A weak grid strength-based online identification-based network-constructing flexible interconnection device adaptive virtual impedance control method and device

Abstract

The application discloses a kind of based on weak grid strength online identification's network type flexible interconnection device adaptive virtual impedance control method and device, belong to distribution control technical field.It includes: the voltage, current, frequency, active power, reactive power and DC bus voltage of network point of network type flexible interconnection device are collected, and the weak grid strength evaluation of network point is calculated;Identify the operating state of the network type flexible interconnection device;Virtual resistance, virtual reactance and virtual impedance phase angle are generated;Adaptive virtual impedance is generated;Determine the corrected voltage reference value;According to the corrected voltage reference value, generate modulation signal to control the network type flexible interconnection device operation;After entering fault recovery state, gradually exit the control process of adaptive virtual impedance according to the preset recovery law.The application can accurately reflect the grid strength state of access point, adapt to different operating environments, and can improve the stable operation ability of the device under weak grid.

Description

Technical Field

[0001] This invention relates to an adaptive virtual impedance control method and device for a grid-type flexible interconnection device based on online identification of weak grid strength, belonging to the fields of power electronic converter control technology, new power system grid-type control technology, and distribution network flexible interconnection technology. Background Technology

[0002] With the large-scale integration of new energy power generation, energy storage systems, and power electronic equipment into distribution networks, the power grid is gradually exhibiting characteristics of low inertia, low short-circuit capacity, and weak damping. Traditional grid-following converters rely on phase-locked loops to operate following the grid voltage, which can easily lead to problems such as decreased synchronization stability, power oscillations, and insufficient voltage support at the grid connection point under weak grid conditions, voltage dips, and fault disturbances. Grid-following control technology, by controlling the converter to simulate the external characteristics of synchronous power sources, enables it to actively establish voltage and frequency, and has become an important technical direction for supporting the stable operation of new power systems.

[0003] Flexible interconnection devices, as a type of distribution network interconnection equipment based on power electronic converters, can realize power regulation, voltage support, and energy sharing between different feeders, transformer substations, microgrids, or AC / DC systems. Network-type flexible interconnection devices can maintain voltage source characteristics during grid-connected operation, islanded operation, fault ride-through, and power restoration, improving the operational flexibility and power supply reliability of the distribution network.

[0004] However, grid-connected flexible interconnection devices are essentially controlled voltage sources. When connected to weak or ultra-weak power grids, they are prone to problems such as voltage drops at the grid connection point, excessive output current, active and reactive power oscillations, and post-fault recovery surges due to the low equivalent short-circuit ratio at the grid connection point, high line impedance, and insufficient damping. Especially under asymmetrical fault conditions, negative-sequence and zero-sequence components further exacerbate current imbalance and power oscillations.

[0005] Existing technologies typically employ methods such as current limiting, virtual impedance, virtual admittance, or control mode switching to address fault overcurrent and stability issues in grid-connected converters. Among these, current limiting is simple to implement but may weaken the voltage source characteristics of grid-connected devices; fixed virtual impedance can suppress fault current to some extent but is difficult to adapt to different grid strengths and fault depths; adaptive virtual impedance methods based on current thresholds often rely on current exceeding limits as the primary triggering condition, failing to fully utilize grid connection point voltage, current, frequency, power oscillations, and DC bus status for a comprehensive evaluation of weak grid strength.

[0006] Therefore, there is an urgent need for a control method that can identify the strength of a weak grid online based on the real-time operating status of the grid connection point, and adaptively adjust the phase angle of virtual resistance, virtual reactance and virtual impedance based on the strength of the weak grid, so that the grid-type flexible interconnection device can still maintain stable operation under conditions of weak grid, voltage drop, asymmetrical fault and power oscillation. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to overcome the defects of the prior art and provide an adaptive virtual impedance control method and device for network-type flexible interconnection devices based on online identification of weak grid strength.

[0008] To solve the above-mentioned technical problems, the present invention is implemented using the following technical solution.

[0009] In a first aspect, the present invention discloses an adaptive virtual impedance control method for a network-type flexible interconnection device based on online identification of weak grid strength, comprising the following steps:

[0010] Collect voltage, current, frequency, active power, reactive power, and DC bus voltage at the grid connection point of the flexible interconnection device.

[0011] Based on the voltage, current, frequency, active power, reactive power, and DC bus voltage, calculate the weak grid strength evaluation quantity at the grid connection point.

[0012] The operating status of the grid-type flexible interconnection device is identified based on the weak grid strength evaluation quantity. The operating status includes normal grid connection status, weak grid operation status, voltage drop status, power oscillation status, and fault recovery status.

