Overhead power line
Towers for support of the lines are made of wood (as-grown or laminated), steel or aluminum (either lattice structures or tubular poles), concrete, and occasionally reinforced plastics.
A major goal of overhead power line design is to maintain adequate clearance between energized conductors and the ground so as to prevent dangerous contact with the line, and to provide reliable support for the conductors, resilience to storms, ice loads, earthquakes and other potential damage causes.
[2] The weight of the conductor must be supported, as well as dynamic loads due to wind and ice accumulation, and effects of vibration.
Depending on the design criteria for a particular line, semi-flexible type structures may rely on the weight of the conductors to be balanced on both sides of each tower.
Insulators must support the conductors and withstand both the normal operating voltage and surges due to switching and lightning.
The number of disks is chosen based on line voltage, lightning withstand requirement, altitude, and environmental factors such as fog, pollution, or salt spray.
Strain insulators must be strong enough mechanically to support the full weight of the span of conductor, as well as loads due to ice accumulation, and wind.
Additionally, the mass of polymer insulators (especially in higher voltages) is approximately 50% to 30% less than that of a comparative porcelain or glass string.
Aluminum is used because it has about half the weight of a comparable resistance copper cable (though larger diameter due to lower specific conductivity), as well as being cheaper.
The sag of the conductor (vertical distance between the highest and lowest point of the curve) varies depending on the temperature and additional load such as ice cover.
While the composite core is nonconductive, it is substantially lighter and stronger than steel, which allows the incorporation of 28% more aluminum (using compact trapezoidal-shaped strands) without any diameter or weight penalty.
The added aluminum content helps reduce line losses by 25 to 40% compared to other conductors of the same diameter and weight, depending upon electric current.
The power lines and their surroundings must be maintained by linemen, sometimes assisted by helicopters with pressure washers or circular saws which may work three times faster.
However this work often occurs in the dangerous areas of the Helicopter height–velocity diagram,[12][13][14] and the pilot must be qualified for this "human external cargo" method.
Transmission higher than 132 kV poses the problem of corona discharge, which causes significant power loss and interference with communication circuits.
Older lines may use surge arresters every few spans in place of a shield wire; this configuration is typically found in the more rural areas of the United States.
By protecting the line from lightning, the design of apparatus in substations is simplified due to lower stress on insulation.
On some power lines for very high voltages in the former Soviet Union, the ground wire is used for PLC systems and mounted on insulators at the pylons.
[19] Low voltage overhead lines may use either bare conductors carried on glass or ceramic insulators or an aerial bundled cable system.
Feeder stations at regular intervals along the overhead line supply power from the high-voltage grid.
Overhead lines are also occasionally used to supply transmitting antennas, especially for efficient transmission of long, medium and short waves.
In the area surrounding the overhead lines it is dangerous to risk interference; e.g. flying kites or balloons, using ladders or operating machinery.
General aviation, hang gliding, paragliding, skydiving, balloon, and kite flying must avoid accidental contact with power lines.
Some power lines are marked with obstruction markers, especially near air strips or over waterways that may support floatplane operations.
[citation needed] The demonstration used damp hemp cords suspended by silk threads (the low resistance of metallic conductors not being appreciated at the time).
In the same year the overhead line traversing of the Strait of Messina went into service in Italy, whose pylons served the Elbe crossing 1.
This was used as the model for the building of the Elbe crossing 2 in the second half of the 1970s which saw the construction of the highest overhead line pylons of the world.
Earlier, in 1952, the first 380 kV line was put into service in Sweden, in 1000 km (625 miles) between the more populated areas in the south and the largest hydroelectric power stations in the north.
In 2002 the building of the highest overhead line commenced in China, the Yangtze River Crossing, its two 346.5 m (1,137 ft) high suspension towers beginning service in 2004.
A short length of a power line (less than 80 km) can be approximated with a resistance in series with an inductance and ignoring the shunt admittances.
