A method for identifying the oscillation instability frequency of a grid-connected inverter

Abstract

The application provides a method for identifying the oscillation instability frequency of a grid-connected inverter, comprising: constructing a small-signal control model of the grid-connected inverter considering the fluctuation of the DC side voltage; determining a feedback loop formed by the grid impedance, a phase-locked loop and a current inner loop controller based on the small-signal control model of the grid-connected inverter; and determining target control parameters based on the feedback loop. The method provided by the application can further obtain a closed-loop transfer function representing the positive and negative feedback loop effect by constructing the small-signal control model of the grid-connected inverter considering the fluctuation of the DC side voltage, can explain the internal mechanism of the stability change of the grid-connected inverter from the perspective of the feedback loop effect, and thus provides a basis for the parameter setting of the current inner loop controller of the grid-connected inverter, the identification of the harmonic oscillation risk and the stable operation control.

Description

Technical Field

[0001] This application relates to the field of stability analysis and control parameter tuning technology for grid-connected inverters, and in particular to a method for identifying the oscillation instability frequency of grid-connected inverters. Background Technology

[0002] With the high proportion of renewable energy generation such as solar and wind power being integrated into the grid, grid-connected inverters, as a crucial interface between renewable energy generation systems and the power grid, play a vital role in the reliability of new power systems through their safe and stable operation. However, under conditions of weak grids or high grid impedance, grid-connected inverters are affected by the coupling effects of multiple factors, including phase-locked loops, voltage outer loops, current inner loops, and grid impedance. This makes their dynamic characteristics more complex and prone to problems such as harmonic oscillations, broadband oscillations, and grid current distortion.

[0003] Existing grid-connected inverters typically employ a dual closed-loop control structure consisting of an outer loop for DC bus voltage and an inner loop for AC output current. The parameters of the inner current loop controller are a crucial factor affecting the stability of the grid-connected inverter. Existing research indicates that variations in the inner current loop controller parameters affect the output impedance, phase margin, and harmonic oscillation characteristics of the grid-connected inverter. However, current technology lacks a clear explanation for the underlying mechanism by which the stability of the grid-connected inverter exhibits a nonlinear pattern of initial enhancement followed by weakening, or initial instability followed by stability and then instability again, after increasing the inner current loop controller parameters.

[0004] Furthermore, when a grid-connected inverter becomes unstable, the harmonic oscillation frequency of its output current may be distributed in the mid-frequency range or shift to the high-frequency range. Existing analysis methods mostly determine the stability of the system from the perspective of impedance modeling, eigenvalue analysis, or simulation verification. However, there is still a lack of intuitive and effective explanations for why the harmonic oscillation frequency range shifts significantly with changes in the parameters of the current inner loop controller. Further, the grid impedance, phase-locked loop, and current inner loop controller form a closed-loop feedback loop in the grid-connected inverter control block diagram. This feedback loop may exhibit a negative feedback effect that is beneficial to stability or a positive feedback effect that is detrimental to stability. If the effects of this positive and negative feedback loop and their variation with controller parameters cannot be accurately identified, it is difficult to reasonably tune the parameters of the current inner loop controller and to promptly assess the potential harmonic oscillation risk of the grid-connected inverter. Summary of the Invention

[0005] This application provides a method for identifying the oscillation instability frequency of a grid-connected inverter. To solve the above-mentioned technical problems, this application adopts the following technical method: This application provides a method for identifying the oscillation instability frequency of a grid-connected inverter, including: Construct a small-signal control model for a grid-connected inverter that considers DC-side voltage fluctuations; Based on the small-signal control model of the grid-connected inverter, the feedback loop formed by the grid impedance through the phase-locked loop and the current inner loop controller is determined. Based on the feedback loop, the target control parameters are determined.

[0006] Optionally, the step of determining the feedback loop formed by the grid impedance through the phase-locked loop and the current inner loop controller based on the small-signal control model of the grid-connected inverter includes: Based on the small-signal control model of the grid-connected inverter, and combined with the main circuit model of the grid-connected inverter, the outer loop of DC bus voltage, the inner loop controller of current, the phase-locked loop structure, and the PWM sampling and calculation delay model, a block diagram of the small-signal control of the grid-connected inverter is constructed. The small-signal control block diagram of the grid-connected inverter is extracted to determine the feedback loop formed by the grid impedance through the phase-locked loop and the current inner loop controller.

