CN112611914A - Voltage fluctuation analysis method and system for photovoltaic direct current grid-connected point - Google Patents

Voltage fluctuation analysis method and system for photovoltaic direct current grid-connected point Download PDF

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CN112611914A
CN112611914A CN202011292801.1A CN202011292801A CN112611914A CN 112611914 A CN112611914 A CN 112611914A CN 202011292801 A CN202011292801 A CN 202011292801A CN 112611914 A CN112611914 A CN 112611914A
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frequency
voltage waveform
band
connected point
current grid
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董玮
张军军
刘美茵
姚广秀
徐亮辉
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State Grid Corp of China SGCC
State Grid Zhejiang Electric Power Co Ltd
China Electric Power Research Institute Co Ltd CEPRI
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State Grid Corp of China SGCC
State Grid Zhejiang Electric Power Co Ltd
China Electric Power Research Institute Co Ltd CEPRI
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R23/00—Arrangements for measuring frequencies; Arrangements for analysing frequency spectra
    • G01R23/16—Spectrum analysis; Fourier analysis
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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Abstract

The invention relates to the field of photovoltaic direct-current grid-connected power quality, and particularly provides a voltage fluctuation analysis method and system for a photovoltaic direct-current grid-connected point, aiming at solving the technical problem of determining harmonic components in the voltage of the photovoltaic direct-current grid-connected point. The method specifically comprises the following steps: sampling to obtain a voltage waveform of a photovoltaic direct current grid-connected point in a power grid; decomposing the voltage waveform of the photovoltaic direct-current grid-connected point into a low-frequency-band voltage waveform, a medium-frequency-band voltage waveform and a high-frequency-band voltage waveform based on a wavelet transformation method; and carrying out flicker measurement on the low-frequency-band voltage waveform to obtain the instantaneous visual sensitivity corresponding to the photovoltaic direct-current grid-connected point voltage waveform, and respectively analyzing the medium-frequency-band voltage waveform and the high-frequency-band voltage waveform by using an FFT analysis method to obtain a medium-frequency harmonic component and a high-frequency harmonic component corresponding to the photovoltaic direct-current grid-connected point voltage waveform. The technical scheme provided by the invention avoids the phenomenon of inaccurate harmonic analysis caused by mutual influence of different frequency band frequencies in the traditional analysis process, and improves the accuracy of harmonic analysis.

Description

Voltage fluctuation analysis method and system for photovoltaic direct current grid-connected point
Technical Field
The invention relates to the field of photovoltaic direct-current grid-connected power quality, in particular to a voltage fluctuation analysis method and system for a photovoltaic direct-current grid-connected point.
Background
The quality of the direct current power directly influences the control effect of the coordination control method in the direct current system and the control performance of the coordination control equipment.
The method comprises the following steps of defining parameters such as direct current harmonic, flicker, voltage sag and the like in a document 'a direct current microgrid bus voltage fluctuation classification and inhibition method review' published in Wangchengshan, Li micro, Wangsheng, Mengning, Yanglan and the like and a document 'a direct current distribution electric energy quality research review' published in Yao steel, Jifeic, Yinxiang, Zhouyidan, Wanfenghua and the like, and shows that the parameters are key indexes for evaluating the direct current electric energy quality.
Due to solar irradiance fluctuation, on-off of a power electronic switch and introduction of an AC/DC converter, voltage fluctuation of a photovoltaic direct current grid-connected point can be caused, and the influence of the voltage fluctuation on the photovoltaic direct current grid-connected point is different; solar irradiance fluctuation generally enables a photovoltaic direct current grid-connected point to generate power fluctuation, and is mainly reflected in a low-frequency part; the on-off of the power electronic switch can cause the photovoltaic direct-current grid-connected point voltage to generate a switching ripple wave which is mainly reflected in a high-frequency part; the introduction of the AC/DC converter can cause harmonic resonance of a photovoltaic direct current grid-connected point, which is mainly reflected in a medium-frequency part. The reason for voltage fluctuation of the photovoltaic direct-current grid-connected point is difficult to distinguish by adopting the traditional FFT algorithm and a fluctuation analysis method based on the peak value;
in addition, the traditional FFT algorithm and the fluctuation analysis method based on the peak value have the phenomenon that the frequencies of different frequency bands influence each other in the harmonic component extraction process, so that the accuracy of harmonic analysis is insufficient, and the subsequent power quality test and evaluation work is not facilitated to be developed.
Accordingly, there is a need in the art for a new voltage fluctuation analysis scheme for photovoltaic dc grid-connected points to solve the above problems.
Disclosure of Invention
In order to overcome the above-mentioned drawbacks, the present invention is proposed to provide a method and a system for analyzing voltage fluctuation of a photovoltaic dc grid-connected point, which solve or at least partially solve the technical problems of insufficient accuracy of harmonic analysis and unclear reasons for generating voltage fluctuation of the photovoltaic dc grid-connected point.
In a first aspect, a method for analyzing voltage fluctuation of a photovoltaic dc grid-connected point is provided, where the method includes:
sampling to obtain a voltage waveform of a photovoltaic direct current grid-connected point in a power grid;
decomposing the voltage waveform of the photovoltaic direct-current grid-connected point into a low-frequency-band voltage waveform, a medium-frequency-band voltage waveform and a high-frequency-band voltage waveform based on a wavelet transformation method;
and carrying out flicker measurement on the low-frequency-band voltage waveform to obtain the instantaneous visual sensitivity corresponding to the photovoltaic direct-current grid-connected point voltage waveform, and respectively analyzing the medium-frequency-band voltage waveform and the high-frequency-band voltage waveform by using an FFT analysis method to obtain a medium-frequency harmonic component and a high-frequency harmonic component corresponding to the photovoltaic direct-current grid-connected point voltage waveform.
In one technical solution of the voltage fluctuation analysis method for a photovoltaic dc grid-connected point, the decomposing a voltage waveform of the photovoltaic dc grid-connected point into a low-frequency-band voltage waveform, a medium-frequency-band voltage waveform, and a high-frequency-band voltage waveform based on a wavelet transform method includes:
calculating the wavelet decomposition layer number J and the medium-high frequency boundary layer number K based on the sampling frequency of the voltage waveform of the photovoltaic direct-current grid-connected point;
photovoltaic power generation system in power grid by utilizing wavelet decomposition functionPerforming J-layer wavelet decomposition on the voltage waveform of the direct current grid-connected point to obtain a profile coefficient AJAnd a detail coefficient DJ…DK…D1；
For the profile coefficient AJCoefficient of detail DJ…DK-1And a detail coefficient DK…D1Respectively carrying out wavelet reconstruction to obtain low-frequency-band voltage waveforms, medium-frequency-band voltage waveforms and high-frequency-band voltage waveforms corresponding to the voltage waveforms of the photovoltaic direct-current grid-connected points;
wherein the frequency range of the low frequency band is [0, F1) The frequency range of the intermediate frequency band is [ F ]1,F2) The frequency range of the high frequency band is [ F ]2,F3]，fs≥2F3，0＜F1＜F2＜F3，F1Is a predetermined frequency boundary value between the low frequency band and the middle frequency band, F2Is a predetermined frequency boundary value, F, between the middle frequency band and the high frequency band3The frequency range of the voltage waveform of the photovoltaic direct current grid-connected point in the power grid is the preset upper limit value of the frequency of the high frequency band
Figure BDA0002784408680000021
Profile coefficient AJIn a frequency range of
Figure BDA0002784408680000022
Detail coefficient DKIn a frequency range of
Figure BDA0002784408680000023
fsAnd K belongs to (1-J) of the sampling frequency of the voltage waveform of the photovoltaic direct-current grid-connected point in the power grid.
