Reducing rotor speed variations of floating wind turbines by compensation of non-minimum phase zeros

Applying a land-based designed pitch controller on a floating wind turbine may cause severe instability. A common strategy to overcome this problem is to reduce the closed-loop bandwidth of the pitch control system. In so doing, the generator speed variation increases possibly leading to shutdowns due to overspeeds. This study uses a parallel path modification to avoid instability without increasing the generator speed variation. The results of comprehensive simulations and load calculations carried out on the 5MW NREL offshore wind turbine installed on the OC3-Hywind spar buoy are presented. These demonstrate that by using the proposed method it is possible to apply the land-based designed pitch controller on its floater-based equivalent.