Subsynchronous oscillation suppression method and device of grid-connected wind power system based on nonlinear inverse sliding mode control

Abstract

The invention discloses a non-linear inverse sliding mode control-based subsynchronous oscillation suppression method and device for a grid-connected wind power system, and the method comprises the steps: constructing a non-linear inverse controller through the non-linear characteristics of a direct-driven wind turbine generator, and reconstructing the input and output relation of the system; designing a sliding mode surface considering amplitude limiting and a switching control law on the nonlinear inverse controller to realize robust tracking control of electromagnetic power and mechanical side rotating speed; therefore, the subsynchronous oscillation under the flexible direct-current grid-connected condition is actively suppressed.

Description

Technical Field

[0001] This invention relates to the field of oscillation suppression in power systems with a high proportion of new energy sources, and particularly to a method and apparatus for suppressing subsynchronous oscillations in grid-connected wind power systems based on nonlinear inverse sliding mode control. Background Technology

[0002] With the development of wind power, the problem of subsynchronous oscillation in grid-connected wind farm projects both domestically and internationally has become increasingly prominent. Subsynchronous oscillation in actual wind farm grid-connected projects can lead to power quality issues, easily causing turbine shutdowns, equipment damage, and may even induce shaft torsional vibration in nearby thermal power units, triggering protection trips. Therefore, researching corresponding subsynchronous oscillation suppression strategies for direct-drive wind farms connected to flexible DC grid systems has significant practical engineering implications.

[0003] Currently, the subsynchronous oscillation suppression strategies for grid-connected wind power systems are mainly divided into three categories: 1) Add a flexible AC power transmission system, a renewable energy hydrogen production unit, and its additional damping control; 2) Add subsynchronous damping control to the converter on the turbine side or grid side of the wind farm; 3) Optimize the parameters of the existing controller in the system.

[0004] While adding FACTS devices or introducing renewable energy-based hydrogen production systems can achieve rapid response and effectively suppress subsynchronous oscillations, their overall economic efficiency remains relatively low. Meanwhile, currently widely adopted additional subsynchronous damping control strategies and parameter optimization-based controller designs largely rely on model construction after approximating the system linearization. However, wind farms connected to the grid via flexible DC systems represent a typical highly nonlinear problem. When the actual operating point deviates significantly from the initial assumed conditions, the aforementioned linearized model-based methods are prone to performance degradation, and their vibration suppression effect and control stability may be significantly affected.

[0005] In existing technologies, the grid-side converter and flexible DC rectifier of wind turbines in flexible DC grid-connected systems generally adopt the following control structure: the grid-side converter adopts a constant DC voltage and constant reactive power control strategy to control the DC voltage and output reactive power of the full-power converter, with the inner loop being a current loop and the outer loop being a DC voltage loop; the flexible DC rectifier adopts a constant AC voltage control strategy to provide a constant AC voltage to the wind farm, with the inner loop also being a current loop and the outer loop being a voltage loop.

[0006] When a direct-drive wind farm transmits power via flexible DC, a complex multi-stage dynamic coupling occurs within the system, linking the wind turbine mechanical side, the turbine-side converter, the high-voltage DC current control loop, and the DC link. The PI control strategy commonly used in wind turbine converters in traditional flexible DC grid-connected structures exhibits significant limitations in this operating scenario: linear PI controllers struggle to effectively identify and suppress oscillations of specific frequency components, lack sufficient robustness to uncertainties such as wind speed disturbances and dynamic changes in the high-voltage DC system, and exhibit lag in rapidly tracking electromagnetic power to reference values. This allows oscillation energy to easily accumulate within the turbine, making it difficult for the wind turbine to actively provide the necessary additional damping to the power system.

[0007] To improve the operational stability of wind farms under uncertain output conditions, some research schemes rely on static synchronous compensators to construct H-based systems. ∞ Theoretical robust controllers are available. However, such control strategies are difficult to fully characterize the inherent strong nonlinearity of flexible DC grid-connected systems, and their implementation cost is relatively high, which limits their application in large-scale engineering projects. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a method and device for suppressing subsynchronous oscillations in grid-connected wind power systems based on nonlinear inverse sliding mode control, which can actively suppress subsynchronous oscillations under flexible DC grid connection conditions.

[0009] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A method for suppressing subsynchronous oscillations in grid-connected wind power systems based on nonlinear inverse sliding mode control, comprising: A nonlinear inverse controller is constructed using the nonlinear characteristics of a direct-drive wind turbine to reconstruct the input-output relationship of the system. A sliding mode surface and a switching control law are designed on the nonlinear inverse controller to achieve robust tracking control of electromagnetic power and mechanical side speed.

