Virtual synchronous generator transient control method and system

Abstract

This invention discloses a method and system for transient control of a virtual synchronous generator (VSG). This system incorporates virtual impedance control into the control loop, using the fault current as a criterion for determining whether to activate virtual impedance control. If the fault current exceeds a set threshold, virtual impedance control is activated and the appropriate virtual impedance value is selected based on the fault drop depth. The virtual impedance is adaptively adjusted at different fault drop depths to limit the VSG fault current. This system can simultaneously limit fault overcurrent and ensure power angle stability.

Description

Technical Field

[0001] The present invention relates to a new energy power generation control technology, and in particular to a virtual synchronous generator transient control method and system. Background Art

[0002] As new energy generation technologies continue to mature, the penetration of inverter-based renewable energy in power grids is rapidly increasing. This has led to a decrease in the proportion of synchronous generators in the grid, resulting in a reduction in grid inertia, which can jeopardize the safe and stable operation of the grid during disturbances. Virtual Synchronous Generation (VSG), which combines the characteristics of synchronous generators and inverters, can simulate the dynamic characteristics of synchronous generators and provide inertia support, and has become widely accepted.

[0003] Although VSGs benefit from the characteristics of synchronous generators, they also face stability issues under various disturbances, especially transient stability under grid fault conditions. Transient synchronous instability and fault overcurrent are two thorny issues facing VSGs during grid faults, which seriously threaten the safe and stable operation of the system. However, existing research regards transient synchronous instability and fault overcurrent as two independent issues that cannot be guaranteed at the same time, and existing control strategies often focus on one and lose the other. How to balance transient synchronous stability with reliable inverter current limiting is the key to whether the VSG system can achieve reliable fault ride-through and is a technical problem that needs to be solved urgently. Summary of the Invention

[0004] The technical problem to be solved by this invention is to address the shortcomings of existing technologies and provide a method and system for transient control of a virtual synchronous generator. This method fully utilizes the advantages of the VSG system, utilizing virtual impedance and the coordination of additional control parameters to improve the VSG system's pause stability and achieve reliable fault current limiting. When a power grid fault occurs, the virtual synchronous generator can ensure that the fault current does not exceed the limit and maintain synchronous operation with the power grid.

[0005] To solve the above technical problems, the technical solution adopted by the present invention is: a method for transient control of a virtual synchronous generator, wherein the virtual synchronous generator includes an inverter; the inverter is connected to a transformer through a filter circuit; the transformer is connected to a power grid; and the method includes the following steps:

[0006] S1, detecting the inverter output current. If the output current exceeds a set threshold, proceeding to step S2;

[0007] S2. Determine whether an equilibrium point exists;

[0008] S3. If the equilibrium point exists, the offset Δω of the virtual synchronous generator angular frequency is used as the input of the proportional controller; otherwise, the offset Δω of the virtual synchronous generator angular frequency is used as the input of the integral controller; wherein the proportional coefficient of the proportional controller is Kp , the integral coefficient of the integral controller is K i / s; the output of the proportional controller and the output of the integral controller are the additional power ΔP; Δω=ω-ω N ,ω N and ω are the rated angular frequency and inverter output angular frequency respectively; J is the virtual inertia, D p is the damping coefficient;

[0009] S4, when the reference active power P ref When it is equal to the sum of the actual output active power P and the additional power ΔP, the process ends; otherwise, step S3 is repeated.

[0010] In the present invention, when the reference active power P ref When the sum of the actual output active power P and the additional power ΔP is equal, the solution of the virtual synchronous generator reshapes the second-order swing equation as follows: At this time, the virtual power angle δ has a steady-state solution, the phase angle difference between the inverter and the grid is a fixed value and they maintain synchronous operation, realizing transient synchronous stability control of the VSG system.

[0011] Furthermore, in the present invention, the specific implementation process of determining whether a balance point exists includes:

[0012] When the critical virtual impedance X v,cr1 Greater than the critical virtual impedance X when the equilibrium point exists v,cr2 There is no equilibrium point;

[0013] When the critical virtual impedance X v,cr1 Less than or equal to the critical virtual impedance X when the equilibrium point exists v,cr2 There is a balance point.

