A method for calculating the wind deflection of a transmission tower-line system

Abstract

The present invention provides a method for calculating the wind deflection of a transmission tower-line system, belonging to the technical field of electric power engineering. This technical solution studies the characteristics and formation mechanism of the gentle wind field of typical transmission line segments in Qinghai under specific topographical and geomorphic conditions. Based on a large amount of wind field data under micro-topographical conditions, combined with wind tunnel tests and theoretical calculations, it analyzes the wind deflection mechanism of typical tower structures and the possible occurrence laws of wind damage tripping and even tower collapse. Based on the data from theoretical research and wind tunnel tests, a wind deflection calculation model for the transmission tower-line system under micro-topographical conditions is established, achieving breakthroughs and innovations in the wind deflection mechanism and prevention errors of transmission lines. The present invention is conducive to improving the technical level of disaster prevention and mitigation of the power system, providing theoretical guidance for the subsequent erection of transmission lines in areas with similar topographical and geomorphic conditions in Qinghai, and thus improving the safety and stability of the power system.

Description

Technical Field

[0001] The present invention relates to the technical field of electric power engineering, and further relates to the analysis, prevention and control of wind-induced flashover faults of high-voltage transmission lines, and specifically relates to a method for calculating the wind deflection of a transmission tower-line system. Background Art

[0002] Wind deflection is one of the main factors causing faults in the operation of transmission lines, and the work of preventing wind deflection has always been emphasized by the operation departments of State Grid Corporation. In recent years, wind deflection of transmission lines in Qinghai has occurred frequently, causing faults such as line tripping, arc burns, and wire breaks. The prevention and control of wind deflection need to be strengthened urgently. The environment where the transmission lines with wind deflection faults are located is usually mainly mountainous areas or windy weather. Once the local climate conditions cannot be deeply analyzed during the line design, it will lead to inconsistencies between the tower head dimensions and the standard requirements, resulting in frequent occurrence of wind-induced flashover faults.

[0003] In some micro-topography areas in Qinghai, once in a strong wind environment, it is extremely easy to cause the occurrence of squall line winds. Under the action of squall line winds, the spatial distance between the insulator string and the tower will decrease. Once the minimum voltage requirement for discharge cannot be met, flashover will occur. Currently, the wind-induced flashover faults of high-voltage transmission lines are directly related to disaster meteorological conditions. Especially in the weather with strong winds mixed with thunderstorms and heavy rains, it is more likely to cause wind-induced flashover faults. Moreover, when the wind deflection fault occurs, the insulation strength of the transmission line will also show a downward trend. Under the action of strong wind weather, rainwater on the wire will form a directional intermittent water line along the wind direction. Once it is in the same direction as the discharge flashover path, it will cause the discharge voltage of the air gap to show a downward trend, which is also an important factor for the occurrence of wind deflection of the line. Moreover, in some local areas, at the positions of wind gaps and air ducts, the wind force is relatively concentrated, and these micro-meteorologies are also extremely likely to cause wind deflection faults.

[0004] The occurrence of wind deflection is mostly related to strong winds, and this has also been found in inspections of areas with multiple wind deflection fault discharges. Strong winds at the site usually break or uproot large trees, and under the action of such strong winds, wind deflection flashovers of transmission lines occur. Because once the conductor is under the action of strong winds, it will deviate from the specified standard position to a certain extent, and due to the reduction of the gap between the insulator string and the tower head, the spatial electric field intensity is enhanced, resulting in local high electric field intensity occurring at the tips of the conductor fittings and the tower body, leading to discharge at this position. Since the occurrence of wind deflection discharge in the transmission line is due to the reduction of the air gap distance between the conductor and the tower pole or between the conductors under the action of strong winds, and once the electrical strength of this gap distance does not match the operating voltage specified by the system, a discharge accident will occur. In order to better prevent the occurrence of wind deflection faults, it is necessary to strengthen in multiple aspects such as the design wind speed design margin, construction and installation technology, and the dimensions of the tower head. Establishing a wind deflection calculation model for the transmission tower-line system under microtopographic conditions and accurately calculating the wind deflection displacement are the basis for the above design and analysis work. Summary of the Invention

[0005] The technical problem to be solved by the present invention is: how to establish a wind deflection calculation model for the transmission tower-line system under microtopographic conditions and accurately calculate the wind deflection displacement.

[0006] To achieve the above technical objectives, the present invention adopts the following technical solutions:

[0007] A method for calculating the wind deflection of a transmission tower-line system. In this method, the maximum sag of the transmission line is calculated by the following formula :

[0008] ;

[0009] In the above formula, is the span, is the self-weight per unit length of the conductor and ground wire, is the horizontal tension of the overhead line, is the elevation angle, is the specific weight of the conductor and ground wire, 0 is the horizontal stress at each point of the conductor and ground wire;

[0010] The sag at any point is calculated by the following formula :

[0011] ;

[0012] In the above formula, is the horizontal distance from one side of the tower pole, is the span, is the maximum sag;

[0013] The horizontal wind deflection displacement at any point is calculated by the following two formulas respectively and the vertical wind deflection displacement at this point :

[0014] ; In the above two formulas, is the length of the insulator string, is the wind deflection angle of the conductor, is the wind deflection angle of the insulator string.

