A fault detection method for a flexible direct current power transmission system and related apparatus

Abstract

The application discloses a fault detection method and related device of a flexible direct current power transmission system. The method adopts a voltage gradient algorithm to detect whether the flexible direct current power transmission system has a fault. When the system has a fault, a fault type is distinguished based on a ground model voltage polarity through a sampled voltage signal, a high-frequency impedance difference ratio of a rectification side and an inversion side in the flexible direct current power transmission system is determined based on the voltage signal and a sampled current signal, a fault direction is distinguished based on the high-frequency impedance difference ratio, and finally a fault detection result is determined according to the fault type and the fault direction. It can be seen that the fault polarity is selected based on the ground model voltage polarity, and the intra-zone and extra-zone faults are accurately distinguished through the high-frequency impedance difference ratio, so that the reliability of the fault detection of the flexible direct current power transmission system is improved.

Description

Technical Field

[0001] This application relates to the field of power fault detection technology, and in particular to a fault detection method and related device for a flexible DC transmission system. Background Technology

[0002] Traveling wave protection is a protection measure that uses the traveling wave characteristics of voltage and current that occur when a short circuit occurs in a transmission line to identify faults and initiate protection.

[0003] Traditional traveling wave protection methods for flexible DC transmission systems based on Modular Multilevel Converters (MMCs), such as differential current protection and low-frequency quantity protection, suffer from limitations in fault detection, including limited response speed, insufficient anti-interference capability, and poor adaptability, which affect the accuracy of fault location. Therefore, improving the reliability of fault detection has become an urgent problem to be solved. Summary of the Invention

[0004] This application provides a fault detection method and related device for a flexible DC transmission system, which improves the reliability of fault detection by using the polarity of the ground mode voltage and the high-frequency impedance difference ratio.

[0005] In a first aspect, embodiments of this application provide a fault detection method for a flexible DC transmission system, the method comprising:

[0006] Based on a preset voltage gradient algorithm, the system determines whether a fault has occurred in the flexible DC transmission system by real-time collecting voltage signals from the rectifier side and the inverter side.

[0007] If the flexible DC transmission system experiences a fault, the time of the fault occurrence is obtained;

[0008] Based on a preset time window, a first sampling dataset corresponding to the rectifier side and a second sampling dataset corresponding to the inverter side are acquired at the time of the fault occurrence; both the first sampling data and the second sampling data include voltage signals and current signals.

[0009] A preset mode transformation is performed on the voltage signals in the first sampled dataset to obtain a first ground mode voltage set; the preset mode transformation is performed on the voltage signals in the second sampled dataset to obtain a second ground mode voltage set.

[0010] The target fault type is determined based on the first ground mode voltage set and the second ground mode voltage set; the target fault type includes one of the following: positive grounding, negative grounding, and bipolar short circuit.

[0011] Perform a preset wavelet transform on the first sampled dataset to obtain a first high-frequency component set; perform the preset wavelet transform on the second sampled dataset to obtain a second high-frequency component set;

[0012] The first high-frequency impedance difference ratio corresponding to the rectifier side is determined based on the first high-frequency component set; the second high-frequency impedance difference ratio corresponding to the inverter side is determined based on the second high-frequency component set.

[0013] The target fault direction is determined based on the first high-frequency impedance difference ratio and the second high-frequency impedance difference ratio; the target fault direction includes one of the following: external fault and internal fault.

[0014] The target fault detection result is determined based on the target fault type and the target fault direction.

[0015] Secondly, embodiments of this application provide a fault detection device for a flexible DC transmission system. The fault detection device for the flexible DC transmission system includes: a fault detection module, a fault data acquisition module, a data collection module, a mode transformation module, a fault type determination module, a wavelet transform module, a data processing module, a fault direction determination module, and a fault detection result generation module.

[0016] The fault detection module is used to determine whether a fault has occurred in the flexible DC transmission system based on a preset voltage gradient algorithm and the voltage signals on the rectifier side and inverter side collected in real time in the flexible DC transmission system.

[0017] The fault data acquisition module is used to acquire the time of fault occurrence if a fault occurs in the flexible DC transmission system.

[0018] The data acquisition module is used to acquire, based on a preset time window, a first sampling dataset corresponding to the rectifier side and a second sampling dataset corresponding to the inverter side at the time of the fault occurrence; both the first sampling data and the second sampling data include voltage signals and current signals.

[0019] The mode transformation module is used to perform a preset mode transformation on the voltage signals in the first sampled dataset to obtain a first ground mode voltage set; and to perform the preset mode transformation on the voltage signals in the second sampled dataset to obtain a second ground mode voltage set.

[0020] The fault type determination module is used to determine the target fault type based on the first ground mode voltage set and the second ground mode voltage set; the target fault type includes one of the following: positive grounding, negative grounding, and bipolar short circuit;

[0021] The wavelet transform module is used to perform a preset wavelet transform on the first sampled dataset to obtain a first high-frequency component set; and to perform the preset wavelet transform on the second sampled dataset to obtain a second high-frequency component set.

[0022] The data processing module is used to determine the first high-frequency impedance difference ratio corresponding to the rectifier side based on the first high-frequency component set; and to determine the second high-frequency impedance difference ratio corresponding to the inverter side based on the second high-frequency component set.

[0023] The fault direction determination module is used to determine the target fault direction based on the first high-frequency impedance difference ratio and the second high-frequency impedance difference ratio; the target fault direction includes one of the following: external fault and internal fault;

[0024] The fault detection result generation module is used to determine the target fault detection result based on the target fault type and the target fault direction.

[0025] Thirdly, embodiments of this application provide an electronic device, including: a processor, a memory, a communication interface, and one or more programs, wherein the one or more programs are stored in the memory and configured to be executed by the processor, and the programs include instructions for performing the steps in the first aspect of embodiments of this application.

[0026] Fourthly, embodiments of this application provide a computer-readable storage medium storing a computer program for electronic data interchange, wherein the computer program causes a computer to perform some or all of the steps described in the first aspect of embodiments of this application.

[0027] Fifthly, embodiments of this application provide a computer program product, wherein the computer program product includes a non-transitory computer-readable storage medium storing a computer program operable to cause a computer to perform some or all of the steps described in the first aspect of embodiments of this application. The computer program product may be a software installation package.

[0028] It can be seen that the embodiments of this application have the following beneficial effects:

[0029] By implementing the embodiments of this application, a voltage gradient algorithm is used to detect whether a fault has occurred in a flexible DC transmission system. When a fault occurs, the fault type is determined based on the polarity of the ground mode voltage using the sampled voltage signal. The high-frequency impedance difference ratio between the rectifier and inverter sides of the flexible DC transmission system is determined based on the voltage signal and the sampled current signal. The fault direction is then determined based on the high-frequency impedance difference ratio. Finally, the fault detection result is determined based on the fault type and fault direction. It is evident that by achieving fault polarity selection based on the ground mode voltage polarity and accurately distinguishing between faults within and outside the fault zone using the high-frequency impedance difference ratio, the reliability of fault detection in flexible DC transmission systems is improved. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of this application or the background art, the accompanying drawings used in the embodiments of this application or the background art will be described below.

[0031] Figure 1 This is a schematic diagram of the structure of a flexible DC transmission system provided in an embodiment of this application;

[0032] Figure 2 This is a schematic diagram illustrating the analysis of positive fault impedance characteristics provided in an embodiment of this application;

[0033] Figure 3 This is a schematic diagram illustrating the analysis of reverse fault impedance characteristics provided in an embodiment of this application;

[0034] Figure 4a This is a schematic diagram of a single-pole ground fault in a DC fault domain equivalent circuit provided in an embodiment of this application;

[0035] Figure 4b This is a schematic diagram of a bipolar ground fault in a DC fault domain equivalent circuit provided in an embodiment of this application;

[0036] Figure 5 This is a flowchart illustrating a fault detection method for a flexible DC transmission system provided in an embodiment of this application;

[0037] Figure 6 This is a simulation diagram of a high-frequency equivalent impedance model provided in an embodiment of this application;

[0038] Figure 7 This is a diagram showing the calculated impedance curves of the protection on both sides when there is a bipolar short-circuit fault in an area with a fault distance of 300km, as provided in the embodiments of this application.

[0039] Figure 8 This is a diagram showing the calculated impedance curves of the protection on both sides when a single-pole grounding fault occurs within a 100km area, as provided in the embodiments of this application.

[0040] Figure 9This is a diagram showing the calculated impedance curves of the protection on both sides during a bipolar metallic short-circuit fault at the inverter side outlet, as provided in an embodiment of this application.

[0041] Figure 10 This is a diagram showing the calculated impedance curves of the protection on both sides when a single-pole grounding fault occurs outside the rectifier side zone, as provided in the embodiments of this application.

[0042] Figure 11 This is a schematic diagram of the structure of a fault detection device for a flexible DC transmission system provided in an embodiment of this application;

[0043] Figure 12 This is a schematic diagram of the structure of an electronic device provided in an embodiment of this application. Detailed Implementation

[0044] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present application.

[0045] The terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish different objects, not to describe a specific order. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to these processes, methods, products, or apparatuses.

[0046] 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.

[0047] The following describes the relevant content, concepts, meanings, technical issues, technical solutions, and beneficial effects involved in the embodiments of this application.

