Georgia Transmission Corp.'s new Delta Cat tower reduces right-of-way footprint and facilitates helicopter maintenance.
Georgia is one of the fastest-growing states in the United States, and this year's electric demand is expected to be nearly double what it was 20 years ago. Fortunately, planners of the state's electric grid began preparing well in advance for future power needs.
At the beginning of this decade, four new 500-kV lines and numerous supporting 115-kV and 230-kV lines were mapped out for the integrated transmission system (ITS) in the largest update to the bulk system since the 1980s. The ITS is a 17,500-mile (28,164-km) network of transmission lines jointly planned and operated by Georgia Power, Georgia Transmission Corp. (GTC), MEAG Power and Dalton Utilities.
GTC, which builds and maintains high-voltage infrastructure for 39 of the state's 42 electric membership cooperatives, and Georgia Power were each slated to build two 500-kV lines. In 2004, GTC began work on its first 500-kV line in more than 15 years in an effort to fortify the grid in east-central Georgia.
Because it had been so many years since the utility's last 500-kV construction project, GTC evaluated the possibility of upgrading the tower design. Rather than settle for minor modifications, GTC assessed a complete redesign that could solve several issues: conductor blowout limiting span length, tree contacts, flashover outages caused by bird contamination, and live-line maintenance capabilities. The project team also saw an opportunity to improve the tower's efficiency, both in electric performance and capital costs.
While a project team studied, routed and planned the line, a parallel collaboration was under way to design and fabricate a new tower featuring a delta configuration with self-supporting lattice. Known as the Delta Cat, the tower top includes additional supports that give the delta window the appearance of a cat's head. The two complex processes were integrated, and the project was delivered early and under budget.
Existing Tower Designs
Horizontal tower design came about in the 1950s and has been used widely throughout the United States ever since. Current configurations in Georgia were developed by Georgia Power in the 1960s with very little computer modeling. Alabama Power introduced a delta family tower design in the 1970s.
These designs allowed for limited live-line maintenance. Within a quarter span of the tower, a helicopter could not operate safely between the overhead ground wire and the conductor. Under extreme wind conditions, the horizontal configuration also presented span limitations with blowout approaching the 150-ft (46-m) right-of-way.
Embarking on a New Design
In late 2004, the ITS agreed to the need for GTC's 39-mile (63-km) 500-kV line in east-central Georgia to connect substations in Thomson and Warthen. GTC began to think early in the process about reworking the existing tower design because of its limitations. Planners approached Georgia Power and discovered its standards team had already initiated a research study to evaluate construction costs of an updated design. The two organizations formed a joint working group and later brought in consulting engineers from Black & Veatch to consult on the design and procurement process.
Georgia Power had recently completed cost comparisons for upgraded 500-kV lines. Models projected the costs for four different configurations: updated horizontal, the Alabama Power delta, single-pole delta and horizontal on guyed H-frames. For an estimated US$40 million project, the Alabama Power delta configuration represented a $3 million cost savings — nearly an 8% reduction in the cost of the project. Subsequently, GTC took the lead on furthering the redesign effort with a goal of implementing the new delta design on Thomson-Warthen.
With slight modifications to the Alabama Power delta configuration, tree clearance and conductor blowout issues could be resolved. But rather than stop there, planners assessed the tower from the ground up, literally accounting for everything from the tower footprint to its material makeup to the impact of the phase configuration on long-term maintenance.
Improving Access for Maintenance
From May 2006 to September 2006, the joint working group initiated meetings with maintenance professionals from GTC, Georgia Power and Alabama Power. Also attending were stakeholders from research and development companies, tool and hardware manufacturers, and other contributors to high-voltage-line building and maintenance. Objectives for the new line from a maintenance perspective were to allow for bare-hand hot stick and helicopter live-line maintenance. Based on these requests, designers enlarged the delta window and raised the overhead shield wire attachment points to create additional space between phases to allow a helicopter access between shield wires and conductors.
Maintenance professionals said bird-caused contamination is a major issue from a performance standpoint. In fact, for GTC, this contamination is the single most-common cause of outages on 500-kV lines. Birds can coat insulators, which can cause a conductive path in high humidity or misty conditions. In extreme cases, contamination causes arcing over the insulator, known as flashovers. To date, several methods, including plywood and corrugated plastic piping, have been installed to shield insulators from birds.
Analysis of the tower series led designers to consider additions. The legacy series running angle structure accommodated line angles up to 15 degrees. After that, double-deadend structures had to be used. Additionally, the heavy deadend structure in the legacy series was designed to accommodate up to 115-degree line angles. Thus, introduction of a larger running angle structure and a smaller double dead-end structure allowed for more efficiency (i.e., reduced total project steel weight).
Not only did engineers make use of a more-complete family of angle structures, they also provided interchangeable angle members between English and metric units in design and detailing. While smaller members that bear less weight are often fabricated in lower-grade 36-ksi steel, engineers specifying the tower materials found that it was more cost effective to build all members of the Delta Cat with higher-grade 50-ksi steel usually reserved for the larger members.
Similar to the Alabama Power delta, the GTC Delta Cat phase configuration brings the outside phases closer together and the center phase higher, allowing for greater span length with respect to conductor blowout and reduced tree contacts.
Designers also included a new feature on the towers: a metal platform to protect insulators from bird contamination. Dubbed the “buzzard shield,” the platform traps contamination on its solid surface, preventing flashover outages.
Underlying the big picture of improving each tower's physical functionality was the potential to improve electrical performance behaviors. The geometric configuration of the delta design improves phase voltage and current balancing, and provides an ancillary benefit of reducing electric and magnetic fields. GTC also opted to switch from porcelain to glass insulators, which proved less expensive for this project.
