Overhead power line
An overhead power line is a structure used in electric power transmission and distribution to transmit electrical energy along large distances. It consists of one or more conductors suspended by towers or poles. Since the surrounding air provides good cooling, insulation along long passages, and allows optical inspection, overhead power lines are generally the lowest-cost method of power transmission for large quantities of electric energy.
Construction
Towers for support of the lines are made of wood, steel or aluminum, concrete, and occasionally reinforced plastics. The bare wire conductors on the line are generally made of aluminum, though some copper wires are used in medium-voltage distribution and low-voltage connections to customer premises. 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.Today, some overhead lines are routinely operated at voltages exceeding 765,000 volts between conductors, with even higher voltages possible in some cases.
Classification by operating voltage
Overhead power transmission lines are classified in the electrical power industry by the range of voltages:- Low voltage – less than 1000 volts, used for connection between a residential or small commercial customer and the utility.
- Medium voltage – between 1000 volts and 69 kV, used for distribution in urban and rural areas.
- High voltage, used for sub-transmission and transmission of bulk quantities of electric power and connection to very large consumers.
- Extra high voltage – from 345 kV, up to about 800 kV, used for long distance, very high power transmission.
- Ultra high voltage – higher than 800 kV. The Financial Times reported UHV lines are a "game changer", making a global electricity grid potentially feasible. StateGrid said that compared to conventional lines, UHV enables the transmission of five times more power, over six times the distance.
Structures
Each structure must be designed for the loads imposed on it by the conductors. The weight of the conductor must be supported, as well as dynamic loads due to wind and ice accumulation, and effects of vibration. Where conductors are in a straight line, towers need only resist the weight since the tension in the conductors approximately balances with no resultant force on the structure. Flexible conductors supported at their ends approximate the form of a catenary, and much of the analysis for construction of transmission lines relies on the properties of this form.
A large transmission line project may have several types of towers, with "tangent" towers intended for most positions and more heavily constructed towers used for turning the line through an angle, dead-ending a line, or for important river or road crossings. 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. More rigid structures may be intended to remain standing even if one or more conductors is broken. Such structures may be installed at intervals in power lines to limit the scale of cascading tower failures.
Foundations for tower structures may be large and costly, particularly if the ground conditions are poor, such as in wetlands. Each structure may be stabilized considerably by the use of guy wires to counteract some of the forces applied by the conductors.
Image:Einebenenleitung.jpg|thumb|Low-profile power lines near an airfield. Note the red and white transmission towers and overhead wire markers on the right.
Power lines and supporting structures can be a form of visual pollution. In some cases the lines are buried to avoid this, but this "undergrounding" is more expensive and therefore not common.
For a single wood utility pole structure, a pole is placed in the ground, then three crossarms extend from this, either staggered or all to one side. The insulators are attached to the crossarms. For an "H"-type wood pole structure, two poles are placed in the ground, then a crossbar is placed on top of these, extending to both sides. The insulators are attached at the ends and in the middle. Lattice tower structures have two common forms. One has a pyramidal base, then a vertical section, where three crossarms extend out, typically staggered. The strain insulators are attached to the crossarms. Another has a pyramidal base, which extends to four support points. On top of this a horizontal truss-like structure is placed.
A grounded wire is sometimes strung along the tops of the towers to provide lightning protection. An optical ground wire is a more advanced version with embedded optical fibers for communication. Overhead wire markers can be mounted on the ground wire to meet International Civil Aviation Organization recommendations.
Some markers include flashing lamps for night-time warning.
Circuits
A single-circuit transmission line carries conductors for only one circuit. For a three-phase system, this implies that each tower supports three conductors.A double-circuit transmission line has two circuits. For three-phase systems, each tower supports and insulates six conductors. Single phase AC-power lines as used for traction current have four conductors for two circuits. Usually both circuits operate at the same voltage.
In HVDC systems typically two conductors are carried per line, but in rare cases only one pole of the system is carried on a set of towers.
