CN119312608A - Temperature stability assessment method for high voltage DC cables based on XLPE conductivity - Google Patents
Temperature stability assessment method for high voltage DC cables based on XLPE conductivity Download PDFInfo
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- 229920003020 cross-linked polyethylene Polymers 0.000 title claims abstract description 88
- 239000004703 cross-linked polyethylene Substances 0.000 title claims abstract description 88
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- 238000013097 stability assessment Methods 0.000 title claims description 20
- 238000004088 simulation Methods 0.000 claims abstract description 35
- 239000011810 insulating material Substances 0.000 claims abstract description 24
- 238000012360 testing method Methods 0.000 claims abstract description 17
- 238000010586 diagram Methods 0.000 claims abstract description 15
- 238000012512 characterization method Methods 0.000 claims abstract description 13
- 230000001052 transient effect Effects 0.000 claims abstract description 10
- 238000009413 insulation Methods 0.000 claims description 34
- 239000004020 conductor Substances 0.000 claims description 21
- 238000011156 evaluation Methods 0.000 claims description 15
- 238000005259 measurement Methods 0.000 claims description 9
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- 239000012774 insulation material Substances 0.000 claims description 4
- 239000002184 metal Substances 0.000 claims description 4
- 238000004364 calculation method Methods 0.000 claims description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims 1
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- 239000006227 byproduct Substances 0.000 description 1
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Abstract
一种基于XLPE电导率的高压直流电缆温度稳定评估方法，该方法中，通过电阻率测试测得XLPE绝缘材料在不同温度下的直流电流；根据所述直流电流计算得到相应的电导率和对应的温度稳定性表征指标，以评估高压直流电缆XLPE绝缘材料的温度稳定性，利用有限元分析软件对高压直流电缆XLPE绝缘材料的仿真模型计算得到温度分布切面图，根据瞬态内部加压后被测电缆和参考电缆护套的温度差，评估工程实际中高压直流电缆XLPE绝缘材料的热稳定性。本方法高效且准确性高。
A method for evaluating the temperature stability of a high-voltage direct current cable based on XLPE conductivity, wherein the direct current of the XLPE insulating material at different temperatures is measured by resistivity testing; the corresponding conductivity and the corresponding temperature stability characterization index are calculated based on the direct current to evaluate the temperature stability of the XLPE insulating material of the high-voltage direct current cable; a temperature distribution cross-section diagram is obtained by calculating the simulation model of the XLPE insulating material of the high-voltage direct current cable using finite element analysis software; and the thermal stability of the XLPE insulating material of the high-voltage direct current cable in the actual project is evaluated based on the temperature difference between the measured cable and the reference cable sheath after transient internal pressurization. The method is efficient and highly accurate.
Description
Technical Field
The invention relates to the technical field of cable temperature detection, in particular to a high-voltage direct-current cable temperature stability assessment method based on XLPE conductivity.
Background
Crosslinked polyethylene (XLPE) has found wide application in ac and dc power cables due to its excellent electrical, heat resistance and mechanical properties. Along with the development of economy and society and the increase of electricity demand, the application voltage grade of XLPE insulated cables is also increased, and aiming at the challenge of higher voltage grade, the current cable structure design mostly adopts an increased insulation thickness. The increase of the insulation thickness makes the cross-linking byproducts difficult to discharge on one hand, and increases the temperature difference between the inner layer and the outer layer of insulation on the other hand, and because of the sensitivity of the electrical conductivity of XLPE material to temperature, the electric field distortion in insulation is aggravated, and the insulation failure probability is increased. Therefore, the XLPE insulating material facing to higher voltage level needs to be additionally inspected for conductivity and temperature stability of the material on the basis of inspecting the electrical and mechanical properties of the XLPE insulating material. However, a method for simply and efficiently judging the thermal stability of XLPE materials is still lacking.
The above information disclosed in the background section is only for enhancement of understanding of the background of the invention and therefore may contain information that does not form the prior art that is already known to a person of ordinary skill in the art.
