CN111650479A - IRC (infrared radiation correlation) and electrothermal accelerated aging test method - Google Patents
IRC (infrared radiation correlation) and electrothermal accelerated aging test method Download PDFInfo
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Abstract
An IRC and electric heating accelerated aging test method belongs to the technical field of power cables. The invention aims to adopt a method of applying large current to enable thermal aging to be closer to the actual operation condition, and provides a set of new accelerated aging test scheme, and an IRC and electric heating accelerated aging test method for simultaneously carrying out electric aging, thermal aging and electric heating combined aging on cables. The isothermal relaxation current test of the power cable comprises the following steps: a pre-processing phase, a polarization phase, a transient short-circuit phase and a measurement phase. According to the invention, the research on the cable insulation performance by three test modes of electricity, heat and electricity-heat combination is carried out simultaneously, and the change of the aging factor of the cable after different accelerated aging modes is analyzed by an isothermal relaxation current method, so that the method has important significance on the research on the cable insulation performance.
Description
Technical Field
The invention belongs to the technical field of power cables.
Background
With the rapid development of national economy and the continuous improvement of modernization construction in China, the power consumption requirements of various industries are continuously increased, and a power cable gradually becomes one of important devices for a power system to transmit high-power electric energy. The cross-linked polyethylene (XLPE) power cable has excellent heat resistance, mechanical property and electrical property, and simple manufacturing process, and is widely applied to power transmission and transformation systems. At present, the laying number of XLPE power cables in China accounts for 99% of the total laying number of the cables, and the XLPE power cables are the main components of the cables. Due to the limitations of the manufacturing process and the cable laying conditions of the insulating layer of the power cable, and the influence of a plurality of factors such as electricity, heat, chemistry, machinery, environment and the like on the cable after the cable is put into operation, the insulating property of the insulating layer can be reduced along with the increase of the operation time of the cable, and the cable is gradually aged to reach the end of the service life. The XLPE cable which is put into operation in the 80 th century in China already serves 30a, the expected design service life is achieved, and the problem of replacement is faced, so that the insulation research of the cable plays a vital role in ensuring the safe operation of the cable and improving the economic benefit of electric power.
The dielectric loss method and the voltage-resistant method are common test methods for testing the insulating property of the cable at present. The dielectric loss method measures the dielectric loss angle through harmonic analysis, but under the influence of various factors, the fundamental frequency of the power system fluctuates to a certain extent, and the harmonic analysis measurement of the dielectric loss angle has a large error. The voltage endurance method can directly evaluate the insulation performance of the cable, but the method is destructive and cannot accurately distinguish the influence of local defects and overall defects on the insulation performance. The method is a nondestructive test method, the insulation performance of the cable is evaluated by measuring the relaxation current, carrying out data analysis and calculating the aging factor, and the laying operation history of the cable is not required, so the method is a commonly used insulation performance evaluation method in the world at present.
The electric heating accelerated aging test is one of important means for researching the insulating property of the cable. When the cable body is subjected to thermal aging, a test method of an aging box is mostly adopted, so that the thermal aging condition of the cable in actual operation cannot be fully simulated.
Disclosure of Invention
The invention aims to adopt a method of applying large current to enable thermal aging to be closer to the actual operation condition, and provides a set of new accelerated aging test scheme, and an IRC and electric heating accelerated aging test method for simultaneously carrying out electric aging, thermal aging and electric heating combined aging on cables.
