CN115356534A - Method for reducing frequency spectrum leakage during power grid current FFT harmonic extraction - Google Patents
Method for reducing frequency spectrum leakage during power grid current FFT harmonic extraction Download PDFInfo
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Abstract
The invention discloses a method for reducing frequency spectrum leakage during power grid current FFT harmonic extraction, which relates to the technical field of power system harmonic detection and comprises the following steps: s1, converting a sine wave into a square wave by adopting a hardware phase-locked loop method; s2, the FPGA starts counting at the first rising edge, stops counting at the next rising edge and calculates the voltage frequency of the real-time power grid; s3, the FPGA adjusts the sampling frequency of an external AD chip to Fs = Fz × N according to the real-time power grid frequency Fz; when the voltage frequency of the power grid fluctuates, the number of sampling points in one power frequency period is constant to N data, so that the FFT harmonic extraction is carried out, and most of frequency spectrum leakage is reduced. The invention adopts a method for dynamically calculating the real-time frequency of the power grid, adopts variable sampling frequency to sample signals, ensures that the number of points N is a constant, and achieves the effect of synchronous sampling.
Description
Technical Field
The invention relates to the technical field of harmonic detection of power systems, in particular to a method for reducing frequency spectrum leakage during power grid current FFT harmonic extraction.
Background
The rated frequency of a power system in China is 50Hz, and under the completely ideal condition, the current has single and fixed frequency and specified voltage amplitude. With the widespread use of power electronic devices and nonlinear elements and the increasing capacity, the problem of harmonic pollution of power systems caused by the power electronic devices and the nonlinear elements becomes more serious and more complex. Non-integer and fractional subharmonics exist in a power grid in a large quantity, great pollution is brought to the power grid, and the safe and economic operation of a power system is seriously influenced. The harmonic wave influences the normal operation of various electrical equipment, and the influence of the harmonic wave on the motor not only causes additional loss, but also generates mechanical vibration, noise and overvoltage, so that the iron loss of the transformer is increased, and local serious overheating can occur. The harmonic wave causes the overheating, insulation aging and service life shortening of equipment such as a capacitor, a cable and the like, so that the equipment is damaged; the harmonic waves can cause local parallel resonance and series resonance in the power grid, so that the harmonic waves are amplified, the influence on equipment is greatly increased, and even serious accidents are caused; the harmonic wave causes additional harmonic loss of elements in the power grid, the efficiency of power generation, transmission and electric equipment is reduced, and a large amount of 3-order harmonic current flows through a neutral line to overheat the line and even cause fire; the harmonic waves may cause interference to neighboring communication systems, reducing communication quality, and even causing information loss and failure of the communication systems.
The harmonic detection analysis method commonly used at present is Fast Fourier Transform (FFT). Although the FFT algorithm can realize accurate analysis and detection of the integer harmonics, the FFT algorithm has severe spectrum leakage and fence phenomena for detection of non-integer harmonics, so that the amplitude, phase angle and frequency of the detected harmonics have large errors and cannot meet the requirement of detection accuracy. The fence problem can be effectively solved by using an interpolation correction algorithm, but the measurement error caused by the spectrum leakage phenomenon cannot be eliminated. Spectral leakage errors can be greatly reduced by using a windowing algorithm.
Reference is made to the prior patent (application number 201811475977.3) and a method for detecting spectrum leakage in power quality monitoring is disclosed, and the method comprises four steps of discretization, windowing a signal through a rectangular window, performing Fourier transform and Euler change, and judging the spectrum leakage condition when the signal is subjected to Fourier transform according to a spectrum leakage coefficient.
The above patent adopts the FFT windowing interpolation weighting method to solve the spectrum leakage problem, but the method is complex in design and large in software calculation amount, and the algorithm can only reduce errors and has inherent errors.
In the power grid, when harmonic voltage or harmonic current is extracted, the sampling frequency Fs and the power grid frequency Fz are determined, and the number of sampling points is N = Fs/Fz.
And the national standard fluctuation grade of the power grid frequency is specified as follows:
the frequency grade A grade is less than or equal to +/-0.05 Hz; grade B is less than or equal to +/-0.5 Hz; grade C is less than or equal to +/-1 Hz.
