Electric heating aging test method and device for simulating space charge effect of cable terminal

Abstract

The invention discloses an electrothermal aging test method and device for simulating the space charge effect of a cable terminal. The method comprises the following steps: S1, treating a cable sample; s2, constructing an electric heating aging platform; s3, carrying out five aging stage tests on the cable sample; s4, carrying out space charge detection on the cable samples at different aging stages; s5, analyzing correlation between a space charge accumulation rule and insulation degradation based on a charge mobility formula and an injection current formula; according to the invention, the actual working condition of the cable terminal is accurately simulated through the electrothermal collaborative aging platform, and the limitation that the space charge evolution law is difficult to reproduce in the traditional method is overcome; a weighted all-phase averaging method is adopted to process signals, and an arc-shaped sensor and a frequency domain deconvolution algorithm are combined, so that interference is effectively filtered out, and the detection stability and accuracy are improved; through correlation of charge accumulation and insulation degradation processes, an insulation failure mechanism is disclosed, fault early warning is realized, and theoretical and test support is provided for design optimization, life evaluation and operation and maintenance of the cable terminal.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of state monitoring and fault early warning of cable terminal, in particular to an electro-thermal aging test method and device for simulating space charge effect of cable terminal. BACKGROUND

[0002] With the rapid development of urban power distribution network, cross-linked polyethylene cable has become the core equipment for medium and high voltage power transmission due to its excellent insulation performance and mechanical properties. The insulation state of cable terminal, as a key accessory connecting cable and other equipment, directly affects the reliability of power grid operation. Field operation experience shows that cable terminal is a high-failure part, and insulation breakdown accidents are closely related to the accumulation of space charge, electric field distortion and material aging in long-term operation. Therefore, accurately simulating the space charge effect of cable terminal under actual working conditions and deeply exploring the correlation mechanism between space charge and insulation deterioration are of great significance to improve the design level of cable terminal and optimize the operation and maintenance strategy. However, the existing technology still has some shortcomings:

[0003] Firstly, traditional cable terminal aging test mostly uses single stress, such as only applying voltage or temperature, which is difficult to reproduce the complex environment of "electric stress-thermal stress" coupling in actual operation. Under actual working conditions, cable terminal not only bears power frequency voltage, but also generates radial temperature gradient due to load current. The synergistic effect of the two stresses can significantly change the accumulation law of space charge. Single stress test cannot reflect the dynamic evolution of space charge under real working conditions, leading to deviation in the study of insulation aging mechanism and difficulty in supporting accurate fault prediction.

[0004] Secondly, existing space charge detection mostly uses flat panel sensors, which have poor adaptability to the arc-shaped structure of cable terminal stress cone. Poor contact can easily lead to signal distortion. Under alternating voltage, the space charge signal changes dynamically with phase, and traditional signal processing methods cannot effectively filter out interference components such as alternating charge and interface charge, resulting in insufficient detection accuracy. In addition, the attenuation and dispersion of sound waves in insulating medium further reduce the reliability of the signal, restricting the accurate extraction of space charge characteristic parameters.

[0005] Finally, existing technologies mostly focus on the detection of space charge or the macroscopic observation of insulation aging, lacking systematic research on the internal correlation between the two. The injection, migration and accumulation of space charge are micro-charge behaviors, while insulation deterioration is manifested as macro-performance degradation. The two are related through micro-mechanisms such as charge traps and molecular chain rupture. Traditional methods fail to quantitatively combine the dynamic characteristics of space charge such as mobility and injection current with the insulation deterioration process, making it difficult to reveal the deep mechanism of space charge-induced insulation failure and provide theoretical support for the design optimization and life assessment of cable terminal.

[0006] Therefore, it is necessary to design an electrothermal aging test method and device for simulating space charge effects of a cable terminal. SUMMARY

[0007] The present application aims to provide an electrothermal aging test method and device for simulating space charge effects of a cable terminal, to solve the problems in the background art that traditional cable terminal aging tests usually use single stress, such as only applying voltage or temperature, which is difficult to reproduce the complex environment of "electrical stress-thermal stress" coupling in actual operation; in actual working conditions, the cable terminal not only bears the power frequency voltage, but also generates a radial temperature gradient due to the load current, and the synergistic effect of the two stresses can significantly change the accumulation law of space charge; single stress test cannot reflect the dynamic evolution of space charge under real working conditions, leading to deviation in the study of insulation aging mechanism, making it difficult to support accurate fault warning, and the existing space charge detection mostly uses flat sensors, which have poor adaptability to the arc-shaped structure of the stress cone of the cable terminal, and are prone to signal distortion due to poor contact, and the space charge signal changes dynamically with the phase under alternating voltage, and the traditional signal processing method cannot effectively filter out interference components such as alternating charge and interface charge, resulting in insufficient detection accuracy; in addition, the attenuation and dispersion of sound waves in insulating media further reduce the reliability of the signal, restricting the accurate extraction of space charge characteristic parameters, and the existing technology mostly focuses on the detection of space charge or the macroscopic observation of insulation aging, lacking systematic research on the internal correlation between the two; the injection, migration and accumulation of space charge are micro-charge behaviors, while insulation degradation is manifested as macro-performance degradation, and the two are related through micro-mechanisms such as charge traps and molecular chain rupture; the traditional method fails to quantitatively combine the dynamic characteristics of space charge such as mobility and injection current with the insulation degradation process, making it difficult to reveal the deep mechanism of space charge-induced insulation failure, and unable to provide theoretical support for the design optimization and life assessment of the cable terminal.

