The Orlando Utilities Commission (OUC) is a water and electric municipal utility with approximately 240 sq miles (622 sq km) of service area, and 150,000 electric and water customers. The bulk of OUC's power generation facilities are east of Orlando, and are connected to the transmission network by two 230-kV transmission lines between the Stanton Energy Center (SEC) and the Indian River Plant, and two 115 kV transmission lines between the Indian River Plant and the Pershing Substation. All four transmission lines share a common corridor and right-of-way.

The construction of a second unit at the SEC Power Plant resulted in the decision to build a new 230-kV transmission line. The new line from the SEC substation to the Taft substation was necessary to ensure that a single corridor-related outage would not separate OUC's generation resources from the urban load center.

Design Criteria The new 230-kV Stanton-Taft transmission line is 19.2 miles (30.9 km) long with approximately 13.5 miles (21.7 km) of new construction connecting to 5.7 miles (9.2 km) of existing transmission line. Because the new line was associated with the construction of SEC Unit 2, OUC included the environmental permitting of the transmission line in the supplemental site certification under Florida's State Environmental Impact Statement approval process.

The preliminary design and conditions of certification called for the new double-circuit transmission line to be constructed using tubular steel structures with only one circuit to be installed initially. The load flow studies indicated that 954 kcmil (483 sq mm) ACSR/AW cardinal conductor would be adequate for all future contingencies.

The OUC received approval of the supplemental certification from the Florida Department of Environmental Protection in December 1991, subject to numerous conditions of certification, which included:

Requirements for submitting final design drawings (plans and cross-sections) for each area of wetland fill or clearing.

Using low-soil impact equipment to clear forested wetlands.

Using best management practices for controlling soil erosion on slopes, including seeding and mulching within 72 hours of final grading. Providing wetland mitigation to offset the wetland loss and habitat degradation resulting from transmission line construction.

Preparing wetland impact/assessment, including descriptions of the nature and condition of the wetlands, acreage, type and quality, and aerial photos.

Preparing wetland mitigation plans for the type of wetland impacted along with documentation to assure reasonable success of planned wetland creation where required.

The project team had determined the structure locations and identified key hole clearing and wetland mitigation areas from aerial photography, but final design and certification were required before construction could begin.

The preliminary environmental impact analysis submitted with the supplemental site certification for Stanton Unit 2 indicated that approximately 13.19 acres (5.34 ha) of forested wetland would be cleared and 4.12 acres (1.67 ha) of wetland would be filled as a result of transmission line construction. One of the objectives for the design team was to reduce the wetlands impacts as much as possible during the final design.

In addition, to minimizing environmental impacts, the design team looked for improvement opportunities in the standards used for transmission-line design by OUC. The broad objectives for this review were to reduce costs and EMF levels and to maintain reliability.

Line Layout and Optimization In reviewing the proposed line layout, the project team decided that they could make some improvements if they could design a special corner structure that would avoid going around the water storage area and reduce the line length by about a 1/2 mile (0.8 km). A similar review of existing structure locations resulted in the elimination of a few short span structures and a reduction of the proposed number of overall structures from 87 to 77, which consequently reduced foundation quantities, structure maintenance pads and wetland fills.

A family of structure types allowed each structure to be designed as close as possible to its maximum design strength (Figs. 1 & 2). Design details for each of the nine structure families is shown in Table 1.

The project team also analyzed structure configurations for minimum EMF effects. They determined that a 10.5 ft (3.2 m) phase-to-phase compacted vertical separation would be optimum for the tangent structures. The double-circuit angle structure designs included analysis of two-shaft and single-shaft concepts. Results showed that it was economically and environmentally more advantageous to use the single-shaft, double-circuit concept (Fig. 3).

The team based minimum insulator length designs on 60 Hz operation with assumed moderate to heavy contamination levels, corresponding to 16 American National Standards Institute (ANSI) 52-3 class porcelain insulators. The insulator and hardware assemblies with electrical design data for tangent horizontal-vee assemblies is shown in Fig. 5.

Using a probability of 0.01 (one outage/100 operations), the switching surge air gap was chosen to be 5 ft (1.5 m). Electrical clearance envelopes using 6 psf (287 Pa) most frequent wind and switching surge air gap of 5 ft (1.5 m), 36 psf (1.7 kPa) extreme wind with 60 Hz air gap of 2 ft (0.6 m). Long and short crossarm lengths on angle structures were set using insulator swing conditions.