[0013] Based on the weak grid strength evaluation quantity and the operating status, virtual resistance, virtual reactance and virtual impedance phase angle are dynamically generated;

[0014] An adaptive virtual impedance is generated based on the virtual resistance, virtual reactance, and virtual impedance phase angle.

[0015] The adaptive virtual impedance is introduced into the grid-type control voltage reference value to obtain the corrected voltage reference value;

[0016] A modulation signal is generated based on the corrected voltage reference value to control the operation of the network-type flexible interconnect device;

[0017] After detecting that the network-type flexible interconnect device has entered the fault recovery state, the adaptive virtual impedance control process is gradually exited according to the preset recovery law.

[0018] Furthermore, the weak grid strength evaluation quantity is calculated based on at least two of the following: equivalent short-circuit ratio, voltage drop index, frequency deviation index, power oscillation index, and DC bus voltage deviation index.

[0019] Furthermore, the formula for calculating the adaptive virtual impedance is as follows:

[0020] ;

[0021] In the formula, For adaptive virtual impedance, For virtual resistance, For virtual reactance, j is the imaginary unit;

[0022] ;

[0023] In the formula, Based on virtual resistance, For evaluating the strength of weak power grids, To exceed the output current limit, K is the active power oscillation amplitude, and k1, k2, and k3 are virtual resistance adjustment coefficients.

[0024] ;

[0025] In the formula, Based on virtual reactance, The degree of voltage drop at the grid connection point, The reactive power deviation is represented by m1, m2, and m3, which are virtual reactance adjustment coefficients.

[0026] The phase angle of the adaptive virtual impedance is:

[0027] ;

[0028] In the formula, The phase angle for adaptive virtual impedance, The phase angle is the equivalent line impedance. This is a voltage sag correction term. This is a stability margin correction term.

[0029] Furthermore, the formula for the corrected voltage reference value is:

[0030] ;

[0031] In the formula, This is the corrected voltage reference value. The original voltage reference value generated for network-type control. For grid connection point current, This is a voltage sag compensation item. This is the power oscillation damping term.

[0032] Furthermore, under asymmetrical fault conditions, the formula for the corrected voltage reference value is:

[0033] ;

[0034] In the formula, This is the corrected voltage reference value. It is a positive sequence current. It is a negative sequence current. It is the zero-sequence current. This is a voltage sag compensation item. For power oscillation damping term, For positive sequence virtual impedance, For negative sequence virtual impedance, This is the zero-sequence virtual impedance;

[0035] ; ; ;

[0036] In the formula, For positive sequence virtual resistance, For negative sequence virtual resistance, For zero-sequence virtual resistance, For positive sequence virtual reactance, For negative sequence virtual reactance, This is the zero-sequence virtual reactance.

[0037] Furthermore, the virtual resistance of the negative-sequence virtual impedance is not less than the virtual resistance of the positive-sequence virtual impedance, and the virtual resistance of the zero-sequence virtual impedance is not less than the virtual resistance of the positive-sequence virtual impedance.

[0038] Furthermore, the candidate parameters in the process of calculating the adaptive virtual impedance are subject to the following constraints:

[0039] ; ; ; ; ;

[0040] In the formula, Z v,min Z v,max These are the lower and upper limits of the virtual impedance amplitude, respectively; R v,min R v,max These are the lower and upper limits of the virtual resistance, respectively; X v,min X v,max These are the lower and upper limits of the virtual reactance, respectively; θ min θ max These are the lower and upper limits of the virtual impedance phase angle, respectively; ρ R ρ X t represents the upper limit of the rate of change of virtual resistance and virtual reactance, respectively, and t is time.

[0041] Furthermore, when a candidate parameter exceeds the stability margin region, it is subjected to amplitude limiting, boundary projection, or order reduction correction to ensure that the candidate parameter always remains within the stable feasible region.

[0042] Furthermore, the preset recovery law is that the virtual impedance exits smoothly according to exponential decay, ramp decay, or piecewise decay.

[0043] Secondly, the present invention also discloses an adaptive virtual impedance control device for a network-type flexible interconnection device based on online identification of weak grid strength, comprising:

[0044] The data acquisition module is used to collect the voltage, current, frequency, active power, reactive power, and DC bus voltage at the grid connection point of the grid-connected flexible interconnection device.

[0045] The calculation module is used to calculate the weak grid strength evaluation quantity at the grid connection point based on the voltage, current, frequency, active power, reactive power and DC bus voltage.

