A method for analyzing transient stability of a grid-forming converter during off-grid process, medium and processor

Abstract

The application discloses a network-constructing type converter off-grid process transient stability analysis method, medium and processor, relates to the power electronic technology field, and the method comprises the steps of S1, for the off-grid switching scene, establishing the off-grid control model of the network-constructing type converter, and the off-grid control model at least includes the active loop equation of virtual synchronous machine control;S2, based on the active loop equation, the frequency offset value generated by the local load power and the reference power mismatch of the network-constructing type converter in off-grid mode is calculated;S3, the initial kinetic energy item corresponding to the frequency offset is combined with the equal-area rule, and the transient stability critical judgment condition considering the influence of initial frequency offset is constructed;S4, whether the network-constructing type converter is unstable in the grid-connected mode is judged according to the transient stability critical judgment condition, if the transient stability critical condition is met, it is determined that the transient stability is met;otherwise, it is determined that the transient instability is met.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to a method, medium, and processor for transient stability analysis of grid-connected and disconnected converters during grid-connected processes. Background Technology

[0002] With the widespread integration of wind power, photovoltaic (PV), and energy storage equipment into power systems, converters, as key AC / DC conversion devices, can convert DC to AC, injecting renewable energy into the grid. The uneven regional distribution of wind and PV resources necessitates local consumption of electricity generated by these sources to enhance my country's local renewable energy absorption capacity. Compared to grid-connected energy storage converters, grid-connected energy storage converters exhibit voltage source characteristics on their AC side, enabling independent power supply to local loads and providing islanded power supply capabilities. This maximizes the local absorption capacity of wind and solar renewable energy in the central and western regions. Due to the uncertainty of renewable energy output or local load power consumption, grid-connected energy storage converters often require off-grid / on-grid switching to compensate for power balance requirements. According to relevant Chinese standards, the voltage frequency, phase, and amplitude on both sides of the grid-connected isolating switch must be below certain values ​​before the switch can be closed to avoid overcurrent issues caused by voltage inconsistencies. However, even if the grid-connected converter achieves successful pre-synchronization, the frequency offset caused by the mismatch between the local load power and the reference power of the grid-connected converter will worsen the transient stability during the off-grid switching process, resulting in transient instability after the pre-synchronization controller is bypassed.

[0003] Currently, the analysis of transient stability of grid-connected converters during grid-connection switching mainly employs methods based on the equal area rule. This method calculates the acceleration and deceleration areas defined by the active power-power angle curve in grid-connected mode to determine whether the system can resynchronize after the pre-synchronization controller is disconnected. Specifically, existing analyses typically assume that at the instant the pre-synchronization controller is disconnected, the converter's initial angular frequency matches the grid's rated angular frequency, i.e., the initial frequency offset is zero. Based on this, the initial kinetic energy term is calculated, and then the magnitudes of the acceleration and deceleration areas are compared to determine transient stability. The aforementioned existing technologies have the following drawbacks: Existing analytical methods neglect the frequency shift caused by the mismatch between local load power and reference power in off-grid mode for grid-connected converters. In actual operation, the converter independently supplies power to the local load in off-grid mode. When the active power consumed by the local load is not equal to the converter's active power reference value, the off-grid steady-state frequency will deviate from the rated value. That is, before the pre-synchronization controller is disconnected, the converter already has a certain frequency shift. Existing technologies do not consider the impact of this initial frequency shift when establishing transient stability criteria. Because the initial frequency shift is ignored, the stability region estimated by the existing equal-area method is too large and cannot accurately reflect the instability risk of the actual power angle trajectory after the pre-synchronization controller is disconnected. Summary of the Invention

