An internal defect detection system for EPR heat-shrinkable cable terminals and its detection method

Abstract

The present invention discloses an internal defect detection system for an EPR heat-shrinkable cable terminal and its detection method, belonging to the technical field of high voltage and insulation, so as to overcome problems such as poor popularization, poor penetration, great influence by environmental temperature, and damage to the object to be measured. The system includes: a femtosecond laser and a beam splitter arranged from left to right along the terahertz pulse path, a detection optical path system and an excitation optical path system arranged in parallel after the beam splitter, and also includes a lock-in amplifier and an acquisition terminal connected to each other. The method includes sample preparation and installation, defect detection, data collection and processing, and result application. The combination of the system and method of the present invention is for the broadening of the application of terahertz non-destructive visualization detection technology to EPR heat-shrinkable cable terminals. The unique combination of structure and method can clearly establish the terahertz time-domain spectral characteristics of internal defects in EPR heat-shrinkable cable terminals, and finally be applied in reverse to realize non-destructive visualization detection of internal defects in composite cables.

Description

Technical Field

[0001] The present invention belongs to the technical field of high voltage and insulation, and particularly relates to an internal defect detection system for an EPR heat-shrinkable cable terminal and a detection method thereof. Background Art

[0002] EPR heat-shrinkable cable terminals have been widely used in medium and low voltage power systems and traction power supply systems due to their small size, light weight, easy installation, wide application range and other characteristics. However, during the installation process, they are easily affected by factors such as installation technology, operating conditions and external force damage; in the later use and long-term operation, their application scenarios are more diverse and they often operate under harsh conditions. Therefore, during long-term operation, under the combined action of rapid cooling and heating alternation, transient overvoltage impact and long-term high-frequency vibration, the internal insulation layers are separated, micro air gaps are generated, and partial discharge is triggered; as the air gaps continue to increase and gradually form a discharge channel, the internal insulation layer material will decompose due to discharge ablation; as the air gaps further grow and expand and the discharge ablation carbide continues to accumulate, the discharge degree continues to intensify, the air gaps continue to grow and finally a through-discharge channel is formed, resulting in discharge breakdown.

[0003] At present, the detection methods for EPR heat-shrinkable cable terminals are limited to the dielectric loss tangent angle test, partial discharge detection, infrared temperature measurement, etc. adopted during various levels of maintenance. However, these detections cannot detect internal insulation delamination and other defects caused by special working conditions of the cable terminal, and it is difficult to meet the actual needs.

[0004] In the prior art, the non-destructive detection methods for internal defects of cable terminals mainly include: CT scanning and X-ray scanning technology based on high-energy rays, which have been applied to equipment such as GIS switchgear, cable terminals, and insulators. However, due to the poor detection effect of X-rays, etc. on delamination and cracks, and the generation of high-energy free radiation, which is harmful to the human body, it is difficult to promote. Other detection methods also cannot be applied to the detection of composite cable terminals due to their respective defects. For example, in the ultrasonic method, the energy of ultrasonic waves attenuates greatly in composite materials with strong sound absorption properties and it is difficult to penetrate thick material structures; the infrared wave method is greatly affected by the ambient temperature; the laser speckle imaging method requires a high-energy laser light source and will cause damage to the object to be measured.

[0005] Based on the problems in the above background art, there is an urgent need for a detection system and a detection method specifically for internal defects of EPR heat-shrinkable cable terminals to overcome problems such as poor promotion, poor penetration, great influence of ambient temperature, and damage to the object to be measured. Summary of the Invention

[0006] The purpose of the present invention is to provide an internal defect detection system for an EPR heat-shrinkable cable terminal and a detection method thereof to solve various problems in the above background art.

