wind_power_in_power_systems

486 Transmission Systems for Offshore Wind Farms

With an increasing distance to shore, reactive power compensation will be required at
both ends of the cable (i.e. at the offshore substation, too). A 400 MW offshore wind farm
with two 150 kV, 120 km, cable systems, for instance, will require 150 MVAR compensa-
tion offshore as well as onshore (Eriksson et al., 2003). Furthermore, the maximum rating
of AC cables is currently limited to about 200 MW per three-phase cable, based on a
voltage rating of 150–170 kV, compensation at both ends of the cable and a maximum
cable length of 200 km. That means that a 1000 MW wind farm would need five cables for
the connection to shore, and that does not even take possible requirements for redundancy
into account.

For shorter distances, higher voltage ratings of up to 245 kV might be possible, which
would increase the maximum rating to 350–400 MW (Rudolfsen, 2001; Rudolfsen,
2002). A three-core, 1000 mm2, conductor cable operated at a system voltage of
230 kV with a maximum of Æ10 % voltage variation between no load and full load
can, for instance, transmit 350 MW over 100 km with losses of 4.3 %, or 300 MW over
200 km with losses of 7.3 %, if the cable is compensated equally at both ends (Rudolfsen,
2001). Higher voltage levels of up to 400 kV are under development, which may allow a
maximum transmission of 1200 MVA over a maximum distance of 100 km. It is, how-
ever, uncertain when such solutions will become commercially available.

Finally, the main disadvantage of HVAC solutions must be mentioned: with increas-
ing wind farm size and distance to shore, load losses will increase significantly. Also, an
increase in the transmission voltage level will lead to larger and more expensive equip-
ment (e.g. transformers) as well as more expensive submarine cables. Hence, an increase
in the voltage level is often only justifiable if an increase in capacity is required.(2)

22.3.2 Line-commutated converter based high-voltage direct-current
transmission

The advantage of LCC based HVDC connections is certainly their proven track record.
The first commercial LCC HVDC link was installed in 1954 between the island of
Gotland and the Swedish mainland. The link was 96 km long, 20 MW rated and used
a 100 kV submarine cable. Since then, LCC based HVDC technology has been installed
in many locations in the world, primarily for bulk power transmission over long
geographical distances and for interconnecting power systems, such as for the different
island systems in Japan and New Zealand (Hammons et al., 2000). Other well-known
examples for conventional HVDC technology are:

. the 1354 km Pacific Interie DC link, with a rating of 3100 MW at a DC voltage of
Æ500 kV;

. the Itaipu link between Brazil and Paraguay, rated at 6300 MW at a DC voltage of
Æ600 kV (two bipoles of 3150 MW).

(2) The capacity charging current of long HVAC cable increases linearly with the voltage level as well as linearly
with the lengths of the cable. Hence, for a given wind farm capacity (e.g. 200 MW) the losses related to the
capacity charging current will be higher for a 245 kV solution than for a 150 kV system design.

Wind Power in Power Systems 487

Shore line

Offshore substation

145 kV, 50 Hz Onshore converter
station

Offshore wind farm 380 kV, 50 Hz Onshore
network
Statcom F
F 380 kV
HFF Single-phase F
three-winding F
Integrated return F
cable converter
transformer

500 MW
500 kV
1000 A

Three-phase HFF
two-winding

converter
transformer

Figure 22.4 Basic configuration of a 500 MW wind farm using a line-commutated converter
(LCC) high-voltage direct-current (HVDC) system with a Statcom (for a configuration for a
1100 MW wind farm using an LCC HVDC system with diesel generators on the offshore
substation, see Kirby et al., 2002). Note: F ¼ filter; HFF ¼ high-frequency filter; the statcom
can be replaced with a diesel generator
Source: based on Cartwright, Xu and Saase, 2004.

However, there is no experience regarding LCC based HVDC offshore substations or
onshore in combination with wind power.

A thyristor based LCC HVDC transmission system consists of the following main
components: an AC based collector system within the wind farm; an offshore substation
with two three-phase two-winding converter transformers as well as filters and either a
Statcom or a diesel generator that supply the necessary short-circuit capacity; DC
cable(s); and an onshore converter station with a single-phase three-winding converter
transformer as well as the relevant filters (Cartwright, Xu and Saase, 2004; Kirby et al.,
2002; see also Figure 22.4).

