A high voltage resistant lead structure

Abstract

The present invention discloses a high-voltage lead structure, including a lead cable body, a protective resistor, a wrapped insulation layer and a stress cone; the lead cable body includes a cable conductor and a cable insulation shielding layer; the lead cable body is provided with a breaking point, and the protective resistor is connected in series with the cable conductors on both sides at the segment; the protective resistor and the lead cable body within a set length range on both sides thereof are provided with a wrapped insulation layer on the periphery, and stress cones are provided at both ends of the wrapped insulation layer. The lead cable has a high withstand voltage peak value, a large withstand voltage range, and good insulation performance, and can be applied to various high-voltage output lines or high-voltage test circuits, has strong withstand voltage capability, and good insulation effect, and can ensure the safety performance of power transmission and the normal operation of related tests, thereby obtaining reliable measurement results.

Description

Technical Field

[0001] The invention relates to the technical field of voltage-resistant insulation design of electrical cables, in particular to a high-voltage-resistant lead wire structure. Background Art

[0002] Polymers have become an indispensable material in the electrical industry because of their excellent electrical properties, such as high resistivity, low dielectric loss, and high electrical breakdown strength. They are usually used to insulate components in electrical systems from each other and from the ground, and it is hoped that the material has the highest possible volume resistivity. The volume resistivity of insulating materials often changes with ambient temperature and applied electric field. For example, the volume resistivity of cross-linked polyethylene used in the insulation layer of high-voltage DC cables changes significantly with temperature and electric field, sometimes by several orders of magnitude, which poses a huge challenge to the design of high-voltage DC cable insulation structures. Under DC voltage, the electric field distribution of the cable insulation layer depends on the volume resistivity of the cross-linked polyethylene. Changes in the volume resistivity will cause serious distortion of the electric field of the cable insulation layer. In addition, the characteristics of the insulating material itself and changes in environmental conditions also have a great influence on the volume resistance. Therefore, accurately measuring and reasonably evaluating the volume resistivity of insulating materials has become a technical problem that must be overcome when designing the insulation structure of DC cables.

[0003] As one of the important instruments for measuring the volume resistivity of materials, high resistance meters are widely used to measure the insulation resistance of insulating materials, electrical products, and various components. They have the advantages of high measurement accuracy, stable performance, and simple operation. The maximum range of the instrument is up to 1017Ω. However, there are still problems in practical applications, such as low test voltage. The maximum output voltage is only 1kV, which cannot meet the testing requirements of insulating materials under actual operating voltage. In order to study the relationship between the volume resistivity of insulating materials and the electric field, it is necessary to provide a test structure that can provide a higher output voltage and a wider output voltage range. However, this requires a lead that can withstand high voltage between the voltage output end and the test structure to ensure that the volume resistivity test is carried out normally under high voltage, thereby ensuring the reliability of the results. Summary of the invention

[0004] The object of the present invention is to provide a high voltage resistant lead structure which can be suitable for high voltage output, especially DC high voltage output, has high withstand voltage peak value, large withstand voltage range and good insulation performance.

[0005] The technical solution adopted by the present invention is: a high-voltage resistant lead structure, including a lead cable body, a protective resistor, a wrapped insulation layer and a stress cone; the lead cable body includes a cable conductor and a cable insulation shielding layer;

[0006] The lead cable body is provided with a dividing point, and the protection resistor is connected in series with the cable conductors on both sides at the dividing point;

[0007] A wrapping insulation layer is arranged around the protective resistor and the lead cable body within a set length range on both sides thereof, and stress cones are arranged at both ends of the wrapping insulation layer.

[0008] Preferably, the conical surface of the stress cone is a curved surface which is concave inwardly toward the location of the protective resistor.

[0009] Preferably, the resistance of the protection resistor is R>U max / I max , where U max is the maximum output voltage flowing through the high voltage lead and the protection resistor, I max Maximum output current for popular high voltage leads and protection resistors.

