A device and method for measuring high frequency harmonics in direct current transmission

Abstract

The present invention proposes a device and method for measuring high-frequency harmonics of direct current transmission, which belongs to the technical field of high-frequency harmonic measurement. The measuring device comprises: a digital-to-analog conversion module, a photoelectric conversion module and a main control module; the main control module comprises a first processor and a second processor, the first processor is used to obtain an initial direct current voltage signal and a rated voltage signal, and the second processor is used to perform data calculation on the initial direct current voltage signal and the rated voltage signal obtained by the first processor and obtain a high-frequency harmonic signal. The present invention provides a miniaturized and portable measuring device that can solve sampling errors and realize accurate measurement of high-frequency harmonics.

Description

Technical Field

[0001] The invention belongs to the technical field of high-frequency harmonic measurement, and in particular relates to a device and method for measuring high-frequency harmonics in direct current transmission. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] With the gradual advancement of the construction of new power systems, photovoltaic, wind power and other new energy power generation has entered a period of rapid development. At present, new energy power generation is mainly transmitted in the form of direct current transmission. Due to the particularity of new energy power generation, when direct current is transmitted, a large number of high-frequency signals will appear in the direct current transmission system, which has repeatedly caused damage to primary equipment such as direct current voltage transformers and converter transformers. The mechanism of high-frequency signals has yet to be studied, but the measurement of high-frequency voltage signals is now a must.

[0004] At present, the DC voltage signal of the DC converter station is mainly acquired by the DC electronic voltage transformer, which is mainly composed of a DC voltage divider, a secondary voltage divider, a remote module and a merging unit. The DC voltage divider consists of a high-voltage arm and a low-voltage arm. The DC voltage divider uses a precision resistor voltage divider based on equipotential shielding technology to sense the DC voltage, and uses a parallel capacitor voltage divider to equalize the voltage and ensure the frequency characteristics; the secondary voltage divider converts the low-voltage signal output by the DC voltage divider into multiple signals for processing by multiple remote modules.

[0005] At present, the harmonic measurement of the converter station mainly relies on the DC control device to obtain the digital signal from the DC voltage electronically and then calculate it, or the high-frequency harmonic measurement device is specially configured to obtain the voltage signal from the merging unit of the DC electronic voltage transformer and then calculate the spectrum. However, the output frequency of the existing ultra-high voltage DC electronic voltage transformer merging unit is only 10kHz, and it is impossible to collect the spectrum above 3kHz; at the same time, the cutoff frequency of the low-pass filter of the remote module of each DC voltage transformer manufacturer is inconsistent, and the high-frequency signal is greatly attenuated, and accurate high-frequency measurement cannot be achieved. The method of temporarily installing a DC voltage transformer to output a small analog voltage signal and then installing a high-frequency measurement device for analog quantity acquisition to achieve the DC voltage high-frequency signal measurement has the disadvantages of being very costly, and most converter stations have no place to install it, and cannot be applied on a large scale. Therefore, the existing DC high-frequency harmonic measurement method has the disadvantages of being unable to collect higher spectrum, inaccurate high-frequency measurement caused by inconsistent cutoff frequencies of remote modules and attenuation of high-frequency signals, high cost, and can only measure a certain specific interval voltage, and cannot be portable and used in multiple places, low measurement efficiency, and poor cost performance. Summary of the invention

[0006] To overcome the above-mentioned deficiencies of the prior art, the present invention proposes a device and method for measuring high-frequency harmonics of direct current transmission, which can solve the problem of measurement errors, achieve accurate measurement of high-frequency signals, and provide a miniaturized and portable measurement device.