[0007] To solve the above problems, the technical solution of the present invention is as follows:

[0008] An internal defect detection system for EPR heat-shrinkable cable terminals, comprising: a femtosecond laser and a beam splitter arranged from left to right along the terahertz pulse path, a detection optical path system and an excitation optical path system arranged in parallel after the beam splitter, and further comprising a lock-in amplifier and an acquisition terminal connected to each other;

[0009] The detection optical path system includes a delay control unit, a photoconductive emitter, a mirror eight, and a two-degree-of-freedom fast moving platform arranged in sequence from left to right along the optical path; a photoconductive detector is arranged above the mirror eight; a sample of the EPR heat-shrinkable cable is arranged on the two-degree-of-freedom platform, and the sample can freely move up and down in the vertical direction and freely rotate in the horizontal direction of the two-degree-of-freedom platform;

[0010] The excitation optical path system includes a lens, and the lens is arranged above the photoconductive detector;

[0011] The photoconductive detector is connected to the lock-in amplifier.

[0012] Further, the mirror eight is inclined at 45° from left to right, the side that can transmit the terahertz pulse faces the photoconductive emitter, and the side that can transmit the terahertz pulse and can reflect the terahertz pulse from the sample to be measured faces the photoconductive detector.

[0013] Further, a mirror one is arranged on the optical path of the femtosecond laser and the beam splitter; a mirror six is arranged on the optical path of the beam splitter and the lens; a mirror two, a mirror three, a mirror four, a mirror five, and a mirror seven are arranged in sequence on the optical path of the beam splitter and the photoconductive emitter.

[0014] Further, the delay control unit is arranged between the mirror three and the mirror four.

[0015] Further, the femtosecond laser is a titanium-sapphire femtosecond pulse laser, its central wavelength is 810 nm, the pulse width is less than 100 fs, the repetition frequency is 80 MHz, and the output power is 960 mW.

[0016] Further, a DC voltage bias is applied to the photoconductive emitter; no DC voltage bias is applied to the photoconductive detector.

[0017] Further, the sample sequentially includes from outside to inside: a heat-shrinkable tube with umbrella skirts, a stress tube, a heat-shrinkable tube, a stress tube, and a main insulation; its internal defects include: insulation delamination defects, insulation moisture defects, and powder-like discharge product accumulation defects.

[0018] The detection method of the above internal defect detection system for EPR heat-shrinkable cable terminals is divided into the following steps:

[0019] Step A, sample preparation and installation:

[0020] Install the sample in a two - degree - of - freedom platform, and debug the relative positions among the photoconductive emitter, mirror eight, sample, and photoconductive detector so that the sample is set at the focal plane of the photoconductive emitter and the photoconductive detector. During the whole test, the sample can move freely up and down in this focal plane.

[0021] Step B: Defect detection:

[0022] Turn on the femtosecond laser. The generated femtosecond laser excites terahertz pulses. Under the action of the beam splitter, the terahertz pulses travel in two paths. In the path of the excitation optical system, the terahertz pulses directly enter the photoconductive detector through the optical path mirror six and the focusing lens, carrying the time - domain signal, frequency - domain amplitude, and phase information.

[0023] In the path of the detection optical system, the terahertz pulses pass through the delay control unit, that is, optical path mirror two, optical path mirror three, optical path mirror four, optical path mirror five, and optical path mirror seven, and enter the photoconductive emitter. Then, they enter the sample to be tested through mirror eight. The terahertz pulses incident on the sample to be tested are scanned and then reflected back to the photoconductive detector through mirror eight, carrying the time - domain signal, frequency - domain amplitude, and phase information.

[0024] The photoconductive detector receives the two paths of terahertz pulses, converts the terahertz pulses carrying the time - domain signal, frequency - domain amplitude, and phase information into corresponding current information, and then transmits it to the lock - in amplifier to obtain the magnitude and direction of the driving current of the terahertz pulse signal on the photoconductive antenna. At the same time, through noise reduction and amplification, the signal - to - noise ratio is improved, and the information is then transmitted to the acquisition terminal.

[0025] Continuously move the sample to be tested, repeat the above steps, conduct a full - range point - by - point two - dimensional scan of the sample to be tested, and store the time - domain waveform of the terahertz pulses sampled by the photoconductive detector. The obtained scan results enter step C for collection and processing.

[0026] Step C: Data collection and processing:

[0027] The acquisition terminal analyzes and processes all the signals collected in step B, that is, performs a Fourier transform on the time - domain waveform of the terahertz pulses to obtain three - dimensional matrix data in the frequency domain. During the signal processing, select the time - position amplitude, time - domain maximum value, and delay time of the time - domain pulse for imaging, and select the amplitude of the frequency points and the superposition value of all frequency - point amplitudes in the frequency domain for imaging.

