Zero sequence behaviour of a double-circuit overhead line

• The paper deals with the zero sequence impedances in double-circuit overhead lines.
• A systematic comparison with IEC 60909-2 formulae has been presented in the paper.
• IEC 60909-2 adds zero sequence self and mutual impedances between the two circuits.
• Zero sequence mutual impedance should not be included in the self impedance.
• IEC 60909-2 has errors up to 20% in the phase-to-ground short circuit current.

Positive, negative and zero sequence series impedances (in Ω/km units) are the first step for power flow and short circuit analyses and are also necessary for setting the distance relays. The paper is devoted to the computation of zero sequence series impedance (in Ω/km units) (self and mutual) in double-circuit overhead lines (OHLs) with one or more earth wires. A systematic comparison with matrix approaches demonstrates that IEC 60909-2 formula adds the zero sequence self and the zero sequence mutual impedance between the two circuits. This approximation of having a unique value of zero sequence impedance is misleading and gives great errors when phase-to-ground short circuit occurs along a circuit. It is worth noting that the zero sequence mutual impedance between parallel circuits installed on the 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 (more than 18%, depending upon the spacing between the circuits) the phase-to-ground fault current for a fault occurrence along one circuit of the double circuit.