A current-limiting control method and device for a network-forming converter and a medium

Abstract

The application discloses a current-limiting control method and device of a grid-forming converter and a medium, relates to the field of power grids, and solves the problem of unstable delay of current-limiting control based on current. The current-limiting control method is suitable for a multi-machine system connected in parallel to a power grid in a distributed manner. The voltage parameter of a power grid common coupling point is taken as a core to determine the fault degree, and a self-adaptive virtual impedance current-limiting circuit based on voltage is built in combination with the current parameter of the power grid side, so that the virtual impedance parameter is dynamically adjusted and a voltage deviation term is generated, and automatic switching between normal and fault conditions is realized. Each grid-forming converter is taken as an independent intelligent agent, information interaction between the intelligent agents and a distributed consistency algorithm are used, the voltage deviation term is converted into a voltage and frequency compensation term, the compensation term is embedded into a bottom virtual synchronous machine power control loop, the output voltage amplitude and angular frequency are corrected and converted into a three-phase reference voltage, a PWM wave is generated through PI control to drive the converter, and the adjustment scheme is more stable.

Description

Technical Field

[0001] This application relates to the field of power grids, and in particular to a current limiting control method, device and medium for a grid-type converter. Background Technology

[0002] The rapid development of distributed renewable energy has led to a gradual increase in the proportion of power electronic converters in new power systems. Grid-type converters, as renewable energy interface devices, can provide reliable voltage and frequency support to the system, effectively addressing the problem of weakened inertia in traditional power grids caused by increased renewable energy penetration. However, grid-type converters have weak short-circuit current tolerance, therefore current-limiting measures need to be implemented during grid faults to ensure safe and stable operation.

[0003] Current current limiting methods include those based on additional hardware and those based on software control. Hardware-based methods increase hardware costs and impact economic efficiency. While mode switching can effectively limit current in software-based methods, it can lead to the loss of voltage source characteristics in grid-connected inverters during faults and introduce stability risks in weak grids. Current saturation methods are prone to power loop saturation, causing oscillations and instability. In virtual impedance methods, fixed virtual impedances are difficult to adapt to changing fault conditions, and phase jumps may trigger overcurrent again during fault recovery. Current-adaptive virtual impedances, due to dynamic interaction with the fault current, experience impedance fluctuations, reducing transient response speed and current limiting reliability, and still suffer from control delay issues.

[0004] Therefore, how to provide a more stable current limiting control method is a technical problem that urgently needs to be solved by those in this field. Summary of the Invention

[0005] The purpose of this application is to provide a current limiting control method, device and medium for grid-type converters to solve the problem of instability in current limiting control based on current.

[0006] To address the aforementioned technical problems, this application provides a current-limiting control method for a grid-connected converter. This method pre-establishes a hierarchical control architecture for a multi-machine system of a distributed, parallel-connected grid-connected converter, including bottom-level control and secondary control. Specifically, virtual impedance is adjusted according to the degree of grid fault to limit fault current. Secondary control coordinates the voltage, frequency, and power distribution of the multi-machine system. The coordination results of the secondary control are embedded into the bottom-level virtual synchronous machine power control loop to drive the grid-connected converter. The method includes: Collect voltage parameters at the common coupling point of the power grid and current parameters on the grid side, and determine the degree of power grid fault based on the voltage parameters; An adaptive virtual impedance current limiting circuit based on voltage is established. The virtual impedance parameters of the current limiting circuit are adjusted according to the determined degree of grid fault, and a voltage deviation term is generated by combining the collected grid-side current parameters. The voltage deviation term is input to the secondary control layer to construct a distributed secondary control strategy based on intelligent agents. Each grid-type converter is used as an intelligent agent. Through information interaction between intelligent agents and a distributed consensus algorithm, voltage compensation term and frequency compensation term adapted to multi-machine system are obtained. The voltage compensation term and frequency compensation term are embedded into the virtual synchronous machine power control loop at the bottom layer of each grid-type converter to correct the output voltage amplitude and angular frequency parameters of the virtual synchronous machine controller and generate a three-phase reference voltage. The three-phase reference voltages are sequentially conditioned by a PI-controlled voltage loop and a current loop to generate a PWM wave, which drives the grid-type converter to operate.

[0007] Optionally, in the above-mentioned current limiting control method for grid-connected converters, collecting voltage parameters at the grid common coupling point and grid-side current parameters, and determining the degree of grid fault based on the voltage parameters, includes: Extract the actual voltage at the common coupling point With the rated voltage of the power grid U n ; judge Is it valid? If so, then a power grid fault is determined. If not, the power grid is considered to be in normal operating condition; Correspondingly, the virtual impedance parameters of the current-limiting circuit are adjusted according to the determined degree of power grid fault, including: If a fault is determined to have occurred in the power grid, the virtual resistance and virtual reactance are determined based on the first formula and the second formula, respectively; if the power grid is determined to be in normal operating condition, the virtual resistance is set to 0, and the virtual reactance is set to 0 synchronously with the virtual resistance. The first formula is: ; The second formula is: ; In the formula, Indicates virtual resistance; Indicates virtual reactance; Indicates the scaling factor; This indicates the set current threshold. This represents the three-phase voltage at the common coupling point. d Axial components; This represents the three-phase voltage at the common coupling point. q Axial components; This is represented as the reference voltage for a multi-machine system of a grid-connected converter. The impedance ratio represents the virtual impedance.

[0008] Optionally, in the current limiting control method for the above-mentioned grid-type converter, the voltage deviation term is determined according to the third formula; The third formula is: ; In the formula, This represents the voltage deviation term; Indicates the magnitude of the grid-side current; s This represents the Laplace operator.

[0009] Optionally, in the above-mentioned current limiting control method for grid-type converters, a distributed secondary control strategy based on intelligent agents is constructed, including: Assign a unique index identifier to each network converter and define the information interaction weight coefficients between the agents. a ij The fixed gain coefficient of the dominant grid-type converter b i Set the voltage- and power-related gain adjustment factor; The voltage deviation term is introduced into the calculation model of the secondary control. Combined with the real-time operating parameters of each agent, a distributed consensus algorithm is used to construct the solution formulas for the voltage compensation term and the frequency compensation term. The fixed gain coefficient of the dominant grid-type converter is... b i The value is 1, and the fixed gain coefficient of other grid-type converters is... b i The value is 0; The information interaction weight coefficient is determined by the fourth formula. a ij ; ; In the formula, λ It is a positive number. m i and m j The first i Taiwan and the j The number of network converters associated with the Taiwan-type network converter.

