A comprehensive weight reduction design method for power transformers applicable to low-frequency power transmission systems

Abstract

The present invention relates to a comprehensive weight reduction design method for a power transformer applicable to a low-frequency power transmission system, including: determining various original technical data according to the design specification; determining a reference scheme for the transformer design based on the various original technical data; performing magnetic circuit design optimization for the reference scheme; performing conductive design optimization for the scheme after magnetic circuit design optimization; performing insulation design optimization for the scheme after conductive design optimization to obtain the final design scheme of the transformer. Compared with the prior art, the present invention optimally considers various aspects such as core material, winding design, insulation design, heat dissipation system, leakage reactance, and leakage magnetic field design, and can reliably achieve the comprehensive weight reduction of the transformer.

Description

Technical Field

[0001] The present invention relates to the technical field of transformer design, and in particular to a comprehensive weight reduction design method for a power transformer applicable to a low-frequency power transmission system. Background Art

[0002] With the gradual depletion of fossil energy and the progressive deterioration of the global environment and climate, the low-carbon transformation of the energy structure has become an irresistible trend, and the development and utilization of new energy have become an important development direction. Among them, offshore wind power generation has the advantages of high power generation utilization hours, no occupation of land resources, and small environmental impact, and is an important part of the current development in the field of renewable energy power generation. In recent years, the planning and commissioning of a large number of offshore wind farms have led to the increasing tension of resources in the offshore sea areas, and the development of offshore wind power towards the deep sea is an important direction for the future.

[0003] At present, there are mainly three widely concerned deep-sea wind power transmission technologies, namely: high-voltage alternating current (HVAC) transmission, high-voltage direct current (HVDC) transmission, and low-frequency alternating current (LFAC) transmission technology. Among them, HVAC and HVDC have been widely used in offshore wind power transmission scenarios, but both have some technical defects: HVAC technology is limited by the long-distance submarine cable transmission limit, and HVDC technology has the problem of difficult fault current switching. Therefore, some scholars have pointed out that the LFAC technology (Fractional Frequency Transmission System, FFTS, also known as the fractional frequency power transmission technology), which draws on the idea of DC transmission frequency change and takes advantage of the zero-disconnection of AC transmission, may provide a more economical and effective solution for large-scale deep-sea wind power grid connection. The flexible low-frequency AC transmission technology is a new and efficient AC transmission technology that flexibly selects a suitable frequency between 0 and 50 Hz with the help of power electronics technology to improve the grid transmission capacity and flexible control ability.

[0004] Although the theoretical research and application of low-frequency AC power transmission technology have developed rapidly, for example, some low-frequency power transmission demonstration projects have been built in China, including the 220 kV Zhongbu-Tingshan flexible low-frequency power transmission demonstration project in Hangzhou, Zhejiang, which is oriented to the urban power grid, and the State Grid Zhejiang Taizhou 35 kV flexible low-frequency power transmission demonstration project for low-frequency fan grid connection. However, there are still many obstacles in the engineering application of low-frequency AC power transmission technology in the scenario of long-distance and large-capacity offshore wind power transmission. Low frequency, high voltage level and large capacity bring difficulties to the research and selection of offshore main transformers. If only the conventional idea of increasing the iron core cross-section and winding turns is used to transform the transformer, the weight and volume of the offshore low-frequency main transformer will increase significantly, bringing great challenges to the load and layout of the offshore substation and the economy of the overall offshore wind power low-frequency transmission scheme. In addition, relying only on adjusting the design method, the optimization degree of the transformer volume and weight is extremely limited. Summary of the Invention

[0005] The purpose of the present invention is to overcome the defects of the above-mentioned existing technologies and provide a comprehensive weight reduction design method for power transformers applicable to low-frequency power transmission systems. Optimization considerations are made from multiple aspects such as core materials, winding design, insulation design, heat dissipation systems, leakage reactance and leakage magnetic field design, and the comprehensive weight reduction of the transformer can be reliably achieved.