The method comprises the following steps:
the isothermal relaxation current is a third-order decay exponential function and is composed of three relaxation current components with different time constants
In the formula I0Is the direct leakage current, τ, when the current decays to steady state1Is the depolarization constant, τ, of the interface between the semiconducting layer and the insulation2Is the interfacial depolarization constant, τ, between the amorphous and crystalline regions3Is the interfacial depolarization constant between XLPE and hydrated salts induced by aging, a1、a2、a3Are respectively the corresponding depolarization intensities, tauiRepresenting the time of electron movement, i.e. the relaxation time, when tauiThe larger the corresponding relaxation process time, aiRepresents the trap density wheniThe larger the corresponding relaxation current, the better the aging factor A is proposed for better measuring the aging state of the cable
Wherein the content of the first and second substances,
the isothermal relaxation current test steps of the power cable are as follows:
A. a pretreatment stage: the outer shielding layers stripped from the two ends of the cable are cleaned by absolute ethyl alcohol, so that the influence of surface dirt on the relaxation current is reduced, and the test accuracy is improved;
B. and (3) polarization stage: applying 1000V direct current high voltage to a XLPE cable core, reliably grounding a metal shield, and polarizing the cable for 1800 s;
C. and (3) transient short circuit stage: after the polarization is finished, the high-voltage direct-current power supply is disconnected, the cable core and the metal shield are short-circuited through a 10M omega resistor, the short-circuit time is 5s, and the influence of surface free charge on the measurement is reduced;
D. and (3) a measuring stage: collecting and recording relaxation current by using a Keysight b2985A type electrometer, disconnecting a short-circuit switch, connecting a cable core with the electrometer through a 50M omega protection resistor, wherein the measurement time is 1800s, and ensuring quiet test environment in the measurement process so as to reduce the influence of noise on a measurement result;
when the same cable is tested repeatedly, the test interval is kept at least over 1h, and the cable is ensured to be restored to the initial state;
accelerated aging test protocol:
the high-voltage equipment used in the test is a series resonance voltage-withstanding device, the device mainly comprises a variable frequency power supply, an excitation transformer and an adjustable inductor, the adjustable inductor is adjusted to form series resonance with a cable capacitor to generate high voltage, the equipment can generate 500kV alternating current voltage at most and can generate enough voltage required by electrical aging of the test, the equipment generating the large current is a large current temperature rise test device, the equipment can generate 4000A current at most, and the equipment is provided with a temperature measuring device, so that the temperature of a cable core and an insulating layer of the cable can be monitored in real time, and the thermal aging of the XLPE cable is completed;
applying high voltage to a prepared sample M1 for electrical aging, simultaneously applying high voltage and large current to M2 for electrothermal combined aging, applying large current to M3 for thermal aging, dividing an accelerated aging test into two test stages, after the test of each stage is finished, performing an isothermal relaxation current test to measure relaxation current, fitting measurement data, calculating an aging factor, and researching the change of the aging factor, wherein the high voltage applied to the samples M1 and M2 in the first stage is 1.3U0, the currents applied to the samples M2 and M3 make the temperature of a cable core reach 90 ℃, and the test current is increased in the second stage to make the temperature of the cable cores of the M2 and M3 reach 100 ℃, and the voltage applied to the cables M1 and M2 is still 1.3U0, and the ideal state of the test is that the voltage and the current are simultaneously increased in the second stage, but the voltage cannot be continuously increased under the limitation of test conditions, so that the voltage is still 1.3U 2.
According to the invention, the research on the cable insulation performance by three test modes of electricity, heat and electricity-heat combination is carried out simultaneously, and the change of the aging factor of the cable after different accelerated aging modes is analyzed by an isothermal relaxation current method, so that the method has important significance on the research on the cable insulation performance. The invention adopts a set of new accelerated aging test scheme, three samples cut from the same 66kV XLPE cable are subjected to combined electrical, thermal and electrical-thermal aging respectively, the insulation performance of the cable samples after two-stage accelerated aging tests is researched through an isothermal relaxation current test, and the following conclusion is obtained:
(1) the IRC method is used as a non-destructive test method, the repeatability of relaxation current is good during the test, the test result is accurate, and the change of the insulation performance of the XLPE cable insulation layer can be represented by the change of the aging factor.
(2) After the first-stage accelerated aging test was completed, the aging factor of the electrical aging test samples increased by 3.70%, while that of the thermal aging test samples increased by 4.82%. After the second stage accelerated aging test stage is completed, the aging factor of the electrical aging test sample is increased by 4.21%, and the aging factor of the thermal aging test sample is increased by 23.42%. It is shown that during operation of XLPE cables, especially during overload, the thermal stress "contributes" more to aging than the electrical stress, and that the effect on the cable insulation performance increases dramatically at higher temperatures.
