CN111077375B - On-line identification method of power grid impedance under the influence of background harmonics based on frequency domain subtraction - Google Patents

Abstract

The invention discloses an on-line identification method of power grid impedance under the influence of background harmonics based on frequency domain subtraction. The system studied by the method includes a DC side voltage source, a three-phase full-bridge inverter circuit, a three-phase LC filter, a three-phase Grid impedance and three-phase grid. This method uses one sampling, injects a disturbance component whose frequency is equal to the background frequency f, and then samples again, and estimates the magnitude of the grid impedance under the influence of background harmonics only by detecting the voltage and current of the PCC point, which is then used for inverter adaptive control. , improve the stability of the grid-connected system without increasing the cost.

Description

On-line identification method of power grid impedance under the influence of background harmonics based on frequency domain subtraction

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 background harmonics.

Background technique

As photovoltaic power generation, wind power generation and other new energy power generation occupy a larger and larger proportion in the power system, and the local consumption capacity of new energy power generation is limited, long-distance power transmission is required, and the line inductive reactance increases. For the inverter, the larger line impedance will reduce the stability of the grid-connected inverter, or even cause instability. Therefore, the inverter should have the ability to estimate the grid impedance online.

In addition, due to the diversity and uncertainty of the load carried by the power grid, the power grid also contains abundant background harmonics in addition to the fundamental wave. These background harmonics make the traditional disturbance injection method have large errors in measurement, and may even lead to wrong estimation of grid impedance. Based on the above two points, it is very necessary to accurately measure the power grid impedance considering the background harmonics.

At present, there have been many academic papers on the online identification of power grid impedance, such as:

1. The article titled "Grid Impedance Estimation Method Based on Complex Filter and Non-characteristic Subharmonic Injection" ("Power Grid Technology", No. 10, 2013, pp. 2796-2801). The grid impedance estimation method uses the injection of non-characteristic harmonics and the complex filter to extract the injected harmonic response, but the influence of background harmonics in the grid is not considered.

2. Titled "Grid impedance estimation for islanding detection and adaptivecontrol of converters", A.Ghanem, M.Rashed, M.Sumner, M.A.Elsayes, and I.I.I.Mansy, IET Power Electronics, 2017:1279-1288 ("Grid-based "Islanding Detection and Adaptive Control of Converters Based on Impedance Estimation", "IET Journal of Power Electronics", published online in 2017), by sampling the switching frequency level voltage and current, and using the circuit equation when the switch is turned on and off to estimate the grid impedance , although this method can ignore the influence of background harmonics in the power grid, but it requires high-precision sensor sampling, which is not conducive to engineering applications.

3. "Identification Method of Grid-connected Inverter Grid Impedance Considering Grid Background Harmonics" published on August 9, 2019 by Chinese invention patent document (publication number CN 110112776 A), which proposes a method of injecting high voltage into the grid. The power grid impedance is measured by using a complex filter to extract high-frequency harmonic components. Although this method takes into account the influence of background harmonics in the grid, the invention ignores the influence of possible lower frequency background harmonics by injecting a high frequency signal.

Based on the above literature, the existing technology has the following shortcomings:

1. The existing grid impedance estimation methods based on non-characteristic harmonic disturbance injection do not consider the influence of background harmonics contained in the grid. It is necessary to study the grid impedance estimation method under the condition that the background harmonics cannot be ignored.

2. The existing method for considering the influence of background harmonics adopts the injection, extraction and analysis of high-frequency harmonic voltage and current to avoid the aliasing effect between the background harmonic frequency and the injection frequency, so it is necessary to Study the impedance estimation method when the background harmonic frequency interacts with the injection frequency.

SUMMARY OF THE INVENTION

The invention proposes an on-line identification method of power grid impedance under the influence of background harmonics based on frequency domain subtraction. The method considers the background harmonics contained in the power grid and does not need to inject high-frequency signals to estimate the power grid impedance. The grid impedance estimated by the method is accurate, and can be used for the adaptive control of the inverter to improve the stability of the grid-connected system without increasing the cost.

