Comparison
of ground-based measurements of near-surface temperature (blue) and
satellite based records of mid-tropospheric temperature (red: UAH; green: RSS) from 1979 to 2010. Trends plotted 1982-2010.
Atmospheric temperature trends from 1979-2016 based on satellite measurements; troposphere above, stratosphere below.
Satellite datasets show that over the past four decades the troposphere has warmed and the stratosphere has cooled. Both of these trends are consistent with the influence of increasing atmospheric concentrations of greenhouse gases.
Measurements
Satellites
do not measure temperature directly. They measure radiances in various
wavelength bands, which must then be mathematically inverted to obtain
indirect inferences of temperature.
The resulting temperature profiles depend on details of the methods
that are used to obtain temperatures from radiances. As a result,
different groups that have analyzed the satellite data have produced
differing temperature datasets.
The satellite time series is not homogeneous. It is constructed
from a series of satellites with similar but not identical sensors. The
sensors also deteriorate over time, and corrections are necessary for
orbital drift and decay.
Particularly large differences between reconstructed temperature series
occur at the few times when there is little temporal overlap between
successive satellites, making intercalibration difficult.
Land
surface temperature anomalies for a given month compared to the
long-term average temperature of that month between 2000-2008.
Sea
surface temperature anomalies for a given month compared to the
long-term average temperature of that month from 1985 through 1997.
Infrared
radiation can be used to measure both the temperature of the surface
(using "window" wavelengths to which the atmosphere is transparent), and
the temperature of the atmosphere (using wavelengths for which the
atmosphere is not transparent, or measuring cloud top temperatures in
infrared windows).
Weather satellites have been available to infer sea surface temperature (SST) information since 1967, with the first global composites occurring during 1970. Since 1982, satellites have been increasingly utilized to measure SST and have allowed its spatial and temporal
variation to be viewed more fully. For example, changes in SST
monitored via satellite have been used to document the progression of
the El Niño-Southern Oscillation since the 1970s.
Over land the retrieval of temperature from radiances is harder, because of inhomogeneities in the surface. Studies have been conducted on the urban heat island effect via satellite imagery. By using the fractal technique, Weng, Q. et al. characterized the spatial pattern of urban heat island. Use of advanced very high resolution infrared satellite imagery can be used, in the absence of cloudiness, to detect density discontinuities (weather fronts) such as cold fronts at ground level. Using the Dvorak technique, infrared satellite imagery can be used to determine the temperature difference between the eye and the cloud top temperature of the central dense overcast of mature tropical cyclones to estimate their maximum sustained winds and their minimum central pressures.
Stratospheric
temperature measurements are made from the Stratospheric Sounding Unit
(SSU) instruments, which are three-channel infrared (IR) radiometers.
Since this measures infrared emission from carbon dioxide, the
atmospheric opacity is higher and hence the temperature is measured at a
higher altitude (stratosphere) than microwave measurements.
Since 1979 the Stratospheric sounding units (SSUs) on the NOAA
operational satellites have provided near global stratospheric
temperature data above the lower stratosphere.
The SSU is a far-infrared
spectrometer employing a pressure modulation technique to make
measurement in three channels in the 15 μm carbon dioxide absorption
band. The three channels use the same frequency but different carbon
dioxide cell pressure, the corresponding weighting functions peaks at
29 km for channel 1, 37 km for channel 2 and 45 km for channel 3.
The process of deriving trends from SSUs measurement has proved
particularly difficult because of satellite drift, inter-calibration
between different satellites with scant overlap and gas leaks in the
instrument carbon dioxide pressure cells. Furthermore since the
radiances measured by SSUs are due to emission by carbon dioxide
the weighting functions move to higher altitudes as the carbon dioxide
concentration in the stratosphere increase.
Mid to upper stratosphere temperatures shows a strong negative trend
interspersed by transient volcanic warming after the explosive volcanic
eruptions of El Chichón and Mount Pinatubo, little temperature trend has been observed since 1995.
