CN117405972A - Method for measuring broadband harmonic waves of offshore wind power network - Google Patents
Method for measuring broadband harmonic waves of offshore wind power network Download PDFInfo
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Abstract
The invention provides a method for measuring broadband harmonic waves of an offshore wind power network, which comprises the following steps: based on the wave recording data, a DFT method is used for solving the frequency spectrum coefficient, a frequency spectrum image of the wave recording data is obtained, and frequency, amplitude and phase information of the fundamental wave and each subharmonic are extracted; judging the frequency range of the inter-harmonic according to the frequency spectrum coefficient of the wave recording data and the frequency spectrum image, carrying out phase rotation on the DFT coefficient, eliminating the influence of frequency spectrum leakage by utilizing the characteristic of opposite phase of the leakage side lobe after rotation, and solving the frequency, amplitude and phase information of the inter-harmonic; data are arranged, and characteristic information of each subharmonic/inter-harmonic in the actually measured wave recording data is obtained; and determining the frequency, amplitude and phase obtained characteristics of the harmonic wave and the inter-harmonic wave under different voltage levels according to the harmonic wave emission characteristics of different voltage levels. The invention can eliminate the mutual interference of the leakage frequency spectrum and accurately separate out the inter-harmonic component.
Description
Technical Field
The invention relates to the technical field of offshore wind power, in particular to a method for measuring broadband harmonic waves of an offshore wind power network.
Background
The harmonic wave generated by the large-scale offshore wind farm is interacted with the background harmonic wave of the power grid, so that the problems of harmonic wave amplification and exceeding standard can be caused, and serious threat is formed to the safety and stability of the power grid and the economic operation of the power utilization load. The fan converter adopts a pulse width modulation technology, and offshore wind power is connected with a grid through the power electronic converter to introduce a large amount of wide frequency domain and high frequency harmonic waves; meanwhile, the offshore wind farms are generally integrated into a power grid through cable collection, the cable distribution capacitance is large, when the harmonic frequency emitted by the offshore wind farms is increased, the inductive capacity coupling effect of the cable distribution parameters is not negligible, so that the caused harmonic amplification can influence the safe and stable operation of the power grid, and the events such as high oil seepage resistance, equipment tripping and the like are caused, so that the safe and stable operation of the power grid and equipment is seriously influenced.
The harmonic wave/inter-harmonic wave is usually generated by the traditional nonlinear load and power electronic equipment, a thyristor rectifying device, a frequency conversion device and the like usually emit a large amount of harmonic wave, and a thyristor alternating current speed regulating device, an arc type load and the like usually emit a large amount of inter-harmonic wave. Under the new power system, the harmonic emission presents new characteristics of "wide frequency domain", "high frequency", "wide distribution", and the harmonic emission characteristics are also becoming diversified, such as: the wind power plant side harmonic voltage level, the included angle between the harmonic voltage and the current phase and the harmonic impedance have different characteristics at different voltage levels. Therefore, the harmonic emission characteristics of the offshore wind power are not clear, and the key point of researching the harmonic emission characteristics of the offshore wind power is to accurately measure the harmonic voltage/harmonic current of each important node of the wind power plant.
Harmonic measurement is the detection of harmonic signal frequency, harmonic amplitude and phase parameters. Early harmonic detection techniques mostly detect harmonic components based on analog filtering principles. The method has the advantages of simple circuit realization, low manufacturing cost, low output impedance and easy control of quality factors. However, the filter center frequency is sensitive to element parameters, is greatly influenced by external environment, is difficult to obtain ideal amplitude frequency and phase frequency characteristics, and the detection accuracy is greatly influenced by power grid frequency fluctuation, and the detected harmonic wave contains more fundamental wave components. With the increasing requirements of harmonic detection of power systems and the increasing maturity of new harmonic detection methods, the method is not preferred. Inter-harmonic measurement techniques are a difficulty of the present subject. The existing harmonic detection device is mostly based on a phase-locked loop synchronous sampling technology, can track the frequency of a power grid in real time, and equally distributes sampling points in a fundamental wave period, and on the basis, harmonic components can be extracted rapidly and accurately by referring to IEC harmonic detection standards and recommended DFT algorithm. The whole period truncation of the harmonic component means the non-whole period truncation of the inter-harmonic component, and the inter-harmonic spectrum can generate spectrum leakage phenomenon, which is likely to interfere with each other and even interfere with the extraction of the harmonic component. On the premise of unknown inter-harmonic frequency, the mutual interference of the leakage frequency spectrum is eliminated, and the inter-harmonic component is difficult to accurately separate.
