Cbip Manual Ehv Networks

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M A N U A L,Q N... :..... " ':R E t l A B L E F A . U l I ~ \ [ : E A R A N C . ....' . . . . . . ..:.,.

B ACK-U PPROTEGE [IC )N :~ O: .':". '. . - ., .U H V T R A N S M I . . . . . . : "':.';in ::,::::;:.,::>:::

ISO 9001 :2000Central Board of Irrigation and Power


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Manualon

RELIABLE FAULT CLEARANCEAND BACK-UP PROTECTIONOF EHV AND UHVTRANSMISSION NETWORKS

Publication No. 296

EditorsG.N. MathurB.S. Palki

Kuldip Singh

CENTRAL BOARD OF IRRIGATION AND POWERNew Delhi September 2005


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2005ISBN 81 7336303X

"Reproduction of art ic les in pub li ca ti on in any form is permiss ib le sub ject t() properacknowledgement and inl imal ion to il18 publishers Tho publishers have taken utmostca, _, to avoid errors in the pubilcation However , tho publisi 'lcrs are in no way responsib lefor the authentici ly cf data or 'nformation f jiven by the cr)nt ribulors,"Central Board of Irrigalion and PowerMalcila Marg, ChanakyapuriNew Doihi 110021PI10f1e ' 9111,2611 5984, 2,68B 0557Fax '91, ,26'11 6347E-mail. cbip@cbip,orp/cbipl@vsnLcomWeb www.cbip Of\)

LIST OF WORKING GROUP MEMBERSChairman

Shri Bapuj i S, PalldVice Presiden: (reclln%oY)and Vice Chairman (Technical Committee)Power lechnologios Div is ionAB8 Limitod

Plot No.5&6, Phase l l. PH, No.~)806pecnya tndustr ia l Area, Banualore560 058MfJmI,Jers

snrt S.K Ray MohapatraDr Directo r (SETD)Central Elec1ricity AuthorityRoom No, 712(N), Sowa ShawanRK Puram, New Dcl il1-110066

Stlri Maia Prasad5 (100 V inay KhandGomli NagarLucknow,226010Slld Vikas Sak serraGeneral ManagerPower Grid Corp, of ind ia L td,Plot No2, Sector 29Gurgaon122001, HaryanaShri S.V. Appa SarmaOy General M'mager(Substation qroup)Power G rid Corp. of india Ltdp ro t No2, sec to r 29Gurgaon122001, HaryanaStlri A.K. Guptanoo (PE-Eleclrical}National Therrnal Power Corp, LtdEngineering Off ice Complex (EOC)Plot No. A-8A, Sector " 2.4Noida-201301Shri Pramod KumarDy General Manager (PEElec/rical)Nat iona l The rmal Power Corp. I 'dEnginoering 011ico Complex (EOC)Plot No, A-SA, Seclor 24Noida-201301Ms. M.S. RadhaChief Design EiJgineer(Eleclrical)Nat ional Th.ermal Powor Corp. Ltd,Eng inee ri ng Off ice Complex (FOC)Plol No, A-BA, Soctor 24Noida-Z01301

Stlri D.A. SatheGeneral MimagerHie Tata Powe r Company Ltd,HOJI1l,ay Tholmal Power SlalionMabel Houd, C!lcrnburMwnbai400074Shri S.G_PatkiAssistant Ganem! MI.lflagerTno Tata Power Company Ud,TrOll lt lay Thormal Power StationM,lhui PO(ld, Ci1(HT1burMumbai-400014Shri P.ThangaveluO,ie! DJgineer (egC)Tamil Nad() Elect rici ty Hoard51b Block, Elnct rica l AvonueU02 , Anna SalaiG!1C1Hmi (;00002A nornu 10Shri R.S. AlagappanExecutive Engineer! P&C'1(ll11i\ Nadu Eleclricity Board51h Block, Elect rica l Avanue,802, Anna Salai, Chennai-600002Prof. T.S.M. RaoIZ83 Sec lQr A . Pocke t L l-Vasanl K(l!\jNeV I Dcll'i-11 00/0

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Dr. M.L KothariProfessorOep!!. 01 Electrical EnnineeringIndian Institute of TechnolorJYHauz Khas, New Delhi110016Shr i R.N. Kau!Heea of DeparlmentProtection & -jestingNorth Delhi Power Limited33 kv Griej Su()stil:ionNea r Lal Bha i Gurudwaranani !3agh, Delhi-l10034S!l ri M.K. Choudharyfield. OMDelhi Transco l tdG123, SakelNew Deihi-l10017Shr! U,G. t.eleChic! Engineer (}!Vue 08M Zone)Maharasli!ri) State Electrici ty BoardAdrn, BlJiieJirl[), Piot No. 5Sector 17, VashiNavi MUlT'bai400103Shri Ashok ThaparDeputy Director IPlfD, PSCBhaxra De"ls Manaqerncnl f30ardSLDC Complex , Indw; lr ia l A rea PhaseChandigarh-160019Shri F13jivKrishnanAssistant Vice Presidentf lead S(lbs1ation Automation (Technolo[JY)A!35 LimitedPlot No. 5&6, Pilase11Peenya Industria! ArNlPB. No. 5806f3angalore-:5600"S

Shri Srinivas PatkarASSistant Vice President (Technical)Communication SystemsAB B Limited22-A, Shah Indust rial EslatoOlf, VONa Desa i Road , Andher i (West )Mumbai-4000:)3Dr. G. SambandanEx-General ManagerEasun Reyro ll f" L imited98, Sipcot industr ia l ComplexH05ur-635 126 (Tamil Nadu)Shrl J.P. laijuALSTOM Ltd.19/1, G.ST Road, PallavaramCh,mnai-600 043Shri B.C. BadiyaSr;Manager ..MarketingPower Transrni,~sion & DistributionSiemens U(i., Plot No. GA, Sector 18Mi_\l'u!i Industria: AmaGUfPil0r1-1220 15

Reliable clearance of a fault, when it occurs in thepower sy:tem network, is cri tical for maintaining powersystem stability, for avoiding system collapse and for bettergrid operation.This manualas the name suggests deals withthe subject of reliable fault clearance andways of providingmain and back-up protection to various power systemelements in EHV and UHV transmission networks.

Wemust not assume that all elements in the fautt clearance systemalwaysoperate correctly. Protection relaysmayfailto operate or mayoperatewhen they shouldnot. Switchingdevicesmay fail to interruptthefault current.It isa commonpracticein EHVand UHVnetworksto duplicatethe protectionsystems. In addition back-up protection is provided to operate when a powersystem fault is not cleared, or abnormal condition is not detected, in therequiredtime because of failure or inabitityof themain protections to operateor failure of the appropriate circuit breaker(s) to trip,S)!providing back-up protection, it is possible to reduce the risk ofwide spreaddisturbances when a protection relay or a switching device failsto operate. The main protection and the back-up protection may reside indifferentsubstations (remote back-up) or in the same substation (local back-up). In case of local back-up, it can be a substation local back-up and/orcircuit local back-up protection.The requirementsof the back-up protection cannot be independent ofthe requirements of the entire fault clearance system. Back-up protection isan important function of the protection system and its design needs to becoordinated with the design of the main protection. Therefore the protectionengineershouldworkin closecooperationwith systemplannersand designers.Even though back-up protection is the focus of this manual, anydiscussions concerning it cannot be separated from main protection. Thusthe manual deals with issues involved in complete protection system viz.,mainprotection,duplicate protectionand back-upprotectionfor various power

system elements in a comprehensive way.The elements covered are Lines!Cables. Power transformers. Shunt reactors, Bus bars and Breaker fai lprotection.The manual also reviews brief ly current practices and makesrecommendations for selection and application of various main and back-up

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FOREWORD

Shri G.K MathurSccrctsrvCerMal Board of Irrigation and PowerMalcha Marg. ChanakyapuriNew Delhi -II OO?1

Shri Kuldtp SinghDirector (Energy)Cen tral Boa rd of I rr igat ion and Powe rMalcha Marq, CnanakvapuriNew D8111;-110021Shri S.K. BatraM,magor (hocl)('ical)Central Goard 01 I r ri ga (ion and PowerMalclla Marg, ChanakyapuriNew Deiill-l10021

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protec tions. A care fu l s tudy o f the manual wi ll help unders tand the reasonsbehind many of the practices cur rent ly fol lowed and help ut il ities nne-tunethem. Ialso notice that i n a number of places reference has been made toanother CSIP manual (No . 214) t it led "Protection of Generators, GeneratorTransformers and 220 kV and 400 kV Networks". The two manualscomplement each other.

Jam sure that the present publi cati on wi ll prove to be a very usefulguide for the Power Ut il ities , Manu facturers, and concerned engineers,particularly to those who have so far been deprived of a uni fied documen t onthe subjec t, and i tg ives them an ins ight to a number a t aspects that shouldhelp them in manag ing the rel iab il it y and avai labi li ty of protection systembetter.

Iappreciate the efforts made by Central Board of I rr igat ion and Powerin bringing out this document and congratulate them for their initiat ive. I a lsoappreciate the efforts made by Working Group in bringing out thiscomprehens ive document and commend the invaluable cont ribution of themembers in bringing out this manual.With Bos! Wishes

.~~Bhanu BhushanMember, CERC

September 2005New Delhi

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PREFACEWith Indian economy growing and expectations of

industries and consumers using electri city increasing,prov id ing rel iab le power supply is becoming increasinglyimportant To meet this chal lenge new power stations arebeing added and T&D networks are continuously beingstrengthened. Formation of nat ional grid is one major step inthis direction.

Protec tion and Automat ion system prov ided in the power system hasan important role to play in meeting the challenge of providing reliable powersupply. Within this, the protect ion system for various power system elementsplays a major role. Two of the impor tan t requ iremen ts on protec tion systemare dependabili ty and security to providereliabte fault clearance.

Toincrease the reliabili ty of faull clearance system in the event of fai lureof main protection or circuit breaker, duplicate main protect ion and back-upprotect ions are provided. Even though protect ion pract ices to be followed incase of EHV and UHV networks are well establi shed and documented, yeta need for separate document on the subjec t o f back-up protec tion to serveas guide to utili ti es in India has long been fett.

In October 2001, CBIP organized 2nd Internat ional Conference on"Integrated Protection, Control and Communication, Experience, Benefits andTrends". Dur ing the valedictory session o f this conference, i t was dec idedthat CBIP should constitute a working group to come out with a report onBACK-UP PROTECTION to serve as a guide to uti li ties in India. Accordingly,CBrp const ituted a Working Group under the chairmanship of Shn B.S. Palki,Regular Member CIGRE Study Committee 85 (Protec tion & Automat ion)from ABS Limited, Bangalore and experts in the fieJd drawn from variousuti li ties, manufacturers and academic institu tions in India.

