Transformer core state diagnosis method based on magnetostrictive characteristic and magnetic flux distribution

Abstract

This invention relates to a method for diagnosing the condition of a transformer core based on magnetostrictive properties and magnetic flux distribution. It includes: (1) establishing a database of magnetostrictive properties of commonly used silicon steel sheet materials; (2) arranging vibration measurement points in the tank considering the main magnetic flux distribution; (3) measuring and calculating the per-unit values ​​of vibration acceleration at each measurement point under different excitation conditions; (4) determining the per-unit value ratio of vibration; and (5) measuring and providing early warning of the core and its frame clamping structure during operation. This invention combines specific material magnetostrictive property data with product vibration characteristics, fully considering the influence of material properties; it considers the specific measurement point arrangement for core vibration under the main magnetic flux distribution, accurately judging the stability or abnormality of vibration distribution and mechanical structure, providing a reference for fault location; and it establishes the correspondence between specific measured vibrations under different excitation voltages, material properties, and core clamping structures, realizing overall mechanical condition judgment and fault location.

Description

Technical Field

[0001] This invention relates to the field of transformer mechanical condition diagnosis, and specifically to a method for diagnosing the condition of transformer cores based on magnetostrictive properties and magnetic flux distribution. Background Technology

[0002] The core and its frame are crucial components of the transformer's mechanical structure, serving as the framework for the magnetic circuit and the transformer body. Structurally, the core's clamping device not only makes the laminated magnetic conductor a mechanically complete structure but also houses insulated coils that support the leads. It houses almost all the internal components of the transformer and bears the weight of the windings. During transient events such as transformer short-circuit impacts, the reaction force of the windings impacts the core frame, potentially causing changes in the core frame's mechanical clamping structure. Simultaneously, the core is also the main magnetic flux path, converting electrical energy from the primary circuit into magnetic energy and then its own magnetic energy into electrical energy for the secondary circuit, acting as the medium for energy conversion. Under special operating conditions such as normal no-load operation, load operation, and overexcitation, it generates vibration and heat. Its insulation, grounding, and mechanical stability are interconnected, and its overall condition can be indirectly judged through its mechanical state. The mechanical stability of the core and its frame structure directly affects the safe operation of the transformer.

[0003] The clamping structure of the iron core is a fastening structure that makes the laminated magnetic conductors a whole, and it should meet the following requirements:

[0004] (1) The frame of the clamping structure shall bear the clamping force, the weight of the lifting body and the mechanical force generated when the transformer is short-circuited, so as to ensure the electromagnetic performance of the silicon steel sheet. (2) The clamping structure shall be able to reliably clamp the coil, support the lead wire, arrange the insulation of the transformer body, and have a transformer body positioning device. (3) The clamping force shall be uniform, the edge of the iron core sheet shall not be warped, the joint shall be tight, and the excitation noise shall be low. (4) In order to reduce the eddy current loss generated by leakage flux in the structural components and prevent the iron core from being grounded at multiple points, the structural components shall be separated from the iron core body by insulating components, and the structural components shall not link the main magnetic flux to form a short circuit turn, but the clamping components and the side screw or side beam can form a closed loop, link the zero sequence magnetic flux and flow the zero sequence current. (5) The insulating components shall be provided with oil channels for heat dissipation.

[0005] The silicon steel sheets used in laminated iron cores have magnetostrictive properties. Different grades of silicon steel sheets have different magnetostrictive curves. The characteristics of the silicon steel sheets used in production can be obtained through performance testing. The magnetostriction of the silicon steel sheet material during excitation is the root cause of core vibration.

[0006] Insufficiency of existing technology:

[0007] (1) Mechanical vibration is calculated and analyzed by methods such as finite element method. However, the accuracy of the calculation is affected by material properties and specific structures such as transformer pull plate, clamp, pull belt, and binding belt, as well as bottom positioning and lamination process. The accuracy of the calculation is affected by many factors and errors.

[0008] (2) There are many discussions on the influence of material properties on transformer mechanical vibration and noise. However, the differences in measured values ​​are often not considered in detail in calculations. There are few studies on the specific impact of changes in material properties on transformer body vibration and the correlation between material properties and manufacturing quality.

