Electrical collector system options for largeoffshore wind farms

G. Quinonez-Varela, G.W. Ault, O. Anaya-Lara and J.R. McDonald

Abstract: The work presented is part of the EC FP6 DOWNVIND project activities, which focuson the requirements and implications of very large offshore wind farms. A comparative analysis ofvarious design options for the electrical collector system of large offshore wind farms is presented,and the advantages and disadvantages in terms of their steady-state performance and economics arediscussed. The case under consideration is that of a proposed 1-GW wind farm located off thenortheast coast of Scotland. The impact on power losses and voltage level changes on the collectorsystem busbars are investigated under various operating conditions. Contingency conditions oflosing one of the cables to the hub end are also explored for collector system designs with redun-dant cables. Finally, the authors introduce an alternative design, based on conceptual ringedarrangements, and its advantages are illustrated and discussed.

1 Introduction

T is estimated that the UK has over 33% of the totalEuropean potential offshore wind resource, particularly farinto the North Sea coastline of Scotland [1]. Hence, offshorewind generation is expected to be a major contributortowards the UK Government’s 2010 target of 10% of elec-tricity from renewables.

Up to now, most offshore wind farms installed worldwidehave been relatively small and electrically simple toconnect. They generally follow existing practices used inonshore installations. However, as the power capacity offuture offshore wind farms increases, the adequacy of thewind farm electrical system design becomes critical as theefficiency, cost, reliability and performance of the overallwind farm will depend to a great extent on the electricalsystem design.

The electrical system concerns all those components thatenable the integration of the wind turbine to the grid supplypoint (e.g. generating units, switchgear, transformers, inter-turbine and transmission cables, power electronic conver-ters etc.).

The overall function of the electrical system is to collectpower from individual wind turbines, to transmit it to shoreand to convert it to the appropriate grid voltage and fre-quency. Electrical collector systems can be designed usingdifferent layouts depending on the wind farm size and thedesired level of collector reliability. Typical configurationscan be:

a) siting all of the wind turbines on a single series circuit(i.e. a string), which has been used in several small offshorewind farms;

# The Institution of Engineering and Technology 2007

doi:10.1049/iet-rpg:20060017

Paper first received 4th December 2006 and in revised form 8th February 2007

The authors are with the Institute for Energy and Environment, University ofStrathclyde, 204 George Street, Glasgow G1 1XW, UK

E-mail: gustavo.quinonez@eee.strath.ac.uk

IET Renew. Power Gener., 2007, 1, (2), pp. 107–114

b) distributing the wind turbines over several strings,allowing the use of lower-rated equipment (which is appli-cable to larger wind farms) andc) a system providing a redundant path for the string powerflow by establishing a looped circuit between the windturbines.

For this investigation, four conceptual designs (alreadyproposed in the open literature), namely radial, single-sidedring, double-sided ring, and star, are analysed and comparedin terms of economics and steady-state performance usingPSS/E. The impact on power losses and voltage levelchanges on the collector system busbars are investigatedunder various operating conditions (taken into accountredundancy issues). A new collector system design is alsoproposed and evaluated.

2 Proposed 1 GW wind farm

The proposed offshore wind farm consists of 200 wind tur-bines rated at 5 MW, with projected layout as illustrated inFig. 1. The seven-turbine arrangement was decided afterinvestigating the effect on geographic layout of the site,the total length and current carrying capacity of cablingand existing turbine cable entry limitations.

The offshore voltage level within the collector system is33 and 132 kV for the offshore hub-to-shore transmissionlink. The step-up transformers at each wind turbine are690 V/33 kV rated at 6.25 MVA, and the transmissiontransformers at the main hub are 33/132 kV rated at 200MVA. The cable runs between wind turbines are 900 mlong, and the cable runs from the turbine nearest the hubto the hub itself are 5 km long. The 33 kV cables in eachstring have tapered ratings (and thus cross sectionalareas), and various standard cable types are used (from95 to 2000 mm2). Copper conductors and XLPE insulationcables allow capacity ratings from 15.4 to 75.2 MVA,which are required for the various collector designsinvestigated.

