Current transformer residual magnetism detection method based on high-frequency square waves

Abstract

The invention discloses a current transformer residual magnetism detection method based on a high-frequency square wave, and the method comprises the steps: controlling a magnetism detection voltage unit to output a high-frequency square wave through an upper computer, and achieving the bidirectional depth saturation of a current transformer in a single magnetism detection period; the data acquisition unit detects the secondary side current of the current transformer and transmits current data to the upper computer; the upper computer processes the collected current signal, and obtains the size and direction of a second harmonic by using FFT (Fast Fourier Transform); and according to the transfer function of the secondary harmonic and the transformer residual magnetism, calculating to obtain the magnitude and direction of the transformer residual magnetism. According to the method, bidirectional deep symmetric saturation of the iron core in a single period is achieved, harmonic signals exist in a secondary side loop, the harmonic signals are extracted and analyzed, and the residual magnetism condition of the iron core can be obtained. According to the invention, high residual magnetism detection precision can be realized, the residual magnetism direction can be discriminated, the residual magnetism condition can be obtained in a short time, and the method has the characteristic of high efficiency.

Description

Technical Field

[0001] The invention relates to the field of power system detection devices, and in particular to a current transformer residual magnetism detection method based on high-frequency square waves. Background Art

[0002] As a device used to monitor line current in power systems, current transformers provide an important guarantee for the safe and stable operation of power systems. Affected by the hysteresis effect, current transformers will have residual magnetism after running for a period of time, which will promote the saturation of the current transformer, causing the secondary current to be distorted compared to the primary current, thus failing to reflect the true current situation in the power grid line. In severe cases, it will cause the subsequent relay protection device to malfunction or delay operation, which will ultimately seriously affect the safety and stability of the power grid.

[0003] A method for detecting residual magnetism of a transformer based on a high-frequency square wave is disclosed in the Chinese patent application No. 202310113412.5. The residual magnetism of the transformer is calculated based on the relevant data by charging the transformer and obtaining the corresponding magnetization parameters. This method requires magnetizing the core. After the process is completed, the residual magnetism of the original core is changed, so this detection method does not make much sense; it undergoes forward magnetization, verifies saturation, and then reverse magnetization and verifies saturation. The whole process takes a long time, and the operation is cumbersome and has poor practicality. Summary of the invention

[0004] In order to overcome the above defects, the present invention provides a current transformer residual magnetism detection method based on high-frequency square waves, which not only does not change the original residual magnetism, but also only needs to provide a preset magnetic detection voltage to achieve detection, which is convenient, efficient and has high accuracy.

[0005] To achieve the above object, the present invention adopts the following scheme: A current transformer residual magnetism detection method based on high frequency square wave, comprising the following steps: (1) The host computer controls the magnetic detection voltage to output a high-frequency square wave, so that the current transformer can achieve bidirectional deep saturation within a single magnetic detection cycle; (2) Collect the secondary current of the current transformer and transmit the collected current signal to the host computer; (3) The host computer processes the collected current signal and uses FFT to obtain the magnitude and direction of the second harmonic of the current; (4) Apply pressure to the secondary side of the current transformer to change the residual magnetism of the core. Repeat steps (1) to (3) to obtain the transfer function between the second harmonic of the current and the residual magnetism of the core. The magnitude and direction of the residual magnetism of the transformer are calculated based on the transfer function.

[0006] Furthermore, the step (1) is specifically as follows: the host computer controls the magnetic detection voltage to output the excitation voltage, and when the amplitude of the changing magnetic flux caused by the excitation is greater than the saturation magnetic flux, the mutual inductor core flux linkage realizes bidirectional deep saturation in a single cycle, and the saturation depth is 3-10 times; the excitation voltage is obtained by a DC power supply through a single-phase inverter outputting a controllable voltage high-frequency waveform, and the bridge arm shutdown of the single-phase inverter is controlled by a signal provided by the host computer; the excitation voltage is a high-frequency square wave voltage, and the integral form of the law of electromagnetic induction realizes the four processes of forward magnetization, forward demagnetization, reverse magnetization, and reverse demagnetization of the core, and the duration is respectively one-quarter of a cycle.

