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9 mars 2011 3 09 /03 /mars /2011 04:18

A high-voltage, direct current (HVDC) electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses  (Dell XPS M1210 Batteryhttp://www.hdd-shop.co.uk .

For shorter distances, the higher cost of DC conversion equipment compared to an AC system may be warranted where other benefits of direct current links are useful.

The modern form of HVDC transmission uses technology developed extensively in the 1930s in Sweden at ASEA Dell Studio XPS 1640 Battery .

Early commercial installations included one in the Soviet Union in 1951 between Moscow and Kashira, and a 10-20 MW system between Gotland and mainlandSweden in 1954.The longest HVDC link in the world is currently the Xiangjiaba-Shanghai 2,071 km (1,287 mi) 6400 MW link connecting theXiangjiaba Dam to Shanghai, in the People's Republic of China Dell Vostro 1710 Battery .

In 2012, the longest HVDC link will be the Rio Madeira link connecting theAmazonas to the São Paulo area and the length of the DC line is over 2,500 km (1,600 mi).

High voltage is used for electric power transmission to reduce the energy lost in the resistance of the wires Dell KM958 battery .

For a given quantity of power transmitted, higher voltage reduces the transmission power loss. The power lost as heat in the wires is proportional to the square of the current. So if a given power is transmitted at higher voltage and lower current, power loss in the wires is reduced  Dell Studio 1555 battery .

Power loss can also be reduced by reducing resistance, for example by increasing the diameter of the conductor, but larger conductors are heavier and more expensive  Sony VGP-BPS13 battery .

High voltages cannot easily be used for lighting and motors, and so transmission-level voltages must be reduced to values compatible with end-use equipment. Transformers are used to change the voltage level in alternating current (AC) transmission circuits  Sony VGP-BPS13/B battery .

The competition between the direct current (DC) of Thomas Edison and the AC of Nikola Tesla and George Westinghouse was known as the War of Currents, with AC becoming dominant. Practical manipulation of DC voltages became possible with the development of high power electronic devices such as mercury arc valves and Sony VGP-BPS13/S battery ,

more recently, semiconductor devices such as thyristors, insulated-gate bipolar transistors (IGBTs), high power MOSFETs and gate turn-off thyristors (GTOs).

The first long-distance transmission of electric power was demonstrated using direct current in 1882 at the Miesbach-Munich Power Transmission, but only 2.5 kW was transmitted  Sony VGP-BPS13A/B battery .

An early method of high-voltage DC transmission was developed by the Swiss engineer René Thury  and his method was put into practice by 1889 in Italy by the Acquedotto De Ferrari-Galliera company. This system used series-connected motor-generator sets to increase voltage  Sony VGP-BPS13B/B battery .

Each set was insulated from ground and driven by insulated shafts from a prime mover. The line was operated in constant current mode, with up to 5,000 volts on each machine, some machines having double commutators to reduce the voltage on each commutator  Sony VGP-BPL9 battery .

This system transmitted 630 kW at 14 kV DC over a distance of 120 km.The Moutiers-Lyon system transmitted 8,600 kW of hydroelectric power a distance of 124 miles, including 6 miles of underground cable. The system used eight series-connected generators with dual commutators for a total voltage of 150,000 volts between the poles, and ran from about 1906 until 1936 Sony VGP-BPS13B/B battery .

Fifteen Thury systems were in operation by 1913  Other Thury systems operating at up to 100 kV DC operated up to the 1930s, but the rotating machinery required high maintenance and had high energy loss. Various other electromechanical devices were tested during the first half of the 20th century with little commercial success  Sony VGP-BPL15 battery .

One conversion technique attempted for conversion of direct current from a high transmission voltage to lower utilization voltage was to charge series-connected batteries, then connect the batteries in parallel to serve distribution loads  Dell Inspiron E1505 battery .

While at least two commercial installations were tried around the turn of the 20th century, the technique was not generally useful owing to the limited capacity of batteries, difficulties in switching between series and parallel connections, and the inherent energy inefficiency of a battery charge/discharge cycle  Dell Latitude E6400 battery .

The grid controlled mercury arc valve became available for power transmission during the period 1920 to 1940. Starting in 1932, General Electric tested mercury-vapor valves and a 12 kV DC transmission line, which also served to convert 40 Hz generation to serve 60 Hz loads, at Mechanicville, New York HP Pavilion dv6000 Battery .

In 1941, a 60 MW, +/-200 kV, 115 km buried cable link was designed for the city of Berlin using mercury arc valves (Elbe-Project), but owing to the collapse of the German government in 1945 the project was never completed. The nominal justification for the project was that, during wartime, a buried cable would be less conspicuous as a bombing target SONY VAIO VGN-FZ Battery .

The equipment was moved to the Soviet Union and was put into service there.

Introduction of the fully static mercury arc valve to commercial service in 1954 marked the beginning of the modern era of HVDC transmission  SONY VAIO VGN-FZ18 Battery .

A HVDC-connection was constructed by ASEA between the mainland of Sweden and the island Gotland. Mercury arc valves were common in systems designed up to 1975, but since then, HVDC systems use only solid-state devices. From 1975 to 2000, line-commutated converters (LCC) using thyristor valves were relied on  SONY VAIO VGN-FZ21E Battery .

According to senior engineer Dr Vijay Sood, the next 25 years may well be dominated by force commutated converters, beginning with capacitor commutated converters (CCC) followed by self commutating converters which have largely supplanted LCC use  SONY VAIO VGN-FW31J Battery - 5200mAh/7800mAh/10400mAh, 11.10V .

Since use of semiconductor commutators, hundreds of HVDC sea-cables have been laid and worked with high reliability, usually better than 96% of the time  SONY VAIO VGN-FW31J Battery - 5200mAh/7800mAh/10400mAh, 11.10V .

The advantage of HVDC is the ability to transmit large amounts of power over long distances with lower capital costs and with lower losses than AC. Depending on voltage level and construction details, losses are quoted as about 3% per 1,000 km. High-voltage direct current transmission allows efficient use of energy sources remote from load centers SONY VAIO VGN-FW21E Battery .

