A method for transient stability analysis of doubly-fed wind power system considering flux linkage security constraints

Abstract

The application discloses a network-constructing doubly-fed wind power transient stability analysis method considering flux linkage safety constraint. The method establishes a transient mathematical model which explicitly considers rotor voltage limiting amplitude dynamics, reveals a new transient instability mechanism triggered by voltage limiting amplitude, which is different from traditional current limiting amplitude, due to considering flux linkage safety constraint. Based on the model, the application quantitatively determines the action law of positive sequence virtual impedance and negative sequence current injection strategy on critical clearing time, perfects the stability analysis theory of network-constructing doubly-fed wind power generator under specific fault conditions, and provides accurate theoretical basis and engineering guidance for improving fault ride-through capability and optimizing control parameters.

Description

Technical Field

[0001] This invention belongs to the field of wind power generation control technology, specifically relating to a transient stability analysis method for grid-connected doubly fed wind power systems that considers magnetic flux safety constraints. Background Technology

[0002] Among the many types of new energy, the wind power industry has developed particularly rapidly. Doubly fed induction generators (DFIGs) have become the absolute mainstay of the onshore wind power market, accounting for nearly 70% of the market share, thanks to their flexible variable speed operation, high economy, and high reliability (such as no risk of demagnetization).

[0003] Traditional doubly-fed induction generator (DFIG) wind turbines, based on phase-locked loop (PLL) grid-following control technology, are essentially a current source control mode. In this mode, wind power passively "follows" the grid, inherently lacking sufficient support for system voltage and frequency. Their stable operation highly depends on a robust grid environment dominated by synchronous generators. However, with the development of large-scale renewable energy bases such as the Gobi Desert, the composition of the power system is undergoing fundamental changes: the proportion of synchronous power sources is continuously decreasing, and the system is exhibiting a "weak grid" form with low inertia and low short-circuit ratios. This transformation presents traditional grid-following wind turbines with a series of new challenges, such as wide-frequency oscillations and cascading grid disconnections. To address this severe situation, academia and industry are actively exploring grid-based control, proactively providing voltage and frequency support to the system, thereby ensuring the safety and stability of high-proportion renewable energy power systems. Against this backdrop, grid-based wind power technology has emerged and developed rapidly.

[0004] In existing technologies, research on the transient stability of grid-connected DFIGs under grid disturbances has become a key focus, but it mainly concentrates on the behavior of the units under current limiting mode. For example, the literature [Transient stability analysis of grid-connected DFIGs equipped with ring current limiters. IEEE Transactions on Industrial Electronics, 2025, Vol. 72, No. 11: 11332-11336] analyzes the impact of virtual impedance parameters on transient stability under current limiting mode, while the literature [Transient modeling and post-fault stability analysis of grid-connected DFIGs considering rotor-side current limiting. IEEE Transactions on Power Electronics, 2025, Vol. 40, No. 9: 11979-11984] compares the performance differences of different current limiters (ring current limiters and priority current limiters) after a fault. To simplify the analysis, these models usually ignore the dynamics of the inner current loop of the rotor-side converter and assume that it can perfectly track the current reference command. This simplification method is only considered effective when analyzing symmetrical fault conditions.

[0005] However, the validity of the above simplified assumptions is challenged during asymmetric faults because the inherent slip characteristics of doubly-fed induction generators (DFIGs) induce a large negative-sequence electromotive force (EMF) in the rotor windings due to the negative-sequence flux linkage. To counteract this EMF, the rotor-side converter requires extremely high control margins, and its output voltage is very likely to reach physical limits, i.e., enter voltage limiting mode. Once voltage limiting occurs, the inner current loop will saturate and trigger the anti-saturation circuit, causing it to be unable to accurately track the current command. In other words, the "perfect tracking assumption" fails, which means that stability criteria derived from traditional current limiting behavior may no longer be applicable under such conditions.

