Circuit Breaker Ratings A Primer for Protection Engineers

Transcription

1 Circui Breaker Raings A Primer for Proecion Engineers Bogdan Kaszenny, Schweizer Engineering Laboraories, Inc. Joe Rosron, Souhern Saes, LLC Absrac This paper explains he asymmerical shor-circui inerruping curren raing for high-volage circui breakers. The paper eaches how he decaying dc componen in he asymmerical faul curren affecs he breaker, and i explains how he X/R raio and he relay operaing ime affec he asymmerical curren breaker raing. The paper briefly inroduces, and illusraes wih field cases, several ulra-high-speed proecion principles ha can operae in jus a few milliseconds. The paper hen explains how o derae a breaker for he relay operaing ime ha is shorer han he sandard reference value of 0.5 cycle. The paper calculaes he raing loss due o fas ripping and suggess ha applying cusomary margins when selecing breakers may be sufficien o miigae he effec of ulra-high-speed relays wihou he need o replace breakers. I. INTRODUCTION The four major componens of any proecion sysem are insrumen ransformers, proecive relays, circui breakers (CBs), and conrol power circuis. Curren and volage insrumen ransformers supply inpu signals o proecive relays. Proecive relays provide a wide range of proecion funcions, including bu no limied o shor-circui proecion. When ripped by proecive relays, breakers inerrup he faul curren o isolae he affeced zone from he res of he power sysem. In high-volage applicaions, he differenial and direcional comparison schemes, as well as he underreaching disance and overcurren elemens, provide insananeous proecion agains shor circuis. Wih securiy as he paramoun performance facor, he faser and more dependable he proecion sysem, he beer. Wih all oher facors equal, a faser relay is always preferred over a slower one. Insrumen ransformers creae well-recognized challenges for proecive relays. Designed for he fundamenal frequency componen, insrumen ransformers may inroduce ransien errors. Capaciively coupled volage ransformers (CCVTs) generae slowly-decaying componens in heir oupu volages ha challenge boh speed and securiy of disance proecion elemens, especially in weak sysems. Curren ransformers (CTs) may saurae due o high currens or long-lasing decaying direc curren componen (dc) offse in he primary curren. Proecive relay designers and praciioners have a good grasp of hese insrumen ransformer ransiens, limiaions, and failure modes. For example, we know how o derae a CT o accoun for he acual CT burden, dc offse (X/R raio), residual flux, or low-frequency operaion. In conras o heir focus on insrumen ransformers, relay praciioners pay less aenion o he oher componen of he proecion sysem he circui breaker. This paper aims a closing his gap and inroducing proecion praciioners o he basics of breaker raing. Manufacurers specify he faul curren inerruping capaciy of heir breakers for a se of reference condiions including, among oher facors, volage, frequency, decaying dc offse in he faul curren, relay operaing ime, emperaure, and aliude. This paper eaches he basics of how breakers are specified and explains rules for deraing breakers for operaing condiions ha differ from he sandard reference values. Special aenion is given o he decaying dc offse in he shor-circui curren and he relay operaing ime. As per curren sandards, he faul curren inerruping raing of a breaker accouns for he asymmerical faul curren inerrupion; i.e., i accouns for he decaying dc offse in he faul curren. The decaying dc componen is ime varying. I subsides and makes he curren inerrupion an easier ask wih he passing of ime. The key sandards for CBs, ANSI/IEEE C37.04 [1] and he IEC counerpar IEC [], use he X/R raio of 17 (60 Hz sysem) and he 0.5-cycle relay operaing ime o esablish a reference condiion for he decaying dc componen. Wih hese assumpions, he sandards ask breaker manufacurers o specify he nameplae inerruping raing for an asymmerical curren. As a resul, breaker applicaions are simplified because he users can direcly apply he nameplae raing wihou exra calculaions if heir relays are no faser han 0.5 cycle and heir sysem X/R raio is a or below 17. How does one derae a breaker for relay operaing imes ha are faser han 0.5 cycle or a sysem X/R raio higher han 17? Breaker praciioners rouinely derae breakers for sysems wih higher X/R raios. Hisorically, however, he 0.5-cycle relay operaing ime was rarely quesioned, and oday, users normally do no derae breakers o accoun for specific relay operaing imes. Today, new ypes of relays have emerged ha operae faser han 0.5 cycle [3]. Applicaion of hese relays calls for evaluaing breaker raings. This paper is a primer for proecion engineers, and i eaches how he breaker raing depends on he X/R raio and he relay operaing ime (Secions II and III). I briefly discusses relay operaing imes and he new principles ha allow reducing operaing imes o jus a few milliseconds (Secion IV). The paper hen inroduces, explains, and illusraes he breaker deraing formula for ulra-fas ripping

2 imes (Secion V). The paper analyzes he impac of he relay operaing ime (faser and slower han he reference value of 0.5 cycle) for a few breaker inerruping imes. The paper shows ha he changes in he breaker raings due o ulra-fas relay operaion are wihin ypical margins applied by breaker praciioners. II. CIRCUIT BREAKER SPECIFICATION CONVENTION Requiremens and specificaions for power circui breakers and circui swichers have been esablished in various sandards over he years. These sandards are principally he ANSI/IEEE sandards, C37.04, C37.06, and C37.09, and he IEC counerpar, IEC The sandard for circui swichers is ANSI/IEEE C We briefly summarize several key specificaions and explain heir purpose and applicaion [1] [] [4]. 1) Normal Operaing Condiions These specificaions refer o environmenal condiions, primarily he ambien emperaure and he aliude. The ANSI sandards specify a emperaure range beween 30 C ( F) and +40 C (104 F) and an aliude below 1,000 m (3,300 f). ) Raed Power Frequency Sysem frequency has a significan impac on he inerruping capabiliy of a breaker because i dicaes he rae of change of he curren near he curren zero crossing. The breakers are specified a eiher 60 Hz or 50 Hz, and hey need o be deraed for operaion a differen frequencies. 3) Maximum Operaing Volage This raing specifies he maximum line-o-line rms volage for a breaker. The ANSI and IEC sandards differ slighly on he nominal values hey recommend. For example, he IEC may lis 55 kv while ANSI may lis 550 kv. These differences resul from he raed nework volage pracices in various pars of he world. 4) Raed Volage Range K-Facor This raing originaed wih older breaker echnologies (such as oil and air magneic breakers) in which he inerruping capabiliy is inversely proporional o he operaing volage. The K-facor is he raio of he raed maximum volage o he lowes operaing volage for which he inverse relaionship beween he operaing volage and he inerruping curren holds rue. The K-facor is a limi for deraing he inerruping curren for a varying operaing volage. Older breaker echnologies had significanly higher curren inerruping capabiliy a lower volage; hence, breakers were essenially consan MVA-raed faul clearing devices. A he ime hey were mos common, he sandards used he concep of he raed (symmerical) shorcircui curren and allowed deraing based on he operaing volage. Today s breaker echnology (SF 6) does no have his same characerisic: he increase in he curren inerrupion capabiliy a lower operaing volages is usually raher small and as such is frequenly ignored. The K-facor, herefore, does no apply o modern breakers. 