A double-layer composite insulation isothermal relaxation current testing device and an aging evaluation model thereof

Abstract

The utility model relates to a kind of double-layer composite insulation isothermal relaxation current testing device and its aging evaluation model, belong to measurement field.It includes power module, test module and computer.Power module includes DC high voltage source and normally open switch;Test module includes semiconductive sheet, cable main insulation and matching insulation flat sample, ground electrode, single-pole double-throw switch and pico-ampere current meter;The isothermal relaxation current I1 of single-layer insulation is measured by time-sharing, and its aging factor is evaluated using three-exponential decay model;The isothermal relaxation current I2 of double-layer composite insulation is measured, and the comprehensive aging factor of composite insulation is calculated using four-exponential equivalent decay model, and then the comprehensive aging degree of multi-medium composite insulation suitable for cable joint is evaluated.The isothermal relaxation current of single-layer main insulation and the composite insulation formed by it is synchronously tested at the same time, which improves the testing efficiency and provides a qualitative reference for the aging evaluation technical index of high-voltage laboratory and power cable accessory testing.

Description

Technical Field

[0001] This invention belongs to the field of measurement, and particularly relates to an apparatus and method for testing or analyzing the properties of materials by measuring their chemical or physical properties. Background Technology

[0002] High-voltage DC polymer insulated cables, due to their superior performance and ease of installation, have been widely used since their invention and have become the main carrier of electrical energy transmission, an important component of the power system.

[0003] However, polymer-insulated power cables are subject to electrical, thermal, chemical, and natural factors during operation, causing their insulation to age and greatly impacting the safety and reliability of the power grid. Assessing the insulation status of power cables after they are put into operation is a difficult task for cable operation and maintenance personnel, but it is also a necessary technical support for managers to make cable replacement decisions.

[0004] The most widely used offline methods for detecting cable aging, both domestically and internationally, include partial discharge method, ultra-low frequency dielectric loss method, and step-by-step voltage increase method. These methods have all achieved varying degrees of success, but they all require comparison with the historical operating data of the cable for research. The isothermal relaxation current method can also accurately assess the aging status of polymer power cables that were laid earlier or for which there is no historical operating data.

[0005] Currently, the isothermal relaxation method has made some progress in the research of high-voltage cables. For example, the fitting parameters of the third-order exponential decay model for the aging of cross-linked polyethylene (XLPE) cables are used to calculate the aging factor and thus assess the aging degree of the cable insulation.

[0006] The invention patent CN 104749503 B, authorized on August 25, 2017, discloses "a method for determining the aging condition of XLPE cable insulation." The method first determines whether aging has occurred by measuring the partial discharge of a first cable sample. If no aging is detected, the isothermal relaxation current is calculated for a second cable sample to obtain the aging factor. The activation energy of cross-linked polyethylene is calculated for a third cable sample. Then, using both the aging factor and the activation energy of cross-linked polyethylene as common criteria, the method determines whether the cable to be tested has reached an aging state and the severity of that state. This technical solution uses the isothermal relaxation current method and the activation energy method to comprehensively determine the aging condition of cable insulation, overcoming the shortcomings of using a single indicator to assess the aging condition of XLPE insulated cables, which can lead to significant errors. This provides a more accurate evaluation of the aging condition of XLPE cable insulation. The technical solution uses the isothermal relaxation current method to evaluate the insulation state of a 110kV cross-linked polyethylene cable before and after actual operational aging, and analyzes the relationship between the aging factor and the cable aging state. However, it can only perform isothermal relaxation current tests on single-layer or composite insulation separately, and the testing process is relatively complex. It is suitable for testing in a test laboratory, but not very suitable for testing in cable burial sites.

[0007] The invention patent CN 110231511 B, with an authorization announcement date of June 29, 2021, discloses an "Isothermal Relaxation Current Detection Device Based on Separate Measurement Loop Method," comprising: a high-voltage module, a low-voltage sampling module, a control module, and a current detection module. The high-voltage module is connected to the core of the high-voltage cable, the low-voltage sampling module is connected to the shielding layer of the high-voltage cable, the control module is connected to the control terminal of the high-voltage module to control the polarization or discharge process, and the current detection module is connected to the output terminal of the low-voltage sampling module to record and store the current signal. A curve showing the current signal changing over time is then plotted to obtain the isothermal relaxation current. In this technical solution, based on the isothermal relaxation current theory, a third-order exponential decay model is used to nonlinearly fit the measured depolarization current, analyze the weights of various depolarization current components in the cable insulation, and use this to assess the aging state of the cable insulation. It can eliminate the influence of high-voltage insulation relaxation current in cable insulation depolarization testing, realize the true measurement of isothermal relaxation current of cable insulation, improve the anti-interference ability of current signal, and enhance the reliability of measurement results, thus meeting the needs of on-site measurement of cable insulation relaxation current. However, it only performs continuous testing on single-layer or composite insulation, and can only perform isothermal relaxation current testing on single-layer or composite insulation separately at the same time. If it is necessary to simultaneously evaluate the aging degree of single-layer insulation and the composite insulation it forms, it is often necessary to replace the sample and repeat the test, resulting in low sampling efficiency.

[0008] In reality, the weakest point in power cable insulation is usually at the joint connecting two cables. Under an applied DC electric field, the joint is more prone to breakdown due to the accumulation of space charge. The interface between the main insulation and the mating insulation of cable accessories accumulates a large amount of space charge due to differences in conductivity and dielectric constant, resulting in a more complex electric field distribution at the interface and accelerating the aging of the composite insulation.

[0009] Traditionally, laboratory tests based on isothermal relaxation current typically involve continuous testing of single-layer or composite insulation. This method often has low sampling efficiency, as it can only perform isothermal relaxation current tests on single-layer or composite insulation at the same time. If it is necessary to assess the aging degree of both single-layer insulation and the composite insulation that it forms with, it is often necessary to replace the sample and repeat the test.

