Grid-connected inverter low voltage ride through control method and system under asymmetric fault

Abstract

The invention belongs to the technical field of power systems, and provides a grid-connected inverter low-voltage ride-through control method and system under an asymmetric fault, and the method comprises the steps: obtaining a real-time grid-connected point voltage, and judging whether a power grid is in an asymmetric fault or not; judging whether the grid-connected point voltage is qualified or not, if not, generating a current instruction by a low-voltage ride-through module by taking positive and negative sequence voltage support as a target; judging whether current out-of-limit and direct-current side voltage oscillation out-of-limit occur in the generated current instruction, and if the current out-of-limit does not occur and the direct-current side voltage oscillation limit is met, normally generating a positive-sequence and negative-sequence reactive current instruction; if current out-of-limit occurs or a direct current side voltage oscillation limit is not satisfied, generating a positive sequence reactive current instruction, and adjusting a negative sequence reactive current instruction; and on the basis of a grid-connected inverter current inner ring, generating a gate control signal of the grid-connected inverter by using the generated current instruction or the positive sequence reactive current instruction and the negative sequence reactive current instruction, thereby completing low-voltage ride-through control of the grid-connected inverter under the asymmetric fault.

Description

Technical Field

[0001] The present invention belongs to the technical field of power systems, and in particular relates to a low voltage ride-through control method and system for a grid-connected inverter under an asymmetric fault. Background Art

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

[0003] As the penetration rate of new energy sources continues to increase, the number of power electronic converters connected to the power grid is increasing. The control system of power electronic devices is susceptible to voltage sags, which can cause the grid-connected system to be disconnected from the grid and reduce the safety and reliability of the system. Grid-connected inverters should have low voltage ride-through capabilities and be able to provide certain voltage support for the grid in the event of a fault. Most voltage drops in power systems are caused by asymmetric faults. Therefore, studying the low voltage ride-through control of grid-connected inverters under asymmetric fault conditions is of great significance to the safe and reliable operation of power systems.

[0004] According to the inventors' understanding, the current low voltage ride-through control method for grid-connected inverters under asymmetric faults has the following problems:

[0005] In the current research on low voltage ride-through control methods for grid-connected inverters under asymmetric faults, most studies focus on improving the positive sequence voltage at the grid connection point, ignoring the impact of the negative sequence voltage on the grid; at the same time, during an asymmetric fault in the grid, the active power output by the grid-connected inverter has a double frequency oscillation, resulting in an oscillation of the same frequency on the DC side voltage, which can easily damage the DC side capacitor and affect its long-term life. Therefore, the effective control method for the grid-connected inverter should take into account the positive and negative sequence voltages supporting the grid connection point, while suppressing the oscillation of the DC side voltage to achieve a multi-objective control effect. Summary of the invention

[0006] To solve the above problems, the present invention proposes a low voltage ride-through control method and system for a grid-connected inverter under an asymmetric fault. Based on the influence of the positive and negative sequence currents of the grid-connected inverter on the grid side during the asymmetric fault, the overcurrent output by the inverter is limited and the voltage oscillation on the DC side is suppressed to ensure accurate positive and negative sequence voltage support at the grid connection point.

[0007] According to some embodiments, a first solution of the present invention provides a low voltage ride-through control method for a grid-connected inverter under an asymmetric fault, which adopts the following technical solution:

[0008] A low voltage ride-through control method for a grid-connected inverter under an asymmetric fault, comprising:

[0009] Obtain the real-time grid connection point voltage to determine whether the grid is in an asymmetric fault;

[0010] Determine whether the obtained grid-connected point voltage is qualified. If not, enter the low voltage ride-through module of the grid-connected inverter and generate current instructions with positive and negative sequence voltage support as the target;

[0011] Determine whether the generated current command exceeds the current limit and whether the DC side voltage oscillation exceeds the limit. If the current does not exceed the limit and the DC side voltage oscillation limit is met, the positive sequence reactive current command and the negative sequence reactive current command are generated normally; if the current exceeds the limit or the DC side voltage oscillation limit is not met, the positive sequence reactive current command is generated first, and then the negative sequence reactive current command is adjusted;

[0012] Based on the current inner loop of the grid-connected inverter, the generated current command or the positive-sequence reactive current command and the negative-sequence reactive current command are used to generate a gating signal for the grid-connected inverter, thereby completing the low voltage ride-through control of the grid-connected inverter under an asymmetric fault.

