A wind turbine grid-connected grid-forming adaptive smooth switching control method

Abstract

This invention discloses an adaptive and smooth switching control method for wind turbines between grid-connected and grid-connected modes, belonging to the field of power switching control technology. This method uses a recursive least squares algorithm to identify grid impedance online and calculates the confidence lower bound of the short-circuit ratio to improve the conservatism of weak grid identification. It dynamically calculates the critical short-circuit ratio for grid-connected control and the grid strength stability margin by combining operating point parameters and phase-locked loop parameters. A fifth-order polynomial weighting function is used to achieve smooth fusion of current commands from grid-connected and grid-connected controllers, and state pre-synchronization eliminates the impact during switching. DC voltage control weights are smoothly transferred with equal weights, and power balance constraints ensure DC voltage stability within the switching interval. This invention enables adaptive and smooth switching of wind turbines between grid-connected and grid-connected modes, avoiding abrupt changes in control state and current surges, and improving the system's operational reliability under weak grid conditions.

Description

Technical Field

[0001] This invention belongs to the field of power switching control technology, specifically, it relates to an adaptive and smooth switching control method for wind turbine generators and grid connection. Background Technology

[0002] With the global energy structure transformation and the advancement of carbon neutrality goals, wind power, as one of the renewable energy technologies with the greatest potential for large-scale development, has seen its installed capacity in the power system continue to grow rapidly. As the core equipment of wind power generation systems, the grid connection control technology of wind turbines directly affects the safe and stable operation of the power grid and the capacity for renewable energy absorption. In the field of wind turbine grid connection control, grid-connected control and grid-connected control are two main control modes, each with different technical characteristics and applicable scenarios.

[0003] Grid-following control, also known as grid-following control, is currently the most widely used grid-connected control method for wind turbines. This control mode tracks the grid voltage phase and frequency in real time through a phase-locked loop (PLL) and achieves decoupled control of active and reactive power based on grid voltage orientation. Grid-following control has the advantages of fast dynamic response and high control accuracy, enabling efficient and stable grid-connected operation of wind turbines under strong grid conditions. However, with the continuous increase in wind power penetration, the grid strength at the connection point is gradually weakening due to the large number of wind turbines connected to the grid, especially in long-distance transmission, offshore wind power grid connection, or weak grid areas, where the grid exhibits high impedance characteristics. Under weak grid conditions, grid-following control faces severe stability challenges. The interaction between the PLL and grid impedance may cause subsynchronous oscillations, loss of synchronization, and other problems, which in severe cases can lead to wind turbine disconnection from the grid, affecting the safe operation of the power system.

[0004] Grid-based control, also known as grid-connected control, employs virtual synchronous generator technology to enable wind turbines to autonomously establish voltage and frequency, allowing them to operate independently without strong grid support or support weak grid voltage. By simulating the inertia and damping characteristics of synchronous generators, grid-based control provides virtual inertia support to the grid, enhancing the power system's disturbance immunity, and is particularly suitable for grid-connected operation in weak grid environments. However, the operating efficiency of grid-based control under strong grid conditions is generally lower than that of grid-connected control, and its dynamic response characteristics differ, making it difficult to maintain optimal performance under all operating conditions.

[0005] Given the differences in operating conditions between strong and weak power grids, a single control mode is insufficient to achieve optimal operation of wind turbines under all grid conditions. Therefore, adaptive switching between grid-following and grid-connecting control modes has become an important technical direction for improving the adaptability of wind turbines across all operating conditions. However, existing mode switching methods have several shortcomings: First, the online assessment accuracy of grid strength is insufficient. Traditional methods often use the static short-circuit ratio as a criterion, failing to consider the uncertainty of grid impedance identification, resulting in conservatism or lag in weak grid identification. Second, the mode switching criteria are singular, failing to comprehensively consider the impact of factors such as wind turbine operating point and phase-locked loop parameters on the stability boundary. Third, the switching process lacks effective smoothing measures; sudden changes in controller state may lead to current surges, DC voltage fluctuations, or even system instability. Fourth, the handover of DC voltage control between the grid-side converter and the turbine-side converter lacks continuity, potentially causing control conflicts or control gaps. Summary of the Invention

