Nanometer composite paper-covered wire for low-loss insulation energy-saving transformer

Abstract

The utility model relates to electrical insulation wire technical field, concretely relates to a kind of nanometer composite paper-covered wire for low-loss insulation energy-saving transformer.The paper-covered wire includes successively from inside to outside: conductor, inner shielding layer, functional gradient insulation layer and outer protective layer.The functional gradient insulation layer is made of nanometer composite insulation paper, and is divided into three layers structure of inner insulator sublayer, middle insulator sublayer and outer insulator sublayer, and porosity gradually improves from inside to outside, density gradually reduces, and three-dimensional mesh reinforcing rib is embedded in middle insulator sublayer.The above structural design can meet the requirement of inner layer resistance high electric field intensity and outer layer anti-environmental aging, and rough interface is introduced between layers to enhance binding force to suppress partial discharge;Its gradient porous insulation structure is beneficial to transformer oil infiltration and internal heat dissipation channel formation, reduces insulation dielectric loss and conductor temperature rise, thereby improves the insulation reliability and energy efficiency of transformer operation.

Description

Technical Field

[0001] This utility model relates to the field of electrical insulated conductor technology, specifically to a nano-composite paper-insulated wire for a low-loss, energy-saving transformer. Background Technology

[0002] The insulation type of transformer winding conductors directly affects the transformer's performance and reliability. Currently, common winding conductor insulation types include enameled wire and paper-insulated wire. Enameled wire is a winding wire made by coating the conductor surface with multiple layers of insulating enamel film. While the technology is mature, it suffers from high processing costs, limited insulation performance, and poor safety in use. In contrast, paper-insulated wire is made by wrapping insulating paper around bare conductors after annealing and softening, and it is widely used, especially in oil-immersed transformers. Oil-impregnated paper insulation, due to the excellent electrical insulation and heat resistance properties of the paper after impregnation with transformer oil, has become one of the main insulation methods in high-voltage, high-capacity transformers.

[0003] However, existing paper-insulated winding wire insulation structures still have many shortcomings. For example, traditional paper-insulated wires typically employ a homogeneous multi-layered paper insulation structure, with the inner and outer layers having essentially the same material and density, failing to meet the different functional requirements of the inner and outer layers. One existing improvement is to increase the number or thickness of the insulating paper layers. For instance, Chinese patent CN202976912U discloses a paper-insulated copper flat wire, in which the conductor is covered by a double-layered insulating paper consisting of a first insulating paper layer and a second insulating paper layer, with the second insulating paper layer covering the outside of the first insulating paper layer to enhance insulation performance. While this double-layer paper insulation improves insulation strength to some extent, it still presents the following problems: First, the inner layer insulation closest to the conductor still needs to withstand the highest electric field strength, while the outer layer insulation needs to withstand oil corrosion and environmental aging. Using the same materials and density for both layers makes it difficult to simultaneously meet the requirements for both the electrical performance of the inner layer and the environmental tolerance of the outer layer. Second, the paper layers typically have smooth surfaces with weak interlayer bonding. During long-term operation, temperature changes and electromagnetic vibrations may cause the paper layers to loosen or partially detach, forming air gaps and inducing partial discharge hazards. Third, the traditional paper insulation layer structure is dense and uniform, making it difficult for transformer oil to penetrate into each layer. Heat dissipation of the winding conductors mainly relies on the external oil cooling, resulting in a single heat conduction path. This limits the conductor's cooling efficiency, leading to higher winding temperature rise and losses, which is detrimental to the energy-saving operation of the transformer. Furthermore, existing paper-insulated wires also face the problem of paper layer deformation and even wear under mechanical stress when winding transformer coils, especially in large-capacity transformers where electromagnetic vibrations can accelerate the aging and damage of the insulation paper.

