Method and device for optimizing virtual impedance current limiting parameters of network-constructing converter

Abstract

This invention discloses a method and apparatus for optimizing virtual impedance current limiting parameters in a grid-type converter, belonging to the field of power electronic converter control technology. The method includes: establishing a nonlinear dynamic model of the grid-type converter containing a fixed virtual impedance; introducing active power-frequency droop control, reactive power-voltage droop control, and virtual impedance control into the control system of the grid-type converter; during voltage reference generation, correcting the internal voltage reference value of the grid-type converter through virtual impedance, so that the internal voltage reference value is dynamically adjusted according to the output current change; performing parameter scanning in a two-dimensional parameter space composed of the virtual impedance amplitude and virtual impedance phase angle to obtain the variation law of the peak output current and steady-state current of the grid-type converter with the virtual impedance parameters; dividing the virtual impedance amplitude change process into three stages: a small impedance region, a medium impedance region, and a large impedance region; and determining the optimized design result of the virtual impedance amplitude and impedance angle in the medium impedance region with the optimization objective of minimizing the steady-state current; and verifying the transient synchronous stability of the virtual impedance parameters based on the phase plane analysis method.

Description

Technical Field

[0001] This invention belongs to the field of power electronic converter control technology, specifically relating to a method and device for optimizing virtual impedance current limiting parameters of grid-type converters. Background Technology

[0002] As the global energy structure shifts towards cleaner and lower-carbon energy sources, the installed capacity of renewable energy continues to grow, and the power system is evolving from a traditional synchronous generator-dominated model to one with a high proportion of power electronics. The large-scale integration of new energy sources significantly reduces the system's equivalent inertia and damping level, and enhances the dynamic coupling between power electronic devices, posing greater challenges to the stability of the power grid under fault and disturbance conditions. Currently widely used grid-following control relies on phase-locked loops (PLLs) to obtain the grid phase angle and only possesses current control capabilities. In weak grids, it is difficult to provide stable voltage and frequency support, easily leading to oscillations or even grid disconnection. Therefore, grid-following converters that can actively establish voltage and frequency references and provide inertia support have become an important technical means to support the stable operation of high-proportion renewable energy power systems.

[0003] However, due to the overcurrent capability limitations of power devices, converters must trigger current-limiting control to protect power devices when large disturbances occur in the grid, such as voltage dips or short-circuit faults. The intervention of current-limiting control alters the converter's equivalent output impedance and power angle dynamic characteristics, potentially weakening power synchronization capabilities and affecting system transient stability. Among existing grid-connected converter current-limiting strategies, virtual impedance technology has received widespread attention due to its simple structure and the elimination of the need for direct current saturation treatment. By introducing an additional voltage drop proportional to the output current into the voltage reference, virtual impedance can change the grid connection point voltage instantaneously during a fault, thereby suppressing current surges and maintaining the converter's voltage source characteristics. However, under large disturbances such as voltage dips, virtual impedance not only affects the amplitude characteristics of the output current but also alters the amplitude and phase of the grid connection point voltage, thus impacting the dynamic evolution of the power angle and the system stability boundary.

[0004] In existing research, the design of virtual impedance parameters typically relies on empirical values ​​or trial-and-error adjustments, lacking systematic theoretical analysis and parameter design methods. Especially under large disturbances such as grid faults, the influence of virtual impedance parameters on output current variations and system dynamics exhibits significant nonlinear characteristics. The comprehensive mechanism by which virtual impedance parameters affect current limiting capability, grid connection point voltage variations, and system stability has not been fully elucidated. Therefore, further research on the design and optimization methods of virtual impedance parameters is necessary.

[0005] In summary, there is a lack of theoretical guidance for the design of virtual impedance current limiting parameters of grid-connected converters under large disturbances such as grid voltage drops. To address this issue, this invention proposes an optimization method for virtual impedance current limiting parameters of grid-connected converters based on three-stage characteristics. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a method and apparatus for optimizing virtual impedance current limiting parameters in grid-type converters, thereby solving the problems in existing technologies.

