Current transformer dimensioning © Siemens AG 2006 CT dimensioning Trainer: Claus Wagner PTD PA15 Tel. 0911-433-8193 Fax 0911-433-8592 Siemens AG 2006 Power Transmission and Distribution © Page 2 Feb 07 Claus Wagner Current Transformer i1 (primary current) w2 i1 w2 R (burden) i1 i1* = w 2 L0 iron losses Siemens AG 2006 Power Transmission and Distribution © Page 3 Feb 07 Claus Wagner R Current Transformer Saturation L0 knee point voltage High L0 for normal operation L0=0 due to CT saturation U=R*i1* i1 i1*= w 2 i1 i1*= w 2 L0=very high value Normal operation R U - the current flows through the current transformer of the relay L0=0 Saturation - the relay measures no current Siemens AG 2006 Power Transmission and Distribution © Page 4 Feb 07 Claus Wagner R Equivalent current transformer circuit I1 jX1 I2 I1 R1 N1 N2 N1 N2 U2 I2 R2 Im P1 P2 jX2 S1 Zm Zb S2 Ideal CT X1 = Primary leakage reactance R1 = Primary winding resistance X2 = Secondary leakage reactance Z0 = Magnetizing impedance R2 = Secondary winding resistance Zb = Secondary load Note: Normally the leakage fluxes X1 and X2 can be negelected Siemens AG 2006 Power Transmission and Distribution © Page 5 Feb 07 Claus Wagner Current transformer, simplified replica circuit jX2 1 : N2 i2 i1 i'1 = N2 1 R2 i1 LW L2<< LW im ZB R2 i2 RB Siemens AG 2006 Power Transmission and Distribution © Page 6 Feb 07 Claus Wagner Current transformer: Phase displacement () and current ratio error () I1 . I1 N1 N2 N1 I2 N2 I2 Zb Im Siemens AG 2006 Power Transmission and Distribution © Page 7 Feb 07 Claus Wagner Current transformer saturation Saturation during steady-state current Saturation during offset current Siemens AG 2006 Power Transmission and Distribution © Page 8 Feb 07 Claus Wagner CT saturation Currents and magnetising IP t saturation flux t IS t Siemens AG 2006 Power Transmission and Distribution © Page 9 Feb 07 Claus Wagner Current transformer magnetising and de-magnetising P B t B BR BMax. BR e BMax B BMax t TS BR BR m m t Siemens AG 2006 Power Transmission and Distribution © Page 10 Feb 07 Claus Wagner Current transformer Course of flux in the case of non-successful auto-reclosure B BMax tF1 =duration of 1st fault t tF1 tSP tF2 tSP dead time tF2 =duration of 2nd fault Siemens AG 2006 Power Transmission and Distribution © Page 11 Feb 07 Claus Wagner Current transformer magnetising curve and point of remanence I B II III up to 80% < 10% negligible H = im w I: closed iron core (TPX) II: core with anti-remanence air-gaps (TPY) III: Linearised core (TPZ) Siemens AG 2006 Power Transmission and Distribution © Page 12 Feb 07 Claus Wagner Current transformers TPX und TPY Course of the flux with non-successful auto-reclosure closed iron core (TPX) BR core with antiremanence air-gaps (TPY) BR tF1 tSP tF2 t Siemens AG 2006 Power Transmission and Distribution © Page 13 Feb 07 Claus Wagner Current transformer with linearised core (TPZ), Course of the flux with non-successful auto-reclosure P t S B m t tF1 tSP tF2 t Siemens AG 2006 Power Transmission and Distribution © Page 14 Feb 07 Claus Wagner Example for a weak CT saturation 5P20, 200/1A, 15VA, Rct = 1Ω, R’b = 1.22Ω, 11·I/INCT ; TP = 100ms 10P10, 2000/1A, 10VA, Rct = 2Ω, R’b = 0.54Ω, 11·I/INCT ; TP = 100ms Siemens AG 2006 Power Transmission and Distribution © Page 15 Feb 07 Claus Wagner Current Transformer Identification 10 P 20; 15VA Transformer Nominal Power Nominal Overcurrent factor Core type: P = Protection M = Measurement Fault limits in % at n x IN Attention! M or n<5 means measurement-core Not suitable for protection application! Siemens AG 2006 Power Transmission and Distribution © Page 16 Feb 07 Claus Wagner Standard Accuracy Classes 5P and 10P Accuracy class Current Error at rated primary current % Phase Displacement at rated primary current minutes Composite error at rated accuracy limit % 5P +/- 1 +/- 60 5 10P +/- 3 not defined 10 Siemens AG 2006 Power Transmission and Distribution © Page 17 Feb 07 Claus Wagner Accuracy limiting factor with various burdens Begin of saturation defined by a max Um