A method for contact deterioration recognition grading assessment of low voltage cable branch boxes

Abstract

The application belongs to the technical field of intelligent operation and maintenance and state diagnosis of low-voltage power distribution equipment, and discloses a contact deterioration identification grading evaluation method for a low-voltage cable branch box. The method first determines key monitoring nodes and collects multi-source operation parameters, and establishes a reference thermal network model according to the device type; then, the thermal network reference temperature and abnormal residual of the key nodes are calculated, a contact deterioration index is constructed, and the equivalent contact resistance is updated; finally, the deterioration positioning and deterioration grade determination are performed, and the evaluation results and maintenance suggestions are output. The application converts the residual between the measured temperature and the reference temperature into a contact deterioration index with physical meaning through the device type parameterized thermal network model, can effectively distinguish the abnormal temperature rise caused by the poor contact from the normal load temperature rise, realizes the accurate positioning of the contact deterioration position and the quantitative evaluation of the deterioration grade, and provides decision support for the preventive maintenance of the low-voltage cable branch box.

Description

Technical Field

[0001] This invention belongs to the field of intelligent operation and maintenance and condition diagnosis technology of low-voltage power distribution equipment. Specifically, it relates to a method for identifying and classifying the contact deterioration of low-voltage cable branch boxes. It is applicable to the identification, anomaly location, and maintenance classification assessment of contact deterioration of low-voltage power distribution equipment such as strip fuse branch boxes, knife fuse switch branch boxes, and molded case circuit breaker branch boxes. Background Technology

[0002] During long-term operation, the contact resistance of low-voltage cable distribution boxes is prone to gradual increase due to factors such as loose bolts, oxidation of contact surfaces, loss of elasticity, aging of conductive components, and environmental humid heat corrosion. When the contact resistance increases, the temperature rise of local nodes usually occurs before obvious faults or power outages. Therefore, early identification of contact degradation is of great significance for the preventive maintenance of low-voltage power distribution equipment.

[0003] Existing monitoring solutions for low-voltage power distribution equipment mostly employ fixed temperature threshold alarms or single-point over-temperature judgment methods. While these methods are simple to implement, they only reflect whether the temperature has exceeded the limit at the current moment and are difficult to identify the formation process of contact degradation. Especially under conditions of high ambient temperature, short-term overload, or large load fluctuations, the normal load rise and abnormal contact heating show similarities in surface temperature. If judgment is based solely on absolute temperature, false alarms or missed alarms are likely to occur.

[0004] If common fault classification algorithms are directly applied to identify contact degradation in cable branch boxes, the following shortcomings usually still exist: On the one hand, different device structures, such as strip fuses, knife fuse switches, and molded case circuit breakers, have significant differences in thermal inertia, location of heating points, and sensitivity of contact surfaces. If the differences in device types are not considered, the stability of the diagnostic results is poor. On the other hand, if the thermal network reference state is lacking and classification is based solely on the original temperature sequence, it is difficult to explain "why the temperature rise belongs to contact degradation rather than load rise".

[0005] The applicant's previous work proposed a low-voltage cable branch box with critical node temperature prediction and early warning functions. This branch box focuses on predicting future hotspot temperatures using thermal network constraints and multi-source operating parameters. However, in actual operation and maintenance, in addition to "whether overheating is possible in the future," it is also necessary to directly answer "where is the current contact degradation located, to what extent, and what measures should be taken." Therefore, this invention, based on the previous critical node temperature prediction and early warning functions, further configures online contact degradation identification and grading assessment functions to output degradation location, degradation degree, and maintenance recommendations, thus complementing the previous temperature prediction function.