[0006] The purpose of the present invention can be realized through the following technical solutions: A comprehensive weight reduction design method for power transformers applicable to low-frequency power transmission systems includes the following steps:

[0007] S1. Determine each original technical data according to the design task book;

[0008] S2. Determine the reference scheme for transformer design according to each original technical data;

[0009] S3. Optimize the magnetic circuit design for the reference scheme;

[0010] S4. Optimize the conductive design for the scheme after magnetic circuit design optimization;

[0011] S5. Optimize the insulation design for the scheme after conductive design optimization to obtain the final design scheme of the transformer.

[0012] Furthermore, each original technical data in step S1 includes rated capacity, rated voltage, whether there is a voltage regulation requirement, rated frequency, number of phases, winding connection group label, rated performance parameters, rated operating conditions, cooling method, temperature rise limit, insulation level, noise level, and the rated performance parameters include no-load loss, load loss, no-load current, and short-circuit impedance.

[0013] Furthermore, step S2 specifically includes the following steps:

[0014] S21. Select the silicon steel sheet grade, the working magnetic density of the core column, and the core structure form;

[0015] S22. Calculate the core diameter and select the cross-sections of the core column and yoke;

[0016] S23. Conduct the winding and insulation design;

[0017] S24. Calculate the impedance voltage.

[0018] Further, the specific process of step S21 is as follows: Using the minimum magnetic polarization intensity reached by the core under an alternating magnetic field with a peak value of 800 A / m as the design saturation magnetic density, which is 1.82 - 1.91 T;

[0019] Considering that the transformer can output the rated current at 105% of the rated voltage, and for some special usage cases, it is also required to operate at no more than 110% of the rated voltage, the design magnetic density of the transformer is controlled at and below. The selection range of is 1.55 - 1.74 T. Among them, for medium and small transformers, the selection range of is 1.55 - 1.65 T; for large transformers,

[0020] The transformer core serves as the main magnetic flux path. For single-phase transformers, a single-phase three-column structure is adopted; for three-phase transformers, when the core diameter exceeds 1000 mm, a three-phase five-column structure is selected.

[0021] Further, the specific process of step S22 is as follows:

[0022] Calculate the core diameter, that is: (mm);

[0023] where is the core diameter empirical coefficient. For copper coil windings and cold-rolled silicon steel sheets, 51 - 57 is selected; is the capacity per column of the transformer;

[0024] The core lamination factor is 0.96 - 0.97;

[0025] For medium, large, and extra-large transformers, the core column cross-section selects a multi-stage rectangular cross-section of the circumscribed circle, and a multi-stage elliptical cross-section is used as the yoke cross-section.

[0026] Further, the specific process of step S23 is as follows:

[0027] For the structural form of the winding coil, it includes:

[0028] Layered coils are applicable to low-voltage coils with small capacities, and multi-layered coils are applicable to high-voltage coils with small capacities;

[0029] Spiral coils are applicable to coils with low voltage and large current or voltage regulating coils;

[0030] Interleaved coils: applicable to high-voltage coils with high voltage;

[0031] Interleaved continuous coils belong to the coil structure of partial compensation;

[0032] Inner shielded coils add shielded wires in the line segments to increase their longitudinal capacitance and improve the impulse distribution. With this structure, the high-voltage windings of high-voltage large-capacity transformers can be wound with transposed conductors;

[0033] In order to reduce the load loss of the transformer, the windings are wound with oxygen-free copper wires. The windings of large power transformers use transposed conductors, which can significantly reduce the load loss, reduce the temperature rise of the winding hot spots, improve the mechanical strength of the winding, make the structure more compact, and make the coil processing more convenient;

[0034] The current density of the coil wire mainly depends on the load loss, the temperature rise of the coil, and the dynamic and thermal stability during the sudden short circuit of the secondary side of the transformer. The current density of the copper wire of the low-voltage coil is at 4.5 and below, and the current density of the copper wire of the high- and medium-voltage coils is at 3 ;

[0035] The turn insulation of the high-voltage coil is 1.35 mm, the medium-voltage is 0.6 mm, and the low-voltage is 0.45 mm; the high-voltage oil duct is 5 mm, the medium-voltage oil duct is 4 mm, and the low-voltage oil duct is 3.5 mm; the phase distance is greater than 120 mm. For a three-phase five-column core, the distance from one coil to the side column needs to be increased, greater than 100 mm;

[0036] The power frequency transformer selects an A-level insulation system, calculates its stable temperature rise under rated load, and makes its value not exceed the corresponding limit.