The 132 kV cables to shore are three-core, 800 mm2,XLPE insulated with a length of 25 km. In order to assess

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different reactive power requirements from the wind tur-bines steady-state models of both, doubly fed induction gen-erators (DFIG) and conventional fixed-speed inductiongenerators (FSIG) were used.

3 Electrical collector system designs

There are various arrangements for wind farm collectorsystems employed in existing offshore wind farms andother conceptual designs have been proposed by the off-shore wind community [2–7]. Taking these variousarrangements, the authors identified four basic designs forthe proposed 1 GW wind farm, as discussed below.

Fig. 2

Fig. 1 Proposed wind farm layout

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3.1 Radial design

The most straightforward arrangement of the collectorsystem in a wind farm is a radial design (Fig. 2a), inwhich a number of wind turbines are connected to asingle cable feeder within a string. The maximum numberof wind turbines on each string feeder is determined bythe capacity of the generators and the maximum rating ofthe subsea cable within the string. This design offers thebenefits of being simple to control and also inexpensivebecause the total cable length is smaller and tapering ofcable capacity away from the hub is possible. The majordrawback of this design is its poor reliability as cable orswitchgear faults at the hub end of the radial string havethe potential to prevent all downstream turbines fromexporting power.

The radial design has been adopted for the 160 MWHorns Rev offshore wind farm in Denmark [8, 9], and ithas been proposed for many other offshore wind farms atthe planning stage, such as the 640 MW Krieger’s Flak[10] and the 420 MW Cape Wind offshore wind farms inSweden and USA, respectively [11]. Radial designs arealso considered in offshore wind farm investigations avail-able in the literature [12–14].

3.2 Single-sided ring design

With some additional cabling, ringed layouts can addresssome of the security of supply issues of the radial designby incorporating a redundant path for the power flowwithin a string. The additional security comes at theexpense of longer cable runs for a given number of wind tur-bines, and higher cable rating requirements throughout thestring circuit.

A single-sided ring design, illustrated in Fig. 2b, requiresan additional cable run from the last wind turbine (i.e. G7)to the hub. This cable must be able to handle the full powerflow of the string (i.e. 35 MW in a 5 MW seven-turbinestring) in the event of a fault in the primary link to the

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hub end (denoted by the open breaker B1 in Fig. 2b). Aninitial feasibility study commissioned by the DOWNVINDconsortium recommended and utilised this design for the1 GW wind farm collector system. This configuration hasbeen also presented in [5].

In this study, the redundant circuits are considered to beavailable for both normal operation and duringcontingencies.

3.3 Double-sided ring design

Fig. 2c illustrates a double-sided ring design [2–5], wherethe last wind turbine in one string is interconnected to thelast wind turbine in the next string (e.g. G7 to G8 asshown in Fig. 2c). If the full output power of the wind tur-bines in one of the strings were to be diverted through theother string, then the cable at the hub end of the latterneeds to be sized for the power output of double thenumber of wind turbines.

Similarly to the previous design, the redundant circuits(i.e. strings interconnection cables) were considered to bein service during normal operation.

Regardless of the benefits of redundancy offered by thesingle and double-sided ringed designs, it is reported thatmost internal power networks of existing offshore windfarms have very little redundancy or none at all [9].However, most of these wind farms are small scale(,100 MW), where the probability of a fault and the associ-ated costs are lower than the costs associated with additionalequipment. In the case of large-scale wind farms(.100 MW), this situation may change, particularly in off-shore installations where the repair downtimes are signifi-cantly longer compared to those onshore.

3.4 Star design

The star design, illustrated in Fig. 2d, has also been citedin [2, 7] as a means to reduce cable ratings and to providea high level of security for the wind farm as a whole(since one cable outage only affects one wind turbines ingeneral). Voltage regulation along the cables betweenwind turbines is also likely to be better in this design.However, there is additional expense in longer diagonalcable runs and some short sections of higher-ratedcabling, but it is not likely that this will be significant(especially in formations of nine turbines). Essentially, themajor cost implication of this arrangement is the morecomplex switchgear requirement at the wind turbine in thecentre of the star (e.g. G5 in Fig. 2d ).

4 Steady-state performance of conceptualdesigns

A number of power system studies, namely power losses,voltage level changes and contingencies, have been con-ducted to evaluate the steady-state performance of thevarious collector system designs using the industry standardpower system simulation package PSS/E.

