Technical collection. Cahier technique. no Current transformers: how to specify them. P. Fonti

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1 Techical collectio Cahier techique o. 194 Curret trasformers: how to specify them P. Foti

2 "Cahiers Techiques" is a collectio of documets iteded for egieers ad techicias, people i the idustry who are lookig for more i-depth iformatio i order to complemet that give i product catalogues. Furthermore, these "Cahiers Techiques" are ofte cosidered as helpful "tools" for traiig courses. They provide kowledge o ew techical ad techological developmets i the electrotechical field ad electroics. They also provide better uderstadig of various pheomea observed i electrical istallatios, systems ad equipmets. Each "Cahier Techique" provides a i-depth study of a precise subject i the fields of electrical etworks, protectio devices, moitorig ad cotrol ad idustrial automatio systems. The latest publicatios ca be dowloaded from the Scheider Electric iteret web site. Code: Sectio: Experts' place Please cotact your Scheider Electric represetative if you wat either a "Cahier Techique" or the list of available titles. The "Cahiers Techiques" collectio is part of the Scheider Electric s "Collectio techique". Foreword The author disclaims all resposibility subsequet to icorrect use of iformatio or diagrams reproduced i this documet, ad caot be held resposible for ay errors or oversights, or for the cosequeces of usig iformatio ad diagrams cotaied i this documet. Reproductio of all or part of a "Cahier Techique" is authorised with the prior coset of the Scietific ad Techical Divisio. The statemet "Extracted from Scheider Electric "Cahier Techique" o...." (please specify) is compulsory.

3 o. 194 Curret trasformers: how to specify them Paola FONTI INPG egieer (Istitut Natioal Polytechique de Greoble). Graduate i Joied Merli Geri i 1981 as cosultat ad head of the Medium Voltage Export egieerig ad desig departmet. She is curretly resposible for the MV project completio ad teder support group for Scheider Electric. ECT 194 first issue, February 2000 Cahier Techique Scheider Electric o. 194 / p.1

4 Lexico I f : maximum through curret crossig a protected area. I s : curret threshold settig. k : omial accuracy limit factor (ALF) of a CT (associated with its accuracy load). k r : real ALF of a CT associated with its real load. P i : (=R ct I 2 ). Iteral losses of the CT at I. P : (=R I 2 ). Accuracy power of the CT. P r : (=R r I 2 ). Real load cosumptio of the CT at I. R L : wirig resistace. R p : protectio relay resistace. ALF: accuracy limit factor. CT: curret trasformer. Overratig of a CT: selectio of a CT whose primary I is greater tha the I immediately greater tha the load I. Matchig, auxiliary or iterposig CT: low voltage CTs istalled at the secodary of the mai CTs for correctig a ratio ad/or the curret phase shift. SF: security factor. Cahier Techique Scheider Electric o. 194 / p.2

5 Curret trasformers: how to specify them Electrical power maagemet requires implemetatio of data processig uits able to moitor etworks or equipmet ad, as applicable, to iitiate the appropriate actios... Data set by curret trasformers are processed by protectio, cotrol ad moitorig uits that sed sigals to operate switchgear ad/or iformatio to a supervisory uit or to a cetral cotrol room. The task, cosistig of idetifyig ad ratig the curret trasformers ad associatig them with the protectio ad/or meterig uits, has always give rise to problems, both for electrical egieers (oversizig the characteristics) ad for the maufacturer (radom feasibility, excessive sizig, high costs). This documet does ot cover the techical demostratios amply referred to i the literature (see Cahiers Techiques o. 164 ad 170). Its purpose is to remid users of a few simple rules eablig the best possible defiitio of the secodary characteristics of a curret trasformer (CT) accordig to the protectios ad applicatios cocered. Practically, it offers costructive assistace for techicias who have reached a dead ed: c either because they do ot possess the ecessary iformatio, c or because the results of their desig have led to curret trasformers that caot be maufactured by the potetial suppliers. Cotets 1 Itroductio p. 4 2 Network disturbaces ad protectios 2.1 Disturbaces p Protectios p. 7 3 Curret trasformers 3.1 Remider of ferromagetic trasformers p No-magetic trasformers p CT maufacturig ad implemetatio p Choosig CTs accordig to protectios 4.1 Choosig CT ALF accordig to protectios p. 15 ad applicatios 4.2 Characterisig CTs accordig to applicatios p Special case of differetial protectio p Distace protectios p CT specificatio examples 5.1 Motor feeder protectios p Trasformer feeder protectios p Trasformers differetial protectio p Differetial protectio for busbars (87B) p Coclusio p. 31 Bibliography p. 32 Cahier Techique Scheider Electric o. 194 / p.3

6 1 Itroductio The desig of MV ad HV electrical etworks is a complex udertakig that must take ito accout the eeds to be satisfied, i.e.: c safety of people ad equipmet, c cotiuity of supply, c istallatio ad operatig costs. The desiger uses the load poits, their simultaeity coefficiet ad the above-metioed criteria to draw up the sigle-lie diagram for the istallatio (see fig. 1 ). He must the select the earthig system, defie the busways, calculate the fault currets, defie the protectio system (discrimiatio, choice of protectios, see Cahier Techique o. 174). Figure 2 shows a example of protectios chose for the stadby supply i figure 1. G 63 kv Staby supply 10 MVA 2 MVA 10 MVA 20 kv Bak Crushig 5 kv Processig Mills LV Shaft o.1, level 300 LV 5 kv Shaft o. 2, level 500 Fig. 1 : example of a geeral sigle-wire diagram for a mie. Cahier Techique Scheider Electric o. 194 / p.4

