Low-frequency transformer and offshore wind power generation system

Abstract

The utility model relates to a low-frequency transformer and an offshore wind power generation system. The low-frequency transformer is of a three-phase five-column structure and comprises an iron core which comprises a main column, a side column and an iron yoke; the winding comprises a high-voltage winding, a medium-voltage winding and a low-voltage winding, and the high-voltage winding of each phase is arranged on the corresponding main column of each phase in a sleeving manner; each phase of medium-voltage winding and each same-phase high-voltage winding are concentrically sleeved on each corresponding phase of main column; each phase of low-voltage winding and each high-voltage winding and each medium-voltage winding which are in the same phase are concentrically sleeved on the corresponding phase of main column and are arranged below each high-voltage winding and each medium-voltage winding which are in the same phase, and insulation intervals are arranged between each phase of low-voltage winding and the corresponding phase of medium-voltage winding and between each phase of low-voltage winding and each phase of high-voltage winding; the core column, the high-voltage winding, the medium-voltage winding and the low-voltage winding are arranged in the oil tank, and cooling oil is contained in the oil tank. According to the low-frequency transformer and the offshore wind power generation system, the magnetic saturation risk of the transformer can be reduced, and the performance of the transformer is improved.

Description

Technical Field

[0001] The utility model relates to the technical fields of low-frequency transformers and offshore wind turbines, and particularly relates to a low-frequency transformer and an offshore wind power generation system. Background Art

[0002] Offshore wind power refers to the technology of generating electricity by using wind energy resources in the ocean. Compared with onshore wind power, sea breeze is not affected by terrain and surface roughness, and the wind energy resources are more abundant. Offshore wind power has the advantages of higher and more stable wind speed, so it has higher power generation efficiency. As an important part of clean energy, offshore wind power has developed rapidly. Offshore wind turbines are connected to an offshore substation through a power collection network. Since the power generation voltage of offshore wind turbines is usually low, it is necessary to step up the voltage through a transformer in the substation and then transmit the power to the land through a submarine cable. A low-frequency transformer refers to a transformer used to reduce the frequency of electric energy transmission, and is commonly used in scenarios such as far-offshore wind farms or other long-distance power transmission. After reducing the frequency, the capacitive effect of the submarine cable can be reduced, thereby reducing losses and improving the efficiency of the power transmission system.

[0003] In the traditional high-voltage AC power transmission technology under the power frequency condition of 50Hz, in terms of submarine cable transportation, due to the skin effect of the wire, when the current is too large, the surface current of the wire will be much higher than the internal current; and when the power transmission system transmits a large-capacity power, a capacitive rise effect will occur, that is, the reactive power compensation added in the power transmission line becomes larger. Reducing the frequency of the transformer can reduce the influence of the skin effect and the capacitive rise effect in the large-scale deep-sea and far-sea wind power transportation. However, when the operating frequency is reduced, in order to avoid transformer magnetic saturation, it is necessary to increase the number of turns of the coil or increase the cross-sectional area of the iron core, thereby increasing the volume and cost of the transformer. Summary of the Utility Model

[0004] Based on this, in view of the problem that the risk of transformer magnetic saturation is relatively large when reducing the skin effect and the capacitive rise effect by reducing the transformer frequency, it is necessary to provide a low-frequency transformer and an offshore wind power generation system that can reduce the risk of transformer magnetic saturation.

[0005] In a first aspect, a low-frequency transformer is provided. The transformer has a three-phase five-column structure and includes: an iron core, including three main columns, two side columns, and four yokes at the top and bottom. Among them, the main columns, the side columns, and the yokes form a frame structure through connecting pieces;

[0006] A winding, including a high-voltage winding, a medium-voltage winding and a low-voltage winding, wherein each phase of the high-voltage winding is respectively sleeved on the corresponding main column of each phase; each phase of the medium-voltage winding and the high-voltage windings of the same phase are concentrically sleeved on the corresponding main columns of each phase; each phase of the low-voltage winding and the high-voltage windings and the medium-voltage windings of the same phase are concentrically sleeved on the corresponding main columns of each phase, and are arranged below the high-voltage windings and the medium-voltage windings of the same phase, and an insulation interval is arranged between each phase of the low-voltage winding and the corresponding medium-voltage winding and high-voltage winding of each phase;

[0007] An oil tank, wherein the iron core, the high-voltage winding, the medium-voltage winding and the low-voltage winding are arranged in the oil tank, and insulating oil is contained in the oil tank.

