591-D-1181

The 138 and 230-kilovolt takeoff structures (figs. 155 and 159) and the transformer structure (fig. 166) were designed for the loads as shown on the drawings, and also for a loading condition of no ice and a wind of 90 miles per hour at 60° F. The 230-kilovolt takeoff structure and the 69-kilovolt bus structure (fig. 164) were previously designed for use in the Missouri River Basin project. These structures were analyzed as multiple-bay, single-story portal frames in the transverse direction and as cantilevers in the longitudinal direction.

The maximum stresses in the individual space frame-type towers, such as the 138kilovolt bus towers and the transformer circuit towers (figs 162 and 163), were obtained by combining the loads from unit stress diagrams for the various loading conditions, vertical loads, dead loads, and horizontal torsional shears produced by unbalanced longitudinal wires. The transformer circuit towers were designed originally for use in the Missouri River Basin project.

The design requirements for the switchyard steel structures are consistent with the requirements of Design Standards No. 10.1/

The wires of the switchyard approach spans of transmission lines and transformer circuits were slacked off to approximately one-half to two-thirds of the maximum tensions used throughout the lines in order to effect economies in the switchyard structures. This was permissible because the switchyard approach spans were not critical with respect to electrical clearances to the lower level of strain buses.

Stringing instructions to be observed in erection of conductors and ground wires were calculated to insure that the design loads of the structures would not be exceeded (figs. 168 and 169). Tensions for conductors and ground wires within the yards were established by the following considerations:

(1) Strength of wires, insulators, and hardware.

(2) Limitation of tension at normal temperatures to avoid vibration in the wires.

(3) Economy of the supporting structures.

(4) Electrical clearances.

(5) Avoidance of unsightly sags at maximum temperatures considering the weights of insulators and taps.

B. Electrical Design

128. Requirements and General Description. The Flaming Gorge Switchyard serves loads at 230, 138, 69, 24.9, and 12.5 kilovolts and also serves as a switching station for the 138-kilovolt system.

The 138-kilovolt structure has six bays, each 32 feet wide. The bays include five outgoing circuits and one bus tie: one circuit to the 138- to 69-kilovolt transformer, one circuit to the 230- to 138-kilovolt transformer, one circuit to the powerplant, and two circuits to the Vernal Substation. One bay is provided for the bus tie power circuit breaker. Space is available for the addition of three more bays in the future for outgoing circuits.

Only one bay is used in the 230-kilovolt structure, having a width of 48 feet. Space is available for the addition of three more bays in the future for outgoing circuits.

The 69-kilovolt structure has three bays, each 22 feet wide, with three outgoing circuits: one circuit to the 138- to 69-kilovolt transformer, one circuit to the 67- to 24.9kilovolt transformer, and one circuit to Bridger Valley. Space is available for the addition of one more bay in the future for an outgoing circuit.

The 24.9-kilovolt structure has two bays, each 15 feet wide, and has one outgoing circuit to Moon Lake. Space is available for the addition of two more bays in the future for outgoing circuits.

1/Transmission Structures, Design Standards No. 10, Bureau of Reclamation, March 8,

1965.