4.1 Power losses

Key issues associated with power losses are the value of lostenergy and the power factor at which the cables are oper-ated. In addition, further losses within the collectorsystem may be introduced depending on the turbine technol-ogy employed.

The per-turbine MVA losses within a string of the col-lector system were calculated according to (1). The

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formulation weighs the summed full power rating of the tur-bines in a string against the simulated power flows of activeand reactive power (P and Q) arriving from a string to theoffshore main hub

SL ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi(PE � PF)2 þ (QE � QF)2

qNWT

(1)

where SL is the per-turbine MVA losses at a specific windspeed and power factor, PE and PF are the summed andsimulated flow of active power from a string connected tothe offshore main hub, QE and QF are the summed and simu-lated flow of reactive power to a string from to the main huband NWT is the number of turbines in a string.

Fig. 3 illustrates the per-turbine MVA losses against windspeed, at various wind turbine operating power factors, for aradial collector system design. For the 1 GW wind farmwith seven-turbine strings, the maximum losses correspondsto an uncompensated FSIG (typical PF ¼ 0.87 lagging)accounting for 1.38 MVA per string. Conversely, theminimum losses (0.83 MVA losses per string) are attainedwhen the turbine operates at unity power factor.

A comparison of the per-turbine MVA losses againstwind speed (for PF ¼ 0.87 lagging and PF ¼ 1.0) for thefour conceptual collector designs is presented in Fig. 4.

Fig. 3 Per-turbine MVA power losses against wind speed in aradial string for PF ¼ 0.87 and PF ¼ 1

Fig. 4 Per-turbine MVA power losses against wind speed in thefour conceptual collector designs for PF ¼ 0.87 and PF ¼ 1

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It can be seen that the single-sided ring design produces thesmallest amount of losses. Compared with the radial design,the power losses are minimised in the order of 45%.Correspondingly, the losses of the double-sided ring andstar designs are also lower than in a radial design (18 and4%, respectively).

For the power flow simulations, the redundant circuit inthe single-sided ring design was considered to be in continu-ous operation; hence, the calculation of total losses in eachstring considered the losses due to power flows through boththe primary and the redundant circuit. Similarly, in thedouble-sided ring the interlinking cable (between G7 andG8 in Fig. 2) was also assumed to be in continuousoperation.

4.2 Voltage levels

Under normal operation, the voltage levels throughout thecollector system must be within permissible limits asdefined by the grid codes [15]. In general, these arewithin +10% of the rated voltage (e.g. 33 kV). Voltageregulation equipment shall act to adjust the busbar voltagesby continuous modulation of the wind farm reactive power,within its reactive power range, or by tap-changing of thecollector hub transformers. For the conceptual collectordesigns investigated (with single-hub connection), theresults indicate that voltage levels do not vary significantlywith the type of design. The maximum discrepancy amongvoltage levels is less than 0.3%.

4.3 Redundancy

The purpose of redundancy in a wind farm collector systemis to keep connected as many wind turbines as possibleduring a contingency. Experience has demonstrated thatrepairing times required in offshore wind farms are signifi-cantly longer than those in onshore installations. Forexample, according to [5], the estimated repair time of afailed subsea power cable may vary from 720 h duringsummer to 2160 h in the winter.

Fig. 5 illustrates the per-turbine MVA power lossesagainst wind speed for PF ¼ 0.87 lagging and PF ¼ 1 insingle- and double-sided ring collector designs operatingunder a contingency. The contingency under considerationis the sudden disconnection of the primary cable due to adisturbance (this is characterised by opening the circuit

Fig. 5 MVA power losses against wind speed for conceptualringed collector system designs (during contingency), withPF ¼ 0.87 and PF ¼ 1.0

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breaker B1 in Fig. 2b and c). Under this condition, thelosses in a double-sided ring design increase by a multipleof approximately four times in comparison with the losseswhen both cable links to the hub end are operating(i.e. circuit breaker B1 closed, Fig. 2c). In the case of asingle-sided ring collector system, the losses increaseapproximately by a factor of 3 (Fig. 5).

