Transient synchronous stability judgment method and product of network-forming inverter parallel system

Abstract

The invention relates to the field of power system stability evaluation, in particular to a transient synchronous stability judgment method and product of a network-forming inverter parallel system.The transient synchronous stability judgment method comprises the steps that firstly, a simplified circuit model during parallel operation of multiple inverters is built, and based on a control strategy, a control model of a single inverter is determined; thirdly, listing a branch current equation and a coupling bus voltage equation through the Kirchhoff theorem to obtain an output active power model of each inverter; thirdly, establishing a virtual power angle dynamic nonlinear mathematical equation of the inverter, drawing a three-dimensional power-virtual power angle curved surface, and determining a power balance point of the system by analyzing the curved surface; and finally, by solving the transient synchronous stability judgment equation, whether transient synchronous instability occurs in the system is determined, and the specific instability condition of each inverter is further judged.

Description

Technical Field

[0001] The present invention relates to the field of power system stability assessment, and in particular to a transient synchronous stability judgment method and product for a grid-connected inverter parallel system. Background Art

[0002] In recent years, a large number of renewable energy sources such as wind power and solar power have been connected to the grid through voltage source converters (VSCs) to meet the growing demand for electricity and address environmental pollution issues. In order to cope with the challenges brought by the lack of inertia and voltage support capabilities, converters with grid formation (GFM) control functions have been proposed. Grid-forming inverters calculate the output power at the common coupling point and use the power difference to achieve self-synchronization. Grid-forming inverters can be equivalent to controlled voltage sources, and can actively provide voltage and frequency support for the power system, and can achieve smooth switching between grid-connected and off-grid modes.

[0003] At present, the stability of grid-connected inverters under grid disturbances has received increasing attention. Most of these works focus on studying the small signal stability of grid-connected inverters or the transient stability of a single grid-connected inverter under large disturbances such as line faults and voltage drops. Existing research rarely involves the transient stability of a multi-machine system composed of multiple grid-connected inverters, and transient synchronous instability will cause problems such as overcurrent and offline.

[0004] With the massive development of renewable energy, it is necessary to study the transient instability problem of multiple grid-connected inverters in parallel in the grid-connected mode. Establishing a large signal model of multiple new energy grid-connected inverters in parallel can quantitatively characterize the dynamic behavior of the inverter during the fault, reveal the transient synchronization instability mechanism and further optimize the design and operation of the system. The research and application of transient synchronization stability judgment methods are of great significance for maintaining safe operation during system faults.

[0005] Therefore, it is necessary to analyze the transient synchronization stability judgment method of multiple parallel systems of new energy grid-connected inverters. Summary of the invention

[0006] The technical problem to be solved by the present invention is the problem of transient synchronization instability when multiple grid-connected inverters are operated in parallel in the event of a grid fault or disturbance. The purpose is to provide a method and product for judging the transient synchronization stability of a grid-connected inverter parallel system, which realizes real-time evaluation of the system synchronization stability in a complex grid environment and effective control during faults, thereby ensuring safe and stable operation of the system.

[0007] The present invention is achieved through the following technical solutions:

[0008] A method for determining transient synchronization stability of a grid-connected inverter parallel system, comprising:

[0009] Based on the characteristics of the inverter output port, a simplified circuit model of the grid-connected inverter parallel system in the grid-connected mode is constructed;

[0010] Based on the control strategy, determine the control model of a single grid-connected inverter;

[0011] Write down the branch current equations and coupled bus voltage equations in the simplified circuit model of the grid-connected inverter parallel system in the grid-connected mode, and obtain the output active power model of each inverter;

[0012] Combining the inverter output active power model and the control model of a single grid-connected inverter, a dynamic nonlinear mathematical equation of the virtual power angle of the inverter in a faulty system of multiple grid-connected inverters is established.

[0013] Draw a three-dimensional power-virtual power angle surface, and then determine the power balance point of the two grid-type inverters;

[0014] By solving the system transient synchronous stability judgment equation, it is judged whether the system has transient synchronous instability, and the specific instability situation of each grid-type inverter is determined based on the judgment result.