[0006] The purpose of this invention is to provide an adaptive and smooth switching control method for wind turbines and the grid, which mainly solves the problems of insufficient accuracy in online assessment of grid strength and single mode switching criteria in the prior art.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A wind turbine and grid-connected adaptive smooth switching control method includes the following steps:

[0009] S1, collects the three-phase voltage and three-phase current signals at the grid connection point, and obtains them through coordinate transformation. The components are identified, and the grid impedance parameters are identified online. The confidence lower bound of the short-circuit ratio is calculated based on the identification results.

[0010] S2, the critical short-circuit ratio for grid control is calculated based on the stability parameters of the wind turbine grid control;

[0011] S3, define the grid strength stability margin as the ratio of the confidence lower bound of the short-circuit ratio to the critical short-circuit ratio for grid control;

[0012] S4, set the threshold for switching from following the network to building the network and the threshold for switching from building the network to following the network, and use hysteresis and delay judgment logic to determine the mode switching timing;

[0013] S5, during the execution mode switch, the network controller and the network construction controller run in parallel to complete the pre-synchronization of the controller state; define the normalized switching time, use a fifth-order polynomial weighting function to continuously transfer control rights; and synchronously transfer DC voltage control rights.

[0014] S6 executes control commands through current limiting, current inner loop, voltage limiting, anti-integral saturation, and PWM modulation;

[0015] S7 feeds back the grid connection operation results to the grid strength assessment layer, mode decision layer and grid parameter scheduling module to form a closed-loop control.

[0016] Further, in step S1, a recursive least squares algorithm with a forgetting factor is used to identify the grid impedance parameters online, obtaining estimated grid resistance and inductance values; the specific expressions are as follows:

[0017]

[0018]

[0019] in, For the parameter estimation vector, The recursive gain matrix is... Let covariance matrix be the variance matrix. Forgetting factor; ; and These are the equivalent resistance of the power grid and the equivalent reactance of the power grid, respectively. Indicates the first The regression matrix at each sampling time point, Represents the identity matrix that matches the dimension of matrix operations. Indicates the first The model residual or identified noise vector at each sampling time.

[0020] Further, in step S1, the expression for the lower confidence bound of the short-circuit ratio is:

[0021]

[0022]

[0023] In the formula, This represents the lower confidence bound of the short-circuit ratio. Indicates the short-circuit ratio at the grid connection point. The standard deviation of the short-circuit ratio estimate. , is the confidence coefficient and These are the grid connection point rated voltage and the wind turbine rated capacity, respectively. This represents the estimated equivalent reactance of the power grid; This represents the estimated equivalent resistance of the power grid.

[0024] Further, in step S2, the stability parameters include grid impedance magnitude, unit active and reactive power operating points, grid voltage, impedance ratio, and phase-locked loop parameters; the calculation process for the grid-connected control critical short-circuit ratio is as follows:

[0025] Define the power grid impedance ratio :

[0026]

[0027] Define the equivalent current at the operating point :

[0028]

[0029] In the formula, This represents the d-axis component of the grid-connected current at the current steady-state operating point. This represents the q-axis component of the grid-connected current at the current steady-state operating point.

[0030] The minimum synchronization voltage stiffness is defined based on the minimum natural frequency and minimum damping ratio requirements of the phase-locked loop:

[0031]

[0032] Therefore, the critical short-circuit ratio for grid control is obtained:

[0033]

[0034] In the formula, This represents the equivalent voltage amplitude of the power grid. This represents the minimum synchronization voltage stiffness required to meet the minimum natural frequency and minimum damping ratio requirements of a phase-locked loop. and These are the proportional and integral coefficients of the phase-locked loop, respectively. and These are the lowest natural frequency and the lowest damping ratio, respectively.

