An insulation testing device without external discharge interference

Abstract

The application discloses an insulation test device without external discharge interference, which comprises a box body, an electrode, an insulation sleeve, a glass window, a valve, a vacuum pump, a vacuum drying box and an external liquid circulating device. The size of the electrode and the insulation sleeve of the device is designed. Firstly, the initial size of the electrode and the insulation sleeve and the voltage value to be applied are set, then the electric field intensity at each place around the electrode is calculated, and then the corona condition of the electrode in the environment is analyzed by using the Tousson criterion, the breakdown condition of the electrode in the environment is analyzed by using the streamer criterion, and finally whether the discharge occurs near the electrode is verified according to the calculation result. If the discharge occurs, the size of the electrode and the insulation sleeve needs to be adjusted. In addition, the glass window of the device is used for observing the discharge phenomenon of the insulation test, the valve is connected with the vacuum pump, the vacuum drying box and the external liquid circulating device respectively, and is used for realizing the vacuum liquid injection and the external liquid circulating functions. The application can verify the insulation performance of the insulation test device in various environments such as multiple altitudes, multiple temperatures and multiple humidities, and ensures that the device itself does not occur discharge. In addition, the device adopts the vacuum liquid injection mode, so that the liquid material entering the device does not contain bubbles and impurities such as precipitates, the circulating heating mode makes the liquid in the device be heated more uniformly, and the dispersion of the liquid material insulation test is reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of high-voltage insulation technology, and in particular to an insulation test device capable of vacuum liquid injection and external liquid circulation without external discharge interference. BACKGROUND

[0002] With the development of the power industry, more and more electrical equipment is applied in the fields of military industry, aerospace, medical treatment, etc. The insulation of electrical equipment is mostly made of high polymer materials with high electrical strength, but local discharge may be caused by defects in the insulation materials. The initial local discharge energy is relatively weak, and the damage to the insulation material is small and not easy to detect. However, long-term local discharge can cause insulation deterioration, and even lead to insulation system breakdown, thereby causing safety accidents such as personal injury and property damage. In engineering, local discharge measurement is one of the routine tests of equipment, and in the laboratory, insulation test of equipment or test piece to obtain its local discharge characteristics is one of the important means to find defects in equipment structure and process.

[0003] The insulation test first needs to ensure that the test circuit itself does not produce discharge to ensure the accuracy of the test results, so the structure with small curvature radius such as electrode and the insulation distance of high voltage to ground need to be considered in the design of the test platform. At present, simulation calculation has become an important reference method for designing structure. The existing calculation model mostly adopts the method of directly calculating the electric field strength near the structure, and combines with the experience criterion to judge. However, this method has poor intuitiveness, and the experience formula is greatly affected by the external environment, which may cause large errors in the judgment result. Therefore, a more accurate calculation method is needed to provide a reference for determining the structure parameters of the test device. In addition, when the insulation test of liquid-containing insulation material is carried out, the traditional method of pouring liquid material into the device is easy to produce bubbles or impurities such as doped precipitates, which causes the dispersion of the insulation test results, and when the test of the influence of temperature on the insulation performance of liquid material is carried out, the external heating method makes the liquid unevenly heated, and the test result is not accurate enough. Therefore, an insulation test device capable of realizing vacuum liquid injection and uniform heating of liquid needs to be designed. SUMMARY

[0004] In order to ensure that the insulation test device itself does not produce discharge and to reduce the dispersion of the test results of liquid material, the present application provides an insulation test device capable of vacuum liquid injection and external liquid circulation without external discharge interference. The electrode size parameters of the test device are designed through simulation calculation, and the changes of the test environment are simulated by combining with the Peek formula to verify the insulation performance of the device in various environments such as multiple altitudes, multiple temperatures and multiple humidities, so as to ensure that the device does not produce discharge when carrying out insulation test. In addition, the vacuum liquid injection method makes the liquid material entering the device not contain bubbles and impurities such as precipitates, and the circulating heating method makes the liquid in the device more uniformly heated, thereby reducing the dispersion of the insulation test of liquid material.

