FDTD simulation considering an AC operating voltage for air-insulation substation in terms of lightning protective level

The accuracy of lightning overvoltage evaluation of electrical equipment has been improved based on circuit analysis such as the Electro-magnetic Transients Program, but analyzing phenomena whose plane wave propagation (TEM) mode cannot be assumed remains unsolved. Since numerical analysis of electromagnetic fields such as the finite-difference time-domain (FDTD) method directly solves Maxwell´s equations, it is an appropriate means for solving this issue. In this paper, the lightning surge waveform due to the back-flashover is analyzed using the FDTD method; bearing detailed modeling of transmission towers and incoming substation lines in mind. An open end of a circuit breaker of a 500 kV air-insulated substation was selected as the subject of analysis, and ac operation voltage was considered as an analysis condition. Following FDTD analysis, a 150 kA lightning stroke to the first tower, which is the standard lightning stroke of the lightning protection design, does not cause back-flashover, but a 200 kA lightning stroke to the first tower and a 150 kA lightning stroke to the second did do so. The flashover occurred in the later timing than the result of conventional circuit analysis, and the rise of the lightning surge was also slower. The voltage waveform of the open end of the circuit breaker reached its peak after repeating reflections, and the overvoltage value became lower than the circuit analysis. According to the FDTD analysis, the required withstand voltage value after the waveform evaluation with a 200 kA lightning stroke is 1,337 kV, which is lower than the present lightning impulse withstand voltage (LIWV) 1,550 kV. The lightning stroke current 200 kA is the value adopted in the lightning protection design of the UHV system. Therefore, the existing air-insulated substation equipment is considered highly capable of withstanding overvoltage at the time of a back-flashover.