[0007] Optionally, the phase-locked loop structure includes a current feedback loop introduced at the current reference value and a current feedback loop introduced at the modulation signal.

[0008] Optionally, the target control parameters are determined based on the feedback loop; Based on the feedback loop, an equivalent transfer function model of each link in the inner current loop of the grid-connected inverter is constructed. Based on the equivalent transfer function model, a closed-loop transfer function is constructed to characterize the positive and negative feedback loop effects. The closed-loop transfer function is used to characterize the positive and negative feedback loop effects formed by the grid impedance through the phase-locked loop and the current inner loop controller in the forward channel of the grid-connected inverter current inner loop. Based on the closed-loop transfer function, the amplitude-phase-frequency characteristics of the closed-loop transfer function are determined under different current inner-loop controller parameters; Based on the amplitude-phase-frequency characteristics, the target control parameters are determined.

[0009] Optionally, the feedback loop includes a q-axis loop and a d-axis loop, and the construction of equivalent transfer function models for each link in the grid-connected inverter current inner loop based on the feedback loop includes: The q-axis loop of the feedback loop is equivalently included in the d-axis loop, and the feedback loop is transformed to construct the equivalent transfer function model of each link in the inner current loop of the grid-connected inverter.

[0010] Optionally, determining the amplitude-phase-frequency characteristics of the closed-loop transfer function under different current inner-loop controller parameters based on the closed-loop transfer function includes: By using the current inner loop controller parameters in the closed-loop transfer function as variables, the amplitude-phase-frequency characteristics of the closed-loop transfer function under different current inner loop controller parameters are determined.

[0011] Optionally, determining the target control parameters based on the amplitude-phase frequency characteristics includes: If the amplitude-phase frequency characteristic of the closed-loop transfer function corresponding to the current inner loop controller parameter does not have a positive feedback effect region, or the positive feedback effect region does not cover the harmonic oscillation frequency generated by the grid-connected inverter, then the current inner loop controller parameter shall be used as the target control parameter; the positive feedback effect region is the frequency range in the phase frequency characteristic of the grid-connected inverter closed-loop transfer function where a negative phase shift occurs.

[0012] This application has the following beneficial effects: The method proposed in this application, by constructing a small-signal control model for a grid-connected inverter that considers DC-side voltage fluctuations, further obtains a closed-loop transfer function characterizing the positive and negative feedback loop effects. This method can explain the intrinsic mechanism of grid-connected inverter stability changes from the perspective of feedback loop effects, thereby providing a basis for the parameter tuning of the grid-connected inverter current inner loop controller, the identification of harmonic oscillation risks, and the stable operation control. Attached Figure Description

[0013] Figure 1 A flowchart illustrating a method for identifying the oscillation instability frequency of a grid-connected inverter, provided in an embodiment of this application; Figure 2 This is a schematic diagram of the grid-connected inverter system structure provided in the embodiments of this application; Figure 3 This is a block diagram of the grid-connected inverter control provided in the embodiments of this application; Figure 4 This is a block diagram of the grid-connected inverter current inner loop control provided in the embodiments of this application; Figure 5 This is an equivalent control block diagram of the inner loop current of a grid-connected inverter provided in an embodiment of this application; Figure 6 This is the closed-loop transfer function provided in the embodiments of this application when the grid impedance changes. A schematic diagram of the amplitude and phase frequency characteristics; Figure 7 This is a waveform diagram of the output current of a grid-connected inverter with a grid impedance of 11mH, provided in an embodiment of this application. Figure 8 This is a schematic diagram of FFT analysis of the output current of a grid-connected inverter with a grid impedance of 11mH provided in an embodiment of this application. Figure 9 This is a waveform diagram of the output current of a grid-connected inverter with a grid impedance of 13mH, provided in an embodiment of this application. Figure 10 This is a schematic diagram of FFT analysis of the output current of a grid-connected inverter with a grid impedance of 13mH provided in an embodiment of this application; Detailed Implementation

[0014] To facilitate understanding by those skilled in the art, the present application will be further described below in conjunction with embodiments and accompanying drawings. The content mentioned in the embodiments is not intended to limit the present application.