Disclosure of Invention
The invention provides a XLPE conductivity-based high-voltage direct current cable temperature stability assessment method, which is characterized in that in order to meet the working temperature range of cable insulation, the temperature stability of a high-voltage direct current cable insulation material is assessed by selecting the ratio of high-temperature conductivity to low-temperature conductivity, a thermal stability assessment index is obtained by calculating the conductivity at a measured set temperature, a temperature distribution tangent plane diagram in a cable model is obtained by combining finite element simulation software, and the temperature stability of a crosslinked polyethylene direct current cable is further assessed according to the temperature distribution tangent plane diagram, so that the temperature stability is assessed simply, conveniently and efficiently.
The XLPE conductivity-based high-voltage direct current cable temperature stability evaluation method comprises the following steps:
s1, measuring direct current of XLPE insulating materials at different temperatures through resistivity tests;
s2, calculating corresponding conductivity and corresponding temperature stability characterization indexes according to the direct current to evaluate the temperature stability of the XLPE insulating material of the high-voltage direct current cable, wherein the temperature stability characterization indexes are determined by the following formula:
,
wherein: is a temperature stability index; the high-temperature conductivity is respectively represented by the maximum allowable temperature of XLPE insulation or the maximum operating temperature of a high-voltage direct-current cable; The low-temperature conductivity represents room temperature or the lowest insulation temperature of the cable;
S3, calculating a temperature distribution tangent plane diagram of a simulation model of the XLPE insulating material of the high-voltage direct-current cable by utilizing finite element analysis software, 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 tested cable and the reference cable sheath after the transient internal pressurization.
In the high-voltage direct-current cable temperature stability assessment method based on XLPE conductivity, in step S1, the resistivity test at least comprises a direct-current high-voltage source, an electrode system, an ammeter, a data acquisition system and a temperature control device, wherein the data acquisition system is arranged between the high-voltage ammeter and the electrode system, and is used for realizing data acquisition through a computer for measuring time control and range selection and recording direct-current data in real time.
In the XLPE conductivity-based high-voltage direct current cable temperature stability evaluation method, a shielding system is arranged outside the electrode system to prevent electromagnetic interference in the measurement process, and an incubator is arranged outside the shielding system to realize temperature control in the measurement.
In the XLPE conductivity-based high-voltage direct current cable temperature stability assessment method, the maximum allowable XLPE insulation temperature or the maximum operation temperature of the high-voltage direct current cable is 90 ℃ or 70 ℃ respectively, and the room temperature or the minimum cable insulation temperature is 30 ℃.
In the XLPE conductivity-based high-voltage direct-current cable temperature stability evaluation method, the corresponding conductivity is calculated and determined by the following formula:
Wherein I v is leakage current, d is sample thickness, U is externally applied direct current voltage, S is effective area of measuring electrode.
In the XLPE conductivity-based high-voltage direct current cable temperature stability assessment method, finite element simulation software is used for establishing a simulation model of an XLPE insulation material of a high-voltage direct current cable, component parameters, a heat source, an initial temperature value and an initial potential value of the simulation model are set, a temperature change trend of a transient simulation cable after internal pressurization is combined, a reference cable sheath temperature and a measured cable sheath temperature are obtained according to a temperature section diagram, and the thermal stability of the high-voltage direct current is assessed according to the temperature difference of the reference cable sheath temperature and the measured cable sheath temperature.
In the XLPE conductivity-based high-voltage direct-current cable temperature stability assessment method, the simulation model sequentially comprises a conductor, a conductor shielding layer, an XLPE insulating layer, an insulating shielding layer, a water-resisting layer, a metal sheath and an outer sheath from inside to outside, and central lines of all the components are overlapped;
the component parameters of the simulation model include density, heat capacity, heat conductivity, conductivity and dielectric constant.
In the XLPE conductivity-based high-voltage direct-current cable temperature stability assessment method, heat sources in the simulation model are respectively joule heat generated by conductor current and heat generated by insulation leakage current, the joule heat generated by the conductor current depends on cable conductor loss, heating power per unit volume of heat generated by the insulation leakage current is gamma E 2, g is conductivity, and E is field intensity.
The initial temperature of the simulation model is between the highest power cable operation temperature and the room temperature, and the initial temperatures of all layers are the same.
In the XLPE conductivity-based high-voltage direct current cable temperature stability evaluation method, initial potential of the simulation model conductor shielding layer and the initial potential of the simulation model insulation layer are 0, and the simulation model heat transfer mode selects solid heat transfer.