In one technical solution of the voltage fluctuation analysis method for a photovoltaic dc grid-connected point, the calculating a wavelet decomposition layer number J and a medium-high frequency boundary layer number K based on a sampling frequency of a voltage waveform of the photovoltaic dc grid-connected point includes:
the number of wavelet decomposition layers J is calculated as follows:
Figure BDA0002784408680000024
calculating the number K of medium-high frequency boundary layers according to the following formula:
Figure BDA0002784408680000025
in the formula (I), the compound is shown in the specification,
Figure BDA0002784408680000026
in order to round up the symbol,
Figure BDA0002784408680000027
to round the symbol down.
In a technical solution of the voltage fluctuation analysis method for a photovoltaic dc grid-connected point, the obtaining of the instantaneous sensitivity corresponding to the voltage waveform of the photovoltaic dc grid-connected point by performing flicker measurement on the low-frequency-band voltage waveform includes:
substituting the low-frequency voltage waveform into the flicker measuring instrument to obtain the instantaneous visual sensitivity output by the flicker measuring instrument.
In a technical scheme of the voltage fluctuation analysis method of the photovoltaic direct-current grid-connected point, the medium-frequency harmonic component is an n-th harmonic effective value of a voltage waveform of the photovoltaic direct-current grid-connected point;
the high-frequency harmonic component is an m-order harmonic effective value of a voltage waveform of a photovoltaic direct-current grid-connected point;
wherein n is an interval
Figure BDA0002784408680000031
M is an interval
Figure BDA0002784408680000032
Integer value of (1), F1Is a predetermined frequency boundary value between the low frequency band and the middle frequency band, F2Is a predetermined frequency boundary value, F, between the middle frequency band and the high frequency band3Is a predetermined upper limit value of the high band frequency, FZIs the grid reference frequency.
In a technical solution of the voltage fluctuation analysis method for a photovoltaic dc grid-connected point, the analyzing the mid-band voltage waveform and the high-band voltage waveform by using an FFT analysis method respectively to obtain a mid-frequency harmonic component and a high-frequency harmonic component corresponding to the voltage waveform of the photovoltaic dc grid-connected point includes:
determining the effective value H of the nth harmonic wave of the voltage waveform of the photovoltaic direct-current grid-connected point according to the following formulan：
Figure BDA0002784408680000033
In the formula (I), the compound is shown in the specification,
Figure BDA0002784408680000034
the frequency of the frequency spectrum corresponding to the voltage waveform of the intermediate frequency band is Fzn+iFG1Of the spectral line of (1), FG1For the frequency interval of the spectral lines in the spectrum corresponding to the mid-band voltage waveform, i e ∈ [ -k, k]K is a first preset value, and the number of spectral lines contained in the n-th harmonic subset is 2k + 1;
determining the m-order harmonic effective value HH of the voltage waveform of the photovoltaic direct-current grid-connected point according to the following formulam：
Figure BDA0002784408680000035
In the formula, CfFor the amplitude of the line of frequency F in the spectrum corresponding to the high-band voltage waveform, FG2For the frequency interval of each spectral line in the frequency spectrum corresponding to the high-band voltage waveform, f is within the mFz-βFG2,mFz+(β+1)FG2]Beta is a second preset value, and the number of spectral lines included in the m-th harmonic band is 2 beta.
In a second aspect, a voltage fluctuation analysis system of a photovoltaic dc grid-connected point is provided, which includes:
the sampling module is used for sampling to obtain the voltage waveform of a photovoltaic direct-current grid-connected point in a power grid;
the wavelet transformation module is used for decomposing the voltage waveform of the photovoltaic direct-current grid connection point into a low-frequency-band voltage waveform, a medium-frequency-band voltage waveform and a high-frequency-band voltage waveform based on a wavelet transformation method;
and the voltage waveform analysis module is used for carrying out flicker measurement on the low-frequency-band voltage waveform to obtain the instantaneous visual sensitivity corresponding to the photovoltaic direct-current grid-connected point voltage waveform, and respectively analyzing the medium-frequency-band voltage waveform and the high-frequency-band voltage waveform by utilizing an FFT analysis method to obtain a medium-frequency harmonic component and a high-frequency harmonic component corresponding to the photovoltaic direct-current grid-connected point voltage waveform.
In a technical solution of the voltage fluctuation analysis system of the photovoltaic dc grid-connected point, the wavelet transform module includes:
the calculating unit is used for calculating the wavelet decomposition layer number J and the medium-high frequency boundary layer number K based on the sampling frequency of the voltage waveform of the photovoltaic direct-current grid-connected point;
a wavelet decomposition module for performing J-layer wavelet decomposition on the voltage waveform of the photovoltaic direct current grid-connected point in the power grid by using a wavelet decomposition function to obtain a profile coefficient AJAnd a detail coefficient DJ…DK…D1；
Wavelet reconstruction module for the profile coefficient AJCoefficient of detail DJ…DK-1And a detail coefficient DK…D1Respectively carrying out wavelet reconstruction to obtain low-frequency-band voltage waveforms, medium-frequency-band voltage waveforms and high-frequency-band voltage waveforms corresponding to the voltage waveforms of the photovoltaic direct-current grid-connected points;
wherein the frequency range of the low frequency band is [0, F1) The frequency range of the intermediate frequency band is [ F ]1,F2) The frequency range of the high frequency band is [ F ]2,F3]，fs≥2F3，0＜F1＜F2＜F3，F1Is a predetermined frequency boundary value between the low frequency band and the middle frequency band, F2Is a predetermined frequency boundary value, F, between the middle frequency band and the high frequency band3The frequency range of the voltage waveform of the photovoltaic direct current grid-connected point in the power grid is the preset upper limit value of the frequency of the high frequency band
Figure BDA0002784408680000041
Profile coefficient AJIn a frequency range of
Figure BDA0002784408680000042
Detail coefficient DKIn a frequency range of
Figure BDA0002784408680000043
fsAnd K belongs to (1-J) of the sampling frequency of the voltage waveform of the photovoltaic direct-current grid-connected point in the power grid.
