A transformer magnetic bias resistance capacity detection method based on hysteresis loop shape

Abstract

The application discloses a transformer magnetic bias resistance capability detection method based on a hysteresis loop shape, belongs to the technical field of high-voltage test methods, and comprises the following steps: keeping the rated voltage of a transformer unchanged, opening a primary side, inputting a controllable square wave power supply to a secondary side, calculating the change law of a magnetic field intensity and a magnetic flux density within one period of the input controllable square wave power supply, drawing a core hysteresis loop according to the result, and calculating the direct-current magnetic bias resistance capability of the transformer core according to the shape parameters of the core hysteresis loop. Through the steps of controllable square wave voltage source injection, current and voltage detection, magnetic flux density and magnetic field intensity calculation, transformer hysteresis loop drawing and important parameter acquisition, the poor operability of the transformer direct-current magnetic bias resistance capability detection is solved, the transformer excitation magnetic bias resistance capability requirement is reliably executed, and the operation reliability of the transformer is improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of high voltage test method, and particularly relates to a transformer DC bias withstand capability detection method based on a hysteresis loop shape. BACKGROUND

[0002] With the development of super-high voltage power grid, the application of large-capacity power transformers is increasing, which puts forward higher requirements on the reliability and rapidity of electrical equipment protection operation in industrial applications. The transformer is an important equipment for modern industrial development, and the core part composed of ferromagnetic material directly affects its electrical performance. During the transformer switching process, voltage ratio, DC resistance measurement and no-load test and other operations are performed, which causes residual magnetism in the core. The existence of residual magnetism can cause transformer core DC bias phenomenon, aggravate the saturation degree of the core, cause the excitation impedance to decrease significantly, the excitation current to increase, and then cause the transformer loss to increase, the vibration to intensify, the local overheating and other problems, and cause the transformer damage, which seriously threatens the stable operation of the power system.

[0003] Therefore, it is necessary to detect the DC bias withstand capability of the transformer. The existing transformer DC bias withstand capability checking method is to test by injecting DC at power frequency, which requires two transformers with the same capacity and performance as the tested product, and the test power supply capacity needs to be much larger than the no-load capacity of the transformer, and the test conditions are relatively high, which is not feasible in the field checking. At present, there is a lack of an easy-to-calculate transformer DC bias withstand capability detection method. SUMMARY

[0004] The purpose of the present application is to overcome the limitations of the existing transformer DC bias withstand capability checking method based on the above-mentioned prior art, and to provide a transformer DC bias withstand capability detection method based on the shape of the hysteresis loop.

[0005] In order to achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0006] A transformer bias withstand capability detection method based on the shape of the hysteresis loop, characterized in that it comprises the following steps:

[0007] S1: keeping the rated voltage of the transformer unchanged, opening the primary side, and inputting a controllable square wave power supply to the secondary side;

[0008] S2: calculating the change law of the magnetic field intensity and the magnetic flux density within one period of the input controllable square wave power supply;

[0009] S3: drawing the core hysteresis loop according to the result of S2;

[0010] S4: calculating the transformer core DC bias withstand capability according to the shape parameters of the core hysteresis loop.

[0011] The above technical solution includes the following steps:

[0012] A1: Connecting the positive and negative saturation points on the hysteresis loop forms the first straight line;

[0013] A2: Draw a tangent line between the zero point and the positive saturation point of the hysteresis loop using the slope of the first straight line to form the second straight line;

[0014] A3: Draw a tangent line on the part of the hysteresis loop between the positive and negative saturation points and where the magnetic flux density is negative, using the slope of the first straight line, to form the third straight line;

[0015] A4: Draw an ellipse with the length of the first line segment as the major axis and the sum of the lengths of the second and third lines to the first line as the minor axis;

[0016] A5: Determine the magnetic tolerance by calculating the eccentricity of the ellipse.

[0017] In the above technical solution, the excitation curve of the iron core is plotted with magnetic field strength as the abscissa and magnetic flux density as the ordinate, and the values ​​of positive and negative saturation points are obtained from the excitation curve.

