GFM converter fault ride-through control method and system

Abstract

The invention provides a GFM converter fault ride-through control method and system. According to the method, when the voltage drop of a power grid is detected, the power angle stability and the voltage supporting capability during a fault period are enhanced by adaptively adjusting active and reactive power reference values; meanwhile, when the output current exceeds a set threshold value, the virtual resistor and the reactance are dynamically injected, the transient impact current is rapidly suppressed, and dual limitation on the fault current is achieved. The control system comprises a voltage and current acquisition unit, a fault detection module, a self-adaptive power instruction control module, a dynamic virtual impedance control module and an instruction generation module, and all the modules operate cooperatively to form a complete closed-loop control structure. According to the method, the operation stability and the power grid supporting capacity of the GFM converter in weak power grid and fault scenes can be improved, and the requirements of power grid specifications such as GC0137 and the like are met.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronics and new energy grid-connected control, and in particular to a GFM converter fault ride-through control method and system based on adaptive power instructions and dynamic virtual impedance. Background Art

[0002] With the widespread access to renewable energy generation, the inertial support capacity of the power system has significantly decreased, resulting in a weakening of the system's frequency regulation capability and an increasingly prominent voltage stability problem. Grid-forming (GFM) converters, due to their controllable voltage source characteristics, have become an important direction for improving system stability and support capabilities. Among them, the control method based on the virtual synchronous generator (VSG) principle simulates the operating behavior of traditional synchronous generators and demonstrates good adaptability in voltage support and frequency regulation, and has become a research hotspot in GFM control technology. Compared with traditional power decoupling control methods, VSG control can achieve active response to grid frequency and nearby voltage, and has stronger adaptability to weak grids.

[0003] In response to the operating behavior of GFM converters during grid faults, studies have proposed various fault current limiting strategies to suppress overcurrent problems. Existing methods mainly fall into three categories: first, switching the GFM control to a grid-following control mode during a fault to improve current control performance; second, directly limiting the reference value of the current inner loop to achieve precise current amplitude control while maintaining the GFM operating mode; and third, introducing virtual impedance technology to achieve current limiting by increasing the equivalent output impedance. Although the above methods can alleviate the risk of overcurrent to a certain extent, they mostly focus on the physical suppression of current amplitude and rarely systematically consider the impact of the current limiting process on the system power angle stability and voltage support capability, making it difficult to achieve comprehensive dynamic performance assurance during fault ride-through.

[0004] When a short circuit or severe voltage drop occurs in the power grid, GFM converters are unable to withstand currents significantly exceeding their rated values, often causing them to shut down or even cause large-scale grid disconnections, threatening the safety and stability of the entire power system. Therefore, the current technical challenge lies in ensuring the safe operation of the converter while simultaneously achieving rapid current limiting during a fault, and balancing system power angle stability and voltage support capabilities to enhance its fault ride-through capability. Summary of the Invention

[0005] In order to solve the problems of the prior art, the present invention provides a method and system for controlling fault ride-through of a GFM converter. The technical solution is as follows:

[0006] In one aspect, a GFM converter fault ride-through control method is provided, comprising the following steps:

[0007] (1) During the grid-connected operation of the GFM converter, the grid connection point voltage and the GFM converter output current are collected in real time. When the grid connection point voltage drops below 0.9 times the rated voltage, the fault ride-through control process is started;

[0008] (2) Start adaptive power command control and dynamically adjust the active power reference value and reactive power reference value according to the per-unit value of the grid connection point voltage, where:

[0009] The reactive power reference value is generated according to the voltage drop level in accordance with the GC0137 specification.

[0010] The active power reference value is corrected according to the voltage reference value and the allowable apparent power to maintain a stable power angle during the fault period;

[0011] (3) While executing adaptive power command control, if it is detected that the output current of the GFM converter is greater than 1.2 times the rated current, the dynamic virtual impedance control module is activated to construct a virtual resistance and virtual reactance based on the difference between the output current and the current threshold to suppress the transient impact current during the fault occurrence and clearing process;

[0012] (4) When the output current of the GFM converter recovers to a value not greater than the current threshold, the dynamic virtual impedance control module is turned off and the adaptive power command control is continued until the fault current meets the grid support requirements and the fault ride-through control process is terminated.

