AC vs DC Cable Transmission for Offshore

AC Vs DC Cable Transmission for Offshore Wind farm

1.

Seminar Report-‘09

INTRODUCTION

The application of wind energy throughout the world is growing fast. The development of larger, more efficient turbines is opening up new frontiers in wind energy generation in the form of large offshore wind farms. The use of high-voltage direct current (HVDC) technology can fully realize the potential of these developments. Wind farms located located offshore offshore are planne planned d becaus becausee of higher higher averag averagee wind wind speeds speeds at sea and space limitations on-shore. Offshore wind farms will be different from their onshore counterparts for several reasons. The turbines will on average have a larger diameter and rated powers, the the farm farm will will be diffi difficu cult lt to acces accesss duri during ng peri period odss with with high high wind winds, s, erect erectio ion n and and maintenance will be more expensive, the turbine noise will probably not be an important issue, and a submarine electrical connection to shore will be required.

Figure 1: Offshore Wind farm

As the increasing demand, offshore wind power characterized by larger size of the farms, 200 – 1000 MW are planned, and increasing distances from the grid. Cable transmission is the only solution for the transmission from the farm to the shore. Often land cables are also required to reach a sufficiently strong interconnection point in the grid. Together this gives transmission distances of 50 – 100 km. The electrical system concerns

Dept. of Electrical & Electronics Engineering

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the electrical power components between the generator shaft and the grid connection and it concerns the way these components are interconnected and operated. Its function is to conver convertt mechan mechanica icall power power to electri electricc power, power, to collect collect electr electric ic power power from from indivi individua duall turbines, to transmit it to the shore and to convert to an appropriate voltage and frequency.

The system consists amongst other of generators, cables, transformers and power electronic converters. Systems are mainly characterized by the type of voltage used within the farm and for the shore connection (AC or DC) and the frequency of the electrical signals (fixed or variable). Some efficient way of configuration to collect the electric power from individual wind turbines and to transmit this power to an on-shore high-voltage power system node has been discussed here. The inventory concerns both constant and variable speed wind turbines and transmission by AC and DC cable networks.

Background

Wind power is the world’s fastest growing energy source. By 2020, 12% of the world’s demand of electricity will be produced by wind. Recent trends are a move from onshore to offshore, the up scaling of wind turbine size (to 3-5 MW), and the integration of land and marine-based networks. A major challenge is connecting a variable energy source to a distant grid demanding power stability.


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2.


WIND POWER

Wind power is the conversion of wind energy into a useful form of energy, such as electricity, using wind turbines. At the end of 2008, worldwide nameplate capacity of wind powered generators was 121.2 gigawatts (GW). In 2008, wind power produced about 1.5% of worldwide electricity usage; and is growing rapidly, having doubled in the three years between 2005 and 2008. Several countries have achieved relatively high levels of wind power power penetr penetrati ation, on, such such as 19% of statio stationar nary y electri electricity city produc productio tion n in Denmar Denmark, k, 11% in Spain and Portugal, and 7% in Germany and the Republic of Ireland in 2008. As of May 2009, eighty countries around the world are using wind power on a commercial basis. Large-scale Large-scale wind farms are connected connected to the electric power transmissio transmission n network; network; smaller facilities are used to provide electricity to isolated locations. Utility companies increasingly buy back surplus electricity produced by small domestic turbines. Wind energy as a powe powerr sour source ce is attr attrac acti tive ve as an alte altern rnat ativ ivee to foss fossil il fuel fuelss, beca becau use it is plentiful, renewable, widely distributed, clean, and produces no greenhouse gas emissions. However, the construction of wind farms is not universally welcomed due to their visual impact and other effects on the environment. Wind power is non-dispatchable, meaning that for economic operation, all of the available output must be taken when it is available. Other resources, such as hydropower, and standard load management techniques must be used to match supply with demand. The intermittency of wind seldom creates problems when using wind power to supply a low proportion of total demand. Where wind is to be used for a mode moderat ratee fract fractio ion n of dema demand nd such such as 40%, 40%, addi additi tion onal al cost costss for for comp compen ensa sati tion on of intermittency are considered to be modest.

2.1

Effect on power grid

The intermittency of wind power and other renewable power sources creates issues in power grids, which expect some supplied power to have a certain degree of constancy and reliability to satisfy baseline demand while other supplied power must respond to variations in demand.


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One proposed solution in Europe is to create a super grid of interconnected wind farms. This large-scale array of dispersed wind farms would be located in different wind regimes, reducing the overall variation in power output.

2.2

Development

To develop a wind farm, a suitable location is first identified. Good locations for wind farms should have fast steady steady winds winds and be near transmission transmission lines. Land parcels on which wind turbines will be located then must be leased from the land owners. The wind resource resource must then be evaluated evaluated using data recorded recorded by onsite onsite meteorolog meteorological ical towers. The wind farm project must then be financed and constructed.

2.3

Types

Depending Depending on the location location of the turbines turbines installed, installed, the wind farms can be classified as Onshore, Nearshore, Airborne, and Offshore. Among these, onshore windfarms became very popular because of its comparative low initial investment and pollution free energy. Thus such windfarms were constructed in site where continuous unidirectional flow of wind is available. But the increasing increasing demand demand and non availability of suitable lands, the world is now going for offshore wind farms.

2.3.1

Onshore

Onshore turbine installations in hilly or mountainous regions tend to be on ridgelines generally three kilometers or more inland from the nearest shoreline. This is done to exploit the so-called topographic acceleration as the wind accelerates over a ridge. The additional wind speeds gained in this way make a significant difference to the amount of energy that is produced. Great attention must be paid to the exact positions of the turbines (a process


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known as micro-siting) because a difference of 30 m can sometimes mean a doubling in output.

