Grid-Connected Pre-Synchronization Control Method and System for Grid-Forming Converters Based on Second-Order LADRC

Abstract

The present invention discloses a grid-forming converter grid connection pre-synchronization control method and system based on a second-order LADRC. The method includes: deriving a controlled object according to the active power loop of the VSG; performing an inverse Laplace transform on the controlled object; designing a second-order LADRC, including a linear extended observer and a linear state error feedback controller; using the bandwidth of the linear extended observer and the bandwidth of the linear state error feedback controller as adjustment factors for control adjustment to achieve grid connection pre-synchronization of the grid-forming converter. The control strategy based on second-order LADRC pre-synchronization proposed by the present invention has the advantages of fast synchronization speed, small overshoot of voltage and power, and less dependence on system parameters, and can meet the functional requirements of different working scenarios.

Description

Technical Field

[0001] The present invention belongs to the field of power grids, and relates to grid-forming converter grid-connection pre-synchronization control, specifically to a grid-forming converter grid-connection pre-synchronization control method and system based on a second-order LADRC. Background Art

[0002] As an interface for distributed microgrids to access the power grid, the grid-connection control strategy of grid-forming converters is directly related to the stable operation of the power grid. Currently, grid-forming converters (GFMs) mainly use virtual synchronous generator control (VSG) to provide inertia and damping for the power grid. However, when distributed microgrids are directly switched on and connected to the grid, it will cause a large voltage impact on the power grid. Currently, the main pre-synchronization controls are pre-synchronization based on a phase-locked loop and pre-synchronization control methods based on coordinate transformation.

[0003] The pre-synchronization control based on a phase-locked loop obtains the voltage difference and phase angle difference between the grid side and the converter outlet, and sends them to the active power loop and reactive power loop controllers of the VSG for secondary regulation. However, it takes a certain amount of time for the phase-locked loop to start and achieve stability, which affects the pre-synchronization speed of the system.

[0004] The pre-synchronization control based on coordinate transformation can achieve zero-error tracking of the grid voltage through a PI controller, which can avoid the cumbersome and adverse effects of the phase-locked loop. However, it has a high dependence on system parameters, and the calculation amount is large and requires a high-performance digital processor to implement, increasing the hardware cost.

[0005] Therefore, in the pre-synchronization control of existing converters, the two requirements of "low dependence on system parameters" and "fast synchronization speed" cannot be achieved simultaneously.

[0006] Therefore, a new technical solution is needed to solve this problem. Summary of the Invention

[0007] Object of the Invention: In order to overcome the deficiencies in the prior art, the present invention provides a grid-forming converter grid-connection pre-synchronization control method and system based on a second-order LADRC, which solves the problems of high dependence on system parameters and slow synchronization speed existing in the existing pre-synchronization control.

[0008] Technical Solution: To achieve the above object, the present invention provides a grid-forming converter grid-connection pre-synchronization control method based on a second-order LADRC, including the following steps:

[0009] S1: Derive the controlled object according to the active power loop of the VSG;

[0010] S2: Perform Laplace inverse transformation on the controlled object;

[0011] S3: Design a second-order LADRC, including a linear extended state observer and a linear state error feedback controller;

[0012] The linear extended state observer is used to observe the output of the controlled system and the total disturbance, and provide them to the linear state error feedback controller;

[0013] The linear state error feedback controller is used to control the input of the controlled system according to the system reference value and the observed data provided by the linear extended state observer, and feedback the error data to the linear extended state observer;

[0014] S4: Take the bandwidth of the linear extended state observer and the bandwidth of the linear state error feedback controller as the adjustment factors for control adjustment to achieve pre-synchronization of the grid-connected converter for grid connection.

[0015] Furthermore, the expression of the controlled object in step S1 is:

[0016]

[0017] where represents the current system angular frequency, is the difference between the current angular frequency and the reference angular frequency, is the rated angular frequency, represents the moment of inertia, represents the damping coefficient, E is the grid-side voltage, U is the voltage at the converter outlet side, X is the impedance between the converter and the grid side; s is the complex frequency component obtained through Laplace transform; K p is the droop coefficient of the active power loop; is the input output transfer function; is the variable value.

[0018] Furthermore, in step S2, perform the inverse Laplace transform on the controlled object to obtain:

[0019]

[0020] where t is the time; thus, convert the s-domain to the time-domain, proving that it is applicable to second-order active disturbance rejection control.

