A method for realizing wide-range cable on-line fault diagnosis and positioning through frequency conversion technology

Abstract

The application discloses a method for realizing wide-range cable online fault diagnosis and positioning through frequency conversion technology, selects a spread spectrum sequence for fault detection, determines a center frequency of the spread spectrum code and a spread spectrum code length, generates a high-frequency detection signal through bpsk modulation, adjusts the center frequency of the spread spectrum code, generates a plurality of medium-high-frequency detection signals with the center frequencies from low to high, generates a low-frequency sinusoidal signal as a low-frequency fault diagnosis signal, couples the low-frequency fault diagnosis signal into a cable to be detected and couples a receiving end, performs fault diagnosis on the cable, if a fault is diagnosed, couples a spread spectrum time domain reflectometry (SSTDR) detection signal into the cable to be detected, receives a reflected signal, performs attenuation compensation on the reflected signal according to a channel frequency attenuation law, judges a fault distance through a reflected wave head after correlation operation, and sequentially reduces the signal from high frequency to low frequency until a fault position is positioned. The method removes the influence of a detection blind area and attenuation on detection, and realizes wide-range cable online fault diagnosis and positioning.

Description

Technical Field

[0001] This invention belongs to the field of cable testing technology, specifically relating to a method for achieving wide-range online fault diagnosis and location of cables through frequency conversion technology. Background Technology

[0002] In aviation and civil power distribution systems, cable fault detection and location have received significant attention from researchers over the past decade. In aerospace systems, power lines and data transmission lines operate in complex environments. Cables are subjected to prolonged vibrations from aircraft in flight and friction with hard surfaces such as aircraft structures, damaging the surface insulation. Simultaneously, bending, moisture, radiation, and high temperatures cause conductors to age and degrade insulation, ultimately leading to short circuits, open circuits, and intermittent faults. This can jeopardize the normal operation of aerospace power supply systems and may even cause onboard fires or other catastrophic accidents. In civil power distribution systems, cables are affected by external pressure, water immersion, chemical corrosion, temperature, humidity, and friction, leading to insulation aging and even theft. This can also result in open circuits, short circuits, and intermittent faults, causing large-scale power outages and even cable fires. Therefore, accurate online prediction, detection, and location of cable faults not only improve system operational safety but also significantly save maintenance time and costs, possessing significant practical importance and application value.

[0003] Currently, the main methods for cable fault location both domestically and internationally are impedance method (bridge method), traveling wave method, acoustomagnetic method, and reflection method. Among these, the reflection method is widely used due to its higher detection accuracy, but its detection range is relatively low, often within 500 meters. To increase the detection range, domestic scholars have adopted a signal compensation method based on the frequency domain reflection method to address the issue of increased signal attenuation with increasing distance. This method compensates for the actual detection curve, effectively improving the accuracy of fault diagnosis for long-distance high-voltage cables. However, this method cannot be applied to online detection. Some scholars have addressed the low accuracy of the traveling wave method for detecting long-distance faults by utilizing the principle of multiple emission and refraction of fault waves to calibrate the wave velocity and improve the accuracy of fault diagnosis for long-distance cables. However, this method is currently only based on theoretical simulation and has not been experimentally verified. Some foreign scholars have addressed the attenuation problem of reflected signals by using a Long Short-Term Memory (LSTM) regression network. This involves first training the normal waveform and then compensating for the attenuated waveform. However, this method cannot be used for online fault location.

[0004] In summary, most current methods for cable fault location suffer from limited detection ranges. While some researchers have addressed this issue, the following problems persist: 1. Increasing the detection range reduces accuracy. 2. Wide-range fault location cannot be performed online. Therefore, a method capable of enabling wide-range online cable fault diagnosis and location is urgently needed. Summary of the Invention

[0005] The main objective of this invention is to overcome the shortcomings and deficiencies of the prior art and to propose a method for wide-range online fault diagnosis and location of cables through frequency conversion technology.

