Transient stability evaluation and enhancement method for grid-forming converter parallel operation system

Abstract

The application discloses a grid-construction converter parallel system transient stability evaluation and enhancement method, and belongs to the technical field of power system transient stability evaluation; the evaluation method comprises the following steps: a differential-algebraic model of a hybrid system is established and is converted into a state space form, time variables are eliminated by performing quotient operation on equations to establish a time domain separation reduced-order model; a multivariable alternating iteration algorithm is used to solve implicit space trajectory integral equation groups, and a generalized transient kinetic energy is used to find a farthest point of a phase orbit, so that instantaneous generalized acceleration torque at the point is calculated to quantitatively evaluate transient stability margin of the system. The enhancement method comprises the following steps: a frequency deviation signal of a local phase-locked loop of the grid-construction converter is collected, and an active current reference value is dynamically reduced by a feedback controller to adjust output current. The application breaks through the order limitation of a traditional equal-area criterion, realizes accurate evaluation of a high-order hybrid system, and the control strategy can spontaneously reduce acceleration torque without station communication, thereby improving system synchronous stability.

Description

Technical Field

[0001] This invention belongs to the field of power system transient stability assessment technology, specifically involving a method for transient stability assessment and enhancement of grid-connected converter parallel systems. Background Technology

[0002] With the continuous expansion of renewable energy grid integration, voltage source converters (VSCs), represented by grid-following (GFL) and grid-forming (GFM) converters, have become key interface equipment in modern power systems. Under large disturbance conditions, VSCs exhibit highly complex nonlinear characteristics, posing a severe challenge to the transient stability assessment of power systems. In particular, when GFL and GFM converters are operated in parallel, the dynamic coupling and interaction between multiple converters make the analysis of transient out-of-step mechanisms even more difficult.

[0003] In existing technologies, a relatively mature theoretical system has been established for transient stability analysis of a single converter. The equal area criterion (EAC) has been introduced to reveal the mechanism of transient loss of synchronization, the iterative EAC method has been proposed to consider the influence of nonlinear damping, and the Lyapunov direct method and Hamilton's theory have been used to rigorously define the attraction domain. However, these methods all face significant limitations when applied to multi-converter hybrid systems: the traditional EAC is only applicable to second-order systems and cannot handle the complex dynamics of high-order multi-machine systems; the Lyapunov direct method is extremely difficult to construct energy functions for variable structure systems; the one-dimensional phase trajectory method requires independent analysis of each converter and cannot capture the dynamics of multi-machine interaction; the two-dimensional phase trajectory method remains only at the qualitative level and lacks rigorous quantitative stability criteria.

[0004] Specifically, the existing transient stability analysis techniques for hybrid parallel systems with grid-connected and grid-connected configurations mainly have the following technical problems: (1) The traditional equal area criterion is only applicable to second-order systems and cannot quantitatively handle the complex high-order nonlinear dynamic coupling between GFL and GFM converters, making it difficult to accurately assess the transient stability margin of the hybrid system; (2) Existing stability enhancement strategies usually rely on precise global parameters such as line impedance, grid voltage amplitude, and GFL output current, which are highly dependent on real-time inter-station communication. Communication delays and parameter uncertainties may trigger erroneous current support actions, further exacerbating the risk of transient instability; (3) Existing research has not fully revealed the quantitative influence mechanism of key factors such as GFL output current amplitude, grid voltage drop depth, GFM active power reference value, and PLL parameters on the transient stability margin of the hybrid system.

[0005] To address this, a method for evaluating and enhancing the transient stability of a grid-type parallel system is proposed. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a method for evaluating and enhancing the transient stability of grid-connected converter parallel systems, thereby solving the problems in existing technologies.

