Sequence impedance modeling and stability analysis method for a heterogeneous new energy integrated power system

Abstract

The present invention discloses a sequence immittance modeling and stability analysis method for a heterogeneous renewable energy grid-connected power system, belonging to the field of power system control technology. The method performs two-port equivalence on various converters in the heterogeneous renewable energy grid-connected power system and adds voltage disturbances to the AC port and the DC port; further, the control loops and nonlinear circuits of the various converters under the current response of the voltage disturbance are modeled to obtain the initial transfer function of the sequence immittance model corresponding to each frequency coupling component, fully considering the interaction between the grid-connected / connected converters, revealing the frequency coupling law of the converter, and improving the accuracy of the sequence immittance model; then, the corrected sequence immittance circuit equation is used to correct the initial transfer function of the sequence immittance model corresponding to each frequency coupling component, to obtain the corrected transfer function of the sequence immittance model of each frequency coupling component, and accurately perform linear modeling on the sequence immittance of the heterogeneous renewable energy grid-connected power system.

Description

Technical Field

[0001] The present invention belongs to the field of power system control technology, and more specifically, relates to a sequence immittance modeling and stability analysis method for a heterogeneous renewable energy grid-connected power system. Background Art

[0002] With the rapid development of renewable energy power generation technologies, the proportion of new energy power electronics in my country's installed power generation capacity continues to rise. Consequently, the grid-connected performance requirements for renewable energy stations are also gradually increasing. Currently, most renewable energy systems connected to the grid use grid-following control, which relies on traditional synchronous generators to provide a stable voltage source. This makes it difficult to adapt to future grid scenarios with a high proportion of renewable energy integration. Heterogeneous renewable energy grid-connected power systems will be connected to both grid-following and grid-connecting converters.

[0003] Traditional small-signal modeling methods are difficult to directly linearize for heterogeneous renewable energy grid-connected power systems. If a synchronously rotating coordinate system is used for small-signal linearization, the physical meaning of the model becomes obscured. Current stability analysis of heterogeneous renewable energy systems presents significant challenges. Existing methods primarily focus on the aggregation of individual renewable energy units, lacking detailed understanding of the internal processes within renewable energy units and the influence of system-wide parameters on heterogeneous, multi-unit renewable energy grid-connected systems. Analyzing the stability of multi-unit grids using aggregation models fails to reveal internal stability. Current methods for multi-unit aggregation include dq impedance aggregation and sequence impedance aggregation. Dq axis impedance aggregation requires obtaining the steady-state operating point phase angles at all dq modeling phase angles to generate a phase angle conversion matrix, which is difficult for practical system applications. Furthermore, sequence impedance aggregation fails to consider the stability between units within the system and focuses solely on the stability of the grid connection point, posing a challenge for stability analysis of heterogeneous renewable energy grid-connected systems. Direct matrix calculation methods exist for large systems, but obtaining the full system network matrix for direct stability analysis is difficult to achieve in today's large systems. Therefore, this approach requires further simplification and cannot be directly applied.

[0004] In summary, the converter's ability to adapt to the stability of strong and weak grid connections is universally and critically important for the reliable operation and absorption of large-scale renewable energy access. Further research is needed to explore the stability modeling and analysis methods for multi-station stations where multiple converters are connected to the grid. Summary of the Invention

[0005] In response to the above-mentioned defects or improvement needs of the prior art, the present invention provides a method for sequence impedance modeling and stability analysis of a heterogeneous renewable energy grid-connected power system, which aims to solve the technical problem that traditional modeling methods are difficult to directly perform linear modeling of a heterogeneous renewable energy grid-connected power system with low computational complexity and high accuracy.

[0006] To achieve the above objectives, according to one aspect of the present invention, a method for sequence immittance modeling of a heterogeneous renewable energy grid-connected power system is provided, comprising:

[0007] S1: Performing two-port equivalence on a grid-following converter and a grid-forming converter in a heterogeneous renewable energy grid-connected power system to obtain two-port networks corresponding to the grid-following converter and the grid-forming converter, respectively; the two-port networks include a control loop, a nonlinear circuit, an AC port, and a DC port; adding voltage disturbances to the AC port and the DC port to obtain current responses to the voltage disturbances;

[0008] S2: performing sequence immittance modeling on the control loops of the grid-following converter and the grid-forming converter under the current response of the voltage disturbance to obtain a sequence immittance model of the grid-following control loop and a sequence immittance model of the grid-forming control loop;

[0009] S3: performing frequency AC / DC coupling analysis on the nonlinear circuits of the grid-following converter and the grid-forming converter under the current response of the voltage disturbance to obtain multiple frequency coupling components; and modeling the initial transfer function of the sequence immittance model corresponding to each frequency coupling component;

[0010] S4: combining the circuit equation of the nonlinear circuit with the control equations of the sequence immittance model of the mesh-following control loop and the sequence immittance model of the mesh-forming control loop, removing the zero-sequence component in the circuit equation to obtain a corrected sequence immittance circuit equation;

[0011] S5: using the modified sequence immittance circuit equation to modify the initial transfer function of the sequence immittance model corresponding to each frequency coupling component, to obtain a modified transfer function of the sequence immittance model corresponding to each frequency coupling component.

[0012] In one embodiment, the synchronous control in the control loop of the grid-following converter and the grid-forming converter is modeled as follows:

[0013]

[0014] in, is the phase angle in the frequency domain, J is the inertia coefficient of the virtual synchronous control loop, D p is the damping coefficient of the virtual synchronous control loop, is the voltage disturbance applied to the AC port, is the current disturbance after the voltage disturbance added to the AC port, ω0 is the initial rotation angular velocity, ω1 is the fundamental frequency, U1 * 、I1 * are the conjugates of the initial phasors of voltage and current, respectively.

[0015] In one embodiment,

[0016] Using the formula Modeling the initial transfer function of the sequence immittance model corresponding to each frequency coupling component;

[0017] Among them, x(t) is the time domain value of the x variable, y(t) is the time domain value of the y variable, is the steady-state value of the x variable at the steady-state operating point, is the superimposed small signal value under small perturbations of the x variable, is the steady-state value of the y variable at the steady-state operating point, is the superimposed small signal value under small perturbation of the y variable, is the small signal value after linearization of the Z(t) harmonic, is the superimposed small signal value under small perturbation of y variable, Y l is the steady-state value of the y variable at the steady-state operating point, where l is used to represent the number of components of the y variable itself, X k is the steady-state value of the variable at the steady-state operating point, is the superimposed small signal value under small perturbation of the variable, where k is used to represent the number of components of the x variable itself; ω p is the disturbance frequency, ω1 is the fundamental frequency, is the kth harmonic component of the x variable, is the kth harmonic component of Y.