[0009] (3) There is little discussion on the analysis of the fault modes of loosening and abnormal mechanical state of the iron core in combination with material properties and transformer main magnetic flux distribution. Summary of the Invention

[0010] The purpose of this invention is to address the shortcomings of existing technologies by providing a method for diagnosing the condition of transformer cores based on magnetostrictive properties and magnetic flux distribution.

[0011] To achieve the above objectives, the technical solution of the present invention is: a method for diagnosing the condition of a transformer core based on magnetostrictive characteristics and magnetic flux distribution, comprising the following steps:

[0012] (1) Establish a database of magnetostrictive properties of commonly used silicon steel sheet materials;

[0013] The analysis of the transformer uses the measured magnetostriction characteristic curve of the transformer material. The measurement range of the magnetostriction characteristic curve is not less than 1.9T magnetic flux density. The magnetostriction curve is fitted with data using a polynomial form as shown in formula (1), with a fitting coefficient > 0.99. The material properties are calculated per unit value based on 1.7T magnetic flux density, including at least the measured values ​​of magnetic flux density B of 1.7, 1.75, 1.8, 1.85, 1.9, and 1.95T. The per unit value is calculated based on the magnetostriction rate at each vibration measurement value of 1.7T, λ1.7*=1, λ1.8*=λ1.8 / λ1.7, λ1.9*=λ1.9 / λ1.7, ... When the measurement data of the measurement points is insufficient within the measurement range, the fitted curve is used to complete the data.

[0014] λ=k1*B 6 +k2*B 5 +k3*B 4 +k4*B 3 +k5*B 2 +k6*B+k0 (1)

[0015] In the formula: k0~k6 are polynomial coefficients.

[0016] (2) Arrangement of vibration measuring points in oil tank considering the distribution of main magnetic flux;

[0017] Vibration measuring points are arranged on the transformer tank wall at the horizontal position of the upper and lower yoke joints and the middle of the side yoke, corresponding to the main column and the side column on the top of the tank, with the corresponding transformer body position in perspective.

[0018] Estimate the state of the transformer core joints, considering the joints δ1 between the side yoke and the main column, and δ2 between the upper and lower main yokes and the main column. The magnetic flux distribution in the three-phase five-column core is as follows: main column fluxes Ф1, Ф2, and Ф3 are in the same direction as the side yokes Ф4 and Ф5, and upper and lower main yokes Ф6 and Ф7. Ф3 and Ф6 reach their peak values ​​in the same direction at Ф3, while Ф3 and Ф4 reach their peak values ​​in opposite directions. At the peak value of Ф3, Ф3, Ф4, and Ф6 are at their peak values. Considering the concentration of magnetic flux at the core corners, use λ. p-p To estimate the expansion and contraction at the joint, assuming the angle between the joint and the horizontal direction is 45°, the magnetostriction coefficients λ1 and λ2 at joints δ1 and δ2 are:

[0019] λ1=(L1+L2)*λ p-p *sin(45°) (2)

[0020] λ2=(L1+L3)*λ p-p *sin(45°) (3)

[0021] In the formula, the core height is L1, the side yoke length is L2, the upper and lower main yoke lengths are L3, and λ p-p This represents the peak-to-peak value of the magnetostriction.

[0022] The core joint structure and process vary between different products and manufacturers. The magnetic flux density at the joint is bent, resulting in significant vibration. Uniformly clamping the silicon wafers at this point is one of the challenges in structural design and process, and this is a point of concern regarding vibration.

[0023] (3) Measure the per-unit value of vibration acceleration at each measuring point under different excitation conditions;

[0024] Measure the peak vibration acceleration at various distributed measuring points under different excitation conditions, including at least the main magnetic flux density of 1.7, 1.75, 1.8, 1.85, 1.9, and 1.95 T. Calculate the per-unit value based on the vibration acceleration measurement value of 1.7 T at each point. 1.7 * =1,a 1.8 * =a 1.8 / a 1.7 a 1.9 * =a 1.9 / a 1.7 , ...;

[0025] The main cause of core vibration is the magnetostriction of the silicon steel sheets; based on the principle of electromagnetic induction and the magnetostrictive properties of ferromagnetic materials, the vibration acceleration 'a' caused by the magnetostriction of the core can be obtained. c for:

[0026]

[0027] In the formula, L represents the original size of the ferromagnetic material; λ c The magnetostriction of the silicon steel sheet is given by the excitation voltage U; U is the amplitude of the power supply voltage; N is the number of turns in the primary winding; S is the cross-sectional area of ​​the core; B c ω represents the magnetic flux density under the core excitation voltage; ω represents the power supply frequency.

