Abstract 1. INTRODUCTION

Transcription

1 Towards Optmzed Dstrbuto Protecto Des F. Soud ad K. Tomsovc School of Electrcal Eeer ad Computer Scece Washto State Uversty Pullma, WA 9964 Abstract Tradtoally, dstrbuto systems eeer has focused o smplfed des crtera uded by dvdual utlty practces ad experece. These dess were ot optmzed but dd satsfy specfc reulatory oals for system performace. Two developmets are valdat ths approach: dereulato of the dustry ad rapd mprovemets techoloy. Dereulato forces utltes to more carefully cosder the cost of provd relable servce ad to be cocered wth the customer cost of relablty. At the same tme, mcroprocessor based devces have reatly expaded the capabltes of protecto apparatus. Further, low cost hh speed computers have redered computer aded des tools feasble. I ths paper, dstrbuto feeder protecto s formulated as a relablty problem. Dstrbuto relablty dces approprate for protecto des are dscussed. Idces whch are mpacted by protecto des are detfed. The authors propose a ew approach to optmz overall dstrbuto system protecto des.. INTRODUCTION Dereulato ad competto s forc utltes to mata a acceptable level of servce relablty whle carefully cotroll captal ad mateace expedtures []. Costs may crease as a utlty attempts to mprove or mata exst relablty levels. At the same tme, the customer costs assocated wth outaes ca be evaluated [2]. I ths evromet, ay relablty mprovemet project must be examed terms of a overall cost/beeft rato. The cost/beeft aalyss relates captal ad mateace expedtures to the customer moetary losses ad utlty reveue losses assocated wth outaes. Relablty dces are oe tool utltes use to prortze captal ad mateace expedtures. A recet survey dcates that the most commo dces used the dustry are customer based dces. These customer based dces are System Averae Iterrupto Freuecy Idex (SAIFI), System Averae Iterrupto Durato dex (SAIDI), Customer Averae Iterrupto Durato Idex (CAIDI) ad Averae System Avalablty Idex (ASAI) [3]. The usae of these dces vares by utlty but the most commo justfcatos are [4]: Fursh maaemet wth performace data reard the ualty of customer servce o the electrcal system as a whole. Idetfy substatos ad crcuts wth substadard performace ad to ascerta the cause. Fursh maaemet wth performace data reard the ualty of customer servce for each operat area. Provde relablty hstory of dvdual crcuts for dscusso wth curret or prospectve customers. Satsfy reulatory report reuremets. Provde a bass to establsh servce cotuty crtera. Fursh maaemet wth performace data reard the ualty of customer servce for each voltae level. Ad the purchase decsos of replacemet parts ad ew eupmet. Provde data for a eeer comparso of electrcal system performace amo coset compaes. Obta the optmum mprovemet relablty per dollar expeded for des, mateace, ad operat prorams. Note, some utltes use load based dces such as, Averae System Iterrupto Freuecy Idex (ASIFI) ad Averae System Iterrupto durato Idex (ASIDI). Sce calculato of the SAIFI ad ASIFI dces rely drectly o the umber, type ad locato of the protectve devces, ths paper wll dscuss optmzato of the effectveess of protectve devces order to mmze the SAIFI ad ASIFI dces. The approach allows ether a mmum cost soluto to acheve a desred relablty oal or a maxmzed effectveess ve a cost costrat. 2. DISTRIBUTION PROTECTION I eeral, protecto of a dstrbuto system cossts of a crcut breaker wth protectve relays ad a automatc reclos relay at the substato, ad a umber of protectve devces wth varous characterstcs o the ma feeder ad laterals. The most commoly used devces are le reclosers, terrupters, sectoalzers ad fuses. These devces provde varous type of protecto ad are brefly dscussed the follow. A crcut breaker has fault terrupto ad automatc reclos capabltes. Automatc reclos esures that follow a temporary fault, servce for the customers wll be restored less tha a few mutes. Tradtoally, protectve ad automatc reclos devces are cosst of

