bis- phenol F as well as glycidyl ethers derived from Phenol-Novolak- resms or cresol-Novolak-resms .

Cycloaliphatic epoxy resins are for example hexahydro-o-phthalic acid-bis-glycidyl ester, hexahydro-m-phthalic acid-bis-glycidyl ester or hexahydro-p-phthalic acid-bis-glycidyl ester. Also aliphatic epoxy resins, for example 1 , 4-butane-diol diglycidyl- ether, may be used as a component for the composition of the present invention.

Preferred within the present invention are also aromatic and/or cycloaliphatic epoxy resins which contain at least one, preferably at least two, ammoglycidyl group in the molecule. Such epoxy resins are known and for example described in WO 99/67315. Pre-- ferred compounds are those of formula (VI) :

D = O SO2 CO CH2 C(CH3)2 C(CF3)2 n = Zero or 1

Especially suitable aminoglycidyl compounds are N, N-diglycidyl- aniline, N, N-diglycidyltoluidine, N, N, N ' ,N ' -tetraglycidyl-1, 3- diammobenzene, N, N, N ' , N ' -tetraglycidyl-1, 4-diammobenzene, N,N,N',N' -tetraglycidylxylylendiamme, N _r N, N ' , N ' -tetraglycidyl- 4,4' -diammodiphenylraethane, N, N, N ' , N ' -tetraglycidyl-3, 3 ' -diethyl- 4, 4 ' -diammodiphenylmethane, N, N, N ' , N ' -tetraglycidyl-3, 3 ' - diammodiphenylsulfone, N, N ' -Dimethyl-N, N ' -diglycidyl-4 , 4 ' -diammodiphenylmethane, N, N, N' , N ' -tetraglycidyl-alfa, alfa ' -bis ( 4- ammophenyl) -p-diisopropylbenzene and N, N, N' ,N ' -tetraglycidyl- alfa, alfa ' -bis- (3, 5-dimethyl-4 -ammophenyl ) -p-diisopropyl- benzene. Preferred aminoglycidyl compounds are also those of formula (VII) : I)

or of formula (VI I I ) :

N ({ % 1 ζ J> 1 <f )) N < ^vl ")

Further aminoglycidyl compounds which can be used according to the present invention are described m e.g. Houben-Weyl, Methoden der Organischen Chemie, Band E20, Makromolekulare Stoffe, Georg Thieme Verlag Stuttgart, 1987, pages 1926-1928.

Hardeners are known to be used in epoxy resins. Hardeners are for example hydroxyl and/or carboxyl containing polymers such as carboxyl terminated polyester and/or carboxyl containing acrylate- and/or methacrylate polymers and/or carboxylic acid anhydrides. Useful hardeners are further cyclic anhydrides of aromatic, aliphatic, cycloaliphatic and heterocyclic poly- carbonic acids. Preferred anhydrides of aromatic polycarbonic

acids are phthalic acid anhydride and substituted deπvates thereof, benzene-1, 2, 4, 5-tetracarbonic acid dianhydride and substituted deπvates thereof. Numerous further hardeners are from the literature.

The optional hardener can be used m concentrations within the range of 0.2 to 1.2, equivalents of hardening groups present, e.g. one anhydride group per 1 epoxide equivalent. However, often a concentration within the range of 0.2 to 0.4, equiva- lents of hardening groups is preferred.

As optional additives the composition may comprise further at least a curing agent (accelerant) for enhancing the polymerization of the epoxy resin with the hardener, at least one wetting/dispersing agent, plasticizers, antioxidants, light absorbers, as well as further additives used in electrical applications .

Curing agents for enhancing the polymerization of the epoxy resin with the hardener are for example tertiary amines, such as benzyldimethylamine or amine-complexes such as complexes of tertiary amines with boron trichloride or boron trifluoπde; urea derivatives, such as N-4-chlorophenyl-N' ,N ' -dimethylurea (Monu- ron) ; optionally substituted imidazoles such as imidazole or 2- phenyl-imidazole. Preferred are tertiary amines. Other curing catalyst such as transition metal complexes of cobalt (III), copper, manganese, (II), zinc in acetylacetonate may also be used, e.g. cobalt acetylacetonate (III) . The amount of catalyst used is a concentration of about 50-1000 ppm by weight, calcu- lated to the composition to be cured.

Wetting/dispersing agents are known per se for example m the form of surface activators; or reactive diluents, preferably epoxy-containing or hydroxyl-contaming reactive diluents; thixotropic agents or resinous modifiers. Known reactive diluents for example are cresylglycidylether, diepoxyethyl-1, 2-

benzene, bisphenol A, bisphenol F and the diglycidylethers thereof, diepoxydes of glycols and of polyglycols, such as neo- pentylglycol-diglycidylether or tπmethylolpropane-diglycidyl- ether . Preferred commercially available wettmg/dispersmg agents are for example organic copolymers containing acidic groups, e.g. Byk® W-9010 having an acid value of 129 mg KOH/g) . Such wetting/ dispersing agents are preferably used m amounts of 0.5% to 1.0% based on the filler weight.

