Quantification method and system for fault voltage support capability of grid-forming converter

Abstract

The present application belongs to the technical field of transient stability evaluation of converters, and provides a fault voltage support capability quantification method and system for grid-connected converters, which combines the switching condition of the ring current limiter, deduces the voltage-phasor equation at the common grid coupling point of the grid-connected converter during the fault period under the ring current limiting strategy, obtains the voltage-phasor three-dimensional surface graph of the grid-connected converter system based on the voltage-phasor equation, analyzes the maximum voltage of the common bus of the grid-connected converter under the ring current limiting strategy under different voltage drop scenarios, and adjusts the phasor control link based on the voltage; the phasor control link is applied to the grid-connected synchronous control link, the voltage drop during the fault period of the grid-connected converter is reduced, and the grid-connected converter can ensure the maximum power output during the fault period. The present application significantly improves the voltage support capability of the grid-connected converter during the fault period.

Description

Technical Field

[0001] This invention belongs to the field of transient stability assessment technology for converters, specifically relating to a method and system for quantifying the fault voltage support capability of grid-connected converters. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] Grid-type inverters, by mimicking the rotor motion equations and excitation circuits of synchronous machines, can provide virtual inertia and damping to the system. This allows the voltage amplitude and phase of the inverter source to remain constant during disturbances, exhibiting voltage source characteristics and effectively improving the grid's immunity. However, during grid voltage dips, the fault current in grid-type inverters can easily exceed the overcurrent threshold of semiconductor components, necessitating current-limiting control to protect the grid-type inverter.

[0004] As one of the most important current limiting strategies, the ring current limiter works by controlling the output current amplitude when the reference current value output by the voltage loop of a grid-connected inverter exceeds the maximum allowable current amplitude. This is achieved by adding a current limiter between the outer voltage loop and the inner current loop, thus limiting the current through the current loop. Under ring current limiting, the grid-connected inverter can be equivalent to a constant voltage source connected in series with an equivalent impedance. During grid voltage dips, the ring current limiting strategy switches the grid-connected inverter from constant voltage mode to current limiting mode, preventing the fault current from exceeding the overcurrent threshold of the semiconductor components, thereby achieving current limiting.

[0005] For grid-connected converters, the main working principle is to achieve synchronization with the AC power grid using a phase-locked loop (PLL). Specifically, the PLL tracks the voltage phase at the point of common coupling (PCC) and uses this as a reference voltage phase to maintain synchronization with the grid, thus achieving grid "following." Overall, it exhibits current source characteristics. The grid-connected converter control system mainly consists of three parts: a power control loop, a current control loop, and a PLL. In engineering practice, the bandwidth of the power control loop is generally designed to be one-tenth of the bandwidth of the current control loop; simultaneously, the bandwidth of the current control loop is generally designed not to exceed one-fifth of the converter's switching frequency. Based on this, the converter control structure adopted in this invention mainly includes: a voltage source converter, a PLL, a current control loop, and an equivalent power grid.

[0006] In the actual grid-connected operation of new power systems, the system is usually composed of grid-connected converters and grid-building converters operating in parallel and in coordination. The grid-building converters are responsible for establishing voltage and frequency, thus providing active support for the system; the grid-connected converters can achieve fast and accurate power tracking through phase-locked loops. The two work together to support the new power system.

[0007] However, for grid-connected systems, the presence of current limiting strategies can cause a voltage drop in the event of a three-phase short-circuit fault in the grid-connected power electronic devices, which is detrimental to ensuring device stability. Furthermore, the voltage variation pattern on the common bus of the grid-connected converter during a fault under the ring current limiting strategy remains unclear. Therefore, there is a lack of research on how to accurately quantify and improve the fault voltage support capability of grid-connected systems under ring current limiting. Summary of the Invention

[0008] To address the aforementioned problems, this invention proposes a method and system for quantifying the fault voltage support capability of grid-connected converters. This invention can accurately quantify the voltage at the common grid coupling point of grid-connected converters under different voltage drops, and significantly improve the voltage support capability of grid-connected converters during faults through power angle control.

