IntroductionInsulating materials

Types of cables Laying of cables

 Electrostatic stress in a single core cable

 Dielectric loss and loss tangent of a cable

 Cables for D.C transmission

 Heating of cables

 Current carrying capacity of cables

     

  Tests on   Electrical Materials

  Testing Underground Cables –   Extracts from Indian Standards

OBJECTIVE TYPE QUESTIONS 

HOME takes you to the start page after you have read these  Topics. Start page has links to other topics.

UNDERGROUND CABLES- OBJECTIVE TYPE QUESTIONS

1. Void formation occurs in

a. XLPE cablesb. Oil-filled cables c. Oil-impregnated paper cables

Ans: (c)

2.        Insulation resistance of a cable 20 km long is 1 Meg-ohm. Two cable lengths, 20 km and 10 km, are connected in parallel. The insulation resistance of the parallel combination is

a. 1 Meg-ohmb. 0.5 Meg-ohmc. 0.666 Meg-ohm

Ans: (c)

3.        If the voltage applied to the core and sheath of a cable is halved , the reactive power generated by the cable will be

a.        Halved

b.       1/4 th of the original value

c.        doubled

Ans. b

4.        The dielectric field intensity at a point within the dielectric of a cable

a.        Is constant

b.       Increases with increase of distance of the point from the centre of the cable

c.        decreases with increase of distance of the point from the centre of the cable

Ans. c

5.        Three insulating materials with identical maximum working stress and permittivities of 2.5, 3 and 4 are used in a single-core cable. The location of the materials with respect to the cable core will be

a.        2.5,3,4

b.       3,2.5,4

c.        4,3,2.5

d.       4,2.5,3

Ans. c

6.        Three insulating materials with breakdown strengths of 2.5,3 and 3.5 are used in a single-core cable. If the factor of safety for the materials is 5, the location of   the materials with respect to the core of the cables will be

a.        2.5,3,3.5

b.       3,2.5,3.5

c.        3.5,3,2.5

d.       3.5,2.5,3

Ans. c

7.        If   is the loss angle of a cable , its power factor is

a.        sin

b.       cos

c.         p .f. is independent of 

d.       p. f depends on but not as in ' a' or 'b'

Ans. a

8.        Match List A with List B.

List A                                                                                  List B

Voltage range                                                   Critical design factor for cable

I.                     up to 33 kV                                         p. Thermal instability                                          

II.                   33-132 kV                                          q. Ionization

III.                 above 132 kV                                      r. Impulse strength

The correct matching is

a.        Ip  IIq  IIIr

b.       Iq IIp IIIr

c.        Iq IIr  IIIp

Ans. c

9.        If C1 is the capacitance between any two cores of  a 3-core cable, and C2  is the capacitance between any core and the sheath, then the measured value of the capacitance between any two cores with the third core isolated is equal to

a.        C1C2/(2C1+C2)

b.       0.5(3C1+C2)

c.        3C2

Ans.  b

10.     Sheaths are provided in cables to

a.        Provide proper insulation

b.       Provide mechanical strength

c.        Prevent ingress of moisture

TOP

Introduction

A considerable amount of transmission & distribution, especially in urban areas is carried out by means of underground cables. In order to preserve amenities of both town and countryside the electricity supply authorities resort to underground transmission &

distribution. Underground transmission is more expensive than the overhead alternative.

Insulating materials

Dielectric properties of cable insulation:

1.        high insulation resistance

2.        high dielectric strength

3.        good mechanical properties

4.        immune to attacks by acids & alkalies

5.        non-hygroscopic

Commonly used insulating materials are:

a)       Oil-impregnated paper

b)       Vulcanized India rubber(V.I.R)

c)       Polyvinyl chloride(P.V.C)

d)       SF6 gas

e)       Cross-linked polythene(XLPC)

Void formation

Voids (small pockets of air or gas) are formed in the insulation where constituent parts of the cable are expanded and contracted to different extents with heat evolved on load

cycles. The stress across the voids is high and breakdown results.

TOP

Types of cables

1.        Single-core cable

2.        Three-core cable

(a)     Belted -type construction

(b)     H-type construction

3.        Oil-filled cable

4.        Gas-filled cable- consisting of a conductor supported in a rigid external pipe which is filled with a gas under pressure- usually SF6 at 3* atmospheric pressure

5.        XLPE cables

Laying of cables

a)       Direct in the soil

b)       In ducts or troughs

c)       In circular ducts or pipes

d)       In air

Electrostatic stress in a single core cable

The potential gradient is maximum at the surface of the conductor. Potential gradient art any point at a distance x from the centre of the conductor is

G= V/ [x ln (R/r)]

 Where R is the inner radius of the sheath and, and r is the radius of the conductor.

Grading of cables

a)       Capacitance grading

b)       Intersheath grading

Dielectric loss and loss tangent of a cable

 The dielectric loss, due to leakage and hysterisis effects in the dielectric, is usually expressed in terms of the loss angle,:

= 90-

where is the dielectric power factor angle.

Dielectric loss  = C V2  tan ,

Where

C= capacitance to neutral

V= phase voltage

A typical value of tan lies in the range 0.002 to 0.003. In low voltage cables the dielectric loss is negligible, but is appreciable in EHV cables.

Cables for D.C transmission

Owing to the absence of periodic charging currents with direct voltage, high voltage cables will play an increasingly important role in D.C transmission links. In A.C cable a power factor of 0.003 can be represented by a loss resistance of 3*10 12 ohm -cm. The D.C resistivity of the same dielectric would be greater than 10 14  ohm-cm. Hence the loss in the dielectric on D.C. is only about 3 % of that on A.C. Whereas the electric stress distribution in A.C cables is determined by the dielectric capacitance, in D.C cables it is determined by the electric resistance of the dielectric. The electric resistivity of the conventional dielectrics is very temperature dependent; for oil-impregnated cable, for example, the resistivity at 20 deg. C is 100 times that at 60 deg. C. In D.C cable, thermal considerations not only determine the rating but also influence the electric stress distribution in the dielectric. The electrical resistivity also varies with the electric stress. Instead of electric stress decreasing through the dielectric from the conductor to the sheath, in D.C cables the stress increases and can be larger at the sheath. This is known as stress inversion and can lead to troubles at terminations and joints where the longitudinal stresses are created.

Explain the phenomenon of void formation in cables? Why is the void subjected to excessive potential gradient?

Void formation does not take place in oil-filled cable -why?

Heating of cables

The temperature rise of cable depends on the following factors:

1.        The production of heat within the external periphery of the cable.

2.        The conveyance of the heat as far as the periphery - that is, up to the boundary of the surrounding medium

3.        The conveyance of the heat through this medium, and therefore away from the cable.

4.        The current rating of the cables.

5.        The nature of the load, i.e. whether continuous or intermittent; not infrequently the rating under short-circuit conditions has to be considered.

Heat production

Within the cable, there are three sources of heat:

1.        I2 R loss in  conductors

2.        Dielectric loss

3.        Sheath & armour loss

 What is the equivalent circuit for calculating sheath losses?

Why is cross bonding of sheaths done?

Current carrying capacity of cables

 The limiting factor in current rating is the temperature to which the insulation nearest the conductor can be raised without suffering deterioration.

The allowable values of temperature rise for different types of cables may be obtained from the manufacturer's data books.

The current rating I of a cable neglecting dielectric losses is given by

I = SQRT[ ( -a)/{nR {S1 + (1+)(S2 +G)}}]   , Amp.

Where

= Core temperature

a = ambient temperature

n = number of conductors

R = Resistance of each conductor

= Sheath loss/core loss

S1 = thermal resistance of the dielectric

S2 = thermal resistance of the protective covering

G = thermal resistance of the ground

         Write down the expressions for computing the various thermal resistance components.

         What is the effect of temperature on dielectric loss? Modify the formula for current rating considering the effect of dielectric loss

         Discuss the factors affecting the short-circuit rating of an underground cable

         Name the types of cable used for different voltage levels

TOP

Tests on Electrical Materials

Type Tests – Tests carried out to prove conformity with the specifications. These are intended to prove the general qualities and design of a given type of manufactured item.

Routine Tests-Tests carried out on each part/item manufactured to check parameters (as per requirements0, which are likely to vary during production.

Acceptance Tests- Tests carried out on samples taken at random from offered lot of manufactured item for the purpose of acceptance of lot.

PVC INSULATED CABLES up to and including 1.1 kV  [IS: 1554(part-1)-1988]

TYPE TESTSNo. Type test Purpose

a Tests on conductor

1.        Annealing test (for copper)

2.        Tensile test (for Aluminium)

3.        Wrapping test  (for Aluminium)

4.        Resistance test

To check softness of wire

To check strength of Al wire

To check hardness of Al wire

To check cross-section of the conductor

b Tests for armouring wires/strips To check electrical , mechanical and chemical properties of armouring wire/strip

c Test for thickness of insulation and sheath

To check capability of insulation  to withstand

voltage and its mechanical strength

d Physical test for insulation & sheath1. Tensile strength & elongation at break

To check mechanical stress and strain during manufacturing and bending

2. Ageing in air oven To check physical & chemical changes  in insulation  due to heat with age

3.  Shrinkage test To prevent problem in termination

4.  Hot deformation To check resistance against deformation due to heat & mechanical pressure

5.  Loss of mass in air oven To check physical & chemical changes  in insulation due to heat  and time

6.  Heat shock test To check ability of cable against overheating

7. Thermal stability To check thermal effecte Insulation resistance test To check uniformities of 

insulation in dielectricf High voltage test(Water

immersion test)To check ability of cable in water during service

g High voltage test at room temperature

To check ability of cable against high voltage  during service

h Flammability test To check flame retardant properties

OPTIONAL TYPE TESTS

No. Optional type test Purposea Cold bend test To check effect of low

temperature  during  bending

b Cold impact test To check effect of low temperature on outer sheath in terms of hardness & softness

c Armour resistance test (or other than mining cables

To check  electrical properties of armouring  wire/strip

ROUTINE TESTSNo. Test Purpose

a Resistance test To check cross-section of the conductor

b High voltage test at room temperature

To check ability of cable against high voltage  during service

c Armour resistance test (for mining cables)

To check conductivity of  armouring materials

TOP

ACCEPTANCE TESTSThe following type tests are taken as acceptance tests: Type test Nos. ,  a1, a2, a3,

a4,c,d1, e, and g

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   

SCALE OF SAMPLING

No. of Drums in a Lot

No. of Drums  to be taken as sample

Permissible No. of Defectives

Up to 50 2 051 to 100 5 0101 to 300 13 0301 to 500 20 1

501 and above 32 2

CROSS –LINKED POLYETHYLENE INSULATED PVC SHEATHED CABLES [XLPE from 66 to 220 kV][IS: 7098 (Part-3)-1988]

TYPE TESTS:

No. Type Test Purpose

a

Tests on conductor

1. Annealing Test (for Cu)To check softness of wire

2. Resistance Test To check cross-section of the conductor

b Physical tests on insulation

1. Test  for thickness & dimensions of insulation

To check capability of insulation to withstand voltage and its mechanical strength

2. Tensile strength & elongation at break To check mechanical stress & strain during manufacturing & bending

3. Thermal ageing in oven To check physical & chemical changes  in insulation  due to heat with age

4. Hot set test To check cross-linking of insulating material

5. Shrinkage test To prevent problem in termination

6.Void & contaminants test To check voids & contaminants

c Resistivity test for semi conducting layers To check resistance of semi-conducting layer

d

Test for concentric metallic screen

i)         Test for concentric  metallic screen

ii)       Test for concentric copper tape

To check capacity against short circuit

e Thickness of  metallic sheathTo check capability of insulation  to withstand voltage and its mechanical strength

f

Tests for armouring material

1. DimensionsTo check that dimensions are within limits

2.Tensile strength & elongation at break To check mechanical stress and strain during manufacturing and bending

3. Wrapping test To check mechanical strength during bending

4. Resistivity test To check resistance of armouring material

g Physical tests for outer sheath

1 Measurement of thickness To check mechanical strength

2 PVC Sheath To know the material used

1. Tensile strength & elongation at break To check mechanical stress and strain during manufacturing and bending

2.  Thermal Ageing in air ovenTo check physical & chemical changes  in sheath  due to heat with age

3. Loss of mass To check physical & chemical changes  in insulation due to heat and time

4. Heat shock test To check ability of cable against overheating

5. Hot deformation test To check resistance against deformation due to heat & mechanical pressure

6. Shrinkage test To prevent problem in termination

7. Thermal Stability To check thermal effect

3. PE SHEATH To know the material used

1. Carbon black content To know the % of carbon

2. Tensile strength & elongation at break before & after ageing

To check mechanical stress and strain during manufacturing and bending

3. Hot deformation To check resistance against deformation due to heat & mechanical pressure

h Flammability test (for PVC outer sheathed cable only)

To check flame retardant  properties

j Water tightness test To check penetration of water in cable

k 1. Thermal ageing on complete cable sample To check physical &chemical changes  in cable due to heat with age

2. Tensile strength & elongation at break for insulation & outer sheath

To check mechanical stress and strain during manufacturing and bending

3. Resistivity test for semi- conducting layers To know resistance of semi-conducting layer

m Bending test followed by P. D. test To check bending radius during bending while installation & handling

n Dielectric power factor measurement at ambient temperature

To check rupturing capacity & voids

p Dielectric power factor measurement at elevated temperature

To check impurities & voids

q Load cycle test followed by P.D To check capacity of cable  under loading

measurement conditions

r Impulse withstand test followed by HV test To check ability of insulating material to withstand  lightning voltage

NOTES: Tests from  (n) to  ( r ) shall be performed  successively  on the same test sample of complete cable, not less than 10 m length between test accessories

Tests at (p) and (q)  may be carried out on different samples.

OPTIONAL TYPE TEST:

No. Type  Test Purpose

1 Cold impact test for outer sheath To check effect of low temperature  on outer sheath  in terms of hardness & softness

ROUTINE TESTS

No. Routine  Test Purpose

a Conductor resistance test To check cross-section of the conductorb P. D. test To check small voids and cavities in

insulationc HV test To check ability of cable in service

ACCEPTANCE TESTS- 

No. Acceptance   Test Purpose

1 Measurement of capacitance To check impurities  & voids

The following Type Tests will be used as Acceptance Tests:

a1,a2, b1, b4,b6,e, g1

The following Routine  Tests will be used as Acceptance Tests:

b, c

Partial discharge test shall be carried out on full drum  length

SCALE OF SAMPLING

                               

No. of Drums in a Lot

No. of Drums  to be taken as sample

Permissible No. of Defectives

Up to 25 3 026 to 50 5 051 to 100 8 0101 to 300 13 1

301 and above 20 1

DRUMS FOR EECTRIC CABLES [IS: 10418-1982]

The tests under TYPE, ROUTINE and ACCEPTANCE categories are not specified in the Indian Standards. However, the following checks shall be made on DRUMS & their

components.

CHECKS FOR CONSTRUCTION OF DRUM:

S. No.

Description Purpose

1 Mechanical strength (a)     Transverse loading test

(b)     Impact test

(c)      Barrel batten test

2 Flange & outside surface Free from protruding materials or Projections or unevenness capable of damaging the cable/hands

3 Flanges (Main Discs)  construction

a)       For dia. Up to 1600mm- 2 ply OR 3 ply construction.

b)       For dia. Above 1600 mm- 2 full ply OR 3 full ply plus  1 segmental layer construction

(Segments shall not be less than six)

Width of middle plank (Minimum)

For flange dia up to 700mm-             100mm

Dia 701 mm-1600mm-                          150 mm

Dia above 1600 mm-    200 mm

4 Barrel end- supports Shall be complete circular discs or of various segments. Securely fixed to inside of flanges by nailing

5 Barrel middle-supports Shall be complete circular construction of single/two ply layers (at 90 0) OR of various segments (Only for drums having transverse above 1000 mm).