[0004] In summary, there is an urgent need for a new type of paper-insulated wire that can simultaneously meet the requirements of inner layer electric field strength and outer layer environmental durability in terms of structure, and has strong interlayer bonding and unobstructed heat dissipation channels, in order to solve the above-mentioned technical problems. Utility Model Content

[0005] The technical problem this invention aims to solve is to provide a paper-insulated conductor with an innovative structure that simultaneously improves its electrical insulation performance, mechanical stability, and heat dissipation capacity, thereby overcoming the shortcomings of existing technologies such as difficulty in achieving balanced performance of inner and outer layers, easy delamination between layers, and poor heat dissipation. To this end, this invention provides a low-loss, energy-saving, insulated nanocomposite paper-insulated wire for transformers, the technical solution of which is as follows: Overall layered structure: The nanocomposite paper-insulated wire comprises, from the inside out, a conductor 1, an inner shielding layer 2, a functionally graded insulation layer 3, and an outer protective layer 4. The conductor 1 is a conventionally shaped copper conductor and serves as the base of the paper-insulated wire. The outer surface of the conductor 1 is tightly covered with an inner shielding layer 2, which is a strip-shaped layer made of semi-conductive material. Outside the inner shielding layer 2, a functionally graded insulation layer 3 is wrapped, composed of multiple layers of insulating paper with properties varying in a gradient along the thickness direction. Finally, an outer protective layer 4 is wrapped around the outermost layer of the functionally graded insulation layer 3, providing overall protection.

[0006] Functionally graded insulating layer 3: The functionally graded insulating layer 3 is the core part of this utility model, and is composed of three layers of insulating paper stacked sequentially: inner insulator layer 3a, middle insulator layer 3b, and outer insulator layer 3c. Each layer of insulating paper differs in material formulation and manufacturing process, resulting in a radially gradient distribution of pore structure within its unit volume. The inner insulator layer 3a is directly wrapped around the inner shielding layer 2. It has the fewest unit volume pores and can be obtained through high-pressure compaction during the papermaking process. Its main function is to withstand the high electric field stress around the conductor 1 and provide the strongest initial insulation strength. The middle insulator layer 3b is wrapped around the outer side of the inner layer and has a three-dimensional mesh reinforcing rib 3d embedded inside. This reinforcing rib is formed by pre-weaving high-strength insulating fiber material into a mesh and then composited with pulp. This gives the middle layer extremely high mechanical strength and resistance to deformation, which is used to resist the tensile and bending stress during coil winding and to prevent interlayer friction and insulation wear caused by vibration during operation. The outer insulator layer 3c is wrapped around the outer side of the middle layer and has the most unit volume pores. It has a loose and porous structure. During the papermaking process, the fiber ratio and pressing process can be controlled to form larger gaps, which facilitates the full penetration and flow of transformer oil, forming an excellent oil-paper insulation composite system. At the same time, this porous structure can provide buffering when the conductor is subjected to impact or thermal expansion and contraction, improving the overall insulation's impact resistance and tear resistance.

[0007] Interlayer rough interface design: To improve the adhesion between insulating layers, this invention designs the interfaces of each layer's contact surface as non-smooth contact surfaces with a certain degree of roughness. This rough interface can be achieved by introducing micro-protruding fibers, granular coatings, or maintaining a rough fiber surface on the insulating paper layer surface without calendering. The rough interface structure increases the actual contact area and mechanical interlocking force between layers, significantly improving the bonding strength between layers and effectively preventing the separation of insulating paper layers due to thermal expansion and contraction or mechanical vibration. In addition, the non-uniform interface makes the surface discharge path more tortuous and longer, greatly improving the surface electrical resistance of the insulation and suppressing the occurrence of partial discharge.

[0008] Interlaced winding structure: In another embodiment, the functionally graded insulating layer 3 is not fabricated layer by layer, but rather multiple insulating paper tapes with different properties are spirally wrapped around the inner shielding layer 2 at a certain angle to form a multi-layer insulating covering structure. Specifically, three types of insulating paper tapes corresponding to the functions of the inner insulator layer 3a, middle insulator layer 3b, and outer insulator layer 3c can be selected and sequentially overlapped and wound from the inside out, with each type of paper tape overlapping each other and alternating in the axial direction. Through this interlaced spiral wrapping, the functional layers are evenly distributed in the axial direction of the paper wrapping wire, without obvious interface delamination, thereby further enhancing the compactness and mechanical stability of the overall structure and ensuring that the interlayer bonding of the paper tapes is firm and not easily loosened.

[0009] Outer protective layer 4: The outermost layer of the paper-insulated wire is a very thin outer protective layer 4, which is composed of dense insulating paper. Its surface is calendered to form a smooth and hard shell structure. This outer protective layer 4 mainly gives the paper-insulated wire excellent environmental protection performance: it has good oil resistance, moisture resistance and scratch resistance, thereby ensuring that the internal gradient insulation structure is not damaged by external factors during subsequent manufacturing, transportation and operation.

