110-Kv. transmission line for Oak Grove development of Portland Railway, Light and Power company

This paper covers the design of the 110-kv. transmission line to transmit energy from the new Oak Grove development of the Portland Railway, Light and Power Company to the city of Portland, Oregon, a distance of 54 miles. The company now has three hydroelectric developments aggregating 52,000 kw. supplied from the same general watershed from which energy is transmitted to Portland over five 60-kv. circuits, only one of which is on steel towers. The problem was to design a transmission line to most economically handle the three 30,000-kv-a. units as installed at the new development and to tie in with the existing transmission system. The following factors were considered in determining the design: 1. Transmission voltage 2. Conductor capacity 3. Maximum conductor tension 4. Assumed maximum mechanical loading conditions 5. Type of supporting structures Consideration was also given to the physical characteristics of the country through which the line must be built. For 14.8 miles the line will be located in the United States Government Forest Reserve, which is very heavily timbered, and accordingly cost of clearing will be very high, averaging $15,000 a mile, even after credit for merchantable timber is allowed. This fact, outside of other considerations, required a minimum number of circuits. To reduce clearing costs as much as possible, the line was located along the river gorge necessitating 55 angles, a few as great as 60 deg., and 28 river crossing spans, vearying from 400 to 1800 feet. After considering various types of supporting structures, three types of double circuit, heavy rigid steel towers were decided upon as follows: Type A towers, to be used on tangents up to 800-foot spans, also on curves up to 10 deg., the spans not exceeding 500 feet; suspension insulators to be used. Type B towers, to be used on tangents up to 1700-foot spans, also on curves from 8 to 30 deg., the spans not exceeding 800 feet for the larger curves; suspension insulators to be used. Type C towers, to be used at dead-ends and curves over 30 deg. Towers are designed for 250,000, cir. mil. copper conductors, and maximum loading conditions of one-half inch of ice and eight pounds per square foot wind pressure at 0 deg. fahr. were assumed. Each type of tower was designed to withstand the maximum loading conditions plus 50 per cent without exceeding the elastic limit of the steel, and the towers were subjected to an actual field test. Special steel having an elastic limit of 45,000 pounds per square inch and a minimum elongation of 22 per cent was used in the towers. In order to reduce the number of insulators and deadend positions, use was made of brackets pivoted to the cross arms so that they would be free to swing in the line but rigid across the line. 12-inch brackets are provided for the Type A towers on 8 deg. curves and 24-inch brackets on Type B towers with 30 deg. curves so as to maintain a three-foot clearance between the conductor and steel on that side of the tower where the conductors pull the insulator strings up and toward the cage. The use of these brackes reduced the total cost of the insulators and hardware fully 25 per cent. Tables are included giving resultant simultaneous loading and weights and dimensions for each type of tower.