[0048] Please see Figure 1 , Figure 1 This is a schematic diagram of the structure of a flexible DC transmission system provided in an embodiment of this application, as shown below. Figure 1As shown, the flexible DC transmission system includes a rectifier side and an inverter side, which form a bipolar structure. The flexible DC transmission system also includes DC transmission lines connecting the rectifier side and the inverter side. Both the rectifier side and the inverter side are equipped with MMC converters (L in the figure). MMC In addition to current-limiting reactors, the main function of MMC converters on the rectifier side is to convert AC power into DC power and transmit it to the DC transmission line. On the inverter side, the main function of MMC converters is to convert DC power transmitted from the DC transmission line into AC power and integrate it into the AC power grid. Among them, current-limiting reactors are mainly used to suppress the harmonics of DC current and the rise rate of fault current.

[0049] DC transmission lines are used to transmit electrical energy. There are multiple fault points on the line, such as the internal fault point (F1), the near-field fault point on the inverter side (F2), and the near-field fault point on the rectifier side (F3). These fault points are typical locations where faults may occur. They are used to simulate the changes in the electrical quantities of the system when faults occur at different locations, thereby providing scenario support for fault detection.

[0050] Key electrical quantity monitoring points for flexible DC transmission systems are deployed at the outlets of each busbar. The busbar outlet refers to the end where the converter (including the rectifier-side converter and the inverter-side converter) connects to the DC transmission line, as shown in the figure. Based on these electrical quantity monitoring points, the positive and negative voltages (u) on the rectifier side can be monitored. RP / u RN ) and positive and negative currents (i RP / i RN This electrical quantity monitoring point can also monitor the positive and negative voltages (u) on the inverter side. IP / u IN ) and positive and negative currents (i IP / i IN By monitoring changes in electrical quantities such as voltage and current on the rectifier and inverter sides through electrical quantity monitoring points, and analyzing these electrical quantities, it is possible to determine the type and direction of faults.

[0051] It should be noted that the positive direction refers to the busbar pointing towards the DC transmission line. On the rectifier side, the positive direction is the rectifier-side busbar pointing towards the DC transmission line; on the inverter side, the positive direction is the inverter-side busbar pointing towards the DC transmission line. Defining the positive direction is to ensure a unified standard for direction determination when analyzing the direction of fault traveling wave propagation and calculating high-frequency impedance difference ratios and other protection-related logic. This allows for accurate differentiation between the positive direction region of the protection (e.g., within the line section) and the negative direction region (e.g., outside the line section, such as the converter back side).

[0052] To better understand the embodiments of this application, the correlation between fault direction and impedance response is analyzed.

[0053] Please see Figure 2 , Figure 2 This is a schematic diagram illustrating the analysis of forward fault impedance characteristics provided in an embodiment of this application, combined with... Figure 1 When a fault occurs in the rectifier-side protection zone (such as point F1), the traveling wave propagation process is as follows: Figure 2 As shown.

[0054] On the rectifier side, when a forward fault occurs, the fault point is located in the direction from the busbar towards the DC transmission line, i.e., on the line side. Looking in from the rectifier-side busbar outlet, the fault is equivalent to connecting a fault voltage source u to the DC transmission line. F1 .

[0055] In the diagram, Z R R represents the equivalent impedance of the rectifier-side converter; R represents the location of the bus outlet on the rectifier side, which is a monitoring point used to monitor electrical quantities such as voltage and current at the bus outlet on the rectifier side.

[0056] As shown in the figure, when the incident wave of the nth fault arrives, the voltage u at the measuring point is... R With current i R The following conditions must be met:

[0057]

[0058] In the above formula, u R Represents the voltage at the measuring point; n represents the nth incident wave; u Rbi u represents the voltage measured during the i-th incident wave measurement. Rfi This represents the voltage measured after the i-th incident wave is reflected; i R Indicates the current at the measuring point; i Rbi This represents the current measured in the i-th incident wave; i Rfi This represents the current measured after the i-th incident wave is reflected.

[0059] Therefore, the measured impedance Z mR This can be deduced as:

[0060]

[0061] In the above formula, Z mR The measured impedance at monitoring point R; β R Represents the reflection coefficient, usually 1. Z c Z represents the surge impedance of a DC transmission line. R This represents the equivalent impedance of the rectifier-side converter.

[0062] It can be seen that the measured impedance of a forward fault (including those within and outside the protection zone) depends only on the equivalent impedance Z on the back side of the protection (rectifier-side converter or inverter-side converter). RSubsequently, the direction of the fault (in-zone fault or out-of-zone fault) can be determined based on the impedance characteristics during a forward fault.

[0063] Please see Figure 3 , Figure 3 This is a schematic diagram illustrating the analysis of reverse fault impedance characteristics provided in an embodiment of this application, combined with... Figure 1 When a fault occurs in the rectifier-side protection zone (such as point F3), the traveling wave propagation process is as follows: Figure 3 As shown.

[0064] On the rectifier side, when a reverse fault occurs, the fault point is located on the busbar in the direction away from the DC transmission line, i.e., on the converter side, where the fault voltage source u F3 It acts on the back side of the rectifier bus. Looking in from the rectifier bus outlet, the traveling wave generated by the fault propagates in the opposite direction to that of a forward fault.

[0065] In the diagram, Z R Z represents the equivalent impedance of the rectifier-side converter; R represents the bus outlet position on the rectifier side, which is a monitoring point used to monitor electrical quantities such as voltage and current at the bus outlet on the rectifier side; N represents the bus outlet position on the inverter side, which is another monitoring point used to monitor electrical quantities such as voltage and current at the bus outlet on the inverter side; Z I This represents the equivalent impedance of the inverter-side converter.

[0066] When 0 ≤ t < 2τ (τ is the propagation delay of the traveling wave), the measured impedance Z mR for:

[0067]

[0068] In the above formula, Z mR Indicates the measured impedance of monitoring point R; i Rf1 Z represents the current measured after the first incident wave is reflected. c This represents the wave impedance of a DC transmission line.

[0069] Because DC transmission lines are relatively long, there is generally only one wavefront within the protection time window during a reverse-direction fault. Therefore, the equivalent circuit for a reverse-direction fault under rectifier-side protection can be obtained as the surge impedance Z of the DC transmission line. c Since DC fault current does not cross zero, DC circuit breakers have to deal with very large fault currents, which makes DC circuit breakers expensive and difficult to make with large capacity. Flexible DC transmission systems usually need to reliably detect faults within 2 to 3 ms after a fault occurs.

[0070] It can be seen that when a reverse fault occurs, a second incident wave will not appear within 2-3 ms. Therefore, the measured impedance is the surge impedance Z of the DC transmission line. cSubsequently, the direction of the fault (in-zone fault or out-of-zone fault) can be determined based on the impedance characteristics during the reverse fault.

[0071] To better understand the embodiments of this application, the polarity selection criteria are analyzed below. In the field of protection for flexible DC transmission systems, polarity selection criteria are the basis or rules for determining the specific polarity of a fault, i.e., whether it is a positive ground fault, a negative ground fault, or a bipolar short-circuit fault.

[0072] It should be noted that electromagnetic coupling exists between bipolar circuits, which is generally decoupled into line-mode and ground-mode components using the following formula:

[0073]

[0074] In the above formula, F0 and F1 represent the ground model component and the linear model component, respectively; F p F n These represent the positive and negative voltages (or currents), respectively.

[0075] It should be noted that, based on the boundary conditions satisfied when a fault occurs, such as the constraint relationships of electrical quantities like voltage and current at the fault point under different fault types, the equivalent circuit of the traveling wave in the composite mode domain for the corresponding fault type can be drawn. The composite mode domain refers to the domain in which the electrical quantities of a multiphase or multi-pole system are decomposed into different modes through mode transformation, facilitating the analysis of traveling wave propagation characteristics. These equivalent circuits can intuitively reflect the propagation and reflection patterns of traveling waves during a fault. Based on these patterns, the characteristics of electrical quantities under different fault types can be studied, thus providing theoretical support for protection logic such as fault pole selection and area identification, and accurately determining the fault type and direction.

[0076] Please see Figure 4a , Figure 4a This is a schematic diagram of a single-pole ground fault in a DC fault domain equivalent circuit provided in an embodiment of this application. Figure 4a As shown in the figure, in the single-pole ground fault scenario illustrated in the figure, Z... c1 Z c0 These represent the line-mode impedance and the ground-mode impedance, respectively; U f1 I f1 Representing line-mode voltage and line-mode current respectively; U f0 I f0 Representing ground mode voltage and ground mode current respectively; U N R is the rated voltage; f This is the transition resistance.

[0077] Please see Figure 4b , Figure 4b This is a schematic diagram of a bipolar ground fault in a DC fault domain equivalent circuit provided in an embodiment of this application. Figure 4b As shown in the figure, in the bipolar ground fault scenario illustrated in the diagram, some components, such as... Figure 4a As shown, this is consistent with the previous description and will not be repeated here.

[0078] Therefore, the following fault selection criteria can be obtained:

[0079]

[0080] In the above formula, u0 represents the zero-sequence voltage, which is used for fault selection to determine whether it is a positive fault, a bipolar short circuit, or a negative fault; s represents the variable related to complex frequency domain analysis.