2 three relays for phase faults, oe relay for roud faults ad oe automatc reclos relay. Today, a sle mcroprocessor based relay allows tradtoal protecto fuctos as well as advaced protecto, cotrol ad motor fuctos. Fault locat ad fault record capabltes help to reduce restorato tme dur a permaet outae by detfy a faulted secto o a tmely maer. A le recloser wth fault terrupto ad automatc reclos capabltes are overhead pole mouted or roud padmouted devces. Moder reclosers are cotrolled by a mcroprocessor based devce. A le recloser wll automatcally restore customers for a temporary fault ad ca be set to save fuses for ay temporary fault o the load sde of the fuse. Iterrupters are smlar to le reclosers but do ot have automatc reclos capablty. These devces are used for uderroud dstrbuto crcuts. A sectoalzer s a protectve devce that automatcally solates faulted sectos. Sce a sectoalzer does ot have a fault terrupto capablty, t must be appled cojucto wth a backup recloser or a breaker. A sectoalzer couts the umber of the source sde devce operatos after ses a fault o ts load sde. After a predetermed umber of couts, the faulted secto of the crcut s solated. A fuse s the most expesve protectve devce avalable for overcurret protecto o a dstrbuto crcut. Fuses have fault terrupto capabltes but do ot have automatc reclos capabltes. I addto to tradtoal expulso fuses, curret lmt ad electroc fuses provde addtoal capabltes. A curret lmt fuse s a better choce f there s a cocer for ether rass fres dry areas, ose eerated dur fuse operato, excessve fault duty or eupmet damae due to hh fault currets. A electroc fuse offers varous tme-curret characterstc curves, provdes curret lmtato for certa applcatos ad has a hher cotuos curret rat capablty. 3. DISTRIBUTION RELIABILITY The prmary causes of faults o a dstrbuto crcut are lht, tree cotact, amals ad eupmet falure. A survey from thrtee utlty compaes over a two year perod [5] dcates that 79% of the faults are phase to roud, 85% of the recorded faults are temporary ad the outae rates o laterals are reater tha the ma feeder. To prevet outaes o a dstrbuto feeders, utltes take varous prevetve actos, clud stall lht arresters to prevet flashover, tree trmm o a reular bass, stall amal uards ad mata eupmet o a reular bass. Stll, prevetve measures wll ot fully elmate the occurrece of outaes ad thus, protectve devces are stalled to detect outaes ad to solate sectos of the dstrbuto crcut. Protecto devces prevet or mmze damae to eupmet, allow reater safety measures ad mprove servce relablty. The relablty dces are sestve to the umber, type ad locato of protectve as well as a utltes restorato practce. A crcut breaker or a le recloser wll mmze umber of the customer affected by a permaet outae ad automatcally restore power for a temporary outae. Some of the mcroprocessor based devces provde fault data whch helps to locate ad solate a trouble secto a tmely maer. The sectoalzer mproves relablty dces oly for permaet faults. Sce a sectoalzer works cojucto wth a breaker or le recloser, customers o the breaker or a le recloser secto wll also experece a temporary outae for ay temporary fault o the sectoalzer secto. Iterrupter mpacts o relablty dces are essetally oly o permaet faults sce early all of the faults o uderroud crcuts are permaet faults. Therefore, a terrupter does ot have automatc reclos capablty. Fally, a fuse does ot have automatc reclos capablty ad thus, temporary faults are treated the same as permaet faults. The addtoal tme reured for replac a blow fuse must be cluded the restorato tmes. I eeral, all of the above devces mpact the customer terrupto freuecy ad durato. Remote cotrolled sectoalz ad maual cotrolled sectoalz devces wll also decrease the terrupto durato but do ot have a effect o the terrupto freuecy. 4. PROTECTION IMPROVEMENTS Utltes are cotuously search for ovel des ad operato practces to mprove servce relablty ad reduce captal ad mateace expedtures. For example, creas the dstrbuto operato voltae allows loer feeder leth ad more customers o the feeder. Loer feeders wth more customers wll crease the terrupto freuecy ad terrupto durato whch s ormally addressed by stall a lare umber of protecto devces. The formulato here ad our earler work shows that poorly located protecto devces may actually result worse relablty [6]. Aother practce to mprove servce relablty s to use a fuse sav scheme. A fuse sav scheme allows temporary faults o the load sde of the fuse to operat a source sde recloser or crcut breaker. Ths scheme mproves permaet terrupto freuecy ad terrupto durato but the temporary terrupto freuecy wll crease. Note as electroc loads crease, customers may be less tolerat of mometary faults. Us a sle phase automatc protectve devce stead of a fuse mproves permaet terrupto freuecy ad durato. Sle phase trps wth three phase lockout wll also mprove temporary terrupto freuecy. Whle ths last scheme has bee proposed, there are some cocers that rema to be addressed wth reards to

