A method for detecting residual magnetism in power transformer core based on local hysteresis loop

Abstract

The present invention provides a method for detecting residual magnetism in a power transformer core based on local hysteresis loops. The method first establishes a simulation model of the transformer core to be tested in finite element software. Secondly, positive and negative DC excitations are applied to the windings on one side of the core, and the winding current on one side and the induced voltage waveform on the other side of the transformer are detected. Positive and negative local hysteresis loops are detected, and the residual magnetism direction is identified based on the shape characteristics of the local hysteresis loops. Finally, the relationship between the area of the positive and negative local hysteresis loops and the residual magnetism under different residual magnetisms is analyzed. A relationship between the residual magnetism and the area of the positive local hysteresis loop is obtained through data fitting. Furthermore, the residual magnetism value of the transformer core to be tested can be determined when the area of the positive local hysteresis loop is known. This method effectively solves the problem of difficulty in measuring the residual magnetism of the power transformer core and simultaneously enables the determination of the residual magnetism direction.

Description

Technical Field

[0001] The present invention relates to the technical field of magnetic properties of transformer core materials, and in particular to a method and an experimental platform for detecting residual magnetism in a power transformer core based on a local hysteresis loop. Background Art

[0002] When the power transformer is reclosed, due to the presence of residual magnetism, a large excitation surge current will be generated, resulting in frequent failure of the transformer to close. The presence of surge current will accelerate the saturation of the iron core, increase the leakage flux of the transformer, cause overheating of the oil tank, promote the aging of the insulation paper and the decomposition of the transformer oil, and affect the life of the transformer; at the same time, the high-order harmonics in the surge current will also cause the grid voltage to rise or fall sharply, reduce the power quality of the grid, and affect the normal operation of other electrical equipment. In order to reduce the impact of the excitation surge current on the grid, the iron core is demagnetized before the transformer is closed. The key to demagnetization is to determine the size and direction of the residual magnetism. Reference [1] studied the method of detecting residual magnetism based on the first peak of the excitation surge current. This method needs to be coordinated with the control of the closing angle. When the residual magnetism of the iron core is large, the transformer fails to close, which will increase the difficulty of peak current detection. Reference [2] studied the use of an impedance analyzer to detect the excitation inductance of the iron core after power failure, and proposed a method of detecting the residual magnetism of the iron core based on the excitation inductance. It can be applied to field measurements, but the inductance value detected by the impedance analyzer is a gradually decaying variable, which increases the difficulty of detecting the inductance value. CN103675728A proposes detecting the residual magnetism of the iron core based on the current value at a specific point in the transient current when DC excitation is applied. However, this requires establishing an accurate mathematical model of the iron core, and different residual magnetism calculation formulas can be obtained at different measurement times, reducing the accuracy of the calculation. Given this, there is still no effective and accurate residual magnetism detection method. Researching a method for accurately detecting residual magnetism and quickly and effectively demagnetizing the transformer core to ensure safe and reliable operation of the power grid has important scientific and theoretical significance and engineering application value.

[0003] References:

[0004] [1] Li Yong, Jin Mingliang, Li Haitao, et al. Research on residual magnetism measurement method of power transformer [J]. Power System Protection and Control, 2019, 47(15): 1-6.

[0005] [2]Wei C, Li Summary of the Invention

[0006] The purpose of the present invention is to provide a method and experimental platform for detecting residual magnetism of power transformer core based on local hysteresis loop, so as to solve the problems existing in the above-mentioned background technology. First, a simulation model of the transformer core to be tested is established in finite element software; secondly, positive and negative DC excitations are applied to the windings on one side of the core respectively, the winding current on one side of the transformer and the induced voltage waveform of the other side of the winding are detected, and the positive and negative local hysteresis loops are measured. The residual magnetism direction is identified according to the shape characteristics of the local hysteresis loop; finally, the relationship between the area of the positive and negative local hysteresis loops and the residual magnetism under different residual magnetism is analyzed, and the relationship between the residual magnetism and the area of the positive local hysteresis loop is obtained by data fitting, so that the residual magnetism value of the transformer core to be tested can be determined when the area of the positive local hysteresis loop is known.

[0007] The technical solution adopted by the present invention to solve the technical problem is:

[0008] A method for detecting residual magnetism of a power transformer core based on a local hysteresis loop, the method comprising the following steps:

[0009] The first step is to establish a simulation model of the transformer core to be tested in the finite element software;

[0010] The iron core simulation model established has two windings. During residual magnetism detection, one winding is connected to the DC measurement excitation, while the other winding is open-circuited.