[0010] Compared with the prior art, the present invention has the following beneficial effects: (1) This utility model introduces a layered structure with gradually decreasing density from the inside to the outside in the functionally graded insulation layer, so that the inner insulation layer has high density to withstand strong electric field and the outer insulation layer has high porosity to resist oil immersion and environmental aging, thereby simultaneously meeting the requirements of the electrical strength of the inner layer and the environmental resistance of the outer layer, solving the problem that traditional paper-insulated wire cannot meet the different needs of the inner and outer layers.

[0011] (2) The rough interlayer interface design of this utility model greatly improves the adhesion and mechanical interlocking between the insulating paper layers, eliminating interlayer air gaps and loosening. Even under long-term thermal cycling and electromagnetic vibration, the insulating layers can be tightly bonded, effectively preventing partial discharge problems caused by interlayer cracking and improving the overall reliability of the insulation system.

[0012] (3) The gradient porous insulation structure of this utility model provides multiple paths for heat conduction and oil cooling of the conductor. The inner dense paper directly conducts heat, while the outer porous paper allows transformer oil to penetrate into the interior and carry away heat, forming a composite heat dissipation mechanism of solid conduction + oil circulation. Compared with the traditional dense paper-insulated wire which only conducts heat through a single path, the structure provided in this paper significantly improves the cooling effect of the winding, reduces the operating temperature rise of the conductor, thereby reducing winding line loss and insulation dielectric loss, improving the operating efficiency of the transformer, and having a significant energy-saving effect.

[0013] (4) The fiber mesh reinforcing ribs embedded in the middle insulator layer of this utility model greatly enhance the tensile strength and rigidity of the paper layer, enabling the paper-wrapped wire to withstand repeated bending and stretching without cracking or deformation during the winding process. At the same time, when the transformer is subjected to short-circuit electrodynamic forces and AC vibrations during operation, the reinforced insulation structure can effectively avoid interlayer relative wear and fatigue damage, ensuring the structural stability of long-term operation.

[0014] (5) The calendered outer protective layer of this invention not only improves the paper-insulated wire's ability to resist oil immersion and humidity, but also provides an additional mechanical protection layer. During coil assembly and subsequent operation and maintenance, the outer protective layer can prevent scratches and impacts from damaging the internal insulation and reduce the accelerating effect of external forces on insulation aging. Overall, the addition of the outer protective layer further extends the service life of the paper-insulated wire and improves operational reliability. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the layered structure of the low-loss insulating nanocomposite paper-sheathed wire of this utility model; Figure 2 This is a schematic diagram of the functionally graded insulating layer in Embodiment 2 of this utility model, which is formed by multiple insulating paper tapes spirally wrapped together to form a layered structure.

[0016] The diagram is labeled as follows: 1-Conductor, 2-Inner shielding layer, 3-Functionally graded insulation layer (3a-Inner insulator layer, 3b-Middle insulator layer, 3c-Outer insulator layer, 3d-Three-dimensional mesh reinforcing rib), 4-Outer protective layer. Detailed Implementation

[0017] The structure and usage of this utility model will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, unless otherwise specified, the features of the following embodiments can be combined with each other to obtain new implementation schemes.

[0018] Example 1: Basic Structure and Manufacturing See Figure 1As shown, the nanocomposite paper-insulated wire provided in this embodiment comprises, from the inside out, a conductor 1, an inner shielding layer 2, a functionally graded insulation layer 3, and an outer protective layer 4. The conductor 1 is the substrate of this product, preferably made of a conductive metal material with high conductivity. Its cross-sectional shape can be circular, rectangular flat, or foil-like, depending on the design requirements of the transformer winding. After softening and annealing treatment, the surface of the conductor 1 is smooth and burr-free to reduce electric field distortion and prevent puncture of the insulation protection. A rough surface structure (3.0-5.0 μm) is used at the interlayer contact surfaces between the conductor 1, inner shielding layer 2, functionally graded insulation layer 3, and outer protective layer 4 to increase the interlayer contact area and friction, thereby improving interlayer bonding strength, effectively preventing delamination between insulation layers, and extending the creepage path for surface discharge.