[0015] Preferably, in this method, the wind load acting on the transmission line is calculated by the following formula:

[0016] ;

[0017] In the above formula, is the wind load perpendicular to the line direction, is the standard value of the reference wind pressure, is the wind pressure non-uniformity coefficient, is the wind pressure height change coefficient, is the shape coefficient of the line, is the wind load adjustment coefficient, is the diameter of the conductor cross-section, is the horizontal span, is the angle between the wind load and the line;

[0018] Wherein:

[0019] ;

[0020] is the wind speed at a reference height of 10 m.

[0021] Preferably, the wind pressure non-uniformity coefficient is determined according to the design basic wind speed in accordance with Table 1; the wind pressure height change coefficient is determined in accordance with Table 1; the shape coefficient of the line , when the conductor diameter is less than 17 mm or in the case of icing, take , when the conductor diameter is greater than or equal to 17 mm, take ; The wind load adjustment coefficient of the conductors of 500 kV and 750 kV lines is determined according to the design basic wind speed in accordance with Table 2;

[0022] Table 1

[0023]

[0024] Table 2

[0025]

[0026] Preferably, in this method, the computational fluid dynamics process includes:

[0027] 1) Establish the governing equations;

[0028] 2) Determine the initial conditions and boundary conditions;

[0029] 3) Divide the grid and generate the computational points;

[0030] 4) Establish the discrete equations;

[0031] 5) Determine the control parameters;

[0032] 6) Solve the discrete equations;

[0033] 7) Judge whether it converges. When it converges, execute the following step 8). When it does not converge, return to the above step 2);

[0034] 8) Output the calculation results.

[0035] Preferably, the Eulerian description of the hydrodynamic governing equations is shown as the following four equations:

[0036] In the above four equations, is the fluid density, is the flow time; is the viscous stress component existing on the surface of the infinitesimal element; is the velocity vector, and its components in the three coordinate directions are respectively , , ; represents the pressure on the fluid infinitesimal element.

[0037] Preferably, the micro-topographic conditions where the object calculated by this method is located include: pass-type terrain, high mountain watershed-type terrain, water vapor enhancement-type terrain, terrain uplift-type terrain, and canyon wind tunnel-type terrain.

[0038] Preferably, in this method, the expression of the inlet boundary condition of the micro-topographic wind field includes the turbulent kinetic energy expression and the dissipation rate expression. Among them, the turbulent kinetic energy expression is as follows:

[0039] ; The dissipation rate expression is as follows:

[0040] ; In the above two equations, represents the turbulence intensity at height under, is the turbulent integral scale, with a value of 0.09, and a value of 0.42.

[0041] Preferably:

[0042]

[0043]

[0044] In the above two formulas, is the gradient wind height, under B-type terrain = 5 m.

[0045] Preferably, in this method, the surface roughness of the micro-topography wind field is described by the roughness height parameter The roughness height parameter and the roughness length satisfy the following relationship:

[0046] ; where is determined by the roughness specification of B-type terrain.

[0047] Preferably, after the calculation, the calculation results are verified through the wind tunnel test of the transmission tower-line system.

[0048] The present invention provides a method for calculating the wind deflection of a transmission tower-line system. This technical solution studies the characteristics and formation mechanism of the gentle wind field of typical transmission line segments in Qinghai under specific topographies and landforms. Based on a large amount of wind field data under micro-topography conditions, combined with wind tunnel tests and theoretical calculations, it analyzes the wind deflection mechanism of typical tower structures and the possible occurrence of wind damage tripping and even tower collapse rules. Based on the data of theoretical research and wind tunnel tests, a wind deflection calculation model of the transmission tower-line system under micro-topography conditions is established, achieving breakthroughs and innovations in the wind deflection mechanism and prevention of transmission lines. The present invention is beneficial to improving the technical level of disaster prevention and mitigation of the power system, providing theoretical guidance for the subsequent erection of transmission lines in areas with similar topographies and landforms in Qinghai, and thus improving the safety and stability of the power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 is the CFD numerical calculation flow chart in the present invention.

[0050] Figure 2 is a schematic diagram of the pass-type terrain.

[0051] Figure 3 is a schematic diagram of the high mountain watershed type terrain.

[0052] Figure 4 is a schematic diagram of the water vapor enhancement type terrain.

[0053] Figure 5 It is a schematic diagram of terrain uplift type terrain.

[0054] Figure 6 It is a schematic diagram of canyon air duct type terrain.

[0055] Figure 7 It is a verification result diagram of the self - holding property of the default wind profile formula.

[0056] Figure 8 It is a verification result diagram of the self - holding property of the modified wind profile.

