Analysis system and method for determining amplitude-frequency oscillation transmission of grid-connected inverter under control of VSG

Abstract

The invention discloses an analysis system and method for judging amplitude-frequency oscillation transmission of a grid-connected inverter under VSG control, and belongs to the technical field of virtual synchronous generator control. The analysis system comprises a frequency self-stabilization closed-loop model, a voltage self-stabilization closed-loop model and a stabilization loop model; each of the frequency self-stabilization closed-loop model and the voltage self-stabilization closed-loop model comprises a forward channel and a feedback channel, and the stabilization loop model comprises a forward channel and two feedback channels. According to the method, a new physical perspective is provided for analysis of an oscillation transmission mechanism between the frequency and the voltage of the virtual synchronous generator, and occurrence conditions of oscillation transmission are explained from the physical perspective; the invention further provides an analysis method for judging the oscillation transmission of the grid-connected inverter under the control of the VSG, and the amplitude-frequency oscillation propagation generation condition between the grid-connected inverter under the control of the VSG and a large power grid can be judged more visually and clearly.

Description

Technical Field

[0001] The present invention mainly relates to the technical field of virtual synchronous generator control, and specifically to a system and method for determining the amplitude-frequency oscillation transmission analysis of a grid-connected inverter under VSG control. Background Art

[0002] The principle of low frequency oscillation (LFO) of voltage and frequency when grid-connected virtual synchronous generators are running has recently become a research hotspot in this field. The grid-connected operation stability of system frequency and voltage and LFO will limit the development of virtual synchronous generator technology, such as frequency regulation capability, inertial support, frequency and voltage support, and improvement of short-circuit ratio. Therefore, it is of great significance to study the mechanism and physical meaning of LFO and other types of instability phenomena.

[0003] Existing studies on VSG stability mainly focus on power coupling mechanism, LFO and frequency / voltage stability. However, the mechanism of oscillation propagation (OP) and the physical meaning of frequency / voltage interaction behavior are rarely studied. The traditional frequency / voltage closed-loop model cannot determine the coupling mechanism and OP relationship between frequency and voltage in VSG systems. Summary of the invention

[0004] The technical solution of the present invention aims at the technical problem that the existing technical solutions are too single, and provides a solution that is significantly different from the existing technologies. It mainly provides a system and method for determining the amplitude-frequency oscillation transfer analysis of the grid-connected inverter under VSG control, so as to solve the technical problem that the traditional frequency / voltage closed-loop model proposed in the above background technology cannot determine the coupling mechanism and OP relationship between frequency and voltage in the VSG system.

[0005] The technical solution adopted by the present invention to solve the above technical problems is:

[0006] A system for determining the amplitude-frequency oscillation transfer analysis of a grid-connected inverter under VSG control, comprising:

[0007] The frequency self-stabilizing closed-loop model includes a forward channel and a feedback channel. The forward channel includes the transfer coupling term from frequency to active power, inertia and damping terms, and the transfer coupling term from active power to frequency. The feedback channel is unit negative feedback.

[0008] The voltage self-stabilization closed-loop model includes a forward channel and a feedback channel. The forward channel includes a voltage-to-reactive power transfer coupling term, a virtual inertia and damping term, and a reactive power-to-voltage transfer coupling term. The feedback channel is a unit negative feedback.

[0009] A stable loop model is proposed, including a forward channel and two feedback channels. The forward channel includes an integral link, a transfer coupling term from frequency to RoCoV, and a transfer coupling term from RoCoV to voltage. The first feedback channel includes a transfer coupling term from voltage to RoCoV, and the second feedback channel includes a transfer coupling term from voltage to RoCoF (frequency change rate) and a transfer coupling term from frequency to RoCoF.

[0010] Specifically, in the frequency self-stabilizing closed-loop model, the small signal value of the frequency reference value is compared with the small signal amount of the frequency, and then the small signal amount of the frequency is obtained through the frequency-to-active power transfer coupling term, the virtual inertia damping term, and the active power-to-frequency transfer coupling term. The frequency small signal amount is then fed back and compared with the small signal amount of the frequency reference value to form a closed-loop model.

