busbar design
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David Chapman
Copper Development Association
david.chapman@copperdev.co.uk
Copper Development Association
Busbar Design Basics
www.leonardo-energy.org
Copper Development Associationwww.cda.org.uk
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
Materials for Busbar Systems
www.leonardo-energy.org
Copper Development Associationwww.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
www.leonardo-energy.org
Copper Development Associationwww.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
www.leonardo-energy.org
Copper Development Associationwww.cda.org.uk
25.0
25.166.7
vPcv
Power dissipated by convection is given by:
where is the temperature rise above ambientv is the vertical height of the surface
Busbar System Current Ratings
Pcv1Pcv2v
www.leonardo-energy.org
Copper Development Associationwww.cda.org.uk
Power dissipation per metre length v Vertical height
0
40
80
120
160
0 50 100 150 200
Vertical height (mm)
Po
we
r d
iss
ipa
tio
n p
er
me
tre
len
gth
(W
)Convection (for 80 C temperature rise)
Busbar System Current Ratings
www.leonardo-energy.org
Copper Development Associationwww.cda.org.uk
Power dissipated by radiation is given by:
841
42 107.5 TTPr
wherePr is the power dissipated per square metreT2 is the working temperature, KT1 is the ambient temperature, K is the emissivity
Busbar System Current Ratings
Pr
Pr
www.leonardo-energy.org
Copper Development Associationwww.cda.org.uk
The emissivity, , 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
Busbar System Current Ratings
www.leonardo-energy.org
Copper Development Associationwww.cda.org.uk
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!
Busbar System Current Ratings
www.leonardo-energy.org
Copper Development Associationwww.cda.org.uk
Busbar System Current Ratings
Pr
Pr
Pr
Pr
Pr Pr
No radiation heat loss from internal surfaces
www.leonardo-energy.org
Copper Development Associationwww.cda.org.uk
Radiation power dissipation v Working temperature
0
20
40
60
80
100
273 323 373
Absolute Working Temperature (K)
Po
we
r d
iss
ipa
tio
n (
W p
er
sq
m)
Radiation (for 30 C ambient)
Busbar System Current Ratings
www.leonardo-energy.org
Copper Development Associationwww.cda.org.uk
for 100 mm vertical height, 30 C ambientTotal power dissipation v Working temperature
0
100
200
300
400
500
600
700
800
900
1000
40 60 80 100 120 140
Working temperature (C)
Po
wer
dis
sip
ati
on
(W
per
sq
m)
Convection
Radiation
Total
Busbar System Current Ratings
www.leonardo-energy.org
Copper Development Associationwww.cda.org.uk
RIP 2a
lIP
2
The power generated by current in the busbar is:
where
is the resistivity of the material
a is the cross sectional area
l is the length
Busbar System Current Ratings
www.leonardo-energy.org
Copper Development Associationwww.cda.org.uk
At some temperature, the heat generated in the
busbar is equal to the total heat lost by convection
and radiation.
rci PPP
Busbar System Current Ratings
www.leonardo-energy.org
Copper Development Associationwww.cda.org.uk
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
Minimum size calculation
www.leonardo-energy.org
Copper Development Associationwww.cda.org.uk
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
Best economic sizing
www.leonardo-energy.org
Copper Development Associationwww.cda.org.uk
Best economic sizingCost of bar
0
5
10
15
0 100 200 300 400 500 600
Cross sectional area (sqmm)
Co
st o
f b
ar p
er
me
tre
($)
www.leonardo-energy.org
Copper Development Associationwww.cda.org.uk
Best economic sizingCost of losses - 5 years
0
10
20
30
0 100 200 300 400 500 600
Cross sectional area (sqmm)
Co
st
of
los
se
s o
ve
r 5
ye
ars
($
)
www.leonardo-energy.org
Copper Development Associationwww.cda.org.uk
Economic Sizing of BusbarsTotal cost - 5 years
0
10
20
30
0 100 200 300 400 500 600
Cross sectional area (sqmm)
Lif
etim
e co
st p
er m
etre
($)
Cost of bar
Cost of loss
Total
www.leonardo-energy.org
Copper Development Associationwww.cda.org.uk
Busbar calculation software
www.leonardo-energy.org
Copper Development Associationwww.cda.org.uk
Busbar calculation software
www.leonardo-energy.org
Copper Development Associationwww.cda.org.uk
Busbar calculation software
www.leonardo-energy.org
Copper Development Associationwww.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
www.leonardo-energy.org
Copper Development Associationwww.cda.org.uk
Summary of Busbar Material CharacteristicsHigh 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
David Chapman
Copper Development Association
david.chapman@copperdev.co.uk
Copper Development Association
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