[0006] To achieve the objectives of this invention, the following technical solution is specifically adopted:

[0007] A method for wide-range online fault diagnosis and location of cables using frequency conversion technology, characterized by comprising:

[0008] Select a spreading sequence for fault detection, determine the center frequency and length of the spreading code, and generate an m-sequence; determine the highest center frequency of the SSTDR detection signal based on the detection conditions, and generate the SSTDR detection signal through bpsk modulation;

[0009] Based on the actual object being detected, and considering the maximum detection length and the frequency attenuation pattern of the SSTDR detection signal, the minimum center frequency of the SSTDR detection signal is determined.

[0010] Adjust the center frequency of the spreading code to generate several SSTDR detection signals with center frequencies ranging from high to low, and determine the maximum detection length and detection blind zone corresponding to different frequencies based on the frequency attenuation law of the SSTDR detection signals.

[0011] A low-frequency sinusoidal signal is generated as a low-frequency fault diagnosis signal;

[0012] The low-frequency fault diagnosis signal is coupled into the cable under test and received to perform fault diagnosis on the cable.

[0013] If a fault is diagnosed, the SSTDR detection signals are coupled to the cable under test in descending order of center frequency to receive the reflected signals. The reflected signals are then compensated for based on the channel frequency attenuation law. The distance to the fault is determined by the reflected wavefront after relevant calculations until the fault location is found.

[0014] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0015] This invention is based on the SSTDR detection method and combined with frequency conversion technology to realize wide-range online fault diagnosis and location of cables. First, the cable fault is initially diagnosed by low-frequency detection signal. The SSTDR detection signal is coupled to the cable under test and the reflected signal is received. At the same time, the reflected signal is compensated for to address the signal attenuation problem. The fault distance is determined by the reflected wavefront after relevant calculations. The signal is gradually reduced from high frequency to low frequency for frequency conversion detection until the fault location is located.

[0016] The advantages of this invention are:

[0017] 1. Combining low-frequency diagnostics with high-frequency localization reduces operational steps and lowers testing costs;

[0018] 2. By adopting frequency conversion technology, a wide range of online cable fault diagnosis and location can be achieved, which solves the problem of short detection length in the current reflection method location technology.

[0019] 3. Attenuation compensation technology is adopted to solve the problem of large channel attenuation caused by the increase of detection distance, thereby improving the accuracy and precision of wide-range cable online diagnosis and positioning.

[0020] 4. It can be widely applied to cable fault location in different environments, such as power supply systems in aerospace, ships, mines, and tunnels. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a flowchart of the method for wide-range online fault diagnosis and location of cables using frequency conversion technology in Example 1;

[0023] Figure 2 The relationship between signal frequency and attenuation rate for 300-meter three-core cables of different lengths;

[0024] Figure 3 Equivalent circuit diagram of lumped parameters for the "short line";

[0025] Figure 4 Actual waveform diagram for preliminary fault diagnosis of 300 open circuit fault;

[0026] Figure 5 Equivalent circuit diagram for "long-line" distributed parameters;

[0027] Figure 6 The waveform diagram of the actual correlation operation of SSTDR before attenuation compensation;

[0028] Figure 7 The actual correlation waveform of SSTDR after attenuation compensation

[0029] Figure 8 Waveform diagram for locating a 300-meter open circuit fault using the frequency conversion method. Detailed Implementation

[0030] The present invention will now be clearly and completely described in conjunction with the embodiments and accompanying drawings.

[0031] Example 1

[0032] This embodiment provides a method for wide-range online fault diagnosis and location of cables using frequency conversion technology, such as... Figure 1 As shown, it includes the following steps:

[0033] S1: Select the spreading sequence for fault detection. Based on the bandwidth of the operating voltage of the cable under test, determine the minimum value of the initial value of the spreading code spectrum, thus obtaining the frequency f of the spreading code. Based on the length of the cable under test, determine the order n of the spreading code. The maximum distance measurement length of the nth order spreading code is... Where v is the signal transmission rate in the cable. After determining the center frequency and length of the spreading code, an m-sequence that meets the requirements is generated. Based on the highest clock frequency of the AD conversion, the minimum number of sampling points required for one sine wave cycle is determined, and the highest center frequency f1 of the SSTDR detection signal is obtained. Finally, the SSTDR detection signal is generated through bpsk modulation.