[0015] To solve the above technical problems, such as Figure 1 As shown, this application proposes a method for identifying the oscillation instability frequency of a grid-connected inverter, comprising: Step S101: Construct a small-signal control model for the grid-connected inverter that considers DC-side voltage fluctuations; like Figure 2 As shown, the grid-connected inverter system includes a DC-side power injection source, a DC bus capacitor, an inverter bridge, an LCL filter, a common grid connection point, grid impedance, and grid voltage. Among these, Indicates the power injection source. This represents the DC bus capacitance. This represents the actual value of the DC bus voltage. , , Construct an LCL filter, This is a passive damping series resistor used to suppress the resonant spikes of an LCL filter. For the inverter bridge output current, For grid-connected output current, Indicates the voltage at the common grid connection point. Indicates the power grid impedance. This indicates the mains voltage. (Control circuit section) Indicates pulse width modulation gain. This represents the grid synchronization phase angle output by the phase-locked loop. This represents the transfer function of the inner-loop PI controller. , Indicates the modulated signal. , These represent the grid-connected output current. The d-axis and q-axis components; , These represent the grid-connected current reference values ​​for the d-axis and q-axis, respectively. Furthermore, and These represent the active power and reactive power output of the grid-connected inverter, respectively. and These represent the reference values ​​for active power and reactive power, respectively. and Let represent the transfer functions of the active power and reactive power controllers, respectively, and s be the Laplace operator.

[0016] The transfer function of the voltage outer loop PI controller is: (1) The transfer function of the current inner-loop PI controller is: (2) in, , These represent the proportional and integral coefficients of the voltage outer-loop PI controller, respectively. , These represent the proportional and integral coefficients of the current inner-loop PI controller, respectively.

[0017] Assuming the DC-side power input equals the inverter-side power output, and considering the impact of DC-side voltage fluctuations while neglecting inverter bridge power losses, a grid-connected small-signal model of the relationship between the DC bus voltage, inverter bridge output current, and modulation signal is obtained. The mathematical expression corresponding to this grid-connected small-signal model is: (3) Right now: (4) in: (5) Indicates pulse width modulation gain. , , , Represents the steady-state value of the corresponding physical variable. , , , This represents the small-signal perturbation value of the corresponding physical variable.

[0018] Step S102: Based on the small-signal control model of the grid-connected inverter, determine the feedback loop formed by the grid impedance through the phase-locked loop and the current inner loop controller; Based on the small-signal control model of the grid-connected inverter obtained above, and combining the main circuit model of the grid-connected inverter, the outer loop of the DC bus voltage, the inner loop controller of the current, the phase-locked loop structure, and the PWM sampling and calculation delay model, a block diagram of the small-signal control of the grid-connected inverter is constructed, as follows: Figure 3 As shown.

[0019] The phase-locked loop (PLL) structure takes into account the small-signal disturbances introduced by the PLL structure. The feedback loop introduced by the PLL structure at the current reference value is as follows: (6) The phase-locked loop (PLL) structure also includes a feedback loop introduced at the modulation signal: (7) in: (8) In the formula, This indicates a PI controller for a phase-locked loop control circuit. , These are the proportional and integral coefficients of the PI controller. This represents the d-axis amplitude of the voltage at the common grid connection point.

[0020] The PWM sampling and delay calculation model is as follows: (9) in, Indicates the sampling period.

[0021] In the grid-connected inverter control block diagram, the transfer function models of each component include: (10) (11) (12) in, , , The transfer function model in the control block diagram of a grid-connected inverter. Representing the power grid impedance model, It is the identity matrix. () is the admittance matrix of inductor L2. This is the inverse of the admittance matrix of inductor L2. Let be the admittance matrix of inductor L1. This is the inverse of the admittance matrix of inductor L1. Here is the impedance matrix of the capacitor and resistor, ( It is the inverse matrix of the impedance matrix of capacitors and resistors.

