A fault identification method, device and equipment of a flexible AC transmission line and a medium

Abstract

The application discloses a fault identification method, device and equipment of a flexible alternating current (AC) feeder, and a medium, and belongs to the field of power system protection. The method is as follows: equivalent circuits of a flexible direct current (DC) system in different modes are topologically connected according to boundary conditions of single-phase ground faults to construct a multi-mode equivalent network model; a fault excitation is applied at a preset fault point of the multi-mode equivalent network model, a theoretical current response of a preset protection installation under the fault excitation is calculated, and theoretical transient characteristic values are extracted from the theoretical current response; electrical signals at the preset protection installation are collected in real time, when it is detected that the electrical signals meet preset starting conditions, an actual current response of the preset protection installation is acquired, and actual transient characteristic values are extracted from the actual current response; the actual transient characteristic values are compared with the theoretical transient characteristic values, and the fault of the flexible AC feeder is identified according to a comparison result. The application can improve the accuracy of fault identification of the flexible AC feeder.

Description

Technical Field

[0001] This application relates to the field of power system protection, and in particular to a fault identification method, device, equipment and medium for a flexible DC AC incoming line. Background Technology

[0002] With the expansion of new energy grid integration, flexible DC transmission systems are increasingly widely used in power grids. Flexible DC systems are typically connected to the AC grid via AC incoming lines, and AC-side faults directly affect the converter's operating status and the stability of the DC system. Especially under asymmetrical fault conditions such as single-phase grounding, the system current and voltage exhibit significant transient changes. Failure to quickly and accurately identify AC incoming line faults in flexible DC systems may lead to converter shutdown, power fluctuations, or even system instability. Therefore, researching AC incoming line fault identification methods suitable for the characteristics of flexible DC systems is of significant engineering importance.

[0003] However, existing flexible DC AC incoming line protection methods have significant limitations in practical applications. On the one hand, current differential protection relies on communication channels, resulting in high construction costs and susceptibility to synchronization errors and delays, leading to poor accuracy in fault identification. On the other hand, the control strategy of flexible DC converters limits the amplitude of fault currents and alters negative-sequence impedance characteristics, causing the amplitude and phase relationship of the fault current to deviate from the fault distribution patterns of traditional power systems. This leads to a decrease in the sensitivity of differential protection, distance protection, and overcurrent protection based on measured characteristics, or even maloperation, thus affecting the accuracy of final fault identification. Furthermore, the zero-sequence component introduced by the modulation strategy also interferes with the accuracy of ground fault detection. Therefore, improving the accuracy of fault identification for flexible DC AC incoming lines has become a core technical problem that urgently needs to be solved. Summary of the Invention

[0004] This application provides a fault identification method, apparatus, equipment, and medium for flexible DC AC incoming lines, which can solve the problem of low accuracy in fault identification of flexible DC AC incoming lines in the prior art.

[0005] Some embodiments of this application provide a fault identification method for flexible DC AC input lines, including: The equivalent circuits of the flexible DC system under different modes are topologically connected according to the boundary conditions of a single-phase ground fault to construct a multi-mode equivalent network model. By applying fault excitation to a preset fault point in the multi-mode equivalent network model, the theoretical current response of the preset protection installation location under the fault excitation is calculated based on the multi-mode equivalent network model, and theoretical transient characteristic values ​​are extracted from the theoretical current response. The modes include a first line mode, a second line mode, and a zero mode. The electrical signals of the flexible DC system at the preset protection installation location are collected in real time. When the electrical signals are detected to meet the preset start-up conditions, the actual transient characteristic values ​​are extracted from the actual current response in the electrical signals. The actual transient characteristic value is compared with the theoretical transient characteristic value to obtain the comparison result, and the fault of the flexible DC AC input line is identified based on the comparison result.

[0006] Compared with existing technologies, the above embodiments have the following beneficial effects: This application constructs a multi-mode equivalent network model by connecting the equivalent circuits of the flexible DC system under different modes such as the first-line mode, the second-line mode, and the zero-mode according to the single-phase grounding fault boundary conditions. This allows for a complete characterization of the modal coupling relationship of the system under fault conditions, accurately reflecting the transient response mechanism of the flexible DC system under a single-phase grounding fault from a theoretical perspective. Based on this, by applying fault excitation at a preset fault point to calculate the theoretical current response at the protection installation location and extracting theoretical transient characteristic values, a fault theoretical characteristic benchmark consistent with the system's physical characteristics is established. Furthermore, after a fault occurs, the transient characteristic values ​​of the actual current response are extracted and compared with the theoretical transient characteristic values. Since the comparison objects are derived from theoretical calculations under the same system structure and the same fault boundary conditions, the impact of the converter control strategy on the fault current amplitude limitation and phase change can be effectively reduced, improving the accuracy of identifying asymmetrical faults such as single-phase grounding, thereby enhancing the reliability of flexible DC AC incoming line fault identification. This approach can effectively overcome the influence of the flexible DC converter control strategy on the amplitude and phase relationship of the fault current, reduce the risk of misjudgment caused by zero-sequence interference introduced by the modulation strategy, and thus significantly improve the accuracy and reliability of flexible DC AC incoming line fault identification.

[0007] Furthermore, the step of connecting the equivalent circuits of the flexible DC system in different modes according to the boundary conditions of a single-phase ground fault to construct a multi-mode equivalent network model includes: Using the trapezoidal integral method, each power element from the AC side to the DC side of the flexible DC system is discretized to obtain the equivalent circuit of Begeron circuit under different modes corresponding to each power element. Based on the physical topology of the flexible DC system, the various Begeron equivalent circuits under the same mode are interconnected to form a primary network model under each mode. The primary network models under each mode are connected in series at the preset fault point according to the boundary conditions to obtain the multimodal equivalent network model.

[0008] Compared to existing technologies, the above embodiments have the following advantages: By employing the trapezoidal integral method to discretize each power component from the AC side to the DC side and establishing a Bergeron equivalent model, the continuous system is transformed into a recursively computeable discrete network model, thereby ensuring the numerical stability and accuracy of transient process calculations. Furthermore, the equivalent model is projected onto the modal domain space using the Kelenberg transformation, and primary network models for each mode are constructed according to the physical topology, allowing the electrical characteristics of the system under different modes to be decomposed and expressed. Finally, the modal networks are connected in series according to the single-phase ground fault boundary conditions, accurately reflecting the coupling relationships between multiple modes under fault conditions. This improves the accuracy of fault modeling, provides a reliable foundation for subsequent theoretical current response calculations, and enhances fault identification accuracy from the source.

[0009] Further, the primary network model includes: a first primary network model corresponding to the first linear mode, a second primary network model corresponding to the second linear mode, and a third primary network model corresponding to the zero mode; the step of connecting the primary network models under each mode in series at the preset fault point according to the boundary conditions to obtain the multimodal equivalent network model includes: By using the Kelvin transform, the boundary conditions are transformed to the modal domain to obtain the modal domain boundary conditions of the fault point in different modes. Based on the modal boundary conditions where the fault point is located in different modes, the outflow end of the first primary network model is connected to the inflow end of the second primary network model, and the outflow end of the second primary network model is connected to the inflow end of the third primary network model to obtain the multimodal equivalent network model.

[0010] Compared with existing technologies, the above embodiments have the following advantages: By transforming the boundary conditions to the mode domain space and sequentially connecting the primary network models corresponding to the first line mode, the second line mode, and the zero mode in the mode domain, the constraint relationships of single-phase grounding faults in different modes are clearly expressed. This method avoids the complex coupling errors caused by direct coupling in the phase domain, makes the modal voltage and current relationships at the fault point clearer, and is conducive to accurately characterizing the modal component change features caused by the fault, thereby improving the expression accuracy of the multimodal equivalent network model under single-phase grounding fault conditions and enhancing the accuracy of subsequent theoretical response calculations.