[0028] The above conducts imaging detection on the sample and obtains two - dimensional matrix data in the frequency domain to establish a database, which is used as the operation basis for step D.

[0029] Step D: Result application:

[0030] The EPR heat-shrinkable cable terminal sample to be detected is defined as sample A. Repeat steps A to C for sample A for detection, and use the database established in step C to inversely judge the type, degree and parameters of the hidden defects of sample A.

[0031] Furthermore, the correspondence between the imaging detection of the sample in step C and the acquisition of the two-dimensional matrix data in the frequency domain is as follows:

[0032] (a) When there is an insulation delamination defect, it can cause the separation of the contact surfaces of each insulation layer of the cable terminal, and then form an air gap between the insulation layers; reflected in the terahertz image, it is that the time-domain spectrum and frequency-domain spectrum of the terahertz wave change significantly, forming specific peaks, and the peak spectrum segment of the frequency-domain spectrum is at a wave number of 30 cm -1 to 45 cm -1 of the wave number.

[0033] By reverse operation, based on the fast imaging and image reconstruction technology using the internal terahertz time-domain signal, frequency-domain amplitude and phase of sample A, the 2D or 3D terahertz image of this EPR heat-shrinkable cable terminal sample can be obtained. According to the difference in the peak values of the time-domain spectrum and frequency-domain spectrum between the reference sample and the sample with insulation delamination defects, it can be judged whether there is an air gap delamination defect in sample A, the parameters of the internal defect, and the degree of the internal defect.

[0034] (b) When there is an insulation moisture absorption defect, it will cause a small amount of moisture between the insulation layers of the cable terminal; reflected in the terahertz image, it is that the time-domain spectrum and frequency-domain spectrum of the terahertz wave change significantly, forming specific peaks, and the peak spectrum segment of the frequency-domain spectrum is at a wave number of 35 cm -1 to 55 cm -1 of the wave number.

[0035] By reverse operation, based on the fast imaging and image reconstruction technology using the internal terahertz time-domain signal, frequency-domain amplitude and phase of sample A, the 2D or 3D terahertz image of this EPR heat-shrinkable cable terminal sample can be obtained. According to the difference in the peak values of the time-domain spectrum and frequency-domain spectrum between the reference sample and the defective sample, it can be judged whether there is an insulation moisture absorption defect in sample A, and the parameters of the crack defect and the degree of the crack defect.

[0036] (c) When there is an accumulation defect of powdery discharge products, it will cause impurities between the insulation layers of the cable terminal; reflected in the terahertz image, it is that the time-domain spectrum and frequency-domain spectrum of the terahertz wave change significantly, forming specific peaks, and the peak spectrum segment of the frequency-domain spectrum is at a wave number of 55 cm -1 to 80 cm -1 of the wave number.

[0037] By performing reverse operations, based on the fast imaging and image reconstruction techniques using the internal terahertz time-domain signal, frequency-domain amplitude, and phase of sample A, a 2D or 3D terahertz image of the EPR heat-shrinkable cable terminal sample can be obtained. According to the differences in the peaks of the time-domain spectrum and frequency-domain spectrum between the reference sample and the defective sample, it is possible to determine whether there are defects such as the accumulation of powdery discharge products in sample A, the parameters of the broken grid defect, and the degree of the broken grid defect.

[0038] The beneficial effects of the present invention are as follows:

[0039] (1) In the system of the present invention, a femtosecond laser is set to generate terahertz pulses, which are split into two paths by a beam splitter. One path of terahertz pulses serves as the excitation light, and the other path of terahertz pulses serves as the detection light. The photoconductive detector receives the two paths of terahertz pulses, converts the terahertz pulses with time-domain signal, frequency-domain amplitude, and phase information into corresponding current information, and then transmits it to the lock-in amplifier to obtain the magnitude and direction of the driving current of the terahertz pulse signal on the photoconductive antenna. At the same time, through noise reduction and amplification, the signal-to-noise ratio is improved; finally, it enters the acquisition terminal to collect and process the current information of each position of the sample to be measured.