[0007] To achieve the above objectives, one or more embodiments of the present invention provide the following technical solutions:

[0008] In a first aspect, a device for measuring high-frequency harmonics in direct current transmission is disclosed, comprising:

[0009] Digital-to-analog conversion module, photoelectric conversion module and main control module;

[0010] The input of the digital-to-analog conversion module is a digital signal configured by the host computer, and the output is a rated voltage signal;

[0011] The photoelectric conversion module acquires the optical signal and converts it into an initial DC voltage signal, which is then transmitted to the main control module;

[0012] The main control module includes a first processor and a second processor, the first processor is used to obtain an initial DC voltage signal and a rated voltage signal, and the second processor is used to perform data calculation on the initial DC voltage signal and the rated voltage signal obtained by the first processor and obtain a high-frequency harmonic signal.

[0013] In a second aspect, a method for measuring high-frequency harmonics in direct current transmission is disclosed, comprising:

[0014] Acquire a rated voltage signal and an initial DC voltage signal;

[0015] Reconstructing the sampling time of the initial DC voltage signal to obtain a sampling sequence;

[0016] Based on the sampling sequence and the rated voltage signal, the cutoff frequency is obtained by offline testing, and the high frequency measurement is performed online to obtain the first high frequency signal;

[0017] The first high-frequency signal is subjected to harmonic attenuation compensation according to the cut-off frequency to obtain a high-frequency harmonic signal.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] The method for measuring high-frequency harmonics in direct current transmission proposed by the present invention compensates for each harmonic of the high-frequency harmonics according to the cut-off frequency of the low-pass filter and the attenuation of the frequency domain function to obtain accurate high-frequency components.

[0020] The main lobe spectrum is obtained after DC filtering, and the phase tracking frequency change method is used to achieve accurate calculation of high-frequency signals.

[0021] The closed-loop test method of D / A output and digital input is used to automatically measure the cutoff frequency of the remote module. The automatic test uses the curve approximation method to sweep the frequency, which can not only obtain the accurate cutoff frequency but also reduce the frequency points of the sweep frequency, thereby improving the test efficiency.

[0022] Taking advantage of the characteristics of the dual remote modules asynchronously sampling random crystal oscillators, data reorganization is used to increase the sampling rate of the DC voltage transformer.

[0023] The measuring device proposed in the present invention adopts a high constant temperature crystal oscillator as the main frequency of the entire data system, and then adopts the hardware precise frequency division of FPGA as the sampling clock of the D / A output circuit, and simultaneously obtains the precise time of the digital sampling moment to ensure the clock accuracy of the entire system and reduce the influence of time error on the frequency output accuracy.

[0024] The measuring device is battery powered and has a dual-core CPU to achieve miniaturization and portability.

[0025] Advantages of additional aspects of the present invention will be given in part in the following description, and in part will become obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0027] Figure 1 This is a diagram of the architecture of the device for measuring high-frequency harmonics in direct current transmission described in the first embodiment of the present invention.

[0028] Figure 2 This is a flow chart of the method for measuring high-frequency harmonics in direct current transmission described in Embodiment 2 of the present invention.

[0029] Figure 3 This is a schematic diagram of the principle of the DC voltage transformer described in the second embodiment of the present invention.

[0030] Figure 4 Schematic diagram of the second-order low-pass filter circuit described in the second embodiment of the present invention. DETAILED DESCRIPTION

[0031] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0032] It should be noted that the terms used herein are for describing specific embodiments only and are not intended to be limiting of exemplary embodiments according to the present invention.

[0033] In the absence of conflict, the embodiments of the present invention and the features of the embodiments may be combined with each other.

[0034] Embodiment 1

[0035] In one or more embodiments, a device for measuring high frequency harmonics in direct current transmission is disclosed, such as Figure 1 As shown, specifically including:

[0036] Digital-to-analog conversion module, photoelectric conversion module, main control module, crystal oscillator module, debugging module, host computer and power supply.

