A fault current limiting and fast recovery method suitable for grid-forming converters

Abstract

The application discloses a fault current limiting and fast recovery method suitable for grid-forming converters. Firstly, in the fault transient stage, the virtual internal potential of the grid-forming converter is used to calculate the virtual impedance by the difference between the virtual internal potential and the grid voltage to prevent the overcurrent of the converter; after entering the steady state stage, the size of the virtual impedance is adjusted according to the three-phase current amplitude to ensure the maximum utilization of the converter capacity during the low-voltage fault; in addition, during the low-voltage fault ride-through, the virtual internal potential is adaptively limited according to the three-phase current amplitude and the total impedance of the system, thereby accelerating the recovery process after the fault is cleared. The application can not only ensure that the converter does not overcurrent during the low-voltage fault ride-through of the grid-forming converter, but also realize the maximum utilization of the converter capacity during the fault, and further accelerate the recovery process after the fault is cleared.

Description

Technical Field

[0001] This invention relates to the field of battery energy storage conversion technology, and in particular to a fault current limiting and fast recovery method applicable to grid-type converters. Background Technology

[0002] Grid-based control dynamically adjusts the output voltage amplitude and phase by simulating the electromechanical characteristics of a synchronous generator, possessing autonomous voltage and frequency regulation, inertia support, and damping compensation capabilities. Unlike grid-following control, which relies on grid phase synchronization, the grid-based converter is equivalent to a low-impedance voltage source, does not depend on external voltage support, and can still operate stably in extremely weak grid environments. This control strategy effectively compensates for the inherent shortcomings of grid-following control, plays a crucial supporting role in new power systems with large-scale integration of renewable energy, and has broad application prospects.

[0003] The voltage source characteristics of grid-connected energy storage converters provide crucial support for new power systems. However, during a sudden voltage drop in the grid, the difference between the converter's internal potential and the grid voltage increases dramatically because the converter needs to maintain a constant internal potential. This leads to a surge in output current. Excessive fault inrush current can directly damage components and even cause permanent converter failure. Grid voltage dips can also induce transient instability in the system. Simultaneously, to enhance the reactive power support capability of the energy storage system under fault conditions, grid-connected energy storage converters need to continuously output a certain amount of reactive power to the faulty grid, thereby effectively supporting the grid connection voltage.

[0004] Traditional low-voltage ride-through control strategies for grid-connected energy storage converters often employ fixed virtual impedance and circular limiting. With the former, using a large virtual impedance makes it difficult to fully utilize the converter's capacity during faults, resulting in reduced reactive power output; while using a small virtual impedance can easily cause overcurrent in the converter during grid voltage dips, leading to damage to the switching transistors. Circular limiting, on the other hand, causes the converter to exhibit current-source characteristics during faults, resulting in the loss of grid-connected characteristics. These issues constitute the core challenges of grid-connected control strategies. Research into fault current-limiting control technology for grid-connected energy storage systems is urgently needed. Furthermore, during the recovery phase after fault clearance, traditional current-limiting control strategies have a slow dynamic response speed, requiring a considerable amount of time for the system to return to normal operation, which is insufficient to meet the demands of scenarios with extremely high power quality requirements, such as data centers and precision manufacturing. Therefore, research into fault current-limiting and rapid recovery control technologies for grid-connected energy storage systems is urgently needed to ensure the safe and stable operation of new power systems. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method and system for current limiting and rapid recovery of grid-connected converters under low-voltage faults, in order to address the shortcomings of the existing technology, thereby achieving current limiting of the grid-connected converter under low-voltage faults and maximizing the capacity utilization of the converter and accelerating the speed of fault recovery.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a fault current limiting and fast recovery method suitable for grid-type converters, comprising: Step S1: Obtain the grid voltage and determine whether the system has entered a fault state; Step S2: If so, change the system power command based on the grid voltage, divide the fault stage into transient stage and steady-state stage, and calculate the adaptive virtual impedance of the corresponding stage according to the grid voltage and current amplitude; calculate the adaptive virtual internal potential by combining the three-phase voltage, current and system impedance. Step S3: Combining the calculated adaptive virtual impedance and adaptive virtual internal potential, the SPWM modulation signal is obtained after voltage and current closed loop.