[0006] In summary, existing technologies mainly focus on current limiting modes under symmetrical faults, while in-depth research is lacking on transient instability problems caused by a novel mechanism of rotor voltage limiting triggered by considering flux linkage safety constraints under asymmetrical faults. The coupling of stator-side network control, rotor-side safety constraints, and nonlinear coordinate transformations brings significant difficulties to accurate modeling under this operating condition, and a complete analytical system for describing and analyzing this phenomenon has not yet been established domestically or internationally. Summary of the Invention

[0007] In view of the above, the present invention provides a transient stability analysis method for grid-connected doubly-fed induction generator (DFIG) wind turbines that consider flux linkage safety constraints. This method accurately reveals the transient instability mechanism caused by rotor voltage limiting due to flux linkage safety constraints, improves the analysis of the instability boundary under this specific operating condition, and provides a theoretical basis and parameter optimization guidance for improving the fault ride-through capability of grid-connected DFIG wind turbines.

[0008] A transient stability analysis method for grid-connected doubly-fed induction generator (DFIG) wind power considering flux safety constraints includes the following steps:

[0009] (1) A transient mathematical model of a grid-type doubly fed wind turbine is established. This model characterizes the dynamic characteristics of rotor voltage limiting under asymmetric faults with consideration of flux safety constraints.

[0010] (2) Draw the power angle curve of the grid-type doubly fed wind turbine according to the transient mathematical model, and then establish a transient stability criterion based on the transient instability characteristics reflected by the power angle curve;

[0011] (3) Based on the transient stability criterion, the virtual impedance and negative sequence current injection strategy are key factors in controlling the length of the critical clearing time. Optimizing these two control parameters can improve the transient stability of the grid-type doubly fed wind turbine.

[0012] Furthermore, the positive sequence control of the grid-type doubly-fed wind turbine adopts a grid-type control strategy based on droop control. This control strategy introduces a feedforward term inside the cascaded voltage and current loops to accelerate the dynamic response, while using a circular limiter to limit the rotor current and rotor voltage. During asymmetrical faults, the negative sequence control of the grid-type doubly-fed wind turbine relies on a dual synchronous rotating coordinate system phase-locked loop to separate the positive and negative sequence components and accurately orient the stator negative sequence voltage.

[0013] Furthermore, when the rotor voltage limiting function is activated, the grid-type doubly fed wind turbine is equivalent to a circuit consisting of a variable voltage source in series scaling virtual impedance and a variable equivalent inductance. Since the negative sequence component adopts a grid-based control strategy, the grid-type doubly fed wind turbine exhibits hybrid dual-sequence characteristics: it acts as both a positive sequence voltage source and a negative sequence current source.

[0014] Furthermore, the expression for the transient mathematical model in step (1) is as follows:

[0015]

[0016]

[0017] in: E This represents the internal potential output from the outer loop of the network. Indicates the grid voltage. μ As an intermediate variable, Represents virtual impedance. Represents the angular velocity of the power grid. Indicates the inductance of the power grid. and These represent the stator inductance and rotor inductance of the DFIG, respectively. This represents the magnetizing inductance of the DFIG. Indicates the slip ratio. σ This represents the leakage flux coefficient of DFIG. j The imaginary unit, This represents the stator positive sequence current of the DFIG. and These represent the proportional gain coefficients of the PI (proportional-integral) control in the voltage loop and current loop, respectively. This represents the rotor voltage limiting coefficient.

[0018] Furthermore, the rotor voltage limiting coefficient The calculation expression is as follows:

[0019]

[0020]

[0021] in: ηAs an intermediate variable, Represents virtual inductance. Re represents the virtual resistance, and Re() represents taking the real part. Represents the angular velocity of the power grid. For the positive sequence work angle of DFIG, This represents the maximum amplitude of the rotor voltage.