[0046] The determination module is used to identify the operating status of the grid-type flexible interconnection device based on the weak grid strength evaluation quantity. The operating status includes normal grid connection status, weak grid operation status, voltage drop status, power oscillation status, and fault recovery status.

[0047] The generation module is used to dynamically generate virtual resistance, virtual reactance, and virtual impedance phase angle based on the weak grid strength evaluation quantity and the operating status; and to generate adaptive virtual impedance based on the virtual resistance, virtual reactance, and virtual impedance phase angle.

[0048] The correction module is used to introduce the adaptive virtual impedance into the network-type control voltage reference value to obtain the corrected voltage reference value;

[0049] A modulation module is used to generate a modulation signal based on the corrected voltage reference value to control the operation of the network-type flexible interconnect device;

[0050] The exit module is used to gradually exit the adaptive virtual impedance control process according to a preset recovery rule after detecting that the network-type flexible interconnect device has entered the fault recovery state.

[0051] The beneficial effects achieved by this invention are as follows:

[0052] 1) This invention integrates grid connection point voltage, current, frequency, power oscillation and DC bus voltage status to construct a weak grid strength evaluation quantity, which can more accurately reflect the grid strength status at the access point of the grid-type flexible interconnection device.

[0053] 2) This invention dynamically adjusts the virtual resistance, virtual reactance, and virtual impedance phase angle based on the weak grid strength evaluation quantity, so that the grid-type flexible interconnection device can adapt to different operating environments such as strong grid, weak grid and ultra-weak grid.

[0054] 3) This invention enhances system damping through virtual resistance, improves voltage support capability through virtual reactance, and balances fault current suppression and voltage drop support through virtual impedance phase angle correction, thereby improving the stable operation capability of the device under weak power grid conditions.

[0055] 4) This invention is applicable to two-port or multi-port flexible interconnection devices, as well as to grid-type soft switching devices, modular multilevel converters and energy storage type flexible interconnection devices equipped with energy storage units. Attached Figure Description

[0056] Figure 1 This is a flowchart of an adaptive virtual impedance control method for a network-type flexible interconnection device based on online identification of weak grid strength, according to the present invention.

[0057] Figure 2 This is a schematic diagram of the main circuit structure of the network-type flexible interconnection device of the present invention;

[0058] Figure 3 This is a flowchart illustrating the calculation of the weak grid strength evaluation quantity of this invention;

[0059] Figure 4 This is a block diagram for generating adaptive virtual impedance parameters in this invention;

[0060] Figure 5 This is a block diagram of the positive-sequence, negative-sequence, and zero-sequence virtual impedance sequencing control of the present invention;

[0061] Figure 6 This is a block diagram for correcting the reference value of the grid-type control voltage in this invention;

[0062] Figure 7 This is a schematic diagram of the virtual impedance stability margin constraint of the present invention;

[0063] Figure 8 This is a schematic diagram of the virtual impedance smooth exit curve during the fault occurrence and fault recovery process of the present invention. Detailed Implementation

[0064] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and should not be used to limit the scope of protection of the present invention.

[0065] Example 1: This example introduces an adaptive virtual impedance control method for a network-type flexible interconnection device based on online identification of weak grid strength, including the following steps:

[0066] S1. Real-time acquisition of operating electrical quantities of network-type flexible interconnected devices;

[0067] S2. Calculate the weak grid strength evaluation quantity based on the aforementioned operating electrical quantities;

[0068] S3. Identify the operating status of the grid-type flexible interconnection device based on the weak grid strength evaluation quantity;

[0069] S4. Dynamically generate adaptive virtual impedance parameters based on the operating status; the adaptive virtual impedance increases with the increase of the weak grid strength evaluation quantity to suppress power oscillation and grid connection point voltage drop, and improve the stability of the grid-type flexible interconnection device under weak grid conditions.

[0070] S5. Introduce the adaptive virtual impedance parameter into the reference value of the grid-type control voltage;

[0071] S6. Apply stability margin constraints and rate of change limits to the virtual impedance parameters;

[0072] S7. Generate a modulation signal based on the corrected voltage reference value to control the operation of the flexible interconnection device;

[0073] S8. When the fault is detected to be cleared or the weak grid strength evaluation value is restored to below the preset recovery threshold, the grid-type flexible interconnection device is determined to enter the fault recovery state. In the fault recovery state, the enhanced adaptive virtual impedance control is gradually withdrawn according to the preset recovery law, so that the virtual impedance is smoothly restored to the steady state value.

[0074] In this embodiment, the operating electrical quantities collected in step S1 include at least one of the following: grid connection point three-phase voltage, grid connection point three-phase current, grid connection point frequency, active power, reactive power, DC bus voltage, DC bus voltage change rate, and energy storage unit state of charge.

