A method for calculating the shock overpressure generated by the explosion of a cable joint

Abstract

The present invention discloses a method for calculating the shock overpressure generated by the explosion of a cable joint, including: Step 1: Establish a simulation model of a single-phase grounding fault of a cable with an electric arc, and obtain the power value of the fault short-circuit arc on the simulation model; Step 2: Establish a multi-physics field coupling model of the temperature field-fluid field-solid field of the cable intermediate joint corresponding to the size, and use the obtained power value of the fault short-circuit arc as the input source of the equivalent heat source; Step 3: Solve the temperature distribution, flow velocity distribution and pressure distribution in the multi-physics field coupling model of the temperature field-fluid field-solid field to obtain the pressure difference inside and outside the joint metal protective shell, which is the shock overpressure generated by the joint explosion. This method can provide a basis for the development of explosion-proof boxes for cable joints and the improvement of explosion-proof measures. By designing explosion-proof according to the calculated shock overpressure, it is possible to avoid insufficient shell strength and waste of production materials and costs in the production of explosion-proof shells.

Description

Technical Field

[0001] The present invention relates to the technical field of cable joint fault analysis and research, and particularly relates to a method for calculating the shock overpressure generated by the explosion of a cable joint. Background Art

[0002] With the continuous increase of the national electricity demand, the operating load of cable lines is constantly increasing. As an important accessory, the number of intermediate cable joints is also increasing. In recent years, short - circuit faults caused by arc breakdown of cable joints in 110 kV and above lines in various regions have occurred frequently. Among them, for tunnel cables, due to the relatively sufficient air and space outside the joints, the impact force and harm generated during the formation of explosion or fire are greater than those of directly buried cables. It will not only affect the operation of its own line, but also damage the surrounding lines, further expanding the fault harm, and at the same time endangering the safety of operation and maintenance personnel.

[0003] To prevent the explosion accidents of high - voltage cable intermediate joints, it is necessary to improve the joint structure and increase the explosion - proof performance of the product. The explosion of the joint is caused by the combustion of arc - high - temperature plasma in the metal protective shell of the joint to form a closed cavity, and the pressure inside the cavity rises sharply under the action of high temperature to form a shock overpressure. At present, the calculation methods for the shock overpressure generated by the explosion of cable joints are relatively scarce. The mainstream explosion - proof products and explosion - proof measures in the market lack sufficient theoretical basis and mainly rely on empirical means. For example, a certain manufacturer achieves the explosion - proof purpose by enhancing the shell strength of the metal protective shell, outer protective shell, etc. The designed shell strength can withstand an impact pressure of 1 MPa, but the source of the specific pressure value lacks theoretical basis. There are also some manufacturers that fill with filling glue containing good arc - extinguishing materials to achieve the explosion - proof effect by shortening the arc combustion time and reducing the volume of the air gap.

[0004] For different lines, calculate the maximum short - circuit energy that may be generated during the actual operation of the line, and further obtain the maximum shock overpressure that may be generated by the explosion of the joint. Based on this, develop an explosion - proof protective shell. This can not only avoid the insufficient shell strength of the explosion - proof protective shell, but also prevent the waste of materials and reduce costs during the production of the protective shell. It conforms to the development trend of calculating high voltages in the development of the power grid. However, the existing technology lacks the calculation of the explosion energy of high - voltage cable joints. Summary of the Invention

[0005] The purpose of the present invention is to solve the problem that the existing technology lacks the calculation of the explosion energy of high - voltage cable joints, and provide a method for calculating the shock overpressure generated by the explosion of a cable joint, so as to provide a basis for the development of explosion - proof boxes for cable joints and the improvement of explosion - proof measures. Designing explosion - proof according to the calculated shock overpressure can avoid insufficient shell strength and waste of production materials and costs in the production of explosion - proof shells.

[0006] To achieve the above - mentioned purpose, the technical solution of the present invention is as follows:

[0007] A method for calculating the shock overpressure generated by the explosion of a cable joint, comprising:

[0008] Step 1: Establish a simulation model of a single-phase grounding fault of a cable with an electric arc, and obtain the power value of the fault short-circuit arc on the simulation model;

[0009] Step 2: Establish a multi-physics coupling model of temperature field - fluid field - solid field for the intermediate joint of the corresponding size of the cable, and use the obtained power value of the fault short-circuit arc as the input source of the equivalent heat source;

[0010] Step 3: Solve the temperature distribution, flow velocity distribution and pressure distribution in the multi-physics coupling model of temperature field - fluid field - solid field to obtain the pressure difference inside and outside the joint metal protective shell, which is the shock overpressure generated by the joint explosion.