[0006] Therefore, there is an urgent need to provide a technical solution that can combine the internal thermal network relationship of low-voltage cable branch boxes, the component type parameter table, and operating condition information to identify and classify the contact degradation status online, so as to improve the accuracy, interpretability, and engineering applicability of contact anomaly identification. Summary of the Invention

[0007] To overcome the problems of existing contact anomaly identification methods, such as reliance on fixed thresholds, difficulty in distinguishing between overload temperature rise and contact degradation temperature rise, lack of device type adaptability, and inability to classify and assess the degree of degradation, this invention proposes a contact degradation identification and classification assessment method for low-voltage cable branch boxes. The branch box, through its built-in thermal network analysis and condition diagnosis program, not only establishes a parameterized thermal network model of the device type based on the branch box structure to obtain the reference thermal state of each key node, but also constructs a contact degradation index using the residual between the measured temperature and the reference temperature, the temperature rise rate, load response consistency, and the influence of humidity. Finally, the degradation index is mapped to the equivalent contact resistance and the classification assessment result to achieve contact anomaly location and degradation degree determination. To solve the above technical problems, the technical solution of this invention is as follows:

[0008] A method for identifying and grading contact degradation in low-voltage cable branch boxes, comprising the following steps:

[0009] S1. Identify the key monitoring nodes of the low-voltage cable branch box and collect multi-source operating parameters;

[0010] S2. Establish a reference thermal network model based on the device type;

[0011] S3. Calculate the thermal network reference temperature and abnormal residuals of critical nodes;

[0012] S4. Construct the contact degradation index and update the equivalent contact resistance;

[0013] S5. Perform degradation location and degradation level determination;

[0014] S6. Output contact degradation assessment results and maintenance recommendations.

[0015] Furthermore, the multi-source operating parameters include at least the main bus current, branch current, ambient temperature inside the enclosure, relative humidity inside the enclosure, enclosure door status, and temperature information of the main switch contacts, busbar connection points, branch device inlet terminals, and branch device outlet terminals, as well as the current branch device type.

[0016] Furthermore, for the three types of branch structures—strip fuse type, knife fuse type, and molded case circuit breaker type—the device type parameter table is called to establish a thermal network model consisting of environmental nodes, air nodes inside the box, main knife switch contact nodes, busbar connection point nodes, branch incoming terminal nodes, branch outgoing terminal nodes, and shell nodes. The corresponding node thermal capacity, inter-node thermal resistance, node-environment thermal resistance, and initial contact resistance are set.

[0017] Furthermore, the parameters of the reference thermal network model are determined as follows:

[0018] First, based on the branch box structural drawings, device mounting structure, conductor cross-section, and shell geometry, lumped parameter hot nodes are defined; for the i-th node, the heat capacity is calculated according to... Determined; among them, For the first The mass of the conductor or structural component corresponding to each node. This represents the specific heat capacity of the corresponding material; for adjacent nodes and The heat conduction path between adjacent nodes and The heat conduction path between nodes, and the thermal resistance between nodes according to Determined; among them, For nodes With nodes The equivalent heat transfer path length between them The equivalent thermal conductivity of the material along the heat transfer path. This is the equivalent heat transfer cross-sectional area; for the heat transfer path between the node and the environment, the environmental thermal resistance is calculated as follows: It is confirmed that, among them, For the first The overall heat transfer coefficient of each node to the air inside the box or the external environment. For the corresponding heat transfer area; the initial contact resistance for critical contact nodes. First, initial values ​​are given based on the rated current and structural heating mechanism. Then, temperature samples under normal operating conditions are used to determine the values. , and Perform joint calibration to generate a reference thermal network parameter table for the corresponding device type;

[0019] Device type codes are not directly used as arbitrary empirical amplification factors in the judgment, but are used as parameter table indexes. For the three types of devices, namely strip fuses, knife fuse switches and molded case circuit breakers, they correspond to different initial contact resistance ranges, node thermal capacities, thermal resistance paths and degradation threshold ranges.

[0020] Furthermore, a three-digit unique thermal code can be used to represent the device type, where the code for a strip fuse is [1,0,0], the code for a knife fuse is [0,1,0], and the code for a molded case circuit breaker is [0,0,1].

[0021] Furthermore, S3 obtains the reference temperature of each key node based on the reference thermal network model established in step S2, combined with the current, ambient temperature, humidity and door status at the current moment; then compares the corresponding measured temperature with the reference temperature to obtain the temperature residual of each key node, and extracts the temperature rise rate term and load response consistency term within the most recent preset time window.

[0022] No. Key nodes at time The temperature residual is calculated according to the following formula:

[0023] ,

[0024] in, For the first Key nodes at time The measured temperature The reference temperature output by the thermal network model;

[0025] The temperature rise rate term was extracted using a combination of the most recent 5-minute window and the most recent 15-minute window.

