Method for calculating surface breakdown field strength of high-voltage direct-current insulator

Abstract

The invention provides a method for calculating the surface breakdown field intensity of a high-voltage direct-current insulator. The method comprises the following steps: 1) calculating a direct-current air gap discharge field intensity expected value Ed; 2) designing the specification of the tested insulation sample according to the Ed obtained in the step 1); 3) loading the tested insulation sample into a test device for breakdown test to obtain a lightning impulse voltage and direct current voltage superposed breakdown voltage U0; and 4) calculating the superposition breakdown field strength E of the lightning impulse voltage and the direct-current voltage of the insulation sample according to the U0. The complete method for calculating the superposition breakdown field intensity of the lightning impulse voltage and the direct-current voltage is provided, the specification of the tested insulation sample is reasonably determined based on the expected value of the negative polarity direct-current air gap discharge field intensity, then the field intensity is calculated, and the field intensity design reference of the direct-current insulator is provided for the design of the high-voltage direct-current insulator.

Description

Technical Field

[0001] This invention belongs to the field of high-voltage DC insulator technology, specifically relating to a method for calculating the surface breakdown field strength of a high-voltage DC insulator. Background Technology

[0002] GIL (Gas Insulated Line) is a high-voltage, high-current power transmission equipment that uses gas insulation and has its casing and conductor arranged coaxially. Its conductors are made of aluminum alloy tubing, and its casing is enclosed by aluminum alloy coils, similar to the coaxial busbar in SF6 gas-insulated metal-enclosed switchgear (GIS). The main advantages of GIL include immunity to harsh weather and special terrain conditions, efficient use of space resources, reduced electromagnetic interference, increased current carrying capacity, low failure rate, and ease of maintenance. Insulators are crucial components within GILs (Gas Insulators), used to secure conductors within the casing. Their structural shape influences surface charge distribution, thereby affecting their insulation performance. The breakdown field strength resulting from the superposition of lightning impulse voltage and DC voltage is a critical parameter for designing the shape of insulators used in high-voltage DC GILs. However, current technology only provides a benchmark for the surface breakdown field strength of insulators used in high-voltage AC GILs, lacking a mature and standardized benchmark for the surface breakdown field strength of high-voltage DC insulators. Design often relies on empirical values, failing to develop a unified and standardized design standard.

[0003] Therefore, there is an urgent need to design a method for calculating the surface breakdown field strength of high-voltage DC insulators, so as to provide a design benchmark for the field strength of DC insulators. Summary of the Invention

[0004] To address the above technical problems, this invention provides a method for calculating the surface breakdown field strength of high-voltage DC insulators, providing strong support for the shape design of high-voltage DC GIL insulators.

[0005] The technical solution of this invention is: a method for calculating the surface breakdown field strength of a high-voltage DC insulator, comprising the following steps: 1) Calculate the expected value E of the DC air gap discharge field strength. d ; 2) Based on E obtained in step 1), d Design the specifications of the insulation sample to be tested; 3) Load the insulation sample to be tested into the test device for breakdown test, and obtain the breakdown voltage U0 by superimposing the lightning impulse voltage and the DC voltage; 4) Calculate the breakdown field strength E of the lightning impulse voltage and DC voltage superimposed on the insulation sample based on U0.

[0006] Preferably, step 1) specifically includes: 1.1) Calculate the 50% breakdown field strength E of the air gap under a negative polarity lightning impulse voltage.50% The unit is kV / mm, and the specific formula is as follows: E 50% =63p+2.4; In the formula, p is the absolute pressure, in MPa; 1.2) According to E 50% Calculate the AC breakdown field strength E in the air gap a Unit: kV / mm; E a = E 50% / 1.3; 1.3) Based on E a Calculate the expected value E of the negative polarity DC air gap discharge field strength. d Unit: kV / mm; E d =E a / M; The value of M in the formula ranges from 1.2 to 1.4.

[0007] Preferably, in step 2), the insulation sample to be tested is cylindrical, and the diameter L and height H of the insulation sample are determined. The diameter L is smaller than the distance from the side of the insulation sample to the inner wall of the testing device; The height H is based on the expected experimental voltage U. d And the expected flashover field strength E on the surface of the insulating component τ Specifically: First, calculate the expected flashover field strength E on the surface of the insulating component. τ =E d / 2; The height H must satisfy: H≥U d / E τ ; In the formula, Ud is the expected breakdown test voltage of the insulating component, Ud=Eτ×h, and h is the height of the insulating platform.

