Fault current limiting method and control system for AHO type virtual oscillator networking based on threshold current control

Abstract

The invention discloses a fault current limiting method for AHO type virtual oscillator networking based on threshold current control. The fault current limiting method comprises the steps of S1, collecting output current and power grid voltage of a networking converter in real time; s2, generating a modulation voltage reference value based on the output current and the power grid voltage by using an AHO type virtual oscillator; s3, according to the modulation voltage reference value, a reverse current control loop is constructed through reverse calculation, and a corresponding current reference value is calculated through reverse calculation to serve as an input signal of the current control loop; and S4, detecting whether the current reference value exceeds a threshold current or not in real time, and if the current reference value exceeds the threshold current, reconstructing a modulation voltage through a current control loop so as to limit the output current of the network construction converter to a safe range. Based on the method, rapid and accurate fault current suppression is realized, meanwhile, the transient stability of the system is maintained, and the low-voltage ride-through capability of the network construction converter is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power electronics and new energy grid-connected control, and particularly relates to a threshold current control-based fault current limiting method and control system for AHO-type virtual oscillator grid-forming. BACKGROUND

[0002] With the rapid development of high-proportion renewable energy power systems, grid-forming converters become the core equipment for stable operation of new power systems due to their ability to actively support grid voltage and frequency. However, in the grid fault scenario, grid-forming converters need to consider the dual goals of fault current suppression and transient stability maintenance. Traditional methods such as current saturation clamping and virtual impedance control can limit short-circuit current, but may cause system power angle instability or voltage collapse due to excessive sacrifice of dynamic characteristics. At the same time, virtual oscillator control (VOC) has become a research hotspot due to its excellent synchronization performance and decentralized control characteristics. Unlike droop control and virtual synchronous generator, virtual oscillator control is not based on the physical mechanism of traditional synchronous machines, but is based on nonlinear oscillator dynamics, which designs the control system of the inverter as a nonlinear oscillator. Among them, the Andronov-Hopf (AHO) type virtual oscillator is widely used due to its simple structure and excellent limit cycle characteristics. However, the control structure of the AHO-type virtual oscillator is fundamentally different from traditional grid-forming strategies, making it difficult to directly use conventional current limiting methods, which limits its fault current limiting capability. Related research is still in the exploratory stage. Specifically, the existing AHO-type virtual oscillator has the following significant defects in fault current limiting:

[0003] 1) The AHO-type virtual oscillator directly generates a modulation signal through voltage oscillation, and traditional current inner loop limiting cannot directly intervene its nonlinear dynamic characteristics, resulting in poor current limiting effect.

[0004] 2) Indirect current limiting strategies such as virtual impedance method have slow response speed, large current impact at the initial stage of fault, and difficulty in balancing dynamic response and current limiting effect, which threatens device safety and system stability. SUMMARY

[0005] To overcome the above-mentioned deficiencies of the existing AHO-type virtual oscillator in fault current limiting, the present application provides a threshold current control-based fault current limiting method and control system for AHO-type virtual oscillator grid-forming, which uses a current limiting control circuit to achieve rapid and accurate fault current suppression while maintaining system transient stability and improving the low voltage ride-through capability of grid-forming converters.

[0006] According to an aspect of the present application, a threshold current control-based fault current limiting method for AHO-type virtual oscillator grid-forming is provided, comprising:

[0007] Step S1: Real-time acquisition of output current i of grid-connected converter Lfαβ and grid voltage v PCCαβ ;

[0008] Step S2: Generating modulation voltage reference value v αβ based on the output current and grid voltage by using AHO type virtual oscillator

[0009] Step S3: According to the modulation voltage reference value, constructing inverse current control loop by inverse calculation to calculate corresponding current reference value i Lfαβref as the input current of current control loop, where, , C(s) represents proportional controller, C(s)= K p , K p is design parameter;

[0010] Step S4: Real-time detection of whether the current reference value exceeds threshold current, if it exceeds threshold current, reconstructing modulation voltage by current control loop to limit the output current of the grid-connected converter to safe range.