[0057] Figure 9 It is a schematic diagram of conductor wind deflection displacement.

[0058] Figure 10 It is a schematic diagram of the research idea of wind load in the field of structural engineering.

[0059] Figure 11 It is a physical picture of the wind tunnel test section.

[0060] Figure 12 It is a physical picture of the console of the wind tunnel test.

[0061] Figure 13 It is a comparison diagram of the theoretical value and the measured value of the wind speed of the transmission tower - line system in the turbulent wind field of typical landforms. Specific implementation manners

[0062] The specific implementation manners of the present invention will be described in detail below. To avoid excessive unnecessary details, the well - known structures or functions will not be described in detail in the following embodiments. The approximate language used in the following embodiments can be used for quantitative expressions, indicating that certain changes in quantity are allowed without changing the basic functions. Unless otherwise defined, the technical and scientific terms used in the following embodiments have the same meaning as commonly understood by those skilled in the art to which the present invention belongs.

[0063] 1. High - precision wind field simulation of micro - terrain

[0064] 1.1 Computational fluid dynamics theory

[0065] Computational Fluid Dynamic, abbreviated as CFD, is an analysis method and means for solving fluid flow, heat transfer effects, and flow field - related parameters based on a computer platform. The basic principle of CFD is: dividing the overall continuous fluid calculation domain in three - dimensional space into extremely small calculation units. Taking the finite element as the smallest calculation scale, solving the fluid mechanics control equations on the micro - elements, and finally obtaining the flow field characteristics. The basic process of CFD numerical calculation is as Figure 1 shown.

[0066] All forms of fluid flow always follow the basic physical conservation laws, including the conservation laws of mass, momentum, energy, and components. The object of study in this invention is the micro-topography wind field, without considering the heat transfer and chemical component parts, that is, the energy conservation equation and the component mass conservation equation do not participate in the calculation. In the default inertial rectangular coordinate system, the Euler description of the fluid mechanics control equation is as follows:

[0067] (1) (2)

[0068] (3)

[0069] (4)

[0070] In the formula, is the fluid density, is the flow time. Due to the viscous action of molecules, there are viscous stress components on the surface of the microelement body, denoted as . is the velocity vector, and its components in the three coordinate directions are respectively , , ; represents the pressure on the fluid microelement; formula (1) is the mass conservation continuity equation, and formulas (2), (3), and (4) are the momentum conservation equations.

[0071] The core idea of CFD numerical calculation is to discretize the cells in the computational domain and then discretize the control equations. The control equations are discretely solved by three discretization methods: the finite difference method (FDM), the finite element method (FEM), and the finite volume method (FVM). Among them, the finite volume method is the most widely used discretization method in most commercial CFD software. In this invention, the Fluent module in the ANSYS workbench platform is used to solve by the finite volume method.

[0072] The finite volume method divides the computational domain into microelement control volumes and discretely solves the differential equations on each microelement volume. Compared with the finite difference method, the finite volume method has no requirement for the orthogonality of the discrete grid. Therefore, a large number of unstructured grids can be used to solve complex flow field model problems. In actual calculation, due to the small amount of grid calculation, the memory space occupied is small, and there is a condition to divide the flow field details more finely.

[0073] 1.2 Main types of micro-topography

[0074] Transmission lines built in micro-topography and micro-meteorology areas will be affected by the local airflow operating environment and may be adversely affected by extreme wind speeds or increased icing beyond the design. Typical micro-topography and micro-meteorology can be roughly divided into five types.

[0075] (1) Passage type Figure 2 ). In the passes formed between the long mountains, the airflow shows a concentrated acceleration phenomenon. When the power transmission line in the mountainous area crosses the pass, the narrow channel effect will cause the lateral wind speed to increase significantly, and there is also the possibility of a small increase in ice cover.

[0076] (2) High mountain watershed type Figure 3 ). When the line corridor crosses the watershed, the windward slope terrain lifting effect generates strong winds full of water vapor, which can easily lead to the collapse of the windward slope line tower and disconnection. At the top of the mountain and the windward slope, the adiabatic expansion process occurs along the slope under the action of wind, so that the supercooled water is cooled out of the air and attached to the conductor, which aggravates the icing of the conductor. The windward and leeward slopes of the watershed have certain vertical wind speeds, which have a certain impact on the force on the conductor.

[0077] (3) Water vapor growth type ( Figure 4 If the transmission line is near a large river or lake, due to the high humidity in the air all year round, once a cold front arrives or the temperature drops below 0°C, it is easy for the overhead transmission wires to be severely covered with ice at all angles, increasing the burden on the wires and insulator strings.

[0078] (4) Terrain uplift type Figure 5 ). It is generally a terrain step in a plain landform, or a basin edge with one side lower and the other side higher, or a stepped hilly slope. Because the basin is low and has sufficient water vapor and high air humidity, cold air easily rises along the hillside with the wind. Similar to the watershed type terrain, clouds and fog form on the top of the hillside, and when the temperature drops during the cold wave, severe ice will appear. At the same time, the terrain-lifting micro-topography will produce obvious vertical wind speed due to the airflow climbing effect, which has a great impact on the force on the conductor.

