CN111077375B  Online identification method for power grid impedance under background harmonic influence based on frequency domain subtraction  Google Patents
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 CN111077375B CN111077375B CN201911280166.2A CN201911280166A CN111077375B CN 111077375 B CN111077375 B CN 111077375B CN 201911280166 A CN201911280166 A CN 201911280166A CN 111077375 B CN111077375 B CN 111077375B
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 G01—MEASURING; TESTING
 G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
 G01R27/00—Arrangements for measuring resistance, reactance, impedance, or electric characteristics derived therefrom
 G01R27/02—Measuring real or complex resistance, reactance, impedance, or other twopole characteristics derived therefrom, e.g. time constant
 G01R27/16—Measuring impedance of element or network through which a current is passing from another source, e.g. cable, power line
Abstract
The invention discloses a power grid impedance online identification method under the influence of background harmonic waves based on frequency domain subtraction. According to the method, through once sampling, the disturbance component with the frequency equal to the background frequency f is injected and then sampled again, and the magnitude of the power grid impedance under the influence of background harmonic waves is estimated only by detecting the voltage and the current of the PCC point, so that the method is used for selfadaptive control of the inverter, the stability of a gridconnected system is improved, and meanwhile, the cost is not increased.
Description
Technical Field
The invention belongs to the field of power grid quality analysis and signal processing, and particularly relates to an online identification method for accurately measuring power grid impedance through harmonic injection under the condition of considering background harmonic.
Background
With the fact that new energy power generation such as photovoltaic power generation and wind power generation occupies larger and larger proportion in a power system and the fact that the local consumption capacity of the new energy power generation is limited, longdistance power transmission is needed, and line inductive reactance is increased. For the inverter, the larger line impedance may reduce the stability of the gridconnected inverter, and may even cause instability. The inverter should therefore have the capability of estimating the grid impedance online.
In addition, due to the diversity and uncertainty of loads carried by the power grid, the power grid also contains abundant background harmonics besides fundamental waves. These background harmonics cause large errors in the conventional disturbance injection method measurement and may even result in erroneous grid impedance estimation. Based on the above two points, it is necessary to accurately measure the grid impedance under the condition of considering the background harmonic wave.
At present, there are many academic papers on online identification of grid impedance, for example:
1. an article entitled "a method for estimating the impedance of a power grid based on a complex filter and injection of noncharacteristic subharmonics" ("power grid technology", 2013, No. 10, pages 27962801). By adopting the method for estimating the power grid impedance by injecting the noncharacteristic subharmonic and extracting the injection harmonic response by using the complex filter, the influence of background harmonic existing in the power grid is not considered.
2. An article entitled "Grid impedance estimation for island detection and adaptive control of converters", a.ghamem, m.rashed, m.Sumner, m.a.Elsayes, and I.I.I.Mansy, IET Power Electronics, 2017: 1279.
3. The invention patent document (publication number CN 110112776 a) in china, which is published in 2019, 8, 9, proposes a method for measuring the grid impedance by injecting a highfrequency voltage signal into the grid and extracting a highfrequency harmonic component by using a complex filter, in "a method for identifying the grid impedance of a gridconnected inverter considering the background harmonic of the grid". Although this method takes into account the background harmonic effects in the grid, the invention ignores the effects of lower frequency background harmonics that may be present by injecting a high frequency signal.
In view of the above documents, the prior art has the following disadvantages:
1. the existing power grid impedance estimation method based on noncharacteristic subharmonic disturbance injection does not consider the influence of background harmonics contained in a power grid, and the power grid impedance estimation method under the condition that the background harmonics are not negligible needs to be researched.
2. In the existing method for considering the influence of background harmonic, injection, extraction and analysis of highfrequency harmonic voltage and current are adopted, and aliasing influence between background harmonic frequency and injection frequency is avoided, so that an impedance estimation method in the interaction of the background harmonic frequency and the injection frequency is necessary to be researched.
