Transient stability evaluation method and device for wind turbine generator

Abstract

The invention relates to the technical field of transient stability analysis of a power system, in particular to a transient stability evaluation method and device for a wind turbine generator, and the method comprises the steps: constructing a full-order model of a control loop of a to-be-evaluated wind turbine generator, and carrying out the transient stability analysis of the full-order model of the control loop; and performing simulation aiming at different voltage sags, different phase-locked loop control parameters, different power grid line impedances and different grid-connected point d-axis current instructions of the to-be-evaluated wind turbine generator, and performing simulation verification on the transient stability of the to-be-evaluated wind turbine generator. According to the derived maximum estimation attraction domain and the property thereof, the transient stability quantitative evaluation index capable of intuitively reflecting the initial state of the transient process of the new energy unit and the boundary of the maximum estimation attraction domain is constructed, and the relationship between the evaluation index value and the state quantity change of the transient process of the unit is qualitatively and quantitatively analyzed; and the transient stability margin of the unit is quickly and visually reflected.

Description

Technical Field

[0001] This application relates to the field of transient stability analysis technology for power systems, and in particular to a method and apparatus for transient stability assessment of wind turbine generators. Background Technology

[0002] Meanwhile, with the continuous expansion of the scale of new energy power generation units connected to the grid, the overall stability of the system has become more fragile, and the difficulty of control has increased significantly. This places higher demands on grid protection and the control of new energy units. How to ensure the safe and stable operation of new energy power generation systems under weak grid conditions has become one of the focal issues for researchers.

[0003] Current power system stability analysis faces several challenges: First, new energy generating units exhibit significant characteristics such as high-order power, strong nonlinearity, low inertia, weak damping, strong coupling, and multi-timescale interaction. This makes the system's state variables highly susceptible to significant changes when subjected to disturbances, resulting in extremely fragile transient stability. Second, as the installed capacity of new energy sources continues to increase, the power grid will inevitably weaken. The stability issues arising from the interaction between grid-connected converters and the grid under weak grid conditions become more pronounced. Furthermore, the complexity of the grid-connected converter's own control system and the interactive coupling between different control components make it difficult to analyze the mechanisms of instability in the grid-connected converter system, severely restricting the safe and stable operation of new energy power systems. Therefore, the transient stability of new energy power generation systems under weak grid conditions has become a key research focus, urgently requiring in-depth exploration of effective transient stability analysis methods to ensure the safe and stable operation of the power system under the current large-scale integration of new energy sources. Summary of the Invention

[0004] To achieve the above objectives, this application provides the following technical solution: According to a first aspect of the present invention, the present invention claims protection for a method for evaluating the transient stability of wind turbine generators, comprising: A full-order model of the control loop of the wind turbine to be evaluated is constructed, and transient stability analysis is performed on the full-order model of the control loop. Simulations were performed on the wind turbine under different voltage drops, different phase-locked loop control parameters, different grid line impedances, and different grid connection point d-axis current commands to verify the transient stability of the wind turbine under evaluation.

[0005] Furthermore, the construction of a full-order model of the control loop of the wind turbine to be evaluated, and the transient stability analysis of the full-order model of the control loop, further includes: When performing transient stability analysis on the full-order model of the control loop, the grid impedance is purely inductive, R g =0, and the new energy grid-connected operation is at unity power factor, Ioqref =0; R g =Grid resistance, I oqre The q-axis component of the current loop setpoint; The impact of the current loop on the transient synchronization stability of the system and the impact of voltage sag depth and control parameters on the transient synchronization stability of the system are obtained, and the range of parameter values ​​for system transient stability is determined. The following transient stability conditions must be met during transient stability analysis: The system equilibrium point still exists after a transient event occurs; After a transient event occurs, a stable equilibrium point exists among the system's equilibrium points; The initial state of the system lies within the attraction domain of the stable equilibrium point; If any of the above three conditions are not met, the synchronous stability of the constructed wind turbine unit to be evaluated cannot be guaranteed.

[0006] Furthermore, the system equilibrium point still exists after the transient event occurs, specifically including: Analyzing the small-signal stability of the constructed wind turbine to be evaluated, by setting the derivative of the state variables to 0, the equilibrium point x can be obtained. full_e =[x 1e ,x 2e ,…x 12e ]: (1) From equation (1), we can see that the equilibrium point x full_e =[x 1e ,x 2e ,…x 12e [Equilibrium Point 1, Equilibrium Point 2, ..., Equilibrium Point 12]; δ e The energy function deviation angle; ω PLL L is the phase-locked loop angular frequency; g For network inductance; U g This refers to the voltage amplitude of the power grid. I odref and I oqref These are the dq-axis components of the current loop setting; R g For grid resistance; i odref and i oqref These are the dq-axis components of the transient current loop setting value; L f For filter inductance; R d C represents the d-axis component of the resistance. f ω0 is the filter capacitor; ω0 is the angular frequency at the grid connection point. Equilibrium point x full_e The condition for existence is δe Existence, i.e., |R g i oqref +ω PLL L g i odref | g This condition is also the condition for the existence of the equilibrium point in the reduced-order model analysis.

[0007] Furthermore, the existence of stable equilibrium points among the system equilibrium points after a transient event also includes: Small-signal stability analysis of the system; determination and analysis of the parameter domain for small-signal stability of the system. When performing small-signal stability analysis on the system, the stability of the equilibrium point after the grid voltage sag can be evaluated by Lyapunov's first theorem, and the full-order model expression (1) can be linearized as follows: (2) In the formula, J is x full In x full_e The Jacobian matrix of the linear model expanded at point .