[0011] Further, in step S2, the formula of modulation voltage reference value is:

[0012] ,

[0013] where, v α , v β represent components of modulation voltage reference value on α axis and β axis respectively, K v represents voltage amplification factor of grid-connected converter, K i represents current amplification factor, v c represents capacitor voltage, i L represents inductor current, ε represents amplification factor of inductor current i L , ξ represents design parameter of oscillator convergence speed, ω0 represents rated value of voltage angular frequency, θ is rotation angle, i Lf_α , i Lf_β represent components of output current of grid-connected converter on α axis and β axis respectively, i Lfα_ref , i Lfβ_ref represent instantaneous current reference value of grid-connected converter on α axis and β axis respectively, v pcc_α , v pcc_β represent components of grid voltage on α axis and β axis respectively.

[0014] Further, the step S4 comprises: detecting the current amplitude of the current reference value in real time, and judging whether the current reference value exceeds a threshold current; if the current reference value does not exceed the threshold current, the output modulation voltage is equal to the modulation voltage reference value; if the current reference value exceeds the threshold current, reconstructing the modulation voltage according to the threshold current to equivalent series virtual resistance, and reconstructing the virtual impedance with constant resistance according to the equivalent series virtual resistance.

[0015] Further, the modulation voltage is reconstructed according to the threshold current, and the formula is:

[0016] ,

[0017] wherein, wherein, v mαβ represents the reconstructed modulation voltage value; i Lfαβref_s represents the threshold current.

[0018] Further, the modulation voltage is reconstructed according to the threshold current to equivalent series virtual resistance, and the formula is:

[0019] ,

[0020] ,

[0021] wherein, I max represents the amplitude of the threshold current; R v represents the equivalent series virtual resistance of the reconstructed modulation voltage; and σ represents the current limiting coefficient, .

[0022] Further, the virtual impedance with constant resistance is reconstructed based on the equivalent series virtual resistance, and the formula is:

[0023] ,

[0024] ,

[0025] wherein, is the reconstructed voltage value after current limiting, is the reconstructed grid voltage after current limiting, is the virtual impedance with constant resistance after current limiting.

[0026] Further, the fault current limiting method further comprises: analyzing the system transient stability of the AHO type virtual oscillator network based on the power angle diagram.

[0027] Further, the fault current limiting method further comprises: performing coordinate transformation on the output current and grid voltage of the grid-connected converter through an abc / αβ coordinate transformation module, and performing inverse transformation on the modulation voltage through an αβ / abc coordinate transformation module to generate a PWM modulation control signal; the PWM modulation control signal is used to regulate the output current of the grid-connected converter, so that the output current of the grid-connected converter is limited to a safe range.

[0028] According to an aspect of the present application, a control system of an AHO type virtual oscillator grid based on threshold current control is provided, comprising:

[0029] An AHO model virtual oscillator is configured to generate a modulation voltage reference value v Lfαβ based on the output current i PCCαβ of the grid-connected converter and the grid voltage v αβ ;

[0030] A current limiting control module is configured to construct an inverse current control loop through inverse calculation according to the modulation voltage reference value, and inversely calculate a corresponding current reference value i Lfαβref as an input signal of the current control loop, and detect whether the current reference value exceeds a threshold current in real time through a current limiting module, and if the threshold current is exceeded, reconstruct the modulation voltage through the current control loop, so that the output current of the grid-connected converter is limited to a safe range; wherein, C(s) represents a proportional controller, C(s)= K p , K p is a design parameter.

[0031] Further, the control system further comprises an abc / αβ coordinate transformation module and an αβ / abc coordinate transformation module, the abc / αβ coordinate transformation module is configured to perform coordinate transformation on the output current and grid voltage of the grid-connected converter in real time, and the αβ / abc coordinate transformation module is configured to perform inverse transformation on the modulation voltage to generate a PWM modulation control signal; the PWM modulation control signal is used to regulate the output current of the grid-connected converter, so that the output current of the grid-connected converter is limited to a safe range.

[0032] The above technical solution activates the current limiting mode immediately when the current reference value exceeds the threshold current, constructs an inverse current control loop through inverse calculation, inversely calculates a corresponding current reference value as an input signal of the current control loop, and reconstructs the modulation voltage to equivalent series virtual resistance, so as to realize rapid and accurate suppression of fault current, and based on transient stability analysis of the power angle diagram, ensures stable operation of the system under severe voltage drop.