[0035] Furthermore, in step S4, the set threshold for mesh transformation is... And the threshold of the network to the network satisfy:

[0036] ;

[0037] When the generator unit is in grid-connected mode, and:

[0038]

[0039] Duration reached At that time, the network switching from the follow network to the construct network is initiated; where, Indicates the strength stability margin of the power grid;

[0040] When the unit is in network construction mode, and:

[0041]

[0042] Duration reached Meanwhile, when the shadow phase-locked loop completes synchronization, it is allowed to return to the network mode;

[0043] Mode switching should also meet DC voltage, speed, and current constraints:

[0044]

[0045]

[0046] In the formula, Indicates the DC bus voltage. This indicates the mechanical angular velocity of the wind turbine rotor. This represents the AC output current vector of the grid-side converter.

[0047] Further, in step S5, the controller state pre-synchronization process is as follows:

[0048] Before switching from following the network to building the network, map the network building controller state to the current following network operating point:

[0049]

[0050]

[0051]

[0052]

[0053] In the formula, This indicates the internal phase angle of the network control. This indicates the phase angle of the phase-locked loop output. This indicates that the internal angular frequency is controlled by the network structure. This represents the output angular frequency of the phase-locked loop. This indicates the internal voltage amplitude of the grid control system. This represents the voltage vector at the grid connection point, and its magnitude represents the voltage amplitude at the grid connection point. This represents the active power reference value of the virtual synchronization control loop of the network controller. This represents the actual active power output of the wind turbine at the grid connection point. Indicates virtual damping. Indicates the rated angular frequency of the power grid. This represents the reference current vector generated by the network controller. This represents the reference current vector generated by the grid controller. Indicates the start time of mode switching;

[0054] Before switching from network construction to network establishment, the phase-locked loop (PLL) operates in shadow mode, and its integral state is initialized as follows:

[0055]

[0056] In the formula, This represents the integral state variable of the phase-locked loop PI controller. This represents the q-axis component of the grid-connected voltage in the synchronous rotating coordinate system of the phase-locked loop.

[0057] Furthermore, in step S5, the handover control of DC voltage control is as follows:

[0058] Define DC voltage error:

[0059]

[0060] In the formula, This indicates the reference value for the DC bus voltage;

[0061] The same weights used during mode switching are applied to the handover of control.

[0062]

[0063]

[0064] In the formula, Indicates the output power of the grid-side converter. This indicates the steady-state reference power corresponding to the DC bus being in a power balance state at the start of mode switching. This indicates a smooth weight switching process within the network. This indicates the DC voltage power regulation gain of the grid-side converter. Indicates the machine-side input power. Indicates the DC voltage power regulation gain of the machine-side converter;

[0065] Combining DC bus power balance, we can obtain:

[0066]

[0067] In the formula, This represents the equivalent capacitance of the DC bus. This represents the steady-state operating point value of the DC bus voltage. Indicates DC bus voltage error Rate of change with respect to time; when and During this time, the DC voltage error remains convergent throughout the entire switching interval, thereby avoiding control conflicts or control gaps between the grid-side and machine-side controllers.

[0068] Compared with the prior art, the present invention has the following beneficial effects:

[0069] (1) This invention identifies grid impedance parameters online using a recursive least squares algorithm and introduces a lower bound on the short-circuit ratio confidence level as a mode switching criterion, effectively overcoming the problems of insufficient accuracy in grid strength assessment and inadequate conservatism in weak grid identification in traditional methods. Specifically, the recursive least squares algorithm with a forgetting factor can track grid impedance changes in real time, while the lower bound on the short-circuit ratio confidence level introduces a statistical confidence interval based on the identification results, avoiding delayed exit from the grid-following mode due to parameter estimation errors, thereby improving the reliability and conservatism of weak grid identification and providing more accurate grid strength information for mode switching decisions.

