Improving wind farm output predictability by means of a soft cut-out strategy
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Improving wind farm output predictability by means of a soft cut-out strategy Ervin Bossanyi & John King
description
Improving wind farm output predictability by means of a soft cut-out strategy. Ervin Bossanyi & John King. 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 - PowerPoint PPT Presentation
Transcript of Improving wind farm output predictability by means of a soft cut-out strategy
Improving wind farm output predictability by means of a soft cut-out strategyErvin Bossanyi & John King
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
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
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)
Ramped cut-out reduces number and severity of power ramps
• Ramps in wind farm output near cut-out wind speed• Results from 1982 paper
-1.0to -0.9
-0.9to -0.8
-0.8to -0.7
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Most severe
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Normal cut-out Ramped cut-out
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
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
Ramp design (steady state)
• Ramp defined in terms of pitch angle so as not to use wind speed signal
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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.
o Large torque variations needed to provide a lot of damping, buto Small additional damping sufficient to bring down tower base Mx without too much
increase in shaft Mxo Damping torque obtained by passing the acceleration signal through a 4th order filter
Fatigue loadsFull set of fatigue load cases run to obtain lifetime fatigue loads with and without the ramp control
Bending moment Material Base case Ramp % Change
Blade root Mx (in plane) (kNm) GRP 10079.4 10092.5 0.1
Blade root My (out of plane) (kNm) GRP 8668.4 8728.4 0.7
Blade root Mz (torsional) (kNm) GRP 166.0 166.1 0.1
Hub Mx (low speed shaft torque) (kNm)
Steel1949.2 2052.9 5.3
Stationary Hub My (kNm) Steel 8569.7 8666.9 1.1
Stationary Hub Mz (kNm) Steel 8606.1 8681.8 0.9
Yaw bearing Mx (side-side) (kNm) Steel 2143.0 2253.2 5.1
Yaw bearing My (nodding) (kNm) Steel 8945.5 9034.7 1.0
Yaw bearing Mz (torsional) (kNm) Steel 9415.3 9495.8 0.9
Tower base Mx (side-side) (kNm) Steel 27414.5 28480.3 3.9
Tower base My (overturning) (kNm) Steel 55219.0 55789.1 1.0
Tower base Mz (torsional) (kNm) Steel 9415.2 9495.7 0.9
Annual energy output (GWh) - 22.9 23.1 0.9
Extreme loads (IEC 61400-1 edition 3)
Blade rootRotating Hub
Stationary Hub
Yaw bearing Tower base
Mx 0.2% 0.2% 0.2% 0.8% 39.1%
My 0.0% 0.0% 0.0% 0.0% 0.0%
Mz 0.0% 0.0% 10.1% 2.9% 2.9%
Mxy 0.0% 0.0% 0.0%
Myz 0.0% 0.0%
Fx 0.0% 0.0% 0.0% 0.0% 0.0%
Fy -3.7% -3.2% 40.9% 39.5% 14.9%
Fz 0.0% -2.7% -4.8% -1.2% -0.5%
Fxy 0.0% 0.0% 0.0%
Fyz -4.7% -4.7%
Table of ultimate loads – design-driving loads are shown in BOLD
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
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
