Stuttgart Wind Energy (SWE) @ Institute of Aircraft Design Evaluation of control methods for floating offshore wind turbines Wei Yu, Frank Lemmer, David Schlipf, Po Wen Cheng, Bart Visser, Harmen Links, Neelabh Gupta, Sabrina Dankemann, Bernardino Counago, Jose Serna
Background & Motivation EU Horizon project: TELWIND Cost reduction for floating offshore turbine Evolved spar concept Telescopic tower Local and low cost material usage: Concrete Simpler manufacturing and installation processes How great is the impact of on FOWTs? What makes controlling FOWTs difficult? [esteyco] How well do the state-of-art control methods work? University of Stuttgart, Stuttgart Wind Energy (SWE) @ Institute of Aircraft Design
What makes controlling FOWTs difficult? Physical: Negative aerodynamic damping Proportional gain: Kp Integral gain: Kp/Ti Applying conventional on-shore to FOWT leads to the instability problem Larsen, T. J., and Hanson, T. D., 7. A method to avoid negative damped low frequent tower vibrations for a floating, pitch controlled wind turbine. Journal of Physics: Conference Series, 75(1), p. 173. University of Stuttgart, Stuttgart Wind Energy (SWE) @ Institute of Aircraft Design 3.1.18 3
What makes controlling FOWTs difficult? Control theory: Right-half-plane-zero (RHPZ) Blade-pitch Transfer Function G(s) Gen-speed RRRRRRRRRR K(s) Wind turbine G(s) ss GG ss = ss.3. Open-loop transfer function pole-zero stable unstable Closed-loop with different gains at 16m/s.3 stable. unstable.1 1.1 j [s -1 ] σσ + jjjj 5 Ae t sin( t+ ) j [s -1 ] low gain median gain high gain -.1-5 5 1 15 -.1-5 1 15 -. -. -.3 -.3 -. -.1.1..3 [s -1 ] -.3 -.3 -. -.1.1..3 [s -1 ] University of Stuttgart, Stuttgart Wind Energy (SWE) @ Institute of Aircraft Design 4
How good do the state-of-art s work? Selection of theoretical methods Different control methods used for FOWT by modifing : Single-input-single-output (SISO): Detuning / scheduled detuning Multi-input-single-output (MISO): Ptfm damper - feedback of Ptfm-Pitch to Blade-Pitch Multi-input-single-output (MIMO): Compensator - feedback of Ptfm-Pitch to Generator torque Evaluation tool: + Ptfm Damper ββ Linear analysis: simplified linear mdoel with 5 DOF (SLOW) Coupled aero-hydro-servo-elastic nonlinear model (Bladed v4.7) TT gg Compensator ββ University of Stuttgart, Stuttgart Wind Energy (SWE) @ Institute of Aircraft Design 5
SISO: Detuning Simple approach.8.6 on-shore Detuning Kp [rad/(rad/s)].4. 1 14 16 18 4 1DOF Drivetrain: second order differential system Wind speed [m/s] II ddrrrrrrrr φφ + MM aaaaaaaa KK PP φφ + MM aaaaaaaa KK PP TT ii φφ = 1 Eigen-frequency of the drivetrain motion should be lower than the Ptfm eigen-frequency Ti [s] Detuning method could lead to negative gains at higher wind speed -1 1 14 16 18 4 Wind speed [m/s] University of Stuttgart, Stuttgart Wind Energy (SWE) @ Institute of Aircraft Design 6
SISO: Detuning Scheduling at different wind speeds Closed-loop with different gains at 16m/s.3. stable unstable Stable with higher gain at 4m/s.8.6 on-shore Detuning j [s -1 ].1 -.1 low gain median gain high gain Closed-loop with different gains at 4m/s.3. stable unstable Kp [rad/(rad/s)].4. Detuning scheduled.1 -. -.3 -.3 -. -.1.1.-.1.3 j [s -1 ] low gain median gain high gain 1 14 16 18 4 Wind speed [m/s] [s -1 ] -. 1 -.3 -.3 -. -.1.1..3 [s -1 ] Ti [s] RHPZ problem differs from the operating wind speed, thus detuning should be applied according to the operating point -1 1 14 16 18 4 Wind speed [m/s] University of Stuttgart, Stuttgart Wind Energy (SWE) @ Institute of Aircraft Design 7
