Error processing method for low-frequency current transformer

Abstract

The invention discloses an error processing method for a low-frequency current transformer, which belongs to the technical field of error calculation and comprises the following steps: calculating and judging whether the error of the low-frequency current transformer is within a preset accuracy level range or not; when the current exceeds the preset accuracy level range, inputting the actually measured secondary side response current of the low-frequency current transformer into a pre-deduced inversion model calculation formula, and calculating and outputting the primary side excitation current after error correction; the calculation formula of the inversion model is based on a current transformer forward circuit containing the nonlinear characteristics of an iron core and is obtained through derivation by adopting a discrete state quantity numerical algorithm. The problems that the error of the low-frequency current transformer is too large and the electric energy metering accuracy is reduced can be solved.

Description

Technical Field

[0001] This invention belongs to the field of error calculation technology, and specifically relates to an error processing method for low-frequency current transformers. Background Technology

[0002] To address the challenges of long-distance transmission and grid connection of large-capacity offshore wind power, low-frequency AC transmission technology has developed rapidly, providing an economical and efficient option for long-distance offshore wind power transmission. However, with the deployment of more electronic equipment and the occurrence of phenomena such as geomagnetic storms, high-voltage direct current transmission systems, and line faults, a certain proportion of DC component will be generated in the distribution network, causing current transformer core saturation, resulting in a surge in excitation current and distortion of the secondary current waveform. Under low-frequency transmission conditions, this severely affects the accuracy of power metering during transmission and distribution.

[0003] Therefore, it is crucial to calculate and analyze the error of current transformers under low-frequency operating conditions due to DC bias. However, existing technologies either only verify the physical error of the current transformer without mathematically modeling the error or analyzing the effects of DC bias and resistive-inductive loads; or they only consider the effect of DC bias, modeling the secondary load as a purely resistive load without incorporating the influence of resistive-inductive loads on the error.

[0004] Furthermore, as a crucial metering device in the power grid, the performance of current transformers under low-frequency conditions directly impacts the accuracy of energy metering during flexible low-frequency transmission. However, current research on transformer measurement accuracy, both domestically and internationally, is largely based on power frequency, with limited literature on low-frequency transformers. Moreover, since low-frequency transformers typically operate within a specific low-frequency range rather than a single frequency, different frequency conditions will affect the magnitude of the transformer's error. Therefore, it is essential to ensure that the transformer's error meets accuracy class requirements throughout this frequency range. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for processing errors in low-frequency current transformers, thereby solving the problem of excessive errors in low-frequency current transformers and reducing the accuracy of power metering.

[0006] To solve the above-mentioned technical problems, the present invention is implemented using the following technical solution:

[0007] This invention provides a method for processing errors in low-frequency current transformers, comprising: calculating and determining whether the error of the low-frequency current transformer is within a preset accuracy range; when it exceeds the preset accuracy range, inputting the measured secondary side response current of the low-frequency current transformer into a pre-derived inversion model calculation formula, calculating and outputting the primary side excitation current after error correction; the inversion model calculation formula is based on the forward circuit of the current transformer containing the nonlinear characteristics of the iron core, and is derived using a discrete state quantity numerical algorithm.

[0008] The aforementioned low-frequency current transformer error processing method, wherein the forward circuit of the current transformer containing the nonlinear characteristics of the iron core is the equivalent circuit of the low-frequency current transformer, constructed based on the Γ model, includes the following components connected in sequence: a first ideal transformer on the left, an excitation branch in the middle, a short-circuit resistor and primary and secondary leakage inductance, and a second ideal transformer on the right; the primary side of the first ideal transformer is connected to the primary side external circuit; an excitation branch is connected in parallel to the secondary side of the first ideal transformer, and the short-circuit resistor, primary and secondary leakage inductance, and the primary side of the second ideal transformer are connected in series to form a series branch, and the excitation branch is connected in parallel with the series branch, wherein the excitation branch is composed of a nonlinear excitation inductor characterizing the saturation characteristics of the iron core and an iron loss resistor connected in parallel; the secondary side of the second ideal transformer is connected to the secondary side external circuit.

[0009] The aforementioned low-frequency current transformer error processing method, wherein the measured secondary side response current of the low-frequency current transformer is input into the pre-derived inversion model calculation formula, and the primary side excitation current is calculated and output, includes: obtaining the measured secondary side response current and the secondary side load impedance of the secondary side external circuit;

[0010] Based on the experimental setup of the current transformer forward circuit, relevant parameters in the current transformer forward circuit are obtained. These parameters include: short-circuit resistance, primary and secondary leakage inductance, iron loss resistance, and incremental inductance used to construct the excitation curve. The relevant parameters and the measured secondary response current are input into the pre-derived inversion model calculation formula to calculate and output the primary excitation current. The turns ratio of the primary and secondary sides of the first ideal transformer is K, and the turns ratio of the primary and secondary sides of the second ideal transformer is 1. The current primary excitation current at the current moment... The inversion model calculation formula is as follows:

[0011] ,

[0012] In the formula, For the current moment Measured secondary response current; Z L This is the secondary load impedance; This represents the current secondary load voltage. This represents the primary voltage of the second ideal transformer at the current moment. For the current moment, the first and second side leakage sensation L σ Leakage inductance current; For the current moment, the first and second side leakage sensation L σ Leakage inductance voltage; For short-circuit resistance; This represents the voltage across the excitation branch at the current moment. Iron loss resistance; This represents the resistive component of the excitation branch current at the current moment. This represents the flux linkage value at the current moment. The excitation curve; This represents the inductive component of the excitation branch current at the current moment. This represents the primary side excitation current at the current moment.

[0013] The aforementioned low-frequency current transformer error processing method, wherein the step of establishing an experiment based on the forward circuit of the current transformer to obtain relevant parameters in the forward circuit of the current transformer includes:

[0014] A short-circuit experiment was conducted based on the forward circuit of the current transformer, and the short-circuit resistance was calculated. and secondary side leakage L σ During the short-circuit test: A short circuit is applied to the primary side, and an AC power supply is applied to the secondary side, with the voltage gradually increasing until the secondary current of the low-frequency current transformer reaches its rated value. The rated value of the secondary current of the low-frequency current transformer is recorded as the effective value I of the secondary current. k The effective value of the applied secondary voltage U k and the measured secondary active power P k ;

[0015] short-circuit resistor and secondary side leakage L σ The calculation formulas are as follows: , ,

[0016] In the formula, Z k The short-circuit impedance is the magnitude of the total leakage impedance of the secondary winding. f is the frequency of the applied AC power supply;

[0017] Based on the open-circuit test established by the forward circuit of the current transformer, the iron loss resistance R of the excitation branch is calculated. m During the open-circuit test: the primary side is open-circuited, and an AC power supply is applied to the secondary side. The secondary side voltage rises from zero to its rated value. The rated value of the secondary side voltage of the low-frequency current transformer is recorded as the effective value U0 of the applied secondary side voltage. The effective value I0 of the secondary side current and the active power P0 of the secondary side are also measured. Iron loss resistance R m The calculation formula is: ;

[0018] A deep saturation test is established based on the forward circuit of the current transformer. Multiple sets of incremental inductance and excitation current data pairs are measured, and numerical integration is performed on the data pairs to obtain the excitation curve f that characterizes the nonlinear properties of the iron core. Lm When the current transformer operates in the saturation region, the magnetizing inductance L m It is the incremental inductance L m_s The depth saturation test includes:

