CN112083294A - Method for nondestructive evaluation of silicon rubber cable joint state by utilizing ultrasonic sound velocity - Google Patents
Method for nondestructive evaluation of silicon rubber cable joint state by utilizing ultrasonic sound velocity Download PDFInfo
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Abstract
The invention discloses a method for nondestructively evaluating the state of a silicon rubber cable joint by utilizing ultrasonic sound velocity, which comprises the steps of judging the operation history of the silicon rubber cable joint and evaluating the aging state of the silicon rubber cable joint by utilizing the ultrasonic sound velocity, establishing a relation curve of the ultrasonic sound velocity of insulation of the silicon rubber cable joint and the equivalent operation age limit of the silicon rubber cable joint at 90 ℃, and indicating the ascending change relation of the ultrasonic sound velocity of an insulation layer of the silicon rubber cable joint along with the increase of the operation time and the aging degree of the cable joint.
Description
Technical Field
The invention belongs to the technical field of power cable detection, and particularly relates to a method for nondestructively evaluating the joint state of a silicon rubber cable by utilizing ultrasonic sound velocity.
Background
The power cable is an important medium for transmitting electric energy, the development of the power cable at home and abroad has a history of hundreds of years, and the running state of the power cable plays a crucial role in the stable running of a power grid. The silicon rubber is widely applied to prefabricated high-voltage cable joints, is used as a main insulating material, and has the characteristics of high elasticity, high and low temperature resistance, excellent electrical performance and the like. Because the cable system has compact structure, high requirements on field installation process and easy introduction of defects, the failure rate of the cable joint is far higher than that of the cable body, and the requirement on the insulation performance of the cable joint is higher. According to statistics data of faults of 110kV and above cable systems in recent ten years by power grid companies, if external force damage is not counted, the fault proportion of cable accessories reaches 85.5%. The cable joint is most used in cable accessories, and is a weak point of electrical insulation and mechanical performance of a cable line and an important node of the cable line for improving operation reliability. Therefore, improving the reliability of the cable joint plays an important role in ensuring safe and stable operation of the power cable system.
At present, high-voltage cable lines which are put into operation in 20 th century in China continuously operate for more than 30 years, the number of silicon rubber cable connectors with the operation time of more than 20 years is huge, and power management departments pay great attention to the states of the cable connectors with long operation time. Therefore, how to accurately and effectively evaluate the insulation state of the silicon rubber cable joint is significant for the stable operation of the power grid. Many scholars at home and abroad have researches on how to evaluate the aging state of the silicone rubber cable joint, and mainly focus on researching the relationship between the material characteristics and the aging state. However, since most domestic and foreign researches are based on laboratory researches, it is difficult to perform on-site detection and analysis on the state of the cable.
At present, most of methods for researching the relationship between the characteristics and the aging state of the insulating material of the high-voltage silicon rubber cable joint belong to destructive experiments, namely the experiments can cause unrecoverable damage to a sample, such as a tensile test, a differential calorimetric scan, a breakdown test, a partial discharge test and the like, so that the possibility of the field use of the research and test means is limited to a certain extent. While the result change is not obvious for the conventional electrical nondestructive test (relative dielectric constant, volume resistivity), and the method is difficult to be used for evaluating the operation state of the silicon rubber cable joint, as shown in fig. 1.
Disclosure of Invention
The invention provides a method for nondestructively evaluating the joint state of a silicon rubber cable by utilizing ultrasonic sound velocity, which is used for carrying out on-site detection and analysis on the state of the cable.
In order to achieve the purpose, the method for nondestructively evaluating the state of the silicon rubber cable joint by utilizing the ultrasonic sound velocity comprises the steps of carrying out an accelerated aging test on a brand-new cable joint of the same type as the silicon rubber cable joint to be evaluated at a test temperature higher than 90 ℃, measuring the ultrasonic sound velocity of an insulating layer of the silicon rubber cable joint with different aging times at the test temperature to obtain the ultrasonic sound velocity of the silicon rubber cable joint with different operation years at 90 ℃, then measuring the ultrasonic sound velocity of the actually operated silicon rubber cable joint to be evaluated on site, and determining the equivalent operation years of a detected object at 90 ℃ according to the ultrasonic sound velocity of the silicon rubber cable joint measured on site.