The greatest cooling occurred in the tropical stratosphere consistent with enhanced Brewer-Dobson circulation under greenhouse gas concentrations increase.
Lower stratospheric cooling is mainly caused by the effects of ozone depletion with a possible contribution from increased stratospheric water vapor and greenhouse gases increase. There has been a decline in stratospheric temperatures, interspersed by warmings related to volcanic eruptions. Global Warming theory suggests that the stratosphere should cool while the troposphere warms.
Top of the stratosphere (TTS) 1979–2006 temperature trend.
The long term cooling in the lower stratosphere occurred in two
downward steps in temperature both after the transient warming related
to explosive volcanic eruptions of El Chichón and Mount Pinatubo,
this behavior of the global stratospheric temperature has been
attributed to global ozone concentration variation in the two years
following volcanic eruptions.
Since 1996 the trend is slightly positive
due to ozone recovery juxtaposed to a cooling trend of 0.1K/decade that
is consistent with the predicted impact of increased greenhouse gases.
The table below shows the stratospheric temperature trend from
the SSU measurements in the three different bands, where negative trend
indicated cooling.
Channel
Start
End Date
STAR v3.0
Global Trend (K/decade)
TMS
1978-11
2017-01
−0.583
TUS
1978-11
2017-01
−0.649
TTS
1979-07
2017-01
−0.728
Microwave (tropospheric and stratospheric) measurements
From 1979 to 2005 the microwave sounding units (MSUs) and since 1998 the Advanced Microwave Sounding Units on NOAA polar orbiting weather satellites have measured the intensity of upwelling microwave radiation from atmospheric oxygen. The intensity is proportional to the temperature of broad vertical layers of the atmosphere.
Upwelling radiance is measured at different frequencies; these
different frequency bands sample a different weighted range of the
atmosphere.
Figure 3 (right) shows the atmospheric levels sampled by
different wavelength reconstructions from the satellite measurements,
where TLS, TTS, and TTT represent three different wavelengths.
Other microwave measurements
A different technique is used by the Aura spacecraft, the Microwave Limb Sounder, which measure microwave emission horizontally, rather than aiming at the nadir.
Temperature measurements are also made by occultation of GPS signals. This technique measures the refraction of the radio signals from GPS satellites by the Earth's atmosphere, thus allowing vertical temperature and moisture profiles to be measured.
Temperature measurements on other planets
Planetary
science missions also make temperature measurements on other planets
and moons of the solar system, using both infrared techniques (typical
of orbiter and flyby missions of planets with solid surfaces) and
microwave techniques (more often used for planets with atmospheres).
Infrared temperature measurement instruments used in planetary missions
include surface temperature measurements taken by the Thermal Emission Spectrometer (TES) instrument on Mars Global Surveyor and the Diviner instrument on the Lunar Reconnaissance Orbiter; and atmospheric temperature measurements taken by the composite infrared spectrometer instrument on the NASA Cassini spacecraft.
Microwave atmospheric temperature measurement instruments include the Microwave Radiometer on the Juno mission to Jupiter.
Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high-quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films.
In typical CVD, the wafer (substrate) is exposed to one or more volatileprecursors, which react and/or decompose on the substrate surface to produce the desired deposit. Frequently, volatile by-products are also produced, which are removed by gas flow through the reaction chamber.
The term chemical vapour deposition was coined 1960 by John M. Blocher, Jr. who intended to differentiate chemical from physical vapour deposition (PVD).
CVD is practiced in a variety of formats. These processes generally
differ in the means by which chemical reactions are initiated.
Classified by operating conditions:
Atmospheric pressure CVD (APCVD) – CVD at atmospheric pressure.
Low-pressure CVD (LPCVD) – CVD at sub-atmospheric pressures. Reduced pressures tend to reduce unwanted gas-phase reactions and improve film uniformity across the wafer.
