As today’s utility-scale wind turbines become taller to reach 

wind resources found at greater heights, their structures 

are becoming more complex and their components more 

flexible and lighter weight. As the components become more 

flexible, it is imperative to control the way they move and interact 

to prevent damage and possible system failures. 

Many large commercial turbines still use classical controllers 

based on a single input and a single output. While adequate for 

controlling the “stiff” machines of the past, these methods are 

inadequate for stabilizing future multimegawatt turbines. The 

challenges facing today’s wind turbine designers is to capture the 

maximum amount of energy with minimal structural loading for 

minimal cost. 

To meet these future challenges, researchers at NREL’s National 

Wind Technology Center (NWTC) are studying conventional 

turbine component controls such as blade pitching, new 

components such as twist-coupled blades, and advanced devices 

such as micro-tabs to develop innovative rotor control strategies 

to mitigate unwanted aerodynamic loads at the rotor hub.

Flow visualization tests conducted in the NASA Ames wind tunnel using smoke emitted from 

the tips of the turbine helped researchers determine the extent of the wake under a limited 

set of conditions. NREL/PIX 09996

Design of control algorithms for wind turbines must account for 

multiple control objectives. Wind turbines are complex, nonlinear, 

dynamic systems forced by aerodynamic, gravitational, inertial,

centrifugal, and gyroscopic loads. Turbine rotors are subjected 

to a complicated 3-D turbulent wind inflow field with embedded 

coherent vortices that drive fatigue loads and reduce the turbine’s 

lifetime. Multimegawatt turbines require active control and 

damping systems that mitigate fatigue loads, maintain stability, 

and allow maximum energy capture in a complex environment.

Controls Advanced Research Turbines (CART)

The test facilities at the NWTC include two controls advanced 

research turbines (CARTs). Testing new control schemes on these 

machines is a critical step before new control systems can be 

implemented in commercial machines.

The two turbines are horizontal-axis Westinghouse WTG-600 

models that were formerly deployed in a commercial wind farm 

Systems and Controls Analysis and Testing

Harvesting more wind energy with advanced controls technology

NWTC researcher works in the hub of the 3-bladed Advanced Controls Wind Turbine at the 

NWTC. Photo by Lee Jay Fingersh, NREL/PIX 14902

National Wind Technology Center

NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.
in Hawaii for 10 years. After the turbines were shipped to the 

NWTC, engineers installed modifications that enabled them to use 

the turbines to test new control schemes for reducing loads on 

wind turbine components. 

Modifications include: 

•	 A new pitch system: The original hydraulically actuated pitch

system was replaced with a high-speed electromechanical 

pitch system that enables high bandwidth independent pitch 

control of the blades. 

•	 A new generation system: The new generation system

incorporates power electronics that make it possible to 

control the turbine in either full variable-speed mode or 

as a constant-speed machine. The new control system allows 

almost complete flexibility in the type of control that can be 

implemented. 

•	 Added custom instrumentation: A variety of instrumentation

was added to allow a high degree of flexibility in the type of 

control algorithm that can be implemented. 

•	 A three-bladed rotor: The two-bladed rotor on one machine

was replaced with a three-bladed rotor to allow researchers 

to test new control schemes applicable only to three-bladed 

machines. 

For more information, contact the National Wind Technology 

Center at 303-384-6900.

Helpful Web Sites

The National Wind Technology Center

www.nrel.gov/wind

Department of Energy Wind and Hydropower 

Technologies Program

www1.eere.energy.gov/windandhydro 

An aerial photo of the two NWTC Controls Advanced Research Turbines (CART). 

Photo by Lee Jay Fingersh, NREL/PIX15736

National Renewable Energy Laboratory

15013 Denver West Parkway, Golden, Colorado 80401 • 303-275-3000 • www.nrel.gov

NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency and Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

NREL/FS-5000-45687 • January 2010

Printed with a renewable-source ink on paper containing at least 50% wastepaper, including 10% post consumer waste.

Photo credits for page 1 masthead: PIX15758, PIX15005, PIX12415, and PIX14691.