[0008] Preferably, step 3) specific steps: 3.1) Load the insulation sample into the test apparatus, and heat the upper part of the insulation platform to the target temperature, while keeping the lower part of the insulation platform at room temperature; 3.2) First, apply a negative DC voltage. After the initial negative DC voltage of Ud reaches the preset time, apply a positive voltage of 2.39Ud. Then, apply the positive voltage step by step according to the preset voltage value until breakdown occurs. Record the breakdown voltage. Replace the insulation sample and repeat the test at least 5 times. Take the arithmetic mean of all the obtained values ​​to obtain the breakdown voltage U0 of the lightning impulse voltage and the DC voltage superimposed.

[0009] Preferably, in step 4), the calculation process for the surface breakdown field strength E of the insulating sample is as follows: 4.1) First calculate the withstand voltage of the insulation component Utds based on the 50% flashover voltage measured during the superposition test. Utds = U0 × (1-3σ) × N, where σ = 0.05, which is the standard deviation of the lightning impulse voltage, and N is the design margin, with a value range of 0.7 to 1. 4.2) The allowable value of the tangential field strength on the surface of the insulating component under the superimposed voltage is obtained by calculating the electric field under the Utds voltage.

[0010] Preferably, the target temperature in step 3.1) is in the range of 100 to 120 degrees Celsius.

[0011] Preferably, the preset time in step 3.2) is not less than 40,000 seconds, and the preset voltage value ranges from 5 to 15 kV.

[0012] Preferably, step 4.2) is the calculation method for the allowable value E of the tangential electric field strength on the surface of the insulating component under superimposed voltage: Take the relative permittivity ε of the insulator I =4.95, The relative permittivity of the insulating gas is ε0 = 1.002. Based on boundary conditions: ; Solve the domain control equations of the entire electrostatic field using equation (1): ; Equation (2) Relationship between charge density and current density of insulating sample: ; In equation (2): ρ V Let K be the charge density of the insulating sample, t be time, and K be the time. VI The conductivity of the insulating sample, Charge density of insulating sample; Equation (3) Gauss's law in electrostatic fields: ; In equation (3) ρ + ρ is the positive and negative charge density. - ρ is the positive and negative charge density. V The charge density of the insulating sample; Equation (4) Connection conditions at the dielectric interface: ; In equation (4), D1 is the gas-side electric displacement vector, D2 is the insulating stage-side electric displacement vector, n is the unit normal vector of the boundary, and ρ s The free charge density at the interface; Formula (5) for calculating electric displacement vector: ; Solving equations (1), (2), (3), (4), and (5) simultaneously yields the breakdown field strength E resulting from the superposition of the lightning impulse voltage and the DC voltage.

[0013] The beneficial effects of this invention are that it proposes a complete method for calculating the breakdown field strength of the superimposed lightning impulse voltage and DC voltage. Based on the expected value of the negative polarity DC air gap discharge field strength, the specifications of the insulation sample under test are reasonably determined, and then the field strength is calculated, providing a field strength design benchmark for DC insulators for high voltage DC insulator design. Attached Figure Description

[0014] Figure 1 This is a flowchart of the present invention. Figure 2 This is a schematic diagram of the structure of the testing device of the present invention. Figure 3 yes Figure 2 A schematic diagram of the local method in section A. Detailed Implementation

[0015] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0016] In the description of this invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "vertical", "horizontal", "inner", "outer", "front", "back", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0017] Figure 1 The method described above is a calculation method for the surface breakdown field strength of a high-voltage DC insulator, which includes the following steps: 1) Calculate the expected value E of the DC air gap discharge field strength. d ; 2) Based on E obtained in step 1), d Design the specifications of the insulation sample to be tested; 3) Load the insulation sample to be tested into the test device for breakdown test, and obtain the breakdown voltage U0 by superimposing the lightning impulse voltage and the DC voltage; 4) Calculate the breakdown field strength E of the lightning impulse voltage and DC voltage superimposed on the insulation sample based on U0.

[0018] In this embodiment, step 1) specifically includes: 1.1) Calculate the 50% breakdown field strength E of the air gap under a negative polarity lightning impulse voltage. 50% The unit is kV / mm, and the specific formula is as follows: E 50% =63p+2.4 kV / mm; In the formula, p is the absolute pressure, in MPa, which is the SF6 pressure of the environment, typically 0.4 to 0.7 MPa. 1.2) According to E 50% Calculate the AC breakdown field strength E in the air gap a Unit: kV / mm; E a = E 50% / 1.3; 1.3) Based on E a Calculate the expected value E of the negative polarity DC air gap discharge field strength. d Unit: kV / mm; E d =E a / M; The value of M in the formula ranges from 1.2 to 1.4. The larger the value of M, the smaller the expected field strength, and the calculated value is conservative.