[0015] Specifically, the simplified circuit model of the grid-connected inverter parallel system constructed in the grid-connected mode is:

[0016] The two grid-type inverters are respectively connected to the filter inductor and the filter capacitor, the output current I1 of one grid-type inverter is connected to the coupling bus through the line admittance Y1, and the output current I2 of the other grid-type inverter is connected to the coupling bus through the line admittance Y2;

[0017] The grid connection point between the line admittance Y1 and the coupled bus is PCC1, and the grid connection point between the line admittance Y2 and the coupled bus is PCC2;

[0018] The branch current I3 on ​​the coupling bus is gathered and output to the main grid through the line admittance Y3.

[0019] Specifically, the active control links of the two grid-connected inverters adopt the power synchronization control strategy, and the control model of a single grid-connected inverter is: θ i =∫(w g +K i (P ref,i -P i ))dt, where θ i is the output voltage phase angle of the i-th grid-connected inverter, w g is the rated angular frequency, K i is the power synchronization coefficient of the i-th grid-connected inverter, P ref,i is the reference active power of the i-th grid-connected inverter, P iis the output active power of the i-th grid-connected inverter, i=1,2, ∫(·)dt is the integral over time;

[0020] Specifically, based on Kirchhoff's theorem, the three branch current equations are written as Among them, V o1 is the output voltage vector of the first grid-connected inverter, V o2 is the output voltage vector of the second grid-connected inverter, Y j is the jth line admittance, j = 1, 2, 3, E is the grid voltage vector, V BUS is the coupled bus voltage,

[0021] In the case of pure inductance circuit, the formula can be simplified to obtain the output active power model of two grid-connected inverters. Among them, E0 is the voltage amplitude on the grid side, δ1 is the virtual power angle of the first grid-type inverter, and δ2 is the virtual power angle of the second grid-type inverter.

[0022] Specifically, a dynamic nonlinear mathematical equation of the virtual power angle is established based on the active power model and the control model: is the first-order derivative of δ1, is the first-order derivative of δ2.

[0023] Optionally, the three-dimensional power-virtual power angle surface is a P-δ surface drawn based on the output active power model and the dynamic nonlinear mathematical equation of the virtual power angle. The Z-axis of the three-dimensional power-virtual power angle surface is the active power, the X-axis is the virtual power angle δ1, and the Y-axis is the virtual power angle δ2.

[0024] Specifically, the method for determining the power balance point of two grid-connected inverters includes:

[0025] Obtaining a first contour line generated by the intersection of the reference active power of the first grid-forming inverter and the three-dimensional power-virtual power angle surface;

[0026] Obtaining a second contour line generated by the intersection of the reference active power of the second grid-forming inverter and the three-dimensional power-virtual power angle surface;

[0027] Place the first contour line and the second contour line in the same two-dimensional coordinate system, where the X axis of the two-dimensional coordinate system is the virtual power angle δ1, and the Y axis is the virtual power angle δ2;

[0028] The intersection of the first contour line and the second contour line is identified, and the intersection position is the power balance point.

[0029] Optionally, the system transient synchronous stability judgment equation is:

[0030] If the judgment equation has a feasible solution, the system is judged to be transiently synchronously stable; if there is no feasible solution, the system is judged to be transiently synchronously unstable.

[0031] Specifically, if it is determined that the grid-connected inverter has transient synchronous instability during a fault, the instability situation is determined as follows:

[0032] If the three-dimensional power-virtual power angle surface has no intersection with the reference active power, it is judged that the grid-type inverter has lost synchronization with the system;

[0033] If there is no intersection between the first contour line and the second contour line in the two-dimensional coordinate system δ1-δ2, it is judged that the system has transient synchronous instability.

[0034] A computer program product includes a computer program / instruction, which, when executed by a processor, implements the transient synchronization stability judgment method of a grid-connected inverter parallel system as described above.