Figure 2: Onshore Windfarm

2.3. 2.3.2 2

Near Nearsh shor oree

Figure 3: Nearshore Windfarm


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Nearshore turbine installations are on land within three kilometers of a shoreline or on water within ten kilometers of land. These areas are good sites for turbine installation, because of wind produced by convection due to differential heating of land and sea each day. Wind speeds in these zones share the characteristics of both onshore and offshore wind, depending on the prevailing wind direction.

2.3. 2.3.3 3

Airb Airbor orne ne

Airborne wind turbines would eliminate the cost of towers and might also be flown in high speed winds at high altitude. No such systems are in commercial operation.

2.3. 2.3.4 4

Offs Of fsho hore re

Figure 4: Offshore wind turbines

Offshore wind development zones are generally considered to be ten kilometers or more from land. Offshore wind turbines are less obtrusive than turbines on land, as their apparent size and noise is mitigated by distance. Because water has less surface roughness than land (especially deeper water), the average wind speed is usually considerably higher over open water. Capacity factors (utilisation rates) are considerably higher than for onshore and nearshore locations. In stormy areas with extended shallow continental shelves, turbines are practical to install


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3.


OFFSHORE WINDFARMS

Offshore wind farms are likely to be larger than those on shore as the economies of scale in offshore projects are more significant. Compared to onshore wind power, offshore wind power is more complex and costly to install and maintain but also has several key advantages. Winds are typically stronger and more stable at sea, resulting in significantly higher production per unit installed. Wind turbines can also be bigger than on land because it is easier to transport very large turbine components by sea. Offshore wind development zones zones are genera generally lly consid considere ered d to be ten kilometer kilometerss or more more from from land. land. Offsho Offshore re wind wind turb turbin ines es are less less obtr obtrus usiv ivee than than turb turbin ines es on land land,, as thei theirr appa apparen rentt size size and and nois noisee is mitiga mitigated ted by distan distance. ce. Because Because water water has less less surface surface roughn roughness ess than than land land (espec (especial ially ly deeper water), the average wind speed is usually considerably higher over open water. Capacity Capacity factors (utilisation (utilisation rates) are considerab considerably ly higher higher than for onshore onshore and nearshore nearshore locations. In stormy areas with extended shallow continental shelves, turbines are practical to install.

Offs Offsho hore re inst instal allat latio ion n is more more expe expens nsiv ivee than than onsh onshor oree but but this this depe depend ndss on the the attributes of the site. Offshore towers are generally taller than onshore towers once the submerged height is included. Offshore foundations may be more expensive to build. Power transmission from offshore turbines is through undersea cable, often using high voltage dire direct ct curre current nt opera operati tion on if sign signifi ifica cant nt dist distan ance ce is to be cove covere red. d. Offs Offsho hore re salt saltwa wate ter r environments also raise maintenance costs by corroding the towers

Offshore wind turbines will probably continue to be the largest turbines in operation, since the high fixed costs of the installation are spread over more energy production, reduci reducing ng the averag averagee cost. cost. Turbin Turbinee compon component entss (rotor (rotor blades blades,, tower tower sectio sections) ns) can be transported by barge, making large parts easier to transport offshore than on land, where turn turn clearan clearances ces and under underpas passs clearan clearances ces of availa available ble roads roads limit limit the size size of turbin turbinee components that can be moved by truck. Similarly, large construction cranes are difficult to move to remote wind farms on land, but crane vessels easily move over water. Offshore


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wind farms tend to be quite large, often involving over 100 turbines. These wind farms are likely to be located some distance from the shore. 3.1

Indian Scenario

At the end of September 2007, India had 7660 MW of wind generating capacity and is the fourth largest market in the world. Indian Wind Energy Association has estimated that with the current level of technology, the ‘on-shore’ potential for utilization of wind energy for electricity generation is of the order of 65,000 MW. There are about a dozen wind pumps pumps of variou variouss design designss provid providing ing water water for agricu agricultu lture, re, affores afforestati tation, on, and domest domestic ic purposes, all scattered over the country. The wind farms are predominantly present in the states states of Tamil Tamil Nadu, Nadu, Maharas Maharashtr htra, a, Karnat Karnataka aka and Gujarat Gujarat..

Other Other states states like like Andhra Andhra

Pradesh, Rajasthan, Kerala and Madhya Pradesh have a very good potential.

3.2

Transmission Issues for Offshore Wind Farms

Electric energy generated by offshore wind generating facilities requires one or more submarine cables to transmit the power generated to the onshore utility grid that services the end-users of this renewable energy source. Because the power from the wind turbines is generated as an alternating current (AC) and the on-shore transmission grid is AC, the most straightforward technical approach is to use an AC cable system connection to facilitate this interconnection. Present state-of-the-art and the most cost effective AC technology for this type type of interco interconne nnectio ction n is solid solid dielec dielectric tric (also (also called called extrud extruded ed dielec dielectric tric or polyme polymeric ric insulated) cable, usually with cross- linked polyethylene (XLPE) insulation. This is the cable system technology presently used for all offshore wind farms constructed to date (all of which are located in Europe) primarily as a result of: ease of interconnection, installation, and maintena maintenance; nce; operatio operational nal reliab reliabilit ility; y; and cost effecti effectiven veness ess..