[0019] The outer surface of conductor 1 is tightly covered with an inner shielding layer 2. The inner shielding layer 2 is a thin layer of semi-conductive material wrapped around the conductor, the thickness of which can be appropriately selected according to the conductor diameter, for example, 0.1–0.2 mm. The material of the inner shielding layer 2 has a moderate volume resistivity; for example, it can be a semi-conductive paper tape or polyester film doped with conductive particles. Preferably, the conductive particles are specially treated carbon black, carbon nanotubes, or metal oxide nanoparticles, which are uniformly dispersed in the substrate, giving the inner shielding layer 2 a certain degree of conductivity rather than being a complete conductor; a typical resistivity range is, for example, 10 Ω·cm. 3 ~10 6 Ω •cm. This semiconductive shielding layer design effectively homogenizes the potential distribution on the surface of conductor 1: when high voltage is applied to conductor 1, the inner shielding layer 2 homogenizes the electric field on the surface of conductor 1, eliminating potential electric field distortion and tip effects, and preventing excessively strong local electric fields from breaking down adjacent insulation. Furthermore, the inner shielding layer 2 also mitigates the rate of change of the potential gradient between conductor 1 and the insulating paper, effectively introducing a gradient transition between conductor 1 and the main insulation, which helps to increase the partial discharge initiation voltage of the entire insulation system.

[0020] Attached immediately to the inner shielding layer 2 is the functionally graded insulating layer 3. The functionally graded insulating layer 3 is composed of three layers of insulating paper with different properties stacked sequentially: the inner insulator layer 3a is closest to conductor 1, the middle insulator layer 3b is in the middle, and the outermost insulator layer 3c is on the outermost side. The material structure and function of these three layers are described below: The inner insulator layer 3a directly coats the outside of the semi-conductive inner shielding layer 2 and is made of highly densified insulating paper material. Specifically, the inner insulator layer 3a undergoes high-pressure pressing and calendering processes during papermaking, resulting in a tightly packed paper fiber structure and a porosity of less than 5%, thereby improving its dielectric constant and dielectric strength. This layer material can be made from high-quality electrical insulating pulp, such as 100% sulfate wood pulp or purified cellulose fibers, and may incorporate a small amount of nano-scale high-dielectric-constant fillers, such as nano-silica (SiO2) or alumina (Al2O3), uniformly dispersed in the pulp to further enhance the paper's insulation properties and heat resistance. Through these processes, the resulting inner insulator layer 3a possesses extremely high initial insulation strength, providing a solid first line of insulation even when high-frequency and impulse voltages are applied to conductor 1.

[0021] The middle insulator layer 3b closely covers the outer side of the inner insulator layer 3a and serves as the intermediate reinforcing layer of the functionally graded insulation layer 3. The unique feature of the middle insulator layer 3b lies in its embedded three-dimensional mesh reinforcing ribs 3d. These three-dimensional mesh reinforcing ribs 3d are fiber mesh structures pre-fabricated before papermaking, preferably using high-strength, heat-resistant insulating fiber materials such as aramid fiber paper, Nomex fiber mesh, or glass fiber cloth, woven or non-woven into a porous mesh sheet. This mesh sheet has a three-dimensional interconnected pore structure, and its thickness is approximately proportional to the middle of the total thickness of the middle insulator layer 3b. During the manufacturing of the middle insulator layer 3b, the pre-fabricated fiber mesh ribs are placed in a pulp fiber suspension and paper is formed together, allowing the pulp fibers to interweave and deposit through the mesh. After drying and pressing, the fiber mesh is firmly embedded within the paper layer. With the introduction of the three-dimensional mesh reinforcing ribs 3d, the mechanical properties of the middle insulator layer 3b are significantly improved: its tensile strength, tear strength, and dimensional stability are significantly higher than those of unreinforced paperboard. In practical applications, transformer winding fabrication requires repeatedly bending and winding paper-insulated wire onto the core or frame. During this process, the conductors and insulation layers must withstand tensile and bending stresses. Adding a fiber-reinforced middle layer of insulating paper prevents the paper layer from cracking or deforming during winding, thus preventing it from maintaining its shape. Furthermore, after the transformer is put into operation, the windings may experience micro-vibrations and impacts due to electromagnetic forces. Under long-term mechanical stress, traditional pure paper insulation layers may wear or fatigue. However, the middle insulator layer 3b of this invention, with its reinforced structure, is robust and resistant to deformation, effectively resisting insulation damage caused by vibration and friction, maintaining interlayer strength, and thus extending insulation life.