Ultrahigh vacuum CVD (UHVCVD) – CVD at very low pressure, typically below 10−6Pa (≈ 10−8torr). Note that in other fields, a lower division between high and ultra-high vacuum is common, often 10−7 Pa.
Sub-atmospheric CVD (SACVD) – CVD at sub-atmospheric pressures. Uses tetraethyl orthosilicate (TEOS) and ozone to fill high aspect ratio Si structures with silicon dioxide (SiO2).
Most modern CVD is either LPCVD or UHVCVD.
Classified by physical characteristics of vapor:
Aerosol assisted CVD (AACVD) – CVD in which the precursors are
transported to the substrate by means of a liquid/gas aerosol, which can
be generated ultrasonically. This technique is suitable for use with
non-volatile precursors.
Direct liquid injection CVD (DLICVD) – CVD in which the precursors
are in liquid form (liquid or solid dissolved in a convenient solvent).
Liquid solutions are injected in a vaporization chamber towards
injectors (typically car injectors). The precursor vapors are then
transported to the substrate as in classical CVD. This technique is
suitable for use on liquid or solid precursors. High growth rates can be
reached using this technique.
Classified by type of substrate heating:
Hot wall CVD – CVD in which the chamber is heated by an external
power source and the substrate is heated by radiation from the heated
chamber walls.
Cold wall CVD – CVD in which only the substrate is directly heated
either by induction or by passing current through the substrate itself
or a heater in contact with the substrate. The chamber walls are at room
temperature.
Plasma-enhanced CVD (PECVD) – CVD that utilizes plasma to enhance chemical reaction rates of the precursors.
PECVD processing allows deposition at lower temperatures, which is
often critical in the manufacture of semiconductors. The lower
temperatures also allow for the deposition of organic coatings, such as
plasma polymers, that have been used for nanoparticle surface
functionalization.
Remote plasma-enhanced CVD (RPECVD) – Similar to PECVD except that
the wafer substrate is not directly in the plasma discharge region.
Removing the wafer from the plasma region allows processing temperatures
down to room temperature.
Combustion chemical vapor deposition
(CCVD) – Combustion Chemical Vapor Deposition or flame pyrolysis is an
open-atmosphere, flame-based technique for depositing high-quality thin
films and nanomaterials.
Hot filament CVD (HFCVD) – also known as catalytic CVD (Cat-CVD) or
more commonly, initiated CVD, this process uses a hot filament to
chemically decompose the source gases.
The filament temperature and substrate temperature thus are
independently controlled, allowing colder temperatures for better
absorption rates at the substrate and higher temperatures necessary for
decomposition of precursors to free radicals at the filament.
Rapid thermal CVD (RTCVD) – This CVD process uses heating lamps or
other methods to rapidly heat the wafer substrate. Heating only the
substrate rather than the gas or chamber walls helps reduce unwanted
gas-phase reactions that can lead to particle formation.
Vapor-phase epitaxy (VPE)
Photo-initiated CVD (PICVD) – This process uses UV light to
stimulate chemical reactions. It is similar to plasma processing, given
that plasmas are strong emitters of UV radiation. Under certain
conditions, PICVD can be operated at or near atmospheric pressure.
Laser chemical vapor deposition
(LCVD) - This CVD process uses lasers to heat spots or lines on a
substrate in semiconductor applications. In MEMS and in fiber production
the lasers are used rapidly to break down the precursor gas—process
temperature can exceed 2000 °C—to build up a solid structure in much the
same way as laser sintering based 3-D printers build up solids from
powders.
Uses
CVD is
commonly used to deposit conformal films and augment substrate surfaces
in ways that more traditional surface modification techniques are not
capable of. CVD is extremely useful in the process of atomic layer deposition at depositing extremely thin layers of material. A variety of applications for such films exist. Gallium arsenide is used in some integrated circuits (ICs) and photovoltaic devices. Amorphous polysilicon is used in photovoltaic devices. Certain carbides and nitrides confer wear-resistance.
