An epoxy resin cast dry-type transformer based on venturi effect and porous matrix heat dissipation

Abstract

The utility model discloses an epoxy resin pouring dry type transformer based on venturi effect and porous matrix heat dissipation relates to dry type transformer technical field, including winding and the iron core of being located winding center, is equipped with the clamp piece at the both ends of iron core, and the clamp piece is connected with the mounting base, and winding includes high voltage winding and the low voltage winding of being located high voltage winding inboard, and high voltage winding and / or low voltage winding inside embeds the unit heat dissipation channel of porous matrix formula, and unit heat dissipation channel is the independent unit air duct component group of integrally formed of several, and the flow channel of unit air duct component is the gradient variable cross section structure of following venturi pipe principle, and the flow channel of unit air duct component is by inlet section, throat and outlet section smooth connection and is composed, and the cross section area of inlet section is greater than the cross section area of throat. The utility model can under the premise of guaranteeing even enhancing winding mechanical strength, improve its inside heat dissipation efficiency greatly, effectively restrain hot spot temperature rise.

Description

Technical Field

[0001] This utility model belongs to the field of dry-type transformer technology, specifically relating to an epoxy resin cast dry-type transformer based on the Venturi effect and porous matrix heat dissipation. Background Technology

[0002] Epoxy resin cast dry-type transformers are widely used in power supply locations with extremely high safety and reliability requirements, such as urban power grids, hospitals, commercial centers, and high-rise buildings, due to their excellent fire resistance, explosion resistance, environmental friendliness, and ease of maintenance. Their basic structure mainly includes an iron core, high-voltage windings, low-voltage windings, and clamping components, with a ring-shaped main heat dissipation channel formed between the high-voltage and low-voltage windings.

[0003] Currently, heat dissipation in this type of transformer mainly relies on the main cooling air channel between the high-voltage and low-voltage windings, using natural convection or forced air cooling to remove the heat generated during operation. However, with the continuous increase in transformer capacity and power density, the temperature rise during operation, especially the temperature rise of hot spots inside the windings, has become a key factor restricting its performance and lifespan. Since the low-voltage winding is usually located on the inside, heat is not easily dissipated, making its heat dissipation problem particularly prominent. Existing main cooling air channel methods have low airflow organization efficiency and limited heat dissipation area, making it difficult to effectively reduce the overall temperature inside the windings.

[0004] To address the aforementioned issues, existing technologies have introduced designs that incorporate a second air duct within the winding, such as honeycomb or parallel straight-through channels. While these designs increase the heat dissipation area to some extent, they still have significant shortcomings: First, simple straight air ducts cannot effectively guide and optimize the internal airflow, resulting in predominantly laminar airflow and low heat exchange efficiency. Second, while increasing the heat dissipation area, straight air duct designs often sacrifice the mechanical strength of the winding, affecting its short-circuit withstand capability and overall reliability. Therefore, there is an urgent need for an innovative heat dissipation structure design that can significantly improve the internal heat dissipation efficiency while ensuring or even enhancing the mechanical strength of the winding, effectively suppressing hot spot temperature rise, and thus meeting the high-capacity, high-reliability, and long-life operation requirements of modern power grids for dry-type transformers. Utility Model Content

[0005] The purpose of this invention is to provide an epoxy resin cast dry-type transformer based on the Venturi effect and porous matrix heat dissipation, thereby overcoming the defects in the prior art. By designing the internal air duct structure of the winding, the heat dissipation capacity of the winding is significantly improved, and the temperature rise is effectively reduced. Secondly, while achieving efficient heat dissipation, the mechanical strength and short-circuit withstand capability of the winding are guaranteed and even enhanced.

[0006] The specific technical solution is as follows: An epoxy resin cast dry-type transformer based on the Venturi effect and porous matrix heat dissipation includes windings and an iron core located at the center of the windings. Clamps are provided at both ends of the iron core, and the clamps are connected to mounting bases. The windings include a high-voltage winding and a low-voltage winding located inside the high-voltage winding. Porous matrix-style unit heat dissipation channels are embedded inside the high-voltage winding and / or the low-voltage winding. Each unit heat dissipation channel is composed of several integrally formed independent unit air duct components. The flow channel of each unit air duct component has a gradient variable cross-section structure following the Venturi tube principle. The flow channel of each unit air duct component is smoothly connected by an inlet section, a throat, and an outlet section. The cross-sectional area of ​​the inlet section is larger than that of the throat, forming a gradient contraction variable cross-section structure to generate a Venturi acceleration effect on the flowing cooling medium.

