A virtual synchronous machine overcurrent capability enhancement system and method

Abstract

The application provides a virtual synchronous machine overcurrent capacity enhancement system and method, and relates to the technical field of power electronics and new energy power generation grid connection control, wherein the system comprises a main power channel, a transient overcurrent channel, an energy regulation unit and a control driving unit, the main power channel comprises a three-phase inverter and an LCL filter; the transient overcurrent channel comprises a thyristor, a current-limiting inductor and a current-limiting reactor; the energy regulation unit comprises a chopper resistor circuit and a super capacitor module, the chopper resistor circuit and the super capacitor module are connected in parallel to the DC bus of the virtual synchronous machine, the chopper resistor circuit comprises a chopper IGBT and a braking resistor connected in series; and the control driving unit is used for controlling the virtual synchronous machine, the thyristor and the chopper IGBT in stages according to the system operating state. The application can overcome the defects that the current limiting of the existing virtual synchronous machine is too strict and the transient energy cannot be fully released when overloaded, so as to improve the transient overcurrent capacity of the virtual synchronous machine.

Description

Technical Field

[0001] This application relates to the fields of power electronics and grid-connected control technology for new energy power generation, and in particular to a virtual synchronous machine overcurrent capacity enhancement system and method. Background Technology

[0002] With the increasing proportion of new energy installed capacity, traditional synchronous generators are gradually being replaced by power electronic converters, significantly weakening the inertia and damping of the power grid and leading to increasingly prominent issues in system frequency and voltage stability. Therefore, Virtual Synchronous Generator (VSG) control technology has been widely researched and applied. This technology simulates the rotor dynamic equations and electromagnetic characteristics of a synchronous generator at the control level, enabling grid-connected converters to possess the inertial response and damping characteristics of a synchronous generator, thereby improving the dynamic stability of the power system.

[0003] However, traditional virtual synchronous generators (VSGs) have significant shortcomings in transient overcurrent capability. Their output current is limited by the safe operating area of ​​the main power devices (such as IGBTs or SiCMOSFETs), requiring hard current limiting protection to prevent device breakdown due to overcurrent. This prevents VSGs from providing transient support current several times their rated current under conditions such as grid short circuits and transient impacts, unlike real synchronous generators. The result is that the current is "flattened," power cannot be effectively released, and the system cannot absorb transient power surges, passively exiting voltage source behavior. This severely impacts the fault ride-through (FRT) capability of grid-connected devices and the system's support capacity.

[0004] The fundamental reason for the weak overload capacity of virtual synchronous machines lies in their "power electronics nature" and "control limiting mechanism." Traditional synchronous machines naturally possess energy buffering capabilities in their mechanical and electromagnetic characteristics: rotor inertia can temporarily release or absorb energy, and magnetic field saturation effects and armature reactions naturally suppress overcurrent. However, the "synchronous behavior" of virtual synchronous machines is entirely simulated by control algorithms, and their energy path is highly dependent on electronic devices and control commands, lacking physical inertia or magnetic field coupling. The current capability of main power switching devices (such as IGBTs) is typically only slightly higher than the rated value by 1.1–1.5 times; exceeding this range will trigger a rapid rise in junction temperature or device breakdown. Therefore, to ensure reliability, the VSG controller must execute hard current limiting within tens of microseconds, resulting in an overcurrent characteristic of "immediate clipping" rather than "decaying over time" transient behavior. This electronic protection logic determines that the short-term overload capacity of virtual synchronous machines is extremely weak, only 1.1–1.2 times the rated current, far lower than the ability of real synchronous machines to output 3–6 times the rated current in a short time.

[0005] To address these issues, existing research has attempted to limit current at the software control level, protecting main power devices through virtual impedance or limiting control. However, the effectiveness remains limited by the device's inherent load-carrying capacity, making it difficult to achieve "controllable and biomimetic" overcurrent characteristics. Furthermore, while using higher current-rated power modules can increase the limiting current, the cost and size increase exponentially, resulting in extremely poor economic efficiency in engineering applications. Therefore, existing virtual synchronous machines still exhibit the characteristics of "fragile power supplies" when facing transient faults, failing to provide effective current support and transient energy absorption.

[0006] Therefore, how to overcome the shortcomings of existing virtual synchronous machines in that the current limiting is too strict and the transient energy cannot be fully released under overload, so as to improve the transient overcurrent capability of virtual synchronous machines, is a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0007] To address the aforementioned technical problems, this application provides a virtual synchronous machine overcurrent capability enhancement system, which overcomes the shortcomings of existing virtual synchronous machines, such as overly strict current limiting and insufficient release of transient energy under overload conditions, thereby improving the transient overcurrent capability of the virtual synchronous machine. This application also provides a virtual synchronous machine overcurrent capability enhancement method, which achieves the same technical effect.

[0008] The first objective of this application is to provide a system for enhancing the overcurrent capability of a virtual synchronous machine.

[0009] The aforementioned objective of this application is achieved through the following technical solution:

[0010] A virtual synchronous machine overcurrent capability enhancement system includes: a main power channel, a transient overcurrent channel, an energy regulation unit, and a control drive unit, wherein:

[0011] The main power channel includes a three-phase inverter and an LCL filter. The three-phase inverter is used to convert DC power into AC power output. The output AC power is connected to the AC power grid after passing through the LCL filter in series.

[0012] The transient overcurrent channel includes a thyristor, a current-limiting inductor, and a current-limiting reactance. The thyristor is connected in series with the current-limiting inductor, and the thyristor is also connected in series with the current-limiting reactance. One end of the transient overcurrent channel closest to the current-limiting reactance is connected to the AC power grid, and the other end is connected to the DC bus of the virtual synchronous machine.

[0013] The energy regulation unit includes a chopper resistor circuit and a supercapacitor module. The chopper resistor circuit and the supercapacitor module are connected in parallel to the DC bus of the virtual synchronous machine. The chopper resistor circuit includes a chopper IGBT and a braking resistor connected in series.

[0014] The control drive unit is used to perform phased control on the virtual synchronous machine, the thyristor, and the chopper IGBT according to the system operating status.

[0015] Preferably, in the virtual synchronous machine overcurrent capability enhancement system, the control drive unit includes: a fault detection module, a hybrid switch drive module, and a chopper current limiting control module, wherein:

[0016] The fault detection module is used to sample the output current and grid connection point voltage from the AC side of the three-phase inverter, identify grid faults based on the output current and grid connection point voltage, and determine the fault level based on preset multi-level fault thresholds;

[0017] The hybrid switch drive module is connected to the gate of the fully controlled power switching device of the three-phase inverter and the gate of the thyristor, respectively.

