Reduction of teeter angle excursions for a two-bladed downwind rotor using cyclic pitch control

During the design of a two-bladed turbine a very important load reducing feature is the teeter mechanism. This mechanism enables a free rigid body motion of the blades relative to the hub in such a way that bending moments between rotor and hub are removed around the teeter axis. Due to the practical issues such as the clearance between blade tip and tower, the teeter angles must be limited as much as possible. In classical literature this is recommended by using a delta-3 coupling – a changed orientation of the teeter axis that results in a direct coupling between teeter angle and blade pitch. In this paper the basic principals of the teeter mechanism as well as the influence of delta-3 coupling are briefly presented. Three alternative methods to limit the teeter excursion are presented and evaluated. Both methods are based on cyclic pitch through the pitch servo system. The first method is based on a PI-control of the measured teeter angle and the second is based on the measured teeter angle velocity whereas the third is a combination of the two. All methods enable the full load reducing facilities as the original teetering system without delta-3, but show a significant reduction of teeter angle excursion during normal operation and extreme gust situations. The interaction of the teeter based cyclic pitch controller and the teeter dynamics is investigated and tuning methods for the controllers are presented. The simulations are performed with the multibody based aero elastic code HAWC2, which is state-ofthe-art within aero elastic modelling of wind turbines. The turbine used in the simulations is a fictitious twobladed downwind edition of the pitch regulated 5MW reference turbine used in the IEA Annex 23 benchmark. The two-bladed turbine The turbine is based on the fictitious 5MW pitch regulated fictitious turbine used in the IEA Annex 23 benchmark project [1]. This turbine is a 3-bladed turbine and therefore changed into a two bladed for the investigation in this paper. The most important part of the conversion to a two-bladed turbine is to keep a constant solidity by scaling the blade chord by a factor of 1.5. This ensures that the overall performance is identical for the two turbines except for the tip loss effect [2], which is slightly higher for the two-bladed compared to a three-bladed turbine. It also enables the use of the original collective pitch controller for power regulation. Further on, the turbine is converted to a down wind configuration in order to increase the tip to tower clearance, see Figure 1. The rotor diameter is 126m and the tower height 90m. Figure 1: Illustration of original 3-bladed and corresponding 2-bladed turbine. The two-bladed turbine is in a downwind configuration. Since the blade chord is scaled by a factor of 1.5 the stiffness properties also change accordingly. If we assume that the blade is built with a main spar as illustrated in Figure 2 the section modulus is roughly speaking proportional to the thickness, height and width of the spar (3).