The Working Group, as a result of their untiring and sincere efforts ofmore than th ree years, has come ou t wi th a comprehensive documen t ti tled,"Manual on Reliable fault clearance and back-up protect ion of EHV and UHVtransmission networks". Over the period of last three years there wereseven formal meetings of the group, each las ting fo r two days and severalinfo rmal meet ings and discussions to ar rive at the structure o f the manua land finalise its contents.

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The manual g ives basic concepts of back-up protect ion, requirementsplaced on the protection system from the power system and includesdiscussions on main and back-up protection systems for lines/ Cables,Power transformers, Shunt reactors, Bus bars and Breaker fai l protect ion.Discussions on DC system and communication system, which form importantelements of the protect ion system, are also inc luded . Use of per fo rmanceindices to assess dependab il it y and secur ity of protection system to serveas tool to make cont inuous improvemen ts is a new concep t for our ut il itiesand th is is covered in a separate chapter . In each chapter suggestions andrecommendat ions have been made for the guidance of the uti li ties to reviewthe cur rent prac tices and make changes and adopt wherever necessary.

The quantum of technical work involved in complet ion of th is manualcould not have been accomplished without the unt ir ing efforts and invaluablecon tr ibut ion of the members o f the Exper t Group & CBIP. The Cent ra! Boardof Irrigation and Power thankfully acknowledges contribution of aUthe membersof the Working Group drawn from various uti li ties, academic institu tions andmanufacturers for their valuable contri butions. Special thanks are dueto Shri B.S. Palki, Chairman of tile Expert Group .or tho tremendous inputand direction given in f inalizing the manual. He also formulated tho draft ofthis manual and then incorporated the suggest ions/feedback received fromthe various members. The contribution of Shri Mala Prasad, Shri VikasSaksena f rom POWEFlGHID, Shr i S.G. Patki f rom Tata Power , Shr i S.K. RayMohapatra from Central Electricity Authority and Shri Pramod Kumar from NTPCfor their activo involverneot in giving final shape 10 the manual needs prominentmention.

It is hoped that this manual wil l help the protect ion engineers of variousutililies in India in managing the availability and reliability of the fault clearancesystem in transmiss ion ne tworks in the vol taqe range f rom 132 kV 10765 kV

September 2005New DelhiG.N. Mathur

SecretaryCentral Board 01Irr igat ion and Power

CONTENTSForeword (v)Preface (vii)Chapter 1 IntroductionChapter 2 Fundamentals 5Chapter 3 Requiremenls 13Chapter 4 Performance Indices 19Chapter 5 L ines and Cables 25Chapter 6 Power Transformers 63Chapter 7 Shunt Heaclors 75Chapter 8 Bus Bars 85Chapter 9 DC Aux il ia ry Supply Systems 95Chapter 10 : Switching Devices 101Chapter 11 : Communication Systems 111Chapter 12 ; References 129

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1!1I

CHAPTER 1INTRODUCTION

The Indian power system is growing steadily. To match with the growing demand,transmission system is also expanding with 765 kV AC lines overlaid on existing400 kV system, new long distance HVDC links in hybrid mode and providing seriescompensation including TCSC wherever feasible on existing 400 kV and 220 kVlines etc. With the development 01 regional grids and interregional l ies to formultimately national and intemational grids, the power system is becoming more andmore complex.

Along with this growth, requirement of high availability and reiiab!e operationof large generating plants with EHV and UH V transmission network assumetremendous importance in maintaining power system stabil ity for better gridoperation. To realize the vision of "ReliatJlc, Affordable and Quality Power for All"somo changes wil! be required in our approach towards integrated protection andautomation system planning.

The protection and automation system used in tho network plays an importantrole in mee1ing this requirement, preventing system collapse during major systemdisturbances, reducfng outage time and minimizing the possibility of damage to ttlemachines and equipment. Ut il ities should install protect ion systems that aredependable. Hero, dependability is the probabil ity of not failing to clear a powersystem fault Of abnormality.

The manual deals predominant ly with this subject viz.: how to enhancedependability of fault clearance system.

When a fault occurs in tho network a protective relay may fail to operata or acircuit breaker may fail to open and interrupt the faul t current. Such tal lures of aprotective relay or a switching device may prevent proper clearance of the faulL

The addit ion of a second main protect ion increases the avai labi lity anddependability of rault clearance system. In addition, tho provision of back-upprotection that operates independently of specified devices in the main protectionsystem enhances this further. It can be genellly said that provision of second rnair,protection and back-up protections enhances the dependability 01 the fauit clearancesystem.

Whi le the provision of second main protection and back-up protect ionenhances the dependabil ity of fault clearance system it increases the costs.Therefore thoro is ollen a tendency to choose to run the power systern without areserve protection and adequate back-up protection in the network. At voliaqe levelsof 220 kV and below this isolton the casco


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2 Manual onflefmblo F:wit Cle:Hanceand Back-Up Protection ofEflV and UfiV Transmission NetworksChapl'" 1 ; Introduction 3

The dis turbance costs inc lude the foHowing :

While preparing the manual the working group has used a report on thissubject prepared by CIGRE working group SC 34; WG:01 ti tled "neliabl e faultclearance and back-up p rotec tion" . I t has been a valuable source of Informat ion tothe working group entrusted with preparation of this manual.

Ano the r documen t refer red whi le prepar ing this i s CBIP manual (No 274 ) t it led"Protection of Generators, Generator Transformers and 220 kV and 400 kVNetworks". Reference has been made to this particularly while makingrecommendat ions to uti li ties in Ind ia .

Since t he number of shunt f aults is high, consequences of failure to clear afault may be ser ious and r esul ts in hi gh disturbance costs that are far greater thanthe costs involved in providing additional protections.

Costs associated with the risk of injUlY to people and damage to third partyproperty Costs associated with the risk of damage to power lines and other powerapparatus

Costs assoc ia ted with the customer outages Costs associated with the voltage disturbances Costs associat ed with the customers' complai nts and ill will.

Therefore the ut il it ies should base the decisions for provision of addi ti onalprotection and back-up protection considering the costs involved in terms ofdisturbances l eadi ng towards unclear ed faults Recommendations made in thismanual have been done considering the abovo. It is further suggested that theprotect ion enqinear should keep this aspect i n mind while reViewing protectionsystem in any given case.

The manual is structured as under.Chapter 2 def ines some Iundarneata l concepts of back-up protect ion,Chapter 3 descr ibes brief ly the externa l requirements for pro tect ive sys temstha t cou ld come in future f rom external author it ie s, governmen tal and o theragnnc ies, equ ipment manufac turers , insurance companies , s tandardizationorganizations, ulili lies and customers.

Chapter 4 defines some pertormance indices for protection devices andswitching devices.Chapter 5 to 8 deals with the diff erent power system components such as:transmission circuits (overhead line.. and underground cables) powertransformers, bus bars and shunt reactors. Each chapter contains,requirements on the protection system, pract ices of prot ection and a finalsection on back- up protection.Chapter 9 deals with lho DC auxiliary supply system and redundancy in theDC supply system.

Chapter 10 deals with the performance of switching devices and circuit-breaker failure protection. Chapter 11 deals with reliability and redundancy of telecommunicationsystems.

CI1C1pter12 9iv()S somo references.

In conclusion i t can be said tha t even though back- up protect ion is the focus ofthis manual, any discussion concerning it cannot be separated from the Mainprotect ion and thus the manual dea ls wi th i ssues involved for comp lete protect ionsys tem inc luding Main and Back-up protect ion. Appropria te recommendat ions havebeen made i n various places, where it is foil that they are departures from thecurrent practices.

Finally it i s hoped tha t the manual wi ll help the protect ion engineers of var iousuti lities in India in managing t hn availability and roriabilit y of tho fault ctearancosystem in transmission networks in the voltage range from 132 kV to765 kV.


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CHAPTER 2FUNDAMENTALS

. ~IfIIj~!

A back-up protection is intended to operate when a power system fault is notcleared, or an abnormal condition is not detected. in the required time because offailure or inabHity ot main protections to operate or failure of the appropriate circuit-breakerts) to trip. The back-up protection, by definition, is slower than mainprotection, Back-up protection is installed to improve the dependability of the faultclearance system. Here, dependability is the probability of not failing to clear apower system fault or abnormality,

Back-up protection shall operate when main protection fails to clear a fault. Insuch a case, the protection may not operate correctly, tile circuit breaker may notreceive any tripping command Of the circuit breaker may fail 10 open and interruptthe tautt current Such fai lures of a protect ive relay or a switching device mayprevent proper clearance of the fault.

Sometimes a second main protection or duplicate protection, intended tooperate ~f the m~in protGcti{)n oy~lej I 'di~sto opera Ie or !s temporarily out of serviceis provided. This, however, should no! be mixed up with back-up protection. Thesecond main protection is there 10 increase the dependability of normal fault clearingmechanism and it must always operate very selectlvely, while the back-up protectionmay operate with less selectivity because it operates after some lime delay.

The requirements on oack-up protection cannot be indepenUe'l! of therequirements on the entire faul! clearance system.

Use of elementary form of the single-fai lure cri terion is often cone whi leplanning protection system arrangement H requires that the Iailure of anyonecomponent in a fault clearance system shoulc not result in a complete failuro [0clear a power system fault or abnormality.

Back-up protection is an important function of the protection system, and itsdesign needs to be coordinated with the dosiUn of the main protect ion. In rhrsprocess, it is suggested that the protection enginoer should work closely wilh thopower system planners and designers.

Tho system planner should inform tile protection engineer regardingassumptions made during system design and requirements on fauit clearancesystem at various voltage levels in the system. HE)must inform him of the needs ofthe protection system that must fulfill, as lor example. ti,e IotaI fault clearance tllne.The protection engineer must also be farnillar with the lo/Iowing :

System requirements


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6 Mo1lwa/ On nolNlb/o Faull CIUaril( l( ',e and Back Up Pro/eel/on ofEfiV,mti IHIV Transmission NetworksThe sys tem des ign cri te riaThe plant specificationsThe failure rates of the protected p lantTho requirements of performance ind ices of the fau lt c learance sys temThe requirements of rel iabi li ty o f pro tect ion equipment

The probabilit y that a switching devi ce f ails to int err upt the fault current .