5) Raed Dielecric Srengh This group of raings is specified by a series of ess, each relaing o ypical power sysem overvolage ransiens, ha a breaker needs o pass. These ess include condiions such as low-frequency overvolage (nominal frequency, we and dry condiions), lighning impulse (basic impulse level), chopped wave, bias es, and swiching impulse. 6) Raed Transien Recovery Volage Transien Recovery Volage (TRV) relaes o he abiliy of he breaker insulaing medium o recover is insulaing properies afer curren inerrupion. A breaker needs o recover is insulaion for he specified TRV waveform across is erminals. The sandards consider his waveform a funcion of he sysem alone and neglec any ineracion beween he sysem and he breaker. TRV is a complex requiremen ha depends on he sysem condiions such as faul ype. The sandards specify several TRV waveforms (condiions) assuming differen faul and sysem scenarios such as erminal fauls or shor line fauls. 7) Raed Coninuous Curren This value relaes o he breaker s hermal design and he allowable emperaure rise from he losses dissipaed across he primary conac and connecion resisances. This raing needs o be considered in relaion o he ambien emperaure. 8) Raed Shor-Circui Curren This value refers o he maximum rms symmerical shorcircui curren (he curren wihou any decaying dc componen) ha can be safely inerruped by he breaker. Hisorically, his specificaion was used o convey he breaker s oal inerruping capaciy, neglecing he impac of he decaying dc componen and leaving o he user he deraing for asymmerical shorcircui curren. The symmerical shor-circui curren raing was ofen considered wih he MVA raing, allowing deraing for operaion a lower volages (consan MVA raing means higher curren capabiliy a lower operaing volage). New breaker echnologies (SF 6) do no allow higher curren raings a lower operaing volages. In addiion, oday he sandards accoun for decaying dc offse in he shor-circui curren, specifying he asymmerical curren raing. 9) Asymmerical Currens These specificaions relae o asymmery in he shor-circui curren, including he following: a decaying dc offse in he shor-circui curren during fauls wih a breaker closed (we describe asymmerical shor-circui curren raing in Secion III); close and lach curren (peak making curren or peak asymmerical closing curren), which refers o a condiion of closing ono a faul; and shor ime curren, which relaes o he hermal curren carry capabiliies for exernal fauls, i.e., wihou opening he breaker. 10) Duy Cycles These specificaions relae o muliple breaker operaions in various sequences, such as Open Close-Open (O-CO) and furher duies of O-CO-CO. Breaker duy cycles, especially wih he older breaker echnologies, have significan residual effecs afer inerrupion such ha hey reduce he faul clearing

3 capabiliy wih repeiive breaking and closing in a rapid succession. Significan deraing needs o ake place for exended duy cycles, such as when using muliple-sho auoreclosing. 11) Capaciive Swiching An addiional se of requiremens relaes o swiching capaciive loads, such as capacior banks, back-o-back capacior banks, and long cables or lines. III. CURRENT INTERRUPTING RATING A. Curren Inerruping Raing Convenion As we briefly explained in Secion II, he following facors impac he breaker shor-circui inerruping capabiliy: Hisorically, he symmerical curren raing was specified following he consan MVA raing principle of he oil and magneic air breakers. The symmerical shor-circui raing could be deraed for he acual operaing volage (Curren Raing = MVA / Operaing Volage) wihin he limis of he K-facor. Also, he users had o derae he symmerical raing for any specific asymmerical curren condiion. Asymmerical curren raing is now used as a sandard raing, assuming he reference X/R raio of 17 and he relay operaing ime of 0.5 cycle. Peak closing curren raing and duy cycles also impac he overall applicabiliy of a given breaker in any given locaion in he grid. To undersand he impac of asymmerical currens on breaker operaion, we need o undersand he iming diagram for he shor-circui curren inerrupion. Referring o Fig. 1, we recognize he following ime insances and ime inervals: The faul iniiaion ime sars he diagram. The faul curren begins o rise a ha momen, and if i conains a decaying dc offse, he curren waveform will have he maximum possible dc offse a ha ime. The wors-case scenario is he fully offse waveform wih he dc offse iniially maching he peak value of he symmerical ac componen. The relay operaing ime (or a release delay) is he ime inerval i akes for a relay proecing he apparaus o operae and issue a rip command o he breaker. This rip command is in he form of he rip coil curren, and herefore, i includes rip-raed relay oupus or inerposing relays as required. Hisorically, 1-cycle relay operaion was considered ypical. The sandards assume 0.5-cycle relay operaing imes for specifying he asymmerical breaker raing. In Secion VI, we discuss he relay operaing ime in more deail. The breaker opening ime (or mechanical ime) is he ime i akes for he breaker o open he conacs enough o sar drawing an arc across he primary conacs. This ime is measured from he sar of he rip curren in he breaker rip coil o he momen he primary conacs sar o arc. The conac paring ime is an inerval beween he faul incepion and he primary conacs saring o arc. According o he breaker sandards, he shor-circui curren a his specific poin in ime (including boh he ac and dc componens) is he primary facor conrolling he asymmerical curren raing of he breaker. The arcing ime is a ime of arcing; i.e., a ime beween paring of he primary conacs unil he following curren zero crossing ( cycles) a which ime he curren is normally inerruped. The clearing ime is measured from he faul incepion unil he las pole of he breaker inerrups he curren. The breaker inerruping ime is a fracion of he clearing ime beween he breaker acuaion and he end of he clearing process. This ime is he breaker operaing ime. Faul Iniiaion Fig. 1. Relay Operaing Time (Release Time) Breaker Acuaion Conac Paring Time Faul clearing sequence. Opening Time (Mechanical Time) Faul Clearing Time Primary Conacs Paring CB Inerruping Time Arcing Time Final Arc Exincion Time The curren level when he primary conacs par and sar drawing an arc is he key facor for breaker inerruping raings. This curren includes he ac shor-circui curren componen and a decaying dc componen value. B. Impac of DC Offse on Inerruping Raing When he breaker conacs move apar, an elecric arc (composed of highly ionized plasma) bridges he space beween conacs. The breaker mus remove he arc plasma energy before a successful inerrupion can occur a he following curren zero crossing. For a given ac shor-circui curren componen, he level of decaying dc componen increases he faul curren peak values. This in urn resuls in an increase in he degree of plasma ionizaion in he arc jus prior o he curren zero where he inerrupion can ake place (see Fig. ). The level of he peak curren for he las peak before he inerrupion is he single mos significan variable conrolling he breaker s abiliy o clear fauls. A faul wih a dc componen is more difficul o clear han a symmerical curren. Such a faul requires deraing he breaker symmerical capabiliy so ha he peak curren is mainained wihin he breaker design limis o assure a successful inerrupion.