[0011] Specifically, in the voltage self-stabilization closed-loop model, the small signal value of the voltage reference value is compared with the small signal amount of the voltage, and the small signal amount of the voltage is obtained through the voltage-to-reactive transfer coupling term and the virtual inertia damping term. The small signal amount of the voltage is then fed back and compared with the small signal value of the voltage reference value to form a closed-loop model.

[0012] Specifically, in the amplitude-frequency stabilization closed-loop model, the small signal value of the RoCoF reference value is compared with the transfer coupling feedback value of the small signal voltage, and then the small signal frequency is obtained through the transfer coupling term from RoCoF to frequency. The small signal frequency is compared with the transfer coupling term of voltage through the transfer coupling term from frequency to RoCoV to obtain RoCoV, and is compared with the transfer coupling term of voltage through the feedback of the transfer coupling term from frequency to RoCoF to obtain RoCoF. RoCoV then obtains the small signal voltage through the transfer coupling term from RoCoV to voltage, and the small signal voltage is fed back through two branches to form a closed-loop model.

[0013] The present invention also provides a method for determining the amplitude-frequency oscillation transfer analysis of a grid-connected inverter under VSG control. Based on the above-mentioned system for determining the amplitude-frequency oscillation transfer analysis of a grid-connected inverter under VSG control, by determining the frequency ISIM and the voltage ISIM in the frequency self-stabilizing closed-loop model, the voltage self-stabilizing closed-loop model, and the self-stabilizing loop model, it is gradually determined whether frequency and voltage oscillations and transfer oscillations occur.

[0014] Specifically, the method for determining the amplitude-frequency oscillation transmission of the grid-connected inverter under VSG control includes the following steps:

[0015] Step 1: Establish the model transfer functions of frequency self-stabilization closed-loop model, voltage self-stabilization closed-loop model, frequency ISIM, and voltage ISIM;

[0016] Step 2: Determine whether the frequency self-stabilizing closed-loop model and the voltage self-stabilizing closed-loop model are stable; if G of(s) is stable, jump to step 3.1; otherwise, the unstable frequency self-stabilizing loop (FSSL) will cause frequency oscillation; if G oE If (s) is stable, then jump to step 3.2; otherwise, the unstable voltage self-stabilizing loop (VSSL) will cause VM oscillation;

[0017] Step 3.1: Determine G oE (s) Is it stable? If it is stable, the frequency is stable; otherwise, jump to step 4.1;

[0018] Step 3.2: Determine G of (s) Is it stable? If so, the voltage is stable; otherwise, jump to step 4.2;

[0019] Step 4.1: Determine the oscillation transfer coefficient k of the frequency ISIM Ef Is it greater than or equal to 1? If it is less than 1, the voltage oscillation will not be transmitted, and the frequency will be stable; otherwise, it will trigger the oscillation transmission and cause frequency oscillation;

[0020] Step 4.2: Determine the oscillation transfer coefficient k of the voltage ISIM fE Is it greater than or equal to 1? If it is lower than 1, the frequency oscillation will not be transmitted and the voltage can remain stable; otherwise, the oscillation will be transmitted, causing voltage oscillation.

[0021] Furthermore, in step 1, the transfer function of the self-stabilizing closed-loop model is The transfer function of the voltage self-stabilization closed-loop model is: The transfer function of frequency ISIM is The transfer function of the voltage ISIM is

[0022] Among them, g1 is the transfer coupling term from frequency to active power, g2 is the transfer coupling term from active power to frequency, D p is the inertia and damping term, k = 1 / (2π); g4 is the transfer coupling from voltage to reactive power, g3 is the transfer coupling from reactive power to voltage, D q are virtual inertia and damping terms; G1 is the integral link, M1 is the transfer coupling term from frequency to RoCoF, and M2 is the transfer coupling term from voltage to RoCoF; G2 is the transfer coupling term from RoCoV to voltage, M3 is the transfer coupling term from frequency to RoCoV (voltage change rate), and M4 is the transfer coupling term from voltage to RoCoV.