[0035] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0036] The present invention first constructs a simplified circuit model when multiple inverters are operated in parallel, and determines the control model of a single inverter based on the control strategy; then, the branch current equation and the coupled bus voltage equation are listed through Kirchhoff's theorem to obtain the output active power model of each inverter; then, a dynamic nonlinear mathematical equation of the virtual power angle of the inverter is established, and a three-dimensional power-virtual power angle surface is plotted, and the power balance point of the system is determined by analyzing the surface; finally, by solving the transient synchronization stability judgment equation, it is determined whether the system has transient synchronization instability, and the specific instability situation of each inverter is further judged.

[0037] The present invention proposes a transient synchronization stability judgment method for a parallel system of multiple grid-connected inverters in a grid-connected mode during a fault process, which fills the gap in the research field of transient synchronization stability of a parallel system of multiple inverters. Based on the established dynamic mathematical model of power angle, the synchronization stability characteristics of a parallel system of multiple grid-connected inverters in a grid-connected mode during a transient fault process can be quantitatively analyzed. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The accompanying drawings illustrate exemplary embodiments of the present invention and, together with the description thereof, are used to explain the principles of the present invention. These drawings are included to provide a further understanding of the present invention, and the accompanying drawings are included in and constitute a part of this specification and do not constitute a limitation of the embodiments of the present invention.

[0039] Figure 1 It is a model of a grid-connected inverter parallel system in a grid-connected mode according to the transient synchronous stability judgment method of the grid-connected inverter parallel system described in the present invention.

[0040] Figure 2 It is an inverter control model based on power synchronization control according to the present invention.

[0041] Figure 3 It is a three-dimensional active power-virtual power angle surface diagram according to the present invention.

[0042] Figure 4 is a two-dimensional virtual power angle diagram according to the present invention.

[0043] Figure 5 It is the simulated waveform of the active power and power angle of the inverter under the symmetrical drop of the grid voltage according to the present invention. DETAILED DESCRIPTION

[0044] To make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and implementation methods. It is understood that the specific implementation methods described herein are only used to explain the relevant content, rather than to limit the present invention.

[0045] It should also be noted that, for the convenience of description, only the parts related to the present invention are shown in the drawings.

[0046] In the absence of conflict, the embodiments and features of the embodiments of the present invention may be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0047] Embodiment 1

[0048] A method for determining transient synchronization stability of a grid-connected inverter parallel system, comprising:

[0049] S1. Based on the characteristics of the inverter output port, a simplified circuit model of the grid-connected inverter parallel system in the grid-connected mode is constructed; based on the control strategy, the control model of a single grid-connected inverter is determined.

[0050] The inverter output port characteristics refer to the electrical characteristics of the inverter at its output port, such as voltage, current, and power output. The control strategy refers to the control method or algorithm adopted to achieve stable operation of the system. In the inverter system, common control strategies include power control, voltage control, frequency control, etc.

[0051] S2. Use Kirchhoff's theorem to write the branch current equations and coupled bus voltage equations in the simplified circuit model of the grid-connected inverter parallel system in the grid-connected mode, and obtain the output active power model of each inverter.

[0052] The branch current equation is based on Kirchhoff's current law (KCL) and describes the distribution of current in different branches of the circuit. The coupled bus voltage equation is based on Kirchhoff's voltage law (KVL) and describes the distribution of voltage on the coupled bus.

[0053] S3. Combine the inverter output active power model and the control model of a single grid-type inverter to establish a dynamic nonlinear mathematical equation for the virtual power angle of the inverter in a faulty system of multiple grid-type inverters. The virtual power angle refers to a parameter used to analyze the dynamic behavior of the inverter, representing the phase difference between the inverter output voltage and the grid voltage.

[0054] S4. Draw a three-dimensional power-virtual power angle surface, and then determine the power balance point of the two grid-type inverters. The power balance point refers to the state where both inverters reach their respective reference active powers at a specific power angle. By drawing the surface graph, the impact of different power angles on the inverter output power can be intuitively observed, and the stable balance point of the system can be found.

[0055] S5. Combine the virtual power angle dynamic nonlinear mathematical equation established in step S3 with the power balance point in step S4, and solve the system transient synchronization stability judgment equation to determine whether the system has transient synchronization instability, and determine the specific instability of each grid-type inverter based on the judgment result. Transient synchronization instability refers to the phenomenon that the inverter cannot resynchronize after a short disturbance (such as a power grid failure), resulting in system instability.

