Chapter 1: Modelling Power System Components

Line Resistance

It is very well known that the dc resistance of a wire is given by



where ρ is the resistivity of the wire in Ω - m, l is the length in meter and A is the cross sectional area in m2 . Unfortunately however the resistance of an overhead conductor is not the same as that given by the above expression. When alternating current flows through a conductor, the current density is not uniform over the entire cross section but is somewhat higher at the surface. This is called the skin effect and this makes the ac resistance a little more than the dc resistance. Moreover in a stranded conductor, the length of each strand is more that the length of the composite conductor. This also increases the value of the resistance from that calculated in (1.1).

Finally the temperature also affects the resistivity of conductors. However the temperature rise in metallic conductors is almost linear in the practical range of operation and is given by




where R1 and R2 are resistances at temperatures t1 and t2 respectively and T is a constant that depends on the conductor material and its conductivity. Since the resistance of a conductor cannot be determined accurately, it is best to determine it from the data supplied by the manufacturer