Grid-connected inverter synchronization stability improvement method based on backstepping method

Abstract

This invention discloses a method for improving the synchronization stability of a grid-connected inverter based on backstepping, comprising: setting a low-pass filter in the active power-frequency droop control loop of the grid-connected inverter; establishing a corresponding state model based on the dynamic characteristics of the low-pass filter; constructing intermediate state variables according to the state model; and recursively deriving the control input expression based on the intermediate state variables using the backstepping control method to obtain a control law that satisfies the Lyapunov stability condition; introducing the control law into the active power-frequency droop control loop to adjust the output frequency of the grid-connected inverter, thereby optimizing the active power-frequency droop control strategy of the grid-connected inverter. This invention only requires superimposing the designed controller on the synchronization loop to effectively enhance the dynamic stability of the system and avoid instability and oscillations caused by factors such as temperature drift and component aging.

Description

Technical Field

[0001] This invention relates to the field of grid-connected control of new energy sources, and specifically to a method for improving the synchronous stability of grid-connected inverters based on the backstepping method. Background Technology

[0002] With the increasing severity of global climate change and the depletion of fossil fuels, large-scale grid integration of new energy sources has become an inevitable trend. In this process, grid-connected inverters, due to their ability to actively construct grid voltage and frequency and provide necessary inertia and damping support for the system, are increasingly becoming key in the field of new energy power generation.

[0003] Grid-connected inverters typically employ droop control, autonomously regulating frequency and voltage and distributing power by simulating the operating characteristics of a synchronous generator. However, when the inverter's output active power exceeds a set value, the controller increases the output voltage frequency to reduce power, thereby decreasing the power angle and causing the output power to drop. Under low grid impedance conditions, any minute voltage disturbance will be amplified into a significant current change. The negative impedance characteristic combined with the low impedance network forms a positive feedback loop: a small disturbance triggers a sharp current fluctuation, which in turn affects the voltage; the voltage change, in turn, further amplifies the current change through negative impedance control, and so on, easily leading to voltage and current oscillations, or even system instability.

[0004] To address the instability issue of grid-connected inverters in low-impedance environments, common methods for improving stability mainly include:

[0005] (1) Virtual impedance control: A virtual impedance element is introduced into the control loop. The voltage reference value is corrected by the voltage drop generated by the output current on the virtual impedance, thereby reshaping the output impedance characteristics of the inverter and enhancing its damping in weak grids or specific frequency bands. However, the value of the virtual impedance needs to be balanced between stability and dynamic performance, and may bring a certain output voltage loss.

[0006] (2) Improved virtual synchronous machine control: When non-ideal operating conditions such as grid faults are detected, parameters such as virtual inertia and virtual damping are dynamically adjusted, and current limiting measures are combined to suppress power oscillations and overcurrent. However, the increase of virtual parameters will affect the dynamic response speed of the system, and the control effect largely depends on the accurate detection of the grid status.

[0007] (3) Intelligent adaptive control: Based on an improved deep learning network, a mapping relationship between grid impedance changes and control parameters is established to achieve online parameter self-tuning, enabling the system to remain stable under different grid strengths. However, this method has a relatively complex algorithm, requires a large amount of offline data to train the model, and faces challenges in actual deployment due to high real-time requirements and high computational consumption.

[0008] Furthermore, this includes methods that apply backstepping to inverter control. When applying backstepping to inverter control, calculations and designs are typically performed directly based on the main circuit model. This often leads to a high-order nonlinear system, placing extremely high demands on the microprocessor's computing power and making real-time control difficult. In addition, the performance of backstepping controllers heavily depends on the accuracy of the mathematical model. Factors such as component aging, parameter drift, and losses in the inverter system can all affect the model's matching degree, thus threatening system stability. Moreover, controllers often need to be redesigned for inverter systems with different parameters, resulting in poor versatility.