[0007] The objective of this invention can be achieved through the following technical solutions: The transient stability assessment method for grid-connected converter parallel systems includes the following steps: A differential-algebraic model of a grid-type parallel system is established, which includes: the phase-locked loop synchronization dynamic equation of the grid-type converter, the power synchronization control dynamic equation of the grid-type converter, and the network power flow equation. Define the power angle of the grid converter, the power angle deviation of the grid converter, and the angular frequency deviation, and convert the differential-algebraic model into a state-space form; by performing quotient operations on the equations in the state-space form to eliminate the time variable, establish a time-domain separation reduced-order model, and obtain the implicit spatial trajectory integral equations for the power angle of the grid converter and the power angle deviation of the grid converter. Using the stable equilibrium point at the time of disturbance as the lower limit of integration, a multivariate alternating iterative algorithm is used to numerically solve the implicit spatial trajectory integral equations, thereby obtaining the system spatial transient trajectory characterized by the power angle distribution function of the grid-type converter and the frequency distribution function of the grid-type converter. Based on the system's spatial transient trajectory, a generalized transient kinetic energy expression is constructed. The point at which the generalized transient kinetic energy is zero, i.e., the angular frequency deviation is zero, is found to be the farthest point of the phase track. The instantaneous generalized acceleration torque at the farthest point of the phase track is calculated and used as a transient stability margin index to quantitatively evaluate the transient synchronization stability of the system.

[0008] Furthermore, the phase-locked loop synchronization dynamic equation of the grid converter is as follows: The power synchronization control dynamic equation of the grid-type converter in, and These are the integral and proportional gains of the PLL, respectively. This is the synchronization gain of the PSC; This represents the q-axis component of the GFL port voltage. Active power injected into the grid for GFM; This is a reference value for active power. To compensate for the power angle deviation of the grid converter; The power angle of the grid-type converter; and The power angle deviation is constructed using nodal analysis and Kirchhoff's circuit laws. Harmony and angle The analytic algebraic representation of .

[0009] Furthermore, the implicit spatial trajectory integral equations are as follows: Where h1, h2, and h3 are nonlinear coefficient functions, For generalized accelerating torque, This is the transient damping coefficient. For the GFM power angle evolution rate, To account for the angular frequency deviation of the mesh converter.

[0010] Furthermore, the iterative update formula of the multivariate alternating iterative algorithm is: Where i is the current iteration step, and These are the initial values ​​of the power angle deviation and the initial power angle at the stable equilibrium point at the moment the disturbance occurs, respectively. It is a positive value; during algorithm initialization, the initial frequency distribution function is set. It is set to a preset positive value, and the initial work angle distribution function is set to a constant value. The iteration stops when the difference between the distribution functions of two adjacent iterations is less than a preset accuracy threshold.

[0011] Furthermore, the transient stability margin index The calculation formula is: in, The power angle deviation value is the value at the farthest point of the phase track; the transient stability criterion is: if the farthest point of the phase track... The transient stability margin index exists and satisfies If <0, the system is transiently stable; if the phase track is farthest point It does not exist, or is the farthest point of the phase. The transient stability margin index exists and satisfies If the value is greater than 0, the system will temporarily lose synchronization.

[0012] A method for enhancing transient stability in a grid-connected converter parallel system includes the following steps: Step 1: Acquire the frequency deviation signal output from the local phase-locked loop of the grid converter; Step 2: Introduce the frequency deviation signal into the active current reference channel of the grid converter, and generate a dynamic current correction amount through the feedback controller; Step 3: Use the dynamic current correction amount to reduce the original active current reference value of the grid converter and generate the corrected actual active current reference value. Step 4: Adjust the active output current of the grid converter according to the actual active current reference value to change the generalized transient acceleration torque during large disturbances.

[0013] Furthermore, the formula for calculating the actual active current reference value is as follows: in, This is the corrected reference value for the actual active current. This is the original active current reference value; Loop gain; and These are the proportional and integral gains of the adaptive loop, respectively. This refers to the frequency deviation of the PLL output.

[0014] Furthermore, the feedback controller is a proportional controller, a proportional-integral controller, or an active disturbance rejection controller.

[0015] Furthermore, the grid-type converter in the grid-type parallel system adopts a power synchronization control strategy or a virtual synchronous generator control strategy.

[0016] An electronic device includes: a processor, a memory, a communication interface, and a communication bus, wherein the processor, the memory, and the communication interface communicate with each other through the communication bus; The memory is used to store at least one executable instruction that causes the processor to perform operations corresponding to the transient stability assessment method described above and the transient stability enhancement method as described in any one of claims 6-9.