In one technical solution of the voltage fluctuation analysis system of the photovoltaic dc grid-connected point, the calculation unit is configured to:
the number of wavelet decomposition layers J is calculated as follows:
Figure BDA0002784408680000044
calculating the number K of medium-high frequency boundary layers according to the following formula:
Figure BDA0002784408680000045
in the formula (I), the compound is shown in the specification,
Figure BDA0002784408680000046
in order to round up the symbol,
Figure BDA0002784408680000047
to round the symbol down.
In one technical solution of the voltage fluctuation analysis system of the photovoltaic dc grid-connected point, the voltage waveform analysis module includes a flicker measurement unit and a harmonic analysis unit, and the flicker measurement unit is configured to:
substituting the low-frequency voltage waveform into the flicker measuring instrument to obtain the instantaneous visual sensitivity output by the flicker measuring instrument.
In a technical solution of the voltage fluctuation analysis system of the photovoltaic dc grid-connected point, the medium frequency harmonic component is an n-th harmonic effective value of a voltage waveform of the photovoltaic dc grid-connected point;
the high-frequency harmonic component is an m-order harmonic effective value of a voltage waveform of a photovoltaic direct-current grid-connected point;
wherein n is an interval
Figure BDA0002784408680000051
M is an interval
Figure BDA0002784408680000052
Integer value of (1), F1Is a predetermined frequency boundary value between the low frequency band and the middle frequency band, F2Is a predetermined frequency boundary value, F, between the middle frequency band and the high frequency band3Is a predetermined upper limit value of the high band frequency, FZIs the grid reference frequency.
In a technical solution of the voltage fluctuation analysis system of the photovoltaic dc grid-connected point, the analyzing the mid-band voltage waveform and the high-band voltage waveform by using an FFT analysis method respectively to obtain the mid-frequency harmonic component and the high-frequency harmonic component corresponding to the voltage waveform of the photovoltaic dc grid-connected point includes:
determining the effective value H of the nth harmonic wave of the voltage waveform of the photovoltaic direct-current grid-connected point according to the following formulan：
Figure BDA0002784408680000053
In the formula (I), the compound is shown in the specification,
Figure BDA0002784408680000054
the frequency of the frequency spectrum corresponding to the voltage waveform of the intermediate frequency band is Fzn+iFG1Of the spectral line of (1), FG1For the frequency interval of the spectral lines in the spectrum corresponding to the mid-band voltage waveform, i e ∈ [ -k, k]K is a first preset value, and the number of spectral lines contained in the n-th harmonic subset is 2k + 1;
determining the m-order harmonic effective value HH of the voltage waveform of the photovoltaic direct-current grid-connected point according to the following formulam：
Figure BDA0002784408680000055
In the formula, CfFor the amplitude of the line of frequency F in the spectrum corresponding to the high-band voltage waveform, FG2For the frequency interval of each spectral line in the frequency spectrum corresponding to the high-band voltage waveform, f is within the mFz-βFG2,mFz+(β+1)FG2]Beta is a second preset value, and the number of spectral lines included in the m-th harmonic band is 2 beta.
One or more technical schemes of the invention at least have the following beneficial effects:
in the technical scheme of the invention, the voltage waveform of a photovoltaic direct current grid-connected point in a power grid is obtained by sampling; decomposing the voltage waveform of the photovoltaic direct-current grid-connected point into a low-frequency-band voltage waveform, a medium-frequency-band voltage waveform and a high-frequency-band voltage waveform based on a wavelet transformation method; and carrying out flicker measurement on the low-frequency-band voltage waveform to obtain the instantaneous visual sensitivity corresponding to the photovoltaic direct-current grid-connected point voltage waveform, and respectively analyzing the medium-frequency-band voltage waveform and the high-frequency-band voltage waveform by using an FFT analysis method to obtain a medium-frequency harmonic component and a high-frequency harmonic component corresponding to the photovoltaic direct-current grid-connected point voltage waveform. The voltage fluctuation of the direct-current grid-connected point is decomposed into voltage waveforms of a high frequency band, a medium frequency band and a low frequency band by utilizing a wavelet decomposition technology, and the voltage waveforms of the three frequency bands are respectively analyzed, so that the phenomenon of inaccurate harmonic analysis caused by mutual influence of different frequency band frequencies in the traditional analysis process is avoided, the accuracy of the harmonic analysis is improved, and the subsequent power quality test and evaluation work is favorably carried out.
In the technical scheme of the invention, the reasons for the voltage fluctuation of the photovoltaic direct-current grid-connected point can be distinguished, and the direct-current power quality change trend caused by different factors can be effectively evaluated.
Drawings
Embodiments of the invention are described below with reference to the accompanying drawings, in which:
FIG. 1 is a flow chart of a voltage fluctuation analysis method of a photovoltaic direct current grid-connected point;
FIG. 2 is a schematic diagram of the process of J-layer wavelet transform in the embodiment of the present invention;
FIG. 3 is a schematic diagram of an analysis of a low band voltage waveform in an embodiment of the invention;
FIG. 4 is a diagram of a spectrum sample obtained by FFT modification of a mid-band voltage waveform according to an embodiment of the present invention;
FIG. 5 is a diagram of spectral samples obtained by FFT modification of a high-band voltage waveform in an embodiment of the present invention;
FIG. 6 is a schematic diagram of a sampled original voltage signal according to an embodiment of the present invention;
FIG. 7 shows a obtained by performing 7-layer wavelet decomposition on an original voltage signal according to an embodiment of the present invention7And D1～D7A schematic diagram of (a);
FIG. 8 shows a pair A in the example of the present invention7、D3～D7And D1～D2Respectively carrying out wavelet reconstruction to obtain a low-frequency-band voltage waveform, a medium-frequency-band voltage waveform and a high-frequency-band voltage waveform schematic diagram;
FIG. 9 shows a low band voltage waveform u according to an embodiment of the present invention1(t) a schematic view of the instantaneous acuity obtained by performing a flicker measurement;
FIG. 10 is a diagram of a mid-band voltage waveform u according to an embodiment of the present invention2(t) obtaining a schematic diagram of the intermediate frequency component by performing FFT variation;
FIG. 11 is a graph of voltage waveform u for a high band in an embodiment of the present invention3(t) performing FFT to obtain a high-frequency component diagram;
fig. 12 is a structural diagram of a voltage fluctuation analysis system of a photovoltaic direct current grid-connected point.
Detailed Description
Some embodiments of the invention are described below with reference to the accompanying drawings. It should be understood by those skilled in the art that these embodiments are only for explaining the technical principle of the present invention, and are not intended to limit the scope of the present invention.