[0011] The second step is to determine the residual magnetism direction of the simulation model:

[0012] First, current excitation of different magnitudes is applied to the preset residual magnetism winding of the transformer core simulation model to obtain different preset residual magnetism values, and the winding on one side of the transformer core simulation model close to the core is selected as the preset residual magnetism winding;

[0013] Secondly, positive and negative short-term DC excitations were applied based on different preset residual magnetisms to measure the current waveform of the transformer winding on one side and the induced voltage waveform of the winding on the other side, thereby obtaining the positive and negative local hysteresis loop waveforms.

[0014] By analyzing the shape characteristics of the local hysteresis loops in two directions under different residual magnetization conditions, the direction of the residual magnetization of the current transformer core simulation model to be tested is determined. That is, the shape of the local hysteresis loop close to a triangle is the positive direction of the residual magnetization, and the shape of the local hysteresis loop close to a quadrilateral is the negative direction of the residual magnetization;

[0015] The third step is to determine the relationship between the residual magnetism of the transformer core under test and the positive local hysteresis loop area:

[0016] Loading current excitations of different magnitudes on the preset residual magnetization winding side of the transformer core simulation model to be tested to obtain residual magnetism of different magnitudes, loading positive and negative short-term small DC excitations on the measurement winding of the transformer core simulation model to be tested to obtain positive and negative local hysteresis loops under different residual magnetization conditions, determining the direction of the residual magnetization of the simulation model, and obtaining the area A1 of the positive local hysteresis loop;

[0017] The area of the positive local hysteresis loop under different remanent magnetization conditions is fitted to obtain the remanent magnetization B r The relationship between the positive local hysteresis loop, B r =f(A1);

[0018] The fourth step is to apply short-term small DC excitation in two different directions to the transformer core to obtain the local hysteresis loop under the excitation, and determine the residual magnetism direction of the core according to the shape characteristics of the hysteresis loop, and then determine the residual magnetism direction according to the residual magnetism B. r The residual magnetism value of the transformer core to be tested is obtained by using the relationship between the positive local hysteresis loop and the residual magnetism value of the transformer core to be tested.

[0019] The short-term small DC excitation signal means that when the excitation is applied, the rate of change of the initial residual magnetism of the iron core must be ensured to be within 5%.

[0020] The preset remanence value range is 0.2 to 0.7 times the saturation magnetic flux density of the ferromagnetic material, and the remanence in the simulation model of the transformer core to be tested is preset within this range.

[0021] The transformer core to be tested is a square core, and the core dimensions are:

[0022] Each core piece is 80mm wide, the core lamination thickness is 20mm, the side length of the overall inner square of the core is 400mm, and the side length of the overall outer square of the core is 560mm.

[0023] A local hysteresis loop-based residual magnetism detection experimental platform for power transformer cores measures the residual magnetism in the transformer core under test. The platform requires measuring instruments including a signal generator, a power amplifier, a fluxmeter, a digital oscilloscope, a current probe, and a PC for control. A signal generator is used to apply a short-term, low-voltage DC excitation to one winding of the transformer core under test, which is then amplified by the power amplifier. The fluxmeter tracks and records changes in the magnetic flux in the core during the measurement process. Current sensors and voltage sensors collect the current waveforms of one winding and the induced voltage waveforms of the other winding, respectively, and display the waveforms on a digital oscilloscope.

[0024] The specific experimental process of the experimental platform is:

[0025] (1) The transformer core to be tested is fully demagnetized, and then a large current is applied to the transformer core winding to be tested, and then the excitation is removed to preset the residual magnetism of the core; during the excitation application process, the magnetic flux in the transformer core to be tested is tracked in real time with a flux meter to determine the size of the preset residual magnetism value generated in the transformer core to be tested;

[0026] (2) Determine the direction of residual magnetism in the transformer core to be tested:

[0027] In step (1), a short-time small DC excitation signal in two different directions consistent with the simulation is applied to the primary winding of the transformer core to be tested with preset residual magnetism, a current signal is detected by a current sensor, a voltage signal is detected by a voltage sensor, and the signals are displayed on a digital oscilloscope; the shape characteristics of the local hysteresis loop in the two directions are analyzed to determine the direction of the residual magnetism, that is, if the shape of the local hysteresis loop is close to a triangle, it is positive residual magnetism, and if the shape of the local hysteresis loop is close to a quadrilateral, it is negative residual magnetism;

[0028] (3) Calculate the residual magnetism of the transformer core to be tested:

[0029] Obtain the positive local hysteresis loop area under the excitation of step (2), and then calculate the residual magnetization B r The residual magnetism value in the transformer core to be tested is calculated using the relationship between the positive local hysteresis loop and the calculated residual magnetism value.