(3) In the two-stage accelerated aging test, the aging factors of the electrothermal combined aging test sample are respectively increased by 13.04% and 37.52%, and are far greater than the aging factors of the electric and thermal single-stress acting samples, which shows that in the aging process of the XLPE power cable, the electric stress and the thermal stress have a synergistic effect, and the aging speed under the dual-stress synergistic effect is far faster than that under the single-stress effect.
Drawings
FIG. 1 is a sample numbering scheme;
FIG. 2 is a test sample cable;
FIG. 3 is a circuit diagram of an accelerated aging test;
FIG. 4 is a sample isothermal relaxation current measurement and fitted curve before unaged;
FIG. 5 is a sample isothermal relaxation current measurement and fitted curve after one stage of aging;
FIG. 6 is a measurement and fitted curve of isothermal relaxation current after the sample has aged for two stages;
FIG. 7 is the aging factor change after the two-stage accelerated aging test.
Detailed Description
The method comprises the following steps:
the isothermal relaxation current is a third-order decay exponential function and is composed of three relaxation current components with different time constants
In the formula I0Is the direct leakage current, τ, when the current decays to steady state1Is the depolarization constant, τ, of the interface between the semiconducting layer and the insulation2Is the interfacial depolarization constant, τ, between the amorphous and crystalline regions3Is the interfacial depolarization constant between XLPE and hydrated salts induced by aging, a1、a2、a3Are respectively the corresponding depolarization intensities, tauiRepresenting the time of electron movement, i.e. the relaxation time, when tauiThe larger the corresponding relaxation process time, aiRepresents the trap density wheniThe larger the corresponding relaxation current, the better the aging factor A is proposed for better measuring the aging state of the cable
Wherein the content of the first and second substances,
the isothermal relaxation current test steps of the power cable are as follows:
A. a pretreatment stage: the outer shielding layers stripped from the two ends of the cable are cleaned by absolute ethyl alcohol, so that the influence of surface dirt on the relaxation current is reduced, and the test accuracy is improved;
B. and (3) polarization stage: applying 1000V direct current high voltage to a XLPE cable core, reliably grounding a metal shield, and polarizing the cable for 1800 s;
C. and (3) transient short circuit stage: after the polarization is finished, the high-voltage direct-current power supply is disconnected, the cable core and the metal shield are short-circuited through a 10M omega resistor, the short-circuit time is 5s, and the influence of surface free charge on the measurement is reduced;
and (3) a measuring stage: collecting and recording relaxation current by using a Keysight b2985A type electrometer, disconnecting a short-circuit switch, connecting a cable core with the electrometer through a 50M omega protection resistor, wherein the measurement time is 1800s, and ensuring quiet test environment in the measurement process so as to reduce the influence of noise on a measurement result;
when the same cable is tested repeatedly, the test interval is kept at least over 1h, and the cable is ensured to be restored to the initial state;
accelerated aging test protocol:
the high-voltage equipment used in the test is a series resonance voltage-withstanding device, the device mainly comprises a variable frequency power supply, an excitation transformer and an adjustable inductor, the adjustable inductor is adjusted to form series resonance with a cable capacitor to generate high voltage, the equipment can generate 500kV alternating current voltage at most and can generate enough voltage required by electrical aging of the test, the equipment generating the large current is a large current temperature rise test device, the equipment can generate 4000A current at most, and the equipment is provided with a temperature measuring device, so that the temperature of a cable core and an insulating layer of the cable can be monitored in real time, and the thermal aging of the XLPE cable is completed;
applying high voltage to a prepared sample M1 for electrical aging, simultaneously applying high voltage and large current to M2 for electrothermal combined aging, applying large current to M3 for thermal aging, dividing an accelerated aging test into two test stages, after the test of each stage is finished, performing an isothermal relaxation current test to measure relaxation current, fitting measurement data, calculating an aging factor, and researching the change of the aging factor, wherein the high voltage applied to the samples M1 and M2 in the first stage is 1.3U0, the currents applied to the samples M2 and M3 make the temperature of a cable core reach 90 ℃, and the test current is increased in the second stage to make the temperature of the cable cores of the M2 and M3 reach 100 ℃, and the voltage applied to the cables M1 and M2 is still 1.3U0, and the ideal state of the test is that the voltage and the current are simultaneously increased in the second stage, but the voltage cannot be continuously increased under the limitation of test conditions, so that the voltage is still 1.3U 2.