[0079] (5) Canyon wind channel type Figure 6 ). The line crosses the canyon, and the two banks are very high and steep. The narrow channel effect generates a large and long-lasting local lateral wind speed, which will lead to a significant increase in the wind load on the transmission line.

[0080] 1.3 Introduction to micro-topography wind field

[0081] (1) Inlet boundary conditions

[0082] The setting of the inlet boundary condition is a core parameter in CFD analysis. To verify the correctness of the simulated wind field, it is necessary to ensure the self-preservation of the boundary condition. The self-preservation of the boundary condition is defined as follows: the inlet boundary wind profile needs to remain unchanged throughout the flow in the flow field before reaching the target. Through the self-preservation of the boundary condition, three methods for maintaining the inlet profile have been obtained. By adjusting the coefficients of the inlet profile expression, or adjusting the empirical coefficients of the turbulence model, or using a modified wall function, the inlet wind profile can be maintained.

[0083] The inlet wind profile refers to the combination of the mean wind profile and the turbulent wind profile.

[0084] The turbulent profile mainly includes two parameters: turbulent kinetic energy and dissipation rate. In general mean flow field analysis, it is assumed that turbulence is isotropic. The expressions for the initial turbulent kinetic energy and dissipation rate are:

[0085] (5)

[0086] (6)

[0087] where represents the turbulence intensity at height and is the turbulence integral scale. There is no clear formula for the turbulence intensity and integral scale in the current Chinese code. According to the research results of the present invention, takes the value of 0.09, and

[0088] (7)

[0089] (8)

[0090] In the formula is the gradient wind height, and under B-type terrain = 5 m. The inlet wind profile of the computational domain is calculated using the default formula, and the wind speed profile at 1 km is selected. The comparison results are as Figure 7 shown.

[0091] From Figure 7 it can be seen that the eddy viscosity value parameter of the fluent default turbulence formula is relatively large, and a large deformation occurs in the near-surface wind profile before it touches the mountain. The horizontal wind speed increases to a certain extent within 200 m above the ground; while above 200 m above the ground, the wind speed decreases relatively. Due to the high eddy viscosity value in the near-surface area, the profile wind speed tends to be evenly distributed. The expression for the eddy viscosity value in the turbulence model is:

[0092] (9)

[0093] Equation (9) shows that the eddy viscosity value is proportional to the square of the turbulent kinetic energy and inversely proportional to the turbulent dissipation rate. By adjusting the parameter coefficients of the turbulent kinetic energy and the turbulent dissipation rate, the influence of the eddy viscosity value on the near-surface wind field is reduced. After adjusting the coefficients, the self-preservation of the wind profile is verified, and the following boundary condition expressions are obtained after iterative adjustment:

[0094] (10)

[0095] (11)

[0096] Take the formulas (10) and (11) after coefficient adjustment as the inlet boundary conditions in the numerical model. Extract the wind speed profile at the position 1 km in front of the micro-topography mountain body, and compare it with the inlet wind speed profile. The results are as Figure 8 shown. The results show that by adjusting the turbulent kinetic energy and the turbulent dissipation rate, the near-surface eddy viscosity value can be reduced, so that the defined wind profile can be more accurately applied to the mountain body terrain, and the self-preservation of the wind profile is relatively stable. The inlet boundary condition settings will be adopted in the numerical calculation processes of the flat terrain wind field and the micro-topography wind field of the present invention.

[0097] (2) Surface roughness

[0098] When analyzing the characteristics of the mountain terrain wind field, the definition of surface roughness in aerodynamics is adopted: due to the influence of the surface undulation or the geometric shape of the landform itself, the position where the wind speed is zero on the wind speed profile is higher than the ground surface height of 0, and this height is defined as the aerodynamic roughness. Generally, the roughness length

[0099] is used to describe the surface roughness. In the present invention, the roughness height parameter is used to describe the surface roughness, and there is the following conversion relationship between this parameter and the roughness length :

[0100] (12)

[0101] The initial surface roughness height value is taken as 0. Since the present invention analyzes the terrain and the wind field, the surface roughness cannot be ignored, and the parameter here needs to be modified. Determine according to the roughness specification of B-type landform , and then convert to the roughness height according to Equation (12). In order to reflect the difference in surface roughness between the flat terrain surface and the micro-topography mountain body surface in the present invention, considering the influence of the actual ground vegetation, the surface roughness of the mountain body is set to 1.5 times that of the flat terrain surface, that is, the rough heights of the mountain body and the flat ground surface are taken as 1.2 m and 0.8 m respectively.