[0019] A hybrid AC / DC power supply is applied to the secondary side external circuit. The DC power supply outputs a DC bias current, and the AC power supply outputs an AC signal; the amplitude and frequency of the AC signal are maintained. The following steps are repeated continuously, adjusting the core saturation level by changing the DC bias current multiple times, to obtain a series of data pairs: (I m_s1 L m_s1 ), (I m_s2 L m_s2 ), ..., (I m_sx L m_sx ):

[0020] By applying a DC bias current I b-dc1 Drive the iron core to saturation and record the excitation current operating point as I. m_s1 =I b-dc1 ;

[0021] Maintain DC bias current I b-dc1 On the premise that it remains unchanged, add a frequency of AC signal i used for measurement b-ac ;Measure the voltage u across the excitation branch m1 (t) and the total current i flowing through m1 (t); for the measured u m1 (t) and i m1 (t) Perform Fast Fourier Transform analysis to extract the fundamental frequency. Voltage amplitude U m_f1 and current amplitude I m_f1 ;

[0022] Based on fundamental frequency Voltage amplitude U m_f1 and current amplitude I m_f1 Calculate the incremental inductance L of the excitation branch m_s1 Excitation current operating point I m_s1 Incremental inductance L in the excitation branch m_s1 The calculation formula is:

[0023] ,

[0024] In the formula, L is the frequency of the AC signal in the AC / DC hybrid power supply. m_s1 The excitation curve is on the horizontal axis I m_s1 The slope at the point; the vertical axis of the excitation curve represents the flux linkage value.

[0025] The aforementioned low-frequency current transformer error handling method, at the current moment, the primary and secondary side leakage inductance L σ Leakage inductance voltage The calculation includes: solving for the leakage inductance voltage using the fourth-order Adams-Bushforth method in discrete state quantity algorithms. The calculation formula is:

[0026] ,

[0027] In the formula, Δt is the time step; For the second moment, the feeling of leakage L σ Leakage inductance current; For the current moment, the first and second side leakage sensation L σ Leakage inductance current; For the first and second side leakage sensations of the previous moment L σ Leakage inductance voltage; For the first two moments, the second and third moments of side leakage L σ Leakage inductance voltage; For the first three moments, there is a feeling of leakage at the first and second intervals. σ The leakage inductance voltage; where u Lσ (0), u Lσ (1) u Lσ (2) All are set to 0.

[0028] The aforementioned low-frequency current transformer error processing method, the current flux linkage value The calculation includes: using the trapezoidal integral method in the discrete state quantity algorithm to calculate the flux linkage value at the current time. The flux linkage value at the current time is equal to the sum of the flux linkage values ​​at the current time. The calculation formula is:

[0029] ,

[0030] In the formula, This is the flux linkage value from the previous moment; This is the flux linkage increment for this time step; This represents the voltage across the excitation branch at the previous moment.

[0031] The aforementioned low-frequency current transformer error processing method, wherein calculating and determining whether the low-frequency current transformer error is within a preset accuracy class range includes:

[0032] Acquire the theoretical and actual secondary current data of the low-frequency current transformer. The theoretical secondary current data considers the presence of resistive-inductive load and DC bias while ignoring the nonlinear characteristics of the core. The actual secondary current data considers the presence of resistive-inductive load, DC bias, and the nonlinear characteristics of the core. Calculate the ratio error and phase error of the low-frequency current transformer based on the theoretical and actual secondary current data. Compare the calculated ratio error and phase error of the low-frequency current transformer with preset accuracy class ranges to determine whether the error of the low-frequency current transformer is within the preset accuracy class range.

[0033] The aforementioned low-frequency current transformer error processing method obtains the theoretical secondary current data of the low-frequency current transformer, including:

[0034] The expressions for the primary current i1(t) and secondary current i2(t) considering DC bias and inductive load are established as follows:

[0035] ,

[0036] ,

[0037] In the formula, I dc I is the amplitude of the DC bias current. ac I represents the effective value of the AC component of the primary current, ω is the current angular frequency, t is the time variable, and I is the effective value of the AC component of the primary current. 21 β1 is the effective value of the secondary current, and β2 is the phase angle of the secondary current considering the resistive-inductive load.

[0038] Excitation current i e The expression for (t) is:

[0039] ,

[0040] In the formula, K is the turns ratio of the primary winding and the secondary winding of the current transformer.

[0041] According to the excitation current i e The expression for (t) is used to obtain the DC component I contained in the excitation current. DC Fundamental frequency AC component amplitude I e1 and phase angle φ e1 The expression:

[0042] ,

[0043] ,

[0044] ,

[0045] I DC I e1 and φe1 This is the theoretical secondary current data for a low-frequency current transformer.

[0046] The aforementioned low-frequency current transformer error processing method obtains the actual secondary current data of the low-frequency current transformer, including:

[0047] The BH curve of the core material is simplified; the simplified BH curve consists of two linear segments with slopes μ1 and μ2, respectively; μ1 is the permeability of the unsaturated core, and μ2 is the permeability of the saturated core; the intersection of the two segments is the knee point of the BH curve of the ferromagnetic material; the magnetic flux density at the knee point is denoted as B. k ;

[0048] Taking into account DC bias and resistive-inductive load, the resultant magnetic flux B inside the iron core is established. h (t) and magnetic field strength H m The expression for (t):

[0049] ,

[0050] ,

[0051] in, ,

[0052] In the formula, B ac B represents the amplitude of the AC component of the magnetic flux density in the iron core under DC bias. dc H represents the amplitude of the DC component. u This is called the knee magnetic field strength, H. s H represents the magnetic field strength corresponding to the peak value of the core magnetic flux density. bias θ is the DC magnetic field strength generated by the DC component of the magnetic flux density; θ is the phase difference between the peak point of the magnetic flux density in the iron core and the origin; α is the flux angle, which increases with the increase of DC current, and when the DC current is zero, α is approximately zero.

[0053] According to B h (t) and H m The excitation current is obtained from the expression of (t). The expression is given, and the DC component is separated from it. fundamental frequency AC component amplitude and phase angle The expression:

[0054] ,

[0055] ,

[0056] ,

[0057] In the formula, N2 is the number of turns of the secondary winding, L is the average magnetic circuit length of the core, and γ1 is the load impedance angle; , and This is the actual secondary current data of the low-frequency current transformer.

[0058] The aforementioned deep learning-based template matching method, which calculates the ratio error and angle error of the low-frequency current transformer based on theoretical and actual secondary current data, includes:

[0059] Assuming all components of the excitation current obtained by the two methods are equal, we solve for the magnitude ratio k2 and phase difference β1 of the currents on both sides, and then, using the error definition, we solve for the expressions for the ratio difference ε and the angle difference δ:

[0060] make , , We can obtain the system of equations:

[0061] ,

[0062] ,

[0063] ,

[0064] Since the flux angle α is normally less than 65°, the relationship between (α-0.5sin2α) and (sinα-αcosα) is approximately linear at 1.9 times, that is: ,

[0065] Will Substituting into the system of equations and simplifying, we get:

[0066] ,

[0067] In the formula, T represents the DC percentage, and its value is I. dc / I ac i u The knee excitation current referred to the secondary side is H. u L / N2; Defined as the effective value of the AC component of the primary current referred to the secondary side, its value is I. ac / K; P is the excitation ratio, its value is The magnitude of the excitation ratio P is related to the knee point of the BH curve. Once the core material is determined, the excitation ratio P is also determined.