[0014] By determining the existence of equilibrium points, the present invention can accurately distinguish between two types of transient synchronous instability (TPI) (Type I and Type II). These two types of TPI have different transient behaviors. For Type II TPI, an integral controller is used to autonomously search for a new equilibrium point; for Type I TPI, a proportional controller is used to ensure that the system converges to the original equilibrium point without changing the system's transmission characteristics.

[0015] Furthermore, in the present invention, the critical virtual impedance X v,cr1 The determination process includes:

[0016] 1) Initialize the virtual impedance value;

[0017] 2) Use the virtual power angle δ to calculate the inverter output current I at the current time fault,t , if the inverter output current I fault,tIf the fault current is greater than the maximum tolerable fault current, the virtual impedance value is increased by the set step size;

[0018] 3) Repeat step 2) until the inverter output current is less than or equal to the maximum tolerable fault current, and the fault current limiting critical virtual impedance is obtained, and the process ends.

[0019] The critical virtual impedance value determined by the iterative algorithm of the present invention can ensure that the output current does not exceed the limit under different fault depths, thereby ensuring the reliable operation of the inverter.

[0020] Furthermore, in the present invention, the critical virtual impedance X when the equilibrium point exists is v,cr2 The determination process includes:

[0021] a) Initialize the virtual impedance value;

[0022] b) calculating the actual output active power of the inverter at the current time using the virtual power angle δ, and if the actual output active power is greater than the reference active power, increasing the virtual impedance value by a set step size;

[0023] c) Repeat step b) and when the actual output active power is less than or equal to the reference active power, obtain the critical virtual impedance when the balance point exists, and end.

[0024] The present invention can determine the critical voltage V for distinguishing two types of transient instability by comparing the virtual impedance when the equilibrium point exists with the fault current limiting virtual impedance. cr When the fault voltage drops below V g Greater than the critical voltage V cr When the fault voltage drops to a level lower than the critical voltage, the system has a balance point under the set current-limiting virtual impedance value, but type I transient synchronous instability may occur.

[0025] The calculation formula of virtual power angle δ is: in, are the second-order derivative and first-order derivative of the virtual power angle δ, respectively. The virtual power angle can describe the transient synchronization behavior of traditional inverters and analyze the transient synchronization stability between the inverter and the power grid.

[0026] In step S1, the calculation formula of the virtual synchronous generator output current I is: Among them, R g is the transmission line resistance, X g is the transmission line reactance, X v is the virtual impedance, V g is the grid voltage, δ is the virtual power angle, V ref =V N +D q (Qref -Q), Q ref and Q are the reference reactive power and the actual output reactive power respectively; V ref and V N are the reference voltage value and the rated voltage value respectively, D q Step S1 can accurately calculate the output current of the virtual synchronous generator and reliably monitor whether the system has a fault.

[0027] The calculation formula of the actual output active power P of the virtual synchronous generator is:

[0028]

[0029] Among them, R eq 、X eq are the total resistance and total reactance of the virtual synchronous generator line, V g is the grid voltage, X g is the transmission line reactance, X v is the virtual impedance, V pcc is the grid connection point voltage, and δ is the virtual power angle.

[0030] In step S1, if the output current does not exceed the set threshold, the output reference voltage of the inverter is set to V ref cosθ ref +jV ref sinθ ref , calculate the difference between the output reference voltage of the inverter and the grid connection point voltage, and the difference is controlled by PI to achieve the grid connection point voltage tracking the output reference voltage without difference.

[0031] In step S1, if the output current exceeds the set threshold, the output reference voltage of the inverter is set to V ref cosθ ref +j(V ref sinθ ref -X v I); calculating the difference between the output reference voltage of the inverter and the grid-connected point voltage, which is controlled by PI to achieve the grid-connected point voltage tracking the output reference voltage; wherein, X v is the virtual impedance value, I is the virtual synchronous generator output current (inverter output current). The output reference voltage of the inverter is set to V ref cosθ ref +j(V ref sinθ ref -X v I) indicates that the system starts the virtual impedance control link. The introduction of virtual impedance changes the reference voltage setting value from V ref cosθ ref +jV refsinθ ref Reduce V ref cosθ ref +j(V ref sinθ ref -X v I), thereby reducing the inverter output current and achieving reliable current limiting.

