A method for realizing low-frequency-band oscillation suppression of a grid-connected inverter based on voltage feedforward

Abstract

The application provides a grid-connected inverter low-frequency-band oscillation suppression implementation method based on voltage feedforward, relates to the technical field of power electronic technology and power converter performance, and is implemented through the steps of selecting a voltage feedforward module, establishing a mathematical model of the voltage feedforward module, designing parameters of the mathematical model of the voltage feedforward module, connecting the established mathematical model and the voltage feedforward module with the designed parameters to an output end of the grid-connected inverter, so that a virtual impedance is connected in parallel to the output end of the grid-connected inverter. The application connects a virtual impedance in parallel to the output end of the grid-connected inverter through voltage feedforward, changes the intersection frequency of the impedance of the grid-connected inverter and the impedance of the power grid and improves the phase margin, enhances the low-frequency-band stability of the system, and thus suppresses the low-frequency-band oscillation phenomenon of the system.

Description

Technical Field

[0001] This invention relates to the technical field of power converter performance in power electronics, and in particular to a method for suppressing low-frequency oscillations in grid-connected inverters based on voltage feedforward. Background Technology

[0002] With the increasing penetration rate of new energy sources, the broadband oscillation risk faced by grid-connected inverters and its potential impact area are also expanding. These broadband oscillations, originating from the interaction between the grid-connected inverter and the power grid, have become a major hidden danger to the safe operation of new energy power systems. In mild cases, they may damage power electronic equipment; in severe cases, they may lead to large-scale grid disconnection of new energy power generation units, disruption of the overall system operation, and even forced disconnection of some grid areas. Analyzing and researching the stability of the interaction between grid-connected inverters and the power grid, as well as oscillation suppression strategies, will help ensure the safety and stability of inverter grid connection and promote the development of new energy power generation systems.

[0003] Grid impedance is not negligible in the following scenarios: 1. Long-distance power transmission: Long-distance transmission lines increase the resistance and inductance of the grid, resulting in higher grid impedance. 2. Weak grids: Small grid capacity or large loads near grid connection points lead to high local grid impedance. 3. Distributed generation: Distributed power sources such as wind power and photovoltaics may have grid impedance that cannot be ignored due to their remote connection points.

[0004] When the grid impedance is not negligible, the phase angle disturbance deviation of the phase-locked loop (PLL) output will affect the current control system, and consequently the output modulation of the current loop and the output current of the grid-connected inverter. The frequency domain characteristics of the PLL will affect the low-frequency impedance characteristics of the grid-connected inverter. Increases in the PLL control bandwidth or grid impedance will lead to a decrease in the stability margin at the intersection frequency, resulting in low-frequency oscillations. The PLL is a control system used to synchronize the phase of the system. In a grid-connected inverter, the PLL is used to detect and track the phase of the grid voltage, ensuring that the inverter output is synchronized with the grid. Phase angle disturbance deviation refers to the deviation of the PLL output phase angle relative to the grid phase angle. This deviation may be caused by grid fluctuations or noise. The current loop is a closed-loop control element in the current control system, achieving stable output through feedback control of the current. The intersection frequency is the intersection of the open-loop gain curve and the 0dB line. It reflects the frequency response characteristics of the system; at this frequency, the system's phase margin is minimal. Increased grid impedance will affect the frequency domain characteristics of the PLL, thus affecting the stability of its output phase angle. Phase-locked loop (PLL) output phase angle disturbances can cause errors in the current control system, affecting the regulation accuracy of the current loop. Increased PLL control bandwidth or grid impedance can reduce the stability margin at the intersection frequency, leading to low-frequency oscillations. These low-frequency oscillations cause instability in the output current of the grid-connected inverter, increased harmonic distortion in the power system, and decreased overall system reliability.

[0005] In simple terms, when there is a large impedance in a power system, the control system is easily affected by disturbances, leading to system instability and oscillations. These oscillations can affect the stability and efficiency of the power system.