[0022] In the small-signal control block diagram of the grid-connected inverter, it can be seen that the grid impedance, through the phase-locked loop and the inner current loop controller, forms a closed-loop feedback loop in the grid-connected inverter control system. This feedback loop, located within the inner current loop, affects the phase-frequency characteristics of the grid-connected inverter control loop. To further analyze the impact of this feedback loop on the stability of the grid-connected inverter, this embodiment extracts the feedback loop from the overall small-signal control block diagram and forms it as shown below. Figure 4 The diagram shown is a block diagram of the inner loop control of the grid-connected inverter current.

[0023] exist Figure 4 middle, , The expression is: (13) in, , , , They represent The four elements of the forward channel two-dimensional matrix.

[0024] Step S103: Determine the target control parameters based on the feedback loop.

[0025] exist Figure 4 In the grid-connected inverter current inner loop control block diagram shown, considering that the DC bus voltage outer loop mainly acts on the d-axis, to improve the accuracy of the analysis, this embodiment equivalently includes the q-axis loop into the d-axis loop and performs an equivalent transformation on the current inner loop control block diagram, resulting in the following: Figure 5 The diagram shows the equivalent control block diagram of the grid-connected inverter's inner current loop. By performing an equivalent transformation on this diagram, the equivalent transfer function models of each element in the grid-connected inverter's inner current loop are obtained: (14) in, Indicates the fundamental angular frequency of the power grid. This represents the inductance of the power grid.

[0026] Combination Figure 5 The equivalent control block diagram of the grid-connected inverter current inner loop, and the equivalent transfer function models of each link in the grid-connected inverter current inner loop, are used to further construct a closed-loop transfer function characterizing the positive and negative feedback loop effects: (15) The closed-loop transfer function It is used to characterize the positive and negative feedback loop effects formed by the grid impedance through the phase-locked loop and the current inner loop controller in the forward path of the current inner loop of the grid-connected inverter.

[0027] In this embodiment, if If the phase frequency characteristics show a negative phase shift region, then this region corresponds to the positive feedback effect region, indicating that the grid-connected inverter has a risk of harmonic oscillation; if the positive feedback effect region disappears, then the feedback loop exhibits a negative feedback loop effect, which is beneficial to the stable operation of the grid-connected inverter.

[0028] Therefore, changing the grid impedance, phase-locked loop controller parameters, or current inner loop controller parameters can all alter the closed-loop transfer function. The phase frequency characteristics of the inverter affect its stability.

[0029] In a preferred embodiment, the parameters of the current inner loop controller are used as variables to obtain the results under different current inner loop controller parameters. The amplitude and phase frequency characteristics.

[0030] When the parameters of the inner current loop controller are relatively small and fall within the first range... The phase frequency characteristics of the grid-connected inverter exhibit a negative phase shift region in the mid-frequency range. This region is the area where the positive feedback effect occurs, and it corresponds to the frequency range in the phase frequency characteristics of the grid-connected inverter's closed-loop transfer function where the negative phase shift occurs. If the harmonic oscillation frequency of the grid-connected inverter's output current falls into this region, it can easily induce instability in the grid-connected inverter.

[0031] When the parameters of the inner current loop controller increase to a certain range and fall within the second interval... When the positive feedback effect disappears in the phase frequency characteristics, the feedback loop formed by the current inner loop controller exhibits a negative feedback loop effect, and the grid-connected inverter can maintain stable operation.

[0032] When the parameters of the inner current loop controller increase further and fall into the third range... The phase frequency characteristics reappear in the negative phase shift region, and the positive feedback effect region shifts from the mid-frequency band to the high-frequency band. At this time, the grid-connected inverter may exhibit high-frequency harmonic oscillation.

[0033] Therefore, this embodiment can be based on The phase-frequency characteristics reveal that the stability of the grid-connected inverter exhibits a nonlinear change pattern of first instability, then stability, and then instability again as the parameters of the inner current loop controller increase.

[0034] Based on the above amplitude-phase frequency characteristic analysis, it can be seen that under different current inner loop controller parameters... Based on the phase frequency characteristics, the current inner loop controller parameters that eliminate the positive feedback effect region or meet the preset stable operation requirements for harmonic oscillation risk are selected as the target control parameters.