[0102] (3) Turbulence Model

[0103] The Navier-Stokes equations, abbreviated as N-S equations (Navier-Stokes equations), are the equations of motion that describe the momentum conservation of viscous incompressible fluids. When using the unsteady N-S equations to simulate and solve fluids, due to the need for finite element analysis, the solution of the flow field equations will be discretized into extremely small time and space steps for separate solutions, resulting in a huge CPU computational load, which does not meet the requirements of engineering calculations. It is necessary to simplify the N-S equations by introducing the Reynolds time-averaged simulation method. However, in the obtained time-averaged physical quantity control equations, the Reynolds stress is unknown and unsolvable. To make the equations solvable, it is necessary to introduce an assumed model to assist in the solution, and the model created under this assumption is the turbulence model. There are three commonly used turbulence models for numerical simulation of the flow field in Fluent: standard Model, Model and Model.

[0104] The standard Model is the most common model in engineering flow field calculations. It belongs to the semi-empirical formula type model. Its accuracy and convergence meet the requirements of engineering calculations for fully turbulent type flows, but the results of calculating strongly separated flows, large curvature flows, and strongly pressured gradient flow conditions are not accurate enough.

[0105] The Model adds a turbulence vortex equation compared to the standard Model. The RNG theory includes an analytical formula for the viscosity of low Reynolds number flows, making this model have higher accuracy in analyzing near-wall surfaces than the standard model. The Model has higher accuracy in complex shear flows, swirling flows, and separated flow fields.

[0106] The Model adds a turbulence viscosity formula compared to the standard Model and adds a transport equation for the dissipation rate. This model can more accurately predict occasions such as rotational flows, boundary layer flows with strong adverse pressure gradients, and flow separations. For the simulation of wind field conditions in the atmospheric boundary layer, it is widely considered that the turbulence model is more accurate. In the Fluent settings of the present invention, the turbulence model is used for solution.

[0107] (4) Determination of Computational Domain Parameters

[0108] After determining the values of several key parameters, the calculation domain of the numerical model can be established. According to the topographic features of the pass - hillside and the actual GPS ranging parameters, the micro - topography of this section is modeled through the ANSYS - Design Modeler platform. In the present invention, the maximum height of the micro - topography mountain is 1 km, the maximum length of the micro - topography mountain is 9.8 km, and the maximum width of the micro - topography mountain is 7.1 km. To ensure that the fluid in the calculation domain can fully develop and avoid the redundant influence brought by the flow field boundary, the height of the CFD calculation domain is taken as 5 times the mountain height, the length of the calculation domain is 8 times the mountain length, the width is taken as 5 times the mountain width, and the blockage ratio is controlled below 5%.

[0109] 2. Wind - induced deflection calculation model of transmission line - tower system

[0110] Under the action of wind load, the overhead transmission line and insulator string will deviate from the initial position, which may cause the distance between the conductor and the tower body to become smaller, or cause the air gap between the conductor and surrounding trees and building facilities to decrease. When the distance between the conductor and other objects fails to reach the electrical safety distance, accidents such as wind - induced flashover or even discharge to objects will occur. According to the actual operation experience of mountainous area lines, the forms of wind - induced tripping accidents are mostly the wind - induced tripping of individual line sections or individual towers in the micro - topography area, and there are also damages to the conductor - ground wire and insulators, while tower collapse and galloping are less. Therefore, to ensure the normal operation of transmission lines in the micro - topography area, it is crucial to accurately estimate the wind - induced deflection parameters of the insulator string and transmission line. Combining the micro - topography wind field obtained by numerical calculation and the finite - element model of the tower - line system, the wind - induced response of the tower - line system under micro - topography is studied.

[0111] 2.1 Wind load calculation of transmission tower - line system

[0112] The design of transmission towers usually considers the action of static wind load as the main load, and the dynamic wind load is equivalent to the static wind load for loading. The external forces acting on the transmission tower - line system are divided into two main parts: one is the wind load acting on the tower members of the pole tower, which is calculated at different heights and applied to the iron tower in segments; the other is the wind load acting on the transmission line, and then the load is transmitted to the cross - arm of the transmission tower through the insulator string. The concentrated load on the transmission iron tower is mainly the force exerted on the cross - arm insulator and the conductor, and the magnitude of the force depends on the load calculation parameters of the conductor.

[0113] For the three - tower two - line system model of N45 - N46 - N47, under the action of wind load, the wind load on the transmission conductor is the main load, and the wind load on the transmission tower is the secondary load. Here, the wind load on the insulator string is ignored. The present invention determines that the wind load acting on the transmission conductor is calculated according to (Equation 13).

[0114] (13)

[0115] (14)

[0116] Wherein, —— Wind load perpendicular to the line direction ( );

[0117] —— Standard value of reference wind pressure ( );

[0118] —— Wind pressure unevenness coefficient;

[0119] —— Wind pressure height variation coefficient;

[0120] —— Shape coefficient of the line;

[0121] —— Wind load adjustment coefficient;

[0122] —— Diameter of the conductor cross-section. If it is a bundled conductor, the sum of the diameters is taken ( );

[0123] —— Horizontal span ( );

[0124] —— Angle between the wind load and the line (°);

[0125] —— Wind speed at a reference height of 10 m ( ).

