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About Engineering & Technology ReferenceEngineering & Technology (E&T) Reference is a new world-class technical resource for engineers, providing authoritative technical knowledge and solutions

It’s being developed by the Institution of Engineering and Technology to help engineers and researchers better understand and solve their day-to-day technical challenges and improve engineering ‘know-how’ through its growing collection of practical engineering articles and real-world case studies.

More practical than other reference productsArticles and case studies written for E&T Reference are by practising engineers and researchers, many of whom are leaders in their field. Case studies include contributions from leading organisations, revealing lessons learned and solutions adopted from past projects, providing real-world engineering insight.

Each article and case study aims to cover:

Key technology issues in specific subject areas How the technology works and the technical challenges Case studies, lessons learned and solutions adopted

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1 More practical than traditional reference sources - sharing the experience and ‘lessons learned’ from practising engineers and researchers, all subject matter experts in their field.

2 The ‘go-to’ resource for trustworthy solutions - avoiding irrelevant search engine results, users click straight to original, peer-reviewed technical reference content, guaranteeing high quality and relevance.

What makes E&T Reference different?

Early published articles are available now to purchase on www.ietdl.org/etr (IET Member rate £20 each, Non Member rate £25 each). All IET Members qualify for free access to articles using their available Knowledge Pack credits (www.theiet.org/credits).

Share your knowledgeHave you been involved in a project in any of the subject areas below?

If you have technical insights and learned lessons that you think would benefit others in industry, then we would like to hear from you. Power GenerationThermal/fossil fuels, hydro, wind (offshore and onshore), geothermal, piezoelectric (kinetic), storage (batteries, fuel cells), nuclear, photovoltaic/solar, biomass and energy from waste, transmission and distribution, smart grids and energy security.

Built EnvironmentBIM, intelligent buildings, energy efficiency, vehicle infrastructures, safety issues, resource efficiency.

Design & ProductionAdditive manufacturing/3D printing, energy consumption, inclusive design, sustainable manufacturing, robotics and autonomous systems, ‘innovation to commercialisation’.

Information & CommunicationsCyber security, cloud computing, IoT, control engineering, data centres, green ICT, mobile data, next generation networks, quality software, wireless and spectrum.

TransportRail, autonomous vehicles, hybrid/electric vehicles,security and risk, asset management, marine engineering, intelligent transportation systems, aerospace.

Contributions from leading organisations include:

Airbus Defence and Space, Alstom Grid, Arup, Clearwater Compliance, Context Information Security, CGI, Cisco System Inc, Esteel UK, Frazer-Nash, Gaelectric, Gamesa Electric, Infigen Energy, LEEDCo, LNEG, MAHLE Powertrain, Modern Grid Solutions, Milsoft Utility Solutions, Mott MacDonald, Mouchel Infrastructure Services, National Grid, NCC Group, Network Rail, Parsons Brinckerhoff, Siemens Plc, TRL, Tetra Tech, Westermo, Zero Carbon Futures plus many more.

3 Get up to speed quickly in a variety of topics - bridging multiple subject areas, E&T Reference will allow readers to acquire technical knowledge in subjects outside their usual area of expertise.

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first time, new challenges had to be solved during the

first projects.

First, there is the question of control. Most HVDC

schemes are designed to be connected into relatively

‘strong’ AC power systems, where the HVDC convert-

er is only a comparatively minor contributor. In such

systems, the role of the HVDC converter is to track

the AC system voltage and ‘lock on’ to it, playing a

subordinate role. Normally, such power systems

contain many large synchronous generators and it is

these generators that effectively ‘create’ the grid.However, when the HVDC converter is collecting

power from an islanded AC system, as is the case

for an offshore wind park, the situation is quite differ-

ent. First of all, there are no synchronous generators

directly connected to the offshore AC grid. Even

where the wind turbines use synchronous machines,

the machines are de-coupled from the offshore grid

Fig. 4 Architecture of typical present-day HVDC-connected offshore wind farm

Fig. 5 Structure of Dolwin 3 project

IET Engineering & Technology Reference

The Use of High-Voltage Direct Current Transmission forOffshore Wind Projects

Eng. Technol. Ref., pp. 1–12doi: 10.1049/etr.2014.0001

5

& The Institution of Engineering and Technology 2014

construction in German waters. The first of these(‘Borwin 1’) used a two-level converter but all of the sub-sequent connections have used variants of the MMC.One of the most recently ordered is the 900 MW, ±320kV, ‘Dolwin 3’ project, which was awarded to AlstomGrid in February 2013. Many more such projects areplanned in Germany, UK, elsewhere in Europe and inthe USA. Each of the systems built to date has a ‘radial’architecture – the HVDC scheme exports power fromone cluster of typically 100–200 turbines and is the onlyroute to shore for the power generated by that cluster.Today’s offshore wind turbines are normally regardedas ‘AC’ machines because they are arranged toproduce a 690 V AC output, but in reality this descrip-tion is misleading because the largest turbines (5 MWand above) are almost all of the ‘full converter’ type.This means that the generator, instead of being con-nected directly to the grid, is instead connected via aback-to-back power electronic converter whichallows the rotor and grid frequencies to be decoupled.This is necessary to maximise the efficiency of theturbine across a range of wind speeds. The converterin the turbine is similar in concept to a smallVSC-HVDC link and normally uses IGBTs in a two-levelor three-level converter configuration, with PWM.As shown in Figs. 4 and 5, the turbine output voltageof 690 V AC is stepped up to 33 kV to connect to the