The invention is further illustrated and verified as follows:
1 sample treatment
A 66kV XLPE cable is selected as a test sample, the cable is defined as a sample M, a phase voltage U0 is 38kV, and basic parameters of the sample are as follows: the cable core is copper and the cross-sectional area is 800mm2The diameter of the conductor is 18.4mm, the thickness of the shielding layer is 1.33mm, the thickness of the main insulation is 13.5mm, and the total length is 9.6 m. Considering the influence of test variables, in order to reduce the test variables, M is evenly cut into three sections of new samples with equal length, the length of each section of cable is 3.2M, the samples are respectively subjected to electric aging, thermal aging and electric-thermal combined aging, and the samples are numbered according to the figure 1. If M1 represents sample 1 without aging, MA1 represents sample 1 after one stage of aging, and MB1 represents sample 1 after two stages of aging.
In order to prevent creepage along the surface between the high-voltage outer semiconductor shielding layer and the cable core grounding part in the high-voltage process, each test sample is made as shown in fig. 2, and the total length of the sample is 320 cm. Through calculation, the safe creepage distance along the surface is larger than 100cm, so that 100cm of each semiconductor shielding layer at the left end and the right end of the stripped cable leaks the XLPE insulating layer.
2 principle and scheme of the test
2.1 isothermal relaxation Current test
2.1.1 principle of isothermal relaxation current
Isothermal relaxation current theory suggests that electron trap levels exist in a polymer body containing impurities and defects, and electrons generate current during the process of moving from the traps to the conduction band. By studying the movement of microscopic electrons and microscopic electrons, the relation between current and time can be used for representing the distribution condition of electron defect traps in the insulation. The isothermal relaxation current method is to perform direct current pressurization on the crosslinked polyethylene power cable at the same temperature and measure the relation between the short circuit depolarized current I of the cable and the time t.
The isothermal relaxation current is a third-order decay exponential function and is composed of three relaxation current components with different time constants. It is expressed in the form:
in the formula I0Is the direct leakage current, τ, when the current decays to steady state1Is the depolarization constant, τ, of the interface between the semiconducting layer and the insulation2Is the interfacial depolarization constant, τ, between the amorphous and crystalline regions3Is the interfacial depolarization constant between XLPE and hydrated salts induced by aging, a1、a2、a3Respectively the corresponding depolarization intensity. Tau isiRepresenting the time of electron movement, i.e. the relaxation time, when tauiThe larger the corresponding relaxation process time. a isiRepresents the trap density wheniThe larger the corresponding relaxation current. In order to better measure the aging state of the cable, an aging factor A is provided
Wherein the content of the first and second substances,
2.1.2 isothermal relaxation Current test protocol
The test respectively measures the isothermal relaxation current before and after accelerated aging, and the specific isothermal relaxation current test steps of the 66kV XLPE power cable are as follows
A. A pretreatment stage: the outer shielding layers stripped from the two ends of the cable are cleaned by absolute ethyl alcohol, so that the influence of surface dirt on the relaxation current is reduced, and the test accuracy is improved.
B. And (3) polarization stage: the XLPE cable core applies 1000V direct current high voltage, and metal shielding reliably ground connection carries out the polarization to the cable, and the polarization time is 1800 s.
C. And (3) transient short circuit stage: after the polarization is finished, the high-voltage direct-current power supply is disconnected, the cable core and the metal shield are in short circuit through a 10M omega resistor, the short circuit time is 5s, and the influence of surface free charge on measurement is reduced.