[0007] The objective of this invention can be achieved through the following technical solutions: The method for optimizing the virtual impedance current limiting parameters of a grid-type converter includes the following steps: A nonlinear dynamic model of a grid-type converter with fixed virtual impedance is established, and active power-frequency droop control, reactive power-voltage droop control, and virtual impedance control are introduced into the control system of the grid-type converter. Based on the aforementioned virtual impedance control loop, during the voltage reference generation process, the internal voltage of the grid-type converter is corrected by the virtual impedance, so that the internal voltage reference value is dynamically adjusted according to the output current change, thereby constructing a system model that couples differential equations and algebraic equations. For the constructed system model, under the same grid voltage drop conditions, parameter scanning is performed in a two-dimensional parameter space composed of virtual impedance amplitude and virtual impedance phase angle to obtain the law of change of peak output current and steady-state current of grid-type converter with virtual impedance parameters. Based on the variation of the peak output current and steady-state current with the virtual impedance parameter, the virtual impedance amplitude variation process is divided into three stages: small impedance region, medium impedance region, and large impedance region. In the three phases defined above, the medium impedance region is selected as the virtual impedance parameter optimization range, and the optimization objective is to minimize the steady-state current. Within the medium impedance region, the optimized design results of the virtual impedance amplitude and impedance angle are determined. The transient synchronous stability of the virtual impedance parameters corresponding to the optimized design results is verified using the phase plane analysis method.

[0008] Furthermore, the nonlinear dynamic model of the grid-type converter consists of the power angle difference δ and the output angular frequency. ω and grid connection point voltage amplitude V o The dynamic equations of a third-order dynamic system are expressed as follows: in, δ For the difference in effort angle, ω For the output angular frequency, ω 0 is the rated angular frequency. ω p This is the cutoff frequency of the active low-pass filter. ω q This is the cutoff frequency of the reactive power low-pass filter. Kp This is the active power droop coefficient. K q This is the reactive power droop factor. P This is the measured value of active power. P ref This is a reference value for active power. V o The voltage amplitude at the grid connection point. V n This is the rated voltage amplitude. Q This is the measured value of reactive power. Q ref This is a reference value for reactive power.

[0009] Furthermore, during the voltage reference generation process, the corrected internal voltage of the grid-type converter satisfies the following expression: in, V in ∠ δ in For internal voltage phasors, R v , X v These are virtual resistance and virtual inductance, respectively. V o ∠ δ o For the grid connection point voltage phasor, I g ∠ φ i This is the output current phasor.

[0010] Furthermore, the system model coupling the differential equation and the algebraic equation is as follows: in, Z g For the equivalent impedance of the power grid, φ g The equivalent impedance angle of the power grid. Z v ∠ φ v This is a virtual impedance.

[0011] Furthermore, in the medium impedance region, the grid connection point voltage is effectively reconstructed, and the system operating point approaches the theoretical optimal operating point. The region, the theoretically optimal operating point The expression is: in, P o Active powerX g The reactance is the equivalent impedance of the power grid. E The voltage amplitude of the power grid. The difference in angle of work.

[0012] Furthermore, in the low impedance region, the system operating point is determined by the power outer loop control. The optimal impedance angle selected in this stage satisfies the condition that the virtual impedance voltage drop is in phase with the output current, so that the internal voltage of the grid-connected converter is approximately in phase with the grid connection point voltage, thereby reducing the driving voltage difference.

[0013] Furthermore, within the medium impedance region, the virtual impedance phase angle range that enables the steady-state current to be kept to a minimum is 20° to 70°.

[0014] The virtual impedance current limiting parameter optimization device for grid-type converters executes the above method, including: The model building and control introduction module is used to establish a nonlinear dynamic model of a grid-type converter with fixed virtual impedance, and to introduce active power-frequency droop control, reactive power-voltage droop control and virtual impedance control into the control system of the grid-type converter. The voltage reference correction module is used to correct the internal voltage reference value of the grid converter through virtual impedance during the voltage reference generation process, based on the virtual impedance control link, so that the internal voltage reference value is dynamically adjusted according to the output current change, thereby constructing a system model that couples differential equations and algebraic equations. The parameter scanning and pattern acquisition module is used to perform parameter scanning on the constructed system model under the same grid voltage drop conditions, within a two-dimensional parameter space composed of virtual impedance amplitude and virtual impedance phase angle, to obtain the pattern of the peak output current and steady-state current of the grid-type converter changing with the virtual impedance parameters. The stage feature extraction module is used to divide the virtual impedance amplitude change process into three stages: small impedance region, medium impedance region, and large impedance region, based on the law of the change of the peak output current and steady-state current with the virtual impedance parameter. The parameter optimization module is used to select the medium impedance region as the virtual impedance parameter optimization range in the three divided stages, and determine the optimized design results of virtual impedance amplitude and impedance angle within the medium impedance region with the optimization objective of minimizing steady-state current. The closed-loop verification module is used to perform transient synchronization stability verification on the virtual impedance parameters corresponding to the optimized design results based on the phase plane analysis method.

