A method for detecting residual magnetism of current transformers based on high-frequency square waves

Abstract

The present invention discloses a method for detecting the residual magnetism of a current transformer based on a high-frequency square wave. This method controls the magnetizing voltage unit by a host computer to output a high-frequency square wave, so as to achieve bidirectional deep saturation of the current transformer within a single magnetizing period; the data acquisition unit detects the magnitude of the current on the secondary side of the current transformer and transmits the current data to the host computer; the host computer processes the acquired current signal and uses FFT to obtain the magnitude and direction of the second harmonic; according to the transfer function between the second harmonic and the residual magnetism of the transformer, the magnitude and direction of the residual magnetism of the transformer are calculated. By realizing bidirectional deep symmetric saturation of the iron core within a single period, there will be harmonic signals in the secondary side circuit. By extracting and analyzing them, the residual magnetism situation of the iron core can be obtained. The present invention can achieve high precision in residual magnetism detection, can distinguish the direction of residual magnetism, can obtain the residual magnetism situation in a short time, and has the characteristics of high efficiency.

Description

Technical Field

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

[0002] As a device used to monitor line current in the power system, the current transformer provides an important guarantee for the safe and stable operation of the power system. Affected by the hysteresis effect, the current transformer 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 distort compared with the primary side, thus unable to reflect the true situation of the current in the power grid line. In severe cases, it will cause subsequent relay protection devices to malfunction or delay action, ultimately seriously affecting the safety and stability of the power grid.

[0003] Chinese Patent Application No. 202310113412.5 discloses a method for detecting residual magnetism of a high-frequency square wave current transformer. By charging the transformer and obtaining the corresponding magnetization parameters, the residual magnetism of the transformer is calculated based on relevant data. This method requires magnetizing the iron core, and after the process ends, the original residual magnetism situation of the iron core is changed. Therefore, this detection method does not have much significance; going through forward magnetization, verifying saturation, then reverse magnetization, verifying saturation, the whole process takes a long time and the operation is rather cumbersome, with poor practicability. Summary of the Invention

[0004] In order to overcome the above defects, the present invention provides a method for detecting residual magnetism of a current transformer based on high-frequency square waves, which not only does not change the original residual magnetism, but 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 solution adopted by the present invention is as follows: A method for detecting residual magnetism of a current transformer based on high-frequency square waves, comprising the following steps:

[0006] (1) The host computer controls the magnetic detection voltage to output a high-frequency square wave, so that the current transformer realizes bidirectional deep saturation within a single magnetic detection cycle;

[0007] (2) Collect the secondary current of the current transformer and transmit the collected current signal to the host computer;

[0008] (3) The host computer processes the collected current signal and uses FFT to obtain the magnitude and direction of the current second harmonic;

[0009] (4) Apply voltage to the secondary side of the current transformer to change the residual magnetism of the iron core. After repeating steps (1)-(3), obtain the transfer function between the current second harmonic and the residual magnetism of the iron core, and calculate the magnitude and direction of the residual magnetism of the transformer according to the transfer function.

[0010] Further, the specific steps of step (1) are as follows: The host computer controls the magnetic inspection voltage to output the exciting voltage. When the amplitude of the changing magnetic flux caused by the excitation is greater than the saturation magnetic flux, the magnetic chain of the transformer core realizes bidirectional deep saturation within a single cycle, and the saturation depth is taken to be 3 - 10 times; the exciting voltage is obtained by outputting a controllable high-frequency voltage waveform through a single-phase inverter from a DC power supply, and the turn-off of the bridge arm of the single-phase inverter is controlled by a signal provided by the host computer; the exciting voltage is a high-frequency square wave voltage, and the four processes of forward magnetization, forward demagnetization, reverse magnetization, and reverse demagnetization of the core are realized by the integral form of Faraday's law of electromagnetic induction, and the duration of each process is one-quarter of a cycle.