[0007] Furthermore, according to the magnetization characteristics of the core flux of the current transformer, when the core is close to saturation, its magnetic permeability will decrease; on the contrary, when the saturation degree of the core decreases, the magnetic permeability will increase accordingly, that is, the value of the magnetic permeability is negatively correlated with the saturation of the core flux. If the core flux changes bidirectionally symmetrically within a single cycle, then the magnetic permeability will also show bidirectional symmetrical changes; however, since the magnetic permeability has no direction, it manifests as a single-phase periodic change, so the periodic change frequency of the magnetic permeability is twice that of the voltage source.

[0008] Specifically, the voltage expression of the high-frequency square wave output by the magnetic detection voltage unit in step (1) is as follows: ; in, is the transient value of the excitation voltage during the forward magnetization process; It is the transient value of the excitation voltage during the forward demagnetization and reverse magnetization process; is the transient value of the excitation voltage during the reverse demagnetization process; is the excitation voltage amplitude; is the cycle length; For motivation time.

[0009] Specifically, the expression of the secondary current of the current transformer in step (2) is as follows: ; In the formula It is the magnetic flux of the residual magnetism in the iron core. and are the DC component amplitude and each harmonic component amplitude of the magnetic resistivity, is the magnetic field strength of the current transformer core, is the number of turns of the secondary winding, is the core cross-sectional area, is the secondary current, is the equivalent magnetic path length of the current transformer core.

[0010] Furthermore, the expression of the second harmonic of the current in step (3) is as follows: ; in, is the second harmonic component of the secondary current; is the amplitude of the second harmonic component of the magnetoresistance.

[0011] Specifically, the expression of the transfer function between the second harmonic of the current and the residual magnetism of the transformer in step (4) is as follows: ; in is the remanence coefficient of the second harmonic of the secondary current of the transformer.

[0012] Furthermore, the transfer function of the second harmonic of the current and the residual magnetism of the core is obtained in the step (4), specifically: after the magnetization operation, there is a harmonic signal in the secondary circuit, the harmonic signal of the secondary circuit is extracted and analyzed, and the residual magnetism of the core of the current transformer is determined according to the transfer function; since there is a linear relationship between the residual magnetism of the core and the harmonic signal, sampling is performed under different residual magnetism conditions through preliminary experiments to obtain corresponding harmonic signals, and the least squares method is used to fit the two to obtain a linear transfer function, and then the magnitude and direction of the residual magnetism of the core are solved when the harmonic signal is obtained.

[0013] Specifically, the direction of the core residual magnetism is determined by the phase information of the harmonic signal. If the second harmonic phase is positive, the residual magnetism direction at this time is defined as the positive direction of the core flux linkage; if the second harmonic phase is negative, the residual magnetism direction at this time is defined as the negative direction of the core flux linkage.

[0014] The present invention also provides a current transformer residual magnetism detection device based on high-frequency square waves, comprising a magnetism detection voltage unit, a data acquisition unit and a host computer; the host computer is connected to the magnetism detection voltage unit and the data acquisition unit, the magnetism detection voltage unit is connected to the secondary side of the current transformer, and the secondary side of the current transformer is connected to the host computer through the data acquisition unit; specifically: The magnetic detection voltage unit includes a DC voltage source and a bridge inverter circuit. The bridge circuit is controlled by a host computer to perform specific modulation on the output voltage of the DC voltage source. The unit is used to control the amplitude and frequency of the high-frequency square wave when the conduction and closing of each bridge arm are controlled by a control signal transmitted by the host computer, that is, to control the output voltage of a specific waveform, frequency, and amplitude, and to realize the magnetization operation, demagnetization operation, and reverse magnetization operation of the current transformer. The data acquisition unit includes a current sampling resistor, which is used to collect the secondary side current of the current transformer, transmit the collected current data to the host computer, perform harmonic analysis and extract the secondary side signal therein; The host computer chip is used to realize the output control of specific voltage and the processing of collected signals. The transfer function of harmonics and residual magnetism is calculated according to the processing results, and finally the residual magnetism size and direction of the current transformer are obtained.

