Fan grid-connected oscillation early warning method and system considering impedance time variation and medium

Abstract

The invention relates to the technical field of intelligent power grids, and discloses a fan grid-connected oscillation early warning method and system considering impedance time variation and a medium, and the method comprises the steps: firstly, determining an impedance ratio model of a fan grid-connected system, and carrying out the calculation based on a generalized Nyquist stability criterion to obtain a Nyquist curve of an impedance ratio matrix; and then correcting the early warning margin of the criterion on the basis of the criterion to obtain a modified fan impedance expression and impedance ratio matrix. After the impedance time-varying characteristic is considered, the Nyquist curve presents a curve band with a certain width, and the curve band is used as an oscillation early warning impedance criterion. For impedance change differences caused by time-varying characteristics under different operation conditions, the width of a Nyquist curve band also changes, the potential oscillation risk degree of the system is evaluated by using the width of the curve band, a time-varying characteristic index CTI of an impedance criterion is introduced to quantify the width of the curve band, and the quantified curve band width is used as an oscillation risk evaluation index. And early recognition and early warning of the oscillation trend of the fan grid-connected system are realized.

Description

Technical Field

[0001] This invention belongs to the field of smart grid technology, especially power system impedance stability analysis technology, and specifically relates to a wind turbine grid-connected oscillation early warning method, system and medium that takes into account impedance time-varying characteristics. Background Technology

[0002] As the penetration rate of new energy sources in the new power system continues to increase, the power system is showing characteristics of high power electronics, the dynamic processes inside the system are becoming increasingly complex, and the problem of power system oscillation and stability is gradually becoming prominent.

[0003] Unlike traditional low-frequency oscillations and sub / supersynchronous oscillations, the large-scale integration of new energy units will cause power system stability problems to exhibit characteristics of wide frequency domain, strong time-varying nature, and wide-area propagation. Furthermore, the complex control links of wind turbine units lead to an extremely complex dynamic interaction process between them and the AC grid, with more obvious coupling effects, resulting in grid connection stability problems.

[0004] Currently, impedance-based stability analysis is an effective method for analyzing this problem. Existing wind turbine grid-connected impedance analyses are typically designed for specific system operating conditions or single oscillation modes. However, the impedance characteristics of wind farms are highly susceptible to factors such as wind speed and control strategy switching during operation. Furthermore, the power system operates under complex and variable conditions, often involving long-distance power transmission via series or parallel compensation lines. The interaction between renewable energy farms and the grid often exhibits time-varying characteristics. Therefore, traditional impedance analysis methods struggle to adapt to time-varying operating conditions and multi-mode oscillations, lack robustness, and suffer from inadequate handling of critical stability states, failing to provide timely early warnings of oscillations.

[0005] Existing research divides the wind turbine-grid interaction system into two subsystems: the source and the grid. This approach, based on considering the control structure and parameter characteristics of each subsystem and accurately establishing their impedance models, applies the generalized Nyquist stability criterion to the impedance ratio of the two subsystems to determine the stability of the grid-connected system. However, during operation, wind turbines are affected by uncertainties such as environmental conditions and control methods, causing their impedance characteristics to change, i.e., exhibiting time-varying impedance characteristics. Although these changes are short-term and sometimes self-recovering, they can still lead to frequent entry into critical instability states. Summary of the Invention

[0006] Given that existing impedance analysis methods cannot adapt to time-varying operating conditions and multi-mode oscillations, lack robustness, and suffer from inadequate handling of critical stability states, thus failing to accurately provide oscillation early warning, this invention aims to provide a wind turbine grid-connected oscillation early warning method that takes into account impedance time-varying characteristics. This method fully considers the dynamic characteristics of impedance changes over short time scales, introduces an oscillation risk assessment index into the stability criterion, sets oscillation early warning criteria, and appropriately relaxes the stability discrimination standard to allow for a certain degree of flexibility and adaptability. This prevents frequent triggering of stability protection mechanisms, significantly improves the accuracy of assessing real oscillation risks under complex operating environments, solves the problem of repeated protection triggering caused by impedance time-varying characteristics, and accurately issues oscillation early warning signals.

[0007] According to a first aspect of the present invention, a method for early warning of grid-connected oscillation of wind turbines that takes into account time-varying impedance is provided, comprising:

[0008] Step 1: Based on frequency coupling, establish the impedance ratio model of the wind turbine grid-connected system, and calculate the Nyquist curve of the impedance ratio matrix through the generalized Nyquist stability criterion.