[0022] Furthermore, the transient stability criterion in step (2) is as follows: after an asymmetrical fault occurs, the power angle curve of the grid-type doubly-fed wind turbine shifts from the SEP point (Stable Equilibrium Point) to the fault trajectory, and the power angle begins to accelerate due to the active power control effect; when the fault duration is less than the critical clearing time, the power angle of the grid-type doubly-fed wind turbine can recover to the original SEP point after the fault is cleared; when the fault duration is greater than the critical clearing time, the power angle will cross the UEP point (Unstable Equilibrium Point) and stabilize again at a new SEP point, but this transient process is accompanied by severe power backfeed, which can lead to generator damage, malfunction of protection devices, and even system instability.

[0023] Furthermore, the calculation expression for the critical purge time is as follows:

[0024]

[0025] Where: CCT represents the critical clearance time. The work angle represents the point of stable equilibrium. The work angle represents the point of unstable equilibrium. This represents the proportional gain coefficient of the droop control active power loop. This indicates the active power command value. To correspond to the positive sequence work angle The active power below.

[0026] Furthermore, in step (3), based on multiple sets of control parameter values ​​for virtual impedance and negative sequence current injection and their corresponding critical clearing time data, the influence of these two control parameters on the length of the critical clearing time is obtained through fitting or inductive analysis:

[0027] ① An increase in virtual impedance will lead to a shortening of the critical clearing time, showing a negative correlation, and the adverse effect of virtual resistance on transient stability is greater than that of virtual inductance.

[0028] ② There is an effective negative sequence current injection phase angle range. Within this range, appropriately increasing the negative sequence current injection can prolong the critical clearing time, thereby improving the transient stability of the grid-type doubly fed wind turbine.

[0029] Furthermore, the effective negative sequence current injection phase angle range is 45°~135°, with the optimal phase angle selected as 90°.

[0030] This invention establishes, for the first time, a transient model of a grid-connected doubly-fed induction generator (DFIG) wind turbine that considers the dynamics of rotor voltage limiting under asymmetrical faults. It reveals a novel transient instability mechanism triggered by voltage limiting, distinct from traditional current limiting, due to consideration of flux linkage safety constraints. Furthermore, based on this model, this invention provides a quantitative method for systematically analyzing the impact of positive-sequence virtual impedance and negative-sequence current injection strategies on transient stability, accurately determining the influence of each control parameter on the critical clearing time. In addition, this invention improves the stability analysis theory of grid-connected DFIG wind turbines under specific fault conditions, providing precise theoretical basis and engineering guidance for enhancing their fault ride-through capability and optimizing control parameter design. Attached Figure Description

[0031] Figure 1 This is a flowchart illustrating the transient stability analysis method for grid-connected doubly-fed wind power according to the present invention.

[0032] Figure 2 A schematic diagram of the topology and control system structure for grid-connected doubly-fed wind turbine generators.

[0033] Figure 3 This is a schematic diagram of the positive and negative sequence equivalent circuit of a grid-type doubly fed wind turbine under asymmetrical fault conditions.

[0034] Figure 4 This is a schematic diagram of the power angle curves of a grid-type doubly fed wind turbine under normal operating conditions and asymmetrical fault conditions.

[0035] Figure 5 This is a contour projection diagram showing the effect of virtual impedance on the critical clearing time.

[0036] Figure 6 This is a schematic diagram of the relationship between the critical clearing time and the negative sequence current injection strategy. Detailed Implementation

[0037] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0038] like Figure 1 As shown, this embodiment provides a transient stability analysis method for grid-connected doubly-fed induction generator (DFIG) wind power considering flux safety constraints, including the following steps:

[0039] Step S1: Establish a transient mathematical model of a grid-type doubly fed wind turbine generator. This model characterizes the rotor voltage limiting dynamics under asymmetric faults, considering flux safety constraints.