Disclosure of Invention
The invention provides an online identification method for power grid impedance under the influence of background harmonic waves based on frequency domain subtraction. The power grid impedance estimated by the method is accurate, and the method can be used for adaptive control of the inverter, so that the stability of a gridconnected system is improved, and meanwhile, the cost is not increased.
The object of the invention is thus achieved. The invention provides a grid impedance online identification method under the influence of background harmonic waves based on frequency domain subtraction.A main circuit topological structure of a gridconnected inverter applying the method comprises a directcurrent side voltage source, a threephase fullbridge inverter circuit, a threephase LC filter, threephase grid impedance and a threephase grid, wherein the directcurrent side voltage source is connected with the threephase fullbridge inverter circuit, and the threephase fullbridge inverter circuit is connected with the threephase grid impedance through the threephase LC filter and then is connected with the threephase grid;
regularly carry out online discernment to the electric wire netting impedance under the influence of background harmonic, set for one in advance promptly and discern interval time T, discern interval time T and arrive, start sampling, disturbance injection, secondary sampling, electric wire netting impedance and discern on line:
specifically, the steps of the online identification period of the power grid impedance under the influence of background harmonic waves are as follows:
step 1, setting the grid connection of an inverter to be in a stable working state, sampling the voltage of an Aphase output end of a threephase LC filter capacitor for one time, and recording the voltage as a PCC point Aphase voltage U_{PCC_A1}Sampling the Aphase current flowing through the threephase power grid impedance for one time, and recording as the Aphase current I of the PCC point_{PCC_A1}；
Step 2, setting that the power grid contains background harmonic waves, recording the frequency of the background harmonic waves as background frequency f, and carrying out phase voltage U on the PCC points A phase collected in the step 1_{PCC_A1}Fourier analysis is carried out on the frequency point of the background frequency f to obtain a harmonic voltage vector with the frequency f when no disturbance is injectedWherein U is_{1f}Is a harmonic voltage vector of frequency f without injected disturbanceModulus of (e), theta_{u1f}Is a harmonic voltage vector of frequency f without injected disturbanceA phase angle; for the PCC point A phase current I acquired in the step 1_{PCC_A1}Fourier analysis is carried out at the f frequency point to obtain a harmonic current vector with the frequency of f when disturbance is not injectedWherein I_{1f}Is a harmonic current vector of frequency f when no disturbance is injectedThe value of the modulus of the (c) component,is a harmonic current vector of frequency f when no disturbance is injectedThe phase angle of (d);
step 3, injecting disturbance components with the frequency equal to the background frequency f into the inverter modulation wave signals, performing secondary sampling on the voltage of the Aphase output end of the threephase LC filter capacitor, and recording the voltage as a secondary PCC point Aphase voltage U_{PCC_A2}Secondly sampling the Aphase current flowing through the threephase power grid impedance and recording the Aphase current as a secondary PCC point Aphase current I_{PCC_A2}；
Step 4, carrying out phase voltage U of the secondary PCC point A phase obtained in the step 2_{PCC_A2}Fourier analysis is carried out on frequency points of background frequency f to obtain harmonic voltage vector with frequency f when injection disturbance is carried outWherein U is_{2f}Is a voltage vector of frequency f at the time of injection of a disturbanceModulus of (e), theta_{u2f}Is a harmonic voltage vector of frequency f when injecting a disturbanceThe phase angle of (d); for the secondary PCC point A phase current I obtained in the step 2_{PCC_A2}Fourier analysis is carried out on frequency points of background frequency f to obtain harmonic current vector with frequency f when disturbance is injectedWherein I_{2f}Is a harmonic current vector of frequency f when injecting a disturbanceThe value of the modulus of the (c) component,is a harmonic current vector of frequency f when injecting a disturbanceThe phase angle of (d);
step 5, recording the numerical value of the threephase power grid impedance as an impedance value Z, and obtaining a harmonic voltage vector with the frequency f when disturbance is not injected in the step 2And a harmonic current vector of frequency f when no disturbance is injectedThe harmonic voltage vector with the frequency f during injection disturbance obtained in the step 4And the harmonic current vector with frequency f when injecting disturbanceCalculating the impedance value Z according to the following formula:
