Short-circuit Calculations
ABB Ltd 03-03
Talk will cover
Short circuit calculation Demo of DOCWin
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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)
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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, total fault current = 2 x 11.5kA or nearly 25kA This gives upstream power short circuit :
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3 x 22kV x 25 kA = 1000 MVA
Consultants method
1. Upstream Network :
Psc : Upstream power short circuit in MVA : 1000 MVA Zup = U 2 / Psc = 400 2 / 1000 = 0.16 m
22kV/400V 1MVA 5% 22kV 1000MVA
2. 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 = 0.38 m/m Xc = 0.24 m/m
(ZT2 RT2)
400V
3. Cable :
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Tabel mV/A/m of cable [R + JX] :
Rc = R m/m x Lm / Xc = X m/m x Lm / 3 3
IF = ?
Consultants method
Impendance upstream network (Zup) = 0.16 m Impendance of transformer (ZT) = 8 m
22kV/400V 1MVA 5% 22kV 1000MVA
Impendance of cable : Rc = 0.38 x 50 / V3 = 10.98 m Xc = 0.24 x 50 / V3 = 6.94 m
Zc = (10.98 2 + 6.94 2) = 12.98 m 400 Fault current at A = 3 x (0.16 + 8) = 28.3 kA
4 x 1c 120mm2 XLPE flat touching on cable tray length 50m 120mm2 cable data Rc = 0.38 m/m Xc = 0.24 m/m
400V
400
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Fault current at B
= 3 x (0.16 + 8 + 12.98)
= 10.94 kA
IF = ?
Consultants method
To show why fault level at HV side can be ignored. Per unit impedance of 22kV source = 1.0 MVA 1000 MVA = 0.001 This value is insignificant compared to the transformer and cable impedance.
22kV 1000MVA 22kV/400V 1MVA 5%
4 x 1c XLPE flat touching on cable tray length 50m 120mm2 cable data Rc = 0.38 m/m Xc = 0.24 m/m 400V 120mm2
Hence the difference in total impedance up to the point of fault is negligible.
IF = ?
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US Consultants method
Sometimes Consultant is very thorough. Because UL standard 1561 allows the marked impedance of a transformer to vary + 10%, therefore the trend is to calculate worse case.
4 x 1c XLPE flat touching on cable tray length 50m 120mm2 cable data Rc = 0.38 m/m Xc = 0.24 m/m 120mm2 22kV 1000MVA 22kV/400V 1MVA 5%
Isc = fault current / 0.9
= 28.3kA / 0.9
400V
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= 31. 4 kA
IF = ?
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 = 394.9 V Ib = 600.0 A Cosphi = 0.91 I"k LLL = 28.2 kA
I>
-QF5
4 x 1c XLPE flat touching on cable tray length 50m 120mm2 cable data Rc = 0.38 m/m Xc = 0.24 m/m
-B1 Df = 1.00 V = 391.3 V Ib = 600.0 A Cosphi = 0.91 I"k LLL = 23.9 kA
120mm2
-WC1 12x(1x120)+4x(1x70)+1G70 dV = 0.63 % Ib = 600.0 A Iz = 669.2 A L = 50 m
400V
-QF3 S7S 1600 PR211-I R1600
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-QF4 S7S 1600 PR211-I R1600
IF = ?
-L1 Sr = 415.69 kVA Cosphi = 0.90 Ir = 600.0 A UF = 100% dV = 2.18 %
Short-circuit current
General rules
in accordance with the rules in articles IEC 364-434.2, 434.3 and IEC 364533.2, 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. the maximum prospective short-circuit current determines : the breaking capacity (Icu) of the circuit-breakers Icu than prospective Icc, the making capacity of the devices, the thermal and electrodynamic withstand of the ducts and switchgear. the minimum prospective short-circuit determines : choice of trip units (curve) and fuse when : - protection of persons depends on it (TN-IT), - cables are very long, - the source is impedant (set).
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in all cases, protection must be compatible with the cable heat stress I2dt K2 S2
Short-circuit current
General rules
for whatever type of short-circuit current (min. or max.), 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.S2
t= k2 S2
I2
t duration in s, S cross-sectional area in mm2 I effective short-circuit current in A expressed, for a.c., as the r.m.s. value, 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.
