A method and system for converter transient control under AC system fault scenarios

Abstract

The application provides a converter transient control method and system under an alternating current system fault scene, the method comprising: obtaining and calculating equivalent reactance values of the alternating current system according to voltage sampling values and current sampling values at a plurality of sampling times; determining a current fault type of the alternating current system according to the equivalent reactance values and preset reactance reference values; if the fault type is a first fault type, determining active given values and virtual inductance values of the converter according to fault depth, so that the converter maintains transient stability; if the fault type is a second fault type, switching a control mode of the converter to a current limiting mode according to real-time output current of the converter, determining current compensation angles of the converter before and after fault removal according to fault removal power angle and stable balance power angle of the converter respectively, so that the converter maintains transient stability in the current limiting mode, and the operation stability of the converter under the alternating current system fault scene is improved.

Description

Technical Field

[0001] This application relates to the field of power system transient stability control technology, and in particular to a converter transient control method and system under AC system fault scenarios. Background Technology

[0002] With the large-scale integration of new energy sources and power electronic equipment, the inertia and short-circuit capacity of power systems are continuously decreasing, and the power grid is shifting from being dominated by synchronous machines to being dominated by power electronics. In weak grids and high-penetration new energy scenarios, voltage and frequency fluctuations are significantly aggravated during faults, and the traditional stabilization mechanism relying on synchronous machines is insufficient to guarantee the safe operation of the system. Grid-connected converters with voltage source characteristics and the ability to actively establish grid voltage and frequency are gradually becoming key supporting equipment, and their transient control performance directly affects fault ride-through capability and grid recovery process. In existing projects, many devices still adopt grid-connected control structures, tracking the grid phase and adjusting active and reactive power through phase-locked loops (PLLs). This type of control can achieve good performance when the grid is strong and the disturbance is small, but it is prone to problems such as PLL lockout, current overshoot, and DC bus overvoltage when there is a deep voltage drop, large frequency deviation, or insufficient short-circuit capacity, and cannot provide effective inertia and voltage support.

[0003] To protect device safety, existing technologies typically incorporate current limiting or current-limiting mechanisms into grid-connected converters, such as current saturation, virtual impedance current limiting, and current feedback in voltage references. While these methods can suppress overcurrent to some extent, they often use fixed limiting thresholds and uniform control parameters, lacking specific differentiation based on fault depth, fault type, and grid strength differences. Furthermore, existing transient control strategies tend to focus on a single objective, making it difficult to simultaneously ensure equipment current safety, maintain grid characteristics, and achieve rapid and stable recovery after a fault under different fault depths. Some solutions also fail to adequately consider actual operating conditions such as asymmetrical faults and multiple units operating in parallel, potentially leading to abnormal power flow redistribution and power angle oscillations between converters. In summary, existing transient control technologies for grid-connected converters with current limiting do not adequately consider differences in fault depth, and the coordination between current limiting and grid control is weak. Summary of the Invention

[0004] To address the aforementioned technical problems, this application provides a converter transient control method and system under AC system fault scenarios, thereby improving the operational stability of the converter under AC system fault scenarios.

[0005] In a first aspect, embodiments of this application provide a converter transient control method under AC system fault scenarios, including: Obtain the voltage sampling value and AC side current sampling value at the grid connection point at several sampling times; The equivalent reactance of the AC system is calculated based on each of the voltage and current sample values. The current fault depth is calculated based on the equivalent reactance value and the preset reactance reference value, and then the current fault type of the AC system is determined based on the fault depth. If the fault type is the first fault type, then the active power setpoint and virtual inductance value of the converter are determined according to the fault depth so that the converter can maintain transient stability. If the fault type is the second fault type, the control mode of the converter is switched to the current limiting mode according to the real-time output current of the converter. In the current limiting mode, the fault clearing power angle and the stable balance power angle of the converter are determined by the equal area rule based on the power angle equation of the converter. Then, the current compensation angle of the converter before and after fault clearing is determined according to the fault clearing power angle and the stable balance power angle, so as to maintain the transient stability of the converter in the current limiting mode based on the current compensation angle.

[0006] This application provides a transient control method for converters under AC system fault scenarios. By real-time acquisition of voltage and current samples at the grid connection point, and calculation of the equivalent reactance of the AC system based on this data, the fault depth is assessed and the fault type is determined. Finally, differentiated control strategies are adopted according to the fault type. Compared with existing technologies, this embodiment, through dynamic calculation of equivalent reactance and fault depth, can accurately identify the real-time state of the power grid, thereby achieving accurate differentiation of fault types and providing a scientific basis for subsequent control strategy selection. Secondly, for the first fault type (e.g., a minor fault), by adjusting the active power setpoint and virtual inductance value of the converter, transient stability is maintained in constant voltage control mode, effectively avoiding the problem of insufficient voltage and power support caused by conservative current limiting in traditional methods. For the second fault type (e.g., a deep fault), the system can quickly switch to current limiting mode and ensure that the converter maintains transient stability under current limiting by calculating the current compensation angle before and after fault clearance. This embodiment not only improves the operational stability of the converter at different fault depths, but also significantly enhances the voltage and frequency support capabilities of the power grid during faults, helping to reduce the risk of system oscillations and transient instability.

[0007] Furthermore, the step of calculating the equivalent reactance value of the AC system based on each of the voltage sample values ​​and each of the current sample values ​​includes: The average voltage sample value at each sampling time is calculated based on the first and second adjacent voltage sample values ​​at each adjacent sampling time of each voltage sample value. The average current sample value at each sampling time is calculated based on the first and second adjacent current sample values ​​at each adjacent sampling time of each current sample value. The current difference value at each sampling time is calculated based on each of the first adjacent current sample values, each of the second adjacent current sample values, and the sampling interval. The equivalent reactance of the AC system is calculated by fitting the average voltage sample value, the average current sample value, and the current difference value using the least squares method.

[0008] This application provides a method for calculating the equivalent reactance of an AC system. By collecting voltage and current data at adjacent sampling times, the method calculates the average voltage sample value, the average current sample value, and the current difference value, and then solves for the equivalent reactance value based on the least squares fitting method. In this embodiment, by using data from adjacent sampling times for averaging and differencing, sampling noise and instantaneous fluctuations can be effectively smoothed, thereby reducing calculation errors and improving the accuracy of the equivalent reactance value. Secondly, the least squares fitting method, as a classic mathematical optimization method, can extract the best fitting result from multiple sets of sampled data, further enhancing the adaptability of reactance calculation to complex power grid environments. This high-precision reactance value provides a solid data foundation for subsequent fault depth assessment and fault type judgment, ensuring that the control system can identify changes in the power grid state in a timely and accurate manner. Furthermore, this method also possesses strong real-time performance and robustness, and can operate stably under different sampling frequencies and power grid conditions, avoiding misjudgments caused by anomalies at a single sampling point, thus laying a solid foundation for the implementation of subsequent differentiated control strategies.

[0009] Furthermore, the step of calculating the current fault depth based on the equivalent reactance value and a preset reactance reference value, and then determining the current fault type of the AC system based on the fault depth, includes: The current fault depth is calculated by dividing the equivalent reactance value by the reactance reference value. If the fault depth is less than a first preset threshold, then the current fault type of the AC system is determined to be the first fault type; If the fault depth is less than the second preset threshold, then the current fault type of the AC system is determined to be the second fault type; The first preset threshold and the second preset threshold are determined based on the power angle characteristic curves of the converter at different fault depths. The first fault type is that there is a stable equilibrium point in the power angle characteristic curve of the converter, and the second fault type is that there is no stable equilibrium point in the power angle characteristic curve of the converter.

[0010] This application clarifies the specific process of calculating fault depth based on equivalent reactance and a preset reactance benchmark value, and distinguishing fault types according to fault depth. First, by comparing the equivalent reactance value with the benchmark value, the system can quantify the severity of the fault, thereby achieving an objective assessment of the fault depth. This quantification method overcomes the limitations of traditional methods that rely on experience or fixed thresholds, making fault judgment more scientific and accurate. Second, based on whether the fault depth exceeds the preset threshold, the system classifies the fault type into Category 1 (with a stable equilibrium point) and Category 2 (without a stable equilibrium point). This classification method is directly related to the converter's power angle characteristic curve, providing a clear physical basis for the selection of control strategies. By adopting differentiated control measures for different fault types, the system can maintain network characteristics and provide voltage support during mild faults, and promptly limit current and protect equipment safety during deep faults. Furthermore, the setting of the preset threshold is based on the analysis of the converter's power angle characteristic curves at different fault depths, ensuring the rationality and applicability of the classification standard and avoiding control malfunctions or failures due to improper threshold settings. Overall, this embodiment provides key decision-making basis for the adaptive control of grid-type converters through refined fault classification, and improves the operational stability of the converters under AC system fault scenarios.

[0011] In one possible implementation, if the fault type is a first fault type, determining the active power setpoint and virtual inductance value of the converter based on the fault depth to maintain transient stability of the converter includes: The active power setpoint of the converter is calculated based on the fault depth, the real-time electromotive force amplitude output by the converter, the real-time voltage amplitude of the grid connection point, the equivalent reactance value of the grid connection point, and the preset balance point power angle threshold. Establish the energy function of the AC system after introducing a virtual inductor, and determine the trigger condition for the converter to switch from the constant voltage control mode to the current limiting mode based on the energy function; The virtual inductance value for maintaining the constant voltage control mode is calculated based on the triggering conditions. Based on the active power setpoint and the virtual inductance value, the converter is controlled to maintain transient stability in constant voltage control mode.