This technology requires comparatively large converter stations both on- and off-
shore, as well as auxiliary service at the offshore converter station. The auxiliary service
has to keep up a rather strong AC system at the offshore converter in order to enable the
operation of the line-commutated converters even during periods with no or very little
wind. This auxiliary offshore service will be most likely supplied by diesel generator(s)
(Kirby et al., 2002). Another solution, also referred to as hybrid HVDC, comprises a
LCC based HVDC converter and a Statcom, see Cartwright et al., 2004. The Statcom
provides the necessary commutation voltage to the HVDC converter and reactive power
compensation to the offshore network during steady state, dynamic and transient conditions.

The total conversion efficiency from AC to DC and back to AC using the two
converters (offshore and onshore) lies in the range of 97–98 % and depends on the
design details of the converter stations. A system design with 98 % efficiency will have
higher investment costs compared with a design with lower efficiency. Hence, the
advantage of an LCC HVDC solution are comparatively low losses (i.e. 2–3 % for a

488 Transmission Systems for Offshore Wind Farms

500 MW transmission over 100 km, including the losses in both converters but excluding
consideration of the requirements for the auxiliary service at the offshore converter
station). In addition, the higher transmission capacity of a single cable compared with
HVAC transmission or VSC based transmission can be an advantage for very large
offshore wind farms.

The requirement for a strong network offshore (and onshore) as well as the need for
comparatively large offshore substation converter stations, however, reduce the like-
lihood of that LCC based HVDC solutions will be considered for small to medium-sized
wind farms (i.e. those that are rated at less than about 300 MW).

22.3.3 Voltage source converter based high-voltage direct-current
transmission

VSC based HVDC technology is gaining more and more attention. It is marketed by
ABB under the name HVDC Light and by Siemens under the name HVDC Plus. This
comparatively new technology has only become possible as a result of important
advances in high-power electronics, namely, in the development of insulated gate
bipolar transistors (IGBTs). In this way, pulse-width modulation (PWM) can be used
for the VSCs as opposed to thyristor based LCCs used in the conventional HVDC
technology.

The first commercial VSC based HVDC link was installed by ABB on the Swedish
island of Gotland in 1999. It is 70 km long, with 60 MVA at Æ80 kV. The link was
built mainly in order to provide voltage support for the large amount of wind power
installed in the South of Gotland (see also Chapter 13). Between 1999 and 2000, a small
demonstration project with 8 MVA at Æ9 kV was built in Tjæeborg, Denmark. The
project is unique, as the link is used to connect a wind farm (three wind turbines with a
total capacity of 4.5 MW) to the Danish power system (Skytt, Holmberg and Juhlin,
2001). In 2000, a link – 65 km long, with 3 Â 60 MVA at Æ80 kV – was built in Australia
between the power grids of Queensland and New South Wales. A second link was
installed in Australia in 2002. With a length of 180 km, this connection is the longest
VSC based HVDC link in the world. It has a capacity of 200 MVA at a DC voltage of
Æ150 kV. In the USA, a 40 km submarine HVDC link was installed between Connecti-
cut and Long Island in 2002. The link operates at a DC voltage of Æ150 kV and has a
capacity of 330 MVA. For 2005, ABB plans the first installation of a VSC offshore
converter station for the offshore Troll A platform in Norway. The link will be 67 km
long with a rating of 2 Â 41 MVA at Æ60 kV (Eriksson et al., 2003).

A VSC based HVDC transmission system consists of the following main components:
an AC based collector system within the wind farm; an offshore substation with the
relevant converter(s); DC cable pair(s); and an onshore converter station [Ha¨ usler and
Owman, 2002; Eriksson et al., 2002; see also Figure 22.5].

A VSC based HVDC link does not require a strong offshore or onshore AC network;
it can even start up against a nonload network. This is possible because in a VSC the
current can be switched off, which means that there is no need for an active commuta-
tion voltage. Furthermore, the active and reactive power supply can be controlled
independently. In addition, it is important to mention that the VSC based HVDC

Wind Power in Power Systems 489

connection is usually not connected to ground. Therefore, the VSC based HVDC always
needs two conductors (cables).

VSCs use IGBT semiconductor elements with a switching frequency of approximately
2 kHz. This design results in comparatively high converter losses of up to 2 % per
converter station. Research currently focuses on how to reduce these losses. The
advantage of the high switching frequency are low harmonic levels and, hence a reduced
need for filters. Also, the rating per converter is presently limited 300–350 MW, while
the cable rating at Æ150 kV is 600 MW. Hence, if more power than 300 MW is to be
transferred, the number of converter stations has to be increased [see Figure 22.5(a)]. It
is, however, expected that the available converter rating will be increased to approxi-
mately 500 MW by the market leader in the near future. Figure 22.5(b) shows the
possible design for a 500 MW offshore wind farm based on converter stations with a
500 MW rating.