[0010] To solve the above-mentioned technical problems, another technical solution adopted by the present invention is as follows: A subsynchronous oscillation suppression device for a grid-connected wind power system based on nonlinear inverse sliding mode control includes a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, it implements the steps of the aforementioned subsynchronous oscillation suppression method for a grid-connected wind power system based on nonlinear inverse sliding mode control.

[0011] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A computer-readable storage medium having stored thereon computer program instructions, which, when executed by a processor, implement the steps of the above-described method for suppressing subsynchronous oscillations in a grid-connected wind power system based on nonlinear inverse sliding mode control.

[0012] The beneficial effects of this invention are as follows: It proposes a method for suppressing subsynchronous oscillations in a direct-drive wind farm connected to a flexible DC grid system based on a combination of nonlinear inverse control and sliding mode control. The method utilizes the nonlinear characteristics of the direct-drive wind turbine to construct a nonlinear inverse control framework, reshapes the system input-output relationship, and designs a sliding mode surface and switching control law that consider amplitude limiting to achieve robust tracking control of electromagnetic power and mechanical side speed, thereby actively suppressing subsynchronous oscillations under flexible DC grid connection conditions. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the structure of a direct-drive wind turbine connected to a flexible DC grid system according to an embodiment of the present invention; Figure 2 This is a flowchart illustrating the steps of a method for suppressing subsynchronous oscillations in a grid-connected wind power system based on nonlinear inverse sliding mode control, according to an embodiment of the present invention. Figure 3 This is a schematic diagram of the control structure of the wind turbine grid-side converter under a nonlinear inverse controller according to an embodiment of the present invention; Figure 4 This is a schematic diagram of the control structure of the flexible DC rectifier under a nonlinear inverse controller according to an embodiment of the present invention; Figure 5 This is a schematic diagram of the control structure of the wind turbine grid-side converter under a nonlinear reverse sliding mode controller according to an embodiment of the present invention; Figure 6 This is a schematic diagram of the control structure of the flexible DC rectifier under a nonlinear reverse sliding mode controller according to an embodiment of the present invention; Figure 7 It is a root locus comparison diagram of the next synchronous oscillation mode at different wind speeds; Figure 8 It is a comparison diagram of the root locus of the next synchronous oscillation mode under different DC capacitor voltages; Figure 9 This is a comparison chart of the transient response of the system when the DC capacitor of the wind turbine suddenly increases by 10%; Figure 10 This is a comparison chart of the transient response of the system when the DC capacitor of the wind turbine suddenly decreases by 10%. Figure 11 This is a comparison diagram of the transient responses of nonlinear inverse control and nonlinear inverse sliding mode control during a three-phase short-circuit ground fault. Figure 12 This is a schematic diagram of a subsynchronous oscillation suppression device for a grid-connected wind power system based on nonlinear inverse sliding mode control, according to an embodiment of the present invention. Detailed Implementation

[0014] To explain in detail the technical content, objectives, and effects of the present invention, the following description is provided in conjunction with the embodiments and accompanying drawings.

[0015] The subsynchronous oscillation suppression method, apparatus, and computer-readable storage medium of the grid-connected wind power system based on nonlinear inverse sliding mode control described in this application are applicable to solving the subsynchronous oscillation problem of direct-drive wind farms connected to flexible DC grid systems. The following detailed embodiments illustrate this: In one alternative implementation, such as Figure 2 As shown, a method for suppressing subsynchronous oscillations in a grid-connected wind power system based on nonlinear inverse sliding mode control includes: A nonlinear inverse controller is constructed using the nonlinear characteristics of a direct-drive wind turbine to reconstruct the input-output relationship of the system. A sliding mode surface and a switching control law are designed on the nonlinear inverse controller to achieve robust tracking control of electromagnetic power and mechanical side speed.

[0016] In another alternative implementation, the construction of the nonlinear inverse controller using the nonlinear characteristics of the direct-drive wind turbine includes: Establish a dynamic model of a direct-drive wind farm connected to a flexible DC grid system; Design a nonlinear inverse controller based on the dynamic model.

