A parameter-stable-bound-based virtual impedance design method for grid-following inverters

Abstract

The application discloses a virtual impedance design method of a grid-following type inverter based on parameter stability boundaries, and the method comprises the following steps: step 1: analyzing system parameters of a power grid system constituted by the grid-following type inverter, and obtaining initial values, actual values and stability boundary values of the system parameters influencing stability of the power grid system; and step 2: calculating a virtual impedance R v The method disclosed by the application is based on parameter stability boundaries to quantitatively calculate impedance values of the accessed impedance, and the calculation step is more simple; the system equivalent resistance of the system parameter stability boundary and the instability boundary is used to determine the impedance values of the impedance and select the system parameter having a greater influence on the system stability as a basis for calculating the impedance values, so that the required impedance values can be quantitatively calculated, and the result is more accurate.

Description

Technical Field

[0001] This invention belongs to the field of power electronics technology, and in particular relates to a virtual impedance design method for grid-connected inverters based on parameter stability boundaries. Background Technology

[0002] With the rapid development of renewable energy sources such as wind power and photovoltaics, the penetration rate of power electronic converters, which are important interfaces for grid connection of new energy, in the power system is constantly increasing, and the development of modern power systems is gradually showing a trend of "high efficiency and high performance".

[0003] Most grid-connected inverters for new energy sources are grid-connected inverters, whose external characteristics are that of a current source. They achieve efficient utilization of distributed power sources by directly controlling the output current, but lack frequency and voltage support capabilities. In 100% renewable energy islanded microgrid systems dominated by power electronic converters, due to their low inertia, weak damping characteristics, and lack of stable frequency and voltage support from the main grid, improper system parameter settings may cause the phase-locked loop (PLL) of the grid-connected inverter to become unstable due to lack of damping. This prevents the grid-connected inverter from synchronizing with the grid-connected inverter, ultimately causing the islanded microgrid system to fail to maintain stable operation. Summary of the Invention

[0004] The purpose of this invention is to provide a virtual impedance design method for grid-connected inverters based on parameter stability boundaries. This method can quantitatively design the virtual impedance, resulting in more accurate results. Introducing virtual impedance into grid-connected inverters effectively avoids instability due to lack of damping caused by improper grid system parameter settings.

[0005] Therefore, the present invention provides a virtual impedance design method for grid-connected inverters based on parameter stability boundaries, comprising the following steps:

[0006] Step 1: Analyze the system parameters of the power grid system composed of grid-connected inverters, and obtain the initial values, actual values ​​and stability boundary values ​​of each system parameter that affect the stability of the power grid system;

[0007] Step 2: Calculate the virtual impedance R introduced into the grid-connected inverter based on the initial, actual, and stability boundary values ​​of each system parameter. v The calculation formula is as follows:

[0008]

[0009] In the formula, var 1N ,......,var nN These are the system parameters var1, ..., var1 that affect system stability in a grid system composed of grid-connected inverters. n The initial value; var 1s,......,var ns These are the system parameters var1, ..., var. n The actual value that var can take; 1max ,......,var nmax These are the system parameters var1, ..., var. n To maintain the system at stable boundary values, k v This is the virtual impedance constant.

[0010] Another approach to the virtual impedance design method for grid-connected inverters based on parameter stability boundaries provided by this invention is as follows: This method is used to design the virtual impedance R introduced into the grid-connected inverter. v The virtual impedance R is designed to... v This is used to change the output of a grid-connected inverter; the grid-connected inverter is used to construct a power grid system, which includes a grid-connected inverter, a grid-connected inverter, and a tie line; a sequence impedance model is established for the grid-connected inverter, the grid-connected inverter, the inverter tie line impedance, and the constant load in the constructed power grid system; the Nyquist stability criterion is obtained; and the stability boundary values ​​of each system parameter and the system equivalent resistance of the instability boundary are calculated as virtual impedance R. v The first step in the design: Calculate the virtual impedance R based on the system's equivalent resistance at the stability and instability boundaries of each system parameter. v As virtual impedance R v The second step in the design process.

