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Short Circuit Currents According to IEC 60909

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Welcome
JUNE 2021
Technical Sharing Session
Lionel Ng, Global Training Specialist
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ABB. All rights reserved.
Short Circuit Calculation
IEC 60909
Short Circuit Calculation
IEC 60909
IEC 60909 Part 0 to 4
• Part 0
Calculation of currents
• Part 1
Factors for the calculation of short circuit currents according to Part 0.
• Part 2
Data of electrical equipment for short-circuit current calculations
• Part 3
Currents during two separate simultaneous line-to-earth short circuits and partial short-circuit currents
flowing through earth
• Part 4
Examples for the calculation of short-circuit currents
June 7, 2021
Slide 3
Short Circuit Calculation
IEC 60909
IEC 60909-0
Short-circuit currents in three-phase a.c. systems
• Terms and definitions
• Short-circuit currents
• Calculating Assumptions
• Short-circuit impedances of electrical equipment
June 7, 2021
Slide 4
Short Circuit Calculation
IEC 60909
Terms & Definitions
• Short-circuit
“Accidental or intentional conductive path between two or more conductive parts forcing the electric
potential differences between these conductive parts to be equal or close to zero”
• Short-circuit current
“Overcurrent resulting from a short-circuit in an electric system”
• Prospective short-circuit current
“Current that would flow if the short circuit were placed by an ideal connection of negligible impedance
without any change of the supply”
June 7, 2021
Slide 5
Short Circuit Calculation
IEC 60909
Short-circuit currents
June 7, 2021
Slide 6
Short Circuit Calculation
IEC 60909
Short-circuit currents
June 7, 2021
Slide 7
Short Circuit Calculation
IEC 60909
Short-circuit currents
June 7, 2021
Slide 8
Short Circuit Calculation
IEC 60909
Calculation assumptions
The calculation of maximum and minimum short-circuit currents is based on the following simplifications.
• For the duration of the short circuit there is no change in the type of short circuit involved, that is, a three-phase
short-circuit remains three-phase and a line-to-earth short circuit remains line-to-earth during the time of short
circuit.
• For the duration of the short circuit, there is no change in the network involved.
• The impedance of the transformers is referred to the tap-changer in main position.
• Arc resistances are not taken into account.
• Shunt admittances of non-rotating loads shall be neglected in the positive-, the negative and the zero-sequence
system.
• Line capacitances shall be neglected in the positive- and negative-sequence system. Line capacitances in the zerosequence system shall be taken into account in low-impedance
• Earthed networks having an earth-fault factor (see IEC 60027-1) higher than 1,4.
• Magnetising admittances of transformers shall be neglected in the positive and negative sequence system.
June 7, 2021
Slide 9
Short Circuit Calculation
IEC 60909
Method of calculation
Equivalent voltage source at the short-circuit location
June 7, 2021
Slide 10
Short Circuit Calculation
IEC 60909
Short-circuit impedances of electrical equipment
June 7, 2021
Slide 11
Short Circuit Calculation
IEC 60909
Short-circuit impedances of two-winding transformer
The positive-sequence short-circuit impedances of two-winding transformers ZT = RT + jXT with and without on-load
tap-changer can be calculated from the rated transformer data as follows:
June 7, 2021
Slide 12
Short Circuit Calculation
IEC 60909
Short-circuit impedances of Overhead lines and cables
The positive-sequence short-circuit impedance ZL = RL + jXL may be calculated from the conductor data, such as the
cross-sections and the centre-distances of the conductors.