[0017] In another alternative implementation, the establishment of a dynamic model for a direct-drive wind farm connected to a flexible DC grid system includes: Establish a dynamic mathematical model for the grid-side converter of the wind turbine; Establish a dynamic mathematical model for a flexible DC rectifier; The design of the nonlinear inverse controller based on the dynamic model includes: The dynamic mathematical models of the wind turbine grid-side converter and the flexible DC rectifier are respectively expressed as affine nonlinear system forms through system coordinate transformation; The zero-dynamic design method is used to divide the state variables of the wind turbine grid-side converter and flexible DC rectifier after coordinate transformation into external state variables and internal state variables. A nonlinear inverse controller is designed for the external state variables of the wind turbine grid-side converter and the flexible DC rectifier, transforming the affine nonlinear system form into a nonlinear inverse controller form. The nonlinear state feedback is determined based on the dynamic mathematical model of the wind turbine grid-side converter and flexible DC rectifier in the form of the nonlinear inverse controller. The affine nonlinear system form is transformed into a linear system form based on the external state variables, internal state variables, and nonlinear state feedback.

[0018] In this embodiment, by performing partial nonlinear inversion on the wind turbine grid-side converter and flexible DC rectifier, and utilizing coordinate transformation, the high-order nonlinear system is converted into a controllable linear external dynamic, thereby achieving complete decoupling between the electromagnetic power control quantity and the current loop. This method effectively solves the limitation of traditional PI control in handling nonlinear problems, ensuring stable operation of the system over a wide wind speed range.

[0019] In practice: (1) Establish a dynamic model of a direct-drive wind farm connected to a flexible DC grid system. Step 1: The system state variables of the wind turbine grid-side converter are The control input variable is According to Kirchhoff's current law, the dynamic equation of the DC voltage of the wind turbine grid-side converter can be obtained as follows: (1) In the formula: This refers to the DC bus voltage of the back-to-back converter for the wind turbine. , These represent the DC input and output currents, respectively; C is the DC capacitor of the back-to-back converter for the wind turbine.

[0020] Step 2: Express the DC voltage dynamic equation in the form of control input variables. Based on the power balance of the AC and DC sides of the wind turbine grid-side converter, we can obtain: (2) In the formula: , The output current of the grid-side converter of the wind turbine are respectively , Axial components; , The output voltages of the grid-side converters of the wind turbines are respectively , Axial components.

[0021] Step 3: In equation (2) Substituting the expression into equation (1), we can obtain the DC voltage dynamic equation in the form of converter control input variables as follows: (3) Step 4: According to Kirchhoff's voltage law, the dynamic equation for the outlet current of the wind turbine grid-side converter can be obtained as follows: (4) In the formula: Synchronous angular velocity; , These are the capacitors of the wind turbine collector circuit. Voltage , Axial component; L g The filter inductor is located at the outlet side of the grid-side converter. Equations (3) and (4) form the dynamic mathematical model of the wind turbine grid-side converter.

[0022] Step 5: The system state variables of the flexible DC rectifier are , , , , The control input variable is , Similar to the derivation of the dynamic mathematical model of the wind turbine grid-side converter, based on Kirchhoff's voltage and current laws and the AC / DC system structure of the flexible DC rectifier, the dynamic mathematical model of the flexible DC rectifier, expressed in the form of control input variables, is as follows: (5) In the formula: , Flexible DC / AC filter capacitors Voltage , Axial components; , These are the currents flowing from the wind farm into the flexible DC system. , Axial components; , These are the currents flowing through the phase reactors, respectively. , Axial components; , These are the output voltages of the flexible DC rectifier. , Axial components; DC capacitor for flexible DC system The voltage on; For the DC-side input current of the flexible DC system; For the switching power loss of the flexible DC rectifier; L c This is the equivalent inductance of the AC-side phase reactor of the flexible DC-DC rectifier.

[0023] (2) Design of nonlinear inverse controller Step 1: Based on the control input u and control output y, the dynamic mathematical models (3)(4)(5) of the wind turbine grid-side converter and flexible DC rectifier are expressed as affine nonlinear system forms, as shown in equations (6)(7): (6) (7) In the affine nonlinear system of the wind turbine grid-side converter (6) Dimension of state variables Control input variables , The grid-side converter of the wind turbine controls the DC voltage and Shaft current, controlling output variable . and The specific expression is as follows:

[0024]

[0025] In the affine nonlinear system of the flexible DC rectifier, equation (7) Dimension of state variables Control input variables , Flexible DC rectifier control , Shaft AC voltage, controlling output variables: .

[0026] and The specific expression is as follows:

[0027] Step 2: Based on the definition of the relational degree of a multi-input multi-output system, for the affine nonlinear system of the wind turbine grid-side converter, we can obtain: (8) In the formula, L represents the Lie derivative.

