CN115236404B - Grid-connected inverter port impedance self-measurement method - Google Patents
Grid-connected inverter port impedance self-measurement method Download PDFInfo
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Abstract
The invention discloses a self-measuring method of the port impedance of an inverter, which is characterized in that two groups of linearly independent measuring parameters are obtained by applying disturbance at a reference current and changing the impedance of a power grid side, and complex frequency domain harmonic components obtained by discrete Fourier transformation of the measuring parameters are substituted into an expression of the self-measuring algorithm of the port impedance of the inverter, so that the self-measurement of the impedance of the inverter is realized. Therefore, the traditional inverter impedance measurement method is improved, an external harmonic injection device or a circuit is required to be additionally arranged, the measurement cost is reduced, a large amount of calculation is not required, the measurement efficiency is quickened, and the measurement frequency can be adjusted.
Description
Technical Field
The invention belongs to the technical field of grid-connected inverter port impedance measurement, and particularly relates to a grid-connected inverter port impedance self-measurement method.
Background
With the development of power electronics technology, impedance stability analysis methods based on impedance analysis methods are widely used. How to effectively acquire the impedance characteristics of the grid-connected inverter and the power grid to judge the stability of the grid-connected inverter system becomes a current research hot spot. Therefore, in order to solve the matching problem between the impedance of the grid-connected inverter and the impedance of the grid, various methods for measuring the impedance of the grid-connected inverter are sequentially proposed. The common impedance measurement methods of grid-connected inverters are of two types: active and passive measurements.
The active measurement method is to inject a series of harmonic disturbance signals into the grid-connected inverter system, and measure the impedance characteristics of the grid-connected inverter by measuring the voltage signal and the current signal response generated at the point of common coupling after disturbance, so that the active measurement method is critical to how to stably inject the harmonic disturbance signals into the system and accurately extract response signals from the system. The invention discloses a grid-connected inverter equivalent impedance measurement method based on disturbance injection in a power distribution network, which is disclosed in China patent application No. CN109932568A published 14/06/2019, wherein the method injects disturbance in the power distribution network and detects voltage and current at a public coupling point through a frequency analyzer. However, this method requires an additional harmonic injection device and a frequency analyzer to analyze the signal, increasing the complexity and cost of the measurement. In the chinese patent application of grid-connected inverter impedance measurement device with publication number CN110554242a published on 10/06 2019, an impedance measurement method based on a harmonic voltage signal generation circuit is disclosed, which comprises transforming a reference harmonic voltage signal through a transformer isolator to obtain an injection harmonic voltage signal electrically isolated from the reference harmonic voltage signal and applied to two terminals of the grid-connected inverter, and processing corresponding voltage and current information to calculate the equivalent impedance of the inverter.
The method comprises the steps of calculating the impedance of an inverter system by utilizing harmonic disturbance generated during operation of nonlinear equipment and calculating the voltage and current parameters of the system by a mathematical calculation method, and performing related impedance measurement by utilizing the harmonic characteristics of the inverter by a passive measurement method. The invention discloses an on-line single-phase grid-connected inverter weak network impedance measurement method based on RPWM in China patent application No. CN109839540A published on the year 06 and 04 in 2019, which combines a passive measurement method and an active measurement method, performs passive measurement in real time, performs active measurement by applying disturbance when a disturbance condition is met, takes the weak network impedance as the active measurement after the disturbance is applied and the passive measurement when the disturbance condition is met, performs disturbance according to the current distortion degree, but only aims at a single-phase system, does not improve a three-phase system, has more judgment conditions and has larger calculated amount.
Disclosure of Invention
The invention aims to overcome the defects of the prior art, provides a self-measuring method for the port impedance of a grid-connected inverter,
In order to achieve the purpose, the port impedance self-measurement method of the grid-connected inverter is characterized by comprising the following steps of:
The invention aims to overcome the defects of the prior art, and provides a photovoltaic inverter port impedance self-measuring method which does not need a special harmonic injection device or circuit, reduces the measuring cost and does not need a large amount of calculation.