[0033] Compared with the prior art, the present application has the following advantages:

[0034] (1) The AHO type virtual oscillator of the present application generates a modulation signal through a current limiting control circuit, which can realize accurate fault current suppression while maintaining system transient stability, and improve the low voltage ride-through capability of the network converter.

[0035] (2) The present application realizes rapid and accurate suppression of fault current by reconstructing the modulation voltage to equivalent series virtual resistance, and the response time is shortened to milliseconds, which is superior to the virtual impedance method.

[0036] (3) The present application is suitable for multi-inverter parallel system and supports decentralized control without additional communication or global reference signal. BRIEF DESCRIPTION OF DRAWINGS

[0037] In order to more clearly illustrate the technical solutions of the embodiments of the present application or the prior art, the drawings used in the embodiments or prior art description will be briefly introduced. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0038] Figure 1 A flow chart of a threshold current control based AHO type virtual oscillator network fault current limiting method is provided for the embodiments of the present application.

[0039] Figure 2 A network control block diagram based on AHO type virtual oscillator is provided for the embodiments of the present application.

[0040] Figure 3 A control block diagram of AHO virtual oscillator is provided for the embodiments of the present application.

[0041] Figure 4 A control block diagram of threshold current control is provided for the embodiments of the present application.

[0042] Figure 5 An equivalent circuit diagram after system current limiting is provided for the embodiments of the present application.

[0043] Figure 6 A parameter setting diagram of simulation verification is provided for the embodiments of the present application.

[0044] Fig. 7(a) is a simulation waveform diagram of the key states of the system under simulation condition 1 provided by the embodiments of the present application.

[0045] Fig. 7(b) is a simulation waveform diagram of the real output voltage and output current under simulation condition 1 provided by the embodiments of the present application.

[0046] Fig. 8(a) is a simulation waveform diagram of the key states of the system under simulation condition 2 provided by the embodiments of the present application.

[0047] Fig. 8(b) is a simulation waveform diagram of real output voltage and output current under simulation condition 2 according to an embodiment of the present application.

[0048] Fig. 9(a) is a simulation waveform diagram of system key states under simulation condition 3 according to an embodiment of the present application.

[0049] Fig. 9(b) is a simulation waveform diagram of real output voltage and output current under simulation condition 3 according to an embodiment of the present application.

[0050] Fig. 10(a) is a simulation waveform diagram of system key states under simulation condition 4 according to an embodiment of the present application.

[0051] Fig. 10(b) is a simulation waveform diagram of real output voltage and output current under simulation condition 4 according to an embodiment of the present application. DETAILED DESCRIPTION

[0052] It should be noted that:

[0053] The terms "comprise" and "have" and any variations thereof in the specification and in the claims and the above mentioned attached drawings, are intended to cover a non-exclusive inclusion, for example, a process, method, system, product or apparatus that includes a list of steps or units not necessarily limited to those clearly listed, but can include other not expressly listed or inherent to such processes, methods, products or apparatus.

[0054] The block diagrams shown in the drawings are merely functional entities, not necessarily corresponding to physically independent entities. That is, these functional entities can be implemented in the form of software, or in one or more hardware modules or integrated circuits, or in different networks and / or processor devices and / or microcontroller devices. The flowcharts shown in the drawings are only exemplary descriptions, not necessarily including all contents and operations / steps, and not necessarily executed in the described order. For example, some operations / steps can be further decomposed, and some operations / steps can be combined or partially combined, so the actual execution order can be changed according to the actual situation.

[0055] In order to make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present application. In addition, the technical features in each of the embodiments or in a single embodiment provided by the present application can be combined with each other at will to form new technical solutions, and such combination is not restricted by the order of steps and / or structure mode, but should be based on the fact that it can be realized by those of ordinary skill in the art. When the combination of technical solutions appears to be contradictory or unachievable, it should be considered that the combination of technical solutions does not exist and is not within the scope of protection required by the present application.

[0056] Please refer to the accompanying drawings Figure 1 and Figure 2 The present application provides a threshold current control based AHO type virtual oscillator network construction fault current limiting method, specifically comprising the following steps:

[0057] Step S1: Real-time acquisition of output current and grid voltage of network construction converter.