[0010] Optionally, in the current limiting control method for the above-mentioned grid-type converter, the voltage compensation term is determined by the fifth formula; The fifth formula is: ; In the formula, This refers to the voltage compensation term; This represents a positive gain adjustment factor; i and j For indexes of grid-type converters; Indicates the relationship with the first i The set of indices of adjacent grid-type converters to the tandem grid-type converter; Indicates the first j , i Average bus voltage of the grid-type converter; This represents a positive gain adjustment factor; , Indicates the first j , i Reactive power-voltage weighting factor for grid-type converters; Indicates the first i, j The output reactive power of the grid-type converter; The frequency compensation term is determined using the sixth formula; The sixth formula is: ; In the formula, This refers to the frequency compensation term; , This represents a positive gain adjustment factor; Indicates the first j , i The actual angular frequency of the grid-type converter; This represents the reference angular frequency of a multi-machine system in a grid-type converter. Indicates the first j , i Active power-frequency weighting factor for grid-type converters; , Indicates the first j , i The output active power of the grid-type converter.

[0011] Optionally, in the current limiting control method for the above-mentioned grid-type converter, the average bus voltage of the grid-type converter is determined according to the seventh formula; The seventh formula is: ; In the formula, V i For the first i Output voltage amplitude of the grid-type converter c ave A positive gain adjustment factor. N i In order to be with the first i The set of indices of adjacent grid-type converters to the tandem grid-type converter.

[0012] Optionally, in the current limiting control method of the above-mentioned grid-type converter, the voltage compensation term and frequency compensation term are embedded in the virtual synchronous machine power control loop at the bottom layer of each grid-type converter to correct the output voltage amplitude and angular frequency parameters of the virtual synchronous machine controller and generate a three-phase reference voltage. The output voltage amplitude and phase angle are obtained based on the voltage compensation term, the frequency compensation term, and the eighth formula group. The eighth formula group is: ; In the formula, V i and θ i The first i The voltage amplitude and phase angle output by the VSG controller of the grid-type converter. V ni and ω ni The first i Rated voltage amplitude and angular frequency of the grid-type converter. P refi and Q refi The first i Active power reference values ​​and reactive power reference values ​​for the grid-type converter. J i For rotational inertia, D i The damping coefficient is... m Pi and n Qi These are the active power-frequency weighting factor and the reactive power-voltage weighting factor, respectively. The three-phase reference voltage is obtained based on the voltage amplitude and phase angle output by the VSG controller and the ninth formula group; The ninth formula group is: ; In the formula, v ai,ref , v bi,ref and v ci,ref They represent a , b , c Phase reference voltage.

[0013] To address the aforementioned technical problems, this application also provides a current-limiting control device for a grid-connected converter. This device pre-establishes a hierarchical control architecture for a multi-machine system of a distributed, parallel-connected grid-connected converter, including bottom-level control and secondary control. The virtual impedance is adjusted according to the degree of grid fault to limit the fault current. Secondary control coordinates the voltage, frequency, and power distribution of the multi-machine system. The coordination results of the secondary control are embedded into the bottom-level virtual synchronous machine power control loop to drive the grid-connected converter. The device includes: The acquisition module is used to acquire voltage parameters at the common coupling point of the power grid and current parameters on the grid side, and to determine the degree of power grid fault based on the voltage parameters. The virtual establishment module is used to establish an adaptive virtual impedance current limiting circuit based on voltage. It adjusts the virtual impedance parameters of the current limiting circuit according to the determined degree of grid fault and generates a voltage deviation term by combining the collected grid-side current parameters. The analysis module is used to input the voltage deviation term to the secondary control layer, construct a distributed secondary control strategy based on intelligent agents, take each grid-type converter as an intelligent agent, and obtain voltage compensation term and frequency compensation term adapted to multi-machine system through information interaction between intelligent agents and distributed consensus algorithm. The compensation module is used to embed the voltage compensation item and frequency compensation item into the virtual synchronous machine power control loop at the bottom layer of each grid-type converter, correct the output voltage amplitude and angular frequency parameters of the virtual synchronous machine controller, and generate a three-phase reference voltage. The adjustment module is used to generate a PWM wave by sequentially conditioning the three-phase reference voltage through a PI-controlled voltage loop and a current loop, and then drive the grid-type converter to operate through the PWM wave.

[0014] To address the aforementioned technical problems, this application also provides a current limiting control device for a grid-type converter, comprising: Memory, used to store computer programs; A processor is used to implement the steps of the current limiting control method for the grid-type converter described above when executing the computer program.

[0015] To address the aforementioned technical problems, this application also provides a computer-readable storage medium storing a computer program, which, when executed by a processor, implements the steps of the current limiting control method for the grid-type converter described above.

[0016] The current limiting control method for grid-connected converters provided in this application is designed for multi-machine systems of grid-connected converters connected in parallel to the power grid. It uses the voltage parameters of the common coupling point of the power grid as the core criterion, combined with the current parameters of the power grid side to determine the degree of grid fault. It can adaptively adjust the virtual impedance based on voltage drops, achieving automatic switching between normal and fault conditions. An adaptive virtual impedance current limiting circuit based on voltage is constructed, and the virtual impedance parameters of the circuit are dynamically adjusted according to the fault degree. Simultaneously, a voltage deviation term is generated by combining the current from the power grid side, and the adjustment result of the current limiting circuit is converted into a quantized electrical signal that can be transmitted to the secondary control layer. Each grid-connected converter is treated as an independent intelligent agent. Through information interaction between intelligent agents and a distributed consensus algorithm, the voltage deviation term is converted into voltage compensation and frequency compensation terms adapted to the multi-machine system. Therefore, the grid-connected converter can maintain voltage source characteristics and synchronous operation during faults, while ensuring system frequency and voltage stability and accurate proportional power distribution. The compensation term obtained from the secondary control is embedded into the underlying virtual synchronous generator (VSG) power control loop. The compensation term corrects the output voltage amplitude and angular frequency of the VSG controller, converting the corrected VSG output into a three-phase reference voltage. This voltage is then used by PI control to generate a PWM wave to drive the grid-type converter. Since the current-based virtual impedance method designs the virtual impedance based on the magnitude of the fault current, it is prone to fluctuations in the virtual impedance value during faults, affecting system stability. In contrast, the voltage-based virtual impedance method of this application utilizes voltage dips to generate virtual impedance values, avoiding control instability caused by virtual impedance fluctuations. Therefore, the regulation scheme of this application is more stable.