Fifty-third of the "power supply business rules" is that under the normal condition of the power system, the allowable deviation of the power supply frequency is:
1. the installed capacity of the power grid is 300 kilo kilowatts or more, and is +/-0.2 Hz; 2. the installed capacity of the power grid is less than 300 ten thousand kilowatts and is +/-0.5 Hz. 3. In abnormal conditions of the power system, the allowable deviation of the power supply frequency should not exceed ± 1.0 hz.
We assume that in the worst case the deviation is ± 1Hz, i.e. the grid frequency is 49Hz to 51 Hz. N = Fs/(Fz ± 1) according to formula (1); if the sampling frequency is fixed, the sampling frequency is not exactly one grid cycle, and the synchronous sampling effect cannot be achieved.
Disclosure of Invention
The invention aims to provide a method for reducing spectrum leakage during power grid current FFT harmonic extraction, and the frequency spectrum leakage is generated mainly because the period of a sampled signal is changed, so that a discretely sampled signal X (t) can be periodically cut off, namely the number N of sampling points is not exactly one period, so as to solve the problems in the background.
In order to solve the technical problems, the invention is realized by the following technical scheme:
the invention discloses a method for reducing frequency spectrum leakage during power grid current FFT harmonic extraction, which comprises the following steps:
s1, converting a sine wave into a square wave by adopting a hardware phase-locked loop method;
s2, the FPGA starts counting at the first rising edge, stops counting at the next rising edge and calculates the voltage frequency of the real-time power grid;
s3, the FPGA adjusts the sampling frequency of an external AD chip to be Fs = Fz × N according to the real-time power grid frequency Fz; when the voltage frequency of the power grid fluctuates, the number of sampling points in one power frequency period is constant to N data, so that when FFT harmonic extraction is carried out, most of frequency spectrum leakage is reduced;
the intelligent Bridge circuit comprises a matlab simulation model, and is characterized in that the matlab simulation model comprises a Sine Wave module (Sine Wave), a Controlled Voltage Source module (Controlled Voltage Source), a Voltage measuring module (Voltage measuring), a Current measuring module (Current measuring), a display module (Scope 1), a Universal Bridge module (Universal Bridge), a setting module (powergui), an inductance device (Series RLC Branch 2), a capacitance device (Series RLC Branch 1) and a resistance device (Series RLC Branch), wherein the Sine Wave module is used for outputting Sine waves and is electrically connected with the Controlled Voltage Source module, the Controlled Voltage Source module converts input signals of the Sine Wave module into an equivalent Voltage Source, the Controlled Voltage Source module is electrically connected with the Voltage measuring module, the Voltage measuring module is used for ideal Voltage Measurement, the Voltage measuring module is electrically connected with the Universal Bridge module, the Universal Bridge module serves as a Universal power converter, the Voltage measuring module is electrically connected with the display module, the display module is used for displaying simulation signals produced by the simulation, the Controlled Voltage measuring module is used for ideal Voltage Measurement, the Voltage measuring module is electrically connected with the Universal Bridge module, the Current measuring module is used for simulating and the Current measuring module, and the Voltage measuring module is used for setting parameters of the closed Current measuring module, the Universal Bridge module and the Universal Bridge module.
Preferably, the matlab simulation model connects a sinusoidal signal generated by the sinusoidal wave module to a signal input port of the controlled voltage source module, drives the controlled voltage source to generate voltage, and connects a voltage source output to an input of the general bridge module, the voltage measurement module and the current measurement module are respectively connected in parallel and in series to the circuit, and the general bridge module connects an output direct current voltage to an input of the resistance load after passing through the LC filter circuit.
The invention has the following beneficial effects:
the invention adopts a method for dynamically calculating the real-time frequency of the power grid, adopts variable sampling frequency to sample signals, ensures that the number of points N is a constant, and then the N-point sampling is just a complete power grid period, thereby achieving the effect of synchronous sampling.
The method has the advantages of simple design, small calculated amount and small error, and can accurately extract the target harmonic current.