[0030] Compare the preset remanence value with the calculated remanence value under the same conditions to analyze the error in measuring remanence.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] The outstanding substantive feature of the present invention is that the method of the present invention proposes a method for measuring the residual magnetism in the transformer core based on the local hysteresis loop. First, a simulation model of the transformer core to be measured is established in finite element software, and the changing trends of the positive and negative local hysteresis loop waveforms under different residual magnetisms and positive and negative short-term small DC excitations are analyzed. The direction of the residual magnetism is determined based on the shape characteristics of the positive and negative local hysteresis loops. The relationship between the residual magnetism and the area of the positive local hysteresis loop is obtained by data fitting, which is the empirical formula for calculating the residual magnetism. An experimental platform is built based on the residual magnetism prediction method, and the relative error of the measurement results is analyzed. In addition, the proposed residual magnetism detection method not only has a simple empirical formula to calculate the residual magnetism in the core, but is also easy to operate in practice. At the same time, according to the magnetic circuit analysis method, it can be seen that this method can be applied to any core with a closed magnetic circuit and has universal applicability.

[0033] The significant advancements of the present invention are:

[0034] (1) The present invention addresses the difficulty in measuring residual magnetism in transformer cores. The method studies the relationship between the residual magnetism of the core and the measured local hysteresis loop after applying positive and negative DC excitations, and develops a method for detecting the magnitude and direction of residual magnetism. The present invention effectively solves the problem of difficulty in measuring the magnitude of residual magnetism in power transformer cores and simultaneously enables determination of the direction of residual magnetism.

[0035] (2) The principle of the method of the present invention is simple and clear, and the test and measurement operation is simple and easy.

[0036] (3) The method of the present invention has the characteristics of high efficiency and high precision. It can repeatedly verify and analyze the real-time residual magnetism of the transformer core, making the measurement of the residual magnetism of the transformer core more accurate. In particular, when the residual magnetism is relatively small (0.5-0.8T), the relative error of the residual magnetism detection can still be controlled within 3.5%, with high precision. It avoids the problem of inaccurate detection when the local hysteresis loop slope method is used in the prior art because the positive and negative slopes are very close in the case of small residual magnetism. The present application is applicable to both large and small residual magnetism and is more intuitive and simple.

[0037] (4) The method of the present invention has universal applicability and can be widely used in the measurement and calculation of residual magnetism of other power equipment with closed magnetic circuit cores. It is an indispensable technology for effectively eliminating the influence of residual magnetism of the cores of power equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The present invention will be further described below with reference to the accompanying drawings and examples.

[0039] Figure 1 It is a schematic block diagram of the operation process of the method of the present invention.

[0040] Figure 2 Schematic diagram of the dimensions of a square transformer core selected in an embodiment of the present invention.

[0041] Figure 3 3 is a model diagram of a square iron core to be tested established in the finite element method in an embodiment of the present invention.

[0042] Figure 4 This is the relationship between the positive local hysteresis loop and the remanence established by the square core to be tested in the embodiment of the present invention.

[0043] Figure 5 This is the determination of the residual magnetism direction of the square iron core to be tested in the embodiment of the present invention.

[0044] Figure 6 It is a residual magnetism measurement experimental platform in the embodiment of the present invention. DETAILED DESCRIPTION

[0045] Figure 1 The illustrated embodiment shows that the operation process of the present invention is:

[0046] The first step is to accurately model the transformer core to be tested in finite element method and establish a simulation model of the transformer core to be tested.

[0047] The second step is to determine the residual magnetism direction of the simulation model: First, different current excitations are applied to the preset residual magnetism winding of the transformer core simulation model to obtain different preset residual magnetism values, and the winding on the side close to the core of the transformer core simulation model is selected as the preset residual magnetism winding;

[0048] Secondly, positive and negative short-term DC excitations were applied based on different preset residual magnetisms to measure the current waveform of the transformer winding on one side and the induced voltage waveform of the winding on the other side, thereby obtaining the positive and negative local hysteresis loop waveforms.