[0009] Step 2: Based on the time-varying characteristics of wind turbine impedance, the generalized Nyquist stability criterion is modified with a margin to determine the oscillation warning trigger zone. The wind turbine impedance expression and impedance ratio matrix, taking into account the time-varying impedance, are obtained to determine the oscillation warning criterion.

[0010] Step 3: Based on the influence of impedance time-varying characteristics on the Nyquist curve under different operating conditions, introduce a time-varying index for impedance criteria, and use this index as the basis for assessing the oscillation risk at the current operating point of the system, so as to achieve early warning of wind turbine grid-connected oscillation.

[0011] As an optional implementation, step 2, which involves margin-correcting the generalized Nyquist stability criterion to determine the oscillation warning trigger zone, includes:

[0012] Define a stability margin radius r, ;

[0013] Using any point on the Nyquist curve as the center, and... With radius , a series of dynamic circular regions are formed on the complex plane, which define the system oscillation early warning trigger area;

[0014] And, when When a point is surrounded by the Nyquist curve but lies within the dynamic circular region, the system is determined to be in an oscillation warning state, indicating that although the system is not completely unstable, there is a clear risk of oscillation and instability.

[0015] As an optional implementation, after taking into account the time-varying characteristics of impedance, the Nyquist curve is represented as a curve band, which is extended into an oscillation risk band.

[0016] Since the impact of impedance time-varying characteristics on the Nyquist curve varies under different operating conditions, resulting in changes in the width of the Nyquist curve, a time-varying index based on the impedance criterion of curve band width is introduced to quantify the impedance time-varying characteristics and to assess the oscillation risk of the system at the current operating point.

[0017] As an optional implementation, the calculation process of the time-varying index of the impedance criterion includes:

[0018] Based on the time-varying characteristics of impedance, the maximum value of the impedance loop ratio matrix sensitivity is... Represented as:

[0019] ;

[0020] In the formula, the subscript correspond eigenvalues ; This indicates that, given the magnitude of the time-varying amplitude, The maximum possible deviation;

[0021] definition The frequency corresponding to the intersection of the eigenvalue and the real axis is the crossover frequency. ;

[0022] use The Nyquist band was calculated at the cross-frequency. The processing width is used as an indicator for oscillation risk assessment, namely the time-varying index (CTI) of the impedance criterion:

[0023] .

[0024] According to a second aspect of the present invention, a computer system is provided, comprising:

[0025] One or more processors;

[0026] The memory stores operable instructions that, when executed by the one or more processors, cause the one or more processors to perform operations, including the aforementioned process of performing the wind turbine grid-connected oscillation early warning method that takes into account impedance time-varying characteristics.

[0027] According to a third aspect of the present invention, a computer-readable medium for storing a computer program is provided, characterized in that the computer program includes instructions / instruction sets executable by one or more processors, wherein the instructions / instruction sets, when executed by the one or more processors, implement the aforementioned method for early warning of grid-connected oscillations of wind turbines taking into account impedance time-varying characteristics.

[0028] Combining the above embodiments with the wind turbine grid-connected oscillation early warning method considering impedance time-varying characteristics, this invention addresses the shortcomings of existing impedance analysis methods in adapting to wind turbine impedance time-varying scenarios, such as inaccurate identification of critical stable states and frequent maloperation of protection systems. By introducing a robust quantitative expression based on the impedance criterion, the early warning margin of the criterion is enhanced, resulting in a modified wind turbine impedance expression and impedance ratio matrix. After considering the impedance time-varying characteristics, the Nyquist curve presents as a "curve band" with a certain width. Due to the limited amplitude of the wind turbine impedance time-varying change over a short time scale, the banded region of the Nyquist curve will exhibit narrow band characteristics. Based on this, the oscillation early warning impedance criterion is defined. The width of the Nyquist curve band also changes according to the differences in impedance changes caused by time-varying characteristics under different operating conditions. The width of the curve band can be used to assess the potential oscillation risk of the system. Therefore, the time-varying index (CTI) of the impedance criterion is introduced to quantify the width of the curve band as an oscillation risk assessment indicator, thereby achieving early identification and early warning of oscillation trends in the wind turbine grid-connected system, enabling the wind turbine grid-connected system to have risk buffering capabilities. Meanwhile, based on the proposed impedance time-varying index, a quantitative assessment of the system oscillation risk under different operating conditions is achieved, providing operators with clear early warning signals and decision-making basis.