[0023] Furthermore, in step 4.1, In step 4.2, Where j represents the imaginary unit, ω cRepresents the zero-crossing frequency of the open-loop Bode plot.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] (1) This invention proposes a new model to explain the oscillation propagation (OP) mechanism between frequency and voltage in VSG system. The main contributions of this invention include:

[0026] 1) It overcomes the shortcomings of the traditional frequency / voltage closed-loop model that it cannot determine the coupling mechanism and OP relationship between frequency and voltage in the VSG system.

[0027] 2) It breaks through the concept of power coupling in VSG systems and proposes a modeling method to clarify the mechanism of OP. In addition, it also proposes the view that voltage stability originates from its self-closed loop or is dynamically transmitted by frequency, and the view that frequency stability originates from its self-closed loop or is dynamically transmitted by voltage.

[0028] 3) The proposed stability-inducing loop model provides a new physical perspective on the OP between frequency and voltage.

[0029] (2) The present invention provides a new physical perspective for analyzing the oscillation transmission mechanism between the frequency and voltage of a virtual synchronous generator, and explains the conditions for the occurrence of oscillation transmission from a physical perspective. The present invention also proposes an analysis method for determining the oscillation transmission of a grid-connected inverter under VSG control, which can more intuitively and clearly determine the conditions for the occurrence of amplitude-frequency oscillation propagation between a grid-connected inverter under VSG control and a large power grid.

[0030] The present invention will be explained in detail below with reference to the accompanying drawings and specific embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 The topological structure and control strategy of the GF-VSG grid connection in the embodiment of the present invention, wherein J represents inertia and D represents damping;

[0032] Figure 2 1 is a self-stabilizing closed-loop model in an embodiment of the present invention, (a) is FSSLM, (b) is VSSLM;

[0033] Figure 3 It is a stabilizing loop model in the embodiment of the present invention;

[0034] Figure 4 It is a flow chart of the VSG system frequency, dynamic voltage self-stabilization and oscillation propagation judgment process in the embodiment;

[0035] Figure 5 The results of the self-stabilizing loop model in the case test in the embodiment, (a) is FSSLM, (b) is VSSLM;

[0036] Figure 6 The results of the stable loop model in the case test in the embodiment, (a) is F-ISIM, (b) is V-ISIM;

[0037] Figure 7 Graphs showing the experimental results in the embodiments, (a)-(d) correspond to the results under the working conditions of cases 1-4, respectively. DETAILED DESCRIPTION

[0038] To facilitate understanding of the present invention, the present invention will be described more comprehensively below with reference to the relevant drawings. Several embodiments of the present invention are given in the drawings, but the present invention can be implemented in different forms and is not limited to the embodiments described in the text. On the contrary, these embodiments are provided to make the content disclosed in the present invention more thorough and comprehensive.

[0039] Taking the virtual synchronous generator (VSG system) as an example, Figure 1 The topology and control scheme of the grid-based virtual synchronous generator (GF-VSG) system are presented. PCU is the abbreviation of “power computing unit”. By simulating the dynamic characteristics of the synchronous generator, the rotor swing equation of the VSG is described as:

[0040]

[0041] Among them, J p , D p , J q and D q are the control coefficients of VSG; δf is the frequency offset (FO) between VSG and the grid, E is the output voltage of VSG; f g Grid frequency; π represents pi; df / dt represents the frequency change rate; dE / dt represents the voltage change rate; U g Represents the grid voltage amplitude; P rf Represents the active power reference value; Q rf represents reactive power reference value; P represents active power; Q represents reactive power.

[0042] The circuit model of VSG can be derived as follows:

[0043]

[0044] in:

[0045] Z=R+sL,L=L f +L g , R = R f +R g .

[0046] Among them, Id represents the d-axis current component in the synchronous rotating inertial reference coordinate system; I q represents the q-axis current component in the synchronous rotating inertial reference coordinate system; δθ represents the difference between the virtual synchronous machine output phase angle and the grid phase; ω represents the virtual synchronous machine output angular frequency; U represents the grid voltage; s represents the Laplace operator; L f Indicates the inductance value of the virtual synchronous machine output filter; L g Represents the equivalent inductance of the power grid; R f Represents the equivalent parasitic resistance of the output filter; R g Indicates the equivalent resistance value of the power grid.