[0026] The load response consistency item Construct it according to the following formula:

[0027]

[0028] in, In order to be with the first The loop current corresponding to each key node. To prevent tiny positive numbers with a denominator of zero, This is the load response consistency threshold.

[0029] Furthermore, S4 constructs a contact degradation index based on the temperature residual, temperature rise rate, load response consistency term, and humidity correction term of each key node; and then updates the degradation state variable and equivalent contact resistance of the corresponding key node based on the contact degradation index.

[0030] The humidity correction item Determine as follows:

[0031] .

[0032] in, For a moment The relative humidity inside the box, Humidity threshold;

[0033] The weighting coefficients are determined either through offline calibration of historical samples or through optimization using the validation set.

[0034] Contact Deterioration State Variables Update according to the following formula:

[0035] .

[0036] in, For smoothing coefficients, The upper limit of the state variables;

[0037] The first The equivalent contact resistance of each key node is updated as follows:

[0038] .

[0039] No. Contact degradation index of key nodes Determine as follows:

[0040] ,

[0041] in, For the node temperature rise rate term, For humidity correction, , , and These are the weighting coefficients. and These are the residual threshold and the rate threshold, respectively.

[0042] Furthermore, in S5, the degradation state variables, equivalent contact resistance or contact degradation index corresponding to the main switch contact node, busbar connection node, branch incoming terminal node and branch outgoing terminal node are compared to determine the key node with the most significant degradation at present, and the normal, mild degradation, moderate degradation and severe degradation levels are output according to the preset classification rules.

[0043] Degradation level is based on degradation state variables Temperature residual The duration of the residual exceeding the threshold is determined jointly, and the time is defined as follows:

[0044]

[0045] in, To continuously satisfy The number of sampling points, The sampling period;

[0046] The grading determination shall be performed according to the following rules: when or At 1 minute, it is judged to be normal; when and At 1 minute, it was determined to be a slight degradation; when and At 1 minute, it was determined to be moderately degraded; when and At 1 minute, it was determined to be severely degraded.

[0047] Furthermore, S6 outputs the corresponding status assessment results based on the deteriorated nodes and deterioration levels obtained in step S5, and generates suggestions for re-inspection, tightening, key inspection, or power outage maintenance.

[0048] This invention has the following characteristics and beneficial effects:

[0049] This invention does not simply judge whether the measured temperature exceeds the limit. Instead, it first establishes a parameterized reference thermal network for the device type, then converts the residual between the measured temperature and the reference temperature into a physically meaningful contact degradation index, and continuously identifies the contact degradation state by updating the equivalent contact resistance online. Compared with traditional fixed temperature threshold methods, this invention can not only identify whether there is a contact anomaly, but also distinguish between normal high-load rise and abnormal temperature rise caused by poor contact, and further output the degradation location and degradation level, which facilitates operation and maintenance personnel to formulate targeted maintenance plans. Attached Figure Description

[0050] Figure 1 A flowchart for an online identification method for contact degradation in low-voltage cable branch boxes.

[0051] Figure 2 This is a schematic diagram of the reference thermal network structure for key nodes of a low-voltage cable branch box.

[0052] Figure 3 This is a comparison chart of the critical node temperatures and reference temperatures under normal operating conditions, short-term overload conditions, and poor contact conditions.

[0053] Figure 4 This is a schematic diagram showing the changes in contact degradation index and equivalent contact resistance over time.

[0054] Figure 5 This is a schematic diagram illustrating the classification and determination of different levels of contact degradation.

[0055] Figure 6 A comparison chart showing the difference between normal load temperature rise and temperature rise due to poor contact. Detailed Implementation

[0056] It should be noted that, unless otherwise specified, the embodiments and technical features described in this invention can be combined with each other.

[0057] 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 a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0058] like Figure 1 As shown, the present invention provides a method for identifying and classifying contact degradation in a low-voltage cable branch box, comprising the following steps:

[0059] S1. Identify the key monitoring nodes of the low-voltage cable branch box and collect multi-source operating parameters;

[0060] The multi-source operating parameters include at least the main bus current, branch current, ambient temperature inside the enclosure, relative humidity inside the enclosure, enclosure door status, and temperature information of the main switch contacts, busbar connection points, branch device inlet terminals, and branch device outlet terminals, while also identifying the current branch device type.