[0017] The beneficial effects of this invention are: 1. This invention first establishes a differential-algebraic model of a grid-type parallel system. By eliminating time variables through integral substitution of the equations, a time-domain separated reduced-order model is constructed. A multivariate alternating iterative algorithm is introduced to solve the implicit integral equation system, thereby proposing a high-order iterative equal-area criterion. This overcomes the order limitation of the traditional equal-area criterion, and can reasonably reduce the analysis dimension of high-order nonlinear systems while retaining the key characteristics of multi-machine interactive dynamics. Thus, it realizes the quantitative evaluation of the transient stability margin of the grid-type hybrid parallel system.

[0018] 2. This invention proposes a communication-free adaptive transient current control strategy. This strategy only collects the frequency deviation signal output by the local phase-locked loop of the grid converter, and generates a dynamic current correction value through the feedback controller to adjust the actual active current reference value. Since this strategy mainly uses local measurement information for adaptive adjustment, it reduces the dependence on real-time communication between converter stations and global system parameters such as line impedance and grid voltage amplitude. This reduces the probability of erroneous support actions triggered by communication delays or parameter uncertainties, and significantly enhances the transient synchronization stability of the hybrid system.

[0019] 3. Based on the high-order iterative equal area criterion, this invention maps phase trajectory evolution into generalized transient kinetic energy and uses the instantaneous generalized acceleration torque sign at the farthest point as the stability quantification criterion. Thus, it can analyze the influence of key parameters such as grid voltage drop depth, grid-connected output current amplitude, grid-connected active power reference value, and phase-locked loop integral gain on acceleration area and stability margin, providing theoretical support and quantitative reference for parameter optimization design, transient operation boundary planning, and synchronous risk warning of hybrid systems. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of the circuit structure of the grid-type parallel system of the present invention; Figure 2 This is a diagram of the communication-free adaptive transient current control architecture of the present invention; Figure 3 This is a hardware architecture diagram of the hardware-in-the-loop experiment of this invention; Figure 4 This is a schematic diagram of the transient response under different grid voltage drops; Figure 5This is a schematic diagram of the transient response at E=0.23 pu when the active power setpoint of GFM increases to 120kW; Figure 6 This is a schematic diagram of the transient response at E=0.23 pu when the GFL current setpoint is increased to 120A; Figure 7 This is a schematic diagram of the transient response under different PLL integral gains; Figure 8 Is A schematic diagram of the transient response of the proposed adaptive GFL current control under PU. Detailed Implementation

[0022] 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 embodiments of the present invention, and not all embodiments. 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.

[0023] Example 1 The transient stability assessment method for grid-connected converter parallel systems includes the following steps: Step S1: Establish the differential-algebraic model of the grid-type parallel system, which includes the phase-locked loop synchronization dynamic equation of the grid-type converter, the power synchronization control dynamic equation of the grid-type converter, and the network power flow equation. As shown in Figure 1, the grid-connected parallel system includes one GFL converter and one GFM converter. The GFL converter is equivalent to a current source, and its output frequency is controlled by a phase-locked loop (PLL) to track the grid frequency in real time. The GFM converter is equivalent to a voltage source, and its output frequency is regulated by power synchronization control (PSC) to provide frequency support to the grid.

[0024] These two converters are connected to a common coupling bus via line impedance, and the common bus is connected to an infinite power grid via transmission line impedance. A grid node analysis model is established: the power angle deviation of the GFL is defined. The power angle of the GFM is the difference between the output phase of the phase-locked loop (PLL) and the phase of the grid voltage. This represents the difference between the output phase of the Power Synchronization Control (PSC) and the grid phase. The synchronization dynamic equations for the two converters are as follows: In the formula, and These are the integral and proportional gains of the PLL, respectively. This is the synchronization gain of the PSC; This represents the q-axis component of the GFL port voltage. Active power injected into the grid for GFM; This is a reference value for active power.