For relatively relatively small

generating capacity wind farms it has been sufficient to bring the power to shore at the same voltage used to interconnect the wind turbine generators (WTG), typically 33 kilovolt (kV). As the energy generating capacity of the wind farm increases, however, use of submarine cables in this voltage class for the connection to shore would require a prohibitively large


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number of cables and would lead to high line losses and excessive voltage drops combined with unnecessary sea-bed disturbance to accommodate installation of many cables. One solution is to step up the wind farm transmission voltage from the WTG production and collection voltage of 33 kV to a higher AC voltage suitable for transmission to shore. This requires an offshore substation platform containing step-up transformers. The first wind farm large enough to require this approach is the 160 MW Horns Rev Wind Farm commissioned for operation in December 2002 in Denmark.

Corona is another factor. Corona discharge is the creation of ions in a fluid (such as air) by the presence of a strong electric field. 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 radiofrequency interference, generate toxic compounds such as oxides of nitrogen and ozone, and bring forth 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 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. (Particles of diffe differe rent nt pola polarit rities ies have have a diff differe erent nt mean mean-fr -free ee path path.) .) Nega Negati tive ve coro corona nass gene genera rate te 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.

Earthing, Earthing, particularly particularly for lightning protection, protection, will need to be addressed as offshore offshore structures structures may be more exposed to positive positive polarity lightning strokes. Positive Positive downward downward lightning has higher peak currents and charge transfer, and is likely to be more destructive than the more common negative downward strike. Coupled with the difficulties of offshore access, this may lead to a much higher economic benefit of improved lightning protection.


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Also for directly connected wind farms with 33 kV collection circuits, some form of reactive power compensation/voltage control may be required. It will, of course, be cheaper to locate this on land.

4.

HIGH HIGH VOLT VOLTAG AGE E DIRE DIRECT CT CURR CURREN ENT T (H (HVD VDC C)

A high-voltage, direct current (HVDC) electric power transmission system uses direct curr curren entt for for the the bulk bulk tran transm smis issi sion on of elec electr tric ical al powe power, r, in cont contra rast st with with the the more more common alternating current systems. For long-distance distribution, HVDC systems are less expensive and suffer lower electrical losses. 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. Early commercial installations included one in the Soviet Union in 1951 between Moscow and Kashira, and a 10-20 MW system between Gotland and mainland Sweden Sweden in 1954. 1954. The The longest longest HVDC link in the world is currently currently the IngaShaba 1,700 km, 600 MW link connecting the Inga Dam to the Shaba copper mine, in the Democratic Republic of Congo.

4.1 4.1

High High volt voltag agee tran transm smis issi sion on

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, 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,


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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 Westinghou Westinghouse se was known as the War War of Currents, Currents, with AC emerging emerging victorious victorious.. Practical Practical manipulation of DC voltages only became possible with the development of high power electro electronic nic device devicess such such as mercur mercury y arc valves valves and later later semico semicondu nducto ctorr devices devices,, such such as thyristors, insulated-gate

bipolar

transistors (IGBTs),

high

power

capable capable MOSFETs MOSFETs (power (power metal–oxide metal–oxide–semic –semicondu onductor ctor field-effect field-effect transistors transistors)) and gate turn-off thyristors (GTOs).

4.2 4.2

Hist Histor ory y of of H HVD VDC C tra trans nsmi miss ssio ion n

Figure 5: 150 KV mercury arc valve converter for transmitting AC hydropower voltage to long distance.

The first long-distance transmission of electric power was demonstrated using direct curren currentt in 1882 1882 at the Miesba Miesbachch-Mun Munich ich Power Power Transm Transmiss ission ion,, but only only 2.5 kW was transmitted. An early method of high-voltage DC transmission was developed by the Swiss engi engine neer er Rene Rene Thur Thury y and and his his meth method od was was put put into into pract practic icee by 1889 1889 in Ital Italy y by the the Company. y. This This syste system m used used series series-co -conne nnected cted motormotor Acquedo Acquedotto tto de FerrariFerrari-Gall Galliera iera Compan genera generator tor sets sets to increas increasee voltag voltage. e. Each Each set was insula insulated ted from from ground ground and driven 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


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the voltage on each commutator. This system transmitted 630 kW at 14 kV DC over a distan distance ce of 120 km The The Moutier Moutiers-L s-Lyon yon system system transmi transmitted tted 8,600 8,600 kW of hydroe hydroelec lectric tric power a distance of 124 miles, including 6 miles of underground cable. The system used eight series-connected generators with dual commutators c ommutators for a total voltage of 150,000 volts between the poles, and ran from about 1906 until 1936. 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. One conversion technique attempted for conversion of direct current from a high transmission voltage to lower utilization voltage was to charge seriesconnected batteries, then connect the batteries in parallel to serve distribution loads. 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 grid contro controlle lled d mercur mercury y arc valve valve became became availa available ble for power power transmi transmissi ssion on 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. In 1941, a 60 MW, +/-200 kV, 115 km buried buried cable cable link link was design designed ed for the city city of Berlin Berlin using using mercur mercury y arc valves valves (Elbe(ElbeProject), 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. The equipment was moved to the Soviet Union and was put into service there.

Introd Introduct uction ion of the fully-s fully-stati taticc mercur mercury y arc valve valve to commer commercial cial servic servicee in 1954 1954 marked the beginning of the modern era of HVDC transmission. 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 only soli solidd-st stat atee devi device ces. s. From From 1975 1975 to 2000 2000,, line line-co -comm mmut utate ated d conv conver erter terss (LCC (LCC))


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using thyristor valves were relied on. According to experts such as Vijay Sood, the next 25 years may well be dominated by force commutated converters, beginning with capacitor commut commutativ ativee conver converters ters (CCC) (CCC) follow followed ed by self self commut commutatin ating g conver converters ters which which have have largely supplanted LCC use. Since use of semiconductor commutators, hundreds hundreds of HVDC sea-cables have been laid and worked with high reliability, usually better than 96% of the time.

