Method and system for detecting early fault of transformer by using third harmonic of leakage magnetic field

Abstract

The invention relates to the field of fault diagnosis and protection of power equipment, in particular to a method and a system for detecting an early fault of a transformer by using third harmonic of a leakage magnetic field, comprising the following steps of: calculating leakage magnetic induction intensity, and establishing a three-dimensional analytical calculation model of the leakage magnetic field; calculating a setting value of a protection criterion by using the leakage magnetic field three-dimensional analytical calculation model, and designing a sensor arrangement scheme; leakage magnetic field signals are monitored in real time based on a sensor, and a fault protection mechanism is established; a leakage magnetic field distribution three-dimensional analytical model considering three-phase windings is established according to an actual transformer winding structure, the occurrence time of a turn short fault is accurately positioned, a measuring point measurement scheme located at the upper, middle and lower positions of a longitudinal gap between the windings is provided according to longitudinal symmetry of leakage magnetic field distribution, accuracy and sensitivity are improved, and the accuracy and the reliability of a measurement result are improved. In the turn-to-turn short circuit starting criterion, a fault characteristic quantity is constructed by using a relative difference value between a fault transient quantity and a normal quantity before a half cycle, so that the protection is not influenced by excitation inrush current and an external fault, and the quickness is superior to that of the traditional longitudinal differential protection.

Description

Technical Field

[0001] The invention relates to the field of fault diagnosis and protection of electric power equipment, and in particular to a method and system for detecting early transformer faults by utilizing third harmonic of leakage magnetic field. Background Art

[0002] The existing research on leakage magnetic field is based on the simulation or mathematical model of the single-phase winding of the transformer, and does not consider the influence of the mutual leakage inductance of the other two-phase winding on the leakage magnetic field. When no-load closing and out-of-zone short circuit occur, the actual leakage magnetic field distribution is very different from the distribution obtained by the single-phase winding mathematical model. The three-dimensional analytical calculation model of the leakage magnetic field taking into account the three-phase winding of the transformer proposed in this patent can accurately calculate the leakage magnetic field distribution under normal operation, early faults, and out-of-zone faults.

[0003] The existing technology still has a gap in fault detection using harmonic information in transformer electromagnetic quantities. When harmonic current passes through the transformer winding, it will form a winding harmonic leakage magnetic field. The distribution of the harmonic leakage magnetic field will change in the early stage of transformer faults. This patent uses the change of the harmonic component of the leakage magnetic field as a fault feature and proposes a detection criterion based on the principle of the third harmonic differential of the leakage magnetic field to identify early faults.

[0004] In view of the deficiencies in the prior art, the present invention proposes a three-dimensional analytical calculation model of the leakage magnetic field taking into account the three-phase winding of the transformer. The model can accurately simulate and calculate the leakage magnetic field distribution characteristics of the transformer under normal operation, early fault and out-of-zone fault conditions. Based on the electromagnetic coupling effect of the three-phase winding, the present invention accurately describes the spatiotemporal distribution characteristics of the leakage magnetic field through analytical calculation methods, and introduces the analysis of harmonic leakage magnetic field for the first time. The change of the harmonic component of the leakage magnetic field is used as a fault feature, and a fault identification criterion based on the third harmonic differential principle is developed. Compared with traditional methods, this model can not only significantly improve the calculation accuracy of the leakage magnetic field distribution, but also capture the slight changes in the harmonic leakage magnetic field in the early fault stage, thereby achieving high-sensitivity detection of faults. Summary of the invention

[0005] In view of the above-mentioned problems, the present invention is proposed.

[0006] To solve the above technical problems, the present invention provides the following technical solutions: a method for detecting early transformer faults using the third harmonic of the leakage magnetic field, comprising: calculating the leakage magnetic induction intensity and establishing a three-dimensional analytical calculation model of the leakage magnetic field; calculating the setting value of the protection criterion using the three-dimensional analytical calculation model of the leakage magnetic field and designing a sensor layout plan; and establishing a fault protection mechanism based on real-time monitoring of the leakage magnetic field signal by the sensor.

[0007] As a preferred embodiment of the method for detecting early faults of a transformer by using the third harmonic of the leakage magnetic field according to the present invention, wherein: establishing a three-dimensional analytical calculation model of the leakage magnetic field includes calculating the magnetic induction intensity generated by a unit current on a conductor loop, obtaining the leakage magnetic induction intensity generated by a single segment, using the superposition property of the leakage magnetic field to calculate the leakage magnetic induction intensity generated by different windings and different segments, obtaining the total leakage magnetic induction intensity of a single phase, and superimposing the leakage magnetic induction intensities of three phases to obtain the distribution of the third harmonic leakage magnetic flux.

[0008] As a preferred embodiment of the method for detecting early faults of a transformer by using the third harmonic of the leakage magnetic field according to the present invention, wherein: the distribution of the third harmonic leakage magnetic flux includes that the amplitude of the third harmonic of the leakage magnetic flux density is expressed as

[0009]

[0010] where B 3ym is the amplitude of the third harmonic of the leakage magnetic flux density;

[0011] The amplitude of the third harmonic of the leakage magnetic flux is expressed as

[0012]

[0013] where Φ 3ym is the amplitude of the third harmonic of the leakage magnetic flux.

[0014] As a preferred embodiment of the method for detecting early faults of a transformer by using the third harmonic of the leakage magnetic field according to the present invention, wherein: calculating the setting value of the protection criterion includes calculating the setting value of the inter-turn short circuit criterion and the setting value of the winding deformation criterion;

[0015] Calculating the setting value of the inter-turn short circuit criterion includes constructing a fault steady-state component by using the difference between the fault steady-state quantity and the normal quantity, expressed as

[0016] ΔΦ″ 3dif (t) = Φ 3dif (t) - Φ 3dif (t - 2.2T)

[0017] ΔB″ 3cen (t) = B 3cen (t) - B 3cen (t - 2.2T)

[0018] Calculating the setting value according to avoiding the three-phase short circuit outside the zone is expressed as

[0019]

[0020] where Φ set3 is the setting value of the fault steady-state component of the magnetic differential, and B set2 is the setting value of the fault steady-state component of the magnetic center, is the reliability coefficient;

[0021] The setting values of the winding deformation criterion include the upper-end deformation setting value, the lower-end deformation setting value, and the magneto-differential criterion setting value, expressed as

[0022]

[0023]

[0024] where is the reliability coefficient, and n% is the minimum axial winding deformation degree that actually needs to be detected.

