Control Systems Design

From Wind wiki

All wind turbines are designed with some sort of power control. There are different ways to control aerodynamic forces on the turbine rotor and thus to limit the power in very high winds in order to avoid damage to the wind turbine.

Passive Stall Control

The simplest, most robust and cheapest control method is the stall control (passive control), where the blades are bolted onto the hub at a fixed angle. The fixed-blade pitch is chosen so that the turbine reaches its maximum or rated power at the desired wind speed. The design of the rotor aerodynamics causes the rotor to stall (lose power) when the wind speed exceeds a certain level. Thus, the aerodynamic power on the blades is limited. Such slow aerodynamic power regulation causes less power fluctuations than a fast-pitch power regulation. Some drawbacks of the method are lower efficiency at low wind speeds, no assisted startup and variations in the maximum steady-state power due to variations in air density and grid frequencies.

Active Pitch Control

Another type of control is active pitch control, where the blades can be turned out or into the wind as the power output becomes too high or too low, respectively. Generally, the advantages of this type of control are good power control, increased energy capture, assisted startup and emergency stop. The pitch change system has to act rapidly. It must change at the rate of 5 degrees per second or better in order to limit power excursions due to gusts enveloping the whole rotor to an acceptable value. From an electrical point of view, good power control means that at high wind speeds the mean value of the power output is kept close to the rated power of the generator. Some disadvantages are the extra complexity arising from the pitch mechanism and the higher power fluctuations at high wind speeds. The instantaneous power will, because of gusts and the limited speed of the pitch mechanism, fluctuate around the rated mean value of the power.

Active Stall Control

The third possible control strategy is the active stall control. As the name indicates, the stall of the blade is actively controlled by pitching the blades. At low wind speeds the blades are pitched similar to a pitch-controlled wind turbine, in order to achieve maximum efficiency. At high wind speeds the blades go into a deeper stall by being pitched slightly into the direction opposite to that of a pitch-controlled turbine. The active stall wind turbine achieves a smoother limited power, without high power fluctuations as in the case of pitch-controlled wind turbines. This control type has the advantage of being able to compensate variation in air density. The combination with the pitch mechanism makes it easier to carry out emergency stops and to startup the wind turbine.

Yaw Control

As most horizontal axis wind turbines employ a yaw drive mechanism to keep the turbine headed into the wind, the use of the same mechanism to yaw the turbine out of wind to limit power output is obviously an attractive one. However, there are two factors which militate against the rapid response of such a system to limit power: first, the large moment of inertia of the nacelle and rotor about the yaw axis, the second, the cosine relationship between the component of wind speed perpendicular to the rotor disc and the yaw angle. The latter factor means that, at small initial yaw angles, yaw changes of, say, ten degrees only bring about reductions in power of a few percent, whereas blade pitch changes of this magnitude can easily halve the power output. Thus active yaw control is only practicable for variable speed machines where the extra energy of a wind gust can be stored as rotor kinetic energy until the yaw drive has made the necessary yaw correction.