A current-limiting control method of a grid-forming converter, a grid-forming SVG and a medium

Abstract

The application relates to the technical field of networked converter control, and discloses a current-limiting control method for a networked converter, a networked SVG and a medium, which comprises the following steps: obtaining a current-related quantity of the networked converter; when the current-related quantity exceeds a preset threshold, generating a virtual impedance based on the current-related quantity; combining the virtual impedance with a cubic quantity of a measured current to form a virtual voltage drop; and correcting a voltage reference instruction of the networked converter by using the virtual voltage drop to inhibit the output current of the networked converter. The cubic virtual impedance feedback adopted by the application has a cubic surge in the voltage drop change rate with the increase of the current. At a critical moment in the initial stage of overcurrent, the nonlinear characteristic can amplify the voltage drop compensation amount, so that the voltage reference value is rapidly reduced, thereby greatly shortening the current-limiting response time and limiting the maximum peak value of the transient current.

Description

Technical Field

[0001] This invention relates to the field of grid-type converter control technology, specifically to a current limiting control method for a grid-type converter, a grid-type SVG, and a dielectric. Background Technology

[0002] Driven by the "dual carbon" goals, my country is accelerating the construction of a new power system dominated by new energy sources such as photovoltaics and wind power. However, with the continuous increase in the installed capacity of new energy sources and the rapid development of distributed generation, the power system is exhibiting characteristics such as low inertia and weak damping, making system stability issues increasingly prominent. In large-scale new energy power plants, static var generators (SVG) with high dynamic regulation capabilities are typically configured to maintain voltage stability at the grid connection point.

[0003] Traditional grid-connected SVGs typically rely on external grid voltage for synchronization, providing voltage support only under quasi-steady-state conditions, which is insufficient to meet the requirements of wide-bandwidth dynamic voltage support. In contrast, grid-connected SVGs can actively establish voltage and frequency, exhibiting external characteristics similar to synchronous generators, and have advantages such as fast response speed and strong support capability, thus attracting widespread attention in recent years.

[0004] However, when the power grid experiences significant disturbances such as voltage dips or short circuits, the grid connection point voltage drops sharply. Because the grid-connected SVG maintains its voltage source characteristics, a large instantaneous voltage difference will arise between its internal controlled potential and the grid connection point voltage. Due to the voltage support inertia of the grid-connected control, this voltage difference directly acts on the loop impedance, thereby triggering a severe transient overcurrent surge. If not effectively limited, this will lead to transient synchronous instability of the system and may even cause overheating and damage to the power electronic switching devices inside the SVG.

[0005] Existing inverter current limiting strategies mainly include two types: one is to directly limit the current reference value through a limiter, and the other is to introduce a linear virtual impedance so that the voltage command decreases linearly with the current. However, direct limiting weakens the voltage control loop's ability to regulate the system, easily leading to frequency synchronization mismatch and transient instability between the new energy unit and the SVG; while the traditional linear virtual impedance strategy has limited response speed in the early stage of a fault, making it difficult to form a sufficient equivalent voltage drop in time, resulting in a still high peak transient inrush current.

[0006] Therefore, there is an urgent need to design a control strategy that can maintain network stability while achieving fast and effective flow limiting. Summary of the Invention

[0007] This invention provides a current limiting control method for a grid-type converter, a grid-type SVG, and a dielectric. By changing the current in the traditional virtual impedance control to a nonlinear cubic current, the reference voltage drop speed during transient overcurrent is accelerated without increasing any hardware cost, thereby achieving faster and more efficient current suppression and ensuring the safe and stable operation of the grid-type SVG.

[0008] This invention is achieved through the following technical solution:

[0009] A current limiting control method for a grid-type converter includes:

[0010] Obtain the current-related quantities of the grid-type converter;

[0011] When the current correlation quantity exceeds a preset threshold, a virtual impedance is generated based on the current correlation quantity;

[0012] A virtual voltage drop is formed by combining the virtual impedance with the cube of the measured current.

[0013] The voltage reference command of the grid converter is corrected using the virtual voltage drop to suppress the output current of the grid converter.

[0014] As an optimization, the current-related quantity is the d-axis current reference command value output by the outer voltage loop. and q-axis current reference command value Composite vector magnitude .

[0015] As an optimization, the generation of virtual impedance specifically includes:

[0016] If the magnitude of the synthesized vector Less than the preset overcurrent threshold The virtual resistance is 0;

[0017] If the magnitude of the synthesized vector Greater than the overcurrent threshold According to the magnitude of the synthesized vector With the overcurrent threshold The difference is used to generate a virtual resistance through a linear multiple, and then a virtual reactance is generated according to a preset impedance ratio coefficient.

