CNC Variable Frequency Current Transformer Calibration Device and Method
By using a CNC variable frequency current transformer calibration device, which utilizes a high-precision CNC variable frequency electronic source module and digital control circuit, the problems of heavy weight, poor repeatability, and weak anti-interference ability of current transformer field calibration devices are solved, and high-precision, stable, and portable current transformer measurement is realized.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- SHANXI INSTR TRANSFORMER ELECTRIC MEASURING EQUIP CO LTD
- Filing Date
- 2022-12-15
- Publication Date
- 2026-07-31
AI Technical Summary
Existing field calibration devices for current transformers are heavy, have poor repeatability, and weak anti-interference capabilities, resulting in inaccurate measurement results.
A numerically controlled variable frequency current transformer calibration device is adopted. A high-precision numerically controlled variable frequency electronic source module provides measurement signals. Combined with isolation protection and data acquisition modules, the error measurement of the current transformer under test is realized through digital control circuit. A high-precision AD chip and digital frequency conversion technology are used, and a zero-crossing sampling algorithm is employed to reduce the influence of electromagnetic interference.
The device has been miniaturized and lightweighted, improving measurement accuracy and anti-interference capabilities, enabling stable measurements in complex electromagnetic environments, and simplifying the operation process.
Smart Images
Figure CN116008895B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of instrument transformer testing technology, specifically relating to a numerically controlled variable frequency current transformer calibration device and method, and more particularly to a device and method for calibrating current transformer errors using a high-precision numerically controlled variable frequency electronic source module. Background Technology
[0002] The initial current transformer calibration device consisted of a transformer calibrator, a current standard, a current booster, and a voltage regulator. The current standard and current booster were bulky and heavy, making them inconvenient to carry and transport. Although they offered high testing accuracy and repeatability, their efficiency was low in field calibration environments. Therefore, portable field current calibration devices were developed domestically in the 1990s, using voltage to calibrate current and extrapolating the admittance to calibrate the current transformer's error. However, the developed field calibration devices still have the following problems:
[0003] (1) It is still relatively heavy. Although it is much more convenient than the initial calibration device, it still contains many voltage transformer coils as standard transformers, which often does not achieve the purpose of portability;
[0004] (2) Poor repeatability. When the core of a current transformer is eccentric with one turn, the impact on the error is small as long as the core is not locally saturated. However, when the core of a voltage transformer is eccentric with one turn, the induced electromotive force may change significantly, thus affecting the measured error. Repeated measurements lead to a large numerical deviation.
[0005] (3) Weak anti-interference ability. The test of 1-turn induced electromotive force is easily affected by the surrounding electromagnetic field, which affects the accuracy of the ratio error compensation value and leads to inaccurate error test results. Summary of the Invention
[0006] Therefore, the purpose of this invention is to provide a numerically controlled variable frequency current transformer calibration device, which aims to solve the technical defects of existing current transformer field calibration devices, such as heavy weight, poor repeatability, and weak anti-interference ability.
[0007] Another objective of this invention is to provide a numerically controlled variable frequency current transformer calibration method, which aims to overcome the shortcomings of existing on-site calibration methods for current transformers, such as susceptibility to interference from surrounding electromagnetic fields, resulting in inaccurate error test results, and the inability to achieve one-click measurement.
[0008] To achieve the above-mentioned objectives, the technical solution adopted by the present invention is as follows:
[0009] A numerically controlled variable frequency current transformer calibration device, comprising a current transformer under test, and:
[0010] The control module sends control commands to the digitally controlled frequency converter electronic power source module to provide the DC or AC voltage required for the measurement signal, based on the parameters of the current transformer under test, and sends control commands to the data acquisition module for synchronous sampling. At the same time, it reads the acquired data sent by the data acquisition module, processes it, and obtains the error of the current transformer under test.
[0011] An isolation protection module is connected to the primary side of the current transformer under test and is used to control the voltage data of the primary side of the current transformer under test within an acceptable range.
[0012] The numerically controlled variable frequency electronic power source module receives control commands from the control module, outputs the secondary side DC voltage or AC voltage required for testing to the current transformer under test, and simultaneously outputs a sine wave signal to the data acquisition module for conversion into a square wave trigger signal.
[0013] The data acquisition module receives control commands from the control module and sinusoidal signals output from the CNC variable frequency electronic source module. It converts these sinusoidal signals into square wave trigger signals to trigger the acquisition, thereby maintaining synchronous sampling of the voltage data on the primary side of the current transformer under test. The acquired voltage data is then uploaded to the control module.
[0014] Furthermore, the isolation protection module includes:
[0015] The fuse module is connected to the primary side of the current transformer under test and is used to protect the entire measurement circuit.
