Cable condition monitoring method based on distributed fibre optic sensing

Abstract

This invention discloses a cable condition monitoring method based on distributed optical fiber sensing, belonging to the field of distributed optical fiber sensing technology. The method involves laying distributed sensing optical fibers along the cable to be monitored and connecting them to an electrical quantity acquisition unit. Narrowband optical pulses and frequency-modulated optical pulses are injected to acquire backscattered light signals, cable voltage traveling wave signals, cable current traveling wave signals, and cable grounding circulating current signals. Temperature response frames and strain response frames are obtained by framing the cables and aligning them with the electrical quantity signals at the same time scale to obtain data for the same source segment. Based on the mileage change direction of the temperature response frames and strain response frames, and the arrival polarity of the voltage and current traveling waves, candidate sections of cable anomalies are determined. The temperature response frames, strain response frames, and grounding circulating current signals of the candidate sections are marked, and the cable condition monitoring results are output. This method enables joint interpretation of optical temperature and strain data with electrical transient data within the same spatial segment and time section.

Description

Technical Field

[0001] This invention relates to the field of distributed optical fiber sensing technology, specifically a cable condition monitoring method based on distributed optical fiber sensing. Background Technology

[0002] In cable condition monitoring, distributed fiber optic sensing utilizes the sensing fiber laid along the cable to acquire backscattered light signals. The temperature or strain distribution along the fiber is obtained by demodulating the intensity or phase changes of the backscattered light. Electrical quantity acquisition devices acquire cable voltage traveling waves, current traveling waves, and grounding circulating current signals through voltage transformers, current transformers, and grounding circulating current sensors. Existing distributed fiber optic monitoring typically uses narrowband optical pulses for temperature sensing alone, or frequency-modulated optical pulses for strain sensing alone. Temperature and strain data are not acquired synchronously in time and space. Fiber optic mileage only reflects the propagation distance of light in the fiber, and its correspondence with cable conductor sections, insulation sections, and accessory sections relies on manual marking on-site, lacking a unified phase identifier and mileage interval mapping. When temperature and strain data are output from different demodulation channels, the time marking methods may differ, and spatial location is only represented by fiber length, failing to directly correspond to the cable conductor, insulation, and accessory sections. After on-site commissioning, as the difference between the cable accessory location and the fiber optic cable laying length accumulates, the deviation between fiber optic mileage and the actual cable location further increases. Existing electrical traveling wave ranging and grounding current monitoring record independently using a local sampling clock, and the sampling time is not synchronized with the fiber optic sensing frame. The change direction of temperature and strain responses along the fiber optic mileage contains information about the location of abnormal sections, and the arrival polarity of voltage and current traveling waves at the cable's beginning and end contains information about the direction of travel wave propagation. However, existing solutions do not jointly compare these directional characteristics in the same time section and the same spatial segment. The arrival polarity of traveling waves is affected by the fault location, the agreed measurement direction at the beginning and end of the line, and reflection and refraction, resulting in a non-unique direction. Relying solely on electrical traveling waves cannot reliably pinpoint abnormal sections. Local anomalies in the cable may simultaneously cause changes in temperature, strain, and traveling wave signals. Changes in the ambient temperature of the section, pressure on the outer sheath, or line operation can also produce similar responses in a single sensor. A single temperature threshold or a single traveling wave measurement is insufficient to distinguish the source of the anomaly, easily leading to misselection or omission of candidate anomaly sections.

[0003] The aforementioned problems make it impossible to use the directional characteristics of temperature, strain, and electrical traveling wave signals to jointly determine abnormal sections when cable condition monitoring is subject to inconsistent time synchronization and spatial mapping of multi-source data. Summary of the Invention

[0004] This invention provides a cable condition monitoring method based on distributed optical fiber sensing, which integrates distributed optical fiber temperature, strain and cable voltage traveling wave, current traveling wave and grounding circulation information under a unified time scale and spatial segment, and determines the candidate segments of cable anomalies based on the direction of temperature change, the direction of strain change and the polarity of the traveling wave arrival.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a cable condition monitoring method based on distributed optical fiber sensing, comprising:

[0006] Distributed sensing optical fibers are laid along the cable to be monitored and connected to an electrical quantity acquisition unit. Narrowband optical pulses and frequency-modulated optical pulses are injected to acquire backscattered light signals, cable voltage traveling wave signals, cable current traveling wave signals, and cable grounding circulating current signals.

[0007] The backscattered light signal is divided into fiber segments according to fiber optic mileage, and the fiber optic mileage and phase relationship are mapped to the cable conductor insulation accessories to obtain segment mapping data.

[0008] The segment mapping data is framed to obtain temperature response frames and strain response frames. The temperature response frames and strain response frames are aligned with the cable voltage traveling wave signal, cable current traveling wave signal and cable grounding circulating current signal according to the same time scale to obtain the same source segment data.

[0009] Based on the mileage change direction of temperature response frames and strain response frames in the data of the same source section, and the arrival polarity of cable voltage traveling wave signal and cable current traveling wave signal, candidate sections of cable anomalies are determined.

[0010] The temperature response frame, strain response frame, and cable grounding circulation signal of the candidate sections of the cable abnormality are marked, and the cable condition monitoring results are generated and output.

[0011] Preferably, the step of laying distributed sensing optical fibers along the cable to be monitored and connecting them to an electrical quantity acquisition unit, injecting narrowband optical pulses and frequency-modulated optical pulses, and acquiring backscattered light signals, cable voltage traveling wave signals, cable current traveling wave signals, and cable grounding circulating current signals includes:

[0012] Distributed sensing optical fibers are bundled to the outer sheath of the cable to be monitored in a three-phase twisted manner.

[0013] Electrical quantity acquisition units are installed at both the beginning and end of the cable to be monitored, and the electrical quantity acquisition units detect the zero-crossing moment of the power frequency voltage of the cable to be monitored.

[0014] Narrowband optical pulses and frequency-modulated optical pulses are alternately introduced into the distributed sensing optical fiber at the zero-crossing moment of the power frequency voltage.

[0015] The backscattered light signal is marked with the optical pulse number and fiber optic mileage, and the cable voltage traveling wave signal, cable current traveling wave signal and cable grounding circulating current signal are marked with the sampling time, so as to obtain the backscattered light signal with number, the cable voltage traveling wave signal with number, the cable current traveling wave signal with number and the cable grounding circulating current signal with number.

