Provisional Microgrid Planning

Transcription

1 1 rovsonal Mcrogrd lannng Amn Khodae, Senor Member, IEEE Absrac The opmal plannng problem of provsonal mcrogrds, as a new class of mcrogrds, s nvesgaed n hs paper. Unlke radonal mcrogrds, provsonal mcrogrds do no possess he slandng capably and are dependen on one or more elecrcally conneced mcrogrds, called coupled mcrogrds, for slandng purposes. rovsonal mcrogrds can be consdered as enablers of rapd negraon of renewable energy resources n dsrbuon neworks whle a he same me provdng economc benefs for local consumers and envronmenal benefs for he enre sysem. The provsonal mcrogrd plannng problem s defned and formulaed n hs paper, consderng ancpaed neracons beween he provsonal mcrogrd, he coupled mcrogrd, and he uly grd, durng grd-conneced and slanded modes. Robus opmzaon s used o effcenly consder physcal and fnancal unceranes n he problem. Numercal smulaons sudy a es provsonal mcrogrd for explorng s mers, as well as demonsrang s benefs when compared o deploymen of a radonal mcrogrd. Index Terms rovsonal mcrogrd, plannng, slanded operaon, renewable dsrbued generaor, uncerany, robus opmzaon. Indces b h Index for hour Index for day Index for DERs Index for year NOMENCLATURE Ses W Se of renewable energy resources,, Se of prmal varables U 1 2 Se of unceran parameers arameers CC Annualzed nvesmen cos of generang uns d Dscoun rae D Load demand Raed power of DERs CM Flow lm beween he provsonal mcrogrd and he coupled mcrogrd M Flow lm beween he provsonal mcrogrd and he uly grd Coeffcen of presen-worh value A. Khodae s wh he Deparmen of Elecrcal and Compuer Engneerng, Unversy of Denver, Denver, CO USA (emal: amn.khodae@du.edu). CM Energy purchase prce from he coupled mcrogrd M Energy purchase prce from he uly grd u Bnary slandng varable (1 f grd-conneced, 0 f slanded) v Value of los load (VOLL) Varables LS Load curalmen DER oupu power CM Coupled mcrogrd power M Uly grd power C Toal plannng cos Q Toal annual operaon cos x DER nvesmen sae (1 f nsalled, 0 oherwse) Dual varable Λ rojeced operaon cos n he nvesmen problem M I. INTRODUCTION ICROGRIDS are becomng vable alernaves o cenralzed generaon and bulk ransmsson of power by offerng a localzed power generaon, regulaon, and consumpon. There are a dverse se of benefs semmed from mcrogrds, ncludng bu no lmed o, enhanced relably by enablng self-healng, enhanced reslency by respondng o exreme evens and uly grd supply nerrupons, ncreased effcency by reducng losses, deferred ransmsson and dsrbuon upgrades by provdng a local supply of loads, and enhanced negraon of responsve and adjusable loads. Enhancng a rapd negraon of renewable energy resources, however, canno be consdered as one of he benefs of mcrogrds. As defned by he U.S. Deparmen of Energy, a mcrogrd s a group of nerconneced loads and dsrbued energy resources (DERs) wh clearly defned elecrcal boundares ha acs as a sngle conrollable eny wh respec o he grd and can connec and dsconnec from he grd o enable o operae n boh grd-conneced or sland modes [1]. Based on hs defnon, a mcrogrd mus be able o operae n slanded mode o seamlessly supply local loads. The negraon of renewable energy resources, however, would no help wh hs requremen as renewable energy resources are unconrollable, hence, mcrogrd developers would prefer o ulze dspachable energy resources, such as CHs, for mcrogrd local generaon raher han renewable energy resources. On he oher hand, he large-scale cenralzed negraon of renewable energy resources s on he rse o reduce generaon

2 2 relaed greenhouse gas emssons, address envronmenal concerns, and ensure energy secury and susanably by he dversfcaon of energy resources. Broader negraon of renewable energy s specfcally argeed n he Uned Saes by sae and governmenal mandaes whch am a enforcng he envronmenal agenda by mgang greenhouse gasses generaed by exhauson of fossl fuels and comba clmae change. The Renewable orfolo Sandards (RS) s a good example of hese mandaes whch demands he elecrc power provders o supply a specfc amoun of elecrc power from renewable energy resources. A wdespread deploymen of hese vable resources, however, s subjec o several sgnfcan burdens ncludng fnancal and echncal. From a fnancal perspecve, he capal cos of renewable energy resources s much hgher han radonal energy resources. Alhough ed wh nsgnfcan operaon coss, he large capal cos of hese resources and he assocaed long reurn on nvesmen s consdered as a major drawback. The fnancal ssue, however, s becomng less of a concern as he echnologcal advances are causng consderable prce drops n renewable echnologes. Moreover, a varey of polces and regulaons are appled by he saes and governmens o suppor nvesmens on renewable energy resources. Some of examples n he Unes Saes nclude publc benef funds for renewable energy (whch are obaned by levyng small axes on elecrcy raes), oupu-based envronmenal regulaons (whch ordan emsson lms n order o encourage elecrc producers o ncrease effcency and conrol ar