[0081] It can be seen that under a positive grounding fault, the ground mode voltage traveling wave is negative; under a bipolar grounding fault, the ground mode voltage traveling wave is zero; under a negative grounding short circuit, the ground mode voltage traveling wave is positive. Therefore, the fault polarity can be selected by the sign of the ground mode voltage traveling wave.

[0082] Please see Figure 5 , Figure 5 This is a flowchart illustrating a fault detection method for a flexible DC transmission system provided in an embodiment of this application. The method includes, but is not limited to, the following steps:

[0083] S101. Based on a preset voltage gradient algorithm, determine whether the flexible DC transmission system has experienced a fault according to the voltage signals on the rectifier side and the inverter side collected in real time in the flexible DC transmission system.

[0084] In this embodiment of the application, the preset voltage gradient algorithm refers to a pre-set calculation model used to capture the transient change characteristics of voltage signals, which is used to determine whether a fault has occurred in the flexible DC transmission system.

[0085] In a specific embodiment, the positive and negative bus voltage signals can be continuously collected by signal monitoring devices deployed at the rectifier side bus outlet and the inverter side bus outlet according to a preset sampling frequency. The preset sampling frequency can be 50kHz, i.e., the sampling period is 20μs.

[0086] The system uses real-time voltage signals from the rectifier and inverter sides to determine whether a fault has occurred in the flexible DC transmission system, based on a preset voltage gradient algorithm.

[0087] This application embodiment achieves rapid and accurate fault detection by combining an improved preset voltage gradient algorithm with voltage signals from the rectifier and inverter sides of the flexible DC transmission system. On the one hand, high-frequency sampling and gradient calculation can quickly capture voltage transient changes caused by faults, meeting the speed requirements of fault detection in flexible DC transmission systems, i.e., a preliminary fault determination needs to be made within 2-3 ms. On the other hand, by coordinating the determination of voltage signals from both the rectifier and inverter sides, misjudgments caused by noise and transient interference from a single signal can be effectively avoided, improving the reliability of fault detection.

[0088] Optionally, the above step of determining whether a fault has occurred in the flexible DC transmission system based on a preset voltage gradient algorithm and the voltage signals on the rectifier side and inverter side acquired in real time in the flexible DC transmission system may include the following steps:

[0089] A101. Obtain the first voltage signal set of the rectifier side and the second voltage signal set of the inverter side within a preset time period;

[0090] A102. Determine the first voltage gradient based on the first voltage signal set and the preset voltage gradient calculation expression;

[0091] A103. Determine the second voltage gradient based on the second voltage signal set and the preset voltage gradient calculation expression;

[0092] A104. If the absolute value of the first voltage gradient is greater than a preset voltage gradient threshold, and the absolute value of the second voltage gradient is greater than the preset voltage gradient threshold, then it is determined that a fault has occurred in the flexible DC transmission system; the end time of the preset time period is the time when the fault occurs.

[0093] In this embodiment of the application, the preset time period refers to a pre-set continuous time interval with the fault occurrence time as the termination time. Its duration must cover the shortest time for the fault traveling wave to propagate to the rectifier side and the inverter side. For example, in this embodiment of the application, the preset time period is set to 6 sampling cycles, which corresponds to a sampling frequency of 50kHz and a duration of 120μs.

[0094] The first voltage signal set refers to the collection of all voltage sample values ​​collected by the monitoring device at the rectifier-side bus outlet within a preset time period. The second voltage signal set refers to the collection of all voltage sample values ​​collected by the monitoring device at the inverter-side bus outlet within the same preset time period. Both the first and second voltage signal sets include continuous sample data of the positive and negative bus voltages, and the sampling times correspond one-to-one.

[0095] In a specific embodiment, by monitoring the voltage sampling process of the rectifier side and the inverter side in real time, when a suspected fault fluctuation is detected in the voltage signal on either side, the current time is used as the termination time, and six sampling cycles are traced back to obtain the first voltage signal set of the rectifier side and the second voltage signal set of the inverter side within the preset time period.

[0096] Next, the first voltage gradient can be determined based on the first voltage signal set and the preset voltage gradient calculation expression; the second voltage gradient can be determined based on the second voltage signal set and the preset voltage gradient calculation expression.

[0097] The preset voltage gradient calculation expression can be as follows:

[0098]

[0099] In the above formula, This represents the voltage gradient on the rectifier side or inverter side at the end of the preset time period, i.e., at the current sampling time k; u(kj) represents the voltage sample value of the first j sampling cycles at the current sampling time k.

[0100] Next, the calculated first and second voltage gradients are compared with preset voltage gradient thresholds. If the absolute value of the first voltage gradient is greater than the preset voltage gradient threshold, and the absolute value of the second voltage gradient is also greater than the preset voltage gradient threshold, then a fault is determined to have occurred in the flexible DC transmission system. At this point, the end time of the preset time period is set as the time when the fault occurred. The preset voltage gradient threshold is pre-set based on the maximum normal operating transient voltage fluctuation value of the flexible DC transmission system, and can be set to 5% of the rated voltage.

[0101] It is evident that by synchronously acquiring the voltage signal sets from the rectifier and inverter sides, uniformly calculating the voltage gradient, and performing collaborative threshold determination on both sides, the reliability of fault detection can be improved.

[0102] S102. If the flexible DC transmission system experiences a fault, the time of the fault occurrence is obtained.

[0103] In a specific embodiment, if a fault occurs in the flexible DC transmission system, the fault occurrence time is obtained. The fault occurrence time refers to the initial time point when the flexible DC transmission system switches from the normal operation state to the fault state, corresponding to the first sampling time when the fault traveling wave first arrives at the monitoring point and causes the voltage signal to show a sudden change that conforms to the fault characteristics.

[0104] S103. Based on a preset time window, obtain the first sampled dataset corresponding to the rectifier side and the second sampled dataset corresponding to the inverter side at the time the fault occurs.

[0105] In this embodiment, both the first sampling data and the second sampling data include voltage signals and current signals.

[0106] In this embodiment, the preset time window refers to a pre-defined continuous time interval centered on the fault occurrence time. For example, in this embodiment, the preset time window is from 2ms before the fault occurrence time to 2ms after the fault occurrence time. The preset time window can be set based on the propagation characteristics of the transient traveling wave of a fault in a flexible DC transmission system. Since the propagation speed of the fault traveling wave in a DC line is approximately the speed of light, the propagation distance of the traveling wave within 2ms can completely capture the characteristic information of the initial transient traveling wave of the fault.

[0107] The first sampling dataset refers to the set of all electrical quantity sample values ​​collected by the monitoring device deployed at the rectifier-side bus outlet within a preset time window; the second sampling dataset refers to the set of all electrical quantity sample values ​​collected by the monitoring device at the inverter-side bus outlet within the same preset time window. Electrical quantity sample values ​​may include voltage signals and current signals.

[0108] In a specific embodiment, the start and end times of a preset time window are determined based on the time of the fault occurrence. The positive and negative voltage signal sampling values ​​and positive and negative current signal sampling values ​​from the monitoring devices on the rectifier side and inverter side are extracted from the start and end times respectively according to the time interval parameters to obtain the first sampling dataset and the second sampling dataset.

[0109] S104. Perform a preset mode transformation on the voltage signals in the first sampled dataset to obtain a first ground mode voltage set; perform the preset mode transformation on the voltage signals in the second sampled dataset to obtain a second ground mode voltage set.

[0110] In this embodiment, the preset mode transformation refers to a pre-defined linear transformation algorithm used to decompose the positive and negative voltage signals of a bipolar DC system into ground mode voltage (zero mode voltage) and line mode voltage. Based on the preset mode transformation, complete decoupling of the ground mode component and the line mode component can be achieved without changing the transient characteristics of the signal.

[0111] In a specific embodiment, after obtaining the first sampled dataset, the positive voltage signal set and the negative voltage signal set are separated from the first sampled dataset. For the voltage signals in the first sampled dataset, the positive and negative voltage signals corresponding to each sampling time are substituted into a preset mode transformation formula to calculate the ground mode voltage at that time. After traversing all sampling times, the first ground mode voltage set can be obtained. Performing a preset mode transformation on the voltage signals in the second sampled dataset yields the second ground mode voltage set.

[0112] Optionally, the above step of performing a preset mode transformation on the voltage signals in the first sampled dataset to obtain a first ground-mode voltage set may include the following steps:

[0113] A401. Extract each voltage signal from the first sampled dataset to obtain a third voltage signal set; each voltage signal includes a positive voltage signal and a negative voltage signal.

[0114] A402. Decouple the third voltage signal set based on the preset positive and negative decoupling expression to obtain the first ground mode voltage set.

[0115] In this embodiment of the application, the preset positive and negative decoupling expressions are as follows:

[0116]

[0117] In the above formula, F0 and F1 represent the ground mode voltage and line mode voltage, respectively; F p F n These represent the positive and negative voltages, respectively.

[0118] In a specific embodiment, extracting each voltage signal from the first sampled dataset yields a third voltage signal set, wherein each voltage signal includes a positive voltage signal and a negative voltage signal. Then, decoupling the third voltage signal set based on a preset positive-negative decoupling expression yields a first ground-mode voltage set.

[0119] It is evident that by processing the voltage signal in the first sampled dataset through preset mode transformation, the accuracy of ground mode voltage extraction can be guaranteed, thus ensuring the reliability of fault detection.

[0120] S105. Determine the target fault type based on the first ground mode voltage set and the second ground mode voltage set.

