A method for grading and positioning of multi-point grounding fault of DC system of hydropower plant

Abstract

This invention provides a hierarchical location method for multi-point grounding faults in the DC system of a hydropower plant, relating to the field of DC insulation fault monitoring. It solves the problem of misjudgment and inability to achieve uninterrupted power-off location in existing technologies when leakage current coupling occurs in multi-point faults. In this method, if a multi-point grounding fault exists in the DC system, an equivalent circuit equation for the multi-point grounding is established, and the equivalent grounding resistance of each potential grounding point is calculated decoupled. A feeder fault discrimination matrix is ​​constructed to screen out faulty feeder branches. The cable is divided into several independent sections, and leakage current differences are collected. An AC test signal is injected, and based on the response characteristics, the faulty cable section is identified. The terminals of each DC device connected to the end are traversed, and their insulation resistance to ground is matched with the equivalent grounding resistance to locate the fault point. This invention can accurately and synchronously identify and locate multi-point composite grounding faults under complex operating conditions in hydropower plants, significantly shortening fault handling time and effectively ensuring the safe and stable operation of the unit.

Description

Technical Field

[0001] This invention relates to the field of DC insulation fault monitoring technology in power systems, specifically to a method for hierarchical location of multi-point grounding faults in a hydropower plant's DC system. Background Technology

[0002] The DC system of a hydropower plant provides uninterrupted operating power for core facilities such as unit speed regulation, excitation, gate opening and closing, relay protection, automatic devices, and monitoring equipment. A ground fault in the DC system can easily cause protective devices to malfunction or fail to operate. Especially when two or more ground faults occur, in severe cases, it can lead to a short circuit in the DC power supply, directly causing unplanned unit shutdowns and posing a significant safety hazard to the hydropower plant. Therefore, quickly and accurately diagnosing and locating ground faults in the DC system is a crucial step in ensuring the safe and stable operation of a hydropower plant.

[0003] Currently, existing DC grounding fault diagnosis technologies have significant shortcomings in practical applications. Traditional line-by-line troubleshooting requires disconnecting each DC feeder individually, which can easily cause instantaneous power loss to operating protection and automatic devices. Since large-scale line-by-line operations are strictly limited during hydropower plant unit operation, this method is extremely risky and time-consuming. While balanced bridge insulation monitoring devices are widely used, they can only monitor the overall insulation degradation of the busbar and are mainly suitable for single-point grounding faults. When two or more grounding points occur in the system, the leakage currents at the positive and negative grounding points will cancel each other out, causing the busbar insulation voltage to tend to balance, and the device cannot issue any fault alarms, thus creating a serious monitoring blind spot.

[0004] Conventional single-frequency signal injection location schemes can typically only locate faulty feeders, failing to accurately segment cables hundreds of meters long in underground hydropower plant tunnels, nor effectively distinguish between moisture-induced grounding in the middle of the cable and grounding at the terminal of the equipment at the end. Furthermore, under the complex harmonic interference generated by the excitation system and frequency converters, misjudgments and omissions are highly likely for high-resistance multi-point grounding faults. In addition, existing hierarchical location technologies are mostly geared towards short-branch DC circuits in substations, failing to fully consider the unique and complex operating conditions of hydropower plants. Underground hydropower plant tunnels are constantly exposed to high humidity, with extremely long cable laying distances, and include dozens of branch circuits such as unit auxiliary control, gate control, and leakage drainage, resulting in frequent high- and low-resistance composite multi-point grounding faults. Existing methods severely lack multi-point decoupling algorithms and scientific hierarchical progressive location processes for such complex operating conditions.

[0005] In summary, existing troubleshooting and location technologies are no longer sufficient to meet the practical needs of rapid, safe, and accurate segmented location of multi-point grounding faults in hydropower plants. Faced with the unique and complex operating conditions of hydropower plants, including long distances, multiple branches, high humidity, and strong harmonic interference, the industry urgently requires an innovative location method that can operate without power interruption, effectively decouple multi-point faults, and progressively narrow down the fault range. This new method needs to overcome the limitations of traditional single-point monitoring and single-frequency injection to adapt to the complex DC network topology of hydropower plants, thereby completely eliminating the safety hazards caused by multi-point grounding in the DC system and effectively ensuring the continuous and reliable operation of hydropower plant units. Summary of the Invention

[0006] The purpose of this invention is to address the problems of existing DC system grounding detection technologies, which are prone to misjudgment when multiple faults are coupled by leakage current, and cannot achieve accurate segmented location without power interruption in the long cable, multi-branch, and high-humidity environments of hydropower plants. This invention proposes a hierarchical location method for multi-point grounding faults in hydropower plant DC systems. It constructs a four-level progressive hierarchical location architecture encompassing busbar identification, feeder screening, cable segmentation, and equipment terminal positioning, and establishes a multi-point grounding equivalent resistance decoupling model to eliminate leakage current coupling interference. Simultaneously, it combines dual-frequency alternating signal injection and multi-source data fusion technology to achieve anti-interference online detection under complex operating conditions. This invention can accurately and synchronously identify and locate multi-point composite grounding faults under complex operating conditions in hydropower plants without interrupting DC power supply, significantly shortening fault handling time and effectively ensuring the safe and stable operation of the units.