[0013] Furthermore, in step (2), the reactive power reference value Q * ref Adaptive adjustment is done as follows:

[0014] When the grid voltage per unit value U s When it is lower than 0.9pu, the preliminary reactive current injection value is calculated first, and its expression is:

[0015] i qref0 =-1.4578U s +1

[0016] Compensation is performed on the basis of the preliminary reactive current injection value to obtain the final injected reactive current, which is expressed as follows:

[0017] i qref1 =K q (0.9-U s )+(-1.4578U s +1)

[0018] Reactive power reference value Q* ref The final calculation expression is:

[0019]

[0020] Among them, K q is the reactive current compensation coefficient.

[0021] Furthermore, in step (2), the active power reference value P * ref The generation methods include:

[0022] Based on the allowable apparent power S after the power grid fault fault , active power reference value P * ref Adaptive adjustment is performed according to the following formula:

[0023]

[0024] Where: S fault is the allowable apparent power after a grid fault, and its calculation formula is as follows:

[0025]

[0026] Where: U n is the system rated voltage; S N is the rated capacity of the system.

[0027] Furthermore, the construction method of the dynamic virtual impedance in step (3) includes:

[0028] When the converter output current I s Exceeding the current threshold I th When the dynamic virtual impedance control module is started, a virtual resistance R proportional to the current difference is constructed. Ⅵ and virtual reactance X Ⅵ , and its calculation formula is:

[0029]

[0030] X VI =n X / R R VI

[0031] Among them, m R is the virtual impedance coefficient, n X / R is the reactance ratio of the virtual impedance, I th The current threshold is set to 1.2 times the rated current.

[0032] Furthermore, the exit method of the dynamic virtual impedance control in step (4) includes:

[0033] When the GFM converter outputs current I s Recover to below the current threshold I th When the virtual resistor R Ⅵ With virtual reactance X Ⅵ , so that the dynamic virtual impedance value returns to zero, that is:

[0034] Z Ⅵ =0

[0035] Wherein, the current threshold I th The virtual impedance is set to 1.2 times the rated current of the converter. The virtual impedance is activated only in the transient phase of the fault occurrence and clearing process and automatically exits in the steady state to avoid affecting the power regulation in normal operation.

[0036] In another aspect, a control system for implementing fault ride-through control of a GFM converter is provided, comprising:

[0037] Voltage and current acquisition unit, used to collect grid connection point voltage and GFM converter output current in real time;

[0038] A fault detection module is used to determine whether the grid connection point voltage drops below a set threshold and trigger a fault ride-through control process when the condition is met;

[0039] Adaptive power command control module, used to adjust the active power reference value and reactive power reference value based on the per-unit value of the grid connection point voltage to meet the grid support requirements and maintain power angle stability during faults;

[0040] Dynamic virtual impedance control module, which is used to construct virtual resistance and virtual reactance proportional to the current difference when detecting that the output current of the GFM converter exceeds the set current threshold, thereby suppressing transient inrush current during fault occurrence and clearing;

[0041] An instruction generation module is used to generate a control instruction according to the output parameters of the adaptive power instruction control module and the dynamic virtual impedance control module to drive the GFM converter to perform fault ride-through control.

[0042] Furthermore, the adaptive power instruction control module includes:

[0043] The reactive power command generation unit is used to generate a preliminary reactive current injection value according to the voltage drop degree when the per-unit value of the grid connection point voltage is lower than 0.9pu, and adjust the target reactive power reference value in combination with the compensation coefficient;

[0044] The active power instruction correction unit is used to dynamically correct the active power reference value based on the relationship between the voltage reference value and the apparent power allowed during the fault period, so as to improve the power angle stability during the fault response process.