[0034] S2: Based on the actual object being detected, the signal frequency attenuation law is obtained through simulation or experimentation, and the minimum center frequency f of the SSTDR detection signal under the maximum detection length is estimated. n .

[0035] S2-1: Obtain the frequency attenuation law of the SSTDR detection signal through simulation or experiment;

[0036] The amplitude of the signal at the beginning of the cable is U1, and the amplitude of the voltage at the end of the cable is recorded as U2. α is defined as the attenuation rate of the voltage amplitude of the transmitted signal from the beginning to the end of the cable. The relationship between frequency and attenuation rate at different lengths was obtained through simulation or experimentation. Attenuation curves for different lengths were plotted, and the attenuation formula was fitted: α=a×f×e bL ×100%, where f is the center frequency of the detection signal, L is the cable length, and a and b are constants, obtained by fitting actual data.

[0037] S2-2: Determine the maximum detection length of the cable under test. Based on the attenuation curve, if α ≤ 30%, the signal attenuation is considered to be within the detectable range. Estimate the minimum center frequency f of the SSTDR detection signal at the maximum detection length. n ;

[0038] S3: Adjust the center frequency of the spreading code to generate several SSTDR detection signals with center frequencies ranging from high to low.

[0039] The frequency of the modulated sine wave is reduced by using the Direct Digital Synthesis (DDS) method, which doubles the readout step size each time, and the original signal frequency is divided into 2... n The proportion decreases, generating several sine waves with frequencies ranging from high to low, namely f1, f2, f3…f nAdjust the frequency of the m-sequence chip to be the same as the frequency of the modulating sine wave; after BPSK modulation, generate several SSTDR detection signals with the same center frequency as the frequency of the modulating sine wave, whose center frequencies are f1, f2, f3…f n .

[0040] S4: Determine the maximum detection length and detection blind zone for different frequencies based on the channel frequency attenuation law.

[0041] S4-1: Based on the attenuation curve, α ≤ 30% is considered to indicate that the signal attenuation is within the detectable range, with center frequencies of f1, f2, f3…f n The detection signal corresponds to a maximum detection length of l b1 l b2 l b3 …l bn ;

[0042] S4-2: The SSTDR detection waveform has side lobes. Due to the influence of these side lobes, the accuracy of cable fault detection and location is greatly reduced. Therefore, the cable length that can be measured within this side lobe period is defined as the minimum cable length that this method can detect, which is the measurement blind zone. It can be defined by the following formula: Where f is the signal center frequency, v is the wave velocity, l is the detection blind zone length, and the center frequencies are f1, f2, f3…f n The detection signal corresponds to a detection blind zone of l a1 l a2 l a3 …l an .

[0043] S4-3: Center frequencies are f1, f2, f3…f n The detection signal corresponds to the detection range as follows: l a1 ~l b1 l a2 ~l b2 l a3 ~l b3 …l an ~l bn .

[0044] S5: Generate a low-frequency sinusoidal signal as a low-frequency fault diagnosis signal, couple the low-frequency fault diagnosis signal to the cable under test and receive it, and perform fault diagnosis on the cable.

[0045] The length of the cable under test is typically tens to thousands of meters. When the wavelength of its carrier signal is much larger than the actual length of the cable, the cable loop can be equivalent to a lumped parameter circuit. In a lumped parameter circuit, the current is only related to time and is independent of its spatial distribution. When a fault point appears in the cable, the signal will not be reflected at the fault point, and the current in the line will change instantly.

[0046] To avoid the detection signal wavelength being similar to the actual length of the cable, the signal frequency is generally not more than 10kHz. At the same time, the detection signal needs to be different from the original signal in the cable. Taking all factors into consideration, the low-frequency detection signal frequency is determined to be f0, which is generally set to 5kHz-10kHz.