[0121] It should be noted that in flexible DC transmission systems, different fault types will cause the ground mode voltage to exhibit specific change patterns. For example, when there is a positive ground fault, the ground mode voltage will show a negative amplitude change, when there is a negative ground fault, the ground mode voltage will show a positive amplitude change, and when there is a bipolar short circuit fault, the ground mode voltage will approach zero due to the cancellation of the positive and negative potential changes.

[0122] In this application embodiment, the target fault type includes one of the following: positive grounding, negative grounding, and bipolar short circuit.

[0123] In a specific embodiment, the target fault type of the flexible DC transmission system can be determined based on the obtained first ground mode voltage set and second ground mode voltage set.

[0124] It is evident that by utilizing the sensitivity of ground mode voltage to fault polarity, the three core fault types can be accurately distinguished, avoiding the problem of load fluctuation interference when using traditional current signal-based judgment.

[0125] Optionally, the above step of determining the target fault type based on the first ground mode voltage set and the second ground mode voltage set may include the following steps:

[0126] A501. Determine the average value of the first ground mode voltage corresponding to the first ground mode voltage set;

[0127] A502. Determine the average value of the second ground mode voltage corresponding to the second ground mode voltage set;

[0128] A503. If both the average value of the first ground mode voltage and the average value of the second ground mode voltage are less than a negative preset ground mode voltage threshold, then the target fault type is determined to be positive grounding; the preset ground mode voltage threshold is a positive value.

[0129] A504. If both the average value of the first ground mode voltage and the average value of the second ground mode voltage are greater than the preset ground mode voltage threshold, then the target fault type is determined to be negative grounding.

[0130] A505. If the average value of the first ground mode voltage and the average value of the second ground mode voltage are both greater than or equal to the negative value of the preset ground mode voltage threshold, and the average value of the first ground mode voltage and the average value of the second ground mode voltage are both less than or equal to the preset ground mode voltage threshold, then the target fault type is determined to be the bipolar short circuit.

[0131] In this embodiment, the preset ground mode voltage threshold refers to a pre-set reference value used to distinguish between normal ground mode voltage fluctuations and fault ground mode voltage characteristics. The value is positive and is determined based on the maximum steady-state fluctuation value of the ground mode voltage during normal operation of the flexible DC transmission system. For example, it can be set to 3%-5% of the rated voltage of the flexible DC transmission system.

[0132] In a specific embodiment, the average value of the first ground mode voltage corresponding to the first ground mode voltage set is determined, and the average value of the second ground mode voltage corresponding to the second ground mode voltage set is determined.

[0133] If the average value of the first ground mode voltage and the average value of the second ground mode voltage are both less than the negative preset ground mode voltage threshold, it indicates that the ground mode voltages on both the rectifier side and the inverter side show a significant negative shift, which is consistent with the ground mode voltage characteristics caused by the change in the positive and negative poles relative to the earth potential during a positive ground fault. Therefore, the target fault type is determined to be positive ground fault.

[0134] If the average value of the first ground mode voltage and the average value of the second ground mode voltage are both greater than the preset ground mode voltage threshold, it indicates that the ground mode voltages on both the rectifier side and the inverter side show a significant positive offset. Matching the ground mode voltage change pattern of the negative grounding fault, the target fault type is determined to be negative grounding.

[0135] If the average value of the first ground mode voltage and the average value of the second ground mode voltage are both within the range of the negative value of the preset ground mode voltage threshold and the preset ground mode voltage threshold, it indicates that the ground mode voltages on both the rectifier side and the inverter side have not exceeded the normal fluctuation range, which is consistent with the characteristic that the positive and negative potential changes cancel each other out during a bipolar short circuit fault, and the target fault type is determined to be a bipolar short circuit.

[0136] During the judgment process, if a situation arises where the average value on one side matches a certain type of fault characteristic, but the average value on the other side does not, for example, the average value of the first ground mode voltage is less than the negative preset ground mode voltage threshold, but the average value of the second ground mode voltage is greater than the preset ground mode voltage threshold, the sliding average value of the ground mode voltage on the rectifier side and the inverter side can be recalculated based on the sliding window. The changing trend of the sliding average value can be used to assist in the verification, eliminate one-sided anomalies caused by signal transmission delay or interference, and ensure the consistency and reliability of the fault type judgment results.

[0137] It is evident that by calculating the standardized average value of the ground mode power supply and performing bilateral collaborative threshold determination, the accuracy of fault type determination can be improved.

[0138] S106. Perform a preset wavelet transform on the first sampled dataset to obtain a first high-frequency component set; perform the preset wavelet transform on the second sampled dataset to obtain a second high-frequency component set.

[0139] In this embodiment, the preset wavelet transform refers to a pre-defined wavelet analysis algorithm used to decompose transient electrical quantity signals. In this embodiment, the Morlet wavelet can be selected as the basis wavelet to extract high-frequency components.

[0140] In a specific embodiment, a first high-frequency component set can be obtained by performing a preset wavelet transform on the first sampled dataset. A second high-frequency component set can be obtained by performing a preset wavelet transform on the second sampled dataset. The high-frequency components in both the first and second high-frequency component sets include voltage high-frequency components and current high-frequency components.

[0141] Optionally, the above step of performing a preset wavelet transform on the first sampled dataset to obtain the first high-frequency component set may include the following steps:

[0142] A601. Perform a preset wavelet transform on the voltage signal in the first sampled dataset to obtain a first voltage high-frequency component set; each first voltage high-frequency component in the first voltage high-frequency component set corresponds to a frequency.

[0143] A602. Perform a preset wavelet transform on the current signal in the first sampled dataset to obtain a first high-frequency current component set; each first high-frequency current component in the first high-frequency current component set corresponds to a frequency.

[0144] A603. Extract the high-frequency components corresponding to the preset frequency bands from the first voltage high-frequency component set and the first current high-frequency component set respectively to obtain the second voltage high-frequency component set and the second current high-frequency component set.

[0145] A604. Integrate the second voltage high-frequency component set and the second current high-frequency component set to obtain the first high-frequency component set.

[0146] In this embodiment, the preset frequency band is a pre-defined frequency range that matches the transient traveling wave characteristics of the fault. For example, it can be set to 3kHz-4.5kHz. The calculation error is minimized in this frequency band, thus improving the accuracy of fault analysis.

[0147] In a specific embodiment, a first voltage high-frequency component set is obtained by performing a preset wavelet transform on the voltage signal in the first sampled dataset. Each high-frequency component in the first voltage high-frequency component set corresponds to a frequency. Similarly, a first current high-frequency component set is obtained by performing a preset wavelet transform on the current signal in the first sampled dataset. Each high-frequency component in the first current high-frequency component set corresponds to a frequency.

[0148] Next, high-frequency components corresponding to preset frequency bands are extracted from the first voltage high-frequency component set and the first current high-frequency component set, respectively, to obtain the second voltage high-frequency component set and the second current high-frequency component set. Integrating the second voltage high-frequency component set and the second current high-frequency component set yields the first high-frequency component set.

[0149] It is evident that by performing wavelet transform on the sampled data and effectively eliminating irrelevant frequency signals based on preset frequency bands, the extracted high-frequency components are focused on the core transient features of the fault, reducing calculation errors and improving the accuracy and stability of fault area identification.

[0150] S107. Determine the first high-frequency impedance difference ratio corresponding to the rectifier side based on the first high-frequency component set; determine the second high-frequency impedance difference ratio corresponding to the inverter side based on the second high-frequency component set.

[0151] In this embodiment, the first high-frequency impedance difference ratio refers to a ratio parameter calculated based on the first high-frequency component set, reflecting the difference in high-frequency impedance changes at the rectifier-side bus outlet. Its core calculation logic revolves around the relative relationship between the high-frequency impedance difference before and after the fault and the reference impedance, and is used to quantitatively evaluate the impedance characteristic changes under transient fault conditions on the rectifier side. Similarly, the second high-frequency impedance difference ratio can be used to quantitatively evaluate the impedance characteristic changes under transient fault conditions on the reverse side.

[0152] In a specific embodiment, the first high-frequency impedance difference ratio corresponding to the rectifier side can be determined based on the first high-frequency component set. The second high-frequency impedance difference ratio corresponding to the inverter side can be determined based on the second high-frequency component set.

[0153] Optionally, the above step of determining the first high-frequency impedance difference ratio corresponding to the rectifier side based on the first high-frequency component set may include the following steps:

[0154] A701. Determine the measurement impedance based on the high-frequency voltage component and high-frequency current component corresponding to each high-frequency component in the first high-frequency component set to obtain a measurement impedance set; each measurement impedance corresponds to a frequency.

[0155] A702. The first high-frequency impedance difference ratio is determined based on the measured impedance set, the preset modular multilevel converter equivalent impedance, the preset line wave impedance, and the preset high-frequency impedance difference ratio calculation expression.

[0156] In this embodiment of the application, the measured impedance refers to the real-time impedance value calculated by the basic definition of impedance based on the high-frequency voltage component and the high-frequency current component at the same frequency and the same sampling time in the first high-frequency component set. It can directly reflect the electrical impedance state at the rectifier side bus outlet at that frequency, and each measured impedance corresponds one-to-one with the frequency of the high-frequency component.