The total efficiency of the two converter stations of a VSC based HVDC system is less
than that of a LCC HVDC system. There is hardly any information on the total system
efficiency published, but at the International Workshops on Transmission Networks for

Offshore substation Shore line

Onshore converter
station

150 kV

30 kV 150 kV

Offshore wind farm 300 MVA 300 MVA Bipolar cable pair 300 MVA Onshore
Rating: 600 MW network

+/–150 kV 600 MVA

300 MVA Bipolar cable pair 400 kV
Rating: 600 MW

+/–150 kV

300 MVA 300 MVA

(a)

Offshore substation Shore line
Offshore wind farm
Onshore converter station Onshore
30 kV network

500 MVA 500 MVA Bipolar cable pair 500 MVA 500 MVA
Rating: 600 MW
150 kV 150 kV 400 kV
+/–150 kV

(b)

Figure 22.5 (a) A 600 MW wind farm using two voltage source converter (VSC); high-voltage
direct-current (HVDC) systems, each converter station with a 300 MW rating. (b) A 500 MW wind
farm using one VSC HVDC system based on a converter station with a 500 MW rating
Source: for part (a) based on Eriksson et al., 2003.

490 Transmission Systems for Offshore Wind Farms

Offshore Wind Farms of 2000 to 2003 the total efficiency, including both converter
stations, was reported to lie in the range of 90–95 %.

Other often-emphasised advantages of a VSC based HVDC solution is the capability
of four-quadrant operation, the reduced number of filters required, black-start cap-
ability and the possibility of controlling a number of variables such as reactive power,
apparent power, harmonics and flicker when feeding the power system from a VSC
(Burges et al., 2001; Ko¨ nig, Luther and Winter, 2003).

22.3.4 Comparison

The following comparison of the three different technical transmission solutions is
divided into technical, economic and environmental issues.

22.3.4.1 Technical issues

The relevant technical issues are: rating, losses, size of offshore installation, grid impact
and implementation issues.

Rating
Presently, AC cables have a maximum rating of about 200 MW per three-phase cable.
This rating is based on a voltage level of 150–170 kV, compensation at both ends of the
cable and a maximum cable length of around 200 km. For shorter distances, voltage
ratings may increase to 245 kV, which would raise the maximum rating to 350 MW over
a maximum of 100 km, or 300 MW over 150–200 km (Rudolfsen, 2001). Bipolar cable
pairs for VSC based HVDC, in comparison, can have a maximum rating of 600 MW for
a voltage level of Æ150 kV, independent of the cable length. Currently, only converter
stations with a maximum rating of 300–350 MW are in operation, which means that two
converter stations would be needed for the full utilisation of the maximum cable rating.
Converter stations with a maximum rating of 500 MW, however, are announced to be
becoming available in the near future, which will reduce the number of cables required
for VSC based HVDC solutions (see Table 22.2).

Table 22.2 Number of cables needed for different wind farms and different technical solutions

Wind farm HVAC LCC HVDC VSC HVDC (150 kV)
capacity (MW), (150 kV)
100 km transmission 150 kV, 450 kV, 300 MW 500 MW
distance 2 bipolar monopolar CS rating, CS rating,
3 cable bipolar cable bipolar cable
300 5 cable
500 6
900 1þ1 1 1þ1 1þ1
1200 2þ2 1 2þ2 1þ1
4þ4 2 3þ3 2þ2
5þ5 2 4þ4 3þ3

Note: CS ¼ converter station; HVAC ¼ high-voltage alternating-current; HVDC ¼ high-voltage
direct-current; LCC ¼ line-commutated converter; VSC ¼ voltage source converter.

Wind Power in Power Systems 491

For LCC based HVDC, the cable and converter ratings are not limiting factors
regarding the maximum capacity (< 1000 MW) of the offshore wind farms that are
presently under discussion.

Table 22.2 shows the number of cables required for wind farms with different total
capacity. It can be seen that a 1200 MW offshore wind farm requires up to six cables for
a HVAC link, but only two for an LCC HVDC solution. The VSC HVDC solution
would require 3–4 cables.

The number of required cables will influence the total investment costs. It should be
taken into account, though, that overall system reliability may increases if two or more
cables are used. In this respect, it should be noted that the different cables should follow
different cable routes to maximise reliability benefits. This may not always be feasible,
though.