4.3 4.3

Rect Rectif ifyi ying ng and and inv inver erti ting ng syst system emss

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 each of the the two two DC rail rails. s. Howe Howeve ver, r, with with a phas phasee chan change ge only only every every sixt sixty y degr degrees ees,, 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 seconda secondary, ry, establ establish ishing ing a thirty thirty degree degree phase phase differe difference nce betwee between n the two sets sets of three 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.


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5.


HVDC VDC TR TRANSM ANSMIS ISSI SIO ON BA BASE SED D ON ON VSC VSC’S ’S

Voltage Source Converters (VSC) have for the first time been used for HVDC transmission in a real network. Experience from the design and commissioning of the transmission shows that the technology has now reached the stage where it is possible to build build high high voltag voltagee conver converters ters utilizi utilizing ng Insula Insulated ted Gate Gate Bipola Bipolarr Transi Transisto stors rs (IGBTs (IGBTs). ). Operation and system tests have proved that the properties that have been discussed for many years regarding VSCs for HVDC are a reality now. They include independent control of active and reactive power, operation against isolated ac. networks with no generation of their own, very limited need of filters and no need of transformers for the conversion process. This is only the first installation of VSC for HVDC.

The development of semiconductors and control equipment is presently very rapid and it is evident that this technology technology will play an important role in the future expansion expansion of electric transmission and distribution systems.

VSC based HVDC Transmission Layout.

5.1 5.1 Conv Conver erte terr Te Tech chno nolo logi gies es 5.1. 5.1.1 1 HVDC HVDC Clas Classi sicc


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Using HVDC to interconnect two points in a power grid, in many cases is the best economic economic alternative, alternative, and furthermore furthermore it has excellent environmental environmental benefits. benefits. The HVDC technology (High Voltage Direct Current) is used to transmit electricity over long distances by overhead transmission lines or submarine cables. It is also used to interconnect separate power systems, where traditional alternating current (AC) connections can not be used. ABB pioneered the HVDC technology and is the undisputed world leader in the HVDC field.

In a high voltage direct current (HVDC) system, electric power is taken from one point in a thre three-p e-pha hase se AC netw networ ork, k, conv convert erted ed to DC in a conv convert erter er stat statio ion, n, tran transm smit itte ted d to the the receiving point by an overhead line or cable and then converted back to AC in another conv convert erter er stat statio ion n and and injec injecte ted d into into the the rece receiv ivin ing g AC netw networ ork. k. Typi Typica cally lly,, an HVDC HVDC transmission has a rated power of more than 100 MW and many are in the 1,000 - 3,000 MW range. HVDC transmissions are used for transmission of power over long or very long dist distan ances ces,, beca becaus usee it then then beco become mess econ econom omic icall ally y attra attracti ctive ve over over conv conven enti tion onal al AC lines. With an HVDC system, the power flow can be controlled rapidly and accurately as to both the power level and the direction. This possibility is often used in order to improve the performance and efficiency of the connected AC networks.

5.1.2 HVDC Light

The Pulse Width Modulated Voltage Source Converter a close to ideal component in the transmission network. From a system point of view it acts as a motor or generator without mass that can control active and reactive power almost instantaneously. Conven Conventio tional nal HVDC HVDC conver converter ter techno technolog logy y is based based on the use use of line-co line-commu mmutat tated ed or phase-commutated converters (PCC). With the appearance of high switching frequency components, such as IGBTs (Insulated Gate Bipolar Transistor) it becomes advantageous to bui build ld VSC VSC (Vol (Volta tage ge Sour Source ce Conv Conver erte ters rs)) usin using g PW PWM M (Pul (Pulse se Widt Width h Modu Modula lati tion on)) Technology.


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The key part of the HVDC Light converter consists of an IGBT valve bridge. No special converter transformers are necessary between the valve bridge and the AC-grid. Aconverter reactor can separate the fundamental frequency from the raw PWM waveform. If the desired DC voltage does not match the AC system voltage, a normal AC transformer may be used in addition to the reactor. A small shunt AC-filter is placed on the AC-side of the reactor. On the DC-side there is a DC capacitor that serves as a DC filter too.

5.1 5.1.2.1 .2.1

Puls ulse wi width dth mo modu dula lattion ion te techn hno olog logy fo for HV HVDC Lig Light

This is an entirely different concept compared with the classical HVDC converter. In the PWM bridge switching very fast between two fixed voltages creates the AC-voltage. The desired fundamental frequency voltage is formed through low pass filtering of the high frequency pulse modulated voltage.


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VSC three phase converter

The PWM pattern and the corresponding power frequency voltage of a VSC converter

With PWM it is possible to create any phase angle or amplitude (up to a certain limit) by changing the PWM pattern, which can be done almost instantaneous. Hereby PWM offers the

possibility

to

control

both

acti ctive

and

rea reactive

power

independently.

This This make makess the the Puls Pulsee Widt Width h Modu Modulat lated ed Volt Voltag agee Sour Source ce Conv Conver erter ter a clos closee to ideal ideal component in the transmission network. From a system point of view it acts as a motor or generator generator without without mass that can control control active and reactive reactive power almost instantaneou instantaneously. sly.


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Furthermore, it does not contribute to the short circuit power as the AC current can be controlled.