[0025] By applying Kirchhoff's circuit laws and node analysis, the derivation is... and about and The parsing expression is as follows: Align the global coordinate system with the grid voltage. The output current of the GFL and the internal voltage of the GFM are respectively and ,in and These are the current magnitude and power factor angle of the GFL, respectively. Let be the internal voltage amplitude of the GFM. The voltage expression can be obtained by performing nodal analysis on node Bus-C: In the formula, This is the equivalent parallel impedance between the GFM branch and the power grid branch, and and These are the dimensionless partial pressure coefficients, and their phase angles are respectively... and .

[0026] The terminal voltage of the GFL is Define lumped impedance And project it onto the local synchronous coordinate system. On the axis (phase shift is) Taking the imaginary part, the feedback variable of the phase-locked loop (PLL) can be obtained: Assume series coupling impedance ,Will Expression Substitution And calculate We can obtain: The aforementioned controller dynamics and network power flow can be unified into a higher-order differential-algebraic model, which can be used to describe the transient synchronization process of a hybrid parallel system.

[0027] Step S2: Define the power angle of the grid converter, the power angle deviation of the grid converter, and the angular frequency deviation, and convert the differential-algebraic model into a state-space form; by performing quotient operations on the equations in the state-space form to eliminate the time variable, establish a time-domain separation reduced-order model to obtain the implicit spatial trajectory integral equations for the power angle of the grid converter and the power angle deviation of the grid converter. Define the angular frequency deviation of GFL The above high-order system can be reorganized into a standard third-order state-space form: In the formula, h1, h2, and h3 are nonlinear coefficient functions that capture the complex electromagnetic power interactions mapped from the physical impedance network. For generalized accelerating torque, This is the transient damping coefficient. This represents the evolution rate of the GFM power angle.

[0028] The core innovation of this invention lies in eliminating the time variable through integral substitution, mapping the dynamic coupling in the time domain to a spatial algebraic relationship. The third equation is divided by the frequency. Eliminate the time variable: Integrating the above equation yields the implicit spatial trajectory function of the GFM power angle: Similarly, multiply both sides of the acceleration equation by Integrating, we obtain the GFL frequency distribution function: The two indefinite integral equations described above constitute a spatial mathematical model that is completely decoupled from the time dimension, accurately describing the state variables. and , The trajectory relationship between them.

[0029] Step S3: Using the stable equilibrium point at the time of disturbance as the lower limit of integration, the implicit spatial trajectory integral equations are numerically solved alternately using a multivariate alternating iterative algorithm to obtain the system spatial transient trajectory characterized by the power angle distribution function of the grid-type converter and the frequency distribution function of the grid-type converter. Since the aforementioned implicit nonlinear equations cannot be solved analytically, this invention proposes a numerical iterative algorithm based on the High-Order Iterative Equal Area Criterion (HITEAC). Its core idea is to continuously update the spatial distribution function through alternating multivariate iterations, projecting the complex high-order dynamics onto a simplified one-dimensional model. In space.

[0030] In each iteration step i, the algorithm updates the current distribution function using the trajectory calculation result from step (i-1). The stable equilibrium point at the moment of the disturbance is... and () represents the lower limit of integration, and the current observation point Assuming an upper limit, the iterative update formula is: The algorithm initialization (i=0) is based on the post-perturbation steady-state assumption, and sets... The value should be a sufficiently small positive value (to avoid a singular denominator). The iteration continues until both the frequency and the GFM power angle distribution function meet the predetermined accuracy threshold. and .

[0031] This invention reveals the convergence mechanism of the HITEAC algorithm: a multivariable cross-coupled negative feedback mechanism. Specifically, if... If underestimated, the denominator decreases, leading to Overrated; overrated The network algebra constraints are transmitted to the generalized accelerating torque h1, so that... Overrated; overrated In the next iteration, the increased denominator suppresses the accumulation of space integrals, leading to It then shifts to underestimation. Through this alternating underestimation / overestimation decaying oscillation, the iterative sequence asymptotically converges globally.

[0032] Step S4: Construct a generalized transient kinetic energy expression based on the system's spatial transient trajectory, find the farthest point of the phase track when the generalized transient kinetic energy is zero, i.e., the angular frequency deviation is zero, and calculate the instantaneous generalized acceleration torque at the farthest point of the phase track as a transient stability margin index to quantitatively evaluate the transient synchronization stability of the system.

