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 (commonly
multiples of three) suspended by towers or utility
poles. Since most of the insulation is
provided by air, overhead power lines are generally the lowest-cost method of
power transmission for large quantities of electric energy.
Towers for support of the lines are
made of wood (as-grown or laminated), steel (either lattice structures or
tubular poles), concrete, aluminum, and occasionally reinforced plastics. The
bare wire conductors on the line are generally made of aluminum (either plain
or reinforced
with steel, or composite materials such as
carbon and glass fiber), 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, resilient to storms, ice load,
earthquakes and other potential causes of damage.[1] Today 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 (LV) – less than 1000 volts, used for connection between a residential or small commercial customer and the utility.
- Medium voltage (MV; distribution) – between 1000 volts (1 kV) and to about 33 kV, used for distribution in urban and rural areas.
- High voltage (HV; subtransmission less than 100 kV; subtransmission or transmission at voltage such as 115 kV and 138 kV), used for sub-transmission and transmission of bulk quantities of electric power and connection to very large consumers.
- Extra high voltage (EHV; transmission) – over 230 kV, up to about 800 kV, used for long distance, very high power transmission.
- Ultra high voltage (UHV) – higher than 800 kV.
Structures
Structures for overhead lines take a
variety of shapes depending on the type of line. Structures may be as simple as
wood poles directly set in the earth, carrying one or more cross-arm
beams to support conductors, or "armless" construction with
conductors supported on insulators attached to the side of the pole. Tubular
steel poles are typically used in urban areas. High-voltage lines are often
carried on lattice-type steel towers
or pylons. For remote areas, aluminum towers may be placed by helicopters.
Concrete poles have also been used.[1]
Poles made of reinforced plastics are also available, but their high cost
restricts application.
Each structure must be designed for
the loads imposed on it by the conductors.[1]
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.[1]
A large transmission line project
may have several types of towers, with "tangent"
("suspension" or "line" towers, UK) towers intended for most
positions and more heavily constructed towers used for turning the line through
an angle, dead-ending (terminating) 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.[1]
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.
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 cable called a static
line 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.[2]
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 rarely 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 works can be more difficult, as either work in close proximity
of high voltage or switch-off of 2 circuits is required. In case of failure,
both systems can be affected.
Insulators
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 (69 kV in North
America) both types are commonly used.[1]
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 (silicone rubber
based) are seeing increasing usage.[3]
China has already developed polymer insulators having a highest system voltage
of 1100kV and India is currently developing a 1200kV (highest system voltage)
line which will initially be charged with 400kV to be upgraded to a 1200kV
line.[citation needed]
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.[4]
Porcelain insulators may have a
semi-conductive glaze finish, so that a small current (a few milliamperes)
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 (especially in higher voltages) 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
Aluminum-conductor steel-reinforced (ACSR) cables are primarily used for medium- and
high-voltage lines, and may also be used for overhead services to individual
customers. Aluminum cable is used because it has about half the weight of a
comparable resistance copper cable (though larger diameter due to lower
fundamental conductivity), as well as being cheaper.[1]
Some copper cables are still used, especially at lower voltages and for
grounding.
While larger conductors may 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.[1]
Since a conductor is a flexible
object with uniform weight per unit length, the geometric shape of a conductor
strung on towers approximates that of a catenary.
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. A minimum overhead clearance must be maintained for safety.
Since the temperature of the conductor increases with increasing heat produced
by the current through it, it is sometimes possible to increase the power
handling capacity (uprate) by changing the conductors for a type with a lower
coefficient of thermal expansion or a higher allowable operating temperature.
One such conductor that offers
reduced thermal sag is known as aluminum conductor composite core (ACCC). In
lieu of steel core strands that are often used to increase overall conductor
strength, the ACCC conductor uses a carbon and glass fiber core 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 (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 electrical current.
The ACCC conductor's reduced thermal sag allows it to carry up to twice the
current ("ampacity") compared to all-aluminum conductor (AAC) or
ACSR.
