A STUDY ON MOTION CHARACTERISTICS OF WIND TURBINE ON A FLOATING PLATFORM IN BLADE PITCH CONTROL MALFUNCTION

Wind power generation has been paid much attention all over the world with the increasing crisis awareness of climate change mainly caused by excessive fuel consumption. For real wind turbine, each blade pitch angle is controlled not only to keep output power constant depending on fluctuation of wind speed but also to avoid the failure of generating system due to over speed of the rotor rotation. Nowadays, a few demonstration projects for Floating Offshore Wind Turbine (FOWT) are under way. It is essential to improve the safety of FOWT. For that, it will be more required to conduct tank test with scale model including control system so as to provide the behavior similar to the actual FOWT. However, almost every institute has never incorporated control system in scale model. This is because every institute faces to the difficulty of incorporating control system in scale model in terms of weight. The larger FOWT would be from now on, the more necessary it is to lighten scale model in consideration of the capacity of existing facilities. Therefore, we conducted 1/100th- scale model tests with control system using a geometrically scaled model of the National Renewable Energy Laboratory (NREL) 5MW reference wind turbine. As for this model, a new control system consisting of bevel gears was developed. Owing to this system, the mass of the scaled turbine is 41% lighter than the conventional one. In addition, this control system realized the shortening of control period in reference to scaling down of the actual control period. This tank test showed this control system is enough effective to keep rotor rotation frequency. Moreover, this paper clarifies the motion of FOWT in control malfunction. If blade pitch angle abruptly changes due to the failure of control system, the number of the rotor rotation increases and floating platform inclines greatly. This great inclination causes the sudden rise of mooring tension and bending moment on the base of tower. This phenomenon jeopardizes the base of tower or the mooring line. We call this phenomenon "Pitch Error". This is because of the rapid increase of thrust. In addition to this, the influence of nacelle's inertia induces the inclination of FOWT. In case of offshore, we have to pay attention to nacelle's inertia caused by inclination because the base of tower is not fixed. FOWT would be more severe than onshore in control malfunction. Finally, how dangerous Pitch Error would be for real FOWT is discussed.