The XLPE conductivity-based high-voltage direct current cable temperature stability assessment method comprises the steps of determining the sheath temperature according to the outside temperature of a temperature distribution section graph obtained through simulation, prolonging to 240 h when the reference cable sheath temperature is the sheath temperature of a cable under the condition that the conductor is not powered when the cable is fully loaded, simulating to obtain the sheath temperature change condition of the measured cable in at least 120 h when the measured cable is fully loaded and the stabilized conductor is powered by the sheath temperature under the condition of 1.45U 0, and judging that the measured cable does not meet the thermal stability if the temperature difference between the measured cable and the reference cable is smaller than 4 ℃ in the last 96 h and smaller than 2 ℃ in the last 72 h, wherein the thermal stability is good, and the measured cable is not met if the temperature difference requirement is still not met.
Compared with the prior art, the method has the advantages that the characterization index calculated by measuring the conductivities at different temperatures is evaluated, the experimental cost of evaluating the temperature stability of the cable insulating material is obviously reduced, the speed and the efficiency of evaluating the temperature stability of the crosslinked polyethylene cable are greatly improved, and the thermal stability test is carried out by combining the finite element simulation software cable model, so that the high-efficiency and accurate evaluation of the temperature stability of the high-voltage direct-current cable is realized.
Drawings
Various other advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiments. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention. It is evident that the figures described below are only some embodiments of the invention, from which other figures can be obtained without inventive effort for a person skilled in the art. Also, like reference numerals are used to designate like parts throughout the figures.
In the drawings:
FIG. 1 is a schematic flow chart of a method for evaluating the temperature stability of a crosslinked polyethylene DC cable provided by the invention;
FIG. 2 is a schematic diagram of a resistivity and conductivity testing system in accordance with the present invention;
FIG. 3 is a schematic representation of resistivity system dimensions in accordance with the present invention;
FIG. 4 is a schematic diagram of a DC cable simulation model in the present invention;
FIG. 5 is a sectional view of a temperature distribution calculated by a cable simulation model in the present invention;
FIG. 6 is a schematic diagram of XLPE current versus time (30 ℃ C., 20 kV/mm) provided by an exemplary embodiment of the present invention;
FIG. 7 is a graphical representation of XLPE conductivity as a function of temperature provided by an exemplary embodiment of the present invention;
FIG. 8 is a schematic representation of the activation energy of XLPE according to an exemplary embodiment of the present invention;
Fig. 9 is a schematic diagram of an electric field distribution inside a cable insulation according to an exemplary embodiment of the invention.
The invention is further explained below with reference to the drawings and examples.
Detailed Description
Specific embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. While specific embodiments of the invention are shown in the drawings, it should be understood that the invention may be embodied in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.
It should be noted that certain terms are used throughout the description and claims to refer to particular components. Those of skill in the art will understand that a person may refer to the same component by different names. The specification and claims do not identify differences in terms of components, but rather differences in terms of the functionality of the components. As used throughout the specification and claims, the terms "include" and "comprise" are used in an open-ended fashion, and thus should be interpreted to mean "include, but not limited to. The description hereinafter sets forth a preferred embodiment for practicing the invention, but is not intended to limit the scope of the invention, as the description proceeds with reference to the general principles of the description. The scope of the invention is defined by the appended claims.
For the purpose of facilitating an understanding of the embodiments of the present invention, reference will now be made to the drawings, by way of example, and specific examples of which are illustrated in the accompanying drawings.
As shown in fig. 1 to 9, the XLPE conductivity-based high voltage direct current cable temperature stability evaluation method includes the steps of:
s1, measuring direct current of XLPE insulating materials at different temperatures through resistivity tests;
s2, calculating corresponding conductivity and corresponding temperature stability characterization indexes according to the direct current to evaluate the temperature stability of the XLPE insulating material of the high-voltage direct current cable, wherein the temperature stability characterization indexes are determined by the following formula:
,
wherein: is a temperature stability index; the high-temperature conductivity is respectively represented by the maximum allowable temperature of XLPE insulation or the maximum operating temperature of a high-voltage direct-current cable; The low-temperature conductivity represents room temperature or the lowest insulation temperature of the cable;
S3, calculating a temperature distribution tangent plane diagram of a simulation model of the XLPE insulating material of the high-voltage direct-current cable by utilizing finite element analysis software, 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 tested cable and the reference cable sheath after the transient internal pressurization.