[0008] In one embodiment, the operating frequency of the low-frequency transformer is not greater than 20 Hz.

[0009] In one embodiment, the number of turns of each high-voltage winding is fixed.

[0010] In one embodiment, insulating parts are arranged in the insulation interval.

[0011] In one embodiment, it further includes a high-voltage outgoing line device, a medium-voltage outgoing line device and a low-voltage outgoing line device, wherein the high-voltage outgoing line device and the medium-voltage outgoing line device include cables, and the low-voltage outgoing line device is an air cable box or a plug-and-play structure.

[0012] In one embodiment, it further includes a positioning part, and the positioning part is connected to the inner wall of the iron core and the oil tank for restricting the relative movement of the iron core and the oil tank.

[0013] In one embodiment, the main column, the side column and the yoke are composed of stacked grain-oriented cold-rolled silicon steel sheets.

[0014] In one embodiment, the low-voltage winding is a continuous winding.

[0015] In one embodiment, the connecting part includes clamp parts, an upper beam and side beams; wherein, the clamp parts are connected to the main columns, the side columns and the yoke of each phase through bolts for fixing the main columns, the side columns and the yoke, the upper beam is fixedly connected to the tops of the main columns and side columns, and the side beams are perpendicular to the upper beam to form a frame structure for providing side support for the iron core and the winding.

[0016] In a second aspect, a marine wind power generation system is provided. The marine wind power generation system is connected to the power grid and includes:

[0017] A wind turbine and a fan voltage output subsystem; and,

[0018] The low-frequency transformer described in the first aspect or any embodiment of the first aspect is used to step up the electric energy output by the wind turbine and then transmit it to the power grid.

[0019] For the above-mentioned low-frequency transformer and the offshore wind power generation system, the three-phase five-column structure can effectively balance the magnetic flux distribution in the transformer. The side columns and the main columns form a complete magnetic circuit through connectors, reducing magnetic flux leakage, optimizing the magnetic flux path of the iron core, improving the efficiency of the transformer, reducing local overheating and losses. The concentric arrangement of the high-voltage winding, medium-voltage winding, and low-voltage winding on the same-phase main column can reduce the coupling reactance and leakage magnetic flux between the windings, thereby reducing electromagnetic losses. It can also better control the electric field distribution between the windings, reduce the local electric field intensity, improve the withstand voltage level and safety of the transformer. Placing the low-voltage winding below the high-voltage and medium-voltage windings and setting an insulation interval between the low-voltage winding and the high-voltage and medium-voltage windings can improve the magnetic coupling relationship between the windings, making the impedance of the medium-voltage winding and the high-voltage winding to the low-voltage winding increase significantly, greatly enhancing the short-circuit resistance of the low-voltage winding. It can also effectively manage the heat distribution of the windings, reduce the thermal coupling between windings of different voltage levels, reduce the temperature rise, and maintain the stability of the windings under the action of thermal expansion or mechanical stress. Thus, the magnetic saturation risk of the transformer can be reduced, and the performance of the transformer can be improved. Brief Description of the Drawings

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0021] Figure 1 Schematic diagram of the winding structure of the low-frequency transformer provided by an embodiment of the present invention;

[0022] Figure 2 Schematic diagram of the structure of the low-frequency transformer provided by an embodiment of the present invention;

[0023] Figure 3 Schematic diagram of the iron core structure of the low-frequency transformer provided by an embodiment of the present invention.

[0024] To make the above and other purposes, features, and advantages of the present invention more obvious and understandable, the descriptions of the attached symbols are as follows:

[0025] 1: Iron core, 2: Medium-voltage winding, 3: High-voltage winding, 4: Low-voltage winding, 5: Insulating end ring, 6: Oil tank, 7: Low-voltage cable box, 8: Medium-voltage outgoing line device, 9: Medium-voltage neutral point outgoing line device, 10: High-voltage outgoing line device, 11: Positioning part, 12: Upper and lower clamping parts, 13: Upper yoke, 14: Lower yoke, 15: Main column, 16: Side column, 17: Upper beam, 18: Lower beam. Detailed implementation manners

[0026] To make the above objects, features and advantages of the present utility model more obvious and understandable, the following detailed description of the specific implementation manners of the present utility model will be given in conjunction with the accompanying drawings. Many specific details are set forth in the following description in order to fully understand the present utility model. However, the present utility model can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without departing from the connotation of the present utility model. Therefore, the present utility model is not limited by the specific embodiments disclosed below.

