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The longest HVDC link in the world is currenlty the Inga-Shaba 1700 km 600 MW link connecting the Inga Dam to the Shaba copper mine, in the Democratic Republic Of Congo . ADVANTAGES OF HIGH VOLTAGE TRANSMISSION High voltage is used for transmission to reduce the energy lost in the resistance of the wires. For a given quantity of power transmitted, higher voltage reduces the transmission power loss. Power in a circuit is proportional to the current, but the power lost as heat in the wires is proportional to the square of the current. However, power is also proportional to voltage, so for a given power level, higher voltage can be traded off for lower current. Thus, the higher the voltage, the lower the power loss. Power loss can also be reduced by reducing resistance, commonly achieved by increasing the diameter of the conductor; but larger conductors are heavier and more expensive. High voltages cannot be easily used in lighting and motors, and so transmission-level voltage must be reduced to values compatible with end-use equipment. The Transformer , which only works with alternating current, is an efficient way to change voltages. The competition between the DC of Thomas Edison and the AC of Nikola Tesla and George Westinghouse was known as the War Of Currents , with AC emerging victorious. Practical manipulation of DC voltages only became possible with the development of high power electronic devices such as Mercury Arc Valve s and later semiconductor devices, such as Thyristor s, IGBT s, MOSFET s and GTO s. HISTORY OF HVDC TRANSMISSION An early method of high-voltage DC transmission was developed by the Swiss engineer Rene Thury.Donald Beaty et al, "Standard Handbook for Electrical Engineers 11th Ed.", McGraw Hill, 1978 This system used series-connected Motor-generator sets to increase voltage. 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 5000 volts on each machine, some machines having double commutators to reduce the voltage on each commutator. An early example of this system was installed in 1889 in Italy by the ''Acquedotto de Ferrari-Galliera'' company. This system transmitted 630 kW at 14 kV DC over a distance of 120 km.http://www.myinsulators.com/acw/bookref/histsyscable/ Other Thury systems operating at up to 100 kV DC operated up until 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.Shaping the Tools of Competitive Power http://www.tema.liu.se/tema-t/sirp/PDF/322_5.pdf One technique attempted for conversion of direct current from a high voltage useful for transmission to lower utilization voltages was to charge series-connected Batteries , then connect the batteries in parallel to serve distribution loads. Thomas P. Hughes, ''Networks of Power'' 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. The grid controlled and was put into service there.http://www.ieee.org/organizations/history_center/Che2004/DITTMANN.pdf Introduction of the fully-static mercury arc valve to commercial service in 1954 marked the beginning of the modern era of HVDC transmission. Mercury arc valves were common in systems designed up to 1975, but since then, HVDC systems use only Solid-state Device s. ADVANTAGES OF HVDC OVER AC TRANSMISSION The huge advantage of HVDC is the ability to transmit large amounts of power over very long distances at much lower capital costs and with much lower losses than AC. Losses are quoted as about 3% per 1000 km. This has given rise to proposals to for example generate between 10-25% of Europe’s electricity in Concentrating Solar Power Stations located in North African deserts, and feeding it to Europe via HVDC lines (see Trans-Mediterranean Renewable Energy Cooperation ). Airtricity is promoting a combined wind farm and HVDC network to link up UK and Western Europe. In a number of applications the advantages of HVDC makes it the preferred option over AC transmission. Examples include:
Long undersea cables have a high Capacitance . While this has minimal effect for DC transmission, the current required to charge and discharge the capacitance of the cable causes additional power losses when the cable is carrying AC. In addition, AC power is lost to Dielectric losses. 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. This voltage determines the insulation thickness and conductor spacing. This allows existing transmission line corridors to be used to carry more power into an area of high power consumption, which can lower costs. Increased stability of power systems Because HVDC allows power transmission between unsynchronised AC distribution systems, it can help increase system stability, by preventing Cascading Failure s from propagating from one part of a wider power transmission grid to another, whilst still allowing power to be imported or exported in the event of smaller failures. This has caused many power system operators to contemplate wider use of HVDC technology for its stability benefits alone. DISADVANTAGES 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 powerline, and the cost of the inverters may not be offset by reductions in line construction cost. In contrast to AC systems, realizing multiterminal systems is complex, as is expanding existing schemes to multiterminal systems. Controlling power flow in a multiterminal DC system requires good communication between all the terminals; power flow must be actively regulated by the control system instead of by the inherent properties of the transmission line. COSTS OF HIGH VOLTAGE DC TRANSMISSION Normally manufacturers such as Siemens and ABB do not state specific cost information of a particular project since this is a commercial matter between the manufacturer and the client. Costs vary widely depending on the specifics of the project such as: power rating, circuit length, overhead vs. underwater route, land costs, and AC network improvements required at either terminal. A detailed evaluation of DC vs AC cost may be required where there is no clear technical advantage to DC alone and only economics drives the selection. However some practitioners have given out some information that can be reasonably well relied upon:
