Analysis of the Grounding for the Substation Under Very Fast Transient Using Improved Lossy Thin-Wire Model for FDTD

A thin-wire model of conductors with arbitrary cross sections for a finite-difference time-domain (FDTD) method is proposed for the study of lightning transients in electrical systems. The proposed thin-wire model considers both the frequency-dependent characteristics of the wires and the influences of the surrounding media. The model represents a wire using four FDTD cells with an equivalent side length and modified material parameters. A rational equivalent circuit is added at the center edge of the FDTD cells to represent the internal impedance of the wire. An analytical formula for determining the equivalent side length and the modified material parameters of the FDTD cells are also presented. The proposed formula gives a new prospect to build the thin-wire model in FDTD. In this model, the equivalent side length can be set arbitrarily and much larger than the original wire, which can greatly reduce the computation time. Meanwhile, the method can be extended to the thin-wire structure with arbitrary cross sections. The model is verified with various measurements. Finally, a practical grounding grid of a substation with external water supply is analyzed.