[0007] Optionally, in step S1, the specific implementation process for determining whether a fault state has been entered includes: S11: Acquire the phase voltage at the common grid connection point PCC of the grid-connected converter. and conduct dq The voltage is transformed and the grid voltage is obtained as shown in the following formula: ; In the formula, θ The phase of the grid voltage. The three-phase voltage d-axis components, The three-phase voltage q-axis component. This refers to the grid voltage. S12: Determine the mains voltage Whether the value is less than 0.9 pu determines whether the system has entered a fault state, as shown in the following formula: ; Optionally, the specific implementation process of step S2 includes: Step S21: Determine the active and reactive power commands for the fault phase based on the collected grid voltage. As shown in the following formula: ; In the formula: P set It is the rated active power set in the system; S nom This refers to the system's rated capacity. U gN It is the rated voltage of the power grid; ; Step S22: Calculate the adaptive virtual impedance during the fault transient phase. R v , X v As shown in the following formula: ; In the formula, R v For virtual resistance, L v For virtual inductance, This is the fault current limit value. R g , L g This refers to the network-side impedance. n This represents the ratio of virtual inductive reactance to virtual resistance. For virtual internal potential and grid voltage The difference; Step S23: Determine the duration of the fault transient phase. t tran As shown in the following formula: ; in, J Let the system's rotational inertia be... D p The damping coefficient; Step S24: After the fault transient process ends, the system enters the fault steady-state process. The initial value of the virtual impedance in this process inherits the virtual impedance value at the instant of the end of the fault transient period. A state machine is set up, and the state is switched according to the magnitude of the three-phase current and the virtual impedance. Different states correspond to different adaptive virtual impedance calculation methods, as shown in the following formula: ; In the formula, k P This is the proportionality coefficient. The integral coefficient is... L v0 The virtual inductance at the instant the fault transient ends; This is the maximum fault current. This represents the minimum fault current. in, The value is determined by the following formula: ; In the formula, This refers to the rated current amplitude during steady-state operation. Step S25: When the grid voltage rises above 0.9 pu, the system fault steady-state process ends, and the adaptive virtual impedance is recalculated using the formula in step S22. The duration is the same as that calculated in step S23. t tran Then the entire fault process ended; Step S26: During the entire fault period, based on the adaptive virtual impedance, line impedance, and three-phase current amplitude calculated in the above steps, and combined with the real-time grid voltage amplitude, calculate the adaptive virtual internal potential range, as shown in the following formula, and limit the internal potential output by the reactive power loop to this range: ; In the formula, k E The constant represents the adjustable coefficient. This is the ratio of the fault voltage drop amplitude to the preset current limit value. This represents the maximum virtual internal potential. This represents the minimum virtual internal potential.

[0008] Optionally, the specific implementation process of step S3 includes: the converter adopts virtual synchronous machine control, and during the fault period, the virtual internal potential of its reactive power loop output is... E Limit the calculation in step S26 [ E min , E max Within the interval, combined with the phase output of the active power loop The virtual synchronous machine three-phase voltage is obtained, and the difference is calculated with the line voltage at the common grid connection point PCC. After processing by the adaptive virtual impedance calculated in step S22 or S24, the reference value of the converter three-phase current is generated. Then, after feedback of the actual phase current of the converter, it is sent to the quasi-PR controller for processing to generate the SPWM modulation signal. After comparison with the carrier signal, the PWM signal is generated to drive each switch of the converter.

[0009] Compared with the prior art, the beneficial effects of the present invention are as follows: 1) The present invention uses an adaptive virtual impedance method to limit the fault current within a controllable range and realize the maximum capacity utilization of the grid-connected converter during the fault period to output more reactive power to support the grid voltage.

[0010] 2) Since the current command is not directly limited, the voltage source characteristics of the grid-type converter are retained.

[0011] 3) This invention uses an adaptive virtual internal potential method to accelerate the fault recovery process of the converter after the grid voltage recovers. Attached Figure Description