[0028] Step 3: Due to the matrix Since it is nonsingular, the overall relational degree of the affine nonlinear system of the wind turbine grid-side converter is: The dimension of the system state variables is less than that shown in equation (6). Therefore, the grid-side converter of the wind turbine can achieve partial nonlinear inversion. Furthermore, similar tests show that the overall relational degree of the affine nonlinear system of the flexible DC rectifier is... The rectifier system dimension is less than that of equation (7). It can also achieve some nonlinear inverses.

[0029] Step 4: For wind turbine grid-side converter systems and flexible DC rectifier systems that can be partially nonlinearly inverted, a zero-dynamic design method can be adopted to reduce the system order and computational complexity in the design of nonlinear inverted controllers. The basic principle of the zero-dynamic design method is to divide the dynamic behavior of the system under study into two parts: external dynamics and internal dynamics. From the perspective of application, it is usually required that the external dynamics have good dynamic quality while ensuring stability, while the internal dynamics only need to ensure stability. Therefore, the state variables after coordinate transformation of the wind turbine grid-side converter and flexible DC rectifier systems can be divided into two parts, as shown in equations (9) and (10): (9) (10) In the formula: , These respectively represent the wind turbine grid-side converter and flexible DC rectifier system regarding... , coordinate transformation function; , These represent the new state variables of the wind turbine grid-side converter and flexible DC rectifier system after coordinate transformation, respectively; the subscript o indicates the external state variable, and its dimension is equal to the system relation degree; the subscript i indicates the internal state variable, and its dimension is equal to the system dimension minus the system relation degree.

[0030] Step 5: The output variables of the wind turbine grid-side converter and the flexible DC rectifier are respectively and External state variables of wind turbine grid-side converters and flexible DC rectifiers They are respectively: (11) (12) Step 6: The dimension of the internal state variables is equal to the system dimension minus the system relation degree. Through appropriate coordinate transformation, the internal state variables of the wind turbine grid-side converter and flexible DC rectifier are... They are respectively: (13) Step 7: From an application perspective, a nonlinear inverse controller needs to be designed for the external dynamics to ensure stability and good dynamic performance. Based on the coordinate transformation of equations (11) and (12), the dynamic equations (6) and (7) of the original system can be transformed into the forms of equations (14) and (15): (14) (15) Step 8: According to the principle of linear control, in order to achieve linearization, let:

[0031] The nonlinear state feedback (which reflects the relationship between the original system's control input variables and the nonlinear inverse controller's pre-control variables) can be obtained as follows: (16) (17) Step 9: Based on coordinate transformation equations (11) and (12) and nonlinear state feedback equations (16) and (17), the original affine nonlinear system equations (6) and (7) can be transformed into linear system equations (18) and (19): (18) (19) In the formula: This refers to the pre-controlled variables of the external system after coordinate transformation of the wind turbine grid-side converter (it can also be defined as a new control input variable); These are the pre-controlled variables of the external system after coordinate transformation of the flexible DC rectifier.

[0032] Step 10: From equations (16) and (17), it can be seen that the nonlinear inverse controller control laws of the wind turbine grid-side converter and the flexible DC rectifier are as shown in equations (20) and (21), respectively: (20) (twenty one) In the formula: , The dq-axis modulated wave signal of the wind turbine grid-side converter under a nonlinear inverse controller; , This is the dq-axis modulated wave signal of the flexible DC rectifier under a nonlinear inverse controller. After inverse dq transformation, the dq-axis modulated wave signal is modulated by PWM to generate trigger pulses, which control the on and off of the switching devices of the wind turbine grid-side converter and the flexible DC rectifier.

[0033] Step 11: From equations (20) and (21), it can be seen that the nonlinear inverse controller control law only contains the pre-control variable. It has not yet been determined. This is because the DC voltage controlled by the grid-side converter of the wind turbine is... Shaft current, flexible DC rectifier control , For shaft AC voltage, the pre-control variable in the control law of the nonlinear inverse controller in current research is generally designed in the following linear form: (twenty two) (twenty three) In the formula: all values ​​with the subscript "ref" are given values ​​of the relevant variables; , For the proportional and integral coefficients of the DC voltage control of the grid-side converter of the wind turbine; , For wind turbine grid-side converters Shaft current control proportional and integral coefficients; , For flexible DC rectifier positioning Shaft voltage proportional and integral coefficients; , For flexible DC rectifier positioning Shaft voltage proportionality and integral coefficient.