[0035] Specifically, when a certain current inner loop controller parameter corresponds to When there is no positive feedback effect region in the phase frequency characteristics, or when the positive feedback effect region does not cover the harmonic oscillation frequency that the grid-connected inverter may generate, the current inner loop controller parameter is determined as the target control parameter, and the grid-connected inverter is controlled according to the target control parameter.

[0036] By employing the above methods, this embodiment can avoid the risk of harmonic oscillations caused by excessively small or large parameters of the inner current loop controller, thereby improving the stable operation capability of the grid-connected inverter under weak grid conditions.

[0037] Verification section: A three-phase LCL grid-connected inverter system was built in MATLAB / Simulink. The main parameters are shown in Table 1.

[0038] Table 1 Main System Parameters ; To verify the method of this application, simulation analysis was performed on the constructed positive and negative feedback loop effect model based on the grid-connected inverter system parameters. Among them, Figure 6 The closed-loop transfer function when the grid impedance changes. A schematic diagram of the amplitude and phase frequency characteristics.

[0039] observe Figure 6 It can be seen that when the grid impedance increases from 11mH to 13mH, the closed-loop transfer function... The phase-frequency response curve exhibits a negative phase shift region in the range of 118.9Hz to 173.1Hz, which is the region where the positive feedback effect occurs. This indicates that as the grid impedance increases, the feedback loop formed by the grid impedance through the phase-locked loop and the current inner loop controller will exhibit a positive feedback loop effect in the current inner loop of the grid-connected inverter, thereby reducing the stability of the grid-connected inverter and potentially inducing harmonic oscillations in the output current.

[0040] To further verify Figure 6 The theoretical analysis results shown present the output current waveforms of the grid-connected inverter under different grid impedances and their FFT analysis results. Among them, Figure 7 and Figure 8 for Output current waveform of grid-connected inverter and its FFT analysis Figure 9 and Figure 10 for Output current waveform of grid-connected inverter and its FFT analysis.

[0041] observe Figure 7 and Figure 8 It can be seen that when the grid impedance is 11mH, the output current waveform of the grid-connected inverter is relatively stable, with no obvious harmonic oscillations, and its total harmonic distortion (THD) is 3.53%, indicating that the grid-connected inverter can maintain stable operation at this time. Observation Figure 9 and Figure 10 It can be seen that when the grid impedance further increases to 13mH, the output current waveform of the grid-connected inverter exhibits significant distortion, displaying harmonic oscillation. According to the FFT analysis results, the total harmonic distortion (THD) of the output current increases to 5.74% at this point, with the harmonic oscillation frequency and its coupling frequency being 175Hz and 75Hz, respectively. Transforming this to the dq coordinate system, the corresponding harmonic oscillation frequency is 125Hz.

[0042] Depend on Figure 6 and Figure 7 The comparison shows that Figure 7 middle The corresponding harmonic oscillation frequency in the dq coordinate system is 125Hz, which is exactly located at...Figure 6 The positive feedback effect is shown within the range of 118.9Hz to 173.1Hz. This indicates that when the output current harmonic oscillation frequency falls within this range... When the positive feedback effect is in effect, the stability of the grid-connected inverter will decrease, and the output current will generate more severe harmonic distortion.

[0043] Therefore, through Figure 6 middle Amplitude and phase frequency response analysis and Figure 7 Output current waveform and Figure 8 The FFT analysis results verify that the method proposed in this invention can effectively characterize the impact of grid impedance changes on the stability of grid-connected inverters and explain the cause of harmonic oscillations in the output current of grid-connected inverters.

[0044] In summary, the method proposed in this application, by constructing a small-signal control model for a grid-connected inverter that considers DC-side voltage fluctuations, further obtains a closed-loop transfer function characterizing the positive and negative feedback loop effects. This method can explain the intrinsic mechanism of grid-connected inverter stability changes from the perspective of feedback loop effects, thereby providing a basis for the parameter tuning of the grid-connected inverter current inner loop controller, the identification of harmonic oscillation risks, and the stable operation control.