[0056] Embodiment 2

[0057] Refer to the instruction manual Figure 1 As shown in the figure, the simplified circuit model of the grid-connected inverter parallel system constructed in the grid-connected mode is:

[0058] The two grid-type inverters are connected to the filter inductor and the filter capacitor respectively. The output current I1 of one grid-type inverter is connected to the coupling bus through the line admittance Y1, and the output current I2 of the other grid-type inverter is connected to the coupling bus through the line admittance Y2; the grid connection point of the line admittance Y1 and the coupling bus is PCC1, and the grid connection point of the line admittance Y2 and the coupling bus is PCC2; the branch current I3 on ​​the coupling bus is converged and output to the main grid through the line admittance Y3.

[0059] Refer to the instruction manual Figure 2 As shown in the figure, the active control links of the two grid-connected inverters adopt the power synchronization control strategy. The active control link generates the phase of the voltage reference value, and the amplitude command of the voltage reference is given as a constant. Then, the drive signal is generated through the voltage and current loop, and the control function is realized through the PWM generator. Figure 2 The control model of a single grid-connected inverter is: i =∫(wg +K i (P ref,i -P i ))dt, where θ i is the output voltage phase angle of the i-th grid-connected inverter, w g is the rated angular frequency, K i is the power synchronization coefficient of the i-th grid-connected inverter, P ref,i is the reference active power of the i-th grid-connected inverter, P i is the output active power of the i-th grid-connected inverter, i=1,2, ∫(·)dt is the integral over time.

[0060] Embodiment 3

[0061] This embodiment is a further detailed supplement and elaboration of the technical solution of the present invention based on the above-mentioned embodiment 1 and embodiment 2. In this embodiment, based on Kirchhoff's theorem, the three branch current equations are listed: Among them, V o1 is the output voltage vector of the first grid-connected inverter, V o2 is the output voltage vector of the second grid-connected inverter, Y j is the jth line admittance, j = 1, 2, 3, E is the grid voltage vector, V BUS To couple the bus voltage, add the three branch current equations and shift the terms to get the coupled bus voltage equation:

[0062] Substituting the coupled bus voltage equation into the I1 and I2 current equations yields the actual expression:

[0063]

[0064] Furthermore, the inductance of the line impedance is usually greater than the resistance, so the line impedance is considered to be purely inductive. The output active power can be obtained by multiplying the inverter voltage vector by the current vector and taking the real part. Under the pure inductive line, the formula can be simplified to obtain the output active power model of the two grid-type inverters: Among them, E0 is the voltage amplitude on the grid side, δ1 is the virtual power angle of the first grid-type inverter, and δ2 is the virtual power angle of the second grid-type inverter.

[0065] Embodiment 4

[0066] As another preferred embodiment of the present invention, this embodiment is a further detailed supplement and elaboration of the technical solution of the present invention based on the above-mentioned embodiment 1, embodiment 2 or embodiment 3. In this embodiment, by combining the power synchronization control mathematical model of the inverter and the output active power model, the virtual power angle dynamic nonlinear mathematical equation of the two inverters in step S3 can be obtained.

[0067] Specifically, a dynamic nonlinear mathematical equation of the virtual power angle is established based on the active power model and the control model: is the first-order derivative of δ1, is the first-order derivative of δ2.

[0068] Embodiment 5

[0069] As another preferred embodiment of the present invention, this embodiment is a further detailed supplement and elaboration of the technical solution of the present invention based on the above-mentioned embodiment 1, embodiment 2, embodiment 3 or embodiment 4. In this embodiment, based on the active power model established in step S2, a three-dimensional active power-virtual power angle surface is drawn with the Z axis as active power, the X axis as δ1, and the Y axis as δ2, as shown in FIG. Figure 3 The P-δ surface represents the output active power of the inverter under different values ​​of δ1 and δ2, and intuitively shows the influence of δ1 and δ2 on the output active power of the inverter.