[0058] In step S1, the output current and grid voltage of the network construction converter are real-time acquired, and the real-time acquired output current i Lf and grid voltage v PCC of the network construction converter are subjected to coordinate transformation through an abc / αβ coordinate transformation module, to obtain the component i Lf on the α axis and the component i Lf_α on the β axis of the output current i Lf_β , and the component v PCC on the α axis and the component v PCC_α on the β axis of the grid voltage v PCC_β .

[0059] Step S2: Generation of a modulation voltage reference value by an AHO type virtual oscillator based on the output current and the grid voltage.

[0060] In step S2, as shown in Figure 3 , the component v PCC on the α axis and the component v PCC_α on the β axis of the grid voltage v PCC_β , the component i Lf on the α axis and the component i Lf_α on the β axis of the output current i Lf_β , active power P0 and reactive power Q0 are input to the AHO model virtual oscillator, to output a modulation voltage reference value v αβ(Including the component of the modulation voltage reference value on the α axis v) α and the component v on the β axis β The following section will further describe the specific steps involved in generating the modulation voltage reference value using the AHO model virtual oscillator.

[0061] (1) The AHO model virtual oscillator includes a current reference module, which converts the grid voltage v PCC The component v on the α axis PCC_α and the component v on the β axis PCC_β The active power P0 and reactive power Q0 are input to the current reference module, and the current reference module outputs the instantaneous current reference value i. Lfαβref Its expression is as follows:

[0062]

[0063] Among them, i Lfα_ref Indicates the instantaneous current reference value i Lfαβref The component on the α-axis, i Lfβ_ref Indicates the instantaneous current reference value i Lfαβref The instantaneous current reference value on the β-axis, v PCC_α Indicates grid voltage v PCC The component on the α-axis, v PCC_β Indicates grid voltage v PCC The components on the β axis are P0 representing active power, Q0 representing reactive power, and V0 representing the rated line voltage of the grid converter.

[0064] (2) Using the instantaneous current reference value and the output current of the grid converter, a set of controlled sources related to the system parameters is calculated, and the expression is as follows:

[0065] ,

[0066] Where u1 and u2 are external signal inputs, a set of controlled sources related to system parameters; θ is the rotation angle, used to compensate for the effects of network impedance, and is usually set as the network impedance angle. For systems that are mainly inductive, θ is generally taken as π / 2; K i This represents the current amplification factor.

[0067] (3) The nonlinear controlled voltage source and current source are calculated using controlled sources u1 and u2 respectively, and their expressions are as follows:

[0068] ,

[0069] Among them, v m and i mThese correspond to the nonlinear controlled voltage source and current source, respectively. ξ represents the design parameter for the oscillator convergence speed, ω0 represents the rated voltage angular frequency, and α represents the square of the radius of the AHO limiting loop, corresponding to the square of the rated line voltage of the grid converter ||V0||. 2 x1 is v c x2 is εi L C and L represent the capacitor and inductor components in the virtual oscillator circuit, respectively. c i represents the voltage across capacitor C. L ε represents the inductor current through the inductor element L, and ε represents the inductor current i. L The magnification factor.

[0070] (4) Utilizing nonlinear controlled voltage and current sources v m and i m Calculate the inductor current i L and capacitor voltage v c Its expression is as follows:

[0071] ,

[0072] It should be noted that the above expression is the state equation of the virtual oscillator, which is constructed based on Kirchhoff's voltage law and current law.

[0073] (5) Also because v a = K v v c v β = K v εi L The control equation for the AHO virtual oscillator is expressed as follows:

[0074] ,

[0075] Among them, v α v β K represents the components of the modulation voltage reference value on the α and β axes, respectively. v K represents the voltage amplification factor of the grid converter. i V represents the current amplification factor. c Indicates the capacitor voltage, i L ε represents the inductor current, and i represents the inductor current. L The amplification factor, ξ represents the design parameter for the oscillator convergence speed, ω0 represents the rated value of the voltage angular frequency, θ is the rotation angle, and i Lf_α i Lf_β These represent the components of the output current of the grid converter on the α-axis and β-axis, respectively. Lfα_ref i Lfβ_ref These represent the instantaneous current reference values ​​of the grid converter on the α-axis and β-axis, respectively.pcc_α v pcc_β These represent the components of the grid voltage on the α-axis and β-axis, respectively.

[0076] Step S3: Based on the modulation voltage reference value, construct the reverse current control loop through reverse calculation, and calculate the corresponding current reference value as the input signal of the current control loop.