6 Stretchers (Core carrier planks) To be provided for drum sizes of 1206 mm and above

7 Tolerances in mm mmDrum flange dia ,up to & including 1600 mmAbove 1600 mm

+/- 20+/-30

Flange thickness up to & including 1600 mmAbove 1600 mm

+/- 06+/-09

 Barrel dia. up to & including 1600 mmAbove 1600 mm

+/- 20+/-30

Overall & transverse widths +/- 25Barrel battens thickness +/-3Stretchers thickness +/-3Centre hole dia. with bush 0 to+2Centre hole dia. without  bush 0 to +5

Other standards are:

CROSS –LINKED POLYETHYLENE INSULATED PVC SHEATHED CABLES[XLPE up to 3.3 kV][IS:7098 (Part-1)-1988]

CROSS –LINKED POLYETHYLENE INSULATED PVC SHEATHED CABLES[XLPE from 3.3to 33 kV][IS:7098 (Part-2)-1988]

Summary:The physical and electrical properties of crosslinked polyethylene (XLPE) and ethylene propylene rubber (EPR) are compared in the context of their use in transmission class cables. Results indicate that the 138-kV XLPE cable has AC withstand/breakdown strength at least 25% higher than the 150-kV EPR cable. The XLPE cable exhibits about 70% higher impulse strength than the EPR cable. The loss factor of the XLPE cable is at least 20 times lower than that of EPR cable. Thus with XLPE cables, the yearly energy savings can be on the order of 15 MWh/cct. km for a 69-kV system, 52 MWh/cct. km for a 138-kV system and 127 MHh/cct. km for a 230-kV system.<>

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Electric power transmissionFrom Wikipedia, the free encyclopedia

Jump to: navigation, search"Electric transmission" redirects here. For Vehicle transmissions, see Diesel-electric transmission.

Transmission lines

Electric power transmission is the bulk transfer of electrical energy, a process in the delivery of electricity to consumers. A power transmission network typically connects power plants to multiple substations near a populated area. The wiring from substations to customers is referred to as electricity distribution, following the historic business model separating the wholesale electricity transmission business from distributors who deliver the electricity to the homes.[1] Electric power transmission allows distant energy sources (such as hydroelectric power plants) to be connected to consumers in population centers, and may allow exploitation of low-grade fuel resources such as coal that would otherwise be too costly to transport to generating facilities.

Usually transmission lines use three phase alternating current (AC). Single phase AC current is sometimes used in a railway electrification system. High-voltage direct current systems are used for long distance transmission, or some undersea cables, or for connecting two different ac networks.

Electricity is transmitted at high voltages (110 kV or above) to reduce the energy lost in transmission. Power is usually transmitted as alternating current through overhead power lines. Underground power transmission is used only in densely populated areas because of its higher cost of installation and maintenance when compared with overhead wires,and the difficulty of voltage control on long cables.

A power transmission network is referred to as a "grid". Multiple redundant lines between points on the network are provided so that power can be routed from any power plant to any load center, through a variety of routes, based on the economics of the transmission path and the cost of power. Much analysis is done by transmission companies to determine the maximum reliable capacity of each line, which, due to system stability considerations, may be less than the physical or thermal limit of the line. Deregulation of electricity companies in many countries has led to renewed interest in reliable economic design of transmission networks. However, in some places the gaming of a deregulated energy system has led to disaster, such as that which occurred during the California electricity crisis of 2000 and 2001.[2]

Diagram of an electrical system.

Contents

[hide] 1 Overhead transmission 2 Underground transmission 3 History 4 Bulk power transmission

o 4.1 Grid input o 4.2 Losses o 4.3 Transmission grid exit

5 High-voltage direct current 6 Limitations 7 Control

o 7.1 Load balancing o 7.2 Failure protection

8 Communications 9 Electricity market reform 10 Merchant transmission 11 Health concerns 12 Government policy 13 Special transmission

o 13.1 Grids for railways o 13.2 Radio frequency power transmission o 13.3 Superconducting cables o 13.4 Single wire earth return o 13.5 Wireless power transmission

14 Cyber-warfare 15 Records 16 See also 17 Notes 18 Further reading

19 External links

[edit] Overhead transmission

Overhead conductors are not covered by insulation. The conductor material is nearly always an aluminum alloy, made into several strands and possibly reinforced with steel strands. Copper was sometimes used for overhead transmission but aluminum is lower in weight for equivalent performance, and much lower in cost. Overhead conductors are a commodity supplied by several companies worldwide. Improved conductor material and shapes are regularly used to allow increased capacity and modernize transmission circuits. Conductor sizes range from 12 mm² (#6 American wire gauge) to 750 mm² (1,590,000 circular mils area), with varying resistance and current-carrying capacity. Thicker wires would lead to a relatively small increase in capacity due to the skin effect, that causes most of the current to flow close to the surface of the wire.

United States power transmission grid consists of 300,000 km of lines operated by 500 companies.

Today, transmission-level voltages are usually considered to be 110 kV and above. Lower voltages such as 66 kV and 33 kV are usually considered sub-transmission voltages but are occasionally used on long lines with light loads. Voltages less than 33 kV are usually used for distribution. Voltages above 230 kV are considered extra high voltage and require different designs compared to equipment used at lower voltages.

Since overhead transmission lines are uninsulated, design of these lines requires minimum clearances to be observed to maintain safety. Adverse weather conditions of high wind and low temperatures can lead to power outages: wind speeds as low as 23 knots (43 km/h) can permit conductors to encroach operating clearances, resulting in a

flashover and loss of supply.[3] Oscillatory motion of the physical line can be termed gallop or flutter depending on the frequency and amplitude of oscillation.

[edit] Underground transmission

Electric power can also be transmitted by underground power cables instead of overhead power lines. They can assist the transmission of power across:

Densely populated urban areas Areas where land is unavailable or planning consent is difficult Rivers and other natural obstacles Land with outstanding natural or environmental heritage Areas of significant or prestigious infrastructural development Land whose value must be maintained for future urban expansion and rural

development

Some other advantages of underground power cables:

Less subject to damage from severe weather conditions (mainly wind and freezing)

Greatly reduced emission, into the surrounding area, of electromagnetic fields (EMF). All electric currents generate EMF, but the shielding provided by the earth surrounding underground cables restricts their range and power. See section below, "Health concerns".

Underground cables need a narrower surrounding strip of about 1- 10 meters to install, whereas an overhead line requires a surrounding strip of about 20- 200 meters wide to be kept permanently clear for safety, maintenance and repair.

Underground cables pose no hazard to low flying aircraft or to wildlife, and are significantly safer as they pose no shock hazard (except to the unwary digger).

Some disadvantages of underground power cables:

Undergrounding is more expensive, since the cost of burying cables at transmission voltages is several times greater than overhead power lines, and the life-cycle cost of an underground power cable is two to four times the cost of an overhead power line.[4] According to the British Stakeholder Advisory Group on ELF EMFs[5], the cost is around GBP 10M/km, compared to GBP 0.5-1M/km for overhead lines. This is mainly due to the limit of the physical properties of the insulation placed during installation, keeping the runs to hundreds of meters between splices, which are most commonly placed in manholes or splice-boxes for repairs.

Whereas finding and repairing overhead wire breaks can be accomplished in hours, underground repairs can take days or weeks[6], and for this reason redundant lines are run.

Operations are more difficult since the high reactive power of underground cables produces large charging currents and so makes voltage control more difficult.[7]

The advantages can in some cases outweigh the disadvantages of the higher investment cost, and more expensive maintenance and management.

Most high-voltage underground cables for power transmission that are currently sold on the market are insulated by a sheath of cross-linked polyethylene (XLPE). Some cable may have a lead or aluminum jacket in conjunction with XLPE insulation to allow for fiber optics to be seamlessly integrated within the cable. Before 1960, underground power cables were insulated with oil and paper and ran in a rigid steel pipe, or a semi-rigid aluminum or lead jacket or sheath. The oil was kept under pressure to prevent formation of voids that would allow partial discharges within the cable insulation. There are still many of these oil-and-paper insulated cables in use worldwide. Between 1960 and 1990, polymers became more widely used at distribution voltages, mostly EPDM (ethylene propylene diene M-class); however, their relative unreliability - particularly early XLPE - resulted in a slow uptake at transmission voltages. While cables of 330kV are commonly constructed using XLPE, this has occurred only in recent decades.

[edit] History

Main article: History of electric power transmission

New York City streets in 1890. Besides telegraph lines, multiple electric lines were required for each class of device requiring different voltages.

In the early days of commercial use of electric power, transmission of electric power at the same voltage as used by lighting and mechanical loads restricted the distance between generating plant and consumers. In 1882 generation was with direct current, which could not easily be increased in voltage for long-distance transmission. Different classes of loads – for example, lighting, fixed motors, and traction (railway) systems – required different voltages, and so used different generators and circuits. [8]

Due to this specialization of lines and because transmission was so inefficient that generators needed to be close by their loads, it seemed at the time that the industry would

develop into what is now known as a distributed generation system with large numbers of small generators located nearby their loads. [9]

In 1886 in Great Barrington, Massachusetts, a 1kV AC distribution system was installed. That same year, AC power at 2kV, transmitted 30 km, was installed at Cerchi, Italy. At an AIEE meeting on May 16, 1888, Nikola Tesla delivered a lecture entitled A New System of Alternating Current Motors and Transformers, describing the equipment which allowed efficient generation and use of polyphase alternating currents. The transformer, and Tesla's polyphase and single-phase induction motors, were essential for a combined AC distribution system for both lighting and machinery. Ownership of the rights to the Tesla patents was a key commercial advantage to the Westinghouse Company in offering a complete alternating current power system for both lighting and power.

Nikola Tesla's Alternating current polyphase generators on display at the 1893 World's Fair in Chicago. Tesla's polyphase innovations revolutionized transmission.

Regarded as one of the most influential innovations for the use of electricity, the "universal system" used transformers to step-up voltage from generators to high-voltage transmission lines, and then to step-down voltage to local distribution circuits or industrial customers[10]. By a suitable choice of utility frequency, both lighting and motor loads could be served. Rotary converters and later mercury-arc valves and other rectifier equipment allowed DC load to be served by local conversion where needed. Even generating stations and loads using different frequencies could be interconnected using rotary converters. By using common generating plants for every type of load, important economies of scale were achieved, lower overall capital investment was required, load factor on each plant was increased allowing for higher efficiency, a lower cost for the consumer and increased overall use of electric power.

By allowing multiple generating plants to be interconnected over a wide area, electricity production cost was reduced. The most efficient available plants could be used to supply the varying loads during the day. Reliability was improved and capital investment cost was reduced, since stand-by generating capacity could be shared over many more customers and a wider geographic area. Remote and low-cost sources of energy, such as

hydroelectric power or mine-mouth coal, could be exploited to lower energy production cost. [10]

The first transmission of three-phase alternating current using high voltage took place in 1891 during the international electricity exhibition in Frankfurt. A 25 kV transmission line, approximately 175 kilometers long, connected Lauffen on the Neckar and Frankfurt.

Voltages used for electric power transmission increased throughout the 20th century. By 1914 fifty-five transmission systems each operating at more than 70 kV were in service. The highest voltage then used was 150 kV. [11]

The rapid industrialization in the 20th century made electrical transmission lines and grids a critical part of the economic infrastructure in most industrialized nations. Interconnection of local generation plants and small distribution networks was greatly spurred by the requirements of World War I, where large electrical generating plants were built by governments to provide power to munitions factories; later these plants were connected to supply civil load through long-distance transmission. [12]

[edit] Bulk power transmission

Engineers design transmission networks to transport the energy as efficiently as feasible, while at the same time taking into account economic factors, network safety and redundancy. These networks use components such as power lines, cables, circuit breakers, switches and transformers.

A transmission substation decreases the voltage of incoming electricity, allowing it to connect from long distance high voltage transmission, to local lower voltage distribution. It also reroutes power to other transmission lines that serve local markets. A transmission substation may include phase-shifting or voltage regulating transformers. This is the PacifiCorp Hale Substation, Orem, Utah.

Transmission efficiency is improved by increasing the voltage using a step-up transformer, which reduces the current in the conductors, while keeping the power transmitted nearly equal to the power input. The reduced current flowing through the conductor reduces the losses in the conductor and since, according to Joule's Law, the losses are proportional to the square of the current. Halving the current makes the transmission loss one quarter the original value.

A transmission grid is a network of power stations, transmission circuits, and substations. Energy is usually transmitted within the grid with three-phase AC. DC systems require relatively costly conversion equipment which may be economically justified for particular projects. Single phase AC is used only for distribution to end users since it is not usable for large polyphase induction motors. In the 19th century two-phase transmission was used, but required either three wires with unequal currents or four wires. Higher order phase systems require more than three wires, but deliver marginal benefits.

The synchronous grids of Eurasia.

The capital cost of electric power stations is so high, and electric demand is so variable, that it is often cheaper to import some portion of the needed power than to generate it locally. Because nearby loads are often correlated (hot weather in the Southwest portion of the United States might cause many people there to turn on their air conditioners), electricity must often come from distant sources. Because of the economics of load balancing, wide area transmission grids now span across countries and even large portions of continents. The web of interconnections between power producers and consumers ensures that power can flow, even if a few links are inoperative.

The unvarying (or slowly varying over many hours) portion of the electric demand is known as the "base load", and is generally served best by large facilities (and therefore efficient due to economies of scale) with low variable costs for fuel and operations, i.e. nuclear, coal, hydro. Renewables such as solar, wind, ocean/tidal, etc. are not considered "base load" but can still add power to the grid. Smaller and higher cost sources, such as combined cycle or combustion turbine plants fueled by natural gas are then added as needed.

A high-power electrical transmission tower.

Long-distance transmission of electricity (thousands of kilometers) is cheap and efficient, with costs of US$ 0.005 to 0.02 per kilowatt-hour (compared to annual averaged large producer costs of US$ 0.01 to US$ 0.025 per kilowatt-hour, retail rates upwards of US$ 0.10 per kilowatt-hour, and multiples of retail for instantaneous suppliers at unpredicted highest demand moments).[13] Thus distant suppliers can be cheaper than local sources (e.g. New York City buys a lot of electricity from Canada). Multiple local sources (even if more expensive and infrequently used) can make the transmission grid more fault tolerant to weather and other disasters that can disconnect distant suppliers.

Long distance transmission allows remote renewable energy resources to be used to displace fossil fuel consumption. Hydro and wind sources can't be moved closer to populous cities, and solar costs are lowest in remote areas where local power needs are minimal. Connection costs alone can determine whether any particular renewable alternative is economically sensible. Costs can be prohibitive for transmission lines, but various proposals for massive infrastructure investment in high capacity, very long distance super grid transmission networks could be recovered with modest usage fees.

[edit] Grid input

At the generating plants the energy is produced at a relatively low voltage between about 2300 volts and 30,000 volts, depending on the size of the unit. The generator terminal voltage is then stepped up by the power station transformer to a higher voltage (115 kV to 765 kV AC, varying by country) for transmission over long distances.

[edit] Losses

Transmitting electricity at high voltage reduces the fraction of energy lost to resistance. For a given amount of power, a higher voltage reduces the current and thus the resistive losses in the conductor. For example, raising the voltage by a factor of 10 reduces the

current by a corresponding factor of 10 and therefore the losses by a factor of 100, provided the same sized conductors are used in both cases. Even if the conductor size

(cross-sectional area) is reduced x10 to match the lower current the losses are still

reduced x10. Long distance transmission is typically done with overhead lines at voltages of 115 to 1,200 kV. At extremely high voltages, more than 2,000 kV between conductor and ground, corona discharge losses are so large that they can offset the lower resistance loss in the line conductors.

Transmission and distribution losses in the USA were estimated at 7.2% in 1995 [2], and in the UK at 7.4% in 1998. [3]

As of 1980, the longest cost-effective distance for electricity was 7,000 km (4,000 miles), although all present transmission lines are considerably shorter.[4]

In an alternating current circuit, the inductance and capacitance of the phase conductors can be significant. The currents that flow in these components of the circuit impedance constitute reactive power, which transmits no energy to the load. Reactive current flow causes extra losses in the transmission circuit. The ratio of real power (transmitted to the load) to apparent power is the power factor. As reactive current increases, the reactive power increases and the power factor decreases. For systems with low power factors, losses are higher than for systems with high power factors. Utilities add capacitor banks and other components throughout the system — such as phase-shifting transformers, static VAR compensators, physical transposition of the phase conductors, and flexible AC transmission systems (FACTS) — to control reactive power flow for reduction of losses and stabilization of system voltage.

[edit] Transmission grid exit

At the substations, transformers reduce the voltage to a lower level for distribution to commercial and residential users. This distribution is accomplished with a combination of sub-transmission (33 kV to 115 kV, varying by country and customer requirements) and distribution (3.3 to 25 kV). Finally, at the point of use, the energy is transformed to low voltage (100 to 600 V, varying by country and customer requirements - see Mains power systems).