[0070] Figure 3 In the figure, the reference active power plane of the inverter intersects with its own P-δ surface to form an isoline. Any set of δ1 and δ2 falling on the isoline can make the output active power of the inverter reach the reference value. Figure 4 It represents placing the contour lines of the two inverters in the same two-dimensional coordinate system (X axis is δ1, Y axis is δ2), Figure 4 It can be found that the intersection point of the two sets of contour lines is the power balance point of the system. At this point, the active power of the two inverters can reach the reference value and the system power is balanced.

[0071] The three-dimensional power-virtual power angle surface is a P-δ surface drawn based on the output active power model and the dynamic nonlinear mathematical equation of the virtual power angle. The Z-axis of the three-dimensional power-virtual power angle surface is the active power, the X-axis is the virtual power angle δ1, and the Y-axis is the virtual power angle δ2.

[0072] The method for determining the power balance point of two grid-connected inverters includes:

[0073] Obtaining a first contour line generated by the intersection of the reference active power of the first grid-forming inverter and the three-dimensional power-virtual power angle surface;

[0074] Obtaining a second contour line generated by the intersection of the reference active power of the second grid-forming inverter and the three-dimensional power-virtual power angle surface;

[0075] Place the first contour line and the second contour line in the same two-dimensional coordinate system, where the X axis of the two-dimensional coordinate system is the virtual power angle δ1, and the Y axis is the virtual power angle δ2;

[0076] The intersection of the first contour line and the second contour line is identified, and the intersection position is the power balance point.

[0077] Embodiment 6

[0078] This embodiment is a further detailed supplement and elaboration of the technical solution of the present invention on the basis of the above-mentioned embodiment 1, embodiment 2, embodiment 3, embodiment 4 or embodiment 5.

[0079] In this embodiment, there are two situations in which the transient synchronization instability occurs in the parallel system of two inverters during a fault:

[0080] Case 1) When the grid-side voltage drops symmetrically, the active power output by each inverter to the grid decreases accordingly, causing the reference active power to be greater than the maximum total active power that the inverter can transmit. This causes the inverter to lose synchronization with the system. From the three-dimensional P-δ diagram, it can be seen that the reference active power plane has no intersection with the P-δ surface.

[0081] Case 2) The P-δ surfaces of the two inverters can still intersect with their respective reference active power planes, but the fault causes the power balance of the system to be broken, and there cannot be a set of common δ1 and δ2 that allow the two inverters to reach their respective reference active powers at the same time. The two-dimensional δ1-δ2 diagram shows that there is no intersection between the contour lines obtained by the intersection of the two sets of inverters, and the system suffers from transient synchronous instability.

[0082] Combining the dynamic nonlinear mathematical equation of virtual power angle established in step S3 and the power balance relationship in step S4, a method for judging the transient synchronous stability of the system can be obtained. That is, if there is a feasible solution to the binary linear nonlinear equation group, the system can be stable. The judgment equation is: If the judgment equation has a feasible solution, the system is judged to be transiently synchronously stable; if there is no feasible solution, the system is judged to be transiently synchronously unstable.

[0083] In addition, in order to ensure that the two sets of power angles obtained are stable power angles (still stable after a small disturbance), it is also necessary to satisfy that the partial derivative of P1 with respect to δ1 and the partial derivative of P2 with respect to δ2 are greater than zero: Among them, EP refers to a small disturbance or offset (Epsilon Perturbation), which is used to test the stability of the system when the power angle changes slightly.

[0084] If it is determined that the grid-connected inverter has transient synchronous instability during a fault, the instability situation is determined as follows:

[0085] If the three-dimensional power-virtual power angle surface has no intersection with the reference active power, it is judged that the grid-type inverter has lost synchronization with the system;

[0086] If there is no intersection between the first contour line and the second contour line in the two-dimensional coordinate system δ1-δ2, it is judged that the system has transient synchronous instability.

[0087] Embodiment 7

[0088] The correctness of the above-mentioned embodiments 1 to 6 is verified through specific examples. For a system with two grid-connected inverters in parallel, three sets of values ​​are taken for the reference active power of the two inverters according to the proposed judgment method to realize the three possible transient processes under the symmetrical voltage drop fault of the grid, thereby verifying the correctness of the transient synchronization stability judgment method.