[0006] To address the aforementioned low-frequency oscillation problem, current technologies introduce virtual impedance to provide additional damping in the low-frequency range, thereby reshaping the system's low-frequency impedance characteristics. These technologies primarily include the following types:

[0007] 1. Directly introduce virtual impedance into the phase-locked loop to suppress the negative damping effect of the phase-locked loop. However, this method has the following disadvantages: (1) Increased complexity: Introducing virtual impedance increases the complexity of the control system, especially when the virtual impedance parameters need to be precisely adjusted. Complex control algorithms and more adjustment parameters may increase the difficulty of design and debugging, and the effect of virtual impedance depends on the accurate estimation of system parameters (such as grid impedance, load characteristics, etc.). If these parameters are not accurately estimated, virtual impedance may not achieve the expected effect, or may even introduce additional instability. (2) Deterioration of dynamic response: Although virtual impedance can improve the stability of the system, it may also lead to a deterioration of the dynamic response of the system. Excessive virtual impedance may slow down the response speed of the system, affecting the speed and accuracy of the system.

[0008] 2. Design virtual impedance based on the output impedance characteristics of the target converter. However, this method has the following disadvantages: (1) It requires high accuracy of the model. This method is very dependent on the accurate modeling of the output impedance characteristics of the target converter. If the model is inaccurate, the effect of the virtual impedance design may be affected, resulting in the system performance being worse than expected. (2) It has poor robustness. When the grid conditions or load change, the pre-designed virtual impedance may not be able to adapt to these changes, resulting in a decrease or instability in system performance. (3) The design is complex.

[0009] 3. Full voltage feedforward: This method directly feeds back changes in the grid voltage to the inverter's control system, enabling the inverter to adjust its output voltage in real time to compensate for grid voltage fluctuations. This means that when the grid voltage changes, the inverter's output voltage adjusts accordingly to reduce the impact of grid fluctuations. However, this method weakens the inverter's impedance because the rapid adjustment of the output voltage to compensate for grid voltage changes means that the inverter's response to output current changes is partially "distributed" to addressing grid voltage variations. This distribution weakens the inverter's ability to regulate load current changes, thus reducing output impedance. Low output impedance means a weakened resistance to external disturbances, potentially causing stability issues in the control loop, especially under conditions of significant grid fluctuations.

[0010] In summary, the existing technology of introducing additional damping for the low-frequency band has the following problems:

[0011] 1. Introducing additional damping may be effective in certain frequency bands, but its improvement on the overall stability of the system is limited, especially when the frequency range is wide.

[0012] 2. To achieve additional damping, additional hardware or complex software control algorithms are required, which increases the complexity of system design and implementation.

[0013] 3. Introducing additional damping often increases the power loss of the system, thereby reducing the overall efficiency of the inverter.

[0014] 4. In order to enhance the stability of the low-frequency band, the dynamic response performance of the system may be sacrificed, resulting in poor performance of the system when dealing with rapid load changes.

[0015] Based on this, the present invention is proposed. Summary of the Invention

[0016] The purpose of this invention is to provide a method for suppressing low-frequency oscillations in grid-connected inverters based on voltage feedforward. By using voltage feedforward to connect a virtual impedance in parallel at the output of the grid-connected inverter, the frequency of the intersection point between the inverter impedance and the grid impedance is changed and the phase margin is increased, thereby enhancing the stability of the system in the low-frequency range and suppressing the low-frequency oscillation phenomenon.

[0017] To achieve the above objectives, the present invention provides the following technical solution:

[0018] A method for suppressing low-frequency oscillations in grid-connected inverters based on voltage feedforward includes:

[0019] S100 selects a voltage feedforward module;

[0020] S200 establishes a mathematical model for the voltage feedforward module;

[0021] S300 design voltage feedforward module mathematical model parameters;

[0022] The S400 connects a voltage feedforward module with an established mathematical model and completed parameter design to the output of the grid-connected inverter, thereby creating a virtual impedance in parallel at the output of the grid-connected inverter.

[0023] The present invention provides a preferred embodiment in which, in step S100, the voltage feedforward module is selected, and the voltage feedforward module adopts a bandpass filter.