The power sys tem p lanne r should he lp the p rotec tion eng ineer in formulat ingthe requirements of the fault clearance system. Points to be considered are ihestabili ty of tho power system and t he type of bus bar arrangement and switchingscheme . P rotect ion eng ineers and the powe r system deSigners mus t co-ordina te thedemands on faul t clearance system, as also the pe rformance of stat ion equipmentand strike a balance between technical and economical benefits and the risksassoc ia ted with mak ing the protect ion sys tem more complex ,2.1 FAULT CLEARANCE SYSTEMSThe basic task o f any fau lt c lea rance sys tem is 10 detect a spor:ifiec! c i a ,s ; :; o f P0WO,sys tem fau lts and abnonnaii iies and to d isconnect t tl (; assoc ia ted item of substationor plant from the rest of the power system. Figure 2.1 shows the components of afau lt c loarancs sys tem. Here TE stands for telepro tect ion equipment.

The f aulty componom should be disconnected as f ast as possiol o and wit hminimum disturbance to the consumers and minimum damage to the powerapparatus , An essential property of the fau lt c learance sys tem is rel iabi li ty . Rel iabi li tyo f p ro tec tion includes dependabi li ty and secu ri ty of protect ion. Faul ! ana lys is andrelay co-ordmanon are important issues for the rel iabiLly o f pro tect ion sys tems,

A power system fault is a power system abnorrnal.ty iha: mvoi vos, or is theresul t of, tailur e 01 primar y equipment and normally requires t he immediated isconnection of the faulty eqUipment f rom the rest of the powe r system by trippingof the appropriate ci rcuit breakers. Power system faults can be shun t, se ries orcombination faults.A non-power sys lem fau l! t ri pping is an unwan ted t ripping of a c ircu it b reake ras a resu lt o f faul ts, other t il an the powe r sys tem fau lt s. The unwan led ope rat ions o f

a protect ion in the absence of a power system faul! or the tr ipping of a breaker dueto o ther seconda ry equipmen t fai lu re or due to human error are examples of non-power sys tem fau l! .

III.iii~

Ctiepter 2 : Fundamentals

Protection System

7

Circu i Breakerr----, __- ~Cifcwl!3reaker~-A(",ch~]Iw~nl

2.2 BACK-UP PROTECTIONAll elements in the fault clearance system do not always operate ,correctly.,-. !t t 0 or ate or may oper ate when they are no! required to~~~~~~~O~~~I~~i~;n:Vi~()S ~1a~ fai l to interrupt the fau!! current. Common ~ra~t lce !~

'. I I rot oct ion syst ems operating i n parallel. 8ack~up pr o eo iOn. k>to use SO\ era p . er s'stem fau lt is not c leared, or abnormal condi tionIf1ten~~cttto lo~e~~t~h:~:;u~r~~~ime ~ecause of failure or inability of other protectionsIS no e ec e , ' he f It Ih. main fault clear ance system e.g,10 kPerale or becbalusreroOmfpoTm~r~~e~~~e i~ntheecab le from relay to the CB trip coi lbro en wire if) ca e. .or fai lu re of the appropria te c ircu i1 breaker(s ) to trip.

Sv rov ict ing back -up protect ion , i t i s possibk, . to reduce the ~iSkaris:ng out o fs ituat io~ ~hen a p ro tec tion relay or a SWi tching dev ice fal ls to operate. T tl ( back upprotections can be claSSified as under.Remote back up. dT !The main protection and the back-up protection may reside in . IirereosubstationsLocal back-up . .. 't _) 0 "Ibstation. 1 t' o 'rle back-un pr'Ao('tIDn reSide In ,10 san t. .J . c .he main pro eL,on an , .. c ., ' ". ..The local back up protect ions can be f lJ rlher c lass il ieo as under.


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8 A1anual o n F ic fi .a bfe Fau lt C icar an r. :u nnd Back,Up P ro te ct io n o fE/IVand UHV rransmission Networks

Circuil tocat back-up protoctionThe protection senses the same current a.nd voltage as the mainprotection.Substation local backup protectionThe protection uses current different from the one used by the mainprotecnon,

Ideal back-up protection should be completely independent of the mainprotect ion. Current trans formers, vol tage trans formers, aux il ia ry tripp ing relay , tripcoi ls and auxi li ary DC supp ly systems should be dupl icated. Th is ideal cond it ion israrely a ttained in pract ice, The foHowing compromises are usual ly made: There! is only one current transformer but it has several cores. One core andi ts assoc iated secondary w ind inO energise each p rotect ion. Some t imes oneCT secondary winding foeds more than one protect ion.

Common vol tage t rans formers am nor riai ly used because dup li ca tion wouldinvo lvG a considerable increase in cost , because o f the vol tage t ransforme rsthemsel ves, and because of the increased space that would have to beprovided. Since Socu ri ly of the VT ou tpu t i s vi tal , i t i s desi ,, Jb l, , tha t t il e supp lyto oach protee lion is ei the r f rom separate co res o r sepa rately fused as doseto the VT as poss ible and cont inuously supervi sed by a relay that will givealarm on fai lu ro o f the supply and, whe re app rop riate, p revent an unwantedopenalion.

Trip supplies to the two pr otections shou!cl be separ ately fused. Duplication oftripp ing bal le ries and of trip coi is on c ircu it breakers is sometnnes provided.2,3 REMOTE BACK-UP PROTECTIONHernote back up protect ion is the Ideal form 01back-up protoct ion, in sys tems wherei t can funct ion p roper ly . Second i lnd thinj zones o f distance relays are examp les o fremote back up. SO"lCt irnes var iat ions of the I ! , feed al 1110 rc-not o bus bar s mayvir1ua!Jy proven! (he application of remote back-up proleclion.

Tho advanlaQO o f remo le back -up is Ihat i t i s cGrnp ietel y independent o! theprot ect ion r elays, currenl transformers and voiiage transf ormers of the mainproloclion syslcrn. 11 is a lso indepc(l(Jent o f tno aux il ia ry DC supp ly system and thob reakers in Ihe subslal ion. There a re har (j ly iF'}' ilar[Jware f ai lu res thai can af fectboth lho rna in pro tect ion and (110backup protection. The $011in9 of Hemo!o Back-upp rolec! lOf ) relays to cover the fault ou[o; ide the 7 !rs! zone o f p rotect ion beyond Iherernole bus bars duo to in feeds 10 t tle faull frem other par aiiDi sources are quit ecornpficalc:d {lno' sornot:rncs less scJcclive

Ctieptor 2: FUf{{jamontafs 9

2.4 CIRCUIT LOCAL BACK UP PROTECTIONThe ci rcui t l ocal back up p rotect ion uses (he ,lame current an~ vo ltage a~ the ma i~rotec tion. Delayed direc tional or non d irec tional over current and earth fauII ,relaysfhat are provided in the same c ircu it are example .. ; o f CirCUit local back up protect ion.

2. 5 SUBSTATION LOCAL BACK UP PROTECTIONA substat ion back up protect ion is a umo-dolayed p rotect ion provided in the samesubstation but normally fed by a CT different from the one toedinq tho rnamprotection, Over current protections provided rn the incomin9 feeders proViding tac~up to protections in the outgoing feeders 111 a substation IS one examp e asubstation local back-up protection. /I must be noted that Ifl a meshed netw~rk ~may be difficult to obtain back up protection of EHV lines by means of substatiolocal back up protection.

In EHV substat ions i t is possible to provide substat ion local back up protect.onby reverse looking e lements of d is tance relay .

A B

H[ij3_J=[5J=DF-=f~~J ..l _ - - -=c-=- [~J '- -=[6J: -c=:=cQ ~]-2,6 DUPLICATED MAIN PROTECT IONSIn EHV and UHV networks i! is common pract ice to use duplica led l ine protect ions ,viz: Mainl Main 2 protections. Since the current transformers, the vO~laget ranstormers and the b reakers are t il e expens ive components In 1h le : ~ l l J , ! 1 clearancesystem it may be dil licu ll to Jus ti fy the cost for i tlel f, d ll !) IIC


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10 Manual on ffe!l~'Jb!u Faul! Clc~1ranceandl1ack-UpProtection ofFf1Vand UNV Transmission NetworksBlock-S represents the lelopro tect ion equipment for Main- 1Block-a represents the tetepro toct lon equipment for Main-2

f3Iock-5 represen ts the telecommun icat ion equipmen t for Main-1Block-C represents the telecommunication equipment for Main-2.

2.7 BREAKER FAILURE PROTECTIONBreaker fai lure p rotect ion is pa r! of the loca l back -up protect ion. The breake r fai lureprotection has to trip the adjacent breakers when the main breaker does notinterrupt the fault current. The most common, and simplest, breaker failureprotection consists of a timer , which t he protecti on st arts when it operat es. If thefau lt cur rent pe rsi sts for longe r l ime than the set tl ng o f the t imer , the breake r fai lureprotection gives a tri p command 10 adjacent breakers. Figure 2. 3 shows the basi cdecision process in any br eaker failur e protection. The ret rip si gnal shown here i soptional.

Yes

Fig, 2_3 Flow (jragram f()f b-roaKer jalluf(! proWctlon

~:8r~I Failure II Protection Ii..

Yn s

Nor~T~~1I Adjac",n' J~_~eakef(S)2.8 RECOMMENDATION FOR APPLICATION OF SINGLE FAILURE

CRITERIONIt is recommended 10 apply the single- failure cr iter ion in the planni ng of the faullc learance sys tem. An elementary form of the s ingle~fa ilure cri te rion requires that thefailuw of anyone component in a faul! clearance system should not result in acomplete fail ure t o clear a power system fault or abnormali ty. The single-f ailurecri te rion can be applied as fol lows:

Assume lha! the powe r system is ei ti tor in Its normal S\vi !ch ing s tale or tha t onel ine is out o f service

Cflapier 2 : Fundamentals 11

Assume that a power system fault occurs on the power system. Consider t hefollowing types of faults.Three-phase fauftPhase-to-earth tau!tPhase-to-phase faultOpen conductor or broken conductor fau lt

Assume that there is a tault in the fault clearance system (in substationelements). Consider one of the fol lowing types of faults in the f ault clearancesystem.loss of input from a vol tage trans former,los s o f inpu t from a current transformer,A fai lu re to operate of a protec tion relay,A blown DC fuse ,An interrupt ion of a tripp ing c ircu itA failur e of remote end communicat ion (especially li ne differ ent ia!protection)A fai lure to operate of a swHching device.