[0021] Furthermore, the linear extended state observer in step S3 includes , and three observation points and , and three gain coefficients, is used to observe the output y of the system, For observing the differential of the output y, For observing the total disturbance of the system, 、 and Three gain coefficients are used to perform real-time estimation and compensation on the uncertain dynamics of the system.

[0022] Furthermore, the expression of the linear extended observer in the step S3 is as follows:

[0023] Write the controlled object as ; Define as the total disturbance of the system, where u and y are the input and output of the system respectively, is the derivative of the output y, w represents the external disturbance, represents the damping characteristic of the control system for the change rate of the output y, reflects the recovery characteristic of the output y tending to the equilibrium state, b is the gain coefficient of the input and the influence on the system state, is its estimated value;

[0024] Define the following state variables as:

[0025]

[0026] Among them, represents the output y, represents the derivative of the output y, represents the total disturbance of the system, represents the derivative of the total disturbance of the system;

[0027] The corresponding linear extended state observer is:

[0028]

[0029] Among them, is the combined input of the LESO, is the observer error feedback gain matrix to be designed, , , ; z represents the observed value of x, represents the derivative of z, represents the observed value of the output y

[0030] Furthermore, the linear state error feedback controller in the step S3 uses TD control to observe the state differential signal, and the TD control law is:

[0031]

[0032] Among them, 、 are the proportional gain coefficient and the differential gain coefficient of the linear state error feedback controller, respectively, and v is the reference value given by the system.

[0033] Further, the bandwidth of the linear state error feedback controller in the step S4 The design and operation include:

[0034] When the system is stable, ; obtain the closed-loop transfer function of the input and output:

[0035]

[0036] wherein, represents the output, represents the quantity after the reference input r undergoes the Laplace transform;

[0037] The pole configuration of the linear state error feedback controller is designed through , is reasonably selected, and by selecting an appropriate pole configuration, the system is made stable.

[0038] Further, the bandwidth of the linear extended observer in the step S4 The design and operation include:

[0039]

[0040] wherein, represents the characteristic polynomial of the above formula; I represents the identity matrix;

[0041] After parameterization, the poles of the linear extended observer are configured on the bandwidth , then there is:

[0042]

[0043] In the present invention, the bandwidth of the linear state error feedback controller and the bandwidth of the linear extended observer are used as adjustment factors for control and adjustment in the following ways:

[0044] One is to make the system quickly converge to the desired state by adjusting the bandwidth , avoid voltage overshoot, and improve the stability of the system.

[0045] The second is to quickly capture the dynamic changes of the system through the bandwidth , timely track unknown disturbances, accelerate the pre-synchronization speed of the microgrid, and reduce the dependence on system parameters.

[0046] The third is to adjust , , to optimize the control performance.

[0047] The present invention also provides a grid-connected pre-synchronization control system for a network-forming converter based on a second-order LADRC, including:

[0048] A controlled object establishment module for deriving the controlled object and performing Laplace inverse transformation;

[0049] An LADRC design module for designing a second-order LADRC, including a linear extended state observer and a linear state error feedback controller;

[0050] A pre-synchronization module for using the bandwidth of the linear extended state observer and the bandwidth of the linear state error feedback controller as adjustment factors for control adjustment to achieve grid-connected pre-synchronization of the network-forming converter.

[0051] The control strategy based on second-order LADRC pre-synchronization proposed by the present invention has the advantages of fast synchronization speed, small voltage and power overshoot, and less dependence on system parameters, and can meet the functional requirements of different working scenarios.

[0052] Beneficial effects: Compared with the prior art, the present invention has the following advantages:

[0053] 1. By designing an extended state observer, the present invention can estimate and compensate the unknown disturbances and uncertain dynamics of the system in real time, and has higher robustness during operation.

[0054] 2. The method of the present invention can directly control the input and output of the controlled system without complex parameter tuning, reduces the deterioration of control performance caused by inaccurate parameters, and enhances the reliability of system operation.

[0055] 3. The present invention has a fast state observation and feedback mechanism and can complete the pre-synchronization process of a distributed microgrid in an extremely short time.