Further, the method comprises the following steps:
and 4, testing the surface echo signal and the bottom echo signal of the insulating layer of the to-be-evaluated silicon rubber cable joint in actual operation on site, calculating the ultrasonic sound velocity of the silicon rubber insulating layer, and comparing the ultrasonic sound velocity with the corresponding relation curve of the cable joint operation age at 90 ℃ and the ultrasonic sound velocity of the silicon rubber insulating layer obtained in the step 3 to obtain the equivalent operation age of the actually-operated silicon rubber cable joint at 90 ℃.
Further, before step 2, the surface of the sample is washed and dried.
Further, in step 2, when the ultrasonic pulse is transmitted, the ultrasonic probe is placed on the surface of the sample, and a coupling agent is arranged between the ultrasonic probe and the surface of the sample.
Further, in step 2, the frequency of the ultrasonic pulse transmitted to the sample is 1MHz-5 MHz.
Further, in step 2, at the same test time point, for the surface echo and the bottom echo signals of a plurality of different positions of the test sample, calculating an average value of time difference values of the surface echo and the bottom echo of each test point of the test sample at that time, calculating an average ultrasonic sound velocity of the test sample at each test time point according to the distance between the surface and the bottom of the test sample and the average value of the time difference values of the surface echo and the bottom echo, and taking the average ultrasonic sound velocity as the ultrasonic sound velocity of the test sample.
In step 1, the temperature R in the aging test is 90+10N, and N is a positive integer.
Further, in step 2, measurements are taken every other period T, which is 14.3 days.
Compared with the prior art, the invention has at least the following beneficial technical effects:
the invention establishes a corresponding relation curve of the ultrasonic sound velocity of the insulation of the silicon rubber cable joint and the equivalent operation life of the insulation at 90 ℃, then obtains the ultrasonic sound velocity data of the insulation of the service silicon rubber cable joint by a field nondestructive ultrasonic testing technology, judges the operation history of the silicon rubber cable joint by comparing the relation curves and evaluates the aging state of the silicon rubber insulation. The detection and analysis method provided by the invention is a nondestructive evaluation technology, can evaluate the insulation state of the cable joint under the condition of not damaging the structure of the service cable joint, can be used for on-site detection and analysis, is simple to operate, has obvious result change, provides convenience for judging the operation history of the silicon rubber cable joint and evaluating the insulation state, provides convenience for the operation and maintenance of a high-voltage power cable line, and has important significance for the safe and reliable operation of a power grid. The invention can quickly measure and analyze the material properties under different aging states by utilizing the ultrasonic sound velocity of the solid medium.
Furthermore, before the step 2, the surface of the sample is cleaned and dried, so that the surface of the sample is free of impurities, and the test accuracy is improved.
Furthermore, in the step 2, when the ultrasonic pulse is transmitted, the ultrasonic probe is placed on the surface of the sample, so that the calculation is convenient.
Furthermore, in the step 2, the frequency of the ultrasonic pulse transmitted to the sample is 1MHz-5MHz, so that the large attenuation in the ultrasonic transmission process is avoided, and the peak point is easy to measure.
Further, in step 2, at the same test time point, 5-10 surface echo and bottom echo signals at different positions are tested on the sample, the average value of the time difference between the surface echo and the bottom echo of each test point of the sample at the time is calculated, the average ultrasonic sound velocity of the sample at each test time point is calculated according to the distance between the surface and the bottom of the sample, and the average ultrasonic sound velocity is used as the ultrasonic sound velocity of the sample, so that the measurement accuracy is improved.
