Photovoltaic grid-connected inverter control method and device under extremely weak power grid

Abstract

The invention relates to the technical field of power electronics, in particular to a photovoltaic grid-connected inverter control method and device under an extremely weak power grid, and can solve the problem that the photovoltaic grid-connected inverter cannot be controlled under the scene that a large amount of distributed power generation is accessed to a certain extent. A traditional grid-connected inverter adopting unity power factor control can cause the problem of low-frequency oscillation due to insufficient reactive power. According to the method, firstly, an extended state observer is adopted to realize real-time online measurement of network side impedance, and then reactive current iq is subjected to adaptive compensation by adding an active current feedforward mode, so that the reactive power of the system is improved within the range of meeting the voltage modulation degree limitation, and further, the voltage modulation degree of the system is improved under any power grid connection strength. And the stability of the grid-connected converter is improved. The method is easy to implement, the principle is clear, the stability of the grid-connected inverter system under the extremely weak power grid is greatly improved, and the power grid adaptability of the grid-connected inverter system is improved.

Description

Technical Field

[0001] The present application relates to the field of power electronics technology, and in particular to a method and device for controlling a photovoltaic grid-connected inverter under an extremely weak power grid. Background Art

[0002] With the increasing application of distributed generation and its large-scale access to the grid, the grid impedance has gradually increased, accompanied by the problem of large fluctuations in grid impedance, making the grid present the characteristics of a weak grid or even an extremely weak grid. In order to describe the strength of the grid, the short circuit ratio (SCR) is generally used as a descriptive indicator, which is defined as the ratio of the AC short-circuit capacity to the rated capacity of the grid-connected device. Usually, in a weak grid, the SCR value is 2 to 3, and in an extremely weak grid, the SCR value is less than 2.

[0003] Therefore, accurate grid impedance information is crucial for the operation of grid-connected inverters. Under extremely weak grid conditions, real-time monitoring of grid impedance is required, and adaptive current control is performed according to the size of grid impedance to ensure reliable and efficient operation of the system. In addition, the measurement and analysis of grid impedance can also quickly detect grid faults. At present, the main methods for online impedance measurement include injecting harmonics and applying reactive disturbances. Different existing measurement methods have different characteristics and are not adaptable. Most methods cannot achieve real-time online measurement of grid-side impedance. In addition, in the implementation process, certain harmonic disturbances or reactive disturbances need to be injected. However, disturbances are omnipresent for the system, and it is difficult to distinguish whether the disturbance is self-injected or external interference. If the grid-side impedance changes in real time, it is also difficult to further measure in real time.

[0004] Since extremely weak power grids are intermittent, random and have output fluctuations, they seriously threaten the safe and stable operation of the power grid and affect the grid-connected consumption of new energy. Therefore, it is necessary to further carry out relevant research on the low-frequency oscillation mechanism and stability operation control strategy of grid-connected inverters under extremely weak power grids. At present, the research on stability control strategies of extremely weak power grids mainly focuses on the optimization design of control systems, namely outer loop control, inner loop control and phase-locked loop (PLL).

[0005] However, the stability control method of extremely weak power grids involves many links and complex coupling relationships in the inverter oscillation behavior under extremely weak power grids. Therefore, the mathematical models in the above analysis are much more complicated than the models that only consider the phase-locked loop and the current inner loop under weak power grids. For these complex extremely weak power grid conditions, it is difficult to continue to use the analytical analysis method of weak power grids for analysis. Although some conclusions that can guide the setting of control parameters are obtained, these conclusions cannot well explain the mechanism of low-frequency oscillation of inverters under extremely weak power grids, and it is also difficult to derive a simple and effective stability improvement method. Summary of the invention

[0006] In order to overcome the problem that the original impedance strategy is inaccurate and cannot monitor in real time and overcome the low-frequency oscillation of the grid-connected inverter under an extremely weak power grid, the present application provides a photovoltaic grid-connected inverter control method and device under an extremely weak power grid.

[0007] The embodiment of the present application is implemented as follows:

[0008] In a first aspect, the present application provides a photovoltaic grid-connected inverter control method under an extremely weak power grid, comprising:

[0009] The grid-side impedance is measured and the grid connection strength SCR is calculated according to the grid-side impedance.