[0047] The active power and reactive power are calculated as follows:

[0048]

[0049] like Figure 2 As shown in the figure, the self-stabilizing closed-loop model in this embodiment, (a) is the frequency self-stabilizing closed-loop model (FSSLM), and (b) is the voltage self-stabilizing closed-loop model (VSSLM). The frequency self-stabilizing closed-loop model (FSSLM) includes a forward channel and a feedback channel. The forward channel includes the frequency-to-active power transfer coupling term (g1), the inertia and damping term (D p ), active to frequency transfer coupling term (g2), k = -1, the feedback channel is unit negative feedback; the voltage self-stabilization closed-loop model (VSSLM) includes a forward channel and a feedback channel. The forward channel includes the voltage to reactive transfer coupling term (g4), the virtual inertia and damping term (D q ), the transfer coupling term from reactive power to voltage (g3), and the feedback channel is unit negative feedback.

[0050] Specifically, in the frequency self-closed-loop model, the small signal value of the frequency reference value is compared with the small signal amount of the frequency, and then the small signal amount of the frequency is obtained through the frequency-to-active power transfer coupling term, the virtual inertia damping term, and the active power-to-frequency transfer coupling term. The frequency small signal amount is then fed back and compared with the small signal amount of the frequency reference value to form a closed-loop model; in the voltage self-stabilization closed-loop model, the small signal value of the voltage reference value is compared with the small signal amount of the voltage, and the small signal amount of the voltage is obtained through the voltage-to-reactive power transfer coupling term and the virtual inertia damping term. The voltage small signal amount is then fed back and compared with the small signal value of the voltage reference value to form a closed-loop model.

[0051] By combining (1), (2) and (3), we can obtain the small signal model of frequency acceleration, voltage acceleration and voltage amplitude (VM):

[0052]

[0053] Among them, M1, M2, M3 and M4 are the coupling terms of frequency acceleration, voltage acceleration, FO (frequency offset) and voltage small signal value. The frequency acceleration here is defined as the rate of change of frequency (RoCoF, i.e. a f = df / dt), voltage acceleration is defined as the rate of change of voltage (RoCoV, i.e. a E =dE / dt).

[0054] like Figure 3 As shown, the stabilizing loop model in this embodiment. The model includes a forward channel and two feedback channels, the forward channel includes an integral link (G1), a transfer coupling term from frequency to RoCoV (voltage change rate) (M3), and a transfer coupling term from RoCoV to voltage (G2, integral link), the first feedback channel includes a transfer coupling term from voltage to RoCoV (M4), and the second feedback channel includes a transfer coupling term from voltage to RoCoF (frequency change rate) (M2) and a transfer coupling term from frequency to RoCoF (M1).

[0055] Specifically, in the amplitude-frequency induced stable closed-loop model, the small signal value of the RoCoF reference value is compared with the transfer coupling feedback value of the small signal voltage, and then the small signal frequency is obtained through the transfer coupling term from RoCoF to frequency. The small signal frequency is compared with the transfer coupling term of voltage through the transfer coupling term from frequency to RoCoV to obtain RoCoV, and is compared with the transfer coupling term of voltage through the transfer coupling term feedback from frequency to RoCoF to obtain RoCoF. RoCoV is then fed back through the transfer coupling term from RoCoV to voltage to obtain the small signal voltage, and the small signal voltage is then fed back through two branches to form a closed-loop model.

[0056] By rearranging Figure 2 , we can get the frequency self-stabilizing closed-loop model (FSSLM) and voltage self-stabilizing closed-loop model (VSSLM) as:

[0057]

[0058] in, g2=1 / (Jp*ωn*s), g3=1 / (Jq*s), g4=-B4, k=1 / (2π). FSSLM is defined as a closed-loop model describing the coupling between RoCoF and frequency, but does not include voltage. VSSLM is defined as a closed-loop model including dynamic RoCoV and voltage, but does not include frequency dynamics.

[0059] Although FSSLM and VSSLM can analyze frequency self-stability and voltage self-stability and their margins, it is difficult to evaluate the dynamic coupling and oscillation transfer between frequency and VM. Therefore, an induction stabilization loop model (ISIM) is proposed to illustrate the coupling effect and oscillation transfer between frequency and VM. The ISIM is obtained as:

[0060]

[0061] Among them G _Ef (s) is defined as frequency ISIM (F-ISIM), G _fE (s) is defined as voltage ISIM (V-ISIM). Using F-ISIM and V-ISIM, the principle of oscillation transmission and coupling effect between frequency and VM can be explained. The judgment flow chart of self-stabilization and oscillation transmission is shown in Figure 4 shown.