[0012] This application embodiment, focusing on the first fault type, details how to achieve transient stability of the converter in constant voltage control mode. First, by combining multiple parameters such as fault depth, real-time electromotive force amplitude, and grid connection point voltage amplitude to calculate the active power setpoint, the system can dynamically adjust the converter's power output to maintain optimal operating conditions under different fault depths, effectively avoiding power imbalance or overload problems caused by traditional fixed parameter control. Second, the introduction of a virtual inductor can simulate the grid impedance characteristics, optimizing the dynamic interaction between the converter and the grid, while energy function analysis provides a theoretical basis for control mode switching, ensuring a smooth and oscillating switching process. Furthermore, this embodiment further optimizes the control parameters by calculating the virtual inductance value required to maintain constant voltage control mode, enabling the converter to provide necessary voltage support during faults while preventing current overshoot and equipment damage. This refined parameter adjustment and mode switching mechanism significantly improves the converter's transient response performance and stability maintenance capability under mild fault scenarios, achieving efficient and stable operation in constant voltage control mode.

[0013] Furthermore, if the fault type is the first fault type, in addition to determining the active power setpoint and virtual inductance value of the converter based on the fault depth, the voltage droop coefficient of the converter is also determined based on the fault depth. Then, based on the active power setpoint, the virtual inductance value, and the voltage droop coefficient, the converter is controlled to maintain transient stability in constant voltage control mode.

[0014] This application further introduces a voltage droop coefficient as a control parameter, which, in conjunction with the active power setpoint and the virtual inductance value, jointly regulates the converter's operating state. Specifically, the introduction of the voltage droop coefficient enables the converter to dynamically adjust its output characteristics according to changes in grid voltage, enhancing the system's adaptability to voltage fluctuations. During faults, grid voltage may fluctuate frequently; by appropriately setting the droop coefficient, the converter can respond more flexibly to voltage changes and maintain voltage stability at the grid connection point. Secondly, by using the voltage droop coefficient in conjunction with the active power setpoint and the virtual inductance value, multi-parameter joint optimization control is achieved, further improving the accuracy and robustness of the constant voltage control mode. This multi-parameter coordination mechanism not only helps maintain the converter's grid connection characteristics under minor faults but also allows for a smooth transition during fault recovery, reducing secondary disturbances in voltage and frequency. By determining the droop coefficient in conjunction with the fault depth, the system can more accurately match the control requirements under different fault scenarios, further enhancing the converter's dynamic performance and stability maintenance capability in constant voltage mode.

[0015] Furthermore, after the converter is controlled to maintain transient stability in constant voltage control mode for a first preset time based on the active power setpoint and the virtual inductance value, it is determined whether the current fault of the AC system has been repaired. If the fault has not been repaired, the active power setpoint and virtual inductance value of the converter are updated, and the converter is subjected to the next round of transient control based on the updated active power setpoint and virtual inductance value.

[0016] This application further provides a parameter update mechanism. By setting a first preset duration as a stable holding period, the system can periodically assess the fault status during the fault's duration, avoiding control failure or equipment overload risks due to unrepaired faults. This periodic detection mechanism enhances the control system's adaptability to long-term faults. Secondly, if the fault is not repaired, the system updates the converter's active power setpoint and virtual inductance value, and performs the next round of transient control based on the new parameters. This dynamic parameter update strategy enables the control system to adjust the control strategy according to real-time changes in the fault, improving the system's adaptability and robustness. Furthermore, by iteratively optimizing the control parameters, the system can continuously optimize the converter's operating state during the fault's duration, reducing energy loss and equipment stress, and extending equipment lifespan. This mechanism also ensures a smooth transition of control parameters during fault repair, avoiding system oscillations or instability caused by sudden parameter changes. Overall, this embodiment, by introducing fault status detection and parameter update cycles, achieves continuous optimization and dynamic adjustment of the control strategy, significantly improving the stable operation capability and fault recovery efficiency of grid-type converters under long-term fault scenarios.

[0017] In one possible implementation, if the fault type is a second fault type, the control mode of the converter is switched to a current limiting mode based on the real-time output current of the converter. In the current limiting mode, the fault clearing power angle and the stable balance power angle of the converter are determined based on the power angle equation of the converter using the equal area rule. Then, the current compensation angle of the converter before and after fault clearing is determined based on the fault clearing power angle and the stable balance power angle, respectively, so that the converter can maintain transient stability in the current limiting mode based on the current compensation angle. This includes: Obtain the real-time output current and real-time power angle of the converter; If the real-time output current is greater than the preset maximum output current value, the control mode of the converter is switched to the current limiting mode according to the real-time output current of the converter. Based on the power angle equations of the converter during and after a fault, the fault clearing power angle and the stable equilibrium power angle are calculated using the equal area criterion. The first current compensation angle of the converter is calculated based on the real-time power angle, the fault clearing power angle, and the preset balance point power angle threshold. The second current compensation angle of the converter is calculated based on the real-time power angle, the stable balance power angle, and the current active power setpoint of the converter. Based on the first current compensation angle and the second current compensation angle, the converter is controlled to maintain transient stability in the current limiting mode.

[0018] This application embodiment, targeting the second fault type, details how to achieve transient stability control by real-time monitoring of the converter output current, switching to current limiting mode, and calculating the current compensation angle based on the power angle before and after fault clearing. First, by real-time monitoring of the output current and comparing it with a preset maximum value, the system can promptly detect overcurrent risks and quickly switch to current limiting mode, effectively preventing equipment damage due to overcurrent. This rapid response mechanism improves system safety and reliability. Second, calculating the fault clearing power angle and stable balance power angle based on the equal area criterion provides a theoretical basis for determining the current compensation angle, ensuring the dynamic stability of the converter in current limiting mode. By calculating the current compensation angle during and after the fault, the system can optimize the current control strategy for different stages, reducing power oscillations and voltage fluctuations during transient processes. Furthermore, this embodiment also calculates the compensation angle by combining factors such as real-time power angle and balance point power angle threshold, controlling the converter to maintain transient stability in current limiting mode based on the current compensation angle, improving the control accuracy and fault ride-through capability of the converter in current limiting mode. Overall, the embodiments of this application achieve efficient current limiting and transient stability maintenance in deep fault scenarios by integrating real-time monitoring, mode switching and power angle optimization control, providing key technical support for the safe and stable operation of grid-type converters under extreme fault conditions.

[0019] Furthermore, controlling the converter to maintain transient stability in current-limiting mode based on the first current compensation angle and the second current compensation angle includes: The converter is controlled to maintain transient stability in current limiting mode according to the first current compensation angle until the fault is cleared. When the fault is cleared, the converter is controlled to switch from the current limiting mode to the constant voltage control mode according to the second current compensation angle.

[0020] This application further refines the control process in current-limiting mode, clarifying how to control the converter's operating state during and after a fault based on the first and second current compensation angles, respectively. First, during a fault, this embodiment controls the converter according to the first current compensation angle to ensure transient stability in current-limiting mode, while effectively suppressing overcurrent and protecting equipment safety. This targeted control strategy optimizes the dynamic response during a fault, reducing power and voltage fluctuations. Second, when the fault is cleared, the system controls the converter to switch back to constant-voltage control mode according to the second current compensation angle, achieving a smooth transition from current-limiting mode to normal operation mode. This transition mechanism avoids potential current surges or voltage spikes during mode switching, improving the system's recovery speed and stability. Furthermore, by distinguishing the compensation angles during and after a fault, the system can more accurately adapt to the control requirements of different stages, improving the coordination and effectiveness of the overall control strategy. This phased control method also enhances the system's adaptability to the fault recovery process, ensuring that the converter can quickly return to its optimal operating state after fault repair, significantly improving the operational stability of grid-type converters in deep fault scenarios.

[0021] Secondly, embodiments of this application provide a converter transient control system under AC system fault scenarios, including an acquisition module, a reactance calculation module, a fault judgment module, a first control module, and a second control module; The acquisition module is used to acquire the voltage sampling value of the grid connection point and the current sampling value of the AC side at several sampling times; The reactance calculation module is used to calculate the equivalent reactance value of the AC system based on each of the voltage sampling values ​​and each of the current sampling values. The fault determination module is used to calculate the current fault depth based on the equivalent reactance value and the preset reactance reference value, and then determine the current fault type of the AC system based on the fault depth. The first control module is used to determine the active power setpoint and virtual inductance value of the converter according to the fault depth if the fault type is the first fault type, so as to keep the converter in transient stability. The second control module is used to switch the control mode of the converter to the current limiting mode according to the real-time output current of the converter if the fault type is the second fault type. In the current limiting mode, the fault clearing power angle and the stable balance power angle of the converter are determined by the equal area rule based on the power angle equation of the converter. Then, the current compensation angle of the converter before and after fault clearing is determined according to the fault clearing power angle and the stable balance power angle, so as to maintain the transient stability of the converter in the current limiting mode based on the current compensation angle.

[0022] Furthermore, the reactance calculation module calculates the equivalent reactance value of the AC system based on each of the voltage sample values ​​and each of the current sample values, including: The average voltage sample value at each sampling time is calculated based on the first and second adjacent voltage sample values ​​at each adjacent sampling time of each voltage sample value. The average current sample value at each sampling time is calculated based on the first and second adjacent current sample values ​​at each adjacent sampling time of each current sample value. The current difference value at each sampling time is calculated based on each of the first adjacent current sample values, each of the second adjacent current sample values, and the sampling interval. The equivalent reactance of the AC system is calculated by fitting the average voltage sample value, the average current sample value, and the current difference value using the least squares method. Attached Figure Description

[0023] Figure 1 A flowchart illustrating a converter transient control method under AC system fault scenarios provided in this application embodiment; Figure 2 This is a schematic diagram of the GFM-VSC grid-connected system in a converter transient control method under an AC system fault scenario provided in an embodiment of this application.

