XLPE conductivity-based high-voltage direct-current cable temperature stability evaluation method

Abstract

A high-voltage direct-current cable temperature stability evaluation method based on XLPE conductivity comprises the steps that direct currents of an XLPE insulating material at different temperatures are measured through resistivity testing; calculating corresponding conductivity and corresponding temperature stability characterization indexes according to the direct current to evaluate the temperature stability of the high-voltage direct-current cable XLPE insulating material, and calculating a simulation model of the high-voltage direct-current cable XLPE insulating material by using finite element analysis software to obtain a temperature distribution section diagram; and evaluating the thermal stability of the XLPE insulating material of the high-voltage direct-current cable in engineering practice according to the temperature difference between the sheath of the tested cable and the sheath of the reference cable after transient internal pressurization. The method is efficient and high in accuracy.

Description

Technical Field

[0001] The invention relates to the technical field of cable temperature detection, and in particular to a temperature stability evaluation method for a high-voltage direct current cable based on XLPE conductivity. Background Art

[0002] Cross-linked polyethylene (XLPE) has been widely used in AC and DC power cables due to its excellent electrical, heat-resistant and mechanical properties. With the development of the economy and society and the increase in electricity demand, the application voltage level of XLPE insulated cables has also increased. In response to the challenges of higher voltage levels, the current cable structure design mostly adopts increased insulation thickness. On the one hand, the increase in insulation thickness makes it difficult to discharge cross-linked by-products, and on the other hand, it increases the temperature difference between the inner and outer layers of the insulation. Due to the sensitivity of the conductivity of XLPE materials to temperature, the electric field distortion in the insulation is aggravated, increasing the probability of insulation failure. Therefore, for XLPE insulation materials with higher voltage levels, it is necessary to additionally examine the temperature stability of the material's conductivity on the basis of examining its electrical and mechanical properties. However, there is still a lack of a simple and efficient method to judge the thermal stability of XLPE materials.

[0003] The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art. Summary of the invention

[0004] The invention provides a temperature stability evaluation method for a high-voltage direct current cable based on XLPE conductivity. To meet the working temperature range of cable insulation, a ratio of high-temperature conductivity to low-temperature conductivity is selected to evaluate the temperature stability of a high-voltage direct current cable insulation material. A thermal stability evaluation index is calculated according to the conductivity measured at a set temperature. A temperature distribution cross-sectional diagram in a cable model is obtained in combination with finite element simulation software. The temperature stability of a cross-linked polyethylene direct current cable is further evaluated according to the temperature distribution cross-sectional diagram, so that the temperature stability is simply and efficiently evaluated.

[0005] A method for evaluating the temperature stability of a high-voltage DC cable based on XLPE conductivity includes: S1. The DC current of XLPE insulation material at different temperatures is measured by resistivity test; S2. Calculate the corresponding conductivity and the corresponding temperature stability characterization index according to the DC current to evaluate the temperature stability of the XLPE insulation material of the high-voltage DC cable. The temperature stability characterization index is determined by the following formula: , Where: It is an indicator of temperature stability; is the high temperature conductivity, which represents the maximum allowable temperature of XLPE insulation or the maximum operating temperature of high voltage DC cable; It is the low temperature conductivity, representing the room temperature or the lowest temperature of the cable insulation; S3. The temperature distribution cross-section diagram is obtained by calculating the simulation model of the XLPE insulation material of the high-voltage DC cable using finite element analysis software. According to the temperature difference between the sheath of the tested cable and the reference cable after transient internal pressurization, the thermal stability of the XLPE insulation material of the high-voltage DC cable in the actual project is evaluated.

[0006] In the temperature stability evaluation method of a high-voltage DC cable based on XLPE conductivity, in step S1, the resistivity test includes at least a DC high-voltage source, an electrode system, an ammeter, a data acquisition system and a temperature control device. The data acquisition system is arranged between the high-current meter and the electrode system, and data acquisition is realized by a computer for measurement time control and range selection, and the DC current data is recorded in real time.