In a number of applications HVDC is more effective than AC transmission. Examples include:

  • Undersea cables, where high capacitance causes additional AC losses. (e.g., 250 km Baltic Cable between Sweden and Germany, the 600 km NorNedcable between Norway and the Netherlands, and 290 km Basslink between the Australian mainland and Tasmania SONY VAIO VGN-NR11S/S Battery )
  • Endpoint-to-endpoint long-haul bulk power transmission without intermediate 'taps', for example, in remote areas
  • Increasing the capacity of an existing power grid in situations where additional wires are difficult or expensive to install
  • Power transmission and stabilization between unsynchronised AC distribution systems SONY VAIO VGN-NR11M/S Battery
  • Connecting a remote generating plant to the distribution grid, for example Nelson River Bipole
  • Stabilizing a predominantly AC power-grid, without increasing prospective short circuit current SONY VAIO VGN-NR11Z/S Battery
  • Reducing line cost. HVDC needs fewer conductors as there is no need to support multiple phases. Also, thinner conductors can be used since HVDC does not suffer from the skin effect
  • Facilitate power transmission between different countries that use AC at differing voltages and/or frequencies SONY VAIO VGN-NR11Z/T Battery
  • Synchronize AC produced by renewable energy sources

Long undersea high voltage cables have a high electrical capacitance, since the conductors are surrounded by a relatively thin layer of insulation and a metal sheath  Sony VAIO VGN-FZ21E Battery .

The geometry is that of a long co-axial capacitor. Where alternating current is used for cable transmission, this capacitance appears in parallel with load. Additional current must flow in the cable to charge the cable capacitance, which generates additional losses in the conductors of the cable  Sony VAIO VGN-FW21E Battery .

Additionally, there is a dielectric loss component in the material of the cable insulation, which consumes power.

When, however, direct current is used, the cable capacitance is charged only when the cable is first energized or when the voltage is changed; there is no steady-state additional current required Sony VAIO VGN-NR11S/S Battery .

For a long AC undersea cable, the entire current-carrying capacity of the conductor could be used to supply the charging current alone. The cable capacitance issue limits the length and power carrying capacity of AC cables. DC cables have no such limitation, and are essentially bound by only Ohm's Law  Sony VAIO VGN-NR11Z/S Battery .

Although some DC leakage current continues to flow through the dielectric insulators, this is very small compared to the cable rating and much less than with AC transmission cables  Sony VAIO VGN-NR11M/S Battery .

HVDC can carry more power per conductor because, for a given power rating, the constant voltage in a DC line is lower than the peak voltage in an AC line. The power delivered is defined by the root mean square (RMS) of an AC voltage, but RMS is only about 71% of the peak voltage  Sony VAIO VGN-NR11Z/T Battery .

The peak voltage of AC determines the actual insulation thickness and conductor spacing. Because DC operates at a constant maximum voltage, this allows existing transmission line corridors with equally sized conductors and insulation to carry more power into an area of high power consumption than AC, which can lower costs SONY VAIO VGN-FZ180E Battery .

Because HVDC allows power transmission between unsynchronized AC distribution systems, it can help increase system stability, by preventing cascading failures from propagating from one part of a wider power transmission grid to another SONY VAIO VGN-FZ220E Battery .

Changes in load that would cause portions of an AC network to become unsynchronized and separate would not similarly affect a DC link, and the power flow through the DC link would tend to stabilize the AC network. The magnitude and direction of power flow through a DC link can be directly commanded SONY VAIO VGN-FZ340E Battery ,

and changed as needed to support the AC networks at either end of the DC link. This has caused many power system operators to contemplate wider use of HVDC technology for its stability benefits alone.

The disadvantages of HVDC are in conversion, switching, control, availability and maintenance SONY VAIO VGN-FZ430E Battery .

HVDC is less reliable and has lower availability than AC systems, mainly due to the extra conversion equipment. Single pole systems have availability of about 98.5%, with about a third of the downtime unscheduled due to faults. Fault redundant bipole systems provide high availability for 50% of the link capacity, but availability of the full capacity is about 97% to 98% SONY VAIO VGN-FZ460E Battery .

The required static inverters are expensive and have limited overload capacity. At smaller transmission distances the losses in the static inverters may be bigger than in an AC transmission line. The cost of the inverters may not be offset by reductions in line construction cost and lower line loss  SONY VAIO VGN-FZ480E Battery .

With two exceptions, all former mercury rectifiers worldwide have been dismantled or replaced bythyristor units. Pole 1 of the HVDC scheme between the North and South Islands of New Zealand still uses mercury arc rectifiers, as does Pole 1 of the Vancouver Island link in Canada  SONY VAIO VGN-FZ4000 Battery .

Both are currently being replaced - in New Zealand by a new thyristor pole and in Canada by a three-phase AC link.

In contrast to AC systems, realizing multiterminal systems is complex, as is expanding existing schemes to multiterminal systemsSONY VAIO VGN-FZ31E Battery .

Controlling power flow in a multiterminal DC system requires good communication between all the terminals; power flow must be actively regulated by the inverter control system instead of the inherent impedance and phase angle properties of the transmission line  SONY VAIO VGN-FZ31B Battery .

Multi-terminal lines are rare. One is in operation at the Hydro Québec - New England transmission from Radisson to Sandy Pond. Another example is the Sardinia-mainland Italy link which was modified in 1989 to also provide power to the island of CorsicaSONY VAIO VGN-FZ31J Battery .

High voltage DC circuit breakers are difficult to build because some mechanism must be included in the circuit breaker to force current to zero, otherwise arcing and contact wear would be too great to allow reliable switching SONY VAIO VGN-FZ31M Battery .

Operating a HVDC scheme requires many spare parts to be kept, often exclusively for one system as HVDC systems are less standardized than AC systems and technology changes faster  SONY VAIO VGN-FZ31Z Battery .

