The current distribution may be non-uniform because of another effect known as proximity effect.
Consider a two wire line as shown in the Fig. 1.
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| Fig. 1 |
Let each of the line conductor is assumed to be divided into 3 sections having equal cross sectional area. Three parallel loops are formed by the pairs xx', yy' and zz'. The flux linking loop xx' is least and it increases for the remaining loops. Thus the inductance of inner loop is less. Thus the current density is highest at inner edges of the conductor. Due to this non-uniform distribution of current, the effective conductor resistance increases. As the distance between the conductors goes on reducing, this distribution of current becomes more and more non-uniform.
For normal spacing of overhead lines this proximity effect is negligible. For underground cables this effect is significant as the conductors are located close to each other.
The proximity effect also depends on the same factors as that of skin effect.
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