ELECTRICAL DEVICE COMPRISING THE SAME Technical Field

The present invention relates to an electrically insulating composite material comprising polyacrylonitrile fibrid and to an electrical device comprising the same.

Backgr ound

Insulation of oil-filled distribution and power transformers may be made from cellulose, polymer paper and pressboard. The cellulose papers or pressboards are mainly used in transformers with relatively lower thermal stability requirements, and polymer papers or pressboards are mainly for transformers with relatively higher thermal stability requirements. Nomex from Dupont and Thermal shield from 3M are typical commercially available polymer papers or pressboards. Generally speaking, cellulose papers or pressboards are more extensively used than polymer ones. The major reason is that the cost of cellulose paper and pressboard is much lower than those made of polymer. For certain applications, the combination of cellulose and polymer paper and pressboard are also used for the balance of thermal stability and cost. The cellulose and polymer raw materials are generally converted by sheet machines to paper or pressboard, which is further converted to different insulation elements used in transformers. These elements include conductor with wrapped paper, spacers, barriers, pressrings and strips.

Cellulose pressboards are mostly made from cellulose pulp fiber. The cellulose pulp fiber is prepared and then dried in sheets, by heating and pressing at the same time. Polymer pressboards are mostly made from polymer fiber and polymer fibrid. The polymer fiber for paper and pressboard preparation is short fiber which is generally made of normal continuous fiber with regular diameter, the short polymer fiber could be treated by further beating to develop their sheetmaking properties. The polymer fibrid, a type of fibrous particle used for binding, is with irregular shape and made from polymer solution, the normal process for polymer fibrid preparation is to get the polymer solution in a polar organic solvent first, then get the suspension of polymer fibrid by putting the polymer solution into the high shearing coagulation bath with the mixture of water and polar organic solvent as coagulation bath and finally get the fibrid by washing and filtrating the suspension of polymer fibrid in water several times to remove the polar organic solvent and the water. The polymer fibrid could be treated by further beating to develop their sheetmaking properties.

Although it is possible to prepare polymer papers or pressboards from polymer fibrid directly without any polymer fiber, the polymer fiber is critical for mechanical property of the final product. Thus, the normal polymer paper and pressboard contains both polymer fiber and polymer fibrid. Cellulose based insulation material has very good insulation properties in combination with oil (e.g. resistance to streamers). However, the mechanical property, especially the compressibility of cellulose pressboard is not good enough. It is very important to improve the mechanical property, especially the compressibility of cellulose pressboard. Further due to the cost, it's impossible to simply replace cellulose by Nomex pressboard. A pressboard with both good compressibility and acceptable cost is required in industry.

WO 03/104559 discloses paper for use in transformers where the wood pulp fiber has been mixed with a polyhydroxy polymer, such as polyvinyl alcohol, as a dry strength additive.

US 2004/140072 discloses a paper of cellulose fibers, a polymeric binder and aramid fibers for use in transformers. The polymeric binder may be polyvinyl alcohol, again as a dry strength additive.

US 2002/012759 relates to a moulding base paper used for forming paper vessels such as a cup or tray for foods and various industrial products. The paper comprises a synthetic resin layer on at least one surface thereof.

WO 92/105311 relates to cellulosic pulp fiber products bonded by a resin containing polyhydroxy acid with improved wet and dry strengths, and their preparation method thereof. It is necessary to soften the resin polymer by heating to bond the cellulosic fiber or to melt the polymer to impregnate and coat the cellulosic pulp fibers. EP 0 304 693 relates to a flame retardant electrical laminate prepared by impregnating a base material with a halogen-containing unsaturated polyester resin which is prepared by dissolving a halogen-containing unsaturated polyester into a polymerizable monomer, and then curing the halogen-containing unsaturated polyester resin. EP 0 623 936A1 discloses a heat-resistant electrical insulation paper, which is based on melamine resin fibers and polymer fibrils which acts as a binder for the fibers.

Some research work has been carried out to improve the properties of cellulose paper or pressboard, while these materials are still too compressible and not rigid enough, there is still a need to provide an electrically insulating composite material in the form of a paper or a pressboard with an improved comprehensive property and improved key properties, such as the compressibility.