[0045] Furthermore, the dynamic virtual impedance control module includes:

[0046] A current threshold judgment unit is used to judge whether the output current of the GFM converter exceeds a set current threshold;

[0047] A virtual impedance construction unit is used to construct corresponding virtual resistance and virtual reactance according to the current difference when the output current exceeds the threshold, and inject them into the control loop to limit the transient impact current;

[0048] The impedance removal unit is used to automatically turn off the injection control of the virtual impedance after the output current drops below the current threshold.

[0049] Furthermore, the instruction generation module generates control instructions adapted to the power control loop of the GFM converter by collecting voltage and current information, integrating the active and reactive power reference values ​​output by the adaptive power instruction control module, and the virtual impedance parameters output by the dynamic virtual impedance control module, and is used to drive the inverter to perform current regulation and voltage support response.

[0050] Furthermore, the control system is deployed in a GFM converter control platform that supports grid-connected operation, and is suitable for responding to grid voltage sag or short-circuit fault conditions, meeting the GC0137 standard's control requirements for reactive support response time, current limiting capability, and transient stability.

[0051] The technical solution provided by the embodiment of the present invention has the following beneficial effects:

[0052] The present invention provides a GFM converter fault ride-through control method and system. By combining adaptive power command control with dynamic virtual impedance regulation, it can effectively achieve output current limiting control and power angle stabilization regulation of the converter when a voltage drop or short circuit fault occurs in the power grid, thereby improving the system's transient stability and fault response performance.

[0053] Specifically, by setting the voltage drop threshold to activate the adaptive power command control module, the active and reactive power reference values ​​are dynamically adjusted according to the fault voltage level, so that the GFM converter can quickly inject reactive current into the grid while ensuring power balance, meeting the response speed and amplitude requirements of the grid support standard. At the same time, through the dynamic virtual impedance module, when the fault current exceeds the threshold, virtual resistance and virtual reactance proportional to the current difference are injected, enhancing the system damping and equivalent impedance, and significantly suppressing the inrush current during the fault occurrence and clearing stages.

[0054] In addition, the system module proposed in the present invention has a clear structure and reasonable functional layering. It can realize the coordinated control of the entire process of rapid acquisition of voltage and current, fault judgment, power instruction correction, virtual impedance injection and final control instruction generation, ensuring that the converter continues to operate in grid connection during faults and avoids the risk of grid disconnection. It is suitable for grid-connected converter control platforms in scenarios with a high proportion of new energy grid connection, and has good engineering application value and promotion prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0056] Figure 1 This is an overall control block diagram of the fault ride-through control strategy for the GFM converter in Example 1 of the present invention;

[0057] Figure 2 This is a diagram of the minimum reactive current injection requirement under the GC0137 grid specification in Example 1 of the present invention;

[0058] Figure 3 This is a schematic diagram of the principle structure of dynamic virtual impedance control in Example 1 of the present invention;

[0059] Figure 4 This is a control flow chart of the GFM fault ride-through control method in Example 1 of the present invention;

[0060] Figure 5 Schematic diagram of the module structure of the GFM converter fault ride-through control system in Example 2 of the present invention;

[0061] Figure 6 This is a simulation waveform diagram of the three-phase short circuit output of the GFM converter under different fault impedances in an application example of the present invention;

[0062] Figure 7 This is a comparison chart of the output current of the GFM converter under different fault impedances in the application example of the present invention and the GC0137 standard;

[0063] Figure 8 This is a comparison chart of transient responses under traditional current limiting control and the control strategy of the present invention in an application example of the present invention. DETAILED DESCRIPTION

[0064] To make the objectives, technical solutions and advantages of the present invention more clear, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0065] To facilitate understanding of the present invention, the technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments. The present invention relates to a method and system for controlling fault ride-through of a GFM converter based on adaptive power commands and dynamic virtual impedance, including control method steps for optimizing current response and stability during a fault, and a control system structure design with coordinated operation of various functional modules.