4 Curren Assume DC a he paring ime offses he peak ha follows AC Componen DC Componen Peak Curren Shor Circui Curren Zero Crossing curren. In oher words, he breaker can claim 100%/1.51 = 66% of is symmerical raing as is asymmerical raing. Fig. 3 plos he asymmery facor S and he percenage reciprocal of S. The percenage reciprocal ells us he fracion of he symmerical raing ha may be claimed as he asymmerical raing for a given dc componen conen. 1.8 Fig.. Conac Paring Time DC Arcing Time Time DC componen increases he peak curren prior o zero crossing. For example, if he dc componen is 80 percen of he ac value, he peak value of he faul curren wih boh he ac and dc componens is 1.8 imes he ac peak value, or he ac rms value is 1.80 =.55. The rms value of he curren wih boh ac and dc componens in his example is he geomerical sum of he ac rms value and he dc rms value. The dc rms value is he same as he dc value iself, i.e., 0.8 imes he ac rms value. Therefore: I RMS = = 1.51 pu (1) Boh he ANSI [1] and IEC [] sandards recommend calculaing he rms value of he combined ac and dc componens and using i for deraing breakers for he decaying dc offse. The sandards define an asymmery facor S as follows: S = 1 + DC% 100 Consider hree sample daa poins for illusraion. Wih no dc componen presen (DC% = 0), he asymmery facor is 1. This means, he symmerical and asymmerical raings are he same, as one would expec if he faul curren does no conain any dc componen. Wih he dc componen being half of he ac componen (DC% = 50%), he asymmery facor is 1.. This means he symmerical raing needs o be percen higher han he ac componen in he asymmerical curren o mainain breaker margins for inerruping his asymmerical curren. In oher words, he breaker can claim 100%/1. = 8% of is symmerical raing as is asymmerical raing. Wih a dc componen of 0.8 (DC% = 80%), he asymmery facor is This means, he symmerical raing needs o be 51 percen higher han he ac componen in he asymmerical () S-Facor DC Level, % Fig. 3. Asymmery facor S (blue) and asymmerical raing o symmerical raing raio (red) as funcions of he dc level in he faul curren. The sandards assume he wors-case scenario, in which he iniial dc offse is he highes possible value, i.e., 100 percen of he ac value (fully offse case). Furher, he sandards assume a single exponenially decaying dc offse. Therefore, he iniial dc value decays wih ime () as follows: Asymmerical Raing / Symmerical Raing DC%() = 100% e T DC (3) Where T DC is he decaying ime consan. For any given power sysem frequency (f), he ime consan depends on he sysem X/R raio: T DC = L R = 1 πf X R = 1 cycle π X R The sandards [1] and [] specify an X/R of 17 as he reference value for he asymmerical raing, which resuls in a decaying ime consan of.71 cycles. In oher words, he sandards assume a condiion when he dc componen compleely decays in abou 8.5 cycles (hree ime consans). When using (3) we mus consider ime () o be he conac paring ime. This ime is he sum of he relay operaing ime and he breaker opening (mechanical) ime (see Fig. 1). The laer is a breaker parameer and herefore can be lef ou of he sandards. The former is an independen facor. Sandards [1] and [] specify he relay operaing ime of 0.5 cycle as he reference condiion for he asymmerical raing. Fig. 4 plos he S-facor as a funcion of he conac paring ime assuming he sandard T DC ime consan for he X/R is 17. The sandards allow breaker manufacurers o neglec he asymmery and es wih symmerical currens for S < 1.1. (4)

5 S-Facor Fig. 4. of Reference Relay Time (0.5 Cycle) Mechanical Time Paring Time (Cycles) Conac Paring Time, ms Asymmery facor S as a funcion of conac paring ime for an X/R C. Curren Inerruping Raing Margins Circui breakers are expensive pieces of equipmen. If a breaker is pushed beyond is design limis, i could no only fail and need o be replaced bu is failure o clear a faul would rigger breaker failure proecion and ripping of poenially large numbers of breakers. This would resul in de-energizing many loads and generaors. Power sysems slowly evolve as equipmen is added or replaced. This changes he shor-circui levels and X/R raios. In addiion, a ypical shor-circui calculaion would have accuracy of 5 percen if no worse (due o he limied accuracy of models and parameers). As a resul, breaker praciioners apply hefy margins in breaker raings. I is no uncommon o have a 0-percen margin in he asymmerical curren raing. This margin allows for sysem growh and may be reduced over ime. IV. RELAY OPERATING TIME Typically, proecive relays provide shor-circui proecion based on he fundamenal frequency componens in volages and currens associaed wih he proeced apparaus. The elecromechanical relay echnology brough in unavoidable filering hrough mechanical ineria o proecive relaying. Solid-sae (saic) relays allowed relay designers a choice of how much filering o apply, bu hese relays did no gain a wide-spread adopion because of he success of he relay echnology ha followed he microprocessor-based relay. Early microprocessor-based relay designers were forced o use phasors o afford lower sampling raes and o provide a wide range of funcions wih he limied processing power available a he ime. This phasor-based approach coninues oday. As a resul, i is a common expecaion ha highperformance proecive relays operae in abou one power sysem cycle [5]. High-speed elemens available in some relays use less filering for faser operaion, and ypically