During the contingency, a double-sided ring forms a14-turbine string whilst a single-sided ring remains a seven-turbine string. None of these cases exceeds the +10% per-missible voltage limits. For example, the maximum voltagelevel difference between normal and contingency operationis 2 and 1.7% for a single- and double-sided ring collectorsystem, respectively.

5 Economic assessment of conceptual designs

When compared with onshore wind farms, the investmentcosts and final cost of electricity produced in an offshoreinstallation are higher. The additional expenditure of sub-marine foundations, turbine installation, subsea plant andgrid connection constitute the major differences. Forexample, it is stated in the open literature that windturbine cost in an onshore wind farm represents approxi-mately 75% of total investment, whilst for an offshorewind farm this percentage is only about 30–50%.

Typically, the collector system has been seen as a minorcontributor to the total investment cost of wind farms,especially in small-scale installations with radial collectors.However, new designs for future large-scale offshore instal-lations can significantly change this situation. Presently,there is a debate among the offshore wind communityregarding the value of redundancy required in the collectorsystem to maximise the energy yield, and the impact it mayimpose on the capital costs of the wind farm. Clearly, themajor concern is related to the cost of supplementarysubsea cabling, either in terms of extra length or higherratings.

The economic assessment carried out in this paper coversthe following components: Wind turbines, 33 kV switch-gear (e.g. remote operated isolators, generator incomers,hub transformer feeders, hub bus tie and installations),33 kV cables (cross-sections of 95, 185, 500, 630 and2000 mm2 dependent of the design) and cable trenching,33/132 transformer and the full costs of installation of allequipment. Costs were available from manufacturers andconsultant within the DOWNVIND consortium, andadditional cable cost information was obtained from [6].

The capital costs of the sub-sea cable system, overall col-lector system and the wind farm were estimated accordingto (2)–(4)

CCB ¼ CCA(u)NCAlCA (2)

CCS ¼ (CSWNSW) þ (CTXNTX) þ CCB (3)

CWF ¼ (CWTNWT) þ CCS (4)

where CCB, CCS and CWF are the total capital cost of thecable system, overall collector system and the wind farmrespectively, CCA(u) is the cost of sub-sea cable (i.e. costper km and installation) as a function of its cross sectionalarea u, NCA and lCA are the total number and length ofcable sections for each cable type (i.e. of an specific u),CSW and CTX are the cost of switchgear components andtransformers (including unit and installation cost), respect-ively, NSW and NTX are total number of switchgear andtransformers units, CWT is the unit cost of wind turbinesand NWT is total number of turbines in the wind farm.

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Fig. 6 illustrates a comparison of the cost per installedkW for the four conceptual collector designs addressed inthis paper. The results show that a significant economic dis-parity exists among ringed and non-ringed arrangements.The double-sided ring array has the highest investmentcost accounting for £175.3 per installed kW, which rep-resents an expenditure in excess of two times the cost of atypical radial collector system (£83.3 per installed kW).For a single-sided ring design, the cost is £131.4 perinstalled kW, which is 58% more expensive than that of atypical radial collector. In the case of a star design, thecost of £81.3 per installed kW represents a cost reductionof around 3% in comparison with a radial arrangement.

Fig. 6 also compares the explicit cost of subsea cables,where the difference among the four conceptual designs isevident. It can be seen that double-sided and single-sideddesigns have a cost of £120.2 and £74.2 per installed kW,respectively. The cable cost in the radial and star designsis £30.4 and £24.8 per installed kW, respectively.Therefore the cable investment in a double-sided ring isapproximately four times the cost of a typical radial collec-tor, whereas for a single-sided ring design this investment isabout 2.5 times that of a radial design.

Fig. 7 illustrates a comparison of the conceptual collec-tors and their impact on the total wind farm investment(taking into account offshore transmission system andwind turbines). It can be seen that the implementationof a ringed collector increments the total wind farm

Fig. 6 Capital cost of the four conceptual collector designs

Fig. 7 Impact on the total wind farm capital costs of the fourconceptual collector designs

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capital cost per installed kW by approximately 11% for adouble-sided design, and 6% for a single-sided, compareda typical radial array. With the option of a star design, theinvestment cost remains practically the same as a typicalradial design (the difference is approximately 0.2%).