Losses
For HVAC transmission, the power loss depends to a large extent on the length and
characteristics of the AC cable (i.e. losses increase significantly with distance; see also
Santjer, Sobeck and Gerdfes, 2001). The losses of HVDC connections show only a very
limited correlation with the length of the cable, depending on the efficiency of the
converter stations. As explained above, the efficiency of LCC stations is usually higher
than that of VSCs. This means that for short distances the losses from a HVAC link are
lower than those from a HVDC connection, owing to the comparatively high converter
losses. There is, however, a distance X where the distance-related HVAC losses reach
similar levels to those of HVDC links (see Figure 22.6). For distances larger than X,
losses in the HVDC solution are lower than those for HVAC links. The distance X
depends on the system configuration (e.g. on cable type and voltage levels) but is,
however, usually longer for VSC HVDC than for LCC HVDC technology.

The critical distance X depends very much on the individual case. For medium-sized
wind farms of around 200 MW, the critical distance X for HVAC compared with VSC
HVDC is around 100 km, for instance. This means that an HVAC link shows lower
losses for distances less than 100 km, but if the distance exceeds 100 km a VSC HVDC
solution will result in lower losses.

Loss HVAC
HVDC

DC
converter

losses

X
Critical distance
Cable Length

Figure 22.6 Comparison of losses for high-voltage alternating current (HVAC) and high-voltage
direct current (HVDC)

492 Transmission Systems for Offshore Wind Farms

Size of offshore substation
Different technical transmission solutions have widely divergent requirements regarding
the size of the offshore substation (Kirby, Xu and Siepman, 2002). In general, the size of
an AC offshore substation will be only about a third of the size of the corresponding
HVDC solution, owing to the significant space required by the converter stations. For
onshore HVDC converter stations, LCC based converter stations need considerably
more space than do VSC based systems. Eriksson et al. (2003) argue that a 300 MW VSC
offshore converter station requires a space of approximately 30 Â 40 Â 20 m (width Â
length  height). Regarding VSC converter stations, it is important to remember that
the maximum possible rating at present is 300 MVA; hence a larger capacity demand
will require multiple VSC converter stations. For very large capacity requirements
(>> 300 MW), the possible advantages of VSC based solutions regarding space require-
ments compared with LCC solutions may be significantly reduced.

Grid impact
Owing to the considerable rating of offshore wind farms, the impact of the entire
offshore wind farm system on the onshore power system has to be taken into account
(i.e. the type of wind turbines, the transmission technology and the grid interface
solution). It is also important to consider that some of the countries that expect a
significant development of offshore wind farms already have an onshore network with
a significant amount of onshore wind power (e.g. on Germany, see Chapter 11; on
Denmark, see Chapter 10).

Transmission network operators in Denmark and Germany, for instance, have there-
fore already defined new grid connection requirements for connecting wind farms to the
transmission system. These regulations are also binding for offshore wind farms (for
details, see Chapter 7). Other transmission network operators currently prepare similar
requirements. The new regulations will try to help the onshore network to remain stable
during faults. An example of such requirements is that a wind farm will have to be able
to reduce the power output to 20 % below rated capacity within 2 s of the onset of a
fault. After the fault, the wind farm output has to return to the prefault level within 30 s.

During the past few years, a number of studies have been conducted on the impact of
the different transmission solutions on the grid and their capability to comply with the
new grid connection requirements (see Bryan et al., 2003; Cartwright, Xu and Saase,
2004; Eriksson et al., 2003; Gru¨ nbaum et al., 2002; Ha¨ usler and Owman 2002; Henschel
et al., 2002; Kirby, Xu and Siepman, 2002; Kirby et al., 2002; Ko¨ nig, Luther and Winter,
2003; Martander 2002; Schettler, Huang and Christl, 2000; Søbrink et al., 2003). It can
be concluded from these studies that the grid impact depends very much on the
individual case (i.e. the grid impact depends on the detailed design of the various
solutions). The manufacturers of the various technical transmission solutions currently
develop appropriate system designs to minimise the grid impact and to comply with the
new grid connection requirements. Many of the above-cited studies are performed by or
in cooperation with manufacturers of the various technical transmission solutions in
order to demonstrate their technical capabilities. In other words, possible drawbacks of
certain technical solutions regarding grid integration are minimised with additional
equipment. Hence the main decision criteria for or against a certain technical solution
will be based mainly on the overall system economics, which should include the cost for

Wind Power in Power Systems 493

the additional equipment. There will certainly be more in-depth research in this area
over the next years, such as the EU project, DOWNVIND (http://www.downvind.com).

