A compact overhead transmission line requires a smaller right of way than a standard overhead powerline. Conductors must not get too close to each other. This can be achieved either by short span lengths and insulating crossbars, or by separating the conductors in the span with insulators. The first type is easier to build as it does not require insulators in the span, which may be difficult to install and to maintain.
Compact transmission lines may be
designed for voltage upgrade of existing lines to increase the power that can
be transmitted on an existing right of way.[9]
Low
voltage
Low voltage overhead lines may use
either bare conductors carried on glass or ceramic insulators or an aerial bundled cable
system. The number of conductors may be anywhere between four (three phase plus
a combined earth/neutral conductor - a TN-C earthing system) up to as many as
six (three phase conductors, separate neutral and earth plus street lighting
supplied by a common switch).
Train
power
Overhead lines or overhead wires are
used to transmit electrical energy to trams, trolleybuses or trains. Overhead
line is designed on the principle of one or more overhead wires situated over
rail tracks. Feeder stations at regular intervals along the overhead line
supply power from the high-voltage grid. For some cases low-frequency AC is
used, and distributed by a special traction
current network.
Further
applications
Overhead lines are also occasionally
used to supply transmitting antennas, especially for efficient transmission of
long, medium and short waves. For this purpose a staggered array line is often
used. Along a staggered array line the conductor cables for the supply of the
earth net of the transmitting antenna are attached on the exterior of a ring,
while the conductor inside the ring, is fastened to insulators leading to the
high-voltage standing feeder of the antenna.
Use
of area under overhead power lines
Use of the area below an overhead
line is restricted because objects must not come too close to the energized
conductors. Overhead lines and structures may shed ice, creating a hazard.
Radio reception can be impaired under a power line, due both to shielding of a
receiver antenna by the overhead conductors, and by partial discharge at
insulators and sharp points of the conductors which creates radio noise.
In the area surrounding overhead
lines it is dangerous to risk interference; e.g. flying kites or balloons,
using ladders or operating machinery.
Overhead distribution and
transmission lines near airfields are often marked on maps, and the
lines themselves marked with conspicuous plastic reflectors, to warn pilots of
the presence of conductors.
Construction of overhead power
lines, especially in wilderness areas, may have significant environmental
effects. Environmental studies for such projects may consider the effect of
bush clearing, changed migration routes for migratory animals, possible access
by predators and humans along transmission corridors, disturbances of fish
habitat at stream crossings, and other effects.
History
The first transmission of electrical
impulses over an extended distance was demonstrated on July 14, 1729 by the
physicist Stephen Gray.
The demonstration used damp hemp cords suspended by silk threads (the low
resistance of metallic conductors not being appreciated at the time).
However the first practical use of
overhead lines was in the context of telegraphy. By 1837
experimental commercial telegraph systems ran as far as 20 km (13 miles).
Electric power transmission was accomplished in 1882 with the first
high-voltage transmission between Munich and Miesbach (60 km). 1891 saw the construction of the first
three-phase alternating current
overhead line on the occasion of the International Electricity Exhibition in Frankfurt, between Lauffen and Frankfurt.
In 1912 the first 110 kV-overhead
power line entered service followed by the first 220 kV-overhead power line in
1923. In the 1920s RWE AG built the first overhead line for this voltage and in
1926 built a Rhine crossing with the pylons of Voerde, two masts 138 meters high.
In 1953, the first 345 kV line was
put into service in the United
States. In Germany in 1957 the first 380
kV overhead power line was commissioned (between the transformer station and
Rommerskirchen). 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 400 kv line was put into service in Sweden, in 160 km (100 miles) between the more populated
areas in the south and the largest hydroelectric power stations in the north.
Starting from 1967 in Russia, and also in the USA and Canada, overhead lines
for voltage of 765 kV were built. In 1982 overhead power lines were built in
Russia between Elektrostal and the power station at Ekibastusz, this was a
three-phase alternating current line at 1150 kV (Powerline Ekibastuz-Kokshetau). In 1999, in Japan the first powerline designed for 1000
kV with 2 circuits were built, the Kita-Iwaki Powerline.
In 2003 the building of the highest overhead line commenced in China, the Yangtze River Crossing.
Mathematical
analysis
An overhead power line is one
example of a transmission line.
At power system frequencies, many useful simplifications can be made for lines
of typical lengths. For analysis of power systems, the distributed resistance,
series inductance, shunt leakage resistance and shunt capacitance can be
replaced with suitable lumped values or simplified networks.
Short
and medium line model
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. It is important to note that
this value is not the total impedance of the line, but rather the series impedance
per unit length of line. For a longer length of line (80–250 km), a shunt
capacitance is added to the model. In this case it is common to distribute half
of the total capacitance to each side of the line. As a result, the power line
can be represented as a two-port
network, such as ABCD parameters.[10]
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