[0011] Furthermore, the said Step 1 includes:

[0012] 101: Establish a line system, including outgoing line, branch line, substation, overhead line or cable line, lightning protection line;

[0013] 102: Establish a cable body and cable joint model according to the cable body and joint of different voltage levels, cross-sections and models;

[0014] 103: Establish an electric arc model for the single-phase grounding fault of the cable system, and establish arc black box models with different applicable ranges to analyze the resistance characteristics of the arc process;

[0015] 104: Study the short-circuit voltage, current and resistance conditions of the near-area short-circuit faults on different outgoing lines, and the short-circuit voltage, current and resistance conditions of the single-phase grounding short-circuits at different positions on the same line;

[0016] 105: Calculate the magnitude of the arc power according to the arc current and arc voltage.

[0017] Furthermore, the said Step 2 includes:

[0018] 201: Establish a two-dimensional axisymmetric geometric model in finite element simulation software according to the structural dimensions of the cable joint and the body, and add material properties to each structure in the two-dimensional axisymmetric geometric model;

[0019] 202: Set boundary conditions, set the axial end face as an adiabatic boundary, and the interface between the surface of the cable joint and the body and the air as a natural convection boundary;

[0020] 203: Perform the coupled solution of the temperature field and the fluid field. The temperature-related data calculated by the temperature field interface is used for the flow field calculation interface, and at the same time, the fluid velocity and pressure data calculated by the flow field interface are used in the temperature field calculation interface.

[0021] Furthermore, step 3 further includes: obtaining the stress on the joint metal protective shell according to the action area of the shock overpressure.

[0022] Furthermore, the shock overpressure is calculated as follows:

[0023] First, calculate the short-circuit energy when a single-phase ground short circuit occurs in the cable line, and then combine the temperature field-fluid field-solid field multi-physical field coupling model to obtain the shock overpressure value generated by the arc.

[0024] Furthermore, the calculation method of the shock overpressure considers the working conditions and fault conditions of the cable joint during actual operation.

[0025] Furthermore, in the case of a fault, when a short-circuit arc is generated after the cable joint is broken down, the energy of the arc plasma acts on the closed cavity of the limited space, and then an explosion occurs.

[0026] Furthermore, the simulation model of the single-phase ground fault of the cable with arc is established on the PSCAD / EMTDC software.

[0027] Furthermore, the temperature field-fluid field-solid field multi-physical field coupling model of the intermediate joint of the corresponding size of the cable is established on the COMSOL finite element simulation software.

[0028] Furthermore, in step 201, the material properties include density, thermal conductivity, and constant-pressure heat capacity.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0030] This method can provide a basis for the development of the explosion-proof box for cable joints and the improvement of explosion-proof measures. By designing the explosion-proof according to the calculated shock overpressure, it is possible to avoid insufficient shell strength and waste of manufacturing materials and costs when making the explosion-proof shell. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a flowchart of the method for calculating the shock overpressure generated by the explosion of a cable joint provided by an embodiment of the present invention;

[0032] Figure 2 It is a 110kV system simulation model on the PSCAD / EMTDC software;

[0033] Figure 3 It is a schematic diagram of the 110kV cable geometric model;

[0034] Figure 4 is the equivalent model of the cable joint circuit;

[0035] Figure 5 is the arc control module of the Cassie model;

[0036] Figure 6 is the sine waveform of the simulated arc power;

[0037] Figure 7 is the simulated pressure distribution. Specific implementation manners

[0038] Embodiment:

[0039] The technical solution of the present invention will be further described below in conjunction with the drawings and embodiments.