[0040] The network-type DFIG topology and control system studied in this embodiment are as follows: Figure 2 As shown, where Indicates the rotor angle. This represents the angle obtained from the positive-sequence active power cycle. Indicates the positive sequence phase-locked angle. Indicates the negative sequence phase-locked angle. Indicates the three-phase voltage of the stator. Indicates the three-phase stator current. This indicates the positive sequence command for rotor voltage under three-phase conditions. This indicates a negative sequence command for rotor voltage under three-phase conditions, 1 / s This represents a unity-gain integrator. and These represent the integral gains of the voltage control loop and the current control loop, respectively. The positive-sequence control employs a droop-based network control strategy, introducing a feedforward term within its cascaded voltage and current closed loops to accelerate dynamic response. Furthermore, the system simultaneously uses circular limiters for current and voltage limiting; its control logic can be expressed as follows:

[0041]

[0042] in: and These represent the rotor current commands at the input and output of the current limiter, respectively. and These represent the rotor voltage commands at the input and output of the voltage limiter, respectively. Indicates the rotor positive sequence current limit. Indicates the rotor positive sequence voltage limit. This represents the rotor current limiting coefficient. This represents the rotor voltage limiting coefficient.

[0043] During asymmetrical faults, negative-sequence control relies on dual-synchronous rotating coordinate system phase-locked loops to separate positive and negative-sequence components and precisely align the stator negative-sequence voltage. To enhance the positive-sequence transient stability of DFIG systems under asymmetrical faults, existing fault ride-through control strategies typically prioritize increasing positive-sequence active power output at the expense of negative-sequence voltage suppression. Modern grid-type equipment possesses strong overcurrent capabilities; therefore, for shallow asymmetrical faults, the likelihood of positive-sequence current limiting being triggered is low, allowing for... The simplified value is 1.

[0044] Once the rotor voltage limiting is triggered, the cascaded closed loop can no longer accurately track its commands. To prevent over-saturation of the integrators in the current and voltage loops, they must be reset to zero. Furthermore, due to the introduction of feedforward terms, the outputs of the voltage and current controllers are approximately zero in steady state. Therefore, the integral action can be ignored during fault periods, and the dynamics of the voltage and current control loops can be expressed as:

[0045]

[0046]

[0047] in: and These represent the proportional gains of the voltage control loop and the current control loop, respectively. This represents the magnetizing inductance of the DFIG. E This represents the internal potential output by the outer loop. Represents virtual impedance. Indicates the stator positive sequence current. Indicates the stator positive sequence voltage. Indicates the rotor positive sequence current. Indicates the rotor positive sequence voltage. This indicates the angular velocity of the power grid.

[0048] Combining the above with the physical equations of a grid-type doubly-fed wind turbine system, its expression is as follows:

[0049]

[0050]

[0051] in: Indicates the grid voltage. Indicates the inductance of the power grid. This indicates the DFIG stator inductance. Indicates rotor inductance, Indicates the slip ratio. σ This represents the leakage flux coefficient.

[0052] From this, the following relationship can be derived:

[0053]

[0054]

[0055] in: μ It is an intermediate variable.

[0056] The above equation shows that during a fault, when the rotor voltage limiting function is activated, the grid-connected doubly-fed induction generator (DFIG) system can be considered as a circuit consisting of a variable voltage source with series-scaled virtual impedance and a variable equivalent inductance. Since the negative sequence component is typically controlled in grid-fed mode to improve control performance, the DFIG system exhibits hybrid dual-sequence characteristics: it acts as both a positive-sequence voltage source and a negative-sequence current source, such as... Figure 3 As shown, where This indicates the positive-sequence stator output current. This indicates the negative sequence stator output current.

[0057] However, it is obvious Still an unknown variable, the rotor voltage amplitude is set to the maximum limit under fault conditions. The expression can be solved as:

[0058]

[0059]

[0060] in: η As an intermediate variable, For virtual inductance, For virtual resistance, It is a positive sequence angle.

[0061] In summary, the closed-loop dynamic characteristics of the grid-type doubly fed wind turbine under rotor voltage limiting mode are characterized by the above formula.

[0062] Step S2: Based on the above transient mathematical model, determine the power angle curve of the unit, and based on the transient instability characteristics reflected by the power angle curve, establish a transient stability criterion for calculating the critical clearance time.

