(3) Economy of the supporting structures. (5) Avoidance of unsightly sags at maximum temperatures considering the weights of insulators and taps. In the conductor stringing operation the powerplant spans were strung prior to stringing backup spans between the rim towers and switchyard approach towers. This eliminated any undue loads on the switchyard approach towers from possible rim tower deflections resulting from powerplant span loads. Owing to changes in power commitments, the 138-kilovolt switchyard and corresponding transformer circuits were deleted from the specifications requirements. B. ELECTRICAL 111. REQUIREMENTS AND GENERAL DESCRIPTION. The switchyard is composed of 345-, 230-, 69-, 24.9-, and 4.16-kilovolt structures and space for a future 138-kilovolt structure within a fenced area located on the canyon rim approximately 850 feet southwest of the right dam abutment. The elevation of the switchyard is approximately 3,885 feet above sea level. The transformer circuits extend from the transformer deck of the powerplant to the switchyard and consist of three 345- and one 230-kilovolt overhead lines. The general arrangement of the switchyard is shown on figure 216 and that of the transformer circuits on figure 217. The switching arrangement and principal equipment ratings and procurement data are shown on figure 218. 112. GENERAL DESIGN. The main buswork for the 345- and 230-kilovolt installations is of the strain-type design, providing for an ultimate breaker and one-half switching arrangement. Initial operation is essentially as ring buses. The typical general arrangement of these installations is shown on figures 219 and 220, and the arrangement of the main autotransformer bank installation is shown on figures 221 and 222. The buswork for the 69 and 25-kilovolt installations is of the rigid-type design with main and transfer buses. The general arrangement of these installations including the 69/25-kilovolt power transformer installation and power supplies for station service and other auxiliaries is shown in plan on figures 223 and 224. Typical details of the takeoffs on the transformer deck and tower connections for the transformer circuits are shown on figures 225 and 226. The control and low-voltage power cables are run in cable trenches through the switchyard. The location and details of the cable trenches are shown on figure 227. The cable trenches have removable aluminum plate covers throughout their entire length. The control cables and alternate station-service power supply run from the powerplant to the switchyard by means of the cable tunnel which joins the cable trench at the north end of the switchyard. 113. INSULATION, COORDINATION, AND LIGHTNING AND SWITCHING SURGE PROTECTION. The electrical insulation and protective devices were selected and coordinated to provide a safe margin of insulation strength above the maximum abnormal voltages permitted by the protective equipment during lightning, switching, and short-circuit surges. All 345-, 230-, and 69-kilovolt lines and circuits are protected by two overhead ground wires terminated on peaks of the structures above the conductor takeoff levels, except the portion of the four transformer circuits between the powerplant and the canyon rim towers. This portion of the transformer circuits was considered to be adequately protected from lightning by the Glen Canyon Bridge, under which they pass, and the rim towers. Additional overhead ground wires have been provided to shield all buswork which would not otherwise be shielded. Lightning arresters are mounted on or adjacent to all equipment having wound-type internal construction and all insulated cables having voltage ratings exceeding 600 volts, to dissipate lightning surges. 114. GROUNDING SYSTEM. The protective grounding system in the switchyard consists of 3/4-inch by 20-foot copperweld ground rods installed around the perimeter of the switchyard and around each lightning arrester installation. The ground rods are tied together by a network of 500,000-circular-mil A.W.G. bare copper cable buried 6 to 18 inches below grade. All equipment and steel structures, including the fence, are connected to the ground mat. A ground bus consisting of two 500,000-circular-mil A.W.G. copper cables extends through the cable tunnel to connect the switchyard ground mat with the powerplant ground mat. The switchyard key grounding plan is shown on figure 228. |