wherein  Z  is a module value of the impedance value Z, and is a phase angle of the impedance value Z, R_{g}Is a resistive component of the impedance value Z, L_{g}Is the inductive component of the impedance value Z, j being the unit of an imaginary number, j^{2}＝1。
Compared with the prior art, the invention has the beneficial effects that:
1. according to the online identification method for the power grid impedance under the influence of the background harmonic wave based on frequency domain subtraction, the power grid impedance can be accurately estimated by detecting the voltage and the current of the PCC point, and then the online identification method is used for selfadaptive control of an inverter and improves the stability of a gridconnected system;
2. according to the method, the harmonic voltage and current amplitude phase angle is measured and calculated twice by analyzing the characteristics of the voltage and current amplitude phase angle of the PCC point, so that the influence of background harmonic waves possibly existing in a power grid on power grid impedance identification is solved;
3. the method for identifying the power grid impedance on line under the influence of the background harmonic wave based on the frequency domain subtraction is only improved on the algorithm of the conventional power electronic converter system, and additional equipment such as a sensor and the like is not required to be added;
drawings
Fig. 1 is a topological diagram of a main circuit of a gridconnected inverter to which the method for online identification of grid impedance under the influence of background harmonics based on frequency domain subtraction is applied.
Fig. 2 is a comparative analysis diagram of the influence of power grid background harmonics on impedance measurement.
Fig. 3 is a diagram of the modulus value  Z  and the impedance angle of the measured grid impedance Z.
Fig. 4 is an enlarged partial view of the area a in fig. 3.
FIG. 5 shows the inductance L of the network impedance Z_{g}Part and resistance R_{g}Partial test patterns.
Fig. 6 is an enlarged partial view of region B in fig. 5.
Detailed Description
The present embodiment will be described in detail below with reference to the accompanying drawings.
Fig. 1 is a topology structure diagram of a main circuit of a gridconnected inverter to which the present invention is applied, and as can be seen, the topology structure includes a dcside voltage source 10, a threephase fullbridge inverter circuit 20, a threephase LC filter 30, a threephase grid impedance 40, and a threephase grid 50.
The direct current side voltage source 10 is connected with a threephase fullbridge inverter circuit 20, and the threephase fullbridge inverter circuit 20 is connected with a threephase power grid impedance 40 through a threephase LC filter 30 and then is connected to a threephase power grid 50. In FIG. 1, V_{dc}Is a DC side voltage source 10, L_{f}Is a bridge arm side inductance, C, of a threephase LC filter 30_{f}A filter capacitor of the threephase LC filter 30, R a passive damping resistor of the threephase LC filter 30, R_{g}Is the resistance, L, in the threephase line impedance 40_{g}Being the inductance in the threephase Grid impedance 40, Grid is the threephase Grid 50.
The main circuit parameters in this embodiment are: voltage V at DC side_{dc}800V, 380V/50Hz of rated output line voltage of the inverter, 100kW of rated power of the inverter and L of filter inductance_{f}0.56mH, filter capacitance C_{f}And the passive damping resistor r is 270uF/0.3 omega, and the inductance part L in the threephase power grid impedance_{g}0.1mH and a resistive portion R_{g}＝0.1Ω。
The method for identifying the power grid impedance under the influence of the background harmonic wave based on the frequency domain subtraction regularly identifies the power grid impedance under the influence of the background harmonic wave on line, namely presetting an identification interval time T, starting primary sampling, disturbance injection, secondary sampling and power grid impedance online identification when the identification interval time T is up. In this embodiment, T is selected to be 16 fundamental period periods T_{0}。
Specifically, the steps of the online identification period of the power grid impedance under the influence of background harmonic waves are as follows:
step 1, setting the grid connection of the inverter to be in a stable working state, sampling the voltage of an Aphase output end of a capacitor of a threephase LC filter 30 for one time, and recording the voltage as a PCC point Aphase voltage U_{PCC_A1}The phase a current flowing through the threephase grid impedance 40 is sampled once and is recorded as a PCC point phase a current I_{PCC_A1}；