[0009] According to some embodiments, the present invention adopts the following technical solution: A method for quantifying the fault voltage support capability of a grid-connected converter includes the following steps: The grid-type inverter converter adopts virtual synchronous machine control. Under normal operating conditions, it uses voltage and current dual closed loop to achieve constant voltage mode control, and under fault conditions, it uses ring current limiting to limit fault current. The grid-connected inverter converter tracks the voltage phase at the common coupling point through a phase-locked loop as a reference voltage phase to maintain synchronization with the grid. The dynamic reactive current increment is adjusted through a current control loop to respond to changes in the grid connection point voltage. Based on the switching conditions of the ring current limiter, the voltage-power angle equation at the common grid coupling point of the ring-limited grid-connected converter during the fault period is derived. Based on the voltage-power angle equation, a three-dimensional voltage-power angle surface diagram of the grid converter system is obtained. The maximum voltage of the common bus of the grid converter under the ring current limiting strategy under different voltage drop scenarios is analyzed, and the power angle control loop is adjusted based on this voltage. By applying a power angle control element to the grid synchronization control stage, the voltage drop during faults of grid-connected converters is reduced, ensuring that grid-connected converters can guarantee maximum power output during faults.

[0010] As an alternative implementation, the process of using virtual synchronous machine control for the grid-connected inverter converter includes: The grid-connected inverter converter section in the grid-connected inverter is controlled by a virtual synchronous machine, and through dual closed-loop control, it exhibits voltage source characteristics. The virtual synchronous machine control equation is expressed as:

[0011]

[0012] Where d is the differential operator; t For time; i The phase difference between the inverter and the power grid in a grid-connected inverter is called the power angle. oh and oh 0 represents the per-unit actual angular frequency and rated angular frequency of the grid-type inverter converter, respectively; oh base The base value of angular frequency; J This is the virtual inertia coefficient; D The damping coefficient; P ref and P c These are the active power reference value and actual active power of the grid-type inverter converter, respectively.

[0013] As an alternative implementation, the process of using ring current limiting to restrict fault current under fault conditions includes: during a fault, the grid-connected inverter converter section of the grid-connected converter uses a ring current limiting device to limit the fault current, and the specific current limiting strategy is as follows:

[0014] In the formula, I c This refers to the actual current of the grid-type converter; The current reference value provided for the outer voltage loop of the grid-type converter; This is the reference value for the actual input current in the inner current loop; I cmax is the current threshold of the grid-type converter; e is the base of the natural logarithm.

[0015] As an alternative implementation, for grid-connected converters, when a three-phase symmetrical fault occurs and the positive-sequence component of the grid connection point voltage is lower than a predetermined percentage of the nominal voltage, the grid-connected equipment should have dynamic reactive power support capability. The dynamic reactive current increment should respond to changes in the grid connection point voltage and satisfy the following formula:

[0016] In the formula, △ I q This refers to the dynamic reactive current increment during grid equipment fault ride-through. K q This refers to the reactive current control coefficient during fault ride-through. U FM To match the voltage at the network equipment terminals, I N This refers to the rated current of the grid-connected equipment.

[0017] As an alternative implementation, for grid-connected converters, during voltage dips, the reactive current output by the grid-connected equipment to the power system is the sum of the output reactive current during normal operation before the voltage dip and the dynamic reactive current increment. The maximum output capacity of the reactive current of the grid-connected equipment is not less than a set multiple of the rated current of the grid-connected equipment. From the moment the voltage dip occurs at the grid connection point, the rise time of the dynamic reactive current of the grid-connected equipment is not greater than a set threshold. From the moment the voltage at the grid connection point recovers to more than 90% of the nominal voltage, the grid-connected equipment exits the dynamic reactive current increment within a set time.

[0018] As an alternative implementation method, the process of deriving the voltage-power angle equation at the common grid coupling point of the ring-limited / grid-connected converter during a fault, in conjunction with the switching conditions of the ring current limiter, includes: Considering the switching conditions of the ring current limiter, under the ring limiting effect, the grid-type converter is equivalent to a voltage source. U ref Connect an equivalent resistance R e Through line resistance R 1 and reactance X 1. Connected to the system, during a fault, the current equation expression for the grid-type converter section under the ring current limiting strategy is obtained through the main circuit equation of the grid-type converter section in the system. That is, the main circuit current equation when the grid-type converter is in current limiting mode is as follows:

[0019] In the formula, U ref and U ref These are the voltage vector and magnitude at the grid-type converter, respectively; U p and U p These are the voltage reference vector and magnitude at the common bus at the parallel connection point of the grid-type converter and the grid-connected converter, respectively. f The voltage phase on the common bus; d The power angle of the grid-type converter; R e The system's equivalent resistance; R 1 represents the line resistance of the grid-type converter; X 1 represents the line reactance of the grid-type converter; j is an imaginary number; For the analysis of the grid converter system, the main circuit equation under the ring limiting condition during the fault period is:

[0020] In the formula, U g This refers to the voltage amplitude of the power grid.I L This refers to the current between the common grid-connected coupling point of the grid-connected converter and the power grid for both grid-connected and network-type converters. Z g To determine the impedance of the line between the common busbar of the grid-connected converter and the power grid; according to Kirchhoff's laws, I L The sum of the currents flowing to the coupling point of the grid-connected converter and the grid-connected converter is used to obtain the voltage amplitude at the common grid-connected coupling point of the grid-connected converter and the grid-connected converter. U p Phase f and equivalent virtual resistance R e about d and i The expressions are as follows: ; ; .

[0021] As an alternative implementation, the process of obtaining the three-dimensional voltage-power angle surface diagram of a grid-connected converter system based on the voltage-power angle equation includes: given the power angle of the grid-connected converter... d and the phase of the line current of the grid-type converter i At that time, calculate the voltage on the corresponding common bus. U p ,Will d and i The ranges are taken from -200° to 360°, resulting in... U p about i and d A 3D diagram.

[0022] As an alternative implementation method, the process of analyzing the maximum voltage of the common bus under the ring current limiting strategy of the grid-type converter in different voltage drop scenarios, and adjusting the power angle control loop based on this voltage, includes: determining the voltage on the common bus during the fault period after the fault occurs through the voltage-power angle three-dimensional surface. U p maximum value U pmax This value serves as a benchmark to quantify the fault voltage support capability of the grid-connected converter during a fault. Simultaneously, based on this benchmark value, a power angle control loop is applied to reduce the drop in fault voltage and improve the fault voltage support capability of the grid-connected converter.

[0023] As an alternative implementation, the process of applying the power angle control element in the grid synchronization control element includes: determining the voltage on the common bus. U p maximum value U pmax Subsequently, a new control loop was added to the synchronization loop of the grid converter control structure. The control loop is as follows:

[0024] in. d c The output of this control loop for the power angle d negative feedback, k P The proportional coefficient for the control link, V p To ensure the common voltage on the common bus of the grid-connected converter and the grid-connected converter, V max for V p The maximum value.

[0025] A fault voltage support capability quantification system for grid-connected converters includes: The grid-connected inverter converter control module is configured to use virtual synchronous machine control for the grid-connected inverter converter. Under normal operating conditions, it uses voltage and current dual closed loops to achieve constant voltage mode control, and under fault conditions, it uses ring current limiting to limit the fault current. The grid-connected inverter converter module is configured to track the voltage phase of the common coupling point through a phase-locked loop as a reference voltage phase to maintain synchronization with the grid. The dynamic reactive current increment is adjusted through a current control loop to respond to changes in the grid connection point voltage. The voltage-power angle equation derivation module is configured to derive the voltage-power angle equation at the common grid-connected coupling point of the grid-connected converter under ring current limit during a fault, in conjunction with the switching conditions of the ring current limiter. The common bus maximum voltage calculation module is configured to obtain a three-dimensional voltage-power angle surface diagram of the grid-connected converter system based on the voltage-power angle equation, analyze the maximum voltage of the common bus under the ring current limiting strategy of the grid-connected converter under different voltage drop scenarios, and adjust the power angle control loop based on this voltage. The synchronization control module is configured to apply a power angle control element to the grid synchronization control stage, reducing voltage drops during grid-connected converter faults and ensuring maximum power output during faults.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows: First, this invention establishes the basic working principle of the grid-type converter under ring limiting and the basic control principle of the grid-connected converter. It derives the voltage-power angle equation on the common bus of the parallel system of grid-connected / connected converters under ring limiting during a fault, and determines the different fault scenarios of the fault voltage of the grid-connected / connected inverter converter under different degrees of fault under ring limiting.