[0024] Figure 3 A schematic diagram of the power angle characteristic curves of GFM-VSC at different fault depths provided in the embodiments of this application; Figure 4 This application provides a schematic diagram of the system potential energy distribution under a Type I fault, taking into account the effect of current limiting. Figure 5 This is a schematic diagram illustrating the changes in the system's attraction domain under different control parameters, as provided in the embodiments of this application.

[0025] Figure 6 A schematic diagram of the system potential energy distribution considering virtual reactance under a Type I fault, provided for an embodiment of this application; Figure 7 This is a schematic diagram illustrating the root locus variation of the GFM-VSC system under different control parameters, as provided in the embodiments of this application. Figure 8 This is a schematic diagram of the power angle characteristic curve of the system under the optimal current compensation angle configuration provided in the embodiments of this application; Figure 9 This is a schematic diagram of a system simulation waveform without optimization strategy under Type I fault, provided as an embodiment of this application. Figure 10This is a schematic diagram of the system phase without optimization strategy under Type I fault, provided in an embodiment of this application. Figure 11 A schematic diagram of system simulation waveforms for applying a converter transient control method under a Type I fault, provided in an embodiment of this application. Figure 12 A schematic diagram of the system phase for applying a converter transient control method under a Type I fault, provided in an embodiment of this application. Figure 13 This is a schematic diagram of a system simulation waveform without optimization strategy under Type II fault, provided in an embodiment of this application. Figure 14 A schematic diagram of system simulation waveforms for applying a converter transient control method under a type II fault, provided in an embodiment of this application. Figure 15 This is a schematic diagram of the structure of a converter transient control system under an AC system fault scenario, provided as an embodiment of this application. Detailed Implementation

[0026] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0027] It should be noted that the step numbers in this document are only for the convenience of explaining the specific embodiments and are not intended to limit the order in which the steps are performed. In the description of this application, 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. Therefore, a feature specified as "first" or "second" may explicitly or implicitly include one or more of that feature.

[0028] Example 1: like Figure 1 As shown, Embodiment 1 provides a converter transient control method under AC system fault scenarios, including steps S1-S5: Step S1: Obtain the voltage sampling value and AC side current sampling value of the grid connection point at several sampling times; Step S2: Calculate the equivalent reactance value of the AC system based on each of the voltage sampling values ​​and each of the current sampling values; Step S3: Calculate the current fault depth based on the equivalent reactance value and the preset reactance reference value, and then determine the current fault type of the AC system based on the fault depth; Step S4: If the fault type is the first fault type, then determine the active power setpoint and virtual inductance value of the converter according to the fault depth, so as to keep the converter in transient stability. Step S5: If the fault type is the second fault type, the control mode of the converter is switched to the current limiting mode according to the real-time output current of the converter. In the current limiting mode, the fault clearing power angle and the stable balance power angle of the converter are determined by the equal area rule based on the power angle equation of the converter. Then, the current compensation angle of the converter before and after fault clearing is determined according to the fault clearing power angle and the stable balance power angle, so as to maintain the transient stability of the converter in the current limiting mode based on the current compensation angle.

[0029] This application provides a transient control method for converters under AC system fault scenarios. By real-time acquisition of voltage and current samples at the grid connection point, and calculation of the equivalent reactance of the AC system based on this data, the fault depth is assessed and the fault type is determined. Finally, differentiated control strategies are adopted according to the fault type. Compared with existing technologies, this embodiment, through dynamic calculation of equivalent reactance and fault depth, can accurately identify the real-time state of the power grid, thereby achieving accurate differentiation of fault types and providing a scientific basis for subsequent control strategy selection. Secondly, for the first fault type (e.g., a minor fault), by adjusting the active power setpoint and virtual inductance value of the converter, transient stability is maintained in constant voltage control mode, effectively avoiding the problem of insufficient voltage and power support caused by conservative current limiting in traditional methods. For the second fault type (e.g., a deep fault), the system can quickly switch to current limiting mode and ensure that the converter maintains transient stability under current limiting by calculating the current compensation angle before and after fault clearance. This embodiment not only improves the operational stability of the converter at different fault depths, but also significantly enhances the voltage and frequency support capabilities of the power grid during faults, helping to reduce the risk of system oscillations and transient instability.

[0030] In a preferred embodiment, the converter is a grid-connected voltage source converter (GFM-VSC), and the dynamic mathematical model of the GFM-VSC grid-connected system is as follows: Figure 2 As shown, the grid-connected system consists of a main circuit and a control system. The main circuit includes a DC power supply, converter, filter, PCC, and AC grid; the control system includes an active power loop, a reactive power loop, and an inner voltage-current loop. The active power loop regulates the frequency and active power output, the reactive power loop regulates the voltage amplitude, and the inner loop generates dq coordinate system control quantities and implements grid-connected control via PWM. The active power control equation of the GFM-VSC is: in, The active power setpoint; P is the active power reference value; P is the output active power. This is the frequency droop coefficient; The damping coefficient; ω represents the system's rated angular velocity; J represents the virtual inertia; ω represents the virtual angular velocity. This is the phase angle of the converter output voltage.

[0031] The reactive power control equation is: in, The given value for reactive power; This is a reference value for reactive power. E represents the output reactive power; E represents the output voltage. This is the system's rated voltage; K is the voltage droop coefficient; K is the voltage integral coefficient. This represents the dynamic process of the reactive power-voltage regulation link.

[0032] Furthermore, the power angle characteristic curves of GFM-VSC at different fault depths are as follows: Figure 3 As shown. Curve III corresponds to normal operating conditions, while curves I and II correspond to the power angle characteristics at different fault depths; curves I and III are related to the active power command. The points intersect, with points a and c being stable equilibrium points (SEP), and points b and d being unstable equilibrium points (UEP). If the power angle of the system exceeds UEP during a fault, transient instability will occur. Therefore, the power angle corresponding to UEP (δ) is... UEPⅠ δ UEPⅢ ) is defined as the critical work angle.

[0033] Based on the presence or absence of a SEP (Self-Extended Precipitation Effect), faults are divided into two categories: if a SEP still exists during the fault period (corresponding to...) Figure 2 Curve I in the diagram is defined as a Type I fault; if no SEP (Situational Effort Point) occurs during the fault period (corresponding to curve II), it is defined as a Type II fault. This application focuses on constructing a unified transient stability analysis and control framework around these two typical fault scenarios.

[0034] For Type I fault scenarios, to analyze the transient stability of GFM-VSC during the fault process, this embodiment constructs an energy function V(δ, ω, E) based on the Lyapunov method. According to the Lassalle invariance principle, the energy function is constructed as follows: in The energy function constructed for the system; The phase angle of the converter output voltage; This refers to virtual angular velocity; Rated angular velocity; This refers to the output voltage amplitude of the converter. This is virtual inertia; These are the active and reactive power setpoints, respectively; For grid-connected equivalent reactance; This is the reactive power droop coefficient; This is the rated voltage amplitude.

[0035] The derivative of the energy function with respect to time can characterize the damping properties of the system: in Its time derivative; This is the voltage regulation coefficient.

[0036] The converter system can be modeled as a dissipative energy system with respect to δ, ω, and E. Considering the limited overcurrent capacity of the GFM-VSC, CLC (current limiting mode) may be triggered both during and after a fault. The triggering condition is as follows: In the formula: Ug represents the voltage amplitude of the power grid during the fault; Ug represents the voltage amplitude of the power grid.

[0037] Potential energy distribution of the GFM-VSC system under Type I fault, taking into account the effect of current limiting, such as Figure 4 As shown. When GFM-VSC triggers CLC, the derivative of the energy function is: Once the operating point enters the flow-limited region, it is no longer possible to directly prove the dissipation of the system.

[0038] The potential energy distribution of the system mainly depends on , , and The scope of the traffic restriction area is the same as Related. To improve the potential energy distribution, this invention introduces a virtual inductance L. v Adjusting the system's equivalent reactance alters the energy function and the location and shape of the current-limiting region. Introducing L... v Then, the energy function is rewritten as: Accordingly, the CLC triggering conditions during and after the fault are modified as follows: in This refers to the output voltage amplitude of the converter. The power angle between the converter output voltage and the grid voltage; For grid-connected equivalent reactance; This is the inductive reactance corresponding to the virtual impedance; For virtual inductance; The system angular frequency; This represents the maximum allowable current.

[0039] Plot the changes in the system's attraction domain under different control parameters, such as... Figure 5 As shown. Figure 5 (a) to (d) give the reactive power droop coefficients under Type I faults. Virtual reactance L v Active power setpoint and reactive power setpoint Impact on the field of attraction: Increase or L v Reducing the size of the attraction field can increase the attraction field and decrease the distance between the SEP and UEP. Increasing the size will shrink the attraction field and increase the distance between the SEP and UEP. The impact on the attraction field is relatively small. Therefore, by reducing... and L v and increase This can effectively improve the transient stability margin of the system.

[0040] Further, the system potential energy distribution diagrams corresponding to different virtual reactance configurations under Type I faults can be obtained, such as... Figure 6 As shown. Figure 6 This indicates that the introduction of L v This can reduce the area of ​​the flow restriction, which is beneficial to improving transient stability. However, at the same time, L v There is a contradiction in the impact on the attraction domain and the flow restriction area: a larger L v While this helps suppress current limiting triggering, it weakens the attraction domain. Therefore, without triggering CLC, the smallest possible L should be chosen. v This allows for a trade-off between current limiting risk and transient stability. Therefore, when the fault type is determined to be the first fault type, the active power setpoint and virtual inductance value of the converter can be determined based on the above conclusion and the fault depth, so that the converter can maintain transient stability in constant voltage control mode.

