Busbar-Design.pdf

7/27/2019 Busbar-Design.pdf

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David Chapman

Copper Development Association

[email protected]


Busbar Design Basics


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www.leonardo-ener .or


www.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


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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 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


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)



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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


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





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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!





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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)



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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



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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 +=



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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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Cost of bar

0

5

10

15

0 100 200 300 400 500 600

Cross sectional area (sqmm)

Costofbarpermetre($)


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Cost of losses - 5 years

0

10

20

30

0 100 200 300 400 500 600


Costoflossesover5years($)


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Economic Sizing of Busbars

Total cost - 5 years

0

10

20

30

0 100 200 300 400 500 600


Lifetimecostp

ermetre($)

Cost of bar

Cost of loss

Total


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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


[email protected]


Documents

Busbar-Design.pdf