Step 2, setting that the power grid contains background harmonic waves, recording the frequency of the background harmonic waves as background frequency f, and carrying out Aphase voltage on the PCC points acquired in the step 1U_{PCC_A1}Fourier analysis is carried out on the frequency point of the background frequency f to obtain a harmonic voltage vector with the frequency f when no disturbance is injectedWherein U is_{1f}Is a harmonic voltage vector of frequency f without injected disturbanceModulus of (e), theta_{u1f}Is a harmonic voltage vector of frequency f without injected disturbanceA phase angle; for the PCC point A phase current I acquired in the step 1_{PCC_A1}Fourier analysis is carried out at the f frequency point to obtain a harmonic current vector with the frequency of f when disturbance is not injectedWherein I_{1f}Is a harmonic current vector of frequency f when no disturbance is injectedThe value of the modulus of the (c) component,is a harmonic current vector of frequency f when no disturbance is injectedThe phase angle of (d);
step 3, injecting disturbance components with the frequency equal to the background frequency f into the inverter modulation wave signal, performing secondary sampling on the voltage of the Aphase output end of the capacitor of the threephase LC filter 30, and recording the voltage as the secondary PCC point Aphase voltage U_{PCC_A2}The phaseA current flowing through the threephase grid impedance 40 is subsampled and recorded as the secondary PCC point phaseA current I_{PCC_A2}；
Step 4, carrying out phase voltage U of the secondary PCC point A phase obtained in the step 2_{PCC_A2}Fourier analysis is carried out on frequency points of background frequency f to obtain the harmonic of frequency f when the disturbance is injectedWave voltage vectorWherein U is_{2f}Is a voltage vector of frequency f at the time of injection of a disturbanceModulus of (e), theta_{u2f}Is a harmonic voltage vector of frequency f when injecting a disturbanceThe phase angle of (d); for the secondary PCC point A phase current I obtained in the step 2_{PCC_A2}Fourier analysis is carried out on frequency points of background frequency f to obtain harmonic current vector with frequency f when disturbance is injectedWherein I_{2f}Is a harmonic current vector of frequency f when injecting a disturbanceThe value of the modulus of the (c) component,is a harmonic current vector of frequency f when injecting a disturbanceThe phase angle of (d);
step 5, recording the numerical value of the threephase power grid impedance 40 as an impedance value Z, and obtaining a harmonic voltage vector with the frequency f when disturbance is not injected in the step 2And a harmonic current vector of frequency f when no disturbance is injectedThe harmonic voltage vector with the frequency f during injection disturbance obtained in the step 4And the harmonic current vector with frequency f when injecting disturbanceCalculating the impedance value Z according to the following formula:
wherein  Z  is a module value of the impedance value Z, and is a phase angle of the impedance value Z, R_{g}Is a resistive component of the impedance value Z, L_{g}Is the inductive component of the impedance value Z, j being the unit of an imaginary number, j^{2}＝1。
For example, the validity of the invention is verified by simulation verification.
Fig. 2 is a comparative analysis diagram of the influence of power grid background harmonics on impedance measurement. As can be seen from fig. 2, the power grid impedance is measured by using the conventional disturbance injection method, and it can be seen that the power grid impedance can be accurately measured when the background harmonic is not considered; however, background harmonic waves appear suddenly on the power grid, so that the impedance of the power grid is measured suddenly, and errors occur in measurement.
Fig. 3 is a diagram of a measured modulus  Z  and impedance angle of the grid impedance Z, and fig. 4 is an enlarged portion of the area a in fig. 3. As can be seen from fig. 3 and 4, by measuring the background harmonic information, the influence of the background harmonic on impedance identification is eliminated, and the obtained power grid impedance is relatively accurate.
FIG. 5 shows the inductance L of the network impedance Z_{g}Part and resistance R_{g}A partial test chart, fig. 6 is an enlarged partial view of the area B in fig. 5. As can be seen from fig. 5 and 6, the method of the present invention can also accurately measure the magnitudes of the inductive component and the resistive component in the power grid impedance.