[0040] Refer to Figure 1 As shown, the method for calculating the shock overpressure generated by the explosion of the cable joint provided in this embodiment mainly includes the following steps:

[0041] Step 1: Establish a simulation model of a single-phase grounding fault of a cable with an arc, study the magnitudes of the arc current and voltage of a short-circuit fault in the near area of the cable system, and obtain the power value of the fault short-circuit arc on the simulation model;

[0042] Step 2: Establish a multi-physical field coupling model of the temperature field - fluid field - solid field of the intermediate joint corresponding to the cable, and use the obtained power value of the fault short-circuit arc as the input source of the equivalent heat source;

[0043] Step 3: Solve the temperature distribution, flow velocity distribution and pressure distribution in the multi-physical field coupling model of the temperature field - fluid field - solid field to obtain the pressure difference inside and outside the joint metal protective shell, which is the shock overpressure generated by the joint explosion.

[0044] It can be seen that this method can provide a basis for the development of the cable joint explosion-proof box and the improvement of explosion-proof measures. Designing explosion-proof according to the calculated shock overpressure can not only avoid insufficient shell strength but also avoid wasting production materials and costs in the production of the explosion-proof shell.

[0045] Specifically, the above Step 1 includes the following sub-steps:

[0046] 101: Establish a line system, including outgoing lines, branch lines, substations, overhead lines or cable lines, lightning protection lines; at the same time, parameters such as the voltage level, capacity, and model size of various equipment need to be considered;

[0047] 102: Establish a cable body and cable joint model according to the cable body and joint with different voltage levels, cross-sections and models;

[0048] 103: Establish an arc model for single-phase grounding faults in cable systems, and establish arc black box models with different applicable ranges to analyze the resistance characteristics of the arc process;

[0049] 104: Study the short-circuit voltage, current, and resistance of near-zone short-circuit faults on different outgoing lines, as well as the short-circuit voltage, current, and resistance of single-phase grounding short-circuits at different positions on the same line;

[0050] 105: Calculate the magnitude of the arc power based on the arc current and arc voltage. The calculation expression is as follows:

[0051] P = u arc × i

[0052] In the formula, u arc is the arc voltage, i is the arc current, and P is the arc power.

[0053] In this way, through the above steps, the power value of the fault short-circuit arc can be accurately obtained on the simulation model.

[0054] Specifically, step 2 above includes:

[0055] 201: According to the structural dimensions of the cable joint and the main body, establish a two-dimensional axisymmetric geometric model in finite element simulation software, and add material properties to each structure in the two-dimensional axisymmetric geometric model. The material properties include density, thermal conductivity, and constant pressure heat capacity.

[0056] 202: Set boundary conditions. Set the axial end face as an adiabatic boundary, and the interface between the surface of the cable joint and the main body and the air as a natural convection boundary, with a convection coefficient of 6 W / (m 2 ·K); the air temperature is 25 °C.

[0057] 203: Perform coupled solution of the temperature field and the fluid field. The temperature-related data calculated by the temperature field interface is used for the flow field calculation interface, and at the same time, the fluid velocity, pressure, and other data calculated by the flow field interface can also be used in the temperature field calculation interface.

[0058] In this way, through the above steps, a cable main body and cable joint model can be accurately established.

[0059] Preferably, step 3 above further includes: obtaining the stress on the joint metal protective shell based on the action area of the shock overpressure to design the explosion-proof of the joint metal protective shell, which can not only avoid insufficient shell strength but also avoid wasting production materials and costs in making the explosion-proof shell.

[0060] Specifically, the above simulation model of single-phase grounding fault of cables with electric arcs is established on the PSCAD / EMTDC software. The above temperature field-fluid field-solid field multi-physical field coupling model of the cable corresponding size intermediate joint is established on the COMSOL finite element simulation software.

[0061] The above shock overpressure is obtained through the following calculation method: First, calculate the short-circuit energy when a single-phase grounding short circuit occurs in the cable line, and then combine with the COMSOL finite element simulation software to obtain the shock overpressure value generated by the electric arc. In addition, this calculation method takes into account the working conditions and fault conditions of the cable joint during actual operation. The fault condition is that when the cable joint is punctured and a short-circuit arc is generated, the huge energy of the high-temperature arc plasma acts on the confined cavity of the limited space, and then an explosion occurs.

[0062] The following further illustrates this method in combination with an application scenario example:

[0063] Taking the 110kV system as an example, calculate the shock overpressure generated by the explosion of the cable joint in combination with the method steps disclosed above. The specific steps are as follows:

[0064] Step 1: Establish a simulation model of single-phase grounding fault of 110kV cables with electric arcs on the PSCAD / EMTDC software, study the magnitudes of the arc current and voltage of the short-circuit fault in the near area of the cable system, and further obtain the power value of the fault short-circuit arc.