[0027] Secondly, this invention determines the three-dimensional voltage-power angle image of the grid-connected inverter during the fault period under different voltage drop scenarios through the voltage-power angle equation. Combined with the maximum fault voltage and the corresponding power angle control link, it accurately quantifies the fault voltage support capability of the grid-connected inverter system under current limitation.

[0028] Third, the present invention considers a method for quantifying and improving the fault voltage support capability of grid-connected inverters under the ring current limiting strategy. This method can be applied to online evaluation of the fault voltage support capability of grid-connected inverter systems under power limiting and current limiting, thereby improving the fault voltage support capability of grid-connected inverters and ensuring successful fault ride-through of new energy equipment.

[0029] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, preferred embodiments are described below in detail with reference to the accompanying drawings. Attached Figure Description

[0030] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0031] Figure 1 This is a flowchart of the method of the present invention; Figure 2 Main circuit block diagram of a dual-unit grid-connected inverter system; Figure 3 The control block diagram of the ring current limiting strategy for the grid-connected converter in a dual-machine grid-connected inverter system; Figure 4 Block diagram of the control structure of the grid-connected converter in a dual-machine grid-connected inverter system; Figure 5 Fault voltage U p about i and d 3D surface plot; Figure 6 A structural diagram of the added power angle control circuit; Figure 7(a) is a schematic diagram of the fault voltage characteristics of the front / grid converter system with the addition of the power angle control loop when the ring limiting voltage drops to 0.5 pu. Figure 7(b) is a schematic diagram of the fault voltage characteristics of the follow / grid converter system after the ring limiting voltage drops to 0.5 pu. Detailed Implementation

[0032] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0033] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0034] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0035] Where there is no conflict, the embodiments and features described in this application may be combined with each other.

[0036] Example 1 A method for quantifying and improving the fault voltage support capability in the ring current limiting of a grid-connected converter, such as... Figure 1 As shown, it includes the following steps: S1: The grid-type inverter converter adopts virtual synchronous machine control and uses voltage and current dual closed loop to achieve constant voltage mode control under normal operating conditions. In case of fault, ring current limiting is used to accurately limit the fault current.

[0037] S2: The control system of the grid-connected inverter mainly consists of three parts: power control loop, current control loop and phase-locked loop. The phase-locked loop tracks the voltage phase at the common coupling point PCC and uses it as a reference voltage phase to maintain synchronization with the grid.

[0038] S3: Derive the voltage-power angle equations at the common grid coupling point of the ring-type limit converter during a fault.

[0039] S4: Based on the voltage-power angle equation, the voltage-power angle three-dimensional surface diagram of the grid-connected converter system is obtained, thereby analyzing the maximum voltage of the common bus of the grid-connected converter under the ring current limiting strategy under different voltage drop scenarios, and adjusting the power angle control loop based on this voltage.

[0040] S5: Apply a power angle control element to the grid synchronization control stage to reduce voltage drop during grid-connected converter faults, ensuring maximum power output during faults and improving the fault voltage support capability of grid-connected converters.

[0041] Thus far, the quantification and improvement of the fault voltage support capability of the grid-type converter under ring current limiting has been successfully completed.

[0042] like Figure 2 As shown, the method provided in this embodiment is applied to a dual-machine grid-connected inverter system.

[0043] In step S1, as follows Figure 3 As shown, in a grid-connected inverter, the inverter section employs virtual synchronous machine control. Through dual closed-loop control, it exhibits voltage source characteristics. The virtual synchronous machine control equation is expressed as: (1) (2) In the formula, d is the differential operator; t For time; i The phase difference between the inverter and the power grid in a grid-connected inverter is called the power angle. oh and oh 0 represents the per-unit actual angular frequency and rated angular frequency of the grid-type inverter converter, respectively; oh base The base value of angular frequency; J This is the virtual inertia coefficient; D The damping coefficient; P ref and P c These are the active power reference value and actual active power of the grid-type inverter converter, respectively.

[0044] During a fault, the grid-connected inverter converter section of the grid-connected converter uses a ring current limiting device to limit the fault current. The specific current limiting strategy is as follows: (3) In the formula, I c This refers to the actual current of the grid-type converter; The current reference value provided for the outer voltage loop of the grid-type converter; This is the reference value for the actual input current in the inner current loop; I cmax is the current threshold of the grid-type converter; e is the base of the natural logarithm.