[0032] As an inventive concept, the present invention also provides a virtual synchronous generator transient control system, which includes:

[0033] one or more processors;

[0034] A memory having one or more programs stored thereon, which, when executed by the one or more processors, enables the one or more processors to implement the steps of the above method of the present invention.

[0035] Compared with the existing technology, the present invention offers the following advantages: The present invention's transient control method for a virtual synchronous generator based on virtual impedance and frequency feedforward introduces virtual impedance control into the control loop, using the fault current magnitude as a criterion for determining whether virtual impedance control should be activated. If the fault current exceeds a set threshold, virtual impedance control is activated and the corresponding virtual impedance value is selected based on the fault drop depth. The virtual impedance magnitude is adaptively adjusted at different fault drop depths to limit the VSG fault current. Under large-disturbance faults, the present invention can simultaneously limit fault overcurrent and ensure power angle stability. By coordinating virtual impedance control with additional transient control, a two-level coordinated large-disturbance control strategy is constructed. This method achieves stable current limiting while addressing the two types of transient instability risks introduced by virtual impedance. Furthermore, the present method improves dynamic performance during and after fault clearance, while ensuring the voltage and frequency support capabilities of the virtual synchronous generator. The present method can also be applied to other grid-connected inverters and can be further extended to fault current limiting and transient power angle control in scenarios such as multi-machine grid connection and island microgrids. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is the VSG control system structure of an embodiment of the present invention;

[0037] Figure 2 This is the block diagram of the adaptive VSG transient control of the present invention;

[0038] Figure 3 The iterative algorithm of the embodiment of the present invention is used to obtain the critical virtual impedance X of the fault current limiting under different fault depths. v,cr1 flow chart;

[0039] Figure 4 The iterative algorithm of the embodiment of the present invention is used to obtain the critical virtual impedance X of the equilibrium point loss under different fault depths. v,cr2flow chart;

[0040] Figure 5 is the critical virtual impedance X for fault current limiting under different fault depths in the embodiment of the present invention. v,cr1 There is a critical virtual impedance X with the equilibrium point v,cr2 curve;

[0041] Figure 6 This is a flow chart of the adaptive VSG transient control method according to an embodiment of the present invention.

[0042] FIG7 (1) and FIG7 (2) are simulation waveforms of the virtual synchronous generator grid-connected system after adopting the traditional control strategy and the control strategy of the embodiment of the present invention respectively when the type I transient synchronous instability occurs;

[0043] FIG8 (1) and FIG8 (2) are simulation waveforms of the virtual synchronous generator grid-connected system after adopting the traditional control strategy and the control strategy of the embodiment of the present invention respectively during type II transient synchronous instability. DETAILED DESCRIPTION

[0044] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0045] Figure 1 The following is the structure diagram of the VSG control system. In the main circuit, the inverter, filter circuit, and transformer are connected in sequence, and the transformer is connected to the grid. L and Z T are the transmission line impedance and transformer impedance respectively. f and C f It is the output LC filter of the inverter. The DC side of the inverter is usually connected to the energy storage unit or DC bus control is adopted, so the DC side voltage U dc But it is considered constant. Grid voltage V g =V g ∠θ g =V g cosθ g +jV g sinθ g , V g ∠θ g Indicates that the grid voltage amplitude is V g , the phase angle is θ g . Inverter output voltage V pcc =V pcc ∠θpcc =V pcc cosθ g +jV pcc sinθ g , V pcc ∠θ pcc Indicates that the inverter output voltage amplitude is V pcc , the phase angle is θ pcc The control part mainly includes the active power control link (APC), the reactive power control link (RPC), the virtual impedance control link (VI), the reference voltage generation link (RV), and the voltage and current control link (IC). APC and RPC are mainly used to control the active and reactive power of the virtual synchronous generator, which can be expressed as follows:

[0046]

[0047] V ref =V N +D q (Q ref -Q) (2)

[0048] In the following formulas, ω N and ω are the rated angular frequency and the output angular frequency of the virtual synchronous generator respectively; P ref and P are the reference active power and actual output active power respectively; Q ref and Q are the reference reactive power and the actual output reactive power respectively; V ref and V N are reference voltage and rated voltage respectively; D p and D q are the damping coefficient and droop coefficient respectively; J is the virtual inertia; θ ref is the reference phase angle.