[0066] The control method specifically includes the following process: first, during the grid-connected operation of the GFM converter, the grid connection point voltage and the GFM converter output current are collected in real time; when it is detected that the grid connection point voltage drops below a set threshold (such as 0.9 times the rated voltage), the fault ride-through control process is triggered; then, the adaptive power command control module is started to dynamically generate a reactive power reference value according to the voltage drop degree, and the active power reference value is adjusted according to the apparent power limit condition of the grid fault to ensure the power angle stability during the fault; if the converter output current exceeds the set current threshold (such as 1.2 times the rated current), the dynamic virtual impedance control module is started to inject a virtual resistance and virtual reactance proportional to the current difference to suppress transient impact current; finally, a control command is generated according to the adjusted power command and virtual impedance parameters to drive the converter into the fault ride-through operation state.

[0067] The control system comprises a voltage and current acquisition unit, a fault detection module, an adaptive power command control module, a dynamic virtual impedance control module, and a command generation module. These modules are interconnected according to a control flow, enabling them to jointly complete the full control process during grid faults, including current detection, fault identification, power command correction, virtual impedance injection, and final control command output. This system is suitable for deployment in a grid-connected GFM converter control platform and meets the GC0137 standard's technical specifications for current limiting, response time, and voltage support.

[0068] The above method and system are described in detail below in conjunction with embodiments.

[0069] Example 1

[0070] The present invention proposes a fault ride-through control method for a GFM converter based on adaptive power instructions and dynamic virtual impedance, which includes transient active power control and transient reactive power control. At the same time, dynamic virtual impedance control is introduced to enable the converter to operate stably and flexibly control the current during the fault period.

[0071] The overall control block diagram of the fault ride-through control strategy is as follows: Figure 1 As shown, the inverter output voltage is U c , the output current is I s , the point of common coupling (PCC) voltage is U s, the filter resistance and inductance between the inverter and PCC are R f , L f , the filter capacitor at PCC is C f , the equivalent resistance and inductance of the line are L g 、R g The inverter's power synchronous control loop uses VSG control to generate virtual voltage amplitude and phase, where P * ref is the active power reference value, P e is the PCC output active power, Q * ref is the reactive power reference value, Q e is the reactive power output by PCC; D is the damping coefficient, J is the moment of inertia, ω is the system angular velocity, ω0 is the synchronous angular velocity; θ is the virtual internal potential phase angle, E is the virtual internal potential amplitude, U * is the voltage base value. E dq is the dq-axis component of the virtual internal potential; u sdq is the dq axis component of the PCC voltage; i sdq is the dq-axis component of the inverter output current.

[0072] Among them, adaptive power command and dynamic virtual impedance control are used to limit the steady-state component of current during fault period and the impact current of transient process respectively. According to the GC0137 specification, in order to ensure that the converter can still provide support for the power grid during short-circuit fault, when the grid voltage drops below 0.9 times the rated value, the adaptive power command control is activated, and the active power reference value and reactive power reference value are changed according to the current amplitude limit requirements, limiting the steady-state component of current during fault period and injecting reactive power into the power grid. When the VSG output current I s When the current exceeds a given threshold of 1.2 pu, the dynamic virtual impedance activates. This virtual impedance is used only to limit inrush current during fault conditions and to clear transients. When the fault current reaches a stable level, the virtual impedance automatically shuts down. These two functions work together to flexibly limit VSG current during a fault.

[0073] The fault ride-through control method includes the following parts:

[0074] 1. Adaptive Power Command

[0075] According to the GC0137 specification, when the PCC voltage amplitude drops below 90%, reactive current is injected, and the response time is required to be less than 5ms. The reactive current injection rate requirement is also given. Figure 2 shown.

[0076] according to Figure 2In order to ensure that the steady-state output current of the inverter runs within the allowable range during the fault period, the active and reactive power commands of the grid-side inverter are set from P ref0 , Q ref0 Switch to P ref1 , Q ref1 In order to enable the GFM converter to provide voltage support for the system during voltage sag, the GFM injects reactive current into the system during the fault period according to Figure 2 The required value (per unit value) is

[0077] i qref0 =-1.4578U s +1 (1)

[0078] Where: U s is the per-unit value of the PCC voltage.

[0079] Further compensation of reactive current is performed to obtain the final expression of GFM injected reactive current:

[0080] i qref1 =K q (0.9-U s )+(-1.4578U s +1) (2)

[0081] Where: K q is the reactive current compensation coefficient.

