Busbar Design
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Busbar Design Basics David Chapman Copper Development Association [email protected]
Copper Development Association
Materials for Busbar Systems High electrical conductivity • only silver is better than copper
Good thermal conductor • heat reaches surface quickly
Strong (at working temperature) • to withstand short circuit stresses • low creep
Easy to joint • resistant to corrosion Copper Development Association www.leonardo-
www.cda.org.uk
Busbar System Current Ratings
Busbar ratings are determined only by the maximum desired working temperature
At working temperature: heat generated = heat lost
Copper Development Association www.leonardo-
www.cda.org.uk
Busbar System Current Ratings Busbar ratings depend on: • Working and ambient temperatures • Heat lost from the busbar – by convection – by radiation • Heat generated in the busbar
Copper Development Association www.leonardo-
www.cda.org.uk
Busbar System Current Ratings Power dissipated by convection is given by:
7.66 θ Pcv = 0.25 v
1.25 Pcv2
Pcv1
v
where θ is the temperature rise above ambient v is the vertical height of the surface
Copper Development Association www.leonardo-
www.cda.org.uk
Busbar System Current Ratings Convection (for 80 C temperature rise) Power dissipation per metre length v Vertical height
Power dissipation per metre length (W)
160
120
80
40
0 0
50
100
150
200
Vertical height (mm)
Copper Development Association www.leonardo-
www.cda.org.uk
Busbar System Current Ratings Power dissipated by radiation is given by:
Pr = 5.7ε
(
4 T2
4 − T1
) ×10
Pr
−8 Pr
where Pr is the power dissipated per square metre T2 is the working temperature, K T1 is the ambient temperature, K ε is the emissivity Copper Development Association www.leonardo-
www.cda.org.uk
Busbar System Current Ratings The emissivity, ε, describes how effectively the surface radiates heat For a perfectly polished surface, the value is close to zero - a very poor radiator For a matt black surface, the value is close to 1 - a very good radiator
Copper Development Association www.leonardo-
www.cda.org.uk
Busbar System Current Ratings Bright copper has an emissivity of about 0.1 During use, the emissivity of the copper surface increases - and the current rating increases - as the copper darkens to reach an emissivity value of about 0.7 Tin plated copper has an emissivity of about 0.3 to 0.5 But painting bars reduces the current rating! Copper Development Association www.leonardo-
www.cda.org.uk
Busbar System Current Ratings Pr
Pr
Pr
Pr
Pr
Pr
No radiation heat loss from internal surfaces
Copper Development Association www.leonardo-
www.cda.org.uk
Busbar System Current Ratings Radiation (for 30 C ambient) Radiation power dissipation v Working temperature
Power dissipation (W per sqm)
100
80
60
40
20
0 273
323
373
Absolute Working Temperature (K)
Copper Development Association www.leonardo-
www.cda.org.uk
Busbar System Current Ratings for 100 mm vertical height, 30 C ambient Total power dissipation v Working temperature
Power dissipation (W per sqm)
1000 900
Convection
800
Radiation
700
Total
600 500 400 300 200 100 0 40
60
80
100
120
140
Working temperature (C)
Copper Development Association www.leonardo-
www.cda.org.uk
Busbar System Current Ratings The power generated by current in the busbar is:
2
P=I R
ρl P=I a 2
where ρ is the resistivity of the material
a is the cross sectional area l is the length Copper Development Association www.leonardo-
www.cda.org.uk
Busbar System Current Ratings At some temperature, the heat generated in the busbar is equal to the total heat lost by convection and radiation.
Pi = [ Pc ] + [ Pr ]
Copper Development Association www.leonardo-
www.cda.org.uk
Minimum size calculation Calculation method: 1
Select working and ambient temperatures
2
Assume initial current density of 8 amps/mm2
3
Find appropriate size in standard range
4
Calculate heat generated due to current
5
Calculate heat loss at working temperature
6
If 4>5, increase size and return to 4 When 4=<5, this is smallest possible size Copper Development Association www.leonardo-
www.cda.org.uk
Best economic sizing The most economic size gives the lowest lifetime cost. It is the minimum total cost of •material •installation and •energy costs over the circuit lifetime.
