Introduction to Wind Turbine Technology - Small Wind Turbines: Analysis, Design, and Application - page 18

Fig. 1.9 The internal layout of the Vestas V80 2 MW turbine (from product brochure

downloaded from http://www.vestas.com/ , 10 Apr 2010)

relays for shorting the generator for electrical braking. The Skystream is unusual in
that it relies entirely on electronic braking for overspeed control. Figure 1.11
shows the inverter for grid-connection. For battery charging, often as part of a
remote power system, a small turbine does not need an inverter. Indeed, most
small turbines are not supplied with an inverter.
Pitch adjustment is generally held to be too expensive for small turbines, but
some mechanism of overspeed protection is required. Many small turbines are
designed to turn the rotor out of the wind in high wind to limit the power—see
Exercise 1.2—for protection. This can be done by furling in the horizontal plane or
by pitching in the vertical plane. Furling is the more common. The rotor and gen-
erator axis is displaced horizontally from the yaw axis (the pivot to the tower) and the
resulting yaw moment due to turbine thrust helps to collapse the tail fin at sufficiently
high wind speed. There are a number of possible problems with furling, such as:

• The difficulty in achieving furling consistently at the same windspeed for a
variety of load cases,
• Furling may cause a non-zero yaw at lower windspeeds with the consequent
reduction in power output, and
• The transient forces associated with furling may be large.