[0010] In addition, aging assessment methods for single-layer insulation have been widely proven to be correlated with the service life of cables in the field, among which the third-order exponential decay model is widely used. However, there are few reports on aging assessment methods for double-layer composite insulation in high-voltage laboratories.

[0011] Reports on cable breakdown accidents at engineering sites indicate that insulation breakdown occurs at the interface between double-layer composite insulation due to aging. Assessing the overall aging of double-layer composite insulation seems to be more meaningful for the reference of cable insulation performance and service life. However, no research has been reported on the assessment model of the overall aging of double-layer composite insulation based on isothermal relaxation current.

[0012] In practical power equipment, composite insulation structures are frequently present, such as at cable accessories and joints. Due to the different electrical parameters of the composite insulation, a large amount of charge accumulates at the contact interface, leading to electric field distortion and significantly accelerating the degradation of the composite insulation. Therefore, exploring an aging assessment method for multilayer insulation is of significant practical importance.

[0013] Furthermore, combining multiple assessment methods to evaluate electrical equipment can better reflect the comprehensive and objective insulation status of the equipment. Summary of the Invention

[0014] The technical problem to be solved by this invention is to provide a testing device for isothermal relaxation current of double-layer composite insulation and its aging assessment model. It can simultaneously measure the isothermal relaxation current of single-layer and double-layer composite insulation. Its matching double-layer composite insulation aging assessment model—a fourth-order exponential decay model—can assess the overall aging degree of the double-layer composite insulation of power cable joints. It can simultaneously test the isothermal relaxation current of a single-layer polymer insulation and a double-layer composite insulation composed of this polymer insulation, comprehensively reflecting the overall aging degree of multi-dielectric composite insulation similar to cable joints, and greatly improving the testing efficiency of isothermal relaxation current.

[0015] The technical solution of this invention is: to provide a test device for isothermal relaxation current of double-layer composite insulation and its aging assessment model, including a power supply module, a test module and a computer, characterized in that:

[0016] The power module includes a DC high voltage source, and a "one-to-two" output switch is connected to the output terminal of the DC high voltage source;

[0017] The test module is equipped with a first test circuit and a second test circuit; each test circuit includes at least a semiconductive thin film and a corresponding ground electrode; the input terminals of the two test circuits are respectively connected to the two output terminals of the output switch; the output terminals of the two test circuits are connected to the data input terminal of the computer via a single-pole double-throw switch, or respectively, via a picoampere ammeter, or connected to the ground electrode.

[0018] The power supply terminal and the single-pole double-throw switch share the same ground.

[0019] The computer is equipped with isothermal relaxation current data recording and processing software, which can synchronously test and record the isothermal relaxation current of the test samples in the first test circuit and the second test circuit in a time-division manner.

[0020] The sampling duration or switching frequency of the single-pole double-throw switch for the isothermal relaxation current signal of the test sample in the first and second test circuits is controlled by the computer.

[0021] The test device for isothermal relaxation current of the double-layer composite insulation is tested according to the following mode:

[0022] 1) Prepare a double-layer composite insulating flat plate sample made of the same material as the power cable joint;

[0023] 2) The double-layer composite insulation plate sample includes a first cable insulation test sample part and a second cable insulation test sample part; wherein the first cable insulation test sample part includes the main insulation layer and the matching insulation layer of the cable, and the second cable insulation test sample part only includes the main insulation layer of the cable.

[0024] 3) The first cable insulation test specimen portion of the double-layer composite insulated flat plate specimen is placed in the first test circuit; the second cable insulation test specimen portion of the double-layer composite insulated flat plate specimen is placed in the second test circuit.

[0025] 4) By controlling the switching action of the single-pole double-throw switch, the isothermal depolarization current of the first cable insulation test sample and the second cable insulation test sample is measured in a time-sharing manner.

[0026] 5) The computer acquires the isothermal relaxation current signal I1 of the second cable insulation test specimen and the isothermal relaxation current signal I2 of the first cable insulation test specimen.

[0027] 6) The isothermal relaxation current signal I1 of the second cable insulation test specimen was fitted using a third-order exponential decay model to evaluate the single-layer aging factor of the second cable insulation test specimen.

[0028] 7) The isothermal relaxation current aging assessment model of double-layer composite insulation is used to fit the isothermal relaxation current signal I2 of the first cable insulation test specimen. The aging degree of the composite insulation is assessed by the double-layer composite insulation aging factor of the first cable insulation test specimen. Based on this, the aging degree of the composite insulation structure of the cable joint is approximated.

[0029] 8) Obtain the aging degree of the double-layer composite insulation medium, provide a basis for judging the aging degree of multi-medium composite insulation at power cable joints, provide a theoretical basis for on-site dielectric aging prediction and periodic maintenance, and provide a reference for aging assessment technical indicators for high-voltage laboratories and power cable accessory testing.

[0030] Specifically, the following preparatory work should be carried out before conducting the isothermal relaxation current test on composite insulation:

[0031] a) Before performing the isothermal relaxation current test, a short-circuit treatment is required, and the short-circuit treatment time should be maintained for at least 6 hours to remove the influence of surface charge on the relaxation current test results.

[0032] b) The placement requirements for the first cable insulation test specimen during the test process are as follows: the length of the main insulation layer in the first cable insulation test specimen must be greater than that of the matching insulation layer, and the length ratio of the main insulation layer to the matching insulation layer in the first cable insulation test specimen must be at least greater than 1.5, so as to avoid the influence caused by lateral current drift.

[0033] c) Check whether the grounding electrode of the DC high voltage source and the single-pole double-throw switch is grounded;

[0034] d) Check and debug whether the built-in isothermal relaxation current test system in the computer can work properly and whether the system can identify and accurately record the current data during the pressurization process;

[0035] e) Check whether the switching control of the single-pole double-throw switch is set up correctly and whether the relay can switch normally.