[0075] In this embodiment, in step S2, the controller calculates the equivalent grid impedance and equivalent short-circuit ratio based on the collected electrical quantities. Then, it integrates voltage drop index, frequency deviation index, power oscillation index, and DC bus deviation index to obtain the weak grid strength evaluation quantity. The weak grid strength evaluation quantity is calculated as follows:

[0076] ;

[0077] Among them, G w F is a quantity for evaluating the strength of weak power grids. SCR To weaken the equivalent short-circuit ratio index, F U For voltage sag indicators, F f F is the frequency deviation index. P F is a power oscillation index. dcThis represents the DC bus voltage deviation index, where a1, a2, a3, a4, and a5 are weighting coefficients. Where:

[0078] ;

[0079] SCR eq (k) represents the equivalent short-circuit ratio in the k-th sampling period; SCR th The threshold for determining a weak power grid; SCR min To preset the minimum short-circuit ratio; U N Rated voltage; S N Rated capacity of the network-type flexible interconnection device; Z eq (k) represents the equivalent grid impedance. F SCR The larger the value, the lower the equivalent short-circuit ratio, and the weaker the power grid.

[0080] ;

[0081] U pcc (k) represents the effective value of the grid connection point voltage, F U = 0 indicates that the voltage has not dropped, F U The larger the value, the more severe the voltage drop.

[0082] ;

[0083] f(k) is the real-time frequency at the grid connection point, f N For the rated frequency, Δf max F represents the maximum permissible frequency deviation. f The larger the value, the more significant the frequency deviation and the stronger the system disturbance.

[0084] , ;

[0085] ;

[0086] P(k) represents the active power at the current sampling time. P is the average active power within the sliding window. N This represents the active power baseline value corresponding to the rated active power or rated capacity; N is the number of sampling points in the sliding window. P The larger the value, the greater the active power oscillation amplitude.

[0087] ;

[0088] U dc (k) represents the DC bus voltage, U dc,ref The reference value for the DC bus voltage is ΔU. dc,max The maximum permissible DC bus voltage deviation. Fdc The larger the value, the more pronounced the energy imbalance at the DC bus.

[0089] In this embodiment, the equivalent grid impedance is calculated as follows:

[0090] ;

[0091] Among them, Z eq (k) represents the equivalent grid impedance corresponding to the kth sampling period, ΔU pcc (k) represents the voltage change at the grid connection point, ΔI pcc (k) represents the change in current at the grid connection point, and ε is a small positive number to prevent the denominator from being zero.

[0092] In this embodiment, the equivalent short-circuit ratio is calculated as follows:

[0093] ;

[0094] Among them, SCR eq (k) is the equivalent short-circuit ratio, U N For the rated voltage, S N Rated capacity for network-type flexible interconnection devices.

[0095] In this embodiment, in step S4, the controller dynamically generates virtual resistance, virtual reactance, and virtual impedance phase angle based on the weak grid strength evaluation and operating status. The virtual resistance is mainly used to enhance damping and suppress power oscillations, while the virtual reactance is mainly used to improve voltage support capability and limit fault current amplitude. The adaptive virtual impedance is:

[0096] ;

[0097] Among them, Z v For adaptive virtual impedance, R v X is a virtual resistance. v This is a virtual reactance.

[0098] In this embodiment, the virtual resistor is generated as follows:

[0099] ;

[0100] Among them, R v0 The basic virtual resistor, ΔI is the output current limit, and ΔP is the output current limit. osc K is the active power oscillation amplitude, and k1, k2, and k3 are adjustment coefficients.

[0101] In this embodiment, the virtual reactance is generated as follows:

[0102] ;

[0103] Among them, X v0 The basic virtual reactance is defined as follows: ΔU is the voltage drop at the grid connection point, ΔQ is the reactive power deviation, and m1, m2, and m3 are the adjustment coefficients.

[0104] In this embodiment, the virtual impedance phase angle is:

[0105] ;

[0106] In this embodiment, the virtual impedance phase angle is corrected based on the equivalent line impedance phase angle, voltage sag correction term, and stability margin correction term:

[0107] ;

[0108] Where, θ line The phase angle of the equivalent line impedance, Δθ u For voltage sag correction, Δθ s This is a stability margin correction term; Corresponding target phase angle, Corresponding to the actual phase angle, the actual control objective is to make and Consistent or nearly consistent.

