A method and apparatus for detecting and demagnetizing residual magnetism in the core of an electromagnetic current transformer.

Abstract

This invention discloses a method and apparatus for detecting and demagnetizing residual magnetism in the core of an electromagnetic current transformer, comprising: acquiring current and voltage signals; performing harmonic analysis on the current signal; performing integration processing on the voltage signal; performing phase decomposition on the even-order harmonic components based on the fundamental phase of the magnetic flux to extract the residual magnetism-related components; determining the normalized residual magnetism index based on the residual magnetism-related components and the odd-order harmonic components; using a controllable current source connected in parallel with the secondary circuit of the electromagnetic current transformer; and adjusting the polarity and amplitude of the controllable DC component using the normalized residual magnetism index as a closed-loop feedback signal to make the normalized residual magnetism index approach zero. This invention can detect the presence and level of residual magnetism in the core online without shutting down the electromagnetic current transformer or interrupting its metering and protection functions, and automatically implement closed-loop active demagnetization when the detected residual magnetism exceeds a safety threshold, restoring the core to a symmetrical magnetization working state without residual magnetism.

Description

Technical Field

[0001] This application relates to the field of power system instrument transformer condition monitoring and maintenance technology, and in particular to a method and device for detecting and demagnetizing residual magnetism in the core of an electromagnetic current transformer. Background Technology

[0002] Electromagnetic current transformers are the most basic measurement and protection devices in power systems. Their function is to transform a large primary current into a standard small secondary current using a fixed transformation ratio, for use by metering instruments and relay protection devices. The core of an electromagnetic current transformer is made of stacked silicon steel sheets with high magnetic permeability. During normal operation, the core operates in the linear region of the magnetization curve, and the magnetic flux density oscillates symmetrically between positive and negative directions with the alternation of the primary current. The secondary current can accurately reflect the waveform and amplitude of the primary current.

[0003] However, during power system operation, the iron core inevitably encounters conditions that lead to residual magnetism. Typical scenarios include the iron core being pushed into a deep saturation region at the moment a short-circuit fault is cleared, resulting in the magnetic flux being "frozen" at a value deviating from zero, thus forming residual magnetism; the magnetization curve being asymmetrically shifted due to a current containing a DC component flowing through the primary side, leaving residual magnetism; and the introduction of residual magnetism by mechanical vibration and external magnetic field interference during the transportation and installation of instrument transformers. Residual magnetism in the iron core causes the magnetization operating point to deviate from the origin, resulting in an asymmetry in the available magnetization margin between the positive and negative half-cycles. When another short-circuit fault occurs in the system, the iron core may rapidly saturate on the same side as the residual magnetism, causing severe distortion of the secondary current waveform, which in turn can lead to the failure or maloperation of relay protection devices, seriously threatening the safe operation of the power grid.

[0004] Among the relevant residual magnetism treatment technologies, existing methods mainly include three types: offline AC demagnetization, primary-side controllable high-current demagnetization, and permanent magnet bias compensation. Offline AC demagnetization involves applying a gradually decaying AC excitation to the secondary winding after the instrument transformer is taken out of service. While reliable, it requires shutting down the instrument transformer, making it difficult to obtain a suitable downtime window for important transmission lines and critical substations where power outages are not permitted. Furthermore, since residual magnetism is generated randomly, the problem remains uncontrollable for months or even years between maintenance shutdowns. Primary-side controllable high-current demagnetization also requires taking the instrument transformer out of service and necessitates dedicated high-power power supply equipment, resulting in high operating costs and safety risks. Permanent magnet bias compensation embeds permanent magnets in the iron core to generate a fixed magnetic bias, but the magnitude and direction of the residual magnetism differ after each fault. The fixed bias cannot adapt to changing residual magnetism conditions, making the compensation effect unreliable.

[0005] The more fundamental problem is that all of the above methods lack the ability to detect the presence and level of residual magnetism. Maintenance personnel cannot know whether the core of the operating instrument transformer retains residual magnetism or whether the level of residual magnetism is high enough to affect transient performance. Currently, the only way to determine the presence of residual magnetism is based on empirical speculation, which is neither accurate nor timely. Summary of the Invention

[0006] In view of the above problems, this application provides a method and device for detecting and demagnetizing residual magnetism in the core of an electromagnetic current transformer. It can detect the presence and level of residual magnetism in the core online without shutting down the electromagnetic current transformer or interrupting its metering and protection functions. When the residual magnetism exceeds the safety threshold, it automatically implements closed-loop active demagnetization to restore the core to a symmetrical magnetization working state without residual magnetism.

[0007] In some embodiments, while the electromagnetic current transformer is in operation, current and voltage signals from the secondary side of the electromagnetic current transformer are acquired; harmonic analysis is performed on the current signal to extract even-order and odd-order harmonic components; the voltage signal is integrated to reconstruct the core flux waveform and determine the fundamental phase of the flux; based on the fundamental phase of the flux, phase decomposition is performed on the even-order harmonic components to extract the remanent magnetization correlation components; based on the remanent magnetization correlation components and the odd-order harmonic components, a normalized remanent magnetization index is determined; when the normalized remanent magnetization index exceeds the demagnetization initiation threshold, a controllable DC component opposite to the remanent magnetization direction is injected into the secondary winding through a controllable current source connected in parallel with the secondary circuit of the electromagnetic current transformer; and the polarity and amplitude of the controllable DC component are adaptively adjusted using the normalized remanent magnetization index as a closed-loop feedback signal to make the normalized remanent magnetization index approach zero. Specifically, by utilizing the phase mapping information of even harmonics from the iron core, the expected phase direction of the even harmonics from the iron core is determined based on the fundamental phase of the magnetic flux, and the even harmonic components are projected along the expected phase direction to obtain the remanent magnetization correlation components.

[0008] In this embodiment, utilizing the nonlinear characteristics of the core magnetization curve, when residual magnetism exists in the core, the magnetization operating point deviates from the origin, causing the central symmetry of the BH curve to be broken. The even-order harmonic components generated in the excitation current serve as characteristic marker signals of residual magnetism. By acquiring existing current and voltage signals on the secondary side, residual magnetism detection can be completed online without shutting down the current transformer. Simultaneously, by using flux fundamental phase reference and orthogonal projection technology, the even-order harmonics caused by residual magnetism in the core are effectively distinguished from those caused by the primary side load. Combined with closed-loop demagnetization control based on normalized residual magnetism index feedback, an integrated automatic closed-loop detection and demagnetization is achieved.