[0015] The beneficial effects of the present invention are as follows: The present invention outputs a stable voltage with a specific amplitude and controllable ripple by using a DC voltage source, and outputs a square wave voltage with a specific amplitude and frequency through a bridge circuit inverter. The bridge circuit is controlled by a MOS tube, and the control signal output by the host computer (MCU) is amplified by the MOS tube control chip and then transmitted to the bridge circuit to control the conduction and shutdown of the relevant bridge arm. The control signal carrier frequency is required to be much larger than the frequency of the output square wave, and the carrier ratio can be more than ten, which can control the duty cycle of the output square wave, thereby changing the output square wave amplitude. The voltage output by the inverter circuit is connected to both sides of the transformer to achieve the magnetization of the internal iron core, and it is required to achieve bidirectional deep saturation. The current on the secondary side of the transformer is sampled and harmonics are extracted to obtain more accurate residual magnetism information, including size and direction. In the entire magnetic detection cycle, the amount of magnetization and demagnetization is consistent, and the original residual magnetism size can be unchanged after detection. It can effectively deal with the protection malfunction problem caused by residual magnetism and bias magnetism in the operation of the power system, and has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 The system block diagram of the residual magnetism detection based on high frequency square wave of the present invention; Figure 2 It is a specific circuit diagram of residual magnetism detection based on high-frequency square wave of the present invention; Figure 3 It is a principle diagram of the residual magnetism detection method based on high-frequency square wave of the present invention; Figure 4 The figure is a flowchart of the residual magnetism detection method based on high-frequency square wave of the present invention. DETAILED DESCRIPTION

[0017] Here, exemplary embodiments are described in detail, and examples thereof are shown in the accompanying drawings. When the following description refers to the drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements. The implementations described in the following exemplary embodiments do not represent all implementations consistent with the present application.

[0018] The terms used in this application are for the purpose of describing specific embodiments only and are not intended to limit this application. The singular forms of "a", "said" and "the" used in this application and the appended claims are also intended to include plural forms unless the context clearly indicates other meanings. It should also be understood that the term "and / or" used herein refers to and includes any or all possible combinations of one or more associated listed items.

[0019] Combination Figure 1 and Figure 2 As shown, Example 1 The present invention discloses a current transformer residual magnetism detection device based on high-frequency square waves, comprising a magnetism detection voltage unit, a DC voltage source 1 outputs voltage, and outputs a high-frequency square wave voltage via a single-phase bridge inverter circuit 2, wherein the on and off of each bridge arm is controlled by a host computer 7, thereby realizing the control of the amplitude and frequency of the high-frequency square wave; the function of a data acquisition unit 3 can be undertaken by a sampling resistor, collecting a current transformer secondary side current signal and transmitting it to the host computer 7, entering a harmonic analysis module 6 for harmonic analysis, and extracting the secondary side signal therefrom; the host computer 7 is responsible for controlling the magnetism detection voltage operation and the related processing of the detection signal, and obtains the transfer function of the harmonic and the residual magnetism by calculation, and finally obtains the residual magnetism of the transformer.

[0020] The host computer 7 is connected to both the magnetic detection voltage unit and the data acquisition unit 3 . The magnetic detection voltage unit is connected to the secondary side of the current transformer 5 . The secondary side of the current transformer 5 is connected to the host computer 7 through the data acquisition unit 3 .

[0021] The magnetic detection voltage unit comprises a DC voltage source 1 and a single-phase bridge inverter circuit 2 .

