Background
In the mid-2000s, the Public Utility Commission of Texas, along with the Electric Reliability Council of Texas (ERCOT), coordinated an extensive construction program of transmission lines to support the expansion of renewable wind energy sources in western Texas, referred to as the Competitive Renewable Energy Zones (CREZ). CREZ required the construction of 2400 mi (3862 km) of transmission lines, built by nine different transmission service providers (TSPs). Cross Timbers Transmission (CTT) was one of the TSPs in the CREZ program involved in designing, building, and operating 235 mi (378 km) of double-circuit, 345-kV transmission lines, a substation, and a one series compensation 345-kV station. ERCOT established the line capacity requirements, and each TSP was allowed to design its respective structure configurations and conductor selection.
CTT performed the conductor evaluations, applying its design criteria for loadings, structure configurations, and capacity requirements, and selected a two-bundle 1590-kcmil ACSS Falcon conductor for its 345-kV design. Preliminary studies were performed for various structure configurations and types, including lattice steel towers, steel monopoles, and a hybrid configuration of monopole tangents and lattice towers for angle structures. Including the considerations for foundation installation, the hybrid option was chosen.
Route selection identified the need for a half-mile (0.805-km) river crossing of the Prairie Dog Town Fork of the Red River. The new line route paralleled an existing wood H-frame, 138-kV line, which had several structures located in the river itself. Based on permitting concerns, it was decided that the new line would need to span the river and avoid in-river structures. Note the very flat terrain at the crossing location (Figure 27), which necessitated that all ground clearances be maintained by tower height only—that is, no elevation difference at the banks of the river.
Figure 27. River crossing showing flat terrain
The resulting span was 2400 ft (732 m). Another constraint was that the tower heights could not exceed 200 ft (61 m) to avoid FAA consideration. This span length and limiting height constraint required additional examination of conductors to select an appropriate conductor to provide the needed capacity as well as the strength to support the span and limitations on the permissible sag. Figure 28 shows the crossing structure being erected in sections.
Figure 28. Tower erection showing cross-arm geometry
Conductor Selection
For the crossing span, CTT considered three different conductor constructions, as follows:
- ACSS/HS285: ACSS conductor with higher strength steel core to provide mechanical support and reduced sag
- ACCC: composite core conductor
- ACCR: composite core conductor
Table 5 provides a comparison for the different conductors considered for the river crossing showing the relative numbers for the maximum tension, MOT, sag at MOT, sag under extreme ice, and tower heights required to maintain clearances. Note that the sag for extreme ice is close to or exceeds the sag at MOT for several of the conductors considered. The comparison starts with the ACSS Falcon, which is the conductor on the remainder of the line. At the time the project was being executed, CTC did not manufacture a Falcon stranding, so a Lapwing was offered as the ACCC conductor. Based on the comparison, the ACCR Pecos was chosen for the river crossing. Note that if the ACCC Falcon had been available, it also would have been a competitive solution.
Table 5. Alternative conductor comparison
| ACSS Falcon | ACSS/HS285 Falcon | ACCC Lapwing | ACCC Falcon | ACCR Pecos | |
|---|---|---|---|---|---|
| Max tension (lb) | 24,000 | 28,000 | 27,000 | 29,000 | 32,000 |
| MOT (°C) | 200 | 200 | 200 | 200 | 200 |
| Sag at MOT (ft) | 171 | 142 | 131 | 121 | 106 |
| Sag at extreme ice (ft) | 165 | 140 | 148 | 139 | 117 |
| Tower heights for clearance (ft) | 235 | 205 | 210 | 200 | 185 |
Construction
The final river crossing configuration included the crossing span and an adjacent span on either side of the river to a dead-end structure for the conductor bundle change from ACCR to ACSS (Figure 29).
Figure 29. River crossing towers with the conductor being strung
Both the nominal transmission line construction and the river crossing used an HTLS conductor selection. The ACSS was applied for the open terrain portion of the line where its lower sag at MOT offered a competitive solution. The river crossing required a higher strength conductor to accommodate the long span and the height limitation to avoid FAA permitting requirements. This long span was also driven by sag requirements incurred under extreme ice loading when the projected sag governed over sag conditions at MOT. In the river crossing application, the added strength afforded by the ACCR conductor type met the project requirement.
Bibliography
Cannon, D., “River Crossing.” T&D World, July 2015.
Cannon, D., and Chapman, N., “Application of HTLS Conductor for a 2400’ River Crossing Span.” TSDOS, 2015.