Method and system for improving small disturbance stability of converter grid-connected system

Abstract

The invention discloses a method and system for improving the small disturbance stability of a converter grid-connected system, and relates to the technical field of electrical equipment and electrical engineering, and the method comprises the steps: respectively constructing sequence impedance models of a network-following converter and a network-constructing converter in the converter grid-connected system; the method comprises the following steps: acquiring voltage and current information of a grid-connected part in a grid-connected converter grid-connected system, and analyzing the stability of the grid-connected converter grid-connected system by adopting an impedance analysis method based on a sequence impedance model of the grid-connected converter; according to a stability judgment result, when the system is unstable, connecting the network-forming converter, enabling modulation voltage waveform information in a control link of the network-forming converter to pass through a feed-forward link of the network-forming converter so as to support a control loop of the network-forming converter, and combining a sequence impedance model of the network-forming converter, so as to obtain the stability of the network-forming converter. Constructing a sequence impedance model of the supported following grid type converter; and based on the supported sequence impedance model, an impedance analysis method is adopted to verify the improvement of the stability of the grid-connected system of the grid-connected converter.

Description

Technical Field

[0001] The present invention relates to the field of electrical equipment and electrical engineering technology, and in particular to a method and system for improving the small disturbance stability of a converter grid-connected system. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] Currently, a growing number of renewable energy generation equipment are emerging on both the generation and consumption sides of power systems, and power electronics are playing an increasingly important role in power grids. Since renewable energy generation mostly outputs direct current (DC), these devices generally require power electronics to convert DC to AC before they can be integrated into the grid. Converters, as power electronics for DC-AC conversion, are widely used in renewable energy power systems. However, the integration of a large number of power electronics devices will partially alter the properties of traditional power grids. Furthermore, coupling between power electronics devices through the grid exacerbates the complexity of power system oscillations, ultimately increasing the oscillation frequency range. Furthermore, as the degree of power electronics in power systems continues to increase, operating environments and operating conditions become increasingly complex. Converter system stability issues have become a potential major hazard. Therefore, in-depth research on stability analysis models, instability mechanisms, and the dominant influencing factors of grid-connected renewable energy generation systems not only provides theoretical guidance for improving system stability but also plays a vital role in ensuring the safe and stable operation of power electronics systems and promoting grid development.

[0004] In renewable energy grid-connected systems, grid-following converters rely on grid voltage synchronization. When operating in weak grids (e.g., those with large grid equivalent inductance), they are easily affected by grid operation, leading to system oscillations and even instability. Existing methods, such as impedance analysis, eigenvalue analysis, and time-domain simulation, have been used to analyze the stability and instability mechanisms of these systems. However, due to their inherent drawbacks (e.g., impedance analysis suffers from insufficient mechanism interpretation, complex control links, and frequency coupling responses; eigenvalue analysis relies on linearized models, involves complex high-order system calculations, and lacks direct guidance for parameter optimization; and time-domain simulation analysis lacks direct insight into underlying mechanisms and is computationally intensive and time-consuming), they are unable to effectively optimize grid-following converter parameters, effectively address their significant influence on grid equivalent inductance and feedforward coefficient, and thus struggle to ensure stable operation in weak grids. Summary of the Invention

[0005] In order to address the deficiencies of the above-mentioned prior art, the present invention provides a method and system for improving the small-disturbance stability of a grid-connected converter system, utilizing a grid-forming converter to actively support a grid-following converter, that is, supporting and improving the stability of the grid-following converter through a feedforward link using information in the control link of the grid-forming converter, thereby achieving improvement in the small-disturbance stability of the grid-connected system.

[0006] In a first aspect, the present invention provides a method for improving the small disturbance stability of a converter grid-connected system.

[0007] A method for improving small disturbance stability of a converter grid-connected system, comprising: Sequence impedance models of grid-following converters and grid-forming converters in the grid-connected converter system are constructed respectively; Collect voltage and current information at the grid-connected point of the grid-connected converter system, and analyze the stability of the grid-connected converter system using the impedance analysis method based on the sequence impedance model of the grid-connected converter. Based on the stability judgment results, the grid-type converter is connected when the system becomes unstable. The modulated voltage waveform information in the control link of the grid-type converter is passed through the feedforward link of the grid-following converter to support the control loop of the grid-following converter. Combined with the sequence impedance model of the grid-type converter, a supported sequence impedance model of the grid-following converter is constructed. Based on the supported sequence impedance model, the impedance analysis method is used to verify the improvement of the grid-connected system stability of the grid-following converter.

[0008] In a second aspect, the present invention provides a system for improving the small disturbance stability of a converter grid-connected system.