In the description of the present invention, a "module" or "processor" may include hardware, software, or a combination of both. A module may comprise hardware circuitry, various suitable sensors, communication ports, memory, may comprise software components such as program code, or may be a combination of software and hardware. The processor may be a central processing unit, microprocessor, image processor, digital signal processor, or any other suitable processor. The processor has data and/or signal processing functionality. The processor may be implemented in software, hardware, or a combination thereof. Non-transitory computer readable storage media include any suitable medium that can store program code, such as magnetic disks, hard disks, optical disks, flash memory, read-only memory, random-access memory, and the like. The term "a and/or B" denotes all possible combinations of a and B, such as a alone, B alone or a and B. The term "at least one A or B" or "at least one of A and B" means similar to "A and/or B" and may include only A, only B, or both A and B. The singular forms "a", "an" and "the" may include the plural forms as well.
Some terms to which the present invention relates are explained first.
The instantaneous visual sensitivity is described by the subjective visual reflection of the luminance fluctuation caused by the voltage fluctuation to people and starting the change of the instantaneous value of the flicker intensity, and is the result of the action of comprehensive factors such as the frequency, the waveform, the size and the like of the voltage fluctuation.
In the prior art, the traditional FFT algorithm and a fluctuation analysis method based on the peak value are adopted, so that the reason for the voltage fluctuation of a photovoltaic direct-current grid-connected point is difficult to distinguish, and the evaluation of the direct-current power quality change trend caused by different factors is not facilitated; in addition, the traditional FFT algorithm and the fluctuation analysis method based on the peak value have the phenomenon that the frequencies of different frequency bands influence each other in the harmonic component extraction process, so that the accuracy of harmonic analysis is insufficient, and the subsequent power quality test and evaluation work is not facilitated to be developed.
Therefore, the traditional method cannot meet the requirement of accurate analysis;
in the embodiment of the invention, the voltage fluctuation of the photovoltaic direct-current grid-connected point is decomposed into voltage waveforms of a high frequency band, a middle frequency band and a low frequency band by utilizing a wavelet decomposition technology, and the voltage waveforms of the three frequency bands are respectively analyzed, so that the phenomenon of inaccurate harmonic analysis caused by mutual influence of different frequency band frequencies in the traditional analysis process is avoided, the accuracy of the harmonic analysis is improved, the subsequent power quality test and evaluation work is facilitated, meanwhile, the reasons of voltage fluctuation generation of the photovoltaic direct-current grid-connected point can be distinguished, and the direct-current power quality change trend caused by different factors can be effectively evaluated.
Referring to fig. 1, fig. 1 is a schematic flow chart illustrating main steps of a voltage fluctuation analysis method of a photovoltaic direct current grid-connected point according to an embodiment of the present invention. As shown in fig. 1, the method for analyzing voltage fluctuation of a photovoltaic dc grid-connected point in the embodiment of the present invention mainly includes the following steps:
step S101: sampling to obtain a voltage waveform of a photovoltaic direct current grid-connected point in a power grid;
in one embodiment, the voltage waveform of a photovoltaic direct current grid point in a power grid is collected at a sampling rate of 18.2 KHZ.
Step S102: decomposing the voltage waveform of the photovoltaic direct-current grid-connected point into a low-frequency-band voltage waveform, a medium-frequency-band voltage waveform and a high-frequency-band voltage waveform based on a wavelet transformation method;
in the embodiment, the number of wavelet decomposition layers J and the number of middle-high frequency boundary layers K are calculated based on the sampling frequency of the voltage waveform of the photovoltaic direct-current grid-connected point;
performing J-layer wavelet decomposition on voltage waveform of photovoltaic direct-current grid-connected point in power grid by using wavelet decomposition function to obtain profile coefficient AJAnd a detail coefficient DJ…DK…D1；
For the profile coefficient AJCoefficient of detail DJ…DK-1And a detail coefficient DK…D1Respectively carrying out wavelet reconstruction to obtain low-frequency-band voltage waveforms, medium-frequency-band voltage waveforms and high-frequency-band voltage waveforms corresponding to the voltage waveforms of the photovoltaic direct-current grid-connected points;
wherein the frequency range of the low frequency band is [0, F1) The frequency range of the intermediate frequency band is [ F ]1,F2) The frequency range of the high frequency band is [ F ]2,F3]，fs≥2F3，0＜F1＜F2＜F3，F1Is a predetermined frequency boundary value between the low frequency band and the middle frequency band, F2Is a predetermined frequency boundary value, F, between the middle frequency band and the high frequency band3The frequency range of the voltage waveform of the photovoltaic direct current grid-connected point in the power grid is the preset upper limit value of the frequency of the high frequency band
Figure BDA0002784408680000081
Profile coefficient AJIn a frequency range of
Figure BDA0002784408680000082
Detail coefficient DKIn a frequency range of
Figure BDA0002784408680000083
fsAnd K belongs to (1-J) of the sampling frequency of the voltage waveform of the photovoltaic direct-current grid-connected point in the power grid.
In this embodiment, the number of wavelet decomposition layers J is calculated as follows:
Figure BDA0002784408680000084
calculating the number K of medium-high frequency boundary layers according to the following formula:
Figure BDA0002784408680000085
in the formula (I), the compound is shown in the specification,
Figure BDA0002784408680000086
in order to round up the symbol,
Figure BDA0002784408680000087
to round the symbol down.
Referring to FIG. 2, the voltage waveform of the photovoltaic DC grid-connected point in the power grid has a frequency range of
Figure BDA0002784408680000088
The voltage waveform is subjected to first-layer wavelet decomposition to obtain a corresponding profile coefficient A1And a detail coefficient D1Coefficient of profile A1And a detail coefficient D1Respectively in the frequency range of
Figure BDA0002784408680000089
And
Figure BDA00027844086800000810
for the profile coefficient A1Performing a second-layer wavelet decomposition to obtain a corresponding profile coefficient A2And a detail coefficient D2Coefficient of profile A2And a detail coefficient D2Respectively in the frequency range of
Figure BDA00027844086800000811
And
Figure BDA00027844086800000812
recursion in turn, to the profile coefficient AJ-1Performing wavelet decomposition of the J-th layer to obtain corresponding profile coefficient AJAnd a detail coefficient DJCoefficient of profile AJAnd a detail coefficient DJRespectively in the frequency range of
Figure BDA00027844086800000813
And
Figure BDA00027844086800000814
finally obtaining a profile coefficient A after J-layer wavelet decompositionJAnd a detail coefficient DJ…DK…D1。
In one embodiment, the low band has a frequency range of [0,50Hz ] for the low band, a frequency range of [50Hz,2.1kHz ] for the middle band, and a frequency range of [2.1kHz,8.9kHz ] for the high band]50Hz is the frequency boundary value between the preset low frequency band and the middle frequency band, 2.1kHz is the frequency boundary value between the preset middle frequency band and the preset high frequency band, and 8.9kHz is the upper limit value of the preset high frequency band frequencyAccording to Shannon's sampling theorem, in order to satisfy the analysis requirement of high frequency covering 8.9kHz, the sampling rate fsValues greater than 18kHz were used.