[0037] Among them, the input of the digital-to-analog conversion module (D / A conversion module) is a digital signal configured by the host computer, and the output is an analog voltage signal; the digital-to-analog conversion module is connected to the first processor FPGA through an SPI bus. In this embodiment, the D / A conversion module adopts ADI's AD5764CSUZ 4-channel 16-bit DA chip with a signal output range of ±10V. According to the frequency and accuracy curve of the chip, a sampling output frequency of 20kHz is selected, which can meet the accuracy of the output small voltage and reduce the superposition of high-frequency signals caused by discrete errors.

[0038] The photoelectric conversion module is used to receive the optical signals output by the two remote module DC voltage transformers collected by the optical fiber serial port, and the photoelectric conversion module converts the optical signals into electrical signals and transmits them to the main control module;

[0039] The main control module includes a first processor and a second processor. Specifically, the first processor is an FPGA, and the second processor is an ARM core. The FPGA and the ARM core are connected through an on-chip bus. The FPGA is used for data acquisition and data transmission. The data acquisition includes an initial DC voltage signal of the DC voltage. The ARM core is used for data calculation, data generation, human-computer interaction, and program downloading and debugging on the data collected by the FPGA.

[0040] In this embodiment, the main control module adopts Xilinx's ZYNQ 7020 chip, which is a dual-processor architecture of FPGA+ARM. The FPGA has good real-time performance and is responsible for external data collection and data transmission. The ARM core is responsible for data calculation, data generation, human-computer interaction, and program downloading and debugging.

[0041] In this embodiment, the crystal oscillator module uses an oven controlled crystal oscillator (OCXO) as the clock of the entire measuring device, and the frequency accuracy level can reach 5ppm, ensuring the stability of frequency measurement during secondary sampling of high-frequency signals.

[0042] In this embodiment, the host computer is a computer for realizing human-computer interaction, which is connected to the ARM through an off-chip bus and is used to input a rated voltage signal, which is a digital signal of a voltage value; the host computer also includes a liquid crystal display, which is used to output high-frequency harmonic measurement results.

[0043] In this embodiment, the debugging module uses an RJ45 debugging interface, which is connected to a host computer to download program software in the measuring device, read the recorded data stored in the device, and output the measurement results to the host computer for analysis and obtaining a test report.

[0044] The measuring device is powered by a rechargeable battery with a capacity of up to 11200mA. It can be used for 24 hours on a single charge. Combined with a dual-core CPU, the measuring device can be miniaturized and portable.

[0045] Embodiment 2

[0046] In one or more embodiments, a method for measuring high frequency harmonics in direct current transmission is disclosed, such as Figure 2 As shown, the following steps are included:

[0047] Step 1: Obtain an initial DC voltage signal.

[0048] Before the test, ARM obtains the test data through the host computer and passes it to FPGA to control the D / A chip to output the analog voltage signal. The analog voltage signal is a high-frequency small voltage signal. Then, according to the high-frequency small voltage signal, the DC voltage signal and frequency response curve of the remote module of the DC voltage transformer are collected through the optical fiber serial port. The principle of the DC voltage transformer is as follows Figure 3 shown.

[0049] After the test is completed, the two voltage signals of the DC voltage transformer are collected through the dual-fiber serial port to obtain the initial DC voltage signal.

[0050] Step 2: reconstruct the data of the sampling time of the initial DC voltage signal to obtain a reconstructed sampling sequence.

[0051] The sampling characteristics of the random crystal oscillator between the two remote modules are used to reconstruct the data of the two sampling signals. The specific data reconstruction method is as follows:

[0052] Assume that the arrival time of the nth sampling point of remote module 1 is t 1n , the arrival time of the nth sampling point of remote module 2 is t 2n , the crystal oscillator time difference is Δt. As time goes by, when Δt can always be at a certain moment, t 2n Located in 1nWhen the time is between the nth point and the n+1th point, the data of remote module 1 and the data of remote module 2 are combined to form a new k Sampling sequence x k , x k t k The sampling value at a time point. Using Lagrange interpolation, the expression for data reconstruction is:

[0053]

[0054] Where t is the current sampling time; t k is the original sampling time point; t k-1 is the sampling point at the moment before the original sampling moment; t k+1 is the sampling point at the next moment after the original sampling moment; x k is the original sampling sequence; x k-1 is the sampling sequence of the moment before the original sampling moment; x k+1 is the sampling sequence after the original sampling moment; P(x) is the reconstructed sampling sequence.