[0027] In the description of the present utility model, it should be understood that the orientation or positional relationship indicated by the terms "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present utility model and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation to the present utility model.

[0028] In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include at least one such feature. In the description of the present utility model, the meaning of "a plurality" is at least two, such as two, three, etc., unless otherwise specifically and clearly defined.

[0029] In the present utility model, unless otherwise clearly specified and defined, the terms "installation", "connection", "connection", "fixation", etc. should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or integrated; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two elements or the interaction relationship between two elements, unless otherwise clearly defined. For those of ordinary skill in the art, the specific meanings of the above terms in the present utility model can be understood according to specific situations.

[0030] In the present utility model, unless otherwise clearly defined and limited, the first feature being "on" or "under" the second feature may be that the first and second features are in direct contact, or the first and second features are in indirect contact through an intermediate medium. Moreover, the first feature being "above", "over" and "on top of" the second feature may be that the first feature is directly above or obliquely above the second feature, or merely indicates that the horizontal height of the first feature is higher than that of the second feature. The first feature being "under", "beneath" and "underneath" the second feature may be that the first feature is directly below or obliquely below the second feature, or merely indicates that the horizontal height of the first feature is less than that of the second feature.

[0031] It should be noted that when an element is referred to as being "fixed to" or "disposed on" another element, it can be directly on the other element or there may also be an intermediate element. When an element is considered to be "connected to" another element, it can be directly connected to the other element or there may be an intermediate element at the same time. The terms "vertical", "horizontal", "upper", "lower", "left", "right" and similar expressions used herein are only for the purpose of illustration and do not represent the only implementation.

[0032] In the traditional high-voltage AC power transmission technology under the 50Hz power frequency condition, in terms of submarine cable transportation, due to the skin effect of the wire, when the current is too large, the surface current of the wire will be much higher than the internal current; and when the power transmission system transmits a large-capacity power, a capacitance rise effect will occur, that is, the reactive power compensation increased in the power transmission line becomes larger. Reducing the frequency of the transformer can reduce the influence of the skin effect and the capacitance rise effect in the large-scale deep-sea and far-sea wind power transportation. However, when the operating frequency is reduced from 50Hz to 20Hz, in order to avoid transformer magnetic saturation, it is necessary to increase the number of turns of the coil or increase the cross-sectional area of the iron core, thereby increasing the volume and cost of the transformer. In view of the above problems, the present utility model provides a low-frequency transformer, which can reduce the magnetic saturation risk of the transformer, improve the performance of the transformer, reduce the volume and weight of the transformer, and enhance the reliability of the transformer. Figure 1 Schematic diagram of the winding structure of the low-frequency transformer provided by an embodiment of the present utility model; Figure 2 Schematic diagram of the structure of the low-frequency transformer provided by an embodiment of the present utility model. Refer to Figure 1 and Figure 2 , the low-frequency transformer provided by an embodiment of the present utility model is of a three-phase five-column structure, including:

[0033] An iron core, including three main columns, two side columns and four yokes at the top and bottom respectively. Among them, the main columns, side columns and yokes form a frame structure through connecting pieces

[0034] A winding, including a high-voltage winding, a medium-voltage winding, and a low-voltage winding. Among them, each phase of the high-voltage winding is sleeved on the corresponding main column of each phase; each phase of the medium-voltage winding and the high-voltage windings of the same phase are concentrically sleeved on the corresponding main columns of each phase; each phase of the low-voltage winding and the high-voltage windings and medium-voltage windings of the same phase are concentrically sleeved on the corresponding main columns of each phase, and are arranged below the high-voltage windings and medium-voltage windings of the same phase. Moreover, an insulation interval is provided between each phase of the low-voltage winding and the corresponding medium-voltage winding and high-voltage winding of each phase.

[0035] An oil tank, wherein the iron core, the high-voltage winding, the medium-voltage winding, and the low-voltage winding are arranged in the oil tank, and insulating oil is contained in the oil tank.