[0011] Further, the step of applying a fault excitation at a preset fault point in the multimodal equivalent network model and calculating the theoretical current response at the preset protection installation location under the fault excitation based on the multimodal equivalent network model includes: In the multimodal equivalent network model, fault excitations corresponding to single-phase grounding faults are injected into each of the preset fault points to form fault excitation conditions. Under the fault excitation condition, the node voltage equations of each node in the first primary network model are established, and the node voltage equations are solved by recursive calculation to obtain the voltage values ​​of each node at each time. Obtain the Beerleon equivalent parameters of each branch in the first primary network model, and calculate the theoretical current value of the branch where the protection installation is located at each time based on the voltage values ​​of each node and the Beerleon equivalent parameters of the branch where the node is located, so as to obtain the theoretical current response.

[0012] Compared with existing technologies, the above embodiments have the following advantages: After applying fault excitation to the multimodal equivalent network model, the node voltage equations of the first primary network model are first established, and the node voltages are solved recursively based on the discretized Bergeron equivalence relation. Then, the theoretical current value at the protection installation location is calculated based on the branch parameters, so that the calculation of the theoretical current response follows a clear voltage-current physical derivation path. This method avoids the errors that may be caused by directly approximating the current calculation, can accurately reflect the voltage and current coupling relationship in the fault transient propagation process, improves the accuracy of the theoretical current response calculation, and thus provides a reliable basis for accurately extracting theoretical transient characteristics.

[0013] Further, the extraction of theoretical transient characteristic values ​​from the theoretical current response includes: By sampling from the theoretical current response at a preset sampling frequency, the current component sequence of the first line mode is obtained; Perform continuous wavelet transform on the current component sequence to obtain the time-frequency diagram; The time-frequency graph is subjected to transient extraction transform to obtain the time-frequency spectrum; The theoretical wavefront count at several preset frequencies is extracted based on the time spectrum, and the average of the theoretical wavefront count is used as the theoretical transient characteristic value.

[0014] Compared to existing technologies, the above embodiments have the following advantages: By performing continuous wavelet transform on the theoretical current response, time-frequency distribution characteristics are obtained, and the number of wavefronts at a specific frequency is identified through transient extraction transform, enabling the theoretical transient characteristic values ​​to reflect the high-frequency transient change patterns in the initial stage of a fault. Compared to relying solely on amplitude or phasor characteristics, this method can extract more sensitive transient information, improve the identification capability of asymmetrical faults such as single-phase grounding, and thus enhance the matching accuracy of theoretical characteristics with actual fault characteristics.

[0015] Further, the electrical signal includes: a first three-phase voltage signal; the detection that the electrical signal meets the preset start-up conditions includes: The first three-phase voltage signal is divided into two second three-phase voltage signals. The zero-mode voltage change signal is obtained based on the difference between the two second three-phase voltage signals. The zero-mode voltage abrupt change signal is processed by exponential weighted moving average to obtain the zero-mode voltage characteristic value at the current detection time; When the zero-mode voltage characteristic value is greater than or equal to the preset start-up threshold, the electrical signal is determined to meet the preset start-up conditions.

[0016] Compared with existing technologies, the above embodiments have the following advantages: By decomposing the three-phase voltage signal and constructing a zero-mode voltage mutation signal, and using exponential weighted moving average processing to obtain the zero-mode voltage characteristic value, the start-up criterion pays more attention to the mutation information in the initial stage of the fault, while suppressing random fluctuations and non-fault interference components introduced by the modulation strategy. This method can improve the stability of the start-up judgment while ensuring sensitivity, avoid false triggering or missed triggering, and provide a reliable premise for accurately obtaining the actual current response.

[0017] Further, the extraction of actual transient characteristic values ​​from the actual current response includes: Using the Kelenberger transform, the actual current response is converted to the mode domain. The first line mode current signal corresponding to the first line mode and the second line mode is obtained from the converted actual current response. Based on the two first line mode current signals, the second line mode current signal is obtained. The envelope signal is obtained by combining the Hilbert transform with the second line-mode current signal; The envelope signal is subjected to nonlinear combination operations by a preset nonlinear instantaneous operator to obtain an energy feature sequence that characterizes the instantaneous energy change of the signal. Based on a preset energy threshold, the number of effective wavefront mutation points in the energy feature sequence within a preset time window is identified, and the number is used as the actual transient feature value.

[0018] Compared to existing technologies, the above embodiments have the following advantages: By converting the actual current response to the mode domain and weighting and fusing the first line mode current, the modal components related to fault propagation can be enhanced, while irrelevant mode interference can be weakened. Furthermore, by obtaining the envelope signal through Hilbert transform and extracting energy mutation features using nonlinear instantaneous operators, the actual transient characteristic values ​​can highlight wavefront mutation information. This processing improves the sensitivity of fault transient wavefront identification, helps to accurately count the number of effective wavefronts, thereby improving the reliability of comparing actual and theoretical characteristics, and further enhancing the accuracy of AC incoming line fault identification.

[0019] Another embodiment of this application provides a fault identification device for a flexible DC AC input line, including: a theoretical feature extraction module, an actual feature extraction module, and a fault identification module; The theoretical feature extraction module is used to connect the equivalent circuits of the flexible DC system under different modes according to the boundary conditions of a single-phase ground fault, construct a multi-mode equivalent network model, apply fault excitation to a preset fault point in the multi-mode equivalent network model, calculate the theoretical current response of the preset protection installation location under the fault excitation based on the multi-mode equivalent network model, and extract theoretical transient feature values ​​from the theoretical current response; the modes include a first line mode, a second line mode, and a zero mode; The actual feature extraction module is used to collect electrical signals of the flexible DC system at a preset protection installation location in real time, and extract actual transient feature values ​​from the actual current response in the electrical signal when the electrical signal is detected to meet the preset start-up conditions. The fault identification module is used to compare the actual transient characteristic value with the theoretical transient characteristic value, obtain the comparison result, and identify the fault of the flexible DC AC input line based on the comparison result.

[0020] Another embodiment of this application also provides a terminal device, including: a processor, a memory, and a computer program stored in the memory and configured to be executed by the processor. When the processor executes the computer program, it implements the steps of the fault identification method for flexible DC AC input lines as described in this application.

[0021] Another embodiment of this application also provides a computer-readable storage medium item, including: a stored computer program, which, when the computer program is running, controls the device where the computer-readable storage medium is located to perform the steps of the fault identification method for flexible DC AC input lines of this application. Attached Figure Description

[0022] To more clearly illustrate the technical solution of this application, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0023] Figure 1 This is a flowchart illustrating a fault identification method for a flexible DC AC input line provided in some embodiments of this application; Figure 2 This is a side view of a true bipolar flexible straight-line converter provided in some embodiments of this application; Figure 3 This application provides a multi-mode equivalent network model for phase A short-circuit ground faults in some embodiments. Figure 4 This is a schematic diagram showing the current and mutation rate of the first line mode at the line preset protection installation location provided in some embodiments of this application; Figure 5 This is a schematic diagram of the transient extraction and transformation processing results provided in some embodiments of this application; Figure 6 This is a schematic diagram of theoretical wavefront extraction at a frequency of 5 kHz provided in some embodiments of this application; Figure 7 These are schematic diagrams of voltage and current waveforms before and after a fault provided in some embodiments of this application; Figure 8 This is a schematic diagram of the startup criteria provided in some embodiments of this application; Figure 9 The following are Hilbert transform processing diagrams provided in some embodiments of this application; Figure 10 This is a diagram of the modulation signal envelope processing provided in some embodiments of this application; Figure 11 This is a diagram showing the number of measured wavefronts extracted based on the Teager energy operator in some embodiments of this application; Figure 12 This application provides a schematic diagram of the structure of a fault identification device for a flexible DC AC input line in some embodiments. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions of this application will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0025] Unless otherwise defined, 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 application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0026] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0027] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0028] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0029] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0030] In the description of the embodiments of this application, unless otherwise expressly specified and limited, technical terms such as "installation," "connection," "joining," and "fixing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. For those skilled in the art, the specific meaning of the above terms in the embodiments of this application can be understood according to the specific circumstances.