[0068] The errors of a current transformer include ratio error and phase error. According to the definitions of ratio error and phase error, we can obtain:

[0069] ,

[0070] ,

[0071] In the formula, φ1 is the phase angle of the primary current and φ2 is the phase angle of the secondary current.

[0072] The expressions for calculating the ratio difference ε and angle difference δ of a current transformer are as follows:

[0073] ,

[0074] ,

[0075] By comparing the expression for the difference ε with the "+" sign and performing monotonicity analysis, it can be seen that the value of the difference function gradually increases with the DC proportion T. When T increases to a certain value, the difference ε will change from a negative value to a positive value. However, in reality, due to the presence of DC, the secondary current of the current transformer is smaller than normal, and the difference should be negative. Therefore, the expression with the "+" sign does not match the actual situation and is discarded.

[0076] The final expressions for the ratio difference ε and angle difference δ are:

[0077] ,

[0078] .

[0079] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0080] The low-frequency current transformer error processing method of the present invention first calculates and determines whether the error of the low-frequency current transformer is within a preset accuracy range. When the error is within the preset accuracy range, the secondary response current measured by the low-frequency current transformer and the calculated primary excitation current can be used directly. When the error is outside the preset accuracy range, the primary excitation current cannot be accurately calculated from the secondary response current measured by the low-frequency current transformer. Directly using the distorted secondary response current will not yield an accurate primary excitation current, which will seriously affect the accuracy of power metering in the power transmission and distribution process. Therefore, the low-frequency current transformer error processing method of the present invention inputs the distorted secondary response current into the pre-derived inversion model calculation formula, calculates and outputs the error-corrected primary excitation current, and solves the problem of excessive low-frequency current transformer error reducing the accuracy of power metering.

[0081] Specifically, the advantages of the low-frequency current transformer error processing method of the present invention include:

[0082] 1. The inversion model calculation formula of the low-frequency current transformer error processing method proposed in this invention is based on the forward circuit of the current transformer containing the nonlinear characteristics of the iron core, and is derived using a discrete state quantity numerical algorithm. This discrete quantity numerical calculation method can solve the problem that the measured distortion current on the secondary side is a discrete quantity and difficult to process. The low-frequency current transformer error processing method proposed in this invention can substitute the discrete values ​​of the measured secondary side distortion current into the inversion mathematical model to calculate the primary side current value point by point, thus realizing the inversion solution of the distortion current of the low-frequency current transformer under DC bias.

[0083] 2. The present invention provides a method for accurately calculating the error of a low-frequency current transformer. This method involves acquiring theoretical and actual secondary current data of the low-frequency current transformer. The theoretical secondary current data considers the presence of resistive-inductive load and DC bias while ignoring the nonlinear characteristics of the core. The actual secondary current data considers the presence of resistive-inductive load, DC bias, and the nonlinear characteristics of the core. Based on the theoretical and actual secondary current data, the ratio error and phase error of the low-frequency current transformer are calculated. The calculated ratio error and phase error are then compared with preset accuracy ranges to determine whether the error of the low-frequency current transformer is within the preset accuracy range.

[0084] 3. In the error calculation and judgment of the low-frequency current transformer error processing method proposed in this invention, the influence of DC bias magnitude on error under low-frequency operating conditions is effectively considered by setting the DC ratio.

[0085] 4. In the error calculation and judgment of the low-frequency current transformer error processing method proposed in this invention, the problem of the influence of load power factor angle change on error under low-frequency operating conditions is effectively considered.

[0086] 5. In the error calculation and judgment of the low-frequency current transformer error processing method proposed in this invention, a simplified core BH curve is adopted, which reduces the complexity of electromagnetic calculation of the core in the saturated and unsaturated regions. Attached Figure Description

[0087] Figure 1 This is a schematic diagram of the structure of a low-frequency current transformer according to Embodiment 1 of the present invention;

[0088] Figure 2 This is a schematic flowchart of a low-frequency current transformer error processing method according to Embodiments 1 and 2 of the present invention;

[0089] Figure 3 This is a simplified BH characteristic curve of the iron core affected by DC bias magnetization, which is a low-frequency current transformer error processing method according to Embodiment 2 of the present invention.

[0090] Figure 4 This is a schematic diagram of the forward circuit model of a current transformer established by a low-frequency current transformer error processing method according to Embodiment 2 of the present invention.

[0091] Figure 5 This is a schematic diagram of a deep saturation test principle for a low-frequency current transformer error processing method according to Embodiment 2 of the present invention;

[0092] Figure 6 This is a curve showing the ratio difference as a function of DC in a low-frequency current transformer error processing method according to Embodiment 2 of the present invention;

[0093] Figure 7 This is a curve showing the angle difference versus DC variation of a low-frequency current transformer error processing method according to Embodiment 2 of the present invention;

[0094] Figure 8 This is a curve showing the ratio difference as a function of the load impedance angle in a low-frequency current transformer error processing method according to Embodiment 2 of the present invention.

[0095] Figure 9 This is a curve showing the angle difference as a function of the load impedance angle in a low-frequency current transformer error processing method according to Embodiment 2 of the present invention.

[0096] Figure 10 This is a comparison chart of the inversion solution results of a low-frequency current transformer error processing method according to Embodiment 2 of the present invention;

[0097] Explanation of reference numerals in the attached figures:

[0098] 1- Primary winding conductor; 2- Iron core; 3- Secondary winding coil; 4- Inductive load connected to the secondary side. Detailed Implementation

[0099] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the embodiments and specific features in the embodiments are detailed descriptions of the technical solution of the present application, rather than limitations thereof. In the absence of conflict, the embodiments and technical features in the embodiments can be combined with each other.

[0100] In this article, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Additionally, the character " / " in this article generally indicates that the preceding and following related objects have an "or" relationship.

[0101] The present invention will be further illustrated below with reference to the accompanying drawings and specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. After reading this invention, any modifications of the invention in various equivalent forms by those skilled in the art will fall within the scope defined by the appended claims.

[0102] Example 1:

[0103] This embodiment introduces a method for handling errors in low-frequency current transformers, including: calculating and determining whether the error of the low-frequency current transformer is within a preset accuracy range; when it exceeds the preset accuracy range, inputting the measured secondary response current of the low-frequency current transformer into a pre-derived inversion model calculation formula, calculating and outputting the primary excitation current after error correction; the inversion model calculation formula is based on the forward circuit of the current transformer containing the nonlinear characteristics of the iron core, and is derived using a discrete state quantity numerical algorithm.

[0104] like Figure 1 As shown, in order to study the error characteristics of a current transformer under low-frequency operating conditions affected by DC bias, this invention takes a 220kV low-frequency current transformer as an example for analysis. Figure 1 As shown.

[0105] The primary winding conductor 1 of this current transformer is a high-voltage cable, which passes through the center of the iron core 2. The secondary winding coil 3 consists of 400 turns wound around the outside of the iron core 2, and an insulating film is laid inside and outside the coil. The secondary winding coil 3 is connected to a resistive-inductive load 4, which is composed of a resistive load and an inductive load. The rated parameters of this current transformer are shown in Table 1.