[0036] Among them, the iron core of a three-phase five-column iron-core transformer can be composed of five columns and four yokes. The main column refers to the columnar structure for winding the windings. Each main column is wound with the high-voltage and low-voltage windings of one phase, respectively carrying the three-phase current, distributed at both ends and the middle position of the iron core. Two side columns can be arranged between the two main columns, and no windings are arranged on the side columns. The magnetic circuit is formed by connecting the yokes, which is used to help share the magnetic flux and maintain the magnetic circuit balance and reduce the leakage magnetic loss when the load is unbalanced or there is a short-circuit fault. The windings on the main columns can be set as star or delta according to actual needs, and this application does not make any limitations in this regard. The high-voltage winding and the medium-voltage winding are respectively wound on different parts of the main column, and can be a concentric winding design, with the high-voltage winding on the outside and the medium-voltage winding on the inside, to reduce the loss caused by electromagnetic coupling. To ensure the safe operation of the transformer, good insulation is required between the windings and the iron core. Therefore, the transformer adopts an oil-immersed structure. The oil in the oil tank not only provides insulation, but also helps with heat dissipation. Placing the low-voltage winding below the medium-voltage and high-voltage windings can reduce the overall volume of the windings, thereby reducing the volume and weight of the transformer. Since the frequency of the transformer decreases, it causes the cross-sectional area of the transformer iron core to increase and the number of turns of the coil to increase, thus increasing the weight and volume of the transformer. The principle is shown in the following formula:

[0037] e = 4.44 * f * Bm * S

[0038] W = U / e

[0039] If the inter-turn electromotive force e remains unchanged and the maximum magnetic flux density Bm has little fluctuation in value, when the frequency f decreases, the corresponding core area S will increase, that is, the diameter of the core becomes larger, the volume of the transformer becomes larger, and the weight of the transformer becomes larger. When the core diameter becomes larger and the number of turns of the coil remains unchanged, the diameter of the coil wound around the core becomes larger, and the corresponding resistance loss will also increase. If the core area S remains unchanged and the maximum magnetic flux density Bm has little fluctuation in value, when the frequency f decreases, the corresponding inter-turn electromotive force e becomes smaller, and since the voltage U is a given value, the number of turns W of the coil obtained becomes larger. The more turns there are, the higher the coil height becomes, the reactance height increases, the corresponding core height increases, the volume of the transformer becomes larger, and the weight becomes larger. Also, because the impedance of the transformer is a given value, which limits the core diameter and the reactance height, considering comprehensively, when the frequency of the transformer decreases, the cross-section of the transformer core will become larger, the number of turns of the coil will become more, and the volume and weight will increase.

[0040] The above-mentioned low-frequency transformer can effectively balance the magnetic flux distribution in the transformer through a three-phase five-column structure. The side columns and the main columns form a complete magnetic circuit through connecting parts, reducing magnetic flux leakage, optimizing the magnetic flux path of the core, improving the efficiency of the transformer, reducing local overheating and losses. The concentric arrangement of the high-voltage winding, medium-voltage winding, and low-voltage winding on the same-phase main column can reduce the coupling reactance and leakage magnetic flux between the windings, thereby reducing electromagnetic losses. It can also better control the electric field distribution between the windings, reduce the local electric field intensity, improve the withstand voltage level and safety of the transformer. Placing the low-voltage winding below the high-voltage and medium-voltage windings and setting an insulation interval between the low-voltage winding and the high-voltage and medium-voltage windings can improve the magnetic coupling relationship between the windings, making the impedance of the medium-voltage winding and the high-voltage winding to the low-voltage winding both increase significantly, greatly enhancing the short-circuit resistance ability of the low-voltage winding. It can also effectively manage the heat distribution of the windings, reduce the thermal coupling between windings of different voltage levels, reduce the temperature rise, and maintain the stability of the windings under the action of thermal expansion or mechanical stress, thereby reducing the magnetic saturation risk of the transformer and improving the performance of the transformer.

[0041] In an exemplary embodiment, the operating frequency of the low-frequency transformer is not greater than 20 Hz. Using a 20 Hz frequency instead of the commonly used 50 Hz transformer, when the frequency of the transformer decreases, the skin effect and capacitance rise effect of the transformer will weaken accordingly, enhancing the benefit of submarine cable transportation and the reliability of the transmission line.

[0042] In an exemplary embodiment, the number of turns of each high-voltage winding is fixed. When the number of turns of the high-voltage winding is fixed, its output voltage is also fixed and does not change with the operating state or external conditions. This can simplify the structure of the winding, reduce the manufacturing cost and maintenance complexity, and without setting moving parts such as tap switches, it can improve the overall reliability of the transformer.

[0043] In an exemplary embodiment, an insulating member is provided in the insulating space. The insulating member can be made of materials such as insulating cardboard.