[0036] Furthermore, the length of the main insulating layer is preferably 200-300 mm, the length of the mating insulating layer is preferably 100 mm, and the widths of the main insulating layer and the mating insulating layer are the same.

[0037] Specifically, when the DC high voltage source is powered on and the voltage is applied during the test, the output switch is closed. At this time, the DC high voltage is simultaneously applied to the semiconductive film of the first test circuit and the second test circuit. If the single-pole double-throw switch is closed in position 1, the semiconductive film, main insulation layer, mating insulation layer and ground electrode in the second test circuit form a circuit. At this time, the computer records the isothermal relaxation current signal I2 of the double-layer composite insulation of the first cable insulation test sample.

[0038] After t s After a certain time, the single-pole double-throw switch is switched to position 2. At this time, the output of the first test circuit is disconnected, and the semiconductive film, main insulation layer and ground electrode in the second test circuit form a new circuit. At this time, the output terminal of the single-pole double-throw switch is connected to the computer via a picoampere ammeter. The computer records the single-layer insulation isothermal relaxation current signal I1 of the second cable insulation test sample.

[0039] Similarly, after the next t s After a certain time, the relay switches back to 1 and continues to acquire the single-layer insulation isothermal relaxation current signal I1. The above steps are repeated until the entire pressure test process is over. At this time, two isothermal relaxation signals I1 and I2 of I(pA)-t(min) can be obtained.

[0040] Specifically, for the isothermal relaxation current signal I1 of the second cable insulation test specimen, which only includes the main insulation layer of the cable, a third-order exponential decay model is used for curve fitting. Based on the current components at different times t of the fitted data, the aging factor of the main insulation is evaluated.

[0041]

[0042] Based on the above formula, we can obtain and τ 11 τ 12 τ 13；

[0043] Among them, I 10 It is the steady-state current when the main insulation depolarization current reaches relative equilibrium, I 11 It is the polarization current I caused by the polarization at the interface between the semiconductor and the main insulator. 12 It is the polarization current generated at the interface between the amorphous and crystal structures, I 13 The polarization current is caused by the metal salts and hydrated ions at the interface due to aging. 1i Overall, τ reflects the total trap density within the medium at different stages. 11 τ represents the decay time constant of the polarization current caused by polarization at the interface between the semiconductor and the main insulator. 12 τ represents the decay time constant of the polarization current generated at the interface between the amorphous and crystal materials. 13The time decay constant of the polarization current generated by metal salts and hydrated ions at the interface due to representation;

[0044] To assess the insulation condition of a cable using the measured isothermal relaxation current, the aging factor A of a single layer of insulation is calculated according to the following formula. single :

[0045]

[0046] in:

[0047]

[0048]

[0049] In the formula, a i , τ i The parameters obtained by fitting the corresponding third-order exponential decay model, a i It can approximately reflect the trap density inside the material, τ i It approximately reflects the depth of traps inside the material;

[0050] Q2(τ2) represents the relevant parameters between the internal crystal and amorphous interface of the insulation, and Q3(τ3) represents the relevant parameters of the various polarizations caused by the aging of the insulation.

[0051] Furthermore, the relevant parameter Q3(τ3) of various polarizations caused by insulation aging increases with the degree of aging. The magnitude of the aging factor directly corresponds to the insulation quality of the cable. Therefore, the aging factor A... single Aging factor A can reflect the aging degree of a single insulating material. single The higher the value, the more severe the aging of the main insulation layer of the cable.

[0052] Specifically, for the isothermal relaxation current signal I2 of the first cable insulation test specimen containing the main insulation layer and the mating insulation layer, the correlation parameter Q2(τ) between the internal crystal and amorphous interface of the insulation is fitted at different times t. 22 ), the relevant parameter Q3(τ) for various polarizations caused by insulation aging 23 And the relevant parameter Q4(τ) of charge polarization at the intermediate interface of the double-layer composite insulation. 24 An aging assessment model based on the isothermal relaxation current of double-layer composite insulation is used to fit and evaluate the comprehensive aging factor of the double-layer composite insulation. This comprehensive aging factor is then used to assess the overall aging degree of the double-layer composite insulation. The isothermal relaxation current aging assessment model is a fourth-order exponential decay model.

[0053]

[0054] Based on the above formula, we can obtain and τ 21 τ 22 τ 23 τ 24 ;

[0055] Among them, I 20 It is the value of the isothermal relaxation current of the double-layer composite insulation when it reaches steady state, I 21 It is the polarization current at the interface between the semiconductor and the main insulator, I 22 It is the polarization current at the interface between the amorphous and crystal structures, I 23 It is the polarization current caused by aging of metal salts and hydrated ions, I 24 It is the polarization current caused by the interfacial charge of the double-layer composite insulating dielectric; a 2i τ 2i Related to the material properties of the dielectric, a 2i This reflects the trap density within the medium, τ 2i Reflects the depth of the traps within the medium, I 22 and its corresponding time constant τ 22 The related parameters remain basically unchanged;

[0056] To assess the insulation condition of cables using measured isothermal relaxation currents, the comprehensive aging factor A, which includes the main insulation layer and the cooperating insulation layer of the cable, is calculated according to the following formula. double :

[0057]

[0058] in:

[0059]

[0060]

[0061]

[0062] Where Q2(τ) 22 Q3(τ) is a parameter relating the internal crystal and amorphous interface of an insulating material. 23 Q4(τ) is a parameter related to the various polarizations caused by insulation aging. 24 ) is a parameter related to the charge polarization of the intermediate interface of the double-layer composite insulation.