[0007] In the temperature stability evaluation method of a high-voltage DC cable based on XLPE conductivity, a shielding system is arranged outside the electrode system to prevent electromagnetic interference during the measurement process; and a constant temperature box is arranged outside the shielding system to realize temperature control during the measurement.

[0008] In the temperature stability evaluation method of a high-voltage DC cable based on XLPE conductivity, the maximum allowable temperature of XLPE insulation or the maximum operating temperature of the high-voltage DC cable is 90°C or 70°C, respectively, and the room temperature or the minimum temperature of the cable insulation is 30°C.

[0009] In the temperature stability evaluation method of a high-voltage DC cable based on XLPE conductivity, the corresponding conductivity is calculated by the following formula:

[0010] Where: I v is the leakage current; d is the specimen thickness; U is the externally applied DC voltage; S To measure the effective area of ​​the electrode.

[0011] In the described method for evaluating the temperature stability of a high-voltage direct current cable based on XLPE conductivity, a simulation model of the XLPE insulating material of the high-voltage direct current cable is established using finite element simulation software, and component parameters, heat sources, initial temperature values, and initial potential values ​​of the simulation model are set; the temperature change trend after the internal pressurization of the cable is simulated in combination with transient simulation, the reference cable sheath temperature and the measured cable sheath temperature are obtained according to the temperature section diagram, and the thermal stability of the high-voltage direct current is evaluated according to the temperature difference between the two.

[0012] In the temperature stability evaluation method of a high-voltage DC cable based on XLPE conductivity, the simulation model is sequentially composed of a conductor, a conductor shielding layer, an XLPE insulating layer, an insulating shielding layer, a water-blocking layer, a metal sheath and an outer sheath from the inside to the outside, and the center lines of the components coincide; The component parameters of the simulation model include density, heat capacity, thermal conductivity, electrical conductivity, and dielectric constant.

[0013] In the temperature stability evaluation method of a high-voltage DC cable based on XLPE conductivity, the heat sources in the simulation model are the Joule heat generated by the conductor current and the heat generated by the insulation leakage current. The Joule heat generated by the conductor current depends on the cable conductor loss, and the heat generated by the insulation leakage current has a unit volume heat generation power of γE 2 , g is the conductivity ， E For field strength.

[0014] The initial temperature of the simulation model is between the maximum operating temperature of the power cable and the room temperature, and the initial temperature of each layer is the same.

[0015] In the temperature stability evaluation method of a high-voltage direct current cable based on XLPE conductivity, the initial potential of the conductor shielding layer and the insulating layer of the simulation model is 0; and the heat transfer mode of the simulation model selects solid heat transfer.

[0016] In the temperature stability evaluation method of a high-voltage DC cable based on XLPE conductivity, the sheath temperature is determined according to the outer temperature of the temperature distribution section diagram obtained by simulation; the reference cable sheath temperature is the sheath temperature when the cable is fully loaded and the conductor is not subjected to voltage; the measured cable sheath temperature is the sheath temperature when the cable is fully loaded and stable and the conductor is subjected to 1.45 U 0, and the temperature change of the sheath of the tested cable is simulated within at least 120 hours; if the temperature difference between the tested cable and the reference cable sheath is less than 4°C in the last 96 hours and less than 2°C in the last 72 hours, it means that the thermal stability is good. If the above conditions are not met, the test is extended to 240 hours. If the temperature difference requirement is still not met, it is considered that the tested cable does not meet the thermal stability requirements.

[0017] Compared with the prior art, the present invention has the following advantages: the present invention performs evaluation by calculating the characterization index by measuring the conductivity at different temperatures, thereby significantly reducing the experimental cost of evaluating the temperature stability of cable insulation materials, greatly improving the speed and efficiency of evaluating the temperature stability of cross-linked polyethylene cable insulation, and combining the finite element simulation software cable model to perform thermal stability testing, thereby achieving efficient and accurate evaluation of the temperature stability of high-voltage DC cables. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] By reading the detailed description of the preferred specific embodiments below, various other advantages and benefits of the present invention will become clear to those of ordinary skill in the art. The drawings in the specification are only for the purpose of illustrating the preferred embodiments and are not considered to be limitations of the present invention. Obviously, the drawings described below are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can also be obtained based on these drawings without creative work. Moreover, the same reference numerals are used to represent the same components throughout the drawings.