specify operaing imes beween 0.5 cycle and 1 cycle. Bu hese elemens are less dependable, and hey operae as acceleraors for he phasor-based elemens. Many saic 4 relays were specified wih 0.5-cycle operaing imes, bu heir securiy was someimes problemaic. Inerposing and lockou relays also play a role in he discussion on he relay operaing ime. Hisorically, proecive relays in high-volage applicaions did no rip breakers direcly, bu hey acuaed inerposing or lockou relays. Some older breakers required rip currens as high as 0 A and hese higher currens called for more robus conacs han were commonly available in proecive relays. These inerposing relays ypically operaed in o 6 ms or in abou 0.5 cycle. Therefore, even if he proecive relay operaed in a few milliseconds, he breaker acuaion ime was no shorer han abou 0.5 cycle. Because of he specified relay operaing imes, acual inservice operaing ime records, and he slowing-down role of he inerposing relays, he indusry seled on an assumpion ha breakers will no be ripped faser han in abou 0.5 cycle for a shor circui. Hence, he reference poin of 0.5 cycle for he relay operaing ime in he breaker sandards [1] []. Today, we need o revisi his assumpion. A. Eliminaion of Inerposing and Lockou Relays Many microprocessor-based relays incorporae rip-raed oupus. These oupus have he curren make and carry raings, as well as he volage raings, ha allow hem o be direcly conneced o breaker rip coils, assuming he 5a breaker conacs ake care of inerruping he curren. Today s breakers require rip currens a he level of abou 5 A, making he applicaion of ripping direcly from he proecive relay oupus even more pracical. Some of hese oupus allow mechanical posiion reenion even upon he loss of power o he relay, hus permiing eliminaion of sand-alone lockou relays. Some applicaions provide lockou via an inerlocking logic raher han mechanical posiion reenion. Ye oher applicaions rely on he operaor s procedures and imers o preven reclosing raher han relying on lockou relays. As a resul, an increasing number of new insallaions (and rerofis) eliminae he inerposing and mechanical lockou relays o improve he overall reliabiliy of he proecion sysem and o lower he maerial and labor coss [6]. Some of hese rip-raed oupus use semiconducors and can close in a shor fracion of a millisecond. This creaes anoher benefi faser ripping. Because of his rend of ripping direcly from he rip-raed oupus of microprocessor-based relays, he relay operaing imes are shorened by several milliseconds, or by abou 0.5 cycle. The assumpion ha a breaker will never be acuaed faser han in 0.5 cycle becomes quesionable. B. High-Performance Relays Using Naurally Secure Proecion Principles A few proecion principles are inherenly secure and herefore can be very fas. These principles, when implemened on a low-laency relay plaform wih semiconducor-based ripraed oupus, can issue a rip signal o a breaker in abou 0.5 cycle for high-curren inernal fauls when breaker

6 inerruping raings are challenged. These principles are as follows: 1) Bus Differenial Proecion High-impedance bus differenial schemes are inherenly very secure. When combined wih a low-laency overcurren or overvolage elemen specifically designed o work wih signals expeced in hese schemes, he high-impedance bus differenial scheme may operae well below 0.5 cycle. Modern low-impedance differenial schemes incorporae fas and dependable exernal faul deecors [7] ha provide excellen securiy for CT sauraion during exernal fauls. In applicaions o bus proecion, hese differenial schemes can herefore operae very fas, especially when implemened on low-laency relay hardware wih semiconducor-based ripraed oupus. ) High-Se Unresrained Transformer Differenial Proecion Unlike bus differenial schemes, ransformer differenial schemes need o rule ou magneizing inrush as a cause of he differenial signal before hey can operae. Harmonic-based inrush deecion is ypically used. This mehod of dealing wih inrush requires abou 1 cycle o release he ransformer differenial relay o operae on an inernal faul. Today, waveshape-based inrush deecion mehods [8] are adoped, and hey need only abou 0.5 cycle o rule ou inrush during heavy inernal fauls. High-se unresrained ransformer differenial elemens differeniae beween fauls and inrush based on he differenial curren level alone. Recenly, improved versions of he unresrained ransformer differenial logic have been inroduced, such as he mehod described in [8]. This logic compares he unipolar (inrush) vs. bipolar (many inernal fauls) naure of he differenial curren and allows ripping in abou 0.5 cycle. Some relays allow insananeous (samplebased wih minimum or no filering) high-se unresrained differenial operaion or even operaion based on he rae of change of he differenial curren. As a resul, ransformer differenial relay operaing imes shorer han 0.5 cycle are becoming possible. This is especially rue for high-curren inzone fauls ha are no limied by he impedance of he ransformer. 