[0013] As a further technical limitation, when the generated current command does not exceed the current limit, it is determined whether the current command is within a reasonable range of DC side oscillation. Specifically, when the DC side voltage oscillation amplitude calculated by the current command provided by the grid-connected inverter exceeds the set value, the current command issued by the grid-connected inverter does not meet the DC side voltage oscillation limit.

[0014] As a further technical limitation, in the process of determining whether the generated current command exceeds the current limit, the current inner loop of the grid-connected inverter is set to the normal operating area. If the current command transmitted to the current inner loop exceeds the normal operating area, it is determined that the current command does not meet the output current limit, that is, the current exceeds the limit.

[0015] As a further technical limitation, when a negative sequence component appears in the obtained grid connection point voltage, it is determined that the grid is in an asymmetric fault state.

[0016] As a further technical limitation, when the obtained grid-connected point voltage is unqualified, the grid-connected inverter adopts constant power control.

[0017] As a further technical limitation, when the positive sequence voltage at the grid connection point drops below 0.9 pu, the grid connection point voltage is unqualified.

[0018] According to some embodiments, a second solution of the present invention provides a low voltage ride-through control system for a grid-connected inverter under an asymmetric fault, which adopts the following technical solution:

[0019] A low voltage ride-through control system for a grid-connected inverter under an asymmetric fault, comprising:

[0020] An acquisition module is configured to acquire the real-time grid connection point voltage and determine whether the grid is in an asymmetric fault;

[0021] A first judgment module is configured to judge whether the obtained grid-connected point voltage is qualified. If it is unqualified, it enters the low voltage ride-through module of the grid-connected inverter and generates a current instruction with positive and negative sequence voltage support as the target;

[0022] The second judgment module is configured to judge whether the generated current command exceeds the current limit and whether the DC side voltage oscillation exceeds the limit. If the current does not exceed the limit and the DC side voltage oscillation limit is met, the positive sequence reactive current command and the negative sequence reactive current command are normally generated; if the current exceeds the limit or the DC side voltage oscillation limit is not met, the positive sequence reactive current command is preferentially generated, and then the negative sequence reactive current command is adjusted;

[0023] The control module is configured to generate a gating signal for the grid-connected inverter based on the generated current instruction or positive-sequence reactive current instruction and negative-sequence reactive current instruction of the grid-connected inverter current inner loop, so as to complete the low voltage ride-through control of the grid-connected inverter under asymmetric fault.

[0024] According to some embodiments, a third solution of the present invention provides a computer-readable storage medium, which adopts the following technical solution:

[0025] A computer-readable storage medium stores a program, which, when executed by a processor, implements the steps of the low voltage ride-through control method for a grid-connected inverter under an asymmetric fault as described in the first solution of the present invention.

[0026] According to some embodiments, a fourth solution of the present invention provides an electronic device, which adopts the following technical solution:

[0027] An electronic device comprises a memory, a processor and a program stored in the memory and running on the processor, wherein when the processor executes the program, the steps in the low voltage ride-through control method of a grid-connected inverter under an asymmetric fault as described in the first solution of the present invention are implemented.

[0028] According to some embodiments, a fifth solution of the present invention provides a computer program product, which adopts the following technical solution:

[0029] A computer program product includes software codes, wherein the program in the software codes executes the steps in the low voltage ride-through control method for a grid-connected inverter under an asymmetric fault as described in the first solution of the present invention.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] The present invention takes into account the maximum current limit of the inverter output and the DC side voltage oscillation limit during the low-power run-through period, based on the impact of the positive and negative sequence currents of the grid-connected inverter on the grid side during an asymmetric fault, and simultaneously issues positive and negative sequence current instructions to limit the overcurrent of the inverter output, suppress the DC side voltage oscillation, and ensure accurate positive and negative sequence voltage support at the grid connection point. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The drawings in the specification that constitute a part of this embodiment are used to provide a further understanding of this embodiment. The schematic embodiments of this embodiment and their descriptions are used to explain this embodiment and do not constitute improper limitations on this embodiment.