[0018] As an optimization, the expression for the virtual resistance is: ;

[0019] The expression for the virtual reactance is: ;

[0020] In the formula, For the generated virtual resistance, This is the virtual impedance current limiting ratio coefficient. This is the impedance ratio coefficient.

[0021] As an optimization, the cube of the measured current is: and ,in, , Obtained by transformation of the measured grid-connected current dq;

[0022] The virtual pressure drop includes the d-axis virtual pressure drop. and q-axis virtual voltage drop The calculation formula is:

[0023] ;

[0024] .

[0025] As an optimization, the revised voltage reference command and The expression is:

[0026] ;

[0027] In the formula, , These are the initial voltage reference commands for the d-axis and q-axis, respectively.

[0028] As an optimization, the grid-type converter adopts DC capacitor self-synchronization control, which controls the DC voltage by adjusting the internal frequency of the converter.

[0029] As an optimization, when the current-related quantity falls below the preset threshold, the virtual impedance automatically returns to zero, and the grid-type converter returns to normal operation.

[0030] The present invention also discloses a grid-type SVG, which employs the aforementioned current limiting control method for a grid-type converter.

[0031] The present invention also discloses a computer-readable storage medium storing a computer program thereon, which, when executed by a processor, implements the aforementioned current limiting control method for a grid-type converter.

[0032] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0033] This invention proposes a current limiting control method for grid-type SVG based on the cubic impedance of current, which can effectively solve the problem of overcurrent instability of traditional grid-type SVG under large transient disturbances.

[0034] During normal system operation, the virtual impedance branch is zero and does not interfere with control. The network-type SVG maintains strong voltage and frequency support through its DC capacitor self-synchronization architecture. When a large disturbance (overcurrent) occurs in the system, the cubic virtual impedance acts as a current limiter. Compared to traditional linear virtual impedance, the cubic virtual impedance feedback used in this invention exhibits a cubic surge in voltage drop rate with increasing current. In the critical initial stage of overcurrent, this nonlinear characteristic amplifies the voltage drop compensation, causing the voltage reference value to decrease rapidly, thereby greatly shortening the current limiting response time and limiting the maximum peak value of the transient current.

[0035] The algorithm of this invention is simple to implement and does not require modification of the main circuit topology. It effectively improves the current limiting effect and transient response characteristics by only making nonlinear innovations in the control logic, that is, while ensuring that the network self-synchronization mechanism does not collapse. It has extremely high engineering application and economic value. Attached Figure Description

[0036] The accompanying drawings, which are included to provide a further understanding of embodiments of the invention and form part of this application, do not constitute a limitation thereof. In the drawings:

[0037] Figure 1 This is a flowchart of the control method of the present invention.

[0038] Figure 2 This is a block diagram of the main circuit and overall control structure of a grid-type SVG based on DC capacitor self-synchronization.

[0039] Figure 3 This is a block diagram illustrating the specific implementation principle of conventional virtual impedance current limiting control.

[0040] Figure 4 This is a block diagram illustrating the specific implementation principle of the cubic virtual impedance current limiting control of the present invention.

[0041] Figure 5 The waveform diagram is a simulation of a fault in a network-type SVG without a current limiting strategy.

[0042] Figure 6 The waveform diagram is a simulation diagram of a fault in a network-type SVG using traditional virtual impedance current limiting control.

[0043] Figure 7 The above is a simulation waveform diagram of a fault in a network-type SVG using the cubic virtual impedance current limiting control of the present invention. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings. The illustrative embodiments and descriptions of the present invention are only used to explain the present invention and are not intended to limit the present invention.

[0045] This embodiment 1 provides a current limiting control method for a grid-type converter, such as... Figure 1 As shown. The core of this method lies in replacing the traditional linear current with the measured cubic current during the fault transient process. This cubic current, combined with the virtual impedance, forms a nonlinear virtual voltage drop, thereby rapidly lowering the voltage reference command in the initial stage of overcurrent and achieving fast current limiting. The entire control process requires no mode switching and automatically recovers after the fault, balancing the dynamic support capability of the network-type SVG with the requirements for transient safe operation.

[0046] The specific implementation of the present invention will be described in detail below in conjunction with the three stages of steady state, fault, and recovery.

[0047] (1) Steady-state operation stage

[0048] When the power grid is in a steady state or under slight disturbances, and the system is operating stably, the grid-type SVG aims to maintain the voltage and frequency support at the PCC point. At this time, the virtual current limiting mechanism is in standby mode (i.e., the virtual impedance is zero and does not interfere with normal control), and the SVG uses DC capacitor self-synchronization control to achieve grid construction.