[0109] In this embodiment, under asymmetrical fault or three-phase imbalance conditions, the grid connection point voltage and current are decomposed into positive-sequence, negative-sequence, and zero-sequence components, generating positive-sequence virtual impedance, negative-sequence virtual impedance, and zero-sequence virtual impedance respectively:

[0110] ; ; ;

[0111] in, For positive sequence virtual impedance, For negative sequence virtual impedance, For zero-sequence virtual impedance, For positive sequence virtual resistance, For negative sequence virtual resistance, For zero-sequence virtual resistance, For positive sequence virtual reactance, For negative sequence virtual reactance, This refers to zero-sequence virtual reactance. Positive-sequence virtual impedance is used to maintain positive-sequence voltage support, negative-sequence virtual impedance is used to suppress negative-sequence current and unbalanced power oscillations, and zero-sequence virtual impedance is used to suppress zero-sequence current in ground faults or four-wire systems. This invention introduces sequential control of positive-sequence, negative-sequence, and zero-sequence virtual impedances under asymmetrical fault conditions, which can improve voltage support capability and negative-sequence and zero-sequence current suppression capability under unbalanced fault conditions.

[0112] In this embodiment, the corrected grid-type voltage reference value is:

[0113] ;

[0114] Among them, E ref The original voltage reference value, I, is generated for network-type control. pcc For the grid connection point current, ΔE u For voltage sag compensation, ΔE d This is the power oscillation damping term.

[0115] In this embodiment, under asymmetrical fault conditions, after introducing adaptive virtual impedance, the controller generates a corrected voltage reference value based on the virtual impedance voltage drop, voltage sag compensation term, and power oscillation damping term:

[0116] ;

[0117] in, It is a positive sequence current. It is a negative sequence current. It is the zero-sequence current.

[0118] In this embodiment, the virtual impedance parameter is subject to the following constraints:

[0119] ; ; ; ; ;

[0120] Among them, Z v,min Z v,max These are the lower and upper limits of the virtual impedance amplitude, respectively; R v,min R v,max These are the lower and upper limits of the virtual resistance, respectively; X v,min X v,max These are the lower and upper limits of the virtual reactance, respectively; θ min θ max These are the lower and upper limits of the virtual impedance phase angle, respectively; ρ R ρ X These represent the upper limits of the rate of change for virtual resistance and virtual reactance, respectively. When the candidate parameters calculated by the controller exceed the stability margin region, they are projected and corrected to ensure that the parameters always remain within the stable feasible region. Therefore, the adaptive virtual impedance control proposed in this invention does not simply increase impedance, but rather performs synergistic optimization of parameters based on theoretical stability constraints, thereby balancing voltage support, fault current limiting, and system stability. This invention sets stability margin constraints and rate of change limits to avoid small-signal oscillations, negative damping, or secondary impacts after fault clearance caused by sudden changes in virtual impedance parameters or improper parameter settings.

[0121] In this embodiment, after the fault is cleared or the weak power grid strength is restored, the virtual impedance is smoothly deactivated in the following manner:

[0122] ;

[0123] Among them, Z v,steady For steady-state virtual impedance, t c λ represents the fault clearing time, and λ is the recovery coefficient.

[0124] Example 2, based on the same inventive concept as Example 1, introduces an adaptive virtual impedance control method for a network-type flexible interconnection device based on online identification of weak grid strength, including the following steps.

[0125] S1. Real-time acquisition of electrical quantities of network-type flexible interconnected devices:

[0126] like Figure 2 As shown, the grid-type flexible interconnection device includes at least two AC ports, at least one voltage source converter, a common DC bus, a DC bus capacitor, a filter, electrical quantity sensors, and a controller. Optionally, the grid-type flexible interconnection device further includes an energy storage unit and a bidirectional DC-DC converter.

[0127] The controller collects the three-phase voltage u at the grid connection point in real time. pcc,abc Three-phase current i at grid connection point pcc,abc Grid connection frequency f, active power P, reactive power Q, DC bus voltage U dc DC bus voltage change rate dU dc / dt, State of charge (SOC) of the energy storage unit.

[0128] S2. Calculate the weak grid strength evaluation quantity:

[0129] like Figure 3 As shown, the controller calculates the equivalent grid impedance and equivalent short-circuit ratio based on the collected electrical quantities. Then, it integrates voltage drop, frequency deviation, power oscillation, and DC bus deviation indices to obtain the weak grid strength evaluation quantity G. w .

[0130] ;

[0131] When G w The larger the value of G, the weaker the grid strength at the grid connection point and the more severe the disturbance; when G... w The smaller the value, the higher the grid strength and the more stable the operation at the grid connection point.