For an 8 GW 40 km link laid under the English Channel, the following are approximate primary equipment costs for a 2000 MW 500 kV bipolar conventional HVDC link (exclude way-leaving, on-shore reinforcement works, consenting, engineering, insurance, etc.) SONY VAIO VGN-FZ38M Battery

  • Converter stations ~£110M
  • Subsea cable + installation ~£1M/km

So for an 8 GW capacity between England and France in four links, little is left over from £750M for the installed works SONY VGP-BPS8 Battery .

Add another £200–300M for the other works depending on additional onshore works required.

Components

Most of the HVDC systems in operation today are based on Line-Commutated Converters. Early static systems used mercury arc rectifiers, which were unreliable SONY VGP-BPS13/S Battery .

Two HVDC systems using mercury arc rectifiers are still in service (As of 2008). The thyristor valve was first used in HVDC systems in the 1960s. The thyristor is a solid-state semiconductor device similar to the diode, but with an extra control terminal that is used to switch the device on at a particular instant during the AC cycle  SONY VGP-BPS13A/B Battery .

The insulated-gate bipolar transistor (IGBT) is now also used, forming a Voltage Sourced Converter, and offers simpler control, reduced harmonics and reduced valve cost  SONY VGP-BPS13B/B Battery .

Because the voltages in HVDC systems, up to 800 kV in some cases, exceed the breakdown voltages of the semiconductor devices, HVDC converters are built using large numbers of semiconductors in series SONY VGP-BPS13A/S Battery .

The low-voltage control circuits used to switch the thyristors on and off need to be isolated from the high voltages present on the transmission lines. This is usually done optically. In a hybrid control system, the low-voltage control electronics sends light pulses along optical fibres to the high-side control electronics  SONY VGP-BPS13AS Battery .

Another system, called direct light triggering, dispenses with the high-side electronics, instead using light pulses from the control electronics to switch light-triggered thyristors (LTTs).

A complete switching element is commonly referred to as a valve, irrespective of its construction Dell Inspiron 1320n Battery .

Rectifying and inverting systems

Rectification and inversion use essentially the same machinery. Many substations (Converter Stations) are set up in such a way that they can act as both rectifiers and inverters  Dell Inspiron 1464 Battery .

At the AC end a set of transformers, often three physically separated single-phase transformers, isolate the station from the AC supply, to provide a local earth, and to ensure the correct eventual DC voltage. The output of these transformers is then connected to a bridge rectifier formed by a number of valves  Dell Inspiron 1564 Battery .

The basic configuration uses six valves, connecting each of the three phases to each of the two DC rails. However, with a phase change only every sixty degrees, considerable harmonics remain on the DC rails Dell Inspiron 1764 Battery .

An enhancement of this configuration uses 12 valves (often known as a twelve-pulse system). The AC is split into two separate three phase supplies before transformation. One of the sets of supplies is then configured to have a star (wye) secondary, the other a delta secondary, establishing a thirty degree phase difference between the two sets of three phases Dell Studio 1450 Battery .

With twelve valves connecting each of the two sets of three phases to the two DC rails, there is a phase change every 30 degrees, and harmonics are considerably reduced.

In addition to the conversion transformers and valve-sets, various passive resistive and reactive components help filter harmonics out of the DC rails Dell Studio 1457 Battery .

Monopole and earth return

In a common configuration, called monopole, one of the terminals of the rectifier is connected to earth ground. The other terminal, at a potential high above or below ground, is connected to a transmission line Dell Latitude D610 Battery .

The earthed terminal may be connected to the corresponding connection at the inverting station by means of a second conductor.

If no metallic conductor is installed, current flows in the earth between the earth electrodes at the two stations. Therefore it is a type of single wire earth return  Toshiba NB100 Battery .

The issues surrounding earth-return current include:

  • Electrochemical corrosion of long buried metal objects such as pipelines Toshiba Satellite M65 battery
  • Underwater earth-return electrodes in seawater may produce chlorine or otherwise affect water chemistry.
  • An unbalanced current path may result in a net magnetic field, which can affect magnetic navigational compasses for ships passing over an underwater cable  Toshiba Satellite M60 battery .

These effects can be eliminated with installation of a metallic return conductor between the two ends of the monopolar transmission line. Since one terminal of the converters is connected to earth, the return conductor need not be insulated for the full transmission voltage which makes it less costly than the high-voltage conductor Dell Latitude D830 Battery .

Use of a metallic return conductor is decided based on economic, technical and environmental factors.

Modern monopolar systems for pure overhead lines carry typically 1,500 MW. If underground or underwater cables are used, the typical value is 600 MW Dell Latitude D620 Battery .

Most monopolar systems are designed for future bipolar expansion. Transmission line towers may be designed to carry two conductors, even if only one is used initially for the monopole transmission system. The second conductor is either unused, used as electrode line or connected in parallel with the other (as in case of Baltic-Cable) Dell Inspiron Mini 10 Battery .

Bipolar

In bipolar transmission a pair of conductors is used, each at a high potential with respect to ground, in opposite polarity. Since these conductors must be insulated for the full voltage, transmission line cost is higher than a monopole with a return conductor Sony VGN-FW11S Battery .

However, there are a number of advantages to bipolar transmission which can make it the attractive option.

  • Under normal load, negligible earth-current flows, as in the case of monopolar transmission with a metallic earth-return. This reduces earth return loss and environmental effects Sony VGN-FW11M Battery .
  • When a fault develops in a line, with earth return electrodes installed at each end of the line, approximately half the rated power can continue to flow using the earth as a return path, operating in monopolar mode Sony VGN-FW139E/H battery .
  • Since for a given total power rating each conductor of a bipolar line carries only half the current of monopolar lines, the cost of the second conductor is reduced compared to a monopolar line of the same rating Dell Latitude E5400 Battery .
  • In very adverse terrain, the second conductor may be carried on an independent set of transmission towers, so that some power may continue to be transmitted even if one line is damaged Dell Latitude E4200 Battery .

A bipolar system may also be installed with a metallic earth return conductor.

Bipolar systems may carry as much as 3,200 MW at voltages of +/-600 kV. Submarine cable installations initially commissioned as a monopole may be upgraded with additional cables and operated as a bipole Dell Inspiron 300M Battery .

