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1. Abstract This paper describes a method for deriving a transmission tower model for EMTP lightning overvoltage studies from a numerical electromagnetic simulation result obtained by the FDTD (Finite Difference Time Domain) method. The FDTD simulation carried out in this paper takes into account the following items which have been ignored or over-simplified in previously-presented simulations: (i) resistivity of the ground soil; (ii) arms, major slant elements, and foundation structures of the tower; (iii) development speed of the lightning return stroke. For validation purpose a pulse test of a 500-kV transmission tower is simulated, and a comparison with the measured result shows that the present FDTD simulation gives a sufficiently accurate result. Using this validated FDTDbased simulation method the insulator-string voltages of a tower for a lightning stroke are calculated, and based on the simulation result the parameter values of the proposed tower model for EMTP studies are determined in a systematic way. Since previously-presented models include trial-and-error process in the parameter determination, it can be said that the proposed model is more general in this regard. As an illustrative example, the 500-kV transmission tower mentioned above is modeled, and it is shown that the derived model closely reproduces the FDTD simulation result. 2. Main Results Figure 1 shows the simulation arrangement used in an FDTD simulation for obtaining the tower current IT , the ground-wire current IG, and upper-, middle-, and lower-phase insulator-string voltages VH1, VH2, and VH3. The simulation was carried out under a realistic condition as mentioned in the abstract above. The calculated result is shown in Fig. 2.

A Tower Model for Lightning Overvoltage Studies Based on the Result of an FDTD Simulation