The 3D model construction method of overhead transmission line based on GIM

With the continuous progress of power grid digitization, Building Information Model (building information modeling, BIM) technology can no longer meet the application display requirements of power grid engineering. Power grid information model (grid information model, GIM) technology uses parameterization to build a 3D model, which reduces redundancy and is conducive to the digital transfer of the project. This paper briefly analyzes the structure of the GIM data model. It carries out research on 3D parametric modeling of overhead transmission line ground wire, tower, foundation, and insulator string based on GIM. It uses a 3D graphics engine to realize the visual rendering of ontology 3D models. On this basis, a 3D scene construction method of overhead transmission line engineering based on GIM is proposed, which provides reliable technical support for digital power grid applications.


Introduction
In 2002, Philip Bernstein first put forward the concept of BIM.BIM has been widely used in the construction industry, and its meaning has been gradually expanded to three levels: BIM model, BIM modeling, and building information management.With the help of the IFC standard [1], the application and promotion of BIM technology realize the exchange of design data between different software [2].In contrast, the international standard of the Industrial Foundation Classification of Construction Industry (Industry Foundation Class, IFC4) lacks relevant content on power grid engineering, which is not conducive to the promotion and application of power grid digital construction.
GIM was first proposed in 2013, which mainly refers to the digital information model of power grid engineering, which digitizes the constituent elements of the power grid based on GIS and integrates the information of each element in the whole life cycle with the information model as the carrier, so as to realize the efficient, accurate and comprehensive application of information [3].GIM is a standard data system that runs through the design, construction, debugging, and operation stages of the power grid construction project.It originates from the design stage and generates matching graphic model and attribute model data by means of digital forward design technology; moreover, it enriches and improves continuously in the follow-up stage and finally realizes the digital transfer and application of the project [4].In recent years, the power grid industry has proposed to comprehensively carry out the three-dimensional design of power transmission and transformation engineering, which puts forward new requirements for the development of design technology.The 3D design of the power transmission and transformation project started relatively late, especially the 3D design of overhead transmission line engineering (hereinafter referred to as "line engineering").It differs from the design of field stations, such as power generation and substation engineering.First, the depth, convenience, and sharing degree of 3D modeling of wire, tower, and other ontology in line engineering must be matched with the practical application.Second, there is a close relationship between ontology and geographic information of line corridors [5].The construction of a three-dimensional model of line engineering is the basis of three-dimensional design and its subsequent stage of work.
This paper analyzes the GIM data standard and studies the 3D parametric modeling method of line engineering ontology.On this basis, the 3D scene of line engineering is constructed.The hierarchy and references of the line project GIM file are shown in Figure 2. 1) There is a unique project file list, project.cbm, in the root directory of the CBM folder of the GIM file, which is the entrance to the whole GIM model.It stores the engineering information and references to the first level (full line) engineering model cbm file.The other *.cbm files contain a five-level system.The first-level Full-line level line contains several second-level segments, and the second-level segment contains several third-level stretch segments.The third-level tension section of the tensile segment grade contains several fourth-level equipment groups (tower group, conductor group, crossover group).The Device group level fourth-level equipment group (tower group, conductor group, crossover group) contains a number of materials, equipment, and facilities.Each level *.cbm file corresponds to a property file *.fam.The DEV file reference for the device is stored in the device-level *.cbm file.

GIM data model structure
2) The DEV folder contains two types of files, one is the file *.fam, which describes the physical model, and the other is the file *.fam which describes the properties of the physical model.*.dev can refer to *.phm (combined model) or other *.dev, *.fam file format related to the corresponding device.The number of attributes and field names stored in *.fam files corresponding to different physical models are different, which generally include design parameters, design freeze parameters, product parameters, construction parameters, test parameters, and operation and inspection parameters.
3) *.phm files are included in the PHM folder.*.phm stores references to *.mod, *.stl model files, or other *.phm, and describes the spatial transformation matrix parameters.
4) The MOD folder contains *.mod files or *.stl files, which are geometric information describing geometric model units, including basic entity descriptions and parametric descriptions.*.mod describes the corresponding information through xml nodes, entity identifies solid objects, and solid objects contain information about geometry.

3D parametric modeling of the overhead transmission line body
Three-dimensional modeling of line engineering ontology mainly refers to the construction process of main models of engineering equipment and facilities, such as ground wire, tower, insulator string, foundation, and so on.Based on the three-dimensional parametric modeling method based on GIM, by describing the key parameters of different types of ontology, the computer generates a three-dimensional model in accordance with the design logic.It has high efficiency, truly reflects the relationship between equipment components, and can assemble the existing primitive models with strong reusability, which accords with the forward design process advocated by the current three-dimensional design technology.From the point of view of meeting the requirements of 3D design, according to the complexity of the model, the 3D model of power grid engineering can be divided into three types: general model, product model, and assembly model, as shown in Table 2

Production and processing
Related professional software format In the process of transmission line engineering design, two types of general model and product model are mainly used.Except for the insulator string product model available *.stl file, the other models are required to be parameterized in *.mod format.