[0004] To address the issue of frequency offset caused by the mismatch between local load power and the reference power of grid-connected converters, which negatively impacts transient stability during grid-connection / off-grid switching, this invention provides a transient stability analysis method, medium, and processor for grid-connected converters during grid-connection / off-grid switching. This method solves the problem of inaccurate judgments caused by existing analysis methods failing to consider the initial frequency offset. The specific technical solution is as follows: A transient stability analysis method for grid-connected converters during off-grid switching includes the following steps: S1. Establishing an off-grid control model for the grid-connected converter in off-grid switching scenarios, the off-grid control model includes at least the active power loop equations of the virtual synchronous machine control; S2. Calculating the frequency offset value of the grid-connected converter in off-grid mode due to the mismatch between local load power and reference power based on the active power loop equations; S3. Constructing transient stability critical judgment conditions that take into account the influence of the initial frequency offset by combining the initial kinetic energy term corresponding to the frequency offset with the equal area rule; S4. Determining whether the grid-connected converter is unstable in grid-connected mode according to the transient stability critical judgment conditions. If the transient stability critical conditions are met, it is determined to be transiently stable; otherwise, it is determined to be transiently unstable.

[0005] Preferably, in the grid-connected mode scenario, the expression for the active power loop equation is: In the formula, R L The resistance of the local load in the parallel equivalent circuit. X L The inductive reactance of the local load in the parallel equivalent circuit, X C The capacitive reactance of the local load in the parallel equivalent circuit. U GFM The output voltage amplitude of the converter. U g The phase voltage amplitude of the power grid. P e This refers to the actual active power output of the converter.

[0006] Preferably, based on the active power loop equation, the frequency offset of the grid-connected converter in off-grid mode due to the mismatch between local load power and reference power is calculated, including: obtaining the active power reference value, inverter output voltage amplitude, local load equivalent resistance, active power loop damping coefficient, and rated angular frequency based on the steady-state balance relationship that the output active power of the grid-connected converter equals the active power consumed by the local load in off-grid mode; calculating the active power consumed by the local load based on the grid-connected converter and the equivalent resistance of the local load, and calculating the difference between the active power reference value and the active power consumed by the local load; and calculating the frequency offset value at the moment the pre-synchronization controller is disconnected based on the difference between the active power reference value and the active power consumed by the local load, the damping coefficient, and the rated angular frequency.

[0007] Preferably, the expression for the frequency offset is: In the formula, P ref This is a reference value for active power. D The damping coefficient is... ω N For the rated frequency, U GFM R is the output voltage amplitude of the converter. L This is the local load resistor.

[0008] Preferably, the initial kinetic energy term corresponding to the frequency offset is combined with the equal area rule to construct a transient stability critical judgment condition that takes into account the influence of the initial frequency offset. This includes: obtaining the virtual inertia coefficient of the grid-connected converter, the frequency offset value at the moment the pre-synchronization controller is disconnected, the rated angular frequency, the active power reference value, the functional relationship between the output active power and the power angle in grid-connected mode, the power angle value corresponding to the stable equilibrium point, and the power angle value corresponding to the unstable equilibrium point; calculating the initial kinetic energy term based on the virtual inertia coefficient and the frequency offset value; calculating the first integral value based on the active power reference value, the output active power, and the rated angular frequency within the power angle range from zero power angle to the stable equilibrium point; calculating the corrected acceleration energy value based on the initial kinetic energy term and the first integral value; calculating the deceleration energy value based on the active power reference value, the output active power, and the rated angular frequency within the power angle range from the stable equilibrium point to the unstable equilibrium point; and setting the transient stability critical judgment condition based on the relationship that the corrected acceleration energy value is less than or equal to the deceleration energy value.

[0009] Preferably, the acquisition of the power angle corresponding to the stable equilibrium point and the power angle corresponding to the unstable equilibrium point includes: determining all candidate power angle values ​​that satisfy the equilibrium condition based on the balance condition that the active power reference value equals the output active power; acquiring the corresponding power angle from the candidate power angle values ​​as the stable equilibrium point based on the monotonicity that the output active power increases with the power angle; and acquiring the corresponding power angle from the candidate power angle values ​​as the unstable equilibrium point based on the monotonicity that the output active power decreases with the power angle.