[0010] According to the voltage triangle relationship, the reactive current that needs to be compensated under different SCRs is derived while keeping the voltage at the PCC point constant.

[0011] Limit the active current;

[0012] A small signal model of LCL grid-connected inverter is established and the stability is analyzed.

[0013] In a possible implementation, the expression of the grid connection strength SCR is:

[0014]

[0015] Among them, |Zg| is the measured grid-side impedance modulus, Un is the effective value of the grid-side line voltage, and PN is the rated power on the AC side.

[0016] In a possible implementation, the magnitude of the reactive current that needs to be compensated is expressed as follows:

[0017]

[0018] Among them, n is the ratio of the reactive current to be compensated to the rated current, and m is the ratio of the active current to the rated current at this time.

[0019] In a possible implementation, the expression for limiting the active current is:

[0020]

[0021] Among them, SCR represents the grid connection strength, and mmax represents the ratio of the active current limit value to the rated current.

[0022] In a possible implementation, when the active current approaches the rated value, the reactive current required to be compensated becomes larger in order to ensure the stable operation of the grid-connected system. When the reactive current to be compensated becomes larger, the voltage at the PCC point will also increase. If the compensation is too much, it will exceed the limit range of the voltage modulation.

[0023] According to the two limiting conditions, the magnitude of the reactive current that needs to be compensated is obtained.

[0024] In a possible implementation, when the active current increases to a certain extent, the reactive current that needs to be compensated will also increase, which will cause the total current to exceed the rated current, and the active current needs to be limited.

[0025] In a possible implementation, the grid-side impedance is identified through an extended state observer to obtain the connection strength of the grid in real time.

[0026] In a second aspect, the present application provides a photovoltaic grid-connected inverter control device under an extremely weak power grid, comprising:

[0027] An observation module is used to obtain the value of the grid-side impedance through an extended state observer, and then calculate the grid connection strength SCR;

[0028] The derivation module is used to derive the reactive current that needs to be compensated under different SCRs under the premise of keeping the voltage at the PCC point constant according to the voltage triangle relationship;

[0029] A limiting module is used to limit the active current;

[0030] The analysis module is used to establish the small signal model of the LCL type grid-connected inverter and analyze the stability.

[0031] The technical solution provided by this application can at least achieve the following beneficial effects:

[0032] 1: Traditional impedance detection technologies are mostly offline detection, or online monitoring but require the injection of certain harmonics or reactive power. There is a problem that the real-time grid-side impedance value cannot be obtained, and even the detection accuracy is insufficient. The method based on the extended state observer proposed in this application can ensure that the amplitude and phase angle of the grid-side impedance can be obtained in real time without injecting other quantities, laying the foundation for improving the adaptability of the power grid;

[0033] 2: The reactive current adaptive compensation control strategy based on active current feedforward proposed in this application can ensure that the grid-connected inverter can operate stably within a wide range of grid connection strength. The principle of this method is clear and easy to understand, the understanding of the limiting factors is clearer, and the adaptability to the grid is strong. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, a brief introduction will be given below to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0035] Figure 1 It is a flow chart of a photovoltaic grid-connected inverter control method under an extremely weak power grid shown in an exemplary embodiment of the present application;

[0036] Figure 2 is a control block diagram of a photovoltaic grid-connected inverter control method under an extremely weak power grid shown in an exemplary embodiment of the present application;

[0037] Figure 3 It is a schematic diagram of the voltage triangle relationship before and after complete reactive power compensation in an extremely weak power grid, shown in an exemplary embodiment of the present application;

[0038] Figure 4 is a schematic diagram of a curve relationship between a compensated reactive current n and an active current m shown in an exemplary embodiment of the present application;

[0039] Figure 5 is a schematic diagram of a curve relationship between a capacitor voltage vc" and the compensated reactive power n' after reactive power compensation, shown in an exemplary embodiment of the present application;

[0040] Figure 6 It is a structural schematic diagram of a photovoltaic grid-connected inverter control device under an extremely weak power grid shown in an exemplary embodiment of the present application.

