AckermannTh. (ed) Wind Power in Power Systems

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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 us ually 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 dist ance-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 dist ance 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.

Cable Length

Loss

Critical distance

HVAC

HVDC

converter

losses

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

direct current (HVDC)

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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 co nverter 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  40  20 m (width 

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 alrea dy 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 current ly 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 ha s 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; Kir by, 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-ci ted 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 shou ld include the cost for

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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 . Durin g 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 pro ject. 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 sho re. 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 alrea dy

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 onsho re. 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; Holdswort h, Jenkins and Strbac, 2001; Lundberg, 2003; Martander,

2002). As it is rather difficult to obtain good inpu t data, in particular regarding the

various costs but also regarding the converter losses of HVDC solutions, significant

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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 maxi mum length of 200 km are very similar, with the DC cable

probably having a slight cost advantage over the AC cable. However, the investment

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

capacity per system

200 MW at 150 kV

350 MW at 245 kV

1200 MW 350 MW

500 MW announced

Voltage level Up to 245 kV Up to 500 kV Up to 150 kV

Does transmission

capacity depend on

distance?

Yes No No

Total system losses Depends on

distance

2–3 % (plus

requirements for

ancillary services

offshore)

4–6 %

Does it have Black-start

capability?

Yes No Yes

Level of faults High compared

with HVDC

solutions

Low compared

with HVAC

Low compared with

HVAC

Technical capability for

network support

Limited Limited Wide range of

possibilities

Are offshore substations

in operation?

Yes No Planned (2005)

Space requirements for

offshore substations

Small Depends on

capacity;

converter is larger

than VSC

Depends on capacity;

converter is smaller

than LCC but larger

than HVAC

substation

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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 usuall y considered the most economic solution. With a larger dist ance, 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, eithe r 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.

50 100 150 200 250 300

100

200

300

400

500

600

700

800

900

1000

HVAC (245 kV) or

VSC based

HVDC

VSC based HVDC

HVAC

(up to170 kV)

VSC based HVDC or LCC based HVDC

LCC based HVDC

VSC based HVDC

HVAC or

VSC

based

HVDC

Capacity (MW)

Distance (km)

HVAC (245 kV) or

VSC based HVDC

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

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Offshore wind farms between 350 and 600 MW

For a maximum size of 400 MW and a comparatively short dist ance 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 solut ions 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 be nefits.

22.3.4.3 Environmental issues

From an environmental perspective, two main issues are of interest: the magnetic field

of the submarine cables as wel l 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 environmental ly. The impac t 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 gen erators on offshore platforms

combined with a significant diesel storage capacity might cause environmental concerns.

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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 app lied 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 offs hore wind turbines. This is

6 kV

~16

PCC

Wind farm

132 kV~16

132 kV

~50 Hz

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.

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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 win d turbines equipped with a back-to-back (AC/DC/AC) converter it is

theoretically possible to separate the co nverter 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 aerody namic 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 efficie ncy

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,

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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

15 kV DC

PCC

Wind farm

150 kV DC

15/150 kV

DC/DC

15 kV DC

DC/AC converter

Onshore

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.

PCC

150 kV DC

DC/AC converter

onshore

DCG

Figure 22.10 DC wind farm design based on wind turbines with DC generators (DCGs). Note:

PCC ¼point of common coupling

Source: based on Lundberg, 2003.

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‘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 exist ing 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 geographic al 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 furt her 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 W ind Farms, Stockholm (2004), Steinberger-

Wilckens emphasised that it is actually more economic to transmit the energy to shore

by electric transmis sion 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

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