Fault Identification Method and System for Distributed Photovoltaic Distribution Networks under Low Voltage Ride-Through Conditions

Abstract

This invention belongs to the field of fault identification and provides a method and system for fault identification in distributed photovoltaic distribution networks under low-voltage ride-through conditions. The method involves acquiring the three-phase voltage and current at the protection installation point or point of common coupling; when inverter-side operating parameters are available, at least one auxiliary operating parameter is acquired; the three-phase voltage and current are decomposed using the symmetrical component method to extract positive-sequence, negative-sequence, and zero-sequence components for low-voltage ride-through state identification; if a low-voltage ride-through state is identified, positive and negative-sequence current reference values ​​are calculated; based on the reference values, the negative-sequence absorption control response is estimated using phasor form; based on the negative-sequence absorption control response, fault characteristic correction calculations are performed; using the corrected fault characteristics, fault category identification, direction determination, and section location are performed; based on the fault category, direction, section, and low-voltage ride-through state, the protection action is determined; otherwise, parameter acquisition continues. This invention achieves accurate fault diagnosis and protection.

Description

Technical Field

[0001] This invention belongs to the field of fault identification, specifically relating to a method and system for fault identification in distributed photovoltaic distribution networks under low voltage ride-through conditions. 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] With the large-scale integration of new energy sources into the distribution network, distributed photovoltaic power is gradually transforming from a low-penetration, low-impact power source into a crucial power source influencing the distribution of fault currents and the behavior of protection systems in the distribution network. Traditional distribution networks are mostly single-source radial structures, with fault currents primarily supplied by upstream system power sources. The magnitude, direction, and phase angle of the fault current at the protection installation point are relatively fixed. Therefore, traditional current protection, directional protection, and distance protection can identify faults and perform protection actions based on current magnitude, power direction, negative sequence components, or measured impedance.

[0004] However, distributed photovoltaic (PV) power generation connected to the grid via power electronic inverters exhibits significantly different fault responses compared to synchronous generators. Existing research indicates that the short-circuit current of inverter-type power sources such as PV and energy storage during faults is characterized by limited amplitude, controlled phase, and a dynamic process determined by the controller. This typically contradicts the traditional assumption that fault current is naturally determined by the power source potential and line impedance. Further research suggests that the short-circuit current amplitude of PV and energy storage power sources is usually limited to 1.2 to 1.5 times the rated current, and its phase angle is jointly determined by reactive power support strategies, power direction, and current limiting conditions. This leads to decreased sensitivity, failure to operate, or false operating risks in traditional current protection and differential protection systems.

[0005] In asymmetrical fault scenarios, negative sequence quantities are often crucial for fault identification and direction determination. Traditional protection methods typically assume that asymmetrical faults such as phase-to-phase short circuits, two-phase grounding, and single-phase grounding generate significant negative sequence currents and voltages. Therefore, the fault direction and phase identification can be determined using the negative sequence current amplitude, negative sequence impedance angle, or negative sequence power direction. Existing research on the direction determination of two-phase short circuits in photovoltaic distribution networks is mostly based on the negative sequence current suppression assumption. That is, the photovoltaic inverter suppresses the negative sequence current during the fault, making the photovoltaic branches in the negative sequence network approximately open-circuited, thereby utilizing the difference in negative sequence current amplitude under forward and reverse faults to achieve direction determination.

[0006] In practical applications, negative sequence current and negative sequence voltage should reflect the characteristics of the fault itself in asymmetrical faults such as phase-to-phase short circuits, two-phase grounding, and single-phase grounding. However, under the control of Low Voltage Ride Through (LVRT) technology, the inverter actively generates negative sequence reactive power absorption current based on the negative sequence voltage at the grid connection point. If the measured negative sequence current is directly used for direction determination or fault identification, it is impossible to distinguish between the actual fault quantity and the control quantity, which can easily lead to misjudgment of the fault direction or deviation in fault segment location.