[0126] The wind pressure unevenness coefficient shall be determined according to the design basic wind speed in accordance with Table 1; the wind pressure height variation coefficient shall be determined in accordance with Table 1; the shape coefficient of the line , when the conductor diameter is less than 17 mm or in icing conditions (regardless of the conductor diameter), shall be taken, and when the conductor diameter is greater than or equal to 17 mm, shall be taken; the wind load adjustment coefficient of the conductors of 500 kV and 750 kV lines shall be determined according to the design basic wind speed in accordance with Table 2.

[0127] Table 1 Wind pressure unevenness coefficient and wind load adjustment coefficient

[0128]

[0129] Table 2 Wind pressure height variation coefficient for B-class ground roughness

[0130]

[0131] 2.2 Calculation of Wind-induced Displacement of Conductors

[0132] (1)Theory of Wind-induced Calculation of Conductors

[0133] Under strong wind conditions, the transmission conductors will shift a certain distance from the original state due to the continuous action of the average wind field. When the wind-induced displacement of the conductors in the span is too large, it is possible that the electrical safety distance between the overhead conductors and surrounding trees, buildings, other towers and other facilities is insufficient, resulting in discharge. Therefore, attention should be paid to the maximum wind-induced displacement of the conductors under strong wind conditions, especially under micro-meteorological conditions.

[0134] There are mountains, buildings, trees, etc. near the side conductors of the transmission line. The conductors in the span need to be checked for the maximum wind-induced displacement according to the maximum wind condition or icing condition to meet the clearance distance requirements.

[0135] Calculation of the Maximum Sag , which generally appears at the middle position of the span and can be measured from the cross-section diagram or calculated by formula.

[0136] (15)

[0137] ——Span, unit: m, obtained from the cross-section diagram or the bill of materials

[0138] ——Self-weight per unit length of the conductor and ground wire, unit: N / m

[0139] ——Horizontal tension of the overhead line, unit: N

[0140] ——Height difference angle, unit: °, obtained from the height difference between the front and rear conductor and ground wire suspension points and the span;

[0141] ——Specific weight of the conductor and ground wire, unit: N / m·mm2

[0142] 0——Horizontal stress at each point of the conductor and ground wire, unit: N / mm2

[0143] Calculation of Sag at Any Point , which can be measured from the cross-section diagram or calculated by formula.

[0144] (16)

[0145] ——Horizontal distance from one side of the tower, unit: m;

[0146] —— Span, unit: m, (i.e., f max )—— Maximum sag, unit: m.

[0147] Then the horizontal wind deflection displacement at any point is

[0148] (17)

[0149] The vertical wind deflection displacement at this point is

[0150] (18)

[0151] Where is the length of the insulator string, is the wind deflection angle of the conductor, is the wind deflection angle of the insulator string,

[0152] (2) Numerical simulation and comparison analysis with specifications

[0153] Compare and calculate the maximum wind deflection displacement of the conductor:

[0154] Condition 1: Based on the line parameters and specification formulas, calculate the maximum wind deflection displacements of the conductors in two spans under the action of horizontal wind loads respectively;

[0155] Condition 2: Based on the finite element model of the transmission tower-line structure, only load the horizontal wind load on the conductor, and numerically solve the maximum wind deflection displacements of the conductors in two spans;

[0156] Condition 3: Based on the finite element model of the transmission tower-line structure, according to the CFD wind field data of the micro-topography, load the horizontal wind loads under the micro-topography conditions in four sections, and numerically solve the maximum wind deflection displacements of the conductors in two spans;

[0157] Condition 4: Based on the finite element model of the transmission tower-line structure, according to the CFD wind field data of the micro-topography, simultaneously load the horizontal wind loads and vertical wind loads under the micro-topography conditions in four sections, and numerically solve the maximum wind deflection displacements of the conductors in two spans;

[0158] Calculation of the maximum wind deflection displacement of the conductor in the X direction in Conditions 2, 3, and 4 ( Figure 9 ) and the simulation results are shown in Table 3.

[0159] Table 3 Calculation of the maximum wind deflection displacement of the conductor (unit: m)

[0160]

[0161] As can be seen from the above table, the maximum wind deflection displacement of the conductor calculated in working condition 1 is relatively safe compared with the simulation calculation value in working condition 2; in working condition 3, the horizontal wind load of the micro-topography is considered. Compared with the wind load of the flat terrain in working condition 2, due to the acceleration effect of the micro-topography wind field, the maximum wind deflection displacement of the conductor has increased significantly, with the increase range being about 18-19%; and after considering the vertical wind speed of the micro-topography, the maximum wind deflection displacement of the conductor in working condition 4 has increased significantly by 23%-36% compared with that in working condition 2. It can be seen that the vertical wind speed brought by the climbing effect of the micro-topography airflow has an obvious adverse effect on the maximum wind deflection displacement of the conductor.