[0157] The preset equivalent impedance of the modular multilevel converter is a pre-set equivalent impedance parameter of the modular multilevel converter in the high-frequency band. It is obtained by offline simulation calculation based on the converter topology and component parameters. Its value changes only slightly with frequency and can be regarded as a constant value within the preset frequency band.

[0158] The preset line surge impedance is a pre-set surge impedance parameter of the DC transmission line in the high-frequency band. It is determined by the distributed inductance and distributed capacitance of the line and remains stable within the preset frequency band.

[0159] A preset high-frequency impedance difference ratio calculation expression is used to quantify the degree of deviation between the measured impedance and the reference impedance. The expression is as follows:

[0160]

[0161] In the above formula, K represents the high-frequency impedance difference ratio; n represents the number of frequency points involved in the calculation, corresponding to the n discrete frequencies selected within the preset frequency band; i is the frequency point number, ranging from 1 to n, corresponding to the 1st to the nth discrete frequency; Z MMC (f i Z represents the preset modular multilevel converter (MMC) equivalent impedance at the i-th frequency point; m (f i Z represents the measured impedance at the i-th frequency point; c (f i ) represents the preset line wave impedance at the i-th frequency point.

[0162] It should be noted that when n=1, K=(Z MMC -Z m ) / (Z c -Z m ), characterized in Z MMC and Z c Between, Z m Which number is closer to the value? In a positive direction fault, the numerator is 0 and K is 0. In a negative direction fault, the denominator is 0 and K is infinity. When Z... m =(Z MMC +Z c When K = 1 / 2, this is the critical case where the difference ratios are equal, and a protection criterion of K < 1 for positive direction faults can be constructed. To improve the reliability of the protection criterion, the K value can be extended to the average of the K values ​​at n different frequencies.

[0163] In a specific embodiment, the measurement impedance can be determined based on the high-frequency voltage component and high-frequency current component corresponding to each high-frequency component in the first high-frequency component set, thus obtaining a measurement impedance set. Each measurement impedance corresponds to a frequency.

[0164] The first high-frequency impedance difference ratio can be determined based on the measured impedance set, the preset equivalent impedance of the modular multilevel converter, the preset line wave impedance, and the preset high-frequency impedance difference ratio calculation expression.

[0165] It is evident that by introducing the preset equivalent impedance of the modular multilevel converter and the preset line wave impedance as benchmarks, the calculation of the high-frequency impedance difference ratio is more in line with the actual topology characteristics of the flexible DC transmission system, avoiding the benchmark drift problem caused by relying solely on real-time data, and significantly improving the reliability of the first high-frequency impedance difference ratio.

[0166] S108. Determine the target fault direction based on the first high-frequency impedance difference ratio and the second high-frequency impedance difference ratio.

[0167] In this embodiment, the target fault direction refers to the area to which the fault occurs. The target fault direction includes one of the following: an external fault or an internal fault. An internal fault refers to a fault located within the monitored DC transmission line section, specifically the line segment between the rectifier-side bus outlet and the inverter-side bus outlet. An external fault refers to a fault located outside this line section, such as inside the rectifier-side converter, inside the inverter-side converter, or other related lines.

[0168] In a specific embodiment, the target fault direction can be determined based on the ratio of the first high-frequency impedance difference to the ratio of the second high-frequency impedance difference.

[0169] Optionally, the above step of determining the target fault direction based on the first high-frequency impedance difference ratio and the second high-frequency impedance difference ratio may include the following steps:

[0170] A801. If the first high-frequency impedance difference ratio is less than or equal to 1, and the second high-frequency impedance difference ratio is less than or equal to 1, then the target fault direction is determined to be a fault within the area.

[0171] A802. If the first high-frequency impedance difference ratio is greater than 1, and / or the second high-frequency impedance difference ratio is greater than 1, then the target fault direction is determined to be the fault outside the zone.

[0172] In this embodiment of the application, in a flexible DC transmission system, if an intra-zone fault occurs, the fault point is located within the protection and monitoring line section. The high-frequency impedance change caused by the fault transient is affected by the combined influence of the line wave impedance and the converter equivalent impedance, resulting in the high-frequency impedance difference ratio between the rectifier side and the inverter side being within a small range. However, when an extra-zone fault occurs, the fault point is located outside the protection and monitoring line section, and the impact of the fault transient on the high-frequency impedance on both sides is significantly different. At least one side's high-frequency impedance difference ratio will exceed the set range. In this embodiment of the application, 1 is used as a preset fixed judgment threshold, wherein the judgment conditions are as follows:

[0173]

[0174] In the above formula, K R This represents the ratio of high-frequency impedance differences on the rectifier side, i.e., the first high-frequency impedance difference ratio; K I This represents the ratio of high-frequency impedance differences on the inverter side, i.e., the second high-frequency impedance difference ratio.

[0175] In a specific embodiment, if the first high-frequency impedance difference ratio is less than or equal to 1, and the second high-frequency impedance difference ratio is less than or equal to 1, that is, the high-frequency impedance difference ratios of the rectifier side and the inverter side are both in a small range, then the target fault direction is determined to be an intra-zone fault.

[0176] If the first high-frequency impedance difference ratio is greater than 1, and / or the second high-frequency impedance difference ratio is greater than 1, that is, the high-frequency impedance difference ratio on at least one side will exceed the set range, then the target fault direction is determined to be an external fault.

[0177] It is evident that by simplifying the fault direction determination process, reducing computational complexity and response delay, and meeting the speed requirements of fault handling in flexible DC transmission systems, the fault direction can be accurately and efficiently identified based on the high-frequency impedance difference ratio.

[0178] S109. Determine the target fault detection result based on the target fault type and the target fault direction.

[0179] In a specific embodiment, the target fault type is determined, namely positive grounding, negative grounding, and bipolar short circuit; the target fault direction is determined, namely intra-zone fault and extra-zone fault. The target fault detection result can be determined based on the target fault type and the target fault direction. For example, if the target fault type is positive grounding and the target fault direction is intra-zone fault, then the target fault detection result is positive grounding fault (intra-zone); if the target fault type is bipolar short circuit and the target fault direction is extra-zone fault, then the target fault detection result is bipolar short circuit fault (extra-zone).

[0180] The integrated target fault detection results can be stored in a fault record database for fault tracing and system operation and maintenance analysis. At the same time, auxiliary information such as the occurrence time of the corresponding fault and the sampled dataset can also be stored in the database.

[0181] It is evident that integrating the two key pieces of information, fault type and fault direction, into structured detection results improves the reliability of the detection results.

[0182] In one possible embodiment, a high-frequency equivalent impedance model of the MMC converter station (either the rectifier side or the inverter side) was established and simulated for verification. The fault was set to occur at 1 second, the simulation step size was set to 2 μs, and considering that the sampling frequency of existing DC line protection is generally high, the sampling frequency was set to 50 kHz. The Morlet wavelet was selected as the fundamental wavelet to extract high-frequency components. The preset time window was set to 2 ms before and after the fault occurrence, and the preset ground mode voltage threshold was set to 20 kV.

[0183] Please see Figure 6 , Figure 6 This is a simulation diagram of a high-frequency equivalent impedance model provided in an embodiment of this application, such as... Figure 6As shown, by using wavelet transform to perform frequency domain transformation on the fault voltage and current measured at the monitoring point, the high-frequency impedance at the moment of maximum high-frequency voltage can be calculated, thereby obtaining the calculated amplitude-frequency curve of the measured impedance. Simulation results show that the MMC converter station has the smallest error within the range of 3kHz to 4.5kHz. Therefore, the preset frequency band for calculating the measured impedance can be set to 3kHz-4.5kHz.

[0184] Please combine Figure 1 The verification is now conducted by setting fault points on the DC transmission line, such as internal fault points (F1), near-field fault points on the inverter side (F2), and near-field fault points on the rectifier side (F3). At the same time, the fault is set to occur at 1 second.

[0185] In one possible embodiment, to verify the fault within the zone, a fault is set at the fault point F1, with the fault types being bipolar short circuit and unipolar grounding, respectively. Based on the signal monitoring devices on the rectifier side and the inverter side, the theoretical amplitude-frequency curve of the measured impedance during the corresponding fault within the zone is obtained.

[0186] For example, a metallic ground fault occurs 300 km from the rectifier side. Wavelet transform is used to perform a frequency domain transformation on the fault voltage and current measured at the monitoring point. The high-frequency impedance at the moment of maximum high-frequency voltage is calculated, thus obtaining the measured impedance and calculated impedance curves for the protection systems on both sides. Please refer to [link to relevant documentation]. Figure 7 , Figure 7 This is a diagram showing the calculated impedance curves of the protection systems on both sides for a bipolar short-circuit fault within a 300km fault distance area, provided in an embodiment of this application. Figure 7 As can be seen, the amplitude-frequency curves calculated from the measured impedances on both the rectifier and inverter sides are close to the theoretical amplitude-frequency curves for forward faults. Specifically, the high-frequency impedance difference on the rectifier side is greater than that of K. R =0.0118, inverter-side high-frequency impedance distance ratio K I =0.0172, which meets the fault criterion for intra-zone faults under bipolar short circuit, so it can be judged as an intra-zone bipolar short circuit fault.