[0022] The outer insulator layer 3c covers the middle insulator layer 3b, forming the outermost insulation layer of the functionally graded insulation layer 3. The outer insulator layer 3c adopts a design approach opposite to the inner layer, namely, maximizing its porosity to form a loose, porous structure. During its manufacturing process, compaction steps are deliberately reduced, or a certain proportion of volatile particles are introduced to form pores in the paper, ensuring a high pore volume fraction in the finished paper. Although the absolute dielectric strength of the resulting outer insulator layer 3c is slightly lower than that of the highly dense inner layer, it exhibits excellent overall performance under oil immersion conditions. First, the higher porosity allows transformer oil to quickly and fully penetrate into the pores of the outer paper, immersing the entire functionally graded insulation layer 3 in oil and forming a stable oil-paper hybrid insulation structure. The dielectric constant of the oil-filled paper layer is increased, and the electric field distribution is more uniform. Even if small air gaps exist locally, the oil will fill them to suppress partial discharge. Second, the porous structure of the outer paper provides a certain degree of compressibility and buffering effect.

[0023] When the winding is subjected to short-term mechanical impact, the fibrous pores of the outer layer can deform appropriately to absorb energy, protecting the more brittle inner insulation layer from damage. Similarly, during thermal expansion and contraction, the loose structure can accommodate minute dimensional changes in the inner layers, preventing tearing due to stress concentration. Finally, due to thorough oil immersion, the thermal conductivity and heat dissipation performance of the outer insulator layer 3c are also improved, which helps to dissipate heat from the middle and inner layers to the external oil flow, reducing the temperature gradient between insulation layers. This is particularly important for the windings of large transformers, as it can slow down the aging of hot spots and improve operational reliability.

[0024] The three layers of insulating paper material described above together constitute the functionally graded insulating layer 3. Each layer utilizes improved electrical insulating paper and is further modified using nanocomposite technology to optimize the paper layer's performance. For example, adding inorganic particles such as nano-silica and nano-zinc oxide to the pulp not only improves the paper layer's heat resistance but also reduces dielectric loss and hygroscopicity, microscopically enhancing the insulating paper's resistance to electrical, thermal, and moisture stresses. It should be noted that the thickness ratio and material formulation of each insulator layer can be adjusted and optimized according to the voltage level and operating environment of the target transformer to minimize the overall diameter and loss of the paper-insulated wire while meeting insulation requirements.

[0025] Continue to refer to Figure 1The outermost layer of the functionally graded insulation layer 3 is covered by an outer protective layer 4. The outer protective layer 4 is essentially the "sheath" of the paper-insulated wire, and in this embodiment, it is formed by wrapping a dense insulating paper tape with a thickness of 0.05–0.1 mm. The selected paper tape material can be calendered electrical insulating paper or similar film composite paper, with a smooth surface (roughness less than 1 μm) and a dense interior without large pores. This paper tape is tightly wound around the outer insulator layer 3c, with multiple turns to ensure no exposed gaps, forming a uniform protective layer. The outer protective layer 4 has three main functions: First, it provides protection against oil immersion. Although the outer insulator layer 3c itself is designed to be oil-resistant, prolonged immersion in hot oil will still cause aging. The outer surface is in direct contact with the oil, thus slowing down the oil aging rate of the inner main insulating paper; second, it provides humidity isolation. During storage, transportation, or major overhaul and maintenance, transformers may be exposed to air, and ambient moisture will gradually penetrate the insulating paper. The dense structure of the outer protective layer 4 can, to a certain extent, prevent moisture from entering, keeping the internal insulation dry and preventing the paper insulation from becoming damp and breaking down. Thirdly, it provides mechanical protection. During winding processing, handling, and installation, the paper-insulated wire may rub against tools or other objects. The smooth and hard outer protective layer 4 can resist scratches and abrasions and absorb some impact, preventing stress from directly acting on the relatively soft inner outer insulator layer 3c, thus avoiding damage to the internal insulation structure. This protection is particularly important in large-capacity transformer windings because on-site assembly may require operations such as bushing, parallel winding, and lead welding. Without a robust outer protective layer, the internal paper layer can easily be torn during these operations, creating potential hazards. With the presence of the outer protective layer 4, even minor surface damage will not affect the protection of the internal main insulation.