[0016] An electrical signal conversion module, connected to the fuse module, is used to convert the primary voltage signal into a current signal;
[0017] An isolation coil, the primary terminal of which is connected to an electrical signal conversion module, is used to proportionally control the current signal within a set range;
[0018] The sampling resistor is connected to the secondary terminal of the isolation coil and is used to convert the current signal into a sampled primary voltage signal.
[0019] Preferably, the ratio of the isolation coil is 1 / 500.
[0020] Furthermore, the numerically controlled variable frequency electronic power source module includes:
[0021] The D / A conversion module is connected to the control module. It receives the direct wave signal or initial square wave signal provided by the control module according to the parameters of the current transformer under test. It converts the direct wave signal into an analog direct wave signal as the DC voltage signal required for the test, and converts the initial square wave signal into a sine wave signal as the AC voltage signal required for the test. At the same time, the sine wave signal is also input into the data acquisition module.
[0022] A voltage regulator module is connected to the D / A conversion module to provide a reference voltage for the D / A conversion module;
[0023] The first analog filtering module is connected to the D / A conversion module and is used to output analog straight wave signals and sine wave signals after removing noise.
[0024] A phase adjustment module, connected to the first analog filter module, is used to adjust the phase of the sinusoidal signal output by the first analog filter module to keep it consistent with the initial phase.
[0025] The feedback adjustment module, connected to the phase adjustment module, is used to reduce the temperature drift caused by the increase in circuit temperature and maintain the stability of the output signal.
[0026] A power amplifier module, connected to the feedback adjustment module, is used to obtain an AC voltage signal with the required amplitude and frequency for testing.
[0027] The voltage-to-current conversion module is connected to the first analog filter module and is used to receive the analog direct wave signal and output the DC voltage signal required for testing.
[0028] Preferably, the frequency of the AC voltage signal output by the CNC variable frequency electronic power source module is 5-60Hz.
[0029] Furthermore, the data acquisition module includes:
[0030] The signal amplification module is connected to the sampling resistor and simultaneously receives the sine wave signal emitted by the digitally controlled frequency conversion electronic source module. It is used to amplify the acquired voltage signal and the sine wave signal and ensure that the signal waveform is not distorted.
[0031] The second analog filtering module is connected to the signal amplification module and is used to remove noise carried during the signal amplification process.
[0032] The frequency divider module is connected to the second analog filter module and the phase-locked loop module to form a combination, which is used to convert the sinusoidal signal input to the signal amplification module into a square wave trigger signal for triggering acquisition.
[0033] The digital-to-analog converter module, connected to the frequency divider module, also receives a square wave trigger signal and voltage data of the current transformer under test, and is used to collect the voltage data of the primary side of the current transformer under test when the square wave trigger signal is triggered.
[0034] A digital filtering module, connected to the digital-to-analog converter module, is used to remove high-order harmonics from the voltage data acquired and converted by the digital-to-analog converter module.
[0035] More preferably, when the square wave trigger signal is triggered, the voltage data of the primary side of the current transformer under test is collected. Specifically, the triggering method is to trigger the rising edge of the square wave trigger signal. After the rising edge is triggered, the digital-to-analog conversion module performs the first acquisition. After the first acquisition, a second acquisition is performed after 1 / 4 cycle of the current frequency. The results of the two acquisitions are calculated by the digital-to-analog conversion module to obtain the ratio difference and phase difference under the current transformation ratio.
[0036] In addition, the numerically controlled variable frequency current transformer calibration device disclosed in this invention also includes:
[0037] The host computer has a built-in instruction input module and display screen. It connects to the control module via an RS232 communication module or a USB communication module and sends control commands to the control module. At the same time, it reads the measurement data in the control module and displays the error of the current transformer under test.
[0038] Furthermore, the present invention also provides a method for calibrating a digitally controlled variable frequency current transformer, comprising the following steps:
[0039] First, input the relevant parameters of the current transformer under test, such as the transformation ratio, rated load, lower limit load and rated power factor, through the LCD screen;
[0040] Then, the control module controls the output of DC current by the CNC variable frequency electronic source module according to the parameters, and measures the DC resistance R of the secondary winding of the current transformer under test.
[0041] Furthermore, the control module controls the CNC variable frequency electronic power source module to output AC voltage to the secondary winding of the current transformer under test according to the parameters, and boosts the voltage to the secondary induced electromotive force corresponding to 5% of the rated current. The primary side voltage is measured to determine whether the turns ratio is correct, and the total impedance Z of the secondary winding of the current transformer under test is obtained according to formula (5). 02 :
[0042] Z 02 =R + Z (Formula 5)
[0043] In the formula, Z is the rated load or lower limit load of the current transformer under test, and R is the DC resistance of the secondary winding of the current transformer under test.
[0044] Next, the voltage is increased to the point of maximum permeability of the secondary winding of the current transformer under test. At this point, the voltage is the highest and the error and admittance are the smallest, which can most accurately reflect the no-load error of the transformer under test. The test current excitation admittance Y1 and the no-load error ε of the transformer are calculated according to formulas (6) and (7). k :
[0045] Y1=G1+jB1 (Formula 6)
[0046] In the formula, G1 is the conductance of the current transformer under test, and B1 is the susceptance of the current transformer under test.