[0187] For example, a single-pole ground fault is set at a distance of 100km from the rectifier side, and the measured impedance and calculated impedance curves of the protection on both sides are obtained. Please refer to [link / reference]. Figure 8 , Figure 8 This is a diagram showing the calculated impedance curves of the protection systems on both sides of a single-pole grounding fault within a 100km area, as provided in this application embodiment. Figure 8 As can be seen, the amplitude-frequency curves calculated from the measured impedances on both the rectifier and inverter sides are close to the theoretical amplitude-frequency curves for forward faults. Specifically, the high-frequency impedance distance ratio K on the rectifier side is... R =0.0138, and the high-frequency impedance distance ratio K on the inverter side is also 0.0138. I =0.0290, which meets the fault criteria within the zone, therefore it can be determined as a fault within the zone.

[0188] In one possible embodiment, to verify the fault outside the zone, a fault is set at fault point F2 or fault point F3, with the fault types being bipolar short circuit and unipolar grounding, respectively. Based on the signal monitoring devices on the rectifier side and the inverter side, the theoretical amplitude-frequency curve of the measured impedance under the corresponding fault outside the zone is obtained.

[0189] For example, a bipolar metallic short-circuit fault is constructed at the inverter-side outlet F2, and the measured impedance and calculated impedance curves of the protection on both sides are obtained. Please refer to [link to relevant documentation]. Figure 9 , Figure 9 This is a diagram showing the calculated impedance curves of the protection on both sides during a bipolar metallic short-circuit fault at the inverter side outlet, as provided in an embodiment of this application. Figure 9 As can be seen, the amplitude-frequency curve calculated by measuring the impedance on the rectifier side is close to the theoretical amplitude-frequency curve of the positive direction fault, and the high-frequency impedance difference on the rectifier side is greater than K. R =0.0497, judged as a forward fault. The amplitude-frequency curve calculated by measuring the inverter side impedance is close to the theoretical amplitude-frequency curve for a reverse fault, and the high-frequency impedance distance ratio K on the inverter side is... I =15225, which is determined to be a reverse direction fault, so it can be determined to be an external fault.

[0190] For example, a single-pole ground fault is simulated at F3 outside the rectifier side zone, and the measured impedance and calculated impedance curves of the protection on both sides are obtained. Please refer to [link to relevant documentation]. Figure 10 , Figure 10 This is a diagram showing the calculated impedance curves of the protection on both sides during a single-pole ground fault outside the rectifier side region, as provided in the embodiments of this application. Figure 10 As can be seen, the amplitude-frequency curves calculated from the measured impedance on the inverter side are close to the theoretical amplitude-frequency curves for forward faults, while the rectifier side shows a significant difference from the theoretical amplitude-frequency curves for forward faults. Specifically, the high-frequency impedance difference on the rectifier side is greater than that of K. R =276.41, and the high-frequency impedance difference ratio on the inverter side is K I =0.0142, which meets the fault criteria for outside the zone, so it can be determined that it is a fault outside the rectifier side.

[0191] Based on the simulation results above, the theoretical amplitude-frequency curve of the measured impedance during a fault can accurately determine whether a fault is inside or outside the fault zone. Faults inside the fault zone have approximately the same waveform curve, while faults outside the fault zone have significantly different waveform curves. Table 1 below shows the simulation results for faults at different locations and with different transition resistances. As shown in Table 1, K... R K represents the ratio of high-frequency impedance differences on the rectifier side. I A represents the ratio of high-frequency impedance differences on the inverter side. u0 This represents the average ground mode voltage. It is evident that the principle of this application provides sufficient margin for reliable operation when high-resistance faults occur at different locations both inside and outside the zone.

[0192] Table 1. Fault simulation results at different locations and with different transition resistances.

[0193]

[0194] In summary, by implementing the embodiments of this application, a voltage gradient algorithm is used to detect whether a fault has occurred in a flexible DC transmission system. When a fault occurs, the sampled voltage signal is used to determine the fault type based on the ground mode voltage polarity. The high-frequency impedance difference ratio between the rectifier and inverter sides of the flexible DC transmission system is determined based on the voltage signal and the sampled current signal. The fault direction is then determined based on the high-frequency impedance difference ratio. Finally, the fault detection result is determined based on the fault type and fault direction. It is evident that by achieving fault polarity selection based on the ground mode voltage polarity and accurately distinguishing between faults within and outside the fault zone using the high-frequency impedance difference ratio, the reliability of fault detection in flexible DC transmission systems is improved.

[0195] Please see Figure 11 , Figure 11 This is a schematic diagram of the structure of a fault detection device for a flexible DC transmission system provided in an embodiment of this application. The fault detection device 200 for the flexible DC transmission system includes: a fault detection module 201, a fault data acquisition module 202, a data acquisition module 203, a mode transformation module 204, a fault type determination module 205, a wavelet transform module 206, a data processing module 207, a fault direction determination module 208, and a fault detection result generation module 209.

[0196] The fault detection module 201 is used to determine whether a fault has occurred in the flexible DC transmission system based on a preset voltage gradient algorithm and the voltage signals on the rectifier side and the inverter side collected in real time in the flexible DC transmission system.

[0197] The fault data acquisition module 202 is used to acquire the time of fault occurrence if a fault occurs in the flexible DC transmission system.

[0198] The data acquisition module 203 is used to acquire, based on a preset time window, a first sampling dataset corresponding to the rectifier side and a second sampling dataset corresponding to the inverter side at the time of the fault occurrence; both the first sampling data and the second sampling data include voltage signals and current signals.

[0199] The mode transformation module 204 is used to perform a preset mode transformation on the voltage signals in the first sampled dataset to obtain a first ground mode voltage set; and to perform the preset mode transformation on the voltage signals in the second sampled dataset to obtain a second ground mode voltage set.

[0200] The fault type determination module 205 is used to determine the target fault type based on the first ground mode voltage set and the second ground mode voltage set; the target fault type includes one of the following: positive grounding, negative grounding, and bipolar short circuit;

[0201] The wavelet transform module 206 is used to perform a preset wavelet transform on the first sampled dataset to obtain a first high-frequency component set; and to perform the preset wavelet transform on the second sampled dataset to obtain a second high-frequency component set.

[0202] The data processing module 207 is used to determine the first high-frequency impedance difference ratio corresponding to the rectifier side based on the first high-frequency component set; and to determine the second high-frequency impedance difference ratio corresponding to the inverter side based on the second high-frequency component set.

[0203] The fault direction determination module 208 is used to determine the target fault direction based on the first high-frequency impedance difference ratio and the second high-frequency impedance difference ratio; the target fault direction includes one of the following: external fault and internal fault;

[0204] The fault detection result generation module 209 is used to determine the target fault detection result based on the target fault type and the target fault direction.

[0205] Optionally, in determining whether a fault has occurred in the flexible DC transmission system based on the voltage signals on the rectifier side and the inverter side acquired in real time in the flexible DC transmission system according to the preset voltage gradient algorithm, the fault detection module 201 is further specifically used for:

[0206] Acquire the first voltage signal set of the rectifier side and the second voltage signal set of the inverter side within a preset time period;

[0207] The first voltage gradient is determined based on the first voltage signal set and the preset voltage gradient calculation expression;

[0208] The second voltage gradient is determined based on the second voltage signal set and the preset voltage gradient calculation expression;

[0209] If the absolute value of the first voltage gradient is greater than a preset voltage gradient threshold, and the absolute value of the second voltage gradient is greater than the preset voltage gradient threshold, then it is determined that a fault has occurred in the flexible DC transmission system; the end time of the preset time period is the time when the fault occurs.

[0210] Optionally, in the process of performing a preset mode transformation on the voltage signals in the first sampled dataset to obtain a first ground mode voltage set, the mode transformation module 204 is further specifically used for:

[0211] Extract each voltage signal from the first sampled dataset to obtain a third voltage signal set; each voltage signal includes a positive voltage signal and a negative voltage signal;

[0212] The third voltage signal set is decoupled based on a preset positive and negative decoupling expression to obtain the first ground mode voltage set.

[0213] Optionally, in determining the target fault type based on the first ground mode voltage set and the second ground mode voltage set, the fault type determination module 205 is further specifically configured to:

[0214] Determine the average value of the first ground mode voltage corresponding to the first ground mode voltage set;

[0215] Determine the average value of the second ground mode voltage corresponding to the second ground mode voltage set;

[0216] If both the average value of the first ground mode voltage and the average value of the second ground mode voltage are less than a negative preset ground mode voltage threshold, then the target fault type is determined to be positive grounding; the preset ground mode voltage threshold is a positive value.

[0217] If both the average value of the first ground mode voltage and the average value of the second ground mode voltage are greater than the preset ground mode voltage threshold, then the target fault type is determined to be negative grounding.

[0218] If the average value of the first ground mode voltage and the average value of the second ground mode voltage are both greater than or equal to the negative value of the preset ground mode voltage threshold, and the average value of the first ground mode voltage and the average value of the second ground mode voltage are both less than or equal to the preset ground mode voltage threshold, then the target fault type is determined to be the bipolar short circuit.

[0219] Optionally, in the step of performing a preset wavelet transform on the first sampled dataset to obtain a first high-frequency component set, the wavelet transform module 206 is further specifically used for:

[0220] A preset wavelet transform is performed on the voltage signal in the first sampled dataset to obtain a first voltage high-frequency component set; each first voltage high-frequency component in the first voltage high-frequency component set corresponds to a frequency.