[0029] It should be understood that all combinations of the foregoing concepts and the additional concepts described in more detail below may be considered part of the inventive subject matter of this disclosure, provided that such concepts do not contradict each other. Furthermore, all combinations of the claimed subject matter are considered part of the inventive subject matter of this disclosure.

[0030] The foregoing and other aspects, embodiments, and features of the teachings of the present invention will be more fully understood from the following description in conjunction with the accompanying drawings. Other additional aspects of the invention, such as features and / or beneficial effects of exemplary embodiments, will become apparent from the following description or may be learned through practice of specific embodiments according to the teachings of the present invention. Attached Figure Description

[0031] The accompanying drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component shown in the various figures may be denoted by the same reference numeral. For clarity, not every component is labeled in each figure. Embodiments of various aspects of the invention will now be described by way of example and with reference to the accompanying drawings.

[0032] Figure 1 This is a flowchart illustrating a wind turbine grid-connected oscillation early warning method that takes into account impedance time-varying characteristics according to an embodiment of the present invention.

[0033] Figure 2 This is a schematic diagram of the grid-side topology of a doubly fed induction generator (DFIG) according to an embodiment of the present invention.

[0034] Figure 3 This is a schematic diagram of the machine-side topology of a doubly fed induction generator (DFIG) according to an embodiment of the present invention.

[0035] Figure 4 This is a schematic diagram of the equivalent circuit of the wind power-grid interconnection system according to an embodiment of the present invention.

[0036] Figure 5 This is a schematic diagram of the impedance criterion considering time-varying impedance in an embodiment of the present invention.

[0037] Figure 6 This is a schematic diagram comparing the generalized Nyquist criterion and the oscillation warning impedance criterion in an embodiment of the present invention.

[0038] Figure 7 This is a schematic diagram of the oscillation warning impedance criterion results after adjusting the dominant parameters in an embodiment of the present invention. Detailed Implementation

[0039] To better understand the technical content of the present invention, specific embodiments are described below in conjunction with the accompanying drawings.

[0040] Various aspects of the invention are described in this disclosure with reference to the accompanying drawings, which illustrate numerous illustrative embodiments. The embodiments of this disclosure are not necessarily intended to encompass all aspects of the invention. It should be understood that the various concepts and embodiments described above, as well as those described in more detail below, can be implemented in any of many ways, because the concepts and embodiments disclosed herein are not limited to any particular implementation. Furthermore, some aspects of the invention disclosed may be used alone or in any suitable combination with other aspects of the invention disclosed.

[0041] {Example 1}

[0042] Combination Figure 1 As shown, the wind turbine grid-connected oscillation early warning method considering impedance time-varying characteristics according to an embodiment of the present invention includes:

[0043] Step 1: Based on frequency coupling, establish the impedance ratio model of the wind turbine grid-connected system, and calculate the Nyquist curve of the impedance ratio matrix through the generalized Nyquist stability criterion.

[0044] Step 2: Based on the time-varying characteristics of wind turbine impedance, the generalized Nyquist stability criterion is modified with a margin to determine the oscillation warning trigger zone. The wind turbine impedance expression and impedance ratio matrix, taking into account the time-varying impedance, are obtained to determine the oscillation warning criterion.

[0045] Step 3: Based on the influence of impedance time-varying characteristics on the Nyquist curve under different operating conditions, introduce a time-varying index for impedance criteria, and use this index as the basis for assessing the oscillation risk at the current operating point of the system, so as to achieve early warning of wind turbine grid-connected oscillation.

[0046] As an optional embodiment, a harmonic linearization method is used to establish the impedance ratio model of the wind turbine grid-connected system.

[0047] For example, consider a two-input, two-output impedance ratio model, where the wind turbine sequence impedance model... The expression is as follows:

[0048] ;

[0049] In the formula, the superscripts “11” and “22” represent the positive and negative sequence impedances of the DFIG, respectively, and “12” and “21” represent the positive sequence coupling impedance and negative sequence coupling impedance of the DFIG, respectively.

