A Stability Control Method for Grid-Type Direct-Drive Wind Turbines Based on Inertial Synchronization

Abstract

This application discloses a stability control method for grid-connected direct-drive wind turbines based on inertial synchronization. The method includes: establishing a small-signal model of the grid-side inverter; obtaining the stability relationship between the inertial transfer link parameters and the small-signal model; determining the lead compensation link coefficient based on the complex torque coefficient method and the stability relationship; and using the lead compensation link coefficient to control the grid-side inverter. This application's solution directly introduces the lead compensation link coefficient into the inertial transfer link on the turbine side of the direct-drive wind turbine. By changing the damping characteristics of the system brought about by the inertial transfer link, it further improves the stability of the system after the introduction of the inertial transfer link and provides the ability to quickly provide active power support.

Description

Technical Field

[0001] This application relates to the field of new energy power generation, and in particular to the stability control of direct-drive wind turbine units. Background Technology

[0002] Currently, most grid-connected direct-drive wind turbines are based on power synchronization control. Classified by control structure, grid-connected control methods include droop control, virtual synchronous machine (VSG) control, and inertial synchronization control. Droop control and VSG control rely on DC voltage stability, meaning they lack the ability to actively stabilize DC voltage. Since direct-drive wind turbines rely on wind power, which is often volatile, inertial synchronization control, which also considers DC voltage stability, is more suitable for engineering applications of direct-drive wind turbines.

[0003] To increase system inertia, inertial synchronous control type direct-drive wind turbines often introduce an inertial transfer mechanism on the turbine side to utilize the turbine's energy. However, analysis shows that the introduction of the inertial transfer mechanism can negatively impact system stability to some extent. Traditional stability improvement methods add a third-order inertial stabilizing element to the inertial transfer mechanism, but this inevitably leads to the direct-drive wind turbine failing to respond promptly to active power support changes in grid frequency. Summary of the Invention

[0004] This application provides a method for stable control of grid-connected direct-drive wind turbines based on inertial synchronization, which can achieve timely active power support when the grid frequency changes.

[0005] In a first aspect, embodiments of this application provide a method for stabilizing and controlling a grid-type direct-drive wind turbine based on inertial synchronization, including:

[0006] Establish a small-signal model of the grid-side inverter and obtain the stability relationship between the parameters of the inertial transfer link and the small-signal model;

[0007] The coefficients of the lead compensation element are determined based on the complex moment coefficient method and the stability relationship.

[0008] The grid-side inverter is controlled using the lead compensation coefficient.

[0009] Preferably, the step of establishing a small-signal model of the grid-side inverter and obtaining the stability relationship between the inertial transfer link parameters and the small-signal model includes:

[0010] The relationship between the overall state variables of the direct-drive wind turbine and the grid angular frequency is as follows:

[0011] (1)

[0012] (2)

[0013] (3)

[0014] The eigenvalues ​​of its state transition matrix A are obtained from equation (3), and the relationship between the parameters of the inertial transfer link and the stability of the small-signal model is determined.

[0015] Where ΔX is the overall state variable of the direct-drive fan based on inertial synchronous control, and ω g Let ω be the grid angular frequency, E be the 10th-order identity matrix, and λ be the eigenvalues ​​of the state transition matrix.

[0016] Preferably, determining the lead compensation coefficient based on the complex moment coefficient method and stability relationship includes:

[0017] Obtain the initial transfer function of the change in active power on the machine side of the direct-drive fan with respect to the change in DC voltage;

[0018] By taking the negative sign in the analysis of the change in active power on the machine side and the complex torque coefficient, the expression for the complex torque coefficient is obtained.

[0019] The coefficients of the lead compensation element are obtained from the expression for the complex moment coefficient.

[0020] Preferably, the initial transfer function includes:

[0021] (4)

[0022] (4) Where K c ω is the inertial transfer coefficient. r0 i represents the steady-state value of the output voltage angular frequency of the synchronous motor. sq0 Ls is the steady-state value of the q-axis current of the machine-side rectifier; T is the equivalent inductance; G is the time constant of the inertial element; r The transfer function of the outer power loop PI element of the machine-side converter; G c This is the transfer function of the PI element in the inner current loop of the machine-side converter. s is the complex frequency, u sq0 ∆δ is the steady-state value of the q-axis voltage of the machine-side rectifier. dc For the linearized virtual power angle of the grid-side inverter; u mq0 This represents the steady-state value of the q-axis voltage of the machine-side rectifier.

