The proliferation of power producers around Houston, Texas, U.S., is creating a heavy demand for transmission line access to transfer power to the open market. It is important to consider options for increasing overall line capacity, in that the existing lines already are experiencing heavy electric loads. These options include new construction, thermal upgrading of existing conductors, reconductoring or using bundled conductors.

New technological advancements are allowing engineers to model and analyze existing lines as well as to design new lines. New surveying techniques and improved software allow the user to model a line in three dimensions. The software integrates surveying, imagery, drafting, procurement and construction details to establish realism in the preliminary stages of design. Engineers can then overlay actual photographs to explore any construction issues.

For existing lines, modeling provides the ability to evaluate the line's capacity and to determine the feasibility for thermally upgrading the circuit. Upgrading implies re-rating the current-carrying capacity of a line to allow a higher permissible conductor temperature that would permit increased loading without impairing clearances because of increased sag. Beyond the thermal upgrading of a line is the possibility that existing structures have sufficient safety factor in their design. This means that there is an untapped mechanical capacity available to allow for restringing with larger, heavier, single or bundled conductors.

Modeling a Transmission Line

The modeling of a transmission line involves data regarding terrain, geography and clearance restrictions. This requires a thorough survey of the line's route. New technology, of which aerial surveying has become the method of choice, has made it easy to obtain survey data quickly and efficiently. By using technologies involving photogrammetry or laser surveying for data collection, engineers can obtain x, y and z coordinates with great precision. This process is especially valuable when it is necessary to collect large amounts of data in a short period.

Once the survey data have been collected, modeling the transmission line in three dimensions can proceed using PLS-CADD software by Power Line Systems Inc. (PLS) of Madison, Wisconsin, U.S. The software translates the survey data into a readable format using data points that are feature coded with labels as conductors, structures, attachment points, trees, roads, structure centers and points of intersection. These labels enable the software to check clearance violations during the modeling process. The feature coded data points can be filtered in and out of the software to display certain aspects of the transmission-line information at different stages of the modeling.

To analyze structures for their capacity characteristics, use other PLS programs such as:

  • Tower, for modeling and analysis of lattice towers

  • Pole, for modeling and analyzing wood, steel and concrete poles.

Each of these programs interfaces directly with PLS-CADD, enabling the designer/engineer to simultaneously determine multiple loading scenarios and the design capacity of the structure by entering clearance violations and other design criteria into the software.

In existing transmission-line design, structures are spotted in accordance with the survey data. In new transmission-line design, several options exist where PLS-CADD allows for optimization, or the optimal spotting of structures based on parameters that include the cost of the structure. PLS-CADD also considers terrain restrictions, such as marshland or expensive urban areas to spot the new transmission line. The engineer always has the option to spot each tower manually by selecting structure locations based on his or her experience and familiarity with the selected route.

Once structures are modeled and correctly positioned, the conductors are strung three-dimensionally on the structures with conductor specifications entered into the PLS-CADD along with insulator/clamp length, capacity and weight information. Design criteria are always based on regional code requirements. All of these conditions are integrated to create an efficient and cost-effective advanced line design. Upon completion of the line design, plan and profile drawings are automatically generated, including actual imagery and all necessary construction information with a total bill of materials and current pricing included.

Reliant Energy Uses Modeling

Houston-based Reliant Energy has experienced large increases in demand on its transmission system. The system is tasked to a level never before anticipated because of deregulation and the desire of independent power producers to generate and sell power in the marketplace. Several new cogeneration plants located around the Houston ship channel provide electricity and steam to adjacent plants, and sell additional generation capacity on the open market. The large concentration of petrochemical facilities in the area produces one of the largest electric-load densities in the nation. Although transmission of power to adjacent plants is not a problem, the heavy load density created when transmission of large blocks of power onto the grid has become a significant issue for the company.

To address the problem, Reliant retained Burns & McDonnell Engineering Inc. (Kansas City, Missouri, U.S.) to survey and model hundreds of miles of transmission lines. When the project began in November 1999, it consisted of 600 miles (966 km) of line; by January 2001, it had grown to more than 1500 miles (2414 km) of line.

The surveying, performed by Aerotech, LLC (Bessemer, Alabama, U.S.), incorporated the use of helicopter-based lasers, which were installed in a laser pod mounted under the belly. At about 500 to 750 ft (152 to 229 m) above ground, the helicopter flies over the top of the line with the laser scanning sideways. In conjunction with the helicopter survey, a fixed-wing aircraft, at about 2000 ft (610 m), provides aerial photogrammetry of the route. A minimal amount of traditional ground surveying was subcontracted to a local firm.