[0007] Preferably, the high-voltage winding and the low-voltage winding are spaced apart to form an annular main heat dissipation channel, and multiple silicone rods are arranged circumferentially within the main heat dissipation channel to support and buffer the high-voltage and low-voltage windings.

[0008] Preferably, the unit air duct component is made of a high thermal conductivity composite material, which is based on epoxy resin and filled with thermally conductive fillers such as boron nitride, alumina or silicon carbide with a high volume fraction. Its thermal conductivity is higher than that of ordinary winding casting resin.

[0009] Preferably, the inner wall of the unit air duct is provided with a spiral flow guiding structure, and the spiral flow guiding structure and the inner wall of the unit air duct are integral structures formed by one-time printing, and the material is the same as that of the air duct body.

[0010] Preferably, the inner wall of the unit air duct is provided with a spiral flow guiding structure, which is an independent hollow spiral tube or a central body component with spiral fins made of high thermal conductivity insulating ceramic materials such as aluminum nitride or beryllium oxide.

[0011] Preferably, the ratio of the inlet cross-sectional area to the throat cross-sectional area of ​​the unit air duct is 1.5:1 to 3:1.

[0012] Preferably, the inlet contraction angle of the unit air duct is 20° to 45°.

[0013] Preferably, the outlet section of the unit air duct is provided with a diffusion section, the outlet cross-sectional area of ​​the diffusion section is larger than its throat cross-sectional area, and its diffusion angle is 5° to 15°.

[0014] Preferably, the cross-sectional shape of the unit air duct is elliptical or rounded rectangle, with its major axis aligning with the circumferential direction of the winding, so as to obtain the maximum heat dissipation surface area while minimizing the weakening of the radial mechanical strength of the winding.

[0015] Preferably, the high-voltage winding adopts a layered and segmented structure of Litz wire harness winding, and the high-voltage terminals are led out through a brazing process; the low-voltage winding adopts a strip conductor or multi-strand Litz wire winding, and the low-voltage busbar is led out through welding.

[0016] Compared with existing technologies, this utility model has the following beneficial effects: 1. Significantly Improved Heat Dissipation Efficiency: This invention applies the Venturi effect to heat dissipation within the winding. The gradient variable cross-section structure actively accelerates the cooling airflow, disrupting the thermal boundary layer. Combined with the swirling flow induced by the spiral guide structure and the large porous matrix heat dissipation area, a synergistic heat dissipation mechanism of "acceleration-turbulence-large area heat exchange" is formed, fundamentally overturning the traditional laminar flow heat dissipation mode of straight air channels, and greatly improving heat dissipation efficiency.

[0017] 2. Effectively suppresses hot spot temperature rise: The porous matrix layout allows the heat dissipation channels to penetrate deep into the heat source concentration area inside the winding and can evenly remove heat, significantly reducing the average temperature rise of the winding and the temperature of local hot spots, thereby improving the transformer's overload capacity and long-term operational reliability.

[0018] 3. Superior Mechanical Strength and Reliability: The prefabricated unit duct components are scientifically designed and possess excellent mechanical properties. Encased in high-quality epoxy resin as inserts, they form a robust whole with the windings. This not only does not weaken the original structure, but the internal support structure also acts as a reinforcing rib, ensuring the product's excellent short-circuit withstand capability and mechanical strength.

[0019] 4. Flexible design and wide applicability: This utility model can flexibly select the arrangement position of the heat dissipation channel and the specific form of the spiral flow guiding structure according to different heat dissipation and strength requirements, and is suitable for transformer products of various capacities and voltage levels, with a wide range of applications. Attached Figure Description

[0020] To more clearly illustrate the technical solutions of the embodiments of this utility model, the drawings used in the description of the embodiments will be briefly introduced below. In all the drawings, similar elements or parts are generally identified by similar reference numerals. The elements or parts in the drawings are not necessarily drawn to scale.

