Abstract: A hydraulic speed control device for limiting the speed of a wind turbine rotor by controlling the deployment of rotor braking devices includes four fluidicly interconnected elements: one or more fluid cylinders, a relief valve, an accumulator, and a check valve. Each of the fluid cylinders is mechanically coupled between a blade and an associated rotor braking device, and controls the movement of the rotor braking device between its aligned and braking positions. The relief valve opens to fluidicly connect the fluid cylinders and the accumulator when the rotational speed of the rotor exceeds a limit value. The accumulator receives fluid through the relief valve when the relief valve opens, thereby reducing the fluid pressure in the fluid cylinders and allowing the rotor braking devices to move from their aligned positions to their braking positions.