Assumption University Thailand EE4503 Electrical Systems Design Short-circuit Current 1.

Assumption University Thailand

EE4503Electrical Systems Design

Short-circuit Current

1


Topics

• Term & Definition

• Equivalent Circuits

– Voltage Source

– Impedance

• Short-circuit Calculation

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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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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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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


Terms & DefinitionComponents: DC & AC

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http://ts-dep.web.cern.ch/ts-dep/groups/el/el_tc/netcomput/neplan0.html


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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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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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


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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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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Equivalent CircuitsImpedance: Cable

and

• is resistance per length

• X is reactance per length

• L is length

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Equivalent CircuitsImpedance: Cable (calculation)

•

– : geometric mean distance between conductor

– : frequency

– : number of bundled cables

– : radius of conductor. If bundled cable,

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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

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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


Equivalent CircuitsImpedance: table for Busway

and

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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

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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


Equivalent CircuitsImpedance: Transformer

and

– , , : rated voltage, current and apparent power

– : copper loss in the transformer

– : percentage impedance of a transformer

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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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• 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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• 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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• Find the three phase short-circuit current: for a system connecting to and a network feeder

– 22kV 3 phase

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• Find the feeder impedance: for a system

connecting to and a network feeder

– 22kV 3 phase

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• 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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Assumption University Thailand EE4503 Electrical Systems Design Short-circuit Current 1.

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