[0063] Furthermore, the aforementioned Q3(τ) 23 ) and Q3(τ 24 The value of Q3(τ) increases with the degree of aging; therefore, Q3(τ) 23 ) and Q3(τ 24 ) and Q2(τ22 It can reflect the aging degree of the double-layer composite insulation medium, including the main insulation layer and the cooperating insulation layer of the cable.

[0064] Specifically, the isothermal relaxation current testing device and its aging assessment model for double-layer composite insulation are used to test the isothermal relaxation current of the cable insulation before and after aging. The process of testing the isothermal relaxation current includes the following steps:

[0065] (1) Pre-processing before testing:

[0066] (2) Electromagnetic shielding of the cable under test:

[0067] (3) Cable polarization:

[0068] (4) Instantaneous short circuit process:

[0069] (5) Measure the isothermal relaxation current.

[0070] The testing device and aging assessment model for isothermal relaxation current of double-layer composite insulation described in this invention can simultaneously test the isothermal relaxation current of both the single-layer main insulation layer and the resulting double-layer composite insulation layer, thereby improving the testing efficiency of isothermal relaxation current. The aging assessment model for isothermal relaxation current of double-layer composite insulation is used to evaluate the overall aging degree of the composite insulation, providing a qualitative basis for judging the aging degree of multi-dielectric composite insulation at power cable joints. This helps improve the reliability and stability of cable system operation, and also provides a basis for cable life extension, safety review, and technical transformation, improving the efficiency of cable aging management and providing a reference for cable aging management.

[0071] Compared with the prior art, the advantages of the present invention are:

[0072] 1. The technical solution of the present invention simultaneously measures the isothermal relaxation current of single-layer and double-layer composite insulation, and can simultaneously test the isothermal relaxation current of a single-layer polymer insulation and a double-layer composite insulation composed of such polymer insulation, which can greatly improve the testing efficiency of isothermal relaxation current.

[0073] 2. The technical solution of the present invention uses an aging assessment model of isothermal relaxation current of double-layer composite insulation (fourth-order exponential decay model) to evaluate the aging factor of double-layer composite insulation, thereby reflecting the overall aging degree of multi-medium composite insulation similar to cable joints. It can evaluate the overall aging degree of double-layer composite insulation of power cable joints.

[0074] 3. The technical solution of this invention provides an assessment method for evaluating the aging of multi-dielectric composite insulation using the "aging assessment model of isothermal relaxation current of double-layer composite insulation—fourth-order exponential decay model," and proposes a method for calculating the comprehensive aging factor A of double-layer composite insulation.double The concept can reflect the aging degree of double-layer composite insulation media, providing a basis for judging the aging degree of multi-media composite insulation at power cable joints, providing a theoretical basis for on-site media aging prediction and regular maintenance, and providing a certain reference for aging assessment technical indicators of high-voltage laboratories and power cable accessories testing.

[0075] 4. The technical solution of the present invention provides an isothermal relaxation current testing system with dual time zone control. Through the frequency switching control mode of single-pole double-throw, after performing isothermal relaxation current testing on single-layer insulation, isothermal relaxation current testing on double-layer composite insulation can be performed on the same sample at the same time. It can realize accurate recording and necessary processing of isothermal relaxation current signals of single-layer main insulation and its constituent double-layer composite insulation.

[0076] 5. The technical solution of the present invention employs an aging factor A of double-layer composite insulation. double This method is used to assess the aging degree of double-layer composite insulation, providing a basis for judging the aging degree of multi-dielectric composite insulation at power cable joints. Aging factor A double The size of this parameter directly corresponds to the different degrees of aging of the multi-medium interface of the power cable joint. When this parameter is greater than a certain reference range, it is considered that the composite insulation has undergone serious aging, and the cable is considered to have the risk of aging breakdown, which requires the attention of relevant maintenance personnel.

[0077] 6. The technical solution of this invention introduces multiple insulation aging assessment indicators, which can improve the accuracy of insulation aging assessment. Therefore, establishing a sound evaluation standard is crucial for the insulation aging assessment of electrical equipment. In addition, the use of assessment methods that combine electrical and non-electrical quantities, as well as multiple electrical quantities, can make a more comprehensive judgment on the insulation condition of power equipment.

[0078] 7. The technical solution of this invention helps to improve the reliability and stability of cable system operation, while providing a basis for cable life extension, safety review, and technical transformation, improving the efficiency of cable aging management, and providing a guiding reference for cable aging management. Attached Figure Description

[0079] Figure 1 This is a schematic diagram of the invention of a single / double-layer isothermal relaxation current testing device;

[0080] Figure 2 This is a block diagram of the testing steps of the present invention;

[0081] Figure 3 This is a schematic diagram illustrating the steps for testing the isothermal relaxation current before and after cable insulation aging.

[0082] In the figure, 1 is the output switch of the DC high voltage source; 2 is a semi-conductive thin film; 3 is a double-layer composite insulating plate sample; 4 is the ground electrode; 5 is a single-pole double-throw switch; 6 is a picoampere ammeter; 7 is a computer; and 8 is the ground electrode. Detailed Implementation

[0083] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0084] Figure 1 This is a schematic diagram of the structure of the double-layer isothermal relaxation current testing device of the present invention.

[0085] The technical solution of this invention provides a testing device for isothermal relaxation current of a double-layer composite insulation structure and its aging evaluation model. The composition and connection method of the testing device are as follows: Figure 1 As shown.

[0086] Figure 1 The double-layer isothermal relaxation current testing device of the present invention includes a power supply module, a testing module, and a computer (also known as a data processing PC). The power supply module includes a DC high-voltage source, and a "one-to-two" output switch 1 is connected to the output end of the DC high-voltage source. The testing module is equipped with two test circuits (referred to as a single-layer test circuit and a composite insulation test circuit, respectively). Each test circuit includes at least a semiconductive film 2 and a corresponding ground electrode 4. The input ends (i.e., the semiconductive films) of the two test circuits are respectively connected to the two output ends (a1 and a2, respectively) of the output switch 1. The output ends (i.e., the ground electrodes) of the two test circuits are connected to either the data input end of the computer 7 via a picoampere ammeter 6 or the ground electrode 8 via a single-pole double-throw switch 5.