In order to achieve the purpose, the port impedance self-measurement method of the grid-connected inverter is characterized by comprising the following steps of:
(1) For a grid-connected inverter system, applying a disturbance signal with frequency of f p to a reference current at a current loop control input end of the grid-connected inverter, wherein at the moment, the impedance on the power grid side Zg(s) =zg 1(s);
(2) Sampling the reference current after the disturbance signal is injected and the phase voltage and the phase current output by the inverter by using sampling equipment to obtain a reference current i r1 (t), a phase voltage u c1 (t) and a phase current i c1 (t);
(3) Performing harmonic extraction on a reference current i r1 (t), a phase voltage u c1 (t) and a phase current i c1 (t) by using discrete Fourier transform, and converting a harmonic component from a time domain to a frequency domain to obtain a first group of complex frequency domain harmonic components: reference current complex frequency domain harmonic component I r1(s), phase voltage complex frequency domain harmonic component U c1(s), phase current complex frequency domain harmonic component I c1(s);
(4) Changing the impedance Zg(s) =zg 2(s) at the power grid side, and repeating the steps (2) - (3) of sampling, harmonic extraction and time-frequency conversion to obtain a second group of complex frequency domain harmonic components: reference current complex frequency domain harmonic component I r2(s), phase voltage complex frequency domain harmonic component U c2(s), phase current frequency domain harmonic component I c2(s);
(5) Inverter port impedance Z inv(s) is calculated from the two sets of complex frequency domain harmonic components:
The invention aims at realizing the following steps:
According to the photovoltaic inverter port impedance self-measurement method, two groups of linearly independent measurement parameters are obtained by applying disturbance at a reference current and changing the power grid side impedance, and complex frequency domain harmonic components obtained by discrete Fourier transformation of the measurement parameters are substituted into an inverter port impedance self-measurement algorithm expression to realize self-measurement of the inverter impedance. Therefore, the traditional inverter impedance measurement method is improved, an external harmonic injection device or a circuit is required to be additionally arranged, the measurement cost is reduced, a large amount of calculation is not required, the measurement efficiency is quickened, and the measurement frequency can be adjusted.
Drawings
FIG. 1 is a flow chart of one embodiment of a method for self-measuring port impedance of a grid-connected inverter according to the present invention;
FIG. 2 is a Norton equivalent model of an inverter of a grid-connected inverter system after equivalent transformation;
FIG. 3 is a schematic diagram of a discrete Fourier transform;
FIG. 4 is a schematic diagram of one phase of a three-phase grid-tie inverter system;
FIG. 5 is an experimental result of inverter port impedance measurement under single current control;
Fig. 6 is an experimental result of measuring the port impedance of the inverter under the control of adding active damping.
Detailed Description
The following description of the embodiments of the invention is presented in conjunction with the accompanying drawings to provide a better understanding of the invention to those skilled in the art. It is to be expressly noted that in the description below, detailed descriptions of known functions and designs are omitted here as perhaps obscuring the present invention.
FIG. 1 is a flow chart of an embodiment of a method for self-measuring port impedance of a grid-connected inverter according to the present invention.
In this embodiment, as shown in fig. 1, the method for self-measuring the port impedance of the grid-connected inverter is characterized by comprising the following steps:
Step S1: applying a disturbance signal at grid-side impedance Zg(s) =zg 1(s)
For a grid-connected inverter system, a disturbance signal with the frequency of f p is applied to a reference current at a current loop control input end of the grid-connected inverter, and at the moment, the impedance Zg(s) =zg 1(s) of the grid side. Wherein s represents a signal, and the signal to be(s) is a signal represented by complex numbers
Step S2: collecting reference current and phase voltage and phase current output by inverter
And sampling and acquiring the reference current after the disturbance signal is injected and the phase voltage and the phase current output by the inverter by using sampling equipment to obtain a reference current i r1 (t), a phase voltage u c1 (t) and a phase current i c1 (t). Where t represents time.
Step S3: harmonic extraction and time-frequency conversion to obtain a first group of complex frequency domain harmonic components
Carrying out harmonic extraction on a reference current i r1 (t), a phase voltage u c1 (t) and a phase current i c1 (t) by using discrete Fourier transform, and converting a harmonic component from a time domain to a frequency domain to obtain a first group of complex frequency domain harmonic components: reference current complex frequency domain harmonic component I r1(s), phase voltage complex frequency domain harmonic component U c1(s), phase current complex frequency domain harmonic component I c1(s);
Step S4: changing the impedance of the power grid side and the amplitude of the disturbance signal, and repeating the steps S2 to S3 to obtain a second group of complex frequency domain harmonic components
Changing the impedance Zg(s) =zg 2(s) at the power grid side, and repeating the steps (2) - (3) of sampling, harmonic extraction and time-frequency conversion to obtain a second group of complex frequency domain harmonic components: reference current complex frequency domain harmonic component I r2(s), phase voltage complex frequency domain harmonic component U c2(s), phase current complex frequency domain harmonic component I c2(s);
step S5: calculating inverter port impedance from two sets of complex frequency domain harmonic components
Inverter port impedance Z inv(s) is calculated according to the following equation:
In the invention, in order to obtain the port impedance characteristic of the grid-connected inverter, the invention further comprises the following steps:
Step S6: repeating the steps S1-S5 to obtain the port impedance characteristic of the grid-connected inverter
And (3) scanning the frequencies in the frequency range to be tested, and repeating the steps S1 to S5 to obtain the impedance of the grid-connected inverter under all frequencies, thereby obtaining the port impedance characteristic of the grid-connected inverter.