[0044] In an exemplary embodiment, it further includes a high-voltage outgoing line device, a medium-voltage outgoing line device, and a low-voltage outgoing line device. Among them, the high-voltage outgoing line device and the medium-voltage outgoing line device include cables, and the low-voltage outgoing line device is an air cable box or a plug-and-play structure. The high-voltage and low-voltage outgoing lines can adopt cable plug connectors for outgoing lines, and the outgoing line distance between the coil and the tank wall is reduced to reduce the volume and weight of the oil tank.

[0045] In an exemplary embodiment, it further includes a positioning member, which is connected to the inner walls of the iron core and the oil tank and is used to limit the relative movement between the iron core and the oil tank. The number and position of the positioning members can be set according to the actual size and structure of the transformer to fix the iron core at multiple points, reduce the deformation caused by factors such as vibration during operation, and thus improve the reliability of the transformer.

[0046] In an exemplary embodiment, the main column, the side column, and the yoke are stacked and composed of grain-oriented cold-rolled silicon steel sheets. Grain-oriented cold-rolled silicon steel sheets refer to those that have been specially treated during the rolling process so that the grains are mainly arranged along a specific direction with excellent magnetic properties (usually along the rolling direction of the silicon steel sheet). In this direction, the magnetic permeability is relatively high and the iron loss is relatively low. This kind of material has excellent magnetic properties and low iron loss characteristics. Using grain-oriented cold-rolled silicon steel sheets to stack and form the core column of the transformer can significantly reduce the iron loss, increase the magnetic permeability, reduce the noise, increase the magnetic flux density, and save materials and costs.

[0047] In an exemplary embodiment, the low-voltage winding is a continuous winding. A continuous winding means that the wire is wound continuously from one end to the other end without intermediate joints or breaks. The continuous winding can adopt structures such as layer winding or disc winding. After the first layer of the wire is wound, it is directly connected to the next layer until all layers of the winding are completed. Each layer of the wire is wound along the length direction of the iron core, and electrical insulation is maintained between the wire layers. Adopting a continuous winding structure for the low-voltage winding can reduce electromagnetic interference, simplify the manufacturing process, and improve the electrical insulation performance.

[0048] In an exemplary embodiment, the connecting member includes a clamping member, an upper beam, and a side beam. Among them, the clamping member is bolted to each of the main columns, the side columns, and the yoke to fix the main columns, the side columns, and the yoke. The upper beam is fixedly connected to the tops of each of the main columns and the side columns. The side beam is perpendicular to the upper beam to form a frame structure for providing lateral support for the iron core and the windings. The transformer iron core adopts a three-phase five-column structure, made of high-permeability silicon steel sheets, with a fully inclined multi-stage joint and non-hole lashing. Each step of the iron core column is tightened with round wooden spacers. The iron core adopts a frame structure composed of a clamping member, an upper beam, a pad foot, a tie strap, and a side beam, which can ensure the roundness of the iron core shape and improve its firmness.

[0049] In an exemplary embodiment, a low-voltage transformer for offshore wind power is provided, and its structure is as Figure 1 shown. Among them, the iron core 1 adopts a three-phase five-column structure, which is the magnetic circuit and installation skeleton of the transformer and is laminated with high-quality grain-oriented cold-rolled silicon steel sheets. The medium-voltage winding 2 is the medium-voltage winding. The high-voltage winding 3 is the high-voltage winding. The high-voltage with tap-changing structure is improved to a separate high-voltage structure. Compared with the conventional high-voltage structure with tap-changing, the separate high-voltage structure is more stable and has higher reliability. The low-voltage winding 4 is the low-voltage winding, and the winding type is a continuous structure. The low voltage is of small capacity. Since the low-voltage winding 4 is arranged below the medium-voltage winding 2 and the high-voltage winding 3, by changing the relative positions of the low-voltage winding 4, the medium-voltage winding 2, and the high-voltage winding 3 in the magnetic field, and then changing the magnetic coupling relationship between the low-voltage winding 4, the medium-voltage winding 2, and the high-voltage winding 3, the impedances of the medium-voltage winding 2 to the low-voltage winding 4 and the high-voltage winding 3 to the low-voltage winding 4 are both significantly increased, and the short-circuit resistance of the low-voltage winding is greatly improved. And by placing the low-voltage winding 4 below the medium-voltage winding 2 and the high-voltage winding 3 and restricting the low-voltage leads in the gear position with pads, it not only reduces the volume and mass of the transformer windings but also ensures that the low-voltage position does not move, reducing the impact caused by excessive vibration during the operation of offshore wind power. The insulating end ring 5 is an insulating end ring made of insulating materials such as cardboard and is arranged between the low-voltage winding 4, the medium-voltage winding 2, and the high-voltage winding 3. Compared with the conventional overhead outgoing line, the cable outgoing line structure does not need to consider the insulation distance of the bushing, the layout of the outgoing line device is relatively compact, and the external dimensions can be effectively reduced. The medium-voltage neutral point outgoing line device 9 is similar to the medium-voltage outgoing line device 8. The medium voltage is a cable outgoing line, reducing the external dimensions of the transformer. The high-voltage outgoing line device 10 is similar to the medium-voltage outgoing line device 8 and the medium-voltage neutral point outgoing line device 9. The high voltage is a cable outgoing line, reducing the external dimensions of the transformer. The iron core 1 and the oil tank 6 are tightly limited by a plurality of positioning devices 11 to make the iron core 1 and the oil tank 6 an integral structure, avoiding the impact caused by excessive vibration during the operation of offshore wind power.