[0089] Figure 5 The power angle simulation waveforms of the two inverters after a symmetrical grid drop fault occurs. Figure 5 (a) to Figure 5 (c) corresponds to the three situations that both inverters can maintain stability, the first inverter has no balance point, and both inverters have balance points but no common power angle. Figure 5 (a)- Figure 5 (c) It can be seen that when the system meets the proposed transient synchronization stability judgment method, both inverters can maintain transient stability during the fault; on the contrary, when it does not meet the requirements, transient synchronization instability may occur in one or both inverters. This verifies the correctness of the theoretical analysis.

[0090] Embodiment 8

[0091] A computer-readable storage medium stores a computer program, which, when executed by a processor, implements the transient synchronization stability judgment method of the grid-connected inverter parallel system.

[0092] Without loss of generality, computer readable media may include computer storage media and communication media. Computer storage media include volatile and non-volatile, removable and non-removable media implemented in any method or technology for storing information such as computer readable instruction data structures, program modules or other data. Computer storage media include RAM, ROM, EPROM, EEPROM, flash memory or other solid-state storage technology, CD-ROM, DVD or other optical storage, cassettes, magnetic tapes, disk storage or other magnetic storage devices. Of course, those skilled in the art will appreciate that computer storage media are not limited to the above. The above-mentioned system memory and mass storage devices can be collectively referred to as memory.

[0093] A computer program product includes a computer program / instruction, which, when executed by a processor, implements the above-mentioned transient synchronization stability judgment method for a grid-connected inverter parallel system.

[0094] A computer program product includes a computer program or set of instructions for performing specific tasks or achieving specific functions. These programs or instructions are designed to be executed by a processor to implement a series of predefined steps or operations. The program product may be stored in various forms of computer storage media, such as memory, hard disk, solid-state drive, optical disk or other forms of digital storage devices. It may exist in the form of compiled binary code or in the form of scripts or bytecodes that can be executed by an interpreter. The program product uses carefully designed algorithms and logical instructions to enable the processor to process data in a specific order and manner to complete various functions such as data analysis, user interaction, device control, etc.

[0095] In the description of this specification, the description with reference to the terms "one embodiment / method", "some embodiments / methods", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment / method or example are included in at least one embodiment / method or example of the present application. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment / method or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments / methods or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments / methods or examples described in this specification and the features of the different embodiments / methods or examples, unless they are contradictory.

[0096] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In the description of this application, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0097] It should be understood by those skilled in the art that the above embodiments are only for the purpose of clearly illustrating the present invention, and are not intended to limit the scope of the present invention. For those skilled in the art, other changes or modifications may be made based on the above invention, and these changes or modifications are still within the scope of the present invention.

Claims

1. A method for determining transient synchronization stability of a grid-connected inverter parallel system, characterized in that: include: Based on the characteristics of the inverter output port, a simplified circuit model of the grid-connected inverter parallel system in the grid-connected mode is constructed; Based on the control strategy, determine the control model of a single grid-connected inverter; Write down the branch current equations and coupled bus voltage equations in the simplified circuit model of the grid-connected inverter parallel system in the grid-connected mode, and obtain the output active power model of each inverter; Combining the inverter output active power model and the control model of a single grid-connected inverter, a dynamic nonlinear mathematical equation of the virtual power angle of the inverter in a faulty system of multiple grid-connected inverters is established. Draw a three-dimensional power-virtual power angle surface, and then determine the power balance point of the two grid-type inverters; By solving the system transient synchronous stability judgment equation, it is judged whether the system has transient synchronous instability, and the specific instability situation of each grid-type inverter is determined based on the judgment result.