polluon), ne energy meerng (o allow prosumers o sell her excess generaon back o he uly for reducng energy paymens as well as ransmsson and dsrbuon charges), feed-n arffs (ha encourage renewable energy developmen by requrng elecrc ules o make long-erm paymens for he power fed by renewable energy developers ono he uly grd), propery assessed clean energy (n whch he cos of renewable energy nsallaons or ncreasng energy effcency s refunded o resdenal properes nsead of ndvdual borrowers, so encouragng propery owners o nves n renewable energy mprovemens), and oher fnancal ncenves (ncludng grans, loans, rebaes, and ax creds ha are offered o encourage he renewable energy deploymen) [2]. From a echncal perspecve, here are sgnfcan drawbacks n negraon of renewable energy resources o he elecrc power sysem. Major sources of renewable energy,.e., wnd and solar, are sgnfcanly dependen on meeorologcal facors. These resources are hghly unpredcable and cause consderable varably n power generaon. Two major characerscs of renewable generaon are nermency (.e., no always avalable - such as solar generaon whch s no avalable durng nghme), and volaly (.e., consan flucuaons from seconds o mnues o hours - such as wnd generaon ha depends on he speed and avalably of wnd, or solar generaon whch could radcally change as he cloud cover changes). To enable an effcen negraon of renewable energy resources, sysem planners have radonally consdered backup generaon for smoohng ou he generaon varably. Backup generaons ypcally offer a fas response o generaon changes; common examples are fas-response gas uns, hydro uns, demand managemen, and energy sorage sysems. In hese cases, here s a chance ha he rue value of he backup generaon nsallaon canno be acheved, snce he backup s ypcally used for he sole purpose of coordnang he renewable generaon. Hence, s cos wll be added o he already large capal cos of he renewable energy resource and furher queson he economc vably of he deploymen. Anoher major ssue ha resuls from he generaon varably of hese resources s ha he nerrupon n power supply from he uly grd canno be fully compensaed by hese resources as he generaon canno be conrolled. In oher words, hese resources, f deployed sand-alone, canno ensure generaon relably. A vable alernave o backup generaon, whle a he same guaraneeng relably, s o deploy mcrogrds. Combnng hese wo ssues, can be saed ha: 1) Mcrogrds could play a vable role n ensurng a rapd and wdespread negraon of renewable energy resources n dsrbuon neworks by provdng a flexble backup generaon and addressng he prevalng echncal consrans, and 2) Mcrogrd developers are no n favor of renewable energy resources snce hese resources are assocaed wh hgher capal coss and also canno be used for slandng purposes o relably supply crcal loads when he supply of power from he uly grd s nerruped. The second pon s a more decsve facor n mcrogrd deploymens as can be seen n curren mcrogrd deploymens. Islandng s he mos salen feaure of he mcrogrds whch represens he mcrogrd capably o be dsconneced from he man grd n case of upsream dsurbances or volage flucuaons [3]-[7]. Ths capably s ensured by he ulzaon of dspachable dsrbued generaors (DGs), energy sorage, and demand managemen, whch ogeher could be scheduled o supply he peak crcal load when dsconneced from he uly grd [8]. To address hs conflc,.e., o benef from mcrogrd o enable rapd negraon of renewable energy resources, he concep of provsonal mcrogrds has been proposed [9]. rovsonal mcrogrds are defned as a group of nerconneced loads and renewable DGs wh clearly defned elecrcal boundares ha acs as a sngle eny wh respec o he grd bu requres addonal generaon from elecrcally conneced mcrogrds o enable o operae n sland modes. Based on hs defnon, provsonal mcrogrds could ulze a hgh percenage of renewable energy resources whou concernng abou slandng requremens. When slandng s needed, he provsonal mcrogrd would be dsconneced from he uly grd and rely on s own generaon, as well as generaon of he coupled mcrogrd, o supply local loads. rovsonal mcrogrds am a achevng he objecves of addressng he economc and relably needs of elecrcy consumers wh less crcal and sensve loads, procurng dsrbuon nework flexbly offered by exsng mcrogrds, and ensurng a rapd and wdespread deploymen of renewable energy resources n dsrbuon neworks. The concep of