[0037] Furthermore, the specific process of step S24 is as follows:

[0038] The impedance voltage includes two components - the resistance voltage drop and the reactance voltage drop. For large transformers, the resistance voltage drop is negligible. The impedance voltage calculation formula is: , where f is the frequency, IW is the rated current × the total number of turns at the main tap, is the electromotive force per turn, H is the average reactance height of the coil, ∑D is the leakage magnetic width, ρ is the Rockwell coefficient, K is the additional reactance coefficient.

[0039] Further, step S3 specifically reduces the cross-sectional area of the iron core based on the magnetization performance of ferromagnetic materials by adopting the idea of increasing the designed magnetic density, and uses ultra-high magnetic induction grain-oriented silicon steel sheets with a magnetic induction of ≥ 1.93 T. Considering that the transformer can output rated current at 105% of the rated voltage, the designed magnetic density is controlled at .

[0040] Further, step S4 specifically optimizes the conductive design by increasing the current density of the winding copper wire.

[0041] Further, step S5 specifically uses synthetic ester MIDEL 7131 as the transformer insulating oil to reduce the insulation distance and optimize the insulation design.

[0042] Compared with the prior art, the present invention has the following advantages:

[0043] The present invention first determines each original technical data according to the design task book; then determines the benchmark scheme for the transformer design according to each original technical data; and then sequentially optimizes the magnetic circuit design, conductive design, and insulation design for the benchmark scheme to obtain the final design scheme of the transformer. Thus, considering aspects such as the iron core material, winding design, insulation design, heat dissipation system, and leakage magnetic design optimization, the comprehensive weight reduction of the transformer can be reliably achieved.

[0044] When the present invention performs the magnetic circuit optimization design, considering that the low-frequency design of the transformer increases the cross-sectional area of the iron core and the number of winding turns, which in turn leads to an increase in the overall volume and weight of the transformer, based on the magnetization performance of ferromagnetic materials, the idea of increasing the designed magnetic density is adopted to reduce the cross-sectional area of the iron core: using ultra-high magnetic induction grain-oriented silicon steel sheets with a magnetic induction of ≥ 1.93 T. Considering that the transformer can output rated current at 105% of the rated voltage, the designed magnetic density is controlled at .

[0045] When the present invention performs the conductive optimization design, a higher heat resistance grade is adopted to increase the temperature rise limit value of the transformer, allowing a higher temperature rise limit value for the windings of the transformer. The winding copper wire uses a higher current density, and the current density of both the high-voltage coil and the medium-voltage coil can be increased to 4.5 or so, achieving a smaller heat dissipation area, thereby reducing the weight and volume of the transformer.

[0046] When conducting insulation optimization design for the present invention, it is considered that during the normal operation of the transformer, the transformer oil duct plays a very important role, mainly including heat dissipation and insulation. The design and application of MIDEL 7131 as the transformer insulating oil can reduce the oil duct between turns of the high- and medium-voltage winding coils to 3.5 mm, enabling the lightweight and miniaturization of the low-frequency transformer itself. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 is the schematic diagram of the method flow of the present invention;

[0048] Figure 2 is the design flow chart of the reference scheme;

[0049] Figure 3 is the schematic diagram of the magnetic circuit design optimization process;

[0050] Figure 4 is the schematic diagram of the conductive design optimization process;

[0051] Figure 5 is the schematic diagram of the insulation design optimization process;

[0052] Figure 6 is the schematic diagram of the application framework of this scheme. DETAILED DESCRIPTION OF THE INVENTION

[0053] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0054] Embodiment

[0055] As Figure 1 shown, a comprehensive weight reduction design method for a power transformer applicable to a low-frequency power transmission system includes the following steps:

[0056] S1. Determine each original technical data according to the design task book;

[0057] S2. Determine the reference scheme for the transformer design according to each original technical data;

[0058] S3. Conduct magnetic circuit design optimization for the reference scheme;

[0059] S4. Conduct conductive design optimization for the scheme after magnetic circuit design optimization;

[0060] S5. Conduct insulation design optimization for the scheme after conductive design optimization to obtain the final design scheme of the transformer.