[0009] Therefore, in order to solve the problems of the traditional backstepping method's strong dependence on accurate mathematical models and computational complexity, a method for improving the synchronization stability of grid-connected inverters based on the backstepping method is needed. The dynamic stability of the system can be effectively enhanced by simply superimposing the designed controller on the synchronization loop, thus avoiding instability and oscillation caused by factors such as temperature drift and component aging. Summary of the Invention

[0010] In view of this, the purpose of this invention is to overcome the defects in the prior art and provide a method for improving the synchronous stability of grid-connected inverters based on the backstepping method. The dynamic stability of the system can be effectively enhanced by simply superimposing the designed controller on the synchronization loop, thus avoiding instability and oscillation caused by factors such as temperature drift and component aging.

[0011] The present invention provides a method for improving the synchronization stability of grid-connected inverters based on the backstepping method, comprising:

[0012] A low-pass filter is set in the active power-frequency droop control loop of the grid-connected inverter, and a corresponding state model is established based on the dynamic characteristics of the low-pass filter.

[0013] Based on the state model, intermediate state variables are constructed, and based on the intermediate state variables, the control input expression is recursively derived using the backstepping control method to obtain the control law that satisfies the Lyapunov stability condition.

[0014] By introducing a control law into the active power-frequency droop control loop, the output frequency of the grid-connected inverter is adjusted, thereby optimizing the active power-frequency droop control strategy of the grid-connected inverter.

[0015] Furthermore, the state model is determined according to the following formula:

[0016] ;

[0017] in, This represents the phase angle difference between the inverter and the power grid. It is a time variable; The inverter frequency; The power grid frequency; This is the cutoff frequency of the low-pass filter; This is the droop coefficient; This is a reference value for active power. This represents the actual active power.

[0018] Furthermore, the intermediate state variables include the phase angle difference between the inverter and the power grid. Differential with respect to time And the difference between the inverter frequency and the grid frequency Differential with respect to time .

[0019] Furthermore, by recursively deriving the control input expression using the backstepping control method, a control law satisfying the Lyapunov stability condition is obtained, specifically including:

[0020] Set error variable : ;in, This is a reference value for the phase angle difference between the inverter and the power grid;

[0021] right Differentiate: ;

[0022] Take the first-order Lyapunov function: (1)

[0023] Differentiate equation (1): ;

[0024] According to Lyapunov stability theory, only when The system is stable when the derivative is less than zero;

[0025] make: ;in, For any number greater than zero, if equal If so, the system is stable;

[0026] Set error variable : ;

[0027] right Differentiate: (2)

[0028] Will and Substituting the expression into equation (2), we get:

[0029] ;

[0030] Take the second-order Lyapunov function: ;

[0031] right Differentiate: (3)

[0032] Will and Substituting the expression into equation (3), we get:

[0033] ;

[0034] make: ;in, It is any constant greater than zero;

[0035] Finally, the control law is obtained:

[0036] .

[0037] Furthermore, the control law is introduced into the active power-frequency droop control loop, specifically including:

[0038] The actual active power output by the control law As the actual power reference quantity in active power-frequency droop control.

[0039] The beneficial effects of this invention are as follows: This invention discloses a method for improving the synchronization stability of grid-connected inverters based on the backstepping method. By introducing a mathematical model of the active power-frequency droop link into the control design process and deriving an additional controller (control law) for the synchronization loop based on Lyapunov's stability theorem, the dynamic characteristics of the system are significantly improved while maintaining its overall structure. This method can maintain system stability even under operating conditions with high grid strength and low impedance, effectively suppressing frequency jumps and reducing the risk of instability. Attached Figure Description

[0040] The present invention will be further described below with reference to the accompanying drawings and embodiments:

[0041] Figure 1 This is a schematic diagram of a traditional power-frequency droop loop control.

[0042] Figure 2 A schematic diagram of the power-frequency droop loop control after adding the control law of this invention;

[0043] Figure 3 The voltage, current, and frequency waveforms at the grid connection point are shown in the case of a strong grid without the addition of the control law of this invention.