[0022] The data acquisition unit includes a data acquisition unit 3 and a harmonic analysis module 6.

[0023] The present invention adopts a DC voltage source 1 to output a stable voltage with a specific amplitude and controllable ripple, and then inverts and outputs a square wave voltage with a specific amplitude and frequency through a single-phase bridge inverter circuit 2. The single-phase bridge inverter circuit 2 is controlled by a MOS tube, and the control signal output by the host computer 7 is transmitted to the single-phase bridge inverter circuit 2 after power amplification via the MOS tube control chip to control the on and off of the relevant bridge arm. The control signal carrier frequency is required to be much larger than the frequency of the output square wave, and the carrier ratio can be taken to be more than ten, which can control the duty cycle of the output square wave, thereby changing the output square wave amplitude. Specifically, if the DC voltage source 1 is selected to be 200V, it is necessary to control the output to be a 100V, 40Hz square wave signal, control the bridge arm to form an output waveform with a period of 0.025s, and the carrier adopts 10kHz, which can better fit the square wave. At the same time, in order to achieve the control of the amplitude of the output square wave voltage to be 100V, it is necessary to control the duty cycle of the output waveform to be 50% of the original duty cycle. The voltage output by the single-phase bridge inverter circuit 2 is connected to both sides of the transformer to realize the magnetization of the internal core 4, requiring bidirectional deep saturation. The data acquisition unit 3 samples the secondary side current of the current transformer 5, and enters the harmonic analysis module to extract the harmonics, thereby obtaining the relevant residual magnetism information.

[0024] like Figure 3 As shown, the present invention also discloses a current transformer residual magnetism detection method based on high frequency square wave, the method comprising the following steps: Step (1), the host computer controls the magnetic detection voltage unit to output a high-frequency square wave, so that the current transformer can achieve bidirectional deep saturation within a single magnetic detection cycle; Step (2), the data acquisition unit detects the current magnitude of the secondary side of the current transformer and transmits the current data to the host computer; Step (3), the host computer processes the collected current signal and uses FFT to obtain the magnitude and direction of the second harmonic; Step (4), applying pressure on the secondary side of the current transformer to change the residual magnetism of the core, repeating steps (1) to (3), and calculating the magnitude and direction of the residual magnetism of the current transformer based on the transfer function between the second harmonic and the residual magnetism of the current transformer.

[0025] Specifically, in step (1), the host computer controls the magnetic detection voltage unit to output a high-frequency square wave, and the specific expression formula of its output voltage is: ; in, is the transient value of the excitation voltage during the forward magnetization process; It is the transient value of the excitation voltage during the forward demagnetization and reverse magnetization process; is the transient value of the excitation voltage during the reverse demagnetization process; is the excitation voltage amplitude; is the cycle length; For motivation time.

[0026] The real-time flux linkage in the current transformer is: ; in, is the transient value of magnetic flux in the forward magnetization process; It is the transient value of magnetic flux in the process of forward demagnetization and reverse magnetization; It is the transient value of magnetic flux in the reverse demagnetization process; It is the magnetic flux remaining in the iron core.

[0027] Step (2) and step (3) collect the secondary side current of the current transformer and transmit the collected current signal to the host computer 7; the host computer 7 processes the collected current signal and obtains the magnitude and direction of the second harmonic of the current by using FFT; specifically includes the following contents: The magnetic permeability of the current transformer core is a function of twice the detection frequency, which does not contain odd harmonics about the detection frequency; by controlling the flux change mode, the magnetic permeability function is made an even function, so there is only a cosine component in the Fourier series; the magnetic permeability of the current transformer core is expanded into a Fourier series: ; in, is the instantaneous value of the core magnetic permeability; is the DC component of the core permeability; is the amplitude of each harmonic component of the core permeability; is the excitation voltage frequency.

