P591-421-5049.

An alternate station-service supply line, at 12.5 kilovolts, takes off on the northwest corner of the powerplant building at elevation 5679, and is carried on the 138-kilovolt transformer circuit towers C1-T1 and C1-T2, as shown on figure 153. From tower C1-T2, this line is carried on wooden poles to the switchyard.

The switchyard is located near the west abutment of the dam approximately at elevation 6000. This location is the only reasonably level site available in the vicinity of the dam for the relative large area required for the yard. It allows ample space for locating the several transmission line approach spans to the 230-, 138-, 69-, and 25-kilovolt yards.

126. Steel Structures for Transformer Circuits. (a) Protective Canopy.--During construction for the powerplant completion, the contractor erected a temporary safety net between the dam and the roof of the powerplant to protect the workmen on the transformer deck from possible falling construction materials and tools during work above on the dam and elevator shaft.

Shortly before the completion of contract, it was decided that a permanent installation should be provided to protect the buses and electrical equipment on the transformer deck from falling objects thrown from the crest of the dam.

The protective canopy, as shown on figure 158, was designed for a snow load of 25 pounds per square foot, based on assumed accumulation of wet snow to a depth of a foot or more, and for the structure dead load, with a safety factor of 1.65. The canopy was furnished and installed under an order for change in the completion contract.

An alternative solution, extending the net to the powerplant e-line wall, and applying the above loading to the greatly increased span and areas, would have required a much more massive supporting framework with an accompanying increase in dead load, as well as greater erection problems.

Also, a solution employing a system of cables for a net support to cover the whole deck was considered and discarded, for the reason that necessary height and strength of cable anchorages above the powerplant roof could not be accommodated by the superstructure and roof design.

(b) Jack Bus Bracket. --Because of the off-center location required on the variablecurved surface of the dam, it was decided to provide concrete pier noses projecting from the face of the dam with their end faces in a plane parallel to the powerplant wall, as shown on figure 150. This concept simplified the steel structure design for the jack bus bracket to handle the three conductor phases per transformer (nine wires in all). The length of the bracket was determined by the transformer spacing and required jack bus phase spacing.

The bracket was designed as a three-bay frame, with a 1, 000-pound horizontal wire tension (3, 000 pounds per bay) and for dead load plus 50 pounds per foot live load vertically. Deflections both horizontally and vertically were investigated and compensated for in the design. The horizontal expansion and contraction of this long bracket necessitated the use of a slotted hole or sliding connection on the ends of the bracket, as shown on figure 151.

The two center brackets were designed as cantilevers with the loads as imposed on them by the above bracket plus dead load.

(c) Main Bus and Takeoff Brackets. -- These brackets, as shown on figure 152, were located so that the phase-to-phase electrical clearance between the main bus and the jack bus conductors would be adequate under all conditions of loading.

The brackets were designed as cantilevers in both the horizontal and vertical planes. The direct line tension was 3, 000 pounds per conductor applied at 37° vertical and 37° horizontal line angles, with 1/2-inch radial ice load and 8 pounds per foot windload at 0° F. Also considered was 1-1/2-inch radial rime (ice at 40 pounds per cubic foot) with a 4-pound windload at 30° F., structure dead load, and a windload of 50 pounds per foot applied horizontally.

Figure 158. --Protective canopy above steel structures for transformer circuits.

(a) Transformer Circuit Towers C1-T1, C1-T2, and C1-13. -- Refer to figures 6 and 154. Prior to determining the outlines of the structures, a sag-tension study was made to determine the conductor and ground-wire spacings, the critical electrical sideswing clearances to the canyon wall between towers C1-T1 and C1-T2, clearance to the ground surface, and for the generally accepted elliptical movement in space from possible "galloping" at midspan.

Using the physical properties of a 3/8-inch-diameter steel ground wire and 795, 000circular-mil ACSR (45/7) conductors intact, conditions were considered with 1-1/2-inch radial ice (40 pounds per cubic foot) and a 4-pound wind load at 30° F.; and no ice with a wind of 90 miles per hour at 60° F. Also considered were broken-wire conditions, including full dead-end loading (all wires off on one side) with 1/2-inch ice (57 pounds per cubic foot), 8-pound windload at 0° F., and no ice at 100° F. The rime condition was considered only for the two spans between the powerplant and tower C1-T2.

Overhead ground wires were not used between the powerplant and tower C1-T1 because of the canyon depth and the assumption that the tower would provide adequate lightning protection for this particular span.

The above conditions were also used for the 12.5-kilovolt circuit using the physical properties of a No. 4/0 ACSR conductor.

Maximum horizontal tensions, in pounds, with 1/2-inch of ice and a windload of 8 pounds per foot at 0° F., were as follows:

The maximum stress in any one member was obtained by combining loads from unit stress diagrams for the various conditions of loadings as discussed above, vertical loads, dead loads, and horizontal torsional shears produced by torsional moments of unbalanced or broken wires.

A safety factor of 1.65 was applied to the maximum member stress obtained from the above loads.

Transformer circuit tower C1-T3 is a 70-foot, type DTH, transmission line tower. This tower, with crossarms on one side only, accommodates the 90° line angle between the switchyard and the paralleling transformer circuit. The conductor attachments change from a horizontal configuration at tower C1-T2 to vertical configuration at tower C1-T3 and then back to horizontal at the switchyard. This loading, plus the addition of several ground wires, was checked to insure that the structure design was adequate.

127. Steel Switchyard Structures. The switchyard structures are of hot-dip galvanized structural steel, field assembled with bolts. The major structures, figures 155 and 159 through 166, were designed to withstand the loads indicated on the drawings as imposed by conductors, overhead ground wires, wind and dead loads, and the loads imposed by the supported electrical equipment. The minor structures, such as shown in figure 167, were designed for wind and dead loads plus the weight of the supported electrical equipment.

The 138-kilovolt switchyard was newly designed for use at Flaming Gorge. The several structure outlines were determined by the electrical clearance and load requirements, and were a compromise between the 115-kilovolt and 161- or 230-kilovolt structures. The 138-kilovolt takeoff structure was also checked for temperature stresses due to the number of continuous bays. A factor of safety of 1.50 was used in this design.

Figure 159. --Switchyard 230-kilovolt takeoff structure--General plan and elevations.