5.1.2.2

Active an and re reactive po power co control

The fundamental frequency voltage across the converter reactor defines the power flow between the AC and DC sides. Changing the phase angle between the fundamental frequency frequency voltage generated generated by the converter converter (Ug) and the voltage voltage on the AC bus controls controls the active power flow between the converter and the network. The reactive power flow is determined by the amplitude of Ug, which is controlled by the width of the pulses from the converter bridge. The control is performed by the MACH2 system developed by ABB. All functions for control, supervision and protection of the stations are implemented in software running in a family of microprocessor circuit boards.

5.1.2.3

Station Design

The majority of equipment in a HVDC Light is delivered in enclosures and tested at factory before shipment. For example the IGBT valves, the control equipment, the valve cooling equipment and the station service are all delivered in enclosures. This simplifies the civil works and also makes the installation installation and commissioning commissioning faster than for a traditional traditional converter. The HVDC Light concept lends itself to a modular standardized design with a high degree of factory testing.


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5.1. 5.1.3 3 HVDC HVDC-P -Plu luss

Siemens Power Transmission and Distribution (PTD) launched a high-voltage direct curre current nt tran transm smis issi sion on (HVD (HVDC) C) syst system em on the the mark market et,, base based d on a new new gene generat ratio ion n of converters converters using using voltage-sour voltage-sourced ced converter converter (VSC) technology. technology. The HVDC Plus system is suitable for direct current links up to the 1,000 MW power range where line-commutated conver converter terss are still still used used exclus exclusive ively ly today. today. In contras contrastt to line-co line-commu mmutat tated ed conver converter ter technology, the HVDC-Plus system operates with power semiconductors which have both turn-on and turn-off capability. As a result, the commutation processes in the converter are indepe independe ndent nt from from the AC system system voltag voltage. e. Next Next to other other applic applicatio ations ns the transmi transmissi ssion on system allows the low-loss transport of electrical energy from offshore wind farms to the coast and the economical and environmentally-friendly supply of power to oil platforms from the AC system on the mainland.

The high-voltage direct-current transmission system HVDC Plus makes use of all the advant advantage agess offere offered d by self-co self-commu mmutate tated d voltag voltage-so e-sourc urced ed conver converter ter techno technolog logy. y. This This includes grid access to very weak AC systems as well as supplying passive networks. Active and reactive power can be controlled independently. The capability of very rapid control and protection actions of the converter makes the system highly dynamic, which is


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necessary especially for AC faults and system disturbances. Last but not least, the blackstart capability function enables the HVDC system to restart a collapsed network.

HVDC Plus operates with an innovative multilevel converter concept, which offers in comparison to existing VSC solutions additional significant benefits. Amongst others these are low losses losses due to low switchin switching g frequen frequencie cies, s, full full modula modularr design design and therew therewith ith a straightforward scalability. In addition to the operation as back-to-back link and as cable trans transmi miss ssio ion, n, HVDC HVDC Plus Plus can can also also be used used in comb combin inati ation on with with over overhe head ad line lines. s. Offshore wind farms in the power range of a few hundred megawatts usually demand for parti particula cularly rly high high requir requireme ements nts of power power transmi transmissi ssion. on. Many Many wind wind farms farms are locate located d offsho offshore re over over a hundre hundred d kilome kilometer terss from from the AC syste system m on the coast. coast. This general generally ly exceeds the economical and technical limits of AC-based cable transmission systems and calls for new DC transmission concepts, for example based on the HVDC Plus system. Oil platforms, which have a high power demand, also require a high level of power quality for the transmission if they are to be supplied from the mainland and not locally as in the past. Power delivery from the mainland not only increases the availability of the electric supply on the drilling rigs but also renders the maintenance and servicing work unnecessary for for the the smal smalll powe powerr plan plants ts curr curren entl tly y used used on the the plat platfo form rms. s. This This also also elim elimin inat ates es environmentally harmful CO2 and NOX emissions from the small power plants usually used at sea.

Submar Submarine ine cables cables are used used exclus exclusive ively ly for power power transmi transmissi ssion on across across the sea. sea. However, However, the transport transport of power in the form of alternating alternating current via cable is limited to a length of about 80 to 120 kilometers for technical and economical reasons, depending on the power to be transmitted. For this reason, direct-current transmission is the preferred solution.

5.2 5.2 Conf nfiiguratio tions

5.2.1 Monopole Monopole and earth earth re return turn


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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 earthed 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



Electrochemical corrosion of long buried metal objects such as pipelines



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.

These These effects effects can be eliminat eliminated ed with with instal installati lation on of a metalli metallicc return return conduc conductor tor betwe between en the two ends of the monopola monopolarr transmi transmissi ssion on line. line. Since Since one terminal terminal of the converters is connected to earth, the return conductor need not be insulated for the full transmissio transmission n voltage voltage which makes it less costly than the high-voltage high-voltage conductor. conductor. Use of a metalli metallicc return return conduc conductor tor is decide decided d based based on econom economic, ic, techni technical cal and enviro environme nmenta ntall 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.

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 or used as electrode line or connected in parallel with the other (as in case of Baltic-Cable).

5.2. 5.2.2 2 Bipo Bipola larr

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


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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.

Figure 6: Bipolar system pylons of the HVDC Cable



Under Under normal normal load, load, neglig negligible ible earth-c earth-curr urrent ent flows, flows, as in the case case of monopo monopolar lar

transmission with a metallic earth-return. This reduces earth return loss and environmental effects. 

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. 

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. 

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.

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.