[0047] ε k =f k +jδ k (Formula 7)
[0048] In the formula, f k δ represents the amplitude error of the tested current transformer at its maximum permeability. k denoted as the phase error of the current transformer under test at maximum permeability; j represents the imaginary number in the complex function and is the imaginary sign;
[0049] The no-load error ε of the test current transformer is calculated by measuring the primary voltage. k And the excitation admittance Y1, take the no-load error ε k The real part f k The amplitude error compensation value is calculated based on the real part G1 of the excitation admittance Y1 according to formula (8):
[0050] Δf=f k +RG1 (Formula 8)
[0051] Furthermore, the device was tested at 1%, 5%, 20%, 100%, and 120% of the rated current corresponding to the rated load and lower limit load, respectively, i.e., at the corresponding secondary induced electromotive force. The admittance Y2 was then measured and the amplitude and phase errors of the tested current transformer at the rated load and lower limit load were calculated according to formula (9):
[0052] ε=-Z 02 Y² + Δf = f + jδ (Formula 9)
[0053] In the formula, f is the amplitude error of the current transformer under rated load and lower limit load, and δ is the phase error of the current transformer under rated load and lower limit load.
[0054] Finally, the control module transmits the error of the tested current transformer to the LCD screen via the RS232 communication module, or the data measured by the control module can be read on the host computer via the USB communication module or the RS232 communication module.
[0055] The beneficial effects of this invention are:
[0056] This invention uses a numerically controlled frequency conversion electronic power source module to replace the standard coil attached to the field verification device for current transformers. During the verification process, the numerically controlled frequency conversion electronic power source module provides the measurement signal, which effectively reduces the weight and size of the instrument and makes it more convenient for field operations.
[0057] The CNC variable frequency electronic power source module uses a high-precision D / A chip to ensure the accuracy of the electronic power source. The feedback adjustment module can effectively solve the problem of unstable output signal caused by temperature drift due to long-term operation of power amplifier modules. The frequency conversion function of the CNC variable frequency electronic power source module can prevent the detection device from being interfered with by fundamental frequency noise in the complex electromagnetic interference of the test site, making the output signal more stable. At the same time, it can also increase the voltage during the test and improve its measurement accuracy.
[0058] This invention employs isolation protection technology. The voltage feedback signal of the tested current transformer is converted into a current signal through a fuse module and then through an electrical signal conversion module. If the voltage is too high, the fuse module will break the circuit to protect the equipment. Then the current signal enters the isolation coil, and the amplitude becomes 1 / 500 of the original signal, which can effectively protect the data acquisition and processing module.
[0059] In this invention, the data acquisition and processing module uses a 24-bit high-precision, programmable, synchronous data acquisition chip to sample the output signal of the current transformer under test. Moreover, the digital filtering adopts a high-frequency digital signal processing chip, which can effectively improve the frequency response, phase response, noise response, bandwidth expansion, etc. caused by the limited sampling rate. Digital filtering of the data stream after digital-to-analog conversion can use either finite impulse response filtering or infinite impulse response filtering to interpolate and sample the data. Compared with analog filtering, point-to-point analysis of the data can be performed, and the obtained data is more accurate, making the measurement results more accurate. Attached Figure Description
[0060] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.
[0061] Figure 1 This is a schematic diagram of the structure of the present invention;
[0062] Figure 2 yes Figure 1 Schematic diagram of the structure of the intermediate isolation protection module;
[0063] Figure 3 yes Figure 1 Schematic diagram of the structure of the CNC variable frequency electronic power source module;
[0064] Figure 4 yes Figure 1 A schematic diagram of the data acquisition module structure. Detailed Implementation
[0065] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.
[0066] Because current field testing devices for current transformers still suffer from drawbacks such as heavy weight, poor repeatability, and weak anti-interference capabilities, in order to solve these problems, based on the current transformer testing regulations, and while ensuring that the original accuracy remains unchanged, the inventors of this application have designed a working mode that does not require a built-in standard transformer and relies on a high-precision signal acquisition and conversion circuit and a digitally controlled frequency conversion electronic source to achieve error measurement of the tested current transformer by building a digital control circuit.
[0067] The digital control circuit constructed by this invention is as follows: Figure 1 As shown, the numerically controlled variable frequency current transformer calibration device includes: LCD screen 1, host computer 2, RS232 communication module 3, USB communication module 4, control module 5, data acquisition module 6, numerically controlled variable frequency electronic power source module 7, current transformer under test 8, and isolation protection module 9.