[0049] Figure 2 The diagram shows the circuit topology and control structure of a meshed SVG. This meshing strategy achieves effective control of the DC voltage by adjusting the internal frequency of the SVG. The control structure is as follows:

[0050] ;

[0051] In the formula: It is the internal frequency of the SVG. It is a frequency reference value. This is the DC voltage reference value. It is the voltage of the DC bus capacitor. For the inertia simulation coefficient, It is the damping coefficient. It is the DC voltage tracking coefficient.

[0052] As can be seen from this control structure, the grid-connected SVG actively constructs voltage and frequency references by simulating the inertia and damping characteristics of a synchronous generator, thereby providing rapid reactive power support during grid disturbances. However, it is precisely this "voltage source" characteristic that causes a huge voltage difference between its internal potential and the grid voltage when the grid connection point voltage drops sharply, leading to overcurrent risk. This is the contradiction that this invention aims to solve: how to achieve effective transient current limiting while maintaining grid-connected characteristics.

[0053] During the transient process of SVG grid connection, its active power comes from the energy stored in the DC-side capacitor. If the power loss of the capacitor itself is ignored, the active power released by the DC-side capacitor during the transient period is equal to the active power output by the SVG. This dynamic relationship can be expressed as:

[0054] ;

[0055] in, It refers to the size of the DC capacitor; It is the power output of the DC capacitor; This refers to the power injected into the power source to the left of the DC capacitor. However, in an SVG, no other power source is typically installed to the left of the DC capacitor, therefore... =0.

[0056] Combining the above formula, we can obtain:

[0057] ;

[0058] Where: equivalent inertia coefficient Equivalent damping coefficient DC voltage coefficient This formula reflects the active power and frequency characteristics under SVG self-synchronization control, and can be divided into grid-connected synchronization term and DC voltage tracking term. Since DC voltage tracking dynamics typically have a fast time response, this tracking term can be ignored, and the formula simplifies to:

[0059] ;

[0060] Grid-type SVG employing self-synchronization control exhibits active-frequency response characteristics similar to synchronous generators, and therefore may also experience similar transient instability phenomena under large disturbances. Furthermore, due to the complex control methods of power electronic equipment, its transient behavior becomes even more complex. Therefore, additional current-limiting mechanisms are required to ensure equipment safety.

[0061] (2) Fault current limiting stage

[0062] When the mains voltage experiences a severe drop or short-circuit fault, the reference current value output by the outer voltage loop rises sharply. Define the composite vector magnitude: .

[0063] when Greater than or equal to the set security threshold At this time, the converter faces the risk of overcurrent damage. In this situation, the control method of this invention can achieve current limiting through nonlinear virtual impedance current limiting control. It should be noted that the safety threshold... The selection of current limit should take into account the current withstand capability of the power device (usually 1.2 to 1.5 times the rated current) and the transient stability margin of the system. It should not be set too low, which may lead to frequent false triggering, nor set too high, which may lead to current limiting failure.

[0064] Figure 3 This is a block diagram illustrating the specific implementation principle of conventional virtual impedance current limiting control.

[0065] Virtual impedance only takes effect when the current exceeds the safety threshold, so it will not affect normal operation. It is obtained from the following formula:

[0066] ;

[0067] ;

[0068] In the formula, For the generated virtual resistance, This is the virtual impedance current limiting ratio coefficient. The set safety threshold. For the generated virtual reactance, This is the current-limiting virtual impedance ratio. The scaling factor is... The selection of the value determines the sensitivity of the current limiting: an excessively large value may cause excessive voltage drop, affecting reactive power support capability; an excessively small value will result in insufficient current limiting effect. It is generally recommended to set it between 0.05 and 0.2.

[0069] The voltage drop model of conventional virtual impedance is expressed as follows:

[0070] ;

[0071] In the formula, , It is obtained by transforming the measured grid-connected current dq.

[0072] As can be seen from this linear model, the voltage drop generated by the conventional virtual impedance is linearly related to the measured current. In the initial stage of overcurrent, when the current just exceeds the threshold, the voltage drop is small and has a limited effect on pulling down the voltage reference command. Therefore, the current limiting response has an inherent delay and is prone to the first current spike.