[0063] The energy angle curve of a grid-type DFIG can be drawn by the following formula:

[0064]

[0065] in: This indicates the positive sequence output active power. It represents the conjugate of the stator positive sequence current.

[0066] like Figure 4 As shown, the maximum permissible power angle offset is defined as the power angle difference between the stable equilibrium point (SEP) and the unstable equilibrium point (UEP). After the fault occurs, the operating point shifts from the SEP point to the fault trajectory, and due to the active power control, the rotor power angle begins to accelerate.

[0067] Critical purge time is a key indicator for evaluating transient stability. For the grid-type doubly-fed induction generator system with droop control in this embodiment, its critical purge time is determined by the following expression:

[0068]

[0069] Where: CCT represents the critical clearance time. Indicates the work angle at the stable equilibrium point. Indicates the work angle at an unstable equilibrium point. This represents the proportional coefficient of the droop control active loop.

[0070] When the fault duration is less than the CCT, the DFIG can recover to the original SEP point after the fault is cleared. Conversely, when the fault duration is greater than the CCT, the power angle will cross the UEP point. Although the system will re-stabilize at a new stable equilibrium point (NewSEP), this transient process is accompanied by severe power backfeed, which may lead to generator damage, malfunction of the protection system, or even system-wide instability.

[0071] Step S3: Based on the transient stability criterion, positive-sequence virtual impedance and negative-sequence current injection are selected as the control parameters to be analyzed. The corresponding critical purge time series are calculated, and the influence of the control parameters on the critical purge time is determined.

[0072] Figure 5 The dependence of the critical clearing time on the virtual impedance parameter is explained, where ΔT represents the difference in CCT as the parameter changes. The study shows that the critical clearing time CCT is related to the virtual resistance. and virtual inductance Both showed a negative correlation. However, quantitative analysis revealed a significant difference in the degree of influence between the two: when the virtual inductance... When the voltage is increased from 0.2 pu to 0.3 pu, the change in critical clearing time is negligible; in contrast, the virtual resistance... An equal increase in the value of results in a sharp reduction of the critical purge time by 0.397 seconds. This result indicates that the critical purge time significantly affects the virtual resistance. Its sensitivity (i.e., partial derivative) is much greater than its sensitivity to virtual inductance. The sensitivity of the virtual resistance was thus confirmed. The adverse effects on positive-sequence transient stability are more pronounced.

[0073] While reducing virtual impedance is beneficial for improving transient performance, it must be noted that the complete removal of virtual impedance poses a threat to the small-signal stability of grid-connected DFIGs, especially under conditions of varying grid strength. Therefore, the tuning of virtual impedance parameters must seek an optimization and trade-off between ensuring steady-state operation requirements and meeting transient fault ride-through capabilities.

[0074] based on Figure 5 A quantitative analysis of the dependence of CCT on virtual impedance parameters reveals that CCT exhibits a negative correlation with both virtual resistance and virtual inductance (among others). The sensitivity is significantly higher than This implementation proposes a two-parameter lower limit clamping setting approach that prioritizes transient stability. Unlike the traditional design approach that significantly increases virtual impedance to limit fault current, this approach explicitly states that, while ensuring the basic controllability of the DFIG network, the impedance should be brought close to its theoretical lower limit. By eliminating redundant virtual impedance components, the system output impedance is reduced to the maximum extent, thereby significantly delaying the power angle divergence process under fault and obtaining the maximum critical clearance time margin.

[0075] The specific parameters are determined according to the following criteria: For the worst-case weakly connected operating condition of the system, small-signal eigenvalue analysis is performed, and the minimum critical resistance value that allows the system to maintain oscillatory convergence (i.e., the real part of the dominant pole is negative and it has the minimum damping ratio) is selected as the minimum critical resistance value. Set a value to avoid its high sensitivity reduction effect on transient stability.

[0076] To further explore The slight gain in transient stability brought about by reducing the power from 0.3 pu to 0.2 pu will be set at a lower limit for maintaining its function in this implementation. This lower limit is determined solely by the power decoupling condition, i.e., satisfying the inductive dominance constraint as follows:

[0077]

[0078] in: This indicates the setting factor (3~5 is recommended), and no additional current limiting impedance margin is reserved.

