A renewable energy extraction device, such as a wind turbine generator includes a turbine driving a hydraulic pump and a variable displacement hydraulic motor driving an electrical generator connected directly to an electricity grid. The hydraulic motor employs electronically controlled valves operated to select the net displacement of working chambers of the hydraulic motor on each successive cycle of working chamber volume. In the event of an electric grid fault causing the maximum absorbable torque of the electrical generator to collapse, the electronically controlled valves are controlled to substantially reduce the rate of displacement of working fluid by the hydraulic motor, rapidly reducing the torque exerted on the generator rotor. This has the benefit of avoiding pole slip which could otherwise cause serious damage. During the fault, the rate of displacement of working fluid by the hydraulic motor is controlled to maintain the phase and frequency of rotation of the generator rotor in synchrony with the electricity grid so that electricity generation can resume rapidly once the grid failure is rectified. Excess working fluid displaced by the hydraulic pump is stored in an accumulator. When the maximum amount has been stored pressurised fluid is discharged through a throttle to avoid damage but maintain pressure within the hydraulic transmission so that electricity generation can resume rapidly if the grid failure is rectified. If the fault persists, the turbine blades are feathered to reduce power input and if the fault persists for a further period of time, the energy extraction device shuts down.