Environmental Engineering Reference
In-Depth Information
the requirement stated in Section 2.4.3 where the tip speed ratio
λ
=
Ω
R
/
U
should be fi xed
at
λ
opt
, the value at which
C
p
is a maximum. For each wind turbine design
λ
opt
is different,
e.g. in Figure 2.8,
C
p
is a maximum when
7 .
Unfortunately, synchronous generators and, to all intents and purposes, cage rotor asyn-
chronous generators, operate at a fi xed speed, say at
λ
≈
1
in Figure 4.42, and a turbine driving
either generator would track a locus such as
bb
. Only at wind speed
U
2
would the wind turbine
operate at
ω
opt
with the consequence that for higher and particularly lower wind speeds less
than maximum energy would be extracted. Operation at variable speed (to track locus
aa
)
with either synchronous or asynchronous generators is possible through the use of power
electronic converters. Due to the losses of such converters the additional energy captured by
a wind turbine operating at variable rather than fi xed speed is typically less than 10% and
would not alone justify the extra expense. However, there are other reasons why variable
speed operation is desirable.
λ
Fixed Versus Variable Speed - Dynamics
The vast majority of the world's electricity is generated by synchronous machines directly
connected to their respective power systems. This confi guration works very well when the
prime mover (usually an engine or a steam, gas or water turbine) provides a steady torque to
the generator. Wind, however, is turbulent and this translates directly into fl uctuations in
drivetrain torque. Using a directly connected synchronous generator in a wind turbine would
form too rigid a coupling between the mechanical and electrical systems. Wind gusts would
cause large mechanical stresses in the turbine and, depending on the nature of the electrical
grid, large fl uctuations in the power fed into the electrical system.
The main advantages of variable speed wind turbines in terms of dynamics are:
The total inertia of the aerodynamic rotor, the gearbox (if there is one) and the electrical
generator act as an energy buffer, smoothing out the wind turbulence. Transient torques
and rapid variation in electrical power as well as stresses in the drivetrain are greatly
reduced
Lower structural loads and lighter foundations are other advantages of particular impor-
tance in offshore applications
Additional advantages are:
The power electronics may also be capable of regulating the reactive power fl ow in the
network.
Noise is reduced, when operating at low wind speeds.
The drawbacks of variable speed are the extra complexity of the generator (in some
schemes) and of the necessary power electronic hardware, all of which increase cost and
possibly reduce reliability.
To allow for wind gusts, virtually all wind turbines have generators or generator systems
that provide some degree of speed variation.