[edit] High-voltage direct current

Main article: High-voltage direct current

High voltage direct current (HVDC) is used to transmit large amounts of power over long distances or for interconnections between asynchronous grids. When electrical energy is required to be transmitted over very long distances, it is more economical to transmit using direct current instead of alternating current. For a long transmission line, the lower losses and reduced construction cost of a DC line can offset the additional cost of converter stations at each end. Also, at high AC voltages, significant (although economically acceptable) amounts of energy are lost due to corona discharge, the capacitance between phases or, in the case of buried cables, between phases and the soil or water in which the cable is buried.

HVDC links are sometimes used to stabilize against control problems with the AC electricity flow. In other words, to transmit AC power as AC when needed in either direction between Seattle and Boston would require the (highly challenging) continuous real-time adjustment of the relative phase of the two electrical grids. With HVDC instead the interconnection would: (1) Convert AC in Seattle into HVDC. (2) Use HVDC for the three thousand miles of cross country transmission. Then (3) convert the HVDC to locally synchronized AC in Boston, and optionally in other cooperating cities along the transmission route. One prominent example of such a transmission line is the Pacific DC Intertie located in the Western United States.

[edit] Limitations

The amount of power that can be sent over a transmission line is limited. The origins of the limits vary depending on the length of the line. For a short line, the heating of conductors due to line losses sets a "thermal" limit. If too much current is drawn, conductors may sag too close to the ground, or conductors and equipment may be damaged by overheating. For intermediate-length lines on the order of 100 km (60 miles), the limit is set by the voltage drop in the line. For longer AC lines, system stability sets the limit to the power that can be transferred. Approximately, the power flowing over an AC line is proportional to the sine of the phase angle of the voltage at the receiving and transmitting ends. Since this angle varies depending on system loading and generation, it is undesirable for the angle to approach 90 degrees. Very approximately, the allowable product of line length and maximum load is proportional to the square of the system voltage. Series capacitors or phase-shifting transformers are used on long lines to improve stability. High-voltage direct current lines are restricted only by thermal and voltage drop limits, since the phase angle is not material to their operation.

Up to now, it has been almost impossible to foresee the temperature distribution along the cable route, so that the maximum applicable current load was usually set as a compromise between understanding of operation conditions and risk minimization. The availability of industrial Distributed Temperature Sensing (DTS) systems that measure in real time temperatures all along the cable is a first step in monitoring the transmission system capacity. This monitoring solution is based on using passive optical fibers as temperature sensors, either integrated directly inside a high voltage cable or mounted externally on the cable insulation. A solution for overhead lines is also available. In this case the optical fiber is integrated into the core of a phase wire of overhead transmission lines (OPPC). The integrated Dynamic Cable Rating (DCR) or also called Real Time Thermal Rating (RTTR) solution enables not only to continuously monitor the temperature of a high voltage cable circuit in real time, but to safely utilize the existing network capacity to its maximum. Furthermore it provides the ability to the operator to predict the behavior of the transmission system upon major changes made to its initial operating conditions.

[edit] Control

To ensure safe and predictable operation the components of the transmission system are controlled with generators, switches, circuit breakers and loads. The voltage, power, frequency, load factor, and reliability capabilities of the transmission system are designed to provide cost effective performance for the customers.

[edit] Load balancing

The transmission system provides for base load and peak load capability, with safety and fault tolerance margins. The peak load times vary by region largely due to the industry mix. In very hot and very cold climates home air conditioning and heating loads have an effect on the overall load. They are typically highest in the late afternoon in the hottest part of the year and in mid-mornings and mid-evenings in the coldest part of the year. This makes the power requirements vary by the season and the time of day. Distribution system designs always take the base load and the peak load into consideration.

The transmission system usually does not have a large buffering capability to match the loads with the generation. Thus generation has to be kept matched to the load, to prevent overloading failures of the generation equipment.

Multiple sources and loads can be connected to the transmission system and they must be controlled to provide orderly transfer of power. In centralized power generation, only local control of generation is necessary, and it involves synchronization of the generation units, to prevent large transients and overload conditions.

In distributed power generation the generators are geographically distributed and the process to bring them online and offline must be carefully controlled. The load control signals can either be sent on separate lines or on the power lines themselves. To load balance the voltage and frequency can be used as a signaling mechanism.

In voltage signaling the variation of voltage is used to increase generation. The power added by any system increases as the line voltage decreases. This arrangement is stable in principle. Voltage based regulation is complex to use in mesh networks, since the individual components and setpoints would need to be reconfigured every time a new generator is added to the mesh.

In frequency signaling, the generating units match the frequency of the power transmission system. In Droop speed control, if the frequency decreases, the power is increased. (The drop in line frequency is an indication that the increased load is causing the generators to slow down.)

Wind turbines, v2g and other distributed storage and generation systems can be connected to the power grid, and interact with it to improve system operation.

[edit] Failure protection

Under excess load conditions, the system can be designed to fail gracefully rather than all at once. Brownouts occur when the supply power drops below the demand. Blackouts occur when the supply fails completely.

Rolling blackouts, or load shedding, are intentionally-engineered electrical power outages, used to distribute insufficient power when the demand for electricity exceeds the supply.

[edit] Communications

Operators of long transmission lines require reliable communications for control of the power grid and, often, associated generation and distribution facilities. Fault-sensing protection relays at each end of the line must communicate to monitor the flow of power into and out of the protected line section so that faulted conductors or equipment can be quickly de-energized and the balance of the system restored. Protection of the transmission line from short circuits and other faults is usually so critical that common carrier telecommunications are insufficiently reliable, and in remote areas a common carrier may not be available. Communication systems associated with a transmission project may use:

Microwaves Power line communication Optical fibers

Rarely, and for short distances, a utility will use pilot-wires strung along the transmission line path. Leased circuits from common carriers are not preferred since availability is not under control of the electric power transmission organization.

Transmission lines can also be used to carry data: this is called power-line carrier, or PLC. PLC signals can be easily received with a radio for the long wave range.

Optical fibers can be included in the stranded conductors of a transmission line, in the overhead shield wires. These cables are known as OPGW or Optical Ground Wire. Sometimes a standalone cable is used, ADSS or All Dielectric Self Supporting cable, attached to the transmission line cross arms.

Some jurisdictions, such as Minnesota, prohibit energy transmission companies from selling surplus communication bandwidth or acting as a telecommunications common carrier. Where the regulatory structure permits, the utility can sell capacity in extra dark fibers to a common carrier, providing another revenue stream.

[edit] Electricity market reform

It has been suggested that this article or section be merged into Electricity market. (Discuss)

Some regulators regard electric transmission to be a natural monopoly [14] [15] and there are moves in many countries to separately regulate transmission (see Electricity market).

Spain was the first country to establish a Regional Transmission Organization. In that country transmission operations and market operations are controlled by separate companies. The transmission system operator is Red Eléctrica de España (REE) and the wholesale electricity market operator is Operador del Mercado Ibérico de Energía - Polo Español, S.A. (OMEL) [5]. Spain's transmission system is interconnected with those of France, Portugal, and Morocco.

In the United States and parts of Canada, electrical transmission companies operate independently of generation and distribution companies.

[edit] Merchant transmission

Merchant transmission is an arrangement where a third party constructs and operates electric transmission lines through the franchise area of an unrelated utility. Advocates of merchant transmission[who?] claim that this will create competition to construct the most efficient and lowest cost additions to the transmission grid. Merchant transmission projects typically involve DC lines because it is easier to limit flows to paying customers.

The only operating merchant transmission project in the United States is the Cross Sound Cable from Long Island, New York to New Haven, Connecticut, although additional projects have been proposed.

There is only one unregulated or market interconnector in Australia: Basslink between Tasmania and Victoria. Two DC links originally implemented as market interconnectors Directlink and Murraylink have been converted to regulated interconnectors. NEMMCO

A major barrier to wider adoption of merchant transmission is the difficulty in identifying who benefits from the facility so that the beneficiaries will pay the toll. Also, it is difficult for a merchant transmission line to compete when the alternative transmission lines are subsidized by other utility businesses.[16]

[edit] Health concerns

Main article: Health effects of electric power transmission

The preponderance of evidence suggests that the low-power, low-frequency, electromagnetic radiation associated with household current does not constitute a short or long term health hazard. Although there seems to be a small statistical correlation between various diseases and living near power lines, and some biophysical mechanisms for the promotion of cancer have been proposed (such as the electric fields around powerlines attracting aerosol pollutants), none have been substantiated.[17]

[edit] Government policy

Historically, local governments have exercised authority over the grid and have significant disincentives to take action that would benefit states other than their own. Localities with cheap electricity have a disincentive to making interstate commerce in electricity trading easier, since other regions will be able to compete for local energy and drive up rates. Some regulators in Maine for example do not wish to address congestion problems because the congestion serves to keep Maine rates low.[18] Further, vocal local constituencies can block or slow permitting by pointing to visual impact, environmental, and perceived health concerns. In the US, generation is growing 4 times faster than transmission, but big transmission upgrades require the coordination of multiple states, a multitude of interlocking permits, and cooperation between a significant portion of the 500 companies that own the grid. From a policy perspective, the control of the grid is balkanized, and even former Energy secretary Bill Richardson refers to it as a "third world grid". There have been efforts in the EU and US to confront the problem. The US national security interest in significantly growing transmission capacity drove passage of the 2005 energy act giving the Department of Energy the authority to approve transmission if states refuse to act. However, soon after using its power to designate two National Interest Electric Transmission Corridors, 14 senators signed a letter stating the DOE was being too aggressive[19].

[edit] Special transmission

[edit] Grids for railways

In some countries where electric trains run on low frequency AC (e.g. 16.7 Hz and 25 Hz) power, there are separate single phase traction power networks operated by the railways. These grids are fed by separate generators in some traction powerstations or by traction current converter plants from the public three phase AC network.

[edit] Radio frequency power transmission

Main article: Radio frequency power transmission

Radio and television broadcasters use specialized transmission lines to carry the output of high-power transmitters to the antenna.

[edit] Superconducting cables

High-temperature superconductors promise to revolutionize power distribution by providing lossless transmission of electrical power. The development of superconductors with transition temperatures higher than the boiling point of liquid nitrogen has made the concept of superconducting power lines commercially feasible, at least for high-load applications. [20] It has been estimated that the waste would be halved using this method, since the necessary refrigeration equipment would consume about half the power saved

by the elimination of the majority of resistive losses. In one hypothetical future system called a SuperGrid, the cost of cooling would be eliminated by coupling the transmission line with a liquid hydrogen pipeline.

Superconducting cables are particularly suited to high load density areas such as the business district of large cities, where purchase of an easement for cables would be very costly. [6]

[edit] Single wire earth return

Main article: Single wire earth return

Single wire earth return (SWER) or single wire ground return is a single-wire transmission line for supplying single-phase electrical power for an electrical grid to remote areas at low cost. It is principally used for rural electrification, but also finds use for larger isolated loads such as water pumps, and light rail. Single wire earth return is also used for HVDC over submarine power cables.

[edit] Wireless power transmission

Main article: Wireless energy transfer

Every radio transmitter emits power wirelessly. Both Nikola Tesla and Hidetsugu Yagi attempted to devise systems for large scale wireless power transmission. Tesla claimed to have succeeded.[21][22][23][24][25] Yagi also proposed a similar concept, but the engineering problems proved to be more onerous than conventional systems. His work, however, led to the invention of the Yagi antenna.

Another form of wireless power transmission has been studied for transmission of power from solar power satellites to the earth. A high power array of microwave transmitters would beam power to a rectenna. Major engineering and economic challenges face any solar power satellite project.

Another form is the operation of a crystal radio powered by the radio station it is tuned to; however, the energetic efficiency is extremely low. Small scale wireless power was demonstrated as early as 1831 by Michael Faraday. By 1888, Heinrich Rudolf Hertz had proven that natural radio waves exist and can be captured.

[edit] Cyber-warfare

The Federal government of the United States admits that the power grid is susceptible to cyber-warfare.[26][27] The United States Department of Homeland Security works with industry to identify vulnerabilities and to help industry enhance the security of control system networks, the federal government is also working to ensure that security is built in as we develop the next generation of 'smart grid' networks.[28] On April 8, 2009, it is believed that China or Russia have infiltrated the U.S. electrical grid and left behind

software programs that could be used to disrupt the system, according to current and former national-security officials.[29][30] China denies intruding into the U.S. electrical grid.[31][32] The North American Electric Reliability Corporation (NERC) has issued a public notice that warns that the electrical grid is not adequately protected from cyber attack.[33] One counter measure the U.S. should consider is disconnecting the power grid from the Internet to decrease the likelihood of attack.[34][35] Massive power outages caused by a cyber attack, would cause a crisis making it difficult for the government and emergency workers to respond to critical concerns leading to national trauma.[36]

[edit] Records

Highest capacity system: 6,300 MW HVDC Itaipu (Brazil) (±600 kV DC)[37]

Highest transmission voltage (AC): 1,150 kV on Powerline Ekibastuz-Kokshetau (Kazakhstan)

Highest pylons: Yangtze River Crossing (height: 345 m (1,132 ft)) Longest power line: Inga-Shaba (length: 1,700 kilometres (1,056 mi)) Longest span of power line: 5,376 m (17,638 ft) at Ameralik Span Longest submarine cables:

o NorNed , North Sea - (length of submarine/underground cable: 580 kilometres (360 mi))

o Basslink , Bass Strait - (length of submarine/underground cable: 290 kilometres (180 mi), total length: 357.4 kilometres (222 mi))

o Baltic-Cable , Baltic Sea - (length of submarine/underground cable: 249 kilometres (155 mi), total length: 261 kilometres (162 mi))

[edit] See also

Energy portal

Look up grid electricity in Wiktionary, the free dictionary.

Energy portal

Dynamic demand (electric power)

Demand response Distributed generation Double-circuit

transmission line Electricity distribution Electricity market Electricity pylon Electromagnetic

Transients Program (EMTP)

Mains electricity Miesbach-Munich Power Transmission Off-the-grid , living without public utility Overhead power line Power line communications (PLC) Power System Harmonics Power outage Submarine power cable Traction current Traction power network Three-phase electric power V2G

Flexible AC transmission system (FACTS)

Geomagnetically induced current, (GIC)

Green power grid Grid-tied electrical

system High-voltage direct

current (HVDC) Infrastructure

Load profile

Wheeling (electric power transmission) Wireless energy transfer

25Hz Power Transmission System

[edit] Notes

1. ̂ (pdf) A Primer on Electric Utilities, Deregulation, and Restructuring of U.S. Electricity Markets. United States Department of Energy Federal Energy Management Program (FEMP). 2002-05. http://www1.eere.energy.gov/femp/pdfs/primer.pdf. Retrieved on 2008-12-27.

2. ̂ Staff Report: PRICE MANIPULATION IN WESTERN MARKETS DOCKET NO. PA02-2-000. United States Department of Energy Federal Energy Regulatory Commission. 2003-03-26. http://www.ferc.gov/industries/electric/indus-act/wec/enron/summary-findings.pdf. Retrieved on 2008-12-27.

3. ̂ Hans Dieter Betz, Ulrich Schumann, Pierre Laroche (2009). Lightning: Principles, Instruments and Applications. Springer, pp. 202-203. ISBN 9781402090783. Retrieved on 2009-05-13.

4. ̂ Edison Electric Institute - Underground Vs. Overhead Distribution Wires: Issues to Consider

5. ̂ "SAGE first interim assessment: Power Lines and Property, Wiring in Homes, and Electrical Equipment in Homes"

6. ̂ Should Power Lines be Underground?7. ̂ Weedy, Stephenson, and others8. ̂ Hughes9. ̂ National Council on Electricity Policy (pdf). Electricity Transmission: A

primer. http://www.oe.energy.gov/DocumentsandMedia/primer.pdf.10. ^ a b Thomas P. Hughes (1993). Networks of Power: Electrification in Western

Society, 1880-1930. Baltimore: Johns Hopkins University Press. p. 119-122. ISBN 0801846145. http://books.google.com/books?id=g07Q9M4agp4C&pg=PA122&lpg=PA122&dq=westinghouse+%22universal+system%22&source=bl&ots=BAyz1BrjNU&sig=xkSMfJqxs1H3dm1YMsrXx4vt4L0&hl=en&sa=X&oi=book_result&resnum=1&ct=result#PPA122,M1.