[0005] To achieve the above object, the present application adopts the following technical solutions:

[0006] The insulating test device without external discharge interference comprises a box body 1, left and right sides of the box body 1 are respectively provided with a left high-voltage electrode 2 and a right high-voltage electrode 3, the two electrodes extend into the inside of the box body and are electrically isolated from the box body through a left insulating sleeve 4 and a right insulating sleeve 5 respectively; a gas collecting box 6 is connected to the top of the box body cover, the gas collecting box 6 is connected to a liquid storage tank 7, so as to ensure that the discharge gas generated during the test does not cause excessive pressure in the box body or even explosion; a left upper tank valve 8, a right upper tank valve 9 and a negative pressure gauge 10 are arranged above the liquid storage tank 7, the left upper tank valve 8 is connected to a vacuum pump 11 and is used to extract the gas in the device to create a vacuum environment; a front bottom valve 14 and a rear bottom valve 15 are arranged at the bottom of the box body, one end of an external liquid circulating device 12 is connected to the right upper tank valve 9, and the other end is connected to the front bottom valve 14, so as to realize the external liquid circulation function; the rear bottom valve 15 extends into the liquid insulating material which has been dried through a hose, and the liquid injection and discharge functions can be realized by changing the air pressure; the negative pressure gauge 10 is used to display the pressure in the device, so as to facilitate the control of the liquid injection time; a front transparent window 16 and a rear transparent window 17 are arranged at the front and rear sides of the box body 1 and are used to observe the discharge phenomenon.

[0007] The insulating test device without external discharge interference can realize the vacuum liquid injection function, and the specific steps are as follows: first, the liquid insulating material which has been dried is placed in a vacuum drying box 13, the valve of the vacuum drying box 13 is opened to communicate with the outside air; then, the left upper tank valve 8 is opened, all other valves are closed, and the vacuum pump 11 is opened to perform the vacuum extraction treatment on the box body 1; when the negative pressure gauge 10 shows that the air pressure in the box body 1 is lower than-0.09, the left upper tank valve 8 and the vacuum pump 11 are closed, the rear bottom valve 15 which extends into the vacuum drying box 13 is opened, and the liquid insulating material is injected into the box body 1 under the action of the air pressure; when it is observed that the liquid surface enters the gas collecting box 6, the rear bottom valve 15 is closed, and the vacuum liquid injection is completed.

[0008] The external liquid circulating device 12 is temperature-adjustable and has a circulating pump, and can realize the external liquid circulation function, and the specific steps are as follows: the front bottom valve 14 and the right upper tank valve 9 are opened, all other valves are closed, the heating temperature of the external liquid circulating device 12 is set, and the circulating pump is opened, so that the heated liquid enters the box body 1 from the front bottom valve 14 and returns to the external liquid circulating device 12 from the right upper tank valve 9; when the temperature of the liquid at the top of the box body 1 is the same as that of the liquid at the bottom, it is considered that the external liquid circulation is completed, and the insulating test of the liquid insulating material is performed in this environment.

[0009] The left high-voltage electrode 2 and the right high-voltage electrode 3 of the device are tested by using the Townsend theory and the streamer theory to determine whether the left high-voltage electrode 2 and the right high-voltage electrode 3 will generate discharge, and the size of the left high-voltage electrode 2, the right high-voltage electrode 3, the left insulating sleeve 4 and the right insulating sleeve 5 of the device is designed by using the method, including the following steps:

[0010] Step 1: A simulation model of the device composed of the box 1, the left high-voltage electrode 2, the right high-voltage electrode 3, the left insulating sleeve 4 and the right insulating sleeve 5 is constructed, the initial size of the left high-voltage electrode 2, the right high-voltage electrode 3, the left insulating sleeve 4 and the right insulating sleeve 5 and the voltage value to be applied are set, and the electric field intensity at each structure of the device is calculated; if only alternating voltage is needed to be applied, the spatial potential at the peak value of the alternating voltage is calculated by using the Poisson equation shown in formula (1); if direct current voltage or alternating and direct current composite voltage is needed to be applied, the spatial potential containing the direct current voltage is calculated by using the full current equation shown in formula (2);

[0011]

[0012] Wherein, ε r is the relative dielectric constant of air, ε0 is the dielectric constant of vacuum, is the Hamiltonian operator, φ is the spatial potential, ρ is the spatial charge density, γ is the conductivity of the insulating material, and Q is the external source term; after the spatial potential is calculated, the electric field intensity is calculated by using the following formula;

[0013]

[0014] During the calculation, the ground potential at the infinite distance is set, the electrical breakdown characteristics are considered, the material and voltage value of each part are set as required, and φ=0 is set at the grounding point; during the simulation calculation process, the spatial charge distortion is not considered, and only the background electric field intensity is considered;