[0007] Traditional negative-sequence directional elements typically rely on the phase angle relationship between negative-sequence voltage and negative-sequence current, assuming the system's negative-sequence network exhibits approximately inductive characteristics. However, the negative-sequence response of inverter-type power supplies is determined by the control strategy, and different control strategies significantly alter the equivalent negative-sequence impedance angle. Especially under negative-sequence absorption control, the inverter's equivalent negative-sequence impedance is no longer naturally determined by physical reactance, but rather by the negative-sequence voltage-current relationship set by the controller. Therefore, traditional negative-sequence directional criteria, distance protection impedance measurements, and fault line impedance estimations may contain inaccuracies. Summary of the Invention

[0008] To address the aforementioned problems, this invention proposes a fault identification method and system for distributed photovoltaic distribution networks under low voltage ride-through conditions. By constructing locally calculable corrected negative sequence current, negative sequence impedance, control residual, and additional impedance angle, this invention improves the fault identification, direction discrimination, and section location capabilities under limited measurement point conditions. It can separate the actual fault characteristics from the inverter control response, thereby achieving accurate fault diagnosis and protection.

[0009] According to some embodiments, the present invention adopts the following technical solution: A method for fault identification in a distributed photovoltaic distribution network under low voltage ride-through conditions includes the following steps: Obtain the three-phase voltage and current at the protection installation point or common connection point; when the conditions for obtaining inverter-side operating parameters are met, obtain at least one auxiliary operating parameter among the DC bus voltage, active power, and reactive power of the photovoltaic inverter; decompose the three-phase voltage and current using the symmetrical component method, extract the positive sequence, negative sequence, and zero sequence components, and identify the low voltage ride-through state; if it is a low voltage ride-through state, proceed to the next step; otherwise, continue to obtain parameters. Calculate the positive and negative sequence current reference values, estimate the negative sequence absorption control response in phasor form based on the reference values, and perform fault characteristic correction calculation based on the negative sequence absorption control response. Using the corrected fault characteristics, fault type identification, direction determination, and section location are performed. Based on the fault type, direction, section, and low voltage ride-through status, the protection action is determined.

[0010] As an alternative implementation, the process of acquiring three-phase voltage and current at the protection installation point or point of common coupling (PCC), and acquiring at least one auxiliary operating quantity among the DC bus voltage, active power, and reactive power of the photovoltaic inverter when the conditions for acquiring inverter-side operating quantities are available, includes: collecting three-phase voltage and current, as well as the DC bus voltage, active power, and reactive power of the photovoltaic inverter at the protection installation point or PCC, recording them in a set form, and decomposing the fault quantity measured at the protection installation point into a fault body component, an inverter low voltage ride-through control influence component, and measurement error and unmodeled items when negative sequence absorption type low voltage ride-through control is present.

[0011] As an alternative implementation, the process of identifying low-voltage ride-through states includes: defining low-voltage ride-through state variables: ; in: This is the per-unit value of the positive sequence voltage at the grid connection point; Low voltage ride-through threshold; The low-voltage ride-through threshold is defined as follows: It is used to create hysteresis and avoid state jitter near the voltage critical point.

[0012] As an alternative implementation, the process of calculating positive and negative sequence current reference values ​​includes: under low voltage ride-through conditions, the photovoltaic inverter generates positive sequence reactive current reference values ​​based on the degree of positive sequence voltage drop at the grid connection point. ; in, Forward order Reference value for shaft reactive current; This is the positive sequence reactive power support coefficient; This is the inverter's rated current; The reference value for negative sequence reactive power absorption is: ; in, negative order Shaft current reference value; This is the per-unit value of the negative sequence voltage at the grid connection point; It is the negative-order reactive power absorption coefficient; If the inverter's maximum current is The reference value for positive sequence active current is: ; in This is a reference value for active power. This is used to ensure that the radical sign is not negative. This is the per-unit value of the positive sequence voltage at the grid connection point.

[0013] As an alternative implementation, the process of estimating the negative-order absorption control response in phasor form based on the reference value includes: estimating the negative-order absorption control response in phasor form: ; in, To estimate the obtained negative sequence control response current; The proportionality coefficient is a fixed constant, or it can be calibrated according to the location of the measuring point, the transformer ratio, and the current base value. The negative sequence reactive power absorption direction angle, the specific sign of which is determined by the positive direction of the current and the dq coordinates; To protect the negative sequence voltage phasor measured at the installation location or grid connection point.

