1 Ameliorating the Negative Damping in the Dynamic Responses of a Tension Leg Spar-Type Support Structure with a Downwind Turbine Madjid Karimirad Torgeir Moan Author CeSOS Centre Centre for Ships for Ships and and Ocean
2 Outline: Floating wind turbines Tension leg spar-type wind turbine Damping, positive or negative! Controller of a wind turbine Servo-induced response instabilities Ameliorating negative damping Remarks
3 Floating Wind Turbines
4 Tension Leg Spar (TLS) similar to SWAY Downwind
5 Challenge Floating Offshore Wind Turbine is an Aero-Hydro-Servo-Elastic Multi-Body System The coupling is inevitable in such structures as the Aerodynamic and Hydrodynamic damping and excitation forces are highly affected by each other through the relative motions. 1) Power Production 2) Structural integrity FLS, ULS, ALS and SLS Motion responses Structural dynamic response Fault conditions Influencing
6 Damping? Positive or Negative!
7 Controller for a wind turbine The pitch-regulated variable-speed wind turbine is the state-of-the-art wind machine device. Depending on the wind speed, the status of the wind turbine is divided into four regions: Pitch-regulated: Feathering the blades Variable-speed: Shaft rotational speed
8 Controller for a wind turbine 800 Operating wind turbine, active control 700 Thrust (kn) 600 500 400 300 Maximum power Constant power 200 100 0 0 5 10 15 20 25 Relative wind speed (m/sec) Shaft rotational speed Feathering the blades
9 Control-induced negative damping If relative wind speed experienced by the blades increases due to rigid body motion of the system, then, the blades will feather to maintain the rated-power. Thus, the thrust force will decrease, which will introduce negative damping for over-rated wind speed load cases. However, in fixed wind turbines since the frequency of the blade pitch controller is normally less than the frequencies associated with the relative rotor motions induced by the structural responses.
10 Control-induced negative damping, cont. Pitch-regulated: Feathering the blades Variable-speed: Shaft rotational speed Instabilities: very large motions with a wide range of frequencies
11 Tuning the controller A dynamic system defined by only one degree of freedom: the rigid body rotation of the rotor coupled to the aerodynamics and the PI-control of the blade pitch angle. Based on Newton s second law
12 Ameliorating instabilities Efficient to remove the instabilities due to the servo-induced negative damping Very important for: Fatigue limit state (FLS) Power production Drive train loads
13 Ameliorating instabilities, cont. Responses Not Tuned/ Tuned Nacelle surge 14.5 BM at blade root 2.7 BM at tower-spar 4.4 Shaft speed 5.0 Tension 1.6 Power 8.0
14 Tuning effects
15 Conclusions a) Wave-frequency responses not significantly affected by the aerodynamic and controller actions. b) Below-rated wind speed, no negative damping. c) Surge resonant response governs the responses for the rated wind speed due to Negative damping. d) Pitch resonant response is greatly affected by negative damping of the collective blade pitch controller for the over-rated wind speed region. Tuning the controller gains, negative damping could be eliminated. Decrease the resonant responses and improves the power production for the over-rated wind speed cases. At the rated wind speed, the response is governed by the surge resonance, and the tuning effect is less effective. However, for the over-rated wind speed region, because the response is governed by the pitch resonance, tuning is effective at eliminating the negative damping. Thanks for your attention.