Cahier technique no. 194


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1 Collectio Techique... 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 idepth 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 idepth 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 Nomagetic 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 abovemetioed criteria to draw up the siglelie 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 siglewire 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 ooperatig 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 startup ca be serious: usuitability, overratig, ostadard 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 shortcircuit curret ca lead to feasibility problems, overratig ad high CT costs. c O the other had, uderestimatio of the shortcircuit 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 shortcircuit 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 shortcircuits betwee phases or phasetoearth, 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 shortcircuit 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 shortcircuit at oe poit of the etwork may be detected right up to the source. The protectio coordiatio study esures that oly the faulty part of the etwork is deeergised. The protectios relatig to shortcircuits either phasetophase or phasetoearth 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 1S11S2 150/5 2S12S2 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 1S11S2 1 secodary circuit 2S12S2 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 shortcircuited). 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 VA5P20 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 Nomagetic trasformers The output sigal, delivered by the omagetic 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 omagetic 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 omagetic 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 : crosssectio of a omagetic trasformer used i MV. c the CT isulatio level, defied just as for a traditioal CT, c the rated thermal shortcircuit 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 : crosssectioal 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 coordiatio 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 multiprotectio 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 VA5P10, 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 shortcircuit 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 (parallelcoectio 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 VA5P10 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
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