[0011] In another aspect, the present invention provides a grid-connected inverter that is connected to a virtual impedance R. v The virtual impedance R v The virtual impedance design method for grid-connected inverters based on parameter stability boundaries provided by this invention is used to design the inverter.

[0012] A third aspect of the present invention provides an islanded microgrid system comprising the grid-connected inverter provided herein.

[0013] The technical effects achieved by adopting the technical solution of the present invention include at least the following:

[0014] 1) The design method provided by this technical solution is based on the parameter stability boundary to quantitatively calculate the impedance value of the connected impedance, and the calculation steps are simpler.

[0015] 2) This technical solution is based on the order impedance model and uses the Nyquist stability criterion to analyze the system stability. It has the advantages of simple calculation steps and concise and clear calculation results. (Other stability analysis methods, such as the generalized Nyquist stability criterion based on the dq impedance model and eigenvalue analysis based on the state space model, have more complex calculation processes.)

[0016] 3) This technical solution uses the system equivalent resistance of the system parameters at the stability boundary and instability boundary to determine the impedance value and selects the system parameters that have a significant impact on system stability as the basis for calculating the impedance value. It can quantitatively calculate the required impedance value and the result is more accurate.

[0017] 4) An impedance is introduced into the inverter in this technical solution. The impedance value is determined by the values ​​of various system parameters that affect the stability of the grid system formed by the grid inverter. It has a certain degree of self-adaptation, which improves the damping of the grid inverter and effectively avoids the situation where the grid inverter becomes unstable due to lack of damping because of improper grid system parameter settings.

[0018] 5) When the grid-connected inverter is built into the grid system, it can effectively avoid the instability of the grid-connected inverter due to lack of damping caused by improper grid system parameter settings, reduce the impact of the grid-connected inverter output on the steady-state operating point of the grid system, effectively improve the damping of the grid system, and increase the stability of the grid system to a certain extent.

[0019] 6) In this technical solution, the impedance is installed in the control circuit of the grid-connected inverter. The introduced impedance is a secondary device, which avoids the need to modify the power grid system. It has the advantages of convenient installation and maintenance, low equipment requirements and low cost.

[0020] 7) In this technical solution, the impedance is installed in the control circuit of the grid-connected inverter, which is easy to operate and has a relatively lower cost. Attached Figure Description

[0021] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention. It is obvious that the drawings described below are merely some embodiments of the invention, and those skilled in the art can obtain other drawings based on these drawings without any inventive effort. In the drawings:

[0022] Figure 1 This is a circuit topology diagram of the grid-connected inverter control loop disclosed in this invention;

[0023] Figure 2 This is a diagram of the main circuit topology of the power grid system disclosed in this invention;

[0024] Figure 3 This is a diagram of the equivalent impedance network model of this disclosure;

[0025] Figure 4 This is a comparison chart showing the stability improvement effect of the present invention.

[0026] In the attached diagram, PCC represents the grid connection point. Detailed Implementation

[0027] Exemplary embodiments will now be described more fully with reference to the accompanying drawings. However, these exemplary embodiments can be implemented in many forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be comprehensive and complete, and will fully convey the concept of the exemplary embodiments to those skilled in the art. In the drawings, the dimensions of some elements may be exaggerated or modified for clarity. The same reference numerals in the drawings denote the same or similar structures, and therefore their detailed description will be omitted.

[0028] To effectively prevent grid-connected inverters from becoming unstable due to lack of damping caused by improper grid system parameter settings, this invention introduces a virtual impedance R into the grid-connected inverter. v To change the output of the grid-connected inverter; the impedance R v The impedance value is determined by the values ​​of various system parameters that affect the stability of the power grid system formed by the grid inverter.

[0029] In this disclosure, the virtual impedance R v The control loop topology of the control loop circuit is introduced into the grid-connected inverter circuit. Figure 1 An exemplary illustration includes:

[0030] The q-axis branch is configured to include a first arithmetic unit, a first PI regulator for zero steady-state error tracking of the DC component in a synchronous coordinate system, a second arithmetic unit, and a first K-axis regulator for dq decoupling control. dp Module;

[0031] The d-axis branch is configured to include a third arithmetic unit, a second PI regulator for error-free tracking of the DC component in the synchronous coordinate system, a fourth arithmetic unit, and a second K-axis regulator for dq decoupling control. dp Modules; and

[0032] The coordinate transformation module dq / abc is used to transform the input signal from the dq coordinate system to the abc coordinate system.

