A method and device for detecting defects in a cable buffer layer

Abstract

The present invention discloses a method and device for detecting cable buffer layer defects. The method includes the following steps: applying high-frequency alternating voltages with different voltage frequencies to the cable core and collecting the generated current signals; judging whether there are buffer layer defects according to whether there are obvious partial discharge signals, and selecting the frequency corresponding to the signal with the largest partial discharge amount as the frequency used for measuring the position of the cable buffer layer defects; performing buffer layer defect positioning, recording the moment when the high-frequency alternating high voltage is applied, the moment when the partial discharge signal appears at the head end and the end moment, and calculating the position of the cable buffer layer defects; integrating multiple defect position data to obtain the cable defect position; arranging ultrasonic signal acquisition devices on both sides according to the cable buffer layer defect position, applying the high-frequency alternating voltage again, and accurately positioning the buffer layer defect position according to the collected signals. The present invention combines high-frequency current detection and ultrasonic detection to achieve accurate positioning of cable buffer layer defects.

Description

Technical Field

[0001] The present invention relates to a method and device for detecting cable buffer layer defects, belonging to the technical field of cable detection. Background Art

[0002] Under the circumstances of the rapid economic development and the accelerating urbanization process in China, the urban power grid has put forward higher and higher requirements for the power demand and power quality. In this context, XLPE insulated cables have gradually become one of the core power transmission equipment in the underground power transmission system due to their excellent electrical performance, large transmission capacity, convenient installation and maintenance, etc., and have posed more challenges to the operation and maintenance of cable lines. In recent years, as the operation time of cable lines has passed, cable equipment has gradually aged, and cases of cable body failures caused by ablation of the cable buffer layer have emerged one after another. Especially in cable lines with a voltage level of 110 kV and above, the failures caused by ablation of the cable buffer layer account for more than 40% of the total number of their failures, and the number of failures shows a significant increasing trend. Moreover, such cable defects have high latency and a large scope of action. At present, the methods for detecting cable buffer layer failures of XLPE insulated cables are not perfect enough and it is difficult to realize condition assessment. Once a failure occurs, it cannot be repaired locally and only the whole section of the cable can be replaced, which has brought serious impacts on the safe operation of the power grid.

[0003] At the present stage, regarding the research on the cable buffer layer, Deng Shenghua et al. simulated the laying environment conditions of power cables and tested the change of the volume resistivity of the buffer layer under natural moisture absorption of the cable. Huang Yu et al. explored the influence of different temperatures and pressures on the AC and DC resistances of the buffer layer. Wang Chuanbin tested the changes in the dielectric properties and thermal conductivity of the buffer layer. The above research on the electrical and thermodynamic properties such as the volume resistance (resistivity), dielectric constant and thermal conductivity of the cable buffer layer can qualitatively reflect the overall quality of the cable buffer layer, but cannot sensitively reflect the specific defects of the cable buffer layer and accurately identify the location of the defects.

[0004] At present, for the defect detection of the cable buffer layer, the methods can be divided into X-ray detection method and gas detection method. (1) The X-ray detection method mainly relies on the fact that the attenuation ability of the ray and the penetration thickness of each layer structure of the power cable are different, and images with different gray levels of performance will be generated on the negative film. Therefore, the internal structure of the cable can be non-destructively imaged by X-ray, and the defects of the cable buffer layer can be identified by analyzing the image features. However, this method is greatly affected by the detection environment, and it is difficult to distinguish the buffer layer burn point defects between the insulation layer and the shielding layer in the actual cable operation site. Moreover, in order to ensure the imaging quality, many parameters such as the transmission thickness, transmission angle, and sensitivity of the image quality agent need to be adjusted, and the operation difficulty and workload are relatively large, which cannot meet the detection requirements of the in-service cable. Therefore, this method is generally applied to the ex-factory inspection of cable production and manufacturing. (2) The gas detection method is a detection method based on chemical derivatives, which is widely used in the identification of power equipment defects. It analyzes the components and concentrations of the characteristic gases generated by XLPE cables through a gas chromatograph, and analyzes the cable state based on this to detect whether there are defects in the cable buffer layer. However, when different materials are used for the cable buffer layer, the composition of the generated gases is different. Therefore, for different specifications of cables, the characteristic gases need to be measured separately. And the concentration of the characteristic gas is also affected by factors such as temperature and detection distance, and the gas accumulation concentration is insufficient in the initial stage of the cable buffer layer generation.

[0005] The above methods are difficult to effectively identify the early defects of the buffer layer and accurately locate the problems, and cannot play the role of detecting potential hazards in the buffer layer. Summary of the Invention

[0006] In order to solve the above problems, the present invention proposes a cable buffer layer defect detection method and device, which can effectively identify the early defects of the buffer layer, accurately locate them, and detect potential hazards in the buffer layer.

[0007] The technical solution adopted by the present invention to solve its technical problems is as follows:

[0008] In the first aspect, a cable buffer layer defect detection method provided by an embodiment of the present invention includes the following steps:

[0009] Apply high-frequency alternating voltages with different voltage frequencies to the high-voltage cable core, and collect the high-frequency current signals generated by the grounded part of the metal armor layer of the high-voltage cable;

[0010] Detect whether there is a partial discharge signal with a partial discharge amount exceeding 50 pC in the collected current signals at different voltage frequencies. If so, there is a buffer layer defect, and select the frequency corresponding to the signal with the largest partial discharge amount among all signals as the frequency used to measure the position of the cable buffer layer defect;

[0011] Locate the defect of the cable buffer layer according to the frequency used to measure the defect position of the cable buffer layer, record the time t0 when the high-frequency alternating current high voltage is applied, the time t when the partial discharge signal appears at the head end s and the time t m at the end, and calculate the defect position l i of the cable buffer layer;

[0012] Change the frequency of the applied high-frequency alternating current voltage, repeat the above operation k times, and obtain the set V = [l1, l2, l3... l n of the cable defect positions, and integrate the above k defect position data to obtain the cable defect position l;

[0013] Apply the high-frequency alternating current voltage again, collect the ultrasonic signals on both sides of the detected defect position l i of the cable buffer layer, and accurately locate the defect position of the cable buffer layer according to the collected ultrasonic signals.

[0014] As a possible implementation of this embodiment, applying high-frequency alternating current voltages with different voltage frequencies to the high-voltage cable core includes: using the lowest applied frequency ω min as a reference, increasing step by step, and respectively selecting 2ω min , 3ω min ... nω min to apply high-frequency alternating current voltages to the high-voltage cable core in sequence, where n is a positive integer.