[0009] In some embodiments, a synchronous discrete Fourier transform with a window length of power frequency period is performed on the current signal, and the start time of the window is determined by the zero-crossing detection of the voltage signal so that each analysis window contains an integer number of power frequency periods; the even harmonic components are decomposed into two orthogonal components: a remanent magnetization correlation component along the expected phase direction and a load correlation component perpendicular to the expected phase direction.

[0010] In this embodiment, the impact of spectral leakage on the accuracy of weak harmonic extraction is eliminated by synchronous Fourier transform at the voltage zero-crossing point. Orthogonal decomposition separates the mixed signal into two independent contributions from the core source and the load source, ensuring the reliability of the detection results in actual power system environments with complex harmonic backgrounds.

[0011] In some embodiments, the normalized remanence index is the ratio of the amplitude of the second harmonic in the remanence associated component to the amplitude of the third harmonic in the odd harmonic component; during the closed-loop demagnetization process, the current injection parameters are maintained when the absolute value of the normalized remanence index continues to decrease, the polarity is reversed and the amplitude is halved when the index crosses zero, and the amplitude is halved when the absolute value of the index no longer decreases or rebounds, and this process is repeated until the absolute value of the index is lower than the demagnetization completion threshold and continues to exceed the preset completion time.

[0012] In this embodiment, using the third harmonic as the normalization reference eliminates common-mode influences such as primary current amplitude variation, secondary circuit impedance variation, and core temperature variation, ensuring that the index only reflects the degree of asymmetry in the BH curve. The closed-loop iterative strategy of successively halving the amplitude and alternating polarities causes the core magnetization state to oscillate and converge to zero within a gradually decreasing offset range, equivalent to the effect of traditional AC demagnetization but without requiring an external AC signal source. The residual magnetism level after demagnetization can be controlled at an extremely low level.

[0013] In some embodiments, the output of the controllable current source is connected in parallel to the secondary circuit via an inductor. The inductor presents a high impedance to the power frequency AC current to block the AC current from flowing into the controllable current source, and a low impedance to the controllable DC component to allow DC injection. The amplitude of the controllable DC component is limited to a preset proportion of the rated secondary current. The output circuit of the controllable current source is disconnected when a sudden drop in the secondary current to near zero and a sudden rise in the secondary terminal voltage to an abnormally high value are detected.

[0014] In this embodiment, the large inductor achieves AC / DC separation, protecting the constant current source from large current stress while ensuring that the injection circuit does not interfere with the normal AC secondary circuit function; the amplitude limiting and secondary open circuit protection mechanism ensure the safety of the device under various abnormal operating conditions, and the impact of the injected micro DC on the normal metering and protection functions of the transformer is negligible.

[0015] In some embodiments, the normalized remanent magnetization index is converted into an estimated remanent magnetization level of the iron core using remanent magnetization level calibration mapping information. The calibration mapping information is obtained through factory calibration or theoretical calibration based on the typical magnetization curve of the iron core material. Demagnetization is initiated when the normalized remanent magnetization index continuously exceeds the demagnetization start threshold and the duration reaches the preset anti-false triggering time. If the demagnetization process is not completed after the maximum allowable time, the output is turned off and an alarm is issued.

[0016] In this embodiment, the two calibration methods are applicable to newly manufactured current transformers and the retrofitting of existing current transformers, respectively. The anti-false triggering time threshold avoids unnecessary demagnetization operations caused by transient interference, and the timeout protection mechanism ensures the safety of the system in abnormal situations such as excessive calibration deviation or hardware failure.

[0017] According to a second aspect of this application, an electromagnetic current transformer core residual magnetism detection and demagnetization device is provided, comprising a signal acquisition module, a digital signal processing module, and a DC injection module, which respectively correspond to the signal acquisition, harmonic analysis and residual magnetism index calculation, and closed-loop demagnetization control functions in the above method.

[0018] According to a third aspect of this application, an electronic device is provided, including a memory and a processor coupled to the memory, the processor being configured to perform the above-described method based on instructions stored in the memory.

[0019] According to a fourth aspect of this application, a computer-readable storage medium is provided, wherein computer instructions are stored, which, when executed by a processor, implement the above-described method.

[0020] According to a fifth aspect of this application, a computer program product is provided, comprising a computer program that, when executed by a processor, implements the above-described method. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the embodiments will be briefly described below.

[0022] Figure 1 This is a schematic diagram of the main flow of the electromagnetic current transformer core residual magnetism detection and demagnetization method according to an embodiment of this application.

[0023] Figure 2 This is a schematic diagram of the sub-process of harmonic analysis and remanent magnetization correlation component extraction in an embodiment of this application.

[0024] Figure 3 This is a schematic diagram of the closed-loop demagnetization control sub-process of an embodiment of this application.

[0025] Figure 4This is a schematic diagram of the BH curve magnetization characteristics, excitation current waveform, and spectrum of the iron core in a state of no residual magnetism.

[0026] Figure 5 This is a schematic diagram of the BH curve magnetization characteristics, excitation current waveform, and spectrum of an iron core in a state of residual magnetism.

[0027] Figure 6 This is a vector diagram illustrating the principle of second harmonic phase decomposition in an embodiment of this application.

[0028] Figure 7 This is a timing diagram of the active demagnetization closed-loop control process according to an embodiment of this application.

[0029] Figure 8 This is a schematic diagram of the module structure of the electromagnetic current transformer core residual magnetism detection and demagnetization device according to an embodiment of this application.

[0030] Figure 9 This is a schematic diagram of the DC injection constant current source circuit in an embodiment of this application.

[0031] Figure 10 This is a schematic diagram showing the device installation location and system connection according to an embodiment of this application.

[0032] Figure 11 This is a schematic diagram of the electronic device structure according to an embodiment of this application. Detailed Implementation

[0033] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings.

[0034] In the description of this application, it should be understood that the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this application, "multiple" means two or more.

[0035] In the description of this application, the term "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent three cases: A exists alone, A and B exist simultaneously, and B exists alone.

