Current transformer selection guide

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Current transformer selection guideDETERMINATION OFTHE CUSTOMER’S NEED(cont’d)Accuracy limit factor in protection (ALF)The ALF required for the circuit will be determined as follows:■ overcurrent protection independent timeThe relay will operate perfectly provided that:(effective ALF of the CT) > 2 x I rI snI r = the set point of the relayI sn = the rated secondary current of the CTFor a relay with two set-points, we will use the highest set point.For a transformer outgoing, there will usually be a high instantaneousset-point set on 14 I n maximum which means that the necessary effectiveALF is > 28.For a motor feeder, we will generally have a high set-point set on 8 I nmaximum which means that the necessary effective ALF > 16.■ overcurrent protection dependent timeIn every case, refer to the relay vendor’s technical data sheet.For these protections, CT accuracy must be ensured throughoutthe whole relay trip range up to 10 I n which is the highestinstantaneous set-point used.In this case, the effective ALF must be > 20 x I r .Specific cases:if the maximum short-circuit current is greater than or equal to 10 I reffective ALF > 20 xI rI snI r = the set-point of the relayif the maximum short-circuit current is under10 I r , for the 1 set-point:effective ALF > 2 x I cc secondaryI snif the protection has a high instantaneous set-point (never effective onincomings and feeder outgoings):effective ALF > 2 x I r2I snI r2 = instantaneous high moduleset pointdate11/92- B•1•4 -revised08/95page 10
Current transformer selection guideDETERMINATION OFTHE CUSTOMER’S NEED(cont’d)3 - BS SPECIFICATIONS FOR DIFFERENTIAL PROTECTION (CLASS X)Many differential protection relay manufacturers recommend class X CTs.Class X is often requested in the form:V k ≥ a . I f (R ct + R b + R r )The exact formula is given by the manufacturer of the relay.Values which characterise the CTV k = knee-point voltage in voltsa = the coefficient which refers to the asymmetrical configurationR ct = maximum resistance of the secondary winding in OhmsR b = resistance of the loop (i.e. the return line) in OhmsR r = resistance of the relay outside the differential part of the circuit in ohmsI f = the maximum fault current value measured by the CT in thesecondary circuit for a fault outside the zone to be protected in AmpsI f = I ccK nI cc = short-circuit current of the primary circuitK n = CT ratiodate11/92- B•1•4 -revised08/95What is the value for I f when determining V k ?The short-circuit current is selected for the application:- generator differential protection- motor differential protection- transformer differential protection- busbar differential protection■ for a generator differential protection:If I cc is known:relayI cc is the short-circuit current of theCTgenerator aloneI f = I ccIf the I n generator is known:K nit will be evaluated by excess asIf the I n generator is not known:it will be evaluated by excess asI f = 7 x I sn(CT)I sn(CT) = 1 or 5 A■ for a motor differential protection:CTGrelayMCTCTIf the starting current is known:we will useI cc = I startingI f = I ccK nIf the motor I n is known:it will be evaluated by excess asI f = 7 x I n generatorK nI f = 7 x I nIf the motor I n is not known:K nit will also be evaluated by excess as:I f = 7 x I sn(CT)I sn(CT) = 1 or 5 Apage 11
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Current transformer selection guide

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