[0010] Preferably, the thickness of the wrapped insulation layer satisfies the requirement that the radial electric field strength on the surface of the wrapped insulation layer is half of the maximum electric field strength of the insulation shielding layer of the lead cable body, that is:

[0011]

[0012] in, Intermediate

[0013] In the above formula, E is the electric field strength at any point in the insulation shielding layer of the lead cable body; U is the cable insulation withstand voltage; a is the inner radius of the insulation shielding layer; b is the outer radius of the insulation shielding layer; γ is the electric field coefficient; r is the distance from the point where the electric field strength is currently calculated to the axis of the cable conductor;

[0014] Then, the thickness n of the wrapped insulation layer is:

[0015] n=cb.

[0016] Preferably, the end point with the smaller diameter of the stress cone is taken as the origin of the horizontal coordinate, and the height of the cable conductor axis is taken as the origin of the vertical coordinate. The cone curve equation of the stress cone is:

[0017]

[0018] The electric field intensity distribution on the cone surface is:

[0019] In the above formula, x is the horizontal coordinate of any point on the cone surface, r t is the ordinate of the point; E is the electric field strength at any point in the insulation shielding layer of the lead cable body; α is the angle between the tangent and the abscissa of a point on the cone surface, dx and dy are the differential forms of the abscissa and ordinate of the point on the cone surface as the position of the cone surface changes; U is the cable insulation withstand voltage; a is the inner radius of the insulation shielding layer; b is the outer radius of the insulation shielding layer, and γ is the electric field coefficient.

[0020] The present invention also discloses a volume resistivity testing system using the above-mentioned high-voltage lead structure, which comprises a DC high-voltage generator, a high-resistance meter, a temperature-controlled shielding box, and a testing electrode in the temperature-controlled shielding box;

[0021] The test electrode comprises a high-voltage electrode and a test electrode placed in parallel with each other, a sample accommodating interval is provided between the two electrodes, and a protective ring is provided outside the high-voltage electrode; the positive pole of the DC output end of the DC high-voltage generator is connected to the high-voltage electrode through the high-voltage lead wire; the measuring electrode is connected to the test end of the high-resistance meter;

[0022] The temperature-controlled shielding box includes a metal shell and a temperature control module capable of changing the temperature inside the box;

[0023] The negative pole of the DC output terminal of the DC high voltage generator, the protection ring, the metal shell of the temperature control shielding box and the grounding terminal of the high resistance meter are connected and grounded.

[0024] In the above application example of high-voltage leads, a DC high-voltage generator is used to provide test voltage, a temperature-controlled shielding box is used to control the test temperature range and shield noise interference, and a guard ring is used to prevent surface leakage current from interfering with the test signal. The guard ring can be made of stainless steel. The high-voltage lead is used to connect the DC high-voltage generator and the test electrode, wherein the protective resistor can reduce the impact of the fault voltage or current on the electrical components. The high resistance meter uses the DC amplification method to measure the volume resistivity, and the voltage across the standard resistor is measured by the high resistance meter through the current of the sample and then calculated. The temperature-controlled shielding box is provided with a temperature control function to obtain test results under different ambient temperatures. The temperature-controlled shielding box can use the existing oven as the basic structure, and then realize the shielding function by eliminating the pores in the metal shell of the oven, and design the insulation distance inside the oven as needed.

[0025] Preferably, for the insulation sample to be tested with a maximum insulation thickness of 2 mm, the maximum output voltage of the DC high voltage generator is 60 kV, the maximum output current is 2 mA, the ripple factor is ≤ 0.5%, and the protection resistance is greater than 3×10 7 Ω, and is much smaller than the insulation resistance, and the maximum continuous working voltage is greater than 60kV. The insulation resistance of the insulation sample to be tested is about 10 10 Ω, the protection resistor can be preferably 1×10 8 Ω, accuracy: ±0.1%~±10%, non-inductive design, able to withstand higher working voltage.

[0026] Furthermore, a voltage-equalizing ring is provided at one end of the high-voltage lead wire connected to the high-voltage electrode. Since there are sharp points on the end faces of the test electrode and the high-voltage lead wire conductor, corona is easily generated under high voltage, affecting the safety and reliability of the system and interfering with the test signal. The present invention solves the discharge problem at the conductor end by providing a voltage-equalizing ring at the conductor end, ensuring that the system still does not generate corona discharge under extreme conditions.