The object of the present invention is achieved in this way. The invention provides an on-line identification method for power grid impedance under the influence of background harmonics based on frequency domain subtraction. The main circuit topology structure of the grid-connected inverter using the method includes a DC side voltage source, a three-phase full-bridge inverter circuit, a three-phase full-bridge inverter circuit, and a three-phase full-bridge inverter circuit. Phase LC filter, three-phase grid impedance and three-phase grid, the DC side voltage source is connected with the three-phase full-bridge inverter circuit, and the three-phase full-bridge inverter circuit is connected with the three-phase grid impedance through the three-phase LC filter Access to three-phase power grid;

On-line identification of power grid impedance under the influence of background harmonics is performed regularly, that is, an identification interval time T is preset, and when the identification interval time T arrives, first sampling, disturbance injection, second sampling, and online identification of power grid impedance are started:

Specifically, the steps for the online identification cycle of grid impedance under the influence of a background harmonic are as follows:

Step 1, set the inverter grid-connected to be in a stable working state, sample the voltage of the A-phase output terminal of the three-phase LC filter capacitor once, record it as the A-phase voltage U _{PCC_A1} at the PCC point, and measure the A-phase flowing through the three-phase grid impedance. The current is sampled once, and is recorded as the A-phase current I _{PCC_A1} at the PCC point;

其中U_1f是未注入扰动时频率为f的谐波电压矢量

的模值，θ_u1f是未注入扰动时频率为f的谐波电压矢量

相角；对步骤1中采集得到的PCC点A相电流I_{PCC_A1}，在f频率点做傅里叶分析得到未注入扰动时频率为f的谐波电流矢量

其中I_1f是未注入扰动时频率为f的谐波电流矢量

的模值，

是未注入扰动时频率为f的谐波电流矢量

的相角；Step 2: Suppose the power grid contains background harmonics, denote the frequency of the background harmonics as the background frequency f, and perform a Fourier transform on the phase A voltage U _{PCC_A1} of the PCC point collected in step 1 at the frequency point of the background frequency f. The harmonic voltage vector with frequency f when no disturbance is injected is obtained by analysis

where U _1f is the harmonic voltage vector of frequency f when no disturbance is injected

The modulo value of , θ _u1f is the harmonic voltage vector of frequency f when no disturbance is injected

Phase angle; for the phase A current I _{PCC_A1} at the PCC point collected in step 1, perform Fourier analysis at the frequency point f to obtain the harmonic current vector with frequency f when no disturbance is injected

where I _1f is the harmonic current vector of frequency f when no disturbance is injected

the modulo value of ,

is the harmonic current vector of frequency f when no disturbance is injected

the phase angle;

Step 3: Inject a disturbance component whose frequency is equal to the background frequency f on the modulated wave signal of the inverter, re-sample the voltage of the A-phase output terminal of the three-phase LC filter capacitor and record it as the A-phase voltage U at the secondary PCC point. _{PCC_A2} , the A-phase current flowing through the impedance of the three-phase power grid is sampled twice and recorded as the A-phase current I _{PCC_A2} at the secondary PCC point;

其中U_2f是注入扰动时频率为f的电压矢量

的模值，θ_u2f是注入扰动时频率为f的谐波电压矢量

的相角；对步骤2中得到的二次PCC点A相电流I_{PCC_A2}，在背景频率f的频率点做傅里叶分析得到注入扰动时频率为f的谐波电流矢量

其中I_2f是注入扰动时频率为f的谐波电流矢量

的模值，

是注入扰动时频率为f的谐波电流矢量

的相角；Step 4: Perform Fourier analysis on the phase A voltage U _{PCC_A2} at the secondary PCC point obtained in step 2 at the frequency point of the background frequency f to obtain the harmonic voltage vector with frequency f when the disturbance is injected.

where U _2f is the voltage vector of frequency f when the perturbation is injected

The modulo value of , θ _u2f is the harmonic voltage vector of frequency f when the disturbance is injected