[0015] Step 2: Based on the electric field intensity calculation result in step 1, the left high-voltage electrode 2 and the right high-voltage electrode 3 in the environment are analyzed by using the Townsend theory criterion, and the basic criterion is shown in formula (3):

[0016]

[0017] The breakdown condition of the left high-voltage electrode 2 and the right high-voltage electrode 3 in the environment is analyzed by using the streamer theory criterion, and the basic criterion is shown in formula (4):

[0018]

[0019] In the formula, N d is the molecular number density, which is calculated by T and P through N d = p / k B T, kB is the Boltzmann constant; d is the equivalent discharge distance; a g is the collision ionization coefficient, which is the key factor to control the development degree of electron avalanche, and is obtained by Townsend theory g = f(E / N d ); γ is the secondary electron emission coefficient of cathode surface; 1 [cm] in equation (4) is to eliminate the dimension of discharge distance, and requires that d is in centimeter; A is the threshold value of space electric field;

[0020] In the calculation, all the grounding boundary conditions are set as q = NaN, where q is the amount of charge contained on the grounding boundary, and NaN represents not a number, meaning non-number; and the Townsend coefficient is set according to the environment where the electrode is located; in order to reduce the amount of calculation, the threshold electric field E c of the calculation region is selected as the screening parameter for calculation; when the result meets the criterion of Townsend theory, it is considered that non-breakdown type discharge is generated; when the result meets the criterion of streamer theory, it is considered that breakdown type discharge is generated; and when neither of the above criteria is met, it is considered that no discharge is generated;

[0021] Step 3: Considering the influence factors of surface roughness, impurities and air moisture, the calculation result is corrected, in order to be consistent with the actual working condition, the threshold electric field E c of the calculation region in step 2 is replaced by the Peek formula shown in equation (5) to consider the altitude factor:

[0022]

[0023] In the formula: E on is the corona onset field strength of the left high-voltage electrode 2 and the right high-voltage electrode 3 in air; E0 and k are empirical constants; δ is the relative air density; r is the radius of the conductor, in centimeters; m is the surface roughness coefficient;

[0024] In order to match the simulation result with the test result, a correction coefficient K is introduced, 0 ≤ K ≤ 1, to correct E on obtained by the Peek formula:

[0025]

[0026] Based on the corrected calculation results, it is determined whether the device has discharged. The dimensions of the left high-voltage electrode 2, right high-voltage electrode 3, left insulating sleeve 4, and right insulating sleeve 5 are then modified and optimized. First, to ensure experimental safety, it is essential to ensure that no breakdown-type discharge areas appear within the simulation domain. If such areas do appear, the radius of curvature of the left high-voltage electrode 2 and right high-voltage electrode 3 should be increased, or the insulation distance between the left insulating sleeve 4 and right insulating sleeve 5 should be increased, and the process should return to step 1. Second, to make the experimental results more accurate, no non-breakdown-type discharge areas should appear within the simulation domain. If such areas do appear, the dimensions of the left high-voltage electrode 2 and right high-voltage electrode 3 should be fine-tuned using chamfering, grinding, and painting methods, and the process should return to step 1. If it is determined that no discharge has occurred, the left high-voltage electrode 2, right high-voltage electrode 3, left insulating sleeve 4, and right insulating sleeve 5 should be designed according to the current dimensions.

[0027] The discharge includes corona discharge, spark discharge, surface flashover, and breakdown.

[0028] In step 2, the spatial electric field threshold A is a constant, ranging from 15 to 20.

[0029] In step 2, the threshold electric field E of the calculation region is calculated. c The default value is 30kV / cm.

[0030] In step 3, the empirical constant E0 is taken as 33.1 kV / cm when the voltage is positive and 31.0 kV / cm when the voltage is negative; the empirical constant k is taken as 0.24 when the voltage is positive and 0.308 when the voltage is negative; under standard atmospheric pressure, the relative air density δ = 1, and under simulation conditions, the relative air density δ can be taken as 0.7 to 0.9; r is the conductor radius in cm; m is the surface roughness coefficient, which can be taken as 0.6 to 0.8.

[0031] The materials of the box body 1, the left valve 8 above the liquid storage tank, the right valve 9 above the liquid storage tank, the front valve 14 at the bottom of the box body, and the rear valve 15 at the bottom of the box body are all stainless steel; the left high-voltage electrode 2 and the right high-voltage electrode 3 are made of brass; the left insulating sleeve 4 and the right insulating sleeve 5 are made of epoxy resin; and the front transparent window 16 and the rear transparent window 17 are made of high-temperature resistant quartz glass.