[0027] Preferably, the height from the pressure equalizing ring to the temperature-controlled shielding box housing is:

[0028] Definition E c is the initial corona field strength under standard atmospheric conditions, r2 is the radius of the equalizing ring, δ r is the relative density of the gas, m1 is the roughness coefficient of the test electrode surface, m2 is the environmental correction coefficient, and h2 is the height from the grading ring to the shielding box shell; then the corona starting voltage is:

[0029]

[0030] in,

[0031] Under the set pressure ring diameter, when U c If the voltage output range is exceeded and greater than 0-60kV, the corresponding h2 range is the height range from the grading ring to the shielding box shell that satisfies the requirement of no corona discharge in the system. The preferred diameter of the grading ring of the present invention is 1cm, and the distance from the grading ring to the shielding box shell is greater than 10cm, which can meet the requirement of no corona discharge in the system.

[0032] In order to ensure the shielding effect and solve the electromagnetic compatibility problem of the test system, the shielding effectiveness of the temperature-controlled shielding box of the present invention is greater than 90dB, and the thickness of the metal shell of the temperature-controlled shielding box is 0.008mm.

[0033] Beneficial Effects

[0034] Compared with the prior art, the present invention has the following advantages and progress:

[0035] The high-voltage lead structure of the present invention can adapt to higher output voltage and output current, as well as a wider output voltage range, and can be applicable to various high-voltage output lines or high-voltage test circuits. It has strong voltage resistance and good insulation effect, and can ensure the safety performance of power transmission and the normal operation of related tests, thereby obtaining reliable measurement results.

[0036] At the same time, the present invention uses a volume resistivity test system of a high-voltage lead structure, measures the volume resistivity of an insulating sample based on a high resistance meter using a current amplification method, designs a protective resistor, and places a test electrode and a sample in a temperature-controlled shielding box, so that the test of the volume resistivity of the insulating material can adapt to a high-temperature and high-pressure environment, such as when the ambient temperature changes from room temperature to room temperature to 100° C., and the test voltage changes from 0 to 1 kV to 0 to 60 kV, and can withstand a DC voltage of up to 60 kV for a long time; at the same time, the risk of damage to test components caused by sample breakdown is reduced; and a voltage equalizing ring is set at the end of the high-voltage lead, and the air insulation distance in the shielding box is reasonably set, so as to eliminate corona discharge of the test electrode under high voltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 The figure shows a schematic diagram of the high voltage resistant lead structure of the present invention;

[0038] Figure 2 The figure shows a schematic diagram of the electric field distribution of the stress cone in the high voltage lead wire of the present invention;

[0039] Figure 3 The figure shows a schematic diagram of the structure of a volume resistivity test system for a high voltage lead wire according to the present invention;

[0040] Figure 4 Shown Figure 3 Equivalent circuit diagram of the test system. DETAILED DESCRIPTION

[0041] The invention is further described below with reference to the accompanying drawings and specific embodiments.

[0042] Example 1

[0043] Combination Figure 1 and Figure 2 As shown, the high-voltage lead structure of this embodiment includes a lead cable body, a protective resistor 11, a wrapped insulating layer 12 and a stress cone 13; the lead cable body includes a cable conductor 101 and a cable insulating shielding layer 102;

[0044] The lead cable body is provided with a dividing point, and the protection resistor is connected in series with the cable conductors on both sides at the dividing point;

[0045] A wrapping insulation layer is arranged around the protective resistor and the lead cable body within a set length range on both sides thereof, and stress cones are arranged at both ends of the wrapping insulation layer.

[0046] In this embodiment, the resistance of the protection resistor is R>U max / I max , where U max is the maximum output voltage flowing through the high voltage lead and the protection resistor, I max Maximum output current for popular high voltage leads and protection resistors.

