Method for measuring unbalanced grid impedance based on grid forming converters

Abstract

A kind of network converter-based unbalanced grid impedance measurement method, the method first establishes the equivalent circuit model of network converter under negative sequence network, accurately extracts the negative sequence component of point of common coupling voltage and output current by resonance controller;Then using impedance reshaping technology, by switching current feedforward and voltage feedback two control modes, make network converter under negative sequence network respectively present high impedance and low impedance characteristics;Finally, based on the negative sequence voltage and current calculated grid negative sequence impedance measured.The invention solves the problem of insufficient measurement accuracy of traditional method under unbalanced grid conditions, without additional hardware equipment, with the characteristics of simple realization, low cost, strong adaptability, especially suitable for weak grid system containing high proportion of renewable energy.

Description

Technical Field

[0001] This invention belongs to the field of power system and power electronics technology, specifically relating to a method for measuring unbalanced grid impedance based on grid converters, which is particularly suitable for scenarios where distributed power sources and renewable energy are widely integrated in modern power systems. Background Technology

[0002] With the large-scale grid integration of distributed power sources and renewable energy, the structure and dynamic characteristics of power grids are becoming increasingly complex. Especially under unbalanced grid conditions, traditional impedance measurement methods face significant challenges. Existing technologies are mainly based on the assumption of a symmetrical power grid, making it difficult to accurately measure the impedance of the power grid under unbalanced conditions. This leads to increased measurement errors, which in turn affect the operating efficiency and stability of the power grid.

[0003] Traditional impedance measurement methods typically employ the symmetrical component method or harmonic injection method, but these methods perform poorly in unbalanced power grids. The symmetrical component method relies on the assumption of grid symmetry and cannot effectively handle three-phase voltage or current imbalances; while the harmonic injection method may introduce additional interference, affecting the normal operation of the power grid. Furthermore, existing technologies lack accurate modeling and control strategies for negative sequence impedance, making it difficult to achieve high-precision impedance measurements.

[0004] Existing grid-connected converters mostly employ fixed control modes in impedance measurement, which cannot flexibly adapt to dynamic changes in the power grid, especially with limited impedance regulation capabilities under negative-sequence networks. These problems limit the applicability and measurement accuracy of traditional methods in unbalanced power grids. Summary of the Invention

[0005] This invention proposes an unbalanced grid impedance measurement method based on a grid-connected converter, aiming to solve the problems of insufficient accuracy and poor adaptability of traditional impedance measurement techniques under unbalanced grid conditions. This method achieves high-precision measurement of the negative-sequence impedance of the grid by establishing an equivalent model of the negative-sequence network and combining a resonant controller and impedance reshaping technology, while maintaining the stability of the system under positive-sequence network conditions.

[0006] The technical solution of the present invention is as follows:

[0007] A method for measuring the impedance of an unbalanced power grid based on a grid converter, characterized by the following steps:

[0008] S1. Establish the equivalent circuit model of the grid converter under the negative sequence network, and convert the machine side part of the grid converter into a Thevenin circuit with a controlled voltage source and the output impedance in series.

[0009] S2. Extract the negative sequence components of the point of common coupling (PCC) voltage and the grid converter output current through a resonant controller;

[0010] S3. Impedance reshaping technology is employed by switching between current feedforward mode and voltage feedback mode:

[0011] - In current feedforward mode, the grid converter exhibits high impedance characteristics under negative sequence network, which is equivalent to an ideal current source;

[0012] - In voltage feedback mode, the grid converter exhibits low impedance characteristics under negative sequence network, which is equivalent to an ideal voltage source;

[0013] S4 calculates the negative sequence impedance of the power grid based on the negative sequence component extracted in step S2 and the impedance characteristics obtained in step S3.

[0014] S5. Maintain the impedance characteristics of the grid converter under the positive sequence network to ensure normal system operation.

[0015] Furthermore, the establishment of the equivalent circuit model in step S1 includes:

[0016] Establish the sequence component network circuit equations for the grid converter based on Kirchhoff's voltage law;

[0017] The mathematical relationship between negative sequence voltage, grid negative sequence impedance, and grid converter negative sequence output impedance is derived.