[0017] In addition, this application also provides an apparatus and medium that correspond to the current limiting control method of the above-mentioned grid-type converter, with the same effect. Attached Figure Description

[0018] To more clearly illustrate the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 A flowchart of a current limiting control method for a grid-type converter provided in this application; Figure 2 This application provides an overall control principle diagram of a multi-machine parallel grid-connected converter system. Figure 3 This application provides a schematic diagram of an adaptive virtual impedance current limiting control principle based on voltage quantity. Figure 4 An experimental waveform diagram showing a grid voltage drop to 0.6 pu provided in this application embodiment; Figure 5 A structural diagram of a current limiting control device for a grid-type converter provided in this application embodiment; Figure 6 A structural diagram of a current limiting control device for another grid-type converter provided in this application embodiment. Detailed Implementation

[0020] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the protection scope of this application.

[0021] The core of this application is to provide a current limiting control method, device, and medium for a grid-type converter.

[0022] To enable those skilled in the art to better understand the present application, the present application will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0023] This application provides a current-limiting control method for a grid-connected converter. A hierarchical control architecture is pre-established for a multi-machine system of a distributed, parallel-connected grid-connected converter, including a lower-level control and a secondary control. The virtual impedance is adjusted according to the degree of grid fault to achieve fault current limiting. The voltage, frequency, and power distribution of the multi-machine system are coordinated through secondary control. The coordination results of the secondary control are embedded into the lower-level virtual synchronous machine power control loop to drive the grid-connected converter operation. Figure 1 As shown, the method includes: S11: Collect voltage parameters at the common coupling point of the power grid and current parameters on the power grid side, and determine the degree of power grid fault based on the voltage parameters; S12: Establish an adaptive virtual impedance current limiting circuit based on voltage quantity, adjust the virtual impedance parameters of the current limiting circuit according to the determined degree of grid fault, and generate a voltage deviation term by combining the collected grid-side current parameters. S13: Input the voltage deviation term to the secondary control layer, construct a distributed secondary control strategy based on intelligent agents, take each grid-type converter as an intelligent agent, and obtain the voltage compensation term and frequency compensation term adapted to the multi-machine system through information interaction between intelligent agents and distributed consensus algorithm. S14: Embed the voltage compensation term and frequency compensation term into the virtual synchronous machine power control loop at the bottom layer of each grid-type converter, correct the output voltage amplitude and angular frequency parameters of the virtual synchronous machine controller, and generate a three-phase reference voltage. S15: The three-phase reference voltage is sequentially conditioned by the voltage loop and current loop controlled by PI to generate a PWM wave, which drives the grid-type converter to operate.

[0024] The implementation environment of this application is a photovoltaic power station / wind farm grid-connected system containing two or more grid-connected converters. The grid-connected converters serve as interface devices between renewable energy and the power grid, and are connected to the grid common point of coupling (PCC) in a distributed, masterless parallel manner. The original control architecture of the system includes a virtual synchronous machine (VSG) underlying power control that provides voltage / frequency support to the grid. This embodiment adds a secondary control layer on the basis of the original architecture to build a hierarchical control architecture of underlying control and secondary control, without changing the original distributed parallel topology and hardware structure of the system.

[0025] Step S11: Collect the voltage parameters and grid-side current parameters at the grid common coupling point. Determine the degree of grid fault based on the voltage parameters. Through the sampling module of the grid-type converter multi-machine system, collect the electrical parameters of the three-phase voltage and grid-side incoming current at the grid common coupling point in real time. The voltage parameters are the core basis for judgment, and the current parameters provide the basic data for subsequent current limiting calculations. The two are collected synchronously, and the sampling frequency can be matched with the converter control frequency. This embodiment does not impose strict limitations.

[0026] The grid common coupling point (PCC) refers to the connection node between multiple grid-connected converters and the power grid. It is a key node for grid fault voltage drops and fault current transmission. The voltage parameters include at least the actual voltage amplitude at the common coupling point, the d-axis component and the q-axis component of the three-phase voltage, and the grid-side current parameters include at least the grid-side current amplitude. The degree of grid fault is mainly determined by the degree of grid voltage drop, that is, by the ratio of the actual voltage at the common coupling point to the rated voltage of the grid. Generally speaking, when the voltage amplitude is lower than 95% of the rated voltage, it is determined that the grid has experienced a voltage drop fault, triggering subsequent current limiting actions. Otherwise, it is determined to be a normal operating condition of the grid, and no current limiting control needs to be initiated.

[0027] Step S11 enables accurate perception of power grid fault status, providing fault trigger signals for subsequent adaptive current limiting control, and simultaneously collecting the basic electrical parameters required for subsequent current limiting calculations to ensure the pertinence and accuracy of current limiting control.

[0028] Step S12 establishes an adaptive virtual impedance current limiting circuit based on voltage. The virtual impedance parameters of the current limiting circuit are adjusted according to the determined degree of grid fault, and a voltage deviation term is generated by combining the collected grid-side current parameters. The adaptive virtual impedance current limiting circuit based on voltage is built in the hierarchical control architecture through software control logic. This circuit is not a physical hardware circuit, but a virtual impedance control logic module implemented through a control algorithm, integrated into the secondary control layer of the system. It does not represent the addition of additional hardware current limiting devices to the system.