[0009] A system for improving small-disturbance stability of a converter grid-connected system, comprising: A sequence impedance model building module is used to build sequence impedance models of the grid-following converter and the grid-forming converter in the grid-connected converter system respectively; An information acquisition module is used to collect voltage and current information at the grid connection point in the grid-connected system of the grid-following converter; Impedance analysis module, which is used to analyze the stability of the grid-connected system of the grid-connected converter using the impedance analysis method based on the sequence impedance model of the grid-connected converter; The logic switch module is used to connect the active support module according to the stability judgment result if the grid-connected system of the grid-connected converter is unstable, and to turn off the active support module if the grid-connected system is unstable; The active support module is used to connect the grid-type converter, and transmit the modulation voltage waveform information in the control link of the grid-type converter through the feedforward link of the grid-following converter to support the control loop of the grid-following converter, and combine the sequence impedance model of the grid-type converter to construct the supported sequence impedance model of the grid-following converter.

[0010] In a third aspect, the present invention further provides an electronic device comprising: a memory for storing executable instructions; and a processor for implementing the above-mentioned method for improving the small disturbance stability of the converter grid-connected system when executing the executable instructions stored in the memory.

[0011] In a fourth aspect, the present invention further provides a computer-readable storage medium storing executable instructions for causing a processor to execute the executable instructions to implement the above-mentioned method for improving the small disturbance stability of the converter grid-connected system.

[0012] In a fifth aspect, the present invention also provides a computer program product, which includes executable instructions, and the executable instructions are stored in a computer-readable storage medium; wherein, when the processor of the electronic device reads the executable instructions from the computer-readable storage medium and executes the executable instructions, the above-mentioned method of improving the small disturbance stability of the converter grid-connected system is implemented.

[0013] One or more of the above technical solutions have the following beneficial effects: 1. The present invention provides a method and system for improving the small disturbance stability of the converter grid-connected system, fully considering the performance of the grid-connected converter that can still maintain stable output in a weak power grid. In order to solve the oscillation problem that the grid-connected converter is still greatly affected by the equivalent inductance and feedforward coefficient of the power grid after preliminary parameter optimization, the grid-connected converter is used to actively support the grid-connected converter, that is, the voltage information in the control link of the grid-connected converter is used to feedforward support the grid-connected converter, and a positive-sequence impedance model of the supported grid-connected converter is given. The traditional Nyquist curve in the impedance analysis method is used to judge the stability of the new energy power generation grid-connected system. After analysis, it can be seen that the supported grid-connected converter is no longer significantly affected by the above two parameters and can be connected to a weak power grid to work normally, which verifies the effectiveness of the oscillation suppression method proposed in the present invention and can achieve the improvement of the small disturbance stability of the grid-connected system.

[0014] 2. The present invention is aimed at the grid-type converter. While considering the active control link under the virtual synchronous machine control strategy, it also considers its reactive control link, and establishes a more accurate positive and negative sequence broadband impedance model of the grid-type converter.

[0015] 3. The present invention is no longer limited to the grid-type converter supporting the grid-type converter only through the power grid, but supports the stability of the grid-type converter grid-connected system through both the power grid and the control loop. Therefore, it is still effective when the power of the grid-type converter is relatively small, which is more in line with the actual production needs.

[0016] Advantages of additional aspects of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The accompanying drawings, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0018] Figure 1 Flowchart of a method for improving small-disturbance stability of a system by actively supporting a grid-type converter with a grid-type converter in accordance with an embodiment of the present invention; Figure 2 Schematic diagram of the equivalent circuit of the grid-connected system of the grid-following converter according to an embodiment of the present invention; Figure 3 A comparison diagram of Nyquist curves of stability criteria for grid-connected systems of supported front and rear grid-connected converters according to an embodiment of the present invention; Figure 4 1 is a three-phase voltage waveform diagram at the common coupling point of the grid-connected system of the grid-following converter in the simulation experiment in the embodiment of the present invention; Figure 5 A comparison diagram of Nyquist curves for stability criterion of a grid-connected system of a grid-following converter under different grid strengths according to an embodiment of the present invention; Figure 6 This is a structural block diagram of a system for actively supporting a grid-type converter and improving the small-disturbance stability of a grid-type converter in an embodiment of the present invention. DETAILED DESCRIPTION

[0019] It should be noted that the following detailed descriptions are exemplary only and are intended to describe specific embodiments and provide further explanation of the present invention, and are not intended to limit the exemplary embodiments according to the present invention. Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those commonly understood by those of ordinary skill in the art to which the present invention belongs. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0020] Example 1 This embodiment proposes a method in which a grid-forming converter actively supports a grid-following converter to improve the small-disturbance stability of the converter grid-connected system. By constructing sequence impedance models of the grid-forming converter and the grid-following converter, the traditional Nyquist curve in the impedance analysis method is used to judge the stability of the renewable energy power generation grid-connected system. When the system is judged to be unstable, the information in the control link of the grid-forming converter is used to support and improve the stability of the grid-following converter through the feedforward link. At the same time, according to the modeling method used, the improved sequence impedance models of the two types of converters are respectively derived. The same impedance analysis method is then used to prove the effectiveness of the proposed oscillation suppression method. The correctness of the above method is further proved through experiments, and the small-disturbance stability of the grid-connected system can be improved.