[0018] In one embodiment, when a positive sequence voltage disturbance is added to the AC port, the initial transfer function of the sequence immittance model corresponding to each frequency coupling component is:

[0019]

[0020] Among them, Y pp (s) is the positive sequence response self-admittance under positive sequence voltage disturbance, Y pd (s) is the AC-DC coupled admittance of the DC response under positive sequence voltage disturbance, Y pn (s) is the negative sequence response AC transfer admittance under positive sequence voltage disturbance; is the response current of the AC port, is the voltage disturbance at the AC port, is the DC current response of the DC port, is the coupling current of the AC port.

[0021] In one embodiment, when a negative sequence voltage disturbance is added to the AC port, the initial transfer function of the sequence immittance model corresponding to each frequency coupling component is:

[0022]

[0023] Among them, Y nn (s) is the positive sequence response self-admittance under negative sequence voltage disturbance, Y nd (s) is the AC-DC coupled admittance of the DC response under negative sequence voltage disturbance, Y np (s) is the negative sequence response AC transfer admittance under negative sequence voltage disturbance; is the response current of the AC port, is the voltage disturbance at the AC port, is the DC current response of the DC port, is the coupling current of the AC port.

[0024] In one embodiment, when a DC voltage disturbance is added to the DC port, the initial transfer function of the sequence immittance model corresponding to each frequency coupling component is:

[0025]

[0026] Among them, Y dd (s) is the DC response self-admittance under DC disturbance, Y dp (s) is the AC-DC coupled admittance of the AC positive sequence response under DC disturbance, Y pn (s) is the AC transfer admittance of the AC negative sequence response under DC disturbance; is the response current of the AC port, is the voltage disturbance at the AC port, is the DC current response of the DC port, is the coupling current of the AC port.

[0027] In one embodiment, the modified transfer function of the grid-following converter is expressed as:

[0028]

[0029] in, is the response current of the AC port, is the voltage disturbance of the AC port, L is the AC inductance, K d is the feedback coefficient of the inner current loop, V dc Represents the DC voltage value, H c (s-jω1) represents the transfer function of the current inner loop, is the fundamental frequency parameter of the modulation signal, The parameter representing the fundamental frequency of the current, represents the conjugate complex number of the fundamental voltage complex number, ω0 represents the frequency base value, G θ (s-jω1) is the closed-loop transfer function of the phase-locked loop, H va (s-jω1) is the transfer function of the AC voltage outer loop, Hc (s-jω1) is the transfer function of the inner current loop, represents the fundamental complex number of admittance, s represents the Laplace factor, and j represents the 90-degree rotation component in the complex frequency domain.

[0030] In one embodiment, the modified transfer function of the grid-type converter is expressed as:

[0031]

[0032] Among them, L is the AC inductance, K d is the feedback coefficient of the inner current loop, V dc Represents the DC voltage value, H c (s-jω1) represents the transfer function of the inner current loop, is the fundamental frequency parameter of the modulation signal, The parameter representing the fundamental frequency of the current, represents the conjugate complex number of the fundamental current complex number, represents the conjugate complex number of the fundamental voltage complex number, ω0 represents the frequency base value, is the response current of the AC port, H c (s-jω1) is the transfer function of the inner current loop, is the voltage disturbance at the AC port, and M(s-jω1) represents the transfer function of the virtual synchronous control loop.

[0033] According to another aspect of the present invention, a sequence immittance modeling device for a heterogeneous renewable energy grid-connected power system is provided, comprising:

[0034] A disturbance module is used to perform two-port equivalence on a grid-following converter and a grid-forming converter in a heterogeneous renewable energy grid-connected power system to obtain a two-port network corresponding to each of the grid-following converter and the grid-forming converter; the two-port network includes: a control loop, a nonlinear circuit, an AC port, and a DC port; a voltage disturbance is added to the AC port and the DC port to obtain a current response to the voltage disturbance;

[0035] a first modeling module, configured to perform sequence immittance modeling on the control loops of the grid-following converter and the grid-forming converter under the current response of the voltage disturbance, to obtain a sequence immittance model of the grid-following control loop and a sequence immittance model of the grid-forming control loop;

[0036] A second modeling module is configured to perform frequency AC / DC coupling analysis on the nonlinear circuits of the grid-following converter and the grid-forming converter under the current response of the voltage disturbance to obtain a plurality of frequency coupling components; and to model an initial transfer function of a sequence immittance model corresponding to each frequency coupling component;

[0037] a first correction module, configured to simultaneously solve the circuit equation of the nonlinear circuit and the control equations of the sequence immittance model of the mesh-following control loop and the sequence immittance model of the mesh-forming control loop, remove the zero-sequence component in the circuit equation, and obtain a corrected sequence immittance circuit equation;

[0038] The second correction module is used to correct the initial transfer function of the sequence immittance model corresponding to each frequency coupling component using the corrected sequence immittance circuit equation to obtain the corrected transfer function of the sequence immittance model corresponding to each frequency coupling component.

[0039] According to another aspect of the present invention, a stability analysis method for a heterogeneous renewable energy grid-connected power system is provided, comprising:

[0040] The modified transfer function of the sequence immittance model of each frequency coupling component is constructed using the sequence immittance modeling method of the heterogeneous renewable energy grid-connected power system.

[0041] Using the modified transfer function, an immittance model is performed on the grid side to which the grid-following converter and the grid-forming converter are connected, so as to obtain an original grid network without the converter connected;

[0042] Perform stability analysis on the original power grid network, the grid-following converter, and the grid-forming converter respectively. If all are stable, connect the grid-forming converter to the original power grid network to determine whether the power system is stable. If not, it is considered that the power system is unstable due to the grid-forming converter. If stable, connect the grid-following converter to the network after the grid-forming converter is connected to the original power grid network to determine whether the power system is stable. If not, it is considered that the power system is unstable due to the grid-following converter.

