Fault early warning method based on operation state of high-voltage cable

Abstract

The invention relates to the technical field of power fault early warning, in particular to a fault early warning method based on the operation state of a high-voltage cable, which comprises the following steps of: performing polarity abrupt change capture on a difference waveform to obtain a polarity abrupt change moment and an abrupt change direction which are only caused by inter-segment insulation reduction; slope symbol overturning detection is carried out on the temperature one-dimensional distribution of the temperature of the protection layer in the time window along the axial direction of the cable, and a unique overturning point position at which the temperature distribution slope is changed from positive to negative or from negative to positive is obtained; and collecting the transient increment of the thickness of an oxide layer formed on the surface of the copper braided wire due to early arc volatilization, and when the transient increment of the thickness of the oxide layer exceeds a local energy density threshold value obtained by converting the position of the overturning point, determining that an internal insulation defect exists, and sending a fault early warning signal. According to the method, the thickness of the microscopic oxide layer on the surface of the copper braided wire is collected in real time, the transient increment of the thickness of the oxide layer is quantified, and early warning triggering is carried out in combination with the stability of multiple sampling states.

Description

Technical Field

[0001] This invention relates to the field of power fault early warning technology, and in particular to a fault early warning method based on the operating status of high-voltage cables. Background Technology

[0002] High-voltage cables, as a critical component of power transmission systems, are directly related to the stability of the power grid and the continuity of power supply. However, due to factors such as insulation aging, water treeing, and structural stress concentration during long-term operation, high-voltage cables are prone to insulation degradation in localized areas, leading to serious faults such as breakdown and short circuits. Traditional fault identification technologies mostly rely on power frequency overcurrent protection devices or cable sheath circulating current monitoring. However, these technologies are often only triggered after a substantial breakdown or metal-to-metal contact has occurred, exhibiting significant lag and failing to meet the need for early intervention based on "fault precursors."

[0003] In recent years, some studies have attempted to introduce cable sheath temperature detection or partial discharge monitoring methods for early warning, but these still face many challenges in practical applications. On the one hand, partial discharge signals are severely affected by electromagnetic interference in the field, making stable extraction difficult. On the other hand, the sheath temperature changes slowly, often making it difficult to distinguish between normal load fluctuations and early fault heat sources. Furthermore, existing technologies mostly use fixed thresholds for early warning judgments, ignoring the influence of different cable structures, path directions, and arc heat transfer processes on energy deposition effects, resulting in a high false positive rate and a lack of specificity and interpretability. Summary of the Invention

[0004] This invention provides a fault early warning method based on the operating status of high-voltage cables. It integrates multi-dimensional sensing information during cable operation and has a novel early warning method with refined spatial positioning and material response analysis capabilities. It can not only identify the precursor characteristics of faults in time before they spread to system-level damage, but also dynamically construct a judgment threshold with physical basis by combining the actual structure and energy propagation path, thereby improving the accuracy and adaptability of the early warning system.

[0005] A fault early warning method based on the operating status of high-voltage cables includes the following steps: S1. During the operation of the cable line under energization, the instantaneous difference of the sheath current between two adjacent coaxial cable sections in the cross-interconnection grounding circuit of the metal sheath is sampled. The difference waveform is obtained by using the through-core current transformer in the cross-interconnection box for short-circuit protection. The polarity change is captured by the difference waveform to obtain the time and direction of the polarity change caused only by the decrease in inter-section insulation. S2, based on the abrupt change time and direction of the abrupt change output by S1, on the continuous temperature measuring cable already laid on the surface of the metal sheath within the same cross-interconnection segment, with the abrupt change time as the zero point, a fixed length of time window is intercepted backward, and the slope sign reversal detection is performed on the one-dimensional temperature distribution of the sheath temperature along the cable axis within the time window to obtain the unique reversal point position where the temperature distribution slope changes from positive to negative or from negative to positive. S3 maps the flip point position output by S2 to the copper busbar of the corresponding phase in the cross-connect box. On the exposed copper braided wire that was originally used for voltage equalization between the copper busbar and the box wall, the transient increase in the oxide layer thickness formed by early arc volatilization on the surface of the copper braided wire is collected. When the transient increase in oxide layer thickness exceeds the local energy density threshold calculated from the flip point position, it is determined to be an internal insulation defect and a fault early warning signal is issued.

[0006] Optionally, during the energized operation of the cable line, synchronous sampling technology is used to simultaneously collect the current in the metal sheath grounding leads of two adjacent coaxial cable sections in the cross-interconnection grounding circuit; inside the cross-interconnection box, the secondary outputs of two through-core current transformers originally used for short-circuit protection, which are respectively installed on the grounding leads on both sides, are connected to the input terminal of a differential amplifier circuit, and the output of the differential amplifier circuit is the instantaneous difference waveform of the sheath current.

[0007] Optionally, the polarity change capture includes monitoring and analyzing the difference waveform, capturing occasional pulse signals above the steady-state power frequency induced current difference baseline by setting a dynamic threshold, recording the moment when the pulse first exceeds the dynamic threshold as the polarity change moment, and determining the change direction according to the positive or negative polarity of the pulse.