[0014] As an alternative implementation method, the process of calculating fault characteristic correction based on the negative sequence absorption control response includes: defining the corrected negative sequence fault current as: ; Define negative-order control residuals: ; in, Used to replace the original negative sequence current for fault identification and direction determination; Used to characterize the degree of influence of negative-order absorption control on fault characteristics, when When the measured negative sequence current is less than the set value, it indicates that the measured negative sequence current is mainly contributed by the controller's negative sequence absorption behavior; when When the value is greater than the predetermined value, it indicates the presence of a real fault negative sequence component; Define the corrected negative sequence impedance: ; Define the corrected negative sequence impedance angle: .

[0015] As an alternative implementation method, the process of identifying fault categories using the corrected fault characteristics includes: For asymmetrical faults, define the zero-sequence current proportionality characteristic: ; Define the zero-sequence voltage proportionality characteristic: ; in: To prevent positive numbers with excessively small denominators; according to , and the corrected negative sequence current, To protect the negative sequence voltage phasor measured at the installation location or grid connection point, This is the corrected negative sequence fault current. Zero-sequence voltage, It is the zero-sequence current; Make the following judgment: like and Greater than or equal to the set value, and , If the value is less than the set value, it is determined to be a phase-to-phase short circuit; like , , , If all values ​​are greater than the predetermined value, it is determined to be a two-phase ground fault; like , If the voltage is greater than or equal to the set value and the single-phase voltage drops significantly, it is determined to be a single-phase ground fault. If the current amplitude is less than the set value and , or If an anomaly occurs, it is determined to be a high-resistance fault or a weak fault characteristic event.

[0016] As an alternative implementation method, the process of determining direction using the corrected fault characteristics includes: The direction criterion is expressed as: ; Otherwise, it is determined to be a reverse direction fault or a locked direction element, and should be used. A corrected negative sequence impedance is constructed to avoid the control current misleading the direction criterion. This is the corrected negative sequence impedance angle. , These are the minimum and maximum impedance angle values ​​set, respectively.

[0017] As an alternative implementation method, the process of locating the fault section using the corrected fault characteristics includes: if the negative sequence equivalent impedance of the protection installation point is known... and line unit negative sequence impedance The estimated fault distance is: ; For the fault line impedance recovery method based on single-ended impedance measurement, the corrected additional impedance angle is constructed as follows: ; in: The corrected fault current; To protect the current measured at the installation location and calculate the corrected fault line impedance: ; in, To protect the measured impedance at the installation location; To measure the impedance angle; The line impedance angle; To control the additional impedance angle after response decoupling.

[0018] A fault identification system for distributed photovoltaic distribution networks under low voltage ride-through conditions includes: The low-voltage ride-through status identification module is configured to acquire the three-phase voltage and current at the protection installation point or common coupling point; when the conditions for acquiring inverter-side operating quantities are met, it acquires at least one auxiliary operating quantity among the DC bus voltage, active power, and reactive power of the photovoltaic inverter, decomposes the three-phase voltage and current using the symmetrical component method, extracts positive sequence, negative sequence, and zero sequence components, and performs low-voltage ride-through status identification. If it is a low-voltage ride-through status, the fault feature correction module is called; otherwise, parameter acquisition continues. The fault characteristic correction module is configured to calculate positive and negative sequence current reference values, estimate the negative sequence absorption control response in phasor form based on the reference values, and perform fault characteristic correction calculations based on the negative sequence absorption control response. The fault diagnosis module is configured to use the corrected fault characteristics to identify the fault type, determine the direction, and locate the section. Based on the fault type, direction, section, and low voltage ride-through status, it determines the protection action.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention can improve the accuracy of asymmetric fault feature extraction. Traditional methods directly determine direction based on the original negative-sequence current or negative-sequence impedance angle, while this invention estimates the negative-sequence control response. The corrected negative sequence fault current is obtained by subtracting the control component from the measured negative sequence current. It can more accurately reflect the characteristics of the fault itself; by combining the corrected negative sequence characteristics with the zero sequence characteristics and positive sequence voltage drop characteristics, it can be used to identify asymmetrical faults such as phase-to-phase short circuits, two-phase grounding, and single-phase grounding, avoiding misjudgment by traditional fault classification methods in inverter-type power supply scenarios due to controlled current characteristics, and improving the reliability of asymmetrical fault identification.