Of course, it is not necessary for any product in which the invention is practiced to achieve all of the above-described advantages at the same time.
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In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the description below are only some embodiments of the present invention, and it is obvious for those skilled in the art that other drawings can be obtained according to the drawings without creative efforts.
FIG. 1 is a flow chart of the method of operation of the present invention;
FIG. 2 is a diagram of matlab simulation model of the present invention;
FIG. 3 is a schematic diagram of a spectrum analysis of a real-time grid voltage with a frequency of 49Hz and a sampling frequency of 10k according to the first embodiment;
FIG. 4 is a schematic diagram of a spectrum analysis of a real-time grid voltage frequency of 49Hz and a sampling frequency of 9.8k according to the first embodiment;
FIG. 5 is a schematic diagram of a frequency spectrum analysis of a real-time grid voltage frequency of 50Hz and a sampling frequency of 10k according to the first embodiment;
FIG. 6 is a schematic diagram of a spectrum analysis of a real-time grid voltage with a frequency of 51Hz and a sampling frequency of 10k according to the first embodiment;
fig. 7 is a schematic diagram of a spectrum analysis of a real-time grid voltage frequency of 51Hz and a sampling frequency of 10.2k according to the first embodiment.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Example one
Please refer to fig. 1 and fig. 2: the invention discloses a method for reducing frequency spectrum leakage during power grid current FFT harmonic extraction, which comprises the following steps:
s1, converting a sine wave into a square wave by adopting a hardware phase-locked loop method;
s2, the FPGA starts counting at the first rising edge, stops counting at the next rising edge and calculates the voltage frequency of the real-time power grid;
s3, the FPGA adjusts the sampling frequency of an external AD chip to be Fs = Fz × N according to the real-time power grid frequency Fz; when the voltage frequency of the power grid fluctuates, the number of sampling points in one power frequency period is constant to N data, so that when FFT harmonic extraction is carried out, most of frequency spectrum leakage is reduced;
still include matlab simulation model, matlab simulation model comprises Sine Wave module (Sine Wave), controlled Voltage Source module (Controlled Voltage Source), voltage Measurement module (Voltage Measurement), current Measurement module (Current Measurement), display module (Scope 1), general Bridge module (Universal Bridge), setting module (powergui), inductance element (Series RLC Branch 2), capacitance element (Series RLC Branch 1) and resistance element (Series RLC Branch), the Sine Wave module is used for exporting the Sine Wave, the Sine Wave module is connected with Controlled Voltage Source module electricity, controlled Voltage Source module converts Sine Wave module's input signal into an equivalent Voltage Source, controlled Voltage Source module and Voltage Measurement module electricity are connected, voltage Measurement module is used for ideal Voltage Measurement, voltage Measurement module and general Bridge module electricity are connected, general Bridge module has acted as a general three-phase power converter, voltage Measurement module and display module electricity are connected, the display module is used for the production the simulation signal and Measurement module and Voltage Measurement module are connected, the Bridge module is used for the setting of common Voltage Measurement module and common Voltage Measurement module, the common Voltage Measurement module is connected with the display module electricity, the common Voltage Measurement module is used for the Measurement of Current Measurement module and the common Voltage Measurement module is connected with the common Voltage Measurement module, the common Voltage Measurement module is connected with the set up with the common Voltage Measurement module, the common Voltage Measurement module is connected with the common Voltage Measurement module.
Furthermore, the matlab simulation model enables a sine wave module to generate a sine signal to be connected to a signal input port of a controlled voltage source module, drives the controlled voltage source to generate voltage, enables a voltage source output to be connected to a universal bridge module to be input, enables a voltage measuring module and a current measuring module to be connected in parallel and in series to be connected to a circuit, and enables an output direct current voltage to be connected to a resistor load after passing through an LC filter circuit through the universal bridge module.
Further, the inductive device L =1e-3 (H); the capacitive device C =3300e-6 (F); the resistance device R =50 Ω.