[0016] Preferably, the step of dividing the backscattered light signal into fiber segments according to fiber optic mileage and mapping it to cable conductor insulation accessories based on the relationship between fiber optic mileage and phase to obtain segment mapping data includes:

[0017] The fiber optic segments corresponding to the backscattered light signals are divided into fiber optic sections based on the location of the splice box and the installation location of the cable accessories to be monitored in the distributed sensing fiber optic cable.

[0018] During the laying of the cable to be monitored, record the phase identification and mileage interval between each fiber optic section and the cable conductor section, cable insulation section, and cable accessory section;

[0019] Backscattered light signals are collected according to fiber segments, and each fiber segment is assigned a cable phase and accessory identifier using phase identifiers and mileage intervals to obtain segment mapping data.

[0020] Preferably, the step of dividing the segment mapping data into frames to obtain temperature response frames and strain response frames, and aligning the temperature response frames and strain response frames with the cable voltage traveling wave signal, cable current traveling wave signal, and cable grounding circulating current signal according to the same time scale, to obtain data of the same source segment, includes:

[0021] Separate the narrowband optical pulse backscattering component and the frequency-modulated optical pulse backscattering component from the backscattered optical signal in the segment mapping data;

[0022] The backscattering component of the narrowband optical pulse is sliced ​​into time windows according to fiber segments to obtain temperature response frames;

[0023] The backscattered components of the frequency-modulated optical pulse are subjected to interference phase demodulation along the same fiber segment to obtain the strain response frame.

[0024] Align the time stamps of the temperature response frame and strain response frame with the sampling time stamps of the cable voltage traveling wave signal, cable current traveling wave signal and cable grounding circulating current signal to obtain data from the same source section.

[0025] Preferably, determining the candidate cable anomaly section based on the mileage change direction of the temperature response frame and strain response frame in the data of the same source section, and the arrival polarity of the cable voltage traveling wave signal and the cable current traveling wave signal, includes:

[0026] In the data of the same source segment, the temperature response frames of adjacent fiber segments are compared sequentially along the fiber optic mileage to obtain the temperature change direction sequence.

[0027] By sequentially comparing the strain response frames of adjacent fiber segments along the fiber optic mileage, a sequence of strain change directions is obtained.

[0028] Extract the arrival times of the cable voltage traveling wave signal and the cable current traveling wave signal at the beginning and end of the cable to be monitored, and determine the arrival polarity;

[0029] The segments whose temperature change direction sequence, strain change direction sequence, and arrival polarity all point to the same optical fiber segment are identified as candidate segments for cable anomalies.

[0030] Preferably, the step of marking the temperature response frame, strain response frame, and cable grounding circulating current signal of the candidate abnormal cable section, and generating and outputting the cable condition monitoring results, includes:

[0031] Extract the heating and cooling sequence of temperature response frames, the stretching and compression sequence of strain response frames, and the number of phase reversals of the cable grounding circulating current signal within the candidate sections of the cable anomaly.

[0032] The heating / cooling sequence, stretching / compression sequence, and number of phase reversals are combined into a segment marking code;

[0033] The section marking code is bound to the fiber optic mileage and cable phase of the candidate cable anomaly section to generate a section status identifier;

[0034] The cable status monitoring results are output according to the fiber optic mileage and cable phase in the section status identifier.

[0035] Preferably, the step of triggering narrowband optical pulses and frequency-modulated optical pulses to alternately enter the distributed sensing optical fiber based on the zero-crossing moment of the power frequency voltage includes:

[0036] Before the zero-crossing moment of the power frequency voltage arrives, the narrowband optical pulse emission time is set to the zero-crossing moment of the positive half-cycle of the power frequency voltage;

[0037] Set the frequency-modulated optical pulse emission time to the zero-crossing time of the negative half-cycle of the power frequency voltage;

[0038] A guard interval is set between narrowband optical pulses and frequency-modulated optical pulses, and the guard interval is greater than the optical round-trip time between adjacent splice boxes in the distributed sensing fiber.

[0039] Narrowband optical pulses and frequency-modulated optical pulses are emitted cyclically according to the power frequency voltage cycle, and the sequence number of each emitted optical pulse is written into the sequenced backscattered light signal.

[0040] Preferably, the step of recording the phase markers and mileage intervals between each fiber optic segment and the cable conductor segment, cable insulation segment, and cable accessory segment during the laying of the cable to be monitored includes:

[0041] When the cable to be monitored is laid, interval marks are set along the length of the cable to be monitored, and the fiber optic mileage of the distributed sensing fiber and the cable phase of the cable to be monitored are recorded at each interval mark.

[0042] Map the start and end positions of the cable conductor section, cable insulation section, and cable accessory section to the fiber optic mileage between adjacent interval markers, respectively.

[0043] The phase identifier and mileage interval of each fiber segment are generated based on the mapping results, and the phase identifier and mileage interval are written into the segment mapping data.

[0044] Preferably, the step of interferometric phase demodulation of the backscattered components of the frequency-modulated optical pulse along the same fiber segment to obtain the strain response frame includes:

[0045] The backscattered component of the frequency-modulated optical pulse is mixed with the local reference light to obtain the difference frequency interference signal;

[0046] The difference frequency interference signal is truncated by sliding window according to the same fiber segment, and the phase shift of the interference peak in each sliding window is extracted;

[0047] The phase shift is accumulated along the fiber optic mileage to obtain the strain response frame;

[0048] The strain response frame is marked with the corresponding fiber segment and sampling time, and then written into the same source segment data.

[0049] Preferably, determining the arrival polarity of the extracted cable voltage traveling wave signal and cable current traveling wave signal at the arrival times of the beginning and end of the cable to be monitored includes:

[0050] The polarity reversal edges of the cable voltage traveling wave signal and the cable current traveling wave signal are detected at the beginning and end of the cable to be monitored, respectively.

[0051] Record the arrival time of each polarity reversal edge relative to the zero-crossing moment of the power frequency voltage;

[0052] Compare the arrival times of the traveling wave signals of the cable voltage at the beginning and end of the cable, and compare the arrival times of the traveling wave signals of the cable current at the beginning and end of the cable.

[0053] The arrival polarity is determined by the arrival times of the traveling wave signals of the cable voltage at the beginning and end, the arrival times of the traveling wave signals of the cable current at the beginning and end, and the direction of the polarity reversal edge.