[0020] This invention can improve the reliability of fault identification, direction discrimination, and section location. Based on the corrected characteristic quantities, it can be used for asymmetric fault type identification, fault direction discrimination, and section location. Compared with the method of directly using the original negative sequence current or the original negative sequence impedance, this invention can reduce the misleading effect of inverter negative sequence absorption control on traditional protection quantities and improve the reliability of directional elements and fault location.

[0021] This invention is applicable to improving the protection adaptability of distribution networks with a high proportion of distributed photovoltaic (PV) power. It can be implemented based on inverter internal control parameters or estimated based on local voltage and current measurements at the protection installation location. It is suitable for distribution automation terminals, feeder protection devices, PV grid-connected controllers, or new energy power plant monitoring systems. As new grid connection standards increasingly demand higher requirements for negative sequence current control, dynamic reactive power support, and low-voltage ride-through capability, this invention provides technical support for fault diagnosis, fault measurement, and protection adaptability improvement in distribution networks with a high proportion of distributed PV power.

[0022] 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

[0023] 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.

[0024] Figure 1 A flowchart illustrating the implementation of a method in one embodiment; Figure 2 This is a diagram of the positive and negative sequence dual closed-loop dq decoupling control structure of a distributed photovoltaic grid-connected inverter according to one embodiment; Figure 3 This is a topology diagram of a two-feeder power distribution system for distributed photovoltaic access, according to one embodiment. Detailed Implementation

[0025] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0026] 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.

[0027] 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.

[0028] Where there is no conflict, the embodiments and features described in this application may be combined with each other.

[0029] Example 1 A fault identification method for distributed photovoltaic distribution networks under low voltage ride-through conditions is implemented according to the following process: "data acquisition—sequence component extraction—low voltage ride-through state identification—positive and negative sequence current reference value calculation—negative sequence control response estimation—fault characteristic correction—fault identification and direction determination—section location". Figure 1 As shown, it includes the following steps: (1) Decomposition of measured quantities First, collect three-phase voltage and current, as well as operating parameters such as DC bus voltage, active power, and reactive power of the photovoltaic inverter at the protection installation point or common connection point (PCC), and record them as follows: (1) in: These are the positive-sequence, negative-sequence, and zero-sequence voltages, respectively. These represent the positive-sequence, negative-sequence, and zero-sequence currents, respectively. To protect the measured impedance at the installation location; Active power and reactive power; This is the DC bus voltage; When negative-sequence absorption-type low-voltage ride-through control is present, the fault quantity measured at the protection installation point can be decomposed into: (2) in: For the faulty body component; The influence component of inverter low voltage ride-through control; For measurement error and unmodeled terms; for negative sequence current channels, the above equation can be further expressed as: (3) in: To protect the measured negative sequence current at the installation location; The negative sequence current is caused by the actual fault itself. The negative sequence current generated by the negative sequence absorption control; To measure noise and unmodeled errors.

[0030] (2) Sequence component extraction and low voltage ride-through state identification The three-phase voltage and current are decomposed using the symmetrical component method. For any three-phase quantity... , , Its chronological order Negative order and zero order The components are: (4) in: ; Can represent voltage or current .

[0031] For phase-to-phase short-circuit faults, the following are usually present: (5) Therefore, positive and negative sequence quantities are mainly used in phase-to-phase short circuit scenarios; for two-phase-to-ground or single-phase-to-ground faults, zero sequence quantities are also introduced for fault type identification.

[0032] Define low voltage ride-through state variables : (6) in: This is the per-unit value of the positive sequence voltage at the grid connection point; Low voltage ride-through threshold; The low-voltage ride-through threshold. It is used to create hysteresis and avoid state jitter near the voltage critical point.

[0033] when When the value is 1, it enters the low voltage ride-through state.

