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10 Lidars and wind turbine control –
Part 2
Eric Simley 1 , Fiona Dunne 1 , Jason Laks 2 , and Lucy Y. Pao 1
1 Department of Electrical, Computer, and Energy Engineering, University of
Colorado, Boulder, CO, USA
2 National Renewable Energy Laboratory, Golden, CO, USA
10.1 Introduction
Modern utility-scale wind turbines are typically controlled using yaw, generator torque, and
blade pitch actuation (Pao and Johnson, 2011). The yaw motor is used to align the rotor with
thewinddirectionforthepurposeofmaximizingpowercapture.Generatortorqueiscontrolled
duringbelow-rated(region2)operationtomaximizepowercapturebymaintainingtheoptimal
tip speed ratio. In above-rated conditions (region 3), the blades are pitched to regulate rotor
speed and power capture by limiting the amount of aerodynamic torque produced by the
rotor. In addition to regulating rotor speed, region 3 control systems are often designed to
reduce structural loads on the turbine caused by turbulence.
A block diagram of a wind turbine’s feedback control system is shown in Fig. 131. In region
2, feedback measurements of generatorspeed are used to maintain the optimal tip speed ratio
by adjusting generator torque. The generator torque command is traditionally set equal to
the square of the generator speed multiplied by a constant whose value is chosen to maintain
the optimal tip speed ratio (Pao and Johnson, 2011). If the wind speed is above the turbine’s
rated wind speed, the region 3 controller employs blade pitch control to decrease the fraction
oftheavailablepowerinthewindextracted bytherotor.Feedbackmeasurementsofgenerator
speed are input to a feedback blade pitch controller designed to both regulate rotor speed
to the rated value and reduce structural loads caused by fluctuations in wind speed, which
act as a disturbance to the wind turbine. With a single feedback measurement of generator
speed, all of the blades are usually pitched together using a strategy called collective pitch
(CP) control. When additional feedback measurements are available, such as the blade root
bending moment at each blade, the blades can be pitched separately using individual pitch
(IP) control (Bossanyi, 2005). Utilizing separate pitch commands for each blade allows for
the reduction of loads due to variations in wind speed across the rotor disk, especially blade
loads at the once-per-revolution (1P) frequency.
A drawback to feedback control is that the wind disturbance must first act on the turbine
before a corrective control action can be made based on a feedback measurement. To address
this delay, feedforward control has been proposed, whereby a preview measurement of the
Portions of this chapter originally appeared in Dunne et al. (2012), Simley and Pao (2013a), Simley
and Pao (2013b), and Laks et al. (2013). The American Institute of Aeronautics and Astronautics holds the
copyright for Dunne et al. (2012) and Simley and Pao (2013a) and the American Automatic Control Council
holds the copyright for Simley and Pao (2013b) and Laks et al. (2013).
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Figure 131: Feedback control scenario.
192 DTU Wind Energy-E-Report-0029(EN)
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