3) Sub-Bus and Swich-Ono-Faul Proecion Sub-bus proecion deecs fauls on a piece of buswork beween one or wo closed breakers and he opened line disconnec swich in a emporary bus configuraion. A ypical case is wo breakers ha are closed o mainain he ring-bus or he breaker-and-a-half configuraion while he line or oher conneced apparaus is ou of service. In dual-breaker applicaions sub-bus proecion is bes accomplished by enabling a low-se differenial overcurren elemen when he disconnec swich is open. Wih securiy inheren in he differenial principle, differenial-based sub-bus proecion is very fas. If a simple overcurren elemen operaing on he summed currens is used (unresrained differenial), a shor ime delay may be needed o accoun for CT sauraion during exernal fauls. Swich-ono-faul (SOTF) proecion deecs fauls on a line being energized, boh genuine fauls as well as swiching errors such as closing he breaker on safey grounds. I is accomplished by enabling a low-se overcurren elemen for a shor ime afer he breaker closes, if he breaker was open for some ime and he line-side volage was no presen confirming he line was no already energized from he opposie erminal. Boh hese proecion schemes may work wih exremely high muliples of pickup (he operaing curren may be many imes higher han he pickup seing). The SOTF pickup seing may be especially low for shor lines ha do no draw large charging currens when energized. Therefore, he sub-bus and SOTF proecion schemes may operae very fas. We are aware of field cases of SOTF schemes ha have operaed as fas as ms. C. New Line Proecion Principles Recenly, new line proecion principles [5] found heir way ino producs [3]. These principles are based on incremenal quaniies (ime-domain (TD) elemens) and raveling waves (TWs). This subsecion briefly reviews hese new proecion elemens: direcional elemens (TD3 and TW3), a disance elemen (TD1), and a differenial scheme (TW87); i also illusraes heir operaing imes wih field cases. 1) TD3 Direcional Elemen To realize he TD3 direcional elemen, a ime-domain relay calculaes an incremenal replica curren ( i Z) as a volage drop resuling from he incremenal curren ( i) a he relay locaion hrough an RL circui wih uniy impedance (1 Ω) [5]. As Fig. 5 shows, he incremenal replica curren is direcly proporional o he incremenal volage ( v) a he relay locaion. For forward fauls, he incremenal replica curren and he incremenal volage are of opposie polariies (Fig. 5a). They are of maching polariies for reverse fauls (Fig. 5b). (a) (b) i F v F i Z i RELAY v RELAY i RELAY v RELAY Fig. 5. TD3 direcional elemen operaing principle for forward (a) and reverse (b) fauls [9]. When implemening he TD3 elemen, he relay [3] uses six measuremen loops (hree ground loops and hree phase loops) o cover all faul ypes; calculaes and inegraes an operaing orque; and applies adapive hresholds for enhanced sensiiviy, speed, and securiy [5]. i Z i F v F

7 The ime-domain relay [3] uses he TD3 elemen in he POTT scheme, o supervise he TD1 elemen, and in some applicaions, o supervise he TW87 scheme. Fig. 6 shows a faul record for a single-line-o-ground faul on a 500 kv, 69.9 mi series-compensaed line in a 60 Hz sysem. The faul was mi from he local erminal. The local and remoe TD3 elemens assered in 1.5 ms and. ms, respecively. This applicaion uses a direc fiber channel for he POTT scheme wih a communicaions laency as shor as abou 0.6 ms including processing he ransmied and received packes by he wo relays. Because of he exremely fas asserion of he direcional elemens, he low-laency POTT channel, and he relaively low POTT overcurren rip supervision seings, he POTT scheme operaed in.8 ms and. ms, a he local and remoe erminals respecively. This relay [3] has semiconducor-based rip-raed oupus ha closed is less han 10 µs. If conneced direcly o he breakers, his relay would have acuaed he breakers as early as. ms ino he faul (his insallaion is in a monioring mode and does no rip breakers a his ime). (a) (b) Acual Volage Change Local Bus Local Bus v v i Calculaed Volage Change i m1 m1 Calculaed Volage Change Remoe Bus Acual Volage Change Remoe Bus Fig. 7. TD1 elemen operaing principle for in-zone (a) and ou-of-zone (b) fauls [9]. When implemening he TD1 elemen, he ime-domain relay [3] uses six measuremen loops o cover all faul ypes, and i applies an insananeous prefaul volage a he reach poin as a resraining signal for sensiiviy and speed. To appreciae he TD1 operaing ime, refer o Fig. 6. The faul is wihin he local erminal TD1 reach. The TD1 elemen operaed in 3.9 ms. Therefore, even if he POTT channel were no available for his case, he relay would sill have operaed in 3.9 ms using he communicaions-independen TD1 elemen. Fig. 8 shows anoher field case of TD1 operaion for a single-line-o-ground faul on a 110 kv, km line in a 50 Hz sysem. The faul was wihin he TD1 reach. The TD1 elemen operaed in 1.8 ms for his faul. The operaing ime is parially credied o he magneic volage ransformers, which responded quickly o he volage change. Fig. 6. Field case example showing he operaion of he ulra-high-speed incremenal-quaniy and TW elemens and he POTT scheme. The local and remoe erminals are labeled 1 and, respecively. ) TD1 Disance Elemen To realize he TD1 disance elemen, a ime-domain relay calculaes as is operaing signal, he change in he insananeous volage a he inended reach poin using he incremenal replica curren, incremenal volage, and line RL parameers. The elemen operaing condiion is derived from he observaion ha he prefaul volage is he highes possible value of he volage change a he faul poin. Wih reference o Fig. 7, if he calculaed volage change a he reach poin is higher han he prefaul volage a he reach poin, he faul mus be closer han he se reach, m 1. If his is rue and he TD3 elemen assers forward, he TD1 elemen operaes [5]. Fig. 8. Field case example for he TD1 elemen.