11. ̂ Bureau of Census data reprinted in Hughes, pp. 282-28312. ̂ Hughes, pp. 293-295

13. ̂ "Present Limits of Very Long Distance Transmission Systems"14. ̂ Raghuvir Srinivasan (August 15, 2004). "Power transmission business is a

natural monopoly". The Hindu Business Line. The Hindu. http://www.thehindubusinessline.com/iw/2004/08/15/stories/2004081501201300.htm. Retrieved on 2008-01-31.

15. ̂ Lynne Kiesling (August 18, 2003). "Rethink the Natural Monopoly Justification of Electricity Regulation". Reason Foundation. http://www.reason.org/commentaries/kiesling_20030818b.shtml. Retrieved on 2008-01-31.

16. ̂ Fiona Woolf (February 2003). Global Transmission Expansion. Pennwell Books. pp. 226, 247. ISBN 0-87814-862-0.

17. ̂ Please see Electromagnetic radiation and health#Health effects of electric power transmission for details and references.

18. ̂ National Council on Electricity Policy (pdf). Electricity Transmission: A primer. p. 32 (41 in pdf). http://www.oe.energy.gov/DocumentsandMedia/primer.pdf.

19. ̂ [|Wald, Matthew] (2008-08-27). Wind Energy Bumps Into Power Grid’s Limits. New York Times. p. A1. http://www.nytimes.com/2008/08/27/business/27grid.html?_r=2&ref=business&oref=slogin. Retrieved on 2008-12-12.

20. ̂ Jacob Oestergaard et al., Energy losses of superconducting power transmission cables in the grid, [1]

21. ̂ "The Transmission of Electrical Energy Without Wires," Electrical World , March 5, 1904

22. ̂ Norrie, H. S., "Induction Coils: How to make, use, and repair them". Norman H. Schneider, 1907, New York. 4th edition.

23. ̂ Electrical Experimenter, January 1919. pg. 61524. ̂ Tesla: Man Out of Time By Margaret Cheney. Page 174.25. ̂ Martin, T. C., & Tesla, N. (1894). The inventions, researches and writings of

Nikola Tesla, with special reference to his work in polyphase currents and high potential lighting. New York: The Electrical Engineer. Page 188.

26. ̂ BBC: Spies 'infiltrate US power grid'27. ̂ CNN: Video28. ̂ Reuters: US concerned power grid vulnerable to cyber-attack29. ̂ Electricity Grid in U.S. Penetrated By Spies30. ̂ Fox News: Video31. ̂ Xinhua: China denies intruding into the U.S. electrical grid32. ̂ China Daily: 'China threat' theory rejected33. ̂ NERC Public Notice34. ̂ ABC News: Video35. ̂ The Raw Story: Disconnect electrical grid from Internet, former terror czar

Clarke warns36. ̂ Fox News: Video37. ̂ "Energy Systems, Environment and Development". Advanced Technology

Assessment Systems (Global Energy Network Institute) (Issue 6). Autumn 1991. http://www.geni.org/globalenergy/library/technical-articles/transmission/united-

nations/center-for-science-and-technology-for-development/advanced-technology-assessment-system/energy-systems-environment-and-development.shtml. Retrieved on 2008-12-27.

[edit] Further reading

Grigsby, L. L., et al. The Electric Power Engineering Handbook. USA: CRC Press. (2001). ISBN 0-8493-8578-4

Thomas P. Hughes , Networks of Power: Electrification in Western Society 1880-1930, The Johns Hopkins University Press,Baltimore 1983 ISBN 0-8018-2873-2, an excellent overview of development during the first 50 years of commercial electric power

Westinghouse Electric Corporation, "Electric power transmission patents; Tesla polyphase system". (Transmission of power; polyphase system; Tesla patents)

Pansini, Anthony J, E.E., P.E. undergrounding electric lines. USA Hayden Book Co, 1978. ISBN 0-8104-0827-9

[edit] External links

Japan: World's First In-Grid High-Temperature Superconducting Power Cable System

A Power Grid for the Hydrogen Economy: Overview/A Continental SuperGrid Global Energy Network Institute (GENI) - The GENI Initiative focuses on linking

renewable energy resources around the world using international electricity transmission.

Union for the Co-ordination of Transmission of Electricity (UCTE) , the association of transmission system operators in continental Europe, running one of the two largest power transmission systems in the world

Non-Ionizing Radiation, Part 1: Static and Extremely Low-Frequency (ELF) Electric and Magnetic Fields (2002) by the IARC -- Link Broken.

A Simulation of the Power Grid - The Trustworthy Cyber Infrastructure for the Power Grid (TCIP) group at the University of Illinois at Urbana-Champaign has developed lessons and an applet which illustrate the transmission of electricity from generators to energy consumers, and allows the user to manipulate generation, consumption, and power flow.

Underground transmission

Electric power can also be transmitted by underground power cables instead of overhead power lines. They can assist the transmission of power across:

Densely populated urban areas Areas where land is unavailable or planning consent is difficult

Rivers and other natural obstacles Land with outstanding natural or environmental heritage Areas of significant or prestigious infrastructural development Land whose value must be maintained for future urban expansion and rural

development

Some other advantages of underground power cables:

Less subject to damage from severe weather conditions (mainly wind and freezing)

Greatly reduced emission, into the surrounding area, of electromagnetic fields (EMF). All electric currents generate EMF, but the shielding provided by the earth surrounding underground cables restricts their range and power. See section below, "Health concerns".

Underground cables need a narrower surrounding strip of about 1- 10 meters to install, whereas an overhead line requires a surrounding strip of about 20- 200 meters wide to be kept permanently clear for safety, maintenance and repair.

Underground cables pose no hazard to low flying aircraft or to wildlife, and are significantly safer as they pose no shock hazard (except to the unwary digger).

Some disadvantages of underground power cables:

Undergrounding is more expensive, since the cost of burying cables at transmission voltages is several times greater than overhead power lines, and the life-cycle cost of an underground power cable is two to four times the cost of an overhead power line.[4] According to the British Stakeholder Advisory Group on ELF EMFs[5], the cost is around GBP 10M/km, compared to GBP 0.5-1M/km for overhead lines. This is mainly due to the limit of the physical properties of the insulation placed during installation, keeping the runs to hundreds of meters between splices, which are most commonly placed in manholes or splice-boxes for repairs.

Whereas finding and repairing overhead wire breaks can be accomplished in hours, underground repairs can take days or weeks[6], and for this reason redundant lines are run.

Operations are more difficult since the high reactive power of underground cables produces large charging currents and so makes voltage control more difficult.[7]

The advantages can in some cases outweigh the disadvantages of the higher investment cost, and more expensive maintenance and management.

Most high-voltage underground cables for power transmission that are currently sold on the market are insulated by a sheath of cross-linked polyethylene (XLPE). Some cable may have a lead or aluminum jacket in conjunction with XLPE insulation to allow for fiber optics to be seamlessly integrated within the cable. Before 1960, underground power cables were insulated with oil and paper and ran in a rigid steel pipe, or a semi-rigid aluminum or lead jacket or sheath. The oil was kept under pressure to prevent

formation of voids that would allow partial discharges within the cable insulation. There are still many of these oil-and-paper insulated cables in use worldwide. Between 1960 and 1990, polymers became more widely used at distribution voltages, mostly EPDM (ethylene propylene diene M-class); however, their relative unreliability - particularly early XLPE - resulted in a slow uptake at transmission voltages. While cables of 330kV are commonly constructed using XLPE, this has occurred only in recent decades.

[edit] History

Main article: History of electric power transmission

New York City streets in 1890. Besides telegraph lines, multiple electric lines were required for each class of device requiring different voltages.

In the early days of commercial use of electric power, transmission of electric power at the same voltage as used by lighting and mechanical loads restricted the distance between generating plant and consumers. In 1882 generation was with direct current, which could not easily be increased in voltage for long-distance transmission. Different classes of loads – for example, lighting, fixed motors, and traction (railway) systems – required different voltages, and so used different generators and circuits. [8]

Due to this specialization of lines and because transmission was so inefficient that generators needed to be close by their loads, it seemed at the time that the industry would develop into what is now known as a distributed generation system with large numbers of small generators located nearby their loads. [9]

In 1886 in Great Barrington, Massachusetts, a 1kV AC distribution system was installed. That same year, AC power at 2kV, transmitted 30 km, was installed at Cerchi, Italy. At an AIEE meeting on May 16, 1888, Nikola Tesla delivered a lecture entitled A New System of Alternating Current Motors and Transformers, describing the equipment which allowed efficient generation and use of polyphase alternating currents. The transformer, and Tesla's polyphase and single-phase induction motors, were essential for a combined

AC distribution system for both lighting and machinery. Ownership of the rights to the Tesla patents was a key commercial advantage to the Westinghouse Company in offering a complete alternating current power system for both lighting and power.

Nikola Tesla's Alternating current polyphase generators on display at the 1893 World's Fair in Chicago. Tesla's polyphase innovations revolutionized transmission.

Regarded as one of the most influential innovations for the use of electricity, the "universal system" used transformers to step-up voltage from generators to high-voltage transmission lines, and then to step-down voltage to local distribution circuits or industrial customers[10]. By a suitable choice of utility frequency, both lighting and motor loads could be served. Rotary converters and later mercury-arc valves and other rectifier equipment allowed DC load to be served by local conversion where needed. Even generating stations and loads using different frequencies could be interconnected using rotary converters. By using common generating plants for every type of load, important economies of scale were achieved, lower overall capital investment was required, load factor on each plant was increased allowing for higher efficiency, a lower cost for the consumer and increased overall use of electric power.

By allowing multiple generating plants to be interconnected over a wide area, electricity production cost was reduced. The most efficient available plants could be used to supply the varying loads during the day. Reliability was improved and capital investment cost was reduced, since stand-by generating capacity could be shared over many more customers and a wider geographic area. Remote and low-cost sources of energy, such as hydroelectric power or mine-mouth coal, could be exploited to lower energy production cost. [10]

The first transmission of three-phase alternating current using high voltage took place in 1891 during the international electricity exhibition in Frankfurt. A 25 kV transmission line, approximately 175 kilometers long, connected Lauffen on the Neckar and Frankfurt.

Voltages used for electric power transmission increased throughout the 20th century. By 1914 fifty-five transmission systems each operating at more than 70 kV were in service. The highest voltage then used was 150 kV. [11]

The rapid industrialization in the 20th century made electrical transmission lines and grids a critical part of the economic infrastructure in most industrialized nations. Interconnection of local generation plants and small distribution networks was greatly spurred by the requirements of World War I, where large electrical generating plants were built by governments to provide power to munitions factories; later these plants were connected to supply civil load through long-distance transmission. [12]

[edit] Bulk power transmission

Engineers design transmission networks to transport the energy as efficiently as feasible, while at the same time taking into account economic factors, network safety and redundancy. These networks use components such as power lines, cables, circuit breakers, switches and transformers.

A transmission substation decreases the voltage of incoming electricity, allowing it to connect from long distance high voltage transmission, to local lower voltage distribution. It also reroutes power to other transmission lines that serve local markets. A transmission substation may include phase-shifting or voltage regulating transformers. This is the PacifiCorp Hale Substation, Orem, Utah.

Transmission efficiency is improved by increasing the voltage using a step-up transformer, which reduces the current in the conductors, while keeping the power transmitted nearly equal to the power input. The reduced current flowing through the conductor reduces the losses in the conductor and since, according to Joule's Law, the losses are proportional to the square of the current. Halving the current makes the transmission loss one quarter the original value.

A transmission grid is a network of power stations, transmission circuits, and substations. Energy is usually transmitted within the grid with three-phase AC. DC systems require relatively costly conversion equipment which may be economically justified for particular projects. Single phase AC is used only for distribution to end users since it is not usable for large polyphase induction motors. In the 19th century two-phase transmission was used, but required either three wires with unequal currents or four wires. Higher order phase systems require more than three wires, but deliver marginal benefits.

The synchronous grids of Eurasia.

The capital cost of electric power stations is so high, and electric demand is so variable, that it is often cheaper to import some portion of the needed power than to generate it locally. Because nearby loads are often correlated (hot weather in the Southwest portion of the United States might cause many people there to turn on their air conditioners), electricity must often come from distant sources. Because of the economics of load balancing, wide area transmission grids now span across countries and even large portions of continents. The web of interconnections between power producers and consumers ensures that power can flow, even if a few links are inoperative.

The unvarying (or slowly varying over many hours) portion of the electric demand is known as the "base load", and is generally served best by large facilities (and therefore efficient due to economies of scale) with low variable costs for fuel and operations, i.e. nuclear, coal, hydro. Renewables such as solar, wind, ocean/tidal, etc. are not considered "base load" but can still add power to the grid. Smaller and higher cost sources, such as combined cycle or combustion turbine plants fueled by natural gas are then added as needed.

A high-power electrical transmission tower.

Long-distance transmission of electricity (thousands of kilometers) is cheap and efficient, with costs of US$ 0.005 to 0.02 per kilowatt-hour (compared to annual averaged large producer costs of US$ 0.01 to US$ 0.025 per kilowatt-hour, retail rates upwards of US$ 0.10 per kilowatt-hour, and multiples of retail for instantaneous suppliers at unpredicted

highest demand moments).[13] Thus distant suppliers can be cheaper than local sources (e.g. New York City buys a lot of electricity from Canada). Multiple local sources (even if more expensive and infrequently used) can make the transmission grid more fault tolerant to weather and other disasters that can disconnect distant suppliers.

Long distance transmission allows remote renewable energy resources to be used to displace fossil fuel consumption. Hydro and wind sources can't be moved closer to populous cities, and solar costs are lowest in remote areas where local power needs are minimal. Connection costs alone can determine whether any particular renewable alternative is economically sensible. Costs can be prohibitive for transmission lines, but various proposals for massive infrastructure investment in high capacity, very long distance super grid transmission networks could be recovered with modest usage fees.

[edit] Grid input

At the generating plants the energy is produced at a relatively low voltage between about 2300 volts and 30,000 volts, depending on the size of the unit. The generator terminal voltage is then stepped up by the power station transformer to a higher voltage (115 kV to 765 kV AC, varying by country) for transmission over long distances.

[edit] Losses

Transmitting electricity at high voltage reduces the fraction of energy lost to resistance. For a given amount of power, a higher voltage reduces the current and thus the resistive losses in the conductor. For example, raising the voltage by a factor of 10 reduces the

current by a corresponding factor of 10 and therefore the losses by a factor of 100, provided the same sized conductors are used in both cases. Even if the conductor size

(cross-sectional area) is reduced x10 to match the lower current the losses are still reduced x10. Long distance transmission is typically done with overhead lines at voltages of 115 to 1,200 kV. At extremely high voltages, more than 2,000 kV between conductor and ground, corona discharge losses are so large that they can offset the lower resistance loss in the line conductors.

Transmission and distribution losses in the USA were estimated at 7.2% in 1995 [2], and in the UK at 7.4% in 1998. [3]

As of 1980, the longest cost-effective distance for electricity was 7,000 km (4,000 miles), although all present transmission lines are considerably shorter.[4]

In an alternating current circuit, the inductance and capacitance of the phase conductors can be significant. The currents that flow in these components of the circuit impedance constitute reactive power, which transmits no energy to the load. Reactive current flow causes extra losses in the transmission circuit. The ratio of real power (transmitted to the load) to apparent power is the power factor. As reactive current increases, the reactive

power increases and the power factor decreases. For systems with low power factors, losses are higher than for systems with high power factors. Utilities add capacitor banks and other components throughout the system — such as phase-shifting transformers, static VAR compensators, physical transposition of the phase conductors, and flexible AC transmission systems (FACTS) — to control reactive power flow for reduction of losses and stabilization of system voltage.

[edit] Transmission grid exit

At the substations, transformers reduce the voltage to a lower level for distribution to commercial and residential users. This distribution is accomplished with a combination of sub-transmission (33 kV to 115 kV, varying by country and customer requirements) and distribution (3.3 to 25 kV). Finally, at the point of use, the energy is transformed to low voltage (100 to 600 V, varying by country and customer requirements - see Mains power systems).

[edit] High-voltage direct current

Main article: High-voltage direct current

High voltage direct current (HVDC) is used to transmit large amounts of power over long distances or for interconnections between asynchronous grids. When electrical energy is required to be transmitted over very long distances, it is more economical to transmit using direct current instead of alternating current. For a long transmission line, the lower losses and reduced construction cost of a DC line can offset the additional cost of converter stations at each end. Also, at high AC voltages, significant (although economically acceptable) amounts of energy are lost due to corona discharge, the capacitance between phases or, in the case of buried cables, between phases and the soil or water in which the cable is buried.