[0082] In summary, the inverter reactive power reference value is adaptively adjusted according to the following rules:

[0083]

[0084] The decrease in grid voltage causes the VSG output active power to deviate from the reference value. Therefore, the basic idea of ​​adaptive active power command control is to adjust the active power command according to the grid voltage U g and PCC voltage U s , dynamically adjust the active power reference value, eliminate active power imbalance, maintain a constant power angle during the fault period to shorten the transient response process, and improve the power angle stability. According to formula (3), the reference value expression of active power can be obtained as

[0085]

[0086] Where: S fault is the allowable apparent power after a grid fault, and its calculation formula is as follows.

[0087]

[0088] Where: U n is the system rated voltage; SN is the rated capacity of the system.

[0089] 2. Dynamic Virtual Impedance

[0090] Sudden changes in grid voltage during the instant of fault occurrence and clearance can cause large transient current surges and non-periodic components in the inverter. Due to the low bandwidth of the outer loop, voltage control cannot quickly suppress these transient current surges at the moment of fault occurrence. The aforementioned current limiting strategy based on adaptive adjustment of the power reference value only considers the steady-state component of the fault current and is not ideal for suppressing the surge current during the transient process of fault occurrence and clearance.

[0091] The surge current in the transient process is caused by the attenuated DC component. Based on this, the present invention proposes a dynamic virtual impedance method to limit transient overcurrent. The specific principle is as follows: Figure 3 shown.

[0092] By adding a dynamic virtual impedance link consisting of inverter load current negative feedback to the voltage and current dual-loop control, the equivalent impedance in the system is increased, thereby reducing overcurrent when a fault occurs. Unlike the traditional virtual impedance method that uses a constant impedance, the dynamic virtual impedance method sets the impedance value to an adaptive change proportional to the overcurrent. The specific expression is as follows

[0093]

[0094] X VI =n X / R R VI (7)

[0095] Where: m R is the virtual impedance coefficient; I th is the set current threshold; n X / R is the impedance ratio of the virtual impedance input.

[0096] From equations (6) and (7), it can be seen that the activation of the dynamic virtual impedance depends on the output current I s When I s >I th When R Ⅵ and X Ⅵ and the current difference I s -I th Proportional to the current, it changes adaptively, otherwise Z Ⅵ = 0. Set the current threshold I th It is set to 1.2 times the rated value, slightly larger than the amplitude of the steady-state component of the current during the fault period, to ensure that the dynamic virtual impedance will not be activated after the transient component of the current decays to 0 during normal operation and fault periods. That is, the dynamic virtual impedance is only used to achieve rapid suppression of transient current.

[0097] In summary, the GFM fault-crossing control flow chart is as follows: Figure 4 As shown in the figure, the system first detects whether a short-circuit fault has occurred in the power grid. When the grid voltage drops to the maximum bus voltage fluctuation threshold, the system adaptively adjusts the power command to ensure power angle stability and suppress steady-state fault current during the fault. If the current exceeds the set safety threshold, a dynamic virtual reactance is used to suppress transient inrush current at the moment of fault occurrence and removal, and the steady-state fault current is suppressed through power loop regulation.

[0098] Example 2

[0099] This embodiment provides a system for implementing fault ride-through control of a GFM converter. The system is built based on a grid-connected inverter control platform and is suitable for abnormal grid-connected conditions such as grid voltage sag or short-circuit faults, meeting the response requirements of the GC0137 standard for grid support capabilities.

[0100] like Figure 5 As shown, the system includes the following modules:

[0101] 1. Voltage and current acquisition unit

[0102] The voltage and current acquisition unit is used to collect the grid connection point (PCC) voltage and the output current of the GFM converter in real time, and sends the collected data to the fault detection module, the adaptive power command control module, and the dynamic virtual impedance control module as the input basis for each module. This unit is generally composed of an isolated sampling module, a voltage and current transmitter, and a high-precision ADC conversion circuit, with a millisecond sampling period.