[0019] In the attached picture: Figure 1 It is a schematic flow chart of the method for evaluating the temperature stability of a cross-linked polyethylene DC cable provided in the present invention; Figure 2 It is a schematic diagram of the resistivity and conductivity testing system in the present invention; Figure 3 A schematic diagram of the resistivity system dimensions in the present invention; Figure 4 It is a schematic diagram of a DC cable simulation model in the present invention; Figure 5 It is a temperature distribution cross-sectional diagram calculated by the cable simulation model in the present invention; Figure 6 A schematic diagram of the change of XLPE current over time (30°C, 20 kV / mm) provided for an exemplary embodiment of the present invention; Figure 7 A schematic diagram of the change of XLPE conductivity with temperature provided by an exemplary embodiment of the present invention; Figure 8 A schematic diagram of the activation energy of XLPE provided by an exemplary embodiment of the present invention; Fig. 9 A schematic diagram of the electric field distribution inside the cable insulation provided by an exemplary embodiment of the present invention.

[0020] The present invention is further explained below in conjunction with the accompanying drawings and embodiments. DETAILED DESCRIPTION

[0021] The specific embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although the specific embodiments of the present invention are shown in the accompanying drawings, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided in order to enable a more thorough understanding of the present invention and to fully convey the scope of the present invention to those skilled in the art.

[0022] It should be noted that certain words are used in the specification and claims to refer to specific components. Those skilled in the art should understand that technicians may use different nouns to refer to the same component. This specification and claims do not use the difference in nouns as a way to distinguish components, but use the difference in the functions of the components as the criterion for distinction. As mentioned throughout the specification and claims, "including" or "comprising" is an open term, so it should be interpreted as "including but not limited to". The subsequent description of the specification is a preferred embodiment of the present invention, but the description is based on the general principles of the specification and is not intended to limit the scope of the present invention. The scope of protection of the present invention shall be determined by the attached claims.

[0023] To facilitate understanding of the embodiments of the present invention, further explanation will be given below by taking specific embodiments as examples in conjunction with the accompanying drawings, and each of the accompanying drawings does not constitute a limitation on the embodiments of the present invention.

[0024] like Figures 1 to 9 As shown, the temperature stability evaluation method of high-voltage DC cable based on XLPE conductivity includes the following steps: S1. The DC current of XLPE insulation material at different temperatures is measured by resistivity test; S2. Calculate the corresponding conductivity and the corresponding temperature stability characterization index according to the DC current to evaluate the temperature stability of the XLPE insulation material of the high-voltage DC cable. The temperature stability characterization index is determined by the following formula: , Where: It is an indicator of temperature stability; is the high temperature conductivity, which represents the maximum allowable temperature of XLPE insulation or the maximum operating temperature of high voltage DC cable; It is the low temperature conductivity, representing the room temperature or the lowest temperature of the cable insulation; S3. The temperature distribution cross-section diagram is obtained by calculating the simulation model of the XLPE insulation material of the high-voltage DC cable using finite element analysis software. According to the temperature difference between the sheath of the tested cable and the reference cable after transient internal pressurization, the thermal stability of the XLPE insulation material of the high-voltage DC cable in the actual project is evaluated.

[0025] In a preferred embodiment of the high-voltage DC cable temperature stability evaluation method based on XLPE conductivity, in step S1, the resistivity test includes at least a DC high-voltage source, an electrode system, an ammeter, a data acquisition system and a temperature control device. The data acquisition system is arranged between the high-current meter and the electrode system, and data acquisition is realized by a computer for measurement time control and range selection, and the DC current data is recorded in real time.

[0026] In a preferred embodiment of the temperature stability evaluation method of a high-voltage DC cable based on XLPE conductivity, a shielding system is provided outside the electrode system to prevent electromagnetic interference during the measurement process; and a constant temperature box is provided outside the shielding system to achieve temperature control during the measurement.