[0008] Among them, A 11 ~A 22 for: A 11 =

[0009] A 12 =

[0010] A 21 =

[0011] A 22 =

[0012] Therefore, J can be obtained. 12×12 In x full_e If the eigenvalues ​​at λ are such that the necessary and sufficient condition for the existence of a stable equilibrium point is that for each eigenvalue λ... i (i=1,2,…,12) all have negative real parts; k p k is the proportional gain of the phase-locked loop (PLL); i k represents the integral coefficients of the phase-locked loop; pc k is the proportionality coefficient of the current loop. ic T is the integral coefficient of the current loop; v T is the voltage filtering time constant; d is the total controller delay; GVF is the voltage feedforward coefficient.

[0013] Furthermore, obtaining the impact of the current loop on the transient synchronization stability of the system also includes:​ The impact of the current loop on the transient synchronization stability of the system is addressed through U g and I odref The parameter domain reflects the range of parameters that ensure the transient stability of the system. This is determined by iterating through the parameter values ​​within a certain range and using the ode23tb solver in Matlab to obtain the phase trajectory of the system under different parameters. The trajectory is then extended beyond (-π-δ). e ,π-δ e The parameter values ​​within the specified range are determined to be unstable. The range of parameter values ​​for voltage drop and output current reference that satisfy the transient stability of the full-order model is plotted. Matlab was used to fit the region boundary, and the parameter domain that ensures the transient stability of the system was plotted. Solve the corresponding parameter domains for other parameters of interest, including at least the current loop parameter domain, the phase-locked loop parameter domain, or other parameter domains in the control loop.

[0014] Furthermore, obtaining the impact of voltage sag depth and control parameters on the transient synchronization stability of the system also includes: To plot the field of attraction (DOA), given a set of parameters, the DOA can be obtained by iterating through the initial values ​​of δ and Δω within a certain range. U g The drop is negatively correlated with the system's stability region, U g The larger the drop, the smaller the system stability region, U g The deeper the drop, the smaller the stability range. I odref The value of I is negatively correlated with the stability of the system. odref The larger the setting, the smaller the range of phase angles that the system can maintain stability. Phase-locked loop proportional coefficient k p k is positively correlated with the system's stability region. p The larger the value, the larger the system stability region, and the larger the integral coefficient k of the phase-locked loop. i The value of k is negatively correlated with the stability of the system. i The larger the setting, the smaller the range of phase angles that the system can maintain stability. Current loop proportionality coefficient k pc It has little correlation with the system's stability region, k pc When the value of k changes, the system's stability region remains unchanged, and when k... pc When the value increases, it will exceed the stable range of the small signal, and the attraction domain will disappear directly. At this time, the system will not be able to operate stably. Current loop integral coefficient k ic The values ​​of the phase-locked loop (PLL) are not strongly correlated with the system's stability region, and changes in their values ​​do not affect the size of the stability region. Furthermore, the values ​​of the PLL parameters determine the size of the attraction region.

[0015] Furthermore, determining the range of parameter values ​​for system transient stability also includes: The magnitude of the grid voltage drop is negatively correlated with the current loop PI transient parameter domain. The greater the grid voltage drop, the larger the value should be, and the smaller the phase-locked loop PI transient parameter domain should be. Grid-connected d-axis setpoint I odref The transient parameter domain of the phase-locked loop (PLL) is negatively correlated with I. odref The larger the value, the smaller the PI transient parameter domain of the phase-locked loop; Grid voltage drop amplitude and grid connection d-axis setting value I odref When changed, the PI parameter domain of the current loop changes little; Under the premise that the current loop parameters satisfy small-signal stability, changes in the current loop parameters do not affect the size of the system's attraction domain. If the current loop parameters do not satisfy small-signal stability, then the system is transiently unstable. Within the current loop parameter domain, if the current loop bandwidth does not meet the order reduction condition, ignoring the current loop dynamics will lead to misjudgment, i.e., the system has become unstable.

[0016] Furthermore, simulations are performed on the wind turbine under different voltage sags, different phase-locked loop control parameters, different grid line impedances, and different d-axis current commands at the grid connection point to verify the transient stability of the wind turbine under evaluation. This also includes: The system performs simulation analysis under grid voltage dips, simulation analysis under different phase-locked loop proportional coefficients, simulation analysis under different phase-locked loop integral coefficients, simulation analysis under different grid line impedances, and simulation analysis under different grid connection point d-axis current commands. Quantitative analysis and simulation verification of the transient stability of the phase-locked loop and the transient stability of the current loop are performed.

[0017] According to a second aspect of the present invention, the present invention claims protection for a transient stability assessment device for wind turbine generators, comprising: One or more processors; A memory storing one or more programs, which, when executed by one or more processors, enable the processors to implement the transient stability assessment method for wind turbine generators.

[0018] This application relates to the field of power system transient stability analysis technology, and particularly to a method and apparatus for transient stability assessment of wind turbine generators. The method involves constructing a full-order model of the control loop of the wind turbine generator to be assessed, and performing transient stability analysis on the full-order model of the control loop. Simulations are performed on the wind turbine generator under different voltage dips, different phase-locked loop control parameters, different grid line impedances, and different d-axis current commands at the grid connection point to verify the transient stability of the wind turbine generator. Based on the derived maximum estimated attraction domain and its properties, this invention constructs a transient stability quantitative assessment index that intuitively reflects the initial state of the new energy generator's transient process relative to the boundary of the maximum estimated attraction domain. Qualitative and quantitative analyses are conducted on the relationship between the index values ​​and the changes in the transient process state variables of the generator, quickly and intuitively reflecting the transient stability margin of the generator. Attached Figure Description