[0044] Figure 4 The voltage, current, and frequency waveforms at the grid connection point are shown in the case of a strong grid, as added to the control law of this invention.

[0045] Figure 5A schematic diagram of the current distortion rate at the grid connection point when the power grid is a strong grid without adding the control law of this invention;

[0046] Figure 6 A schematic diagram of the current distortion rate at the grid connection point in the case of a strong grid where the control law of this invention has been added. Detailed Implementation

[0047] The present invention will be further described below with reference to the accompanying drawings, as shown in the figures:

[0048] This embodiment discloses a method for improving the synchronization stability of grid-connected inverters based on the backstepping method, including the following steps:

[0049] S1. Set a low-pass filter in the active power-frequency droop control loop of the grid-connected inverter, and establish the corresponding state model based on the dynamic characteristics of the low-pass filter.

[0050] S2. Construct intermediate state variables based on the state model, and based on the intermediate state variables, recursively derive the control input expression using the backstepping control method to obtain the control law that satisfies the Lyapunov stability condition;

[0051] S3. Introduce the control law into the active power-frequency droop control loop to adjust the output frequency of the grid-connected inverter, thereby optimizing the active power-frequency droop control strategy of the grid-connected inverter.

[0052] This invention significantly improves the dynamic characteristics of a grid-connected inverter by using a backstepping method to design the active power-frequency droop control loop, while maintaining the overall structure. This method is simple in structure, easy to implement, and does not increase additional hardware consumption.

[0053] In this embodiment, in step S1, a low-pass filter is introduced into the traditional active power-frequency droop control loop of the grid-connected inverter. After adding the filter, a second-order differential equation can be obtained, and this second-order differential equation is used as the corresponding state model. The second-order differential equation is:

[0054] ;

[0055] in, This represents the phase angle difference between the inverter and the power grid. It is a time variable; The inverter frequency; The power grid frequency; This is the cutoff frequency of the low-pass filter; This is the droop coefficient; This is a reference value for active power. This represents the actual active power.

[0056] From the above second-order differential equation, the state-space equation can be obtained:

[0057] ;

[0058] In the above state-space equations, let For the external input of the final control law, , They are respectively The derivative and The derivative; that is, the intermediate state variables include the phase angle difference between the inverter and the power grid. Differential with respect to time And the difference between the inverter frequency and the grid frequency Differential with respect to time .

[0059] In this embodiment, step S2 involves recursively deriving the control input expression using the backstepping control method to obtain a control law that satisfies the Lyapunov stability condition. Specifically, this includes:

[0060] Set error variable : ;in, This is a reference value for the phase angle difference between the inverter and the power grid;

[0061] right Differentiate: ;

[0062] Take the first-order Lyapunov function: (1)

[0063] Differentiate equation (1): ;

[0064] According to Lyapunov stability theory, only when When the derivative is less than zero, the system is stable; where the system can be understood as a closed-loop control system based on the grid-connected inverter and its active power-frequency droop control loop.

[0065] make: ;in, For any number greater than zero, if equal If so, the system is stable;

[0066] Set error variable : ;

[0067] right Differentiate: (2)

[0068] Will and Substituting the expression into equation (2), we get:

[0069] ;

[0070] Take the second-order Lyapunov function: ;

[0071] right Differentiate: (3)

[0072] Will and Substituting the expression into equation (3), we get:

[0073] ;

[0074] make: ;in, It is any constant greater than zero;

[0075] Finally, the control law is obtained:

[0076] .

[0077] In this embodiment, step S3, introducing the control law into the active power-frequency droop control loop, specifically includes:

[0078] The actual active power output by the control law This serves as the actual power reference in active power-frequency droop control. In other words, it refers to the actual active power output by the control law constructed in this invention. Instead of the actual power reference in traditional droop control, the control law constructed in this invention introduces the frequency deviation and the phase angle deviation obtained by its integration in the active power-frequency droop control loop.