[0025] As a preferred solution of the method for detecting early faults of a transformer by using the third harmonic of the leakage magnetic field according to the present invention, wherein: the designed sensor arrangement scheme includes arranging measuring points at positions on the outer diameter length from the winding axis and the axial height of the winding within the iron core window.

[0026] As a preferred solution of the method for detecting early faults of a transformer by using the third harmonic of the leakage magnetic field according to the present invention, wherein: the real-time monitoring of the leakage magnetic field signal includes collecting the leakage magnetic field data of the measuring points, calculating the third harmonic amplitude signal in real time by using the short-time Fourier transform, and calculating the relative value of the fault transient component, the fault steady-state quantity, and the fault steady-state component.

[0027] As a preferred solution of the method for detecting early faults of a transformer by using the third harmonic of the leakage magnetic field according to the present invention, wherein: the fault protection mechanism includes that if the limit is exceeded, the turn-to-turn short-circuit protection is started, and median filtering is performed after a delay of one and a half cycles, and it is judged whether it is a turn-to-turn short-circuit fault through the setting value;

[0028] If the limit is not exceeded, it means that the signal has not changed suddenly, and median filtering is directly performed, and it is judged whether it is a deformation fault through the setting value;

[0029] If it is judged as a fault, the fault phase is located and a fault report is sent.

[0030] To solve the above technical problems, the present invention provides the following technical solution: a system for the method of detecting early faults of a transformer by using the third harmonic of the leakage magnetic field, characterized in that it includes: a model establishment module, a sensor arrangement module, and a fault protection module;

[0031] The model establishment module calculates the leakage magnetic induction intensity and establishes a three-dimensional analytical calculation model of the leakage magnetic field;

[0032] The sensor arrangement module calculates the setting value of the protection criterion by using the three-dimensional analytical calculation model of the leakage magnetic field and designs a sensor arrangement scheme;

[0033] The fault protection module monitors the leakage magnetic field signal in real time based on a sensor and establishes a fault protection mechanism.

[0034] A computer device includes a memory and a processor. The memory stores a computer program, and when the processor executes the computer program, the steps of the method for detecting early faults of a transformer using the third harmonic of the leakage magnetic field as described above are implemented.

[0035] A computer-readable storage medium stores a computer program thereon. When the computer program is executed by a processor, the steps of the method for detecting early faults of a transformer using the third harmonic of the leakage magnetic field as described above are implemented.

[0036] The beneficial effects of the present invention: A three-dimensional analytical model of the leakage magnetic field distribution considering three-phase windings is established according to the actual transformer winding structure. The established model shows that the sensitivity of the leakage magnetic third harmonic signal is higher than that of the electrical quantity harmonic signal. The third harmonic amplitude curve extracted by comprehensively using the short-time Fourier transform and median filtering is far superior to the fundamental wave curve in terms of transient change, and can accurately locate the occurrence time of the turn-to-turn short circuit fault.

[0037] According to the longitudinal symmetry of the leakage magnetic field distribution, a 6-measurement point measurement scheme at the upper, middle, and lower positions of the longitudinal gap between windings is proposed. The relative value of the fault transient component and the fault steady-state component constructed according to the characteristics of the third harmonic amplitude curve have high accuracy and sensitivity in the case of slight turn-to-turn short circuit and winding deformation faults, and can identify winding deformation faults of 5% and above and turn-to-turn short circuit faults of 0.995% and above.

[0038] In the turn-to-turn short circuit starting criterion, a fault characteristic quantity is constructed using the relative difference between the fault transient quantity and the normal quantity half a cycle ago, and in the turn-to-turn short circuit criterion, a fault characteristic quantity is constructed using the difference between the fault steady-state quantity and the normal quantity, so that this protection is not affected by inrush current and external faults, and is superior to the traditional longitudinal differential protection in terms of speed. Description of the Drawings

[0039] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for the description of the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0040] Figure 1 It is a flowchart of the method for detecting early faults of a transformer using the third harmonic of the leakage magnetic field provided by an embodiment of the present invention.

[0041] Figure 2 It is a schematic diagram of the analytical model calculation of the method for detecting early faults of a transformer using the third harmonic of the leakage magnetic field provided by an embodiment of the present invention.

[0042] Figure 3 The STFT signal mutation point detection diagram for the method of detecting early faults of transformers using the third harmonic of leakage magnetic field provided by an embodiment of the present invention.

[0043] Figure 4 The wavelet transform signal mutation point detection diagram for the method of detecting early faults of transformers using the third harmonic of leakage magnetic field provided by an embodiment of the present invention.

[0044] Figure 5 The schematic diagram of the calculated value of the axial distribution of leakage magnetic field for the method of detecting early faults of transformers using the third harmonic of leakage magnetic field provided by an embodiment of the present invention.

[0045] Figure 6 The sensor layout diagram and fault phase location logic diagram for the method of detecting early faults of transformers using the third harmonic of leakage magnetic field provided by an embodiment of the present invention.

[0046] Figure 7 The flowchart of harmonic differential protection of leakage magnetic field for the method of detecting early faults of transformers using the third harmonic of leakage magnetic field provided by an embodiment of the present invention.

[0047] Figure 8 The dynamic simulation system device diagram for the method of detecting early faults of transformers using the third harmonic of leakage magnetic field provided by an embodiment of the present invention.

[0048] Figure 9 The ANSYS simulation model of a segmented irregular winding transformer for the method of detecting early faults of transformers using the third harmonic of leakage magnetic field provided by an embodiment of the present invention.

[0049] Figure 10 The experimental waveform diagram of the radial leakage magnetic field at the upper measuring point under different degrees of winding deformation for the method of detecting early faults of transformers using the third harmonic of leakage magnetic field provided by an embodiment of the present invention.

[0050] Figure 11 The amplitude curve diagram of the third harmonic of the radial leakage magnetic field at the upper measuring point under different degrees of winding deformation for the method of detecting early faults of transformers using the third harmonic of leakage magnetic field provided by an embodiment of the present invention.

[0051] Figure 12 The amplitude curve diagram of the third harmonic of the magnetic differential momentum and magnetic center quantity under 0.995% turn-to-turn short circuit at the lower end for the method of detecting early faults of transformers using the third harmonic of leakage magnetic field provided by an embodiment of the present invention.

[0052] Figure 13 The experimental waveform diagram of the radial leakage magnetic flux at the upper and lower measuring points during no-load closing for the method of detecting early faults of transformers using the third harmonic of leakage magnetic field provided by an embodiment of the present invention.