The effective resistance per unit length RL’ of overhead lines at the conductor temperature 20 °C may be calculated
from the nominal cross-section qn and the resistivity ρ :
June 7, 2021
Slide 13
Short Circuit Calculation
IEC 60909
Calculation of Short circuit currents
The equivalent network is solved according to standard electro-technical rules (circuits in series / in parallel)
Zknet
ZcMV
ZTR
ZcLV
Ik
Ik =
June 7, 2021
Slide 14
Un
3  (Z knet + Z cMV + Z TR + Z cLV )
Un
Short Circuit Calculation
Basic Principles
Calculation of Short Circuit Currents
Distribution network
– It is necessary to know the network short-circuit power
• From 250MVA to 500MVA Un  30kV
• From 700MVA to 1500MVA Un > 30kV
• According to IEC 60076-5
Z knet
June 7, 2021
Un2
=
=
Sknet
Slide 15
Un
3  Iknet
Short Circuit Calculation
Basic Principles
Calculation of Short Circuit Currents
Generator short circuit impedance
– It is necessary to know the
• Rated apparent power Sn
• Rated voltage Un
• Subtransient reactance X”d
from 10% to 20% smooth rotor (isotropic machines)
from 15% to 30% salient pole rotor (anisotropic machines)
"
2
x
U
X "d = d  n
100 Sn
June 7, 2021
Slide 16
Short Circuit Calculation
Basic Principles
Calculation of Short Circuit Currents
Transformers
– It is necessary to know the
• Rated apparent power Sn
• Primary rated voltage V1n
• Secondary rated voltage V2n
• Short-circuit voltage uk%(Between 4% to 8%)
• According to IEC60076-5
Z TR
June 7, 2021
Slide 17
uk % U22n
=

100 Sn
Short Circuit Calculation
Basic Principles
Calculation of Short Circuit Currents
Asynchronous motors
• In case of short-circuit it functions as a generator
with a x”d from 20% to 25%
• a current equal to 4-6 times the In can be assumed
as contribution to the short-circuit
• the minimum criteria for taking into consideration
the phenomenon
(Ik short-circuit without motor contribution)
June 7, 2021
Slide 18
Short Circuit Calculation
Basic Principles
Calculation of Short Circuit Currents
Example
Sk = 500 MVA
UMV= 20 kV
ZcMV = 0.01 
Sn = 400 kVA
U= 20/0.4 kV
uk% = 4 %
ZcLV = 0.06 
June 7, 2021
Slide 19
U
Short Circuit Calculation
Basic Principles
Calculation of Short Circuit Currents
Example
U
Z u20 kV
Z u 400 V
(
(
)
Sk = 500 MVA
UMV= 20 kV
Zu = 0.32 m
2
U2
20  10 3
=
=
= 0.8
Sk
500  10 6
)
2
(
400 )
= Z u20 kV 
(20000 )2
= 3.2  10 −4 
V1
V2
N1
N2
I2
I1
P
V
R
June 7, 2021
Slide 20
I
V
I
Short Circuit Calculation
Basic Principles
Calculation of Short Circuit Currents
Example
U
Z cMV 400 V
June 7, 2021
2
(
400 )
= Z cMV 20 kV 
(20000 )2
Slide 21
Zu = 0.32 m
= 4  10 −6 
ZcMV = 0.01 
ZcMV = 410-3 m
Short Circuit Calculation
Basic Principles
Calculation of Short Circuit Currents
Example
U
Zu = 0.32 m
ZcMV = 410-3 m
2
(400 ) = 0.016
uk % UnLV
4
ZT =
=

100 Sn
100 400  10 3
2
June 7, 2021
Slide 22
Sn = 400 kVA
U= 20/0.4 kV
uk% = 4 %
ZT = 0.016 
Short Circuit Calculation
Basic Principles
Calculation of Short Circuit Currents
Example
U
Zu = 0.32 m
ZcMV = 410-3 m
ZT = 0.016 
ZcLV = 0.061 m
June 7, 2021
Slide 23
ZcLV = 0.061 m
Short Circuit Calculation
Basic Principles
Calculation of Short Circuit Currents
Example
Sk = 500 MVA
UMV= 20 kV
U
Zu = 0.32 m
ZcMV = 410-3 m
Ik =
Ik =
June 7, 2021
Un
3  (Z u + Z cMV + Z T )
Un
ZT = 16 m
 14.1kA
3  (Z u + ZcMV + ZT + ZcLV )
Slide 24
ZcLV = 0.061 m
 13.6kA
Un = 400 V
Short Circuit Calculation
Basic Principles
Common Management Methods
Substation with single transformer
Typical single line diagrams
June 7, 2021
Slide 25
Short Circuit Calculation
Basic Principles
Common Management Methods
Substation with two transformers
(One as spare for the other)
– The use of mechanical interlock between two ACBs
June 7, 2021
Slide 26
Short Circuit Calculation
Basic Principles
Common Management Methods
Substation with two transformers which operate in
parallel
June 7, 2021
Slide 27
Short Circuit Calculation
Basic Principles
Common Management Methods
Substation with two transformers which operate
simultaneously on two separate half-busbars. (Two
incoming with bus-coupler)
Mechanical Interlock between three ACBs
June 7, 2021
Slide 28
Short Circuit Calculation
Basic Principles
Management philosophy for the protections
To ensure continuity of service to sound parts of the
installation
– Circuit breaker L1 must trip and clear the fault
June 7, 2021
Slide 29
Short Circuit Calculation
Basic Principles
Management philosophy for the protections
To ensure continuity of service to sound parts of the
installation
– Circuit breaker I LV1 must trip and clear the fault
– If CLV is closed I LV2 must trip as well. Depending if CLV is
an ACB or just a bus coupler.