[0033] q-axis reference current I qref The q-axis current component I at the grid connection point in a synchronously rotating coordinate system q After being introduced into the first arithmetic unit, the signal is fed into the second arithmetic unit via the first PI regulator. The first K... dpThe signal output from the module is introduced into the second arithmetic unit; the d-axis reference current I dref The d-axis current component I at the grid connection point in a synchronously rotating coordinate system d After being introduced into the third arithmetic unit, the signal is fed into the fourth arithmetic unit via the second PI regulator, and the second K... dp The signal output from the module is introduced into the fourth arithmetic unit; the outputs of the second and fourth arithmetic units are respectively introduced into the coordinate transformation module dq / abc to output the modulated wave e. abc .

[0034] The inverter is introduced with a virtual impedance R. v To change the output of the grid-connected inverter, the coordinate transformation module dq / abc outputs a modulated wave e. abc Introducing the fifth arithmetic unit and i oabc R v The calculation yields the impedance R v The final modulated wave is obtained The impedance R connected v The output modulated wave then becomes:

[0035]

[0036] e abc For the unconnected virtual impedance R v Modulation wave with grid-type inverter; To connect the virtual impedance R v Modulation wave with grid inverter; i oabc This refers to the three-phase output current of the grid-connected inverter.

[0037] In this disclosure, the virtual impedance R v Obtained via the following expression:

[0038]

[0039] In the formula, var 1N ,......,var nN These are the system parameters var1, ..., var1 that affect system stability in a grid system composed of grid-connected inverters. n The initial value; var 1s ,......,var ns These are the system parameters var1, ..., var. n The actual value that var can take; 1max ,......,var nmax These are the system parameters var1, ..., var. n To maintain the system at stable boundary values, k vThis represents the virtual impedance constant. The system parameters var1, ..., var... are given. n The initial values ​​are the initial values ​​of various system parameters that can maintain the stable operation of the system.

[0040] To ensure the virtual impedance R of the connection v It will not cause instability in the power grid system. When selecting the value of the virtual impedance constant, the virtual impedance R should be such that... v The impedance value satisfies:

[0041]

[0042] In the formula: R eq1 ,......,R eqn For each system parameter var1,......,var n The equivalent loop resistance of the power grid system at the moment of instability is R. eq1 ,......,R eqn =Re[Z eq1 ],......,Re[Z eqn ];Z eq1 ...Z eqn They are obtained using the following expressions respectively:

[0043] Z eq =Z load +(Z 11 +Z g11 ) / / ...... / / (Z 1i +Z g1i ) / / (Z 21 +Z g21 ) / / ...... / / (Z 2j +Z g2j )

[0044] In the formula: Z load For the sequence resistance of a constant load in a power grid system, (Z) 11 +Z g11 ) / / ...... / / (Z 1i +Z g1i (Z) represents the parallel impedance of the i grid-type inverter branches used to construct the power grid system; 21 +Z g21 ) / / ...... / / (Z 2j +Z g2j ) represents the parallel impedance of j grid-connected inverter branches in the power grid system.

[0045] The main circuit topology of the grid system constructed by this grid-connected inverter includes i grid-connected inverter branches, j root-connected inverter branches, inverter tie-line impedance, and sequence impedance of constant loads, such as... Figure 2 As shown. Based on this power grid system and combined with the following steps, the virtual impedance R is calculated. v A more detailed description of the impedance values ​​will be provided.