[0015] As a possible implementation of this embodiment, the lowest frequency ω min is preferably 10 kHz, n = 10, and the highest frequency is 100 kHz.

[0016] As a possible implementation of this embodiment, calculating the defect position l i of the cable buffer layer includes:

[0017] Calculate the relationship between the defect position l i of the cable buffer layer and the time t s at the head end of the cable:

[0018] l i = v(t s - t0)

[0019] where t0 represents the time when partial discharge occurs in the cable buffer layer, and v represents the propagation wave velocity of the partial discharge signal at the cable armor layer;

[0020] Calculate the relationship between the distance from the defect position of the cable buffer layer to the end of the cable and the time t m at the end of the cable:

[0021] L - l i = v(tm -t0)

[0022] Combining the above two relationships and eliminating the time t0 when the cable buffer layer defect occurs, the cable buffer layer defect position l is obtained. i ;

[0023]

[0024] Where L is the cable length, t m Indicates the time when the partial discharge is transmitted to the end of the cable, t s represents the moment at the beginning of the cable, and v represents the transmission wave velocity of the partial discharge signal at the cable armor layer.

[0025] As a possible implementation of this embodiment, the k defect position data are integrated to obtain the cable defect position l, including:

[0026] Calculate the average value of the defect location data With standard deviation S l :

[0027]

[0028]

[0029] By setting the confidence interval to 100(1-α)%, the data range ξ is determined:

[0030]

[0031]

[0032] Eliminate bad data, obtain data points that meet the data range and establish a new data set V', and calculate the average value of the data in the new data set V' as the final data result l.

[0033] As a possible implementation of this embodiment, the accurate positioning of the cable buffer layer defect position according to the collected ultrasonic signal includes:

[0034] Performing denoising on the collected ultrasonic signal;

[0035] Defects are accurately located based on the denoised ultrasonic signal.

[0036] As a possible implementation of this embodiment, the denoising process of the collected ultrasonic signal includes:

[0037] Taking ultrasonic sensor i as reference, D = τ j -τ i , the signal x detected by the two ultrasonic sensors i(t) and x j The relationship between (t) and the cross - correlation function are respectively as follows:

[0038]

[0039] where n i (t) and n j (t) represents the noise in the signals of two ultrasonic sensors, and s(t) represents the ultrasonic signal generated by partial discharge;

[0040]

[0041] where τ represents the signal time shift number, and the τ value corresponding to the peak of the cross - correlation function is the time difference of the two received signals to be found;

[0042] Transform the signal into the frequency domain, then perform weighted operation on its power spectrum, and finally transform it back to the time domain to detect the peak. The formula is:

[0043]

[0044] where P ij (ω) represents the cross - power spectrum of the two detection signals, and ψ ij (ω) represents the weighting component.

[0045] As a possible implementation of this embodiment, the ψ ij (ω) representing the weighting component includes:

[0046] ψ ij (ω) as the weighting component is determined by the SCOT weighting function:

[0047]

[0048] where represents the noise power spectrum of sensor i, represents the noise power spectrum of sensor j, and P ss (ω) represents the power spectrum of the ultrasonic signal source.

[0049] As a possible implementation of this embodiment, the precise defect location based on the denoised ultrasonic signal includes:

[0050] Calculate according to the time difference of the signals monitored by each ultrasonic sensor. The relationship between the time differences between each ultrasonic sensor and the 1st ultrasonic sensor when receiving signals is as follows:

[0051]

[0052] Convert the solution of the above equations into the solution of constrained optimization:

[0053]

[0054]

[0055] where (x, y, z) is the actual position of the cable buffer layer defect, and (x k , y k , z k ) is the position of the k-th ultrasonic sensor, and τ k1 is the time difference between the signals received by the k-th ultrasonic sensor and the 1st ultrasonic sensor.

[0056] In a second aspect, a cable buffer layer defect detection device provided by an embodiment of the present invention includes:

[0057] A signal acquisition module, configured to apply high-frequency alternating voltages with different voltage frequencies to the high-voltage cable core and acquire high-frequency current signals generated by the grounded part of the metal armor layer of the high-voltage cable;

[0058] A buffer layer defect judgment module, configured to detect whether there is a partial discharge signal with a partial discharge amount exceeding 50 pC in the acquired current signals at different voltage frequencies. If so, there is a buffer layer defect, and the frequency corresponding to the signal with the largest partial discharge amount among all signals is selected as the frequency used to measure the position of the cable buffer layer defect;

[0059] A defect position calculation module, configured to perform cable buffer layer defect positioning according to the frequency used to measure the position of the cable buffer layer defect, record the high-frequency AC high-voltage application time t0, the time t s when the partial discharge signal appears at the head end, and the time t m at the tail end, and calculate the cable buffer layer defect position l i ;

[0060] A defect position integration module, configured to change the frequency of the applied high-frequency alternating voltage, repeat the above operations k times, obtain a set V = [l1, l2, l3... l n of cable defect positions, and integrate the above k defect position data to obtain the cable defect position l;

[0061] A buffer layer defect positioning module, configured to apply a high-frequency alternating voltage again, collect ultrasonic signals on both sides of the detected cable buffer layer defect position l i , and accurately position the cable buffer layer defect position according to the collected ultrasonic signals.

[0062] In a third aspect, a cable buffer layer defect detection system provided by an embodiment of the present invention includes: a high-frequency high-voltage AC power supply device, a high-frequency current signal acquisition device, an ultrasonic signal acquisition device, an industrial control computer, and a terminal data cloud platform. The output end of the high-frequency high-voltage AC power supply device is connected to the core of the high-voltage cable to be tested. The signal acquisition end of the high-frequency current signal acquisition device is connected to the grounded part of the metal armor layer. The ultrasonic signal acquisition device is arranged on the high-voltage cable to be tested. The output ends of the high-frequency current signal acquisition device and the ultrasonic signal acquisition device are respectively connected to the signal input end of the industrial control computer. The control output end of the industrial control computer is respectively connected to the control ends of the high-frequency high-voltage AC power supply device, the high-frequency current signal acquisition device, and the ultrasonic signal acquisition device. The terminal data cloud platform is communicatively connected to the industrial control computer; the terminal data cloud platform includes the cable buffer layer defect detection device as described above.

[0063] As a possible implementation manner of this embodiment, the grounding method of the metal armor layer is to ground both ends simultaneously; the installation method of the high-frequency current signal acquisition device is to install it in phases. Considering the cross-interconnection situation of the cable line, it is ensured that the high-frequency current signal acquisition devices at both ends of the same phase are installed at both ends of the metal armor layer of the same-phase cable.