[0040] The principle of action of the beam splitter in the system is that after a beam of light is projected onto the coated glass, the beam is divided into two beams through reflection and refraction; the lens in the system is used to focus the terahertz pulses for excitation reflected from the beam splitter; the photoconductive emitter is used to receive one path of terahertz pulses for detection from the femtosecond laser; the photoconductive detector is used to receive the two paths of terahertz pulses.

[0041] (2) The two-degree-of-freedom platform facilitates the all-round detection of the samples of the EPR heat-shrinkable cable and is convenient for accurately collecting the states and data of different positions of the samples; the setting of the beam splitter splits the terahertz pulses emitted by the femtosecond laser to ensure the working processes of the photoconductive emitter and the photoconductive detector; the optical path mirrors are set as required. On the one hand, they are used for time delay. The function of time delay is to change the overlapping position of the terahertz pulses and the femtosecond laser pulses on the test antenna, so as to sample and scan the entire terahertz time-domain pulse signal; on the other hand, they are used to reflect the terahertz pulses in the optical path and change their propagation directions.

[0042] (3) The terahertz pulse signal adopted by the method of the present invention is an electromagnetic wave with special properties such as low energy consumption, strong penetrability, high signal-to-noise ratio, and fingerprint spectrum. Compared with measurement means such as ultrasonic waves, X-rays, and infrared measurements, it has advantages such as non-contact, non-destructive, and high penetrability, and has unique advantages in the field of non-metallic materials.

[0043] Moreover, terahertz waves are extremely sensitive to molecular changes in materials. Coincidentally, there are obvious differences in molecular structure and types between the air gaps and discharge decomposition products that occur during the operation of EPR heat-shrinkable cable terminations and the adjacent insulating layer materials. At the same time, the special structure of EPR heat-shrinkable cable terminations without shielding and armor is extremely suitable for terahertz detection methods, and can efficiently and accurately diagnose internal defects of EPR heat-shrinkable cable terminations.

[0044] (4) The combination of the system and method of the present invention is an expansion of the application of terahertz non-destructive visualization detection technology to EPR heat-shrinkable cable terminations. The unique combination of structure and method can clearly establish the terahertz time-domain spectral characteristics of internal defects in EPR heat-shrinkable cable terminations, and finally be applied in reverse to achieve non-destructive visualization detection of internal defects in composite cables, with good promotion prospects. Brief Description of the Drawings

[0045] Figure 1 It is a schematic structural diagram of a system for detecting internal defects of an EPR heat-shrinkable cable termination;

[0046] Figure 2 It is a schematic diagram of the structural stratification of a sample from the outside to the center in a system for detecting internal defects of an EPR heat-shrinkable cable termination.

[0047] The reference numerals are as follows: 11 - femtosecond laser; 12 - first mirror; 13 - beam splitter; 14 - second mirror; 15 - third mirror; 16 - fourth mirror; 17 - fifth mirror; 18 - photoconductive emitter; 19 - sixth mirror; 110 - sample; 111 - lens; 112 - seventh mirror; 113 - lock-in amplifier; 114 - acquisition terminal; 115 - eighth mirror; 116 - two-degree-of-freedom platform; 117 - photoconductive detector; 118 - delay control unit. Detailed Embodiments

[0048] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0049] Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the present invention that is claimed, but merely represents selected embodiments of the present invention.

[0050] It should be noted that similar reference numerals and letters denote similar items in the following figures. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0051] In the present invention, unless otherwise clearly defined and limited, the terms "connected", "coupled", and other terms should be understood in a broad sense. For example, they can be directly connected or indirectly connected through an intermediate medium, and can be the internal connection of two components or the interaction relationship between two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances. Embodiment

[0052] As Figure 1-2 As shown, an internal defect detection system for an EPR heat-shrinkable cable terminal includes: a femtosecond laser 11 and a beam splitter 13 arranged from left to right along the terahertz pulse path, a detection optical path system and an excitation optical path system arranged in parallel after the beam splitter 13, and also includes a lock-in amplifier 113 and an acquisition terminal 114 connected to each other;