[0049] By analyzing the shape characteristics of the local hysteresis loops in two directions under different residual magnetization conditions, it is found that the positive local hysteresis loop is triangular and the negative local hysteresis loop is quadrilateral. Therefore, the direction of the residual magnetization of the current transformer core simulation model under test can be determined based on the approximate shape of the local hysteresis loop. That is, the local hysteresis loop shape close to a triangle indicates the positive direction of the residual magnetization, and the local hysteresis loop shape close to a quadrilateral indicates the negative direction of the residual magnetization.

[0050] The third step is to fit the empirical formula for measuring the residual magnetism of the core to be tested: Under different residual magnetism, the relationship between the local hysteresis loop area and the residual magnetism of the simulation model of the transformer core to be tested is analyzed. The negative local hysteresis loop area and the residual magnetism are nonlinear, while the positive local hysteresis loop area and the residual magnetism show a linear relationship. The relationship between the residual magnetism and the positive local hysteresis loop area is fitted, which is the empirical formula for measuring the residual magnetism of the core to be tested.

[0051] Step 4: Conduct residual magnetism test analysis on the core to be tested:

[0052] By applying short-term small DC excitation in two different directions to the transformer core under test, the positive direction of the residual magnetism of the transformer core under test is determined according to the shape of the local hysteresis loop, and the area of the local hysteresis loop in the positive direction is obtained. Then, using the relationship established in the third step, the residual magnetism value of the transformer core under test with the known residual magnetism direction is determined.

[0053] Build an experimental platform for the residual magnetism of the transformer core to be tested, determine the direction and size of the residual magnetism of the transformer core to be tested, and perform error analysis.

[0054] Figure 2 The illustrated embodiment shows a schematic diagram of the dimensions of the closed magnetic circuit transformer core selected in the embodiment of the present invention.

[0055] The dimensions of the selected transformer core are: each core piece is 80 mm wide, the core lamination thickness is 20 mm, the square core is an annular cube as a whole, the side length of the square inside the core is 400 mm, and the side length of the square outside the core is 560 mm.

[0056] Figure 3 The illustrated embodiment shows a model diagram of a square transformer core to be tested established by finite element method in an embodiment of the present invention.

[0057] In finite element simulation, there are two windings: the primary winding of the transformer, which is mainly used to apply short-term, low-voltage DC excitation to the iron core, and the secondary winding, which operates in an open circuit. When DC excitation is applied to the primary winding, the current waveform of the primary winding and the voltage waveform of the secondary winding can be detected separately. In addition, in finite element simulation, preset residual magnetism is also required. In an actual transformer, the inside is low voltage and the outside is high voltage. When superimposed, the preset residual magnetism winding is close to the iron core. That is, the preset residual magnetism winding uses a measurement winding close to the iron core. In this embodiment, the primary winding is used as the preset residual magnetism winding for illustration.

[0058] Figure 4 The illustrated embodiment shows that the relationship between the residual magnetism and the positive local hysteresis loop area established in the embodiment of the present invention is close to linear, and the magnitude of the residual magnetism can be detected by the relationship between the two.

[0059] Figure 5 The illustrated embodiment shows that in the embodiment of the present invention, when DC excitation of different polarities is applied to the primary winding under different remanences, positive and negative local hysteresis loops are obtained when the preset remanence is 0.832T. The shape characteristics of the two local hysteresis loops are analyzed to determine the direction of the remanence. That is, the local hysteresis loop shape close to a triangle indicates the positive direction of the remanence, and the shape close to a quadrilateral indicates the negative direction of the remanence.

[0060] Figure 6The embodiment shown shows an experimental platform for residual magnetism measurement in an embodiment of the present invention. The residual magnetism detection method of the present invention is verified on the constructed experimental platform, and the measurement errors of the preset residual magnetism and the calculated residual magnetism are analyzed. To measure the residual magnetism in the transformer core to be tested, the measuring instruments required by the experimental platform include a signal generator 1, a power amplifier 2, a fluxmeter 3, a digital oscilloscope 4 and a current probe and a PC computer control; the fluxmeter is connected to the transformer core 5 to be tested, the fluxmeter is connected to the secondary side, the excitation is given to the primary side of the transformer, and the secondary side is used to measure the induced voltage; the signal generator sends a DC excitation signal, which is amplified by the power amplifier and applied to both sides of the primary winding of the core, and the secondary winding is open-circuited; the fluxmeter is used to track and record the changes in the magnetic flux in the core during the excitation application process; the measured current and voltage signals are collected by the current sensor and the voltage sensor and displayed on the digital oscilloscope 4; H is obtained from the current, B is obtained from the voltage, and a local hysteresis loop is formed after BH is plotted.