[0021] Figure 1 The diagram shows a top view (a) and a side view (b) of the high-voltage and low-voltage windings of the dry-type transformer of this utility model.

[0022] Figure 2 for Figure 1 An enlarged schematic diagram of structure 3 in the medium and low voltage winding.

[0023] Figure 3This is a cross-sectional view of the high-voltage and low-voltage windings and the core structure of a dry-type transformer.

[0024] Figure 4 The diagram shows a single unit air passage without spiral fins and two unit air passage structures with different spiral fins.

[0025] Figure 5 This is a schematic diagram of the inlet contraction angle α and the outlet diffusion angle β of a unit air duct.

[0026] Figure 6 Top view of the overall structure of an epoxy resin cast dry-type transformer.

[0027] Figure 7 A schematic diagram of a dry-type transformer cast with epoxy resin.

[0028] Explanation of key figure labels: High-voltage winding 1; low-voltage winding 2; low-voltage unit heat dissipation channel 3; high-voltage unit heat dissipation channel 4; fixed silicone rod 5; main heat dissipation channel 6; iron core 7; first spiral fin 8; second spiral fin 9; low-voltage busbar 10; high-voltage terminal 11; high-voltage connecting piece 12; high-voltage connecting rod 13; high-voltage tap 14; upper clamp 15; lower clamp 16; unit air duct component 17; mounting base 18; unit air duct inlet 19; unit air duct throat 20; unit air duct outlet 21. Detailed Implementation

[0029] The technical solutions of the present utility model will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only some embodiments of the present utility model, and not all embodiments. Based on the embodiments of the present utility model, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the protection scope of the present utility model.

[0030] In the description of this utility model, "several" means one or more, "multiple" means two or more, "greater than," "less than," and "exceeding" are understood to exclude the stated number, while "above," "below," and "within" are understood to include the stated number. If the terms "first," "second," and "third" are used in the description, they are for descriptive purposes and to distinguish technical features, and should not be construed as indicating or implying relative importance, or implicitly indicating the number of indicated technical features, or implicitly indicating the sequential relationship of the indicated technical features.

[0031] In the description of this utility model, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "setting" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this utility model based on the specific circumstances. The embodiments of this utility model will now be described based on its overall structure.

[0032] Reference Figures 1-7 A dry-type epoxy resin cast transformer based on the Venturi effect and porous matrix heat dissipation includes windings and an iron core 7 located at the center of the windings. The iron core 7 has clamps at both ends, and the clamps are connected to the mounting base 18. The windings include a high-voltage winding 1 and a low-voltage winding 2 located inside the high-voltage winding 1. The high-voltage winding 1 and / or the low-voltage winding 2 are embedded with porous matrix-type unit heat dissipation channels. The unit heat dissipation channels are composed of several integrally formed independent unit air duct components 17. The unit heat dissipation channels are divided into low-voltage unit heat dissipation channels 3 of the low-voltage winding and high-voltage unit heat dissipation channels 4 of the high-voltage winding. Preferably, the unit air duct components 17 are pre-formed integrally by photopolymerization 3D printing technology, and fixed in the mold in a specific three-dimensional array before the windings are cast. They are then cured and encapsulated by epoxy resin vacuum casting. The flow channel of unit air duct component 17 is a gradient variable cross-section structure following the Venturi tube principle. The flow channel of unit air duct component 17 is smoothly connected by an inlet section 19, a throat 20, and an outlet section 21. The cross-sectional area of ​​the inlet section 19 is larger than that of the throat 20, forming a gradient contraction variable cross-section structure to generate a Venturi acceleration effect on the flowing cooling medium. It should be noted that a Venturi tube is a device used to measure fluid flow rate. Its working principle is based on Bernoulli's principle, generating flow velocity changes by altering the pipe cross-sectional area. The clamps include an upper clamp 15 and a lower clamp 16. Furthermore, the winding is integrally molded using modified epoxy resin through a vacuum casting process. The iron core magnetic circuit system of this invention adopts a three-phase, three-column structure, made of stacked high-permeability cold-rolled grain-oriented silicon steel sheets. The cross-section of the 7 iron core columns adopts a multi-level approximately circular design, effectively optimizing the magnetic circuit distribution and significantly reducing iron loss and noise levels.