[0047] In this embodiment, the thickness of the wrapped insulation layer satisfies the requirement that the radial electric field strength on the surface of the wrapped insulation layer is half of the maximum electric field strength of the insulation shielding layer of the lead cable body, that is:

[0048]

[0049] in, Intermediate

[0050] In the above formula, E is the electric field strength at any point in the insulation shielding layer of the lead cable body; U is the cable insulation withstand voltage; a is the inner radius of the insulation shielding layer; b is the outer radius of the insulation shielding layer; γ is the electric field coefficient; r is the distance from the point where the electric field strength is currently calculated to the axis of the cable conductor;

[0051] Then, the thickness n of the wrapped insulation layer is:

[0052] n=cb.

[0053] In this embodiment, the stress cone surface is a curved surface that is concave inwardly toward the location of the protection resistor.

[0054] With the end point with the smaller diameter of the stress cone as the origin of the horizontal coordinate and the height of the cable conductor axis as the origin of the vertical coordinate, the cone curve equation of the stress cone is:

[0055]

[0056] The electric field intensity distribution on the cone surface is:

[0057] In the above formula, x is the horizontal coordinate of any point on the cone surface, r t is the ordinate of the point; E is the electric field strength at any point in the insulation shielding layer of the lead cable body; α is the angle between the tangent and the abscissa of a point on the cone surface, dx and dy are the differential forms of the abscissa and ordinate of the point on the cone surface as the position of the cone surface changes; U is the cable insulation withstand voltage; a is the inner radius of the insulation shielding layer; b is the outer radius of the insulation shielding layer, and γ is the electric field coefficient.

[0058] Example 2

[0059] Combination Figure 3 and Figure 4 , the volume resistivity testing system of the high-voltage lead wire described in Example 1 is applied, comprising a DC high-voltage generator, a high-resistance meter, a temperature-controlled shielding box, and a testing electrode in the temperature-controlled shielding box;

[0060] The test electrode comprises a high-voltage electrode and a test electrode placed in parallel with each other, a sample accommodating interval is provided between the two electrodes, and a protective ring is provided outside the high-voltage electrode; the positive pole of the DC output end of the DC high-voltage generator is connected to the high-voltage electrode through the high-voltage lead wire; the measuring electrode is connected to the test end of the high-resistance meter;

[0061] The temperature-controlled shielding box includes a metal shell and a temperature control module capable of changing the temperature inside the box;

[0062] The negative pole of the DC output terminal of the DC high voltage generator, the protection ring, the metal shell of the temperature control shielding box and the grounding terminal of the high resistance meter are connected and grounded.

[0063] When used, the DC high voltage generator is used to provide the test voltage, the temperature-controlled shielding box is used to control the test temperature range and shield noise interference, and the protection ring is used to prevent the surface leakage current from interfering with the test signal. The protection ring can be made of stainless steel. The high voltage lead is used to connect the DC high voltage generator and the test electrode, where the protection resistor can reduce the impact of the fault voltage or current on the electrical components. The high resistance meter uses the DC amplification method to measure the volume resistivity. The current through the sample is measured by the high resistance meter to measure the voltage across the standard resistor and then calculate it. The temperature-controlled shielding box is set with a temperature control function to obtain test results under different ambient temperatures. The temperature-controlled shielding box can use the existing oven as the basic structure, and then realize the shielding function by eliminating the pores in the metal shell of the oven, and design the insulation distance inside the oven as needed.

[0064] The volume resistivity formula is:

[0065] Among them, ρ v - volume resistivity, unit Ω·m; U - test voltage, unit kV; I - test current, unit μA; h - thickness of insulating material sample, unit m; A - measuring electrode ( Figure 2 The effective area of ​​the middle electrode 3), in m 2 , d1-diameter of measuring electrode, in m; g-width of the gap between measuring electrode and guard electrode, in m.

[0066] The following is a description of each component in the system.

[0067] 1. DC high voltage generator

[0068] The main function of the DC generator in the entire system is to provide a DC high voltage to the insulation sample to be tested. The DC high voltage generator must consider three important indicators: maximum output voltage, maximum output current and ripple factor.