[0023] Preferably, the expression for the complex moment coefficient is:

[0024] (5)

[0025] (5) Where K c ω is the inertial transfer coefficient. r0 The steady-state value of the output voltage angular frequency of the synchronous motor; u sq0 i is the steady-state value of the q-axis voltage of the machine-side rectifier; sq0Ls is the steady-state value of the q-axis current of the machine-side rectifier; T is the equivalent inductance; G is the time constant of the inertial element; r The transfer function of the outer power loop PI element of the machine-side converter; G c The transfer function of the PI element in the inner current loop of the machine-side converter; H(s) = K m +jΩD m K m and D m ω represents the synchronous torque coefficient and damping torque coefficient corresponding to the machine-side rectifier, respectively; j represents the imaginary unit in the complex number; Ω represents the dominant oscillation frequency of the system; s is the complex frequency; and the time constant of the inertial element is T = 0.1. r0 u is the steady-state value of the rotor speed of the permanent magnet synchronous motor. mq0 ∆δ is the steady-state value of the q-axis voltage of the machine-side rectifier. dc This is the linearized virtual power angle for the grid-side inverter.

[0026] The beneficial effects of the technical solutions provided in some embodiments of this application include at least the following: establishing a small-signal model of the grid-side inverter, obtaining the stability relationship between the inertial transfer link parameters and the small-signal model, determining the lead compensation link coefficient based on the complex torque coefficient method and the stability relationship, and using the lead compensation link coefficient to control the grid-side inverter. This application's solution directly introduces the lead compensation link coefficient into the machine-side inertial transfer link of the direct-drive wind turbine, thereby changing the damping characteristics brought to the system by the inertial transfer link, further improving the stability of the system after the introduction of the inertial transfer link, and enabling it to quickly provide active power support. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 A flowchart illustrating a method for stabilizing a grid-type direct-drive wind turbine based on inertial synchronization, provided for an embodiment of this application;

[0029] Figure 2 This application describes the overall control strategy for the grid-side inverter in a direct-drive wind turbine grid-connected system.

[0030] Figure 3 A comparison of simulation waveforms before and after the introduction of a stabilizing controller in the embodiments of this application;

[0031] Figure 4 This is a comparison of simulation waveforms under different stabilization controllers in the embodiments of this application. Detailed Implementation

[0032] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings.

[0033] The terms "first," "second," "third," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish different objects, not to describe a specific order. 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 apparatus 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 apparatuses.

[0034] Traditional stability improvement methods add a third-order inertial stabilization element to the inertia transfer stage to enhance system stability. However, this inevitably leads to the direct-drive wind turbine failing to respond to active power support in a timely manner when the grid frequency changes. The active power support of the turbine-side converter only begins to respond when the active power support of the grid-side inverter inertial synchronous response ends, which also leads to a reduction in the peak value of active power support.

[0035] This invention directly introduces second-order lead-lag compensation into the inertia transfer stage of a direct-drive wind turbine. By altering the damping characteristics of the inertia transfer stage, the stability of the system is further improved. Compared to traditional methods, the turbine side is more sensitive to changes in grid frequency, responding and increasing active power almost instantly upon frequency drop, with a larger peak value supported by the active power.

[0036] Reference Figure 1 To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings.

[0037] Step S100: Establish a small-signal model of the grid-side inverter and obtain the stability relationship between the parameters of the inertial transfer link and the small-signal model;

[0038] To simplify the analysis, the influence of reactive power loops, damping controllers, and machine-side stabilization mechanisms is ignored; that is, the grid-side inverter of the direct-drive wind turbine grid-connected system operates in constant voltage mode. A small-signal model is established, and the impact of inertia transfer mechanisms on its small-signal stability is analyzed. The steps include:

[0039] Step 1: Ignoring the effects of reactive power loops, damping controllers, and machine-side stabilization, i.e., assuming the grid-side inverter of the direct-drive wind turbine grid-connected system operates in constant voltage mode to simplify the analysis, its small-signal model is established as follows:

[0040] (1)

[0041] (1) In the formula, ΔX is the overall state variable of the direct-drive fan based on inertial synchronous control, and ω g Let be the angular frequency of the power grid. And we have:

[0042] (2)

[0043] Step 2: Calculate the eigenvalues ​​of the state transition matrix A using equation (3), determine the stability of the system, and study the inertial transfer parameter K. c Impact on system stability.

[0044] (3)

[0045] (3) In the formula, E is the tenth-order identity matrix; λ is the eigenvalue of the state transition matrix.