[0028] And the inverse of the magnetic permeability is expanded into a Fourier series: ; in, and They are the amplitude of the DC component and each harmonic component of the magnetic resistivity respectively.

[0029] According to Ampere's circuit theorem and the definition formula of magnetic flux: ; in, is the magnetic field strength of the current transformer core; is the number of turns of the secondary winding, is the cross-sectional area of ​​the core; is the secondary current; is the equivalent magnetic path length of the current transformer core.

[0030] The secondary current expression is obtained: ; in, is the excitation voltage frequency; is the equivalent magnetic path length of the current transformer core.

[0031] Its even harmonic components are the desired harmonic signals. Among the harmonics of the secondary current of the current transformer, the amplitude of the second harmonic is the largest, which can be expressed as: ; in, is the second harmonic component of the secondary current; is the amplitude of the second harmonic component of the magnetoresistance.

[0032] like Figure 4 As shown, the above process is the complete process of single residual magnetism detection, and the final data obtained is the size of the second harmonic of the current under a certain residual magnetism condition. The output voltage of the single-phase bridge inverter circuit 2 is controlled to change the residual magnetism of the core, and the above steps are repeated again to obtain the transfer function of the second harmonic of the current and the residual magnetism of the core, which is expressed as: ; in is the remanence coefficient of the second harmonic of the secondary current of the transformer.

[0033] Therefore, before detecting the unknown residual magnetism, the above steps need to be performed as a preliminary experiment to obtain the transfer function between the second harmonic of the current and the residual magnetism of the core, and calculate the unknown residual magnetism based on the measured harmonic size.

[0034] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A current transformer residual magnetism detection method based on high frequency square wave, characterized in that: The following steps are involved: (1) The host computer controls the magnetic detection voltage to output a high-frequency square wave, so that the current transformer can achieve bidirectional deep saturation within a single magnetic detection cycle; (2) Collect the secondary current of the current transformer and transmit the collected current signal to the host computer; (3) The host computer processes the collected current signal and uses FFT to obtain the magnitude and direction of the second harmonic of the current; (4) Apply pressure to the secondary side of the current transformer to change the residual magnetism of the core. Repeat steps (1) to (3) to obtain the transfer function between the second harmonic of the current and the residual magnetism of the core. The magnitude and direction of the residual magnetism of the transformer are calculated based on the transfer function.

2. A current transformer residual magnetism detection method based on high frequency square wave according to claim 1, characterized in that: The step (1) is specifically as follows: the host computer controls the magnetic detection voltage to output the excitation voltage. When the amplitude of the changing magnetic flux caused by the excitation is greater than the saturation magnetic flux, the mutual inductor core flux linkage realizes bidirectional deep saturation in a single cycle, and the saturation depth is 3-10 times; the excitation voltage is obtained by outputting a controllable voltage high-frequency waveform through a single-phase inverter via a DC power supply, and the bridge arm shutdown of the single-phase inverter is controlled by a signal provided by the host computer; the excitation voltage is a high-frequency square wave voltage, and the integral form of the law of electromagnetic induction realizes the four processes of forward magnetization, forward demagnetization, reverse magnetization, and reverse demagnetization of the core, and the duration is respectively one-quarter of a cycle.

3. The method for detecting residual magnetism of a current transformer based on a high frequency square wave according to claim 1, characterized in that: According to the magnetization characteristics of the core flux of the current transformer, when the core is close to saturation, its magnetic permeability will decrease; on the contrary, when the saturation degree of the core decreases, the magnetic permeability will increase accordingly, that is, the value of the magnetic permeability is negatively correlated with the saturation of the core flux. If the core flux changes bidirectionally symmetrically within a single cycle, then the magnetic permeability will also show bidirectional symmetrical changes; however, since the magnetic permeability has no direction, it manifests as a single-phase periodic change, so the periodic change frequency of the magnetic permeability is twice that of the voltage source.