[0028] (4) Judgment of vibration per-unit value ratio;

[0029] Based on vibration acceleration a c The proportional relationship between the vibration acceleration and the power supply voltage amplitude U, and the decrease in vibration acceleration after the silicon steel sheets in the iron core are restricted by being tightly bound, are analyzed and judged based on the ratio K of magnetostriction to acceleration. The values ​​of K under different excitation voltages are as follows:

[0030] K 1.7 =λ 1.7 * / a 1.7 * ,

[0031] K 1.75 =λ 1.75 * / a 1.75 * ,

[0032] K 1.8 =λ 1.8 * / a 1.8 * ,

[0033] K 1.85 =λ 1.85 * / a 1.85 * ,

[0034] K 1.9 =λ 1.9 * / a 1.9 * ,

[0035] K 1.95 =λ 1.95 * / a 1.95 *,

[0036] The K value reflects the limiting effect of the core clamping structure on magnetostrictive vibration. A K value > 2 indicates that the magnetostrictive rate of the vibration acceleration at this measuring point is limited to less than 50% under the clamping state. When the K value at the measuring point is < 2, the magnetostriction at the measuring point is not effectively limited, and the clamping structure or clamping force may be insufficient.

[0037] (5) Measurement and early warning of the core and its frame clamping structure during operation:

[0038] During transformer operation, the transformer is subjected to lightning strikes, short-circuit current strikes, and overexcitation. The winding clamping state and core frame of the transformer may become loose under the excitation of large electrodynamic force or excitation magnetic flux. The vibration measurement method in steps (1) to (4) is compared with the vibration measurement point acceleration at the beginning of operation. The vibration acceleration change rate at the measurement point is <50%, and the K value is >2, indicating that the core frame clamping state is good.

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] (1) Combine specific material magnetostrictive property data with product vibration characteristics, and fully consider the influence of material properties;

[0041] (2) Consider the specific measurement point arrangement of the core vibration under the main magnetic flux distribution, accurately judge the vibration distribution and the stability or abnormality of the mechanical structure, and provide a reference for fault location.

[0042] (3) Establish the correspondence between the material properties and the core clamping structure under different excitation voltages for specific vibration measurements, so as to realize the overall mechanical condition judgment and fault location. Attached Figure Description

[0043] Figure 1 This invention provides a diagnostic process for the mechanical condition of transformer cores based on the magnetostriction and main magnetic flux distribution characteristics of materials.

[0044] Figure 2 This indicates the location of the core frame corresponding to the vibration measurement point of the oil tank.

[0045] Figure 3 It has a three-phase, five-column core with magnetic flux distribution.

[0046] Figure 4 This is the waveform of the flux curve of a three-phase five-limb transformer.

[0047] Figure 5 The magnetostriction curve of a certain grade of silicon steel sheet with a thickness of 0.23 mm is shown.

[0048] Figure 6 A comparison chart of per-unit values ​​for vibration acceleration and magnetostriction.

[0049] Figure 7 The ratio K of magnetostriction / vibration acceleration at measurement points 1-6 is given. Detailed Implementation

[0050] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings.

[0051] like Figure 1 As shown, this invention provides a method for diagnosing the condition of a transformer core based on magnetostrictive properties and magnetic flux distribution, comprising the following steps:

[0052] (1) Establish a database of magnetostrictive properties of commonly used silicon steel sheet materials;

[0053] (2) Arrangement of vibration measuring points in oil tank considering the distribution of main magnetic flux;

[0054] (3) Measure and calculate the per-unit value of vibration acceleration at each measuring point under different excitation conditions;

[0055] (4) Judgment of vibration per-unit value ratio;

[0056] (5) Measurement and early warning of the core and its frame clamping structure during operation.