Summary

According to the present invention, there is provided an electrically insulating composite material which comprises fiber and fibrid, wherein the fibrid comprises polyacrylonitrile fibrid. According to one aspect of the present invention, the fibrid may further comprise polyethylene terephthalate fibrid, poly(metaphenylene isophthamide) fibrid, polyethylene naphthalate fibrid, polybutylene terephthalate fibrid or combinations thereof. According to another aspect of the present invention, wherein the fibrid in the electrically insulating composite material may have a specific surface area of 3 m 2 /g to 80 m 2 /g, preferably 10 m ² /g to 40 m ² /g.

According to one embodiment of the present invention, the fiber in the electrically insulating composite material comprises at least one of the following fibers: cellulose fiber, polyethylene terephthalate fiber, polyacrylonitrile fiber, poly(metaphenylene isophthamide) fiber, and glass fiber. According to another embodiment of the present invention, the fibers may have a length of 1 mm to 20 mm, preferably 2 mm to 10 mm, and more preferably 3 mm to 7 mm. According to one embodiment of the present invention, the fiber is present in the electrically insulating composite material in an amount of from 10 to 99 wt , preferably 40 wt to 95 wt , the fibrid is present in the electrically insulating composite material in an amount of from 1 wt to 90 wt , preferably 5 wt to 60 wt , based on the total weight of the electrically insulating composites material.

According to one embodiment of the present invention, the amount of the polyacrylonitrile fibrid is from 1 wt to 90 wt , preferably 5 wt to 60 wt , based on the total weight of the electrically insulating composite material. According to another embodiment of the present invention, the amount of the polyethylene terephthalate fibrid is from 0 wt to 20 wt , the amount of the poly(metaphenylene isophthamide) fibrid is from 0 wt to 10 wt , the amount of polyethylene naphthalate fibrid is from 0 wt to 60 wt and the amount of polybutylene terephthalate fibrid is from 0 wt to 20 wt , based on the total weight of the electrically insulating composite material.

According to one embodiment of the present invention, the amount of the cellulose fiber is from 0 wt to 99 wt , preferably 40 wt to 95 wt , the amount of the polyethylene terephthalate fiber is from 0 wt to 70 wt , the amount of polyacrylonitrile fiber is from 0 wt to 60 wt , the amount of poly (metaphenylene isophthamide) fiber is from 0 wt to 10 wt and the amount of glass fiber is from 0 wt to 20 wt , based on the total weight of the composite material. According to one embodiment of the present invention, the electrically insulating composite material may further comprise a nano filler and the said nano filler may be selected from nano silica, nano alumina or their mixture.

According to one embodiment of the present invention, the electrically insulating composite material is in the form of a paper or a pressboard. Another aspect of the present invention relates to an electrical device comprising the above electrically insulating composite material, such as an electrical transformer or an electrical motor. Yet another aspect of the present invention relates to the use of polyacrylonitrile fibrid for preparing an electrically insulating composite material. Such composite material in turn can be used to prepare electrical devices, such as a transformer or an electrical motor.

The inventors have found unexpectedly that, by introducing polyacrylonitrile fibrid into the fibrid, it is possible to provide a high voltage insulating material with improved comprehensive property. Specifically, the composite material displays excellent compressibility as compared with cellulose pressboard, the compressibility could be lowered to 3.5% and the reversible compressibility could be enhanced to 78%. Without being bound to any theory, it is believed that the decrease in compressibility is partly due to the intrinsic excellent compressibility of polyacrylonotrile, and partly due to the good interface of polyacrylonitrile fibrid with other materials, such as the cellulose fiber.

Brief Description of the Drawings

Embodiments will be described, by way of example, with reference to the accompanying drawings, in which: Fig 1 is a SEM image of the polyacrylonitrile fibrid which can be used in the electrically insulating composite material in accordance with the present invention.

Fig 2 is a SEM image of the polyethylene terephthalate fiber which can be used in the electrically insulating composite material in accordance with the present invention.

Fig 3 is a schematic flow chart of an embodiment of a method according to the present invention.