[0041] Reference numerals:

[0042] 1. Observation module; 2. Derivation module; 3. Limiting module; 4. Analysis module. DETAILED DESCRIPTION

[0043] In order to make the purpose, implementation mode and advantages of the present application clearer, the exemplary implementation mode of the present application will be clearly and completely described below in conjunction with the drawings in the exemplary embodiments of the present application. Obviously, the described exemplary embodiments are only part of the embodiments of the present application, not all of the embodiments. It should be understood that the specific embodiments described here are only used to explain the present application and are not used to limit the present application.

[0044] It should be noted that the brief description of terms in this application is only for the convenience of understanding the embodiments described below, and is not intended to limit the embodiments of this application. Unless otherwise specified, these terms should be understood according to their common and usual meanings.

[0045] The terms "first", "second", "third", etc. in the specification and claims of this application and the above drawings are used to distinguish similar or similar objects or entities, and do not necessarily mean to limit a specific order or sequence, unless otherwise noted. It should be understood that the terms used in this way can be interchangeable under appropriate circumstances.

[0046] The terms "comprises," "comprising," and "having," and any variations thereof, are intended to cover but not exclude inclusion, for example, a product or device comprising a list of components is not necessarily limited to all the components expressly listed but may include other components not expressly listed or inherent to such product or device.

[0047] Before explaining the photovoltaic grid-connected inverter control method under an extremely weak power grid provided in the embodiment of the present application, the application scenario and implementation environment of the embodiment of the present application are first introduced.

[0048] With the increasing application of distributed generation and its large-scale access to the power grid, the grid impedance has gradually increased, accompanied by the problem of large fluctuations in grid impedance, making the grid present the characteristics of a weak grid or even an extremely weak grid. In order to describe the strength of the grid, the short circuit ratio (SCR) is generally used as a descriptive indicator, which is defined as the ratio of the AC short-circuit capacity to the rated capacity of the grid-connected device. Usually, in a weak grid, the SCR value is 2 to 3, and in an extremely weak grid, the SCR value is less than 2. Therefore, accurate grid impedance information is crucial for the operation of the grid-connected inverter. Under extremely weak grid conditions, it is necessary to monitor the grid impedance in real time, and to perform adaptive current control according to the size of the grid impedance to ensure reliable and efficient operation of the system. In addition, the measurement and analysis of grid impedance can also quickly detect grid faults. At present, the main methods for online impedance measurement include injecting harmonics and applying reactive disturbances. For example:

[0049] 1) Zhang Yufeng, Yang Shuying, Xie Zhen et al. published an article titled "Research on Online Measurement Technology of Grid Impedance of High-Power Grid-connected Inverters Based on Multi-sinusoidal Signal Injection" in the 10th issue of Volume 44 of the Proceedings of the CSEE in 2024. A multiple sine wave method was proposed. The multiple sine wave method can be used to achieve grid impedance measurement in a wide frequency range, and multiple sine waves are synthesized into a time domain signal to achieve impedance measurement in a specified frequency band. This method retains the advantages of the small signal injection method. However, the number of levels of multiple sine waves is too large, which places higher requirements on the controller. At the same time, multiple sine waves often have higher time domain amplitudes, which can easily cause nonlinear effects on the system.

[0050] 2) "Céspedes M, Sun J. Online grid impedance identification for adaptive control of grid-connected inverters." published at the 2012 Energy Conversion Conference and Expo (ECCE) conducted relevant research on the single pulse injection method, which was used to achieve online measurement of broadband grid impedance. However, the spectrum of the single pulse method is uncontrollable and has low flexibility. At the same time, when the grid impedance is small, the time domain amplitude needs to be increased to improve the measurement accuracy, but too large a time domain amplitude will affect the stable operation of the system.

[0051] 3) "Choi DK, Kang DH, Lee K B. A novel gain scheduling method for distributed power generation systems with a LCL-filter by estimating grid impedance." published at the 2010 IEEE International Symposium on Industrial Electronics. This paper proposes to measure the grid impedance by applying PQ power disturbance, that is, adding a disturbance value to the benchmark of active power and reactive power controlled by PQ, measuring the voltage and current values ​​at two different working points at the PCC point, and calculating the grid impedance at this time. The measurement accuracy of this method is affected by the PQ change. Moreover, it is limited to three-phase inverters using PQ control, and its application scope has certain limitations.