In some countries like Germany most power lines with voltages above 100 kV are implemented as double, quadruple or in rare cases even hextuple power line as rights of way are rare. Sometimes all conductors are installed with the erection of the pylons; often some circuits are installed later. A disadvantage of double circuit transmission lines is that maintenance can be difficult, as either work in close proximity of high voltage or switch-off of two circuits is required. In case of failure, both systems can be affected.
The largest double-circuit transmission line is the Kita-Iwaki Powerline.
Insulators
must support the conductors and withstand both the normal operating voltage and surges due to switching and lightning. Insulators are broadly classified as either pin-type, which support the conductor above the structure, or suspension type, where the conductor hangs below the structure. The invention of the strain insulator was a critical factor in allowing higher voltages to be used.At the end of the 19th century, the limited electrical strength of telegraph-style pin insulators limited the voltage to no more than 69,000 volts. Up to about 33 kV both types are commonly used. At higher voltages only suspension-type insulators are common for overhead conductors.
Insulators are usually made of wet-process porcelain or toughened glass, with increasing use of glass-reinforced polymer insulators. However, with rising voltage levels, polymer insulators is a lower cost, shorter life span option. China has already developed polymer insulators having a highest system voltage of 1100 kV and India is currently developing a 1200 kV line which will initially be charged with 400 kV to be upgraded to a 1200 kV line.
Suspension insulators are made of multiple units, with the number of unit insulator disks increasing at higher voltages. The number of disks is chosen based on line voltage, lightning withstand requirement, altitude, and environmental factors such as fog, pollution, or salt spray. In cases where these conditions are suboptimal, longer insulators must be used. Longer insulators with longer creepage distance for leakage current, are required in these cases. 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.
Porcelain insulators may have a semi-conductive glaze finish, so that a small current passes through the insulator. This warms the surface slightly and reduces the effect of fog and dirt accumulation. The semiconducting glaze also ensures a more even distribution of voltage along the length of the chain of insulator units.
Polymer insulators by nature have hydrophobic characteristics providing for improved wet performance. Also, studies have shown that the specific creepage distance required in polymer insulators is much lower than that required in porcelain or glass. Additionally, the mass of polymer insulators is approximately 50% to 30% less than that of a comparative porcelain or glass string. Better pollution and wet performance is leading to the increased use of such insulators.
Insulators for very high voltages, exceeding 200 kV, may have grading rings installed at their terminals. This improves the electric field distribution around the insulator and makes it more resistant to flash-over during voltage surges.
Conductors
The most common conductor in use for transmission today is aluminum conductor steel reinforced. Also seeing much use is all-aluminum-alloy conductor. Aluminum is used because it has about half the weight of a comparable resistance copper cable, as well as being cheaper.Copper was more popular in the past and is still in use, especially at lower voltages and for grounding.
While larger conductors lose less energy due to lower electrical resistance, they are more costly than smaller conductors. An optimization rule called Kelvin's Law states that the optimum size of conductor for a line is found when the cost of the energy wasted in the conductor is equal to the annual interest paid on that portion of the line construction cost due to the size of the conductors. The optimization problem is made more complex by additional factors such as varying annual load, varying cost of installation, and the discrete sizes of cable that are commonly made.
Since a conductor is a flexible object with uniform weight per unit length, the shape of a conductor strung between two towers approximates that of a catenary. The sag of the conductor varies depending on the temperature and additional load such as ice cover. A minimum overhead clearance must be maintained for safety. Since the temperature and therefore length of the conductor increase with increasing current through it, it is sometimes possible to increase the power handling capacity by changing the conductors for a type with a lower coefficient of thermal expansion or a higher allowable operating temperature.
Such conductors which offer reduced thermal sag are known as composite core conductors or type 4 HTLS conductors, and include brand names from various manufacturers such as ACCC, ACCR, HVCRC and Losag. In lieu of steel core strands that are often used to increase overall conductor strength, HTLS conductors use a composite core made principally of carbon fiber that offers a coefficient of thermal expansion about 1/10 of that of steel. While the composite core is nonconductive, it is substantially lighter and stronger than steel, which allows the incorporation of 28% more aluminum 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 carbon core conductor's reduced thermal sag allows it to carry up to twice the current compared to all-aluminum conductor or ACSR.
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, and the pilot must be qualified for this "human external cargo" method.