[0045] In step S2, as Figure 5 As shown, the main working principle of a grid-connected inverter is to achieve synchronization with the AC power grid using a phase-locked loop (PLL). Specifically, the PLL tracks the voltage phase at the point of common coupling (PCC) and uses this as a reference voltage phase to maintain synchronization with the grid, thus achieving grid "following." Overall, it exhibits current source characteristics. Its control system mainly consists of three parts: a power control loop, a current control loop, and a PLL. Based on this, the converter control structure adopted in this invention mainly includes: a voltage source converter, a PLL, a current control loop, and an equivalent power grid.

[0046] For grid-connected converters, when a three-phase symmetrical fault occurs and the positive sequence component of the grid connection point voltage is lower than 80% of the nominal voltage, the grid-connected equipment should have dynamic reactive power support capability. The dynamic reactive current increment should respond to the change in grid connection point voltage and should satisfy the following formula: (4) In the formula, △ I q This refers to the dynamic reactive current increment during grid equipment fault ride-through. K q This refers to the reactive current control coefficient during fault ride-through. U FM To match the voltage at the network equipment terminals, I N This refers to the rated current of the grid-connected equipment.

[0047] During a voltage dip, the reactive current output of grid-connected equipment to the power system should be the sum of the normal operating reactive current before the voltage dip and the dynamic reactive current increment. The maximum reactive current output capacity of the grid-connected equipment should not be less than 1.05 times its rated current. From the moment the voltage dip occurs at the grid connection point, the rise time of the dynamic reactive current of the grid-connected equipment should not exceed 60ms. From the moment the grid connection point voltage recovers to more than 90% of the nominal voltage, the grid-connected equipment should exit the dynamic reactive current increment within 40ms.

[0048] In step S3, such as Figure 4 As shown, considering the switching conditions of the ring current limiter, under the ring limiting effect, the grid-type converter is equivalent to a voltage source. U ref Connect an equivalent resistance R e Through line resistance R 1 and reactance X1. Connected to the system. Therefore, for a grid-connected converter system, during a fault, the current equation expression for the grid-connected converter section under the ring current limiting strategy can be obtained from the main circuit equation of the grid-connected converter section in the system. That is, the main circuit current equation when the grid-connected converter is in current limiting mode is as follows: (5) In the formula, U ref and U ref These are the voltage vector and magnitude at the grid-type converter, respectively; U p and U p These are the voltage reference vector and magnitude at the common bus at the parallel connection point of the grid-type converter and the grid-connected converter, respectively. f The voltage phase on the common bus; d The power angle of the grid-type converter; R e The system's equivalent resistance; R 1 represents the line resistance of the grid-type converter; X 1 represents the line reactance of the grid-type converter; j is an imaginary number.

[0049] The equivalent resistance R e The calculation process is as follows. By analyzing equation (5), the equivalent resistance can be obtained. R e The specific expression is as follows: (6) For the analysis of the grid converter system, the main circuit equation under the ring limiting condition during the fault period is as follows: (7) In the formula, U g This refers to the voltage amplitude of the power grid. I L This refers to the current between the common grid-connected coupling point of the grid-connected converter and the power grid for both grid-connected and network-type converters. Z g This refers to the impedance of the line between the common busbar of the grid-connected converter and the power grid. According to Kirchhoff's laws, ... I L The sum of the currents flowing to the coupling point of the grid-connected converter and the network-connected converter respectively satisfies the following equation: (8) In the formula, I FL For grid-connected converter line current amplitude; iTo match the phase of the line current in the grid converter, substituting equation (8) into the main circuit equation (7), the main circuit equation can be extended to the following: (9) By decomposing and transforming equation (7), we obtain the following related expressions: (10) (11) By combining equations (6), (8), and (9), we can obtain a set of equations, which gives the voltage amplitude at the common grid coupling point of the grid-connected converter and the grid-connected converter. U p Phase f and equivalent virtual resistance R e about d and i The expressions are as follows: (12) (13) (14) The voltage-phase equation on the common bus is shown in equation (12).