[0033] Figure 1 This is a flow chart of a low voltage ride-through control method for a grid-connected inverter under an asymmetric fault in Embodiment 1 of the present invention;

[0034] Figure 2 It is a structural schematic diagram of a low voltage ride-through control method for a grid-connected inverter under an asymmetric fault in Embodiment 1 of the present invention;

[0035] Figure 3 This is a result diagram showing that neither the current limit nor the DC side voltage limit is exceeded when the positive and negative sequence voltage support is achieved in the first embodiment of the present invention;

[0036] Figure 4 This is a diagram of adjustment results when the current limit is exceeded but the DC side voltage does not exceed the limit value when the positive and negative sequence voltage support is achieved in the first embodiment of the present invention;

[0037] Figure 5 This is a diagram of adjustment results when the DC side voltage limit is exceeded but the output current does not exceed the limit value when the positive and negative sequence voltage support is achieved in the first embodiment of the present invention;

[0038] Figure 6 This is a diagram of adjustment results when both the DC side voltage limit and the current limit are exceeded when the positive and negative sequence voltage support is achieved in the first embodiment of the present invention;

[0039] Figure 7 This is a structural block diagram of a low voltage ride-through control system for a grid-connected inverter under an asymmetric fault in the second embodiment of the present invention. DETAILED DESCRIPTION

[0040] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0041] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present application belongs.

[0042] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0043] In the present invention, terms such as "upper", "lower", "left", "right", "front", "back", "vertical", "horizontal", "side", "bottom", etc. indicate directions or positional relationships based on the directions or positional relationships shown in the accompanying drawings. They are relational words determined only for the convenience of describing the structural relationships of the various parts or elements of the present invention, and do not specifically refer to any part or element in the present invention and should not be understood as limitations on the present invention.

[0044] In the present invention, terms such as "fixed connection", "connected", "connection", etc. should be understood in a broad sense, indicating that it can be fixedly connected, integrally connected or detachably connected; it can be directly connected or indirectly connected through an intermediate medium. For relevant scientific research or technical personnel in this field, the specific meanings of the above terms in the present invention can be determined according to specific circumstances, and they cannot be understood as limitations on the present invention.

[0045] In the absence of conflict, the embodiments of the present invention and the features of the embodiments may be combined with each other.

[0046] Embodiment 1

[0047] Embodiment 1 of the present invention introduces a low voltage ride-through control method for a grid-connected inverter under an asymmetric fault.

[0048] like Figure 1 A low voltage ride-through control method for a grid-connected inverter under an asymmetric fault is shown, comprising the following steps:

[0049] Step S01: monitor the grid connection point voltage in real time to determine whether the grid connection point voltage is qualified; if it is unqualified, enter the grid-connected inverter low voltage ride-through module and enter step S02;

[0050] Step S02: determine whether the current command issued by the grid-connected inverter meets the output current limit. If not, enter the positive sequence voltage priority control module. If it meets, enter step S03;

[0051] Step S03: determine whether the current command issued by the grid-connected inverter meets the DC side voltage oscillation limit. If not, enter the positive sequence voltage priority control module. If yes, enter step S04.

[0052] Step S04: according to the current over-limit and DC side voltage over-limit situations determined in step S02 and step S03, four scenarios are divided and current instructions are generated and adjusted, and then step S05 is entered;

[0053] Step S05: inputting the generated current command into the inverter current inner loop, and the current inner loop generates a gating signal for the inverter, thereby realizing low voltage ride-through control under asymmetric faults.

[0054] like Figure 2 As shown, in this embodiment, during an asymmetric fault, the controller first uses the sequence component extraction and voltage amplitude calculation functions to decompose the voltage and current output by the grid-connected point into positive and negative sequence components and calculate their amplitudes. The current reference value calculation function is the low voltage ride-through method proposed in the invention. The current reference value generated by the current reference value calculation function is input into the current inner loop of the grid-connected inverter to generate a gating signal for the input inverter, thereby realizing low voltage ride-through control during an asymmetric fault.