[0057] The following is a detailed implementation process of the present invention.

[0058] (1) Establish a database of magnetostrictive properties of commonly used silicon steel sheet materials.

[0059] The analysis of specific transformer products uses the measured magnetostriction characteristic curve of the product material. The measurement range of the magnetostriction characteristic curve shall not be less than 1.9T magnetic flux density. The magnetostriction curve is fitted with data in a polynomial form, formula (1), with a fitting coefficient > 0.99. The material properties are calculated per unit value based on 1.7T magnetic flux density, including at least the measured values ​​of magnetic flux density B of 1.7, 1.75, 1.8, 1.85, 1.9, and 1.95T. The per unit value is calculated based on the magnetostriction rate at each vibration measurement value of 1.7T, λ1.7*=1, λ1.8*=λ1.8 / λ1.7, λ1.9*=λ1.9 / λ1.7, ... When the measurement point data is insufficient within the measurement range, the fitted curve can be used to supplement the data.

[0060] λ=k1*B 6 +k2*B 5 +k3*B 4 +k4*B 3 +k5*B 2 +k6*B+k0 (1)

[0061] In the formula: k0~k6 are polynomial coefficients. (1)

[0062] (2) Arrangement of vibration measuring points in oil tank considering the main magnetic flux distribution

[0063] Vibration measuring points are arranged on the oil tank wall at the horizontal positions of the upper and lower yoke joints and the middle of the side yoke, corresponding to the iron core, and at the vertical positions of the main column and side column on the top of the oil tank. These points are positioned in perspective relative to the corresponding positions on the tank body, avoiding the reinforcing iron of the oil tank. The locations of the vibration measuring points are shown in the attached figure. Figure 2 .

[0064] Estimate the condition of the transformer core joints, including the joint δ1 between the side yoke and the main column, and the joints δ2 between the upper and lower main yokes and the main column. The magnetic flux distribution of the three-phase five-limb core is shown in the attached figure. Figure 3 The phase relationships between the main column fluxes Ф1, Ф2, Ф3 and the side yokes Ф4, Ф5, and the upper and lower main yokes Ф6, Ф7 are shown in the attached figure. Figure 4 Ф3 and Ф6 reach their peak values ​​in the same direction at Ф3, while Ф3 and Ф4 reach their peak values ​​in opposite directions at Ф3. At this moment, Ф3, Ф4, and Ф6 are essentially at their peak values. Considering that the magnetic flux density is concentrated at the core corner, λ is used. p-p To estimate the expansion and contraction at the joint, assuming the angle between the joint and the horizontal direction is 45°, the magnetostriction coefficients λ1 and λ2 at joints δ1 and δ2 are:

[0065] λ1=(L1+L2)*λ p-p *sin(45°) (2)

[0066] λ2=(L1+L3)*λ p-p *sin(45°) (3)

[0067] In the formula, the core height is L1, the side yoke length is L2, the upper and lower main yoke lengths are L3, and λ p-p This represents the peak-to-peak value of the magnetostriction.

[0068] The core joint structure and process vary between different products and manufacturers. The magnetic flux density at the joint is bent, resulting in significant vibration. Uniformly clamping the silicon wafers at this point is one of the challenges in structural design and process, and this is a point of concern regarding vibration.

[0069] Considering manufacturing deviations, the joint control values ​​δ1 and δ2 should not be too small, and space should be left for magnetostriction. Vibration acceleration has a higher value in this area. It is recommended that the joint gap control values ​​δ1>λ1+Δ and δ2>λ2+Δ, where Δ is the margin value.

[0070] (3) Measure the per-unit value of vibration acceleration at various measuring points under different excitation conditions.

[0071] Measure the peak vibration acceleration at various distributed measuring points under different excitation conditions, including at least the main magnetic flux density of 1.7, 1.75, 1.8, 1.85, 1.9, and 1.95 T. Calculate the per-unit value based on the vibration acceleration measurement value of 1.7 T at each point.1.7 * =1,a 1.8 * =a 1.8 / a 1.7 a 1.9 * =a 1.9 / a 1.7 , ...;

[0072] The main cause of core vibration is the magnetostriction of the silicon steel sheets. Based on the principle of electromagnetic induction and the magnetostrictive properties of ferromagnetic materials, the vibration acceleration 'a' caused by the magnetostriction of the core can be calculated. c for:

[0073]

[0074] In the formula, L represents the original size of the ferromagnetic material; λ c The magnetostriction of the silicon steel sheet is given by the excitation voltage U; U is the amplitude of the power supply voltage; N is the number of turns in the primary winding; S is the cross-sectional area of ​​the core; B c ω represents the magnetic flux density under the core excitation voltage; ω represents the power supply frequency.