[0012] Figure 1A flowchart illustrating a fault current limiting and fast recovery method for a grid-type converter provided in an embodiment of the present invention; Figure 2 This is a diagram of the main circuit topology of the angle-type high-voltage direct-connected chain energy storage converter provided in an embodiment of the present invention. Figure 3 This is a block diagram of the overall control structure of the angle-type high-voltage direct-connected chain energy storage converter provided in an embodiment of the present invention; Figure 4 The adaptive virtual impedance current limiting fault transient steady-state timing diagram provided in the embodiments of the present invention; Figure 5 A schematic diagram of the adaptive virtual impedance fault current limiting strategy based on a state machine provided in this embodiment of the invention; Figure 5 (a) is a diagram of the state machine decision criteria; Figure 5 (b) is a state machine state classification diagram; Figure 5 (c) is a state machine state classification diagram; Figure 6 This is a block diagram of the adaptive fault current limiting and fast recovery control technology for a grid-type energy storage converter provided in an embodiment of the present invention; Figure 7 The simulation waveform diagram of power grid slump fault under a fixed virtual impedance (small) is provided for the embodiments of the present invention; Figure 7 (a) is a diagram of the three-phase voltage waveforms; Figure 7 (b) shows the three-phase current waveforms; Figure 7 (c) is the waveform diagram of the total active power of the three phases; Figure 7 (d) is the waveform diagram of the total reactive power of the three phases; Figure 7 (e) is the waveform diagram of the power angle; Figure 7 (f) is the angular frequency waveform of the virtual synchronizer; Figure 8 The simulation waveform diagram of power grid slump fault under a fixed virtual impedance (relatively large) provided in the embodiments of the present invention; Figure 8 (a) is a diagram of the three-phase voltage waveforms; Figure 8 (b) shows the three-phase current waveforms; Figure 8 (c) is the waveform diagram of the total active power of the three phases; Figure 8 (d) is the waveform diagram of the total reactive power of the three phases; Figure 8 (e) is the waveform diagram of the power angle; Figure 8 (f) is the angular frequency waveform of the virtual synchronizer; Figure 9 Simulation waveform of grid dip fault under adaptive fault current limiting and fast recovery control technology of grid-type energy storage converter provided in the embodiment of the present invention; Figure 9 (a) is a diagram of the three-phase voltage waveforms; Figure 9 (b) shows the three-phase current waveforms; Figure 9(c) is the waveform diagram of the total active power of the three phases; Figure 9 (d) is the waveform diagram of the total reactive power of the three phases; Figure 9 (e) is the waveform diagram of the power angle; Figure 9 (f) is the angular frequency waveform of the virtual synchronizer; Figure 9 (g) is the adaptive virtual impedance waveform diagram; Figure 10 The simulation waveform diagram of the power grid slump fault without adaptive E-limiting is provided for the embodiments of the present invention. Figure 10 (a) is a diagram of the three-phase voltage waveforms; Figure 10 (b) shows the three-phase current waveforms; Figure 10 (c) is the waveform diagram of the total active power of the three phases; Figure 10 (d) is the waveform diagram of the total reactive power of the three phases; Figure 10 (e) is the waveform diagram of the power angle; Figure 10 (f) is the angular frequency waveform of the virtual synchronizer; Detailed Implementation

[0013] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0014] like Figure 1 As shown, this application proposes a fault current limiting and fast recovery method applicable to grid-type converters, including: Step S1: Obtain the grid voltage and determine whether the system has entered a fault state; The proposed grid-type converter is a delta-connected high-voltage direct-chain energy storage converter, and its main circuit topology is as follows: Figure 2 As shown, the delta-type medium-voltage direct-connected chain energy storage system includes 3N conversion submodules, AC filter inductors, AC contactors, and fuses. Each conversion submodule includes a battery energy storage unit, a DC-side LC filter, and an H-bridge inverter. The battery energy storage unit is connected to the DC-side LC filter; the DC-side LC filter is connected to the H-bridge inverter; the A, B, and C phases are each composed of N cascaded submodules; the three-phase terminals are connected in a delta configuration via AC filter inductors, and then connected to the AC power grid via fuses and AC contactors. For an ideal three-phase power grid, The line voltage at the grid connection point of the angle-connected high-voltage direct-connected energy storage converter. R g , Lg These are the grid-side resistor and the grid-side inductor, respectively. This refers to the three-phase power grid line current. This is the zero-sequence circulating current inside the converter. This represents the phase current. L s For AC side filter inductance, This represents the phase voltage of the converter. S dc This is a switch between the DC-side battery and the H-bridge. This is the open-circuit voltage of the battery cluster. Input current to the battery cluster, DC-side filter capacitor C The current, This is the DC link current.

[0015] S11: Acquire the phase voltage at the common grid connection point PCC of the grid-connected converter. v x (x = a,b,c), and perform dq Transform and obtain the grid voltage; See Figure 3 The overall control structure block diagram of the angled high-voltage direct-connected chain energy storage system. P e This refers to the real-time three-phase active power of the energy storage system. This is the rated angular frequency of the three-phase power grid. J For rotational inertia, D p The damping coefficient is... This is the phase reference value generated by the active power loop; This refers to the real-time three-phase reactive power of the energy storage system. k q The reactive power integral coefficient, E 0 represents the initial internal potential of the virtual synchronous machine. E ref This is the virtual internal potential reference value output by the reactive power loop. R v , L v These are virtual resistance and virtual inductance, respectively. These are the real-time line voltage and phase current of the energy storage system, respectively. This is the current reference value output by the voltage-current loop. Reference current With actual current The difference, G QPR(s) As a quasi-PR controller, This is the modulated waveform for each phase. It obtains the phase voltages of the AC power grid. v x(x=a, b, c), through dq Transform and calculate its amplitude to obtain the grid voltage U g As shown in equation (1): (1) In the formula, θ The phase of the grid voltage. The d-axis components of the three-phase voltage ， This represents the q-axis component of the three-phase voltage.