[0103] 2. Fault detection module

[0104] The fault detection module receives the grid connection point voltage data from the acquisition unit and determines whether the voltage drops below a preset threshold (such as 0.9 pu). If the voltage drop condition is met, it outputs a start signal to trigger the entire fault ride-through control process.

[0105] 3. Adaptive power command control module

[0106] The module receives grid connection point voltage and output current data, and after detecting a voltage drop, executes a power command adjustment process, including:

[0107] Reactive power command generation submodule: Based on the voltage drop at the grid connection point and in accordance with standard requirements (GC0137), it generates a preliminary reactive current command, introduces a reactive compensation coefficient on this basis, and outputs a target reactive power reference value;

[0108] Active power command correction submodule: Dynamically corrects the active power reference value based on the relationship between voltage and system fault apparent power limit to maintain power angle stability during faults.

[0109] 4. Dynamic virtual impedance control module

[0110] The module includes the following sub-functional units:

[0111] Current threshold judgment submodule: used to judge whether the current converter output current exceeds the threshold I th (set to 1.2 times the rated current);

[0112] Virtual impedance construction submodule: In I s >I th When the condition is met, the current difference (I s -I th ) Real-time calculation of virtual resistance R Ⅵ With virtual reactance X Ⅵ , and injected into the control loop;

[0113] Impedance removal submodule: When the current drops below the safety threshold, the virtual impedance is automatically removed to ensure that the system efficiency is not affected under normal operating conditions.

[0114] 5. Instruction generation module

[0115] This module integrates the power reference output from the adaptive power command control module and the virtual impedance parameters from the dynamic virtual impedance control module to output control commands for driving the GFM converter. These commands may include the inner-loop voltage target, the outer-loop current limiter, and the phase-angle synchronous compensation value, ensuring accurate and rapid fault response.

[0116] The connections and interactions between the above modules are as follows:

[0117] The voltage and current acquisition units provide input data to the fault detection module, the adaptive power command control module and the dynamic virtual impedance control module respectively;

[0118] The start signal output by the fault detection module synchronously triggers the action logic of the power command module and the virtual impedance module;

[0119] The outputs of the adaptive power command control module and the dynamic virtual impedance control module serve as the input of the command generation module;

[0120] The output of the instruction generation module is connected to the GFM converter main controller to achieve closed-loop control and regulation of the converter.

[0121] The system can be deployed in the GFM converter control platform that supports grid-connected operation. Through modular design, it achieves rapid response, hierarchical control and parallel processing. It is particularly suitable for the bidirectional suppression task of fault current in renewable energy power generation scenarios and has good scalability and engineering practicality.

[0122] Application Examples

[0123] In order to verify the effectiveness of the fault ride-through control strategy proposed in this paper, a Matlab / Simulink Figure 1 The main simulation parameters of the VSG grid-connected simulation model are shown in Table 1.

[0124] Table 1 VSG grid-connected simulation model parameters

[0125]

[0126] 1. Comparative Simulation Verification of GFM Fault Ride-Through Characteristics and GC0137 Power Grid Specifications

[0127] Simulation conditions: A permanent three-phase ground fault with different fault impedances is set in the PCC of the GFM converter at 3s. The GFM fault reactive current injection is compared with the minimum reactive injection current in the GC0137 grid code standard.

[0128] Simulation results: The simulation waveforms of permanent three-phase ground fault in GFM converter under different fault impedances are as follows: Figure 6 The steady-state output results are shown in Table 2; the total fault current peak value (I s ) and fault reactive current peak value (I q ) and GC0137 power grid specification standard Figure 7 shown.

[0129] Table 2 GFM output steady-state results under different fault impedance conditions

[0130]

[0131] like Figure 7 As shown in Figure 1, the GC0137 standard specifies the minimum reactive power that a GFM converter must provide under low voltage conditions. Figure 6 、 Figure 7 As can be seen from Table 2, under the above test conditions, the GFM converter adopts the proposed fault ride-through control strategy based on adaptive power command and dynamic virtual impedance, and the system operates normally in the green shaded area, which complies with the GC0137 grid specification standard.