Power lines sometimes have spherical
markers to meet International Civil Aviation Organization recommendations.[5]
Bundled
conductors
At very high voltages, bundle
conductors are used to reduce corona
losses. Bundle conductors consist of several parallel cables connected at
intervals by spacers, often in a cylindrical configuration. For 220 kV
lines, two-conductor bundles are usually used, and for 380 kV lines
usually three or even four. American Electric Power[6]
is building 765 kV lines using six conductors per phase in a bundle.
Spacers must resist the forces due to wind, and magnetic forces during a
short-circuit.
Advantages
At
extra high voltage, the electric field gradient
at the surface of a single conductor is high enough to ionize air, which loses
power and generates both audible noise and interference with communication systems. The field surrounding a bundle of conductors is similar to
the field that would surround a single, very large conductor—this produces
lower gradients which mitigates issues associated with high field strength.
When transmitting alternating current, bundle conductors also avoid the
reduction in ampacity of a single large conductor due to the skin effect.
A bundle conductor also has lower reactance, compared to a single conductor. Additionally, bundled
conductors cool themselves more efficiently due to the increased surface area
of the conductors, further reducing line losses.In addition to reducing Corona
losses and improving the skin effect, conductor bundling also reduces line
inductance. Low line inductance is highly desired because it reduces reactive
current flow, line heating, and voltage drop across transmission lines. For a
non-bundled transmission line, two parameters of transmission lines affect the
inductance: the geometric mean radius, D, and the equivalent conductor radius,
rx. The geometric mean radius is the geometric mean of the distance
between phases in a bundled or non-bundled transmission line. For example, a
3-phase system with equal line spacing d and conductors arranged in a straight
line has a geometric mean radius where 2d represents the distance between the
two outermost phases. The conductor radius rx is the effective
radius of a single conductor. The equation for inductance is then:
Usually, the rx value is
tabulated because it depends on the exact composition of the conductor and
inductive properties that result—these are hard to describe analytically
especially in the case of composite conductors. Typical values of rx
range from 6 to 18 mm.[7]
For a bundled transmission cable, two additional factors affect the line
inductance: the bundle diameter DB and the geometric arrangement of
the bundle. These two parameters can be used to calculate an effective bundled
cable radius, DBE.[8]
Ground
wires
Overhead power lines are often
equipped with a ground conductor (shield wire or overhead earth wire). The
ground conductor is usually grounded (earthed) at the top of the supporting
structure, to minimize the likelihood of direct lightning strikes to the phase
conductors. In circuits with earthed neutral,
it also serves as a parallel path with the earth for fault currents. Very
high-voltage transmission lines may have two ground conductors. These are
either at the outermost ends of the highest cross beam, at two V-shaped mast
points, or at a separate cross arm. 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. Shield wires on transmission lines may include optical
fibers (optical ground wires/OPGW), used for communication and control of the power
system.
At some HVDC converter stations, the
ground wire is used also as the electrode line to connect to a distant
grounding electrode. This allows the HVDC system to use the earth as one
conductor. The ground conductor is mounted on small insulators bridged by
lightning arrestors above the phase conductors. The insulation prevents
electrochemical corrosion of the pylon.
Medium-voltage distribution lines
may also use one or two shield wires, or may have the grounded conductor strung
below the phase conductors to provide some measure of protection against tall
vehicles or equipment touching the energized line, as well as to provide a
neutral line in Wye wired systems.
On some power lines for very high
voltages in the former Soviet Union, the ground wire is used for PLC-radio
systems and mounted on insulators at the pylons.
Insulated
conductors and cable
Overhead insulated cables are rarely
used, usually for short distances (less than a kilometer). Insulated cables can
be directly fastened to structures without insulating supports. An overhead line
with bare conductors insulated by air is typically less costly than a cable
with insulated conductors.
A more common approach is
"covered" line wire. It is treated as bare cable, but often is safer
for wildlife, as the insulation on the cables increases the likelihood of a
large-wing-span raptor to survive a brush with the lines, and reduces the
overall danger of the lines slightly. These types of lines are often seen in
the eastern United States and in heavily wooded areas, where tree-line contact
is likely. The only pitfall is cost, as insulated wire is often costlier than
its bare counterpart. Many utility companies implement covered line wire as
jumper material where the wires are often closer to each other on the pole,
such as an underground riser/pothead,
and on reclosers, cutouts and the likes.
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