[0027] In a preferred implementation of the method for evaluating the temperature stability of a high-voltage DC cable based on XLPE conductivity, the maximum allowable temperature of the XLPE insulation or the maximum operating temperature of the high-voltage DC cable is 90°C or 70°C, respectively, and the room temperature or the minimum temperature of the cable insulation is 30°C.

[0028] In a preferred embodiment of the method for evaluating the temperature stability of a high-voltage DC cable based on XLPE conductivity, the corresponding conductivity is calculated by the following formula:

[0029] Where: I v is the leakage current; d is the specimen thickness; U is the externally applied DC voltage; S To measure the effective area of ​​the electrode.

[0030] In a preferred embodiment of the temperature stability evaluation method for a high-voltage DC cable based on XLPE conductivity, a simulation model of the XLPE insulation material of the high-voltage DC cable is established using finite element simulation software, and the component parameters, heat source, initial temperature value, and initial potential value of the simulation model are set; the temperature change trend after the internal pressurization of the cable is combined with transient simulation, and the reference cable sheath temperature and the measured cable sheath temperature are obtained according to the temperature section diagram, and the thermal stability of the high-voltage DC current is evaluated according to the temperature difference between the two.

[0031] In a preferred embodiment of the temperature stability evaluation method of a high-voltage DC cable based on XLPE conductivity, the simulation model is sequentially composed of a conductor, a conductor shielding layer, an XLPE insulating layer, an insulating shielding layer, a water-blocking layer, a metal sheath and an outer sheath from the inside to the outside, and the center lines of the components coincide; The component parameters of the simulation model include density, heat capacity, thermal conductivity, electrical conductivity, and dielectric constant.

[0032] In a preferred embodiment of the temperature stability evaluation method of a high-voltage DC cable based on XLPE conductivity, the heat sources in the simulation model are the Joule heat generated by the conductor current and the heat generated by the insulation leakage current. The Joule heat generated by the conductor current depends on the cable conductor loss. The heat generated by the insulation leakage current has a unit volume heat power of γE 2 , g is the conductivity ，EFor field strength.

[0033] The initial temperature of the simulation model is between the maximum operating temperature of the power cable and the room temperature, and the initial temperature of each layer is the same.

[0034] In a preferred implementation of the method for evaluating temperature stability of a high-voltage DC cable based on XLPE conductivity, the initial potential of the conductor shielding layer and the insulating layer of the simulation model is 0; and the heat transfer mode of the simulation model selects solid heat transfer.

[0035] In a preferred embodiment of the temperature stability evaluation method of a high-voltage DC cable based on XLPE conductivity, the sheath temperature is determined according to the outer temperature of the temperature distribution cross-section diagram obtained by simulation; the reference cable sheath temperature is the sheath temperature of the conductor when the cable is fully loaded and no voltage is applied; the measured cable sheath temperature is the sheath temperature of the conductor when the cable is fully loaded and stable and 1.45 U 0, and the temperature change of the sheath of the tested cable is simulated within at least 120 hours; if the temperature difference between the tested cable and the reference cable sheath is less than 4°C in the last 96 hours and less than 2°C in the last 72 hours, it means that the thermal stability is good. If the above conditions are not met, the test is extended to 240 hours. If the temperature difference requirement is still not met, it is considered that the tested cable does not meet the thermal stability requirements.

[0036] In one embodiment, the method comprises the following steps: Step 1: measuring the DC current of the XLPE insulation material at different temperatures by using a resistivity and conductivity test system; Step 2: According to the measured DC current, the corresponding conductivity and the corresponding temperature stability characterization index are calculated, and the temperature stability of the high-voltage DC cable insulation material can be evaluated accordingly; Step 3: Use finite element simulation software to simulate the DC cable model to obtain a temperature distribution cross-section diagram. According to the temperature difference between the tested cable and the reference cable sheath after transient internal pressurization, the thermal stability of the high-voltage DC cable in actual engineering is evaluated.