[0031] Existing flexible DC AC incoming line protection methods have significant limitations in practical applications. On the one hand, current differential protection relies on communication channels, resulting in high construction costs and susceptibility to synchronization errors and delays, leading to poor accuracy in fault identification. On the other hand, the control strategy of flexible DC converters limits the amplitude of fault currents and alters negative-sequence impedance characteristics, causing the amplitude and phase relationship of the fault current to deviate from the fault distribution patterns of traditional power systems. This leads to a decrease in the sensitivity of differential protection, distance protection, and overcurrent protection based on measured characteristics, or even maloperation, thus affecting the accuracy of final fault identification. Furthermore, the zero-sequence component introduced by the modulation strategy also interferes with the accuracy of ground fault detection.

[0032] Please refer to Figure 1 To address the issue of low accuracy in fault identification of flexible DC AC input lines in existing technologies, this application provides a fault identification method for flexible DC AC input lines, applicable to, for example... Figure 2 The true bipolar flexible straight-line converter transformer AC side line shown is as follows: Figure 2 As shown, the voltage and current signal measurement points for line MN are located at point M. In this embodiment, it is set... Always on the line f A phase A short-circuit ground fault (i.e., single-phase ground fault) occurred at location 1, where T1 is the first transformer, T2 is the second transformer, M0 is the local protection device for the flexible DC AC incoming line, and Z... s1 and Z s2 Let L1 be the power supply equivalent impedance of a 1-mode network, L1 be an inductor, and S be the power supply.

[0033] Furthermore, the fault identification method for a flexible DC AC input line provided in this application embodiment includes the following steps S101 to S103: S101: Connect the equivalent circuits of the flexible DC system under different modes according to the boundary conditions of a single-phase ground fault to construct a multi-mode equivalent network model. Apply fault excitation to a preset fault point in the multi-mode equivalent network model, calculate the theoretical current response of the preset protection installation location under the fault excitation based on the multi-mode equivalent network model, and extract the theoretical transient characteristic value from the theoretical current response. The modes include a first line mode, a second line mode, and a zero mode.

[0034] Furthermore, in some embodiments of this application, the first line mode is a 1-mode, the second line mode is a 2-mode, and the zero mode is a 0-mode. The 1-mode, 2-mode, and 0-mode networks are network models of the flexible DC system or power components in their respective modes. For example, the 1-mode network of the flexible DC system is the first primary network model, the 2-mode network of the flexible DC system is the second primary network model, and the 0-mode network of the flexible DC system is the third primary network model.

[0035] Furthermore, in some embodiments of this application, the step of connecting the equivalent circuits of the flexible DC system in different modes according to the boundary conditions of a single-phase ground fault to construct a multimodal equivalent network model includes: Using the trapezoidal integral method, each power element from the AC side to the DC side of the flexible DC system is discretized to obtain the equivalent circuit of Begeron circuit under different modes corresponding to each power element. Based on the physical topology of the flexible DC system, the various Begeron equivalent circuits under the same mode are interconnected to form a primary network model under each mode. The primary network models under each mode are connected in series at the preset fault point according to the boundary conditions to obtain the multimodal equivalent network model.

[0036] Preferably, in some embodiments of this application, the power components include: transformers, inductors, capacitors, lines, and other power components in the flexible DC system from the AC power supply to the DC line side. By performing equivalent calculations on each power component, the Bergeron equivalent circuits for each power component in the system are obtained.

[0037] Preferably, in some embodiments of this application, when constructing the Bergeron equivalent circuit of the transformer, according to the transformer's wiring method and physical characteristics, it is equivalent to a Bergeron equivalent circuit consisting of an equivalent conductance matrix and a historical equivalent current source connected in parallel under 1-mode, 2-mode, and 0-mode networks, respectively. The construction steps include the following T111 to T112: T111: Based on the transformer winding connection method (YNd) and magnetic circuit characteristics, determine the physical equivalent path for each mode. For mode 1 and mode 2 networks, both the primary and secondary sides of the transformer have paths, and the magnetic circuit coupling is tight. In transient calculations, the model is represented as the series coupling of the primary leakage inductance and the secondary equivalent leakage inductance. For the network corresponding to mode 0, based on the YNd connection characteristics, the primary neutral point is grounded, and there is a zero-sequence current path to ground. The secondary side is delta connected, with circulating current flowing internally, but there is no zero-sequence path externally, and it is an open circuit (infinite impedance) to the external network, retaining only the primary leakage inductance.

[0038] T112: Based on the physical equivalent path described in T111, the following transformer port voltages are obtained. With current Differential relation: (1); (2); in, The current moment; , They are respectively Port current and voltage at any given time; , They are respectively The time transformer T (specifically refers to the transformer in the circuit) The primary and secondary port voltages of ( ); , They are respectively The current at the primary and secondary terminals of the transformer at any given moment; , They are respectively The self-inductance of the primary and secondary sides of the transformer at any given time; This refers to the mutual inductance between the primary and secondary edges.

[0039] For both sides of the above formula (1) By integrating using the trapezoidal rule, the differential equation is transformed into a difference equation, yielding: (3); in, For time step; It is the inverse matrix of the inductance matrix; , They are respectively The port current and voltage at any given time.

[0040] Among them, for primary and secondary edge coupling in 1-mode and 2-mode networks, Represented as: (4); For the network corresponding to 0 mode, it is in an open-circuit state with respect to the external network (impedance is infinite), and only the primary leakage inductance is retained. , Represented as: (5); Transforming the discretized equations into Norton equivalent circuit form, we obtain: (6); in, This is the equivalent conductance matrix of the transformer; For the primary side of the transformer The equivalent current source vector at time t; For the secondary side of the transformer The equivalent current source vector at time t.

[0041] Further obtain Current source at all times and Current source at all times Relationship: (7); This completes the task. Construction of the Bergeron equivalent circuit for time transformer T.

[0042] Preferably, in some embodiments of this application, when constructing the Bergeron equivalent circuit of the inductor element, the inductor element is... The equivalent value is a resistance calculated from the equivalent value. and equivalent current source Parallel-connected equivalent circuits of the Begeron circuit.

[0043] According to inductor The relationship between the voltage difference across the terminals and the rate of change of the current flowing through them can be summarized as follows: (8); in, , Inductor Left node 1 and right node 2 are Voltage at time, For a moment Current flowing through the inductor The current flows from node 1 to node 2 in the reference direction.

[0044] For both sides of equation (8) Integrating within the inner, we get: (9); in, For a moment Current flowing through the inductor The current flows from node 1 to node 2 in the reference direction.

[0045] According to the trapezoidal rule, we can obtain from equation (9): (10); Substituting equation (10) into equation (9), we obtain the Norton equivalent circuit expression for the circuit: (11); The equivalent system of Bergeron equations is obtained by rearranging the equations: (12); in, for The equivalent current source at a given time.