Disclosure of Invention
The invention solves the problems that the current measurement of the broadband harmonic wave of the offshore wind power network is greatly influenced by the environment and has low measurement precision, and provides the measurement method of the broadband harmonic wave of the offshore wind power network, which can eliminate the mutual interference of the leakage frequency spectrum and accurately obtain and separate the inter-harmonic component.
In order to achieve the above purpose, the following technical scheme is provided:
a method for measuring broadband harmonic waves of an offshore wind power network comprises the following steps:
s1: based on the wave recording data, a DFT method is used for solving the frequency spectrum coefficient, a frequency spectrum image of the wave recording data is obtained, and frequency, amplitude and phase information of the fundamental wave and each subharmonic are extracted;
s2: judging the frequency range of the inter-harmonic according to the frequency spectrum coefficient of the wave recording data and the frequency spectrum image, carrying out phase rotation on the DFT coefficient, eliminating the influence of frequency spectrum leakage by utilizing the characteristic of opposite phase of the leakage side lobe after rotation, and solving the frequency, amplitude and phase information of the inter-harmonic;
s3: data are arranged, and characteristic information of each subharmonic/inter-harmonic in the actually measured wave recording data is obtained;
s4: and determining the frequency, amplitude and phase obtained characteristics of the harmonic wave and the inter-harmonic wave under different voltage levels according to the harmonic wave emission characteristics of different voltage levels.
According to the method, the influence of spectrum leakage is eliminated by utilizing the characteristic of opposite leakage side lobe phases after rotation, and frequency, amplitude and phase information of inter-harmonics are accurately solved. The frequency, amplitude and phase information of the accurate harmonic wave are beneficial to the source management of the harmonic wave, the source management of the harmonic wave is the most economical mode, a power grid company formulates corresponding harmonic wave rewarding and punishing measures according to harmonic wave responsibility quantification results, and the power grid company can greatly reduce funds required by the power grid company for managing the power quality problem of the power grid by adopting fine cost for users with excessive harmonic wave emission or requiring the users to install harmonic wave management devices and other harmonic wave management and control schemes.
Preferably, the harmonic emission characteristics of different voltage levels comprise harmonic emission characteristics of a fan outlet, a box-type transformer outlet, a cable collection point and a new energy station grid-connected point.
Preferably, the step S1 specifically includes the following steps:
assuming that the signal of the recorded wave data detected by the harmonic monitoring device is x (t), the signal x (t) contains M frequency components, and the amplitude of the kth frequency component is A k Frequency f k The phase isThe measured signal can be expressed as:
according to IEC standard, rectangular window w (n) with 10 fundamental wave frequency lengths is adopted for discrete sampling, and the discrete signal y [ n ] obtained by sampling is:
the above-mentioned materials are converted into frequency domain by means of discrete time Fourier transform so as to obtain y n]Spectral envelope function Y [ e ] jω ]The method comprises the following steps:
the spectrum of the DFT is the sampling of the DTFT envelope, which willSubstituting the above formula yields the DFT transform formula of the windowing function:
in the middle of For DFT resolution, using DFT on a finite length signal is equivalent to automatically applying a rectangular window function to the finite length signal to truncate, where the rectangular window function is:
the DFT transform of the rectangular window function is:
finally, Y [ n ] is obtained as follows:
the above formula is the frequency domain corresponding signal of the time domain discrete sampling signal.
Preferably, the harmonic monitoring device uses a phase-locked loop to synchronize sampling wave-recording data.