[0046] Step 1: Establish sequence impedance models for the mid-grid inverter, grid-connected inverter, inverter tie-line impedance, and constant load in the constructed power grid system. Obtain the Nyquist stability criterion and calculate the stability boundary values ​​and instability boundary values ​​of each system parameter. Specifically, this step includes the following sub-steps:

[0047] Step 1.1: Establish the sequence impedance model Z of the i grid-connected inverters constituting the power grid system. 11 ,......,Z 1i ; Sequence impedance model of a grid-connected inverter Z 21 ,......,Z 2j Sequence impedance model Z of interconnection lines in grid-type inverters g11 ,......,Z g1i The sequence impedance model Z of the interconnect line of the grid inverter g21 ,......Z g2j and the sequence impedance model Z for constant load load ;

[0048] Step 1.2: Based on the sequence impedance model in Step 1.1, establish the equivalent impedance network model of the power grid system, such as... Figure 3 As shown, the expression for the grid connection point output current is obtained as follows:

[0049]

[0050]

[0051] When H 11 (s),......,H 1k When neither (s) has a positive real pole, the stability of the power grid system depends on H. 21 (s),......,H 2l (s) and H3(s), H 21 (s),......,H 2l Both H(s) and H3(s) can be considered as closed-loop transfer functions, and their stability depends on the open-loop transfer function L. m1 (s),......,L ml (s) and L m (s), when L m1 (s),......,L ml (s) and L m(s) When the Nyquist stability criterion is satisfied, the power grid system is stable. Based on this, the initial system parameters affecting the stability of the power grid system are obtained, such as: inverter tie-line inductance L. gi Reference value I of d-axis output current of grid-connected inverter drefi Constant load impedance Z load wait;

[0052] Step 1.3: Modify the initial system parameters obtained in Step 1.2 (e.g., inverter tie line inductance L). gi Reference value I of d-axis output current of grid-connected inverter drefi Constant load impedance Z load The system parameters (etc.) are used to determine the stability of the power grid system under different parameter values ​​according to the Nyquist stability criterion. The system parameters that have a significant impact on the stability of the power grid system are identified and labeled as: var1, ..., var n Determine the limiting values ​​of each system parameter that maintain the stability of the power grid system as the stability boundary values, and label them as: var 1max ,......,var nmax Stability boundary values ​​can be identified based on the results of system stability analysis.

[0053] Step 1.4: Calculate the equivalent loop impedance Z under the stability boundary values ​​of each system parameter. eq Equivalent loop impedance Z eq The real part of the equation is the equivalent loop resistance R of the power grid system at the moment when the power grid system becomes unstable due to various system parameters. eq The stability boundary is the limit for the power grid system to maintain stability, and it is also the boundary for the power grid system to become unstable. In other words, when a certain parameter affecting the system is configured to a certain value, the power grid system will remain stable. However, if the value is exceeded, the system will become unstable. The set parameter value is both the boundary value that ensures the stability of the system and the boundary value that causes the system to become unstable.

[0054] In this disclosure, the system parameter is determined to be a system parameter that has a significant impact on the stability of the power grid system if the following condition is met: when a stable system becomes unstable due to improper setting (or modification) of a certain parameter, that parameter should be classified as having a significant impact on the stability of the system.

[0055] Among them, the equivalent loop impedance Z under each system parameter eq Calculated using the following expression:

[0056] Z eq =Z load +(Z 11 +Z g11 ) / / ...... / / (Z 1i +Z g1i ) / / (Z21 +Z g21 ) / / ...... / / (Z 2j +Z g2j )

[0057] In the formula: Z load For the sequence resistance of a constant load in a power grid system, (Z) 11 +Z g11 ) / / ...... / / (Z 1i +Z g1i (Z) represents the parallel impedance of the i grid-connected inverter branches; 21 +Z g21 ) / / ...... / / (Z 2j +Z g2j ) represents the parallel impedance of j grid-connected inverter branches.

[0058] For example, calculating the stability boundary value var of the kth critical system parameter. kmax The sequence impedance Z of the under-grid inverter 11k ,......,Z 1ik And the sequence impedance Z of the grid-connected inverter 21k ,......,Z 2jk And the corresponding tie-line sequence impedance Z g11k ,......,Z g1ik and Z g21k ,......Z g2jk At this time, the equivalent loop impedance of the system is Z. eqk The calculation formula is as follows:

[0059] Z eqk =Z load +(Z 11k +Z g11k ) / / ...... / / (Z 1ik +Z g1ik ) / / (Z 21k +Z g21k ) / / ...... / / (Z 2jk +Z g2jk The corresponding system equivalent resistance is R. eqk =Re[Z eqk ], that is, the system equivalent loop impedance Z eqk The real part.