[0053] Figure 14 The curve of the third harmonic amplitude of the leakage magnetic flux at the lower measurement point and the relative value curve of the fault transient component during no-load closing of the method for detecting early faults of a transformer using the third harmonic of the leakage magnetic field provided by an embodiment of the present invention. Specific Embodiments

[0054] In order to make the above objects, features, and advantages of the present invention more obvious and understandable, the following will describe the specific embodiments of the present invention in detail with reference to the accompanying drawings of the specification. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0055] In the following description, many specific details are set forth in order to fully understand the present invention. However, the present invention can also be implemented in other ways different from those described herein. Those skilled in the art can make similar extensions without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0056] Example 1, referring to Figures 1-7 , which is an embodiment of the present invention. The method for detecting early faults of a transformer using the third harmonic of the leakage magnetic field includes:

[0057] S1: Calculate the leakage magnetic induction intensity and establish a three-dimensional analytical calculation model of the leakage magnetic field.

[0058] It should be noted that establishing a three-dimensional analytical calculation model of the leakage magnetic field includes calculating the magnetic induction intensity generated by a unit current on the conductor ring to obtain the leakage magnetic induction intensity generated by a single segment, using the superposition property of the leakage magnetic field to calculate the leakage magnetic induction intensity generated by different windings and different segments, obtaining the total leakage magnetic induction intensity of a single phase, and superimposing the leakage magnetic induction intensities of the three phases to obtain the distribution of the third harmonic leakage magnetic flux.

[0059] Furthermore, in order to make the established analytical model universal, a non-regular segmented winding transformer is used as the research object, and its size is the same as that of the dynamic simulation transformer below. The specific parameters are shown in Table 1. A rectangular coordinate system is established with the geometric center of the iron core column of phase B as the origin, and its two-dimensional front view is as Figure 2 shown. Since most of the magnetic force line loops in the leakage magnetic field pass through the air medium, the superposition property of the leakage magnetic field can be used to calculate the leakage magnetic induction intensity generated by different windings and different segments respectively, and finally add them up to obtain the total leakage magnetic induction intensity, as shown in S1 in Figure 1 .

[0060] Table 1 Structural Parameters of Non-regular Winding Transformer

[0061]

[0062] Taking a certain segment k of the B-phase high-voltage winding as the analysis object, the segment is divided into a series of conductor layers in the axial direction. A single conductor layer with a thickness of dz is taken and divided into a series of conductor rings in the radial direction. The Biot-Savart law can be used to find the magnetic induction intensity generated by a unit current on the conductor ring, and finally the leakage magnetic induction intensity generated by segment k is obtained by integration.

[0063] The radial leakage magnetic induction intensities of all segments of the B-phase are superimposed to obtain:

[0064]

[0065] If the transformer structure function is denoted as,

[0066]

[0067] Then the structure functions of the A-phase and C-phase windings can be obtained from the transformer structure function through coordinate transformation:

[0068] G a (x0,y0,z0) = G(x0,y0 + L c ,z0)

[0069] G c (x0,y0,z0) = G(x0,y0 - L c ,z0)

[0070] where L c is the center distance of the iron core column.

[0071] The leakage magnetic induction intensity can be expressed as the product of the current function and the structure function, indicating that the harmonic leakage magnetic component is mainly generated by the harmonic current component of the winding. From the generation mechanism, the primary side winding current can be divided into the exciting component and the grid side component, and the secondary side winding current can be divided into the exciting component and the load side component, expressed as,

[0072]

[0073] However, for the convenience of analysis, it is assumed that the above non-sinusoidal winding current waveforms are all known. Substitute their Fourier expansion formulas into the leakage magnetic induction intensity superposition expression, and then superimpose the radial leakage magnetic formulas of the three phases, that is, the radial leakage magnetic induction intensity distribution considering the three-phase windings at any point is obtained, expressed as,

[0074]

[0075] If the third harmonic term is extracted, the third harmonic leakage magnetic distribution formula is obtained, expressed as,

[0076]

[0077] When inter-turn short circuit occurs in the winding, the ampere-turn balance is destroyed, and the value of the steady-state current of the inter-turn short circuit in the primary winding approximately satisfies the following relationship:

[0078]

[0079] Where: I'1 is the current of the primary winding; I s is the current of the short-circuited winding; I'2 is the current of the secondary winding; N s is the number of shorted turns; N'1 is the number of turns in the non-shorted area of the primary side; N2 is the number of turns of the secondary side.

[0080] Then the current of the short-circuited winding can be obtained from the current of the primary and secondary windings at this time according to the numerical relationship formula of the steady-state current of the inter-turn short circuit in the primary winding. According to the superposition property of the leakage magnetic field, the leakage magnetic field under inter-turn short circuit can be obtained by adding the normal-phase leakage magnetic field B y_health and the fault-phase leakage magnetic field B y_fault . The method for obtaining the normal-phase leakage magnetic field is the same as that of the normal model, and the fault-phase leakage magnetic field is obtained by superimposing the leakage magnetic fields of the non-shorted section of the fault winding, the shorted section of the fault winding, and the non-fault winding. Assume that the inter-turn short circuit occurs in the kth section of the primary winding of phase A, and its height range is [H 2s , H 1s , and the radial range is [a s , b s . The fault-phase leakage magnetic field is:

[0081]

[0082] Where: G' A is the structure function of the non-shorted area of the primary winding; G As is the structure function of the shorted area of the primary winding; G a is the structure function of the secondary winding.

[0083] From the perspective of the model, the axial winding deformation of the transformer is essentially a change in the winding height, and the winding current is not different. Only need to modify the original height [H 2k , H 1k of the deformed segmented winding to the deformed height [H′ 2k , H′ 1k , and then substitute it into the transformer structure function to obtain the new structure function.

[0084] The expressions for the harmonic leakage magnetic density and the amplitude of the third harmonic of the leakage magnetic flux are as follows:

[0085]

[0086] When using harmonic signals to detect internal faults, the harmonic content rate is a key indicator reflecting the sensitivity. In this embodiment, the harmonic content rate HRB3 of the leakage magnetic third harmonic is analyzed based on the analytical model. For the convenience of analysis, three assumptions are made:

[0087] Primary side winding current i A,B,C and secondary side current i a,b,c are basically in antiphase;

[0088] 1) The third harmonic content rates of the primary and secondary side currents are the same, that is

[0089] I A3m / I A1m = I a3m / I a1m = k

[0090] 3) The primary and secondary side currents are both three-phase symmetrical currents, that is, the phases differ by 120° and the amplitudes are the same.

