Madariaga et al. from the Offshore RenewableEnergy (ORE) Catapult give an overview of existingcost literature from former studies and include inputsfrom industry consultation to provide cost data for allcrucial components. The resulting costs are comparedto realised projects, namelyBorWin2andSylWin1.ETYS15 is the most recent version of theETYS this review refers to. FollowingETYS13as theearliest version containing a detailed cost compilationfor HVDC technology,ETYS15comprises technologycost data based on updated project information.TorbaghanfromtheDelftUniversityofTechnology includes a cost model for the offshoregrid investigations in his doctoral thesis.Assessingoffshore grid investments with a focus on optimisationtechniques, the cost model uses only one parameterto cover all costs incurred by offshore transmissioninvestments.3. Cost parameter set processingThere are two motivations why a uniform cost modelis indispensable in this context:•To apply the cost parameter sets in transmissionexpansion studies•To allow for comparison and evaluation of thedifferent cost parameter setsHence, such a uniform cost model is introduced here,which allows the collected cost parameter sets to beconverted into a common format, and the calculation ofan average parameter set.3.1. Cost modelTransmission grid expansion problems are usuallyformulated by a set of nodes representing connectionpoints, generators, and load centres, as well as a setof branches representing potential transmission linesbetween these nodes. The branch length is calculatedbased on the known bee-line distance between thenodes, either by including a simple markup or with adetailed line route definition. An optimisation algorithmidentifies the connections to invest in and computes thecorresponding transmission capacities, based on a costmodel function assigning investment cost parameters topotential nodes and branches.3
A linear cost model yields significant benefits fortransmission expansion planning problems and theoptimisation algorithms solving them, as computationtime and convergence face severe challenges whencomplex cost models are applied.For this reason,the cost model used in this article is linear.Thiskind of linear model provides an approximation oftheinvestmentcostassociatedwithoffshoregridHVDC infrastructure and yields a reasonable accuracyregarding long-term large-scale transmission expansionstudies.For instance, a detailed justification can befound in .Based on  and , the linear unicorn cost modelfor VSC HVDC transmission investments is defined inEquation (1):Ckest,i=FiXfBkf(lf,pf)+GiXgNkg(pg)+HiXhSkh(ph)(1)ThespecificcostcomponentsinEquation(1)Bkf(lf,pf),Nkg(pg)andSkh(ph)arespecifiedbyEquation (2), Equation (3) and Equation (4):Bkf(lf,pf)=Bklp·lf·pf+pfˆPjBkl·lf+Bk0(2)Nkg(pg)=Nkp·pg+pgˆPjNk0(3)Skh(ph)=Skp·ph+p
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Electric power transmission, Cost overrun, VSC, VSC HVDC