Overhead Line Components: Supports, Insulators, Conductors
Supports
Supports are the elements that support the conductors and other components of an overhead line, separating the ground and the lines. They have to resist compression, bending forces, wind action, and other meteorological conditions.
Classification of Supports
- Alignment Supports: Their function is to support the conductors and ground wires; they are used in straight alignments.
- Angle Supports: Used to support conductors and ground wires at the vertices or angles that the line forms in its path. Besides bending forces, composition stress appears in each direction on this kind of support.
- Anchor Supports: Their purpose is to provide firm points on the line, to limit and prevent its destruction when, for any reason, a support or a conductor breaks.
- Dead-End Supports: Support the stresses produced by the line; they are its greatest resistance anchor point.
- Special Supports: Their function is different from those listed above; they may be used, for example, for crossings over rail lines, rivers, telecommunication lines, or at a bifurcation.
Materials
- Wood:
- Advantages: Lightness and portability; low price compared to concrete and steel.
- Disadvantages: Relatively short lifespan; do not allow the installation of large openings; limited head strength and height.
- Concrete: A construction material composed primarily of aggregate (sand, gravel, stone, and slag), cement, and water. It is used for medium and low voltage.
- Vibrated reinforced concrete post: Vibrated to prevent bubbles from appearing; can be manufactured in all locations.
- Centrifuged reinforced concrete pole: Circular, made in wooden molds, and spun.
- Prestressed reinforced concrete poles: Vibrated concrete with stressed steel wires. When the concrete is hard, the wires are cut, compressing and strengthening the pole.
- Metal: The most used metal is steel, in the form of rolled, T, U, etc. Aluminum-steel and cast iron are also used. To join them, we use rivets, screws, bolts, and solder.
- Tubular poles: Used for installations inside towns.
- Laminated profile metal poles: Exclusively used in low voltage.
- Lattice poles: Mostly used for medium and high voltage.
Cross Arms
Types include: Arch-shaped (with suspended wires), Flag (all wires on the same side), Offset (two wires on one side and the center wire on the other side), Straight (multiple lines in a T-shape), Suspended, and Posed.
Foundations
Foundations help the support counter external forces and hold the poles. To calculate the embedding height: h = 1.3 + (Ht – 8) * 0.1, where H is the total height of the support and h is the embedment height. h cannot be less than 1.3 meters. The sill is the distance between the end of the hole and where the support is set up, with a minimum of 0.2 meters.
Insulators
An electrical insulator is a material whose internal electric charges do not flow freely, and therefore does not conduct an electric current under the influence of an electric field.
Characteristics
- Sufficient Dielectric Strength: The maximum electric field that the insulator can withstand without breaking down.
- Breakdown Voltage: The minimum voltage that causes a portion of the insulator to become an electrical conductor.
- Suitable Arrangement: To increase the flashover voltage.
- Flashover Voltage: The voltage that causes an arc through the air around the surface of the insulator. It is lower than the breakdown voltage. The path between the insulator and ground is called the leakage line.
- Adequate Mechanical Strength: To withstand the stresses demanded by the conductor.
- Resistance to temperature variations.
- Absence of aging.
Materials
Porcelain, Glass, Steatite, and Epoxy.
Types
- Fixed (ARVI12): Attached to the pole.
- Chain/Suspended: Cap-rod, engine, Langstab.
- Special: Fog (deeper ripples, greater contour), Coast (wider and higher).
Tests
- Quality: The normal assay is to break the insulator or try to break it, applying different forces and examining the fracture surfaces. Errors are sought, and the enamel surface is examined, analyzing it with normal and polarized light.
- Flashover: Checks if the flashover voltage is adequate. Electricity is applied under specific conditions (wet, dry, etc.).
- Puncture: Performed under oil, until the insulator breaks.
- Mechanical Assay: Insulators are subjected to stress for 24 hours.
Conductors
Copper
- Advantages: Resists the attack of acids, salts, and sulfates better; can be perfectly welded with tin; better conductor than aluminum.
- Disadvantages: Copper is not abundant in nature; it is heavy and more expensive.
Aluminum
- Advantages: More suitable than copper for high-voltage lines; aluminum is abundant in nature, cheaper, lighter, and has higher mechanical strength.
- Disadvantages: Acids, salts, and sulfates attack aluminum more; can be welded, but under very special conditions and using special welding; it is a worse conductor, so it needs a larger cross-section.
Summary: Sal = 1.6 * Scu; Weight AL = 0.5 * Weight Cu; Mechanical Tension Al = Cu; Price Al = 0.25 * Cu; Stored Heat Al = 1.13 * Heat Cu.
Types
Aluminum-steel conductor (LA), Compacted Aluminum-steel (LAC), Galvanized steel (AC), and Copper.
Composition
Conductors are composed of several aluminum wires of the same or different nominal diameters, and a galvanized steel wire core.
- 1+6: A core of 1 steel wire and a layer of 6 aluminum wires.
- 7+30: A core of 1 steel wire surrounded by 6 steel wires, and 30 aluminum wires in different layers.
- 7+54: A core of 1 steel wire surrounded by 6 steel wires, and 54 aluminum wires in different layers (1, 6, 12, 18, 24).
Imax = Density x Section x Coefficient // Section: s = πr2 x number of wires
Insulated Wire Layers
- Usually an aluminum conductor.
- Semiconductor layer.
- Insulation of cross-linked polyethylene or ethylene-propylene.
- Semiconductor layer.
- Screen: copper strip or copper wire wreath.
- Black vinyl chloride cover.
Skin Effect
(Kelvin effect)
Corona Effect
Caused by electromagnetic forces (attraction and exit of electricity).