[0046] Step 3: Observe the parameter K of the inertial transmission link. c The system eigenvalue distribution when K is 0 is analyzed, and the dominant modes of the system are determined to be 1.42+j314.16 and 1.42-j314.16. Furthermore, the distribution with increasing coefficient K is determined. c The increase of K causes the dominant pole to shift to the right of the imaginary axis, leading to system instability and oscillation. This indicates that the introduction of the inertia transfer element reduces system stability. However, it is worth noting that K... c The change has almost no effect on the system oscillation frequency, which remains at 50Hz (314.16rad / s).

[0047] Step S200: Determine the coefficient of the lead correction element based on the complex moment coefficient method and the stability relationship.

[0048] This paper utilizes the complex torque coefficient method, commonly used in traditional power system analysis of low-frequency oscillations, to analyze the mechanism of system instability caused by inertia transfer. Based on this analysis, an improved stability controller strategy is proposed. The steps include:

[0049] Step 1: Determine H without making any improvements to the inertia transfer process. s When (s)=1, the change in active power ΔP on the machine side of the direct-drive wind turbine. m Regarding the DC side voltage change Δu dc Transfer function:

[0050] (4)

[0051] (4) Where K c ω is the inertial transfer coefficient. r0 The steady-state value of the output voltage angular frequency of the synchronous motor; u sq0 i is the steady-state value of the q-axis voltage of the machine-side rectifier;sq0 L represents the steady-state value of the q-axis current of the machine-side rectifier. s G is the equivalent inductance; T is the time constant of the inertial element; G is the time constant of the inertial element. r The transfer function of the outer power loop PI element of the machine-side converter; G c This is the transfer function of the PI element in the inner current loop of the machine-side converter. s is the complex frequency, u sq0 ∆δ is the steady-state value of the q-axis voltage of the machine-side rectifier. dc This is the linearized virtual power angle for the grid-side inverter.

[0052] Step 2: Considering P m With P e The sign is opposite to that in the rotation equation of a permanent magnet synchronous motor, thus the change in active power ΔP on the machine side of the direct-drive fan is... m In the analysis of the complex moment coefficients with respect to Δδ, a negative sign must be taken, i.e., -ΔP. m =(K m +jΩD m By analyzing the form of Δδ, we obtain the correct expression for its complex moment coefficient:

[0053] (5)

[0054] (5) Where H(s) = K m +jΩD m ; s is the complex frequency; the time constant of the inertial element is T = 0.1. K m and D m ω represents the synchronous torque coefficient and damping torque coefficient corresponding to the machine-side rectifier, respectively; j represents the imaginary unit in a complex number; Ω represents the dominant oscillation frequency of the system. r0 u is the steady-state value of the rotor speed of the permanent magnet synchronous motor. mq0 ∆δ is the steady-state value of the q-axis voltage of the machine-side rectifier. dc This is the linearized virtual power angle for the grid-side inverter.

[0055] Step 3: Observe the Bode plot of the transfer function H(s), noting that if D needs to be made... m A phase angle greater than 0 is required at the oscillation frequency Ω = 50Hz, i.e., a phase angle correction greater than 93°, to enhance system stability. Therefore, a second-order lead-lag compensation is introduced as a stabilizing element in the machine-side converter, the coefficient of the lead compensation element is determined, and an improved strategy for the stability controller is proposed.

[0056] Step S300: Control the grid-side inverter using the lead compensation coefficient.

[0057] In specific implementation, such as Figure 2As shown, the grid-connected system of grid-connected direct-drive wind turbine generator based on inertial synchronous control uses DC voltage to map frequency on the grid side, thereby realizing inertial synchronous control. A reactive power outer loop is constructed based on the relationship between reactive power and voltage. Reactive power control is achieved by introducing a PI controller, while the generator-side converter adopts flux-oriented control in the grid-connected direct-drive wind turbine control strategy. Figure 3 Figure (a) shows the introduction of an inertia transfer mechanism on the aircraft side, with a stabilizing mechanism (H) added to the aircraft side. s Simulation results when (s)=1, at which point the inertia transfer coefficient K c =3. As can be seen from the figure, when the grid frequency drops from 50Hz to 49.5Hz in 6 seconds, the power and DC side voltage of the system continue to oscillate. This shows that the damping controller introduced on the grid side can enhance the stability of the system. However, when the inertia transfer coefficient increases, the direct-drive wind turbine grid-connected system still faces the risk of instability. Figure 3 Figure (b) shows the inertia transfer coefficient K after a second-order lead-lag stabilization element is introduced on the machine side. c =6. As shown in the figure, when the grid frequency drops from 50Hz to 49.5Hz in 6 seconds, the system remains stable and provides considerable active power and inertia support to the grid, thus verifying the effectiveness of the second-order lead-lag stabilization mechanism. Figure 4 As can be seen, under grid frequency drops, the third-order inertial stabilization mechanism, as analyzed theoretically, slows down the active power support of the generator-side converter. It only begins to respond when the active power support of the grid-side inverter's inertial synchronous response ends, resulting in a reduction in the peak value of active power support. Conversely, when the generator-side converter employs a second-order lead-lag stabilization mechanism, it is more sensitive to changes in grid frequency, responding and increasing active power almost instantaneously upon frequency drops, with a larger peak value of active power support. This verifies the correctness of the theoretical analysis and demonstrates the effectiveness and superiority of the second-order lead-lag stabilization mechanism.

