3-D modeling boosts transmission capacity

John E. Lionberger, P.E.,
and Leslie Duke, BSCE,
Burns & McDonnell Engineering

North America’s bulk transmission system is being pushed to the limits. California has been experiencing rolling blackouts due to inadequate generation and system inability to transmit power from producer to consumer. Significant increase in the capacity of the bulk transmission system is required to maintain integrity of the nation’s electric system while allowing producers to transmit power to the consumer.

Impending deregulation and the uncertainty of recovering stranded assets kept utilities from substantially upgrading transmission systems from the late 80s through the 90s. Utilities typically deferred major transmission projects waiting to see how infrastructure costs would be recouped. When the wholesale generation market deregulated, large numbers of generation facilities were built in locations never considered by utility transmission planners. Excess capacity that was typically designed into the bulk transmission system was becoming rapidly consumed by various factors including normal load growth not accompanied by new major line construction and new merchant power plants.

Increasing capacity

There are various methods to increase capacity of the bulk transmission system. The following shows approximate costs per circuit mile for each option.

  • New transmission line
    115-kV, steel pole, double circuit, 715.5 kcmil, $160,000 per circuit mile.
    230-kV, steel pole, double circuit, 1,113 kcmil, $230,000 per circuit mile.
    (costs do not include right-of-way acquisition)
  • Thermal upgrading
    Approximately $7,000 per circuit mile for aerial surveying, modeling and analysis.
    (cost does not include line or R.O.W. modifications)
  • Reconductoring
    115-kV line, to 715.5 kcmil, $80,000 per circuit mile.
    230-kV line, to 1,113 kcmil, $120,000 per circuit mile.
  • Bundling
    115-kV line, 715.5 kcmil, $130,000 per circuit mile.
    230-kV line, 1,113 kcmil, $200,000 per circuit mile.

New transmission line construction is expensive, time consuming, often insufficient for the needs of independent power producers and fiercely opposed by the public and by regulators. The Public Utility Commission in Texas recently rejected a permit for a 345-kV line deemed critical by the Electric Reliability Council of Texas. Cheaper, quicker methods are required to increase system capacity for new plants while new line construction should be reserved for long-term system expansion.

One way to increase line capacity with minimal or no construction is through thermal upgrading. Increasing the thermal rating of a line may be as simple as analyzing the line to see if any capacity remains.

In the past, lines were often rated very conservatively. Using modern techniques to analyze lines often reveals that thermal ratings can be increased with little or no physical modifications.

Reconductoring and bundling of conductors are also cost-effective methods for increasing the rating of an existing line compared to new construction. However, increased loading due to increased conductor weight or stringing tension may require some relatively inexpensive modifications to existing structures.

Modeling

Modeling the transmission line in 3-D is an important step before upgrading. An example of state-of-the-art modeling and engineering software is Power Line System’s PLS-CADD program. Models are generated in PLS-CADD using feature coded data points. These data points are x-y-z coordinates with associated feature codes-identifying labels. Flexible feature-coded data includes conductor attachment points, pole or tower base locations, points on the conductor for constructing the catenary, ground points for constructing ground profiles, and obstacle data for obstacles within the corridor (or under conductor at maximum blowout).

Feature-coded data is derived from either a survey of the line or digitized data from plan and profile drawings. However, data from plan and profile drawings can be questionable. The preferred method for collecting the data is surveying. However, traditional ground surveys are impractical from a cost and time standpoint for a long transmission line or even a short line in a congested urban area, costing up to $30,000 per mile. New technological advances have made aerial surveying a cost-effective, accurate and quick method. The two predominant methods are photogrammetry and laser surveying, each requiring some supplemental ground surveying. Both surveying methods are georeferenced and have similar accuracies. Aerial surveys cost approximately $3,000 per mile and can cover more than 100 miles a day. While the line is being flown by either method, line loading and ambient weather data is recorded, at regular intervals and used to calculate the operating temperature of the conductor during the survey.

After collecting and processing, the surveying, line loading and weather data is entered into PLS-CADD software and used to create a 3-D engineering model of the transmission line that includes terrain, structure, and conductor information. Then, terrain parameters and engineering standards are set. With standards in place, PLS-CADD uses feature-coded structure information to spot structures and subsequently sag in the conductors between dead-end structures. Once the conductor is strung and loading conditions applied, equivalent loads are generated at the insulator attachment points on the structure. These loads are used to evaluate the structural integrity of the transmission line.

Three-dimensional models-preferred to generic as the full effects of transmission line upgrading can be simulated-are generated and actual weather and line-loading data are used to depict line tensions. A completely modeled transmission line can then be analyzed for untapped thermal capacity, underutilized structural strength, and additional clearance allowance.

Case study

Reliant Energy, formerly Houston Light and Power, experienced many of the problems other utilities are facing. Diminishing capacity on their transmission system combined with the proposed addition of several thousand megawatts of new generation within their service territory caused the need for large increases in capacity. Located in the ship channel crowded with petrochemical facilities and burdened with large electric load densities, the company had difficulty transferring large amounts of power onto the grid to sell in the market. Reliant began a survey and modeling project for existing structures.

Burns & McDonnell Engineering Inc. assisted Reliant in the project to cover several hundred miles of their 69- through 345-kV transmission system. The project began in November 1999 and by January 2001 more than 1,500 miles of transmission line had been surveyed and modeled. Aerotec LLC, of Bessemer, Ala., performed aerial surveys using helicopter-based lasers and fixed-wing aircraft. A supplemental ground survey was contracted locally. With coordination of flight planning, map generation, data verification, and archived SCADA data, models were completed upon receipt of the ground survey, weather data and line loading.

Reliant analyzed the models and used a combination of thermal uprating, reconductoring and bundling of conductors. The increased load on the structures from the new conductor size caused overloading on several of their towers. Burns & McDonnell modeled the overloaded towers using

PLS-Tower software and determined required structure modifications. Reliant was well on its way to having the entire system modeled, enhancing the ability to engineer further additions and modifications to their system.

Lionberger, general manager, and Duke, staff civil/structural engineer, are located in the Houston office of Burns & McDonnell Engineering Inc. Contact Lionberger at jlion@burnsmcd.co; Duke at lduke@burnsmcd.com.

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