In the preferred embodiment of the XLPE conductivity-based high-voltage direct-current cable temperature stability assessment method, in step S1, the resistivity test at least comprises a direct-current high-voltage source, an electrode system, an ammeter, a data acquisition system and a temperature control device, wherein the data acquisition system is arranged between the high ammeter and the electrode system, and is used for measuring time control and range selection through computer data acquisition and recording direct-current data in real time.
In the preferred implementation mode of the XLPE conductivity-based high-voltage direct-current cable temperature stability assessment method, a shielding system is arranged outside the electrode system to prevent electromagnetic interference in the measurement process, and a constant temperature box is arranged outside the shielding system to realize temperature control in the measurement process.
In a preferred embodiment of the XLPE conductivity-based high-voltage direct current cable temperature stability assessment method, the XLPE insulation maximum allowable temperature or the high-voltage direct current cable maximum operation temperature is 90 ℃ or 70 ℃ respectively, and the room temperature or the cable insulation minimum temperature is 30 ℃.
In a preferred embodiment of the method for evaluating temperature stability of a high-voltage direct-current cable based on XLPE conductivity, the calculated corresponding conductivity is determined by the following formula:
Wherein I v is leakage current, d is sample thickness, U is externally applied direct current voltage, S is effective area of measuring electrode.
In a preferred implementation mode of the XLPE conductivity-based high-voltage direct current cable temperature stability assessment method, finite element simulation software is used for establishing a simulation model of an XLPE insulation material of a high-voltage direct current cable, component parameters, a heat source, an initial temperature value and an initial potential value of the simulation model are set, a temperature change trend after internal pressurization of a transient simulation cable is combined, reference cable sheath temperature and measured cable sheath temperature are obtained according to a temperature section diagram, and the thermal stability of the high-voltage direct current is assessed according to the temperature difference of the reference cable sheath temperature and the measured cable sheath temperature.
In a preferred embodiment of the XLPE conductivity-based high-voltage direct-current cable temperature stability assessment method, the simulation model sequentially comprises a conductor, a conductor shielding layer, an XLPE insulating layer, an insulating shielding layer, a water-resisting layer, a metal sheath and an outer sheath from inside to outside, wherein the central lines of all the components are overlapped;
the component parameters of the simulation model include density, heat capacity, heat conductivity, conductivity and dielectric constant.
In a preferred embodiment of the XLPE conductivity-based high-voltage direct-current cable temperature stability evaluation method, the heat sources in the simulation model are joule heat generated by conductor current and heat generated by insulation leakage current respectively, the joule heat generated by the conductor current depends on cable conductor loss, heating power per unit volume of heat generated by the insulation leakage current is γe 2, g is conductivity, and E is field intensity.
The initial temperature of the simulation model is between the highest power cable operation temperature and the room temperature, and the initial temperatures of all layers are the same.
In the preferred implementation mode of the XLPE conductivity-based high-voltage direct-current cable temperature stability evaluation method, initial potential of the simulation model conductor shielding layer and initial potential of the simulation model insulation layer are 0, and the simulation model heat transfer mode selects solid heat transfer.
In a preferred embodiment of the XLPE conductivity-based high-voltage direct current cable temperature stability assessment method, the sheath temperature is determined according to the outside temperature of a temperature distribution section graph obtained through simulation, the reference cable sheath temperature is the sheath temperature of a cable under the condition that a conductor is not applied with voltage when the cable is fully loaded, the measured cable sheath temperature is the sheath temperature of the cable under the condition that the conductor is applied with 1.45U 0 after the cable is fully loaded and stable, the simulation obtains the sheath temperature change condition of the measured cable in at least 120 h, if the temperature difference between the measured cable and the reference cable sheath is smaller than 4 ℃ in the last 96 h and smaller than 2 ℃ in the last 72 h, the heat stability is good, if the conditions are not met, the measured cable is prolonged to 240 h, and if the temperature difference requirement still cannot be met, the measured cable is considered to be inconsistent with the heat stability.