[0016] Step S12: Determine the grid voltage U g Whether the value is less than 0.9 pu is used to determine if the system has entered a fault state: Determine whether the system has entered a fault state according to the following formula (2): (2) Step S2: If so, change the system power command based on the grid voltage, divide the fault stage into transient stage and steady-state stage, and calculate the adaptive virtual impedance of the corresponding stage according to the grid voltage and current amplitude; calculate the adaptive virtual internal potential by combining the three-phase voltage, current and system impedance. Step S21: Determine the active and reactive power commands for the fault phase based on the collected grid voltage. ; After a fault is detected, in order to balance the system power angle stability under fault conditions and the voltage support capability of the grid-type energy storage converter, the power reference command is adjusted as follows: the active power reference command is determined based on the grid voltage and the preset low-voltage threshold, and the reactive power reference command is determined based on the grid voltage and the grid voltage rating. (3) (4) In formula (3): U g It is the grid voltage; P ref It is the active power reference command given after the grid voltage drops; P set It is the rated active power set in the system; S nom This refers to the system's rated capacity.

[0017] In equation (4): U gN It is the rated voltage of the power grid; It is based on the reactive power reference command given by the national standard.

[0018] After making the above power reference command adjustment, the reactive power reference command will be... Substitute as Figure 3 A series of calculations are performed in the reactive-voltage control loop shown to obtain the virtual internal potential. E Active power reference command P ref Substitute as Figure 3 In the active-frequency control loop shown, the updated phase angle can be calculated. The modulation signal for dual closed-loop control of input voltage and current can be obtained by calculating both. .

[0019] Because an adaptive impedance is added to the voltage and current dual closed-loop control stage, the virtual internal potential is limited by combining the grid voltage and the total impedance of the system during the low voltage fault of the power grid. This keeps the deviation between the fault steady-state power angle and the normal operating power angle within a small range, thereby shortening the recovery time.

[0020] Step S22: Calculate the adaptive virtual impedance during the fault transient phase. R v , X v As shown in the following formula: See Figure 4 An embodiment of the present invention provides an adaptive virtual impedance current limiting fault transient steady-state timing diagram. When the system enters the fault transient stage, a preset large-amplitude transient virtual impedance is immediately applied. The calculation process of the transient virtual impedance is as follows: To ensure that the output current of the grid-connected energy storage converter is strictly controlled within the fault current limit during grid fault phases. Within this range, combined with the equivalent circuit at the converter output, the virtual resistance... R v With virtual inductance L v Under the operating conditions, the fault current amplitude is stabilized at... That is, calculation based on grid voltage Calculate the virtual internal potential The voltage difference with the grid voltage can establish the following voltage balance relationship: (5) in, R g , L g Given the grid-side impedance, based on the aforementioned voltage balance relationship, the difference can be determined using the set fault current limit value. Directly derive the equivalent total impedance of the system Z a The expression for this is the ratio of the fault voltage drop magnitude to the preset current limit value: (6) Considering the preset ratio of virtual inductive reactance to virtual resistance in the system n (In this example, the value is set to 3). Based on the above formula, the analytical expression for the first adaptive virtual impedance that needs to be applied during the fault transient stage can be derived: (7) In the formula, , This is the angular frequency of the power grid.

[0021] Step S23: Determine the duration of the fault transient phase. t tran As shown in the following formula: The duration of transient virtual impedance activation is not arbitrarily set. If the activation time is too short, the transient phase may not end before switching to a smaller impedance, potentially leading to incomplete suppression of current surges. If the activation time is too long, a larger impedance will be maintained during the fault steady-state phase, weakening the system's reactive power support capability. The core characteristic of the fault transient phase is the dynamic process of the angular velocity deviation change rate converging from the abrupt value triggered by the fault to the steady-state value. Therefore, the threshold of the angular velocity deviation change rate can be chosen as the quantitative indicator for the transient phase, which can intuitively reflect the degree of attenuation of the power angle swing and is strongly correlated with the transient response characteristics of the converter. Therefore, this invention proposes a method based on the angular velocity deviation change rate of the virtual rotor to quantify the duration of the transient phase. The specific implementation process is as follows: First, the equation of motion for the damped virtual rotor is: (8) in, J Let the system's rotational inertia be... D p The damping coefficient is... Let be the system angular velocity. P ref The system active power command. P e This represents the actual total power of the system. The angular frequency is the rated frequency. Solving the first-order linear nonhomogeneous differential equation described above yields the time-domain expression for the angular velocity: (9) In equation (8): for t Angular velocity deviation at time 0 when the fault occurs.

[0022] t A fault occurring at time 0 will cause a sudden change in grid voltage, directly leading to a jump in output power, resulting in a significant power imbalance in the system. (Remaining power) The absolute value of the value increases dramatically, which leads to a sharp jump in the degree of change of the angular velocity deviation at the moment of the fault, corresponding to the system entering the transient fault adjustment phase at this time.

