Improving wind farm output predictability by means of a soft cut-out strategy

1. Improving wind farm output predictability by means of a soft cut-out strategy Ervin Bossanyi & John King

2. Contents • High wind cut-out strategies • Advantages of soft cut-out • Baseline turbine for study • Ramp design (steady state) • Dynamic control design • Fatigue loads • Extreme loads • Conclusions

3. High wind cut-out strategies • Conventional strategies: • Cut-out wind speed (normally 25 m/s) • Cut-out triggered when one of several conditions occurs, for example: • [10-minute] average nacelle anemometer wind speed reaches [25 m/s] • [5-second] gust on nacelle anemometer reaches [35 m/s] • [60-second] average pitch demand exceeds a certain value • etc … • Gentle ramp in pitch/speed/power to reach idling state • Cut back in when [e.g. 5-minute] average nacelle anemometer wind speed falls below [e.g. 23 m/s] (gentle start-up ramp) • Optimisation of parameters […] is a trade-off between energy capture and loads incurred

4. Advantages of soft cut-out • Power curve sharp cutoff at 25 m/s leads to rapid shutdown of whole wind farms in high winds. • Power output predictability is very poor when wind speed is predicted to be around 25 m/s. • Gradual shutdown gives better predictability: • More efficient deployment of backup generation • Power generated has higher value • Could be vital at large penetrations • Not a new idea (1982)

5. Ramped cut-out reduces number and severity of power ramps • Ramps in wind farm output near cut-out wind speed • Results from 1982 paper Normal cut-out Ramped cut-out

6. Baseline turbine for study • Study uses generic 5MW UPWIND reference turbine (126m diameter) with “state-of-the-art” controller: • Peak Cp tracking in low winds up to maximum speed of 12.1 rpm • Speed regulation at 12.1 rpm using torque (below rated) and collective pitch (above rated) • Damping of drive train resonance by modifying torque demand • Combined torque and pitch control to ensure smooth transitions at rated • Fore-aft tower damping by collective pitch in response to measured acceleration • 1P individual pitch control to reduce asymmetric loads especially on rotating components • 2P individual pitch control to reduce 3P fatigue loads on non-rotating components IEC 61400-1 Ed.3 Class 1A wind: 10 m/s mean, Rayleigh distribution

7. Ramp design (steady state) • Ramp power to reach zero at 35 m/s (approximately linear ramp) IEC Class 1 wind: mean 10 m/s, Rayleigh distribution: • 25 m/s exceeded 64.7 hours/year • 35 m/s exceeded 0.6 hours/year • Ramp achieved by reducing both speed and torque, allowing a ‘speed exclusion zone’ to prevent tower excitation by 3P at 47% of nominal speed • Speed reduced to 60% of nominal • Torque then reduced as speed held constant • Speed then decreased quickly to 40% by increasing torque • Speed and torque then reduced together until zero power • Ramp optimised to minimise any load increase or energy decrease • Start of ramp reduced to 23 m/s • Worthwhile to reduce loads • Still achieves more energy capture than without ramp

8. 1.2 1.2 Speed Torque Power Speed Torque Power 1 1 0.8 0.8 Fraction of nominal value Fraction of nominal value 0.6 0.6 0.4 0.4 0.2 0.2 0 0 0 10 20 30 40 0 20 40 60 80 100 Wind speed Pitch angle (deg) Ramp design (steady state) • Ramp defined in terms of pitch angle so as not to use wind speed signal

9. Dynamic control design • Some modifications to the control loops were required • Weighted average used to ramp the upper torque limit from ‘constant power’ towards ‘constant torque’ in high winds (weighting defined as a function of pitch angle). Constant power strategy makes no sense when speed and torque are very small. • Tower side-side vibration becomes severe in high wind due to large pitch angles: some damping introduced by further modification to generator torque in response to measured side-side acceleration. • Large torque variations needed to provide a lot of damping, but • Small additional damping sufficient to bring down tower base Mx without too much increase in shaft Mx • Damping torque obtained by passing the acceleration signal through a 4th order filter

10. Fatigue loads • Full set of fatigue load cases run to obtain lifetime fatigue loads with and without the ramp control

11. Extreme loads (IEC 61400-1 edition 3) Table of ultimate loads – design-driving loads are shown in BOLD

12. Extreme loads: Is this a fair comparison? • Ramp case is operating in 25 – 35 m/s range, where Base case is idling • Ramp case experienced severe loads operating in this region with the extreme turbulence model & emergency shutdown load cases • Base case idling runs not suitable with extreme turbulence model in this region; this would produce larger ultimate loads for Base case, making the Ramp case look more advantageous • Highlights uncertainty in the way standards should be applied to ensure a fair comparison

13. Conclusions • Ramped cut-out strategy proposed to achieve less variability and better predictability of output in high wind conditions • Ramp shape (speed and torque reduction) designed to prevent tower excitation • Careful design of closed loop controller required • Particularly to manage increased side-side vibration • Proposed strategy is feasible • Little change to design load envelope (fatigue & extreme) • No loss of energy output – actually a slight increase • This provides headroom for further optimisation of ramp shape • Comparison of extreme idling and power production loads above the normal cut-out needs careful consideration • Allows increased wind energy penetration, and higher value for the generated energy

14. Thank you for your attentionervin.bossanyi@gl-garradhassan.comjohn.king@gl-garradhassan.com