A compensation method, device and system for direct current bias magnetization of a current transformer

Abstract

This invention provides a method, apparatus, and system for compensating DC bias in a current transformer. The method includes: acquiring secondary-side current waveform data from the current transformer and determining the secondary-side current bias degree based on the waveform data; the secondary-side current bias degree measures the degree to which the secondary current waveform deviates from a preset time axis; determining the primary-side DC component based on the secondary-side current bias degree using a pre-established deviation relationship model between the primary-side DC component and the secondary-side current; the deviation relationship model recording the bias degree between the primary-side DC component and the secondary-side current on the corresponding time axis; and controlling a voltage-controlled current source in the current transformer to generate a reverse DC current to eliminate bias based on the primary-side DC component. This invention uses the secondary-side current bias degree to reflect the magnitude of the primary-side DC component, eliminating the need for additional measuring devices on the primary side and avoiding the cumbersome and difficult aspects of algorithm compensation in modeling, data processing, and algorithm writing.

Description

Technical Field

[0001] This invention relates to the field of DC bias compensation technology, and in particular to a method, apparatus and system for compensating DC bias in current transformers. Background Technology

[0002] With the construction of ultra-high voltage direct current (UHVDC) transmission projects, China's power grid has taken on a hybrid AC / DC grid configuration. When DC transmission operates in a monopolar polarity loop or bipolar unbalanced operation mode, DC currents of up to several thousand amperes flow into the ground. Some of this DC current flows through the neutral-grounded power transformer, forming a loop in the AC system, thereby generating DC bias magnetism in the transformer and current transformer.

[0003] During normal operation, the current transformer operates in the linear region, with a very small excitation current i0. The ratio of the primary current to the secondary current can be considered equal to the turns ratio, and the primary current information can be obtained by measuring the small secondary current. When a DC component exists in the primary current of the current transformer, this component generates a bias flux, altering the operating point of the current transformer and potentially causing saturation and distortion of the secondary current. If the primary current is still calculated based on the turns ratio under these conditions, the measurement error will increase, failing to meet the original measurement accuracy requirements. Therefore, researching compensation methods for DC bias in current transformers is of significant practical importance.

[0004] Currently, there are two main methods for compensating DC bias in current transformers. One method involves calculating the magnetization curve of the material during DC bias, obtaining the magnetic field strength H from the magnetic flux density B and hysteresis curve generated on the primary side, calculating the excitation current i0, and finally correcting the secondary current using a digital algorithm. The other method involves extracting the secondary harmonic content of the secondary current to determine the saturation level of the current transformer. By establishing the relationship between the secondary harmonic content of the secondary current and the magnitude of the DC current on the primary side, a voltage-controlled current source in an external circuit generates a DC current of equal magnitude but opposite direction to eliminate the DC magnetomotive force and achieve the purpose of compensation.

[0005] To calculate the magnetization curve of a material under DC bias, a magnetic field model (JA model or Preisach model) needs to be established for the current transformer to find the operating point of the current transformer core and determine whether it is saturated. The formula for the magnetic field model of the current transformer is complex, requiring the solution of differential equations, and the parameters are difficult to determine accurately. The process is complex and the calculation is tedious. Furthermore, different DC component magnitudes correspond to different magnetization curves. In actual power systems, the DC component is not a constant value. Once the DC component changes, the magnetization curve needs to be recalculated, making it difficult to adapt to practical engineering.

[0006] Extracting the magnitude of the second harmonic of the secondary current requires complex external circuitry, including a standard impedance transformer, phase-sensitive detector circuit, several filters, and negative feedback control circuitry, resulting in a complex structure and high cost. Furthermore, because the fundamental and second harmonic frequencies are close, it is difficult to separate them using simple filtering devices. In addition, a small amount of second harmonic exists in actual power systems, making it difficult to establish an accurate relationship between the second harmonic content on the secondary side and the magnitude of the DC component on the primary side, thus affecting the accuracy of DC bias compensation. Summary of the Invention

[0007] The purpose of this invention is to provide a method, device, and system for compensating DC bias in current transformers, thereby solving the technical problem of how to more easily eliminate DC magnetomotive force in current transformers and improve the working accuracy of current transformers.