[0087] The computer is equipped with isothermal relaxation current data recording and processing software (also known as isothermal relaxation current testing system), which can synchronously test the isothermal relaxation current of single-layer and composite insulation in different time zones.

[0088] The sampling duration of the single-pole double-throw switch for single-layer and composite insulation isothermal relaxation current signals of single-layer and double-layer insulation is controlled by the computer.

[0089] During testing, a double-layer composite insulating plate sample 3, made of the same material as the power cable joint, was first prepared.

[0090] The double-layer composite insulating plate sample includes a first cable insulation test sample part and a second cable insulation test sample part. The first cable insulation test sample part includes the main insulation layer and the matching insulation layer of the cable, while the second cable insulation test sample part only includes the main insulation layer of the cable.

[0091] The first cable insulation test specimen portion of the double-layer composite insulated flat plate specimen is placed in the first test circuit (also known as the composite insulation test circuit), and the second cable insulation test specimen portion of the composite insulated flat plate specimen is placed in the second test circuit (also known as the single-layer test circuit).

[0092] According to such Figure 1 The isothermal relaxation current test was performed on the double-layer composite insulation flat plate sample arrangement shown. The isothermal relaxation current signal I1 of the single-layer insulation and the isothermal relaxation current signal I2 of the double-layer composite insulation sample were recorded by the PC. I1 and I2 were fitted by the third-order exponential decay model and the fourth-order exponential decay model, respectively. The aging degree of the single-layer and composite insulation was evaluated according to the existing single-layer aging factor evaluation method and the double-layer composite insulation aging factor proposed in this invention.

[0093] Specifically, the following preparatory work needs to be done before conducting isothermal relaxation current tests on single / double-layer composite insulation flat plate samples:

[0094] (1) Before the pressure test, a short circuit of 20MPa is required in the vacuum switch cabinet. It is recommended to keep it for at least 6 hours to remove the influence of surface charge on the relaxation current test results.

[0095] (2) The placement requirements for the first cable insulation test specimen (also known as the double-layer composite insulation plate specimen) during the test are as follows: the length of the main insulation layer in the first cable insulation test specimen should be much greater than that of the cooperating insulation layer, and the length ratio of the main insulation layer to the cooperating insulation layer should be at least greater than 1.5 to avoid the influence caused by lateral current drift. It is recommended that the length of the main insulation layer be 200-300mm and the length of the cooperating insulation layer be 100mm, and the thickness of both should be the same.

[0096] (3) Check whether the DC high voltage source and the single-pole double-throw switch share a common ground;

[0097] (4) Check and debug whether the isothermal relaxation current test system built into the computer 7 can work properly and whether the system can identify and accurately record the current data during the pressurization process.

[0098] (5) Check whether the switching control of the single-pole double-throw switch is set up and whether the relay can switch normally.

[0099] During the pressure test, after completing the above-mentioned pre-test preparations, when the power is applied and the pressure is applied, switch 1 is closed. At this time, DC high voltage is simultaneously applied to the semiconductive film (also known as the anode) of the first and second test circuits (the left and right circuits in the figure). If the single-pole double-throw switch is closed in position 1, then the semiconductive film, main insulating layer, mating insulating layer and ground electrode (also known as the cathode) on the left side form a circuit.

[0100] like Figure 1 As indicated by the middle arrow, the computer is recording the isothermal relaxation current signal I2 of the double-layer composite insulation. After t... s After a certain time, the single-pole double-throw switch is switched to position 2. At this time, the left circuit (i.e., the first test circuit) is disconnected, and the semiconductive film, main insulation layer and ground electrode of the right circuit (i.e., the second test circuit) form a new circuit. At this time, the output terminal 2 of the single-pole double-throw switch is connected to the PC terminal via a picoampere ammeter. The computer records the isothermal relaxation current signal I1 of the single-layer insulation.

[0101] Similarly, after the next t s After a certain time, the relay switches back to 1 and continues to acquire the I1 signal. The above steps are repeated until the entire pressure test process is over. At this time, two isothermal relaxation current signals I1 for single-layer insulation and I2 for double-layer composite insulation can be obtained.

[0102] The two current signal curves mentioned above are processed as follows:

[0103] For the I1 curve, a third-order exponential decay model is used for fitting:

[0104]

[0105] Based on the above formula, we can obtain and τ 11 τ 12 τ 13 Among them, I 10 It is the steady-state current when the main insulation depolarization current reaches relative equilibrium, I 11 It is the polarization current I caused by the polarization at the interface between the semiconductor and the main insulator. 12 It is the polarization current generated at the interface between the amorphous and crystal structures, I 13 It is the polarization current generated by metal salts and hydrated ions at the interface due to aging. 1i Overall, τ reflects the total trap density within the medium at different stages. 11 τ represents the decay time constant of the polarization current caused by polarization at the interface between the semiconductor and the main insulator. 12 τ represents the decay time constant of the polarization current generated at the interface between the amorphous and crystal materials. 13 The time decay constant of the polarization current generated by the metal salts and hydrated ions at the interface due to representation.

[0106] The aging factor A of a single layer of insulation can be calculated using the following formula. single :

[0107]

[0108] in:

[0109]

[0110]

[0111] In the formula, a i , τ i The parameters obtained by fitting the corresponding third-order exponential decay model, a i It can approximately reflect the trap density inside the material, τ i It approximately reflects the depth of traps inside the material.