[0054] The technical effects and advantages provided by the present invention in the above technical solution are as follows:

[0055] By laying distributed sensing optical fibers along the cable to be monitored and connecting them to electrical quantity acquisition units, narrowband optical pulses and frequency-modulated optical pulses are alternately injected. The backscattered optical signal is divided into optical fiber segments according to the fiber mileage, and mapped to the cable conductor insulation accessories based on the fiber mileage and phase relationship recorded during laying. After framing the segment mapping data, the backscattered component of the narrowband optical pulse is sliced ​​into time windows according to the optical fiber segment to obtain the temperature response frame, and the backscattered component of the frequency-modulated optical pulse is subjected to interference phase demodulation according to the same optical fiber segment to obtain the strain response frame. The temperature response frame, strain response frame, cable voltage traveling wave signal, cable current traveling wave signal, and cable grounding circulating current signal are aligned with the same time scale to obtain data from the same source segment. This processing method makes the optical temperature, strain data, and electrical transient data correspond to the same cable phase and accessory identifier in the spatial dimension, and to the same sampling section in the temporal dimension. It can avoid time scale deviation and spatial misalignment caused by independent recording by optical sensing devices and electrical quantity acquisition devices. Subsequent directional feature comparisons can directly apply to multi-source data of the same cable segment. After obtaining the data for the same source section, the temperature response frames and strain response frames of adjacent fiber optic sections are compared sequentially along the fiber optic mileage to obtain the temperature change direction sequence and strain change direction sequence. The arrival polarity of the cable voltage traveling wave signal and cable current traveling wave signal at the beginning and end of the cable to be monitored is extracted. The section where the temperature change direction sequence, strain change direction sequence, and arrival polarity all point to the same fiber optic section is identified as a candidate section for cable anomalies. The temperature response frames, strain response frames, and cable grounding circulating current signals of the candidate sections for cable anomalies are then marked, and the monitoring results are output. Since the determination of candidate sections for anomalies is formed by the joint constraints of multiple parameters such as temperature, strain, and traveling wave polarity in spatial direction, the directional change caused by a single temperature fluctuation or local strain disturbance will not directly trigger the section determination. The spurious polarity pointing caused by external environmental disturbances and line reflections is also suppressed, thereby reducing the misselection and omission of anomaly sections, and ensuring that the location of the anomaly section in the output results is consistent with the cable phase and accessory identification. Attached Figure Description

[0056] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this invention. For those skilled in the art, other drawings can be obtained based on these drawings.

[0057] Figure 1 This is a flowchart of a cable condition monitoring method based on distributed optical fiber sensing.

[0058] Figure 2 This is a flowchart of the distributed sensing fiber optic segment mapping data generation process;

[0059] Figure 3This is a flowchart of a method for locating candidate sections of cable anomalies based on temperature, strain, and the polarity of the traveling wave arrival. Detailed Implementation

[0060] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0061] See Figure 1 This invention provides a cable condition monitoring method based on distributed optical fiber sensing. Distributed sensing optical fibers are laid along the cable to be monitored and connected to an electrical quantity acquisition unit. Narrowband optical pulses and frequency-modulated optical pulses are injected to acquire backscattered light signals, cable voltage traveling wave signals, cable current traveling wave signals, and cable grounding circulating current signals. The backscattered light signals are divided into optical fiber segments according to fiber optic mileage. Based on the relationship between fiber optic mileage and phase, these segments are mapped to the cable conductor insulation accessories to obtain segment mapping data. The segment mapping data is framed to obtain temperature response frames and strain response frames. These temperature response frames and strain response frames are aligned with the cable voltage traveling wave signals, cable current traveling wave signals, and cable grounding circulating current signals using the same time scale to obtain co-source segment data. Based on the mileage change direction of the temperature response frames and strain response frames in the co-source segment data and the arrival polarity of the cable voltage traveling wave signals and cable current traveling wave signals, candidate cable anomaly segments are determined. The temperature response frames, strain response frames, and cable grounding circulating current signals of the candidate cable anomaly segments are marked, and the cable condition monitoring results are generated and output.

[0062] In practice, the distributed sensing optical fiber is bundled to the outer sheath of the cable to be monitored using a three-phase twisted method. The distributed sensing optical fiber is arranged along the length of the cable and spirally wound around the outer sheath, maintaining contact with it. The bundling positions of the distributed sensing optical fiber on the outer sheath cover the conductor section, insulation section, and accessory section of the cable. Spaced fixing points are set during bundling, and weather-resistant binding tape is used to secure the distributed sensing optical fiber to the outer sheath. The distance between the fixing points is determined based on the diameter of the outer sheath and the bending radius of the distributed sensing optical fiber.

[0063] Electrical quantity acquisition units are installed at both the beginning and end of the cable to be monitored. These units are connected to the secondary sides of the voltage transformer and current transformer, as well as the grounding circulating current sensor of the cable. The electrical quantity acquisition units detect the zero-crossing moment of the power frequency voltage of the cable as follows: the zero-crossing detection circuit within the electrical quantity acquisition unit compares the power frequency voltage waveform to zero-crossing values, and outputs a trigger pulse corresponding to the zero-crossing moment of the power frequency voltage.

[0064] Before the zero-crossing moment of the power frequency voltage, the narrowband optical pulse emission time is set to the zero-crossing moment of the positive half-cycle of the power frequency voltage, and the frequency-modulated optical pulse emission time is set to the zero-crossing moment of the negative half-cycle of the power frequency voltage. The narrowband optical pulse emission time is set as follows: the optical pulse generator in the distributed optical fiber sensor receives the zero-crossing trigger signal of the positive half-cycle of the power frequency voltage output by the electrical quantity acquisition unit, and initiates narrowband optical pulse emission at the zero-crossing moment of the positive half-cycle of the power frequency voltage according to the trigger signal. Similarly, the frequency-modulated optical pulse emission time is set as follows: the optical pulse generator in the distributed optical fiber sensor receives the zero-crossing trigger signal of the negative half-cycle of the power frequency voltage output by the electrical quantity acquisition unit, and initiates frequency-modulated optical pulse emission at the zero-crossing moment of the negative half-cycle of the power frequency voltage according to the trigger signal.

[0065] A guard interval is set between narrowband optical pulses and frequency-modulated (FM) optical pulses. This guard interval is greater than the round-trip time (RTT) between adjacent junction boxes in the distributed fiber optic sensing system. Specifically, in the timing control of the optical pulse generator in the distributed fiber optic sensing system, after the narrowband optical pulse is emitted, an FM optical pulse is emitted after the guard interval. After the FM optical pulse is emitted, another narrowband optical pulse is emitted after the guard interval. The guard interval is calculated based on the fiber length and refractive index between adjacent junction boxes in the distributed fiber optic sensing system. The RTT is equal to twice the fiber length between adjacent junction boxes divided by the speed of light in the fiber.