Ground conductor modeling
The modeling of ground conductors is generally generated by calculation.In line engineering, the wire is suspended by poles and towers, and the shape is a "catenary".Therefore, the catenary formula can be used for calculation.
The formula for calculating the arc sag point where the coordinate O point is located at the ground wire suspension point A: Maximum sag calculation formula: in the formula, f is the sa g of the wire,  is the height difference angle, x is the horizontal distance from each point of the wire to the ordinate, y is the vertical height from each point of the wire to the Abscissa, 0  is the horizontal stress of each point of the wire,  is the specific load of the wire, l is the span, and h is the height difference.
The connection line of the two ends is divided into enough segments.The arc sag point corresponding to each division point is calculated by using Formula (1) with one endpoint as the origin.The maximum arc sag point is obtained according to Formula (2) and inserted into the arc sag array sequentially.Then, each node is connected in turn, the shape of the ground wire is drawn, and according to the outer diameter size, a three-dimensional model of the ground wire is formed.

Tower modeling
Both the general model and the product model of the tower adopt the parametric model, and the origin of the model is the geometric center of the longest leg.The general model is generally represented as the tower single-line model.The product model is the tower three-dimensional solid model with angle steel or steel pipe style information.In the tower *.mod file, the body, legs, and long and short legs of the tower are represented by the keywords Body, Leg, and SubLeg, respectively.R, P, and G are used as the marks of nodes, members, and hanging points in each part, and each unit occupies a row, as shown in Table 3.
Table 3 G, hang point type, hang point name, a, b, c coordinates relative to the model origin Taking the tower product model as an example, the steps of rendering the model using Direct3D components in the computer are as follows (the basic and insulator string rendering principles are similar): 1) P coordinates of the two ends of the bar are found according to R. 2) All the face vertices of the member model are obtained according to the limb width, limb thickness, and limb orientation, and stored in the structure of vertex numbers and vertex coordinates.All points are stored as an array of vertex positions in this form.3) All faces of the segmented member are triangular patches.The triangle vertex index numbers are stored in a timely structure.All triangular patches are stored as vertex index array in this structure; 4) The vertex position array in 2) and vertex index array information in 3) are transferred into Direct3D 3D model rendering object device; 5) Each vertex is processed, cropped, perspective back culling, rasterization, filtering, pixel processing and other processes within the device, and finally the 3D model is displayed on the screen.6) According to G, the hanging point information of the tower is marked and rendered.

Basic modeling
The tower foundation is generally constructed in the form of a product model.The connection between the outer profile and the tower foot of each foundation type can be decomposed into a number of basic three-dimensional elements (such as cuboids, cylinders, etc.).Therefore, taking the center of the top surface of the foundation model as the origin, the basic element parameters that make up the foundation are described and grouped.The *.mod format data is generated and rendered into 3D solid graphics.

Insulator string modeling
For the three-dimensional model of an insulator string, the preliminary design stage of line engineering requires a general model, with the whole string as a unit, using parametric modeling (*.mod file).The construction drawing design stage is the product model (*.stl file), which is composed of insulator and hardware models.The model contains geometric information, connection information, and attribute parameters.In the process of parametric modeling, the connection point between the insulator string and the tower is taken as the origin.When there are multiple hanging points, the origin is the intersection of the horizontal plane where the highest hanging point is located and the central lead hammer line of the insulator string.The parametric description of insulator string includes type ID, wire split number, split information (arrangement mode, spacing), string purpose (wire string, ground string), string type (overhang string, tension string), V string angle, U string connection length, hardware length, connection number, arrangement mode, insulator information (radius, number of pieces, material, etc.), equalizing ring information (number, height, radius, Location), connection point information, etc., through 3D graphics.

Conclusion
Starting from the data structure of the GIM standard file, this paper studies the 3D parametric modeling, model visualization, and engineering 3D scene construction of line engineering ontology.In practical application, the 3D model construction based on the GIM standard has the following advantages.First, the 3D model is constructed by parameterization, and the model style is edited by changing the parameter values, which can improve the modeling efficiency at the same time.The amount of data storage is reduced for a single structured model.Second, the GIM model standard adopts hierarchical reference, which reduces redundancy.Third, the GIM model highly unifies graphic data and attribute data, which is conducive to data query and retrieval, but also facilitates information extraction and sharing.At present, there are still some shortcomings in the GIM model standard, such as the statistics of engineering quantity and material quantity and the correlation of related cost indicators.At present, the standard is more oriented to the design stage, and the definition and application depth of the model in the stages of construction, debugging, production, and operation is not deep enough.In the next step, data penetration can be realized through flexible information expansion for it to better serve the whole life cycle application of power grid engineering.

Figure 2 .
Figure 2. Reference structure of overhead transmission line engineering model Line project GIM standard format file design uses four file directories, CBM, DEV, PHM, and MOD, for data entity content storage.Each folder stores the corresponding types of files, CBM folder storage *.cbm, *.fam, DEV folder storage *.dev, *.fam, PHM folder storage *.phm, MOD folder storage *.mod, *.stl.
*.stl A file format used to represent triangular meshes in computer graphics applications, which cannot be used in parameterized model files

Table 2 .
. Comparison of three types of 3D models . Description of parameter information of tower model