Polymerization by CVD, perhaps the most versatile of all applications,
allows for super-thin coatings which possess some very desirable
qualities, such as lubricity, hydrophobicity and weather-resistance to
name a few. The CVD of metal-organic frameworks, a class of crystalline nanoporous materials, has recently been demonstrated. Recently scaled up as an integrated cleanroom process depositing large-area substrates, the applications for these films are anticipated in gas sensing and low-κ dielectrics.
CVD techniques are advantageous for membrane coatings as well, such as
those in desalination or water treatment, as these coatings can be
sufficiently uniform (conformal) and thin that they do not clog membrane
pores.
This reaction is usually performed in LPCVD systems, with either pure silane feedstock, or a solution of silane with 70–80% nitrogen. Temperatures between 600 and 650 °C and pressures between 25 and 150 Pa yield a growth rate between 10 and 20 nm per minute. An alternative process uses a hydrogen-based
solution. The hydrogen reduces the growth rate, but the temperature is
raised to 850 or even 1050 °C to compensate. Polysilicon may be grown
directly with doping, if gases such as phosphine, arsine or diborane are added to the CVD chamber. Diborane increases the growth rate, but arsine and phosphine decrease it.
Silicon dioxide
Silicon
dioxide (usually called simply "oxide" in the semiconductor industry)
may be deposited by several different processes. Common source gases
include silane and oxygen, dichlorosilane (SiCl2H2) and nitrous oxide (N2O), or tetraethylorthosilicate (TEOS; Si(OC2H5)4). The reactions are as follows:
SiH4 + O2 → SiO2 + 2 H2
SiCl2H2 + 2 N2O → SiO2 + 2 N2 + 2 HCl
Si(OC2H5)4 → SiO2 + byproducts
The choice of source gas depends on the thermal stability of the substrate; for instance, aluminium
is sensitive to high temperature. Silane deposits between 300 and
500 °C, dichlorosilane at around 900 °C, and TEOS between 650 and
750 °C, resulting in a layer of low- temperature oxide (LTO). However, silane produces a lower-quality oxide than the other methods (lower dielectric strength, for instance), and it deposits nonconformally.
Any of these reactions may be used in LPCVD, but the silane reaction is
also done in APCVD. CVD oxide invariably has lower quality than thermal oxide, but thermal oxidation can only be used in the earliest stages of IC manufacturing.
Oxide may also be grown with impurities (alloying or "doping"). This may have two purposes. During further process steps that occur at high temperature, the impurities may diffuse
from the oxide into adjacent layers (most notably silicon) and dope
them. Oxides containing 5–15% impurities by mass are often used for this
purpose. In addition, silicon dioxide alloyed with phosphorus pentoxide
("P-glass") can be used to smooth out uneven surfaces. P-glass softens
and reflows at temperatures above 1000 °C. This process requires a
phosphorus concentration of at least 6%, but concentrations above 8% can
corrode aluminium. Phosphorus is deposited from phosphine gas and
oxygen:
4 PH3 + 5 O2 → 2 P2O5 + 6 H2
Glasses
containing both boron and phosphorus (borophosphosilicate glass, BPSG)
undergo viscous flow at lower temperatures; around 850 °C is achievable
with glasses containing around 5 weight % of both constituents, but
stability in air can be difficult to achieve. Phosphorus oxide in high
concentrations interacts with ambient moisture to produce phosphoric
acid. Crystals of BPO4 can also precipitate from the flowing
glass on cooling; these crystals are not readily etched in the standard
reactive plasmas used to pattern oxides, and will result in circuit
defects in integrated circuit manufacturing.
Besides these intentional impurities, CVD oxide may contain
byproducts of the deposition. TEOS produces a relatively pure oxide,
whereas silane introduces hydrogen impurities, and dichlorosilane
introduces chlorine.