Check if wi lh Ihe aoove fau!t (s), the f aun cl ear ance system clears the powersystem fau lt a t p re-def ined loca t-or -s cons ider ing (ho fol lowing types of fau lt son system elements:A l ine fau l!A bus bar faul tA fau lt in power trans formerA faul t i n the shunt reac tor

Add second mai n protection or back-up protection until Hw f ault clear ancesys tem clears a l the fau lts.Check if the healthy li nes and healthy items of pian! can wit hsland t he faultcur rent for the above cases. Add second ma in p rotect ion, back-up protec tiono r reinforce the pr imary equ ipment unt il i t wi thstands the faul ! cur rent du ringthe fau lt c learance t ime.The p rotect ion eng inee r mus t car ry out a more detailed analysis 10 check theabove cases under d if ferent ou tage cond it ions for which the system has beenpl anned. Examples of out ages to be considered are outage 01 a Ilno, outage of apower trans former, outage of a generator e tc .


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CHAPTER 3REQUIREMENTS

The protect ion engineer has 10 unde rstand requ iremen ts that are expec tedf rom the protection system before deci ding the arrangements for it Here therequirements can be grouped under two categories; Externa! requir ements Power system requirements

3.1 EXT ERNAL REQ UI REMENT SExternal requir ements for prot ecti ve syst ems encompass a wide range of non-techn ical considera tions put on the protect ion engineer by some externa l authori ties .These considera tions fal l in the fol lowing s ix catngorios :

Flequiremonts imposed by various governmontol and other agonciesregarding safety. Requirements imposed by rna(lufac!urOIS of the pr imary equipment in order 10validate equipment Warranties. Requirements by insurance companies who undervvrlte equipment failures. Legal r equirements to meet "pr udent uHlity practice and industry standar ds" in

the even t tha t p rimary equipment fai lu res resu lt i n personal injury o r p roper1ydamage and legal action is taKen against the util ity by tho parties incurringdamage.Requirements for ll1e safety grounding systems.

General power quality requirements fr om the customerSome of these requir erllonls may not be imposed or fel! at present by tileprotection engineers in India. But as and when these are introduced by the

concerned author it ies to meet the requirements of tile system as a result ofderegulation and restruclur ing comi ng into force, the appr opr iato pr otectionappl icat ion commensurate wi th (he chan ned envi ronmen t wi l! have to made by theconce rned author it ies. Tho ioi lowioq parawaphs g ive some more de tai ls o f theserequirernonls3.L 1 Safety RegulationsEl ectr ical Safety ncgulalions may require Some back~lJP pr otections ThesereqUirernonts are intended to meet lhe min irnum requirernents assoc ia ted >\'ifll publicsafety.


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14 Manual on fleliaNe Fault Cloafanco andBack"Up Ptotacticn 01HIV ,'nd UeW Transmission No(works3.1 .2 Equipment Warrant iesU ti li ti es may obtain war ranty as a pa rt o f pu rchase agreements. The manu facturerhas the respons ibi li ty to replace damaged equipmen t and may requi re the u ti li ty toprovide a min imum level o f pro tect ion.3.1.3 Insurance RequirementsDevices may be covered by insurance. The insurance companies pay the cost ofrep lacing fai led equipment and may require min imum levels of pro tect ion.3.1.4 Industry StandardsI n some cases persona l injury and proper ty damage l iab it il ie s are decided in cou rt .The cour t' s dec ision may be based on a rev iew of p rotect ion standa rds . ! fl he ut il it yhas not met the minimum levels of protection, they may be held liable for theappropriate damages.3.1.5 Prudent Utility PracticeAnother common case against the utility is based on a review of prudent utilitypracl ic e I f one ut il it y uses lowe r leve ls of protec tion than other u ti li ti es, i t i s a rouedthat the uti li ty rs no t fol lOWing pruden ! l Jl iI it y p rac ti ce and they may be he ld Habie forthe appropriate damages.3.1.6 Safety GroundingCare is to be taken to provide for appropriate step and touch potentials duringground faults iJS also control c ircu it t rans ients assoc ia ted with fau ll switching or evennormal operations. Short c learance t ime is advantageous because the risk of e lectricshock is_great ly reduced and the r isk of seve re injury or death is great ly reduced ift he du rat ion o f a cur rent f low throl lg ll t he body is very b rief .3.1.7 Power QualityPower qu,"i ty requirements are changing as competit ion increases. Potentialcust omers wil l start aski ng for reliability data of the elect ricit y supply before theyestablish new plants" Some process industr ies cannot tolerate even very shortsystem disturbanccs.

3 .2 POWER SYSTEM REQUIREMENTSIn general p rotect ion system c ri te ria must mee t the planning and operat ing cr iter iareou ircmen ts whi le mce tlng thn speci fi c mqu iremc! tl ts of the power sys tem e lemen tbe ing protected and p revent ing damago to othor power sys tem e lements supplyingfault current.

Chapter 3:fiequiremonts 15

Over t ri pping o f protect ive system must be l im ited to events where more than(n- t) d irnens ioninq can be tolerated by the system. Fai lu re o f the protect ive systemto operate during fault events must be eliminated by using appropriate back upprotection to fulfHt the requirement of planning criteria,

Duplicate protection and back-up protection must meet ali of the designrequirements of the p lann ing cri te ria at a min ima! cos t.

3.3 NORMS USED TRANSMISSION PLANNING IN INDIAN POWER SYSTEMThe p lanning and ope rat iona l requi remen ts and secur it y standards for the purposeo f t ransmission plann ing for Ind ian power sys tem a re brought out in the Documentt itled "Manual on Transmission Planning Crit eria" issued by Cernral Electr icityAuthor it y, New De lhi in 1992 and this may be referred for more details. Thisdocument may be further subjected to revisions lor updafing in view of systemexpansion.

3.4 PROTECTION CRITERIAProt ecti on crHeri a arc devel oped and derived from the Planning and 0pAfAt'n:Cri teria. The purpose is to meet t he dimensioning requi rements associated wi thf ault s so as to prevent f oss of st abi lity, l oss of synchr onism, voltaqe col lapse,undesi red load shedding o r unaccep tab le f requency and vol taqo excursions. Someu ti li ti es may want 10 do preventive maint enance with ( he pr imary equipment inservice"

Utilities may desire to provide for loss of one system element (such asequipment fa i l ure) dur ing peak toad conditi ons. If this be t he case such planningcriteria should be made known to the protect ion engineer. It i s vi tal that the re is ve ryc lose col laborat ion be tween t il e planning and protec tion eng ineers at the ve ry oar tystages of the projects to identify tho protection requirements for the envisagedprojects.

There are t hree types of crit eria, as ment ioned below, that the protect ivesys tem must meet.Cr iteri a specific to (he equipment wit hin the protective zone must be mel .These are construct ion speci fic requirements SUCrl as tank rupturerequirements of capacitor cans.Cr iteri a specific t o other equipment suppl ying fault current to the faultedelement, These are fault current withstand requirements such as themax imum fau lt current durat ion of a power trans former.Cri te ria speci fic to the s tabi ii ty o f the network . These are topology speci fic trmelimits associated with voltage and transient stability.


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16 ManU/,a Of] nc/i;JbfD , . - : t : w i t C IDa nmco a n d B a ck -U p P rot e c t/ o n 01EHV and U{fV Tronstn.ssion NelwolksPower sys tem fau lt s sub ject t he power generat ing uni ts 10vo! !age excursi ons

and dips. If tho power system fault o ccurs close to the la rge power stations, there isa risk that many power generating units could get disconnected from thetransmission networks. Ttlis means that a correctly cleared fault may cause anoutage i f t he power -generat ing uni t does not w it hs tand severe vol tage d ips. Back-updelayed c learance o f a power system Iau'. close to the power plant may cause asevere outage. General vol tage s tabi li ty requi rements determine the durat ion o f t hefault and are system configuration dependent .

3.5 FAULT CURRENTWITHSTAND CAPABILITYNo utility can speci fy that tho power system e lements sp ecially CT, Isola tors a ndcircuit breakers besides 1 I 1D power t ranstorrners shaH w);hs land only fault currentsassoc ia ted w it h norma l f au lt c learance and take the r isk o f c ir cu it b reaker fai lu res.Ihe system elements are usual ly speci fied 10withstand the fault currents associatedw it h backup c learance t imes o f 05, 1 .0 and 3 .0 seconds

The power system elements must w ithstand both normai rat ed load currentsand fau!! currents spec ificd.n10 raWd poak withstand c urrent and the rated shortt ime vvi thstand current character ize the components. Fault currents are usual ly sma!!i n t he ini ti al s tages o f oevotop rnon t o r power sys tem. Norma ll y. ! hc magni tude c f U ,efault cur rent increases as the system develops. The magnitude of fault currentsi n! luences the d imenSion ing o f t he power sys tem components l ike t rans fo rmers,circuit breake rs and other primary equipment. High sho rt circuit currents a ffectprimarily til() mechanical and tt.er.nal dimonsioning of the power systemcomponents.

The lault clearance s ystem cannot normally influence the peak value of thefnu lt cur rent . The p rope rt ies o f Hw fau lt c learance sys tem can , however , i n! !uencethe durat ion o f t he fau lt cur rent . Thus, t he fau lt c learance t ime inf luences the hea ti ngo f conductor s dur ing d is tu rbances. The p ro tect ion eng inee r mus t know how long thepower system components can withsta nd th e faull cur rents wit hout permanentcamaqo. I n ger ;e ra l, t he components o f t il e power sys tem are speci fi ed to w it hs tandHI() fau! l curron I duration a ssociated with back-up faul t clearanc e a s spec ified innalional and international standards. For example, transformer through faultcapabil ity is ou( fined in ANSlJ lEEE C:l7.12.00 and IEC Publications 76-5.

Tho requirements in respect of Indian Power System for fault willislandcurrents of tho power system elomerus a re broUGht out in the document e ntitled"Manual on Tra nsmission Pla nning Criteria" of Central Electricity Authority, NewDel il l. Somo dotai ls from this am reproduced below.

Ch;;ptcr 3 : Requirements 17

Tal lIe 3.1 Raled breaking current capat li !i ly ofswitchgear ardi f ferent vol tages

3-6 RECOMMENDATIONS FOR SYSTEM PLANNERS AND PROTECTIONENGINEERS

For reasons brought ou t a bove, it is recommended that the system, planners anddes igne rs assess the requi rements for 132 kV, 220 kV, 400 kV and 765 kV systemsand coo rd inate w it h the p ro tect ion eng inee rs to ensure tha t t he~e reqUl r:ments arelu lfilled. Sometimes such requirements may have to be studie d on case -to-casebasis where general guidelines may no! be suffide~t to meet the systemrequi rements and sui table measu res taken . I n v iew of rapid generation expansionsthere could be certain locations in generation complexes where fault level mayexceed the speci fi ed sho rt t ime cur rent rat ing. I nsuch cases sui table measu res a rerequired to be tak en by the sy stem planners to cornam and limit the fault currentlhrr,,'gh effective usc of Fault Current l imiters.