Detailed Description

Embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments are shown. However, other embodiments in many different forms are possible within the scope of the present disclosure. Rather, the following embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

According to some embodiments of the present invention, there is provided an electrically insulating composite material comprising fiber and fibrid, wherein the fibrid comprises polyacrylonitrile fibrid or fibrid mixtures of polyacrylonitrile fibrid with polyethylene terephthalate fibrid, poly(metaphenylene isophthamide) fibrid, polyethylene naphthalate fibrid and polybutylene terephthalate fibrid. Preferably, the fibrid used in the electrically insulating composite material may have a specific surface area of 3 m 2 /g to 80 m 2 /g. According to some embodiments of the present invention, the more preferred fibrid may have a specific surface area of 10 m 27g to 40 m 27g.

Figure 1 shows one polyacrylonitrile fibrid, which can be used in the electrically insulating composite material according to the present invention. From Figure 1 it can be seen that the polyacrylonitrile fibrid may be with an irregular shape.

According to some embodiments of the present invention, the fiber comprises at least one of the following fibers selected from cellulose fiber, polyethylene terephthalate fiber, polyacrylonitrile fiber, poly (metaphenylene isophthamide) fiber, glass fiber or any mixtures thereof. Preferably, the fibers used in the electrically insulating composite material may have a length of 1 mm to 20 mm, preferably 2 mm to 10 mm, and more preferably 3 to 7 mm.

Fig 2 shows one type of polyethylene terephthalate fiber, which can be used in the electrically insulating composite material according to the present invention. It can be seen that the said fiber have a regular diameter.

According to some embodiments of the present invention, the electrically insulation composite material essentially comprises fiber and fibrid. In particular, the fiber is present in the electrically insulation composite material in an amount of from 10 to 99 percent by weight, preferably 40 to 95 wt . The fibrid can be used in an amount of 1 wt to 90 wt , based on the total weight of the electrically insulation composite material. Preferably, the fibrid is used in an amount of 5 wt% to 60 wt%, based on the total weight of the electrically insulation composite material. According to some embodiments of the present invention, the amount of the polyacrylonitrile fibrid is from 1 wt% to 90 wt%, based on the total weight of the electrically insulation composite material. Preferably, the polyacrylonitrile fibrid is used in an amount of 5 wt% to 60 wt%, based on the total weight of the electrically insulation composite material. According to some embodiments of the present invention, the amount of the polyethylene terephthalate fibrid is from 0 wt% to 20 wt%, the amount of the poly(metaphenylene isophthamide) fibrid is from 0 wt% to 10 wt%, the amount of polyethylene naphthalate fibrid is from 0 wt% to 60 wt%, and the amount of polybutylene terephthalate fibrid is from 0 wt% to 20 wt%, based on the total weight of the electrically insulation composite material.

According to some embodiments of the present invention, the amount of the cellulose fibers is from 0 wt% to 99 wt%, preferably 40 wt% to 95 wt%, based on the total weight of the electrically insulation composite material. According to some embodiments of the present invention, the amount of the polyethylene terephthalate fiber is from 0 wt% to 70 wt%, the amount of polyacrylonitrile fiber is from 0 wt% to 60 wt%, the amount of poly (metaphenylene isophthamide) fiber is from 0 wt% to 10 wt% and the amount of glass fiber is from 0 wt% to 20 wt%, based on the total weight of the electrically insulation composite material. According to some embodiments of the present invention, the electrically insulating composite material may further comprise fillers. Preferably, the fillers are nano fillers. In particular to the current invention, the amount of the nano fillers is preferably from 0 wt% to 10 wt%, based on the total weight of the electrically insulation composite material. There is no limitation to the specific types of filler, provided that better dielectric strength and electrical creepage resistance can be obtained. Commonly used nano fillers in the field are nano silica, nano alumina and/or their mixture.

As can be seen from the below Example 1 in which the pressboard comprises 40% polyacrylonitrile fibrid and 60% cellulose fiber based on the total weight of the pressboard, by the combined use of polyacrylonitrile fibrid and cellulose fiber, the pressboard showed excellent compressibility, higher reversible amount of compressibility, and lower moisture content. The compressibility can be improved to 4%, which is much lower than the requirement of 10% in IEC standard. The reversible amount of compressibility is about 60%, which is much higher than 45% in IEC standard. Furthermore, the final pressboard had a moisture content of about 0.9 wt%, which is much lower than that of IEC requirements in IEC 60641-3-1. The lower moisture content can substantially shorten the pretreatment time of pressboard for transformer application, which results in the lower fabrication cost. With the improved compressibility and reversible amount of compressibility, less time is needed for insulation height adjustment during transformer fabrication and the total insulation thickness could also be reduced.