[0029] Among them, the voltage-based adaptive virtual impedance current limiting circuit refers to the control logic module that uses the grid voltage parameter as the adjustment basis to realize the dynamic adaptive adjustment of the virtual impedance parameter. Its core components are virtual resistance and virtual reactance. The virtual impedance parameter refers to the value of the virtual resistance and virtual reactance. The voltage deviation term is a quantized electrical signal calculated by combining the virtual impedance parameter with the grid-side current amplitude. It is the core intermediate variable that transmits the current limiting control requirements to the subsequent secondary control layer.

[0030] If step S11 determines that the power grid is in a fault condition, the virtual resistance of the virtual impedance current limiting circuit is adaptively adjusted according to the voltage parameters, and the virtual reactance is determined by the ratio of the virtual resistance to the preset impedance. If it is determined to be in a normal condition, the virtual resistance is set to 0, and the virtual reactance is set to 0 synchronously with the virtual resistance. Based on the adjusted virtual impedance parameters and the collected power grid current amplitude, a voltage deviation term is generated.

[0031] Step S13 inputs the voltage deviation term to the secondary control layer to construct a distributed secondary control strategy based on intelligent agents. Each grid-type converter is used as an intelligent agent. Through information interaction and distributed consensus algorithm between intelligent agents, voltage compensation term and frequency compensation term adapted to the multi-machine system are obtained. This step refers to inputting the voltage deviation term generated in step S12 to the secondary control layer of the hierarchical control architecture as a current limiting constraint condition and integrating it into the coordinated control of the multi-machine system. Distributed control logic based on intelligent agents is built in the secondary control layer to realize the coordination of voltage, frequency and power of the multi-machine system.

[0032] like Figure 2 , Figure 3 As shown, the secondary control layer is an upper-level control architecture relative to the original VSG underlying power control of the grid-connected converter, mainly undertaking the global coordination control function of the multi-machine system. The agent-based distributed secondary control strategy refers to treating each grid-connected converter as an independent agent, with each agent only interacting with neighboring agents locally, without the need for a central controller to issue global commands. An agent refers to a single independent grid-connected converter, which is the basic unit for information interaction and control execution in a multi-machine parallel system.

[0033] Step S14 embeds the voltage compensation term and frequency compensation term into the virtual synchronous machine power control loop at the bottom layer of each grid-type converter, corrects the output voltage amplitude and angular frequency parameters of the virtual synchronous machine controller and generates a three-phase reference voltage, embeds the voltage compensation term and frequency compensation term generated in step S13 into the corresponding control equations of the original VSG bottom layer power control loop of each grid-type converter, incrementally corrects the output voltage amplitude and angular frequency of the VSG controller, and then generates a three-phase reference voltage to drive the converter based on the corrected parameters.

[0034] Among them, the underlying Virtual Synchronous Generator (VSG) power control loop is the original core control loop of the grid-type converter. It simulates the rotor motion characteristics and active-frequency and reactive-voltage regulation characteristics of a traditional synchronous generator, providing voltage / frequency support and inertial support for the power grid. Its core outputs are voltage amplitude and angular frequency. The virtual synchronous generator controller is the control module that implements the VSG power control logic and is the core control unit of the grid-type converter. The three-phase reference voltage refers to the a, b, and c three-phase voltage reference signals provided for the power conversion circuit of the grid-type converter. It is a key signal connecting the control strategy and equipment execution.

[0035] Step S15: The three-phase reference voltage is sequentially conditioned by a proportional-integral (PI) controlled voltage loop and a current loop to generate a pulse-width modulation (PWM) wave. The PWM wave drives the grid-type converter to operate. The three-phase reference voltage generated in step S14 is input to the closed-loop control module of the grid-type converter. After sequential cascade conditioning by the PI controlled voltage loop and current loop, the conditioned control signal is converted into a PWM wave modulation signal. Finally, the PWM wave drives the power switching devices of the grid-type converter to operate, thereby realizing the execution of the control strategy.

[0036] Among them, PI control refers to proportional-integral control, which is a commonly used closed-loop control algorithm in power electronic converters; the voltage loop is a closed-loop control with the three-phase reference voltage as the given value and the converter output voltage as the feedback, mainly used to ensure the accuracy and stability of the converter output voltage; the current loop is a closed-loop control with the voltage loop output as the given value and the converter output current as the feedback, mainly used to limit the converter output current and improve the dynamic response speed of the system. The two are cascade control structures; PWM wave, or pulse width modulation wave, is the control wave that drives the power electronic power switching devices to switch on and off. By adjusting the duty cycle of the PWM wave, the converter output voltage / current can be precisely adjusted.

[0037] The current limiting control method for grid-connected converters provided in this application is designed for multi-machine systems of grid-connected converters connected in parallel to the power grid. It uses the voltage parameters of the common coupling point of the power grid as the core criterion, combined with the current parameters of the power grid side to determine the degree of grid fault. It can adaptively adjust the virtual impedance based on voltage drops, achieving automatic switching between normal and fault conditions. An adaptive virtual impedance current limiting circuit based on voltage is constructed, and the virtual impedance parameters of the circuit are dynamically adjusted according to the fault degree. Simultaneously, a voltage deviation term is generated by combining the current from the power grid side, and the adjustment result of the current limiting circuit is converted into a quantized electrical signal that can be transmitted to the secondary control layer. Each grid-connected converter is treated as an independent intelligent agent. Through information interaction between intelligent agents and a distributed consensus algorithm, the voltage deviation term is converted into voltage compensation and frequency compensation terms adapted to the multi-machine system. Therefore, the grid-connected converter can maintain voltage source characteristics and synchronous operation during faults, while ensuring system frequency and voltage stability and accurate proportional power distribution. The compensation term obtained from the secondary control is embedded into the underlying virtual synchronous generator (VSG) power control loop. The compensation term corrects the output voltage amplitude and angular frequency of the VSG controller, converting the corrected VSG output into a three-phase reference voltage. This voltage is then used by PI control to generate a PWM wave to drive the grid-type converter. Since the current-based virtual impedance method designs the virtual impedance based on the magnitude of the fault current, it is prone to fluctuations in the virtual impedance value during faults, affecting system stability. In contrast, the voltage-based virtual impedance method of this application utilizes voltage dips to generate virtual impedance values, avoiding control instability caused by virtual impedance fluctuations. Therefore, the regulation scheme of this application is more stable.