[0024] This invention provides a preferred embodiment, in which step S300 designs the mathematical model parameters of the voltage feedforward module, specifically including: selecting the impedance shaping coefficient k. d upper and lower limits of bandpass filter angular frequency ω dh and ω dl To improve impedance characteristics in the low-frequency range.

[0025] The present invention provides a preferred solution. Step S400 connects the established mathematical model and the completed parameter design of the voltage feedforward module to the output of the grid-connected inverter so that a virtual impedance is connected in parallel at the output of the grid-connected inverter. Specifically, this includes: introducing the PCC grid-connected voltage input voltage feedforward module into the current inner loop control and superimposing it on the dq axis component of the modulation signal to realize the virtual impedance being connected in parallel at the output of the grid-connected inverter.

[0026] This invention provides a preferred embodiment for setting the upper and lower limits ω of the bandpass filter's angular frequency. dh With ω dl Set to 50Hz-200Hz.

[0027] This invention provides a preferred embodiment, wherein the impedance shaping coefficient k dThe selection methods include the following: plotting the Nyquist curve of the grid-connected inverter impedance after introducing a bandpass filter, and observing the Nyquist curve as a function of the impedance shaping coefficient k. d The change in impedance shaping coefficient k is found in the Nyquist curve to maximize the system's stability margin. d At the same time, the magnitude of the impedance amplitude is controlled.

[0028] Compared with existing technologies, the above technical solution has the following advantages:

[0029] This invention discloses a method for suppressing low-frequency oscillations in grid-connected inverters based on voltage feedforward. By connecting a virtual impedance in parallel at the output of the grid-connected inverter through voltage feedforward, the frequency of the intersection point between the inverter impedance and the grid impedance is changed and the phase margin is increased, thereby enhancing the stability of the system in the low-frequency range and suppressing the low-frequency oscillation phenomenon. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0031] Figure 1 A flowchart illustrating a method for suppressing low-frequency oscillations in a grid-connected inverter based on voltage feedforward, provided as a specific embodiment of the present invention;

[0032] Figure 2 A control block diagram of a method for suppressing low-frequency oscillations in a grid-connected inverter based on voltage feedforward, provided for a specific embodiment of the present invention;

[0033] Figure 3 The Nyquist curve is shown in a method for suppressing low-frequency oscillations in a grid-connected inverter based on voltage feedforward, provided as a specific embodiment of the present invention. Detailed Implementation

[0034] 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 some embodiments of the present invention, and not all embodiments. 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.

[0035] Please refer to Figure 1 This embodiment of a method for suppressing low-frequency oscillations in a grid-connected inverter based on voltage feedforward includes the following:

[0036] S100 selects a voltage feedforward module. In this embodiment, the voltage feedforward module uses a low-frequency bandpass filter, and two of them are set. The use of bandpass filters avoids the influence on other frequency bands.

[0037] The S200 establishes a mathematical model for the voltage feedforward module, specifically as follows:

[0038] Voltage feedforward module, i.e., bandpass filter H d The expression for (s) is:

[0039]

[0040] In the formula ω dh ω dl These are the upper and lower limits of the bandpass filter's angular frequency, k d Here, is the impedance shaping coefficient. In the transfer function, s represents a complex variable in the complex plane, which is the complex frequency variable in the Laplace transform domain. The Laplace transform is a method for describing linear time-invariant systems; it transforms a function in the time domain into a function in the complex plane, where s is a complex number, usually written as s = σ + jω, where σ is the real part, j is the imaginary unit, and ω is the angular frequency.

[0041] In summary, s is used to analyze and describe the frequency domain characteristics and dynamic response of a system.

[0042] S300 design voltage feedforward module mathematical model parameters.