8 3) TW3 Direcional Elemen The TW3 direcional elemen compares he relaive polariy of he curren TWs and he volage TWs. For a forward even, he wo TWs are of opposie polariies; for a reverse even, hey are of maching polariies [5]. To realize he TW3 elemen, he ime-domain relay [3] filers he TW signals, inegraes a orque calculaed from he curren and volage TWs, and checks he inegraed value a few ens of microseconds ino he faul (see Fig. 9). As a resul, he relay responds o he TW aciviy during he few ens of microseconds following he firs TW. Once assered, he TW3 elemen laches for a shor period of ime o ac as an acceleraor for he dependable TD3 elemen for permissive keying in he POTT scheme. When applied wih CCVTs, he TW3 elemen benefis from he parasiic capaciances across he CCVT uning reacor and sep-down ransformer, which oherwise block he highfrequency TW signals. These capaciances creae a pah for hese signal componens, allowing some volage TW signals o appear a he secondary CCVT erminals. The elemen only needs accurae polariy and iming of he firs volage TW, and herefore, he elemen is suiable for CCVTs despie heir poor reproducion of volage TWs, especially for he second and subsequen TWs. The relay in [3] uses he TW3 elemen o accelerae he permissive key signal in he POTT scheme. (a) (b) line erminals wih a ime separaion ha is less han he TWLPT (see Fig. 10b). To realize he TW87 scheme, he imedomain relay [3] exracs TWs from he local and remoe currens and idenifies he firs TW for each. I hen searches for he exiing TW from he local and remoe currens arriving a he opposie line erminal afer he TWLPT. The relay hen calculaes he operaing and resraining signals from he firs TW and he exiing TW [5]. The TW87 logic applies a facoryseleced magniude pickup level and securiy slope and provides an overcurren rip supervision hreshold for he user. (a) Local TW Remoe TW (b) Local TW + + < TWLPT TWLPT Volage TW Volage TW Remoe TW Curren TW TW3 A Few Tens of µs Curren TW Inegraed Inegraed Torque Torque VTW ITW VTW ITW TW3 Fig. 9. Volage and curren TWs for a forward (a) and reverse (b) faul [9]. To appreciae he TW3 speed, refer o Fig. 6 and observe ha he TW3 elemens assered in 0.1 ms a he remoe erminal (:TW3F). 4) TW87 Differenial Scheme The TW87 differenial scheme compares ime-aligned curren TWs a boh ends of he proeced line. For an exernal faul, a TW ha enered one erminal wih a given polariy leaves he oher erminal wih he opposie polariy exacly afer he known TW line propagaion ime (TWLPT) (see Fig. 10a). For an inernal faul, TWs of maching polariies arrive a boh + Fig. 10. Curren TW iming and polariies for exernal (a) and inernal (b) fauls [9]. Fig. 11 shows a faul record for a single-line-o-ground faul on a 115 kv, 0.65 mi line in a 60 Hz sysem (TWLPT is µs). The faul was 9.4 mi from he local erminal. The TW87 scheme requires a direc fiber channel, which brings he exra benefi of low communicaions laency. Addiionally, in his case, i used relaively low overcurren supervision seings (fas release from he overcurren elemens). As a resul, i operaed in 0.9 ms a boh he local and remoe erminals. Fig. 1 shows he firs curren TWs for he local and remoe erminal of he line (compare wih Fig. 10b). The relay in [3] ha uses hese new line proecion principles has a field rack record of operaing imes in he range of 5 ms, considerably below he 0.5-cycle reference relay operaing ime in he breaker sandards [1] and [].

9 may argue ha a misoperaion is more probable during a faul condiion han during normal seady-sae condiions. Therefore, an exremely low, ye no zero, probabiliy exiss ha a breaker may be ripped a fracion of a millisecond ino a faul if a relay misoperaes. Any misoperaion normally riggers an invesigaion and a correcive acion. However, an imporan quesion is his: Shall a relay misoperaion be allowed o push a breaker beyond is raing, risking breaker failure and resuling in a beaker failure rip and a larger ouage, no o menion he cos, labor, and operaional inconvenience of losing a breaker? In his respec, we srongly advocae having enough margin in he breaker raings o cover he low-probabiliy case of a relay misoperaion a he very beginning of a heavy faul wih a fully offse (asymmerical) curren. V. RATING A CIRCUIT BREAKER FOR THE RELAY OPERATING TIME Fig. 11. Field case example for he TW87 operaion. Labels 1 and correspond o he local erminal, and label 3 corresponds o he remoe erminal. A. Deraing Formulas for Relay Operaing Time To derive a deraing formula accouning for an arbirary relay operaing ime, we follow he S-facor () regarding he asymmerical curren raing for a breaker: I RATED = S I SYM = I SYM 1 + e PART T DC (5) Fig. 1. Local and remoe currens (op) and raveling waves (boom) for he case of Fig. 11. D. Proecion Misoperaions and Breaker Raings Finally, we need o consider he case of relay misoperaions. Modern microprocessor-based relays incorporae exensive self-monioring o deec any inernal failures in boh hardware and daa inegriy, and upon a failure hey fall back gracefully wihou misoperaion while seing an alarm o ensure proper mainenance aenion. Sill, here is a non-zero probabiliy, even if very low, ha a relay may misoperae due o an inernal problem or because of a seing or logic error. We where: I RATED is he raed breaker asymmerical inerruping curren, I SYM is he raed breaker symmerical inerruping curren, PART is he breaker conac paring ime, T DC is he dc offse ime consan (depends on he X/R raio). Equaion (5) effecively specifies an exra margin ha is required for he asymmerical raing as compared wih he symmerical raing for any given conac paring ime and dc offse ime consan. Noe ha he value in he square roo is higher han one, making I RATED higher han I SYM. This means ha o safely inerrup he ac componen of I SYM under he presence of a fully offse dc componen wih a ime consan T DC, he breaker needs o be raed such ha I RATED > I SYM. Or conversely, one can claim ha a breaker wih he symmerical raing of I SYM has he raing of I SYM / S for asymmerical condiions. We divide he conac paring ime ino wo componens: he relay operaing ime ( REL) and he breaker mechanical ime ( MECH), and rewrie (5) as follows: I RATED = I SYM 1 + e REL+ MECH T DC IEEE Sandard C37.04 [1] asks he breaker manufacurers o use 0.5 cycle for he relay operaing ime ( REL = 0.5 cycle) and 45 ms (corresponding o X/R = 17 for 60 Hz sysems) for he (6)