[0021] like Figure 1 As shown, the present embodiment proposes a method for a grid-connected converter to actively support a grid-connected converter to improve the small disturbance stability of the converter grid-connected system, including the following steps: Step S1: constructing sequence impedance models of a grid-following converter and a grid-forming converter in a grid-connected converter system respectively.

[0022] Step S2: collecting voltage and current information at the grid connection point of the grid-connected converter system, and analyzing the stability of the grid-connected converter system using an impedance analysis method based on a sequence impedance model of the grid-connected converter.

[0023] Step S3: Based on the stability judgment result, the grid-type converter is connected when the system is unstable, and the modulation voltage waveform information in the control link of the grid-type converter is passed through the feedforward link of the grid-type converter to support the control loop of the grid-type converter, and the sequence impedance model of the grid-type converter is combined to construct the supported sequence impedance model of the grid-type converter.

[0024] Step S4: Based on the supported sequence impedance model, an impedance analysis method is used to verify the improvement of the stability of the grid-connected system of the grid-following converter.

[0025] The following content introduces the method for improving the small disturbance stability of the grid-connected system proposed in this embodiment in more detail.

[0026] In step S1, sequence impedance models of a grid-following converter and a grid-forming converter are established respectively, including: determining the voltage-current relationship for the circuit topology diagrams of the two types of converters respectively; injecting positive and negative sequence voltage disturbances at the common coupling point of the grid-connected system to obtain the time domain expression of each voltage at the grid-connected point of the converter, and obtaining its frequency domain expression after Fourier transform; for converters under different control strategies, deriving the influence of the disturbance voltage on the grid-connected voltage and current according to the control loop, and after obtaining the quantized voltage and current formula, substituting it into the voltage and current relationship formula at the grid-connected point of the converter and simplifying it, the sequence impedance models of the two types of converters can be obtained.

[0027] Specifically, for the grid-following converter, its sequence impedance model is constructed as follows: Step S1.1: Determine the current-voltage relationship of the filter circuit in the main circuit topology of the grid-type converter, which is: (1) in, is a complex frequency variable, is the equivalent voltage on the DC side of the converter, is the PWM modulation coefficient, is the three-phase current after passing through the converter filter circuit, is the three-phase voltage at the common coupling point between the converter and the grid, Represents the filter inductance and filter resistance of the converter, The final three-phase modulation wave is obtained through the feedforward link.

[0028] Step S1.2: Inject positive and negative sequence voltage disturbances at the common coupling point of the converter grid-connected system to obtain the time domain expression of the current and voltage at the converter grid-connected point. Taking the phase A voltage at the grid-connected point as an example, the time domain expression of the phase A voltage at the converter grid-connected point in equation (1) is: (2) in, are the amplitudes of the system normal voltage, positive sequence disturbance voltage and negative sequence disturbance voltage respectively; is the initial phase angle of the positive sequence disturbance voltage, is the initial phase angle of the negative sequence disturbance voltage; The system stable operating frequency is 50 , and is a series of positive and negative sequence disturbance frequencies.

[0029] At the same time, after the disturbance is added, a current of corresponding frequency will appear in phase A of the system, so the time domain expression of phase A current is: (3) in, are the amplitude of the normal current of the system and the amplitude of the positive-sequence disturbance current and negative-sequence disturbance current generated by the positive-sequence disturbance voltage and negative-sequence disturbance voltage respectively; It is the initial phase angle of the system normal current and positive and negative sequence response current.