[0056] 4. The pre-synchronization control based on LADRC in the present invention relies on an adaptive parameter adjustment mechanism and does not require data transmission through communication. Therefore, when the converter is connected to the grid, the output voltage and frequency can quickly synchronize with the grid. Description of the Drawings

[0057] Figure 1 is a schematic diagram of a pre-synchronization control strategy based on LADRC;

[0058] Figure 2 is a control block diagram of a second-order LADRC;

[0059] Figure 3 is a single-unit distributed microgrid grid-connected simulation system;

[0060] Figure 4 Active power diagram of the converter under different control strategies for operating condition 1;

[0061] Figure 5 Reactive power diagram of the converter under different control strategies for operating condition 1;

[0062] Figure 6 Converter voltage data diagram under different control strategies for operating condition 1;

[0063] Figure 7 Timing diagram of different strategies meeting the pre-synchronization control for operating condition 1;

[0064] Figure 8 Active power diagram of the converter under different control strategies for operating condition 2;

[0065] Figure 9 Reactive power diagram of the converter under different control strategies for operating condition 2;

[0066] Figure 10 Converter voltage data diagram under different control strategies for operating condition 2;

[0067] Figure 11 Timing diagram of different strategies meeting the pre-synchronization control for operating condition 2. Specific implementation manner

[0068] The following further clarifies the present invention in conjunction with the accompanying drawings and specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. After reading the present invention, various equivalent modifications of the present invention by those skilled in the art fall within the scope defined by the appended claims of this application.

[0069] Embodiment 1:

[0070] This embodiment provides a grid-connected pre-synchronization control method for a grid-forming converter based on a second-order LADRC, including the following steps:

[0071] S1: Refer to Figure 1 , according to the active power loop of the VSG, the controlled object can be derived according to Mason's formula, and its expression is:

[0072]

[0073] Wherein, represents the current system angular frequency, is the difference between the current angular frequency and the reference angular frequency, is the rated angular frequency, represents the moment of inertia, represents the damping coefficient, E is the grid-side voltage, U is the converter outlet-side voltage, and X is the impedance between the converter and the grid side; s is the complex frequency component obtained through Laplace transform; K p is the active loop droop coefficient; is the input output transfer function; is the variable value; represents integration, is the converter electromagnetic power, is the rated frequency, is the reference frequency, and pi represents the pi parameter;

[0074] S2: Perform the inverse Laplace transform on the controlled object to obtain:

[0075]

[0076] where t is time; thus, the s-domain is converted to the time-domain, proving that it is applicable to second-order active disturbance rejection control.

[0077] S3: Design a second-order LADRC, including a Linear Extended State Observer (LESO) and a Linear State Error Feedback (LSEF) controller;

[0078] The linear extended state observer is used to observe the output of the controlled system and the total disturbance, and provide it to the linear state error feedback controller;

[0079] The linear state error feedback controller is used to control the input of the controlled system according to the system reference value and the observation data provided by the linear extended state observer, and feedback the error data to the linear extended state observer;

[0080] As Figure 2 shown, f represents the external disturbance, and G(s) represents the transfer function; the linear extended state observer LESO includes 、 and three observation points and 、 and three gain coefficients, is used to observe the output y of the system, is used to observe the differential of the output y, is used to observe the total disturbance of the system, 、 and three gain coefficients are used to perform real-time estimation and compensation on the uncertain dynamics of the system;

[0081] The expression of the linear extended observer is as follows:

[0082] The controlled object is written as ; Define as the total disturbance of the system, where u and y are the input and output of the system respectively, is the derivative of the output y, w represents the external disturbance, represents the damping characteristic of the control system to the change rate of the output y, reflects the recovery characteristic of the output y tending to the equilibrium state, b is the gain coefficient of the input and its influence on the system state, is its estimated value.

[0083] Define the following state variables as:

[0084]

[0085] where, represents the output y, represents the derivative of the output y, represents the total disturbance of the system, represents the derivative of the total disturbance of the system;

[0086]

[0087] where, , , , .

[0088] The corresponding linear extended state observer is:

[0089]

[0090] where, is the combined input of the LESO, is the observer error feedback gain matrix to be designed, z represents the observed value of x, represents the derivative of z, represents the observed value of the output y.

[0091] The linear state error feedback controller uses TD control to observe the state differential signal, and the TD control law is:

[0092]

[0093] where, , are the proportional gain coefficient and the differential gain coefficient of the linear state error feedback controller respectively, and v is the reference value given by the system.

[0094] S4: Take the bandwidth of the linear extended observer and the bandwidth of the linear state error feedback controller as adjustment factors for control adjustment to achieve pre-synchronization of the grid-forming converter for grid connection.