[0011] Further, according to the magnetization characteristics of the magnetic chain of the current transformer core, when the core approaches 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 magnetic chain. If the magnetic chain of the core shows bidirectional symmetric changes within a single cycle, then at this time the magnetic permeability will also show bidirectional symmetric changes; however, since the magnetic permeability has no direction, it is manifested as a single-phase periodic change, so the periodic change frequency of the magnetic permeability is twice that of the voltage source.

[0012] Specifically, the voltage expression of the high-frequency square wave output by the magnetic inspection voltage unit in step (1) is as follows:

[0013] ;

[0014] where is the transient value of the exciting voltage during the forward magnetization process; is the transient value of the exciting voltage during the forward demagnetization and reverse magnetization processes; is the transient value of the exciting voltage during the reverse demagnetization process; is the amplitude of the exciting voltage; is the period length; is the excitation time.

[0015] Specifically, the expression of the current on the secondary side of the current transformer in step (2) is as follows:

[0016] ;

[0017] In the formula is the magnetic chain of the residual remanence of the core, and are respectively the amplitude of the DC component and the amplitudes of the harmonic components of the reluctivity, 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.

[0018] Further, the expression of the current second harmonic in step (3) is as follows:

[0019] ;

[0020] wherein, is the second harmonic component of the secondary side current; is the amplitude of the second harmonic component of the magnetic resistivity.

[0021] Specifically, the expression of the transfer function of the current second harmonic and the residual magnetism of the current transformer in step (4) is as follows:

[0022] ;

[0023] where is the residual magnetism coefficient of the second harmonic of the secondary side current of the current transformer.

[0024] Further, the transfer function of the current second harmonic and the core residual magnetism obtained in step (4) is specifically: after the magnetization operation, there is a harmonic signal in the secondary side circuit. Extract and analyze the harmonic signal of the secondary side circuit, and determine the core residual magnetism of the current transformer according to the transfer function; since there is a linear relationship between the core residual magnetism and the harmonic signal, samples are taken under different residual magnetism conditions through pre-experiments to obtain the corresponding harmonic signals, and the two are fitted by the least squares method to obtain a linear transfer function, and then the magnitude and direction of the core residual magnetism are solved in the case of the harmonic signal.

[0025] Specifically, the direction of the core residual magnetism is judged by the phase information of the harmonic signal. If the phase of the second harmonic is positive, it is specified that the residual magnetism direction is the positive direction of the core magnetic chain at this time; if the phase of the second harmonic is negative, it is specified that the residual magnetism direction is the negative direction of the core magnetic chain at this time.

[0026] The present invention also provides a current transformer residual magnetism detection device based on a high-frequency square wave, including a magnetic inspection voltage unit, a data acquisition unit and a host computer; the host computer is connected to both the magnetic inspection voltage unit and the data acquisition unit, the magnetic inspection 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:

[0027] The magnetic inspection voltage unit includes a DC voltage source and a bridge inverter circuit. The bridge circuit is controlled by the host computer to perform specific modulation on the output voltage of the DC voltage source; it is used to control the conduction and shutdown of each bridge arm by receiving the control signal transmitted by the host computer, so as to control the amplitude and frequency of the high-frequency square wave, that is, to control the output of a voltage with a specific waveform, frequency and amplitude, and to realize the magnetization operation, demagnetization operation and reverse magnetization operation of the current transformer;

[0028] The data acquisition unit includes a current sampling resistor, which is used to collect the current on the secondary side of the current transformer, transfer the collected current data to the upper computer, and perform harmonic analysis and extract the secondary side signal therein.

[0029] The upper computer uses an upper computer chip to achieve the output control of a specific voltage and the processing of the collected signal, calculates the transfer function of harmonics and remanence according to the processing result, and finally obtains the magnitude and direction of the remanence of the current transformer.