Particularly, the pressboard with the combination of 60 wt% polyacrylonitrile fibrid and 40 wt% polyethylene terephthalate fiber showed better result, as shown in Example 2.

Preferably, the pressboard according to the present invention has a thickness of higher than 0.9 mm. More preferably, the thickness of the pressboard is 1-12 mm, and most preferably 1-8 mm. A paper of the present invention has a thickness of less than 0.9 mm, preferably less than 0.8 mm, and more preferably is between 0.05 to 0.5 mm.

The composite material is electrically insulating and is suitable for use as insulation material in an electrical device. The composite material may be, for example, used as electrical insulation in an electrical device, such as in a power transformer, whereby the composite material may be a high voltage insulation material.

As mentioned above, the electrically insulating composite material may have especially beneficial electrically insulating properties in an oily environment. Thus, the electrically insulating composite material may be at least partly soaked in oil. The present invention further provides an electrical device comprising the electrically insulating composite material according to the present invention. The electrical device may be any electrical device which comprises electrical insulation, e.g. an electrical transformer or a conductor of electricity or an electrical motor, which may especially benefit from the composite material, such as with improved mechanical properties, less time is needed for insulation height adjustment during transformer fabrication and the total insulation thickness can be reduced. Especially, the electrical device according to the present invention is an electrical transformer.

The electrically insulating composite material may be in the form of a paper, spacer, barrier, strip or press ring for insulation in or of an electrical device, such as a conductor of electricity, an electrical transformer. The electrically insulating composite material has electrically insulating properties which may be useful in any electrical device, such as for insulating an electricity conduit, but the electrically insulating composite material may be especially advantageous in an oily environment, such as in an electrical transformer. Specifically, the electrically insulating composite material may be used for making electrically insulating spacers in a transformer winding.

An improved electrical device is obtained by using the electrically insulating composite material in accordance with the present invention. In particular, the electrically insulation composite material displays improved mechanical properties, such as an improved compressibility.

Fig 3 is a schematic flow chart of an embodiment of a method 1 according to the present invention, for producing polyacrylonitrile fibrid based electrical insulation material, e.g. for an electrical transformer. Fibrids are provided, see 2, and fibers are also provided, see 3. The fibrids and fibers are then mixed with each other, see 4. A paper press, multi-daylight hot press of the like, is then used for pressing the mixture to provide a pressboard or a presspaper or the like of the composite material discussed herein, see 5. The pressing also comprises heating, see 6, and drying the mixture, see 7, as well as pressing the mixture to the pressboard, see 8. The pressboard was then cooled, see 9. The cooled pressboard may then be cut into desired insulation parts, for example, for use in a transformer or any other electrical device. For instance, a spacer, barrier, strip or press ring for insulation of an electrical transformer, can be produced from the composite material from this invention. Examples

An electrically insulation composite material according to the present invention, which is polyacrylonitrile fibrid based paper or pressboard, was produced and tested for its properties under the IEC (International Electrotechnical Commission) standard 60641-2 and the properties were compared to the corresponding requirements according to IEC 60641-3-1. Below table is part of property requirements in IEC 60641-3-1.

Example 1

A pressboard was made according to the process in Figure 3. The solid materials used in the making of this pressboard were 40 weight percent of polyacrylonitrile fibrid (Shanghai Labon Technical Fiber Co., Ltd) and 60 weight percent of cellulose fiber. This pressboard had a basic weight of 1080 g/m 2 , a thickness of 1 mm and a density of 1.08 g/cm 3.

The final pressboard had a moisture content of about 0.9 wt , which is much lower than that of IEC requirements in IEC 60641-3-1. The tensile strength of the final pressboard is about 120MPa. The compressibility of the final pressboard is about 4% and the reversible amount of compressibility is about 60%. With the improved compressibility and reversible amount of compressibility, less time is needed for insulation height adjustment during transformer fabrication and the total insulation thickness could also be reduced. The electrical strength in mineral oil is about 48kV/mm.