[0010] Preferred active power in grid-connected mode P e The expression is: .

[0011] In the formula, U GFM R is the output voltage amplitude of the converter. L For local load resistance, Ug X represents the grid voltage amplitude. g This refers to the line reactance.

[0012] Preferably, step S4 includes a transient stability critical determination condition to determine the stability boundary of the grid-connected converter in grid-connected mode. By comparing the positional relationship between the initial operating point of the grid-connected converter after the pre-synchronous controller is disconnected and the stability boundary, its transient stability during the off-grid switching process is determined: if the initial operating point is within the stability boundary, it is determined to be transiently stable; otherwise, it is determined to be transiently unstable.

[0013] The present invention also provides a computer-readable storage medium, which includes a stored program, wherein, when the program is running, it controls the device where the computer-readable storage medium is located to execute the above-described transient stability analysis method for grid-connected converters during grid-connection and disconnection processes.

[0014] The present invention also provides a processor for running a program, wherein the program executes the above-described transient stability analysis method for grid-connected converters during grid connection and disconnection processes.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention establishes an off-grid and grid-connected control model that includes active and reactive power loops, and derives the initial frequency offset caused by the mismatch between local load and reference power under off-grid operation. The initial kinetic energy term corresponding to the initial frequency offset is introduced into the traditional equal area rule to obtain the transient stability critical condition that considers the influence of the initial frequency offset. This can more accurately determine the transient stability of the off-grid and grid-connected switching process of grid-connected converters, and solves the problem that the existing methods do not consider the influence of the initial frequency offset, resulting in inaccurate judgment results. It provides a more accurate theoretical basis for the transient stability analysis of off-grid and grid-connected switching of grid-connected converters. Attached Figure Description

[0016] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to scale.

[0017] Figure 1 This is a flowchart of the transient stability analysis method for grid-connected converters during grid disconnection and connection processes in Example 1; Figure 2 This is a diagram showing the grid-connected topology and control strategy of a virtual synchronous machine controlled grid-connected converter in Example 1. Figure 3 This is a phase plane stability domain analysis diagram of the phase angle and power angle change rate in Example 2. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] It should be understood that, when used in this specification and the appended claims, the terms "comprising" and "including" indicate the presence of the described features, integrals, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or collections thereof.

[0020] It should also be understood that the terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise.

[0021] It should also be further understood that the term "and / or" as used in this specification and the appended claims refers to any combination of one or more of the associated listed items and all possible combinations, and includes such combinations.

[0022] Example 1 Please refer to Figure 1 , Figure 1 This is a flowchart illustrating a method for transient stability analysis of a grid-connected converter during grid disconnection and reconnection. This embodiment provides a method for transient stability analysis of a grid-connected converter during grid disconnection and reconnection, which includes the following steps: S1. For the off-grid switching scenario, establish an off-grid control model for the grid-connected converter. The off-grid control model shall include at least the active power loop equations of the virtual synchronous machine control.

[0023] Please refer to Figure 2 , Figure 2 This diagram illustrates the grid-connected topology and control strategy for a virtual synchronous machine (VSC) controlled grid-connected converter. The hardware topology includes a grid-connected converter (VSC) connected to an energy storage system or a new energy DC bus on the DC side, with a DC voltage... U dc The value is controlled to a constant value by the energy storage system. The AC side of the converter passes through an LC filter (filter inductor). L f Filter capacitor C f Connected to the local load and the disconnect switch. The local load is a parallel RLC load, including a resistor. R L ,inductance L L and capacitor C L The other side of the disconnecting switch is connected via the equivalent line impedance. L g Corresponding reactance X g= ω N L g Connect to the power grid U g The disconnect switch is initially open and closes after successful pre-synchronization.