[0023] As the power angle dynamically fluctuates, the active power gradually approaches the given value, the absolute value of the remaining power continuously decreases, and the intensity of the change in angular velocity deviation also decreases accordingly. When the remaining power approaches 0, the angular velocity deviation no longer fluctuates significantly, and its rate of change approaches 0. At this point, the power angle, power, and current all tend to stabilize, and the system enters the fault steady-state stage. Differentiating equation (9) with respect to time, we obtain the rate of change of angular velocity deviation: (10) t At time 0, the fault is triggered, the exponent is 1, and the absolute value of the rate of change of angular velocity deviation reaches its maximum value, corresponding to the rapid adjustment of angular velocity caused by power imbalance in the initial stage of the transient state; as time progresses... t > t 0, the exponential term decays, the absolute value of the rate of change continues to decrease, corresponding to the convergence of residual power and the process of the system approaching steady state; when t When the value is large enough, the exponential term approaches 0, and the rate of change also approaches 0. At this point, the angular velocity deviation stabilizes, and the system enters a steady state.

[0024] Based on the above analysis, the duration of the transient phase of the fault can be calculated using the rate of change of angular velocity deviation, thereby reasonably applying a larger virtual impedance to suppress the large current during the transient phase. The criterion for the end of the transient phase is defined as the absolute value of the rate of change of angular velocity deviation decreasing to 30% of its initial value. (11) Taking the natural logarithm of both sides of the equation and rearranging, we finally obtain the quantitative formula for the duration of the transient phase: (12) Through the above derivation, the quantification of the fault transient duration was completed.

[0025] Step S24: After the fault transient process ends, the system enters the fault steady-state process. The initial value of the virtual impedance in this process inherits the virtual impedance value at the instant of the end of the fault transient period. A state machine is set up, and the state is switched according to the magnitude of the three-phase current and the virtual impedance. Different states correspond to different adaptive virtual impedance calculation methods, as shown in the following formula: When a system fault is detected, the transient impedance is applied for the duration calculated in the previous step. t tran See also: Figure 4The adaptive virtual impedance current limiting fault transient steady-state timing diagram shows that when the fault transient phase ends, the transient impedance generator stops working and the system will enter the fault steady-state phase. At the same time, the fault steady-state generator is started. The initial value of the virtual impedance in the steady-state phase inherits the virtual impedance value at the last moment of the transient phase.

[0026] During the steady-state phase of a fault, to enhance the reactive power support capability of the device, dynamic adjustment of the virtual impedance is necessary: ​​if the current reactive power output is not at its optimal level, the virtual impedance is appropriately reduced to increase the reactive current output and strengthen the voltage support effect on the power grid; if the fault current exceeds the safety threshold, the virtual impedance is increased to suppress the risk of overcurrent. Through this dynamic adjustment mechanism, the virtual impedance can adapt to different operating conditions during the steady-state phase of a fault, improving the reactive power support capability of the device while strictly adhering to current safety constraints, thus meeting national standards.

[0027] To achieve precise dynamic adjustment of virtual impedance during fault conditions, this paper proposes a state machine-based fault adaptive virtual impedance fault current limiting technique, the principle of which is described in [reference needed]. Figure 5 The diagram shown illustrates the principle of an adaptive virtual impedance fault current limiting strategy based on a state machine. This method, based on the real-time status of the power grid fault state, fault current, and virtual impedance, divides the current operating state of the system fault condition into transient and four steady-state operating states. Different virtual impedance control strategies correspond to different states. The specific judgment rules and adaptive virtual impedance adjustment are as follows: Transient: Determined using a voltage surge edge triggering mechanism, with the moment of the grid voltage surge (drop or recovery) as the baseline, and the voltage change occurring after the surge. t tran The system is considered transient within the time window.

[0028] State 1: After the transient phase ends and the system enters steady state, when the maximum amplitude of the fault current is less than 0.9 times the current limit. And virtual inductance L v When the amplitude is less than 0.01 pu, the system is in state one; State 2: After the transient phase ends and the state becomes steady, when the maximum amplitude of the fault current exceeds the current limit... At that time, the system is in state two; State 3: After the transient phase ends and the state becomes steady, when the maximum amplitude of the fault current is at 0.9 times the current limit. To current limit When the interval is within which the system is in state three; State 4 (State Four): After the transient phase ends and the system enters steady state, when the maximum amplitude of the fault current is less than 0.9 times the current limit... When the virtual impedance amplitude is greater than 0.01 pu, the system is in state four.

[0029] The virtual impedance of the virtual impedance generator under fault conditions is adaptively adjusted using equation (13): (13) in, k P This is the proportionality coefficient. The integral coefficient is... L v0 The virtual inductance at the instant the fault transient ends. This is the maximum fault current. This represents the minimum fault current. The value is determined by the following formula: (14) In the formula, This refers to the rated current amplitude during steady-state operation. As can be seen from equation (13), the adaptive virtual impedance corresponding to the current operating state can be calculated based on the current operating state of the system fault.