[0008] To achieve the above-mentioned purpose, the present application provides the following technical solutions:

[0009] In a first aspect, an electrothermal aging test method for simulating space charge effects of a cable terminal is provided, comprising the following steps:

[0010] S1: selecting a 10kV XLPE cable as a sample, stripping the outer semiconductive layer on both sides, leaving the outer semiconductive layer in the middle section, installing a stress cone on the stripped section, installing a corona shield on the exposed end of the core, wrapping the outer semiconductive layer in the middle with aluminum foil and grounding it;

[0011] S2: placing the processed cable sample in a modified dry blast box, i.e. opening a hole in the top of the box, introducing alternating high voltage through a high voltage sleeve and applying it to the core of the cable aging sample, and controlling the temperature in the blast box to form thermal stress, thus completing the construction of the electrothermal aging platform;

[0012] S3: 26.1kV AC voltage is applied to the cable sample, the aging temperature is controlled to be 80 DEG C on the outside and 115 DEG C on the inside of the cable insulation, and five stages of 75 days, 150 days, 225 days, 300 days and 375 days are set;

[0013] S4: The space charge of the cable sample at different aging stages is detected by using an electro-acoustic pulse method, and the detection parameters are as follows: the high-voltage pulse source outputs a 5kV amplitude and a 50ns pulse width pulse, the pulse frequency is 1600Hz, the high-voltage pulse source is triggered at 32 phase points through an AC phase matching circuit, and the space charge signal is collected every power frequency cycle;

[0014] S5: The space charge signal of 32 phases is processed by using a full-phase average method, alternating charges and interface charges are filtered out, non-alternating charge characteristic parameters are extracted, and the space charge accumulation rule is analyzed based on the charge mobility formula and the injection current formula to analyze the correlation between the insulation deterioration.

[0015] As a further technical scheme of the application, in S1, the cable sample parameters are as follows: the cross-sectional area of the copper conductor is 240mm2, the thickness of the inner and outer semiconductive shielding layer is 0.8mm, the thickness of the XLPE main insulation is 4.5mm, the total length is 340mm, and the length of the outer semiconductive layer in the middle is 90mm.

[0016] As a further technical scheme of the application, in S2, the modification of the dry blast box includes that a through hole with a diameter of 50-80mm is formed in the center of the top of the box body, an insulating flange is installed at the edge of the through hole, a high-voltage sleeve is fixed through the flange, and the insulation distance between the sleeve and the box body is ensured to be not less than 100mm.

[0017] As a further technical scheme of the application, in S3, a large current is passed into the wire core through a current riser to form a temperature gradient of 80 DEG C on the outside to 115 DEG C on the inside of the insulation layer, so that the insulation layer is expanded to 5.5mm due to thermal expansion, and the electro-thermal synergistic effect in actual operation is simulated.

[0018] As a further technical scheme of the application, in S4, the working process of the AC phase matching circuit includes:

[0019] 1) A synchronous sinusoidal signal with an amplitude of 5-10V is extracted from the 26.1kV AC voltage through a capacitor voltage divider;

[0020] 2) High-frequency noise is removed through a 10kHz low-pass filter circuit, and a zero-crossing comparator is input to convert it into a square wave signal;

[0021] 3) The square wave signal is delayed for 0-5us after phase correction to trigger the signal generator, and 32 trigger pulses with a period of 500us are output to control the high-voltage pulse source to trigger uniformly within a 360 DEG power frequency cycle.

[0022] As a further technical solution of the present application, in the S5, the specific calculation formula of the full-phase average method is:

[0023]

[0024] Wherein, is the non-alternating charge density average value at the radial position ; is the index of 32 phase points, corresponding to 0°-360°, with an interval of 11.25°; is the space charge density of the th phase point at the radial position ; is the weight coefficient of the th phase point, and satisfies the following formula:

[0025]

[0026] Wherein: is the unweighted average charge density, and the weight coefficient is used to suppress the interference of abnormal fluctuation signals.