AC NETWORK INTERCONNECTIONS AC transmission lines can only interconnect synchronized AC networks that oscillate at the same frequency and in phase. Many areas that wish to share power have unsynchronized networks. The power grids of the and Quebec . Brazil and Paraguay , which share the massive 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. 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. 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. 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 . 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. 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. RECTIFYING AND INVERTING Rectifying and inverting components Early static systems used Mercury Arc Rectifier s, which were unreliable. Nevertheless some HVDC systems using Mercury Arc Rectifier s are still in service in 2005. 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. The Insulated-gate Bipolar Transistor (IGBT) is now also used and offers simpler control and reduced valve cost. Because the voltages in HVDC systems, up to 800 kV in some cases, exceed the Breakdown Voltage s of the semiconductor devices, HVDC converters are built using large numbers of semiconductors in series. 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. 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. Rectifying and inverting systems Rectification and inversion use essentially the same machinery. Many substations are set up in such a way that they can act as both rectifiers and inverters. At the AC end a set of transformers, often three physically separate 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. The basic configuration uses six valves, connecting each of the three phases to each of the DC rails. However, with a phase change only every sixty degrees, considerable harmonics remain on the DC rails. 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. 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. CONFIGURATIONS 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. The Earth ed terminal may or may not 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 . The issues surrounding earth-return current include
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. Use of a metallic return conductor is decided based on economic, technical and environmental factors. Basslink project Modern monopolar systems for pure overhead lines carry typically 1500 MW. Siemens AG - HVDC website If underground or seacables are used the typical value is 600 MW. Most monopolar systems are designed for future bipolar expansion. If overhead power transmission lines are used, the used electricity pylons are often designed to carry two conductors and in many cases they do also. The second conductor is either unused, used as Electrode Line or permanently parallelized with the other (as in case of Baltic-Cable ). 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. However, there are a number of advantages to bipolar transmission which can make it the attractive option.
A bipolar system may also be installed with a metallic earth return conductor. Bipolar systems may carry as much as 3000 MW at voltages of +/-533 kV. Submarine cable installations initially commissioned as a monopole may be upgraded with additional cables and operated as a bipole. Back to back A back-to-back station is a plant in which both static inverters are in the same area, usually even in the same building and the length of the direct current line is only a few meters. HVDC back-to-back stations are used for
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 parallel switching of valves. 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 a Station -to-station Link , where two Inverter / Rectifier stations are connected by means of a dedicated HVDC Link . 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. 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 2000 MW Quebec - New England Transmission system opened in 1992, which is currently the largest multi-terminal HVDC system in the world. ABB HVDC Transmission Québec - New England website Tripole - current-modulating control A newly patented scheme (2004) ( Current modulation of direct current transmission lines ) is useful for converting existing AC transmission lines to HVDC. 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. 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. 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. Combined with the higher average power possible with a DC transmission line for the same line-to-ground voltage, a tripole conversion of an existing AC line could allow up to 80% more power to be transferred using the same transmission right-of-way, towers, and conductors. Some AC lines cannot be loaded to their thermal limit due to system stability, reliability, and reactive power concerns, which would not exist with an HVDC link. The system operates without earth-return current. Since a single failure of a pole converter or a conductor results in only a small loss of capacity and no earth-return current, reliability of this scheme would be high. No time would be lost in switching if a conductor broke. The valves would inherently have an emergency overload rating in bipole mode. This would possibly allow great increase in power transmission with significant effect in congested transmission systems, where consequences of a single line failure limit the allowed loading of other parallel transmission lines. While capital costs are higher than for a bipole conversion operating at the same voltage class, the extra power capability reduces incremental cost per megawatt. Depending on transmission line physical configuration, replacement of insulators may be required to achieve the highest power rating, to insure proper line-to-line clearance distances. As of 2005 no tri-pole conversions are in operation, although a transmission line in India has been converted to bipole HVDC. CORONA DISCHARGE Corona Discharge is the creation of Ion s in a Fluid (such as Air ) by the presence of a strong Electric Field . Electron s are torn from un- Ion ised air, and either the positive ions or else the electrons are attracted to the conductor, whilst 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 lead to arcing. 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. With monopolar transmission the choice of polarity of the energised 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 Particul ate condensation (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. APPLICATIONS 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. Offshore windfarms also require undersea cables, and their Turbine s are unsynchronized. In very long-distance connections between just two points, for example around the remote communities of Siberia , Canada , and the Scandinavia n North, the decreased line-costs of HVDC also makes it the usual choice. Other applications have been noted throughout this article. The development of Insulated Gate Bipolar Transistors (IGBT) and Gate Turn-off Thyristors (GTO) has made smaller HVDC systems economical. 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. One manufacturer calls this concept "HVDC Light" and a second manufacturer calls a similar concept "HVDC PLUS" (Power Link Universal System). 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. The difference lies in the concept of the Voltage-Sourced Converter (VSC) technology whereas "HVDC Light" uses Pulse Width Modulation and "HVDC PLUS" is based on multilevel switching. See List Of HVDC Projects for some notable installations. SEE ALSO
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