[0060] Step 2: Calculate the virtual impedance R based on the system equivalent resistance at the stability and instability boundaries of each system parameter. v This step specifically involves calculating the virtual impedance R using the following expression. v :

[0061]

[0062] In the formula, var 1N ,......,var nN These are the initial values ​​for each system parameter; var 1s ,......,var ns These are the actual values ​​of each system parameter; k v This is the virtual impedance constant.

[0063] Wherein, the virtual impedance constant k v The value of should make the virtual impedance R v satisfy:

[0064]

[0065] In a 100% renewable energy islanded microgrid system dominated by power electronic converters, due to its low inertia, weak damping characteristics, and lack of stable frequency and voltage support from the main grid, improper system parameter settings may cause the phase-locked loop of the grid-connected inverter to become unstable due to lack of damping, making it impossible for the islanded microgrid system to maintain stable operation.

[0066] To address this, the present invention provides a grid-connected inverter. A virtual impedance is introduced into the control loop of this inverter. This virtual impedance is designed based on the values ​​of various system parameters affecting the stability of the grid system formed by the grid-connected inverter, exhibiting a certain degree of self-adaptation. This improves the damping of the grid-connected inverter and effectively avoids instability due to lack of damping caused by improper grid system parameter settings. By configuring the grid-connected inverter of this invention into an islanded microgrid system or other grid systems, the impact of improper grid system parameter settings on the steady-state operating point of the islanded microgrid system due to lack of damping can be reduced, thereby enhancing the stability of the islanded microgrid system.

[0067] The main circuit topology of the islanded microgrid system built with this grid inverter is as follows: Figure 2 This paper uses one grid-connected inverter and one follow-the-grid inverter as examples to illustrate how to determine the virtual impedance value introduced by the follow-the-grid inverter in an islanded microgrid system. Those skilled in the art should understand that using one inverter as an example does not mean there is only one inverter, but rather that it can be configured as i inverters and j inverters.

[0068] The impedance value of the virtual impedance introduced by the grid-connected inverter in an islanded microgrid system is determined by the following steps:

[0069] Step 1: Establish the sequence impedance model Z of one grid-connected inverter 11 The sequence admittance model Z of a grid-connected inverter 21 Sequence impedance model Z of interconnection lines in grid-type inverters g11The sequence impedance model Z of the interconnect line of the grid inverter g21 and the sequence impedance model Z for constant load load ;

[0070] Step 2: Based on the sequence impedance model in Step 1, establish the equivalent impedance network model of the islanded microgrid system, such as... Figure 3 As shown, the expression for the output current at the grid connection point is:

[0071] I PCC =(H 11 ·V s1 +H 21 ·I s1 )·H3

[0072]

[0073] When H 11 When neither (s) has a positive real pole, the stability of the islanded microgrid system depends on H. 21 (s) and H3(s). H 21 Both H(s) and H3(s) can be considered as closed-loop transfer functions, and their stability depends on the open-loop transfer function L. m1 (s) and L m (s). Therefore, when L m1 (s) and L m (s) An islanded microgrid system is stable when it simultaneously satisfies the Nyquist stability criterion.

[0074] Step 3: Modify the parameter values ​​of each initial system parameter obtained in Step 1.2, and determine the stability of the power grid system under different parameter values ​​according to the Nyquist stability criterion. Identify the system parameters that have a significant impact on system stability, namely the inductance L of the grid-connected inverter. g21 and its d-axis output current reference value I dref1 Simultaneously, based on the system stability analysis results, the stability boundary values ​​of each parameter were determined as L. g21max =5mH, I d1max =20A.

[0075] Step 4: Calculate the stability boundary value L for different system parameters g21max I d1max The equivalent loop impedances of the lower system are Z eq1 Z eq2 The corresponding system equivalent resistance is R. eq1 =Re[Z eq1 ]、R eq2 =Re[Z eq2 ];

[0076] Step 5: Based on the parameter stability boundary L g21max Id1max and the system equivalent resistance R at the instability boundary eq1 R eq2 The impedance of the grid-connected inverter is designed to be...

