Influence of Ground Electrical Properties on Lightning-Electromagnetic Fields for Wind Farms and Overhead Lines - Applications of the FDTD Method

Direct and indirect lightning events result in transient disturbances in electric power systems that may cause damages of the components of the power system and interruptions of the power supply. The resistivity and permittivity of the ground have a significant influence on these transient disturbances due to both lightning events. In order to study this influence, five research tasks have been implemented using the finite-difference time-domain method. The 1st and 2nd tasks are relevant to indirect lightning problems where lightning-induced voltages (LIVs) on overhead lines have been investigated due to nearby return strokes on the ground. The 3rd and 4th tasks are related to direct lightning problems where the electromagnetic fields have been studied due to direct stroke to a grounding system and a grounded wind turbine (WT) considering the impact of the horizontal stratification of the ground. The 5th one includes both direct and indirect lightning problems where the electromagnetic fields are studied for a grounded WT struck by a return stroke beside a nearby overhead line.For a proper design of the overhead line insulation, it is required to accurately compute LIVs. The influences of the ground resistivity, permittivity and the rate of rise of lightning current are investigated on the lightning electromagnetic fields and LIVs impinging overhead lines. A formula has been proposed to calculate the peak values of LIVs due to the typical first and subsequent strokes in the IEEE Standard 1410-2010 considering high values of ground resistivity. The proposed formula is applied to calculate the indirect lightning performance in terms of an annual number of flashovers per 100 km of the overhead line. The grounding system is essential for effective lightning protection against direct strokes so the effect of ground resistivity and permittivity on the impedance of grounding systems has been studied. Such study is extended to investigate the lightning-electromagnetic fields on the cable sheath in a wind farm due to a lightning stroke striking a grounded WT. The impact of connecting the grounding systems of the WTs on mitigating these electromagnetic fields has been also investigated. For the horizontally stratified ground case, it is found that the reflections at the boundary between the two layers affect these electromagnetic fields significantly. Since overhead lines may exist nearby WTs, the influence of ground resistivity and permittivity on LIVs and electric fields impinging the line is studied due to a return stroke striking a nearby grounded WT. The first and subsequent strokes are adopted for this study besides the consideration of the horizontally stratified ground case.