[0018] The hybrid switch drive module is used to control the on and off of the fully controlled power switch device under normal grid operation, and to trigger the thyristor to conduct when a grid fault occurs, and to cooperate with the fully controlled power switch device to resume operation after the grid fault is cleared, thereby realizing the exit of the transient overcurrent channel.

[0019] The chopper current limiting control module is connected to the gate of the chopper IGBT;

[0020] The chopper current limiting control module is used to output a corresponding PWM signal to the chopper IGBT according to a preset current target curve during a power grid fault.

[0021] Preferably, in the virtual synchronous machine overcurrent capability enhancement system, the control drive unit is specifically used for:

[0022] During normal operation, the grid connection point voltage V-pcc and output current are sampled in real time. And obtain the equivalent junction temperature parameter Tj of the power semiconductor device;

[0023] Based on the equivalent junction temperature parameter Tj and the output current And a preset device safe operating area model is used to calculate the maximum allowable output current limit I_max(t) that varies with time, which is used to constrain the output current command amplitude of the virtual synchronous machine and control the virtual synchronous machine to work under the preset droop control strategy.

[0024] Entering the overcurrent detection phase, the output current is sampled in real time. After first-order filtering, the filtered current is obtained. ;

[0025] judge Is it less than and If the rate of change is less than or equal to the preset first rate of change threshold, then maintain the normal operation phase.

[0026] judge Does it meet the requirements? ,or If the current exceeds a preset first rate of change threshold, then proceed to the hybrid current limiting stage, adjust the virtual impedance parameter to reduce the system output impedance, and then determine... Is it greater than And whether the Safe Operating Area (SOA) is in an available state; if so, then turn on the thyristor.

[0027] judge Is it greater than Or has the DC bus voltage reached the set threshold? If so, then the deep current limiting stage is entered, and the chopper resistor circuit is turned on.

[0028] judge Whether from Descending to The following should maintain the first set duration, or If the value is less than or equal to a preset second rate of change threshold and remains at that threshold for a second set duration, then the fault recovery phase begins. The chopper resistor circuit and the thyristor are sequentially shut down, and the virtual reactance is restored to a preset virtual reactance rating. ;

[0029] in, , This indicates a preset first current threshold. This indicates a preset second current threshold. Indicates the preset current rating. This indicates the current threshold that allows access to the transient overcurrent channel. Indicates the filter current The rate of change relative to time Indicates filter voltage The rate of change of PVF with respect to time, the filtered voltage pvf is obtained by first-order low-pass filtering of the DC side voltage V_pv.

[0030] Preferably, in the virtual synchronous machine overcurrent capability enhancement system, when the control drive unit performs the adjustment of the virtual impedance parameter to reduce the system output impedance, it is specifically used for:

[0031] The virtual impedance parameter is adjusted according to a preset adjustment formula to reduce the system output impedance. The preset adjustment formula is as follows:

[0032]

[0033] in, This represents the virtual reactance at time t. This represents the output current at time t. This represents the impedance dynamic adjustment coefficient.

[0034] Preferably, in the virtual synchronous machine overcurrent capacity enhancement system, after the control drive unit turns on the thyristor, it introduces the thyristor branch reactance. To adjust the system's equivalent impedance, the specific calculation formula is as follows:

[0035]

[0036] in, This represents the adjusted equivalent impedance of the system. Indicates the main channel impedance. Represents the imaginary unit. Represents angular frequency. This indicates a current-limiting inductor.

[0037] Preferably, in the virtual synchronous machine overcurrent capability enhancement system, after the control drive unit turns on the chopper resistor circuit, the calculation formula for the power dissipation of the chopper resistor circuit is:

[0038]

[0039] in, This represents the power dissipation at time t. This represents the duty cycle of the chopper IGBT at time t. This indicates the resistance value of the braking resistor. This represents the DC bus voltage at time t.

[0040] Preferably, in the virtual synchronous machine overcurrent capability enhancement system, the control drive unit, when performing the restoration of virtual reactance to a preset virtual reactance rated value... When, specifically used for:

[0041] According to the preset recovery formula, the virtual reactance is restored to the preset virtual reactance rated value. The preset recovery formula is as follows:

[0042]

[0043] in, Indicates the time when the fault is cleared. express Virtual reactance at any moment This represents the time recovery constant.

[0044] Preferably, in the virtual synchronous machine overcurrent capability enhancement system, the control drive unit, when executing the operation based on the equivalent junction temperature parameter Tj and the output current... And based on the preset device safe operating area model, when calculating the maximum permissible output current limit I_max(t) that varies with time, it is specifically used for:

[0045] Based on the equivalent junction temperature parameter Tj and the output current And based on the preset device safe working area model, the available safety margin SOA_budget of the current system is calculated;

[0046] Based on the aforementioned safety margin SOA_budget, the maximum permissible output current limit I_max(t) that varies over time is generated.

[0047] Preferably, in the virtual synchronous machine overcurrent capability enhancement system, the control drive unit, when performing the step of determining whether the secure working area (SOA) is in an available state, is specifically used for:

[0048] Based on the real-time sampling of the output current The output current is calculated. rate of change relative to time ;

[0049] judge Is it less than or equal to a preset third rate of change threshold, and Is it less than or equal to? Furthermore, the transient junction temperature calculated by combining the preset device safe operating area model and the thermal model is determined to be less than or equal to the preset upper temperature limit. If it is, the safe operating area SOA is determined to be in an available state; otherwise, the safe operating area SOA is determined to be in an unavailable state.

[0050] The second objective of this application is to provide a method for enhancing the overcurrent capability of a virtual synchronous machine.

[0051] The second objective of this application is achieved through the following technical solution:

[0052] A method for enhancing the overcurrent capability of a virtual synchronous machine, applied to the virtual synchronous machine overcurrent capability enhancement system described above, includes:

[0053] During normal operation, the grid connection point voltage V-pcc and output current are sampled in real time. And obtain the equivalent junction temperature parameter Tj of the power semiconductor device;

[0054] Based on the equivalent junction temperature parameter Tj and the output current And a preset device safe operating area model is used to calculate the maximum allowable output current limit I_max(t) that varies with time, which is used to constrain the output current command amplitude of the virtual synchronous machine and control the virtual synchronous machine to work under the preset droop control strategy.

[0055] Entering the overcurrent detection phase, the output current is sampled in real time. After first-order filtering, the filtered current is obtained. ;

[0056] judge Is it less than and If the rate of change is less than or equal to the preset first rate of change threshold, then maintain the normal operation phase.

[0057] judge Does it meet the requirements? ,or If the current exceeds a preset first rate of change threshold, then proceed to the hybrid current limiting stage, adjust the virtual impedance parameter to reduce the system output impedance, and then determine... Is it greater than And check if the Safe Operating Area (SOA) is available; if so, turn on the thyristor.

[0058] judge Is it greater than Or has the DC bus voltage reached the set threshold? If so, then the deep current limiting stage is entered, and the chopper resistor circuit is turned on.