[0008] On the one hand, a method for compensating DC bias in a current transformer is provided, comprising:

[0009] Collect the secondary current waveform data in the current transformer, and determine the secondary current bias degree based on the secondary current waveform data; wherein, the secondary current bias degree is used to measure the degree to which the secondary current waveform deviates from the preset time axis;

[0010] The primary side DC component is determined based on the secondary side current bias by using a pre-established deviation relationship model between the primary side DC component and the secondary side current; wherein, the deviation relationship model records the bias between the primary side DC component and the secondary side current corresponding to the time axis.

[0011] The voltage-controlled current source in the current transformer is controlled to generate a reverse DC current to eliminate the bias magnetization based on the primary side DC component.

[0012] Preferably, it further includes:

[0013] After acquiring the secondary current waveform data, it is converted into a voltage signal by a preset signal converter, and the high-order harmonic components and high-frequency interference signals in the voltage signal are filtered out by a preset filter. The filtered voltage signal is then arranged into a secondary current bias degree according to the time axis.

[0014] Preferably, determining the secondary current bias degree based on the secondary current waveform data includes:

[0015] The secondary current bias is calculated using the following formula:

[0016]

[0017] Where δ is the secondary current bias, and i2 is the instantaneous value of the secondary current.

[0018] Preferably, the pre-established deviation relationship model between the primary-side DC component and the secondary-side current is obtained through the following process:

[0019] A rated fundamental current is applied to the primary side through an AC / DC power supply, and a DC component is superimposed on the primary side while continuously changing the magnitude of the DC component.

[0020] The DC component of the current transformer in the actual operating environment was simulated multiple times, and the bias degree data of the secondary current corresponding to the DC component was collected in each simulation.

[0021] The bias data of the secondary current is combined with the primary DC component on the corresponding time axis to form a deviation relationship model.

[0022] On the other hand, a compensation device for DC bias of a current transformer is also provided to implement the aforementioned compensation method for DC bias of a current transformer, comprising:

[0023] The bias calculation module is used to collect the secondary current waveform data in the current transformer and determine the secondary current bias based on the secondary current waveform data; wherein, the secondary current bias measures the degree to which the secondary current waveform deviates from a preset time axis.

[0024] The DC component calculation module is used to determine the primary side DC component based on the secondary side current bias degree using a pre-established deviation relationship model between the primary side DC component and the secondary side current; wherein, the deviation relationship model records the bias degree between the primary side DC component and the secondary side current corresponding to the time axis.

[0025] The demagnetizing module is used to control the voltage-controlled current source in the current transformer to generate a reverse DC current to eliminate the bias magnetization based on the primary side DC component.

[0026] Preferably, the bias calculation module is specifically used to convert the secondary current waveform data into a voltage signal through a preset signal converter after acquisition, filter out the high-order harmonic components and high-frequency interference signals in the voltage signal through a preset filter, and compose the secondary current bias by arranging the filtered voltage signal according to the time axis.

[0027] Preferably, the bias calculation module is further configured to calculate the secondary current bias according to the following formula:

[0028]

[0029] Where δ is the secondary current bias, and i2 is the instantaneous value of the secondary current.

[0030] Preferably, it further includes:

[0031] The deviation relationship model establishment module is used to apply a rated fundamental current to the primary side through an AC / DC power supply, superimpose a DC component on the primary side, and continuously change the magnitude of the DC component.

[0032] The DC component of the current transformer in the actual operating environment was simulated multiple times, and the bias degree data of the secondary current corresponding to the DC component was collected in each simulation.

[0033] The bias data of the secondary current is combined with the primary DC component on the corresponding time axis to form a deviation relationship model.

[0034] On the other hand, a compensation system for DC bias of a current transformer is also provided, which achieves compensation for DC bias of the current transformer by means of a compensation device for DC bias of the current transformer.

[0035] In summary, implementing the embodiments of the present invention has the following beneficial effects:

[0036] The present invention provides a method, apparatus, and system for compensating DC bias in current transformers. It uses the secondary-side current bias to reflect the magnitude of the primary-side DC component, offering a simple and effective indicator without requiring additional measuring devices on the primary side. A reverse DC current is generated by a voltage-controlled current source to eliminate the DC magnetomotive force in the current transformer, improving its operating accuracy and fundamentally eliminating the influence of DC bias. The apparatus operates online in real time, providing corresponding compensation based on different DC contents on the primary side. Online compensation avoids the cumbersome and difficult aspects of algorithmic compensation in terms of modeling, data processing, and algorithm writing. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, obtaining other drawings based on these drawings without creative effort still falls within the scope of the present invention.