[0007] The present invention employs the following technical solutions to achieve its objective: A method for hierarchical location of multi-point grounding faults in a hydropower plant's DC system, comprising the following steps: S1. Real-time acquisition of the ground voltage and total ground insulation resistance of the positive and negative busbars of the hydropower plant's DC system, and determination of whether the DC system has a multi-point grounding fault; if a multi-point grounding fault exists, establish the multi-point grounding equivalent loop equation, and decouple the calculation of the equivalent grounding resistance of each potential grounding point. S2. Synchronously collect real-time leakage current data of each feeder outlet of the DC system; based on the equivalent grounding resistance of each grounding point obtained by decoupling, construct a feeder fault discrimination matrix, and filter out feeder branches containing grounding faults according to the feeder fault discrimination matrix. S3. For the selected faulty feeder branches, divide the cable into several independent sections along the cable segmentation nodes, and collect the leakage current difference between the inlet and outlet of each independent section; inject an AC test signal into the DC system, and combine the leakage current difference of each independent section with the response characteristics of the AC test signal to locate the cable section with grounding fault. S4. For the locked faulty cable section, traverse all DC equipment terminals connected to the end of the section; calculate the ground insulation resistance of each DC equipment terminal online, and match the ground insulation resistance with the decoupled equivalent grounding resistance to locate the equipment terminal where the grounding fault occurred.

[0008] Specifically, in step S1, determining whether the DC system has a multi-point grounding fault includes: when only the insulation resistance of a single positive or single negative pole decreases, it is determined to be a single-point grounding fault; if the insulation resistance of multiple points on both the positive and negative poles decreases simultaneously, it is determined that the DC system has a multi-point composite grounding fault; if the insulation resistance of two or more points of the same polarity is lower than a preset fault threshold, it is also determined that the DC system has a multi-point composite grounding fault.

[0009] Specifically, in step S1, the equivalent circuit equations for multi-point grounding are established, and the equivalent grounding resistance of each potential grounding point is calculated in decoupled manner. This includes: assuming the total insulation resistance of the positive pole to ground in the DC system is R+, the total insulation resistance of the negative pole to ground is R-, and the system has m positive grounding points and n negative grounding points; the grounding resistance of each positive grounding point is R+1 to R+m, and the grounding resistance of each negative grounding point is R-1 to Rn; based on R+, R-, R+1 to R+m, and R-1 to Rn, a set of equivalent circuit equations for multi-point grounding is established, and the equivalent grounding resistance of each independent grounding point is solved by the elimination method.

[0010] Furthermore, in step S2, a feeder fault discrimination matrix is ​​constructed, including: the rows of the feeder fault discrimination matrix correspond to the feeder numbers, and the columns correspond to the contribution of the equivalent resistance of each grounding point after decoupling; the elements of the feeder fault discrimination matrix are the deviation between the theoretical calculation value and the measured value of the corresponding feeder leakage current. Based on the feeder fault discrimination matrix, feeder branches containing ground faults are selected, including: when the deviation exceeds a preset noise threshold, the feeder marked in the corresponding row is selected as a feeder branch containing a ground fault.

[0011] Furthermore, in step S3, injecting AC test signals into the DC system includes: using a dual-frequency alternating signal injection method to alternately inject a low-frequency AC test signal with a first preset frequency and a medium-frequency AC test signal with a second preset frequency into the bus of the DC system; collecting the response current amplitude and phase characteristics of each independent section to the low-frequency AC test signal and the medium-frequency AC test signal respectively; and using the collected response current amplitude and phase characteristics to eliminate harmonic interference generated by the frequency converter and excitation device of the hydropower plant.

[0012] Specifically, in step S3, the cable is divided into several independent sections along the cable segmentation nodes, including: for cables in the underground corridor of the hydropower plant, a cable segmentation node is set every second preset distance; for branch cables of indoor equipment of the unit, a cable segmentation node is set every first preset distance; for gate control cables, a cable segmentation node is set every third preset distance; the first preset distance < the second preset distance < the third preset distance.

[0013] Furthermore, in step S3, locking the cable section with grounding fault includes: calculating the insulation attenuation coefficient of each independent section under dual-frequency signals based on the leakage current difference between the inlet and outlet of each independent section, combined with the response current amplitude and phase characteristics of the low-frequency AC test signal and the medium-frequency AC test signal; comparing the calculated insulation attenuation coefficient with a preset attenuation threshold, and locking the independent section with the insulation attenuation coefficient exceeding the attenuation threshold as the cable section with grounding fault.