[0008] Optionally, S1 further includes filtering out pulses whose duration exceeds a preset upper limit caused by external electromagnetic interference or lightning strikes, to ensure that the timing and direction of the sudden change are caused only by the decrease in inter-segment insulation.

[0009] Optionally, S2 receives the abrupt change time from S1, and uses the abrupt change time as the zero point of time to synchronously trigger the reading of the continuous temperature measuring cable laid on the surface of the metal sheath of the same cross-interconnection section; from the zero point of time, a fixed-length time window is extracted, the fixed length being determined based on the thermal conduction time constant of the sheath material and the early thermal diffusion characteristics of the fault arc; within the time window, at a preset sampling interval, the temperature values ​​of a series of measuring points along the cable axis are read from the distributed temperature sensing unit of the continuous temperature measuring cable to form a one-dimensional spatial distribution sequence of the sheath surface temperature along the axial direction.

[0010] Optionally, S2 further includes slope calculation and reversal point detection, specifically including performing data smoothing preprocessing on the one-dimensional spatial distribution sequence, calculating the temperature change slope between adjacent measuring points point by point along the axial direction; performing sign analysis on the calculated slope sequence to identify the unique reversal point where the slope sign changes from positive to negative or from negative to positive, and taking the axial position of the cable corresponding to the unique reversal point as the reversal point position; wherein, the determination of sign reversal must meet the preset spatial continuity condition to exclude false reversals caused by measurement noise.

[0011] Optionally, the preset spatial continuity condition specifically includes: Centered on the reversal point, several adjacent measuring points are selected in both directions before and after the cable axis to form a local continuous spatial region. Within this region, it is determined whether at least one measuring point exhibits a temperature slope change trend consistent with the trend of the reversal point after the reversal. Only when a measuring point with the same slope change trend as the reversal point is confirmed within the local continuous spatial region is the reversal considered spatially continuous, and the reversal point is confirmed as a valid temperature slope reversal point. If no measuring point with a consistent trend is found in the adjacent regions before and after the reversal point, i.e., the slope change occurs only at a single measuring point, the reversal is determined to be a pseudo-reversal caused by measurement noise, localized sporadic disturbances, or data jitter, and is discarded, not used as a basis for subsequent fault location and energy determination. Optionally, S3 specifically includes determining the corresponding cross-connect box and the target copper busbar inside the box by using a mapping model based on the location of the reversal point and the relationship between the cable laying path and the installation location of the cross-connect box; and calculating the local energy density threshold based on the arc propagation path length from the reversal point location to the corresponding target cross-connect box, the diffusion equivalent deposition area, and the initial fault energy polarity contained in the direction of the sudden change.

[0012] Optionally, the acquisition of the transient increase in oxide layer thickness includes installing a non-contact laser rangefinder on the exposed copper braided wire between the target copper busbar and the box wall, aiming at the typical arc-affected area on the surface of the copper braided wire; continuously acquiring microscopic morphology data of the surface of the copper braided wire within a preset monitoring period after capturing the abrupt change moment; and calculating the transient increase in oxide layer thickness caused by early arc metal volatile deposition and oxidation by comparing the characteristic differences of the data acquired before and after the abrupt change moment.

[0013] Optionally, the transient increment of oxide layer thickness collected and calculated is compared with the local energy density threshold obtained by conversion; when the transient increment of oxide layer thickness exceeds the local energy density threshold for the first time, and this state is maintained in several consecutive samplings, it is determined that the internal insulation defect has developed to the warning stage, and the fault early warning signal is immediately triggered and issued.

[0014] The beneficial effects of this invention are: This invention, by acquiring the instantaneous difference waveform of the sheath current in high-voltage cables and combining it with the sign reversal of the temperature slope and changes in the oxide layer of the copper braided wire, achieves full-link perception from electrical signal mutations and thermal diffusion response to material morphology evolution, constructing a fault precursor identification mechanism spanning three physical levels: electrical, thermal, and deformation. This mechanism can identify insulation anomalies that have not yet evolved into breakdown before traditional power frequency short circuit or overcurrent protection operates, improving the early detection capability of cable operation risks and the system's safety redundancy.

[0015] This invention proposes incorporating early arc heat, path direction, energy deposition efficiency, and heat diffusion area into a unified calculation framework. A modified expression is used to dynamically estimate the local energy density threshold for each event through physical modeling. By mapping this energy threshold to an oxide layer thickness standard, the threshold setting is transformed from an empirically determined value to a structure-response-driven approach, enhancing the system's adaptability and versatility under different structural configurations and environmental conditions.

[0016] This invention collects real-time data on the thickness of the micro-oxide layer on the surface of copper braided wire, quantifies the transient increment of the oxide layer thickness, and triggers early warning by combining the stability of multiple sampling states. This effectively reduces the risk of misjudgment caused by sporadic interference at a single point. This method provides clear physical response evidence for early warning of insulation degradation. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only for this invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a schematic diagram of the early warning method according to an embodiment of the present invention; Figure 2 This is a schematic diagram illustrating the acquisition of the flip point position according to an embodiment of the present invention. Detailed Implementation

[0019] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. For some well-known technologies, those skilled in the art may also use other alternative methods to implement the invention. Moreover, the accompanying drawings are only for more specific description of the embodiments and are not intended to specifically limit the present invention.

