Device and method for testing critical breakdown field strength of high-voltage direct-current air gap of conductor

Abstract

The invention discloses a conductor high-voltage direct-current air gap critical breakdown field strength testing device and method, and the device comprises an insulating housing, a power connection conductor, a tested conductor, a grounding housing, and an insulating installation plate. The grounding shell is arranged on the lower portion of the insulating shell, an inner cavity of the grounding shell is communicated with an inner cavity of the insulating shell, and SF6 gas is arranged in the grounding shell. The high-voltage direct-current air gap critical breakdown field strength testing device has the beneficial effects that concentric arrangement of the tested conductor and the grounding shell is ensured through the insulating mounting plate, reliable connection of the power connection conductor and the tested conductor is ensured through the spring contact finger, and the device capable of accurately testing the high-voltage direct-current air gap critical breakdown field strength of the conductor is provided; according to the method, the air gap safety tolerance negative polarity direct current voltage Ud1 during negative polarity direct current voltage is firstly calculated, then the conductor high-voltage direct current air gap critical breakdown field strength is deduced, the method capable of accurately calculating the conductor high-voltage direct current air gap critical breakdown field strength is provided, and reference is provided for designing the shape of an insulator.

Description

Technical Field

[0001] The invention belongs to the technical field of high-voltage test devices, and in particular relates to a device and method for testing the critical breakdown field strength of a conductor high-voltage direct current air gap. Background Art

[0002] A GIL (gas-insulated transmission line) is a high-voltage, high-current power transmission device that utilizes gas insulation and a coaxial arrangement of the casing and conductors. The conductors are made of aluminum alloy tubing, and the casing is enclosed in aluminum alloy coiled sheet metal, similar to the coaxial busbars used in SF6 gas-insulated metal-enclosed switchgear (GIS). The main advantages of GIL include immunity to environmental factors such as harsh climates and unusual terrain, efficient use of space resources, reduced electromagnetic interference, increased current carrying capacity, low failure rates, and easy maintenance.

[0003] Insulators are critical components within GILs, securing conductors within their outer casing. Their structural shape affects surface charge distribution, which in turn impacts insulation performance. However, actual production presents the following technical challenges: Existing insulator shape designs often rely on empirical data, lacking standardized design standards. For reliability, insulator shape design often includes significant margins, resulting in high costs.

[0004] Among them, the critical breakdown field strength of the conductor's high-voltage DC air gap is an important reference value for designing the insulator shape. Therefore, it is urgent to design a device and method that can accurately calculate the critical breakdown field strength of the conductor's high-voltage DC air gap to provide a reference for designing the insulator shape. Summary of the Invention

[0005] In response to the above technical problems, the present invention provides a method and device for testing the critical breakdown field strength of the high-voltage DC air gap of an insulator, providing strong support for the shape design of the insulator.

[0006] The technical solution of the present invention is: a conductor high-voltage DC air gap critical breakdown field strength test device, comprising an insulating shell and a power conductor, and also comprising a conductor to be tested, a grounding shell and an insulating mounting plate;

[0007] The grounding shell is arranged at the lower part of the insulating shell, the inner cavity of the grounding shell is communicated with the inner cavity of the insulating shell, and SF6 gas is arranged inside.

[0008] The power conductor is arranged in the insulating housing, the outer end of the power conductor extends out of the insulating housing and is connected to the high-voltage generating device, the inner end of the power conductor is electrically connected to the conductor under test, the conductor under test is fixed in the grounding housing through the insulating mounting plate, and the grounding housing is connected to the grounding device;

[0009] The width of the air gap between the bottom of the conductor to be measured and the grounding shell is greater than the width of the air gap between the side surface of the conductor to be measured and the grounding shell.

[0010] Preferably, one end of the power connection conductor is inserted into the conductor to be measured, and a plurality of spring contact fingers are provided on the power connection conductor, and the power connection conductor is electrically connected to the conductor to be measured through the plurality of spring contact fingers.

