Today's energy industry faces the same economic pressures as other businesses, with the added factor that many costs are difficult to control. There are not a lot of bargains when it comes to the indexed price of steel, aluminum and other raw materials closely related to the overall cost of a transmission line or substation.

Because of this reality, Avista Utilities is actively seeking areas where it can realize significant savings. One business sector delivering new efficiencies for the utility is advanced computer-aided design tools with interactive 3-D modeling. These advanced computer tools are speeding up the design process, improving labor contracting and construction, and lowering overall project costs.

Uprating to Fiber in the Shield Wire

Avista produces and distributes electricity and provides natural gas to a 30,000-sq mile (77,700-sq km) service territory in eastern Washington, northern Idaho and parts of southern and eastern Oregon for a population of 1.5 million. The utility typically has used microwave and land-line communications to remotely control and monitor its electric transmission grid, but that is changing. Avista has committed to an overall strategy of replacing microwave equipment with fiber-optic cables and routing those cables on existing transmission lines.

This strategy has prompted a significant amount of new design work to modify existing transmission lines. For example, Avista's West of Hatwai project added more than 200 circuit miles (322 km) of fiber-optic cable to meet federal guidelines for communication redundancy, and many of those miles required the addition of optical ground wire (OPGW) to existing transmission towers. To efficiently model and analyze existing transmission structures, as well as design and test modifications, Avista is using digital orthophotography and leading-edge computer software to design and construct upgrades more efficiently.

Today, Avista is adding fiber-optic cable to its 36-mile (58-km), 230-kV North Lewiston-Shawnee transmission line (from Lewiston, Idaho, to Colfax, Washington). The project involves engineering analysis of 270 structures and the evaluation of both OPGW and all-dielectric self-supporting (ADSS) cable segments. Ultimately, this additional fiber-optic circuit will be used as part of a digital communications network to control and monitor all of Avista's bulk transmission lines and substations.

Using Technology to Subdue Complexity

Transmission line design is considerably more complex than many think. In addition to obvious factors like conductor sag and tension, engineers must account for cable contraction and expansion due to temperature changes, determine structure capacities, analyze insulator swing and conductor blowout (wind-induced sway), evaluate conductor ground clearance and mid-span conductor separation, as well as consider many other physical and electrical factors. Avista has demonstrated 3-D modeling is a powerful tool that can be used to analyze a number of factors simultaneously. This approach allows engineers to conduct a majority of the required analysis without the aid of additional ground surveying, field monitoring, or other invasive and time-consuming methods.

The computer-aided design process begins with using light detection and ranging or other aerial photogrammetry techniques to render an accurate terrain model, or a digital elevation model, including ground contour and survey monument information. Avista conducts flights as needed for its service territory and usually finds ways to cost share with federal, state and local agencies, other utilities and resource-based industries. The flights, together with elevation information assembled from control surveys and photogrammetric techniques, become a layer in Avista's ESRI-based geographic information system (GIS). In the Lewiston transmission project, orthophotography and GIS information were imported directly into TL-Pro Design Studio software, a product of Pondera Engineers, recently acquired by Trimble.

Tower locations were superimposed on the terrain models, including information related to structure capacities and conductor attachments. The transmission line design solution is able to recognize tower footprints, and the GIS import feature adds all the information in the appropriate format. Work that used to take several days is now being accomplished in a matter of minutes. The completed 3-D model renders transmission towers projected onto an elevation surface and includes trees, buildings, fence lines, roadways and other physical features.

Once the terrain and structure model is developed, engineers then move forward with evaluating the tower capacities and conductor interactions associated with adding the fiber-optic cable. Without the aid of computer design tools, many of these calculations would require days or weeks to complete. The visual aspects of model-based design are critical to reducing the overall design time and minimizing the impact of design errors. Conventional 2-D flat-profile views cannot provide the wealth of information often required to fully analyze the impact of tower revisions and cable interactions. The software's 3-D rendering tool literally allows the engineers to “fly” along the line route and check conductor ground clearances, evaluate conflicts with physical features, consider the overall aesthetic impact of structures and conductors, and share this information with both internal and external stakeholders. Most importantly, design issues and conflicts that are noted in the office get resolved in the office.

Prior to implementing TL-Pro, Avista used manual design methods — drafting boards, sag templates, hand calculations and a variety of in-house developed computer programs. Though reasonably effective, the design process was slow and tedious, often forcing engineers to use both graphical and computer-aided design tools along with aerial photographs, ground-based survey data and information derived from multiple on-site visits.

The design phase of the North Lewiston-Shawnee project lasted only four months. Previously, this level of project design would have lasted 12 to 18 months.