[0061] Specific experimental process:

[0062] (1) The transformer core to be tested is fully demagnetized, and then a large current excitation is applied to the core winding. The excitation is then removed to preset the residual magnetism of the core. During the excitation application process, the flux in the transformer core is tracked in real time with the help of a flux meter to determine the size of the preset residual magnetism generated in the transformer core to be tested;

[0063] (2) Determine the direction of residual magnetism in the transformer core to be tested:

[0064] A short-term, small DC excitation signal in two different directions, consistent with the simulation, is applied to the primary winding of the transformer core under test. The current signal is detected by the current sensor, and the voltage signal is detected by the voltage sensor, and both signals are displayed on a digital oscilloscope. The shape characteristics of the local hysteresis loop in the two directions are analyzed to determine the direction of the residual magnetism. That is, if the shape of the local hysteresis loop is close to a triangle, it is positive residual magnetism, and if the shape of the local hysteresis loop is close to a quadrilateral, it is negative residual magnetism.

[0065] (3) Calculate the residual magnetism of the transformer core to be tested:

[0066] The relationship between the residual magnetism of the transformer core and the positive local hysteresis loop area is obtained through finite element simulation. r =f(A1). Given a DC excitation in both directions across the primary winding of the transformer core under test, the positive local hysteresis loop area under this excitation is obtained, and the residual magnetization value in the core is calculated. Comparing the preset residual magnetization value with the calculated residual magnetization value under the same conditions reveals that the measured residual magnetization has a small error and high accuracy.

[0067] The present invention determines the range of residual magnetism based on an empirical estimation method, which suggests that the residual magnetism is approximately 0.2 to 0.7 times the saturation flux density of the core. For example, when the saturation flux density of the core is 1.8T, the preset range of residual magnetism is 0.4T-1.36T. Within this range, the residual magnetism of the transformer core under test is preset.

[0068] After the residual magnetism is preset, a positive and reverse short-time small DC excitation is loaded on the primary winding of the simulation model of the transformer core to be tested, respectively, and the current waveform of the primary winding and the open-circuit voltage waveform of the secondary winding are obtained, and the positive and negative local hysteresis loops are obtained. By comparing and analyzing the two local hysteresis loop waveforms, it is found that the shape of the positive local hysteresis loop is close to a triangle, and the shape of the negative local hysteresis loop is close to a quadrilateral. Therefore, the residual magnetism direction of the transformer core to be tested is determined by judging the shape characteristics of the positive and negative local hysteresis loops.

[0069] Example

[0070] The present invention studies the residual magnetism measurement in the transformer core to be measured. In the embodiment, a square transformer core is selected as the research object to verify the residual magnetism measurement method of the present invention.

[0071] The first step is to establish a finite element model of the square core.

[0072] In the finite element software, the transformer core to be tested is accurately modeled. According to the structural dimensions of the transformer core to be tested, a geometric model of the transformer core to be tested is established in the electromagnetic field simulation software, and according to the material characteristic parameters of the transformer core to be tested, the core material properties are set in the electromagnetic field simulation software. Thus, a simulation model of the transformer core to be tested is established in the electromagnetic field simulation software. In this embodiment, in the finite element software COMSOL, according to Figure 2 The square core structure dimensions are shown in the figure. The material properties of the square core are set, and the material model used for the square core is B30P105. The simulation model of the square core is as follows: Figure 3 As shown, the two windings are the primary winding and the secondary winding of the transformer.

[0073] The second step is to determine the direction of the residual magnetism of the square iron core.

[0074] By applying different currents to the core windings, the core generates remanence values of varying magnitudes and directions. The core material is made of B30P105 silicon steel sheet, which has a saturation flux density of 1.8T. Therefore, the preset remanence range in this embodiment is 0.4T-1.36T.

[0075] When the positive direction of the core's remanent magnetism is known, small positive and negative excitation signals are applied to the measurement winding of a square core. The waveforms of the positive and negative transient currents flowing through the circuit are obtained, and the relationship between the remanent magnetism and the positive and negative local hysteresis loops is analyzed. When the core's remanent magnetism varies from 0.4T to 1.36T, the shape of the positive local hysteresis loop approaches a triangle, confirming that the remanent magnetism direction is positive.