[0030] Step S1.3: Use Fourier transform to convert the time domain expression into the frequency domain expression, which can be expressed as: (4) According to the Fourier transform principle, we can get: ; ; ; ; (5) Step S1.4: The control strategy commonly used by grid-following converters is to use a phase-locked loop to set the angle required for coordinate transformation. , the rotation coordinate system obtained by coordinate transformation based on this angle meets the requirements. Based on the above principle, the coordinate change matrix is: (6) In addition, a small perturbation injected by measuring the sequence impedance of the converter will also produce a small deflection angle , which produces a steering angle with the normal voltage The sum is the final rotation reference angle , the coordinate change matrix (6) can be further approximated as: (7) Step S1.5: The sampling delay link in the main circuit topology has a non-negligible delay. In order to accurately characterize the model, a delay function needs to be introduced. and low-pass filter function for: (8) in, is the sampling time, is the low-pass filter cutoff frequency, , then the actual dq axis voltage collected at the common coupling point is for: (9) Step S1.6, assuming positive and negative sequence disturbance voltage cause The transfer functions are ,Right now: (10) According to formula (10), the frequency domain form of formula (9) is as follows: (11) (12) From the grid-following converter phase-locked loop control structure, we can get: (13) in, Represents the transfer function of PI control and integral link. Note that Is considered Substituting equation (12) into equation (13) yields the q-axis voltage ,for: (14) The dq modulated wave obtained by closed-loop control in the dq rotating coordinate system for: (15) Among them, the frequency domain expressions of each element are known, and it can be known from the frequency domain convolution theorem that The frequency domain expression of is: (16) (17) After obtaining the frequency domain expression of the modulation wave in the dq rotating coordinate system, it is necessary to perform the inverse transformation of the Park transform to obtain the three-phase modulation wave in the natural stationary coordinate system. for: (18) Step S1.7, three-phase modulation wave Then the three-phase voltage of the power grid Participating feedforward links (feedforward parameters ) can get the modulation wave for controlling the power electronic switch for: (19) Step S1.8: Based on the frequency domain expressions of voltage and current, the phase-frequency domain expression of the modulation wave can be obtained by analyzing the voltage disturbance in the control link of the grid-following converter in the above steps through phase-locked loop control, current closed-loop control and feedforward control. Taking phase A as an example, according to equations (16) and (17), the three-phase modulation wave can be obtained. and actual control modulation wave The frequency domain expression of is: (20) Step S1.9: Substitute the frequency domain expression of the modulation wave into the voltage-current relationship of the filter circuit and simplify it to obtain the positive and negative sequence impedance model of the grid-following converter. Specifically, through the calculations in the above steps, the frequency domain expressions of all elements in formula (1) have been obtained. Simplifying formula (1) can obtain the frequency domain expressions of the converter's positive and negative sequence impedance as follows: (twenty one) (twenty two) As an implementation method, the positive and negative sequence impedance models of the grid-type converter are derived based on the same idea. The only difference is that when analyzing the phase-frequency domain expression of the modulation wave based on the frequency domain expression of voltage and current, the control strategy of the grid-type converter needs to be considered. That is, the control of voltage disturbances in the active and reactive power control links of the grid-type converter under the virtual synchronous machine control strategy needs to be considered. Through a similar method as above, the positive and negative sequence impedance models of the grid-type converter can be obtained, which can be expressed as: ; .

[0031] In step S2, the impedance analysis method can be used to more intuitively determine whether the system is stable based on the sequence impedance model established in the previous step and the Nyquist criterion in classical control theory. Specifically, the impedance analysis method divides the converter grid-connected system into two parts: the converter and the grid. Both can be represented by a power supply and an impedance in series and parallel. It should be noted that the grid-connected converter exhibits controlled current source characteristics in the grid. The schematic diagram of the grid-connected converter grid-connected system is shown in Figure 1. Figure 2 As shown, specifically, the impedance analysis method is used to analyze the stability of the grid-connected system of the grid-connected converter, including: Step S2.1: The grid-connected current in the grid-connected converter system is: (twenty three) in, It is the current output by the grid-type converter to the grid. It is a controlled current source equivalent to the grid-type converter. is the grid voltage, is the equivalent impedance of the grid-type converter, Represents the grid impedance.

[0032] Step S2.2: Based on the grid-connected current in the grid-connected converter system, the grid-connected system stability criterion is set as the ratio of the grid impedance to the equivalent impedance of the grid-connected converter, that is, , The zero point of is the pole of the closed-loop system. Similarly, The number of zeros is the number of zeros in the s domain The number of circles of the Nyquist curve around the point (0, 0j). Therefore, the stability criterion of the system can be The Nyquist curve can be used to intuitively judge the stability of the system.

[0033] Specifically, system stability is determined by the number of Nyquist curves encircling the point (-1, 0j). The core formula for the Nyquist stability criterion is Z = N + P, where Z is the number of poles in the right half plane of the closed-loop system in the s-domain (closed-loop stability requires Z = 0), N is the number of clockwise encircles of the Nyquist curve around the point (-1, 0j), and P is the number of right half plane poles in the open-loop system in the s-domain. Therefore, to ensure closed-loop stability (i.e., Z = 0), if the open-loop system has no right-half-plane poles (P = 0), the Nyquist curve cannot encircle the point (-1, 0j), and N = 0. If the open-loop system has right-half-plane poles (P > 0), the Nyquist curve must encircle the point (-1, 0j) P times counterclockwise, and N = -P, to ensure system stability. That is, when the Nyquist curve is closer to or surrounds the (-1, 0j) point, the smaller the gain margin and phase margin of the system are, and the system is closer to being unstable or has already become unstable.

[0034] In this embodiment, according to the system stability criterion The Nyquist curve is used to determine the stability of the system, including: (1) Determining a stable state: If the Nyquist curve of the open-loop transfer function does not enclose the critical point (-1, 0j), or if the system encloses the critical point in a manner that satisfies Z = N + P and Z = 0 (i.e., encloses the critical point P times counterclockwise), based on the number of open-loop right-half-plane poles P in the system, then the system is stable. Generally, for systems without right-half-plane open-loop poles, a Nyquist curve that does not enclose the point (-1, 0j) indicates stability.