[0162] 3. Wind tunnel test of transmission tower-line system

[0163] 3.1 Theoretical basis

[0164] In the structural design of transmission tower-line structures, bridges, and building structures, it is only required that the structure has sufficient strength, stiffness, and stability under wind loads, that is, to ensure the safety, comfort, and durability of bridge structures and building structures; (this is different from the design of aircraft - striving to minimize the resistance of the moving air around it), mainly focusing on the flow around sharp corners and separated flows. Therefore, it is called "bluff body aerodynamics". Actual measurements of individual poles, buildings, and bridges have been carried out.

[0165] Since scale geometry leads to a series of problems of physical similarity, similarity criteria are the theoretical basis of wind tunnel tests. It should be noted that due to the geometric scale of the model, some physical phenomena cannot be accurately reflected, such as the Reynolds number effect. Therefore, in actual model test design, a series of trade-offs are needed to ensure that the main problems can be simulated. The research ideas of wind loads in the field of structural engineering are as Figure 10 shown.

[0166] Before conducting a wind tunnel test on a scaled model, it is necessary to design the test model according to the similarity criteria of relevant parameters according to the test purpose. Whether the similarity criteria can be met is the key to whether the results of the model test can be converted to the actual prototype, which is related to the success or failure of the model test. The similarity ratio of the aeroelastic model is determined comprehensively according to the size of the prototype structure, the size of the wind tunnel test section, and the flow field characteristics of the simulated turbulent wind field. Model design generally needs to meet the principles of flow field similarity, dynamic characteristic similarity, and shape similarity. From the requirements of flow similarity and the dynamic characteristics similarity of the structure shape, a series of dimensionless number similarity criteria can be derived from the structural dynamics equation or simple dimensional analysis. The wind tunnel test of the aeroelastic model of the structure in the atmospheric boundary layer is required to meet the dimensionless number similarity as shown in the table, and it is required to meet the dimensionless number similarity shown in the following table.

[0167] Table 4 Requirements for dimensionless parameter similarity

[0168]

[0169] In model tests, as long as these three dimensionless parameters are ensured to be equal for the model and the actual structure, the actual laws of the structure can be truly reflected. This is also the theoretical basis for wind tunnel tests of bridges and structural engineering. Geometric similarity should include geometric shape similarity and similarity of vibration modes and amplitudes. This is of great significance for the application and popularization of the results of structural wind tunnel tests. With the construction and development of high-voltage and extra-high-voltage transmission lines, the height of high-voltage transmission towers and the span of the lines are getting larger and larger. The transmission tower structure is more prone to aeroelastic instability under external excitation, resulting in large-amplitude vibrations of the transmission tower and even tower collapse accidents, seriously threatening the safe and stable operation of the transmission line. Due to the particularity of the transmission tower structure and working conditions, it is not easy to use the method of on-site test research. Therefore, measuring the aerodynamic characteristics and wind-induced dynamic responses of the transmission tower-line structure through aeroelastic model wind tunnel tests is still a relatively economical and effective research method at present.

[0170] The vibration equation of the transmission tower-line structure under pulsating wind loads can generally be expressed as:

[0171] (19)

[0172] In the formula: Ms, Cs, and Ks are the mass matrix, damping matrix, and stiffness matrix of the transmission tower-line structure respectively; , , , are the displacement, velocity, and acceleration vectors of the structure respectively; each term on the left side of the equation represents the inertial force, damping force, and elastic force of the structure itself in turn; the right side of the equation represents the horizontal load acting on the structure, including the aerodynamic force caused by fluid pulsation and the self-excited force generated by the influence of the structure's own motion.

[0173] Since both ends of the equation contain unknowns and cannot be solved directly. Therefore, the following assumptions are introduced to simplify the equation.

[0174] (1) The amplitude of the wind-induced vibration response of the transmission tower structure is much smaller than the characteristic size of its own structure, and it is considered to meet the statically determinate hypothesis condition proposed by Isyumov N. Therefore, the right side of the above formula can be approximately regarded as:

[0175] (20)

[0176] Where: is the aerodynamic force caused by the pulsation of the fluid itself and can be measured through a rigid model wind tunnel test; is the self-excited force generated by the influence of the structure's own motion.

[0177] (2) The self-excited force generated by the motion of the transmission tower itself is considered to be caused by the part of the pulsating wind load close to the main vibration frequency of the transmission tower structure. For After decoupling and ignoring the influence of high-order terms, the first-order approximation expression of the aerodynamic force term can be obtained as follows:

[0178] (21)

[0179] In the formula: , and respectively represent the correlation coefficients between the wind load and the acceleration, velocity and displacement of the transmission tower structure. After sorting, we can get

[0180] (22)

[0181] Where: Ma is the added mass related to the acceleration of the transmission tower structure in the aerodynamic force; Ca is the aerodynamic damping related to the aerodynamic velocity of the transmission tower structure; Ka is the aerodynamic stiffness related to the aerodynamic displacement of the transmission tower structure.