In order to meet the requirement that the upper limit value of the frequency band of the low-frequency band voltage waveform is less than 50Hz, the decomposition layer number of the wavelet decomposition is set, and the wavelet decomposition layer number J is determined by the following formula:
Figure BDA0002784408680000091
in order to meet the requirement that the lower limit value of the frequency band of the high-frequency band voltage waveform is larger than 2.1kHz, the number of middle and high frequency boundary layers of wavelet decomposition is set, and the number K of the high frequency boundary layers is determined by the following formula:
Figure BDA0002784408680000092
in the formula (I), the compound is shown in the specification,
Figure BDA0002784408680000093
in order to round up the symbol,
Figure BDA0002784408680000094
to round the symbol down.
Step S103: and carrying out flicker measurement on the low-frequency-band voltage waveform to obtain the instantaneous visual sensitivity corresponding to the photovoltaic direct-current grid-connected point voltage waveform, and respectively analyzing the medium-frequency-band voltage waveform and the high-frequency-band voltage waveform by using an FFT analysis method to obtain a medium-frequency harmonic component and a high-frequency harmonic component corresponding to the photovoltaic direct-current grid-connected point voltage waveform.
In this embodiment, the low-frequency voltage waveform is substituted into the flicker meter to obtain the instantaneous visual sensitivity of the output of the flicker meter.
In one embodiment, the low-frequency voltage waveform is substituted into the IEC flicker meter to obtain the instantaneous visual sensitivity s (t) output by the IEC flicker meter, and the corresponding process is shown in fig. 3.
In this embodiment, the medium-frequency harmonic component is an n-th harmonic effective value of a voltage waveform of a photovoltaic direct-current grid-connected point;
the high-frequency harmonic component is an m-order harmonic effective value of a voltage waveform of a photovoltaic direct-current grid-connected point;
wherein n is an interval
Figure BDA0002784408680000095
M is an interval
Figure BDA0002784408680000096
Integer value of (1), F1Is a predetermined frequency boundary value between the low frequency band and the middle frequency band, F2Is a predetermined frequency boundary value, F, between the middle frequency band and the high frequency band3Is a predetermined upper limit value of the high band frequency, FZIs the grid reference frequency.
In one embodiment, if F1＝FZ＝50，F＝2100，F3When 8900, n is 1,2, … 40; m is 42,43, … 178.
In the embodiment, the effective value H of the nth harmonic wave of the voltage waveform of the photovoltaic direct current grid-connected point is determined according to the following formulan：
Figure BDA0002784408680000101
In the formula (I), the compound is shown in the specification,
Figure BDA0002784408680000102
the frequency of the frequency spectrum corresponding to the voltage waveform of the intermediate frequency band is Fzn+iFG1Of the spectral line of (1), FG1For the frequency interval of the spectral lines in the spectrum corresponding to the mid-band voltage waveform, i e ∈ [ -k, k]K is a first preset value, and the number of spectral lines contained in the n-th harmonic subset is 2k + 1;
determining the m-order harmonic effective value HH of the voltage waveform of the photovoltaic direct-current grid-connected point according to the following formulam：
Figure BDA0002784408680000103
In the formula, CfFor the amplitude of the line of frequency F in the spectrum corresponding to the high-band voltage waveform, FG2For the frequency interval of each spectral line in the frequency spectrum corresponding to the high-band voltage waveform, f is within the mFz-βFG2,mFz+(β+1)FG2]Beta is a second preset value, and the number of spectral lines included in the m-th harmonic band is 2 beta.
In one embodiment, the mid-band voltage waveform is subjected to FFT variation, the window width of the FFT is 200ms, Hanning window weighting is carried out, and after decomposition, a harmonic spectrum with the interval of 5Hz, namely F, is obtainedG1Referring to fig. 4;
let k take the value of 1, the number of spectral lines contained in the n-th harmonic subset is 3; counting the harmonic subgroups by integral multiples of 50HZ to obtain the nth harmonic effective value H of the voltage waveform of the photovoltaic direct-current grid-connected pointn：
Figure BDA0002784408680000104
FFT is carried out on the high-frequency band voltage waveform, the window width of the FFT is 100ms and is provided with Hanning window weighting, and after decomposition, a harmonic frequency spectrum with the interval of 10Hz, namely F is obtainedG2Referring to fig. 5;
let β be 10, the number of spectral lines included in the m-th harmonic frequency band be 20, and for simplification, the harmonic subgroups are counted by integer multiples of 200HZ, that is, only the harmonic effective values of m 42,46 and 50 … 178 are calculated, and the m-th harmonic effective value HH of the voltage waveform of the photovoltaic direct-current grid-connected point is obtainedm：
Figure BDA0002784408680000105
In the embodiment of the invention, the photovoltaic DC/DC grid-connected device starts to be connected to the grid from the time 0 and enters a steady state, and the voltage of a grid-connected point is 750V. The irradiance is suddenly increased in 1 second, 2 seconds and 3 seconds, the voltage of a grid connection point is suddenly changed at the corresponding moment, and the maximum voltage peak reaches 880V. After the voltage peak is passed, the direct current distribution network is quickly recovered to be stable. As the irradiance increases, the ripple of the dc grid voltage increases, see fig. 6.
In the embodiment, a 0.5-3.5 second time period is selected as an analysis object, the sampling rate of the original voltage waveform is 20k, the frequency range of the low frequency band is the frequency range of the low frequency band [0,39.06Hz ], the frequency range of the middle frequency band is the frequency range of the middle frequency band [39.06Hz,2.5kHz ], the frequency range of the high frequency band is the frequency range of the high frequency band [2.5kHz,10kHz ], 39.06Hz is a preset frequency boundary value between the low frequency band and the middle frequency band, 2.5kHz is a preset frequency boundary value between the middle frequency band and the high frequency band, 10kHz is a preset upper limit value of the high frequency band frequency, the number of wavelet decomposition layers J is calculated to be 7, the number of middle and high frequency boundaries is calculated to be 2, J-layer wavelet decomposition is performed on the original voltage waveform by adopting a Mallat;
decomposing the original voltage waveform into 3 frequency bands after wavelet decomposition, wherein A7 corresponds to a low frequency band, D3-D7 corresponds to a medium frequency band, and D1-D2 correspond to a high frequency band, which is specifically shown in Table 1;
TABLE 1
Corresponding wavelet coefficient
Astable (Low frequency part) 0～39.06Hz A7
Steady state (intermediate frequency part) 39.06Hz～2.5k D3～D7
Steady state (high frequency part) 2.5k～10kHz D1～D2
Respectively carrying out wavelet reconstruction on A7, D3-D7 and D1-D2 to obtain a low-frequency-band voltage waveform u of the original voltage waveform1(t) a mid-band voltage waveform u2(t) and a high band voltage waveform u3(t), see fig. 8;
will u1(t) inputting the IEC flicker meter to obtain the instantaneous visual sensitivity output by the IEC flicker meter, and referring to FIG. 9 specifically; it can be seen from fig. 9 that after the photovoltaic DC/DC grid connection enters the steady state operation, the instantaneous visibility is 0, which indicates that human eyes cannot detect the intensity change of illumination, and after the power rises for 1 second to cause the voltage impact, the instantaneous visibility rapidly rises to reach a maximum value of 254, and then gradually returns to 12, and the change process of the instantaneous visibility caused by the two tests is similar. It can be seen that the instantaneous visual sensitivity makes an effective analysis of irradiance fluctuations.