[0036] In the description of this application, unless otherwise expressly specified and limited, the terms "connection", "linking", "coupled", etc., shall be interpreted broadly. For example, they may refer to electrical connection or mutual communication, direct connection or indirect connection through an intermediate medium, or internal connection between two elements or interaction between two elements.

[0037] I. Basic Knowledge of Residual Magnetism in Electromagnetic Current Transformer Cores

[0038] like Figure 4 , Figure 5 As shown, the core of the electromagnetic current transformer is made of stacked silicon steel sheets with high permeability, and its magnetization curve (BH curve) is essentially nonlinear. During normal operation, the core operates in the approximately linear region of the magnetization curve, and the magnetic flux density oscillates symmetrically between positive and negative directions with the alternation of the primary current. When the core has no residual magnetism, the BH curve is centrally symmetrical about the origin, and the magnetization characteristics of the positive and negative half-cycles are completely symmetrical. At this time, the excitation current harmonics generated by the nonlinearity of the core only contain odd-order components (third, fifth, seventh, etc.) and do not contain even-order components (second, fourth, etc.). This is because the centrally symmetrical nonlinear function does not contain even-order terms in its Fourier expansion.

[0039] When residual magnetism exists in the iron core, the magnetization operating point deviates from the origin, and the magnetic flux density no longer oscillates around zero but alternates around a DC bias equal to the residual magnetism value. This deviation breaks the central symmetry of the BH curve, and the magnetization characteristics of the iron core in the positive and negative half-cycles are no longer symmetrical, resulting in asymmetry in the positive and negative half-cycles of the excitation current waveform. This asymmetrical waveform necessarily contains even-order harmonic components in Fourier decomposition, especially the second harmonic component. The amplitude of the even-order harmonics is directly related to the level of residual magnetism; the larger the residual magnetism, the stronger the asymmetry, and the higher the amplitude of the even-order harmonics. When the residual magnetism is zero, the even-order harmonics tend to disappear. This forms a deterministic physical causal chain: residual magnetism causes the magnetization operating point to shift, which in turn leads to the breaking of the central symmetry of the BH curve, resulting in asymmetry in the positive and negative half-cycles of the excitation current, thus generating even-order harmonics. This causal chain is unidirectional, and the amplitude of the even-order harmonics increases monotonically with the level of residual magnetism, making even-order harmonics a reliable criterion for the presence and level of residual magnetism.

[0040] II. A method for detecting and demagnetizing residual magnetism in the core of an electromagnetic current transformer.

[0041] like Figure 1 As shown in the figure, this application provides a method for detecting and demagnetizing the residual magnetism of an electromagnetic current transformer core. This method can be performed by a detection and demagnetization device installed on the secondary circuit of the transformer. The method includes the following steps.

[0042] Step S101: While the electromagnetic current transformer is not out of operation, collect the current signal and voltage signal on the secondary side of the electromagnetic current transformer.

[0043] In some embodiments, step S101 involves acquiring the precise waveform of the secondary current by installing a precision sampling resistor in series in the secondary circuit of the transformer, and simultaneously acquiring the secondary-side load voltage waveform by connecting a high-input-impedance differential voltage sampling channel in parallel at the secondary terminals of the transformer. The resistance of the precision sampling resistor is extremely small, and its voltage drop has a much smaller impact on the total impedance of the secondary circuit than the allowable load error limit of the transformer. Both the secondary current signal and the load voltage signal are fed into a high-precision dual-channel synchronous analog-to-digital converter to be converted into digital signals for subsequent processing.

[0044] As an example, taking an electromagnetic current transformer with a rated secondary current of 5A as an example, a precision manganese copper wire sampling resistor with a resistance of 10mΩ is selected, which generates a full-scale voltage drop of 50mV under rated current. This voltage drop is amplified by an instrumentation amplifier and then sent to an analog-to-digital converter (ADC). The ADC adopts a dual-channel synchronous sampling method with an effective resolution of 16 bits (corresponding to a signal-to-noise ratio greater than 96dB) and a sampling rate of 10kHz. The 16-bit resolution allows harmonic components with amplitudes only on the order of one ten-thousandth of the fundamental frequency to be digitally captured, and the 10kHz sampling rate provides ample sampling margin relative to the highest sixth harmonic (300Hz) of interest.

[0045] In this embodiment, a sampling resistor with extremely low resistance is used in series to acquire the current waveform, and the impact on the secondary circuit impedance is negligible, ensuring that the metering and protection functions of the current transformer are not disturbed; dual-channel synchronous sampling ensures the time alignment of the current and voltage signals, providing an accurate basis for subsequent phase analysis.

[0046] Step S102: Perform harmonic analysis on the current signal to extract the even-order harmonic components and odd-order harmonic components from the current signal.

[0047] like Figure 2 As shown, in some embodiments, the harmonic analysis in step S102 may further include the following sub-steps.

[0048] Step S201: Perform a synchronous discrete Fourier transform on the acquired secondary current data with a window length of power frequency period. The start time of the window is determined by zero-crossing detection of the load voltage signal to ensure that each analysis window precisely contains an integer number of power frequency periods and eliminate spectral leakage.

[0049] For example, in a 50Hz system, one power frequency cycle is 20ms, corresponding to 200 sampling points (sampling rate 10kHz). In actual analysis, data from the most recent five power frequency cycles (100ms, 1000 sampling points) are used for synchronous Fourier transform to improve the signal-to-noise ratio. Zero-crossing detection determines the precise zero-crossing time by linearly interpolating the voltage signal, aligning the start of the analysis window to the positive zero-crossing time of the voltage.

[0050] Step S202: Extract the complex coefficients of each harmonic from the results of the synchronous Fourier transform, including amplitude and phase information. Specifically, extract the amplitude and phase of the second, third, fourth, and fifth harmonics for later use.

[0051] As an example, suppose the Fourier transform results at a certain moment show: the fundamental frequency (50Hz) amplitude is 5.000A; the second harmonic (100Hz) amplitude is 0.0012A with a phase of 35°; the third harmonic (150Hz) amplitude is 0.0085A with a phase of -15°; the fourth harmonic (200Hz) amplitude is 0.0003A; and the fifth harmonic (250Hz) amplitude is 0.0045A. The second harmonic is the target for subsequent remanence detection, and the third harmonic is used as a normalization reference.