[0075] (4) Vibration per-unit value ratio judgment

[0076] Based on vibration acceleration a c The proportional relationship with U, formula (4), and the vibration acceleration decreases after the silicon steel sheets of the iron core are bound. Based on the ratio of magnetostriction to acceleration K, the clamping state of the iron core is analyzed and judged. Under different excitation voltages, the values ​​of K are as follows:

[0077] K 1.7 =λ 1.7 * / a 1.7 * ,

[0078] K 1.75 =λ 1.75 * / a 1.75 * ,

[0079] K 1.8 =λ 1.8 * / a 1.8 * ,

[0080] K 1.85 =λ 1.85 * / a 1.85 * ,

[0081] K 1.9 =λ 1.9 * / a 1.9 * ,

[0082] K 1.95 =λ 1.95 * / a 1.95 * ,

[0083] The K value reflects the limiting effect of the core clamping structure on magnetostrictive vibration. A K value > 2 or greater indicates that the magnetostrictive rate of the vibration acceleration at this measuring point is limited to less than 50% under the clamping state. When the K value at the measuring point is < 2, the magnetostriction at the measuring point is not effectively limited, and the clamping structure or clamping force may be insufficient.

[0084] (5) Measurement and early warning of the core and its frame clamping structure during operation

[0085] During transformer operation, transformers are subjected to various lightning strikes, short-circuit current surges, overexcitation, etc. The winding clamping state and core frame of the transformer may become loose under the excitation of large electrodynamic forces or magnetic flux density. The vibration measurement method mentioned above can be used to compare the acceleration of the vibration measurement point at the beginning of operation. If the change rate of vibration acceleration at the measurement point is <50% and the K value is >2, it indicates that the core frame clamping state is good at this point.

[0086] Case Study on Clamping Status Analysis of Iron Core and its Frame

[0087] (1) Magnetostriction characteristics of a commonly used silicon steel sheet material

[0088] λ = 1931.7 * B 6 -9848.7*B 5 +18632*B 4 -15856*B 3 +5865*B 2 -817.36*B

[0089] The measurement curves of the magnetostrictive properties of the material are attached. Figure 5 The measurement results for the two samples are shown in Table 1. The arrangement of measuring points on the tank wall is shown in the attached figure. Figure 2 (Figure (a) shows the height position of the oil tank vibration measuring point in perspective view of the iron core and frame (front view),) Figure 2 (b) is Figure 2 (a) Top view, Figure 2 In (a) and (b), points 1-15 are measuring points, all of which are arranged on the surface of the oil tank P, and are connected to the iron core Q (the iron core consists of multiple pieces such as...). Figure 2 (a) shows thick iron sheets with openings stacked together. Figure 2 (b) The corresponding position of the thickness) is shown in Table 2, where the vibration acceleration measured at measuring points 1 to 6 is shown in Table 2. Figure 6 This is a comparison chart of the per-unit average values ​​of vibration acceleration and magnetostriction. Magnetostriction is the vibration characteristic of the material itself, measured without pressure or clamping force. Vibration acceleration is the measured value of the product. The core is assembled from silicon steel sheets into product components, forming an assembly through clamps, binding steel straps, etc. The material is clamped at multiple points. After the magnetostriction characteristic of the material is subjected to clamping force, the vibration expansion rate is restricted. As can be seen from the chart, with the increase of voltage, the increase of vibration at the product measuring point is much smaller than the increase of magnetostriction of the material itself. The ratio of magnetostriction to vibration acceleration at each measuring point shows the constraint of the material characteristics at that point after being subjected to clamping force. Figure 7 The graph represents the ratio of magnetostriction to vibration acceleration, K. This graph clearly shows the ratio of magnetostriction to vibration acceleration, indicating the restriction on magnetostriction after the core is clamped. It can also plot the changes in K values ​​at each measuring point, analyze the restriction on vibration at each point under different voltages, and determine the mechanical clamping state at each point.