[0027] As a further technical solution of the present application, in the S5, the movement of electrons in the dielectric is achieved by the jump conduction between traps, and the movement is achieved by jumping from one trap to an adjacent trap to overcome the potential barrier. The mobility calculation of jump conduction adopts the formula:

[0028]

[0029] Under low field strength, , then it is simplified as:

[0030]

[0031] Wherein, is the elementary charge; is the phonon frequency; is the average distance between traps; is the Boltzmann constant; is the potential barrier between adjacent traps; is the temperature; is the electric field strength.

[0032] As a further technical solution of the present application, in the S5, the electron injection follows the Poole-Frenkel effect, and the emission current formula is:

[0033]

[0034] Wherein, is the temperature; is the Richardson constant; is the electron injection potential barrier; is the Boltzmann constant; is the absolute dielectric constant of the medium; the thermally assisted tunneling effect can be calculated according to the tunneling potential barrier and the mirror charge, and the formula for calculating the thermally assisted tunneling current is:

[0035]

[0036] wherein, is the tunneling probability; is the inverse local radius; is the electrostatic potential; is the blocking surface charge potential barrier.

[0037] As a further technical solution of the present application, in the S5, the total amount of space charge is calculated by integrating the charge density in the radial range of 12.7-14.1 mm, and the axial monitoring is performed in the range of -2.5 cm to 0.5 cm with the inside edge of the stress cone as the zero point, wherein the positive direction is towards the inside of the cable and the negative direction is towards the outside.

[0038] In a second aspect, an electrical-thermal aging test device for simulating the space charge effect of a cable terminal is provided, which comprises a sample assembly, a thermal stress generating unit, an electrical stress generating unit, a space charge detection unit and a data acquisition unit.

[0039] The sample assembly is composed of a 10 kV XLPE cable sample, a stress cone, a anti-corona ball and an aluminum foil grounding layer, the stress cone is adapted to the stripped section of the cable, and the aluminum foil is wrapped around the middle and outer semiconductive layers.

[0040] The thermal stress generating unit comprises a modified dry blast box with an aluminum sample support and a high-voltage bushing at the top, which can maintain a temperature gradient of 80-115℃; the aluminum sample support adopts a hollow grid structure, and the surface is sprayed with a 0.1mm thick polytetrafluoroethylene insulation layer; the height of the support can be adjusted to ensure that the cable sample is placed horizontally and the distance from the inner wall of the blast box is not less than 50mm, ensuring uniform temperature field; the high-voltage bushing adopts an insulator string structure, the length of the end in the box is extended to prevent end flashover, and the creepage distance meets the requirements of the 26.1kV voltage level;

[0041] The electrical stress generating unit comprises a voltage regulator, a step-up transformer, a protective resistor and a coupling capacitor, and outputs 26.1kV AC voltage to the core through the high-voltage bushing;

[0042] The space charge detection unit includes an electroacoustic pulse arc sensor, a 5kV / 50ns high-voltage pulse source, and an AC phase matching circuit. The arc contact surface of the electroacoustic pulse arc sensor has a radius of curvature of 50-100mm, which matches the curvature of the outer surface of the stress cone of the 10kV cable terminal. The contact area is not less than 10cm². It achieves tight adhesion to the stress cone through silicone rubber coupling agent. The coupling layer thickness is 0.1-0.3mm. It is fixed to the stress cone by a buckle. The piezoelectric conversion element of the electroacoustic pulse arc sensor is a polyvinylidene fluoride film. The AC phase matching circuit consists of a capacitive voltage divider, a filter circuit, and a zero-crossing comparator. It can control the phase difference between the pulse trigger and the AC voltage within ±11.25°. The 32 phase points are evenly distributed within the 360° power frequency cycle to ensure uniform sampling.

[0043] The data acquisition unit includes an oscilloscope connected to the detection unit and supporting data processing software. It records the space charge signal at 32 phase points, supports full-phase averaging processing, waveform attenuation correction, and dispersion recovery. The waveform attenuation correction in the data processing software is based on the formula:

[0044]

[0045] in, Radial position Location, frequency The corresponding space charge sound pressure signal amplitude; For reference position Location, frequency The corresponding initial sound pressure signal amplitude; The piezoelectric conversion coefficient of the sensor; This is the frequency-dependent attenuation coefficient; The dispersion coefficient; The coordinates of the radial detection point on the cable; The coordinates of the radial reference starting point of the cable are used; signal recovery is achieved through a frequency domain deconvolution algorithm, where the deconvolution kernel function is the inverse Fourier transform matrix of the sensor's impulse response. The specific implementation of the frequency domain deconvolution algorithm includes the following steps and parameter settings:

[0046] 1) Acquisition of the impulse response of the arc-shaped sensor: The impulse response function is obtained through calibration with a standard sample. Specifically, a 1mm thick polytetrafluoroethylene sheet was selected as the standard sample and placed between the arc-shaped sensor and the metal electrode. A 5kV / 50ns high-voltage pulse was applied, and the time-domain signal output by the arc-shaped sensor was recorded. After noise filtering and normalization using a 50kHz low-pass filter, the impulse response of the arc-shaped sensor was obtained. ;

[0047] 2) Frequency domain deconvolution steps:

[0048] 2.1) Fourier transform the space charge detection raw signal and the impulse response respectively to obtain the frequency domain representation and respectively. ;

[0049] 2.2) Calculate the frequency domain recovery signal , wherein is a regularization parameter used to suppress high-frequency noise amplification.

[0050] 2.3) Inverse Fourier transform to obtain the time domain recovery signal , i.e., complete signal recovery.

[0051] The number of Fourier transform points is 2048 points, and the sampling frequency is consistent with the sampling rate of the oscilloscope.

[0052] Compared with the prior art, the beneficial effects of the electrical thermal aging test method and device for simulating the space charge effect of the cable terminal are:

[0053] By constructing an electrical thermal aging platform, combining specific cable sample processing methods and multi-stage aging parameter settings, the electrical thermal coupling environment in the actual operation of the cable terminal is accurately simulated, breaking through the limitations of traditional single stress tests that cannot reproduce the space charge evolution law under real working conditions, and providing a more practical test basis for research.

[0054] Using the weighted all-phase average method to process the multi-phase space charge signal, in combination with the arc-shaped sensor design suitable for the stress cone and the frequency domain deconvolution signal recovery algorithm, not only effectively filters out interfering charges to extract key characteristic parameters, but also through the synergistic optimization of hardware and algorithms, improves the stability and accuracy of space charge detection, and solves the problems of difficult charge signal recognition and low accuracy in complex electric field environments.

[0055] By combining the space charge detection results with the mobility formula and the injection current formula, the charge accumulation law and the insulation deterioration process are deeply related, which not only enables effective early warning of potential faults in the cable terminal, but also reveals the internal mechanism of space charge-induced insulation failure, providing theoretical and experimental support from phenomena to essence for the design optimization, life assessment, and operation strategy formulation of the cable terminal. BRIEF DESCRIPTION OF DRAWINGS

[0056] Figure 1 is a flowchart of the method of the present application;

[0057] Figure 2 is a schematic diagram of an electrical thermal aging platform;

[0058] ​Figure 3 Schematic diagram of AC phase matching circuit;

[0059] Figure 4 Cable space charge waveform diagram obtained by full phase average method at 16 days of voltage increase;

[0060] Figure 5 Schematic diagram of portable charge measuring device installation;

[0061] Figure 6 Schematic diagram of high-voltage pulse power supply structure;

[0062] Figure 7 Space charge test result diagram. DETAILED DESCRIPTION

[0063] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0064] Please refer to the drawings Figure 1 An embodiment provided by the present application: an electrothermal aging test method for simulating cable terminal space charge effect, comprising the following steps:

[0065] S1: select a 10kV XLPE cable as a sample, remove the outer semiconductive layer on both sides, retain the outer semiconductive layer in the middle section, install a stress cone at the removed section, install a anti-corona ball at the exposed end of the core, wrap the outer semiconductive layer in the middle with aluminum foil and ground it;

[0066] The cable sample parameters are: copper conductor cross-sectional area 240mm², inner and outer semiconductive shielding layer thickness 0.8mm, XLPE main insulation thickness 4.5mm, total length 340mm, and the length of the retained outer semiconductive layer in the middle is 90mm;

[0067] S2: place the processed cable sample in a modified dry blast box, the modification of the dry blast box includes opening a through hole with a diameter of 50-80mm in the center of the top of the box, installing an insulating flange at the edge of the through hole, fixing a high-voltage bushing through the flange, ensuring that the insulation distance between the bushing and the box is not less than 100mm, introducing AC high voltage through the high-voltage bushing and applying it to the core of the cable aging sample, controlling the temperature in the blast box to form thermal stress, and completing the construction of the electrothermal aging platform;

[0068] Specifically, the processed cable sample is placed horizontally on an aluminum sample support in the blast box, the support adopts a hollow grid structure, the surface is sprayed with a 0.1 mm thick polytetrafluoroethylene insulation layer, and the height is adjusted to 150 mm to ensure that the sample is at a distance of ≥50 mm from the wall of the box; the alternating current high voltage is introduced through a high-voltage sleeve and connected to the cable core, the outer semiconductive layer of the cable is connected to the support through an aluminum foil to realize grounding, forming an aging environment with synergistic effect of "electric stress + thermal stress", see Figure 2 ;