[0112] Q2(τ2) represents the relevant parameters between the crystalline and amorphous interfaces within the insulation. These parameters remain largely unchanged during the aging process. Q3(τ3), on the other hand, represents the relevant parameters related to the various polarizations caused by aging. This parameter increases with the degree of aging. The magnitude of the aging factor directly corresponds to the insulation quality of the cable. Therefore, A single It can reflect the aging degree of a single insulating material, A single The larger the value, the more severe the aging of the single layer of insulation.

[0113] In engineering practice, the aging factor of single-layer insulation has been proven to have a direct correlation with material aging. Aging factor A single The aging of the single-layer insulation of the cable is detailed in Table 1.

[0114] Table 1

[0115]

[0116] When the aging factor is greater than 2, the insulation is considered to have undergone serious deterioration, and the residual breakdown voltage of the insulation medium drops significantly.

[0117] Example:

[0118] like Figure 3 As shown in the figure, a 110kV XLPE cable sample was selected, and the isothermal relaxation current of the cable before and after insulation aging was tested. The main steps included:

[0119] (1) Pre-processing before testing:

[0120] The outer shield of the 110kV XLPE cable sample was peeled off to a depth of about 8cm. To reduce the error caused by surface leakage current, the dirt left after peeling off the shield was cleaned with alcohol.

[0121] (2) Electromagnetic shielding of the cable under test:

[0122] Because the depolarization current is extremely small and the cable is easily affected by external electromagnetic interference, a retractable aluminum foil corrugated tube can be used for electromagnetic shielding.

[0123] (3) Cable polarization:

[0124] One end of the cable under test is reliably insulated, and the other end is polarized with a high-voltage DC power supply. The shielding metal is grounded, the conductor is connected to the positive terminal, and the polarization time is 1800s.

[0125] (4) Instantaneous short circuit process:

[0126] To reduce the influence of surface charge on the test results, surface free charge must be eliminated by short-circuiting the cable after measuring the polarization current with a small resistor.

[0127] (5) Measure the isothermal relaxation current:

[0128] The samples before and after aging were subjected to isothermal relaxation current tests. The isothermal relaxation current of each cable was measured and recorded using a computer. The relationship between the aging factor and aging state of each sample was obtained by curve fitting using the third-order exponential decay model mentioned above, as shown in Table 2.

[0129] Table 2

[0130]

[0131] As shown in the table above, when the aging factor of the isothermal relaxation current test for single-layer cable insulation is greater than 2, it indicates that the aging of the dielectric is very severe and the insulation aging condition is extremely poor. When the aging factor is around 2, it indicates that the material has undergone a long period of aging and is in an aged state. When the aging factor is less than 1.8, it indicates that the insulation performance of the material is relatively good. The aging factor calculated by the third-order exponential decay model of isothermal relaxation current shows a strong correlation with the degree of material aging.

[0132] Isothermal relaxation current aging factor A for single-layer insulation single For aging assessment, various countries have established corresponding standards, as shown in Table 3:

[0133] Table 3

[0134]

[0135]

[0136] The table above shows a strong correlation between aging factors and material aging. However, there are currently no clear corresponding standards in my country, so extensive experimental research is still needed to obtain A-level standards suitable for my country's engineering practices. single The criteria are used to guide cable maintenance personnel in my country to better inspect and maintain cable lines.

[0137] For the isothermal relaxation current signal I2 curve of double-layer composite insulation, the technical solution of the present invention uses an aging evaluation model of the isothermal relaxation current of double-layer composite insulation to fit it. The aging evaluation model of the isothermal relaxation current of double-layer composite insulation is a fourth-order exponential decay model.

[0138]

[0139] Based on the above formula, we can obtain and τ 21 τ 22 τ 23 τ 24 .

[0140] Among them, I 20 It is the value of the isothermal relaxation current of the double-layer composite insulation when it reaches steady state, I 21 It is the polarization current at the interface between the semiconductor and the main insulator, I 22 It is the polarization current at the interface between the amorphous and crystal structures, I 23 It is the polarization current caused by aging of metal salts and hydrated ions, I 24 It is the polarization current caused by the interfacial charge of the double-layer composite insulating medium. 2i τ 2i Related to the material properties of the dielectric, a 2i This reflects the trap density within the medium, τ 2i This reflects the depth of traps within the medium. It is assumed that during aging, the relevant parameters of amorphous and crystal interface polarization do not change significantly with increasing aging. 22 and its corresponding time constant τ 22 The related parameters remain largely unchanged. Correspondingly, it is believed that during the aging process, water trees will be generated in the cable due to the effects of electric field, temperature, and hydrated ions, and τ3 will change significantly with the aging process. Furthermore, the technical solution of this invention considers I... 24 The corresponding time constant τ 24 It will also change significantly as the composite insulation ages.

[0141] In summary, the technical solution of this invention uses the following formula to calculate the comprehensive aging factor A of double-layer composite insulation. double :

[0142]

[0143] in:

[0144]

[0145]

[0146]

[0147] Where Q2(τ) 22 Q3(τ) is a parameter relating to the internal crystalline and amorphous interfaces of an insulator; during aging, this parameter remains essentially unchanged. 23 Q4(τ) represents the relevant parameters for various polarizations caused by insulation aging. 24 Q3(τ) represents the relevant parameter of charge polarization at the intermediate interface of the double-layer composite insulation. 23 ) and Q3(τ 24 The value of Q3(τ) increases with the degree of aging; therefore, Q3(τ) 23 ) and Q3(τ 24 ) and Q2(τ 22 It can reflect the aging degree of the double-layer composite insulation medium.