Higher purchase cost - lower running costs Copper Development Association www.leonardo-
www.cda.org.uk
Best economic sizing Cost of bar
Cost of bar per metre ($)
15
10
5
0 0
100
200
300
400
500
600
Cross sectional area (sqmm)
Copper Development Association www.leonardo-
www.cda.org.uk
Best economic sizing Cost of losses - 5 years
Cost of losses over 5 years ($)
30
20
10
0 0
100
200
300
400
500
600
Cross sectional area (sqmm)
Copper Development Association www.leonardo-
www.cda.org.uk
Economic Sizing of Busbars Total cost - 5 years 30
Cost of bar Cost of loss Lifetime cost per metre ($)
Total 20
10
0 0
100
200
300
400
500
600
Cross sectional area (sqmm)
Copper Development Association www.leonardo-
www.cda.org.uk
Busbar calculation software
Copper Development Association www.leonardo-
www.cda.org.uk
Busbar calculation software
Copper Development Association www.leonardo-
www.cda.org.uk
Busbar calculation software
Copper Development Association www.leonardo-
www.cda.org.uk
Further considerations Having calculated the size of the bar, there are three further considerations: • voltage drop • skin effect – increases apparent resistance by reducing effective area - important for: ◊
thick busbars
◊
high frequencies
◊
harmonics generated by non-linear loads
• short circuit current Copper Development Association www.leonardo-
www.cda.org.uk
Summary of Busbar Material Characteristics High Conductivity • low loss, low voltage drop • >101.5 % IACS
Easy Formability • due to small grain size and advanced production technology • easy to bend without surface deformation
Good Flatness • simple reliable jointing
Good Straightness • easy installation, lower joint stress Copper Development Association www.leonardo-
www.cda.org.uk
David Chapman Copper Development Association [email protected]
Copper Development Association
Copper Development Association
Materials for Busbar Systems High electrical conductivity • only silver is better than copper
Good thermal conductor • heat reaches surface quickly
Strong (at working temperature) • to withstand short circuit stresses • low creep
Easy to joint • resistant to corrosion Copper Development Association www.leonardo-
www.cda.org.uk
Busbar System Current Ratings
Busbar ratings are determined only by the maximum desired working temperature
At working temperature: heat generated = heat lost
Copper Development Association www.leonardo-
www.cda.org.uk
Busbar System Current Ratings Busbar ratings depend on: • Working and ambient temperatures • Heat lost from the busbar – by convection – by radiation • Heat generated in the busbar
Copper Development Association www.leonardo-
www.cda.org.uk
Busbar System Current Ratings Power dissipated by convection is given by:
7.66 θ Pcv = 0.25 v
1.25 Pcv2
Pcv1
v
where θ is the temperature rise above ambient v is the vertical height of the surface
Copper Development Association www.leonardo-
www.cda.org.uk
Busbar System Current Ratings Convection (for 80 C temperature rise) Power dissipation per metre length v Vertical height
Power dissipation per metre length (W)
160
120
80
40
0 0
50
100
150
200
Vertical height (mm)
Copper Development Association www.leonardo-
www.cda.org.uk
Busbar System Current Ratings Power dissipated by radiation is given by:
Pr = 5.7ε
(
4 T2
4 − T1
) ×10
Pr
−8 Pr
where Pr is the power dissipated per square metre T2 is the working temperature, K T1 is the ambient temperature, K ε is the emissivity Copper Development Association www.leonardo-
www.cda.org.uk
Busbar System Current Ratings The emissivity, ε, describes how effectively the surface radiates heat For a perfectly polished surface, the value is close to zero - a very poor radiator For a matt black surface, the value is close to 1 - a very good radiator
Copper Development Association www.leonardo-
www.cda.org.uk