[0041] On the other hand, for Type II deep faults, due to the large voltage drop, the port current quickly reaches the limit, and the current limiting function (CLC) is almost inevitably activated during the fault. At this time, the original energy function no longer satisfies the asymptotic stability condition, and the equal area criterion needs to be used to evaluate the transient stability of the system. To characterize the most unfavorable operating condition, it can be assumed that the GFM-VSC is in a current limiting state during the fault and in the transient recovery phase after the fault is cleared, which are denoted as CLC1 mode and CLC2 mode, respectively.

[0042] According to the equal area criterion, for the system to remain transiently stable after experiencing a Type II deep fault, the constraint relationship between the acceleration area and the deceleration area must be satisfied: S1 < S2. The corresponding fault clearing angle δc is the critical clearing angle. S1 and S2 can be calculated using the following formula: Where S1 is the acceleration area; S2 is the deceleration area; The power angle corresponding to the unstable equilibrium point of the system after the fault; This is the fault clearing angle; This represents the maximum value reached by the power angle oscillation after the fault. This represents the critical line power during the fault period. This represents the critical line power after fault clearance. CLC significantly reduces the deceleration area S2 in the post-fault stage, leading to a decrease in the critical clearance angle δc, thus weakening the system's transient stability margin. Since the magnitudes of S1 and S2 are jointly determined by the positions of the power angle characteristic curves in CLC1 and CLC2 modes, and these two curves are closely related to the current limiting angle φ, this embodiment further introduces two levels of current compensation angles ΔφⅠ and ΔφⅡ. By compensating for the current limiting angle at different stages during and after the fault clearance, it is equivalent to shifting the power angle curves in CLC1 and CLC2 modes, thereby achieving the goal of reducing the acceleration area S1 and increasing the deceleration area S2. The corrected power angle equation is: Among them, U g I represents the voltage amplitude of the power grid. max δ is the maximum allowable current; δ is the power angle between the converter output voltage and the grid voltage; ΔφI and ΔφII represent the additional offset of the current phase angle during the fault and after the fault is cleared, respectively; φ is the equivalent impedance angle of the grid connection channel.

[0043] To avoid control lock-up issues, ΔφⅡ must satisfy: Where, δ s φ is the power angle corresponding to the stable equilibrium point; φ is the equivalent impedance angle of the grid-connected channel; arccos represents the inverse cosine function.

[0044] In summary, the transient stability of GFM-VSC is significantly weakened in the presence of CLC: for Type I faults, this can be mitigated by increasing the virtual damping. Reduce active reference and appropriately reduce the virtual reactance L vTo improve system stability margin, the above parameters need to be coordinated, and the current compensation angles ΔφⅠ and ΔφⅡ need to be introduced and configured reasonably. By controlling the acceleration area S1 to be minimized and the deceleration area S2 to be maximized, the current limiting control can be smoothly exited and switched back to CVC (constant voltage control mode) under a suitable power angle, so as to achieve transient stability of GFM-VSC under deep fault conditions and safe recovery after the fault.

[0045] Further, in step S2, calculating the equivalent reactance value of the AC system based on each of the voltage sample values ​​and each of the current sample values ​​includes: The average voltage sample value at each sampling time is calculated based on the first and second adjacent voltage sample values ​​at each adjacent sampling time of each voltage sample value. The average current sample value at each sampling time is calculated based on the first and second adjacent current sample values ​​at each adjacent sampling time of each current sample value. The current difference value at each sampling time is calculated based on each of the first adjacent current sample values, each of the second adjacent current sample values, and the sampling interval. The equivalent reactance of the AC system is calculated by fitting the average voltage sample value, the average current sample value, and the current difference value using the least squares method.

[0046] This application provides a method for calculating the equivalent reactance of an AC system. By collecting voltage and current data at adjacent sampling times, the method calculates the average voltage sample value, the average current sample value, and the current difference value, and then solves for the equivalent reactance value based on the least squares fitting method. In this embodiment, by using data from adjacent sampling times for averaging and differencing, sampling noise and instantaneous fluctuations can be effectively smoothed, thereby reducing calculation errors and improving the accuracy of the equivalent reactance value. Secondly, the least squares fitting method, as a classic mathematical optimization method, can extract the best fitting result from multiple sets of sampled data, further enhancing the adaptability of reactance calculation to complex power grid environments. This high-precision reactance value provides a solid data foundation for subsequent fault depth assessment and fault type judgment, ensuring that the control system can identify changes in the power grid state in a timely and accurate manner. Furthermore, this method also possesses strong real-time performance and robustness, and can operate stably under different sampling frequencies and power grid conditions, avoiding misjudgments caused by anomalies at a single sampling point, thus laying a solid foundation for the implementation of subsequent differentiated control strategies.

[0047] In a preferred embodiment, based on the foregoing analysis, since Type I and Type II faults differ significantly in their fault characteristics and stability mechanisms, this embodiment designs differentiated control strategies for each, integrating them into a unified adaptive transient stability control method: For Type I faults, the system's attraction domain and current-limiting region are dynamically adjusted to avoid triggering CLC during and after the fault; for Type II faults, current compensation angles ΔφⅠ and ΔφⅡ are introduced to reduce the acceleration area S1 and increase the deceleration area S2, respectively, ensuring a smooth transition from CLC to CVC mode after the fault. To improve transient stability at different fault depths without affecting steady-state performance, this embodiment first establishes the activation criteria for the control strategy. To characterize the fault depth of the AC system, the real-time measured value of the AC system's equivalent reactance is calculated using instantaneous impedance measurement. : Where Δt is the sampling step size, and n is the length of the data window required for calculation; is the real-time measured value of the equivalent reactance of the AC system; upcc(k) is the sampled value of the PCC voltage at the grid connection point at the k-th sampling time; This represents the sampled value of the AC side current at the k-th sampling time. For current Discrete derivative with respect to time; k is the sampling point number; , These represent the AC side current sampling values ​​at two adjacent sampling times, , These represent the PCC voltage sample values ​​at two adjacent sampling times.

[0048] Furthermore, in step S3, the calculation of the current fault depth based on the equivalent reactance value and the preset reactance reference value, and the determination of the current fault type of the AC system based on the fault depth, includes: The current fault depth is calculated by dividing the equivalent reactance value by the reactance reference value. If the fault depth is less than a first preset threshold, then the current fault type of the AC system is determined to be the first fault type; If the fault depth is less than the second preset threshold, then the current fault type of the AC system is determined to be the second fault type; The first preset threshold and the second preset threshold are determined based on the power angle characteristic curves of the converter at different fault depths. The first fault type is that there is a stable equilibrium point in the power angle characteristic curve of the converter, and the second fault type is that there is no stable equilibrium point in the power angle characteristic curve of the converter.

[0049] This application clarifies the specific process of calculating fault depth based on equivalent reactance and a preset reactance benchmark value, and distinguishing fault types according to fault depth. First, by comparing the equivalent reactance value with the benchmark value, the system can quantify the severity of the fault, thereby achieving an objective assessment of the fault depth. This quantification method overcomes the limitations of traditional methods that rely on experience or fixed thresholds, making fault judgment more scientific and accurate. Second, based on whether the fault depth exceeds the preset threshold, the system classifies the fault type into Category 1 (with a stable equilibrium point) and Category 2 (without a stable equilibrium point). This classification method is directly related to the converter's power angle characteristic curve, providing a clear physical basis for the selection of control strategies. By adopting differentiated control measures for different fault types, the system can maintain network characteristics and provide voltage support during mild faults, and promptly limit current and protect equipment safety during deep faults. Furthermore, the setting of the preset threshold is based on the analysis of the converter's power angle characteristic curves at different fault depths, ensuring the rationality and applicability of the classification standard and avoiding control malfunctions or failures due to improper threshold settings. Overall, this embodiment provides key decision-making basis for the adaptive control of grid-type converters through refined fault classification, and improves the operational stability of the converters under AC system fault scenarios.

[0050] In a preferred embodiment, when <0.95 When a voltage dip fault is detected in the AC system, the transient control strategy of this invention is activated, wherein... This is the baseline value of the equivalent reactance of the AC system before the fault; when Leq < 0.5Lg, it is determined to be a deep voltage dip fault, i.e., a Type II fault. The equivalent reactance of the AC system measured in real time during normal operation. Compared with the baseline value before the fault The results are largely consistent. Considering measurement errors and fluctuations under normal operating conditions, and to avoid misjudgment, the fault initiation threshold is set to 0.95. That is, when A voltage drop of less than 0.95 indicates a type I voltage dip fault in the AC system. Furthermore, as the fault deepens... Continuously decreasing. When When the power angle curve drops to a certain critical value, it changes from "having a stable equilibrium point (SEP)" to "not having a stable equilibrium point" during the fault period, corresponding to the boundary between Type I and Type II faults mentioned above. Combined with... Figure 3 The power angle characteristic analysis at different fault depths is shown. In this embodiment, the critical value is taken as approximately 0.5. Therefore, when When the voltage drop is less than 0.5, it is determined to be a deep voltage drop fault, i.e., a Type II fault.

[0051] In one possible implementation, in step S4, if the fault type is a first fault type, determining the active power setpoint and virtual inductance value of the converter based on the fault depth to maintain transient stability of the converter includes: The active power setpoint of the converter is calculated based on the fault depth, the real-time electromotive force amplitude output by the converter, the real-time voltage amplitude of the grid connection point, the equivalent reactance value of the grid connection point, and the preset balance point power angle threshold. Establish the energy function of the AC system after introducing a virtual inductor, and determine the trigger condition for the converter to switch from the constant voltage control mode to the current limiting mode based on the energy function; The virtual inductance value for maintaining the constant voltage control mode is calculated based on the triggering conditions. Based on the active power setpoint and the virtual inductance value, the converter is controlled to maintain transient stability in constant voltage control mode.