[0038] According to the above embodiments, specifically, the voltage parameters of the power grid common coupling point and the current parameters of the power grid side are collected, and the degree of power grid fault is determined based on the voltage parameters, including: Extract the actual voltage at the common coupling point With the rated voltage of the power grid U n ; judge Is it valid? If so, then a power grid fault is determined. If not, the power grid is considered to be in normal operating condition; Correspondingly, the virtual impedance parameters of the current-limiting circuit are adjusted according to the determined degree of power grid fault, including: If a fault is determined to have occurred in the power grid, the virtual resistance and virtual reactance are determined based on the first formula and the second formula, respectively; if the power grid is determined to be in normal operating condition, the virtual resistance is set to 0, and the virtual reactance is set to 0 synchronously with the virtual resistance. The first formula is: ; The second formula is: ; In the formula, Indicates virtual resistance; Indicates virtual reactance; Indicates the scaling factor; This indicates the set current threshold. This represents the three-phase voltage at the common coupling point. d Axial components; This represents the three-phase voltage at the common coupling point. q Axial components; This is represented as the reference voltage for a multi-machine system of a grid-connected converter. The impedance ratio represents the virtual impedance.

[0039] This embodiment uses the ratio of the actual voltage at the common coupling point of the power grid to the rated voltage as the basis for fault determination. By fixing the voltage threshold, a binary determination of power grid fault / normal operating conditions is achieved. Then, the virtual resistance and virtual reactance of the adaptive virtual impedance current limiting circuit are adjusted differently for different operating conditions. Under fault conditions, the impedance parameters are determined by calculation through formula. Under normal operating conditions, the impedance parameters are directly set to 0 to achieve automatic start and stop of current limiting control.

[0040] The first formula integrates core parameters such as current threshold, scaling factor, dq-axis components of the three-phase voltage at the common coupling point, and reference voltage of the multi-machine system, enabling the calculated virtual resistance to accurately match the degree of grid fault. The second formula ensures the synergistic adjustment of virtual resistance and virtual reactance through a fixed impedance ratio, avoiding current limiting failure caused by impedance mismatch. The formulaic impedance parameter calculation under fault conditions allows virtual resistance and virtual reactance to adaptively adjust according to the actual voltage state of the grid fault, solving the problem that fixed virtual impedance cannot adapt to dynamic fault conditions, achieving precise limitation of fault current, and confining the fault current within the tolerance range of the grid-connected converter.

[0041] According to the above embodiments, specifically, as follows: Figure 3 As shown, the voltage deviation term is determined according to the third formula; The third formula is: ; In the formula, Indicates the voltage deviation term; Indicates the magnitude of the grid-side current; s This represents the Laplace operator.

[0042] In this embodiment, the determined virtual resistance, virtual reactance, and the collected grid-side current amplitude are substituted into the third formula for complex frequency domain calculation to directly calculate the voltage deviation term.

[0043] The third formula transforms the current-limiting effect of virtual impedance into a quantized voltage signal. The core function of virtual impedance is to limit fault current through impedance voltage drop. It is calculated by the current amplitude on the grid side and the virtual impedance parameters without introducing additional complex parameters. This simplifies the calculation logic, improves the response speed, avoids calculation errors caused by multi-parameter coupling, and ensures the reliability of the voltage deviation term.

[0044] According to the above embodiments, specifically, constructing a distributed secondary control strategy based on intelligent agents includes: Assign a unique index identifier to each network converter and define the information interaction weight coefficients between the agents. a ij The fixed gain coefficient of the dominant grid-type converter b i Set the voltage- and power-related gain adjustment factor; The voltage deviation term is introduced into the calculation model of the secondary control. Combined with the real-time operating parameters of each agent, a distributed consensus algorithm is used to construct the solution formulas for the voltage compensation term and the frequency compensation term. Among these, the fixed gain coefficient of the dominant grid-type converter is... b i The value is 1, and the fixed gain coefficient of other grid-type converters is... b i The value is 0; The information interaction weight coefficient is determined by the fourth formula. a ij ; ; In the formula, λ It is a positive number. m i and m j The first i Taiwan and the j The number of network converters associated with the Taiwan-type network converter.

[0045] In this embodiment, each grid-type converter is treated as an independent intelligent agent. A unique index identifier is configured for each grid-type converter, core control parameters are defined, and the voltage deviation term is introduced into the calculation model of the secondary control. Combined with the real-time operating parameters of each intelligent agent, the solution formulas for the voltage compensation term and frequency compensation term are constructed through a distributed consensus algorithm. The fixed gain coefficient of the dominant converter is set to non-zero gain only for the preset dominant converter, while the gain of the other converters is 0, providing a unified reference benchmark for the distributed system and ensuring the consistency of multi-machine coordination.

[0046] According to the above embodiments, specifically, as follows: Figure 2 , 3 As shown, the voltage compensation term is determined using the fifth formula; The fifth formula is: ; In the formula, Indicates voltage compensation term; This represents a positive gain adjustment factor; i and j For indexes of grid-type converters; Indicates the relationship with the first i The set of indices of adjacent grid-type converters to the tandem grid-type converter; Indicates the first j , i Average bus voltage of the grid-type converter; This represents a positive gain adjustment factor; , Indicates the first j , i Reactive power-voltage weighting factor for grid-type converters; Indicates the first i, j The output reactive power of the grid-type converter; The frequency compensation term is determined using the sixth formula; The sixth formula is: ; In the formula, Indicates the frequency compensation term; , This represents a positive gain adjustment factor; Indicates the first j , i The actual angular frequency of the grid-type converter; This represents the reference angular frequency of a multi-machine system in a grid-type converter. Indicates the first j , i Active power-frequency weighting factor for grid-type converters; , Indicates the first j , i The output active power of the grid-type converter.

[0047] like Figure 2 As shown, this embodiment constructs the solution logic for voltage compensation and frequency compensation terms through integral operations. The calculation of both compensation terms only depends on the local interaction information of adjacent converters, without the need for global data aggregation. Furthermore, integral operations can smooth the fluctuations in the dynamic adjustment process, ensuring the continuity and stability of the compensation terms.