The phase angle of the second PCC point A obtained in step 2, I _{PCC_A2} , do Fourier analysis at the frequency point of the background frequency f to obtain the harmonic current vector with the frequency f when the disturbance is injected

where I _2f is the harmonic current vector at frequency f when the disturbance is injected

the modulo value of ,

is the harmonic current vector of frequency f when the disturbance is injected

the phase angle;

和未注入扰动时频率为f的谐波电流矢量

步骤4中得到的注入扰动时频率为f的谐波电压矢量

和注入扰动时频率为f的谐波电流矢量

计算阻抗值Z,计算式如下：Step 5, record the value of the impedance of the three-phase power grid as the impedance value Z, and use the harmonic voltage vector with frequency f obtained in step 2 when no disturbance is injected.

and the harmonic current vector of frequency f when no disturbance is injected

The harmonic voltage vector of frequency f obtained in step 4 when the perturbation is injected

and the harmonic current vector of frequency f when the disturbance is injected

Calculate the impedance value Z, the formula is as follows:

Among them, |Z| is the modulus value of the impedance value Z, δ is the phase angle of the impedance value Z, R _g is the resistive component of the impedance value Z, L _g is the inductive component of the impedance value Z, j is the imaginary unit, j ² =-1.

Compared with the prior art, the beneficial effects of the present invention are:

1. The method for online identification of power grid impedance under the influence of background harmonics based on frequency domain subtraction according to the present invention can accurately estimate the power grid impedance by detecting the voltage and current of the PCC point, which is then used for inverter adaptive control to improve grid connection. system stability;

2. The present invention solves the influence of the background harmonics that may always exist in the power grid on the power grid impedance identification by analyzing the phase angle characteristics of the voltage and current amplitude at the PCC point, and measuring and calculating the harmonic voltage and current amplitude phase angle twice;

3. The on-line identification method of power grid impedance under the influence of background harmonics based on frequency domain subtraction according to the present invention is only improved on the algorithm of the existing power electronic converter system, without adding additional equipment, such as sensors, etc.;

Description of drawings

FIG. 1 is a topology diagram of a main circuit of a grid-connected inverter using the method for online identification of grid impedance under the influence of background harmonics based on frequency domain subtraction according to the present invention.

Figure 2 is a comparative analysis diagram of the influence of power grid background harmonics on impedance measurement.

Fig. 3 is a graph of the modulus value |Z| and the impedance angle of the measured grid impedance Z.

FIG. 4 is an enlarged partial view of area A in FIG. 3 .

FIG. 5 is a test diagram of the inductance L _g part and the resistance R _g part of the grid impedance Z.

FIG. 6 is an enlarged partial view of area B in FIG. 5 .

Detailed ways

The present embodiment will be described in detail below with reference to the accompanying drawings.

1 is a topological structure diagram of the main circuit of a grid-connected inverter to which the present invention is applied. As can be seen from the figure, the topological structure includes a DC side voltage source 10, a three-phase full-bridge inverter circuit 20, a three-phase LC filter 30, and a three-phase LC filter. Phase grid impedance 40 and three-phase grid 50 .

The DC side voltage source 10 is connected to the three-phase full-bridge inverter circuit 20 , and the three-phase full-bridge inverter circuit 20 is connected to the three-phase grid 50 after being connected to the three-phase grid impedance 40 through the three-phase LC filter 30 . In FIG. 1 , V _dc is the DC side voltage source 10 , L _f is the bridge arm side inductance of the three-phase LC filter 30 , C _f is the filter capacitor of the three-phase LC filter 30 , and r is the passive source of the three-phase LC filter 30 . Damping resistance, R _g is the resistance in the three-phase line impedance 40 , L _g is the inductance in the three-phase grid impedance 40 , and Grid is the three-phase grid 50 .

In this embodiment, the main circuit parameters are: the DC side voltage V _dc is 800V, the rated output line voltage of the inverter is 380V/50Hz, the rated power of the inverter is 100kW, the filter inductance L _f is 0.56mH, the filter capacitor C _f and The passive damping resistance r is 270uF/0.3Ω, the inductance part L _g =0.1mH and the resistance part R _g =0.1Ω in the three-phase power grid impedance.