[0033] The high-voltage connecting piece 12 is mainly used in high-voltage circuits to achieve rapid connection or disconnection between two points; essentially, it is a simple high-voltage switch or connecting element. The high-voltage connecting rod 13 is mainly used inside or between high-voltage equipment to connect two high-voltage conductive components across a certain spatial interval, forming a conductive path. The high-voltage tap changer 14 is typically installed on the windings of high-voltage equipment such as transformers and reactors; by changing the turns ratio of the windings, voltage or impedance regulation is achieved.

[0034] Reference Figure 1 In sections a and b: A gap is maintained between the high-voltage winding 1 and the low-voltage winding 2 to form an annular main heat dissipation channel 6. Multiple silicone rods 5 are spaced circumferentially within the main heat dissipation channel 6 to support and buffer the high and low voltage windings. Preferably, an annular main heat dissipation channel 6 with a width of 40 mm is formed between the high-voltage winding 1 and the low-voltage winding 2. A circular silicone rod 5 with a diameter of 50 mm is placed every 120° along the circumference within this main heat dissipation channel 6. Its Shore A hardness is 60±5, providing elastic support for the windings and effectively buffering short-circuit electrodynamic forces.

[0035] Unit air duct component 17 is made of a high thermal conductivity composite material. This material uses epoxy resin as the matrix and is filled with thermally conductive fillers such as boron nitride, alumina, or silicon carbide in high volume fractions. Its thermal conductivity is higher than that of ordinary winding casting resin. The specific materials are as follows: The printing material is a special high thermal conductivity epoxy resin composite material, which is composed of the following components by weight: 100 parts of bisphenol A type epoxy resin E-51, 85 parts of methyl hexahydrophthalic anhydride MeHHPA curing agent, boron nitride (BN), 150 parts of thermally conductive filler with a particle size of 10 μm, 3 parts of fumed silica A-200 thixotropic agent, and appropriate amounts of defoamer and coupling agent.

[0036] After thorough mixing and vacuum degassing, the material is loaded into the DLP printer's feed tank. The printing layer thickness is set to 50 μm, the ultraviolet wavelength to 405 nm, and the exposure time to 8 seconds per layer. After printing, the molded part is placed in a circulating air drying oven and post-cured according to a stepped program of 80℃ / 2h + 120℃ / 4h + 150℃ / 6h, ultimately obtaining a unit air duct preform 3 with high mechanical strength and dimensional stability. This finished spiral guide structure and the inner wall of the unit air duct are integrally printed in one step, and its material is the same as that of the air duct body. Its structure is as follows: Figure 4 As shown in (c).

[0037] Reference Figures 1-4 The inner wall of the unit air duct is equipped with a spiral flow guiding structure, which can take two forms: The spiral flow guide structure and the inner wall of the unit air duct are integral structures printed in one piece, and their material is the same as that of the air duct body.

[0038] The inner wall of the unit air duct is equipped with a spiral flow guiding structure. This spiral flow guiding structure is an independent, hollow spiral tube or a central body component with spiral fins, made of a high thermal conductivity insulating ceramic material such as aluminum nitride or beryllium oxide. Furthermore, the pitch of the spiral fin structure is 1.5-2.0 times the equivalent diameter of the flow channel, and the helix angle is 40°-55°. This parameter range can effectively disrupt the flow boundary layer, enhance heat transfer, and control pressure loss.

[0039] The component is fixed to the central axis of the unit air duct by ceramic support feet at both ends, maintaining a uniform annular air gap of 0.5mm to 3mm between its outer spiral fins and the inner wall of the unit air duct. This structure facilitates both radiative and convective heat transfer. Preferably, refer to... Figure 4 The spiral fins include a first spiral fin 8 and a second spiral fin 9. The first spiral fin 8 is a high thermal conductivity epoxy resin spiral fin, and the second spiral fin is a ceramic spiral fin.

[0040] The unit's heat dissipation channel itself is a smooth Venturi tube without internal fins. At its center, a second spiral fin 9 made of independent high thermal conductivity ceramic material is fixedly installed via a ceramic support frame. A uniform air gap of 0.5-3.0 mm is maintained between the ceramic fins and the inner wall of the unit's heat dissipation channel, forming a radiation-convection composite heat dissipation channel.

