Grid-connected inverter low-frequency-band oscillation suppression implementation method based on voltage feedforward

Abstract

The invention provides a grid-connected inverter low-frequency-band oscillation suppression implementation method based on voltage feedforward, and relates to the technical field of power converter performance of the power electronic technology. The method comprises the following steps of: selecting a voltage feed-forward module, establishing a voltage feed-forward module mathematical model, designing parameters of the voltage feed-forward module mathematical model, and connecting the established mathematical model and the voltage feed-forward module of which the parameter design is completed to the output end of the grid-connected inverter. And the output end of the grid-connected inverter is connected with a virtual impedance in parallel. The virtual impedance is connected in parallel to the output end of the grid-connected inverter through voltage feedforward, the frequency of the intersection point of the grid-connected inverter impedance and the power grid impedance is changed, the phase margin is improved, the low-frequency-band stability of the system is enhanced, and therefore the low-frequency-band oscillation phenomenon of the system is restrained.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronics technology power converter performance, and in particular to a method for realizing low-frequency oscillation suppression of a grid-connected inverter based on voltage feedforward. Background Art

[0002] With the continuous increase in the penetration rate of new energy, the risk of broadband oscillation faced by grid-connected inverters and their potential impact areas are also showing a trend of expansion. This type of broadband oscillation phenomenon caused by the interaction between grid-connected inverters and the power grid has become a major hidden danger for the safe operation of new energy power systems. In mild cases, power electronic equipment may be damaged; in severe cases, large-scale grid failures of new energy power generation units may occur, the overall operation of the system may be hindered, and even some grid areas may be forced to be disconnected. Analyzing and studying the stability of the interaction between grid-connected inverters and the power grid and the oscillation suppression strategy will help ensure the safety and stability of inverter grid connection and promote the development of new energy power generation systems.

[0003] The grid impedance cannot be ignored in the following scenarios: 1. Long-distance transmission: Long-distance transmission lines increase the resistance and inductance of the grid, resulting in higher grid impedance. 2. Weak grid: The grid capacity is small or the load near the grid connection point is large, resulting in higher local grid impedance. 3. Distributed generation: Distributed power sources such as wind power and photovoltaic power generation have long access points, which may cause the grid impedance to be non-negligible.

[0004] When the grid impedance cannot be ignored, the phase angle disturbance deviation of the phase-locked loop output will affect the current control system, and then affect the output modulation of the current loop and the output current of the grid-connected inverter. The frequency domain characteristics of the phase-locked loop will affect the low-frequency impedance characteristics of the grid-connected inverter. The increase of the phase-locked loop control bandwidth or the grid impedance will cause the stability margin at the intersection frequency to decrease and produce low-frequency oscillation. Among them, the phase-locked loop (PLL) is a control system used to synchronize the phase of the system. In the grid-connected inverter, the PLL is used to detect and track the phase of the grid voltage to ensure that the inverter output is synchronized with the grid. The phase angle disturbance deviation refers to the deviation of the phase angle of the phase-locked loop output relative to the grid phase angle. This deviation may be caused by grid fluctuations or noise. The current loop is a closed-loop control link in the current control system, which controls the current through feedback to achieve stable output. 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, and the phase margin of the system is the smallest at this frequency. The increase of grid impedance will affect the frequency domain characteristics of the phase-locked loop, thereby affecting the stability of its output phase angle. The phase angle disturbance deviation of the phase-locked loop output will cause errors in the current control system and affect the regulation accuracy of the current loop. The increase of the phase-locked loop control bandwidth or the increase of the grid impedance will reduce the stability margin at the intersection frequency, resulting in low-frequency oscillation. Low-frequency oscillation will cause the output current of the grid-connected inverter to be unstable, the harmonic distortion of the power system to increase, and the overall reliability of the system to decrease.

[0005] In simple terms, when there is a large impedance in the power system, the control system is susceptible to disturbances, resulting in system instability and oscillation. Such oscillations can affect the stability and efficiency of the power system.

[0006] In order to solve the above-mentioned low-frequency oscillation problem, the existing technology currently introduces additional damping for the low-frequency band by introducing virtual impedance to reshape the low-frequency impedance characteristics of the system, which mainly includes 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. The introduction of virtual impedance will increase 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, the virtual impedance may not achieve the expected effect and may even introduce additional instability. (2) Deterioration of dynamic response. Although virtual impedance can improve the stability of the system, it may also cause the dynamic response of the system to deteriorate. Excessive virtual impedance may slow down the response speed of the system and affect 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) High requirements for model accuracy. This method relies heavily on 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 system performance that is not as good as expected. (2) Poor robustness. When grid conditions or loads change, the pre-designed virtual impedance may not be able to adapt to these changes, resulting in system performance degradation or instability. (3) Complex design.