[0069] S3: Through the voltage regulator, step-up transformer and coupling capacitor, 26.1 kV alternating voltage is applied to the cable sample, the protection resistance is 10 kΩ, and a large current is passed into the core through the current booster to form a temperature gradient of 80°C outside to 115°C inside the insulation layer, so that the insulation layer expands to 5.5 mm due to heat, simulating the synergistic effect of electric heat in actual operation, and setting five stages of 75 days, 150 days, 225 days, 300 days and 375 days. After each stage, stop to detect space charges;

[0070] S4: The space charge of the cable sample at different aging stages is detected by the electro-acoustic pulse method, and the detection parameters are: the high-voltage pulse source outputs a 5 kV amplitude, 50 ns pulse width pulse, the pulse frequency is 1600 Hz, and the high-voltage pulse source is triggered at 32 phase points through the alternating phase matching circuit. Space charge signals are collected every power frequency cycle;

[0071] The working process of the alternating phase matching circuit includes:

[0072] 1) A synchronous sine signal with an amplitude of 5-10 V is extracted from the 26.1 kV alternating voltage through a capacitor voltage divider;

[0073] 2) The high-frequency noise is removed through a 10 kHz low-pass filter circuit, and a zero-crossing comparator is input to convert it into a square wave signal;

[0074] 3) The square wave signal is delayed for 0-5 μs after phase correction to trigger the signal generator, output 32 cycles of 500 μs trigger pulses, control the high-voltage pulse source to trigger evenly within 360° power frequency cycle, collect 32 phase space charge signals every power frequency cycle, see Figure 3 ;

[0075] S5: The 32-phase space charge signals are processed by the full-phase average method, the alternating charges and interface charges are filtered out, the non-alternating charge characteristic parameters are extracted, the space charge accumulation rule is analyzed based on the charge mobility formula and the injection current formula, and the correlation with insulation degradation is analyzed, see Figure 4 ;

[0076] The specific calculation formula of the full-phase average method is:

[0077]

[0078] wherein, is the non-alternating charge density average value at the radial position is the index of the 32 phase points, corresponding to 0°-360°, with an interval of 11.25°; is the space charge density of the th phase point at the radial position is the weight coefficient of theth phase point, and satisfies the following formula:

[0079]

[0080] wherein, is the unweighted average charge density, and the weight coefficient is used to suppress the interference of abnormal fluctuation signals;

[0081] The movement of electrons in the dielectric is conducted by jumping between traps, and the movement is achieved by jumping from one trap to an adjacent trap to overcome the potential barrier. The mobility calculation of the jump conduction uses the formula:

[0082]

[0083] At low field strength, , then it is simplified as:

[0084]

[0085] wherein, is the elementary charge; is the phonon frequency; is the average distance between traps; is the Boltzmann constant; is the potential barrier between adjacent traps; is the temperature; is the electric field strength;

[0086] The electron injection follows the Poole-Frenkel effect, and the emission current formula is:

[0087]

[0088] wherein, is the temperature; is the Richardson constant; is the electron injection potential barrier; is the Boltzmann constant; is the absolute dielectric constant of the medium; the thermal assisted tunneling effect can be calculated according to the tunneling potential barrier and the mirror charge, and the thermal assisted tunneling current calculation formula is:

[0089] ​

[0090] wherein, is the tunneling probability; is the inverse local radius; is the electrostatic potential; is the blocking surface charge potential barrier;

[0091] The total amount of space charge is calculated by integrating the charge density in the radial range of 12.7-14.1 mm, and the axial monitoring is from -2.5 cm to 0.5 cm with the inside edge of the stress cone as the zero point, wherein the inside of the cable is positive and the outside is negative.

[0092] Another embodiment provided by the application: an electrical-thermal aging test device for simulating the space charge effect of cable terminals, comprising a sample assembly, a thermal stress generating unit, an electrical stress generating unit, a space charge detection unit and a data acquisition unit;

[0093] The sample assembly is composed of a 10 kV XLPE cable sample, a stress cone, a anti-corona ball and an aluminum foil grounding layer. The stress cone is adapted to the stripped section of the cable, and the aluminum foil is wrapped around the middle and outer semiconductive layers and reliably connected to the grounding end.

[0094] The thermal stress generating unit includes a modified dry blast box with an aluminum sample holder and a high-voltage sleeve at the top to maintain a temperature gradient of 80-115°C. The aluminum sample holder has a hollow grid structure with a 0.1 mm thick polytetrafluoroethylene insulating layer sprayed on the surface. The height of the holder can be adjusted to ensure that the cable sample is placed horizontally and the distance from the inner wall of the blast box is not less than 50 mm, ensuring uniform temperature field. The high-voltage sleeve adopts an insulator string structure, the length of the end in the box is extended to prevent end flashover, and the creepage distance meets the requirements of 26.1 kV voltage level.