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A block block diagra diagram m of a bipola bipolarr HVDC HVDC transmi transmissi ssion on system system,, betwee between n two statio stations ns designated A and B. AC - represents an alternating current network CON - represents a converter valve, either rectifier or inverter, TR represents a power transformer, DCTL is the directdirect-cur curren rentt transm transmiss ission ion line line conduc conductor tor,, DCL is a direct direct-cur -curren rentt filter filter induct inductor, or, BP represents a bypass switch, and PM represent power factor correction and harmonic filter networks required at both ends of the link. The DC transmission line may be very short in a back-to-back link, or extend hundreds of miles (km) overhead, underground or underwater. One conductor of the DC line may be replaced by connections to earth ground.

Figure 7: Bipolar HVDC transmission system

5.2. 5.2.3 3 Back Back to to back 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. HVDC back-to-back stations are used for coupling of electricity mains of different frequency and phase number and two network of the same nominal frequency but no fixed phase relationship. The DC voltage in the intermediate circuit can be selected freely at HVDC back-to-back stations because of the short conductor


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length. The DC voltage is as low as possible, in order to build a small valve hall and to avoid series connections of valves. For this reason at HVDC back-to-back stations valves with the highest available current rating are used.

5.2.4 Systems Systems with with trans transmissi mission on lines lines

The most common configuration of an HVDC link is two inverter/rectifier stations connec connected ted by an overhe overhead ad power power line. line. This This is also also a config configura uratio tion n common commonly ly used used in connecting unsynchronised grids, in long-haul power transmission, and in undersea cables. Multi-termina Multi-terminall HVDC links, connecting connecting more than two points, are rare. The configuratio configuration n 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

Transmission system opened in 1992, which is currently the largest multi-terminal HVDC system in the world.

5.2.5 Tripole: Tripole: current current-mod -modulati ulating ng control control

A newl newly y pate patent nted ed sche scheme me (As (As of 2004 2004)) (Cur (Curre rent nt modu modula lati tion on of direc directt curre current nt transmission lines) is intended for conversion of 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 thermal limit, with the parallel monopole monopole always carrying +/- 1 times times its therma thermall limit limit curren current. t. The combin combined ed RMS heatin heating g effect effect is as if each 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 transmissio transmission. n. High currents currents can be circulated circulated through the line conductors conductors even when load demand is low, for removal of ice.

Combined with the higher average power possible with a DC transmission line for the same line-to-groun line-to-ground d voltage, a tripole tripole conversion of an existing existing AC line could allow up to


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80% more power to be transferred using the same transmission right-of-way, towers, and conductors conductors.. Some AC lines cannot be loaded loaded to their thermal thermal limit due to system system stability, reliability, and reactive power concerns, which would not exist with an HVDC link.

The system would operate 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, with no time required for switching. As of 2008, no tri-pole conversions are in operation, although a transmission line in India has been converted to bipole HVDC.

5.3 5.3 Poss Possib ible le Conn Connec ecti tion onss

Three options for connecting an offshore wind farm have been examined to establish the electrical characteristics, feasibility and costs.

5.3.1 5.3.1 Multi Multiple ple 33 kV kV links links

This appears to be the cheapest option for distances offshore up to 20 km and power levels up to 200 MW. Outside these ranges the cable laying costs and electrical losses are the limiting factors. Also it must be recognized that 200 MW is a large injection of power for any distribution circuit to accept and so there may well be limitations imposed by the utility for large wind farms. This option has no wind farm transformers offshore, only the individual turbine transformers. The wind farm is divided into blocks. Each block is fed by its own, 3 core cable from shore. The maximum practical conductor size for operation at 33 kV appears to be 300 mm2, giving a block rating in the range 25 to 30 MW. If necessary, two blocks could be connected by one cable, while the faulty cable is repaired.

5.3.2 Single Single 132 kV kV link and and transfo transformer rmer

This is the simplest system with a higher transmission voltage for the link to the shore. From discussion with cable manufacturers, 132 kV is their preferred voltage rather than 66 kV. However, an offshore substation is r equired, on a separate platform. Also, if the link fails, the whole wind farm is disconnected. This option is expensive for all distances


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out to 30 km and power levels less than 200 MW. Putting a substation offshore for this type of link is novel and the first few would be expensive and require careful monitoring. Thus, it is unlikely to be used for the first offshore wind farms around the UK.

5.4 HVDC link

Advances in power electronic technology have led to the development of HVDC systems at lower ratings than were previously cost effective. For instance, one manufacturer has a system in the range 2 to 200 MW based on IGBT voltage source converters. Typically a 110 MW link including cable, might cost £13M. One advantage of voltage source forced commut commutated ated conver converter ters, s, over over traditi traditiona onall HVDC HVDC curren currentt source source conver converter terss based based on thyristors, is that synchronous rotating machines are not required at each end of the link. The AC connection voltage at the ends does not have to be the same, possibly saving a site transformer at the shore. To date only a demonstration system has been installed and there has yet to be an offshore installation. The technology is in its infancy and further advances are likely. In the longer term, there is the possibility that offshore turbines will be connected at DC offshore, making HVDC links more attractive. The HVDC link uses the technology available at present on the assumption that a marinised version is available. Again a platform is required to house the offshore converter and switchgear, and the whole wind farm is lost if the cable fails. This option is too expensive for distances closer to shore than 25 km and for power levels less than 200 MW. There is also a risk, which will decrease with time, associated with applying this new technology offshore.

6.