[0024] The grid-connected converter employs a virtual synchronous machine (VSG) control system, comprising active power loop, reactive power loop, and voltage-current dual closed loops. The control equations for the active power loop are as follows: In the formula, J For virtual inertia coefficient, D The damping coefficient is... ω N For the rated frequency, P e For the actual active power output of the converter, P ref This is a reference value for active power. ω GFM0 This refers to the output angular frequency of the converter.

[0025] The grid-connected control model also includes the reactive power loop control equation for virtual synchronous machine control. The reactive power loop control equation is used to generate a voltage amplitude reference value based on the deviation between the reactive power reference value and the actual output reactive power.

[0026] The expression for the reactive power loop control equation is: in, U d For the obtained AC voltage loop d Shaft reference voltage, Q ref For reactive power reference, Q e This represents the actual reverse output reactive power. k q This is the reactive power loop droop factor. U N This refers to the rated phase voltage amplitude of the converter.

[0027] Calculate the output current of the grid-type converter respectively I GFM grid-side current component I g Load-side current component I L The expression is as follows: in, U GFM The output voltage amplitude of the converter. U g Let j be the phase voltage amplitude of the power grid, and j be an imaginary number. X g R is the line reactance, and R is the equivalent resistance. X L For the inductive reactance of the local load, I g I represents the grid-side current component of the output current of a grid-connected converter. L For the load-side current component, This is the output current of the converter.

[0028] The instantaneous power expression for a three-phase system is: Based on the grid-side current component of the output current of the grid-type converter I g Load-side current component I L The instantaneous power derivation expression of a three-phase system and the output active power of a grid-connected converter in grid-connected mode. P e With reactive power Q e The expression yields the active power. P e With reactive powerQ e The expression is: In the formula, in the formula, R L The equivalent resistance of the local load. X L Local load equivalent inductive impedance X C Local load equivalent capacity U GFM The output voltage amplitude of the converter. U g This refers to the phase voltage amplitude of the power grid. P e This refers to the actual active power output of the converter.

[0029] Step S2: Based on the active power loop equation, calculate the frequency offset value of the grid-type converter in off-grid mode due to the mismatch between local load power and reference power.

[0030] Specifically, it includes the following steps: Based on the steady-state balance relationship where the output active power of the grid-connected converter in off-grid mode equals the active power consumed by the local load, a reference value for active power is obtained. P ref Converter output voltage amplitude U GFM Local load equivalent resistance R L Active circuit damping coefficient D and ω N Rated frequency; The active power consumed by the local load is calculated based on the grid-type converter and the equivalent resistance of the local load, and the difference between the active power reference value and the active power consumed by the local load is calculated. The frequency offset value at the instant the pre-synchronization controller is cut off is calculated based on the difference between the active power reference value and the active power consumed by the local load, the damping coefficient, and the rated angular frequency.

[0031] The expression for frequency offset is: In the formula, P ref This is a reference value for active power. D The damping coefficient is... ω N For the rated frequency, U GFM R is the output voltage amplitude of the converter. L This is the local load resistor.

[0032] Step S3: Combine the initial kinetic energy term corresponding to the frequency shift with the equal area rule to construct a transient stability critical condition that takes into account the influence of the initial frequency shift.

[0033] The process of a grid-connected converter switching from off-grid to grid-connected mode is as follows: Initially operating in off-grid mode, upon receiving the grid-connected signal Spre, it enters pre-synchronization control, adjusting the voltage phase deviation across the disconnector switch to zero. After successful pre-synchronization, the disconnector switch closes, simultaneously disconnecting the pre-synchronization module, and the converter enters grid-connected mode. Because the converter frequency cannot change abruptly at the moment of pre-synchronization disconnection, it retains the frequency offset value from off-grid mode. This initial frequency offset injects additional kinetic energy into the system, potentially worsening transient stability.

[0034] In the classical equal-area rule (EAC), the transient stability criterion is that the acceleration area is less than the maximum deceleration area. This embodiment obtains the transient stability critical condition by adding an initial kinetic energy term that considers the influence of off-grid mode frequency offset to the equal-area rule.