[0011] Preferably, the surface finish of the conductor to be measured has a value of Ra=6.3 μm, and the width of the air gap between the side surface of the conductor to be measured and the grounded housing is 17.5±0.5 mm.

[0012] A method for testing the critical breakdown field strength of a conductor high-voltage direct current air gap is also provided, comprising the following steps:

[0013] 1) Calculate the expected value E of the DC air gap discharge field strength d

[0014] 1.1) Calculate the 50% breakdown field strength E of the air gap under negative polarity lightning impulse voltage 50％ , unit kV / mm, the specific formula is as follows:

[0015] E 50％ =63p+2.4kV / mm

[0016] Where p is the absolute air pressure in MPa;

[0017] 1.2) Based on the technical manual, calculate the air gap AC breakdown field strength E a , unit kV / mm;

[0018] E a =E 50％ / 1.3

[0019] 1.3): Based on step 1.2), calculate the expected value E of the negative polarity DC air gap discharge field strength d , unit kV / mm;

[0020] E d ≈E a / M

[0021] Wherein, the value range of M is (1.2-1.4), and the value is derived from the technical manual;

[0022] 2) Calculate the expected value of the DC air gap breakdown field strength test voltage

[0023] 2.1) According to the formula for the electric field strength on the surface of the central conductor in a coaxial cylindrical electric field:

[0024]

[0025] Where: E is the allowable lightning impulse field strength on the conductor surface under a certain SF6 gas pressure, unit: kV / mm; U this the lightning impulse withstand voltage in kV; r1 is the radius of the central conductor, in mm; r2 is the radius of the inner cavity of the grounded shell, in mm; reverse deduction yields:

[0026]

[0027] Where, E1 is the design basis of field strength under a certain SF6 gas pressure, and the value is derived from the technical manual;

[0028] 2.3) Calculate the expected value U of the test gap's ability to withstand negative polarity DC voltage under a certain SF6 gas pressure d :

[0029]

[0030] 3) Install the conductor to be tested into the test device and take the expected value U of the negative polarity DC voltage withstand capability of the test gap in step 2.3) d The voltage is applied from 80% to 90% of the value until breakdown. The 50% discharge voltage U of the air gap under different SF6 pressures is obtained. d50％ ;

[0031] 4) By U d50％ Calculate the DC withstand voltage:

[0032] U ds =Ud 50％ X(1-3σ)=0.85Ud 50％

[0033] Where, σ = 0.05, is the standard deviation of lightning impulse voltage;

[0034] Taking into account various deviations and unfavorable factors in manufacturing and operation, the design margin Z is taken, and the value range of Z is: (0, 0.3), and the air gap safety tolerance negative polarity DC voltage U is obtained when the negative polarity DC voltage is applied. d1 :

[0035] U d1 =0.85×(1-Z)×U d50％

[0036] in U d1 The maximum value of the composite field strength of the high voltage electrode under voltage is: the design allowable value of the DC air gap field strength E d1 ;

[0037] Design allowable value E of DC air gap field strength d1 Calculation method: Use COMSOL software and input U d1 , based on the electric field analysis under DC working voltage and calculated according to the current conservation equation.

[0038] Preferably, in step 3), the expected value U of the negative polarity DC voltage tolerance capability of the test gap is d Start applying voltage at 80 to 90% of the rated voltage, gradually increase the voltage by 5 kV, and stay at each voltage point for 1 minute until breakdown.

[0039] Preferably, at least three conductors to be tested with the same specifications are prepared for testing.

[0040] Preferably, the voltage is applied starting from 80 to 90% of the expected value Ud of the negative polarity DC voltage tolerance of the test gap. If the value obtained is not divisible by 5, the previous value divisible by 5, 240 kV, is used and the voltage is applied step by step every 5 kV.