[0095] The electrical stress generating unit includes a voltage regulator, a step-up transformer, a protective resistor and a coupling capacitor, which outputs 26.1 kV AC voltage to the core through the high-voltage sleeve.

[0096] The space charge detection unit includes an arc-shaped sensor of the electro-acoustic pulse method, a 5 kV / 50 ns high-voltage pulse source and an AC phase matching circuit.

[0097] The arc-shaped contact surface of the arc-shaped sensor of the electro-acoustic pulse method has a curvature radius of 50-100 mm, which matches the curvature of the outer surface of the 10 kV cable terminal stress cone. The contact area is not less than 10 cm². The sensor is tightly attached to the stress cone through a silicone coupling agent with a coupling layer thickness of 0.1-0.3 mm. The sensor is fixed to the stress cone by a buckle. The piezoelectric transducer element of the arc-shaped sensor of the electro-acoustic pulse method is a polyvinylidene fluoride film, as described in Figure 5

[0098] ​The high-voltage pulse source outputs a pulse voltage with an amplitude of 5 kV and a pulse width of 50 ns, the rising edge / falling edge is ≤5 ns, and the repetition frequency is 1600 Hz, as shown in FIG. 2. Figure 6 ;

[0099] The AC phase matching circuit is composed of a capacitance divider, a filter circuit and a zero-crossing comparator, and can control the phase difference between the pulse trigger and the AC voltage within ±11.25°, and 32 phase points are uniformly distributed within a 360° power frequency cycle, so as to ensure uniform sampling.

[0100] The data acquisition unit includes an oscilloscope connected to the detection unit and a matching data processing software, which is used to record the space charge signals of the 32 phase points, supports full-phase average processing, waveform attenuation correction and dispersion recovery, and the waveform attenuation correction of the data processing software is based on the formula:

[0101]

[0102] wherein, is the radial position , the frequency corresponding to the space charge sound pressure signal amplitude; is the reference position , the frequency corresponding to the initial sound pressure signal amplitude; is the piezoelectric conversion coefficient of the sensor; is the frequency-dependent attenuation coefficient; is the dispersion coefficient; is the cable radial detection point position coordinate; is the cable radial reference starting point position coordinate; the signal recovery is realized by a frequency domain deconvolution algorithm, and the inverse matrix function of the Fourier transform of the sensor impulse response is used as the deconvolution kernel function, wherein the specific implementation of the frequency domain deconvolution algorithm includes the following steps and parameter settings:

[0103] 1) Arc-shaped sensor impulse response acquisition: the impulse response function is obtained by standard sample calibration, specifically: a polytetrafluoroethylene sheet with a thickness of 1 mm is selected as a standard sample, which is placed between the arc-shaped sensor and the metal electrode, a 5 kV / 50 ns high-voltage pulse is applied, and the time-domain signal output by the arc-shaped sensor is recorded. After being filtered by a 50 kHz low-pass filter and normalized, the arc-shaped sensor impulse response is obtained.

[0104] 2) Frequency domain deconvolution steps:

[0105] 2.1) Fourier transform is performed on the space charge detection original signal and the impulse response to obtain the frequency domain representations and ;

[0106] 2.2) Calculate the frequency domain recovery signal wherein is a regularization parameter for suppressing high frequency noise amplification;

[0107] 2.3) Inverse Fourier transform of to obtain the time domain recovery signal , i.e., signal recovery is completed;

[0108] Wherein the number of Fourier transform points is 2048 points, and the sampling frequency is consistent with the oscilloscope sampling rate.

[0109] Through the above method and device, the cable sample is subjected to aging test for 375 days under the condition of 103 DEG C and 26.1kV, and the results show that the total amount of space charge at the stress cone monotonously increases with the aging time, and reaches 85*10 -4 C, and the correlation between space charge accumulation and insulation deterioration is verified;

[0110] After processing by the all-phase average method, the signal-to-noise ratio of the non-alternating charge characteristic parameter is improved to more than 30dB, and the alternating charge and interface charge interference is effectively filtered out, refer to Figure 7 .

