Patent Description:
Traditionally, oil-filled transformers use mineral oil as the dielectric liquid for both insulating and cooling purposes and cellulose or aramid papers as solid insulation for the conductors, layers and main insulation. Cellulose based materials have low cost and the transformer industry has great experience on cellulose-oil insulation systems over long time. In comparison, aramid insulations are expensive, but they are more suitable for use at high temperatures.

Cellulose insulating materials, have some inherent disadvantages for transformer insulation purposes, e.g., comparatively low dielectric strength, affinity towards water, poor thermal degradation properties and high shrinkage. For transformer insulation design with cellulose materials, these poor characteristics have to be compensated by having a dielectric design criterion with low dielectric stress and/or through the use of excess cellulose insulating materials. Excess cellulose materials in a transformer increases the drying time of the insulation, increases the overall size of the transformer and has a negative influence on the dielectric integrity of the insulation system. Further, some of the cellulose properties like the thermal degradation characteristics and affinity for water also restrict the continuous operation of transformers at higher temperatures (e.g. under overloaded conditions).

In different parts of electrical transformers, insulating material is used to avoid flash-overs and such. This insulating material is typically cellulose based since such a paper or pressboard material is cheap and easy to handle while giving adequate insulation. Examples of insulators in an oil filled transformer are:.

To improve the electrical properties of the insulation material, attempts have been made to use inorganic nanoparticles as a filler material in the solid insulation material e.g. cellulose based ditto.

<CIT> discloses the use of nano-fillers in an aliphatic polyamide insulation material for insulating coils in an oil-filled transformer. The nano-fillers may be titanium dioxide (TiO2), silicon dioxide (SiO2) or aluminium oxide (Al<NUM>O3), or mixtures thereof.

<CIT> discloses a voltage transformer with a solid insulating material comprising polydicyclopentadiene. The insulating material may comprise a nano sized inorganic filler, e.g. siliceous materials, carbonaceous materials, metal hydrates, metal oxides, metal borides, metal nitrides, and mixtures of two or more of the foregoing.

<CIT> discloses a nano-composite insulating material comprising semiconducting nanoparticles that consist in particular of boron nitride nanotubes (BNNT) and are distributed in an electrically insulating insulator, such as cellulose fibres.

<CIT> discloses a battery pack insulator where an insulating element is bound to a tray of plastic or metal to form a box in which the battery may reside. The insulating element comprises a sheet or mat of (e.g. non-woven) inorganic fibres. The inorganic fibres may be mixed with up to <NUM>% of thermoplastic polymeric fibres which act as binder for the inorganic fibres. The sheet or mat of the insulating element has an average sheet thickness as low as about <NUM>.

<CIT> discloses an insulation layer for a distribution transformer. The insulation layer is made up of a cement mixed with inorganic particles, which is reinforced by <NUM>-<NUM>% of inorganic fibres and pressed to a layer of <NUM>-<NUM> mils thickness.

<CIT> discloses an insulation material of an epoxy resin filled with elastomer particles and inorganic particles, for a transformer. The inorganic particle has a very small thermal expansion coefficient, such as silica or alumina. It is said that an excessive addition of the inorganic particle considerably increases the viscosity of the resin to reduce a workability. Further, it may cause a void in the resin hardened material. The void in the resin hardened material is in danger of causing a decrease in insulation properties due to a concentration of an electric field, or a decrease in a mechanical strength.

<CIT> discloses an insulating sheet that can increase handleability and storage stability in an uncured state and, further, can increase heat dissipation characteristics and adhesiveness of the cured article after curing. The insulating sheet is used to make a thermal conductor with thermal conductivity of <NUM> W/m·K or greater adhere to an electrically conductive layer. The insulating sheet contains a polymer with weight average molecular weight of <NUM> or greater, a curable compound that has a molecular weight of <NUM> or less and has an epoxy group or oxetanyl group, a curing agent, and an inorganic filler. The insulating sheet has a content of the inorganic filler between <NUM> wt% and <NUM> vol%. The inorganic filler is preferably spherical or crushed alumina. The crushed filler has an aspect ratio of <NUM>-<NUM>. The insulating sheet is obtained by molding, e.g. by solvent casting or film extrusion, followed by defoaming.

Document "XP002743618 - Flame resistant electrical insulating sheet" discloses a film for use in a flyback transformera base film (e.g. of polyester) having inorganic particles deposited thereon. There is thus not disclosed at least <NUM> wt% inorganic particles bound by a binder.

