Cost Analysis and Comparison of HVAC, LFAC and HVDC for Offshore Wind Power Connection

Transcription

1 1 Cost Analysis and Comparison of, and for Wind Power Connetion X. Xiang, M. M. C. Merlin,. C. Green Imperial College London, UK, {x.xiang14, mihael.merlin7, Keywords: offshore wind power onnetion, low-frequeny AC, frational frequeny AC, Abstrat Low frequeny AC () has been proposed as a means to avoid some of the large onverter station osts of high voltage DC () while delivering some of the benefits in terms of better line or able utilization and its tehnial feasibility has been established. It is said to offer a lower osts than or onventional high voltage AC () for a range of intermediate distanes, with beoming heaper over long distanes. However, the basis for identifying the distane range and extent of ost saving has not been established. Here, ost estimate methodologies are extended for. A diffiulty is the absene of ommerial shemes that an provide pratial examples of osts. In this paper, osts are broken down into onstituent terms and estimates are made from the most similar equipment from other shemes. he apaity limits and power losses assoiated with subsea ables are analyzed for low frequeny ases. For a given power transfer and for eah distane, a hoie of operating voltage, able size and number of parallel iruits is made in order to find the lowest route ost. his yields ost as a funtion of distane that is a non-linear and disontinuous funtion. he ost urves for are ompared with and options. he results for urrent ost estimates show that has a range of route length over whih it is the lowest ost option and but this range narrows and eventually eases to exist for higher power transfer ratings. 1 Introdution Wind energy is regarded as one of the most important renewable soures around the world [1]-[]. In reent years, enormous effort has been invested to progress the tehnologies to onnet offshore wind power beause of the better wind energy potential [3]-[4]. It is well known that and systems, illustrated in Fig. 1 and Fig., are the ommerialized solutions for high power transmission onnetions to generators in offshore areas and they ompete against eah other on ost [5]-[7]. he overall ost of and systems an be separated into the terminal ost and route ost. has the advantage of relatively inexpensive terminal ost whereas has expensive power onverter stations. On the other hand, the route ost in rises muh more sharply with distane than that in beause of the different transmission apability limits. Over short distanes is favored for its lower terminal osts but beyond some threshold distane, the advantage of lower route osts favors. he ross-over point of and osts is reported to be in the region of 6 km for subsea able transmission systems [8][9]. Generator Generator Generator Step-up ransformer 5/6Hz Subsea Cable Fig. 1 Struture of transmission system Step-up ransformer AC/DC Fig. Struture of transmission system LF Step-up ransformer Step-down ransformer Fig. 3 Struture of transmission system It has been noted that low frequeny AC (), also alled frational frequeny transmission system (FFS), suffers less from able shunt suseptane effets than standard AC and so makes more ost effetive use of the able. he transmission frequeny in is usually set as 16.7Hz/Hz, one third of the standard frequeny in (5Hz/6Hz) and above (Hz). Further, requires only one power onverter for frequeny onversion if being used to onnet a generator that is isolated from the main system, Fig. 3. he tehnial feasibility of has been omprehensively analyzed [1]-[15] and a laboratory prototype of an system has also been suessfully demonstrated [16][17]. It is postulated that for some range of distanes should be lower ost than either or [1][13] beause one power onverter at one end will give terminal osts higher than but lower than and route osts in will be rise more steeply than but shallower than. Fig. 4 illustrates ost against distane for for three ases, all of whih use terminal osts and unit distane osts between those of and. In ases 1 and, the ost of ross the ost of before rossing the ost of and so has a range for whih it is heapest. However, in ase 3, the ost of rosses the ost of first and there is no distane for whih it is the preferred hoie. So, knowing that the terminal and unit distane osts of are between those of and is not suffiient to establish that it is ever the lowest ost option, let alone establishing the distane range. Cost analysis for has reeived some attention [1][13][18]- [] but not to the degree need. Some ost analysis and omparison for offshore subsea able transmission systems have been presented [1][19] based on the simple and not wellsupported assumption that both the terminal ost and route ost per unit distane of are around halfway between the osts of and. Under that assumption, the osts of follow ase in Fig. 4 and based on typial osts of Hz Subsea Cable 16.7/Hz Subsea Cable DC/AC AC/AC Step-down ransformer Step-down ransformer Grid Grid Grid