Further, in step 1, the temperature R of the aging test is 90+10N, N is a positive integer, and for the insulating material, the service life of the cable joint insulation is reduced by half for each 10 ℃ rise of the temperature, and when the temperature R of the aging test meets the condition, the calculation is convenient.
Further, in step 2, the period T is 14.3 days, and at 160 ℃, the aging acceleration multiple is 27The aging state at 90 ℃ for 14.3 days is equivalent to the aging state for 5 years, and the calculation is convenient.
Drawings
FIG. 1 is a graph showing the variation of non-destructive electrical parameters at different aging times at 160 ℃;
FIG. 2 is a schematic diagram of a pulse echo method;
FIG. 3 is a graph showing the relationship between the ultrasonic sound velocity of an insulation layer of an A-type silicone rubber cable joint and the equivalent operating time at 90 ℃;
FIG. 4 is a graph showing the relationship between the ultrasonic velocity of sound and the equivalent operating time at 90 ℃ of an insulating layer of a B-type silicone rubber cable joint
In the drawings: 1. sample, 2, sample surface, 3, sample-to-support-platform contact surface, 4, probe, 5, coupling agent.
Detailed Description
In order to make the objects and technical solutions of the present invention clearer and easier to understand. The present invention will be described in further detail with reference to the following drawings and examples, wherein the specific examples are provided for illustrative purposes only and are not intended to limit the present invention.
In the description of the present invention, it is to be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", and the like, indicate orientations or positional relationships based on those shown in the drawings, and are used only for convenience in describing the present invention and for simplicity in description, and do not indicate or imply that the referenced devices or elements must have a particular orientation, be constructed and operated in a particular orientation, and thus, are not to be construed as limiting the present invention. Furthermore, the terms "first", "second" and "first" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of the present invention, "a plurality" means two or more unless otherwise specified. In the description of the present invention, it should be noted that, unless otherwise explicitly specified or limited, the terms "mounted," "connected," and "connected" are to be construed broadly, e.g., as meaning either a fixed connection, a removable connection, or an integral connection; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meanings of the above terms in the present invention can be understood in specific cases to those skilled in the art.
First, the basic principle of the invention
Wave motion is a form of motion of matter, and vibration is the source of wave motion. From a frequency perspective and with respect to the human sensible frequency as a boundary, the wavelength can be divided into infrasonic, audible, ultrasonic, hypersonic, and lattice vibrations. Wherein, ultrasonic wave refers to sound wave with frequency higher than 20 kHz. Ultrasonic waves are mechanical waves, which are the propagation of mechanical vibrations in an elastic medium.
Ultrasonic waves are similar to light waves in some respects, all of which can reflect, refract, diffuse, and be blocked. These properties can be used to perform material testing, such as using ultrasound to detect internal microscopic defects. The detection method adopted by the invention is a pulse echo method. The acoustic impedance of a material, i.e., the product of the material density and the speed of sound, is an important acoustic characteristic. As long as the acoustic impedance of the medium does not change, the propagation of the ultrasonic wave is not disturbed. The change of the acoustic impedance causes the reflection of part of the ultrasonic signal, so that the ultrasonic probe can receive ultrasonic echoes reflected by different interfaces.
Fig. 2 illustrates the ultrasonic path of the pulse echo method, and the ultrasonic probe can generate ultrasonic pulses with a certain frequency, and once the pulses are triggered, the probe 4 can emit ultrasonic waves with a certain frequency. The ultrasonic waves firstly pass through the couplant 5, and once the ultrasonic waves contact the contact surface of the couplant 5 and the sample 1, part of ultrasonic signals are reflected to form a distinct surface wave pulse (wave T); the ultrasonic wave transmitted through the surface 2 of the sample is continuously transmitted and is reflected to form bottom surface pulse wave (wave B) when reaching the contact surface 3 of the sample and the supporting platform, and vaseline or insulating silicone grease can be used as the coupling agent.