[0077] In step S3, as Figure 4 As shown, the modulation voltage reference value, grid voltage, and output current of the grid converter are input to the reverse current control loop. The reverse current control loop is based on the proportional controller C(s) = k P The constructed reverse current control loop outputs an instantaneous current reference value. The formula for generating the current reference value using the reverse current control loop is as follows:

[0078] ,

[0079] Among them, i Lfαβref This represents the instantaneous current reference value, including the instantaneous current reference values ​​of the grid-connected converter on the α-axis and β-axis, i.e., i Lfα_ref and i Lfβ_ref ;v αβ This represents the modulation voltage reference value, including its components v on the α and β axes. α v β ;v PCCαβ (i.e. v) PCC This represents the grid voltage, including its components v on the α and β axes. PCC_α v PCC_β C(s) represents the proportional controller, C(s) = k P K p For design parameters; i Lfαβ (i.e. i Lf () represents the output current of the grid converter, including the components of the output current on the α and β axes. Lf_α i Lf_β .

[0080] It should be noted that the parameter C(s) of the reverse current control loop in step S3 is particularly critical. To ensure accurate cancellation between the current control loop and the reverse current control loop, this invention employs a proportional controller C(s) = k P To simplify the implementation of the reverse current control loop.

[0081] Step S4: Real-time detection of whether the current reference value exceeds the threshold current. If it exceeds the threshold current, the modulation voltage is reconstructed through the current control loop to limit the output current of the grid converter to a safe range.

[0082] In step S4, the current reference value i generated by the reverse current control loop is...Lfαβref The input is sent to the current limiting module, which monitors the current amplitude of the current reference value in real time and determines whether the current amplitude of the current reference value exceeds the threshold current i. Lfαβref_s Current amplitude I max If the current amplitude of the current reference value does not exceed the current amplitude of the threshold current, then the current reference value i Lfαβref As the input signal to the current control loop, the modulation voltage v output by the current control loop is... mαβ Equal to the modulation voltage reference value v αβ If the current amplitude of the current reference value exceeds the current amplitude of the threshold current, the grid converter enters the current limiting mode, and the current limiting module limits the input signal of the current control loop to the threshold current i. Lfαβref_s That is, the threshold current i Lfαβref_s As the input signal of the current control loop, the current control loop reconstructs the modulation voltage based on the threshold current to form an equivalent series virtual resistance, and reconstructs a virtual impedance with a constant resistance based on the equivalent series virtual resistance.

[0083] Finally, the modulation voltage is input to the αβ / abc coordinate transformation module, which performs an inverse transformation on the modulation voltage to generate the PWM modulation control signal for the grid converter. This PWM modulation control signal is then input to the grid converter. Under the regulation of this PWM modulation control signal, the grid converter limits its output current to a safe range. The specific safe range can be adjusted according to actual conditions and is not limited here. Understandably, when the grid voltage drops sharply, the output current of the grid converter will surge. Without current limiting, current overshoot can easily occur, threatening the stability of the system operation.

[0084] Furthermore, the current control loop reconstructs the modulation voltage based on the threshold current, as shown in the formula:

[0085] ,

[0086] Among them, v mαβ This represents the reconstructed modulation voltage value, including the reconstructed modulation voltage value components v on the α and β axes. mα v mβ i Lfαβref_s Represents the threshold current, including the threshold current components i on the α and β axes. Lfαref_s i Lfβref_s The threshold current amplitude is I. max .

[0087] Furthermore, in step S4, the modulation voltage value v mαβ The equivalent virtual resistance R is achieved by dynamically adjusting the current limiting coefficient σ. vReconfiguration. When a low-voltage fault occurs in the system and triggers current limiting, the dynamics of the current control loop are ignored, assuming i Lfαβ =i Lfαβref_s ,get:

[0088] ,

[0089] Among them, I max R represents the magnitude of the threshold current. v The equivalent series virtual resistance R represents the reconstructed modulation voltage. v The current limiting factor σ is dynamically adjusted; σ represents the current limiting factor, which is defined as follows: The current limiting factor σ takes effect after the current limiting is triggered, and is a positive number greater than 0 and less than or equal to 1. Therefore:

[0090] ,

[0091] It should be noted that both sides of the equation above contain the letter i. Lfαβ It is to make i Lfαβ Write it as an expression related to σ for easier subsequent analysis, rather than for the sake of using i on the right side of the equals sign. Lfαβ Iteratively solve for i on the left side of the equals sign Lfαβ .

