Low-frequency oscillation suppression method based on adaptive virtual impedance

Abstract

The invention discloses a low-frequency oscillation suppression method based on self-adaptive virtual impedance, and belongs to the technical field of grid-connected inverter control. The method aims to solve the problem that low-frequency oscillation suppression and power loss cannot be considered in existing virtual synchronous control. The method comprises the following steps: sampling three-phase current of a grid-connected point of a grid-connected inverter to be subjected to low-frequency suppression, and converting the three-phase current into current under an alpha-beta two-phase static coordinate system; filtering through a notch filter, filtering a fundamental frequency current component, and extracting and calculating an effective value of a resonance current component; based on the effective value, adaptively adjusting a virtual resistance value, multiplying the adaptive virtual resistance value by the resonance current component to obtain a virtual voltage compensation amount, and subtracting the virtual voltage compensation amount from the three-phase modulation voltage signal before correction to obtain a corrected three-phase modulation signal; and inputting the corrected three-phase modulation signal into a PWM (Pulse Width Modulation) circuit to generate a driving signal to control the action of a switching device of the grid-connected inverter so as to suppress low-frequency oscillation. The method is mainly used for low-frequency oscillation suppression.

Description

Technical Field

[0001] This invention belongs to the field of grid-connected inverter control technology. Background Technology

[0002] With the rapid development of new energy power generation technologies, grid-connected inverters, as the core equipment connecting new energy power generation systems to the power grid, directly affect the safe and stable operation of the power grid. Virtual Synchronous Generator (VSG) strategies, which can simulate the inertia and damping characteristics of synchronous generators, effectively improve the grid-connected stability of new energy power generation systems and are widely used in grid-connected inverter control.

[0003] However, under virtual synchronous control strategies, the output impedance of the grid-connected inverter and the grid impedance tend to intersect at characteristic curves in the low-frequency region, leading to low-frequency oscillations in the system. In severe cases, this can disrupt the normal operation of the grid and reduce the system's stability margin. Traditional oscillation suppression methods often employ virtual impedance or damping control with fixed parameters. While these methods can alleviate oscillations to some extent, they struggle to balance suppression effectiveness with system power loss: excessively large fixed virtual impedances can lead to increased additional power loss and reduced system efficiency; excessively small virtual impedances cannot effectively suppress oscillations, resulting in limited stability improvement. Therefore, a low-frequency oscillation suppression method that can adaptively adjust parameters and balance suppression effectiveness with power loss is urgently needed. Summary of the Invention

[0004] This invention aims to address the problem in existing virtual synchronization control systems where low-frequency oscillation suppression and power loss cannot be simultaneously addressed. A low-frequency oscillation suppression method based on adaptive virtual impedance is provided.

[0005] The low-frequency oscillation suppression method based on adaptive virtual impedance described in this invention includes:

[0006] Step 1: Sample the three-phase current at the grid connection point of the grid-connected inverter to be suppressed for low frequency, and perform Clark transformation on the sampled three-phase current values ​​to obtain the current in the αβ two-phase stationary coordinate system.

[0007] Step 2: Use a notch filter to filter the current in the αβ two-phase stationary coordinate system, filter out the fundamental frequency current component, and extract the resonant current component. And calculate the resonant current components. Valid values;

[0008] Step 3: Effective value based on resonant current component Adaptive adjustment of virtual resistance value ,when >Preset threshold hour, ,when ≤ hour, ,in, This is a virtual resistance reference value. These are adaptive coefficients;

[0009] Step 4: Adjust the adaptive virtual resistance value With resonant current component Multiply the two to obtain the virtual voltage compensation amount. Subtract this virtual voltage compensation amount from the three-phase modulated voltage signal before correction to obtain the corrected three-phase modulated signal.

[0010] Step 5: Input the corrected three-phase modulation signal into the PWM circuit to generate a drive signal to control the switching devices of the grid-connected inverter, thereby suppressing low-frequency oscillations.

[0011] Furthermore, in this invention, in step one, the current in the αβ two-phase stationary coordinate system is:

[0012]

[0013] , , These represent the three-phase currents of the power grid. , Let α and β represent the currents in the stationary coordinate system of the two phases α and β, respectively.

[0014] Furthermore, in this invention, in step two, the characteristic frequency of the notch filter... It is equal to the angular frequency of the fundamental component of the grid connection point voltage.