3 3 provsonal mcrogrds can also be looked a from he prosumers perspecve. rosumers,.e., elecrcy consumers whch are equpped wh DGs and can parally supply her elecrcy needs, are rapdly ncreasng n he Unes Saes and around he world. These prosumers prmarly deploy renewable DGs whch alhough can be helpful n reducng he elecrcy paymens, her varable naure prevens relably mprovemens. However, f a mcrogrd s avalable and elecrcally conneced o he prosumer, e.g. n he same dsrbuon feeder, he prosumer can rely on a supply of power from he mcrogrd n case of uly grd supply nerrupon, and herefore, mprove s relably. In oher words, prosumers can be elevaed o he saus of provsonal mcrogrds f hey can make a connecon o an exsng mcrogrd o purchase power durng slanded operaon and also ulze a maser conroller o monor and conrol he power exchange wh he coupled mcrogrd. rovsonal mcrogrds could be furher consdered as exensons o mcrogrd clusers. There are several sudes regardng mcrogrd clusers n he leraure from dfferen perspecves, ncludng economc analyss, cooperave schedulng, and effcen conrol. Economc benefs of mcrogrd clusers are dscussed n [10] and [11] where s shown ha her operaon leads o reducon n emssons and end user cos whle addressng he load growh. Mcrogrd clusers enable an effcen energy radng by allowng cooperaon. The sudy n [12] formulaes a coalonal game beween a number of mcrogrds o sudy novel cooperave sraeges n mcrogrd clusers. Smulaon resuls show a reducon n erms of he average power losses relave o he non-cooperave case. An analyss of prce compeon among nerconneced mcrogrds s presened n [13] usng he game heory framework, whch explcly compues Nash Equlbrum and shows s unqueness. The sudy n [14] addresses he case where wo mcrogrds are solaed from he uly grd bu can exchange energy wh each oher n a peero-peer manner amng o mnmze he oal cos resulng from energy generaon and ransfer, whle each mcrogrd sasfes s local power demand. The conrol of he mcrogrd clusers s anoher mporan opc of sudy. In [15] a mcrogrd cluser conrol sysem s proposed and mplemened usng mul-agen sysems for communcaon and conrol among a number of adjacen mcrogrds. The sudy n [16] presens a novel mcrogrd cluser wh a dsrbued conrol orened herarchcal sysem and dsrbued mul-agen sysem archecure. Agens nclude he mcrogrd cluser managemen, mcrogrd conrol, and local agens. A mulagen sysem s a nework sysem composed of a number of loosely coupled agens. These agens are physcally or logcally dspersed, and have some dsnc characerscs, ncludng dsrbued daa, an asynchronous or smulaneous process of compuaon, lack of nformaon and capably of ndvdual problem solvng, and neracon and cooperaon wh oher agens o mprove her problem solvng capably. In [17] he conrol of mcrogrd clusers s performed n hree levels: local mcrosource and load conroller, mcrogrd cenral conroller, and dsrbuon managemen sysem. Conrol of hs sysem s done by a cenral auonomous managemen conroller, whch serves as an nerface o he dsrbuon managemen sysem. In [18] a herarchcal and decenralzed scheme for coordnaed volage suppor and frequency conrol, as well as for sae esmaon for mcrogrd clusers, s proposed. Fuzzy sae esmaon and mcrogrd cluser sae esmaon are furher proposed n [17] and [19]. In [19] conrol funconaly o manage mcro-generaon n mcrogrd clusers s proposed consderng acve loads and energy sorage, subjec o dfferen consrans. Dscussons on mcrogrd clusers consder wo or more mcrogrds whch are capable of operang n he slanded mode whou he need o purchase power from conneced mcrogrds. In hs case, power exchanges are commonly performed o mprove economcs and conrol raher han ensurng a seamless slandng. The concep of provsonal mcrogrds was proposed n [9] accompaned by dealed dscussons on s opmal schedulng problem. Ths paper bulds on ha work o show ha n addon o an economc operaon, provsonal mcrogrds could be consdered as vable alernaves compared o mcrogrds from a leas-cos plannng perspecve. The res of he paper s organzed as follows. Secon II revews he concep of provsonal mcrogrds and furher oulnes he plannng model and decsve facors n he plannng problem. Secon III formulaes he plannng problem whle Secon IV dscusses he applcaon of robus opmzaon o solve he plannng problem. Secon V performs numercal sudes on a es sysem. Dscusson on he feaures of he proposed model and concludng remarks are provded n Secons VI and VII, respecvely. II. LANNING ROBLEM MODEL OUTLINE rovsonal mcrogrds are smlar o mcrogrds as her elecrcal boundares are clearly defned and a maser conroller operaes and conrols avalable resources. Unlke mcrogrds, however, provsonal mcrogrds do no have he ably o be slanded on her own. rovsonal mcrogrds are dependen on one or more elecrcally conneced mcrogrds, called coupled mcrogrds, for operaon n he slanded mode. rovsonal mcrogrds could ulze a hgh percenage of renewable energy resources whou concernng abou slandng requremens. When slandng s needed, he provsonal mcrogrd would be dsconneced from he man grd dsrbuon nework and rely on s own generaon, as well as generaon from he coupled mcrogrd, o supply local loads. I should furher be assumed ha he loads whn he provsonal mcrogrd are no crcal. The connecon o he coupled mcrogrd would provde he requred flexbly o coordnae varable generaon f needed, and also he unused capacy of he coupled mcrogrd would be used n slandng ncdens o ensure adequae supply of local loads. In he grd-conneced mode, he provsonal mcrogrd generaes power by coordnang avalable resources and