[0041] Preferably, the feature is that if discharge occurs during step 3), the voltage is recorded, the conductor under test is processed after opening, and then the voltage is increased starting from the previous voltage of the discharge point to eliminate accidental discharge values until the final discharge voltage is found.

[0042] The beneficial effects of the present invention are: the insulating mounting plate ensures that the conductor to be measured and the grounding shell are arranged concentrically, and the spring contact finger ensures a reliable connection between the power conductor and the conductor to be measured, thereby providing a device that can accurately measure the critical breakdown field strength of the high-voltage direct current air gap of the conductor;

[0043] First calculate the air gap safety tolerance negative polarity DC voltage U when negative polarity DC voltage is applied d1 , and then the critical breakdown field strength of the conductor high-voltage DC air gap is derived, providing a method for accurately calculating the critical breakdown field strength of the conductor high-voltage DC air gap, providing a reference for designing the insulator shape. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 It is a structural schematic diagram of the present invention,

[0045] Figure 2 yes Figure 1 A partial enlarged schematic diagram of part A in the middle.

[0046] In the figure, 1 is an insulating housing, 2 is a conductor connected to the power supply, 21 is a spring contact finger, 3 is an insulating mounting plate, 4 is a conductor to be measured, and 5 is a grounding housing;

[0047] a is the radius of the conductor being measured, and b is the radius of the inner cavity of the grounded shell. DETAILED DESCRIPTION

[0048] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0049] In the description of the present invention, it should be noted that the terms "center", "up", "down", "left", "right", "vertical", "horizontal", "vertical", "horizontal", "inside", "outside", "front", "back" and the like to indicate orientations or positional relationships are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation to the present invention.

[0050] See also Figure 1 and Figure 2 , a conductor high voltage direct current air gap critical breakdown field strength test device, comprising an insulating shell 1 and a power conductor 2, further comprising a conductor to be tested 4, a grounding shell 5 and an insulating mounting plate 3;

[0051] The grounding shell 5 is arranged at the lower part of the insulating shell 4. The inner cavity of the grounding shell 5 is connected with the inner cavity of the insulating shell 1, and SF6 gas is provided inside.

[0052] The power conductor 2 is disposed within the insulating housing 1. The outer end of the power conductor 2 extends out of the insulating housing 1 and is connected to a high-voltage generator. The high-voltage generator is conventional technical equipment, and how to connect it is conventional technology. The inner end of the power conductor 2 is electrically connected to the conductor under test 4. The conductor under test 4 is fixed to the grounding housing 5 through the insulating mounting plate 3. The two have circular cross-sections and are concentrically arranged. The grounding housing 5 is connected to the grounding device.

[0053] The width of the air gap between the bottom of the measured conductor 4 and the grounded shell 5 is greater than the width of the air gap between the side of the measured conductor 4 and the grounded shell 5. In this embodiment, the surface finish of the measured conductor 4 has a value of Ra=6.3 μm, and the lateral width of the air gap between the measured conductor 4 and the grounded shell 5 is 17.5±0.5 mm.

[0054] See also Figure 2 One end of the connecting conductor 2 is inserted into the conductor 4 to be measured. The connecting conductor 2 is provided with a plurality of spring contact fingers 21. The connecting conductor 2 is electrically connected to the conductor 4 to be measured through the plurality of spring contact fingers 21 to ensure the reliability of the connection.

[0055] A method for testing the critical breakdown field strength of a conductor high-voltage direct current air gap is also provided, comprising the following steps:

[0056] 1) Calculate the expected value E of the DC air gap discharge field strength d， Used to determine the insulation grade of the insulation components in the test device and as a reference later;

[0057] 1.1) Calculate the 50% breakdown field strength E of the air gap under negative polarity lightning impulse voltage 50％ , unit kV / mm, the specific formula is as follows:

[0058] E 50％ =63p+2.4kV / mm

[0059] Where p is the absolute air pressure in MPa;

[0060] 1.2) Based on the technical manual and the research results of Shibaura Corporation of Japan in this embodiment, the air gap AC breakdown field strength E is calculated in a coaxial cylindrical electrode. a , unit kV / mm;

[0061] E a =E 50％ / 1.3

[0062] E a Less than E 50％ This is because the AC voltage increases slowly and lasts a long time (large electrical stress).