[0032] The entire housing 1 is mounted on the trolley 18. The interior of the trolley 18 is hollowed out to accommodate various equipment: a vacuum drying oven 13 is placed at the lower rear to hold dried transformer oil; a vacuum pump 11 is placed at the lower front to evacuate the device or the vacuum drying oven; and an external liquid circulation device 12 is placed at the upper front to reduce the overall footprint of the device.

[0033] The beneficial effects achieved by this invention are as follows: Firstly, the method of determining whether the electrodes of the test device discharge using Townsend theory and streamer theory no longer relies on empirical criteria. It also considers factors such as surface roughness, impurities, and air moisture, making the simulation results closer to reality. Furthermore, it verifies the insulation performance of the device in various environments with varying altitudes, temperatures, and humidity levels, ensuring that the device itself does not discharge during insulation tests. Secondly, the device uses a vacuum injection method, ensuring that the injected liquid is free of impurities such as air bubbles and solid precipitates, guaranteeing the purity of the liquid. The use of circulating heating, with the high-temperature liquid entering from the bottom of the device, utilizes the different densities of liquids at different temperatures to cause the hot liquid to flow upwards, resulting in more uniform heating of the liquid within the device and reducing the dispersion of insulation test results. Attached Figure Description

[0034] Figure 1 This is the front view of the entire device.

[0035] Figure 2 This is a right view of the entire device.

[0036] Figure 3 This is a flowchart illustrating the design process for the electrode dimensions of the device.

[0037] Figure 4 This is a simulation model diagram of the device's electrodes and housing.

[0038] Figure 5 The diagram shows the electric field simulation results for the device electrodes and housing.

[0039] Figure 6 The figure shows the simulation results of electrical breakdown detection of the device electrodes and housing.

[0040] Figure 7 This is a physical image of the device.

[0041] In the diagram: 1-Test chamber; 2-Left high-voltage electrode; 3-Right high-voltage electrode; 4-Left insulating sleeve; 5-Right insulating sleeve; 6-Gas collection box; 7-Liquid storage tank; 8-Left valve above the liquid storage tank; 9-Right valve above the liquid storage tank; 10-Negative pressure gauge; 11-Vacuum pump; 12-External liquid circulation equipment; 13-Vacuum drying oven; 14-Front valve at the bottom of the chamber; 15-Rear valve at the bottom of the chamber; 16-Front transparent window; 17-Rear transparent window; 18-Trolley. Detailed Implementation

[0042] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0043] like Figure 1 As shown, this invention discloses an insulation testing device without external discharge interference, such as...Figure 1 As shown, the device body is a cuboid box 1, the left and right sides of the box 1 are respectively provided with left high-voltage electrode 2 and right high-voltage electrode 3, the two electrodes extend into the box interior and are electrically isolated from the box through left side insulating sleeve 4 and right side insulating sleeve 5 respectively; the top cover of the box is communicated with gas collection box 6, the gas collection box 6 is connected with liquid storage tank 7, and the gas generated during the insulation test is ensured to cause excessive pressure in the box or even explosion; the top of the liquid storage tank 7 is provided with left top valve 8, right top valve 9 and negative pressure gauge 10, the left top valve 8 is connected with vacuum pump 11, and is used for extracting the gas in the device to create a vacuum environment; the negative pressure gauge 10 is used to display the pressure inside the device, so as to facilitate the control of the liquid injection time. Figure 2 As shown, the bottom of the box is provided with box bottom front valve 14 and box bottom rear valve 15, the outer liquid circulating device 12 is temperature-adjustable and has a circulating pump, one end of the circulating pump is connected with the right top valve 9 of the liquid storage tank, and the other end is connected with the box bottom front valve 14; the box bottom rear valve 15 extends into the liquid insulation material subjected to drying treatment through a hose, and the liquid insulation material is placed in vacuum drying box 13; in addition, in order to fully observe the discharge phenomenon of the insulation test, front transparent window 16 and rear transparent window 17 are arranged on the front and rear sides of the box 1, and the window material is high-temperature-resistant quartz glass. The entire device is installed on trolley 18, and the interior of the trolley 18 is hollowed out to place various devices: the vacuum drying box 13 is placed at the lower rear; the vacuum pump 11 is placed at the lower front; and the outer liquid circulating device 12 is placed at the upper front.