[0030] Step S25: When the grid voltage rises above 0.9 pu, the system fault steady-state process ends, and the adaptive virtual impedance is recalculated using the formula in step S22. The duration is the same as that calculated in step S23. t tran Then the entire fault process ended; like Figure 4 As shown, when the grid voltage rises to above 0.9 pu, the system fault steady-state process ends. At this time, it is still necessary to calculate the adaptive virtual impedance using formula (7) in step S22 and calculate the transient duration using formula (12) in step S23. t tran This will bring the entire fault process to an end.

[0031] Step S26: During the entire fault period, based on the adaptive virtual impedance, line impedance and three-phase current amplitude calculated in the above steps, combined with the real-time grid voltage amplitude, calculate the adaptive virtual internal potential range, as shown in the following formula, and limit the internal potential output by the reactive power loop to this range. In addition to implementing adaptive virtual impedance during fault periods to achieve current limiting, this invention also requires limiting the internal potential during fault periods. E The size of this value is used to accelerate fault recovery, and the calculation process is as follows: When the grid voltage returns to normal, the energy storage system needs to adjust from the fault steady-state power angle to the system steady-state power angle. The larger the power angle adjustment range, the longer the time it takes for the system to complete power angle adaptation and recover to normal operation, due to the inertial characteristics of the virtual rotor. To avoid the aforementioned problems of large power angle deviation and slow recovery after a fault, it is necessary to properly configure the internal potential. E Limitations. Internal potential. E The smaller the value, the smaller the difference between the power angle under fault conditions and the power angle under steady-state conditions, which helps to shorten the time it takes for the system to recover to normal operating conditions. However, if E If the limitations are too small, the potential difference between the grid-connected energy storage converter and the grid will be weakened, and its ability to output inductive reactive power under grid slip faults will be significantly limited, thereby reducing its support for the grid.

[0032] During the entire fault period, based on the adaptive virtual impedance, line impedance, and three-phase current amplitude calculated according to the above steps, and combined with the real-time grid voltage amplitude, the adaptive virtual internal potential range is calculated as shown in the following formula, and the internal potential output by the reactive power loop is limited to this range: as shown in formula (15): (15) In equation (15), k E The constant represents the adjustable coefficient. This is the ratio of the fault voltage drop amplitude to the preset current limit value. This represents the maximum virtual internal potential. This is the minimum virtual internal potential; determined by trial and error, and used for further adjustment. E .

[0033] This strategy combines the grid voltage and the system's total impedance to limit the internal potential. Essentially, it constrains the difference between the internal potential and the grid voltage within the maximum current amplitude threshold corresponding to the virtual impedance voltage division. (16) This constraint ensures that the output current of the grid-type energy storage converter does not exceed the maximum amplitude limit from the source of the potential difference. It can actively limit the fault recovery inrush current without additional hardware, and at the same time avoids the performance conflict between current limiting and reactive power support under the traditional fixed E limit.

[0034] S3: Combining the calculated adaptive virtual impedance and adaptive virtual internal potential, the SPWM modulation signal is obtained after voltage and current closed loop. In other words, after calculating the virtual internal potential, such as Figure 6 As shown, the converter uses virtual synchronous machine control, and during a fault, it outputs the virtual internal potential of its reactive power loop. E Limit the calculation in step 6) [ E min , Emax Within the interval, combined with the phase output of the active power loop The three-phase voltage of the virtual synchronous machine is obtained, and the difference is calculated with the line voltage at the common grid connection point PCC. After processing by the adaptive virtual impedance calculated in step 2) or 4), the reference value of the three-phase current of the converter is generated. Then, after feedback of the actual phase current of the converter, it is sent to the quasi-PR controller for processing to generate the SPWM modulation signal. After comparison with the carrier signal, the PWM signal is generated to drive each switch of the converter.

[0035] Alternatively, it can be understood as follows: Using the virtual internal potential limiting value from equation (15) above, the virtual internal potential is limited to generate a limited virtual internal potential. Then, based on this virtual internal potential and the adaptive impedance at different stages of the fault, the following steps are performed: Figure 6 The calculation process shown determines the SPWM modulation signal corresponding to the fault stage. The virtual internal potential E and the grid connection point line voltage... The difference is calculated, and the difference is used to generate current commands for each phase through a virtual impedance. , With phase current The difference is sent to the quasi-PR controller. G QPR (s) adjustment, and after amplitude limiting, the modulation wave of each phase is obtained, and finally the SPWM modulation signal corresponding to the fault stage is determined.

[0036] Simulation verification This invention verifies the effectiveness and advancement of the method proposed in the embodiments of the invention by constructing a 35kV / 10MW / 10MWh angled high-voltage direct-connected chain energy storage system in MATLAB / Simulink simulation software.

[0037] See Figure 7 In one embodiment of the present invention, a simulation waveform of a power grid slump fault under a fixed virtual impedance (relatively small) is used. Figure 8 In one embodiment of the present invention, a simulation waveform of a power grid drop fault under a fixed virtual impedance (larger) is used. The peak transient current of the former is 7.36 pu and the peak transient current of the latter is 6.26 pu. It can be seen that the traditional fixed virtual impedance can indeed effectively reduce the impact of fault transient current, and the larger the amplitude of the virtual impedance, the stronger the current limiting effect.