[0034] (3) Calculation of positive and negative sequence current reference values ​​and estimation of control response During low-voltage ride-through, the photovoltaic inverter generates a positive-sequence reactive current reference value based on the degree of positive-sequence voltage drop at the grid connection point: (7) in: Forward order Reference value for shaft reactive current; This is the positive sequence reactive power support coefficient; This is the rated current of the inverter.

[0035] Under asymmetrical fault conditions, the photovoltaic inverter generates a reference value for negative sequence reactive power absorption current based on the negative sequence voltage at the grid connection point, and reduces the negative sequence absorption amplitude in the deep voltage drop range to prioritize the positive sequence reactive power support capability.

[0036] The reference value for negative sequence reactive power absorption is: (8) in: negative order Shaft current reference value; This is the per-unit value of the negative sequence voltage at the grid connection point; It is the negative-order reactive power absorption coefficient.

[0037] It should be noted that the formula given... This mainly represents the amplitude distribution pattern of the negative-sequence reactive power absorption current. Its sign changes from negative sequence to positive sequence. dqThe direction of the coordinate system is defined. If the negative-sequence reactive power absorption direction is defined as negative in the adopted coordinate system... The axis can then be written as: (9) After the reference values ​​for positive-sequence reactive power support and negative-sequence reactive power absorption are determined, the positive-sequence active current is constrained according to the residual current capacity to prevent the inverter output current from exceeding the allowable value.

[0038] If the inverter's maximum current is The reference value for positive sequence active current is: (10) in This is a reference value for active power. , is used to ensure that the radical sign is not negative.

[0039] Equation (10) is a conservative capacity constraint form that prioritizes the use of capacity based on the negative sequence of current absorption; in other embodiments, a unified capacity constraint form based on the current vector magnitude can also be adopted.

[0040] This formula indicates that the negative sequence absorption current is first determined according to formulas (8) and (9), and its current capacity is deducted from the total capacity; the remaining capacity prioritizes the positive sequence reactive power support, and then restricts the positive sequence active current. This process is different from the proportional redistribution of positive and negative sequence reactive currents.

[0041] (4) Fault characteristic correction, direction identification and section location To avoid sign confusion caused by different dq coordinate definitions, this invention uses phasor form to estimate the negative-order absorption control response: (11) in: To estimate the obtained negative sequence control response current; This is the proportionality coefficient, which can be taken as 1, or calibrated according to the location of the measuring point, the transformer ratio, and the current base value. The negative sequence reactive power absorption direction angle, the specific sign of which is determined by the positive direction of the current and the dq coordinates; To protect the negative-sequence voltage phasor measured at the installation point or grid connection point, after estimating the negative-sequence control response, the corrected negative-sequence fault current is defined as: (12) Further define negative-order control residuals: (13) in: Used to replace the original negative sequence current for fault identification and direction determination; Used to characterize the degree of influence of negative-order absorption control on fault characteristics. When When the value is small, it indicates that the measured negative sequence current is mainly contributed by the negative sequence absorption behavior of the controller; when... When the value is large, it indicates the presence of a significant negative sequence component of the actual fault.

[0042] Define the corrected negative sequence impedance: (14) Define the corrected negative sequence impedance angle: (15) The direction criterion can be expressed as: (16) Otherwise, it is judged as a reverse direction fault or a locked direction element. This differs from traditional direct use. The methods differ; this embodiment uses... A corrected negative sequence impedance is constructed to avoid the control current misleading the direction criterion.

[0043] For asymmetrical faults, define the zero-sequence current proportionality characteristic: (17) Define the zero-sequence voltage proportionality characteristic: (18) in: To prevent positive numbers with excessively small denominators; according to , And based on the corrected negative sequence current, the following judgment can be made: like and Greater than or equal to the set value, and , If the value is small, it is determined to be a phase-to-phase short circuit; like , , , If both values ​​are relatively large, it is determined to be a two-phase ground fault; like , If the voltage is greater than or equal to the set value and the single-phase voltage drops significantly, it is determined to be a single-phase ground fault. If the current amplitude is small but , or If an anomaly occurs, it is determined to be a high-resistance fault or a weak fault characteristic event.