[0055] As one or more implementation modes, in step S01, a decoupled double synchronous reference frame phase-locked loop (Decoupled Double Synchronous Reference Frame Phase-Locked Loop, DDSRF-PLL) is used to collect the positive and negative sequence voltages of the grid-connected point, and monitor in real time whether they drop; under an asymmetric fault, that is, when a negative sequence component appears in the grid-connected point voltage during the fault period, if the positive sequence voltage of the grid-connected point drops below 0.9pu, the grid-connected point voltage is determined to be unqualified, and the grid-connected inverter enters a low voltage ride-through control mode.

[0056] As one or more implementation modes, in step S02, when the system determines that the grid-connected point voltage is unqualified, the grid-connected inverter automatically enters the low voltage ride-through control mode. Based on the positive and negative sequence conversion under DDSRF, when an asymmetric fault occurs, the positive sequence relationship between the grid-side voltage, the grid-connected point voltage and the grid-connected inverter output current can be written as:

[0057]

[0058] Among them, v + Represents the positive sequence component of the grid connection point voltage; i + Represents the positive sequence component of the inverter output current; is the positive sequence component of the grid-side voltage; L g With R g are the inductance and resistance values ​​in the circuit; t represents the time.

[0059] Similarly, the negative sequence relation can be written as:

[0060]

[0061] Among them, v - Represents the negative sequence component of the grid connection point voltage; i - Represents the negative sequence component of the inverter output current; is the negative sequence component of the grid-side voltage.

[0062] Transforming the above formula, we can get:

[0063]

[0064] in, is the amplitude of the positive sequence component of the grid-side voltage; V + is the amplitude of the positive sequence component of the grid connection point voltage; ω is the angular frequency at the power frequency of the grid; It is the active and reactive positive sequence current command output by the inverter.

[0065]

[0066] in, is the amplitude of the positive sequence component of the grid-side voltage; V - is the amplitude of the negative sequence component of the grid connection point voltage;

[0067] It is the active and reactive negative sequence current command output by the inverter.

[0068] At this time, the three-phase voltage at the grid connection point can be written as:

[0069]

[0070] Among them, V a 、V b 、V c is the amplitude of the three-phase voltage of a, b, c at the grid connection point; is the phase angle between positive and negative sequence voltages.

[0071] When an asymmetric fault occurs, in order to make the system operate within a safe range, the phase voltage at the grid connection point should be specified with maximum and minimum limits. At the same time, in order to achieve the goal of increasing the positive sequence voltage at the grid connection point and reducing the negative sequence voltage at the grid connection point, the maximum and minimum phase voltages should be relatively close. At this time, the maximum and minimum phase voltages can be expressed as:

[0072]

[0073] Among them, V max With V min are the maximum and minimum values ​​of the grid-connected point voltage respectively; γ max With γ min is the maximum and minimum value of the cosine of the phase angle difference between the positive and negative sequence voltages of the three phases, which can be expressed as:

[0074]

[0075] Solving the above equation can improve the positive sequence voltage of the grid connection point and reduce The low grid-connected point negative sequence voltage is the target, and the reactive current command output by the grid-connected inverter is:

[0076]

[0077]

[0078] in, and They are respectively the reference values ​​of the positive and negative sequence voltages at the grid connection point; they can be specifically expressed as:

[0079]

[0080] in,

[0081] At this time, the reactive current command output by the inverter is generated.

[0082] As one or more implementation modes, in step S02, the three-phase current output by the inverter can be expressed by the positive and negative sequence current in DDSRF as follows:

[0083]

[0084] Among them, I a ,I b ,I c I is the current amplitude of three phases a, b and c output by the grid-connected inverter; + with I - is the positive and negative sequence current amplitude output by the grid-connected inverter; It is the positive and negative sequence current angle output by the grid-connected inverter.

[0085] The three-phase current should meet the following requirements:

[0086] max{I a ,I b ,I c}≤I max ;

[0087] Among them, I max It is the maximum value of the inverter output phase current.