Using FFT variation method to u2(t) analysis was performed to obtain the corresponding harmonic analysis results, see FIG. 10.
It can be seen from fig. 10 that the maximum harmonic frequency of the intermediate frequency band is 100Hz, and the maximum harmonic amplitude reaches 2.1V at the time of 2.4 seconds, mainly due to the intermediate frequency component introduced by the dead zone of the switch of the AC/DC converter.
Using FFT variation method to u3(t) analysis was performed to obtain the corresponding harmonic analysis results, see FIG. 11.
As shown in fig. 11, the maximum harmonic frequency of the high frequency band is 4.9kHz, and the generation factor is to introduce a switching frequency of 5kHz according to detailed analysis, and as the illumination amplitude increases, the harmonic content in the dc side current also increases.
It should be noted that, although the foregoing embodiments describe each step in a specific sequence, those skilled in the art will understand that, in order to achieve the effect of the present invention, different steps do not necessarily need to be executed in such a sequence, and they may be executed simultaneously (in parallel) or in other sequences, and these changes are all within the protection scope of the present invention.
Furthermore, the invention also provides a voltage fluctuation analysis system of the photovoltaic direct current grid-connected point.
Referring to fig. 12, fig. 12 is a main structural block diagram of a voltage fluctuation analysis system of a photovoltaic dc grid-connected point according to an embodiment of the present invention. As shown in fig. 12, the voltage fluctuation analysis system of the photovoltaic dc grid-connected point in the embodiment of the present invention mainly includes a sampling module, a wavelet transform module, and a voltage waveform analysis module.
In some embodiments, one or more of the sampling module, the wavelet transform module, and the voltage waveform analysis module may be combined together into one module. For simplicity, although a processor and memory are not shown in fig. 12, one skilled in the art will appreciate that the voltage fluctuation analysis system of the photovoltaic dc grid-connected point may be part of the processor and/or memory. For example, in some embodiments, one or more of the sampling module, the wavelet transform module, and the voltage waveform analysis module may be part of a processor. In some embodiments, these means may correspond to a portion of the electronic circuitry in the processor performing the signal or data processing, respectively, or may correspond to associated program code stored in a computer readable medium (e.g., a memory). In some embodiments, the sampling module, the wavelet transform module, and the voltage waveform analysis module may also not be part of the current processor, but part of another processor in addition to the current processor. In some embodiments, one or more of the sampling module, the wavelet transform module, and the voltage waveform analysis module may be combined together into one device module.
Specifically, the sampling module may be configured to sample a voltage waveform of a photovoltaic dc grid-connected point in the power grid; in one embodiment, the description of the specific implementation function may be referred to in step S101.
The wavelet transform module may be configured to decompose the photovoltaic direct current grid-connected point voltage waveform into a low-frequency band voltage waveform, a mid-frequency band voltage waveform, and a high-frequency band voltage waveform based on a wavelet transform method; in one embodiment, the description of the specific implementation function may be referred to in step S102.
Specifically, the wavelet transform module may include a calculation unit, a wavelet decomposition module, and a wavelet reconstruction module; the calculation unit may be configured to calculate the number of wavelet decomposition layers J and the number of middle and high frequency boundary layers K based on the sampling frequency of the photovoltaic direct current grid-connected point voltage waveform, and the calculation unit may be configured to perform the following processes:
the number of wavelet decomposition layers J is calculated as follows:
Figure BDA0002784408680000121
calculating the number K of medium-high frequency boundary layers according to the following formula:
Figure BDA0002784408680000122
in the formula (I), the compound is shown in the specification,
Figure BDA0002784408680000123
in order to round up the symbol,
Figure BDA0002784408680000124
to round the symbol down.
The wavelet decomposition module can be configured to perform J-layer wavelet decomposition on the voltage waveform of the photovoltaic direct-current grid-connected point in the power grid by using a wavelet decomposition function to obtain a profile coefficient AJAnd a detail coefficient DJ…DK…D1；
The wavelet decomposition module may be configured to pair profile coefficients aJCoefficient of detail DJ…DK-1And a detail coefficient DK…D1The wavelet reconstruction is carried out respectively, and the wavelet reconstruction is carried out,obtaining low-frequency-band voltage waveforms, medium-frequency-band voltage waveforms and high-frequency-band voltage waveforms corresponding to the voltage waveforms of the photovoltaic direct-current grid-connected points;
wherein the frequency range of the low frequency band is [0, F1) The frequency range of the intermediate frequency band is [ F ]1,F2) The frequency range of the high frequency band is [ F ]2,F3]，fs≥2F3，0＜F1＜F2＜F3，F1Is a predetermined frequency boundary value between the low frequency band and the middle frequency band, F2Is a predetermined frequency boundary value, F, between the middle frequency band and the high frequency band3The frequency range of the voltage waveform of the photovoltaic direct current grid-connected point in the power grid is the preset upper limit value of the frequency of the high frequency band
Figure BDA0002784408680000131
Profile coefficient AJIn a frequency range of
Figure BDA0002784408680000132
Detail coefficient DKIn a frequency range of
Figure BDA0002784408680000133
fsAnd K belongs to (1-J) of the sampling frequency of the voltage waveform of the photovoltaic direct-current grid-connected point in the power grid.
The voltage waveform analysis module can be configured to carry out flicker measurement on the low-frequency-band voltage waveform to obtain instantaneous visual sensitivity corresponding to the photovoltaic direct-current grid-connected point voltage waveform, and respectively analyze the medium-frequency-band voltage waveform and the high-frequency-band voltage waveform by utilizing an FFT analysis method to obtain a medium-frequency harmonic component and a high-frequency harmonic component corresponding to the photovoltaic direct-current grid-connected point voltage waveform; in one embodiment, the description of the specific implementation function may be referred to in step S103.
The specific execution process comprises the following steps: substituting the low-frequency voltage waveform into the flicker measuring instrument to obtain the instantaneous visual sensitivity output by the flicker measuring instrument.