[0106] Table 1 Parameters of Low-Frequency Current Transformers

[0107]

[0108] The present invention provides a low-frequency current transformer error processing method, which consists of two parts. The first part calculates the error of the transformer considering DC bias and resistive-inductive load. If the error is too large, the second part is performed to address the distortion of the measured secondary response current caused by severe bias, which leads to excessive error. Figure 2 The inversion derivation shown is the actual primary side excitation current.

[0109] The first part calculates the error of the current transformer considering DC bias and resistive-inductive load. The specific steps include:

[0110] Step 1: Establish the expressions for the primary side current i1(t) and the secondary side current i2(t) considering DC bias as follows:

[0111] (1)

[0112] (2)

[0113] In the formula, I dc I is the amplitude of the DC bias current. ac I represents the effective value of the AC component of the primary current, ω is the current angular frequency, t is the time variable, and I is the effective value of the AC component of the primary current. 21 β1 is the effective value of the secondary current, and β2 is the phase angle of the secondary current considering the resistive-inductive load.

[0114] Excitation current i e The expression for (t) is:

[0115] (3)

[0116] In the formula, K is the turns ratio of the primary winding and the secondary winding of the current transformer.

[0117] According to the excitation current i e The expression for (t) yields the DC component I contained in the excitation current. DC Fundamental frequency AC component amplitude I e1 and phase angle φ e1 The expression:

[0118] (4)

[0119] (5)

[0120] (6)

[0121] Step 2: Simplify the BH curve of the core material. The BH curve represents the magnetic flux density B. h (t) and magnetic field strength H m The magnetization curves showing the relationship between (t) are as follows: Figure 3 As shown on the right, the simplified BH curve consists of two linear line segments with slopes of μ1 and μ2, respectively.

[0122] Figure 3 Left side, B ac B represents the amplitude of the AC component of the magnetic flux density in the iron core under DC bias. dc For the DC component, B k Let be the magnetic flux density at the knee point. α is defined as the flux angle, which increases with increasing DC current; when the DC current is zero, α is approximately zero. θ is the phase difference between the peak point of the core's magnetic flux density and the origin. μ1 is the permeability of the unsaturated core, and μ2 is the permeability of the saturated core. H u H represents the magnetic field strength at the knee point. s H represents the magnetic field strength corresponding to the peak value of the core magnetic flux density.bias Let B be the intensity of the DC magnetic field generated by the DC component of the magnetic flux density. The intersection of the two line segments is the knee point of the BH curve for the ferromagnetic material, and the magnetic flux density at the knee point is denoted as B. k .

[0123] Step 3: Taking into account DC bias and resistive-inductive load, establish the resultant magnetic flux density B inside the iron core. h (t) and magnetic field strength H m The expression for (t) is:

[0124] (7)

[0125] (8)

[0126] in, (9)

[0127] Step 4: Based on magnetic flux density B h (t) and magnetic field strength H m The excitation current is obtained from the expression of (t). The expression is derived, and the DC component in the excitation current is separated. fundamental frequency AC component amplitude and phase angle The expressions are as follows:

[0128] (10)

[0129] (11)

[0130] (12)

[0131] In the formula, N2 is the number of turns in the secondary winding, L is the average magnetic circuit length of the core, and γ1 is the load impedance angle.

[0132] Step 5: Make all components of the excitation current obtained by the two methods equal, and solve for the amplitude ratio k2 and phase difference β1 of the current converted values ​​on both sides:

[0133] Right now , , We can obtain the system of equations:

[0134] (13)

[0135] (14)

[0136] (15)

[0137] Since the flux angle α is normally less than 65°, the relationship between (α-0.5sin2α) and (sinα-αcosα) is approximately linear at 1.9 times, that is:

[0138] (16)

[0139] Substituting equation (16) into equation (13), we get:

[0140] (17)

[0141] Substituting equation (17) into equation (14) and squaring both sides of the equation, we get:

[0142] (18)

[0143] Divide both sides of equation (18) by I. ac 2 / K 2 We can obtain:

[0144] (19)

[0145] Simplifying equation (15) yields:

[0146] (20)

[0147] Substituting equation (20) into equation (19) and simplifying, we get:

[0148] ,(twenty one)

[0149] Extracting the root yields:

[0150] ,(twenty two)

[0151] By applying the formula on the left side of equation (19), we can obtain:

[0152] ,(twenty three)

[0153] Extracting the root yields:

[0154] ,(twenty four)

[0155] Substituting equation (22) into equation (24), we get:

[0156] (25)

[0157] Finally, the amplitude ratio k2 and phase difference β1 of the currents on both sides are obtained:

[0158] (26)

[0159] In the formula, T is defined as the DC percentage, and its value is I. dc / I ac i u Defined as the knee excitation current referred to the secondary side, its value is H. u L / N2; Defined as the effective value of the AC component of the primary current referred to the secondary side, its value is I. ac / K;P is defined as the excitation ratio, and its value is The magnitude of the excitation ratio P is related to the knee point of the BH curve. Once the core material is determined, the excitation ratio P is also determined.

[0160] The errors of a current transformer include ratio error and phase error. According to the definitions of ratio error and phase error, we can obtain:

[0161] (27)

[0162] (28)

[0163] In the formula, φ1 is the phase angle of the primary current and φ2 is the phase angle of the secondary current.

[0164] The expressions for calculating the ratio difference ε and angle difference δ of a current transformer are as follows:

[0165] (29)

[0166] (30)

[0167] By performing a monotonicity analysis on the mathematical model when the sign in equation (29) is "+", it can be seen that the ratio difference function value gradually increases with the DC ratio T. When T increases to a certain value, the ratio difference will change from negative to positive. However, in reality, due to the presence of DC, the secondary current of the current transformer is smaller than normal, and the ratio difference should be negative. Therefore, the mathematical model when the sign is "+" does not match the actual situation and is discarded.

[0168] (31)

[0169] , (32).

[0170] Example 2

[0171] Based on the same inventive concept as Embodiment 1, this embodiment introduces a second part of a low-frequency current transformer error processing method. When the calculated error exceeds a preset accuracy level, the measured secondary side response current of the low-frequency current transformer is input into the pre-derived inversion model calculation formula to calculate and output the primary side excitation current, including:

[0172] Step 6: Obtain the measured secondary-side response current and the secondary-side load impedance of the external circuit.

[0173] Step 7: Establish an experiment based on the pre-constructed forward circuit of the current transformer to obtain the relevant parameters in the forward circuit of the current transformer. The relevant parameters include: short-circuit resistance, primary and secondary leakage inductance, iron loss resistance, and incremental inductance used to construct the excitation curve.