[0088] If the requirements are not met, the positive-sequence reactive current is output with the maximum positive-sequence voltage as the target. The positive-sequence reactive current instruction is the same as step S02. At the same time, the negative-sequence reactive current instruction is adjusted to meet the inverter output current output limit; if the requirements are met, enter step S04.

[0089] As one or more implementation modes, in step S03, in an asymmetric fault, the output power oscillation of the grid-connected inverter may cause the oscillation of the DC side voltage. The relationship between the voltage oscillation and the active power oscillation may be expressed as:

[0090]

[0091] in, is the amplitude of the oscillation term on the DC side; is the oscillation amplitude of the grid-connected inverter output power; C dc is the DC side capacitance value; V dc is the DC side voltage value.

[0092] In order to stabilize the DC side voltage and reduce the DC side voltage oscillation, the active power oscillation output by the inverter under asymmetric faults should be suppressed. In DDSRF, the oscillation amplitude of the inverter output active power can be expressed as:

[0093]

[0094] If the DC side voltage oscillation amplitude calculated by the current command in step S02 does not meet the requirements, firstly, the positive sequence reactive current reference value is obtained with the goal of increasing the positive sequence voltage, and then the reactive current command is calculated according to the oscillation limit requirement, and then the process proceeds to step S04. If the DC side voltage oscillation amplitude calculated by the current command in step S02 meets the requirements, the process proceeds to step S05.

[0095] Based on the current limit and the DC side voltage oscillation limit, the following four scenarios can be summarized: Scenario 1 is that the current command calculated by step S02 makes the inverter not exceed the current limit value nor the DC side voltage limit value during the low voltage ride-through period. At this time, the current command of the inverter is:

[0096]

[0097] Scenario 2 is that the current command calculated in step S02 causes the inverter to exceed the current limit but not exceed the DC side voltage limit during low voltage ride-through. At this time, the current command of the inverter is:

[0098]

[0099] Among them, ε max It is the maximum cosine value of the angle between the positive and negative sequence currents.

[0100] Scenario 3 is that the current command calculated by step S02 makes the inverter not exceed the current limit but exceed the DC side voltage limit during the low voltage ride-through period. At this time, the current command of the inverter is:

[0101]

[0102] Scenario 4 is that the current command calculated in step S02 causes the inverter to exceed both the current limit and the DC side voltage limit during low voltage ride-through. At this time, the current command of the inverter is:

[0103]

[0104] in, is the oscillation limit of the DC side voltage; P is the active power base value output by the grid-connected inverter; go to step S05.

[0105] Based on the current commands in the above four scenarios, the generated current commands are input into the inverter current inner loop, and the inverter gating signal is generated by the current inner loop, thereby realizing low voltage ride-through control under asymmetric faults.

[0106] Figure 3 The simulation results of the proposed method under asymmetric fault conditions, where neither the current limit nor the DC side voltage limit is exceeded, show the comparison results with the traditional positive sequence voltage control method (BPSC). Figure 3 (a) is the phase voltage result diagram, which shows that the proposed low voltage ride-through method can control the maximum phase voltage to 0.95pu and the minimum phase voltage to 0.75pu. Figure 4 As shown in (c), the positive sequence voltage V + From 0.86pu to 0.82pu, the negative sequence voltage V - From 0.16pu to 0.13pu. Compared with BPSC, the proposed multi-objective low voltage ride-through control only sacrifices 0.46% of the positive sequence voltage support capability, but improves 25% of the negative sequence voltage support capability.

[0107] Figure 4 The simulation results of the proposed method in the current limit scenario are shown in Figure 2. Figure 4 As shown in (b), the inverter overcurrent is effectively suppressed and the inverter capacity is fully utilized. Figure 5 The proposed method is effective in suppressing the DC side voltage oscillation during asymmetric faults. Figure 5 As shown in (c) in the figure, the DC side voltage oscillation is well suppressed to avoid damaging the capacitors in the DC link. Figure 5 (a) and Figure 5 As shown in (b), before and after adjusting the current reference value, the phase current does not exceed the current limit.