[0092] It can be seen that after the circular current limiting strategy is triggered, an equivalent current limiting circuit with a value of R is connected in series. v The virtual resistor R. When the current limiting policy is not triggered, this virtual resistor R... v The virtual resistance R is 0. v The virtual resistance R is related to σ. v The resistance value changes over time during transients, which is inconvenient for analyzing the fault ride-through capability of grid-type converters. Therefore, by reconfiguring v... αβ and v PCCαβ To address the issue of virtual resistance being affected by system operating conditions and time, the above equation can be transformed to obtain:

[0093] ,

[0094] In the formula, v αβ_s and v PCCαβ_s R is the reconstructed voltage value after current limiting. v_cons This is the virtual impedance reconstructed after current limiting. Specifically, v αβ_s It is a defined virtual internal potential of a grid converter, v PCCαβ_s It is the sampled value v PCCαβ Amplifying the quantity by (1-σ) / σ times, we have:

[0095] ,

[0096] Its corresponding current-limited equivalent circuit is as follows: Figure 5 As shown, the internal potential of the grid converter is v. αβ_s Virtual impedance R v_cons Constant. Although the output current is saturated at this point, the output voltage phase angle of the grid converter is still determined by the internal virtual voltage v. αβ_s Under control, the grid converter continued to operate as a voltage source. It should be noted that this method is applicable to multi-inverter parallel systems, where each inverter achieves self-synchronization through distributed control, without requiring global communication or a unified reference signal.

[0097] In this embodiment, the current limiting method further includes analyzing the transient stability of the AHO-type virtual oscillator network based on the power angle phase diagram to ensure a stable equilibrium point exists during a fault and to restore the system to its initial operating state after the fault is cleared. Understandably, if the system does not converge to a fixed equilibrium point, it will oscillate continuously and cannot operate stably. To verify the effectiveness of this invention, a system was built in Matlab / Simulink. Figure 2 The system model shown has the following parameter settings: Figure 6 As shown. Figures 7(a) to 10(b) Simulation waveforms for four different low-voltage operating conditions are shown.

[0098] Operating Condition 1: As shown in Figure 7(a), the grid voltage v PCC The current drops sharply to 0.75 pu at t = 1.0s and remains there for 0.5s. At the moment of the fault, the output current i of the AHO grid converter... Lf The current is immediately limited to 1.2 pu. During the fault, the power angle quickly reaches a new steady state, the system remains transiently stable, and the output active power decreases while reactive power increases. After the fault is cleared, the current immediately returns to normal, and the system returns to its original steady state. Figure 7(b) shows the actual output voltage v under operating condition 1. PCC With output current i Lf Waveform.

[0099] Operating Condition 2: As shown in Figures 8(a) and 8(b), the grid voltage v PCC The voltage drops sharply to 0.75 pu at t=1.0s and remains there for 1.735s. At the time of the fault, the output current i of the grid converter... Lf The voltage quickly reached the 1.2 pu limit. Throughout the fault, the system maintained stable operation. Once the voltage recovered, the output current i... Lf The fault immediately returned to the normal range, and the system quickly recovered to its stable operating point before the fault. Throughout the process, the grid converter controlled by the AHO-type virtual oscillator demonstrated excellent fault ride-through capability and transient stability.

[0100] Operating Condition 3: As shown in Figures 9(a) and 9(b), the grid voltage vPCC The current drops sharply to 0.2 pu at t=1.0s and remains there for 0.2s. The output current i of the grid converter controlled by the AHO virtual oscillator... Lf The system quickly reached the 1.2 pu limit. During the fault, the system power angle exhibited a dynamic characteristic of first rising rapidly and then slowly decreasing. Before the power angle fully stabilized, the grid voltage recovered, at which point the converter immediately released the current limit and exited current limiting. After a brief transient adjustment, the system successfully returned to its stable operating state before the fault.