[0061] The sampling period for multi-source operating parameters is 1 minute;

[0062] S2. Establish a reference thermal network model based on the device type;

[0063] For three types of branch structures—strip fuse type, knife fuse type, and molded case circuit breaker type—the device type parameter table is called to establish a thermal network model consisting of environmental nodes, air nodes inside the box, main knife switch contact nodes, busbar connection point nodes, branch incoming terminal nodes, branch outgoing terminal nodes, and shell nodes. The corresponding node thermal capacity, inter-node thermal resistance, node-environment thermal resistance, and initial contact resistance are set.

[0064] The parameters of the reference thermal network model are determined as follows: First, lumped parameter thermal nodes are defined based on the branch box structure drawings, device mounting structure, conductor cross-section, and shell geometry; for the i-th node, the heat capacity is determined according to... Determined; among them, For the first The mass of the conductor or structural component corresponding to each node (unit: kg). This refers to the specific heat capacity of the corresponding material (unit: J / (kg·K)). The unit is J / K. For adjacent nodes and The heat conduction path between nodes, and the thermal resistance between nodes according to Determined; among them, For nodes With nodes Equivalent heat transfer path length between them (unit: m). It is the equivalent thermal conductivity of the material along the heat transfer path (unit: W / (m·K)). Equivalent heat transfer cross-sectional area (unit: m²). The unit is K / W. For the heat transfer path between the node and the environment, the ambient thermal resistance is calculated as follows: Confirmed. Among them, For the first The overall heat transfer coefficient of each node to the air inside the box or the external environment (unit: W / (m²·K)). For the corresponding heat exchange area (unit: m²). The unit is K / W. Initial contact resistance for critical contact nodes. First, initial values ​​are given based on the rated current and structural heating mechanism. Then, temperature samples under normal operating conditions are used to determine the values. , and Joint calibration is performed to generate a reference thermal network parameter table for the corresponding device type.

[0065] The device type code is not directly used as an arbitrary empirical amplification factor in the judgment, but is used as an index in the parameter table. For the three types of devices, namely strip fuses, knife fuses, and molded case circuit breakers, there are different initial contact resistance ranges, node thermal capacities, thermal resistance paths, and degradation threshold ranges. More preferably, a three-digit unique thermal code can be used to represent the device type, where the code for strip fuses is [1,0,0], the code for knife fuses is [0,1,0], and the code for molded case circuit breakers is [0,0,1].

[0066] S3. Calculate the thermal network reference temperature and abnormal residuals of critical nodes;

[0067] Based on the reference thermal network model established in step S2, and combined with the current, ambient temperature, humidity and door status at the current moment, the reference temperature of each key node is obtained; then the corresponding measured temperature is compared with the reference temperature to obtain the temperature residual of each key node, and the temperature rise rate term and load response consistency term within the most recent preset time window are extracted.

[0068] No. Key nodes at time The temperature residual is calculated according to the following formula:

[0069] ,

[0070] in, For the first Key nodes at time The measured temperature This is the reference temperature output by the thermal network model.

[0071] The temperature rise rate term was extracted using a combination of the most recent 5-minute window and the most recent 15-minute window.

[0072] The load response consistency item Construct it according to the following formula:

[0073]

[0074] in, In order to be with the first The loop current corresponding to each key node. To prevent tiny positive numbers with a denominator of zero, This is the load response consistency threshold. Therefore, when the node temperature rise cannot be fully explained by the square term of the loop current, it is considered that there is an abnormal contact trend of heating beyond the normal load.

[0075] S4. Construct the contact degradation index and update the equivalent contact resistance;

[0076] Based on the temperature residual, temperature rise rate, load response consistency term, and humidity correction term of each key node, a contact degradation index is constructed; then, the degradation state variables and equivalent contact resistance of the corresponding key node are updated based on the contact degradation index.

[0077] The humidity correction item It can be determined as follows:

[0078] .