[0019] Figure 1 A flowchart illustrating the transient stability assessment method for wind turbine generators claimed in this application. Figure 2 The U-shaped transient stability assessment method for wind turbine generators claimed in the embodiments of this application g Schematic diagram showing the relationship between drop and changes in the system's stability domain; Figure 3 The k-method for transient stability assessment of wind turbine generators claimed in the embodiments of this application. p / k i A schematic diagram illustrating the changes in the system's stability domain under changing conditions; Figure 4 The k-method for transient stability assessment of wind turbine generators claimed in the embodiments of this application. pc / k ic A schematic diagram illustrating the changes in the system's stability domain under changing conditions; Figure 5 The U-shaped transient stability assessment method for wind turbine generators claimed in the embodiments of this application g Schematic diagram of the transient parameter domain of the phase-locked loop PI during changes; Figure 6 The transient stability assessment method for wind turbine generators claimed in this application is I. odref Schematic diagram of the transient parameter domain of the phase-locked loop PI during changes; Figure 7 The U-shaped transient stability assessment method for wind turbine generators claimed in the embodiments of this application g Schematic diagram of the transient parameter domain of the current loop PI during change; Figure 8 The transient stability assessment method for wind turbine generators claimed in this application is I. odrefSchematic diagram of the transient parameter domain of the current loop PI during change; Figure 9 The grid voltage drop and reduction k of the transient stability assessment method for wind turbine generators claimed in this application are described in the embodiments of this application. p The diagram below shows the working point settings. Detailed Implementation

[0020] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0021] The terms "first," "second," and "third" in this application are for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first," "second," or "third" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified. All directional indications (such as up, down, left, right, front, back, etc.) in the embodiments of this application are only used to explain the relative positional relationships and movements between components in a specific orientation (as shown in the figures). If the specific orientation changes, the directional indications also change accordingly. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to these processes, methods, products, or devices.

[0022] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0023] According to the first embodiment of the present invention, referring to Figure 1 This invention claims protection for a transient stability assessment method for wind turbine generators, comprising: A full-order model of the control loop of the wind turbine to be evaluated is constructed, and transient stability analysis is performed on the full-order model of the control loop. Simulations were performed on the wind turbine under different voltage drops, different phase-locked loop control parameters, different grid line impedances, and different grid connection point d-axis current commands to verify the transient stability of the wind turbine under evaluation.

[0024] Furthermore, the construction of a full-order model of the control loop of the wind turbine to be evaluated, and the transient stability analysis of the full-order model of the control loop, further includes: When performing transient stability analysis on the full-order model of the control loop, the grid impedance is purely inductive, R g =0, and the new energy grid-connected operation is at unity power factor, I oqref =0; R g =Grid resistance, I oqre The q-axis component of the current loop setpoint; The impact of the current loop on the transient synchronization stability of the system and the impact of voltage sag depth and control parameters on the transient synchronization stability of the system are obtained, and the range of parameter values ​​for system transient stability is determined. The following transient stability conditions must be met during transient stability analysis: The system equilibrium point still exists after a transient event occurs; After a transient event occurs, a stable equilibrium point exists among the system's equilibrium points; The initial state of the system lies within the attraction domain of the stable equilibrium point; If any of the above three conditions are not met, the synchronous stability of the constructed wind turbine unit to be evaluated cannot be guaranteed.

[0025] Specifically, in this embodiment: Condition 1: The system equilibrium point still exists after a transient event: The existence of a system equilibrium point is a fundamental prerequisite for the system to maintain stability. The existence of an equilibrium point determines whether a system is in a steady state; if no equilibrium point exists after a transient event, the system will definitely diverge.

[0026] Condition 2: A stable equilibrium point should exist among the system's equilibrium points after a transient event: In addition to confirming the existence of an equilibrium point, it must be ensured that this equilibrium point is stable, meaning the system can return to this equilibrium point when subjected to a small disturbance. The existence condition of a stable equilibrium point can be solved through small-signal analysis, such as the range of system parameters. Meeting this condition ensures a stable equilibrium point.

[0027] Condition 3: The initial state of the system lies within the attraction domain of the stable equilibrium point: Based on the first two conditions, according to Lyapunov stability theory, when a transient event occurs, the system's state variables must also lie within the attraction domain of the stable equilibrium point. The attraction domain is the set of all initial states that can guarantee the system's convergence to the stable equilibrium point. Only when the initial state lies within the attraction domain of the stable equilibrium point can the system gradually recover to the stable equilibrium point after a large disturbance, ultimately maintaining transient synchronization stability. If the state variables exceed the attraction domain of the stable equilibrium point, the system may become unstable, leading to desynchronization. Therefore, it is necessary to solve for the system's attraction domain and the parameter domain that guarantees system stability, ensuring a comprehensive assessment of the impact of the system's initial state and parameter values ​​on the system's transient stability.

[0028] Furthermore, the system equilibrium point still exists after the transient event occurs, specifically including: Analyzing the small-signal stability of the constructed wind turbine to be evaluated, by setting the derivative of the state variables to 0, the equilibrium point x can be obtained. full_e =[x 1e ,x 2e ,…x 12e ]: (1) From equation (1), we can see that the equilibrium point x full_e =[x 1e ,x 2e ,…x 12e [Equilibrium Point 1, Equilibrium Point 2, ..., Equilibrium Point 12]; δ e The energy function deviation angle; ω PLL L is the phase-locked loop angular frequency; g For network inductance; U g This refers to the voltage amplitude of the power grid. I odref and I oqref These are the dq-axis components of the current loop setting; R g For grid resistance; i odref and i oqref These are the dq-axis components of the transient current loop setting value; L f For filter inductance; R d C represents the d-axis component of the resistance. f ω0 is the filter capacitor; ω0 is the angular frequency at the grid connection point. Equilibrium point x full_e The condition for existence is δ e Existence, i.e., |R g i oqref +ω PLL L g iodref | g This condition is also the condition for the existence of the equilibrium point in the reduced-order model analysis.

[0029] Furthermore, the existence of stable equilibrium points among the system equilibrium points after a transient event also includes: Small-signal stability analysis of the system; determination and analysis of the parameter domain for small-signal stability of the system. When performing small-signal stability analysis on the system, the stability of the equilibrium point after the grid voltage sag can be evaluated by Lyapunov's first theorem, and the full-order model expression (1) can be linearized as follows: (2) In the formula, J is x full In x full_e The Jacobian matrix of the linear model expanded at point .

