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Computer-aided design of ã li ransmission nes Alain H. Peyrot University of Wisconsin Madison WI 53706 USA Eric M. Peyrot Power Line Systems Madison WI 53705 USA Thomas Carton Electricite de France Paris France This paper gives an overview of structural and geometric design con- cepts for electrical power lines. It presents a new integrated com- puterized environment for generating new designs or evaluating existing ones. In that environment, productivity can be radically improved and the risk of errors is reduced. New design concepts that achieve greater economy can be implemented. The manner in which such diverse utility groups as surveyors, planners, engineers and CAD technicians interact with each other can be improved. Electrical parameters or power lines are only discussed insofar as they affect the line geometry. Keywords: transmission lines, CAD An electric power line is a very complex electrical and structural system for the transport of energy over large distances. The sole purpose of the structural system is to support a given amount of aluminium and keep its distance from the ground and nearby obstacles above specified values. The amount of aluminium in a conduc- tor is dictated by the system planners based on anticipated energy transfer needs. Figure 1 summarizes the broad categories and sub- categories of design parameters normally considered. The line route is a corridor made up of straight line segments between corner points (line angle points). An ~angle structure' is required at each angle point: its loading is primarily affected by the cable tensions. Structures between angle points are 'tangent structures' which are not affected by the cable tensions, but by the wind and weight spans which they must support. New line route locations can be optimized by comparing total installed costs of alternatives. However, there is over- whelming pressure in developed areas of Europe and the US to reuse existing right-of-ways through upgrading. The pressure is often imposed by environmental con- cerns and prohibitive acquisition costs. Once selected, a route is completely described by its geometry and soil characteristics, traditionally in the form of plan and pro- file paper drawings. This article shows the advantages of using a GIS-type electronic representation of the terrain and the line corridor. The line voltage affects its future electrical losses, electric and magnetic fields are required clearances to ground and obstacles. The climate 0141-0296/93/040229-09 © 1993 Butterworth-Heinemann Ltd in the area of the line is the basis for the calculation of design loads to resist wind, snow or temperature changes. This is done through minimum code requirements ~, or better, by using recently proposed reliability-based procedures 23. The security of a line is related to the ability of the utility to restore service quickly after failure. This requires that the possibility of cascading failure be minimized and that a capability for rapid restoration be in place. Long cascades can be limited by specifying adequate longitudinal strength at Route Voltage Climate Security Ruling span Conductor Structure families Spotting Construction/maintence Environment, right-of-way, line angles, profile, obstacles, soil properties Standard values, losses, required clearances, Electrical effects Wind, ice, snow, temperature, codes, reliability Anticascading measures, conse- quences of failure Range of spans Type, bundling, size, tension Structural concepts and materials, top geometry, insulation, grounding type-tangent, angle, dead end detailed components design heights, foundations Structure locations on terrain Safety, equipment Figure Line design parameters Engng. Struct. 1993, Volume 15, Number 4 9 Computer aided design of transmission lines: A. H. Peyrot et al. every structure or by inserting stronger structures at fixed intervals. The ruling span is a single theoretical span length which is used to calculate cable tensions for various loading conditions. The cable in the ruling span simulates the behaviour of the entire length of cable in several spans between adjacent dead ends in the real line. At the preliminary design stage, the ruling span is an assumption. Once a line is spotted the actual ruling span can be calculated. If the assumed value is not within 10% of the actual value, the assumed ruling span is nor- mally revised. This is a trial and error process best handled by computer. The conductor is the most impor- tant part of the line. It typically account for 35% of the total cost of a line 4~. Conductor selection or optimiza- tion is therefore a very important design step. The selec- tion of conductor tension involves a trade-off between the increased costs of angle structures (affected by cable tensions) and the decreased costs of tangent structures (smaller sags requiring smaller heights) at higher con- ductor tensions. Excessive tension should be avoided as it increases the conductor's vulnerability to fatigue from aeolian vibrations. The calculation of cable tensions is not a trivial matter as it must take into account nonlinear behaviour due to creep and nonrecoverable stretch under large loads ~'7. Transmission structures are manufactured from all the primary construction materials: steel, wood and con- crete. The structural schemes can be self-supporting or guyed. A structure top geometry is dictated by insulation considerations, often based on an accepted risk of flashover and concerns for electric and magnetic fields. Structures of a particular configuration and strength are normally grouped in a family The family includes dif- ferent structure types: tangent structures of different heights, some angle structures capable of specific line angles (say 0 ° -5 °, 5 ° -45 °, and above 45 °) and dead- end structures. It is uneconomical to include too many different types of a family. In design calculations, the strength of a structure can be checked by comparing its actual wind and weight spans to corresponding allowable values or by comparing calculated stress resultants in all or a few critical components to allowable values. This is discussed further in connection with the contents of structures' files. Current methods for the detailed design of structural components and founda- tions are available in guides and standards s ~2 Structure locations on the terrain have traditionally been determined by an experienced 'spotter' using a template-based graphical method. This manual method is clumsy and is replaced by either interactive computer spotting or automatic spotting. Optimized automatic spotting is a complex subject which is discussed in a companion paper in this issue~. Construction and maintenance operations should be well defined at the line design stage. They affect the choice of structure through design loads and geometry. The design items in Figure 1 include a very large amount of information which needs to be accessed or manipulated by different people: system planners for the voltage selection; electrical engineers for losses and insulation co-ordination; surveyors, drafters, archeologists and environmentalists for the route selec- tion; structural and geotechnical engineers for structural and foundation design; and operation personel for con- struction and maintenance matters. Traditionally, this information has been handled on paper, causing ineffi- ciency and risk of errors. This article shows how the information can be organized for efficient computer pro- cessing. Access to many design items may be needed for the verification of a single component. For example, the force or moment in a particular member of an angle structure depends on the model of that structure and its supported loads. The loads themselves depend on the location of the structure (line angle, geometries of back and ahead spans), the specified climatic conditions, the ruling span, the cable tensions under the climatic condi- tions and other load assumptions. Some design parameters are imposed by nature, codes or standards t, others can be selected by the designer. The selection may be aided by optimization algorithms similar to those discussed in detail elsewhere ~ The need for integration The use of computer programs to assist designers of electric power lines is widespread. There are numerous programs for calculating loads, performing sag-tension calculations, analysing structures, spotting the structures on a ground profile, producing plan and profile sheets, etc. However, within one orgamzation, line design pro- grams are often separate entities, possibly having been acquired over periods of time by different groups. As a result, the programs may not interact well with each other: there is potential for inefficiency and risk of error when the output information from one program is manually converted to input for another program. With the recent developments of powerful microcom- puters or workstations, mouse-based interactive graphics, electronic surveying and mature CAD systems, all at increasingly affordable costs, it is now possible to integrate all the line design tools into a single computer environment. An engineer sitting at a single workstation can start with a map of a proposed line route, build a complete three-dimensional model of the line, perform all the engineering calculations and finally enable the automatic production of plan and profile sheets and printed reports. All the work can be done interactively, using mouse-based interactive graphics. Such an environment was developed by the authors on MS-DOS machines (386 or better). Computing integration and increased power also pro- mote new ways of performing certain engineering calculations. Many existing programs are computerized implementations of older manual procedures which required simplifying assumptions. Without the con- straints 6f simplifying assumptions better procedures can now be used to obtain cheaper designs. Basic design concept: three-dimensional model of entire line The overall guiding concept behind the new computer environment is accessibility to a detailed three- dimensional model of an entire line and its right-of-way. Such a model should include a detailed representation of the terrain under the line, all structures, all insulators, and all cables in all spans. Building and modifying the model should be done interactively, preferably through 230 Engng. Struct. 1993, Volume 15, Number 4 Computer aided design of transmission lines: A. H. Peyrot et al. the use of a mouse. The three-dimensional approach requires managing a considerable amount of data. Therefore, extreme care was taken to break all input data into standard libraries of cables, structures, loads, clearance criteria, etc. The items in the various struc- tures and cables libraries are treated as objects which are mouse selected and which can be interconnected to other objects. Figure 2 shows the three-dimensional model at the centre of the environment. Surrounding it are the various input, output and processing blocks which are discussed in this paper. Terrain representation A three-dimensional GIS-type representation of the ter- rain was adopted for its versatility and compatibility with modern electronic surveying equipment and mapp- ing concepts. Terrain points and other points of interest (such as obstacles) are described by their co-ordinates X,Y,Z) in a global co-ordinate system and an attribute A (or feature code). The attribute is used to define the characteristics of a particular point or obstacle. It is used as a pointer to tables