[0047] A low-frequency detection signal is injected into the detection circuit. When the cable is open-circuited, the equivalent resistance of the circuit approaches infinity, and the received signal amplitude decreases. When the cable is short-circuited, the equivalent resistance of the circuit is much lower than normal, and the received signal amplitude increases. Based on the magnitude of the received signal amplitude, a preliminary diagnosis can be made as to whether a cable fault has occurred.

[0048] S6: If a cable fault is diagnosed in S4, the SSTDR detection signal with a center frequency of f1 is coupled to the cable under test, the reflected signal is received, and the reflected signal is attenuated according to the channel frequency attenuation law. The fault distance is determined by the reflected wavefront after relevant calculations.

[0049] The length of the cable under test is typically tens to thousands of meters. When the wavelength of the carrier signal is equal to or less than the actual length of the cable, the cable loop can be considered equivalent to a distributed parameter circuit. This circuit consists of distributed resistance, distributed inductance, distributed conductance, and distributed capacitance. In a distributed parameter circuit, the current is related not only to time but also to its spatial distribution. When a fault point appears in the cable, the detection signal is reflected at the fault point. When an open-circuit fault occurs in the cable under test, the peaks of both the reflected and incident waves are positive; while when a short-circuit fault occurs, the peak of the incident wave is positive and the peak of the reflected wave is negative. By identifying the peak points of the incident and reflected waves, the time interval Δt between them can be measured. Combined with the signal propagation speed v in the cable, the fault distance can be calculated.

[0050] The SSTDR detection signal with a center frequency of f1 is coupled to the cable under test. If the fault point is identified and the fault location is located, the fault detection is completed; if the fault point is not identified, proceed to step S7.

[0051] S7: Perform frequency conversion detection, and gradually reduce the center frequency of the SSTDR detection signal. Based on the correspondence between the signal center frequency, the maximum detection length, and the detection blind zone in S4, start detection from the high-frequency signal f2. First, detect short-distance faults. If no fault point is found, reduce the signal center frequency to detect long-distance faults until the fault location is located.

[0052] Example 2

[0053] This example focuses on a 300m three-core cable with a working voltage of 220V three-phase AC. The fault point is set at 300m, and frequency conversion technology is used to detect and locate the cable fault.

[0054] A method for wide-range online fault diagnosis and location of cables using frequency conversion technology includes the following steps:

[0055] S1: After determining the center frequency and length of the spreading code, generate an m-sequence that meets the requirements. In this embodiment, the highest clock frequency of the AD conversion is 500MHz, and the minimum number of sampling points required for one sine wave cycle is 8. The highest center frequency of the SSTDR detection signal is f1 = 62.5MHz. Finally, the SSTDR detection signal is generated by bpsk modulation.

[0056] S2: Based on the actual detection object, the frequency attenuation law of the SSTDR detection signal is obtained through simulation or experimentation, and the minimum center frequency f of the detection signal under the maximum detection length is estimated. n .

[0057] S2-1: Obtain the frequency attenuation law of the SSTDR detection signal through simulation or experiment;

[0058] The amplitude of the signal at the beginning of the cable is U1, and the amplitude of the voltage at the end of the cable is recorded as U2. α is defined as the attenuation rate of the voltage amplitude of the transmitted signal from the beginning to the end of the cable. In this embodiment, simulation is used to first extract the channel parameters of the three-core cable through finite element analysis, build a channel model, and simulate the relationship between frequency and attenuation rate for cable lengths of 10m, 50m, 100m, 500m, and 1000m. Attenuation curves for different lengths are then plotted. Figure 2 As shown, an experimental attenuation rate test was conducted on a 300m cable. The relationship between frequency and attenuation rate over the 300m length was obtained, and the attenuation formula was fitted: α=a×f×e bL ×100%, based on experimental data, we can obtain a = 4.7 × 10⁻⁶. -7 b = 4.7 × 10 -3 .