[0079] in, For a moment The relative humidity inside the box, This is the humidity threshold. In this embodiment, The humidity can be preferably set to 72%. Therefore, when the relative humidity inside the chamber does not exceed the humidity threshold, the humidity correction term is 0; when the relative humidity inside the chamber rises to above the humidity threshold, the humidity correction term increases with the increase of humidity.

[0080] The weighting coefficients can be determined offline based on historical samples or through optimization using a validation set. In this embodiment, the nodes at the branch inlet and branch outlet can be preferably selected. , , , For the main switch contacts and busbar connection points, it can maintain... , and The dominant role remains unchanged, and the humidity weighting The value was lowered to 0.02 to reflect the differences in the sensitivity of different nodes to hot and humid environments.

[0081] Furthermore, contact the degraded state variables Update according to the following formula:

[0082] .

[0083] in, For smoothing coefficients, This represents the upper limit of the state variables.

[0084] Furthermore, the first The equivalent contact resistance of each key node is updated as follows:

[0085] .

[0086] No. Contact degradation index of key nodes Determine as follows:

[0087] .

[0088] in, For the node temperature rise rate term, For humidity correction, , , and These are the weighting coefficients. and These are the residual threshold and the rate threshold, respectively.

[0089] S5. Perform degradation location and degradation level determination;

[0090] By comparing the degradation state variables, equivalent contact resistance, or contact degradation index corresponding to the main switch contact node, busbar connection node, branch incoming terminal node, and branch outgoing terminal node, the critical node with the most significant degradation is determined, and the normal, mild degradation, moderate degradation, and severe degradation levels are output according to the preset classification rules.

[0091] The degradation level can be based on degradation state variables. Temperature residual The residual duration is determined jointly. The duration of the residual exceeding the threshold is defined as:

[0092]

[0093] in, To continuously satisfy The number of sampling points, The sampling period is denoted as . In this embodiment, The preferred time is 1 minute.

[0094] Furthermore, in this embodiment, the hierarchical determination can be performed according to the following rules: when or At 1 minute, it is judged to be normal; when and At 1 minute, it was determined to be a slight degradation; when and At 1 minute, it was determined to be moderately degraded; when and At a certain time, it is determined to be severely degraded. Therefore, the amplitude information of the degraded state variable and the abnormal duration information can be combined for grade determination.

[0095] S6. Output contact degradation assessment results and maintenance recommendations;

[0096] Based on the deteriorated nodes and deterioration levels obtained in step S5, the corresponding status assessment results are output, and suggestions for re-inspection, tightening, key inspection, or power outage maintenance are generated.

[0097] The following description is based on specific embodiments.

[0098] Step 1: Determine the target audience and set basic parameters

[0099] The DFW low-voltage cable branch box was selected as the implementation object. The rated current of the main busbar of the branch box is 630A, the rated current of the branch circuit is 250A, the rated operating voltage is 400V, the insulation voltage is 690V, the frequency is 50Hz, the enclosure is a 304 stainless steel outdoor enclosure, and the protection level is IP44. The branch circuit protection structure can be a strip fuse, a knife fuse switch, or a molded case circuit breaker.

[0100] To improve the interpretability of contact degradation diagnosis results under different internal structures, a device type parameter table is established in this embodiment. Strip fuses, knife fuses, and molded case circuit breakers correspond to different initial contact resistance ranges, node thermal capacities, thermal resistance paths, and degradation grading thresholds, respectively. The device type adopts a unique thermal encoding input diagnostic model, where strip fuses are encoded as [1,0,0], knife fuses are encoded as [0,1,0], and molded case circuit breakers are encoded as [0,0,1].

[0101] Step 2: Establish a reference heat network model

[0102] like Figure 2 As shown, the low-voltage cable branch box is abstracted into environmental nodes, internal air nodes, main switch contact nodes, busbar connection point nodes, branch line inlet nodes, branch line outlet nodes, and casing nodes. Each node is connected by thermal resistance and a corresponding heat capacity is set to characterize the inertia of node temperature changes. The reference heating power of each key node is positively correlated with the square of the corresponding circuit current.