[0052] 4) "Gu H, Guo X, Wang D, et al. Real-time grid impedance estimation technique for grid-connected power converters." published in the 2012 IEEE International Symposium on Industrial Electronics proposed to use the switching subharmonics in the voltage and current of the grid-connected inverter to measure the grid impedance. However, in practical applications, the content of switching subharmonics is very small, making the measurement result inaccurate, and only the impedance information at the switching frequency can be measured, which cannot fully represent the impedance information of the entire frequency range.

[0053] 5) "Cobreces S, Rodriguez P, Pizarro D, et al. Complex-space recursive least squares power system identification." published in the 2007 Power Electronics Experts Conference proposed to measure the PCC point voltage and current at different operating points and use the least squares method to recursively estimate the optimal value of the grid impedance. However, this method requires multiple recursive iterations, has a large amount of calculation, and has low calculation accuracy.

[0054] 6) “Hoffmann N, Fuchs F W. Minimal Invasive Equivalent Grid Impedance Estimation in Inductive–Resistive Power Networks Using Extended Kalman Filter.” published in the Journal of Power Electronics Society in 2014 proposed a parameter estimation method based on Kalman filter. This method transforms the problem of identifying grid impedance into a mathematical problem, and uses the disturbance information of the power network itself to write the Kalman covariance matrix parameters to estimate the grid impedance.

[0055] 7) "Ciobotaru M, Agelidis V, Teodorescu R. Line impedance estimation using model based identification technique." published in the 2011 European Power Electronics and Applications Conference proposed a quasi-passive method for measuring grid impedance. First, the limit range of typical grid impedance is determined, a grid impedance model value is assumed, and then the estimated value of grid impedance is gradually adjusted to approximate the actual value. However, this method requires knowing both the PCC point voltage and the grid voltage, and the assumption that the grid voltage is constant cannot be met in practical applications.

[0056] In summary, the existing research on impedance measurement has the following problems:

[0057] 1) Different methods have different characteristics and are not adaptable, and most methods cannot achieve real-time online measurement of grid-side impedance;

[0058] 2) In the implementation process of the above methods, it is necessary to inject certain harmonic disturbances or reactive disturbances. However, disturbances are omnipresent in the system, and it is difficult to distinguish whether the disturbance is self-injected or external interference. If the grid-side impedance changes in real time, it is also difficult to further measure in real time.

[0059] Since extremely weak power grids are intermittent, random, and have output fluctuations, they seriously threaten the safe and stable operation of the power grid and affect the grid-connected consumption of new energy. Therefore, it is necessary to further carry out relevant research on the low-frequency oscillation mechanism and stability operation control strategy of grid-connected inverters under extremely weak power grids. At present, the research on stability control strategies for extremely weak power grids mainly focuses on the optimization design of control systems, namely outer loop control, inner loop control, and phase-locked loop (PLL). In terms of optimization of inner and outer loop control, the existing methods are mainly based on voltage feedforward and shaping of inverter output admittance. For example:

[0060] 1) Yang Ling, Chen Yandong, Chen Zhiyong et al. published the article "Phase Angle Compensation Control Method for Three-Phase Grid-Connected System Considering Phase-Locked Loop Influence under Weak Grid" in the 20th issue of Volume 38 of the Proceedings of the CSEE in 2018. A phase angle compensation control method was added on the basis of capacitor voltage feedforward. The output admittance of the grid-connected inverter was changed by PCC voltage feedforward, which solved the problem of system instability caused by the large PLL bandwidth and ensured the dynamic response speed of the system.

[0061] 2) Cao Ziheng, Xiao Xianyong, Ma Junpeng et al. published the article "Improved capacitor current feedback active damping strategy for improving the robustness of LCL grid-connected inverters" in "High Voltage Technology" Vol. 46, No. 11 in 2020. An improved capacitor current feedback active damping strategy based on phase advance compensation is proposed, that is, a phase advance compensation link is added to the capacitor current feedback branch to reduce the influence of control delay on the active damping equivalent impedance, thereby expanding the effective damping area.