[0059] judge Whether from Descending to The following should maintain the first set duration, or If the value is less than or equal to a preset second rate of change threshold and remains at that threshold for a second set duration, then the fault recovery phase begins. The chopper resistor circuit and the thyristor are sequentially shut down, and the virtual reactance is restored to a preset virtual reactance rating. ;

[0060] in, , This indicates a preset first current threshold. This indicates a preset second current threshold. Indicates the preset current rating. This indicates the current threshold that allows access to the transient overcurrent channel. Indicates the filter current The rate of change relative to time Indicates filter voltage The rate of change of PVF with respect to time, the filtered voltage pvf is obtained by first-order low-pass filtering of the DC side voltage V_pv.

[0061] The above technical solution includes a main power channel comprising a three-phase inverter and an LCL filter. The three-phase inverter converts DC power to AC power output, and the output AC power is connected to the AC power grid after passing through the series LCL filter, realizing normal bidirectional energy transmission. The transient overcurrent channel includes a thyristor, a current-limiting inductor, and a current-limiting reactor. The thyristor is connected in series with the current-limiting inductor and also in series with the current-limiting reactor. The end of the transient overcurrent channel closest to the current-limiting reactor is connected to the AC power grid, and the other end is connected to the DC bus of the virtual synchronous machine. When a transient overcurrent signal is detected, the thyristor conducts, providing a low-impedance path for the system, allowing the DC energy storage to release energy through the inductor to support the AC side. The energy regulation unit includes a chopper resistor circuit and a supercapacitor module. The chopper resistor circuit and the supercapacitor module are connected in parallel to the DC bus of the virtual synchronous machine. The chopper resistor circuit includes a chopper IGBT and a braking resistor connected in series. By controlling the duty cycle of the chopper IGBT, it actively consumes energy on the DC bus, thereby precisely controlling the DC side voltage drop curve and "shaping" and limiting the amplitude and attenuation characteristics of the AC side fault current from the source. The control drive unit is used to perform staged control of the virtual synchronous machine, thyristors, and chopper IGBT according to the system operating status. This allows the system to temporarily reduce the equivalent reactance during transient overcurrent, forming a short-time overcurrent response consistent with flux linkage conservation, thereby achieving a 3-5 times performance enhancement in transient current while avoiding device overheating or bus overvoltage. Unlike the pure limiting protection of existing VSGs, the above technical solution can dynamically adjust the system's equivalent reactance and bus voltage under energy conservation constraints, realizing short-time power response and system self-protection functions at the synchronous machine level.

[0062] In summary, the above technical solution can overcome the shortcomings of existing virtual synchronous machines, such as excessively strict current limiting and insufficient release of transient energy under overload, thereby improving the transient overcurrent capability of virtual synchronous machines. Attached Figure Description

[0063] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0064] Figure 1 This is a schematic diagram of the structure of a virtual synchronous machine overcurrent capability enhancement system according to an embodiment of this application;

[0065] Figure 2This is a schematic diagram of the overall topology of a virtual synchronous machine overcurrent capability enhancement system according to an embodiment of this application;

[0066] Figure 3 This is a flowchart illustrating a method for enhancing the overcurrent capability of a virtual synchronous machine according to an embodiment of this application. Detailed Implementation

[0067] To enable those skilled in the art to better understand the technical solutions in this application, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0068] In the embodiments provided in this application, it should be understood that the disclosed methods and systems can be implemented in other ways. The system embodiments described below are merely illustrative. For example, the division of units and modules is only a logical functional division, and in actual implementation, there may be other division methods, such as: multiple units or modules can be combined, or integrated into another system, or some features can be ignored or not executed. In addition, the coupling, direct coupling, or communication connection between the various components shown or discussed can be through some interfaces, indirect coupling or communication connection of devices or modules, and can be electrical, mechanical, or other forms.

[0069] In addition, each functional unit in the various embodiments of this application can be integrated into a single processor, or each unit can be a separate device, or two or more units can be integrated into a single device; each functional unit in the various embodiments of this application can be implemented in hardware or in the form of hardware plus software functional units.

[0070] Those skilled in the art will understand that all or part of the steps of the following method embodiments can be implemented by program instructions and related hardware. The aforementioned program instructions can be stored in a computer-readable storage medium. When the program instructions are executed, they perform the steps of the following method embodiments. The aforementioned storage medium includes various media capable of storing program code, such as mobile storage devices, read-only memory (ROM), magnetic disks, or optical disks.

[0071] It should be understood that the use of terms such as "system," "device," "unit," and / or "module" in this application is merely one method of distinguishing different components, elements, parts, sections, or assemblies at different levels. However, if other terms can achieve the same purpose, they may be replaced by other expressions.

[0072] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this application, "a plurality of" or "several" means two or more, unless otherwise explicitly specified.

[0073] If a flowchart is used in this application, it is used to illustrate the operations performed by the system according to embodiments of this application. It should be understood that the preceding or following operations are not necessarily performed precisely in sequence. Instead, the steps can be processed in reverse order or simultaneously. Furthermore, other operations can be added to these processes, or one or more steps can be removed from them.

[0074] It should also be noted that, in this document, terms such as “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that an article or device that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such an article or device. Without further limitation, an element defined by the phrase “comprising one…” does not exclude the presence of other identical elements in the article or device that includes the aforementioned element.

[0075] The embodiments in this application are written in a progressive manner.

[0076] like Figure 1 As shown, this application provides a virtual synchronous machine overcurrent capability enhancement system, including: a main power channel 1, a transient overcurrent channel 2, an energy regulation unit 3, and a control drive unit 4, wherein:

[0077] Main power channel 1 includes a three-phase inverter and an LCL filter. The three-phase inverter is used to convert DC power into AC power output. The output AC power is connected to the AC power grid after passing through the series LCL filter.

[0078] The transient overcurrent channel 2 includes a thyristor, a current-limiting inductor, and a current-limiting reactance. The thyristor is connected in series with the current-limiting inductor and also in series with the current-limiting reactance. One end of the transient overcurrent channel 2 closest to the current-limiting reactance is connected to the AC power grid, and the other end is connected to the DC bus of the virtual synchronous machine.

[0079] The energy regulation unit 3 includes a chopper resistor circuit and a supercapacitor module. The chopper resistor circuit and the supercapacitor module are connected in parallel to the DC bus of the virtual synchronous machine. The chopper resistor circuit includes a chopper IGBT and a braking resistor connected in series.

[0080] The control drive unit 4 is used to perform phased control of the virtual synchronous machine, thyristor and chopper IGBT according to the system operating status.