[0052] This application embodiment, focusing on the first fault type, details how to achieve transient stability of the converter in constant voltage control mode. First, by combining multiple parameters such as fault depth, real-time electromotive force amplitude, and grid connection point voltage amplitude to calculate the active power setpoint, the system can dynamically adjust the converter's power output to maintain optimal operating conditions under different fault depths, effectively avoiding power imbalance or overload problems caused by traditional fixed parameter control. Second, the introduction of a virtual inductor can simulate the grid impedance characteristics, optimizing the dynamic interaction between the converter and the grid, while energy function analysis provides a theoretical basis for control mode switching, ensuring a smooth and oscillating switching process. Furthermore, this embodiment further optimizes the control parameters by calculating the virtual inductance value required to maintain constant voltage control mode, enabling the converter to provide necessary voltage support during faults while preventing current overshoot and equipment damage. This refined parameter adjustment and mode switching mechanism significantly improves the converter's transient response performance and stability maintenance capability under mild fault scenarios, achieving efficient and stable operation in constant voltage control mode.

[0053] Furthermore, if the fault type is the first fault type, in addition to determining the active power setpoint and virtual inductance value of the converter based on the fault depth, the voltage droop coefficient of the converter is also determined based on the fault depth. Then, based on the active power setpoint, the virtual inductance value, and the voltage droop coefficient, the converter is controlled to maintain transient stability in constant voltage control mode.

[0054] This application further introduces a voltage droop coefficient as a control parameter, which, in conjunction with the active power setpoint and the virtual inductance value, jointly regulates the converter's operating state. Specifically, the introduction of the voltage droop coefficient enables the converter to dynamically adjust its output characteristics according to changes in grid voltage, enhancing the system's adaptability to voltage fluctuations. During faults, grid voltage may fluctuate frequently; by appropriately setting the droop coefficient, the converter can respond more flexibly to voltage changes and maintain voltage stability at the grid connection point. Secondly, by using the voltage droop coefficient in conjunction with the active power setpoint and the virtual inductance value, multi-parameter joint optimization control is achieved, further improving the accuracy and robustness of the constant voltage control mode. This multi-parameter coordination mechanism not only helps maintain the converter's grid connection characteristics under minor faults but also allows for a smooth transition during fault recovery, reducing secondary disturbances in voltage and frequency. By determining the droop coefficient in conjunction with the fault depth, the system can more accurately match the control requirements under different fault scenarios, further enhancing the converter's dynamic performance and stability maintenance capability in constant voltage mode.

[0055] In a preferred embodiment, for Type I faults, by adjusting , and The system's attraction domain is altered. As discussed in the preceding system stability analysis, active power imbalance is the root cause of system instability. Therefore, this embodiment appropriately reduces the active power setpoint during the fault process. : In the formula: The active power setpoint is adjusted during the fault; k is the grid voltage sag depth, k=L eq / L g After clearing the fault Restore the initial settings to ensure the system's steady-state power regulation performance.

[0056] While meeting transient stability requirements, the control parameters must also satisfy small-signal stability constraints. The control parameters can be plotted based on the small-signal model of the GFM-VSC system. and The effect of changes on the root locus of the system, such as Figure 7 As shown: With As the number of elements increases, the eigenvalues ​​corresponding to the dominant modes of the system move further away from the imaginary axis, resulting in enhanced damping characteristics; with... As the value increases, the characteristic roots move closer to the imaginary axis, and the transient characteristics of the system deteriorate. This indicates that... and The requirements for steady-state and transient performance are consistent. Therefore, a larger value is selected during the initial system design. and smaller This allows for the simultaneous consideration of both steady-state and transient performance; for example, it can be set as follows: , Based on this, the size of the fault depth is adaptively increased. The value of is adjusted to further improve the transient performance of the system.

[0057] Regarding virtual inductance To avoid triggering CLC during and after a fault, this embodiment dynamically increases the virtual inductance after a fault occurs: in, and These are the converter output currents during and after the fault, respectively. and These represent the corresponding virtual inductances; μ is the attenuation coefficient (μ = –e(t – tc), where tc is the fault clearing time), to ensure that the converter gradually returns to its stable operating point after the fault is cleared. The above expression is used to establish the correspondence between the converter output current and the virtual inductance, and to determine the minimum virtual inductance value that meets the current limiting requirements.

[0058] To avoid triggering CLC, the converter output current should not exceed [a certain value]. The minimum required virtual inductance is: ; Furthermore, after the converter is controlled to maintain transient stability in constant voltage control mode for a first preset time based on the active power setpoint and the virtual inductance value, it is determined whether the current fault of the AC system has been repaired. If the fault has not been repaired, the active power setpoint and virtual inductance value of the converter are updated, and the converter is subjected to the next round of transient control based on the updated active power setpoint and virtual inductance value.

[0059] This application further provides a parameter update mechanism. By setting a first preset duration as a stable holding period, the system can periodically assess the fault status during the fault's duration, avoiding control failure or equipment overload risks due to unrepaired faults. This periodic detection mechanism enhances the control system's adaptability to long-term faults. Secondly, if the fault is not repaired, the system updates the converter's active power setpoint and virtual inductance value, and performs the next round of transient control based on the new parameters. This dynamic parameter update strategy enables the control system to adjust the control strategy according to real-time changes in the fault, improving the system's adaptability and robustness. Furthermore, by iteratively optimizing the control parameters, the system can continuously optimize the converter's operating state during the fault's duration, reducing energy loss and equipment stress, and extending equipment lifespan. This mechanism also ensures a smooth transition of control parameters during fault repair, avoiding system oscillations or instability caused by sudden parameter changes. Overall, this embodiment, by introducing fault status detection and parameter update cycles, achieves continuous optimization and dynamic adjustment of the control strategy, significantly improving the stable operation capability and fault recovery efficiency of grid-type converters under long-term fault scenarios.

[0060] In one possible implementation, in step S5, if the fault type is the second fault type, the control mode of the converter is switched to current limiting mode based on the real-time output current of the converter. In the current limiting mode, the fault clearing power angle and stable balance power angle of the converter are determined based on the power angle equation of the converter using the equal area rule. Then, the current compensation angle of the converter before and after fault clearing is determined based on the fault clearing power angle and the stable balance power angle, respectively, so that the converter maintains transient stability in the current limiting mode based on the current compensation angle. This includes: Obtain the real-time output current and real-time power angle of the converter; If the real-time output current is greater than the preset maximum output current value, the control mode of the converter is switched to the current limiting mode according to the real-time output current of the converter. Based on the power angle equations of the converter during and after a fault, the fault clearing power angle and the stable equilibrium power angle are calculated using the equal area criterion. The first current compensation angle of the converter is calculated based on the real-time power angle, the fault clearing power angle, and the preset balance point power angle threshold. The second current compensation angle of the converter is calculated based on the real-time power angle, the stable balance power angle, and the current active power setpoint of the converter. Based on the first current compensation angle and the second current compensation angle, the converter is controlled to maintain transient stability in the current limiting mode.

[0061] This application embodiment, targeting the second fault type, details how to achieve transient stability control by real-time monitoring of the converter output current, switching to current limiting mode, and calculating the current compensation angle based on the power angle before and after fault clearing. First, by real-time monitoring of the output current and comparing it with a preset maximum value, the system can promptly detect overcurrent risks and quickly switch to current limiting mode, effectively preventing equipment damage due to overcurrent. This rapid response mechanism improves system safety and reliability. Second, calculating the fault clearing power angle and stable balance power angle based on the equal area criterion provides a theoretical basis for determining the current compensation angle, ensuring the dynamic stability of the converter in current limiting mode. By calculating the current compensation angle during and after the fault, the system can optimize the current control strategy for different stages, reducing power oscillations and voltage fluctuations during transient processes. Furthermore, this embodiment also calculates the compensation angle by combining factors such as real-time power angle and balance point power angle threshold, controlling the converter to maintain transient stability in current limiting mode based on the current compensation angle, improving the control accuracy and fault ride-through capability of the converter in current limiting mode. Overall, the embodiments of this application achieve efficient current limiting and transient stability maintenance in deep fault scenarios by integrating real-time monitoring, mode switching and power angle optimization control, providing key technical support for the safe and stable operation of grid-type converters under extreme fault conditions.

[0062] Furthermore, controlling the converter to maintain transient stability in current-limiting mode based on the first current compensation angle and the second current compensation angle includes: The converter is controlled to maintain transient stability in current limiting mode according to the first current compensation angle until the fault is cleared. When the fault is cleared, the converter is controlled to switch from the current limiting mode to the constant voltage control mode according to the second current compensation angle.

[0063] This application further refines the control process in current-limiting mode, clarifying how to control the converter's operating state during and after a fault based on the first and second current compensation angles, respectively. First, during a fault, this embodiment controls the converter according to the first current compensation angle to ensure transient stability in current-limiting mode, while effectively suppressing overcurrent and protecting equipment safety. This targeted control strategy optimizes the dynamic response during a fault, reducing power and voltage fluctuations. Second, when the fault is cleared, the system controls the converter to switch back to constant-voltage control mode according to the second current compensation angle, achieving a smooth transition from current-limiting mode to normal operation mode. This transition mechanism avoids potential current surges or voltage spikes during mode switching, improving the system's recovery speed and stability. Furthermore, by distinguishing the compensation angles during and after a fault, the system can more accurately adapt to the control requirements of different stages, improving the coordination and effectiveness of the overall control strategy. This phased control method also enhances the system's adaptability to the fault recovery process, ensuring that the converter can quickly return to its optimal operating state after fault repair, significantly improving the operational stability of grid-type converters in deep fault scenarios.

[0064] In a preferred embodiment, for Type II faults, the present invention reconstructs the CLC control law: ; Where, Δφ=k Ⅰ ΔφⅠ+k Ⅱ ΔφⅡ, when the system is running in CLC1 mode k Ⅰ =1, k Ⅱ =0; k is running in CLC2 mode Ⅰ =0, k Ⅱ =1; k is running in CVC mode Ⅰ =0, k Ⅱ =0.