[0044] If the negative sequence equivalent impedance of the protection installation point is known and line unit negative sequence impedance Then the fault distance can be estimated as: (19) Furthermore, for the fault line impedance recovery method based on single-ended impedance measurement, a corrected additional impedance angle can be constructed: (20) in: The corrected fault current; To protect the current measured at the installation location, and to calculate the corrected fault line impedance: (twenty one) in: To protect the measured impedance at the installation location; To measure the impedance angle; The line impedance angle; This method is used to control the additional impedance angle after response decoupling. It can be applied to fault location and distance protection criterion correction.

[0045] The method in this embodiment can be applied. Figure 2 The distributed photovoltaic grid-connected inverter control structure shown includes a photovoltaic array, a DC-DC boost stage, a DC unloading circuit, a DC bus, a three-phase DC / AC inverter bridge, an LCL filter, and a PCC. The control system includes MPPT control under normal operating conditions, a DC voltage / active power outer loop, a positive-sequence reactive power support outer loop, and positive-sequence and negative-sequence current inner loops under low-voltage ride-through conditions. Upon entering low-voltage ride-through mode, the MPPT exits or locks out, and the positive-sequence branch... Generate positive-sequence reactive power support reference values, and negative-sequence branches according to... and The negative sequence reactive power absorption reference value is generated from the drop interval, and the final current reference value is obtained through the capacity constraint module.

[0046] use Figure 3 The two-feeder power distribution system shown can verify the method provided in this embodiment. The system consists of the upstream power grid, main transformer T1, feeder 1, feeder 2, distributed photovoltaic access points, loads, and multiple protection devices. (See figure...) , , "etc." represents different candidate fault points. This indicates the protection installation point or direction protection unit. This topology can be used to verify the effectiveness of the proposed fault characteristic correction, direction discrimination and section location methods under different fault locations, different fault types and different photovoltaic output conditions.

[0047] Example 2 A method for fault identification in distributed photovoltaic distribution networks under low voltage ride-through conditions, which differs from Embodiment 1 in that, in addition to employing... , , In addition to the corrected features, negative sequence power correction, positive and negative sequence phase angle offset, transient energy residual, or DC bus disturbance features can also be used.

[0048] The negative sequence power correction can be defined as: (twenty two) The superscript * indicates conjugate. The active and reactive components can be used to determine the direction of the fault.

[0049] The positive and negative sequence phase angle offset can be defined as: (twenty three) Used for joint determination of fault type and direction.

[0050] The transient energy residual can be defined as: (twenty four) Used for transient fault identification and high-resistance fault detection.

[0051] The characteristics of DC bus disturbance can be defined as follows: (25) Used to identify the degree of power imbalance between upstream and downstream stages during a fault.

[0052] (2) Alternative scheme for direction criterion In addition to using the corrected negative sequence impedance angle, the corrected negative sequence power direction, the corrected negative sequence current amplitude, or the corrected impedance region can also be used as direction criteria.

[0053] The direction of the corrected negative sequence power can be expressed as: (26) The direction of the fault is determined by its symbol and threshold.

[0054] The criterion for the direction of the negative sequence current amplitude after correction can be expressed as: (27) Otherwise, it is judged as a reverse fault or lockout.

[0055] The corrected impedance interval criterion can be expressed as: (28) in, This is a preset positive fault impedance region.

[0056] (3) Alternative solutions for segment location: In addition to using the corrected negative sequence impedance for segment location, feature library matching or a fusion of model-driven and data-driven methods can also be used.

[0057] Location based on corrected negative sequence impedance: (29) The feature library-based localization method can establish the following feature library: (30) in, Indicates the candidate segment, Indicates the candidate fault type. Online calculation: (31) Pick The smallest segment is designated as the fault segment. This embodiment is applicable not only to a single 2MW or 4MW distributed photovoltaic unit, but also to active distribution network scenarios where multiple distributed photovoltaic units are connected in parallel, photovoltaic and energy storage are jointly connected, or grid-connected or grid-built inverters are connected.