[0081] Specifically, the thyristors in transient overcurrent channel 2 can be anti-parallel thyristors or bidirectional thyristors, combined with... Figure 2 As shown, Pcc represents the Point of Common Coupling; V-pcc represents the voltage at the PCC; Z-grid represents the grid impedance; the LCL filter includes the inverter-side inductor Lf, the filter capacitor Cf, the grid-side inductor Lg, If represents the filter current, i.e., the current flowing through the inverter-side inductor Lf; and Cdc represents the DC bus capacitance. Taking the thyristor in transient overcurrent channel 2 as a bidirectional thyristor as an example, its corresponding... Figure 2 The bidirectional SCR in the figure is used to provide a bidirectional transient overcurrent path during system faults; Xs represents the current-limiting reactor connected in series with the thyristor (bidirectional SCR) to limit the rate of current rise through the thyristor channel during transient overcurrents.

[0082] The GFM controller, or grid-forming inverter (GFM), is the core control unit used to simulate the voltage source characteristics of a synchronous generator. During normal operation, it is responsible for maintaining the stability of the grid connection point voltage and frequency; during faults, it is responsible for generating current reference commands.

[0083] A hybrid current limiter refers to a composite current limiting algorithm. It combines "circular / elliptical limiting" (used to limit the magnitude of the current vector) with "VI characteristics" (droop control characteristics used to adjust reactive current output based on voltage drop depth).

[0084] Z_virt refers to the software-simulated impedance introduced in the control algorithm, rather than the physical impedance. It is used to increase the equivalent damping of the system, suppress current surges and power oscillations during faults, and improve the transient stability of the system.

[0085] I_ref refers to the target current command signal that the inverter should output, calculated by the controller.

[0086] The anti-integral saturation channel refers to the anti-integral saturation logic in the PI controller. When the system enters a limiting state (such as fault current limiting), this logic is used to freeze or decay the integral term to prevent the integrator from continuously accumulating errors, which could lead to system overshoot or instability, and to ensure that the system can quickly and smoothly transition from fault recovery.

[0087] SOA Sensing Budget Analyzer refers to a model-based safe operating area estimation module. It calculates the instantaneous junction temperature (Tj) and current change rate (di / dt) of the device in real time based on real-time sampled current, voltage, and thermal models, and compares them with the device's safe operating limits.

[0088] SCR trigger / cut-off and phase synchronization refers to the drive logic unit of the thyristor. It performs phase synchronization based on the zero-crossing signal of the grid connection point voltage to achieve voltage-oriented control, ensuring that the thyristor turns on in the correct half-cycle and turns off naturally at zero crossing.

[0089] The three-phase inverter is specifically a three-phase full-bridge inverter, with each bridge arm employing fully controlled power switching devices (such as IGBTs and SiC MOSFETs). During normal operation, it executes PWM modulation to achieve active and reactive power control of a virtual synchronous machine. Main power channel 1 consists of the three-phase inverter and an LCL filter, enabling normal bidirectional energy transfer. Its DC bus dynamic equation is:

[0090]

[0091] in, This represents the DC bus capacitance. Indicates the DC bus voltage. This indicates the current threshold that allows access to transient overcurrent channel 2. Indicates inverter current. This represents the chopper branch current. This formula describes the basic law of energy distribution in the DC bus and provides a basis for subsequent transient energy analysis.

[0092] The transient overcurrent channel 2 includes a thyristor, a current-limiting inductor, and a current-limiting reactance. The thyristor is connected in series with the current-limiting inductor and also in series with the current-limiting reactance. One end of the transient overcurrent channel 2 closest to the current-limiting reactance is connected to the AC power grid, and the other end is connected to the DC bus of the virtual synchronous machine. The thyristor in the transient overcurrent channel 2 can be an anti-parallel thyristor or a bidirectional thyristor. The thyristor constitutes the main conducting device of the transient overcurrent channel 2, providing a bypass path for current when a system fault occurs. When an overcurrent signal is detected, the thyristor is triggered to conduct by the control drive unit 4, providing a low-impedance path to the main circuit, allowing the DC energy storage to release energy through the inductor to support the AC side. The current-limiting reactance is used to limit the rate of current rise through the thyristor channel during transient overcurrent. The system energy relationship can be simplified as follows:

[0093]

[0094] in, Indicates the DC-side input power. Indicates AC output power. This represents system losses. The equation shows that, under the constraint of energy conservation, the energy required for transient overcurrent is released by the DC energy storage unit.

[0095] Energy regulation unit 3 includes a chopper resistor circuit and a supercapacitor module. The chopper resistor circuit and the supercapacitor module are connected in parallel to the DC bus of the virtual synchronous machine. The supercapacitor module is used to provide the energy required for instantaneous high-power pulses during transient processes. The chopper resistor circuit includes a chopper IGBT and a braking resistor connected in series. It is used to dissipate excess energy on the DC side during system transient or fault conditions. This circuit actively consumes energy on the DC bus by controlling the duty cycle of the chopper IGBT, thereby precisely controlling the voltage drop curve of the DC side and "shaping" and limiting the amplitude and attenuation characteristics of the AC side fault current from the source.

[0096] The control drive unit 4 is used to perform phased control on the virtual synchronous machine, thyristor and chopper IGBT according to the system operating status, so that the system can temporarily reduce the equivalent reactance during transient overcurrent, forming a short-time overcurrent response consistent with the magnetic flux conservation, thereby enhancing the performance of transient current, while avoiding device overheating or bus overvoltage.

[0097] In some embodiments, the control drive unit 4 includes: a fault detection module, a hybrid switch drive module, and a chopper current limiting control module, wherein:

[0098] The fault detection module is used to sample the output current and grid connection point voltage from the AC side of the three-phase inverter. It identifies grid faults based on the output current and grid connection point voltage, and determines the fault level based on preset multi-level fault thresholds. This module draws on the concept of graded current limiting and can preset multi-level fault thresholds.

[0099] The hybrid switch drive module is connected to the gate of the fully controlled power switching device and the gate of the thyristor of the three-phase inverter respectively. The hybrid switch drive module is used to control the on and off of the fully controlled power switching device under normal grid operation, and to trigger the thyristor to conduct when a grid fault occurs, so as to access the transient overcurrent channel 2. After the grid fault is cleared, it cooperates with the resumption of operation of the fully controlled power switching device to realize the exit of the transient overcurrent channel 2.

[0100] The chopper current limiting control module is connected to the gate of the chopper IGBT. During grid faults, the module outputs a corresponding PWM signal to the chopper IGBT based on a preset current target curve. Specifically, the current target curve is a current modulation signal calculated based on the DC bus voltage deviation.