[0063] 1.3): Based on step 1.2), calculate the expected value E of the negative polarity DC air gap discharge field strength d , unit kV / mm;

[0064] E d ≈E a / M

[0065] Wherein, the value range of M is (1.2-1.4), and the value is derived from the technical manual;

[0066] The insulating shell 1 has a lightning impulse resistance of Uth≥550kV and a power frequency voltage of Ug≥230kV / 1min, and the casing shed is made of high temperature vulcanized silicone rubber HTV.

[0067] 2) Calculate the expected value of the DC air gap breakdown field strength test voltage to facilitate the efficient start of the breakdown test,

[0068] 2.1) According to the formula for the electric field strength on the surface of the central conductor in a coaxial cylindrical electric field:

[0069]

[0070] Where: E is the allowable lightning impulse field strength on the conductor surface under a certain SF6 gas pressure, unit: kV / mm; U th is the lightning impulse withstand voltage in kV; r1 is the radius of the central conductor, in mm; r2 is the radius of the inner cavity of the grounded shell, in mm; reverse deduction yields:

[0071]

[0072] Where, E1 is the design basis of field strength under a certain SF6 gas pressure, and the value is derived from the technical manual;

[0073] 2.3) Calculate the expected value U of the test gap's ability to withstand negative polarity DC voltage under a certain SF6 gas pressure, based on the technical manual. d， Here we take integers:

[0074]

[0075] 3) Install the conductor to be tested into the test device and take the expected value U of the negative polarity DC voltage withstand capability of the test gap in step 2.3) d The voltage is applied from 80% to 90% of the value until breakdown. The 50% discharge voltage U of the air gap under different SF6 pressures is obtained. d50％ ;

[0076] 4) 50% discharge voltage U from air gap d50％ The electric field of the test piece under the condition is calculated to obtain the corresponding field strength, and then the corresponding design allowable field strength is formulated according to the field strength;

[0077] byU d50％ Calculate the DC withstand voltage:

[0078] U ds =Ud 50％ X(1-3σ)=0.85Ud 50％

[0079] Where, σ = 0.05, is the standard deviation of lightning impulse voltage;

[0080] Taking into account various deviations and unfavorable factors in manufacturing and operation, the design margin Z is taken, and the value range of Z is: (0, 0.3), and the air gap safety tolerance negative polarity DC voltage U is obtained when the negative polarity DC voltage is applied. d1 :

[0081] U d1 =0.85×(1-Z)×U d50％

[0082] in U d1 The maximum value of the composite field strength of the high voltage electrode under voltage is: the design allowable value of the DC air gap field strength E d1 ;

[0083] Design allowable value E of DC air gap field strength d1 Calculation method: Use COMSOL software and input U d1 , based on the electric field analysis under DC working voltage and the current conservation equation. The calculation process is conventional technology. When the calculation method is obtained, technicians in this field can calculate based on the actual values obtained, and the details will not be repeated here.

[0084] In step 3), the expected value of the negative polarity DC voltage tolerance capability U from the test gap is dStart applying voltage at 90% of the discharge voltage, increasing the voltage by 5 kV, and dwell at each voltage point for 1 minute until breakdown occurs. If discharge occurs during this process, record the voltage, open the electrode, treat the electrode, and then continue applying voltage from the voltage level above the discharge point. Eliminate any incidental discharges until the final discharge voltage is determined.

[0085] Prepare at least three conductors of the same specifications for testing to eliminate errors in processing and assembly.