[0020] This invention proposes a method to detect signs of failure before high-voltage cables are completely damaged. It primarily detects early abnormal signals by monitoring subtle changes already present in the cable, providing early warning without requiring damage, power outages, or significant deployment costs.

[0021] When a cable operates for an extended period or is exposed to moisture, its internal insulation may gradually deteriorate. During this process, the sheath current between adjacent cable segments may experience a sudden change in direction. This process is analogous to a doctor listening to a heartbeat with a stethoscope. We use the existing current sensors in the cable cross-connection system to monitor the current fluctuations in real time. Once a sudden change in current direction is detected, it indicates a potential problem, and the moment and direction of this anomaly are recorded.

[0022] Next, we examine the temperature distribution map of the cable surface where the current anomaly occurred. Starting from the moment the current jump, we look back over a short period of time. If, during this period, the temperature trend of the cable suddenly changes from getting hotter to suddenly getting colder, or vice versa, it indicates that a certain area of ​​the cable may be in a transitional state from healthy to dangerous. We should accurately mark this location.

[0023] Finally, we check near the copper wires in the cable junction box for any signs of slight burning. Specifically, we look to see if the oxide layer on the surface of the copper wires has suddenly thickened. When an abnormal arc occurs inside the cable, it releases some heat and chemicals, which can cause the copper surface to turn black and thicken. If this change exceeds the range that the abnormal temperature location we identified earlier can explain, we can be fairly certain that this is a genuine case of internal insulation degradation in the cable, and issue an early warning in time.

[0024] In summary: Instead of waiting for a cable to fail before issuing an alarm, the system detects early signs of a fault by observing changes in current, temperature trends, and minute signs of oxidation, thus providing an early warning before the cable is completely damaged.

[0025] like Figures 1-2 As shown, a fault early warning method based on the operating status of high-voltage cables includes the following steps: S1. During the operation of the cable line under energization, the instantaneous difference of the sheath current between two adjacent coaxial cable sections in the cross-interconnection grounding circuit of the metal sheath is sampled. The difference waveform is obtained by using the through-core current transformer in the cross-interconnection box for short-circuit protection. The polarity change is captured by the difference waveform to obtain the time and direction of the polarity change caused only by the decrease in inter-segment insulation.

[0026] S11, Sampling and Differential Waveform Acquisition: In a cross-interconnection grounding structure, the metallic sheaths of two adjacent cable segments are typically connected to the cross-interconnection box via grounding down conductors. Under normal circumstances, these two cable segments have similar power frequency induction currents. However, when local insulation deteriorates, micro-disturbances are introduced into the induced current of their sheaths. Based on this, this invention employs a synchronous sampling method, simultaneously sampling the current in the grounding down conductors of these two sheath segments. This ensures that the collected data has strict time alignment characteristics, thereby making the subsequently calculated difference reliable and real-time, avoiding misjudgments caused by asynchrony.

[0027] Two grounding down conductors each have a through-core current transformer (CT) running through them. Their function is to sense and output the AC current signal in the corresponding conductor. This signal is reflected as a voltage on the secondary side (output) of the transformer. Connecting the outputs of these two transformers to a differential amplifier circuit allows for real-time calculation of the instantaneous difference in sheath current on both sides, outputting a dynamically changing voltage signal, the so-called difference waveform. This waveform contains the pulse signal characteristics caused by insulation degradation. The core task of the differential amplifier is to retain the difference between the two signals, suppress common noise components, and provide a stable, linearly amplified output for subsequent abrupt change detection.

[0028] Specifically, during the energized operation of cable lines, we use synchronous sampling technology to monitor the current in the metal sheath grounding leads of adjacent coaxial cable sections in a cross-connected grounding loop. and Simultaneous data acquisition is performed; the secondary outputs of the two through-core current transformers, which are respectively connected to the grounding leads on both sides, are collected. , Waveform of instantaneous difference in sheath current output from differential amplifier circuit : ;in, This represents the waveform of the instantaneous difference in sheath current. This represents the secondary voltage signal of the through-core current transformer. G represents the gain constant of the differential amplifier, a fixed proportional coefficient used to amplify the difference portion of the input signal. G should not be too large, otherwise it may cause the amplifier output to saturate and lose its linear response capability when the amplitude of the sudden signal is high. At the same time, it should not be too small, otherwise weak pulse signals will not be distinguishable from background noise. The gain of the differential amplifier is usually set between 10 and 100 V / V, that is, the output voltage is 10 to 100 times the input voltage difference. In practical applications, we can first collect a difference waveform during a typical operating period, select the maximum gain that is not distorted and can distinguish small disturbances, and use a variable gain amplifier to dynamically adjust G to adapt to the amplitude changes of the signal under different operating conditions. Initially, G can be set to 20, and then fine-tuned according to the signal amplification degree during subsequent debugging.

[0029] A through-core current transformer is a sensing device that uses the principle of electromagnetic induction to convert a large primary current into a small secondary current (or voltage) output. It is used for indirect measurement of high voltage or large current in power systems. The primary side refers to the conductor passing through the transformer core; in this design, this is the grounding lead of the metal sheath. The secondary side refers to the output terminal of the induction coil inside the transformer. Through this coil, the current transformer (CT) induces a voltage signal or a small current signal proportional to the primary current. In this invention, the secondary output of the transformer is a voltage signal representing the change in sheath current, as described above. .