i'1 N2 i1 1 Ri i2 Um,N = I2max,N (Ri+RN) Um.N = I2max,b (Ri+Rb) in both cases the max. voltages are the same: Lm im Um RN / Rb I2max,N (Ri RN ) I2max,b (Ri Rb ) I2max,b I2max,N I2max,b IN k ALF k ALF , N (R i R N ) (R i R b ) K 'SSC K SSC (Ri RN ) (Ri Rb ) I2max,N (Ri RN ) I N (Ri Rb ) (R i R N ) (R i R b ) n' n N (R i R N ) (R i R b ) Siemens AG 2006 Power Transmission and Distribution © Page 18 Feb 07 Claus Wagner Kneepoint voltage CT design according to BS 3938 / IEC 60044-1 (2000) The design values according to IEC cl. P can be approximately transferred into the IEC class PX (BS class X) standard definition by following formula: VK R b R i I sn K ssc 1.3 Example: IEC 60044 600/1, 5P10, 15 VA, Rct = 4 Ω IEC PX or BS cl. X: VK 15 4 110 V 146 V, 1.3 R ct 4 Ω Siemens AG 2006 Power Transmission and Distribution © Page 19 Feb 07 Claus Wagner CT Saturation: Under Steady State Conditions Example: CT1 CT2 1000/1 100/1 Current transformer data 5P20 20VA Pi = 4 VA P_rel = 0,2 VA l = 200m (lead length) A = 4mm2 (10 x IN) R lead (100 x IN) 200m 2 0,018 2 2ρ l mm 1,8 2 A 4mm k ALF 20 10 kA 20 4 80 24 => CT saturates at 80 x IN CT1 does not saturate CT2 saturates Siemens AG 2006 Power Transmission and Distribution © Page 21 Feb 07 Claus Wagner DC Displacement U Short circuit in Voltage-maximum no displacement (DC element) Short circuit in zero-crossing Complete displacement (DC element) Siemens AG 2006 Power Transmission and Distribution © Page 22 Feb 07 Claus Wagner Current Transformer Flux due to DC offset Flux Secondary Current Primary Current The DC component in the current causes a severe increase of flux A CT that does not saturate due to steady state currents can saturate due to DC offset Siemens AG 2006 Power Transmission and Distribution © Page 23 Feb 07 Claus Wagner Current Transformer Flux due to DC offset - Transient Factor BAC KTF = Flux BDC BAC KTF = 1 + S BDC If n' > Primary/Secondary Current IF ICT If n' > KTF IF ICT S S P P S then the CT does not saturate if there is no DC offset then the CT does not saturate if there is DC offset With: P = Primary System Time Constant S = Secondary CT Time Constant Siemens AG 2006 Power Transmission and Distribution © Page 24 Feb 07 Claus Wagner Typical Transient Factors and Time Constants Transient Factor X/R Time Constant Generator/Transformer 30 - 50 95 - 160 ms Major 330 kV Busbars 25 - 30 75 - 90 ms 330 kV line Major 132 kV Busbars 132 kV line Major 66 kV Busbars 66 kV line 10 15 - 20 3 10 - 15 5 - 10 30 ms 45 - 60 ms 10 ms 30 - 45 ms 15 - 30 ms L = R = X R 1 2 f Siemens AG 2006 Power Transmission and Distribution © Page 25 Feb 07 Claus Wagner Current Transformer Time to saturation due to DC offset KTF tM = 30,00 KTF = 1 + SP P S e -tM P -tM -e S 25,00 20,00 tM = 60 ms tM = Time to Saturation tM = 40 ms P = Primary system time constant tM = 20 ms S = CT secondary time constant (5 s) 15,00 10,00 tM = 10 ms 5,00 0,00 0 20 40 60 80 100 120 140 160 P ms 180 Siemens AG 2006 Power Transmission and Distribution © Page 26 Feb 07 Claus Wagner CT requirements of Siprotec: SIP 2006 Glossary of used abbreviations (according to IEC 60044-6, as defined) Kssc = K’ssc = Ktd = Iscc max = Ipn = Isn = Rct = Rb = R’b = TP = VK = Rrelay = rated symmetrical short-circuit current factor (example: CT cl. 5P20 Kssc = 20) effective symmetrical short-circuit current factor transient dimensioning factor maximum symmetrical short-circuit current CT rated primary current CT rated secondary current secondary winding d.c. resistance at 75°C (or other specified temperature) rated resistive burden Rlead + Rrelay = connected resistive burden primary time constant (net time constant) kneepoint voltage (r.m.s.) relay burden Siemens AG 2006 Power Transmission and Distribution © Page 27 Feb 07 Claus Wagner CT requirements of Siprotec O/C, Line diff Siemens AG 2006 Power Transmission and Distribution © Page 28 Feb 07 Claus Wagner SIP 2006: CT requirements of Siprotec Trafo, Busbar and Distance protection Siemens AG 2006 Power Transmission and Distribution © Page 29 Feb 07 Claus Wagner