[0048] The implementation method of this embodiment accurately calculates voltage and frequency compensation terms, which not only completes the transmission of current limiting constraints to the underlying control, but also ensures the voltage and frequency stability and reasonable power distribution of the multi-machine parallel system. It is a key link connecting the coordination of secondary control and the execution of underlying control.

[0049] According to the above embodiments, specifically, the average bus voltage of the grid-type converter is determined according to the seventh formula; The seventh formula is: ; In the formula, V i For the first i Output voltage amplitude of the grid-type converter c ave A positive gain adjustment factor. N i In order to be with the first i The set of indices of adjacent grid-type converters to the tandem grid-type converter.

[0050] This embodiment combines the difference between the output voltage of a single grid-type converter and the average bus voltage of adjacent converters, and uses the seventh formula to dynamically calculate the average bus voltage. The core is to generate a reference quantity that reflects the voltage coordination state of multiple machines, providing a foundation for the accurate solution of voltage compensation terms.

[0051] like Figure 2 As shown, based on the actual output voltage amplitude of the i-th grid-connected converter, the difference between its voltage and the average bus voltage of adjacent converters is introduced as the adjustment basis. The influence intensity of the difference is controlled by a gain adjustment factor, and the adjustment process is smoothed through integral calculation to finally obtain the average bus voltage of the converter. This embodiment does not strictly limit the specific value of the gain adjustment factor and can be adjusted according to the number of parallel units and the system voltage level. Accurate calculation of the average bus voltage provides a reliable voltage reference for voltage compensation.

[0052] According to the above embodiments, specifically, voltage compensation terms and frequency compensation terms are embedded in the virtual synchronous machine power control loop at the bottom layer of each grid-type converter, correcting the output voltage amplitude and angular frequency parameters of the virtual synchronous machine controller and generating a three-phase reference voltage; such as Figure 3 As shown: The output voltage amplitude and phase angle are obtained based on the voltage compensation term, the frequency compensation term, and the eighth formula group. The eighth formula group is: ; In the formula, V i and θ i The first iThe voltage amplitude and phase angle output by the VSG controller of the grid-type converter. V ni and ω ni The first i Rated voltage amplitude and angular frequency of the grid-type converter. P refi and Q refi The first i Active power reference values ​​and reactive power reference values ​​for the grid-type converter. J i For rotational inertia, D i The damping coefficient is... m Pi and n Qi These are the active power-frequency weighting factor and the reactive power-voltage weighting factor, respectively. The three-phase reference voltage is obtained based on the voltage amplitude and phase angle output by the VSG controller and the ninth formula group; The ninth formula group is: ; In the formula, v ai,ref , v bi,ref and v ci,ref They represent a , b , c Phase reference voltage.

[0053] In this embodiment, the voltage compensation term and frequency compensation term obtained from the secondary control layer are embedded into the power control loop of the underlying virtual synchronous machine (VSG). The output voltage amplitude and angular frequency of the VSG are corrected by formula set, and then a three-phase reference voltage is generated based on the corrected parameters to achieve the synergy between current limiting constraints and grid support characteristics.

[0054] First, the voltage compensation term is integrated into the reactive power-voltage control logic of the VSG. Combining the converter's rated voltage, the difference between the reactive power reference value and the actual output reactive power, the output voltage amplitude is obtained by correcting the values ​​using the corresponding formula in the eighth formula group. Similarly, the frequency compensation term is embedded into the VSG's active power-frequency control logic. Combining the rated angular frequency, the difference between the active power reference value and the actual output active power, and core VSG parameters such as moment of inertia and damping coefficient, the output angular frequency is obtained by correcting the values. The phase angle is then obtained through operations between the angular frequency and the Laplace operator. Based on the corrected voltage amplitude and phase angle, the three-phase reference voltages (a, b, and c) are generated using the ninth formula group. The generation of the three-phase reference voltages based on the corrected voltage amplitude and phase angle ensures the accuracy of the converter's output voltage and avoids three-phase imbalance through a fixed phase difference design, providing a reliable basis for subsequent PWM wave generation.

[0055] An experiment was conducted with four grid-connected converters connected in parallel, under the condition that the grid voltage dropped to 0.6 pu. Figure 4 An experimental waveform diagram showing a grid voltage drop to 0.6 pu is provided as an embodiment of this application. Figure 4 As shown, the control method designed in this invention can quickly and effectively limit fault current, maintain grid frequency stability, and at the same time ensure system frequency, voltage stability, and accurate proportional power distribution.

[0056] In the above embodiments, the current limiting control method for grid-type converters has been described in detail. This application also provides embodiments corresponding to the current limiting control device for grid-type converters. It should be noted that this application describes the embodiments of the device part from two perspectives: one is based on the functional module, and the other is based on the hardware.

[0057] From the perspective of functional modules Figure 5 A structural diagram of a current limiting control device for a grid converter provided in this application embodiment is shown below. Figure 5 As shown, a current-limiting control device for a grid-connected converter pre-establishes a hierarchical control architecture for a distributed, parallel-connected grid-connected multi-machine system of grid-connected converters, including a lower-level control and a secondary control. The device adjusts the virtual impedance according to the degree of grid fault to achieve fault current limiting. The secondary control coordinates the voltage, frequency, and power distribution of the multi-machine system. The coordination results of the secondary control are embedded into the lower-level virtual synchronous machine power control loop to drive the grid-connected converter operation. The device includes: The acquisition module 21 is used to acquire voltage parameters at the common coupling point of the power grid and current parameters on the power grid side, and to determine the degree of power grid fault based on the voltage parameters. The virtual establishment module 22 is used to establish an adaptive virtual impedance current limiting circuit based on voltage. It adjusts the virtual impedance parameters of the current limiting circuit according to the determined degree of grid fault and generates a voltage deviation term by combining the collected grid-side current parameters. Analysis module 23 is used to input the voltage deviation term to the secondary control layer, construct a distributed secondary control strategy based on intelligent agents, take each grid-type converter as an intelligent agent, and obtain the voltage compensation term and frequency compensation term adapted to the multi-machine system through information interaction between intelligent agents and distributed consensus algorithm. Compensation module 24 is used to embed voltage compensation terms and frequency compensation terms into the virtual synchronous machine power control loop at the bottom layer of each grid-type converter, correct the output voltage amplitude and angular frequency parameters of the virtual synchronous machine controller, and generate a three-phase reference voltage. The adjustment module 25 is used to generate a PWM wave by conditioning the three-phase reference voltage through the voltage loop and current loop controlled by PI in sequence, and then drive the grid-type converter to operate through the PWM wave.