[0069] 1.1. Maximum output voltage

[0070] For the insulation sample to be tested with a maximum insulation thickness of 2mm, the test electric field is required to be no less than 30kV / mm, and it can be determined that the maximum output voltage of the DC high voltage generator is 60kV.

[0071] 1.2. Maximum output current

[0072] The maximum output current mainly depends on the charge and discharge current of the sample and capacitor during the voltage increase and decrease process. The sample capacitance calculation formula is the general capacitance calculation formula: According to the formula, the maximum capacitance of the insulation sample to be tested is about 0.3nF. Considering the maximum voltage increase or decrease rate of 120kV / s, according to the sample charging current formula: It can be concluded that the maximum charging current is 0.36 mA. In order to ensure sufficient margin of the DC high voltage generator and in combination with the actual situation of commercial DC generator products, the maximum output current of the DC high voltage generator is selected as 2 mA in this embodiment.

[0073] 1.3 Ripple coefficient

[0074] The ripple current caused by the ripple voltage may interfere with the signal current. When the ripple factor is 0.5%, the ripple current is much smaller than the test current, so the interference with the signal current can be almost ignored.

[0075] To sum up, the main technical requirements of DC high voltage generator are: output DC 60kV; maximum current 2mA; ripple factor ≤0.5%.

[0076] 2. High voltage lead and protection resistor

[0077] 2.1. Protective resistor

[0078] refer to Figure 3 In order to ensure that the short-circuit current generated when the sample breaks down is not greater than the maximum output current, according to the maximum output voltage of the DC high-voltage generator of 60kV and the maximum output current of 2mA, it can be deduced that the protection resistance is greater than 3×10 7 Ω. Depending on the sample size, the insulation resistance is about 10 10 Ω. To ensure that the voltage on the sample is almost equal to the output voltage of the DC high voltage generator, the protection resistance R<<R 样品 , that is, the protection resistance is much smaller than the insulation resistance.

[0079] Therefore, the resistance of the protection resistor in this embodiment is selected to be 1×10 8 Ω, to ensure that it can withstand higher working voltage, the maximum continuous working voltage of the protection resistor is greater than 60kV, the resistance accuracy is: ±0.1% ~ ±10%, and a non-inductive design is adopted.

[0080] 2.2 Insulation structure of protective resistor and high voltage lead

[0081] The present invention cuts off the high-voltage lead and connects a protective resistor in the middle, which can reduce the risk of damage to the test components caused by sample breakdown. In order to ensure that the high-voltage lead can withstand 60kV DC high voltage for a long time, it is also necessary to design a wrapped insulation layer and a stress cone around the protective resistor and the high-voltage lead.

[0082] 2.2.1 Thickness of the wrapped insulation layer

[0083] According to the design principle of electrical insulation structure, the electric field distribution in DC cable insulation can be expressed as follows:

[0084]

[0085] Among them, the intermediate

[0086] Where, E is the electric field strength on the surface of the conductor connection sleeve; U is the withstand voltage of the cable insulation; a is the inner radius of the cable body insulation; b is the outer radius of the cable body insulation; γ is the electric field coefficient; r is the distance from the point where the electric field strength is currently calculated to the axis of the cable body;

[0087] When the outer diameter of the wrapped insulation layer is c, the radial electric field strength on the surface of the wrapped insulation layer is taken as half of the maximum electric field strength of the high-voltage lead cable body, that is:

[0088]

[0089] The thickness of the insulation wrapped is:

[0090] n=cb

[0091] 2.2.2 Stress cone surface

[0092] refer to Figure 3 As shown, the axial electric field intensity E at any point on the stress cone surface is t for:

[0093]

[0094] in

[0095] Figure 3 In the equation, since r = b + dy, we have dr = dy, and we have:

[0096]

[0097] Integrate both sides of the above formula:

[0098]

[0099] The equation of the stress cone curve is:

[0100]

[0101] When the variable r = c, the length of the stress cone is:

[0102]

[0103] From this the shape of the stress cone can be calculated.