[0035] (2) Determining an unstable state: If the Nyquist curve surrounds the critical point (-1, 0j), where the surround is clockwise in the absence of an open-loop pole in the right half plane, or the surround method causes the closed-loop pole to fall in the right half plane, then the system is in an unstable state.

[0036] As an implementation method, an oscilloscope is used to record the three-phase voltage and current waveforms of the grid-connected system of the grid-type converter in the simulation experiment; the Nyquist curve obtained by the above-mentioned impedance analysis method and the collected voltage and current waveforms are combined to jointly judge the stability of the grid-connected system; if both show that the system is in an unstable state, the grid-type converter is connected to support the control loop of the grid-type converter via the grid-type converter feedforward link, that is, step S3 is executed.

[0037] The Nyquist curve obtained by the integrated impedance analysis method and the collected voltage and current waveforms are used to jointly determine the stability of the grid-connected system, further improving the accuracy and reliability of stability judgments. Specifically, based on the impedance analysis method (i.e., the Nyquist curve), when the shape of the Nyquist curve and its encirclement of the (-1, 0j) point indicate the presence of a right-half-plane pole in the closed-loop system, the system is considered unstable according to the Nyquist criterion. When the Nyquist curve meets the stability criterion (i.e., all closed-loop system poles are in the left-half-plane), the system is considered stable. Furthermore, based on the collected voltage and current waveforms (i.e., experimental or observational), the system is considered unstable when the oscillation amplitude continues to increase (i.e., the voltage and / or current waveforms exhibit oscillations with increasing amplitude), the waveforms diverge (i.e., the waveforms deviate from the normal operating range and even approach infinity), or the system fails to operate normally (i.e., it cannot maintain normal voltage, frequency, or power output, and may even cause protective tripping). Conversely, when the voltage and current waveforms remain within the normal range, even when disturbed, the waveforms quickly decay and return to a steady state, without exhibiting sustained or divergent oscillations, the system is considered stable. By combining these two criteria, the system is ultimately deemed unstable only when both theoretical analysis (Nyquist curve) and waveform observations consistently indicate instability. This more rigorous and reliable approach avoids the limitations of a single method.

[0038] In step S3, the grid-forming converter is connected to support the control loop of the grid-type converter via the grid-type converter feedforward link, and the sequence impedance model of the supported grid-type converter is derived based on this modification, and the effectiveness of this method is then verified through impedance analysis and an oscilloscope in step S4.

[0039] Among them, the interconnected grid-type converter transmits the modulation voltage waveform information in the control link of the grid-type converter through the feedforward link of the grid-type converter to support the control loop of the grid-type converter, including: Step S3.1: From the grid-following converter control block diagram, we can see that the modulation wave of the voltage and current double closed loop output is The final modulation wave can only be obtained through grid voltage information feedforward. , taking phase A as an example, it can be expressed as: (twenty four) Step S3.2, since the positive sequence voltage disturbance in the grid voltage will be fed through the feedforward coefficient Affects the stability of the grid-connected system of the grid-following converter. To this end, the original grid voltage information (i.e., the feedforward voltage information) in the feedforward link of the grid-following converter is replaced by the three-phase voltage information output by the reactive power control link of the grid-forming converter. Similarly, taking phase A as an example, it can be expressed as: (25) Step S3.3: Due to The positive sequence disturbance in the system is processed by the control link, so it is smaller than the positive sequence disturbance in the power grid, and contains some characteristics of the grid-type converter, which can theoretically weaken The influence of the entire positive sequence impedance makes the grid-type converter have certain characteristics of the grid-type converter. Therefore, after the feedforward grid voltage information is replaced, the modulation wave frequency domain expression of the grid-type converter is updated, and the supported grid-type converter A phase modulation wave frequency domain expression is for: (26) Step S3.4, substitute the updated modulation wave frequency domain expression into the voltage-current relationship of the filter circuit in the main circuit topology of the grid-type converter, and simplify the positive sequence impedance model of the grid-type converter after the active support of the grid-type converter. Specifically, substitute formula (26) into formula (1) to obtain the positive sequence impedance model of the grid-type converter after support for: (27) For the convenience of writing, assume that A in the numerator and B in the denominator are: (28) In step S4, the supported grid-following converter is connected to the grid equivalent inductance In the power grid, since the Nyquist curve under the positive sequence impedance criterion is always closer to the judgment point than the Nyquist curve under the negative sequence impedance criterion, similarly, only the positive sequence impedance of the converter is considered. According to step S2, the stability criterion of the grid-connected system with supported grid-type converter is: (29) Finally, the supported grid-connected converter is followed by the grid-connected system stability criterion Compared with the original system stability criterion The s-domain Nyquist curve is compared with the Figure 3 shown.