[0035] Specifically, the virtual inertia coefficient of the grid-type converter is obtained. J Frequency offset value at the instant of pre-synchronization controller cut-off, rated frequency ω N Active power reference value P ref The functional relationship between output active power and power angle in grid-connected mode, and the power angle value corresponding to the stable equilibrium point. δ S and the work angle value corresponding to the unstable equilibrium point δ US。

[0036] Calculate the initial kinetic energy term based on the virtual inertia coefficient and frequency offset. ; Within the power angle range from zero power angle to the stable equilibrium point, the first integral value is calculated based on the active power reference value, the output active power, and the rated angular frequency: ; The corrected acceleration energy value is calculated based on the initial kinetic energy term and the first integral value: ; Within the range from the stable equilibrium point power angle to the unstable equilibrium point power angle, the deceleration energy value is calculated based on the active power reference value, the output active power, and the rated frequency. ; Based on the relationship that the corrected acceleration energy value is less than or equal to the deceleration energy value, a transient stability critical determination condition is set.

[0037] The expression for the transient stability critical condition is: in, For the initial kinetic energy term, To accelerate from the initial power angle (power angle 0 after successful pre-synchronization) to the stable equilibrium point δ S During the process, the work done by excess mechanical power (i.e., the acceleration area). From δ S The work done by the portion of electromagnetic power that exceeds mechanical power during the process of reaching the unstable equilibrium point δUS (i.e., the maximum deceleration area). δ S This indicates the power angle corresponding to the stable equilibrium point of a grid-connected converter in grid-connected mode. δ US This indicates the power angle corresponding to the unstable equilibrium point of a grid-connected converter in grid-connected mode. P ref This is a reference value for active power. P e This refers to the actual output active power. ω N For the rated frequency, J For virtual inertia coefficient, This is a virtual power angle.

[0038] Furthermore, obtaining the work angle corresponding to the stable equilibrium point and the work angle corresponding to the unstable equilibrium point includes the following steps: Based on the balance condition that the active power reference value equals the output active power, i.e. P ref = P e Determine all candidate values ​​of the work angle that satisfy this balance condition; Based on the monotonicity of the output active power increasing with the increase of the power angle, i.e., d P e / d δ> 0. Obtain the corresponding work angle from the candidate work angle values ​​as the stable equilibrium point; Based on the monotonicity of the output active power decreasing as the power angle increases, i.e., d P e / d δ< 0, obtain the corresponding power angle from the candidate power angle values ​​as the unstable equilibrium point.

[0039] S4. Determine whether the grid-connected converter is unstable in grid-connected mode according to the transient stability critical determination condition. If the transient stability critical condition is met, it is determined to be transiently stable; otherwise, it is determined to be transiently unstable.

[0040] Specifically, the transient stability critical determination condition is used to determine the stability boundary of the grid-connected converter in grid-connected mode. By comparing the positional relationship between the initial operating point of the grid-connected converter after the pre-synchronization controller is disconnected and the stability boundary, its transient stability during the off-grid switching process is determined: if the initial operating point is within the stability boundary, it is determined to be transiently stable; otherwise, it is determined to be transiently unstable.

[0041] In summary, the transient stability analysis method for grid-connected converters during grid-connection and off-grid operation provided in this embodiment introduces the initial frequency offset in off-grid mode into the transient stability analysis process. By modifying the equal area rule, a transient stability critical condition considering the influence of initial kinetic energy is obtained. This method can more accurately determine the transient stability of grid-connected converters during grid-connection and off-grid switching, providing a theoretical basis for parameter design and grid-connected control of grid-connected converters in practical engineering, and improving the success rate of grid-connection and off-grid switching and system operation stability.

[0042] Example 2 To demonstrate the practical effectiveness of the transient stability analysis method for grid-connected converters during grid disconnection and reconnection in Example 1, this example uses a simulation platform to build a corresponding grid-connected converter grid disconnection and reconnection system model, and verifies the transient stability under different operating conditions. The simulation results are as follows: Figure 3 As shown, Figure 3 This is a phase plane stability domain analysis diagram showing the rate of change of phase angle and power angle.