[0062] 3) “Z. Lin and X. Ruan, A Three-Phase Adaptive Active Damper for Improving the Stability of Grid-Connected Inverters Under Weak Grid”, published in IEEE Applied Power Electronics Conference and Exposition 2019 (APEC), Anaheim, CA, USA, 2019, pp. 1084-1089. (“A three-phase adaptive active damper for improving the stability of grid-connected inverters under weak grids”) In order to solve the influence of digital control delay on traditional impedance shaping, a harmonic current reference compensation unit is introduced to enable the active damper to accurately simulate the virtual resistance within the unity gain crossover frequency of the current loop.

[0063] 4) “Y. Tang, J. Fang, X. Li and H. Li, Reshaping Quadrature-Axis Impedance of Three-Phase Grid-Connected Converters for Low-Frequency Stability Improvement”, 2018 International Power Electronics Conference, Niigata, Japan, 2018, pp. 3910-3915. This paper transforms the stability of the grid-connected inverter into an evaluation method of the ratio of grid impedance to inverter impedance. By optimizing the control system, the inverter impedance is reshaped into a low-frequency positive resistance to offset the negative impact of the PLL on system stability.

[0064] In summary, the existing research on extremely weak power grid stability control has the following problems:

[0065] 1) Since the inverter oscillation behavior under extremely weak power grid involves many links and complex coupling relationships, the mathematical models in the above analysis are much more complicated than the models that only consider the phase-locked loop and the current inner loop under weak power grid. For these complex extremely weak power grid conditions, it is difficult to continue to use the analytical analysis method of weak power grid for analysis.

[0066] 2) Although some conclusions that can guide the setting of control parameters are obtained, these conclusions cannot well explain the mechanism of low-frequency oscillation of inverters under extremely weak power grids, and it is also difficult to derive simple and effective methods to improve stability.

[0067] Based on this, the present application provides a photovoltaic grid-connected inverter control method and device under an extremely weak power grid, and the expanded state observer method measures the grid-side impedance in real time; the second problem to be solved is to overcome the low-frequency oscillation problem of the grid-connected inverter under an extremely weak power grid. The original technology is complex to implement and is no longer applicable when the power grid is extremely weak. Therefore, it is necessary to propose a grid-connected inverter control strategy with stronger grid adaptability from a theoretical level.

[0068] Next, the technical solution of the present application and how the technical solution of the present application solves the above-mentioned technical problems will be described in detail through embodiments and in combination with the accompanying drawings. The embodiments may be combined with each other, and the same or similar concepts or processes may not be repeated in some embodiments. Obviously, the described embodiments are part of the embodiments of the present application, not all of them.

[0069] Figure 1 It is a flow chart of a photovoltaic grid-connected inverter control method under an extremely weak power grid shown in an exemplary embodiment of the present application.

[0070] In an exemplary embodiment, Figure 1 As shown, a method for controlling a photovoltaic grid-connected inverter under an extremely weak power grid is provided. In this embodiment, the method may include the following steps:

[0071] Step 100: Measure and obtain the value of the grid-side impedance, and calculate the grid connection strength SCR according to the grid-side impedance.

[0072] Step 200: Based on the voltage triangle relationship, derive the size of the reactive current that needs to be compensated under different SCRs while keeping the voltage at the PCC point constant.

[0073] Step 300: Limit the active current.

[0074] Step 400: Establish a small signal model of the LCL type grid-connected inverter and analyze the stability.

[0075] Figure 2 is a control block diagram of a photovoltaic grid-connected inverter control method under an extremely weak power grid shown in an exemplary embodiment of the present application, Figure 3 is a schematic diagram of the voltage triangle relationship before and after complete reactive power compensation in an extremely weak power grid, shown in an exemplary embodiment of the present application. Figure 4 is a schematic diagram of a curve relationship between a compensated reactive current n and an active current m shown in an exemplary embodiment of the present application, Figure 5 1 is a schematic diagram of a curve relationship between a capacitor voltage vc" and the compensated reactive power n' after reactive power compensation, shown in an exemplary embodiment of the present application.