[0052] In this embodiment, the Fourier transform synchronized by the voltage zero-crossing point ensures the precise alignment of the analysis window with the power frequency cycle, eliminates the spectral leakage caused by non-integer period truncation, and enables the weak even-order harmonic components to be accurately extracted.

[0053] Step S103: Integrate the voltage signal to reconstruct the core flux waveform and determine the fundamental phase of the flux.

[0054] In some embodiments, according to Faraday's law of electromagnetic induction, the voltage at the secondary winding terminals is proportional to the rate of change of the core flux over time; therefore, the core flux waveform is proportional to the integral of the terminal voltage over time. A digital signal processor performs numerical integration on the acquired load voltage waveform (e.g., using the trapezoidal integration method) to obtain the core flux waveform in real time. A Fourier transform is then performed on the integrated flux waveform to extract the phase angle of its fundamental component as the fundamental phase of the flux.

[0055] As an example, assuming the fundamental amplitude of the secondary terminal load voltage is 2.5V and the phase is 0°, the fundamental phase of the magnetic flux obtained after integration is -90° (the integration operation causes a 90° phase lag). This fundamental phase of the magnetic flux will serve as the reference for subsequent phase decomposition.

[0056] In this embodiment, the magnetic flux waveform is reconstructed by digital integration of the voltage signal, and the information on the magnetic flux state inside the iron core is obtained by utilizing the existing voltage sampling signal. No additional magnetic flux sensor is required, which simplifies the hardware and provides the reference benchmark required for phase decomposition.

[0057] Step S104: Based on the fundamental phase of the magnetic flux, perform phase decomposition on the even harmonic components and extract the remanent magnetic correlation components.

[0058] like Figure 6 As shown, in some embodiments, step S104 may further include the following sub-steps.

[0059] Step S203: Using the phase mapping information of the even harmonics from the core source, the expected phase direction of the even harmonics from the core source is determined based on the fundamental phase of the magnetic flux. The phase mapping information of the even harmonics from the core source includes the mapping relationship between the fundamental phase of the magnetic flux and the expected phase direction of the even harmonics from the core source. This mapping relationship is uniquely determined by the shape of the BH curve of the core material and is a stable constant angle relationship for a given core material.

[0060] In some embodiments, the phase mapping relationship can be obtained through type testing calibration before the transformer leaves the factory: a known residual magnetism is applied to the core under controlled conditions, and the angular relationship between the corresponding second harmonic excitation current phase and the fundamental phase of the magnetic flux is measured and stored. In other embodiments, the phase mapping relationship can also be automatically acquired and stored through a short learning process when the device is first put into operation.

[0061] As an example, suppose that for a certain type of grain-oriented silicon steel core, the known angular difference between the expected phase direction of the second harmonic excitation current caused by residual magnetism and the phase of the fundamental magnetic flux is Δφ = 87°. If the current fundamental magnetic flux phase is -90°, then the expected phase direction of the second harmonic from the core is -90° + 87° = -3°.

[0062] Step S204: Project the even-order harmonic component (taking the second harmonic as an example) along the expected phase direction, and use the projected component as the remanent magnetization correlation component; at the same time, use the projected component perpendicular to the expected phase direction as the load correlation component.

[0063] like Figure 6 As shown, this operation can be understood in the form of vector decomposition: the measured second harmonic vector in the secondary current is decomposed into a projection component along the "expected direction of the core source" (residual magnetism-related component) and a projection component in the perpendicular direction (load-related component). This orthogonal decomposition is similar to the active and reactive power decomposition commonly used in power systems, using a known reference phase to separate the mixed signal into contributions from two independent sources.

[0064] As an example, continuing the previous discussion, assume that the second harmonic amplitude extracted from the secondary current is 0.0012A and the phase is 35°, while the expected phase direction from the iron core source is -3°. Projecting this second harmonic vector along the -3° direction, the amplitude of the remanent magnetization correlation component is obtained as follows:

[0065] 0.0012×cos(35°-(-3°))=0.0012×cos(38°)≈0.000945A; the amplitude of the load-related component is 0.0012×sin(38°)≈0.000739A. Where 0.000945A is the estimated value of the second harmonic contribution from the core remanence.

[0066] In this embodiment, phase orthogonal decomposition utilizes the deterministic phase relationship between the even harmonics from the iron core and the fundamental magnetic flux, effectively separating the even harmonics caused by the residual magnetism of the iron core from the even harmonics caused by the primary load, thus solving the core problem of residual magnetism detection in actual power systems with complex harmonic backgrounds.

[0067] Step S105: Determine the normalized remanence index based on the remanence correlation component and the odd harmonic component.

[0068] In some embodiments, the normalized remanence index is the ratio of the amplitude of the second harmonic in the remanence-related component to the amplitude of the third harmonic in the odd harmonic component. The third harmonic is mainly generated by the nonlinearity of the iron core (it exists regardless of the presence of remanence). The third harmonic in the primary load is generally small or non-existent (in a symmetrical three-phase system, the third harmonic is blocked by the star connection). Therefore, the amplitude of the third harmonic can be used as a normalized reference for the degree of nonlinearity of the iron core.

[0069] As an example, continuing the previous case, if the second harmonic amplitude in the remanence correlation component is 0.000945A and the third harmonic amplitude is 0.0085A, then the normalized remanence index is 0.000945 / 0.0085≈0.111. A positive value for this index indicates that the remanence direction is positive.

[0070] In this embodiment, the normalization operation based on the third harmonic eliminates many common-mode influencing factors: changes in primary current amplitude (changes in load size cause changes in the core's working magnetic flux density, resulting in proportional changes in the amplitudes of all harmonics), changes in the total impedance of the secondary circuit (affecting the absolute amplitudes of all harmonics), and changes in core temperature (affecting the overall shape of the BH curve, thus affecting the absolute amplitudes of all harmonics). The normalized index only reflects the degree of asymmetry in the BH curve, i.e., the remanence level, and exhibits good stability and comparability under different operating conditions.