[0030] The beneficial effects of the present invention are as follows:

[0031] In the present invention, a stable voltage with a specific amplitude and controllable ripple is output by a DC voltage source, and a square wave voltage with a specific amplitude and frequency is output through the inversion of a bridge circuit. The bridge circuit is controlled by MOS transistors, and the control signal output by the upper computer (MCU) is amplified in power through a MOS transistor control chip and then transferred to the bridge circuit to control the conduction and cut-off of the relevant bridge arms. The carrier frequency of the control signal is required to be much greater 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 and thus change the amplitude of the output square wave. The voltage output through the inversion circuit is connected to both sides of the transformer to achieve the magnetization of the internal iron core, and bidirectional deep saturation is required to be achieved. The current on the secondary side of the sampling transformer is sampled and its harmonics are extracted, and more accurate remanence information, including magnitude and direction, can be obtained. During the entire magnetic detection cycle, the amount of magnetization and demagnetization is the same, and the original remanence magnitude can be unchanged after detection, which can effectively address the problem of misoperation of protection caused by remanence and bias magnetization during the operation of the power system, and has good application prospects. Description of the Drawings

[0032] Figure 1 It is the system block diagram of the remanence detection based on high-frequency square wave of the present invention;

[0033] Figure 2 It is the specific circuit diagram of the remanence detection based on high-frequency square wave of the present invention;

[0034] Figure 3 It is the schematic diagram of the principle of the remanence detection method based on high-frequency square wave of the present invention;

[0035] Figure 4 It is the flow block diagram of the remanence detection method based on high-frequency square wave of the present invention. Detailed Embodiments

[0036] Here, the exemplary embodiments will be described in detail, and the examples are shown in the drawings. When the following description relates to the drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present application.

[0037] 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 "a", "the", and "said" used in this application and the appended claims are also intended to include the plural forms unless the context clearly dictates otherwise. It should also be understood that the term "and / or" used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items.

[0038] Combined with Figure 1 and Figure 2 as shown, Embodiment 1

[0039] The present invention discloses a residual magnetic detection device for a current transformer based on a high-frequency square wave, including a magnetic detection voltage unit. The direct current voltage source 1 outputs a voltage, which is output as a high-frequency square wave voltage through a single-phase bridge inverter circuit 2, wherein the conduction and shutdown of each bridge arm are controlled by the host computer 7, thereby realizing the control of the amplitude and frequency of the high-frequency square wave; the function of the data acquisition unit 3 can be borne by a sampling resistor, collecting the current signal on the secondary side of the current transformer and transmitting it to the host computer 7, and entering the harmonic analysis module 6 for harmonic analysis to extract the secondary side signal therein; the host computer 7 is responsible for controlling the magnetic detection voltage operation and related processing of the detection signal, and obtaining the transfer function between the harmonic and the residual magnetism through calculation, and finally obtaining the residual magnetism of the current transformer.

[0040] 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, and the secondary side of the current transformer 5 is connected to the host computer 7 through the data acquisition unit 3.

[0041] The said magnetic detection voltage unit includes a direct current voltage source 1 and a single-phase bridge inverter circuit 2.

[0042] The said data acquisition unit includes a data acquisition unit 3 and a harmonic analysis module 6.

[0043] The present invention uses a DC voltage source 1 to output a stable voltage with a specific amplitude and controllable ripple. After inversion by a single-phase bridge inverter circuit 2, a square-wave voltage with a specific amplitude and frequency is output. Among them, the single-phase bridge inverter circuit 2 is controlled by MOS transistors. The control signal output by the upper computer 7 is amplified in power by a MOS transistor control chip and then transmitted to the single-phase bridge inverter circuit 2 to control the conduction and cut-off of the relevant bridge arms. The carrier frequency of the control signal is required to be much greater 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 amplitude of the output square wave. Specifically, if the DC voltage source 1 is selected as 200V and an output of a 100V, 40Hz square-wave signal is required, the control bridge arm forms an output waveform with a period of 0.025s, and the carrier is 10kHz, which can fit the square wave well. At the same time, in order to achieve a controlled output square-wave voltage amplitude of 100V, the duty cycle of the output waveform needs to be controlled at 50% of the original duty cycle. The voltage output by the single-phase bridge inverter circuit 2 is connected to both sides of the current transformer to realize the magnetization of the internal iron core 4, and bidirectional deep saturation is required to be achieved. The data acquisition unit 3 samples the secondary-side current of the current transformer 5 and enters the harmonic analysis module to extract harmonics therefrom, and then relevant remanence information is obtained.