[0030] Among them, A 11 ~A 22 for: A 11 =

[0031] A 12 =

[0032] A 21 =

[0033] A 22 =

[0034] Therefore, we can obtain the result from J 12×12 In x full_e If the eigenvalues ​​at λ are such that the necessary and sufficient condition for the existence of a stable equilibrium point is that for each eigenvalue λ... i (i=1,2,…,12) all have negative real parts; k p k is the proportional gain of the phase-locked loop (PLL); i k represents the integral coefficients of the phase-locked loop; pc k is the proportionality coefficient of the current loop. ic T is the integral coefficient of the current loop; v T is the voltage filtering time constant; d is the total controller delay; GVF is the voltage feedforward coefficient.

[0035] In this embodiment, U is used g Decreasing from 1 p.u. to 0.4 p.u. and I odref The J12×12 eigenvalue trajectory distribution (U) increasing from 0p.u. to 2.5pu. g When I changes​odref Take 1p.u., I odref When U changes g Take 1p.u). With U g As the value gradually decreases, the dominant eigenvalue of the system continuously shifts towards the imaginary axis, eventually reaching U. g When it drops to 0.4 pu, it intersects the imaginary axis, that is, at U g When U = 0.4 pu, the system loses small-signal stability. g At a value of 0.43 pu, the system still maintains small-signal stability, but simulation results show that at U g During the drop from 1.0 pu to 0.43 pu, the renewable energy grid-connected system actually showed signs of transient instability. This indicates that meeting only the first two conditions is insufficient to fully guarantee system stability; even when small-signal stability is met, system instability may still occur. A similar situation also occurred. odref In the case of mutation, with I odref As the setpoint is gradually increased, the dominant eigenvalue of the system continuously shifts towards the imaginary axis, but the system's eigenvalues ​​remain in the left half-plane, maintaining small-signal stability. When I... odref At 2.4 pu, the system still maintains small-signal stability, at I odref During the process of the sudden increase from 1.0 PU to 2.4 PU, the new energy grid-connected system has actually shown signs of transient instability.

[0036] In addition to the grid voltage U mentioned above g Grid-connected current setting value I ref Line inductance L g Changes in current loop parameters can affect system stability, and inappropriate current loop parameters can also worsen the small-signal stability of new energy grid connection. The eigenvalue trajectory distribution under different current loop parameters shows that inappropriate PI parameters can cause the system eigenvalues ​​to shift to the right half-plane, meaning that the equilibrium point under these parameters cannot maintain small-signal stability, leading to system instability.

[0037] In this embodiment, compared to the parameter domain of the phase-locked loop, the parameter range of the current loop varies with U. g and I odref The changes did not show significant fluctuations. This indicates that the parameters of the phase-locked loop (PLL) have a more significant impact on the small-signal stability of the system compared to the parameters of the current loop.

[0038] While small-signal stability is a prerequisite for a stable equilibrium point, merely satisfying small-signal stability does not guarantee the existence of transient instability. Therefore, more transient stability criteria should be considered. Furthermore, since the full-order model introduces control loop effects, transient instability caused by current loop parameters must also be considered during analysis.

[0039] Furthermore, obtaining the impact of the current loop on the transient synchronization stability of the system also includes: The impact of the current loop on the transient synchronization stability of the system is addressed through U g and I odref The parameter domain reflects the range of parameters that ensure the transient stability of the system. This is determined by iterating through the parameter values ​​within a certain range and using the ode23tb solver in Matlab to obtain the phase trajectory of the system under different parameters. The trajectory is then extended beyond (-π-δ). e ,π-δ e The parameter values ​​within the specified range are determined to be unstable. The range of parameter values ​​for voltage drop and output current reference that satisfy the transient stability of the full-order model is plotted. Matlab was used to fit the region boundary, and the parameter domain that ensures the transient stability of the system was plotted. Solve the corresponding parameter domains for other parameters of interest, including at least the current loop parameter domain, the phase-locked loop parameter domain, or other parameter domains in the control loop.

[0040] Furthermore, obtaining the impact of voltage sag depth and control parameters on the transient synchronization stability of the system also includes: To plot the field of attraction (DOA), given a set of parameters, the DOA can be obtained by iterating through the initial values ​​of δ and Δω within a certain range. U g The drop is negatively correlated with the system's stability region, U g The larger the drop, the smaller the system stability region, U g The deeper the drop, the smaller the stability range. I odref The value of I is negatively correlated with the stability of the system. odref The larger the setting, the smaller the range of phase angles that the system can maintain stability. Phase-locked loop proportional coefficient k p k is positively correlated with the system's stability region. p The larger the value, the larger the system stability region, and the larger the integral coefficient k of the phase-locked loop. i The value of k is negatively correlated with the stability of the system. i The larger the setting, the smaller the range of phase angles that the system can maintain stability. Current loop proportionality coefficient k pc It has little correlation with the system's stability region, k pcWhen the value of k changes, the system's stability region remains unchanged, and when k... pc When the value increases, it will exceed the stable range of the small signal, and the attraction domain will disappear directly. At this time, the system will not be able to operate stably. The integral coefficient kic of the current loop has little correlation with the system's stability region. Changing its value does not affect the size of the stability region. Furthermore, the values ​​of the phase-locked loop parameters determine the size of the attraction region.

[0041] In this embodiment, after the system starts from the zero state (0,0), since the initial point is located at U... g Within DOA1 plotted under the condition =1p.u., it will first converge to the first steady-state point A under this condition; after the transient event occurs, U g =0.45pu, at this point the attraction region of the system becomes DOA2, and due to the change in conditions, the system is no longer in equilibrium, and the operating point will migrate to a new steady-state point. As can be seen from the figure, the initial value of the system when the transient event occurs is point A, and A is located at U. g The system is within the attraction region DOA2 of 0.45 pu, therefore, after the transient event, the system can converge to the next stable operating point B, that is, the system is within U. g It is transiently stable when it drops from 1 p.u. to 0.45 p.u.