[0045] In some embodiments, this application also provides a computer system including a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the steps in the above-described method embodiments.

[0046] This application also provides a computer-readable storage medium for storing a computer program. This computer-readable storage medium can be applied to a computer device, and the computer program causes the computer device to execute the corresponding processes in the methods described above in the embodiments of this application; for brevity, further details are omitted here.

[0047] The above embodiments are preferred implementations of this application. In addition, this application can be implemented in other ways. Any obvious substitutions without departing from the concept of this technical solution are within the protection scope of this application.

[0048] To facilitate understanding by those skilled in the art of the improvements made by this application compared to the prior art, some of the accompanying drawings and descriptions have been simplified, and for clarity, some other elements have been omitted from this application. Those skilled in the art should realize that these omitted elements may also constitute the content of this application.

Claims

1. A method for identifying the oscillation instability frequency of a grid-connected inverter, characterized in that, include: Construct a small-signal control model for a grid-connected inverter that considers DC-side voltage fluctuations; Based on the small-signal control model of the grid-connected inverter, the feedback loop formed by the grid impedance through the phase-locked loop and the current inner loop controller is determined. Based on the feedback loop, the target control parameters are determined.

2. The method according to claim 1, characterized in that, The method based on the small-signal control model of the grid-connected inverter determines the feedback loop formed by the grid impedance via a phase-locked loop and a current inner-loop controller; including: Based on the small-signal control model of the grid-connected inverter, and combined with the main circuit model of the grid-connected inverter, the outer loop of DC bus voltage, the inner loop controller of current, the phase-locked loop structure, and the PWM sampling and calculation delay model, a block diagram of the small-signal control of the grid-connected inverter is constructed. The small-signal control block diagram of the grid-connected inverter is extracted to determine the feedback loop formed by the grid impedance through the phase-locked loop and the current inner loop controller.

3. The method according to claim 2, characterized in that, The phase-locked loop structure includes a current feedback loop introduced at the current reference value and a current feedback loop introduced at the modulation signal.

4. The method according to claim 3, characterized in that, The target control parameters are determined based on the feedback loop; Based on the feedback loop, an equivalent transfer function model of each link in the inner current loop of the grid-connected inverter is constructed. Based on the equivalent transfer function model, a closed-loop transfer function is constructed to characterize the positive and negative feedback loop effects. The closed-loop transfer function is used to characterize the positive and negative feedback loop effects formed by the grid impedance through the phase-locked loop and the current inner loop controller in the forward channel of the grid-connected inverter current inner loop. Based on the closed-loop transfer function, the amplitude-phase-frequency characteristics of the closed-loop transfer function are determined under different current inner-loop controller parameters; Based on the amplitude-phase-frequency characteristics, the target control parameters are determined.

5. The method according to claim 4, characterized in that, The feedback loop includes a q-axis loop and a d-axis loop. Based on the feedback loop, the equivalent transfer function model of each link in the inner current loop of the grid-connected inverter is constructed, including: The q-axis loop of the feedback loop is equivalently included in the d-axis loop, and the feedback loop is transformed to construct the equivalent transfer function model of each link in the inner current loop of the grid-connected inverter.

6. The method according to claim 4, characterized in that, The step of determining the amplitude-phase-frequency characteristics of the closed-loop transfer function under different current inner-loop controller parameters based on the closed-loop transfer function includes: By setting the current inner loop controller parameters in the closed-loop transfer function as variables, the amplitude-phase-frequency characteristics of the closed-loop transfer function under different current inner loop controller parameters are determined.

7. The method according to claim 6, characterized in that, The determination of the target control parameters based on the amplitude-phase frequency characteristics includes: If the amplitude-phase frequency characteristic of the closed-loop transfer function corresponding to the current inner loop controller parameter does not have a positive feedback effect region, or the positive feedback effect region does not cover the harmonic oscillation frequency generated by the grid-connected inverter, then the current inner loop controller parameter shall be used as the target control parameter; the positive feedback effect region is the frequency range in the phase frequency characteristic of the grid-connected inverter closed-loop transfer function where a negative phase shift occurs.

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