[0066] Narrowband and frequency-modulated (FM) optical pulses are cyclically emitted according to the power frequency voltage cycle. These pulses alternately enter the distributed sensing fiber at the zero-crossing point of the power frequency voltage. During the positive half-cycle of the power frequency voltage, a narrowband optical pulse enters the distributed sensing fiber; during the negative half-cycle, an FM optical pulse enters. One power frequency voltage cycle consists of one narrowband optical pulse emission and one FM optical pulse emission, with the two pulses alternating in time.

[0067] The backscattered light signal is labeled with the optical pulse number and fiber optic mileage. The backscattered light signal is received by a photodetector in the distributed fiber optic sensor, and the backscattered light signal output by the photodetector enters the data acquisition unit of the distributed fiber optic sensor. The data acquisition unit in the distributed fiber optic sensor labels the optical pulse number according to the synchronization signal output by the optical pulse generator in the distributed fiber optic sensor. The optical pulse number corresponds one-to-one with each transmitted narrowband optical pulse or frequency-modulated optical pulse. The data acquisition unit in the distributed fiber optic sensor calculates the fiber optic mileage corresponding to the backscattered light signal based on the time difference between the optical pulse emission time and the backscattered light signal sampling time, as well as the propagation speed of light in the distributed sensing fiber, and labels the fiber optic mileage onto the backscattered light signal.

[0068] The sampling times are marked for the cable voltage traveling wave signal, cable current traveling wave signal, and cable grounding circulating current signal. The electrical quantity acquisition unit samples the cable voltage traveling wave signal, cable current traveling wave signal, and cable grounding circulating current signal according to the sampling clock, which is synchronized with the clock in the distributed fiber optic sensor. Each sampling point corresponds to a sampling time, and the sampling time is written into the corresponding signal data.

[0069] The system obtains numbered backscattered light signals, numbered cable voltage traveling wave signals, numbered cable current traveling wave signals, and numbered cable grounding circulating current signals. In the numbered backscattered light signals, the optical pulse sequence number is used to distinguish the backscattered light signals corresponding to narrowband optical pulses and frequency-modulated optical pulses, and the fiber optic mileage is used to locate the reflection position on the distributed sensing fiber. The sampling time in the numbered cable voltage traveling wave signals, numbered cable current traveling wave signals, and numbered cable grounding circulating current signals is used for time-scale alignment with the backscattered light signals.

[0070] The sequence number of each transmitted optical pulse is written into the numbered backscattered optical signal. The optical pulse sequence number is synchronously stored in the data acquisition unit of the distributed optical fiber sensor along with the backscattered optical signal. The optical pulse sequence number increments with each transmission and is written into the data frame header or data segment of the backscattered optical signal.

[0071] In specific implementation, please refer to Figure 2Based on the locations of splice boxes in the distributed sensing fiber optic cable and the installation locations of the cable accessories to be monitored, the fiber optic mileage corresponding to the backscattered light signal is divided into fiber segments. The splice box location in the distributed sensing fiber optic cable is the physical splice point, and the fiber optic mileage corresponding to each splice box location is recorded during the laying of the distributed sensing fiber optic cable. The installation locations of the cable accessories to be monitored are the start and end locations of the cable accessory segments on the cable to be monitored, and the cable accessory segments include intermediate joints and terminations. Based on the mileage values ​​of the splice box locations and the installation locations of the cable accessories to be monitored, the interval between two adjacent mileage boundary points is considered as one fiber segment, thus dividing the fiber optic mileage corresponding to the backscattered light signal into multiple fiber segments. Each fiber segment has a fiber optic mileage start point and a fiber optic mileage end point, and fiber segments do not overlap.

[0072] During the laying of the cable to be monitored, interval markers are set along the length of the cable. These markers are placed on the surface of the outer sheath of the cable and can be physical or electronically recorded. The spacing between adjacent markers is determined based on the installation location of the cable accessories and the location of the splice cassette in the distributed sensing fiber. At each interval marker, the fiber optic mileage of the distributed sensing fiber and the cable phase of the cable to be monitored are recorded. The fiber optic mileage is obtained through optical time-domain reflectometry (OTDR) in the distributed fiber optic sensor, and the cable phase is obtained from the construction records. The records at each interval marker are written into the record table corresponding to the segment mapping data.

[0073] The start and end positions of the cable conductor section, cable insulation section, and cable accessory section are mapped to the fiber optic mileage between adjacent interval markers, respectively. The cable conductor section refers to the section containing the cable conductor in the cable to be monitored; the cable insulation section refers to the section containing the cable insulation layer; and the cable accessory section refers to the section containing the cable accessories. Based on the construction layout diagram of the cable to be monitored, the start and end positions of the cable conductor section, cable insulation section, and cable accessory section along the length of the cable to be monitored are obtained. The start and end positions of the cable conductor section, the cable insulation section, and the cable accessory section are mapped to the fiber optic mileage interval between adjacent interval markers. During the mapping process, the relative positional relationship between the cable conductor section, cable insulation section, and cable accessory section and the distributed sensing fiber remains unchanged.

[0074] Based on the mapping results, phase identifiers and mileage intervals are generated for each optical fiber segment. The phase identifier includes cable phase (A, B, or C) and accessory identifier, which distinguishes whether the optical fiber segment corresponds to a cable accessory segment. The mileage interval represents the start and end points of the optical fiber segment's mileage. The mapped mileage intervals of the cable conductor, cable insulation, and cable accessory segments are mapped to the optical fiber segments of the distributed sensing fiber, generating a phase identifier and mileage interval for each segment. The phase identifier and mileage interval are then written into the segment mapping data. The segment mapping data is a structured data record; each record contains the optical fiber segment number, optical fiber mileage start point, optical fiber mileage end point, cable phase, and accessory identifier.

[0075] Backscattered optical signals are aggregated according to fiber segments. Each backscattered optical signal contains fiber mileage markers. Based on these markers, the backscattered optical signals are assigned to corresponding fiber segments. Specifically, the allocation method involves comparing the fiber mileage markers of the backscattered optical signals with the mileage intervals of each fiber segment. If a marker falls within the mileage interval of a fiber segment, the backscattered optical signal data point is assigned to that fiber segment. The aggregated backscattered optical signals are organized according to fiber segments, with each fiber segment corresponding to a set of backscattered optical signal data points.

