GIS application for power transmission line siting: An llustrative case

GIS application for power transmission line siting: An llustrative case

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Tyeb Pervaiz
GIS Development
Analyses such as the selection of suitable areas, the optimum path finding, the profile analyses, the engineering design of towers and wires, and the cost estimation can be done using GIS.

Analyses such as the selection of suitable areas, the optimum path finding, the profile analyses, the engineering design of towers and wires, and the cost estimation can be done using GIS.

In the last few decades, the electric power industries have been developing power transmission systems to follow up with the rapid growth of the power demand. On the other hand, the suitable site for new transmission lines has been getting restricted, because of development of rural areas and the growing concern over environmental issues. Analyses such as the selection of suitable areas, the optimum path finding, the profile analyses, the engineering design of towers and wires, and the cost estimation can be done using GIS. This will help planners and engineers in the environmental and engineering analyses for transmission line siting.

Application System
In general, the process of the planning and design of transmission lines consists of the following 5 phases.

  • Planning: The master guidelines of route constructions are settled based on the long-range power supply plan. The outline is determined for each transmission line planned, which includes voltage, number of lines, starting and ending substations.
  • Survey: Information about natural environment, geological features, local communities and regulations in the area of interest etc. is collected in this phase and several alternative routes are compared in terms of environmental impact, technical issues and cost of construction.
  • Basic Route: Basic route is determined by the position of each tower along the proposed routes and interference of radio wave caused by the transmission line is estimated. All this is done using a medium scale toposheet.
  • Detailed Route: A photogrammetric surveying is performed along the basic route. Based on the results, the detailed position of each tower is determined in the large scale (1:2,000) . And engineering design process follows, which includes the determination of tower type, tower height and supporting devices, and the cost estimation.
  • Route for Implementation: In this phase, the detailed field surveying is performed along the determined route. The towers, wires and basement of towers are designed.

The computer system developed here supports phase 1 to 4, and consists of 5 subsystems. Figure 1 shows the outline of the system

Each project execution components has been described below:

Data Entry System This system installs, checks and edits the geographical database used in all other subsystems. The database includes Topographical Maps in 1:50,000 (raster images), Environmental Information (Coverages), Land Information Database (Governmental boundaries, roads, railroads, rivers, lakes (coverages), altitude (GRID), Photogrammetric Maps in 1:2,000 scale (raster images), DTM (GRID) etc.

Route Zone Evaluation Supporting System The purpose of this subsystem is to select the zone, called the “route zone”, which is considered suitable for a transmission line in terms of environmental impact and regulations. The route zone is determined in the medium scale of about 1:50,000.

The functions of this system are:

  • Display and plot environmental database
  • Create the optimum route
  • Create suitability map
  • Create aerial view
  • Estimate the construction cost

Basic Route Evaluation Supporting System The position of each tower is determined interactively referencing the topographical maps, the suitability maps and the optimum route computed above. Figure 2 displays the functions of the subsystem.

Detailed Route Evaluation Supporting System Once the basic route is determined, the photogrammetric surveying is performed along the route to make planimetric, topographic maps, and DTMs in the large scale of 1:2,000. Based on the database, the detailed position of each tower is determined. Furthermore, the system has various functions to perform the engineering analyses. Figure 3 shows the functions of this subsystem.


Wave Interference Evaluation Supporting System A transmission line may cause the interference in television and microwave communication. The purpose of this subsystem is to predict the wave interference caused by the determined transmission line. The system has functions as shown in Figure 4.

Methodologies
Suitability Map and Optimum Route The existing process of the site selection is evaluated and as per the need, the environmental information can be classified into categories like, natural, social and technical environment. Each of these categories contains layers as mentioned below.

  • Environment containing habitats of endangered species, national parks, etc
  • Social environment containing view points, scenic areas, cultural assets, temples, shrines, agricultural promotion area, forest area, cities, district for urban planning, airports, etc.
  • Technical environment contained faults, dangerous district for collapse, areas of snowfall, thunderstorm, salty breeze, wind pressure, etc.

An index should be introduced to represent the relative difficulty of the route construction, based on the experience of engineers. The index may be defined as:

Index = 4 [Negative Control Point (The route must not pass through)]

Index = 3 [Route construction is strictly regulated, or has a great impact.]

Index = 2 [Route construction may be permitted, or has a moderate influence.]

Index = 1 [Route construction has a slight influence.]

Index = 0 [No problem for route construction.]

The index number can be assigned to each polygon feature of the environmental database, according to the difficulty of route construction. For point or line features, a sort of buffer procedure can be performed using GRID function, and the index number can be assigned according to the distance from the center. The suitability map can be created by overlaying and summing up all indexed layers. This procedure will utilize GRID functions, and is shown in Figure 5.

The resulted suitability map can be considered to represent a sort of impedance for route construction. Using GRID COSTDISTANCE functions, the optimum route can be computed based on the suitability map as shown in Figure 5.

Determine Tower Height In the detailed route, height of each tower can be determined to minimize the cost of construction of the whole route. It can be accomplished as shown in Figure 6.

  • First, the possible combination of tower height should be computed for each span. The required clearance of wire should be ensured over the ground and structures.
  • Then, the combinations of tower height are joined along the whole route. The result is constructed as a network model with a turntable. The cost of each turn is computed, which depends on the tower height and the tower type.
  • Finally, the optimum path can be derived using NETWORK PATH command, based on the cost at each turn. The resulted path corresponds to the most cost-effective tower height combination.

Profile along the Detailed Route Once the detailed route is determined, the profile along the route can be plotted as one of the final results. This plot includes the profile along the route, the structures under wires, the required clearance, the dip of wires, and the towers. Figure7 shows an example of the profile plot. Those features are plotted using specific command modules of the GIS software. The curves of structures, clearance and wire dip can be computed.

Estimate the Construction Cost The cost estimation can be accomplished using a spreadsheet package. The specifications of towers and wires, can be read by the spreadsheet. Further, in-house can be used to create a ledger of the route or to estimate the construction cost.

Conclusion
Several extensions can be incorporated into the system. The extensions include the automatic siting of each tower, the use of the result of detailed-field surveying and the simulation of realistic views using CAD/GIS packages.

Reference
Masahiko Murata,Systems Engineering Center, PASCO Corporation, TOKYO, JAPAN