[0058] Example 3 A fault identification system for distributed photovoltaic distribution networks under low voltage ride-through conditions includes: The low-voltage ride-through status identification module is configured to acquire the three-phase voltage and current at the protection installation point or common coupling point; when the conditions for acquiring inverter-side operating quantities are met, it further acquires at least one auxiliary operating quantity among the DC bus voltage, active power, and reactive power of the photovoltaic inverter, decomposes the three-phase voltage and current using the symmetrical component method, extracts positive sequence, negative sequence, and zero sequence components, and performs low-voltage ride-through status identification. If it is a low-voltage ride-through status, the fault feature correction module is called; otherwise, parameter acquisition continues. The fault characteristic correction module is configured to calculate positive and negative sequence current reference values, estimate the negative sequence absorption control response in phasor form based on the reference values, and perform fault characteristic correction calculations based on the negative sequence absorption control response. The fault diagnosis module is configured to use the corrected fault characteristics to identify the fault type, determine the direction, and locate the section. Based on the fault type, direction, section, and low voltage ride-through status, it determines the protection action.

[0059] 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.).

[0060] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0061] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0062] 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 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0063] 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 fault identification in distributed photovoltaic distribution networks under low voltage ride-through conditions, characterized in that, Includes the following steps: Obtain the three-phase voltage and current at the protection installation point or common connection point; when the conditions for obtaining inverter-side operating parameters are met, obtain at least one auxiliary operating parameter among the DC bus voltage, active power, and reactive power of the photovoltaic inverter; decompose the three-phase voltage and current using the symmetrical component method, extract the positive sequence, negative sequence, and zero sequence components, and identify the low voltage ride-through state; if it is a low voltage ride-through state, proceed to the next step; otherwise, continue to obtain parameters. Calculate the positive and negative sequence current reference values, estimate the negative sequence absorption control response in phasor form based on the reference values, and perform fault characteristic correction calculation based on the negative sequence absorption control response. Using the corrected fault characteristics, fault type identification, direction determination, and section location are performed. Based on the fault type, direction, section, and low voltage ride-through state, the protection action is determined. The process of estimating the negative-order absorption control response in phasor form based on the reference value includes: estimating the negative-order absorption control response in phasor form: in, To estimate the obtained negative sequence control response current; The proportionality coefficient is a fixed constant, or it can be calibrated according to the location of the measuring point, the transformer ratio, and the current base value. The negative sequence reactive power absorption direction angle, the specific sign of which is determined by the positive direction of the current and the dq coordinates; To protect the negative sequence voltage phasor measured at the installation location or grid connection point; The process of calculating fault characteristic correction based on the negative sequence absorption control response includes: defining the corrected negative sequence fault current as: ； Define negative-order control residuals: ； in, Used to replace the original negative sequence current for fault identification and direction determination; Used to characterize the degree of influence of negative-order absorption control on fault characteristics, when When the measured negative sequence current is less than the set value, it indicates that the measured negative sequence current is mainly contributed by the controller's negative sequence absorption behavior; when When the value is greater than the predetermined value, it indicates the presence of a real fault negative sequence component; Define the corrected negative sequence impedance: ； Define the corrected negative sequence impedance angle: 。 2. The method for fault identification of distributed photovoltaic distribution networks under low voltage ride-through conditions as described in claim 1, characterized in that, The process of acquiring three-phase voltage and current at the protection installation point or point of common coupling (PCC), and acquiring at least one auxiliary operating quantity among the DC bus voltage, active power, and reactive power of the photovoltaic inverter when the conditions for acquiring inverter-side operating quantities are met, includes: collecting three-phase voltage and current, as well as the DC bus voltage, active power, and reactive power of the photovoltaic inverter at the protection installation point or PCC, recording them in set form, and decomposing the fault quantity measured at the protection installation point into the fault body component, the inverter low voltage ride-through control influence component, and the measurement error and unmodeled items when negative sequence absorption type low voltage ride-through control is present.

3. The method for fault identification of distributed photovoltaic distribution networks under low voltage ride-through conditions as described in claim 1, characterized in that, The process of identifying low-voltage ride-through states includes: defining low-voltage ride-through state variables: ； in: This is the per-unit value of the positive sequence voltage at the grid connection point; Low voltage ride-through threshold; The low-voltage ride-through threshold is defined as follows: It is used to create hysteresis and avoid state jitter near the voltage critical point.