[0042] in U g As the drop depth increases further, the situation changes when the drop reaches 0.42 pu. It can be seen that after the transient event occurs, U... g When PU = 0.42, the attraction domain DOA2 no longer includes the initial operating point A of the system after the transient event. Therefore, the system diverges after the transient event. This process shows that the resilience of a system to transient events depends on whether the system's attraction domain includes the steady-state operating point before the transient event. This implies that improving the transient stability of a system can be achieved by expanding the range of its attraction domain.

[0043] To explore the difference between the attraction domain of the reduced-order model and the full-order model, it was found that there is a significant difference in the DOA range between the reduced-order model and the full-order model.

[0044] To analyze the impact of various parameters in the system on DOA, a graph was plotted. Figures 2 to 4 The picture x 1 represents the phase difference between the grid voltage and the voltage at the grid connection point, also known as the power angle. x 2 is the output of the phase-locked loop integration channel. Figure 2 As can be seen from this, U g The drop is negatively correlated with the system's stability region, U g The larger the drop, the smaller the system's stability region, and it can be obtained from U gFrom 1.0 PU to 0.70 PU and U g The shrinking of the stable range from 0.70 pu to 0.44 pu shows that U g The deeper the drop, the smaller the stability range. Furthermore, I odref The value of I is negatively correlated with the stability of the system. odref The larger the setting, the smaller the range of phase angles the system can maintain stability.

[0045] Depend on Figure 3 It can be seen that the proportional coefficient k of the phase-locked loop p k is positively correlated with the system's stability region. p The larger the value, the larger the system stability region. Simultaneously, due to... Figure 2 It can be seen that the integral coefficient k of the phase-locked loop i The value of k is negatively correlated with the stability of the system. i The larger the setting, the smaller the range of phase angles the system can maintain stability.

[0046] Depend on Figure 4 It can be seen that the current loop proportionality coefficient k pc It has little correlation with the system stability region; when the value of kpc changes, the system stability region remains basically unchanged, and when k... pc When the value increases further, it exceeds the small-signal stability range, and the attraction domain disappears directly. At this point, the system does not operate stably, which aligns with the conclusion that small-signal stability is a prerequisite for system transient stability. Simultaneously, due to... Figure 4 It can be seen that the integral coefficient k of the current loop ic The values ​​of these parameters have little correlation with the system's stability region; changes in their values ​​have virtually no impact on the size of the stability region. Furthermore, the values ​​of the phase-locked loop parameters determine the size of this attraction region.

[0047] Based on the above analysis, the following conclusions can be drawn: there is a close correlation between the grid voltage, the d-axis current setpoint, and the phase-locked loop (PLL) PI parameters and the system's stability region, playing a crucial role in the system's stability. The system's attraction region remains almost unchanged when the current loop parameters are altered. Specifically, U... g Drop range, Iodref, k p / k i There is a clear positive correlation between it and the transient stability of the system.

[0048] Furthermore, determining the range of parameter values ​​for system transient stability also includes: The magnitude of the grid voltage drop is negatively correlated with the current loop PI transient parameter domain. The greater the grid voltage drop, the larger the value should be, and the smaller the phase-locked loop PI transient parameter domain should be. Grid-connected d-axis setpoint I odrefThe transient parameter domain of the phase-locked loop (PLL) is negatively correlated with I. odref The larger the value, the smaller the PI transient parameter domain of the phase-locked loop; Grid voltage drop amplitude and grid connection d-axis setting value I odref When changed, the PI parameter domain of the current loop changes little; Under the premise that the current loop parameters satisfy small-signal stability, changes in the current loop parameters do not affect the size of the system's attraction domain. If the current loop parameters do not satisfy small-signal stability, then the system is transiently unstable. Within the current loop parameter domain, if the current loop bandwidth does not meet the order reduction condition, ignoring the current loop dynamics will lead to misjudgment, i.e., the system has become unstable.

[0049] In this embodiment, by Figure 5 It can be seen that the magnitude of the grid voltage sag is negatively correlated with the PI transient parameter domain of the current loop; the greater the grid voltage sag, the larger the value should be, and the smaller the PI transient parameter domain of the phase-locked loop should be. Furthermore, from... Figure 6 It can be seen that the grid-connected d-axis setpoint I odref The transient parameter domain of the phase-locked loop (PLL) is negatively correlated with I. odref The larger the value is set, the smaller the PI transient parameter domain of the phase-locked loop becomes.

[0050] Depend on Figure 7 and Figure 8 It can be seen that the voltage drop amplitude of the grid is related to the grid connection d-axis setting value I. odref When the parameters are changed, the PI parameter domain of the current loop does not change significantly.

[0051] Furthermore, simulations are performed on the wind turbine under different voltage sags, different phase-locked loop control parameters, different grid line impedances, and different d-axis current commands at the grid connection point to verify the transient stability of the wind turbine under evaluation. This also includes: The system performs simulation analysis under grid voltage dips, simulation analysis under different phase-locked loop proportional coefficients, simulation analysis under different phase-locked loop integral coefficients, simulation analysis under different grid line impedances, and simulation analysis under different grid connection point d-axis current commands. Quantitative analysis and simulation verification of the transient stability of the phase-locked loop and the transient stability of the current loop are performed.

[0052] In this embodiment, during system simulation analysis under grid voltage dips, based on the correlation analysis results between system parameters and synchronization stability, it is known that when other parameters remain constant, the lower the grid voltage, the lower the system stability. Therefore, the system operation under different voltage dip levels is simulated. Let I... odref =0.6pu,k p =100, k i=9000, obtaining the feasible region of system parameters. The system operating points are set as A→B→C→D→E, that is, the grid voltage drops from 1 p.u. to 0.8 p.u., 0.6 p.u., 0.4 p.u., and 0.2 p.u. According to the theoretical analysis results, the system can remain stable at the four operating points A, B, C, and D, while operating point E is an unstable operating point, and the system experiences synchronous instability.