[0009] 3. Full voltage feedforward, that is, by directly feeding back the changes in grid voltage to the inverter's control system, the inverter can 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 will be adjusted accordingly to reduce the impact of grid fluctuations, but this method will weaken the inverter impedance because when the grid voltage changes, the inverter output voltage will quickly adjust to compensate for this change, which means that the inverter's ability to respond to output current changes is partially "dispersed" to respond to grid voltage changes. This dispersion weakens the inverter's ability to regulate load current changes, thereby reducing output impedance. Low output impedance means that the inverter's resistance to external disturbances is weakened, which may cause stability problems in the control loop, especially when grid conditions change greatly.

[0010] In summary, the prior art method of introducing additional damping for the low frequency band has the following problems:

[0011] 1. Introducing additional damping may be effective in certain specific frequency bands, but it has limited effect on improving the overall stability of the system, 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. The introduction of additional damping tends to increase 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, causing the system to perform poorly when dealing with rapid load changes.

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

[0016] The purpose of the present invention is to provide a method for realizing low-frequency oscillation suppression of a grid-connected inverter based on voltage feedforward. A virtual impedance is connected in parallel at the output end of the grid-connected inverter through voltage feedforward to change the intersection frequency of the grid-connected inverter impedance and the grid impedance and improve the phase margin, thereby enhancing the low-frequency stability of the system and suppressing the low-frequency oscillation phenomenon of the system.

[0017] To achieve the above object, the present invention provides the following technical solutions:

[0018] A method for implementing low-frequency oscillation suppression of a grid-connected inverter based on voltage feedforward, comprising:

[0019] S100 selects the voltage feed-forward module;

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

[0021] S300 designs the mathematical model parameters of the voltage feedforward module;

[0022] S400 connects a voltage feedforward module with an established mathematical model and completed parameter design to the 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.

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

[0024] The present invention provides a preferred solution, step S300 designs the mathematical model parameters of the voltage feedforward module, specifically including: selecting the impedance shaping coefficient k d 、Bandpass filter angular frequency upper and lower limits ω dh and ω dl To improve the low frequency impedance characteristics.

[0025] The present invention provides a preferred solution, in which step S400 connects a voltage feedforward module with an established mathematical model and completed parameter design to the 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. Specifically, the PCC grid-connected voltage is input into the voltage feedforward module and then introduced into the current inner loop control, and superimposed on the dq-axis components of the modulation signal, so as to realize the virtual impedance being connected in parallel to the output end of the grid-connected inverter.

[0026] The present invention provides a preferred solution, which sets the upper and lower limits of the angular frequency of the bandpass filter ω dh With ω dl Settings from 50Hz-200Hz.

[0027] The present invention provides a preferred solution, the impedance shaping coefficient k dThe selection includes the following methods: draw the Nyquist curve of the grid-connected inverter impedance after the bandpass filter is introduced, and observe the Nyquist curve with the impedance shaping coefficient k d The impedance shaping factor k that maximizes the system stability margin is found in the Nyquist curve. d While controlling the impedance amplitude.

[0028] Compared with the prior art, the above technical solution has the following advantages:

[0029] The present invention discloses a method for realizing low-frequency oscillation suppression of a grid-connected inverter based on voltage feedforward. A virtual impedance is connected in parallel to the output end of the grid-connected inverter through voltage feedforward, so as to change the intersection frequency of the grid-connected inverter impedance and the grid impedance and improve the phase margin, thereby enhancing the low-frequency stability of the system and suppressing the low-frequency oscillation phenomenon of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0031] Figure 1 A flowchart of a method for implementing low-frequency oscillation suppression of a grid-connected inverter based on voltage feedforward provided in a specific embodiment of the present invention;

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

[0033] Figure 3 A Nyquist curve diagram used in a method for implementing low-frequency oscillation suppression of a grid-connected inverter based on voltage feedforward provided in a specific embodiment of the present invention. DETAILED DESCRIPTION

[0034] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] Please refer to Figure 1 , a method for implementing low-frequency oscillation suppression of a grid-connected inverter based on voltage feedforward in this embodiment 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 provided. The bandpass filter is used to avoid affecting other frequency bands.