In one embodiment, the method comprises the steps of:
step one, measuring direct current of XLPE insulating materials at different temperatures through a resistivity and conductivity testing system;
Calculating corresponding conductivity and corresponding temperature stability characterization indexes according to the measured direct current, and accordingly evaluating the temperature stability of the high-voltage direct current cable insulating material;
and thirdly, simulating the direct current cable model by using finite element simulation software to obtain a temperature distribution tangent plane graph, and evaluating the thermal stability of the high-voltage direct current cable in engineering practice according to the temperature difference between the tested cable and the reference cable sheath after the transient internal pressurization.
Step one:
In one embodiment, the resistivity testing system in the first step is shown in fig. 2, and mainly includes a dc high voltage source, a temperature control system, a three-electrode system, a protection resistor, a high resistance meter, and a computer. And a direct-current high-voltage source is adopted, the output voltage is 0-40 kV, and the ripple coefficient is 0.1%. A current limiting resistor of approximately 15mΩ is used. And an electrometer is adopted, and the current measurement range is 1 nA-21 mA. The data acquisition is realized through a computer and is used for measuring time control and range selection and recording current data in real time. The reference resistivity test system is connected with a circuit according to the working principle, the test sample is wiped in advance during test to ensure the surface to be clean, then corresponding voltage is loaded, the leakage current was recorded by a high resistance meter, a computer, and specific parameters of the resistivity system were as shown in fig. 3, thereby calculating the conductivity of the material. Sheet samples of dimensions 100 mm x 100 mm x 0.2 mm were taken. The polarization process affects the value of the conducted current, which takes a certain time to reach a steady state.
To determine the test time, fig. 6 shows the XLPE current trend over time at 30 ℃, 20 kV/mm (near the insulation internal field strength of the high voltage dc cable). It can be clearly seen that after 1000 s, there is no significant change in current. Therefore, in this embodiment, 1860 s is selected as the measurement time, the current value of the last 60 s is selected to calculate the resistivity of the sample, and the experimental temperature is selected based on the actual temperature range of the cable insulation.
In one embodiment, the direct current at different temperatures, specifically, the high temperature conductivity test temperature was selected to be 70 ℃, the test field strength was selected to be 20 kV/mm, and the XLPE direct current at different temperatures was shown in table 1 according to the direct current measured in step one.
Table 1 two XLPE materials direct current at 30 ℃ and 70 °c
Step two:
In one embodiment, specifically, the measuring electrode effective area is 0.000491m 2, and XLPE dc conductivity is calculated using equation (1) as shown in table 2.
Table 2 conductivity of two XLPE materials at different temperatures
Based on the measured conductivities at different temperatures, XLPE temperature stability characterization G was calculated using formula (2) and is shown in table 3.
Table 3 XLPE temperature stability characterization index G
In this example, XLPE1 has a characterization parameter greater than XLPE2, indicating that XLPE1 has better temperature stability than XLPE2.
XLPE is predominantly electron-hopping in conduction under experimental conditions, according to the mechanism of conduction. The density of local states in an amorphous dielectric is large, whereas electrons are mostly in local states, so that inter-local state electron migration is the primary process, i.e., electrons migrate from one local state to another by way of a jump barrier, which is visually described as a jump migration. The probability of electron jump between two adjacent local states is related to the space distance a and the potential barrier u 0 between the two states, and when the electron thermal vibration frequency is set to be v, mobility can be obtained as follows:
(4)
Further, considering the conductive carrier concentration n and the charged amount q, a conductivity formula in the case of electron hopping conductivity can be obtained:
(5)
taking the logarithm of the left side and the right side of the formula (5) to obtain:
(6)
therefore, when the logarithm of the conductivity is taken as the ordinate and the reciprocal of the absolute temperature is taken as the abscissa, a straight line can be obtained, and the activation energy can be calculated from the slope of the straight line.
In this example, the activation energy was calculated by further fitting the conductivities of two different XLPE materials at different temperatures as shown in fig. 7, as shown in fig. 8. It can be seen that XLPE1 has a smaller activation energy and is believed to have a smaller change in conductivity with temperature, i.e. the material has a higher temperature stability.