[0030] S12, Polarity Abruptness Capture: After acquiring the difference waveform, we need to identify the polarity abruptness signal caused by the inter-segment insulation degradation. This signal is characterized by its extremely short duration, significant amplitude jump above the baseline fluctuation, and occasional occurrence only during insulation performance abrupt changes. It is easily overwhelmed by other interferences in the field. Therefore, a dynamic threshold mechanism is introduced to ensure that the abruptness signal can be reliably captured while excluding non-target interference sources.

[0031] Regarding the Real-time monitoring and signal analysis are performed by introducing dynamic thresholds. The dynamic threshold is calculated in real time based on the trend of the difference waveform over a recent period and is used to determine polarity abrupt change events. The dynamic threshold is defined as follows: ;in, It is the moving average of the waveform over a recent period, i.e., the steady-state reference of the background current, with a time length of approximately 200 ms. It represents the sliding standard deviation within the corresponding window, used to measure the degree of fluctuation of the waveform within that interval. This is the sensitivity coefficient, an empirically set value, with a range of 2-5. In actual operation, the difference waveform may experience short-term jumps due to various factors, such as transient disturbances caused by lightning strikes, instantaneous startup of large inductive loads, and switching of high-voltage equipment. Although these disturbances also have high signal amplitudes, their duration is usually longer than that of transient penetrating disturbances caused by insulation degradation. Therefore, a time constraint mechanism is introduced, namely, an upper limit on the duration, which is set when the following conditions are met. That is, a polarity mutation is considered to have occurred, and this moment is recorded as the polarity mutation moment. , and according to The sign of the mutation determines the direction of the mutation: like , then it is a positive mutation; like Then it is a negative mutation.

[0032] In addition, set a duration limit. Power frequency disturbances and lightning interference often last longer than 30 ms, while penetrating current disturbances caused by insulation degradation are generally concentrated between 3 and 10 ms. Setting an upper limit helps filter out slow-changing irrelevant interferences and focuses on the characteristics of extremely short arc mutations. Therefore, a value of 5–15 ms is recommended, with an initial value of 10 ms. Duration Upper Limit Used to filter out non-insulation fault pulse interference caused by lightning strikes or large electromagnetic disturbances, ensuring the final recording... Both the direction of the abrupt change and the direction of the change are caused by a decrease in inter-segment insulation.

[0033] S2, based on the abrupt change time and direction output by S1, on the continuous temperature measuring cable already laid on the surface of the metal sheath within the same cross-interconnection segment, with the abrupt change time as the zero point, a fixed-length time window is intercepted backward, and the slope sign reversal detection is performed on the one-dimensional temperature distribution of the sheath along the cable axis within the time window to obtain the unique reversal point position where the temperature distribution slope changes from positive to negative or from negative to positive.

[0034] S21, Data Reception and Time Window Extraction: The goal is to accurately extract a representative temperature response time window using the detected abrupt change as an anchor point, thereby determining whether the abrupt change is accompanied by actual heat diffusion, and further, whether it is an early internal insulation problem in the cable. To ensure that the temperature acquisition has temporal causality and physical responsiveness, a method is proposed to extract a fixed-length time window starting from the abrupt change point. The length of this time window is not arbitrarily set, but is calculated based on the actual thermal response duration during the process.

[0035] Specifically, S21 receives the mutation time signal output from the polarity mutation capture module described in S1. ,by This abrupt change serves as the synchronous trigger point for temperature data acquisition; starting from this point, a fixed-length time window is extracted along the time axis. Among them, the fixed time window length Indicates from The duration of continuous temperature change observation is determined by a fixed time window that is neither too short (to miss actual heat diffusion) nor too long (to introduce excessive irrelevant fluctuations). This involves comprehensively considering two types of thermal response characteristics, including the thermal conductivity time constant of the cable sheath material. and the characteristic time of the thermal diffusion front of the fault arc Therefore, the joint setting is represented as: ;in: Length of the temperature sampling time window: The thermal conduction time constant of the sheath metal material along the cable axis represents the typical time required for heat to diffuse from a local hot spot to the surface of the temperature measuring cable within the cable's metal sheath. It is affected by the thermal conductivity of the sheath material, the sheath thickness and radius, and the length of the heat diffusion path, and is typically between 0.1 and 0.4 seconds. An approximate calculation can be made using a one-dimensional unsteady-state heat conduction model, as follows: ;in, This refers to the length of the heat conduction path, which can be understood as the distance from the arc point to the temperature measuring cable. Let be the thermal diffusivity of the material, which is approximately for copper. Aluminum is .

[0036] The response time of early arc heat diffusion on the sheath surface represents the typical response time of heat generated by the arc, starting from the partial discharge point, diffused to the sheath surface through the air gap or local insulation medium. Essentially, it is the response delay time between heat source triggering and the start of surface heating. Using experimental measurement methods, the response delay of the surface temperature measuring cable after arc triggering can be observed through small-sample high-voltage discharge experiments.