[0018] In the above technical solution, the magnetic flux density corresponding to the ordinate of the tangent point of the second straight line and the third straight line that are tangent to the excitation curve is obtained respectively, and the average value of the absolute values ​​of the magnetic flux density corresponding to the two points is the magnetic field strength of the ellipse.

[0019] In the above technical solution, by detecting and recording the changes in the excitation current flowing through the winding and the voltage across the winding over time during the entire pressurization process, the time-varying laws of the magnetic field strength and magnetic flux density are calculated, thereby drawing the hysteresis loop.

[0020] In the above technical solution, calculations are performed in the following manner.

[0021] The magnetic field strength is: ,

[0022] The magnetic flux density is: ,

[0023] Where: L is the straight line connecting the positive and negative saturation points. For excitation current, For winding voltage, The DC resistance on the winding, It is the number of turns in the winding. It is the area where the magnetic flux is perpendicular to the direction of the magnetic field.

[0024] In the above technical solution, the circuit between the primary side and the secondary side is equivalent to: the DC resistance of the windings connected in series, the leakage inductance of the windings, and the equivalent resistance of the magnetizing inductance and eddy current loss connected in parallel.

[0025] In the above technical solution, the voltage drop on the leakage inductance of the winding side is ignored in the equivalent circuit, which is equivalent to a short circuit. The eddy current loss is ignored and the equivalent current is equivalent to an open circuit.

[0026] In the above technical solution, the voltage is applied to the secondary side by continuously inputting a high-level voltage for one power frequency cycle and a low-level voltage for one and a half power frequency cycles in sequence, and then the voltage application is stopped.

[0027] In the above technical solution, the input frequency of one power frequency cycle is set to a square wave voltage of 50Hz.

[0028] The design principle of this invention is as follows:

[0029] The fundamental principle of this invention lies in establishing a quantitative relationship between the hysteresis characteristics of a transformer core and its DC bias tolerance. Ferromagnetic materials exhibit hysteresis under the influence of an alternating magnetic field, and the shape characteristics of their hysteresis loop (such as the position of the saturation point and changes in slope) directly reflect the magnetization process of the material. When a transformer has DC bias, the operating point of the core will shift, causing a regular change in the shape of the hysteresis loop.

[0030] This invention extracts key geometric parameters of the hysteresis loop and constructs a specific geometric model (elliptical model), transforming the abstract magnetic property problem into a quantifiable geometric morphology analysis problem, thereby achieving indirect detection of bias magnetic tolerance.

[0031] Controllable square wave excitation principle: A continuous high-level voltage can magnetize the iron core to a positive saturation state, and the subsequent low-level voltage (negative voltage) causes the iron core to reverse magnetize from the positive saturation point to the negative saturation state. This excitation method can ensure that the iron core undergoes a complete magnetization process from the initial state → positive saturation → negative saturation within a single test cycle, thereby drawing a complete hysteresis loop containing both positive and negative saturation points.

[0032] Electromagnetic parameter calculation principle: Based on the law of electromagnetic induction and Ampere's circuital law, it is calculated by measuring the voltage of the winding. and excitation current Calculate the magnetic field strength inside the iron core. and magnetic flux density . The physical meaning of the calculation formula is that the magnetic field strength is proportional to the excitation current, reflecting the magnetomotive force per unit length of the magnetic circuit. The physical meaning of the calculation formula is that the magnetic flux density is the integral of the induced electromotive force over time, reflecting the magnetic flux per unit area. The voltage drop across the winding resistance is subtracted in the calculation to obtain the pure induced electromotive force.

[0033] Hysteresis Loop Geometric Modeling and Parameter Extraction (Elliptical Model) Principle: The nonlinear and asymmetric characteristics of the hysteresis loop are approximated using the geometric features of an ellipse. The specific steps are as follows:

[0034] The baseline is determined (the first straight line): connecting the positive saturation point A (H1, B1) and the negative saturation point B (H2, B2) of the hysteresis loop, this straight line reflects the average magnetization characteristics of the iron core in the saturation range, and its slope is related to the equivalent permeability of the iron core.

