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WEEK 13: DISTRIBUTION: EQUIPMENT Sections: Structures | Pole Lines | Conductors | Transformers | Voltage Control | Protection Considerations Traditional overhead construction embodied one or more crossarms on each pole for mounting primary insulators, distribution transformers, surge arresters, cutouts, etc. New concepts include the introduction and wide use of armless construction wherein insulators and equipment are directly mounted to the poles. Other ideas were also introduced, since 1960s, such as use of shorter poles, cabled secondaries and services, strategic use of steel poles to reduce or eliminate need for guy wires and fewer circuits per pole. Poles Wood poles have been used almost universally for overhead distribution lines because of the abundance of the material, ease of handling, and cost. The life of wood poles is materially extended with wood preservatives, and cedar, pine and fir are most commonly used. Refer to Support Structures: Wood Poles for details. Specifications and dimensions for wood poles are grouped into classes based on their circumference at a location six (6) feet from the butt. Poles are give class and length are designed to have approximately the same load-carrying capacity regardless of species. In making calculations, it is assumed that the pole is used as a simple cantilever and that the maximum fiber stress in the pole due to the bending moment will occur at the ground level. Concrete poles reinforced with steel were originally employed chiefly for street lighting standards. Some concrete poles have been used for general distribution as well, usually with a minimum of attached wires and apparatus. Steel poles, ordinarily set in concrete, are also employed for street lighting systems, More recently, in a more ornamental form and bolted to concrete foundations, they have been used for parkways lighting, and to a limited extent, for distribution where appearance demands. Loading Poles carrying overhead distribution lines are subjected to vertical and horizontal forces, of which some are continuous and others are applied only under abnormal or occasional conditions. Normal vertical forces are the weight of the wires, transformers, and other support equipment: voltage regulators or capacitor banks, and there are less than normal horizontal forces in many cases. The equipment also presents additional traverse loading due to wind pressure. Abnormal vertical load is imposed when wires are coated with ice, which may increase their normal weight 200 to 400%. Therefore, poles to be used for supporting equipment usually are specified to be of better class than those supporting conductors only. Normal horizontal forces acting upon a pole are the unbalanced component of wire tension at turns and corners, the side pull of service drops, and the horizontal component of weight when the pole is not vertical. Abnormal horizontal stresses are imposed by wind pressure, by breakage of conductors, or by failure of supporting guys. Traverse forces are imposed on a pole when there is a change of direction of the line, i.e., where the conductors on either side of the pole form an angle. These forces are offset by guys when practicable. The loading of the structure, including guys, is considered to be the resultant load equal to the traverse wind load and the load imposed by the conductors due to the change in direction. The most severe condition ordinarily encountered is for right angle turns, when divergence from the straight line form a 45° angle. In such cases tensions should be reduced as much as practicable at the corner pole by shortening spans and transferring part a of the stress to guy wires. Stresses at dead-end poles are treated similarly. Strength of Wood Poles must be sufficient to withstand the traverse forces, such as wind pressure, on the pole and conductors; unbalanced pull on conductors when they are broken; and side pull on curves and corners where guys cannot be provided. These forces place the fiber on the wood under tension, and the load which a pole will carry is determined by the inherent strength of its wood fiber under tension and the moment of force. |