[0033] Based on the law of conservation of energy, the phase trajectory evolution is mapped as a generalized transient kinetic energy. The integrator kernel contains three physically independent interactive components: term (I) is the synchronous component, term (II) represents the GFM power angle coupling component, and term (III) is the transient damping component. Term (I) + term (II) is equivalent to the generalized accelerating torque h1, and term (III) corresponds to h2·ω1.

[0034] Based on the law of conservation of energy, at the farthest point At the point where kinetic energy is exhausted (total deceleration area equals acceleration area), the instantaneous frequency deviation ω1 is zero. The stability margin index η is defined as the instantaneous generalized acceleration torque at the farthest point. The transient stability criterion is: if the farthest point If the farthest point exists and η < 0, then the system has sufficient restoring torque and the system is stable; if the farthest point does not exist or η > 0, then the kinetic energy breaks through the potential barrier and the system loses its steps.

[0035] This invention, based on HITEAC, quantitatively assesses the impact of key parameters on transient stability margin through two-dimensional phase trajectory analysis, specifically including: 1) Voltage sag depth E: The deeper the voltage sag, the greater the voltage drop depth. The more severe the abnormal uplift, the faster the area expands, and the more compressed the stability margin. 2) GFL output current amplitude Increased current directly amplifies the dominant synchronizing component, accelerating the expansion of the area. 3) GFM Active Power Reference : Increasing the term (II) in the energy expression further increases the acceleration area; 4) PLL integral gain Increased energy accumulation during large disturbances leads to increased phase-track oscillations and reduced stability margin.

[0036] In other embodiments, the synchronous control of the GFM converter is not limited to power synchronous control (PSC), and a virtual synchronous generator (VSG) control strategy can also be adopted, while the core idea of ​​stability margin assessment remains unchanged. The PI controller in the adaptive current control strategy can be replaced with other forms of negative feedback controllers, such as proportional controllers or autoreactive controllers, the core of which is to use local frequency deviation to achieve current negative feedback; Furthermore, this method can be extended to multi-machine systems with multiple GFL and GFM converters connected in parallel, and the framework of the iterative algorithm is scalable.

[0037] Example 2 In this embodiment, based on the transient energy conversion principle revealed by HITEAC, a communication-free adaptive transient current control strategy for GFL converters is proposed to enhance the transient stability of grid-to-grid parallel systems. The specific steps include: Step 1: Acquire the frequency deviation signal output from the local phase-locked loop of the mesh converter. ; Step 2: Convert the frequency deviation signal An active current reference channel similar to that of the grid converter is introduced, and a dynamic current correction is generated through a feedback controller; Step 3: Use the dynamic current correction amount to reduce the original active current reference value of the grid converter and generate the corrected actual active current reference value. Step 4: Adjust the active output current of the grid converter according to the actual active current reference value to change the generalized transient acceleration torque during large disturbances; Specifically, such as Figure 2 As shown, this communication-free adaptive transient current control strategy utilizes only the frequency deviation information of the GFL converter's local PLL for adaptive negative feedback adjustment, requiring no inter-station communication or global system parameters. By controlling the frequency deviation... An active current reference channel is introduced, and a dynamic current correction is generated by a proportional-integral (PI) controller: In the formula, This is the original active current reference value; Loop gain; and These are the proportional and integral gains of the adaptive loop, respectively. This refers to the frequency deviation of the PLL output.

[0038] When a power grid failure leads to the risk of system acceleration and loss of synchronization ( >0), adaptive supplementary loop fast response, dynamically reduces the actual active current reference value of GFL, directly limiting the first swing period The excessive transient rise achieves a "peak-shaving" effect on the generalized transient acceleration torque. The acceleration area is significantly reduced and the deceleration margin is expanded, enabling the system to effectively dissipate excess kinetic energy during faults. The originally divergent phase paths are forcibly pulled back to a stable manifold, significantly improving the transient synchronization stability of the hybrid system.