[0064] As a possible implementation manner of this embodiment, each high-frequency current signal acquisition device in a phase includes two electrical signal sensors, a data acquisition unit, and a data conversion unit. The two electrical signal sensors are respectively installed at the grounded parts of the metal armor layers at both ends of the same-phase cable, and are used to sense the high-frequency current signals generated by the partial discharge of the cable buffer layer. The electrical signal sensor is preferably a Rogowski coil, and the passband frequency range of the current transformer should include the change range of the pulse signal frequency and the partial discharge signal frequency.

[0065] The upper limit f1 and the lower limit f2 of the conduction frequency of the current sensor are determined by the mutual inductance M of the coil, the self-inductance L of the coil S 、the equivalent resistance R of the coil S 、the equivalent stray capacitance C of the coil S and R is the calibration of the integral resistance of the coil. The specific formula is as follows:

[0066]

[0067]

[0068] The conduction frequency of the current sensor is preferably 10 kHz to 1.2 MHz.

[0069] The described data acquisition unit is connected to the electrical signal sensor through a radio frequency coaxial cable, and is used to extract effective partial discharge signals from the signals collected by the electrical signal sensor through a filter circuit, mixing amplification, and high-speed sampling, and transmit the electrical signals to the data conversion unit. The described data conversion unit converts the electrical signals into optical signals and transmits the optical signals to the industrial control computer.

[0070] As a possible implementation of this embodiment, the industrial control computer receives the optical signals transmitted by the data conversion unit and uploads the optical signals to the terminal data cloud platform through TCP / IP to achieve synchronous online monitoring of the partial discharge signals at both ends of the cable.

[0071] As a possible implementation of this embodiment, the terminal data cloud platform saves the signals uploaded by the industrial control computer and extracts eigenvalue, and the extraction of signal characteristics includes the amplitude of the signal, the partial discharge amount of the partial discharge signal, and the moment when the signal wavefront reaches the cable grounding point.

[0072] To more accurately extract the moment when the signal wavefront reaches the cable grounding point, the signal wavefront is calibrated. The method for calibrating the signal wavefront is to use the moment of the maximum value of the modulus of the wavelet function w(ω) as the signal mutation moment, that is, the traveling wave front moment.

[0073] The method for obtaining the wavelet function w(ω) is as follows: First, perform Fourier transform on the continuous signal x(t) transmitted by the signal filtering part to obtain the mother function ψ(ω), and calculate the wavelet function w(ω) by calculating the continuous signal x(t) and the conjugate function ψ * (ω) of the mother function ψ(ω). The calculation formula is as follows:

[0074]

[0075] As a possible implementation of this embodiment, the high-frequency high-voltage AC power supply device includes a power supply part, an inverter part, a resonance part, and a boosting part. The power supply part is connected to the inverter part and can output a DC voltage to the inverter part, and the DC voltage can be adjusted steplessly. The high-frequency high-voltage AC power supply device adjusts the amplitude of the high-frequency AC high voltage finally applied to the cable core by adjusting the magnitude of the DC voltage transmitted by the power supply part. The inverter part is a full-bridge MOSFET inverter, which consists of four MOSFET switches and is synchronously driven and controlled by a square wave signal from the driver. It inversely converts the DC voltage transmitted by the power supply part into an AC voltage and then transmits it to the resonance part. The resonance part is an RLC resonance, which consists of a tunable capacitor, a tunable inductor, and a resistor connected in series, and controls the frequency by adjusting the parameters of the capacitor and the inductor, modulates the AC voltage into a high-frequency voltage, and transmits it to the boosting part.

[0076] The adjustable capacitor and the adjustable inductor are used to control the frequency of the final high-frequency AC voltage, and their relationship satisfies the following formula:

[0077]

[0078] For the high-frequency high-voltage AC power supply device, the adjustable range of the output high-frequency AC power supply frequency is preferably 10 kHz to 100 kHz.

[0079] The boosting part includes a high-voltage high-frequency high-power boosting transformer, which can boost the high-frequency voltage transmitted by the resonance part and finally transmit it to the cable core.

[0080] The adjustable capacitor is preferably a ceramic disc capacitor, and the iron core of the adjustable inductor is preferably an anti-saturation nanocrystalline magnetic core.

[0081] The iron core of the high-voltage high-frequency high-power boosting transformer adopts a nanocrystalline magnetic core, and the boosting ratio is not less than 1000 times.

[0082] As a possible implementation manner of this embodiment, the ultrasonic signal acquisition device includes an ultrasonic sensor array and an optical fiber connector; the ultrasonic sensor array is composed of four ultrasonic sensors, and the installation position of the ultrasonic sensors is preferably installed at the defect position l of the cable buffer layer i At horizontal distances of 15 m and 30 m.

[0083] The optical fiber connector can convert the optical signal transmitted by the ultrasonic sensor array into an electrical signal and transmit it to the industrial control computer.

[0084] The technical solution of the embodiment of the present invention can have the following beneficial effects:

[0085] The present invention uses the method of inducing partial discharge by high-frequency AC voltage to detect the defects of the cable buffer layer, which can identify the defects at the initial stage of the formation of the cable buffer layer defects, and has a preventive effect on the cable faults caused by the buffer layer defects. The present invention induces partial discharge in the cable buffer layer defects, locates the partial discharge signal, and calibrates the defect position of the cable buffer layer, which can effectively reduce the workload of power personnel for detection and troubleshooting. Compared with the traditional acquisition device, the high-frequency signal acquisition device adopted by the present invention has a smaller signal distortion rate and higher sensitivity to signal time acquisition. The present invention removes bad data through data screening, reduces the influence of bad data on the final measurement result, and can improve the accuracy of cable buffer layer defect location.

