Short Circuit Calculation

Short-circuit Calculations© ABB Ltd – 03-03 Talk will cover Short circuit calculation Demo of DOCWin © ABB SACE – BM - 2 Three phase 66/22kV Primary 75 MVA Transformer Secondary Tertiary 66kV 22kV IF = V/ 3 (Zt + Zs) = 1.1035 p.u IF = 22kV/ 1.1035 = 11.5 kA 3 Zs can be neglected, % impedance voltage between pri & sec windings = 17.1% (given) © ABB SACE – BM - 3 Zt = Usc x E2 100 x (MVA) = 17.1 x (22)2 100 x (75) Three phase 66/22kV Primary 75 MVA Transformer Secondary Tertiary 66kV 22kV Normally PG operates the transformers in parallel.5kA or nearly 25kA This gives upstream power short circuit : © ABB SACE – BM .4 3 x 22kV x 25 kA = 1000 MVA . total fault current = 2 x 11. 5  Tabel mV/A/m of cable [R + JX] : Rc = R mΩ/m x Lm / Xc = X mΩ/m x Lm / 3 3 IF = ? . Upstream Network : Psc : Upstream power short circuit in MVA : 1000 MVA Zup = U 2 / Psc = 400 2 / 1000 = 0.38 m/m Xc = 0. Transformer : ZT = Usc x U2 / Sn x 10-3 RT = Wc x XT = U2 / Sn x 10-3 Usc :short circuit voltage (%) Wc : Copper Loss (W) 4 x 1c 120mm2 XLPE flat touching on cable tray length 50m 120mm2 cable data Rc = 0.24 m/m A (ZT2 – RT2)  400V B 3. Cable : © ABB SACE – BM .16 mΩ 22kV/400V 1MVA 5% 22kV 1000MVA   2.Consultant’s method 1. 16 + 8) = 28.24 m/m A  400V B  400 © ABB SACE – BM .98 mΩ 400 Fault current at A = 3 x (0.16 + 8 + 12.38 x 50 / V3 = 10.38 m/m Xc = 0.6 Fault current at B = 3 x (0.16 mΩ Impendance of transformer (ZT) = 8 mΩ 22kV/400V 1MVA 5% 22kV 1000MVA  Impendance of cable : Rc = 0.94 2) = 12.98) = 10.3 kA  4 x 1c 120mm2 XLPE flat touching on cable tray length 50m 120mm2 cable data Rc = 0.Consultant’s method Impendance upstream network (Zup) = 0.24 x 50 / V3 = 6.98 2 + 6.94 mΩ Zc = (10.94 kA IF = ? .98 mΩ Xc = 0. 24 m/m 400V 120mm2 A Hence the difference in total impedance up to the point of fault is negligible.7 . 22kV 1000MVA 22kV/400V 1MVA 5%   4 x 1c XLPE flat touching on cable tray length 50m 120mm2 cable data Rc = 0.   IF = ? B © ABB SACE – BM .Consultant’s method To show why fault level at HV side can be ignored.001 This value is insignificant compared to the transformer and cable impedance.0 MVA 1000 MVA = 0.38 m/m Xc = 0. Per unit impedance of 22kV source = 1. 4 kA IF = ? .9  400V B  © ABB SACE – BM . therefore the trend is to calculate ‘worse case’.9 = 28.24 m/m 120mm2 22kV 1000MVA 22kV/400V 1MVA 5%   A Isc = fault current / 0.US Consultant’s method Sometimes Consultant is very thorough.3kA / 0.38 m/m Xc = 0. Because UL standard 1561 allows the marked impedance of a transformer to vary + 10%.8 = 31. 4 x 1c XLPE flat touching on cable tray length 50m 120mm2 cable data Rc = 0. 9 V Ib = 600.2 A L = 50 m   400V -QF3 S7S 1600 PR211-I R1600 B   © ABB SACE – BM .90 Ir = 600.18 % .Use ABB DOCwin software 22kV 1000MVA U -U1 Vref = 22000 V LLL / IT Plf = 374 kW Qlf = 181 kvar  22kV/400V 1MVA 5%  -TM1 Vr2 = 400 V Sn = 1000 kVA 2nd: LLLN / TN-S  -B3 Df = 1.00 V = 391.2 kA  I> -QF5 4 x 1c XLPE flat touching on cable tray length 50m 120mm2 cable data Rc = 0.9 -QF4 S7S 1600 PR211-I R1600 IF = ?  L -L1 Sr = 415.0 A Cosphi = 0.3 V Ib = 600.38 m/m Xc = 0.91 I"k LLL = 23.0 A Cosphi = 0.00 V = 394.0 A Iz = 669.69 kVA Cosphi = 0.91 I"k LLL = 28.63 % Ib = 600.24 m/m -B1 Df = 1.9 kA 120mm2 A  -WC1 12x(1x120)+4x(1x70)+1G70 dV = 0.0 A UF = 100% dV = 2. 434.2. © ABB SACE – BM .the source is impedant (set).10  in all cases.Short-circuit current General rules  in accordance with the rules in articles IEC 364-434.3 and IEC 364533.  the thermal and electrodynamic withstand of the ducts and switchgear. the maximum prospective short-circuit current at the origin of the circuit and the minimum prospective short-circuit current at the end of the circuit must be determined for each circuit.2. . protection must be compatible with the cable heat stress ∫ I2dt ≤ K2 S2 .  the maximum prospective short-circuit current determines :  the breaking capacity (Icu) of the circuit-breakers Icu ≥ than prospective Icc.protection of persons depends on it (TN-IT).  the minimum prospective short-circuit determines :  choice of trip units (curve) and fuse when : .  