It must, however, be mentioned that both HVDC technologies have a significant
advantage over an HVAC solution: HVDC technologies significantly reduce the fault
contribution to the onshore power network. Expensive upgrades of existing onshore
equipment such as transformers and switchgears may thus become unnecessary. In
addition, VSC based HVDC technology has the capability of providing ancillary
services to the onshore network (e.g. providing active as well as reactive power supply
and voltage control). This capability could also be used within the offshore AC net-
works for controlling the reactive power in the network, for instance. However, HVAC
or an LCC based HVDC solution in combination with additional equipment (e.g. an
SVC or a Stacom) might be able to provide similar benefits.

Implementation
Many wind farms will be built in two steps: at first, there will be a small number of wind
turbines and then a larger second phase. Therefore, it is important to point out that
XLPE cables can be used for AC as well as for HVDC links. During the first step, an AC
solution may be applied as a VSC based HVDC system will not be economic because of
the small size of this first phase of the project. In the second phase, the AC system will be
converted to an HVDC system, which requires a converter station onshore and offshore,
and possibly more cables to shore. The existing cable, however, may be incorporated
into the HVDC link. This approach might be seen in some German offshore wind farms.
The first phase with the AC solution will comprise a wind farm with a capacity of 50–
100 MW. The distance to the onshore grid may be around 60–100 km. During the
second phase, up to 1000 MW may be added. At that point, the link from the offshore
wind farm has to be extended to the 380 kV onshore network (an extra 35 km), and,
most likely, HVDC technology will be used, incorporating the cable that was already
installed during phase one.

Summary
In Table 22.3 the three standard transmission solutions are briefly compared. The
technical capabilities of each system can probably be improved by adding additional
equipment to the overall system solution.

22.3.4.2 Economic issues

When comparing HVAC and HVDC links, the total system cost for equivalent energy
transmission over a similar distance should be considered. The total system cost com-
prises investment costs and operating costs, including transmission losses and converter
losses. Investment costs change with rating and operating costs (i.e. losses) and with the
distance from a strong network connection point onshore. Therefore the economic
analysis has to be carried out based on specific cases. Over the past years, a number
of studies have been conducted (e.g. Burges et al., 2001; CA-OWEA, 2001; Ha¨ usler and
Owman, 2002; Holdsworth, Jenkins and Strbac, 2001; Lundberg, 2003; Martander,
2002). As it is rather difficult to obtain good input data, in particular regarding the
various costs but also regarding the converter losses of HVDC solutions, significant

494 Transmission Systems for Offshore Wind Farms

Table 22.3 Summary of transmission solutions: high-voltage alternating-current (HVAC)
transmission, line-commutated converter (LCC) based high-voltage direct-current (HVDC)
transmission and voltage source converter (VSC) based HVDC transmission

Transmission solution

HVAC LCC based HVDC VSC based HVDC

Maximum available 200 MW at 150 kV $1200 MW 350 MW
capacity per system 350 MW at 245 kV 500 MW announced
Up to 245 kV Up to Æ500 kV Up to Æ150 kV
Voltage level Yes No No
Does transmission
Depends on 2–3 % (plus 4–6 %
capacity depend on distance requirements for
distance? ancillary services Yes
Total system losses Yes offshore)
No Low compared with
Does it have Black-start High compared HVAC
capability? with HVDC Low compared
solutions with HVAC Wide range of
Level of faults Limited possibilities
Limited Planned (2005)
Technical capability for Yes
network support No Depends on capacity;
Small converter is smaller
Are offshore substations Depends on than LCC but larger
in operation? capacity; than HVAC
converter is larger substation
Space requirements for than VSC
offshore substations

differences between the different solutions can be found. In the following, we will
present some general economic conclusions (for a summary, see Figure 22.7). It should
be emphasised, though, that the results are very specific to the individual cases. Also, the
economic impact of a possible 500 MW converter station rating for the VSC based
HVDC solution is not considered, because the relevant data are currently not available.
More detailed research in this area will certainly be performed in the near future (e.g.
within the proposed EU project, DOWNVIND).