[0046] In equation (12) Substitution The definition is as follows: The recursive formula for the current source at any given time: (13); Organized Current source at any time and Relationship between current sources at different times: (14); Thus, we have obtained At any moment, inductor The equivalent circuit of the Begeron circuit.

[0047] Preferably, in some embodiments of this application, when constructing the Bergeron equivalent circuit of the capacitor element, the capacitor element is... The equivalent value is a resistance calculation. and equivalent current source Parallel equivalent calculation circuits.

[0048] According to capacitor elements The relationship between the current flowing through the device and the voltage difference across its terminals can be summarized as follows: (15); According to the trapezoidal rule, we can obtain Current source at any time and Relationship between current sources at different times: (16); in, Capacitor element The capacitance value.

[0049] The above is the result. At any given moment, the capacitor element The equivalent circuit of the Begeron circuit.

[0050] Preferably, in some embodiments of this application, after obtaining the equivalent Begeron circuits corresponding to each power component, a Begeron equivalent circuit (i.e., a multimode equivalent network model) of the flexible DC system from the AC power supply to the DC line side is constructed. Utilizing the principle of impedance matching, a large resistor is connected in parallel at the DC-side outlet of the VSC converter to ground, constructing a reflection-free boundary to ignore the traveling wave reflection effect of subsequent lines. A short-circuit ground fault is set on the AC side A-phase, and a multimode equivalent network model containing the corresponding networks of mode 1, mode 2, and mode 0 is constructed. Specific steps include the following T121 to T122: T121: Perform modal decoupling and independent network construction. Based on the Kelenberger transform, the three-phase coupled transmission system is decoupled into three independent single-phase modal networks: mode 1 network (i.e., the first primary network model), mode 2 network (i.e., the second primary network model), and mode 0 network (i.e., the third primary network model).

[0051] According to the superposition principle, the electrical quantity after a fault = the normal component before the fault + the superimposed component caused by the fault. In order to analyze the superimposed component network separately, the normal power supply on the system side is regarded as a short circuit, and only the line parameters are retained.

[0052] For each mode of network, as shown in step T111, a lossless Begeron transmission line model is used for equivalence, transforming the distributed parameter transmission line into a series structure of wave impedance and historical current source. Specifically, wave impedance for mode 1 and mode 2 is calculated using line mode parameters, while wave impedance for mode 0 is calculated using zero-mode parameters.

[0053] T122: For a short-circuit ground fault f1 in phase A, the wave equation of the circuit is decoupled into independent modal components according to the Kelvin transform. Based on the fault point boundary conditions, the topological connection relationship of the 1-mode network, 2-mode network and 0-mode network connected in series at the fault point is obtained.

[0054] During fault feature extraction, considering the severe dispersion and attenuation of the zero-mode component during transmission through the ground loop, resulting in significant wavefront distortion detected at the far end, the traveling wave transmission details of the zero-mode in the spatial dimension are ignored. Only its boundary constraint effect at the fault point is retained, and the equivalent impedance is used. This replaces the representation of the 0-mode network (i.e., the third primary network model). For the 2-mode network, since its wave process is highly symmetrical with the 1-mode network, a constant wave impedance load is used at the fault point. Instead, a 2-mode network (i.e., the second primary network model) is used. In this case, an additional excitation source is inserted at the fault point to simulate the transient process. This source uses a step voltage model, with the amplitude taken as the negative of the voltage instantaneously before the A-phase fault, and the negative terminal pointing towards the reference ground, as shown below. Figure 3 As shown. To simulate the instantaneous effect of a fault, an additional fault excitation source (i.e., fault excitation) is inserted into the series circuit. The excitation source is a step voltage source, the voltage amplitude of which is the opposite of the voltage of phase A instantaneously before the fault occurs, and the negative terminal of the voltage source faces the reference ground.

[0055] in, Figure 3 middle, The voltage of each node in the 1-mode network; For power supply; and These are the combined equivalent impedances of the power supply in a 1-mode network. and Inductor components Equivalent calculation of resistance and equivalent current source, , , These are the equivalent resistance and controlled voltage source of node M to the left of transmission line MN in a mode 1 network, respectively. , , These are the equivalent resistance and controlled voltage source of node N to the right of transmission line MN in a mode 1 network, respectively. and Transformer elements of a 1-mode network Equivalent resistance and equivalent current source on the primary side; and Transformer elements of a 1-mode network Equivalent resistance and equivalent current source on the secondary side; and Transformer elements of a 1-mode network Equivalent resistance and equivalent current source on the primary side; and Transformer elements of a 1-mode network Equivalent resistance and equivalent current source on the secondary side; The wave impedance boundary value of the 2-mode network; The wave impedance boundary value for the 0-mode network; For fault transition resistance; To provide additional fault excitation sources.

[0056] By employing the trapezoidal integral method to discretize each power component from the AC side to the DC side and establishing a Bergeron equivalent model, the continuous system is transformed into a recursively computeable discrete network model, thus ensuring the numerical stability and accuracy of transient process calculations. Furthermore, the equivalent model is projected onto the mode space using the Kelvin transform, and primary network models for each mode are constructed according to the physical topology, allowing for the decomposition and expression of the electrical characteristics of the system under different modes. Finally, the modal networks are connected in series according to the single-phase ground fault boundary conditions, accurately reflecting the coupling relationships between multiple modes under fault conditions. This improves the accuracy of fault modeling, provides a reliable foundation for subsequent theoretical current response calculations, and enhances fault identification accuracy from the source.

[0057] Further, in some embodiments of this application, the primary network model includes: a first primary network model corresponding to the first linear mode, a second primary network model corresponding to the second linear mode, and a third primary network model corresponding to the zero mode; the step of connecting the primary network models under each mode in series at the preset fault point according to the boundary conditions to obtain the multimodal equivalent network model includes: By using the Kelvin transform, the boundary conditions are transformed to the modal domain to obtain the modal domain boundary conditions of the fault point in different modes. Based on the modal boundary conditions where the fault point is located in different modes, the outflow end of the first primary network model is connected to the inflow end of the second primary network model, and the outflow end of the second primary network model is connected to the inflow end of the third primary network model to obtain the multimodal equivalent network model.

[0058] Preferably, in some embodiments of this application, step T121 includes: A phase A short-circuit ground fault occurred on the transmission line. f 1. At this point, the boundary conditions at the fault point are: (twenty one); According to equation (22), the Kalenberg transformation is performed to transform the fault boundary conditions to the module domain, resulting in the module domain boundary conditions: (twenty two); (twenty three); in, , as well as A phase A short-circuit ground fault occurred on the transmission line. f At time 1, the voltages of phases A, B, and C at terminal M; , as well as A phase A short-circuit ground fault occurred on the transmission line. fAt time 1, the currents of phases A, B, and C at terminal M; , as well as A phase A short-circuit ground fault occurred on the transmission line. f At time 1, the voltages of mode 0, mode 1, and mode 2 at terminal M; , as well as A phase A short-circuit ground fault occurred on the transmission line. f At time 1, the current of M terminal 0 mode, 1 mode, and 2 mode.

[0059] According to equation (23), the short-circuit ground fault in phase A is obtained. f In step 1, the 0-mode, 1-mode, and 2-mode Begeron networks should be connected in series at the fault point. The fault point ports of the 1-mode network, the 2-mode network, and the 0-mode network should be connected in series end to end. That is, the outflow end of the 1-mode network is connected to the inflow end of the 2-mode network, and the outflow end of the 2-mode network is connected to the inflow end of the 0-mode network, forming a single series flow loop.