[0039] Example 3 In this embodiment, the evaluation method proposed in Embodiment 1 and the enhancement method proposed in Embodiment 2 are simulated and verified through specific hardware-in-the-loop experiments; the experimental parameters are shown in Table 1 below: Table 1 Experimental Parameter Table The hardware setup for this experiment is as follows: Figure 3 As shown, the main circuit topology of the GFM-GFL converter parallel system is simulated by RT-LAB, and the converter control is implemented through RTU-BOX based on TMS320F28377d.

[0040] Figure 4The transient response under two voltage dip conditions is illustrated. A large disturbance is applied to the grid terminal, causing a voltage dip; the system... The PU can maintain synchronization (e.g.) Figure 4 (as shown in (a)); but in Loss of synchronization (LOS) during pu, specifically as follows: Figure 4 As shown in (b) in the figure; this perfectly matches the boundary derived based on HITEAC.

[0041] Figure 5 The load effect was verified: at the critical sag voltage Under pu, set the active power setpoint of GFM. Increasing the power to 120 kW would expand the acceleration area and exceed the available deceleration margin, thus triggering loss of synchronization (LOS), which is consistent with HITEAC's prediction.

[0042] Figure 6 The sensitivity of GFL current injection was evaluated: in pu under, will Increasing the rated current from 100A to 120A showed that such an adjustment forced the phase trajectory to diverge, confirming that excessive current injection compresses the stability margin.

[0043] Figure 7 The effect of phase-locked loop (PLL) damping was analyzed: adjusting the integral parameters of the PLL, Figure 7 (a) and (b) in the text are respectively... Setting them to 50 and 150 resulted in an increase. Will weaken The damping is manifested as a larger overshoot and a longer oscillation time.

[0044] The proposed adaptive current control strategy is effective against severe voltage dips. This was verified using PU. Figure 4 As shown in (b), without compensation, the system experienced rapid loss of synchronization (LOS). However, after enabling the proposed adaptive transient current control strategy, Figure 8 The GFL output current is displayed according to... Dynamic reshaping was performed, the acceleration area was compressed, and the divergent phase trajectory was pulled back into the stable manifold, thus successfully restoring synchronization at the same fault depth that previously caused the loss of synchronization.

[0045] The methods of the present invention can be implemented in hardware, firmware, or as software or computer code that can be stored in a recording medium (such as a CD-ROM, RAM, floppy disk, hard disk, or magneto-optical disk), or as computer code originally stored on a remote recording medium or a non-transitory machine-readable medium and subsequently stored on a local recording medium, downloaded via a network. Thus, the methods described herein can be processed by software stored on a recording medium using a general-purpose computer, a dedicated processor, or programmable or dedicated hardware (such as an ASIC or FPGA). It is understood that the computer, processor, microprocessor controller, or programmable hardware includes storage components (e.g., RAM, ROM, flash memory, etc.) capable of storing or receiving software or computer code that, when accessed and executed by the computer, processor, or hardware, implements the methods described herein. Furthermore, when a general-purpose computer accesses the code used to implement the methods shown herein, the execution of the code transforms the general-purpose computer into a dedicated computer for performing the methods shown herein.

[0046] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A method for evaluating the transient stability of a grid-connected converter parallel system, characterized in that, Includes the following steps: A differential-algebraic model of a grid-type parallel system is established, which includes: the phase-locked loop synchronization dynamic equation of the grid-type converter, the power synchronization control dynamic equation of the grid-type converter, and the network power flow equation. Define the power angle of the grid converter, the power angle deviation of the grid converter, and the angular frequency deviation, and convert the differential-algebraic model into a state-space form; by performing quotient operations on the equations in the state-space form to eliminate the time variable, establish a time-domain separation reduced-order model, and obtain the implicit spatial trajectory integral equations for the power angle of the grid converter and the power angle deviation of the grid converter. Using the stable equilibrium point at the time of disturbance as the lower limit of integration, a multivariate alternating iterative algorithm is used to numerically solve the implicit spatial trajectory integral equations, thereby obtaining the system spatial transient trajectory characterized by the power angle distribution function of the grid-type converter and the frequency distribution function of the grid-type converter. Based on the system's spatial transient trajectory, a generalized transient kinetic energy expression is constructed. The point at which the generalized transient kinetic energy is zero, i.e., the angular frequency deviation is zero, is found to be the farthest point of the phase track. The instantaneous generalized acceleration torque at the farthest point of the phase track is calculated and used as a transient stability margin index to quantitatively evaluate the transient synchronization stability of the system.