[0101] In other embodiments, the system operation states include the following stages: normal operation stage, overcurrent detection stage, hybrid current limiting stage, deep current limiting stage, and fault recovery stage. Through phased dynamic control, the system can achieve rapid current limiting, energy redistribution, and smooth recovery during overload and fault conditions. Control drive unit 4 is specifically used for:

[0102] During normal operation, the grid connection point voltage V-pcc and output current are sampled in real time. And obtain the equivalent junction temperature parameter Tj of the power semiconductor device;

[0103] Based on the equivalent junction temperature parameter Tj and output current And a preset device safe operating area model is used to calculate the maximum allowable output current limit I_max(t) that varies with time, which is used to constrain the output current command amplitude of the virtual synchronous machine and control the virtual synchronous machine to work under the preset droop control strategy.

[0104] Specifically, during normal operation, the control drive unit 4 samples the grid connection point voltage V-pcc and output current in real time. The equivalent junction temperature parameter Tj of the power semiconductor device is obtained. The grid connection point voltage V-pcc primarily serves as a phase reference. When the grid connection point voltage V-pcc is detected to be greater than 0, the drive unit 4 triggers the upper bridge arm thyristor of the bypass switch; when the grid connection point voltage V-pcc is detected to be less than or equal to 0, the drive unit 4 triggers the lower bridge arm thyristor of the bypass switch. The equivalent junction temperature parameter Tj is used to characterize the thermal stress state of the fully controlled power switching device in the three-phase inverter under the current operating conditions. The equivalent junction temperature parameter Tj can be indirectly obtained through a device temperature model or a current-loss mapping relationship; this application does not impose specific limitations on this. Then, based on the equivalent junction temperature parameter Tj and the output current... Based on a pre-defined Secure Operating Area (SOA) model, the available safety margin SOA_budget for the current system is calculated. SOA_budget reflects the residual current and thermal capacity that the power device can withstand without exceeding the SOA. Furthermore, based on SOA_budget, a time-varying maximum allowable output current limit I_max(t) is generated. I_max(t) constrains the output current command amplitude of the virtual synchronizer in subsequent control phases to prevent the power device from entering overcurrent or overheating states. The virtual synchronizer is controlled to operate according to a pre-set standard droop control strategy to maintain voltage and frequency stability. The output voltage reference model is:

[0105]

[0106]

[0107] in, Indicates the output voltage. This represents the internal potential of the virtual synchronizer. Indicates the output current. Represents virtual impedance. Indicates virtual resistance. Represents virtual reactance. This represents the imaginary unit. At this stage, the virtual reactance... Maintain at the preset virtual reactance rating. The main inverter handles all power transmission. The system current satisfies:

[0108]

[0109] in, This indicates the main channel impedance. The main power channel 1 and the energy storage unit are in an energy balance state, and the thyristor and chopper branch remain closed.

[0110] Then, the overcurrent detection phase begins, where the output current is sampled in real time. After first-order filtering, the filtered current is obtained. ;

[0111] Specifically, the control drive unit 4 samples the output current in real time, and obtains a smooth filtered current after first-order filtering. The calculation formula is as follows:

[0112]

[0113]

[0114] in, This represents the output current at the k-th sampling time. This represents the filter current at the k-th sampling time. This represents the filter current at the (k-1)th sampling time. Represents the filter coefficients. Indicates the sampling period. This represents the filtering time constant.

[0115] Next, the control drive unit 4 performs a graded judgment based on the relationship between the filtered current and the preset threshold:

[0116] judge Is it less than and If the rate of change is less than or equal to the preset first rate of change threshold, then maintain the normal operation phase.

[0117] Specifically, This indicates the preset first current threshold, the specific value of which can be set based on actual needs; Represents the filter current The rate of change relative to time is used to characterize the degree of abrupt change in current. When < and When the value is less than or equal to the preset first rate of change threshold, the system is determined to be in normal operation state, maintains the normal operation stage, and does not enter the mixed current limiting stage. The specific value of the preset first rate of change threshold can be set based on actual needs.

[0118] judge Does it meet the requirements? ,or If the current exceeds a preset first rate of change threshold, then proceed to the hybrid current limiting stage, adjust the virtual impedance parameter to reduce the system output impedance, and then determine... Is it greater than And check if the Safe Operating Area (SOA) is available; if so, turn on the thyristor.

[0119] Specifically, This indicates a preset second current threshold. > , The specific value can be set based on actual needs; when satisfy Then it enters the mixed current limiting phase, or When the preset first rate of change threshold is reached, if a sudden current disturbance or short-circuit fault is detected in the system, it will enter the hybrid current limiting stage, adjusting the virtual impedance parameter to reduce the system output impedance. Then, a judgment is made. Is it greater than And whether the Secure Workspace (SOA) is in an available state, among which, This indicates the current threshold that allows access to transient overcurrent channel 2, which can be set based on actual needs. Determining whether the Safe Operating Area (SOA) is available specifically includes: based on real-time sampled output current. The output current is calculated. rate of change relative to time ;judge Is it less than or equal to a preset third rate of change threshold, and Is it less than or equal to? Furthermore, the transient junction temperature calculated using the preset device safe operating area (SOA) model and thermal model is checked against the preset upper temperature limit. If the SOA is less than or equal to the preset upper temperature limit, it is determined that the SOA is in a usable state; otherwise, it is determined that the SOA is not in a usable state. The specific values ​​of the preset third rate of change threshold and the preset upper temperature limit can be set based on actual needs. Less than or equal to Alternatively, if the secure working area SOA is not available, maintain the rate limiting adjustment or strengthen the rate limiting control strategy (increase Z_virt). Greater than Furthermore, when the safe working area (SOA) is in an available state, the thyristor is turned on to connect to the transient overcurrent channel 2.

[0120] judge Is it greater than Or has the DC bus voltage reached the set threshold? If so, then the deep current limiting stage is entered, and the chopper resistor circuit is turned on.

[0121] Specifically, when Greater than At that time, or when the DC bus voltage rises to a set threshold. At this point, the system enters the deep current limiting phase, and the chopper resistor circuit is activated to dissipate excess energy on the DC side. A threshold value is set within this threshold. The specific value can be set based on actual needs.

[0122] judge Whether from Descending to The following should maintain the first set duration, or If the value is less than or equal to a preset second rate of change threshold and remains at that threshold for a second set duration, then the fault recovery phase begins. The chopper resistor circuit and thyristor are sequentially shut down, and the virtual reactance is restored to the preset virtual reactance rating. ;

[0123] Specifically, , This indicates the preset current rating, ensuring a smooth transition of the system under different overload levels and preventing false triggering. Indicates filter voltage The rate of change of PVF with respect to time, the filter voltage PVF is obtained by applying a first-order low-pass filter to the DC-side voltage V_pv. The DC-side voltage V_pv can be obtained by sampling and then performing a first-order low-pass filter to obtain the filtered voltage. PVF, and calculate its voltage change rate based on discrete sampling results, using the following formula: ≈ ( pvf(k)- pvf(k-1)) / T_s; where T_s is the sampling period, and k represents the current sampling time. from Descending to If the first set duration is maintained, the fault recovery phase can be initiated, or When the voltage change rate is less than or equal to the preset second rate of change threshold and remains at the second set duration, it can be considered that the voltage change rate has entered the slow change range. This is used to characterize the end of the system transient process and the transition of DC-side energy exchange from a violent state to a stable state, thus entering the fault recovery stage. The recovery actions of shutting down the chopper resistor circuit and the thyristor are executed sequentially, and the virtual reactance is restored to the preset virtual reactance rating. Conversely, if the DC side is determined to be in a significant transient change phase, the control drive unit 4 maintains the current current limiting or energy dissipation control strategy until the above criteria are met. The first set duration, the second set duration, and the preset second rate of change threshold can be set based on actual needs.