6.1 6.1

HVDC CA CABLE TR TRANSMISSION

Adva Advant ntag ages es of of HVDC HVDC ove overr AC tra trans nsmi miss ssio ion n

The advantage advantage of HVDC is the ability ability to transmit transmit large amounts 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. Long Long unders undersea ea cables cables have have a high high capaci capacitanc tance. e. While While this this has minimal minimal effect effect for DC


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transmission, the current required to charge and discharge the capacitance of the cable causes additional I 2 R 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. In AC power, the root root mean square (RMS) (RMS) voltage measuremen measurementt is considered considered the standard, standard, but RMS is only only abou aboutt 71% 71% of the the peak peak volt voltag age. e. The The peak peak volt voltag agee of AC dete determ rmin ines es the the actu actual al insulation thickness and conductor spacing. Because DC operates at a constant maximum voltage without RMS, this allows existing transmission line corridors with equally sized cond conduc ucto tors rs and and insu insula lati tion on to carry carry 100% 100% more more powe powerr into into an area area of high high powe power r consumption than AC, which can lower costs.

Because HVDC allows power transmission between unsynchronised AC distribution system systems, s, it can help help increas increasee system system stabil stability ity,, by preven preventin ting g cascad cascading ing failure failuress from from propagatin propagating g from one part of a wider power transmission transmission grid to another. another. 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, 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.

6.2

Disadvantages

The disadv disadvant antage agess of HVDC HVDC are in conver conversio sion, n, switch switching ing and contro control. l. Furthe Further r operat operating ing an HVDC HVDC scheme scheme requir requires es keepin keeping g many many spare spare parts, parts, which may be used used exclusively in one system as HVDC systems are less standardized than AC systems and the used technology changes fast.


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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 trans transmi miss ssio ion n line line.. The The cost cost of the the inve invert rters ers may may not not be offs offset et by redu reduct ctio ions ns in line line construction cost and lower line loss. With two exceptions, all former mercury rectifiers worldwide have been dismantled or replaced by thyristor 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.

In cont contras rastt to AC syst system ems, s, reali realizi zing ng mult multit iterm ermin inal al syst system emss is comp comple lex, x, as is expand expanding ing existi existing ng scheme schemess to multit multitermi erminal nal system systems. s. Contro Controllin lling g power power flow 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. 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. 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 Corsica. For cable links longer than 40-50 km, DC provides lower Investment costs. The saving gained from installing only one DC cable instead of three AC cables more than compensates for the cost of the AC/DC converter stations.

• DC cable cable transm transmiss ission ionss have have lower lower losses losses than than a corresp correspond onding ing AC cable cable link. link. The converter station losses are normally as low as 0.6% per station, and DC cable losses are only around 0.3-0.4% per 100 km. • Long AC cables produce high amounts of reactive power requiring shunt reactors at both ends ends.. In extr extrem emee cases cases the the react reactiv ivee curr curren entt may may seri seriou ousl sly y reduc reducee the the acti active ve power power transmission capability. These drawbacks do not arise in a DC cable. • DC links can connect two asynchronous power grids in cases where it is impossible or impracticable to establish a synchronous interconnection.


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• The high controllability of the DC system can be exploited to improve the operating conditions of the interconnected grids.

6.3 AC Cable Cable tran transmi smissi ssion on Vs. Vs. DC DC Cable Cable transm transmiss ission ion

AC Cable

DC Cable

High Higher er cabl cablee cos costs ts:: mor moree cab cable less nee neede ded d

Lowe Lowerr cab cable le cost costs: s: fewe fewerr cab cable less nee neede ded d

Lower power power transfer transfer capabilities capabilities (per cable) cable) Higher Higher power transfer transfer capabilities capabilities (per cable) cable) Capacity reduction due to charging currents

No additional costs of the converter stations

No capacity reduction by charging currents Additional costs involved due to the requirement of converter stations at either end of the transmission line

7.

APPLICATIONS

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

Unde Unders rsea ea cables cables,, where where high high capa capaci cita tance nce caus causes es addi additi tion onal al AC loss losses es.. (e.g. (e.g.,,

250 km Baltic Cable between Sweden and Germany) 

Endpoint-to-endpoint long-haul bulk power transmission without intermediate 'taps',

for example, in remote areas


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Increasing the capacity of an existing power grid in situations where additional wires



are difficult or expensive to install Power Power transmi transmissi ssion on and stabil stabilizat ization ion betwee between n unsync unsynchro hronis nised ed AC distri distribut bution ion



systems Connecting a remote generating plant to the distribution grid, for example Nelson



River Bipole Stabilizing Stabilizing a predominan predominantly tly AC power-grid, power-grid, without without increasing increasing prospectiv prospectivee short short



circuit current 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 Synchronize AC produced by renewable energy sources.



8.

COST AND ECONOMICS FOR HVDC CABLE TRANSMISSION

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 However some practitioners practitioners have given out some informatio information n that can be reasonably well relied upon:


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For For an 8 GW 40 km link link laid laid unde underr the the Engl Englis ish h Chan Channe nel, l, the the foll follow owin ing g are are approximate primary equipment costs for a 2000 MW 500 kV bipolar conventional HVDC link 

Converter stations ~£110M



Subsea cable + installation ~£1M/km While choosing the technology used for conversion of AC and DC based on VSC’s,

the the HVDC HVDC Ligh Lightt is an attr attract activ ivee solu soluti tion on for for offs offsho hore re wind wind powe powerr when when HVDC HVDC is considered for offshore wind power, the most attractive technology is the voltage source technology in ABB version named HVDC Light. The evidence is in the table describing the differences between the technologies.

Figure 8: Development of HVDC Light ratings and l osses

The new converter design gives the following benefits:



Well-proven semiconductor technology with large number of components in

service with identical voltage. 

Simple and robust converter design.



Good dynamic properties.