2 and, the ost-effetive range for able-based in an offshore wind appliation would be about 3km- 15km. However, this simple approximation should be questioned sine there are reasons to believe that the offshore platform for an LF transformer and an onshore AC/AC HV power onverter would be greater than this mid-point. Meanwhile, although a able operated at 16.7Hz requires less reative urrent than at 5 Hz, the AC voltage waveform is subjet to a peak/effetive ratio underutilization ompared to DC and so route osts are unlikely to be the midpoint either. Further, a simple onstant ost per unit distane is known to be only an approximation of ost that is loser to being quadrati. Cost (5/6Hz) 3 route terminal route terminal (HZ) route 1 terminal Distane Fig. 4 hree basi possibilities Fig. 5 Relationships between different for ost kinds of osts his paper sets out to analyze the osts of in enough detail to identify the ross-over points for its osts with and and to identify the ost-effetive range of from the best available data. he ost estimation algorithm ompares, for eah distane, different hoies of operating voltage and hoies of numbers of parallel ables in order to meet the speified power transfer at minimum ost for that tehnology. his yields osts that are a non-linear and disontinuous funtion of distane. In Setion, the major fators in ost analysis are outlined in general, and the osteffetive range omparison for able-based offshore wind farms (COWF) onnetion is disussed in detail in Setion 3, whih is the main body of this paper. he onlusions are drawn in Setion 4, and future work is illustrated in the last Setion. Major Fators in Cost Analysis A omprehensive ost analysis for transmission routes is omplex to ondut in detail in analytial form. It needs to take many fators into onsideration, suh as the apital ost, maintenane ost, power losses, equipment reliability and so on. For a new approah like, this analysis is more diffiult to omplete beause there are no ommerial projets that have established design limits and yielded ost data. o make this analysis feasible, some minor fators need be negleted and soures of ost data from omparable individual equipment items should be sought. For and systems, several ost estimation methods have been reported [1]-[5] that an serve as a starting point for this analysis. Cost data suitable for use in these methods have been reported for some ommerial transmission projets [8][9][6]-[8]. It is ommon to separate out the apital ost () and the apitalized ost of power losses (LC) as the major fators within the overall system ost. able I lists the osts of eah of, and under the headings of and LC. hese items will be analyzed in detail in next Setions. able I Main apital ost and power losses ost in, and step-up transformer plant and platform (ompound). Cables and ompensation. step-down transformer plant and ompound. LF step-up transformer plant and platform (ompound). Cables and ompensation. AC/AC onverter station plant inluding valves, transformers and filters. onverter station plant and platform (ompound) inluding valves, transformers and filters. Cables. onverter station plant inluding valves, transformers and filters. 3 Cost-effetive Range Analysis LC transformer losses. Cables losses. transformer losses. LF transformer losses Cables losses. AC/AC onverter losses. AC/DC onverter losses. Cables losses. DC/AC onverter losses. For subsea able based offshore wind farms (COWF) onnetion, voltage-soured onverter (VSC-) is hosen as the DC option in this paper for analysis and omparison. he overall ost (C) of a transmission system an be separated into and LC but also into the terminal ost (C) whih is independent of distane and route ost (RC) whih is a funtion of distane. Fig. 5 illustrates this deomposition and a further deomposition into terminal apital ost (), terminal losses ost (LC), route apital ost (R) and route power losses ost (). Interpreting able I osts for COWF in terms of Fig. 5 yields that the onsists of the offshore platform and plant ost () and onshore plant ost () and the R onsists of the able ost (CBC) and ompensation ost (). he ost analysis of eah part in Fig. 5 will be introdued in the following parts respetively. 3.1 Cost Analysis in and VSC- he estimations for and, (1)-(4), follow the empirial formulas in ommerial projets report [6], and the relevant variable desription and assumptions are summarized in Appendix A.1. = FC + [1 + d( n )] ( f + p ) n SS = S (1).7513 =.61S () = FC + [1 + d( nc )] C nc SSC = S (3) =.8148S (4) o alulate the able ost and ompensation ost in, able transmission apability needs be analyzed first. In subsea able transmission, parallel apaitive suseptane is the major parameter limiting ative power transmission due to the lose proximity of sea water. he reative power, Q C, produed by apaitive harging urrent, an be expressed as = Vn π fnc l (5) o provide more spae for ative power urrent and therefore expand able transmission apability, an eonomi option is to distribute ompensation power evenly at both ends of ables. With this onfiguration, able transmission apability, P C, ould be given by (6), and the ompensation ost in,, an be indiated as (7).