[0124] In some embodiments, when the control drive unit 4 performs the adjustment of the virtual impedance parameter to reduce the system output impedance, it is specifically used for:

[0125] The virtual impedance parameter is adjusted according to a preset adjustment formula to reduce the system output impedance. The preset adjustment formula is as follows:

[0126]

[0127] in, This represents the virtual reactance at time t. This represents the output current at time t. This represents the impedance dynamic adjustment coefficient.

[0128] In this embodiment, when a transient overcurrent is detected, the control drive unit 4 first adjusts the virtual impedance parameter to rapidly reduce the system output impedance and improve the current response capability. By controlling the virtual reactance to decrease, the system current is accelerated to rise, thus establishing short-term energy support conditions.

[0129] In some embodiments, after the control drive unit 4 turns on the thyristor, the thyristor branch reactance is introduced. To adjust the system's equivalent impedance, the specific calculation formula is as follows:

[0130]

[0131] in, This represents the adjusted equivalent impedance of the system. Indicates the main channel impedance. Represents the imaginary unit. Represents angular frequency. This indicates a current-limiting inductor.

[0132] when At this time, the system transient current amplifies significantly, and the energy stored on the DC side is released to the AC side through a low-impedance channel. The peak value of the transient current is approximately 3 to 5 times the rated current, and its ratio can be determined by the following formula:

[0133]

[0134] in, This indicates the conduction current (transient peak current). This indicates the turn-off current (steady-state current). This represents the system's equivalent reactance. It can be seen that the virtual impedance and the thyristor branch work together to form a "hardware-software combined" current-limiting channel, which enhances transient output capability while maintaining system voltage stability.

[0135] In some embodiments, after the control drive unit 4 turns on the chopper resistor circuit, the formula for calculating the power dissipation of the chopper resistor circuit is:

[0136]

[0137] in, This represents the power dissipation at time t. This represents the duty cycle of the chopper IGBT at time t. This indicates the resistance value of the braking resistor. This represents the DC bus voltage at time t. By adjusting... This enables dynamic stability of bus voltage and control of energy consumption.

[0138] In some embodiments, when The current drops to the preset first current threshold. The following will maintain the first set duration or When the rate of change is less than or equal to a preset second rate of change threshold and remains at a second set duration, the control drive unit 4 sequentially shuts down the chopper resistor circuit and the thyristor, while simultaneously restoring the virtual reactance to the preset virtual reactance rated value. The recovery process follows a first-order smoothing law. Specifically, the control drive unit 4 restores the virtual reactance to the preset virtual reactance rating. When, specifically used for:

[0139] According to the preset recovery formula, the virtual reactance is restored to the preset virtual reactance rated value. The preset recovery formula is:

[0140]

[0141] in, This indicates the time when the fault is cleared, that is, the moment when the system detects that the grid connection point voltage has recovered to the normal operating threshold range, and determines that the aforementioned conditions for entering the 'fault recovery phase' are met. express Virtual reactance at any moment This represents the time recovery constant. During this process, the system current and voltage gradually return to steady state, avoiding secondary shocks and achieving smooth grid-connected recovery.

[0142] To achieve transient overcurrent capability in virtual synchronous generator (VSG) type devices, two physical conditions must be met simultaneously: first, a sufficient instantaneous energy source must be available; second, a low-impedance path must be available to allow large currents to pass through. This application achieves both conditions by using a hybrid switch (thyristor branch) and a chopper branch.

[0143] (a) Energy source: Controlled release of DC energy storage:

[0144] When a system fault or heavy load occurs, the output power of the three-phase inverter increases sharply. Control drive unit 4 allows the DC bus voltage to drop briefly to release energy. The change in DC-side energy can be expressed as:

[0145]

[0146] in, Indicates within the time interval [ , Internal DC side energy change Indicates AC output power. This indicates the power consumed by chopping energy. This indicates system losses.

[0147] Based on the energy storage element model, we can obtain:

[0148]

[0149] in, Indicates bus capacitance. Indicates supercapacitor, Indicates the battery terminal voltage. Indicates the battery terminal current. This represents the initial steady-state voltage of the DC bus before the fault occurred.

[0150] During this stage, the system maintains the output current by releasing energy through DC energy storage, while the chopper resistor circuit adjusts the duty cycle. Controlling the rate of energy dissipation:

[0151]

[0152] This controlled release and absorption mechanism ensures the stability of the bus voltage and the safety of power devices.

[0153] (II) Impedance dynamic reconstruction and energy transfer:

[0154] After overcurrent detection, control drive unit 4 triggers the thyristor branch to conduct, establishing a low-impedance energy path to rapidly release the DC stored energy. At this point, the total equivalent impedance of the system is:

[0155]

[0156] in, This represents the adjusted equivalent impedance of the system. Indicates the main channel impedance. The virtual reactance is adjusted by the control drive unit 4; This is the reactance of the thyristor branch. Because... Much smaller than the main channel impedance, the system's equivalent impedance after conduction The significant decrease allows the transient current to rapidly increase to 3 to 5 times the rated current. This indicates that the virtual impedance reduction of the control layer and the low-resistance path of the physical branch work synergistically to achieve rapid energy transfer.

[0157] (III) Conservation of magnetic flux and transient energy balance:

[0158] From the perspective of motor equivalence, a virtual synchronous machine can be approximated as a combination of a controlled voltage source and an inductive load. Under brief disturbances, the internal flux linkage of the system remains essentially conserved, and the relationship is as follows:

[0159]

[0160] in, Indicates the d-axis flux linkage. Indicates the d-axis inductance. This is the equivalent excitation flux linkage. Represents the d-axis current. Represents electromotive force. It represents angular frequency.

[0161] Based on the electromagnetic energy balance relationship:

[0162]

[0163] It can be seen that when the equivalent impedance When the flux linkage is reduced, the d-axis current... The current will inevitably increase. Therefore, the system achieves transient current amplification by reducing impedance, while the DC-side energy storage unit provides energy compensation to maintain the dynamic balance between the output power of the virtual synchronous machine and the flux linkage energy.