[0058] Since the embodiments of the apparatus and the embodiments of the method correspond to each other, please refer to the description of the embodiments of the method for the embodiments of the apparatus, which will not be repeated here.

[0059] Figure 6 A structural diagram of another current-limiting control device for a grid-type converter provided in this application embodiment is shown below. Figure 6 As shown, the current limiting control device of the grid-type converter includes: a memory 30 for storing computer programs; The processor 31 is used to execute a computer program to implement the steps of the method for obtaining user operation habit information as described in the above embodiment (current limiting control method for grid-type converter).

[0060] The current limiting control device for the grid converter provided in this embodiment may include, but is not limited to, mobile terminals, personal computers, workstations, etc.

[0061] The processor 31 may include one or more processing cores, such as a quad-core processor or an octa-core processor. The processor 31 may be implemented using at least one of the following hardware forms: Digital Signal Processor (DSP), Field-Programmable Gate Array (FPGA), or Programmable Logic Array (PLA). The processor 31 may also include a main processor and a coprocessor. The main processor, also known as the Central Processing Unit (CPU), is used to process data in the wake-up state; the coprocessor is a low-power processor used to process data in the standby state. In some embodiments, the processor 31 may integrate a Graphics Processing Unit (GPU), which is responsible for rendering and drawing the content to be displayed on the screen. In some embodiments, the processor 31 may also include an Artificial Intelligence (AI) processor, which handles computational operations related to machine learning.

[0062] The memory 30 may include one or more computer-readable storage media, which may be non-transitory. The memory 30 may also include high-speed random access memory and non-volatile memory, such as one or more disk storage devices or flash memory devices. In this embodiment, the memory 30 is used to store at least the following computer program 301, which, after being loaded and executed by the processor 31, is capable of implementing the relevant steps of the current limiting control method for the grid-type converter disclosed in any of the foregoing embodiments. In addition, the resources stored in the memory 30 may also include an operating system 302 and data 303, and the storage method may be temporary or permanent storage. The operating system 302 may include Windows, Unix, Linux, etc. The data 303 may include, but is not limited to, the data involved in implementing the current limiting control method for the grid-type converter.

[0063] In some embodiments, the current limiting control device of the grid converter may further include a display screen 32, an input / output interface 33, a communication interface 34, a power supply 35, and a communication bus 36.

[0064] Those skilled in the art will understand that Figure 6 The structure shown does not constitute a limitation on the current limiting control device for a grid converter and may include more or fewer components than shown.

[0065] The current limiting control device for a grid-type converter provided in this application includes a memory and a processor. When the processor executes the program stored in the memory, it can implement the following method: a current limiting control method for a grid-type converter.

[0066] Finally, this application also provides an embodiment corresponding to a computer-readable storage medium. The computer-readable storage medium stores a computer program, which, when executed by a processor, implements the steps described in the above embodiment of the current limiting control method for a grid-type converter.

[0067] It is understood that if the methods in the above embodiments are implemented as software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of this application, in essence, or the part that contributes to the prior art, or all or part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and executes all or part of the steps of the methods described in the various embodiments of this application. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0068] The computer-readable storage medium provided in this embodiment stores a computer program. When the processor executes the program, it can implement the following method: a current limiting control method for a grid-type converter.

[0069] The current limiting control method, apparatus, and medium for grid-type converters provided in this application have been described in detail above. The various embodiments in the specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since it corresponds to the method disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to in the method section. It should be noted that those skilled in the art can make several improvements and modifications to this application without departing from the principles of this application, and these improvements and modifications also fall within the protection scope of this application.

[0070] It should also be noted that, in this specification, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

Claims

1. A current limiting control method for a grid-type converter, characterized in that, A hierarchical control architecture is pre-established for a multi-machine grid-connected grid converter system, including bottom-level control and secondary control. The architecture involves adjusting virtual impedance based on the degree of grid fault to limit fault current, coordinating voltage, frequency, and power distribution within the multi-machine system through secondary control, and embedding the coordination results of the secondary control into the bottom-level virtual synchronous machine power control loop to drive the grid-connected converter operation. The method includes: Collect voltage parameters at the common coupling point of the power grid and current parameters on the grid side, and determine the degree of power grid fault based on the voltage parameters; An adaptive virtual impedance current limiting circuit based on voltage is established. The virtual impedance parameters of the current limiting circuit are adjusted according to the determined degree of grid fault, and a voltage deviation term is generated by combining the collected grid-side current parameters. The voltage deviation term is input to the secondary control layer to construct a distributed secondary control strategy based on intelligent agents. Each grid-type converter is used as an intelligent agent. Through information interaction between intelligent agents and a distributed consensus algorithm, voltage compensation term and frequency compensation term adapted to multi-machine system are obtained. The voltage compensation term and frequency compensation term are embedded into the virtual synchronous machine power control loop at the bottom layer of each grid-type converter to correct the output voltage amplitude and angular frequency parameters of the virtual synchronous machine controller and generate a three-phase reference voltage. The three-phase reference voltages are sequentially conditioned by a PI-controlled voltage loop and a current loop to generate a PWM wave, which drives the grid-type converter to operate.

2. The current limiting control method for a grid-type converter according to claim 1, characterized in that, Collect voltage parameters at the grid's common coupling point and grid-side current parameters, and determine the degree of grid fault based on the voltage parameters, including: Extract the actual voltage at the common coupling point With the rated voltage of the power grid U n ; judge Is it valid? If so, then a power grid fault is determined. If not, the power grid is considered to be in normal operating condition; Correspondingly, the virtual impedance parameters of the current-limiting circuit are adjusted according to the determined degree of power grid fault, including: If a fault is determined to have occurred in the power grid, the virtual resistance and virtual reactance are determined based on the first formula and the second formula, respectively; if the power grid is determined to be in normal operating condition, the virtual resistance is set to 0, and the virtual reactance is set to 0 synchronously with the virtual resistance. The first formula is: ； The second formula is: ; In the formula, Indicates virtual resistance; Indicates virtual reactance; Indicates the scaling factor; This indicates the set current threshold. This represents the three-phase voltage at the common coupling point. d Axial components; This represents the three-phase voltage at the common coupling point. q Axial components; This is represented as the reference voltage for a multi-machine system of a grid-connected converter. The impedance ratio represents the virtual impedance.