4 4 neracs wh he uly grd for power ransfer o supply local loads, whle n he slanded mode dsconnecs from he uly grd dsrbuon nework and ransfers power wh he coupled mcrogrd. The provsonal mcrogrd and he coupled mcrogrd would operae smulaneously n he slanded mode n response o uly grd falures and/or volage flucuaons, where s assumed ha he connecon beween he provsonal mcrogrd and he coupled mcrogrd wll be mananed durng slandng. The provsonal mcrogrd would furher rely on he coupled mcrogrd for frequency regulaon and volage conrol. The coupled mcrogrd would rea he provsonal mcrogrd as a load, smlar o how reas s local loads. There are no any conrollable devces n he provsonal mcrogrd, so a coordnaed conrol beween wo mcrogrds would no seem necessary. Sgnfcan economc and relably benefs, whch wll sem from he power ransfer, are momenous drvers n mananng he connecon beween he provsonal mcrogrd and he coupled mcrogrd n slanded mode. The coupled mcrogrd would benef by sellng s excess generaon o he provsonal mcrogrd and he provsonal mcrogrd could relably supply s local loads, reduce load curalmen, and ncrease operaonal relably. Alhough s assumed ha provsonal mcrogrd loads are no crcal, hs capably would preven undesred load curalmens. Fg. 1 depcs he provsonal and coupled mcrogrds power exchanges durng grd-conneced and slanded modes. Assumng ha boh mcrogrds are conneced o he same upsream HV subsaon,.e., he marke prce for boh mcrogrds s he same, and also mcrogrds are conneced o he uly grd wh adequaely large lne capaces ha can handle he mum local load, can be observed ha here would be no power exchange beween he mcrogrds durng he grd-conneced mode. To clarfy he ssue, suppose ha he marke prce s lower han he prce of power ransfer from he coupled mcrogrd o he provsonal mcrogrd. In hs case, he provsonal mcrogrd would prefer o purchase power from he uly grd raher han he coupled mcrogrd. Also f has excess generaon, he coupled mcrogrd would no be neresed n purchasng he power from he provsonal mcrogrd, as could be acqured from he uly grd a a lower prce. The same observaon s vald when he marke prce s hgher han he prce of power ransfer from he coupled mcrogrd. In hs case, he provsonal mcrogrd would be wllng o purchase power from he coupled mcrogrd, however, he coupled mcrogrd would sell s excess generaon, f any, o he uly grd as would resul n a hgher benef. Consderng hese, here would be no power exchange beween he provsonal mcrogrd and he coupled mcrogrd n he grd-conneced mode as would reduce her ndvdual economc benefs. Uly grd (normal operaon) Coupled Mcrogrd rovsonal Mcrogrd Uly grd (dsurbance) Coupled Mcrogrd rovsonal Mcrogrd Fg. 1 rovsonal mcrogrd operaon durng grd-conneced (lef) and slanded (rgh) operaon modes In mos of he operang hours, he power ransfer wh he uly grd helps supply local loads. In mnor and nfrequen slanded hours, he power ransfer wh he coupled mcrogrd combned wh he local generaon enables supplyng local loads. The coupled mcrogrd s desgned o compleely supply s crcal loads a peak hours. Therefore, he coupled mcrogrd would normally have unused capacy n boh grdconneced and slanded modes. The coupled mcrogrd excess generaon, beyond s load, would help he provsonal mcrogrd o supply local loads durng slanded operaon. If suffcen generaon s no avalable o fully supply loads, he provsonal mcrogrd wll cural some of s load o guaranee load-supply balance. The possbly of load curalmen mus be consdered n he provsonal mcrogrd desgn process as he cos of relably. A second approach, whch wll be more favorable for boh mcrogrds, s o follow negoaed capacy and prce values of power ransfer durng slanded modes. If he capacy and he prce are negoaed,.e., boh mcrogrds agree beforehand on he mum avalable power ransfer as well as prce per kwh of ransfer durng slanded modes, here would be no uncerany for power exchange. The coupled mcrogrd benefs from hs agreemen as would sell s excess generaon o he provsonal mcrogrd durng slanded modes; he provsonal mcrogrd benefs from hs agreemen as would reduce load curalmens f adequae local generaon s no avalable. Ths mehod s consdered n hs paper for provsonal mcrogrd plannng, where case sudes are furher performed o show he sensvy of plannng resuls o negoaed capacy and prce values. A mulple me-scale plannng problem s proposed, comprsng he provsonal mcrogrd long-erm nvesmen (.e., annual) and shor-erm operaon (.e., hourly). Any oher schedulng resoluon can be seleced based on he developer s dscreon whou loss of generaly n he proposed model. By he selecon of an hourly schedulng, schedules are obaned based on hourly operaon and also he slandng duraon s consdered as an neger mulple of one hour. Shorer me perods could be employed o more accuraely capure rapd changes n load and renewable generaon as well as shorer slandng duraons. The selecon of a proper me perod for schedulng represens a radeoff beween he soluon accuracy and he compuaon me. Shorer me perods would analyze more daa and provde more accurae soluons whle ncreasng compuaon requremens. Smulaneous consderaon of annual nvesmen wh shorerm operaon follows he exensve work n power sysem dynamc plannng, such as n [20]-[24].