[0038] Figure 1 This is a schematic diagram of the structure of a current transformer according to an embodiment of the present invention.

[0039] Figure 2 This is a schematic diagram of a magnetization curve in an embodiment of the present invention.

[0040] Figure 3 This is a schematic diagram of the main flow of a method for compensating DC bias of a current transformer according to an embodiment of the present invention.

[0041] Figure 4 This is a schematic diagram of a DC bias magnetization experiment in an embodiment of the present invention.

[0042] Figure 5 This is a schematic diagram of the PSCAD simulation model in an embodiment of the present invention.

[0043] Figure 6 This is the secondary current waveform under rated conditions in an embodiment of the present invention.

[0044] Figure 7 This is the secondary current waveform after superimposing a DC component in an embodiment of the present invention.

[0045] Figure 8 This is the cubic spline interpolation result in an embodiment of the present invention.

[0046] Figure 9 The waveforms of the secondary side current before and after compensation are shown in the embodiments of the present invention.

[0047] Figure 10 This is a schematic diagram of a compensation device for DC bias of a current transformer according to an embodiment of the present invention. Detailed Implementation

[0048] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings.

[0049] like Figure 1 The principle structure of the current transformer shown is as follows: Figure 2 The magnetization curve of the core material is shown. When the current transformer is working, there is a magnetic flux density B in the core. According to the magnetization curve of the material, the corresponding magnetic field strength H is given.

[0050] According to Ampere's circuital law:

[0051] Hl=N1i1-N2i2

[0052] In the formula, N1 is the number of primary turns, N2 is the number of secondary turns; i1 is the primary current, i2 is the secondary current; and l is the average length of the magnetic circuit of the current transformer.

[0053] As shown in the above equation and the magnetization curve, when the magnetic flux density B is too high, it corresponds to entering the nonlinear region of the magnetization curve, i.e., the current transformer enters saturation. Since the slope of the BH curve is very small at this time, a small change in the magnetic flux density B will lead to a large change in the magnetic field strength. The greater the magnetic field strength, the greater the excitation current, and the greater the difference between the primary and secondary currents. Coupled with the nonlinearity of the magnetization curve, the waveform of the secondary current is prone to distortion.

[0054] Under steady-state conditions, the magnetic flux generated by the fundamental or harmonic waves on the primary side is a sinusoidal waveform, symmetrical about the time axis, and the magnetization loop of the material also exhibits symmetry. If the magnetic flux density B generated is too large due to the excessive amplitude of the fundamental or harmonic waves, leading to saturation, the secondary current will also be distorted. However, due to the symmetry about the time axis, the distorted secondary current also exhibits the characteristic of symmetrical positive and negative half-cycle waveforms.

[0055] The presence of a DC component causes a positive or negative shift in the original sinusoidal magnetic flux density waveform, effectively altering the original symmetrical operating point and disrupting the symmetry about the time axis. This is achieved with a positively biased DC magnetic flux φ. DC Taking the example of a half-cycle of primary current in the positive direction, the magnetic flux density B is generated. + Will with φ DC The superposition of these components increases the actual working magnetic flux of the iron core, pushing it into the saturation region. For the primary current of the negative half-axis, this has a demagnetizing effect, while the negative half-axis still operates in the linear region. The larger the positive DC component, the easier it is for the working magnetic flux of the positive half-cycle to enter the nonlinear region of the material's magnetization curve, generating a larger magnetic field strength H in the iron core. According to Ampere's circuital law, the greater the magnetic field strength, the larger the excitation current i0, resulting in a spike in the positive half-cycle of the secondary current and severe waveform distortion. The negative half-cycle remains in the linear region, with no waveform distortion; it is still part of a complete sine wave, shifted in the positive direction.