A bipolar scheme can be implemented so that the polarity of one or both poles can be changed. This allows the operation as two parallel monopoles. If one conductor fails, transmission can still continue at reduced capacity. Losses may increase if ground electrodes and lines are not designed for the extra current in this mode Dell Vostro A840 Battery .

To reduce losses in this case, intermediate switching stations may be installed, at which line segments can be switched off or parallelized. This was done at Inga–Shaba HVDC  Dell Studio 1737 battery .

Back to back

A back-to-back station (or B2B for short) is a plant in which both static inverters and rectifiers are in the same area, usually in the same building. The length of the direct current line is kept as short as possible Dell Inspiron E1505 battery .

HVDC back-to-back stations are used for

  • coupling of electricity mains of different frequency (as in Japan; and the GCC interconnection between UAE [50 Hz] and Saudi Arabia [60 Hz] under construction in ±2009-2011)  Dell RM791 battery
  • coupling two networks of the same nominal frequency but no fixed phase relationship (as until 1995/96 in Etzenricht, Dürnrohr, Vienna, and theVyborg HVDC scheme).
  • different frequency and phase number (for example, as a replacement for traction current converter plants) Dell XPS M1530 battery

The DC voltage in the intermediate circuit can be selected freely at HVDC back-to-back stations because of the short conductor length. The DC voltage is as low as possible, in order to build a small valve hall and to avoid series connections of valves Dell XPS M2010 battery .

For this reason at HVDC back-to-back stations valves with the highest available current rating are used .

Systems with transmission lines

The most common configuration of an HVDC link is two inverter/rectifier stations connected by an overhead power line Acer Aspire One battery .

This is also a configuration commonly used in connecting unsynchronised grids, in long-haul power transmission, and in undersea cables.

Multi-terminal HVDC links, connecting more than two points, are rare  Toshiba Satellite P10 Battery .

The configuration of multiple terminals can be series, parallel, or hybrid (a mixture of series and parallel). Parallel configuration tends to be used for large capacity stations, and series for lower capacity stations. An example is the 2,000 MW Quebec - New England Transmissionsystem opened in 1992, which is currently the largest multi-terminal HVDC system in the world SONY VGN-FZ210CE Battery .

Tripole: current-modulating control

A scheme patented in 2004 (Current modulation of direct current transmission lines) is intended for conversion of existing AC transmission lines to HVDC  Dell Precision M70 Battery .

Two of the three circuit conductors are operated as a bipole. The third conductor is used as a parallel monopole, equipped with reversing valves (or parallel valves connected in reverse polarity). The parallel monopole periodically relieves current from one pole or the other, switching polarity over a span of several minutes Toshiba Satellite L305 Battery .

The bipole conductors would be loaded to either 1.37 or 0.37 of their thermal limit, with the parallel monopole always carrying +/- 1 times its thermal limit current. The combined RMS heating effect is as if each of the conductors is always carrying 1.0 of its rated current Toshiba Satellite T4900 Battery .

This allows heavier currents to be carried by the bipole conductors, and full use of the installed third conductor for energy transmission. High currents can be circulated through the line conductors even when load demand is low, for removal of ice Toshiba PA3399U-2BRS battery .

As of 2005, no tri-pole conversions are in operation, although a transmission line in India has been converted to bipole HVDC.

Cross-Skagerrak consists of 3 poles, from which 2 are switched in parallel and the third uses an opposite polarity with a higher transmission voltage  Toshiba Satellite A200 Battery .

A similar arrangement is HVDC Inter-Island, but it consists of 2 parallel-switched inverters feeding in the same pole and a third one with opposite polarity and higher operation voltage.

Corona discharge is the creation of ions in a fluid (such as air) by the presence of a strong electric field Toshiba Satellite 1200 Battery.

Electrons are torn from neutral air, and either the positive ions or the electrons are attracted to the conductor, while the charged particles drift. This effect can cause considerable power loss, create audible and radio-frequency interference, generate toxic compounds such as oxides of nitrogen and ozone, and bring forth arcing Toshiba Satellite M300 Battery .

Both AC and DC transmission lines can generate coronas, in the former case in the form of oscillating particles, in the latter a constant wind. Due to the space charge formed around the conductors, an HVDC system may have about half the loss per unit length of a high voltage AC system carrying the same amount of power SONY VGP-BPS13A/Q Battery .

With monopolar transmission the choice of polarity of the energized conductor leads to a degree of control over the corona discharge. In particular, the polarity of the ions emitted can be controlled, which may have an environmental impact on particulate condensation  SONY VGP-BPS13A/Q Battery .

(particles of different polarities have a different mean-free path.) Negative coronas generate considerably more ozone than positive coronas, and generate it further downwind of the power line, creating the potential for health effects. The use of a positive voltage will reduce the ozone impacts of monopole HVDC power lines SONY VGP-BPS13B/Q Battery .

Overview

The controllability of current-flow through HVDC rectifiers and inverters, their application in connecting unsynchronized networks, and their applications in efficient submarine cables mean that HVDC cables are often used at national boundaries for the exchange of power SONY VGP-BPS13B/Q Battery

(in North America, HVDC connections divide much of Canada and the United States into several electrical regions that cross national borders, although the purpose of these connections is still to connect unsynchronized AC grids to each other). Offshore windfarms also require undersea cables, and their turbines are unsynchronized SONY VGP-BPS13/Q Battery .

In very long-distance connections between just two points, for example around the remote communities of Siberia, Canada, and the Scandinavian North, the decreased line-costs of HVDC also makes it the usual choice. Other applications have been noted throughout this article SONY VGP-BPS13/Q Battery .

AC network interconnections

AC transmission lines can interconnect only synchronized AC networks that oscillate at the same frequency and in phase. Many areas that wish to share power have unsynchronized networks SONY VGP-BPS21A/B Battery .

The power grids of the UK, Northern Europe and continental Europe are not united into a single synchronized network. Japan has 50 Hz and 60 Hz networks. Continental North America, while operating at 60 Hz throughout, is divided into regions which are unsynchronised: East, West, Texas, Quebec, and Alaska  SONY VGP-BPS21/S Battery .