[0065] At the same fault clearing angle To minimize the acceleration area S1, P should be minimized. CLC1 satisfy: At this time, the current compensation angle ΔφⅠ is configured as follows: When configuring ΔφⅡ, it is necessary to maximize the deceleration area S2 while avoiding the converter locking in CLC2 mode. Therefore, when P... CLC2 SEP and switching angle δ s When they coincide, the optimal configuration of ΔφⅡ can be obtained: Finally, the comprehensive expression for the current compensation angle Δφ is: Where, δ c For fault clearing angle, δ SEP δs is the power angle corresponding to the stable equilibrium point during the fault, and δs is the power angle corresponding to the stable equilibrium point after the fault (i.e., the stable equilibrium power angle); P CLC1 This is the critical power curve during the fault, where S1 and S2 are the acceleration and deceleration areas, respectively; U gF U represents the voltage amplitude of the power grid during the fault. g I represents the voltage amplitude of the power grid after the fault is cleared. max The maximum allowable output current of the converter; ΔφI and ΔφII are the current compensation angles corresponding to Type I and Type II faults, respectively, and Δφ is the comprehensive configuration value of the current compensation angle; φ is the equivalent impedance angle of the grid connection channel; P ref This is a reference value for active power; k I and k I The selection coefficients corresponding to the two types of fault compensation strategies are 0 or 1: when the fault is determined to be type I, kI=1 and kII=0; when the fault is determined to be type II, kI=0 and kII=1; arccos represents the inverse cosine function.

[0066] Figure 8 The power angle characteristic curve of the system under the optimal current compensation angle configuration, after adopting the optimal current limiting angle derived in this embodiment in CLC1 and CLC2 modes respectively, can significantly reduce the acceleration area, increase the deceleration area and improve the ultimate fault clearing angle, while effectively avoiding the system locking in CLC2 mode.

[0067] In a preferred embodiment, to verify the effectiveness of the above theoretical analysis and the proposed control strategy under different fault depths, the following steps are taken: Figure 1 The GFM-VSC grid-connected system shown was tested for fault ride-through and transient stability using a grid-type converter employing the control strategy of this invention. The main circuit of the system includes a grid-type voltage source converter GFM-VSC, a parallel filter branch, a grid-connected reactor, and the connected AC grid. The converter control section adopts the adaptive transient control strategy based on dynamic parameter adjustment and optimal current compensation angle configuration described in this invention. The main electrical and control parameters of the system are shown in Table 1, where the converter current limit is set to I_max = 2.0 pu.

[0068] Table 1 System Parameters Based on the above parameters, the GFM-VSC operates according to the aforementioned active / reactive droop control and current limiting logic. When the output current reaches the limit value I_max, CLC is triggered, and the converter switches from voltage source mode to controlled current source mode. To examine the adaptability of the method provided in this application under different fault depths, this embodiment sets two representative types of grid voltage sag faults: Type I fault: a stable equilibrium point (SEP) still exists during the fault; Type II fault: no SEP exists during the fault, and CLC triggering is unavoidable. Comparative simulations are performed under the two conditions of not using the control strategy of this application and using the control strategy of this application, comparing the transient response characteristics of key variables such as output voltage, current, power angle, and phase trajectory.

[0069] I. Simulation Example of Type I Fault (Medium Voltage Drop with SEP) In this scenario, the simulated power grid experiences a moderate voltage drop while a SEP (Self-Effective Power Buffer) still exists during the fault, which falls under the Type I fault definition of this application. Specifically, after the system stabilizes, the AC side voltage drops to 0.6 pu at t = 1 s, and the fault duration is 200 ms.

[0070] 1. Operating conditions where the control strategy of this invention is not adopted.

[0071] Simulations were performed while maintaining traditional network control parameters and without introducing the dynamic parameter adjustment and virtual inductor optimization strategies described in this invention. The results are as follows: Figure 9 and Figure 10 As shown.

[0072] like Figure 9 As shown, when a substantial fault with a voltage drop of 0.6 pu occurs at t = 1 s, the output current of the GFM-VSC rises sharply. After the fault is cleared, the CLC is triggered, and the output current is limited to around 2.0 pu, with the converter entering controlled current source operation mode. Simultaneously, a significant overshoot occurs in the output voltage, with a peak value of approximately 1.5 pu, and the grid connection voltage support capability decreases significantly, indicating that the system experienced transient instability during the fault recovery phase.

[0073] like Figure 10 The diagram shows the corresponding system phase trajectory. After the fault occurs, the system operating point migrates from the steady-state equilibrium point a to point b under the fault condition, and the transient energy increases significantly. After the fault is cleared, the energy at the operating point continues to accumulate, crossing the unstable equilibrium point UEP. The corresponding transient energy exceeds the critical value, and eventually moves away from the original SEP along the c–d trajectory, unable to return to the original attraction domain. This reflects that the system has lost transient stability under type I fault conditions.

[0074] Therefore, if the control parameters and current limiting behavior are not coordinated, the CLC will destroy the original attraction domain structure after a fault, resulting in voltage overshoot, power angle instability and loss of voltage support capability.

[0075] 2. Operating conditions under which the control strategy of this invention is employed.

[0076] Under the same fault conditions, the dynamic parameter adjustment strategy designed for Type I faults in this invention is used to optimize the active power setpoint and virtual inductance online. Based on the aforementioned energy function analysis and current limiting constraints, the following calculations are obtained: P′ref = 0.592 pu, L v1 _min = 2.72 mH, L v2 _min = 3.33 mH.

[0077] The simulation was performed under these parameter configurations, and the results are as follows: Figure 11 and Figure 12 As shown. Figure 11 As shown, during the fault occurrence and clearing process, the GFM-VSC output voltage remained basically near the pre-fault rated value, the power angle fluctuation amplitude was significantly reduced, the output current did not exceed the limit value of 2.0 pu throughout the entire process, the CLC was not triggered, the overcurrent phenomenon was effectively suppressed, and the converter always maintained the voltage source characteristics and provided stable voltage support to the grid. Figure 12 As shown, after adopting the strategy of this invention, the system phase trajectory always lies within the attraction domain, and the operating point evolves along the trajectory a–b–c–d. After the fault is cleared, it returns to the vicinity of the stable equilibrium point SEP and gradually converges. This indicates that the system achieves transient stable operation and smooth recovery under Type I faults, verifying the effectiveness of this invention in reducing P. ref 1. Choose D appropriately q And improve L online v The design philosophy is to expand the attraction domain and avoid CLC triggering.

[0078] II. Simulation Example of Type II Fault (Deep Voltage Drop without SEP) In this scenario, a deep voltage drop occurs in the simulated power grid. During the fault, there is no stable equilibrium point (SEP), and CLC triggering is unavoidable, classifying it as a Type II fault as defined in this invention. Specifically, after the system stabilizes, the AC side voltage drops to 0.1 pu at t = 1 s, with the fault duration being 130 ms.

[0079] 1. Operating conditions where the control strategy of this invention is not adopted.

[0080] Simulations were performed under traditional control parameters, and the results are as follows: Figure 13 As shown.

[0081] Before the fault occurred, the system operating angle was δ. SEP= 0.459 rad, the theoretical calculated value is 0.460 rad, the two are basically consistent, indicating that the modeling and parameter tuning under steady state are accurate. After the fault occurred and was cleared, because the current limiting angle and switching conditions during CLC were not specifically designed, the system could not exit the current limiting mode after running in CLC1 and CLC2 modes, and was eventually locked in CLC2 mode.

[0082] Simulation results show that after the fault ends, the system stabilizes around δ = −1.391 rad, corresponding to a theoretically calculated value of −1.345 rad. At this point, the GFM-VSC exhibits typical controlled current source characteristics: the output current is continuously limited to 2.0 pu, the grid-connected port voltage rises to approximately 1.51 pu, and the converter loses its normal voltage regulation capability and grid-connected characteristics. These results indicate that in a deep fault type II scenario, relying solely on traditional CLC logic without current compensation angle and mode switching boundary design can easily lead to CLC2 mode lock-in, causing the converter to remain in a current-limited state for an extended period, severely threatening system safety.

[0083] 2. Operating conditions under which the control strategy of this invention is employed.

[0084] Under the same voltage drop depth, the current compensation angle configuration and mode switching strategy based on the equal area criterion designed for Type II faults in this invention are adopted. According to the aforementioned dynamic power angle analysis and equal area criterion, the following can be obtained: critical fault clearing angle: δc = 0.887 rad; CLC to CVC switching angle: δs = 0.767 rad; the optimal current compensation angle in CLC1 and CLC2 modes is uniformly expressed as: Δφ = −0.473 kⅠ − 1.514 kⅡ.

[0085] The simulation was performed under the above configuration, and the results are as follows: Figure 14 As shown. After a fault occurs, the system first enters CLC1 mode, where a compensation angle ΔφⅠ is introduced to minimize the acceleration area S1. After the fault is cleared, the system enters CLC2 mode, where a compensation angle ΔφⅡ is introduced to maximize the deceleration area S2. Simultaneously, the design of ΔφⅡ ensures that the stable equilibrium point of CLC2 mode coincides with the switching angle δs, thus guaranteeing that the CLC→CVC mode switching is triggered within a suitable power angle range. Simulation results show that the system smoothly switches from CLC2 mode to CVC mode at t ≈ 1.21 s, subsequently gradually recovering its voltage regulation capability, and the power angle δ converges back to δ SEP = 0.459 rad, consistent with the pre-fault value. Throughout the deep fault, the system did not exhibit CLC mode lockout, and the voltage support capability was effectively restored, verifying the correctness and effectiveness of the ΔφⅠ and ΔφⅡ configuration criteria and the CLC→CVC switching boundary conditions derived in this invention.