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III

CHAPTER 4PERFORMANCE INDICES

The task of fault clearance system is to detect power system faults andabnormal it ie s, ident if y the faul ty Hem of the plant , and inter rupt the !aul t cu rrents asquickiy and reasonabl e. !n order to assess quanti tati vel y the r eliabil ity of f aultclearance sys tems, there have to be a set o f pe rformance indices. For thi s pu rpose ,certain indices for protection devices and sWi tch ing dev ices a re recommended andthese are descr ibed below.

4.1 CORRECT PERFORMANCETo describe what is meant by correct performance and what is incorrectperformance by a protect ive sys tem consider the fol lowing three Gases:

Cons ide r a fau lt i n the powe r system lor which the p rotect ion sys tem providedshould ope rate. I I the p ro tec tion sys tem operates and issues a QmCCl trippingcommand, i t shou ld bo taken as cor rect pcriornFlnCC and if it dOGS no! issuetri pping command and operates incorrectl y it should be t aken as incorrectperformance

Consider a fault in the power system for which the protection should notoperate In thi s case the p rotec tion system operates incor rect ly if it issues atripp ing command and it operates correct ly if it retra ins from issuing a tripp ingcommand.

e Cons ide r a case when ne ither a power sys tem faul t no r an abno rma li ty existsand theref ore the pr otection should not oper ate. ln this case the protectionsystem ope rates incor rec tl y i f i t i ssues a t ri pp ing command and i t ope ratescorrect ly i f i t refra ins tram issuing a tripp ing command.4 .2 D EPENDABI LI TY O F PRO TECT IO NSThe dependabili ty of protection is defined as the probabili ty f or a prot ect ion of nothaving a fai iure to ope ra te under ( li ven cond it ions for a g iven t ime interval . An indexo f dependabi li ty should measure the abi li ty of the p ro tec tion sys tem to ope rate whenillNe is a power sys tem fau lt for which the protect ion shaH operate .

Index, D of dependabi li ty is def inee j as fol lows:," '{f N N ! Nf) J . ~./ v I . N" + - y N , X,


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20 Manual onfleilablo Faull Ctonranco and Back-Up Protection 01ENVand UflV Transmission NetworksHere

NIis the number of failures to operate at internal power system faultsN. isthe number of internal power system faultsN, is the number of correct operations dl/ring the given time interval.The performance index 0 is to measure the porformance of protection whenthere is a power system fault,

4.3 SECURITY OF PROTECTIONSThe Security of protection is defined as the probability for a protection of not haVingan unwanted operation for a given t ime interval. An index of securi ty shouldmeasure the ability of the protection not tooperate when it should not.

Index, S of security is (jefined as follows:

s iV"A r c " + N 'JWhere

N o is the number of correct operationsN" is the number of unwanted operations of the protection during the giventime interval.Here N o is sum of N:m and N , , , " , where N w , is the number of unwantedoperations of tho protection in a given time interval without any power system fault

or abnormality and N uw is number of operations of a protection in a given timeinterval When there is a power system fault or abnormality for which protectionshould no t have operated,

The performance index, S is to measure the ability of protection of not havingan unwanted operation in a given time interval. It measures the performance ofprotection both When there is a power system fault or abnormality and when there isno power system faulL

4.4 RELIABILITY OF PROTECTIONSThe mliabili ly of a protection system is defined as the probability that a protectioncan perform a required function under given conditions for a given time intervar, ThereJiabili ty of protection is the abil ity of not having an incorrect operation, It is thecombined ability of not having a failure to operate and of not having an unwantedoperation

Cnapter 4 : Performance Indices 21Index, R of reliability is defined as follows:

where,Nt is the number of correct operationsN is the number of incorrect operations during the given time interval.Here N is sum of NI and N", where N, is the number cf failures to operate on

internal power system faul ts in a given time interval and Nu IS the number ofunwanted operations of the protection during the given time Interval.

4.5 DEPENDABILITY OF SWITCHING DEVICESThe dependability of a switching device is defined as the probaoitity of not having afailure to interrupt the fault current when thedevice has received a tnp command

The dependability of a switching device is defined as follows:

l) '""t V , . +~Vj

whereN" is the number of correct responses when the device has received an

operate command.N, is the number of failure to respond.

4.6 FAULT CLEARANCE TIMEThe performance indices described above measures the probability of not ha~ing, afaiJureto opera te, the abilily of not haVing an unwanted operation and the probabilityof breaking or making the fault current. The periormance Indices ,dO not expticittymeasure the speed of operation, For measunng the speed of operation follOWing ISsuggested:

Fault clearance lime, Tt is defined as:T~ "MaxlT,,}Wht'rc,i I, ,;V


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22 ManU:l! on Heli i :~/) I ( )Pa l il l C l e a ra n c e and . l fJck-Up Protection off::NVand UllV Transmission NetworksHere, T, ; i s the fau lt clearance t ime at terminal ( i) of the protected sect ion , andN is the number of termina ls of the protected section.Equation below defines the f ault clearance lime, TI.i at l~nninal (i) of theprotected section:

Here, T r, i s the operat ing t ime of the p rotec tion system at termina l (i), and To "is m e operat ion l ime of the swi tching dev ice a t terminal ( i) . Wi len necessary, Triinc ludes the operating t ime of the teteoro tect ion channel .Figures 4. 1 and 4.2 illustrat e the concept 01 the favl! clearance time, TI andthe fau lt c learance t ime, Tn at the termina l (i) oj the protected section.

4. 7 TYPICAL EXAMPLE FOR CALCULATION OF VARIOUS PERFORMANCEINDICESPerformance of Transformer and Heactor pro tect ions during a g iven l ime interva l

Dependabilit, D c . " N,N, IN!1 1 ' , .

l300: 1.0nOtO

S(curit,I"S N .. DO ~.086713(H 17+3iV,..Rc!iilbiliry, R ~. '

/V ~ + N ,1 \ 1 ~

N, . + Nt + N." -f Nu130 "-0.86713 0 + ()+ 17 + 3

Ctisptet


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24 Manual Oil neltable raull Clearance and BackUp Protection ofI IVand UNV Transmission Networks

Jt is recommended that once this practi ce is adopted and sufficient data iscollected, a system of benchmarking the indices be introduced by ut ilities andactions be taken f or improvement as cons idered necessa ry. The approach shou ldh9 to compare the indices collected in a particular period with the targets set,analyse why the re a re di fferences and wha t act ions can be taken to b ridge the gapsand then take act ions for imp rovement . These ac tions can be regarding rev ision inrelay setti ngs, bett er maintenance practices, moderni zing and ret rofi tting ofswi lching and p rotect ion sys tem e tc, as fel t app rop riate. The targets can then be seth igher and further act ions be taken for improvement.

While evaluating this data apart from looking at protection devices andswitching devices, attention shoul d also be given t o remaining part of tho fauJ!c lear ing sys tem viz ., D.C. suppl ies, telepro tect ion s igna ling, hea lthiness of tripp ingcables, termination etc. Thi s will hctp 10 impr ove the design of tauit clearancesystem.

If CHAPTER 5

LINES AND CABLESI n protect ion o f t ransmission l ines bo th dependabi li ty and secur it y of protec tion areof paramount importance. Any uncleared fault may create unwanted t ripping andlead to grid disturbances.

Transmission circuit back-up protection caters for failure of any mainprotect ion system to c lea r any fau lt that i t i s expected to c lear . A p rotect ion funct ionthat off ers back-up for most faults may also pr ovide main protection for some faultconditions. However there could be some known limitations of the operatingpr inciples o f the main protect ion hav ing rest ri ct ions in relay set ti ngs that cou ld beapplied to the main protection.

Two main protections COUld be j usti fied on the basis of being able to keep animportant transmission circuit in service with one sot of pr otection remaining inse rvi ce whi le second set of protect ion is taken under maintenance. Where two ma inproteclion systems are justified one may take (he opportunity to select mainpro tect ion sys tems that cover each other 's l irnHat ions .

The requ iremen ts of overhead l ine and cab le protec tion systems vary great ly,due to the exposure of transmission ci rcui ts to a wide var iety of environmentalhazards and are subjected to the wide variations in the format, usage andconstruction methodologies o f t ransmiss ion c ircu its. The type of pro tect ion s igna ling(te leorotec tlon) or data communication sys tems required to work with the protect ionsystems will also influence protection scheme requirements.

As discussed in chapter 2, back-up protection might be provided in one ormore of the fol lowing forms. Circuit local back-up. Substation local back-up. Remote back-up.5.1 TYPICAL TRANSMISSION CIRCUIT ARRANGEMENTSThis sec tion o ffers a b rief overview of common t ransmission ci rcui t a rrangementsand physica l construct ion [ac tors which might have some inf luence on the exposureof transmiss ion c ircu its to fau lts and there fore on the selec tion 01back-up protect ionschemes. Transmission circuit construction can be considered in three maincategories.

Ovorhead constructionUnderground cab!"! construct.onCornposire construction.


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26 MarlUdi onFkliablc Faull Ctcerance andBack-Up Protection ofENVand WIV Transmission Networks5 .1 .1 Ove rhead T ransmission C ircui tsPhysical a rrangements for ove rhead conductor suppo rt vary depending on vol tagelevel and cost considerations.. The el ectrical parameters of resi stance, r eact ance and capacitance are mai nlyInf luenced by the s ize and type of conductors. conductor configura tion and geometrywith respect to each ot her and wit h respect t o ground along with the ear th wires ontop of towers

The thermal balance of elect rical heat input and heat di ssipation governsci rcuit l oad curr ent capacit y. Heat input s ar e fr om 1 2 R l os ses, solar rad ia tion andsolar conduction. Heat dissi pations are through radiation and conduction, whichdepends on ambient temperature , wind veloc ity and chm factors .