[0108] like Figure 6 As shown, to achieve the maximum positive and negative sequence voltage support effect, the phase current exceeds the current limit, and the DC link voltage oscillates beyond the limit, as shown in Figure 6 (a) and Figure 6 By adjusting the current reference value, Figure 6 (b) It can be seen that the maximum phase current is equal to the current limit This indicates that the inverter capacity has been fully utilized. Figure 6 As shown in (c) in FIG. 5 , after applying the modified current reference value, the DC side voltage oscillation can be effectively suppressed.

[0109] This embodiment takes into account the maximum current limit of the inverter output and the DC side voltage oscillation limit during the low-voltage breakover period, and based on the impact of the positive and negative sequence currents of the grid-connected inverter on the grid side during an asymmetric fault, simultaneously issues positive and negative sequence current instructions to limit the overcurrent of the inverter output, suppress the DC side voltage oscillation, and ensure accurate positive and negative sequence voltage support at the grid connection point.

[0110] Embodiment 2

[0111] Embodiment 2 of the present invention introduces a low voltage ride-through control system for a grid-connected inverter under an asymmetric fault.

[0112] like Figure 7 A low voltage ride-through control system for a grid-connected inverter under an asymmetric fault is shown, comprising:

[0113] An acquisition module is configured to acquire the real-time grid connection point voltage and determine whether the grid is in an asymmetric fault;

[0114] A first judgment module is configured to judge whether the obtained grid-connected point voltage is qualified. If it is unqualified, it enters the low voltage ride-through module of the grid-connected inverter and generates a current instruction with positive and negative sequence voltage support as the target;

[0115] The second judgment module is configured to judge whether the generated current command exceeds the current limit and whether the DC side voltage oscillation exceeds the limit. If the current does not exceed the limit and the DC side voltage oscillation limit is met, the positive sequence reactive current command and the negative sequence reactive current command are normally generated; if the current exceeds the limit or the DC side voltage oscillation limit is not met, the positive sequence reactive current command is preferentially generated, and then the negative sequence reactive current command is adjusted;

[0116] The control module is configured to generate a gating signal for the grid-connected inverter based on the generated current instruction or positive-sequence reactive current instruction and negative-sequence reactive current instruction of the grid-connected inverter current inner loop, so as to complete the low voltage ride-through control of the grid-connected inverter under asymmetric fault.

[0117] The detailed steps are the same as those of the low voltage ride-through control method for the grid-connected inverter under an asymmetric fault provided in the first embodiment, and will not be repeated here.

[0118] Embodiment 3

[0119] Embodiment 3 of the present invention provides a computer-readable storage medium.

[0120] A computer-readable storage medium stores a program thereon, which, when executed by a processor, implements the steps in the low voltage ride-through control method for a grid-connected inverter under an asymmetric fault as described in Embodiment 1 of the present invention.

[0121] The detailed steps are the same as those of the low voltage ride-through control method for the grid-connected inverter under an asymmetric fault provided in the first embodiment, and will not be repeated here.

[0122] Embodiment 4

[0123] A fourth embodiment of the present invention provides an electronic device.

[0124] An electronic device comprises a memory, a processor and a program stored in the memory and running on the processor, wherein when the processor executes the program, the steps in the low voltage ride-through control method of a grid-connected inverter under an asymmetric fault as described in Embodiment 1 of the present invention are implemented.

[0125] The detailed steps are the same as those of the low voltage ride-through control method for the grid-connected inverter under an asymmetric fault provided in the first embodiment, and will not be repeated here.

[0126] Embodiment 5

[0127] Embodiment 5 of the present invention provides a computer program product.

[0128] A computer program product includes software codes, wherein the program in the software codes executes the steps in the low voltage ride-through control method for a grid-connected inverter under an asymmetric fault as described in the first embodiment of the present invention.

[0129] The detailed steps are the same as those of the low voltage ride-through control method for the grid-connected inverter under an asymmetric fault provided in the first embodiment, and will not be repeated here.

[0130] The above description is only a preferred embodiment of the present embodiment and is not intended to limit the present embodiment. For those skilled in the art, the present embodiment may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present embodiment shall be included in the protection scope of the present embodiment.