[0164] In the above embodiment, the main power channel 1 includes a three-phase inverter and an LCL filter. The three-phase inverter is used to convert DC power into AC power output. The output AC power is connected to the AC power grid after passing through the series LCL filter, realizing normal bidirectional energy transmission. The transient overcurrent channel 2 includes a thyristor, a current-limiting inductor, and a current-limiting reactance. The thyristor is connected in series with the current-limiting inductor and also in series with the current-limiting reactance. One end of the transient overcurrent channel 2 near the current-limiting reactance is connected to the AC power grid, and the other end is connected to the DC bus of the virtual synchronous machine. When a transient overcurrent signal is detected, the thyristor is turned on, providing a low-impedance path for the system, allowing the DC energy storage to release energy through the inductor to support the AC side. Energy regulation unit 3 includes a chopper resistor circuit and a supercapacitor module. The chopper resistor circuit and the supercapacitor module are connected in parallel to the DC bus of the virtual synchronous machine. The chopper resistor circuit includes a chopper IGBT and a braking resistor connected in series. By controlling the duty cycle of the chopper IGBT, it actively consumes energy on the DC bus, thereby precisely controlling the DC side voltage drop curve and "shaping" and limiting the amplitude and attenuation characteristics of the AC side fault current from the source. Control drive unit 4 is used to perform phased control of the virtual synchronous machine, thyristors, and chopper IGBT according to the system operating status. This allows the system to temporarily reduce the equivalent reactance during transient overcurrent, forming a short-time overcurrent response consistent with flux conservation, thereby achieving a 3-5 times increase in transient current performance while avoiding device overheating or bus overvoltage. Unlike the pure limiting protection of existing VSGs, the above embodiment can dynamically adjust the system's equivalent reactance and bus voltage under energy conservation constraints, realizing short-time power response and system self-protection functions at the synchronous machine level.

[0165] In summary, the above embodiments can overcome the shortcomings of existing virtual synchronous machines, such as excessively strict current limiting and insufficient release of transient energy under overload, thereby improving the transient overcurrent capability of virtual synchronous machines.

[0166] like Figure 3 As shown, in another embodiment of this application, a method for enhancing the overcurrent capability of a virtual synchronous machine is provided, applied to the virtual synchronous machine overcurrent capability enhancement system described above, comprising:

[0167] S101. During normal operation, the grid connection point voltage V-pcc and output current are sampled in real time. And obtain the equivalent junction temperature parameter Tj of the power semiconductor device;

[0168] S102. Based on equivalent junction temperature parameter Tj and output current And a preset device safe operating area model is used to calculate the maximum allowable output current limit I_max(t) that varies with time, which is used to constrain the output current command amplitude of the virtual synchronous machine and control the virtual synchronous machine to work under the preset droop control strategy.

[0169] S103. Enter the overcurrent detection stage and sample the output current in real time. After first-order filtering, the filtered current is obtained. ;

[0170] S104. Judgment Is it less than and If the rate of change is less than or equal to the preset first rate of change threshold, then maintain the normal operation phase.

[0171] S105. Judgment Does it meet the requirements? ,or If the current exceeds a preset first rate of change threshold, then proceed to the hybrid current limiting stage, adjust the virtual impedance parameter to reduce the system output impedance, and then determine... Is it greater than And check if the Safe Operating Area (SOA) is available; if so, turn on the thyristor.

[0172] S106. Judgment Is it greater than Or has the DC bus voltage reached the set threshold? If so, then the deep current limiting stage is entered, and the chopper resistor circuit is turned on.

[0173] S107. Judgment Whether from Descending to The following should maintain the first set duration, or If the value is less than or equal to a preset second rate of change threshold and remains at that threshold for a second set duration, then the fault recovery phase begins. The chopper resistor circuit and thyristor are sequentially shut down, and the virtual reactance is restored to the preset virtual reactance rating. ;

[0174] in, , This indicates a preset first current threshold. This indicates a preset second current threshold. Indicates the preset current rating. This indicates the current threshold that allows access to the transient overcurrent channel. Represents the filter current The rate of change relative to time Indicates filter voltage The rate of change of PVF with respect to time, the filter voltage pvf is obtained by first-order low-pass filtering of the DC side voltage V_pv.

[0175] In other embodiments of this application, the specific implementation of the step of adjusting the virtual impedance parameter to reduce the system output impedance in the above-described method for enhancing the overcurrent capability of a virtual synchronous machine includes:

[0176] The virtual impedance parameter is adjusted according to a preset adjustment formula to reduce the system output impedance. The preset adjustment formula is as follows:

[0177]

[0178] in, This represents the virtual reactance at time t. This represents the output current at time t. This represents the impedance dynamic adjustment coefficient.

[0179] In other embodiments of this application, in the above-described method for enhancing the overcurrent capability of a virtual synchronous machine, the virtual reactance is restored to a preset virtual reactance rating. The specific implementation methods of the steps include:

[0180] According to the preset recovery formula, the virtual reactance is restored to the preset virtual reactance rated value. The preset recovery formula is:

[0181]

[0182] in, Indicates the time when the fault is cleared. express Virtual reactance at any moment This represents the time recovery constant.

[0183] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A virtual synchronous machine overcurrent capability enhancement system, characterized by, include: The system comprises a main power channel, a transient overcurrent channel, an energy regulation unit, and a control drive unit, wherein: The main power channel includes a three-phase inverter and an LCL filter. The three-phase inverter is used to convert DC power into AC power output. The output AC power is connected to the AC power grid after passing through the LCL filter in series. The transient overcurrent channel includes a thyristor, a current-limiting inductor, and a current-limiting reactance. The thyristor is connected in series with the current-limiting inductor, and the thyristor is also connected in series with the current-limiting reactance. One end of the transient overcurrent channel closest to the current-limiting reactance is connected to the AC power grid, and the other end is connected to the DC bus of the virtual synchronous machine. The energy regulation unit includes a chopper resistor circuit and a supercapacitor module. The chopper resistor circuit and the supercapacitor module are connected in parallel to the DC bus of the virtual synchronous machine. The chopper resistor circuit includes a chopper IGBT and a braking resistor connected in series. The control drive unit is used to perform phased control on the virtual synchronous machine, the thyristor and the chopper IGBT according to the system operating status. Specifically, the control drive unit is used for: In the normal operation stage, the grid-connected point voltage V-pcc and output current I-pcc are sampled in real time and the equivalent junction temperature parameter T-j of the power semiconductor device is obtained. Based on the equivalent junction temperature parameter Tj and the output current And a preset device safe operating area model is used to calculate the maximum allowable output current limit I_max(t) that varies with time, which is used to constrain the output current command amplitude of the virtual synchronous machine and control the virtual synchronous machine to work under the preset droop control strategy. Entering the overcurrent detection phase, the output current is sampled in real time. After first-order filtering, the filtered current is obtained. ; judge Is it less than and If the rate of change is less than or equal to the preset first rate of change threshold, then maintain the normal operation phase. judge Does it meet the requirements? ,or If the current exceeds a preset first rate of change threshold, then proceed to the hybrid current limiting stage, adjust the virtual impedance parameter to reduce the system output impedance, and then determine... Is it greater than And whether the Safe Operating Area (SOA) is in an available state; if so, then turn on the thyristor. determining whether greater than or whether the dc bus voltage reaches a set threshold if so, entering a deep current limiting phase, turning on the chopper resistor circuit judge Whether from Descending to The following should maintain the first set duration, or If the value is less than or equal to a preset second rate of change threshold and remains at that threshold for a second set duration, then the fault recovery phase begins. The chopper resistor circuit and the thyristor are sequentially shut down, and the virtual reactance is restored to a preset virtual reactance rating. ; in, , This indicates a preset first current threshold. This indicates a preset second current threshold. Indicates the preset current rating. This indicates the current threshold that allows access to the transient overcurrent channel. Indicates the filter current The rate of change relative to time Indicates filter voltage The rate of change of PVF with respect to time, the filtered voltage pvf is obtained by first-order low-pass filtering of the DC side voltage V_pv.