[0132] S3. Identify the operating status of network-type flexible interconnection devices:

[0133] The controller is based on the weak grid strength evaluation quantity Gw The current operating status of the grid-connected flexible interconnection device is identified by measuring the degree of voltage drop at the grid connection point, the degree of output current exceeding the limit, and the amplitude of power oscillation. The operating status includes normal grid connection status, weak grid operation status, voltage drop status, asymmetrical fault status, power oscillation status, and fault recovery status.

[0134] When a three-phase imbalance or asymmetrical fault is detected, the controller performs positive-sequence, negative-sequence, and zero-sequence decomposition on the grid connection point voltage and current to obtain... .

[0135] S4. Generate adaptive virtual impedance parameters:

[0136] like Figure 4 As shown, the controller dynamically generates virtual resistance, virtual reactance, and virtual impedance phase angle based on the weak grid strength evaluation quantity and operating status.

[0137] ;

[0138] ;

[0139] ;

[0140] Virtual resistance is mainly used to enhance damping and suppress power oscillations, while virtual reactance is mainly used to improve voltage support capability and limit fault current amplitude. In a preferred embodiment, the virtual impedance phase angle is corrected according to the equivalent line impedance phase angle, so that the virtual impedance parameters meet both the weak grid support requirements and the small-signal stability constraints.

[0141] S5. Perform sequence control of positive-sequence, negative-sequence, and zero-sequence virtual impedance:

[0142] like Figure 5 As shown, under asymmetrical fault or three-phase unbalanced conditions, the controller generates positive-sequence, negative-sequence, and zero-sequence virtual impedances, respectively.

[0143] ; ; ;

[0144] Among them, positive sequence virtual impedance is used to maintain positive sequence voltage support, negative sequence virtual impedance is used to suppress negative sequence current and unbalanced power oscillation, and zero sequence virtual impedance is used to suppress zero sequence current in ground faults or four-wire systems.

[0145] S6. Correct the reference value of the grid-type control voltage:

[0146] like Figure 6 As shown, the grid-type controller first generates the original voltage reference value E based on active-frequency control, reactive-voltage control, or virtual synchronous machine control. refAfter introducing adaptive virtual impedance, the controller generates a corrected voltage reference value based on the virtual impedance voltage drop, voltage sag compensation term, and power oscillation damping term. .

[0147] ;

[0148] Under asymmetrical fault conditions, the controller calculates the positive-sequence, negative-sequence, and zero-sequence virtual impedance voltage drops respectively, and synthesizes them into a voltage reference correction.

[0149] S7. Stability Margin Constraints and Modulation Signal Generation:

[0150] The virtual impedance parameter is constrained as follows:

[0151] ; ; ; ; ;

[0152] like Figure 7 As shown, Figure 7 This is a schematic diagram illustrating the stability margin constraint of the virtual impedance parameter in this invention. The horizontal axis in the diagram represents the virtual resistance R. v The vertical axis represents the virtual reactance X. v Based on the existence of the equilibrium point, small-signal stability, and large-signal transient stability analysis, the stability margin region of the virtual impedance parameter can be obtained. The shaded area represents the feasible region of the parameter, and the adaptive trajectory represents the adjustment path of the virtual impedance parameter during the process of changes in the strength of the weak power grid.

[0153] If the calculated virtual impedance parameter exceeds the stability margin region, the controller performs amplitude limiting, boundary projection, or order reduction processing on the virtual impedance parameter. The voltage reference value after stability margin constraint is input to the modulation module, which generates PWM, SVM, or nearest-level modulation signals to drive the power switching devices in the grid-type flexible interconnect device.

[0154] Therefore, the adaptive virtual impedance control proposed in this invention does not simply increase the impedance, but rather performs synergistic optimization of parameters based on theoretical stability constraints, thereby taking into account voltage support, fault current limiting, and system stability.

[0155] S8, Fault Recovery and Graceful Exit:

[0156] When the controller detects that the grid connection point voltage has recovered, the output current is below the threshold, the power oscillation amplitude has decreased, and the weak grid strength evaluation value is less than the preset recovery threshold, the controller enters the fault recovery state.

[0157] In the fault recovery state, the virtual impedance smoothly exits according to exponential decay, ramp decay, or piecewise decay. This smooth exit mechanism avoids voltage surges, current surges, and secondary power oscillations caused by sudden changes in virtual impedance after fault clearance.

[0158] To verify the effectiveness of the adaptive virtual impedance control method for a network-type flexible interconnection device based on online identification of weak grid strength according to an embodiment of the present invention, a case study analysis was conducted using relevant information and parameters from a real system, as shown in Table 1.