[0060] This application transforms boundary conditions to the mode domain space and sequentially connects the primary network models corresponding to the first line mode, the second line mode, and the zero mode within the mode domain, thus clearly expressing the constraint relationships of single-phase grounding faults in different modes. This approach avoids the complex coupling errors caused by direct coupling in the phase domain, making the modal voltage and current relationships at the fault point clearer. This facilitates the accurate characterization of the modal component changes caused by the fault, thereby improving the expression accuracy of the multimodal equivalent network model under single-phase grounding fault conditions and enhancing the accuracy of subsequent theoretical response calculations.

[0061] Furthermore, in some embodiments of this application, the step of applying a fault excitation at a preset fault point in the multimodal equivalent network model and calculating the theoretical current response at a preset protection installation location under the fault excitation based on the multimodal equivalent network model includes: In the multimodal equivalent network model, fault excitations corresponding to single-phase grounding faults are injected into each of the preset fault points to form fault excitation conditions. Under the fault excitation condition, the node voltage equations of each node in the first primary network model are established, and the node voltage equations are solved by recursive calculation to obtain the voltage values ​​of each node at each time. Obtain the Beerleon equivalent parameters of each branch in the first primary network model, and calculate the theoretical current value of the branch where the protection installation is located at each time based on the voltage values ​​of each node and the Beerleon equivalent parameters of the branch where the node is located, so as to obtain the theoretical current response.

[0062] Preferably, in some embodiments of this application, the voltage value of each node is calculated and obtained through the following steps: due to the node voltage By connecting the fault point to the 2-mode network and the 0-mode network, and according to Thevenin's theorem, the following formula can be derived: (twenty three); According to Kirchhoff's current theorem, the current excitation matrix of each node in the equivalent circuit is obtained. .

[0063] (twenty four); According to equation (25), the admittance matrix is ​​obtained. .

[0064] (25); The voltage at each node can be calculated using equation (26).

[0065] (26); Preferably, in some embodiments of this application, the theoretical current response is calculated through the following steps: calculating the current time at the installation location of the line protection. hour 1-mode current : (27); in, For the current moment Protective installation location The rated current, and the rated current calculated at multiple times constitute the theoretical current response; Let M be the equivalent resistance of node M to the left of transmission line MN in a 1-mode network, and also the equivalent parameter of the Bergeron network.

[0066] This application, after applying fault excitation to a multimodal equivalent network model, first establishes the node voltage equations of the first primary network model, and then solves for the node voltages using a recursive method based on the discretized Bergeron equivalence relation. Finally, it calculates the theoretical current value at the protection installation location based on branch parameters, ensuring that the calculation of the theoretical current response follows a clear voltage-current physical derivation path. This method avoids errors that may arise from directly approximating the current calculation, accurately reflects the voltage-current coupling relationship during fault transient propagation, improves the accuracy of the theoretical current response calculation, and thus provides a reliable foundation for accurately extracting theoretical transient characteristics.

[0067] Furthermore, in some embodiments of this application, the extraction of theoretical transient characteristic values ​​from the theoretical current response includes: By sampling from the theoretical current response at a preset sampling frequency, the current component sequence of the first line mode is obtained; Perform continuous wavelet transform on the current component sequence to obtain the time-frequency diagram; The time-frequency graph is subjected to transient extraction transform to obtain the time-frequency spectrum; The theoretical wavefront count at several preset frequencies is extracted based on the time spectrum, and the average of the theoretical wavefront count is used as the theoretical transient characteristic value.

[0068] Preferably, in some embodiments of this application, the step of sampling from the theoretical current response using a preset sampling frequency to obtain the current component sequence of the first line mode includes: setting an AC side A-phase short-circuit ground fault, and extracting a time window starting from the current moment. The line protection installation location was obtained. 1-mode current .by Sampling frequency Sampling interval for time window Internal current signal Sampling is performed, and the sampling points within the time window are used to construct the AC side-line mode current component sequence. .

[0069] (28); in, The number of sampling points within the time window. = ;set up for A time series composed of sampling points.

[0070] Preferably, in some embodiments of this application, after obtaining the 1-mode current component sequence, a transient extraction transform method based on continuous wavelet transform is used to extract the current components in... The number of mutation points within the zone is used as the threshold, multiplied by 0.85, to determine whether a fault occurs within or outside the zone. (Refer to...) Figure 4 The diagram shows the modulus current and mutation rate at the line protection installation point.

[0071] Preferably, in some embodiments of this application, performing continuous wavelet transform on the current component sequence to obtain a time-frequency diagram includes: obtaining a 1-mode current component sequence from the Bergeron equivalent circuit of the system. Perform continuous wavelet transform according to the following formula: (29); in, Scale factor; The translation factor; For the mother wavelet function; The complex conjugate of the mother wavelet function; Normalization factor; This is the result of continuous wavelet transform.

[0072] Preferably, in some embodiments of this application, the transient extraction transform of the time-frequency graph to obtain the time-frequency spectrum includes: The transient extraction transformation is performed using the following formula: (30); (31); in, For the signal at scale and time The instantaneous angular frequency; The partial derivatives of the wavelet coefficients with respect to time; For Dirac function; The result of the transient extraction transformation, such as Figure 5 As shown.

[0073] Preferably, in some embodiments of this application, the theoretical number of wavefronts at several preset frequencies is determined by the following formula: (32); in, ; =1 kHz, =5 kHz =10 kHz; The theoretical number of wavefronts at frequencies of 1 kHz, 5 kHz, and 10 kHz; It is a counting function; The detection threshold; The arrival time of the wave head; Sampling frequency The sampling interval; where the theoretical wavefronts detected at 5 kHz are as follows: Figure 6 As shown.

[0074] Preferably, in some embodiments of this application, the theoretical transient characteristic value is calculated using the following formula: (33); in, The theoretical number of wavefronts at frequencies of 1 kHz, 5 kHz, and 10 kHz, respectively; This represents the theoretical transient eigenvalue.

[0075] Preferably, in some embodiments of this application, after obtaining the theoretical transient characteristic value, the detection threshold is set by the following formula for fault determination.

[0076] (34); in, For the detection threshold, For the reliability coefficient, in this embodiment, the reliability coefficient is... =0.85, =31, =26.

[0077] In this embodiment, the sampling frequency is set. =50 kHz (sampling interval is 20) ), The time window is 10 ms. The number of sampling points within the time window is 500. The number of wavefronts detected at 1 kHz, 5 kHz, and 10 kHz are 22, 34, and 37, respectively, which means the theoretical number of wavefronts is 31. The detection threshold is set to 26.

[0078] This application obtains the time-frequency distribution characteristics by performing continuous wavelet transform on the theoretical current response, and identifies the number of wavefronts at specific frequencies through transient extraction transform, enabling the theoretical transient characteristic values ​​to reflect the high-frequency transient change patterns in the initial stage of a fault. Compared to relying solely on amplitude or phasor characteristics, this method can extract more sensitive transient information, improve the identification capability of asymmetrical faults such as single-phase grounding, and thus enhance the matching accuracy of theoretical characteristics with actual fault characteristics.

[0079] S103: Real-time acquisition of electrical signals of the flexible DC system at the preset protection installation location; when the electrical signal is detected to meet the preset start-up conditions, extraction of actual transient characteristic values ​​from the actual current response in the electrical signal.

[0080] Furthermore, in some embodiments of this application, the electrical signal includes: a first three-phase voltage signal; the detection that the electrical signal meets a preset start-up condition includes: The first three-phase voltage signal is divided into two second three-phase voltage signals. The zero-mode voltage change signal is obtained based on the difference between the two second three-phase voltage signals. The zero-mode voltage abrupt change signal is processed by exponential weighted moving average to obtain the zero-mode voltage characteristic value at the current detection time; When the zero-mode voltage characteristic value is greater than or equal to the preset start-up threshold, the electrical signal is determined to meet the preset start-up conditions.