[0061] In step S1, before the formal electromagnetic calculation starts, the original conditions for transformer design should be determined according to the design specification first. The main items include: rated capacity, rated voltage, whether there is a voltage regulation requirement, rated frequency, number of phases, winding connection group label, rated performance parameters (no-load loss, load loss, no-load current, short-circuit impedance, etc.), rated operating conditions, cooling method, temperature rise limit, insulation level, noise level, etc.

[0062] In step S2, the reference scheme refers to the transformer design scheme at 50 Hz power frequency under the same conditions. This scheme is a low-frequency and lightweight optimization transformation of a 50 Hz power frequency transformer. As Figure 2 shown, the main contents are:

[0063] 1) Select the silicon steel sheet grade, working magnetic density of the core column, and core structure form

[0064] When designing a transformer, usually the minimum magnetic polarization intensity reached by the core under an alternating magnetic field of 800 A / m (peak value) is used as the design saturation magnetic density. For the currently most commonly used cold-rolled silicon steel sheets, generally it is 1.82 - 1.91 T.

[0065] Considering that the transformer can output the rated current at 105% of the rated voltage, for some special operating conditions, it is also required to operate at no more than 110% of the rated voltage. The design magnetic density of the transformer is generally controlled at and below. In current designs the selection range is 1.55 - 1.74 T. For medium and small transformers, it is generally 1.55 - 1.65 T; for large transformers, it is generally 1.7 - 1.74 T.

[0066] The transformer core is the path of the main magnetic flux. For a single-phase transformer, generally a single-phase three-column structure is adopted; for a three-phase transformer, when the core diameter exceeds 1000 mm, in order to enable the transformer to be transported within the transformer factory and on the highway outside the transformer factory, a three-phase five-column structure is selected in the design.

[0067] 2) Calculate the core diameter and select the cross-sections of the core column and yoke

[0068] Currently in the design, an empirical formula is used to calculate the core diameter, that is: (mm), where —— Empirical coefficient of the core diameter, select 51 - 57 for copper coil windings and cold-rolled silicon steel sheets; —— Capacity per column of the transformer, kVA.

[0069] The core lamination factor is an important factor affecting the effective cross-sectional area of the core column. Currently, due to the continuous improvement of the quality of silicon steel sheets and the continuous improvement of core processing and binding processes, a value of 0.96 - 0.97 can be achieved.

[0070] For medium, large, and extra-large transformers, the circumscribed circle multi-stage rectangular cross-section is selected for the core column cross-section. In order to make the magnetic flux distribution uniform and simplify the process, transformer manufacturers often use multi-stage elliptical cross-sections as the yoke cross-sections during design.

[0071] 3) Winding and insulation design

[0072] The structural forms of winding coils usually include the following:

[0073] Layered coils: Generally applicable to low-voltage coils with small capacities, and multi-layered coils are applicable to high-voltage coils with small capacities.

[0074] Helical coils: Applicable to coils with low voltage and large current or regulating coils.

[0075] Continuous coils: Have a wide range of applications, high mechanical strength, but require high winding technology.

[0076] Interleaved coils: Since it increases the longitudinal capacitance, it thus improves the impulse distribution and is applicable to high-voltage coils of high voltage.

[0077] Interleaved continuous coils: Belong to the coil structure of partial compensation.

[0078] Inner shielded coils: Shielding wires are added in the line segments to increase their longitudinal capacitance and improve the impulse distribution. Using this structure enables the high-voltage windings of high-voltage large-capacity transformers to be wound with transposed conductors.