[0159] Table 1. Relevant electrical and control parameters of network-type flexible interconnection devices

[0160]

[0161] Where: SCR is the equivalent short-circuit ratio, R g / X g The line impedance ratio, t f t is the time when the fault occurs. c t is the fault clearing time. r To restore the completion time, 𝜆 is the smooth exit coefficient.

[0162] After the fault occurs, such as Figure 8 As shown, the grid connection point voltage drops from 1.0 pu to approximately 0.48 pu, while the output current rises to approximately 1.45 pu. The controller detects the weak grid strength assessment quantity G. w After increasing, the virtual impedance amplitude |Z is rapidly increased. v This increases the virtual impedance from 0.18 pu to approximately 0.80 pu. With the increased virtual impedance, the fault current is limited to within acceptable limits, and the grid connection voltage is somewhat supported.

[0163] After the fault is cleared, the controller does not immediately drop the virtual impedance to its initial value, but gradually exits according to an exponential law:

[0164] ;

[0165] After the fault was cleared, the virtual impedance smoothly exited according to an exponential law, and both voltage and current returned to approximately t. r The system recovers to steady state around 4.20 s. Simulation results show that the method of this invention can achieve voltage support and current limiting control during faults, and can avoid secondary impacts and recover oscillations after a fault.

[0166] Example 3, based on the same inventive concept as other examples, introduces an adaptive virtual impedance control device for a network-type flexible interconnection device based on online identification of weak grid strength, comprising:

[0167] The data acquisition module is used to collect the voltage, current, frequency, active power, reactive power, and DC bus voltage at the grid connection point of the grid-connected flexible interconnection device.

[0168] The calculation module is used to calculate the weak grid strength evaluation quantity at the grid connection point based on the voltage, current, frequency, active power, reactive power and DC bus voltage.

[0169] The determination module is used to identify the operating status of the grid-type flexible interconnection device based on the weak grid strength evaluation quantity. The operating status includes normal grid connection status, weak grid operation status, voltage drop status, power oscillation status, and fault recovery status.

[0170] The generation module is used to dynamically generate virtual resistance, virtual reactance, and virtual impedance phase angle based on the weak grid strength evaluation quantity and the operating status; and to generate adaptive virtual impedance based on the virtual resistance, virtual reactance, and virtual impedance phase angle.

[0171] The correction module is used to introduce the adaptive virtual impedance into the network-type control voltage reference value to obtain the corrected voltage reference value;

[0172] A modulation module is used to generate a modulation signal based on the corrected voltage reference value to control the operation of the network-type flexible interconnect device;

[0173] The exit module is used to gradually exit the adaptive virtual impedance control process according to a preset recovery rule after detecting that the network-type flexible interconnect device has entered the fault recovery state.

[0174] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0175] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0176] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0177] These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable apparatus for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0178] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. An adaptive virtual impedance control method for network-type flexible interconnection devices based on online identification of weak grid strength, characterized in that, Includes the following steps: Collect voltage, current, frequency, active power, reactive power, and DC bus voltage at the grid connection point of the flexible interconnection device. Based on the voltage, current, frequency, active power, reactive power, and DC bus voltage, calculate the weak grid strength evaluation quantity at the grid connection point. The operating status of the grid-type flexible interconnection device is identified based on the weak grid strength evaluation quantity. The operating status includes normal grid connection status, weak grid operation status, voltage drop status, power oscillation status, and fault recovery status. Based on the weak grid strength evaluation quantity and the operating status, virtual resistance, virtual reactance and virtual impedance phase angle are dynamically generated; An adaptive virtual impedance is generated based on the virtual resistance, virtual reactance, and virtual impedance phase angle. The adaptive virtual impedance is introduced into the grid-type control voltage reference value to obtain the corrected voltage reference value; A modulation signal is generated based on the corrected voltage reference value to control the operation of the network-type flexible interconnect device; After detecting that the network-type flexible interconnect device has entered the fault recovery state, the adaptive virtual impedance control process is gradually exited according to the preset recovery law.

2. The adaptive virtual impedance control method for network-type flexible interconnection devices based on online identification of weak grid strength as described in claim 1, characterized in that, The weak grid strength evaluation quantity is calculated based on at least two of the following: equivalent short-circuit ratio, voltage drop index, frequency deviation index, power oscillation index, and DC bus voltage deviation index.

