POWER SYSTEM FAULTSPOWER SYSTEM FAULTSPower substations as a target of study consists of some elementslikeTransmissionLines,BusBars,Power Transformers, Outgoing Feeders, and Bus Couplers. Regardlessoftedesignandtesystematicpre!enti!e maintenanceproceduresinstituted,failuresdueto abnormal or fault conditions do occurFaultareintolerablepowerconditions"otertan steady#state or rated ones$ to wic te power system or re%uirement are sub&ected.General causes of power system faults '# Fault Current(ealty insulation in te e%uipment is sub&ected to eiter transient o!er !oltages of small time duration due to switcing and ligtning strokes, direct or indirect. Failure of insulation may be appened, resulting in !ery ig fault current. Tis current may be more tan ') times te rated or nominal current of te e%uipment.*# +nsulation ,ging,ging of power e%uipments may cause breakdown of its insulation e!en at normal power fre%uency !oltage.General cause of power system faults cont!"-# ./ternal Causes./ternal ob&ect suc as bird, kite, or tree branc are considered as e/ternal cause of fault. Tese ob&ects may span one conductor and ground causing single line to ground fault "pase#eart$ or span two conductors causing pase#pase fault Fault EffectsTe fault must be cleared as fast as possible. 0any e%uipments may be destroyed if te fault is not cleared rapidly. Te dangerous of te faults depends on te type of te fault, as e/ample te tree pase sort circuit is te most dangerous fault because te sort circuit current is ma/imum. 1ome of te effects of sort circuit current are listed ere under.Fault Effects'# 2ue to o!ereating and te mecanical forces de!eloped by faults, electrical e%uipments suc as bus bars, generators, transformers will be damaged*# 3egati!e se%uence current arises from unsymmetrical faults will lead to o!ereating.-# 4oltage profiles may be reduced to unacceptable limits as a result of faults. , fre%uency drop may lead to instabilityFault TypesFault can be classified due t as:1- Permanent2- TransientOr due to PARTICIPATED PHAE as1- P!ase-Eart!2- P!ase-P!ase"- P!ase-P!ase-Eart!#- T!ree-P!ase or T!ree-P!ase-Eart!#roa! cate$or%&at%on of Fault TypesFault may be categori5ed broadly into * types61ymmetrical or balanced faults,symmetrical or unbalanced faults#roa! cate$or%&at%on of Fault TypesBalanced (3) FaultsR,R. 6# 0a&ority of Faults are 7nbalanced C,71.1 6#'. 1ystem .nergisation wit 0aintenance .artingClamps still connected.*. '8 Faults de!eloping into -8 Faults L%ne ' L%ne ' L%ne ()"3 Phase; Ia + Ib + Ic = 0 and Va = Vb = Vc#roa! cate$or%&at%on of Fault Types-8 F,7LT1 0,9 B. R.PR.1.3T.2 B9 '8 C+RC7+TTis is !alid because system is maintained in a B,L,3C.2 state during te fault4oltages e%ual and '*): apartCurrents e%ual and '*): apartPower 1ystem Plant 1ymmetricalPase +mpedances .%ual0utual +mpedances .%ual1unt ,dmittances .%ual#roa! cate$or%&at%on of Fault TypesUnbalanced FaultsUn*alance! Faults may *e class%f%e! %ntoS+U,T FAULTS an! SER-ES FAULTS.S+U,T FAULTS/L%ne to Groun!L%ne to L%neL%ne to L%ne to Groun!#roa! cate$or%&at%on of Fault Types#roa! cate$or%&at%on of Fault TypesCauses :1$ Insulation %rea&do'n2$ (i)!tnin) Disc!ar)es and ot!er O*er*olta)es"$ +ec!anical Dama)eDuring unbalanced faults, symmetry of system Is lost therefore single phase representation is no longer Valid#roa! cate$or%&at%on of Fault TypesSERIES FAULTSOR OPEN CIRCUIT:in)le P!ase O,en CircuitDouble P!ase O,en CircuitCauses :1$ %ro&en Conductor2$ O,eration of Fuses"$ +alo,eration of in)le P!ase Circuit %rea&ers C0aracter%st%cs of FaultsA fault is c!aracteri-ed b.:+a)nitude of t!e fault currentPo'er factor or ,!ase an)le of t!e fault currentT!e ma)nitude of t!e fault current de,ends u,on:T!e ca,acit. and ma)nitude of t!e )eneratin) sources feedin) into t!e fault C0aracter%st%cs of Faults T!es.stemim,edanceu,tot!e,ointoffaultor source im,edance be!ind t!e faultT.,e of fault.stem)roundin)/numberandsi-eofo*er!ead )round 'iresFaultresistanceorresistanceoft!eeart!int!ecase of )round faults and arc resistance in t!e case of bot! ,!ase and )round faultsC0aracter%st%cs of FaultsT!e ,!ase an)le of t!e fault current is de,endent u,on:For ,!ase faults: - t!e nature of t!e source and connected circuits u, to t!e fault location andFor )round faults: - t!e t.,e of s.stem )roundin) in addition toabo*e0 ,ecess%ty for fault calculat%onsFault calculations are done ,rimaril. for t!e follo'in)6To determine t!e ma1imum fault current at t!e ,oint of installation of a circuit brea&er and to c!oose a standard ratin) for t!e circuit brea&er 2ru,turin)$To select t!e t.,e of circuit brea&er de,endin) u,on t!e nature and t.,e of fault0,ecess%ty for fault calculat%onsTo determine t!e t.,e of ,rotection sc!eme to be de,lo.ed0To select t!e a,,ro,riate rela. settin)s of t!e ,rotection sc!eme0To co-ordinate t!e rela. settin)s in t!e o*erall ,rotection sc!eme of t!e s.stem,ecess%ty for fault calculat%onsT!e calculation is not onl. limited to ,resent s.stem re3uirements but also meet:T!e future e1,ansion sc!emes of t!e s.stem suc! as addition of ne' )eneratin) unitsConstruction of ne' transmission lines to e*acuate ,o'er0Construction of ne' lines to meet t!e load )ro't! and or Construction of interconnectin) tie lines,ecess%ty for fault calculat%ons%asicall./ t!ere are t'o a,,roac!es to fault calculations0 T!ese are:2a$ Actual reactance or im,edance met!od2b$ Percenta)e reactance or im,edance met!od or ,er unit 2,0u$ reactance or im,edance met!od0+ac!ine and Transformer im,edance or reactance are al'a.s noted in ,ercenta)e *alues on t!e name,late0 Hence t!e latter met!od is considered for our calculation0Per Unit SystemPower system quantities such as voltage, current and impedance are often expressed in per unit or percent of specifed values.Per unit quantities are calculated as: Value BaseValue ActualValue Per Unit=Per Unit Values basepuSSS=basepuIII=basepuVVV=basepuZZZ=ZZ2basebasebasepuVSZZ = =pubase2basepu baseZSVZ Z = = ZConversion of Per Unit ValuesPer Unit Sste!Per Unit SystemUsually, the nominal apparent power (S and nominal voltage (! are ta"en as the #ase values for power (Sbase and voltage (Vbase.$he #ase values for the current (Ibase and impedance (Zbase can #e calculated #ased on the frst two #ase values."##$ Zbaseactual$ =ZZ$he percent impedancee.g. in a synchronous generator with %&.' "! as its nominal voltage, instead of saying the voltage is %(.)( "!, we say the voltage is *.+ p.u.Simplifed: ,oncerns a#out using phase or line voltages are removed in the per-unit system .ctual values of /, 0, and 01 for lines, ca#les, and other electrical equipment typically phase values. 2t is convenient to wor" in terms of #ase !. (#ase volt-amperes %&&BBBB B BVSII V S==( )BBBBBB B BSVIVZZ I V2& '&= ==(Usuall% t)e &*p)ase SB +r MVAB an, line*t+*line VB +r -VB are selecte,(IB an, ZB ,epen,ent +n SB an, VB $he impedance of individual generators 3 transformer, are generally in terms of percent4per unit #ased on their own ratings.2mpedance of transmission line in ohmic value5hen pieces of equipment with various di6erent ratings are connected to a system, it is necessary to convert their impedances to a per unit value expressed on the same #ase.( )= = ZVSZZZoldBoldBoldBoldpu2( )= = ZVSZZZnewBnewBnewBnewpu2ne.Bne.BV base /+lta0e ne. 1S base p+.erne. +n t)e i!pe,ance unitperne. t)e benewpuZ+l,B+l,BV base /+lta0e 1S base p+.er+n t)e i!pe,ance unitpert)e beoldpuZ"22)an0e +3 Base2=newBoldBoldBnewBoldpunewpuVVSSZ Z=oldBnewBoldpunewpuSSZ ZFr+! 4"5 an, 425% t)e relati+ns)ip bet.een t)e +l, an, t)e ne. per unit /alueI3 t)e /+lta0e base are t)e sa!e%2)an0e +3 Base 25 425 45 45 45 4 5 46 6new baseold baseold basenew baseold pu new puKVKVMVAMVAZ Z =7ther #ase quantites :-". in"! . &8!.,urrent 9ase7hms in8!. ("!: 2mpedance 9ase####(##= == =7Per Unit !alues;.ctual !alue 9ase !alue,urrent Unit Per ("!8!.. :::: 2mpedance Unit Per