[0079] By using the above dual lower limit tuning, the system operates in the low impedance region of the impedance plane, thereby maximizing the critical clearing time in the physical limit.

[0080] Figure 6 The effect of rotor negative sequence current injection on the critical purge time is demonstrated, and this effect depends on the magnitude of the injected current. With phase angle Analysis shows that when a negative sequence current is injected into the phase angle... Within the range of 45° to 135° (preferably 90°), increasing the injection current effectively suppresses negative sequence voltage, thereby extending the critical clearing time (CCT), which indicates enhanced positive sequence transient stability. Conversely, at other injection angles, the injection behavior has the opposite effect, leading to a shorter CCT and deterioration of transient stability.

[0081] From an engineering practice perspective, a key consideration is that overly aggressive negative-sequence current injection may exacerbate the transient overcurrent level of a grid-type DFIG. Nevertheless, strategically injecting a negative-sequence current of moderate magnitude to suppress the adverse effects of negative-sequence electromotive force is a feasible solution to improve the unit's transient fault ride-through performance.

[0082] based on Figure 6 The non-monotonic influence of the rotor negative-sequence current phase on the CCT is shown in this embodiment. This implementation proposes a phase-locked and amplitude-limited negative-sequence current injection strategy. First, the positive gain phase sector for transient stability is defined; that is, based on the analysis conclusions, the phase of the negative-sequence current command is... Strictly constrained within the range of [45°, 135°], with 90° as the optimal dominant injection angle, to ensure that the injection behavior always suppresses the negative sequence electromotive force, thereby extending the CCT. Injection is strictly prohibited in non-gain sectors (especially angles that deteriorate the CCT), thus fundamentally avoiding the risk of the control strategy in the opposite direction of disrupting system stability.

[0083] Based on the established injection phase, in order to balance fault ride-through capability and equipment overcurrent risk, the positive sequence current output level of the grid-type DFIG during a fault should be monitored in real time. Combined with the converter's maximum allowable instantaneous current and the transient instability caused by not triggering rotor voltage limits, the injection of negative sequence current is dynamically calculated. The control system sets the injection amplitude as follows:

[0084]

[0085] in: This indicates the actual current limit value of the rotor converter. This indicates the actual current limit value of the rotor converter. Indicates the magnitude of the positive sequence rotor current. Indicates the stator negative sequence voltage. This indicates the magnitude of the rotor positive sequence voltage.

[0086] The above standard ensures that negative sequence injection is only performed when the converter still has a current margin. This fully utilizes the physical potential of the equipment to suppress negative sequence electromotive force and improve transient stability. It also completely eliminates transient overcurrent disconnection accidents caused by blindly pursuing performance through hard constraints.

[0087] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. Those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. A transient stability analysis method for grid-connected doubly-fed induction generator (DFIG) wind power considering flux linkage safety constraints, characterized in that, Includes the following steps: (1) A transient mathematical model of a grid-connected doubly-fed induction generator (DFIG) is established. This model characterizes the dynamic characteristics of rotor voltage limiting under asymmetric faults, considering flux safety constraints. The expression of the transient mathematical model is as follows: in: E This represents the internal potential output from the outer loop of the network. Indicates the grid voltage. μ As an intermediate variable, Represents virtual impedance. Represents the angular velocity of the power grid. Indicates the inductance of the power grid. and These represent the stator inductance and rotor inductance of the DFIG, respectively. This represents the magnetizing inductance of the DFIG. Indicates the slip ratio. σ This represents the leakage flux coefficient of DFIG. j The imaginary unit, This represents the stator positive sequence current of the DFIG. and These represent the proportional gain coefficients of the PI control in the voltage loop and current loop, respectively. Indicates the rotor voltage limiting coefficient; (2) Draw the power angle curve of the grid-type doubly fed wind turbine according to the transient mathematical model, and then establish a transient stability criterion based on the transient instability characteristics reflected by the power angle curve; (3) Based on the transient stability criterion, the virtual impedance and negative sequence current injection strategy are key factors in controlling the critical clearing time. Optimizing these two control parameters can improve the transient stability of grid-connected doubly-fed induction generators. Specifically, based on multiple sets of control parameter values ​​for virtual impedance and negative sequence current injection and their corresponding critical clearing time data, the influence of these two control parameters on the critical clearing time can be obtained through fitting or inductive analysis: ① An increase in virtual impedance will lead to a shortening of the critical clearing time, showing a negative correlation, and the adverse effect of virtual resistance on transient stability is greater than that of virtual inductance. ② There is an effective negative sequence current injection phase angle range. Within this range, appropriately increasing the negative sequence current injection can prolong the critical clearing time, thereby improving the transient stability of the grid-type doubly fed wind turbine.