[0091] Due to the nth harmonic component of the radial leakage flux:

[0092]

[0093] It is composed of the addition of sinusoidal quantities with the same frequency, so it can be transformed into phasor analysis and the amplitude expression can be obtained through complex number calculation. After the three-phase symmetrical current is expanded into Fourier series, the fundamental wave belongs to the positive sequence group and the third harmonic belongs to the zero sequence group. Since the fundamental wave dominates in the current, it can be considered that the phase relationship between the fundamental wave and the whole is the same, that is, the phases of the primary side and the secondary side are basically in antiphase, while the phase relationship between the primary side and the secondary side of the third harmonic needs to be determined by the specific waveform, but the sum of the two has a definite range.

[0094] Therefore, the amplitude of the fundamental wave is:

[0095]

[0096] For the third harmonic, the amplitudes in the two cases of in-phase and antiphase between the primary side and the secondary side, which are the maximum and minimum, are respectively:

[0097]

[0098] Substitute the coordinate values (0, -480, 175) of the measuring point outside the upper end of phase A into the calculation, and the numerical values of each structure function are:

[0099] {G A , G B , G C , G a , G b , G c} = {-22.86, -0.81, -0.13, -0.54, -0.26, -0.04} * 10 -5

[0100] And considering that the fundamental wave of the primary and secondary sides basically satisfies I a1m / IA1m = N1 / N2 = 1.45

[0101] And based on assumption (2), the leakage magnetic flux third harmonic content rate can be obtained as follows:

[0102]

[0103] All are greater than the harmonic content rate k of the current.

[0104] S2: Calculate the setting value of the protection criterion using the three-dimensional analytical calculation model of the leakage magnetic field, and design the sensor layout scheme.

[0105] It should be noted that calculating the setting value of the protection criterion includes calculating the setting value of the turn-to-turn short circuit criterion and the setting value of the winding deformation criterion;

[0106] The calculation of the setting value of the turn-to-turn short circuit criterion includes constructing the fault steady-state component using the difference between the fault steady-state quantity and the normal quantity, expressed as,

[0107] ΔΦ″ 3dif (t) = Φ 3dif (t) - Φ 3dif (t - 2.2T)

[0108] ΔB″ 3cen (t) = B 3cen (t) - B 3cen (t - 2.2T)

[0109] And calculating the setting value according to avoiding the out-of-zone three-phase short circuit situation, expressed as,

[0110]

[0111] Among them, Φ set3 is the setting value of the magnetic differential fault steady-state component, B set2 is the setting value of the magnetic center fault steady-state component, is the reliability coefficient;

[0112] The calculation of the setting value of the winding deformation criterion includes the upper end deformation setting value, the lower end deformation setting value, and the magnetic differential criterion setting value, expressed as,

[0113]

[0114] Among them, is the reliability coefficient, and n% is the minimum axial winding deformation degree that actually needs to be detected.

[0115] Furthermore, according to the principle of magnetic flux continuity, the leakage magnetic lines of force of the transformer winding are closed loops that link with the winding. Therefore, for any closed surface, the magnetic lines of force entering must be equal to the magnetic lines of force exiting. Also, since the leakage magnetic field distribution exhibits a certain symmetry along the horizontal axis in the middle of the winding under normal operating conditions, the differential protection of the leakage magnetic field can be constituted by measuring the magnetic induction intensities (one entering and one exiting) at two symmetric points passing through the same magnetic lines of force within the closed space. The leakage magnetic field is generated by the winding current. As long as the symmetry of the current density distribution across the winding cross-section remains unchanged, when through-current flows through both ends of the winding (such as magnetizing inrush current, out-of-zone fault), the magnetic lines of force passing through the two measuring points are still equal in magnitude and opposite in direction, and the differential quantity is 0. However, when winding deformation or internal turn-to-turn faults occur, the symmetry of the current density distribution and the leakage magnetic field distribution is disrupted, resulting in a large differential quantity.

[0116] To improve the reliability of this embodiment, the differential quantity is constructed by measuring the leakage magnetic flux passing through the triangular regions at symmetric positions on the upper and lower ends. Three sensors are placed at the vertices of an equilateral triangle with a side length of 30 mm to measure the leakage magnetic induction intensity. If an xy coordinate system is established on the plane of the triangle, and the B y value is used as the z-axis, then the leakage magnetic field on the measurement domain of this plane can be described by a surface in the rectangular coordinate system. Similar to the linear interpolation that can be performed on a straight line in a plane, a plane can be constructed using three vertices P1, P2, and P3 for "plane interpolation" in space.

[0117] In this way, the originally unknown leakage magnetic field distribution B y (x, y) is replaced by a simple plane equation, and the calculation formula for the leakage magnetic flux can be approximated as:

[0118] Φ y =∫∫ ∑ B y (x, y)·dS≈∫∫ ∑ (-A'x - B'y + D')dS

[0119] In the formula, A', B', and D' are the coefficients of the plane equation, which are obtained from the coordinate values of the three vertices and the leakage magnetic induction intensity, and can be expressed as:

[0120]

[0121] The four third-harmonic protection characteristic quantities used in this embodiment are the upper magnetic quantity Φ 3up , the lower magnetic quantity Φ 3dw , the central magnetic quantity Φ 3cen , and the magnetic differential quantity Φ 3dif , which are expressed as,

[0122] Φ 3dif =Φ 3up -kΦ 3dv

[0123] Among them, k is a compensation coefficient for compensating the inconsistent amplitudes of the leakage fluxes at the upper and lower ends.

[0124] As a time-frequency joint analysis method (Time-Frequency Analysis), the Short-Time Fourier Transform (STFT) has the advantages of taking into account both time-domain and frequency-domain resolutions, having a clear physical meaning of the transform, and a small amount of computation, and is suitable for analyzing the transient disturbance signals of the leakage magnetic field. For STFT, the signal mutation means an increase in high-frequency components. When the window function moves to the transition section where the turn-to-turn short circuit occurs, the amplitudes of the high-order harmonics will show a changing rule of increasing first and then decreasing. Therefore, the amplitude curve of the third harmonic obtained by STFT will present a very large transient peak at the moment when the turn-to-turn short circuit occurs, generally lasting for one cycle T (=0.02 s), and then stabilizing at a new value. This characteristic is beneficial to real-time fault detection.