With the Delta Cat design nearly complete on paper and with bid parameters in hand, GTC initiated the procurement process in search of an entity experienced with in-house design, detail, testing and fabrication. It quickly became apparent the United States and Canada did not have a single company with adequate in-house resources for all the services being sought.
GTC and Georgia Power evaluated prequalification bids from 11 international companies from as far away as India and Turkey. Eight were invited to submit proposals, five proposals were received and two were short-listed for consideration. In 2007, the project team hired Mexico-based SAE to design, test and fabricate 158 towers for the Thomson-Warthen line.
Tower testing services went international, as well. The Delta Cat tower testing took place at SAE in Belo Horizonte, Brazil. Beginning in December 2007, every tower type was tested to ensure it could bear the physical load necessary.
To test the towers, the various tower designs were fitted with an array of cables and subjected to a mechanical stress test that simulated extreme conditions. The test cases simulated ice and wind loads on the conductors and towers.
Small- and large-angle structures exhibited numerous premature failures under the tension of the cables. Revisions and multiple tests were necessary to finalize the design. GTC concluded the failures were a result of modeling or fabrication errors, or a combination of the two factors. Tests concluded in September 2008. The total cost for tower design, fabrication and testing was $7.2 million.
GTC initiated a comprehensive outreach effort to inform key stakeholders about the line through public meetings, close contact with elected officials and negotiations with individual property owners. Distribution cooperatives Washington EMC and Jefferson Energy assisted in executing the communications strategy. GTC used the EPRI-GTC Siting Methodology developed at the beginning of the decade to scientifically evaluate the study area and identify preferred corridors in consideration of the built and natural environment as well as engineering requirements.
Construction encountered no public controversy and very little landowner opposition when construction began. Only 2% of 124 acquired land parcels required the use of eminent domain. The route runs through rural areas of middle Georgia, home to expanses of timber and working farms. The majority of easements were negotiated with these types of commercial landowners.
Because of this limited need for condemnation proceedings, GTC completed the land acquisition in less than a year at a cost of $3.9 million. These factors contributed to a 25% savings from the estimated acquisition costs, representing about 10% of total savings for the project.
Irby Construction was the primary contractor of the Thomson-Warthen line and contributed further to cost savings through active project management on the ground. Even when it meant deviating from standard procedure, decision making by project managers was based heavily on cost effectiveness. Because of the size and scope of the project, GTC assumed management over processes typically overseen by contractors and decided to employ direct procurement on other aspects of the construction.
GTC sought competitive soil sample analysis to assist in determining the materials necessary to stabilize the tower. The comparison provided a clearer picture of the consistency of the terrain and impacted the design and strategy for pouring the tower foundations.
The project team assessed the terrain and determined the amount of rock necessary to establish construction access. GTC bought rock directly and managed the placement on-site. This hands-on approach led to a $1 million cost savings.
Debris cleared from the right-of-way was chipped and used for stabilization. The excess was burned, a practice regarded as environmentally sound in rural areas and preferable to hauling debris through wetlands and other sensitive areas.
Additional on-site inspectors worked the length of the line so the portions of the project that were miles apart were advanced simultaneously. A third-party inspector also conducted a comprehensive inspection of the line by helicopter and found very few construction issues. During this inspection, a new X-ray technique was used to check the integrity of conductor splices along the line.
GTC procurement also worked closely with wire vendor Southwire to tap into the commodities market at optimum times. Buying aluminum after the commodity market price dropped slashed a half-million dollars from the cost of 358 miles (576 km) of wire.
A Fine Result
Before the project began, GTC estimated a cost of $60 million. Instead, work to complete the line totaled $47 million. The final cost was about $1.2 million per mile, 22% under budget. The project team finished the line eight months early. It will be energized in the summer of 2010.
Given the situation — that it was GTC's first project of this scope in nearly 20 years and a new tower design was incorporated — it might have seemed unrealistic to predict an early and under-budget delivery. Through a concerted effort, a commitment to communication and a willingness to rethink the possibilities, GTC delivered the Delta Cat tower — the new 500-kV tower for Georgia.
Gregory Starks (email@example.com) is a project manager for bulk transmission line projects for Georgia Transmission Corp. His 30 years of industry experience ranges from substation design for both transmission and distribution to construction and project management. For the last decade, Starks has overseen substations and transmission projects ranging from 115 kV to 500 kV. Starks holds a BSEE degree from the Georgia Institute of Technology. He is a member of the IEEE and a registered professional engineer.
Herb Payne (firstname.lastname@example.org) is a project manager for bulk transmission line projects for Georgia Transmission Corp. He entered the utility industry in 1977. His experience includes work as a design engineer, project engineer, project manager and department manager for transmission lines, substations and construction inspection. Payne holds a BS degree in architectural engineering from the University of Texas at Austin. He is a member of the American Society of Civil Engineers and a registered professional engineer.
Quan He Fan (email@example.com) is the principal engineer of transmission line design at Georgia Transmission Corp. He has been working in transmission line design ranging from 46 kV to 735 kV, both in the U.S. and internationally, since 1982. He graduated from Shanghai Jiao Tong University with a bachelor's degree in engineering mechanics in 1982. He also holds master's degrees in applied mathematics and civil/structural engineering from University of Waterloo and McGill University in Canada, respectively. He is a registered professional engineer.
Alabama Power www.alabamapower.com
Companies mentioned in this article:
Black & Veatch www.bv.com
Dalton Utilities www.dutil.com
Georgia Power www.georgiapower.com
Georgia Transmission Corp. www.gatrans.com
Irby Construction www.occusoft.com
Jefferson Energy www.jeffersonenergy.com
MEAG Power www.meagpower.org
SAE Towers www.saetowers.com
Washington EMC www.washingtonemc.com