5 5 III. LANNING ROBLEM FORMULATION The objecve of he provsonal mcrogrd plannng problem s o mnmze he oal plannng cos (1) subjec o operaonal consrans (2)-(6). The oal plannng cos, here shown wh C, ncludes he renewable DG nvesmen cos, provsonal mcrogrd operaon cos, and he cos of unserved energy. The plannng problem s formulaed as follows: mnmzec CC x h b h b h b v CM, bh CM, bh M, bh M, bh bh LS bh Subjec o: bh M, bh CM, bh LSbh Dbh b, h, (2) M M u u b, h, bh M, bh bh CM, bh (1 ubh ) CM, bh CM, bh (1 ubh ) b, h, (4) bh (1) (3) ˆ x, b, h, (5) bh 0 LSbh Dbh b, h, (6) The objecve s mnmzed over he se of prmal varables, where ={x, CM, M, LS}. The nvesmen cos of renewable DGs s a funcon of her generang capacy. I s assumed ha he operaon cos assocaed wh renewable DGs s neglgble, hus he mcrogrd operaon cos ncludes he cos of energy purchase from he uly grd plus he cos of energy purchase from he coupled mcrogrd. The cos of energy purchase s defned as he amoun of purchased energy mes he assocaed prce a he pon of connecon. A snglesep prce curve s consdered for energy purchase from he coupled mcrogrd. The uly grd power M would be negave f he provsonal mcrogrd s exporng s excess power o he uly grd (pad a marke prce). The coupled mcrogrd power CM would be negave f he provsonal mcrogrd s exporng s excess power o he coupled mcrogrd (pad a he negoaed prce). The cos of unserved energy, whch represens he provsonal mcrogrd relably n supplyng local loads, s defned as he load curalmen quany mes he value of los load (VOLL). VOLL s he energy prce for compensang curaled consumers, whch depends on several facors ncludng he ypes of consumers, quanes and duraons of curalmens, and he me of ouages. A hgher VOLL corresponds o more crcal loads [25]-[26]. The objecve s evaluaed n erms of dscouned coss, where dscoun raes are ncorporaed n he presenworh cos componens,.e., 1 (1 d). I should be 1 consdered ha only renewable DGs are consdered for nsallaon n he provsonal mcrogrd, herefore, here would be no nsallaon and operaon cos assocaed wh dspachable DGs and he energy sorage n he objecve. The nvesmen cos s defned annually whle operaon coss are calculaed hourly n he plannng horzon. The power balance equaon (2) ensures ha he provsonal mcrogrd load wll be suppled by he local generaon, he power from he uly grd, and he power from he coupled mcrogrd. A load curalmen varable s added o he load balance equaon o compensae generaon shorages n slanded modes. The load curalmen wll be zero durng grdconneced operaon as can be assumed ha adequae power can be suppled from he uly grd o fully supply local loads. The provsonal mcrogrd power ransfer wh he uly grd n he grd-conneced mode s lmed by he flow lm of he assocaed connecng lne (3), and wll be zero n slanded mode, where u=0. The provsonal mcrogrd power ransfer wh he coupled mcrogrd wll be resrced by negoaed capacy lms durng slanded modes (4), and wll be zero durng grd-conneced mode, where u=1. The renewable generaon s obaned based on a forecas, and wll be se o zero f he assocaed renewable DG s no seleced for nsallaon (5). Fnally, he hourly load curalmen s consdered nonnegave and resrced o parcpang loads (6). The modelng of he provsonal mcrogrd power ransfer wh he uly grd and he coupled mcrogrd are conssen wh dscussons n Secon II. IV. ROBUST OTIMIZATION The proposed provsonal mcrogrd plannng problem s subjec o several unceranes,.e., facors whch are havng a major nfluence on plannng decsons bu are no under conrol of he mcrogrd developer or canno be predced wh cerany. Forecass assocaed wh loads, marke prces, nondspachable generaon, as well as slandng ncdens can be consdered as prevalng unceranes n he plannng problem. Moreover, he nformaon assocaed wh he coupled mcrogrd could be unceran whch ncludes he avalable unused capacy n slanded operaon and he generaon prce. However, as dscussed, s assumed ha he nformaon from he coupled mcrogrd wll be negoaed and known n advance, hence removng he assocaed uncerany. To effcenly manage unceranes, a robus opmzaon mehod wll be adoped [27]-[31]. Fg. 2 depcs he flowchar of he proposed provsonal mcrogrd plannng model where he orgnal plannng problem s decomposed o a maser problem and a subproblem. Maser roblem (nvesmen) Deermne opmal renewable DG for deploymen Opmaly cu Renewable DG nvesmen plan Subproblem (operaon) Deermne wors-case opmal operaon Fg. 2 roposed provsonal mcrogrd plannng model The maser problem, whch s an nvesmen problem, deermnes he leas-cos canddae renewable DGs o be