Claims

1. A method for controlling low voltage ride-through of a grid-connected inverter under an asymmetric fault, characterized in that: include: Obtain the real-time grid connection point voltage to determine whether the grid is in an asymmetric fault; Determine whether the obtained grid-connected point voltage is qualified. If not, enter the low voltage ride-through module of the grid-connected inverter and generate current instructions with positive and negative sequence voltage support as the target; Determine whether the generated current command exceeds the current limit and whether the DC side voltage oscillation exceeds the limit. If the current does not exceed the limit and the DC side voltage oscillation limit is met, then generate the positive sequence reactive current command and the negative sequence reactive current command normally. If the current exceeds the limit or does not meet the DC side voltage oscillation limit, the positive sequence reactive current command is generated first, and then the negative sequence reactive current command is adjusted; Based on the current inner loop of the grid-connected inverter, the generated current command or the positive-sequence reactive current command and the negative-sequence reactive current command are used to generate a gating signal for the grid-connected inverter, thereby completing the low voltage ride-through control of the grid-connected inverter under an asymmetric fault.

2. A method for controlling low voltage ride-through of a grid-connected inverter under an asymmetric fault as claimed in claim 1, characterized in that: When the generated current command does not exceed the current limit, it is determined whether the current command is within a reasonable range of DC side oscillation. Specifically, when the DC side voltage oscillation amplitude calculated from the current command provided by the grid-connected inverter exceeds the set value, the current command issued by the grid-connected inverter does not meet the DC side voltage oscillation limit.

3. A method for controlling low voltage ride-through of a grid-connected inverter under an asymmetric fault as claimed in claim 1, characterized in that: In the process of judging whether the generated current command exceeds the current limit, the current inner loop of the grid-connected inverter is set to the normal working area. If the current command transmitted to the current inner loop exceeds the normal working area, it is judged that the current command does not meet the output current limit, that is, the current exceeds the limit.

4. A method for controlling low voltage ride-through of a grid-connected inverter under an asymmetric fault as claimed in claim 1, characterized in that: When a negative sequence component appears in the obtained grid connection point voltage, it is determined that the power grid is in an asymmetric fault state.

5. A method for controlling low voltage ride-through of a grid-connected inverter under an asymmetric fault as claimed in claim 1, characterized in that: When the obtained grid-connected point voltage is unqualified, the grid-connected inverter adopts constant power control.

6. A method for controlling low voltage ride-through of a grid-connected inverter under an asymmetric fault as claimed in claim 1, characterized in that: When the positive sequence voltage at the grid connection point drops below 0.9pu, the grid connection point voltage is unqualified.

7. A low voltage ride-through control system for a grid-connected inverter under asymmetric fault, characterized in that: include: An acquisition module is configured to acquire the real-time grid connection point voltage and determine whether the grid is in an asymmetric fault; A first judgment module is configured to judge whether the obtained grid-connected point voltage is qualified. If it is unqualified, it enters the low voltage ride-through module of the grid-connected inverter and generates a current instruction with positive and negative sequence voltage support as the target; A second judgment module is configured to judge whether the generated current command has a current over-limit and whether a DC side voltage oscillation has exceeded the limit, and if the current does not exceed the limit and the DC side voltage oscillation limit is met, a positive sequence reactive current command and a negative sequence reactive current command are normally generated; If the current exceeds the limit or does not meet the DC side voltage oscillation limit, the positive sequence reactive current command is generated first, and then the negative sequence reactive current command is adjusted; The control module is configured to generate a gating signal for the grid-connected inverter based on the generated current instruction or positive-sequence reactive current instruction and negative-sequence reactive current instruction of the grid-connected inverter current inner loop, so as to complete the low voltage ride-through control of the grid-connected inverter under asymmetric fault.

8. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, the steps of the low voltage ride-through control method of a grid-connected inverter under an asymmetric fault as described in any one of claims 1-6 are implemented.

9. An electronic device comprising a memory, a processor, and a computer program stored in the memory and running on the processor, characterized in that: When the processor executes the program, the steps of the low voltage ride-through control method of the grid-connected inverter under an asymmetric fault are implemented as described in any one of claims 1-6.

10. A computer program product comprising software code, characterized in that The program in the software code executes the steps of the low voltage ride-through control method for a grid-connected inverter under an asymmetric fault as described in any one of claims 1-6.

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