3 backfeed ad arc extcto. Recetly may utltes have appled advaced supervsory cotrol ad data acusto systems (SCADA) to the dstrbuto system. Such a dstrbuto automato scheme mproves servce relablty ad wll help better utlze the dstrbuto system capacty. 5. PROPOSED METHODOLOGY Ths secto formulates the optmzato problem. For a varato o ths formulato see [6]. 5. Optmzato problem The proposed techue formulates a bary proramm optmzato to detfy type of the protectve devces o a dstrbuto feeder. The oal of ths techue s to select the specfc umber of the protectve devces to be stalled at the predetermed locatos o a dstrbuto feeder ether to mmze the SAIFI or ASIFI dces or to acheve a desred performace level these devces. Locatos ca be selected based o specfc utlty practces, such as, umber of customers, or based o eeer judmet ad the feeder cofurato. A dstrbuto feeder cossts of a ma secto ad several lateral taps. Deped o a utlty s practces, the lateral taps ca be dvded to varous cateores whch we label oe, two ad three. A cateory oe lateral s short ad has lmted exposure (usually less tha three spas). Due to cost, ths type of lateral wll ot be fused. A cateory two lateral s usually loer tha about three spas ad wll have a sle fuse at the tap. A cateory three lateral s heavly loaded or lo, ad varous types of protectve devces ca be stalled o the tap or at other pots. I the SAIFI or ASIFI dces calculato, the cateory oe lateral falure rate ad the umber of the customers ca be treated as a part of the ma feeder. The effect of cateory two laterals the SAIFI or ASIFI dces level s costat uless a fuse sav scheme s appled. Cosder F. whch ca be a ma feeder or a cateory three lateral wth predetermed locatos for stall protectve devces. For a ma feeder, a crcut breaker s located at locato. For other locatos o the ma feeder ad cateory three laterals there may be le reclosers, sectoalzers or fuses for a overhead crcut ad terrupters or fuses for a uderroud crcut. Sce effects of the breakers, reclosers, sectoalzers ad terrupters are the same the SAIFI or ASIFI dces calculato, these are treated detcally the optmzato. 2 F. A typcal ma or lateral feeder wth several sectos The follow basc assumptos are used the calculato of the SAIFI or ASIFI dces whch are represetatve of preset utlty practces. The feeder s operated as a radal feeder. The umber ad type of customers as well as total coected load are kow for each dvdual secto. Falure rate for a le secto cludes coected eupmet falure rates. Multple faults are ot cosdered ad all falures are repared before the ext fault occurs. All laterals have bee assed to be oe of the three pre-defed cateores. A breaker s located at the substato. Falure rate for uderroud sectos are 00% permaet. 5.2 Bary proramm formulato Iteer proramm seeks to maxmze or mmze a fucto of may varables subject to costrats clud varables whch must be teers. Ay such problem ca be approached by the follow bary (zero-oe) proramm problem [6] m cx = such that ax bj, for jî{... m} ad x Î 0, = { } where c are cost coeffcets, a ad b j are parameters descrb the costrats, ad m s the umber of costrats. The SAIFI dex for a dstrbuto feeder s defed as N SAIFI = l N where l s the falure rate ad N s the umber of customers secto, N T s the total umber of customers o the feeder. The umerator of (6) ca be broke dow to the cotrbutos from the ma feeder ad each lateral as T b+ l N = A = where a s the umber of cateory three laterals, b s the umber of cateory two laterals, ad the frst term, =, s the cotrbuto from the ma feeder. The for each ma feeder or lateral () (2) (3)

4 A = l N + x N j k2 = j= = j= j, k Î{ 234,,, } (4) where s the umber of the locatos o the ma feeder or lateral, l s the permaet falure rate ad s the temporary falure rate for secto of, ad N j s the umber of customers for secto j of ad f there s a fuse at locato, the the varable x k2 =. Note, the dex k represets the type of the crcut, k = s a three phase overhead, k = 2 s a sle phase overhead, k = 3 s a three phase uderroud ad k = 4 s a sle phase uderroud crcut. For the cateory two laterals, a fuse wll be stalled at the tap wth o other protectve devces so devce may be stalled at each locato. Fally, f a fuse s stalled at locato, the a le recloser devce wll ot be allowed for ay possble locato o the load sde of the fuse. Fally, there may be stuatos where oly a partcular devce may be stalled Cost lmtatos ad relablty objectves Wth cost lmtatos, there may be a lmted umber of three phase ad sle phase devces whch ca be expressed as x = = (9) ( ) A = l + N Î a + 2L a + b + (5) = = Euato (5) s costat for a ve feeder sce the protecto has bee already determed, so