HVDC links are sometimes used to stabilize against control problems with the AC electricity flow. In other words, to transmit AC power as AC when needed in either direction between Seattle and Boston would require the (highly challenging) continuous real-time adjustment of the relative phase of the two electrical grids. With HVDC instead the interconnection would: (1) Convert AC in Seattle into HVDC. (2) Use HVDC for the three thousand miles of cross country transmission. Then (3) convert the HVDC to locally synchronized AC in Boston, and optionally in other cooperating cities along the transmission route. One prominent example of such a transmission line is the Pacific DC Intertie located in the Western United States.

[edit] Limitations

The amount of power that can be sent over a transmission line is limited. The origins of the limits vary depending on the length of the line. For a short line, the heating of conductors due to line losses sets a "thermal" limit. If too much current is drawn,

conductors may sag too close to the ground, or conductors and equipment may be damaged by overheating. For intermediate-length lines on the order of 100 km (60 miles), the limit is set by the voltage drop in the line. For longer AC lines, system stability sets the limit to the power that can be transferred. Approximately, the power flowing over an AC line is proportional to the sine of the phase angle of the voltage at the receiving and transmitting ends. Since this angle varies depending on system loading and generation, it is undesirable for the angle to approach 90 degrees. Very approximately, the allowable product of line length and maximum load is proportional to the square of the system voltage. Series capacitors or phase-shifting transformers are used on long lines to improve stability. High-voltage direct current lines are restricted only by thermal and voltage drop limits, since the phase angle is not material to their operation.

Up to now, it has been almost impossible to foresee the temperature distribution along the cable route, so that the maximum applicable current load was usually set as a compromise between understanding of operation conditions and risk minimization. The availability of industrial Distributed Temperature Sensing (DTS) systems that measure in real time temperatures all along the cable is a first step in monitoring the transmission system capacity. This monitoring solution is based on using passive optical fibers as temperature sensors, either integrated directly inside a high voltage cable or mounted externally on the cable insulation. A solution for overhead lines is also available. In this case the optical fiber is integrated into the core of a phase wire of overhead transmission lines (OPPC). The integrated Dynamic Cable Rating (DCR) or also called Real Time Thermal Rating (RTTR) solution enables not only to continuously monitor the temperature of a high voltage cable circuit in real time, but to safely utilize the existing network capacity to its maximum. Furthermore it provides the ability to the operator to predict the behavior of the transmission system upon major changes made to its initial operating conditions.

[edit] Control

To ensure safe and predictable operation the components of the transmission system are controlled with generators, switches, circuit breakers and loads. The voltage, power, frequency, load factor, and reliability capabilities of the transmission system are designed to provide cost effective performance for the customers.

[edit] Load balancing

The transmission system provides for base load and peak load capability, with safety and fault tolerance margins. The peak load times vary by region largely due to the industry mix. In very hot and very cold climates home air conditioning and heating loads have an effect on the overall load. They are typically highest in the late afternoon in the hottest part of the year and in mid-mornings and mid-evenings in the coldest part of the year. This makes the power requirements vary by the season and the time of day. Distribution system designs always take the base load and the peak load into consideration.

The transmission system usually does not have a large buffering capability to match the loads with the generation. Thus generation has to be kept matched to the load, to prevent overloading failures of the generation equipment.

Multiple sources and loads can be connected to the transmission system and they must be controlled to provide orderly transfer of power. In centralized power generation, only local control of generation is necessary, and it involves synchronization of the generation units, to prevent large transients and overload conditions.

In distributed power generation the generators are geographically distributed and the process to bring them online and offline must be carefully controlled. The load control signals can either be sent on separate lines or on the power lines themselves. To load balance the voltage and frequency can be used as a signaling mechanism.

In voltage signaling the variation of voltage is used to increase generation. The power added by any system increases as the line voltage decreases. This arrangement is stable in principle. Voltage based regulation is complex to use in mesh networks, since the individual components and setpoints would need to be reconfigured every time a new generator is added to the mesh.

In frequency signaling, the generating units match the frequency of the power transmission system. In Droop speed control, if the frequency decreases, the power is increased. (The drop in line frequency is an indication that the increased load is causing the generators to slow down.)

Wind turbines, v2g and other distributed storage and generation systems can be connected to the power grid, and interact with it to improve system operation.

[edit] Failure protection

Under excess load conditions, the system can be designed to fail gracefully rather than all at once. Brownouts occur when the supply power drops below the demand. Blackouts occur when the supply fails completely.

Rolling blackouts, or load shedding, are intentionally-engineered electrical power outages, used to distribute insufficient power when the demand for electricity exceeds the supply.

[edit] Communications

Operators of long transmission lines require reliable communications for control of the power grid and, often, associated generation and distribution facilities. Fault-sensing protection relays at each end of the line must communicate to monitor the flow of power into and out of the protected line section so that faulted conductors or equipment can be quickly de-energized and the balance of the system restored. Protection of the transmission line from short circuits and other faults is usually so critical that common

carrier telecommunications are insufficiently reliable, and in remote areas a common carrier may not be available. Communication systems associated with a transmission project may use:

Microwaves Power line communication Optical fibers

Rarely, and for short distances, a utility will use pilot-wires strung along the transmission line path. Leased circuits from common carriers are not preferred since availability is not under control of the electric power transmission organization.

Transmission lines can also be used to carry data: this is called power-line carrier, or PLC. PLC signals can be easily received with a radio for the long wave range.

Optical fibers can be included in the stranded conductors of a transmission line, in the overhead shield wires. These cables are known as OPGW or Optical Ground Wire. Sometimes a standalone cable is used, ADSS or All Dielectric Self Supporting cable, attached to the transmission line cross arms.

Some jurisdictions, such as Minnesota, prohibit energy transmission companies from selling surplus communication bandwidth or acting as a telecommunications common carrier. Where the regulatory structure permits, the utility can sell capacity in extra dark fibers to a common carrier, providing another revenue stream.

[edit] Electricity market reform

It has been suggested that this article or section be merged into Electricity market. (Discuss)

Some regulators regard electric transmission to be a natural monopoly [14] [15] and there are moves in many countries to separately regulate transmission (see Electricity market).

Spain was the first country to establish a Regional Transmission Organization. In that country transmission operations and market operations are controlled by separate companies. The transmission system operator is Red Eléctrica de España (REE) and the wholesale electricity market operator is Operador del Mercado Ibérico de Energía - Polo Español, S.A. (OMEL) [5]. Spain's transmission system is interconnected with those of France, Portugal, and Morocco.

In the United States and parts of Canada, electrical transmission companies operate independently of generation and distribution companies.

[edit] Merchant transmission

Merchant transmission is an arrangement where a third party constructs and operates electric transmission lines through the franchise area of an unrelated utility. Advocates of merchant transmission[who?] claim that this will create competition to construct the most efficient and lowest cost additions to the transmission grid. Merchant transmission projects typically involve DC lines because it is easier to limit flows to paying customers.

The only operating merchant transmission project in the United States is the Cross Sound Cable from Long Island, New York to New Haven, Connecticut, although additional projects have been proposed.

There is only one unregulated or market interconnector in Australia: Basslink between Tasmania and Victoria. Two DC links originally implemented as market interconnectors Directlink and Murraylink have been converted to regulated interconnectors. NEMMCO

A major barrier to wider adoption of merchant transmission is the difficulty in identifying who benefits from the facility so that the beneficiaries will pay the toll. Also, it is difficult for a merchant transmission line to compete when the alternative transmission lines are subsidized by other utility businesses.[16]

[edit] Health concerns

Main article: Health effects of electric power transmission

The preponderance of evidence suggests that the low-power, low-frequency, electromagnetic radiation associated with household current does not constitute a short or long term health hazard. Although there seems to be a small statistical correlation between various diseases and living near power lines, and some biophysical mechanisms for the promotion of cancer have been proposed (such as the electric fields around powerlines attracting aerosol pollutants), none have been substantiated.[17]

[edit] Government policy

Historically, local governments have exercised authority over the grid and have significant disincentives to take action that would benefit states other than their own. Localities with cheap electricity have a disincentive to making interstate commerce in electricity trading easier, since other regions will be able to compete for local energy and drive up rates. Some regulators in Maine for example do not wish to address congestion problems because the congestion serves to keep Maine rates low.[18] Further, vocal local constituencies can block or slow permitting by pointing to visual impact, environmental, and perceived health concerns. In the US, generation is growing 4 times faster than transmission, but big transmission upgrades require the coordination of multiple states, a multitude of interlocking permits, and cooperation between a significant portion of the 500 companies that own the grid. From a policy perspective, the control of the grid is balkanized, and even former Energy secretary Bill Richardson refers to it as a "third world grid". There have been efforts in the EU and US to confront the problem. The US

national security interest in significantly growing transmission capacity drove passage of the 2005 energy act giving the Department of Energy the authority to approve transmission if states refuse to act. However, soon after using its power to designate two National Interest Electric Transmission Corridors, 14 senators signed a letter stating the DOE was being too aggressive[19].

[edit] Special transmission

[edit] Grids for railways

In some countries where electric trains run on low frequency AC (e.g. 16.7 Hz and 25 Hz) power, there are separate single phase traction power networks operated by the railways. These grids are fed by separate generators in some traction powerstations or by traction current converter plants from the public three phase AC network.

[edit] Radio frequency power transmission

Main article: Radio frequency power transmission

Radio and television broadcasters use specialized transmission lines to carry the output of high-power transmitters to the antenna.

[edit] Superconducting cables

High-temperature superconductors promise to revolutionize power distribution by providing lossless transmission of electrical power. The development of superconductors with transition temperatures higher than the boiling point of liquid nitrogen has made the concept of superconducting power lines commercially feasible, at least for high-load applications. [20] It has been estimated that the waste would be halved using this method, since the necessary refrigeration equipment would consume about half the power saved by the elimination of the majority of resistive losses. In one hypothetical future system called a SuperGrid, the cost of cooling would be eliminated by coupling the transmission line with a liquid hydrogen pipeline.

Superconducting cables are particularly suited to high load density areas such as the business district of large cities, where purchase of an easement for cables would be very costly. [6]

[edit] Single wire earth return

Main article: Single wire earth return

Single wire earth return (SWER) or single wire ground return is a single-wire transmission line for supplying single-phase electrical power for an electrical grid to remote areas at low cost. It is principally used for rural electrification, but also finds use

for larger isolated loads such as water pumps, and light rail. Single wire earth return is also used for HVDC over submarine power cables.

[edit] Wireless power transmission

Main article: Wireless energy transfer

Every radio transmitter emits power wirelessly. Both Nikola Tesla and Hidetsugu Yagi attempted to devise systems for large scale wireless power transmission. Tesla claimed to have succeeded.[21][22][23][24][25] Yagi also proposed a similar concept, but the engineering problems proved to be more onerous than conventional systems. His work, however, led to the invention of the Yagi antenna.

Another form of wireless power transmission has been studied for transmission of power from solar power satellites to the earth. A high power array of microwave transmitters would beam power to a rectenna. Major engineering and economic challenges face any solar power satellite project.

Another form is the operation of a crystal radio powered by the radio station it is tuned to; however, the energetic efficiency is extremely low. Small scale wireless power was demonstrated as early as 1831 by Michael Faraday. By 1888, Heinrich Rudolf Hertz had proven that natural radio waves exist and can be captured.

[edit] Cyber-warfare

The Federal government of the United States admits that the power grid is susceptible to cyber-warfare.[26][27] The United States Department of Homeland Security works with industry to identify vulnerabilities and to help industry enhance the security of control system networks, the federal government is also working to ensure that security is built in as we develop the next generation of 'smart grid' networks.[28] On April 8, 2009, it is believed that China or Russia have infiltrated the U.S. electrical grid and left behind software programs that could be used to disrupt the system, according to current and former national-security officials.[29][30] China denies intruding into the U.S. electrical grid.[31][32] The North American Electric Reliability Corporation (NERC) has issued a public notice that warns that the electrical grid is not adequately protected from cyber attack.[33] One counter measure the U.S. should consider is disconnecting the power grid from the Internet to decrease the likelihood of attack.[34][35] Massive power outages caused by a cyber attack, would cause a crisis making it difficult for the government and emergency workers to respond to critical concerns leading to national trauma.[36]

[edit] Records

Highest capacity system: 6,300 MW HVDC Itaipu (Brazil) (±600 kV DC)[37]

Highest transmission voltage (AC): 1,150 kV on Powerline Ekibastuz-Kokshetau (Kazakhstan)

Highest pylons: Yangtze River Crossing (height: 345 m (1,132 ft)) Longest power line: Inga-Shaba (length: 1,700 kilometres (1,056 mi)) Longest span of power line: 5,376 m (17,638 ft) at Ameralik Span Longest submarine cables:

o NorNed , North Sea - (length of submarine/underground cable: 580 kilometres (360 mi))

o Basslink , Bass Strait - (length of submarine/underground cable: 290 kilometres (180 mi), total length: 357.4 kilometres (222 mi))

o Baltic-Cable , Baltic Sea - (length of submarine/underground cable: 249 kilometres (155 mi), total length: 261 kilometres (162 mi))

[edit] See also

History

Westinghouse Early AC System 1887 (US patent 373035)

A power transformer developed by Lucien Gaulard and John Dixon Gibbs was demonstrated in London in 1881, and attracted the interest of Westinghouse. They also exhibited the invention in Turin in 1884, where it was adopted for an electric lighting system. Many of their designs were adapted to the particular laws governing electrical distribution in the UK.[citation needed]

In 1882, 1884, and 1885 Gaulard and Gibbs applied for patents on their transformer; however, these were overturned due to prior arts of Nikola Tesla and actions initiated by Sebastian Ziani de Ferranti.

Ferranti went into this business in 1882 when he set up shop in London designing various electrical devices. Ferranti bet on the success of alternating current power distribution early on, and was one of the few experts in this system in the UK. In 1887 the London Electric Supply Corporation (LESCo) hired Ferranti for the design of their power station at Deptford. He designed the building, the generating plant and the distribution system. On its completion in 1891 it was the first truly modern power station, supplying high-voltage AC power that was then "stepped down" for consumer use on each street. This basic system remains in use today around the world. Many homes all over the world still have electric meters with the Ferranti AC patent stamped on them.

William Stanley, Jr. designed one of the first practical devices to transfer AC power efficiently between isolated circuits. Using pairs of coils wound on a common iron core, his design, called an induction coil, was an early transformer. The AC power system used today developed rapidly after 1886, and includes key concepts by Nikola Tesla, who subsequently sold his patent to George Westinghouse. Lucien Gaulard, John Dixon Gibbs, Carl Wilhelm Siemens and others contributed subsequently to this field. AC systems overcame the limitations of the direct current system used by Thomas Edison to distribute electricity efficiently over long distances even though Edison attempted to discredit alternating current as too dangerous during the War of Currents.

The first commercial power plant in the United States using three-phase alternating current was at the Mill Creek hydroelectric plant near Redlands, California in 1893 designed by Almirian Decker. Decker's design incorporated 10,000-volt three-phase transmission and established the standards for the complete system of generation, transmission and motors used today.

The Jaruga power plant in Croatia was set in operation on 28 August 1895 in 20'00 hours,[clarification needed] three days after the Niagara Falls plant, becoming the second commercial hydro power plant in the world. The two generators (42 Hz, 550 kW each) and the transformers were produced and installed by the Hungarian company Ganz. The transmission line from the power plant to the City of Šibenik was 11.5 kilometres (7.1 mi) long on wooden towers, and the municipal distribution grid 3000V/110 V included six transforming stations.

Alternating current circuit theory evolved rapidly in the latter part of the 19th and early 20th century. Notable contributors to the theoretical basis of alternating current calculations include Charles Steinmetz, James Clerk Maxwell, Oliver Heaviside, and many others. Calculations in unbalanced three-phase systems were simplified by the symmetrical components methods discussed by Charles Legeyt Fortescue in 1918.

[edit] Transmission, distribution, and domestic power supply

Main articles: Electric power transmission and Electricity distribution

AC voltage may be increased or decreased with a transformer. Use of a higher voltage leads to significantly more efficient transmission of power. The power losses in a conductor are a product of the square of the current and the resistance of the conductor, described by the formula P = I2R. This means that when transmitting a fixed power on a given wire, if the current is doubled, the power loss will be four times greater.