[0035] Tangent positioning (second and third lines): Find points on the hysteresis loop where the slope is the same as the first line (i.e., the rate of change is consistent with the average rate of change in the saturation range). These points are usually located in the nonlinear region of the magnetization curve. The tangents at points C (H3, B3) and D (H4, B4) are parallel to the first line. These two points reflect the critical positions where the permeability changes significantly during magnetization.

[0036] Ellipse construction: using the length of the line segment between positive and negative saturation points As the major axis of the ellipse, the sum of the perpendicular distances from the second and third lines to the first line. Construct an ellipse with its minor axis as the minor axis. The flatness (eccentricity) of this ellipse reflects both the "width" and "asymmetry" of the hysteresis loop.

[0037] Endurance criterion: A larger eccentricity results in a flatter ellipse, meaning a narrower hysteresis loop, potentially indicating lower hysteresis loss in the core material, lower sensitivity to DC bias, and stronger endurance. Conversely, a smaller eccentricity results in a rounder ellipse, potentially indicating a thicker hysteresis loop, higher hysteresis loss, and weaker endurance. Simultaneously, the average magnetic field strength corresponding to tangency points C and D... It can also be used as an auxiliary criterion, reflecting the magnetic field strength required to reach a specific magnetization state.

[0038] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0039] This invention collects data on the changes in transformer excitation current and winding voltage over time, calculates time-varying data on magnetic field strength and magnetic flux density, plots BH curves, extracts key parameters from the curves, establishes the relationship between magnetic field strength and parameters, and tests the bias magnetic tolerance capability.

[0040] This invention solves the problem of poor operability in detecting the DC bias magnetic tolerance capability of transformers by injecting a controllable square wave voltage source, detecting current and voltage, calculating magnetic flux density and magnetic field strength, drawing the transformer hysteresis loop, and collecting important parameters of the curve. This ensures that the requirements for the excitation bias magnetic tolerance capability of transformers are reliably executed and improves the operational reliability of transformers. Attached Figure Description

[0041] To more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of the present invention and should not be considered as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort. In the drawings:

[0042] Figure 1 This is a flowchart illustrating this embodiment;

[0043] Figure 2 This is the equivalent circuit for detecting the DC bias magnetic withstand capability of the transformer in this embodiment;

[0044] Figure 3 This is a schematic diagram of the square wave controllable source voltage waveform in this embodiment;

[0045] Figure 4a This is a graph showing the real-time change in the magnetic flux density of the transformer core from 0 to 0.02 seconds in this embodiment. Figure 4b This is a graph showing the real-time change in the magnetic flux density of the transformer core from 0.02 to 0.06 seconds in this embodiment.

[0046] Figure 5a This is a graph showing the real-time change of the transformer core excitation current from 0 to 0.02 seconds in this embodiment. Figure 5b This is a real-time variation curve of the transformer core excitation current from 0.02 to 0.06 seconds in this embodiment.

[0047] Figure 6 This is a diagram showing the hysteresis loop curve path of the transformer core in this embodiment;

[0048] Figure 7 This is a flowchart illustrating the process of drawing an ellipse in this embodiment;

[0049] Figure 8 This is a schematic diagram of the hysteresis loop of the transformer core in this invention.

[0050] The attached diagram shows the markings and corresponding component names:

[0051] R dc L is the DC resistance on the winding. σ For leakage inductance of this side winding, R e L is the equivalent resistance for eddy current losses. m For the magnetizing inductor, i ex (t) is the excitation current, i m (t) represents the flow through L m The magnetizing current, i e e(t) is the equivalent current of eddy current loss, e(t) is the induced electromotive force, and u(t) is the voltage applied to the winding. Detailed Implementation

[0052] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings. The illustrative embodiments and descriptions of the present invention are only used to explain the present invention and are not intended to limit the present invention.

[0053] Example 1

[0054] The target of this embodiment is a single transformer, not two transformers with the same capacity and performance as in the traditional test, and the test method used is not the traditional method of injecting DC at power frequency.