[0079] In order to reduce the load loss of the transformer, windings are wound with oxygen-free copper wires. The windings of large power transformers use transposed conductors, which can significantly reduce the load loss, reduce the temperature rise of the winding hot spots, improve the mechanical strength of the winding, make the structure more compact, and simplify the coil processing.

[0080] The current density of the coil conductors mainly depends on the load loss, the temperature rise of the coil, and the dynamic and thermal stability during a sudden short circuit on the secondary side of the transformer. Generally, the current density of the copper conductors in low-voltage coils is 4.5 and below, and the current density of the copper conductors in high- and medium-voltage coils is about 3 or so.

[0081] The turn insulation of the high-voltage coil uses 1.35 mm, the medium-voltage uses 0.6 mm, and the low-voltage uses 0.45 mm. The high-voltage oil duct is 5 mm, the medium-voltage oil duct is 4 mm, and the low-voltage oil duct is 3.5 mm. The phase distance needs to be greater than 120 mm. For a three-phase five-column core, the distance from a coil to the side column needs to be increased, greater than 100 mm. Power transformers generally operate continuously for a long time. During design, A-level insulation systems are usually selected for power-frequency transformers, and the stable temperature rise under rated load is calculated to ensure that its value does not exceed the limit in Table 1.

[0082] Table 1 Temperature Rise Limit Table

[0083]

[0084] 4) Impedance voltage

[0085] The impedance voltage includes two components: resistance voltage drop and reactance voltage drop. The resistance voltage drop is generally very small and can be ignored for large transformers. The impedance voltage calculation formula: ; Where: f ——Frequency, Hz; IW ——Rated current × total number of turns at the main tap (data on the same side); ——Electromotive force per turn, (V / turn); H ——Average reactance height of the coil; ∑D——Leakage magnetic width; ρ ——Rochelle coefficient; K ——Additional reactance coefficient.

[0086] The standard short-circuit impedance values of two-winding and three-winding transformers and autotransformers are shown in Table 2 and Table 3 (deviation ±10%).

[0087] Table 2 Standard Short-Circuit Impedance of Two-Winding Power Transformers

[0088]

[0089] Table 3 Standard Short-Circuit Impedance Values of Two-Winding and Autotransformers

[0090]

[0091] The process of step S3 is as Figure 3 shown. The low-frequency design of the transformer increases the cross-sectional area of the iron core and the number of winding turns, which in turn leads to an increase in the overall volume and weight of the transformer. Based on the magnetization performance of ferromagnetic materials, the idea of increasing the design magnetic density is adopted to reduce the cross-sectional area of the iron core. Using ultra-high magnetic induction grain-oriented silicon steel sheets with ≥1.93 T, considering that the transformer can output the rated current at 105% of the rated voltage, the design magnetic density can be controlled at .

[0092] The process of step S4 is as follows Figure 4 shown. By adopting a higher heat resistance level and increasing the temperature rise limit of the transformer, a higher temperature rise limit for the windings of the transformer can be allowed. The copper wire of the windings can then adopt a higher current density, and the current density of both the high-voltage coil and the medium-voltage coil can be increased to about 4.5 .

[0093] The process of step S5 is as follows Figure 5 shown. During the normal operation of the transformer, the transformer oil duct plays a very important role, mainly including heat dissipation and insulation. By applying MIDEL 7131 as the transformer insulating oil, the oil ducts between the turns of the high- and medium-voltage winding coils can be reduced to 3.5 mm

[0094] This solution takes into account that flexible low-frequency power transmission essentially still belongs to AC power transmission, and the voltage level can be adjusted through an AC transformer. The induced electromotive force of the transformer is shown as follows . Taking 20 Hz as an example, when a 50 Hz power frequency transformer operates at 20 Hz, assuming that the iron core will not saturate, if an induced electromotive force equal to that in the power frequency operating state is to be excited, the maximum working magnetic density in the transformer iron core needs to be increased to 2.5 times the original value. However, in the actual operation process, the iron core will be severely saturated, resulting in a sharp increase in no-load loss and no-load current, an increase in the iron core temperature rise, and in severe cases, the transformer will be burned out. Therefore, when designing, it is necessary to increase the iron core cross-section and the number of turns to solve this problem, but this will cause an increase in the iron core, windings, and even the overall volume and weight of the transformer. Therefore, for the weight reduction of low-frequency transformers, this solution mainly starts from aspects such as iron core material, winding design, insulation design, heat dissipation system, and leakage magnetic field design optimization, as Figure 6 shown