[0040] The above solution of this embodiment can effectively improve the stability of the system. The principle is to introduce a grid-forming converter (GFM) and support the control loop of the grid-following converter (GFL) through its feedforward link.

[0041] Specifically, the grid-following converter (GFL) has certain limitations: the traditional grid-following converter relies on the voltage and frequency signals of the grid for synchronization and operation. When the grid is weak (such as high impedance) or there are other GFL interactions, its own phase-locked loop (PLL) may be easily disturbed, resulting in unstable control and even oscillation. In fact, the GFL is essentially a current source, which requires a voltage source for stable operation. The grid-forming converter (GFM) is different from the grid-following converter. It can independently form and maintain AC voltage and frequency, similar to a traditional synchronous generator. It is essentially a voltage source that can provide inertia and damping, thereby showing better stability under weak grid conditions. On this basis, the grid-forming converter is connected to support the control loop of the grid-following converter through the grid-following converter feedforward link, and the stability is improved through feedforward support. The principle is: (1) Providing voltage support and stable reference: When the grid-following converter system detects instability, the grid-forming converter is connected through the feedforward link. As a voltage source, the GFM can provide a stable, high-quality voltage reference for the GFL, which helps the GFL's phase-locked loop operate more robustly, reducing its sensitivity to external grid disturbances, thereby suppressing the generation and amplification of oscillations.

[0042] (2) Reshaping the system equivalent impedance: Stability issues often arise from the interaction of impedances between the converter and the grid or load. The connection of the grid-type converter, especially the support provided by the feedforward method in its control loop, actually changes the equivalent impedance characteristics of the entire system at the grid connection point. GFM can provide effective damping and adjust its output impedance to improve the overall impedance characteristics of the system within the critical frequency range, thereby moving the pole of the closed-loop system from the right half plane (i.e., the unstable region) to the left half plane (i.e., the stable region). In this embodiment, the effect of the impedance reshaping is analyzed by deriving the sequence impedance model of the supported grid-type converter.

[0043] (3) Providing inertia and damping: In power systems, inertia and damping are crucial for suppressing oscillations. Grid-connected converters can simulate the inertia of rotating machinery and provide effective damping, which is particularly important for suppressing high-frequency oscillations in systems dominated by power electronics. Through feedforward support, the GFM can effectively transfer this inertia and damping characteristics to the GFL, thereby improving the dynamic stability of the overall system.

[0044] Therefore, this embodiment introduces a grid-forming converter and supports the grid-following converter through a feedforward link. This is equivalent to providing an active support point that can form voltage and reshape impedance when the system faces the risk of instability. Through precise parameter design and adaptive mechanism, it ensures that the grid-forming converter can robustly cope with drastic changes in the equivalent inductance of the grid, thereby completely solving the problem that the grid-following converter is greatly affected by the equivalent inductance of the grid and the feedforward coefficient, and significantly enhancing the stability of the entire grid-connected system.

[0045] The correctness of the above method proposed in this embodiment is further verified through the following specific example analysis.

[0046] Carry out simulation experiment in MATLAB / Simulink according to the above ideas: The output waveform of the simulation experiment is as follows: Figure 4 As shown, it can be divided into three stages. The first stage is that the unsupported grid-following converter, which has been optimized in the first section but has no support, is connected to an ideal grid with an equivalent inductance of 5mH. At 0.5 seconds, the equivalent inductance of the ideal grid suddenly changes to 10mH, and the grid strength is greatly reduced. At 1 second, the stable grid-forming converter supports the grid-following converter that is oscillating with the grid at this time. From the waveform results, it can be seen that the supported grid-following converter can be connected to a weak grid and operate stably, which is consistent with the Figure 3 The analysis results are consistent, indicating that the proposed method is correct.

[0047] The model parameters of the grid-following converter are shown in Tables 1 and 2. It should be noted that since the response speed of the voltage outer loop is much lower than that of the current inner loop, the reference value of the current inner loop can be used in the modeling. Considered to be constant, the converter output active power is: (30) in, The value of is the current amplitude at the common coupling point, is the d-axis voltage in the dq rotating coordinate system, Generally take zero, so we can use To determine the active power output of the converter.

[0048] Table 1 Grid-following converter circuit topology parameters

[0049] Table 2 Parameters of the control link of the grid-following converter

[0050] According to existing technology, the stability of the grid-connected system of the grid-connected converter is closely related to the strength of the grid to which it is connected. The strength of the grid can be determined by the equivalent inductance of the grid. express, The larger the value, the weaker the power grid strength. The smaller the value, the stronger the grid strength. For the naturally static grid parameters, the positive and negative sequence impedances are equal and their values ​​are Furthermore, the criteria for the equivalent inductance of different power grids are The s-domain Nyquist curve is as follows Figure 5 As shown in the figure, we can see that as the equivalent inductance of the power grid increases As the power grid becomes weaker, the stability criterion of the grid-connected system of the grid-connected converter is of s The Nyquist curve of the domain gradually surrounds the point (-1, 0j), and the stability gradually deteriorates. It can be seen from the local enlarged image that It can be regarded as the stability edge of the system. If the strength of the power grid continues to weaken, the system will no longer be stable.