[0174] Step 8: Input the relevant parameters and the measured secondary side response current into the pre-derived inversion model calculation formula, calculate and output the primary side excitation current;

[0175] The specific steps include:

[0176] Step 7: Construct the forward circuit of the current transformer based on the Γ model, such as... Figure 4 As shown, it consists of a line containing iron loss resistance R m and the nonlinear excitation inductance L characterizing the saturation properties of the iron core m Excitation branch, short-circuit resistance R k First and secondary side leakage sensation L σ Secondary load impedance Z L And two ideal transformers, one on the left and one on the right, used to calculate the intermediate excitation branch and connect to the external circuit. The primary and secondary turns ratio of the first ideal transformer on the left is N1:N, and the primary and secondary turns ratio of the second ideal transformer on the right is N:N2, where N1 / N=K and N / N2=1. The secondary load impedance Z L These are data that have been directly measured or calculated beforehand. The secondary side of the second ideal transformer on the right is connected to the external circuit on the secondary side. The secondary load impedance of the external circuit on the secondary side is Z. L To simplify the calculation, an impedance is used to replace the entire secondary external circuit.

[0177] like Figure 4 The equivalent circuit shown is constructed based on the Γ model and includes, in sequence: a first ideal transformer on the left, an excitation branch in the middle, a short-circuit resistor and primary and secondary leakage inductances, and a second ideal transformer on the right; the primary side of the first ideal transformer is connected to the primary side external circuit; an excitation branch is connected in parallel to the secondary side of the first ideal transformer, and the short-circuit resistor, primary and secondary leakage inductances, and the primary side of the second ideal transformer are connected in series to form a series branch, and the excitation branch is connected in parallel with the series branch, wherein the excitation branch is composed of a nonlinear excitation inductor characterizing the core saturation characteristics and an iron loss resistor connected in parallel; the secondary side of the second ideal transformer is connected to the secondary side external circuit.

[0178] Step 8: Based on the experimental setup of the current transformer forward circuit, obtain the relevant parameters in the current transformer forward circuit, including:

[0179] A short-circuit experiment was conducted based on the forward circuit of the current transformer, and the short-circuit resistance was calculated. and secondary side leakage L σ Based on the open-circuit test established by the current transformer forward circuit, the iron loss resistance R of the excitation branch is calculated. m A deep saturation test was established based on the current transformer forward circuit. Multiple sets of incremental inductance and excitation current data pairs were measured, and an excitation curve f characterizing the nonlinear characteristics of the iron core was constructed. Lm ;

[0180] (1) Short circuit test

[0181] When the primary side is short-circuited, a lower AC voltage is applied to the secondary side. Due to the primary side short circuit, the induced voltage on the primary side of the first ideal transformer is zero, therefore its secondary voltage is also zero. For example... Figure 4 As shown, almost all of the applied voltage drops across the short-circuit resistor. and secondary side leakage L σ The internal impedance is composed of a small component, while the current flowing through the excitation branch is very small and can be ignored. At this point, the forward circuit of the current transformer can be simplified to a circuit consisting of a component... and L σ A series circuit.

[0182] During the short-circuit test: A short circuit is applied to the primary side, and an AC power supply is applied to the secondary side, with the voltage gradually increasing until the secondary current reaches its rated value. In this embodiment, the rated secondary current is 5A. When the secondary current reaches its rated value, which is the effective value I of the secondary current... k When the current is 5A, the rated value of the secondary current of the low-frequency current transformer is recorded as the effective value I of the secondary current. k The effective value of the applied secondary voltage U k and the measured secondary active power P k ;

[0183] short-circuit resistor and secondary side leakage L σ The calculation formulas are as follows:

[0184] (33)

[0185] (34)

[0186] In the formula, Z k The short-circuit impedance is the magnitude of the total leakage impedance of the secondary winding. f is the frequency of the applied AC power supply; in this embodiment, f is 25Hz.

[0187] (2) Open circuit test

[0188] With the primary side open, an AC voltage is applied to the secondary side, such as... Figure 4 As shown, at this time, the secondary current, that is, the magnetizing current, flows entirely through the magnetizing branch and does not flow into the secondary winding of the first ideal transformer. Because L m It is inductive and only consumes reactive power. Therefore, the active power P0 measured in the open-circuit experiment can be considered to be entirely consumed by the iron loss resistance R. m Above. By measuring the voltage, current, and power at this point, the iron loss resistance R of the excitation branch can be calculated. m .

[0189] During the open-circuit test: the primary side is open-circuited, and an AC power supply is applied to the secondary side. The secondary side voltage rises from zero to the rated value. In this embodiment, the rated capacity of the current transformer is 10VA, and the rated secondary current is 5A. The rated capacity is divided by the rated secondary current to obtain the rated secondary voltage of 2V. When the secondary side voltage rises from zero to 2V, the rated value of the secondary side voltage of the low-frequency current transformer is recorded as the effective value of the applied secondary side voltage U0. When U0 is 2V, the effective value of the secondary side current I0 and the active power P0 of the secondary side are measured.

[0190] Iron loss resistance R m The calculation formula is:

[0191] (35)

[0192] In the formula, P0 is the active power on the secondary side measured in the open-circuit experiment.

[0193] (3) Extraction of parameters for depth saturation segment

[0194] When the current transformer operates in the non-saturation region, the magnetizing inductance L m It is the average inductance, with very high permeability, and the magnetizing inductance L m Large and constant; when the current transformer operates in the saturation region, the magnetizing inductance L m It is the incremental inductance L m_s The permeability decreases sharply, and the magnetizing inductance L m It is very small and decreases as the current increases. A hybrid AC / DC power supply is used to perform saturation testing on the current transformer: a stable DC flux is established in the core through a DC power supply, shifting the core's static operating point to the saturation region, forming a DC bias; based on this DC bias, a small-amplitude AC signal from an AC power supply is superimposed. This small AC signal only causes minor changes near the current operating point of the saturation curve. Under the combined effect of the AC / DC power supply, the incremental inductance of the excitation branch can be measured; the incremental inductance of the excitation branch is the slope of the excitation curve at that current operating point. For example... Figure 5As shown, during the deep saturation test, a mixed AC / DC power supply is applied to the secondary side external circuit of the second ideal transformer, while the primary side external circuit of the first ideal transformer is open. The DC power supply outputs a DC bias current to control the degree of saturation, and the AC power supply outputs a small AC signal for measurement. The steps of the deep saturation test include:

[0195] S1: By applying a DC bias current I b-dc1 This drives the iron core to a specific saturation state. At this point, the operating point of the excitation current is I. m_s1 =I b-dc1 That is, the point on the horizontal axis of the excitation curve;

[0196] S2: Maintaining DC bias current I b-dc1 On the premise that it remains unchanged, add a frequency of AC signal with very small amplitude i b-ac ;Measure the voltage u across the excitation branch m1 (t) and the total current i flowing through m1 (t); for the measured u m1 (t) and i m1 (t) Perform Fast Fourier Transform analysis to extract the fundamental frequency. Voltage amplitude U m_f1 and current amplitude I m_f1 ;

[0197] Based on fundamental frequency Voltage amplitude U m_f1 and current amplitude I m_f1 Calculate the incremental inductance L of the excitation branch m_s1 Excitation current operating point I m_s1 Incremental inductance L in the excitation branch m_s1 The calculation formula is:

[0198] (36)

[0199] In the formula, L is the frequency of the AC signal in the AC / DC hybrid power supply. m_s1 The excitation curve is on the horizontal axis I m_s1 The slope at the point; the vertical axis of the excitation curve represents the flux linkage value;

[0200] S3: Adjust the applied DC bias current to a new value I. b-dc2 I b-dc2 Not equal to I b-dc1 At this point, the operating point of the excitation current becomes I. m_s2 =I b-dc2 Maintain the amplitude and frequency of the AC signal. The process remains unchanged; step S2 is repeated to calculate the new incremental inductance L of the excitation branch. m_s2 ;

[0201] By repeatedly changing the DC bias current, a series of data pairs were obtained: (I m_s1 L m_s1 ), (I m_s2 L m_s2 ), ..., (I m_sx L m_sx ).