[0073] Figure 4 This is an equivalent block diagram of the cubic virtual impedance control module of the present invention. The method for obtaining the virtual impedance is the same as described above. and (Calculation method). Unlike conventional practices, this invention proposes using... and This is to accelerate the enhancement of the virtual voltage drop during faults. The voltage drop model of the cubic virtual impedance is expressed as:

[0074] ;

[0075] From a mathematical perspective, the equivalent impedance of a traditional virtual impedance is a constant, while the equivalent virtual impedance of this invention is a non-linear value that varies with the current. Specifically, when the measured current... When it increases, The voltage increases at a cubic rate, so the rate of change of the virtual voltage drop is much greater than in the linear case. For example, when the current increases from 1.0 pu to 2.0 pu: the linear voltage drop increases by a factor of 1, while the cubic voltage drop increases by a factor of 8. This non-linear high-gain characteristic rapidly reduces the voltage reference command within the first millisecond of a fault occurrence, greatly limiting further current increases.

[0076] At this point, the updated SVG voltage reference command (i.e., the value fed into the inner voltage loop) becomes:

[0077] ;

[0078] It is particularly noteworthy that this invention eliminates the need for mode switching between conventional control and current limiting control. When the current exceeds the threshold, the cubic term automatically generates a voltage drop; when the current falls back below the threshold, the cubic term automatically returns to zero. This "spontaneous" current limiting mechanism avoids the transient impact and control delay caused by mode switching, which is one of the key advantages of this invention.

[0079] (3) Fault clearance and recovery phase

[0080] If the grid fault is cleared, the system voltage begins to recover, and the grid-connected current gradually decreases. Once the current is detected to have safely decreased and stabilized within the threshold, the virtual impedance automatically returns to zero, thus smoothly exiting the current-limiting state and maintaining the grid-type voltage support state. This recovery process also requires no manual intervention or mode switching; the system can automatically return to normal operation, demonstrating the excellent control autonomy of this invention.

[0081] Based on the above analysis, the specific steps of the network-based SVG current limiting control method based on the cubic current virtual impedance of the present invention are as follows:

[0082] Step 1: Obtain the current-related quantities of the grid-type converter.

[0083] Real-time acquisition of the d-axis current reference command value of the voltage outer loop output and q-axis current reference command value Calculate the magnitude of the composite vector:

[0084] ;

[0085] Will With preset overcurrent threshold In comparison, if Virtual resistance ;like ,but: ;

[0086] Then according to the preset impedance ratio coefficient Calculate virtual reactance:

[0087] , This is the virtual impedance current limiting ratio coefficient.

[0088] Step 2: When the current-related quantity exceeds a preset threshold, a virtual impedance is generated based on the current-related quantity; the virtual impedance is combined with the cube of the measured current to form a virtual voltage drop.

[0089] Collect measured grid-connected current , Construct cubic quantities and Calculate the virtual voltage drop:

[0090] ;

[0091] .

[0092] Step 3: Use the virtual voltage drop to correct the voltage reference command of the grid converter in order to suppress the output current of the grid converter.

[0093] Revised voltage reference command and The expression is:

[0094] ;

[0095] In the formula, , These are the initial voltage reference commands for the d-axis and q-axis, respectively.

[0096] The revised voltage reference command is sent into the voltage inner loop.

[0097] By utilizing the aforementioned cubic nonlinear voltage drop, the output voltage amplitude is forcibly reduced during overcurrent, enabling rapid current limiting without mode switching. After fault clearance, when Falling back to The following times, The virtual impedance automatically returns to zero, and the system resumes reactive power support.

[0098] The effectiveness of the proposed current limiting control method based on cubic current virtual impedance is verified through specific examples below.

[0099] A simulation model of a grid-connected system based on a DC capacitor self-synchronizing SVG was built in the MATLAB / Simulink environment. The main parameters of the model are shown in Table 1.

[0100] At t = 0.5s, a symmetrical three-phase short-circuit ground fault was set on the low-voltage side of the transformer. The fault duration was set to 0.5s (i.e., the fault was cleared and restored at t = 1s) to simulate the severe transient impact conditions of the power grid. To comprehensively evaluate the performance of the strategy proposed in this invention, the following three comparative schemes were designed for simulation study: no current limiting measures (baseline group), conventional linear virtual impedance current limiting control (control group), and the cubic virtual impedance current limiting control proposed in this invention (experimental group).

[0101] Table 1 Main parameters of the network-type SVG system

[0102]

[0103] Figure 5 The image shows the simulation waveform of a grid-connected SVG fault without current limiting. Because no current limiting strategy was implemented, the grid current increased rapidly after the fault occurred, reaching a peak current of 9 pu, and exhibited significant oscillations, taking a considerable amount of time to stabilize.