[0053] The detection optical path system includes a delay control unit 118, a photoconductive emitter 18, a mirror eight 115, and a two-degree-of-freedom fast moving platform 116 arranged in sequence from left to right along the optical path; a photoconductive detector 117 is provided above the mirror eight 115; a sample 110 of the EPR heat-shrinkable cable is provided on the two-degree-of-freedom platform 116, and the sample 110 can freely move up and down in the vertical direction and freely rotate in the horizontal direction of the two-degree-of-freedom platform 116;

[0054] The excitation optical path system includes a lens 111, and the lens 111 is provided above the photoconductive detector 117;

[0055] The photoconductive detector 117 is connected to the lock-in amplifier 113.

[0056] The specific setting method is as described below:

[0057] The mirror eight 115 is inclined at 45° from left to right, and the side that can transmit the terahertz pulse faces the photoconductive emitter 18, and the side that can transmit the terahertz pulse and can reflect the terahertz pulse from the sample 110 to be measured faces the photoconductive detector 117.

[0058] A mirror one 12 is provided on the optical path of the femtosecond laser 11 and the beam splitter 13; a mirror six 19 is provided on the optical path of the beam splitter 13 and the lens 111; a mirror two 14, a mirror three 15, a mirror four 16, a mirror five 17, and a mirror seven 112 are sequentially provided on the optical path of the beam splitter 13 and the photoconductive emitter 18; the delay control unit 118 is provided between the mirror three 15 and the mirror four 16.

[0059] The specific configuration parameters of the equipment and samples are as follows:

[0060] The femtosecond laser 11 is a titanium sapphire femtosecond pulse laser with a central wavelength of 810 nm, a pulse width less than 100 fs, a repetition frequency of 80 MHz, and an output power of 960 mW.

[0061] The photoconductive emitter 18 is biased with a DC voltage; the photoconductive detector 117 is not biased with a DC voltage.

[0062] The sample 110 includes, from outside to inside: a heat shrinkable tube with an umbrella skirt, a stress tube, a heat shrinkable tube, a stress tube, and main insulation; its internal defects include: insulation delamination defects, insulation moisture absorption defects, and powder-like discharge product accumulation defects.

[0063] The detection method of the above EPR heat shrink cable terminal internal defect detection system is divided into the following steps:

[0064] Step A: Sample preparation and installation:

[0065] Install the sample 110 in the two-degree-of-freedom platform 116, and adjust the relative positions among the photoconductive emitter 18, the mirror eight 115, the sample 110, and the photoconductive detector 117 so that the sample 110 is set at the focal plane of the photoconductive emitter 18 and the photoconductive detector 117, and during the whole test, the sample 110 can move freely up and down in this focal plane.

[0066] Step B: Defect detection:

[0067] Turn on the femtosecond laser 11 to generate femtosecond laser to excite terahertz pulses. Under the action of the beam splitter 13, the terahertz pulses travel in two paths. The terahertz pulses directly send the terahertz pulses with time-domain signals, frequency-domain amplitude and phase information into the photoconductive detector 117 through the optical path mirror six 19 and the focusing lens 111 in the path of the excitation optical path system.

[0068] The terahertz pulses pass through the delay control unit 118 in the path of the detection optical path system, that is, the optical path mirror two 14, the optical path mirror three 15, the optical path mirror four 16, the optical path mirror five 17, and the optical path mirror seven 112, enter the photoconductive emitter 18, pass through the mirror eight 115 and enter the sample 110 to be tested. After scanning the terahertz pulses incident on the sample 110 to be tested, the terahertz pulses with time-domain signals, frequency-domain amplitude and phase information are reflected by the mirror eight 115 to the photoconductive detector 117.

[0069] The photoconductive detector 117 receives two paths of terahertz pulses, converts the terahertz pulses with time-domain signals, frequency-domain amplitude and phase information into corresponding current information, and then transmits it to the lock-in amplifier 113 to obtain the magnitude and direction of the driving current of the terahertz pulse signal on the photoconductive antenna. At the same time, through noise reduction and amplification, the signal-to-noise ratio is improved, and the information is then transmitted to the acquisition terminal 114.

