Numerical electromagnetic field analysis on measuring methods of tower surge impedance

The dependence of the tower surge impedance on the measuring methods is studied with the help of NEC-2. The tower surge impedance measured by the direct method varies dependent on the arrangement of the current lead wire. The arrangement of a vertical current lead wire and a horizontal voltage measuring wire gives about 20% higher value than that measured with a horizontal current lead wire in extension to a voltage measuring wire. The former gives about 150 Ω for a typical double-circuit transmission tower and it is almost equal to the value given by the estimating method recommended by the IEEE Working Group. The tower surge impedance evaluated from the current splitting ratio at the tower top is more than 20% higher than that characterized by the direct method when the current is injected into the connecting point from a simulated vertical lightning stroke. On the other hand, the tower surge impedance evaluated by the refraction method is about 10% lower than that characterized by the direct method