Since the power transmitted is equal to the product of the current and the voltage (assuming no phase difference), the same amount of power can be transmitted with a lower current by increasing the voltage. Therefore it is advantageous when transmitting

large amounts of power to distribute the power with high voltages (often hundreds of kilovolts).

High voltage transmission lines deliver power from electric generation plants over long distances using alternating current. These lines are located in eastern Utah.

However, high voltages also have disadvantages, the main one being the increased insulation required, and generally increased difficulty in their safe handling. In a power plant, power is generated at a convenient voltage for the design of a generator, and then stepped up to a high voltage for transmission. Near the loads, the transmission voltage is stepped down to the voltages used by equipment. Consumer voltages vary depending on the country and size of load, but generally motors and lighting are built to use up to a few hundred volts between phases.

The utilization voltage delivered to equipment such as lighting and motor loads is standardized, with an allowable range of voltage over which equipment is expected to operate. Standard power utilization voltages and percentage tolerance vary in the different mains power systems found in the world.

Modern high-voltage, direct-current electric power transmission systems contrast with the more common alternating-current systems as a means for the efficient bulk transmission of electrical power over long distances. HVDC systems, however, tend to be more expensive and less efficient over shorter distances than transformers. Transmission with high voltage direct current was not feasible when Edison, Westinghouse and Tesla were designing their power systems, since there was then no way to economically convert AC power to DC and back again at the necessary voltages.

Three-phase electrical generation is very common. Three separate coils in the generator stator are physically offset by an angle of 120° to each other. Three current waveforms are produced that are equal in magnitude and 120° out of phase to each other.

If the load on a three-phase system is balanced equally among the phases, no current flows through the neutral point. Even in the worst-case unbalanced (linear) load, the

neutral current will not exceed the highest of the phase currents. Non-linear loads (e.g. computers) may require an oversized neutral bus and neutral conductor in the upstream distribution panel to handle harmonics. Harmonics can cause neutral conductor current levels to exceed that of one or all phase conductors.

For three-phase at utilization voltages a four-wire system is often used. When stepping down three-phase, a transformer with a Delta (3-wire) primary and a Star (4-wire, centre-earthed) secondary is often used so there is no need for a neutral on the supply side.

For smaller customers (just how small varies by country and age of the installation) only a single phase and the neutral or two phases and the neutral are taken to the property. For larger installations all three phases and the neutral are taken to the main distribution panel. From the three-phase main panel, both single and three-phase circuits may lead off.

Three-wire single phase systems, with a single centre-tapped transformer giving two live conductors, is a common distribution scheme for residential and small commercial buildings in North America. This arrangement is sometimes incorrectly referred to as "two phase". A similar method is used for a different reason on construction sites in the UK. Small power tools and lighting are supposed to be supplied by a local center-tapped transformer with a voltage of 55 V between each power conductor and earth. This significantly reduces the risk of electric shock in the event that one of the live conductors becomes exposed through an equipment fault whilst still allowing a reasonable voltage of 110 V between the two conductors for running the tools.

A third wire, called the bond (or earth) wire, is often connected between non-current-carrying metal enclosures and earth ground. This conductor provides protection from electric shock due to accidental contact of circuit conductors with the metal chassis of portable appliances and tools. Bonding all non-current-carrying metal parts into one complete system ensures there is always a low electrical impedance path to ground sufficient to carry any fault current for as long as it takes for the system to clear the fault. This low impedance path allows the maximum amount of fault current, causing the overcurrent protection device (breakers, fuses) to trip or burn out as quickly as possible, bringing the electrical system to a safe state. All bond wires are bonded to ground at the main service panel, as is the Neutral/Identified conductor if present.

[edit] AC power supply frequencies

The frequency of the electrical system varies by country; most electric power is generated at either 50 or 60 Hz. See List of countries with mains power plugs, voltages and frequencies. Some countries have a mixture of 50 Hz and 60 Hz supplies, notably Japan.

A low frequency eases the design of low speed electric motors, particularly for hoisting, crushing and rolling applications, and commutator-type traction motors for applications such as railways, but also causes a noticeable flicker in incandescent lighting and an objectionable flicker in fluorescent lamps. 16⅔ Hz power is still used in some European

rail systems, such as in Austria, Germany, Norway, Sweden and Switzerland. The use of lower frequencies also provided the advantage of lower impedance losses, which are proportional to frequency. The original Niagara Falls generators were built to produce 25 Hz power, as a compromise between low frequency for traction and heavy induction motors, while still allowing incandescent lighting to operate (although with noticeable flicker); most of the 25 Hz residential and commercial customers for Niagara Falls power were converted to 60 Hz by the late 1950s, although some 25 Hz industrial customers still existed as of the start of the 21st century.

Off-shore, military, textile industry, marine, computer mainframe, aircraft, and spacecraft applications sometimes use 400 Hz, for benefits of reduced weight of apparatus or higher motor speeds.

[edit] Effects at high frequencies

A direct, constant current flows uniformly throughout the cross-section of the (uniform) wire that carries it. With alternating current of any frequency, the current is forced towards the outer surface of the wire, and away from the center. This is because an electric charge which accelerates (as is the case of an alternating current) radiates electromagnetic waves, and materials of high conductivity (the metal which makes up the wire) do not allow propagation of electromagnetic waves. This phenomenon is called skin effect.

At very high frequencies the current no longer flows in the wire, but effectively flows on the surface of the wire, within a thickness of a few skin depths. The skin depth is the thickness at which the current density is reduced by 63%. Even at relatively low frequencies used for high power transmission (50–60 Hz), non-uniform distribution of current still occurs in sufficiently thick conductors. For example, the skin depth of a copper conductor is approximately 8.57 mm at 60 Hz, so high current conductors are usually hollow to reduce their mass and cost.

Since the current tends to flow in the periphery of conductors, the effective cross-section of the conductor is reduced. This increases the effective AC resistance of the conductor, since resistance is inversely proportional to the cross-sectional area in which the current actually flows. The AC resistance often is many times higher than the DC resistance, causing a much higher energy loss due to ohmic heating (also called I2R loss).

[edit] Techniques for reducing AC resistance

For low to medium frequencies, conductors can be divided into stranded wires, each insulated from one other, and the individual strands specially arranged to change their relative position within the conductor bundle. Wire constructed using this technique is called Litz wire. This measure helps to partially mitigate skin effect by forcing more equal current flow throughout the total cross section of the stranded conductors. Litz wire is used for making high Q inductors, reducing losses in flexible conductors carrying very high currents at lower frequencies, and in the windings of devices carrying higher radio

frequency current (up to hundreds of kilohertz), such as switch-mode power supplies and radio frequency transformers.

[edit] Techniques for reducing radiation loss

As written above, an alternating current is made of electric charge under periodic acceleration, which causes radiation of electromagnetic waves. Energy that is radiated represents a loss. Depending on the frequency, different techniques are used to minimize the loss due to radiation.

[edit] Twisted pairs

At frequencies up to about 1 GHz, wires are paired together in cabling to form a twisted pair in order to reduce losses due to electromagnetic radiation and inductive coupling. A twisted pair must be used with a balanced signalling system, where the two wires carry equal but opposite currents. The result is that each wire in the twisted pair radiates a signal that is effectively cancelled by the other wire, resulting in almost no electromagnetic radiation.

[edit] Coaxial cables

At frequencies above 1 GHz, unshielded wires of practical dimensions lose too much energy to radiation, so coaxial cables are used instead. A coaxial cable has a conductive wire inside a conductive tube. The current flowing on the inner conductor is equal and opposite to the current flowing on the inner surface of the outer tube. This causes the electromagnetic field to be completely contained within the tube, and (ideally) no energy is radiated or coupled outside the tube. Coaxial cables have acceptably small losses for frequencies up to about 20 GHz. For microwave frequencies greater than 20 GHz, the dielectric losses (due mainly to the dissipation factor of the dielectric layer which separates the inner wire from the outer tube) become too large, making waveguides a more efficient medium for transmitting energy.

[edit] Waveguides

Waveguides are similar to coax cables, as both consist of tubes, with the biggest difference being that the waveguide has no inner conductor. Waveguides can have any arbitrary cross section, but rectangular cross sections are the most common. Because waveguides do not have an inner conductor to carry a return current, waveguides cannot deliver energy by means of an electric current, but rather by means of a guided electromagnetic field. Although surface currents do flow on the inner walls of the waveguides, those surface currents do not carry power. Power is carried by the guided electromagnetic fields. The surface currents are set up by the guided electromagnetic fields and have the effect of keeping the fields inside the waveguide and preventing leakage of the fields to the space outside the waveguide.

Waveguides have dimensions comparable to the wavelength of the alternating current to be transmitted, so they are only feasible at microwave frequencies. In addition to this mechanical feasibility, electrical resistance of the non-ideal metals forming the walls of the waveguide cause dissipation of power (surface currents flowing on lossy conductors dissipate power). At higher frequencies, the power lost to this dissipation becomes unacceptably large.

[edit] Fiber optics

At frequencies greater than 200 GHz, waveguide dimensions become impractically small, and the ohmic losses in the waveguide walls become large. Instead, fiber optics, which are a form of dielectric waveguides, can be used. For such frequencies, the concepts of voltages and currents are no longer used.

[edit] Mathematics of AC voltages

A sine wave, over one cycle (360°). The dashed line represents the root mean square (RMS) value at about 0.707

Alternating currents are accompanied (or caused) by alternating voltages. An AC voltage v can be described mathematically as a function of time by the following equation:

,

where

is the peak voltage (unit: volt),

is the angular frequency (unit: radians per second)

o The angular frequency is related to the physical frequency, (unit = hertz), which represents the number of cycles per second , by the equation

.

is the time (unit: second).

The peak-to-peak value of an AC voltage is defined as the difference between its positive peak and its negative peak. Since the maximum value of sin(x) is +1 and the minimum value is −1, an AC voltage swings between + Vpeak and − Vpeak. The peak-to-peak voltage, usually written as Vpp or VP − P, is therefore Vpeak − ( − Vpeak) = 2Vpeak.

[edit] Power and root mean square

The relationship between voltage and the power delivered is

where R represents a load resistance.

Rather than using instantaneous power, P(t), it is more practical to use a time averaged power (where the averaging is performed over any integer number of cycles). Therefore, AC voltage is often expressed as a root mean square (RMS) value, written as Vrms, because

For a sinusoidal voltage:

The factor is called the crest factor, which varies for different waveforms.

For a triangle wave form centered about zero

For a square wave form centered about zero

[edit] Example

To illustrate these concepts, consider a 230 V AC mains supply used in many countries around the world. It is so called because its root mean square value is 230 V. This means that the time-averaged power delivered is equivalent to the power delivered by a DC voltage of 230 volts. To determine the peak voltage (amplitude), we can rearrange the above equation to:

For our 230 V AC, the peak voltage Vpeak is therefore , which is about 325 V. The

peak-to-peak value of the 230 V AC is double that, at about 650 V.

Note that some countries use a frequency of 50 hertz, while others use a frequency of 60 hertz. The calculation to convert from RMS voltage to peak voltage is independent of the frequency.

[edit] See also

6+9

Electrical breakdownFrom Wikipedia, the free encyclopedia

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Electrical breakdown in an electric discharge showing the ribbon-like plasma filaments from a Tesla coil.

The term electrical breakdown has several similar but distinctly different meanings. The term can apply to the failure of an electric circuit. Alternately, it may refer to a rapid reduction in the resistance of an electrical insulator that can lead to a spark jumping around or through the insulator. This may be a momentary event (as in an electrostatic discharge), or may lead to a continuous arc discharge if protective devices fail to interrupt the current in a high power circuit.

Contents

[hide] 1 Electrical system failure 2 Failure of electrical insulation 3 Disruptive devices 4 Mechanism

o 4.1 Voltage-Current relation o 4.2 Corona breakdown

5 See also

6 References

[edit] Electrical system failure

The most common meaning is related to automobiles and is the failure of an electric circuit or associated device resulting in a loss of vehicle function (a breakdown). Common problems include battery discharge, alternator failure, broken wires, blown fuses, etc.

[edit] Failure of electrical insulation

The second meaning of the term is more specifically a reference to the breakdown of the insulation of an electrical wire or other electrical component. Such breakdown usually results in a short circuit or a blown fuse. This occurs at the breakdown voltage. Actual insulation breakdown is more generally found in high-voltage applications, where it sometimes causes the opening of a protective circuit breaker. Electrical breakdown is often associated with the failure of solid or liquid insulating materials used inside high voltage transformers or capacitors in the electricity distribution grid. Electrical breakdown can also occur across the strings of insulators that suspend overhead power lines, within underground power cables, or lines arcing to nearby branches of trees. Under sufficient electrical stress, electrical breakdown can occur within solids, liquids, gases or vacuum. However, the specific breakdown mechanisms are significantly different for each, particularly in different kinds of dielectric medium. All this leads to catastrophic failure of the instruments.

[edit] Disruptive devices

A disruptive device is a device that has a dielectric, whereupon being stressed beyond its dielectric strength, has an electrical breakdown. This results in the sudden transition of part of the dielectric material from an insulating state to a highly conductive state. This transition is characterized by the formation of an electric spark, and possibly an electric arc through the material. If this occurs within a solid dielectric, physical and chemical changes along the path of the discharge will cause permanent degradation and significant reduction in the material's dielectric strength. A spark gap is a type of disruptive device that uses a gas or fluid dielectric between spaced electrodes. Unlike solid dielectrics, liquid or gaseous dielectrics can usually recover their full dielectric strength once current flow (through the plasma in the gap) has been externally interrupted.

[edit] Mechanism

Electrical breakdown occurs within a gas (or mixture of gases, such as air) when the dielectric strength of the gas(es) is exceeded. Regions of high electrical stress can cause nearby gas to partially ionize and begin conducting. This is done deliberately in low

pressure discharges such as in fluorescent lights (see also Electrostatic Discharge) or in an electrostatic precipitator.

Partial electrical breakdown of the air causes the "fresh air" smell of ozone during thunderstorms or around high-voltage equipment. Although air is normally an excellent insulator, when stressed by a sufficiently high voltage (an electric field strength of about 3 x 106V/m[1]), air can begin to break down, becoming partially conductive. If the voltage is sufficiently high, complete electrical breakdown of the air will culminate in an electrical spark or arc that bridges the entire gap. While the small sparks generated by static electricity may barely be audible, larger sparks are often accompanied by a loud snap or bang. Lightning is an example of an immense spark that can be many miles long. The color of the spark depends upon the gases that make up the gaseous media.

Electric discharge showing the lightning-like plasma filaments from a Tesla coil. (Click image for detail)

If a fuse or circuit breaker fails to interrupt the current through a spark in a power circuit, current may continue, forming a very hot electric arc. The color of an arc depends primarily upon the conductor materials (as they are vaporized and mix within the hot plasma in the arc). Although sparks and arcs are usually undesirable, they can be useful in everyday applications such as spark plugs for gasoline engines, electrical welding of metals, or for metal melting in an electric arc furnace.

[edit] Voltage-Current relation

Voltage-current relation before breakdown

Before breakdown, there is a non-linear relation between voltage and current as shown in figure. In region 1, there are free ions that can be accelerated by the field and induce a current. These will be saturated after a certain voltage and give a constant current, region 2. Region 3 and 4 are caused by ion avalanche as explained by the Townsend discharge mechanism.

[edit] Corona breakdown

Partial breakdown of the air occurs as a corona discharge on high voltage conductors at points with the highest electrical stress. As the dielectric strength of the material surrounding the conductor determines the maximum strength of the electric field the surrounding material can tolerate before becoming conductive, conductors that consist of sharp points, or balls with small radii, are more prone to causing dielectric breakdown. Corona is sometimes seen as a bluish glow around high voltage wires and heard as a sizzling sound along high voltage power lines. Corona also generates radio frequency noise that can also be heard as 'static' or buzzing on radio receivers. Corona can also occur naturally at high points (such as church spires, treetops, or ship masts) during thunderstorms as St. Elmo's Fire. Although corona discharge is usually undesirable, until recently it was essential in the operation of photocopiers (Xerography) and laser printers. Many modern copiers and laser printers now charge the photoconductor drum with an electrically conductive roller, reducing undesirable indoor ozone pollution. Additionally, lightning rods use corona discharge to create conductive paths in the air that point towards the rod, deflecting potentially-damaging lightning away from buildings and other structures.[2]

Corona discharge ozone generators have been used for more than 30 years in the water purification process. Ozone is a toxic gas, even more potent than chlorine. In a typical drinking water treatment plant, the ozone gas is dissolved into the filtered water to kill bacteria and viruses. Ozone also removes the bad odours and taste from the water. The main advantage of ozone is that the overdose (residual) decomposes to gaseous oxygen well before the water reaches the consumer. This is in contrast with chlorine which stays in the water and can be tasted by the consumer.