[0081] Preferably, in some embodiments of this application, when acquiring the first three-phase voltage signal, the three-phase voltage at point M on the AC side of the converter transformer is detected in real time, and the current detection time is used as the reference. Based on this, the time window is... The voltage signal, at a sampling rate The three-phase voltage signals within this time window are acquired. The A, B, and C phase voltage signals acquired within this time window are used to construct a three-phase voltage signal matrix. (i.e., the first three-phase voltage signal), where the matrix It will be updated in real time over time.

[0082] (35); in, The number of sampling points included within the time window, where ; , , The three-phase voltages at the busbar are A, B, and C, respectively. The voltage signal obtained by constant measurement; , , The three-phase voltages at the busbar are A, B, and C, respectively. The voltage signal obtained by measuring at any time.

[0083] In this example, the sampling rate is set. =50kHz, time window Take 2 cycles, =40 ms, therefore the number of sampling points within the time window It is 2000.

[0084] Preferably, in some embodiments of this application, the step of dividing the first three-phase voltage signal into two second three-phase voltage signals and obtaining the zero-mode voltage change signal based on the difference between the two second three-phase voltage signals includes: In this embodiment, the time window for sampling the signal is Divide it into two parts: and From the three-phase voltage signal matrix The signal values ​​within the two time windows are extracted and used to construct matrices. and (i.e., the second and third phase voltage signals).

[0085] (36); (37); matrix and Perform the subtraction operation as shown in equation (38) to obtain the matrix. That is, at the current detection time The voltage fluctuations within the previous two cycles are shown below.

[0086] (38); (39); in, , , These represent the three phases A, B, and C at the busbar at the current detection time. Voltage fluctuation; For time windows and The number of sampling points contained within.

[0087] Preferably, in some embodiments of this application, the zero-mode voltage abrupt change signal is processed by an exponentially weighted moving average to obtain the zero-mode voltage characteristic value at the current detection time, including: For matrix Each column (i.e., the three-phase voltage signal at each moment within these two cycles) is transformed according to equation (40) to obtain a vector. Among them, vectors It is a matrix composed of zero-mode voltage abrupt change signals.

[0088] (40); Where, the range of values ​​for t0 is: .

[0089] (41); in, In order to be in The instantaneous change in zero-mode voltage at time t; for The zero-mode voltage change at time t is sampled at the following number of points: .

[0090] For zero-mode voltage mutation vector According to equation (43), an exponentially weighted moving average is performed to obtain the result at each time step. Weighted zero-mode voltage eigenvalues Let it be the starting criterion, as shown below: (42); (43); in, For vectors middle The zero-mode voltage change value at time t. For the first Normalized exponential weighted coefficients for each sampling point , Corresponding to the closest time within the sliding window The sampling point with the largest weight; The length of the sliding window; It is the attenuation factor; In order to be in The zero-mode voltage characteristic value at time t.

[0091] Through the above formula, if Not satisfied If the AC side of the converter transformer is in normal operating condition, then it is considered to be in normal operating condition; if satisfy If the AC side of the converter transformer is in an abnormal operating state, i.e., the preset start-up conditions are met, and the current detection time is recorded as the fault occurrence time. In this embodiment The voltage is set to 270V, and the duration of two cycles is 0.04s. For example... Figure 7 The image shows the voltage and current waveforms before and after the fault. Figure 8 As shown, the activation criterion The calculated value is 2951.1V, which meets the startup criterion.

[0092] This application decomposes the three-phase voltage signal and constructs a zero-mode voltage abrupt change signal. It then employs an exponentially weighted moving average to obtain the zero-mode voltage characteristic value, making the start-up criterion more focused on the abrupt change information in the initial stage of a fault, while suppressing random fluctuations and non-fault-related interference components introduced by the modulation strategy. This method can improve the stability of the start-up judgment while ensuring sensitivity, avoiding false triggering or missed triggering, and providing a reliable basis for accurately obtaining the actual current response.

[0093] Furthermore, in some embodiments of this application, the step of extracting actual transient characteristic values ​​from the actual current response includes: Using the Kelenberger transform, the actual current response is converted to the mode domain. The first line mode current signal corresponding to the first line mode and the second line mode is obtained from the converted actual current response. Based on the two first line mode current signals, the second line mode current signal is obtained. The envelope signal is obtained by combining the Hilbert transform with the second line-mode current signal; The envelope signal is subjected to nonlinear combination operations by a preset nonlinear instantaneous operator to obtain an energy feature sequence that characterizes the instantaneous energy change of the signal. Based on a preset energy threshold, the number of effective wavefront mutation points in the energy feature sequence within a preset time window is identified, and the number is used as the actual transient feature value.

[0094] Preferably, in some embodiments of this application, when collecting the actual current response, the fault time recorded in the above embodiments is used. Based on this, set the time window size to [value]. sampling rate The three-phase voltage and current at terminal M of the AC side line of the converter transformer are sampled to obtain the actual current response: Use sampling rate For time windows The three-phase voltage and current at terminal M are sampled, and a matrix is ​​constructed using the sampling points within the time window. .

[0095] (44); in, , , , , , These are the three-phase voltage and current at terminal M, respectively. , , Together they constitute the actual current response; , , , , , They are respectively The three-phase voltage and current at terminal M are measured at all times. The number of sampling points within the time window. .

[0096] In this embodiment, the sampling frequency is set. =50 kHz (sampling interval is 20) ), time window For 1 cycle, The time window is 10 ms, and the number of sampling points within the time window is 1000.

[0097] Preferably, in some embodiments of this application, the step of using Kelenberger transform to convert the actual current response to the mode domain, obtaining the first line mode current signal corresponding to the first line mode and the second line mode from the converted actual current response, and obtaining the second line mode current signal based on the two first line mode current signals includes: For matrix The matrix is ​​obtained by performing a Kelenberg transformation. The voltage and current components of the M-terminal 1 and 2 modes are denoted as row vectors. , , , , and These are the first line-mode current signals corresponding to mode 1 and mode 2, respectively; the voltage and current components of mode 0 are denoted as row vectors. , .

[0098] (45); (46); in, The specific representation of a matrix is ​​as follows: (47); in, , , , , , The three-phase voltage and current at terminal M are respectively The result of performing the Karenbel transformation at any given time.

[0099] Extracting the matrix The fourth and fifth lines use the second half of the time window. Subtract the first half of the time window from the elements The elements yield two entirely new row vectors. , .

[0100] (48); (49); in, The current at terminal M is modulo 1. The current at terminal M is modulo 2.

[0101] Transform each element of the fault components of the M-terminal current in both mode 1 and mode 2 according to equation (50) to obtain a new row vector. The reference current signal used to characterize mode 1 (i.e., the second line mode current signal). It should be noted that the "first" or "second" in the first and second line mode current signals does not represent the current signal of mode 1 (i.e., the first line mode) or mode 2 (i.e., the second line mode). The first line mode current signal includes the current signals corresponding to both mode 1 and mode 2, while the second line mode current signal is the reference current signal corresponding to mode 1.

[0102] (50); (51); in, =0.6, =0.4.

[0103] Preferably, in some embodiments of this application, the envelope signal is obtained by combining the Hilbert transform with the second line-mode current signal, including: right The synthesized current is obtained by sampling. ,Will As the target of the subsequent Hilbert transform, For vectors The One sampling point.