Fig. 2 is a graph of impedance model of the grid-tie inverter system of the present invention.
As shown in fig. 2, the inverter is denoted as I r(s)Tc(s), where I r(s) is a reference current, T c(s) is an inverter system closed loop transfer function, phase current is denoted as I c(s), inverter port impedance is denoted as Z inv(s), grid side impedance is denoted as Z g(s), and grid side voltage is denoted as U g(s).
As can be seen from fig. 2, in order to obtain the impedance characteristics of the inverter port impedance Z inv(s), it is necessary to perform disturbance signal injection to the grid-connected inverter system, considering that the inverter port impedance Z inv(s) is simultaneously acted by the reference current I r(s) which is an internal excitation signal of the inverter system and the grid-side voltage U g(s) which is an external signal. The invention considers the feasibility analysis of the impedance self-test principle based on the internal excitation signal, namely the reference current I r(s), and because the impedance measurement method of the internal harmonic injection method is different from the external harmonic injection method, the influence of the internal excitation signal, namely the reference current I r(s), is not negligible when the impedance of an inverter port is measured, and the influence of the external signal, namely the power grid side voltage U g(s), is also influenced, and the influence of the internal excitation signal, namely the reference current I r(s), and the external signal, namely the power grid side voltage U g(s), on the impedance measurement of the inverter port is considered when the internal excitation signal, namely the reference current I r(s), and the external signal, namely the power grid side voltage U g(s) are simultaneously acted.
On the basis of analyzing the self-test method principle of the port impedance of the grid-connected inverter system, two groups of linearly independent harmonic components are needed to be obtained simultaneously in order to solve the port impedance characteristic Z inv(s) of the inverter system, because the grid-connected inverter system is used for equivalent impedance models to linear models in modeling, only two groups of linearly dependent equations can be obtained only by changing the amplitude or the phase of the internal excitation signal I r, and further the port impedance Z inv(s) of the inverter can not be solved.
In order to obtain two sets of linear independent equations, after analyzing factors influencing the measured inverter port impedance, it can be found that the inverter port impedance is only related to the structural parameters, the filter parameters and the control parameters of the inverter, is unrelated to the circuit parameters outside the grid-connected inverter system, and the external circuit parameters of the grid-connected inverter system are changed without changing the inverter port impedance characteristics. Based on the analysis, the invention provides a method for changing the impedance of the power grid side, after the impedance of the power grid side is changed, two groups of linearly independent signals can be obtained after the same harmonic signals are injected, and the alternative method for changing the impedance of the power grid side is to change the values of a series inductance parameter, a parallel capacitance parameter and a resistance parameter, and respectively measure the reference signals and response signals of the system before and after changing the impedance of the power grid.
The equation set including the two sets of linearly independent equations is shown in equation (1).
First, the relation between Z inv(s) and I r(s),Ic(s),Uc(s) needs to be solved, T c(s) in the equation set needs to be eliminated, and the formula (1) is expanded to obtain the formula (2).
And (3) obtaining the formula (3) after performing the term shifting treatment on the equation set.
Dividing the equation set of the formula (3) by two results in the expression (4) not including T c(s).
After the arithmetic processing of the expression (4), an expression (5) about the measured variable with respect to the inverter port impedance Z inv(s) can be obtained.
Fig. 3 is a schematic diagram of a discrete fourier transform.
In fig. 3, reference current i r1(t)、ir2 (t), phase voltage u c1(t)、uc2 (t), and phase current i c1(t)、ic2 (t) are all represented as input signals x (t), and sin (ω 0t)、cos(ω0 t) is a sine and cosine signal of the output phase of the grid-synchronous phase-locked loop.
As shown in fig. 3, the input signal x (t) is multiplied by the sine and cosine signal sin (ω 0t)、cos(ω0 t), then is respectively subjected to periodic averaging, and then the real part (Re) and the imaginary part (Im) are respectively taken, so that the amplitude |u| and the phase |u of the response signal which jointly act to generate the frequency point to be measured are obtained, and the corresponding frequency domain harmonic component is obtained.
Examples
Fig. 4 is a schematic diagram of one phase of a three-phase grid-tie inverter system.