[0050] In step S4, the voltage-phase equation (10) on the common bus is analyzed, and the corresponding equation set (6), (8), and (9) are written in MATLAB to obtain the following results: U p and R e about i and d Numerical relationships and three-dimensional graphs, such as Figure 5 As shown, when the power angle of a given grid-type converter is... d and the phase of the line current of the grid-type converter i At that time, the voltage on the corresponding common bus can be calculated. U p .Will d and i The range is taken from -200° to 360° respectively, and then we can get U p about i and d A three-dimensional diagram. Using the voltage-power angle three-dimensional surface, the voltage on the common bus during the fault period can be obtained after the fault occurs. U p maximum value U pmaxThis value serves as a benchmark to quantify the fault voltage support capability of the grid-connected converter during a fault. Simultaneously, this benchmark value is used to apply power angle control, thereby reducing the voltage drop during a fault and improving the fault voltage support capability of the grid-connected converter.

[0051] In step S5, the voltage on the common bus is determined. U p maximum value U pmax Subsequently, in order to reduce the voltage on the common bus... U p To mitigate voltage drops and enhance the fault voltage support capability of grid-connected power electronic devices, a new control element has been added to the synchronization stage of the grid-connected converter control structure, such as... Figure 6 As shown, the main principle of the control loop is as follows: (15) in d c The output of this control loop for the power angle d negative feedback, k P The proportional coefficient for the control link, V p To ensure the common voltage on the common bus of the grid-connected converter and the grid-connected converter, V max for V p The maximum value.

[0052] The main principle of this control loop is to apply feedback to the existing power angle. When the existing common bus voltage... U p After a voltage drop occurs, it will be lower than the voltage on the common bus. U p maximum value U max The difference between the two is compared with the proportionality coefficient. k P Multiplying them together will give the output. d c When the original power angle is combined with the negative feedback output of the additional control circuit, d c The power angle of the grid-type converter d Then it will decrease, from the previously obtained voltage. U p about i and d A 3D surface plot, voltage on the common bus. U pThis will reduce voltage drops, thereby maximizing the fault voltage support capability of grid-connected power electronic devices.

[0053] As shown in Figures 7(a) and 7(b), by adjusting the angle of force... i By adding a power angle control element, the power angle can be controlled. i By keeping the voltage within an appropriate range, the voltage drop of the grid-connected converter can be controlled under different current limiting strategies, ensuring the normal operation of the system and improving its fault voltage support capability under different voltage drop scenarios, thereby significantly improving the low voltage ride-through capability of the new power system.

[0054] Example 2 A fault voltage support capability quantification system for grid-connected converters includes: The grid-connected inverter converter control module is configured to use virtual synchronous machine control for the grid-connected inverter converter. Under normal operating conditions, it uses voltage and current dual closed loops to achieve constant voltage mode control, and under fault conditions, it uses ring current limiting to limit the fault current. The grid-connected inverter converter module is configured to track the voltage phase of the common coupling point through a phase-locked loop as a reference voltage phase to maintain synchronization with the grid. The dynamic reactive current increment is adjusted through a current control loop to respond to changes in the grid connection point voltage. The voltage-power angle equation derivation module is configured to derive the voltage-power angle equation at the common grid-connected coupling point of the grid-connected converter under ring current limit during a fault, in conjunction with the switching conditions of the ring current limiter. The common bus maximum voltage calculation module is configured to obtain a three-dimensional voltage-power angle surface diagram of the grid-connected converter system based on the voltage-power angle equation, analyze the maximum voltage of the common bus under the ring current limiting strategy of the grid-connected converter under different voltage drop scenarios, and adjust the power angle control loop based on this voltage. The synchronization control module is configured to apply a power angle control element to the grid synchronization control stage, reducing voltage drops during grid-connected converter faults and ensuring maximum power output during faults.

[0055] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of one or more computer-usable storage media (including, but not limited to, disk storage, etc.) containing computer-usable program code. CD - ROM It takes the form of a computer program product implemented on (such as optical memory, etc.).

[0056] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure one One or more processes and / or boxes Figure one A device that provides the functions specified in one or more boxes.

[0057] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure one One or more processes and / or boxes Figure one The function specified in one or more boxes.

[0058] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure one One or more processes and / or boxes Figure one The steps of the function specified in one or more boxes.

