CALCULATION OFELECTRO DYNAMICFORCES ON BUSBARS IN LV SYSTEMS

In distribution boards copper busbars and busbar supports are used. These two forms acomplete busbar system is categorized as below:

• According to the shape of conductor• According to the desing of each phase• According to the copper busbar quantity per phase• According to the type of insulating support (rool, comb, barrel type supports)

The distance between supports must be calculated according to the short circuit (kA) value, during the busbar design.

1) GENERAL INFORMATIONS BEFORE CALCULATION

• Shape and dimesions of the busbar (meter)• n = number of busbars per phase• Isc rms value = effective short circuit• Fault will be 2 phase short circuit or 3 phase short circuit• dph = distance between two different phase centers (meter)• Configuration of a busbar β = β1 all busbars belongs to the same phase β = β2 one busbar belong to the one phase• Elastic limit of copper, Rp0,2 (N/m2) Rp0,2 = 125x106 N/m2 (for 1050 alluminium) Rp0,2 = 250x106 N/m2 (for copper)• Mechanical withstand of support Rm (N/m2), • Cross-section of support subjected to tensile stress Sm (m2).

2) CALCULATION OF FORCES

Each conductor is effected by the forces below:• Forces belong to the other phases• Forces belong to the same phase conductorAt first, we calculate forces of 2nd and 3rd phases on the 1st phase conductors:

F1/ℓ = Cfault x 2.10-7 x k1 (2,2 Isc )2 x 1

dph

0,87 if it is 3 phase short circuit 1 if it is 2 phase short circuit

You can use Dwight table to find k1 coefficient by using and values

1. phase 2. phase 3. phase

d12 a d1 ℓ1

b h

d13 dph

Drawing 1 - Busbar Design

b = h

k1 = f (h, ℓ1, dph)

k2 = f (a, b, d1)

Now, we calculate the forces between bars of the same phase:

F2 / ℓ = Ʃi F21,i / ℓ i = 1, 2, 3,....n

F21,i / ℓ = 2.10 -7 k21,i

(2,2 Isc/n)2 • 1

d1

Example; k21,2 = ( d12 , b )

a a k21,3 = ( d13 , b )

a a

dph a

ba

from Dwight table

3) CALCULATION OF THE DISTANCE BETWEEN SUPPORTS

σc = + 8Ƶ 8Ƶ0

d1 = distance between two busbar support (m)

Ƶ0 = resistance module of a bar in (m3)

β1 = 0,73 simple support coefficient β2 = 0,5 flush mounting coefficient

σm = 1,5 Rp0,2 max stress that busbar can withstand

Ƶ0 = ba2

Z = nƵ0 6

d1

2 = σm

d2 = Rm.Sm

σc α (F1/ℓ)

d < min (d1, d2) must be selected.

β1 (F1/ℓ) d12 β2 (F2/ℓ) d1

2

a

a

a

b

b

b

b0.2

0.3

0.4

1.2

1.1

1.3

1.4

0.5

0.6

0.7

0.8

0.9

1

k

Çizelge 2 - k sabitinin b/a ve d/a değerlerine göre seçimi( Dwight çizelgesi )

2 3 4 5 6 8 10 20 40 60 80 100 200 300

1

0.5

0.01....0.2ab

2

5

10

1520

3040 50 60

8010

0

Drawing 2 - Dwight coefficients Table