[0111] In summary, by constructing an aging platform with electric-thermal synergy, combined with a specific cable sample processing method and multi-stage aging parameter setting, the present application realizes accurate simulation of the electric-thermal coupled environment in the actual operation of the cable terminal, breaks through the limitations of traditional single stress tests that cannot reproduce the space charge evolution law under real working conditions, and provides a more practical test basis for research;

[0112] The all-phase average method with weight is used to process the multi-phase space charge signal, and the arc-shaped sensor design matched with the stress cone and the frequency domain deconvolution signal recovery algorithm are used, which not only effectively filters out the interference charge to extract the key characteristic parameters, but also improves the stability and accuracy of the space charge detection through the synergistic optimization of hardware and algorithm, solving the problems of difficult charge signal recognition and low accuracy in complex electric field environment;

[0113] Combining the space charge detection results with the mobility formula and the injection current formula, the charge accumulation law and the insulation deterioration process are deeply related, which not only can realize effective early warning of potential faults of the cable terminal, but also can reveal the internal mechanism of space charge causing insulation failure, providing theoretical and experimental support from phenomenon to essence for the design optimization, life assessment and operation strategy formulation of the cable terminal.

[0114] It will be apparent to those skilled in the art that the application is not limited to the details of the above-exemplified embodiments and that the present application can be implemented in other particular forms without departing from the spirit or essential characteristics of the present application. The embodiments should therefore be considered in all respects as illustrative and not restrictive, the scope of the application being indicated by the appended claims rather than by the above description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. No reference signs in the claims should be considered as limiting the scope of the claims with respect to the figures of the patent document.

Claims

1. A method for simulating the space charge effect at cable terminals using electrothermal aging testing, characterized in that: Includes the following steps: S1: Select a 10kV XLPE cable as a sample, remove the outer semiconducting layers on both sides, retain the outer semiconducting layer in the middle section, install stress cones on the stripped section, install anti-corona balls on the exposed ends of the conductors, and wrap the middle outer semiconducting layer with aluminum foil and ground it. S2: Place the treated cable sample in the modified drying blower box, that is, make a hole in the top of the box, introduce AC high voltage through the high voltage bushing and apply it to the core of the cable aging sample, while controlling the temperature inside the blower box to form thermal stress, and complete the construction of the electrothermal aging platform. S3: Apply 26.1kV AC voltage to the cable sample, control the aging temperature to 80℃ on the outside and 115℃ on the inside of the cable insulation, and set five stages: 75 days, 150 days, 225 days, 300 days and 375 days. S4: Space charge detection was performed on cable samples at different aging stages using the electroacoustic pulse method. The detection parameters were: a high-voltage pulse source outputting a 5kV amplitude, a 50ns pulse width, and a pulse frequency of 1600Hz. The high-voltage pulse source was controlled to trigger at 32 phase points through an AC phase matching circuit, and the space charge signal was collected in each power frequency cycle. S5: The space charge signal of 32 phases is processed by the full-phase averaging method to filter out alternating charge and interface charge, extract non-alternating charge characteristic parameters, and analyze the correlation between space charge accumulation and insulation degradation based on the charge mobility formula and injection current formula.

2. The electrothermal aging test method for simulating the space charge effect of cable terminals according to claim 1, characterized in that: In S1, the cable sample parameters are as follows: copper conductor cross-sectional area 240mm², inner and outer semiconductive shielding layer thickness 0.8mm, XLPE main insulation thickness 4.5mm, total length 340mm, with an outer semiconductive layer length of 90mm retained in the middle.

3. The electrothermal aging test method for simulating the space charge effect of cable terminals according to claim 1, characterized in that: In S2, the modification of the drying blower box includes opening a through hole with a diameter of 50-80mm in the center of the top of the box, adding an insulating flange to the edge of the through hole, and fixing the high-voltage bushing through the flange to ensure that the insulation distance between the bushing and the box is not less than 100mm.

4. The electrothermal aging test method for simulating the space charge effect of cable terminals according to claim 1, characterized in that: In S3, a large current is introduced into the wire core through the current booster to create a temperature gradient of 80°C on the outside to 115°C on the inside of the insulation layer, causing the insulation layer to expand to 5.5mm due to thermal expansion, simulating the electrothermal synergy in actual operation.

5. The electrothermal aging test method for simulating the space charge effect of cable terminals according to claim 1, characterized in that: In step S4, the operation of the AC phase matching circuit includes: 1) Extract a synchronous sine wave signal with an amplitude of 5-10V from the 26.1kV AC voltage using a capacitor divider; 2) After high-frequency noise is removed by a 10kHz low-pass filter circuit, the signal is converted into a square wave signal by a zero-crossing comparator. 3) After the square wave signal is phase-corrected and delayed by 0-5μs, it triggers the signal generator and outputs 32 trigger pulses with a period of 500μs, controlling the high voltage pulse source to trigger uniformly within the 360° power frequency cycle.

6. The electrothermal aging test method for simulating the space charge effect of cable terminals according to claim 1, characterized in that: In S5, the specific calculation formula for the full-phase averaging method is as follows: in, Radial position The average non-alternating charge density at that location; The index consists of 32 phase points, corresponding to 0°-360°, with an interval of 11.25°. For the first Each phase point is located in the radial position. Space charge density at that location; Let be the weight coefficient for the i-th phase point, and satisfy the following formula: in: The unweighted average charge density is used to suppress interference from abnormal fluctuation signals.

