Method for introducing virtual harmonic impedance of network construction converter under coupling impedance constraint

Abstract

The invention relates to the technical field of converter electric energy quality treatment, in particular to a virtual harmonic impedance introduction method of a network construction converter under coupling impedance constraint. The method mainly comprises the following steps: considering coupling impedance between a converter and a power grid, establishing a harmonic domain equivalent model of the converter for constructing the network, and writing KCL and KVL equations in parallel; according to different feedback quantities, an output harmonic voltage equation of the converter after passing through the virtual harmonic impedance and the influence of the size of the virtual harmonic impedance on the harmonic current and voltage output by the grid side harmonic wave are obtained; the output current of the converter is used as feedback quantity, and a virtual harmonic impedance introduction scheme is designed according to the coupling impedance value. According to the method, a virtual harmonic impedance parameter design scheme under the coupling impedance constraint is constructed, the problem that the virtual harmonic impedance cannot be accurately configured is effectively solved, and the treatment effect that the content of each harmonic of the current and voltage harmonic at the power grid side is 70% or above when the nonlinear load is connected is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of converter power quality management, and in particular to a method for introducing virtual harmonic impedance into a grid-connected converter under coupling impedance constraints. Background Art

[0002] As the global energy transition accelerates, the penetration of renewable energy sources, such as wind power and photovoltaics, in power systems continues to rise. Unlike traditional power generation methods, renewable energy generation requires power electronics to connect to the grid. However, the integration of a high proportion of power electronics significantly alters the characteristics of traditional power grids, leading to a decrease in grid strength and a weakening trend. To address this, researchers have proposed applying grid-based control technology to distributed energy grid-connected systems, enhancing system inertia and damping support by simulating the characteristics of synchronous generators.

[0003] The addition of numerous nonlinear loads has significantly increased the proportion of harmonic currents, severely deteriorating grid power quality. Current research indicates that grid-following converters, as controlled current sources, can directly mitigate grid harmonics through harmonic current compensation strategies. However, grid-forming converters, as voltage sources, require output voltage regulation to mitigate harmonics. To address this, researchers have proposed virtual harmonic impedance technology. This technology adjusts the converter's equivalent output harmonic impedance characteristics to produce a grid-connected current that meets harmonic compensation requirements.

[0004] However, existing research still faces a key challenge: the mechanism by which different feedback levels introduce virtual harmonic impedance remains unclear. The current mainstream approach uses the converter's output harmonic current to introduce negative inductance, but this approach has inherent flaws. If the virtual impedance parameter is set too high, it will not only fail to effectively suppress harmonics, but may also exacerbate grid-connected power quality issues. Furthermore, existing parameter design methods lack theoretical support, making it difficult to accurately configure virtual harmonic impedance. Summary of the Invention

[0005] To address the shortcomings of the aforementioned prior art, the present invention proposes a method for introducing virtual harmonic impedance into a grid-connected converter under coupled impedance constraints. This method establishes a harmonic domain equivalent circuit for the grid-connected converter and systematically analyzes the impact of different feedback quantities (converter output current / grid current) on the grid-side harmonic current and voltage. Specifically, the method considers the impact of the coupled impedance between the converter and the grid on the design of virtual harmonic impedance parameters. A virtual harmonic impedance parameter design scheme that incorporates line parameters is constructed, effectively addressing the inability to accurately configure virtual harmonic impedance. Based on this theoretical framework, a virtual harmonic impedance optimization scheme using converter output current as the feedback quantity is further proposed to suppress grid current and voltage harmonics.

[0006] The present invention is achieved through the following technical solutions:

[0007] A method for introducing virtual harmonic impedance of grid-connected converters under coupled impedance constraint is proposed, which includes the following steps:

[0008] S1: Considering the coupling impedance between the converter and the grid, establish a harmonic domain equivalent model of the grid-connected converter and write the KCL and KVL equations.

[0009] S2: Based on different feedback quantities, the equation for the harmonic voltage output by the converter after passing through the virtual harmonic impedance is obtained, as well as the impact of the virtual harmonic impedance value on the harmonic current and voltage of the grid-side harmonic output.

[0010] S3: Using the converter output current as the feedback quantity and the coupling impedance value as the input, a virtual harmonic impedance introduction scheme is designed.

[0011] Furthermore, S1 includes the following sub-steps:

[0012] S1-1: The harmonic domain equivalent model of the grid-connected converter with nonlinear loads assumes that the grid has no harmonic voltage and considers the influence of the coupling impedance between the converter and the grid.