[0132] 2. Simulation Verification of the Effectiveness of GFM Fault Ride-Through Control Strategy

[0133] Simulation conditions: A permanent three-phase ground short-circuit fault is set in the PCC of the GFM converter at 3s. The transient responses of the VSG grid-connected system under the traditional current limiting control (i.e., limiting control based on current saturation) and the fault ride-through control strategy proposed in this invention are compared.

[0134] Simulation results: The transient responses of the VSG grid-connected system under the traditional current limiting control and the control strategy proposed by the present invention are as follows: Figure 8 shown.

[0135] Depend on Figure 8 It can be seen that under traditional current limiting control, the maximum transient impact amplitude of the inverter output current is 2.3pu, and the output current can only be limited to 1.4pu. In addition, after the fault, the system power output fluctuates greatly, while the active and reactive power reference values ​​remain unchanged. The system is always in the acceleration stage, causing the power angle difference between the GFM converter and the system to continue to increase and causing the frequency to increase sharply.

[0136] When the control strategy proposed by the present invention is adopted, U s When the power drops to 0.9 pu, the VSG control system enters the fault ride-through control mode. According to equations (3) and (4), the adaptive active and reactive power command values ​​are calculated respectively. The GFM converter outputs reactive power of 0.88 pu to the system, which can provide a certain voltage support for the system. The power angle and frequency of the system fluctuate very little during the fault period, and basically maintain the rated value unchanged, thus achieving fault ride-through. Figure 8 From the output current waveform, it can be seen that the maximum transient impact amplitude of the inverter output current at the moment of the fault is only 1.4pu. During the fault period, the output current is limited to 1.2pu. Compared with the traditional control strategy, the harmonic content of voltage and current is very small.

[0137] In summary, the GFM converter fault ride-through control method and control system based on adaptive power instructions and dynamic virtual impedance provided by the present invention can collaboratively complete the adaptive adjustment of active and reactive power and the dynamic suppression of transient current when a short-term voltage drop or short-circuit fault occurs in the power grid, effectively improving the operating stability of the converter during the fault and the power grid support capability. The control method realizes the dual constraints of the steady state and transient response of the fault current. The system structure realizes the hierarchical coordination of data acquisition, fault identification, instruction calculation and control execution through modular design, and has good response speed and engineering adaptability. Through simulation verification, the proposed strategy complies with the GC0137 power grid specification standard, can significantly suppress transient impact current, maintain the stability of the system power angle, and has good promotion value and application prospects.

[0138] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A GFM converter fault ride-through control method, characterized in that: The steps include: (1) During the grid-connected operation of the GFM converter, the grid connection point voltage and the GFM converter output current are collected in real time. When the grid connection point voltage drops below 0.9 times the rated voltage, the fault ride-through control process is started; (2) Start adaptive power command control and dynamically adjust the active power reference value and reactive power reference value according to the per-unit value of the grid connection point voltage, where: The reactive power reference value is generated according to the voltage drop level in accordance with the GC0137 specification. The active power reference value is corrected according to the voltage reference value and the allowable apparent power to maintain a stable power angle during the fault period; (3) While executing adaptive power command control, if it is detected that the output current of the GFM converter is greater than 1.2 times the rated current, the dynamic virtual impedance control module is activated to construct a virtual resistance and virtual reactance based on the difference between the output current and the current threshold to suppress the transient impact current during the fault occurrence and clearing process; (4) When the output current of the GFM converter recovers to a value not greater than the current threshold, the dynamic virtual impedance control module is turned off and the adaptive power command control is continued until the fault current meets the grid support requirements and the fault ride-through control process is terminated.

2. The method according to claim 1, characterized in that The reactive power reference value Q in step (2) * ref Adaptive adjustment is done as follows: When the grid voltage per unit value U s When it is lower than 0.9pu, the preliminary reactive current injection value is calculated first, and its expression is: i qref0 =-1.4578U s +1 Compensation is performed on the basis of the preliminary reactive current injection value to obtain the final injected reactive current, which is expressed as follows: i qref1 =K q (0.9-U s )+(-1.4578U s +1) Reactive power reference value Q * ref The final calculation expression is: Among them, K q is the reactive current compensation coefficient.