2. A method for determining transient synchronization stability of a grid-connected inverter parallel system according to claim 1, characterized in that: The simplified circuit model of the grid-connected inverter parallel system constructed in the grid-connected mode is: The two grid-type inverters are respectively connected to the filter inductor and the filter capacitor, the output current I1 of one grid-type inverter is connected to the coupling bus through the line admittance Y1, and the output current I2 of the other grid-type inverter is connected to the coupling bus through the line admittance Y2; The grid connection point between the line admittance Y1 and the coupled bus is PCC1, and the grid connection point between the line admittance Y2 and the coupled bus is PCC2; The branch current I3 on ​​the coupling bus is gathered and output to the main grid through the line admittance Y3.

3. A method for determining transient synchronization stability of a grid-connected inverter parallel system according to claim 2, characterized in that: The active control links of the two grid-connected inverters adopt the power synchronization control strategy. The control model of a single grid-connected inverter is: i =∫(w g +K i (P ref,i -P i ))dt, where θ i is the output voltage phase angle of the i-th grid-connected inverter, w g is the rated angular frequency, K i is the power synchronization coefficient of the i-th grid-connected inverter, P ref,i is the reference active power of the i-th grid-connected inverter, P i is the output active power of the i-th grid-connected inverter, i=1,2, ∫(·)dt is the integral over time.

4. A method for determining transient synchronization stability of a grid-connected inverter parallel system according to claim 3, characterized in that: Based on Kirchhoff's theorem, write the three branch current equations Among them, V o1 is the output voltage vector of the first grid-connected inverter, V o2 is the output voltage vector of the second grid-connected inverter, Y j is the jth line admittance, j = 1, 2, 3, E is the grid voltage vector, V BUS is the coupled bus voltage, Under the pure inductance circuit, the formula is simplified to obtain the output active power model of two grid-connected inverters. Among them, E0 is the voltage amplitude on the grid side, δ1 is the virtual power angle of the first grid-type inverter, and δ2 is the virtual power angle of the second grid-type inverter.

5. A method for determining transient synchronization stability of a grid-connected inverter parallel system according to claim 4, characterized in that: Based on the active power model and control model, the dynamic nonlinear mathematical equation of the virtual power angle is established: is the first-order derivative of δ1, is the first-order derivative of δ2.

6. A method for determining transient synchronization stability of a grid-connected inverter parallel system according to claim 4, characterized in that: The three-dimensional power-virtual power angle surface is a P-δ surface drawn based on the output active power model and the dynamic nonlinear mathematical equation of the virtual power angle. The Z-axis of the three-dimensional power-virtual power angle surface is the active power, the X-axis is the virtual power angle δ1, and the Y-axis is the virtual power angle δ2.

7. A method for determining transient synchronization stability of a grid-connected inverter parallel system according to claim 6, characterized in that: The method for determining the power balance point of two grid-connected inverters includes: Obtaining a first contour line generated by the intersection of the reference active power of the first grid-forming inverter and the three-dimensional power-virtual power angle surface; Obtaining a second contour line generated by the intersection of the reference active power of the second grid-forming inverter and the three-dimensional power-virtual power angle surface; Place the first contour line and the second contour line in the same two-dimensional coordinate system, where the X axis of the two-dimensional coordinate system is the virtual power angle δ1, and the Y axis is the virtual power angle δ2; The intersection of the first contour line and the second contour line is identified, and the intersection position is the power balance point.

8. A method for determining transient synchronization stability of a grid-connected inverter parallel system according to claim 7, characterized in that: The transient synchronous stability judgment equation of the system is: If the judgment equation has a feasible solution, the system is judged to be transiently synchronously stable; if there is no feasible solution, the system is judged to be transiently synchronously unstable.

9. A method for determining transient synchronization stability of a grid-connected inverter parallel system according to claim 8, characterized in that: If it is determined that the grid-connected inverter has transient synchronous instability during a fault, the instability situation is determined as follows: If the three-dimensional power-virtual power angle surface has no intersection with the reference active power, it is judged that the grid-type inverter has lost synchronization with the system; If there is no intersection between the first contour line and the second contour line in the two-dimensional coordinate system δ1-δ2, it is judged that the system has transient synchronous instability.

10. A computer program product comprising a computer program / instructions, characterized in that When the computer program / instruction is executed by a processor, the transient synchronization stability judgment method of a grid-type inverter parallel system as described in any one of claims 1 to 9 is implemented.

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