Determination of Minimum Air Clearances for a 420kV Novel Unibody Composite Cross-Arm

One of the most important requirements of any overhead line tower is determining the air clearances between live parts and earthed parts such as phase conductor and tower structure. In contrast to traditional steel lattice towers, the recently introduced fully composite pylon is completely made of non-conductive materials and the air clearances along the unibody cross-arm of the pylon depend on corresponding insulation lengths. The first step in electrical design of fully composite pylon is dimensioning the air clearances which, are required to specify the mechanical and material designs.
This paper presents the insulation coordination studies to determine minimum required air clearances on the unibody cross-arm. The procedure and relevant equations to calculate minimum air clearances to avoid flashover between phases’ conductors as well as top phase conductor and shield wire are based on the theoretical formulae reported in EN 50341-1. Considering differences in gap types, the calculated values of air clearances are compared with the recommended values by CIGRÉ TB 72, EN 50341-1 and IEC 60071-1, 2 and as a result one empirical value and one calculated value are determined as phase-to-earth and phase-to-phase air clearances, respectively. Subsequently, internal clearances at the tower and within the span are specified according to the three load cases (maximum conductor temperature, wind and ice) given in EN 50341-3, NNAs - DK.