[0086] The present invention adopts the method of combining high-frequency current detection and ultrasonic detection to achieve precise positioning of cable buffer layer defects. Description of the Drawings

[0087] Figure 1It is a flowchart of a method for detecting defects in a cable buffer layer shown according to an exemplary embodiment;

[0088] Figure 2 It is a schematic diagram of a device for detecting defects in a cable buffer layer shown according to an exemplary embodiment;

[0089] Figure 3 It is a schematic diagram of the distribution of each layer inside the cable shown according to an exemplary embodiment;

[0090] Figure 4 It is a cable distribution parameter model shown according to an exemplary embodiment;

[0091] Figure 5 It is a schematic diagram of an equivalent geometric model of a cable buffer layer defect shown according to an exemplary embodiment;

[0092] Figure 6 It is a radial electric field distribution diagram of a cable with a defective buffer layer at 50 Hz shown according to an exemplary embodiment;

[0093] Figure 7 It is a radial electric field distribution diagram of a cable with a defective buffer layer at 100 kHz shown according to an exemplary embodiment;

[0094] Figure 8 It is a connection diagram of a pulse generating device and a high-frequency signal acquisition device shown according to an exemplary embodiment;

[0095] Figure 9 It is an equivalent current diagram of a current transformer shown according to an exemplary embodiment;

[0096] Figure 10 It is a schematic diagram of the cross-connected situation of a cable shown according to an exemplary embodiment. Detailed implementation manners

[0097] The present invention will be further described below in conjunction with the drawings and embodiments:

[0098] To clearly illustrate the technical features of the present solution, the present invention will be elaborated in detail below through specific implementation manners and in conjunction with its drawings. The following disclosure provides many different embodiments or examples for implementing different structures of the present invention. To simplify the disclosure of the present invention, the components and settings of specific examples are described below. In addition, the present invention may repeat reference numerals and / or letters in different examples. This repetition is for the purpose of simplification and clarity, and does not itself indicate the relationship between the various embodiments and / or settings discussed. It should be noted that the components illustrated in the drawings are not necessarily drawn to scale. The present invention omits the description of well-known components and processing techniques and processes to avoid unnecessarily limiting the present invention.

[0099] As shown Figure 1 A method for detecting defects in a cable buffer layer provided by an embodiment of the present invention includes the following steps:

[0100] Apply high-frequency alternating voltages with different voltage frequencies to the core of the high-voltage cable, and collect the high-frequency current signals generated at the grounded part of the metal armor layer of the high-voltage cable;

[0101] Detect whether there is a partial discharge signal with a partial discharge amount exceeding 50 pC in the collected current signals at different voltage frequencies. If so, there is a buffer layer defect, and select the frequency corresponding to the signal with the largest partial discharge amount among all signals as the frequency used to measure the defect position of the cable buffer layer;

[0102] Locate the defect of the cable buffer layer according to the frequency used to measure the defect position of the cable buffer layer, record the application time t0 of the high-frequency alternating high voltage, the time t s when the partial discharge signal appears at the head end, and the time t m at the end, and calculate the defect position l i of the cable buffer layer;

[0103] Change the frequency of the applied high-frequency alternating voltage, repeat the above operations k times, obtain the set V = [l1, l2, l3... l n of cable defect positions, and integrate the above k defect position data to obtain the cable defect position l;

[0104] Apply the high-frequency alternating voltage again, collect the ultrasonic signals on both sides of the detected defect position l i of the cable buffer layer, and accurately locate the defect position of the cable buffer layer according to the collected ultrasonic signals.

[0105] As a possible implementation manner of this embodiment, applying high-frequency alternating voltages with different voltage frequencies to the core of the high-voltage cable includes: taking the lowest applied frequency ω min as a reference, increasing step by step, and respectively selecting 2ω min , 3ω min ... nω min to apply high-frequency alternating voltages to the core of the high-voltage cable in sequence, where n is a positive integer.

[0106] As a possible implementation manner of this embodiment, the lowest frequency ω min is preferably 10 kHz, n = 10, and the highest frequency is 100 kHz.

[0107] As a possible implementation manner of this embodiment, calculating the defect position l i of the cable buffer layer includes:

[0108] Calculate the defect position of the cable buffer layer as l iAt the beginning of the cable, time t s Relationship:

[0109] l i =v(t s -t0)

[0110] Among them, t0 represents the moment when partial discharge occurs due to the cable buffer layer defect, and v represents the transmission wave velocity of the partial discharge signal at the cable armor layer;

[0111] Calculate the distance from the cable buffer layer defect position to the cable end and the time t at the cable end m Relationship:

[0112] L i =v(t m -t0)

[0113] Combining the above two relationships and eliminating the time t0 when the cable buffer layer defect occurs, the cable buffer layer defect position l is obtained. i ;

[0114]

[0115] Where L is the cable length, t m Indicates the time when the partial discharge is transmitted to the end of the cable, t s represents the moment at the beginning of the cable, and v represents the transmission wave velocity of the partial discharge signal at the cable armor layer.

[0116] As a possible implementation of this embodiment, the k defect position data are integrated to obtain the cable defect position l, including:

[0117] Calculate the average value of the defect location data With standard deviation S l :

[0118]

[0119]

[0120] By setting the confidence interval to 100(1-α)%, the data range ξ is determined:

[0121] Z~N(0,1)

[0122]

[0123] Eliminate bad data, obtain data points that meet the data range and establish a new data set V', and calculate the average value of the data in the new data set V' as the final data result l.

[0124] As a possible implementation of this embodiment, the precise positioning of the defect location of the cable buffer layer according to the collected ultrasonic signals includes:

[0125] Denoise the collected ultrasonic signals;

[0126] Perform precise defect positioning according to the denoised ultrasonic signals.

[0127] As a possible implementation of this embodiment, the denoising of the collected ultrasonic signals includes:

[0128] Taking ultrasonic sensor i as a reference, D = τ j -τ i The relationship and cross-correlation function between the signals x i (t) and x j (t) detected by the two ultrasonic sensors are respectively:

[0129]

[0130] where n i (t) and n j (t) represent the noise in the signals of the two ultrasonic sensors, and s(t) represents the ultrasonic signal generated by partial discharge;

[0131]

[0132] where τ represents the signal time shift number, and the τ value corresponding to the peak of the cross-correlation function is the time difference of the two received signals to be obtained;

[0133] Transform the signal to the frequency domain, then perform power spectrum weighting operation on it, and finally transform it back to the time domain to detect the peak value. The formula is:

[0134]

[0135] where P ij (ω) represents the cross-power spectrum of the two detected signals, and ψ ij (ω) represents the weighting component.

[0136] As a possible implementation of this embodiment, the ψ ij (ω) representing the weighting component includes:

[0137] ψ ij (ω) as the weighting component is determined by the SCOT weighting function:

[0138]

[0139] where represents the noise power spectrum of sensor i, denotes the noise power spectrum of sensor j, and P ss (ω) denotes the power spectrum of the ultrasonic signal source.

[0140] As a possible implementation of this embodiment, the accurate defect positioning based on the denoised ultrasonic signal includes:

[0141] Calculating according to the time difference of the signals monitored by each ultrasonic sensor. The relationship between the time differences of each ultrasonic sensor and the 1st ultrasonic sensor receiving the signal is as follows:

[0142]

[0143] Converting the solution of the above equations into the solution of constrained optimization:

[0144]

[0145]

[0146] where (x, y, z) is the actual position of the cable buffer layer defect, and (x k , y k , z k ) is the position of the kth ultrasonic sensor, and τ k1 is the time difference between the kth ultrasonic sensor and the 1st ultrasonic sensor receiving the signal.