7 Permaet isulatio moitor a 49 Thermal image 46 Negative sequece 87T Differetial 51V 32P Voltage restraied O/C Reverse active power 32Q Field loss (max. reactive power) 27 Udervoltage 59 Overvoltage 81 Over ad uder frequecy 59N Zero sequece voltage Fig. 2 : protectios of a MV uit module. The protectio pla must specify the operatig or o-operatig coditios for all the protectios durig a fault ad durig ormal operatio (trasiets). It must idicate the protectio settigs. However, the pla rarely idicates the characteristics of the protectio iput circuit ad other data ecessary to specify the curret trasformers (CT). This is because it is ofte very hard for the desiger to collect all the ecessary data. The cosequeces o idustrial start-up ca be serious: usuitability, overratig, o-stadard specificatios ad high costs, additio of matchig CTs, last miute CT chages, postpoemet of delivery, commissioig, productio times, etc. More serious still, icorrect defiitio ca lead to malfuctios i the protectio chael causig destructio of equipmet or, worse still, dager for the operator. A few examples: c Overestimatio of the short-circuit curret ca lead to feasibility problems, overratig ad high CT costs. c O the other had, uderestimatio of the short-circuit curret ca lead to failure to detect the fault, thus destroyig the equipmet, placig the operator i dager ad geeratig operatig dowtime. c A output power or accuracy error ca result i a malfuctio or i failure to trip of the protectio devices, thus destroyig the equipmet, placig the operator i dager ad geeratig operatig dowtime. c A error i defiig the accuracy class of a meterig widig will lead to icorrect eergy billig ad thus a loss of icome for the electrical utility or the customer. c Etc. Cahier Techique Scheider Electric o. 194 / p.5

8 The aim of this documet is to provide assistace with defiig curret trasformers. Before eterig the heart of the subject, a remider is give of the iformatio ecessary to defie a CT (see the table i figure 3 ). Necessary iformatio Abbreviatios Uits Isulatio level U kv Nomial short-circuit curret I sc ka Duratio (1 to 3 secods) t s Nomial primary curret I p A Number of secodary widigs (1 to 3) For each secodary widig: c what type c associated protectio or meterig ad settig c output power P output VA (relay ad wirig cosumptio) c accuracy factor v protectio ALF v meterig SF c Nomial secodary curret (1 or 5 A) I s A Fig. 3 : iformatio ecessary to specify a CT with a sigle primary. Cahier Techique Scheider Electric o. 194 / p.6

9 2 Network disturbaces ad protectios 2.1 Disturbaces A MV or HV electrical etwork is disturbed: c exceptioally by lightig overvoltages, by temperature rises further to overloads or followig violet short-circuits betwee phases or phase-to-earth, c more frequetly, ad more aturally, by switchig overvoltages (e.g. capacitor eergisatio) or atural trasiet coditios (e.g. motor startig or power trasformer switchig) resultig i high but temporary overcurrets. The cosequeces of disturbaces Major disturbaces, such as short-circuit currets, ca result i serious damage: c fatigue or deterioratio of etwork compoets, c dager for people, c loss of supply ad productio, etc. It is thus ecessary to provide the relevat protectio devices with the right iformatio to esure prompt actio, as the greater the damage, the loger ad more costly the repairs ad the heavier the losses. However, trasiet ad ormal disturbaces are a ecessary evil ad istallatios must be desiged to withstad them. Furthermore, the curret trasformer/protectio pair(s) must ot cause uisace trippig. Elimiatio of faults Permaet moitorig of etwork electrical values by reliable ad properly rated curret trasformers supplyig protectio relays allows rapid isolatio of the faulty area. These relays must igore trasiet ad ormal disturbaces but systematically trip whe a destructive fault has to be elimiated. 2.2 Protectios Défiitio of protectios c Fuctio of protectios The protectio fuctios of a etwork are iteded to moitor oe or more parameters of the istallatio, for example: currets, voltage, temperature, frequecy, etc. These values are permaetly measured ad compared with setpoits or thresholds beyod which the situatio is defied as abormal ad dagerous. Whe a fault occurs, the protectio device issues a trippig sigal. The, i order to durably isolate the faulty part, it prevets reclosig util the device has bee repaired. It ca also geerate a alarm to iform maiteace persoel ad eable them to take the ecessary actio. c The techologies With the particularly rapid growth of electroics techology, protectio relays, origially electromechaical, have become static devices: aalogue electroic the digital electroic thaks to microprocessors. These devices perform icreasigly sophisticated fuctios ad are more ad more ofte referred to as processig uits. Digital techology is becomig widespread for all applicatios (etwork compoets or protected loads). This techology has a ample data processig capacity, which allows cotrol ad moitorig liked to protectio fuctios ad commuicatio to a supervisory uit or a cetralised cotrol system. These uits are ormally supplied with a stadard protectio, cotrol ad idicatio programme, thus eablig them to be used without extra studies or programmig. Oly parameters have to be set o commissioig (e.g. the protectio settigs). They are desiged to fully meet applicatio eeds. All you have to do is select the versio correspodig to the fuctios available for each applicatio. Some examples of applicatios are: v trasformers, v geerators, v capacitors, v motors, v substatios, v etc. Cahier Techique Scheider Electric o. 194 / p.7

10 These uits icorporate meters such as ammeters, voltmeters, wattmeters, kilowatt hour meters, etc. with fewer wirig ad compact space requiremets (see fig. 4 ). Furthermore, their reduced power requiremets (less demadig tha those of electromagetic relays), requirig less powerful CTs, make them more ecoomic. Whe combied with protectio fuctios ad cotrol logic, they also display alarm ad operatig messages. c The curret trasformers The CT characteristics are defied accordig to the techology selected for the processig uit ad the scheduled fuctios (protectio, meterig, cotrol ad moitorig, idicatio). c Implemetatio Choice of protectio settigs is very tricky. It calls for thorough kowledge of parameter levels accordig to whether the disturbace is ormal or trasiet or caused by a fault that must be elimiated. Furthermore, it is commo kowledge that a short-circuit at oe poit of the etwork may be detected right up to the source. The protectio co-ordiatio study esures that oly the faulty part of the etwork is deeergised. The protectios relatig to short-circuits either phase-to-phase or phase-to-earth use oe of the followig discrimiatio types, as applicable: v overcurret, v time, v logic, v differetial, v directioal, (see Cahiers Techiques o. 174 ad 113). Electromagetic techique Multifuctioal digital system Fig. 4 : simplificatio ad savigs provided by a multifuctioal digital system (protectio - automatio - meterig) compared with the former electromagetic techique. Cahier Techique Scheider Electric o. 194 / p.8