[0037] Step 1: In one embodiment, the resistivity testing system in step 1 is as follows: Figure 2As shown in the figure, it mainly includes a DC high-voltage source, a temperature control system, a three-electrode system, a protective resistor, a high-resistance meter and a computer. A DC high-voltage source is used, the output voltage is 0~40 kV, and the ripple factor is 0.1%. A current-limiting resistor of about 15MΩ is used. An electrometer is used, and the current measurement range is 1 nA~21mA. Data acquisition is achieved through a computer, which is used for measurement time control and range selection, and real-time recording of current data. Refer to the working principle of the resistivity test system to connect the circuit. During the test, wipe the sample in advance to ensure its surface is clean, and then load the corresponding voltage. The leakage current is recorded through a high-resistance meter and a computer. The specific parameters of the resistivity system are as follows. Figure 3 As shown, the conductivity of the material is calculated. A sheet sample with a size of 100 mm × 100 mm × 0.2 mm was selected. The polarization process will affect the value of the conduction current, and it takes a certain amount of time for the current to reach a stable state.

[0038] To determine the test time, Figure 6 The variation trend of XLPE current over time at 30°C and 20 kV / mm (close to the internal field strength of high-voltage DC cable insulation) is given. It can be clearly seen that after 1000 s, the current has no obvious change. Therefore, this embodiment selects 1860 s as the measurement time, selects the current value of the last 60 s to calculate the resistivity of the sample, and the selection of the experimental temperature is based on the actual temperature range of the cable insulation.

[0039] In one embodiment, the DC current at different temperatures is measured according to step 1. Specifically, the high temperature conductivity test temperature is 70° C., the test field strength is 20 kV / mm, and the XLPE DC current at different temperatures is shown in Table 1.

[0040] Table 1 DC current of two XLPE materials at 30℃ and 70℃

[0041] Step 2: In one embodiment, specifically, the effective area of ​​the measuring electrode is 0.000491 m 2 The DC conductivity of XLPE is calculated using formula (1) as shown in Table 2.

[0042] Table 2 Electrical conductivity of two XLPE materials at different temperatures

[0043] According to the measured conductivity at different temperatures, the XLPE temperature stability characterization index G is calculated using formula (2) as shown in Table 3.

[0044] Table 3 XLPE temperature stability characterization index G

[0045] In this embodiment, the characterization parameters of XLPE1 are greater than those of XLPE2, which indicates that the temperature stability of XLPE1 is better than that of XLPE2.

[0046] According to the conductivity mechanism, XLPE is mainly electron hopping conductivity under experimental conditions. The localized state density in amorphous dielectrics is very large, and electrons are mostly in localized states. Therefore, electron migration between localized states is the main process, that is, electrons migrate from one localized state to another localized state through jumping potential barriers. This process is vividly described as jumping migration. The probability of electrons jumping between two adjacent localized states is related to the spatial distance between the two states. a Potential Barrier u 0, when the electronic thermal vibration frequency is set to v When , the mobility can be obtained as: (4) Furthermore, considering the concentration of conductive carriers n And the amount of charge q , we can get the conductivity formula for electron hopping conductivity: (5) Taking the logarithm of both sides of equation (5) yields: (6) Therefore, if the logarithm of conductivity is used as the ordinate and the reciprocal of absolute temperature is used as the abscissa, a straight line can be obtained, and the activation energy can be calculated by the slope of the straight line.

[0047] In this embodiment, the conductivity of two different XLPE materials at different temperatures is measured. Figure 7 As shown, further fitting calculations can be performed to obtain the activation energy, such as Figure 8 It can be seen that XLPE1 has a small activation energy, and it is believed that its conductivity changes less with temperature, that is, the material has a high temperature stability.

[0048] According to the DC cable electric field design, the insulation layer of the cable will present a Laplace electric field with high inside and low outside, as well as a temperature difference with high inside and low outside. However, due to the influence of temperature and electric field on XLPE, the conductivity in the insulation layer presents a distribution with high inside and low outside. In the case of a DC electric field, the conductivity will directly affect the distribution of the electric field in the insulating material. Under the combined effect of the above reasons, the electric field on the inside of the insulating material will be relatively reduced, while the electric field on the outside will be relatively increased. In certain cases, there will even be a phenomenon of "electric field reversal", which will cause the electric field in the actual insulating material to deviate from the designed electric field, increasing the risk of cable failure. Fig. 9The electric field distribution of the cable insulation layer using the above two XLPE as insulation is given in the figure. Although both of them have the "electric field reversal" phenomenon, the "electric field reversal" degree and maximum field strength of XLPE1 with lower activation energy and better temperature stability are smaller. This shows that the use of activation energy can measure the temperature stability of materials and guide their actual use.