[0043] The S400 connects a voltage feedforward module, based on an established mathematical model and pre-designed parameters, to the output of the grid-connected inverter, thereby creating a virtual impedance in parallel at the inverter's output. Please refer to [reference needed]. Figure 2 In this embodiment, the voltage feedforward module is used to detect the PCC grid-connected voltage V. gabc Subsequently, in the inner current loop control, the feedforward signal is superimposed on the dq-axis components, realizing a virtual impedance in parallel at the inverter output. This changes the frequency of the intersection of the inverter impedance and the grid impedance, improves the phase margin, and suppresses low-frequency oscillations. Where i d and i q This represents the current component in a rotating coordinate system, i d It is generally related to the active power of the inverter, i q Related to reactive power. m a m b m c These represent modulation signals, which control the switching devices of the inverter.

[0044] Figure 2 θ PLL i is the phase-locked angle; dqref idq v dq Given the current, sampling current, and sampling voltage along the dq axis in the rotating coordinate system; where i dref and i qref These represent the reference values ​​for the d-axis and q-axis currents, respectively, and represent the desired active power output current and reactive power output current. This is achieved by adjusting i... d / i q Make it track i dref i qref To achieve control of active and reactive power; m dq m abc The modulation signals are in rotating and stationary coordinate systems; G i (s) is a current loop controller; v gabc This refers to the PCC grid-connected voltage. The PCC is the connection point between the grid-connected inverter and the power grid. At this point, the inverter's output current and the grid voltage intersect. The grid impedance and inverter output impedance measured at the PCC are crucial for analyzing system stability. When the frequency response curves of the inverter's output impedance and the grid impedance intersect at a certain frequency, that frequency is called the intersection frequency. PCC grid-connected voltage v d v q After H d (s) is then introduced into the inner current loop control and superimposed on the modulation signal. dq m on the axis component d m q Therefore, the form of the virtual impedance is the same as that of the feedforward function H. d (s) is relevant. Current feed function H d (s) is a low-frequency bandpass filter, which will primarily affect the low-frequency impedance characteristics of the positive and negative sequence equivalent impedances. This can be achieved by selecting an appropriate impedance shaping coefficient k. d upper and lower limits of bandpass filter angular frequency ω dh With ω dl It can improve impedance characteristics in the low-frequency range.

[0045] Regarding current inner loop control and current loop controller, for i d and i q The control is current inner-loop control. Current inner-loop control is an overall control strategy, while the current loop controller is the specific component that implements this strategy. Current inner-loop control refers to the part of the control system used to regulate the inverter's output current. Its goal is to ensure that the actual output current can quickly and accurately track the reference value i. dref and i qref The current loop controller is Gi(s).

[0046] The mathematical model parameters for designing the voltage feedforward module in step S300 specifically include the following: Since the phase-locked loop mainly affects the impedance characteristics in the frequency band below 200Hz, in order to improve the system stability margin in the low-frequency range of 50Hz-200Hz, the upper and lower limits ωd of the bandpass filter's angular frequency are... h With ω dl Set it to this range (50Hz-200Hz). Plot the Nyquist curve of the grid-connected inverter impedance after introducing the bandpass filter. Figure 3 As shown, the Nyquist plot is a tool in frequency response analysis used to evaluate the stability of linear time-invariant systems. It displays the trajectory of the poles and zeros of the system's open-loop transfer function in the complex plane as a function of frequency. The Nyquist plot can be used to determine the stability of a closed-loop system and provides information about gain and phase margins. The stability of a system can be determined using the Nyquist plot according to the Nyquist stability criterion. Specifically, the stability of the closed-loop system can be determined by observing whether the curve encircles the point -1 + j0. The Nyquist plot can be observed as a function of the impedance shaping factor k. d To maximize the system's stability margin, it's crucial to determine the appropriate impedance shaping factor based on the changes in impedance. Simultaneously, the impedance amplitude must not be excessively large, otherwise it will reduce the grid-connected inverter's output voltage under the same conditions, degrading its power supply characteristics and hindering energy output. The impedance amplitude should be selected while ensuring system stability margin. Typically, the system stability margin requires a phase margin of at least 45 degrees and a gain margin of at least 6 dB. The impedance amplitude should not exceed 10%-20% of the grid-connected inverter's rated output impedance. The specific range needs to be determined based on the characteristics of the inverter and the power grid.