Example 2 A pressboard was made according to the process in Figure 3. The solid materials used in the making of this pressboard were 60 weight percent of polyacrylonitrile fibrid and 40 weight percent of polyethylene terephthalate fiber (Woongjin Chemical Co., Ltd). This pressboard had a basic weight of 2420 g/m 2 , a thickness of 2 mm and a density of 1.21 g/cm 3. The final pressboard had a moisture content of about 0.5 wt%; the tensile strength of the final pressboard is about 100 MPa; the compressibility of the final pressboard is about 3.5% and the reversible amount of compressibility is about 78%. The electrical strength in mineral oil is about 38kV/mm.

Example 3 A pressboard was made according to the process in Figure 3. The solid materials used in the making of this pressboard were 10 weight percent of polyacrylonitrile fibrid and 90 weight percent of cellulose fiber. This pressboard had a basic weight of 3600 g/m , a thickness of 3 mm and a density of 1.20 g/cm ³ .

The final pressboard had a moisture content of about 1.4 wt%; the tensile strength of the final pressboard is about 130 MPa; the compressibility of the final pressboard is about 5.0% and the reversible amount of compressibility is about 60%. The electrical strength in mineral oil is about 40kV/mm.

Example 4

A pressboard was made according to the process in Figure 3. The solid materials used in the preparation of this pressboard were 20 weight percent of polyacrylonitrile fibrid, 70 weight percent of cellulose fiber and 10 weight percent of polyethylene terephthalate fiber. This pressboard had a basic weight of 2800 g/m , a thickness of 2.5 mm and a density of 1.12 g/cm ³ .

The final pressboard had a moisture content of about 1.1 wt%; the tensile strength of the final pressboard is about 120 MPa; the compressibility of the final pressboard is about 4.5% and the reversible amount of compressibility is about 60%. The electrical strength in mineral oil is about 37kV/mm.

Example 5 A pressboard was made according to the process in Figure 3. The solid materials used in the preparation of this pressboard were 50 weight percent of polyacrylonitrile fibrid, 10 weight percent of poly (metaphenylene isophthamide) fibrid (Yantai Tayho Advanced Materials Co., Ltd), 40 weight percent of polyethylene terephthalate fiber. This pressboard had a basic weight of 2800 g/m 2 , a thickness of 2.5 mm and a density of 1.12 g/cm 3. The final pressboard had a moisture content of about 1.4 wt%; the tensile strength of the final pressboard is about 120 MPa; the compressibility of the final pressboard is about 3.6% and the reversible amount of compressibility is about 77%. The electrical strength in mineral oil is about 35kV/mm. Example 6

A pressboard was made according to the process in Figure 3. The solid materials used in the preparation of this pressboard were 40 weight percent of polyacrylonitrile fibrid, 20 weight percent of polyethylene terephthalate fibrid cellulose fiber and 40 weight percent of polyacrylonitrile fiber (Shanghai Labon Technical Fiber Co., Ltd). This pressboard had a basic weight of 2300 g/m 2 , a thickness of 2.0 mm and a density of 1.15 g/cm 3.

The final pressboard had a moisture content of about 0.9 wt%; the tensile strength of the final pressboard is about 120 MPa; the compressibility of the final pressboard is about 4.0% and the reversible amount of compressibility is about 75%. The electrical strength in mineral oil is about 36kV/mm.

Examples 7

A paper was made according to the process in Figure 3. The solid materials used in the making of this pressboard were 50 weight percent of polyacrylonitrile fibrid and 50 weight percent of polyethylene terephthalate fiber. This paper had a basic weight of 36 g/m , a thickness of 0.05 mm and a density of 0.72 g/cm .

The final paper had a moisture content of about 0.6 wt%; the tensile strength of the final pressboard is about 80 MPa; The electrical strength in mineral oil is about 50kV/mm.

Example 8

A pressboard was made according to the process in Figure 3. The solid materials used in the preparation of this pressboard were 10 weight percent of polyacrylonitrile fibrid, 20 weight percent of polyethylene naphthalate fibrid, 70 weight percent of cellulose fiber. This pressboard had a basic weight of 2320 g/m , a thickness of 2 mm and a density of 1.16 g/cm ³ .

The final pressboard had a moisture content of about 0.8 wt%; the tensile strength of the final pressboard is about 120 MPa; the compressibility of the final pressboard is about 3.7% and the reversible amount of compressibility is about 75%. The electrical strength in mineral oil is about 35kV/mm.