[0050] Sequence impedance model of the same type of power grid subsystem The expression is as follows:

[0051] ;

[0052] In the formula, The system oscillation frequency, 50Hz Indicates the equivalent inductance on the grid side;

[0053] Therefore, the impedance ratio matrix of the wind turbine grid-connected system is:

[0054] .

[0055] Based on the frequency coupling characteristics, the system's impedance ratio has a 2*2 matrix structure. The generalized Nyquist criterion (GNC) can be applied to calculate the Nyquist curves of the impedance ratio matrix eigenvalues, as shown below:

[0056]

[0057] If and only if , The Nyquist curves do not enclose When the signal is at a certain point, the system's small signal is stable; otherwise, the system's small signal is unstable.

[0058] As an optional embodiment, step 2, which involves performing margin correction on the generalized Nyquist stability criterion to determine the oscillation warning trigger zone, includes:

[0059] Define a stability margin radius r, ;

[0060] Using any point on the Nyquist curve as the center, and... With radius , a series of dynamic circular regions are formed on the complex plane, which define the system oscillation early warning trigger area;

[0061] And, when When a point is surrounded by the Nyquist curve but lies within the dynamic circular region, the system is determined to be in an oscillation warning state, indicating that although the system is not completely unstable, there is a clear risk of oscillation and instability.

[0062] As an optional embodiment, in step 2, the wind turbine impedance after taking into account the time-varying impedance is corrected to:

[0063]

[0064] In the formula, The wind turbine impedance, taking into account the time-varying characteristics of impedance, This represents the time-varying deviation of the impedance. , These represent the time-varying amplitude and phase, respectively.

[0065] The impedance ratio matrix is ​​corrected to:

[0066]

[0067] in, Used to reflect the change in impedance loop ratio, corresponding to the dynamic circular region, its maximum value is equal to the radius of the dynamic circular region.

[0068] As an optional embodiment, after taking into account the time-varying characteristics of impedance, the Nyquist curve is represented as a curve band, which is extended into an oscillation risk band;

[0069] Since the impact of impedance time-varying characteristics on the Nyquist curve varies under different operating conditions, resulting in changes in the width of the Nyquist curve, a time-varying index based on the impedance criterion of curve band width is introduced to quantify the impedance time-varying characteristics and to assess the oscillation risk of the system at the current operating point.

[0070] As an optional embodiment, in step 3, the calculation process of the time-varying index of the impedance criterion includes:

[0071] Based on the time-varying characteristics of impedance, the maximum value of the impedance loop ratio matrix sensitivity is... Represented as:

[0072] ;

[0073] In the formula, the subscript correspond eigenvalues ; This indicates that, given the magnitude of the time-varying amplitude, The maximum possible deviation;

[0074] definition The frequency corresponding to the intersection of the eigenvalue and the real axis is the crossover frequency. ;

[0075] use The Nyquist band was calculated at the cross-frequency. The processing width is used as an indicator for oscillation risk assessment, namely the time-varying index (CTI) of the impedance criterion:

[0076] .

[0077] Therefore, through the implementation of the wind turbine grid-connected oscillation early warning system considering impedance time-varying characteristics in the above embodiments of the present invention, the impedance ratio model of the wind turbine grid-connected system is first determined, and the Nyquist curve of the impedance ratio matrix is ​​calculated based on the generalized Nyquist stability criterion. Furthermore, robust quantitative characterization is added to the impedance criterion, and the modified wind turbine impedance expression and impedance ratio matrix are obtained by correcting the early warning margin of the criterion. After considering the impedance time-varying characteristics, the Nyquist curve presents as a curve band with a certain width.

[0078] Because the amplitude of the time-varying impedance of wind turbines is limited over a short timescale, the banded region of the Nyquist curve will exhibit narrow-band characteristics. Therefore, in the scheme of this invention, its bandwidth is used as the basis for defining the oscillation early warning impedance criterion. The width of the Nyquist curve band also varies depending on the differences in impedance changes caused by time-varying impedance under different operating conditions. Therefore, in the embodiments of this invention, the width of the curve band is used to assess the potential oscillation risk of the system. The time-varying impedance index (CTI) is introduced to quantify the width of the curve band as an oscillation risk assessment indicator, thereby achieving early identification and early warning of oscillation trends in the wind turbine grid-connected system.

[0079] {Example 2}

[0080] In this embodiment, a doubly-fed induction generator (DFIG) grid-connected system will be used as an example, combined with the attached... Figure 2-7 Specific embodiments of the present invention will be described in detail below.

