How does ring earth electrode effect to wind turbine?

This report performs an electromagnetic transient analysis on an earthing system of a wind turbine to confirm the effect of a ring earth electrode. The main aim of the present analysis is to search the accurate combination of the ring earth electrode(s) and the vertical rods. As described in IEC TR61400-24, the ring earth electrode is recommended for wind turbine earthing. The technical report recommends additional electrodes if the radius of the turbine foundation is less than the minimum radius stated in IEC 61024-1 (presently IEC 62305-3). But the additional electrodes given by IEC 62035-3 are originally considered for larger earthing system of houses or buildings. Then, is it really suitable for a wind turbine with smaller foundation area? What is the best combination of the ring electrode and vertical rods to realise the effective lightning protection for a wind turbine? These questions need to be immediately resolved to establish safe countermeasures of wind turbines against lightning incidents. As the result of the analysis using finite difference time domain (FDTD) method, it becomes clear that there exist inaccurate combination of ring electrode and vertical rods even if it obeys IEC 62035 recommendation. It suggests that requirement level of earthing in IEC 61400-24 should be improved for accurate lightning protection of wind turbines.