[0095] The design and operation of the bandwidth of the linear state error feedback controller include:

[0096] When the system is stable, ; obtain the closed-loop transfer function of the system input and output:

[0097]

[0098] where, represents the output, represents the quantity after the reference input r undergoes Laplace transformation;

[0099] Design the pole placement of the linear state error feedback controller through , for reasonable selection, and make the system stable by selecting appropriate pole placement.

[0100] The design and operation of the bandwidth of the linear extended observer include:

[0101] According to modern control theory, when the (A - LC) eigenvalues are less than 0, as time increases, the system error will converge to 0.

[0102]

[0103] where, represents the characteristic polynomial of the above formula; I represents the identity matrix;

[0104] After parameterization, place the poles of the linear extended observer on the bandwidth , then there is:

[0105]

[0106] The bandwidth of the linear state error feedback controller and the bandwidth of the linear extended observer The ways of using them as adjustment factors for control adjustment include:

[0107] One is to adjust the bandwidth to make the system quickly converge to the desired state, avoid voltage overshoot, and improve the stability of the system.

[0108] The second is to design a larger observer bandwidth Quickly capture the dynamic changes of the system, timely track unknown disturbances, accelerate the pre-synchronization speed of the microgrid, and reduce the dependence on system parameters.

[0109] Thirdly, adjust in real time according to different working conditions 、 , and optimize the control performance.

[0110] Embodiment 2:

[0111] Based on the pre-synchronization control method provided in Embodiment 1, this embodiment provides a grid-forming converter grid-connection pre-synchronization control system based on a second-order LADRC, including:

[0112] A controlled object establishment module, which is used to deduce the controlled object and perform Laplace inverse transformation;

[0113] An LADRC design module, which is used to design a second-order LADRC, including a linear extended observer and a linear state error feedback controller;

[0114] A pre-synchronization module, which is used to use the bandwidth of the linear extended observer and the bandwidth of the linear state error feedback controller as adjustment factors for control adjustment to achieve grid-connection pre-synchronization of the grid-forming converter.

[0115] Embodiment 3:

[0116] In order to verify the effectiveness and effect of the solution of the present invention, the following simulation experiments are carried out:

[0117] As Figure 3 shown, a single-unit distributed microgrid grid-connection simulation system is built, where represents the converter-side impedance, is the voltage at the point of common coupling; Xv is the impedance between the converter and the grid side, P V is the active power transmitted from the converter to the grid, Qv is the reactive power transmitted from the converter to the grid, is the grid-side voltage, is the grid-side impedance, VSG represents the virtual synchronous generator; Grid represents the grid; S g is the grid-connection closing switch;

[0118] The parameter settings during its simulation are shown in Table 1, where represents the converter-side capacitor.

[0119] Table 1 Simulation parameters

[0120]

[0121] Operating condition 1: When the distributed microgrid operates in island mode with a load of 5 kW / 0.5 kVar, at 0.2 s of system operation, the active power reference value increases by 10 kW and the reactive power reference value increases by 1 kVar. At 0.5 s, a pre-synchronization closing signal is given to the system. The simulation time of the entire system is 2 s, and the simulation waveforms are as shown in Figures 4 - 7 shown.

[0122] It can be seen that during the transition time of pre-synchronization closing at 0.5 s, the active power, reactive power, and voltage at the output of the converter based on the second-order LADRC are significantly smoother than those based on coordinate transformation during pre-synchronization, without large impacts. It can be seen from Figures 4 - 6 that the pre-synchronization speed based on the second-order LADRC is significantly faster than that based on coordinate transformation. Figure 7

[0123] Operating condition 2: When the distributed microgrid operates in island mode, at 0.2 s, the load suddenly increases by 5 kW. At 0.5 s, a pre-synchronization closing signal is given to the system. The active power reference value is 10 kW. The simulation time of the entire system is 2 s, and the simulation waveforms are as shown in Figures 8 - 11 shown.

[0124] Figures 8 - 10 It can be seen that the active power deviation of the pre-synchronization output based on the second-order LADRC is only 0.1%, while the active power deviation of the pre-synchronization output based on coordinate transformation is 2.37%. Moreover, both the power and voltage transitions based on the second-order LADRC are relatively smooth at the moment of closing. It can be seen from Figure 11 that the pre-synchronization control based on the second-order LADRC quickly meets the requirements of pre-synchronization grid connection.