[0015] A computer storage medium storing a readable program that, when executed, instructs a computing device to perform the virtual impedance current limiting parameter optimization method for a grid-type converter as described above.

[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, which causes the processor to perform the operation corresponding to the virtual impedance current limiting parameter optimization method for grid-type converters described above.

[0017] The beneficial effects of this invention are: 1. This invention establishes a nonlinear dynamic model of a grid-connected converter with a fixed virtual impedance. Virtual impedance is introduced during the voltage reference generation process to correct the internal voltage, constructing a system model coupling differential and algebraic equations. By making explicit the complex nonlinear coupling relationship between the internal voltage, power angle, frequency, and grid connection point voltage, the intrinsic physical relationship between the virtual impedance parameter and the output current and system power angle dynamics is revealed. This breaks through the limitations of existing technologies where virtual impedance parameter design has long relied on empirical values ​​or trial-and-error methods, enhancing the systematic nature and theoretical guiding significance of parameter design.

[0018] 2. Under the same grid voltage drop conditions, this invention performs a two-dimensional parameter scan using the virtual impedance amplitude and impedance angle as variables. Based on the steady-state current variation with amplitude, it creatively divides the virtual impedance process into three stages: a low-impedance region, a medium-impedance region, and a high-impedance region. This division not only clarifies the physical mechanism of current limiting in each stage, but more importantly, it effectively avoids the high-impedance region, which is highly sensitive to changes in impedance angle and easily triggers overcurrent or even system instability. Simultaneously, it accurately locates the medium-impedance region where the grid connection point voltage is effectively reconstructed. The impedance angle in this region has strong fault tolerance, greatly improving engineering feasibility.

[0019] 3. In analyzing the peak current pattern, this invention explicitly utilizes the characteristic that "peak currents all reach their minimum value under purely resistive virtual impedance conditions," and combines this with the goal of minimizing steady-state current to optimize parameters within the S2 region. Since the voltage drop generated by the purely resistive virtual impedance is completely in phase with the output current, this method can directly weaken the voltage amplitude at the grid connection point, thereby minimizing the effective drive voltage difference. This allows the converter to most effectively suppress fault current surges within physical limits when large disturbances occur in the power grid, significantly improving the safety of power devices and the system.

[0020] 4. This invention goes beyond simply finding the minimum current parameter. In the final step, it employs phase plane analysis to verify the transient synchronization stability of the optimized virtual impedance parameters, observing whether the phase plane trajectory converges to a stable equilibrium point. This verification step forms a complete closed-loop design. It thoroughly solves the industry pain point in existing technologies where "the intervention of current limiting control changes the equivalent output impedance, potentially weakening power synchronization capability and leading to system transient instability." Ultimately, it achieves the goal of effectively limiting fault current while ensuring the stable operation of the grid-connected converter under grid disturbance conditions. Attached Figure Description

[0021] 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.

[0022] Figure 1 This is a schematic diagram of the control structure of the grid-type converter with virtual impedance according to the present invention; Figure 2 This is a schematic diagram of the equivalent model of the grid-type converter with virtual impedance according to the present invention; Figure 3 This is a schematic diagram of the phasor relationship of the grid-type converter with virtual impedance according to the present invention; Figure 4 This is a schematic diagram showing the relationship between the peak and steady-state output current of the present invention and the virtual impedance phase angle. Figure 5 This is a schematic diagram of the phasor analysis of the influence of virtual impedance at different stages of the present invention; Figure 6 For the present invention | Z v Schematic diagram of the experimental current waveform when |=0.5; Figure 7 For the present invention | Z v Schematic diagram of the experimental current waveform when |=2; Figure 8 For the present invention | Z v Schematic diagram of the experimental current waveform when |=3; Figure 9 For the present invention | Z v Schematic diagram of the experimental current waveform when |=3.2; Figure 10 For the present invention | Z v A schematic diagram of the phase plane trajectory of the system when |=0.5 and the impedance angle is at its optimal value. Detailed Implementation

[0023] 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.