[0034] Step 12: From equations (20)-(23), it can be seen that the control structure of the wind turbine grid-side converter and flexible DC rectifier under the nonlinear inverse controller is as follows: Figure 3 , 4 As shown.

[0035] In another alternative implementation, a sliding mode surface considering amplitude limiting and a switching control law are designed on the nonlinear inverse controller to achieve robust tracking control of electromagnetic power and mechanical side speed, including: Design of a nonlinear inverse sliding mode controller considering amplitude limiting constraints Step 1: First, perform soft limiting on the given values ​​of DC voltage and converter output current. Limit the reference values ​​through a saturation circuit, as shown in equations (24) and (25): (twenty four) (25) In the formula For the safe operating range of the DC bus, This is the rated current of the converter.

[0036] Step 2: For the grid-side converter of the wind turbine, since the controlled objects are DC voltage and q-axis current, a linear sliding surface can be selected as shown in equation (26): (26) Step 3: To reduce chattering, a sliding mode control law combining the constant velocity reaching law and the saturation function is adopted, as shown in Equation (27). The sliding mode reaching law simplifies the solution to the sliding mode variable structure control problem and reduces chattering. Simultaneously, the saturation function further reduces system chattering.

[0037] (27) In the formula, constant This indicates that the system's motion point approaches the switching surface. The speed. The smaller the value, the less chattering occurs when the moving point reaches the switching surface, but the slower the approach speed.

[0038] Step 4: Consider the requirements for approach speed and chatter amplitude to select the appropriate method. Saturation function The expression is: (28) In the formula, The neighborhood is the boundary layer of the sliding mode switching surface. The value of is generally between -0.5 and +0.5.

[0039] Step 5: From equations (26) and (27), it can be seen that when using sliding mode control to design the pre-control variable in the nonlinear inverse controller of the wind turbine grid-side converter of equation (20), the pre-control variable can be obtained as follows: (29) Step 6: Similarly, when using sliding mode control to design the pre-control variable in the nonlinear inverse controller of the flexible DC rectifier (21), the pre-control variable can be obtained. for: (30) In the formula, constant , The coefficients of the constant velocity reaching law for sliding mode control of flexible DC rectifiers, and the sliding surface. , .

[0040] Step 7: Combining the nonlinear inverse controller and sliding mode control, substituting equations (29) and (30) into equations (20) and (21), we can obtain the equivalent control law of the nonlinear inverse sliding mode control as follows: (31) (32) From equations (31) and (32), it can be seen that the control structure of the wind turbine grid-side converter and flexible DC rectifier under nonlinear reverse sliding mode control is as follows: Figure 5 , 6 As shown.

[0041] In this embodiment, a sliding mode surface is constructed on the linearized structure after nonlinear inversion. A constant velocity approaching law combined with a saturation function is used to effectively reduce chattering. Rapid additional damping injection is achieved through sliding mode control terms, thereby significantly improving the system's robustness to wind speed disturbances, DC capacitance changes, and fault disturbances, ensuring the stability and reliability of system operation.

[0042] Meanwhile, sliding mode control is used to compensate for the sensitivity of nonlinear inverse to changes in system parameters, while nonlinear inverse improves the response efficiency of sliding mode control. The two form a mutually reinforcing synergistic control structure, which significantly improves the subsynchronous oscillation mode damping characteristics of the system, thereby improving the dynamic performance and stability of the overall system.

[0043] Simulation verification was performed on the above implementation method: Step 1: To evaluate the performance of nonlinear inverse sliding mode control in suppressing subsynchronous oscillations in a direct-drive wind farm connected to a flexible DC grid system, based on... Figure 1 The direct-drive wind farm shown is connected to a flexible DC grid system. Small-signal models and PSCAD / EMTDC simulation models of the system under dual closed-loop PI control, nonlinear inverse controller, and other conditions are established.

[0044] Step 2: Based on the small-signal model and time-domain simulation model under PI control and nonlinear inverse controller, evaluate the subsynchronous oscillation suppression effect of the nonlinear inverse controller under different wind speeds. Define the wind turbine grid-side nonlinear inverse controller as one that uses a nonlinear inverse controller in the wind turbine grid-side converter and a traditional dual-closed-loop PI control in the flexible DC rectifier; define the flexible DC rectifier nonlinear inverse controller as one that uses a traditional dual-closed-loop PI control in the wind turbine grid-side converter and a nonlinear inverse controller in the flexible DC rectifier. Set different wind speeds under traditional dual-closed-loop PI control (PI), wind turbine grid-side nonlinear inverse control, and flexible DC rectifier nonlinear inverse control, respectively. The root locus of the system's subsynchronous oscillation mode is as follows: Figure 7 As shown. By Figure 7 It can be seen that the nonlinear inverse controller on the grid side of the wind turbine is less affected by wind speed than the PI control for subsynchronous oscillation damping, and has a good effect on improving subsynchronous oscillation damping.