[0014] Specifically, in step S4, the ground insulation resistance value is matched with the decoupled equivalent grounding resistance, including: obtaining the equivalent resistance range of each independent grounding point output by the multi-point grounding equivalent loop equation in step S1; comparing and matching the ground insulation resistance value of each DC equipment terminal obtained by traversing and calculating with the equivalent resistance range; when the ground insulation resistance value of a certain DC equipment terminal falls within a certain equivalent resistance range, the DC equipment terminal is located as the equipment terminal point where a grounding fault has occurred.

[0015] Specifically, after step S4, the method further includes step S5: summarizing the hierarchical positioning results of primary bus fault identification, secondary feeder fault branch screening, tertiary cable section segmentation and positioning, and quaternary equipment terminal positioning; distinguishing between bus full-area faults, feeder branch faults, cable section faults, and single or multiple point faults of equipment terminals, and pushing alarm information according to the severity of the fault; outputting the coordinate position, grounding resistance value, and corresponding fault type of all grounding points.

[0016] Preferably, after step S5, the method further includes step S6: storing the multi-point grounding locations, grounding resistance values, corridor environmental humidity, and cable operating time obtained in this location into a fault sample library; iteratively updating the noise threshold and insulation attenuation judgment threshold of the feeder fault discrimination matrix based on historical data in the fault sample library; and adaptively adjusting the threshold values ​​of various parameters for multi-point grounding fault discrimination according to the humid conditions of the hydropower plant in different seasons.

[0017] In summary, due to the adoption of this technical solution, the beneficial effects of this invention are as follows: This invention, by constructing a multi-level progressive hierarchical positioning architecture and a multi-point grounding equivalent resistance decoupling model, completely overcomes the limitations of traditional DC grounding detection technology, which is prone to misjudgment and missed judgment when multiple-point fault leakage current coupling occurs. This invention can accurately decouple various multi-point composite grounding faults, effectively eliminate insulation measurement distortion caused by mutual cancellation of leakage currents, and significantly improve the accuracy and reliability of fault identification under complex DC network topologies.

[0018] This invention enables uninterrupted online detection from the entire busbar area to the accurate pinpointing of equipment terminals, completely eliminating the high-risk traditional circuit-pulling troubleshooting operation. This fundamentally avoids the power loss problem of protection and automatic devices during operation caused by disconnecting feeders one by one, and completely eliminates the safety hazards of unit protection malfunction or failure to operate during the troubleshooting process, effectively ensuring the continuous, safe and stable operation of hydropower plant units.

[0019] Addressing the complex operating conditions unique to hydropower plants, such as long underground corridors, multi-branch auxiliary control circuits, and consistently high humidity, this invention combines dual-frequency alternating signal injection and cable segmentation technology to effectively overcome harmonic interference generated by frequency converters and excitation devices. This invention not only significantly improves the sensitivity of identifying high-resistance grounding faults in humid environments but also accurately pinpoints the fault to specific cable segments and equipment terminals, greatly shortening the time required for fault diagnosis and resolution.

[0020] Furthermore, this invention introduces an iterative optimization mechanism for fault data, deeply integrating historical location data with environmental parameters to achieve adaptive dynamic adjustment of the discrimination threshold. This intelligent evolution capability enables it to perfectly adapt to the changes in different seasons and humid conditions in hydropower plants, maintaining extremely high detection accuracy and engineering practicality, and providing reliable technical support for the intelligent operation and maintenance of hydropower plant DC systems. Attached Figure Description

[0021] The technical solution of this invention is described in detail with reference to the following figures, specifically including three figures as follows: Figure 1 This is a schematic diagram illustrating the overall process of the multi-point grounding fault hierarchical location method of the present invention; Figure 2 This is a schematic diagram illustrating the application principle of the feeder fault discrimination matrix in the method of the present invention; Figure 3 This is a schematic diagram showing the layout of cable segment detection nodes in the underground corridor in the method of the present invention. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0023] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0024] Example A method for hierarchical location of multi-point grounding faults in a hydropower plant's DC system. Figure 1 A brief overview of the overall process of this method is shown below and can be viewed concurrently; the key steps of this method can be summarized as follows: S1. Real-time acquisition of ground voltage and total ground insulation resistance of the positive and negative busbars of the hydropower plant's DC system to determine whether there are multiple grounding faults in the DC system; if there are multiple grounding faults, establish the equivalent loop equation for multiple grounding and decouple the calculation of the equivalent grounding resistance of each potential grounding point. S2. Synchronously collect real-time leakage current data of each feeder outlet of the DC system; based on the equivalent grounding resistance of each grounding point obtained by decoupling, construct a feeder fault discrimination matrix, and filter out feeder branches containing grounding faults according to the feeder fault discrimination matrix. S3. For the selected faulty feeder branches, divide the cable into several independent sections along the cable segmentation nodes, and collect the leakage current difference between the inlet and outlet of each independent section; inject AC test signals into the DC system, and combine the leakage current difference between each independent section with the response characteristics of the AC test signals to locate the cable section with grounding fault. S4. For the locked faulty cable section, traverse all DC equipment terminals connected to the end of the section; calculate the ground insulation resistance of each DC equipment terminal online, and match the ground insulation resistance with the equivalent grounding resistance obtained by decoupling to locate the equipment terminal where the grounding fault occurred.