[0071] In some embodiments, pre-stored calibration curves can be used to convert the normalized remanence index into an estimated core remanence level. The calibration curve reflects the correspondence between the normalized index value and the actual remanence level (expressed as magnetic flux density in Tesla), typically exhibiting an approximately linear relationship. Calibration curves can be obtained through either factory calibration or theoretical calibration. Factory calibration involves applying a known DC bias to the core using specialized equipment during the transformer's factory testing phase, placing the core at different known remanence levels while simultaneously measuring the corresponding normalized index values. Multiple sets of data are then fitted into a calibration curve and stored in the device's non-volatile memory. Theoretical calibration, based on typical BH curve data of the core material, calculates theoretical values ​​of the normalized index at different remanence levels through numerical simulation. This method eliminates the need for individual testing and is suitable for scenarios involving retrofitting existing transformers.

[0072] As an example, continuing the previous example, we assume that the calibration curve shows that the remanence level corresponding to the normalized index value of 0.111 is about 30% of the core saturation flux density. That is, if the core saturation flux density is 1.8T, the estimated remanence is about 0.54T.

[0073] Step S106: When the normalized residual magnetism index exceeds the demagnetization start threshold, a controllable DC component opposite to the residual magnetism direction is injected into the secondary winding through a controllable current source connected in parallel with the secondary circuit of the electromagnetic current transformer.

[0074] In some embodiments, the demagnetization initiation threshold is set as a normalized index value corresponding to a remanence level reaching 20% ​​of the saturation flux density. This threshold provides a safety margin of 100% relative to the critical remanence level (40% of the saturation flux density) that leads to deterioration of protection performance, ensuring that demagnetization is completed before the remanence develops to a dangerous level.

[0075] In some embodiments, a mechanism to prevent false triggering is also included: the demagnetization process is only initiated when the normalized residual magnetization index continuously exceeds the demagnetization start threshold and the duration reaches the preset false triggering duration (which can be set to 5 seconds), so as to avoid unnecessary demagnetization operations caused by transient interference.

[0076] The polarity of the injected DC is determined by the polarity of the normalized remanence index. A positive index indicates a positive remanence direction, and the injected DC is negative, and vice versa. The initial amplitude of the injected DC is set to a preset proportion of the rated secondary current.

[0077] As an example, assuming the normalized index value corresponding to the demagnetization start threshold is 0.073 (corresponding to a remanence level of 20% of the saturation magnetic flux density), and the current normalized remanence index is 0.111 (greater than 0.073), the continuous exceedance time has reached 5 seconds, therefore the system determines to start demagnetization. A positive index value indicates that the remanence direction is positive, and the injection direction is determined to be negative DC. The injection start amplitude is set to 0.05% of the rated secondary current 5A, i.e., 2.5mA.

[0078] In this embodiment, the setting of the demagnetization start threshold enables the system to trigger demagnetization before the residual magnetism level may affect the transient performance of the transformer. The anti-false triggering time threshold avoids the interference of transient conditions (such as load changes and system transient oscillations) on the demagnetization determination. The injected DC amplitude is much smaller than the rated secondary current and its impact on the normal function of the transformer is negligible.

[0079] Step S107: Using the normalized remanence index as a closed-loop feedback signal, adaptively adjust the polarity and amplitude of the controllable DC component so that the normalized remanence index approaches zero.

[0080] like Figure 3As shown, in some embodiments, the closed-loop demagnetization control in step S107 may further include the following sub-steps.

[0081] Step S301: Determine the initial injection polarity (opposite to the index polarity) based on the polarity of the normalized remanence index, set the initial injection amplitude to a preset ratio of the rated secondary current (which can be set to 0.05%), and start the controllable current source output.

[0082] Step S302: After the injection is started, the normalized remanence index value is updated once every preset feedback period (e.g., every 100 milliseconds).

[0083] Step S303: Observe the changing trend of the absolute value of the normalized remanence index and perform corresponding adjustment operations: If the absolute value of the normalized remanence index continues to decrease after the controllable DC component is injected, determine to maintain the current polarity and amplitude and continue injection; If the absolute value of the normalized remanence index no longer decreases or rebounds, determine to halve the injection amplitude; If the normalized remanence index crosses zero (polarity reversal), determine to reverse the injection polarity and halve the injection amplitude.

[0084] Step S304: Repeat the iterative process from steps S302 to S303.

[0085] Step S305: Determine if demagnetization is complete: If the absolute value of the normalized residual magnetism index remains stable below the demagnetization completion threshold and continues for more than a preset completion time (e.g., ten seconds), demagnetization is confirmed to be complete. The controllable current source output is turned off, the online monitoring mode is switched back, and information such as the time of this demagnetization event, the initial residual magnetism level, and the demagnetization time are recorded. The demagnetization completion threshold is set to 1 / 10 of the demagnetization start threshold, corresponding to 2% of the saturation magnetic flux density.

[0086] Step S306, Abnormal Handling: If the demagnetization process continues for more than the preset maximum allowable time (e.g., sixty seconds) and the demagnetization completion condition is not met, it is determined to be a demagnetization abnormality. The controllable current source output is turned off, the monitoring mode is switched back, and an alarm signal is issued to prompt the operation and maintenance personnel to intervene and check.

[0087] like Figure 7The following diagram illustrates a complete demagnetization process. Assume the initial normalized remanence index is +0.111 (corresponding to approximately 30% of the saturation magnetic flux density). After the system injects -2.5mA DC, the normalized index gradually decreases, dropping from +0.111 to +0.035 after about 3 seconds. The rate of decrease slows as the index approaches zero, and after approximately 5 seconds, it crosses zero at +0.005, becoming -0.003 (index crosses zero). At this point, the system reverses the injection polarity to the positive direction and halves the amplitude to 1.25mA. After injecting 1.25mA in the positive direction, the index rises from -0.003 to -0.001 before crossing zero again to +0.001. The system then reverses the polarity again to the negative direction and halves the amplitude once more to 0.625mA. This process is repeated several times, and the oscillation amplitude of the index gradually decreases. After approximately 12 seconds, the absolute value of the indicator stabilized below 0.002 (below the demagnetization completion threshold of 0.0073), and after remaining below this threshold for more than 10 seconds, demagnetization was deemed complete, and DC injection was shut off. The entire process took approximately 22 seconds.