[0068] exist Figure 1 In this process, the host computer 2 communicates bidirectionally with the control module 5 via the RS232 communication module 3 and the USB communication module 4, sending a verification command to the control module 5 to verify the tested current transformer 8. The LCD screen 1 can be integrated into the host computer 2 or used as a standalone external command input device. If the LCD screen 1 is used as a standalone external command input device, it communicates bidirectionally with the control module 5 via the RS232 communication module 3. When the control module 5 receives an external verification command, it sends a control command to the CNC variable frequency electronic power source module to provide the DC or AC voltage required for the measurement signal, and a control command to the data acquisition module to synchronize sampling. The CNC variable frequency electronic power source module 7 receives the control command from the control module and outputs the secondary side DC or AC voltage required for the test to the current transformer under test. At the same time, it outputs a sine wave signal to the data acquisition module for conversion into a square wave trigger signal. After the voltage on the primary side of the current transformer under test 8 passes through the isolation protection module 9, the voltage on the primary side of the current transformer under test is controlled within the collectable range. Then, the data acquisition module 6 converts the sine wave signal output by the CNC variable frequency electronic power source module into a square wave trigger signal for triggering acquisition, thereby ensuring synchronous sampling of the voltage data on the primary side of the current transformer under test. The data is then uploaded to the control module 5, and the control module 5 displays the data on the LCD screen 1 or the upper device 2 through the RS232 communication module 3 or the USB communication module 4.
[0069] In the aforementioned control structure, this invention, based on the principle of low-voltage extrapolation of current transformers, utilizes a high-precision CNC frequency conversion electronic source module to replace the traditional instrument's built-in standard. To ensure the accuracy of the verification results, this design employs a high-precision AD chip, digital frequency conversion technology, and a zero-crossing sampling algorithm. Simultaneously, to ensure stable transmission between the measured signal and the acquisition circuit and to protect the digital circuit, this design incorporates a fuse module and an isolation protection module. Furthermore, in the aforementioned... Figure 1 In the structure shown, the CNC variable frequency electronic power source module inputs the voltage on the secondary side of the current transformer, and the data acquisition and processing module acquires the output voltage on the primary side of the current transformer. The error of the current transformer under test is obtained through processing and calculation. Furthermore, the CNC variable frequency current transformer calibration device of this invention can also obtain information such as the current transformer ratio, excitation curve, and volt-ampere characteristics. In the entire test process, the control module serves as the main control core, coordinating and controlling the entire workflow and related communications. The test results are transmitted to the host computer for display.
[0070] Furthermore, in the above Figure 1 The structure shown includes a digitally controlled variable frequency electronic power supply module that enables AC / DC conversion and frequency conversion. The maximum AC output voltage is 120V, with a power of 30VA and an AC frequency output of 5Hz to 60Hz, achieving an accuracy of 0.02%. The maximum DC current output is 2A. The entire calibration equipment features fully automatic one-button measurement, is small in size, lightweight, has strong anti-interference capabilities, is portable, and easy to operate.
[0071] The following will be combined with the appendix Figure 2-4 The composition and operation of each module are described in detail.
[0072] like Figure 2 As shown, the isolation protection module 9 includes: a fuse module 9-1, an electrical signal conversion module 9-2, an isolation coil 9-3, and a sampling resistor 9-4.
[0073] Since the secondary winding of the current transformer under test needs to be raised to the maximum permeability during the test, the voltage at this point is equivalent to the voltage under 200% to 3000% of the rated current of the current transformer under test, which poses a certain danger to the data acquisition module. Therefore, it needs to be isolated and protected first.
[0074] Among them: the fuse module is connected to the primary side of the current transformer under test. When the voltage signal of the current transformer under test exceeds the equipment voltage threshold, the fuse module blows the circuit, effectively protecting the subsequent circuits.
[0075] An electrical signal conversion module, connected to the fuse module, is used to convert the primary-side voltage signal into a current signal. For example, Figure 2As shown, the resistors R1, R2, R3, R4, R5, and R6 in the electrical signal conversion module all have a resistance value of 10K.
[0076] An isolation coil, with its primary terminals P1 and P2 connected to an electrical signal conversion module, is used to control the current signal within a set range at a ratio of 500 / 1. The transformation ratio of the isolation coil is 1 / 500.
[0077] A sampling resistor, connected to the secondary terminals S1 and S2 of the isolation coil, is used to convert the current signal into a sampled primary-side voltage signal. This sampled primary-side voltage signal is input to the data acquisition module for processing. The sampling resistor R7 has a resistance of 250Ω.
[0078] like Figure 3 As shown, the numerically controlled variable frequency electronic power source module includes: a voltage stabilizing module 7-1, a D / A conversion module 7-2, a voltage-to-current conversion module 7-3, a phase adjustment module 7-4, a feedback adjustment module 7-5, a power amplification module 7-6, and a first analog filter module 7-7.
