Adjustable Speed Generators
Modern high-power wind turbines are capable
of adjustable speed operation. Key
advantages of adjustable speed generators
(ASGs) compared to fixed-speed generators
(FSGs) are:
They are cost effective and provide
simple pitch control; the controlling
speed of the generator (frequency)
allows the pitch control
time constants to become longer,
reducing pitch control complexity
and peak power requirements. At
lower wind speed, the pitch angle is
usually fixed. Pitch angle control is
performed only to limit maximum
output power at high wind speed.
They reduce mechanical stresses; gusts of wind can
be absorbed, i.e., energy is stored in the mechanical
inertia of the turbine, creating an “elasticity” that reduces
torque pulsations.
They dynamically compensate for torque and power
pulsations caused by back pressure of the tower. This
back pressure causes noticeable torque pulsations at a
rate equal to the turbine rotor speed times the number
of rotor wings.
They improve power quality; torque pulsations can
be reduced due to the elasticity of the wind turbine
system. This eliminates electrical power variations,
i.e., less flicker.
They improve system efficiency; turbine speed is adjusted
as a function of wind speed to maximize output
power. Operation at the maximum power point can be
realized over a wide power range. Fig. 2 illustrates
typical output power-speed curves as a function of turbine
speed and wind speed. As a result, energy efficiency
improvement up to 10% is possible (Fig. 3).
They reduce acoustic noise, because low-speed operation
is possible at low power conditions.
Inverter output filters and EMI filters are rated for 1
p.u. output power, making filter design difficult and
costly.
Converter efficiency plays an important factor in total
system efficiency over the entire operating range.