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LECTURE NOTES

More About the Control of Wind Turbines

Niall McMahon © 2013

Context

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These Notes

These notes contain more about the control of wind turbines. See Wind Energy Explained for expanded descriptions of some of the material.

(Review also last year's lecture on control.)

Wind Turbine Control Systems

Wind turbines are equipped with control systems of varying complexity. Control systems are responsible for monitoring system health, ensuring that the machine operates safely and ensuring that the machine operates effectively. This involves measuring quantities, making decisions based on this information and implementing the decisions by actuating mechanical or electrical systems.

At one end of the spectrum, there are purely mechanistic control systems - think of Watt's flyball governor for steam. At the other end, advanced computer systems take charge. In between, there are ladder-logic systems and electronic logic systems. Control systems are built to implement control algorithms based on an over-arching control philosophy.

Control systems must allow fail-safe operation, i.e. no matter what happens with the system and in the environment, the machine will end up in a parked configuration. For safety, control of forces is critical.

Design of Control Systems

In order to design a control system, the engineer must have a complete understanding of the proposed machine, the forces that will act on it and the conditions necessary to maintain safe and efficient operation. A mathematical model is very useful at this stage. The engineer will develop a control framework or philosophy, i.e. how control will be effected (e.g. with a powered pitch system, aerodynamic overspeed protection, a disk brake and CPU), and then, more specifically an implementation of control algorithms using this framework. The implementation will be tested rigorously using both simulation and experiment.

Good system models, such as that which you worked on as part of the second assignment, are important.

Forces Acting On a Wind Turbine

These forces include: static (non-rotating); steady (rotating); cyclic; transient; impulsive; stochastic; resonance-induced.

Static forces are the steady loads acting on a structure. For example, the main part of the force exerted by a wind of constant velocity or the gravitational force pulling components downwards. Steady, rotating forces, arise from fixed speed rotation or similar phenomena. Cyclic forces vary periodically and are associated with yaw motion, small mass imbalances or steady wind shear across the rotor, among other things. Transient forces vary non-periodically in response to some short duration event, e.g. braking. Impulsive forces are large transient forces that have a very short duration. An emergency braking event, for example, could involve in impulse force. Stochastic forces are unsteady forces that are unpredictable on small time-scales. These include, for example, forces arising from atmospheric turbulence. Finally, resonance-induced forces occur when some part the machine oscillates at a frequency close to a natural frequency of one or more components.

Quantities Frequently Monitored by Advanced Control Systems

Modern wind turbine control systems monitor a large number of system parameters. These can include the wind speed, the wind direction, the air temperature, the rotor's rate of rotation, the generator's rate of rotation, other system speeds (e.g. in the gearbox), system temperatures (e.g. oils, generator windings), vibration of the rotor, nacelle and other components; in short the health of the electrical sub-system and other sub-systems (e.g. hydraulic or pneumatic).

Open and Closed Loop Control

Overview

Open loop contollers do not have feedback mechanisms. The control system is designed to actuate in response to a particular input, usually a change in wind speed; the desired outcome is pre-programmed but no explicit measurement is made of the actual outcome. If the system components do not perform as expected, maybe not achieving the desired pitch angle due to wear and tear, no compensatory action can be taken. There is no feedback.

With a closed loop system, on the other hand, the controller compares the actual outcome with the desired outcome, i.e. an explicit measurement of the actual outcome is made. If a closed-loop system is well designed, it may be possible for the controller to compensate for a malfunction.

Closed-loop control often involves powered actuation, e.g. servo-motors.

Example: A Passive Pitch Control System

A passive pitch system can be imagined that uses (i) the aerodynamic pitching moment about the blade axis, (ii) the pitching moment associated with the blade mass and (iii) the pitching moment associated with a restoring spring. The net torque acting about the axis of a blade during operation will be a sum of these moments, i.e.

Mtotal = Maerodynamic + Mmass + Mspring

The angular acceleration of the blade about its long axis, θ'', that results from this net moment can be written, in a simple way as,

θ'' = Mtotal / I

Where I is the moment of inertia of the blade about its long axis. Other quantities, e.g. angular velocity and position, can be derived from this.

As outlined, this system combines closed- and open- loop elements.

Legals

Please see here.

Most material © Niall McMahon. See legals and disambiguation for more detail. Don't forget that opinions expressed here are not necessarily shared by others, including my employers.