[0182] 3.2 Practical basis

[0183] So far, the technology that can fully simulate the atmospheric boundary layer in the wind tunnel is not sufficient. This requires that the structural characteristics of the model be as similar as possible to the prototype, and at the same time, the test flow field of the actual wind field be as similar as possible. The wind tunnel test can be carried out in the double-speed low-speed return boundary layer wind tunnel of North China Electric Power University. The physical objects in the test section of the wind tunnel are as shown in Figure 11 shown, the physical object of the operation console is as shown in Figure 12 shown, and the comparison diagram of the theoretical value and the measured value of the wind speed of the transmission tower-line system in the typical terrain turbulent wind field is as shown in Figure 13 shown.

[0184] The above has described the embodiments of the present invention in detail, but the described content is only the preferred embodiments of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the scope of the application of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for calculating the wind deflection of a transmission tower-line system, characterized in that, In this method, the maximum sag f of the transmission line is calculated by the following formula m :[[]]END]] In the above formula, l is the span, g is the self-weight per unit length of the conductor and ground wire, Τ is the horizontal tension of the overhead line, is the elevation angle, γ is the specific weight of the conductor and ground wire, and σ0 is the horizontal stress at each point of the conductor and ground wire; The sag f at any point is calculated by the following formula x :[[]]END]] In the above formula, x is the horizontal distance from one side of the pole tower, l is the span, and f m is the maximum sag; The horizontal wind deflection displacement \(x\) and the vertical wind deflection displacement \(y\) of any point are calculated respectively by the following two formulas: In the above two equations, λ is the length of the insulator string, and ξ is the wind deflection angle of the conductor. is the wind deflection angle of the insulator string.

2. The wind deflection calculation method for a transmission tower-line system according to claim 1, characterized in that In this method, the wind load acting on the transmission line is calculated by the following formula: W x = α·ω0·μ z ·μ s ·β c ·d·L p ·sin 2 θ; In the above formula, W x is the wind load perpendicular to the line direction, ω0 is the standard value of the reference wind pressure, α is the wind pressure non-uniformity coefficient, μ z is the wind pressure height change coefficient, μ s is the shape coefficient of the line, β c is the wind load adjustment coefficient, d is the diameter of the conductor cross-section, L p is the horizontal span, and θ is the angle between the wind load and the line; Where: ω0 = v 2 / 1600; \(v\) is the wind speed at a reference height of 10 m.

3. A method for calculating the wind deflection of a transmission tower-line system according to claim 1, characterized in that, In this method, the computational fluid dynamics process includes: 1) Establish the governing equations; 2) Determine the initial conditions and boundary conditions; 3) Divide the grid and generate the calculation points; 4) Establish the discrete equations; 5) Determine the control parameters; 6) Solve the discrete equations; 7) Judge whether it converges. When it converges, execute step 8) below. When it does not converge, return to step 2) above; 8) Output the calculation results.

4. A method for calculating the wind deflection of a transmission tower-line system according to claim 3, characterized in that, The Eulerian description of the fluid mechanics governing equations is shown in the following four formulas: In the above four equations, ρ is the fluid density, t is the flow time; τ is the viscous stress component existing on the surface of the infinitesimal element; is the velocity vector, and its components in the three coordinate directions are u, v, and w respectively; p represents the pressure on the fluid infinitesimal element.

5. A method for calculating the wind deflection of a transmission tower-line system according to claim 1, characterized in that, The micro-topographic conditions where the object calculated by this method is located include: pass-type terrain, high mountain watershed-type terrain, water vapor augmentation-type terrain, terrain uplift-type terrain, canyon wind tunnel-type terrain.

6. The wind deflection calculation method for a transmission tower-line system according to claim 1, characterized in that, In this method, the inlet boundary condition expressions of the micro-topographic wind field include the turbulent kinetic energy expression and the dissipation rate expression. The turbulent kinetic energy expression is as follows: The dissipation rate expression is as follows: In the above two equations, I(z) represents the turbulence intensity at height z, and L u is the turbulence integral scale, C u takes a value of 0.09, and K takes a value of 0.

42.

7. A method for calculating wind deflection of a transmission tower-line system according to claim 6, characterized in that In the above two equations, z g is the gradient wind height. Under B-type terrain, z b = 5 m.

8. A method for calculating the wind deflection of a transmission tower-line system according to claim 1, characterized in that, In this method, the surface roughness of the micro-topography wind field is described by the roughness height parameter k s and the roughness height parameter k s and the roughness length z0 satisfy the following relationship: k s = 20z0; where \(z_0\) is determined by the roughness specification of Class B terrain.

9. A method for calculating the wind deflection of a transmission tower-line system according to claim 1, characterized in that, After the calculation is completed, the calculation results are verified by a wind tunnel test of the transmission tower-line system.

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