2. The system as recited in claim 1, wherein, The control drive unit includes: a fault detection module, a hybrid switch drive module, and a chopper current limiting control module, wherein: The fault detection module is used to sample the output current and grid connection point voltage from the AC side of the three-phase inverter, identify grid faults based on the output current and grid connection point voltage, and determine the fault level based on preset multi-level fault thresholds; The hybrid switch drive module is connected to the gate of the fully controlled power switching device of the three-phase inverter and the gate of the thyristor, respectively. The hybrid switch drive module is used to control the on and off of the fully controlled power switch device under normal grid operation, and to trigger the thyristor to conduct when a grid fault occurs, and to cooperate with the fully controlled power switch device to resume operation after the grid fault is cleared, thereby realizing the exit of the transient overcurrent channel. The chopper current limiting control module is connected to the gate of the chopper IGBT; The chopper current limiting control module is used to output a corresponding PWM signal to the chopper IGBT according to a preset current target curve during a power grid fault.

3. The system as recited in claim 1, wherein, When the control drive unit performs the adjustment of the virtual impedance parameter to reduce the system output impedance, it is specifically used for: The virtual impedance parameter is adjusted according to a preset adjustment formula to reduce the system output impedance. The preset adjustment formula is as follows: ； wherein, represents a virtual reactance at time t, represents an output current at time t, represents an impedance dynamic adjustment coefficient.

4. The system as described in claim 3, characterized in that, After the control drive unit turns on the thyristor, it introduces the thyristor branch reactance. To adjust the system's equivalent impedance, the specific calculation formula is as follows: ； wherein, represents the adjusted system equivalent impedance, represents the main channel impedance, represents the imaginary unit, represents the angular frequency, represents the current limiting inductance.

5. The system as recited in claim 1, wherein, After the control drive unit turns on the chopper resistor circuit, the formula for calculating the power dissipation of the chopper resistor circuit is as follows: ； in, This represents the power dissipation at time t. This represents the duty cycle of the chopper IGBT at time t. This indicates the resistance value of the braking resistor. This represents the DC bus voltage at time t.

6. The system as described in claim 3, characterized in that, The control drive unit performs the operation of restoring the virtual reactance to the preset virtual reactance rating. When, specifically used for: recover the virtual reactance to a preset virtual reactance rating value according to a preset recovery formula wherein the preset recovery formula is: ； wherein denotes the moment of failure removal, denotes the virtual reactance at the moment, denotes the time recovery constant.

7. The system as described in claim 1, characterized in that, The control drive unit executes the output current based on the equivalent junction temperature parameter Tj. And based on the preset device safe operating area model, when calculating the maximum permissible output current limit I_max(t) that varies with time, it is specifically used for: based on the equivalent junction temperature parameter T-j, the output current and a preset device safe operating area model, a safe operating area SOA_budget available for the current system is calculated. Based on the aforementioned safety margin SOA_budget, the maximum permissible output current limit I_max(t) that varies over time is generated.

8. The system as recited in claim 1, wherein, The control drive unit, when performing the step of determining whether the Secure Office Area (SOA) is in an available state, is specifically used for: According to the real-time sampled output current , the output current is calculated ; the rate of change with respect to time judge Is it less than or equal to a preset third rate of change threshold, and Is it less than or equal to? Furthermore, the transient junction temperature calculated by combining the preset device safe operating area model and the thermal model is determined to be less than or equal to the preset upper temperature limit. If it is, the safe operating area SOA is determined to be in an available state; otherwise, the safe operating area SOA is determined to be in an unavailable state.

9. A method for virtual synchronous machine overcurrent capability enhancement, characterized in that, The system applied to the virtual synchronous machine overcurrent capability enhancement system according to any one of claims 1-8 includes: During normal operation, the grid connection point voltage V-pcc and output current are sampled in real time. And obtain the equivalent junction temperature parameter Tj of the power semiconductor device; Based on the equivalent junction temperature parameter Tj and the output current And a preset device safe operating area model is used to calculate the maximum allowable output current limit I_max(t) that varies with time, which is used to constrain the output current command amplitude of the virtual synchronous machine and control the virtual synchronous machine to work under the preset droop control strategy. Entering the overcurrent detection phase, the output current is sampled in real time. After first-order filtering, the filtered current is obtained. ; determining whether less than and whether less than or equal to a preset first change rate threshold, if yes, maintaining the normal operation stage judge Does it meet the requirements? ,or If the current exceeds a preset first rate of change threshold, then proceed to the hybrid current limiting stage, adjust the virtual impedance parameter to reduce the system output impedance, and then determine... Is it greater than And check if the Safe Operating Area (SOA) is available; if so, turn on the thyristor. judge Is it greater than Or has the DC bus voltage reached the set threshold? If so, then the deep current limiting stage is entered, and the chopper resistor circuit is turned on. judge Whether from Descending to The following should maintain the first set duration, or If the value is less than or equal to a preset second rate of change threshold and remains at that threshold for a second set duration, then the fault recovery phase begins. The chopper resistor circuit and the thyristor are sequentially shut down, and the virtual reactance is restored to a preset virtual reactance rating. ; in, , This indicates a preset first current threshold. This indicates a preset second current threshold. Indicates the preset current rating. This indicates the current threshold that allows access to the transient overcurrent channel. Indicates the filter current The rate of change relative to time Indicates filter voltage The rate of change of PVF with respect to time, the filtered voltage pvf is obtained by first-order low-pass filtering of the DC side voltage V_pv.

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