[0076] Therefore, when the direction of the residual magnetism of the iron core is unknown, the shape characteristics of the local hysteresis loop are judged according to a given residual magnetism. If the local hysteresis loop is triangular, it is in the positive direction of the residual magnetism, and if the local hysteresis loop is quadrilateral, it is in the negative direction of the residual magnetism.

[0077] The third step is to calculate the empirical formula for the residual magnetism of the square iron core.

[0078] Different magnitudes of current excitation are loaded on the winding side of the transformer core simulation model to be tested to obtain different magnitudes of residual magnetism. According to the residual magnetism direction of the simulation model determined in the second step, a short-term small DC excitation is applied to the primary winding of the transformer core simulation model to be tested to obtain local hysteresis loops under different residual magnetism conditions; the fitting relationship between residual magnetism and local hysteresis loop area is analyzed, which is the empirical formula for calculating residual magnetism, which is B r =f(A1)=-0.02291A1+1.5117.

[0079] Step 4: Build an experimental platform for the square iron core.

[0080] (1) An experimental platform for the core to be tested was constructed to experimentally verify the proposed method for measuring residual magnetism in transformer cores based on local hysteresis loops. The residual magnetism of the core was preset, and the range of the preset residual magnetism of the core was basically consistent with the simulation range, which was 0.4T-1.36T.

[0081] (2) The experimental platform built is as follows Figure 5 As shown in the figure, when a small positive and negative excitation signal (voltage excitation 0.1V) is applied to the measurement winding, the current sensor's current probe collects the current signal in the measurement circuit, and the voltage sensor detects the open-circuit voltage signal of the transformer core secondary winding. After data processing, a local hysteresis loop waveform is plotted, and the shape characteristics of this waveform are observed to determine the direction of residual magnetization. Through simulation and experimental analysis, it is found that the shape characteristics of the positive local hysteresis loop are close to a triangle, so the direction of residual magnetization is determined to be positive.

[0082] (3) Calculate the size of the residual magnetism in the square iron core. According to the empirical formula obtained in the third step, the residual magnetism in the square iron core is calculated. When a positive short-term small DC excitation signal with an effect of less than 5% on the residual magnetism is applied to the primary winding of the transformer, a positive local hysteresis loop waveform is obtained. Then the corresponding positive local loop area is calculated, and this value is substituted into the empirical formula in the third step to calculate the size of the residual magnetism of the iron core. By comparing the calculated residual magnetism value with the actual residual magnetism value tracked in real time by the fluxmeter, the relative error is within 3.5%, as shown in Table 1. When the residual magnetism is large, the measurement error is small and the accuracy can reach 0.8%. This shows that the detection method can accurately detect large residual magnetism values. Compared with the methods for measuring residual magnetism in existing literature, it has the advantages of high detection accuracy, easy judgment of residual magnetism direction, and simple operation.

[0083] Table 1. Experimental measurement data

[0084]

[0085] Any matters not described in the present invention are applicable to the prior art.

Claims

1. A method for detecting residual magnetism in the core of a power transformer based on a local hysteresis loop, characterized in that: The steps of this detection method are: The first step is to establish a simulation model of the transformer core to be tested in the finite element software; The iron core simulation model established has two windings. During residual magnetism detection, one winding is connected to the DC measurement excitation, while the other winding is open-circuited. The second step is to determine the residual magnetism direction of the simulation model: First, current excitation of different magnitudes is applied to the preset residual magnetism winding of the transformer core simulation model to obtain different preset residual magnetism values, and the winding on one side of the transformer core simulation model close to the core is selected as the preset residual magnetism winding; Secondly, positive and negative short-term DC excitations were applied based on different preset residual magnetisms to measure the current waveform of the transformer winding on one side and the induced voltage waveform of the winding on the other side, thereby obtaining the positive and negative local hysteresis loop waveforms. By analyzing the shape characteristics of the local hysteresis loops in two directions under different residual magnetization conditions, the direction of the residual magnetization of the current transformer core simulation model to be tested is determined. That is, the shape of the local hysteresis loop close to a triangle is the positive direction of the residual magnetization, and the shape of the local hysteresis loop close to a quadrilateral is the negative direction of the residual magnetization; The third step is to determine the relationship between the residual magnetism of the transformer core under test and the positive local hysteresis loop area: Loading current excitations of different magnitudes on the preset residual magnetization winding side of the transformer core simulation model to be tested to obtain residual magnetism of different magnitudes, loading positive and negative short-term small DC excitations on the measurement winding of the transformer core simulation model to be tested to obtain positive and negative local hysteresis loops under different residual magnetization conditions, determining the direction of the residual magnetization of the simulation model, and obtaining the area A1 of the positive local hysteresis loop; The area of the positive local hysteresis loop under different remanent magnetization conditions is fitted to obtain the remanent magnetization B r The relationship between the positive local hysteresis loop, B r =f(A1); The fourth step is to apply short-term small DC excitation in two different directions to the transformer core to obtain the local hysteresis loop under the excitation, and determine the residual magnetism direction of the core according to the shape characteristics of the hysteresis loop, and then determine the residual magnetism direction according to the residual magnetism B. r The residual magnetism value of the transformer core to be tested is obtained by using the relationship between the positive local hysteresis loop and the residual magnetism value of the transformer core to be tested.