The ultrasonic sound velocity of the insulating material is closely related to characteristic parameters such as density and elastic modulus, and the change of the microstructure in the material in the heat aging process can be accurately reflected. For solid media, the theoretical value v of the propagation velocity of the ultrasonic longitudinal waveLMay be represented by formula (1). In ultrasonic detection, the return time of the surface wave T and the bottom wave B is recorded, and since the propagation paths of the wave T and the wave B are twice the thickness of the detection sample, the calculation can be performed by the equation (2)And (3) ultrasonic sound velocity v of each detection point of the cable joint.
Where E is the Young's modulus of the sample, ρ is the density of the sample, and σ is the Poisson's ratio of the sample.
Wherein d is the thickness of the sample detection point, tBIs the bottom wave B return time, tTThe surface wave T return time.
In addition, the maximum service temperature of the silicon rubber cable joint is 90 ℃, and according to the accelerated aging rule, the service life of the insulation of the cable joint is reduced by half for every 10 ℃ rise of the temperature of the insulation material. According to the rule, the aging states of the cable joint with different operation ages at 90 ℃ can be equalized by using an accelerated aging experiment (namely, the aging temperature is increased), so that the corresponding relation between the ultrasonic sound velocity of the insulation of the silicon rubber cable joint and the equivalent operation age at 90 ℃ is established, and then the operation history of the actually operated silicon rubber cable joint can be judged and the state of the actually operated silicon rubber cable joint can be evaluated by measuring the ultrasonic sound velocity of the silicon rubber cable joint on site.
Second, technical scheme
The invention provides a nondestructive testing evaluation method for the operation history and state of a silicon rubber cable joint. The ultrasonic sound velocity of the insulation of the silicon rubber cable joint at each aging stage is accurately calculated according to a formula by transmitting and receiving ultrasonic echoes reflected from the surface and the bottom surface of the insulation layer of the silicon rubber cable joint, and a relation curve of the ultrasonic sound velocity of the insulation of the silicon rubber cable joint and the equivalent operating life of the insulation of the silicon rubber cable joint at 90 ℃ is established. And then, acquiring echo signals of the cable joint on site, calculating the ultrasonic sound velocity of the silicon rubber insulating layer, and evaluating the 90 ℃ equivalent operating life and the insulating state of the silicon rubber cable joint by comparing the ultrasonic sound velocity with the relation curve.
The specific technical scheme is as follows:
and 3, sampling according to periods (sampling 1 or 2 periods per period): after a sample is naturally cooled to room temperature, a 1MHz-5MHz ultrasonic probe is arranged on the surface of a cable joint section to transmit ultrasonic pulses and receive surface echo signals and bottom echo signals of a sample insulating layer, and a sample in the same aging stage tests 5-10 echo signals at different positions;
and 4, calculating the average value of the time difference values of the surface echo and the bottom echo of the cable joint silicon rubber insulation layer test point in the same aging time, and calculating the average ultrasonic sound velocity of the silicon rubber cable joint insulation layer in each period according to the distance between the surface and the bottom of the cable joint test point.
and 6, testing surface and bottom echo signals of different positions of the insulating layer 5-10 of the actually-operated silicon rubber cable joint of the same model on site, obtaining the average ultrasonic sound velocity of the silicon rubber insulating layer under the same operation time according to the method described in the step 4 and the step 5, comparing the average ultrasonic sound velocity with a relational graph to judge the equivalent operation life of the silicon rubber insulating layer of the main cable joint at 90 ℃ by comparing the average ultrasonic sound velocity with the relational graph (if no special description is provided, the theoretical life is referred to as the designed life of the silicon rubber cable joint when the main cable joint is operated at 90 ℃), and judging the operation history of the silicon rubber cable joint and evaluating the aging state according to the comparison of the equivalent operation.