Plasticizers, antioxidants, light absorbers, as well as further additives used in electrical applications are known in the art and are not critical.

The insulating composition made from epoxy resin is made simply by mixing all components, optionally under vacuum, in any desired sequence and curing the mixture by heating. If a fiber- remforced composite system comprising at least one layer of reinforcing fiber roving cloth is to be manufactured, the insulating composition made from epoxy resin is made simply by mixing all components, optionally under vacuum, m any desired sequence and then adding it to the at least one layer of reinforcing fiber roving cloth.

Preferably the hardener and the curing agent are separately added before curing. The curing temperature is preferably within the range of 50 ⁰ C to 280 ⁰ C, preferably within the range of 100°C to 200°C. Curing generally is possible also at lower temperatures, whereby at lower temperatures complete curing may last up to several days, depending also on catalyst present and its concentration.

The casting of the unhardened monomer composition into the mold so that the mold becomes filled and the at least one layer of reinforcing material in the mold becomes fully impregnated is preferably done by using vacuum techniques, preferably m combination with the application of pressure. Vibration methods

may also be applied in order to improve impregnation and minimizing air bubbles. The non-hardened insulating resin composition specifically is preferably applied by using vacuum casting or automated pressure gelation (APG) manufacturing processes optionally under the application of vacuum or resin transfer molding or vacuum assisted resin transfer molding, also to remove all moisture and air bubbles from the insulating composition. The encapsulating composition may then be cured by any method known in the art by heating the composition to the desired curing temperature.

The electrical insulation system according to the present invention is useful for the production of electrical gas- insulated applications, especially electrical gas-insulated switchgear applications, preferably gas-insulated metal enclosed electrical applications such as pressurized gas-insulated switchgear stations (GIS) , or life-tank breakers, dead-tank breaker and related applications.

The electrical insulation system according to the present invention may also be used for example m the production of transformers, 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. The following example illustrates the invention.

Example 1 The epoxy resin compositions Formulation A and Formulation B are made from the components as given m Table 1. The compositions are prepared by thoroughly mixing the epoxy resin, the hardener and the accelerator at a temperature of 8O ⁰ C. Then the mixture is outgassed under vacuum at 80 ⁰ C. The mixture is then transferred into a mold using vacuum impregnation to produce insulating rods and operating rods by adding the uncured epoxy

composition to the mould comprising a dry winding of a roving (each time made from PEN-fiber, or PET-fiber) . The composition is then cured for ten hours at 140 ⁰ C.

Definition of raw materials:

CY 228 bisphenol A epoxy resin

HY 918 modified carboxylic anhydride

EPC 845 modified tertiary amine

PEN-fiber type T112-110 from Performance Fibers GmbH

PET-fiber type T711 from Performance Fibers GmbH

Table 1:

Comparison between PET and PEN reinforced rods

Test results show that the mechanical properties of PEN-fiber reinforced components made from Formulation A are superior compared to the mechanical properties of PET-fiber reinforced components made from Formulation B. Tensile tests carried out on PET and PEN reinforced epoxy rods show that there is a significant increase in stiffness and load to failure (Figure 1) .

Example 2

The epoxy resin compositions Formulation C and Formulation D are made from the components as given m Table 2. The compositions are prepared by thoroughly mixing the epoxy resin, the hardener and the accelerator at a temperature of 80 ⁰ C. Then the mixture is outgassed under vacuum at 80°C. The mixture is then transferred into a mold using vacuum impregnation to produce insula- ting rods and operating rods by adding the uncured epoxy composition to the mould comprising a dry winding of a roving (each time made from Vectran®, or Kevlar fibres) . The composition is then cured for ten hours at 14O ⁰ C. Definition of raw materials:

CY 228 bisphenol A epoxy resin

HY 918 modified carboxylic anhydride

EPC 845 modified tertiary amine

Vectran® a copolyester from p-hydroxybenzoic acid and 6-hydroxy-2- naphthoic acid in the form of a fiber roving from Kuraray

America, Inc. Kevlar Aramid fiber roving from DuPont

Table 2:

CY228 HY918 EPC 845 Reinforcing fiber

Formulation C 100 85 2 Vectran®

Formulation D 100 85 2 Kevlar

Comparison between Vectran® and Kevlar reinforced rods The moisture uptake of Vectran is significantly lower compared to Kevlar. A drying step prior to impregnation of the reinforcing fibres is only required in the case of Kevlar. The substitution of Kevlar with Vectran® makes the production of insulators more efficient and the product more suited for electrical insulation application, due to a lower moisture content .