[0041] The ratio of the inlet cross-sectional area to the throat cross-sectional area of ​​the unit air duct is 1.5:1 to 3:1.

[0042] Reference Figure 5 The inlet contraction angle α of the unit air duct is 20° to 45°. The transition angle α at the junction of the inlet contraction angle of 19-20° is designed to be 25-35°.

[0043] Reference Figure 4 The outlet section 21 of the unit air duct is provided with a diffusion section. The outlet cross-sectional area of ​​the diffusion section is larger than the cross-sectional area of ​​its throat 20, and its diffusion angle β is 5° to 15°.

[0044] Reference Figure 1 and Figure 4 The cross-sectional shape of the unit air duct is preferably elliptical or rounded rectangle, with its major axis aligned with the circumferential direction of the winding. This maximizes the heat dissipation surface area while minimizing the reduction in radial mechanical strength of the winding. This optimizes heat dissipation performance while ensuring mechanical reliability.

[0045] The high-voltage winding 1 adopts a layered and segmented structure with Litz wire harness winding, and the high-voltage terminal 11 is led out through a brazing process. Furthermore, the conductive coil in the high-voltage winding 1 is wound with composite insulated Litz wire of temperature resistance class H, and has a layered and segmented structure, consisting of 8 layers and 12 segments. The inner and outer leads of the winding and the tap leads connected to the tap 14 use silver brazing filler metal with a silver content of 2%, and achieve a reliable connection with the high-voltage terminal 11 through high-frequency induction brazing under high-purity argon gas protection. The weld is full and the resistivity is lower than that of the base material. The low-voltage winding 2 is wound with a strip conductor or multi-strand Litz wire, and the low-voltage busbar 10 is led out through welding. Furthermore, the conductive coil in the low-voltage winding 2 is wound with T2 type oxygen-free copper foil, with a copper foil thickness of 0.8 mm, and the layers are isolated by 0.15 mm thick NOMEX® insulating paper. Its first and last turns are connected to the low-voltage interface busbar 10 via copper flexible connecting pieces with a rated current density of 2.8A / mm², and a 350A high-current energy storage welding machine is used to achieve a low-resistance, high-strength electrical connection.

[0046] Preparation method: The aforementioned prefabricated unit air duct components were fixed in a honeycomb matrix arrangement around the pre-wound low-voltage winding 2 within a mold using a precision nylon positioning fixture, ensuring that all air duct inlets faced the same direction. After mold closing, vacuum casting was performed. A conventional anhydride-cured epoxy resin system with a thermal conductivity of ~0.2 W / (m·K) was used for casting. Casting was completed under a vacuum of -0.1 MPa and 65°C, followed by stepped temperature curing. After molding, the unit air duct and the winding body are firmly bonded together as a single unit using epoxy resin. The thermal conductivity of the unit heat dissipation air duct can reach 2-3 times that of conventional casting resin, effectively improving heat transfer efficiency. The high-voltage winding was prepared using the same method.

[0047] Preferably, in the porous matrix heat dissipation system, the spacing of the heat dissipation air ducts 4 in each unit is 1.8-2.2 times the equivalent diameter of the unit. This arrangement can maximize the heat dissipation area while ensuring structural strength.

[0048] This invention optimizes the cooling airflow organization through a venturi channel structure and, combined with the expanded heat exchange area provided by a porous matrix, establishes a highly efficient thermal management system that significantly improves the product's heat dissipation performance while ensuring insulation performance and structural reliability.

[0049] The foregoing description of specific exemplary embodiments of the present invention is for illustrative and explanatory purposes. These descriptions are not intended to limit the present invention to the precise forms disclosed, and it is obvious that many changes and variations can be made based on the above teachings. Although embodiments of the present invention have been shown and described, these specific embodiments are merely explanations of the present invention and are not intended to limit the invention. The specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. The purpose of selecting and describing exemplary embodiments is to explain the specific principles of the present invention and its practical application, so that those skilled in the art, after reading this specification, can make modifications, substitutions, variations, and various choices and changes to the embodiments as needed without departing from the principles and spirit of the present invention, provided that such modifications, substitutions, variations, and choices and changes are within the scope of the claims of the present invention and are protected by patent law.