[0081] Appendix Figure 2 and attached Figure 3 These are the DFIG network-side topology and the machine-side topology, which include phase-locked angles. The acquisition of voltage and current coordinates, as well as voltage and current control circuits.

[0082] Step 1: The DFIG grid-connected system consists of two subsystems connected in parallel: the source side and the grid side. Their equivalent circuits are shown in the attached diagram. Figure 4 As shown.

[0083] A DFIG sequence impedance model is established using the harmonic linearization method. Its structure is as follows:

[0084]

[0085] In the formula, the superscripts “11” and “22” represent the positive and negative sequence impedances of the DFIG, respectively, and “12” and “21” represent the positive sequence coupling impedance and negative sequence coupling impedance of the DFIG, respectively.

[0086] Next, the sequence impedance model of the same type of power grid subsystem is determined. When the three-phase impedance of the power grid is balanced, the port of the power grid subsystem has a single-input single-output characteristic and there is no frequency coupling phenomenon. It is defined by the following expression:

[0087]

[0088] In the formula, The system oscillation frequency, 50Hz This represents the equivalent inductance on the grid side.

[0089] Through impedance model and Describe the port characteristics of the source / network subsystems separately. In this case, the stability of the DFIG grid-connected system depends on the impedance ratio. / The stability and impedance ratio structure are as follows:

[0090]

[0091] Due to the frequency coupling characteristics, the system's impedance ratio has a 2*2 matrix structure, requiring the application of the generalized Nyquist criterion (GNC). The Nyquist curves for calculating the eigenvalues ​​of the impedance ratio matrix are shown below:

[0092]

[0093] If and only if , The Nyquist curves do not enclose When the signal is at a certain point, the system's small signal is stable; otherwise, the system's small signal is unstable.

[0094] Step 2: Considering the uncertainties of DFIG operation due to factors such as wind speed and control method, i.e., the existence of impedance time-varying characteristics, the generalized Nyquist warning criterion retains a certain margin, specifically:

[0095] Set a stability margin radius Using any point on the generalized Nyquist curve as the center, and... A series of dynamic circular regions are formed on the complex plane with radius [radius]; these circular regions represent the robustness requirements of the impedance criterion and are defined as the system's "oscillation warning trigger zone". (See attached diagram.) Figure 5 The figure shows the improved generalized Nyquist stability criterion. The blue circle represents the robustness requirement of the impedance criterion, which is the oscillation warning trigger zone of the system.

[0096] when When a point is surrounded by the Nyquist curve but lies within this dynamic circular region (blue area), the system is determined to be in an early warning state, indicating that although the system is not completely unstable, there is a clear risk of oscillation and instability.

[0097] The robustness quantification of the improved criterion is reflected in the DFIG impedance expression, as follows:

[0098]

[0099] In the formula, This represents the DFIG impedance taking into account its time-varying characteristics. This represents the time-varying deviation of the impedance. , These represent the time-varying amplitude and phase, respectively.

[0100] Therefore, after considering the time-varying characteristics of the impedance, the impedance ratio matrix can be expressed as:

[0101]

[0102] in, This reflects the change in the impedance loop ratio, which is a quantitative representation of the robustness of the impedance criterion, corresponding to the attached... Figure 4 The maximum value of the blue warning area is equal to the radius of the blue circle.

[0103] Therefore, after taking into account the time-varying characteristics of impedance, the Nyquist curve will be represented in the form of a "curved band", which expands into an oscillation risk band, ensuring that the improved impedance criterion has high robustness.

[0104] It should be noted that, since the time-varying amplitude of the DFIG impedance is not large over a short time scale, the banded region of the Nyquist curve will exhibit narrow-band characteristics.

[0105] Therefore, the impedance criterion that takes into account the time-varying nature of impedance will only classify curves with small overmargins as stable and issue a warning signal, and will not misclassify obvious instability in extreme cases as stable, thus ensuring that the improved impedance criterion has high robustness. In the embodiments of the present invention, it is defined as the oscillation warning impedance criterion.

[0106] Step 3: Since the DFIG impedance changes caused by time-varying characteristics are different under different operating conditions, the width of the Nyquist curve band is also different. The width of the curve band can be used to characterize the degree of system oscillation risk. The time-varying index (CTI) of the impedance criterion is introduced to quantify it. The CTI value can be used as a key early warning indicator to assess the probability of system oscillation under the current operating conditions.