6 Proposed electrical collector system design

According to the steady-state results presented in Section 4,a single-sided ring arrangement can be propose as the betteroption for a large offshore collector system as it achievesfewer losses in both, normal and contingency operation; italso offers greater security than a typical radial system.However, the results of the economic assessment presentedin Section 5 have shown that the cost of the associatedredundant circuit required by this design is very likely tobecome an economic barrier, particularly when comparedwith the cheaper option of a typical radial design.

Bearing this in mind, the authors investigated an alterna-tive design in order to minimise the overall cost of theringed collector system whilst maintaining a goodsteady-state performance when compared with a typicalradial design. The proposed collector system, illustrated inFig. 8, has been derived from the two conceptual ringedarrangements. It consists of four groups of seven-turbinestrings linked together via a cable connection at each ofthe outmost wind turbines in the strings.

The four-string arrangement was selected to maintainappropriate security levels and consistency (and uniformityamong the strings) with the number of seven-turbine strings(i.e. forming 7 four-string groups out of the total 28 strings)and the geographic layout of the wind farm (Fig. 1).

One redundant cable circuit returning from one of theoutmost wind turbines to the main hub is also incorporated.

The redundant circuit is designed to potentially deliverthe full power output of a failing string within the four-string arrangement (i.e. carrying 35 MW). The probabilityof two strings failing simultaneously was considered suffi-ciently small to avoid an uprated-cable capacity and theadditional cost it implies.

6.1 Steady-state comparison

Fig. 9 compares the per-turbine MVA losses of the pro-posed design with those of the conceptual radial and single-sided designs. The results show that power losses in theproposed design, at nominal wind speed, are approximately18% lower than in a typical radial array.

The cable electrical losses were calculated consideringmultidirectional nature of the power flows. It must be

Fig. 8 Proposed design for a large offshore wind farm collectorsystem

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noted that these losses are still higher than in a single-sidedring array.

Fig. 10 compares the per-turbine MVA power losses ofthe proposed design and the two conceptual ringed arraysduring a contingency (i.e. losing the primary link to thehub). The comparison shows that power losses in thealternative arrangement are significantly lower than thoseattained in the two conceptual ringed designs.

6.2 Economic comparison

The capital cost of the proposed collector system design incomparison with a typical radial and a conceptual single-sided ring is shown in Fig. 11.

The results illustrate that the capital cost of the proposeddesign is significantly lower than the conceptual ringedarrangement and of competitive cost in comparison to atypical radial design.

7 Quantification of energy generation and losses

An existing dataset comprising 20-year historic wind speedrecorded at Wick, Scotland (40 km north of the 1 GW off-shore site) was used to assess the energy yield of the windfarm and the losses associated with the various collectordesigns considered in this investigation.

Fig. 9 MVA power losses against wind speed with PF ¼ 0.87and PF ¼ 1.0 including the proposed design

Fig. 10 MVA power losses against wind speed with PF ¼ 0.87and PF ¼ 1.0 including the proposed design (during contingency)

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The dataset provides monthly distributions of wind speedover the whole 20-year period, i.e. the hourly occurrence ofwind within specific wind speed bands.

As the original data was logged at 10 m height, the windspeeds had to be corrected to 90 m (typical hub-height of the5 MW wind turbines) using established techniques [16], asillustrated in (5)

U ¼ Uo

h

ho

� �a(5)

where U is the corrected wind speed at height h, Uo is themeasured wind speed at height ho and a is the coefficientof roughness.

The annual mean wind speed calculated at 90 m height is7.7 m/s, with the highest mean of 9.4 m/s during the monthof February and the lowest mean of 5.9 m/s during themonth of August (Fig. 12)

The energy yield of the wind farm is calculated by com-bining the ‘binned’ annual distribution of wind speeds withthe turbine power curve, as expressed in (2)

ET ¼Xni¼1

[H(Un)P(Un)NWT] (6)

where H(Un) is the number of hours in wind speed bin n(bins of 0.5 m/s were used in this assessment) during theyear, P(Un) is the turbine power output at the wind speedbin and NWT is the total number of turbines in the wind farm.