Claims

1. An epoxy resin cast dry-type transformer based on the Venturi effect and porous matrix heat dissipation, comprising windings and an iron core (7) located at the center of the windings, wherein clamps are provided at both ends of the iron core (7), and the clamps are connected to a mounting base (18), characterized in that, The winding includes a high-voltage winding (1) and a low-voltage winding (2) located inside the high-voltage winding (1). The high-voltage winding (1) and / or the low-voltage winding (2) are embedded with a porous matrix-type unit heat dissipation channel. The unit heat dissipation channel is composed of several integrally formed independent unit air duct components (17). The flow channel of the unit air duct component (17) is a gradient variable cross-section structure following the Venturi tube principle. The flow channel of the unit air duct component (17) is smoothly connected by an inlet section (19), a throat (20) and an outlet section (21). The cross-sectional area of ​​the inlet section (19) is larger than the cross-sectional area of ​​the throat (20), forming a gradient contraction variable cross-section structure, which is used to generate a Venturi acceleration effect on the cooling medium flowing through it.

2. The epoxy resin cast dry-type transformer based on the Venturi effect and porous matrix heat dissipation according to claim 1, characterized in that, The high-voltage winding (1) and the low-voltage winding (2) are spaced apart to form an annular main heat dissipation channel (6). Multiple silicone rods (5) are arranged circumferentially in the main heat dissipation channel (6) to support and buffer the high and low voltage windings.

3. The epoxy resin cast dry-type transformer based on the Venturi effect and porous matrix heat dissipation according to claim 1, characterized in that, The unit air duct component (17) is made of a high thermal conductivity composite material. The material is based on epoxy resin and filled with a high volume fraction of thermally conductive filler. The thermally conductive filler is boron nitride, aluminum oxide or silicon carbide, and its thermal conductivity is higher than that of ordinary winding casting resin.

4. The epoxy resin cast dry-type transformer based on the Venturi effect and porous matrix heat dissipation according to claim 3, characterized in that, The inner wall of the unit air duct is provided with a spiral flow guiding structure. The spiral flow guiding structure and the inner wall of the unit air duct are integral structures formed by one-time printing, and their material is the same as that of the air duct body.

5. The epoxy resin cast dry-type transformer based on the Venturi effect and porous matrix heat dissipation according to claim 3, characterized in that, The inner wall of the unit air duct is provided with a spiral flow guiding structure, which is an independent hollow spiral tube or a central body component with spiral fins made of high thermal conductivity insulating ceramic materials such as aluminum nitride or beryllium oxide.

6. The epoxy resin cast dry-type transformer based on the Venturi effect and porous matrix heat dissipation according to claim 4, characterized in that, The ratio of the inlet cross-sectional area to the throat cross-sectional area of ​​the unit air duct is 1.5:1 to 3:

1.

7. The epoxy resin cast dry-type transformer based on the Venturi effect and porous matrix heat dissipation according to claim 6, characterized in that, The inlet contraction angle α of the unit air duct is 20° to 45°.

8. The epoxy resin cast dry-type transformer based on the Venturi effect and porous matrix heat dissipation according to claim 1, characterized in that, The outlet section (21) of the unit air duct is provided with a diffusion section, the outlet cross-sectional area of ​​the diffusion section is larger than the cross-sectional area of ​​its throat (20), and its diffusion angle β is 5° to 15°.

9. The epoxy resin cast dry-type transformer based on the Venturi effect and porous matrix heat dissipation according to claim 1, characterized in that, The cross-sectional shape of the unit air duct is elliptical or rounded rectangle, and its major axis is arranged in accordance with the circumferential direction of the winding, so as to obtain the maximum heat dissipation surface area while minimizing the weakening of the radial mechanical strength of the winding.

10. The epoxy resin cast dry-type transformer based on the Venturi effect and porous matrix heat dissipation according to claim 1, characterized in that, The high-voltage winding (1) adopts a layered and segmented structure with Litz wire harness winding and leads out the high-voltage terminal (11) through brazing process. The low-voltage winding (2) adopts a strip conductor or multi-strand Litz wire winding and leads out the low-voltage busbar (10) through welding.

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