[0055] The sampling time of the dual remote modules is random, and there is a certain error between their crystal oscillators, so the time when they arrive at the measuring device is inconsistent and their arrival time will also rotate over time. The measuring device is resampled twice through data reconstruction to improve the sampling accuracy.

[0056] Since the output sampling frequency of the remote module of the DC electronic transformer is generally 50kHz, and the high-frequency signal to be measured is generally between 10k and 20kHz, the sampling rate should be at least 4 times the measured spectrum according to the Fourier integral accuracy. Therefore, the sampling characteristics of the random crystal oscillator between the dual remote modules can reconstruct the data of the two sampling signals to ensure the measurement accuracy of the high-frequency signal.

[0057] Step 3: Based on the sampling sequence, the cutoff frequency is obtained by offline testing, and the high-frequency measurement is performed online to obtain the first high-frequency signal.

[0058] Step 301: Offline testing uses a frequency sweep method to obtain the cutoff frequency of a second-order low-pass filter circuit. The structure of the second-order low-pass filter circuit is as follows: Figure 4 As shown;

[0059] The frequency domain function of the second-order low-pass filter circuit is:

[0060]

[0061] in, is the frequency domain function, is the angular frequency, is an imaginary unit, is the resistance value of the second-order low-pass filter circuit, is the capacitance value of the second-order low-pass filter circuit; the cut-off frequency is equal The frequency value at .

[0062] The frequency sweep method is specifically a curve approximation method. The measuring device sends an analog voltage signal at a rated voltage according to the D / A chip, and then collects the digital signal of the remote module to obtain the attenuation value of the frequency point. According to the magnitude of the attenuation value, the output frequency is adjusted according to the frequency domain function. The purpose of the adjustment is to measure the attenuation to The frequency value when When it is lower than , the frequency is increased, and when it is lower than , the frequency is decreased. Two frequency values ​​are preset and then the binary method is gradually approached. , and finally obtain the accurate cutoff frequency.

[0063] Since the sampling rate of the remote module of the DC voltage transformer is 50kHz, its sampling cutoff frequency is generally around 15kHz. However, since there is no precise matching of resistors and capacitors, approximate values ​​are generally used when designing second-order low-pass filters. Therefore, the precise cutoff frequency can be obtained through the sweep frequency method.

[0064] Step 302: Perform high-frequency measurement in an online test and obtain a first high-frequency signal, specifically:

[0065] During online testing, the main component of the signal is the DC component, and the spectrum range of the superimposed signal is not determined. Therefore, the main lobe spectrum is used to perform high-frequency measurement.

[0066] Step 3021, taking data of a data window of 80ms, calculating the DC component by the mean value method and filtering out the DC component, thereby obtaining a waveform that is symmetrical up and down;

[0067] Step 3022, obtain the fundamental frequency by using the zero-crossing moment around zero point, perform Fourier transform on the obtained fundamental frequency, and obtain the accurate frequency by using the fundamental frequency phase tracking method;

[0068] The sinusoidal signal is transformed by the Fourier algorithm. As the data window moves, the corresponding real and imaginary parts obtained by the Fourier algorithm satisfy the identity:

[0069]

[0070] in, f is the true frequency, is the assumed rated frequency, is the frequency change rate, is the imaginary part of the current voltage, is the imaginary part of the voltage calculated last time, is the real part of the current voltage, is the real part of the voltage calculated last time.