[0147] Thereby optimizing the final ultrasonic positioning result and improving the positioning accuracy.

[0148] As Figure 2 shown, corresponding to the above method, an embodiment of the present invention also provides a cable buffer layer defect detection device, including:

[0149] A signal acquisition module, configured to apply high-frequency alternating voltages with different voltage frequencies to the high-voltage cable core and acquire high-frequency current signals generated by the grounded part of the metal armor layer of the high-voltage cable;

[0150] A buffer layer defect judgment module, configured to detect whether there is a partial discharge signal with a partial discharge amount exceeding 50 pC in the acquired current signals at different voltage frequencies. If so, there is a buffer layer defect, and select the frequency corresponding to the signal with the largest partial discharge amount among all signals as the frequency used to measure the position of the cable buffer layer defect;

[0151] A defect position calculation module, configured to perform cable buffer layer defect positioning according to the frequency used to measure the position of the cable buffer layer defect, record the application time t0 of the high-frequency alternating high voltage, the appearance time t s at the head end of the partial discharge signal, and the time t m at the end, and calculate the cable buffer layer defect position l i ;

[0152] The defect location integration module is used to change the frequency of the applied high-frequency alternating voltage, repeat the above operation k times, and obtain the set V = [l1, l2, l3…l n , and integrate the above k defect location data to obtain the cable defect location l;

[0153] The buffer layer defect location module is used to apply the high-frequency alternating voltage again, collect the ultrasonic signals on both sides of the detected cable buffer layer defect location l i and accurately locate the cable buffer layer defect location according to the collected ultrasonic signals.

[0154] Based on the above cable buffer layer defect detection device, an embodiment of the present invention further provides a cable buffer layer defect detection system, including: a high-frequency high-voltage AC power supply device, a high-frequency current signal acquisition device, an ultrasonic signal acquisition device, an industrial control computer, and a terminal data cloud platform. The output end of the high-frequency high-voltage AC power supply device is connected to the core of the high-voltage cable to be measured. The signal acquisition end of the high-frequency current signal acquisition device is connected to the grounded part of the metal armor layer. The ultrasonic signal acquisition device is arranged on the high-voltage cable to be measured. The output ends of the high-frequency current signal acquisition device and the ultrasonic signal acquisition device are respectively connected to the signal input ends of the industrial control computer. The control output end of the industrial control computer is respectively connected to the control ends of the high-frequency high-voltage AC power supply device, the high-frequency current signal acquisition device, and the ultrasonic signal acquisition device. The terminal data cloud platform is communicatively connected to the industrial control computer; the terminal data cloud platform includes the cable buffer layer defect detection device as described above.

[0155] The internal structure of the cable is as Figure 3 shown, from the inside to the outside are the conductor, insulation layer, insulation shield, buffer layer, aluminum sheath, and external sheath respectively.

[0156] According to the cable structure, a distributed parameter model of the cable is established, as Figure 4 shown, where C1 is the equivalent radial capacitance of the insulation layer and the insulation shield layer, R1 is the equivalent axial resistance of the insulation shield layer, R2 is the equivalent axial resistance of the cable buffer layer. If there is a defect in the cable buffer layer, it will be reflected in the distributed parameter model as an equivalent radial capacitance C2 in series with R2.

[0157] The axial resistance is calculated by the following formula:

[0158]

[0159] The radial resistance is calculated by the following formula:

[0160]

[0161] where ρ is the resistivity, L is the cable length, r b is the outer boundary diameter, r a is the inner boundary diameter.

[0162] The insulating layer can be regarded as a cylindrical capacitor. According to the calculation formula of the cylindrical capacitor, ignoring the influence of the edge effect, its calculation formula is as follows, where ε0 is the absolute permittivity, ε1 is the relative permittivity, L is the cable length, D b is the outer boundary diameter, D a is the inner boundary diameter:

[0163]

[0164] The schematic diagram of the equivalent geometric model of the defect in the cable buffer layer is as shown in Figure 5 the figure. The inner circle radius can be replaced by (r1 + δ), and the calculation formula of its equivalent parameters is:

[0165]

[0166] where θ is the central angle corresponding to the contact surface; ε r is the relative permittivity of the white powder; ε0 is the absolute permittivity; W is the width of the contact surface.

[0167] According to the derivation based on the above distributed parameter model, the induced voltage U at the center of the defect area of the cable buffer layer j satisfies the following formula conditions, where n is the number of defects in the cable buffer layer:

[0168]

[0169] It can be seen from the above formula that the induced voltage at the center of the defect area of the cable buffer layer is related to the voltage frequency applied to the cable core. When a high-frequency voltage is used, the amplitude of the induced voltage is similar to the amplitude of the applied high-frequency voltage. Therefore, it can induce partial discharge in the cable buffer layer defect, and thus through the detection of the partial discharge signal, the prevention and investigation of the cable buffer layer defect can be realized.

[0170] Subsequently, a simulation study was carried out on the cable buffer layer defect, and the results also demonstrated that compared with the 50Hz working voltage under the cable operating conditions, the high-frequency alternating voltage is more likely to cause partial discharge in the cable buffer layer defect. Assuming that there is a slight poor contact in the cable buffer layer, that is, there is a 0.1mm air gap between the cable buffer layer and the cable metal armor layer, an alternating voltage with a frequency of 50Hz and 100kHz is applied to the cable core respectively, and the voltage amplitude is 100kV. The radial electric field distribution of the cable under the 50Hz alternating voltage is as shown in Figure 6 the figure, as shown by Figure 6It can be seen that a tiny electric field intensity appears at the air gap of the cable buffer layer, with a peak value of approximately 0.042 kV / cm. Local discharge will not be triggered under this electric field. When an AC high voltage of 100 kHz is applied to the cable core, with the same high voltage amplitude of 100 kV, the radial electric field distribution of the cable in this case is as Figure 7 shown. It can be seen from Figure 7 that a peak of electric field intensity appears at the air gap position of the cable buffer layer. The peak value of this electric field is approximately 32 kV / cm, which is sufficient to break down the buffer layer defect and cause local discharge.

[0171] Based on the above principle, the process of detecting cable buffer layer defects using the cable buffer layer defect detection system of the present invention is as follows:

[0172] First, install a high-frequency high-voltage AC power supply device on the core of the high-voltage cable, and install a high-frequency current signal acquisition device on the grounded part of the metal armor layer, as Figure 8 shown.