According to the design of the direct current cable electric field, laplace electric field with high inside and low outside and temperature difference with high inside and low outside can be displayed in the insulating layer of the cable, but due to the influence of temperature and electric field on XLPE, the conductivity in the insulating layer shows distribution of high inside and low outside. Under the condition of a direct current electric field, the electric conductivity can directly influence the distribution of an electric field in the insulating material, under the combined action of the reasons, the electric field at the inner side in the insulating material is relatively reduced, the electric field at the outer side is relatively increased, and even the phenomenon of 'electric field inversion' occurs under certain conditions, so that the electric field in the actual insulating material is deviated from the designed electric field, and the risk of cable failure is increased. Fig. 9 shows the electric field distribution of the cable insulation layer using the two XLPE as insulation, wherein the two XLPE1 has lower activation energy and better temperature stability, and smaller degree of electric field inversion and smaller maximum field strength, although the two electric field inversion phenomena occur. The use of activation energy is described as being able to measure the temperature stability of the material and guide the actual use.
Therefore, the temperature stability evaluation method provided by the invention can obviously reduce the experimental cost of cable insulation temperature stability evaluation, and greatly improve the speed and efficiency of crosslinked polyethylene cable insulation temperature stability evaluation.
Step three:
In this embodiment, a dc cable model is built in simulation software according to the 500 kV dc cable model structure.
Further, the corresponding density, heat capacity and conductivity can be obtained according to the specific types of the components, the density and heat capacity are generally obtained according to factory calibration values, the heat conductivity coefficient of the components is measured through a heat conduction measuring instrument, and the relative dielectric constant is measured through a broadband frequency spectrum.
The reference cable conductor parameter is set as 401W/(m.K), 8940: 8940 kg/m 3, 385: 385J/(kg.K), 57142875S/m conductivity and 1 relative dielectric constant;
the parameter setting of the reference cable shielding layer comprises that the heat conductivity coefficient is 0.28W/(m.K), the density is 1120 kg/m 3, the heat capacity is 2700J/(kg.K), the conductivity is 0.1S/m, and the relative dielectric constant is 100;
The reference cable water-blocking layer parameter is set to have a heat conductivity coefficient of 0.08W/(m.K), a density of 200 kg/m 3 and a heat capacity of 60J/(kg.K);
wherein, the reference cable metal sheath parameter setting is that the heat conductivity coefficient is 250W/(m.K), the density is 8940 kg/m 3, and the heat capacity is 385J/(kg.K);
Wherein, the reference cable outer sheath parameter is set as that the heat conductivity coefficient is 0.29W/(m.K), the density is 968 kg/m 3, and the heat capacity is 2532J/(kg.K);
Wherein the reference cable XLPE insulation layer parameter is set to have a thermal conductivity of 0.285W/(m.K), a density of 980 kg/m 3, a heat capacity of 2200J/(kg.K), a relative dielectric constant of 2.2, in particular, a conductivity satisfying formula (3), beta of 1.0413, gamma 0 of XLPE1 and XLPE2 being set to 2.91×10 -29 S/m and 2.94×10 -31 S/m, respectively, alpha being set to 0.0608 and 0.0767, respectively, and XLPE1 being less dependent on temperature than XLPE 2. Equation (3) assumes that the dielectric is uniform and considers a single value of parameters α and β, but the parameters α and β depend on temperature and field strength, respectively. In normal operation, the values of α and β will vary throughout the insulation. Therefore, the effects of these stresses and temperatures must be considered, whereby the uncertainty in the alpha and beta calculations is about 3%.
In this embodiment, the parameter settings of each component of the tested cable are consistent with the standard cable.
Further, initial temperatures of all parts of the cable model are set to 303.15K, a heat source in the cable model is set to be Joule heat of conductor current and insulation leakage current to generate heat, the former is 17300W/m 3, the unit volume of heating power is gamma E 2, 1.45U 0, namely 725 kV transient voltage is applied to the outer side of the conductor during thermal stability test, simulation time is selected from 0 to 120 h, measurement interval is selected from 12 h, the temperature of a cable sheath to be measured is determined at the last 96 h according to a temperature section diagram, and test results are shown in Table 4.
Table 4 reference cable and measured cable jacket temperature and temperature differential
Continuous table 4
Referring to table 4, the temperature difference between XLPE1 and the reference cable sheath in the last 96 h and the temperature difference between XLPE2 and the reference cable sheath in the last 72 h are less than 4 ℃ and less than 2 ℃ respectively, which indicate that the thermal stability is good, while the temperature difference between XLPE2 and the reference cable sheath in the last 72 h is greater than 2 ℃ and extends to 240 h, and the temperature difference requirement cannot be met yet, and the XLPE2 measured cable is not considered to meet the thermal stability standard and is consistent with the evaluation result of the temperature stability characterization index.