[0053] When the grid voltage drops to 0.8 pu and 0.6 pu, the system can quickly track the reference current input. By adjusting the phase-locked loop (PLL) frequency, the VSC (Voltage Control System) is synchronized with the grid, and the grid-connected voltage stabilizes quickly, enabling stable system operation. When the grid voltage drops to 0.4 pu, the system experiences slight oscillations at the moment of the voltage drop, but ultimately remains stable. The increased time required for the grid-connected voltage Uo to switch from stabilization to stability indicates a significant decrease in system stability. When the grid voltage drops to 0.2 pu, the PLL fails to lock onto the grid phase, the PLL angle deviation increases rapidly, and the system experiences synchronization instability. The VSC output voltage E, output current Io, and grid-connected voltage Uo diverge.

[0054] Simulation analysis under different phase-locked loop proportional coefficients (PLLs) keeps other parameter settings the same as in the system simulation analysis under grid voltage dips. The PLL proportional coefficient k... p Set as k p =50, thus obtaining the system parameter domain. (Refer to...) Figure 9 It can be seen that k p The voltage drop decreases, reducing the system's stability parameter domain. In the system simulation analysis under grid voltage dips, the original stable operating point D(0.4, 0.5) now becomes an unstable operating point, meaning the system experiences synchronous instability at point D. The simulation is still performed using A→B→C→D→E as the set operating point.

[0055] Simulation results show that when the grid voltage drops to 0.8 pu and 0.6 pu, the system can quickly track the reference current input, adjust the PLL frequency to keep the VSC synchronized with the grid, and the grid connection point voltage can quickly stabilize, achieving stable system operation. When the grid voltage drops to 0.4 pu, the PLL output oscillates, the system always has a frequency deviation, and the VSC output voltage E, output current Io, and grid connection point voltage Uo oscillate at low frequencies, causing system instability. When the grid voltage drops to 0.2 pu, the PLL cannot lock the grid phase, the PLL frequency deviation increases rapidly, the system experiences synchronization instability, and the VSC output voltage E, output current Io, and grid connection point voltage Uo diverge.

[0056] In simulations with different phase-locked loop (PLL) integral coefficients, other parameter settings were kept the same as in the system simulation under grid voltage dips. The PLL integral coefficient ki was set to ki=50, yielding the system parameter domain. Decreasing ki increased the system stability parameter domain. The system operating points were set as F→G→H, representing a grid voltage drop from 1 p.u. to 0.5 p.u., and then to 0.1 p.u. According to theoretical analysis, the system remained stable at operating points F and G, but operating point H was unstable, indicating synchronous instability.

[0057] Simulation results show that when the grid voltage drops to 0.5 pu, the system can quickly track the reference current input, adjust the PLL frequency to keep the VSC synchronized with the grid, and the grid connection point voltage can quickly stabilize, achieving stable system operation. When the grid voltage drops to 0.1 pu, the PLL cannot lock the grid phase, the PLL frequency deviation increases rapidly, the system experiences synchronization instability, and the VSC output voltage E, output current Io, and grid connection point voltage Uo diverge.

[0058] When performing simulation analysis under different power grid line impedances, I is taken odref =0.6pu,k p =1000, ki=18000, thus obtaining the feasible region of the system parameters. In the line inductance L... g When the grid voltage is 0.4 pu, the system operating point is set from A to B, meaning the grid voltage drops from 1 p.u. to 0.8 pu; at the line inductance L g When the power consumption is 0.54 PU, the system is set to run at point C. According to theoretical analysis, the system can remain stable at points A and B, but point C is an unstable operating point, and the system experiences synchronous instability.

[0059] In the simulation results, when the system is operating at operating point A, it can quickly track the reference current input, and the grid connection point voltage can stabilize rapidly. When the system moves from operating point A to operating point B, i.e., the grid voltage drops to 0.8 pu, the system oscillates at the moment of the grid voltage drop, and the stabilization process is longer compared to operating point A, but it can still remain stable in the end. When the system is operating at operating point C, the phase-locked loop cannot lock the grid phase, the phase-locked loop angle deviation increases rapidly, the system experiences synchronous instability, and the VSC output voltage E, output current Io, and grid connection point voltage Uo diverge.

[0060] When performing simulation analysis under different grid connection point d-axis current commands, L is taken g =0.6pu,k p=700, ki=900, obtaining the feasible region of system parameters. When the grid connection point d-axis current command is set to 0.3pu, the system operating point A is set; when the grid connection point d-axis current command is set to 0.9pu, the system operating point B is set. According to the theoretical analysis results, the system can remain stable at point A, but operating point B is an unstable operating point, and the system experiences synchronous instability.

[0061] In the simulation results, when the system is operating at operating point A, the system can quickly track the reference current input, and the grid connection point voltage can quickly stabilize. When the system moves from operating point A to operating point C, that is, when the grid connection point d-axis current command increases from 0.3pu to 0.9pu, the phase-locked loop cannot lock the grid phase, the phase-locked loop angle deviation increases rapidly, the system experiences synchronous instability, and the VSC output voltage E, output current Io, and grid connection point voltage Uo diverge.

[0062] When performing quantitative analysis and simulation verification of the transient stability of the phase-locked loop, the obtained phase-locked loop parameter domain is verified.

[0063] Take L g =0.4pu, U g =1.0→0.42pu, I odref =1.0pu,k pc =0.165, kic=1.65, obtaining the transient parameter domain of the system's phase-locked loop, and taking the operating point (k p =0.22, ki=1.2), (k p =0.22, ki=1.8) and (k p =0.19, ki=1.8). Based on the quantitative analysis results of transient stability, the system is in (k p =0.22, ki=1.2) points and (k p =0.22, ki=1.8) can remain stable at the working point (k p =0.19, ki=1.8) is an unstable operating point, and the system has experienced synchronous instability.