Preferably, the principle of using phase-locked loop to synchronously sample wave-recording data by the harmonic monitoring device is as follows:
taking k=k 1 The length T of the window function w (n) when synchronous sampling is adopted by a phase-locked loop w For the period of harmonic signalsInteger multiple and->Is an integer only>Time DFT coefficient->But->When Y [ n ]]＝0。
Preferably, the step S2 specifically includes the following steps: make phase rotation and recordThe method comprises the following steps:
delta [ n ] in the above]For the influence of the current non-concerned frequency harmonic wave and the current concerned harmonic wave negative frequency component, record is madeZ[n]The preparation method has the following properties: n-theta k When > 0 or when n-theta k Sign (Z (n)) = -sign (Z (n+1)) when < 0.
The beneficial effects of the invention are as follows: and eliminating the influence of spectrum leakage by utilizing the characteristic of opposite phase of the leakage sidelobes after rotation, and accurately solving the frequency, amplitude and phase information of the inter-harmonic wave. The frequency, amplitude and phase information of the accurate harmonic wave are beneficial to the source management of the harmonic wave, the source management of the harmonic wave is the most economical mode, a power grid company formulates corresponding harmonic wave rewarding and punishing measures according to harmonic wave responsibility quantification results, and the power grid company can greatly reduce funds required by the power grid company for managing the power quality problem of the power grid by adopting fine cost for users with excessive harmonic wave emission or requiring the users to install harmonic wave management devices and other harmonic wave management and control schemes.
Drawings
FIG. 1 is a flow chart of an embodiment;
FIG. 2 is a block diagram of a system for acquiring power grid signal data based on phase-locked loop synchronous sampling according to an embodiment;
FIG. 3 is a graph of example DFT and DTFT magnitude spectra;
FIG. 4 is a diagram illustrating DFT values after phase rotation adjustment according to an embodiment.
Detailed Description
Examples:
the embodiment provides a method for measuring broadband harmonic waves of an offshore wind power network, and referring to fig. 1, the method comprises the following steps:
s1: based on the wave recording data, a DFT method is used for solving the frequency spectrum coefficient, a frequency spectrum image of the wave recording data is obtained, and frequency, amplitude and phase information of the fundamental wave and each subharmonic are extracted;
s2: judging the frequency range of the inter-harmonic according to the frequency spectrum coefficient of the wave recording data and the frequency spectrum image, carrying out phase rotation on the DFT coefficient, eliminating the influence of frequency spectrum leakage by utilizing the characteristic of opposite phase of the leakage side lobe after rotation, and solving the frequency, amplitude and phase information of the inter-harmonic;
s3: data are arranged, and characteristic information of each subharmonic/inter-harmonic in the actually measured wave recording data is obtained;
s4: and determining the frequency, amplitude and phase obtained characteristics of the harmonic wave and the inter-harmonic wave under different voltage levels according to the harmonic wave emission characteristics of different voltage levels.
The periodic characteristics of a voltage/current distortion signal determine that the energy of each distortion component is concentrated in the frequency domain, and the distortion component has frequency point characteristics in the frequency domain. The distortion components having frequency point characteristics are mainly harmonic and inter-harmonic components. The DFT method converts a time domain signal into a frequency domain signal, and separates distortion components from the time domain signal by frequency decomposition, thereby realizing detection of harmonic components and inter-harmonic components. The DFT method has the characteristics of high calculation speed, good instantaneity, good stability and the like, is the most widely applied steady signal frequency domain analysis method at present, and is also an IEC standard IEC61000-4-7:2009 recommended mathematical method for power system harmonic and inter-harmonic detection. The principle of separating harmonic components from inter-harmonic components in distorted signals by using a DFT method is as follows:
assuming that the signal of the recorded wave data detected by the harmonic monitoring device is x (t), the signal x (t) contains M frequency components, and the amplitude of the kth frequency component is A k Frequency f k The phase isThe measured signal can be expressed as:
according to IEC standard, rectangular window w (n) with 10 fundamental wave frequency lengths is adopted for discrete sampling, and the discrete signal y [ n ] obtained by sampling is:
the formula (3-2) is converted into a frequency domain through Discrete time Fourier transform (Discrete-time Fourier Transform, DTFT) to obtain y [ n ]]Spectral envelope function Y [ e ] jω ]The method comprises the following steps:
the spectrum of the DFT is the sampling of the DTFT envelope, which willSubstitution formula (3-3) yields the DFT-transformed formula of the windowing function:
in the formula (3-4) Is the DFT resolution. The adoption of DFT on the finite length signal is equivalent to the automatic application of a rectangular window function to the finite length signal for truncation, wherein the rectangular window function is as follows:
the DFT transform of the rectangular window function is:
bringing formula (3-6) into formula (3-4) gives Y [ n ] as:
equation (3-7) is the frequency domain corresponding signal of the time domain discrete sampling signal.