[0044] The electrodes of the device are tested whether they can produce discharge (corona discharge, spark discharge, surface flashover, breakdown, etc.) by using Townsend theory and streamer theory, and the size of the electrodes and the insulating sleeve is designed by this method, and the design flow chart is as shown in Figure 3 As shown, the specific steps include the following steps:

[0045] Step 1: Set the initial size of the device electrode and the voltage value to be applied, construct the device simulation model by equal ratio, and calculate the electric field intensity at each structure of the device. If only alternating voltage is needed to be applied, Poisson equation shown in formula (1) is used to calculate the space potential at the peak value time of the alternating voltage; if direct current voltage or alternating current and direct current composite voltage is needed to be applied, the full current equation shown in formula (2) is used to calculate the space potential containing direct current voltage.

[0046]

[0047] Wherein, ε r is the relative permittivity of air, ε0 is the vacuum permittivity, is the Hamiltonian operator, φ is the space potential, ρ is the space charge density, γ is the conductivity of the insulation material, and Q is the external source term. After the space potential is calculated, the electric field intensity can be calculated by the following formula.

[0048]

[0049] In the calculation, the infinite distance is set as the ground potential, the electrical breakdown characteristics are considered, the material and voltage values of each part are set as required, and φ = 0 is set at the grounding point. In the simulation calculation process, spatial charge distortion is not considered, and only the background electric field strength is considered.

[0050] Step 2: Based on the electric field strength calculation result in step 1, the breakdown condition of the device in the environment is analyzed by using the Townsend theory criterion, and the basic criterion is shown in formula (3):

[0051]

[0052] The breakdown condition of the device in the environment is analyzed by using the streamer theory criterion, and the basic criterion is shown in formula (4):

[0053]

[0054] In the formula, N d is the molecular number density, which can be calculated by the gas temperature T and the gas pressure P through N d = p / k B T, k B is the Boltzmann constant; d is the equivalent discharge distance; α g is the collision ionization coefficient, which is a key factor controlling the degree of electron avalanche development, and is obtained from the Townsend theory (α g = f (E / N d )); γ is the cathode surface secondary electron emission coefficient. In formula (4), 1 [cm] is to eliminate the dimension of the discharge distance, and d is required to be in units of centimeters; A is the spatial electric field threshold (constant), which can be taken as 15-20.

[0055] In the calculation, all the grounding boundary conditions are set as q = NaN (not a number), where q is the charge amount contained on the grounding boundary, NaN represents not a number, and means not a number; and the Townsend coefficient is set according to the environment of the electrode. In order to reduce the calculation amount, the threshold electric field E c of the calculation region (that is, only the region exceeding E c participates in the criterion calculation) is selected as the screening parameter for calculation, and the default value of E c is 30 kV / cm. The electrode model is calculated, and when the result meets the Townsend theory criterion, it is considered that non-breakdown type discharge occurs; when the result meets the streamer theory criterion, it is considered that breakdown type discharge occurs; and when neither of the above criteria is met, it is considered that no discharge occurs.

[0056] Step 3: Considering the influence factors such as surface roughness, impurities, air moisture, etc., the calculation result is corrected, and in order to be consistent with the actual working condition, the threshold electric field Ec Alternatively, consider the altitude factor:

[0057]

[0058] In the formula: E on δ represents the corona induction field strength of the left high-voltage electrode 2 and the right high-voltage electrode 3 in air; E0 and k are empirical constants, E0 = 33.1 kV / cm (positive electrode) or 31.0 kV / cm (negative electrode), k = 0.24 (positive electrode) or 0.308 (negative electrode); δ is the relative air density, δ = 1 under standard atmospheric pressure, and δ can be taken as 0.7 to 0.9 under simulation conditions; r is the conductor radius in cm; m is the surface roughness coefficient, which can be taken as 0.6 to 0.8.

[0059] To ensure a more accurate match between the simulation and experimental results, a correction coefficient K (0 ≤ K ≤ 1) is introduced for the E obtained from the Peek formula. c Make corrections:

[0060]

[0061] Determine whether the device is discharging based on the corrected calculation results. If it is determined that a discharge has occurred, the electrode size needs to be adjusted and the process should return to step 1. If it is determined that no discharge has occurred, the electrode and insulating sleeve should be designed according to the current size.