[0025] The core principle of this embodiment is to deconstruct the complex DC network topology into four physical levels: bus, feeder, cable section to equipment terminal, and integrate the multi-point grounding equivalent resistance decoupling model and dual-frequency alternating signal injection technology to achieve in-depth extraction of multi-source fault characteristics and elimination of coupling interference.

[0026] This embodiment can fill the technical gap in engineering terms of the inability to accurately locate multi-point composite grounding faults in long-distance, multi-branch, and high-humidity corridor environments of hydropower plants. Moreover, it fundamentally eliminates the risk of unit protection maloperation or failure to operate caused by troubleshooting operations, and greatly improves the safety of DC system operation and maintenance and the efficiency of fault handling.

[0027] The following section will elaborate on and explain each step of the hierarchical progression in this embodiment in detail, taking into account specific application scenarios.

[0028] I. Comprehensive bus fault detection Step S1 corresponds to the stage of comprehensive fault identification for the primary busbar. The insulation monitoring host first collects the ground voltage of the positive and negative buses of the hydropower plant's DC system in real time, and calculates the total insulation resistance of the positive and negative buses to ground. An insulation warning threshold is preset within the DC system; in this embodiment, it is preferably set to 50kΩ. When the overall insulation resistance of either polarity is detected to be lower than this preset threshold, a ground fault is initially determined to exist in the DC system, thereby triggering the subsequent diagnostic and decoupling analysis process.

[0029] After initially determining the presence of a grounding fault, the fault type is further accurately distinguished based on the attenuation characteristics of the insulation resistance. When the insulation resistance of only a single positive or negative pole decreases, it is determined to be a single-point grounding fault; if the insulation resistance decreases at multiple points on both the positive and negative poles simultaneously, it is determined to be a multi-point composite grounding fault of opposite polarities in the DC system; if the insulation resistance of two or more points of the same polarity is lower than a preset fault threshold, it is determined to be a multi-point composite grounding fault of the same polarity in the DC system. This embodiment, through this logical discrimination, can complete the preliminary characterization of fault types across the entire domain.

[0030] To eliminate insulation measurement distortion caused by the mutual cancellation of leakage currents from multiple faults, this embodiment establishes an equivalent loop equation for multi-point grounding and decouples the calculation of the equivalent grounding resistance of each potential grounding point. In this process, the total insulation resistance of the DC system's positive terminal to ground is set as R+, and the total insulation resistance of the negative terminal to ground is set as R-. It is assumed that the system has m positive grounding points and n negative grounding points. The grounding resistance of each positive grounding point is set from R+1 to R+m, where 1 to m are merely serial numbers. Similarly, the grounding resistance of each negative grounding point is set from R-1 to Rn. Based on the above total insulation resistance and the resistance parameters of each independent grounding point, a set of equivalent loop equations for multi-point grounding is constructed.

[0031] In this embodiment, it is assumed that the DC system has two positive grounding points and two negative grounding points simultaneously, corresponding to the four grounding point resistances R+1, R+2, R-1, and R-2, respectively. The insulation monitoring host collects multiple sets of positive and negative bus voltage data to ground by alternately switching on the internal positive and negative detection resistors with known resistance values. Based on Kirchhoff's current law, the total leakage current to ground of the positive bus is equal to the sum of the leakage currents at each positive grounding point. That is, the positive-to-ground voltage divided by the total positive-to-ground insulation resistance R+ is equal to the positive-to-ground voltage divided by the sum of R+1 and R+2, respectively. The same applies to the negative circuit.

[0032] To accurately determine the resistance of these four unknown grounding points, this embodiment combines the aforementioned multiple sets of bus-to-ground voltage data with the initial leakage current data synchronously collected at each feeder outlet as boundary constraints to construct a system of multivariate nonlinear equations containing the resistances of these four grounding points. Subsequently, matrix elimination is used to reduce the order of these multivariate equations and iteratively solve them, thereby decoupling and obtaining the equivalent resistance values ​​of these four independent grounding points. This calculation method based on simultaneous elimination using multi-source data effectively eliminates measurement interference caused by the mutual cancellation of leakage currents during multi-point grounding, enabling the insulation monitoring host to accurately obtain the resistance value of each actual grounding point, ultimately achieving high-precision fault identification across the entire primary busbar area.

[0033] II. Screening of Faulty Feeder Branches Step S2 corresponds to the secondary feeder fault branch screening stage. First, it's necessary to acquire real-time operating data for each feeder branch of the entire plant's DC system. Taking a hydropower plant with a total of 32 DC feeders as an example, each feeder's outgoing terminal is equipped with a switchable high-precision leakage current transformer. These high-precision leakage current transformers allow for the simultaneous acquisition of real-time leakage current data for all 32 branches, providing reliable basic data support for subsequent accurate fault screening.