3 3 1 PC = SC Qoff = SC ( ) = ( 3 VnIssn) ( Vn π fnc l) (6) = Qoff OFC + Qon ONC =. Vn π fnc l (7) he relevant parameters of some ommon ables in are listed in Appendix able I, and the able ost in, CBC, an be alulated by CBC = t l n (8) he ables ost in VSC- option have substantial advantages over AC solutions ( and ), sine DC voltage waveform is not subjet to a peak/effetive ratio underutilization. For the same able physial onditions, the transmission apability ratio of DC ables to AC ables an be given by Pmax DC VndInd VnaIna = = = 1 (9) Pmax AC 3 VnaIna pf 3 VnaIna pf 3 In (9), it an be seen that, for a given power transfer, DC option need only two polar ables while AC solutions need three. he able ost in VSC- would be muh smaller than that in AC options. If the reative power and skin effet are taken into alulation, this advantage ould be more distint. he speifi parameters of some ommon ables are listed in Appendix able II, and (8) an be also used to alulate the able ost in VSC- system. With (1)-(4) (7) and (8), the main apital ost of and VSC- an be expressed as (1) and (11) = CBC.7513 = 5+.45S +.61S +. Vn π fnc l + t l n (1) = + + CBC = 5+.11S S + t l n (11) he power losses ost (LC) in transmission system is an aumulative ost related to operation time, op, and energy prie, E op. he terminal power losses ost, LC, and route power losses ost,, in are given by (1)-(14) [6] LC = S pf (1 η ) δ E (1) off offt op op S pf ηofft = 3 r l n δ E n 3V n op op (13) S pf ηofft LCon = [ S pf ηofft 3 r l n ] (1 ηont ) op δ E (14) op n 3V n Under the value assumptions in Appendix A.1, the power losses ost of, LC, an be simplified as (15). And with some adjustments for effiieny parameters, the power losses ost of VSC- are given by (16). LC = LC + + LC =.911S off on.994s.994s r l n [.994 S r l n] (15) LC = LC + + LC =.61S off on.988s.988s r l n [.988 S r l n] (16) he overall ost of and VSC- an be obtained as (17) and (18).7513 C = + LC = 5+.45S +.61S + t l n.994s +. V π f C l +.911S r l n n n.994s [.994 S r l n] (17) C = 5+.11S S + t l n +.61S S.988S r l n [.988 S r l n] (18) 3. Cost Analysis in Sine there is still no ommerial transmission to date, the offshore platform and plant ost,, onshore plant ost,, power losses ost, LC, and able parameters in need be made from the most similar equipment from and VSC- atual projets [1][4][6]. First of all, the LF step-up transformers, theoretially, should be three times larger of that in. But onsidering that the performane of many operation fators, suh as heat dissipation, voltage isolation, system reliability and so on, ould be improved with equipment volume inreasing, the pratial weight, volume and ost of this LF transformer ould be dereased to about twie of the standard transformers at best ondition [1][18][][9], where the platform fixed ost keeps the same with situation, and the platform and plant variable osts inrease to twie. hus, the offshore platform and plant ost in,, an be estimated by = FC + [1 + d( n )] ( f + p ) n SS = 5 +.9S (19) Seondly, the topologies of onshore AC/AC onverter station in ould have several options, suh as lyloonverter, bak to bak Modular Multilevel Converter (MMC), matrix onverter and so on [13][3]-[33], but all of these topologies need more ative swithes and reative omponents than their one DC/AC ounterpart [13][33]. hus, the lowest onshore plant ost and its power losses ost, would be not less than a L- onshore option. Its apital ost is given by () [1] L =.596S () Further, aording to (5), it an be seen that the reative power produed by harging urrent is proportional to transmission frequeny. As a result, the required offshore and onshore ompensation power in will be theoretially one third of that in, and (7) an be still used for the ompensation ost alulation in. hanks to the one third harging urrent and lower skin effet, the ables in transmission ould have stronger transmission apability than that in. he unit prie of able in is assumed to be same with. Based on the simulation and experimental data [1][16][18], the eletrial parameters of some ommon ables in are presented in Appendix able III, and (8) an also be used to estimate the able ost in. As for transformer power losses ost, despite the fat that the volume of LF transformer would be at least two times larger than standard one, the ore losses per unit ould be redued thanks to the one third frequeny. Based on the analysis in [34], the effiieny of LF transformer would be very lose to standard transformer. hus, the power losses ost of, LC, an be simplified as (1) with some effiient assumptions in Appendix A.1.