Brazil and Paraguay, which share the enormous Itaipu hydroelectric plant, operate on 60 Hz and 50 Hz respectively. However, HVDC systems make it possible to interconnect unsynchronized AC networks, and also add the possibility of controlling AC voltage and reactive power flow  SONY VGP-BPS21B Battery .

A generator connected to a long AC transmission line may become unstable and fall out of synchronization with a distant AC power system. An HVDC transmission link may make it economically feasible to use remote generation sites  SONY VGP-BPS21A Battery .

Wind farms located off-shore may use HVDC systems to collect power from multiple unsynchronized generators for transmission to the shore by an underwater cable.

In general, however, an HVDC power line will interconnect two AC regions of the power-distribution grid SONY VGP-BPS21 Battery .

Machinery to convert between AC and DC power adds a considerable cost in power transmission. The conversion from AC to DC is known as rectification, and from DC to AC as inversion  Sony VGP-BPS21A/B Battery .

Above a certain break-even distance (about 50 km for submarine cables, and perhaps 600–800 km for overhead cables), the lower cost of the HVDC electrical conductors outweighs the cost of the electronics Sony VGP-BPS21/S Battery .

The conversion electronics also present an opportunity to effectively manage the power grid by means of controlling the magnitude and direction of power flow. An additional advantage of the existence of HVDC links, therefore, is potential increased stability in the transmission grid Sony VGP-BPS21A Battery .

A number of studies have highlighted the potential benefits of very wide area super grids based on HVDC since they can mitigate the effects of intermittency by averaging and smoothing the outputs of large numbers of geographically dispersed wind farms or solar farms Sony VGP-BPS21 Battery .

Czisch's study concludes that a grid covering the fringes of Europe could bring 100% renewable power (70% wind, 30% biomass) at close to today's prices. There has been debate over the technical feasibility of this proposal  and the political risks involved in energy transmission across a large number of international borders  Sony Vaio PCG-5G2L Battery .

The construction of such green power superhighways is advocated in a white paper that was released by the American Wind Energy Association and theSolar Energy Industries Association  Sony Vaio PCG-5G3L Battery .

In January 2009, the European Commission proposed €300 million to subsidize the development of HVDC links between Ireland, Britain, the Netherlands, Germany, Denmark, and Sweden, as part of a wider €1.2 billion package supporting links to offshore wind farms and cross-border interconnectors throughout Europe Sony Vaio PCG-5J1L Battery .

Meanwhile the recently founded Union of the Mediterranean has embraced a Mediterranean Solar Plan to import large amounts of concentrating solar power into Europe from North Africa and the Middle East Sony Vaio PCG-5K2L Battery .

oltage Sourced Converters (VSC)

The development of insulated gate bipolar transistors (IGBT) and gate turn-off thyristors (GTO) has made smaller HVDC systems economical  Sony Vaio PCG-5J2L Battery .

These may be installed in existing AC grids for their role in stabilizing power flow without the additional short-circuit current that would be produced by an additional AC transmission line. The manufacturer ABB calls this concept "HVDC Light", while Siemens calls a similar concept "HVDC PLUS" (Power Link Universal System)  Sony Vaio PCG-5K1L Battery .

They have extended the use of HVDC down to blocks as small as a few tens of megawatts and lines as short as a few score kilometres of overhead line. There are several different variants of Voltage-Sourced Converter (VSC) technology: most "HVDC Light" installations use pulse width modulation but the most recent installations, along with "HVDC PLUS", are based on multilevel switching Sony Vaio PCG-6S2L Battery .

An electrode line is an electrical connection from a static inverter plant of a high voltage direct current transmission system to the earth grounding electrode (cathode or anode). The grounding electrode is for the prevention of electro-chemical corrosion and for the avoidance of electromagnetic influence Sony Vaio PCG-6S3L Battery ,

located some kilometers away from the static inverter station, so for this a separate line connection is necessary. This can be implemented depending upon the situation of the electrode (ashore or in the sea) as ground cables, as overhead line, or as a combination of ground cable and overhead line  Sony Vaio PCG-6V1L Battery .

Electrode lines are mostly used with systems that have an operating voltage of approx. 10-20kV (medium voltage range).

A transmission tower (colloquially termed an electricity pylon in the United Kingdom and parts of Europe, an ironmanin Australia Sony Vaio PCG-6W1L Battery ,

and a hydro tower in parts of Canada) is a tall structure, usually a steel lattice tower, used to support an overhead power line. They are used in high-voltage AC and DC systems, and come in a wide variety of shapes and sizes. Typical height ranges from 15 to 55 metres (49 to 180 ft),  although heights in excess of 300 metres (980 ft) exist Sony Vaio PCG-6W2L Battery .

In addition to steel, other materials may be used, including concrete and wood.

Four major functions of transmission towers are in use:  suspension towers, terminal towers, tension towers, and transposition towers. Some transmission towers combine these basic functions Sony Vaio PCG-6W3L Battery .

Transmission towers and their overhead power lines are often considered to be a form of visual pollution. Methods to reduce the visual impact include undergrounding.

"Transmission tower" is the name for the structure used in the industry in the United Kingdom, United States, and other English-speaking countries Sony Vaio PCG-7111L Battery .

The term "pylon" comes from the basic shape of the structure, an obelisk-like structure which tapers toward the top, and is mostly used in the United Kingdom and parts of Europe in everyday colloquial speech. This term is rarely, if ever, used in the United States, as the word "pylon" is commonly used for a multitude of other things, mostly for traffic cones Sony Vaio PCG-7112L Battery .

In Canada, the term "hydro tower" comes from the name of local hydroelectric power utility companies.

Three-phase electric power systems are used for high and extra-high voltage AC transmission lines (50 kV and above). The towers must be designed to carry three (or multiples of three) conductors Sony Vaio PCG-7113L Battery .

The towers are usually steel lattices or trusses (wooden structures are used in Canada, Germany, and Scandinavia in some cases) and the insulators are either glass or porcelain discs or composite insulators using silicone rubber or EPDM rubber material assembled in strings or long rods whose lengths are dependent on the line voltage and environmental conditions Sony Vaio PCG-7133L Battery .

