Busbar-Design.pdf
Transcript of Busbar-Design.pdf
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David Chapman
Copper Development Association
Copper Development Association
Busbar Design Basics
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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
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Busbar System Current Ratings
Busbar ratings are determined only by the
maximum desired working temperature
At working temperature:
heat generated = heat lost
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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
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25.0
25.166.7
v
Pcv
=
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
Pcv1Pcv2 v
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Power dissipation per metre length v Vertical height
0
40
80
120
160
0 50 100 150 200
Vertical height (mm)
Powerdissipationpermetrelength(W)
Convection (for 80 C temperature rise)
Busbar System Current Ratings
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Power dissipated by radiation is given by:
84
1
4
2107.5
= TTP
r
wherePr is the power dissipated per square metre
T2 is the working temperature, KT1 is the ambient temperature, K
is the emissivity
Busbar System Current Ratings
Pr
Pr
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The emissivity, , describes how effectivelythe 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 to1 - a very good radiator
Busbar System Current Ratings
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Bright copper has an emissivity of about 0.1
During use, the emissivity of the coppersurface increases - and the current rating
increases - as the copper darkens to reach anemissivity value of about 0.7
Tin plated copper has an emissivity of about0.3 to 0.5
But painting bars reduces the current rating!
Busbar System Current Ratings
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Busbar System Current Ratings
Pr
Pr
Pr
Pr
Pr Pr
No radiation heat loss
from internal surfaces
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Radiation power dissipation v Working temperature
0
20
40
60
80
100
273 323 373
Absolute Working Temperature (K)
Powerdissipation(Wp
ersqm)
Radiation (for 30 C ambient)
Busbar System Current Ratings
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for 100 mm vertical height, 30 C ambient
Total 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)
Powerdissipation
(Wp
ersq
Convection
Radiation
Total
Busbar System Current Ratings
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RIP2
=
a
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
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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
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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=
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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.
Higherpurchase cost - lower running costs
Best economic sizing
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Best economic sizing
Cost of bar
0
5
10
15
0 100 200 300 400 500 600
Cross sectional area (sqmm)
Costofbarpermetre($)
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Best economic sizing
Cost of losses - 5 years
0
10
20
30
0 100 200 300 400 500 600
Cross sectional area (sqmm)
Costoflossesover5years($)
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Economic Sizing of Busbars
Total cost - 5 years
0
10
20
30
0 100 200 300 400 500 600
Cross sectional area (sqmm)
Lifetimecostp
ermetre($)
Cost of bar
Cost of loss
Total
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Busbar calculation software
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Busbar calculation software
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Busbar calculation software
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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
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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
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David Chapman
Copper Development Association
Copper Development Association