[0071] In principle, the Fourier algorithm can accurately calculate the true frequency by adding one more point. It should be noted that if the real parts of two consecutive calculations are close, the frequency value may have a large error. Therefore, according to the geometric theorem, the algorithm is further optimized as follows:

[0072]

[0073] Among them, M is the number of 1 / 8 cycle sampling points, for i The imaginary part of the voltage at the moment, for i-1 The imaginary part of the voltage at the moment, for i The real part of the voltage at the moment, for i-1 The real part of voltage at time instant.

[0074] To further improve the frequency measurement algorithm, the measured frequency is calculated , resample the initial DC voltage signal twice, repeat the above frequency measurement calculation, compare the calculated frequency with the last assumed rated frequency, if the error between the two is less than the ideal error value, it is considered that the calculated frequency is the true frequency value; otherwise, use the current calculated frequency as the assumed rated frequency and repeat the above steps iteratively until the true frequency is obtained.

[0075] According to the calculated true frequency, the initial DC voltage signal is resampled twice and then Fourier transformed, thereby reducing the influence of high-frequency measurement error caused by frequency offset and finally obtaining the baseband signal.

[0076] Because the high-frequency signal of UHV DC exists in the form of 6N times harmonics, after the fundamental frequency is obtained, the first high-frequency signal near the fundamental frequency is calculated based on the fundamental frequency signal on both sides of the fundamental frequency with the f±50*6N spectrum.

[0077] Step 4: Perform harmonic attenuation compensation on the first high-frequency signal according to the cutoff frequency to obtain a high-frequency harmonic signal.

[0078] Since the high-frequency signal is attenuated in the remote module, harmonic compensation is performed according to the frequency domain function of the low-pass filter circuit:

[0079]

[0080] Among them, the cut-off frequency is H(jω) equal to , the solution is ωR1C1=0.375.

[0081] According to the cutoff frequency measured in step 301, the product of R1C1 is calculated, and the value is substituted into the frequency domain function to solve the attenuation value of each spectrum, and the high-frequency harmonic signal is finally obtained by reverse calculation of the high-frequency signal, wherein the reverse calculation is the high-frequency signal of each spectrum divided by the attenuation value, and the high-frequency harmonic signal includes the precise value of each frequency spectrum of the high-frequency signal of the DC system, thereby realizing the precise measurement of high-frequency harmonics in the DC transmission system.

[0082] Step 5: Draw a frequency-amplitude correlation curve based on the amplitude of each spectrum in the high-frequency harmonic signal. The user can see the amplitude of each spectrum more intuitively and complete the high-frequency harmonic measurement of the DC system voltage. At the same time, the spectrum and amplitude of the three largest amplitudes are displayed separately to achieve an alarm effect.

[0083] The description of each embodiment in the above embodiments has different emphases. For parts not described in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0084] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A device for measuring high-frequency harmonics of direct current transmission, characterized in that: include: Digital-to-analog conversion module, photoelectric conversion module and main control module; The input of the digital-to-analog conversion module is a digital signal configured by the host computer, and the output is a rated voltage signal; The photoelectric conversion module acquires the optical signal and converts it into an initial DC voltage signal, which is then transmitted to the main control module; The main control module includes a first processor and a second processor, the first processor is used to obtain an initial DC voltage signal and a rated voltage signal, and the second processor is used to perform data calculation on the initial DC voltage signal and the rated voltage signal obtained by the first processor and obtain a high-frequency harmonic signal, specifically: data is reconstructed at the sampling time of the initial DC voltage signal to obtain a sampling sequence; based on the sampling sequence and the rated voltage signal, an offline test is performed to obtain a cutoff frequency, and an online test is performed to perform high-frequency measurement to obtain a first high-frequency signal; and harmonic attenuation compensation is performed on the first high-frequency signal according to the cutoff frequency to obtain a high-frequency harmonic signal.

2. A device for measuring high-frequency harmonics in direct current transmission according to claim 1, characterized in that: It also includes a crystal oscillator module; The crystal oscillator module uses a high constant temperature crystal oscillator as the clock of the entire measuring device.