[0125] Take the Figure 3 leakage magnetic flux signal at the lower measuring point of the winding when 2 turns at the lower end of the winding shown in Figure 3 (a) are subjected to STFT transformation. The fault occurrence moment is at point 262 and lasts for 16.5 cycles. The window function of STFT is selected as the blackman window, with a window width of one cycle. The amplitude curves of the third, fifth harmonics and the fundamental wave obtained by the transformation are as shown in

[0126] (b), (c), (d). Figure 3 The wavelet transform is another means of detecting local mutation points of the signal. After the signal is subjected to wavelet transform, the absolute value of the wavelet coefficient corresponding to the mutation point is often very large. Apply the DB6 wavelet function to decompose the leakage magnetic flux signal in Figure 4 (a) at 3 scales to obtain the wavelet coefficients D1, D2, D3 at each scale as shown in

[0127] It can be seen that the fault moment determined by the transient peak of the third harmonic amplitude curve is consistent with the mutation moment of the original signal waveform, and the detection ability at the fault starting moment is better than that of the fifth harmonic. The fundamental wave component shows a monotonically decreasing slope in the turn-to-turn short circuit transition section, and the start and end moments of the fault cannot be accurately located, and the sensitivity of detecting the turn-to-turn short circuit fault is also not as good as that of the third harmonic. Since the mutation degree of the original signal waveform is not severe enough at the fault starting moment, the modulus maxima of the three wavelet coefficient curves are not obvious at this moment, and the start and end moments of the fault cannot be accurately detected. Therefore, compared with the wavelet transform, STFT has stronger detection ability and higher sensitivity for the leakage magnetic field mutation signal under turn-to-turn short circuit. To sum up, the third harmonic component after STFT is selected in this embodiment to analyze the leakage magnetic field mutation signal.

[0128] When a turn - to - turn short - circuit fault occurs in a transformer, the third - harmonic amplitude curve under the turn - to - turn short - circuit can be divided into: a normal section, a fault transient section, and a fault steady - state section. The corresponding third - harmonic amplitudes are called the normal quantity, the fault transient quantity, and the fault steady - state quantity. In this embodiment, a fault characteristic quantity is constructed using the relative difference between the fault transient quantity and the normal quantity half a cycle ago, which is called the relative value of the fault transient component and is expressed as

[0129]

[0130] The corresponding turn - to - turn short - circuit starting criterion is:

[0131] ΔX′%>X set

[0132] where X = Φ 3up ,Φ 3dv ,B 3cen ,X set is the corresponding setting value.

[0133] After a turn - to - turn short - circuit occurs in the winding, the difference between the third - harmonic fault steady - state quantity and the normal quantity at the measuring point near the fault is significant. Therefore, a fault characteristic quantity is constructed using the difference between the fault steady - state quantity and the normal quantity a certain time ago, which is called the fault steady - state component. Since there are slight ripples in the stable section of the actually measured third - harmonic curve of the leakage magnetic field, which is not conducive to the operation of the protection criterion, median filtering is used to smooth the stable section of the curve after STFT. The calculation width of the median filtering takes the number of sampling points N within one cycle, and the median filtering value of any sampling point is obtained from this point and the previous N - 1 sampling points. Assume that the turn - to - turn short - circuit starts at time t2, then after half a cycle (t = t3) the curve enters the fault steady - state section, and the first effective median filtering point is calculated at time t4. In addition, to avoid the side - lobe interference beside the transient spike, the time interval is finally taken as 2.2T, and the fault steady - state component is expressed as

[0134] ΔX″(t) = X(t)-X(t - 2.2T)

[0135] The corresponding turn - to - turn short - circuit criterion is:

[0136] |ΔΦ′ 3dif |>Φ set3

[0137] ΔB″ 3cen >B set2

[0138] where X = Φ 3dif ,B 3cen ,Φ set3 ,B set2 is the corresponding setting value.

[0139] Transformer winding deformation is generally caused by the repeated action of the electrodynamic force generated by the fault current and belongs to an accumulative fault. Therefore, there is only the steady-state quantity of the fault. When the winding is axially compressed and deformed, the amplitude curve of the third harmonic on the near-compressed side decreases, while the change on the non-compressed side is very small. Therefore, the criterion for winding deformation is:

[0140] Φ′ 3up <Φ′ set1

[0141] Φ″ 3dv <Φ′ set2

[0142] |Φ″ 3dif |>Φ′ set3

[0143] Among them, Φ' set1 、Φ' set2 、Φ' set3 are the corresponding setting values.

[0144] Furthermore, the design of the sensor layout scheme includes arranging measurement points at the positions of the outer diameter length and the axial height of the winding from the axis of the winding in the iron core window.

[0145] To determine the position of the leakage magnetic field measurement with the highest sensitivity, the leakage magnetic induction intensity distribution curves on the vertical paths at 110% of the outer diameter length from the axis of the winding inside the iron core window, inside the iron core window, and outside the iron core window when the current in phase A reaches the peak are calculated respectively using the leakage magnetic field analysis model proposed above, as shown in Figure 5 (a). It can be seen from the calculation results in Figure 5 (b) that the overall value of the leakage magnetic induction intensity inside the window is greater than that on the outside and the front, and extreme values appear at -155.7 mm and 155.7 mm, indicating that these two places can best reflect the change of the current. In addition, due to the irregularity of the winding structure, the symmetric position of the leakage magnetic field is not at the 1 / 2 height of the winding, but at 27.26 mm, where the leakage magnetic field is close to 0 under normal conditions and is not 0 during internal faults. Therefore, the installation positions of the upper, middle, and lower measurement points are at 110% of the outer diameter length from the axis of the winding inside the iron core window, -155.7 mm, 27.26 mm, and 155.7 mm on the axial height of the winding.

[0146] S3: Based on the real-time monitoring of the leakage magnetic field signal by the sensor, a fault protection mechanism is established.

[0147] It should be noted that the real-time monitoring of the leakage magnetic field signal includes collecting the leakage magnetic field data of the measurement points, calculating the third harmonic amplitude signal in real time using the short-time Fourier transform, and calculating the relative value of the fault transient component, the fault steady-state quantity, and the fault steady-state component.

[0148] Furthermore, the fault protection mechanism includes that if the limit is exceeded, the inter-turn short-circuit protection is activated, and median filtering is performed after a delay of one and a half cycles, and it is judged whether it is an inter-turn short-circuit fault through the setting value;

[0149] If the limit is not exceeded, it indicates that the signal has not mutated, and median filtering is directly performed, and it is judged whether it is a deformation fault through the setting value;

[0150] If it is judged as a fault, the fault phase is located and a fault report is sent.