[0125] In summary, it can be seen that the pre-synchronization control strategy based on the second-order LADRC proposed in the present invention not only shortens the grid connection closing time of the converter, reduces the voltage impact and power impact at the moment of closing, but also significantly reduces the influence of external disturbances on the system operation stability, reduces the dependence on system parameters, and provides a strong guarantee for the reliable grid connection of the distributed microgrid.​​

Claims

1. A grid-connected pre-synchronization control method for a grid-forming converter based on second-order LADRC, characterized in that, It includes the following steps: S1: Derive the controlled object according to the active power loop of the VSG; S2: Perform the inverse Laplace transform on the controlled object; S3: Design a second-order LADRC, including a linear extended state observer and a linear state error feedback controller; The linear extended state observer is used to observe the output and the total disturbance of the controlled system and provide them to the linear state error feedback controller; The linear state error feedback controller is used to control the input of the controlled system according to the system reference value and the observation data provided by the linear extended state observer, and feedback the error data to the linear extended state observer; S4: Use the bandwidth w0 of the linear extended observer and the bandwidth w of the linear state error feedback controller as adjustment factors for control adjustment to achieve pre-synchronization of the grid-forming converter for grid connection; c ​ The expression of the controlled object in step S1 is: Among them, ω represents the current system angular frequency, Δω is the difference between the current angular frequency and the reference angular frequency, ω n is the rated angular frequency, J represents the moment of inertia, D represents the damping coefficient, E is the grid-side voltage, U is the converter outlet-side voltage, X is the impedance between the converter and the grid side; s is the complex frequency component; K p is the active loop droop coefficient; G ω-Δω is the transfer function that inputs Δω and outputs ω; K D is the variable value; In step S2, perform the inverse Laplace transform on the controlled object to obtain: where t is time.

2. A grid-connected pre-synchronization control method for a grid-forming converter based on second-order LADRC according to claim 1, characterized in that, In step S3, the linear extended state observer includes three observation points z1, z2, and z3 and three gain coefficients β1, β2, and β3. z1 is used to observe the output y of the system, z2 is used to observe the differential of the output y, z3 is used to observe the total disturbance of the system, and the three gain coefficients β1, β2, and β3 are used to perform real-time estimation and compensation on the uncertain dynamics of the system.

3. A grid-connected pre-synchronization control method for a network-forming converter based on second-order LADRC according to claim 2, characterized in that, The expression of the linear extended state observer in step S3 is as follows: Write the controlled object as Let be defined as the total disturbance of the system, where u and y are the input and output of the system respectively, is the derivative of the output y, w represents the external disturbance, a1 represents the damping characteristic of the control system for the change rate of the output y, a2 reflects the recovery characteristic of the output y tending to the equilibrium state, b is the gain coefficient of the input and its influence on the system state, and b0 is its estimated value; Define the following state variables as: where x1 represents the output y, x2 represents the derivative of the output y, x3 represents the total disturbance of the system, and h represents the derivative of the total disturbance of the system; The corresponding linear extended state observer is: where, u c = [u y] T is the combined input of LESO, L = [β1 β2 β3] T is the observer error feedback gain matrix to be designed, B = [0 b 0] T , C = [0 0 1] T ; z represents the observed value of x, represents the derivative of z, represents the observed value of the output y.

4. A grid-connected pre-synchronization control method for a grid-forming converter based on second-order LADRC according to claim 3, characterized in that In step S3, the linear state error feedback controller uses TD control to observe the state differential signal, and the TD control law is: u0 = k p (v - z1) - k d z2 where k p and k d are the proportional gain coefficient and the derivative gain coefficient of the linear state error feedback controller respectively, and v is the reference value given by the system.

5. A grid-connected pre-synchronization control method for a grid-forming converter based on second-order LADRC according to claim 4, characterized in that The bandwidth w of the linear state error feedback controller in the step S4 c is designed and operated to include: When the system is stable, Find the closed-loop transfer function of the input and output: where y(s) represents the output, and r(s) represents the quantity of the reference input r after Laplace transformation; The pole placement of the designed linear state error feedback controller is achieved through k d = 2w c is selected, and the system is made stable by choosing the pole placement.

6. A grid-connected pre-synchronization control method for a network-forming converter based on second-order LADRC according to claim 5, characterized in that In step S4, the design and operation of the bandwidth w0 of the linear extended state observer include: λ(s) = |sI - (A - LC)| = s 3 + β1s 2 + β2s + β3 = (s + ω0) 3 where λ(s) represents the characteristic polynomial of the above formula; I represents the identity matrix; After parameterization, configure the poles of the linear extended state observer on the bandwidth w0, then there is:

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