[0062] Following the steps outlined above, a model of the experimental setup is created in COMSOL software as follows: Figure 4 As shown, the internal dimensions of the enclosure are 240*240*210 (mm), the wall thickness is 10mm, the insulation distance between the two electrodes and the outside of the enclosure is set to 120mm, and the outermost radius of curvature of the electrodes is 50mm. First, the electric field is simulated using the current module built into COMSOL software. An 80kV AC voltage is applied to one electrode, and an 80kV DC voltage is applied to the other electrode. The enclosure walls and infinity are set to ground. The simulation results of the electric field are shown below. Figure 5 As shown. Then, the electrical breakdown detection module built into the COMSOL software is used to simulate whether there is discharge. In equation (2), γ is taken as 0.07, and in equation (3), A is taken as the software's built-in constant of 17.7. The electrode surface is set as the cathode, the wall condition is set as "disappearance", and the Townsend coefficient is set as "dry air". The simulation results are as follows. Figure 6 As shown in the figure, there is no corona discharge or streamer discharge on the outside of the device at this size, which meets the requirements for being an insulating device.

[0063] Subsequently consider the method of realizing the vacuum injection and external liquid circulation function of the device. The specific steps of realizing the vacuum injection are as follows: first, place the liquid insulation material after drying treatment in the vacuum drying box 13, open the valve of the vacuum drying box 13 to communicate with the outside air; then open the left upper valve 8 of the liquid storage tank, close all other valves and open the vacuum pump 11 to perform vacuumizing treatment on the box 1, when the negative pressure table 10 shows that the air pressure inside the box 1 is lower than-0.09, close the left upper valve 8 of the liquid storage tank and the vacuum pump 11, open the rear bottom valve 15 of the box inserted into the vacuum drying box 13, and the liquid insulation material can be injected into the box 1 under the action of air pressure. When it is observed that the liquid surface enters the gas collection box 6, close the rear bottom valve 15 of the box, and the vacuum injection is completed.

[0064] The specific steps of realizing the external liquid circulation are as follows: open the front bottom valve 14 of the box and the right upper valve 9 of the liquid storage tank, close all other valves, set the heating temperature of the external liquid circulation equipment 12 and open the circulation pump, so that the heated liquid enters the box 1 from the front bottom valve 14 of the box and returns to the external liquid circulation equipment 12 from the right upper valve 9 of the liquid storage tank. When the temperature of the liquid above and below in the box 1 is the same, it is considered that the external liquid circulation is completed, and the insulation test of the liquid insulation material can be carried out in this environment.

[0065] The physical diagram designed according to the above idea is shown in Figure 7 The effect of the device is verified in actual operation, the pressure in the box can be extracted to at least 133 Pa, no bubbles are generated during the liquid injection process; after setting the external liquid circulation equipment to 80℃ heating and circulating for 15 minutes, the liquid temperature above and below the box is measured, and it is found that it is in the range of 79.6-80.3℃, which proves that the device has good practical effect.

[0066] The above is the preferred embodiment of the present application, it should be pointed out that for those skilled in the art, various changes and variations can be made without departing from the principles of the present application, any modification, equivalent replacement, improvement, etc. made should be regarded as the protection scope of the present application.