[0148] The technical solution of this invention provides a qualitative basis for judging the aging degree of multi-dielectric composite insulation at power cable joints, with aging factor A. double The magnitude of this parameter directly corresponds to the varying degrees of aging at the multi-dielectric interface of the power cable joint. When this parameter exceeds a certain reference range, it is considered that the composite insulation has undergone severe aging, indicating a risk of aging-induced breakdown in the cable, requiring attention from relevant maintenance personnel. Given the current scarcity of field data for assessing the aging of multi-dielectric composite insulation, this invention provides a method for evaluating the aging of multi-dielectric composite insulation using a "fourth-order exponential decay model," offering a theoretical basis for predicting dielectric aging and conducting regular maintenance in the field.

[0149] The technical solution of this invention can simultaneously test the isothermal relaxation current of the single-layer main insulation layer and the double-layer composite insulation layer it forms, greatly improving the testing efficiency of isothermal relaxation current. It also proposes an aging assessment model using the isothermal relaxation current of double-layer composite insulation to evaluate the overall aging degree of the composite insulation, providing a qualitative basis for judging the aging degree of multi-dielectric composite insulation at power cable joints, improving the reliability and stability of cable system operation, and providing a basis for cable life extension, safety review, and technical transformation. This improves the efficiency of cable aging management and provides a reference for cable aging management.

[0150] This invention can be widely used in the fields of predicting dielectric aging and managing regular maintenance of power cables.

Claims

1. A testing device for isothermal relaxation current of double-layer composite insulation, comprising a power supply module, a testing module, and a computer, characterized in that: The power module includes a DC high voltage source, and a "one-to-two" output switch is connected to the output terminal of the DC high voltage source; The test module is equipped with a first test circuit and a second test circuit; each test circuit includes at least a semiconductive thin film and a corresponding ground electrode; the input terminals of the two test circuits are respectively connected to the two output terminals of the output switch; the output terminals of the two test circuits are connected to the data input terminal of the computer via a single-pole double-throw switch, or respectively, via a picoampere ammeter, or connected to the ground electrode. The power supply terminal and the single-pole double-throw switch share the same ground. The computer is equipped with isothermal relaxation current data recording and processing software, which can synchronously test and record the isothermal relaxation current of the test samples in the first test circuit and the second test circuit in a time-division manner. The sampling duration or switching frequency of the single-pole double-throw switch for the isothermal relaxation current signal of the test sample in the first and second test circuits is controlled by the computer. The test device for isothermal relaxation current of the double-layer composite insulation is tested according to the following mode: 1) Prepare a double-layer composite insulating flat plate sample made of the same material as the power cable joint; 2) The double-layer composite insulation plate sample includes a first cable insulation test sample part and a second cable insulation test sample part; wherein the first cable insulation test sample part includes the main insulation layer and the matching insulation layer of the cable, and the second cable insulation test sample part only includes the main insulation layer of the cable. 3) The first cable insulation test specimen portion of the double-layer composite insulated flat plate specimen is placed in the first test circuit; the second cable insulation test specimen portion of the double-layer composite insulated flat plate specimen is placed in the second test circuit. 4) By controlling the switching action of the single-pole double-throw switch, the isothermal depolarization current of the first cable insulation test sample and the second cable insulation test sample is measured in a time-sharing manner. 5) The computer acquires the isothermal relaxation current signal I1 of the second cable insulation test specimen and the isothermal relaxation current signal I2 of the first cable insulation test specimen; 6) The isothermal relaxation current signal I1 of the second cable insulation test specimen was fitted using a third-order exponential decay model to evaluate the single-layer aging factor of the second cable insulation test specimen. 7) The isothermal relaxation current aging assessment model of double-layer composite insulation is used to fit the isothermal relaxation current signal I2 of the first cable insulation test specimen. The aging degree of the composite insulation is assessed by the double-layer composite insulation aging factor of the first cable insulation test specimen. Based on this, the aging degree of the composite insulation structure of the cable joint is approximated. 8) Obtain the aging degree of the double-layer composite insulation medium, provide a basis for judging the aging degree of multi-medium composite insulation at power cable joints, provide a theoretical basis for on-site dielectric aging prediction and regular maintenance, and provide a reference for aging assessment technical indicators for high-voltage laboratories and power cable accessory testing.

2. The testing device for isothermal relaxation current of double-layer composite insulation according to claim 1, characterized in that... Before conducting the isothermal relaxation current test on composite insulation, the following preparatory work should be performed: a) Before performing the isothermal relaxation current test, a short-circuit treatment is required, and the short-circuit treatment time should be maintained for at least 6 hours to remove the influence of surface charge on the relaxation current test results. b) The placement requirements for the first cable insulation test specimen during the test process are as follows: the length of the main insulation layer in the first cable insulation test specimen must be greater than that of the matching insulation layer, and the length ratio of the main insulation layer to the matching insulation layer in the first cable insulation test specimen must be at least greater than 1.5, so as to avoid the influence caused by lateral current drift. c) Check whether the grounding electrode of the DC high voltage source and the single-pole double-throw switch is grounded; d) Check and debug whether the built-in isothermal relaxation current test system in the computer can work properly and whether the system can identify and accurately record the current data during the pressurization process; e) Check whether the switching control of the single-pole double-throw switch is set up correctly and whether the relay can switch normally.

3. The testing device for isothermal relaxation current of double-layer composite insulation according to claim 2, characterized in that: The main insulating layer has a length of 200-300 mm, the mating insulating layer has a length of 100 mm, and the main insulating layer and the mating insulating layer have the same width.

