Error in propagation velocity due to staircase approximation of an inclined thin wire in FDTD surge Simulation

This paper presents the result of a study on the error in propagation velocity introduced by the staircase approximation of a thin wire in the finite difference time domain (FDTD) surge simulation. The FDTD method directly solves Maxwell's equations by discretizing the space of interest into cubic cells. Thus, it is suitable for solving very-fast surge phenomena which cannot be dealt with by conventional techniques based on the lumped- and distributed-parameter circuit theories. However, FDTD has a limitation that the shape of a conductive object must be modeled by a combination of sides of cells with forced zero electric fields. This indicates that a thin wire, one of the most important components in the surge simulation, results in a staircase approximation, if the wire is not parallel to any of the coordinate axes used for the discretization. A staircase approximation gives a slower propagation velocity due to the zigzag path which is longer than the actual length of the wire. For precise simulations, the error in propagation velocity has to be clarified quantitatively. In this paper, extensive simulations are carried out to obtain the velocity versus inclination characteristic, and it is deduced that the maximum error in propagation velocity is less than 14%.