[0104] Sampling sequence of fault current at M terminal We perform a Hilbert transform to extract the signal envelope. First, we define the frequency response of the ideal Hilbert transform in the frequency domain. : (52); in, It is the frequency response function. The imaginary unit, This is the digital angular frequency. The corresponding unit impulse response in the time domain is obtained through the inverse discrete-time Fourier transform. : (53); in, The input signal is the unit impulse response of the Hilbert transform. The result after Hilbert transformation : (54); in, The result of the Hilbert transform, This is an auxiliary variable used in convolution summation. The specific processing procedure is as follows: Figure 9 As shown.

[0105] An analytic signal can be constructed as follows: (55); Among them, analytic signal The signal is a complex number, and its real part is the input signal. The imaginary part is a Hilbert transform. .

[0106] signal amplitude envelope It can be obtained by calculating the modulus of the analytical signal: (56); The process of envelope demodulation of modulated signals based on Hilbert transform is as follows: Figure 10 .

[0107] Preferably, in some embodiments of this application, the envelope signal is subjected to nonlinear combination operations using a preset nonlinear instantaneous operator to obtain an energy feature sequence characterizing the instantaneous energy change of the signal, including: By applying the Teager energy operator to the envelope signal using the following formula, an energy feature sequence characterizing instantaneous energy changes in the signal can be obtained: (57); The obtained energy sequence It exhibits an extreme value at the moment of wavefront arrival. For the energy sequence... Perform peak search, count the number of peaks exceeding a preset threshold, and record them as the measured wavefront count. ,like Figure 11 As shown.

[0108] This application enhances the modal components related to fault propagation and weakens irrelevant modal interference by converting the actual current response to the modal domain and weighting and fusing the first line mode current. Furthermore, it obtains the envelope signal through Hilbert transform and extracts energy mutation features using nonlinear instantaneous operators, enabling the actual transient characteristic values ​​to highlight wavefront mutation information. This processing improves the sensitivity of fault transient wavefront identification, helps to accurately count the number of effective wavefronts, thereby improving the reliability of comparing actual and theoretical characteristics and further enhancing the accuracy of AC incoming line fault identification.

[0109] S104: Compare the actual transient characteristic value with the theoretical transient characteristic value, obtain the comparison result, and identify the fault of the flexible DC AC input line based on the comparison result.

[0110] In this embodiment, the theoretical number of wavefronts The value is 31, the detection threshold is 26, and the actual number of wavefronts is calculated according to equation (59). The value is 29, therefore it can be determined that a phase A short-circuit ground fault has occurred on the AC side of the flexible straight-line converter transformer.

[0111] In summary, the fault identification method for flexible DC AC input lines provided in this application has the following advantages: (1) This application adopts the single-ended quantity protection principle, which only needs to use the voltage and current information collected at the protection installation point to complete the fault identification, completely getting rid of the dependence on the communication link and completely eliminating the risk of protection failure caused by communication delay or packet loss; at the same time, only 10ms of fault data window needs to be intercepted to perform fault identification, realize millisecond-level rapid identification and removal of faults, and effectively ensure the safety of converter station equipment.

[0112] (2) This application no longer relies on the magnitude of the power frequency current, but captures the singularity characteristics of the waveform in the time domain at the moment of fault occurrence. Even when the fault current is limited by the converter, the waveform abrupt change singularity point generated by the fault is still significant, thereby greatly improving the sensitivity of the protection under the weak feed characteristics of the flexible DC system.

[0113] (3) This application uses the Bergeron method to accurately establish the AC side model of the flexible DC system, and obtains the characteristic value of the number of singular points under faults in the zone through theoretical derivation. Based on this, the action threshold is set and faults outside the zone are excluded. This effectively avoids the interference of steady-state control strategies such as third harmonic modulation on the protection criteria, and ensures that the protection device has extremely high reliability in distinguishing faults in the zone from faults outside the zone, thus achieving a unity of high sensitivity and high selectivity.

[0114] like Figure 12 As shown, based on the above-mentioned method embodiments, an embodiment of this application provides a fault identification device for a flexible DC AC input line, including: a theoretical feature extraction module 201, an actual feature extraction module 202, and a fault identification module 203; The theoretical feature extraction module 201 is used to connect the equivalent circuits of the flexible DC system under different modes according to the boundary conditions of a single-phase ground fault, construct a multi-mode equivalent network model, apply fault excitation to a preset fault point in the multi-mode equivalent network model, calculate the theoretical current response of the preset protection installation location under the fault excitation based on the multi-mode equivalent network model, and extract theoretical transient feature values ​​from the theoretical current response; the modes include a first line mode, a second line mode, and a zero mode; The actual feature extraction module 202 is used to collect electrical signals of the flexible DC system at a preset protection installation location in real time, and extract actual transient feature values ​​from the actual current response in the electrical signal when the electrical signal is detected to meet the preset start-up conditions. The fault identification module 203 is used to compare the actual transient characteristic value with the theoretical transient characteristic value, obtain the comparison result, and identify the fault of the flexible DC AC input line based on the comparison result.

[0115] In summary, the fault identification device for flexible DC AC incoming lines provided in this application has the following advantages compared to the prior art: This application constructs a multi-mode equivalent network model by connecting the equivalent circuits of the flexible DC system under different modes such as the first line mode, the second line mode, and the zero mode according to the single-phase grounding fault boundary conditions. This allows for a complete characterization of the modal coupling relationship of the system under fault conditions, accurately reflecting the transient response mechanism of the flexible DC system under a single-phase grounding fault from a theoretical perspective. Based on this, by applying fault excitation at a preset fault point to calculate the theoretical current response at the protection installation location and extracting theoretical transient characteristic values, a fault theoretical characteristic benchmark consistent with the system's physical characteristics is established. Furthermore, after a fault occurs, the transient characteristic values ​​of the actual current response are extracted and compared with the theoretical transient characteristic values. Since the comparison objects are derived from theoretical calculations under the same system structure and the same fault boundary conditions, the impact of the converter control strategy on the fault current amplitude limitation and phase change can be effectively reduced, improving the identification accuracy of asymmetrical faults such as single-phase grounding, thereby enhancing the reliability of fault identification for flexible DC AC incoming lines. This approach can effectively overcome the influence of the flexible DC converter control strategy on the amplitude and phase relationship of the fault current, reduce the risk of misjudgment caused by zero-sequence interference introduced by the modulation strategy, and thus significantly improve the accuracy and reliability of flexible DC AC incoming line fault identification.

[0116] It is understood that the above-described device embodiments correspond to the method embodiments of this application, and can implement the fault identification method for flexible DC AC input lines provided by any of the above-described method embodiments of this application.

[0117] It should be noted that the device embodiments described above are merely illustrative, and some or all of the modules can be selected to achieve the purpose of this embodiment according to actual needs. Furthermore, in the accompanying drawings of the device embodiments provided in this application, the connection relationships between modules indicate that they have communication connections, which can specifically be implemented as one or more communication buses or signal lines. Those skilled in the art can understand and implement this without any creative effort.

[0118] Based on the above embodiments of the fault identification method for flexible DC AC input lines, another embodiment of this application provides a terminal device, which includes a processor, a memory, and a computer program stored in the memory and configured to be executed by the processor. When the processor executes the computer program, it implements the fault identification method for flexible DC AC input lines of any embodiment of this application.

[0119] For example, in this embodiment, the computer program can be divided into one or more modules, which are stored in the memory and executed by the processor to complete this application. The one or more module units may be a series of computer program instruction segments capable of performing a specific function, which describe the execution process of the computer program in the terminal device.

[0120] The terminal device may be a desktop computer, laptop, handheld computer, or cloud server, etc. The terminal device may include, but is not limited to, a processor and a memory.

[0121] The processor can be a Central Processing Unit (CPU), or other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. A general-purpose processor can be a microprocessor or any conventional processor. The processor is the control center of the terminal device, connecting all parts of the terminal device via various interfaces and lines.