[0079] The CNC variable frequency electronic power source module first generates an initial square wave signal and a straight wave signal of a certain frequency based on the relevant parameters of the current transformer under test. The initial square wave signal is then converted into a sine wave signal, which is then amplified and filtered multiple times to ensure that the capacity and accuracy of the electronic power source meet the design requirements.
[0080] The D / A conversion module is connected to the control module. It receives the direct wave signal or initial square wave signal provided by the control module according to the parameters of the current transformer under test. It converts the direct wave signal into an analog direct wave signal as the DC voltage signal required for the test, and converts the initial square wave signal into a sine wave signal as the AC voltage signal required for the test. At the same time, the sine wave signal generated by the D / A conversion module is also input into the data acquisition module.
[0081] A voltage regulator module is connected to the D / A conversion module to provide a reference voltage for the D / A conversion module.
[0082] The first analog filtering module is connected to the D / A conversion module and is used to output analog straight wave signals and sine wave signals after noise removal.
[0083] The phase adjustment module is connected to the first analog filter module and is used to adjust the phase of the sinusoidal signal output by the first analog filter module to keep it consistent with the initial phase.
[0084] The feedback adjustment module, connected to the phase adjustment module, is used to reduce the temperature drift caused by the increase in circuit temperature and maintain the stability of the output signal.
[0085] The power amplifier module, connected to the feedback adjustment module, is used to obtain an AC voltage signal with the required amplitude and frequency for testing.
[0086] The voltage-to-current conversion module is connected to the first analog filter module and is used to receive the analog direct wave signal and output the DC voltage signal required for testing.
[0087] After the initial square wave signal is converted into a sine wave signal, two outputs are generated. One output passes through the first analog filter circuit 7-7, the phase adjustment circuit 7-4, the feedback adjustment circuit 7-5, and the power amplifier circuit 7-6 to obtain an AC voltage signal that meets the amplitude and frequency requirements. The other output enters the data acquisition module.
[0088] After the direct wave signal is converted into an analog direct wave signal, it passes through the first analog filter circuit 7-7 and then enters the voltage-current conversion circuit 7-3 to obtain the DC voltage required for testing.
[0089] In order to ensure sampling accuracy, this invention uses a digitally controlled variable frequency electronic source module. The output signal is not a 50Hz power frequency signal but a 5-60Hz signal. This can prevent the sampling signal from being interfered with by fundamental frequency noise, making the output signal more stable. At the same time, it can also increase the voltage during testing and improve its measurement accuracy.
[0090] exist Figure 3 In this circuit, voltage regulator module 7-1 uses model PW6206, D / A converter module 7-2 uses model PCM1606E2K24, voltage-current converter module 7-3 uses model TIP142, phase adjustment module 7-4 uses model HMC649A, feedback adjustment module 7-5 uses model CA3140, power amplifier module 7-6 uses model SGL160N60UFD, and the first analog filter circuit uses model OPA227 or AD7501.
[0091] like Figure 4 As shown, the data acquisition module includes: a signal amplification module 6-1, a second analog filtering module 6-2, a frequency divider module 6-3, a phase-locked loop module 6-4, a digital-to-analog conversion module 6-5, and a digital filtering module 6-6.
[0092] The data acquisition module of this invention adopts a zero-crossing acquisition method. The sine wave output by the CNC variable frequency electronic source module is divided into two paths: one path serves as the input voltage of the tested current transformer, and the other path serves as a square wave trigger signal to trigger the data acquisition module. The triggering method is based on the rising edge of the square wave trigger signal. After the rising edge trigger, the data acquisition module performs the first acquisition. After the first acquisition, a second acquisition is performed after 1 / 4 cycle of the current frequency. The results of the two acquisitions are processed by the data acquisition module to obtain the ratio difference and phase difference under the current transformation ratio. The algorithm is as follows:
[0093] After sampling at the zero-time and 1 / 4-cycle time of the electron source output, the following equations can be obtained:
[0094]
[0095]
[0096] Where x(t) is the expression for the change of the primary feedback signal of the current transformer with time, A is the amplitude, and w is the angular velocity. Let t be the initial phase and t be the time.
[0097] Since the frequency of the electronic source output signal is known, ω is also known. When acquiring data at the zero-crossing point, it can be assumed that the acquisition occurs at time t1 = 0. The magnitude of the feedback signal from the primary terminal of the current transformer acquired at the zero-crossing point is A1, i.e.
[0098]
[0099] After 1 / 4 of the electronic source output signal period, it can be assumed that the data is acquired at time t2 = π / 2. The magnitude of the acquired feedback signal from the primary terminal of the current transformer is A2.
[0100]
[0101] Solving equations (3) and (4) simultaneously will yield the amplitude A and initial phase of the primary feedback signal of the current transformer. This allows us to determine the amplitude error and phase error of the current transformer under test.