[0049] For other control links, if the virtual impedance control link is not added, the reference phase angle θ generated by APC and RPC is ref With reference voltage V ref After the reference voltage generation link (RV), the inverter output reference voltage V1 = V2 = V ref ∠θ ref =V ref cosθ ref +jV ref sinθ ref If the virtual impedance control link (VI) is started, the inverter output reference voltage is V1 = V2-X v I=V ref ∠θ ref -jX v I=V ref cosθ ref +j(Vref sinθ ref -X v I). The reference voltage V1 of the inverter output and the actual output voltage of the inverter are jointly input into the voltage-current control link (IC) to effectively control the output voltage and current of the inverter.

[0050] In the embodiment of the present invention, in order to describe the synchronization stability between the inverter and the power grid, the virtual power angle δ = θ is defined ref -θ g .

[0051] Combining equations (1) and (2), the virtual synchronous generator can be described as a second-order swing equation with respect to the virtual power angle δ, as shown in equation (3).

[0052]

[0053] Where are the first derivative and the second derivative of the virtual power angle δ respectively.

[0054] According to the power transmission characteristics, the output power of the inverter can be calculated by equation (4), where R eq , X eq are the total line resistance and the total reactance respectively.

[0055]

[0056] In the embodiment of the present invention, in order to take into account fault current limiting and transient synchronization stability, a virtual synchronous generator transient control method based on virtual impedance and frequency feedforward as shown in Figure 2 is proposed. This method effectively limits the fault current through the adaptive change of the virtual impedance; for transient power angle stability, the offset of the angular frequency Δω is negatively fed forward, and an additional power ΔP is generated through a PI controller. The additional power ΔP can adaptively adjust the imbalance between the parameter active power P ref and the actual output active power P, thereby solving the risk of two types of transient synchronization instability induced by the virtual impedance. Specifically, when P ref > P, the excess power will drive the system to "accelerate", making Δω > 0, resulting in an additional power ΔP > 0, equivalently increasing P until the power reaches balance; when P ref < P, the deficit power will drive the system to "decelerate", making Δω < 0, resulting in an additional power ΔP < 0, equivalently decreasing P until the power reaches balance.

[0057] For fault current limiting, virtual impedance control is introduced into the control loop, and the magnitude of the fault current is used as the basis for judging whether to start the virtual impedance value control. According to Kirchhoff's law, the output current I of the inverter can be expressed as:

[0058]

[0059] Where R g is the transmission line resistance; X g is the reactance of the transmission line; X v is the virtual impedance; V g is the grid voltage; I max Maximum tolerable fault current. In the embodiment of the present invention, the parameter values ​​are expressed in per unit values, I max The general value is 1.2pu. If the fault current exceeds the set threshold, the virtual impedance control starts and the corresponding virtual impedance value is selected in combination with the fault drop depth. sag The virtual impedance is adaptively adjusted to limit the VSG fault current. To ensure reliable startup of the virtual impedance, the current threshold is generally set to be less than the maximum tolerable fault current, and the current threshold is set to 1p.u. The fault drop depth indicates the degree to which the grid voltage deviates from the rated value, V sag =1-V g .

[0060] The virtual impedance size under different fault drop depths is mainly realized based on the iterative algorithm, and its algorithm block diagram is as follows: Figure 3 First, the virtual impedance X v Initialize to a small value, namely X v,0 Using the second-order nonlinear state equation (3) of VSG, the virtual power angle δ is solved by the MATLAB command "ode45". The fault current I at different times is derived using the virtual power angle δ. fault,t Then, determine I fault,t and the maximum tolerable fault current. fault,t is greater than the maximum tolerable fault current, then the virtual impedance X v Increase. X v Increase by 0.01 in each iteration until the fault current is less than the maximum tolerable fault current. Repeat this process to determine the fault current limiting critical value X at different fault depths. v,cr1 .