[0077]

[0078] In the formula, L g21N I dref1N L respectively g21 I dref1 The initial value of L; g21s I d1s The system parameters L are respectively g21 I dref1 The actual value of k; v Let be the virtual impedance constant, and its value should be such that the virtual impedance satisfies:

[0079]

[0080] The virtual impedance R is determined through the above steps. v The impedance value is then introduced into the control loop of the grid-connected inverter, such as... Figure 1 As shown, the modulation wave of the improved grid-connected inverter becomes:

[0081]

[0082] e abc Unconnected impedance R v Modulation wave with grid-type inverter; For the connection impedance R v Modulation wave with grid inverter; i oabc This refers to the three-phase output current of the grid-connected inverter.

[0083] Changing the virtual impedance constant makes the virtual impedance R v Different, such as Figure 4 As shown, the constant coefficient k of different virtual impedances is... v Below is a comparison of the simulation waveforms of an islanded microgrid system when the reference value of the d-axis output current of the grid-connected inverter is changed. In the figure, δ2 is the phase angle difference of the local dq coordinate axis between the grid-connected and grid-connected inverters (based on the grid-connected inverter), f is the output frequency of the phase-locked loop of the grid-connected inverter, Id is the d-axis output current of the grid-connected inverter, and the solid line represents k. v The simulated waveform with k = 0.5, the dashed line represents k. v The simulation waveform is equal to 0.1. Increase the reference value I of the d-axis output current of the grid-connected inverter. dref1 This increases the current from 14A to the stability boundary of 20A. A comparison is made between different virtual impedance constant coefficients k. v From the simulation waveform below, we can see that k v The larger the impedance R, the greater the impedance R.v The larger the value of k, the better the improvement in system stability. v When = 0.1, due to impedance R v <max{|R eq1 |,|R eq2 |}, causing the reference value I of the d-axis output current of the grid-connected inverter to be related to the value of the reference value of the output current. dref1 Reaching the stable boundary I d1max When the current is 20A, it cannot provide sufficient damping support for the system, ultimately leading to instability in the islanded microgrid system. When k... v When the value is 0.5, the system receives sufficient damping support, and the simulated waveform of the system tends to stabilize after a small amount of damped oscillation, and the system can maintain stable operation.

[0084] In this disclosure, the grid-connected inverter is a VSG-based grid-connected inverter, and the grid-following inverter is a current-controlled grid-following inverter. Both are configured to include: a DC power supply (V... dc11 ......V dc1i V dc21 ......V dc2i The converter VSC converts DC to AC, providing grid-connected current and grid-connected point voltage, and the filter inductor (L) f11 ......L f1i / L f21 ......L f2i ), inverter impedance (R) f11 ......R f1i / R f21 ......R f2i ), filter capacitor (C) f11 ......C f1i / C f21 ......C f2i The power grid it constructs is also configured to include tie-line impedance (R). g11 ......R g1i / R g11 ......R g2i ), tie line inductance (L) g11 ......L g1i / L g21 ......L g2i and constant load impedance Z load .

[0085] Of course, it can also be configured as other types of network topology and follow-up network topology.

[0086] The virtual impedance R disclosed herein v It is introduced into the grid-connected inverter in an independent manner, avoiding interference with the primary equipment of the power grid system (not connected to impedance R).v The grid structure and grid-connected inverters are modified to have the advantages of convenient installation and maintenance, and lower equipment requirements and costs.

[0087] It should be understood that any parts not described in detail in this specification belong to the prior art. Although specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, those skilled in the art should understand that these are merely illustrative examples, and various modifications or variations can be made to these embodiments without departing from the principles and essence of the present invention. The scope of the present invention is defined only by the appended claims.