From the DFT analysis of the signal, it is known that correctly determining the frequency of the sampled signal is a critical issue for data acquisition. Because the power grid frequency fluctuates in real time, in order to track the power grid frequency, the existing harmonic monitoring device mostly adopts a phase-locked loop technology, and a frequency multiplication circuit is formed by referring to the phase-locked loop and an external frequency divider in fig. 2, a measured power grid signal is subjected to band-pass filtering of a band-pass filter and shaping of a zero-pass comparator, and a signal output by the zero-pass comparator and a signal output by a voltage-controlled oscillator and subjected to frequency division by the frequency divider are subjected to phase comparison in a phase discriminator. The amplitude of the direct current signal output by the phase discriminator is proportional to the phase difference, the output of the phase discriminator controls the frequency of the voltage-controlled oscillator after passing through the low-pass filter, so that the frequency of the voltage-controlled oscillator output after being divided by the frequency divider is close to the frequency of the signal to be measured until the frequency difference disappears, the loop is locked, and the output frequency of the voltage-controlled oscillator is exactly N times that of the signal to be measured. The voltage controlled oscillator output is used to control the sample/hold and to start the a/D converter. After the A/D conversion is finished, an interrupt request signal is sent to the microcomputer, and after the microcomputer responds to the interrupt, the A/D conversion result is read in, so that N sampling points can be uniformly distributed in one whole cycle of the power grid, and synchronous sampling is realized.
The mathematical principle of the phase-locked loop synchronous sampling technology for eliminating spectrum leakage is as follows: to simplify the problem, take k=k 1 The length T of the window function w (n) when the phase-locked loop synchronous sampling technology is adopted for single-frequency signal analysis w For the period of harmonic signalsInteger multiple and->Is an integer only>Time DFT coefficient->But->Time Y [ n ]]=0. When the phase-locked loop synchronous sampling technology is adopted, the harmonic spectrum energy is concentrated without leakage phenomenon, and the harmonic spectrum accurately reflects the frequency, amplitude and phase angle information of the harmonic signal.
The harmonic detection device based on the phase-locked loop synchronous sampling technology can ensure the whole period interception of harmonic components, but because inter-harmonic signal frequency distribution is wide and is not integral multiple of fundamental wave signals, the inter-harmonic components can be intercepted by an irregular period, so that the frequency spectrum leakage phenomenon occurs. As shown in FIG. 3, by means of the full-period synchronous sampling of the phase-locked loop technology, the fence effect can be effectively overcome by measuring the harmonic component, and the frequency spectrum of the system does not show leakage. The frequency domain form of the inter-harmonic wave cannot accurately reflect the time domain characteristics of the inter-harmonic wave due to the non-whole period interception of the inter-harmonic wave component, the frequency, the amplitude and the phase of the inter-harmonic wave component cannot be accurately measured based on the current harmonic wave detection standard of the DFT algorithm, and leakage side lobes of the inter-harmonic wave component can possibly influence the frequency spectrum of the harmonic wave component so as to interfere the accurate measurement of the harmonic wave component.
The harmonic detection device based on the phase-locked loop technology cannot meet the full period sampling of harmonic components and inter-harmonic components at the same time in the time domain, so that the frequency spectrum of the harmonic/inter-harmonic components is corrected based on fundamental synchronous sampling from the frequency domain so as to achieve accurate estimation of the harmonic/inter-harmonic components, and the properties of the harmonic/inter-harmonic frequency spectrum need to be studied first. It was observed that both the positive and negative frequency components of the harmonic/inter-harmonic spectrum in (3-7) contain phase factorsAnd the phase factor changes with line position changes to affect the phase of the spectral coefficients. Make phase rotation, record->The method comprises the following steps:
delta [ n ] in the above]Is the effect of the current non-frequency harmonic of interest and the current harmonic negative frequency component of interest. Recording deviceZ[n]The preparation method has the following properties: n-theta k When > 0 or when n-theta k Sign (Z (n)) = -sign (Z (n+1)) when < 0.