Typically, one or two ground wires are placed on top to intercept lightning and harmlessly divert it to ground.

In some countries, towers for high and extra-high voltage are usually designed to carry two or more electric circuits. For double circuit lines in Germany, the "Danube" towers or, more rarely, the "fir tree" towers are usually used  Sony Vaio PCG-7Z2L Battery .

If a line is constructed using towers designed to carry several circuits, it is not necessary to install all the circuits at the time of construction.

Some high voltage circuits are often erected on the same tower as 110 kV lines  Sony Vaio PCG-8Y1L Battery .

Paralleling circuits of 380 kV, 220 kV and 110 kV-lines on the same towers is common. Sometimes, especially with 110 kV circuits, a parallel circuit carries traction lines for railway electrification.

High-voltage direct current (HVDC) transmission lines are either monopolar or bipolar systems  Sony Vaio PCG-8Y2L Battery .

With bipolar systems a conductor arrangement with one conductor on each side of the tower is used. For single-pole HVDC transmission with ground return, towers with only one conductor can be used. In many cases, however, the towers are designed for later conversion to a two-pole system  Sony Vaio PCG-8Z1L Battery .

In these cases, conductors are installed on both sides of the tower for mechanical reasons. Until the second pole is needed, it is either grounded, or joined in parallel with the pole in use. In the latter case the line from the converter station to the earthing (grounding) electrode is built as underground cable  Sony Vaio PCG-8Z2L Battery .

Towers used for single phase AC railway traction lines are similar in construction to those towers used for 110 kV-three phase lines. Steel tube or concrete poles are also often used for these lines. However, railway traction current systems are two-pole AC systems, so traction lines are designed for two conductors (or multiples of two, usually four, eight, or twelve) SONY VAIO VGN-FW31J Battery.

As a rule, the towers of railway traction lines carry two electric circuits, so they have four conductors. These are usually arranged on one level, whereby each circuit occupies one half of the crossarm. For four traction circuits the arrangement of the conductors is in two-levels and for six electric circuits the arrangement of the conductors is in three levels SONY VAIO VGN-FW31E Battery .

With limited space conditions, it is possible to arrange the conductors of one traction circuit in two levels. Running a traction power line parallel to a high voltage transmission line for three-phase AC on a separate crossarm of the same tower is possible SONY VAIO VGN-FW31B Battery .

If traction lines are led parallel to 380 kV-lines, the insulation must be designed for 220 kV, because in the event of a fault, dangerous overvoltages to the three-phase alternating current line can occur. Traction lines are usually equipped with one earth conductor. In Austria, on some traction circuits, two earth conductors are used SONY VAIO VGN-FW31M Battery .

Before transmission towers are even erected, prototype towers are tested at tower testing stations. There are a variety of ways they can then be assembled and erected   SONY VAIO VGN-FW31Z Battery :

  • They can be assembled horizontally on the ground and erected by push-pull cable. This method is rarely used, however, because of the large assembly area needed.
  • They can be assembled vertically (in their final upright position) SONY VAIO VGN-FW38M Battery .
  • Very tall towers, such as the Yangtze River Crossing, were assembled in this way.
  • A jin-pole crane can be used to assemble lattice towers.This is also used for utility poles.
  • Helicopters can serve as aerial cranes for their assembly in areas with limited accessibility SONY VAIO VGN-AW41JF Battery .
  • Towers can also be assembled elsewhere and flown to their place on the transmission right-of-way.

Electricity pylons often have an identification number or code placed on the pole in the form of a sign, an identification plate, painted numbers, or anything else the electric company chooses SONY VAIO VGN-AW41JF/H Battery .

These tags are usually marked with the names of the line (either the terminal points of the line or the internal designation of the power company) and the tower number. This makes identifying the location of a fault to the power company that owns the tower easier SONY VAIO VGN-AW41MF Battery .

Transmission towers, much like other steel lattice towers including broadcasting or cellphone towers, are marked with signs which discourage public access due to the danger of the high voltage. Often this is accomplished with a sign warning of the high voltage; other times the entire access point to the transmission corridor is marked with a sign SONY VAIO VGN-AW41MF/H Battery .

Some countries require that lattice steel towers be equipped with a barbed wire barrier approximately 3 metres (9.8 ft) above ground in order to deter unauthorized climbing. Such barriers can often be found on towers close to roads or other areas with easy public access, even where there is not a legal requirement SONY VAIO VGN-FW30B Battery .

In the United Kingdom, all such towers are fitted with barbed wire.

Sometimes (in particular on steel lattice towers for the highest voltage levels) transmitting plants are installed, and antennas mounted on the top above or below the overhead ground wire SONY VAIO VGN-FW50B Battery .

Usually these installations are for mobile phone services or the operating radio of the power supply firm, but occasionally also for other radio services, like directional radio . Thus transmitting antennas for low-power FM radio and television transmitters were already installed on pylons  SONY VAIO VGN-FW51B/W Battery .

On the Elbe Crossing 1tower, there is a radar facility belonging to the Hamburg water and navigation office.

For crossing broad valleys, a large distance between the conductors must be maintained to avoid short-circuits caused by conductor cables colliding during storms  SONY VAIO VGN-FW51MF Battery .

To achieve this, sometimes a separate mast or tower is used for each conductor. For crossing wide rivers and straits with flat coastlines, very tall towers must be built due to the necessity of a large height clearance for navigation. Such towers and the conductors they carry must be equipped with flight safety lamps and reflectors SONY VAIO VGN-FW51MF/H Battery .

Two well-known wide river crossings are the Elbe Crossing 1 and Elbe Crossing 2. The latter has the tallest overhead line masts in Europe, at 227 metres (745 ft) tall. In Spain, the overhead line crossing pylons in the Spanish bay of Cádiz have a particularly interesting construction SONY VAIO VGN-FW51ZF Battery .