[0059] S2-2: Determine the maximum detection length of the cable under test as 300m. Based on the attenuation curve, α≤30% is considered to be within the detectable range of signal attenuation. Estimate the minimum SSTDR detection signal center frequency f under the maximum detection length. n =488KHz;

[0060] S3: Adjust the center frequency of the spreading code to generate several SSTDR detection signals with center frequencies ranging from high to low. Reduce the modulation sine wave frequency using the Direct Digital Synthesis (DDS) method, doubling the reading step size each time, and adjusting the original signal frequency according to 2... nThe ratio is reduced, generating eight sine waves with frequencies ranging from high to low: 62.5MHz, 31.25MHz, 15.6MHz, 7.9MHz, 3.95MHz, 1.98MHz, 0.98MHz, and 488kHz. The frequency of the m-sequence chip is adjusted to be the same as the frequency of the modulated sine wave. After BPSK modulation, several SSTDR detection signals with the same center frequency as the modulated sine wave are generated: 62.5MHz, 31.25MHz, 15.6MHz, 7.9MHz, 3.95MHz, 1.98MHz, 0.98MHz, and 488kHz.

[0061] S4: Determine the maximum detection distance and detection blind zone corresponding to different frequencies based on the channel frequency attenuation law.

[0062] S4-1: Based on the attenuation curve, α≤30% is considered to be within the detectable range of signal attenuation. The maximum detection distances corresponding to SSTDR detection signals with center frequencies of 62.5MHz, 31.25MHz, ...488KHz are 5m, 10m, ...500m, respectively.

[0063] S4-2: The SSTDR detection waveform has side lobes. Due to the influence of these side lobes, the accuracy of cable fault detection and location is greatly reduced. Therefore, the cable length that can be measured within this side lobe period is defined as the minimum distance that this method can detect, which is the measurement blind zone. It can be defined by the following formula: Where f is the signal center frequency and v is the electromagnetic wave velocity in the cable, typically 2 × 10⁻⁶. 8 m / s, l is the detection blind zone length, and the detection signals with center frequencies of 62.5MHz, 31.25MHz, ... 488KHz correspond to detection blind zones of 1.6m, 3.2m ... 204m.

[0064] S4-3: The detection ranges corresponding to SSTDR detection signals with center frequencies of 62.5MHz, 31.25MHz, ... 488KHz are: 1.6m~5m, 3.2m~10m... 204m~500m.

[0065] S5: Generate a low-frequency sinusoidal signal as a low-frequency fault diagnosis signal, couple the low-frequency fault diagnosis signal to the cable under test and receive it, and perform fault diagnosis on the cable.

[0066] The length of the cable under test is typically from tens of meters to several kilometers. When the wavelength of its carrier signal is much larger than the actual length of the cable, the cable loop can be equivalent to a lumped parameter circuit, such as... Figure 3 As shown. In a lumped parameter circuit, the current is only related to time and not to its spatial distribution. When a fault occurs in the cable, the signal will not be reflected at the fault point, and the current in the line will change instantly.

[0067] To avoid the detection signal wavelength being equivalent to the actual cable length, the signal frequency should not exceed 10kHz. Simultaneously, the detection signal needs to be distinct from the existing signal in the cable. The operating signal is 50Hz AC, containing its higher harmonics; therefore, the detection signal must be higher than 1kHz. This embodiment experimentally tests a KVVP2-22 cable. The highest signal frequency is determined when the cable loop can be equivalent to a lumped parameter circuit. Considering channel attenuation, the low-frequency detection signal frequency is determined to be f0 = 8kHz.

[0068] An 8kHz low-frequency detection signal is injected into the detection circuit. When the cable is open-circuited, the equivalent resistance of the circuit approaches infinity, and the received signal amplitude decreases. When the cable is short-circuited, the equivalent resistance of the circuit is much lower than normal, and the received signal amplitude increases. Based on the magnitude of the received signal amplitude, a preliminary diagnosis of whether a cable fault has occurred is made. In this embodiment, an open-circuit fault is set at 300m, and the diagnostic waveform is as follows. Figure 4 As shown.

