Subsynchronous oscillation suppression method and system for grid-connected inverter

Abstract

The invention discloses a grid-connected inverter subsynchronous oscillation suppression method and system, and the method comprises the steps: enabling the impedance of a grid-connected inverter to be represented as a single-input single-output matrix in an order reduction manner, and obtaining a sensitive frequency band of the grid-connected inverter with an oscillation risk based on the impedance ratio criterion of the single-input single-output matrix; key parameters influencing the stability of a sensitive frequency band are obtained through sensitivity analysis, and parameter compensation is performed on the sensitive frequency band through a single-resonance controller, so that subsynchronous oscillation caused by a grid-connected inverter is inhibited. The subsynchronous oscillation suppression method and system for the grid-connected inverter are high in universality and wide in application range, can still be applied when control parameters of the grid-connected inverter change, and can enhance the stability of the inverter for a subsynchronous frequency band.

Description

Technical Field

[0001] The present invention relates to the technical field of grid-connected inverter control, and in particular to a method and system for suppressing subsynchronous oscillation of a grid-connected inverter. Background Art

[0002] As the proportion of new energy in the power system increases, grid-connected inverters, as the interface between new energy and the power grid, are often connected to the grid through longer lines and transformers, which reduces the stability of the PCC (Point of Critical Coupling). At the same time, due to the characteristics of the inverter controller, its output impedance presents a capacitive negative resistance characteristic in certain frequency bands, and there may be a risk of RLC circuit resonance with the inductive line impedance.

[0003] In the process of impedance modeling and analysis, the traditional grid-connected inverter has the problem of frequency coupling, which means that the impedance in the synchronous rotating coordinate system needs to be represented by a second-order multiple-input multiple-output (MIMO) matrix. The analysis of system stability requires the use of the generalized Nyquist theorem to analyze the augmented impedance ratio matrix, which greatly increases the complexity of the analysis and makes it difficult to analyze the key parameters that affect stability. Relevant literature has proposed the conditions for reducing the MIMO matrix to a single-input single-output (SISO) matrix. The SISO matrix not only has multiple criteria for stability judgment, but also can analyze the influence and sensitivity of various physical quantities on impedance, thereby more clearly showing the influence of various physical quantities on stability.

[0004] Common oscillation suppression strategies include parameter setting, virtual impedance, etc. However, due to the existence of frequency coupling effect, these common oscillation suppression strategies are difficult to improve the stability of a certain frequency band without affecting the performance of other frequency bands, and may cause unexpected negative effects when used. Summary of the invention

[0005] In order to solve the above problems, the present invention proposes a method and system for suppressing subsynchronous oscillation of a grid-connected inverter, which has strong versatility and a wide range of applications, and can accurately determine the subsynchronous oscillation risk of the grid-connected inverter and suppress the oscillation.

[0006] The technical solution adopted by the present invention is as follows:

[0007] On the one hand, the present invention provides a method for suppressing subsynchronous oscillation of a grid-connected inverter, comprising:

[0008] The impedance of the grid-connected inverter is reduced to a single-input single-output matrix, and the sensitive frequency band of the grid-connected inverter with oscillation risk is obtained based on the impedance ratio criterion of the single-input single-output matrix.

[0009] The key parameters that affect the stability of the sensitive frequency band are obtained through sensitivity analysis, and the parameters of the sensitive frequency band are compensated through a single resonant controller, thereby suppressing the subsynchronous oscillation caused by the grid-connected inverter.

[0010] Furthermore, the impedance of the grid-connected inverter is reduced to a single-input single-output matrix, including: in a dq coordinate system, i.e., a two-phase rotating coordinate system, a controller model based on a symmetric phase-locked loop and a current loop is established for the grid-connected inverter, and the impedance of the grid-connected inverter in the dq coordinate system is obtained based on the controller model.

[0011] Furthermore, the impedance ratio criterion based on the single-input single-output matrix obtains the sensitive frequency band where the grid-connected inverter has an oscillation risk, including: equating the ratio of the grid-connected inverter impedance to the line impedance to an open-loop transfer function, analyzing the open-loop transfer function through the Nyquist stability criterion, and analyzing the amplitude margin and the phase margin through the Bode diagram, thereby obtaining the sensitive frequency band where the grid-connected inverter has an oscillation risk.