[0038] Furthermore, the advantages of a larger virtual impedance are also evident during the grid voltage recovery phase. Figure 7 During the voltage recovery instant, the active power surge reached 4.38 pu, the reactive power surge reached 5.59 pu, the power angle fluctuation reached 6.27 rad, and the virtual angular frequency fluctuation reached 357 rad / s; while Figure 7The active power surge was 3.66 pu, the reactive power surge was 4.51 pu, the maximum power angle fluctuation was 6.15 rad, and the peak angular frequency was 352 rad / s. At the same time, the fault recovery time was shortened from about 60 ms to 55 ms, indicating that a larger virtual impedance can further reduce the power, power angle and frequency fluctuations during fault recovery and accelerate the system recovery speed.

[0039] However, the drawbacks of a fixed virtual impedance also become apparent in the steady-state phase. Figure 7 The steady-state fault current is 1.3 pu, while Figure 8 The steady-state fault current is only 1.18 pu. As the virtual impedance amplitude increases, the steady-state fault current is excessively restricted, and the reactive power support capability of the device is significantly weakened. Figure 8 The reactive power support level is relatively Figure 7 The voltage drop is approximately 9.2%, which fails to fully utilize its voltage support function for the power grid. This simulation result is completely consistent with the theoretical analysis above: although a fixed virtual impedance can enhance the transient current limiting effect, a large amplitude setting will sacrifice the fault steady-state reactive power support performance, making it difficult to meet the dual requirements of transient current limiting and steady-state power grid support.

[0040] See Figure 9 The adaptive control logic can be clearly seen from the change curve of the virtual inductance. At the moment of fault triggering, the virtual impedance rises rapidly to strengthen transient current limiting. After the system enters the fault steady state stage, the virtual impedance gradually decreases. By reducing the equivalent virtual impedance, the steady state fault current is increased. Therefore, it ultimately achieves both strong transient current limiting and ensures reactive power support capability in the steady state stage.

[0041] During the fault recovery phase, the advantages of this strategy are even more significant, with a fault recovery time of only 48ms, compared to... Figure 7 The 60ms time was reduced by 20%, compared to Figure 8 The 55ms time was shortened by 12.73%; at the same time, the active power surge at the moment of voltage recovery was only 1.19 pu and the reactive power surge was only 1.33 pu, with the surge amplitude far lower than that of the previous 55ms time. Figure 7 -4.39 PU (active power), -5.99 PU (reactive power) and Figure 8 -3.69 PU (active power) and -4.51 PU (reactive power). In addition... Figure 9 The corresponding maximum power angle fluctuation is only 2.103 rad, and the peak virtual angular frequency is 330 rad / s, compared to Figure 7 The maximum fluctuation of the power angle is 6.27 rad and the peak virtual angular frequency is 357 rad / s. Figure 8 The maximum fluctuation of the power angle is 6.15 rad and the peak virtual angular frequency is 352 rad / s. Figure 9The maximum fluctuation of the corresponding power angle and the peak value of the virtual angular frequency are significantly reduced, and both can quickly converge to the steady-state initial value.

[0042] In summary, based on Figures 7 to 9 Simulation analysis shows that the adaptive virtual impedance fault current limiting technology proposed in this paper solves the overcurrent risk under smaller fixed virtual impedance current limiting and the problem of weak reactive power support under larger fixed virtual impedance current limiting, thus achieving dual performance optimization of current limiting safety and grid support.

[0043] See Figure 10 This invention presents a simulated waveform of a grid slump fault under an adaptive virtual impedance fault current limiting strategy without adaptive E-limiting. With fixed E-limiting, the system takes approximately 68ms to return to stability after the grid voltage recovers; while... Figure 9 Adaptive E After implementing the restriction policy, the fault recovery time was only 48ms, an improvement of approximately 30% in recovery speed, significantly shortening the duration of system instability after a fault. Simultaneously, during fault recovery, adaptive... E The regulatory effect of restrictions is also more excellent. Figure 9 The impact amplitude at the lower angle is relatively large Figure 10 The power angle impact amplitude was reduced by 9.28%, and the fluctuation amplitude of the virtual angular frequency peak was also reduced by 0.5%. During the recovery process, the fluctuations of power angle and frequency were smoother, and the system was able to converge to the steady-state initial value more quickly.

[0044] In summary, the method proposed in this invention can further accelerate the fault recovery speed of grid-type energy storage converters based on adaptive virtual impedance fault current limiting, while reducing the impact of state variables during the recovery phase, thus providing effective support for a smooth transition after system faults.

[0045] In some embodiments, the present invention also provides a computer system including a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the steps in the above-described method embodiments.