[0045] Step 3: Based on the small-signal model and time-domain simulation model under PI control and nonlinear inverse controller, evaluate the subsynchronous oscillation suppression effect of the nonlinear inverse controller under different DC voltages of the wind turbine grid-side converter. With a wind speed of 8 m / s, different DC voltage setpoints for the wind turbine grid-side converter are set under traditional dual-closed-loop PI control, wind turbine grid-side nonlinear inverse controller, and flexible DC rectifier nonlinear inverse controller. The root locus of the subsynchronous oscillation mode of the direct-drive wind farm via the flexible DC grid-connected system is shown below. Figure 8 As shown. By Figure 8 The eigenvalue analysis results show that, compared with PI control, the subsynchronous oscillation mode of the wind turbine grid-side nonlinear inverse controller is farther from the imaginary axis on the left side of the complex plane, and the subsynchronous oscillation damping is significantly increased.

[0046] Step 4: In order to evaluate the robust stability of the nonlinear inverse sliding mode control to parameter perturbations or external disturbances, firstly, the value of the DC capacitor of the wind turbine is changed to simulate parameter perturbations, and then a short-circuit fault is set to simulate external disturbances.

[0047] With a wind speed of 8 m / s, and the DC capacitor of the wind turbine suddenly increasing and decreasing by 10% at t=1s, the response curves of the system DC capacitor voltage and grid connection point voltage under the nonlinear inverse controller and nonlinear inverse sliding mode control on the wind turbine grid side are as follows: Figure 9 , 10 As shown.

[0048] Depend on Figure 9 , 10 It can be seen that when the DC capacitor of the wind turbine changes, the DC capacitor voltage and grid connection point voltage of the nonlinear inverse sliding mode control can recover to stability faster than the nonlinear inverse control of the wind turbine grid side, and the overshoot is smaller, showing stronger robustness to parameter perturbations.

[0049] Step 5: Set the wind speed to 8 m / s, and assume a three-phase short-circuit ground fault occurs at the grid connection point at t=1s (fault duration 50 ms). The transient response curves of the system under the nonlinear inverse control and nonlinear inverse sliding mode control on the wind turbine grid side are as follows: Figure 11 As shown. By Figure 11 It can be seen that when a short-circuit fault occurs, nonlinear inverse sliding mode control, compared with nonlinear inverse control on the grid side of the wind turbine, enables the DC voltage of the wind turbine and the voltage at the grid connection point to recover and stabilize more quickly, reduces the impact of transient DC voltage and grid connection point voltage fluctuations on the system, and shows stronger robustness to external disturbances.

[0050] In another alternative implementation, such as Figure 12 As shown, a subsynchronous oscillation suppression device for a grid-connected wind power system based on nonlinear inverse sliding mode control includes a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, it implements the steps of the subsynchronous oscillation suppression method for a grid-connected wind power system based on nonlinear inverse sliding mode control described in any of the above embodiments.

[0051] In another alternative embodiment, a computer-readable storage medium stores computer program instructions thereon, which, when executed by a processor, implement the steps of the subsynchronous oscillation suppression method for a grid-connected wind power system based on nonlinear inverse sliding mode control as described in any of the above embodiments. In summary, this invention provides a method and apparatus for suppressing subsynchronous oscillations in grid-connected wind power systems based on nonlinear inverse sliding mode control. Nonlinear inverse control transforms the nonlinear system into a linear system form based on coordinate transformation and feedback of system state variables, enabling decoupled control within a sufficiently large domain of the state space without requiring additional equipment. While nonlinear inverse control can eliminate system nonlinearity, it is based on a system model and therefore has the drawback of being sensitive to parameter perturbations or external disturbances, limiting its application in grid-connected wind power systems with complex operating environments. Sliding mode control, on the other hand, exhibits strong robustness to parameter perturbations and external disturbances. Applying sliding mode control to a linear system after nonlinear inverse coordinate transformation can compensate for the shortcomings of nonlinear inverse control.