[0086] Example 1

[0087] Calculate the critical breakdown field strength of the high-voltage DC air gap of the conductor at a pressure of 0.3 MPa. The radius of the conductor under test is 50 mm, and the radius of the inner cavity of the grounded shell is 67.5 mm.

[0088] Considering the simplicity of calculation, the decimal places are rounded to two digits.

[0089] 1) Calculate the expected value E of the DC air gap discharge field strength d

[0090] 1.1) Calculate the 50% breakdown field strength E of the air gap under negative polarity lightning impulse voltage 50％ , unit kV / mm,

[0091] E 50％ =63×0.3+2.4kV / mm=21.3kV / mm

[0092] 1.2) Calculate the air gap AC breakdown field strength E a , unit kV / mm;

[0093] E a =E 50％ / 1.3=16.38kV / mm

[0094] 1.3): Based on step 1.2), calculate the expected value E of the negative polarity DC air gap discharge field strength d , unit kV / mm;

[0095] E d ≈E a / 1.2=13.65kV / mm

[0096] The M value of 1.2 is derived from the technical manual;

[0097] 2) Calculate the expected value of the DC air gap breakdown field strength test voltage 2.1)

[0099]

[0100] Where, E1 is the design basis of SF6 field strength at 0.3MPa pressure, and the value is derived from the technical manual;

[0101] 2.3) Calculate the expected value U of the test gap's ability to withstand negative polarity DC voltage under a certain SF6 gas pressure d :

[0102]

[0103] 3) Install the conductor to be tested into the test device and take the expected value U of the negative polarity DC voltage withstand capability of the test gap in step 2.3) d 80% of the value is 153.6kV. Take the previous digit divisible by 5, 150kV, and increase the voltage step by step by 5kV, pausing at each voltage point for 1 minute until breakdown occurs. If discharge occurs during this process, record the voltage, open the electrode, treat it, and then increase the voltage again from the voltage level above the discharge point. Eliminate accidental discharge values until the final discharge voltage is found. The test results show that the 50% discharge voltage U of the air gap under 0.3MPa SF6 pressure is d50％ ;

[0104] 4) By U d50％ Calculate the DC withstand voltage:

[0105] U ds =Ud 50％ X(1-1.5)=0.85Ud 50％

[0106] Where, σ = 0.05, is the standard deviation of lightning impulse voltage;

[0107] Taking into account various deviations and unfavorable factors in manufacturing and operation, the design margin is 0.15, and the air gap safety tolerance negative polarity DC voltage U is obtained when the negative polarity DC voltage is applied. d1 :

[0108] U d1 =0.85×0.85×U d50％

[0109] in U d1 The maximum value of the composite field strength of the high voltage electrode under voltage is: the design allowable value of the DC air gap field strength E d1 ;

[0110] Design allowable value E of DC air gap field strength d1 Calculation method: Use COMSOL software and input U d1 , based on the electric field analysis under DC working voltage and calculated according to the current conservation equation.

[0111] Five conductors of the same specification are used for testing, and the final result is the arithmetic mean.

[0112] Example 2

[0113] Calculate the critical breakdown field strength of the high-voltage DC air gap of the conductor at a pressure of 0.6 MPa. The radius of the conductor under test is 50 mm, and the radius of the inner cavity of the grounded shell is 67.5 mm.

[0114] Considering the simplicity of calculation, the decimal places are rounded to two digits.