Corona discharges are also used to modify the surface properties of many polymers. An example is the corona treatment of plastic materials which allows paint or ink to adhere properly.

[edit] See also

Coaxial cableFrom Wikipedia, the free encyclopedia

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RG-59 flexible coaxial cable.A: outer plastic sheathB: copper screenC: inner dielectric insulatorD: copper core

Coaxial cable, or coax, is an electrical cable with an inner conductor surrounded by a tubular insulating layer typically of a flexible material with a high dielectric constant, all of which are surrounded by a conductive layer (typically of fine woven wire for flexibility, or of a thin metallic foil), and finally covered with a thin insulating layer on the outside. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis. Coaxial cable is used as a transmission line for radio frequency signals, in applications such as connecting radio transmitters and receivers with their antennas, computer network (Internet) connections, and distributing cable television signals. One advantage of coax over other types of transmission line is that in an ideal coaxial cable the electromagnetic field carrying the signal exists only in the space between the inner and outer conductors. This allows coaxial cable runs to be installed next to metal objects such as gutters without the power losses that occur in other

transmission lines, and provides protection of the signal from external electromagnetic interference.

Coaxial cable should not be confused with other shielded cable used for carrying lower frequency signals such as audio signals. Shielded cable is similar in that it consists of a central wire or wires surrounded by a tubular shield conductor, but it is not constructed with the precise conductor spacing needed to function efficiently as a radio frequency transmission line.

Contents[hide]

1 How it works 2 Description 3 Signal propagation 4 Connectors 5 Important parameters

o 5.1 Physical parameters o 5.2 Fundamental electrical parameters o 5.3 Derived electrical parameters o 5.4 Significance of impedance

6 Issues o 6.1 Signal leakage o 6.2 Ground loops o 6.3 Induction

6.3.1 Transformer effect o 6.4 Common mode current and radiation o 6.5 Miscellaneous

7 Standards o 7.1 References for this section

8 Uses 9 Types

o 9.1 Hard line o 9.2 Radiating o 9.3 RG/6 o 9.4 Triaxial cable o 9.5 Twin-axial cable o 9.6 Biaxial cable o 9.7 Semi-rigid

10 Interference and troubleshooting 11 History 12 See also

13 References

[edit] How it works

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Coaxial cable cutaway

Like an electrical power cord, coaxial cable conducts AC electric current between locations. Like these other cables, it has two conductors, the central wire and the tubular shield. At any moment the current is traveling outward from the source in one of the conductors, and returning in the other. However, since it is alternating current, the current reverses direction many times a second. Coaxial cable differs from other cable because it is designed to carry radio frequency current. This has a frequency much higher than the 50 or 60 Hz used in mains (electric power) cables, reversing direction millions to billions of times per second. Like other types of radio transmission line, this requires special construction to prevent power losses:

If an ordinary wire is used to carry high frequency currents, the wire acts as an antenna, and the high frequency currents radiate off the wire as radio waves, causing power losses. To prevent this, in coaxial cable one of the conductors is formed into a tube and encloses the other conductor. This confines the radio waves from the central conductor to the space inside the tube. To prevent the outer conductor, or shield, from radiating, it is connected to electrical ground, keeping it at a constant potential.

The dimensions and spacing of the conductors are uniform. Any abrupt change in the spacing of the two conductors along the cable tends to reflect radio frequency power back toward the source, causing a condition called standing waves. This acts as a bottleneck, reducing the amount and quality of the transmitted power. To hold the shield at a uniform distance from the central conductor, the space between the two is filled with a plastic dielectric. Manufacturers specify a minimum bend radius, to prevent kinks that would cause reflections. The connectors used with coax hold the correct spacing through the body of the connector.

Each type of coaxial cable has a characteristic impedance (resistance) depending on its dimensions and construction, which is the ratio of the voltage to the current in the cable. In order to prevent reflections at the destination end of the cable from

causing standing waves, any equipment the cable is attached to must present an impedance equal to the characteristic impedance. Thus the equipment "appears" electrically similar to a continuation of the cable, preventing reflections. Common values of characteristic impedance for coaxial cable are 50 and 75 ohms.

[edit] Description

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Coaxial cable design choices affect physical size, frequency performance, attenuation, power handling capabilities, flexibility, strength and cost. The inner conductor might be solid or stranded; stranded is more flexible. To get better high-frequency performance, the inner conductor may be silver plated. Sometimes copper-plated iron wire is used as an inner conductor.

The insulator surrounding the inner conductor may be solid plastic, a foam plastic, or may be air with spacers supporting the inner wire. The properties of dielectric control some electrical properties of the cable. A common choice is a solid polyethylene (PE) insulator, used in lower-loss cables. Solid Teflon (PTFE) is also used as an insulator. Some coaxial lines use air (or some other gas) and have spacers to keep the inner conductor from touching the shield.

There is variety in the shield. Conventional coaxial cable has braided copper wire forming the shield. This allows the cable to be flexible, but it also means there are gaps in the shield layer, and the inner dimension of the shield varies slightly because the braid cannot be flat. Sometimes the braid is silver plated. For better shield performance, some cables have a double-layer shield. The shield might be just two braids, but it is more common now to have a thin foil shield covered by a wire braid. Some cables may invest in more than two shield layers. Other shield designs sacrifice flexibility for better performance; some shields are a solid metal tube. Those cables cannot take sharp bends, as the shield will kink, causing losses in the cable. Many Cable television (CATV) distribution systems use such "hard line" cables, as they provide a lower signal loss.

The insulating jacket can be made from many materials. A common choice is PVC, but some applications may require fire-resistant materials. Outdoor applications may require the jacket to resist ultraviolet light and oxidation. For internal chassis connections the insulating jacket may be omitted.

Connections at the ends of coaxial cables are usually made with RF connectors.

[edit] Signal propagation

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Open wire transmission lines have the property that the electromagnetic wave propagating down the line extends into the space surrounding the parallel wires. These lines have low loss, but also have undesirable characteristics. They cannot be bent, twisted or otherwise shaped without changing their characteristic impedance, causing reflection of the signal back toward the source. They also cannot be run along or attached to anything conductive, as the extended fields will induce currents in the nearby conductors causing unwanted radiation and detuning of the line. Coaxial lines solve this problem by confining the electromagnetic wave to the area inside the cable, between the center conductor and the shield. The transmission of energy in the line occurs totally through the dielectric inside the cable between the conductors. Coaxial lines can therefore be bent and moderately twisted without negative effects, and they can be strapped to conductive supports without inducing unwanted currents in them. In radio-frequency applications up to a few gigahertz, the wave propagates primarily in the transverse electric magnetic (TEM) mode, which means that the electric and magnetic fields are both perpendicular to the direction of propagation. However, above a certain cutoff frequency, transverse electric (TE) and/or transverse magnetic (TM) modes can also propagate, as they do in a waveguide. It is usually undesirable to transmit signals above the cutoff frequency, since it may cause multiple modes with different phase velocities to propagate, interfering with each other. The outer diameter is roughly inversely proportional to the cutoff frequency. A propagating surface-wave mode that does not involve or require the outer shield but only a single central conductor also exists in coax but this mode is effectively suppressed in coax of conventional geometry and common impedance. Electric field lines for this TM mode have a longitudinal component and require line lengths of a half-wavelength or longer.

The outer conductor can also be made of (in order of decreasing leakage and in this case degree of balance): double shield, wound foil, woven tape, braid. The ohmic losses in the conductor increase in this order: Ideal conductor (no loss), superconductor, silver, copper. It is further increased by rough surface (in the order of the skin depth, lateral: current hot spots, longitudinal: long current path) for example due to woven braid, multistranded conductors or a corrugated tube as a conductor) and impurities especially oxygen in the metal (due to a lack of a protective coating). Litz wire is used between 1 kHz and 1 MHz to reduce ohmic losses. Coaxial cables require an internal structure of an insulating (dielectric) material to maintain the spacing between the center conductor and shield. The dielectric losses increase in this order: Ideal dielectric (no loss), vacuum, air, Polytetrafluoroethylene (PTFE), polyethylene foam, and solid polyethylene. It is further increased by impurities like water. In typical applications the loss in polyethylene is comparable to the ohmic loss at 1 GHz and the loss in PTFE is comparable to ohmic losses at 10 GHz. A low dielectric constant allows for a greater center conductor: less ohmic losses. An inhomogeneous dielectric needs to be compensated by a noncircular conductor to avoid current hot-spots.

[edit] Connectors

A coaxial connector (male N-type).Main article: RF connector

Coaxial connectors are designed to maintain a coaxial form across the connection and have the same well-defined impedance as the attached cable. Connectors are often plated with high-conductivity metals such as silver or gold. Due to the skin effect, the RF signal is only carried by the plating and does not penetrate to the connector body. Although silver oxidizes quickly, the silver oxide that is produced is still conductive. While this may pose a cosmetic issue, it does not degrade performance.

[edit] Important parameters

Coaxial cable is a particular kind of transmission line, so the circuit models developed for general transmission lines are appropriate. See Telegrapher's equation.

Schematic representation of the elementary components of a transmission line.

Schematic representation of a coaxial transmission line, showing the characteristic impedance Z0.

[edit] Physical parameters

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Outside diameter of inner conductor, d. Inside diameter of shield, D. Dielectric constant of the insulator, ε. The dielectric constant is often quoted as

the relative dielectric constant εr referred to the dielectric constant of free space ε0: ε = εrε0. When the insulator is a mixture of different dielectric materials (e.g., polyethylene foam is a mixture of polyethylene and air), then the term effective dielectric constant εeff is often used.

Magnetic permeability of the insulator. μ Permeability is often quoted as the relative permeability μr referred to the permeability of free space μ0: μ = μrμ0. The relative permeability will almost always be 1.

[edit] Fundamental electrical parameters

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Shunt Capacitance per unit length, in farads per metre.

Series Inductance per unit length, in Henrys per metre.

Series Resistance per unit length, in ohms per metre. The resistance per unit length is just the resistance of inner conductor and the shield at low frequencies. At higher frequencies, skin effect increases the effective resistance by confining the conduction to a thin layer of each conductor.

Shunt Conductance per unit length, in mhos per metre. The shunt conductance is usually very small because insulators with good dielectric properties are used (a very low loss tangent). At high frequencies, a dielectric can have a significant resistive loss.

[edit] Derived electrical parameters

Characteristic impedance in ohms (Ω). Neglecting resistance per unit length for most coaxial cables, the characteristic impedance is determined from the capacitance per unit length (C) and the inductance per unit length (L). The

simplified expression is ( ). Those parameters are determined from the ratio of the inner (d) and outer (D) diameters and the dielectric constant (ε). The characteristic impedance is given by[1]

Assuming the dielectric properties of the material inside the cable do not vary appreciably over the operating range of the cable, this impedance is frequency independent above about five times the shield cutoff frequency. For typical coaxial cables, the shield cutoff frequency is 600 (RG-6A) to 2,000 Hz (RG-58C).[2]

Attenuation (loss) per unit length, in decibels per meter. This is dependent on the loss in the dielectric material filling the cable, and resistive losses in the center conductor and outer shield. These losses are frequency dependent, the losses becoming higher as the frequency increases. Skin effect losses in the conductors can be reduced by increasing the diameter of the cable. A cable with twice the diameter will have half the skin effect resistance. Ignoring dielectric and other losses, the larger cable would halve the dB/meter loss. In designing a system, engineers consider not only the loss in the cable, but also the loss in the connectors.

Velocity of propagation , in meters per second. The velocity of propagation depends on the dielectric constant and permeability (which is usually 1).

Cutoff frequency is determined by the possibility of exciting other propagation modes in the coaxial cable. The average circumference of the insulator is π(D + d) / 2. Make that length equal to 1 wavelength in the dielectric. The TE01 cutoff frequency is therefore

. Peak Voltage Outside diameter, which dictates which connectors must be used to terminate the

cable.

[edit] Significance of impedance

The best coaxial cable impedances in high-power, high-voltage, and low-attenuation applications were experimentally determined in 1929 at Bell Laboratories to be 30, 60, and 77 Ω respectively. For an air dielectric coaxial cable with a diameter of 10 mm the attenuation is lowest at 77 ohms when calculated for 10 GHz.[1] The curve showing the power handling maxima at 30 ohms can be found here:[2]

CATV systems were one of the first applications for very large quantities of coaxial cable. CATV is typically using such low levels of RF power that power handling and high voltage breakdown characteristics were totally unimportant when compared to attenuation. Moreover, many CATV headends used 300 ohm folded dipole antennas to receive off the air TV signals. 75 ohm coax made a nice 4:1 balun transformer for these antennas as well as presented a nice attenuation specification. But this is a bit of a red herring, when normal dielectrics are added to the equation the best loss impedance drops

down to values between 64 and 52 ohms. Details and a graph showing this effect can be found here: [3][citation needed] 30 Ω cable is more difficult to manufacture due to the much larger center conductor and the stiffness and weight it adds.

The arithmetic mean between 30 ohms and 77 ohms is 53.5, the geometric mean is 48 ohms. The selection of 50 ohms as a compromise between power handling capability and attenuation is generally cited as the reason for the number.

One reference to a paper presented by Bird Electronic Corp as to why 50 ohms was chosen can be found here: [4]

50 Ohms works out well for other reasons such as it corresponds very closely to the drive impedance of a half wave dipole antenna in real environments, and provides an acceptable match to the drive impedance of quarter wave monopoles as well. 73 Ω is an exact match for a centre fed dipole aerial/antenna in free space (approximated by very high dipoles without ground reflections).

RG-62 is a 93 ohm coaxial cable. It is purported that RG-62 cable was originally used in mainframe computer networks. (1970's / early 1980s). It was the cable used to connect the terminals to the terminal cluster controllers. Later some manufacturers of LAN equipment such as ARCNET adopted RG-62 as a standard. It has the lowest capacitance per unit length when compared to other coaxial cables of similar size. Capacitance is the enemy of square wave data transmission and is much more important than power handling or attenuation specifications in these environments.

All of the components of a coaxial system should have the same impedance to reduce internal reflections at connections between components. Such reflections increase signal loss and can result in the reflected signal reaching a receiver with a slight delay from the original. In analog video or TV systems this visual effect is commonly referred to as ghosting. (see Impedance matching)

[edit] Issues

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[edit] Signal leakage

Signal leakage is the passage of electromagnetic fields through the shield of a cable and occurs in both directions. Ingress is the passage of an outside signal into the cable and can result in noise and disruption of the desired signal. Egress is the passage of signal intended to remain within the cable into the outside world and can result in a weaker signal at the end of the cable and radio frequency interference to nearby devices.

For example, in the United States, signal leakage from cable television systems is regulated by the FCC, since cable signals use the same frequencies as aeronautical and radionavigation bands. CATV operators may also choose to monitor their networks for leakage to prevent ingress. Outside signals entering the cable can cause unwanted noise and picture ghosting. Excessive noise can overwhelm the signal, making it useless.

An ideal shield would be a perfect conductor with no holes, gaps or bumps connected to a perfect ground. However, a smooth solid copper shield would be heavy, inflexible, and expensive. Practical cables must make compromises between shield efficacy, flexibility and cost, such as the corrugated surface of hardline, flexible braid, or foil shields. Since the shields are not perfect conductors, electric fields can exist inside the shield, thus allowing radiating electromagnetic fields to go through the shield.

Consider the skin effect. The magnitude of an alternating current in an imperfect conductor decays exponentially with distance beneath the surface, with the depth of penetration being proportional to the square root of the resistivity. This means that in a shield of finite thickness, some small amount of current will still be flowing on the opposite surface of the conductor. With a perfect conductor (i.e., zero resistivity), all of the current would flow at the surface, with no penetration into and through the conductor. Real cables have a shield made of an imperfect, although usually very good, conductor, so there will always be some leakage.