11 3 Curret trasformers 3.1 Remiders of ferromagetic trasformers Istrumet ad protectio CTs Curret trasformers are used to supply iformatio to the protective relays ad/or curret, power ad eergy meterig istrumets. For this purpose they must supply a secodary curret proportioal to the primary curret flowig through them ad must be adapted to etwork characteristics: voltage, frequecy ad curret. They are defied by their ratio, power ad accuracy class. Their class (accuracy as a fuctio of CT load ad of overcurret) is chose accordig to the applicatio. c A protectio CT must saturate sufficietly high to allow a relatively accurate measuremet of the fault curret by the protectio whose operatig threshold ca be very high. Curret trasformers are thus expected to have a Accuracy Limit Factor (ALF) that is usually fairly high. Note that the associated relay must be able to withstad high overcurrets. c A istrumet CT requires good accuracy aroud the omial curret value. The meterig istrumets do ot eed to withstad currets as high as the protectio relays. This is why the istrumet CTs, ulike the protectio CTs, have the lowest possible Safety Factor (SF) i order to protect these istrumets through earlier saturatio. c Some CTs have secodary widigs dedicated to protectio ad meterig. These istrumet ad protectio CTs are govered by stadard IEC (i Frace NF C ). The matchig of CTs with protectio relays calls for a thorough kowledge of CTs. The followig sectio gives a few remiders of CTs correspodig to this use. Characterisatio of CTs c A example of a protectio CT: v rated primary curret: 200 A, v rated secodary curret: 5 A. 15 VA 5P 10 accuracy limit factor = 10 accuracy class = 5P accuracy power = 15 VA Its accuracy load: P = 15 VA Its accuracy limit factor is ALF = 10 For I = ALF. I, its accuracy is 5% (5P), (see fig. 5 ) To simplify, for the protectio CT give i example, the ratio error is less tha 5% at 10 I, if the real load cosumes 15 VA at I. However these data are ot sufficiet. Also, it is useful to kow the stadard values. CT serial umber with year of maufacture Network voltage characteristics Rated isulatio voltage: 17.5 kv Power frequecy withstad voltage: 38 kv 1 m 50Hz Impulse withstad voltage: 95 kv peak CT type Applicable CT stadard Network curret characteristic I th : 25 ka/1 s I dy : 62.5 ka peak trasformateur de courat - curret trasformer ,5/38/95 kv 50 Hz Ith 25 ka 1 s rapport bores ratio termials 150/5 1S1-1S2 150/5 2S1-2S2 RCF 2 / B type CEI orme stadard Idy 62,5 ka ext. % classe VA FS ou FLP 15 class 0, P Safety factor (SF) Accuracy limit factor (ALF) Ratio 1 primary circuit 1 secodary circuit 1S1-1S2 1 secodary circuit 2S1-2S2 Accuracy power Fig. 5 : example of the ameplate of a curret trasformer with two secodaries. Accuracy class Cahier Techique Scheider Electric o. 194 / p.9

12 c A few defiitios v Rated (omial) primary curret I 1 Defied by stadards, it is chose from the discrete values: A ad their decimal multiples. v Rated (omial) secodary curret I 2 Equals 1 or 5 A. v Ratio (I 1 / I 2 ) The primary ad secodary currets are stadard, thus these values are discrete. v Accuracy load Load value o which the accuracy coditios are based. v Rated (omial) accuracy power P Expressed i VA, it is the apparet power supplied to the secodary circuit for the omial (rated) secodary curret ad the accuracy load. The stadard values are: VA. v Real power P r I this Cahier Techique, it is the power correspodig to the real load cosumptio of the CT at I. v Accuracy class This class defies the error limits guarateed o the ratio ad o the phase shift i specified power ad curret coditios. For the omial 5P ad 10P classes, the table i figure 6 defies these limits. v Special accuracy class Class X is a class defied by British stadard BS It must also be defied i the future stadard IEC uder the ame of class PX. This class specifies the miimum value of the kee poit voltage V k of the CT. It also imposes a maximum value of R ct (CT secodary widig resistace). Sometimes, it specifies the maximum value of the magetisig curret I o at kee poit voltage. If we cosider the magetisig curve V(I o ) of the CT, the kee poit voltage V k is defied as the poit o this curve from which a 10% icrease i voltage causes a 50% icrease i the magetisig curret I o. Class X correspods to a better meterig accuracy tha classes 5P ad eve more so 10P (see fig. 7 ). It is always possible to fid a equivalece betwee a CT defied i class X ad a 5P CT or i some cases eve a 10P CT (refer to Cahier Techique o. 195 which deals with equivaleces). v Real accuracy factor (F p or K r ) This is the ratio betwee the overcurret correspodig to the omial error ad the rated curret of the CT whe the real load is differet from the omial load. v Accuracy limit factor (ALF or K ) This is the ratio betwee the omial overcurret (e.g. 10 I ) ad the rated curret (I ). v Short time withstad curret Expressed i ka, this is the maximum curret I th that ca be withstood for oe secod (whe the secodary is short-circuited). It represets the 10P 5P X V S V S2 V S1 V k I S Fig. 7 : voltages correspodig to differet CT classes. I o Accuracy Curret error for Phase shift for the Composite error for the class the omial curret omial curret accuracy limit curret as as a % Miutes Cetiradias a % 5P ± 1 ± 60 ± P ± 3 10 Fig. 6 : errors o the module ad the phase at omial curret accordig to stadard IEC Cahier Techique Scheider Electric o. 194 / p.10