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Short-circuit current
General rules
values of k for common materials, 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 60C rubber 85C rubber 90C thermosetting impregnated paper mineral - conductor - sleeves and seals pvc 60C rubber 85C rubber 90C thermosetting impregnated paper
k 115/103 141 134 143 108
115 135 76/68 93 89 94 71
Aluminum
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note : where two values of limiting final temperature and k are given, the lower value relates to 2 cables having conductors with a cross-sectional area greater than 300mm
Short-circuit current
Short-circuit calculation procedure
upstream Scc
HV/LV transformer rating
Usc (%)
Isc at transformer terminals - power factor, - coincidence factor, - duty factor - foreseeable expansion factor conductor characteristics busbars : length, width, thickness. cables : type of insulation single-core or multicore, length, cross-section. environment : ambient temperature, installation method, number of contiguous circuits. breaking capacity inst. trip setting Isc of main LV switchboard outgoers breaking capacity inst. trip setting Isc at head of secondary switchboards breaking capacity inst. trip setting Isc at head of final switchboards breaking capacity inst. 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
- feeder - current ratings - voltage drops
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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
m)
U ZI U ZI
B
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Short-circuit current
The various short-circuits currents
three-phase fault ZL
ZSC
~
ZL ZL
I sc3 =
U/ 3 Zsc
phase-to-phase fault ZL
ZSC
~
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U ZSC
I sc2 =
ZL
U 2. Zsc
= 0,86. Isc3
Short-circuit current
The various short-circuits currents
phase-to-neutral fault
ZL ZSC
ZLn
V
ZLn
I sc1 =
U/ 3 Zsc + ZLn
= 0,5. Isc3
phase-to-earth fault
ZL
ZSC
~
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V Z(0)
I sc(0) =
U/ 3 Zsc + Z(0)
Z(0)
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 . Zk
k k
the composition method : which may be used when the characteristics of the power supply are not known U IscB = IscA . U + Zc . IscA
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the conventional method : which can be used when the impedance or the Isc in the installation upstream of the given circuit are not known, to calculate the Isc at the end of a line
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
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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), limited thermal withstand. n protection characteristics : long time protection acting quickly (<15s) for an overload of 1.5 ln, low short time protection (< 2ln).
250 KVA 400 xd = 30%
load shedding non-priority priority
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n value of Icc3 at the terminals of a set
Icc3 =
(1)
Sn V3 Un
1 Xd
; Xd (1) = transient reactance expressed as (30%)
The value of the transient reactance should be check to genset manufacture
Short-circuit current
The case of a generator
n subtransient reactance : xd in %
Xd =
Un Sn
x.xd
if no info = 20 %
n transient reactance : xd in %
Xd =
Un Sn
x.xd if no info = 30 %
n zero sequence reactance : xo in %
Xo =
Un Sn
x.xo
if no info = 6 %
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Note : to be checked to the manufacturer
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. cable heating is reduced hence longer cable life. electrodynamic forces reduced, thus electric contacts less likely to be deformed or broken. measuring equipment situated near an electric circuit less affected the cascading technique offers substantial savings on equipment, enclosures and design by using lower rated devices downstream.
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Short-circuit current
Principle of limitation
i u U arc
prospective current limited current
arc voltage
t network voltage
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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.
kA peak
55
without limitation
25
with limitation
0
Isc prospective Isc peak
30
kA rms
example : system prospective = 30 kA rms, = 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
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6 x 106
energy let through during half cycle with limitation
30
kA rms
Short-circuit current
Definition : discrimination
discrimination (selectivity), 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, and by that device alone.
no discrimination discrimination
CB1
CB1
CB2
CB2
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CB1 and CB2 open
only CB2 open
why is discrimination useful ? Discrimination contributes to continuity of service, a necessity in many industrial, 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
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D1
Short-circuit current
Full or restricted current discrimination
case of full discrimination
2 i dt
case of restricted discrimination
i dt
D2
D2
D1
D1 I
D2
Is
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Icc
D1
Short-circuit current
Improvement (continued)
n zone selective interlocking
D1
logic relay
I D2
logic relay
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D3
logic relay
III
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.g. 0,1 =2 0,05
tD 10ms
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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.
the principle of cascading has been recognised by the IEC 364-434.3 standard cascading can only be checked by laboratory tests and the possible
combinations can be specified only by the circuit breaker manufacturer.
comments : the upstream CB acts as a barrier against short-circuit currents.
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.
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