[0061] Existing research shows that virtual impedance has an adverse effect on transient synchronization stability. Excessive virtual impedance can induce two types of transient synchronization instability. One type is transient synchronization instability with the existence of a balance point (Type I); the other type is transient synchronization instability without the existence of a balance point (Type II). Figure 4 The iterative algorithm is used to calculate the inverter output power P using formula (4), and the critical virtual impedance X when the equilibrium point exists can be determined. v,cr2 Fault current limiting critical virtual impedance X at different fault depths v,cr1 There is a critical virtual impedance X with the equilibrium point v,cr2 Curves such as Figure 5As shown, when the fault voltage drops to level V g Greater than the critical voltage V cr When the fault current limiting critical virtual impedance X v,cr1 There is a critical virtual impedance X greater than the equilibrium point v,cr2 At this time, there is no equilibrium point in the system under the set fault current limiting virtual impedance value, resulting in type II transient synchronous instability; when the fault voltage drops below the critical voltage, the fault current limiting critical virtual impedance X v,cr1 There is a critical virtual impedance X below the equilibrium point v , cr2 At this time, there is a balance point in the system under the set current-limiting virtual impedance value, but type I transient synchronous instability may occur.

[0062] To address the two types of transient synchronous instabilities caused by virtual impedance, frequency feedforward is used in APC to generate compensation power through PI control. This compensation power reshapes the second-order swing equation and adaptively adjusts the imbalance between electromagnetic and mechanical power. The reshaped second-order swing equation can be expressed as:

[0063]

[0064] From (6), we can see that through frequency feedforward, virtual damping term and virtual torque term are introduced into the traditional swing equation. The virtual damping term can increase the system damping by the integral coefficient K i The introduced virtual torque can adaptively adjust and compensate P ref The virtual torque can reshape the virtual power angle curve and adaptively find a new balance point. Therefore, the integral system K i It can solve the type II transient synchronous instability under the condition of no equilibrium point; while the proportional system K p It has no effect on the equilibrium point, but can increase the deceleration area equivalently by increasing the system damping, thereby avoiding type I transient synchronous instability.

[0065] Based on the above analysis, the embodiment of the present invention uses the adaptive VSG transient control method coordinated with virtual impedance and frequency feedforward as follows Figure 6 , the method includes three levels:

[0066] The first stage detects the fault current. If the fault output current exceeds the rated value, that is, I>1.0pu, the system is determined to have a fault.

[0067] The second level detects the fault depth, starts the virtual impedance, and uses the iterative algorithm to calculate the critical virtual impedance value Xv of the fault current limiting to determine the existence of the balance point. Figure 5 As shown, when the fault voltage drops to level V g Greater than the critical voltage V crWhen there is no equilibrium point in the system under the set virtual impedance value; when the fault voltage drops to a level V g less than the critical voltage V cr there is an equilibrium point in the system under the set virtual impedance value at this time.

[0068] For the third-level starting frequency feedforward control, when the equilibrium point is lost due to the virtual impedance, Figure 2 switch S1 in i is switched to position 1, and the K ref control loop is activated. When P ref > P, at this time Δω> 0, the virtual torque is greater than 0, and P is equivalently increased. Due to the existence of the integral controller, the virtual torque continuously increases with Δω until the power reaches balance; when P ref < P, the analysis is the same. Therefore, the integral control parameter can adjust the virtual torque according to the frequency change, and the power is balanced, so that the second-order swing equation has a steady-state solution, thereby realizing the adaptive search for the equilibrium point and solving the type-II transient synchronous instability under the condition of no equilibrium point; when the virtual impedance does not cause the loss of the equilibrium point, Figure 2 switch S1 in p is switched to position 2, and the K p control loop is activated, and the system damping increases, enabling the system to have a steady-state operating point, thereby avoiding type-I transient synchronous instability.

[0069] The VSG control method of the embodiment of the present invention can not only ensure that the inverter remains synchronized with the power grid when a serious fault occurs in the power grid, but also prevent the power electronic inverter from being damaged by overcurrent. In addition, the transient performance during and after the fault is also improved.