[0024] Example 1 like Figure 1 As shown, the grid-type converter system of this invention includes a voltage source converter, a filter inductor, a grid equivalent impedance, and a grid voltage source. The control system incorporates active power-frequency droop control, reactive power-voltage droop control, and virtual impedance control. By correcting the internal voltage reference using virtual impedance, the output current can be regulated and limited under grid voltage disturbance conditions.

[0025] The virtual impedance current limiting parameter optimization method for grid-type converters based on three-stage characteristics described in this invention includes the following steps: (1) establishing a nonlinear dynamic model of the grid-type converter; (2) establishing a virtual impedance voltage correction model; (3) establishing a differential-algebraic system model containing virtual impedance; (4) constructing a virtual impedance parameter scanning space; (5) determining the theoretical optimal operating point; (6) identifying the three-stage characteristics of the output current; (7) determining the virtual impedance parameter optimization range; and (8) verifying transient synchronization stability.

[0026] The above eight steps will be explained in detail below with reference to the accompanying drawings: like Figure 1 The diagram shown is a schematic of the grid-type converter system described in this invention. The system includes a voltage source converter and a filter inductor. L f Filter capacitor C f Equivalent impedance of power grid Z g and grid voltage source E ∠0. The converter output, after passing through a filter inductor and capacitor, is connected to the grid voltage via the grid impedance. The control system adopts a typical grid-based control architecture, including active power-frequency droop control, reactive power-voltage droop control, and virtual impedance control. The grid impedance is typically inductive, i.e. X g >> R g The grid impedance angle can be approximated as purely inductive. Under this condition, the active power exchanged between the converter and the grid... P and reactive power Q It can be simplified to: The nonlinear dynamic equations of a grid-type converter can be expressed as the following set of third-order differential equations: To improve the system's overcurrent capability, a virtual impedance element is introduced in the voltage reference generation. Figure 2 A simplified equivalent model of the grid-type converter considering virtual impedance is presented. Based on this equivalent model, the nonlinear dynamics of the grid-type converter can be expressed as the following system of differential-algebraic equations: like Figure 2 The diagram shown is a schematic representation of the equivalent model of the GFM-VSC with virtual impedance according to the present invention. In this equivalent model, the grid-type converter can be equivalent to an internal voltage source. V in ∠ δ in And through virtual impedance Z v It is connected to the power grid.

[0027] like Figure 3 The diagram shown illustrates the phasor relationship of the GFM-VSC with virtual impedance according to the present invention. Figure 3 As can be seen, the virtual impedance generates an additional voltage drop under the influence of the output current, causing a shift in amplitude and phase between the internal voltage reference and the PCC voltage, thereby altering the power characteristics and power angle dynamics. In the phasor plane, the internal voltage... V in Grid connection point voltage V o and grid voltage E The two satisfy a phasor relationship, and the voltage drop across the virtual impedance is I· Z v The system output current is determined by the driving voltage difference between the internal voltage and the grid voltage; therefore, the virtual impedance parameter will directly affect the position of the system operating point in the phasor plane.

[0028] To investigate the influence of virtual impedance parameters on output current, under the same grid voltage drop conditions, the virtual impedance amplitude | Z v | Phase angle with virtual impedance φ v Perform a two-dimensional parameter scan and record the peak output current. I peak and steady-state current I os The simulation was performed in MATLAB, and the system dynamic equations were solved using the numerical integrator ode45. The step size for each variable during the parameter sweep was set to 1×10⁻⁶.−3 For each combination of virtual impedance parameters, the current waveform is obtained by solving the system's time-domain response, and the corresponding waveform is extracted. I peak and I os For subsequent analysis. The grid current is expressed as: As attached Figure 4 The diagram shown illustrates the relationship between the peak and steady-state output current values ​​of the present invention and the virtual impedance phase angle. Figure 4 (a) Figure 4 (b) and Figure 4 (c) in the figure corresponds to the virtual impedance magnitude | Z v | These are the current variation curves for values ​​of 0.5, 2, and 3. Figure 4 Figure (d) shows a magnified view of the steady-state current variation. As can be seen from the figure, under different virtual impedance amplitudes, both the peak output current and the steady-state current exhibit a clear pattern as the virtual impedance phase angle changes.