[0058] The embodiments described above are merely preferred embodiments of this application and are not intended to limit the scope of this application. Any modifications and improvements made by those skilled in the art to the technical solutions of this application without departing from the spirit of this application should fall within the protection scope defined by the claims of this application.

Claims

1. A method for stable control of a grid-type direct-drive wind turbine based on inertial synchronization, characterized in that, include: Establish a small-signal model of the grid-side inverter and obtain the stability relationship between the parameters of the inertial transfer link and the small-signal model; The coefficients of the lead compensation element are determined based on the complex moment coefficient method and the stability relationship. The grid-side inverter is controlled using a lead compensation coefficient; wherein, establishing a small-signal model of the grid-side inverter and obtaining the stability relationship between the inertial transfer parameters and the small-signal model includes: The relationship between the overall state variables of the direct-drive wind turbine and the grid angular frequency is as follows: （1） （2） （3） The eigenvalues ​​of its state transition matrix A are obtained from equation (3), and the relationship between the parameters of the inertial transfer link and the stability of the small-signal model is determined. Where ΔX is the overall state variable of the direct-drive fan based on inertial synchronous control, and ω g Let ω be the grid angular frequency, E be the 10th-order identity matrix, and λ be the eigenvalues ​​of the state transition matrix.

2. The method as described in claim 1, characterized in that, The determination of the lead compensation coefficient based on the complex moment coefficient method and stability relationship includes: Obtain the initial transfer function of the change in active power on the machine side of the direct-drive fan with respect to the change in DC voltage; By taking the negative sign in the analysis of the change in active power on the machine side and the complex torque coefficient, the expression for the complex torque coefficient is obtained. The coefficients of the lead compensation element are obtained from the expression for the complex moment coefficient.

3. The method as described in claim 2, characterized in that, The initial transfer function includes: (4) (4) Where K c ω is the inertial transfer coefficient. r0 i represents the steady-state value of the output voltage angular frequency of the synchronous motor. sq0 Ls is the steady-state value of the q-axis current of the machine-side rectifier; T is the equivalent inductance; G is the time constant of the inertial element; r The transfer function of the outer power loop PI element of the machine-side converter; G c The transfer function of the PI element in the inner current loop of the machine-side converter; s is the complex frequency, u sq0 ∆δ is the steady-state value of the q-axis voltage of the machine-side rectifier. dc For the linearized virtual power angle of the grid-side inverter; u mq0 This represents the steady-state value of the q-axis voltage of the machine-side rectifier.

4. The method as described in claim 2, characterized in that, The expression for the complex moment coefficient is: （5） (5) Where K c ω is the inertial transfer coefficient. r0 The steady-state value of the output voltage angular frequency of the synchronous motor; u sq0 i is the steady-state value of the q-axis voltage of the machine-side rectifier; sq0 Ls is the steady-state value of the q-axis current of the machine-side rectifier; T is the equivalent inductance; G is the time constant of the inertial element; r The transfer function of the outer power loop PI element of the machine-side converter; G c The transfer function of the PI element in the inner current loop of the machine-side converter; H(s) = K m +jΩD m K m and D m These are the synchronous torque coefficient and damping torque coefficient corresponding to the machine-side rectifier, respectively; j represents the imaginary unit in the complex number; Ω represents the dominant oscillation frequency of the system; s is the complex frequency; the inertial element time constant T = 0.1; ω r0 u is the steady-state value of the rotor speed of the permanent magnet synchronous motor. mq0 ∆δ is the steady-state value of the q-axis voltage of the machine-side rectifier. dc This is the linearized virtual power angle for the grid-side inverter.

Images