[0026] Example 2: Interlaced paper tape spiral wrapping structure This embodiment provides another structural form of the present invention, which has the same function and purpose as Embodiment 1, but differs in its implementation. See also Figure 2As shown, in this embodiment, the functionally graded insulation layer 3 is formed by interlacing and spirally winding three types of insulating paper tapes with different properties, rather than by layer-by-layer forming. Specifically, three types of insulating paper tapes are prepared: the first is a high-density paper tape, corresponding to the function of the inner insulator layer 3a in Embodiment 1; the second is a reinforcing paper tape containing fiber mesh, corresponding to the function of the middle insulator layer 3b; and the third is a porous paper tape, corresponding to the function of the outer insulator layer 3c. The widths of the three paper tapes can be the same or different, generally several times the diameter of conductor 1, so that there is a certain degree of overlap during spiral winding. During wrapping, the first type of high-density paper tape is first wound around the inner shielding layer 2 at a spiral angle of 45°, leaving a small gap between adjacent turns; then the second type of reinforcing paper tape and the third type of porous paper tape are wound around it in sequence, and the various paper tapes can overlap each other to cover part of the previous layer. This multi-layer spiral wrapping is performed alternately until the required total thickness of the insulation layer is achieved. To ensure tight bonding of multiple paper tapes, a certain tension can be applied to the winding wire during the wrapping process, and the wrapping angle and position of the paper tapes can be appropriately controlled to ensure uniform distribution. After wrapping, the entire insulation layer can be vacuum dried and shaped to form a certain hydrogen bond between the paper tape layers or to fix them with a small amount of transformer oil as a bonding medium. In practical applications, the winding will eventually be impregnated with oil in a vacuum. Through the above-mentioned staggered wrapping process, the resulting functionally graded insulation layer 3 achieves the interweaving and fusion of various functional paper layers in the axial direction, without obvious interlayer interfaces, thus avoiding air gaps and stress concentration problems that may be caused by interface delamination. At the same time, the winding of multiple paper tapes makes the entire insulation structure spirally stacked, which is more adaptable to the slight deformation of the winding under thermal shock than a single integral paper tube structure, without cracking. In addition, the periodic alternation of different performance paper tapes in the axial direction during the wrapping process helps to make the distribution of functional materials more uniform on any cross-section of the winding wire, which is beneficial to the performance consistency of long-length windings. The three paper tape materials and functions used in this embodiment correspond to layers 3a, 3b, and 3c in Embodiment 1, respectively, thus achieving similar electrical and mechanical performance improvements as in Embodiment 1. The two implementation schemes can be selected based on manufacturing conditions and application requirements: when traditional paper-wrapped wire manufacturing processes are preferred, the integrally formed gradient paper layer structure of Embodiment 1 can be used; when further improving interlayer integration and wrapping flexibility is desired, the staggered spiral wrapping structure of this embodiment can be used.

[0027] Example 3: Application in transformer windings This embodiment describes the specific application effect and usage method of the paper-insulated wire of this utility model in transformer windings. When manufacturing high-voltage transformer windings, the nano-composite paper-insulated wire obtained in Embodiment 1 or Embodiment 2 above can be used as the winding conductor. First, calculate the required conductor length and number of turns according to the transformer design requirements, and then wind the paper-insulated wire of this utility model onto the coil frame. Because the paper-insulated wire of this utility model has high flexibility and mechanical strength, it can withstand the necessary tension and bending during the winding process without breaking, and the interlayer insulation paper will not shift or wrinkle due to excessive tension, thereby ensuring the dimensional accuracy and compactness of the winding coil. After completing the winding and corresponding lead connection, the winding and paper-insulated wire are vacuum dried and then placed in a vacuum oil immersion tank to immerse in transformer oil. Because the outer layer (3c layer) of the functionally graded insulation layer 3 of this utility model has a porous structure, during the oil immersion process, the transformer oil can quickly penetrate and fill the pores of the paper layer, so that the entire insulation layer is filled with insulating oil without residual air bubbles. This is crucial for the insulation reliability of large high-voltage windings. Traditional dense paper-insulated wires often suffer from air bubbles and moisture that are difficult to completely remove during oil impregnation due to the unavoidable micropores and moisture within the paper. This invention, however, utilizes a gradient pore design to ensure more thorough and complete oil impregnation. After impregnation, the insulation layer of the paper-insulated wire fuses with the oil, forming a stable "oil-paper insulation system," significantly improving insulation withstand voltage and durability.