[0029] The optimal point of the theory V b Determined by the active power balance condition: Meanwhile, the steady-state current amplitude is proportional to the geometric distance between the operating point and the grid voltage phasor in the phasor plane: Based on the parameter scan results, the variation of output current with virtual impedance amplitude was analyzed. Under the same steady-state operating conditions, the grid voltage was applied down to 0.6. E The peak and steady-state characteristics of the output current were examined. (From the attached...) Figure 4 As shown in the curves illustrating the variation of output current with virtual impedance amplitude and impedance angle, it can be observed that the steady-state current exhibits a distinct phased characteristic with respect to the virtual impedance amplitude, which can be divided into three phases: like Figure 5 The diagram shown illustrates the phasor analysis of the influence of virtual impedance at different stages of this invention. To clarify the criteria for dividing the regions into low-impedance region S1, medium-impedance region S2, and high-impedance region S3, this invention is based on the system phasor relationship and the operating point relative to the theoretical optimum. V b The degree of offset is used to determine the operating state under different virtual impedance amplitude conditions. The system voltage phasor relationship can be expressed as follows: With the virtual impedance amplitude | Z v | changes, Vin ∠ φ in Corresponding to the function of the outer power loop, virtual impedance voltage drop I g Z v The effect of the above phasor superposition gradually increases, thereby changing the output voltage phasor. V o The position in the phasor plane. When the system's operating point gradually approaches or deviates from the theoretical optimum. V b This allows for the determination of transition boundaries for different impedance ranges. Specifically, when the virtual impedance magnitude increases to the point that the output voltage phasor begins to approach the theoretical optimum... V b When the impedance is near the threshold, the corresponding impedance magnitude can be used as the transition threshold between S1 and S2; when the virtual impedance further increases and the internal voltage... V in When significant compression or shift occurs, resulting in a substantial increase in the system's sensitivity to the impedance angle, the corresponding impedance magnitude can be used as the transition threshold between S2 and S3. Based on the above criteria, the virtual impedance process can be divided into the following three stages.

[0030] Figure 5 (a) in the diagram is a schematic diagram of the phasor relationship in stage S1. Stage S1: Low impedance region. The virtual impedance amplitude is relatively small in this region, corresponding to... Figure 4 (a) in the middle | Z v When |=0.5, the grid connection point voltage has not been fully reconfigured, and the operating point is mainly determined by the power outer loop control. This is because the system is difficult to move to its theoretical optimum. V b The steady-state current decreases only slightly. Figure 4 As can be seen in (a), the output current is quite sensitive to changes in the impedance angle in this region, and the peak current reaches its minimum under pure resistance conditions. In this stage, the optimal impedance angle satisfies... φ v = δ - φ i This condition ensures that the virtual impedance voltage drop is in phase with the current, making the internal voltage approximately phase-aligned with the PCC voltage, thereby reducing the driving voltage difference.

[0031] Figure 5 (b) shows the phasor relationship diagram for stage S2. Stage S2: Medium Impedance Region. In this region, the virtual impedance amplitude is moderate, corresponding to... Figure 4 (b) in the middle | Z v When |=2, the grid connection point voltage is effectively reconstructed, and the operating point can naturally approach the theoretical optimum.V b .from Figure 4 As shown in (b), the output current in this region can maintain its theoretical minimum value over a wide impedance angle range (approximately 20°~70°), forming a distinct "flat bottom" characteristic, indicating that the impedance angle has strong fault tolerance within this range. This stage represents the optimal design range, where the steady-state current maintains its minimum value over a large angle range.

[0032] Figure 5 (c) in the diagram is a phasor relationship diagram for stage S3. Stage S3: High Impedance Region. The virtual impedance is relatively large in this region, corresponding to... Figure 4 (c) in | Z v When |=3, the virtual impedance has a dominant effect on the grid connection point voltage, the external power control loop contributes little to the steady-state current, and the internal voltage... V in It eventually tends to a steady-state value. From Figure 4 (c) and Figure 4 As can be seen from (d) in the figure, although the system can still reach the theoretical optimum in the S3 stage. V b However, the requirements for the impedance angle are very strict, and optimal current limiting can only be achieved within an extremely narrow impedance angle range. If the angle is not chosen properly, it can easily lead to overcurrent or even system instability.