Example 1
The accelerated aging at 160 ℃ is carried out on the A-type silicon rubber cable connector, and the accelerated aging multiple is 27And therefore, setting 14.3 days per cycle (equivalent to 5 years at 90 ℃), namely setting the ultrasonic sound velocity obtained by testing at 160 ℃ for accelerated ageing for 14.3d to be equal to the ultrasonic sound velocity obtained by running the A-type silicone rubber cable joint at 90 ℃ for 5 years, setting 6 aging cycles in total, and testing unaged and aged samples. And 3-5, carrying out ultrasonic testing on the insulating layer of the silicon rubber cable connector according to the steps of the technical scheme to obtain the ultrasonic sound velocity of the silicon rubber insulation of the sample under different aging times. The table of the relationship between the aging time at 160 ℃ and the ultrasonic sound velocity of the insulating layer of the silicone rubber cable joint is shown in table 1. According to the accelerated aging rule, a relation curve of the equivalent operation age of the cable joint and the ultrasonic sound velocity of the silicon rubber insulating layer at 90 ℃ is obtained by back-stepping and piecewise fitting, and is shown in fig. 3. Wherein a state when the silicone rubber cable joint is aged for 28.5 days at 160 ℃ corresponds to a state of being aged for 10 years at 90 ℃, wherein a state when the silicone rubber cable joint is aged for 42.8 days at 160 ℃ corresponds to a state of being aged for 15 years at 90 ℃, wherein a state when the silicone rubber cable joint is aged for 57.0 days at 160 ℃ corresponds to a state of being aged for 20 years at 90 ℃, wherein a state when the silicone rubber cable joint is aged for 71.3 days at 160 ℃ corresponds to a state of being aged for 25 years at 90 ℃, and wherein a state when the silicone rubber cable joint is aged for 85.5 days at 160 ℃ corresponds to a state of being aged for 30 years at 90 ℃. And the design service life of the silicone rubber cable joint is 30 years.
TABLE 1160 deg.C TABLE FOR RELATIVE TIME OF INSULATING LAYER OF B-TYPE SILICONE RUBBER CABLE JOINT AND ULTRASONIC SOUND SPEED
Comparing the ultrasonic sound velocity obtained by the test of the A-type silicon rubber cable joint which is actually operated on site with the curve of the figure 3, the operation history of the cable joint can be judged and the aging state can be evaluated.
According to the method described in step 6 in the technical scheme, the A-type silicone rubber cable joint which runs for 5 years in a certain place is tested, the ultrasonic sound velocity is measured to be 983.2m/s, and compared with the figure 3, the 90 ℃ equivalent running life of the cable joint can be judged to be between 0 and 5 years. Calculated according to the piecewise linear fitting curve of fig. 3, the 90 ℃ equivalent running time is 5- (984.13-983.2)/(984.13-975.32) × 5 is 4.47 years, and the 90 ℃ equivalent service life is slightly less than the actual service life, which indicates that the cable joint is in a normal running state. Under the condition that the integral operation condition is not changed, the designed service life of the silicon rubber cable connector can reach 90 ℃.
Example 2
In this embodiment, table 1 in embodiment 1 is used to perform an ultrasonic test on a type a cable joint which runs for 12 years in a certain place, the ultrasonic sound velocity is measured to be 993.74m/s, and compared with fig. 3, it can be judged that the 90 ℃ equivalent running life of the cable joint is between 10 and 15 years. According to the piecewise linear fitting curve calculation of fig. 3, the equivalent operating time at 90 ℃ is 15- (995.57-993.74)/(995.57-990.75) × 5 is 13.10 years, and it can be judged that the equivalent operating life at 90 ℃ is slightly longer than the actual operating life, but still ranges from 10 to 15 years, which indicates that the actual operating environment of the cable joint is severe, and there may be an overload operating history, so the aging state is slightly severe compared with the cable joint design condition, and the operating state of the cable joint needs to be focused on in subsequent operations.