4 4.994S LC =.911S r l n S [.994 S r l n] (1) Combining (7) (8) and (19)-(1), the overall ost of, C, an be written as () C = 5 +.9S +.596S +. V π f C l + t l n n n.994s +.911S r l n [.994S.994S r l n ] 3.3 Case Study for Lower Power Rating () Aording to (6), the able transmission apability in AC systems would slump nonlinearly with transmission distane inreasing. With the parameters in Appendix able I and able III, Fig. 6 and Fig. 7 are skethed to show the detailed transmission apability of some ommon ables in and. Firstly, it is easy to see that has a lear advantage over due to the one third transmission frequeny and lower skin effet. More importantly, it an be found that, for AC systems, as long as the required ative transmission power is set, different transmission distane requirements ould result in different optimal able hoies in order to meet the speified power transfer at minimum ost. As for VSC-, beause DC ables would not suffer from the harging urrent, one unitary able hoie ould satisfy all distane analysis. 1 Cable Capability Pmax (MW) kV, mm with ompensation 4kV, mm without ompensation kv, 1mm with ompensation kv, 1mm without ompensation 13kV, 1mm with ompensation 13kV, 1mm without ompensation kV, mm with ompensation 4kV, mm without ompensation kv, 1mm with ompensation kv, 1mm without ompensation 13kV, 1mm with ompensation 13kV, 1mm without ompensation Fig. 6 ransmission apability Fig. 7 ransmission apability of some ommon ables in of some ommon ables in.6 GW is seleted for the lower power rating ase study. After omparing different able hoies of operating voltage and parallel numbers in Appendix able I, the minimum ost options for eah distane in are listed in able II. For this power rating ase study, one unitary able hoie in ould satisfy minimum ost for all distane (-4km) analysis as VSC-. he able hoies in and VSC- are given in able III and able IV. able II Cable hoies in for.6 GW transmission Distane l (km) Voltage V n (kv) (mm ) Capability per set (MW) Number of sets n Cable Capability Pmax (MW) able III Cable hoie in for.6 GW transmission Distane l Voltage V n Capability per set Number of (km) (kv) (mm ) (MW) sets n able IV Cable hoie in VSC- for.6 GW transmission Distane l Voltage V n Capability per set Number of (km) (kv) (mm ) (MW) sets n -4 ± Aording to (17) and able II, Fig. 8 and Fig. 9 are obtained to show the.6 GW ost analysis. here are 8 step points in graphs beause of the different optimal able hoies for different distane analysis, whih leads to an approximately quadrati overall ost as a funtion of distane. Similarly, with (18) and able IV, () and able III, the ost analysis for VSC- and is presented in Fig. 1 and Fig. 11 respetively. he overall osts of both and keep linear relationships with distane in this power rating, but the step points will ome out in as long as the power rating and distane keep rising LC C =.119l +.813l+59.41(M ) R= Fig. 8.6 GW ost analysis Fig. 1.6 GW ost analysis LC VSC- C =.9l (M ) Fig. 9.6 GW step points Fig GW ost analysis Fig. 1.6 GW omparison Fig GW detailed omparison With the ost analysis in Fig. 8-Fig. 11, the omparison result is illustrated in Fig. 1. Zooming in the area from 67km to 115km, the detailed result is given in Fig. 13. First of all, it is shown that there exists a ost-effetive range, about 3km, for in the intermediate distane for this.6 GW omparison. Further, it is also lear to see that the terminal ost is higher than but lower than VSC-, and the route ost of is steeper than VSC- but smoother than, whih orresponds to the general expetations in Setion I. Meanwhile, aording to Fig. 1, it an be found that C C.38M /km.34m /km.9m /km 1.78M /km LC 46.1M.83M /km 3.53M /km 3.37M /km 3.76M /km C =1.51l+17.8(M ) VSC