[0088] In this embodiment, the physical effect of the closed-loop decreasing oscillation demagnetization strategy is equivalent to the traditional offline AC demagnetization method, which causes the iron core to converge to the origin along a gradually narrowing hysteresis loop. However, the implementation method is completely different. It is achieved through a series of alternating polarity and decreasing amplitude DC bias steps. The polarity and amplitude of each bias step are determined by closed-loop feedback rather than blindly executing according to a preset waveform. Therefore, the demagnetization efficiency is higher and the risk of overcompensation is lower. Throughout the demagnetization process, the amplitude of the injected DC component is always maintained below one-thousandth of the rated secondary current, and its impact on the metering accuracy and protection performance of the transformer is negligible.

[0089] In some embodiments, the method has three operating modes: initial calibration mode, online monitoring mode, and active demagnetization mode. The initial calibration mode is performed once after the device is first put into operation or after the current transformer is replaced. It collects data, calculates the third harmonic reference, and determines the expected phase direction of the second harmonic originating from the core. After calibration, it automatically switches to online monitoring mode. Online monitoring mode is the normal operating mode. It continuously executes the detection process of steps S101 to S105 at a preset frequency (e.g., ten times per second) and compares the index values ​​with the demagnetization start threshold. Simultaneously, it stores the index values ​​and estimated residual magnetism level in the historical record area (which can store data from the last thirty days) once per minute for trend analysis and maintenance queries. Active demagnetization mode is automatically entered when the demagnetization start conditions are met. It executes the demagnetization control process of steps S106 to S107. After demagnetization is completed or an abnormality occurs, it switches back to online monitoring mode.

[0090] III. A device for detecting and demagnetizing residual magnetism in the core of an electromagnetic current transformer.

[0091] like Figure 8As shown in the figure, this application provides a device for detecting and demagnetizing the residual magnetism of an electromagnetic current transformer core. This device is packaged in a separate, compact chassis and installed near the secondary terminal box of the transformer or in a substation protection cabinet. The device includes the following functional modules.

[0092] A signal acquisition module is used to acquire the current and voltage signals on the secondary side of the electromagnetic current transformer while the transformer is in operation. In some embodiments, the signal acquisition module includes a precision sampling resistor connected in series in the secondary circuit, a high input impedance voltage sampling channel connected in parallel to the secondary terminals, and a dual-channel synchronous analog-to-digital converter. The precision sampling resistor is a precision manganese copper wire sampling resistor, the resistance value of which is selected according to the rated secondary current, and the secondary current waveform is obtained through the voltage drop across it. The voltage sampling channel is connected to the analog-to-digital converter through a high-impedance voltage divider. The two channels share the same analog-to-digital converter chip to achieve dual-channel synchronous sampling, ensuring the synchronization of current and voltage signals.

[0093] The digital signal processing module is used to perform harmonic analysis on the current signal to extract even-order and odd-order harmonic components, to perform integration processing on the voltage signal to reconstruct the core flux waveform and determine the fundamental phase of the flux, to perform phase decomposition on the even-order harmonic components based on the fundamental phase of the flux to extract remanent magnetization correlation components, and to determine the normalized remanent magnetization index based on the remanent magnetization correlation components and the odd-order harmonic components. In some embodiments, the digital signal processing module is a microcontroller-based embedded processing board. The microcontroller has a built-in floating-point arithmetic unit that can efficiently execute synchronous Fourier transform, phase decomposition, normalized index calculation, and demagnetization control iterative algorithms. The chip's built-in flash memory and random access memory are sufficient to store calibration curve data, operating parameters, and historical records. The digital signal processing module is also used to adaptively adjust the polarity and amplitude of the controllable DC component so that the normalized remanence index approaches zero, using the normalized remanence index as a closed-loop feedback signal; it uses the phase mapping information of the even harmonics from the core source to determine the expected phase direction of the even harmonics from the core source according to the fundamental phase of the magnetic flux, and projects the even harmonic components along the expected phase direction to obtain the remanence correlation component.

[0094] The DC injection module includes a controllable current source connected in parallel with the secondary circuit, used to inject a controllable DC component opposite to the direction of residual magnetism into the secondary winding when the normalized residual magnetism index exceeds the demagnetization initiation threshold.

[0095] like Figure 9As shown, in some embodiments, the core component of the DC injection module is a bipolar programmable constant current source. The output of this constant current source is connected to the secondary circuit in parallel through a large inductor. The large inductor presents extremely high impedance to the power frequency AC secondary current, effectively blocking the AC current from flowing into the constant current source and avoiding the constant current source from being subjected to high current stress; the impedance to the DC component of the constant current source output is zero, and the DC component can be injected into the secondary circuit without obstruction. The constant current source circuit adopts a linear topology instead of a switching topology to avoid switching ripple interference with harmonic measurements, specifically implemented as a transistor constant current circuit controlled by an operational amplifier. The digital signal processing module outputs a current setpoint through a high-precision digital-to-analog converter. This voltage signal serves as the reference input of the operational amplifier, and the output of the operational amplifier drives a power transistor. A precision sampling resistor is connected in series with the source of the transistor, and the voltage across the sampling resistor is fed back to the inverting input of the operational amplifier to form a closed-loop constant current control. The polarity of the output current is reversed by a relay switching circuit with an H-bridge structure.

[0096] In some embodiments, the DC injection module is equipped with the following safety protection mechanisms: if the digital signal processing module detects a sudden drop in secondary current to near zero and a sudden rise in secondary terminal voltage to an abnormally high value in the secondary circuit open-circuit detection logic, it immediately disconnects and locks the output circuit of the constant current source; the hardware limit of the output current of the constant current source is set to 1% of the rated secondary current, and even if the software abnormal injection current cannot exceed this limit; the power supply of the constant current source comes from an independent low-voltage isolated power supply, and the insulation requirements between the power supply and the secondary circuit of the transformer meet the corresponding voltage level requirements.

[0097] In some embodiments, the precision sampling resistor installed in series in the secondary circuit is also utilized by the constant current source circuit. When the output current of the constant current source flows through the sampling resistor, the DC voltage drop generated on it can be synchronously collected by the analog-to-digital converter to verify whether the actual injected DC amplitude is consistent with the set value of the digital-to-analog converter, thereby realizing the dual verification of the injected current.

