METHOD OF CONTROLLING A BLADE PITCH ANGLE OF A WIND TURBINE BY USE OF A HYDRAULIC SYSTEM

It is described a method of controlling a pitch angle of at least one blade of a wind turbine by use of a hydraulic system, the hydraulic system comprising at least one reservoir configured to store a hydraulic fluid, and at least one pump configured to supply the hydraulic fluid from the reservoir to at least one accumulator, if a hydraulic fluid pressure in the accumulator falls below a lower threshold value and till the hydraulic fluid pressure in the accumulator exceeds an upper threshold value. The accumulator is configured to store the pressurized hydraulic fluid supplied by the pump and to supply the pressurized hydraulic fluid to at least one pitch control cylinder of the hydraulic system via at least one output valve of the hydraulic system. The pressurized hydraulic fluid in the pitch control cylinder drives at least one piston to change the pitch angle of the blade. The method comprises steps of determining a target outflow of the pressurized hydraulic fluid from the accumulator to the pitch control cylinder; determining a difference between the determined target outflow of the pressurized hydraulic fluid from the accumulator to the pitch control cylinder and a predetermined inflow of the hydraulic fluid from the reservoir to the accumulator; and supplying the hydraulic fluid from the reservoir to the accumulator by the pump regardless whether or not the hydraulic fluid pressure in the accumulator is fallen below the lower threshold value, if the determined target outflow is larger than the predetermined inflow.