Large scale access to wind farms would have a significant impact on the static voltage stability of power grid. Taking the extended single machine infinite bus system with direct drive wind turbines as an example, the equivalent model of the system with direct drive wind turbines was analyzed by two static power flow calculation methods, and the analytical formula of bus voltage at the grid connected point of the system was derived after wind power was connected. The simulation models of the extended single machine infinite bus system with direct drive wind turbines and the actual power grid system were established in BPA. The theoretical calculation results and simulation results were compared. The permeability of wind power, the grid connected position of wind turbines, the load access ratio and the load access position of the bus voltage at the grid connected point were analyzed under different control modes. The influences of the factors on the static voltage stability of the system were discussed.