[0055] In this embodiment, a single transformer is used with the primary side open and the secondary side inputting a controllable square wave voltage with alternating positive and negative values ​​for voltage application. The excitation current flowing through the winding and the voltage across the winding are detected and recorded as curves over time. The magnetic flux density and magnetic field strength at the geometric center of the core are calculated as curves over time. The hysteresis loop of the core from time 0 to positive saturation and then to negative saturation is plotted, and the coordinates of the magnetic field strength and magnetic flux density corresponding to the positive and negative saturation points, the distance between the two points, and the slope of the line are obtained. The DC bias magnetic withstand capability of the transformer is detected based on the hysteresis loop parameters.

[0056] This implementation example Figure 1 As shown, this embodiment includes four steps: drawing the equivalent circuit, inputting a controllable square wave voltage, drawing the core hysteresis loop, and calculating the DC bias magnetic withstand capability of the transformer. The specific process is as follows.

[0057] Step S1: Connect the circuit components such as the transformer to be tested. The transformer is a 110kV main transformer with an open primary side and a controllable square wave power supply connected to the secondary side. Draw the equivalent circuit.

[0058] In the equivalent circuit, such as Figure 2 As shown, from the secondary side to the primary side, there are interconnected winding DC resistance, winding leakage inductance, and interconnected excitation inductance and eddy current loss equivalent resistance.

[0059] Step S2: Input a controllable square wave power supply to the secondary side of the equivalent circuit. The square wave voltage waveform is as follows: Figure 3 As shown, the controllable square wave power supply is set to a high level for one full power frequency cycle, then a low level for one and a half power frequency cycles, and then the voltage is stopped. The entire power frequency is set to 50Hz.

[0060] Step S3: Based on S2, detect and record the changes in the excitation current flowing through the winding and the voltage across the winding over time throughout the entire process, and calculate the change in magnetic field strength over time as follows: Figure 5a , Figure 5b The variation of magnetic flux density B with time is calculated as follows:Figure 4a , Figure 4b As shown.

[0061] Step S4: Establish a coordinate system based on the obtained magnetic field strength and magnetic flux density, and draw the hysteresis loop of the iron core as shown below. Figure 6 As shown, record the shape parameters: record the coordinates corresponding to the positive and negative saturation points, and determine the angle between the line connecting the positive and negative saturation points and the x-axis.

[0062] Step S5: Calculate the DC bias magnetic withstand capability of the transformer core using shape parameters. Parameter calibration is as follows: Figure 8 As shown.

[0063] In this embodiment, considering the very small leakage inductance of the current transformer in the equivalent circuit and the slowly changing DC voltage applied, the voltage drop across its leakage inductance can be ignored and treated as a short circuit. The equivalent resistance of the eddy current loss in the transformer core in this embodiment... The current is quite large, so its eddy current loss equivalent current can be ignored, and this branch can be regarded as an open circuit.

[0064] In this embodiment, to satisfy the above method, the actual parameters of the equivalent circuit are designed as follows:

[0065] The transformer core satisfies the following equation constraint, enabling electromagnetic energy conversion.

[0066]

[0067] Where: D is the electric displacement vector. Let E be the dielectric constant, E be the electric field strength, and B be the magnetic flux density. H is the magnetic permeability of the medium, H is the magnetic field strength, and J is the current density. E is the electrical conductivity, and E is the electric field strength.

[0068] In this embodiment, the design of the controllable square wave power supply is as follows:

[0069] The design power frequency for the transformer under AC voltage is: The voltage period is ;

[0070] Therefore, the duration of the positive high level (high level) in this embodiment for: ;

[0071] The duration of the negative high level (low level) in this embodiment for: ;

[0072] The magnetic field strength in this embodiment is calculated as follows:

[0073]

[0074] The magnetic flux density in this embodiment is calculated as follows:

[0075]

[0076] Where: L is the straight line connecting the positive and negative saturation points. For excitation current, For winding voltage, The DC resistance on the winding, It is the number of turns in the winding. It is the area where the magnetic flux is perpendicular to the direction of the magnetic field.