[0051] At this time, the logic judgment program or module connects the support of the mesh converter to the control loop of the mesh converter. The parameters of the mesh converter are shown in Tables 3 and 4 below.

[0052] Table 3 Grid-type converter circuit topology parameters

[0053] Table 4 Parameters of control link of grid-type converter

[0054] The waveform recorded by the experiment (i.e. Figure 4 ) and the Nyquist curve comparison of the grid-following converter before support (i.e. Figure 3 ) It can be seen that the proposed method can actively provide damping capability for the grid-following converter connected to the weak power grid and effectively suppress the system oscillation phenomenon.

[0055] Example 2 This embodiment provides a system for improving the small disturbance stability of a converter grid-connected system, such as Figure 6 As shown, including: A sequence impedance model building module is used to build sequence impedance models of the grid-following converter and the grid-forming converter in the grid-connected converter system respectively; An information acquisition module is used to collect voltage and current information at the grid connection point in the grid-connected system of the grid-following converter; Impedance analysis module, which is used to analyze the stability of the grid-connected system of the grid-connected converter using the impedance analysis method based on the sequence impedance model of the grid-connected converter; The logic switch module is used to connect the active support module according to the stability judgment result if the grid-connected system of the grid-connected converter is unstable, and to turn off the active support module if the grid-connected system is unstable; The active support module is used to connect the grid-type converter, and transmit the modulation voltage waveform information in the control link of the grid-type converter through the feedforward link of the grid-following converter to support the control loop of the grid-following converter, and combine the sequence impedance model of the grid-type converter to construct the supported sequence impedance model of the grid-following converter.

[0056] Example 3 This embodiment provides an electronic device, including: a memory for storing executable instructions; and a processor for implementing the above method provided in this embodiment when executing the executable instructions stored in the memory.

[0057] Example 4 This embodiment further provides a computer-readable storage medium storing executable instructions. When the executable instructions are executed by a processor, the processor will be caused to execute the above method provided in this embodiment.

[0058] Example 5 This embodiment provides a computer program product including executable instructions, which are computer instructions stored in a computer-readable storage medium. When a processor of an electronic device reads the executable instructions from the computer-readable storage medium and executes the executable instructions, the electronic device performs the method provided in this embodiment.

[0059] The steps involved in the above embodiments 2 to 5 correspond to those in embodiment 1. For detailed implementation, please refer to the relevant description of embodiment 1. The term "computer-readable storage medium" should be understood as a single medium or multiple media that includes one or more instruction sets; it should also be understood to include any medium that can store, encode, or carry an instruction set for execution by a processor and cause the processor to perform any method of the present invention.

[0060] Those skilled in the art will appreciate that the modules or steps of the present invention described above can be implemented using a general-purpose computer device. Alternatively, they can be implemented using program code executable by a computing device, which can then be stored in a storage device and executed by the computing device. Alternatively, they can be fabricated into separate integrated circuit modules, or multiple modules or steps can be fabricated into a single integrated circuit module for implementation. The present invention is not limited to any specific combination of hardware and software.

[0061] The above description is only a preferred embodiment of the present invention. Although the specific implementation of the present invention is described in conjunction with the accompanying drawings, it does not limit the scope of protection of the present invention. Those skilled in the art should understand that based on the technical solution of the present invention, various modifications or variations that can be made by those skilled in the art without creative work are still within the scope of protection of the present invention.

Claims

1. A method for improving the small disturbance stability of a converter grid-connected system, characterized in that: include: Sequence impedance models of grid-following converters and grid-forming converters in the grid-connected converter system are constructed respectively; Collect voltage and current information at the grid-connected point of the grid-connected converter system, and analyze the stability of the grid-connected converter system using the impedance analysis method based on the sequence impedance model of the grid-connected converter. Based on the stability judgment results, the grid-type converter is connected when the system becomes unstable. The modulated voltage waveform information in the control link of the grid-type converter is passed through the feedforward link of the grid-following converter to support the control loop of the grid-following converter. Combined with the sequence impedance model of the grid-type converter, a supported sequence impedance model of the grid-following converter is constructed. Based on the supported sequence impedance model, the impedance analysis method is used to verify the improvement of the grid-connected system stability of the grid-following converter.