[0019] The value is rounded to one decimal place.

[0020] In this embodiment, the insulation sample to be tested in step 2) is cylindrical, and the diameter L and height H of the insulation sample are determined to ensure that the test discharge occurs on the surface of the insulation component and not in the SF6 air gap of the prototype. The diameter L is smaller than the distance from the side of the insulation sample under test to the inner wall of the testing device; see the testing device section. Figure 2 This is a standard piece of equipment, and its specific structure and working principle will not be described in detail.

[0021] Height H is based on the expected experimental voltage U d And the expected flashover field strength E on the surface of the insulating component τ Specifically: First, calculate the expected flashover field strength E on the surface of the insulating component. τ =E d / 2; The height H must satisfy: H≥U d / E τ To ensure it is securely placed on the insulating platform; In the formula U d Ud = E is the expected breakdown test voltage for the insulating component. τ ×h, where h is the height of the insulating platform, i.e., the distance between the two discharge electrodes. See [reference needed]. Figure 3 .

[0022] In this embodiment, step 3) is specifically as follows: 3.1) Load the insulation sample into the test apparatus, and heat the upper part of the insulation platform to the target temperature, while keeping the lower part of the insulation platform at room temperature; 3.2) First, apply a negative DC voltage, with an initial voltage value of U. d After the negative DC voltage reaches the preset time, a value of 2.39U is first applied. d Apply a positive voltage, and then gradually increase the positive voltage according to the preset voltage value until breakdown occurs. Record the breakdown voltage, replace the insulation sample, and repeat the test at least 5 times. Take the arithmetic mean of all the obtained values ​​to obtain the breakdown voltage U0, which is the sum of the lightning impulse voltage and the DC voltage.

[0023] In this embodiment, step 4) involves the calculation of the surface breakdown field strength E of the insulating sample: 4.1) First calculate the withstand voltage U of the insulation component based on the 50% flashover voltage measured during the superposition test. tds = U0×(1-3σ)×N,where σ=0.05 ,is the standard deviation of lightning impulse voltage, and N is the design margin, with a value range of 0.7 to 1; 4.2) In U tds The allowable value E of the design tangential field strength on the surface of the insulating component under superimposed voltage is obtained by calculating the electric field under voltage.

[0024] In this embodiment, the target temperature in step 3.1) is in the range of 100 to 120 degrees Celsius.

[0025] In this embodiment, the preset time mentioned in step 3.2) is not less than 40,000 seconds, and the preset voltage value ranges from 5 to 15 kV.

[0026] Derivation process: Step 4.2) Calculation method for the allowable value E of the tangential field strength on the surface of the insulating component under superimposed voltage: Take the relative permittivity ε of the insulator I =4.95, and the relative permittivity of the insulating gas ε0 = 1.002, both of which are constants that do not change with environmental factors such as temperature and electric field; Based on boundary conditions, the potential expressions for the high-voltage side (conductor), the grounding side (shell), and the normal component of the electric displacement vector on the artificial boundary are as follows: ; Solve the domain control equations of the entire electrostatic field using equation (1): ; Derivation of Equation (2): According to the current continuity equation, the relationship between charge density and current density of the insulating sample is as follows: ; It is derived from the divergence operation rules: ; According to the differential form of Gauss's flux theorem: ; That is, derive equation (2). ; Equation (3) Gauss's law in electrostatic fields: ; In equation (3) ρ + ρ is the positive and negative charge density. - ρ is the positive and negative charge density. V The charge density of the insulating sample; Equation (4) Connection conditions at the dielectric interface: ; In equation (4), D1 is the gas-side electric displacement vector, D2 is the insulating stage-side electric displacement vector, n is the unit normal vector of the boundary, and ρ s The free charge density at the interface; that is, the difference in normal electric displacement on both sides is equal to the surface charge density. Formula (5) for calculating electric displacement vector: ; is the relative permittivity of the insulator and the gas; The various physical fields are coupled with each other. Solving the equations (1), (2), (3), (4), and (5) simultaneously is a complex process. In practice, it is usually based on software calculation, which yields the breakdown field strength E of the superimposed lightning impulse voltage and DC voltage.

[0027] The electric field strength on the insulator or metal surface is calculated by measuring the actual discharge voltage.