[0037] The scaling factor is empirical, and a range of 1.0-1.5 is recommended. In real-world environments, thermal responses vary, cable structures and thermal resistance may be uneven, and the sampling system may have a certain response delay. Therefore, this invention recommends a value of 1.2, which allows for an additional 20% observation margin based on the physical time constant.

[0038] S22, One-dimensional temperature distribution acquisition: When a polarity change is detected in the cable, it is necessary to further confirm whether this change is accompanied by a real temperature response in order to determine whether it is a substantial thermal event caused by an insulation problem. To this end, within a set time window, temperature data needs to be continuously read from the cable surface along its length to observe whether heat accumulates along the cable axis at a certain location to form a hot spot. The goal is to construct a one-dimensional axial temperature distribution image of the cable sheath surface at a certain moment for subsequent hot spot slope analysis and fault location.

[0039] Specifically, this includes fixed time windows. Inside, along the axial direction of the cable , with a preset spatial sampling interval The temperature values ​​of the distributed temperature sensing units on the continuous temperature measuring cable are read sequentially at each reading point. A temperature value is recorded at each location. All temperature values ​​are arranged in order of position to obtain a temperature sequence: This formula means: starting from the initial position Start, at every A temperature measurement point is collected at a distance of meters, and a total of N+1 points are collected, each with a temperature value. Indicates the axial position of the cable Temperature value at that location, This marks the starting point for temperature acquisition within the time window. Indicates the interval between measuring points along the cable axis. This represents the total number of measurement points. Indicates the first The actual spatial position of each measuring point relative to the starting point.

[0040] S23, Slope Calculation and Flip Point Detection: Having obtained the one-dimensional temperature distribution data along the cable axis in S22, the next step is to determine whether the temperature distribution exhibits a typical trend of first rising and then falling or first falling and then rising. This implies the existence of local hot spots or energy focusing areas in a certain section of the cable, corresponding to the physical thermal response of early insulation faults. The key to identifying such trends is to find the reversal point of the temperature change slope, that is, the location where the temperature change along the cable length changes from rising to falling or vice versa, indicating the center of the hot spot.

[0041] S231, for temperature sequences After smoothing the data and removing random noise using the moving average or median filtering method, the slope sequence of temperature change between adjacent measuring points is calculated point by point. ; Indicates the location of the cable arrive The temperature change trend between these two points is the core indicator for judging whether the temperature is rising or falling.

[0042] S232, slope sequence Convert to a symbolic sequence: ; in, This is the slope tolerance threshold, used to avoid small noise disturbances affecting sign determination and to prevent extremely small temperature differences caused by temperature measurement errors, sampling jitter, environmental disturbances, etc., from being misjudged as trend reversals. The value is between 0.2 and 0.5 ℃ / m and can be obtained by analyzing the maximum normal gradient fluctuation under non-fault conditions from historical operating data; for example, if the temperature difference caused by the maximum noise in the sampling environment is 0.1 ℃, If it is 0.5m, then It can be set to 0.2 ℃ / m. If the setting is too small, it will frequently respond to meaningless fluctuations; if it is too large, it may ignore real small hot spots. The above sign conversion eliminates small fluctuations in the numerical value, focusing only on areas with obvious trend changes, and is not swayed by noise disturbances.

[0043] S233, search for the unique sign change in the symbol sequence where a sign changes from positive to negative or vice versa, i.e.: ; And it satisfies the following spatial continuity conditions: ; Position the unique flip point The location of the flip point is defined as the spatial reference anchor point for the center of the fault heat source in subsequent steps.

[0044] The above formula means: at the point where the sign changes Search the vicinity, before and after it. At each position, does there exist at least one point whose symbol is... Same. Among them, The value is the size of the continuous window, ranging from 1 to 3. This is used to prevent misjudgments caused by isolated fluctuations. A value of 1 indicates that at least one point before and after the same direction is required, which is suitable for temperature measurement cables with high spatial resolution. A value of 2 to 3 is more suitable for analysis scenarios with strong noise resistance and can more effectively filter out local anomalies.

[0045] In general, when S233 detects a condition that satisfies: The sign of the slope is flipped, that is, positive becomes negative or negative becomes positive; The flip point passed the continuity verification window; Then the position corresponding to this point As the temperature slope reversal point, it is used for subsequent spatial positioning and is the core output of the entire temperature analysis stage.

[0046] S3 maps the flip point position output by S2 to the copper busbar of the corresponding phase in the cross-connect box. On the exposed copper braided wire that was originally used for voltage equalization between the copper busbar and the box wall, the transient increase in the oxide layer thickness formed by early arc volatilization on the surface of the copper braided wire is collected. When the transient increase in oxide layer thickness exceeds the local energy density threshold calculated from the flip point position, it is determined to be an internal insulation defect and a fault early warning signal is issued.

[0047] S31, Location mapping and energy density threshold conversion: S311, the location of the temperature slope reversal point determined by S2. Based on the cable laying path and the installation structure of the cross-connection box, a pre-set spatial mapping model is used. Determine the cross-connect box number corresponding to the location of the flip point. and its internal target copper busbar number .