[0103] In this embodiment, the reference thermal network model is initialized using a device type parameter table. For the initial contact resistance of critical contact nodes, an initial value is first given based on the rated operating conditions and structural heating characteristics. Then, the node thermal resistance, ambient thermal resistance, and initial contact resistance are jointly corrected using normal operating condition samples to ensure that the node temperature output by the reference thermal network under normal operating conditions is consistent with the actual node temperature change trend.

[0104] Step 3: Constructing the Anomalous Residuals and Load Response Consistency Terms

[0105] In this embodiment, the reference temperature of each key node is first output by the reference thermal network model in step two based on the current main bus current, branch current, ambient temperature, humidity and box door status. Then, it is compared with the corresponding measured temperature to obtain the temperature residual of the main switch contact, busbar connection point, branch inlet and branch outlet.

[0106] Simultaneously, the temperature rise rate terms for each critical node are calculated within the most recent 5-minute and 15-minute windows, and a load response consistency term is further constructed. Since the normal load temperature rise can usually be explained by the loop current square term, while the additional heat generation caused by contact degradation does not change linearly with the load explanation term, when the residual of a node continues to increase under relatively small current changes, it can be considered that the node has a contact anomaly trend.

[0107] Step 4: Calculate the contact degradation index and update the equivalent contact resistance.

[0108] In this embodiment, the temperature residual, temperature rise rate, load response consistency term, and humidity correction term are combined into a contact degradation index. Therefore, the contact degradation index is no longer an abstract empirical quantity, but a state quantity that reflects the node's additional heat intensity, rate of change, and environmental impact.

[0109] In this embodiment, the humidity correction term can be adopted. The structure is constructed in the form of , where RH(k) is the percentage of relative humidity inside the box. For the branch inlet and outlet nodes, the preferred selection can be... , , , For the main switch contacts and busbar connection points, the humidity weight can be appropriately reduced to reflect the difference in humidity and heat sensitivity between the main circuit nodes and the branch circuit contact nodes.

[0110] Subsequently, the contact degradation state variable is updated online based on the contact degradation index, and the equivalent contact resistance is further updated. For the same node, when the residual continues to increase, the temperature rise rate continues to be large, or the load response consistency term continues to be abnormal, the corresponding contact degradation state variable gradually increases, and the equivalent contact resistance increases synchronously.

[0111] Step 5: Perform degradation location and grading assessment

[0112] In this embodiment, the contact degradation state variables of the main switch contact node, busbar connection node, branch incoming terminal node, and branch outgoing terminal node are compared, and the node with the largest degradation state variable is taken as the most likely abnormal node. Simultaneously, based on the magnitude of the degradation state variable, the duration of the temperature residual exceeding the threshold, and whether a continuous upward trend occurs, the abnormal state is classified into mild degradation, moderate degradation, and severe degradation. Specifically, the node that continuously meets the... The number of sampling points is converted into duration; when the duration is less than 3 minutes or When, maintain the normal level; when And if the duration is no less than 3 minutes, it is judged as mild degradation; when And if the duration is not less than 10 minutes, it is judged as moderate degradation; when If the duration is no less than 15 minutes, it is considered a severe degradation.

[0113] For nodes with minor degradation, recommendations for re-inspection and focused patrols can be provided; for nodes with moderate degradation, recommendations for tightening and on-site repairs can be provided; for nodes with severe degradation, recommendations for immediate power outage repairs or component replacement can be provided. Thus, the diagnostic results can directly support operational and maintenance decisions.

[0114] Step Six: Verify the Scheme Design

[0115] Since no physical hardware testing has been conducted at this stage of this embodiment, a system-level digital prototype was built using a Python environment to perform timing simulation verification under normal fluctuation conditions, short-term overload conditions, and poor contact conditions. During verification, the equivalent contact resistance at the branch input or output terminal can be gradually increased under poor contact conditions, and the changes in the contact degradation index, the estimated equivalent contact resistance, and the degradation level output can be observed. The verification results are only used to illustrate the feasibility and relative improvement trend of the technical solution of this invention under the set parameters and operating conditions, and should not be directly equated with the absolute performance under all field operating conditions.

[0116] Preferably, during the verification process, the method of the present invention is compared with a fixed temperature threshold judgment method. If the degradation index output by the method of the present invention remains at a low level under normal high load and short-term overload conditions, but continues to rise and triggers a higher level of degradation assessment under poor contact conditions, it indicates that the method of the present invention can effectively distinguish between normal load temperature rise and additional heat generation caused by poor contact.