[0013] S1-2: Equivalently converting the harmonic source to a Norton model means connecting a constant current harmonic current source in parallel with a Norton equivalent impedance.

[0014] S1-3: Write the KCL and KVL equations to obtain the current-voltage relationship of the harmonic domain equivalent model of the grid-connected converter.

[0015]

[0016] Where s is the Laplace operator, I1 is the harmonic current output by the converter, I2 is the harmonic current on the grid side, I3 is the current passing through the Norton equivalent impedance, and I H U is the output current of the constant current harmonic current source. ho (s) is the converter output harmonic voltage, U hg is the grid-connected harmonic voltage, Z L is the coupling impedance between the converter and the grid, Z g is the grid line impedance, Z H is the Norton equivalent impedance.

[0017] Furthermore, S2 includes the following sub-steps:

[0018] S2-1: Feedback can include the converter output harmonic current I1, grid side harmonic current I2, grid side harmonic voltage U hg , the present invention mainly analyzes I1 and I2.

[0019] S2-2: If the converter output harmonic current I1 is used as the feedback quantity, the converter output harmonic voltage is Uho =-I1Z hvir G(s), Z hvir is the virtual harmonic impedance, G(s) is the voltage-current double closed-loop transfer function, and the relationship between the grid-connected side harmonic voltage, converter output harmonic current and constant current harmonic current can be further obtained.

[0020]

[0021] Based on this, it is possible to determine the impact of the positive and negative values and magnitude of the virtual harmonic impedance on the grid-connected side voltage harmonics when the converter output harmonic current I1 is used as the feedback quantity.

[0022] S2-3: If the grid-side harmonic current I2 is used as the feedback quantity, the converter output voltage is U ho =-I2Z hvir G(s), we can further obtain the relationship between the grid-connected side harmonic voltage, converter output harmonic current and constant current harmonic current.

[0023]

[0024] Based on this, it is possible to determine the impact of the positive and negative values and magnitude of the virtual harmonic impedance on the grid-side voltage harmonics and output current harmonics when the grid-side harmonic current I2 is used as the feedback quantity.

[0025] Furthermore, S3 includes the following sub-steps:

[0026] S3-1: The coupling impedance between the converter and the grid. Since the 6k±1 harmonics are analyzed in the harmonic domain, it can be approximated as an inductor.

[0027] S3-2: According to the analysis of the mechanism of introducing virtual harmonic impedance with the converter output current as the feedback quantity obtained in S2-2, it can be seen that introducing a negative inductance with the same magnitude as the coupling impedance between the converter and the grid can theoretically make Infinity, so that the grid side harmonic voltage is approximately 0, and the grid side harmonic current is also approximately 0.

[0028] The grid-type active power control loop adopts virtual synchronous machine control.

[0029] The beneficial effects of the present invention are:

[0030] This paper constructs a harmonic domain equivalent model for grid-connected converters with nonlinear loads. The paper analyzes the mechanism by which the introduction of virtual harmonic impedances with varying feedback quantities affects grid current and voltage harmonics. It focuses on the impact of the coupling impedance between the converter and the grid on the design of virtual impedance parameters. This paper provides a parameter design mechanism for better introducing virtual harmonic impedances to manage grid-side voltage and current harmonics. Furthermore, a virtual harmonic impedance introduction scheme is proposed, using the converter output current as the feedback quantity and utilizing the coupling impedance value to effectively manage grid current and voltage harmonics. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the following description of the embodiments with reference to the accompanying drawings, in which:

[0032] Figure 1 Schematic diagram of the overall control scheme of the grid-type converter of the present invention;

[0033] Figure 2 It is the harmonic domain equivalent model of the grid-connected converter with uncontrolled rectifier source of the present invention;

[0034] Figure 3 Schematic diagram of a specific scheme for introducing virtual harmonic impedance according to the present invention;

[0035] Figure 4-1 This is a comparison of the voltage waveform before and after the grid-connected side voltage waveform is treated after adding virtual harmonic impedance in the present invention;

[0036] Figure 4-2 This is a comparison of the grid-connected side current waveform before and after the virtual harmonic impedance is added to the control of the present invention; DETAILED DESCRIPTION

[0037] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0038] like Figure 1 As shown in FIG, the grid-connected system of the grid-connected converter of the present invention introduces a schematic diagram of virtual harmonic impedance using the converter output current as feedback quantity, specifically including:

[0039] S1: Considering the coupling impedance between the converter and the grid, establish a harmonic domain equivalent model of the grid-connected converter and write the KCL and KVL equations.