[0102] Specifically, regarding the present invention Figure 4 In the middle, the signal amplification module is connected to the sampling resistor and simultaneously receives the sine wave signal emitted by the digitally controlled frequency conversion electronic source module. It is used to amplify the acquired voltage signal and the sine wave signal and ensure that the signal waveform is not distorted.
[0103] The second analog filtering module is connected to the signal amplification module and is used to remove noise carried during the signal amplification process.
[0104] The frequency divider module, connected to the second analog filter module and the phase-locked loop module, forms a combination to convert the sinusoidal signal input to the signal amplification module into a square wave trigger signal for triggering acquisition.
[0105] The digital-to-analog converter module, connected to the frequency divider module, also receives a square wave trigger signal and voltage data from the current transformer under test. It is used to collect the voltage data of the primary side of the current transformer under test when the square wave trigger signal is triggered.
[0106] A digital filtering module, connected to the digital-to-analog converter module, is used to remove high-order harmonics from the voltage data acquired and converted by the digital-to-analog converter module.
[0107] Furthermore, within the aforementioned data acquisition module, the transmission path sequence of the sinusoidal signal emitted by the CNC variable frequency electronic source module is as follows: signal amplification module 6-1 — second analog filter module 6-2 — frequency divider module 6-3 — phase-locked loop module 6-4 — digital-to-analog converter module 6-5 — digital filter module 6-6. The frequency divider module 6-3 and the phase-locked loop module 6-4 are integrated to generate the square wave trigger signal required by the digital-to-analog converter module 6-5.
[0108] Meanwhile, within the aforementioned data acquisition module, the output path of the voltage signal input by the sampling resistor is: signal amplification module 6-1 — second analog filtering module 6-2 — digital-to-analog conversion module 6-5 — digital filtering module 6-6.
[0109] exist Figure 4 The data acquisition and processing module includes an A / D converter of model TIADS1299, a PLL phase-locked loop circuit of model 74HC4046, a frequency divider circuit of model FS7104, a digital filter circuit of model high-frequency digital signal processing chip DSP TMS320C6000, a second analog filter circuit of model OPA227, AD7501, and a signal amplification circuit of model OP07.
[0110] Furthermore, the frequency conversion of this invention is mainly accomplished by the control module, which employs a field-programmable gate array controller, model FPGAXC7A100TFGG484-2. The chip has a built-in DDSIP core, which consists of three parts: a frequency control word register, a phase accumulator, and a sine / cosine lookup table (S / CoLUT). By controlling the bit width of the phase accumulator and the bit width of the lookup table, sine signals of different frequencies and dynamic ranges can be generated.
[0111] Finally, based on the aforementioned testing device, this invention also provides a testing method for a digitally controlled variable frequency current transformer, comprising the following steps:
[0112] First, input the relevant parameters of the current transformer under test, such as the transformation ratio, rated load, lower limit load and rated power factor, through the LCD screen;
[0113] Then, the control module controls the output of DC current by the CNC variable frequency electronic source module according to the parameters, and measures the DC resistance R of the secondary winding of the current transformer under test.
[0114] Furthermore, the control module controls the CNC variable frequency electronic power source module to output AC voltage to the secondary winding of the current transformer under test according to the parameters, and boosts the voltage to the secondary induced electromotive force corresponding to 5% of the rated current. The primary side voltage is measured to determine whether the turns ratio is correct, and the total impedance Z of the secondary winding of the current transformer under test is obtained according to formula (5). 02 :
[0115] Z 02 =R + Z (Formula 5)
[0116] In the formula, Z is the rated load or lower limit load of the current transformer under test, and R is the DC resistance of the secondary winding of the current transformer under test.
[0117] Next, the voltage is increased to the point of maximum permeability of the secondary winding of the current transformer under test. At this point, the voltage is the highest and the error and admittance are the smallest, which can most accurately reflect the no-load error of the transformer under test. The test current excitation admittance Y1 and the no-load error ε of the transformer are calculated according to formulas (6) and (7). k :
[0118] Y1=G1+jB1 (Formula 6)
[0119] In the formula, G1 is the conductance of the current transformer under test, and B1 is the susceptance of the current transformer under test.
[0120] ε k =f k +jδ k (Formula 7)
[0121] In the formula, f k δ represents the amplitude error of the tested current transformer at its maximum permeability. k denoted as the phase error of the current transformer under test at maximum permeability; j represents the imaginary number in the complex function and is the imaginary sign;
[0122] The no-load error ε of the test current transformer is calculated by measuring the primary voltage. k And the excitation admittance Y1, take the no-load error ε k The real part f kThe amplitude error compensation value is calculated based on the real part G1 of the excitation admittance Y1 according to formula (8):
[0123] Δf=f k +RG1 (Formula 8)
[0124] Furthermore, the device was tested at 1%, 5%, 20%, 100%, and 120% of the rated current corresponding to the rated load and lower limit load, respectively, i.e., at the corresponding secondary induced electromotive force. The admittance Y2 was then measured and the amplitude and phase errors of the tested current transformer at the rated load and lower limit load were calculated according to formula (9):
[0125] ε=-Z 02 Y² + Δf = f + jδ (Formula 9)
[0126] In the formula, f is the amplitude error of the current transformer under rated load and lower limit load, and δ is the phase error of the current transformer under rated load and lower limit load.