[0043] The dashed area in the figure represents the stable region of the grid-connected converter when switching between grid and off-grid, obtained by the traditional equal area rule, while the solid area represents the stable region of the grid-connected converter when switching between grid and off-grid, obtained by the method proposed in this patent.

[0044] from Figure 3 It can be observed that after the pre-synchronization controller is disconnected, the output frequency of the grid-connected converter undergoes a positive abrupt change to the frequency offset value before the off-grid mode. Point A10- represents the state of the grid-connected converter at point A1 in the figure before the pre-synchronization control is disconnected, and point A10+ represents the state of the grid-connected converter at midpoint A1 after the pre-synchronization control is disconnected. The operating point A10+ is within the estimated stability region, but the phase trajectory of the grid-connected converter still exhibits a divergent oscillation pattern, and the final frequency offset first decreases and then increases, leading to system power angle instability.

[0045] The comparison shows that, under the influence of the initial frequency offset, the grid-type converter is effectively injected with an additional acceleration area. Therefore, the obtained stability region is smaller than the stability region estimated by the traditional method. Thus, after the pre-synchronization controller is removed, the operating point of the grid-type converter is outside the stability region. This indicates that the transient stability analysis method of the grid-type converter during the grid-connection and disconnection process can fully quantify the transient instability risk of the grid-type converter during the grid-connection and disconnection process.

[0046] Example 3 This embodiment provides a computer-readable storage medium, which includes a stored program. When the program is executed, it controls the device where the computer-readable storage medium is located to perform the transient stability analysis method for grid-connected and disconnected converters described in the above embodiment.

[0047] This embodiment also provides a processor for running a program, which executes the transient stability analysis method for grid-connected and disconnected converters described in the above embodiment.

[0048] Example 2 This embodiment provides a computer-readable storage medium, which includes a stored program. When the program runs, it controls the device where the computer-readable storage medium is located to execute the transient stability analysis method for grid-connected and disconnected converters in Embodiment 1.

[0049] Example 3 This embodiment provides a processor for running a program, wherein the program executes the transient stability analysis method for the grid-connected and disconnected process of the grid-connected converter in Embodiment 1.

[0050] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered within the scope of the claims and specification of the present invention.

Claims

1. A method for transient stability analysis of grid-connected and disconnected converters during grid-connection processes, characterized in that, Includes the following steps: S1. For the off-grid switching scenario, establish an off-grid control model for the grid-connected converter. The off-grid control model shall include at least the active power loop equations of the virtual synchronous machine control. S2. Based on the active power loop equation, calculate the frequency offset value of the grid-type converter in off-grid mode due to the mismatch between local load power and reference power. S3. Combine the initial kinetic energy term corresponding to the frequency shift with the equal area rule to construct a transient stability critical judgment condition that takes into account the influence of the initial frequency shift. S4. Determine whether the grid-connected converter is unstable in grid-connected mode according to the transient stability critical determination condition. If the transient stability critical condition is met, it is determined to be transiently stable; otherwise, it is determined to be transiently unstable.

2. The transient stability analysis method for grid-connected and disconnected converters according to claim 1, characterized in that, In the grid-connected mode scenario, the expression for the active power loop equation is: In the formula, R L The resistance of the local load in the parallel equivalent circuit. X L The inductive reactance of the local load in the parallel equivalent circuit, X C The capacitive reactance of the local load in the parallel equivalent circuit. U GFM The output voltage amplitude of the converter. U g The phase voltage amplitude of the power grid. P e This refers to the actual active power output of the converter.