However, the use of small amounts of the filler material gives only limited improvements to the electrical properties, while larger amounts markedly worsen the mechanical properties of the insulation material, e.g. making it brittle.

It is an objective of the present invention to provide an improved insulation material for use in electrical devices, having improved electrically insulating properties.

According to an aspect of the present invention, there is provided an electrically insulating inorganic sheet comprising at least <NUM>% by weight inorganic particles, and a binder which binds the particles together to form the sheet. The binder thus makes up less than <NUM>% by weight (wt%) of the inorganic sheet. The sheet is suitable for use as solid insulation in an electrical device, e.g. an oil-filled power transformer, and may e.g. form a layer in an insulating laminate.

According to another aspect of the present invention, there is provided an electrical device comprising an embodiment of the electrically insulating inorganic sheet of the present disclosure, as solid insulation in the electrical device.

According to another aspect of the present invention, there is provided a laminate comprising an electrically insulating inorganic layer laminated to another electrically insulating layer, wherein the electrically insulating inorganic layer comprises at least <NUM>% by weight inorganic particles, and a binder which binds the particles together.

According to another aspect of the present invention, there is provided a method of producing an electrically insulating inorganic sheet for use as solid insulation in an electrical device. The method comprises forming the sheet from a mixture of at least <NUM>% by weight inorganic particles, and a binder which binds the particles together.

The present invention proposes an insulation system e.g. for distribution transformers (DTR), comprising inorganic sheets. The inorganic sheet material may be used as a single material in the insulation system or can be used as a composite (laminate) together with other insulation materials, such which are commonly used in transformers or other electrical devices, e.g. cellulose based paper/pressboard or aramid paper/pressboard. The use of the inorganic sheets gives a higher dielectric strength of the solid insulation which in turn may enable reduced use of materials in the electrical device, thereby reducing its size, as well as reduce the risk of flash-overs.

Further, the drying time may also be reduced since the amount of conventional solid insulation material may be reduced. The inorganic sheet may also have a higher thermal conductivity.

The proposed material solution could also be implemented in other transformer types, e.g. other liquid-filled transformers or dry transformers. However, some of the benefits in these transformers could come from different areas (e.g. conductor insulation allowing higher operation temperatures) than in the case of DTRs.

It is to be noted that any feature of any of the aspects may be applied to any other aspect, wherever appropriate. Likewise, any advantage of any of the aspects may apply to any of the other aspects.

In accordance with the present invention, the electrically insulating inorganic sheet comprises at least <NUM> wt% inorganic particles, and consequently less than <NUM> wt% binder. It is because of the high amount of particles of an inorganic material that the sheet is herein referred to as an inorganic sheet. However, the binder does not have to be inorganic. Rather, the binder may be any suitable binder for binding the inorganic particles to each other, e.g. a curable binder such as epoxy or polyester, or a phenolic resin.

Further, the inorganic sheet may be porous, e.g. able to allow gas and/or liquid to impregnate it and/or to pass through it. This may be advantageous in e.g. a liquid-filled electrical device where the insulation liquid preferably impregnates the solid insulation material to reduce the risk of flash-overs due to air bubbles or pockets in the insulation material. However, in other embodiments, the inorganic sheet may be non-porous and/or impregnable (not allowing gas and/or liquid to impregnate it and/or to pass through it).

<FIG> schematically illustrates an embodiment of an electrical device <NUM>, here in the form of a power transformer which is at least partly liquid-filled, e.g. with a mineral oil or an ester liquid, (schematically illustrated by the wavy oil-air interface indicated in the figure). A transformer is used as an example, but the inorganic sheet of the present invention may be useful also in other electrical devices, e.g. motors or switches/circuit breakers, which are dry or liquid-filled. The transformer in <FIG> is a single-phase transformer, but the discussion is in applicable parts relevant for any type of transformer e.g. a three-phase transformer such as with a three or five legged core. It is noted that the figure is only schematic and provided to illustrate in particular some of the different kinds of solid insulators in which the inorganic sheet of the present invention may be included.

Two neighbouring windings <NUM> (a & b) are shown, each comprising a coil of an electrical conductor <NUM> (a & b) around a core <NUM> (a & b), e.g. a metal core. The cores 103a and 103b are connected and fixed to each other by means of top and bottom yokes <NUM>. This is thus one example set up of a transformer, but any other transformer set up can alternatively be used with the present invention, as is appreciated by a person skilled in the art.