[0018] Furthermore, the resonant controller G R The transfer function expression for (s) is:

[0019]

[0020] In the formula, k s For the resonant gain, ω c For G R The bandwidth of (s) ranges from 5 to 20ad / s, ω2 is twice the fundamental angular frequency, and s represents the Laplace operator.

[0021] Furthermore, the current feedforward mode enables the grid converter to function as an ideal current source in a negative-sequence network, with its equivalent output impedance Z... o Approaching infinity, used to measure the negative sequence voltage v of the PCC. an ;

[0022] The voltage feedback mode makes the grid-connected converter equivalent to an ideal voltage source under negative-sequence network conditions, and its equivalent output impedance Zo approaches zero, which is used to measure the negative-sequence output current i of the grid-connected converter. an .

[0023] Furthermore, the negative sequence impedance Z of the power grid gn for

[0024]

[0025] In the formula, Rg ω1 represents the grid resistance, L represents the grid frequency, and L represents the grid resistance. g This represents the inductance of the power grid.

[0026] Compared with the prior art, the technical effects of the present invention are as follows:

[0027] (1) Improve measurement accuracy: This method can maintain high measurement accuracy and small error over a wide range of impedance ratios.

[0028] (2) The use of a resonant controller can maintain the stability and robustness of the system when the power grid frequency fluctuates.

[0029] (3) Theoretical support: By establishing a relationship model between negative sequence voltage, grid negative sequence impedance and grid converter negative sequence output impedance, a theoretical basis is provided for the adjustment of grid converter impedance. Attached Figure Description

[0030] Figure 1 Block diagram of grid-connected converter system

[0031] Figure 2 Simplified block diagram of grid-connected converter system

[0032] Figure 3 Equivalent control block diagram of the inner loop of the grid converter

[0033] Figure 4 Equivalent topology of grid converter connected to an asymmetric weak grid

[0034] Figure 5 The proposed method for measuring grid impedance under unbalanced power grids

[0035] Figure 6 Zo Bode plots of equivalent impedance of grid converter under two control modes

[0036] Figure 7 Measurement results when the grid impedance-to-inductance ratio R / X = 0.01

[0037] Figure 8 Measurement results when the grid impedance-to-inductance ratio R / X = 0.5

[0038] Figure 9 Measurement results when the grid impedance-to-inductance ratio R / X = 1

[0039] Figure 10 The flowchart of the present invention is a method for measuring the unbalanced grid impedance based on a grid converter. Detailed Implementation

[0040] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings.

[0041] Please see Figure 1 , Figure 1 The diagram shows a grid-connected converter system block diagram. The grid-connected converter is connected to the PCC via an LC filter. Lf and Cf are the filter inductor and filter capacitor, respectively. iL, io, and vo are the filter inductor current, the grid-connected converter output current, and the filter capacitor voltage, respectively. Zg is the equivalent line impedance. The grid-connected converter controller mainly consists of two parts: an outer-loop power calculation and control section, and an inner-loop voltage and current control section.

[0042] Due to the three-phase symmetry of the grid-connected converter system, in the αβ stationary coordinate system, the grid-connected converter can be equivalently modeled as a single-input-single-output system, and its simplified open-loop model is as follows: Figure 2 As shown. The second-order transfer function equation of the controlled object is:

[0043]

[0044]

[0045] In the formula Z Lf (s), Y Cf (s) represent the filter inductance L f impedance and filter capacitor C f Admittance.

[0046] For the inner loop control circuit of a grid converter, the goal is to track the voltage command value output by the outer power loop, such as... Figure 3 As shown. Voltage loop controller G v (s), Current loop controller G c (s) all use PI controllers, G f (s) is a first-order low-pass filter (LPF). The system delay caused by sampling and modulation can be expressed by the linearized transfer function. It means that T in the formula s This refers to the system control cycle.