[0012] Furthermore, obtaining the key parameters affecting the stability of the sensitive frequency band through sensitivity analysis includes: analyzing the influence of relevant parameters on the real part of the impedance of the sensitive frequency band through sensitivity partial derivatives to determine the key parameters affecting the sensitive frequency band.

[0013] Furthermore, the parameter compensation for the sensitive frequency band by using a single resonant controller includes: introducing a single resonant controller in a relevant control link of key parameters, thereby compensating for the stability of the grid-connected inverter in the sensitive frequency band and suppressing subsynchronous oscillation.

[0014] On the other hand, the present invention provides a subsynchronous oscillation suppression system for a grid-connected inverter, comprising:

[0015] The impedance reduction and analysis module is configured to represent the impedance reduction of the grid-connected inverter as a single-input single-output matrix, and obtain a sensitive frequency band of the grid-connected inverter where there is an oscillation risk based on an impedance ratio criterion of the single-input single-output matrix;

[0016] The subsynchronous oscillation suppression module is configured to obtain key parameters affecting the stability of sensitive frequency bands through sensitivity analysis, and to perform parameter compensation on the sensitive frequency bands through a single resonant controller, thereby suppressing the subsynchronous oscillation caused by the grid-connected inverter.

[0017] Furthermore, in the impedance reduction and analysis module, the impedance reduction of the grid-connected inverter is represented as a single-input single-output matrix, including: in the dq coordinate system, i.e., the two-phase rotating coordinate system, a controller model based on a symmetrical phase-locked loop and a current loop is established for the grid-connected inverter, and the impedance of the grid-connected inverter in the dq coordinate system is obtained based on the controller model.

[0018] Furthermore, in the impedance reduction and analysis module, a sensitive frequency band where the grid-connected inverter has an oscillation risk is obtained based on an impedance ratio criterion of a single-input single-output matrix, including: equating the ratio of the grid-connected inverter impedance to the line impedance to an open-loop transfer function, analyzing the open-loop transfer function through the Nyquist stability criterion, and analyzing the amplitude margin and the phase margin through a Bode diagram, thereby obtaining the sensitive frequency band where the grid-connected inverter has an oscillation risk.

[0019] Furthermore, in the subsynchronous oscillation suppression module, key parameters affecting the stability of the sensitive frequency band are obtained through sensitivity analysis, including: analyzing the influence of relevant parameters on the real part of the impedance of the sensitive frequency band through sensitivity partial derivative, and determining the key parameters affecting the sensitive frequency band.

[0020] Furthermore, in the subsynchronous oscillation suppression module, a single resonant controller is used to perform parameter compensation on a sensitive frequency band, including: introducing a single resonant controller in a related control link of key parameters, thereby compensating for the stability of the grid-connected inverter in the sensitive frequency band and suppressing subsynchronous oscillations.

[0021] The beneficial effects of the present invention are:

[0022] The present invention can obtain the sensitive frequency band where the grid-connected inverter has the risk of oscillation through impedance reduction and the impedance ratio criterion of the single-input single-output matrix; the key parameters affecting the stability of the sensitive frequency band can be obtained through sensitivity analysis, and the sensitive frequency band can be parameter-compensated through a single resonant controller, thereby suppressing the subsynchronous oscillation caused by the grid-connected inverter. The present invention can improve the stability of a single frequency band without affecting the performance of other frequency bands. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The present invention is a flow chart of a method for suppressing subsynchronous oscillation of a grid-connected inverter.

[0024] Figure 2 This is the topological diagram of the three-phase grid-connected inverter.

[0025] Figure 3 This is a diagram showing the influence of SISO impedance on frequency coupling effect.

[0026] Figure 4 Theoretical and swept frequency Bode plots of complex impedance.

[0027] Figure 5 Bode plot of impedance ratio.

[0028] Figure 6 This is the FFT analysis diagram of the simulated oscillating current.

[0029] Figure 7 This is the result diagram of sensitivity analysis.

[0030] Figure 8 Figure 2 is the block diagram of a single resonant controller.

[0031] Fig. 9 Block diagram for compensating the proportional parameters of a phase-locked loop.

[0032] Fig.10 Inverter impedance ratio diagram before and after compensating the phase-locked loop ratio parameters

[0033] Fig.11 It is the waveform diagram of grid-connected power. DETAILED DESCRIPTION

[0034] In order to have a clearer understanding of the technical features, purposes and effects of the present invention, the specific implementation methods of the present invention are now described. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention, that is, the embodiments described are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative work are within the scope of protection of the present invention.