[0076] Each fiber optic segment is assigned cable phase and accessory identifiers using phase identifiers and mileage intervals. For each fiber optic segment, the corresponding phase identifier and mileage interval are read from the segment mapping data, and the cable phase and accessory identifiers from the phase identifiers are assigned to the backscattered optical signal data points within that fiber optic segment. The assigned backscattered optical signal data points carry cable phase and accessory identifier information. The set of backscattered optical signal data after assignment processing forms the segment mapping data. The segment mapping data is used for subsequent framing processing and generation of data for segments originating from the same source.

[0077] In practice, the backscattered light signal in the segment mapping data is separated into narrowband optical pulse backscattering components and frequency-modulated optical pulse backscattering components. The backscattered light signal is marked with an optical pulse number, which corresponds one-to-one with each emitted narrowband or frequency-modulated optical pulse. When separating the narrowband and frequency-modulated optical pulse backscattering components, the backscattered light signal data points corresponding to the narrowband optical pulses are assigned to the narrowband backscattered component, and the backscattered light signal data points corresponding to the frequency-modulated optical pulses are assigned to the frequency-modulated optical pulse backscattering component. The narrowband and frequency-modulated optical pulse backscattering components then form independent data sets.

[0078] Narrowband optical pulse backscattering components are sliced ​​into time windows according to fiber segments to obtain temperature response frames. For each fiber segment in the segment mapping data, a time window is set along the fiber mileage direction on the narrowband optical pulse backscattering component. The start point of the time window corresponds to the start point of the fiber segment's mileage, and the end point of the time window corresponds to the end point of the fiber segment's mileage. The narrowband optical pulse backscattering component data segment within each time window constitutes a temperature response frame. The temperature response frame contains backscattered light intensity data, which reflects the change in backscattered light intensity along the fiber segment of the distributed sensing fiber. The temperature response frame is marked with the corresponding fiber segment and sampling time.

[0079] The backscattered component of a frequency-modulated (FM) optical pulse is mixed with a local reference light to obtain a difference-frequency interference (DFE) signal. The FM pulse backscattered component enters one input port of an optical mixer, while the local reference light enters the other input port. The local reference light and the FM pulse are coherent, maintaining a fixed frequency difference. The optical mixer optically mixes the FM pulse backscattered component with the local reference light, outputting the DFE signal. The DFE signal contains interference fringes, and the phase changes of the interference fringes are related to the strain changes in the distributed sensing fiber. The DFE signal is converted into an electrical signal by a photodetector.

[0080] Optionally, the difference-frequency interference signal is truncated using a sliding window within the same fiber segment, and the phase shift of the interference peak within each window is extracted. For each fiber segment, a sliding window is set on the difference-frequency interference signal according to the mileage interval of the fiber segment. The sliding window slides along the fiber mileage direction, with the window length being less than the mileage length of the fiber segment and the window step size being less than the window length. Each window contains a segment of the difference-frequency interference signal. Interference peaks are identified within the difference-frequency interference signal within the window, and the phase values ​​corresponding to the interference peaks are extracted. The phase difference between the interference peaks of two adjacent sliding windows is used as the phase shift, which represents the change in optical path caused by strain in the distributed sensing fiber between adjacent sliding windows.

[0081] The phase shift is accumulated along the fiber optic mileage to obtain the strain response frame. Let the fiber segment number be... Fiber optic segment Contains The first sliding window, the... The phase shift of the interference peak within each sliding window is Then in fiber optic mileage The cumulative strain response frame value at the location is The cumulative relationship is:

[0082]

[0083] in, For fiber optic segment numbering, The sliding window number. For fiber optic segments The total number of sliding windows inside. For fiber optic segments Inner Phase shift of the interference peak of each sliding window For fiber optic segments Inner The strain response frame values ​​accumulated at each sliding window position. For fiber optic segments Inner The fiber optic mileage corresponding to the center of each sliding window. The strain response frame marker corresponds to the fiber segment and sampling time, and is written into the same source segment data.

[0084] The time stamps of the temperature response frame and strain response frame are aligned with the sampling time stamps of the cable voltage traveling wave signal, cable current traveling wave signal, and cable grounding circulating current signal to obtain data for the same source section. The sampling times of the temperature response frame and strain response frame are recorded using the sampling clock in the distributed optical fiber sensor, while the sampling times of the cable voltage traveling wave signal, cable current traveling wave signal, and cable grounding circulating current signal are recorded using the sampling clock in the electrical quantity acquisition unit. The sampling clock in the distributed optical fiber sensor is synchronized with the sampling clock in the electrical quantity acquisition unit. During alignment, based on the sampling time, the data from the temperature response frame, strain response frame, cable voltage traveling wave signal, cable current traveling wave signal, and cable grounding circulating current signal at the same sampling time are combined to form a data record for the same source section. The data record for the same source section includes the optical fiber section number, temperature response frame data, strain response frame data, sampled values ​​of the cable voltage traveling wave signal, sampled values ​​of the cable current traveling wave signal, and sampled values ​​of the cable grounding circulating current signal.

[0085] In specific implementation, please refer to Figure 3Temperature response frames are read sequentially from the source segment data, from smallest to largest fiber optic mileage, between adjacent fiber segments. Each temperature response frame contains backscattered light intensity data. The average value of the temperature response frame data within each fiber segment is calculated to obtain a representative temperature value for that segment. For two adjacent fiber segments, the representative temperature value of the segment with the smaller fiber optic mileage is compared with that of the segment with the larger fiber optic mileage. If the representative temperature value of the segment with the larger fiber mileage is greater than that of the segment with the smaller fiber mileage, the temperature change direction is recorded as the heating direction; if the representative temperature value of the segment with the larger fiber mileage is less than that of the segment with the smaller fiber mileage, the temperature change direction is recorded as the cooling direction; if the representative temperature value of the segment with the larger fiber mileage is equal to that of the segment with the smaller fiber mileage, the temperature change direction is recorded as the level direction. The temperature change directions of all adjacent fiber segments are arranged in order of fiber optic mileage to obtain a temperature change direction sequence.