[0076] In one possible implementation, Figure 2 As shown, the grid-side phase voltage peak eg is taken as 110V, the DC side voltage udc is taken as 300V, the grid-side inductance Lg is taken as 7.2mH, the filter capacitor Cf is taken as 2.9μF, the inverter-side inductance Li is taken as 10.2mH, and the additional inductance L is connected in series at the grid-side inductance as needed to simulate the SCR operating conditions under different situations.

[0077] The photovoltaic grid-connected inverter includes a DC source, an LCL filter, a grid-side line additional impedance and a three-phase grid, and the DC source includes a photovoltaic simulator and a DC-side capacitor;

[0078] The control strategy first adopts an extended state observer to realize real-time online measurement of the grid-side impedance, and then adaptively compensates the reactive current iq by adding active current feedforward, thereby improving the reactive power of the system within the range of satisfying the voltage modulation limit, thereby improving the stability of the grid-connected converter under any grid connection strength.

[0079] The specific implementation steps are as follows:

[0080] Step 1: The grid-side impedance is obtained through the extended state observer, and then the grid connection strength SCR is calculated, which is expressed as:

[0081]

[0082] Where |Zg| is the measured grid-side impedance modulus, Un is the effective value of the grid-side line voltage, and PN is the rated power on the AC side.

[0083] Step 2, according to Figure 3 The voltage triangle relationship shown in the figure deduces that under the premise of keeping the voltage at the PCC point constant, a certain reactive current needs to be compensated under different SCRs to keep the voltage at the PCC point at the grid voltage level, thereby improving the stability of the system's grid-connected operation under weak power grids.

[0084] Depend on Figure 4 It can be seen that when the active current approaches the rated value, the reactive current required to compensate for the stable operation of the grid-connected system will be greater. Figure 5 It can be seen that when the compensated reactive current is larger, the voltage at the PCC point will also increase. If the compensation is too much, it will exceed the limit range of the voltage modulation. At this time, it is necessary to balance the relationship between the two and obtain the size of the reactive current that satisfies both restrictions. The expression is:

[0085]

[0086] In the formula, n is the ratio of the reactive current to be compensated to the rated current, and m is the ratio of the active current to the rated current at this time.

[0087] Step 3: According to step 2, when the active current increases to a certain extent, the reactive current required to be compensated will also increase, which will cause the total current to exceed the rated current. At this time, the active current needs to be limited. The expression is:

[0088]

[0089] Where SCR represents the grid connection strength, and mmax represents the ratio of the active current limit value to the rated current.

[0090] Step 4: Establish a small signal model of the LCL grid-connected inverter and analyze its stability.

[0091] In a possible implementation, the photovoltaic grid-connected inverter control strategy under the extremely weak power grid includes an impedance measurement part, an active current feedforward control part, a DC voltage outer loop control part, a dq axis current inner loop control part and a DSOGI phase-locked loop part. The photovoltaic grid-connected inverter includes a DC source, an LCL filter, a grid-side line additional impedance and a three-phase power grid. The DC source includes a photovoltaic simulator and a DC side capacitor.

[0092] It should be understood that, although the various steps in the flowcharts involved in the above-mentioned embodiments are displayed in sequence according to the instructions, these steps are not necessarily executed in sequence according to the order of the instructions. Unless there is a clear explanation in this article, the execution of these steps is not strictly limited in order, and these steps can be executed in other orders. Moreover, at least a portion of the steps in the flowcharts involved in the above-mentioned embodiments may include multiple steps or multiple stages, and these steps or stages are not necessarily executed at the same time, but can be executed at different times, and the execution order of these steps or stages is not necessarily to be carried out in sequence, but can be executed in turn or alternately with other steps or at least a portion of the steps or stages in other steps.

[0093] Corresponding to the embodiment of the photovoltaic grid-connected inverter control method under the extremely weak power grid described above, adopting the same technical concept, the present application also provides an embodiment of the photovoltaic grid-connected inverter control device under the extremely weak power grid.