[0095] I. Iron Core Material

[0096] 1) Selection of Design Magnetic Density

[0097] Currently, high magnetic induction ( 1.88 T) silicon steel sheets are selected for manufacturing the iron cores of large power transformers. Considering that the weight and volume problems of large-capacity high-voltage offshore low-frequency main transformers have become key issues in the application of offshore wind power low-frequency transmission projects, this solution applies ultra-high magnetic induction ( 1.93 T) silicon steel sheets to further increase the design magnetic density. Considering that the transformer can output the rated current at 105% of the rated voltage, the design magnetic density of the transformer can be selected as .

[0098] 2) Cross-sectional Area of Iron Core Column

[0099] Taking the transformer designed under power frequency conditions (mainly the design of magnetic density, core column diameter, and core column cross-sectional area) as a reference, the core column cross-sectional area of the low-frequency transformer is optimized using ultra-high magnetic induction materials.

[0100] II. Winding Design

[0101] In the design of the transformer winding, the number of turns of its coil and the structural height are mainly considered. For the cabin transformer with limited installation environment, it is required that the number of turns of the transformer winding and the structural height remain unchanged or decrease; for the step-up transformer not restricted by the installation environment, the number of turns of the winding coil and the structural height can be appropriately increased.

[0102] III. Insulation Design

[0103] The main functions of the insulating oil in the oil-immersed transformer are insulation and heat dissipation. According to the types of base oils, it can be divided into 4 categories, namely mineral oil, silicone oil, synthetic ester, and natural ester (commonly known as vegetable oil). The technology of using mineral oil as the transformer insulating oil is mature and has been widely used. Among them, #10, #25, and #45 oils are all commonly used mineral insulating oils in the transformer industry. With the continuous development of the research and application technology of ester-based high flash point insulating oils, ester-based high flash point insulating oil transformers have been considered a new type of safe and environmentally friendly transformer. This scheme uses synthetic ester (MIDEL 7131) as the transformer insulating oil. MIDEL7131 insulating oil is specially developed for traditional oil-immersed transformers and is a safer alternative insulating oil. It has excellent high and low temperature resistance, better fire safety, is more environmentally friendly, and has excellent antioxidant and moisture resistance. Its breakdown voltage is higher than that of mineral oil. Using MIDEL 7131 insulating oil can reduce the insulation size and achieve the lightweight and miniaturization of the low-frequency transformer itself.

[0104] IV. Heat Dissipation System

[0105] Adopt an overall high heat resistance design

[0106] Adopt insulating materials with higher heat resistance grades (B grade, F grade, H grade) to increase the temperature rise limit value of the transformer, which can allow higher temperature rise limit values and smaller heat dissipation areas for the core and winding of the transformer, thereby reducing the weight and volume of the transformer.

[0107] Optimize the heat dissipation layout

[0108] When designing the core, calculate the core temperature rise. Under the condition of meeting the allowable temperature rise of the insulation system: ① Do not set oil ducts inside the core; ② Reduce the number of oil ducts inside the core according to the power frequency situation; ③ Reduce the thickness of the oil ducts.

[0109] When designing the coil winding, reduce the thickness of the inter-turn oil ducts.

[0110] For the cabin modification, the radiators are scattered throughout the cabin to balance the cabin weight.

[0111] Adopt an efficient cooling method to reduce volume and weight

[0112] The oil-immersed transformer adopts a forced oil circulation air-cooling scheme to accelerate the oil flow rate in the transformer, improve the heat dissipation efficiency, and cool the hot oil through a heat exchanger and then send it back into the transformer, reducing the product volume.