In one embodiment, the medium-frequency harmonic component is an nth harmonic effective value of a voltage waveform of a photovoltaic direct-current grid-connected point;
the high-frequency harmonic component is an m-order harmonic effective value of a voltage waveform of a photovoltaic direct-current grid-connected point;
wherein n is an interval
Figure BDA0002784408680000134
M is an interval
Figure BDA0002784408680000135
Integer value of (1), F1Is a predetermined frequency boundary value between the low frequency band and the middle frequency band, F2Is a predetermined frequency boundary value, F, between the middle frequency band and the high frequency band3Is a predetermined upper limit value of the high band frequency, FZIs the grid reference frequency.
In an embodiment, the analyzing the mid-band voltage waveform and the high-band voltage waveform by using an FFT analysis method to obtain a mid-frequency harmonic component and a high-frequency harmonic component corresponding to the voltage waveform of the photovoltaic dc grid-connected point includes:
determining the effective value H of the nth harmonic wave of the voltage waveform of the photovoltaic direct-current grid-connected point according to the following formulan：
Figure BDA0002784408680000136
In the formula (I), the compound is shown in the specification,
Figure BDA0002784408680000141
the frequency of the frequency spectrum corresponding to the voltage waveform of the intermediate frequency band is Fzn+iFG1Of the spectral line of (1), FG1For the frequency interval of the spectral lines in the spectrum corresponding to the mid-band voltage waveform, i e ∈ [ -k, k]K is a first preset value, and the number of spectral lines contained in the n-th harmonic subset is 2k + 1;
determining the m-order harmonic effective value HH of the voltage waveform of the photovoltaic direct-current grid-connected point according to the following formulam：
Figure BDA0002784408680000142
In the formula, CfFor the amplitude of the line of frequency F in the spectrum corresponding to the high-band voltage waveform, FG2For the frequency interval of each spectral line in the frequency spectrum corresponding to the high-band voltage waveform, f is within the mFz-βFG2,mFz+(β+1)FG2]Beta is a second preset value, and the number of spectral lines included in the m-th harmonic band is 2 beta.
The voltage fluctuation analysis system of the photovoltaic dc grid-connected point is used for executing the embodiment of the voltage fluctuation analysis method of the photovoltaic dc grid-connected point shown in fig. 1, and the technical principles, the solved technical problems, and the generated technical effects of the two are similar.
It will be understood by those skilled in the art that all or part of the flow of the method according to the above-described embodiment may be implemented by a computer program, which may be stored in a computer-readable storage medium and used to implement the steps of the above-described embodiments of the method when the computer program is executed by a processor. Wherein the computer program comprises computer program code, which may be in the form of source code, object code, an executable file or some intermediate form, etc. The computer-readable medium may include: any entity or device capable of carrying said computer program code, media, usb disk, removable hard disk, magnetic diskette, optical disk, computer memory, read-only memory, random access memory, electrical carrier wave signals, telecommunication signals, software distribution media, etc. It should be noted that the computer readable medium may contain content that is subject to appropriate increase or decrease as required by legislation and patent practice in jurisdictions, for example, in some jurisdictions, computer readable media does not include electrical carrier signals and telecommunications signals as is required by legislation and patent practice.
Further, it should be understood that, since the modules are only configured to illustrate the functional units of the system of the present invention, the corresponding physical devices of the modules may be the processor itself, or a part of software, a part of hardware, or a part of a combination of software and hardware in the processor. Thus, the number of individual modules in the figures is merely illustrative.
Those skilled in the art will appreciate that the various modules in the system may be adaptively split or combined. Such splitting or combining of specific modules does not cause the technical solutions to deviate from the principle of the present invention, and therefore, the technical solutions after splitting or combining will fall within the protection scope of the present invention.
So far, the technical solution of the present invention has been described with reference to one embodiment shown in the drawings, but it is easily understood by those skilled in the art that the scope of the present invention is obviously not limited to these specific embodiments. Equivalent changes or substitutions of related technical features can be made by those skilled in the art without departing from the principle of the invention, and the technical scheme after the changes or substitutions can fall into the protection scope of the invention.

Claims (10)

1. A voltage fluctuation analysis method of a photovoltaic direct current grid-connected point is characterized by comprising the following steps:
sampling to obtain a voltage waveform of a photovoltaic direct current grid-connected point in a power grid;
decomposing the voltage waveform of the photovoltaic direct-current grid-connected point into a low-frequency-band voltage waveform, a medium-frequency-band voltage waveform and a high-frequency-band voltage waveform based on a wavelet transformation method;
and carrying out flicker measurement on the low-frequency-band voltage waveform to obtain the instantaneous visual sensitivity corresponding to the photovoltaic direct-current grid-connected point voltage waveform, and respectively analyzing the medium-frequency-band voltage waveform and the high-frequency-band voltage waveform by using an FFT analysis method to obtain a medium-frequency harmonic component and a high-frequency harmonic component corresponding to the photovoltaic direct-current grid-connected point voltage waveform.
2. The method for analyzing voltage fluctuation of a photovoltaic dc grid-connected point according to claim 1, wherein the decomposing of the photovoltaic dc grid-connected point voltage waveform into a low-band voltage waveform, a medium-band voltage waveform, and a high-band voltage waveform based on the wavelet transform method comprises:
calculating the wavelet decomposition layer number J and the medium-high frequency boundary layer number K based on the sampling frequency of the voltage waveform of the photovoltaic direct-current grid-connected point;
performing J-layer wavelet decomposition on voltage waveform of photovoltaic direct-current grid-connected point in power grid by using wavelet decomposition function to obtain profile coefficient AJAnd a detail coefficient DJ…DK…D1；
For the profile coefficient AJCoefficient of detail DJ…DK-1And a detail coefficient DK…D1Respectively carrying out wavelet reconstruction to obtain low-frequency-band voltage waveforms, medium-frequency-band voltage waveforms and high-frequency-band voltage waveforms corresponding to the voltage waveforms of the photovoltaic direct-current grid-connected points;
wherein the frequency range of the low frequency band is [0, F1) The frequency range of the intermediate frequency band is [ F ]1,F2) The frequency range of the high frequency band is [ F ]2,F3]，fs≥2F3，0＜F1＜F2＜F3，F1Is a predetermined frequency boundary value between the low frequency band and the middle frequency band, F2Is a predetermined frequency boundary value, F, between the middle frequency band and the high frequency band3The frequency range of the voltage waveform of the photovoltaic direct current grid-connected point in the power grid is the preset upper limit value of the frequency of the high frequency band
Figure FDA0002784408670000011
Profile coefficient AJIn a frequency range of
Figure FDA0002784408670000012
Detail coefficient DKIn a frequency range of
Figure FDA0002784408670000013
fsFor photovoltaic dc grid voltage waveforms in an electric networkThe sampling frequency, K ∈ (1 to J).