[0104] 3. Temperature Control Shielding Box

[0105] 3.1. Design of insulation distance inside the box

[0106] In this embodiment, the plate connected to the test end of the high resistance meter is a cylindrical plate, and the protection ring is a cylindrical ring. The protection ring is used to prevent the surface leakage current from interfering with the test signal. The protection ring can be made of stainless steel.

[0107] Since there are sharp points on the end faces of the test electrode and the high-voltage lead conductor, corona is easily generated under high voltage, affecting the safety and reliability of the system and interfering with the test signal. Therefore, in this embodiment, a voltage equalizing ring is provided at one end of the high-voltage lead connected to the test electrode, which can solve the discharge problem at the end of the conductor and ensure that the system does not generate corona discharge under extreme conditions.

[0108] The corona initial field strength E under standard atmospheric conditions c The empirical expression is:

[0109]

[0110] In the formula, E c is the initial corona field strength, in kV / cm; r2 is the radius of the grading ring, in cm; δ r is the relative density of the gas; m1 is the roughness coefficient of the electrode surface, which is taken as 0.8; m2 is the environmental correction coefficient, which can be estimated as 0.8 in good weather. Assuming that the radius of the pressure equalizing ring is 1 cm, E can be obtained: c , define h2 as the height from the grading ring to the shielding box shell, in cm, then the corona inception voltage can be obtained according to the position of the grading ring:

[0111]

[0112] In this embodiment, r2 = 1 cm, m1 = 0.8, m2 = 0.8, and E c =25kV / cm, and then according to the relationship between the distance from the grading ring to the shielding box and the corona starting voltage, when h2>10cm, U c Exceeding the voltage output range of 0-60kV, that is, greater than 60kV, can meet the system's requirement of no corona discharge.

[0113] 3.2 Shielding effectiveness of temperature-controlled shielding box

[0114] There are many strong interference sources (such as high voltage or high current devices) in the resistivity test process, and there are also some sensitive equipment, so the electromagnetic compatibility problem of the system is more prominent. Electromagnetic shielding is one of the important measures to solve the electromagnetic compatibility problem. The principle of electromagnetic shielding is that the metal shielding body shields the radiation interference by reflecting and absorbing the electromagnetic waves.

[0115] The supplementary requirements for accreditation in the field of electromagnetic compatibility testing in the "Laboratory Accreditation Criteria" stipulate that the shielding effectiveness of the shielding box should reach S>60dB when f=0.014MHz~1MHz; S>90dB when f=1MHz~1000MHz. In this embodiment, the shielding effectiveness is required to be greater than 90dB.

[0116] According to the electromagnetic wave propagation theory:

[0117] When the electromagnetic wave reaches the surface of the shield, due to the discontinuity of impedance at the interface between air and metal, the reflection of the incident wave is:

[0118] R'=168-10×lg(fμ r σ r )

[0119] Where, R' is the reflection loss, unit is dB; μ r is the relative magnetic permeability of the metal; σ r is the relative conductivity of the metal (taking copper as an example σ r is 1); f is the frequency of the electromagnetic wave. This type of reflection does not require the shielding material to have a certain thickness, but only requires discontinuity on the interface.

[0120] ② The energy that is not reflected by the surface and enters the shielding body is attenuated by the shielding material during the process of propagating forward in the body. This is also called absorption; that is:

[0121]

[0122] Where A' is the absorption loss, in dB; t is the thickness of the metal plate, in mm.

[0123] When the remaining energy that has not yet attenuated in the shielding body is transmitted to the other surface of the material, it will encounter the interface of metal-air impedance discontinuity, which will form a second reflection and return to the shielding body. This reflection may have multiple reflections at the interface of two metals. When the above absorption loss A' is greater than 10dB, it can generally be ignored in actual engineering. The theoretical value of the shielding effectiveness S of the shielding box is S=R'+A'. In this embodiment, when the thickness of the metal plate is 0.008mm, the shielding effectiveness requirement can be met.