[0062] Specifically, the present invention also proposes a method for determining the oscillation transmission analysis of a grid-connected inverter under VSG control, such as Figure 4 As shown:

[0063] Step 1: Build G of (s), G oE (s), and G _Ef (s), G _fE (s) model transfer function.

[0064] Step 2: Determine G separately of (s) and G oE (s) is stable. If G of If G is stable, go to step 3.1; otherwise, the unstable frequency self-stabilizing loop (FSSL) will cause frequency oscillation. oE If (s) is stable, then jump to step 3.2; otherwise, the unstable voltage self-stabilizing loop (VSSL) will cause VM oscillation.

[0065] Step 3.1: Determine G oE (s) is stable. If it is stable, the frequency is stable. Otherwise, jump to step 4.1.

[0066] Step 3.2: Determine G of (s) Is it stable? If it is stable, the voltage is stable. Otherwise, jump to step 4.2.

[0067] Step 4.1: Determine |G _Ef (jωc)| is greater than or equal to 1 (0dB). If it is less than 1, the voltage oscillation will not be transmitted and the frequency will be stable. Otherwise, it will trigger the oscillation transmission and cause frequency oscillation.

[0068] Step 4.2: Determine |G _fE (jωc)| is greater than or equal to 1 (0dB). If it is less than 1, the frequency oscillation will not be transmitted and the voltage can remain stable. Otherwise, the oscillation will be transmitted, causing voltage oscillation.

[0069] In order to better demonstrate the feasibility of the present invention, the specific use method of the present invention and its actual effect are explained below in combination with experimental and simulation results.

[0070] First, the oscillation transfer coefficient is defined, which is obtained by F-ISIM and V-ISIM at the key oscillation frequency point, namely:

[0071]

[0072] Among them, k Ef Defined as the voltage oscillation transfer coefficient (VOPC), it can reflect the frequency-induced stability (FIS). fE Defined as the frequency oscillation transfer coefficient (FOPC), it can explain the voltage induced stability (VIS). In the above formula, j represents the imaginary unit, ω c Represents the zero-crossing frequency of the open-loop Bode plot.

[0073] Note: If k Ef ≥1, the voltage oscillation will be transferred to the frequency, resulting in frequency oscillation; if k Ef <1, the voltage oscillation will not be transmitted to the frequency, resulting in no frequency oscillation. However, there are two prerequisites for this oscillation transmission: 1) k Ef ≥1; 2) Voltage oscillation occurs. If k fE ≥1, the frequency oscillation will be transferred to the voltage, resulting in VM oscillation; if k fE <1, the frequency oscillation will not be transmitted to the voltage, resulting in no voltage oscillation. There are two prerequisites for this oscillation propagation: 1) k fE ≥1; 2) The occurrence of frequency oscillation.

[0074] The following is a verification of the stability margin and cut-off frequency (CF) of GFVSG under different working conditions (cases 1-4). The system parameters under different working conditions are shown in Table 1, and the result data are shown in Table 1. of (s) and G oE (s) The obtained FSSLM and VSSLM results are as follows Figure 5 As shown by G _Ef (s) and G _fE (s) The obtained F-ISIM and V-ISIM results are as follows Figure 6 shown.

[0075] Depend on Figure 5 (a) It can be seen that due to the insufficient phase margin (-3°, -2°, -3° and -3°) of Case 1 to Case 4, there is a risk of weakly damped oscillation in frequency. Figure 5In (b), all amplitudes are below 0db, which indicates that there is sufficient gain margin and therefore the system is stable with respect to VSSLM. The results shown in the figure show that the self-stability of frequency is poor, but the self-stability of voltage is good enough. Figure 4 In the medium standard flow chart, the frequency has a high risk of oscillation and the voltage stability should comply with FOPC.

