24-08-2012, 11:29 AM
DOUBLY FED INDUCTION GENERATOR IN A WIND TURBINE
DOUBLY FED INDUCTION GENERATOR IN A WIND TURBINE.pdf (Size: 586 KB / Downloads: 138)
1 INTRODUCTION
In a wind power generating system, it is required that the
generator tracks a prescribed torque-speed profile. Variablespeed
operation is introduced to gain high efficiency in the
generating system. Otherwise the generating system cannot
capture the largest possible energy available from the wind and
the blades of the wind turbine will subject to torsional stress
and windage friction.
The mechanical efficiency in a wind turbine is dependent of the
power coefficient. The power coefficient of the rotating wind
turbine is given by [1] the Cp(l,q) curve, where l is the tip
speed ratio and q is the pitch angle.
Figure 1 Power coefficient Cp(l,q) .
When converters in the rotor circuit provide the doubly fed
induction generator the opportunity to realize adjustable speed,
the maximal mechanical efficiency can be obtained and
operation at the maximum power output can be realized over a
wide power range.
2 TORSIONAL RESONANCE
The rotating shaft system in a wind turbine is divided into
sections. The turbine itself is quite heavy and the machine
rotor is light. The shaft, connecting the generator and the
turbine cannot be assumed to be of infinite stiffness. The
gearbox reduces the stiffness, therefore the shaft will twist as
it transits torque from one end to the other. Typical value of
the resonance frequency of such systems is in the range 1-2
Hz, and for a specific Danish windmill the resonance
frequency is known to be 1.67 Hz [2].
A simple method for modeling the shaft system in MatLab is
shown below. Because the mass of the shaft itself is very
small, seen from the generator, it is reduced to zero. The
inertia at the turbine gives a negative contribution to the
torque when the generator is in generating mode.