3. A device for measuring high-frequency harmonics in direct current transmission according to claim 1, characterized in that: Also includes a power supply; The power supply adopts a rechargeable battery to supply power to the measuring device.

4. A method for measuring high-frequency harmonics in direct current transmission, using the device for measuring high-frequency harmonics in direct current transmission as claimed in any one of claims 1 to 3, characterized in that: include: Acquire a rated voltage signal and an initial DC voltage signal; Reconstructing the sampling time of the initial DC voltage signal to obtain a sampling sequence; Based on the sampling sequence and the rated voltage signal, the cutoff frequency is obtained by offline testing, and the high frequency measurement is performed online to obtain the first high frequency signal; The first high-frequency signal is subjected to harmonic attenuation compensation according to the cut-off frequency to obtain a high-frequency harmonic signal.

5. A method for measuring high-frequency harmonics in direct current transmission as claimed in claim 4, characterized in that: The data reconstruction is specifically as follows: The arrival time of the nth sampling point of remote module 1 is t 1n , the arrival time of the nth sampling point of remote module 2 is t 2n , the crystal oscillator time difference is Δt. When Δt satisfies, t 2n Located in 1n When the time is between the nth point and the n+1th point, the data of remote module 1 and the data of remote module 2 are combined to form a k Sampling sequence x k , x k t k The sampling value at a time point; the expression for data reconstruction is: Where t is the current sampling time; t k is the original sampling time point; t k-1 is the sampling point at the moment before the original sampling moment; t k+1 is the sampling point at the next moment after the original sampling moment; x k is the original sampling sequence; x k-1 is the sampling sequence of the moment before the original sampling moment; x k+1 is the sampling sequence after the original sampling moment; P(x) is the reconstructed sampling sequence.

6. A method for measuring high-frequency harmonics in direct current transmission as claimed in claim 4, characterized in that: The offline test uses a frequency sweep method to obtain the cutoff frequency of the second-order low-pass filter loop.

7. A method for measuring high-frequency harmonics in direct current transmission as claimed in claim 6, characterized in that: The frequency sweeping method is specifically a curve approximation frequency sweeping method. The measuring device collects the digital signal of the remote module according to the rated voltage signal emitted by the digital-to-analog conversion module to obtain the attenuation value, and adjusts its output frequency according to the frequency domain function according to the attenuation value to obtain the precise cutoff frequency.

8. A method for measuring high-frequency harmonics in direct current transmission as claimed in claim 4, characterized in that: The online test for high frequency measurement comprises the following steps: Take the data in the data window within the set time, use the mean method to calculate the DC component and filter out the DC component to obtain a waveform that is symmetrical up and down; The fundamental frequency is obtained by measuring the zero-crossing moment of the waveform around the zero point, and the obtained fundamental frequency is subjected to Fourier transformation, and the first high-frequency signal is obtained according to the fundamental frequency phase tracking method.

9. A method for measuring high-frequency harmonics in direct current transmission according to claim 8, characterized in that: The calculation expression of the fundamental frequency is: in, f is the true frequency, is the assumed rated frequency, M is the number of sampling points for 1 / 8 cycle, for i The imaginary part of the voltage at the moment, for i-1 The imaginary part of the voltage at the moment, for i The real part of the voltage at the moment, for i-1 The real part of voltage at time instant.

10. A method for measuring high-frequency harmonics in direct current transmission according to claim 4, characterized in that: The harmonic attenuation compensation is based on the frequency domain function of the low-pass filter loop: in, is the frequency domain function, is the angular frequency, is an imaginary unit, is the resistance value of the second-order low-pass filter circuit, is the capacitance value of the second-order low-pass filter circuit; the cut-off frequency is equal The frequency value at ; According to the cut-off frequency, the product of R1C1 is calculated and substituted into the frequency domain function to solve the attenuation value of each spectrum, and the high-frequency harmonic signal is obtained by reverse calculation.

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