[0090] Table 1. Measurement and analysis results of magnetostriction of a certain grade 23 silicon steel sheet.

[0091] <![CDATA[λ 1.7 ]]> -20 56 18 54 <![CDATA[λ 1.8 ]]> 168 242 205 52 <![CDATA[λ 1.9 ]]> 708 686 697 16 <![CDATA[λ min ]]> -138 -96 -117 30 <![CDATA[λ p-p ]]> 846 782 814 46 <![CDATA[λ 1.7-1.8 ]]> -188 -186 -187 2

[0092] Table 2 Analysis of Average Values ​​of Clamping Status of Core and Frame

[0093]

[0094] * indicates per-unit value

[0095] Estimate the magnitude of magnetostriction in the iron core corner region:

[0096] L1 = 2800, L2 = 600, L3 = 1500, the magnetostriction is:

[0097] λ1=(2800+600)*814*10-6*sin(45°)=1.96mm

[0098] λ2=(2800+1500)*814*10-6*sin(45°)=2.48mm

[0099] It is recommended that the gaps at δ1 and δ2 be greater than the magnetostriction amount, with a certain margin.

[0100] The K value at measuring point 3 is relatively small across the entire voltage range of 1.03 to 1.16 times, indicating potentially insufficient clamping force. The analysis results for measuring points 2 and 3 are shown in Table 3, and the K value analysis results for all measuring points 1-6 are shown in Table 4. The K value at measuring point 4 deviates significantly from the average value; therefore, a detailed analysis of the clamping structure at this location is necessary, comparing it with measurements from similar and surrounding locations to determine if there is a possibility of excessive pressure or uneven force distribution.

[0101] Table 3. Analysis results of clamping status at measuring points 2 and 3.

[0102]

[0103] * indicates per-unit value

[0104] Table 4. Analysis results of clamping status at measuring points 1-6

[0105]

[0106] The above are preferred embodiments of the present invention. Any changes made to the technical solution of the present invention that do not exceed the scope of the technical solution of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for diagnosing the condition of a transformer core based on magnetostrictive properties and magnetic flux distribution, characterized in that, Includes the following steps: (1) Establish a database of magnetostrictive properties of commonly used silicon steel sheet materials; (2) Based on the main magnetic flux distribution of the three-phase five-column core, vibration measuring points are arranged on the transformer tank wall at the horizontal position of the upper and lower yoke joints and the middle of the side yoke, and at the vertical position of the main column and the side column on the top of the tank, so that each vibration measuring point is in perspective relative to the corresponding transformer body position. (3) Measure and calculate the per-unit values ​​of vibration acceleration at each measuring point under different excitation conditions; (4) Judgment of vibration per-unit value ratio; (5) Vibration measurement and early warning of the core and its frame clamping structure during operation; The specific implementation of step (4) is as follows: Based on vibration acceleration a c The proportional relationship between the vibration acceleration and the power supply voltage amplitude U, and the decrease in vibration acceleration after the silicon steel sheets in the iron core are restricted by being tightly bound, are analyzed and judged based on the ratio K of magnetostriction to acceleration. The values ​​of K under different excitation voltages are as follows: K 1.7 =λ 1.7 * / a 1.7 * , K 1.75 =λ 1.75 * / a 1.75 * , K 1.8 =λ 1.8 * / a 1.8 * , K 1.85 =λ 1.85 * / a 1.85 * , K 1.9 =λ 1.9 * / a 1.9 * , K 1.95 =λ 1.95 * / a 1.95 * , In the above formula, the subscripts represent the K values ​​at main magnetic flux density of 1.7, 1.75, 1.8, 1.85, 1.9, and 1.95 T, respectively. The K value reflects the limiting effect of the core clamping structure on magnetostrictive vibration. A K value > 2 indicates that the magnetostrictive rate at this measuring point is limited to less than 50% under the clamping state. When the K value at the measuring point is < 2, the magnetostriction at the measuring point is not effectively limited, and the clamping structure or clamping force may be insufficient. The specific implementation of step (5) is as follows: During transformer operation, the transformer is subjected to lightning strikes, short-circuit current strikes, and overexcitation. The winding clamping state and core frame of the transformer may become loose under the excitation of large electrodynamic force or excitation magnetic flux. The vibration measurement method in steps (1) to (4) is compared with the vibration measurement point acceleration at the beginning of operation. The vibration acceleration change rate at the measurement point is <50%, and the K value is >2, indicating that the core frame clamping state is good.