[0049] Therefore, the temperature stability evaluation method proposed in the present invention can significantly reduce the experimental cost of cable insulation temperature stability evaluation, and greatly improve the speed and efficiency of cross-linked polyethylene cable insulation temperature stability evaluation.

[0050] Step 3: In this embodiment, a DC cable model is established in the simulation software according to the 500 kV DC cable model structure.

[0051] Furthermore, the corresponding density, heat capacity and electrical conductivity can be obtained according to the specific type of each component. Usually, the density and heat capacity are obtained according to the factory calibration values, the thermal conductivity of each component is measured by a thermal conductivity measuring instrument, and the relative dielectric constant is measured by a broadband spectrum.

[0052] The reference cable conductor parameter settings are: thermal conductivity is 401 W / (m·K), density is 8940 kg / m 3 , heat capacity is 385 J / (kg·K), electrical conductivity is 57142875 S / m, and relative dielectric constant is 1; The reference cable shielding layer parameter settings are: thermal conductivity is 0.28 W / (m·K), density is 1120 kg / m 3 , heat capacity is 2700 J / (kg·K), conductivity is 0.1 S / m, and relative dielectric constant is 100; The reference cable water barrier parameters are as follows: thermal conductivity is 0.08 W / (m·K), density is 200 kg / m 3 , heat capacity is 60 J / (kg·K); The reference cable metal sheath parameter settings are: thermal conductivity is 250 W / (m·K), density is 8940 kg / m 3 , heat capacity is 385 J / (kg·K); The reference cable outer sheath parameter settings are: thermal conductivity is 0.29 W / (m·K), density is 968 kg / m 3 , heat capacity is 2532 J / (kg·K); The reference cable XLPE insulation layer parameter settings are: thermal conductivity is 0.285 W / (m·K), density is 980kg / m 3, heat capacity is 2200 J / (kg·K), relative dielectric constant is 2.2, in particular, the conductivity satisfies formula (3), β 1.0413, XLPE1 and XLPE2 γ 0 are set to 2.91×10 -29 S / m and 2.94×10 -31 S / m, α Set to 0.0608 and 0.0767 respectively, XLPE1 is less dependent on temperature than XLPE2. Equation (3) assumes that the dielectric is uniform and considers the parameter α and β A single value, but the parameter α and β Depends on temperature and field strength respectively. In normal operation, α and β The value of will vary throughout the insulation. Therefore, these stresses and the effects of temperature must be considered, thus α and β The uncertainty in the calculation is about 3%. In this embodiment, parameter settings of various components of the tested cable are consistent with those of the standard cable.

[0053] Furthermore, the initial temperature of each part of the cable model is set to 303.15 K. The heat sources in the cable model are set to the Joule heat of the conductor current and the heat generated by the insulation leakage current. The former is 17300 W / m 3 , the latter has a unit volume heat generation power of γE 2 ; During the thermal stability test, apply 1.45 on the outside of the conductor U 0, i.e. 725 kV transient voltage, the simulation time is selected from 0 to 120 h, and the measurement interval is selected as 12 h. The sheath temperature of the tested cable in the last 96 h is determined according to the temperature section diagram. The test results are shown in Table 4.

[0054] Table 4 Temperature and temperature difference between reference cable and tested cable sheath

[0055] Table 4

[0056] Combined with Table 4, the temperature difference between XLPE1 and the reference cable sheath in the last 96 hours obtained from the temperature section diagram is less than 4°C, and less than 2°C in the last 72 hours, indicating good thermal stability. However, the temperature difference between XLPE2 and the reference cable sheath in the last 72 hours is greater than 2°C, and the temperature difference requirement cannot be met even after being extended to 240 hours. It is believed that the XLPE2 tested cable does not meet the thermal stability standard, which is consistent with the evaluation results of the temperature stability characterization index.