3. The transient stability analysis method for grid-connected and disconnected converters according to claim 1, characterized in that, The calculation of frequency offset in off-grid mode of a grid-connected converter due to the mismatch between local load power and reference power, based on the active power loop equation, includes: Based on the steady-state balance relationship that the output active power of the grid-connected converter in off-grid mode is equal to the active power consumed by the local load, the active power reference value, the inverter output voltage amplitude, the equivalent resistance of the local load, the active loop damping coefficient, and the rated angular frequency are obtained. The active power consumed by the local load is calculated based on the grid-type converter and the equivalent resistance of the local load, and the difference between the active power reference value and the active power consumed by the local load is calculated. The frequency offset value at the instant the pre-synchronization controller is cut off is calculated based on the difference between the active power reference value and the active power consumed by the local load, the damping coefficient, and the rated angular frequency.

4. The transient stability analysis method for grid-connected and disconnected converters according to claim 1, characterized in that, The expression for frequency offset is: In the formula, P ref This is a reference value for active power. D The damping coefficient is... ω N For the rated frequency, U GFM R is the output voltage amplitude of the converter. L This is the local load resistor.

5. The transient stability analysis method for grid-connected converters during grid disconnection and connection processes according to claim 1, characterized in that, The method of combining the initial kinetic energy term corresponding to the frequency shift with the equal area rule to construct a transient stability critical condition that takes into account the influence of the initial frequency shift includes: The virtual inertia coefficient, frequency offset value at the moment of disconnection of the pre-synchronization controller, rated angular frequency, active power reference value, functional relationship between output active power and power angle in grid-connected mode, power angle value corresponding to stable equilibrium point and power angle value corresponding to unstable equilibrium point are obtained for the grid-connected converter. Calculate the initial kinetic energy term based on the virtual inertia coefficient and frequency offset value; Within the power angle range from zero power angle to the stable equilibrium point, the first integral value is calculated based on the active power reference value, the output active power, and the rated angular frequency. The corrected acceleration energy value is calculated based on the initial kinetic energy term and the first integral value; Within the range from the stable equilibrium point power angle to the unstable equilibrium point power angle, the deceleration energy value is calculated based on the active power reference value, the output active power, and the rated angular frequency. Based on the relationship that the corrected acceleration energy value is less than or equal to the deceleration energy value, a transient stability critical determination condition is set.

6. The transient stability analysis method for grid-connected and disconnected converters according to claim 5, characterized in that, The acquisition of the power angle corresponding to the stable equilibrium point and the power angle corresponding to the unstable equilibrium point includes: Based on the balance condition that the active power reference value equals the output active power, determine all candidate power angle values ​​that satisfy this balance condition. Based on the monotonicity of the output active power increasing with the increase of the power angle, the corresponding power angle is obtained from the candidate power angle values ​​as a stable equilibrium point. Based on the monotonicity of the output active power decreasing as the power angle increases, the corresponding power angle is obtained from the candidate power angle values ​​as an unstable equilibrium point.

7. The transient stability analysis method for grid-connected and disconnected converters according to claim 1, characterized in that, The expression for active power in grid-connected mode is: In the formula, U GFM R is the output voltage amplitude of the converter. L For local load resistance, Ug X represents the grid voltage amplitude. g This refers to the line reactance.

8. The transient stability analysis method for grid-connected and disconnected converters according to claim 1, characterized in that, Step S4 includes: The transient stability critical determination condition is used to determine the stability boundary of the grid-connected converter in grid-connected mode. By comparing the positional relationship between the initial operating point of the grid-connected converter after the pre-synchronization controller is disconnected and the stability boundary, its transient stability during the off-grid switching process is determined: if the initial operating point is within the stability boundary, it is determined to be transiently stable; otherwise, it is determined to be transiently unstable.

9. A computer-readable storage medium, characterized in that, The computer-readable storage medium includes a stored program, wherein, when the program is executed, it controls the device containing the computer-readable storage medium to perform the transient stability analysis method for grid-connected and disconnected converters as described in any one of claims 1 to 8.

10. A processor, characterized in that, The processor is used to run a program, wherein the program executes the transient stability analysis method for grid-connected and disconnected converters according to any one of claims 1 to 8.

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