6 6 nsalled, and s formulaed as follows: mnmze IC CC x (7) 1 Subjec o: Q ˆ ˆ ( x xˆ ) (8) h b bh bh The objecve s mnmzed over he se of prmal varables 1 ={x}, and s subjec o he opmaly cu (8) formed n he subproblem. Q s he calculaed objecve value of he subproblem,.e., (9). The opmaly cu ndcaes ha he soluon of he revsed nvesmen plan can lead o a more economcal soluon n he subproblem. The subproblem, whch deals wh opmal operaon of he provsonal mcrogrd, uses he obaned plan o fnd he opmal neracons wh he uly grd and he coupled mcrogrd whle consderng physcal and fnancal unceranes. The objecve of he subproblem comprses he provsonal mcrogrd operaon cos and he cos of unserved energy (9). A wors-case analyss s performed o deermne a robus soluon agans unceranes, where he objecve s mnmzed over he se of prmal varables 2 ={ CM, M, LS} and mzed over he se of unceran parameers U={, D, u}. The subproblem for each plannng year s defned as follows: mzemnq U 2 h b h b h b v bh M, bh M, bh bh CM, bh CM, bh LS Subjec o: bh ˆ bh xˆ bh, b, h (10) and (2)-(6). Consran (10) consders he mpac of he nvesmen sae deermned n he maser problem on renewable generaon. If x=0, he assocaed generaon wll be se o zero, and f x=1, he assocaed generaon wll be equal o he forecased value. Ths consran furher helps deermne he dual mulpler requred for formng he opmaly cu. olyhedral uncerany ses are consdered n whch unceran parameers would choose one of exreme pons of he assocaed uncerany nerval. The soluon of he subproblem s furher used o examne he convergence, whch s based on he proxmy of a lower bound (calculaed n he maser problem) and an upper bound (calculaed n he subproblem). If no converged, he opmaly cu (8) wll be generaed n he subproblem and sen back o he maser problem for revsng curren plannng decsons. Once converged, he plan s consdered fnal. The proposed robus opmzaon follows he formulaon n [32], whle he coupled mcrogrd power ransfer s added as a new varable. V. NUMERICAL SIMULATIONS A provsonal mcrogrd s o be nsalled for a group of (9) elecrcy consumers wh a peak annual load demand of 8.5 MW. The se of canddaes ncludes wo renewable DGs ncludng one aggregaed solar un and one aggregaed wnd un. The raed power for boh renewable DGs s consdered 2 MW, wh annualzed nvesmen coss of $120,000/MW and $180,000/MW for he solar un and he wnd un, respecvely. The load, varable renewable generaon, and marke prce are forecased based on hsorcal daa [33]-[34]. Nne hours of slandng per year s consdered. The mpac of slandng hours on plannng resuls s furher nvesgaed n case sudes. The plannng horzon s 20 years. The problem s mplemened on a 2.4-GHz personal compuer usng CLEX 12.1 [35]. There s no lm on he power ransfer wh he uly grd, where he capacy lm of he lne connecng he provsonal mcrogrd o he uly grd s consdered o be adequaely large o handle any power ransfer. The power ransfer lm wh he coupled mcrogrd durng slanded mode s nally consdered 1 MW wh a negoaed prce of $90/MWh. Followng cases are suded wh a focus on he provsonal mcrogrd plannng based on a varey of decsve facors: Case 1: Base case provsonal mcrogrd plannng Case 2: Impac of he coupled mcrogrd power ransfer capacy and prce Case 3: Impac of he raed power of canddaes Case 4: Impac of he number of slandng hours Case 1: Base case provsonal mcrogrd plannng The provsonal mcrogrd plannng problem s solved consderng unceranes n hourly load, renewable generaon, and marke prce forecass. A ±10 uncerany n load forecass and ±20 uncerany n renewable generaon and marke prce forecass s consdered. The lm on uncerany opon for load and renewable generaon s consdered 1000 hours/year, whle hs lm for he marke prce s se a 2000 hours/year. The slandng s lmed o nne hours n each plannng year. The provsonal mcrogrd plannng soluon would nsall boh renewable DGs 1 and 2. The oal plannng cos s $40,607,816, wh a cos breakdown of $10,007,077 for he nvesmen, $27,821,299 energy purchase from he uly grd, $13,509 energy purchase from he coupled mcrogrd, and $2,765,930 as he cos of unserved energy resuled from load curalmens. The cos of purchasng energy from he uly grd n case he provsonal mcrogrd was no deployed, hence elmnang he energy purchase from he coupled mcrogrd, would be $43,124,190 whch s more han 6% larger han he oal provsonal mcrogrd plannng cos. The dfference n cos ndcaes ha he provsonal mcrogrd deploymen s economcal and he renewable DG nvesmens wll be recompensed from revenues. Ths soluon s furher compared wh he case of a mcrogrd deploymen as suded n [32]. The oal plannng cos of he mcrogrd deploymen s $42,451,534, whch s larger han ha of he provsonal mcrogrd. Ths comparson clearly demonsraes he economc vably of he provsonal mcrogrd when

