Measured and simulated impulse responses of the grounding systems of a pair of wind-turbines connected by a buried insulated wire

•Prediction method of ground characteristics of wind turbines based on numerical electromagnetic field analysis method.•From the analysis, highly accurate transient grounding characteristics of wind turbines can be guessed.•It was possible to accurately acquire the characteristics of the lightning current flowing from the wind turbine to another wind turbine during a lightning strike. In recent years, it is becoming more common to replace old wind turbines and build new wind turbines with larger capacities in Japan. In the grounding design when replacing wind turbines, wind-turbine owners often utilize a part of the grounding electrodes of the conventional wind turbines to achieve lower grounding resistance with low cost. In addition, in order to make the grounding resistance value of the wind turbine lower than 10 Ω specified by IEC 61400–24, the grounding systems of multiple wind turbines are often connected. The ratio of the cost of constructing a grounding system of a wind turbine is not low, the cost is increasing year by year. Therefore, many companies involved in the design of grounding systems are beginning to use numerical electromagnetic field analysis methods in their designs in order to reduce costs. In this paper, measured and simulated impulse responses of the grounding systems of two single wind turbines and a pair of wind turbines connected by a buried insulated wire are introduced, and its characteristics are considered. In the analysis using the FDTD method, it is difficult to take into account the frequency-dependent characteristics of the soil because Maxwell's equations are time-differenced to formulate the equations used in the FDTD method. To construct a highly accurate analysis model, it may be necessary to build a model that takes into account the frequency-dependent characteristics of the soil. The method to construct such frequency-dependent model is also introduced in this paper.