D. And (3) a measuring stage: relaxation currents were collected and recorded using a Keysight model b2985A electrometer. And (5) disconnecting the short-circuit switch, connecting the cable core with an electrometer through a 50M omega protection resistor, and measuring for 1800 s. The test environment is ensured to be quiet during the measurement process so as to reduce the influence of noise on the measurement result.
When the same 66kV XLPE cable is tested repeatedly, the test interval is kept at least over 1h, and the cable is ensured to be restored to the initial state.
2.2 accelerated aging test
2.2.1 accelerated ageing test principle
When the influence of multiple stresses on the aging degree of the cable is researched, the cable needs to be subjected to an aging test. The aging test under the ideal condition is to apply the same electrical stress and thermal stress to the cable as the cable in normal operation and simulate the working state of the cable in normal use, but the aging test time under the working condition is too long and unacceptable. The accelerated aging test is to improve two aging stress levels of electricity and heat, deteriorate the operating environment of the cable, shorten the aging test time and achieve the test purpose.
2.2.2 accelerated aging test protocol
In order to research the influence of multiple stresses on cable aging, single-factor and double-factor accelerated aging tests are respectively carried out, the cable is simultaneously subjected to electric aging, thermal aging and electric-thermal combined aging respectively, and the influence of electric stress, thermal single stress and electric-thermal double stress on cable aging is researched. In the test, high-voltage alternating voltage is applied between the cable insulation and the sheath for electrical aging, the cable core is heated by large current for thermal aging, and the cable insulation and the sheath are simultaneously applied for combined electrical and thermal aging. The specific aging test circuit is shown in the figure.
The high-voltage equipment used in the test is a series resonance voltage-withstanding device, the device mainly comprises a variable-frequency power supply, an exciting transformer and an adjustable inductor, the adjustable inductor and a cable capacitor form series resonance to generate high voltage by adjusting the adjustable inductor, and the equipment can generate 500kV alternating-current voltage at most and is enough for the test to carry out voltage required by electrical aging. The equipment generating the large current is a large current temperature rise test device, the device can generate 4000A of current at most, and the equipment is provided with a temperature measuring device, so that the temperature of a cable core and an insulating layer of the cable can be monitored in real time, and the thermal aging of the XLPE cable is completed.
And applying high voltage to the prepared sample M1 for electrical aging, simultaneously applying high voltage and large current to the sample M2 for electrothermal combined aging, and applying large current to the sample M3 for thermal aging. The accelerated aging test is divided into two test stages, after the test of each stage is completed, an isothermal relaxation current test is carried out to measure the relaxation current, measurement data are fitted, an aging factor is calculated, and the change of the aging factor is researched. The high voltage applied to the first stage samples M1 and M2 was 1.3U0, and the current applied by M2 and M3 brought the core temperature to 90 ℃. And in the second stage, the test current is increased, so that the cable core temperature of the M2 and M3 cables reaches 100 ℃, and the voltage applied to the M1 and M2 cables is still 1.3U 0. The ideal state of the test is that the voltage and the current are simultaneously increased in the second stage test, but the voltage cannot be continuously increased under the limit of the test conditions, so the voltage is still 1.3U 0. According to the test environment of the cable, calculation is carried out through finite element software ANSYS, and mutual verification is carried out with the temperature measured by the large-current temperature rise equipment, so that when the temperature of the cable core is 90 ℃, the current of the cable core is 1420A, and when the temperature of the cable core is increased to 100 ℃, the current of the cable core is 1525A.
The advantages of this test protocol are mainly twofold. On one hand, the test simultaneously carries out electric, thermal and electric-thermal combined aging under the same environment, controls the influence of environmental variables such as temperature, humidity and the like on the test, and increases the accuracy of the test result. On the other hand, the thermal aging of the test is not carried out by changing the external temperature of the cable by using the aging box, but carried out by applying large current on the cable core, so that the thermal aging test more conforms to the heating condition of the cable body and is closer to the actual operation working condition of the cable.