[0127] Finally, the control module transmits the error of the tested current transformer to the LCD screen via the RS232 communication module, or the data measured by the control module can be read on the host computer via the USB communication module or the RS232 communication module.
[0128] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.
Claims
1. A digital numerical frequency variable current transformer calibrating device comprising a tested current transformer, characterized in that, It also includes: The control module sends control commands to the digitally controlled frequency converter electronic power source module to provide the DC or AC voltage required for the measurement signal, based on the parameters of the current transformer under test, and sends control commands to the data acquisition module for synchronous sampling. At the same time, it reads the acquired data sent by the data acquisition module, processes it, and obtains the error of the current transformer under test. An isolation protection module, connected to the primary side of the current transformer under test, is used to control the voltage data of the primary side of the current transformer under test within an acquireable range. The isolation protection module includes a fuse module, an electrical signal conversion module, an isolation coil, and a sampling resistor. The fuse module, connected to the primary side of the current transformer under test, protects the entire measurement circuit. The electrical signal conversion module, connected to the fuse module, converts the primary side voltage signal into a current signal. The primary terminal of the isolation coil is connected to the electrical signal conversion module, proportionally controlling the current signal within a set range, and the transformation ratio of the isolation coil is 1 / 500. The sampling resistor, connected to the secondary terminal of the isolation coil, converts the current signal into a sampled primary side voltage signal. The numerically controlled variable frequency electronic power source module receives control commands from the control module and outputs the secondary side DC voltage or AC voltage required for testing to the current transformer under test. The AC voltage output by the numerically controlled variable frequency electronic power source module is a sine wave signal, which is divided into two paths: one path serves as the secondary side input test voltage of the current transformer under test, and the other path is synchronously output to the data acquisition module as a reference signal for generating a square wave trigger signal. The data acquisition module receives control commands from the control module and sinusoidal signals output from the CNC variable frequency electronic source module. It converts these sinusoidal signals into square wave trigger signals to trigger the acquisition, thereby maintaining synchronous sampling of the voltage data on the primary side of the current transformer under test. The acquired voltage data is then uploaded to the control module. The data acquisition module includes: The signal amplification module is connected to the sampling resistor and simultaneously receives the sine wave signal emitted by the digitally controlled frequency conversion electronic source module. It is used to amplify the acquired voltage signal and the sine wave signal and ensure that the signal waveform is not distorted. The second analog filtering module is connected to the signal amplification module and is used to remove noise carried during the signal amplification process; The frequency divider module is connected to the second analog filter module and the phase-locked loop module to form a combination, which is used to convert the sinusoidal signal input to the signal amplification module into a square wave trigger signal for triggering acquisition. The digital-to-analog converter module, connected to the frequency divider module, also receives a square wave trigger signal and voltage data of the current transformer under test, and is used to collect the voltage data of the primary side of the current transformer under test when the square wave trigger signal is triggered. A digital filtering module, connected to the digital-to-analog converter module, is used to remove high-order harmonics from the voltage data acquired and converted by the digital-to-analog converter module; The transmission path sequence of the sinusoidal signal emitted by the digitally controlled frequency conversion electronic source module in the data acquisition module is as follows: signal amplification module, second analog filter module, frequency divider module, phase-locked loop module, digital-to-analog conversion module, in order to generate the square wave trigger signal required by the digital-to-analog conversion module. The transmission path sequence of the primary side voltage signal input by the sampling resistor within the data acquisition module is as follows: signal amplification module, second analog filtering module, digital-to-analog conversion module, and digital filtering module. When the square wave trigger signal is triggered, the voltage data of the primary side of the current transformer under test is collected. Specifically, the time corresponding to the zero-crossing point of the sinusoidal signal output by the digitally controlled frequency converter electronic source module is taken as the first collection time. After the first collection, after 1 / 4 cycle of the current frequency, the second collection time is taken as the second collection time. Since the frequency of the output signal of the digitally controlled frequency converter electronic source module is known and the angular velocity ω is known, the amplitude and initial phase of the feedback signal of the primary end of the current transformer under test are calculated through the two collection results, and the ratio difference and phase difference under the current ratio are obtained. In addition, the numerically controlled variable frequency electronic power source module includes: The D / A conversion module is connected to the control module. It receives the direct wave signal or initial square wave signal provided by the control module according to the parameters of the current transformer under test. It converts the direct wave signal into an analog direct wave signal as the DC voltage signal required for the test, and converts the initial square wave signal into a sine wave signal as the AC voltage signal required for the test. At the same time, the sine wave signal is also input to the data acquisition module. A voltage regulator module is connected to the D / A conversion module to provide a reference voltage for the D / A conversion module; The first analog filtering module is connected to the D / A conversion module and is used to output analog straight wave signals and sine wave signals after removing noise. A phase adjustment module, connected to the first analog filter module, is used to adjust the phase of the sinusoidal signal output by the first analog filter module to keep it consistent with the initial phase. The feedback adjustment module, connected to the phase adjustment module, is used to reduce the temperature drift caused by the increase in circuit temperature and maintain the stability of the output signal. A power amplifier module, connected to the feedback adjustment module, is used to obtain an AC voltage signal with the required amplitude and frequency for testing. The voltage-to-current conversion module is connected to the first analog filter module and is used to receive the analog direct wave signal and output the DC voltage signal required for the test. The frequency of the AC voltage signal output by the numerically controlled variable frequency electronic power source module is 5-60Hz.