[0052] This scheme combines the advantages of nonlinear inverse control and sliding mode control, proposing a nonlinear inverse controller for a direct-drive wind farm connected to a flexible DC grid system. Compared to traditional dual-closed-loop PI control, this controller offers the following advantages: 1) Although the control structure increases a certain amount of algebraic operations, it reduces the inner loop PI control link on the dq axis control channel of the wind turbine, thus reducing the difficulty of PI parameter tuning; 2) The design of control parameters is more direct and simple because it is based on a system with precise linearization; 3) In terms of performance, since the nonlinearity of the system is taken into account, it has better dynamic performance and a wide range of stable operation capabilities.

[0053] Furthermore, compared with traditional methods using additional damping controllers, this invention requires no additional hardware, has a simple structure, and its control parameters are easy to design, resulting in high economic efficiency and engineering feasibility. Simultaneously, this method effectively suppresses subsynchronous oscillations, significantly improves the system's dynamic response speed and disturbance rejection capability, reduces equipment stress fluctuations and power quality issues, extends equipment lifespan, and ensures safe, stable, and efficient grid-connected operation of wind farms.

[0054] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent modifications made based on the content of the present invention specification and drawings, or direct or indirect applications in related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for subsynchronous oscillation suppression of grid-connected wind power system based on nonlinear inverse sliding mode control, characterized in that, include: A nonlinear inverse controller is constructed using the nonlinear characteristics of a direct-drive wind turbine to reconstruct the input-output relationship of the system. A sliding mode surface with amplitude limiting and a switching control law are designed on the nonlinear inverse controller to achieve robust tracking control of electromagnetic power and mechanical side speed.

2. The method of claim 1, wherein the method is characterized by: The construction of a nonlinear inverse controller utilizing the nonlinear characteristics of a direct-drive wind turbine includes: Establish a dynamic model of a direct-drive wind farm connected to a flexible DC grid system; Design a nonlinear inverse controller based on the dynamic model. 3.The subsynchronous oscillation suppression method for grid-connected wind power system based on nonlinear inverse sliding mode control according to claim 2, characterized in that, The dynamic model for establishing a direct-drive wind farm connected to a flexible DC grid system includes: Establish a dynamic mathematical model for the grid-side converter of the wind turbine; Establish a dynamic mathematical model for a flexible DC rectifier; The design of the nonlinear inverse controller based on the dynamic model includes: The dynamic mathematical models of the wind turbine grid-side converter and the flexible DC rectifier are respectively expressed as affine nonlinear system forms through system coordinate transformation; The zero-dynamic design method is used to divide the state variables of the wind turbine grid-side converter and flexible DC rectifier after coordinate transformation into external state variables and internal state variables. A nonlinear inverse controller is designed for the external state variables of the wind turbine grid-side converter and the flexible DC rectifier, transforming the affine nonlinear system form into a nonlinear inverse controller form. The nonlinear state feedback is determined based on the dynamic mathematical model of the wind turbine grid-side converter and flexible DC rectifier in the form of the nonlinear inverse controller. The affine nonlinear system form is transformed into a linear system form based on the external state variables, internal state variables, and nonlinear state feedback.