[0151] Considering the small probability of simultaneous faults in more than two windings and the economy of the protection scheme, in this paper, a layout scheme of arranging 3 measuring points between the AB-phase windings and between the BC-phase windings in the iron core window, with a total of 14 sensors, is adopted, as Figure 6 (a) shown. Among them, the 3 sensors between the AB phases are called sensor group 1, and between the BC phases is called group 2. Since the three-phase three-column planar transformer takes the B-phase iron core as the axis of symmetry, the leakage magnetic field distribution is symmetric left and right, and the fault criteria of group 1 and group 2 are the same, and the setting values are also the same.

[0152] In the case of only considering a single fault, if only group 1 operates, it indicates that the A-phase winding has a fault; if both group 1 and group 2 operate, it indicates that the B-phase winding has a fault; if only group 2 operates, it indicates that the C-phase winding has a fault, as Figure 6 (b) shown.

[0153] The specific flow chart of the leakage magnetic field harmonic differential protection is as Figure 7 shown. First, the leakage magnetic field data of 6 measuring points are collected by the magneto-optical sensor, then the STFT calculates the amplitude of the third harmonic and further calculates the relative value of the fault transient component. If the limit is exceeded, the inter-turn short-circuit protection is activated, and median filtering is performed after a delay of 1.5 cycles and then it is judged whether it is an inter-turn short fault; if the limit is not exceeded, it indicates that the signal has not mutated, and median filtering is directly performed to judge whether it is a deformation fault. If it is judged as a fault, then according to Figure 6 the discrimination logic locates the fault phase, and finally acts and sends a fault report.

[0154] Embodiment 2 is an embodiment of the present invention, and the present invention provides the following technical solutions: A system for detecting early faults of a transformer by using the third harmonic of the leakage magnetic field includes: a model establishment module, a sensor layout module, and a fault protection module;

[0155] The model establishment module calculates the leakage magnetic induction intensity and establishes a three-dimensional analytical calculation model of the leakage magnetic field;

[0156] The sensor layout module uses the three-dimensional analytical calculation model of the leakage magnetic field to calculate the setting value of the protection criterion and designs the sensor layout scheme;

[0157] The fault protection module monitors the leakage magnetic field signal in real time based on a sensor and establishes a fault protection mechanism.

[0158] This embodiment also provides a computing device applicable to the case of using the third harmonic of the leakage magnetic field to detect early faults of a transformer, including:

[0159] A memory and a processor; the memory is used to store computer-executable instructions, and the processor is used to execute the computer-executable instructions to implement the method for detecting early faults of a transformer using the third harmonic of the leakage magnetic field as proposed in the above embodiment.

[0160] The storage medium proposed in this embodiment and the method for detecting early faults of a transformer using the third harmonic of the leakage magnetic field proposed in the above embodiment belong to the same inventive concept. Technical details not described in detail in this embodiment can be referred to in the above embodiment, and this embodiment has the same beneficial effects as the above embodiment.

[0161] If the above functions are implemented in the form of software function units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, or a part of this technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions for causing a computer device (which can be a personal computer, a server, or a network device, etc.) to execute all or part of the steps of the methods described in various embodiments of the present invention. The aforementioned storage medium includes: various media such as USB flash drives, mobile hard disks, read-only memory (ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), magnetic disks, or optical discs that can store program codes.

[0162] The logic and / or steps represented in the flowchart or described in other ways herein, for example, can be considered as a definite sequence list of executable instructions for implementing logical functions, and can be specifically implemented in any computer-readable medium for use by an instruction execution system, apparatus, or device (such as a computer-based system, a system including a processor, or other systems that can fetch instructions from the instruction execution system, apparatus, or device and execute the instructions), or in combination with these instruction execution systems, apparatus, or devices. For the purposes of this specification, a "computer-readable medium" can be any device that can contain, store, communicate, propagate, or transmit a program for use by or in connection with an instruction execution system, apparatus, or device.

[0163] It should be understood that various parts of the present invention can be implemented by hardware, software, firmware, or a combination thereof. In the above embodiments, multiple steps or methods can be implemented by software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented by hardware, as in another embodiment, any one or a combination of the following techniques well known in the art can be used: discrete logic circuits having logic gate circuits for implementing logical functions on data signals, application specific integrated circuits having appropriate combinational logic gate circuits, programmable gate arrays (PGAs), field programmable gate arrays (FPGAs), etc.

[0164] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

[0165] Example 3, referring to Figures 8-14 , an embodiment of the present invention provides a method for detecting early faults of a transformer by using the third harmonic of the leakage magnetic field. In order to verify the beneficial effects of the present invention, scientific demonstrations are carried out through economic benefit calculations and simulation experiments.

[0166] (1) Establishment of simulation and experiment

[0167] ① Establishment of a dynamic simulation system

[0168] The experimental transformer is a three-phase double-winding dry-type transformer with a Yn / d11 connection. The electrical parameters of the transformer are: rated capacity 50 kVA, rated voltage 1 / 0.4 kV, and the winding parameters referred to the high-voltage side are R T = 0.637 Ω, X T = 1.52 Ω, G T = 0.257 mS, B T = 1.72 mS. The voltage regulation range of the infinite adjustable power supply is 0 - 1.5 kV, and the internal resistance is not considered; line parameters: r1 = 8.835 mΩ / km, l1 = 0.482 mH / km, and the length is 200 km; the adjustable resistive load has an adjustment range of 3.2 - 230.9 Ω.

[0169] The physical object of the dynamic simulation transformer is shown in Figure 8 (a), where there are 7 short-circuit taps on the surface of the high-voltage winding of phase A, and in cooperation with the transformer control cabinet, a slight turn-to-turn fault of turns 1 to 7 in the middle and lower part of the winding can be realized for 500 ms; the high-voltage winding of phase C is a variable-height winding to simulate a slight axial compression fault of the winding. The leakage magnetic field measurement system uses a magneto-optical sensor based on the Faraday magneto-optical effect, which has the advantages of being immune to electromagnetic interference and small volume.Figure 8 (c) The independently developed magnetic protection unit calculates the STFT, smooths, and calculates the protection criterion for the magnetic leakage data from the sensor, and then transmits the magnetic leakage harmonic data and fault reports to the upper computer, so as to realize the real-time online detection of early transformer faults.