[0043] According to another aspect of the present invention, a stability analysis device for a heterogeneous renewable energy grid-connected power system is provided, comprising:

[0044] A disturbance module is used to perform two-port equivalence on a grid-following converter and a grid-forming converter in a heterogeneous renewable energy grid-connected power system to obtain a two-port network corresponding to each of the grid-following converter and the grid-forming converter; the two-port network includes: a control loop, a nonlinear circuit, an AC port, and a DC port; a voltage disturbance is added to the AC port and the DC port to obtain a current response to the voltage disturbance;

[0045] a first modeling module, configured to perform sequence immittance modeling on the control loops of the grid-following converter and the grid-forming converter under the current response of the voltage disturbance, to obtain a sequence immittance model of the grid-following control loop and a sequence immittance model of the grid-forming control loop;

[0046] A second modeling module is configured to perform frequency AC / DC coupling analysis on the nonlinear circuits of the grid-following converter and the grid-forming converter under the current response of the voltage disturbance to obtain a plurality of frequency coupling components; and to model an initial transfer function of a sequence immittance model corresponding to each frequency coupling component;

[0047] a first correction module, configured to simultaneously solve the circuit equation of the nonlinear circuit and the control equations of the sequence immittance model of the mesh-following control loop and the sequence immittance model of the mesh-forming control loop, remove the zero-sequence component in the circuit equation, and obtain a corrected sequence immittance circuit equation;

[0048] A second correction module is used to correct the initial transfer function of the sequence immittance model corresponding to each frequency coupling component using the corrected sequence immittance circuit equation to obtain a corrected transfer function of the sequence immittance model corresponding to each frequency coupling component;

[0049] A judgment module is used to use the modified transfer function to perform impedance modeling on the grid side to which the grid-following converter and the grid-forming converter are connected, and obtain the original grid network without the converter connected; perform stability analysis on the original grid network, the grid-following converter, and the grid-forming converter respectively; if all are stable, connect the grid-forming converter to the original grid network to determine whether the power system is stable; if not, it is regarded as the power system instability caused by the grid-forming converter; if stable, connect the grid-following converter to the network after the grid-forming converter is connected to the original grid network to determine whether the power system is stable; if not, it is regarded as the power system instability caused by the grid-following converter.

[0050] According to another aspect of the present invention, an electronic device is provided, including a memory and a processor, wherein the memory stores a computer program, and when the processor executes the computer program, it implements the steps of the sequence immittance modeling method or stability analysis method for the heterogeneous new energy grid-connected power system.

[0051] According to another aspect of the present invention, a computer-readable storage medium is provided, on which a computer program is stored. When the processor executes the computer program, the steps of the sequence immittance modeling method or stability analysis method of the heterogeneous new energy grid-connected power system are implemented.

[0052] In general, the above technical solutions conceived by the present invention can achieve the following beneficial effects compared with the prior art:

[0053] (1) The present invention provides a sequence immittance modeling method for a heterogeneous renewable energy grid-connected power system, which performs two-port equivalence on various converters in the heterogeneous renewable energy grid-connected power system and adds voltage disturbances to the AC port and the DC port; further, the control loops and nonlinear circuits of the various converters under the current response of the voltage disturbance are modeled to obtain the initial transfer functions of the sequence immittance models corresponding to the various frequency coupling components, fully considering the interaction between the grid-connected / connected converters, revealing the frequency coupling law of the converters, and improving the accuracy of the sequence immittance model; further, the initial transfer functions of the sequence immittance models corresponding to the various frequency coupling components are corrected by using the corrected sequence immittance circuit equation to obtain the corrected transfer functions of the sequence immittance models of the various frequency coupling components, and accurately linearize the sequence immittance modeling of the heterogeneous renewable energy grid-connected power system.

[0054] (2) In this solution, the virtual synchronous control model in the control loops of the grid-following converter and the grid-forming converter is: This approach considers the two main power sources of heterogeneous renewable energy grid-connected systems: grid-following converters and grid-forming converters. It also considers the effect of frequency-domain phase angle under the frequency coupling of grid-following and grid-forming control. Compared with existing renewable energy grid-connected analysis, this approach can fully consider the stability of heterogeneous stations composed of both grid-following and grid-forming converters.

[0055] (3) This scheme uses The initial transfer function of the sequence impedance model corresponding to each frequency coupling component is modeled; it takes into account the feedback coupling and frequency coupling of the converter DC system. Compared with the existing sequence impedance modeling, this method defines the influence of a total of 9 functions. Compared with the traditional two functions and four transfer functions, it can more comprehensively explain the transfer function influence mechanism of the sequence impedance of the grid-type control and explain the role of the impedance modeling.

[0056] (4) In this scheme, when the positive sequence voltage disturbance is added to the AC port, the initial transfer function of the sequence immittance model corresponding to each frequency coupling component is: It takes into account the coupling feedback effect on the grid side and the feedback loop effect on the DC side. Compared with the existing sequence impedance modeling, it can more comprehensively reflect the interactive influence relationship between the converter control and the circuit, and improve the frequency domain modeling accuracy of the positive sequence impedance in the low frequency band.

[0057] (5) In this scheme, when a negative sequence voltage disturbance is added to the AC port, the initial transfer function of the sequence immittance model corresponding to each frequency coupling component is: It takes into account the coupling feedback effect on the grid side and the feedback loop effect on the DC side. Compared with the existing sequence impedance modeling, it can more comprehensively reflect the interactive influence relationship between the converter control and the circuit, and improve the frequency domain modeling accuracy of negative sequence impedance in the low frequency band.

[0058] (6) In this scheme, when a DC voltage disturbance is added to the AC port, the initial transfer function of the sequence immittance model corresponding to each frequency coupling component is: It takes into account the AC / DC coupling characteristics of the converter and can improve the modeling accuracy of DC impedance compared with the existing sequence impedance modeling.

[0059] (7) The modified transfer function of the grid-type converter described in this scheme is expressed as: The zero-sequence variables of the circuit are removed, and the accuracy is higher.