[0202] S4: Based on the fact that the incremental inductance is the differential of the flux linkage with respect to the excitation current, the series of data obtained in step S3 are numerically integrated to reconstruct the excitation curve f, which characterizes the nonlinear properties of the iron core. Lm The integration starts from the operating point of the excitation current being 0, and the initial flux linkage is set. It is 0.

[0203] Excitation curve f Lm This represents the relationship between the flux linkage of the excitation branch and the excitation current that generates that flux linkage. Based on the excitation curve f... Lm and the current flux linkage value You can get The corresponding inductive component value of the excitation current at the current moment .

[0204] Step 8: Derive the inversion model based on the forward circuit model of the current transformer:

[0205] Based on the structural analysis of the forward circuit model of the current transformer, we can conclude that:

[0206] The formula for calculating the secondary load voltage u2 is: (37)

[0207] In the formula, i2 is the measured secondary side response current; Z L This is the secondary load impedance;

[0208] The formula for calculating the primary voltage u3 of the ideal transformer on the right is: (38)

[0209] I felt a slight leakage after one or two flows. σ The leakage inductance current is: (39)

[0210] L from primary and secondary side leakage σ The leakage inductance voltage u can be obtained from the volt-ampere characteristic. Lσ for:

[0211] (40)

[0212] Considering the leakage inductance current i Lσ For discrete data, this embodiment uses the fourth-order Adams-Bushforth method in discrete state quantity algorithms to optimize the solution of transient voltage, aiming to maintain accuracy without significantly increasing the amount of computation.

[0213] Derived leakage inductance voltage u Lσ The calculation formula is shown in equation (41) below, and the specific derivation process is as follows:

[0214] Derivation of the fourth-order Adams-Bushforth method:

[0215] Given the differential equation:

[0216] And it has an initial value: ,

[0217] In t n Nearby Taylor series expansion:

[0218] ,

[0219] Where h is the sampling step size.

[0220] Based on the Adams-Bushforth method, , The second-order Taylor expansion can be expressed as:

[0221] ,

[0222] The formula for calculating the second-order Adams-Bashforth discrete algorithm can be obtained by rearranging the formula:

[0223] ,

[0224] The Adams-Bashforth method is derived based on the Taylor series expansion formula, using difference equations to replace the second derivative. Therefore, by substituting the difference equations into the third and fourth derivatives respectively, we can obtain the higher-order Adams-Bashforth discrete algorithm, as shown in the following two equations.

[0225] The calculation formula for the third-order Adams-Bashforth discrete algorithm:

[0226] Formula for calculating the fourth-order Adams-Bashforth discrete algorithm:

[0227] The fourth-order Adams-Bushforth method in this embodiment:

[0228] Applying the fourth-order Adams-Bushforth method to solve for leakage inductance voltage, the calculation formula of the above fourth-order Adams-Bushforth discrete algorithm is transformed into:

[0229]

[0230] The leakage inductance voltage is obtained from the above formula. Calculation formula:

[0231] (41)

[0232] In the formula, Δt is the time step; For the second moment, the feeling of leakage L σ Leakage inductance current; For the current moment, the first and second side leakage sensation L σ Leakage inductance current; For the first and second side leakage sensations of the previous moment L σ Leakage inductance voltage; For the first two moments, the second and third moments of side leakage L σ Leakage inductance voltage; For the first three moments, there is a feeling of leakage at the first and second intervals. σ The leakage inductance voltage.

[0233] In solving for the transient voltage of a current transformer, the numerical solution is usually performed starting from the instant of switching on. Since the sampling frequency of the secondary voltage acquisition equipment is high, the system is assumed to be approximately in a zero state at the initial moment; therefore, the iterative starting value u is used. Lσ (0), u Lσ (1) u Lσ (2) All values ​​are set to 0 and substituted into equation (41) to calculate the subsequent values.

[0234] Figure 4 In the above, the formula for calculating the voltage u4 across the excitation branch is:

[0235] (42)

[0236] Resistive component of excitation branch current The calculation formula is:

[0237] (43)

[0238] The inductive component of the excitation branch current and the excitation inductance flux exhibit nonlinear characteristics. The calculation formula is:

[0239] (44)

[0240] In the formula, f LmThe nonlinear excitation curve characterizing the excitation branch; flux linkage λ Lm It is a quantity that changes over time. Magnetic flux linkage λ Lm The voltage u4 across the excitation branch is obtained by integrating over time. Since the voltage u4 across the excitation branch is a sampled discrete value, the sampling time point from 0 to the current time t* is... Time step The interval between adjacent time points is calculated using the trapezoidal integral method in discrete state quantity algorithms: In a discrete system, from the previous time point... The change in flux linkage up to the current time t* is the flux linkage increment at this time step. This is obtained by calculating the area under the trapezoid of the voltage curve during that time period: Therefore, the flux linkage value at the current moment is equal to the flux linkage value at the previous moment. Add the flux increment at this time step The current flux linkage value The calculation formula is:

[0241] (45)

[0242] In the formula, This represents the voltage across the excitation branch at the previous moment.

[0243] Based on Kirchhoff's law (KCL), the primary current of a current transformer can be expressed as:

[0244] (46)

[0245] In the formula, i2 is the measured secondary response current; i1 is the primary excitation current to be solved; K is the turns ratio of the primary winding and the secondary winding of the current transformer. ; The inductive component of the excitation branch current; This is the resistive component of the excitation branch current;

[0246] Current primary side excitation current The calculation formulas for the inversion model are summarized as follows:

[0247] (47)

[0248] In the formula, Z represents the measured secondary response current at the current moment. L This is the secondary load impedance; This represents the current secondary load voltage. L represents the primary voltage of the second ideal transformer at the current moment. σ It is a sensation of leakage, either once or twice. For the current moment, the first and second side leakage sensation Lσ Leakage inductance current; For the second moment, the feeling of leakage L σ Leakage inductance current; For the first and second side leakage sensations of the previous moment L σ Leakage inductance voltage; For the current moment, the first and second side leakage sensation L σ Leakage inductance voltage; For short-circuit resistance; This represents the voltage across the excitation branch at the current moment. Iron loss resistance; This represents the resistive component of the excitation branch current at the current moment. This is the flux linkage value from the previous moment; The voltage across the excitation branch at the previous moment; A time step represents the interval between adjacent time points; This represents the flux linkage value at the current moment. The excitation curve; This represents the inductive component of the excitation branch current at the current moment. This represents the primary side excitation current at the current moment.

[0249] To verify the correctness of the method proposed in this invention, a finite element simulation model of the current transformer was established. The current transformer was operated at a frequency of 25Hz, and the external excitation power supply was adjusted to gradually increase the DC proportion T. Curves showing the variation of the ratio error and phase angle error with the DC proportion were obtained and compared with the calculation results of the mathematical model proposed in this invention. The changes in the current transformer with different DC proportions were analyzed. Figure 6 and Figure 7 As shown.