[0122] Based on the above-described method embodiments, another embodiment of this application provides a computer-readable storage medium including a stored computer program, wherein, when the computer program is executed, it controls the device where the computer-readable storage medium is located to execute the fault identification method for flexible DC AC input lines described in any of the above-described method embodiments of this application.

[0123] The modules / units integrated in the device / terminal equipment, if implemented as software functional units and sold or used as independent products, can be stored in a computer-readable storage medium. Based on this understanding, all or part of the processes in the methods of the above embodiments can also be implemented by a computer program instructing related hardware. The computer program can be stored in a computer-readable storage medium, and when executed by a processor, it can implement the steps of the various method embodiments described above. The computer program includes computer program code, which can be in the form of source code, object code, executable files, or certain intermediate forms. The computer-readable medium can include: any entity or device capable of carrying the computer program code, recording media, USB flash drives, portable hard drives, magnetic disks, optical disks, computer memory, read-only memory (ROM), random access memory (RAM), electrical carrier signals, telecommunication signals, and software distribution media, etc.

Claims

1. A fault identification method for a flexible DC AC input line, characterized in that, include: The equivalent circuits of the flexible DC system under different modes are topologically connected according to the boundary conditions of a single-phase ground fault to construct a multi-mode equivalent network model. By applying fault excitation to a preset fault point in the multi-mode equivalent network model, the theoretical current response of the preset protection installation location under the fault excitation is calculated based on the multi-mode equivalent network model, and theoretical transient characteristic values ​​are extracted from the theoretical current response. The modes include a first line mode, a second line mode, and a zero mode. The electrical signals of the flexible DC system at the preset protection installation location are collected in real time. When the electrical signals are detected to meet the preset start-up conditions, the actual transient characteristic values ​​are extracted from the actual current response in the electrical signals. The actual transient characteristic value is compared with the theoretical transient characteristic value to obtain the comparison result, and the fault of the flexible DC AC input line is identified based on the comparison result.

2. The fault identification method for a flexible DC AC input line as described in claim 1, characterized in that, The process of connecting the equivalent circuits of the flexible DC system in different modes according to the boundary conditions of a single-phase ground fault to construct a multimodal equivalent network model includes: Using the trapezoidal integral method, each power element from the AC side to the DC side of the flexible DC system is discretized to obtain the equivalent circuit of Begeron circuit under different modes corresponding to each power element. Based on the physical topology of the flexible DC system, the various Begeron equivalent circuits under the same mode are interconnected to form a primary network model under each mode. The primary network models under each mode are connected in series at the preset fault point according to the boundary conditions to obtain the multimodal equivalent network model.

3. The fault identification method for a flexible DC AC input line as described in claim 2, characterized in that, The primary network model includes: a first primary network model corresponding to the first linear mode, a second primary network model corresponding to the second linear mode, and a third primary network model corresponding to the zero mode; the step of connecting the primary network models under each mode in series at the preset fault point according to the boundary conditions to obtain the multimodal equivalent network model includes: By using the Kelvin transform, the boundary conditions are transformed to the modal domain to obtain the modal domain boundary conditions of the fault point in different modes. Based on the modal boundary conditions where the fault point is located in different modes, the outflow end of the first primary network model is connected to the inflow end of the second primary network model, and the outflow end of the second primary network model is connected to the inflow end of the third primary network model to obtain the multimodal equivalent network model.

4. The fault identification method for a flexible DC AC input line as described in claim 3, characterized in that, The step of applying a fault excitation at a preset fault point in the multimodal equivalent network model and calculating the theoretical current response of the preset protection installation location under the fault excitation based on the multimodal equivalent network model includes: In the multimodal equivalent network model, fault excitations corresponding to single-phase grounding faults are injected into each of the preset fault points to form fault excitation conditions. Under the fault excitation condition, the node voltage equations of each node in the first primary network model are established, and the node voltage equations are solved by recursive calculation to obtain the voltage values ​​of each node at each time. Obtain the Beerleon equivalent parameters of each branch in the first primary network model, and calculate the theoretical current value of the branch where the protection installation is located at each time based on the voltage values ​​of each node and the Beerleon equivalent parameters of the branch where the node is located, so as to obtain the theoretical current response.

5. The fault identification method for a flexible DC AC input line as described in claim 1, characterized in that, The extraction of theoretical transient characteristic values ​​from the theoretical current response includes: By sampling from the theoretical current response at a preset sampling frequency, the current component sequence of the first line mode is obtained; Perform continuous wavelet transform on the current component sequence to obtain the time-frequency diagram; The time-frequency graph is subjected to transient extraction transform to obtain the time-frequency spectrum; The theoretical wavefront count at several preset frequencies is extracted based on the time spectrum, and the average of the theoretical wavefront count is used as the theoretical transient characteristic value.

6. The fault identification method for a flexible DC AC input line as described in claim 1, characterized in that, The electrical signal includes: a first three-phase voltage signal; the detection that the electrical signal meets the preset start-up conditions includes: The first three-phase voltage signal is divided into two second three-phase voltage signals. The zero-mode voltage change signal is obtained based on the difference between the two second three-phase voltage signals. The zero-mode voltage abrupt change signal is processed by exponential weighted moving average to obtain the zero-mode voltage characteristic value at the current detection time; When the zero-mode voltage characteristic value is greater than or equal to the preset start-up threshold, the electrical signal is determined to meet the preset start-up conditions.

7. The fault identification method for a flexible DC AC input line as described in claim 1, characterized in that, Extracting actual transient characteristic values ​​from the actual current response includes: Using the Kelenberger transform, the actual current response is converted to the mode domain. The first line mode current signal corresponding to the first line mode and the second line mode is obtained from the converted actual current response. Based on the two first line mode current signals, the second line mode current signal is obtained. The envelope signal is obtained by combining the Hilbert transform with the second line-mode current signal; The envelope signal is subjected to nonlinear combination operations by a preset nonlinear instantaneous operator to obtain an energy feature sequence that characterizes the instantaneous energy change of the signal. Based on a preset energy threshold, the number of effective wavefront mutation points in the energy feature sequence within a preset time window is identified, and the number is used as the actual transient feature value.

8. A fault identification device for a flexible DC AC input line, characterized in that, include: Theoretical feature extraction module, practical feature extraction module, and fault identification module; The theoretical feature extraction module is used to connect the equivalent circuits of the flexible DC system under different modes according to the boundary conditions of a single-phase ground fault, construct a multi-mode equivalent network model, apply fault excitation to a preset fault point in the multi-mode equivalent network model, calculate the theoretical current response of the preset protection installation location under the fault excitation based on the multi-mode equivalent network model, and extract theoretical transient feature values ​​from the theoretical current response; the modes include a first line mode, a second line mode, and a zero mode; The actual feature extraction module is used to collect electrical signals of the flexible DC system at a preset protection installation location in real time, and extract actual transient feature values ​​from the actual current response in the electrical signal when the electrical signal is detected to meet the preset start-up conditions. The fault identification module is used to compare the actual transient characteristic value with the theoretical transient characteristic value, obtain the comparison result, and identify the fault of the flexible DC AC input line based on the comparison result.

9. A terminal device, characterized in that, The system includes a processor, a memory, and a computer program stored in the memory and configured to be executed by the processor, wherein the processor, when executing the computer program, implements a fault identification method for a flexible DC AC input line as described in any one of claims 1 to 7.

10. A computer-readable storage medium, characterized in that, The computer-readable storage medium includes a stored computer program, wherein, when the computer program is executed, it controls the device containing the computer-readable storage medium to perform a fault identification method for a flexible DC AC input line as described in any one of claims 1 to 7.

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