June 7, 2021
Slide 30
Short Circuit Calculation
Basic Principles
Management philosophy for the protections
To ensure continuity of service to sound parts of the
installation
– Circuit breaker I LV1 must trip and clear the fault
– If CLV is closed I LV2 must trip as well. Depending if CLV is
an ACB or just a bus coupler.
June 7, 2021
Slide 31
Short Circuit Calculation
Basic Principles
Short Circuit Calculations Single Transformer
U
MV Level 22kV 750MVA
Z22kV =
(22k)2
750M
= 0.6453 Ω
4002
Z400V = 0.6453 x
220002
ZTX =
6
100
x
4002
1.6M
= 0.2133 mΩ
1600kVA
22/0.4kV
6%
= 6 mΩ
ZTotal = 6.2133 mΩ
400
Isc =
= 37.168kA
3 x 6.2133m
June 7, 2021
22kV
750MVA
Slide 32
Isc
Short Circuit Calculation
Basic Principles
Short Circuit Calculations two transformers in parallel
MV Level 22kV 750MVA
Z22kV =
(22k)2
750M
= 0.6453 Ω
4002
Z400V = 0.6453 x
220002
6
ZTX =
100
x
4002
1.6M
= 0.2133 mΩ
= 6 mΩ
ZTotal = 6.2133 mΩ
Isc =
400
3 x 6.2133m
1600kVA
22/0.4kV
6%
= 37.168kA
ISC Total = 37.168 x 2 = 74.336kA
June 7, 2021
U
22kV
750MVA
Slide 33
Isc Total
U
Short Circuit Calculation
Basic Principles
Short Circuit Calculations two transformers in parallel
MV Level 22kV 750MVA
Z22kV =
(22k)2
750M
= 0.6453 Ω
4002
Z400V = 0.6453 x
220002
5
ZTX =
100
x
4002
8ooK
= 0.2133 mΩ
= 0.01 Ω
ZTotal = 10.2133 mΩ
Isc =
400
3 x 10.2133m
800kVA
22/0.4kV
5%
= 22.611 kA
ISC Total = 22.611 x 2 = 45.22 kA
June 7, 2021
U
22kV
750MVA
Slide 34
Isc Total
U
Short Circuit Calculation
Basic Principles
Short Circuit Calculations Single Transformer
ZTX =
Isc =
6
100
x
400
3 x 6m
4002
1.6M
U
= 6 mΩ
= 38.49kA
1600kVA
22/0.4kV
6%
Isc
June 7, 2021
Slide 35
22kV
MVA = ???
Short Circuit Calculation
Basic Principles
Time Current curves of Circuit breakers
– L function protection against overload
L
– S function protection against delayed short-circuit
– I function protection against instantaneous shortcircuit
– G function protection against earth-fault
June 7, 2021
Slide 36
S
G
I
Short Circuit Calculation
Basic Principles
Calculation of Short Circuit Currents
Technical Application Paper
June 7, 2021
Slide 37
http://new.abb.com/low-voltage
http://new.abb.com/low-voltage/business/epc
Short Circuit Calculation
Basic Principles
Calculation of Short Circuit Currents
Electrical Installation Handbook
Determination of short-circuit
current downstream of a cable
as a function of the upstream
short-circuit current (Table 6.4).
June 7, 2021
Slide 38
Determination of shortcircuit current
downstream of a cable as
a function of the
upstream short-circuit
current (Table 6.4).
June 7, 2021
Slide 39
Short Circuit Calculation
Basic Principles
EasyPower Short Circuit Calculation
Example
June 7, 2021
Slide 40
Short Circuit Calculation
Basic Principles
Calculation of Short Circuit Currents
Manual Verification
• Example 1
June 7, 2021
Slide 41
June 7, 2021
Slide 42
Short Circuit Calculation
Basic Principles
Calculation of Short Circuit Currents
Manual Verification
•
Example 1
June 7, 2021
Slide 43
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