[0060] (8) This solution provides a sequence impedance modeling device for a heterogeneous renewable energy grid-connected power system, which performs two-port equivalence on various converters in the heterogeneous renewable energy grid-connected power system and adds voltage disturbances to the AC port and the DC port; further, the control loops and nonlinear circuits of various converters under the current response of the voltage disturbance are modeled to obtain the initial transfer functions of the sequence impedance models corresponding to the various frequency coupling components, fully considering the interaction between the grid-connected / connected converters, revealing the frequency coupling law of the converters, and improving the accuracy of the sequence impedance model; further, the initial transfer functions of the sequence impedance models corresponding to the various frequency coupling components are corrected using the corrected sequence impedance circuit equation to obtain the corrected transfer functions of the sequence impedance models of the various frequency coupling components, and accurately linearize the sequence impedance modeling of the heterogeneous renewable energy grid-connected power system.

[0061] (10) This solution provides a stability analysis method for a heterogeneous new energy grid-connected power system, which utilizes the sequence impedance modeling method of the above-mentioned heterogeneous new energy grid-connected power system to obtain a sequence impedance model that is accurate and has low computational complexity, providing an effective analysis means for the stability analysis of renewable energy connected to the power grid; further, the stability analysis of the original power grid network, the grid-type converter, and the grid-forming converter that are not connected are performed separately, and the grid-forming converter and the grid-forming converter are gradually connected to determine the cause of the power system instability; the judgment process has low computational complexity and high accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 This is a flow chart of a sequence immittance modeling method for a heterogeneous renewable energy grid-connected power system provided in Example 1 of the present invention;

[0063] Figure 2a1 is a schematic diagram of a hierarchical analysis process for sequence immittance modeling of a heterogeneous renewable energy grid-connected power system provided in Example 1 of the present invention;

[0064] Figure 2b This is an equivalent circuit diagram of the heterogeneous new energy grid-connected power system provided by Example 1 of the present invention when performing sequence immittance modeling;

[0065] Figure 3 This is a system modeling block diagram of a heterogeneous renewable energy grid-connected power system provided by Example 1 of the present invention when performing sequence immittance modeling;

[0066] Figure 4 Schematic diagram of a stability analysis method for a heterogeneous renewable energy grid-connected power system provided in Example 3 of the present invention;

[0067] Figure 5a 3. This is a graph showing the comparison and modeling results of the grid-following converter and the grid-forming converter, as well as the grid-side equivalent immittance characteristics, provided in Example 3 of the present invention;

[0068] Figure 5b is a Nyquist curve diagram of the grid-connected converter provided by Example 3 of the present invention when SCR=1.75 is connected to the grid;

[0069] Figure 5c This is the Nyquist curve of the grid-connected converter provided by Example 3 of the present invention under multiple machine grid connection at different short-circuit ratios. DETAILED DESCRIPTION

[0070] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0071] Example 1

[0072] like Figure 1As shown, this embodiment provides a sequence immittance modeling method for a heterogeneous renewable energy grid-connected power system, including: S1-S5. S1: Perform two-port equivalence on the grid-type converter and the grid-forming converter in the heterogeneous renewable energy grid-connected power system to obtain a two-port network corresponding to the grid-type converter and the grid-forming converter respectively; the two-port network includes: a control loop, a nonlinear circuit, an AC port and a DC port; a voltage disturbance is added to the AC port and the DC port. S2: Perform sequence immittance modeling on the control loops of the grid-type converter and the grid-forming converter respectively under the current response of the voltage disturbance to obtain a sequence immittance model of the grid-type control loop and a sequence immittance model of the grid-forming control loop. S3: Perform frequency AC / DC coupling analysis on the nonlinear circuits of the grid-type converter and the grid-forming converter respectively under the current response of the voltage disturbance to obtain multiple frequency coupling components; and model the initial transfer function of the sequence immittance model corresponding to each frequency coupling component. S4: Combine the circuit equation of the nonlinear circuit with the control equations of the sequence immittance model of the grid-following control loop and the sequence immittance model of the grid-forming control loop, remove the zero-sequence component from the circuit equation, and obtain the corrected sequence immittance circuit equation. S5: Use the corrected sequence immittance circuit equation to correct the initial transfer function of the sequence immittance model corresponding to each frequency coupling component, and obtain the corrected transfer function of the sequence immittance model corresponding to each frequency coupling component.

[0073] It should be understood that although Figure 1 The steps in the flowchart are shown in sequence as indicated by the arrows, but these steps are not necessarily executed in the order indicated by the arrows. Unless otherwise specified in this document, there is no strict order restriction for the execution of these steps, and these steps can be executed in other orders. In addition, Figure 1 At least part of the steps may include multiple sub-steps or multiple stages. These sub-steps or stages are not necessarily executed at the same time, but can be executed at different times. The execution order of these sub-steps or stages is not necessarily sequential, but can be executed in turn or alternately with other steps or at least part of the sub-steps or stages of other steps.

[0074] In step S1, the simplified process of the dual port is:

[0075]

[0076] s p The reference component representing the sequence is multiplied by the reference component matrix on the basis of phase a. The matrix defines the relationship between the sequences. By multiplying the reference component matrix, the complete components of the other three phases can be obtained.

[0077] Specifically, Figure 2a 1 is a schematic diagram of a hierarchical analysis process for sequence immittance modeling of a heterogeneous renewable energy grid-connected power system provided in Example 1 of the present invention; Figure 2b This is an equivalent circuit diagram of the heterogeneous new energy grid-connected power system provided by Example 1 of the present invention when performing sequence immittance modeling; Figure 3 This is a system modeling block diagram for sequence immittance modeling of a heterogeneous renewable energy grid-connected power system, as provided in Example 1 of the present invention. Sequence immittance modeling of the control loops of both the grid-following and grid-connecting converters under current response to voltage disturbances includes the current control loop, voltage control loop, PLL, and virtual synchronization loop.

[0078] The modeling process of the current control loop in step S2 is calculated as:

[0079]

[0080] Z c (s) represents the equivalent impedance of the grid, H c (s) is the gain of the loop, K d represents the feedback gain of the current loop, and L represents the grid-connected inductance of the converter.

[0081] The modeling process of the voltage control loop in step S2 is calculated as:

[0082]

[0083] Z v (s) represents the equivalent impedance of the grid, H va (s) is the gain of the voltage loop, V dc represents the DC bus voltage gain, and L represents the grid-connected inductance of the converter.

[0084] The modeling process of the PLL phase-locked loop in step S2 is calculated as:

[0085]

[0086] The phase angle in the frequency domain is expressed as H θ (s) is the open-loop gain of the PLL phase-locked PI, T θ (s) is the forward gain of the PLL phase-locked loop, G θ (s) is the closed-loop gain of the PLL phase-locked loop.