of allowable clearances, depending on a particular phase voltage. The attribute is also used to assign different symbols which can be drawn on the computer screen or final paper drawings to identify par- ticular points or obstacles. The assignment of attributes and the corresponding graphical representations are completely flexible. This terrain representation will be referred to as the XYZA model. For spotting purposes and to reduce the amount of ter- rain data which is accessed during a single line evalua- tion, XYZA terrain data are automatically transformed into corridor data which include station, offset, eleva- tion and feature code of the reduced number of points located inside the corridor SOZA representation). The corridor is defined interactively by its width and by selecting with the mouse the comer (alignment) points in the plan view (see Figure 3)._It should be noted that the SOZA terrain representation is vastly superior to current representations by centre and side profiles and associated clearance lines. It enables calculations of detailed vertical and lateral clearances to any object from any phase. It also provides the necessary detailed information for drafting a plan view inside the CAD system. Moving or adding a corner structure in an existing design is not easily handled by most existing programs. The move causes instantaneous changes in stations and offsets, and a change in the centre line profile. However, with the ability to switch back and forth bet- ween XYZA and SOZA terrain representations, a design can be updated automatically following a corner move. The corner move is simply made by pointing at the structure with the mouse and dragging it to its new loca- tion on the plan view as illustrated in Figure 4. In pro- grams that deal only with data on a centre profile, the change described above is very difficult. Design criteria Design criteria affecting the spotting of a line are nor- mally defined by combinations of wind, ice, snow and temperature which must be withstood by the structures or under which minimum clearances should be maintained ~. Such climatic conditions are described in load files. Minimum clearances to terrain points or obstacles are defined for given voltages in a clearance file. Climatic conditions and clearance files are part of libraries that can be shared across several projects. They are created and updated interactively with special input screens. Input Processing Corddor errain Ground + Obmdu T ~Ut Ubrarles t~ -I 8'D MOdelOf I iii:iiArchM~iiiiiiii i i ii: i ii Figure Overview of integrated environment for line design Engng. Struct. 1993, Volume 15, Number 4 231 Computer aided design of transmission lines. A. H. Peyrot et al. Y Alignment Corner / Surveyed Points Profile Corridor Width X ) y Attachment points ã Set I ã Set 2 [] Set 3 Figure Defining alignment by clicking on terrain point with mouse ~lm New Position For ~ ructure #4 ã Before Structure Move © After Structure Move Figure 5 x HT Tower height and top geometry Figure4 Moving a corner point by dragging it with mouse Structures A structure file concentrates in one location all the geometric and mechanical design information (excluding cost) that pertains to a given structure type and height. Costs, which vary with time, are kept in a separate file. The structure file also contains data on its insulators and foundation. More specifically, the struc- ture file contains the following. Structure height and top geometry: So that the positions of any point on any cable in any span be known in space as any structure is added or moved, it is necessary that the cable connection points be well defined. This is done by specifying the height of the structure HT and its top geometry as shown in Figure 5 for a latticed tower. The top geometry includes sets of attachment points for ground wires and conductors. A set of cables is an ensemble of cables with identical properties and ten- sions. For the example of Figure 5 the two ground wires are part of cable set 1, the three cables in the left circuit are part of set 2 and the three cables in the right circuit are part of set 3. The positions of the attachment points /'or a set are described in a local co-ordinate system x,y located at height HT in the transverse plane of the structure. Within one set, the attachment points are numbered consecutively to identify specific phases. Switching phase numbers in otherwise identical struc- tures can be used to model phase transposition. hzsulators: For each attachment set, one connector type must be defined. Clamps, strain insulators, suspension links or insulators, V-strings or horizontal Vs can be used. Rigid post insulators should be treated as exten- sions of the structure, i.e. the tips of the posts are struc- ture attachment points. Connectors and insulators are described by their strength, geometry and allowable swing angles. The inclusion of insulators in the structure file, rather than in a separate insulator file, is dictated by the fact that allowable swing angles are structure specific. Thus, a structure such as that shown in Figure 5 with its insulators attached, is treated as a three- dimensional object which can be located at any point in the corridor by clicking on that point with the mouse. Once the structure is located and its orientation imposed, the attachment points of all the cables (at the bases of the insulators) are perfectly defined in space. Strength: Four different methods are available to describe the strength of a structure in a structure file. Method 1 is the oldest and most common: it is used in traditional manual spotting and by most automatic spotting programs. It relies on the most elementary con- cept of allowable wind and weight spans. For each of 232 Engng. Struct. 1993, Volume 15, Number 4