In summary, the invention points out that the cable model is simulated by utilizing finite element simulation software to obtain a stable distribution section diagram, and according to the temperature difference between the measured cable and the reference cable sheath after the transient internal pressurization, the thermal stability of different high-voltage direct-current cables in engineering practice can be evaluated, and the accuracy of the temperature stability evaluation of the high-voltage direct-current cables is improved.
Although the embodiments of the present invention have been described above with reference to the accompanying drawings, the present invention is not limited to the above-described specific embodiments and application fields, and the above-described specific embodiments are merely illustrative, and not restrictive. Those skilled in the art, having the benefit of this disclosure, may effect numerous forms of the invention without departing from the scope of the invention as claimed.
Claims (7)
1. The XLPE conductivity-based high-voltage direct-current cable temperature stability evaluation method is characterized by comprising the following steps of:
s1, measuring direct current of XLPE insulating materials at different temperatures through resistivity tests;
s2, calculating corresponding conductivity and corresponding temperature stability characterization indexes according to the direct current to evaluate the temperature stability of the XLPE insulating material of the high-voltage direct current cable, wherein the temperature stability characterization indexes are determined by the following formula:
,
wherein: is a temperature stability index; the high-temperature conductivity is respectively represented by the maximum allowable temperature of XLPE insulation or the maximum operating temperature of a high-voltage direct-current cable; The low-temperature conductivity represents room temperature or the lowest insulation temperature of the cable;
S3, calculating a temperature distribution tangent plane diagram of a simulation model of the XLPE insulating material of the high-voltage direct-current cable by utilizing finite element analysis software, 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 tested cable and the reference cable sheath after the transient internal pressurization.
2. The XLPE conductivity-based high voltage dc cable temperature stability assessment method according to claim 1, wherein preferably, in step S1, the resistivity test at least comprises a dc high voltage source, an electrode system, an ammeter, a data acquisition system and a temperature control device, wherein the data acquisition system is arranged between the high ammeter and the electrode system, and the data acquisition is implemented by a computer for measuring time control and range selection, and recording the data of the dc current in real time.
3. The XLPE conductivity-based high-voltage direct current cable temperature stability assessment method is characterized in that a shielding system is arranged outside the electrode system to prevent electromagnetic interference in the measurement process, and a constant temperature box is arranged outside the shielding system to realize temperature control in the measurement.
4. The method for evaluating the temperature stability of the high-voltage direct-current cable based on XLPE conductivity according to claim 1, wherein the maximum allowable temperature of XLPE insulation or the maximum operation temperature of the high-voltage direct-current cable is 90 ℃ or 70 ℃ respectively, and the minimum temperature of room temperature or cable insulation is 30 ℃.
5. The method for evaluating the temperature stability of a high-voltage direct-current cable based on XLPE conductivity according to claim 1, wherein the calculation of the corresponding conductivity is determined by the following formula:
,
Wherein I v is leakage current, d is sample thickness, U is externally applied direct current voltage, S is effective area of measuring electrode.
6. The XLPE conductivity-based high-voltage direct current cable temperature stability assessment method is characterized by establishing a simulation model of an XLPE insulation material of a high-voltage direct current cable by using finite element simulation software, setting component parameters, a heat source, an initial temperature value and an initial potential value of the simulation model, combining the temperature change trend of a transient simulation cable after internal pressurization, obtaining a reference cable sheath temperature and a measured cable sheath temperature according to a temperature section diagram, and assessing the thermal stability of the high-voltage direct current according to the temperature difference of the reference cable sheath temperature and the measured cable sheath temperature.
7. The XLPE conductivity-based high-voltage direct current cable temperature stability assessment method according to claim 6, wherein the simulation model sequentially comprises a conductor, a conductor shielding layer, an XLPE insulating layer, an insulating shielding layer, a water-resistant layer, a metal sheath and an outer sheath from inside to outside, and the central lines of the components are coincident;
the component parameters of the simulation model include density, heat capacity, heat conductivity, conductivity and dielectric constant.
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