[0070] Figures 7(1) and 7(2) show the simulation waveforms of the virtual synchronous generator grid-connected system when type-I transient synchronous instability occurs, respectively using the traditional control and the control strategy of the embodiment of the present invention. A fault occurs in the power grid at 3 s. As can be seen from Figure 7(1), when the traditional control strategy is adopted, the output current exceeds the set threshold and transient synchronous instability occurs. As can be seen from Figure 7(2), when the control strategy of the embodiment of the present invention is adopted, the system realizes reliable current limiting and remains synchronized and stable with the power grid.

[0071] Figures 8(1) and 8(2) show the simulation waveforms of the virtual synchronous generator grid-connected system when type-II transient synchronous instability occurs, respectively using the traditional control and the control strategy of the embodiment of the present invention. A fault occurs in the power grid at 3 s. As can be seen from Figure 8(1), when the traditional control strategy is adopted, the output current exceeds the set threshold and transient synchronous instability occurs. As can be seen from Figure 8(2), when the control strategy of the embodiment of the present invention is adopted, the system realizes reliable current limiting and remains synchronized and stable with the power grid.

[0072] Another embodiment of the present invention provides a control system corresponding to the above-mentioned embodiment 1, including one or more memories, a processor and a computer program stored in the memory; the processor executes the computer program in the memory to implement the steps of the method of the above-mentioned embodiment.

[0073] In some implementations, the memory may be a high-speed random access memory (RAM), and may also include a non-volatile memory, such as at least one disk storage.

[0074] In other implementations, the processor may be a central processing unit (CPU), a digital signal processor (DSP), or other general-purpose processors, which are not limited herein.

[0075] Another embodiment of the present invention provides a computer-readable storage medium corresponding to the method of the above embodiment, on which a computer program / instruction is stored. When the computer program / instruction is executed by a processor, the steps of the method of the above embodiment are implemented.

[0076] Computer readable storage media can be tangible devices that hold and store instructions used by instruction execution devices. Computer readable storage media can be, for example, but not limited to, electronic storage devices, magnetic storage devices, optical storage devices, electromagnetic storage devices, semiconductor storage devices, or any combination thereof.

[0077] Those skilled in the art will appreciate that the embodiments of the present application can be provided as methods, systems, or computer program products. Therefore, the application can adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment in combination with software and hardware. Moreover, the application can adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) that contain computer-usable program code. The scheme in the embodiment of the present application can be implemented in various computer languages, for example, object-oriented programming language Java and literal translation scripting language JavaScript, etc.

[0078] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the steps in the process. Figure 1a process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0079] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operational steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing the instructions executed on the computer or other programmable device for implementing the process. Figure 1 a process or multiple processes and / or boxes Figure 1 A step that specifies a function in one or more boxes.

[0080] Although the preferred embodiments of the present application have been described, those skilled in the art may make additional changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present application.

[0081] Obviously, those skilled in the art may make various changes and modifications to this application without departing from the spirit and scope of this application. Thus, if these modifications and variations of this application fall within the scope of the claims of this application and their equivalents, this application is intended to include these modifications and variations.

Claims

1. A method for transient control of a virtual synchronous generator, wherein the virtual synchronous generator comprises an inverter; the inverter is connected to a transformer via a filter circuit; the transformer is connected to a power grid; and the method is characterized in that: The following steps are involved: S1. Detect the inverter output current. If the output current exceeds the set threshold, proceed to step S2； S2. Determine whether an equilibrium point exists; S3. If the equilibrium point exists, the offset Δω of the virtual synchronous generator angular frequency is used as the input of the proportional controller; otherwise, the offset Δω of the virtual synchronous generator angular frequency is used as the input of the integral controller; wherein the proportional coefficient of the proportional controller is K p , the integral coefficient of the integral controller is K i / s; the output of the proportional controller and the output of the integral controller are the additional power ΔP; Δω=ω-ω N ,ω N and ω are the rated angular frequency and inverter output angular frequency respectively; J is the virtual inertia, D p is the damping coefficient; S4, when the reference active power P ref When it is equal to the sum of the actual output active power P and the additional power ΔP, the process ends; otherwise, step S3 is repeated.