[0040] S2: Based on different feedback quantities, the equation for the harmonic voltage output by the converter after passing through the virtual harmonic impedance is obtained, as well as the impact of the virtual harmonic impedance value on the harmonic current and voltage of the grid-side harmonic output.

[0041] S3: Using the converter output current as the feedback quantity and the coupling impedance value as the input, a virtual harmonic impedance introduction scheme is designed.

[0042] Furthermore, S1 includes the following sub-steps:

[0043] S1-1: The harmonic domain equivalent model of the grid-connected converter with nonlinear loads assumes that the grid has no harmonic voltage and considers the influence of the coupling impedance between the converter and the grid.

[0044] S1-2: Equivalently converting the harmonic source to a Norton model means connecting a constant current harmonic current source in parallel with a Norton equivalent impedance.

[0045] S1-3: Write the KCL and KVL equations to obtain the current-voltage relationship of the harmonic domain equivalent model of the grid-connected converter.

[0046]

[0047] Where s is the Laplace operator, I1 is the harmonic current output by the converter, I2 is the harmonic current on the grid side, I3 is the current passing through the Norton equivalent impedance, and I H U is the output current of the constant current harmonic current source. ho (s) is the converter output harmonic voltage, U hg is the grid-connected harmonic voltage, Z L is the coupling impedance between the converter and the grid, Z g is the grid line impedance, Z H is the Norton equivalent impedance.

[0048] Furthermore, S2 includes the following sub-steps:

[0049] S2-1: Feedback can include the converter output harmonic current I1, grid side harmonic current I2, grid side harmonic voltage U hg , the present invention mainly analyzes I1 and I2.

[0050] S2-2: If the converter output harmonic current I1 is used as the feedback quantity, that is, Figure 3 As shown, the converter output current is extracted through a hybrid generalized integrator to extract harmonics, and a virtual harmonic inductor is introduced under the abc axis. After the closed-loop transfer function G(s), the converter output harmonic voltage is obtained as U ho =-I1Z hvir G(s), Z hvir is the virtual harmonic impedance, and the relationship between the grid-side harmonic voltage, converter output harmonic current and constant current harmonic current can be further obtained.

[0051]

[0052] Based on this, it is possible to determine the impact of the positive and negative values and magnitude of the virtual harmonic impedance on the grid-connected side voltage harmonics when the converter output harmonic current I1 is used as the feedback quantity.

[0053] S2-3: If the grid-side harmonic current I2 is used as the feedback quantity, the converter output harmonic voltage is U ho =-I2Z hvir G(s), we can further obtain the relationship between the grid-connected side harmonic voltage, converter output harmonic current and constant current harmonic current.

[0054]

[0055] Based on this, it is possible to determine the impact of the positive and negative values and magnitude of the virtual harmonic impedance on the grid-side voltage harmonics and output current harmonics when the grid-side harmonic current I2 is used as the feedback quantity.

[0056] Furthermore, S3 includes the following sub-steps:

[0057] S3-1: The coupling impedance between the converter and the grid. Since the 6k±1 harmonics are analyzed in the harmonic domain, it can be approximated as an inductor.

[0058] S3-2: According to the analysis of the mechanism of introducing virtual harmonic impedance with the converter output current as the feedback quantity obtained in S2-2, it can be seen that introducing a negative inductance with the same magnitude as the coupling impedance between the converter and the grid can theoretically make Infinity, so that the grid side harmonic voltage is approximately 0, and the grid side harmonic current is also approximately 0.

[0059] Example 1

[0060] In one embodiment of the present invention, the system topology is as follows: Figure 1 The main circuit parameters are shown in Table 1, the control link parameters are shown in Table 2, and PI control is selected for both the voltage loop and the current loop.

[0061] Table 1 Main circuit parameters

[0062]

[0063] Table 2 Control circuit parameters

[0064]

[0065] The inverter selects the control method derived from the mechanism analysis in S2-2, and sets the introduced negative virtual harmonic impedance to be the same as the absolute value of the coupling impedance. In theory, the grid-side harmonic voltage can be approximated to 0. Figure 4-1This is a comparison of the grid-side voltage waveforms before and after the introduction of virtual harmonic impedance. (a) shows the voltage waveform is clearly distorted before treatment, and (b) shows the voltage waveform is almost sinusoidal after treatment. Figure 4-2 The grid-side current waveforms are compared before and after the introduction of virtual harmonic impedance. The conclusions are similar to the above voltage waveforms.