[0065] The line system of the 110kV system is as Figure 2 shown. This system contains a total of 6 outgoing lines and several branch lines, including a 220kV / 110kV substation and several 110kV / 10kV substations. The line includes two types of lines: overhead lines and cables. The model of the overhead conductor is LGJ240 / 30, the model of the lightning protection line is GJ-50, the conductor cross-section of the cable is 1200mm2, and the three-phase cables are arranged in a triangular formation. Each 220kV / 110kV transformer has a rated capacity of 240MVA and 180MVA, and each 110kV / 10kV transformer has a rated capacity of 63MVA and 40MVA. The active power and reactive power at the end of the line, as well as the lengths of each outgoing line, are as Figure 2 shown.

[0066] The object of study in this embodiment is the 110kV cable system. Currently, all the cables in use are cross-linked polyethylene (XLPE) cables. There is a semi-conductive buffer layer on both the inner and outer sides of the main insulation to avoid electric field concentration. However, there is currently no perfect electromagnetic transient simulation software that can establish the semi-conductive layer, and the thickness of the semi-conductive layer is thin. Therefore, its influence on the actual capacitance of the cable is considered in the main insulation layer, and the dielectric constant of XLPE is converted from the actual 2.3 to 2.6. The geometric parameters of the 110kV cable model are asFigure 2 As shown, the simulated physical parameters are shown in Table 1 respectively.

[0067] The 110 kV cable joint is represented by an RLC equivalent circuit, as Figure 4 shown. In the figure, R1 is the resistance effect between cable joints, with a value of 0.1 mΩ. C1 is the capacitance between the conductor core and the external shield of the stress cone for the straight-through joint, with a value of 384.59 pF; C2 represents the capacitance between the conductor core and the external shield of the rubber insulation, with a value of 177.12 pF; the capacitance at both ends of the shield layer in the insulated joint is equivalent to C0, with a value of 352.83 pF.

[0068] The Cassie arc model is selected to describe the variation characteristics of the grounding resistance of the 110 kV cable system. The control equation of the Cassie model is as follows:

[0069]

[0070] In the formula, the time constant τ c is taken as 300 μs, and the static arc field strength e0 is taken as 30 kV. The control module in the electromagnetic transient simulation software PSCAD is as Figure 5 shown. Finally, the arc power of the simulation result is as Figure 6 shown, and the maximum arc power is 590.58454 MW.

[0071] Step 2: Establish a multi-physics coupling model of temperature field - fluid field - solid field for the 110 kV cable and the intermediate joint of the 220 kV cable with corresponding dimensions through the COMSOL finite element simulation software, and use the arc power obtained in Step 1 as the input source of the equivalent heat source.

[0072] According to the structural dimensions of the cable joint and the body, establish a two-dimensional axisymmetric geometric model in the finite element simulation software. Determine the structure and each structural dimension according to the cross-sectional view and description of the cable body and the cable joint. The main structures of the cable body include conductor, semi-conductive shielding tape, conductor shielding layer, cross-linked polyethylene main insulation, insulation shielding layer, semi-conductive water blocking layer, air gap layer, metal aluminum sheath, and outer sheath; the main structures of the cable joint include compression joint, grading ring, silicone rubber main insulation, semi-conductive shielding layer, metal protective shell, and colloid filling layer. Among them, the silicone rubber main insulation also includes a high-voltage shielding layer and a stress cone structure, which should be drawn separately during modeling. Add material properties to each structure in the model, and the material properties include density, thermal conductivity, and constant-pressure heat capacity.

[0073] Use the equivalent heat source to equivalent the arc thermal effect, and the surface of the cable joint dissipates heat to the outside through convection heat transfer and radiation heat transfer. The mesh is divided into a conventional size. Perform the coupling solution of multi-physics fields.

[0074] The coupling process of the temperature field and the fluid field is as follows: The temperature-related data calculated by the temperature field interface can be used for the fluid field calculation interface, and at the same time, the data such as fluid velocity and pressure calculated by the fluid field interface can also be used in the temperature field calculation interface.

[0075] The coupling process of the temperature field and the solid field is as follows: Temperature affects the relevant parameters of the material in the fluid field, and the fluid field affects the temperature transfer during the change process.