3. The adaptive virtual impedance control method for network-type flexible interconnection devices based on online identification of weak grid strength as described in claim 1, characterized in that, The formula for calculating the adaptive virtual impedance is as follows: ； In the formula, For adaptive virtual impedance, For virtual resistance, For virtual reactance, j is the imaginary unit; ； In the formula, Based on virtual resistance, For evaluating the strength of weak power grids, To exceed the output current limit, K is the active power oscillation amplitude, and k1, k2, and k3 are virtual resistance adjustment coefficients. ； In the formula, Based on virtual reactance, The degree of voltage drop at the grid connection point, The reactive power deviation is represented by m1, m2, and m3, which are virtual reactance adjustment coefficients. The phase angle of the adaptive virtual impedance is: ； In the formula, The phase angle for adaptive virtual impedance, The phase angle is the equivalent line impedance. This is a voltage sag correction term. This is a stability margin correction term.

4. The adaptive virtual impedance control method for network-type flexible interconnection devices based on online identification of weak grid strength according to claim 3, characterized in that, The formula for the corrected voltage reference value is: ； In the formula, This is the corrected voltage reference value. The original voltage reference value generated for network-type control. For grid connection point current, This is a voltage sag compensation item. This is the power oscillation damping term.

5. The adaptive virtual impedance control method for network-type flexible interconnection devices based on online identification of weak grid strength according to claim 3, characterized in that, Under asymmetrical fault conditions, the formula for the corrected voltage reference value is: ； In the formula, This is the corrected voltage reference value. It is a positive sequence current. It is a negative sequence current. It is the zero-sequence current. This is a voltage sag compensation item. For power oscillation damping term, For positive sequence virtual impedance, For negative sequence virtual impedance, This is the zero-sequence virtual impedance; ； ； ； In the formula, For positive sequence virtual resistance, For negative sequence virtual resistance, For zero-sequence virtual resistance, For positive sequence virtual reactance, For negative sequence virtual reactance, This is the zero-sequence virtual reactance.

6. The adaptive virtual impedance control method for network-type flexible interconnection devices based on online identification of weak grid strength according to claim 5, characterized in that, The virtual resistance of the negative-sequence virtual impedance is not less than the virtual resistance of the positive-sequence virtual impedance, and the virtual resistance of the zero-sequence virtual impedance is not less than the virtual resistance of the positive-sequence virtual impedance.

7. The adaptive virtual impedance control method for network-type flexible interconnection devices based on online identification of weak grid strength according to claim 3, characterized in that, The following constraints are applied to the candidate parameters in the process of calculating the adaptive virtual impedance: ； ； ； ； ； In the formula, Z v,min Z v,max These are the lower and upper limits of the virtual impedance amplitude, respectively; R v,min R v,max These are the lower and upper limits of the virtual resistance, respectively; X v,min X v,max These are the lower and upper limits of the virtual reactance, respectively. θ min θ max These are the lower and upper limits of the virtual impedance phase angle, respectively. ρ R ρ X denoted as the upper limit of the rate of change of virtual resistance and virtual reactance, respectively, and t is a time variable.

8. The adaptive virtual impedance control method for network-type flexible interconnection devices based on online identification of weak grid strength according to claim 7, characterized in that, When a candidate parameter exceeds the stability margin region, it is subjected to amplitude limiting, boundary projection, or order reduction correction to ensure that the candidate parameter always remains within the stable feasible region.

9. The adaptive virtual impedance control method for network-type flexible interconnection devices based on online identification of weak grid strength according to claim 1, characterized in that, The preset recovery law is that the virtual impedance exits smoothly according to exponential decay, ramp decay, or segmented decay.

10. An adaptive virtual impedance control device for a network-type flexible interconnection device based on online identification of weak grid strength, characterized in that, include: The data acquisition module is used to collect the voltage, current, frequency, active power, reactive power, and DC bus voltage at the grid connection point of the grid-connected flexible interconnection device. The calculation module is used to calculate the weak grid strength evaluation quantity at the grid connection point based on the voltage, current, frequency, active power, reactive power and DC bus voltage. The determination module is used to identify the operating status of the grid-type flexible interconnection device based on the weak grid strength evaluation quantity. The operating status includes normal grid connection status, weak grid operation status, voltage drop status, power oscillation status, and fault recovery status. The generation module is used to dynamically generate virtual resistance, virtual reactance, and virtual impedance phase angle based on the weak grid strength evaluation quantity and the operating status; and to generate adaptive virtual impedance based on the virtual resistance, virtual reactance, and virtual impedance phase angle. The correction module is used to introduce the adaptive virtual impedance into the network-type control voltage reference value to obtain the corrected voltage reference value; A modulation module is used to generate a modulation signal based on the corrected voltage reference value to control the operation of the network-type flexible interconnect device; The exit module is used to gradually exit the adaptive virtual impedance control process according to a preset recovery rule after detecting that the network-type flexible interconnect device has entered the fault recovery state.

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