Offshore wind farms of up to 200 MW
In general, the investment costs for a bipole DC cable and a single three-core 150 kV
XLPE AC cable with a maximum length of 200 km are very similar, with the DC cable
probably having a slight cost advantage over the AC cable. However, the investment

Wind Power in Power Systems 495

1000 LCC based HVDC
900 VSC based HVDC or LCC based HVDC
800
Capacity (MW) 700 VSC based HVDC
600
500 HVAC (245 kV) or based HVDC
400 VSC based HVDC VSC
300
200 HVAC HVAC or HVAC (245 kV) or
100 (up to170 kV) VSC
VSC based
based

HVDC HVDC

50 100 150 200 250 300
Distance (km)

Figure 22.7 Choice of transmission technology for different wind farm capacities and distances
to onshore grid connection point based on overall system economics (approximation): economics
of high-voltage alternating-current (HVAC) links, line-commutated converter (LCC) based high-
voltage direct-current (HVDC) links and voltage source converter (VSC) based HVDC link

cost of VSC converters is up to 10 times higher than that of an HVAC infrastructure
(e.g. a transformer station). Hence, for maximum distances of approximately 100 km
and a maximum rating of 200 MW, an HVAC link operated at a maximum voltage level
of 170 kV is usually considered the most economic solution. With a larger distance, the
increasing losses in the HVAC link may justify the investment in a VSC based HVDC
solution. For distances between 150 and 250 km, VSC based HVDC and HVAC links
operated at a maximum voltage level of 245 kV are rather close as far as their economics
is concerned. Once a distance of 250 km is exceeded, theoretically only VSC based
HVDC links are technically feasible. HVAC solutions may be technically viable if
compensation is installed along the cables, which may require an offshore platform
for the compensation equipment (Eriksson et al., 2003). A distance of 150 km or more is
not unlikely because strong grid connection points onshore might be at some distance
inland and the distance onshore has to be included in the total transmission distance.

Offshore wind farms between 200 and 350 MW
For wind farms between 200 and 350 MW, either two 150 kV three-core XLPE AC
cables are required or one 245 kV cable. That means that the cost of an HVAC link
increases and a VSC based HVDC solution may become economically competitive.
However, for distances exceeding 100 km, the technical feasibility of current HVAC
solutions based on maximum voltage levels drops to about 300 MW at 200 km.
Hence, VSC based HVDC connections are most likely to be more economic than a
second AC cable.

496 Transmission Systems for Offshore Wind Farms

Offshore wind farms between 350 and 600 MW
For a maximum size of 400 MW and a comparatively short distance to a strong grid
interconnection point, HVAC operated at 245 KV might be a very competitive solution.
For larger capacities, HVAC links will need at least two three-core XLPE AC cables
operated at 245 kV or even three cables operated at 150 kV. VSC based HVDC links, on
the other hand, will still only require one bipole DC cable. Hence, VSC HVDC seems to
be the most economic solution.

Offshore wind farms between 600 and 900 MW
For a wind farm rating of 600 MW or more, VSC HVDC links will also require two
bipole DC cables as well as three converter stations onshore as well as offshore. An LCC
based HVDC link requiress only one DC cable and only one converter station onshore
and one offshore and probably will therefore economically lie very close to a VSC
HVDC solution. However, reliability issues may result in a solution where two cables to
shore are preferred because the risk of losing one cable and consequently the whole
offshore wind farm might be considered too high.

Offshore wind farms larger than 900 MW
For wind farms larger than 900 MW, LCC based HVDC links are probably the most
economic solution. However, as mentioned above, reliability issues may lead to two
independent cable systems to shore. In that case, a VSC based HVDC link will most
likely be the more economic solution.

Summary
It remains to be seen how technical development will affect the economics of the
different solutions in the future. Advocates of VSC based HVDC solutions argue that
cost reduction in power electronics will make this technology cheaper in the near future,
whereas HVAC advocates hope that a future increase in transmission voltage will
provide similar benefits.

22.3.4.3 Environmental issues

From an environmental perspective, two main issues are of interest: the magnetic field
of the submarine cables as well as the number of submarine cables buried in the seabed.

Submarine cables installed and operated from offshore installations to the shore often
pass through very sensitive areas environmentally. The impact of submarine cables on
these areas is therefore often a very important part of the environmental permitting
process. The permit granting authorities will most likely favour the technical solution
with the lowest impact on these sensitive areas, which means a solution with a minimum
number of cables as well as low magnetic fields for the submarine cables. In general,
three-core AC submarine cables have a lower magnetic field than DC submarine cables;
however, AC solutions may require more cables than DC solutions. Hence, it is not
directly obvious which solution will have the lowest environmental impact and is there-
fore very much case-dependent.

In addition, it should be mentioned that diesel generators on offshore platforms
combined with a significant diesel storage capacity might cause environmental concerns.

Wind Power in Power Systems 497

22.4 System Solutions for Offshore Wind Farms

The installation of a large offshore wind farms combined with a transmission link built
for the sole purpose of transmitting the power of an offshore wind farm is different from
typical onshore solutions. Onshore, a wind farm typically is connected to a transmission
or distribution system that is at least partly already in place and services a number of
customers. The design of an onshore installation must take this into account.

