1. The document advertises the opportunity for engineers and subject matter experts to contribute articles and case studies or serve as peer reviewers for a new online technical reference collection called Engineering & Technology Reference (ETR). 2. ETR aims to provide practical solutions to common engineering challenges through real-world articles and case studies written by practicing engineers, with the goal of helping readers better understand and solve technical problems. 3. Contributors and peer reviewers will gain professional recognition and can showcase their work and organizations, while supporting the development of other engineers.

1. YOUR CHANCE TO GET INVOLVED...
ENGINEERING & TECHNOLOGY REFERENCE
Calling all subject experts,
authors and peer reviewers!
A new world-class collection of technical articles and industry case studies
revealing current technological challenges, lessons learned and solutions adopted.
Boost your professional career and get involved www.theiet.org/etr-involve

2. About Engineering & Technology Reference
Engineering & 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 products
Articles 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
Want to showcase your
work alongside other
leading companies?
Go to www.theiet.org/
etr-involve
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).

3. Share your knowledge
Have 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 Generation
Thermal/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 Environment
BIM, intelligent buildings, energy efficiency, vehicle
infrastructures, safety issues, resource efficiency.
Design & Production
Additive manufacturing/3D printing, energy
consumption, inclusive design, sustainable
manufacturing, robotics and autonomous systems,
‘innovation to commercialisation’.
Information & Communications
Cyber security, cloud computing, IoT, control
engineering, data centres, green ICT, mobile data,
next generation networks, quality software, wireless
and spectrum.
Transport
Rail, 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.
4 Will help students get ‘job-ready’ -
providing ‘learning by example’ - real-life
insight into the engineering challenges
and solutions of today.
Full package subscriptions will be available on single-user or multi-user licence.
For further information email ETR@theiet.org.
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 Transmissio
n forOffshore Wind Projects
Eng. Technol. Ref., pp. 1–12
doi: 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, ±320
kV, ‘Dolwin 3’ project, which was awarded to AlstomGrid in February 2013. Many more such projects are
planned in Germany, UK, elsewhere in Europe and in
the USA. Each of the systems built to date has a ‘radial’architecture – the HVDC scheme exports power from
one cluster of typically 100–200 turbines and is the only
route to shore for the power generated by that cluster.
Today’s offshore wind turbines are normally regardedas ‘AC’ machines because they are arranged to
produce a 690 V AC output, but in reality this descrip-tion is misleading because the largest turbines (5 MW
and 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 a
back-to-back power electronic converter which
allows the rotor and grid frequencies to be decoupled.
This is necessary to maximise the efficiency of the
turbine across a range of wind speeds. The converterin the turbine is similar in concept to a small
VSC-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 AC
voltage is stepped up again to an intermediate voltage(e.g. 155 kV in the case of Germany) and fed to a
single HVDC converter platform. The converter trans-
forms this AC voltage into DC, typically with a
voltage of ±320 kV. From the offshore converter,
power is transmitted to land via two DC cables to
the on-shore converter station where the power is
converted back to AC and fed into the national grid.
An important point to note here is that when an
HVDC connection is used, the wind turbines are com-
pletely isolated from the onshore grid and thus form
an electrical ‘island’. This can be an advantage
because 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 to
be performed with care. This is an emerging aspect
of power engineering in which there is considerable
potential for improvement.
Challenges of offshore HVDCAlthough HVDC has been in commercial use for
almost 60 years, it is only in the last few years that it
has 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 Current
Transmission for Offshore Wind Projects
Colin C Davidson MA (Cantab.), CEng, FIET Chief Technology Officer, HVDC, Alstom Grid, Stafford, UK
Abstract
As 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 for
the shore connections from nine offshore wind farms off the coast of Germany. The use of HVDC for offshore
wind connections brings some new challenges, not least of which is that associated with the ‘islanded’ offshore
AC collector grid. However, HVDC brings lower power losses and greater controllability. There are also great
opportunities for inter-connecting multiple offshore wind farms to multiple countries and, eventually, to use
direct current (DC) all the way from the generator to the onshore grid. Significant investment in R&D is
under way for the associated components, such as DC circuit breakers and DC–DC converters, needed to
realise such a scheme.
Introduction
The design, installation and commissioning of an off-
shore wind farm is a complex project involving many
engineering disciplines and high levels of risk.
However, one aspect of such projects which some-
times does not receive as much attention as it deserves
is the transmission infrastructure needed to bring the
generated power to shore. For wind parks located
close to the shore, the necessary transmission infra-
structure is comparatively straightforward and well
understood; however, as the distances to the shore
increase, so too do the challenges of providing a
transmission connection to the onshore grid.
For the power transmission to the shore, just as with
any power transmission on land or under the sea,
there are two alternatives: alternating current (AC)
and direct current (DC). AC is more widely known
and has been used for most of the offshore wind
parks built to date, where distances to the shore are
quite short. AC dominated the power transmission in-
dustry throughout the 20th century, predominantly
because the invention of the transformer allowed
the transmission voltage to be stepped up and down
to enable efficient transmission at high voltage.
However, AC transmission has significant drawbacks
which become increasingly serious as the distances
from 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 become
ever greater in the future.
Choice of AC against DC Power
Transmission
From the very earliest days of electrical power trans-
mission in the late 19th century, it was known that
DC had advantages over AC in certain circumstances.
DC, despite the backing of Thomas Edison, lost the
‘Battle of the Currents’ largely because while AC
could easily be stepped up and down using transfor-
mers, there was no technology available at the time
that could efficiently convert high-voltage AC to high-
voltage DC. Several projects were built in the early
20th century using electromechanical conversion
from AC to DC – essentially using motor-generator
sets – however, the efficiency was poor. In the
1930s and 1940s, mercury arc rectifier technology
was developed sufficiently to perform the necessary
conversion from AC to DC efficiently at high
voltage, and DC transmission began to make a come-
back. In 1954, the first true commercial HVDC
scheme was inaugurated and from that date HVDC
technology has grown steadily in importance as a
niche technology, finally starting to become a main-
stream power transmission technology in the last
decade.
DC transmission has several advantages over AC: it is
cheaper for long-distance bulk power transmission
Reference Article
1st published in June 2014
doi: 10.1049/etr.2014.0001
ISSN 2056-4007
www.ietdl.org
Eng. Technol. Ref., pp. 1–12
doi: 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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The IET is now accepting contributions for
the new Engineering & Technology Reference.
Get involved as an author or peer reviewer
and enjoy the following benefits:
Raise your work profile and Worldwide
readership of your article, including a possible
168,000 IET members
Showcase your organisation’s work alongside
other leading companies
Support the development of your peers and
colleagues by sharing your experience and
technical knowledge
For IET members - article submissions count towards Continuous Professional
Development (CPD) under the IET CPD Monitoring Scheme (www.iet.org/cpd)
“ Writing for E&T Reference has given me recognition in my community
and it’s certainly raised my profile in terms of engaging in a technical area
Dr Siraj Ahmed Shaikh, Cyber Security Lead, Knowledge Transfer Network ”
For further details on how you can get involved, please send in a short
biography or contact the team for more information. ETR@theiet.org
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