7 7 compared o he mcrogrd deploymen or no developmen a all. When comparng o he mcrogrd developmen, some addonal ssues can be aken no consderaon: ) The cos of unserved energy (whch represens he provsonal mcrogrd relably n supplyng local loads) s consdered n he plannng objecve o ensure ha he ssue of relably s quanfed and consdered n he plannng problem. Hence, cusomers would no necessarly swch o a mcrogrd only because load curalmens wll be lower; ) The mcrogrd plannng cos obaned from [32] only ncludes he cos of DER nsallaon and mcrogrd operaon. However, mcrogrds would requre exensve addonal nvesmens for upgradng he mcrogrd dsrbuon nework and deployng a vable and sophscaed maser conroller o effcenly conrol and operae all DERs. These nvesmens could poenally add addonal coss and complexy n deploymen and operaon of mcrogrds, hence makng he provsonal mcrogrd a more suable alernave for cusomers who are no wllng o encouner ha level of nvesmen and complexy; ) Smlar npu daa s used n hs sudy for boh mcrogrd and provsonal mcrogrd deploymens, ncludng smlar VOLL. Snce he provsonal mcrogrd s deployed n areas wh less crcal loads, a smaller VOLL should be consdered, hence reducng he oal plannng cos of he provsonal mcrogrd deploymen and furher demonsrang s economc vably. Table I summarzes he coss assocaed wh he plannng of he mcrogrd, obaned from [32], and he provsonal mcrogrd. From a generaon mx perspecve, he provsonal mcrogrd deploys 100% renewable generaon, compared o he mcrogrd whch reles on only 16% of renewable generaon and he res wll be based on dspachable hermal uns. TABLE I COMARISON OF RESULTS BETWEEN MICROGRID AND ROVISIONAL MICROGRID DELOYMENTS Insallaon lannng Cos Breakdown ($) Type Invesmen Operaon Unserved Toal Mcrogrd [32] 17,012,031 25,439, ,451,534 rovsonal Mcrogrd 10,007,077 27,834,809 2,765,930 40,607,816 Case 2: Impac of he coupled mcrogrd power ransfer capacy and prce The mpac of he coupled mcrogrd power ransfer capacy and prce on he provsonal mcrogrd plannng resuls are furher suded. The power ransfer capacy s changed from 0 MW o 2 MW wh a sep of 0.5 MW. The VOLL s reduced o $2000/MWh o represen less crcal loads whn he provsonal mcrogrd. The provsonal mcrogrd plannng soluon would nsall he renewable DG 1,.e., he solar un. Increasng he power ransfer capacy wh he coupled mcrogrd would no change he nvesmen plan bu would reduce he plannng cos. The reducon n he plannng cos s due o he reduced cos of unserved energy. As he ransfer capacy ncreases he provsonal mcrogrd would be able o purchase addonal energy from he coupled mcrogrd durng slanded hours, hence he load curalmen, and consequenly he cos of unserved energy, would drop. Furher decrease n he plannng cos, however, would no occur for ransfer capaces more han 3.77 MW snce he load would be fully suppled durng slanded hours and he cos of unserved energy would be zero. The sensvy analyss wh respec o he power ransfer prce shows ha ndependen of he prce, he provsonal mcrogrd would purchase power, up o he ransfer capacy, a all slanded hours from he coupled mcrogrd. Ths resul s obvous as would be always more benefcal o purchase power from he coupled mcrogrd han o cural loads. The economc benef s obaned because of he lower prce of he coupled mcrogrd compared o VOLL. Evdenly, f he VOLL s less han he coupled mcrogrd ransfer prce, would be more benefcal o cural loads han o purchase he relavely expensve power from he coupled mcrogrd. Case 3: Impac of he raed power of canddaes In hs case he sensvy of he plannng cos wh respec o he raed power s suded. In he proposed provsonal mcrogrd plannng problem, he raed power of canddae renewable DGs s consdered as an npu o he problem. To consder a varey of values for raed power, he forecased generaon s normalzed and hen scaled up based on he raed power. The VOLL s consdered o be $2000/MWh n hs case. Fg. 3 shows he resuls of he sensvy analyss. As he raed power ncreases, he nvesmen cos wll lnearly ncrease whle he cos of energy purchase from he uly grd wll decrease n a nonlnear fashon. The cos of energy purchase from he coupled mcrogrd and he cos of unserved energy are almos he same n all cases as he number of slandng hours hasn changed and also renewable generaon n he slanded hours s very small. Thus, he plannng cos as shown n Fg. 3 wll be obaned, demonsrang he opmal renewable DG raed power of 1.8 MW. lannng Cos (Mllon $) Raed ower (MW) Fg. 3 rovsonal mcrogrd plannng cos as a funcon of he canddae DGs raed power Case 4: Impac of he number of slandng hours The provsonal mcrogrd plannng problem s solved for a varey of slandng hours. As has been prevously demonsraed n [32], he slandng always occurs a mes ha resuls n he lowes cos of unserved energy,.e., when he