Use of correct l ine parameters is important for proper selli ng of prot ectiverelays. Calculation of overhead line electrical parameters at nominal systemfrequency and line surge impedance are usually performed using dedicatedcomputer software. These calculations may then be verified by carrying outmeasurements on final inst all ati ons t nat could be subjected to f ield t est ing , asgr ound conduct ion ef fects i nfl uence t he zer o sequence surge impedances. Thisinformati on may then be used by protection engi neers to determine opt imum rel aysettings. lhe CIGRE WG 34.04 report titled "App!!c3tlon guide on protection ofcomplex transmission network configurations" gives details of how themeasurement s of line impedance for the selling of distance protecti on and faul tlocators can be carried out.5.1,1.1 Ground WiresGround w ires he lp to reduce tho apparen t tower foot ing res istance . I t may be no tedthat a n HV, FHV and UHV lin(~s need one or more than one ground wires at acert ain height above (he conductors to provide the desir ed shiel dino The towerfoo ting impedance parameters are requ ired to be kep t as low as p ract ic al iy feas ibleand may need special measur es like count erpoi ses and other known methods ofreduci:lg the foolinq impeoancos.

. The economic justification for ground wires, the numbor of ground Wiresrequlr(od and the ir ~Nometry ISmainly determined by ground f lash densi ty , keraunlclevels and seve ri ty of l igh tn ing surges as also topo logy of the l ine p rof il e and lowerheights.Besides reduci ng t ho number of lif lht ning induced faults, gr ound wir es al sohelp to reduce the apparent tower fooling resistance seen during conductorf lashover to ground causing ground fau lts. They provide multiple ground fau lt current

return paths through many lower footings, which hetps in this. The annual outagerate of the t ransmission l ine is requ ired to be !cs 27

As said ear lier individual tower f ooting resi stances can be substanti all yreduced by the use of counterpoises, wh ich are general ly o f stool conduc to rs bur iedunder each t ower with several confi gurat ions in crowtoot layout or continuouscounterpoises connecti ng two or more number of t ower s i n a row. This is usuall ydone whe re so il resi st iv it y i s very h igh.

Even wit h flr ound wire shielding, a l ightning strike to conductors or to eart hwire or to tower peaks of a line may cause an insulator back-flash over leading toline outage. This is due to discharge cur rent flowing through lower impedance andlower footing resistance, which will result in a transient tower voltaqe rise withrespect to power !i nc conductor s. Such voltage r ise could be suf fici ently high tocause a single o r a mul ti -phase back -f la sh f rom a tower to i ts power conduc tors. I t i simportant to appreciate that where a high transmission tower is erected or . highpr of ile qr ound wit h high tower f ooting resistance, ground wires may increase Iheli ghtni ng outage rates. The tr ansmission l ine desi gner s cover this aspect whiledesigning the t ower and conductor confiqur ali on and Btl , of insul ator st rings. Tominimize the risk of outages there should be good coordination between Bil ofinsulator strings and tower footing resistance.

5.12 Underground T ransmission Ci rcu it sThe e lec tr ica l charac ter ist ics o f HV and EHV cables for unde rground t ransmiss ionci rcu it s to ca rry a given load are se t by the phys ica l conf Igu ra tion of the conductorsand the p rope rt ie s o f the insulat ing med ia , wh ich mater ial ly a ffect the capaci tance ofthe cable. The materials used and the voltage raHnn detormines the externaldiameter and the we igh t o f a cable. Heat d is sipat ion wi thin a cab le i s predominan tl ythrough I"R load current losses and this is norrnal ty d issipated by conduct ion throughthe dielectr ic to the soil i n which the cabl e is buried. Cooling may be enhanced byspec ial back -f il ls in cable t renches. Interna l cool inq for EHV Cable may be ob tainedby c irculat inq o il through hol low conductors or through o the r duc ts w ithin the cab le.Cables a re assigned a con tinuous load cur ren t rat ing at reference envi ronmen talconditions.

Cable shun! capacit ance per unit l ength is much hinher than for overheadlines. Charging currents increase with cable voltage ralings. This means thatinductive shunt compensation is commonly required f or EHV cabl es and also forunusually long HV cabl es. A good example of such a scenar ios could be seen fromthe parallel cable r ings at 400 kV in BerEn provided by Power Uti lity BEWAG andalso in the Na tiona l Grid of UK .

Being less prone to tau 't - induc ing env ironmental hazards ltla1l overhead lines,cable faul ts ar e almost excl usively permanent fault s. Consequently, automaticreclosing is no t used for unde rg round t ransnuss ton c ircui ts, In cable c ircui ts, l au l! scan also occur in associated swit chgear and cable j oints. The level of incidence offau lt s in these ISof ten compa rab le to faul ts in the cable i tseH.


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28 Manual on i1e!!8bleFaull ClearanCiJand Back-Up Protoction ofEtJVand unv Transmission Ne/worAsCables do not wi th stand faul t cu rrents lor long pe riods o f t ime ; consequent lythey demand the use of protection without any intentional time delay, This is af eature of uni t protect ion , such as cur rent di fferen, ial protect ion, through pi lot w iresor f ib re opt ic cab les, which is o llen applied to cab le c ircu its.

5.1.3 Composite Transmission CircuitsPlanning st rateg ies and design requi remen ts somet imes demand the creat ion o fcomposi te t ransmission c ircui ts. The typ ical composi te ci rcui t i s a mix o f overheadand underground circuit.

A lthough less frequent, there are cases of non-homogeneous rad ia l overheadl ines, w ith di fferen t types o f conductor or varying conductor geomet ry a long the irroute. This is often the case where a sub-transmission li ne supplies many teedt ransformers, since it i s possible to economize on conductor size as the cir cuitloading decreases from the primary substat ion up to Ihe most remote SUbstat ion.

Wit tl non-homogeneous series impedance for a composi te c ircu it , espec ia llywhe re cab le sec tions are involved, there may be some di fl ic ul ly in set ti ng opt imumimpedance sellings for distance relays and in setting optimum residualcompensat ion for wound Iaufts.Although the transmission lines are or iginally planned to inter connect twosubst ations after a period they ar e altered to suit changi ng requirement s in thet ransmission system. Often , the presence o f mul ti -c ir cui t l ines, mul ti -termina! l inesand trans former tee-o ft 's , are due to these reasons.In some cases, planning strategy may lead to a gradual implementation ofcomplex transmiss ion arrangements, With long interva ls between success ive s tagesof i rnplernontat ion, i t i s no! always possible to eng inew protect ion sys tems at each

slage that will sui! the final configuration, When such changes take place it isrecommended tha t changing, refurbishing or upgrading ot protection systemsharmonizing with the power sys tems already in operation, be carried out.5.2 COMPLEX TRANSMISSION CIRCUITSThe fol lowing types of l ines can be class if ied as complex t ransmission c ircui ts thatoften present special protection diffiClJl!ies :

Paral lel t ransmission lines where two or more three phase transmissioncircuits are arranged on Ihe same tower or follow the same right of way onadjacent toworcMulli -lennina l l ines h i' lv ing three or more termina ls with substantial generat ionbehind each

Chapter 5 : Lines and Cab/as 29

Tapped l ines hav ing one or more termina ls with substantial generat ion behindthem and taps feeding only the load. 1hctap IS done through a step-downtrans former and do not have sufficient current feedback to operate the relays.Composi te l ines whe re system e lements l ike t rans formers, ove rhead l ines orcables are connected together without intermediate circuit breakers,

Series Compensated linesThe protect ion of these complex t ransm!ssion l ines is v :; ry wol !desc ribed in

tho document produced by CIGRE WG 34.04 In 1991 tit led - Apptication gUi de onprotection of complex transmission network configurations".5.2. t Parallel Transmiss ion Circu itsOften, for reasons of economy, two or more circuits are run in pat ane! o~ the samelowers. Transmission reliabil ity i s reduced in the event of rnuit iple CIrCUit faultscaused by l igh tning and cer ta in common mode even ts, such as a tower collapse,which resul t in sus ta ined fau lls. I f insulat ion back-Hash occurs with a d irec t I lght l1 lngs tr ike to a tower, it could result in rnulticircuil faults resulting in10aUtJG(;S.

The possibi li ty of mul ti ple fau! ts occur ring s imul taneously on more than onecircuit ota t ransmiss ion network imposes special demands on protect ion In t erms o fphase se lect ion tor s ingle"poie t ri pp ing, that rnayattect high speed sing le phaseauto-reclosinq,

Wi th double ci rcui t tower const ruct ion, there is relat ive ly st rong inter -ci rcui tmagnetic coupl ing between conductors, which is expressed in terms of var ious inter-c ircu it mutua l impedances for the purposes of short-c ircu it calcu la trons and analYSISof protecti on performance. It is usual ly acceptable to neglect the POSitive andnegat ive sequence mutua l impedances, s ince they are only afraction of ttl;; pos,tlvesequence sel f impedances. I t is somet-rnes necessary to consider t tle coupl inq fromthe pos it ive sec,JOnce network to the zero sequence network when setting sensi tiveresidual over cur rent p rotect ions. L 'ero sequence mutua! Impedance should no t beignored, because o f Its relat ive ly t li uh value and itt possible inf luence on properground fau lt pro tect ion of l ines .

The mu tual coupl ing pe rmutat ions tha t resul t lrom multiple c ircu its, whichshare a common right-a I-way, are complex in nature.

Mutual coupling is not restricted to parallel circuits at tho same vo ltarw leve l o rto circuits which have two common t erminals. In some cases circu.ts may run Inparal iel for pa r! of thei r rou te. Who re thoro rs a pa ral le l run , two ci rcui t" miqt )t lIS",common, double-circuit support towers


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30 Manual on Fkd iiJb fD ( -du ff Ctcarenco and Back-Up Protection ofUW and UlW Tmnsmission NetworksThoro a re some cases o f c ircui ts at di fferen t voJ!age levels shar ing the sametower. This may lead .to inter -system faults. Despite thei r sever ity, it may bo verydifficult to discriminati vely detect such tautts with protection functions that are

dependent on vol tage measurement. The appropria te phase selec tion d if licu li ies areeven grea ter for these l ines . Phase -seg rega ted uni t p rotect ion based on cur rentmeasurement is the best option for both circuits when it is economically andtechnically feasible.. . The main consequence of the mannetic coupling between parallel over headlines IS t he ~odl fi cat lon of the zero sequence vo ltage prof il e du ring a g round faul t onone CrCUIt. 1 he ze ro sequence vol tage prof il e a long any ci rcui t w il l not be en ti rel ydependent on the zero sequence current f lowing in that c ircu it . Ins tead i t wil l dependon zero sequence current f lowing in the paral le l c ircu it (s ).

l! f oJ lows that protect ion funct ions based on voHago measu rement are thosemost likely 10 be affected by zero sequence mutual coupling, as highlighted below: Directional ground fau lt protection

Zero sequence voltage and current signals are those most commonly.employed by ground faul t di rec tiona l cont ro l c iements, A ground faul t on oneCircu it may cause operation of d irec tional ground fau lt pro tect ion on a partiai lyparal lel cr rcuu due to pa rt ial ze ro seqUence mutua l coup ling. This problemdoes not exist for paral le l c ircu its between common substal ion bus bars.Distance protection reach accuracyHie reach of distance protect ion g round faul t clements i s adversely af fe ctedby modi fi cat ion of power sys tem zero sequence VO l t age profi le through zerosequence mutual coupling, If zero sequence currents f lowing in parallolc ircu its are of s irnuar pnaso to tho current in the protected c ircu its, ground fau ltdi stance e lements w! ll under reach. I f the cur ren ts are vi rtual ly in ant i~phase,tho d is tance e lements wil l tend to over reach.