[0115] 1) Calculate the expected value E of the DC air gap discharge field strength d

[0116] 1.1) Calculate the 50% breakdown field strength E of the air gap under negative polarity lightning impulse voltage 50％ , unit kV / mm,

[0117] E 50％ =63×0.6+2.4kV / mm=40.2kV / mm

[0118] 1.2) Calculate the air gap AC breakdown field strength E a , unit kV / mm;

[0119] E a =E 50％ / 1.3=30.92kV / mm

[0120] 1.3): Based on step 1.2), calculate the expected value E of the negative polarity DC air gap discharge field strength d , unit kV / mm;

[0121] E d ≈E a / 1.2=25.77kV / mm

[0122] The M value of 1.2 is derived from the technical manual;

[0123] 2) Calculate the expected value of the DC air gap breakdown field strength test voltage 2.1)

[0125]

[0126] Where, E1 is the design basis of SF6 field strength at 0.6MPa pressure, and the value is derived from the technical manual;

[0127] 2.3) Calculate the expected value U of the test gap's ability to withstand negative polarity DC voltage under a certain SF6 gas pressure d， Take the integer:

[0128]

[0129] 3) Install the conductor to be tested into the test device and take the expected value U of the negative polarity DC voltage withstand capability of the test gap in step 2.3) d90% of the value is 285.3kV. Take the previous digit divided by 5, 285kV, and increase the voltage step by step by 5kV. Stay at each voltage point for 1 minute until breakdown occurs. If discharge occurs during the process, record the voltage, open it, treat the electrode, and then increase the voltage from the voltage level above the discharge point to eliminate accidental discharge values. Until the final discharge voltage is found. The test results show that the 50% discharge voltage U of the air gap under 0.6MPa SF6 pressure is d50％ ;

[0130] 4) By U d50％ Calculate the DC withstand voltage:

[0131] U ds =Ud 50％ X(1-1.5)=0.85Ud 50％

[0132] Where, σ = 0.05, is the standard deviation of lightning impulse voltage;

[0133] Taking into account various deviations and unfavorable factors in manufacturing and operation, the design margin is 0.15, and the air gap safety tolerance negative polarity DC voltage U is obtained when the negative polarity DC voltage is applied. d1 :

[0134] U d1 =0.85×0.85×U d50％

[0135] in U d1 The maximum value of the composite field strength of the high voltage electrode under voltage is: the design allowable value of the DC air gap field strength E d1 ;

[0136] Design allowable value E of DC air gap field strength d1 Calculation method: Use COMSOL software and input U d1 , based on the electric field analysis under DC working voltage and calculated according to the current conservation equation.

[0137] Five conductors of the same specification are used for testing, and the final result is the arithmetic mean.

[0138] The present invention is not limited to the above-mentioned embodiments. On the basis of the technical solutions disclosed in the present invention, those skilled in the art can make some substitutions and modifications to some of the technical features therein according to the disclosed technical content without creative labor, and these substitutions and modifications are all within the protection scope of the present invention.

Claims

1. A conductor high voltage DC air gap critical breakdown field strength test device, comprising an insulating housing and a power conductor, characterized in that: Also includes the conductor under test, grounded enclosure, and insulating mounting plate; The grounding shell is arranged at the lower part of the insulating shell, the inner cavity of the grounding shell is communicated with the inner cavity of the insulating shell, and SF6 gas is arranged inside. The power conductor is arranged in the insulating housing, the outer end of the power conductor extends out of the insulating housing and is connected to the high-voltage generating device, the inner end of the power conductor is electrically connected to the conductor under test, the conductor under test is fixed in the grounding housing through the insulating mounting plate, and the grounding housing is connected to the grounding device; The width of the air gap between the bottom of the conductor to be measured and the grounding shell is greater than the width of the air gap between the side surface of the conductor to be measured and the grounding shell.

2. A conductor high voltage direct current air gap critical breakdown field strength test device according to claim 1, characterized in that: One end of the power-connecting conductor is inserted into the conductor to be measured. A plurality of spring contact fingers are provided on the power-connecting conductor. The power-connecting conductor is electrically connected to the conductor to be measured through the plurality of spring contact fingers.

3. The conductor high voltage direct current air gap critical breakdown field strength test device according to claim 1, characterized in that: The surface finish of the conductor under test has a value of Ra=6.3 μm, and the width of the air gap between the side surface of the conductor under test and the grounding shell is 17.5±0.5 mm.