[0048] S312, based on the arc propagation path length from the flip point to the target copper busbar. Combining the geometric layout parameters of the cross-interconnection structure with the thermal diffusion characteristics of the protective layer material, the equivalent thermal diffusion deposition area A corresponding to this path is calculated; the equivalent area A is estimated using the following formula: ;in, It is the equivalent thermal diffusion radius. This can be further determined by path length. With thermal diffusion angle Estimation: Therefore, in the end: thermal diffusion angle The angle is typically set between 15° and 30°, depending on the material's thermal conductivity and structural layout. S313, Simultaneously, read the mutation direction information determined in S1 and convert it into a mutation direction factor. This is used to distinguish whether the energy projection path affects the current copper busbar, and ultimately to estimate the local energy density threshold. , is represented as: This formula is used to calculate a unit area energy density threshold that matches the actual fault thermal response intensity, serving as a reference value for subsequent judgment on whether the oxide layer thickness on the surface of the copper braided wire reaches the warning standard. Since the heat received by the copper braided wire comes from the early arc heat that propagates along the sheath path from the internal insulation abnormality of the cable, this formula establishes a simplified heat transfer-transmission-deposition relationship from the source (energy release point) to the point of action (copper braided wire) to estimate the intensity of thermal energy deposition that may be generated per unit area.

[0049] In the formula: This represents the local energy density threshold per unit area (J / m²). For reference, the initial unit arc energy released (J) represents the total amount of heat energy released by the arc when the insulation inside the cable breaks down locally. A reference value is set based on laboratory discharge energy or historical field cases. To simplify the calculation, this invention uses the unit discharge energy as the input parameter and standardizes the calculation. This value determines the source strength of the entire heat effect and is physically equivalent to a point heat source.

[0050] The direction factor for abrupt change is +1 for positive and -1 for negative: it indicates whether the arc propagation direction is towards the copper braided wire segment. If the abrupt change direction points towards the target copper busbar, it is +1, indicating that the energy will be projected onto the copper braided wire; if the direction is opposite, it is set to -1. 1 indicates that the path is unlikely to be the main energy channel, and the threshold can be ignored. This item uses the polarity of the current change captured by S1 to determine whether thermal energy may act on the target cable position, which is a structured use of directional information.

[0051] For thermal diffusion deposition area: In this invention, local energy diffuses from the insulation deterioration point inside the cable (i.e. the temperature slope reversal point) through the sheath, shielding layer and other structures along the space to the target copper busbar. The final deposition location of the whole process is the surface of the copper braided wire. Therefore, A is not the total surface area of ​​the copper braided wire, but the energy projection area that ultimately affects the copper braided wire from the fault point through the thermal diffusion process.

[0052] The energy conversion factor considers arc heat transfer efficiency and surface deposition ratio: to prevent delayed warnings due to sensor errors or interference, a bias towards the upper limit of 0.8–0.9 is generally chosen; in actual deployments, if the acquisition accuracy is high, the median value of 0.75 can also be chosen as the balance point. An empirical range of 0.6–0.9 represents the proportion of heat energy that, under ideal conditions, is effectively absorbed, deposited, and leads to oxide layer formation after initial heat transfer to the surface of the copper braided wire.

[0053] The pre-defined spatial mapping model aims to establish the correspondence between the physical coordinates of the cable axis and the spatial layout of the cross-connection equipment. This allows for the accurate location of the cross-connection box and its target copper busbar based on the cable location of the fault heat source, i.e., the slope reversal point. This model is based on the following three types of information: Cable laying path data: including the actual axial length, direction, segment number, start and end points of each cable segment, which can be provided by design drawings or BIM model.

[0054] Cross-connection box layout information: including the layout location of each cross-connection box in the entire line, the box number, and the specific connection relationship of each phase copper busbar in the box.

[0055] The correspondence between the temperature measuring cable and its physical location: that is, the index position of the slope reversal point on the continuous temperature measuring cable should be able to correspond back to the actual axial coordinate of the cable.

[0056] The logic of the mapping process is as follows: Step 1: Based on the temperature measurement point number or coordinates of the reversal point identified in S2, and combined with the starting position and sampling interval of the temperature measuring cable, calculate the absolute axial position of that point on the cable. For example: if the starting position is 0 meters and the sampling interval is 0.25 meters, then the 85th measurement point is located at 21.25 meters.

[0057] Step 2: Locate the cross-connection segment to which this location belongs in the cable route table: Determine the interval based on the "segment start point - segment end point - corresponding cross-connection box number" recorded in the cable laying route table. For example: if 21.25 meters is located in "segment 3: 18 meters - 28 meters, associated cross-connection box number B3", then the reversal point is considered to correspond to cross-connection box B3.

[0058] Step 3: Determine the target copper busbar number corresponding to this point: Inside the cross-connect box, the cables of the three phases are usually introduced to the corresponding busbars, such as phase A, phase B, and phase C busbars. At this point, refer to: The phase of this cable segment; The access topology of this cable segment (which side it enters from, and which busbar it connects to); Internal wiring specifications for the enclosure; This allows us to find the number of the copper busbar that the position corresponds to. For example, the C-phase copper busbar is denoted as C3.

[0059] In actual deployment, this spatial mapping model can be stored and retrieved in the following form; a pre-set database table records the following fields: Temperature measurement section number; Start and end axial positions; Corresponding cross-connect box number; Connect the phase and copper busbar number; Configure rule models in GIS or digital twin systems.