There are usually no customers connected to offshore wind farms or to the transmis-
sion system to shore. Only at the point of common coupling (PCC) onshore (see Figure 22.2)
do grid codes have to be fulfilled. Furthermore, HVDC transmission solutions
decouple the offshore wind farm from the onshore power system. This condition may
allow the application of different wind turbine design concepts (e.g. based on different
generator technologies or different control approaches). The ultimate goal of new
concepts would then be to find the optimal economic solution for the overall system
of wind turbines and transmission system rather than to focus either on the wind
turbines or on the transmission system individually.

In the following, we will present the most interesting system solutions that are
currently under discussion.

22.4.1 Use of low frequency

Schu¨ tte, Gustavsson and Stro¨ m (2001) suggest the use of a lower AC frequency within
the offshore wind farm. Frequencies lower than 50 or 60 Hz are currently used mainly in
electrified railway systems. The railway systems in Germany, Switzerland, Austria,
Sweden and Norway, for instance, use 16 2/3 Hz at 15 kV, Costa Rica uses 20 Hz and
the USA mainly 25 Hz.

Now, if an HVDC transmission link is chosen for an offshore wind farm, the low AC
frequency would be applied only within the collector system of the offshore wind farm.
If an HVAC transmission solution is used, the low AC frequency can be applied in the
internal wind farm network and for the transmission system to shore. Onshore, a
frequency converter station would be required to convert the low frequency of the
offshore network to the frequency of the onshore network (see also Figure 22.8).

The advantages of a low AC frequency approach lie in two areas. First of all, a low
network frequency would allow a simpler design in the offshore wind turbines. This is

Wind farm

6 kV ~132 kV
50 Hz
~16 2/3 Hz
~
132 kV ~16 2/3 Hz ~ PCC

Figure 22.8 Connection of an offshore wind farm using a low AC frequency. Note: PCC ¼ point
of common coupling
Source: based on Schu¨ tte, Gustavsson and Stro¨ m, 2001.

498 Transmission Systems for Offshore Wind Farms

mainly because of the fact that the aerodynamic rotor of a large wind turbine operates rather
slowly (i.e. the rotor of a 3–5 MW turbine has a maximum revolution of 15 to 20 rpm). A
lower AC frequency would therefore allow a smaller gear ratio for wind turbines with a
gearbox, or a reduction of pole numbers for wind turbines with direct-driven generators,
both consequences resulting in lighter turbines that are thus likely to be cheaper. Second, a
low AC frequency will increase the transmission capacity of HVAC transmission links or
the possible maximum transmission distance, as a capacity charging current is significantly
reduced at lower frequency. The disadvantage of this concept is that the transformer size
will increase significantly and therefore transformers will be more expensive.

As far as we know, the idea of low AC frequency for offshore wind farms is currently
not being pursued further by the industry.

22.4.2 DC solutions based on wind turbines with AC generators

When using wind turbines equipped with a back-to-back (AC/DC/AC) converter it is
theoretically possible to separate the converter into an AC/DC converter installed at the
wind turbine, followed by a DC transmission to shore and a DC/AC converter close to
the PCC. In other words, the DC bridge in the converter is replaced with a (VSC based)
HVDC transmission system. As the AC generator usually operates at 690 V and the
(VSC based) HVDC transmission at around 150 kV, an additional DC/DC transformer
(DC/DC switch mode converter/buck booster) is usually required to reach the required
HVDC voltage. The disadvantage of this approach is that if all wind turbines are
connected to the same DC/DC transformer, they will all work at the same operational
speed. This operational speed can vary over time. Large offshore wind farms, however,
will cover such large areas that only a few turbines will be exposed to the same wind
speed at any given time. The operational speed of most wind turbines will not lead to
optimal aerodynamic efficiency. Therefore wind turbines are connected in clusters of
approximately five turbines to the DC/DC transformer (see Figure 22.9). The five
turbines of a cluster will operate at the same speed, which can vary over time. As the
wind speed can also vary between those five turbines, the overall aerodynamic efficiency
of this solution will still be lower than in the case of individual variable speeds at each
turbine. The idea, however, is that the cost benefits of using clusters are larger than the
drawbacks of the lower aerodynamic efficiency.

Variations of the principal design concept are possible. They are discussed in more
detail elsewhere (Courault, 2001; Lundberg, 2003; Macken, Driesen and Belmans, 2001;
Martander, 2002; Pierik et al., 2001, 2004; Weixing and Boon-Teck, 2002, 2003). The
studies cited also include detailed economic analyses of this concept, but with partly
different conclusions. Some companies find this approach interesting and promising
enough to investigate it further.