[0173] Among them, the frequency adjustable range of the output high-frequency AC power supply of the high-frequency high-voltage AC power supply device is preferably from 10 kHz to 100 kHz. This device includes a power supply part, an inverter part, a resonance part, and a boost part.

[0174] The said power supply part is connected to the inverter part and can output a DC voltage to the inverter part. Moreover, the DC voltage can be adjusted steplessly, and the amplitude of the high-frequency AC high voltage finally applied to the cable core can be adjusted by adjusting the magnitude of the DC voltage transmitted by the power supply part.

[0175] The inverter part is a full-bridge MOSFET inverter, which consists of four MOSFET switches and is synchronously driven and controlled by a square wave signal from a driver. It inverses the DC voltage transmitted by the power supply part into an AC voltage and then transmits it to the resonance part.

[0176] The resonance part is an RLC resonance, which consists of a tunable capacitor, a tunable inductor, and a resistor connected in series. Moreover, the tunable capacitor is preferably a ceramic disc capacitor, and the iron core of the tunable inductor is preferably an anti-saturation nanocrystalline magnetic core. The frequency can be controlled by adjusting the parameters of the capacitor and the inductor, modulating the AC voltage into a high-frequency voltage, and transmitting it to the boost part. The relationship between the tunable capacitor and the tunable inductor and the frequency of the finally output high-frequency AC voltage satisfies the following formula:

[0177]

[0178] The boost part includes a high-voltage high-frequency high-power boost transformer, which can raise the high-frequency voltage transmitted by the resonance part and finally transmit it to the cable core. The iron core of this high-voltage high-frequency high-power boost transformer adopts a nanocrystalline magnetic core, and the boost ratio is not less than 1000 times.

[0179] A high-frequency signal acquisition device installed on the grounding part of the cable metal armor layer includes a signal induction part, a signal filtering part, a signal storage and extraction part, and finally a signal analysis part. Among them, the signal induction part is connected to the cable line, measures the pulse signal in the cable and transmits the pulse signal to the signal storage and extraction part. Among them, the signal induction part is connected to the cable line, measures the pulse signal in the cable and transmits the pulse signal to the signal storage and extraction part. It is composed of a Rogowski coil, and the passband frequency range of this signal induction part should include the change range of the pulse signal frequency and the partial discharge signal frequency, and its conduction frequency is preferably 10 kHz to 1.2 MHz. The equivalent circuit of this part is as shown in Figure 9 shown, and its upper conduction frequency limit f1 and lower limit f2 are determined by the mutual inductance M of the coil, the self-inductance L of the coil S , the equivalent resistance R of the coil S , the equivalent stray capacitance C of the coil S and R is the calibration of the coil integration resistance. The specific formula is as follows:

[0180]

[0181]

[0182] The signal filtering part receives the signal measured by the signal induction part, then processes the signal, filters out the electromagnetic interference and background noise in the signal, and transmits the processed signal to the signal storage and extraction part. The main means of the said signal filtering part is to add anti-corona covers at both ends of the cable and use band-pass filters and notch filters. The signal storage and extraction part receives the signal output by the signal filtering part, stores the processed signal, extracts the features in the signal, and conveys the extracted signal information to the signal transmission part.

[0183] The extraction of signal features by the signal storage and extraction part includes the amplitude of the signal, the partial discharge amount of the partial discharge signal, and the wavefront of the calibration signal. Among them, the method of calibrating the wavefront of the signal is to use the maximum value moment of the modulus of the wavelet function w(ω) as the signal mutation moment, that is, the traveling wave wavefront moment. The method of obtaining this wavelet function w(ω) is as follows: First, perform Fourier transform on the continuous signal x(t) transmitted by the signal filtering part to obtain the mother function ψ(ω), and calculate the continuous signal x(t) and the conjugate function ψ of the mother function ψ(ω) * (ω) to obtain the wavelet function w(ω). The calculation formula is as follows:

[0184]

[0185] The signal analysis part is connected to the signal storage and extraction part, calculates and analyzes the signal information transmitted by the signal storage and extraction part, and is used to identify cable defects and locate the defect positions.

[0186] The high-frequency signal acquisition device is installed at the grounding part of the cable metal armor layer, and generally the metal armor layer adopts a grounding method of grounding at both ends simultaneously. And the installation method of the high-frequency current signal acquisition device is phase-by-phase installation, and the cross-interconnection situation of the cable line is considered, such as Figure 10 shown, and it is ensured that the high-frequency current signal acquisition devices at both ends of the same phase are installed at both ends of the metal armor layer of the same-phase cable.

[0187] After the device is installed, start the high-frequency high-voltage AC power supply device, apply a high-frequency AC voltage to the cable core, and the applied voltage amplitude U m is related to the rated voltage U N of the cable, and considering the voltage fluctuation in the power system after the cable is put into operation, so the voltage amplitude U m is 1.1 times the rated voltage U N of the cable.

[0188] Subsequently, check whether obvious partial discharge signals are detected in the high-frequency signal acquisition device: if obvious partial discharge signals exist, then the cable buffer layer does not undergo partial discharge at this frequency; if no obvious partial discharge signals exist, then the cable buffer layer does not undergo partial discharge at this frequency. The obvious partial discharge signal refers to a partial discharge signal with a partial discharge amount exceeding 50 pC.

[0189] If there are defects in the cable buffer layer, then change the frequency of the voltage applied by the high-frequency high-voltage AC power supply device, repeatedly apply high-frequency AC high voltage, and detect partial discharge signals.

[0190] Regarding the above frequency change, based on the lowest frequency ω min applied in the test, increase it step by step, and respectively select 2ω min , 3ω min ... nω min to conduct tests in sequence. And the lowest frequency ω min is preferably 10 kHz, and the maximum applied frequency multiple is preferably 10 times, that is, the highest frequency ω max is 100 kHz.

[0191] If no obvious partial discharge signals are detected in the high-frequency signal acquisition device at all frequencies, then there are no buffer layer defects in this cable; if obvious partial discharge signals exist in the high-frequency signal acquisition device at the applied frequencies, then there are buffer layer defects in this cable, and select the frequency corresponding to the signal with the most obvious partial discharge among all signals as the frequency used to measure the defect position of the cable buffer layer, where the signal with the largest partial discharge amount is used as the signal with the most obvious partial discharge.

[0192] The time t at which the partial discharge signal collected by the high-frequency signal acquisition device is transmitted to the end of the cable m , Cable head end time t s , the total cable length L and the transmission wave velocity v of the partial discharge signal at the cable armor layer are calculated.