[0079] To better understand the control law of this invention, the following further explanation is provided:

[0080] The parameters of a certain grid-connected inverter system were collected, as shown in Table 1.

[0081] Table 1

[0082]

[0083] Substitute the data in Table 1 into the control law calculation formula constructed in this invention. To simplify the calculation, and If all values ​​are set to 1, then we get:

[0084] .

[0085] The location where the control law is added is as follows: Figure 2 As shown, the output of the control law (actual active power) ) replaced Figure 1 The actual power reference value in traditional droop control is used to form a closed-loop control system.

[0086] Set the grid impedance to 0.001mH, and then... Figure 3 It can be seen that, under this impedance, without the control law of this invention, the current frequency at the grid connection point is already completely unstable. However, when the control law of this invention is applied, as... Figure 4 As shown, the current frequency has stabilized. Furthermore, Figure 5 This is a schematic diagram showing the grid current distortion rate without the control law of this invention. Figure 6 This diagram illustrates the grid-connected current distortion rate after incorporating the control law of this invention, at which point the grid connection conditions have been met. Here, THD refers to Total Harmonic Distortion, an indicator that measures the harmonic content in current or voltage relative to the fundamental frequency.

[0087] This invention significantly improves the stability of the power grid under low impedance by adding a control law based on backstepping to the traditional power-frequency droop loop. This method does not require extensive modification to the original control structure, is simple to implement, and improves the stability of the inverter. It is a safe, stable, and worthwhile technical solution to promote.

[0088] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for improving the synchronization stability of grid-connected inverters based on backstepping, characterized in that: Includes the following steps: A low-pass filter is set in the active power-frequency droop control loop of the grid-connected inverter, and a corresponding state model is established based on the dynamic characteristics of the low-pass filter. Based on the state model, intermediate state variables are constructed, and based on the intermediate state variables, the control input expression is recursively derived using the backstepping control method to obtain the control law that satisfies the Lyapunov stability condition. By introducing a control law into the active power-frequency droop control loop, the output frequency of the grid-connected inverter is adjusted, thereby optimizing the active power-frequency droop control strategy of the grid-connected inverter.

2. The method for improving the synchronization stability of grid-connected inverters based on the backstepping method according to claim 1, characterized in that: The state model is determined using the following formula: ； in, This represents the phase angle difference between the inverter and the power grid. It is a time variable; The inverter frequency; The power grid frequency; This is the cutoff frequency of the low-pass filter; This is the droop coefficient; This is a reference value for active power. This represents the actual active power.

3. The method for improving the synchronization stability of grid-connected inverters based on the backstepping method according to claim 1, characterized in that: The intermediate state variables include the phase angle difference between the inverter and the power grid. Differential with respect to time And the difference between the inverter frequency and the grid frequency Differential with respect to time .

4. The method for improving the synchronization stability of grid-connected inverters based on the backstepping method according to claim 1, characterized in that: By recursively deriving the control input expression using the backstepping control method, a control law satisfying the Lyapunov stability condition is obtained, specifically including: Set error variable : ;in, This is a reference value for the phase angle difference between the inverter and the power grid; right Differentiate: ; Take the first-order Lyapunov function: (1) Differentiate equation (1): ; According to Lyapunov stability theory, only when The system is stable when the derivative is less than zero; make: ;in, For any number greater than zero, if equal If so, the system is stable; Set error variable : ; right Differentiate: (2) Will and Substituting the expression into equation (2), we get: ； Take the second-order Lyapunov function: ; right Differentiate: (3) Will and Substituting the expression into equation (3), we get: ； make: ;in, It is any constant greater than zero; Finally, the control law is obtained: 。 5. The method for improving the synchronization stability of grid-connected inverters based on the backstepping method according to claim 1, characterized in that: The control law is introduced into the active power-frequency droop control loop, specifically including: The actual active power output by the control law As the actual power reference quantity in active power-frequency droop control.

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