[0064] Take L g =0.4pu, U g =1.0pu, I odref =1.0→2.3pu,k pc =0.165, kic=1.65, obtaining the transient parameter domain of the system's phase-locked loop, and taking the operating point (k p =0.22, ki=1.2), (k p =0.22, k i =1.8) and (k p =0.19, k i=1.8). Based on the quantitative analysis results of transient stability, the system is in (k p =0.22, k i =1.2) points and (k p =0.22, k i =1.8) can remain stable, the operating point (k) p =0.19, k i =1.8) is an unstable operating point, and the system experiences synchronous instability.

[0065] When performing quantitative analysis and simulation verification of the transient stability of the current loop, the first step is to quantitatively analyze the transient stability of the proportional parameters of the current loop. Let L... g =0.4pu, U g =1.0pu, I odref =2.3pu, k p =20p.u., ki=10p.u., obtain the transient parameter domain of the system current loop, and set the operating point A(k) respectively. pc =0.12, k ic =30), B(k) pc =0.07, k ic =30) and C(k) pc =0.05, k ic =30). According to the quantitative analysis of transient stability, the system can remain stable at points A and B, but the operating point C is an unstable operating point, and the system experiences synchronous instability.

[0066] The simulation results show that when the system is operating at operating point A and point B, the system can quickly track the reference current input and the grid connection point voltage can quickly stabilize. When the system moves from operating point B to operating point C, that is, when the current loop proportional coefficient drops from 0.07 to 0.05, the system experiences synchronous instability, and the output current Io and the grid connection point voltage Uo diverge.

[0067] Subsequently, a quantitative analysis of the transient stability of the current loop integral parameters was performed.

[0068] Take L g =0.4pu, U g =1.0pu, I odref =2.3pu, k p =20, k i =10, thus obtaining the transient parameter domain of the system current loop. The operating point A(k) is set accordingly. pc =0.015, k ic =2), B(k) pc =0.015, k ic =5) and C(k) pc =0.015, k ic=8). According to the quantitative analysis of transient stability, the system can remain stable at points A and B, but the operating point C is an unstable operating point, and the system experiences synchronous instability.

[0069] In the simulation results, when the system is operating at operating point A and point B, the system can quickly track the reference current input and the grid connection point voltage can quickly stabilize. When the system moves from operating point B to operating point C, that is, when the current loop integral coefficient increases from 5 to 8, the system experiences synchronous instability, and the output current Io and the grid connection point voltage Uo diverge.

[0070] According to a second aspect of the present invention, the present invention claims protection for a transient stability assessment device for wind turbine generators, comprising: One or more processors; A memory storing one or more programs, which, when executed by one or more processors, enable the processors to implement the transient stability assessment method for wind turbine generators.

[0071] In the several embodiments provided in this application, it should be understood that the disclosed systems, apparatuses, and methods can be implemented in other ways. For example, the apparatus embodiments described above are merely illustrative; for instance, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be through some interfaces, or indirect coupling or communication connection between apparatuses or units, and may be electrical, mechanical, or other forms.

[0072] Furthermore, the functional units in the various embodiments of this application can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated units described above can be implemented in hardware or as software functional units. The above are merely embodiments of this application and do not limit the patent scope of this application. Any equivalent structural or procedural transformations made based on the description and drawings of this application, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

[0073] The specific embodiments of the invention have been described in detail above, but they are only examples, and this application is not limited to the specific embodiments described above. For those skilled in the art, any equivalent modifications or substitutions to the invention are also within the scope of this application. Therefore, all equivalent changes, modifications, and improvements made without departing from the spirit and principles of this application should be covered within the scope of this application.

Claims

1. A method for evaluating the transient stability of wind turbine generators, characterized in that, include: A full-order model of the control loop of the wind turbine to be evaluated is constructed, and transient stability analysis is performed on the full-order model of the control loop. Simulations were performed on the wind turbine under different voltage drops, different phase-locked loop control parameters, different grid line impedances, and different grid connection point d-axis current commands to verify the transient stability of the wind turbine under evaluation.

2. The transient stability assessment method for wind turbine generators according to claim 1, characterized in that, The process of constructing a full-order model of the control loop of the wind turbine to be evaluated, and performing transient stability analysis on the full-order model of the control loop, further includes: When performing transient stability analysis on the full-order model of the control loop, the grid impedance is purely inductive, R g =0, and the new energy grid-connected operation is at unity power factor, I oqref =0; R g =Grid resistance, I oqre The q-axis component of the current loop setpoint; The impact of the current loop on the transient synchronization stability of the system and the impact of voltage sag depth and control parameters on the transient synchronization stability of the system are obtained, and the range of parameter values ​​for system transient stability is determined. The following transient stability conditions must be met during transient stability analysis: The system equilibrium point still exists after a transient event occurs; After a transient event occurs, a stable equilibrium point exists among the system's equilibrium points; The initial state of the system lies within the attraction domain of the stable equilibrium point; If any of the above three conditions are not met, the synchronous stability of the constructed wind turbine unit to be evaluated cannot be guaranteed.