Claims

1. An insulation test device free from external discharge interference, characterized by, The device includes a box (1), the left and right sides of the box (1) are respectively provided with a left high-voltage electrode (2) and a right high-voltage electrode (3), the two electrodes extend into the inside of the box and are electrically isolated from the box through a left insulating sleeve (4) and a right insulating sleeve (5) respectively; a gas collecting box (6) is connected to the top cover of the box, the gas collecting box (6) is connected to a liquid storage tank (7), so as to ensure that the gas generated by discharge during the test does not cause excessive pressure in the box or even explosion; a left upper valve (8) of the liquid storage tank, a right upper valve (9) of the liquid storage tank and a negative pressure gauge (10) are arranged above the liquid storage tank (7), the left upper valve (8) of the liquid storage tank is connected to a vacuum pump (11) and is used for extracting the gas in the device to create a vacuum environment; a front bottom valve (14) of the box and a rear bottom valve (15) of the box are arranged at the bottom of the box, one end of an external liquid circulating device (12) is connected to the right upper valve (9) of the liquid storage tank (7), the other end of the external liquid circulating device (12) is connected to the front bottom valve (14) of the box, so as to realize the external liquid circulation function; the rear bottom valve (15) of the box extends into the liquid insulating material which has been dried through a hose, and the liquid injection and discharge functions can be realized by changing the air pressure; the negative pressure gauge (10) is used for displaying the pressure in the device, so as to facilitate the control of the liquid injection time; a front transparent window (16) and a rear transparent window (17) are arranged on the front and rear sides of the box (1) and are used for observing the discharge phenomenon. The left high-voltage electrode (2) and the right high-voltage electrode (3) of the device are tested by using the Townsend theory and the streamer theory to determine whether the left high-voltage electrode (2) and the right high-voltage electrode (3) will produce discharge, and the size of the left high-voltage electrode (2), the right high-voltage electrode (3), the left insulating sleeve (4) and the right insulating sleeve (5) of the device is designed by using this method, including the following steps: Step 1: a simulation model of the device composed of the box (1), the left high-voltage electrode (2), the right high-voltage electrode (3), the left insulating sleeve (4) and the right insulating sleeve (5) is constructed, the initial size of the left high-voltage electrode (2), the right high-voltage electrode (3), the left insulating sleeve (4) and the right insulating sleeve (5) and the voltage value to be applied are set, and the electric field intensity at each structure of the device is calculated; if only alternating voltage is required to be applied, the spatial potential at the peak value of the alternating voltage is calculated by using the Poisson equation shown in formula (1); if direct current voltage or alternating current and direct current composite voltage is required to be applied, the spatial potential containing direct current voltage is calculated by using the full current equation shown in formula (2); where ε r is the relative permittivity of air, ε0 is the permittivity of vacuum, is the Hamiltonian, φ is the spatial electric potential, ρ is the spatial charge density, γ is the conductivity of the insulating material, Q is the external source term; after calculating the spatial electric potential, the electric field strength is calculated using the following formula; During the calculation, the infinite distance is set as the ground potential, the electrical breakdown characteristics are considered, the material and voltage value of each part are set as required, and φ=0 is set at the grounding point; during the simulation calculation process, the space charge distortion is not considered, and only the background electric field intensity is considered; Step 2: based on the electric field intensity calculation result in step 1, the streamer theory criterion is used to analyze the streamer conditions of the left high-voltage electrode (2) and the right high-voltage electrode (3) in the environment, and the basic criterion is shown in formula (3): The streamer theory criterion is used to analyze the breakdown conditions of the left high-voltage electrode (2) and the right high-voltage electrode (3) in the environment, and the basic criterion is shown in formula (4): where N d is the molecular number density, calculated by the gas temperature T and pressure P through N d = p / k B T; k B is the Boltzmann constant; d is the equivalent discharge distance; a g is the collision ionization coefficient, which is a key factor controlling the development of the electron avalanche, and is obtained from the Townsend theory a g = f(E / N d ); g is the cathode surface secondary electron emission coefficient; 1 [cm] in equation (4) is to eliminate the dimension of the discharge distance, and requires d to be in units of centimeters; A is the threshold of the space electric field; All ground boundary conditions are set as q = NaN in the calculation, where q is the amount of charge contained on the ground boundary, and NaN represents not a number, meaning non-number; and the Townsend coefficient is set according to the environment in which the electrode is located; in order to reduce the amount of calculation, the threshold electric field E c As a screening parameter for calculation, the device simulation model is calculated, and when the result meets the breakdown criterion of the Townsend theory, it is considered that the non-breakdown type discharge is generated; when the result meets the criterion of the streamer theory, it is considered that the breakdown type discharge is generated; and when neither of the above criteria is met, it is considered that there is no discharge. Step 3: Considering the surface roughness, impurities, and air moisture factors, the calculation results are corrected. In order to conform to the actual working conditions, the threshold electric field E of the calculation region in step 2 is corrected to E using the Peek formula shown in equation (5) c Instead, consider the altitude factor: wherein: E on E0and k are empirical constants; δ is the relative air density; r is the conductor radius in cm; and m is the surface roughness factor. In order to match the simulation results with the test results, a correction coefficient K is introduced, 0≤K≤1, to correct E on The correction is made: According to the modified calculation result, it is judged whether discharge occurs, so that the size of the left high-voltage electrode (2), the right high-voltage electrode (3), the left insulating sleeve (4) and the right insulating sleeve (5) is modified and optimized: first, in order to ensure safety of the test, it must be ensured that there is no area of breakdown type discharge in the simulation domain, if there is, the radius of curvature of the left high-voltage electrode (2) and the right high-voltage electrode (3) or the insulation distance of the left insulating sleeve (4) and the right insulating sleeve (5) is increased and the step 1 is returned; second, in order to make the test result more accurate, there is also no area of non-breakdown type discharge in the simulation domain, if there is, the size of the left high-voltage electrode (2) and the right high-voltage electrode (3) is finely adjusted by chamfering, polishing and painting and the step 1 is returned; if it is judged that no discharge occurs, the left high-voltage electrode (2), the right high-voltage electrode (3), the left insulating sleeve (4) and the right insulating sleeve (5) are designed according to the current size.