[0221] A preset wavelet transform is performed on the current signal in the first sampled dataset to obtain a first high-frequency component set of current; each high-frequency component of the first current in the first high-frequency component set corresponds to a frequency.

[0222] High-frequency components corresponding to preset frequency bands are extracted from the first voltage high-frequency component set and the first current high-frequency component set to obtain the second voltage high-frequency component set and the second current high-frequency component set.

[0223] The first high-frequency component set is obtained by integrating the second voltage high-frequency component set and the second current high-frequency component set.

[0224] Optionally, in determining the first high-frequency impedance difference ratio corresponding to the rectifier side based on the first high-frequency component set, the data processing module 207 is further specifically used for:

[0225] The measurement impedance is determined based on the high-frequency voltage component and high-frequency current component corresponding to each high-frequency component in the first high-frequency component set, thus obtaining a measurement impedance set; each measurement impedance corresponds to a frequency.

[0226] The first high-frequency impedance difference ratio is determined based on the measured impedance set, the preset modular multilevel converter equivalent impedance, the preset line wave impedance, and the preset high-frequency impedance difference ratio calculation expression.

[0227] Optionally, in determining the target fault direction based on the first high-frequency impedance difference ratio and the second high-frequency impedance difference ratio, the fault direction determination module 208 is further specifically used for:

[0228] If the first high-frequency impedance difference ratio is less than or equal to 1, and the second high-frequency impedance difference ratio is less than or equal to 1, then the target fault direction is determined to be a fault within the area.

[0229] If the first high-frequency impedance difference ratio is greater than 1, and / or the second high-frequency impedance difference ratio is greater than 1, then the target fault direction is determined to be the fault outside the zone.

[0230] The fault detection device 200 for flexible DC transmission systems described in this application can detect whether a fault has occurred in the flexible DC transmission system using a voltage gradient algorithm. When a fault occurs, the device determines the fault type based on the ground mode voltage polarity using sampled voltage signals, and determines the high-frequency impedance difference ratio between the rectifier side and the inverter side of the flexible DC transmission system based on the voltage signal and sampled current signal. The fault direction is then determined based on the high-frequency impedance difference ratio, and finally, the fault detection result is determined according to the fault type and fault direction. Therefore, by achieving fault polarity selection based on ground mode voltage polarity and accurately distinguishing between faults within and outside the fault zone using the high-frequency impedance difference ratio, the reliability of fault detection in flexible DC transmission systems is improved.

[0231] Please see Figure 12 , Figure 12This is a schematic diagram of the structure of an electronic device provided in an embodiment of this application. The electronic device may include a processor, a memory, a communication interface, and one or more programs. The processor, memory, and communication interface can be interconnected via a bus. The one or more programs are stored in the memory and configured to be executed by the processor. In this embodiment, the programs include instructions for performing the following steps:

[0232] Based on a preset voltage gradient algorithm, the system determines whether a fault has occurred in the flexible DC transmission system by real-time collecting voltage signals from the rectifier side and the inverter side.

[0233] If the flexible DC transmission system experiences a fault, the time of the fault occurrence is obtained;

[0234] Based on a preset time window, a first sampling dataset corresponding to the rectifier side and a second sampling dataset corresponding to the inverter side are acquired at the time of the fault occurrence; both the first sampling data and the second sampling data include voltage signals and current signals.

[0235] A preset mode transformation is performed on the voltage signals in the first sampled dataset to obtain a first ground mode voltage set; the preset mode transformation is performed on the voltage signals in the second sampled dataset to obtain a second ground mode voltage set.

[0236] The target fault type is determined based on the first ground mode voltage set and the second ground mode voltage set; the target fault type includes one of the following: positive grounding, negative grounding, and bipolar short circuit.

[0237] Perform a preset wavelet transform on the first sampled dataset to obtain a first high-frequency component set; perform the preset wavelet transform on the second sampled dataset to obtain a second high-frequency component set;

[0238] The first high-frequency impedance difference ratio corresponding to the rectifier side is determined based on the first high-frequency component set; the second high-frequency impedance difference ratio corresponding to the inverter side is determined based on the second high-frequency component set.

[0239] The target fault direction is determined based on the first high-frequency impedance difference ratio and the second high-frequency impedance difference ratio; the target fault direction includes one of the following: external fault and internal fault.

[0240] The target fault detection result is determined based on the target fault type and the target fault direction.

[0241] The electronic device described in this application can use a voltage gradient algorithm to detect whether a fault has occurred in a flexible DC transmission system. When a fault occurs, the fault type is determined based on the polarity of the ground mode voltage using sampled voltage signals. The high-frequency impedance difference ratio between the rectifier and inverter sides of the flexible DC transmission system is determined based on the voltage signal and sampled current signal. The fault direction is then determined based on the high-frequency impedance difference ratio. Finally, the fault detection result is determined based on the fault type and fault direction. Therefore, by achieving fault polarity selection based on the ground mode voltage polarity and accurately distinguishing between faults within and outside the fault zone using the high-frequency impedance difference ratio, the reliability of fault detection in flexible DC transmission systems is improved.

[0242] This application also provides a computer-readable storage medium storing a computer program for electronic data interchange, which causes a computer to perform some or all of the steps of any of the methods described in the above method embodiments, wherein the computer includes an electronic device.

[0243] This application also provides a computer program product, which includes a non-transitory computer-readable storage medium storing a computer program operable to cause a computer to perform some or all of the steps of any of the methods described in the above method embodiments. The computer program product may be a software installation package, and the computer may include an electronic device.

[0244] Those skilled in the art will understand that all or part of the processes in the methods of the above embodiments can be implemented by a computer program instructing related hardware. This program can be stored in a computer-readable storage medium, and when executed, it can include the processes described in the above method embodiments. The aforementioned storage medium includes various media capable of storing program code, such as ROM or random access memory (RAM), magnetic disks, or optical disks.

[0245] The steps of the methods or algorithms described in the embodiments of this application can be implemented in hardware or by a processor executing software instructions. The software instructions can consist of corresponding software modules, which can be stored in RAM, flash memory, ROM, EPROM, electrically erasable programmable read-only memory (EEPROM), registers, hard disk, portable hard disk, read-only optical disk (CD-ROM), or any other form of storage medium well known in the art. An exemplary storage medium is coupled to a processor, enabling the processor to read information from and write information to the storage medium. Of course, the storage medium can also be a component of the processor. The processor and storage medium can reside in an ASIC. Furthermore, the ASIC can reside in a terminal device or management device. Alternatively, the processor and storage medium can exist as discrete components in the terminal device or management device.

[0246] Those skilled in the art will recognize that, in one or more of the examples above, the functions described in the embodiments of this application can be implemented, in whole or in part, by software, hardware, firmware, or any combination thereof. When implemented in software, it can be implemented, in whole or in part, in the form of a computer program product. This computer program product includes one or more computer instructions. When these computer program instructions are loaded and executed on a computer, all or part of the processes or functions described in the embodiments of this application are generated. The computer can be a general-purpose computer, a special-purpose computer, a computer network, or other programmable device. The computer instructions can be stored in a computer-readable storage medium or transmitted from one computer-readable storage medium to another. For example, the computer instructions can be transmitted from one website, computer, server, or data center to another via wired (e.g., coaxial cable, fiber optic, digital subscriber line (DSL)) or wireless (e.g., infrared, wireless, microwave, etc.) means. The computer-readable storage medium can be any available medium accessible to a computer or a data storage device such as a server or data center that integrates one or more available media. The available media can be magnetic media (e.g., floppy disks, hard disks, magnetic tapes), optical media (e.g., digital video discs (DVDs)), or semiconductor media (e.g., solid-state disks (SSDs)).

[0247] The modules / units included in the various devices and products described in the above embodiments can be software modules / units, hardware modules / units, or a combination of both. For example, for devices and products applied to or integrated into a chip, all modules / units can be implemented using hardware methods such as circuits, or at least some modules / units can be implemented using software programs that run on a processor integrated within the chip, while the remaining (if any) modules / units can be implemented using hardware methods such as circuits. For devices and products applied to or integrated into a chip module, all modules / units can be implemented using hardware methods such as circuits. Different modules / units can be located in the same component (e.g., chip, circuit module, etc.) or different components of the chip module, or at least some modules / units can be implemented using hardware methods such as circuits. The implementation is achieved through a software program that runs on a processor integrated within the chip module. The remaining modules / units (if any) can be implemented using hardware methods such as circuits. For various devices and products applied to or integrated into terminal equipment, each of their modules / units can be implemented using hardware methods such as circuits. Different modules / units can be located in the same component (e.g., chip, circuit module, etc.) or different components within the terminal equipment. Alternatively, at least some modules / units can be implemented using a software program that runs on a processor integrated within the terminal equipment, while the remaining modules / units (if any) can be implemented using hardware methods such as circuits.

[0248] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the embodiments of this application. It should be understood that the above descriptions are merely specific embodiments of the embodiments of this application and are not intended to limit the protection scope of the embodiments of this application. Any modifications, equivalent substitutions, improvements, etc., made on the basis of the technical solutions of the embodiments of this application should be included within the protection scope of the embodiments of this application.