[0070] (2) This invention comprehensively considers multiple factors such as grid impedance amplitude, unit active and reactive power operating points, grid voltage, impedance ratio, and phase-locked loop parameters to construct a grid-following control critical short-circuit ratio model. Based on this, the grid strength stability margin is defined as the core criterion for mode switching, overcoming the problem that the mode switching criterion in the prior art is singular and fails to fully reflect the stability boundary. At the same time, the hysteresis and delay judgment logic is used to set the grid-following to grid-building threshold and the grid-building to grid-following threshold, which effectively avoids frequent switching when the grid strength fluctuates in the critical region, ensuring the stability and reliability of mode switching decisions.

[0071] (3) This invention uses a fifth-order polynomial weighting function to achieve continuous and smooth fusion of current commands between the network controller and the network construction controller. Combined with the controller state pre-synchronization mechanism, it eliminates current surges and phase jumps during switching, overcoming the problem of lack of smoothing measures and easy system instability in the prior art. The first and second derivatives of the fifth-order polynomial weighting function are both zero at the starting and ending points of the switching, which can ensure the continuity and smoothness of current commands during the handover of control. At the same time, it transforms the current commands of the two controllers into the αβ coordinate system for weighted fusion, avoiding the phase wrapping problem caused by direct interpolation angle.

[0072] (4) This invention uses the same weighting as mode switching to continuously transfer the DC voltage control rights. Combined with DC bus power balance constraints, it ensures that the DC voltage error remains convergent throughout the entire switching interval, overcoming the problem of discontinuous control transfer between the grid-side and turbine-side converters in the prior art, which may cause control conflicts or control gaps. When both the grid-side and turbine-side controllers use positive gain regulation, the DC voltage error always converges within the switching interval, thereby ensuring the stability of the DC voltage during mode switching and improving the operational reliability of wind turbines under weak grid conditions. Attached Figure Description

[0073] Figure 1 This is a flowchart illustrating the method of the present invention. Detailed Implementation

[0074] The present invention will be further described below with reference to the accompanying drawings and embodiments. The embodiments of the present invention include, but are not limited to, the following embodiments.

[0075] like Figure 1 As shown, the wind turbine grid-connected adaptive smooth switching control method disclosed in this invention, in the grid equivalent model construction stage, represents the external grid at the wind turbine grid connection point as an equivalent form of an ideal voltage source and series impedance:

[0076]

[0077] In the formula, , and These are the equivalent resistance, reactance, and inductance of the power grid, respectively. The angular frequency of the power grid. The imaginary unit is used. It is obtained from the three-phase voltage and current at the grid connection point through coordinate transformation. The axis component is used, and an incremental model is employed to eliminate the impact of slow changes in the equivalent voltage of the power grid within a short time window:

[0078]

[0079] in, This represents the d-axis component increment of the grid-connected point voltage at adjacent sampling times (or relative to the reference operating point). This represents the increment of the d-axis component of the grid-connected current. This represents the q-axis component increment of the grid connection point voltage. This represents the increment of the rate of change of the grid-connected current along the q-axis. This represents the modeling error and measurement noise vector of the incremental power grid model.

[0080] Rewriting the above equation in standard linear regression form:

[0081]

[0082] The power grid parameters are updated using a recursive least squares algorithm with a forgetting factor.

[0083]

[0084]

[0085] in, For the parameter estimation vector, The recursive gain matrix is... Let covariance matrix be the variance matrix. It is a forgetting factor.

[0086] The identification results show that:

[0087] .

[0088] The short-circuit ratio at the grid connection point of a wind turbine is defined as:

[0089]

[0090] In the formula, and These are the grid connection point rated voltage and the wind turbine rated capacity, respectively. To avoid delayed exit from grid-connected mode when grid impedance is underestimated, a lower bound for the short-circuit ratio confidence is introduced:

[0091]

[0092] in, The standard deviation of the short-circuit ratio estimate. This is the confidence coefficient. The control system uses... Instead Pattern determination is performed to improve the conservatism and reliability of weak grid identification.