4. The testing device for isothermal relaxation current of double-layer composite insulation according to claim 1, characterized in that... When the DC high voltage source is powered on and tested, the output switch is closed. At this time, the DC high voltage is simultaneously applied to the semiconductive film of the first test circuit and the second test circuit. If the single-pole double-throw switch is closed in position 1, the semiconductive film, main insulation layer, mating insulation layer and ground electrode in the first test circuit form a circuit. At this time, the computer records the isothermal relaxation current signal I2 of the double-layer composite insulation of the first cable insulation test sample. After t s After a certain time, the single-pole double-throw switch is switched to position 2. At this time, the output of the first test circuit is disconnected, and the semiconductive film, main insulation layer and ground electrode in the second test circuit form a new circuit. At this time, the output terminal of the single-pole double-throw switch is connected to the computer via a picoampere ammeter. The computer records the single-layer insulation isothermal relaxation current signal I1 of the second cable insulation test sample. Similarly, after the next t s After a certain time, the relay switches back to 1 and continues to acquire the single-layer insulation isothermal relaxation current signal I2. The above steps are repeated until the entire pressure test process is over. At this time, two isothermal relaxation signals I1 and I2 of I(pA)-t(min) can be obtained.

5. The testing device for isothermal relaxation current of double-layer composite insulation according to claim 1, characterized in that: For the isothermal relaxation current signal I1 of the second cable insulation test specimen, which only includes the main cable insulation layer, a third-order exponential decay model is used for curve fitting. Based on the current components at different times t of the fitted curve, the aging factor of the main insulation is evaluated. Based on the above formula, we can obtain I. 10 I 11 ( ), I 12 ( ), I 13 ( ) and τ 11 τ 12 τ 13； Among them, I 10 It is the steady-state current when the main insulation depolarization current reaches relative equilibrium, I 11 It is the polarization current I caused by the polarization at the interface between the semiconductor and the main insulator. 12 It is the polarization current generated at the interface between the amorphous and crystal structures, I 13 The polarization current is caused by the metal salts and hydrated ions at the interface due to aging. 1i Overall, τ reflects the total trap density within the medium at different stages. 11 τ represents the decay time constant of the polarization current caused by polarization at the interface between the semiconductor and the main insulator. 12 τ represents the decay time constant of the polarization current generated at the interface between the amorphous and crystal materials. 13 The time decay constant representing the polarization current generated by metal salts and hydrated ions at the interface due to aging; To assess the insulation condition of a cable using the measured isothermal relaxation current, the aging factor A of a single layer of insulation is calculated according to the following formula. single : A single in: In the formula, a i , τ i The parameters obtained by fitting the corresponding third-order exponential decay model, a i It can approximately reflect the trap density inside the material, τ i It approximately reflects the depth of traps inside the material; Q2(τ2) represents the relevant parameters between the internal crystal and amorphous interface of the insulation, and Q3(τ3) represents the relevant parameters of the various polarizations caused by the aging of the insulation.

6. The testing device for isothermal relaxation current of double-layer composite insulation according to claim 5, characterized in that: The parameter Q3(τ3), which relates to various polarizations caused by insulation aging, increases with the degree of aging. The magnitude of the aging factor directly corresponds to the insulation quality of the cable. Therefore, the aging factor A... single Aging factor A can reflect the aging degree of a single insulating material. single The higher the value, the more severe the aging of the main insulation layer of the cable.

7. The testing device for isothermal relaxation current of double-layer composite insulation according to claim 1, characterized in that: For the isothermal relaxation current signal I2 of the first cable insulation test specimen containing the main insulation layer and the mating insulation layer, the correlation parameter Q2(τ) between the internal crystal and amorphous interface of the insulation is fitted at different times t. 22 ), the relevant parameter Q3(τ) for various polarizations caused by insulation aging 23 And the relevant parameter Q4(τ) of charge polarization at the intermediate interface of the double-layer composite insulation. 24 An aging assessment model based on the isothermal relaxation current of double-layer composite insulation is used to fit and evaluate the comprehensive aging factor of the double-layer composite insulation. This comprehensive aging factor is then used to assess the overall aging degree of the double-layer composite insulation. The isothermal relaxation current aging assessment model is a fourth-order exponential decay model. ； Based on the above formula, we can obtain I. 20 I 21 ( ), I 22 ( ), I 23 ( ), I 24 ( ) and τ 21 τ 22 τ 23 τ 24 ; Among them, I 20 It is the value of the isothermal relaxation current of the double-layer composite insulation when it reaches steady state, I 21 It is the polarization current at the interface between the semiconductor and the main insulator, I 22 It is the polarization current at the interface between the amorphous and crystal structures, I 23 It is the polarization current caused by aging of metal salts and hydrated ions, I 24 It is the polarization current caused by the interfacial charge of the double-layer composite insulating dielectric; a 2i τ 2i Related to the material properties of the dielectric, a 2i τi reflects the trap density inside the medium, and I reflects the trap depth inside the medium. 22 and its corresponding time constant τ 22 The related parameters remain basically unchanged; To assess the insulation condition of cables using measured isothermal relaxation currents, the comprehensive aging factor A, which includes the main insulation layer and the cooperating insulation layer of the cable, is calculated according to the following formula. double : in: Where Q2(τ) 22 Q3(τ) is a parameter relating the internal crystal and amorphous interface of an insulating material. 23 Q4(τ) is a parameter related to the various polarizations caused by insulation aging. 24 ) is a parameter related to the charge polarization of the intermediate interface of the double-layer composite insulation.

8. The testing device for isothermal relaxation current of double-layer composite insulation according to claim 7, characterized in that: The Q3(τ) 23 ) and Q3(τ 24 The value of Q3(τ) increases with the degree of aging; therefore, Q3(τ) 23 ) and Q3(τ 24 ) and Q2(τ 22 It can reflect the aging degree of the double-layer composite insulation medium, including the main insulation layer and the cooperating insulation layer of the cable.

9. The testing device for isothermal relaxation current of double-layer composite insulation according to claim 1, characterized in that: The isothermal relaxation current testing device for double-layer composite insulation tests the isothermal relaxation current before and after cable insulation aging. The process of testing the isothermal relaxation current includes the following steps: (1) Pre-processing before testing: (2) Electromagnetic shielding of the cable under test: (3) Cable polarization: (4) Instantaneous short circuit process: (5) Measure the isothermal relaxation current.

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