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WEEK 06: TRANSMISSION: OVERHEAD AC Mechanical Design Considerations Sections: Voltage | Conductor | Environment | Line | Mechanical | Support | Accessories | Foundation Types of Supporting Structures Numerous types of structures are used for supporting transmission-line conductors: self-supporting steel towers, guyed steel towers, self-supporting aluminum towers, guyed aluminum towers, self-supporting steel poles, flexible and semiflexible steel towers and poles, rope suspension, wood poles, wood H frames, and concrete poles. The type of supporting structures to use depend upon such factors as the location of the line, importance of the line, desired life of the line, money available for initial investment, cost of maintenance, and availability of material. Structures may support single, double or multiple circuits. The first two types are generally used for transmission lines except in congested areas where right-of-way is very expensive and it is desired to transmit large blocks of power over one line.
Semiflexible Structures have a narrow base in the direction of the line. The ground wires are strung tightly to take up unbalanced loads due to broken conductors and form part of the structural system. In case a conductor breaks, the unbalanced load will be taken up by the ground wires and transmitted by them to the next anchor tower. Guyed Towers are used for single-conductor lines. The self-supporting feature of a tower is replaced by four guys in the direction of the line and with an increase in strength. These towers should withstand a loading criteria to include 100-mi/h winds. Tubular Steel Poles are used on city streets and in congested areas where wide right-of-way cannot be gained. They are used for voltages up to and including 345 kV. Vertical configurations of configurations is used for all high-voltage lines. Insulators maybe side post or suspension on cantilever arms. The poles are spaced approximately 300 ft apart. Wood poles are used extensively on medium- and lower-voltage lines, 69 kV and lower, and are fitted with steel or wood crossarms.
Wood H-frames composed of two poles tied together at the top with wood or steel crossarms are used for higher voltages up to 345 kV. Wood Poles have two desirable advantages: initial economy and natural insulating qualities. Cedar is one of the most durable wood. It is light, strong, gradual taper, and is fairly straight, although full of small knots, but has slow rate of decay. Chestnut is an extremely strong durable wood but crooked, fewer knots. Long leaf southern yellow pine is very strong, straight, has a gradual taper and is usually fairly free of knots. Despite its excellent appearance, the use is limited by its lack of durability. Two factors must be considered in choosing wood poles: length and strength required. The pole length depends on the required clearance above the surface of the ground., the number of crossarms to be attached, and other equipment which maybe installed. Provision should also be made for future additions of crossarms, transformers or other devices. Poles come in standard lengths ranging from 25 to 90 ft in 5-foot differences. Required pole strength is determined by the weight of crossarms, insulators, wires, transformers, and other equipment it must carry, as well as ice or wind loadings. All these forces tend to break a pole at the ground line. It also makes a big difference where the conductor is attached on the pole. The higher above the ground the load is applied, the greater will be the tendency for the pole to break at ground level. A wind pressure of 9 lb per sq. ft. (approximately 67.5 miles per hour) and a temperature of 30°F are used for design purposes. Another factor which contributes to this bending tendency is the force applied by the wires at the poles. Normally, equal spans of wires are suspended from both sides of a pole. However, should a wire span on one side break, then the uneven pulls will tend to pull the pole over. These uneven pulls should be counteracted by guys. Pole Depth Soil conditions, the height of the pole, weight and pull factors must be considered in deciding how deep a pole must be planted in the ground. Pole Gaining is the process of shaving or cutting a pole to receive the crossarms. Two methods of gain are employed. Slab gaining is when a surface of the pole where the crossarms will be attached is merely flattened by shaving off some of the wood to present a slat smooth area. In recess-gained pole, two notches are made on the wood to receive each individual crossarm. Pole roof consist of a cut from one side to the high point of the pole. The gable roof is actually two 45° angles cut on the top of the pole from a distance equal to 1/2 of the pole-top diameter. The slant roof is a 60° angle cut from the side to the top of the pole. The pole roof is made to prevent decay from snow, ice or rain water accumulating on top. Modern, well-preserved poles need only a flat-roof. Concrete poles are used where timber is scarce and where ingredients for making concrete are readily obtainable. They are advantageous in terms of insect damage and other forms of decay prevalent with wood structures. Concrete poles is reinforced with steel, or iron or aluminum mesh to take care of the bending stresses due to wind loads, cable pull, and vertical load. With the increased manufacturing and quality control capability for prestressed concrete poles, and the need for tall structures for narrow rights-of-way or aesthetic requirements, has come the more frequent use of prestressed concrete poles in transmission lines. Structure Protection Protective coating are done on steel structures, to include galvanizing, painting, and weathering. Galvanizing is coating the tower material including the bolts and nuts with an average coat of 2 oz of zinc per square foot of surface. Structures located near industrial plants, if subjected to sulfuric acid and fumes, should not be galvanized. Painting is resorted to for fabricated-steel towers and poles and generally shop-riveted, welded, or special steel poles are painter. Towers near industrial plants in smoky atmosphere should be painted. The base coat should be a mixture of red lead and raw linseed oil. The outer coats maybe of any good all-weather paint. To keep structures in good condition, painting is necessary every 2 or 3 years. A paint made from asphaltum compounds are applied where steel structures are buried in the ground. Weathering steels made from the chemical composition of the materials used where in a few years, a dense dark-brown oxide with a purplish cast forms which becomes a permanent protective coating to all surfaces exposed to the weather. A slight loss of thickness occurs, which eventually stops, as the corrosion rate is nonprogressive. Preservative Treatment is done on wood poles. Pole decay is due to fungus which requires air, moisture, warmth and food for its subsistence. The conditions most favorable to the growth of the fungus are ground at the ground line. The preservative has toxic or antiseptic properties making the wood either poisonous or unfit food for the fungus. There are many wood preservatives, including those using poisonous salts such as copper, mercury, zinc, and arsenic compounds. However, coal-tar creosote and 5% solution of pentachlorophenol (commonly called "penta")in a petroleum distillate are gaining as a standard. The open-tank method, applied to cedar poles. consists of boiling the butts of the poles in a tank of creosote oil, after which the oil is allowed to cool or the poles are transferred to a cold tank of oil. The pressure treatment, applied to pine and fir, the poles are run into a steel cylinder and subjected to a steam treatment for a period of several hours at a temperature which will not damage the wood cells. The pressure is then removed and a vacuum applied. The steam treatment opens up the wood cells and allows the preservative to penetrate. For more information on treated wood, visit: American Wood Preservers Institute. Pole Ground Wires should be installed on all poles, at all voltages, in lightning areas to prevent splitting of poles by lightning and to provide a direct connection to ground and prevent pole burning if an insulator breaks down. Since the ground wire on these lines has a relatively high resistance to ground, the wire can be small as No. 6 galvanized iron and the ground connection can be several wraps of wire around the butt of the pole. |