[0250] Depend on Figure 6 and Figure 7 As can be seen, the finite element simulation results are basically consistent with the mathematical model calculation results, and both the ratio error and the angle error gradually increase with the increase of the DC ratio T. When T is small, the calculation results of the two are in good agreement and show a linear relationship; when T increases, the magnetic circuit saturation effect caused by DC bias makes the error of the current transformer exhibit nonlinear characteristics. Since this paper is based on the assumption that the core magnetic flux density still maintains a sinusoidal fundamental wave change under DC bias, the error increases slightly.

[0251] To further investigate the impact of different load impedance angles on the error of the current transformer under DC bias, the secondary load was kept constant, and the DC ratio was maintained at 0.3%. The magnitude of the load impedance angle was varied, and the simulated error was compared with the theoretical value. The error values ​​calculated in this invention and the finite element simulation results are shown below. Figure 8 and Figure 9 As shown. By Figure 8 and Figure 9 As can be seen, the finite element simulation results are basically consistent with the mathematical model calculation results. The ratio difference gradually increases with the increase of the load impedance angle γ1, while the angle difference gradually decreases with the increase of γ1.

[0252] To further address the severe bias magnetization with significant errors, the derived inversion mathematical model was used to calculate the primary-side current waveform, which was then compared with the actual primary excitation current. The secondary distorted current waveform with a superimposed DC current of 500A was selected for verification. Using the distorted secondary-side current waveform data, the primary-side current was inverted, and the solution results are as follows: Figure 10 As shown. To make the results easier to compare, Figure 10 The secondary current waveform has been attributed to the primary side.

[0253] Figure 10 This indicates that, due to the influence of DC bias, the core is deeply saturated, and the secondary current waveform is distorted, failing to accurately reflect the true primary waveform data. However, this paper solves for the current waveform through inversion, which is consistent with the actual primary current waveform, effectively compensating for the distortion caused by DC bias.

[0254] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0255] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0256] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0257] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0258] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for processing errors in a low-frequency current transformer, characterized in that, include: Calculate and determine whether the error of the low-frequency current transformer is within the preset accuracy class range; When the measured secondary response current of the low-frequency current transformer exceeds the preset accuracy level range, the calculation formula of the pre-derived inversion model is input to calculate and output the primary excitation current after error correction. The calculation formula of the inversion model is based on the forward circuit of the current transformer containing the nonlinear characteristics of the iron core, and is derived by using a discrete state quantity numerical algorithm.

2. The low-frequency current transformer error processing method according to claim 1, characterized in that, The forward circuit of the current transformer containing the nonlinear characteristics of the iron core is the equivalent circuit of the low-frequency current transformer. It is constructed based on the Γ model and includes the following components connected in sequence: the first ideal transformer on the left, the excitation branch in the middle, the short-circuit resistor and the primary and secondary leakage inductance, and the second ideal transformer on the right. The primary side of the first ideal transformer is connected to the external primary circuit. The first ideal transformer has a parallel excitation branch on the secondary side. The short-circuit resistor, the leakage inductance of the primary and secondary sides and the primary side of the second ideal transformer are connected in series to form a series branch. The excitation branch is connected in parallel with the series branch. The excitation branch is composed of a nonlinear excitation inductor that characterizes the saturation characteristics of the iron core and an iron loss resistor connected in parallel. The secondary side of the second ideal transformer is connected to the external circuit of the secondary side.

3. The low-frequency current transformer error processing method according to claim 2, characterized in that, The process of inputting the measured secondary response current of the low-frequency current transformer into the pre-derived inversion model calculation formula to calculate and output the primary excitation current includes: Obtain the measured secondary-side response current and the secondary-side load impedance of the external circuit. Based on the experiment established by the forward circuit of the current transformer, the relevant parameters in the forward circuit of the current transformer are obtained. The relevant parameters include: short-circuit resistance, primary and secondary leakage inductance, iron loss resistance, and incremental inductance used to construct the excitation curve. The relevant parameters and the measured secondary response current are input into the pre-derived inversion model calculation formula to calculate and output the primary excitation current; The turns ratio of the primary and secondary sides of the first ideal transformer is K, and the turns ratio of the primary and secondary sides of the second ideal transformer is 1; the primary excitation current at the current moment... The inversion model calculation formula is as follows: ， In the formula, For the current moment Measured secondary response current; Z L This is the secondary-side load impedance; This represents the current secondary load voltage. This represents the primary voltage of the second ideal transformer at the current moment. For the current moment, the first and second side leakage sensation L σ Leakage inductance current; For the current moment, the first and second side leakage sensation L σ Leakage inductance voltage; For short-circuit resistance; This represents the voltage across the excitation branch at the current moment. Iron loss resistance; This represents the resistive component of the excitation branch current at the current moment. This represents the flux linkage value at the current moment. The excitation curve; This represents the inductive component of the excitation branch current at the current moment. This represents the primary side excitation current at the current moment.

4. The low-frequency current transformer error processing method according to claim 3, characterized in that, The experiment established based on the forward circuit of the current transformer yields relevant parameters in the forward circuit of the current transformer, including: A short-circuit experiment was conducted based on the forward circuit of the current transformer, and the short-circuit resistance was calculated. and secondary side leakage L σ During the short-circuit test: A short circuit is applied to the primary side, and an AC power supply is applied to the secondary side, with the voltage gradually increasing until the secondary current of the low-frequency current transformer reaches its rated value. The rated value of the secondary current of the low-frequency current transformer is recorded as the effective value I of the secondary current. k The effective value of the applied secondary voltage U k and the measured secondary active power P k ; short-circuit resistor and secondary side leakage L σ The calculation formulas are as follows: , , In the formula, Z k The short-circuit impedance is the magnitude of the total leakage impedance of the secondary winding. f is the frequency of the applied AC power supply; Based on the open-circuit test established by the forward circuit of the current transformer, the iron loss resistance R of the excitation branch is calculated. m During the open-circuit test: the primary side is open-circuited, and an AC power supply is applied to the secondary side. The secondary side voltage rises from zero to its rated value. The rated value of the secondary side voltage of the low-frequency current transformer is recorded as the effective value U0 of the applied secondary side voltage. The effective value I0 of the secondary side current and the active power P0 of the secondary side are also measured. Iron loss resistance R m The calculation formula is: ; A deep saturation test is established based on the forward circuit of the current transformer. Multiple sets of incremental inductance and excitation current data pairs are measured, and numerical integration is performed on the data pairs to obtain the excitation curve f that characterizes the nonlinear properties of the iron core. Lm When the current transformer operates in the saturation region, the magnetizing inductance L m It is the incremental inductance L m_s The depth saturation test includes: A hybrid AC / DC power supply is applied to the secondary side external circuit. The DC power supply outputs a DC bias current, and the AC power supply outputs an AC signal; the amplitude and frequency of the AC signal are maintained. The following steps are repeated continuously, adjusting the core saturation level by changing the DC bias current multiple times, to obtain a series of data pairs: (I m_s1 L m_s1 ), (I m_s2 L m_s2 ), ..., (I m_sx L m_sx ): By applying a DC bias current I b-dc1 Drive the iron core to saturation and record the excitation current operating point as I. m_s1 =I b-dc1 ; Maintain DC bias current I b-dc1 On the premise that it remains unchanged, add a frequency of AC signal i used for measurement b-ac ;Measure the voltage u across the excitation branch m1 (t) and the total current i flowing through m1 (t); for the measured u m1 (t) and i m1 (t) Perform Fast Fourier Transform analysis to extract the fundamental frequency. Voltage amplitude U m_f1 and current amplitude I m_f1 ; Based on fundamental frequency Voltage amplitude U m_f1 and current amplitude I m_f1 Calculate the incremental inductance L of the excitation branch m_s1 Excitation current operating point I m_s1 Incremental inductance L in the excitation branch m_s1 The calculation formula is: ， In the formula, L is the frequency of the AC signal in the AC / DC hybrid power supply. m_s1 The excitation curve on the horizontal axis I m_s1 The slope at the point; the vertical axis of the excitation curve represents the flux linkage value.