2. The digital controlled variable frequency current transformer calibrating device according to claim 1, characterized in that, The device also includes: The host computer has a built-in instruction input module and display screen. It connects to the control module via an RS232 communication module or a USB communication module and sends control commands to the control module. At the same time, it reads the measurement data in the control module and displays the error of the current transformer under test.
3. The method for calibrating a digitally controlled variable frequency current transformer, characterized in that, Includes the following steps: First, input the relevant parameters of the current transformer under test, such as the transformation ratio, rated load, lower limit load and rated power factor, through the LCD screen; Then, the control module controls the output of DC current by the CNC variable frequency electronic source module according to the parameters, and measures the DC resistance R of the secondary winding of the current transformer under test. Furthermore, the control module controls the CNC variable frequency electronic power source module to output AC voltage to the secondary winding of the current transformer under test according to the parameters. The frequency of the AC voltage is 5-60Hz. When the voltage is boosted to the secondary induced electromotive force corresponding to 5% of the rated current, the primary side voltage is measured to determine whether the turns ratio is correct. The total impedance Z of the secondary winding of the current transformer under test is obtained according to formula (5). 02 : Z 02 = R + Z Equation (5) In the formula: Z is the rated load or lower limit load of the current transformer under test, and R is the DC resistance of the secondary winding of the current transformer under test; Next, the voltage is increased to the point of maximum permeability of the secondary winding of the current transformer under test. At this point, the voltage is highest and the error and admittance are smallest, which can most accurately reflect the no-load error of the transformer under test. The test current excitation admittance Y1 and the no-load error ε of the transformer are calculated according to formulas (6) and (7). k : Y1=G1+jB1 formula (6) In the formula, G1 is the conductance of the current transformer under test, and B1 is the susceptance of the current transformer under test. ε k = f k + jδ k Equation (7) where f k is the amplitude error of the subject current transformer at maximum permeability, δ k is the phase error of the subject current transformer at maximum permeability; j represents the imaginary number in complex functions, is the imaginary unit; The no-load error ε of the test current transformer is calculated by measuring the primary voltage. k And the excitation admittance Y1, take the no-load error ε k The real part f k The amplitude error compensation value is calculated based on the real part G1 of the excitation admittance Y1 according to formula (8): Δf = f k + RG1 Equation (8) Furthermore, the device was tested at 1%, 5%, 20%, 100%, and 120% of the rated current corresponding to the rated load and lower limit load, respectively, i.e., at the corresponding secondary induced electromotive force. The admittance Y2 was then tested and the amplitude error and phase error of the tested current transformer at the rated load and lower limit load were calculated according to formula (9): ε = -Z 02 Y2+ Δf = f + jδ Equation (9) Where: f is the amplitude error of the current transformer under rated load and lower limit load, and δ is the phase error of the current transformer under rated load and lower limit load; Among them, a synchronous trigger sampling mechanism is adopted: the sinusoidal signal output by the digitally controlled variable frequency electronic source module is divided into two paths. One path is used as the input test voltage of the secondary side of the current transformer under test, and the other path is synchronously output to the data acquisition module as a reference signal for generating the square wave trigger signal. The data acquisition module converts the sinusoidal signal into a square wave trigger signal for trigger acquisition through the frequency divider module and the phase-locked loop module. The zero-crossing point of the sinusoidal signal output by the CNC variable frequency electronic source module is taken as the first acquisition time. The second acquisition time is taken after 1 / 4 cycle of the current frequency after the first acquisition. Since the output signal frequency of the CNC variable frequency electronic source module and the angular velocity ω are known, the amplitude and initial phase of the feedback signal of the primary terminal of the current transformer under test are calculated by the two acquisition results, and the ratio difference and phase difference under the current ratio are obtained. Finally, the control module transmits the error of the tested current transformer to the LCD screen via the RS232 communication module, or the data measured by the control module can be read on the host computer via the USB communication module or the RS232 communication module.