Lower temperature deposition of silicon dioxide and doped glasses
from TEOS using ozone rather than oxygen has also been explored (350 to
500 °C). Ozone glasses have excellent conformality but tend to be
hygroscopic – that is, they absorb water from the air due to the
incorporation of silanol (Si-OH) in the glass. Infrared spectroscopy and
mechanical strain as a function of temperature are valuable diagnostic
tools for diagnosing such problems.
Silicon nitride
Silicon
nitride is often used as an insulator and chemical barrier in
manufacturing ICs. The following two reactions deposit silicon nitride
from the gas phase:
3 SiH4 + 4 NH3 → Si3N4 + 12 H2
3 SiCl2H2 + 4 NH3 → Si3N4 + 6 HCl + 6 H2
Silicon nitride deposited by LPCVD contains up to 8% hydrogen. It also experiences strong tensile stress, which may crack films thicker than 200 nm. However, it has higher resistivity and dielectric strength than most insulators commonly available in microfabrication (1016Ω·cm and 10 MV/cm, respectively).
Another two reactions may be used in plasma to deposit SiNH:
2 SiH4 + N2 → 2 SiNH + 3 H2
SiH4 + NH3 → SiNH + 3 H2
These films have much less tensile stress, but worse electrical properties (resistivity 106 to 1015 Ω·cm, and dielectric strength 1 to 5 MV/cm).
Metals
Tungsten CVD, used for forming conductive contacts, vias, and plugs on a semiconductor device, is achieved from tungsten hexafluoride (WF6), which may be deposited in two ways:
WF6 → W + 3 F2
WF6 + 3 H2 → W + 6 HF
Other metals, notably aluminium and copper, can be deposited by CVD. As of 2010, commercially cost-effective CVD for copper did not exist, although volatile sources exist, such as Cu(hfac)2. Copper is typically deposited by electroplating. Aluminium can be deposited from triisobutylaluminium (TIBAL) and related organoaluminium compounds.
CVD for molybdenum, tantalum, titanium, nickel is widely used. These metals can form useful silicides
when deposited onto silicon. Mo, Ta and Ti are deposited by LPCVD, from
their pentachlorides. Nickel, molybdenum, and tungsten can be deposited
at low temperatures from their carbonyl precursors. In general, for an
arbitrary metal M, the chloride deposition reaction is as follows:
2 MCl5 + 5 H2 → 2 M + 10 HCl
whereas the carbonyl decomposition reaction can happen spontaneously
under thermal treatment or acoustic cavitation and is as follows:
M(CO)n → M + n CO
the decomposition of metal carbonyls is often violently precipitated
by moisture or air, where oxygen reacts with the metal precursor to form
metal or metal oxide along with carbon dioxide.
Many
variations of CVD can be utilized to synthesize graphene. Although many
advancements have been made, the processes listed below are not
commercially viable yet.
Carbon source
The most popular carbon source that is used to produce graphene is
methane gas. One of the less popular choices is petroleum asphalt,
notable for being inexpensive but more difficult to work with.
Although methane is the most popular carbon source, hydrogen is
required during the preparation process to promote carbon deposition on
the substrate. If the flow ratio of methane and hydrogen are not
appropriate, it will cause undesirable results. During the growth of
graphene, the role of methane is to provide a carbon source, the role of
hydrogen is to provide H atoms to corrode amorphous C, and improve the quality of graphene. But excessive H atoms can also corrode graphene. As a result, the integrity of the crystal lattice is destroyed, and the quality of graphene is deteriorated.
Therefore, by optimizing the flow rate of methane and hydrogen gases in
the growth process, the quality of graphene can be improved.
Use of catalyst
The use of catalyst is viable in changing the physical process of
graphene production. Notable examples include iron nanoparticles, nickel
foam, and gallium vapor. These catalysts can either be used in situ
during graphene buildup or situated at some distance away at the deposition area. Some catalysts require another step to remove them from the sample material.
