Active Control Systems for Wind Turbines Avishek Kumar Supervisor: Dr Karl Stol The University of Auckland

Overview Wind Turbines Power Extraction Traditional Control Modern Control Future of Control

Horizontal Axis Wind Turbines Source: US Department of Energy

Large Wind Turbines 5MW (1400-1500 households) 126m Blade Span 12.1 rpm Power controlled by blade pitch Onshore and offshore

Ideal turbine (max. 60% efficient) Power Capture Power [kW] Pwind  w3 Ideal turbine (max. 60% efficient) Torque Control Pitch Control Prated Region 1 Region 2 Region 3 Wind Speed, w [m/s] wcut-in wrated wcut-out Source: Dr. Karl Stol, UoA

Power Generation Control Objectives Region 1: Turbine is stopped Region 2: Maintain constant tip speed ratio to produce maximum power below rated wind speed. Region 3: Maintain rated rotor speed and power. Power Coefficent, Cp (Blade Tip Speed)/Wind Speed, λ

Classical Control Use collective pitch and/or generator torque to adjust rotor speed depending on the region. The torque and pitch controllers work separately.

Loads Become more significant as turbines get larger By reducing loads we can also decrease cost of energy by: Increasing the lifetime of turbines Reducing structural material Reducing maintenance

Loads High Winds Stochastic Winds Vertical Wind Shear and Cross Winds Inertia and Gravity Tower Interference Wind Turbulence and Gusts Source: E Hau

Modern Control Traditional Control Modern Control Single control objective Multiple control objective Single input single output Multi input multi output Controllers work separately Single centralised controller

Modern Control Objectives for the Wind Turbine Maintain Rotor Speed Keep the best tip speed ratios in Region 2 Not exceed rated velocity in Region 3 Have smooth power output Reducing DYNAMIC loading on the turbine. Blade flap Tower fore-aft vibration Drive train torsional vibration

Individual Blade Pitching With modern control (MIMO) we can control the load on each blade individually This now allows mitigation of ASSYMETRIC loading: Wind shear Tower shadow Inertial loads Turbulence across the swept area

Simulation Results Stol, Zhao, Wright (2006), Individual Blade Pitch Control for the Controls Advanced Research Turbine (CART), J. of Solar Energy Eng., Transactions of the ASME, v 128, n 4, Nov, 2006, p 498-505

Model based control tresats a nonlinear system as a linear one The Problems Model based control tresats a nonlinear system as a linear one y x

The Solution Allows control over the entire operating envelope More predictable behaviour Nonlinear Control Increase in the performance of control Increase in safety

Current State of Nonlinear Control Variety of nonlinear controllers are being explored Simulations show successful SISO power control Very little work has been conducted with multiple control objectives systems No work has been conducted (publicly) with Individual Blade Pitching

My Research What: Why: Nonlinear control Individual blade pitching Multiple control objectives Why: Reduce cost of energy More predictable turbine behaviour Safer turbine behaviour

Summary Wind turbines are getting bigger Loads are increasing cost of energy Modern control (Linear) can mitigate loads AND maintain rated power Using individual blade pitch we can mitigate ASSYMETRIC loads Modern Linear Control is only optimal about it’s operating region Nonlinear control aims to apply all the above benefits over the entire operating envelope

Questions?

Wind Energy Facts Wind accounts for currently 2% of our electricity. Global increase of 25% a year for the last 5 years. 321 MW either running or being commissioned in NZ. Current COE is 5.5-7c/kWhr (2005) New Zealand Wind Energy Association. (2007, June 28 - last update). [Online]. Available: http://www.windenergy.org.nz/ [2007, July 11]