3. The method according to claim 1, characterized in that The active power reference value P in step (2) * ref The generation methods include: Based on the allowable apparent power S after the power grid fault fault , active power reference value P * ref Adaptive adjustment is performed according to the following formula: Where: S fault is the allowable apparent power after a grid fault, and its calculation formula is as follows: Where: U n is the system rated voltage; S N is the rated capacity of the system.

4. The method according to claim 1, wherein The construction method of the dynamic virtual impedance in step (3) includes: When the converter output current I s Exceeding the current threshold I th When the dynamic virtual impedance control module is started, a virtual resistance R proportional to the current difference is constructed. Ⅵ and virtual reactance X Ⅵ , and its calculation formula is: X VI =n X / R R VI Among them, m R is the virtual impedance coefficient, n X / R is the reactance ratio of the virtual impedance, I th The current threshold is set to 1.2 times the rated current.

5. The method according to claim 1, wherein The exit method of the dynamic virtual impedance control in step (4) includes: When the GFM converter outputs current I s Recover to below the current threshold I th When the virtual resistor R Ⅵ With virtual reactance X Ⅵ , so that the dynamic virtual impedance value returns to zero, that is: Z Ⅵ ＝0 Wherein, the current threshold I th The virtual impedance is set to 1.2 times the rated current of the converter. The virtual impedance is activated only in the transient phase of the fault occurrence and clearing process and automatically exits in the steady state to avoid affecting the power regulation in normal operation.

6. A system for implementing fault ride-through control of a GFM converter, characterized in that: include: Voltage and current acquisition unit, used to collect grid connection point voltage and GFM converter output current in real time; A fault detection module is used to determine whether the grid connection point voltage drops below a set threshold and trigger a fault ride-through control process when the condition is met; Adaptive power command control module, used to adjust the active power reference value and reactive power reference value based on the per-unit value of the grid connection point voltage to meet the grid support requirements and maintain power angle stability during faults; Dynamic virtual impedance control module, which is used to construct virtual resistance and virtual reactance proportional to the current difference when detecting that the output current of the GFM converter exceeds the set current threshold, thereby suppressing transient inrush current during fault occurrence and clearing; An instruction generation module is used to generate a control instruction according to the output parameters of the adaptive power instruction control module and the dynamic virtual impedance control module to drive the GFM converter to perform fault ride-through control.

7. The system according to claim 6, characterized in that The adaptive power command control module includes: The reactive power command generation unit is used to generate a preliminary reactive current injection value according to the voltage drop degree when the per-unit value of the grid connection point voltage is lower than 0.9pu, and adjust the target reactive power reference value in combination with the compensation coefficient; The active power instruction correction unit is used to dynamically correct the active power reference value based on the relationship between the voltage reference value and the apparent power allowed during the fault period, so as to improve the power angle stability during the fault response process.

8. The system according to claim 6, wherein: The dynamic virtual impedance control module includes: A current threshold judgment unit is used to judge whether the output current of the GFM converter exceeds a set current threshold; A virtual impedance construction unit is used to construct corresponding virtual resistance and virtual reactance according to the current difference when the output current exceeds the threshold, and inject them into the control loop to limit the transient impact current; The impedance removal unit is used to automatically turn off the injection control of the virtual impedance after the output current drops below the current threshold.

9. The system according to claim 6, wherein: The instruction generation module generates control instructions adapted to the GFM converter power control loop by collecting voltage and current information, integrating the active and reactive power reference values ​​output by the adaptive power instruction control module and the virtual impedance parameters output by the dynamic virtual impedance control module. These instructions are used to drive the inverter to perform current regulation and voltage support response.

10. The system according to claim 6, wherein: The control system is deployed in the GFM converter control platform that supports grid-connected operation. It is suitable for responding to grid voltage sag or short-circuit fault conditions and meets the GC0137 standard's control requirements for reactive power support response time, current limiting capability, and transient stability.

Images