The main crossing towers are 158 metres (518 ft) tall with one crossarm atop a frustum framework construction. The longest overhead line spans are the crossing of the Norwegian Sognefjord (4,597 metres (15,082 ft) between two masts) and the Ameralik span in Greenland (5,376 metres (17,638 ft)) SONY VAIO VGN-FW51ZF/H Battery .

In Germany, the overhead line of the EnBW AG crossing of the Eyachtal has the longest span in the country at 1,444 metres (4,738 ft).

In order to drop overhead lines into steep, deep valleys, inclined towers are occasionally used. These are utilized at the Hoover Dam, located in the United States, to descend the cliff walls of the Black Canyon of the Colorado SONY VAIO VGN-FW52JB Battery .

In Switzerland, a NOK pylon inclined around 20 degrees to the vertical is located near Sargans, St. Gallens. Highly sloping masts are used on two 380 kV pylons in Switzerland, the top 32 meters of one of them being bent by 18 degrees to the vertical SONY VAIO VGN-FW54FB Battery .

Power station chimneys are sometimes equipped with crossbars for fixing conductors of the outgoing lines. Because of possible problems with corrosion by the flue gases, such constructions are very rare.

A new type of pylon will be used in the Netherlands starting in 2010 SONY VAIO VGN-FW70DB Battery .

The pylons were designed as a minimalist structure by Dutch architects Zwarts and Jansma. The use of physical laws for the design made a reduction of the magnetic field possible. Also, the visual impact on the surrounding landscape is reduced.

Tower structures can be classified by the way in which they support the line conductors SONY VAIO VGN-FW71DB/W Battery .

Suspension structures support the conductor vertically using suspension insulators. . Strain structures resist net tension in the conductors and the conductors attach to the structure through strain insulators. Dead-end structures support the full weight of the conductor and also all the tension in it, and also use strain insulators SONY VAIO VGN-FW72JGB Battery .

Where the conductors are straight, a tangent tower is used. Angle towers are used where a line must change direction.

Structures are classified as tangent suspension, angle suspension, tangent strain, angle strain, tangent dead-end and angle dead-endSONY VAIO VGN-FW73JGB Battery .

Cross arms and conductor arrangement

Generally three conductors are required per AC 3-phase circuit, although single-phase and DC circuits are also carried on towers. Conductors may be arranged in one plane, or by use of several cross-arms may be arranged in a roughly symmetrical, triangulated pattern to balance the impedances of all three phases SONY VAIO VGN-FW74FB Battery .

If more than one circuit is required to be carried and the width of the line right-of-way does not permit multiple towers to be used, two or three circuits can be carried on the same tower using several levels of cross-arms. Often multiple circuits are the same voltage, but mixed voltages can be found on some structures SONY VAIO VGN-FW81HS Battery .

Wood

Because of the limited height of available trees the maximum height of wooden pylons is limited (approximately 30 metres). In Germany wood pylons are used as a rule only for lines with voltages up to approximately 30 kV SONY VAIO VGN-FW81NS Battery.

In the United States wood poles are used to construct H-frame or K-frame stuctures for voltages up to 345 kV; these can be less costly than steel structures and take advantage of the surge voltage insulating properties of wood  SONY VAIO VGN-FW81S Battery .

Concrete

Because of its durability and ease of manufacturing and installation, many utilities in recent years prefer th

e use of concrete and steel tube pylons over wood and lattice steel pylons for new power lines and pylon replacements  SONY VAIO VGN-FW82DS Battery .

Concrete pylons are used in Germany normally only for lines with operating voltages below 30kV. In exceptional cases concrete pylons are used also for 110kV-lines, as well as for the public grid or for the railway traction current grid  SONY VAIO VGN-FW82JS Battery .

In Switzerland, concrete pylons with heights of up to 59.5 metres (world's tallest pylon of prefabricated concrete at Littau) are used for 380kV-overhead lines. Concrete poles are also used in Canada.

Concrete pylons, which are not prefabricated, are also used for constructions taller than 60 metres SONY VAIO VGN-FW82XS Battery .

One example is a 66 metres tall pylon of a 380 kV powerline near Reutter West Power Plant in Berlin. Such pylons look like industrial chimneys and some of these structures are also used as chimneys. In China some tall pylons of powerline crossings of wide rivers were built of concrete  SONY VAIO VGN-FW83DS Battery

The tallest of these pylons belong to the Yangtze Powerline crossing at Nanjing with a height of 257 metres.

Tubular steel monopole

Poles made of tubular steel generally are assembled at the factory and placed on the right-of-way afterward SONY VAIO VGN-FW83JS Battery .

Because of its durability and ease of manufacturing and installation, many utilities in recent years prefer the use of steel tube and concrete pylons over wood and lattice steel pylons for new power lines and pylon replacements SONY VAIO VGN-FW83XS Battery .

In Germany steel tube pylons are also established predominantly for medium voltage lines, in addition, for high voltage transmission lines or two electric circuits for operating voltages by up to 110 kV. In France steel tube pylons are used frequently also for pylons by 380 kV lines in the USA also for 500 kV lines  SONY VAIO VGN-FW90NS Battery .

Lattice steel

A lattice steel tower is a steel framework construction. Lattice steel towers are used for power lines of all voltages, and are the most common type for high-voltage transmission lines.

A lattice tower is usually assembled at the location where it is to be erected  SONY VAIO VGN-FW90S Battery .

This makes very tall towers possible (up to 100 metres—in special cases even higher, as in the Elbe crossing 1 and Elbe crossing 2). Assembly of lattice steel towers can be done using a crane. Lattice steel towers are generally made of angle-profiled steel beams (L- or T-beams). For very tall towers, trusses are often used SONY VAIO VGN-FW91NS Battery .

Aluminum

Where towers must be placed in inaccessible terrain by helicopters, the extra material cost of aluminum towers will be offset by lower installation cost. Design of aluminum lattice towers is similar to that for steel, but must take into account alumunim's lower Young's modulus  SONY VAIO VGN-FW91S Battery .

Submarine power cables are major transmission cables for carrying electric power below the surface of the water. These are called "submarine" because they usually carry electric power beneathsalt water (arms of the ocean, seas, straits, etc  SONY VAIO VGN-FW92DS Battery .

but it is also possible to use submarine power cables beneath fresh water (large lakes and rivers). Examples of the latter exist that connect the mainland with large islands in the St. Lawrence River.