[0087] As an optional implementation, the virtual synchronization control in the control loops of the grid-following converter and the grid-forming converter is modeled as follows:

[0088]

[0089] in, is the phase angle in the frequency domain, J is the inertia coefficient of the virtual synchronous control loop, D pis the damping coefficient of the virtual synchronous control loop, is the voltage disturbance applied to the AC port, is the current disturbance after the voltage disturbance added to the AC port, ω0 is the initial rotation angular velocity, ω1 is the fundamental frequency, U1 * 、I1 * are the conjugates of the initial phasors of voltage and current, respectively.

[0090] As a preferred embodiment, it should be noted that the frequency coupling of the grid-type and grid-forming converters comes from the nonlinearity of the converter control, but the frequency coupling effect comes from the transmission of the main circuit AC and DC system: dq conversion and PLL, dq conversion will couple out ω p -2ω1 and ω p The +2ω1 component, the PLL control action on the q-axis causes control asymmetry, the DC voltage acts on the d-axis, and the frequency coupling also comes from the modulation action in the main circuit.

[0091] Using the formula Modeling the initial transfer function of the sequence immittance model corresponding to each frequency coupling component;

[0092] Among them, x(t) is the time domain value of the x variable, y(t) is the time domain value of the y variable, is the steady-state value of the x variable at the steady-state operating point, is the superimposed small signal value under small perturbations of the x variable, is the steady-state value of the y variable at the steady-state operating point, is the superimposed small signal value under small perturbation of the y variable, is the small signal value after linearization of the Z(t) harmonic, is the superimposed small signal value under small perturbation of y variable, Y l is the steady-state value of the y variable at the steady-state operating point, where l is used to represent the number of components of the y variable itself, X k is the steady-state value of the variable at the steady-state operating point, is the superimposed small signal value under small perturbation of the variable, where k is used to represent the number of components of the x variable itself; ω p is the disturbance frequency, ω1 is the fundamental frequency, is the kth harmonic component of the x variable, is the kth harmonic component of Y.

[0093] As an optional implementation, when a positive sequence voltage disturbance is added to the AC port, the initial transfer function of the sequence immittance model corresponding to each frequency coupling component is:

[0094]

[0095] Among them, Y pp(s) is the positive sequence response self-admittance under positive sequence voltage disturbance, Y pd (s) is the AC-DC coupled admittance of the DC response under positive sequence voltage disturbance, Y pn (s) is the negative sequence response AC transfer admittance under positive sequence voltage disturbance; is the response current of the AC port, is the voltage disturbance at the AC port, is the DC current response of the DC port, is the coupling current of the AC port.

[0096] As an optional implementation, when a negative sequence voltage disturbance is added to the AC port, the initial transfer function of the sequence immittance model corresponding to each frequency coupling component is:

[0097]

[0098] Among them, Y nn (s) is the positive sequence response self-admittance under negative sequence voltage disturbance, Y nd (s) is the AC-DC coupled admittance of the DC response under negative sequence voltage disturbance, Y np (s) is the negative sequence response AC transfer admittance under negative sequence voltage disturbance; is the response current of the AC port, is the voltage disturbance at the AC port, is the DC current response of the DC port, is the coupling current of the AC port.

[0099] As an optional implementation, when a DC voltage disturbance is added to the DC port, the initial transfer function of the sequence immittance model corresponding to each frequency coupling component is:

[0100]

[0101] Among them, Y dd (s) is the DC response self-admittance under DC disturbance, Y dp (s) is the AC-DC coupled admittance of the AC positive sequence response under DC disturbance, Y pn (s) is the AC transfer admittance of the AC negative sequence response under DC disturbance; is the response current of the AC port, is the voltage disturbance at the AC port, is the DC current response of the DC port, is the coupling current of the AC port.

[0102] As an optional implementation, the modified transfer function of the grid-following converter is expressed as:

[0103]

[0104] in, is the response current of the AC port, is the voltage disturbance of the AC port, L is the AC inductance, K d is the feedback coefficient of the inner current loop, V dc Represents the DC voltage value, H c (s-jω1) represents the transfer function of the inner current loop, is the fundamental frequency parameter of the modulation signal, The parameter representing the fundamental frequency of the current, represents the conjugate complex number of the fundamental voltage complex number, ω0 represents the frequency base value, G θ (s-jω1) is the closed-loop transfer function of the phase-locked loop, H va (s-jω1) is the transfer function of the AC voltage outer loop, H c (s-jω1) is the transfer function of the inner loop of the AC current, represents the fundamental complex number of admittance, s represents the Laplace factor, and j represents the 90-degree rotation component in the complex frequency domain.

[0105] As an optional implementation, the modified transfer function of the grid-type converter is expressed as:

[0106]

[0107] Among them, L is the AC inductance, K d is the feedback coefficient of the inner current loop, V dc Represents the DC voltage value, H c (s-jω1) represents the transfer function of the inner current loop, is the fundamental frequency parameter of the modulation signal, The parameter representing the fundamental frequency of the current, represents the conjugate complex number of the fundamental current complex number, represents the conjugate complex number of the fundamental voltage complex number, ω0 represents the frequency base value, is the response current of the AC port, H c (s-jω1) is the transfer function of the inner loop of the AC current, is the voltage disturbance at the AC port, and M(s-jω1) represents the transfer function of the virtual synchronous control loop.

[0108] Example 2

[0109] This embodiment provides a sequence immittance modeling device for a heterogeneous renewable energy grid-connected power system, including: a disturbance module, a first modeling module, a second modeling module, a first correction module, and a second correction module.

[0110] The disturbance module is used to perform two-port equivalence on the grid-following converter and the grid-forming converter in the heterogeneous renewable energy grid-connected power system, thereby obtaining two-port networks corresponding to the grid-following converter and the grid-forming converter respectively; the two-port networks include: a control loop, a nonlinear circuit, an AC port, and a DC port; and voltage disturbances are added to the AC port and the DC port.

[0111] The first modeling module is used to perform sequence immittance modeling on the control loops of the grid-following converter and the grid-forming converter under the current response of the voltage disturbance, and obtain the sequence immittance model of the grid-following control loop and the sequence immittance model of the grid-forming control loop.