[0059] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made by those skilled in the art without creative effort within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for quantifying the fault voltage support capability of a grid-connected converter, characterized in that, Includes the following steps: The grid-type inverter converter adopts virtual synchronous machine control. Under normal operating conditions, it uses voltage and current dual closed loop to achieve constant voltage mode control, and under fault conditions, it uses ring current limiting to limit fault current. The grid-connected inverter converter tracks the voltage phase at the common coupling point through a phase-locked loop as a reference voltage phase to maintain synchronization with the grid. The dynamic reactive current increment is adjusted through a current control loop to respond to changes in the grid connection point voltage. Based on the switching conditions of the ring current limiter, the voltage-power angle equation at the common grid coupling point of the ring-limited grid-connected converter during the fault period is derived. Based on the voltage-power angle equation, a three-dimensional voltage-power angle surface diagram of the grid converter system is obtained. The maximum voltage of the common bus of the grid converter under the ring current limiting strategy under different voltage drop scenarios is analyzed, and the power angle control loop is adjusted based on this voltage. By applying a power angle control element to the grid synchronization control stage, the voltage drop during faults of grid-connected converters is reduced, ensuring that grid-connected converters can guarantee maximum power output during faults.

2. The method for quantifying the fault voltage support capability of a grid-connected converter as described in claim 1, characterized in that, The process of using virtual synchronous machine control in a grid-connected inverter converter includes: The grid-connected inverter converter section in a grid-connected inverter uses virtual synchronous machine control, employing dual closed-loop control to make it exhibit voltage source characteristics. The virtual synchronous machine control equation is expressed as: Where d is the differential operator; t For time; θ The phase difference between the inverter and the power grid in a grid-connected inverter is called the power angle. ω and ω 0 represents the per-unit actual angular frequency and rated angular frequency of the grid-type inverter converter, respectively; ω base The base value of angular frequency; J This is the virtual inertia coefficient; D The damping coefficient; P ref and P c These are the active power reference value and actual active power of the grid-type inverter converter, respectively.

3. The method for quantifying the fault voltage support capability of a grid-connected converter as described in claim 1, characterized in that, The process of using a ring current limiter to restrict fault current during a fault includes: During a fault, the grid-connected inverter converter section in a grid-connected converter uses a ring current limiter to limit the fault current. The specific current limiting strategy is as follows: In the formula, I c This refers to the actual current of the grid-type converter; The current reference value provided for the outer voltage loop of the grid-type converter; This is the reference value for the actual input current in the inner current loop; I cmax is the current threshold of the grid-type converter; e is the base of the natural logarithm.

4. The method for quantifying the fault voltage support capability of a grid-connected converter as described in claim 1, characterized in that, For grid-connected converters, when a three-phase symmetrical fault occurs and the positive-sequence component of the grid connection point voltage is lower than a predetermined percentage of the nominal voltage, the grid-connected equipment should have dynamic reactive power support capability. The dynamic reactive current increment should respond to the change in grid connection point voltage and satisfy the following formula: In the formula, △ I q This refers to the dynamic reactive current increment during grid equipment fault ride-through. K q This refers to the reactive current control coefficient during fault ride-through. U FM To match the voltage at the network equipment terminals, I N This refers to the rated current of the grid-connected equipment.

5. The method for quantifying the fault voltage support capability of a grid-connected converter as described in claim 1, characterized in that, For grid-connected converters, during voltage dips, the reactive current output by the grid-connected equipment to the power system is the sum of the reactive current output during normal operation before the voltage dip and the dynamic reactive current increment. The maximum output capacity of the reactive current of the grid-connected equipment is not less than a set multiple of the rated current of the grid-connected equipment. From the moment the voltage dip occurs at the grid connection point, the rise time of the dynamic reactive current of the grid-connected equipment is not greater than a set threshold. From the moment the voltage at the grid connection point recovers to more than 90% of the nominal voltage, the grid-connected equipment exits the dynamic reactive current increment within a set time.