[0086] Strain response frames are read sequentially from the source segment data, from smallest to largest fiber optic mileage, from two adjacent fiber segments. Each strain response frame contains a cumulative strain response frame value. The average value of the strain response frame data within each fiber segment is calculated to obtain the representative strain value for that segment. For two adjacent fiber segments, the representative strain value of the segment with the smaller fiber optic mileage is compared with that of the segment with the larger fiber optic mileage. If the representative strain value of the segment with the larger fiber mileage is greater than that of the segment with the smaller fiber mileage, the strain change direction is recorded as the tensile direction; if the representative strain value of the segment with the larger fiber mileage is less than that of the segment with the smaller fiber mileage, the strain change direction is recorded as the compressive direction; if the representative strain value of the segment with the larger fiber mileage is equal to that of the segment with the smaller fiber mileage, the strain change direction is recorded as the neutral direction. The strain change directions of all adjacent fiber segments are arranged in order of fiber optic mileage to obtain a strain change direction sequence.

[0087] At the beginning of the cable to be monitored, the polarity reversal edges of the cable voltage and current traveling wave signals are detected. A first-order differential operation is performed on the cable voltage traveling wave signal to obtain a differential sequence. Zero-crossing points where the difference value changes from positive to negative or vice versa are identified and used as the polarity reversal edges of the cable voltage traveling wave signal. Similarly, a first-order differential operation is performed on the cable current traveling wave signal to obtain a differential sequence. Zero-crossing points where the difference value changes from positive to negative or vice versa are identified and used as the polarity reversal edges of the cable current traveling wave signal. The same method is used to detect the polarity reversal edges of the cable voltage and current traveling wave signals at the end of the cable to be monitored.

[0088] Record the arrival time of each polarity reversal edge relative to the zero-crossing time of the power frequency voltage. When the electrical quantity acquisition unit samples the cable voltage traveling wave signal and the cable current traveling wave signal, it simultaneously records the sampling time of each sampling point. The zero-crossing time of the power frequency voltage is taken as the zero-crossing time of the positive half-cycle of the power frequency voltage. For each polarity reversal edge, read the sampling time corresponding to that polarity reversal edge from the sampled data, calculate the time difference between the sampling time corresponding to that polarity reversal edge and the nearest zero-crossing time of the positive half-cycle of the power frequency voltage, and use this time difference as the arrival time of that polarity reversal edge. The arrival time is set to the zero point of the positive half-cycle of the power frequency voltage.

[0089] Compare the arrival times of the traveling wave signals of the cable voltage at the beginning and end of the cable. Read the arrival time of the polarity reversal edge of the traveling wave signal of the cable voltage detected at the beginning of the cable, and read the arrival time of the polarity reversal edge of the traveling wave signal of the cable voltage detected at the end of the cable. If the arrival time of the traveling wave signal of the cable voltage at the beginning of the cable is earlier than the arrival time of the traveling wave signal of the cable voltage at the end of the cable, then the time when the traveling wave signal of the cable voltage at the beginning of the cable is earlier than the time when it is closer to the end of the cable; if the arrival time of the traveling wave signal of the cable voltage at the beginning of the cable is later than the arrival time of the traveling wave signal of the cable voltage at the end of the cable, then the time when the traveling wave signal of the cable voltage at the beginning of the cable is later than the time when it is closer to the end of the cable. Compare the arrival times of the traveling wave signals of the cable current at the beginning and end of the cable using the same method.

[0090] The arrival polarity is determined based on the arrival times of the traveling wave signals of the cable voltage and current at the beginning and end of the cable, as well as the direction of the polarity reversal edge. The direction of the polarity reversal edge includes a positive reversal direction and a negative reversal direction. The positive reversal direction is the direction of the zero-crossing point where the difference value changes from negative to positive, and the negative reversal direction is the direction of the zero-crossing point where the difference value changes from positive to negative. For cable voltage traveling wave signals, if the arrival time of the cable voltage traveling wave signal at the beginning of the monitored cable is earlier than the arrival time of the cable voltage traveling wave signal at the end of the monitored cable, and the polarity reversal direction of the cable voltage traveling wave signal at the beginning of the monitored cable is positive, and the polarity reversal direction of the cable voltage traveling wave signal at the end of the monitored cable is also positive, then the arrival polarity of the cable voltage traveling wave signal is positive. For cable current traveling wave signals, the same rules are used to determine the arrival polarity of the cable current traveling wave signal.

[0091] Sections where the temperature change direction sequence, strain change direction sequence, and arrival polarity all point to the same fiber optic segment are identified as candidate sections for cable anomalies. Each temperature change direction in the temperature change direction sequence corresponds to a fiber optic segment boundary or a combination of adjacent fiber optic segments; each strain change direction in the strain change direction sequence corresponds to a fiber optic segment boundary or a combination of adjacent fiber optic segments; and the arrival polarity corresponds to the cable segment or fiber optic segment of the cable to be monitored. When a temperature change direction in the temperature change direction sequence points to a certain fiber optic segment, the corresponding strain change direction in the strain change direction sequence also points to the same fiber optic segment, and the arrival polarity points to the same fiber optic segment, that same fiber optic segment is identified as a candidate section for cable anomalies. Pointing to a fiber optic segment means that the anomaly location represented by the temperature change direction, strain change direction, or arrival polarity is located within the mileage range of that fiber optic segment. All fiber optic segments that meet the condition of pointing to the same fiber optic segment are marked as candidate sections for cable anomalies.

[0092] In practice, temperature response frames from multiple consecutive sampling times within the candidate cable anomaly section are read. The average value of the backscattered light intensity data is calculated for each sampling time frame to obtain the section temperature value at each sampling time. The section temperature values ​​are arranged in chronological order of sampling time, and the section temperature values ​​at adjacent sampling times are compared. If the section temperature value at a later sampling time is greater than that at a previous sampling time, a temperature rise event is recorded; if the section temperature value at a later sampling time is less than that at a previous sampling time, a temperature fall event is recorded. The order in which temperature rise and temperature fall events occur within the candidate cable anomaly section is extracted as the temperature rise / fall order. If a temperature rise event occurs before a temperature fall event, the temperature rise / fall order is rising followed by falling; if a temperature fall event occurs before a temperature rise event, the temperature rise / fall order is falling followed by rising.

[0093] Read strain response frames from multiple consecutive sampling times within the candidate section of the cable anomaly. Calculate the average of the cumulative strain response frames for each sampling time to obtain the section strain value at each sampling time. Arrange the section strain values ​​in chronological order of sampling time and compare the strain values ​​at adjacent sampling times. If the strain value at a later sampling time is greater than that at a previous sampling time, record a strain tension event; if the strain value at a later sampling time is less than that at a previous sampling time, record a strain compression event. Extract the order of occurrence of strain tension and strain compression events within the candidate section of the cable anomaly as the tension-compression order. If a strain tension event precedes a strain compression event, the tension-compression order is tension followed by compression; if a strain compression event precedes a strain tension event, the tension-compression order is compression followed by tension.