[0112] The second modeling module is used to perform frequency AC / DC coupling analysis on the nonlinear circuits of the grid-following converter and the grid-forming converter under the current response of the voltage disturbance to obtain multiple frequency coupling components; and to model the initial transfer function of the sequence immittance model corresponding to each frequency coupling component.

[0113] The first correction module is used to simultaneously establish the circuit equation of the nonlinear circuit and the control equations of the sequence immittance model of the grid-following control loop and the sequence immittance model of the grid-forming control loop, remove the zero-sequence component in the circuit equation, and obtain the corrected sequence immittance circuit equation.

[0114] The second correction module is used to correct the initial transfer function of the sequence immittance model corresponding to each frequency coupling component using the corrected sequence immittance circuit equation to obtain the corrected transfer function of the sequence immittance model corresponding to each frequency coupling component.

[0115] The division of each module in the above-mentioned sequence conduction immittance modeling device for heterogeneous renewable energy grid-connected power system is only for illustration. In other embodiments, the sequence conduction immittance modeling device for heterogeneous renewable energy grid-connected power system can be divided into different modules as needed to complete all or part of the functions of the above-mentioned sequence conduction immittance modeling device for heterogeneous renewable energy grid-connected power system.

[0116] The specific limitations of the sequence-conductance immittance modeling apparatus for a heterogeneous renewable energy grid-connected power system can be found in the limitations of the sequence-conductance immittance modeling method for a heterogeneous renewable energy grid-connected power system described above and will not be further elaborated here. Each module in the aforementioned sequence-conductance immittance modeling apparatus for a heterogeneous renewable energy grid-connected power system can be implemented in whole or in part via software, hardware, or a combination thereof. Each of these modules can be embedded in or independent of a processor in a computer device in hardware form, or stored in a computer device memory in software form, allowing the processor to call and execute the corresponding operations of each module.

[0117] Example 3

[0118] like Figure 4As shown, this embodiment provides a stability analysis method for a heterogeneous renewable energy grid-connected power system, comprising: using the sequence immittance modeling method for the heterogeneous renewable energy grid-connected power system provided in Example 1 to construct a modified transfer function of the sequence immittance model of each frequency coupling component; using the modified transfer function to respectively perform immittance modeling on the grid-side system to which the grid-following converter and the grid-forming converter are connected, specifically divided into two methods: if the line connecting the grid side and the source side is long, the immittance analysis of the grid-side system to which each converter is to be connected can be replaced by the immittance of the line; if the line connecting the grid side and the source side is short, the grid-side system to be connected needs to be partitioned, and the obtained immittance is obtained by partitioning the grid-side system. The equivalent calculated immittance of the converter access port is finally obtained, and the original power grid network without the converter is obtained. The goal of this step is to obtain the grid-side immittance of the heterogeneous new energy grid-connected system to be analyzed, so as to facilitate the subsequent stability analysis; the original power grid network, grid-following converter, and grid-forming converter are separately analyzed for stability. If they are all stable, the grid-forming converter is connected to the original power grid network to determine whether the power system is stable. If it is unstable, it is regarded as the power system instability caused by the grid-forming converter; if it is stable, the grid-following converter is connected to the network after the grid-forming converter is connected to the original power grid network to determine whether the power system is stable. If it is unstable, it is regarded as the power system instability caused by the grid-following converter.

[0119] in, Figure 5a 3. This is a graph showing the comparison and modeling results of the grid-following converter and the grid-forming converter, as well as the grid-side equivalent immittance characteristics, provided in Example 3 of the present invention; Figure 5b is a Nyquist curve diagram of the grid-connected converter provided by Example 3 of the present invention when SCR=1.75 is connected to the grid; Figure 5c This is the Nyquist curve of the grid-connected converter under different short-circuit ratios provided by Example 3 of the present invention. The process of performing stability analysis on the power system is as follows:

[0120] The network voltage-current relationship matrix is classified. When listing the node equations, three types of nodes need to be distinguished. The nodes connected to the converter are numbered as {x1, x2, ..., xn} and collectively represented as x. Assume that the power grid contains m voltage sources at nodes {s1, s2, ..., sm}, and represent them as s. The voltage source nodes can be further divided into grid-forming converter nodes and generator nodes, which are respectively denoted as s. GFM With s Ge The remaining nodes of the power grid are denoted as y. The modified hierarchical sequential network voltage and current equations are formed:

[0121]

[0122] Where Y(s) represents the node admittance matrix of each classification node. Vs(s) represents the disturbance voltage of the voltage source node. Vx(s) and Vy(s) represent the voltage response of the non-voltage source node. ix(s) and iy(s) represent the current response of the non-voltage source node. Yxx(s) represents the self-admittance matrix of the x node, and Yyy(s) represents the self-admittance matrix of the and node. Yxy(s) and Yyx(s) represent the mutual admittance matrices between the x and y nodes.

[0123] Furthermore, the network voltage-current relationship matrix is classified, and the three types of nodes need to be treated differently when listing the node equations. Ya(s) is replaced by the converter admittance matrix Yap(s) or Yan(s). The definitions of the other admittance matrices involved can be inferred from the standard node voltage equations. Among them, in the hierarchical stability analysis, Ya(s) represents the converter admittance matrix in different hierarchical stability judgment stages. In the first-level stability judgment, Ya(s) = 0. In the second-level stability judgment, Ya(s) represents the admittance matrix of the access port of the grid-forming converter. In the third-level stability judgment, Ya(s) represents the admittance matrix of the access port of the grid-following converter.

[0124] Among them, the network voltage-current relationship matrix is used to obtain the hierarchical voltage-current equation expression under the proposed hierarchical sequential stability analysis process:

[0125]

[0126]

[0127] Where Y(s) represents the node admittance matrix of each classification node. Vs(s) represents the disturbance voltage of the voltage source node. Vx(s) and Vy(s) represent the voltage response of the non-voltage source node. ix(s) and iy(s) represent the current response of the non-voltage source node. Yxx(s) represents the self-admittance matrix of the x node, and Yyy(s) represents the self-admittance matrix of the and node. Yxy(s) and Yyx(s) represent the mutual admittance matrices between the x and y nodes. Z(s) represents the node impedance matrix of each classification node. Ya(s) represents the admittance matrix of the converter access port node. Ixsum and Iysum represent the sum of the current responses of the x and y nodes.