4. According to the method for detecting residual magnetism of a current transformer based on high-frequency square wave in claim 1, the voltage expression of the high-frequency square wave output by the magnetic detection voltage unit in step (1) is as follows: ； in, is the transient value of the excitation voltage during the forward magnetization process; It is the transient value of the excitation voltage during the forward demagnetization and reverse magnetization process; is the transient value of the excitation voltage during the reverse demagnetization process; is the excitation voltage amplitude; is the cycle length; For motivation time.

5. The method for detecting residual magnetism of a current transformer based on a high frequency square wave according to claim 1, characterized in that: The expression of the secondary current of the current transformer in step (2) is as follows: ； In the formula It is the magnetic flux of the residual magnetism in the iron core. and are the DC component amplitude and each harmonic component amplitude of the magnetic resistivity, is the magnetic field strength of the current transformer core, is the number of turns of the secondary winding, is the core cross-sectional area, is the secondary current, is the equivalent magnetic path length of the current transformer core.

6. The method for detecting residual magnetism of a current transformer based on high-frequency square wave according to claim 1, characterized in that: The expression of the second harmonic of current in step (3) is as follows: ； in, is the second harmonic component of the secondary current; is the amplitude of the second harmonic component of the magnetoresistance.

7. The method for detecting residual magnetism of a current transformer based on high-frequency square wave according to claim 1, characterized in that: The expression of the transfer function between the second harmonic of the current and the residual magnetism of the transformer in step (4) is as follows: ； in is the remanence coefficient of the second harmonic of the secondary current of the transformer.

8. The method for detecting residual magnetism of a current transformer based on high frequency square wave according to claim 1, characterized in that: The transfer function of the second harmonic of the current and the residual magnetism of the core is obtained in the step (4), specifically: after the magnetization operation, there is a harmonic signal in the secondary circuit, the harmonic signal of the secondary circuit is extracted and analyzed, and the residual magnetism of the core of the current transformer is determined according to the transfer function; since there is a linear relationship between the residual magnetism of the core and the harmonic signal, sampling is performed under different residual magnetism conditions through preliminary experiments to obtain the corresponding harmonic signal, and the least squares method is used to fit the two to obtain a linear transfer function, thereby obtaining the harmonic signal condition. Under these circumstances, the magnitude and direction of the residual magnetism in the core can be solved.

9. The method for detecting residual magnetism of a current transformer based on high-frequency square wave according to claim 8, characterized in that: The direction of the core residual magnetism is determined by the phase information of the harmonic signal. If the second harmonic phase is positive, the residual magnetism direction is defined as the positive direction of the core flux linkage; if the second harmonic phase is negative, the residual magnetism direction is defined as the negative direction of the core flux linkage.

10. A current transformer residual magnetism detection device based on high frequency square wave, characterized in that: It includes a magnetic detection voltage unit, a data acquisition unit and a host computer; the host computer is connected to the magnetic detection voltage unit and the data acquisition unit, the magnetic detection voltage unit is connected to the secondary side of the current transformer, and the secondary side of the current transformer is connected to the host computer through the data acquisition unit; specifically: The magnetic detection voltage unit includes a DC voltage source and a bridge inverter circuit. The bridge circuit is controlled by a host computer to perform specific modulation on the output voltage of the DC voltage source. The unit is used to control the amplitude and frequency of the high-frequency square wave when the conduction and closing of each bridge arm are controlled by a control signal transmitted by the host computer, that is, to control the output voltage of a specific waveform, frequency, and amplitude, and to realize the magnetization operation, demagnetization operation, and reverse magnetization operation of the current transformer. The data acquisition unit includes a current sampling resistor, which is used to collect the secondary side current of the current transformer, transmit the collected current data to the host computer, perform harmonic analysis and extract the secondary side signal therein; The host computer chip is used to realize the output control of specific voltage and the processing of collected signals. The transfer function of harmonics and residual magnetism is calculated according to the processing results, and finally the residual magnetism size and direction of the current transformer are obtained.

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