[0069] S6: If a cable fault is diagnosed in S4, considering the attenuation of 300m cable, the SSTDR detection signal with a center frequency of f5 = 3.95MHz is coupled to the cable under test. At this time, the detection range is 25m to 50m. The reflected signal is received, and the attenuation compensation of the reflected signal is performed according to the channel frequency attenuation law. The fault distance is determined by the reflected wavefront after relevant calculations.

[0070] The length of the cable under test is typically from tens of meters to several kilometers. When the wavelength of its carrier signal is equal to or less than the actual length of the cable, the cable loop can be equivalent to a distributed parameter circuit, such as... Figure 5 As shown, this circuit consists of distributed resistance, distributed inductance, distributed conductance, and distributed capacitance. In a distributed parameter circuit, the current is related not only to time but also to its spatial distribution. When a fault occurs in the cable, the detection signal is reflected at the fault point. When an open-circuit fault occurs in the cable under test, the peaks of both the reflected and incident waves are positive; while when a short-circuit fault occurs, the peak of the incident wave is positive and the peak of the reflected wave is negative. By identifying the peak points of the incident and reflected waves, the time interval Δt between them can be measured. Combined with the signal propagation speed v in the cable, the distance to the fault can be calculated. Actual related calculation waveforms are as follows Figure 6 As shown.

[0071] Due to attenuation issues, the reflected wavefront value is relatively small, affecting detection accuracy. This embodiment compensates for the attenuation of the reflected signal based on the channel frequency attenuation law. Specifically, it includes:

[0072] The reflected signal is subjected to a first correlation operation to separate the incident and reflected waves. The amplitude of the reflected wave is then compensated, allowing the reflected wave to... Where U0 is the amplitude of the reflected signal before compensation, and U1 is the amplitude of the reflected wave after compensation, a second correlation operation is performed on the compensated reflected wave to obtain the peak of the reflected wave. The time interval Δt between the peaks of the reflected wave and the incident wave is measured, and then combined with the signal propagation speed v in the cable, the fault distance is calculated again. The waveform of the correlation operation after compensation is as follows Figure 7 As shown.

[0073] The SSTDR detection signal with a center frequency of f5 = 3.95MHz is coupled to the cable under test. If the fault point is identified and the fault location is located, the fault detection is completed; if the fault point is not identified, proceed to step S7.

[0074] S7: Perform frequency conversion detection, successively decreasing the center frequency of the SSTDR detection signal, and repeat step S6. Based on the correspondence between the signal center frequency, maximum detection distance, and detection blind zone in S4, start detection from the high-frequency signal f6 = 1.98MHz. First, detect short-distance faults. If no fault point is found, then decrease the signal center frequency to detect long-distance faults until the fault location is located. In this embodiment, four center frequency SSTDR signals of 3.95MHz, 1.98MHz, 0.98MHz, and 488KHz are selected, and the fault distance is located to be 300.8 meters. Figure 8 As shown.

Claims

1. A method for wide-range online fault diagnosis and location of cables using frequency conversion technology, characterized in that, include: Select a spreading sequence for fault detection, determine the center frequency and length of the spreading code, and generate an m-sequence; The highest center frequency of the SSTDR detection signal is determined based on the detection conditions, and the SSTDR detection signal is generated by bpsk modulation. Based on the actual object being detected, and considering the maximum detection length and the frequency attenuation pattern of the SSTDR detection signal, the minimum center frequency of the SSTDR detection signal is determined. Adjust the center frequency of the spreading code to generate several SSTDR detection signals with center frequencies ranging from high to low, and determine the maximum detection length and detection blind zone corresponding to different frequencies based on the frequency attenuation law of the SSTDR detection signals. A low-frequency sinusoidal signal is generated as a low-frequency fault diagnosis signal; The low-frequency fault diagnosis signal is coupled into the cable under test and received to diagnose the cable fault. If a fault is diagnosed, the SSTDR detection signals are coupled to the cable under test in descending order of center frequency to receive the reflected signals. The reflected signals are then compensated for based on the channel frequency attenuation law. The distance to the fault is determined by the reflected wavefront after relevant calculations until the fault location is found.