As shown in fig. 3-7, the matlab simulation model is used for performing spectrum analysis and comparison on a power grid voltage frequency of 49Hz, 50Hz, and 51Hz, in order to ensure that the number of sampling points is 200 points in one power frequency cycle, and the sampling frequency is 200 × Fs (Fs is the real-time power grid frequency) in real time and is fixed 200 × 500.
Wherein, fig. 3 shows that the real-time grid frequency is 49Hz, and the sampling frequency is 50hz × 200=10k; fig. 4 shows that the real-time grid frequency is 49Hz, and the sampling frequency is dynamically changed to sample according to 49hz × 200=9.8 k;
fig. 5 shows that the real-time grid frequency is 50Hz, and the sampling frequency is 50hz × 200=10 k;
fig. 6 shows that the real-time grid frequency is 51Hz, and the sampling frequency is 50hz × 200 =10k; fig. 7 shows that the real-time grid frequency is 51Hz, and the sampling frequency is dynamically changed according to 51hz × 200= 10.2k;
through data analysis, the problem of frequency spectrum leakage can be effectively solved according to the mode of dynamically changing the sampling frequency of real-time frequency.
In the description herein, references to the description of "one embodiment," "an example," "a specific example" or the like are intended to mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the schematic representations of the terms used above do not necessarily refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.
The preferred embodiments of the invention disclosed above are intended to be illustrative only. The preferred embodiments are not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Obviously, many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and the practical application, to thereby enable others skilled in the art to best understand the invention for and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.
Claims (3)
1. A method for reducing frequency spectrum leakage during power grid current FFT harmonic extraction is characterized in that: the method comprises the following steps:
s1, converting a sine wave into a square wave by adopting a hardware phase-locked loop method;
s2, the FPGA starts counting at the first rising edge, stops counting at the next rising edge and calculates the voltage frequency of the real-time power grid;
s3, the FPGA adjusts the sampling frequency of an external AD chip to be Fs = Fz × N according to the real-time power grid frequency Fz; when the voltage frequency of the power grid fluctuates, the number of sampling points in one power frequency period is constant to N data, so that the FFT harmonic extraction is carried out, and most of frequency spectrum leakage is reduced.
2. The method for reducing the frequency spectrum leakage during the FFT harmonic extraction of the power grid according to claim 1, further comprising a matlab simulation model, wherein the matlab simulation model is composed of a Sine Wave module (Sine Wave), a Controlled Voltage Source module (Controlled Voltage Source), a Voltage Measurement module (Voltage Measurement), a Current Measurement module (Current Measurement), a display module (Scope 1), a Universal Bridge module (Universal Bridge), a setting module (powergui), an inductance device (Series RLC Branch 2), a capacitance device (Series RLC Branch 1) and a resistance device (Series RLC Branch), the Sine Wave module is used for outputting a Sine Wave, the Sine Wave module is electrically connected with the Controlled Voltage Source module, the Controlled Voltage Source module converts an input signal of the Sine Wave module into an equivalent Voltage Source, the Controlled Voltage Source module is electrically connected with the Voltage measuring module, the Voltage measuring module is used for ideal Voltage Measurement, the Voltage measuring module is electrically connected with the general Bridge module, the general Bridge module serves as a general three-phase power converter, the Voltage measuring module is electrically connected with the display module, the display module is used for displaying signals of simulation production, the Controlled Voltage Source module is electrically connected with the Current measuring module, the Current measuring module is used for ideal Current Measurement, the Current measuring module is electrically connected with the display module, the Voltage measuring module is electrically connected with the general Bridge module, an inductance device, a capacitance device and a resistance device form a closed loop together, and the setting module is used for setting an analog type, simulation parameters and preferences.
3. The method for reducing the frequency spectrum leakage during the power grid current FFT harmonic extraction according to claim 2, wherein the matlab simulation model enables a sine wave module to generate a sine signal to be connected to a signal input port of a controlled voltage source module, drives the controlled voltage source to generate voltage, enables a voltage source output to be connected to an input of a universal bridge module, enables a voltage measuring module and a current measuring module to be connected in parallel and in series to a circuit, and enables an output direct current voltage to be connected to an input of a resistance load after passing through an LC filter circuit.
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