The amplitude and phase characteristics of the DFT spectral coefficients after phase rotation are shown in figure 4,
the amplitude and the phase of the frequency spectrum coefficient have mathematical rules, and adjacent side lobes have opposite phases, so that the fence effect can be overcome and the influence of inter-harmonic frequency spectrum leakage can be eliminated by combining the DFT coefficient expression, thereby realizing accurate measurement of harmonic wave/inter-harmonic wave components.
Claims (6)
1. A method for measuring broadband harmonic waves of an offshore wind power network is characterized by comprising the following steps:
s1: based on the wave recording data, a DFT method is used for solving the frequency spectrum coefficient, a frequency spectrum image of the wave recording data is obtained, and frequency, amplitude and phase information of the fundamental wave and each subharmonic are extracted;
s2: judging the frequency range of the inter-harmonic according to the frequency spectrum coefficient of the wave recording data and the frequency spectrum image, carrying out phase rotation on the DFT coefficient, eliminating the influence of frequency spectrum leakage by utilizing the characteristic of opposite phase of the leakage side lobe after rotation, and solving the frequency, amplitude and phase information of the inter-harmonic;
s3: data are arranged, and characteristic information of each subharmonic/inter-harmonic in the actually measured wave recording data is obtained;
s4: and determining the frequency, amplitude and phase obtained characteristics of the harmonic wave and the inter-harmonic wave under different voltage levels according to the harmonic wave emission characteristics of different voltage levels.
2. The method for measuring broadband harmonic waves of an offshore wind power network according to claim 1, wherein the harmonic wave emission characteristics of different voltage levels comprise harmonic wave emission characteristics of a fan outlet, a box-type transformer outlet, a cable collection point and a new energy station grid-connected point.
3. The method for measuring broadband harmonic waves of an offshore wind power network according to claim 1, wherein the step S1 specifically comprises the following steps:
assuming that the signal of the recorded wave data detected by the harmonic monitoring device is x (t), the signal x (t) contains M frequency components, and the amplitude of the kth frequency component is A k Frequency f k The phase isThe measured signal can be expressed as:
according to IEC standard, rectangular window w (n) with 10 fundamental wave frequency lengths is adopted for discrete sampling, and the discrete signal y [ n ] obtained by sampling is:
the above-mentioned materials are converted into frequency domain by means of discrete time Fourier transform so as to obtain y n]Spectral envelope function Y [ e ] jω ]The method comprises the following steps:
the spectrum of the DFT is the sampling of the DTFT envelope, which willSubstituting the above formula yields the DFT transform formula of the windowing function:
in the middle of For DFT resolution, using DFT on a finite length signal is equivalent to automatically applying a rectangular window function to the finite length signal to truncate, where the rectangular window function is:
the DFT transform of the rectangular window function is:
finally, Y [ n ] is obtained as follows:
the above formula is the frequency domain corresponding signal of the time domain discrete sampling signal.
4. A method for measuring broadband harmonic waves of an offshore wind power network according to claim 3, wherein the harmonic monitoring device uses a phase-locked loop to synchronously sample wave recording data.
5. The method for measuring broadband harmonic waves of an offshore wind power network according to claim 4, wherein the principle that the harmonic monitoring device uses phase-locked loops to synchronously sample wave-recording data is as follows:
taking k=k 1 The length T of the window function w (n) when synchronous sampling is adopted by a phase-locked loop w For the period of harmonic signalsInteger multiple and->Is an integer only>Time DFT coefficient->But->When Y [ n ]]＝0。
6. The method for measuring broadband harmonic waves of the offshore wind power network according to claim 5, wherein the step S2 specifically comprises the following steps: make phase rotation and recordThe method comprises the following steps:
delta [ n ] in the above]For the influence of the current non-concerned frequency harmonic wave and the current concerned harmonic wave negative frequency component, record is madeZ[n]The preparation method has the following properties: n-theta k When > 0 or when n-theta k <Sign (Z (n)) = -sign (Z (n+1)) at 0.
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