3 test data processing and analysis
Fig. 4 to 6 are test measurement data and data fitting curves of the isothermal relaxation current test performed on the three samples before aging, after the first stage of aging, and after the second stage of aging, respectively, and tables 1 to 3 are parameters and aging factors obtained by performing third-order exponential fitting on the three samples at each stage according to the measured relaxation current.
As can be seen from fig. 4 to 6, the relaxation current measured by the IRC method has good repeatability as a whole, but since the magnitude of the depolarized current is pA level, which is a weak current signal, after the depolarization current is attenuated to be close to stable, the current is too small, which is greatly influenced by the environment, and a fluctuation condition occurs, but within an acceptable range; meanwhile, after the samples are subjected to two-stage accelerated aging, the initial value of the relaxation current is increased, which is because the number of insulated internal traps is increased and the capability of capturing charges is enhanced in the accelerated aging process of the samples, so that the initial value of the depolarization current is increased.
In order to reduce the test variables, the three cables used in the test were cut from the same cable and of equal length. However, as can be seen from table 1, the aging factor of the sample M2 is 1.9049 at maximum, the aging factor of the sample M3 is 1.8362 at minimum, and the aging factors of the sample cables before aging are different but not much different, because although the test samples are from the same cable, the density and depth of the insulation traps are not completely and uniformly distributed in the cable, that is, the traps at different positions of the insulation layer have a certain difference in the process of putting the cable into operation.
TABLE 1 sample fitting parameters before non-aging and aging factor
TABLE 2 sample fitting parameters and aging factors after one stage of aging
TABLE 3 sample fitting parameters and aging factors after two stages of aging
As can be seen from tables 1 to 3, the fitting parameters of the relaxation currents of the samples in three periods satisfy tau3>τ2>τ1The influence of three polarization modes in the cable on the isothermal relaxation current is different, the influence of the interfacial polarization between the semi-conducting layer and the insulation on the aging factor is minimum, and the influence of the interfacial polarization between XLPE and hydrated salt caused by aging on the aging factor is maximum; at the same time, it can be seen from the table that1Is much smaller than τ2And τ3The values of (d) are mainly due to the fact that during aging of the XLPE cable, the variation of the interfacial polarization between the semiconductive and insulating layers is small, whereas the variation of the interfacial polarization between the amorphous and crystalline regions and between XLPE and the hydrated salt caused by aging is large, so that in calculating the aging factor, τ is calculated2And τ3Plays a major role, tau can be roughly ignored1The influence of the value; as the insulating layer is aged more, the density and depth of traps in the insulating layer are increased continuously, electrons are more difficult to be separated after being trapped by the traps, and therefore, the electrons are influenced by hydrated ions and salts caused by aging, tau3The largest value of (c) is the main factor but not the dominant factor affecting the magnitude of the aging factor, and it can be seen from tables 1 to 3 that not all of τ is3The cable with a large value corresponds to a certain large aging factor.
Comparing tables 1, 2 and 3, the aging factor increased to different extents for all three groups of samples after testing in two stages with different accelerated aging modes. As can be seen from tables 1 and 2, after the first-stage aging test was performed, the aging factor of the sample MA1 subjected to the electrical aging acceleration test was increased by 3.70%, the aging factor of the sample MA2 subjected to the combined electrical and thermal aging test was increased by 13.04%, and the aging factor of the sample MA3 subjected to the thermal aging acceleration test was increased by 4.82%. It can be seen that the effect of the temperature of 90 ℃ on the cable aging is greater than that of the voltage of 1.3U0, namely, the thermal stress has a greater "contribution" to the cable aging relative to the electrical stress during the cable aging process, and the effect on the aging factor is far better than that of the electrical and thermal single stress when the electrothermal double stress is combined. As can be seen from tables 2 and 3, after the second stage accelerated aging test, the aging factors of the combined electric and thermal aging test samples were increased by 4.21%, 37.52% and 23.42% respectively from the first stage. The aging factor change of the electrical aging test sample obtained from data is not large, but the aging factor of the temperature-related thermal aging test sample and the aging factor of the electrical-thermal combined aging test sample are increased sharply, because the voltage of the second stage is still 1.3U0, the temperature of the cable core is increased to 110 ℃, the influence of the temperature change caused by the current exceeding the current-carrying capacity on the cable aging is larger, and the influence of the electrical-thermal combined aging on the thermal aging and the electrical-thermal dual stress on the aging is proved to be much larger than the influence of the single stress on the aging. In order to more visually see the change of the aging factor of each sample after two stages, the graph is a bar chart of the aging factor of each stage.