7. The electrothermal aging test method for simulating the space charge effect of cable terminals according to claim 1, characterized in that: In step S5, the movement of electrons in the dielectric is accomplished through jump conduction between traps, overcoming the potential barrier to jump from one trap to an adjacent trap. The mobility of jump conduction is calculated using the following formula: At low field strength Then it simplifies to: in, It is the elementary charge; The frequency of the phonon; It is the average distance between traps; It is the Boltzmann constant; It is a barrier between adjacent traps; It's temperature; It is the electric field strength.

8. The electrothermal aging test method for simulating the space charge effect of cable terminals according to claim 1, characterized in that: In S5, electron injection follows the Poul-Frenkel effect, and its emission current formula is: in, It's temperature; It is Richardson's constant; It is an electron injection barrier; It is the Boltzmann constant; Let be the absolute dielectric constant of the medium; the thermally assisted tunneling effect can be calculated based on the tunneling barrier and the image charge. The formula for calculating the thermally assisted tunneling current is: in, It is the tunneling probability; It is the reverse local radius; It is electrostatic potential; It is a barrier that blocks surface charge potential.

9. The electrothermal aging test method for simulating the space charge effect of cable terminals according to claim 1, characterized in that: In S5, the total space charge is calculated by integrating the charge density in the radial range of 12.7-14.1 mm, and the axial monitoring is performed in the range of -2.5 cm to 0.5 cm with the inner edge of the stress cone as the zero point, where the direction towards the inside of the cable is positive and the direction towards the outside is negative.

10. An electrothermal aging test apparatus for simulating the space charge effect at cable terminals, characterized in that: It includes a sample assembly, a thermal stress generation unit, an electrical stress generation unit, a space charge detection unit, and a data acquisition unit; The sample assembly consists of a 10kV XLPE cable sample, a stress cone, an anti-corona ball, and an aluminum foil grounding layer. The stress cone is adapted to the stripped section of the cable, and the aluminum foil wraps the middle outer semiconductive layer. The thermal stress generating unit includes a modified drying blower box with an internal aluminum sample holder and a high-voltage bushing on top, which can maintain a temperature gradient of 80-115℃. The aluminum sample holder adopts a hollow grid structure with a 0.1mm thick polytetrafluoroethylene insulation layer sprayed on the surface. The height of the holder is adjustable to ensure that the cable sample is placed horizontally and the distance between it and the inner wall of the blower box is not less than 50mm, thus ensuring a uniform temperature field. The high-voltage bushing adopts an insulator string structure, extending the length of the inner end of the box to prevent end flashover, and the creepage distance meets the requirements of the 26.1kV voltage level. The electric stress generating unit includes a voltage regulator, a step-up transformer, a protective resistor, and a coupling capacitor, and outputs a 26.1kV AC voltage to the conductor through a high-voltage bushing. The space charge detection unit includes an electroacoustic pulse arc sensor, a 5kV / 50ns high-voltage pulse source, and an AC phase matching circuit. The arc contact surface of the electroacoustic pulse arc sensor has a radius of curvature of 50-100mm, which matches the curvature of the outer surface of the stress cone of the 10kV cable terminal. The contact area is not less than 10cm². It achieves tight adhesion to the stress cone through silicone rubber coupling agent. The coupling layer thickness is 0.1-0.3mm. It is fixed to the stress cone by a buckle. The piezoelectric conversion element of the electroacoustic pulse arc sensor is a polyvinylidene fluoride film. The AC phase matching circuit consists of a capacitive voltage divider, a filter circuit, and a zero-crossing comparator. It can control the phase difference between the pulse trigger and the AC voltage within ±11.25°. The 32 phase points are evenly distributed within the 360° power frequency cycle to ensure uniform sampling. The data acquisition unit includes an oscilloscope connected to the detection unit and supporting data processing software. It records the space charge signal at 32 phase points, supports full-phase averaging processing, waveform attenuation correction, and dispersion recovery. The waveform attenuation correction in the data processing software is based on the formula: in, Radial position Location, frequency The corresponding space charge sound pressure signal amplitude; For reference position Location, frequency The corresponding initial sound pressure signal amplitude; The piezoelectric conversion coefficient of the sensor; This is the frequency-dependent attenuation coefficient; The dispersion coefficient; The coordinates of the radial detection point on the cable; The coordinates are the radial reference starting point coordinates of the cable; signal recovery is achieved through a frequency domain deconvolution algorithm, where the deconvolution kernel function is the inverse Fourier transform matrix of the sensor's impulse response.

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