Influence of Highly Resistive Ground Parameters on Lightning-Induced Overvoltages Using 3-D FDTD Method

It is important for the proper insulation design of the distribution system that lightning-induced overvoltages ( LIOVs) are accurately computed. This paper investigates the impact of ground resistivity, considering wide range up to 20 kΩm, on LIOVs impinging an overhead line due to nearby return stroke using the three-dimensional finite difference time domain (3-D FDTD) method. The investigation considers two values of both ground permittivity as well as the rise rate of lightning current. Subsequently, it is inferred that the influence of both ground permittivity and rise rate of lightning current on the peak values of LIOVs depends on the ground resistivity. Furthermore, horizontal conduction and displacement current densities are also calculated on the ground surface under the line to analyze the behavior of ground surface impedance. Consequently, it is deduced that the behavior of the impedance changes from inductive to capacitive at high values of ground resistivity, thus increasing the time to the peak value of LIOVs. The peak values of LIOVs computed using the 3-D FDTD method are compared with those calculated using Darveniza's formula. It has been revealed that this formula considerably underestimates the peak values of LIOVs at high values of ground resistivity. Thereby, an interpretation is presented for the reason of this underestimation from an electromagnetic perspective. Accordingly, Darveniza's formula is appropriately modified to improve the computation accuracy.