4. The method for fault identification of distributed photovoltaic distribution networks under low voltage ride-through conditions as described in claim 1, characterized in that, The process of calculating positive and negative sequence current reference values ​​includes: Under low voltage ride-through conditions, the photovoltaic inverter generates positive sequence reactive current reference values ​​based on the degree of positive sequence voltage drop at the grid connection point. ； in, Forward order Reference value for shaft reactive current; This is the positive sequence reactive power support coefficient; This is the inverter's rated current; The reference value for negative sequence reactive power absorption is: ； in, negative order Shaft current reference value; This is the per-unit value of the negative sequence voltage at the grid connection point; It is the negative-order reactive power absorption coefficient; If the inverter's maximum current is The reference value for positive sequence active current is: ； in This is a reference value for active power. This is used to ensure that the radical sign is not negative. This is the per-unit value of the positive sequence voltage at the grid connection point.

5. The method for fault identification of distributed photovoltaic distribution networks under low voltage ride-through conditions as described in claim 1, characterized in that, The process of identifying fault categories using corrected fault features includes: For asymmetrical faults, define the zero-sequence current proportionality characteristic: ； Define the zero-sequence voltage proportionality characteristic: ； in: To prevent positive numbers with excessively small denominators; according to , and the corrected negative sequence current, To protect the negative sequence voltage phasor measured at the installation location or grid connection point, This is the corrected negative sequence fault current. Zero-sequence voltage, It is the zero-sequence current; Make the following judgment: like and Greater than or equal to the set value, and , If the value is less than the set value, it is determined to be a phase-to-phase short circuit; like , , , If all values ​​are greater than the predetermined value, it is determined to be a two-phase ground fault; like , If the voltage is greater than or equal to the set value and the single-phase voltage drops significantly, it is determined to be a single-phase ground fault. If the current amplitude is less than the set value and , or If an anomaly occurs, it is determined to be a high-resistance fault or a weak fault characteristic event.

6. The method for fault identification of distributed photovoltaic distribution networks under low voltage ride-through conditions as described in claim 1, characterized in that, The process of determining direction using the corrected fault characteristics includes: The direction criterion is expressed as: ； Otherwise, it is determined to be a reverse direction fault or a locked direction element, and should be used. A corrected negative sequence impedance is constructed to avoid the control current misleading the direction criterion. This is the corrected negative sequence impedance angle. , These are the minimum and maximum impedance angle values ​​set, respectively.

7. The method for fault identification of distributed photovoltaic distribution networks under low voltage ride-through conditions as described in claim 1, characterized in that, The process of locating the fault section using the corrected fault characteristics includes: if the negative sequence equivalent impedance of the protection installation point is known... and line unit negative sequence impedance The estimated fault distance is: ； For the fault line impedance recovery method based on single-ended impedance measurement, the corrected additional impedance angle is constructed as follows: in: The corrected fault current; To protect the current measured at the installation location and calculate the corrected fault line impedance: in, To protect the measured impedance at the installation location; To measure the impedance angle; The line impedance angle; To control the additional impedance angle after response decoupling.

8. A fault identification system for distributed photovoltaic distribution networks under low voltage ride-through conditions, using the method described in any one of claims 1-7, characterized in that, include: The low-voltage ride-through status identification module is configured to acquire the three-phase voltage and current at the protection installation point or common coupling point; when the conditions for acquiring inverter-side operating quantities are met, it acquires at least one auxiliary operating quantity among the DC bus voltage, active power, and reactive power of the photovoltaic inverter, decomposes the three-phase voltage and current using the symmetrical component method, extracts positive sequence, negative sequence, and zero sequence components, and performs low-voltage ride-through status identification. If it is a low-voltage ride-through status, the fault feature correction module is called; otherwise, parameter acquisition continues. The fault characteristic correction module is configured to calculate positive and negative sequence current reference values, estimate the negative sequence absorption control response in phasor form based on the reference values, and perform fault characteristic correction calculations based on the negative sequence absorption control response. The fault diagnosis module is configured to use the corrected fault characteristics to identify the fault type, determine the direction, and locate the section. Based on the fault type, direction, section, and low voltage ride-through status, it determines the protection action.

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