[0057] In summary, the present invention points out that by simulating the cable model using finite element simulation software to obtain a steadily distributed cross-sectional diagram, according to the temperature difference between the sheath of the tested cable and the reference cable after transient internal pressurization, the thermal stability of different high-voltage DC cables in actual engineering can be evaluated, thereby improving the accuracy of the temperature stability evaluation of the high-voltage DC cable.

[0058] Although the embodiments of the present invention are described above in conjunction with the accompanying drawings, the present invention is not limited to the above specific embodiments and application fields, and the above specific embodiments are only illustrative and instructive, rather than restrictive. A person of ordinary skill in the art can also make many forms under the guidance of this specification and without departing from the scope of protection of the claims of the present invention, all of which belong to the protection of the present invention.

Claims

1. A method for evaluating the temperature stability of a high-voltage DC cable based on XLPE conductivity, characterized in that: The steps include: S1. The DC current of XLPE insulation material at different temperatures is measured by resistivity test; S2. Calculate the corresponding conductivity and the corresponding temperature stability characterization index according to the DC current to evaluate the temperature stability of the XLPE insulation material of the high-voltage DC cable. The temperature stability characterization index is determined by the following formula: ， Where: It is an indicator of temperature stability; is the high temperature conductivity, which represents the maximum allowable temperature of XLPE insulation or the maximum operating temperature of high voltage DC cable; It is the low temperature conductivity, representing the room temperature or the lowest temperature of the cable insulation; S3. The temperature distribution cross-section diagram is obtained by calculating the simulation model of the XLPE insulation material of the high-voltage DC cable using finite element analysis software. According to the temperature difference between the sheath of the tested cable and the reference cable after transient internal pressurization, the thermal stability of the XLPE insulation material of the high-voltage DC cable in the actual project is evaluated.

2. A method for evaluating temperature stability of a high-voltage DC cable based on XLPE conductivity according to claim 1, characterized in that: Preferably, in step S1, the resistivity test includes at least a DC high voltage source, an electrode system, an ammeter, a data acquisition system and a temperature control device. The data acquisition system is arranged between the high current meter and the electrode system, and realizes data acquisition through a computer for measurement time control and range selection, and records the DC current data in real time.

3. A method for evaluating temperature stability of a high-voltage DC cable based on XLPE conductivity according to claim 2, characterized in that: A shielding system is arranged outside the electrode system to prevent electromagnetic interference during the measurement process; a constant temperature box is arranged outside the shielding system to realize temperature control during the measurement.

4. A method for evaluating temperature stability of a high-voltage DC cable based on XLPE conductivity according to claim 1, characterized in that: The maximum allowable temperature of XLPE insulation or the maximum operating temperature of high-voltage DC cable is 90℃ or 70℃ respectively, and the room temperature or the minimum temperature of cable insulation is 30℃.

5. The method for evaluating the temperature stability of a high-voltage DC cable based on XLPE conductivity according to claim 1, characterized in that: The corresponding conductivity is calculated using the following formula: ， Where: I v is the leakage current; d is the specimen thickness; U is the externally applied DC voltage; S To measure the effective area of ​​the electrode.

6. A method for evaluating temperature stability of a high-voltage DC cable based on XLPE conductivity according to claim 1, characterized in that: Use finite element simulation software to establish a simulation model of XLPE insulation material for high-voltage DC cables, and set component parameters, heat sources, initial temperature values, and initial potential values ​​of the simulation model; Combined with the transient simulation of the temperature change trend after the cable is pressurized, the reference cable sheath temperature and the measured cable sheath temperature are obtained according to the temperature section diagram, and the thermal stability of the high-voltage DC current is evaluated based on the temperature difference between the two.

7. A method for evaluating temperature stability of a high-voltage DC cable based on XLPE conductivity according to claim 6, characterized in that: The simulation model is composed of conductor, conductor shielding layer, XLPE insulation layer, insulation shielding layer, water-blocking layer, metal sheath and outer sheath in order from inside to outside, and the center lines of each component coincide. The component parameters of the simulation model include density, heat capacity, thermal conductivity, electrical conductivity, and dielectric constant.

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