Fig. 4 shows a schematic diagram of one phase of a three-phase grid-tied inverter system, the other two phases being identical. The inverter is connected with an LCL filter formed by an inductor L 1、L2 and a capacitor C, and the grid-connected inverter system adopts a current loop plus active damping control. The active damping control adopts capacitor voltage feedback and capacitor current feedforward, wherein the capacitor voltage feedback is that a reference signal generated by capacitor voltage u c through a controller H f is superimposed on a signal of an original control loop to jointly act on the system; the capacitive current feedforward, i.e. the capacitive C output current i c, is superimposed on the signal of the original control loop via the reference signal generated by the controller H ad. The reference current i ref is subtracted from the current loop to select the current i 1 at the inverter side of the LCL filter or the current i 2 at the power grid side, a difference signal obtained is transmitted to the controller G c, a signal of a control loop is generated, a switching signal is generated through space vector pulse width modulation SV-PWM to control a switching tube, an output signal of a synchronous power grid is generated and is output to the LCL filter, a sinusoidal output signal synchronous with the power grid is obtained, and inversion from a direct current Source (DC Source) to an alternating current Source is achieved. In fig. 4, PCC is a common connection point, u pcc is a grid-side voltage, L g is a grid inductance, and the working principle of the grid-connected inverter system belongs to the prior art, which is not described in detail herein.
In this embodiment, the current control method adopts proportional resonance control, the active damping control method adopts proportional control, an experimental model as shown in fig. 4 is established in the experiment, the output adopts an LCL filter, and the parameters used by the inverter are shown in table 1.
| Description of the invention | Numerical value |
| DC side voltage | 350V |
| Grid frequency | 50Hz |
| Ac side voltage | 220V |
| LCL filter capacitor | 25uf |
| LCL filter inductance | 6mH |
| Switching frequency | 3kHz |
| Current loop proportional gain | 6.3 |
| Current resonance gain | 1973.9 |
| Capacitive current active damping gain | 2.03 |
| Capacitor voltage active damping gain | 0.5 |
TABLE 1
The experimental platform is built in a laboratory environment, an inverter port impedance measurement result under single current control is added into an inverter port impedance measurement experimental result under active damping control.
From fig. 5 and fig. 6, it can be seen that, by using the self-measuring method for the port impedance of the grid-connected inverter, the amplitude-frequency characteristic curve of the port impedance of the grid-connected inverter is basically identical to the theoretical value in the middle-low frequency band, and some deviation occurs in the high frequency band, which indicates that the invention can accurately measure the port impedance of the inverter while realizing the reduction of the measurement cost without needing a large amount of calculation, thereby realizing the purpose of the invention.
While the foregoing describes illustrative embodiments of the present invention to facilitate an understanding of the present invention by those skilled in the art, it should be understood that the present invention is not limited to the scope of the embodiments, but is to be construed as protected by the accompanying claims insofar as various changes are within the spirit and scope of the present invention as defined and defined by the appended claims.
Claims (2)
1. The grid-connected inverter port impedance self-measurement method is characterized by comprising the following steps of:
(1) For a grid-connected inverter system, applying a disturbance signal with frequency of f p to a reference current at a current loop control input end of the grid-connected inverter, wherein at the moment, the impedance on the power grid side Zg(s) =zg 1(s);
(2) Sampling the reference current after the disturbance signal is injected and the phase voltage and the phase current output by the inverter by using sampling equipment to obtain a reference current i r1 (t), a phase voltage u c1 (t) and a phase current i c1 (t);
(3) Performing harmonic extraction on a reference current i r1 (t), a phase voltage u c1 (t) and a phase current i c1 (t) by using discrete Fourier transform, and converting a harmonic component from a time domain to a frequency domain to obtain a first group of complex frequency domain harmonic components: reference current complex frequency domain harmonic component I r1(s), phase voltage complex frequency domain harmonic component U c1(s), phase current complex frequency domain harmonic component I c1(s);
(4) Changing the impedance Zg(s) =zg 2(s) at the power grid side, and repeating the steps (2) - (3) of sampling, harmonic extraction and time-frequency conversion to obtain a second group of complex frequency domain harmonic components: reference current complex frequency domain harmonic component I r2(s), phase voltage complex frequency domain harmonic component U c2(s), phase current complex frequency domain harmonic component I c2(s);
(5) Inverter port impedance Z inv(s) is calculated from the two sets of harmonic complex frequency domain components:
2. the grid-tie inverter port impedance self-measurement method of claim 1, further comprising the steps of:
(6) And (3) scanning the frequencies in the frequency range to be tested, and repeating the steps (1) to (5) to obtain the impedance of the grid-connected inverter under all frequencies, so as to obtain the port impedance characteristic of the grid-connected inverter.
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| CN110112776A (en) * | 2019-05-07 | 2019-08-09 | 郑州轻工业学院 | Consider the gird-connected inverter electric network impedance discrimination method of power grid background harmonics |
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| CN111239491B (en) * | 2019-12-31 | 2021-01-26 | 浙江大学 | Real-time experimental measurement method of generalized impedance using disturbance injection of physical controller |
| US11680558B2 (en) * | 2020-09-16 | 2023-06-20 | General Electric Company | Grid-forming control of inverter-based resource using virtual impedance |
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| CN110112776A (en) * | 2019-05-07 | 2019-08-09 | 郑州轻工业学院 | Consider the gird-connected inverter electric network impedance discrimination method of power grid background harmonics |
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