[0070] Continuously move the sample to be measured 110, repeat the above steps, perform a full-range point-by-point two-dimensional scan on the sample to be measured 110, and store the terahertz pulse time-domain waveform sampled by the photoconductive detector 117. The obtained scan results enter the collection and processing in step C.

[0071] Step C: Data collection and processing:

[0072] The acquisition terminal 114 analyzes and processes all the signals collected in step B, that is, performs a Fourier transform on the terahertz pulse time-domain waveform to obtain three-dimensional matrix data in the frequency domain. During the signal processing, the time position amplitude, the time-domain maximum value, and the delay time of the time-domain pulse are selected for imaging, and the amplitude of the frequency point and the superimposed value of all frequency point amplitudes are selected for imaging in the frequency domain.

[0073] The above performs imaging detection on the sample and obtains two-dimensional matrix data in the frequency domain to establish a database, which is used as the operation basis for step D.

[0074] Step D: Result application:

[0075] For the EPR heat-shrinkable cable terminal sample to be detected, it is defined as sample A. Repeat steps A - C for sample A for detection, and use the database established in step C to inversely judge the type, degree, and parameters of the hidden defects of sample A.

[0076] Specifically: The correspondence between the imaging detection of the sample in step C and the obtained two-dimensional matrix data in the frequency domain is as follows:

[0077] (a) When there is an insulation delamination defect, it can cause the separation of the contact surfaces of each insulation layer of the cable terminal, and then form air gaps between the insulation layers; reflected in the terahertz image, it is that the time-domain spectrum and frequency-domain spectrum of the terahertz wave change significantly, forming specific peaks, and the peak spectrum segment of the frequency-domain spectrum is at a wave number of 30 cm -1 to 45 cm -1 of the wave number.

[0078] By performing reverse operations, based on the fast imaging and image reconstruction techniques using the internal terahertz time-domain signal, frequency-domain amplitude, and phase of sample A, a 2D or 3D terahertz image of the EPR heat-shrinkable cable terminal sample can be obtained. According to the differences in the peaks of the time-domain spectrum and frequency-domain spectrum between the reference sample and the sample with insulation delamination defects, it is possible to determine whether there are air-gap delamination defects in sample A, the parameters of internal defects, and the degree of internal defects.

[0079] (b)When there are insulation moisture defects, it will cause a small amount of moisture between the insulation layers of the cable terminal; this is reflected in the terahertz image as obvious changes in the time-domain spectrum and frequency-domain spectrum of the terahertz wave, forming specific peaks. The peak spectral segment of the frequency-domain spectrum is at a wavenumber of 35 cm -1 to 55 cm -1 in wavenumber.

[0080] By performing reverse operations, based on the fast imaging and image reconstruction techniques using the internal terahertz time-domain signal, frequency-domain amplitude, and phase of sample A, a 2D or 3D terahertz image of the EPR heat-shrinkable cable terminal sample can be obtained. According to the differences in the peaks of the time-domain spectrum and frequency-domain spectrum between the reference sample and the defective sample, it is possible to determine whether there are insulation moisture defects in sample A, the parameters of crack defects, and the degree of crack defects.

[0081] (c)When there are defects caused by the accumulation of powdery discharge products, it will result in impurities between the insulation layers of the cable terminal; this is reflected in the terahertz image as obvious changes in the time-domain spectrum and frequency-domain spectrum of the terahertz wave, forming specific peaks. The peak spectral segment of the frequency-domain spectrum is at a wavenumber of 55 cm -1 to 80 cm -1 in wavenumber.

[0082] By performing reverse operations, based on the fast imaging and image reconstruction techniques using the internal terahertz time-domain signal, frequency-domain amplitude, and phase of sample A, a 2D or 3D terahertz image of the EPR heat-shrinkable cable terminal sample can be obtained. According to the differences in the peaks of the time-domain spectrum and frequency-domain spectrum between the reference sample and the defective sample, it is possible to determine whether there are defects caused by the accumulation of powdery discharge products in sample A, the parameters of broken grid defects, and the degree of broken grid defects.