[0015] Furthermore, in this invention, in step two, the mathematical expression for the notch filter is:

[0016]

[0017] in, For frequency trap width, It is the characteristic frequency, and The angular frequency of the fundamental component of the grid connection point voltage is equal to the frequency of the fundamental component, ensuring accurate filtering of the fundamental frequency component and extraction of the resonant current component i. h And calculate its effective value, where s represents the Laplace operator.

[0018] This invention reshapes the frequency response curve of the inverter's output impedance using a series virtual impedance, ensuring that the corrected output impedance phase frequency curve meets the stability condition of a phase angle greater than -90° at the intersection with the grid impedance. This significantly improves the system's stability margin and effectively suppresses low-frequency oscillations. Based on resonant current adaptive adjustment of the virtual resistance value, it avoids the additional power loss caused by a fixed large resistance. When the oscillation risk is high, the resistance is increased to enhance suppression; when stable, a reference resistance is used to reduce losses, balancing suppression effect with system efficiency. A notch filter is used to specifically extract the resonant current component, damping only the resonant frequency band without affecting the fundamental frequency component transmission, reducing fundamental frequency power loss and ensuring normal system power output. The control logic is clear, and the parameters are flexibly adjustable, adaptable to grid-connected inverters and grid environments with different parameters. It requires no major modification to existing hardware systems, making it easy to implement in engineering and for widespread application. Attached Figure Description

[0019] Figure 1 This is a flowchart of the method described in this invention;

[0020] Figure 2 The system equivalent circuit with virtual impedance;

[0021] Figure 3 Bode plot of the output impedance after series virtual impedance;

[0022] Figure 4 This is a block diagram illustrating the principle of adaptive series virtual impedance control.

[0023] Figure 5 This is the Bode plot of the notch filter. Detailed Implementation

[0024] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0025] Specific implementation method one: Refer to Figure 1 This embodiment specifically describes the low-frequency oscillation suppression method based on adaptive virtual impedance, which includes:

[0026] Step 1: Sample the three-phase current at the grid connection point of the grid-connected inverter to be suppressed for low frequency, and perform Clark transformation on the sampled three-phase current values ​​to obtain the current in the αβ two-phase stationary coordinate system.

[0027] Step 2: Use a notch filter to filter the current in the αβ two-phase stationary coordinate system, filter out the fundamental frequency current component, and extract the resonant current component. And calculate the resonant current components. Valid values;

[0028] Step 3: Effective value based on resonant current component Adaptive adjustment of virtual resistance value ,when >Preset threshold hour, ,when ≤ hour, ,in, This is a virtual resistance reference value. These are adaptive coefficients;

[0029] Step 4: Adjust the adaptive virtual resistance value With resonant current component Multiply the two to obtain the virtual voltage compensation amount. Subtract this virtual voltage compensation amount from the three-phase modulated voltage signal before correction to obtain the corrected three-phase modulated signal.

[0030] Step 5: Input the corrected three-phase modulation signal into the PWM circuit to generate a drive signal to control the switching devices of the grid-connected inverter, thereby suppressing low-frequency oscillations.

[0031] Furthermore, in this invention, in step one, the current in the αβ two-phase stationary coordinate system is:

[0032]

[0033] , , These represent the three-phase currents of the power grid. , Let α and β represent the currents in the stationary coordinate system of the two phases α and β, respectively.

[0034] Furthermore, in this invention, in step two, the characteristic frequency of the notch filter... It is equal to the angular frequency of the fundamental component of the grid connection point voltage;

[0035] Furthermore, in this invention, in step two, the mathematical expression for the notch filter is:

[0036]

[0037] in, For frequency trap width, It is the characteristic frequency, and The angular frequency of the fundamental component of the grid connection point voltage is equal to the frequency of the fundamental component, ensuring accurate filtering of the fundamental frequency component and extraction of the resonant current component i. h And calculate its effective value, where s represents the Laplace operator.