[0170] To analyze the protection operation under complex working conditions such as out-of-zone faults that are difficult to conduct in dynamic simulation experiments, in this embodiment, a three-dimensional simulation model consistent with the dynamic simulation experiment transformer is first established in ANSYS as Figure 9 shown. Then, the magnetic leakage simulation values and experimental values of the upper, middle, and lower measuring points of Group 1 under rated operation and no-load closing are respectively plotted to verify the consistency of the magnetic leakage field distribution between the simulation model and the experimental transformer. In order to quantitatively evaluate the consistency of the model, the Hausdorff distance algorithm is selected to measure the similarity between the experimental curve and the simulation curve, and between the analytical curve and the simulation curve. Taking the percentage HB% of the Hausdorff distance of the magnetic leakage flux experimental curve itself under rated conditions relative to the curve amplitude as the measurement benchmark, the Hausdorff distance percentage H% of the curve group to be verified is compared with it. If H% does not exceed 10HB%, it indicates that the similarity of this group of curves is good. The H% corresponding to the curve group in the above figure is shown in Table 2.

[0171] Table 2 Consistency evaluation of the simulation model and the analytical model

[0172]

[0173] In order to simulate the same harmonic phenomenon as the experimental transformer in the simulation model, in addition to setting the core material parameters according to the magnetization curve of the experimental transformer, a power frequency voltage source in series with a 150Hz voltage source of 0.85% rated voltage is used on the power supply side of the external circuit to simulate the real external power grid.

[0174] The comparison between the experimental values and simulation values of the harmonic content of the magnetic leakage flux at the upper measuring point of Group 1 under rated conditions is shown in Table 3. It can be seen that the difference in the third harmonic content rate between the two is within the allowable range, verifying the accuracy of the established simulation model.

[0175] Table 3 Comparison between experimental values and simulation values of the harmonic content rate of the upper radial magnetic leakage

[0176]

[0177] (2) Dynamic simulation verification

[0178] ① Winding deformation

[0179] To verify the accuracy and high sensitivity of the leakage magnetic field harmonic differential protection proposed in this paper for detecting slight winding deformation, winding deformation tests of 5% and 10% axial compression faults were conducted on the C-phase high-voltage winding of the dynamic simulation transformer. The test sampling curves and third-harmonic amplitude curves at the upper measuring points of Group 2 are as Figure 10 shown.

[0180] As Figure 11 can be seen, the winding deformation criterion can accurately identify winding deformation faults with a fault degree of 5% and above, and the fault quantity is obvious. In practical applications, it can quickly detect early faults and perform maintenance, improving the service life of the transformer. The complete protection verification results are shown in Table 4, and it can be seen that this protection can accurately locate the fault to Phase C.

[0181] Table 4 Protection action conditions for winding deformations of different degrees

[0182]

[0183] A 2-turn, i.e., 0.995%, turn-to-turn short circuit experiment was conducted at the lower end of the A-phase high-voltage winding of the dynamic simulation transformer. The third-harmonic curves of the magnetic differential quantity and the magnetic center quantity plotted based on the data of Sensor Group 1 are as Figure 12 shown. The curve enters the fault steady state section at about 180 ms. The dotted line values in the figure are the sum of the normal value and the setting value at 156 ms, both of which are less than the fault steady state quantity. According to the definition of the fault steady state component, it shows that the protection can accurately identify a slight 0.995% turn-to-turn short circuit. However, since the margin of the curve in the fault steady state section is not large at this time, it indicates that 0.995% is the minimum detectable turn-to-turn short circuit. To further verify the effectiveness of this protection in detecting slight turn-to-turn short circuits, turn-to-turn short circuit experiments with different degrees and positions were conducted on the A-phase winding. The protection action conditions are shown in Table 5. In the table, A lower 0.995% represents a 0.995% turn short at the lower end of Phase A, and A middle 0.498% represents a 0.498% turn short in the middle of Phase A. The relative value of the fault transient component proposed in this embodiment has high sensitivity in various turn-to-turn short circuits, and the fault steady state component increases with the deepening of the fault degree. Since the magnetic differential fault steady state component and the magnetic center fault steady state component do not exceed the setting value during a 0.498% middle turn-to-turn short circuit, this protection scheme can effectively identify turn-to-turn short circuits at the lower end and in the middle with a degree of 0.995% and above.

[0184] Table 5 Protection action conditions for turn-to-turn short circuits of different degrees

[0185]

[0186] ② No-load closing

[0187] During the no-load closing of the transformer, the internal structure is not damaged, and the symmetry of the leakage magnetic field distribution remains unchanged. The harmonic differential protection of the leakage magnetic field should not be affected by inrush current. As Figure 13 shown, the time-domain waveforms of the leakage magnetic fluxes measured at the upper and lower measuring points of the winding during no-load closing maintain symmetry. Calculate the third-harmonic curve of the leakage magnetic flux at the lower measuring point and the relative value curve of its fault transient component, as Figure 14 shown. It can be seen that the relative value of the fault transient component is always less than the threshold, and the inter-turn short-circuit protection does not start. This protection can avoid the influence of no-load closing without blocking.

[0188] To test the fault detection ability of this protection under extreme conditions, a dynamic simulation experiment of no-load closing with different degrees of inter-turn short-circuit is carried out in this embodiment. The protection action conditions are shown in Table 6. When the transformer is energized with an inter-turn short-circuit, the fault characteristic quantities are obvious and increase with the aggravation of the fault degree, and it can act accurately, verifying the effectiveness of this protection under extreme conditions.

[0189] Table 6 Protection action conditions when no-load closing on inter-turn short-circuits with different degrees

[0190]

[0191] Use the consistency simulation model established above to obtain the winding leakage magnetic field data under different types of external fault conditions for fault discrimination. The total simulation duration is 220 ms. All external faults are set at the secondary side outlet of the transformer and occur at 40 - 180 ms. The superimposed inter-turn short-circuit is set on the high-voltage side of phase A and occurs at 80 - 140 ms. The protection action conditions are shown in the following table, where fa represents a ground fault of phase A, fab represents a short-circuit fault between phases AB, and fabc represents a three-phase short-circuit fault. Since external short-circuits also belong to transient faults, the sharp increase in the winding current causes the relative values of the leakage magnetic fault transient components to all exceed the setting value, so the inter-turn short-circuit protection starts; since the external short-circuit current is a traversing current and does not change the spatial distribution of the leakage magnetic field, the steady-state components of the magnetic differential and magnetic center faults are not large, and the inter-turn short-circuit protection does not act; when an external short-circuit is superimposed with an internal inter-turn short-circuit, since the asymmetry degree of the leakage magnetic field develops on the leakage magnetic field excited by a short-circuit current with a very large value, the fault characteristic quantities are more obvious and the sensitivity is higher. Therefore, the above simulation verifies the security of this protection not malfunctioning during external short-circuits and the reliability of not refusing to act when an external short-circuit occurs simultaneously with an inter-turn short-circuit, as shown in Table 7.

