The strategy for controlling the rotor speed of a variable speed wind turbine is to aim at the relation shown in the figure below between the rotational speed of the generator and the generator power. Actually, the wind turbine controller uses such a curve as a lookup table to determine the generator speed reference depending on the generator power.
The curve divides the wind turbine control strategy into 3 operation ranges:
The line A-B ensures that the minimum generator rotational speed is kept at ωmin .
The curve B-C is defined so that the wind turbine controller tracks the maximum possible power in this operation range
The line C-D ensures that the wind turbine rotational speed is limited, which is done by limiting the generator rotational speed to ωmax.
The graphic show a vertical behavior A-B , C-D also vertical and the distance from B-C a smooth curve.
The minimum rotational speed of the generator is 900 rpm in the operation range A-B. What is the value of ωmin ?