[0034] After acquiring real-time leakage current data, a feeder fault discrimination matrix is ​​constructed for in-depth analysis. The application process can be found in [link to documentation]. Figure 2 This is a schematic diagram. The rows of the feeder fault discrimination matrix correspond to the feeder numbers, that is, rows 1 to 32 correspond to feeders 1 to 32 respectively; the columns of the matrix correspond to the leakage current contribution of the equivalent resistance of each grounding point after decoupling in the previous steps.

[0035] It should be noted here that the columns of the matrix actually represent each identified independent grounding point, i.e., a specific fault source. In step S1, the equivalent resistance of multiple independent grounding points is decoupled and calculated, and each column of the matrix corresponds specifically to one of these grounding points. The leakage current contribution mentioned in this embodiment refers to the magnitude of the current that should leak from the grounding point to the system, theoretically calculated using a circuit model based on the equivalent resistance of that specific grounding point and the bus-to-ground voltage. Therefore, the columns of the matrix decompose the complex total fault current into each specific grounding point, which is used to measure the theoretical contribution of each independent grounding point to the overall or local leakage current, thereby providing a reliable theoretical benchmark for subsequent accurate comparison with measured leakage current data.

[0036] In this embodiment, each element in the matrix is ​​set as the deviation between the theoretically calculated value and the measured value of the corresponding feeder leakage current. The theoretically calculated value is the expected leakage current calculated by substituting the equivalent resistance of each independent grounding point obtained from the decoupling steps in the previous steps into the circuit model, while the measured value is the actual leakage current synchronously collected by the high-precision leakage current transformer.

[0037] After calculating the deviation values ​​of all elements in the matrix, a dynamic noise threshold is introduced for fault determination. This dynamic noise threshold is not fixed but is automatically adjusted based on the current unit load status of the hydropower plant and the real-time ambient humidity of the underground corridor. When the deviation value corresponding to a certain row exceeds the dynamic noise threshold set under the current operating conditions, the feeder marked in that row is filtered and marked as a feeder branch containing a grounding fault. Ultimately, based on this method, the DC system can effectively eliminate noise differential current caused by equipment start-up and shutdown and electromagnetic interference, outputting the numbers of all faulty branches, thereby completing the accurate screening of secondary feeders.

[0038] III. Cable Section Division and Positioning Step S3 corresponds to the third-level cable segmentation and positioning stage. First, the selected faulty feeder branches need to be physically segmented. Considering the actual topology of the hydropower plant's DC system cable laying, this embodiment sets up multiple cable segmentation nodes along the cable laying path, and installs segmented current transformers at each node. Preferably, for the branch cables of the unit's indoor equipment, due to the compact space and dense equipment, a cable segmentation node is set at a first preset distance of 50 meters; for the cables in the underground corridor of the hydropower plant, considering the long linear extension of the corridor and the complex environment, a cable segmentation node is set at a second preset distance of 100 meters.

[0039] like Figure 3As shown, the main cable in the underground corridor is divided into multiple continuous independent monitoring sections by current transformers at each segment node. Adjacent sections are connected in series through current transformers to form a chain-like sensing network, which realizes full coverage monitoring of the long-distance corridor cable.

[0040] For long-distance gate control cables, due to their extremely long laying distance and few branches, a cable segmentation node can be set at a third preset distance every 150 meters. This gradient division rule of three preset distances adapts to the cable laying characteristics of different areas of the hydropower plant.

[0041] After the physical division of cable sections was completed, a dual-frequency alternating signal injection method was used to inject AC test signals into the DC system to obtain the deep insulation characteristics of each section. The insulation monitoring host alternately injected a low-frequency AC test signal of 5Hz and a medium-frequency AC test signal of 25Hz into the DC system bus. During the signal injection, the response current amplitude and phase characteristics of each independent section to the low-frequency AC test signal and the medium-frequency AC test signal were collected respectively.

[0042] Because hydropower plants have numerous frequency converters and excitation devices, the complex harmonics they generate can easily cause severe interference to single-frequency detection signals. By simultaneously extracting the response amplitude and phase characteristics at two different frequencies, 5Hz and 25Hz, the differentiated propagation characteristics of dual-frequency signals can be utilized to effectively filter out and eliminate interference from strong harmonics at the site, thereby significantly improving the sensitivity and accuracy of grounding fault identification in humid and high-resistivity environments.

[0043] After obtaining the dual-frequency response characteristics, the insulation status of each independent section is further calculated to pinpoint the fault location. Based on the leakage current difference collected by the current transformers at the inlet and outlet of each independent section, and combined with the response current amplitude and phase characteristics of the aforementioned low-frequency AC test signal and medium-frequency AC test signal, the insulation attenuation coefficient of each independent section under the action of the dual-frequency signal is comprehensively calculated. The calculated insulation attenuation coefficient is then compared with a preset attenuation threshold. When the insulation attenuation coefficient of a certain independent section exceeds the attenuation threshold, that section can be identified as a cable section with a grounding fault.