10 dc offse ime consan (T DC = 45 ms or.71 cycles). Knowing heir symmerical capabiliy (I SYM) and he mechanical ime ( MECH), he manufacurers specify and es he asymmerical raing (I RATED) ha accouns for he reference relay operaing ime and he reference X/R raio. We can use (6) and calculae a deraing facor: a raio of he breaker inerruping raing a an arbirary relay operaing ime, REL, and he nameplae raing applicable o relays ha operae in 0.5 cycle ( 0.5 = 0.5 cycle). I RATED(0.5cycle) = I SYM 1 + e 0.5+ MECH T DC I RATED(REL ) = I SYM 1 + e REL+ MECH T DC The raio of he inerruping curren for an arbirary relay operaing ime o he inerruping curren for he reference 0.5-cycle relay operaing ime is as follows: R = I RATED( REL ) I RATED(0.5cycle) = 1 + e REL+ MECH T DC (7) (8) (9) 1 + e 0.5+ MECH T DC Fig. 13 plos he R-facor for relay operaing imes beween ms and 8 ms, and for hree ypical breaker mechanical imes of 13 ms (wo-cycle breaker), 30 ms (hree-cycle breaker) and 63 ms (five-cycle breaker). The figure assumes he reference X/R raio of 17 (T DC = 45 ms in 60 Hz sysems). Secion V, Subsecion B explains he mehod for esimaing he breaker mechanical ime. We obain R below 1 for he relay operaing imes shorer han 0.5 cycle. R < 1 means he breaker los some capabiliy because of fas ripping. The 1 R value is he penaly for he relay operaing in less han 0.5 cycle. Asymm. Raing Relaive o 0.5 cyc Relay Time, % MECH 63 ms (5-cycle CB) 30 ms (3-cycle CB) 13 ms (-cycle CB) Relay Operaing Time, ms Fig. 13. Deraing curves accouning for fas relay operaion for an X/R of 17 in a 60 Hz sysem. For example, a breaker wih a 30 ms mechanical ime (a hree-cycle breaker) ripped in 3 ms and los abou 3.5 percen of is raing. A breaker wih a 13 ms mechanical ime (wocycle breaker) ripped in ms and los abou 6.5 percen of is raing (a very exreme case for boh he relay and he breaker). A five-cycle breaker wih a 63 ms mechanical ime ripped in ms and los abou 1 percen of is raing. B. Esimaing he Breaker Mechanical Time To apply he deraing formula (9), one needs o know he mechanical ime in (9). You can calculae he mechanical ime from he breaker inerruping ime by subracing he arcing ime wih margin. You can approximae he arcing ime by adding he ime beween consecuive zero-crossings of 0.5 cycle (8.3 ms), accouning for he scaer of zero-crossings beween all hree phases during a hree-phase faul (4. ms), and adding an exra margin. In pracice, a maximum faul currens, a breaker needs o par is conacs 1 15 ms before is raed inerruping ime o develop sufficien inerruper pressure o inerrup he highes curren fauls. Ofen, a 0 ms arcing inerval is used for safey. In oher words, a wo-cycle breaker has a mechanical ime of approximaely 33 ms 0 ms = 13 ms, and a five-cycle breaker has a mechanical ime of abou 63 ms. Anoher way o approximae he breaker mechanical ime is o use he symmerical raing, if known. We can use (6) and solve i for he mechanical ime as follows: MECH = T DC ln 0.5 I RATED cyc (10) I SYM where ln is he naural (base e) logarihm. For example, for he asymmerical raing requiremen of 1.5 imes he symmerical raing in a 60 Hz sysem wih an X/R of 17, he mechanical ime is abou 0 ms; for he asymmerical raing requiremen of 1.3 imes he symmerical raing, he mechanical ime is abou 16 ms; for he asymmerical raing requiremen of 1.1 imes he symmerical raing, he mechanical ime is abou 4 ms. You can also conac your breaker manufacurer o obain a more precise esimae of he mechanical ime. Noe ha slow breakers do no need or have much of an oversizing facor for he dc componen because he arc appears when he dc offse already decayed o a large degree. C. Impac of he X/R Raio and Mechanical Time The deraing facor R (9) includes hree variables: Relay operaing ime, REL. DC offse ime consan ha depends on he X/R raio, T DC. Breaker mechanical ime, MECH. The impac of he relay operaing ime on he breaker raing varies depending on he wo oher facors. Fig. 13 plos he deraing curves for he reference X/R raio of 17 and hree breaker mechanical imes. Fig. 14 and Fig. 15 plo he deraing curves for ime consan values of 100 ms (X/R of 37.7 in a 60 Hz sysem) and 5 ms (X/R of 9.4 in a 60 Hz sysem), respecively. The hree plos show deraing facors for he relay

11 operaing ime, assuming ha he X/R raios of 17, 37.7, and 9.4, respecively, do no change. Should he X/R raio change, he breaker shall be furher deraed. Asymm. Raing Relaive o 0.5 cyc Relay Time, % MECH 63 ms (5-cycle CB) 30 ms (3-cycle CB) ms (-cycle CB) Relay Operaing Time, ms Fig. 14. Deraing curves accouning for fas relay operaion for an X/R of 37.7 (100 ms ime consan) in a 60 Hz sysem. Asymm. Raing Relaive o 0.5 cyc Relay Time, % MECH 63 ms (5-cycle CB) 30 ms (3-cycle CB) ms (-cycle CB) Relay Operaing Time, ms Fig. 15. Deraing curves accouning for fas relay operaion for an X/R of 9.4 (5 ms ime consan) in a 60 Hz sysem. The plos in Fig. 14 and Fig. 15 may seem counerinuiive a firs: he impac of fas ripping for sysems wih long ime consans is smaller han for sysems wih shor ime consans. The long ime consan case in Fig. 14 (large X/R) is less punishing for breakers ripped from fas relays, because he long decay of he dc componen is he dominaing facor in he raing, and he relay operaing ime becomes a secondary facor. In oher words, he dc componen is approximaely as high when he relay operaes very fas (such as in ms) as when i operaes a he reference ime of 0.5 cycle. The shor ime consan case in Fig. 15 seems o be more punishing for very fas relays, bu he deraing does no maer ha much. Breakers are raed for he sandard ime consan. When operaed in a sysem wih a shor ime consan, hese breakers gain some exra margin in raing due o he fas dc decay, and ha margin is removed by relay operaion faser han 0.5 cycle. D. Fas Relays and Slow Relays So far, we have considered relay operaing imes faser han he reference 0.5-cycle value. Fig. 16 plos he deraing curves for relay operaing imes boh faser and slower han 0.5 cycle for hree sample breaker mechanical imes. Asymm. Raing Relaive o 0.5 cyc Relay Time, % MECH 13 ms (-cycle CB) 30 ms (3-cycle CB) 63 ms (5-cycle CB) Relay Operaing Time, ms