[0056] The above analysis shows that the larger the DC component, the greater the operating point offset, and the more the waveform deviates from the time axis. Furthermore, the degree of deviation and the magnitude of the DC component have a monotonically increasing function relationship. If we can define the degree of waveform deviation and establish its relationship with the magnitude of the primary-side DC component, then we can determine the magnitude of the primary-side DC component using the defined degree of deviation.

[0057] like Figure 3 The diagram shown is a schematic representation of an embodiment of a method for compensating DC bias in a current transformer according to the present invention. In this embodiment, the method includes the following steps:

[0058] Step S1: Acquire the secondary current waveform data of the current transformer and determine the secondary current bias degree based on the secondary current waveform data. The secondary current bias degree measures the degree to which the secondary current waveform deviates from a preset time axis. That is, the secondary current signal is converted into a voltage signal by a standard signal converter for subsequent signal processing and calculation. The voltage signal is filtered by a low-pass filter to remove high-order harmonic components and high-frequency interference signals. The bias degree is calculated in the negative feedback control loop, the magnitude of the DC component is obtained from a table, and the voltage-controlled current source is controlled to generate an equal reverse DC current. The DC magnetomotive force is eliminated through the secondary compensation winding, achieving the compensation and correction effect.

[0059] In a specific embodiment, after the secondary current waveform data is acquired, it is converted into a voltage signal by a preset signal converter, and the high-order harmonic components and high-frequency interference signals in the voltage signal are filtered out by a preset filter. The filtered voltage signal is then arranged according to the time axis to form the secondary current bias.

[0060] Specifically, the secondary current bias is calculated using the following formula:

[0061]

[0062] Where δ is the secondary current bias, and i2 is the instantaneous value of the secondary current. The secondary current bias δ ​​is defined to measure the degree to which the secondary current waveform deviates from the time axis.

[0063] Step S2: Determine the primary DC component based on the secondary current bias by using a pre-established deviation relationship model between the primary DC component and the secondary current. The deviation relationship model records the bias between the primary DC component and the secondary current on the corresponding time axis. In other words, establishing the relationship between the magnitude of the primary DC component and the degree of deviation of the secondary current requires prior experimental data acquisition and data fitting.

[0064] In a specific embodiment, the pre-established deviation relationship model between the primary-side DC component and the secondary-side current is obtained through the following process: A rated fundamental current is applied to the primary side using an AC / DC power supply; a DC component is superimposed on the primary side, and the magnitude of the DC component is continuously changed; the DC component in the actual operating environment of the current transformer is simulated multiple times, and the bias degree data of the secondary-side current corresponding to the DC component is collected in each simulation; the bias degree data of the secondary-side current is combined with the primary-side DC component on the corresponding time axis to form the deviation relationship model. Understandably, a schematic diagram of the DC bias experiment is shown below. Figure 4 As shown, the primary side is supplied with a rated fundamental current superimposed with a DC component through an AC / DC power supply. By changing the magnitude of the DC component, the range of DC component variation in the actual operating environment of the current transformer is covered, and the waveform and data of the secondary current are obtained.

[0065] In this process, a piecewise cubic spline function is used for interpolation to fit the relationship between the secondary current bias and the magnitude of the DC component. The DC component is characterized by the percentage of DC to the rated primary current.

[0066] Suppose there are n+1 nodes x on the interval [a, b]. i Satisfying a = x0 <x1<…<x n =b, at node x i The function value at that point is y i =f(x) i If the function S(x) satisfies three conditions:

[0067] 1. In each subinterval [x i-1 ,x i On the [above], S(x) is a cubic polynomial.

[0068] 2. S(x) i )=y i

[0069] 3. On the interval [a, b], the second derivative of S(x) is continuous.

[0070] S(x) is then called the cubic spline interpolation function of the function y = f(x) on the interval [a, b]. The expression for S(x) is:

[0071]

[0072] In the formula, h i =x i+1 -x i .

[0073] Performing cubic spline interpolation requires solving three moment equations:

[0074] μ i M i-1 +2M i +λ i M i+1 =d i i = 1, ..., n-1

[0075] In the formula,

[0076] Step S3: Based on the primary side DC component, control the voltage-controlled current source in the current transformer to generate a reverse DC current to eliminate the bias magnetization. That is, 2. By generating a reverse DC current through the voltage-controlled current source, the DC magnetomotive force in the current transformer is eliminated, the operating accuracy of the current transformer is improved, and the influence of DC bias magnetization is essentially eliminated.