mmz x3 = = x2 = 2 = 2 3 (0) () a + z = A = s euvalet to mmz the SAIFI dex. For the ASIFI dex, the above euatos are modfed by replac the umber of the customers wth the coected kva load o each secto result A = l L + x L j k 2 = j= = j= ( ) j (6), k Î{ 234,,, } (7) A = l + L, Î a + 2L a + b + (8) = = where L s the coected load secto of lateral Costrats The costrats for ths problem are coordato, des ad applcato lmtatos, ad costs. These are summarzed brefly the follow. The relevat detals of the costrats ca be foud [6]. Coordato betwee devces of the same type or dfferet type must be assured. A aalyss of the coordato restrctos s assumed to be completed determ the devce locatos so that t s esured coordato s possble for at least some combato of devces. The des specfcatos reure a breaker wll be stalled at the frst locato. Also oly oe type of the protectve where s the umber of the avalable three phase le reclosers plus the breaker, 2 s the umber of the avalable three phase terrupters ad 3 s the umber of the avalable sle phase le reclosers. Coversely, there may be a stated objectve or oal for the SAIFI ad/or ASIFI dces so that costrats are placed o these dces ad the cost s mmzed. 6. TEST CASE Cosder the smple overhead radal system show Fure 2 wth eht protectve devce locatos. The permaet ad temporary falure rates, the umber of customers ad averae coected load to each secto are show Table. I ths example, four dfferet objectves are aalyzed detfy the type of the protectve devces at eht locatos. These objectves are: (a) mmz SAIFI, (b) mmz ASIFI, (c) mmz cost based o achev less tha 3.0 for SAIFI (Cost #) ad (d) mmz cost based o achev less tha 3.0 for the ASIFI (Cost #2) Phase Phase

5 F. 2 Smple 8-load pot radal system Secto l (f/yr) (f/yr) N Customers L (KVA) Table Compoet data for the system The system must satsfy the follow costrats whch traslate to a total of 7 costrats the proposed formulato: Fuses wll be stalled o ay other locatos for whch a le recloser has ot bee selected. There wll be a breaker locato #. All laterals are cateory three. A le recloser wll ot be allowed o the load sde of the fuse. There are o coordato problems. The optmal soluto for ths problem was foud us a commercal mxed teer lear proramm packae. Table 2 shows the solutos for cases (a)-(d). Note, the values ve for the objectve cases (c) ad (d) are actually the SAIFI ad ASIFI dces. The cost was calculated as sle phase fuse was oe cost ut, a three phase fuse was two cost uts, a sle phase recloser was three cost uts ad a three phase recloser was fve cost uts. The solutos for mmz the SAIFI or ASIFI dces are detcal wth a breaker at locato ad le reclosers at the other locatos. For mmz cost #, the optmal soluto s a breaker at locato, le reclosers at locatos 2, 3 ad 5, ad fuses at the rema locatos. For mmz cost #2, the optmal soluto s a breaker at locato, le reclosers at locatos 2 ad 4, ad fuses at the rema locatos. Objectve Value CB Reclosers Fuses a) SAIFI 2.9 2,3,4,5,6,7,8 Noe b) ASIFI ,3,4,5,6,7,8 Noe c) SAIFI ,3,5 4,6,7,8 d) ASIFI ,4 3,5,6,7,8 proper locatos to stall protectve devces ca reatly ehace ther effectveess. 7. CONCLUSION Ths paper proposes a bary proramm techue to mprove the effectveess of dstrbuto protectve des. The techue detfes type of the protectve devces at the predetermed locatos o a dstrbuto feeder based o the objectve of mmz relablty dces, such as SAIFI, ASIFI, or by mmz cost whle achev desred performace levels. REFERENCES [] S. Burs ad G. Gross, Value of Servce Relablty, IEEE Trasactos o Power Systems, Vol. 5, No. 3, Au. 990, pp [2] R. Brow, S. Gupta, S.S. Vekata, R.D. Chrste ad R. Fletcher, Automated Prmary Dstrbuto System Des: Relablty ad Cost Optmzato, IEEE PES Trasmsso ad Dstrbuto Coferece ad Exposto, Los Aeles, CA, Sept [3] IEEE Work Group o System Des, Tral Use Gude for Electrc Power Dstrbuto Relablty Idces, Report p366, Draft #4. [4] S. Kostyal ad R.Bllto, Status of Dstrbuto System Relablty Evaluato Caada, IEEE Trasacto o Power Systems, Vol. PWRD-, No. 2, Aprl 986, pp [5] IEEE Tutoral, Detecto of Dowed Coductors o Utlty Dstrbuto Systems, 90EH030-3-PWR, IEEE Press. [6] F. Soud ad K. Tomsovc, Optmz Dstrbuto Protecto Us Bary Proramm, submtted to IEEE Trasactos o Power Delvery. [7] C. McMlla, Mathematcal Proramm, Joh Wley & Sos, New York, 970, pp Table 2 Optmal Solutos Ths umercal example shows clearly the addtoal costs ecessary to acheve mprovemets the relablty dces. If a partcular dex s more mportat for some feeder, the ths wll mpact the optmal locato. I eeral, select