[0033] Furthermore, the physical mechanism controlling the steady-state current remains consistent across stages S1-S3, which also determines the peak current. I peak The overall trend of change. The interaction between the virtual impedance magnitude and the angle determines that the system's operating point is close to the optimum. V b The degree to which the peak current is adjusted, thereby regulating the magnitude of the peak current in a manner consistent with the steady-state current trend. From Figure 4 As shown in (a)-(c), under different virtual impedance amplitudes, the peak current reaches its minimum under the condition of a purely resistive virtual impedance. When the virtual impedance is purely resistive, the virtual impedance voltage drop is in phase with the output current, the voltage amplitude at the grid connection point is directly weakened, and the effective driving voltage difference is reduced to the maximum extent, thus the peak current reaches its minimum. In contrast, when the virtual impedance contains a large inductive or capacitive component, its main effect is phase shift rather than pure amplitude attenuation, thereby weakening the current limiting effect and leading to an increase in peak current.

[0034] Based on the three-stage characteristic analysis results, with the goal of minimizing steady-state current, the second-stage S2 region in the virtual impedance parameter space is selected as the virtual impedance parameter optimization interval, and the range of virtual impedance angles that can keep the steady-state current at its minimum value is selected as the optimized design result of the virtual impedance parameters within this region.

[0035] like Figure 6 As shown, this is the present invention | Z v A schematic diagram of the experimental current waveform when |=0.5, where Figure 6 In the diagram, (a) shows the experimental current waveform under purely resistive virtual impedance conditions. Figure 6 (b) in the figure shows the experimental current waveform under the optimal impedance angle condition. When the virtual impedance amplitude is 0.5, the peak current under the purely resistive virtual impedance condition is 19.1A; under the optimal impedance angle condition, the output current amplitude is significantly reduced.

[0036] like Figure 7 As shown, this is the present invention | Z v A schematic diagram of the experimental current waveform when |=2, where, Figure 7 In the diagram, (a) shows the experimental current waveform under purely resistive virtual impedance conditions. Figure 7 Figure (b) shows the experimental current waveform under the optimal impedance angle condition. When the virtual impedance amplitude is 2, the peak current under the purely resistive virtual impedance condition is 17.38A; under the optimal impedance angle condition, the steady-state current reaches the theoretical minimum of 15.7A. The experimental results show that by appropriately selecting the virtual impedance phase angle, the system can effectively reduce the fault current while ensuring stable operation.

[0037] like Figure 8 As shown, this is the present invention | Z v A schematic diagram of the experimental current waveform when |=3, where Figure 8 In the diagram, (a) shows the experimental current waveform under purely resistive virtual impedance conditions. Figure 8 (b) shows the experimental current waveform under the optimal impedance angle condition. When the virtual impedance amplitude is 3, the peak current under the purely resistive virtual impedance condition is 18.1A; under the optimal impedance angle condition, the steady-state current also reaches the theoretical minimum of 15.7A. This result further verifies that the virtual impedance phase angle has a significant impact on the current limiting effect.

[0038] like Figure 9 As shown, this is the present invention | Z v A schematic diagram of the experimental current waveform when |=3.2, where, Figure 9 (a) in the figure represents the experimental results of system instability under purely resistive virtual impedance conditions. Figure 9(b) shows the experimental results of the system remaining stable under the optimal impedance angle condition. When the virtual impedance amplitude increases to 3.2, the system becomes unstable under the purely resistive virtual impedance condition; however, under the optimal impedance angle condition, the system can still maintain stable operation, and the steady-state current reaches the theoretical minimum of 15.7A. This result shows that a reasonable selection of the virtual impedance phase angle can effectively improve the system's stability margin.

[0039] Finally, the transient synchronization stability under optimized parameter conditions is verified using phase plane analysis to determine the dynamic evolution of the power angle of the grid-connected converter under grid voltage disturbance conditions. Specifically, this is achieved by constructing the power angle... δ Output voltage V o With angular frequency ω The phase plane trajectory is used to analyze the evolution path of the system state variables after the disturbance and determine whether it converges to the stable equilibrium point of the system. When the system state trajectory satisfies: When it is assumed that the system state can converge to a stable equilibrium point, where ( δ , ω , V o The ) represents the steady-state operating point of the system. When the phase plane trajectory remains bounded and does not diverge within a preset time interval, it is determined that the phase plane trajectory has converged, indicating that the grid-type converter can maintain transient synchronous and stable operation under the virtual impedance parameter conditions, thus verifying that the virtual impedance parameter can guarantee stable system operation under current-limiting conditions.