13 thermal withstad of the CT to overcurrets (the stadard values are give by the stadards metioed i the appedix). v CT rated voltage This is the rated voltage to which the CT primary is subjected. It is importat to remember that the primary is at HV potetial ad that oe of the termials of the secodary (which must ever be opeed) is ormally earthed. Just as for ay devices, a maximum withstad voltage for oe miute at power frequecy ad a maximum impulse voltage withstad are also defied. Their values are defied by the stadards. For example: for a rated voltage of 24 kv, the CT must withstad 50 kv for 1 miute at 50 Hz ad 125 kv at the impulse voltage. c CT with several secodaries Some CTs may have several secodaries dedicated to protectio or to meterig. The most typical cases are CTs with 2 secodaries, more rarely with 3 secodaries. Physically, these CTs group i the same mould the equivalet of 2 or 3 separate CTs that ca have differet classes ad ratios (see fig. 8 ). Ifluece of the load o the accuracy limit factor Remember that the equivalet simplified diagram of the magetic curret trasformer is show i figure 9. I 1 I o Fig. 9 : CT equivalet diagram. I S R ct I 2 Applied to this diagram, Ohm s law lets us write: V = I 2 (R ct + R), where: R ct : CT secodary widig resistace R: load resistace icludig wirig, v if I 2 = k I ad R = R = P / I 2, V = k I (R ct + R ) (1) ( k = omial ALF) v if I 2 = k I ad R = R p = P r / I 2, V r = k I (R ct + R p ) O figure 10 we ca see that although R p is far smaller tha R, the CT saturatio kee poit is far from beig reached at the declared accuracy limit factor k. The real accuracy limit factor correspodig to the real load (protectio + wirig) ca be calculated. This is ALF r = k r for which the saturatio kee poit voltage V is reached: V = k r I (R ct + R p ) (2) If R p is less tha R, k r is greater tha k (ALF r > ALF) V R V (volts) I 1 V Operatig poit of the CT whe it is loaded at P ad supplied by a curret k I V r Operatig poit of the CT whe it is loaded at P r ad supplied by the same curret k I S 1 S 2 S 3 Fig. 8 : maufacturig priciple of a CT with 3 secodaries (with 3 widigs i the same mould). I or I o I o Fig. 10 : operatig poits of the CT accordig to its load. Cahier Techique Scheider Electric o. 194 / p.11

14 By combiig the equatios (1) ad (2), we fid the followig formula: R + + kr = k ct R kr = R + R, or k P i P ct p Pi + Pr where: P i =R ct I 2 = iteral losses of the CT at I P =R I 2 = CT accuracy power P r =R p I 2 = real load cosumptio of the CT at I. It is obvious that proper operatio of a protectio relay is liked to the behaviour of the associated CT ad to its real load ad ot to the behaviour of the CT associated with a theoretical omial load. Real eeds eable us to determie the miimum accuracy power to be chose. Usig a CT with a load P r < P icreases the ALF. Likewise, the ALF icreases more if the R ct (iteral losses P i ) is low (see fig. 11 ). Calculatio of the real ALF (k r ) of a CT, associated with its real load, esures that the right CT is chose i all traditioal cases. Note: for very demadig protectios (e.g. differetial protectios), curret trasformers are most ofte defied i class X. This class is always defied accordig to the real load of the CT ad to its ow iteral losses. k r 80 P i = 2 VA P i = 5 VA P r Fig. 11 : behaviour of the accuracy limit factor k r = f(p r ) of two CTs of 10 VA-5P20 with differet iteral losses (R ct ) accordig to the real load coected to the secodary. Cahier Techique Scheider Electric o. 194 / p.12

15 3.2 No-magetic trasformers The output sigal, delivered by the o-magetic trasformers (also kow as ROGOWSKI coils) is a voltage proportioal to the derivative of the primary curret. (Lez law: e= d φ dt ) They do ot saturate ad their respose is liear. Cosequetly, they ca be used over wide curret rages: the oly limitatio is the dyamics ad the liearity of the iput circuit of the associated protectio. The techology of the protectio, cotrol ad moitorig uits coected to these omagetic trasformers is of the digital microprocessor type. This techology is able to process sigals of very low amplitude. For a give o-magetic trasformer, i view of the liearity of the output sigal, the omial primary curret is replaced by a wide rage, for example 30 to 300 A. I additio to the advatage of liearity, the use of o-magetic CTs reduces: c risks of error whe choosig primary curret at the desig stage of the istallatio, c the umber of models to be maaged. It also miimises the delivery times. Today these trasformers are seldom used. A stadard (IEC ) should defie them. Scheider Electric has bee usig these trasformers (see fig. 12 ) i associatio with the Sepam protectio, cotrol, moitorig ad meterig uits sice To specify them, all you have to do is idicate: Primary widig 2 - Dielectric scree 3 - Dielectric isulatio 4 - Settig resistace Secodary widig 6 - Secodary widig support 7 - Magetic shieldig Fig. 12 : cross-sectio of a o-magetic trasformer used i MV. c the CT isulatio level, defied just as for a traditioal CT, c the rated thermal short-circuit curret (I th ) ad the dyamic curret (I dy ) set accordig to the same rules as for the CTs, c the utilisatio rage (rated primary curret ad the thermal curret) CT maufacturig ad implemetatio CTs are idustrial products desiged accordig to stadards. They are mass produced, thereby reducig costs ad guarateeig their characteristics. Their live part is duplicate moulded i order to comply with isulatio, temperature rise ad electrodyamic withstad requiremets. The umber of moulds correspodig to a stadard rage is ecessarily limited. Furthermore, the switchgear ad CTs are most ofte istalled i paels that have bee optimised, stadardised ad subjected to qualificatio tests. I this case, use of stadard moulds is compulsory as CTs perform other fuctios such as the bushigs betwee the cable ad the circuitbreaker compartmets (see fig. 13 overleaf). Cosequetly, ay modificatio i volume or shape of a CT results i major study, productio ad test ivestmets. Cahier Techique Scheider Electric o. 194 / p.13

16 To solve special cases, without overcosts or additioal techical risks, it is therefore always advisable to look for solutios i order to fit the mould of stadard CTs. Pael structure is thus maitaied itact. These solutios are: c good ratig of electrical characteristics: for example avoid overratig i power ad ALF, c use of CTs with two or three widigs, c use of relays performig several protectio fuctios with the same curret iformatio. Fig. 13 : cross-sectioal view of the pael ad multifuctioal CT. Cahier Techique Scheider Electric o. 194 / p.14