The direct growth of high-quality, large single-crystalline
domains of graphene on a dielectric substrate is of vital importance for
applications in electronics and optoelectronics. Combining the
advantages of both catalytic CVD and the ultra-flat dielectric
substrate, gaseous catalyst-assisted CVD paves the way for synthesizing high-quality graphene for device applications while avoiding the transfer process.
Physical conditions
Physical conditions such as surrounding pressure, temperature,
carrier gas, and chamber material play a big role in production of
graphene.
Most systems use LPCVD with pressures ranging from 1 to 1500 Pa.However, some still use APCVD.
Low pressures are used more commonly as they help prevent unwanted
reactions and produce more uniform thickness of deposition on the
substrate.
On the other hand, temperatures used range from 800–1050 °C.
High temperatures translate to an increase of the rate of reaction.
Caution has to be exercised as high temperatures do pose higher danger
levels in addition to greater energy costs.
Carrier gas
Hydrogen gas and inert gases such as argon are flowed into the system.
These gases act as a carrier, enhancing surface reaction and improving
reaction rate, thereby increasing deposition of graphene onto the
substrate.
Chamber material
Standard quartz tubing and chambers are used in CVD of graphene.
Quartz is chosen because it has a very high melting point and is
chemically inert. In other words, quartz does not interfere with any
physical or chemical reactions regardless of the conditions.
Methods of analysis of results
Raman spectroscopy, X-ray spectroscopy, transmission electron
microscopy (TEM), and scanning electron microscopy (SEM) are used to
examine and characterize the graphene samples.
Raman spectroscopy is used to characterize and identify the
graphene particles; X-ray spectroscopy is used to characterize chemical
states; TEM is used to provide fine details regarding the internal
composition of graphene; SEM is used to examine the surface and
topography.
Sometimes, atomic force microscopy (AFM) is used to measure local properties such as friction and magnetism.
Cold wall CVD technique can be used to study the underlying
surface science involved in graphene nucleation and growth as it allows
unprecedented control of process parameters like gas flow rates,
temperature and pressure as demonstrated in a recent study. The study
was carried out in a home-built vertical cold wall system utilizing
resistive heating by passing direct current through the substrate. It
provided conclusive insight into a typical surface-mediated nucleation
and growth mechanism involved in two-dimensional materials grown using
catalytic CVD under conditions sought out in the semiconductor industry.
Graphene nanoribbon
In
spite of graphene's exciting electronic and thermal properties, it is
unsuitable as a transistor for future digital devices, due to the
absence of a bandgap between the conduction and valence bands. This
makes it impossible to switch between on and off states with respect to
electron flow. Scaling things down, graphene nanoribbons of less than
10 nm in width do exhibit electronic bandgaps and are therefore
potential candidates for digital devices. Precise control over their
dimensions, and hence electronic properties, however, represents a
challenging goal, and the ribbons typically possess rough edges that are
detrimental to their performance.
Diamond
Free-standing single-crystal CVD diamond discColorless gem cut from diamond grown by chemical vapor deposition
CVD can be used to produce a synthetic diamond
by creating the circumstances necessary for carbon atoms in a gas to
settle on a substrate in crystalline form. CVD of diamonds has received
much attention in the materials sciences because it allows many new
applications that had previously been considered too expensive. CVD
diamond growth typically occurs under low pressure (1–27 kPa; 0.145–3.926 psi; 7.5–203 Torr)
and involves feeding varying amounts of gases into a chamber,
energizing them and providing conditions for diamond growth on the
substrate. The gases always include a carbon source, and typically
include hydrogen as well, though the amounts used vary greatly depending
on the type of diamond being grown. Energy sources include hot filament, microwave power, and arc discharges,
among others. The energy source is intended to generate a plasma in
which the gases are broken down and more complex chemistries occur. The
actual chemical process for diamond growth is still under study and is
complicated by the very wide variety of diamond growth processes used.