[0040] like Figure 10 As shown, this is the present invention | Z v This diagram illustrates the system phase plane trajectory when |=2 and the impedance angle is at its optimal value. As shown in the figure, the system state trajectory eventually converges to a stable equilibrium point, indicating that under the proposed virtual impedance parameter optimization method, the grid-type converter can maintain good transient synchronous stability while achieving current limiting.

[0041] Based on a similar inventive concept, embodiments of the present invention also provide a computer storage medium storing a readable program that, when run by a processor, can execute the above-described method for optimizing virtual impedance current limiting parameters of a grid-type converter.

[0042] Based on a similar inventive concept, this invention provides an electronic device, including: 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, which causes the processor to perform the operation corresponding to the above-described method for optimizing virtual impedance current limiting parameters of a grid-type converter.

[0043] Based on a similar inventive concept, embodiments of the present invention also provide a computer program product, including computer instructions, which instruct a computing device to perform the operation corresponding to the above-described method for optimizing virtual impedance current limiting parameters of a grid-type converter.

[0044] Example 2 In this embodiment, a virtual impedance current limiting parameter optimization device for grid-type converters is proposed, specifically including: The model building and control introduction module is used to establish a nonlinear dynamic model of a grid-type converter with fixed virtual impedance, and to introduce active power-frequency droop control, reactive power-voltage droop control and virtual impedance control into the control system of the grid-type converter. The voltage reference correction module is used to correct the internal voltage reference value of the grid converter through virtual impedance during the voltage reference generation process, based on the virtual impedance control link, so that the internal voltage reference value is dynamically adjusted according to the output current change, thereby constructing a system model that couples differential equations and algebraic equations. The parameter scanning and pattern acquisition module is used to perform parameter scanning on the constructed system model under the same grid voltage drop conditions, within a two-dimensional parameter space composed of virtual impedance amplitude and virtual impedance phase angle, to obtain the pattern of the peak output current and steady-state current of the grid-type converter changing with the virtual impedance parameters. The stage feature extraction module is used to divide the virtual impedance amplitude change process into three stages: small impedance region, medium impedance region, and large impedance region, based on the law of the change of the peak output current and steady-state current with the virtual impedance parameter. The parameter optimization module is used to select the medium impedance region as the virtual impedance parameter optimization range in the three divided stages, and determine the optimized design results of virtual impedance amplitude and impedance angle within the medium impedance region with the optimization objective of minimizing steady-state current. The closed-loop verification module is used to perform transient synchronous stability verification on the virtual impedance parameters corresponding to the optimized design results based on the phase plane analysis method, so as to determine that the virtual impedance parameters can achieve current limiting and maintain stable system operation under grid voltage disturbance conditions.

[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 optimizing virtual impedance current limiting parameters in a grid-type converter, characterized in that, Includes the following steps: A nonlinear dynamic model of a grid-type converter with fixed virtual impedance is established, and active power-frequency droop control, reactive power-voltage droop control, and virtual impedance control are introduced into the control system of the grid-type converter. Based on the aforementioned virtual impedance control loop, during the voltage reference generation process, the internal voltage of the grid-type converter is corrected by the virtual impedance, so that the internal voltage reference value is dynamically adjusted according to the output current change, thereby constructing a system model that couples differential equations and algebraic equations. For the constructed system model, under the same grid voltage drop conditions, parameter scanning is performed in a two-dimensional parameter space composed of virtual impedance amplitude and virtual impedance phase angle to obtain the law of change of peak output current and steady-state current of grid-type converter with virtual impedance parameters. Based on the variation of the peak output current and steady-state current with the virtual impedance parameter, the virtual impedance amplitude variation process is divided into three stages: small impedance region, medium impedance region, and large impedance region. In the three phases defined above, the medium impedance region is selected as the virtual impedance parameter optimization range, and the optimization objective is to minimize the steady-state current. Within the medium impedance region, the optimized design results of the virtual impedance amplitude and impedance angle are determined. The transient synchronous stability of the virtual impedance parameters corresponding to the optimized design results is verified using the phase plane analysis method.