[0044] This embodiment uses a calculation method based on the fusion of leakage current difference and dual-frequency response characteristics to not only locate the physical section where the fault occurred, but also effectively distinguish between moisture grounding caused by dampness in the middle section of the cable due to the corridor and terminal grounding caused by the branch of the equipment at the end of the cable. The entire location process does not require disconnecting the feeder power supply, ensuring uninterrupted power supply to the unit control circuit.

[0045] IV. Precise positioning of equipment terminals Step S4 corresponds to the precise pinpointing stage of the fourth-level equipment terminals. For the faulty cable section identified in the previous steps, a final inspection is conducted at the end-device level. The maintenance terminal connects to the online detection units of all DC equipment terminals connected to the end of the faulty cable section. These DC devices cover the core control and actuators of the hydropower plant, specifically including speed governors, excitation regulators, gate control cabinets, and leakage drainage PLC controllers. By connecting to these online detection units, the insulation resistance to ground of each DC equipment terminal can be measured online without disconnecting the DC feeder power supply, thereby obtaining the true insulation status data of the end-devices.

[0046] After obtaining the measured insulation resistance to ground of each device terminal, it is compared and matched with the equivalent resistance range of each independent grounding point output by the multi-point grounding equivalent loop equation in step S1. Since step S1 has already obtained the theoretical equivalent resistance and its allowable error fluctuation range (equivalent resistance range) of each potential grounding point through decoupling calculation, the insulation resistance to ground of each DC device terminal obtained by traversing the calculation can now be compared with these equivalent resistance ranges one by one. When it is detected that the insulation resistance to ground of a certain DC device terminal falls within a certain equivalent resistance range, it can be confirmed that the terminal matches the theoretically decoupled fault point, thereby accurately locating the DC device terminal as the device terminal where the grounding fault occurred.

[0047] In this embodiment, the DC system, through the aforementioned method, can not only accurately locate faulty terminals but also simultaneously distinguish the nature of different types of grounding faults. When the measured ground insulation resistance of a certain leakage drainage controller terminal is 30kΩ, it is matched to the corresponding equivalent resistance range, and the point is determined to be a high-resistance moisture-induced grounding fault with a resistance between 10kΩ and 50kΩ. If the measured ground insulation resistance of a certain excitation regulator terminal is only 500Ω, it is determined to be a metallic low-resistance grounding fault with a resistance less than 1kΩ. Finally, the specific physical location of all grounding points, the precise grounding resistance value, and the corresponding fault type are summarized and output, providing maintenance personnel with intuitive and detailed fault handling guidance, and thoroughly completing the entire multi-point grounding fault hierarchical location process.

[0048] V. Hierarchical Alarm Output After the precise positioning of the device terminals is completed in step S4, the fault output and hierarchical alarm process can be automatically executed, that is... Figure 1Step S5. In this stage, all hierarchical location results generated from primary busbar fault identification, secondary feeder branch fault screening, tertiary cable section segmentation and location, and quaternary equipment terminal precise positioning are comprehensively summarized. Based on this multi-dimensional location data, the specific fault types currently existing in the DC system are intelligently distinguished, including full-domain busbar faults, feeder branch faults, cable section faults, and single-point or multi-point equipment terminal faults. Subsequently, according to the potential impact of various faults on the safe operation of hydropower plant units, the faults are classified into different severity levels, and corresponding hierarchical alarm push mechanisms are triggered.

[0049] In specific alarm push operations, a four-color hierarchical alarm strategy is adopted to intuitively reflect the urgency of the fault. When a fault is detected across the entire primary busbar, such as a severe decrease in the overall insulation of the positive and negative busbars and the presence of multiple composite grounding points, a red emergency alarm will be triggered immediately, reminding maintenance personnel to take immediate intervention measures. If the fault is located at the secondary feeder branch level, an orange alarm will be pushed, indicating that there is a potential insulation hazard in a specific feeder circuit. When the fault is further refined and locked into a tertiary cable section, a yellow alarm will be issued, indicating that there is moisture or damage to a specific section of cable in an underground corridor or indoor area. For single-point or multi-point faults at the tertiary equipment terminal level, a blue alert alarm will be pushed, informing maintenance personnel of the insulation status of the specific terminal equipment. Through the hierarchical alarm mechanism, maintenance personnel can quickly determine the macroscopic impact range of the fault and the priority of emergency handling based on the alarm color. Furthermore, the number of triggers of the lowest-level alarm can directly determine whether to further trigger the alarm of the next higher level, or different trigger thresholds and conditions can be independently designed for each level of alarm.