Claims (1)
1. An IRC and electrothermal accelerated aging test method is characterized in that:
the isothermal relaxation current is a third-order decay exponential function and is composed of three relaxation current components with different time constants
In the formula I0Is the direct leakage current, τ, when the current decays to steady state1Is the depolarization constant, τ, of the interface between the semiconducting layer and the insulation2Is the interfacial depolarization constant, τ, between the amorphous and crystalline regions3Is the interfacial depolarization constant between XLPE and hydrated salts induced by aging, a1、a2、a3Are respectively the corresponding depolarization intensities, tauiRepresenting the time of electron movement, i.e. the relaxation time, when tauiThe larger the corresponding relaxation process time, aiRepresenting trapsDensity when aiThe larger the corresponding relaxation current, the better the aging factor A is proposed for better measuring the aging state of the cable
Wherein the content of the first and second substances,
the isothermal relaxation current test steps of the power cable are as follows:
A. a pretreatment stage: the outer shielding layers stripped from the two ends of the cable are cleaned by absolute ethyl alcohol, so that the influence of surface dirt on the relaxation current is reduced, and the test accuracy is improved;
B. and (3) polarization stage: applying 1000V direct current high voltage to a XLPE cable core, reliably grounding a metal shield, and polarizing the cable for 1800 s;
C. and (3) transient short circuit stage: after the polarization is finished, the high-voltage direct-current power supply is disconnected, the cable core and the metal shield are short-circuited through a 10M omega resistor, the short-circuit time is 5s, and the influence of surface free charge on the measurement is reduced;
and (3) a measuring stage: collecting and recording relaxation current by using a Keysight b2985A type electrometer, disconnecting a short-circuit switch, connecting a cable core with the electrometer through a 50M omega protection resistor, wherein the measurement time is 1800s, and ensuring quiet test environment in the measurement process so as to reduce the influence of noise on a measurement result;
when the same cable is tested repeatedly, the test interval is kept at least over 1h, and the cable is ensured to be restored to the initial state;
accelerated aging test protocol:
the high-voltage equipment used in the test is a series resonance voltage-withstanding device, the device mainly comprises a variable frequency power supply, an excitation transformer and an adjustable inductor, the adjustable inductor is adjusted to form series resonance with a cable capacitor to generate high voltage, the equipment can generate 500kV alternating current voltage at most and can generate enough voltage required by electrical aging of the test, the equipment generating the large current is a large current temperature rise test device, the equipment can generate 4000A current at most, and the equipment is provided with a temperature measuring device, so that the temperature of a cable core and an insulating layer of the cable can be monitored in real time, and the thermal aging of the XLPE cable is completed;
applying high voltage to a prepared sample M1 for electrical aging, simultaneously applying high voltage and large current to M2 for electrothermal combined aging, applying large current to M3 for thermal aging, dividing an accelerated aging test into two test stages, after the test of each stage is finished, performing an isothermal relaxation current test to measure relaxation current, fitting measurement data, calculating an aging factor, and researching the change of the aging factor, wherein the high voltage applied to the samples M1 and M2 in the first stage is 1.3U0, the currents applied to the samples M2 and M3 make the temperature of a cable core reach 90 ℃, and the test current is increased in the second stage to make the temperature of the cable cores of the M2 and M3 reach 100 ℃, and the voltage applied to the cables M1 and M2 is still 1.3U0, and the ideal state of the test is that the voltage and the current are simultaneously increased in the second stage, but the voltage cannot be continuously increased under the limitation of test conditions, so that the voltage is still 1.3U 2.
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