[0094] Read the cable grounding circulating current signal corresponding to the candidate cable anomaly section. The cable grounding circulating current signal is a discrete data sequence sampled by the electrical quantity acquisition unit within the power frequency voltage cycle. The method for detecting the number of phase reversals in the cable grounding circulating current signal is as follows: The cable grounding circulating current signal is de-DC processed to obtain the de-DC cable grounding circulating current signal; the zero-crossing points where the signs of adjacent sampling points change in the de-DC cable grounding circulating current signal are identified; at each zero-crossing point, it is determined whether the phase of the cable grounding circulating current signal has reversed. If the signal waveform before and after the zero-crossing point changes from the positive half-cycle to the negative half-cycle and the phase jump exceeds half a power frequency cycle, or if the signal waveform before and after the zero-crossing point changes from the negative half-cycle to the positive half-cycle and the phase jump exceeds half a power frequency cycle, then a phase reversal is recorded. The number of phase reversals occurring in the cable grounding circulating current signal corresponding to the candidate cable anomaly section within one power frequency voltage cycle is counted to obtain the number of phase reversals.

[0095] The heating / cooling sequence, the stretching / compression sequence, and the number of phase reversals are combined to form a segment marker code. Let the heating / cooling sequence code be... The stretching and compression sequence is coded as follows: The number of phase reversals is The segment marker code is The combination formula is:

[0096]

[0097] in, This is a code for the heating / cooling sequence, with a value of 0 or 1. When the heating / cooling sequence is cooling followed by heating... Set to 0, when the heating and cooling sequence is heating followed by cooling. Take 1; This is a code for the stretching / compression sequence, with a value of 0 or 1. When the stretching / compression sequence is compression followed by stretching... Set to 0 when the stretching-compression sequence is stretching followed by compression. Take 1; This represents the number of phase reversals, and its value is a non-negative integer. This is the segment marker code, with a value that is a non-negative integer. It also represents the encoding of the heating / cooling sequence in the combined formula. Multiply by a weight of 1, then encode the stretching and compression sequence. Multiply by weight 2, number of phase reversals Multiply by a weight of 4. The weight value is determined by the combination of binary bits. The heating and cooling sequence code occupies the lowest bit, the stretching and compression sequence code occupies the second bit, and the number of phase reversals is expressed starting from the third bit, so that different combinations of heating and cooling sequences, stretching and compression sequences, and the number of phase reversals correspond to different segment mark code values.

[0098] The segment marker code is bound to the fiber optic mileage and cable phase of the candidate cable anomaly segment to generate a segment status identifier. The segment marker code is a value calculated using a combination formula. The fiber optic mileage represents the start and end points of the candidate cable anomaly segment on the distributed sensing fiber. The cable phase is the A, B, or C phase corresponding to the candidate cable anomaly segment. The binding operation is performed within the corresponding fiber optic segment record in the segment mapping data. The segment marker code is written to the fiber optic segment record, and the fiber optic mileage and cable phase are associated with the segment marker code to form the segment status identifier. The segment status identifier includes the segment marker code, the start point of the fiber optic mileage, the end point of the fiber optic mileage, and the cable phase.

[0099] Cable status monitoring results are output according to the fiber optic mileage and cable phase in the segment status identifier. The cable status monitoring results are output in structured data format. Each cable status monitoring result record corresponds to a candidate cable anomaly segment, including the fiber optic mileage start point, fiber optic mileage end point, cable phase, and segment marker code in the segment status identifier. The output order is arranged in ascending order of fiber optic mileage start point, and within the same fiber optic mileage start point, the cable phases are arranged in the order of A, B, and C. Output formats include at least one of the following: display screen, data file storage, and communication interface reporting.

[0100] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application.

Claims

1. A cable condition monitoring method based on distributed optical fiber sensing, characterized in that, include: Distributed sensing optical fibers are laid along the cable to be monitored and connected to an electrical quantity acquisition unit. Narrowband optical pulses and frequency-modulated optical pulses are injected to acquire backscattered light signals, cable voltage traveling wave signals, cable current traveling wave signals, and cable grounding circulating current signals. The backscattered light signal is divided into fiber segments according to fiber optic mileage, and the fiber optic mileage and phase relationship are mapped to the cable conductor insulation accessories to obtain segment mapping data. The segment mapping data is framed to obtain temperature response frames and strain response frames. The temperature response frames and strain response frames are aligned with the cable voltage traveling wave signal, cable current traveling wave signal and cable grounding circulating current signal according to the same time scale to obtain the same source segment data. Based on the mileage change direction of temperature response frames and strain response frames in the data of the same source section, and the arrival polarity of cable voltage traveling wave signal and cable current traveling wave signal, candidate sections of cable anomalies are determined. The temperature response frame, strain response frame, and cable grounding circulation signal of the candidate sections of the cable abnormality are marked, and the cable condition monitoring results are generated and output.

2. The cable condition monitoring method based on distributed optical fiber sensing according to claim 1, characterized in that, The distributed sensing optical fiber laid along the cable to be monitored and connected to the electrical quantity acquisition unit injects narrowband optical pulses and frequency-modulated optical pulses to acquire backscattered light signals, cable voltage traveling wave signals, cable current traveling wave signals, and cable grounding circulating current signals, including: Distributed sensing optical fibers are bundled to the outer sheath of the cable to be monitored in a three-phase twisted manner. Electrical quantity acquisition units are installed at both the beginning and end of the cable to be monitored, and the electrical quantity acquisition units detect the zero-crossing moment of the power frequency voltage of the cable to be monitored. Narrowband optical pulses and frequency-modulated optical pulses are alternately introduced into the distributed sensing optical fiber at the zero-crossing moment of the power frequency voltage. The backscattered light signal is marked with the optical pulse number and fiber optic mileage, and the cable voltage traveling wave signal, cable current traveling wave signal and cable grounding circulating current signal are marked with the sampling time, so as to obtain the backscattered light signal with number, the cable voltage traveling wave signal with number, the cable current traveling wave signal with number and the cable grounding circulating current signal with number.