[0038] The embodiment takes a three-phase grid-connected inverter employing a virtual synchronous control strategy as the application object, combined with Figures 1 to 5 To explain, the power grid's fundamental frequency is 50Hz, corresponding to an angular frequency ω0 = 2π × 50 = 314 rad / s. The specific implementation steps are as follows:

[0039] 1. Current Sampling and Coordinate Transformation: The three-phase current at the grid connection point of the grid-connected inverter is sampled using a current sensor. , , The Clark transformation is used to convert it into current in the αβ coordinate system. , The transformation formula is:

[0040]

[0041] 2. Resonant current component extraction: Design a notch filter and set the frequency trap width. =10 rad / s, characteristic frequency =314 rad / s, its transfer function is .Will , The notch filter is input to filter out the 50Hz fundamental frequency component, resulting in the resonant current component. , Calculate its effective value .when > hour, ;when ≤ hour, ;

[0042] 3. Adaptive Virtual Resistance Calculation: Set the virtual resistance reference value. =5Ω, adaptive coefficient k=2Ω / A, resonant current threshold =0.5A. If the calculated value is... =0.8A>0.5A, then =5 + 2 × 0.8 = 6.6Ω; if =0.3A≤0.5A, then =5Ω.

[0043] 4. Virtual voltage compensation and modulation signal correction: ... respectively , Multiply by this to obtain the virtual voltage compensation amount. = , It is converted into a three-phase virtual voltage compensation quantity through inverse coordinate transformation. , , The three-phase modulation voltage before correction , , Subtract respectively , , The corrected modulation voltage is obtained. , , .

[0044] 5. PWM Drive and Oscillation Suppression: [This section likely refers to a specific feature or function, but without further context, it's difficult to translate accurately.] , , The input PWM module generates PWM drive signals to control the switching of the inverter's IGBT switching devices, adjusts the inverter's output voltage, reshapes the output impedance characteristics, and achieves low-frequency oscillation suppression.

[0045] Verification results:

[0046] Simulation verification of the method in this embodiment shows that without the addition of adaptive virtual impedance, the system exhibits significant oscillations in the low-frequency range (10-50Hz), with a phase margin of only 15°. After incorporating the method of this invention, the phase angle of the output impedance phase-frequency curve at the intersection with the grid impedance is increased to over 30°, the phase margin is significantly improved, the oscillation phenomenon is completely suppressed, the fundamental frequency power loss is reduced by 12%, and the system efficiency is increased to over 96%, verifying the effectiveness and superiority of this invention.

[0047] While the invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the invention. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that different dependent claims and features described herein can be combined in ways different from those described in the original claims. It is also understood that features described in conjunction with individual embodiments can be used in other described embodiments.

Claims

1. A low-frequency oscillation suppression method based on adaptive virtual impedance, characterized in that, include: Step 1: Sample the three-phase current at the grid connection point of the grid-connected inverter to be suppressed for low frequency, and perform Clark transformation on the sampled three-phase current values ​​to obtain the current in the αβ two-phase stationary coordinate system. Step 2: Use a notch filter to filter the current in the αβ two-phase stationary coordinate system, filter out the fundamental frequency current component, and extract the resonant current component. And calculate the resonant current components. Valid values; Step 3: Effective value based on resonant current component Adaptive adjustment of virtual resistance value ,when >Preset threshold hour, ,when ≤ hour, ,in, This is a virtual resistance reference value. These are adaptive coefficients; Step 4: Adjust the adaptive virtual resistance value With resonant current component Multiply by the two to obtain the virtual voltage compensation amount. Subtract this virtual voltage compensation amount from the three-phase modulated voltage signal before correction by the grid-connected inverter to obtain the corrected three-phase modulated signal. Step 5: Input the corrected three-phase modulation signal into the PWM circuit to generate a drive signal to control the switching devices of the grid-connected inverter, thereby suppressing low-frequency oscillations.

2. The low-frequency oscillation suppression method based on adaptive virtual impedance according to claim 1, characterized in that, In step one, the currents in the αβ two-phase stationary coordinate system are: , , These represent the three-phase currents of the power grid. , Let α and β represent the currents in the stationary coordinate system of the two phases α and β, respectively.

3. The low-frequency oscillation suppression method based on adaptive virtual impedance according to claim 1, characterized in that, In step two, the characteristic frequency of the notch filter... It is equal to the angular frequency of the fundamental component of the grid connection point voltage.

4. The low-frequency oscillation suppression method based on adaptive virtual impedance according to claim 1, characterized in that, In step two, the mathematical expression for the notch filter is: in, For frequency trap width, It is the characteristic frequency, and The angular frequency of the fundamental component of the grid connection point voltage is equal to the frequency of the fundamental component, ensuring accurate filtering of the fundamental frequency component and extraction of the resonant current component i. h And calculate its effective value, where s represents the Laplace operator.

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