The method for online identification of power grid impedance under the influence of background harmonics based on frequency domain subtraction according to the present invention regularly performs online identification of power grid impedance under the influence of background harmonics, that is, an identification interval time T is preset, and the identification interval time T reaches, Start primary sampling, disturbance injection, secondary sampling, and online identification of grid impedance. In this embodiment, T is selected to be 16 fundamental wave period periods T ₀ .

Specifically, the steps for the online identification cycle of grid impedance under the influence of a background harmonic are as follows:

Step 1, set the inverter grid-connected to be in a stable working state, sample the voltage of the A-phase output terminal of the three-phase LC filter 30 capacitor once, record it as the A-phase voltage U _{PCC_A1} at the PCC point, and measure the voltage flowing through the three-phase grid impedance 40 . The A-phase current is sampled once, and is recorded as the A-phase current I _{PCC_A1} at the PCC point;

其中U_1f是未注入扰动时频率为f的谐波电压矢量

的模值，θ_u1f是未注入扰动时频率为f的谐波电压矢量

相角；对步骤1中采集得到的PCC点A相电流I_{PCC_A1}，在f频率点做傅里叶分析得到未注入扰动时频率为f的谐波电流矢量

其中I_1f是未注入扰动时频率为f的谐波电流矢量

的模值，

是未注入扰动时频率为f的谐波电流矢量

的相角；Step 2: Suppose the power grid contains background harmonics, denote the frequency of the background harmonics as the background frequency f, and perform a Fourier transform on the phase A voltage U _{PCC_A1} of the PCC point collected in step 1 at the frequency point of the background frequency f. The harmonic voltage vector with frequency f when no disturbance is injected is obtained by analysis

where U _1f is the harmonic voltage vector of frequency f when no disturbance is injected

The modulo value of , θ _u1f is the harmonic voltage vector of frequency f when no disturbance is injected

Phase angle; for the phase A current I _{PCC_A1} at the PCC point collected in step 1, perform Fourier analysis at the frequency point f to obtain the harmonic current vector with frequency f when no disturbance is injected

where I _1f is the harmonic current vector of frequency f when no disturbance is injected

the modulo value of ,

is the harmonic current vector of frequency f when no disturbance is injected

the phase angle;

Step 3: Inject a disturbance component whose frequency is equal to the background frequency f on the modulated wave signal of the inverter, and resample the voltage of the phase A output terminal of the capacitor of the three-phase LC filter 30 and record it as the phase A voltage of the secondary PCC point. U _{PCC_A2} , the A-phase current flowing through the three-phase grid impedance 40 is sampled twice and recorded as the A-phase current I _{PCC_A2} at the secondary PCC point;

其中U_2f是注入扰动时频率为f的电压矢量

的模值，θ_u2f是注入扰动时频率为f的谐波电压矢量

的相角；对步骤2中得到的二次PCC点A相电流I_{PCC_A2}，在背景频率f的频率点做傅里叶分析得到注入扰动时频率为f的谐波电流矢量

其中I_2f是注入扰动时频率为f的谐波电流矢量

的模值，

是注入扰动时频率为f的谐波电流矢量

的相角；Step 4: Perform Fourier analysis on the phase A voltage U _{PCC_A2} at the secondary PCC point obtained in step 2 at the frequency point of the background frequency f to obtain the harmonic voltage vector with frequency f when the disturbance is injected.

where U _2f is the voltage vector of frequency f when the perturbation is injected

The modulo value of , θ _u2f is the harmonic voltage vector of frequency f when the disturbance is injected

The phase angle of the second PCC point A obtained in step 2, I _{PCC_A2} , do Fourier analysis at the frequency point of the background frequency f to obtain the harmonic current vector with the frequency f when the disturbance is injected

where I _2f is the harmonic current vector at frequency f when the disturbance is injected

the modulo value of ,

is the harmonic current vector of frequency f when the disturbance is injected

the phase angle;