[0076] from Figure 6 (a) It can be seen that k Ef is always less than 1, indicating that the voltage oscillation is not transmitted. However, since in Case 4, k fE >1, a positive feedback effect is formed, and the frequency oscillation will be transmitted to the voltage. Although VSSL is stable, it will also cause voltage oscillation in Case 4. However, in Case 1, Case 2, and Case 3, due to k fE <1, the frequency oscillation transmission will be suppressed. Two new findings were obtained from the analysis:

[0077] 1) Unlike the strong coupling effect between active power and reactive power, there is a possibility that the frequency oscillates but the voltage is stable and does not oscillate.

[0078] 2) Although the voltage self-stabilizing loop is stable, the voltage will oscillate due to the frequency oscillation transmission effect. The disclosure of this transmission relationship mechanism is the unique feature of the present invention.

[0079] Specifically, to further illustrate the effectiveness of the present invention, the experimental results are shown below.

[0080] Figure 7 The experimental results are given in Figure 2. Figure 1 The topology and control strategy in the paper are established. The experimental results are consistent with the theoretical analysis results. The light blue curve represents the voltage offset (E-Ug) and the light purple curve represents the frequency offset (f-fg). The offset here is to make the dynamic changes more detailed and clear.

[0081] The present invention is also applicable to the determination of amplitude-frequency oscillation transmission of a grid-connected inverter under any other control technology.

[0082] Compared with the prior art, the grid-connected inverter amplitude-frequency oscillation transfer analysis system proposed in the present invention provides a new physical perspective for the amplitude-frequency coupling effect and oscillation transfer mechanism of the virtual synchronous generator, and determines the dynamic interaction mechanism of the voltage amplitude and the grid frequency from a physical perspective; compared with the traditional eigenvalue analysis method and the impedance criterion analysis method, the grid-connected inverter amplitude-frequency oscillation transfer analysis method proposed in the present invention can more intuitively and clearly determine the conditions for the occurrence of amplitude-frequency oscillations of the grid-connected inverter under the control of the VSG system.

[0083] The above is an exemplary description of the present invention in combination with the accompanying drawings. It is obvious that the specific implementation of the present invention is not limited to the above-mentioned method. As long as such non-substantial improvements are made by adopting the method concept and technical solution of the present invention, or the concept and technical solution of the present invention are directly applied to other occasions without improvement, they are all within the protection scope of the present invention.

Claims

1. A system for determining the amplitude-frequency oscillation transfer of a grid-connected inverter under VSG control, characterized in that: include: The frequency self-stabilizing closed-loop model includes a forward channel and a feedback channel. The forward channel includes the transfer coupling term from frequency to active power, inertia and damping terms, and the transfer coupling term from active power to frequency. The feedback channel is unit negative feedback. The voltage self-stabilization closed-loop model includes a forward channel and a feedback channel. The forward channel includes a voltage-to-reactive power transfer coupling term, a virtual inertia and damping term, and a reactive power-to-voltage transfer coupling term. The feedback channel is a unit negative feedback. A stable loop model is proposed, including a forward channel and two feedback channels. The forward channel includes an integral link, a transfer coupling term from frequency to RoCoV, and a transfer coupling term from RoCoV to voltage. The first feedback channel includes a transfer coupling term from voltage to RoCoV, and the second feedback channel includes a transfer coupling term from voltage to RoCoF (frequency change rate) and a transfer coupling term from frequency to RoCoF.

2. The system for determining the amplitude-frequency oscillation transfer of a grid-connected inverter under VSG control according to claim 1 is characterized in that: In the frequency self-stabilizing closed-loop model, the small signal value of the frequency reference value is compared with the small signal amount of the frequency, and then the small signal amount of the frequency is obtained through the frequency-to-active power transfer coupling term, the virtual inertia damping term, and the active power-to-frequency transfer coupling term. The frequency small signal amount is then fed back and compared with the small signal amount of the frequency reference value to form a closed-loop model.

3. The system for determining the amplitude-frequency oscillation transfer of a grid-connected inverter under VSG control according to claim 1 is characterized in that: In the voltage self-stabilization closed-loop model, the small signal value of the voltage reference value is compared with the small signal amount of the voltage, and the small signal amount of the voltage is obtained through the voltage-to-reactive power transfer coupling term and the virtual inertia damping term. The small signal amount of the voltage is then fed back and compared with the small signal value of the voltage reference value to form a closed-loop model.