2. The method for improving the small disturbance stability of the converter grid-connected system according to claim 1, characterized in that: The construction of the grid-following converter sequence impedance model includes: Determine the current-voltage relationship of the filter circuit in the main circuit topology of the grid-type converter; Positive and negative sequence voltage disturbances are injected at the common coupling point of the converter grid-connected system to obtain the time domain expressions of the current and voltage at the converter grid-connected point, and the time domain expressions are converted into frequency domain expressions using Fourier transform. Based on the frequency domain expressions of voltage and current, the phase-frequency domain expression of the modulation wave is obtained by considering the voltage disturbance in the control link of the grid-following converter through phase-locked loop control, current closed-loop control and feedforward control. The frequency domain expression of the modulation wave is substituted into the voltage-current relationship of the filter circuit to simplify the positive and negative sequence impedance model of the grid-following converter.

3. The method for improving the small disturbance stability of the converter grid-connected system according to claim 1, characterized in that: The construction of the grid-type converter sequence impedance model includes: Determine the current-voltage relationship of the filter circuit in the main circuit topology of the grid-type converter; Positive and negative sequence voltage disturbances are injected at the common coupling point of the converter grid-connected system to obtain the time domain expressions of the current and voltage at the converter grid-connected point, and the time domain expressions are converted into frequency domain expressions using Fourier transform. Based on the frequency domain expressions of voltage and current, the control of voltage disturbance in the active and reactive power control links of the grid-connected converter under the virtual synchronous machine control strategy is considered, and the phase-frequency domain expression of the modulation wave is obtained; The frequency domain expression of the modulation wave is substituted into the voltage-current relationship of the filter circuit to simplify the positive and negative sequence impedance model of the grid-type converter.

4. The method for improving the small disturbance stability of a converter grid-connected system according to claim 1, wherein: The impedance analysis method is used to analyze the stability of the grid-connected system with grid-following converters, including: According to the grid-connected current in the grid-connected converter system, the grid-connected system stability criterion is set as the ratio of the grid impedance to the equivalent impedance of the grid-connected converter; The stability of the system is determined based on the Nyquist curve of the grid-connected system stability criterion.

5. The method for improving the small disturbance stability of the converter grid-connected system according to claim 4, characterized in that: When judging the stability of the grid-connected system, the collected voltage and current waveforms and the Nyquist curve of the grid-connected system stability criterion are used to jointly judge the stability of the grid-connected system: When the system is judged to be in an unstable state based on both judgment methods, the grid-forming converter is connected to support the control loop of the grid-following converter via the grid-following converter feedforward link.

6. The method for improving the small disturbance stability of a converter grid-connected system according to claim 1, wherein: Connecting the grid-type converter, the modulation voltage waveform information in the control link of the grid-type converter is fed through the feedforward link of the grid-type converter to support the control loop of the grid-type converter, including: The original grid voltage information in the feedforward link of the grid-following converter is replaced by the three-phase voltage information output by the reactive power control link of the grid-forming converter; After the feedforward grid voltage information is replaced, the frequency domain expression of the modulation wave of the grid-following converter is updated; The updated frequency domain expression of the modulation wave is substituted into the voltage-current relationship of the filter circuit in the main circuit topology of the grid-type converter, and the positive-sequence impedance model of the grid-type converter after active support by the constructed grid-type converter is simplified.

7. A system for improving the small disturbance stability of a converter grid-connected system, characterized in that: include: A sequence impedance model building module is used to build sequence impedance models of the grid-following converter and the grid-forming converter in the grid-connected converter system respectively; An information acquisition module is used to collect voltage and current information at the grid connection point in the grid-connected system of the grid-following converter; Impedance analysis module, which is used to analyze the stability of the grid-connected system of the grid-connected converter using the impedance analysis method based on the sequence impedance model of the grid-connected converter; The logic switch module is used to connect the active support module according to the stability judgment result if the grid-connected system of the grid-connected converter is unstable, and to turn off the active support module if the grid-connected system is unstable; The active support module is used to connect the grid-type converter, and transmit the modulation voltage waveform information in the control link of the grid-type converter through the feedforward link of the grid-following converter to support the control loop of the grid-following converter, and combine the sequence impedance model of the grid-type converter to construct the supported sequence impedance model of the grid-following converter.

8. An electronic device, characterized in that: include: a memory for storing executable instructions; The processor is configured to implement the method for improving the small disturbance stability of a converter grid-connected system as described in any one of claims 1 to 6 when executing the executable instructions stored in the memory.

9. A computer-readable storage medium, characterized in that Executable instructions are stored, which are used to cause a processor to execute the executable instructions to implement the method for improving the small disturbance stability of the converter grid-connected system according to any one of claims 1 to 6.

10. A computer program product, characterized in that The computer program product includes executable instructions stored in a computer-readable storage medium; When the processor of the electronic device reads the executable instructions from the computer-readable storage medium and executes the executable instructions, the method for improving the small disturbance stability of the converter grid-connected system according to any one of claims 1 to 6 is implemented.

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