2. The transient stability assessment method for a grid-connected converter parallel system according to claim 1, characterized in that, The phase-locked loop synchronization dynamic equation of the grid converter is: The power synchronization control dynamic equation of the grid-type converter in, and These are the integral and proportional gains of the PLL, respectively. This is the synchronization gain of the PSC; This represents the q-axis component of the GFL port voltage. Active power injected into the grid for GFM; This is a reference value for active power. To compensate for the power angle deviation of the grid converter; The power angle of the grid-type converter; and The power angle deviation is constructed using nodal analysis and Kirchhoff's circuit laws. Harmony and angle The analytic algebraic representation of .

3. The transient stability assessment method for a grid-connected converter parallel system according to claim 2, characterized in that, The implicit spatial trajectory integral equations are as follows: Where h1, h2, and h3 are nonlinear coefficient functions, For generalized accelerating torque, This is the transient damping coefficient. For the GFM power angle evolution rate, To account for the angular frequency deviation of the mesh converter.

4. The transient stability assessment method for a grid-connected converter parallel system according to claim 3, characterized in that, The iterative update formula of the multivariate alternating iterative algorithm is: Where i is the current iteration step, and These are the initial values ​​of the power angle deviation and the initial power angle at the stable equilibrium point at the moment the disturbance occurs, respectively. It is a positive value; during algorithm initialization, the initial frequency distribution function is set. It is set to a preset positive value, and the initial work angle distribution function is set to a constant value. The iteration stops when the difference between the distribution functions of two adjacent iterations is less than a preset accuracy threshold.

5. The transient stability assessment method for a grid-connected converter parallel system according to claim 3, characterized in that, The transient stability margin index The calculation formula is: in, The power angle deviation value is the value at the farthest point of the phase track; the transient stability criterion is: if the farthest point of the phase track... The transient stability margin index exists and satisfies If <0, the system is transiently stable; if the phase track is farthest point It does not exist, or is the farthest point of the phase. The transient stability margin index exists and satisfies If the value is greater than 0, the system will temporarily lose synchronization.

6. A method for enhancing transient stability in a grid-connected converter parallel system, characterized in that, Includes the following steps: Step 1: Acquire the frequency deviation signal output from the local phase-locked loop of the grid converter; Step 2: Introduce the frequency deviation signal into the active current reference channel of the grid converter, and generate a dynamic current correction amount through the feedback controller; Step 3: Use the dynamic current correction amount to reduce the original active current reference value of the grid converter and generate the corrected actual active current reference value. Step 4: Adjust the active output current of the grid converter according to the actual active current reference value to change the generalized transient acceleration torque during large disturbances.

7. The method for enhancing transient stability of a grid-connected converter parallel system according to claim 6, characterized in that, The formula for calculating the actual active current reference value is as follows: in, This is the corrected reference value for the actual active current. This is the original active current reference value; Loop gain; and These are the proportional and integral gains of the adaptive loop, respectively. This refers to the frequency deviation of the PLL output.

8. The method for enhancing transient stability of a grid-connected converter parallel system according to claim 6, characterized in that, The feedback controller is a proportional controller, a proportional-integral controller, or an active disturbance rejection controller.

9. The method for enhancing transient stability of a grid-connected converter parallel system according to claim 6, characterized in that, The grid-type converter in the grid-type parallel system adopts a power synchronization control strategy or a virtual synchronous generator control strategy.

10. An electronic device, characterized in that, include: The processor, memory, communication interface, and communication bus are provided, wherein the processor, memory, and communication interface communicate with each other via the communication bus. The memory is used to store at least one executable instruction that causes the processor to perform the operation corresponding to the transient stability evaluation method as described in any one of claims 1-5 and the transient stability enhancement method as described in any one of claims 6-9.

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