Claims

1. A parameter-stable-boundary-based virtual impedance design method for a grid- connected inverter, characterized by, The method comprises the following steps: Step 1: analyzing system parameters of a grid system composed of a grid-connected inverter, and obtaining initial values, actual values and stable boundary values of the system parameters affecting stability of the grid system; Step 2: Calculate the virtual impedance R introduced to the grid-connected inverter according to the initial value, actual value and stable boundary value of each system parameter v The calculation formula is as follows: In the formula, var 1N ,......,var nN are initial values of system parameters var1,......,var n in a grid system composed of grid-connected inverters which affect system stability; var 1s ,......,var ns are actual values of the system parameters var1,......,var n ; var 1max ,......,var nmax are stable boundary values of the system parameters var1,......,var n for keeping the system stable, and k v is a virtual impedance constant. var1,..., var n were determined by the following steps: Step 1.1: Establish the sequence impedance model Z of i grid-connected inverters constituting the grid system 11 ,......,Z 1i ; the sequence impedance model Z of j grid-following inverters 21 ,......,Z 2j ; the sequence impedance model Z of the grid-connected inverter tie line g11 ,......,Z g1i ; the sequence impedance model Z of the grid-following inverter tie line g21 ,......Z g2j ; and the sequence impedance model Z of constant loads load ; Step 1.2: based on the sequence impedance model in step 1.1, establishing an equivalent impedance network model of the grid system, and establishing a grid-connected point output current: When L m1 (s),......,L ml (s) and L m (s) satisfy the Nyquist stability criterion at the same time, the power grid system is stable, and according to this, the initial system parameter values affecting the stability of the power grid system are obtained, the initial system parameter values including the inverter tie-line inductance L gi , the grid-following type inverter d-axis output current reference value I drefi , and the constant load impedance Z load ; Step 1.3: Modify the initial system parameter values ​​obtained in Step 1.2, determine the stability of the power grid system under different parameter values ​​according to the Nyquist stability criterion, and obtain the system parameters that affect the stability of the power grid system, which are labeled as: var1, ..., var n ; The stable boundary value var 1max ,......,var nmax , based on the power grid system stability analysis results to find each parameter to make the system meet the Nyquist stability criterion stable boundary is determined as var 1max ,......,var nmax .

2. The parameter-stable-bound-based grid-connected inverter virtual impedance design method of claim 1, wherein: The virtual impedance constant k v The value of the virtual impedance R v satisfies: In the formula: R eq1 ,......,R eqn are system parameters var1,......,var n , respectively, and Re eq1 ,......,Re eqn = Re[Z eq1 ],......,Re[Z eqn ] are the equivalent loop resistances of the power grid system at the moment of instability, respectively; Z eq1 .....Z eqn are obtained through the following expressions, respectively: Z eq = Z load +(Z 11 +Z g11 ) / / ...... / / (Z 1i +Z g1i ) / / (Z 21 +Z g21 ) / / ...... / / (Z 2j +Z g2j In the formula: Z load is the impedance of the constant load of the power grid system, (Z 11 +Z g11 ) / / ...... / / (Z 1i +Z g1i ) is the parallel impedance of the i grid-constructing inverter branches of the power grid system; and (Z 21 +Z g21 ) / / ...... / / (Z 2j +Z g2j ) is the parallel impedance of the j grid-following inverter branches of the power grid system.