Claims (1)
1. A gridconnected inverter main circuit topological structure applying the method comprises a direct current side voltage source (10), a threephase fullbridge inverter circuit (20), a threephase LC filter (30), a threephase grid impedance (40) and a threephase grid (50), wherein the direct current side voltage source (10) is connected with the threephase fullbridge inverter circuit (20), and the threephase fullbridge inverter circuit (20) is connected with the threephase grid impedance (40) through the threephase LC filter (30) and then is connected with the threephase grid (50);
the method is characterized in that the online identification is carried out on the power grid impedance under the influence of background harmonic wave at regular time, namely, an identification interval time T is preset, the identification interval time T is up, and the online identification of primary sampling, disturbance injection, secondary sampling and power grid impedance is started:
specifically, the steps of the online identification period of the power grid impedance under the influence of background harmonic waves are as follows:
step 1, setting the grid connection of an inverter to be in a stable working state, sampling the voltage of an Aphase output end of a capacitor of a threephase LC filter (30) for one time, and marking as a PCC point Aphase voltage U_{PCC_A1}The phase A current flowing through the threephase network impedance (40) is sampled once and is recorded as the phase A current I of the PCC point_{PCC_A1}；
Step 2, setting that the power grid contains background harmonic waves, recording the frequency of the background harmonic waves as background frequency f, and carrying out phase voltage U on the PCC points A phase collected in the step 1_{PCC_A1}Fourier analysis is carried out on the frequency point of the background frequency f to obtain a harmonic voltage vector with the frequency f when no disturbance is injectedWherein U is_{1f}Is a harmonic voltage vector of frequency f without injected disturbanceModulus of (e), theta_{u1f}Is a harmonic voltage vector of frequency f without injected disturbanceA phase angle; for the PCC point A phase current I acquired in the step 1_{PCC_A1}Fourier analysis is carried out at the f frequency point to obtain a harmonic current vector with the frequency of f when disturbance is not injectedWherein I_{1f}Is a harmonic current vector of frequency f when no disturbance is injectedThe value of the modulus of the (c) component,is a harmonic current vector of frequency f when no disturbance is injectedThe phase angle of (d);
and 3, injecting disturbance components with the frequency equal to the background frequency f into the inverter modulation wave signal, performing secondary sampling on the voltage of the Aphase output end of the capacitor of the threephase LC filter (30) and recording the voltage as the secondary PCC point Aphase voltage U_{PCC_A2}The phaseA current flowing through the threephase network impedance (40) is subsampled and recorded as a secondary PCC point phaseA current I_{PCC_A2}；
Step 4, carrying out phase voltage U of the secondary PCC point A phase obtained in the step 3_{PCC_A2}Fourier analysis is carried out on frequency points of background frequency f to obtain harmonic voltage vector with frequency f when injection disturbance is carried outWherein U is_{2f}Is a voltage vector of frequency f at the time of injection of a disturbanceModulus of (e), theta_{u2f}Is a harmonic voltage vector of frequency f when injecting a disturbanceThe phase angle of (d); for the secondary PCC point A phase current I obtained in the step 3_{PCC_A2}Fourier analysis is carried out on frequency points of background frequency f to obtain harmonic current vector with frequency f when disturbance is injectedWherein I_{2f}Is a harmonic current vector of frequency f when injecting a disturbanceThe value of the modulus of the (c) component,is a harmonic current vector of frequency f when injecting a disturbanceThe phase angle of (d);
step 5, recording the numerical value of the threephase power grid impedance (40) as an impedance value Z, and obtaining a harmonic voltage vector with the frequency f when disturbance is not injected in the step 2And a harmonic current vector of frequency f when no disturbance is injectedThe harmonic voltage vector with the frequency f during injection disturbance obtained in the step 4And the harmonic current vector with frequency f when injecting disturbanceCalculating the impedance value Z according to the following formula:
wherein  Z  is a module value of the impedance value Z, and is a phase angle of the impedance value Z, R_{g}Is a resistive component of the impedance value Z, L_{g}Is the inductive component of the impedance value Z, j being the unit of an imaginary number, j^{2}＝1。
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