[0104] Figure 6 The simulation results are for a traditional linear virtual impedance current limiting control. Figure 6 As can be seen, under the same fault conditions, at the instant of voltage drop, the peak current of the system using ordinary virtual impedance reached 3.7 pu, and the current limiting response time was approximately 0.04 s, roughly two cycles. In steady state with current limiting control in effect, the current amplitude was approximately 2 pu.

[0105] Figure 7 The simulation results are for the cubic virtual impedance current limiting control of this invention. Figure 7 As can be seen, under the same fault conditions, the proposed cubic virtual impedance current limiting strategy results in a system transient current peak of 2.4 pu, which is 35% lower than that of the traditional control. Furthermore, the new method exhibits a shorter duration of the current transient peak and a faster response, approximately 0.01 s. This indicates that the cubic characteristic can generate faster and larger voltage drop compensation in the initial stage of overcurrent. Simultaneously, in the steady state where the current limiting control is effective, the current amplitude is approximately 1.7 pu.

[0106] It is evident that the proposed control method can fully leverage the control advantages of nonlinear functions, achieving faster and more effective current limiting protection than conventional linear virtual impedance, thus greatly enhancing the safety and stability of grid-connected SVG systems.

[0107] Example 2 discloses a grid-connected SVG, which employs the current-limiting control method for the grid-connected converter described in Example 1. Specifically, the grid-connected SVG includes: a main circuit (three-phase full-bridge inverter, LC filter, DC bus capacitor), a sampling circuit (voltage and current sampling), a controller (DSP or FPGA), and a drive circuit. The controller internally contains a computer program that implements the method described in Example 1, which collects grid-connected voltage and current signals in real time, calculates virtual impedance and cubic virtual voltage drop, corrects voltage reference commands, and generates PWM drive signals.

[0108] Example 3 discloses a computer-readable storage medium storing a computer program that, when executed by a processor, implements the current limiting control method for the grid-type converter described in Example 1. The storage medium can be any form of non-volatile storage medium such as ROM, RAM, hard disk, optical disk, or USB flash drive, and is suitable for operation on digital signal processors (DSPs), field-programmable gate arrays (FPGAs), microcontrollers (MCUs), or general-purpose processors.

[0109] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A current limiting control method for a grid-type converter, characterized in that, include: Obtain the current-related quantities of the grid-type converter; When the current correlation quantity exceeds a preset threshold, a virtual impedance is generated based on the current correlation quantity; A virtual voltage drop is formed by combining the virtual impedance with the cube of the measured current. The voltage reference command of the grid converter is corrected using the virtual voltage drop to suppress the output current of the grid converter.

2. The current limiting control method for a grid-type converter according to claim 1, characterized in that, The current-related quantity is the d-axis current reference command value output by the outer voltage loop. and q-axis current reference command value Composite vector magnitude .

3. The current limiting control method for a grid-type converter according to claim 2, characterized in that, The generation of virtual impedance specifically includes: If the magnitude of the synthesized vector Less than the preset overcurrent threshold The virtual resistance is 0; If the magnitude of the synthesized vector Greater than the overcurrent threshold According to the magnitude of the synthesized vector With the overcurrent threshold The difference is used to generate a virtual resistance through a linear multiple, and then a virtual reactance is generated according to a preset impedance ratio coefficient.

4. The current limiting control method for a grid-type converter according to claim 3, characterized in that, The expression for the virtual resistor is: ; The expression for the virtual reactance is: ; In the formula, For the generated virtual resistance, This is the virtual impedance current limiting ratio coefficient. This is the impedance ratio coefficient.

5. The current limiting control method for a grid-type converter according to claim 4, characterized in that, The cube of the measured current is and ,in, , Obtained by transformation of the measured grid-connected current dq; The virtual pressure drop includes the d-axis virtual pressure drop. and q-axis virtual voltage drop The calculation formula is: ； 。 6. The current limiting control method for a grid-type converter according to claim 5, characterized in that, The corrected voltage reference command and The expression is: ； In the formula, , These are the initial voltage reference commands for the d-axis and q-axis, respectively.

7. The current limiting control method for a grid-type converter according to claim 1, characterized in that, The grid-type converter adopts DC capacitor self-synchronization control, and the DC voltage is controlled by adjusting the internal frequency of the converter.

8. The current limiting control method for a grid-type converter according to claim 1, characterized in that, When the current-related quantity falls below the preset threshold, the virtual impedance automatically returns to zero, and the grid-type converter returns to normal operation.

9. A mesh-type SVG, characterized in that, A current limiting control method for a grid-type converter according to any one of claims 1 to 8.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When executed by the processor, the program implements a current limiting control method for a grid-type converter as described in any one of claims 1 to 8.

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