2. An insulation test device free from external discharge interference as claimed in claim 1, characterized in that, The vacuum liquid injection function can be realized, and the specific steps are as follows: first, the liquid insulation material subjected to drying treatment is placed in the vacuum drying box (13), and the valve of the vacuum drying box (13) is opened to communicate with the external air; then, the left upper valve (8) of the liquid storage tank is opened, all other valves are closed, and the vacuum pump (11) is opened to perform vacuumizing treatment on the box body (1); when the negative pressure table (10) shows that the air pressure inside the box body (1) is lower than-0.09, the left upper valve (8) of the liquid storage tank and the vacuum pump (11) are closed, the rear bottom valve (15) of the box body extending into the vacuum drying box (13) is opened, and the liquid insulation material is injected into the box body (1) under the action of air pressure; when it is observed that the liquid surface enters the gas collection box (6), the rear bottom valve (15) of the box body is closed, and the vacuum liquid injection is completed.

3. An insulation test device free from external discharge interference as claimed in claim 1, characterized in that, The outer liquid circulating device (12) is temperature-adjustable and has a circulating pump, and can realize the outer liquid circulating function, and the specific steps are as follows: the front bottom valve (14) of the box body and the right upper valve (9) of the liquid storage tank are opened, all other valves are closed, the heating temperature of the outer liquid circulating device (12) is set, and the circulating pump is opened, so that the heated liquid enters the box body (1) from the front bottom valve (14) of the box body and returns to the outer liquid circulating device (12) from the right upper valve (9) of the liquid storage tank; when the temperature of the upper liquid in the box body (1) is the same as that of the lower liquid, it is considered that the outer liquid circulation is completed, and the insulation test of the liquid insulation material is performed in this environment.

4. An insulation test device free from external discharge interference as defined in claim 1, wherein The discharge includes corona discharge, spark discharge, surface flashover and breakdown.

5. An insulation test device free from external discharge interference as defined in claim 1, wherein In step 2, the space electric field threshold A is a constant, and the range is 15-20.

6. An insulation test device free from external discharge interference as defined in claim 1, wherein In step 2, the threshold electric field E of the region is calculated c with a default value of 30 kV / cm.

7. An insulation test device free from external discharge interference as defined in claim 1, wherein In step 3, the empirical constant E0 is 33.1 kV / cm when the voltage is positive, and is 31.0 kV / cm when the voltage is negative, the empirical constant k is 0.24 when the voltage is positive, and is 0.308 when the voltage is negative; under the standard atmospheric pressure, the relative air density δ=1, and under the simulation condition, the relative air density δ can be 0.7-0.9; r is the radius of the conductor, and the unit is cm; m is the surface roughness coefficient, and can be 0.6-0.

8.

8. An insulation test device free from external discharge interference as defined in claim 1, wherein The box (1) and the liquid storage tank upper left valve (8), the liquid storage tank upper right valve (9), the box bottom front valve (14) and the box bottom rear valve (15) are all made of stainless steel; the left high-voltage electrode (2) and the right high-voltage electrode (3) are made of brass; the left side insulation sleeve (4) and the right side insulation sleeve (5) are made of epoxy resin; the front transparent window (16) and the rear transparent window (17) are made of high-temperature-resistant quartz glass.

9. An insulation test device free from external discharge interference as defined in claim 1, wherein The box (1) is integrally installed on the trolley (18), the trolley (18) is hollowed out inside to place various equipment: the vacuum drying box (13) is placed at the lower rear to store the transformer oil after drying treatment; the vacuum pump (11) is placed at the lower front to perform vacuumizing treatment on the device or the vacuum drying box; the outer liquid circulation equipment (12) is placed at the upper front to reduce the overall floor area of the device.

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