17 4 Choosig CTs accordig to protectios ad applicatios Thorough kowledge of CTs, their possibilities ad their limits is useful oly whe they are associated with a specific protectio relay whose characteristics ad scope of actio regardig the moitored curret rage are kow. The protectio relays istalled o a electrical etwork are defied i the protectio pla. This pla specifies the positio ad settig of the selected protectios. It also defies the positio of the CTs, their ratio ad, more rarely, their power, accuracy ad ALF. I poit of fact, complete specificatio of CTs also requires kowledge of: c the protectio iput impedace, c the wirig impedace, c the protectio operatig thresholds (ormally take ito accout i the protectio co-ordiatio study). Today, most protectios are of the digital techology kid ad are highly accurate. CT accuracy is thus a decisive factor. The type of protectio also affects the required CT accuracy: c a overcurret protectio oly takes the curret value ito accout, c a differetial protectio compares two currets, c a earth fault protectio treats the sum of the three phase currets. 4.1 Choosig CT ALF accordig to protectios Whe choosig a CT out of the stadard CTs, a remider is ecessary of the relatioship likig the omial ALF (liked to R ) ad the real ALF liked to the real load R p : k = k r Rct + Rp Rct + R + R or k = k R ct r Rct + Rp A CT ca supply several differet protectios either separate or grouped i a multi-protectio system (e.g. the Sepam). This leads us to examie the protectios determiig CT sizig. V 2V s V s Operatig poit at k r I Operatig poit at 2 I s max Operatig poit at I s max Defiite time overcurret protectio The threshold I s (protectio settig) ca be set, for example, from 2 to 10 I of the CT if the CT I is the applicatio I. To esure that the CT will ot affect the operatig accuracy of the protectio, it is ecessary to have o saturatio up to the settig poit. But it is usual to take a safety coefficiet of 2 (see fig. 14 ). Thus the ALF r (k r ) at real load will be: k r u 2 I Is if I s = 10 I k r u 20 Example: 200/5 CT - 10 VA-5P10, Load I : 160 A I s = 8 I of load Let us verify if the proposed CT is suitable: Fig. 14 : operatig poits of the CT at maximum threshold. Is of O/C I of CT = = the miimum recommeded ALF r (k r ) is thus: k r u 2 x 6.4 = If the CT load ad its iteral resistace are kow with, for example: R ct + R p = 1 2 (R ct + R ) we obtai: k r = k x 2 = 20 higher tha the miimum value required. Thus the CT is suitable. I m Cahier Techique Scheider Electric o. 194 / p.15

18 Iverse time overcurret protectio If the aim is correct accuracy over the etire relay iverse curve, it is ecessary to kow at what poit it becomes a defiite time curve. For most relays it occurs at the miimum value betwee 20 I s ad 24 I (CT). So, assumig I sc max is the maximum short-circuit curret, the same reasoig as above, usig a safety coefficiet of 1.5, gives: k r mi = miimum value betwee: Is Isc max 30, 36, ad 1.5 I (CT) I (CT) Directioal curret protectio The rules, uless otherwise specified (refer to Cahier Techique o. 181) are the same as for the overcurret protectios. Note that for the three curret protectios described above: c If several curret protectios are supplied by the same CT, it is the oe with the lowest curve (the shortest time delay) for high currets that determies the sizig. c I difficult cases, the safety coefficiet of 2 ca be lowered to 1.5. Earth fault protectio As the protectio is supplied by the vector sum of the secodary currets of 3 CTs coected as per the Nicholso arragemet (see fig. 15 ), it is preferable to use idetical CTs produced by the same maufacturer. However, if there is a DC compoet (eergisatio of a trasformer) or whe a high curret occurs, this arragemet (parallel-coectio of the secodary of the 3 CTs) will deliver a false earth fault curret which may cause uisace trippig of the protectio. To give a example, with the 5P10 CTs, a protectio threshold of 10 % of CT I is a limit below which there is a risk of uisace trippig for defiite time protectios. The CT accuracy limit factor is give by the expressio: k rh > X I. Ihs The safety coefficiet (X) is ormally equal to 6 (give by the relay maufacturers). This correspods to the fact that the CT (associated with the phase with the earth fault) must be able to develop a voltage V h = X I hs (R ct + 2 R L + R h ). Note: c If a CT also supplies a overcurret relay, R h must be replaced by R h + R p. c If the CTs are iitially desiged for overcurret protectio, we recommed you check that they are suitable for supplyig earth fault protectio too. Thus, the k rh of a 100/1 CT - 10 VA-5P10 is give by the expressio: Rct + P/I 2 krh = k. Rct + 2 RL + Rp + Rh Bearig i mid that the impedace of the relay used depeds o the settig of I h (i this case 0.1 A), the digital applicatio gives: R h 1VA 0.1A = ( ) = 2 100, 3+10 k rh = 10 = a value to be compared with the expressio of the eeded k rh : k rh = = 0.6, the CT is suitable. 1 I h > Fig. 15 : the vector sum of the phase currets gives the earth fault curret. Cahier Techique Scheider Electric o. 194 / p.16

19 If the short-circuit curret is very high ad if the protectio time delay is short, to avoid uisace trippig it may be ecessary to add a stabilisig resistace i series with the earth fault relay. Its purpose will be examied i paragraph 4.3. I order to avoid difficulties that may arise with the 3 CT arragemet, it is preferable, wheever possible, to use a toroidal CT aroud the 3 phases (see fig. 16 ). Note that the 3 phases must be placed i the cetre of the toroid to avoid local saturatio of the magetic material. Use of a toroidal CT allows very low operatig thresholds (a few amps) to be chose Differetial protectios Icreasigly used for the protectio of trasformers, rotatig machies ad busbars, they have the advatage of beig fast ad idepedet from the other protectios i terms of discrimiatio. These relays ofte operate durig the trasiet fault curret period. Just as for the earth fault protectios, a aperiodic DC compoet may result i trasiet saturatio of the CTs ad thus geerate a false differetial curret. Sice differetial protectios are tricky to implemet, maufacturers ormally supply the ecessary iformatio to desig ad istall the CTs. Fig. 16 : the toroid placed i (1) or (2) gives the same iformatio but the toroid placed i (1) also moitors the faults located upstream of the toroid (2). Coclusios c It is the high settig of the overcurret protectios that determies the miimum k r (real ALF) to be complied with. c It is the stability of the relay up agaist trasiet pheomea that determies the ALF or class X for earth fault ad differetial protectios. 4.2 Characterisig CTs accordig to applicatios I this cotext the term applicatios meas the compoets of the electrical etwork to be protected: lie icomers ad feeders, trasformers, busbars, geerators, motors, capacitor baks, etc. Each compoet requires the use of several protectios. A example is give by the diagram i figure 17 (overleaf) i which the protectios are idetified by their ANSI code (America Natioal Stadard Istitute). However, to optimise choice of a CT, you eed to kow which protectios it supplies, as well as their settigs, real impedace, short-circuit curret, etc. The difficulty lies i the fact that these values are rarely kow by the CT specifier whe his choice has to be fialised. For this reaso this sectio proposes to study the various applicatios where values ca be defied by excess. They ca therefore be chose without risk as they will ecessarily cover eeds. However, they will lead to a less restrictive defiitio which is more realistic tha the oe proposed by relay suppliers i their catalogues. I poit of fact, each supplier gives some guidelies for choice coverig all the settig rages of his relay associated with CTs assumed to have high iteral losses ad also maximum wirig impedaces. For the various applicatios, we shall list the protectio fuctios commoly used. The most ufavourable protectio will be chose to size the curret trasformers. Note: Iitially, the earth fault ad differetial protectios will ot be take ito accout. Cahier Techique Scheider Electric o. 194 / p.17