Fig. 12 Monthly mean wind speeds from dataset (90 m height)and calculated wind farm energy yield

Fig. 11 Capital cost of the proposed collector system design

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Using the wind speed data and the power curve of the 5MW wind turbine, the energy produced by the proposed 1GW wind farm (200 turbines) was calculated.

For example, in the worst-case power factor scenarioconsidered in this paper (PF ¼ 0.87), an average annualenergy production of 3368 � 103 MVAh (with an activepower component of 2930 GWh) is produced (Fig. 12)without taking into account power losses.

The total energy losses during normal operation were cal-culated and are shown in Fig. 13. The characteristics ofMVA power losses against wind speed for both conceptualdesigns and the proposed design (illustrated in Figs. 3, 4 and9) were approximated via exponential growth models, (7),for each operating power factor considered in this study.

EL ¼Xni¼1

A expUnBð Þ

� �NWT

h i(7)

where EL is the total power losses in MVA, and A and B arethe corresponding constants of the exponential growthfitting function. It should be mentioned that there is aspecific power loss function for each operating powerfactor analysed in Section 4.1. The power losses at windspeeds above 13 m/s were considered constant since theturbine power is regulated to rated power, i.e. 5 MW.

Table 1 summarises the annual MVA energy losses forthe various collector designs and quantifies the percentageof the expected annual energy production of the offshorewind farm for two cases of operating power factor, theworst-case of PF ¼ 0.87 lagging and a typical case ofPF ¼ 0.95 lagging.

These results show that both ringed designs produce lessenergy losses in comparison with radial and star designs.For example, the discrepancy between a radial and a single-sided ring collector system is 41.1 � 103 MVAh (atPF ¼ 0.87), whilst for a double-sided ring, the differenceis 18.2 � 103 MVAh (at PF ¼ 0.87). It can also be observedthat the losses in the proposed design are also lower thanthose in the radial, double-sided ring and star designs.

It must be highlighted that these figures correspond to thecollector system only; therefore the energy losses from the200 MVA step-up transformers in the main hub must beincluded, which according to the authors’ analysis remainconstant at 0.3% of the total energy production for all col-lector designs. Furthermore, additional energy losses mustbe added due to the subsea transmission from the offshore

Fig. 13 Monthly energy losses in wind turbine strings withdifferent collector systems

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main hub to shore, which depends on the type of trans-mission solution (i.e. HVAC or HVDC) [17].

8 Conclusions

This paper has presented a comparative analysis of variousdesign options of electrical collector systems for large off-shore wind farms in terms of their steady-state performanceand economics. It focused on the case of a proposed 1 GWwind farm development within Scotland (EC FP6DOWNVIND project).

The results have demonstrated that the electrical inter-connection arrangement of the wind turbines markedlyimpacts the amount of losses within the collector system.Ringed designs provided fewer power losses duringnormal operation in comparison withradial and stardesigns. In contrast to losses, the voltage levels do notvary significantly with the type of collector design.

The economic assessment illustrated that althoughringed-designs are superior in terms of steady-stateperformance, they involve a higher investment, which isvery likely to be a barrier for the deployment of thesedesigns.

The authors have proposed an alternative collectorsystem design. Apart from the intrinsic advantage of provid-ing redundancy (compared to a radial design), the analysisdemonstrated that this alternative has fewer losses thanmost of the conceptual designs under normal operation; italso has a superior performance than both conceptualringed options during a contingency. It is less expensivethan both conceptual ringed designs with competitivecapital costs in comparison with a typical radial array(without redundancy). Based on these advantages, it isbelieved that the proposed design may be an appropriatetechnical and cost effective solution for the collectorsystem of large offshore wind farms.

9 References

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Table 1: Summary of annual energy losses in collectorsystems

Collector

system

design

Total annual

losses, MVAh � 103Percentage of

annual production, %

PF ¼ 0.87

(lagging)

PF ¼ 0.95

(lagging)

PF ¼ 0.87

(lagging)

PF ¼ 0.95

(lagging)

radial 90.7 63.1 2.7 1.9

single-sided ring 49.6 34.1 1.5 1

double-sided ring 72.5 51.4 2.6 1.9

star 86.0 63.9 2.2 1.5

proposed 71.7 48.1 2.1 1.4

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