A wind turbine power production system with a closed loop hydrostatic transmission system for the transfer of mechanical energy from a wind turbine rotor to an electric generator. The hydrostatic transmission system comprises a closed loop with a pump and a motor connected by tubes or pipes. The assembly of the wind turbine rotor and the pump is arranged for rotating about a vertical axis above a swivel arranged for transferring fluid to and from the motor, fluid reservoir and other components arranged below the swivel.