[0035] Example 1

[0036] At present, common oscillation suppression strategies include parameter setting, virtual impedance, etc. However, due to the existence of frequency coupling effect, these oscillation suppression strategies are difficult to improve the stability of a certain frequency band without affecting the performance of other frequency bands, and may cause unexpected negative effects when used.

[0037] Based on this, this embodiment provides a method for suppressing subsynchronous oscillation of a grid-connected inverter proposed by the present invention, such as Figure 1 As shown, the method includes: reducing the impedance of the grid-connected inverter to a single-input single-output matrix, and obtaining the sensitive frequency band where the grid-connected inverter has an oscillation risk based on the impedance ratio criterion of the single-input single-output matrix; obtaining the key parameters affecting the stability of the sensitive frequency band through sensitivity analysis, and performing parameter compensation on the sensitive frequency band through a single resonant controller, thereby suppressing the subsynchronous oscillation caused by the grid-connected inverter.

[0038] Specifically, the method for suppressing subsynchronous oscillation of a grid-connected inverter of this embodiment can be implemented by the following steps:

[0039] S1: Small signal modeling of the symmetrical phase-locked loop, current loop and L-type inductor filter is performed to obtain the SISO small signal matrix T of the grid-connected voltage and grid-connected current in the dq coordinate system based on the current loop and the symmetrical phase-locked loop. v-i , T I-i ;

[0040] S2: Use T v-i , T I-iWrite down the impedance equation of the inverter in the synchronous rotating coordinate system and solve the equivalent PCC point SISO complex impedance Z pcc , the complex impedance expression is as follows:

[0041]

[0042] Among them, G i (s), and G pll (s) are the current loop transfer function and the symmetrical phase-locked loop transfer function, L is the filter inductance, I d ,I q is the grid-connected current in the rotating coordinate system. Complex vectors representing voltage and current at the grid connection point;

[0043] S3: The ratio of complex impedance to line impedance is equivalent to an open-loop transfer function, and the amplitude margin and phase margin are analyzed by using the Bode diagram to determine the stability of the system in the subsynchronous frequency band;

[0044] S4: Analyze the influence of relevant parameters on the real part of the impedance of the risk frequency band through sensitivity partial derivative, and find out the key parameters affecting the risk frequency band;

[0045] S5: Introduce a single resonant controller in the relevant control links of key factors to achieve the effect of oscillation suppression by compensating system stability in a fixed frequency band without affecting the performance of other frequency bands.

[0046] It should be noted that the impedance of the classic grid-connected inverter is a multiple-input multiple-output (MIMO) impedance, which is not conducive to mathematical analysis. After the impedance is reduced, the system becomes a single-input single-output (SISO) system, and the impedance ratio of its output impedance and line impedance will also be reduced to one dimension, and then the one-dimensional Nyquist stability criterion can be used to judge the system stability, reducing the difficulty of stability analysis.

[0047] In the stability analysis based on impedance ratio, the mechanism of subsynchronous oscillation of grid-connected inverter is roughly as follows: in the context of inductive line impedance on the grid side, when the SISO complex impedance is equivalent to a resistor in series with a capacitor in a certain frequency band, the grid-connected system may experience RLC oscillation, and at this time, the sufficient condition for system stability is that the real part of the impedance is greater than zero. Therefore, when there is a risk of oscillation in the system, it is hoped that by performing sensitivity partial derivative analysis on the subsynchronous frequency band, the key parameters that mainly affect the real part of the impedance in this frequency band can be obtained.

[0048] In this embodiment, considering the coupling between the phase-locked loop and the weak grid, the voltage disturbance at the PCC point will enter the controller system through the phase-locked loop, thereby affecting the system stability. Therefore, it is necessary to consider the inverter impedance under the coupling between the phase-locked loop and the weak grid.

[0049] Figure 2 The topological structure diagram of the three-phase grid-connected inverter, where V A 、V B 、V C is the three-phase PCC point voltage, I A ,I B ,I C is the three-phase grid-connected current, V ga 、V gb 、V gc is the three-phase grid voltage, Z L is the line impedance, L is the inverter side filter, V DC is the DC side voltage of the inverter, G pll (s) is the phase-locked loop transfer function, G c (s) is the power loop transfer function, G i (s) is the current loop transfer function, V dq is the voltage at the PCC point in the synchronously rotating coordinate system, I dq is the grid-connected current in the synchronous rotating coordinate system, I dqref is the reference grid current in the synchronous rotating coordinate system, V invdq is the inverter side output voltage in the synchronous rotating coordinate system, D abc is the modulation ratio.