[0094] Figure 6 It is a structural schematic diagram of a photovoltaic grid-connected inverter control device under an extremely weak power grid shown in an exemplary embodiment of the present application.

[0095] In an exemplary embodiment, Figure 6 As shown, the photovoltaic grid-connected inverter control device under the extremely weak power grid includes:

[0096] Observation module 1, used to obtain the value of grid-side impedance through an extended state observer, and then calculate the grid connection strength SCR;

[0097] The derivation module 2 is used to derive the magnitude of reactive current that needs to be compensated under different SCRs under the premise of keeping the voltage at the PCC point constant according to the voltage triangle relationship;

[0098] A limiting module 3 is used to limit the active current;

[0099] The analysis module 4 is used to establish a small signal model of the LCL type grid-connected inverter and analyze the stability.

[0100] For the specific definition of the photovoltaic grid-connected inverter control device under an extremely weak power grid, please refer to the definition of the photovoltaic grid-connected inverter control method under an extremely weak power grid above, which will not be repeated here. Each module in the above-mentioned photovoltaic grid-connected inverter control device under an extremely weak power grid can be implemented in whole or in part by software, hardware and a combination thereof. The above-mentioned modules can be embedded in or independent of the processor in the computer device in the form of hardware, or can be stored in the memory of the computer device in the form of software, so that the processor can call and execute the operations corresponding to the above modules.

[0101] The technical features of the above embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0102] The above-described embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the attached claims.

Claims

1. A photovoltaic grid-connected inverter control method under extremely weak power grid, characterized in that: include: The grid-side impedance is measured and the grid connection strength SCR is calculated according to the grid-side impedance. According to the voltage triangle relationship, the reactive current that needs to be compensated under different SCRs is derived while keeping the voltage at the PCC point constant. Limit the active current; A small signal model of LCL grid-connected inverter is established and the stability is analyzed.

2. The photovoltaic grid-connected inverter control method under extremely weak power grid according to claim 1, characterized in that: The expression of the grid connection strength SCR is: Among them, |Zg| is the measured grid-side impedance modulus, Un is the effective value of the grid-side line voltage, and PN is the rated power on the AC side.

3. The photovoltaic grid-connected inverter control method under extremely weak power grid as claimed in claim 2, characterized in that: The magnitude of the reactive current that needs to be compensated is expressed as: Among them, n is the ratio of the reactive current to be compensated to the rated current, and m is the ratio of the active current to the rated current at this time.

4. The photovoltaic grid-connected inverter control method under extremely weak power grid as claimed in claim 3, characterized in that: The expression for limiting the active current is: Among them, SCR represents the grid connection strength, and mmax represents the ratio of the active current limit value to the rated current.

5. The photovoltaic grid-connected inverter control method under extremely weak power grid as claimed in claim 4, characterized in that: When the active current approaches the rated value, the reactive current that needs to be compensated will be larger to ensure the stable operation of the grid-connected system. When the reactive current to be compensated is larger, the voltage at the PCC point will also increase. If the compensation is too much, it will exceed the limit range of the voltage modulation. According to the two limiting conditions, the magnitude of the reactive current that needs to be compensated is obtained.

6. The photovoltaic grid-connected inverter control method under extremely weak power grid as claimed in claim 5, characterized in that: When the active current increases to a certain level, the reactive current that needs to be compensated will also increase, causing the total current to exceed the rated current, and the active current needs to be limited.

7. The photovoltaic grid-connected inverter control method under extremely weak power grid according to claim 1, characterized in that: The grid-side impedance is identified through the extended state observer, and the connection strength of the power grid is obtained in real time.

8. A photovoltaic grid-connected inverter control device under extremely weak power grid, characterized in that: include: An observation module is used to obtain the value of the grid-side impedance through an extended state observer, and then calculate the grid connection strength SCR; The derivation module is used to derive the reactive current that needs to be compensated under different SCRs under the premise of keeping the voltage at the PCC point constant according to the voltage triangle relationship; A limiting module is used to limit the active current; The analysis module is used to establish the small signal model of the LCL type grid-connected inverter and analyze the stability.

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