. The tendency for distanco protection to over roach IS of no consequence forc istanco clements, which are intended to ove rreach a protec ted l ine , such as Zones2 or 3. Any tendency to under reach can be compensated for , when setting t he overreach e lements. Consequently, a ll tole-protec tion schemes based on over reach ingelemen ts are SUi table lor protocuon of magnet ica ll y coup led ci rcui ts , when set totake into account unde r reaching du ring u rounci laul ts. Howeve r, where commonimpedance sel lings exist for phase and earth fau lt impedance elements, it should benoted Ihat any compensa tory mcroase in distance elemen t ove r reach may make i tnecessa ry to employ cur rent reversal guard logic in a teicp rotecl ion scheme, Wheret ile over roach ing e lernor-ts are also used to provide Zonc -2. back-up protect ion forend zone fau lts, il compensatory increase in reach may also cmate d iscr iminat ionproblems wilh ph,lS() 10 phase iau it pro tect ion for shorf adpcent lines.

Chapter 5: Linesilnd Cables 31

A poten tial ly prob lema ti c case of Zone -I over reach 10 be noted is when aparaf le l c ircl li ! is switched out and grounded at both termina ls . In such an instance,the Zone-j reach security margin may be greatly reduced or it m:

impedance is to take zero sequence cur rent signaUrom the pa ral le l c ir cu il and use i tt o provide compensat ion. This technique i s not effectrve when parallel cir cuit isdi sconnected and g rounded at bo th ends . FOf thi s reason this method is not used forprotection but used for raul! location.

In general, it is not advantageous to introduce mutual compensation lord is tance protect ion schemes. The acivantaues, ( [any, are usual ly far oulwei(Jhed bythe problems and operational complex it ies that would be introduced. Most d is tanceprotec tion schemes can be set to provide fas t t ri pp ing for faul!s a/onn a multi-circuitlirlQ and be stable for externa l fau lts without employing mutua l compensat ion. Thereare a lso applica tions where i t is impossible 10 access tho current from a rnaqnelicallycoupled ci rcun at one o r both terminals. Ina i rsuch cases tho maximum reach o f f ir s!zone setting f ixecJas 80% or less cou ld be advantageous.

An alternative to implement ing mutua l compensat ion is 10 apply individual zerosequence compcnsanon factor for each zone o f g round tau lt di stance protect ion"Another opt ion is differ ent groups of set l ing par ameters for di fferent oper atingcondi tions of the double c ircu it l ine.

From the above il transpires tilat correct calwlation of ttl(! transmission lineimpe(jance matrix with zero sequence parameters be carried out and proved by fieldtes ts before using them in setting the relays.

5.2.2 Multi-tenninal Transmission circuitsWhenever a t ransmiss Ion ci rcui t has throe o r more ter rninats, i ts protect ion may besubject to adverse ef fects wi th in the p rotected l in! " due 10 the effects of in feeds.Depending on the protect ion operat ing pr inciple, these e ffec ts may be a cause forconcern. Add it ional in feod can increase impedances seen by ci istance relays andout feed can cause di rect ional p rotec tion a t one lermlna l to idenl if y an internal faul tas extema) . Car rier -a ided protect ion w ith ef fi cien t informat ion l ink s wl lt l j udiciousapplica tion of correct rneasurands and protect ion relays should be applied


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32 Manual on fi()!iab!e Fault Clearancf) and Back-Up Protection ofEUV and UfiV Transmission No[wolk$5.2.3 Shunt CompensationThe dist ributed shunt capacitance of a transmis sion line can cause a vOltage risewhen load i s l ower than norma l l oad. Thi s i s t e rmed as Fer rant i E ll ec t.

Switched or non-switched shunt reactors are often used 10 compensat e fo rh igh level s o f shunt capac it ance (or l ong ove rhead l ines o r f or underground cab les.The shunt compensat ion i s i nvar iabl y l ocated a t t ermina l substat ions , whe re i t maybe connected 10 t il e aSS()Cial ed c ir cu it by an i so la to r swi tch o r by a c ir cu it b reaker .The former opt ion i s l ess expensi ve , but i tmay be necessa ry to de-energi se the l ineto i nser t o r swi tch-ot t t he reactor . Somet imes shunt reactor s con trol led by c ir cu itbreakers are connected 10 EHV bus bars. In many o ther cases , shunt reactor s (andmany t imes S ta ti c Var Compensator s w it h i nduc ti ve and capac it ive compensat ionrange are also connected to the tertiary windings of Interconnecting auto-transformers (leT) for control of d ownstream voltage p rofile. With expansion ofpower netwo rk , many l imes !ho l inc l engths get sho rtened and intermediate s ta ti onsare connected with loop in and loop out (L1l0) connect ions. Under such condi tionsthe non~~wi lched l ine reactor s p rovided on the fong l ines a re removed o r rep lacedby bus reactors Under certain conditions it may be advisable to usc the controlledshun t reacto r th at c ould f(,rnain on t ne line for all condition s of operation. Su chapplication Gould be kept in v iew to provuio minimum chang{"s in reactor shi it ing.

Shunt compensation cquipmont must have ils own protective gear. Where ashunt reactor i s connected 10 a transmission circui: '"lnly by isolator switch, itsp ro tect ion requi res a Ias1 and rel iable t rans fe r- tr ippi ng scheme in o rder t o removeremote-end fault current in io"d.

Shunl reactors do no! have a great influence on the selection of linep ro tect ion, excep t whe re d if fe rent ia l o r d ir ec li onal compari son p ro tect ion might berespons ive to l ive- li ne sw' tching o f reactor s. I n such cases , reactor s may have to beexc luded f rom t il e zone o f l ine p ro tect ion through the use o f reactor CT' s i n paral le lwith line C'Ts. It ma y also be necessary to inhibit ani reacto r b ack-up ground faultprotection during single-pole auto rociose sequences.

When sinqle-pole ,ripping and actorccioscre is applied, capacitive andinductive coupling may delay the extinction of the secondary arc and the faultedphase voltage decay rna,' be delayed. This is due to resonance between shuntreactor i nduc tance and tho capac it ance coupl ing to l ive phases a ft er t he b reaker i stripped. This may delay arc extinction for a transient fault and so single-poleautoreclose dead-limes may need to be extended for lirniling tile parameter ofseconda ry a rc cur rent t o a p ract ical value 01about 10 amperes . A Surge Pro tectedNeu tral F leaC lo r usual ly o f 02 !e 0 .4 pu ohmic value i s connected between neu tral o fEHV shunt reactor and the ground" This requires the ratio of zero to positivesequence impedance o f S[lunt reactor be around 0.9 10 l im it t he B IL o f shunt reactorto around 550 kV in i.l 400 kV network.

Chapter 5 : Lil)os and Cablas 33

5.2.4 Series CompensationAs s tabi li ty const ra in ts l im it t he max imum value o f power ang le for l ong l ines , t he lul lload curr en! capacity can only be utilize d if t he t ransmiss ion c ir cu it impedance i sreduced. The surge Impedance Loading (Sll) of a t ransmission lin(~, say 400 kV, isaround 5 t 5 MW whi le the thermal limit of th e same !ine c ould be aro und 800 to 1 200MW under various operating conditions. This is further complicated by shuntreactors dir ectty connected on the lines that modify the SIt. to about 70% of origin a!value. Such conditions always need extra capacitive vars if more power to the lev elof its thermal capacity is required to be pushed into the line. The application ofseries compensation on transmis sion lines is provided for su ch relief. Some of theo ther reasons for app ly ing ser ies compensat ion cou ld be to avo id vol tage col lapse,to optimize load distributi on or to improve qualilY of supp ly apart from improvingtransient slability.

Somet imes Thy ri st or Con trol led Se. tes Capac it or (TCSe) i s used. Some o f thoreasons for t hi s cou ld be power osc il la ti on detec ti on , p revent ion o f sub synch ronousresonance or load f low control .

Ser ies compensat ion has been uni ve rsal lv ann li f' 0 to !):;c::; al ,iii the knownvol tage level i l9h1 from t i kV 10 800 kV .Tho p rol~~tion relaying of suc h lines arequi te compl icated and need to be evo lved a ft er detai led sys tem s tudios "

The combined e f! ec t o f ser ies capac it or s and the ir p ro tect ive cur rent d iversi ondevices on l ine protect ion per formance bn d the irnv8c: oj serie s compensatio n onp ro tect ion o f adjacen t f ines requi re detai led d iscuss ion and i s not covered w it hi n thi sdocument. Document produced by CIGRE WG 34.04 t itled "Application guide onprotect ion of complex t ransmiss ion network configurations' may be reforred for this.

5.3 TRANSMISSION CIRCUIT FAILURE MODES AND RATESThe failure rate of transmission lines range from 0.2 10 more than 10 faults per"100 km per year. The fa ilure rate is a function of keraunic le vel, the insulat ion levelan d the existence of many enviro nmental fa ctors such a s veget ation prOXimity,f ou li ng w it h ove r-grown t rees a ir pol lu ti on Iovot s, vanda li sm and bush f ir es o r f ir es oncrops waste cre ated by the farmers below or very close to lines. Some of the se aredescribed below.

5 .3 .1 Arc ing Fau lt sMany t ypes o f f au lt -i nduc ing events resul t i n t he o reat ion o f an a rc ing fau lt betweenphases of a line, between phases and tower structures or between phases andg round. Fau lt a rcs p resent non -l inea r f au it r es is tance" Esl .ima !cs o f l au i! r es is tancebased on arc length can be obtained using well-knovvf1 Warrinpton formulae orvar ious other empir ical lormulas used in Europe.