[0046] The present invention also provides a computer-readable storage medium for storing a computer program. This computer-readable storage medium can be applied to a computer device, and the computer program causes the computer device to perform the corresponding processes in the methods described above in the embodiments of the present invention; for the sake of brevity, further details are omitted here.

[0047] The above embodiments are preferred implementations of the present invention. In addition, the present invention can be implemented in other ways. Any obvious substitutions without departing from the concept of the present technical solution are within the protection scope of the present invention.

[0048] To facilitate understanding by those skilled in the art of the improvements of this invention over the prior art, some of the accompanying drawings and descriptions have been simplified, and for clarity, some other elements have been omitted from this document. Those skilled in the art should recognize that these omitted elements may also constitute the content of this invention.

Claims

1. A fault current limiting and fast recovery method applicable to grid-type converters, characterized in that, include: Step S1: Obtain the grid voltage and determine whether the system has entered a fault state; Step S2: If so, change the system power command based on the grid voltage, divide the fault stage into transient stage and steady-state stage, and calculate the adaptive virtual impedance of the corresponding stage according to the grid voltage and current amplitude; calculate the adaptive virtual internal potential by combining the three-phase voltage, current and system impedance. Step S3: Combining the calculated adaptive virtual impedance and adaptive virtual internal potential, the SPWM modulation signal is obtained after voltage and current closed loop.

2. The method according to claim 1, characterized in that, In step S1, the specific implementation process for determining whether a fault state has been entered includes: Step S11: Acquire the phase voltage at the common grid connection point PCC of the grid-connected converter. v x (x = a,b,c), and perform dq The voltage is transformed and the grid voltage is obtained as shown in the following formula: ； In the formula, θ The phase of the grid voltage. The d-axis components of the three-phase voltage ， For the q-axis components of the three-phase voltage, This refers to the grid voltage. Step S12: Determine the grid voltage Whether the value is less than 0.9 pu determines whether the system has entered a fault state, as shown in the following formula: 。 3. The method according to claim 1, characterized in that, The specific implementation process of step S2 includes: Step S21: Determine the active and reactive power commands for the fault phase based on the collected grid voltage. As shown in the following formula: ； In the formula: P set It is the rated active power set in the system; S nom This refers to the system's rated capacity. U gN It is the rated value of the power grid voltage; ； Step S22: Calculate the adaptive virtual impedance during the fault transient phase. R v , X v As shown in the following formula: ； In the formula, R v For virtual resistance, L v For virtual inductance, This is the fault current limit value. R g , L g This refers to the network-side impedance. n This represents the ratio of virtual inductive reactance to virtual resistance. For virtual internal potential and grid voltage The difference; Step S23: Determine the duration of the fault transient phase. t tran As shown in the following formula: ； in, J Let the system's rotational inertia be... D p The damping coefficient; Step S24: After the fault transient process ends, the system enters the fault steady-state process. The initial value of the virtual impedance in this process inherits the virtual impedance value at the instant of the end of the fault transient period. A state machine is set up, and the state is switched according to the magnitude of the three-phase current and the virtual impedance. Different states correspond to different adaptive virtual impedance calculation methods, as shown in the following formula: ； In the formula k P This is the proportionality coefficient. The integral coefficient is... L v0 The virtual inductance at the instant the fault transient ends; This is the maximum fault current. This represents the minimum fault current. in, The value is determined by the following formula: ； In the formula, This refers to the rated current amplitude during steady-state operation. Step S25: When the grid voltage rises above 0.9 pu, the system fault steady-state process ends, and the adaptive virtual impedance is recalculated using the formula in step S22. The duration is the same as that calculated in step S23. t tran Then the entire fault process ended; Step S26: During the entire fault period, based on the adaptive virtual impedance, line impedance, and three-phase current amplitude calculated in the above steps, and combined with the real-time grid voltage amplitude, calculate the adaptive virtual internal potential range, as shown in the following formula, and limit the internal potential output by the reactive power loop to this range: ； In the formula, k E This is a constant, representing an adjustable coefficient. This is the ratio of the fault voltage drop amplitude to the preset current limit value. This represents the maximum virtual internal potential. This represents the minimum virtual internal potential.

4. The method according to claim 3, characterized in that, The specific implementation process of step S3 includes: the converter adopts virtual synchronous machine control, and during the fault period, the virtual internal potential of its reactive power loop output is... E Limit the calculation in step S26 [ E min , E max Within the interval, combined with the phase output of the active power loop The virtual synchronous machine three-phase voltage is obtained, and the difference is calculated with the line voltage at the common grid connection point PCC. After processing by the adaptive virtual impedance calculated in step S22 or S24, the reference value of the converter three-phase current is generated. Then, after feedback of the actual phase current of the converter, it is sent to the quasi-PR controller for processing to generate the SPWM modulation signal. After comparison with the carrier signal, the PWM signal is generated to drive each switch of the converter.

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