[0047] This embodiment of the low-frequency oscillation suppression method for grid-connected inverters based on voltage feedforward is mainly achieved by introducing a voltage feedforward element into the control system. The advantages and principles of this method can be explained in detail below:

[0048] 1. Effectively suppresses low-frequency oscillations: Through voltage feedforward, it can quickly respond to and compensate for changes in grid-side voltage, effectively suppressing inverter oscillations in the low-frequency range and improving system stability and reliability.

[0049] 2. Voltage feedforward can compensate in advance when the grid voltage changes, reducing inverter response delay and improving the dynamic response speed and accuracy of the system.

[0050] 3. Optimizing the voltage feedforward circuit can help reduce inverter output harmonics and offset, improve power quality, and meet grid connection specifications.

[0051] 4. Compared with traditional feedback control methods, voltage feedforward can simplify the design and debugging of control systems, reduce implementation costs, and improve system maintainability.

[0052] The above provides a detailed description of a low-frequency oscillation suppression method for grid-connected inverters based on voltage feedforward, as provided in this application. Specific examples have been used to illustrate the principles and implementation methods of this application. The descriptions of the embodiments above are merely for the purpose of helping to understand the method and its core ideas. It should be noted that those skilled in the art can make various improvements and modifications to this application without departing from its principles, and these improvements and modifications also fall within the protection scope of the claims of this application.

Claims

1. A method for suppressing low-frequency oscillations in a grid-connected inverter based on voltage feedforward, characterized in that, include: S100 selects a voltage feedforward module; the voltage feedforward module employs a bandpass filter; S200 establishes a mathematical model for the voltage feedforward module; the expression for the mathematical model of the voltage feedforward module is: In the formula ω dh ω dl These are the upper and lower limits of the bandpass filter's angular frequency, k d is the impedance shaping coefficient; s represents a complex variable in the complex plane, which is the complex frequency variable in the Laplace transform domain. The Laplace transform is a method used to describe linear time-invariant systems. It transforms a function in the time domain into a function in the complex plane, where s is a complex number, written as s = σ + jω, where σ is the real part, j is the imaginary unit, and ω is the angular frequency. S300 design voltage feedforward module mathematical model parameters; including: selecting the impedance shaping coefficient k. d upper and lower limits of bandpass filter angular frequency ω dh and ω dl To improve impedance characteristics in the low-frequency range; S400 connects a voltage feedforward module with an established mathematical model and completed parameter design to the output of the grid-connected inverter, so that a virtual impedance is connected in parallel at the output of the grid-connected inverter; including: introducing the PCC grid-connected voltage input voltage feedforward module into the current inner loop control and superimposing it on the dq axis component of the modulation signal; Impedance shaping coefficient k d The selection methods include the following: plotting the Nyquist curve of the grid-connected inverter impedance after introducing a bandpass filter, and observing the Nyquist curve as a function of the impedance shaping coefficient k. d The change in impedance shaping coefficient k is found in the Nyquist curve to maximize the system's stability margin. d .

2. The method for suppressing low-frequency oscillations in grid-connected inverters based on voltage feedforward as described in claim 1, characterized in that, The bandpass filter is a low-frequency bandpass filter.

3. The method for suppressing low-frequency oscillations in grid-connected inverters based on voltage feedforward as described in claim 1, characterized in that, Two bandpass filters are provided, and the PCC grid-connected voltage v d v q The inputs are respectively fed into two bandpass filters.

4. The method for suppressing low-frequency oscillations in grid-connected inverters based on voltage feedforward as described in claim 1, characterized in that, The upper and lower limits of the bandpass filter's angular frequency ω dh With ω dl Set to 200Hz and 50Hz.

5. The method for suppressing low-frequency oscillations in grid-connected inverters based on voltage feedforward according to claim 1, characterized in that, Impedance shaping coefficient k d The selection also includes the following methods: finding the impedance shaping coefficient k in the Nyquist curve that maximizes the system's stability margin. d At the same time, the magnitude of the impedance amplitude is controlled.

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