Fig. 16. Deraing curves accouning for relay operaion ime faser and slower han he reference 0.5 cycle for an X/R of 17 (45 ms ime consan) in a 60 Hz sysem. For example, when he breaker mechanical ime is 30 ms (a hree-cycle breaker), a ms relay operaing ime penalizes he breaker raing by abou 4 percen compared wih he nameplae. A slow relay operaing in 0 ms rewards he breaker wih he exra 7 percen compared wih he nameplae. As we can see, he impac of he relay operaing ime in boh direcions below and above he assumed 0.5 cycle is no ha dramaic. Also, i should be noed ha his apparenly higherraed capabiliy when using slow relays only applies o he fully asymmerical bus or erminal faul. Oher es duies, such as he shor line faul es, are no affeced by his change in dc asymmery. This higher capabiliy from a slow relay operaion becomes an addiional margin raher han a rue increase in he raed capabiliy. However, if one inenionally (or unknowingly) benefis from he slow proecion ime premium, one may see some issues during occasional fas ripping or afer rerofiing proecive relays. For example, assume a breaker wih a 30 ms mechanical ime is marginal when operaed from a 0 ms relay. When one rerofis he 0 ms relay wih a ms relay, one would lose +7% ( 4%) = 11% of he asymmerical raing in his example. The 11 percen is sill wihin he 0 percen margin recommended for breakers. However, if his breaker does no have a leas an 11 percen margin, i may have issues when i is ripped in ms as compared o 0 ms. E. Is Deraing for Relays Faser Than 0.5 Cycle Needed? An ac breaker can inerrup only a he naural curren zero crossing. For a fully offse curren, he firs curren zero crossing occurs jus before one full cycle (see Fig. 17). Assume ha he shores breaker mechanical ime is 0.5 cycle. If we assume he relay operaing ime o be zero, we may conclude ha his breaker can inerrup a he firs zero crossing and he

12 inerrupion will be concerned wih he level of he firs curren peak. This consiues he absolue wors-case scenario. Curren, pu of ac peak s Peak = 1.91 pu S = 1.63 S = 1.34 nd Peak = 0.4 pu S = Time, ms Fig. 17. Illusraion of why he hird curren peak is criical for he asymmerical breaker raing. The firs curren peak occurs cycle ino he faul (0.5 cycle for symmerical curren and 0.5 cycle for a fully offse curren; assume a more sringen case of 0.5 cycle in his analysis). A he ime, assuming he sandard X/R raio (a dc ime consan of.71 cycles), he dc componen of a fully offse curren is: DC% = 100% e = 91% and he corresponding S-facor is: S = 1 + (0.91) = 1.63 The above S-facor of 1.63 would ensure he absolue worscase raing for an insan relay (0 ms operaing ime) and an insan breaker (mechanical ime below 0.5 cycle). If he breaker sars arcing laer, i may inerrup a he second zero crossing, pas he second curren peak. The second curren peak, however, is very small for a fully offse curren. For he sandard X/R of 17, he firs peak occurs a 0.5 cycle and is 1.91 imes he symmerical componen. The second peak occurs a 0.75 cycle and is only 0.4 imes he symmerical componen (DC% is negaive 76 percen a = 0.75 cycle). Theoreically, a breaker ha inerrups a he second zero crossing deals wih a much smaller peak curren because he dc and ac componens have opposie polariies and hey parially cancel. However, o inerrup a he second zero crossing, he conacs need o par considerably earlier, before he firs zero crossing in his case, a he ime he curren is sill large and falling from he previous peak. This large curren in he early sage of arcing creaes hea and plasma and will make i less likely o inerrup pas he second peak a he second zero crossing. Also, arcing a he ime of he second lower peak generaes lower energy, and his may creae problems for breakers ha depend on arc-generaed energy for inerrupion. If he inerrupion akes place a he nex (hird) zero crossing, he preceding peak occurs a 1.5 cycles and is 1.63 imes he symmerical componen. Wih he above examples, we wan o bring he following aspecs o our discussion: The decaying dc offse in he asymmerical curren elevaes he ac curren peak only a every oher peak. The odd peaks (firs, hird, fifh, and so on) are elevaed while he even peaks (second, fourh, sixh, and so on) are reduced compared wih he peaks of he symmerical componen. A breaker can inerrup only a a curren zero crossing. As a resul, he deraing calculaions may need o be rounded o a discree ime of odd curren peaks (firs, hird, fifh, and so on). If arcing did no sar before he firs zero crossing (in he firs cycle), he mos inense arcing will occur a he hird (no he second) curren peak. The dc componen elevaes he hird peak because he dc and ac componens are of he same polariy and hey add up. The dc value a he ime of he hird peak is lower han a he ime of he second peak. The hird curren peak (assuming he sandard X/R raio) has he S-facor of 1.34, while he firs peak has he S-facor of The fifh peak occurs a 1.75 cycles and has an S-facor of 1.4. If he relay operaing ime is such ha arcing sars before he firs zero crossing, he applicaion calls for an S-facor of If relay operaing ime is such ha arcing sars afer he firs zero crossing bu sufficienly before he second zero crossing, he applicaion calls for an S-facor of If relay operaing ime is such ha arcing sars afer he second zero crossing bu sufficienly before he hird zero crossing, he applicaion calls for an S-facor of 1.4. This discussion may explain why we do no have field cases of breaker failures for breakers properly raed for 0.5-cycle relay operaion when acuaed from SOTF relays, fas bus differenial relays, or during relay misoperaions. We are aware of breaker problems afer faser relays have been insalled. However, hose problems have roos in insufficien breaker raings wih respec o he 0.5-cycle sandard relay operaing ime and no in he acual relay operaing imes being faser han 0.5 cycle. VI. CONCLUSIONS This paper explains he impac of he faul curren dc componen on he breaker asymmerical curren inerruping raing. The asymmerical raing is driven by he curren dc componen level a he ime of conac paring. The longer he dc ime consan, he higher he dc value a he ime of conac paring, and he harder i will be for he breaker o inerrup he curren. Similarly, he faser he relay, he higher he dc value