8 8 load s a s lowes. As he number of slandng hours ncreases, he cos of energy purchase from he coupled mcrogrd as well as he cos of unserved energy ncrease, snce hese coss are drecly assocaed wh he slanded operaon. The cos of energy purchase from he uly grd, on he oher hand, slghly decreases snce he power from he uly grd a slandng hours wll be reduced o zero. In oal, he provsonal mcrogrd plannng cos ncreases as he number of slandng hours ncrease. Consderng hs, he provsonal mcrogrd s more economcal when he number of slandng hours s lmed, or n oher words, he cusomer average nerrupon duraon ndex (CAIDI) s low. Ths s a compleely dfferen resul from he case of mcrogrds. The economc vably of mcrogrds s hghly dependen on he number of slandng hours as a larger number would ncrease savngs and reduce he payback me. Ths s manly due o he hgh cos of unserved energy whch wll no occur when he mcrogrd s deployed. Furhermore, s possble ha for small values of slandng hours he mcrogrd nsallaon would no be economcal. Ths sudy reveals a grea dfference beween he mcrogrd and he provsonal mcrogrd,.e., her benefs n response o slandng hours. The mcrogrd, besdes provdng economc benefs by ulzng local resources, s a vable soluon o consumers relably problems and would sgnfcanly reduce consumers load curalmens. For he provsonal mcrogrd, however, he case s dfferen snce canno be prmarly used for relably purposes. VI. DISCUSSIONS rovsonal mcrogrds are defned o remove slandng consderaons as par of he mcrogrd desgn and plannng process, reduce he DER nvesmen cos, and preven underulzaon of capal-nensve dspachable uns and underdeploymen of renewable energy resources. Consequenly, provsonal mcrogrds could be consdered as vable alernaves o guaranee a wdespread and rapd deploymen of renewable DGs n dsrbuon neworks. Specfc feaures of he provsonal mcrogrds, ganed from he proposed provsonal mcrogrd plannng model and numercal sudes, are lsed as follows: - Increasng renewable DG prolferaon and posvely mpacng he envronmen: Compared o mcrogrds whch prmarly rely on a large percenage of dspachable DGs for slandng purposes and nclude a small percenage of renewable DGs, he provsonal mcrogrds could deploy 100% of her generaon capacy based on renewable DGs, and furher address envronmenal concerns by enablng large and dsrbued peneraon of hese emsson-free resources n dsrbuon neworks. - Economc benefs: rovsonal mcrogrds could be poenally less expensve opons o deploy when compared o mcrogrds for he same se of consumers, and could furher guaranee a faser reurn on nvesmen. The consumer VOLL and CAIDI wll become decsve facors n hs case snce hey could sgnfcanly change he plannng resuls and aler he economc vably of he nvesmens. - Removng he need o enhance dsrbuon nework flexbly: rovsonal mcrogrds wll benef from he avalable flexbly n dsrbuon neworks offered by exsng mcrogrds, whch would furher operae as coupled mcrogrds and benef from he elecrcal connecon o he provsonal mcrogrd. Thus, here would be no need for sysem operaors o renforce he dsrbuon nework flexbly by addonal capalnensve nsallaons and sysem upgrades. - Role of slandng hours: Despe mcrogrds whch are vable opons o address uly grd nerrupons by swchng o an slanded mode and relably supplyng local loads, provsonal mcrogrds are no good opons o be used for slandng purposes. In fac, provsonal mcrogrds wll become less aracve opons as he number of slandng hours ncreases. VII. CONCLUSIONS The provsonal mcrogrd plannng problem was developed and formulaed n hs paper o show he sgnfcance of hs novel concep and furher enable progress oward he nex generaon of nellgen and susanable negraed power grds. rovsonal mcrogrds hold smlar characerscs as mcrogrds, however, do no offer he slandng capably, hence allowng a larger deploymen of renewable DGs. The provsonal mcrogrd plannng problem was solved for a varey of cases, showng he benefs and dfferences from a radonal mcrogrd deploymen. I was shown ha n comparson o mcrogrds, wh a prmary applcaon of mprovng relably for local cusomers and managng he ever ncreasng peneraon of DERs, provsonal mcrogrds would suppor he effcen deploymen of renewable DGs n dsrbuon neworks by leveragng he avalable flexbly offered by already nsalled mcrogrds and benefng from he capacy of coupled mcrogrds durng he slanded operaon. 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