[0093] The stability of grid-connected control depends not only on the grid impedance magnitude, but also on the active and reactive power operating points of the generating units, grid voltage, impedance ratio, and phase-locked loop parameters. Grid impedance ratio is defined as follows:

[0094]

[0095] And the equivalent current at the operating point:

[0096]

[0097] The minimum synchronization voltage stiffness is defined based on the minimum natural frequency and minimum damping ratio requirements of the phase-locked loop:

[0098]

[0099] Therefore, the critical short-circuit ratio for grid control is obtained:

[0100]

[0101] In the formula, This represents the equivalent voltage amplitude of the power grid. and These are the proportional and integral coefficients of the phase-locked loop, respectively. and These are the lowest natural frequency and the lowest damping ratio, respectively.

[0102] Define the power grid strength stability margin:

[0103]

[0104] The core criterion for mode switching: The smaller the value, the closer it is to the stability boundary of the network control distance; The larger the value, the stronger the power grid and the more sufficient the stability margin for grid control.

[0105] Set the threshold for network conversion. And the threshold of the network to the network And satisfy:

[0106]

[0107] When the generator unit is in grid-connected mode, and:

[0108]

[0109] Duration reached When the unit is in network-connected mode, the switch to network-building mode is initiated.

[0110]

[0111] Duration reached At the same time, when the shadow phase-locked loop completes synchronization, it is allowed to return to the network mode.

[0112] Mode switching should also meet DC voltage, speed, and current constraints:

[0113]

[0114]

[0115] Hysteresis thresholds and durations can prevent frequent switching of grid strength when it fluctuates in the critical region.

[0116] In this embodiment, the grid controller employs phase-locked loop-oriented power-current control, and its reference current is:

[0117]

[0118] In the formula This represents the active power reference value under grid-connected control mode. This represents the d-axis reference current generated by the grid controller. This represents the d-axis component of the grid-connected voltage in the synchronous rotating coordinate system of the phase-locked loop. This represents the q-axis reference current generated by the grid controller. This indicates the reactive power reference value under grid-connected control mode;

[0119] The network controller uses virtual synchronization control to establish internal frequency and phase angle:

[0120]

[0121]

[0122] And the internal voltage amplitude is formed through voltage-reactive power control:

[0123]

[0124] in, , , and These are virtual inertia, virtual damping, reactive droop coefficient, and voltage loop time constant, respectively.

[0125] Before switching from following the network to building the network, map the network building controller state to the current following network operating point:

[0126]

[0127]

[0128]

[0129]

[0130] The above initialization ensures that the phase angle, frequency, voltage and current commands of the two types of controllers are consistent at the switching start point, avoiding state jumps when the controller is put into operation.

[0131] Before switching from network construction to network establishment, the phase-locked loop (PLL) operates in shadow mode, and its integral state is initialized as follows:

[0132] .

[0133] Define normalized switching time:

[0134]

[0135] In the formula, Indicates the duration of the mode switch.

[0136] A fifth-order smoothing weight function is used:

[0137]

[0138] This function satisfies the following when switching the start and end points:

[0139]

[0140] To avoid phase wrapping caused by directly interpolating two angles, the current commands of the two controllers are transformed to a unified value. Coordinate system:

[0141]

[0142]

[0143] In the formula, and These represent the network controller and network construction controller at rest, respectively. Reference current vector in coordinate system and These represent the transformation angles using the phase angle of the phase-locked loop output and the phase angle inside the network controller, respectively. to Coordinate rotation transformation matrix. and These respectively indicate that the network controller and the network construction controller are rotating synchronously. Reference current vector in the coordinate system.

[0144] The final reference current is:

[0145]

[0146] When switching from a network to a network structure, Smoothly increase from 0 to 1; during the switch from network construction to network integration. Smoothly decrease from 1 to 0.

