24-11-2012, 02:04 PM
A Seminar on Doubly Fed Induction Generators(DFIG)
Doubly Fed.ppt (Size: 2.01 MB / Downloads: 203)
Turbine’s rotor absorbs the mechanical power fluctuations.
Output power curve is smoother.
Since variable speed operation produces a variable frequency voltage, a power electronic converter must be used.
The Inductance Machine as a Motor
Stator RMF causes the rotor to spin.
As rotor approaches synchronous speed of the rmf, the relative motion becomes less and less.
Thus, an induction machine as a motor always spins somewhat slower than synchronous speed.
The Induction Machine as a Generator
The stator requires excitation current
from the grid if it is grid-connected or by incorporating external capacitors
Wind speed forces generator shaft to exceed synchronous speed
Slip is negative because the rotor spins faster than synchronous speed
Slip is normally less than 1% for grid-connected
WHY INDUCTION GENERATOR?
Simple and robust construction.
Can run independently.
Inexpensive.
Minimal maintenance.
Inherent overload protection.
At high speed, reduces size and weight of machine and filter components.
Torque Control Scheme
To modify the electromagnetic torque of the generator according to wind speed variations.
Driving the system to the required operating point reference.
The reference torque is manipulated to generate a reference value for the rotor current in the q axis.
Voltage Control Scheme
To provide terminal voltage or power factor control using the rotor-side converter.
Reactive power injection for voltage control schemes, the rotor-side converter is likely to be preferred.
It is because the rotor circuit is effectively amplified by a factor of 1/s.
Power Control SCHEME
In this, Peref is determined by the wind turbine P–N characteristic for maximum power extraction from the prevailing wind velocity.
The difference in the magnitude of Pe, from Peref , forms an error signal.
Then it is processed by the compensator to produce the reference value, δgref with respect to the stator voltage vector.