[0050] For the system small signal model that takes into account the power loop, symmetrical phase-locked loop and current loop, such as Figure 2 As shown, the relationship between the phase obtained by the symmetrical phase-locked loop and the grid-connected point voltage is as follows:

[0051]

[0052] The symmetrical phase-locked loop supplements the dimension by introducing the direct-axis grid information, and can compensate the coefficient matrix of small voltage disturbances into a symmetrical matrix, that is, suppress the negative sequence impedance to zero, and finally suppress the frequency coupling when only considering the symmetrical phase-locked loop and the current loop.

[0053] When only the phase-locked loop and the current inner loop are considered, the SISO small signal matrix T of the grid-connected voltage and grid-connected current is obtained. V-i , T I-i as follows:

[0054]

[0055] T I-i =(G i (S)+sL)E

[0056] Where E is the second-order identity matrix, G pll (s) represents the phase-locked loop transfer function, G i (s) represents the current loop transfer function.

[0057] Figure 3 This is the influence diagram of SISO impedance on frequency coupling effect, which mainly includes FFT analysis with 370Hz voltage disturbance added. Figure 3 It can be seen that after adding the 370Hz voltage disturbance, the grid-connected current of the inverter model including the current loop and the symmetrical phase-locked loop only produced a 370Hz response, and the frequency coupling phenomenon did not occur. It is proved that the second-order MIMO impedance can be reduced to the SISO complex impedance.

[0058] Therefore, the inverter impedance equation can be written as follows:

[0059]

[0060] The SISO complex impedance is solved as follows:

[0061]

[0062] Among them, G i (s) and G pll (s) are the current loop transfer function and phase-locked loop transfer function in the Laplace domain, L is the filter inductance, I d ,I q is the grid-connected current in the rotating coordinate system.

[0063] Figure 4 The theory and swept frequency Bode plot of complex impedance, from Figure 4 It can be seen that the theoretical impedance model and the impedance model obtained by frequency sweep are almost the same, which verifies the correctness of the small signal model. Finally, the system stability can be determined by using the SISO complex impedance model.

[0064] Figure 5 The Bode diagram of the impedance ratio is shown in Figure 2. According to the phase margin, the resonant frequency of the system is 70Hz. The FFT analysis of the current during oscillation is obtained through simulation. Figure 6 As shown. Taking the phase-locked loop ratio parameters and current loop ratio parameters as examples, the sensitivity analysis results are shown in Figure 7 As shown, the unnormalized sensitivity of the phase-locked loop proportional parameter is much greater than that of the current loop proportional parameter in the frequency band near 70 Hz, so relatively speaking, the phase-locked loop proportional parameter dominates in this frequency band. After comparing various parameters, it is concluded that the phase-locked loop proportional parameter is the key parameter of subsynchronous oscillation.

[0065] Under the condition of known sensitive frequency bands and key factors, in order to enhance the system stability at 70Hz, the invention simulates the adjustment of key factors by paralleling a single resonant controller. Due to the single-band gain characteristics of the single resonant controller, the invention will not affect other frequency bands.

[0066] like Figure 8 The block diagram of a single resonant controller is shown, and its transfer function is as follows:

[0067]

[0068] Through zero-pole cancellation, the transfer function is reduced from second order to first order, thus realizing a single resonant controller. At the same time, in the SISO system, each physical quantity can find a physical quantity orthogonal to it, that is, the j operation with imaginary units can be realized in the controller, such as X d =jX q The key parameter is k p When the single resonant controller is connected in parallel, the performance of the key parameter in the resonant frequency band tends to the key parameter k b +k p performance, thereby achieving the effect of enhancing stability.

[0069] Fig. 9 Block diagram for compensating the proportional parameters of a phase-locked loop, Fig.10 The inverter impedance ratio diagram before and after compensating the phase-locked loop proportional parameter shows that the amplitude-frequency characteristic curve at 70Hz is far away from the zero decibel line and the system is stable. The phase-locked loop proportional parameter is reduced at t=0.35s, a disturbance is introduced at t=0.4s, and then the suppression strategy is turned on at t=0.8s.