[0028] Example 1 When the absolute pressure is 0.4 MPa Step 1): 1.1) Calculate the 50% breakdown field strength E of the air gap under a negative polarity lightning impulse voltage. 50% , E 50% =63p+2.4 =27.6 kV / mm; In the formula, p is the absolute pressure, in MPa, which is the SF6 pressure of the environment, typically 0.4 to 0.7 MPa. 1.2) According to E 50% Calculate the AC breakdown field strength E in the air gap a , E a = E 50% / 1.3=21.2 kV / mm; 1.3) Based on E a Calculate the expected value E of the negative polarity DC air gap discharge field strength. d , E d =E a / 1.25=16.96 kV / mm; In step 2), the insulation sample being tested is cylindrical. Determine the diameter L and height H of the insulation sample. In this embodiment, step 3) is specifically as follows: 3.1) Load the insulation sample into the test apparatus, and heat the upper part of the insulation platform to 110°C while keeping the lower part of the insulation platform at room temperature; 3.2) First, apply a negative DC voltage, with an initial voltage value of U. d After the negative DC voltage reaches 40000s, a value of 2.39U is first applied. d Apply a positive voltage, then gradually increase the positive voltage by 10kV until breakdown occurs. Record the breakdown voltage. Replace the insulation sample and repeat the test 5 times. Take the arithmetic mean of all the values ​​obtained to obtain the breakdown voltage U0, which is the sum of the lightning impulse voltage and the DC voltage.

[0029] Step 4), the calculation process of the surface breakdown field strength E of the insulating sample: 4.1) First calculate the withstand voltage U of the insulation component based on the 50% flashover voltage measured during the superposition test. tds = U0×(1-3σ)×0.85; 4.2) In U tds The allowable value E of the design tangential field strength on the surface of the insulating component under superimposed voltage is obtained by calculating the electric field under voltage.

[0030] Example 2 When the absolute pressure is 0.7 MPa 1.1) Calculate the 50% breakdown field strength E of the air gap under a negative polarity lightning impulse voltage. 50% , E 50% =63p+2.4 =46.5 kV / mm In the formula, p is the absolute pressure, in MPa, which is the SF6 pressure of the environment, typically 0.4 to 0.7 MPa. 1.2) According to E 50% Calculate the AC breakdown field strength E in the air gap a , E a = E 50% / 1.3=35.8 kV / mm; 1.3) Based on E a Calculate the expected value E of the negative polarity DC air gap discharge field strength. d , E d =E a / 1.4=25.6 kV / mm; In step 2), the insulation sample being tested is cylindrical. Determine the diameter L and height H of the insulation sample. In this embodiment, step 3) is specifically as follows: 3.1) Load the insulation sample into the test apparatus, and heat the upper part of the insulation platform to 115°C while keeping the lower part of the insulation platform at room temperature; 3.2) First, apply a negative DC voltage, with an initial voltage value of U. d After the negative DC voltage reaches 45000s, a value of 2.39U is first applied. d Apply a positive voltage, then gradually increase the positive voltage by 5kV until breakdown occurs. Record the breakdown voltage, replace the insulation sample, and repeat the test 6 times. Take the arithmetic mean of all the obtained values ​​to obtain the breakdown voltage U0, which is the sum of the lightning impulse voltage and the DC voltage.

[0031] Step 4), the calculation process of the surface breakdown field strength E of the insulating sample: 4.1) First calculate the withstand voltage U of the insulation component based on the 50% flashover voltage measured during the superposition test. tds = U0×(1-3σ)×0.8; 4.2) In U tds The allowable value E of the design tangential field strength on the surface of the insulating component under superimposed voltage is obtained by calculating the electric field under voltage.

[0032] This invention proposes a complete method for calculating the surface breakdown voltage field strength of an insulating component under DC voltage and the breakdown field strength superimposed with lightning impulse voltage and DC voltage. Based on the expected value of the negative polarity DC air gap discharge field strength, the specifications of the insulation sample under test are reasonably determined, and then the field strength is reasonably calculated.

[0033] This invention is not limited to the above embodiments. Based on the technical solutions disclosed in this invention, those skilled in the art can make some substitutions and modifications to some of the technical features without creative effort, and all such substitutions and modifications are within the protection scope of this invention.

Claims

1. A method for calculating the surface breakdown field strength of a high-voltage DC insulator, characterized in that, Includes the following steps: 1) Calculate the expected value E of the DC air gap discharge field strength. d ; 2) Based on E obtained in step 1), d Design the specifications of the insulation sample to be tested; 3) Load the insulation sample to be tested into the test device for breakdown test, and obtain the breakdown voltage U0 by superimposing the lightning impulse voltage and the DC voltage; 4) Calculate the breakdown field strength E of the lightning impulse voltage and DC voltage superimposed on the insulation sample based on U0.