[0113] V. Optimization of leakage reactance and leakage magnetic flux design

[0114] Leakage magnetic flux refers to the magnetic field escape caused by the incomplete magnetic circuit in the transformer, resulting in magnetic flux. This part of the magnetic flux does not pass through the iron core or the coil, so it cannot provide effective power to the load and only causes energy loss. The generation of leakage magnetic flux is related to the structure of the transformer. For example, factors such as the gap between the iron core and the coil and the change in the number of coil turns will affect the magnitude of the leakage magnetic flux. To reduce the impact of leakage magnetic flux, the following measures need to be taken:

[0115] Optimize the structural design of the transformer

[0116] By reasonably designing the transformer structure, reduce the gap between the iron core and the coil and the change in the number of coil turns, thereby reducing the generation of leakage magnetic flux.

[0117] Adopt suitable materials

[0118] Select suitable insulating materials, which can reduce the thickness of the insulating layer, thereby narrowing the leakage magnetic flux path and reducing the occurrence of leakage reactance. To verify the effectiveness of this scheme, in this embodiment, weight reduction designs are carried out for single-phase transformers and three-phase transformers respectively. Among them, the product specifications and technical requirements of the single-phase transformer are:

[0119]

[0120] After applying this scheme for optimized design, the basic parameters are compared as follows:

[0121]

[0122] Through the low-frequency and lightweight transformation of the single-phase transformer by this scheme, compared with the power-frequency transformer, the volume increases by 44%; compared with the 20Hz low-frequency transformer designed by the conventional method, the weight is reduced by 15.04% and the load loss increases by 10.5%. Because of the high loss, a high-temperature insulation system is adopted, and the product size is reduced. The product specifications and technical requirements of the three-phase transformer are:

[0123]

[0124] After applying this scheme for optimized design, the basic parameters are compared as follows:

[0125]

[0126] Through this solution, the three-phase transformer is transformed to be lower-frequency and lighter. Compared with the power-frequency transformer, the volume increases by 49.23%; compared with the 20Hz low-frequency transformer designed by the conventional method, the weight is reduced by 18.67%, and the load loss increases by 8.69%.

Claims

1. An integrated weight reduction design method for power transformers applicable to low-frequency power transmission systems, characterized in that, It includes the following steps: S1. Determine each original technical data according to the design task book; S2. Determine the benchmark scheme for transformer design according to each original technical data; S3. Optimize the magnetic circuit design for the benchmark scheme; S4. Optimize the conductive design for the scheme after magnetic circuit design optimization; S5. Optimize the insulation design for the scheme after conductive design optimization to obtain the final design scheme of the transformer; The specific steps of step S2 include the following steps: S21. Select the silicon steel sheet grade, the working magnetic density of the core column and the core structure form; S22. Calculate the core diameter and select the cross-sections of the core column and the yoke; S23. Conduct winding and insulation design; S24. Calculate the impedance voltage; The specific process of the step S21 is as follows: taking the minimum magnetic polarization intensity B reached by the iron core under an alternating magnetic field with a peak value of 800 A / m 800 as the designed saturation magnetic flux density, B 800 is 1.82 to 1.91 T; Considering that the transformer can output the rated current at 105% of the rated voltage, for some special applications, it is also required to operate at no more than 110% of the rated voltage, and the designed magnetic density of the transformer is controlled at and below. The selection range of the designed B m is 1.55 - 1.74 T. Among them, for medium and small transformers, the selection range of B m is 1.55 - 1.65 T; for large transformers, the selection range of B m is 1.7 - 1.74 T; The transformer core serves as the channel for the main magnetic flux. For single-phase transformers, a single-phase three-column structure is adopted; for three-phase transformers, when the core diameter exceeds 1000 mm, a three-phase five-column structure is selected; Specifically, in step S3, based on the magnetization performance of ferromagnetic materials, the idea of increasing the designed magnetic density is adopted to reduce the cross-sectional area of the iron core. Ultra-high magnetic induction grain-oriented silicon steel sheets with B 800 ≥1.93 T are used. Considering that the transformer can output rated current at 105% of the rated voltage, the designed magnetic density is controlled at Specifically, the step S4 is to optimize the conductive design by increasing the current density of the winding copper wire, and increase the current densities of the high-voltage coil and the medium-voltage coil to 4.5 A / mm 2 ; The specific step S5 is to apply synthetic ester MIDEL 7131 as the transformer insulating oil to reduce the insulation distance, reduce the oil ducts between turns of the high- and medium-voltage winding coils to 3.5 mm, and achieve insulation design optimization.