[0070] Since the grid-connected current is an AC quantity, it is not convenient to display it. Therefore, for the convenience of explanation, Fig.11 The power waveform is given by Fig.11 It can be seen that the addition of the oscillation suppression strategy suppresses the subsynchronous oscillation of the system caused by disturbances, proving the effectiveness of the suppression strategy.

[0071] Example 2

[0072] This embodiment provides a subsynchronous oscillation suppression system for a grid-connected inverter, including:

[0073] The impedance reduction and analysis module is configured to represent the impedance reduction of the grid-connected inverter as a single-input single-output matrix, and obtain a sensitive frequency band of the grid-connected inverter where there is an oscillation risk based on an impedance ratio criterion of the single-input single-output matrix;

[0074] The subsynchronous oscillation suppression module is configured to obtain key parameters affecting the stability of sensitive frequency bands through sensitivity analysis, and to perform parameter compensation on the sensitive frequency bands through a single resonant controller, thereby suppressing the subsynchronous oscillation caused by the grid-connected inverter.

[0075] Preferably, in the impedance reduction and analysis module, the impedance reduction of the grid-connected inverter is represented as a single-input single-output matrix, including: in the dq coordinate system, i.e., the two-phase rotating coordinate system, a controller model based on a symmetrical phase-locked loop and a current loop is established for the grid-connected inverter, and the impedance of the grid-connected inverter in the dq coordinate system is obtained based on the controller model.

[0076] Preferably, in the impedance reduction and analysis module, a sensitive frequency band where the grid-connected inverter has an oscillation risk is obtained based on an impedance ratio criterion of a single-input single-output matrix, including: equating the ratio of the grid-connected inverter impedance to the line impedance to an open-loop transfer function, analyzing the open-loop transfer function through the Nyquist stability criterion, and analyzing the amplitude margin and phase margin through a Bode diagram, thereby obtaining the sensitive frequency band where the grid-connected inverter has an oscillation risk.

[0077] Preferably, in the subsynchronous oscillation suppression module, key parameters affecting the stability of the sensitive frequency band are obtained through sensitivity analysis, including: analyzing the influence of relevant parameters on the real part of the impedance of the sensitive frequency band through sensitivity partial derivatives to determine the key parameters affecting the sensitive frequency band.

[0078] Preferably, in the subsynchronous oscillation suppression module, parameter compensation is performed on the sensitive frequency band through a single resonant controller, including: introducing a single resonant controller in the relevant control link of key parameters, thereby compensating the stability of the grid-connected inverter in the sensitive frequency band and suppressing subsynchronous oscillation.

[0079] Example 3

[0080] This embodiment is based on embodiment 1:

[0081] This embodiment provides a computer device, including a memory and a processor, wherein the memory stores a computer program, and when the processor executes the computer program, the method for suppressing subsynchronous oscillation of a grid-connected inverter of Embodiment 1 is implemented. The computer program may be in source code form, object code form, executable file, or some intermediate form.

[0082] Example 4

[0083] This embodiment is based on embodiment 1:

[0084] This embodiment provides a computer-readable storage medium storing a computer program, which, when executed by a processor, implements the method for suppressing subsynchronous oscillation of a grid-connected inverter of Embodiment 1. The computer program may be in source code form, object code form, executable file, or some intermediate form, etc. The storage medium includes: any entity or device capable of carrying computer program code, recording medium, computer memory, read-only memory (ROM), random access memory (RAM), electric carrier signal, telecommunication signal, and software distribution medium, etc. It should be noted that the content contained in the storage medium may be appropriately increased or decreased according to the requirements of legislation and patent practice in the jurisdiction. For example, in some jurisdictions, according to legislation and patent practice, the storage medium does not include electric carrier signals and telecommunication signals.

[0085] It should be noted that, for the aforementioned method embodiments, for the sake of simplicity of description, they are expressed as a series of action combinations, but those skilled in the art should be aware that the present application is not limited by the order of the actions described, because according to the present application, certain steps can be performed in other orders or simultaneously. Secondly, those skilled in the art should also be aware that the embodiments described in the specification are all preferred embodiments, and the actions and modules involved are not necessarily required by the present application.

Claims

1. A method for suppressing subsynchronous oscillation of a grid-connected inverter, characterized in that: include: The impedance of the grid-connected inverter is reduced to a single-input single-output matrix, and the sensitive frequency band of the grid-connected inverter with oscillation risk is obtained based on the impedance ratio criterion of the single-input single-output matrix. The key parameters that affect the stability of the sensitive frequency band are obtained through sensitivity analysis, and the parameters of the sensitive frequency band are compensated through a single resonant controller, thereby suppressing the subsynchronous oscillation caused by the grid-connected inverter.