In evaluating the lightning performance of tangent structures, the team found no significant change in performance for the compacted (10.5 ft vs. 12 ft or 3.2 m vs. 3.7 m) vertical phase spacing at 10 ohm footing resistance. However, EMF field levels were improved (Fig. 4 and Table 2).

The structures were hot-dip galvanized and designed using National Electrical Safety Code (NESC) Light, ASCE extreme wind, snub-off, stringing and deflection loading conditions. The ASCE extreme wind was developed from 95 mph (42.5 m/s) - 23.1 psf basic wind adjusted for height, gust, and exposure to 36 psf (1.7 kPa) with an overload factor of 1.1.

The team members analyzed conductor sags and tensions for spans ranging from 600 to 1000 ft (183 to 305 m). Using preliminary structure locations, they determined a 900-ft (274-m) ruling span was optimum. They selected maximum initial conductor tension of 14,300 lb (6,486 kg) at high wind load considering vibration damper protection levels, NESC and everyday stress limits, and 18% of ultimate conductor breaking strength at 60øF (15.6øC) final. Conductor to ground clearance is to be 27 ft (8.2 m) at 212øF (100øC) final sag. They designed structures to accommodate two future 48-fiber shield wire loads, but initially one 12 fiber OPGW was installed.

Foundation Design The preliminary transmission-line, as submitted for permitting, called for pier foundations for all structure types. After the project teamed analyzed conceptual structure designs along with various foundation designs, it was determined that pier foundations were not an economical solution for the tangent structures SLA1, SLA2, SLA3 and SMA4. Socket-type caissons were selected for the tangent structures and drilled piers for the angles structures. Drilled piers would still be used for the angle structures because these structures have the largest transverse loads.

The change in foundation type alone saved US$100,000 and reduced environmental impact to several wetlands along the transmission-line route. The lower costs and reduced impacts were caused by the reduced volume of excavation and de-watering required for socket-type caisson foundations. Geotechnical information gathered for each structure location was used to establish foundation requirements for each structure type.

The drilled pier foundations were typical for angle and dead end structures. Crews augured the hole and poured concrete around the reinforcing cage. They inserted anchor bolts with their templates in place and then allowed the concrete time to set (Figs. 6 & 7).

The socket type caissons are a section of 3/8 inch thick galvanized steel casing that has been coated inside and out with polyurethane to resist corrosion. Installed driving ears allow the attachment of the vibratory hammer to the casing. In this particular application, crews drove the casing into the ground leaving 1.5 times the diameter exposed (Fig. 8). Then they lowered the pole into the casing and used aligning bolts to adjust for plumb. Finally, they filled the 3 inch (7.6 cm) void between the pole and casing with high-strength grout to allow for uniform load transfer.

Construction The OUC awarded procurement contracts for structures with casings and anchor bolts, conductor, OPGW, insulator and hardware assemblies along with dampers and conductor accessories during 1994. It awarded the construction contract and began construction in February 1995. The total budget for the line construction including access road construction, sales tax, OUC costs and contingencies was US$8.6 million. Actual line costs totaled US$1.6 million (18.6 %) less than the estimated budget, and the line was completed three months ahead of the scheduled April 1, 1996 deadline. TDW

Phil Clark is the director of the Transmission Engineering and Planning Division at the OUC, which he joined in 1970. His responsibilities include transmission system planning, budgeting and responsibility for all transmission and substation capital projects. Clark's experience includes system relaying, metering, SCADA and programmable controller application system retrofits and upgrades. He is a member of the IEEE and has served as chairman of the FCG Relay Subcommittee.

Gungor Yildirim is principal engineer in the Electric and Thermal Plant Facilities Practice of RW Beck, Inc., Orlando, Florida, U.S. He received the BSEE and MSEE from the University of Missouri. He joined RW Beck in 1990 and has more than 27 years of service on numerous projects throughout the world with extensive experience in high voltage and extra high voltage transmission line planning, design, management research and development. He has been responsible for more than 2000 miles (3219 km) of 500-kV line design projects.

Ivan Clark is a senior director of RW Beck, Inc.'s Environmental Services Division, which he joined in 1975. He has the BSEE degree from Kansas State University and is a registered professional engineer in five U.S. states and one U.S. territory. His responsibilities have included project management of planning and design projects, and environmental licensing/studies for electric transmission/generation projects, cogeneration projects and industrial projects.