[0028] After the transformer is put into operation, the advantages of this paper-insulated wire will be further demonstrated: In terms of electrical properties, when conductor 1 is subjected to high voltage, due to the presence of the inner shielding layer 2, the electric field distribution around it is very smooth, without high-gradient electric field concentration, thereby increasing the partial discharge initiation voltage. Even under higher operating voltages or impulse overvoltages, the inner shielding layer 2 can effectively alleviate electric field stress concentration and protect the main insulation from transient voltage breakdown. In addition, the nanocomposite modified insulating paper reduces dielectric loss, generates less heat under power frequency electric fields, and reduces insulation temperature rise, resulting in a decrease in both no-load loss and dielectric loss, which is particularly beneficial for energy-saving transformers. In terms of thermal performance, the temperature rise of this paper-insulated wire during current-carrying operation is lower than that of traditional paper-insulated wire under the same conditions. On the one hand, the density gradient structure allows heat to be conducted to the oil more quickly; on the other hand, the oil immersion in the porous layer forms an "internal cooling channel," enhancing heat convection exchange. This multi-path heat dissipation mechanism ensures a more uniform temperature distribution in each layer of the winding, without hot spots, which helps to delay insulation aging. Under the same current-carrying conditions, the paper-insulated wire of this invention can reduce the conductor temperature rise by more than 5°C. For large-capacity transformers, this can correspondingly improve their overload capacity or reduce the energy consumption of cooling devices. Finally, mechanically, this paper-insulated wire winding can better resist electrodynamic impacts when encountering a power grid short circuit. Short-circuit impacts in conventional transformers may cause winding deformation or even inter-turn short circuits, while this paper-insulated wire, due to the fiber reinforcement in the middle layer, is less prone to crushing of the insulation layer. Furthermore, the interlayer bonding is strong, with no "slippage" phenomenon, resulting in stronger winding geometric stability. According to actual simulation experiments, the geometric dimensions of this paper-insulated wire winding remain almost unchanged after being subjected to a short-circuit impact, far superior to traditional paper-insulated wire windings. Therefore, the application of this invention in transformers significantly improves the winding's fault resistance and long-term reliability.

[0029] It should be noted that the embodiments of this utility model have been described in detail above, but these descriptions are not intended to limit the scope of protection of this utility model. Those skilled in the art can make various modifications and improvements without departing from the principle of this utility model, such as adding additional shielding or protective layers, etc., and these should all be considered equivalent modifications and fall within the scope of protection of this utility model.

Claims

1. A nano-composite paper-insulated wire for a low-loss, energy-saving transformer, characterized in that, It includes a conductor (1), an inner shielding layer (2) covering the outer surface of the conductor (1), a functionally graded insulating layer (3) covering the outer side of the inner shielding layer (2), and an outer protective layer (4) covering the outermost side of the functionally graded insulating layer (3). The functionally graded insulating layer (3) is composed of an inner insulator layer (3a), a middle insulator layer (3b), and an outer insulator layer (3c) stacked sequentially. The porosity structure per unit volume in each layer increases layer by layer, and the middle insulator layer (3b) is embedded with a three-dimensional mesh reinforcing rib (3d).

2. The nanocomposite paper-insulated wire for a low-loss, energy-saving transformer as described in claim 1, characterized in that, The conductor (1) has a cross-sectional shape that is circular, rectangular, or foil-like.

3. The nanocomposite paper-insulated wire for a low-loss, energy-saving transformer as described in claim 1, characterized in that, The interlayer contact surfaces between the conductor (1), the inner shielding layer (2), the functionally graded insulating layer (3), and the outer protective layer (4) adopt a rough surface structure with a roughness of 3-5 μm, while the non-contact surfaces of the outer protective layer (4) adopt a smooth surface structure with a roughness of less than 1 μm.

4. The nanocomposite paper-insulated wire for a low-loss, energy-saving transformer as described in claim 1, characterized in that, The inner insulator layer (3a), the middle insulator layer (3b) and the outer insulator layer (3c) are sequentially and alternately wound around the surface of the inner shielding layer (2) in an oblique spiral manner.

5. The nanocomposite paper-insulated wire for a low-loss, energy-saving transformer as described in claim 4, characterized in that, When the inner insulator layer (3a), the middle insulator layer (3b) and the outer insulator layer (3c) are wound separately, gaps are left between the winding turns, and the next layer is wrapped alternately over the gaps.

6. The nanocomposite paper-insulated wire for a low-loss, energy-saving transformer as described in claim 4, characterized in that, The oblique spiral configuration has an oblique angle of 45° with respect to the conductor (1) as its axis.

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