[0128] Furthermore, the Nyquist stability criterion can be used to analyze system stability. The core objective is to use a simplified Nyquist stability criterion, where the stability condition is that the system does not orbit or pass through the origin in the complex plane, provided that: 1) the inverter is stable when operating under an ideal voltage source; and 2) the grid is stable before the inverter is connected. This eliminates the need to examine open-loop RHP poles. This method leverages the known stability of each component of the system before interconnection, and the loop equation for the Nyquist stability criterion can be calculated from the loop equation:

[0129]

[0130] l(s)=det[I+Z xx (s)Y a (s)];

[0131] Here, I is the identity matrix, suggesting the possibility of studying system stability using a low-dimensional model. The closed-loop stability depends solely on det[I + Zg(s)Ya(s)]. When the first set of grid-connected / constructing converters is connected, the external stability of the grid also guarantees the internal stability of the grid thereafter. This satisfies the requirement to repeat the above analysis process when the next set of converters is connected. This can be repeated without requiring separate modeling of internal stability at any stage. The number of converters and their connection locations to the grid can vary significantly. The grid impedance matrix Zg(s) needs to be reconstructed. This allows for hierarchical analysis of multiple converters.

[0132] This embodiment can accurately characterize the stability of the converter multi-machine grid-connected system, greatly reduce the complexity of stability analysis, realize the hierarchical decoupling of the converter analysis object, discover the frequency domain instability problems in the multi-machine grid-connected system, and improve the stability and adaptability of the converter multi-machine grid-connected system.

[0133] Example 4

[0134] This embodiment provides a stability analysis device for a heterogeneous renewable energy grid-connected power system, including: a disturbance module, a first modeling module, a second modeling module, a first correction module, a second correction module and a judgment module.

[0135] The disturbance module is used to perform two-port equivalence on the grid-following converter and the grid-forming converter in the heterogeneous renewable energy grid-connected power system, thereby obtaining two-port networks corresponding to the grid-following converter and the grid-forming converter respectively; the two-port networks include: a control loop, a nonlinear circuit, an AC port, and a DC port; and voltage disturbances are added to the AC port and the DC port.

[0136] The first modeling module is used to perform sequence immittance modeling on the control loops of the grid-following converter and the grid-forming converter under the current response of the voltage disturbance, and obtain the sequence immittance model of the grid-following control loop and the sequence immittance model of the grid-forming control loop.

[0137] The second modeling module is used to perform frequency AC / DC coupling analysis on the nonlinear circuits of the grid-following converter and the grid-forming converter under the current response of the voltage disturbance to obtain multiple frequency coupling components; and to model the initial transfer function of the sequence immittance model corresponding to each frequency coupling component.

[0138] The first correction module is used to simultaneously establish the circuit equation of the nonlinear circuit and the control equations of the sequence immittance model of the grid-following control loop and the sequence immittance model of the grid-forming control loop, remove the zero-sequence component in the circuit equation, and obtain the corrected sequence immittance circuit equation.

[0139] The second correction module is used to correct the initial transfer function of the sequence immittance model corresponding to each frequency coupling component using the corrected sequence immittance circuit equation to obtain the corrected transfer function of the sequence immittance model corresponding to each frequency coupling component.

[0140] The judgment module is used to use the modified transfer function to model the conductiveness of the grid side to which the grid-following converter and the grid-forming converter are connected, and obtain the original grid network without the converter connected; the original grid network, the grid-following converter, and the grid-forming converter are separately analyzed for stability; if all are stable, the grid-forming converter is connected to the original grid network to determine whether the power system is stable; if not, it is considered that the power system is unstable due to the grid-forming converter; if stable, the grid-following converter is connected to the network after the grid-forming converter is connected to the original grid network to determine whether the power system is stable; if not, it is considered that the power system is unstable due to the grid-following converter.

[0141] Example 5

[0142] This embodiment provides an electronic device including a memory and a processor. The memory stores a computer program. When the processor executes the computer program, it implements the steps of the sequence impedance modeling method for a heterogeneous new energy grid-connected power system provided in Example 1 or the stability analysis method for a new energy grid-connected power system provided in Example 3.

[0143] Example 6

[0144] This embodiment provides a computer-readable storage medium having a computer program stored thereon. When a processor executes the computer program, steps of the sequence impedance modeling method for a heterogeneous new energy grid-connected power system provided in Example 1 or the stability analysis method for a new energy grid-connected power system provided in Example 3 are implemented.

[0145] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A sequence immittance modeling method for a heterogeneous renewable energy grid-connected power system, characterized in that: include: S1: Performing two-port equivalence on a grid-following converter and a grid-forming converter in a heterogeneous renewable energy grid-connected power system to obtain two-port networks corresponding to the grid-following converter and the grid-forming converter, respectively; the two-port networks include a control loop, a nonlinear circuit, an AC port, and a DC port; adding voltage disturbances to the AC port and the DC port to obtain current responses to the voltage disturbances; S2: performing sequence immittance modeling on the control loops of the grid-following converter and the grid-forming converter under the current response of the voltage disturbance to obtain a sequence immittance model of the grid-following control loop and a sequence immittance model of the grid-forming control loop; S3: performing frequency AC / DC coupling analysis on the nonlinear circuits of the grid-following converter and the grid-forming converter under the current response of the voltage disturbance to obtain multiple frequency coupling components; and modeling the initial transfer function of the sequence immittance model corresponding to each frequency coupling component; S4: combining the circuit equation of the nonlinear circuit with the control equations of the sequence immittance model of the mesh-following control loop and the sequence immittance model of the mesh-forming control loop, removing the zero-sequence component in the circuit equation to obtain a corrected sequence immittance circuit equation; S5: using the modified sequence immittance circuit equation to modify the initial transfer function of the sequence immittance model corresponding to each frequency coupling component, to obtain a modified transfer function of the sequence immittance model corresponding to each frequency coupling component; The modified transfer function of the grid-following converter is expressed as: in, is the response current of the AC port, is the voltage disturbance of the AC port, L is the AC inductance, K d is the feedback coefficient of the inner current loop, V dc Represents the DC voltage value, H c (s-jω1) represents the transfer function of the inner current loop, G θ (s-jω1) is the closed-loop transfer function of the phase-locked loop, H va (s-jω1) is the transfer function of the AC voltage outer loop, Y1 represents the fundamental complex number of admittance, s represents the Laplace factor, j represents the 90-degree rotation component in the complex frequency domain; L is the AC inductance; The modified transfer function of the grid-type converter is expressed as: M 1 is the fundamental frequency parameter of the modulation signal, I 1 represents the parameter of the current fundamental frequency, represents the conjugate complex number of the fundamental current complex number, represents the conjugate complex number of the fundamental voltage complex number, ω0 represents the frequency base value, and M(s-jω1) represents the transfer function of the virtual synchronous control loop.

