A power generating apparatus of a renewable energy type which generates power using renewable energy, comprising: an actuated element which is actuated by energy received from a renewable energy source; a rotating shaft which is rotated by the renewable energy received via the actuated element; a hydraulic transmission which comprises a hydraulic pump driven by the rotating shaft and a hydraulic motor driven by pressurized hydraulic fluid supplied from the hydraulic pump through a high pressure manifold; a synchronous generator which is driven by the hydraulic motor to generate power and coupled to a grid without an intervening electronic power converter, the generated power being supplied to the grid; the generator having a field coil having a field strength, a transmission controller which controls the hydraulic transmission in a normal operation mode to adjust the displacement of each of the hydraulic pump and the hydraulic motor such that the rotating shaft rotates at a rotational speed that varies with the flow speed of the renewable energy while keeping the frequency of the electricity generated by the generator on the frequency of the grid during normal operation of the power generating apparatus, wherein the transmission controller comprises: a motor demand determination unit which determines a demand Dm of the displacement of the hydraulic motor taking into account the pressure in the high pressure manifold, and also taking into account a torque limit related to the maximum torque allowable by the generator to prevent pole slip, which torque limit depends on the field strength, and measured generator terminal voltage, characterised in that the motor demand determination unit is configured to limit the rate of change of the displacement of the motor. an actuated element which is actuated by energy received from a renewable energy source; a rotating shaft which is rotated by the renewable energy received via the actuated element; a hydraulic transmission which comprises a hydraulic pump driven by the rotating shaft and a hydraulic motor driven by pressurized hydraulic fluid supplied from the hydraulic pump through a high pressure manifold; a synchronous generator which is driven by the hydraulic motor to generate power and coupled to a grid without an intervening electronic power converter, the generated power being supplied to the grid; the generator having a field coil having a field strength, a transmission controller which controls the hydraulic transmission in a normal operation mode to adjust the displacement of each of the hydraulic pump and the hydraulic motor such that the rotating shaft rotates at a rotational speed that varies with the flow speed of the renewable energy while keeping the frequency of the electricity generated by the generator on the frequency of the grid during normal operation of the power generating apparatus, wherein the transmission controller comprises: a motor demand determination unit which determines a demand Dm of the displacement of the hydraulic motor taking into account the pressure in the high pressure manifold, and also taking into account a torque limit related to the maximum torque allowable by the generator to prevent pole slip, which torque limit depends on the field strength, and measured generator terminal voltage, characterised in that the motor demand determination unit is configured to limit the rate of change of the displacement of the motor. a motor demand determination unit which determines a demand Dm of the displacement of the hydraulic motor taking into account the pressure in the high pressure manifold, and also taking into account a torque limit related to the maximum torque allowable by the generator to prevent pole slip, which torque limit depends on the field strength, and measured generator terminal voltage, characterised in that the motor demand determination unit is configured to limit the rate of change of the displacement of the motor.