[0107] Taking into account the time-varying nature of impedance, the maximum value of the impedance loop ratio matrix sensitivity is... Represented as:

[0108]

[0109] In the formula, the subscript Corresponding eigenvalues , This indicates that, given the magnitude of the time-varying amplitude, The maximum possible deviation. The smaller the value, the smaller the change in the impedance ratio matrix corresponding to the impedance time-varying criterion when the DFIG is affected by time-varying factors such as wind speed.

[0110] The key to system stability lies in The intersection of the eigenvalues ​​with the real axis is defined as the crossover frequency. .

[0111] Furthermore, utilizing The Nyquist curve zone can be calculated. The width of the vicinity is used as an indicator of time-varying characteristics, and its expression is:

[0112]

[0113] In the formula, the subscript Corresponding eigenvalues .

[0114] A larger CTI corresponds to a wider curve band at the crossover frequency, indicating that the repeated changes in system stability caused by time-varying characteristics are more prominent, and the risk of system oscillation is also greater. Therefore, a smaller CTI reduces the likelihood of system oscillation, which is more conducive to improving system stability. This indicator also provides guidance for parameter optimization.

[0115] To verify the technical effect of the present invention, an example simulation was conducted, in conjunction with the appendix. Figure 6-7 The technical solutions provided by the above-described embodiments of the invention are further explained below:

[0116] For appendix Figure 2 and attached Figure 3 The parameters for the DFIG grid-connected system shown are as shown in Table 1: Table 1 Control Parameters and Power Component Parameters

[0117] When considering time-varying characteristics, it is believed that the time scale of the study should be set to the minute level. The wind speed change within the minute-level time scale is usually less than 2 m / s. The effectiveness of the oscillation early warning impedance criterion will be further explained below.

[0118] Appendix Figure 6 This demonstrates the difference between the generalized Nyquist criterion and the oscillation warning impedance criterion. When the wind speed is 6 m / s, the result is determined by the attached... Figure 6 From (a) and (b), we can know that at this time... The Nyquist curve encircles At this point, according to the generalized Nyquist criterion, it is considered unstable; however, when the wind speed changes slightly and drops to 5.8 m / s, The Nyquist curve no longer encloses If the point is then judged as stable, this repeated change in stability judgment is not conducive to the operation of the protection device and cannot provide accurate oscillation warnings.

[0119] From the appendix Figure 6 (c) and (d) show that when the wind speed is 6 m / s, Point located at On the Nyquist curve band, an oscillation warning signal is issued based on the oscillation warning impedance criterion. Stabilization protection measures can be temporarily suspended, avoiding frequent operation of the protection device. At this point, the oscillation risk assessment index CTI is calculated to be 0.7. A relatively high CTI value indicates a higher probability of system oscillation.

[0120] Modify the system parameters as follows =4, =3, =1, =0.5 and =5, resulting in the appendix Figure 7 The oscillation warning impedance criterion results after parameter adjustment shown indicate that neither of the two Nyquist curve bands encloses the other. The value at this point indicates that the system is stable and no oscillation warning signal is issued. The system oscillation risk assessment index (CTI) for this condition is calculated to be 0.15. Compared to the CTI of 0.7 before parameter adjustment, this shows that the system stability is less affected by the time-varying impedance characteristics after parameter modification, resulting in a low oscillation risk. This demonstrates the effectiveness of the oscillation warning impedance criterion and the oscillation risk assessment index.

[0121] The details not described in this specification are existing knowledge known to those skilled in the art. While the invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the invention. Those skilled in the art can make various modifications and refinements without departing from the spirit and scope of the invention. Therefore, the scope of protection of this invention is determined by the claims.

Claims

1. A method for early warning of grid-connected oscillation of wind turbines considering time-varying impedance, characterized in that, include: Step 1: Based on frequency coupling, establish the impedance ratio model of the wind turbine grid-connected system, and calculate the Nyquist curve of the impedance ratio matrix through the generalized Nyquist stability criterion. Step 2: Based on the time-varying characteristics of wind turbine impedance, the generalized Nyquist stability criterion is modified with margin to determine the oscillation warning trigger zone, and the wind turbine impedance expression and impedance ratio matrix after taking into account the time-varying impedance are obtained to determine the oscillation warning criterion. as well as Step 3: Based on the influence of impedance time-varying characteristics on the Nyquist curve under different operating conditions, introduce a time-varying index for impedance criteria, and use this index as the basis for assessing the oscillation risk at the current operating point of the system, so as to achieve early warning of wind turbine grid-connected oscillation.