[0050] Based on the same inventive concept, the present application also provides an offshore wind power generation system. The offshore wind power generation system is connected to the power grid and includes a wind turbine and a fan voltage output subsystem; and a low-frequency transformer as described in any of the above embodiments. The low-frequency transformer is used to step up the electric energy output by the wind turbine and then transmit it to the power grid.

[0051] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope recorded in this specification.

[0052] The above-described embodiments only express several implementation manners of the present utility model. The description is relatively specific and detailed, but it cannot be understood as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the inventive concept of the present utility model, several deformations and improvements can still be made, and these all belong to the protection scope of the present utility model. Therefore, the protection scope of the patent of the present utility model shall be subject to the appended claims.

Claims

1. A low frequency transformer, characterized in that: The transformer is a three-phase five-column structure, comprising: The iron core comprises three main columns, two side columns and four upper and lower iron yokes, wherein the main columns, the side columns and the iron yokes form a frame structure through connecting pieces; Windings, including high-voltage windings, medium-voltage windings and low-voltage windings, wherein the high-voltage windings of each phase are respectively sleeved on the corresponding main poles of each phase; the medium-voltage windings of each phase are concentrically sleeved on the corresponding main poles of each phase with the high-voltage windings of the same phase; the low-voltage windings of each phase are concentrically sleeved on the corresponding main poles of each phase with the high-voltage windings and the medium-voltage windings of the same phase, and are arranged below the high-voltage windings and the medium-voltage windings of the same phase, and an insulating space is provided between the low-voltage windings of each phase and the corresponding medium-voltage windings and high-voltage windings of each phase; An oil tank, in which the core, the high voltage winding, the medium voltage winding and the low voltage winding are arranged, and the oil tank contains insulating oil.

2. The low-frequency transformer according to claim 1, characterized in that: The operating frequency of the low-frequency transformer is no more than 20 Hz.

3. The low-frequency transformer according to claim 1, characterized in that: The number of turns of each high-voltage winding is fixed.

4. The low-frequency transformer according to claim 1, characterized in that: An insulating member is arranged in the insulating interval.

5. The low-frequency transformer according to claim 1, characterized in that: It also includes a high-voltage outlet device, a medium-voltage outlet device and a low-voltage outlet device, wherein the high-voltage outlet device and the medium-voltage outlet device include cables, and the low-voltage outlet device is an air cable box or a plug-in structure.

6. The low-frequency transformer according to claim 1, characterized in that: It also includes a positioning member, which is connected to the iron core and the inner wall of the oil tank and is used to limit the relative movement of the iron core and the oil tank.

7. The low-frequency transformer according to claim 1, characterized in that: The main column, the side column and the iron yoke are composed of stacked grain-oriented cold-rolled silicon steel sheets.

8. The low-frequency transformer according to claim 1, characterized in that: The low voltage winding is a continuous winding.

9. The low-frequency transformer according to claim 1, characterized in that: The connecting parts include clamps, upper beams and side beams; wherein, the clamps connect the main columns, the side columns and the iron yokes of each phase by bolts, and are used to fix the main columns, the side columns and the iron yokes; the upper beams are fixedly connected to the top ends of the main columns and the side columns; the side beams and the upper beams vertically form a frame structure, which is used to provide side support for the iron core and the winding.

10. An offshore wind power generation system, characterized in that: The offshore wind power generation system is connected to a power grid, and includes: Wind generator and wind turbine voltage output subsystems; and, The low-frequency transformer according to any one of claims 1 to 9, wherein the low-frequency transformer is used to boost the electric energy output by the wind turbine and transmit it to the power grid.

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