[0050] Upon triggering a tiered alarm, the human-machine interface screen simultaneously displays detailed fault diagnosis information. The screen not only dynamically displays the complete four-level location tracking path from the busbar to the equipment terminal in the form of a topology map, but also generates and displays a pop-up window showing a complete list of grounding points. This list details the specific physical coordinates of each precisely located grounding point, the accurately calculated grounding resistance value, and the corresponding fault type. For example, the list might show that the terminal coordinates of a leakage drainage control cabinet are cabinet number 15 in area A on the second basement level of the underground corridor, the calculated grounding resistance is 30kΩ, and the fault type is determined to be high-resistance moisture-induced grounding. This comprehensive and visualized information output method provides maintenance personnel with clear and explicit on-site troubleshooting guidance, thus completing the entire process of the multi-point grounding fault tiered location method.

[0051] VI. Iterative Optimization of Fault Data Step S6 corresponds to the fault data iterative optimization stage, which can be found in [reference]. Figure 1Overview. As a preferred embodiment, the multi-dimensional data generated during this fault location process is first deeply collected and persistently stored. Key information obtained from this precise location, such as the coordinates of multiple grounding points, the number of grounding points, the precise grounding resistance values, the humidity of the corridor environment at the time of the fault, and the service life of the corresponding cables, is uniformly uploaded and stored in a cloud-based fault sample database. This operation not only provides a complete digital archive for a single fault investigation but also accumulates a massive and realistic foundation of historical on-site data for the continuous evolution of subsequent algorithm models.

[0052] After accumulating sufficient historical data, an automated iterative update mechanism for parameter thresholds can be initiated. The cloud server will perform batch mining and statistical analysis of historical data in the fault sample database monthly, focusing on evaluating the distribution patterns of historical differential current noise and insulation attenuation characteristics under different operating conditions. Based on the statistical analysis results, the differential current noise threshold in secondary feeder screening and the insulation attenuation judgment threshold in tertiary cable section location will be automatically corrected and iteratively updated. For example, if historical data shows that the normal leakage current of cables in a certain area fluctuates significantly under specific unit loads, the noise threshold for that area will be appropriately relaxed, thereby effectively reducing the false alarm rate caused by electromagnetic interference and making the feeder fault discrimination matrix more closely match the actual operating status of the equipment.

[0053] Furthermore, this embodiment also possesses adaptive adjustment capabilities for the unique seasonal climate of hydropower plants. Underground corridors of hydropower plants are often in extremely high humidity environments during the rainy season or flood season, causing a natural decline in the overall insulation level of cables. In such cases, based on historical high-humidity operating condition characteristics matched in the cloud-based sample database, the insulation warning threshold in the overall identification of the primary busbar is automatically lowered. For example, if the insulation warning threshold is set to 50kΩ during the normal dry season, it can be adaptively lowered to 30kΩ under the high humidity environment of the rainy season. Through a dynamic threshold adjustment strategy based on environmental perception, the sensitivity and accuracy of identifying multi-point high-resistance grounding faults under extreme humid conditions can be significantly improved, achieving intelligent optimization of the entire positioning method.

Claims

1. A method for hierarchical location of multi-point grounding faults in a hydropower plant's DC system, characterized in that, The method includes the following steps: S1. Real-time acquisition of the ground voltage and total ground insulation resistance of the positive and negative busbars of the hydropower plant's DC system, and determination of whether the DC system has a multi-point grounding fault; if a multi-point grounding fault exists, establish the multi-point grounding equivalent loop equation, and decouple the calculation of the equivalent grounding resistance of each potential grounding point. S2. Synchronously collect real-time leakage current data of each feeder outlet of the DC system; based on the equivalent grounding resistance of each grounding point obtained by decoupling, construct a feeder fault discrimination matrix, and filter out feeder branches containing grounding faults according to the feeder fault discrimination matrix. S3. For the selected faulty feeder branches, divide the cable into several independent sections along the cable segmentation nodes, and collect the leakage current difference between the inlet and outlet of each independent section; inject an AC test signal into the DC system, and combine the leakage current difference of each independent section with the response characteristics of the AC test signal to locate the cable section with grounding fault. S4. For the locked faulty cable section, traverse all DC equipment terminals connected to the end of the section; calculate the ground insulation resistance of each DC equipment terminal online, and match the ground insulation resistance with the decoupled equivalent grounding resistance to locate the equipment terminal where the grounding fault occurred.

2. The method for hierarchical location of multi-point grounding faults in a hydropower plant's DC system according to claim 1, characterized in that, In step S1, determining whether the DC system has a multi-point grounding fault includes: when only the insulation resistance of a single positive or single negative pole decreases, it is determined to be a single-point grounding fault; if the insulation resistance of multiple positive and negative poles decreases simultaneously, the DC system is determined to have a multi-point composite grounding fault; if the insulation resistance of two or more poles of the same polarity is lower than a preset fault threshold, the DC system is also determined to have a multi-point composite grounding fault.