2. The method for transient control of a virtual synchronous generator according to claim 1, characterized in that: The specific implementation process of determining whether a balance point exists includes: When the critical virtual impedance X v , cr1 is greater than the critical virtual impedance X when the equilibrium point exists v ,cr2, there is no equilibrium point; When the critical virtual impedance X v , cr1 is less than or equal to the critical virtual impedance X when the equilibrium point exists v ,cr2, there is an equilibrium point.

3. The method for transient control of a virtual synchronous generator according to claim 2, characterized in that: Fault current limiting critical virtual impedance X v ,The determination process of cr1 includes: 1) Initialize the virtual impedance value; 2) Use the virtual power angle δ to calculate the inverter output current I at the current time fault,t , if the inverter output current I fault,t If the fault current is greater than the maximum tolerable fault current, the virtual impedance value is increased by the set step size; 3) Repeat step 2) until the inverter output current is less than or equal to the maximum tolerable fault current, and the fault current limiting critical virtual impedance is obtained, and the process ends.

4. The method for transient control of a virtual synchronous generator according to claim 2, characterized in that: Critical virtual impedance X when the equilibrium point exists v ,The cr2 determination process includes: a) Initialize the virtual impedance value; b) calculating the actual output active power of the inverter at the current time using the virtual power angle δ, and if the actual output active power is greater than the reference active power, increasing the virtual impedance value by a set step size; c) Repeat step b) and when the actual output active power is less than or equal to the reference active power, obtain the critical virtual impedance when the balance point exists, and end.

5. The method for transient control of a virtual synchronous generator according to claim 3 or 4, characterized in that: The calculation formula of virtual power angle δ is: in, are the second-order derivative and first-order derivative of the virtual power angle δ, respectively.

6. The method for transient control of a virtual synchronous generator according to any one of claims 1 to 4, characterized in that: In step S1, the calculation formula of the virtual synchronous generator output current I is: Among them, R g is the transmission line resistance, X g is the transmission line reactance, X v is the virtual impedance, V g is the grid voltage, δ is the virtual power angle, V ref =V N +D q (Q ref -Q), Q ref and Q are the reference reactive power and the actual output reactive power respectively; V ref and V N are the reference voltage value and the rated voltage value respectively, D q is the droop coefficient.

7. The method for transient control of a virtual synchronous generator according to any one of claims 1 to 4, characterized in that: The calculation formula of the actual output active power P of the virtual synchronous generator is: Among them, R eq 、X eq are the total resistance and total reactance of the virtual synchronous generator line, V g is the grid voltage, X g is the transmission line reactance, X v is the virtual impedance, V pcc is the grid connection point voltage, and δ is the virtual power angle.

8. The method for transient control of a virtual synchronous generator according to any one of claims 1 to 4, characterized in that: In step S1, if the output current does not exceed the set threshold, the output reference voltage of the inverter is set to V ref cosθ ref +jV ref sinθ ref , calculating the difference between the output reference voltage of the inverter and the grid connection point voltage, and the difference is controlled by PI to achieve the grid connection point voltage tracking the output reference voltage without difference; θ ref is the reference phase angle, V ref is the reference voltage value.

9. The method for transient control of a virtual synchronous generator according to any one of claims 1 to 4, characterized in that: In step S1, if the output current exceeds the set threshold, the output reference voltage of the inverter is set to V ref cosθ ref +j(V ref sinθ ref -X v I); calculating the difference between the output reference voltage of the inverter and the grid-connected point voltage, the difference is controlled by PI to achieve the grid-connected point voltage tracking the output reference voltage; wherein, X v is the virtual impedance value, I is the inverter output current, θ ref is the reference phase angle, V ref is the reference voltage value.

10. A virtual synchronous generator transient control system, characterized in that: include: one or more processors; A memory having one or more programs stored thereon, which, when executed by the one or more processors, enables the one or more processors to implement the steps of the method according to any one of claims 1 to 9.

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