The conductors <NUM> are insulated from each other and from other parts of the transformer <NUM> by means of the fluid which the transformer contains (e.g. an oil). However, also solid insulators are needed to structurally keep the conductors and other parts of the transformer immobile in their intended positions. Today, such solid phase insulators are typically made of cellulose based pressboard or Nomex™ impregnated by the insulating fluid. In contrast, according to the present invention, an inorganic sheet is used, possibly laminated with sheet(s) of a traditional insulation material e.g. based on cellulose or aramid. The insulators may e.g. be in the form of spacers <NUM> separating turns or discs of a winding <NUM> from each other, axial sticks <NUM> e.g. separating the conductor <NUM> winding <NUM> from its core <NUM> or from another winding <NUM>, winding tables <NUM> separating the windings from other parts of the transformer <NUM> e.g. forming a support or table on which the windings, cores, yokes etc. rest, as well as insulating coating of, or winding around, the conductor <NUM>. In <FIG>, only a few different example insulators are shown for clarity. For instance, a cylinder around a winding, between a winding and its core or between different windings (e.g. between high voltage and low voltage windings), made with the insulating inorganic sheet may be used in some embodiments. Such a cylinder may provide mechanical stability to windings when the conductor is e.g. wound over/onto the cylinder, and it may break the large oil gaps between two windings <NUM> (e.g. low voltage and high voltage winding), which improves the overall insulation strength of the gap between the two windings. In some embodiments, concentric cylinders around the core may be used to separate and insulate different conductor layers of a winding from each other.

The electrical transformer <NUM> may be configured for an operating temperature of at least <NUM>, at least <NUM> or at least <NUM>. As defined in IEC <NUM>-<NUM> (Table <NUM>), the transformer uses high temperature class materials and therefore can be designed with Hybrid insulation system or Semi Hybrid insulation system or mixed insulation system or conventional insulation system. Therefore the insulator in accordance with the present invention can e.g. be in a temperature class of solid insulating materials starting from <NUM> (Class A), to/from <NUM> (class B), to/from <NUM> (class F) or to/from <NUM> (class H).

As discussed above, the electrical transformer may be fluid-filled, for improved insulation and/or heat exchange. The fluid may e.g. be mineral oil, silicon oil, synthetic ester or natural ester, or a gas (in a dry transformer). For high temperature applications, it may be convenient to use an ester oil, e.g. a natural or synthetic ester oil. Preferably, the insulating material and the fluid should not affect each other's properties, and should not react with each other e.g. to dissolve the inorganic sheet.

<FIG> illustrates an embodiment of an inorganic sheet <NUM> in accordance with the present invention. The sheet <NUM> comprises inorganic particles <NUM>, here in the form of fibres but could e.g. be in the form of flakes and a binder <NUM>. For clarity, the figure may give the impression that the main constituent of the sheet <NUM> is the binder <NUM>. However, the binder is less than <NUM> wt% of the sheet <NUM>, typically less than <NUM> wt% or less than <NUM> or <NUM> wt%. The main component of the sheet <NUM> is thus the inorganic particles <NUM> which constitutes at least <NUM> wt% of the sheet <NUM>, e.g. at least <NUM> wt%, such as <NUM> or <NUM> wt%.

The inorganic particles may be of an electrically insulating material from commonly used minerals, e.g., TiO2 (Titanium dioxide), SiO2 (Silicon dioxide), Al<NUM>O3 (Aluminium oxide), BaTiO3 (Barium titanate), SrTiO3 (Strontium titanate), ZrO2 (Zirconium dioxide) and/or BN (Boron nitride). It may be convenient to use inorganic particles <NUM> in the form of fibres since fibres are readily produced and are easy to bind together with a small amount of binder <NUM>. Inorganic fibres may be prepared e.g. by an electrospinning technique. The fibres may in some embodiments have an average length of between <NUM> millimetre and <NUM> centimetre, but may in other embodiments on average be shorter than <NUM> millimetre e.g. nanofibers. The fibres may have an average diameter within the range of between <NUM> nanometre and <NUM> micrometres, typically between <NUM> and <NUM> mikrometres. The fibres may have an aspect ratio (diameter to length) within the range of <NUM>:<NUM> to <NUM>:<NUM>, e.g. <NUM>:<NUM> to <NUM>:<NUM>.

In other embodiments, flakes may be preferred, e.g. of a thickness between <NUM> and <NUM> micrometres, e.g. between <NUM> and <NUM> micrometres, and having an average area within the range of <NUM><NUM> to <NUM><NUM>.

The inorganic sheet <NUM> is thin since it may have better electrical properties and be more bendable. According to the invention, the inorganic sheet <NUM> has a thickness within the range of from <NUM> nanometres to <NUM> micrometres, e.g. from <NUM> to <NUM> micrometres such as from <NUM> to <NUM> micrometres.

