Surge impedance at independent transmission towers characterized by the direct method

Measurement of the surge impedance of a transmission tower by the direct method is studied with the help of 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 double-circuit transmission tower has an impedance of about 150 Omega with the traveling wave speed in the tower being equal to that of light. This value can be applied to the case of a direct lightning hit on the tower top, regardless of the type of lightning, upward or downward. (C) 1998 Scripta Technica.