OVERHEAD DISTRIBUTION LINES TYPICAL STRUCTURE CONFIGURATION

OVERHEAD DISTRIBUTION LINES TYPICAL STRUCTURE CONFIGURATION
What are the typical structures used for overhead lines?

Transmission lines and Overhead Distribution lines may be similar in terms with their functions but the two differs a lot in terms of their constructions.  Transmission lines are designed to carry high voltage transmission and travels longer distances while overhead distribution lines is made to carry medium voltage transmission and travels distances only depending on the size of its scope or depending to the utilities discretions.

In a power distribution system, electrical power leaves the power distribution substation and is distributed to different areas by distribution lines. Distribution lines on the high voltage side of the distribution transformer are called primary distribution lines or primaries. While, secondary are termed to those on the low-voltage side of the distribution transformer are called secondary distribution lines.

Typical overhead distribution lines usually consisting of poles and wires in which where distribution transformers are attached to in order to transform voltage to lower rating that is useable to the customers.  Usually found in a distribution line poles are materials like conductors (ACSR), insulators (polymer or ceramic), cross-arms (where wires are laid into), distribution transformers (which includes fuse cut-outs), surge arresters, power quality devices (capacitors), disconnect switches and others.

Beside distribution lines, secondary lines or low voltage lines are usually also attached to the distribution poles. Some electric utilities even allow telephone companies to use their poles for cable and telecommunication transmission and this is of course with monetary arrangement between the two companies.

Transmission and Distribution are typically in 3-phase circuits and since it is so, several three phase configurations have been formulated each with its one purpose and advantage. Distribution lines may be constructed for single-phase, vee-phase, or three phase configuration. For three-phase, lines can be arranged both in horizontal or vertical set-up.

Some distribution line constructions are also designed to accommodate double-circuit configuration. Double circuit line is the term used in referring to distribution lines which carries two independent feeders/supply lines. One known reason why this exists is to maximize the route of the lines since it would be very impractical to have two different sets of distribution poles to carry each feeder.

The configuration of lines in a distribution system does not necessarily follow a strict step-by-step process but it usually depends on the design engineer that lays-out the construction design according to the needed specification. All of these however, should follow a certain standards relating to live parts clearances and equipment sizing both for effective transmission and personnel safety.

Majority of overhead distribution systems uses poles to support the distribution conductors. Poles used for this purpose basically have three types, wood poles, concrete poles, and steel poles. Wood materials are known to be easily get damaged by extreme weather conditions, that is why poles used for distribution are chemically treated (creosoted and tanalized).  Concrete poles and steels poles have been a great alternative for wooden poles however these two are more expensive.

The rating of equipments to be used in an overhead distribution line construction depends on the utilities chosen voltage level. Typical primary line voltages levels are 2,300, 4,160, 12,470, 13,800, 25,000 and 34,500 volts depending on which distribution voltages a utility uses. With this in mind, proper clearances should be observed and equipments BIL rating should also carefully be considered.

Like any other structures, distribution line design requires proper structure analysis to avoid structure failures due to poor design. The design engineer should carefully consult experts like a civil engineer in proper structure construction since this type of work is a partnership between a civil and electrical engineer.