[0077] In this embodiment, based on the parameters obtained above, the hysteresis loop is plotted. The method for characterizing the hysteresis loop parameters is as follows: Figure 7 As shown:

[0078] A1: Connecting the positive and negative saturation points on the hysteresis loop forms the first straight line;

[0079] Establish a coordinate system with magnetic field strength as the abscissa and magnetic flux density as the ordinate, and plot the real-time excitation curve of the transformer core as follows: Figure 6 As shown, the coordinates of the positive saturation point are recorded as follows: The coordinates of the negative saturation point are ;like Figure 8 As shown, connect the positive and negative saturation points. The line connecting the two points is denoted as the first line L, the length of the line segment between the two points is denoted as l, and the angle between the line and the x-axis is denoted as θ. .

[0080] Therefore, the slope of the first straight line L is: ;

[0081] The functional expression of the first straight line L in the coordinate system is: ;

[0082] Therefore, the slope of the hysteresis loop can be plotted based on the slope of the first straight line L. Curve showing the change in magnetic field strength: .

[0083] A2: Construct a second straight line using the slope of the first straight line and the tangent of the hysteresis loop between the zero point and the positive saturation point;

[0084] The slope of the hysteresis loop as it changes from 0 to the positive saturation point Let point C be the location of the tangent point, and let the coordinates of the tangent point be denoted as . Establish a second straight line, whose function expression in the coordinate system is: .

[0085] A3: Construct a third straight line at the tangent between the zero point and the negative saturation point, using the slope of the first straight line and the hysteresis loop.

[0086] The slope of the hysteresis loop as it changes from 0 to the negative saturation point Let point D be the location of the tangent point, and let the coordinates of the point of tangency be denoted as Establish a third straight line, whose function expression in the coordinate system is: .

[0087] A4: Draw an ellipse with the length of the first line segment as the major axis and the sum of the lengths of the second and third lines to the first line as the minor axis;

[0088] In this embodiment, the first, second, and third straight lines have the same slope and are parallel to each other. Therefore, the distances from the tangency points C and D to the first straight line L are c and d, respectively: , .

[0089] Let the length of the line segment between the positive and negative saturation points be ,

[0090] Let the sum of the distances from the second and third lines to the first line L be denoted as . ,

[0091] by For the long axis, Draw an ellipse with the minor axis as the major axis.

[0092] A5: Determine the magnetic tolerance by calculating the eccentricity of the ellipse;

[0093] In this embodiment, based on the magnetic field environment, the average value of the absolute values ​​of the magnetic field strength at points c and d in the ellipse can be calculated as follows: The corresponding excitation current is .

[0094] In this embodiment, the eccentricity of the ellipse can be expressed geometrically by the magnetic field strength and magnetic flux density as follows:

[0095]

[0096] This embodiment can determine the changes in magnetic field strength and magnetic flux density based on the magnitude of the eccentricity of the drawn ellipse, thereby determining the bias magnetic tolerance of the transformer itself.

[0097] Example 2

[0098] Comparative testing of the bias magnetic withstand capability of the valve-side winding of converter transformers was conducted. The test subjects were two ZHSFPZ-405000 / 500 transformers, numbered T1 and T2. The grid-side winding was open-circuited, and a square wave voltage was applied to the valve-side winding.

[0099] Data Comparison and Findings:

[0100] Transformer T1: Positive saturation point A1 (410, 2.05), negative saturation point B1 (-405, -2.03), tangent point C1 (120, 1.60), tangent point D1 (-115, -1.58);

[0101] Calculations show that: l a1 ≈815.02, l b1 =0.451 + 3.627 = 4.078; Eccentricity e1 ≈ 0.999987

[0102] Transformer T2: Positive saturation point A2 (430, 2.06), negative saturation point B2 (-425, -2.04), tangent point C2 (135, 1.58), tangent point D2 (-130, -1.55);

[0103] Calculations show that: l a2 ≈855.08, l b2 =0.482+3.715=4.197; Eccentricity e2≈0.999985;

[0104] Conclusion: Both transformers have very large eccentricities, indicating excellent bias magnetic withstand capability. The saturation magnetic field strength |H2| of transformer T2 is slightly higher than that of T1, and its minor axis l of the ellipse... b2 The length is slightly longer, resulting in an eccentricity e2 that is slightly less than e1.