[0193] Viewed from the beginning of the cable, the defect position of the cable buffer layer is l i , and the distance from the cable buffer layer defect to the cable end is Ll i Assuming that the moment when partial discharge occurs due to the cable buffer layer defect is t0,

[0194] Therefore, the defect position of the cable buffer layer is l i At the beginning of the cable, time t s The relationship is:

[0195] l i =v(t s -t0)

[0196] The distance from the cable buffer layer defect to the cable end and the time t at the cable end m The relationship is:

[0197] L i =v(t m -t0)

[0198] Combining the two and eliminating the time t0 when the cable buffer layer defect occurs, the cable buffer layer defect position l can be obtained. i The time t when the partial discharge is transmitted to the end of the cable m and the time t at the beginning of the cable s The relationship is as follows:

[0199]

[0200] Then repeat the above cable buffer layer defect location test k times to obtain the set of cable defect locations V = [l1,l2,l3…l n ] and integrate the above k defect position data to obtain the cable defect position l, where the number of repetitions k is not less than 20 times.

[0201] When integrating data, the data range is determined by setting a confidence interval, and the confidence interval is not less than 80%, so as to eliminate bad data, obtain data points that meet the data range and establish a new data set V', and then calculate the average value of the data in the new data set V' as the final data result l. The specific method of determining the data range is to first calculate the average value of the above data With standard deviation S l , and set the confidence interval to 100(1-α)%, and get the range of the data ξ. The specific formula is as follows:

[0202]

[0203]

[0204] Z~N(0,1)

[0205]

[0206] After obtaining the position l of the cable buffer layer defect i Based on the position l i Ultrasonic signal acquisition devices are evenly arranged on both sides of it. Then, the high-frequency high-voltage AC power supply device is started again, and based on the signals measured by the ultrasonic signal acquisition devices, the position of the cable buffer layer defect is accurately located.

[0207] The arranged ultrasonic signal acquisition devices include an ultrasonic sensor array and an optical fiber connector.

[0208] The number of ultrasonic sensors in the ultrasonic sensor array is not less than 10, and they are connected to the conductive optical fiber to transmit the signals to the optical fiber connector. The optical fiber connector is connected to the industrial control computer, which can convert the optical signals transmitted by the ultrasonic sensor array into electrical signals and transmit them to the noise reduction processing part of the terminal data cloud platform. The noise reduction processing part removes the background noise in the electrical signals transmitted by the optical fiber connector and transmits the processed signals to the defect location part.

[0209] The noise reduction processing part performs noise reduction based on the generalized cross-correlation method. Its principle is to assume that the ultrasonic signal generated by partial discharge is s(t), and the signals detected by two ultrasonic sensors are x i (t) and x j (t), and their mutual relationship is:

[0210]

[0211] where n i (t) and n j (t) are the noises in the signals of the two ultrasonic sensors. Taking the i sensor as the reference, let D = τ j -τ i , then the above relational expression can be changed to:

[0212]

[0213] The cross-correlation function of the signals received by the two sensors is expressed as:

[0214]

[0215] where τ is the signal time shift number, and the τ value corresponding to the peak of the cross-correlation function is the time difference of the two received signals to be found.

[0216] First, it transforms the signal into the frequency domain, then performs weighted operation on its power spectrum, and finally transforms it back to the time domain to detect the peak. The formula is as follows:

[0217]

[0218] Where P ij (ω) is the cross-power spectrum of the two detection signals, and ψ ij (ω) is the weighting component determined by the SCOT weighting function. The formula is as follows:

[0219]

[0220] Where is the noise power spectrum of sensor i, is the noise power spectrum of sensor j, and P ss (ω) is the power spectrum of the ultrasonic signal source.

[0221] Finally, for the defect location part, the time difference location method is used to accurately locate the defects in the cable buffer layer. The time difference location method calculates based on the time difference of the signals monitored by each ultrasonic sensor. Among them, (x, y, z) is the actual position of the defect in the cable buffer layer, and (x k , y k , z k ) is the installation position of the k-th sensor, T is the reference sensor, that is, the sensor numbered 1, which is the time required to receive the signal, and τ k1 is the time difference between the k-th sensor and the 1st sensor when receiving the signal. Their relationship is as follows:

[0222]

[0223] Convert the above equations into a hyperboloid equation system, that is:

[0224]

[0225] And since when the number of ultrasonic sensors exceeds 4, the above equation system is a redundant equation, so the solution of the above equation system needs to be converted into a constrained optimization solution, that is:

[0226]

[0227]

[0228] Thus, the final ultrasonic location result is optimized and the location accuracy is improved.

[0229] The present invention combines high-frequency current detection and ultrasonic detection to achieve accurate location of defects in the cable buffer layer.

[0230] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: modifications or equivalent substitutions can still be made to the specific embodiments of the present invention, and any modification or equivalent substitution that does not depart from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A method for detecting cable buffer layer defects, characterized in that, It includes the following steps: Apply high-frequency alternating voltages with different voltage frequencies to the core of the high-voltage cable, and collect the high-frequency current signals generated at the grounded part of the metal armor layer of the high-voltage cable; Detect whether there is a partial discharge signal with a partial discharge amount exceeding 50 pC in the collected current signals at different voltage frequencies. If so, there is a buffer layer defect, and select the frequency corresponding to the signal with the largest partial discharge amount among all signals as the frequency used to measure the position of the buffer layer defect of the cable; Locate the defect of the cable buffer layer according to the frequency used to measure the defect position of the cable buffer layer, and record the moment when the high-frequency AC high voltage is applied t 0. The moment when the partial discharge signal appears at the head end t s and the moment at the end t m , and calculate the defect position of the cable buffer layer l i ; Change the frequency of the applied high-frequency alternating voltage and repeat k times the above operations to obtain a set of cable defect positions V = , and integrate the above k defect position data to obtain the cable defect position l ; Apply the high-frequency alternating voltage again to the detected defect location of the cable buffer layer l i Collect the ultrasonic signals on both sides, and accurately locate the defect location of the cable buffer layer based on the collected ultrasonic signals; Calculating the defect position of the cable buffer layer l i , including: Calculating the defect position of the cable buffer layer is l i The relationship with the cable head end time t s is: Among them, t 0 represents the moment when partial discharge occurs in the cable buffer layer defect, v represents the propagation wave velocity of the partial discharge signal at the cable armor layer; Calculate the relationship between the distance from the defect position of the cable buffer layer to the cable end and the time of the cable end t m as follows: Combining the above two relationships and eliminating the time t0 when the cable buffer layer defect occurs, the cable buffer layer defect position is obtained. l i ; where L represents the cable length, t m represents the moment when the partial discharge reaches the cable end, t s represents the moment at the cable head, v represents the propagation wave velocity of the partial discharge signal at the cable armor layer; Integrating the above-mentioned k defect location data to obtain the cable defect location l , including: Obtain the average value of the defect position data and the standard deviation S l : By setting a confidence interval of 100(1 - α ), the data range is determined ξ : Eliminate bad data, obtain data points that meet the data range and establish a new data set V ’ , and obtain the new data set V ’ The average value of the data in is used as the final data result l .

