assumption university thailand ee4503 electrical systems design short-circuit current 1
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Assumption University Thailand
EE4503Electrical Systems Design
Short-circuit Current
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Assumption University Thailand
Topics
• Term & Definition
• Equivalent Circuits
– Voltage Source
– Impedance
• Short-circuit Calculation
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Assumption University Thailand
Terms & DefinitionShort Circuits in Power Systems
IEC-60909
• Far-from-generator Short Circuit
– Usually in transmission systems / end-users
– Impedance is constant
• Near-to-generator Short Circuit
– System Impedance changes when the short circuit occurred.
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Assumption University Thailand
Terms & DefinitionTypes of Short Circuits
• Three-phase short-circuit
• Double-phase short-circuit without earth/ground connection
• Double-phase short-circuit with earth/ground connection
• Line-to-earth (line-to-ground) short-circuit
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Assumption University Thailand
Terms & DefinitionTypes of Short Circuits
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http://www.globalspec.com/reference/26006/203279/chapter-4-calculation-of-short-circuit-current-in-a-c-three-phase-hv-systems
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Assumption University Thailand
Terms & DefinitionComponents: DC & AC
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http://ts-dep.web.cern.ch/ts-dep/groups/el/el_tc/netcomput/neplan0.html
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Assumption University Thailand
Terms & DefinitionComponents of Short Circuit Currents
• : Initial Symmetrical Short-circuit Current: RMS value of the AC component at the beginning
• : Peak Short-circuit Current: Peak current
(usually happens at the beginning)
• Steady-state Short-circuit Current: RMS
value of the current after the transient period
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Assumption University Thailand
Terms & DefinitionComponents of Short Circuit Currents
• : Symmetrical Short-circuit Breaking Current: RMS of the AC component when the
switching device break the circuit
• : Nominal System Voltage: line to line voltage
(depends on the short circuit type)
• : Initial Symmetrical Short-circuit (Apparent) Power:
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Assumption University Thailand
Equivalent Circuits
• Equivalent Voltage Source:
– : Voltage factor is the (standard) factor to find the maximum and minimum voltage.
Approx.: 1.05/0.95 for LV and 1.1/1.0 for HV
• Impedance:
– Depend on the systems: e.g. cable, transformer, generator etc. 9
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Assumption University Thailand
Equivalent CircuitsImpedance: Network Feeder
• : Voltage at the considered location
• : Initial Symmetrical Short-circuit (Apparent) Power at the considered location
• : Initial Symmetrical Short-circuit Current
• : Voltage Factor
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Assumption University Thailand
Equivalent CircuitsImpedance: Network Feeder
• If transformer exists,
• If there is no specific information regarding the resistance and reactance:
– for overhead cable with the voltage higher than 35kV:
– otherwise: and
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Assumption University Thailand
Equivalent CircuitsImpedance: Cable
and
• is resistance per length
• X is reactance per length
• L is length
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Assumption University Thailand
Equivalent CircuitsImpedance: Cable (calculation)
•
– : geometric mean distance between conductor
– : frequency
– : number of bundled cables
– : radius of conductor. If bundled cable,
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Assumption University Thailand
Equivalent CircuitsImpedance: table for PVC cables
and
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Size () Resistance()
Reactance()
2.5 7.400 0.155
4 4.625 0.141
6 3.083 0.131
10 1.850 0.121
16 1.156 0.113
25 0.740 0.107
35 0.529 0.103
50 0.370 0.100
Electrical Systems Design: ประสิ�ทธิ์�� พิ�ทยพิ�ฒน์�
Size () Resistance()
Reactance()
70 0.264 0.097
95 0.195 0.096
120 0.154 0.094
150 0.123 0.092
185 0.100 0.091
240 0.077 0.090
300 0.062 0.089
400 0.051 0.088
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Assumption University Thailand
Equivalent CircuitsImpedance: table for Busway
and
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Size () Resistance()
Reactance()
225-600 0.1342 0.0350
800 0.0814 0.0216
1000 0.0712 0.0186
1200 0.0568 0.0150
1350 0.0407 0.0112
1600 0.0367 0.0098
2000 0.0292 0.0079
Aluminium
Electrical Systems Design: ประสิ�ทธิ์�� พิ�ทยพิ�ฒน์�
Size () Resistance()
Reactance()
225-600 0.0764 0.0350
800 0.0764 0.0350
1000 0.0623 0.0260
1200 0.0489 0.0261
1350 0.0417 0.0186
1600 0.0328 0.0150
2000 0.0240 0.0112
Copper
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Assumption University Thailand
Equivalent CircuitsImpedance: Transformer
and
– , , : rated voltage, current and apparent power
– : copper loss in the transformer
– : percentage impedance of a transformer
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Assumption University Thailand
Short-circuit CalculationExample from CoE
• Find Symmetrical Short-circuit power
– Balance Three Phase Short-circuit
– Initial Symmetrical Short-circuit Current is 10kA
– Norminal Voltage is 22kV
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Assumption University Thailand
Short-circuit CalculationExample from CoE
• Find Symmetrical Short-circuit power
– Balance Three Phase Short-circuit
– Initial Symmetrical Short-circuit Current is 10kA
– Nominal Voltage is 230kV
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Assumption University Thailand
Short-circuit CalculationExample from CoE
• Find the three phase short-circuit current: for a system connecting to an infinite bus through a transformer
– Power rating 3.15MVA,
– Voltage rating 22kV/400-230V
– Impedance Voltage = 8%
04 then
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Assumption University Thailand
Short-circuit CalculationExample from CoE
• Find the three phase short-circuit current: for a system connecting to and a network feeder
– 22kV 3 phase
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Assumption University Thailand
Short-circuit CalculationExample from CoE
• Find the feeder impedance: for a system
connecting to and a network feeder
– 22kV 3 phase
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Assumption University Thailand
• Electrical Systems Design: ประสิ�ทธิ์�� พิ�ทยพิ�ฒน์�• http://www.openelectrical.org/wiki/index.php?
title=Short_Circuit_Calculation
• Calculation of short-circuit currents: Schneider Electric
Reference
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