2. The integrated weight reduction design method of a power transformer applicable to a low-frequency power transmission system according to claim 1, characterized in that, Each original technical data in step S1 includes rated capacity, rated voltage, whether there is a voltage regulation requirement, rated frequency, number of phases, winding connection group label, rated performance parameters, rated operating conditions, cooling method, temperature rise limit, insulation level, and noise level. The rated performance parameters include no-load loss, load loss, no-load current, and short-circuit impedance.

3. A comprehensive weight reduction design method for a power transformer applicable to a low-frequency power transmission system according to claim 1, characterized in that, The specific process of step S22 is as follows: Calculate the core diameter, i.e.: Among them, K D is the empirical coefficient of the core diameter, and 51-57 is selected for the copper coil winding and cold-rolled silicon steel sheet; S Z is the capacity per column of the transformer; The core lamination factor is 0.96 - 0.97; For medium, large, and extra-large transformers, the cross-section of the core column is selected as a multi-stage rectangular cross-section of the circumscribed circle, and a multi-stage elliptical cross-section is used as the cross-section of the side yoke.

4. A comprehensive weight reduction design method for a power transformer applicable to a low-frequency power transmission system according to claim 3, characterized in that The specific process of step S23 is as follows: For the structural form of the winding coils, it includes: Layered coils, suitable for low-voltage coils with small capacity, and multi-layered coils are suitable for high-voltage coils with small capacity; Spiral coils, suitable for coils with low voltage and large current or voltage regulating coils; Interleaved coils: suitable for high-voltage coils with high voltage; Interleaved continuous coils, which belong to the coil structure of partial compensation; Inner shielded coils, adding shielded wires in the segments to increase their longitudinal capacitance and improve the impulse distribution. With this structure, the high-voltage windings of high-voltage large-capacity transformers can be wound with transposed conductors; In order to reduce the load loss of the transformer, the windings are wound with oxygen-free copper wires. The windings of large power transformers use transposed conductors, which can significantly reduce the load loss, reduce the temperature rise of the winding hot spots, improve the mechanical strength of the winding, make the structure more compact, and make the coil processing more convenient; The current density of the coil conductor mainly depends on the load loss, the temperature rise of the coil, and the dynamic and thermal stability during a sudden short circuit on the secondary side of the transformer. The current density of the copper conductor of the low-voltage coil is 4.5 A / mm 2 and below, and the current density of the copper conductor of the high- and medium-voltage coils is 3 A / mm 2 ; The turn insulation of the high-voltage coil is 1.35 mm, the medium-voltage is 0.6 mm, and the low-voltage is 0.45 mm; the high-voltage oil duct is 5 mm, the medium-voltage oil duct is 4 mm, and the low-voltage oil duct is 3.5 mm; the phase distance is greater than 120 mm, and for the three-phase five-column core, the distance from one coil to the side column needs to be increased, greater than 100 mm; The power frequency transformer selects an A-level insulation system and calculates its stable temperature rise under rated load so that its value does not exceed the corresponding limit.

5. A comprehensive weight reduction design method for a power transformer applicable to a low-frequency power transmission system according to claim 4, characterized in that The specific process of step S24 is as follows: The impedance voltage includes two components - the resistance voltage drop and the reactance voltage drop. For large transformers, the resistance voltage drop can be neglected. The calculation formula for the impedance voltage is as follows: where f is the frequency, IW is the rated current × the total number of turns at the main tap, e t is the electromotive force per turn, H is the average reactance height of the coil, ∑D is the leakage magnetic width, ρ is the Rockwell coefficient, and K is the additional reactance coefficient.

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