New small turbine technologies - Wind Power Generation and Wind Turbine Design - page 402

Thus it should be understood that both speed control and yaw control perfor-
mances are requested to be as high as or higher than LWTs.
Upon this nature, the fi rst issue to decide on the control strategy is whether to adopt
active control or passive control concept. The former is a concept to control the system
using a yaw drive with any kind of active power source such as electric or hydraulic
power actuators, while the latter is to control by natural forces generated by wind
or mechanical forces such as centrifugal force without using any active power source.
The fi ner control system based on active control concept is desirable at one
hand, however, there are another reasons that passive control concept is attractive
at another. The passive control concept gives us the advantage of structural sim-
plicity, which would give greater reliability in general, provides as smart control
measures using natural force without extra control power sources and supplies us
at more economical cost.
In principle both concepts exist in LWT design and large turbines, but the status
of technical tendency is that more complete active control concept with LWTs and
more passive control concept or less active control concept with SWTs. Should
“smart” technologies be really smart, the passive control concept could be also
popular with LWTs in future.
There is an evidence for this. For long, a constant-speed operation system was
technically thought to be most suitable for grid connected systems simply because the
electrical grid has constant frequency and the rotor should be regulated to rotate at
constant speed. But wind will seldom keep blowing constantly at the rated wind speed
at most of the time during the operation of wind turbine. It is much more natural for a
WT to work at any given wind speed with higher effi ciency but with less extra active
power source. The solution was given by “variable operation systems” which employ
a power convertor system such as AC-DC-AC generator system. Electronically
fi ner and faster while mechanically less loaded, is modern technical tendency.
Decision of choosing either of constant-speed operation or variable-speed
operation is also important task.
Figure 5 shows an example of a basic performance of an optimal rotor design.
The rotor is designed by BEM theory at design wind speed of 10 m/s, tip-speed
ratio of 7 m for 10 m of rotor diameter using a thin wing section of Illinois Univer-
sity. Once aerofoil section(s) selected, Reynolds number effect neglected and pitch
angle fi xed, then C P is a function of l = r / V only.

Maximum C P is 0.475 for l = 7 at the design point. When neglecting the effect
of low Reynolds number, as long as l = 7 is kept, the rotor produces power with
maximum aerodynamic effi ciency. As shown in Fig. 5, C P ( l ) will decrease with
constant-speed system (

Ω

D = Const.) as l departs from the design point while
keeping pitch angle constant. As a result,

Ω = Ω

C

≥ 0.4 only for 7.5 m/s < V < 14 m/s

(18 )

On the other hand, with a variable-speed operation rotor, optimal C P can be real-
ized if the optimal rotor speed is properly regulated. In this case, under the varying
natural wind speed, it is desirable to keep l = l opt = 7, or to vary rotor speed in the
manner:

Ω = ( V / R ) l opt .