[0086] Based on the above simulation embodiments, the converter transient control method under AC system fault scenarios provided in this application can be implemented in engineering applications according to the following process: 1. In the GFM-VSC controller, the equivalent reactance Leq of the AC system is estimated based on instantaneous voltage and current measurements, and the fault depth is determined according to the ratio of Leq to line inductance Lg. When Leq is within a predetermined range, it is determined to be a Type I fault, and when Leq is below a deeper threshold, it is determined to be a Type II fault.

[0087] 2. For voltage drop conditions identified as Type I faults, based on the energy function and attraction domain analysis results, the active power setpoint Pref is reduced online, the voltage droop coefficient Dq is increased, and the virtual inductance Lv is increased according to the calculation results during the fault and after the fault is cleared, so as to maximize the system attraction domain and minimize the current limiting region, thereby suppressing overcurrent and maintaining voltage source characteristics without triggering CLC.

[0088] 3. For deep voltage drop conditions identified as Type II faults, derive the critical cut-off angle δc and switching angle δs based on the power angle equation and the equal area criterion, calculate the current compensation angles ΔφⅠ and ΔφⅡ in CLC1 and CLC2 modes, minimize the acceleration area S1 and maximize the deceleration area S2, and make the balance point of CLC2 mode coincide with δs, ensuring that the system has the ability to recover from CLC2 to CVC mode.

[0089] Specifically, when the output current reaches the limit value I_max, CLC is activated. The comprehensive compensation angle expression Δφ = −0.473 kⅠ −1.514 kⅡ is used. When switching between CLC1 and CLC2 modes, ΔφⅠ and ΔφⅡ are automatically switched to achieve dynamic shaping of the power angle trajectory and prevent excessive acceleration area and mode lock-in under deep fault conditions.

[0090] 4. After the fault is cleared, when the system power angle reaches the switching angle δs and the output current and voltage simultaneously meet preset conditions, the system smoothly switches from CLC mode to CVC mode. The preset conditions include: the converter output current does not exceed the allowable upper limit Imax, the grid connection point voltage recovers to within the allowable deviation range of the rated voltage, and these conditions are maintained continuously for a preset time. After the switching is completed, the control parameters adjusted during the fault period are gradually restored to their preset steady-state values. Let any control parameter to be restored be denoted as ξ, and its value during the fault period be ξ. f If the preset steady-state value is ξ0, then its recovery process can be expressed as follows: in, Tξ represents the start time of the switch from CLC mode to CVC mode, where Tξ is the recovery time constant of parameter ξ, and ξ includes P. refD q and L v One or more of the parameters. Through the above gradual recovery method, secondary oscillations caused by sudden changes in control parameters can be avoided, and the system can be guaranteed to smoothly return to the preset steady-state operating point.

[0091] In summary, to achieve the goal of preventing CLC triggering under mild faults and ensuring safe and smooth switching between CLC and CVC modes under deep faults, thereby improving the transient stability of the grid-connected system, this application proposes a converter transient control method under AC system fault scenarios. This method primarily aims to improve the transient stability of GFM-VSCs under large disturbances such as grid voltage drops, addressing the problems of transient instability, mode lock-up, and difficulty in safe recovery after faults caused by easily triggering CLC at different fault depths. By identifying fault depth online, distinguishing between Type I and Type II faults, and implementing dynamic parameter adjustments and current compensation angle optimization control respectively, the GFM-VSC can avoid triggering CLC under mild faults and achieve safe and smooth switching between CLC and CVC modes under deep faults, thus ensuring voltage support and power delivery capabilities under all fault scenarios. Compared with existing technologies, this application has at least the following beneficial effects: (1) By constructing an improved energy function and power angle characteristic analysis framework, the intrinsic influence mechanism of current limiting on transient stability under mild and deep faults is revealed. Based on this, differentiated control objectives for different fault depths are proposed, overcoming the shortcomings of a single control strategy that cannot cover all fault conditions.

[0092] (2) For Type I faults, the embodiments of this application maximize the system attraction domain without sacrificing steady-state control performance by dynamically coordinating the adjustment of multiple parameters such as active power command and virtual impedance, effectively suppressing the false triggering of CLC during and after the fault; for Type II faults, by precisely configuring the current compensation angle associated with the fault depth, the acceleration area is minimized and the deceleration area is maximized, fundamentally improving the survivability and stability margin of GFM-VSC under deep fault conditions.

[0093] (3) The current compensation angle configuration criteria derived in this application ensures the transient stability of the system under type II faults, while strictly providing the boundary conditions for safe switching from CLC2 mode to CVC mode. This completely avoids the problem of CLC mode mis-locking caused by improper current limiting angle configuration after fault clearance, ensures the safe and smooth transition of grid-type converter control mode, and improves the overall operational reliability of the new power system under the condition of high proportion of renewable energy access.

[0094] Example 2: like Figure 15As shown, Embodiment 2 provides a converter transient control system under AC system fault scenarios, including an acquisition module 10, a reactance calculation module 20, a fault judgment module 30, a first control module 40, and a second control module 50; The acquisition module 10 is used to acquire the voltage sampling value of the grid connection point and the current sampling value of the AC side at several sampling times. The reactance calculation module 20 is used to calculate the equivalent reactance value of the AC system based on each of the voltage sampling values ​​and each of the current sampling values. The fault determination module 30 is used to calculate the current fault depth based on the equivalent reactance value and the preset reactance reference value, and then determine the current fault type of the AC system based on the fault depth. The first control module 40 is used to determine the active power setpoint and virtual inductance value of the converter according to the fault depth if the fault type is the first fault type, so as to keep the converter in transient stability. The second control module 50 is used to switch the control mode of the converter to the current limiting mode according to the real-time output current of the converter if the fault type is the second fault type. In the current limiting mode, the fault clearing power angle and the stable balance power angle of the converter are determined by the equal area rule based on the power angle equation of the converter. Then, the current compensation angle of the converter before and after fault clearing is determined according to the fault clearing power angle and the stable balance power angle, so as to maintain the transient stability of the converter in the current limiting mode based on the current compensation angle.

[0095] Furthermore, the reactance calculation module 20 calculates the equivalent reactance value of the AC system based on each of the voltage sample values ​​and each of the current sample values, including: The average voltage sample value at each sampling time is calculated based on the first and second adjacent voltage sample values ​​at each adjacent sampling time of each voltage sample value. The average current sample value at each sampling time is calculated based on the first and second adjacent current sample values ​​at each adjacent sampling time of each current sample value. The current difference value at each sampling time is calculated based on each of the first adjacent current sample values, each of the second adjacent current sample values, and the sampling interval. The equivalent reactance of the AC system is calculated by fitting the average voltage sample value, the average current sample value, and the current difference value using the least squares method.

[0096] Furthermore, the fault determination module 30 calculates the current fault depth based on the equivalent reactance value and a preset reactance reference value, and then determines the current fault type of the AC system based on the fault depth, including: The current fault depth is calculated by dividing the equivalent reactance value by the reactance reference value. If the fault depth is less than a first preset threshold, then the current fault type of the AC system is determined to be the first fault type; If the fault depth is less than the second preset threshold, then the current fault type of the AC system is determined to be the second fault type; The first preset threshold and the second preset threshold are determined based on the power angle characteristic curves of the converter at different fault depths. The first fault type is that there is a stable equilibrium point in the power angle characteristic curve of the converter, and the second fault type is that there is no stable equilibrium point in the power angle characteristic curve of the converter.

[0097] In one possible implementation, if the fault type is a first fault type, the first control module 40 determines the active power setpoint and virtual inductance value of the converter based on the fault depth to maintain transient stability of the converter, including: The active power setpoint of the converter is calculated based on the fault depth, the real-time electromotive force amplitude output by the converter, the real-time voltage amplitude of the grid connection point, the equivalent reactance value of the grid connection point, and the preset balance point power angle threshold. Establish the energy function of the AC system after introducing a virtual inductor, and determine the trigger condition for the converter to switch from the constant voltage control mode to the current limiting mode based on the energy function; The virtual inductance value for maintaining the constant voltage control mode is calculated based on the triggering conditions. Based on the active power setpoint and the virtual inductance value, the converter is controlled to maintain transient stability in constant voltage control mode.

[0098] Furthermore, if the fault type is the first fault type, in addition to determining the active power setpoint and virtual inductance value of the converter based on the fault depth, the voltage droop coefficient of the converter is also determined based on the fault depth. Then, based on the active power setpoint, the virtual inductance value, and the voltage droop coefficient, the converter is controlled to maintain transient stability in constant voltage control mode.

[0099] Furthermore, after the converter is controlled to maintain transient stability in constant voltage control mode for a first preset time based on the active power setpoint and the virtual inductance value, it is determined whether the current fault of the AC system has been repaired. If the fault has not been repaired, the active power setpoint and virtual inductance value of the converter are updated, and the converter is subjected to the next round of transient control based on the updated active power setpoint and virtual inductance value.

[0100] In one possible implementation, if the fault type is a second fault type, the second control module 50 switches the control mode of the converter to a current limiting mode based on the real-time output current of the converter, and determines the current compensation angle of the converter before and after fault clearing based on the fault clearing power angle and the stable balance power angle of the converter, so as to maintain transient stability of the converter in the current limiting mode, including: Obtain the real-time output current and real-time power angle of the converter; If the real-time output current is greater than the preset maximum output current value, the control mode of the converter is switched to the current limiting mode according to the real-time output current of the converter. Based on the power angle equations of the converter during and after a fault, the fault clearing power angle and the stable equilibrium power angle are calculated using the equal area criterion. The first current compensation angle of the converter is calculated based on the real-time power angle, the fault clearing power angle, and the preset balance point power angle threshold. The second current compensation angle of the converter is calculated based on the real-time power angle, the stable balance power angle, and the current active power setpoint of the converter. Based on the first current compensation angle and the second current compensation angle, the converter is controlled to maintain transient stability in the current limiting mode.