3. A parameter-stable-boundary-based virtual impedance design method for a grid-connected inverter, characterized by: The method is used for changing the output of the grid-connected inverter by introducing a virtual impedance R v into the grid-connected inverter v for changing the output of the grid-connected inverter The grid-following inverter is used to construct a power grid system, the constructed power grid system comprising a grid-forming inverter, a grid-following inverter and a tie line; a sequence impedance model of the grid-forming inverter, the grid-following inverter, the inverter tie line impedance and a constant load in the constructed power grid system is established, a Nyquist stability criterion is obtained, and a stable boundary value of each system parameter and a system equivalent resistance of an instability boundary are calculated as a virtual impedance R v A first step of the design; based on the stable boundary and the system equivalent resistance of the instability boundary of each system parameter, the virtual impedance R is calculated v The stable boundary and the system equivalent resistance of the instability boundary of each system parameter are used as the virtual impedance R v A second step of the design, The first process comprises the following sub-steps: Step 1.1: Establish the sequence impedance model Z of i grid-forming inverters constituting the grid system 11 ,......,Z 1i ; the sequence impedance model Z of j grid-following inverters 21 ,......,Z 2j ; the sequence impedance model Z of the grid-forming inverter tie line g11 ,......,Z g1i ; the sequence impedance model Z of the grid-following inverter tie line g21 ,......Z g2j ; and the sequence impedance model Z of constant loads load ; Step 1.2: based on the sequence impedance model in step 1.1, establishing an equivalent impedance network model of the grid system, and obtaining an expression of the grid-connected point output current as follows: When L m1 (s),......,L ml (s) and L m (s) satisfy the Nyquist stability criterion at the same time, the power grid system is stable, and according to this, each initial system parameter affecting the stability of the power grid system is obtained, the initial system parameter values including the inverter tie-line inductance L gi , the grid-following type inverter d-axis output current reference value I drefi , and the constant load impedance Z load ; Step 1.3: Modify the initial system parameter values ​​obtained in Step 1.2, determine the stability of the power grid system under different parameter values ​​according to the Nyquist stability criterion, and obtain the system parameters that affect the stability of the power grid system, which are labeled as: var1, ..., var n The limiting values ​​of each system parameter that maintain the stability of the power grid system are obtained as stability boundary values ​​and denoted as: var 1max ,......,var nmax ; Step 1.4: Calculate the equivalent loop impedance Z of the power grid system under the stability boundary of each system parameter eq , the real part of the equivalent loop impedance Z eq is the equivalent loop resistance R of the power grid system at the moment when the power grid system loses stability under each system parameter eq ; Equivalent loop impedance Z under each system parameter eq is calculated by the following expression: Z eq = Z load + (Z 11 + Z g11 ) / / ... / / (Z 1i + Z g1i ) / / (Z 21 + Z g21 ) / / ... / / (Z 2j + Z g2j ) In the formula: Z load For the sequence resistance of a constant load in a power grid system, (Z) 11 +Z g11 ) / / ...... / / (Z 1i +Z g1i (Z) represents the parallel impedance of the i grid-connected inverter branches; 21 +Z g21 ) / / ...... / / (Z 2j +Z g2j ) represents the parallel impedance of j grid-connected inverter branches; The second process calculates the virtual impedance R based on the stable boundary values of each system parameter using the following expression v , In the formula, var 1N ,......,var nN are initial values of respective system parameters; var 1s ,......,var ns are actual values of respective system parameters; k v is a virtual impedance constant; the value of the virtual impedance constant k v should be such that the virtual impedance R v satisfies:

4. The parameter-stable-boundary-based grid-connected inverter virtual impedance design method of claim 3, wherein, The virtual impedance R v is incorporated into the control loop of the grid-connected inverter.

5. The parameter-stable-boundary-based grid-connected inverter virtual impedance design method of claim 4, wherein, The control loop comprises: a q-axis branch configured to include a first operator, a first PI regulator for zero-static error tracking of a direct current component in a synchronous coordinate system, a second operator, and a first K dp module a d-axis branch configured to include a third operator, a second PI regulator for zero-static error tracking of a direct current component in a synchronous reference frame, a fourth operator, and a second K dp module; and A coordinate transformation module dq / abc; q-axis reference current I qref and the q-axis current component I at the point of common coupling in the synchronous rotating coordinate system q The first K is introduced into the second operator after the first operator and the first PI regulator, respectively. dp The signal output by the module is introduced into the second operator; d-axis reference current I dref and the d-axis current component I at the point of common coupling in the synchronous rotating coordinate system d The third operator is introduced into the fourth operator through the second PI regulator, and the second K dp The signal output by the module is introduced into the fourth operator; The second and fourth arithmetic operators output respectively introduce the coordinate transformation module dq / abc output modulation wave e abc Introducing a fifth arithmetic operator with i oabc R v Arithmetic operation obtains the virtual impedance R v Get the final modulation wave i oabc For the three-phase output current of the grid-connected inverter.

6. A net-follower inverter, characterized by The grid-connected inverter is connected to the virtual impedance R. v The virtual impedance R v The virtual impedance design method for grid-connected inverters based on parameter stability boundaries, as described in any one of claims 1-5, is used to design the inverter.

7. An islanded microgrid system, characterized by, The system comprises the grid-connected inverter of claim 6.

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