3. The current limiting control method for a grid-type converter according to claim 2, characterized in that, The voltage deviation term is determined according to the third formula; The third formula is: ; In the formula, This represents the voltage deviation term; Indicates the magnitude of the grid-side current; s This represents the Laplace operator.

4. The current limiting control method for a grid-type converter according to claim 3, characterized in that, Constructing an agent-based distributed secondary control strategy includes: Assign a unique index identifier to each network converter and define the information interaction weight coefficients between the agents. a ij The fixed gain coefficient of the dominant grid-type converter b i Set the voltage- and power-related gain adjustment factor; The voltage deviation term is introduced into the calculation model of the secondary control. Combined with the real-time operating parameters of each agent, a distributed consensus algorithm is used to construct the solution formulas for the voltage compensation term and the frequency compensation term. The fixed gain coefficient of the dominant grid-type converter is... b i The value is 1, and the fixed gain coefficient of other grid-type converters is... b i The value is 0; The information interaction weight coefficient is determined by the fourth formula. a ij ; ； In the formula, λ It is a positive number. m i and m j The first i Taiwan and the j The number of network converters associated with the Taiwan-type network converter.

5. The current limiting control method for a grid-type converter according to claim 4, characterized in that, The voltage compensation term is determined using the fifth formula; The fifth formula is: ； In the formula, This refers to the voltage compensation term; This represents a positive gain adjustment factor; i and j For indexes of grid-type converters; Indicates the relationship with the first i The set of indices of adjacent grid-type converters to the tandem grid-type converter; Indicates the first j , i Average bus voltage of the grid-type converter; This represents a positive gain adjustment factor; , Indicates the first j , i Reactive power-voltage weighting factor for grid-type converters; Indicates the first i, j The output reactive power of the grid-type converter; The frequency compensation term is determined using the sixth formula; The sixth formula is: ； In the formula, This refers to the frequency compensation term; , This represents a positive gain adjustment factor; Indicates the first j , i The actual angular frequency of the grid-type converter; This represents the reference angular frequency of a multi-machine system in a grid-type converter. Indicates the first j , i Active power-frequency weighting factor for grid-type converters; , Indicates the first j , i The output active power of the grid-type converter.

6. The current limiting control method for a grid-type converter according to claim 5, characterized in that, The average bus voltage of the grid-type converter is determined according to the seventh formula. The seventh formula is: ; In the formula, V i For the first i Output voltage amplitude of the grid-type converter c ave A positive gain adjustment factor. N i In order to be with the first i The set of indices of adjacent grid-type converters to the tandem grid-type converter.

7. The current limiting control method for a grid-type converter according to claim 6, characterized in that, The voltage compensation term and frequency compensation term are embedded into the virtual synchronous machine power control loop at the bottom layer of each grid-type converter to correct the output voltage amplitude and angular frequency parameters of the virtual synchronous machine controller and generate a three-phase reference voltage. The output voltage amplitude and phase angle are obtained based on the voltage compensation term, the frequency compensation term, and the eighth formula group. The eighth formula group is: ； In the formula, V i and θ i The first i The voltage amplitude and phase angle output by the VSG controller of the grid-type converter. V ni and ω ni The first i Rated voltage amplitude and angular frequency of the grid-type converter. P refi and Q refi The first i Active power reference values ​​and reactive power reference values ​​for the grid-type converter. J i For rotational inertia, D i The damping coefficient is... m Pi and n Qi These are the active power-frequency weighting factor and the reactive power-voltage weighting factor, respectively. The three-phase reference voltage is obtained based on the voltage amplitude and phase angle output by the VSG controller and the ninth formula group; The ninth formula group is: ； In the formula, v ai,ref , v bi,ref and v ci,ref They represent a , b , c Phase reference voltage.

8. A current limiting control device for a grid-type converter, characterized in that, A hierarchical control architecture is pre-established for a multi-machine grid-connected converter system with distributed parallel grid access, including bottom-level control and secondary control. The virtual impedance is adjusted according to the degree of grid fault to achieve fault current limiting. Secondary control coordinates the voltage, frequency, and power distribution of the multi-machine system. The coordination results of the secondary control are embedded into the bottom-level virtual synchronous machine power control loop to drive the grid-connected converter operation. The device includes: The acquisition module is used to acquire voltage parameters at the common coupling point of the power grid and current parameters on the grid side, and to determine the degree of power grid fault based on the voltage parameters. The virtual establishment module is used to establish an adaptive virtual impedance current limiting circuit based on voltage. It adjusts the virtual impedance parameters of the current limiting circuit according to the determined degree of grid fault and generates a voltage deviation term by combining the collected grid-side current parameters. The analysis module is used to input the voltage deviation term to the secondary control layer, construct a distributed secondary control strategy based on intelligent agents, take each grid-type converter as an intelligent agent, and obtain voltage compensation term and frequency compensation term adapted to multi-machine system through information interaction between intelligent agents and distributed consensus algorithm. The compensation module is used to embed the voltage compensation item and frequency compensation item into the virtual synchronous machine power control loop at the bottom layer of each grid-type converter, correct the output voltage amplitude and angular frequency parameters of the virtual synchronous machine controller, and generate a three-phase reference voltage. The adjustment module is used to generate a PWM wave by sequentially conditioning the three-phase reference voltage through a PI-controlled voltage loop and a current loop, and then drive the grid-type converter to operate through the PWM wave.

9. A current limiting control device for a grid-type converter, characterized in that, include: Memory, used to store computer programs; A processor, configured to implement the steps of the current limiting control method for a grid-type converter as described in any one of claims 1 to 7 when executing the computer program.

10. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a computer program that, when executed by a processor, implements the steps of the current limiting control method for a grid-type converter as described in any one of claims 1 to 7.

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