[0124] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A high voltage resistant lead structure, characterized in that: It includes a lead cable body, a protective resistor, a wrapped insulation layer and a stress cone; the lead cable body includes a cable conductor and a cable insulation shielding layer; The lead cable body is provided with a breaking point, and the protection resistor is connected in series with the cable conductors on both sides at the breaking point; The protective resistor and the lead cable body within a set length range on both sides thereof are provided with a wrapped insulation layer on the periphery thereof, and stress cones are provided at both ends of the wrapped insulation layer, wherein the conical surface of the stress cone is a curved surface concave inwardly toward the position of the protective resistor; Wherein, the thickness of the wrapped insulation layer is satisfy , and the radial electric field strength on the surface of the wrapped insulation layer is half of the maximum electric field strength of the insulation shielding layer of the lead cable body, that is: ， in, , the intermediate amount ; In the above formula, It is the electric field strength at any point in the insulation shielding layer of the lead cable body; To withstand voltage for cable insulation; is the inner radius of the insulating shield; is the outer radius of the insulating shield, is the electric field coefficient; is the outer radius of the wrapped insulation layer; The distance from the point where the electric field strength is currently calculated to the axis of the cable conductor; With the end point with the smaller diameter of the stress cone as the origin of the horizontal coordinate and the height of the cable conductor axis as the origin of the vertical coordinate, the cone curve equation of the stress cone is: ， The electric field intensity distribution on the cone surface is: , In the above formula, is the horizontal coordinate of any point on the cone surface, is the ordinate of the point; It is the electric field strength at any point in the insulation shielding layer of the lead cable body; is the angle between the tangent direction and the horizontal coordinate of a point on the cone surface, and It is the differential form of the horizontal and vertical coordinates of the points on the cone surface as they change with the position on the cone surface.

2. The high voltage lead structure according to claim 1, characterized in that: The resistance of the protection resistor is ,in is the maximum output voltage flowing through the high voltage lead and the protection resistor, It is the maximum output current flowing through the high voltage lead and the protection resistor.

3. A volume resistivity testing system using the high voltage lead structure according to claim 1 or 2, characterized in that: It includes a DC high voltage generator, a high resistance meter, a temperature-controlled shielding box, and a test electrode in the temperature-controlled shielding box; The test electrode comprises a high-voltage electrode and a test electrode placed in parallel with each other, a sample accommodating interval is provided between the two electrodes, and a protective ring is provided outside the high-voltage electrode; the positive pole of the DC output end of the DC high-voltage generator is connected to the high-voltage electrode through the high-voltage lead wire; the measuring electrode is connected to the test end of the high-resistance meter; The temperature-controlled shielding box includes a metal shell and a temperature control module capable of changing the temperature inside the box; The negative pole of the DC output terminal of the DC high voltage generator, the protection ring, the metal shell of the temperature control shielding box and the grounding terminal of the megger are connected and grounded; A voltage-equalizing ring is provided at one end of the high-voltage lead wire connected to the high-voltage electrode. The height from the voltage-equalizing ring to the shell of the temperature-controlled shielding box is: Under the set pressure ring diameter, when the corona starting voltage If the voltage exceeds the output range of 0-60kV, the height from the corresponding voltage grading ring to the shielding box shell The range is the height range from the grading ring to the shielding box shell to ensure that the system has no corona discharge; among which: ， ， is the initial corona field strength under standard atmospheric conditions, is the radius of the pressure equalizing ring, is the relative density of gas, To test the electrode surface roughness coefficient, is the environmental correction factor.

4. The volume resistivity testing system of the high voltage lead structure according to claim 3 is characterized in that: For the insulation sample to be tested with a maximum insulation thickness of 2 mm, the maximum output voltage of the DC high-voltage generator is 60 kV, the maximum output current is 2 mA, the ripple factor is ≤0.5%, the protection resistance is greater than 3×107 Ω, which is much smaller than the insulation resistance, and the maximum continuous working voltage is greater than 60 kV.

5. The volume resistivity testing system of the high voltage lead structure according to claim 4 is characterized in that: The shielding effectiveness of the temperature-controlled shielding box is greater than 90dB, and the thickness of the metal shell of the temperature-controlled shielding box is 0.008mm.

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