2. The transformer core condition diagnosis method based on magnetostrictive properties and magnetic flux distribution according to claim 1, characterized in that, The specific implementation of step (1) is as follows: The analysis of the transformer uses the measured magnetostriction characteristic curve of the material of this product. The measurement range of the magnetostriction characteristic curve is not less than 1.9T magnetic flux density. The magnetostriction curve is fitted with data using a polynomial form as shown in formula (1), with a fitting coefficient > 0.

99. The material properties are calculated per unit value based on 1.7T magnetic flux density, including at least the measured values ​​of magnetic flux density B of 1.7, 1.75, 1.8, 1.85, 1.9, and 1.95T. The per unit value is calculated based on the magnetostriction rate at each vibration measurement value of 1.7T, λ1.7*=1, λ1.8*=λ1.8 / λ1.7, λ1.9*=λ1.9 / λ1.7, ... When the measurement data of the measurement points is insufficient within the measurement range, the fitted curve is used to complete the data. λ=k1*B 6 +k2*B 5 +k3*B 4 +k4*B 3 +k5*B 2 +k6*B+k0 (1) In the formula: k0~k6 are polynomial coefficients.

3. The transformer core condition diagnosis method based on magnetostrictive properties and magnetic flux distribution according to claim 2, characterized in that, Step (2) is implemented as follows: Estimate the state of the transformer core joints, considering the joints δ1 between the side yoke and the main column, and δ2 between the upper and lower main yokes and the main column. The magnetic flux distribution in the three-phase five-column core is as follows: main column fluxes Ф1, Ф2, and Ф3 are in the same direction as the side yokes Ф4 and Ф5, and upper and lower main yokes Ф6 and Ф7. Ф3 and Ф6 reach their peak values ​​in the same direction at Ф3, while Ф3 and Ф4 reach their peak values ​​in opposite directions. At the peak value of Ф3, Ф3, Ф4, and Ф6 are at their peak values. Considering the concentration of magnetic flux at the core corners, use λ. p-p To estimate the expansion and contraction at the joint, assuming the angle between the joint and the horizontal direction is 45°, the magnetostriction coefficients λ1 and λ2 at joints δ1 and δ2 are: λ1=(L1+L2)* λ p-p *sin(45︒) (2) λ2=(L1+L3)* λ p-p *sin(45︒) (3) In the formula, the core height is L1, the side yoke length is L2, the upper and lower main yoke lengths are L3, and λ p-p This represents the peak-to-peak value of the magnetostriction.

4. The transformer core condition diagnosis method based on magnetostrictive properties and magnetic flux distribution according to claim 3, characterized in that, The specific implementation of step (3) is as follows: Measure the peak vibration acceleration at various distributed measuring points under different excitation conditions, including at least the main magnetic flux density of 1.7, 1.75, 1.8, 1.85, 1.9, and 1.95 T. Calculate the per-unit value based on the vibration acceleration measurement value of 1.7 T at each point. 1.7 * =1,a 1.8 * =a 1.8 / a 1.7 a 1.9 * =a 1.9 / a 1.7 , ...; The main cause of core vibration is the magnetostriction of the silicon steel sheets; based on the principle of electromagnetic induction and the magnetostrictive properties of ferromagnetic materials, the vibration acceleration α caused by the magnetostriction of the core can be obtained. c for: （4） In the formula, L represents the original size of the ferromagnetic material; λ c denoted as the magnetostriction of the silicon steel sheet under the excitation voltage U; U is the amplitude of the power supply voltage. N is the number of turns in the primary winding; S is the cross-sectional area of ​​the core; B c ω represents the magnetic flux density under the core excitation voltage; ω represents the power supply frequency.

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