4. The system for determining the amplitude-frequency oscillation transfer of a grid-connected inverter under VSG control according to claim 1 is characterized in that: In the amplitude-frequency induced stable closed-loop model, the small signal value of the RoCoF reference value is compared with the transfer coupling feedback value of the small signal voltage, and then the small signal frequency is obtained through the transfer coupling term from RoCoF to frequency. The small signal frequency is compared with the transfer coupling term of voltage through the transfer coupling term from frequency to RoCoV to obtain RoCoV, and is compared with the transfer coupling term of voltage through the feedback of the transfer coupling term from frequency to RoCoF to obtain RoCoF. RoCoV then obtains the small signal voltage through the transfer coupling term from RoCoV to voltage, and the small signal voltage is fed back through two branches to form a closed-loop model.

5. A method for determining the amplitude-frequency oscillation transfer analysis of a grid-connected inverter under VSG control, characterized in that: Based on the amplitude-frequency oscillation transfer analysis system for determining the grid-connected inverter under VSG control as described in any one of claims 1-4, it is gradually determined whether frequency and voltage oscillations and transfer oscillations occur by judging the frequency self-stabilization closed-loop model, the voltage self-stabilization closed-loop model, and the frequency ISIM and the voltage ISIM in the self-stabilization loop model.

6. The method for determining the amplitude-frequency oscillation transmission of a grid-connected inverter under VSG control according to claim 5 is characterized in that: The steps include: Step 1: Establish the model transfer functions of frequency self-stabilization closed-loop model, voltage self-stabilization closed-loop model, frequency ISIM, and voltage ISIM; Step 2: Determine whether the frequency self-stabilizing closed-loop model and the voltage self-stabilizing closed-loop model are stable; if G of (s) is stable, jump to step 3.1; otherwise, the unstable frequency self-stabilizing loop (FSSL) will cause frequency oscillation; if G oE If (s) is stable, then jump to step 3.2; otherwise, the unstable voltage self-stabilizing loop (VSSL) will cause VM oscillation; Step 3.1: Determine G oE (s) Is it stable? If it is stable, the frequency is stable; otherwise, jump to step 4.1; Step 3.2: Determine G of (s) Is it stable? If so, the voltage is stable; otherwise, jump to step 4.2; Step 4.1: Determine the oscillation transfer coefficient k of the frequency ISIM Ef Is it greater than or equal to 1? If it is less than 1, the voltage oscillation will not be transmitted, and the frequency will be stable; otherwise, it will trigger the oscillation transmission and cause frequency oscillation; Step 4.2: Determine the oscillation transfer coefficient k of the voltage ISIM fE Is it greater than or equal to 1? If it is lower than 1, the frequency oscillation will not be transmitted and the voltage can remain stable; otherwise, the oscillation will be transmitted, causing voltage oscillation.

7. The method for determining the amplitude-frequency oscillation transmission of a grid-connected inverter under VSG control according to claim 6 is characterized in that: In step 1, the transfer function of the self-stabilizing closed-loop model is The transfer function of the voltage self-stabilization closed-loop model is: The transfer function of frequency ISIM is The transfer function of the voltage ISIM is Among them, g1 is the transfer coupling term from frequency to active power, g2 is the transfer coupling term from active power to frequency, D p is the inertia and damping term, k = 1 / (2π); g4 is the transfer coupling from voltage to reactive power, g3 is the transfer coupling from reactive power to voltage, D q are virtual inertia and damping terms; G1 is the integral link, M1 is the transfer coupling term from frequency to RoCoF, and M2 is the transfer coupling term from voltage to RoCoF; G2 is the transfer coupling term from RoCoV to voltage, M3 is the transfer coupling term from frequency to RoCoV (voltage change rate), and M4 is the transfer coupling term from voltage to RoCoV.

8. The method for determining the amplitude-frequency oscillation transmission of a grid-connected inverter under VSG control according to claim 7 is characterized in that: In step 4.1, In step 4.2, Where j represents the imaginary unit, ω c Represents the zero-crossing frequency of the open-loop Bode plot.

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