[0060] S32, Transient incremental acquisition of oxide layer thickness: In cross-connect boxes, there is often a section of exposed copper braided wire between the copper busbar and the box body, used for electrical connection or equalization protection. Because this copper braided wire lacks insulation, it is one of the areas most susceptible to electric arcs. When an internal insulation anomaly occurs, a weak electric arc may form. These arcs release metal ions and high-temperature heat, leading to the deposition of an oxide layer on the copper surface. This oxide layer manifests at a microscale as a slight bulge or roughening of the copper surface. To capture this change, this invention uses a laser ranging sensor. Inside the target cross-connect box, along the area of ​​the exposed copper braided wire between the copper busbar and the box wall, a non-contact laser ranging sensor is installed, focusing on the area of ​​the copper braided wire surface that is susceptible to electric arcs. (The last sentence appears to be incomplete and possibly refers to a different topic.) As the starting point of the thermal response event, at the moment of abrupt change. The next preset monitoring cycle Inside, a sequence of microscopic morphology data of the copper braided wire surface is continuously scanned and collected. By comparing the differences in surface contour features collected before and after the abrupt change, the transient increment of oxide layer thickness was calculated. : ;in, For a moment The distance from the surface of the lower copper braided wire to the sensor. The monitoring cycle is 5-15 seconds. The reference time interval before the mutation is set to 1 second, which is used for baseline profile sampling. This represents the change in oxide layer thickness caused by the abrupt event.

[0061] S33, in the first two steps, has already completed two important stages: A fault-related local energy density threshold was estimated, which is the thermal energy intensity required per unit area to cause an actual physical response in the copper braided wire oxide layer. Using laser ranging technology, a transient change in the thickness of an oxide layer was collected on the surface of the copper braided wire to observe whether a material surface response had occurred.

[0062] First, the current oxide layer thickness increment is compared with the thickness standard obtained from the aforementioned energy density threshold conversion. If the thickness change of the copper surface has reached or exceeded this standard value, and this state persists in subsequent sampling (i.e., non-random fluctuations), it indicates that the copper braided wire has undergone a stable early thermal damage process. At this point, an early warning signal will be triggered, indicating the presence of insulation defects that may continue to expand.

[0063] Since the aforementioned local energy density threshold is measured in joules per square meter, while the oxide layer is measured in terms of thickness increase, and the units are different, a conversion factor is needed to convert the energy density into an equivalent oxide layer thickness standard. Specifically, this involves converting the transient increment of the oxide layer thickness... The local energy density threshold calculated by S31 The corresponding thickness standard converted Compare the results if the following conditions are met: Furthermore, if this state remains consistent in subsequent consecutive sampling rounds, meaning the above relationship is satisfied in three or more consecutive sampling rounds, then the system determines that the internal insulation defect has developed to a warning stage, and immediately triggers and outputs a fault early warning signal. Among these, ; This is the energy conversion factor for oxide layer growth. It reflects the rate of thickness change on the surface of copper materials due to high-temperature oxidation, ion deposition, or micro-erosion under a unit energy density input. Essentially, it describes the conversion efficiency from thermal energy to material change, taking into account multiple factors such as heat transfer efficiency, material reactivity, oxide layer density, and microstructural changes.

[0064] This factor is a typical empirically calibrated material parameter and needs to be obtained through experimental calibration: under laboratory or field conditions, simulating early-stage electric arc scenarios, it can be obtained through the following method: A known heat input per unit area is applied to the copper braided wire sample, for example, using a controllable pulsed discharge power supply; Control the duration, direction, and intensity of the electric arc, and record the corresponding total energy released; Use a high-precision surface profilometer or in-situ laser rangefinder in the area of ​​action to record the changes in copper surface thickness before and after the electric arc. Energy-thickness response curves were constructed with different thermal input values ​​on the horizontal axis and surface thickness on the vertical axis. Fit a linear relationship to the curve, and the slope is the energy-thickness conversion factor γ for this material structure.

[0065] This invention encompasses any substitutions, modifications, equivalent methods, and solutions made within the spirit and scope of this invention. To provide the public with a thorough understanding of this invention, specific details are described in detail in the following preferred embodiments; however, those skilled in the art will fully understand the invention even without these details. Furthermore, to avoid unnecessary misunderstanding of the essence of this invention, well-known methods, processes, procedures, components, and circuits are not described in detail.

[0066] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for early fault warning based on the operating status of high-voltage cables, characterized in that, Includes the following steps: S1. During the operation of the cable line under energization, the instantaneous difference of the sheath current between two adjacent coaxial cable sections in the cross-interconnection grounding circuit of the metal sheath is sampled. The difference waveform is obtained by using the through-core current transformer in the cross-interconnection box for short-circuit protection. The polarity change is captured by the difference waveform to obtain the time and direction of the polarity change caused only by the decrease in inter-section insulation. S2, based on the abrupt change time and direction of the abrupt change output by S1, on the continuous temperature measuring cable already laid on the surface of the metal sheath within the same cross-interconnection segment, with the abrupt change time as the zero point, a fixed length of time window is intercepted backward, and the slope sign reversal detection is performed on the one-dimensional temperature distribution of the sheath temperature along the cable axis within the time window to obtain the unique reversal point position where the temperature distribution slope changes from positive to negative or from negative to positive. S3 maps the flip point position output by S2 to the copper busbar of the corresponding phase in the cross-connect box. On the exposed copper braided wire that was originally used for voltage equalization between the copper busbar and the box wall, the transient increase in the oxide layer thickness formed by early arc volatilization on the surface of the copper braided wire is collected. When the transient increase in oxide layer thickness exceeds the local energy density threshold calculated from the flip point position, it is determined to be an internal insulation defect and a fault early warning signal is issued.