2. The method for optimizing virtual impedance current limiting parameters of a grid-type converter according to claim 1, characterized in that, The nonlinear dynamic model of the grid-type converter consists of the power angle difference δ and the output angular frequency. ω and grid connection point voltage amplitude V o The dynamic equations of a third-order dynamic system are expressed as follows: in, δ For the difference in effort angle, ω For the output angular frequency, ω 0 is the rated angular frequency. ω p This is the cutoff frequency of the active low-pass filter. ω q This is the cutoff frequency of the reactive power low-pass filter. K p This is the active power droop coefficient. K q This is the reactive power droop factor. P This is the measured value of active power. P ref This is a reference value for active power. V o The voltage amplitude at the grid connection point. V n This is the rated voltage amplitude. Q This is the measured value of reactive power. Q ref This is a reference value for reactive power.

3. The method for optimizing virtual impedance current limiting parameters of a grid-type converter according to claim 1, characterized in that, During the voltage reference generation process, the corrected internal voltage of the grid-type converter satisfies the following expression: in, V in ∠ δ in For internal voltage phasors, R v , X v These are virtual resistance and virtual inductance, respectively. V o ∠ δ o For the grid connection point voltage phasor, I g ∠ φ i This is the output current phasor.

4. The method for optimizing virtual impedance current limiting parameters of a grid-type converter according to claim 3, characterized in that, The system model that couples the differential equation and the algebraic equation is as follows: in, Z g For the equivalent impedance of the power grid, φ g The equivalent impedance angle of the power grid. Z v ∠ φ v This is a virtual impedance.

5. The method for optimizing virtual impedance current limiting parameters of a grid-type converter according to claim 1, characterized in that, In the medium impedance region, the grid connection point voltage is effectively reconstructed, and the system operating point approaches the theoretical optimal operating point. The region, the theoretically optimal operating point The expression is: in, P o Active power X g The reactance is the equivalent impedance of the power grid. E The voltage amplitude of the power grid. The difference in angle of work.

6. The method for optimizing virtual impedance current limiting parameters of a grid-type converter according to claim 1, characterized in that, In the low impedance region, the system operating point is determined by the power outer loop control. The optimal impedance angle selected in this stage satisfies the condition that the virtual impedance voltage drop is in phase with the output current, so that the internal voltage of the grid-connected converter is approximately in phase with the grid connection point voltage, thereby reducing the driving voltage difference.

7. The method for optimizing virtual impedance current limiting parameters of a grid-type converter according to claim 1, characterized in that, Within the medium impedance region, the virtual impedance phase angle range that enables the steady-state current to be kept to a minimum is 20° to 70°.

8. A virtual impedance current limiting parameter optimization device for a grid-type converter, comprising the method described in any one of claims 1-7, characterized in that, include: The model building and control introduction module is used to establish a nonlinear dynamic model of a grid-type converter with fixed virtual impedance, and to introduce active power-frequency droop control, reactive power-voltage droop control and virtual impedance control into the control system of the grid-type converter. The voltage reference correction module is used to correct the internal voltage reference value of the grid converter through virtual impedance during the voltage reference generation process, based on the virtual impedance control link, so that the internal voltage reference value is dynamically adjusted according to the output current change, thereby constructing a system model that couples differential equations and algebraic equations. The parameter scanning and pattern acquisition module is used to perform parameter scanning on the constructed system model under the same grid voltage drop conditions, within a two-dimensional parameter space composed of virtual impedance amplitude and virtual impedance phase angle, to obtain the pattern of the peak output current and steady-state current of the grid-type converter changing with the virtual impedance parameters. The stage feature extraction module is used to divide the virtual impedance amplitude change process into three stages: small impedance region, medium impedance region, and large impedance region, based on the law of the change of the peak output current and steady-state current with the virtual impedance parameter. The parameter optimization module is used to select the medium impedance region as the virtual impedance parameter optimization range in the three divided stages, and determine the optimized design results of virtual impedance amplitude and impedance angle within the medium impedance region with the optimization objective of minimizing steady-state current. The closed-loop verification module is used to perform transient synchronization stability verification on the virtual impedance parameters corresponding to the optimized design results based on the phase plane analysis method.

9. A computer storage medium storing a readable program, characterized in that, When the program runs, it can instruct the computing device to perform the virtual impedance current limiting parameter optimization method for grid-type converters as described in any one of claims 1-7.

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, which causes the processor to perform the operation corresponding to the virtual impedance current limiting parameter optimization method for grid-type converters as described in any one of claims 1-7.

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