[0044] As Figure 3 shown, the present invention also discloses a method for detecting the remanence of a current transformer based on a high-frequency square wave, and the method includes the following steps:

[0045] Step (1), the upper computer controls the magnetizing voltage unit to output a high-frequency square wave to achieve bidirectional deep saturation of the current transformer within a single magnetizing cycle;

[0046] Step (2), the data acquisition unit detects the magnitude of the secondary-side current of the current transformer and transmits the current data to the upper computer;

[0047] Step (3), the upper computer processes the collected current signal and uses FFT to obtain the magnitude and direction of the second harmonic;

[0048] Step (4), a voltage is applied to the secondary side of the current transformer to change the remanence of the iron core, and after repeating steps (1)-(3), the magnitude and direction of the remanence of the current transformer are calculated according to the transfer function between the second harmonic and the remanence of the current transformer.

[0049] Specifically, in step (1), the upper computer controls the magnetizing voltage unit to output a high-frequency square wave, and the specific expression formula for its output voltage is: ;

[0050] wherein, is the transient value of the excitation voltage during the forward magnetization process; is the transient value of the excitation voltage during the forward demagnetization and reverse magnetization processes; is the transient value of the excitation voltage during the reverse demagnetization process; is the amplitude of the excitation voltage; is the period length; is the excitation time.

[0051] The real-time magnetic flux in the current transformer is:

[0052] ;

[0053] where is the transient value of the magnetic flux during the forward magnetization process; is the transient value of the magnetic flux during the forward demagnetization and reverse magnetization processes; is the transient value of the magnetic flux during the reverse demagnetization process; is the magnetic flux of the residual remanence in the iron core.

[0054] In step (2) and step (3), the secondary current of the current transformer is collected, and the collected current signal is transmitted to the host computer 7; the host computer 7 processes the collected current signal and uses FFT to obtain the magnitude and direction of the current second harmonic; specifically, it includes the following content:

[0055] The magnetic permeability of the current transformer iron core is a function of a frequency twice the magnetic inspection frequency, and it does not contain odd harmonics of the magnetic inspection frequency; by controlling the magnetic flux change method, 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 iron core is expanded into a Fourier series:

[0056] ;

[0057] where is the instantaneous value of the magnetic permeability of the iron core; is the DC component of the magnetic permeability of the iron core; is the amplitude of each harmonic component of the magnetic permeability of the iron core; is the excitation voltage frequency.

[0058] And the reciprocal of the magnetic permeability is expanded into a Fourier series:

[0059] ;

[0060] where and are the amplitude of the DC component and the amplitude of each harmonic component of the reluctivity respectively.

[0061] According to Ampere's circuital theorem and the definition formula of magnetic flux:

[0062] ;

[0063] where is the magnetic field strength of the current transformer iron core; is the number of turns of the secondary winding, is the cross-sectional area of the iron core; is the secondary current; is the equivalent magnetic path length of the current transformer iron core.

[0064] The expression of the secondary current is obtained:

[0065] ;

[0066] Among them, is the excitation voltage frequency; is the equivalent magnetic path length of the current transformer iron core.

[0067] The even harmonic component is the harmonic signal sought. Among the harmonics of the secondary current of the current transformer, the amplitude of the second harmonic is the largest, which is expressed as:

[0068] ;

[0069] Among them, is the second harmonic component of the secondary current; is the amplitude of the second harmonic component of the magnetic resistivity.

[0070] As Figure 4 shown, the above process is the complete process of single - time residual magnetic detection, and the finally obtained data is the magnitude of the current second harmonic under a certain residual magnetic condition. By controlling the output voltage of the single - phase bridge inverter 2 to change the residual magnetism of the iron core and repeating the above steps again, the transfer function between the current second harmonic and the iron core residual magnetism can be obtained, which is expressed as:

[0071] ;

[0072] Among them is the residual magnetic coefficient of the second harmonic of the secondary current of the transformer.