[0037] S200 establishes a mathematical model of the voltage feedforward module, specifically:

[0038] Voltage feedforward module, i.e. bandpass filter H d (s) is expressed as:

[0039]

[0040] Where ω dh ,ω dl are the upper and lower limits of the angular frequency of the bandpass filter, k d is the impedance shaping coefficient. In the transfer function, s represents a complex variable on the complex plane, which is a complex frequency variable in the Laplace transform domain. Laplace transform is a method used to describe linear time-invariant systems. It converts a function in the time domain into a function on 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 the system.

[0042] S300 designs mathematical model parameters of voltage feedforward module.

[0043] S400 connects the voltage feedforward module with established mathematical model and parameter design to the output of the grid-connected inverter, so that a virtual impedance is connected in parallel to the output of the grid-connected inverter. Figure 2 , the voltage feedforward module of this embodiment: used to detect the PCC grid-connected voltage V gabc Then, it is introduced into the current inner loop control, and the feedforward signal is superimposed on the dq axis components to realize the virtual impedance in parallel at the inverter output end, change the intersection frequency of the inverter impedance and the grid impedance, improve the phase margin, and suppress low-frequency oscillation. d and i q represents the current component in the rotating coordinate system, i d Generally related to the active power of the inverter, i q Associated with reactive power. m a , m b , m c Represents the modulation signals that control the switching devices of the inverter.

[0044] Figure 2 Medium θ PLL is the phase-locking angle; i dqref 、idq 、v dq is the given current, sampled current, and sampled voltage of the dq axis in the rotating coordinate system; where i dref and i qref They represent the reference values ​​of the d-axis and q-axis currents, respectively, and represent the desired active power output current and reactive power output current. d / i q Make it track i dref i qref , to achieve the control of active power and reactive power; m dq 、m abc is the modulation signal in the rotating coordinate system and the stationary coordinate system; G i (s) is the current loop controller; v gabc is the PCC grid-connected voltage. PCC is the connection point between the grid-connected inverter and the grid. At this point, the output current of the inverter intersects with the grid voltage. The grid impedance and inverter output impedance measured at PCC are the key to analyzing system stability. When the frequency response curves of the inverter output impedance and the grid impedance intersect at a certain frequency, this frequency is the intersection frequency. PCC grid-connected voltage v d 、v q After H d (s) is then introduced into the current inner loop control and superimposed on the modulation signal dq Axis component m d 、m q Therefore, the virtual impedance has the same form as the feedforward function H d (s) is related. When the current feed function H d (s) is a low-frequency bandpass filter, which will mainly affect the low-frequency impedance characteristics of the positive and negative sequence equivalent impedance. By selecting a suitable impedance shaping coefficient k d 、Bandpass filter angular frequency upper and lower limits ω dh With ω dl Can improve low frequency impedance characteristics.

[0045] Regarding the current inner loop control and current loop controller, d and i q The control of the current inner loop is called the current inner loop control. The current inner loop control is an overall control strategy, and the current loop controller is the specific component that implements the strategy. The current inner loop control refers to the part of the control system used to adjust the inverter 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 , and the current loop controller is Gi(s).

[0046] The design of the mathematical model parameters of the voltage feedforward module in step S300 specifically includes the following: Since the phase-locked loop mainly affects the impedance characteristics of the frequency band below 200 Hz, in order to improve the system stability margin in the low frequency band of 50 Hz-200 Hz, the upper and lower limits of the angular frequency of the bandpass filter ωd h With ω dl Set to this range (50Hz-200Hz). Draw the Nyquist curve of the grid-connected inverter impedance after the introduction of 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 shows the trajectory of the poles and zeros of the system's open-loop transfer function in the complex plane as the frequency changes. The Nyquist plot can be used to determine the stability of a closed-loop system and provide information about the gain margin and phase margin. According to the Nyquist stability criterion, the stability of the system can be determined by the Nyquist plot. Specifically, the stability of the closed-loop system can be determined by observing whether the curve surrounds the -1+j0 point. Observe the Nyquist curve as the impedance shaping factor k d The impedance amplitude should be selected on the premise of ensuring the system stability margin. Usually, the system stability margin requires a phase margin of at least 45 degrees and a gain margin of at least 6dB. The impedance amplitude should not exceed 10%-20% of the rated output impedance of the grid-connected inverter. The specific range needs to be determined according to the characteristics of the inverter and the power grid.