5. The low-frequency current transformer error processing method according to claim 4, characterized in that, Current moment, primary and secondary leakage sensation L σ Leakage inductance voltage The calculation includes: solving for the leakage inductance voltage using the fourth-order Adams-Bushforth method in discrete state quantity algorithms. The calculation formula is: ， In the formula, Δt is the time step; For the second moment, the feeling of leakage L σ Leakage inductance current; For the current moment, the first and second side leakage sensation L σ Leakage inductance current; For the first and second side leakage sensations of the previous moment L σ Leakage inductance voltage; For the first two moments, the second and third moments of side leakage L σ Leakage inductance voltage; For the first three moments, there is a feeling of leakage at the first and second intervals. σ The leakage inductance voltage; where u Lσ (0), u Lσ (1) u Lσ (2) All are set to 0.

6. The low-frequency current transformer error processing method according to claim 5, characterized in that, Current flux linkage value The calculation includes: using the trapezoidal integral method in the discrete state quantity algorithm to calculate the flux linkage value at the current time. The flux linkage value at the current time is equal to the sum of the flux linkage values ​​at the current time. The calculation formula is: ， In the formula, This is the flux linkage value from the previous moment; This is the flux linkage increment for this time step; This represents the voltage across the excitation branch at the previous moment.

7. The low-frequency current transformer error processing method according to claim 1, characterized in that, The calculation and determination of whether the error of the low-frequency current transformer is within the preset accuracy class range includes: The theoretical and actual secondary current data of the low-frequency current transformer are obtained. The theoretical secondary current data takes into account the presence of resistive-inductive load and DC bias while ignoring the nonlinear characteristics of the iron core. The actual secondary current data takes into account the presence of resistive-inductive load, DC bias, and the nonlinear characteristics of the iron core. Calculate the ratio difference and angle difference of the low-frequency current transformer based on theoretical and actual secondary current data. The calculated ratio error and angle error of the low-frequency current transformer are compared with the preset accuracy class ranges for ratio error and angle error, respectively, to determine whether the error of the low-frequency current transformer is within the preset accuracy class range.

8. The low-frequency current transformer error processing method according to claim 7, characterized in that, Obtain the theoretical secondary current data of the low-frequency current transformer, including: The expressions for the primary current i1(t) and secondary current i2(t) considering DC bias and inductive load are established as follows: ， ， In the formula, I dc I is the amplitude of the DC bias current. ac I represents the effective value of the AC component of the primary current, ω is the current angular frequency, t is the time variable, and I is the effective value of the AC component of the primary current. 21 β1 is the effective value of the secondary current, and β2 is the phase angle of the secondary current considering the resistive-inductive load. Excitation current i e The expression for (t) is: ， In the formula, K is the turns ratio of the primary winding and the secondary winding of the current transformer. According to the excitation current i e The expression for (t) is used to obtain the DC component I contained in the excitation current. DC Fundamental frequency AC component amplitude I e1 and phase angle φ e1 The expression: ， ， ， I DC I e1 and φ e1 This is the theoretical secondary current data for a low-frequency current transformer.

9. The low-frequency current transformer error processing method according to claim 8, characterized in that, Obtain the actual secondary current data of the low-frequency current transformer, including: The BH curve of the core material is simplified; the simplified BH curve consists of two linear segments with slopes μ1 and μ2, respectively; μ1 is the permeability of the unsaturated core, and μ2 is the permeability of the saturated core; the intersection of the two segments is the knee point of the BH curve of the ferromagnetic material; the magnetic flux density at the knee point is denoted as B. k ; Taking into account DC bias and resistive-inductive load, the resultant magnetic flux B inside the iron core is established. h (t) and magnetic field strength H m The expression for (t): ， ， in, , In the formula, B ac B represents the amplitude of the AC component of the magnetic flux density in the iron core under DC bias. dc H represents the amplitude of the DC component. u This is called the knee magnetic field strength, H. s H represents the magnetic field strength corresponding to the peak value of the core magnetic flux density. bias θ is the DC magnetic field strength generated by the DC component of the magnetic flux density; θ is the phase difference between the peak point of the magnetic flux density in the iron core and the origin; α is the flux angle, which increases with the increase of DC current, and when the DC current is zero, α is approximately zero. According to B h (t) and H m The excitation current is obtained from the expression of (t). The expression is given, and the DC component is separated from it. fundamental frequency AC component amplitude and phase angle The expression: ， ， ， In the formula, N2 is the number of turns of the secondary winding, L is the average magnetic circuit length of the core, and γ1 is the load impedance angle; , and This is the actual secondary current data of the low-frequency current transformer.

10. The template matching method based on deep learning according to claim 9, characterized in that, The calculation of the ratio error and phase angle error of the low-frequency current transformer based on theoretical and actual secondary current data includes: Assuming all components of the excitation current obtained by the two methods are equal, we solve for the magnitude ratio k2 and phase difference β1 of the currents on both sides, and then, using the error definition, we solve for the expressions for the ratio difference ε and the angle difference δ: make , , We can obtain the system of equations: ， ， ， Since the flux angle α is normally less than 65°, the relationship between (α-0.5sin2α) and (sinα-αcosα) is approximately linear at 1.9 times, that is: , Will Substituting into the system of equations and simplifying, we get: ， In the formula, T represents the DC percentage, and its value is I. dc / I ac i u The knee excitation current referred to the secondary side is H. u L / N2; Defined as the effective value of the AC component of the primary current referred to the secondary side, its value is I. ac / K; P is the excitation ratio, its value is The magnitude of the excitation ratio P is related to the knee point of the BH curve. Once the core material is determined, the excitation ratio P is also determined. The errors of a current transformer include ratio error and phase error. According to the definitions of ratio error and phase error, we can obtain: ， ， In the formula, φ1 is the phase angle of the primary current and φ2 is the phase angle of the secondary current. The expressions for calculating the ratio difference ε and angle difference δ of a current transformer are as follows: ， ， By comparing the expression for the difference ε with the "+" sign and performing monotonicity analysis, it can be seen that the value of the difference function gradually increases with the DC proportion T. When T increases to a certain value, the difference ε will change from negative to positive. However, in reality, due to the presence of DC, the secondary current of the current transformer is smaller than normal, and the difference should be negative. Therefore, the expression with the "+" sign does not match the actual situation and is discarded. The expressions for the final ratio difference ε and angle difference δ are: ， 。

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