3. The transient stability assessment method for wind turbine generators according to claim 2, characterized in that, The system equilibrium point still exists after the transient event occurs, specifically including: Analyzing the small-signal stability of the constructed wind turbine to be evaluated, by setting the derivative of the state variables to 0, the equilibrium point x can be obtained. full_e =[x 1e ,x 2e ,…x 12e ]: （1） From equation (1), we can see that, Equilibrium point x full_e =[x 1e ,x 2e ,…x 12e [Equilibrium Point 1, Equilibrium Point 2, ..., Equilibrium Point 12]; δ e The energy function deviation angle; ω PLL L is the phase-locked loop angular frequency; g For network inductance; U g This refers to the voltage amplitude of the power grid. I odref and I oqref These are the dq-axis components of the current loop setting; R g For grid resistance; i odref and i oqref These are the dq-axis components of the transient current loop setting value; L f For filter inductance; R d C represents the d-axis component of the resistance. f ω0 is the filter capacitor; ω0 is the angular frequency at the grid connection point. Equilibrium point x full_e The condition for existence is δ e Existence, i.e., |R g i oqref +ω PLL L g i odref | g This condition is also the condition for the existence of the equilibrium point in the reduced-order model analysis.​ 4. The transient stability assessment method for wind turbine generators according to claim 2, characterized in that, After a transient event occurs, there are stable equilibrium points among the system's equilibrium points, including: Small-signal stability analysis of the system; determination and analysis of the parameter domain for small-signal stability of the system. When performing small-signal stability analysis on the system, the stability of the equilibrium point after the grid voltage sag can be evaluated by Lyapunov's first theorem, and the full-order model expression (1) can be linearized as follows: （2） In the formula, J is x full In x full_e The Jacobian matrix of the linear model expanded at point ; Among them, A 11 ~A 22 for: A 11 = A 12 = A 21 = A 22 = Therefore, J can be obtained. 12×12 In x full_e If the eigenvalues ​​at λ are such that the necessary and sufficient condition for the existence of a stable equilibrium point is that for each eigenvalue λ... i (i=1,2,…,12) all have negative real parts; k p k is the proportional gain of the phase-locked loop (PLL); i k represents the integral coefficients of the phase-locked loop; pc k is the proportionality coefficient of the current loop. ic T is the integral coefficient of the current loop; v T is the voltage filtering time constant; d is the total controller delay; GVF is the voltage feedforward coefficient.

5. The transient stability assessment method for wind turbine generators according to claim 2, characterized in that, The impact of the current loop on the transient synchronization stability of the system also includes: The impact of the current loop on the transient synchronization stability of the system is addressed through U g and I odref The parameter domain reflects the range of parameters that ensure the transient stability of the system. This is determined by iterating through the parameter values ​​within a certain range and using the ode23tb solver in Matlab to obtain the phase trajectory of the system under different parameters. The trajectory is then extended beyond (-π-δ). e ,π-δ e The parameter values ​​within the specified range are determined to be unstable. The range of parameter values ​​for voltage drop and output current reference that satisfy the transient stability of the full-order model is plotted. Matlab was used to fit the region boundary, and the parameter domain that ensures the transient stability of the system was plotted. Solve the corresponding parameter domains for other parameters of interest, including at least the current loop parameter domain, the phase-locked loop parameter domain, or other parameter domains in the control loop.

6. The transient stability assessment method for wind turbine generators according to claim 2, characterized in that, The impact of voltage sag depth and control parameters on the transient synchronization stability of the system also includes: To plot the field of attraction (DOA), given a set of parameters, the DOA can be obtained by iterating through the initial values ​​of δ and Δω within a certain range. U g The drop is negatively correlated with the system's stability region, U g The larger the drop, the smaller the system stability region, U g The deeper the drop, the smaller the stability range. I odref The value of I is negatively correlated with the stability of the system. odref The larger the setting, the smaller the range of phase angles that the system can maintain stability. Phase-locked loop proportional coefficient k p It is positively correlated with the system's stability region, k p The larger the value, the larger the system stability region, and the larger the integral coefficient k of the phase-locked loop. i The value of k is negatively correlated with the stability of the system. i The larger the setting, the smaller the range of phase angles that the system can maintain stability. Current loop proportionality coefficient k pc It has little correlation with the system's stability region, k pc When the value of k changes, the system's stability region remains unchanged, and when k... pc When the value increases, it will exceed the stable range of the small signal, and the attraction domain will disappear directly. At this time, the system will not be able to operate stably. Current loop integral coefficient k ic The values ​​of the phase-locked loop (PLL) are not strongly correlated with the system's stability region, and changes in their values ​​do not affect the size of the stability region. Furthermore, the values ​​of the PLL parameters determine the size of the attraction region.

7. The transient stability assessment method for wind turbine generators according to claim 2, characterized in that, Determining the range of parameter values ​​for system transient stability also includes: The magnitude of the grid voltage drop is negatively correlated with the current loop PI transient parameter domain. The greater the grid voltage drop, the larger the value should be, and the smaller the phase-locked loop PI transient parameter domain should be. Grid-connected d-axis setpoint I odref The transient parameter domain of the phase-locked loop (PLL) is negatively correlated with I. odref The larger the value, the smaller the PI transient parameter domain of the phase-locked loop; Grid voltage drop amplitude and grid connection d-axis setting value I odref When changed, the PI parameter domain of the current loop changes little; Under the premise that the current loop parameters satisfy small-signal stability, changes in the current loop parameters do not affect the size of the system's attraction domain. If the current loop parameters do not satisfy small-signal stability, then the system is transiently unstable. Within the current loop parameter domain, if the current loop bandwidth does not meet the order reduction condition, ignoring the current loop dynamics will lead to misjudgment, i.e., the system has become unstable.

8. The transient stability assessment method for wind turbine generators according to claim 1, characterized in that, Simulations were performed on the wind turbine under different voltage sags, different phase-locked loop control parameters, different grid line impedances, and different d-axis current commands at the grid connection point to verify the transient stability of the wind turbine under evaluation. The simulation also included: The system performs simulation analysis under grid voltage dips, simulation analysis under different phase-locked loop proportional coefficients, simulation analysis under different phase-locked loop integral coefficients, simulation analysis under different grid line impedances, and simulation analysis under different grid connection point d-axis current commands. Quantitative analysis and simulation verification of the transient stability of the phase-locked loop and the transient stability of the current loop are performed.

9. A transient stability assessment device for wind turbine generators, characterized in that, include: One or more processors; A memory having stored one or more programs that, when executed by one or more processors, cause the one or more processors to implement a transient stability assessment method for wind turbines according to any one of claims 1 to 8.

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