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34 Manual Of) R(}/iableFault Clearance andBackUp Protection ofEHVand UHV Transmission Netwolks5.3 .2 Smoke HazardAcci dental or def iberately ini tiated bush fir es ar e quite common dur!~g certai nseasons. Burnll19 of sugarcane waste close to transmission line is one suchexample. Smoke from such f ires may i nduce arcing fault s as a consequence of airIonization. Some faults may be located across insulators or between a pair ofphases and these will not differ much in nature from those i nitiated by lightning.O the rs may be located at the mid-span between towers, f rom the lowes! conductorto ground. Such mid-span faul ts can be h ighly res ist iv e, no t on ly as a resul t o f longarc lengths, but al so as a resul t o f the ground res istance between the point of a rcand the nearest support towers.

Itmay o ften be necessary to provide high resistance g round fau lt protec tion assupp lemen tary main p rotect ion in o rde r to de tec t such faul ts, which may a lso act asback-up protect ion for sohd fau lts and is therefore recommended.e Fau lts resul ti ng f rom bush f ires are usua ll y non -damaging fau lt s, wi th a goodchance of success fu l autorec losure fol lowing fau lt c learance. But the effec tiveness

?f autoreel?se schemes is frequently defeated by the intense ionization of theInsulating air In t he vicini ty of Ihe f ire. l eading to now developing faul ts fol low ing l inerc"enorglsat!on.

5.3.3 Vegetation FaultsThere can be many linofauits to ground, which are caused by rapidly growingvegGtalJOn. Bamboo plant IS one such example. From operat ional expe rience suchf~uJts ar: , known to be of h igh-resistance. The faull presents itself as a ve~ highrCSls!anct .. to ear rn (of the o rde r of several ki lo Ohms) , unt il t he s team and smokeproduced by .thermal energy d issipation in the tree res is tance resul ts in a tree- lengthflashover. This occurs after several seconds""' Gr ound lault .~Iemonts of dist ance relays may not detect t he high r esi stancef,.ul ls and the sensiuve ground fau lt pro tect ion mentioned in the previous paragraphcan also take care of ! llgh res is tance faults due to growi:lg vegetation.5. 3.4 Forms of Overhead line FaultsThe f ailure rates of power lines vary from utility to lIH1ily and f rom year to year, The:nost con:mon type of fault IS p hase to ground fault (of the order of 75% or more)and. most faults are transient In nature. Since most laults involve ground,co~sldera!lon must be gi ven to th:" levels of ground fauft resistance that might beencountered for par ti cu lar appl ic at ions and whethe r the ma in p rotect ion wi ll be abletodetect al l such g round fau lt s. I f not , SUpp lementary protect ion may be requi red tocover high ros lstance fau lt s, The ground faul l elemen ts o f d istance retays should besupplelT1ente ti by a sensi tive res idua l over current relay

Chapter 5.' Lines and C'lbles 35

5 .4 TRANSMISS ION CIRCUIT PROTECTION REQUIREMENTSTo determine the back-up p rotect ion requ iremen ts for t ransmiss ion ci rcui ts it isnecessary a t the f ir st i ns tance to ident if y the main protect ion requi rements for eacho f the system vo ltage leve ! and then to ident if y app li ca tion speci fi c requi rements inrelat ion to the protec ted ci rcui t. Examples o f possible main requi rements are l istedbelow:

Maintaining transient stability 01 the power system Maint aini ng operat ion of power stat ion auxi liary system Avoiding loss of supply to bus bars QualJ!y of supplyExamples of app lica tion speci fic requirements are l is tod below:

A llow for fai lure o f a protection signaling/communication channelAllow protect ion tes ting with the c ircu it in serviceProtect a trans former feederProtect a tapped l ine through a step-oown trans formerProtect a c ircu it with more than two termina lsProtect a c ircu it in ser ies compensated networkDetection of broken conductors/jumpers of overhead tines.Detec tion of h igh res is tance fau lts on overhead l inesOperate within short-t ime thermal rat ings of overhead l ines and cables

Fault detection wit h weak or zer o in feed from one circuit termi nal

Comhinations of main and back-up protection systems should be used toaddress the main and application specific requirements for transmission circuits.5.4.1 Maintaining Transient StabilityCommon requirements for circuits at primary tr:ansmission voltages are themaximum permi tted f aul! clearance time in order to mai ntai n system transi entstah' li ty. A single t ime l im it i s of ten quoted for a par ti cular t ransmission vo ltage !eve l,bu t separate I ir ruts might be quo ted for dHferenl fau l! t ypes . On tho bas is tha t mostprotec tion a rrangements rely on a sing le protect ion system to clear di fferent types ofsolid tautt, a single time limit is usually used as a main design requirement. Anexception sometimes exists to allow slower cl earance of hi gh r esistance groundfaul ts tha I may not be seve re In na ture.

It must be recognized t hat the rnaxrmurn permitted faul! cl ear ance timesdeclared for a p rima ry t ransmission system wi ll no t rema in f ix oci . Power systems


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36 Manual on Roliable Faull Claarance and Back-Up Protection offJ1Vand U,W Transmission Notworksunde rqo constant evo lut ion, In par ti cular , the changes in the locat ion , capaci ty anddesign of generating plant that occur on most power systems demand constantreviews by system planners of the maximum permitted fault clearance times forexist ing t ransmission ci rcu it s, For EHV system the des ired faul t c learance t ime of 5cyc les that inc lude the relay t ime, breaker time and carrier times is ge:1erally applied,It may be desirable for EHV lines to have a maximum total-break time of circuitbreake rs as 2 cycles (40 milliseconds) giving f reedom to optimize the time ofprotective re lay and information links.

When app lying the single lai lure protect ion design cr iter ion, and whe re thereis a potential transi ent stability problem, a second pr otection syst em should beprovi ded which will clear f aults within the maximum permitted time 10 maintaintrans ient s tabi li ty . The back-up protect ion should a lso be ful ly d iscr iminat ive, so thattripp ing of more than one c ircu it does not further impair the power trans fer capabil ityof the transmission system,This means that the second form o fp rotect ion must be ci rcui t- loca l and i tmusthave similar operating speed and discr imination qualities to those offered by thema in protect ion. This form of p rotect ion is retor red to as second-main protect ion orMain-2 protection,As discussed earus r, Main~2 protect ion is not general ly regarded as being

back -up protect ion, The cost of thi s form of p rotect ion is easi ly just if ied in terms ofthe s ingle fai lu re dos ign cri te rion and the costs of possible sys tem col lapse resul tingfrom inadequate back- lip pro tect ion performance. Any addit iona l pro tect ion providedas back-up to two main protection systems (M1 and M2) would be back-upprotect ion for a dual f ai lure cr iter ion. where longe r operat ing t lmo l imi ts would beallowed,5.4.2 Maintaining Operation of Power Station Aux il ia ry SystemDepress ion of power supply vol tages for aux il ia ry p lant in some generat ing s ta tionsmay roduco the station output Maintenance of full generation output may be acr it ica l power system secu ruy factor , In the case of nuclear plant . auxi li ary powersuppl ies are a lso a major fac tor in providing fuj i nuc lear p lant safety and security,

The po tent ial l oss of sys tem goneral ion or the potent ial chal lenges to nuc learplant safety systems may be Iactors which will dictate the longest acceptablec learance l imes for transmiss ion c ircu it fau lts in the v ic in ity o f a power s ta tion ,5-4.3 Avoiding Loss of SupplyWhere remote back-up protect ion or substat ion local back-up protect ion operates foran uncleared tault on one transrnission circuit. thoro will be a complete loss ofsupply to tho bus, which supplies tno faulted Circuit. in the case of multi-bussubstation;, (D[)ut.>!c Bus. Breakeh,n(H{alt bus bars. Double Main and T ransfer bus

eMpler 5 : Lines and Cables 37

o r sing le bus or main and t ransfer bus SUbstat ions) , there wl ll be lOSS 01 supply tosome step-down transformers, which mayor may not result in loss of supply toconsumers. There will also be loss of supply to "'ny radially fed downstreamsubstations. In thscase of a s ingle bus arrangement, there wil l be a co,mple !e loss ofsupp ly to local consumers. The c081s of reduced supply capacity or total loss ofsupply must be considered in relation to t he cost of providing ef fect ive ci rcui t l oca lback-up protect ion to avo id loss .o f bus supply. .5.4.4 Quality of SupplyA utility may enter into agreement with a large industrial consumer to limit thedurat ion of severe vol tage depress ions in order \0 secure a power supply contrac t toindust ries l ike mining indust ry or cost ly con tinuous process indust ry . Qual it y 01supply considerat ions may inf luence declstons not only about the form of back-upprotect ion to be appl ied to a powe r system bu t also other add -ens such as dynamiCvol tage suppo rts and needs to be reviewed as and when those demands are madeby consumers on a case-to-case bas is .

5.5 PROTECTION SCHEMESThe decision of whether or not to provide t ransmission ci rcui t back -l ip p rotect ion, o rthe leve l of back-up sophist icat ion requi red, must be related to the cost o f fai lu re ofmain protection to clear a faul t which il i s expected to clear and to t he exposure tosuch a cost. This means t he decision to provide Main-2 protection for Hxampleshould not be merely based on vol tage leve l but on haw impo rtant the ci rcui t IS andwhat it costs, if a fau lt on the c ircu it is not cleared in t ime. This concept cou ld verywel l app lied to 2 20 I< V and even at 132 kV leve ls

Some consideraticn must be given to performance of main protect lon forcomp lex taul t condi ti ons, such as evo lving faul ts, s imul taneous faul ts and inter -s ystem Iuu lt s. Where such faul ts a re possible, but are of low probabi li ty, there maybe a need to provide back-up protection to cover for any identified crodiblel im itat ions o f the main protect ion systemts) . This aspect shou ld be examined on acase-to-case basis,

In order to determine transmission circuit back-up protection requirements it isnecessary 10 understand and examine al ! poss ible modes o f fai lure and any lauHdetection limitations for main protection schemes.

Listed below are schemes that are normal ly appl ied in India. Some of the maina ttnoutes and l imi tat ions in the schemes are also l isted under them, When applyingthese schemes l! i s essent ial that tho ma in


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38 Manua l (1) Fh > li a bJo F a , ,1 1C leamn ce ! lr l d B a cA '-U p P rot e c ti o n o fE I" iV a n d U I 1V Tm nsm is si on N e tw o rk s

5.5.1 Distance Protection (Without signaling channel)5.5.1.1 Muftizone Distance Protection (With Three or More Zones)Attributese Fast t ri pping at both ends for about 60% of tho protected l ine

Offers remote back-up

Cbip Manual Ehv Networks

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