2. The method for calculating the surface breakdown field strength of a high-voltage DC insulator according to claim 1, characterized in that, Step 1) specifically includes: 1.1) Calculate the 50% breakdown field strength E of the air gap under a negative polarity lightning impulse voltage. 50% The unit is kV / mm, and the specific formula is as follows: AND 50% =63p+2.4; In the formula, p is the absolute pressure, in MPa; 1.2) According to E 50% Calculate the AC breakdown field strength E in the air gap a Unit: kV / mm; AND a =E 50% / 1.3; 1.3) Based on E a Calculate the expected value E of the negative polarity DC air gap discharge field strength. d Unit: kV / mm; E d =E a / M; The value of M in the formula ranges from 1.2 to 1.

4.

3. The method for calculating the surface breakdown field strength of a high-voltage DC insulator according to claim 2, characterized in that, In step 2), the insulation sample being tested is cylindrical; determine the diameter L and height H of the insulation sample. The diameter L is smaller than the distance from the side of the insulation sample to the inner wall of the testing device; The height H is based on the expected experimental voltage U. d And the expected flashover field strength E on the surface of the insulating component τ Specifically: First, calculate the expected flashover field strength E on the surface of the insulating component. τ =E d / 2; The height H must satisfy: H≥U d / E τ ; In the formula U d U is the expected breakdown test voltage for the insulating component. d =E τ ×h, where h is the height of the insulating platform.

4. The method for calculating the surface breakdown field strength of a high-voltage DC insulator according to claim 3, characterized in that, Step 3) Specific steps: 3.1) Load the insulation sample into the test apparatus, and heat the upper part of the insulation platform to the target temperature, while keeping the lower part of the insulation platform at room temperature; 3.2) First apply a negative DC voltage. The initial voltage value is U d After the negative DC voltage reaches the preset time, a value of 2.39U is first applied. d Apply a positive voltage, and then gradually increase the positive voltage according to the preset voltage value until breakdown occurs. Record the breakdown voltage, replace the insulation sample, and repeat the test at least 5 times. Take the arithmetic mean of all the obtained values ​​to obtain the breakdown voltage U0, which is the sum of the lightning impulse voltage and the DC voltage.

5. The method for calculating the surface breakdown field strength of a high-voltage DC insulator according to claim 3, characterized in that, Step 4), the calculation process of the surface breakdown field strength E of the insulating sample: 4.1) First calculate The withstand voltage of the insulator is calculated based on the 50% flashover voltage measured during the superposition test. U tds = U0×(1-3σ)×N,where σ=0.05 ,is the standard deviation of lightning impulse voltage, and N is the design margin, with a value range of 0.7 to 1; 4.2) In U tds The allowable value E of the design tangential field strength on the surface of the insulating component under superimposed voltage is obtained by calculating the electric field under voltage.

6. The method for calculating the surface breakdown field strength of a high-voltage DC insulator according to claim 4, characterized in that, In step 3.1), the target temperature range is 100 to 120 degrees Celsius.

7. The method for calculating the surface breakdown field strength of a high-voltage DC insulator according to claim 4, characterized in that, The preset time mentioned in step 3.2) is not less than 40,000 seconds, and the preset voltage value ranges from 5 to 15 kV.

8. The method for calculating the surface breakdown field strength of a high-voltage DC insulator according to claim 4, characterized in that, Step 4.2) Calculation method for the allowable value E of the tangential field strength on the surface of the insulating component under superimposed voltage: Take the relative permittivity ε of the insulator I =4.95, The relative permittivity of the insulating gas is ε0 = 1.002; Based on boundary conditions: ； Solve the domain control equations of the entire electrostatic field using equation (1): ； Equation (2) Relationship between charge density and current density of insulating sample: ； In equation (2): t is time, K VI The conductivity of the insulating sample, Charge density of insulating sample; Equation (3) Gauss's law in electrostatic fields: ； In equation (3) ρ + ρ is the positive and negative charge density. - ρ is the positive and negative charge density. V The charge density of the insulating sample; Equation (4) Connection conditions at the dielectric interface: ； In equation (4), D1 is the gas-side electric displacement vector, D2 is the insulating stage-side electric displacement vector, n is the unit normal vector of the boundary, and ρ s The free charge density at the interface; Formula (5) for calculating electric displacement vector: ； Solving equations (1), (2), (3), (4), and (5) simultaneously yields the breakdown field strength E resulting from the superposition of the lightning impulse voltage and the DC voltage.

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