和未注入扰动时频率为f的谐波电流矢量

步骤4中得到的注入扰动时频率为f的谐波电压矢量

和注入扰动时频率为f的谐波电流矢量

计算阻抗值Z,计算式如下：In step 5, the value of the impedance 40 of the three-phase power grid is recorded as the impedance value Z, and the harmonic voltage vector with the frequency f when the disturbance is not injected is obtained in step 2.

and the harmonic current vector of frequency f when no disturbance is injected

The harmonic voltage vector of frequency f obtained in step 4 when the perturbation is injected

and the harmonic current vector of frequency f when the disturbance is injected

Calculate the impedance value Z, the formula is as follows:

Among them, |Z| is the modulus value of the impedance value Z, δ is the phase angle of the impedance value Z, R _g is the resistive component of the impedance value Z, L _g is the inductive component of the impedance value Z, j is the imaginary unit, j ² =-1.

Taking an example to verify the effectiveness of the present invention, simulation verification is carried out.

Figure 2 is a comparative analysis diagram of the influence of power grid background harmonics on impedance measurement. It can be seen from Figure 2 that the grid impedance is measured by the conventional disturbance injection method, and it can be seen that the grid impedance can be accurately measured without considering the background harmonics; however, the background harmonics suddenly appear in the grid, resulting in a sudden change in the measured grid impedance, and the measurement is An error will occur.

FIG. 3 is a graph of the modulus value |Z| and the impedance angle of the measured grid impedance Z, and FIG. 4 is an enlarged partial graph of the area A in FIG. 3 . As can be seen in Figure 3 and Figure 4, by measuring the background harmonic information, the influence of the background harmonic on the impedance identification is eliminated, and the obtained grid impedance is relatively accurate.

FIG. 5 is a test diagram of the inductance L _g part and the resistance R _g part of the grid impedance Z, and FIG. 6 is an enlarged partial diagram of the area B in FIG. 5 . It can be seen from FIG. 5 and FIG. 6 that the method of the present invention can also accurately measure the magnitude of the inductive component and the resistive component in the grid impedance.

Claims (1)

1. A grid-connected inverter main circuit topological structure applying the method comprises a direct current side voltage source (10), a three-phase full-bridge inverter circuit (20), a three-phase LC filter (30), a three-phase grid impedance (40) and a three-phase grid (50), wherein the direct current side voltage source (10) is connected with the three-phase full-bridge inverter circuit (20), and the three-phase full-bridge inverter circuit (20) is connected with the three-phase grid impedance (40) through the three-phase LC filter (30) and then is connected with the three-phase 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 A-phase output end of a capacitor of a three-phase LC filter (30) for one time, and marking as a PCC point A-phase voltage U_{PCC_A1}The phase A current flowing through the three-phase 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 injected

Wherein U is_1fIs a harmonic voltage vector of frequency f without injected disturbance

Modulus of (e), theta_u1fIs a harmonic voltage vector of frequency f without injected disturbance

A 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 injected

Wherein I_1fIs a harmonic current vector of frequency f when no disturbance is injected

The value of the modulus of the (c) component,

is a harmonic current vector of frequency f when no disturbance is injected

The 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 A-phase output end of the capacitor of the three-phase LC filter (30) and recording the voltage as the secondary PCC point A-phase voltage U_{PCC_A2}The phase-A current flowing through the three-phase network impedance (40) is subsampled and recorded as a secondary PCC point phase-A 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 out

Wherein U is_2fIs a voltage vector of frequency f at the time of injection of a disturbance

Modulus of (e), theta_u2fIs a harmonic voltage vector of frequency f when injecting a disturbance

The 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 injected

Wherein I_2fIs a harmonic current vector of frequency f when injecting a disturbance

The value of the modulus of the (c) component,

is a harmonic current vector of frequency f when injecting a disturbance

The phase angle of (d);

step 5, recording the numerical value of the three-phase 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 2

And a harmonic current vector of frequency f when no disturbance is injected

The harmonic voltage vector with the frequency f during injection disturbance obtained in the step 4

And the harmonic current vector with frequency f when injecting disturbance

Calculating 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_gIs a resistive component of the impedance value Z, L_gIs the inductive component of the impedance value Z, j being the unit of an imaginary number, j²＝-1。

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