[0147] Subsequently, DC voltage control is continuously transferred, and DC voltage error is defined:

[0148]

[0149] In grid-connected mode, the grid-side converter handles DC voltage control; in integrated grid mode, the generator-side converter or energy storage device handles DC power balancing. Control handover is performed using the same weighting as during mode switching.

[0150]

[0151]

[0152] Combining DC bus power balance, we can obtain:

[0153]

[0154] when and During this time, the DC voltage error remains convergent throughout the entire switching interval, thereby avoiding control conflicts or control gaps between the grid-side and machine-side controllers.

[0155] The merged reference current enters the unified current limiter:

[0156]

[0157] In the formula, This represents the reference current vector before it enters the unified current limiter after being weighted and merged by the network controller and the network structure controller. This indicates the maximum allowable current amplitude of the converter. The limited command passes sequentially through the inner current loop, modulation voltage limiting, anti-integral saturation, and PWM modulation before finally being applied to the grid-side converter. When the current continuously reaches the upper limit or the DC voltage exceeds the allowable range, the weight change rate should be frozen or reduced. Switching should continue only after the system recovers, in order to avoid the simultaneous activation of mode switching and nonlinear current limiting.

[0158] The above embodiments are merely one of the preferred embodiments of the present invention and should not be used to limit the scope of protection of the present invention. Any modifications or refinements made to the main design concept and spirit of the present invention that are not of substantial significance, but solve the same technical problem as the present invention, should be included within the scope of protection of the present invention.

Claims

1. A wind turbine generator and grid-connected adaptive smooth switching control method, characterized in that, Includes the following steps: S1, collects the three-phase voltage and three-phase current signals at the grid connection point, and obtains them through coordinate transformation. Quantity; The power grid impedance parameters are identified online, and the lower bound of the short-circuit ratio is calculated based on the identification results. S2, the critical short-circuit ratio for grid control is calculated based on the stability parameters of the wind turbine grid control. S3, define the grid strength stability margin as the ratio of the confidence lower bound of the short-circuit ratio to the critical short-circuit ratio for grid control; S4, set the threshold for switching from following the network to building the network and the threshold for switching from building the network to following the network, and use hysteresis and delay judgment logic to determine the mode switching timing; S5, when switching execution modes, the network controller and the network construction controller run in parallel to complete the pre-synchronization of the controller state; define the normalized switching time, use a fifth-order polynomial weighting function to continuously transfer control rights; and synchronously transfer DC voltage control rights. S6 executes control commands through current limiting, current inner loop, voltage limiting, anti-integral saturation, and PWM modulation; S7 feeds back the grid connection operation results to the grid strength assessment layer, mode decision layer and grid parameter scheduling module to form a closed-loop control.

2. The wind turbine and grid adaptive smooth switching control method according to claim 1, characterized in that, In step S1, a recursive least squares algorithm with a forgetting factor is used to identify the grid impedance parameters online, obtaining estimated grid resistance and inductance values; the specific expressions are as follows: in, For the parameter estimation vector, The recursive gain matrix is... Let covariance matrix be the variance matrix. Forgetting factor; ; and These are the equivalent resistance of the power grid and the equivalent reactance of the power grid, respectively. Indicates the first The regression matrix at each sampling time point, Represents the identity matrix that matches the dimension of matrix operations. Indicates the first The model residual or identified noise vector at each sampling time.

3. The wind turbine and grid adaptive smooth switching control method according to claim 2, characterized in that, In step S1, the expression for the lower confidence bound of the short-circuit ratio is: In the formula, This represents the lower confidence bound of the short-circuit ratio. Indicates the short-circuit ratio at the grid connection point. The standard deviation of the short-circuit ratio estimate. Here is the confidence coefficient. and These are the grid connection point rated voltage and the wind turbine rated capacity, respectively. This represents the estimated equivalent reactance of the power grid; This represents the estimated equivalent resistance of the power grid.