Low losses



Reduced costs


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A recent study performed by Econnect, regarding the technological options related to the UK offshore projects also use voltage source converters as the HVDC alternative. This study concludes that for the UK projects in a number of projects HVDC would be feasible par parti ticu cular larly ly if “joi “joint nt conn connect ectio ion” n” is appl applie ied. d. “Joi “Joint nt conn connec ecti tion on”” is to comb combin inee the the transmission for a number of wind parks. The total interconnection cost for the studied projects are in the range of 250 kEuro/MW to 370 kEuro/MW with an average cost of 270 kEuro/MW. The joint connection alternative shoved an average of 240 kEuro/MW. The studied range in power and transmission distance is 64 to 1000 MW and 30 – 100 km. The study suggests that HVDC (Voltage source converters) should be considered particularly in the joint connection alternatives that also gives the lowest overall costs. This study does not cover potential transmission reinforcement costs or the cost for power flow equipment. It is clear clear that that the competiti competitiven veness ess of the HVDC altern alternativ ativee increas increases es with with the size size of the projects but also with the transmission distance. The following screening questions based on the above chart are important.



Need for power transmission 200 - 1000 MW



Need for accurate and fast control



Distance more than 50 km



Difficult to obtain permits for OH-lines



Difficult to find/reach interconnection point in the grid



Difficult to build a substation near the coastline (for reactive power compensation)



Weak AC network



Risk for dynamic instability



Power quality issues



Need for grid black start capability



Need for high availabili availability ty although although occurrence occurrence of thunderstorms, thunderstorms,

windstorms windstorms//

hurricanes or heavily icing conditions may apply 

Need for compact offshore module



Risk of low harmonic resonances



Need for fast voltage and reactive power control to enhance network security


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When the HVDC transmission an interesting alternative, a typical case for HVDC Light in an offshore application could be of 350 MW transmissions with 70 km sub sea cable and 30 km land cable

The direct investment investment cost for HVDC Light Light option option including including converters, converters, cables and installation of cable and converters will be in the range of 110 – 140 MEuro. The range is primarily given be differences in installation costs and local market conditions. For the AC cable option there is similarly a range in cost 110 – 140 MEuro. This corresponds in both cases to 310 – 400 kEuro/MW and gives similar results as in the previously quoted study. The two alternatives are thus similar in cost and a detailed study for the individual case will determine the best solution. But other factors should also be considered such that may show beneficial for the HVDC option:



Grid reinforcement costs may be significant in the AC case but are very unlikely for

a HVDC voltage source solution 

Cost for power flow equipment in the AC case



Possibilities to go much further on land with underground cable at very moderate

cost in the HVDC case. 

Increased transmission capacity in existing AC grid (HVDC case).

Obtaining cost information on the different options for different wind farm sizes proved quite difficult as none of the equipment is off-the-shelf. Budget costs and some simple assumptions on scaling have led to the following conclusions.

8.

CONCLUSION

The equipment required for the electrical infrastructure of offshore wind farms is available. The early offshore wind farms are likely to use electrical designs quite similar to those adopted for recent on shore developments. A maximum voltage of 33 kV both for the wind farm collection circuits and for the connection to land is likely in the first instance.


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Howe Howeve ver, r, as larg largee offs offsho hore re inst instal alla lati tion onss are are deve develo lope ped d (>10 (>100 0 MW) MW) then then HVDC HVDC transmission to shore may be more cost effective. Considerable development work is still required required for the large offshore offshore substation substationss or converter stations stations which will be required required for large offshore wind farms.

8.

REFERENCES

1)

http:/ http://en /en.wi .wikip kipedi edia.o a.org/ rg/wik wiki/H i/High igh-vo -volta ltage_ ge_dire direct_ ct_cur curren rent#H t#High igh_vo _voltag ltage_t e_tran ransmi smissi ssion on

2)

http:/ http://en /en.wi .wikip kipedi edia.o a.org/ rg/wik wiki/S i/Sub ubmari marine_ ne_pow power_ er_cabl cable#S e#Subm ubmari arine_ ne_cabl cables_ es_for for_AC _AC

3)

www.ab www.abb.c b.com/ om/caw cawp/g p/gad0 ad0218 2181/1 1/18e6 8e68b7 8b778f 78f900 900952 952c12 c1256e 56e4b0 4b002b 02b25b 25be.as e.aspx px


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4)


http:/ http://en /energ ergyza yzarr.t rr.type ypepad pad.co .com/e m/energ nergyza yzarrn rrnatio ational nalcom com/20 /2009/ 09/03/ 03/ac-d ac-dc-w c-warsars-con contin tinueue part-ii.html

5)

powers powersyst ystems emsdes design ign.co .com/i m/inde ndex.p x.php? hp?opt option ion=com =com_co _conte ntent& nt&view view=art =article icle&id &id=86 =86:pd :pdfffull-magazine-archives&catid=21:content&Itemid=87

6)

http http:/ ://en /en.w .wik ikip iped edia. ia.or org/ g/wi wiki ki/S /Sub ubma marin rine_ e_po power wer_c _cab able le

7)

www.ab www.abb.c b.com/ om/caw cawp/g p/gad0 ad0218 2181/c 1/c125 1256d7 6d7100 1001e0 1e0037 037c12 c1256d 56d080 08002e 02e728 7282.a 2.aspx spx

8)

http:/ http://en /energ ergyza yzarr.t rr.type ypepad pad.co .com/e m/energ nergyza yzarrn rrnatio ational nalcom com/20 /2009/ 09/02/ 02/ind index. ex.htm htmll


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AC vs DC Cable Transmission for Offshore

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