Using CVD, films of diamond can be grown over large areas of
substrate with control over the properties of the diamond produced. In
the past, when high pressure high temperature (HPHT) techniques were
used to produce a diamond, the result was typically very small
free-standing diamonds of varying sizes. With CVD diamond, growth areas
of greater than fifteen centimeters (six inches) in diameter have been
achieved, and much larger areas are likely to be successfully coated
with diamond in the future. Improving this process is key to enabling
several important applications.
The growth of diamond directly on a substrate allows the addition
of many of diamond's important qualities to other materials. Since
diamond has the highest thermal conductivity
of any bulk material, layering diamond onto high heat-producing
electronics (such as optics and transistors) allows the diamond to be
used as a heat sink.
Diamond films are being grown on valve rings, cutting tools, and other
objects that benefit from diamond's hardness and exceedingly low wear
rate. In each case the diamond growth must be carefully done to achieve
the necessary adhesion onto the substrate. Diamond's very high scratch
resistance and thermal conductivity, combined with a lower coefficient of thermal expansion than Pyrex glass, a coefficient of friction close to that of Teflon (polytetrafluoroethylene) and strong lipophilicity would make it a nearly ideal non-stick coating for cookware if large substrate areas could be coated economically.
CVD growth allows one to control the properties of the diamond
produced. In the area of diamond growth, the word "diamond" is used as a
description of any material primarily made up of sp3-bonded
carbon, and there are many different types of diamond included in this.
By regulating the processing parameters—especially the gases
introduced, but also including the pressure the system is operated
under, the temperature of the diamond, and the method of generating
plasma—many different materials that can be considered diamond can be
made. Single-crystal diamond can be made containing various dopants. Polycrystalline diamond consisting of grain sizes from several nanometers to several micrometers can be grown.
Some polycrystalline diamond grains are surrounded by thin, non-diamond
carbon, while others are not. These different factors affect the
diamond's hardness, smoothness, conductivity, optical properties and
more.
Chalcogenides
Commercially, mercury cadmium telluride is of continuing interest for detection of infrared radiation. Consisting of an alloy of CdTe and HgTe, this material can be prepared from the dimethyl derivatives of the respective elements.
Since most of the insulation
is provided by the surrounding air, overhead power lines are generally
the least costly method of power transmission for large quantities of
electric energy.
Towers for support of the lines are made of wood either grown or laminated, steel or aluminum
(either lattice structures or tubular poles), concrete, 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, resilience to storms, ice
loads, earthquakes and other potential damage causes. Today overhead lines are routinely operated at voltages exceeding 765,000 volts between conductors.
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 69 kV, used for distribution in urban and rural areas.
High voltage (HV; subtransmission
less than 100 kV; subtransmission or transmission at voltages 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) – from 345 kV, up to about 800 kV, used for long distance, very high power transmission.
Ultra high voltage (UHV), often associated with ≥ ±800 kVDC and ≥ 1000 kVAC
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. 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.
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" ("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.
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 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, such as 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.
Insulators
Medium-voltage power lines with ceramic insulators in CaliforniaModular
suspension insulators are used for high-voltage lines. The objects
attached to the conductors near the bottom of the insulator are Stockbridge dampers.
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. 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.
China has already developed polymer insulators having a highest system
voltage of 1100 kV and India is currently developing a 1200 kV (highest
system voltage) 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 (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
Sample cross-section of ACSR power line
The most common conductor in use for transmission today is aluminum conductor steel reinforced (ACSR). Also seeing much use is all-aluminum-alloy conductor
(AAAC). Aluminum is used because it has about half the weight and lower
cost of a comparable resistance copper cable. It does, however, require
a larger diameter than copper because of lower specific conductivity. 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 because of their lower electrical resistance, they cost more 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 a smaller conductor is equal to the annual
interest paid on that additional cost of the line construction for a
larger conductor. 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 hanging between two 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 length 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.
Conventional ACSR (left) and modern carbon core (right) conductors
Two such conductors that offer reduced thermal sag are known as composite core conductors (ACCR and ACCC conductor).