3. The cable condition monitoring method based on distributed optical fiber sensing according to claim 1, characterized in that, The process of dividing the backscattered light signal into fiber segments according to fiber optic mileage and mapping it to cable conductor insulation accessories based on the relationship between fiber optic mileage and phase to obtain segment mapping data includes: The fiber optic segments corresponding to the backscattered light signals are divided into fiber optic sections based on the location of the splice box and the installation location of the cable accessories to be monitored in the distributed sensing fiber optic cable. During the laying of the cable to be monitored, record the phase identification and mileage interval between each fiber optic section and the cable conductor section, cable insulation section, and cable accessory section; Backscattered light signals are collected according to fiber segments, and each fiber segment is assigned a cable phase and accessory identifier using phase identifiers and mileage intervals to obtain segment mapping data.

4. The cable condition monitoring method based on distributed optical fiber sensing according to claim 1, characterized in that, The segment mapping data is framed to obtain temperature response frames and strain response frames. These temperature response frames and strain response frames are then aligned with the cable voltage traveling wave signal, cable current traveling wave signal, and cable grounding circulating current signal using the same time scale to obtain data for the same source segment, including: Separate the narrowband optical pulse backscattering component and the frequency-modulated optical pulse backscattering component from the backscattered optical signal in the segment mapping data; The backscattering component of the narrowband optical pulse is sliced ​​into time windows according to fiber segments to obtain temperature response frames; The backscattered components of the frequency-modulated optical pulse are subjected to interference phase demodulation along the same fiber segment to obtain the strain response frame. Align the time stamps of the temperature response frame and strain response frame with the sampling time stamps of the cable voltage traveling wave signal, cable current traveling wave signal and cable grounding circulating current signal to obtain data from the same source section.

5. The cable condition monitoring method based on distributed optical fiber sensing according to claim 1, characterized in that, The process of determining candidate cable anomaly sections based on the mileage change direction of temperature response frames and strain response frames in the data of the same source section, and the arrival polarity of cable voltage traveling wave signals and cable current traveling wave signals, includes: In the data of the same source segment, the temperature response frames of adjacent fiber segments are compared sequentially along the fiber optic mileage to obtain the temperature change direction sequence. By sequentially comparing the strain response frames of adjacent fiber segments along the fiber optic mileage, a sequence of strain change directions is obtained. Extract the arrival times of the cable voltage traveling wave signal and the cable current traveling wave signal at the beginning and end of the cable to be monitored, and determine the arrival polarity; The segments whose temperature change direction sequence, strain change direction sequence, and arrival polarity all point to the same optical fiber segment are identified as candidate segments for cable anomalies.

6. The cable condition monitoring method based on distributed optical fiber sensing according to claim 1, characterized in that, The process of marking the temperature response frame, strain response frame, and cable grounding circulating current signal of the candidate abnormal cable sections, and generating and outputting cable condition monitoring results includes: Extract the heating and cooling sequence of temperature response frames, the stretching and compression sequence of strain response frames, and the number of phase reversals of the cable grounding circulating current signal within the candidate sections of the cable anomaly. The heating / cooling sequence, stretching / compression sequence, and number of phase reversals are combined into a segment marking code; The section marking code is bound to the fiber optic mileage and cable phase of the candidate cable anomaly section to generate a section status identifier; The cable status monitoring results are output according to the fiber optic mileage and cable phase in the section status identifier.

7. The cable condition monitoring method based on distributed optical fiber sensing according to claim 2, characterized in that, The step of triggering narrowband optical pulses and frequency-modulated optical pulses to alternately enter the distributed sensing fiber based on the zero-crossing moment of the power frequency voltage includes: Before the zero-crossing moment of the power frequency voltage arrives, the narrowband optical pulse emission time is set to the zero-crossing moment of the positive half-cycle of the power frequency voltage; Set the frequency-modulated optical pulse emission time to the zero-crossing time of the negative half-cycle of the power frequency voltage; A guard interval is set between narrowband optical pulses and frequency-modulated optical pulses, and the guard interval is greater than the optical round-trip time between adjacent splice boxes in the distributed sensing fiber. Narrowband optical pulses and frequency-modulated optical pulses are emitted cyclically according to the power frequency voltage cycle, and the sequence number of each emitted optical pulse is written into the sequenced backscattered light signal.

8. The cable condition monitoring method based on distributed optical fiber sensing according to claim 3, characterized in that, The process of recording the phase markers and mileage intervals between each fiber optic segment and the cable conductor segment, cable insulation segment, and cable accessory segment during the laying of the cable to be monitored includes: When the cable to be monitored is laid, interval marks are set along the length of the cable to be monitored, and the fiber optic mileage of the distributed sensing fiber and the cable phase of the cable to be monitored are recorded at each interval mark. Map the start and end positions of the cable conductor section, cable insulation section, and cable accessory section to the fiber optic mileage between adjacent interval markers, respectively. The phase identifier and mileage interval of each fiber segment are generated based on the mapping results, and the phase identifier and mileage interval are written into the segment mapping data.

9. The cable condition monitoring method based on distributed optical fiber sensing according to claim 4, characterized in that, The process of interferometric phase demodulation of the backscattered components of the frequency-modulated optical pulse along the same fiber segment to obtain a strain response frame includes: The backscattered component of the frequency-modulated optical pulse is mixed with the local reference light to obtain the difference frequency interference signal; The difference frequency interference signal is truncated by sliding window according to the same fiber segment, and the phase shift of the interference peak in each sliding window is extracted; The phase shift is accumulated along the fiber optic mileage to obtain the strain response frame; The strain response frame is marked with the corresponding fiber segment and sampling time, and then written into the same source segment data.

10. The cable condition monitoring method based on distributed optical fiber sensing according to claim 5, characterized in that, The step of extracting the arrival times of the cable voltage traveling wave signal and the cable current traveling wave signal at the beginning and end of the cable to be monitored, and determining the arrival polarity, includes: The polarity reversal edges of the cable voltage traveling wave signal and the cable current traveling wave signal are detected at the beginning and end of the cable to be monitored, respectively. Record the arrival time of each polarity reversal edge relative to the zero-crossing moment of the power frequency voltage; Compare the arrival times of the traveling wave signals of the cable voltage at the beginning and end of the cable, and compare the arrival times of the traveling wave signals of the cable current at the beginning and end of the cable. The arrival polarity is determined by the arrival times of the traveling wave signals of the cable voltage at the beginning and end, the arrival times of the traveling wave signals of the cable current at the beginning and end, and the direction of the polarity reversal edge.

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