3. The method for analyzing voltage fluctuation of a photovoltaic direct-current grid-connected point as claimed in claim 2, wherein the calculating of the number of wavelet decomposition layers J and the number of middle-high frequency boundary layers K based on the sampling frequency of the voltage waveform of the photovoltaic direct-current grid-connected point comprises:
the number of wavelet decomposition layers J is calculated as follows:
Figure FDA0002784408670000014
calculating the number K of medium-high frequency boundary layers according to the following formula:
Figure FDA0002784408670000015
in the formula (I), the compound is shown in the specification,
Figure FDA0002784408670000021
in order to round up the symbol,
Figure FDA0002784408670000022
to round the symbol down.
4. The method for analyzing voltage fluctuation of a photovoltaic direct-current grid-connected point according to claim 1, wherein the flicker measurement of the low-frequency-band voltage waveform to obtain an instantaneous visual sensitivity corresponding to the photovoltaic direct-current grid-connected point voltage waveform comprises:
substituting the low-frequency voltage waveform into the flicker measuring instrument to obtain the instantaneous visual sensitivity output by the flicker measuring instrument.
5. The method according to claim 1, wherein the medium frequency harmonic component is an nth harmonic effective value of a voltage waveform of the photovoltaic dc grid-connected point;
the high-frequency harmonic component is an m-order harmonic effective value of a voltage waveform of a photovoltaic direct-current grid-connected point;
wherein n is an interval
Figure FDA0002784408670000023
M is an interval
Figure FDA0002784408670000024
Integer value of (1), F1Is a predetermined frequency boundary value between the low frequency band and the middle frequency band, F2Is a predetermined frequency boundary value, F, between the middle frequency band and the high frequency band3Is a predetermined upper limit value of the high band frequency, FZIs the grid reference frequency.
6. The method for analyzing voltage fluctuation of a photovoltaic direct-current grid connection point according to claim 5, wherein the step of analyzing the medium-frequency band voltage waveform and the high-frequency band voltage waveform respectively by using an FFT analysis method to obtain a medium-frequency harmonic component and a high-frequency harmonic component corresponding to the voltage waveform of the photovoltaic direct-current grid connection point comprises the steps of:
determining the effective value H of the nth harmonic wave of the voltage waveform of the photovoltaic direct-current grid-connected point according to the following formulan：
Figure FDA0002784408670000025
In the formula (I), the compound is shown in the specification,
Figure FDA0002784408670000026
the frequency of the frequency spectrum corresponding to the voltage waveform of the intermediate frequency band is Fzn+iFG1Of the spectral line of (1), FG1For the frequency interval of the spectral lines in the spectrum corresponding to the mid-band voltage waveform, i e ∈ [ -k, k]K is a first preset value, and the number of spectral lines contained in the n-th harmonic subset is 2k + 1;
determining the m-order harmonic effective value HH of the voltage waveform of the photovoltaic direct-current grid-connected point according to the following formulam：
Figure FDA0002784408670000027
In the formula, CfFor the amplitude of the line of frequency F in the spectrum corresponding to the high-band voltage waveform, FG2For the frequency interval of each spectral line in the frequency spectrum corresponding to the high-band voltage waveform, f is within the mFz-βFG2,mFz+(β+1)FG2]Beta is a second preset value, and the number of spectral lines included in the m-th harmonic band is 2 beta.
7. A voltage fluctuation analysis system of a photovoltaic direct current grid-connected point is characterized by comprising:
the sampling module is used for sampling to obtain the voltage waveform of a photovoltaic direct-current grid-connected point in a power grid;
the wavelet transformation module is used for decomposing the voltage waveform of the photovoltaic direct-current grid connection point into a low-frequency-band voltage waveform, a medium-frequency-band voltage waveform and a high-frequency-band voltage waveform based on a wavelet transformation method;
and the voltage waveform analysis module is used for carrying out flicker measurement on the low-frequency-band voltage waveform to obtain the instantaneous visual sensitivity corresponding to the photovoltaic direct-current grid-connected point voltage waveform, and respectively analyzing the medium-frequency-band voltage waveform and the high-frequency-band voltage waveform by utilizing an FFT analysis method to obtain a medium-frequency harmonic component and a high-frequency harmonic component corresponding to the photovoltaic direct-current grid-connected point voltage waveform.
8. The system for analyzing voltage fluctuation of a photovoltaic direct current grid-connected point according to claim 7, wherein the wavelet transform module comprises:
the calculating unit is used for calculating the wavelet decomposition layer number J and the medium-high frequency boundary layer number K based on the sampling frequency of the voltage waveform of the photovoltaic direct-current grid-connected point;
a wavelet decomposition module for performing J-layer wavelet decomposition on the voltage waveform of the photovoltaic direct current grid-connected point in the power grid by using a wavelet decomposition function to obtain a profile coefficient AJAnd a detail coefficient DJ…DK…D1；
Wavelet reconstruction module for the profile coefficient AJCoefficient of detail DJ…DK-1And a detail coefficient DK…D1Respectively carrying out wavelet reconstruction to obtain low-frequency-band voltage waveforms, medium-frequency-band voltage waveforms and high-frequency-band voltage waveforms corresponding to the voltage waveforms of the photovoltaic direct-current grid-connected points;
wherein the frequency range of the low frequency band is [0, F1) The frequency range of the intermediate frequency band is [ F ]1,F2) The frequency range of the high frequency band is [ F ]2,F3]，fs≥2F3，0＜F1＜F2＜F3，F1Is a predetermined frequency boundary value between the low frequency band and the middle frequency band, F2Is a predetermined frequency boundary value, F, between the middle frequency band and the high frequency band3The frequency range of the voltage waveform of the photovoltaic direct current grid-connected point in the power grid is the preset upper limit value of the frequency of the high frequency band
Figure FDA0002784408670000031
Profile coefficient AJIn a frequency range of
Figure FDA0002784408670000032
Detail coefficient DKIn a frequency range of
Figure FDA0002784408670000033
fsAnd K belongs to (1-J) of the sampling frequency of the voltage waveform of the photovoltaic direct-current grid-connected point in the power grid.
9. The system according to claim 8, wherein the computing unit is configured to:
the number of wavelet decomposition layers J is calculated as follows:
Figure FDA0002784408670000034
calculating the number K of medium-high frequency boundary layers according to the following formula:
Figure FDA0002784408670000035
in the formula (I), the compound is shown in the specification,
Figure FDA0002784408670000041
in order to round up the symbol,
Figure FDA0002784408670000042
to round the symbol down.
10. The system according to claim 7, wherein the voltage waveform analysis module comprises a flicker measurement unit and a harmonic analysis unit, and the flicker measurement unit is configured to:
substituting the low-frequency voltage waveform into the flicker measuring instrument to obtain the instantaneous visual sensitivity output by the flicker measuring instrument.
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Publication number Priority date Publication date Assignee Title
CN114325057A (en) * 2021-12-31 2022-04-12 重庆大学 PT primary voltage reconstruction method based on inverse black box and inverse electromagnetic dual model
CN114325057B (en) * 2021-12-31 2023-09-19 重庆大学 PT primary voltage reconstruction method based on inverse black box and inverse electromagnetic dual model

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