Claims

1. An internal defect detection system for EPR heat-shrinkable cable terminals, characterized in that, it includes: A femtosecond laser (11) and a beam splitter (13) arranged from left to right along the terahertz pulse path, a detection optical path system and an excitation optical path system arranged in parallel after the beam splitter (13), and also includes a lock-in amplifier (113) and an acquisition terminal (114) connected to each other; A first mirror (12) is provided on the optical path of the femtosecond laser (11) and the beam splitter (13); The detection optical path system includes a delay control unit (118), a photoconductive emitter (18), an eighth mirror (115), and a two-degree-of-freedom fast moving platform (116) arranged in sequence from left to right along the optical path; A photoconductive detector (117) is provided above the eighth mirror (115); a sample (110) of an EPR heat-shrinkable cable is provided on the two-degree-of-freedom platform (116), and the sample (110) can freely move up and down in the vertical direction and freely rotate in the horizontal direction on the two-degree-of-freedom platform (116); The eighth mirror (115) is arranged obliquely at 45° from left to right, the side that can transmit the terahertz pulse faces the photoconductive emitter (18), and the side that can transmit the terahertz pulse and can reflect the terahertz pulse from the sample to be measured (110) faces the photoconductive detector (117); The excitation optical path system includes a lens (111), and the lens (111) is provided above the photoconductive detector (117); the photoconductive detector (117) is connected to the lock-in amplifier (113); A sixth mirror (19) is provided on the optical path of the beam splitter (13) and the lens (111); a second mirror (14), a third mirror (15), a fourth mirror (16), a fifth mirror (17), and a seventh mirror (112) are arranged in sequence on the optical path of the beam splitter (13) and the photoconductive emitter (18); The delay control unit (118) is arranged between the third mirror (15) and the fourth mirror (16); The sample (110) includes, from outside to inside: a heat-shrinkable tube with umbrella skirts, a stress tube, a heat-shrinkable tube, a stress tube, and a main insulation; its internal defects include: insulation delamination defects, insulation moisture defects, and powder-like discharge product accumulation defects.

2. An internal defect detection system for EPR heat-shrinkable cable terminals according to claim 1, characterized in that: The femtosecond laser (11) is a titanium sapphire femtosecond pulse laser, its central wavelength is 810 nm, the pulse width is less than 100 fs, the repetition frequency is 80 MHz, and the output power is 960 mW.

3. An internal defect detection system for EPR heat-shrinkable cable terminals according to claim 1, characterized in that: A DC voltage bias is applied to the photoconductive emitter (18); no DC voltage bias is applied to the photoconductive detector (117).