[0101] Operating Condition 4: As shown in Figures 10(a) and 10(b), the grid voltage v PCC At t=1.0s, the voltage suddenly dropped to 0.2 pu and remained there for 0.625s. Similar to the simulation results under operating condition 3, the power angle of the grid converter gradually tended to a stable value after a rapid increase. The grid voltage v... PCC After returning to normal, the current limiting automatically exits, and the output current i Lf It then fell back. After a brief adjustment, the grid-connected converter system based on the AHO virtual oscillator successfully recovered to its initial stable operating point.

[0102] Furthermore, simulation verification shows that the fault current limiting method for AHO-type virtual oscillator network based on threshold current control provided by the present invention can ensure that the system can limit the current to 1.2 times the rated value and maintain transient stability during the fault period even in the event of a severe fault (voltage drops to 0.2 pu), thus exhibiting good fault ride-through capability.

[0103] Please refer to it again. Figure 1Based on the same technical concept as the aforementioned embodiments, this invention also provides a control system for a grid-connected AHO-type virtual oscillator based on threshold current control. This system is used to quickly respond and activate a current-limiting protection mechanism when a sudden drop in grid voltage is detected, thereby limiting the output current of the grid-connected converter within a safe range. The control system includes an abc / αβ coordinate transformation module, an AHO model virtual oscillator, a current-limiting control module, and an αβ / abc coordinate transformation module. The grid-connected converter, the abc / αβ coordinate transformation module, the AHO model virtual oscillator, the current-limiting control module, and the αβ / abc coordinate transformation module are connected sequentially to form a closed control loop. The current-limiting control circuit includes a reverse current control loop, a current-limiting module, and a current control loop. The abc / αβ coordinate transformation module is used to perform coordinate transformation on the real-time acquired output current of the grid-connected converter and the grid voltage. The AHO model virtual oscillator is used to generate a modulation voltage reference value based on the coordinate-transformed output current and grid voltage. The current limiting control module constructs a reverse current control loop based on the modulation voltage reference value through inverse calculation. It calculates the corresponding current reference value as the input signal for the current control loop and uses the current limiting module to detect in real time whether the current amplitude of the current reference value exceeds a threshold current. If it does, the modulation voltage is reconstructed through the current control loop. The αβ / abc coordinate transformation module performs an inverse transformation on the modulation voltage to generate the PWM modulation control signal for the grid converter. The PWM modulation control signal is used to regulate the output current of the grid converter to limit it to a safe range.

[0104] In summary, this invention relates to a fault current limiting method for grid-connected converters using an AHO-type virtual oscillator based on threshold current control, applicable to grid-connected converters controlled by an AHO-type virtual oscillator. Fault current limiting is activated immediately when the current reference value exceeds the maximum allowable value (i.e., the threshold current). A reverse current control mechanism is constructed through inverse calculation, using the corresponding current reference value as the input signal for the current control loop. The modulation voltage is reconstructed to equivalently series a virtual resistor, achieving rapid and accurate suppression of the fault current. Simultaneously, based on transient stability analysis of the power angle phase diagram, the system maintains stable operation even under severe voltage dips. Compared to traditional current limiting control strategies such as the virtual impedance method, this invention improves the fault ride-through capability of grid-connected converters and is applicable to multi-inverter parallel systems, supporting distributed control.

[0105] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the technical solutions of the embodiments of the present invention.

Claims

1. A fault current limiting method for an AHO-type virtual oscillator network based on threshold current control, characterized in that, include: Step S1: Real-time acquisition of the output current i of the grid converter Lfαβ and grid voltage v PCCαβ ; Step S2: Use an AHO-type virtual oscillator to generate a modulation voltage reference value v based on the output current and the mains voltage. αβ ; Step S3: Based on the modulation voltage reference value, construct a reverse current control loop through reverse calculation, and calculate the corresponding current reference value i in reverse. Lfαβref As the input current of the current control loop, where, C(s) represents the proportional controller, C(s) = K p K p For design parameters; Step S4: Real-time detection of whether the current reference value exceeds the threshold current. If it exceeds the threshold current, the modulation voltage is reconstructed through the current control loop to limit the output current of the grid converter to a safe range.