the making capacity of the devices.cables are very long. . c.  k is a factor that takes into account the resistivity. temperature factor and heat capacity of the conductor material. and the appropriate initial and final temperatures..s.  S cross-sectional area in mm2  I effective short-circuit current in A expressed.S2 t= k2 S2 I2  t duration in s. value.m. as the r.11 . or max.Short-circuit current General rules  for whatever type of short-circuit current (min.). the protection device must clear the Isc within a time t < 5sec that is compatible with the thermal stress that can be withstood by the protected cable ∫ I2dt ≤ k2. for a. © ABB SACE – BM . for calculation of the effects of shortcircuit current limiting initial temperature °C 70 60 85 90 80 70 105 70 60 85 90 80 limiting final temperature °C 160/140 200 220 250 160 160 250 160/140 200 220 250 160 conductor material copper insulation material pve 60°C rubber 85°C rubber 90°C thermosetting impregnated paper mineral .Short-circuit current General rules  values of k for common materials. the lower value relates to 2 cables having conductors with a cross-sectional area greater than 300mm .12  note : where two values of limiting final temperature and k are given.sleeves and seals pvc 60°C rubber 85°C rubber 90°C thermosetting impregnated paper k 115/103 141 134 143 108 115 135 76/68 93 89 94 71 Aluminum © ABB SACE – BM .conductor . installation method.current ratings . • environment : ambient temperature. cross-section. number of contiguous circuits.voltage drops © ABB SACE – BM .duty factor . . breaking capacity inst. trip setting Isc at head of final switchboards breaking capacity inst. trip setting Isc of main LV switchboard outgoers breaking capacity inst.power factor.foreseeable expansion factor conductor characteristics • busbars : length. trip setting Isc at head of secondary switchboards breaking capacity inst.Short-circuit current Short-circuit calculation procedure upstream Scc HV/LV transformer rating Usc (%) Isc at transformer terminals . • cables : type of insulation single-core or multicore. . thickness.feeder . trip setting load rating Isc at end of final outgoers final distribution circuit breaker secondary distribution circuit breaker main circuit breaker main LV switchboard distribution circuit breaker .13 .coincidence factor. width. length. 14 .Short-circuit current Definition  a short-circuit current is an overcurrent resulting from a fault of negligible impedance between points at different potentials in normal service.  Zt = R2 + X2  Icc3 = U = Z U R2 + X2 A Zt Zt mΩ) U ZI U ZI B © ABB SACE – BM . Zsc = 0.86. Isc3 .Short-circuit current The various short-circuits currents  three-phase fault ZL ZSC ~ ZL ZL V I sc3 = U/ 3 Zsc  phase-to-phase fault ZL ZSC ~ © ABB SACE – BM .15 U ZSC I sc2 = ZL U 2. 5. Isc3  phase-to-earth fault ZL ZSC ~ © ABB SACE – BM .16 V Z(0) I sc(0) = U/ 3 Zsc + Z(0) Z(0) .Short-circuit current The various short-circuits currents  phase-to-neutral fault ZL ZSC ZLn ~ V ZLn I sc1 = U/ 3 Zsc + ZLn = 0. to calculate the Isc at the end of a line . U + Zc .17  the conventional method :  which can be used when the impedance or the Isc in the installation upstream of the given circuit are not known. Zk k k  the composition method :  which may be used when the characteristics of the power supply are not known U IscB = IscA . IscA © ABB SACE – BM .Short-circuit current How to calculate a balanced short-circuit  the “impedance method” :  used to calculate fault currents at any point in an installation with a high degree of accuracy Un Un Isck = = 3 R2 + X2 3 . Short-circuit current The case of several transformers in parallel feeding a busbar  what happens with the breaking capacity of each CB D1 D2 D1 D2 D3 D4 D4 © ABB SACE – BM .20 . 21 n value of Icc3 at the terminals of a set Icc3 = (1) Sn V3 Un . X’d (1) = transient reactance expressed as (30%) The value of the transient reactance should be check to genset manufacture .  limited thermal withstand.Short-circuit current The case of a generator n generator set characteristics :  low short-circuit current depending on its transient reactance (2 to 5 ln). n protection characteristics :  long time protection acting quickly (<15s) for an overload of 1.  