2. The method for detecting residual magnetism of a power transformer core based on a local hysteresis loop according to claim 1, characterized in that: The short-time low DC excitation means that when the excitation is applied, the rate of change of the initial residual magnetism of the iron core must be ensured to be within 5%.

3. The method for detecting residual magnetism of a power transformer core based on a local hysteresis loop according to claim 1, characterized in that: The preset remanence value ranges from 0.2 to 0.7 times the saturation magnetic flux density of the ferromagnetic material.

4. The method for detecting residual magnetism of a power transformer core based on a local hysteresis loop according to claim 1, wherein: The transformer core to be tested is a square core with the following dimensions: each core piece is 80 mm wide, the core lamination is 20 mm thick, the side length of the square inside the core is 400 mm, and the side length of the square outside the core is 560 mm.

5. The method for detecting residual magnetism of a power transformer core based on a local hysteresis loop according to any one of claims 1 to 4, characterized in that: The detection method is used for detecting residual magnetism in an iron core with a closed magnetic circuit.

6. A power transformer core residual magnetism detection experimental platform based on local hysteresis loop, characterized in that: Determine the direction and size of the residual magnetism of the transformer core to be tested, and perform error analysis; The measuring instruments required for the experimental platform include a signal generator (1), a power amplifier (2), a fluxmeter (3), a digital oscilloscope (4), a current probe, and a PC computer control; the fluxmeter is connected to the transformer core to be tested (5); the signal generator sends a DC excitation signal, which is amplified by the power amplifier and applied to both sides of the primary winding of the core, and the secondary winding is open-circuited; the fluxmeter tracks and records the change of the magnetic flux in the core during the excitation application process; the measured current and voltage signals are collected by the current sensor and the voltage sensor and displayed on the digital oscilloscope (4); H is obtained from the current, B is obtained from the voltage, and the local hysteresis loop is obtained by plotting BH; The specific experimental process of the experimental platform is: (1) The transformer core to be tested is fully demagnetized, and then a large current is applied to the transformer core winding to be tested, and then the excitation is removed to preset the residual magnetism of the core; during the excitation application process, the magnetic flux in the transformer core to be tested is tracked in real time with a flux meter to determine the size of the preset residual magnetism value generated in the transformer core to be tested; (2) Determine the direction of residual magnetism in the transformer core to be tested: In step (1), a short-time small DC excitation signal in two different directions consistent with the simulation is applied to the primary winding of the transformer core to be tested with preset residual magnetism, a current signal is detected by a current sensor, a voltage signal is detected by a voltage sensor, and the signals are displayed on a digital oscilloscope; the shape characteristics of the local hysteresis loop in the two directions are analyzed to determine the direction of the residual magnetism, that is, if the shape of the local hysteresis loop is close to a triangle, it is positive residual magnetism, and if the shape of the local hysteresis loop is close to a quadrilateral, it is negative residual magnetism; (3) Calculate the residual magnetism of the transformer core to be tested: Obtain the positive local hysteresis loop area under the excitation of step (2), and then calculate the residual magnetization B r The residual magnetism value in the transformer core to be tested is calculated using the relationship between the positive local hysteresis loop and the calculated residual magnetism value. Compare the preset remanence value with the calculated remanence value under the same conditions to analyze the error in measuring remanence.

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