4. The method of claim 3, wherein the method is characterized by: The dynamic mathematical model of the wind turbine grid-side converter is as follows: In the formula, This refers to the DC bus voltage of the back-to-back converter for the wind turbine. DC input, , The output current of the grid-side converter of the wind turbine are respectively , Axial components, , The output voltages of the grid-side converters of the wind turbines are respectively , The axial component, C, represents the DC capacitance of the back-to-back converter of the fan. For synchronous angular velocity, , These are the capacitors of the wind turbine collector circuit. Voltage , Axial component, L g For the filter inductor at the output side of the grid-side converter; The dynamic mathematical model of the flexible DC rectifier is as follows: In the formula, , Flexible DC / AC filter capacitors Voltage , Axial components; , These are the currents flowing from the wind farm into the flexible DC system. , Axial components; , These are the currents flowing through the phase reactors, respectively. , Axial components; , These are the output voltages of the flexible DC rectifier. , Axial components; DC capacitor for flexible DC system The voltage on; For the DC side input current of the flexible DC system; For the switching power loss of the flexible DC rectifier; L c The equivalent inductance of the AC-side phase reactor of the flexible DC-DC rectifier; The affine nonlinear system form of the dynamic mathematical model of the wind turbine grid-side converter is as follows: In the formula, Control input variables , The grid-side converter of the wind turbine controls the DC voltage and Shaft current, controlling output variable ; and The specific expression of the above is as follows: ； The affine nonlinear system form of the dynamic mathematical model of the flexible DC rectifier is as follows: In the formula, , control input variable , , flexible DC rectifier control , shaft AC voltage, control output variable ; and The specific expression is as follows: ； 。 5. The method of claim 4, wherein the method is characterized by: The zero-dynamic design method is used to divide the state variables of the wind turbine grid-side converter and flexible DC rectifier after coordinate transformation into external state variables and internal state variables, including: The state variables of the wind turbine grid-side converter and flexible DC rectifier system after coordinate transformation are divided into two parts: In the formula, , These respectively represent the wind turbine grid-side converter and flexible DC rectifier system regarding... , coordinate transformation function; , These represent the new state variables of the wind turbine grid-side converter and flexible DC rectifier system after coordinate transformation, respectively; the subscript o indicates the external state variable, and its dimension is equal to the system relation degree; the subscript i indicates the internal state variable, and its dimension is equal to the system dimension minus the system relation degree. The output variables of the wind turbine grid-side converter and the flexible DC rectifier are respectively and External state variables of wind turbine grid-side converters and flexible DC rectifiers They are respectively: The internal state variables of the wind turbine grid-side converter and flexible DC rectifier They are respectively: ； The nonlinear inverse controller takes the form of: 。 6. The method for suppressing subsynchronous oscillations in a grid-connected wind power system based on nonlinear inverse sliding mode control according to claim 5, characterized in that, The nonlinear state feedback is: ； The linear system takes the form of: In the formula, These are the pre-controlled variables of the external system after coordinate transformation of the wind turbine grid-side converter; The coordinate transformation of the external system is the pre-controlled variable of the flexible DC rectifier.

7. The method for suppressing subsynchronous oscillations in a grid-connected wind power system based on nonlinear inverse sliding mode control according to claim 6, characterized in that, Based on the nonlinear state feedback, the nonlinear inverse controller control laws for the wind turbine grid-side converter and the flexible DC rectifier can be determined as follows: In the formula: , The dq-axis modulated wave signal of the wind turbine grid-side converter under a nonlinear inverse controller; , The dq-axis modulated wave signal of the flexible DC rectifier under a nonlinear inverse controller; Based on the above nonlinear inverse controller control law, the pre-control variable in the nonlinear inverse controller control law is determined as follows: In the formula: all values ​​marked with ref are given values ​​of the relevant variables; , For the proportional and integral coefficients of the DC voltage control of the grid-side converter of the wind turbine; , For wind turbine grid-side converters Shaft current control proportional and integral coefficients; , For flexible DC rectifier positioning Shaft voltage proportional and integral coefficients; , For flexible DC rectifier positioning Shaft voltage proportionality and integral coefficient.

8. The method for suppressing subsynchronous oscillations in a grid-connected wind power system based on nonlinear inverse sliding mode control according to claim 7, characterized in that, The design of the sliding mode surface and switching control law on the nonlinear inverse controller, which take into account amplitude limiting, includes: Soft-limiting is applied to the setpoints of DC voltage and converter output current: In the formula, For the safe operating range of the DC bus, This is the rated current of the converter; For the wind turbine-side converter, a linear sliding surface is selected: Based on the linear sliding surface, using a sliding control law combining the constant velocity reaching law and the saturation function, we obtain: In the formula, constant This indicates that the system's motion point approaches the switching surface. speed; The expression for the saturation function is: In the formula, The neighborhood is the boundary layer of the sliding mode switching surface; Based on the linear sliding surface and sliding mode control law, when designing the pre-control variables in the nonlinear inverse controller of the wind turbine grid-side converter using sliding mode control, the pre-control variables can be obtained as follows: Based on the linear sliding surface and sliding mode control law, when designing the pre-control variables in the nonlinear inverse controller of the flexible DC rectifier using sliding mode control, the pre-control variables can be obtained as follows: In the formula, constant , The coefficients of the constant velocity reaching law for sliding mode control of flexible DC rectifiers, and the sliding surface. , ; Combining the aforementioned nonlinear inverse controller and sliding mode control, the equivalent control law of the nonlinear inverse sliding mode control can be obtained as follows: 。 9. A subsynchronous oscillation suppression device for a grid-connected wind power system based on nonlinear inverse sliding mode control, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, When the processor executes the computer program, it implements the steps of the subsynchronous oscillation suppression method for a grid-connected wind power system based on nonlinear inverse sliding mode control as described in any one of claims 1 to 8.

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