[0073] Therefore, before detecting the unknown residual magnetism, the above steps need to be carried out as a preliminary experiment to obtain the transfer function between the current second harmonic and the iron core residual magnetism, and calculate the unknown residual magnetism according to the measured harmonic magnitude.

[0074] 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 by the above - mentioned embodiments. What is described in the above - mentioned embodiments and the specification only illustrates the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed. The scope of protection claimed by the present invention is defined by the appended 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 as to realize bidirectional deep saturation of the current transformer within a single magnetic detection cycle; specifically: the host computer controls the magnetic detection voltage to output an excitation voltage, and when the amplitude of the changing magnetic flux caused by the excitation is greater than the saturation magnetic flux, the magnetic flux of the transformer core realizes bidirectional deep saturation within 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; (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) applying pressure to the secondary side of the current transformer to change the core residual magnetism, and repeating steps (1) to (3) to obtain the transfer function between the second harmonic of the current and the core residual magnetism, and calculating the magnitude and direction of the transformer residual magnetism according to the transfer function; the expression of the transfer function between the second harmonic of the current and the transformer residual magnetism is as follows: P rem =K rem I 2nd ; Among them Ψ rem is the magnetic flux of the residual magnetism in the core; K rem is the remanence coefficient of the second harmonic of the secondary current of the transformer; i 2nd is the second harmonic component of the secondary side current.

2. A current transformer residual magnetism detection method based on 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.

3. 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: u s1 =U det (t<1 / 4t det ) u s2 =-U det (1 / 4t det ≤t<3 / 4t det ); u s3 =U det (3 / 4t det ≤t) in, u s1 is the transient value of the excitation voltage during the forward magnetization process; u s2 is the transient value of the excitation voltage during the forward demagnetization and reverse magnetization process; u s3 is the transient value of the excitation voltage during the reverse demagnetization process; U det is the excitation voltage amplitude; t det is the cycle length; t is the excitation time.

4. 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: Where Ψ rem is the magnetic flux of the residual magnetism in the core, MR d and MR n are the DC component amplitude and each harmonic component amplitude of the magnetic resistivity, N2 is the number of turns of the secondary winding, S is the cross-sectional area of ​​the core, i2 is the secondary current, and L eq is the equivalent magnetic circuit length of the current transformer core; U det is the excitation voltage amplitude; ω det is the excitation voltage frequency.

5. 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: Among them, i 2nd is the second harmonic component of the secondary current; rem is the magnetic flux of the residual magnetism in the core; L eq is the equivalent magnetic path length of the current transformer core; N2 is the number of turns of the secondary winding, S is the cross-sectional area of ​​the core; ω det is the excitation voltage frequency; MR1 is the amplitude of the second harmonic component of the magnetic resistivity.

6. 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 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 size of the residual magnetism of the core and the direction of the residual magnetism of the core are solved when the harmonic signal is obtained.

7. A current transformer residual magnetism detection method based on high frequency square wave according to claim 6, 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.

8. 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 inverter 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 forward magnetization, forward demagnetization, reverse magnetization, and reverse demagnetization of the current transformer, and the duration is respectively one-quarter of a cycle; 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 uses a host computer chip to realize the output control of a specific voltage and the processing of the collected signal. The transfer function of the second harmonic of the current and the residual magnetism of the transformer is calculated according to the processing result, and finally the residual magnetism size and direction of the current transformer are obtained; the expression of the transfer function of the second harmonic of the current and the residual magnetism of the transformer is as follows: P rem =K rem I 2nd ; Among them Ψ rem is the magnetic flux of the residual magnetism in the core; K rem is the remanence coefficient of the second harmonic of the secondary current of the transformer; i 2nd is the second harmonic component of the secondary side current.

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