3. The method for hierarchical location of multi-point grounding faults in a hydropower plant's DC system according to claim 1, characterized in that, In step S1, the equivalent circuit equations for multi-point grounding are established, and the equivalent grounding resistance of each potential grounding point is calculated in decoupled manner. This includes: assuming the total insulation resistance of the positive pole to ground in the DC system is R+, the total insulation resistance of the negative pole to ground is R-, and the system has m positive grounding points and n negative grounding points; the grounding resistance of each positive grounding point is R+1 to R+m, and the grounding resistance of each negative grounding point is R-1 to Rn; based on R+, R-, R+1 to R+m, and R-1 to Rn, a set of equivalent circuit equations for multi-point grounding is established, and the equivalent grounding resistance of each independent grounding point is solved by the elimination method.

4. The method for hierarchical location of multi-point grounding faults in a hydropower plant's DC system according to claim 1, characterized in that, In step S2, a feeder fault discrimination matrix is ​​constructed, including: the rows of the feeder fault discrimination matrix correspond to the feeder numbers, and the columns correspond to the contribution of the equivalent resistance of each grounding point after decoupling; the elements of the feeder fault discrimination matrix are the deviation between the theoretical calculation value and the measured value of the corresponding feeder leakage current. Based on the feeder fault discrimination matrix, feeder branches containing ground faults are selected, including: when the deviation exceeds a preset noise threshold, the feeder marked in the corresponding row is selected as a feeder branch containing a ground fault.

5. The method for hierarchical location of multi-point grounding faults in a hydropower plant's DC system according to claim 1, characterized in that, In step S3, injecting AC test signals into the DC system includes: using a dual-frequency alternating signal injection method, alternately injecting a low-frequency AC test signal with a first preset frequency and a medium-frequency AC test signal with a second preset frequency into the bus of the DC system; collecting the response current amplitude and phase characteristics of each independent section to the low-frequency AC test signal and the medium-frequency AC test signal, and using the collected response current amplitude and phase characteristics to eliminate harmonic interference generated by the frequency converter and excitation device of the hydropower plant.

6. The method for hierarchical location of multi-point grounding faults in a hydropower plant's DC system according to claim 5, characterized in that, In step S3, the cable is divided into several independent sections along the cable segmentation nodes, including: for cables in the underground corridor of the hydropower plant, a cable segmentation node is set every second preset distance; for branch cables of indoor equipment of the unit, a cable segmentation node is set every first preset distance; for gate control cables, a cable segmentation node is set every third preset distance; the first preset distance < the second preset distance < the third preset distance.

7. The method for hierarchical location of multi-point grounding faults in a hydropower plant's DC system according to claim 5, characterized in that, In step S3, identifying cable sections with grounding faults includes: calculating the insulation attenuation coefficient of each independent section under dual-frequency signals based on the leakage current difference between the inlet and outlet of each independent section, combined with the response current amplitude and phase characteristics of the low-frequency AC test signal and the medium-frequency AC test signal; comparing the calculated insulation attenuation coefficient with a preset attenuation threshold, and identifying independent sections with insulation attenuation coefficients exceeding the attenuation threshold as cable sections with grounding faults.

8. The method for hierarchical location of multi-point grounding faults in a hydropower plant's DC system according to claim 1, characterized in that, In step S4, the ground insulation resistance value is matched with the decoupled equivalent grounding resistance, including: obtaining the equivalent resistance range of each independent grounding point output by the multi-point grounding equivalent loop equation in step S1; comparing and matching the ground insulation resistance value of each DC equipment terminal obtained by traversing and calculating with the equivalent resistance range; when the ground insulation resistance value of a certain DC equipment terminal falls within a certain equivalent resistance range, the DC equipment terminal is located as the equipment terminal point where a grounding fault has occurred.

9. The method for hierarchical location of multi-point grounding faults in a hydropower plant's DC system according to claim 1, characterized in that, After step S4, the method further includes step S5: summarizing the hierarchical positioning results of primary bus fault identification, secondary feeder fault branch screening, tertiary cable section segmentation and positioning, and quaternary equipment terminal positioning; distinguishing between bus full-area faults, feeder branch faults, cable section faults, and single or multiple point faults of equipment terminals, and pushing alarm information according to the severity of the fault. Output the coordinates of all grounding points, the grounding resistance value, and the corresponding fault type.

10. The method for hierarchical location of multi-point grounding faults in a hydropower plant's DC system according to claim 9, characterized in that, After step S5, the method further includes step S6: storing the multi-point grounding locations, grounding resistance values, corridor environmental humidity, and cable operating time obtained in this location into the fault sample library; and iteratively updating the noise threshold and insulation attenuation judgment threshold of the feeder fault discrimination matrix based on the historical data in the fault sample library. Based on the humid operating conditions of hydropower plants in different seasons, the threshold values ​​of various parameters for multi-point grounding fault detection are adaptively adjusted.

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