[0077] In this embodiment, the effectiveness of the method is verified through a simulation example. A simulation model is built on the PSCAD platform, such as... Figure 5 As shown. Parameters of the current transformer: N1 = 20, N2 = 200, l = 637.7 mm, S = 2601 mm. 2 The primary rated current is I. N =2000A, fundamental frequency f=50Hz; secondary load R2=0.5Ω, L=0.8mH.

[0078] Under rated conditions, the secondary current waveform is as follows: Figure 6 As shown. With 5% and 10% DC components superimposed on the primary side, the secondary side current waveform is as follows. Figure 7As shown in the figure, the secondary current is distorted and deviates from the time axis after the DC component is superimposed. The larger the DC component, the more severe the waveform distortion and the greater the overall deviation from the time axis. Simulation experiments were conducted every 1% within the DC content range of 1% to 20%, and the bias degree under that DC component was calculated. Piecewise cubic splines were used for interpolation, and the results are shown in the figure. Figure 8 As shown. A DC component ranging from 0% to 20% is randomly superimposed on the primary side, and compensation is performed step by step. The secondary side current waveforms before and after compensation are shown below. Figure 9 As shown, before compensation, the ratio error of the current transformer was -10.3% and the angle error was 115.9'. After compensation, the ratio error was 0.17% and the angle error was 1.32'. It can be seen that this method has a good DC bias compensation effect.

[0079] In specific implementations of this invention, such as Figure 10 As shown, a compensation device for DC bias of a current transformer is also provided to implement the aforementioned compensation method for DC bias of a current transformer, comprising:

[0080] The bias calculation module is used to collect the secondary current waveform data in the current transformer and determine the secondary current bias based on the secondary current waveform data; wherein, the secondary current bias measures the degree to which the secondary current waveform deviates from a preset time axis.

[0081] The DC component calculation module is used to determine the primary side DC component based on the secondary side current bias degree using a pre-established deviation relationship model between the primary side DC component and the secondary side current; wherein, the deviation relationship model records the bias degree between the primary side DC component and the secondary side current corresponding to the time axis.

[0082] The demagnetizing module is used to control the voltage-controlled current source in the current transformer to generate a reverse DC current to eliminate the bias magnetization based on the primary side DC component.

[0083] In a specific embodiment, the bias calculation module is specifically used to: after acquiring the secondary current waveform data, convert it into a voltage signal through a preset signal converter, filter out high-order harmonic components and high-frequency interference signals in the voltage signal through a preset filter, and compose the secondary current bias by arranging the filtered voltage signal according to the time axis. The bias calculation module is also used to calculate the secondary current bias according to the following formula:

[0084]

[0085] Where δ is the secondary current bias, and i2 is the instantaneous value of the secondary current.

[0086] Specifically, it also includes:

[0087] The deviation relationship model establishment module is used to apply a rated fundamental current to the primary side through an AC / DC power supply, superimpose a DC component on the primary side, and continuously change the magnitude of the DC component.

[0088] The DC component of the current transformer in the actual operating environment was simulated multiple times, and the bias degree data of the secondary current corresponding to the DC component was collected in each simulation.

[0089] The bias data of the secondary current is combined with the primary DC component on the corresponding time axis to form a deviation relationship model.

[0090] In a specific implementation of the present invention, a compensation system for DC bias of a current transformer is also provided, which achieves compensation for DC bias of the current transformer through a compensation device for DC bias of the current transformer.

[0091] It should be noted that the systems and devices described in the above embodiments correspond to the methods described in the above embodiments. Therefore, the parts of the systems and devices described in the above embodiments that are not described in detail can be obtained by referring to the content of the methods described in the above embodiments, and will not be repeated here.