22.4.3 DC solutions based on wind turbines with DC generators

Finally, the AC generator can be replaced with a DC generator or an AC generator with
an AC/AC–AC/DC converter (another option would be a DC generator with a gearbox
that allows variable-speed operation, as proposed by Voith Turbo GmbH, Crailsheim,

Wind Power in Power Systems 499

Wind farm

G ~= 15 kV DC DC/AC converter
15/150 kV Onshore

DC/DC 150 kV DC = PCC

G ~= = ~
=

G ~=

G ~= 15 kV DC

G ~= == == ==

G ~=

Figure 22.9 DC wind farm design based on wind turbines with AC generators. Note:
PCC ¼ point of common coupling
Source: based on Martander, 2002.

DCG DCG DCG DC/AC converter
onshore

150 kV DC = PCC

DCG DCG DCG ~

DCG DCG DCG

Figure 22.10 DC wind farm design based on wind turbines with DC generators (DCGs). Note:
PCC ¼ point of common coupling
Source: based on Lundberg, 2003.

Germany). One design option for a wind farm would then be similar to the design shown
in Figure 22.7, but without AC/DC converters close to the wind turbines. Another option
is to connect all wind turbines in series in order to obtain a voltage suitable for transmis-
sion (see Figure 22.10). This option, however, would require a DC/AC–AC/DC converter
for a DC generator to allow each turbine to have an individual variable speed. The
advantage of a series connection of DC wind turbines is that it does not require offshore
substations (for a more detailed discussion of this concept, see Lundberg, 2003).

22.5 Offshore Grid Systems

There is a wide range of ideas under discussion in the area of transmission grids for
offshore wind farms. One idea is that of a large offshore grid, often referred to as the

500 Transmission Systems for Offshore Wind Farms

‘DC Supergrid’, for instance. It assumes that an offshore LCC or VSC based HVDC
transmission network can be built. It would range from Scandinavia in the North of
Europe down to France in the South of Europe, with connections to all countries that lie
inbetween, including the United Kingdom and Ireland. All offshore wind farms in the
area would be connected to this supergrid. Such a system is assumed to be able to handle
redundancy, and it might better solve possible network integration issues as it aggre-
gates wind power production distributed over a large geographical area.

The cost of such an offshore network would be enormous. First studies suggest that it
would be more cost-effective to upgrade the existing onshore networks in order to
incorporate the additional offshore wind power than to build an offshore transmission
network (see also PB Power, 2002). The conversion of the existing onshore AC trans-
mission lines to LCC or VSC based HVDC could be a very interesting and useful
approach to upgrading the onshore network. In this way, existing transmission rights-
of-way and infrastructure could be used. This constitutes a major advantage as it is very
difficult to obtain permission to build new transmission infrastructure projects. Switch-
ing to HVDC could at least double the capacity in comparison with existing AC high-
voltage transmission lines.

Initial studies on local offshore grid structures found that interconnecting wind farms
within smaller geographical areas such as between the British Isles (Watson 2002) or off
the shores of Denmark (Svenson and Olsen, 1999) or the Netherlands seems to be
economically more justified. More detailed studies, however, are certainly necessary
for further evaluation.

22.6 Alternative Transmission Solutions

Finally, what alternatives are there for transporting the energy produced by an offshore
wind farm? Some studies focus on the offshore production of hydrogen. This hydrogen
could then be transported to shore via pipeline or even on large ships. The German
government has already indicated that it might not tax any hydrogen produced by
offshore wind farms. In this way, hydrogen would have to compete with petrol (gaso-
line, 95–98 octane); that is, at a price of around e 1.1 per litre, which is the typical price
in Europe. First studies imply that hydrogen produced by offshore wind farms could be
competitive at this price level. However, at the Third International Workshop on
Transmission Networks for Offshore Wind Farms, Stockholm (2004), Steinberger-
Wilckens emphasised that it is actually more economic to transmit the energy to shore
by electric transmission and to produce the hydrogen onshore (see Chapter 23).

22.7 Conclusions

In summary, it can be said that there are many alternatives for the design of the internal
electric system of a wind farm and of the connection to shore. The technically and
economically appropriate solutions depend very much on the specific case. For opera-
tional and economic reasons, though, the long-standing principle held by offshore
engineers should not be discarded lightly: keep offshore installations as simple as
possible. Many of the commonly discussed solutions for the electric system of offshore