2. The transient stability analysis method for grid-connected doubly-fed induction generator (DFIG) wind power considering flux safety constraints according to claim 1, characterized in that: The positive sequence control of the grid-type doubly-fed induction generator (DFIG) adopts a grid-type control strategy based on droop control. This strategy introduces a feedforward term within the cascaded voltage and current loops to accelerate dynamic response, while using a circular limiter to limit the rotor current and rotor voltage. During asymmetrical faults, the negative sequence control of the DFIG relies on a dual synchronous rotating coordinate system phase-locked loop to separate the positive and negative sequence components and accurately orient the stator negative sequence voltage.

3. The transient stability analysis method for grid-connected doubly-fed induction generator (DFIG) wind power considering flux safety constraints according to claim 2, characterized in that: When the rotor voltage limiting function is activated, the grid-type doubly fed wind turbine is equivalent to a circuit consisting of a variable voltage source in series scaling virtual impedance and a variable equivalent inductance. Since the negative sequence component adopts a grid-type control strategy, the grid-type doubly fed wind turbine exhibits hybrid dual-sequence characteristics: it acts as both a positive sequence voltage source and a negative sequence current source.

4. The transient stability analysis method for grid-connected doubly-fed induction generator (DFIG) wind power considering flux safety constraints according to claim 1, characterized in that: The rotor voltage limiting coefficient The calculation expression is as follows: in: η As an intermediate variable, Represents virtual inductance. Re represents the virtual resistance, and Re() represents taking the real part. Represents the angular velocity of the power grid. For the positive sequence work angle of DFIG, This represents the maximum amplitude of the rotor voltage.

5. The transient stability analysis method for grid-connected doubly-fed induction generator (DFIG) wind power considering flux safety constraints according to claim 1, characterized in that: The transient stability criterion in step (2) is: after an asymmetrical fault occurs, the power angle curve of the grid-type doubly fed wind turbine shifts from the SEP point to the fault trajectory, and the power angle begins to accelerate due to the active power control. When the fault duration is less than the critical clearing time, the power angle of the grid-connected doubly fed wind turbine can be restored to the original SEP point after the fault is cleared. When the fault duration is greater than the critical clearing time, the power angle will cross the UEP point and stabilize again at a new SEP point, but this will be accompanied by severe power backfeed, which will lead to generator damage, malfunction of protection devices, and even system instability.

6. The transient stability analysis method for grid-connected doubly-fed induction generator (DFIG) wind power considering flux safety constraints according to claim 1, characterized in that: The calculation expression for the critical purge time is as follows: Where: CCT represents the critical clearance time. The work angle represents the point of stable equilibrium. The work angle represents the point of unstable equilibrium. This represents the proportional gain coefficient of the droop control active power loop. This indicates the active power command value. To correspond to the positive sequence work angle The active power below.

7. The transient stability analysis method for grid-connected doubly-fed induction generator (DFIG) wind power considering flux safety constraints according to claim 1, characterized in that: The effective negative sequence current injection phase angle range is 45°~135°, and the optimal phase angle is selected as 90°.

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