5 SISO: Detuning Trade-off between system stability and control performance 1 1 3.5.1 3.7 15.8.15 5 1. Gm[dB] Pm[deg] cl frequency cl damping 3 1 high stability megien stability low stability TT ii Ti 8 6 4 3.15. 15.5 15 3 1 3 5 1.6.5 5. 15 3.5 3 3 1 3.6.7.8.5 5 15 3.5 15 15 4 6 8 1 1 14 KK pp kp 1-3 1 3.6.7 3.8.5.15.1.5 1 5 1 4 Higher stability is at the cost of the control performance. 1-1 -1 1 University of Stuttgart, Stuttgart Wind Energy (SWE) @ Institute of Aircraft Design 8
j [s -1 ] MISO: Feedback of Ptfm-Pitch to Blade-pitch How does it work? 6 Closed-loop with different dampers at 16m/s 4.3..1 stable unstable no damper soft damper median damper hard damper -.1 -. -.3 -.3 -. -.1.1..3 [s -1 ].1.5 1 16 6 4 soft damper medien damper hard damper Ptfm damper ββ Ptfm-damper can increase the pitch stability, however the trade-off between stability and control performance still exist. 1-1 -1 1 University of Stuttgart, Stuttgart Wind Energy (SWE) @ Institute of Aircraft Design 9
.1 MISO: Feedback of Ptfm-Pitch to Blade-pitch Problem with wave PtfmPitch velocity without wave [rad/s] Ptfm damper ββ.5 Bode plot of the Ptfm-pitch velocity filter -.5 original filtered -.1 3 35 4 45 5 55 6 65 7 Time [s] PtfmPitch velocity with wave [rad/s] Magnitude [db] - -4 low bandwidth medien bandwidth high bandwidth.1 -.1 original filtered 4 4 44 46 48 5 5 54 56 58 6 Time [s] Phase [deg] 5 ptfm-pitch wave -5 1-3 1-1 -1 1 1 1 Frequency [rad/s] Due to the difficulty on filtering out the signal in wave frequencies, Ptfm Damper doesn t work well for Ptfms with pitch eigen-frequency close to the wave frequencies, University of Stuttgart, Stuttgart Wind Energy (SWE) @ Institute of Aircraft Design 1
j [s -1 ] MIMO: Feedback of Ptfm Pitch to Gen Torque How does it work? 6 low cmps medien cmps 4 Closed-loop with different Compensators at 16m/s high cmps.3 stable unstable..1 no compensate low compensate medien compensate high compensate -.1 -. -.3 -.3 -. -.1.1..3 [s -1 ]..1 1 15 15 1 5 1-1 -1 1 TT gg Compensator ββ A RHPZ- Compensator can solve the trade-off problem by moving the positive zero to the left s-place, however will increase the maximum loads on the generator torque. University of Stuttgart, Stuttgart Wind Energy (SWE) @ Institute of Aircraft Design 11
How great is the impact of on FOWTs? Ptfm damper ββ Wind: [1, 16,, 4] m/s, IEC3-A class 4 3 15 1 Wave: Hs 5.7 [m], Tp 11.5 [s] TT gg Compensator ββ 5 1 3.3 Simple detuning.5 1.5..1 Scheduled detuning Scheduled+Damper Scheduled+Compensator 1 6 1 5 55 5 1 45 1 16 4 1-1 -1 1 1
Conclusion System motions and loads are strongly influenced by the. These can be significantly reduced by a well designed. Additional loops can improve the control performance. However, all of the state-of-art approaches have drawbacks. Improvement of control performance in wave frequency region is difficult with current sensor and actuators. University of Stuttgart, Stuttgart Wind Energy (SWE) @ Institute of Aircraft Design 13
Thank you! Wei Viola Yu e-mail yu@ifb.uni-stuttgart.de phone +49 () 711 685-684 fax +49 () 711 685- University of Stuttgart