[0117] Simulation results show that, within the entire time-series sample range set in this embodiment, the overall hierarchical identification accuracy of the method of the present invention is approximately 95.69%, the macro-average F1 value is approximately 0.6453, and the fault node localization accuracy is approximately 98.92%. In contrast, the fixed temperature threshold method has an overall accuracy of approximately 74.73%, a macro-average F1 value of approximately 0.3251, and a fault node localization accuracy of approximately 93.76%. These results demonstrate that, under the simulation conditions of this embodiment, the method of the present invention outperforms the method based solely on temperature thresholds in both contact degradation hierarchical identification and fault node localization.

[0118] Furthermore, under poor contact conditions, the identification accuracy rates for strip fuse structures, knife fuse switch structures, and molded case circuit breaker structures are approximately 97.43%, 98.26%, and 98.26%, respectively, with an average of approximately 97.99%. Under short-term overload conditions, the average identification accuracy rate for the three types of device structures is approximately 85.79%. These results demonstrate that the method of the present invention can not only reliably identify obvious contact degradation states, but also has good distinguishing ability for short-term overload temperature rise scenarios that are easily confused with contact abnormalities.

[0119] like Figure 3 As shown, the deviation between the measured temperature and the reference temperature of key nodes under normal operating conditions and short-term overload conditions is relatively small; for example... Figure 4 As shown, under poor contact conditions, the contact degradation index and the estimated equivalent contact resistance gradually increase over time; as Figure 5 As shown, multi-level degradation can be classified based on degradation state variables and residual duration; for example... Figure 6 As shown, the method of the present invention can distinguish between normal load heating and poor contact heating.

[0120] In summary, the method described in this invention does not rely solely on a single temperature threshold for fault determination. Instead, it forms a closed-loop diagnostic process through a device type parameterized reference thermal network, abnormal residual construction, contact degradation index update, and a graded evaluation mechanism. This approach can more effectively reflect the degradation evolution of key contacts in low-voltage cable branch boxes.

[0121] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for identifying and grading contact deterioration in low-voltage cable branch boxes, characterized in that, Includes the following steps: S1. Identify the key monitoring nodes of the low-voltage cable branch box and collect multi-source operating parameters; S2. Establish a reference thermal network model based on the device type; S3. Calculate the thermal network reference temperature and abnormal residuals of critical nodes; S4. Construct the contact degradation index and update the equivalent contact resistance; S5. Perform degradation location and degradation level determination; S6. Output contact degradation assessment results and maintenance recommendations.

2. The method for identifying and grading contact deterioration in low-voltage cable branch boxes according to claim 1, characterized in that, In S1, the multi-source operating parameters include at least the main bus current, branch current, ambient temperature inside the enclosure, relative humidity inside the enclosure, enclosure door status, and temperature information of the main switch contacts, busbar connection points, branch device inlet terminals, and branch device outlet terminals, as well as the current branch device type.

3. The method for identifying and grading contact deterioration in low-voltage cable branch boxes according to claim 1, characterized in that, In S2, for three types of branch structures—strip fuse type, knife fuse type, and molded case circuit breaker type—the device type parameter table is called to establish a thermal network model consisting of environmental nodes, air nodes inside the box, main knife switch contact nodes, busbar connection point nodes, branch incoming terminal nodes, branch outgoing terminal nodes, and shell nodes. The corresponding node thermal capacity, inter-node thermal resistance, node-environment thermal resistance, and initial contact resistance are set.