collector array. Several collector arrays are brought to-gether onto an AC substation platform where the ACvoltage is stepped up again to an intermediate voltage(e.g. 155 kV in the case of Germany) and fed to asingle HVDC converter platform. The converter trans-forms this AC voltage into DC, typically with avoltage of ±320 kV. From the offshore converter,power is transmitted to land via two DC cables tothe on-shore converter station where the power isconverted back to AC and fed into the national grid.An important point to note here is that when anHVDC connection is used, the wind turbines are com-pletely isolated from the onshore grid and thus forman electrical ‘island’. This can be an advantagebecause it means the offshore grid does not necessar-ily have to run at the same frequency as the onshoregrid. However, the design of the islanded offshoreAC grid has many traps for the unwary and needs tobe performed with care. This is an emerging aspectof power engineering in which there is considerablepotential for improvement.

Challenges of offshore HVDCAlthough HVDC has been in commercial use foralmost 60 years, it is only in the last few years that ithas found application in offshore power transmission.As with any technology applied to a new area for the

Fig. 3 MMC in half-bridge form

IET Engineering & Technology Reference

Colin C Davidson

4& The Institution of Engineering and Technology 2014

Eng. Technol. Ref., pp. 1–12doi: 10.1049/etr.2014.0001

The Use of High-Voltage Direct CurrentTransmission for Offshore Wind Projects

Colin C Davidson MA (Cantab.), CEng, FIET Chief Technology Officer, HVDC, Alstom Grid, Stafford, UK

AbstractAs wind generation is exploited at increasing distances from the shore, traditional alternating current (AC) trans-mission is approaching the limit of technical feasibility and hence high-voltage direct current (HVDC) is needed.HVDC is a well-established alternative technology for power transmission on land, and has now been used forthe shore connections from nine offshore wind farms off the coast of Germany. The use of HVDC for offshorewind connections brings some new challenges, not least of which is that associated with the ‘islanded’ offshoreAC collector grid. However, HVDC brings lower power losses and greater controllability. There are also greatopportunities for inter-connecting multiple offshore wind farms to multiple countries and, eventually, to usedirect current (DC) all the way from the generator to the onshore grid. Significant investment in R&D isunder way for the associated components, such as DC circuit breakers and DC–DC converters, needed torealise such a scheme.

IntroductionThe design, installation and commissioning of an off-shore wind farm is a complex project involving manyengineering disciplines and high levels of risk.However, one aspect of such projects which some-times does not receive as much attention as it deservesis the transmission infrastructure needed to bring thegenerated power to shore. For wind parks locatedclose to the shore, the necessary transmission infra-structure is comparatively straightforward and wellunderstood; however, as the distances to the shoreincrease, so too do the challenges of providing atransmission connection to the onshore grid.

For the power transmission to the shore, just as withany power transmission on land or under the sea,there are two alternatives: alternating current (AC)and direct current (DC). AC is more widely knownand has been used for most of the offshore windparks built to date, where distances to the shore arequite short. AC dominated the power transmission in-dustry throughout the 20th century, predominantlybecause the invention of the transformer allowedthe transmission voltage to be stepped up and downto enable efficient transmission at high voltage.However, AC transmission has significant drawbackswhich become increasingly serious as the distancesfrom the offshore wind park to the shore increase.For this reason, high-voltage direct current (HVDC)[1] is starting to play an important role in offshore

wind projects – a role that is expected to becomeever greater in the future.

Choice of AC against DC PowerTransmissionFrom the very earliest days of electrical power trans-mission in the late 19th century, it was known thatDC had advantages over AC in certain circumstances.DC, despite the backing of Thomas Edison, lost the‘Battle of the Currents’ largely because while ACcould easily be stepped up and down using transfor-mers, there was no technology available at the timethat could efficiently convert high-voltage AC to high-voltage DC. Several projects were built in the early20th century using electromechanical conversionfrom AC to DC – essentially using motor-generatorsets – however, the efficiency was poor. In the1930s and 1940s, mercury arc rectifier technologywas developed sufficiently to perform the necessaryconversion from AC to DC efficiently at highvoltage, and DC transmission began to make a come-back. In 1954, the first true commercial HVDCscheme was inaugurated and from that date HVDCtechnology has grown steadily in importance as aniche technology, finally starting to become a main-stream power transmission technology in the lastdecade.

DC transmission has several advantages over AC: it ischeaper for long-distance bulk power transmission

Reference Article1st published in June 2014doi: 10.1049/etr.2014.0001

ISSN 2056-4007www.ietdl.org

Eng. Technol. Ref., pp. 1–12doi: 10.1049/etr.2014.0001

1& The Institution of Engineering and Technology 2014

See early published articles at www.ietdl.org/etr

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