[0105] Based on the discrimination method described in this embodiment, under the same DC bias conditions, the withstand capability of transformer T1 is slightly better than that of transformer T2. Traditional no-load tests are unlikely to detect this degree of difference. This test result provides valuable data support for the quality control and importance rating of this high-end transformer.

[0106] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for detecting the bias magnetic withstand capability of a transformer based on the shape of a hysteresis loop, characterized in that... Includes the following steps: S1: Keep the rated voltage of the transformer unchanged, open the primary side, and input a controllable square wave power supply to the secondary side; S2: Calculate the variation of magnetic field strength and magnetic flux density within one cycle of the input controllable square wave power supply; S3: Draw the hysteresis loop of the iron core based on the results of S2: Connecting the positive and negative saturation points on the hysteresis loop forms the first straight line. Draw a tangent line between the zero point and the positive saturation point of the hysteresis loop using the slope of the first straight line, thus forming the second straight line. Draw a tangent line to the first straight line on the portion of the hysteresis loop between the positive and negative saturation points, where the magnetic flux density is negative, thus forming the third straight line. Draw an ellipse with the length of the first line segment as the major axis and the sum of the distances from the second and third lines to the first line as the minor axis; S4: Determine the magnetic tolerance by calculating the eccentricity of the ellipse.

2. The method for detecting the bias magnetic withstand capability of a transformer based on the shape of a hysteresis loop according to claim 1, characterized in that: Plot the core excitation curve with magnetic field strength on the x-axis and magnetic flux density on the y-axis, and obtain the values ​​of positive and negative saturation points from the excitation curve.

3. The method for detecting the bias magnetic withstand capability of a transformer based on the shape of a hysteresis loop according to claim 2, characterized in that: Obtain the magnetic flux density corresponding to the ordinate of the tangent points of the second and third straight lines and the excitation curve, respectively. The average of the absolute values ​​of the magnetic flux density at the two points is the magnetic field strength of the ellipse.

4. The method for detecting the bias magnetic withstand capability of a transformer based on the shape of a hysteresis loop according to claim 2, characterized in that: By detecting and recording the changes in the excitation current flowing through the winding and the voltage across the winding over time during the entire pressurization process, the time-varying laws of the magnetic field strength and magnetic flux density are calculated, and the hysteresis loop is plotted.

5. The method for detecting the bias magnetic withstand capability of a transformer based on the shape of a hysteresis loop according to claim 4, characterized in that: The magnetic field strength is: , The magnetic flux density is: , Where: L is the straight line connecting the positive and negative saturation points. For excitation current, For winding voltage, The DC resistance on the winding, It refers to the number of turns in the winding.

6. A method for detecting the bias magnetic withstand capability of a transformer based on the shape of a hysteresis loop, as described in claim 1 or 4, characterized in that... The circuit between the primary and secondary sides can be represented as: the DC resistance of the windings connected in series, the leakage inductance of the windings, and the equivalent resistance of the magnetizing inductance and eddy current loss connected in parallel.

7. The method for detecting the bias magnetic withstand capability of a transformer based on the shape of a hysteresis loop according to claim 6, characterized in that... In the equivalent circuit, ignoring the voltage drop across the leakage inductance on the winding side is equivalent to a short circuit, and ignoring the eddy current loss is equivalent to an open circuit.

8. The method for detecting the bias magnetic withstand capability of a transformer based on the shape of a hysteresis loop according to claim 6, characterized in that... After continuously inputting a high-level voltage for one power frequency cycle and a low-level voltage for one and a half power frequency cycles to the secondary side, the voltage application is stopped.

9. The method for detecting the bias magnetic withstand capability of a transformer based on the shape of a hysteresis loop according to claim 8, characterized in that... Input a square wave voltage with a frequency set to 50Hz for one power frequency cycle.

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