In preparation for lamination of the inorganic sheet <NUM>, it may in some embodiments be provided with dots of a surface binder, for facilitating binding of the inorganic sheet to another electrically insulating sheet to form a laminate, or to bind it to a metal surface (e.g. a conductor). In some embodiments, the surface binder may be of the same material as the binder <NUM> binding the inorganic particles together, or any other suitable surface binder. A laminate comprising an electrically insulating inorganic layer <NUM> may be formed by laminating an electrically insulating inorganic sheet <NUM> to another electrically insulating sheet, or to itself in a roll, or by coating the other electrically insulating sheet with the binder <NUM> and adding the inorganic particles <NUM> thereto thus forming the electrically insulating inorganic sheet/layer <NUM> directly on the other electrically insulating sheet.

<FIG> illustrates an embodiment of a laminate <NUM> formed from a plurality of layers of inorganic sheet(s) <NUM>. The inorganic sheet <NUM> may be too thin to use as it is and may thus conveniently be used as layers in an insulator laminate <NUM>. The laminate <NUM> may e.g. be formed by a plurality of sheets <NUM> stacked on top of each other and bound together by a surface binder, e.g. a curable surface binder such as epoxy or the like. Alternatively, the laminate <NUM> may be formed by a sheet <NUM> being rolled up onto itself to form a laminate roll where a plurality of the layers are made up of the same original sheet <NUM>. Such a laminate roll may e.g. be used to insulate a conductor <NUM>. The laminate <NUM> may in some embodiments have a thickness within the range of <NUM> micrometres to <NUM> centimetre, depending on where the insulator is to be used. Typically, the laminate <NUM> may have a thickness within the range of <NUM> micrometres to <NUM> millimetres, typically <NUM> micrometres to <NUM> millimetres.

<FIG> illustrates an embodiment of a laminate <NUM> formed from at least one layer of the inorganic sheet <NUM> and at least one layer <NUM> of another insulating material, e.g. cellulose or aramid based (or based on another synthetic polymer), to form a composite. The inorganic sheet <NUM> material may in some embodiments be too brittle or otherwise unstable for use in a desired application, in which case the inorganic sheet <NUM> may be laminated to another insulating sheet <NUM>, e.g. an electrically insulating polymer sheet of e.g. cellulose or aramid, to form a laminate <NUM>. As in the laminate of <FIG>, The laminate <NUM> may e.g. be formed by a plurality of sheets <NUM> and <NUM> alternately stacked on top of each other and bound together by a surface binder, e.g. a curable surface binder such as epoxy or the like. Alternatively, the laminate <NUM> may be formed by a sheet <NUM>, laminated to a polymer sheet <NUM> or between two polymer sheets <NUM>, being rolled up onto itself to form a laminate roll. Such a laminate roll may e.g. be used to insulate a conductor <NUM>. It may be convenient that the two externally facing layers of the laminate <NUM> are of a polymer sheet <NUM>, whereby the inorganic sheet(s) <NUM> are protected from the ambient environment of the laminate by the polymer sheets <NUM>.

In some embodiments, the insulator laminate <NUM> is in the form of at least one of a spacer <NUM> between turns or discs of the winding <NUM>, an axial stick <NUM> outside or inside of the winding <NUM> e.g. between the winding and the core <NUM>, a cylinder around a winding, between a winding and its core or between windings, a winding table <NUM> positioned atop of or below the coil winding and a conductor insulation adhered to and surrounding the conductor <NUM> of the winding coil. The electrical conductor <NUM> can e.g. be an electrically conducting wire, thread or strip, which may in some embodiments suitably be insulated by being wound or coated with the inorganic sheet <NUM> or a laminate <NUM> comprising the inorganic sheet in accordance with the present invention. These are examples of insulators in a transformer where the inorganic sheet material of the present disclosure can be beneficially used.

By means of embodiments of the present invention, several advantages may be obtained, depending on the application in which the inorganic sheet <NUM> is used.

Use of less materials in a transformer <NUM> may have the following advantageous effects:.

Claim 1:
An electrically insulating inorganic sheet (<NUM>) comprising at least <NUM>% by weight inorganic particles (<NUM>) in the form of fibres or flakes, and a binder (<NUM>) which binds the particles together to form the sheet, for use as solid insulation in an electrical device (<NUM>), wherein the inorganic sheet (<NUM>) has a thickness within the range of from <NUM> nanometres to <NUM> micrometres.