low short time protection (< 2ln). G 250 KVA 400 x’d = 30% load shedding non-priority priority © ABB SACE – BM .5 ln. 1 X’d . x’d if no info = 30 % n zero sequence reactance : xo in % Xo = Un Sn 2 x.xo if no info = 6 % © ABB SACE – BM .Short-circuit current The case of a generator n subtransient reactance : x”d in % X’’d = Un Sn 2 x.22 Note : to be checked to the manufacturer .x’’d if no info = 20 % n transient reactance : x’d in % X’d = Un Sn 2 x.  cable heating is reduced hence longer cable life.  measuring equipment situated near an electric circuit less affected  the cascading technique offers substantial savings on equipment. thus electric contacts less likely to be deformed or broken.Short-circuit current Limitation : why n Installation of current limiting circuit breakers offers several advantages :  current limiting circuit breakers considerably reduce the undesirable effects of short-circuit currents in an installation.24 . enclosures and design by using lower rated devices downstream.  electrodynamic forces reduced. © ABB SACE – BM . 25 .Short-circuit current Principle of limitation i u U arc prospective current limited current arc voltage t network voltage © ABB SACE – BM . = 55 kA peak Limited value = 25 kA peak prospective Isc limited Isc peak 9x I2 t 106 total energy let through during half cycle without limitation limited Isc © ABB SACE – BM .Short-circuit current What it is limitation : tables to use for applications circuit breaker limitation capability : the limitation capability of a circuit breaker is that characteristic whereby only a current less than the prospective fault current is allowed to flow under short-circuit conditions.26 t 6 x 106 energy let through during half cycle with limitation 0 30 kA rms . kA peak 55 without limitation 25 with limitation 0 Isc prospective Isc peak 30 kA rms  example : system prospective = 30 kA rms. no discrimination discrimination CB1 CB1 CB2 CB2 © ABB SACE – BM . is the coordination of automatic protective devices in such a manner that a fault appearing at a given point in a network is cleared by the protective device installed immediately upstream of the fault.27 CB1 and CB2 open only CB2 open  why is discrimination useful ?  Discrimination contributes to continuity of service. . a necessity in many industrial.Short-circuit current Definition : discrimination  discrimination (selectivity). and by that device alone. commercial or institutional installations. Short-circuit current Current discrimination  by comparing the characteristic operating curves for :  limitation of the downstream circuit breaker (D1) : ∫i2 dt  no tripping energy of the upstream circuit breaker (D2) ∫i2 dt tripping no-tripping D2 © ABB SACE – BM .28 D1 . Short-circuit current Full or restricted current discrimination  case of full discrimination 2 • i dt  case of restricted discrimination i dt • 2 D2 D2 D1 D1 I D2 Is I © ABB SACE – BM .29 Icc D1 . Short-circuit current Improvement (continued) n zone selective interlocking D1 logic relay I D2 logic relay © ABB SACE – BM .30 D3 logic relay III . 05 tD • 10ms © ABB SACE – BM .31 I .g.1 =2 0.Short-circuit current Discrimination between HV fuses and LV circuit breaker HV circuitbreaker imag D +20% 10% "current " safety IF ≥ 1.35 ICB HV/LV transformer D ± 20% I F ± 10% Icc > minimum breaking current of HV circuitbreakers Icc t tF "time" safety tF tCB ≥ 2 e. 0.  the principle of cascading has been recognised by the IEC 364-434. © ABB SACE – BM . They thus allow circuit breakers of lower breaking capacity than the prospective short-circuit current at their point of installation to operate under the stress conditions of normal breaking.Short-circuit current Definition : cascading  cascading is the use of the current limiting capacity of circuit breakers to permit installation of lower rated and therefore lower cost downstream circuit breakers.32 .  comments : the upstream CB acts as a barrier against short-circuit currents.3 standard  cascading can only be checked by laboratory tests and the possible combinations can be specified only by the circuit breaker manufacturer. 33 .© ABB SACE – BM .