4. The method for identifying and grading contact deterioration in low-voltage cable branch boxes according to claim 1, characterized in that, In S2, the parameters of the reference thermal network model are determined as follows: First, based on the branch box structural drawings, device mounting structure, conductor cross-section, and shell geometry, lumped parameter hot nodes are defined; for the i-th node, the heat capacity is calculated according to... Determined; among them, For the first The mass of the conductor or structural component corresponding to each node. This represents the specific heat capacity of the corresponding material; for adjacent nodes and The heat conduction path between nodes, and the thermal resistance between nodes according to Determined; among them, For nodes With nodes The equivalent heat transfer path length between them The equivalent thermal conductivity of the material along the heat transfer path. This is the equivalent heat transfer cross-sectional area; for the heat transfer path between the node and the environment, the environmental thermal resistance is calculated as follows: It is confirmed that, among them, For the first The overall heat transfer coefficient of each node to the air inside the box or the external environment. For the corresponding heat transfer area; the initial contact resistance for critical contact nodes. First, initial values ​​are given based on the rated current and structural heating mechanism. Then, temperature samples under normal operating conditions are used to determine the values. , and Perform joint calibration to generate a reference thermal network parameter table for the corresponding device type; Device type codes are not directly used as arbitrary empirical amplification factors in the judgment, but are used as parameter table indexes. For the three types of devices, namely strip fuses, knife fuse switches and molded case circuit breakers, they correspond to different initial contact resistance ranges, node thermal capacities, thermal resistance paths and degradation threshold ranges.

5. The method for identifying and grading contact deterioration in low-voltage cable branch boxes according to claim 1, characterized in that, In S2, a three-digit unique thermal code can be used to represent the device type, where the code for a strip fuse is [1,0,0], the code for a knife fuse is [0,1,0], and the code for a molded case circuit breaker is [0,0,1].

6. The method for identifying and grading contact deterioration in low-voltage cable branch boxes according to claim 1, characterized in that, S3, based on the reference thermal network model established in step S2, and combined with the current, ambient temperature, humidity, and door status at the current moment, obtains the reference temperature of each key node. Then, the corresponding measured temperature is compared with the reference temperature to obtain the temperature residual of each key node, and the temperature rise rate and load response consistency items within the most recent preset time window are extracted. No. Key nodes at time The temperature residual is calculated using the following formula: ， in, For the first Key nodes at time The measured temperature The reference temperature output by the thermal network model; The temperature rise rate term was extracted using a combination of the most recent 5-minute window and the most recent 15-minute window. The load response consistency item Construct according to the following formula: in, In order to be with the first The loop current corresponding to each key node. To prevent tiny positive numbers with a denominator of zero, This is the load response consistency threshold.

7. The method for identifying and grading contact deterioration in low-voltage cable branch boxes according to claim 1, characterized in that, S4 constructs a contact degradation index based on the temperature residual, temperature rise rate, load response consistency term, and humidity correction term of each key node; then updates the degradation state variable and equivalent contact resistance of the corresponding key node based on the contact degradation index. The humidity correction item Determine as follows: 。 in, For a moment The relative humidity inside the box, Humidity threshold; The weighting coefficients are determined either through offline calibration of historical samples or through optimization using the validation set. Contact Deterioration State Variables Update according to the following formula: 。 in, For smoothing coefficients, The upper limit of the state variables; The first The equivalent contact resistance of each key node is updated as follows: 。 No. Contact degradation index of key nodes Determine as follows: ， in, For the node temperature rise rate term, For humidity correction, , , and These are the weighting coefficients. and These are the residual threshold and the rate threshold, respectively.

8. The method for identifying and grading contact deterioration in low-voltage cable branch boxes according to claim 1, characterized in that, In S5, the degradation state variables, equivalent contact resistance or contact degradation index corresponding to the main switch contact node, busbar connection node, branch incoming terminal node and branch outgoing terminal node are compared to determine the key node with the most significant degradation at present, and the normal, mild degradation, moderate degradation and severe degradation levels are output according to the preset classification rules. Degradation level is based on degradation state variables Temperature residual The duration of the residual exceeding the threshold is determined jointly, and the time is defined as follows: in, To continuously satisfy The number of sampling points, The sampling period; The grading determination shall be performed according to the following rules: when or At 1 minute, it is judged to be normal; when and At 1 minute, it was determined to be a slight degradation; when and At 1 minute, it was determined to be moderately degraded; when and At 1 minute, it was determined to be severely degraded.

9. The method for identifying and grading contact deterioration in low-voltage cable branch boxes according to claim 1, characterized in that, S6 outputs the corresponding status assessment results based on the deteriorated nodes and deterioration levels obtained in step S5, and generates suggestions for re-inspection, tightening, key inspection, or power outage maintenance.

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