5 5 in this power rating, both the terminal ost and route ost per unit distane of approximately lie halfway between and VSC- when distane is shorter than 1km. he ross-over points of with and VSC- are 8km and 17km respetively, and VSC- and break-even point is 87km. he ost omparison result in this lower power ase study is losest to ase in Fig Case Study for Higher Power Rating LC C =.36l l+165.8(M ) R= Fig GW ost analysis Fig GW step points Fig GW ost analysis Fig GW ost analysis LC VSC- C =1.36l (M ) Fig GW omparison Fig GW detailed omparison In the higher power rating ase study, 1.4 GW is hosen for detailed analysis. he individual ost analysis is skethed in Fig. 14-Fig. 17. It an be seen that the overall ost of still shows a nearly quadrati relationship with distane, and the first step point in appears when distane grows to km, making its overall ost beome a non-linear funtion of distane as. With the results in Fig. 14-Fig. 17, Fig. 18 and Fig. 19 are obtained to illustrate the omparison results among these three options. It is shown that there is already no ost-effetive range for in this 1.4 GW omparison. he terminal ost of is loser to VSC- but the route ost is more inlined to. he starting point of ost is near VSC-, but the ost rises sharply as with distane growing. Aording to Fig. 14- Fig. 17, this situation in an be explained by the expensive high power AC/AC onverter station and LF transformer, whih almost approah the terminal ost of VSC-, and the high ost AC able struture is also just slightly lower than. In the higher C C 4.7M /km 4.53M /km LC 5.3M /km 5.6M /km 5.51M /km 5.3M /km 99.49M 6.47M /km 8.44M /km 7.45M /km 8.73M /km C1 =3.8l+44.88(M ) VSC- C =3.35l+44.88(M ) power and higher voltage omparison, DC ables shows more deisive advantages over AC options both in able ost and transmission apability. he break-even point of and VSC- is about 7km. After this distane, DC seems to be the only option in these three options. Case 3 in Fig. 4 is the best hypothesis for the ost omparison result in this higher power rating ase study. 4 Conlusion In order to identify the ross-over points of osts with and and to explore the extent of osteffetive ranges, some ost estimate methodologies are extended in this paper to make the ost analysis and omparison for these three options more feasible and reliable. he ost estimates for eah onstituent terms in are from the most similar equipment from and VSC- ommerial projets, and thus it inreases the reliability and auray of the final omparison results. Meanwhile, the ost estimation algorithm ompares all ommon able hoies in order to guarantee the minimum ost for eah distane analysis. A number of ost analysis and omparison results are presented in ase studies. It indiates that has some ost-effetive ranges between and thanks to the inherent benefits, but these ranges gradually narrows and eventually eases to exist with transmission power inreasing, sine DC system ould demonstrate more lear advantage over AC options in the higher power higher voltage transmission. In addition, the results also show that, with different ondutor hoies for different range analysis, the overall ost of AC system is developed as a quadrati relationship with distane rather than the simple linear one. Appendix A.1 Assumptions List Variable Desription Assumption Value FC offshore transformer platform fixed ost 5M d Cost fator for a different number ( ) of transformers, onverters. n, n C Number of transformers, onverters per offshore platform f offshore transformer platform variable ost. M /MVA p offshore transformer plant variable ost.5 M /MVA f n ransmission frequeny l,l o Cable, OHL transmission distane S S single transformer power rating S ransmission power rating Q off, Q on, onshore ompensation power n,n o Number of able, OHL parallel iruits OFC ompensation ost per unit.5 M /MVAr ONC ompensation ost per unit.15m /MVAr V nd, I nd DC able nominal voltage, urrent V na, I na AC able nominal voltage, urrent pf Power fator 1 η offt, η ont Effiieny of offshore, onshore transformers 99.4% op otal operation hours δ Loss load fator.31 E op Energy prie per unit 5 /MWh FC VSC- offshore onverter station platform fixed ost 5 M C VSC- offshore onverter station variable ost.11 M /MVA S SC VSC- single onverter power rating η offr Effiieny of VSC- offshore retifier station 98.8% η oni Effiieny of VSC- onshore inverter station 98.19% η onl Effiieny of L- onshore inverter station 99.1% η onaa Effiieny of onshore AC/AC onverter station 99.1%

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