2. The method for wide-range online fault diagnosis and location of cables using frequency conversion technology as described in claim 1, characterized in that, The step of determining the highest center frequency of the SSTDR detection signal based on the detection conditions includes: determining the minimum number of sampling points required for one sine wave cycle based on the highest clock frequency of the AD conversion, and calculating the highest center frequency of the SSTDR detection signal.

3. The method for wide-range online fault diagnosis and location of cables using frequency conversion technology as described in claim 1, characterized in that, The step of determining the minimum center frequency of the SSTDR detection signal based on the actual detection object, combined with the maximum detection length and the frequency attenuation law of the SSTDR detection signal, includes: obtaining the frequency attenuation law of the SSTDR detection signal through simulation or experiment based on the actual detection object, and estimating the minimum center frequency of the SSTDR detection signal under the maximum detection length.

4. The method for wide-range online fault diagnosis and location of cables using frequency conversion technology as described in claim 1, characterized in that, The adjustment of the spreading code center frequency to generate several SSTDR detection signals with center frequencies ranging from high to low includes: The frequency of the modulated sine wave is reduced by directly digitally synthesizing the signal, with the reading step size doubled each time, and the original signal frequency divided into 2... n The proportion decreases, generating several sine waves with frequencies ranging from high to low, namely f1, f2, f3…f n ; Adjust the frequency of the m-sequence chip to be the same as the frequency of the modulated sine wave; After BPSK modulation, several SSTDR detection signals with the same center frequency as the modulation sine wave are generated, and their center frequencies are f1, f2, f3…f n Where f1 is the highest center frequency of the SSTDR detection signal, f n This is the lowest center frequency of the SSTDR detection signal.

5. The method for wide-range online fault diagnosis and location of cables using frequency conversion technology as described in claim 1, characterized in that, The determination of the maximum detection length and detection blind zone corresponding to different frequencies based on the SSTDR detection signal frequency attenuation law includes: determining the center frequencies as f1, f2, f3…f based on the channel frequency attenuation law, with a maximum attenuation rate of 30%. n The detection signals correspond to the maximum detection distance l respectively b1 l b2 l b3 …l bn and center frequencies of f1, f2, f3…f n The detection signals correspond to the detection blind zones l a1 l a2 l a3 …l an Where f1 is the highest center frequency of the SSTDR detection signal, f n This is the lowest center frequency of the SSTDR detection signal.

6. The method for wide-range online fault diagnosis and location of cables using frequency conversion technology as described in claim 1, characterized in that, The process of coupling a low-frequency fault diagnosis signal to the cable under test and receiving it to diagnose cable faults includes: At the transmitting end, the low-frequency fault diagnosis signal is coupled to the cable under test through a coupling device; The fault diagnosis signal is coupled and received at the receiving end, and the signal is modulated. Compare the received signal status with the normal signal status of the cable. If the signal amplitude is higher than the normal status, the cable is short-circuited; if the signal amplitude is lower than the normal status, the cable is open-circuited.

7. The method for wide-range online fault diagnosis and location of cables using frequency conversion technology as described in claim 1, characterized in that, The SSTDR detection signals are coupled to the cable under test sequentially in descending order of center frequency. Reflected signals are received, and attenuation compensation is applied to the reflected signals according to the channel frequency attenuation law. The fault distance is determined by the reflected wavefront obtained from the relevant calculations, until the fault location is pinpointed. Specifically: Based on the correspondence between the signal center frequency, the maximum detection length, and the detection blind zone, detection starts from the highest frequency signal. Short-distance faults are detected first. If no fault point is found, the signal center frequency is lowered to detect long-distance faults until the fault location is detected.

8. The method for wide-range online fault diagnosis and location of cables using frequency conversion technology as described in claim 1 or 7, characterized in that, The attenuation compensation of the reflected signal according to the channel frequency attenuation law includes: Perform a correlation operation on the reflected signal; Separate the incident wave and the reflected wave in the reflected signal; Based on the frequency attenuation law, the amplitude of the separated reflected wave is compensated.

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