20 150/ R 51G 3000/5 3000/1 150/6 kv 22.5/30 MVA Zsc = 11 % (30 MVA) YD t 27I 27 87T /1 300/1 a 51G 300/ V /5 200/5 100/5 100/5 100/5 600/ t N N 51N 6 kv/400/230 V 2500 kva Zsc = 7.5 % DY11 51N 400 kva Zsc = 4 % M M 51G 2000 kva I = A 710 kw I = 82.3 A I d / I = 6 T d = 5.2 sec 4500 kw I = 491 A I d = 5.8 I T d = 5 sec 51 50t 315 A 132 kw I = 250 A I d / I = 5 T d = 2 sec M 49 Fig. 17 : example of a protectio pla. Cahier Techique Scheider Electric o. 194 / p.18

21 Applicatios with typical protectios Examiatio of the protectios ormally implemeted for the various applicatios (see fig. 18 ), shows that it is always the high threshold overcurret protectio which is the CT sizig factor. The exceptio to this rule is, however, the motors protected by fuses for which the locked rotor protectio is the most restrictive. To optimise the CTs, the settigs of these protectios eed to be take ito accout. c Lie icomers ad feeders The high threshold overcurret protectio is ormally set betwee 3 i I s i 10 I, hece a k rm u 2 I with at maximum a k rm = 20. Is If a iverse time protectio is used: k r mi = miimum value betwee: Is Isc max 30, 36, ad 1.5 I (CT) I (CT) c Geerator icomer There is o reaso to select a settig > 7 I, give the relatively low short-circuit curret of a geerator, hece a k rm u 14. c Trasformer icomer If we take the example of the protectios dowstream of a HV/MV trasformer, the settig threshold must be lower, for example, tha 70 % of the I sc at the secodary side, i.e. as a iitial approximatio: I sc = 0.7 I Z sc. If we apply the rule: k rm u 2 I, Is it becomes: k rm u Z sc This k rm is maximised ; i reality it is the discrimiatio study of the dowstream etwork that sets the I s ad thus determies the k rm. ANSI Typical protectios Applicatios code Lie Geerator Tras- Tras- Capacitor Motor feeder with icomer icomer former former feeder circuit- cotactor ad feeder icomer feeder breaker + fuses 37 Udercurret c c 46 Ubalace c c c 49 Thermal image c c (c) c c c 50 Istataeous high c c threshold overcurret 50N Istataeous c earth fault 51N Time delayed earth fault c c c c c c c 51N1 Star-star capacitor c ubalace 51LR Locked rotor / c c too log start 51V Voltage restraied c overcurret 51-1 Time delayed low c c c c c threshold overcurret 51-2 Time delayed high c c c c threshold overcurret 66 Number of starts c c 67 Directioal overcurret c c c 67N Directioal earth fault c c c Fig. 18 : protectios implemeted by applicatio, i the most commo cases. Cahier Techique Scheider Electric o. 194 / p.19

22 c Trasformer feeder The operatig curret of the high threshold overcurret protectio (I s ), placed upstream of the trasformer, must be higher tha the curret observed at the primary (I sct ) o a short-circuit at the trasformer secodary termials. As a first approximatio we ca state that: I sct i I Z sc where P I 1 = = omial primary curret, 3 U 1 P = omial power of the power trasformer, U 1 = primary voltage. I fact, the upstream source impedace also helps to limit I sct. We ca thus be certai that: I s i I Z sc If the geeral rule k rm u 2 I applies, Is we obtai: k rm u 2 I 1 I 100. Z sc The typical values for short-circuit impedaces of power trasformers ca rage from 4 % for small trasformers to 20 % for the largest oes (see fig. 19 ). This would result i requirig miimum k rm ragig from 10 I 1 for large trasformers to I 50 I 1 for the smallest oes. I These values may be too difficult to obtai for low rated CTs particularly whe their thermal withstad is high (e.g. 40 ka, 1 s). Therefore, i difficult cases, the problem of feasibility could be solved by overratig the CT primary or by usig a reduced coefficiet (1.5 istead of 2) as described i paragraph 4.1, which gives: k rm u 1.5 I 1 I 100. Z sc If you choose to overrate a CT, you must esure that the settig of the trasformer thermal Trasformer ratig Z sc (MVA) (%) Fig. 19 : typical short-circuit impedaces for power trasformers. protectio is still possible. Otherwise you must pla this protectio dowstream of the trasformer. c Capacitor feeder The time delayed high threshold recommeded by capacitor suppliers is 3 I (0.3 s). If we take I s max = 5 I ; k rm u 10. c Motor feeder The high threshold must be set above the startig curret, which is always i 7 I. If we take I s max = 8 I ; k rm u 16. If the short-circuit protectio is provided by fuses, the the locked rotor / too log start protectio is the most restrictive. Its settig is betwee 2.5 ad 4 I ; k rm u 8. Note: From the above, we have assumed that CT I = applicatio I. If this is ot the case, the resultig k rm must be multiplied by the ratio: I (applicatio). I (CT) The table i figure 20 summarises the miimum ALF to be complied with accordig to the applicatios. Cahier Techique Scheider Electric o. 194 / p.20