[0192] Table 7 Protection action conditions when no-load closing on inter-turn short-circuits with different degrees

[0193]

[0194] The simulation data is used to compare and analyze the action of the protection scheme in this paper and the traditional longitudinal differential current protection under 1.990%, 2.488% short turns at the lower end of phase A and no-load closing at 2.488% short turns at the lower end of phase A. Assuming that the longitudinal differential protection is equipped with an excitation surge current locking device and a secondary circuit disconnection device, the operating current is set according to the maximum unbalanced current when avoiding the short circuit outside the zone, and the value converted to the secondary side of the transformer is 25.16A. When a 1.990% internal turn-to-turn short circuit occurs, the terminal current changes little and the differential current does not exceed the set value, so the longitudinal differential protection does not operate, while the protection in this paper operates; when the no-load closing is at 2.488% short turns, which can operate both, the longitudinal differential protection will be locked for 60ms due to the second harmonic braking element, which is 20ms later than the protection in this paper. The superiority of the leakage magnetic field harmonic differential protection is reflected, and the comparison results of the two protections are shown in Table 8.

[0195] Table 8 Comparison between leakage magnetic harmonic differential protection and traditional longitudinal differential protection

[0196]

Claims

1. A method for detecting early transformer faults using the third harmonic of the leakage magnetic field, characterized in that: include: Calculate the leakage magnetic induction intensity and establish a three-dimensional analytical calculation model of the leakage magnetic field; Use the three-dimensional analytical calculation model of leakage magnetic field to calculate the setting value of protection criterion and design the sensor layout plan; Based on the real-time monitoring of leakage magnetic field signals by sensors, a fault protection mechanism is established.

2. The method for detecting early transformer faults by using the third harmonic of the leakage magnetic field as claimed in claim 1, characterized in that: The establishment of the three-dimensional analytical calculation model of the leakage magnetic field includes calculating the magnetic induction intensity generated by the unitary current on the conductor ring to obtain the leakage magnetic induction intensity generated by a single segment, using the superposition of the leakage magnetic field to calculate the leakage magnetic induction intensity generated by different windings and different segments to obtain the single-phase total leakage magnetic induction intensity, superimposing the leakage magnetic induction intensity of the three phases, and obtaining the third harmonic leakage magnetic distribution.

3. The method for detecting early transformer faults by using the third harmonic of the leakage magnetic field as claimed in claim 2, characterized in that: The third harmonic leakage flux distribution includes: the third harmonic amplitude of the leakage flux density is expressed as: Among them, B 3ym is the third harmonic amplitude of leakage flux density; The third harmonic amplitude of leakage flux is expressed as, Among them, Φ 3ym is the third harmonic amplitude of leakage flux.

4. The method for detecting early transformer failures by using the third harmonic of the leakage magnetic field as claimed in claim 3 is characterized in that: The calculation of the setting value of the protection criterion includes calculating the setting value of the inter-turn short circuit criterion and the setting value of the winding deformation criterion; The calculation of the turn-to-turn short circuit criterion setting value includes constructing a fault steady-state component using the difference between the fault steady-state quantity and the normal quantity, which is expressed as: DF″ 3dif (t)=Φ 3dif (t)-Φ 3dif (t-2.2T) ΔB″ 3cen (t)=B 3cen (t)-B 3cen (t-2.2T) The setting value is calculated according to the three-phase short circuit outside the avoidance zone, expressed as: Among them, Φ set3 is the setting value of the steady-state component of magnetic differential fault, B set2 is the setting value of the steady-state component of magnetic center fault, is the reliability coefficient; The winding deformation criterion setting value includes an upper deformation setting value, a lower deformation setting value, and a magnetic differential criterion setting value, which are expressed as: in, is the reliability coefficient, and n% is the minimum axial winding deformation that actually needs to be detected.

5. The method for detecting early transformer faults by using the third harmonic of the leakage magnetic field as claimed in claim 4, characterized in that: The sensor arrangement scheme includes arranging measuring points at positions within the core window at a distance from the outer diameter of the winding axis and at a height along the winding axis.

6. The method for detecting early transformer failures by using the third harmonic of the leakage magnetic field as claimed in claim 5, characterized in that: The real-time monitoring of leakage magnetic field signals includes collecting leakage magnetic field data of measuring points, calculating the third harmonic amplitude signal in real time by short-time Fourier transform, and calculating the relative value of the fault transient component, the fault steady-state quantity and the fault steady-state component.

7. The method for detecting early transformer failures by using the third harmonic of the leakage magnetic field as claimed in claim 6, characterized in that: The fault protection mechanism includes: if the limit is exceeded, the turn-to-turn short-circuit protection is activated, and the median filter is performed after a delay of one and a half cycles, and the setting value is used to determine whether it is a turn-to-turn short-circuit fault; If the limit is not exceeded, it means that the signal has no sudden change, and median filtering is performed directly to determine whether it is a deformation fault through the setting value; If it is determined to be a fault, the fault phase is located and a fault report is sent.

8. A system using the method for detecting early transformer faults using the third harmonic of leakage magnetic field as claimed in any one of claims 1 to 7, characterized in that: include: Model building module, sensor placement module and fault protection module; The model building module calculates the leakage magnetic induction intensity and establishes a three-dimensional analytical calculation model of the leakage magnetic field; The sensor arrangement module calculates the setting value of the protection criterion by using the three-dimensional analytical calculation model of the leakage magnetic field and designs the sensor arrangement plan; The fault protection module monitors the leakage magnetic field signal in real time based on the sensor and establishes a fault protection mechanism.

9. A computer device comprising a memory and a processor, wherein the memory stores a computer program, wherein: When the processor executes the computer program, the steps of the method for detecting early transformer faults by using the third harmonic of the leakage magnetic field according to any one of claims 1 to 7 are implemented.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the method for detecting early transformer faults by using the third harmonic of the leakage magnetic field according to any one of claims 1 to 7 are implemented.

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