Construction of Grand Coulee Third Power Plant

Construction of Grand Coulee Third Power Plant, begun in July, 1967, will add 3,900 mW to the existing 2,329-mW capacity of Grand Coulee. Major items of work for the total project include modifications to electrical facilities; over 22,000,000 cy yd (17,000,000 cu m) of excavation; removal of 260 ft (79 m) of existing concrete dam; construction of a 200-ft (61-m), 1,275-ft long (389-m) concrete gravity dam containing over 600,000 cu yd (460,000 cu m); construction of a 1,128-ft long (344-m) power plant; and a 500-kV switchyard. Six 40-ft diam (12-m) steel penstocks will deliver 30,000 cfs (840 m³/s) each to three 600-mW and three 700-mW turbine-generator units. For the first three 600-mW units, turbine capacity is 820,000 hp at 285-ft (86.9-m) head) and 72 rpm, and generator capacity is 615,385-kVA, 97.5% power factor; each rotor is 60 ft (18 m) in diameter and weighs 1,936 tons (1,756,000 kg). Upstream control of penstocks is provided by 29-ft by 43.5-ft (8.8-m by 13.3-m) wheel-mounted gates.