2. The fault current limiting method for an AHO-type virtual oscillator network based on threshold current control according to claim 1, characterized in that, In step S2, the formula for calculating the modulation voltage reference value is: ， in, , K represents the components of the modulation voltage reference value on the α and β axes, respectively. v K represents the voltage amplification factor of the grid converter. i V represents the current amplification factor. c Indicates the capacitor voltage, i L ε represents the inductor current, and i represents the inductor current. L The amplification factor, ξ represents the design parameter for the oscillator convergence speed, ω0 represents the rated value of the voltage angular frequency, θ is the rotation angle, and i Lf_α i Lf_β These represent the components of the output current of the grid converter on the α-axis and β-axis, respectively. Lfα_ref i Lfβ_ref These represent the instantaneous current reference values ​​of the grid converter on the α-axis and β-axis, respectively. pcc_α v pcc_β These represent the components of the grid voltage on the α-axis and β-axis, respectively.

3. The fault current limiting method for an AHO-type virtual oscillator network based on threshold current control according to claim 1, characterized in that, Step S4 includes: If the current reference value does not exceed the threshold current, the output modulation voltage is equal to the modulation voltage reference value; If the current reference value exceeds the threshold current, the modulation voltage is reconstructed based on the threshold current to form an equivalent series virtual resistance, and a virtual impedance with a constant resistance is reconstructed based on the equivalent series virtual resistance.

4. The fault current limiting method for an AHO-type virtual oscillator network based on threshold current control according to claim 3, characterized in that, The modulation voltage is reconstructed based on the threshold current, using the following formula: ， Among them, v mαβ Indicates the reconstructed modulation voltage; i Lfαβref_s This represents the threshold current.

5. The fault current limiting method for an AHO-type virtual oscillator network based on threshold current control according to claim 4, characterized in that, The modulation voltage is reconstructed based on the threshold current to equivalently connect a virtual resistance, as shown in the following formula: ， ， Among them, I max R represents the magnitude of the threshold current. v σ represents the equivalent series virtual resistance of the reconstructed modulation voltage; σ represents the current limiting factor. .

6. The fault current limiting method for an AHO-type virtual oscillator network based on threshold current control according to claim 5, characterized in that, Based on the aforementioned equivalent series virtual resistance, a virtual impedance with a constant resistance is reconstructed, using the following formula: ， ， In the formula, This is the reconstructed voltage value after current limiting. This refers to the reconstructed grid voltage after current limiting. This is a virtual impedance with a constant resistance value after current limiting and reconstruction.

7. The fault current limiting method for an AHO-type virtual oscillator network based on threshold current control according to claim 1, characterized in that, The fault current limiting method also includes: analyzing the system transient stability of the AHO-type virtual oscillator network based on the power angle phase diagram.

8. The fault current limiting method for an AHO-type virtual oscillator network based on threshold current control according to claim 1, characterized in that, The fault current limiting method further includes: performing coordinate transformation on the output current and grid voltage of the grid converter acquired in real time through an abc / αβ coordinate transformation module; performing inverse transformation on the modulation voltage through an αβ / abc coordinate transformation module to generate a PWM modulation control signal; and using the PWM modulation control signal to regulate the output current of the grid converter so that the output current of the grid converter is limited to a safe range.

9. A control system for an AHO-type virtual oscillator network based on threshold current control, characterized in that, include: AHO model virtual oscillator is used to simulate the output current i of the grid converter based on real-time acquisition. Lfαβ and grid voltage v PCCαβ Generate modulation voltage reference value v αβ ; The current limiting control module is used to construct a reverse current control loop based on the modulation voltage reference value, and to back-calculate the corresponding current reference value i. Lfαβref As the input signal to the current control loop, the current reference value is monitored in real time by the current limiting module to determine whether it exceeds the threshold current. If it exceeds the threshold current, the modulation voltage is reconstructed through the current control loop to limit the output current of the grid converter to a safe range. C(s) represents the proportional controller, C(s) = K p K p These are design parameters.

10. The control system for an AHO-type virtual oscillator network based on threshold current control according to claim 9, characterized in that, The control system further includes an abc / αβ coordinate transformation module and an αβ / abc coordinate transformation module. The abc / αβ coordinate transformation module is used to perform coordinate transformation on the output current and grid voltage of the grid converter acquired in real time, and the αβ / abc coordinate transformation module is used to perform inverse transformation on the modulation voltage to generate a PWM modulation control signal. The PWM modulation control signal is used to regulate the output current of the grid converter so that the output current of the grid converter is limited to a safe range.

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