[0047] The method for implementing low-frequency oscillation suppression of a grid-connected inverter based on voltage feedforward in this embodiment is mainly implemented by introducing a voltage feedforward link in the control system. The advantages and principles of this method can be described in detail as follows:

[0048] 1. Effectively suppress low-frequency oscillations: Through voltage feedforward, it can quickly respond to and compensate for changes in grid-side voltage, effectively suppress the inverter's oscillations in the low-frequency band, and improve system stability and reliability.

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

[0050] 3. Optimizing the voltage feedforward link can help reduce the harmonics and imbalance of the inverter output, improve the power quality, and meet the regulatory requirements for grid access.

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

[0052] The above is a detailed introduction to a method for implementing low-frequency oscillation suppression of a grid-connected inverter based on voltage feedforward provided by the present application. This article uses specific examples to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the method of the present application and its core idea. It should be pointed out that for ordinary technicians in this technical field, without departing from the principles of the present application, several improvements and modifications can be made to the present application, and these improvements and modifications also fall within the scope of protection of the claims of the present application.

Claims

1. A method for implementing low-frequency oscillation suppression of a grid-connected inverter based on voltage feedforward, characterized in that: include: S100 selects the voltage feed-forward module; S200 establishes a mathematical model of the voltage feedforward module; S300 designs the mathematical model parameters of the voltage feedforward module; S400 connects a voltage feedforward module with an established mathematical model and completed parameter design to the 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.

2. The method for implementing low-frequency oscillation suppression of a grid-connected inverter based on voltage feedforward according to claim 1, characterized in that: In step S100 , a voltage feedforward module is selected, wherein the voltage feedforward module adopts a bandpass filter.

3. The method for implementing low-frequency oscillation suppression of a grid-connected inverter based on voltage feedforward according to claim 2, characterized in that: In step S200, a voltage feedforward module mathematical model is established, and the expression of the voltage feedforward module mathematical model is: Where ω dh ,ω dl are the upper and lower limits of the angular frequency of the bandpass filter, k d is the impedance shaping factor.

4. The method for implementing low-frequency oscillation suppression of a grid-connected inverter based on voltage feedforward according to claim 3 is characterized in that: Step S300 designs the mathematical model parameters of the voltage feedforward module, specifically including: selecting the impedance shaping coefficient k d 、Bandpass filter angular frequency upper and lower limits ω dh and ω dl To improve the low frequency impedance characteristics.

5. The method for implementing low-frequency oscillation suppression of a grid-connected inverter based on voltage feedforward according to claim 1 or 4, characterized in that: Step S400 connects the voltage feedforward module with established mathematical model and completed parameter design to the 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. Specifically, the PCC grid-connected voltage is input into the voltage feedforward module and then introduced into the current inner loop control, and superimposed on the dq axis components of the modulation signal to realize the virtual impedance connected in parallel to the output end of the grid-connected inverter.

6. The method for implementing low-frequency oscillation suppression of a grid-connected inverter based on voltage feedforward according to claim 2, characterized in that: The band-pass filter is a low-frequency band-pass filter.

7. The method for implementing low-frequency oscillation suppression of a grid-connected inverter based on voltage feedforward according to claim 5, characterized in that: The bandpass filter is set to two, PCC grid voltage v d 、v q Input into two bandpass filters respectively.

8. The method for implementing low-frequency oscillation suppression of a grid-connected inverter based on voltage feedforward according to claim 4, characterized in that: Set the upper and lower limits of the bandpass filter angular frequency ω dh With ω dl Settings from 50Hz-200Hz.

9. The method for implementing low-frequency oscillation suppression of a grid-connected inverter based on voltage feedforward according to claim 4, characterized in that: Impedance shaping factor k d The selection includes the following methods: draw the Nyquist curve of the grid-connected inverter impedance after the bandpass filter is introduced, and observe the Nyquist curve with the impedance shaping coefficient k d The impedance shaping factor k that maximizes the system stability margin is found in the Nyquist curve. d .

10. The method for realizing low-frequency oscillation suppression of a grid-connected inverter based on voltage feedforward according to claim 9, characterized in that: Impedance shaping factor k d The selection of also includes the following methods: Find the impedance shaping factor k that maximizes the system stability margin in the Nyquist curve d While controlling the impedance amplitude.

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