Zero sequence behaviour of a double-circuit overhead line

tPositive, negative and zero sequence series impedances (in /km units) are the first step for power flowand short circuit analyses and are also necessary for setting the distance relays. The paper is devoted tothe computation of zero sequence series impedance (in /km units) (self and mutual) in double-circuitoverhead lines (OHLs) with one or more earth wires. A systematic comparison with matrix approachesdemonstrates that IEC 60909-2 formula adds the zero sequence self and the zero sequence mutualimpedance between the two circuits. This approximation of having a unique value of zero sequenceimpedance is misleading and gives great errors when phase-to-ground short circuit occurs along a cir-cuit. It is worth noting that the zero sequence mutual impedance between parallel circuits installed onthe same tower may be as high as 50% (or more) of the zero sequence self impedance of a circuit alone.Consequently, zero sequence mutual impedance between the two circuits cannot be neglected or mixed.MCA (Multiconductor Cell Analysis) shows that the simplified IEC approach underestimates heavily (morethan 18%, depending upon the spacing between the circuits) the phase-to-ground fault current for a faultoccurrence along one circuit of the double circuit.