2. A method for suppressing subsynchronous oscillation of a grid-connected inverter according to claim 1, characterized in that: The impedance of the grid-connected inverter is reduced to a single-input single-output matrix, including: in a dq coordinate system, i.e., a two-phase rotating coordinate system, a controller model based on a symmetrical phase-locked loop and a current loop is established for the grid-connected inverter, and the impedance of the grid-connected inverter in the dq coordinate system is obtained based on the controller model.

3. A method for suppressing subsynchronous oscillation of a grid-connected inverter according to claim 1, characterized in that: The impedance ratio criterion based on the single-input single-output matrix obtains the sensitive frequency band where the grid-connected inverter has an oscillation risk, including: equating the ratio of the grid-connected inverter impedance to the line impedance to an open-loop transfer function, analyzing the open-loop transfer function through the Nyquist stability criterion, and analyzing the amplitude margin and the phase margin through the Bode diagram, so as to obtain the sensitive frequency band where the grid-connected inverter has an oscillation risk.

4. A method for suppressing subsynchronous oscillation of a grid-connected inverter according to claim 1, characterized in that: The obtaining of key parameters affecting the stability of the sensitive frequency band by sensitivity analysis includes: analyzing the influence of relevant parameters on the real part of the impedance of the sensitive frequency band by sensitivity partial derivative, and determining the key parameters affecting the sensitive frequency band.

5. A method for suppressing subsynchronous oscillation of a grid-connected inverter according to claim 1, characterized in that: The method of performing parameter compensation on the sensitive frequency band by using a single resonant controller includes: introducing a single resonant controller into a relevant control link of key parameters, thereby compensating for the stability of the grid-connected inverter in the sensitive frequency band and suppressing subsynchronous oscillation.

6. A subsynchronous oscillation suppression system for a grid-connected inverter, characterized in that: include: The impedance reduction and analysis module is configured to represent the impedance reduction of the grid-connected inverter as a single-input single-output matrix, and obtain a sensitive frequency band of the grid-connected inverter where there is an oscillation risk based on an impedance ratio criterion of the single-input single-output matrix; The subsynchronous oscillation suppression module is configured to obtain key parameters affecting the stability of sensitive frequency bands through sensitivity analysis, and to perform parameter compensation on the sensitive frequency bands through a single resonant controller, thereby suppressing the subsynchronous oscillation caused by the grid-connected inverter.

7. A grid-connected inverter subsynchronous oscillation suppression system according to claim 6, characterized in that: In the impedance reduction and analysis module, the impedance reduction of the grid-connected inverter is represented as a single-input single-output matrix, including: in the dq coordinate system, i.e., the two-phase rotating coordinate system, a controller model based on a symmetrical phase-locked loop and a current loop is established for the grid-connected inverter, and the impedance of the grid-connected inverter in the dq coordinate system is obtained based on the controller model.

8. The subsynchronous oscillation suppression system of a grid-connected inverter according to claim 6, characterized in that: In the impedance reduction and analysis module, a sensitive frequency band where the grid-connected inverter has an oscillation risk is obtained based on an impedance ratio criterion of a single-input single-output matrix, including: equating the ratio of the grid-connected inverter impedance to the line impedance to an open-loop transfer function, analyzing the open-loop transfer function through a Nyquist stability criterion, and analyzing the amplitude margin and the phase margin through a Bode diagram, thereby obtaining the sensitive frequency band where the grid-connected inverter has an oscillation risk.

9. The subsynchronous oscillation suppression system of a grid-connected inverter according to claim 6, characterized in that: In the subsynchronous oscillation suppression module, key parameters affecting the stability of the sensitive frequency band are obtained through sensitivity analysis, including: analyzing the influence of relevant parameters on the real part of the impedance of the sensitive frequency band through sensitivity partial derivative, and determining the key parameters affecting the sensitive frequency band.

10. The subsynchronous oscillation suppression system of a grid-connected inverter according to claim 6, characterized in that: In the subsynchronous oscillation suppression module, a single resonant controller is used to perform parameter compensation on a sensitive frequency band, including: introducing a single resonant controller into a relevant control link of key parameters, thereby compensating for the stability of the grid-connected inverter in the sensitive frequency band and suppressing subsynchronous oscillation.

Images