4. The wind turbine and grid adaptive smooth switching control method according to claim 3, characterized in that, In step S2, the stability parameters include grid impedance amplitude, unit active and reactive power operating points, grid voltage, impedance ratio, and phase-locked loop parameters; the calculation process for the grid-connected control critical short-circuit ratio is as follows: Define the power grid impedance ratio : Define the equivalent current at the operating point : In the formula, This represents the d-axis component of the grid-connected current at the current steady-state operating point. This represents the q-axis component of the grid-connected current at the current steady-state operating point. The minimum synchronization voltage stiffness is defined based on the minimum natural frequency and minimum damping ratio requirements of the phase-locked loop: Therefore, the critical short-circuit ratio for grid control is obtained: In the formula, This represents the equivalent voltage amplitude of the power grid. This represents the minimum synchronization voltage stiffness required to meet the minimum natural frequency and minimum damping ratio requirements of a phase-locked loop. and These are the proportional and integral coefficients of the phase-locked loop, respectively. and These are the lowest natural frequency and the lowest damping ratio, respectively.

5. The wind turbine and grid adaptive smooth switching control method according to claim 4, characterized in that, In step S4, the threshold for converting the network to a web structure is set. And the threshold of the network to the network satisfy: ； When the unit is in grid-connected mode, and: Duration reached At that time, the network switching from the follow network to the construct network is initiated; where, Indicates the strength stability margin of the power grid; When the unit is in network construction mode, and: Duration reached Meanwhile, when the shadow phase-locked loop completes synchronization, it is allowed to return to the network mode; Mode switching should also meet DC voltage, speed, and current constraints: In the formula, Indicates the DC bus voltage. This indicates the mechanical angular velocity of the wind turbine rotor. This represents the AC output current vector of the grid-side converter.

6. The wind turbine and grid adaptive smooth switching control method according to claim 5, characterized in that, In step S5, the controller state pre-synchronization process is as follows: Before switching from following the network to building the network, map the network building controller state to the current following network operating point: In the formula, This indicates the internal phase angle of the network control. This indicates the phase angle of the phase-locked loop output. This indicates that the internal angular frequency is controlled by the network structure. This represents the output angular frequency of the phase-locked loop. This indicates the internal voltage amplitude of the grid control system. This represents the voltage vector at the grid connection point, and its magnitude represents the voltage amplitude at the grid connection point. This represents the active power reference value of the virtual synchronization control loop of the network controller. This represents the actual active power output of the wind turbine at the grid connection point. Indicates virtual damping. Indicates the rated angular frequency of the power grid. This represents the reference current vector generated by the network controller. This represents the reference current vector generated by the grid controller. Indicates the start time of mode switching; Before switching from network construction to network establishment, the phase-locked loop (PLL) operates in shadow mode, and its integral state is initialized as follows: In the formula, This represents the integral state variable of the phase-locked loop PI controller. This represents the q-axis component of the grid-connected voltage in the synchronous rotating coordinate system of the phase-locked loop.

7. The wind turbine and grid adaptive smooth switching control method according to claim 6, characterized in that, In step S5, the handover control of DC voltage control is as follows: Define DC voltage error: In the formula, This indicates the reference value for the DC bus voltage; The same weights used during mode switching are applied to the handover of control. In the formula, Indicates the output power of the grid-side converter. This indicates the steady-state reference power corresponding to the DC bus being in a power balance state at the start of mode switching. This indicates a smooth weight switching process within the network. This indicates the DC voltage power regulation gain of the grid-side converter. Indicates the machine-side input power. Indicates the DC voltage power regulation gain of the machine-side converter; Combining DC bus power balance, we can obtain: In the formula, This represents the equivalent capacitance of the DC bus. This represents the steady-state operating point value of the DC bus voltage. Indicates DC bus voltage error Rate of change with respect to time; when and During this time, the DC voltage error remains convergent throughout the entire switching interval, thereby avoiding control conflicts or control gaps between the grid-side and machine-side controllers.

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