[0098] In some embodiments, the device further includes a communication and human-machine interface module for communicating with the substation integrated automation system and displaying the normalized residual magnetism index, estimated residual magnetism level, and current operating status. The communication and human-machine interface module provides a communication interface for communicating with the substation integrated automation system, and a passive dry contact output for hard-wired transmission of residual magnetism warning and alarm signals. A display screen can be equipped on the chassis panel to display the normalized residual magnetism index value, estimated residual magnetism level, and current operating status in real time.

[0099] In some embodiments, the device further includes a power supply module that receives power from the substation's DC operating power supply and internally converts it into the low-voltage isolated power required by each module.

[0100] like Figure 10As shown, the installation location and system connection of the device are as follows: the primary side of the electromagnetic current transformer is connected to the power line, and the secondary side is led out through the terminal box to the protection and metering equipment. This device is installed in the secondary circuit: a precision sampling resistor is connected in series on a conductor in the secondary circuit, the voltage sampling line is connected in parallel at the secondary terminals, and the DC injection circuit is connected in parallel to the secondary circuit via a large inductor. The installation of the device involves only simple wiring modifications to the secondary circuit and does not affect the primary side or the core of the transformer, making it suitable for upgrading existing transformers in service.

[0101] like Figure 11 As shown, this application provides an electronic device that can serve as the hardware platform for the digital signal processing module in the aforementioned detection and demagnetization device. The electronic device includes a memory and a processor coupled to the memory.

[0102] The memory may include non-volatile memory and / or volatile memory, wherein the non-volatile memory is used to store computer program instructions required to perform the above methods, as well as persistent data such as calibration curve data, operating parameters, and historical records, and the volatile memory is used to provide temporary data storage space during processor program execution.

[0103] The processor is configured to execute all the steps of the electromagnetic current transformer core residual magnetism detection and demagnetization method described in the above method embodiments, based on instructions stored in the memory. The processor can be a microcontroller or a digital signal processor chip, and in some embodiments, it has a built-in floating-point unit to efficiently execute synchronous Fourier transform, phase decomposition, normalization index calculation, and demagnetization control iterative algorithm.

[0104] In some embodiments, the electronic device further includes a communication interface for connecting to an analog-to-digital converter to receive sampled data, connecting to a digital-to-analog converter to output a current setpoint, and connecting to an external communication bus. In some embodiments, the electronic device further includes a general-purpose input / output interface for controlling a constant current source polarity switching relay and outputting an alarm dry contact signal.

[0105] In some embodiments, the processor's built-in flash memory capacity is no less than 1MB, and its random access memory capacity is no less than 192KB, sufficient to store calibration curve data, historical records for the past thirty days, and operating parameters. The processor's computing power meets the real-time requirement of completing the entire calculation process from reading sampled data to outputting normalized indicators at a frequency of more than ten times per second.

[0106] This application also provides a computer-readable storage medium storing computer instructions, which, when executed by a processor, implement the electromagnetic current transformer core residual magnetism detection and demagnetization method described in the above method embodiments. The computer-readable storage medium may include, but is not limited to, flash memory, electrically erasable programmable read-only memory (EEPROM), or other non-volatile storage devices.

[0107] This application also provides a computer program product, including a computer program that, when executed by a processor, implements all the steps described in the above method embodiments. This computer program can be burned into the memory of an embedded processor in firmware form, or it can be stored in an external storage device and loaded and executed by the processor.

[0108] IV. Supplementary Explanation

[0109] In the above embodiments of this application, the numbering of each step is only for ease of explanation and should not be regarded as a limitation on the execution order of the steps. Without violating the technical logic, some steps can be executed in parallel or their order can be adjusted. For example, the harmonic analysis in step S102 and the magnetic flux waveform reconstruction in step S103 can be executed in parallel, with the input data for both coming from the same sampled current data and voltage data, respectively.

[0110] In some embodiments, steps S101 to S105 constitute an online detection process, which can be executed independently to achieve a simple online monitoring function of residual magnetism without performing a demagnetization operation. In other embodiments, steps S101 to S107 constitute a complete closed-loop process of detection and demagnetization.

[0111] The specific values ​​mentioned in the embodiments of this application (such as sampling resistor value of 10mΩ, analog-to-digital converter resolution of 16 bits, sampling rate of 10kHz, injection DC initial amplitude of 2.5mA, demagnetization start threshold corresponding to saturation magnetic flux density of 20%, demagnetization completion threshold corresponding to saturation magnetic flux density of 2%, anti-false triggering time of 5 seconds, demagnetization completion confirmation time of 10 seconds, maximum allowable demagnetization time of 60 seconds, etc.) are all illustrative examples. Those skilled in the art can adjust them according to the specific transformer model, protection configuration requirements and engineering needs. These changes do not depart from the protection scope of this application.

[0112] In the various embodiments of this application, the sequence numbers of the above processes do not imply a specific order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application. Where there is no conflict, the embodiments and features in the embodiments of this application can be combined with each other.

[0113] The above description is merely an embodiment of this application and is not intended to limit the scope of protection of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principle of this application should be included within the scope of the claims of this application.

Claims

1. A method for detecting and demagnetizing the residual magnetism of an electromagnetic current transformer core, characterized in that, include: While the electromagnetic current transformer is in operation, the current and voltage signals on the secondary side of the electromagnetic current transformer are collected. Harmonic analysis is performed on the current signal to extract the even-order harmonic components and odd-order harmonic components from the current signal. The voltage signal is integrated to reconstruct the core flux waveform and determine the fundamental phase of the flux. Based on the fundamental phase of the magnetic flux, the even harmonic components are decomposed to extract the remanent magnetization correlation components. Based on the remanence correlation component and the odd harmonic component, a normalized remanence index is determined; wherein, the normalized remanence index is the ratio of the amplitude of the second harmonic in the remanence correlation component to the amplitude of the third harmonic in the odd harmonic component. When the normalized residual magnetism index exceeds the demagnetization initiation threshold, a controllable DC component opposite to the direction of residual magnetism is injected into the secondary winding through a controllable current source connected in parallel with the secondary circuit of the electromagnetic current transformer; and Using the normalized remanence index as a closed-loop feedback signal, the polarity and amplitude of the controllable DC component are adaptively adjusted so that the normalized remanence index approaches zero. The step of performing phase decomposition on the even harmonic components based on the fundamental phase of the magnetic flux and extracting the remanent magnetization correlation components includes: using the phase mapping information of the even harmonics from the core, determining the expected phase direction of the even harmonics from the core based on the fundamental phase of the magnetic flux; projecting the even harmonic components along the expected phase direction, and using the projected components as the remanent magnetization correlation components; the phase mapping information of the even harmonics from the core includes: the mapping relationship between the fundamental phase of the magnetic flux and the expected phase direction of the even harmonics from the core.