[0047] according to Figure 3 Without considering v oref Under dynamic conditions, the system closed-loop transfer function and the equivalent output impedance of the grid converter are expressed as follows:

[0048]

[0049] In the formula

[0050]

[0051] Therefore, the voltage command value v output by the power loop oref , filter capacitor voltage v o and feeder output current io The relationship between them is

[0052] v o =v oref G vv -i o Z o (9)

[0053] A grid-connected converter system can be divided into two parts: the generator-side part, consisting of the converter and its grid-connected control strategy, and the grid-side part, which includes the remote grid voltage and line impedance. According to Thevenin's theorem, the sequence component network of the generator-side part of the grid-connected converter can be equivalently represented as a circuit structure of a controlled voltage source in series with an impedance. Based on this, an equivalent sequence network model of the grid-connected converter control system is established, such as... Figure 4 As shown. In Figure 4 In this model, 'e' represents the equivalent internal potential of the grid converter. According to this model, the grid voltage v... g The combined effect of the equivalent internal potential e on the series-connected Z g and Z o The current i caused by the above o .

[0054] According to Kirchhoff's voltage law, the sequence component network circuit equation of the grid converter can be expressed as follows:

[0055]

[0056] It can be seen that the system line impedance is composed of the passive impedance Z g and active impedance Z o Composition. Then the PCC voltage can be expressed as:

[0057]

[0058] Because the LPF bandwidth of the power loop is much smaller than 100Hz, the negative sequence voltage command value |v_grid converter generated by the grid converter is... oref (-jω1)|(ω1 represents the fundamental frequency) can be neglected in the analysis, that is, in the negative sequence network, the negative sequence controlled voltage source can be omitted. Therefore, in the negative sequence network model, the output negative sequence current and PCC negative sequence voltage of the grid converter are respectively

[0059]

[0060] In the formula, the subscript "n" represents the negative-order component, which corresponds to the value of the variable at -jω1.

[0061] In summary, by analyzing the negative-sequence voltage characteristics of the grid converter, a relationship model between the negative-sequence voltage, the grid negative-sequence impedance, and the grid converter negative-sequence output impedance was established, providing a theoretical basis for further research on the regulation of negative-sequence impedance. To achieve coordinated optimization of negative-sequence voltage and current, the mathematical expression between negative-sequence voltage and negative-sequence impedance was derived from the equivalent model of the grid converter's negative-sequence network. According to the equivalent circuit modeling analysis, the dynamic characteristics of the negative-sequence voltage are mainly determined by the relationship between the grid negative-sequence impedance and the grid converter negative-sequence output impedance. From equations (12) and (13), it can be seen that when |Z o When (-jω1)|=0 or ∞, the transformation from an ideal voltage source (impedance of 0) to an ideal current source (impedance tending to infinity) can be achieved in a negative sequence network. Based on this principle, the inner loop control block diagram of the redesigned grid converter is shown below. Figure 5 As shown.

[0062] G in the diagram R (s) is a reduced-order resonant controller, mainly used to extract the negative-sequence components of the PCC voltage and the grid converter output current. G R The transfer function expression for (s) is:

[0063]

[0064] In the formula k s For the resonant gain, ω c For G R The bandwidth of (s) is typically taken in the range of 5–20 ad / s, and ω2 is twice the fundamental angular frequency. By selecting an appropriate resonant gain k s and bandwidth ω c It can extract negative sequence components relatively accurately and exhibits strong robustness when facing grid voltage frequency fluctuations.

[0065] By selecting the output current i o Feedforward or capacitor voltage v o Feedback mechanisms are used to reshape the equivalent impedance of grid converters.

[0066] (1) Select the output current i o Feedforward: In a negative-sequence network, the grid-connected converter is equivalent to an ideal current source, and its equivalent impedance is:

[0067]

[0068] (2) Select capacitor voltage v o Feedback: In a negative-sequence network, the grid-connected converter is equivalent to an ideal voltage source, and its equivalent impedance is...

[0069]

[0070] In the formula

[0071]

[0072] This yields the equivalent output impedance Z of the grid converter under the two control modes. o Bird diagram Figure 6 As shown in the figure. It can be seen that when the output current i is used... o With feedforward, the grid-connected converter has an amplitude of 43.28 dB at -50 Hz. At this frequency, the grid-connected converter is equivalent to an ideal current source. The negative sequence voltage of phase A of PCC is extracted and denoted as v. an When using capacitor voltage v o Feedback indicates that the grid-connected converter has an amplitude of -55.68 dB at a frequency of -50 Hz. At this point, the grid-connected converter is equivalent to an ideal voltage source. The negative sequence output current of phase A of the grid-connected converter is extracted and denoted as i. an Furthermore, the change in control mode only affects the negative-sequence network, and the positive-sequence output impedance amplitude of the grid converter stabilizes at -44.04 dB, verifying the frequency selectivity independence and positive-sequence robustness of the proposed strategy. Therefore, the negative-sequence grid impedance Z is obtained from this. gn for