23 k r mii for defiite time overcurret k r mii for iverse time overcurret c I s is kow, all applicatios k r = 2 I Is betwee: 30 Is max, 36, ad 1.5 I (CT) Isc I (CT) c I s is ukow Trasformer feeders k r = 2 I I1 100 Not cocered Zsc I Trasformer icomers, k r = 20 (by excess) betwee: s Isc max 30, 36, ad 1.5, I (CT) I (CT) Lie icomers ad feeders if o defiite time o a secod threshold Capacitor feeders k r = 10 (by excess) Not cocered Circuit-breaker motor feeders k r = 16 Not cocered Cotactor with fuse motor feeders k r = 8 Geerator icomers k r = 14* k r = 14* I this table: I s is the settig curret of the overcurret threshold for which the respose time is shortest for the high currets, I is the CT omial primary curret, I 1 is the power trasformer omial curret, (*) = geeral cases. Fig. 20 : real ALF (k r ) required for overcurret accordig to the applicatios. 4.3 Special case of differetial protectio Although the maufacturers of relays dedicated to differetial protectio impose the ecessary CT secodary characteristics required for proper operatio, it is useful, for uderstadig ad avoidig errors, to possess miimum kowledge of this type of protectio. A differetial protectio moitors a area limited by CTs which measure icomig ad outgoig currets. If the outgoig currets are ot cosistet with the icomig currets, this is ormally because a fault has occurred i the protected area. We shall examie i tur high impedace, pilot wire, percetage differetial ad low impedace differetial protectios with their respective CT requiremets. Accordig to the type of protectio ad its use, relay maufacturers have had to apply a variety of priciples varyig i complexity i order to guaratee the stability of their relays agaist trasiet pheomea likely to cause uisace trippig of this protectio. High impedace differetial protectio This type of protectio is ormally used for protectio of motors, geerators, busbars as well as for restricted earth fault protectio of trasformers. c Geeral This type of protectio is used to protect a area with the same voltage level. Cahier Techique Scheider Electric o. 194 / p.21

24 I healthy coditios, the iput curret i e is idetical to the output curret i s ad thus the differetial curret i d = 0 (see fig. 21 ) A high fault curret ca flow through the moitored area ad cause CT saturatio, hece the risk of uisace trippig of the protectio (o stability). The stability of the relay is obtaied by coectig it i series with a stabilisig resistace R st. This resistace is calculated i such a way that the derived curret i the differetial circuit (R st +R p ) caot reach the relay settig threshold whe the maximum through curret saturates a CT with its DC compoet. This results i: A I e Applicatio (moitored area) i' e i' s i d R st Protectio (R p ) Fig. 21 : high impedace differetial priciple. I s B (1) (R st + R p ) u (R ct + 2 R L ) I ssc Ir where I ssc = maximum through curret observed at the CT secodary, I r = relay secodary settig curret. R st may vary betwee a few ohms ad a few hudred ohms (exceptioally it may be greater tha 1000 ohms). For the relay to operate properly at I r if a fault occurs i the area, the kee poit voltage V k must be greater tha: 2 (R st + R p + R ct + 2 R L ) I r. As a rule R ct + 2 R L are egligible compared with R st + R p thus: (2) V k u 2 I r (R st + R p ). By combiig the equatios (1) ad (2) we fid: (3) V k u 2 I ssc (R ct + 2 R L ). These equatios show that R st (expressio 1) ad V k have greater values whe R ct is high. A high value stabilisig resistace geerates high overvoltages at the CT secodary. Therefore whe overvoltages i excess of 3000 V are aticipated, a protectio by a oliear resistace (ZO) is added. The result of these observatios is that the CTs are optimised if R ct ad V k are as low as possible ad if the through curret (see from the CT secodary, i.e. I scc ) is defied without excess. Whatever the applicatio where the high impedace differetial is used, all the CTs must have: v the same ratio, v the same magetisig curve (same miimum V k ), v the same maximum R ct, ad comply with expressio (3) ; for V k, sice the relevat CTs are ot at the same distace from the relay, take the maximum R L. For this protectio, a maximum value of the magetisig curret I o must also be defied at V k /2 accordig to the required sesitivity. For the relay to detect a curret I r, a voltage V s =V k /2 must be developed at the termials of the parallel-coected CTs; for this purpose, the miimum primary curret I rms really detected by the relay will be I rms = (I r + ρ I o ), where = CT ratio ad ρ = umber of parallel-coected CTs (there may be may of them o a busbar protectio!). c Applicatio to the motor differetial protectio The maximum through curret for which the motor must remai isesitive is i this case the motor startig curret: I ssc = I st (see at the secodary). If you do ot kow this curret I st, you kow that: I st < 7 I motor c Applicatio to the geerator differetial protectio The maximum through curret is i this case the short-circuit curret supplied by this geerator oly. If you kow the geerator subtrasiet reactace X %, the followig will be take: Issc = I 100 ; X If this value is ot kow, X % = 15 will be take Note: the peak voltage at the CT secodary must be calculated usig: I ssc maxi = I geerator + I sc etwork. c Applicatio to the busbar differetial protectio (see fig. 22 ). I this case, the through curret is equal to switchboard I sc : I ssc = switchboard I sc see from the CT secodary. c Applicatio to the restricted earth fault differetial protectio of trasformers (REF) v I the case of figure 23a, this protectio detects the isulatio faults at the trasformer secodary widigs ad up to the CTs located dowstream. Cahier Techique Scheider Electric o. 194 / p.22