2. The method as described in claim 1, characterized in that, The harmonic analysis of the current signal includes: A synchronous discrete Fourier transform with a window length of power frequency period is performed on the current signal, wherein the starting time of the window of the synchronous discrete Fourier transform is determined by the zero-crossing detection of the voltage signal, so that each analysis window contains an integer number of power frequency periods. The phase decomposition of the even-order harmonic components further includes: The even harmonic component is decomposed into two orthogonal components: the component along the expected phase direction is the remanent magnetization-related component, and the component perpendicular to the expected phase direction is the load-related component. The load-related component corresponds to the even harmonic of the primary load source.

3. The method as described in claim 1, characterized in that, The step of using the normalized remanence index as a closed-loop feedback signal to adaptively adjust the polarity and amplitude of the controllable DC component includes: If the absolute value of the normalized remanence index continues to decrease after the injection of the controllable DC component, it is determined to maintain the current polarity and amplitude and continue injection. When the normalized remanence index crosses zero, it is determined that the polarity of the controllable DC component will be reversed and the amplitude of the controllable DC component will be halved. If the absolute value of the normalized remanence index no longer decreases or rebounds, it is determined that the amplitude of the controllable DC component should be halved. Repeat the above adjustment process until the absolute value of the normalized residual magnetism index is lower than the demagnetization completion threshold and continues to exceed the preset completion time, then determine that the demagnetization is complete and turn off the output of the controllable current source. Wherein, the demagnetization completion threshold is lower than the demagnetization start threshold.

4. The method as described in claim 1, characterized in that, Also includes: The output of the controllable current source is connected to the secondary circuit in parallel through an inductor; the inductor presents a high impedance to the power frequency AC current to block the AC current in the secondary circuit from flowing into the controllable current source, and presents a low impedance to the controllable DC component to allow the controllable DC component to be injected into the secondary circuit. The amplitude of the controllable DC component is limited to a preset proportion of the rated secondary current of the electromagnetic current transformer. If the secondary current in the secondary circuit drops to near zero and the secondary terminal voltage rises to an abnormally high value, it is determined that the output circuit of the controllable current source should be disconnected.

5. The method as described in claim 1, characterized in that, Also includes: Using the remanence level calibration mapping information, the estimated remanence level of the iron core is determined according to the normalized remanence index; The remanence level calibration mapping information includes: the mapping relationship between the normalized remanence index and the actual remanence level of the iron core; The residual magnetism level calibration mapping information is obtained through one of the following methods: applying a known magnitude of DC bias magnetism to the core during the current transformer's factory testing phase for factory calibration, or performing theoretical calibration through numerical simulation based on typical magnetization curve data of the core material. If the normalized residual magnetism index continuously exceeds the demagnetization initiation threshold and the duration reaches the preset anti-false triggering time, the demagnetization process is initiated. If the demagnetization process continues for more than the preset maximum allowable time without reaching the demagnetization completion condition, the output of the controllable current source will be turned off and an alarm signal will be issued.

6. A device for detecting and demagnetizing the residual magnetism of an electromagnetic current transformer core, characterized in that, include: The signal acquisition module is used to acquire the current signal and voltage signal on the secondary side of the electromagnetic current transformer while the electromagnetic current transformer is not out of operation. The digital signal processing module is used to: perform harmonic analysis on the current signal to extract even-order harmonic components and odd-order harmonic components; The voltage signal is integrated to reconstruct the core flux waveform and determine the fundamental phase of the flux. The even harmonic components are phase-decomposed according to the fundamental phase of the magnetic flux to extract the remanent magnetic correlation components. And a normalized remanence index is determined based on the remanence correlation component and the odd harmonic component; wherein the normalized remanence index is the ratio of the amplitude of the second harmonic in the remanence correlation component to the amplitude of the third harmonic in the odd harmonic component. The DC injection module includes a controllable current source connected in parallel with the secondary circuit, used to inject a controllable DC component opposite to the direction of residual magnetism into the secondary winding when the normalized residual magnetism index exceeds the demagnetization start threshold. The digital signal processing module is further configured to use the normalized remanence index as a closed-loop feedback signal to adaptively adjust the polarity and amplitude of the controllable DC component, so that the normalized remanence index approaches zero. The digital signal processing module utilizes the phase mapping information of the even-order harmonics from the core source to determine the expected phase direction of the even-order harmonics from the core source based on the fundamental phase of the magnetic flux, and projects the even-order harmonic components along the expected phase direction to obtain the remanence-related component. The phase mapping information of the even-order harmonics from the core source includes the mapping relationship between the fundamental phase of the magnetic flux and the expected phase direction of the even-order harmonics from the core source.

7. The apparatus as claimed in claim 6, characterized in that, The signal acquisition module includes: a precision sampling resistor connected in series in the secondary circuit, used to obtain the secondary current waveform through the voltage drop across the precision sampling resistor; a high input impedance voltage sampling channel connected in parallel to the secondary terminal, used to obtain the secondary load voltage waveform; and a dual-channel synchronous analog-to-digital converter, used to synchronously convert the current signal and the voltage signal into digital signals. The DC injection module also includes an inductor connected in series between the output terminal of the controllable current source and the secondary circuit. The inductor is used to block the flow of power frequency AC current into the controllable current source and allow the controllable DC component to be injected into the secondary circuit. The device also includes a communication and human-machine interface module for communicating with the substation integrated automation system and displaying the normalized residual magnetism index, estimated residual magnetism level, and current operating status.

8. An electronic device, characterized in that, include: Memory; as well as A processor coupled to the memory, the processor being configured to perform the method as described in any one of claims 1 to 5 based on instructions stored in the memory.

9. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores computer instructions that, when executed by a processor, implement the method as described in any one of claims 1 to 5.

10. A computer program product, characterized in that, It includes a computer program that, when executed by a processor, implements the method as described in any one of claims 1 to 5.

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