The gaps or holes, allow some of the electromagnetic field to penetrate to the other side. For example, braided shields have many small gaps. The gaps are smaller when using a foil (solid metal) shield, but there is still a seam running the length of the cable. Foil becomes increasingly rigid with increasing thickness, so a thin foil layer is often surrounded by a layer of braided metal, which offers greater flexibility for a given cross-section.

This type of leakage can also occur at locations of poor contact between connectors at either end of the cable.

[edit] Ground loops

A continuous current flow, even if small, along the imperfect shield of a coaxial cable can cause visible or audible interference. In CATV systems distributing analog signals the potential difference between the coaxial network and the electrical grounding system of a house can cause a visible "hum bar" in the picture. This appears as a wide horizontal distortion bar in the picture that scrolls slowly upward. Such differences in potential can be reduced by proper bonding to a common ground at the house. See ground loop.

[edit] Induction

External current sources like switched-mode power supplies create a voltage across the inductance of the outer conductor between sender and receiver. The effect is less when there are several parallel cables, as this reduces the inductance and therefore the voltage.

Because the outer conductor carries the reference potential for the signal on the inner conductor, the receiving circuit measures the wrong voltage.

[edit] Transformer effect

The transformer effect is sometimes used to mitigate the effect of currents induced in the shield. The inner and outer conductors form the primary and secondary winding of the transformer, and the effect is enhanced in some high quality cables that have an outer layer of mu-metal. Because of this 1:1 transformer, the aforementioned voltage across the outer conductor is transformed onto the inner conductor so that the two voltages can be cancelled by the receiver. Many sender and receivers have means to reduce the leakage even further. They increase the transformer effect by passing the whole cable through a ferrite core sometimes several times.

[edit] Common mode current and radiation

Common mode current occurs when stray currents in the shield flow in the same direction as the current in the center conductor, causing the coax to radiate.

Most of the shield effect in coax results from opposing currents in the center conductor and shield creating opposite magnetic fields that cancel, and thus do not radiate. The same effect helps ladder line. However, ladder line is extremely sensitive to surrounding metal objects which can enter the fields before they completely cancel. Coax does not have this problem since the field is enclosed in the shield. However, it is still possible for a field to form between the shield and other connected objects, such as the antenna the coax feeds. The current formed by the field between the antenna and the coax shield would flow in the same direction as the current in the center conductor, and thus not be canceled, and would actually cause energy to radiate from the coax itself, making it appear to be part of the antenna, affecting the radiation pattern of the antenna and possibly introducing dangerous radio frequency energy into areas near people, with the risk of radiation burns if the coax is being used for sufficiently high power transmissions. A properly placed and sized balun can prevent common mode radiation in coax.

[edit] Miscellaneous

Some senders and receivers use only a limited range of frequencies and block all others by means of an isolating transformer. Such a transformer breaks the shield for high frequencies. Still others avoid the transformer effect altogether by using two capacitors. If the capacitor for the outer conductor is implemented as one thin gap in the shield, no leakage at high frequencies occurs. At high frequencies, beyond the limits of coaxial cables, it becomes more efficient to use other types of transmission line such as wave guides or optical fiber, which offer low leakage (and much lower losses) around 200 THz and good isolation for all other frequencies.

[edit] Standards

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Most coaxial cables have a characteristic impedance of either 50, 52, 75, or 93 Ω. The RF industry uses standard type-names for coaxial cables. Thanks to television, RG-6 is the most commonly-used coaxial cable for home use, and the majority of connections outside Europe are by F connectors.

A series of standard types of coaxial cable were specified for military uses, in the form "RG-#" or "RG-#/U". They date from WW II and were listed in MIL-HDBK-216 published in 1962. These designations are now obsolete. The current military standard is MIL-SPEC MIL-C-17. MIL-C-17 numbers, such as "M17/75-RG214," are given for military cables and manufacturer's catalog numbers for civilian applications. However, the RG-series designations were so common for generations that they are still used, although critical users should be aware that since the handbook is withdrawn there is no standard to guarantee the electrical and physical characteristics of a cable described as "RG-# type". The RG designators are mostly used to identify compatible connectors that fit the inner conductor, dielectric, and jacket dimensions of the old RG-series cables.

Table of RG standards

type

approx.impedan

ce[ohms]

coredielectric

overall diameter

braidvelocity factor

commentstype [in] [mm] in mm

RG-6/U

75 1.0 mmSolid P

E0.185 4.7

0.270

8.4doubl

e0.75

Low loss at high frequency for cable television, satellite television and cable modems

RG-6/UQ

75Solid PE

0.298

7.62 quad

This is "quad shield RG-6". It has four layers of shielding; regular RG-6 only has one or two

RG-8/U

50 2.17 mmSolid PE

0.285 7.20.40

510.3

Amateur radio; Thicknet (10BASE5) is similar

RG-9/U

51Solid PE

0.420

10.7

RG-11/U

75 1.63 mmSolid PE

0.285 7.20.41

210.5 0.66

Used for long drops and underground conduit

RG-58/U

50 0.9 mmSolid PE

0.116 2.90.19

55.0 single

0.66/0.78

Used for radiocommunication and amateur radio, thin Ethernet (10BASE2) and NIM electronics. Common.

RG-59/U

75 0.81 mmSolid PE

0.146 3.70.24

26.1 single 0.66

Used to carry baseband video in closed-circuit television, previously used for cable television. Generally it has poor shielding but will carry an HQ HD signal or video over short distances.

RG-60/U

501.024 m

mSolid PE

0.425

10.8 single

Used for high-definition cable TV and high-speed cable Internet.

RG-62/U

92Solid PE

0.242

6.1 single 0.84

Used for ARCNET and automotive radio antennas.

RG-62A

93 ASP0.24

26.1 single

Used for NIM electronics

RG-174/

U50 0.48 mm

Solid PE

0.100 2.50.10

02.55 single 0.66

Common for wifi pigtails: more flexible but higher loss than RG58; used with LEMO 00 connectors in NIM electronics.

RG-178/

U50

7×0.1 mm

(Ag plated

Cu clad Steel)

PTFE 0.033 0.840.07

11.8 single 0.69

RG-179/

75 7×0.1 mm

PTFE 0.063 1.6 0.098

2.5 single 0.67 VGA RGBHV

U(Ag

plated Cu)

RG-213/

U50

7×0.0296 in Cu

Solid PE

0.285 7.20.40

510.3 single 0.66

For radiocommunication and amateur radio, EMC test antenna cables. Typically lower loss than RG58. Common.

RG-214/

U50

7×0.0296 in

PTFE 0.285 7.20.42

510.8

double

0.66

RG-218

500.195 in

CuSolid PE

0.660 (0.680

?)

16.76 (17.27

?)

0.870

22 single 0.66

Large diameter, not very flexible, low loss (2.5dB/100' @ 400 MHz), 11kV dielectric withstand.

RG-223

50 2.74 mmPE

Foam0.285 7.24

0.405

10.29

Double

RG-316/

U50

7x0.0067 in

PTFE 0.060 1.50.10

22.6 single 0.695

used with LEMO 00 connectors in NIM electronics

PE is Polyethylene; PTFE is Polytetrafluoroethylene; ASP is Air Space Polyethylene[3]

Commercial designations

type

approx.impedanc

e[ohms]

coredielectric

overall diameter

braidvelocity factor

commentstype [in] [mm] in mm

H155 50 0.79

lower loss at high frequency for radiocommunication and amateur radio

H500 50 0.82

low loss at high frequency for radiocommunication and amateur radio

LMR-195

50low loss drop-in replacement for RG-58

LMR-200

50 1.12 mm Cu

PF CF

0.116 2.95 0.195 4.95 0.83 low loss communications,

HDF-200

CFD-200

0.554 dB/meter @ 2.4 GHz

LMR-400

HDF-400

CFD-400

502.74 mm(Cu clad

Al)

PF CF

0.285 7.24 0.405 10.29 0.85

low loss communications, 0.223 dB/meter @ 2.4 GHz[4]

LMR-600

504.47 mm(Cu clad

Al)PF 0.455 11.56 0.590 14.99 0.87

low loss communications, 0.144 dB/meter @ 2.4 GHz

LMR-900

506.65 mm(BC tube)

PF 0.680 17.27 0.870 22.10 0.87

low loss communications, 0.098 dB/meter @ 2.4 GHz

LMR-1200

508.86 mm(BC tube)

PF 0.920 23.37 1.200 30.48 0.88

low loss communications, 0.075 dB/meter @ 2.4 GHz

LMR-1700

5013.39 m

m(BC tube)

PF 1.350 34.29 1.670 42.42 0.89

low loss communications, 0.056 dB/meter @ 2.4 GHz

There are also other designation schemes for coaxial cables such as The URM, CT and WF series

[edit] References for this section

RF transmission lines and fittings. Military Standardization Handbook MIL-HDBK-216, U.S. Department of Defense, 4 January 1962. [5]

Withdrawal Notice for MIL-HDBK-216 2001 Cables, radio frequency, flexible and rigid. Details Specification MIL-DTL-17H,

19 August 2005 (superseding MIL-C-17G, 9 March 1990). [6] Radio-frequency cables, International Standard IEC 60096. Coaxial communication cables, International Standard IEC 61196. Coaxial cables, British Standard BS EN 50117 H. P. Westman et al., (ed), Reference Data for Radio Engineers, Fifth Edition,

1968, Howard W. Sams and Co., no ISBN, Library of Congress Card No. 43-14665

http://www.rfcafe.com/references/electrical/coax-chart.htm Talley Communications CAxT Cable Assembly Specs for MIL-C-17 Coaxial Cable Q.P.L. Times Microwave Systems LMR Wireless Products Catalog

CFD Cable Specifications Specs of RG174/U, RG58C/U etc. RG213/8, RG218, CLX1/4", CLX1/2", CLX7/8", CLX1+5/8" Cable Power &

Impedance Specs Velocity factor of various coaxial cables Pasternack 2009 Catalog Union Copper site with pictures, diagrams, and spec sheet

[edit] Uses

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Short coaxial cables are commonly used to connect home video equipment, in ham radio setups, and in measurement electronics. They used to be common for implementing computer networks, in particular Ethernet, but twisted pair cables have replaced them in most applications except in the growing consumer cable modem market for broadband Internet access.

Long distance coaxial cable is used to connect radio networks and television networks, though this has largely been superseded by other more high-tech methods (fibre optics, T1/E1, satellite). It still carries cable television signals to the majority of television receivers, and this purpose consumes the majority of coaxial cable production.

Micro coaxial cables are used in a range of consumer devices, military equipment, and also in ultra-sound scanning equipment.

The most common impedances that are widely used are 50 or 52 ohms, and 75 ohms, although other impedances are available for specific applications. The 50 / 52 ohm cables are widely used for industrial and commercial two-way radio frequency applications (including radio, and telecommunications), although 75 ohms is commonly used for broadcast television and radio.

[edit] Types

[edit] Hard line

1-5/8" hard line

Hard line is often confused with waveguide but the two are not the same. Hard line is used in broadcasting as well as many other forms of radio communication. It is a coaxial cable constructed using round copper, silver or gold tubing or a combination of such metals as a shield. Some lower quality hard line may use aluminum shielding, aluminum however is easily oxidized and unlike silver or gold oxide, aluminum oxide drastically loses effective conductivity. Therefore all connections must be air and water tight. The center conductor may consist of solid copper, or copper plated aluminum. Since skin effect is an issue with RF, copper plating provides sufficient surface for an effective conductor. Most varieties of hardline used for external chassis or when exposed to the elements have a PVC jacket; however, some internal applications may omit the insulation jacket. Hard line can be very thick, typically at least a half inch or 13 mm and up to several times that, and has low loss even at high power. These large scale hard lines are almost always used in the connection between a transmitter on the ground and the antenna or aerial on a tower. Hard line may also be known by trademarked names such as Heliax (Andrew),[5] or Cablewave (RFS/Cablewave).[6] Larger varieties of hardline may consist of a center conductor which is constructed from either rigid or corrugated copper tubing. The dielectric in hard line may consist of polyethylene foam, air or a pressurized gas such as nitrogen or desiccated air (dried air). In gas-charged lines, hard plastics such as nylon are used as spacers to separate the inner and outer conductors. The addition of these gases into the dielectric space reduces moisture contamination, provides a stable dielectric constant, as well as a reduced risk of internal arcing. Gas-filled hardlines are usually used on high powered RF transmitters such as television or radio broadcasting, military transmitters, as well as high powered amateur radio applications but may also be used on some critical lower powered applications such as those in the microwave bands. Although in the microwave region waveguide is more often used than hard line for transmitter to antenna, or antenna to receiver applications. The various shields used in hardline also differ; some forms use rigid tubing, or pipe, others may use a corrugated tubing which makes bending easier, as well as reduces kinking when the cable is bent to conform. Smaller varieties of hard line may be used internally in some high frequency applications, particularly in equipment within the microwave range, to reduce interference between stages of the device.

[edit] Radiating

Radiating or Leaky Cable is another form of coaxial cable which is constructed in a similar fashion to hard line, however it is constructed with tuned slots cut into the shield. These slots are tuned to the specific RF wavelength of operation or tuned to a specific radio frequency band. This type of cable is to provide a tuned bi-directional "desired" leakage effect between transmitter and receiver. It is often used in elevator shafts, underground, transportation tunnels and in other areas where an antenna is not feasible. One example of this type of cable is Radiax (Andrew).[7]

[edit] RG/6

RG/6 is available in three different types designed for various applications. "Plain" or "house" wire is designed for indoor or external house wiring. "Flooded" cable is infused with heavy waterproofing for use in underground conduit (ideally) or direct burial. "Messenger" may contain some waterproofing but is distinguished by the addition of a steel messenger wire along its length to carry the tension involved in an aerial drop from a utility pole.

[edit] Triaxial cable

Main article: Triaxial cable

Triaxial cable or triax is coaxial cable with a third layer of shielding, insulation and sheathing. The outer shield, which is earthed (grounded), protects the inner shield from electromagnetic interference from outside sources.

[edit] Twin-axial cable

Main article: Twinaxial cabling

Twin-axial cable or twinax is a balanced, twisted pair within a cylindrical shield. It allows a nearly perfect differential signal which is both shielded and balanced to pass through. Multi-conductor coaxial cable is also sometimes used.

[edit] Biaxial cable

Main article: Twin-lead

Biaxial cable, biax or Twin-Lead is a figure-8 configuration of two 50 Ω coaxial cables, externally resembling that of lamp cord, or speaker wire. Biax is used in some proprietary computer networks. Others may be familiar with 75Ω biax which at one time was popular on many cable TV services.

[edit] Semi-rigid

Semi-rigid cable is a coaxial form using a solid copper outer sheath. This type of coax offers superior screening compared to cables with a braided outer conductor, especially at higher frequencies. The major disadvantage is that the cable, as its name implies, is not very flexible, and is not intended to be flexed after initial forming. (See "hard line")

[edit] Interference and troubleshooting

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Coaxial cable insulation may degrade, requiring replacement of the cable, especially if it has been exposed to the elements on a continuous basis. The shield is normally grounded, and if even a single thread of the braid or filament of foil touches the center conductor, the signal will be shorted causing significant or total signal loss. This most often occurs at improperly installed end connectors and splices. Also, the connector or splice must be properly attached to the shield, as this provides the path to ground for the interfering signal.

Despite being shielded, interference can occur on coaxial cable lines. Susceptibility to interference has little relationship to broad cable type designations (e.g. RG-59, RG-6) but is strongly related to the composition and configuration of the cable's shielding. For cable television, with frequencies extending well into the UHF range, a foil shield is normally provided, and will provide total coverage as well as high effectiveness against high-frequency interference. Foil shielding is ordinarily accompanied by a tinned copper or aluminum braid shield, with anywhere from 60 to 95% coverage. The braid is important to shield effectiveness because (1) it is more effective than foil at absorbing low-frequency interference, (2) it provides higher conductivity to ground than foil, and (3) it makes attaching a connector easier and more reliable. "Quad-shield" cable, using two low-coverage aluminum braid shields and two layers of foil, is often used in situations involving troublesome interference, but is less effective than a single layer of foil and single high-coverage copper braid shield such as is found on broadcast-quality precision video cable.

In the United States and some other countries, cable television distribution systems use extensive networks of outdoor coaxial cable, often with in-line distribution amplifiers. Leakage of signals into and out of cable TV systems can cause interference to cable subscribers and to over-the-air radio services using the same frequencies as those of the cable system.

[edit] History