[0066] The changes in harmonic content before and after treatment are shown in Table 3

[0067] Table 3 Changes in the harmonic content of grid-side voltage and current, and converter output current before and after treatment

[0068]

[0069]

[0070] It can be seen that the introduction of a negative virtual harmonic impedance with the same absolute value as the coupling impedance using the output harmonic current as the feedback quantity obtained in accordance with the present invention can achieve a control effect of more than 70% on the harmonic voltage and current on the grid side, thereby improving the economic benefits of microgrid power quality management. At the same time, the output current harmonics of the converter also have the same change trend as the analyzed trend, which proves the correctness of the mechanism analysis and provides a theoretical basis for the subsequent design of virtual harmonic impedance.

[0071] The contents not described in detail in this specification belong to the prior art known to professional and technical personnel in this field. Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments or to replace some of the technical features therein with equivalents. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the scope of protection of the present invention.

Claims

1. A method for introducing virtual harmonic impedance into a grid-connected converter under coupled impedance constraints, characterized by: S1: Considering the coupling impedance between the converter and the grid, establish the harmonic domain equivalent model of the grid converter and write the KCL and KVL equations; S2: Based on different feedback quantities, the equation for the harmonic voltage output by the converter after passing through the virtual harmonic impedance is derived, as well as the impact of the virtual harmonic impedance value on the harmonic current and voltage output on the grid side. S3: Using the converter output current as the feedback quantity and the coupling impedance value as the input, a virtual harmonic impedance introduction scheme is designed.

2. The method for introducing virtual harmonic impedance of a grid-connected converter under coupling impedance constraint according to claim 1 is characterized in that: The following steps are included in S1: S1-1: The harmonic domain equivalent model of the grid-connected converter with nonlinear loads assumes that the grid has no harmonic voltage and considers the influence of the coupling impedance between the converter and the grid; S1-3: Write the KCL and KVL equations to obtain the current-voltage relationship of the harmonic domain equivalent model of the grid-connected converter. Where s is the Laplace operator, I1 is the harmonic current output by the converter, I2 is the harmonic current on the grid side, I3 is the current passing through the Norton equivalent impedance, and I H U is the output current of the constant current harmonic current source. ho (s) is the converter output harmonic voltage, U hg is the grid-connected harmonic voltage, Z L is the coupling impedance between the converter and the grid, Z g is the grid line impedance, Z H is the Norton equivalent impedance.

3. The method for introducing virtual harmonic impedance of a grid-connected converter under coupling impedance constraint according to claim 2, characterized in that: The following steps are included in S2: S2-1: Feedback can include the converter output harmonic current I1, grid side harmonic current I2, grid side harmonic voltage U hg , the present invention mainly analyzes I1 and I2; S2-2: If the converter output harmonic current I1 is used as the feedback quantity, the converter output harmonic voltage is U ho =-I1Z hvir G(s), Z hvir is the virtual harmonic impedance, G(s) is the voltage-current double closed-loop transfer function, and the relationship between the grid-connected side harmonic voltage, converter output harmonic current and constant current harmonic current can be further obtained; Based on this, we can determine the impact of the positive and negative values and magnitude of the virtual harmonic impedance on the grid-connected side voltage harmonics when the converter output harmonic current I1 is used as the feedback quantity; S2-3: If the grid-side harmonic current I2 is used as the feedback quantity, the converter output voltage is U ho =-I2Z hvir G(s), we can further get the relationship between the grid-connected side harmonic voltage, converter output harmonic current and constant current harmonic current; Based on this, it is possible to determine the impact of the positive and negative values and magnitude of the virtual harmonic impedance on the grid-side voltage harmonics and output current harmonics when the grid-side harmonic current I2 is used as the feedback quantity.

4. The method for introducing virtual harmonic impedance of a grid-connected converter under coupling impedance constraint according to claim 3 is characterized in that: The following steps are included in S3: S3-1: The coupling impedance between the converter and the grid. Since the 6k±1 harmonics are analyzed in the harmonic domain, it can be approximated as an inductor. S3-2: According to the analysis of the mechanism of introducing virtual harmonic impedance with the converter output current as the feedback quantity obtained in S2-2, it can be seen that introducing a negative inductance with the same magnitude as the coupling impedance between the converter and the grid can theoretically make Infinity, so that the grid side harmonic voltage is approximately 0, and the grid side harmonic current is also approximately 0.

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