The majority of submarine power cables use high-voltage direct current (HVDC) electric power transmission  SONY VAIO VGN-FW92JS Battery .

This is a form of power transmission that was favored by Thomas Edison long ago, but mostly rejected in the late 19th century in favor of alternating-current (AC) transmission. This is the kind of electricity that is now used to power almost everything that is significant  SONY VAIO VGN-FW93DS Battery .

However, high-voltage alternating-current (HVAC) power lines are sometimes difficult to use, because the electromagnetic interactions between the current and the metal casing of the cable can drive up voltages to unusable peaks - damaging insulation and causing many other problems SONY VAIO VGN-FW93JS Battery .

However, there are several significant engineering advantages in using HVDC to transmit large amounts of electric power underwater.

Direct-current transmission has also undergone a modest revival over dry land, over long distances, because in this case its power losses (due to waste heat) are smaller, and it current flows are easier to control  SONY VAIO VGN-FW139E/H Battery .

A DC power-transmission system can use the earth (including seafloor)s and seawater as the return path for current. However, this method cannot always be used because of deleterious ecological effects of electrochemical reactions below ground and where the electrical conductors are connected to the ground  SONY VAIO VGN-FW145E/W Battery .

The overall length of AC submarine cables is restricted by the capacitance between their active electric conductors and their surrounding metallic shields. If the cable were to be made long enough, thereactive power produced by an AC cable would take up the entire current carrying capacity of the conductor, so no usable power would be transmitted SONY VAIO VGN-FW190EBH Battery.

Therefore, for transmission of large amounts ofelectric power via long submarine cables, DC transmission is nearly-always preferable over AC transmission, because DC cables have no such effect as mentioned above SONY VAIO VGN-FW190EDH Battery.

Electrical wiring in general refers to insulated conductors used to carry electricity, and associated devices. This article describes general aspects of electrical wiring as used to provide power in buildings and structures, commonly referred to as building wiringSONY VAIO VGN-FW21E Battery .

This article is intended to describe common features of electrical wiring that should apply worldwide.

Wiring safety codes

Wiring safety codes are intended to protect people and buildings from electrical shock and fire hazards SONY VAIO VGN-FW21J Battery .

Regulations may be established by city, county, provincial/state or national legislation, sometimes by adopting in amended form a model code produced by a technical standards-setting organization, or by a national standard electrical code.

Electrical codes arose in the 1880s with the commercial introduction of electrical power SONY VAIO VGN-FW21L Battery .

Many conflicting standards existed for the selection of wire sizes and other design rules for electrical installations.

The first electrical codes in the United States originated in New York in 1881 to regulate installations of electric lighting SONY VAIO VGN-FW21M Battery .

Since 1897 the U.S. National Fire Protection Association, a private nonprofit association formed by insurance companies, has published the National Electrical Code (NEC). States, counties or cities often include the NEC in their local building codes by reference along with local differences SONY VAIO VGN-FW21Z Battery .

The NEC is modified every three years. It is a consensus code considering suggestions from interested parties. The proposals are studied by committees of engineers, tradesmen, manufacturer representatives, fire fighters, and other invitees SONY VAIO VGN-FW25T/B Battery .

Since 1927, the Canadian Standards Association (CSA) has produced the Canadian Safety Standard for Electrical Installations, which is the basis for provincial electrical codes. The CSA also produces the Canadian Electrical Code, the 2006 edition of which references IEC 60364  SONY VAIO VGN-FW26T/B Battery

(Electrical Installations for Buildings) and states that the code addresses the fundamental principles of electrical protection in Section 131. The Canadian code reprints Chapter 13 of IEC 60364, and it is interesting to note that there are no numerical criteria listed in that chapter whereby the adequacy of any electrical installation can be assessed .SONY VAIO VGN-FW27/B Battery .

Although the U.S. and Canadian national standards deal with the same physical phenomena and broadly similar objectives, they differ occasionally in technical detail. As part of the North American Free Trade Agreement (NAFTA) program, U.S. and Canadian standards are slowly converging toward each other, in a process known as harmonization  SONY VAIO VGN-FW27/W Battery.

In European countries, an attempt has been made to harmonize national wiring standards in an IEC standard, IEC 60364 Electrical Installations for Buildings. Hence national standards follow an identical system of sections and chapters. However, this standard is not written in such language that it can readily be adapted as a national wiring code  . Neither is it designed for field use by electrical tradesmen and inspectors for testing compliance with national wiring standards. National codes, such as the NEC or CSA C22.1, exemplify the common objectives of IEC 60364, and provide rules in a form that allows for guidance of those installing and inspecting electrical systems  SONY VAIO VGN-FW27T/H Battery .

DKE - the German Commission for Electrical, Electronic and Information Technologies of DIN and VDE - is the German organisation responsible for the promulgation of electrical standards and safety specifications. DIN VDE 0100 is the German wiring regulations document harmonised with IEC 60364 SONY VAIO VGN-FW29/B Battery .

In the United Kingdom wiring installations are regulated by the Institution of Engineering and Technology Requirements for Electrical Installations: IEE Wiring Regulations, BS 7671: 2008, which are harmonised with IEC 60364. The previous edition (16th) was replaced by the current 17th Edition in January 2008  SONY VAIO VGN-FW290JRB Battery .

The 17th edition includes new sections for microgeneration and solar photovoltaic systems. The first edition was published in 1882.

AS/NZS 3000 is an Australian/New Zealand standard, commonly known as the "wiring rules," that specifies the requirements for the selection and installation of electrical equipment and the design and testing of such installations   SONY VAIO VGN-FW290JTB Battery .

The standard is a mandatory standard in both New Zealand and Australia; therefore, all electrical work covered by the standard must comply.

The international standard wire sizes are given in the IEC 60228 standard of the International Electrotechnical Commission. In North America, the American Wire Gauge is used  SONY VAIO VGN-FW290JTH Battery .

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