[0092] In summary, implementing the embodiments of the present invention has the following beneficial effects:

[0093] The present invention provides a method, apparatus, and system for compensating DC bias in current transformers. It uses the secondary-side current bias to reflect the magnitude of the primary-side DC component, offering a simple and effective indicator without requiring additional measuring devices on the primary side. A reverse DC current is generated by a voltage-controlled current source to eliminate the DC magnetomotive force in the current transformer, improving its operating accuracy and fundamentally eliminating the influence of DC bias. The apparatus operates online in real time, providing corresponding compensation based on different DC contents on the primary side. Online compensation avoids the cumbersome and difficult aspects of algorithmic compensation in terms of modeling, data processing, and algorithm writing.

[0094] The above description discloses only preferred embodiments of the present invention and should not be construed as limiting the scope of the present invention. Therefore, equivalent variations made in accordance with the claims of the present invention are still within the scope of the present invention.

Claims

1. A method for compensating DC bias in a current transformer, characterized in that, include: Collect the secondary current waveform data in the current transformer, and determine the secondary current bias degree based on the secondary current waveform data; wherein, the secondary current bias degree is used to measure the degree to which the secondary current waveform deviates from the preset time axis. The primary side DC component is determined based on the secondary side current bias by using a pre-established deviation relationship model between the primary side DC component and the secondary side current; wherein, the deviation relationship model records the bias between the primary side DC component and the secondary side current corresponding to the time axis. The voltage-controlled current source in the current transformer is controlled to generate a reverse DC current to eliminate the bias magnetization based on the primary side DC component. The pre-established deviation relationship model between the primary side DC component and the secondary side current is obtained through the following process: A rated fundamental current is applied to the primary side through an AC / DC power supply, and a DC component is superimposed on the primary side while continuously changing the magnitude of the DC component. The DC component of the current transformer in the actual operating environment was simulated multiple times, and the bias degree data of the secondary current corresponding to the DC component was collected in each simulation. The bias data of the secondary current is combined with the primary DC component on the corresponding time axis to form a deviation relationship model.

2. The method as described in claim 1, characterized in that, Also includes: After acquiring the secondary current waveform data, it is converted into a voltage signal by a preset signal converter, and the high-order harmonic components and high-frequency interference signals in the voltage signal are filtered out by a preset filter. The filtered voltage signal is then arranged into a secondary current bias degree according to the time axis.

3. The method as described in claim 2, characterized in that, Determining the secondary current bias based on the secondary current waveform data includes: The secondary current bias is calculated using the following formula: in, This refers to the secondary current bias. This is the instantaneous value of the secondary current.

4. A compensation device for DC bias magnetism in a current transformer, used to implement the method as described in any one of claims 1-3, characterized in that, include: The bias calculation module is used to collect the secondary current waveform data in the current transformer and determine the secondary current bias based on the secondary current waveform data; wherein, the secondary current bias is used to measure the degree to which the secondary current waveform deviates from the preset time axis. The DC component calculation module is used to determine the primary side DC component based on the secondary side current bias degree using a pre-established deviation relationship model between the primary side DC component and the secondary side current; wherein, the deviation relationship model records the bias degree between the primary side DC component and the secondary side current corresponding to the time axis. The demagnetizing module is used to control the voltage-controlled current source in the current transformer to generate a reverse DC current to eliminate the bias magnetization based on the primary side DC component. The deviation relationship model establishment module is used to apply a rated fundamental current to the primary side through an AC / DC power supply, superimpose a DC component on the primary side, and continuously change the magnitude of the DC component. The DC component of the current transformer in the actual operating environment was simulated multiple times, and the bias degree data of the secondary current corresponding to the DC component was collected in each simulation. The bias data of the secondary current is combined with the primary DC component on the corresponding time axis to form a deviation relationship model.

5. The apparatus as described in claim 4, characterized in that, The bias calculation module is specifically used to convert the secondary current waveform data into a voltage signal through a preset signal converter after acquisition, and to filter out the high-order harmonic components and high-frequency interference signals in the voltage signal through a preset filter, and to compose the secondary current bias by arranging the filtered voltage signal according to the time axis.

6. The apparatus as claimed in claim 5, characterized in that, The bias calculation module is also used to calculate the secondary current bias according to the following formula: in, This refers to the secondary current bias. This is the instantaneous value of the secondary current.

7. A compensation system for DC bias magnetism in a current transformer, characterized in that, The compensation of DC bias of a current transformer is achieved by using a compensation device for DC bias of a current transformer as described in any one of claims 4-6.

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