2. The fault early warning method based on the operating status of high-voltage cables according to claim 1, characterized in that, During the energized operation of the cable line, synchronous sampling technology is used to simultaneously collect the current in the metal sheath grounding leads of two adjacent coaxial cable sections in the cross-interconnection grounding circuit. Inside the cross-interconnection box, the secondary outputs of two through-core current transformers, which were originally used for short-circuit protection and are respectively installed on the grounding leads on both sides, are connected to the input of a differential amplifier circuit. The output of the differential amplifier circuit is the instantaneous difference waveform of the sheath current.

3. The method for early fault warning based on the operating status of high-voltage cables according to claim 1, characterized in that, The polarity change capture includes monitoring and analyzing the difference waveform, capturing occasional pulse signals above the steady-state power frequency induced current difference baseline by setting a dynamic threshold, recording the moment when the pulse first exceeds the dynamic threshold as the polarity change moment, and determining the change direction according to the positive and negative polarity of the pulse.

4. The fault early warning method based on the operating status of high-voltage cables according to claim 3, characterized in that, The S1 further includes filtering out pulses whose duration exceeds a preset upper limit caused by external electromagnetic interference or lightning strikes, to ensure that the timing and direction of the sudden change are caused only by the decrease in inter-segment insulation.

5. The method for early fault warning based on the operating status of high-voltage cables according to claim 1, characterized in that, S2 receives the abrupt change time from S1, and uses the abrupt change time as the zero point of time to synchronously trigger the reading of the continuous temperature measuring cable laid on the surface of the metal sheath of the same cross-interconnection section; from the zero point of time, a fixed-length time window is extracted, the fixed length being determined based on the thermal conduction time constant of the sheath material and the early thermal diffusion characteristics of the fault arc; within the time window, at a preset sampling interval, the temperature values ​​of a series of measuring points along the cable axis are read from the distributed temperature sensing unit of the continuous temperature measuring cable, forming a one-dimensional spatial distribution sequence of the sheath surface temperature along the axial direction.

6. The method for early fault warning based on the operating status of high-voltage cables according to claim 5, characterized in that, S2 further includes slope calculation and reversal point detection. Specifically, after performing data smoothing preprocessing on the one-dimensional spatial distribution sequence, the slope of temperature change between adjacent measuring points is calculated point by point along the axial direction. The calculated slope sequence is then subjected to sign analysis to identify the unique reversal point where the slope sign changes from positive to negative or from negative to positive. The axial position of the cable corresponding to the unique reversal point is taken as the reversal point position. The determination of sign reversal must meet a preset spatial continuity condition to exclude false reversals caused by measurement noise.

7. A fault early warning method based on the operating status of high-voltage cables according to claim 6, characterized in that, The preset spatial continuity conditions specifically include: Centered on the reversal point, several adjacent measuring points are selected in both the forward and backward directions along the cable axis to form a local continuous spatial region. Within the local continuous spatial region, it is determined whether there exists at least one measuring point whose temperature change slope trend is consistent with the change trend of the reversal point after reversal. Only when a measuring point with the same slope change trend as the reversal point is confirmed to exist within the local continuous spatial region is the reversal considered to have spatial continuity, and the reversal point is confirmed as a valid temperature slope reversal point.

8. The method for early fault warning based on the operating status of high-voltage cables according to claim 1, characterized in that, S3 specifically includes determining the corresponding cross-connect box and the target copper busbar inside the box by using a mapping model based on the location of the reversal point and the relationship between the cable laying path and the installation location of the cross-connect box; and calculating the local energy density threshold based on the arc propagation path length from the reversal point to the corresponding target cross-connect box, the diffusion equivalent deposition area, and the initial energy polarity of the fault contained in the direction of the sudden change.

9. A fault early warning method based on the operating status of high-voltage cables according to claim 8, characterized in that, The acquisition of the transient increase in oxide layer thickness involves installing a non-contact laser rangefinder on the exposed copper braided wire between the target copper busbar and the box wall, aiming it at the typical arc-affected area on the surface of the copper braided wire; continuously acquiring microscopic morphology data of the surface of the copper braided wire within a preset monitoring period after capturing the abrupt change moment; and calculating the transient increase in oxide layer thickness caused by early arc metal volatile deposition and oxidation by comparing the characteristic differences of the data acquired before and after the abrupt change moment.

10. A fault early warning method based on the operating status of high-voltage cables according to claim 9, characterized in that, The transient increment of oxide layer thickness collected and calculated is compared with the local energy density threshold obtained by conversion; when the transient increment of oxide layer thickness exceeds the local energy density threshold for the first time, and this state is maintained in several consecutive samplings, it is determined that the internal insulation defect has developed to the warning stage, and the fault early warning signal is immediately triggered and issued.

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