2. The sequence immittance modeling method for a heterogeneous renewable energy grid-connected power system according to claim 1, characterized in that: The synchronous control modeling in the control loops of the grid-following converter and the grid-forming converter are respectively: in, is the phase angle in the frequency domain, J is the inertia coefficient of the virtual synchronous control loop, D p is the damping coefficient of the virtual synchronous control loop, is the voltage disturbance applied to the AC port, is the current disturbance after the voltage disturbance added to the AC port, U1 * 、I1 * are the conjugates of the initial phasors of voltage and current, respectively.

3. The sequence immittance modeling method for a heterogeneous renewable energy grid-connected power system according to claim 1, characterized in that: Using the formula Modeling the initial transfer function of the sequence immittance model corresponding to each frequency coupling component; Among them, x(t) is the time domain value of the x variable, y(t) is the time domain value of the y variable, is the steady-state value of the x variable at the steady-state operating point, is the superimposed small signal value under small perturbations of the x variable, is the steady-state value of the y variable at the steady-state operating point, is the superimposed small signal value under small perturbation of the y variable, is the small signal value after linearization of the Z(t) harmonic, is the superimposed small signal value under small perturbation of y variable, Y l is the steady-state value of the y variable under the steady-state operating point, where l is used to represent the number of components of the y variable itself, X k is the steady-state value of the variable at the steady-state operating point, is the superimposed small signal value under small perturbation of the variable, where k is used to represent the number of components of the x variable itself; ω p is the disturbance frequency, ω1 is the fundamental frequency, X k is the kth harmonic component of the x variable, Y k is the kth harmonic component of Y.

4. The sequence immittance modeling method for a heterogeneous renewable energy grid-connected power system according to claim 3, characterized in that: When a positive sequence voltage disturbance is added to the AC port, the initial transfer function of the sequence immittance model corresponding to each frequency coupling component is: Among them, Y pp (s) is the positive sequence response self-admittance under positive sequence voltage disturbance, Y pd (s) is the AC-DC coupled admittance of the DC response under positive sequence voltage disturbance, Y pn (s) is the negative sequence response AC transfer admittance under positive sequence voltage disturbance; is the DC current response of the DC port under the positive sequence voltage disturbance, is the coupling current of the AC port under the positive sequence voltage disturbance.

5. The sequence immittance modeling method for a heterogeneous renewable energy grid-connected power system according to claim 3, characterized in that: When a negative sequence voltage disturbance is added to the AC port, the initial transfer function of the sequence immittance model corresponding to each frequency coupling component is: Among them, Y nn (s) is the positive sequence response self-admittance under negative sequence voltage disturbance, Y nd (s) is the AC-DC coupled admittance of the DC response under negative sequence voltage disturbance, Y np (s) is the negative sequence response AC transfer admittance under negative sequence voltage disturbance; is the DC current response of the DC port under the negative sequence disturbance, is the coupling current of the AC port under the negative sequence disturbance.

6. The sequence immittance modeling method for a heterogeneous renewable energy grid-connected power system according to claim 3, characterized in that: When a DC voltage disturbance is added to the DC port, the initial transfer function of the sequence immittance model corresponding to each frequency coupling component is: Among them, Y dd (s) is the DC response self-admittance under DC disturbance, Y dp (s) is the AC-DC coupled admittance of the AC positive sequence response under DC disturbance, Y dn (s) is the AC transfer admittance of the AC negative sequence response under DC disturbance.

7. A sequence immittance modeling device for a heterogeneous renewable energy grid-connected power system, characterized in that: A method for modeling a sequence immittance of a heterogeneous renewable energy grid-connected power system according to any one of claims 1 to 6, comprising: A disturbance module is used to perform two-port equivalence on a grid-following converter and a grid-forming converter in a heterogeneous renewable energy grid-connected power system to obtain a two-port network corresponding to each of the grid-following converter and the grid-forming converter; the two-port network includes: a control loop, a nonlinear circuit, an AC port, and a DC port; a voltage disturbance is added to the AC port and the DC port to obtain a current response to the voltage disturbance; a first modeling module, configured to perform sequence immittance modeling on the control loops of the grid-following converter and the grid-forming converter under the current response of the voltage disturbance, to obtain a sequence immittance model of the grid-following control loop and a sequence immittance model of the grid-forming control loop; A second modeling module is configured to perform frequency AC / DC coupling analysis on the nonlinear circuits of the grid-following converter and the grid-forming converter under the current response of the voltage disturbance to obtain a plurality of frequency coupling components; and to model an initial transfer function of a sequence immittance model corresponding to each frequency coupling component; a first correction module, configured to simultaneously solve the circuit equation of the nonlinear circuit and the control equations of the sequence immittance model of the mesh-following control loop and the sequence immittance model of the mesh-forming control loop, remove the zero-sequence component in the circuit equation, and obtain a corrected sequence immittance circuit equation; The second correction module is used to correct the initial transfer function of the sequence immittance model corresponding to each frequency coupling component using the corrected sequence immittance circuit equation to obtain the corrected transfer function of the sequence immittance model corresponding to each frequency coupling component.

8. A stability analysis method for a heterogeneous renewable energy grid-connected power system, characterized in that: include: Constructing a modified transfer function of the sequence immittance model of each frequency coupling component using the sequence immittance modeling method for a heterogeneous renewable energy grid-connected power system according to any one of claims 1 to 6; Using the modified transfer function, an immittance model is performed on the grid side to which the grid-following converter and the grid-forming converter are connected, so as to obtain an original grid network without the converter connected; Performing stability analysis on the original power grid, the grid-following converter, and the grid-forming converter respectively; if all are stable, connecting the grid-forming converter to the original power grid to determine whether the power system is stable; if not, deeming the power system instability caused by the grid-forming converter; If it is stable, the grid-following converter is connected to the network after the grid-forming converter is connected to the original power grid network to determine whether the power system is stable. If it is unstable, it is considered that the power system is unstable due to the grid-following converter.

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