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 electric current. The carbon core conductor's
reduced thermal sag allows it to carry up to twice the current
("ampacity") compared to all-aluminum conductor (AAC) or ACSR.
For transmission of power across long distances, high voltage
transmission is employed. Transmission higher than 132 kV poses the
problem of corona discharge,
which causes significant power loss and interference with communication
circuits. To reduce this corona effect, it is preferable to use more
than one conductor per phase, or bundled conductors.
In addition to reducing corona, audible and radio noise (and associated
electrical losses), bundled conductors also increase the amount of
current that can be carried compared to a single conductor of equal
aluminum content due to the skin effect (for AC lines).
Bundle conductors consist of several parallel cables connected at
intervals by spacers, often in a cylindrical configuration. The optimum
number of conductors depends on the current rating, but typically
higher-voltage lines also have higher current. American Electric Power
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.
Spacer damper for four-conductor bundlesBundle conductor attachment
Bundled conductors reduce the electric fieldgradient in the vicinity of the line, to reduce the possibility of corona discharge. At extra high voltage,
the electric field gradient at the surface of a single conductor is
high enough to ionize air, which wastes power, generates unwanted
audible noise and interferes 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.
The transmission efficiency is improved as loss due to corona effect is
countered.
Bundled conductors cool themselves more efficiently due to the
increased surface area of the conductors, further reducing line losses.
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.
While wind resistance is higher, wind-induced oscillation can be
damped at bundle spacers. The ice and wind loading of bundled conductors
will be greater than a single conductor of the same total cross
section, and bundled conductors are more difficult to install than
single conductors. Aeolian vibration
is generally less pronounced on bundled conductors due to the effect of
spacers and spacer dampers installed at relatively close intervals
along the line.
Ground wires
Aluminum conductor crosslinked polyethylene insulation wire. It is used for 6600V power lines.
Overhead power lines are often equipped with a ground conductor
(shield wire, static 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.
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.
HVDC Fenno-Skan with ground wires used as electrode line
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 like.
Medium-voltage
compact overhead power line mounted on a concrete pole in Thailand. The
appearance is similar to a bundle conductor, but this line consists of
three conductors, which are attached to a single, cross-shaped
porcellain insulator.
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.
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 two (most
likely a phase and neutral) up to as many as six (three phase
conductors, separate neutral and earth plus street lighting supplied by a
common switch); a common case is four (three phase and neutral, where
the neutral might also serve as a protective earthing conductor).
Overhead lines or overhead wires are used to transmit electrical
energy to trams, trolleybuses and trains. Overhead lines are 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. In 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 limited 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 the 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.
Linear parks will usually occupy the area under overhead power lines, to provide easy access, and prevent obstacles.
Aviation accidents
An
aviation obstruction marker on a high-voltage overhead transmission
line reminds pilots of the presence of an overhead line. Some markers
are lit at night or have strobe lights.The Ekibastuz–Kokshetau high-voltage line in Kazakhstan. It was the first commercially used power line which operated at 1150 kV, the highest transmission line voltage in the world.
General aviation, hang gliding, paragliding, skydiving, balloon, and
kite flying must avoid accidental contact with power lines. Nearly every
kite product warns users to stay away from power lines. Deaths occur
when aircraft crash into power lines. Some power lines are marked with
obstruction makers, especially near air strips or over waterways that
may support floatplane operations. The placement of power lines
sometimes use up sites that would otherwise be used by hang gliders.
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 built by The L.E. Myers Co. and put into service by the Ohio Valley Electric Corporation 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 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. Starting from
1967 in Russia, and also in the US and Canada, overhead lines for
voltage of 765 kV were built. In 1982 overhead power lines were built in
Soviet Union between Elektrostal and the power station at Ekibastuz, 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. 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 (800–2500 m), 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 with ABCD parameters.
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The circuit can be characterized as
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