4. A method for testing the critical breakdown field strength of a conductor high voltage DC air gap, characterized in that: The following steps are involved: 1) Calculate the expected value E of the DC air gap discharge field strength d 1.1) Calculate the 50% breakdown field strength E of the air gap under negative polarity lightning impulse voltage 50％ , unit kV / mm, the specific formula is as follows: E 50％ =63p+2.4kV / mm Where p is the absolute air pressure in MPa; 1.2) Based on the technical manual, calculate the air gap AC breakdown field strength E a , unit kV / mm; AND a =And 50％ / 1.3 1.3): Based on step 1.2), calculate the expected value E of the negative polarity DC air gap discharge field strength d , unit kV / mm; HAVE BEEN d ≈E a / M Wherein, the value range of M is (1.2-1.4), and the value is derived from the technical manual; 2) Calculate the expected value of the DC air gap breakdown field strength test voltage 2.1) According to the formula for the electric field strength on the surface of the central conductor in a coaxial cylindrical electric field: Where: E is the allowable lightning impulse field strength on the conductor surface under a certain SF6 gas pressure, unit: kV / mm; U th is the lightning impulse withstand voltage in kV; r1 is the radius of the central conductor, in mm; r2 is the radius of the inner cavity of the grounded shell, in mm; reverse deduction yields: Where, E1 is the design basis of field strength under a certain SF6 gas pressure, and the value is derived from the technical manual; 2.3) Calculate the expected value U of the test gap's ability to withstand negative polarity DC voltage under a certain SF6 gas pressure d : 3) Install the conductor to be tested into the test device and take the expected value U of the negative polarity DC voltage withstand capability of the test gap in step 2.3) d The voltage is applied from 80% to 90% of the value until breakdown. The 50% discharge voltage U of the air gap under different SF6 pressures is obtained. d50％ ; 4) By U d50％ Calculate the DC withstand voltage: U ds =Ud 50％ X(1-3σ)=0.85Ud 50％ Where, σ = 0.05, is the standard deviation of lightning impulse voltage; Taking into account various deviations and unfavorable factors in manufacturing and operation, the design margin Z is taken, and the value range of Z is: (0, 0.3), and the air gap safety tolerance negative polarity DC voltage U is obtained when the negative polarity DC voltage is applied. d1 : AT d1 =0.85×(1-Z)×U d50％ in U d1 The maximum value of the composite field strength of the high voltage electrode under voltage is: the design allowable value of the DC air gap field strength E d1 ; Design allowable value E of DC air gap field strength d1 Calculation method: Use COMSOL software and input U d1 , based on the electric field analysis under DC working voltage and calculated according to the current conservation equation.

5. The method for testing the critical breakdown field strength of a conductor high voltage direct current air gap according to claim 3, wherein: In step 3), the expected value of the negative polarity DC voltage tolerance capability U from the test gap is d Start applying voltage at 80 to 90% of the rated voltage, gradually increase the voltage by 5 kV, and stay at each voltage point for 1 minute until breakdown.

6. The method for testing the critical breakdown field strength of a conductor high voltage direct current air gap according to claim 3, wherein: Prepare at least three conductors of the same specifications for testing.

7. The method for testing the critical breakdown field strength of a conductor high voltage direct current air gap according to claim 5, wherein: Start applying voltage from 80 to 90% of the expected value Ud of the test gap's ability to withstand negative polarity DC voltage. If the value obtained is not divisible by 5, take the previous value divisible by 5, 240kV, and increase the voltage step by step every 5kV.

8. A method for testing the critical breakdown field strength of a conductor high voltage direct current air gap according to any one of claims 3 to 7, characterized in that: Step 3) If discharge occurs midway, record the voltage, open it, treat the conductor under test, and then increase the voltage from the previous voltage level at the discharge point to eliminate accidental discharge values until the final discharge voltage is found.

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