25 1 2 3 R st 87B v I the case of figure 23b, this protectio detects the isulatio faults at the trasformer primary ad advatageously improves the typical earth fault protectio which is sesitive to trasformer violet irush currets ad to the through currets resultig from a dowstream asymmetrical short-circuit. 4 5 Fig. 22 : high impedace busbar differetial protectio. a b R st R st Fig. 23 : restricted earth fault protectio of the secodary [a] or primary [b] widigs of a trasformer. R p R p I this case also, we shall calculate R st ad V k based o the maximum through curret i the CTs for a fault outside the protected area. As a first approximatio, we ca say that this curret is less tha the curret limited by the trasformer impedace, i.e. I through = P U sct 3, where P sct = P 100, (trasformer short-circuit Zsc power); if we kow the upstream short-circuit power (P u ), a more accurate value ca be calculated replacig ( Psct Pu ) P sct with ( Psct + Pu ). The through fault curret must the be coverted ito I ssc see at the CT secodary. Lie or cable differetial protectio with pilot wires (see fig. 24 ). A relay of this type is istalled at each ed of the cable or the lie. O the pilot wires, each relay reproduces a voltage which is the image of the sum: a I 1 + b I 2 + c I 3 + d I h. If the two voltages are differet, both relays trip. NB: Note that the coefficiets a, b, c ad d are differet to esure that all fault types correspod to a sum other tha zero. Cosequetly, the operatig threshold of a two-phase fault or of a phase-to-earth fault is slightly differet accordig to the faulty phase. I this case also, class X CTs are required, ad each maufacturer gives a empirical formula for the miimum kee poit voltage V k. R L R L Lie differetial 87L Pilot wires Lie differetial 87L Fig. 24 : lie or cable differetial protectio with pilot wires. Cahier Techique Scheider Electric o. 194 / p.23

26 A example of the miimum kee poit voltage required: V k mii = 0,5 N k t I (R ct + X R L ) where N, k t ad X are costats associated with the relay respose time, its sesitivity ad its type of coectio. Aother example: V k mii = 50 + I f (R ct + 2 R L ) I where: I is the CT omial secodary curret (1 or 5 A), I f is the through short-circuit curret, see at the CT secodary. The stability of this relay is achieved both by complyig with the required kee poit voltage ad by a percetage differetial operatig threshold which will icrease with the through curret by use of retaiig widigs. The CTs at the eds of the lie must have the same ratio ad must comply with the miimum V k ad maximum I o specified by the maufacturer. However, their magetisig curves ad R ct do ot eed to be idetical. Percetage biased differetial protectio for trasformers The term percetage differetial stems from the fact that the operatig threshold icreases with the through curret. Simple compariso of the currets i each upstream phase with the currets i the same dowstream phases is ot suitable for trasformer differetial protectio. This is because: v the upstream ad dowstream currets of a power trasformer do ot have the same amplitude or the same phase agle, v whe the trasformer is eergised, its magetisig curret is oly see upstream, v the presece of a earthig geerator i the protected area (e.g. earthig the trasformer eutral) ca trip the protectio, while the fault is, for example, located o a dowstream feeder. c Precautios to be take to solve these problems: The aim is to esure that the relay sees upstream ad dowstream currets of the same amplitude ad i phase durig ormal operatig coditios. This ca be achieved by itelliget choice of CT ratio ad coectios. Matchig CTs are used for this purpose ad ofte help to elimiate the zero-sequece curret that could trip the protectio whe a earth fault occurs outside the protected area. However, most of the ew digital relays are able to perform iterally, by parameter settig, the adjustmets ecessary to reset the currets, thus cosiderably simplifyig their implemetatio. Furthermore, it must be oted that all the trasformer differetial relays are immuised to the 2 d order harmoic blockig their operatio whe the trasformer is eergised. c CT voltage V k I 99 % of cases, a class X is requested. The miimum kee poit voltage is imposed ad depeds o the resistace of the secodary widig R ct of the CT ad of its real load R r. More complex specificatios are sometimes metioed, which iclude the X/R ratio of the etwork or the magetisig curret of the power trasformer. However, faced with the problems that users have i obtaiig all these parameters, relay suppliers sometimes provide simplified empirical formulas which lead to a slight oversizig. Examples of miimum kee poit voltage imposed for the Sepam 2000 D02 (Scheider Electric): V k mii = A I b (R ct + 2 R L ) where: 2 R L = total resistace of the secodary wirig, R ct = CT secodary widig resistace, I b = power trasformer omial curret see at the CT secodary, A = costat depedig o trasformer power: Some suppliers take ito accout the through curret, for example: I f ( ( )) V k u 4 ct 3 R L Rp 3 R + + o the star side of the power trasformer, ad V k u 4 I f (R ct + 2 R L + R p ) o the delta side of the power trasformer. The through curret will be defied i the same way as for the restricted earth fault protectio. Note: Use of matchig CTs leads to differet expressios of the kee poit voltage for the mai CTs which must take ito accout the extra load that they represet. Cahier Techique Scheider Electric o. 194 / p.24

27 To coclude, the stability of this protectio is esured by: c the threshold which icreases with the through curret (restrait system), c the right choice of CT kee poit voltage V k, c a system esurig immuity to 2 d order harmoics geerated by irush currets, c the most sophisticated relays are also immue to 5 th order harmoics which occur durig power trasformer overexcitatio (saturatio). Low impedace differetial protectio This protectio is used for busbar differetial protectio. It is very costly ad spacecosumig, as it requires a large umber of modules ad matchig CTs which eed oe or more cubicles accordig to switchboard size (see fig. 25 ). I the case of a double busbar switchboard, the protectio must be cotiually iformed o the positio of the trasfer switches i order to direct the currets of each feeder ad icomer to the relay associated with moitorig of the busbar o which this feeder or icomer is coected. The CTs associated with this sophisticated protectio may have differet ratios. Their secodaries are also defied as class X i most cases. However, as saturatio ca be tolerated, kee poit voltage requiremets are less severe tha for high impedace differetial protectio. B1 C1.2 B2 B3 B4 C3.4 C1.3 C2.4 D1 D2 D3 D4 CMT D1 CMT D2 CM C1.3 CM C1.2/C3.4 CM C2.4 CMT D3 CMT D4 1 B1 B3 Global B2 B4 2 87B B1 87B B3 87B Global 87B B2 87B B4 CMT = curret meterig ad trasfer (for feeders D1, D2, D3 ad D4) CM = curret meterig (for crossig ad i-lie couplers C1.2, C1.3, C2.4 ad C3.4) 1 = trasfer ad meterig 2 = protectio Fig. 25 : example of a low impedace differetial protectio for a double busbar. Cahier Techique Scheider Electric o. 194 / p.25