Busbar System Current Ratings Bright copper has an emissivity of about 0.1 During use, the emissivity of the copper surface increases - and the current rating increases - as the copper darkens to reach an emissivity value of about 0.7 Tin plated copper has an emissivity of about 0.3 to 0.5 But painting bars reduces the current rating! Copper Development Association www.leonardo-
www.cda.org.uk
Busbar System Current Ratings Pr
Pr
Pr
Pr
Pr
Pr
No radiation heat loss from internal surfaces
Copper Development Association www.leonardo-
www.cda.org.uk
Busbar System Current Ratings Radiation (for 30 C ambient) Radiation power dissipation v Working temperature
Power dissipation (W per sqm)
100
80
60
40
20
0 273
323
373
Absolute Working Temperature (K)
Copper Development Association www.leonardo-
www.cda.org.uk
Busbar System Current Ratings for 100 mm vertical height, 30 C ambient Total power dissipation v Working temperature
Power dissipation (W per sqm)
1000 900
Convection
800
Radiation
700
Total
600 500 400 300 200 100 0 40
60
80
100
120
140
Working temperature (C)
Copper Development Association www.leonardo-
www.cda.org.uk
Busbar System Current Ratings The power generated by current in the busbar is:
2
P=I R
ρl P=I a 2
where ρ is the resistivity of the material
a is the cross sectional area l is the length Copper Development Association www.leonardo-
www.cda.org.uk
Busbar System Current Ratings At some temperature, the heat generated in the busbar is equal to the total heat lost by convection and radiation.
Pi = [ Pc ] + [ Pr ]
Copper Development Association www.leonardo-
www.cda.org.uk
Minimum size calculation Calculation method: 1
Select working and ambient temperatures
2
Assume initial current density of 8 amps/mm2
3
Find appropriate size in standard range
4
Calculate heat generated due to current
5
Calculate heat loss at working temperature
6
If 4>5, increase size and return to 4 When 4=<5, this is smallest possible size Copper Development Association www.leonardo-
www.cda.org.uk
Best economic sizing The most economic size gives the lowest lifetime cost. It is the minimum total cost of •material •installation and •energy costs over the circuit lifetime.
Higher purchase cost - lower running costs Copper Development Association www.leonardo-
www.cda.org.uk
Best economic sizing Cost of bar
Cost of bar per metre ($)
15
10
5
0 0
100
200
300
400
500
600
Cross sectional area (sqmm)
Copper Development Association www.leonardo-
www.cda.org.uk
Best economic sizing Cost of losses - 5 years
Cost of losses over 5 years ($)
30
20
10
0 0
100
200
300
400
500
600
Cross sectional area (sqmm)
Copper Development Association www.leonardo-
www.cda.org.uk
Economic Sizing of Busbars Total cost - 5 years 30
Cost of bar Cost of loss Lifetime cost per metre ($)
Total 20
10
0 0
100
200
300
400
500
600
Cross sectional area (sqmm)
Copper Development Association www.leonardo-
www.cda.org.uk
Busbar calculation software
Copper Development Association www.leonardo-
www.cda.org.uk
Busbar calculation software
Copper Development Association www.leonardo-
www.cda.org.uk
Busbar calculation software
Copper Development Association www.leonardo-
www.cda.org.uk
Further considerations Having calculated the size of the bar, there are three further considerations: • voltage drop • skin effect – increases apparent resistance by reducing effective area - important for: ◊
thick busbars
◊
high frequencies
◊
harmonics generated by non-linear loads
• short circuit current Copper Development Association www.leonardo-
www.cda.org.uk
Summary of Busbar Material Characteristics High Conductivity • low loss, low voltage drop • >101.5 % IACS
Easy Formability • due to small grain size and advanced production technology • easy to bend without surface deformation
Good Flatness • simple reliable jointing
Good Straightness • easy installation, lower joint stress Copper Development Association www.leonardo-
www.cda.org.uk
David Chapman Copper Development Association [email protected]
Copper Development Association
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