The preliminary stages of the project required map generation to indicate the location of the transmission lines to be flown. Reliant provided the approximate latitude and longitude of all structures, which were superimposed on digital USGS quad sheets. Flight plans for the helicopter and fixed-wing aircraft were generated using these quads. Prior to the aerial surveying, coordination meetings were held with air-traffic control at Houston's two major airports since several of the lines were in the approach to the airports. Additionally, Reliant's customer-service representatives notified several plants around the ship channel that an aerial survey was under way that would result in low-flying aircraft over their facilities.

Ground personnel from Aerotech reviewed and filtered the raw data obtained from each day's flights and forwarded the data to its office for final processing. Burns & McDonnell received the filtered data, along with weather and line-loading information, within a few days of the flight and proceeded with the modeling process. While field data were being assembled for analyzing physical characteristics of conductors and structures, Reliant's control center was monitoring the ampere loading on all of the project's line segments. For facilities that were not directly monitored by supervisory control and data acquisition (SCADA), calculated data or data from a station demand recorder were utilized. With all of this information, Burns & McDonnell produced models of the lines in PLS-CADD, which were sent to Reliant for analysis.

Often, when thermal up rating provided insufficient increases in capacity, reconductoring or bundling of conductors was considered. For example, the line between the T.H. Wharton Plant and the North Belt Substation, having two 927.2 kcmil ACAR “Greeley” conductors, was reconductored with three 959.6 kcmil ACSS/TW “Suwannee” conductors. At 345 kV, the line's capacity was increased from 1374 MVA to 2907 MVA. In addition, Reliant reconductored its line between the Crosby Substation to the Atacocita Substation, replacing a single 795 kcmil AAC “Arbutus” conductor with two 959.6 kcmil ACSS/TW “Suwannee” conductors to increase the rating at 138 kV from 264 MVA to 776 MVA.

In both cases, the original lines were rated based on conductor temperatures of 120°C (248°F), while the replacements were rated because of conductor temperature of 180°C (356°F). The higher conductor temperatures of the ACSS are feasible because the conductor is made with fully annealed aluminum strands that carry practically no mechanical load. The strands rely on the steel core for its mechanical strength, which minimizes sag at high conductor temperatures. The designation ACSS/TW is described as Aluminum Conductor, Steel Supported with trapezoidal-shaped wires. With existing PLS-CADD models already in place, Reliant could apply new conductoring scenarios and determine how best to achieve required capacities.

Because of increased mechanical loading associated with reconductoring, many lattice towers were in danger of being loaded beyond their design capacity. In these cases, it was necessary to analyze the structures for potential structural failures and, when possible, to modify certain structural components to overcome any structural deficiencies. Because of construction restrictions, the members that can be modified are limited. Obvious construction restraints prevent the replacement of leg members and girts. However, cross bracing and other minor structural components are easily replaced with similar size angles that are simply thicker, minimizing the amount of detailing required. This ability realized significant benefits for many towers designed and constructed in the past before some of the current design software was available. These towers were usually over-designed and were typically able to accommodate the heavier loads imposed by the larger conductors used to increase the line's capacity.

In Conclusion

Technology and ingenuity have partnered to launch a new era in transmission-line design where great strides have been made in both the modeling and analysis of existing lines and the design of new lines. With new surveying techniques, advancements in software and in engineering expertise, transmission-line design can be approached in a new, more cost-effective style. With the current energy market looking uncertain, and with the need to move power in the most cost-effective and efficient way possible, a need exists for fast-tracked, well-designed transmission systems. Modeling these systems provides the ability to maximize transmission capacity.

Close coordination is necessary among all parties involved when undertaking projects that involve modifications of transmission lines when increased capacity is desired. The projects at Reliant Energy exemplify the teamwork that existed among the three companies that worked to solve the problems encountered during the course of the reconstruction. Typical problems included delays in receiving ground-surveying data; laser overheating because of high-ambient temperatures and radio-frequency interference with the helicopter onboard computers. Weekly conference calls were held to discuss the progress and to preempt upcoming problems that might present themselves. The work process was eventually improved to the point where each company's effort was seamlessly incorporated into the final product Reliant required.

John E. Lionberger is the general manager of Burns & McDonnell's Houston, Texas, U.S. office, having previously served as manager of transmission and distribution. He has consulted on a variety of issues involving design and operation of high- and medium-voltage system design, power quality, substation automation and fiber-optic networks. Prior to joining the firm in 1995, Lionberger worked at Central Illinois Public Service Co. as a SCADA and controls engineer. He graduated from Bradley University with a BSEE degree. He is a registered professional engineer in Texas.

Leslie Duke is a structural engineer in the transmission and distribution division's civil/structural department at Burns & McDonnell. Her responsibilities include steel design, standard and site-specific equipment supports and foundation design. In addition, Duke is involved in transmission structure modeling and design and project management. She graduated from Texas Tech University in 1994 with a degree in civil engineering.