2. The method for detecting cable buffer layer defects according to claim 1, characterized in that Applying a high-frequency alternating voltage with different voltage frequencies to the core of a high-voltage cable includes: taking the lowest frequency applied ω min as a reference, increasing step by step, and respectively selecting 2 ω min , 3 ω min ... n ω min to successively apply a high-frequency alternating voltage to the core of the high-voltage cable, where n is a positive integer.

3. The method for detecting cable buffer layer defects according to claim 1, wherein The precise positioning of the cable buffer layer defect according to the collected ultrasonic signals includes: Denoise the collected ultrasonic signals; Perform precise defect positioning according to the denoised ultrasonic signals.

4. The method for detecting cable buffer layer defects according to claim 3, wherein The denoising process of the collected ultrasonic signals includes: Taking the ultrasonic sensor i as a reference, , the signals x i detected by the two ultrasonic sensors x j and the mutual relationship and cross-correlation function between \(s_{i}(t)\) and \(s_{j}(t)\) are respectively: Among them n i (t) and n j (t) represent the noise in the two ultrasonic sensor signals, and s(t) represents the ultrasonic signal generated by partial discharge; where τ represents the signal time shift number, and the value corresponding to the peak of the cross-correlation function is the time difference between the two received signals to be obtained; τ ​ Transform the signal into the frequency domain, then perform weighted operation on its power spectrum, and finally transform it back to the time domain to detect the peak value. The formula is: wherein represents the cross-power spectrum of two detection signals, represents the weighted component.

5. A method for detecting cable buffer layer defects according to claim 4, characterized in that, The represents a weighted component and includes: The weighted component is determined by the SCOT weighting function: wherein represents the noise power spectrum of sensor i, represents the noise power spectrum of sensor j, represents the power spectrum of the ultrasonic signal source.

6. A method for detecting defects in a cable buffer layer according to claim 3, characterized in that, The precise defect positioning according to the denoised ultrasonic signals includes: Calculate according to the time differences of the signals monitored by each ultrasonic sensor. The relationship between the time differences of each ultrasonic sensor and the 1st ultrasonic sensor receiving the signal is as follows: Convert the solution of the above equations into the solution of constrained optimization: Among them, ( x, y, z ), is the actual position of the cable buffer layer defect, ( x k ,y k ,z k ), is the position of the k th ultrasonic sensor, τ k1 is the time difference between the signals received by the k th ultrasonic sensor and the 1st ultrasonic sensor.

7. A cable buffer layer defect detection device, characterized in that, It includes: A signal acquisition module, which is used to apply high-frequency alternating voltages with different voltage frequencies to the core of the high-voltage cable and collect the high-frequency current signals generated at the grounded part of the metal armor layer of the high-voltage cable; A buffer layer defect judgment module, which is used to detect whether there is a partial discharge signal with a partial discharge amount exceeding 50 pC in the collected current signals at different voltage frequencies. If so, there is a buffer layer defect, and select the frequency corresponding to the signal with the largest partial discharge amount among all signals as the frequency used to measure the position of the buffer layer defect of the cable; A defect location calculation module, which is used to locate the defect of the cable buffer layer according to the frequency used to measure the defect location of the cable buffer layer, and record the moment when the high-frequency alternating current high voltage is applied t 0. The moment when the partial discharge signal appears at the head end t s And the moment at the end t m , and calculate the defect location of the cable buffer layer l i ; The defect location integration module is used to change the frequency of the applied high-frequency alternating voltage, repeat k times the above operation to obtain a set of cable defect locations V = , and integrate the above k defect location data to obtain the cable defect location l ; The buffer layer defect location module is used to apply a high-frequency alternating voltage again to the detected defect location of the cable buffer layer l i collect the ultrasonic signals on both sides, and accurately locate the defect location of the cable buffer layer according to the collected ultrasonic signals; Calculating the defect position of the cable buffer layer l i , including: The calculated defect position of the cable buffer layer is l i The relationship with the cable head end time t s is: wherein, t 0 represents the moment when partial discharge occurs in the cable buffer layer, v represents the propagation wave velocity of the partial discharge signal at the cable armor layer; Calculate the relationship between the distance from the defect location of the cable buffer layer to the cable end and the time at the cable end t m as follows: Combining the above two relationships and eliminating the time t0 when the cable buffer layer defect occurs, the cable buffer layer defect position is obtained. l i ; where L represents the cable length, t m represents the moment when the partial discharge reaches the cable end, t s represents the moment at the cable head, v represents the propagation velocity of the partial discharge signal at the cable armor layer; The above-mentioned k integration of the defect location data to obtain the cable defect location l , including: Obtain the average value of the defect location data and the standard deviation S l : By setting a confidence interval of 100(1 - α ), the data range is determined ξ : Eliminate bad data, obtain data points that meet the data range, and establish a new data set V ’ , and calculate the new data set V ’ The average value of the data in is used as the final data result l .

8. A cable buffer layer defect detection system, characterized in that, It includes: A high-frequency high-voltage AC power supply device, a high-frequency current signal acquisition device, an ultrasonic signal acquisition device, an industrial computer, and a terminal data cloud platform. The output end of the high-frequency high-voltage AC power supply device is connected to the core of the high-voltage cable to be measured. The signal acquisition end of the high-frequency current signal acquisition device is connected to the grounded part of the metal armor layer. The ultrasonic signal acquisition device is arranged on the high-voltage cable to be measured. The output ends of the high-frequency current signal acquisition device and the ultrasonic signal acquisition device are respectively connected to the signal input ends of the industrial computer. The control output end of the industrial computer is respectively connected to the control ends of the high-frequency high-voltage AC power supply device, the high-frequency current signal acquisition device, and the ultrasonic signal acquisition device. The terminal data cloud platform is communicatively connected to the industrial computer; the terminal data cloud platform includes the cable buffer layer defect detection device described in claim 7.

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