[0073]

[0074] The grid impedance inductance ratios R / X were set to 0.01, 0.5, and 1, respectively. Other system parameters are shown in Table 1. The calculated grid impedance values ​​are as follows: Figures 7 to 9 As shown, the actual grid impedance is 0.2976Ω, and the relative calculation errors of its amplitude are 2.15%, 2.59%, and 2.49%, respectively. The phase measurement is basically consistent with the actual value. This demonstrates that the proposed grid impedance measurement strategy can maintain high measurement accuracy under different R / X ratios.

[0075] Table 1 System Parameters

[0076]

[0077] The above embodiments are merely preferred embodiments of the present invention. Those skilled in the art can make various modifications and improvements without departing from the principles of the present invention, and these modifications and improvements should also be considered within the scope of protection of the present invention.

Claims

1. A method for measuring the impedance of an unbalanced power grid based on a grid converter, characterized in that, Includes the following steps: S1. Establish the equivalent circuit model of the grid converter under the negative sequence network, and convert the machine side part of the grid converter into a Thevenin circuit with a controlled voltage source and an output impedance in series. Based on Kirchhoff's voltage law, determine the relationship between the output negative sequence current of the grid converter, the PCC negative sequence voltage, the grid negative sequence impedance, and the negative sequence output impedance of the grid converter. S2. Extract the negative sequence components of the PCC voltage and the grid converter output current through the resonant controller; S3. Impedance reshaping technology is used to switch between current feedforward mode and voltage feedback mode respectively: - In current feedforward mode, the grid converter exhibits high impedance characteristics under negative sequence network conditions, equivalent to an ideal current source, and the negative sequence voltage of PCC phase A is extracted. v an ; - In voltage feedback mode, the grid converter exhibits low impedance characteristics under negative sequence network conditions, equivalent to an ideal voltage source, and the negative sequence output current of phase A of the grid converter is extracted. i an ; S4. The negative sequence voltage of phase A of PCC extracted under current feedforward mode v an The negative sequence output current of phase A of the grid converter extracted under voltage feedback mode. i an Calculate the negative sequence impedance of the power grid; S5. Maintain the impedance characteristics of the grid converter under the positive sequence network unchanged to ensure normal system operation.

2. The method for measuring the unbalanced grid impedance based on a grid converter according to claim 1, characterized in that, The establishment of the equivalent circuit model in step S1 includes: Establish the sequence component network circuit equations for the grid converter based on Kirchhoff's voltage law; The mathematical relationship between negative sequence voltage, grid negative sequence impedance, and grid converter negative sequence output impedance is derived.

3. The method for measuring the unbalanced grid impedance based on a grid converter according to claim 1, characterized in that, The resonant controller G R The transfer function expression for (s) is: In the formula, k s For resonant gain, ω c for G R The bandwidth of (s) ranges from 5 to 20 rad / s. ω 2 represents twice the fundamental angular frequency, and s represents the Laplace operator.

4. The method for measuring the unbalanced grid impedance based on a grid converter according to claim 1, characterized in that, The current feedforward mode enables the grid converter to function as an ideal current source in a negative-sequence network, with its equivalent output impedance... Z o Approaching infinity, used to measure the negative sequence voltage of the PCC. v an ; The voltage feedback mode enables the grid-connected converter to function as an ideal voltage source in a negative-sequence network, with its equivalent output impedance Zo approaching zero. This is used to measure the negative-sequence output current of the grid-connected converter. i an .

5. The method for measuring the unbalanced grid impedance based on a grid converter according to claim 1, characterized in that, The negative sequence impedance of the power grid Z gn for In the formula, v an This represents the negative sequence voltage of phase A of the PCC obtained in current feedforward mode. i an This represents the negative sequence output current of phase A of the grid converter extracted under voltage feedback mode. R g Indicates the grid resistance. ω 1 represents the power grid frequency. L g This represents the inductance of the power grid.

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