[0076] The coupling process of the fluid field and the solid field is as follows: After the gas is heated and expands, it generates pressure on the solid, causing the solid to deform, and the change of the solid field will in turn affect the fluid field velocity.

[0077] Step 3: Calculate the temperature distribution, flow velocity distribution and pressure distribution in the solution model to obtain the pressure difference inside and outside the joint metal protective shell, which is the shock overpressure generated by the joint explosion, and further obtain the stress on the joint metal protective shell according to the pressure acting area. The simulated pressure distribution diagram is as Figure 7 shown.

[0078] In summary, the present invention provides a basis for the development of cable joint explosion-proof boxes and the improvement of explosion-proof measures. By designing explosion-proof according to the calculated shock overpressure, it is possible to avoid insufficient shell strength and waste of production materials and costs in the production of explosion-proof shells.

[0079] The above embodiments are only used to illustrate the technical concept and characteristics of the present invention. The purpose is to enable ordinary technical personnel in the field to understand the content of the present invention and implement it accordingly, and it cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the essence of the content of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for calculating the shock overpressure generated by the explosion of a cable joint, characterized in that, Including: Step 1: Establish a simulation model of single-phase grounding fault of a cable with an arc, and obtain the power value of the fault short-circuit arc on the simulation model; Step 2: Establish a multi-physics coupling model of temperature field - fluid field - solid field for the intermediate joint of the corresponding size of the cable, and use the obtained power value of the fault short-circuit arc as the input source of the equivalent heat source; Step 3: Solve the temperature distribution, flow velocity distribution and pressure distribution in the multi-physics coupling model of temperature field - fluid field - solid field to obtain the pressure difference inside and outside the joint metal protective shell, which is the shock overpressure generated by the joint explosion; The said Step 1 includes: 101: Establish a line system, including outgoing line, branch line, substation, overhead line or cable line, lightning protection line; 102: Establish cable body and cable joint models according to the cable body and joint of different voltage levels, cross-sections and models; 103: Establish an arc model for the single-phase grounding fault of the cable system, and establish arc black box models with different applicable ranges to analyze the resistance characteristics of the arc process; 104: Study the short-circuit voltage, current and resistance conditions of near-area short-circuit faults on different outgoing lines, and the short-circuit voltage, current and resistance conditions of single-phase grounding short-circuits at different positions on the same line; 105: Calculate the magnitude of the arc power according to the arc current and arc voltage; The said Step 2 includes: 201: Establish a two-dimensional axisymmetric geometric model in the finite element simulation software according to the structural dimensions of the cable joint and the body, and add material properties to each structure in the two-dimensional axisymmetric geometric model; the material properties include density, thermal conductivity, and constant pressure heat capacity; 202: Set boundary conditions, set the axial end face as an adiabatic boundary, and the interface between the surface of the cable joint and the body and the air as a natural convection boundary; 203: Perform coupled solution of the temperature field and the fluid field. The temperature-related data calculated by the temperature field interface is used for the flow field calculation interface, and at the same time, the fluid flow velocity and pressure data calculated by the flow field interface are used in the temperature field calculation interface; The said Step 3 further includes: Obtain the stress on the joint metal protective shell according to the action area of the shock overpressure; The said shock overpressure is calculated through the following method: First, calculate the short-circuit energy when a single-phase grounding short-circuit occurs in the cable line, and then combine the multi-physics coupling model of temperature field - fluid field - solid field to obtain the shock overpressure value generated by the arc.

2. The method for calculating the shock overpressure generated by the explosion of a cable joint according to claim 1, wherein The calculation method of the said shock overpressure considers the working conditions and fault conditions of the cable joint during actual operation.

3. The method for calculating the shock overpressure generated by the explosion of a cable joint according to claim 2, characterized in that, The said fault condition is that when the cable joint is broken down and a short-circuit arc is generated, the energy of the arc plasma acts on the closed cavity of the limited space, and then an explosion occurs.

4. The method for calculating the shock overpressure generated by the explosion of a cable joint according to claim 1, characterized in that, The simulation model of single-phase grounding fault of the cable with an arc is established on the PSCAD / EMTDC software.

5. The method for calculating the shock overpressure generated by the explosion of a cable joint according to claim 1, characterized in that, The multi-physics coupling model of temperature field - fluid field - solid field for the intermediate joint of the corresponding size of the cable is established on the COMSOL finite element simulation software.

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