4. A detection method for an internal defect detection system for EPR heat-shrinkable cable terminals according to any one of claims 1-3, characterized in that: This method is the following steps: Step A. Sample preparation and installation: Install the sample (110) in the two-degree-of-freedom platform (116), and debug the relative positions among the photoconductive emitter (18), mirror eight (115), sample (110), and photoconductive detector (117) so that the sample (110) is set at the focal plane of the photoconductive emitter (18) and the photoconductive detector (117), and during the whole test, the sample (110) can move freely up and down in this focal plane; Step B. Defect detection: Turn on the femtosecond laser (11) to generate femtosecond laser pulses. Under the action of the beam splitter (13), the femtosecond laser pulses travel in two paths. In the path of the excitation optical system, the terahertz pulses directly send the terahertz pulses with time-domain signals, frequency-domain amplitude, and phase information to the photoconductive detector (117) through the optical path mirror six (19) and the focusing lens (111); In the path of the detection optical system, the terahertz pulses pass through the delay control unit (118), that is, the optical path mirror two (14), optical path mirror three (15), optical path mirror four (16), optical path mirror five (17), and optical path mirror seven (112) and enter the photoconductive emitter (18), pass through the mirror eight (115) and enter the sample to be tested (110). After scanning the terahertz pulses incident on the sample to be tested (110), the terahertz pulses with time-domain signals, frequency-domain amplitude, and phase information are reflected by the mirror eight (115) to the photoconductive detector (117); The photoconductive detector (117) receives the two paths of terahertz pulses, converts the terahertz pulses with time-domain signals, frequency-domain amplitude, and phase information into corresponding current information, and then transmits it to the lock-in amplifier (113) to obtain the magnitude and direction of the driving current of the terahertz pulse signal on the photoconductive antenna. At the same time, through noise reduction and amplification, the signal-to-noise ratio is improved, and the information is then transmitted to the acquisition terminal (114); Continuously move the sample to be tested (110), repeat the above steps, perform a full-range point-by-point two-dimensional scan on the sample to be tested (110), and store the time-domain waveform of the terahertz pulses sampled by the photoconductive detector (117). The obtained scan results enter step C for collection and processing; Step C. Data collection and processing: The acquisition terminal (114) analyzes and processes all the signals collected in step B, that is, performs a Fourier transform on the time-domain waveform of the terahertz pulses to obtain three-dimensional matrix data in the frequency domain. During the signal processing process, the time position amplitude, time-domain maximum value, and delay time of the time-domain pulses are selected for imaging, and the amplitude of the frequency points and the superimposed value of the amplitudes of all frequency points are selected for imaging in the frequency domain; The above performs imaging detection on the sample and obtains two-dimensional matrix data in the frequency domain to establish a database, which is used as the operation basis for step D; Step D. Result application: For the EPR heat-shrinkable cable terminal sample to be detected, define it as sample A. Repeat steps A - C for sample A for detection, and use the database established in step C to inversely judge the type, degree, and parameters of the hidden defects of sample A; Specifically: The correspondence between the imaging detection of the sample in step C and the obtained two-dimensional matrix data in the frequency domain is as follows: (a)When there is an insulation delamination defect, it can cause the separation of the contact surfaces of each insulation layer at the cable terminal, and then form air gaps between the insulation layers; this is reflected in the terahertz image as obvious changes in the time-domain spectrum and frequency-domain spectrum of the terahertz wave, forming specific peaks, and the peak spectral segment of the frequency-domain spectrum is at a wave number of 30 cm -1 to 45 cm -1 ; By performing reverse operations, based on the fast imaging and image reconstruction techniques using the internal terahertz time-domain signal, frequency-domain amplitude, and phase of sample A, a 2D or 3D terahertz image of the EPR heat-shrinkable cable terminal sample can be obtained. According to the differences in the peaks of the time-domain spectrum and frequency-domain spectrum between the reference sample and the sample with insulation delamination defects, it is possible to determine whether there are air-gap delamination defects in sample A, the parameters of the internal defects, and the degree of the internal defects. (b)When there is a defect of insulation moisture absorption, it will cause a small amount of moisture between the insulation layers of the cable terminal; this is reflected in the terahertz image as obvious changes in the time-domain spectrum and frequency-domain spectrum of terahertz waves, forming specific peaks, and the peak spectrum segment of the frequency-domain spectrum is at a wavenumber of -1 from 35 cm -1 to 55 cm By performing reverse operations, based on the fast imaging and image reconstruction techniques using the internal terahertz time-domain signal, frequency-domain amplitude, and phase of sample A, a 2D or 3D terahertz image of the EPR heat-shrinkable cable terminal sample can be obtained. According to the differences in the peaks of the time-domain spectrum and frequency-domain spectrum between the reference sample and the defective sample, it is possible to determine whether there are insulation moisture absorption defects in sample A, the parameters of the crack defects, and the degree of the crack defects. (c)When there are defects in the accumulation of powdery discharge products, impurities will exist between the insulating layers of the cable terminal; this is reflected in the terahertz image as obvious changes in the time-domain spectrum and frequency-domain spectrum of terahertz waves, forming specific peaks, and the peak spectral segment of the frequency-domain spectrum is at a wavenumber of -1 from 55 cm -1 to 80 cm By performing reverse operations, based on the fast imaging and image reconstruction techniques using the internal terahertz time-domain signal, frequency-domain amplitude, and phase of sample A, a 2D or 3D terahertz image of the EPR heat-shrinkable cable terminal sample can be obtained. According to the differences in the peaks of the time-domain spectrum and frequency-domain spectrum between the reference sample and the defective sample, it is possible to determine whether there are powdery discharge product accumulation defects in sample A, the parameters of the broken grid defects, and the degree of the broken grid defects.

Images