Example 3
The accelerated aging of the type B silicon rubber cable connector at 160 ℃ is carried out, and the acceleration multiple is 27The samples were aged for 6 cycles each at 57d (equivalent to 5 years at 90 ℃) and tested unaged and aged samples. According to the method of the step 3-5 of the technical scheme, the silicone rubber is applied to the cable jointAnd carrying out ultrasonic testing on the insulating layer to obtain the ultrasonic sound velocity of the silicon rubber insulation of the sample under different aging times. The table of the relationship between the aging time at 160 ℃ and the ultrasonic sound velocity of the cable joint silicone rubber insulating layer is shown in table 2. According to the accelerated aging rule, a relation curve of the equivalent operation age of the cable joint and the ultrasonic sound velocity of the silicon rubber insulating layer at 90 ℃ is obtained by back-stepping and piecewise fitting, and is shown in fig. 4.
TABLE 2160 deg.C relationship table between B-type silicon rubber cable joint insulation layer aging time and ultrasonic sound velocity
| Aging time (d) | Ultrasonic sound velocity (m · s)-1) |
| 0 | 974.25 |
| 14.3 | 985.74 |
| 28.5 | 991.08 |
| 42.8 | 996.37 |
| 57.0 | 1000.14 |
| 71.3 | 1004.32 |
| 85.5 | 1007.8 |
Ultrasonic testing is carried out on the B-type cable joint which runs for 10 years in a certain place, the ultrasonic sound velocity is measured to be 988.62m/s, and compared with the figure 3, the equivalent running time of the cable joint at 90 ℃ can be judged to be between 5 and 10 years. Calculated according to the piecewise linear fitting curve of fig. 3, the 90 ℃ equivalent running time is 10- (990.75-988.62)/(990.75-984.13) × 5 is 8.39 years, which is less than the actual service time, and thus the actual running environment and the aging state of the cable joint are good, the running environment is within the design running range of the silicone rubber cable joint, and the design service life of the silicone rubber cable joint can reach or even exceed the 90 ℃ under the condition that the overall running condition is not changed.
Example 4
In this embodiment, by using table 2 obtained in example 3 and according to the method described in steps 3 to 5 in the technical scheme, an ultrasonic sound velocity test is performed on a B-type cable joint which has been operated for 20 years in a certain place, and the ultrasonic sound velocity is measured to be 1002.38m/s, and compared with fig. 3, it can be judged that the 90 ℃ equivalent operating life of the cable joint is between 20 and 25 years. Calculated according to the piecewise linear fit curve of fig. 3, the equivalent operating time of the cable joint at 90 ℃ is 25- (1003.6-1002.38)/(1003.6-999.03) × 5 is 24.99 years, which is much longer than the actual operating time, and indicates that the actual operating environment of the cable joint is very severe, and the cable joint may experience an extreme operating environment in the service process or has a severe overload operating history and a severe aging state. Because the cable joint has longer running time, is close to the design service life, has severe ultrasonic sound velocity change and serious performance degradation, the batch of silicon rubber cable joints need to be replaced in time in consideration of maintaining the safe and reliable running of a cable system, and the occurrence of power accidents is avoided.
The invention provides a method for judging the operation history of a silicon rubber cable connector and evaluating the aging state of the silicon rubber cable connector by using ultrasonic sound velocity;
the invention establishes a relation curve of the ultrasonic sound velocity of the insulation of the silicon rubber cable joint and the equivalent operation age of the insulation at 90 ℃, and points out the change relation that the ultrasonic sound velocity of the insulation layer of the silicon rubber cable joint rises along with the increase of the operation time and the aging degree of the cable joint.
The above-mentioned contents are only for illustrating the technical idea of the present invention, and the protection scope of the present invention is not limited thereby, and any modification made on the basis of the technical idea of the present invention falls within the protection scope of the claims of the present invention.
Claims (8)
1. A method for nondestructively evaluating the state of a silicon rubber cable joint by utilizing ultrasonic sound velocity is characterized in that a brand-new cable joint of the same type as a silicon rubber cable joint to be evaluated is subjected to an accelerated aging test at a test temperature higher than 90 ℃, the ultrasonic sound velocity of a silicon rubber cable joint insulating layer with different aging time at the test temperature is obtained by measurement, the ultrasonic sound velocity of the silicon rubber cable joint at different operation years at 90 ℃ is obtained by the equivalent test, then the ultrasonic sound velocity of the silicon rubber cable joint which is actually operated and is to be evaluated is measured on site, and the equivalent operation year of a detected object at 90 ℃ is determined according to the ultrasonic sound velocity of the silicon rubber cable joint measured on site.