[0101] Furthermore, controlling the converter to maintain transient stability in current-limiting mode based on the first current compensation angle and the second current compensation angle includes: The converter is controlled to maintain transient stability in current limiting mode according to the first current compensation angle until the fault is cleared. When the fault is cleared, the converter is controlled to switch from the current limiting mode to the constant voltage control mode according to the second current compensation angle.

[0102] This application provides a transient control system for a converter under AC system fault scenarios. By real-time acquisition of voltage and current samples at the grid connection point, and calculation of the equivalent reactance of the AC system based on this data, the fault depth is assessed and the fault type is determined. Finally, differentiated control strategies are adopted according to the fault type. Compared with existing technologies, this embodiment, through dynamic calculation of equivalent reactance and fault depth, can accurately identify the real-time state of the power grid, thereby achieving accurate differentiation of fault types and providing a scientific basis for subsequent control strategy selection. Secondly, for the first fault type (e.g., a minor fault), by adjusting the active power setpoint and virtual inductance value of the converter, transient stability is maintained in constant voltage control mode, effectively avoiding the problem of insufficient voltage and power support caused by conservative current limiting in traditional methods. For the second fault type (e.g., a deep fault), the system can quickly switch to current limiting mode and ensure that the converter maintains transient stability under current limiting by calculating the current compensation angle before and after fault clearance. This embodiment not only improves the operational stability of the converter under different fault depths but also significantly enhances the voltage and frequency support capability of the power grid during faults, helping to reduce the risk of system oscillation and transient instability.

[0103] For a more detailed explanation of the working principle and procedures of this embodiment, please refer to the relevant description in Embodiment 1.

[0104] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of this application. It should be understood that the above descriptions are merely specific embodiments of this application and are not intended to limit the scope of protection of this application. In particular, it should be noted that any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application for those skilled in the art.

Claims

1. A transient control method for a converter under AC system fault scenarios, characterized in that, include: Obtain the voltage sampling value and AC side current sampling value at the grid connection point at several sampling times; The equivalent reactance of the AC system is calculated based on each of the voltage and current sample values. The current fault depth is calculated based on the equivalent reactance value and the preset reactance reference value, and then the current fault type of the AC system is determined based on the fault depth. If the fault type is the first fault type, then the active power setpoint and virtual inductance value of the converter are determined according to the fault depth so that the converter can maintain transient stability. If the fault type is the second fault type, the control mode of the converter is switched to the current limiting mode according to the real-time output current of the converter. In the current limiting mode, the fault clearing power angle and the stable balance power angle of the converter are determined by the equal area rule based on the power angle equation of the converter. Then, the current compensation angle of the converter before and after fault clearing is determined according to the fault clearing power angle and the stable balance power angle, so as to maintain the transient stability of the converter in the current limiting mode based on the current compensation angle.

2. The converter transient control method under AC system fault scenarios as described in claim 1, characterized in that, The step of calculating the equivalent reactance of the AC system based on each of the voltage sample values ​​and each of the current sample values ​​includes: The average voltage sample value at each sampling time is calculated based on the first and second adjacent voltage sample values ​​at each adjacent sampling time of each voltage sample value. The average current sample value at each sampling time is calculated based on the first and second adjacent current sample values ​​at each adjacent sampling time of each current sample value. The current difference value at each sampling time is calculated based on each of the first adjacent current sample values, each of the second adjacent current sample values, and the sampling interval. The equivalent reactance of the AC system is calculated by fitting the average voltage sample value, the average current sample value, and the current difference value using the least squares method.

3. The converter transient control method under AC system fault scenarios as described in claim 1, characterized in that, The step of calculating the current fault depth based on the equivalent reactance value and a preset reactance reference value, and then determining the current fault type of the AC system based on the fault depth, includes: The current fault depth is calculated by dividing the equivalent reactance value by the reactance reference value. If the fault depth is less than a first preset threshold, then the current fault type of the AC system is determined to be the first fault type; If the fault depth is less than the second preset threshold, then the current fault type of the AC system is determined to be the second fault type; The first preset threshold and the second preset threshold are determined based on the power angle characteristic curves of the converter at different fault depths. The first fault type is that there is a stable equilibrium point in the power angle characteristic curve of the converter, and the second fault type is that there is no stable equilibrium point in the power angle characteristic curve of the converter.

4. The converter transient control method under AC system fault scenarios as described in claim 1, characterized in that, If the fault type is the first fault type, then the active power setpoint and virtual inductance value of the converter are determined according to the fault depth to maintain the transient stability of the converter, including: The active power setpoint of the converter is calculated based on the fault depth, the real-time electromotive force amplitude output by the converter, the real-time voltage amplitude of the grid connection point, the equivalent reactance value of the grid connection point, and the preset balance point power angle threshold. An energy function of the AC system after the introduction of a virtual inductor is established, and the triggering condition for the converter to switch from constant voltage control mode to current limiting mode is determined based on the energy function. The virtual inductance value for maintaining the constant voltage control mode is calculated based on the triggering conditions. Based on the active power setpoint and the virtual inductance value, the converter is controlled to maintain transient stability in constant voltage control mode.

5. The converter transient control method under AC system fault scenarios as described in claim 4, characterized in that, If the fault type is the first fault type, in addition to determining the active power setpoint and virtual inductance value of the converter according to the fault depth, the voltage droop coefficient of the converter is also determined according to the fault depth. Then, based on the active power setpoint, the virtual inductance value and the voltage droop coefficient, the converter is controlled to maintain transient stability in constant voltage control mode.

6. The converter transient control method under AC system fault scenarios as described in claim 4, characterized in that, After the converter is controlled to maintain transient stability in constant voltage control mode for a first preset time based on the active power setpoint and the virtual inductance value, it is determined whether the current fault of the AC system has been repaired. If the fault has not been repaired, the active power setpoint and virtual inductance value of the converter are updated, and the converter is subjected to the next round of transient control based on the updated active power setpoint and virtual inductance value.

7. The converter transient control method under AC system fault scenarios as described in claim 1, characterized in that, If the fault type is the second fault type, the control mode of the converter is switched to the current limiting mode according to the real-time output current of the converter. In the current limiting mode, the fault clearing power angle and the stable balance power angle of the converter are determined based on the power angle equation of the converter using the equal area rule. Then, the current compensation angle of the converter before and after fault clearing is determined based on the fault clearing power angle and the stable balance power angle, respectively, so that the converter can maintain transient stability in the current limiting mode based on the current compensation angle. This includes: Obtain the real-time output current and real-time power angle of the converter; If the real-time output current is greater than the preset maximum output current value, the control mode of the converter is switched to the current limiting mode according to the real-time output current of the converter. Based on the power angle equations of the converter during and after a fault, the fault clearing power angle and the stable equilibrium power angle are calculated using the equal area criterion. The first current compensation angle of the converter is calculated based on the real-time power angle, the fault clearing power angle, and the preset balance point power angle threshold. The second current compensation angle of the converter is calculated based on the real-time power angle, the stable balance power angle, and the current active power setpoint of the converter. Based on the first current compensation angle and the second current compensation angle, the converter is controlled to maintain transient stability in the current limiting mode.

8. The converter transient control method under AC system fault scenarios as described in claim 7, characterized in that, The step of controlling the converter to maintain transient stability in current-limiting mode based on the first current compensation angle and the second current compensation angle includes: The converter is controlled to maintain transient stability in current limiting mode according to the first current compensation angle until the fault is cleared. When the fault is cleared, the converter is controlled to switch from the current limiting mode to the constant voltage control mode according to the second current compensation angle.

9. A transient control system for a converter under AC system fault scenarios, characterized in that, It includes an acquisition module, a reactance calculation module, a fault diagnosis module, a first control module, and a second control module; The acquisition module is used to acquire the voltage sampling value of the grid connection point and the current sampling value of the AC side at several sampling times; The reactance calculation module is used to calculate the equivalent reactance value of the AC system based on each of the voltage sampling values ​​and each of the current sampling values. The fault determination module is used to calculate the current fault depth based on the equivalent reactance value and the preset reactance reference value, and then determine the current fault type of the AC system based on the fault depth. The first control module is used to determine the active power setpoint and virtual inductance value of the converter according to the fault depth if the fault type is the first fault type, so as to keep the converter in transient stability. The second control module is used to switch the control mode of the converter to the current limiting mode according to the real-time output current of the converter if the fault type is the second fault type. In the current limiting mode, the fault clearing power angle and the stable balance power angle of the converter are determined by the equal area rule based on the power angle equation of the converter. Then, the current compensation angle of the converter before and after fault clearing is determined according to the fault clearing power angle and the stable balance power angle, so as to maintain the transient stability of the converter in the current limiting mode based on the current compensation angle.

10. The converter transient control system under AC system fault scenarios as described in claim 9, characterized in that, The reactance calculation module calculates the equivalent reactance value of the AC system based on each of the voltage sample values ​​and each of the current sample values, including: The average voltage sample value at each sampling time is calculated based on the first and second adjacent voltage sample values ​​at each adjacent sampling time of each voltage sample value. The average current sample value at each sampling time is calculated based on the first and second adjacent current sample values ​​at each adjacent sampling time of each current sample value. The current difference value at each sampling time is calculated based on each of the first adjacent current sample values, each of the second adjacent current sample values, and the sampling interval. The equivalent reactance of the AC system is calculated by fitting the average voltage sample value, the average current sample value, and the current difference value using the least squares method.

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