2. The wind turbine grid-connected oscillation early warning method considering impedance time-varying characteristics according to claim 1, characterized in that, In step 1, the impedance ratio model of the wind turbine grid-connected system is established by using the harmonic linearization method.

3. The wind turbine grid-connected oscillation early warning method considering impedance time-varying characteristics according to claim 2, characterized in that, The impedance ratio model of the wind turbine grid-connected system adopts a two-input, two-output impedance ratio model, wherein the wind turbine sequence impedance model... The expression is as follows: ； In the formula, the superscripts "11" and "22" represent the positive and negative sequence impedances of the DFIG, respectively, and "12" and "21" represent the positive sequence coupling impedance and negative sequence coupling impedance of the DFIG, respectively. Sequence impedance model of the same type of power grid subsystem The expression is as follows: ； In the formula, The system oscillation frequency, 50Hz Indicates the equivalent inductance on the grid side; Therefore, the impedance ratio matrix of the wind turbine grid-connected system is: ； The Nyquist curve for calculating the eigenvalues ​​of the impedance ratio matrix if and only if , The Nyquist curves do not enclose When the signal is at a certain point, the system's small signal is stable; otherwise, the system's small signal is unstable.

4. The wind turbine grid-connected oscillation early warning method considering impedance time-varying characteristics according to claim 1, characterized in that, In step 2, the margin correction of the generalized Nyquist stability criterion to determine the oscillation warning trigger zone includes: Define a stability margin radius r, ; Using any point on the Nyquist curve as the center, and... With radius , a series of dynamic circular regions are formed on the complex plane, which define the system oscillation early warning trigger area; And, when When a point is surrounded by the Nyquist curve but lies within the dynamic circular region, the system is determined to be in an oscillation warning state, indicating that although the system is not completely unstable, there is a clear risk of oscillation and instability.

5. The wind turbine grid-connected oscillation early warning method considering impedance time-varying characteristics according to claim 1, characterized in that, In step 2, the wind turbine impedance after taking into account the time-varying impedance is corrected as follows: ； In the formula, The wind turbine impedance, taking into account the time-varying characteristics of impedance, This represents the time-varying deviation of the impedance. , These represent the time-varying amplitude and phase, respectively. The impedance ratio matrix is ​​corrected to: ； in, Used to reflect the change in impedance loop ratio, corresponding to the dynamic circular region, its maximum value is equal to the radius of the dynamic circular region.

6. The wind turbine grid-connected oscillation early warning method considering impedance time-varying characteristics according to claim 5, characterized in that, After taking into account the time-varying characteristics of impedance, the Nyquist curve is represented as a curve band, which expands into an oscillation risk band; Since the impact of impedance time-varying characteristics on the Nyquist curve varies under different operating conditions, resulting in changes in the width of the Nyquist curve, a time-varying index based on the impedance criterion of curve band width is introduced to quantify the impedance time-varying characteristics and to assess the oscillation risk of the system at the current operating point.

7. The wind turbine grid-connected oscillation early warning method considering impedance time-varying characteristics according to claim 5, characterized in that, The calculation process of the time-varying index of the impedance criterion includes: Based on the time-varying characteristics of impedance, the maximum value of the impedance loop ratio matrix sensitivity is... Represented as: ； In the formula, the subscript correspond eigenvalues ; This indicates that, given the magnitude of the time-varying amplitude, The maximum possible deviation; definition The frequency corresponding to the intersection of the eigenvalue and the real axis is the crossover frequency. ; use The Nyquist band was calculated at the cross-frequency. The processing width is used as an indicator for oscillation risk assessment, namely the time-varying index (CTI) of the impedance criterion: 。 8. A computer system, characterized in that, include: One or more processors; A memory that stores operable instructions, which, when executed by the one or more processors, cause the one or more processors to perform operations, including the process of performing the method of any one of claims 1-7.

9. A computer-readable medium for storing computer programs, characterized in that, The computer program includes instructions / instruction set executable by one or more processors, which, when executed by the one or more processors, implement the process of the method described in any one of claims 1-7.

Images