Wind turbines are very vulnerable to lightning because of their height, sharp edges and remote locations often with low soil conductivity. In this paper we present numerical simulations of the impedance of a typical wind turbine grounding geometry. We analyze the effect of interconnecting grounding systems of different wind turbines. IEC TR61400-24 suggests interconnection of grounding electrodes of wind turbines through horizontal electrodes (in the form of insulated or bare conductors) to achieve low steady-state grounding resistance. The analysis takes into account frequency dependent soil parameters. It is shown that the low frequency grounding impedance is reduced by a factor of two or more. However, the reduction is lower at higher frequencies because of the interconnection wire's inductance.