2. The method for nondestructively evaluating the joint state of the silicone rubber cable by using the ultrasonic sound velocity according to claim 1, characterized by comprising the steps of:
step 1, taking a brand-new cable joint with the same type as a silicone rubber cable joint to be evaluated, recording the brand-new cable joint as a sample, and determining the temperature R of an aging test;
step 2, determining a test period T, transmitting ultrasonic pulses to the sample and receiving surface echo signals and bottom echo signals of the insulation layer of the sample at set time intervals from 0 moment to measure until equivalent time of a measuring point at a temperature R reaches the designed service life of the sample, calculating time difference of the surface echo and the bottom echo of each test time point, and calculating the ultrasonic sound velocity of the insulation layer of the silicone rubber cable joint of each period according to the distance between the surface and the bottom of the test point of the cable joint and the time difference of the surface echo and the bottom echo;
step 3, obtaining the ultrasonic sound velocity of the insulation of the silicone rubber cable joint at each test time point through the step 2, reversely deducing the equivalent operation life of the silicone rubber cable joint at 90 ℃ through the aging temperature R, and drawing a corresponding relation curve of the operation life of the silicone rubber cable joint at 90 ℃ and the ultrasonic sound velocity of the silicone rubber insulation layer;
and 4, testing the surface echo signal and the bottom echo signal of the insulating layer of the to-be-evaluated silicon rubber cable joint in actual operation on site, calculating the ultrasonic sound velocity of the silicon rubber insulating layer, and comparing the ultrasonic sound velocity with the corresponding relation curve of the cable joint operation age at 90 ℃ and the ultrasonic sound velocity of the silicon rubber insulating layer obtained in the step 3 to obtain the equivalent operation age of the actually-operated silicon rubber cable joint at 90 ℃.
3. The method for non-destructive evaluation of the joint state of silicone rubber cable by using ultrasonic sound velocity as claimed in claim 1, wherein the surface of the test specimen is cleaned and dried before step 2.
4. The method for non-destructive evaluation of the joint state of the silicone rubber cable by using ultrasonic sound velocity according to claim 1, wherein in the step 2, when the ultrasonic pulse is transmitted, the ultrasonic probe is placed on the surface of the test sample, and the couplant is arranged between the ultrasonic probe and the surface of the test sample.
5. The method for non-destructive evaluation of the joint state of the silicone rubber cable by using the ultrasonic sound velocity according to claim 1, wherein in the step 2, the frequency of the ultrasonic pulse transmitted to the test specimen is 1MHz to 5 MHz.
6. The method for non-destructively assessing the state of a silicone rubber cable joint by using ultrasonic sound velocity according to claim 1, wherein in step 2, for the same test time point, the surface echo and the bottom echo signals of the test sample at a plurality of different positions are calculated, and the average value of the time difference between the surface echo and the bottom echo of each test point of the test sample at that time is calculated, and the average ultrasonic sound velocity of the test sample at each test time point is calculated according to the distance between the surface and the bottom of the test sample and the average value of the time difference between the surface echo and the bottom echo, and the average ultrasonic sound velocity is used as the ultrasonic sound velocity of the test sample.
7. The method for nondestructive evaluation of joint state of silicone rubber cable by utilizing ultrasonic sound velocity as claimed in claim 1, wherein in step 1, the temperature of aging test is R-90 +10N, N is positive integer.
8. The method for nondestructive evaluation of joint condition of silicone rubber cable by utilizing ultrasonic sound velocity according to claim 7, wherein in step 2, measurement is performed every period T, where T is 14.3 days.
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