6. The method for quantifying the fault voltage support capability of a grid-connected converter as described in claim 1, characterized in that, The process of deriving the voltage-power angle equation at the common grid coupling point of the ring-type converter under ring limiting during a fault, based on the switching conditions of the ring current limiter, includes: Considering the switching conditions of the ring current limiter, under the ring limiting effect, the grid-type converter is equivalent to a voltage source. U ref Connect an equivalent resistance R e Through line resistance R 1 and reactance X 1. Connected to the system, during a fault, the current equation expression for the grid-type converter section under the ring current limiting strategy is obtained through the main circuit equation of the grid-type converter section in the system. That is, the main circuit current equation when the grid-type converter is in current limiting mode is as follows: In the formula, U ref and U ref These are the voltage vector and magnitude at the grid-type converter, respectively; U p and U p These are the voltage reference vector and magnitude at the common bus at the parallel connection point of the grid-type converter and the grid-connected converter, respectively. φ The voltage phase on the common bus; δ The power angle of the grid-type converter; R e The system's equivalent resistance; R 1 represents the line resistance of the grid-type converter; X 1 represents the line reactance of the grid-type converter; j is an imaginary number; For the analysis of the grid converter system, the main circuit equation under the ring limiting condition during the fault period is: In the formula, U g This refers to the voltage amplitude of the power grid. I L This refers to the current between the common grid-connected coupling point of the grid-connected converter and the power grid for both grid-connected and network-type converters. Z g To determine the impedance of the line between the common busbar of the grid-connected converter and the power grid; according to Kirchhoff's laws, I L The sum of the currents flowing to the coupling point of the grid-connected converter and the grid-connected converter is used to obtain the voltage amplitude at the common grid-connected coupling point of the grid-connected converter and the grid-connected converter. U p Phase φ and equivalent virtual resistance R e about δ and θ The expressions are as follows: ； ； 。 7. The method for quantifying the fault voltage support capability of a grid-connected converter as described in claim 1, characterized in that, The process of obtaining the three-dimensional voltage-power angle surface diagram of a grid-connected converter system based on the voltage-power angle equation includes: given the power angle of the grid-connected converter... δ and the phase of the line current of the grid-type converter θ At that time, calculate the voltage on the corresponding common bus. U p ,Will δ and θ The ranges are taken from -200° to 360°, resulting in... U p about θ and δ A 3D diagram.

8. The method for quantifying the fault voltage support capability of a grid-connected converter as described in claim 1, characterized in that, The process of analyzing the maximum common bus voltage under different voltage drop scenarios using a ring current limiting strategy in grid-type converters, and adjusting the power angle control loop based on this voltage, includes: determining the voltage on the common bus during the fault period after a fault occurs using a three-dimensional voltage-power angle surface. U p maximum value U pmax This value serves as a benchmark to quantify the fault voltage support capability of the grid-connected converter during a fault. Simultaneously, based on this benchmark value, a power angle control loop is applied to reduce the drop in fault voltage and improve the fault voltage support capability of the grid-connected converter.

9. A method for quantifying the fault voltage support capability of a grid-connected converter as described in claim 1, characterized in that, The process of applying the power angle control element to the grid synchronization control element includes: determining the voltage on the common bus. U p maximum value U pmax Subsequently, a new control loop was added to the synchronization loop of the grid converter control structure. The control loop is as follows: in, δ c The output of this control loop for the power angle δ negative feedback, k P The proportional coefficient for the control link, V p To ensure the common voltage on the common bus of the grid-connected converter and the grid-connected converter, V max for V p The maximum value.

10. A fault voltage support capability quantification system for grid-connected converters, characterized in that, include: The grid-connected inverter converter control module is configured to use virtual synchronous machine control for the grid-connected inverter converter. Under normal operating conditions, it uses voltage and current dual closed loops to achieve constant voltage mode control, and under fault conditions, it uses ring current limiting to limit the fault current. The grid-connected inverter converter module is configured to track the voltage phase of the common coupling point through a phase-locked loop as a reference voltage phase to maintain synchronization with the grid. The dynamic reactive current increment is adjusted through a current control loop to respond to changes in the grid connection point voltage. The voltage-power angle equation derivation module is configured to derive the voltage-power angle equation at the common grid-connected coupling point of the grid-connected converter under ring current limit during a fault, in conjunction with the switching conditions of the ring current limiter. The common bus maximum voltage calculation module is configured to obtain a three-dimensional voltage-power angle surface diagram of the grid-connected converter system based on the voltage-power angle equation, analyze the maximum voltage of the common bus under the ring current limiting strategy of the grid-connected converter under different voltage drop scenarios, and adjust the power angle control loop based on this voltage. The synchronization control module is configured to apply a power angle control element to the grid synchronization control stage, reducing voltage drops during grid-connected converter faults and ensuring maximum power output during faults.

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