Claims

1. A fault detection method for a flexible DC transmission system, characterized in that, The method includes: Based on a preset voltage gradient algorithm, the system determines whether a fault has occurred in the flexible DC transmission system by real-time collecting voltage signals from the rectifier side and the inverter side. If the flexible DC transmission system experiences a fault, the time of the fault occurrence is obtained; Based on a preset time window, a first sampling dataset corresponding to the rectifier side and a second sampling dataset corresponding to the inverter side are acquired at the time of the fault occurrence; both the first sampling data and the second sampling data include voltage signals and current signals. A preset mode transformation is performed on the voltage signals in the first sampled dataset to obtain a first ground mode voltage set; the preset mode transformation is performed on the voltage signals in the second sampled dataset to obtain a second ground mode voltage set. The target fault type is determined based on the first ground mode voltage set and the second ground mode voltage set; the target fault type includes one of the following: positive grounding, negative grounding, and bipolar short circuit. Perform a preset wavelet transform on the first sampled dataset to obtain a first high-frequency component set; perform the preset wavelet transform on the second sampled dataset to obtain a second high-frequency component set; The first high-frequency impedance difference ratio corresponding to the rectifier side is determined based on the first high-frequency component set; the second high-frequency impedance difference ratio corresponding to the inverter side is determined based on the second high-frequency component set. The target fault direction is determined based on the first high-frequency impedance difference ratio and the second high-frequency impedance difference ratio; the target fault direction includes one of the following: external fault and internal fault. The target fault detection result is determined based on the target fault type and the target fault direction; Wherein, determining the first high-frequency impedance difference ratio corresponding to the rectifier side based on the first high-frequency component set includes: The measurement impedance is determined based on the high-frequency voltage component and high-frequency current component corresponding to each high-frequency component in the first high-frequency component set, thus obtaining a measurement impedance set; each measurement impedance corresponds to a frequency. The first high-frequency impedance difference ratio is determined based on the measured impedance set, the preset modular multilevel converter equivalent impedance, the preset line wave impedance, and the preset high-frequency impedance difference ratio calculation expression. The preset high-frequency impedance difference ratio calculation expression is used to quantify the degree of deviation between the measured impedance and the reference impedance. The expression is as follows: In the above formula, Indicates the ratio of high-frequency impedance differences; This indicates the number of frequency points involved in the calculation, corresponding to n discrete frequencies selected within a preset frequency band; The frequency point number ranges from 1 to n, corresponding to the 1st to nth discrete frequencies; The preset modular multilevel converter (MMC) equivalent impedance at the i-th frequency point; Let be the measured impedance at the i-th frequency point; The preset line impedance is given at the i-th frequency point.

2. The method as described in claim 1, characterized in that, The method of determining whether a fault has occurred in the flexible DC transmission system based on a preset voltage gradient algorithm, according to the voltage signals on the rectifier side and the inverter side acquired in real time within the flexible DC transmission system, includes: Acquire the first voltage signal set of the rectifier side and the second voltage signal set of the inverter side within a preset time period; The first voltage gradient is determined based on the first voltage signal set and the preset voltage gradient calculation expression; The second voltage gradient is determined based on the second voltage signal set and the preset voltage gradient calculation expression; If the absolute value of the first voltage gradient is greater than a preset voltage gradient threshold, and the absolute value of the second voltage gradient is greater than the preset voltage gradient threshold, then it is determined that a fault has occurred in the flexible DC transmission system; the end time of the preset time period is the time when the fault occurs.

3. The method as described in claim 1, characterized in that, The step of performing a preset mode transformation on the voltage signals in the first sampled dataset to obtain a first ground mode voltage set includes: Extract each voltage signal from the first sampled dataset to obtain a third voltage signal set; each voltage signal includes a positive voltage signal and a negative voltage signal; The third voltage signal set is decoupled based on a preset positive and negative decoupling expression to obtain the first ground mode voltage set.

4. The method as described in claim 3, characterized in that, The step of determining the target fault type based on the first ground mode voltage set and the second ground mode voltage set includes: Determine the average value of the first ground mode voltage corresponding to the first ground mode voltage set; Determine the average value of the second ground mode voltage corresponding to the second ground mode voltage set; If both the average value of the first ground mode voltage and the average value of the second ground mode voltage are less than a negative preset ground mode voltage threshold, then the target fault type is determined to be positive grounding; the preset ground mode voltage threshold is a positive value. If both the average value of the first ground mode voltage and the average value of the second ground mode voltage are greater than the preset ground mode voltage threshold, then the target fault type is determined to be negative grounding. If the average value of the first ground mode voltage and the average value of the second ground mode voltage are both greater than or equal to the negative value of the preset ground mode voltage threshold, and the average value of the first ground mode voltage and the average value of the second ground mode voltage are both less than or equal to the preset ground mode voltage threshold, then the target fault type is determined to be the bipolar short circuit.

5. The method as described in claim 1, characterized in that, The step of performing a preset wavelet transform on the first sampled dataset to obtain the first high-frequency component set includes: A preset wavelet transform is performed on the voltage signal in the first sampled dataset to obtain a first voltage high-frequency component set; each first voltage high-frequency component in the first voltage high-frequency component set corresponds to a frequency. A preset wavelet transform is performed on the current signal in the first sampled dataset to obtain a first high-frequency component set of current; each high-frequency component of the first current in the first high-frequency component set corresponds to a frequency. High-frequency components corresponding to preset frequency bands are extracted from the first voltage high-frequency component set and the first current high-frequency component set to obtain the second voltage high-frequency component set and the second current high-frequency component set. The first high-frequency component set is obtained by integrating the second voltage high-frequency component set and the second current high-frequency component set.

6. The method as described in claim 1, characterized in that, Determining the target fault direction based on the first high-frequency impedance difference ratio and the second high-frequency impedance difference ratio includes: If the first high-frequency impedance difference ratio is less than or equal to 1, and the second high-frequency impedance difference ratio is less than or equal to 1, then the target fault direction is determined to be a fault within the area. If the first high-frequency impedance difference ratio is greater than 1, and / or the second high-frequency impedance difference ratio is greater than 1, then the target fault direction is determined to be the fault outside the zone.

7. A fault detection device for a flexible DC transmission system, characterized in that, The fault detection device for the flexible DC transmission system includes: a fault detection module, a fault data acquisition module, a data acquisition module, a mode transformation module, a fault type determination module, a wavelet transform module, a data processing module, a fault direction determination module, and a fault detection result generation module. The fault detection module is used to determine whether a fault has occurred in the flexible DC transmission system based on a preset voltage gradient algorithm and the voltage signals on the rectifier side and inverter side collected in real time in the flexible DC transmission system. The fault data acquisition module is used to acquire the time of fault occurrence if a fault occurs in the flexible DC transmission system. The data acquisition module is used to acquire, based on a preset time window, a first sampling dataset corresponding to the rectifier side and a second sampling dataset corresponding to the inverter side at the time of the fault occurrence; both the first sampling data and the second sampling data include voltage signals and current signals. The mode transformation module is used to perform a preset mode transformation on the voltage signals in the first sampled dataset to obtain a first ground mode voltage set; and to perform the preset mode transformation on the voltage signals in the second sampled dataset to obtain a second ground mode voltage set. The fault type determination module is used to determine the target fault type based on the first ground mode voltage set and the second ground mode voltage set; the target fault type includes one of the following: positive grounding, negative grounding, and bipolar short circuit; The wavelet transform module is used to perform a preset wavelet transform on the first sampled dataset to obtain a first high-frequency component set; and to perform the preset wavelet transform on the second sampled dataset to obtain a second high-frequency component set. The data processing module is used to determine the first high-frequency impedance difference ratio corresponding to the rectifier side based on the first high-frequency component set; and to determine the second high-frequency impedance difference ratio corresponding to the inverter side based on the second high-frequency component set. The fault direction determination module is used to determine the target fault direction based on the first high-frequency impedance difference ratio and the second high-frequency impedance difference ratio; the target fault direction includes one of the following: external fault and internal fault; The fault detection result generation module is used to determine the target fault detection result based on the target fault type and the target fault direction; Wherein, determining the first high-frequency impedance difference ratio corresponding to the rectifier side based on the first high-frequency component set includes: The measurement impedance is determined based on the high-frequency voltage component and high-frequency current component corresponding to each high-frequency component in the first high-frequency component set, thus obtaining a measurement impedance set; each measurement impedance corresponds to a frequency. The first high-frequency impedance difference ratio is determined based on the measured impedance set, the preset modular multilevel converter equivalent impedance, the preset line wave impedance, and the preset high-frequency impedance difference ratio calculation expression. The preset high-frequency impedance difference ratio calculation expression is used to quantify the degree of deviation between the measured impedance and the reference impedance. The expression is as follows: In the above formula, Indicates the ratio of high-frequency impedance differences; This indicates the number of frequency points involved in the calculation, corresponding to n discrete frequencies selected within a preset frequency band; The frequency point number ranges from 1 to n, corresponding to the 1st to nth discrete frequencies; The preset modular multilevel converter (MMC) equivalent impedance at the i-th frequency point; Let be the measured impedance at the i-th frequency point; The preset line impedance is given at the i-th frequency point.

8. An electronic device, characterized in that, include: Processor, memory, communication interface, and one or more programs; The one or more programs are stored in the memory and configured to be executed by the processor, the programs including instructions for performing the steps of the method as described in any one of claims 1-6.

9. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a computer program, the computer program including program instructions that, when executed by a processor, cause the processor to perform the method as described in any one of claims 1-6.

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