Surge Impedance of Independent Transmission Tower Characterized by Direct Method

Abstract

Measurement of impedance of a transmission tower by the direct method is investigated by Numerical Electromagnetic Code (NEC-2). The arrangement of the measuring wires in the experiment influences the result, and it is possible to correct the measured tower surge impedance to that for a standard arrangement with a vertical current injection wire and a horizontal voltage measuring wire. By reviewing various experimental results in this way, it is concluded that an independent doublecircuit transmission tower has impedance of about 150Ω with the traveling wave speed in the tower being equal to that of light. This value can be applied to the case of direct hit of lightning to the tower top, regardless of the type of lightning, upward or downward.