1. Power Generation from Renewable Energy Sources
Fall 2013
Instructor: Xiaodong Chu
Office Tel.:

2. Flashbacks of Last Lecture
The power extracted by the wind turbine blades is equal to
Define the ratio of downstream to upstream wind speed
and the rotor efficiency
then
The maximum possible rotor efficiency is (Betz efficiency or Betz’ law)

3. Flashbacks of Last Lecture
Example 6.7 on page 327 of the textbook

4. Flashbacks of Last Lecture
Grid-connected wind turbines use ac generators including synchronous generators and asynchronous induction generators
Synchronous generators spin at a precise rotational speed determined by the number of poles and the frequency needed for the power grid
An exciter, slip rings and brushes are needed, which adds to the maintenance
Most of current world’s wind turbines use induction generators, which do not turn at a fixed speed
The key advantage of induction generators is that their rotors do not require the exciter, slip rings and brushes, so they are less complicated and less expensive

5. Wind Power Systems – Wind Turbine Generators
Classification of AC generators used in wind turbines
Synchronous generators
Wound rotor synchronous generators (WRSG)
Permanent magnet synchronous generators (PMSG)
Induction generators
Squirrel cage induction generators (SCIG)
Wound rotor induction generators (WRIG)

6. Wind Power Systems – Wind Turbine Generators
Squirrel cage induction generators

7. Wind Power Systems – Wind Turbine Generators
Slip of an induction machine is the difference of rotating speed between magnetic field and rotor
when it is in the motor mode, the rotor rotates a little slower than magnetic field whereas in the generator mode, the rotor rotates faster
Example 6.8 on page 333 of the textbook

8. Wind Power Systems – Speed Control for Maximum Power
The energy conversion efficiency of a wind turbine is a function of TSR, i.e., rotor speed should follow wind speed that is random in nature
It is needed to control forces on the rotor and thus to limit the power in very high winds in order to avoid damage to the wind turbine

9. Wind Power Systems – Speed Control for Maximum Power
Turbine blades should change their speed as the wind speed changes for maximum efficiency

10. Wind Power Systems – Speed Control for Maximum Power
The impact of rotational speed adjustment has on delivered power

11. Wind Power Systems – Speed Control for Maximum Power
The generator of a wind turbine may need to spin at a fixed speed in order to deliver current and voltage in phase with the grid
For grid-connected turbines, the challenge is to accommodate variable rotor speed and fixed generator speed
How can this challenge be solved?

12. Wind Power Systems – Speed Control for Maximum Power
Pole-changing induction generators
Induction generators spin at a frequency that is largely controlled by the number of poles
If the number of poles could be changed, the wind turbine could have several operating speeds, approximating multi-step rotational speed adjustment
It is a kind of discrete control

13. Wind Power Systems – Speed Control for Maximum Power
Multiple gearboxes
Some wind turbines have two gearboxes with separate generators attached to each, one high-wind-speed gear ratio and one low-speed gear ratio
It is a kind of discrete control and rare in practical use

14. Wind Power Systems – Speed Control for Maximum Power
Variable-slip induction generators
The slip of induction generators is a function of the dc resistance in the rotor conductors, and by adding variable resistance to the rotor, the amount of slip can range up to around 10%
One way is to have adjustable resistors external to the generator
Use a wound rotor with slip rings and brushes similar to what a synchronous generator has, and more maintenance will be required.
Another way is to mount the resistors and the electronics needed to control them on the rotor
Send signals to the rotor telling how much slip to provide
It is a kind of limited continuous control

15. Wind Power Systems – Speed Control for Maximum Power
Indirect grid connection systems
The wind turbine is allowed to spin at whatever speed needed to deliver the maximum power
When attached to a synchronous or induction generator, the electrical output will have variable frequency, and the generator cannot be directly connected to the utility grid, which is of fixed frequency (50- or 60-Hz)
Variable-speed wind turbines have advantages of higher annual energy production and greatly minimizing the wear and tear on the system caused by rapidly wind speeds
It is a kind of continuous control

16. Wind Power Systems – Speed Control for Maximum Power
Power control: all wind turbines are designed with some sort of power control
The simplest, most robust and cheapest control method is the stall control, where the blades are bolted onto the hub at a fixed angle
The rotor stalls when the wind speed exceeds a certain level
It is a slow aerodynamic power regulation causing less power fluctuations than a fast-pitch power regulation
Drawbacks of the method are lower efficiency at low wind speeds, no assisted startup and variations in the maximum steady –state power due to variations in air density and grid frequencies

17. Wind Power Systems – Speed Control for Maximum Power
Power control: all wind turbines are designed with some sort of power control
The simplest, most robust and cheapest control method is the stall control, where the blades are bolted onto the hub at a fixed angle
The rotor stalls when the wind speed exceeds a certain level
It is a slow aerodynamic power regulation causing less power fluctuations than a fast-pitch power regulation
Drawbacks of the method are lower efficiency at low wind speeds, no assisted startup and variations in the maximum steady –state power due to variations in air density and grid frequencies

18. Wind Power Systems – Speed Control for Maximum Power
Another type of control is pitch control, where the blades can be turned out or into the wind as the power output becomes too high or too low , respectively
Advantages of this type of control are good power control (at high wind speeds the mean value of the power output is kept close to the rated power of the generator), assisted startup and emergency stop
Some disadvantages are the extra complexity arising from the pitch mechanism and the higher power fluctuations at high wind speeds

19. Wind Power Systems – Speed Control for Maximum Power
The third control strategy is the active stall control, where the stall of the blade is actively controlled by pitching the blades
At low speeds the blades are pitched similar to a pitch-controlled wind turbine to achieve maximum efficiency and at high wind speeds the blades go into a deeper stall by being pitched slightly into the direction opposite to that of a pitch-controlled turbine

20. Wind Power Systems – Speed Control for Maximum Power
The most commonly applied wind turbine configurations can be classified both by their ability to control speed and the type of power control they use
Note: The grey zones indicate combinations that are not in use in the wind turbine industry today

21. Wind Power Systems – Speed Control for Maximum Power
Type A: fixed speed
This configuration denotes the fixed-speed wind turbine with an squirrel cage induction generator (SCIG) directly connected to the grid via a transformer
Since the SCIG always draws reactive power from the grid, this configuration uses a capacitor bank for reactive power compensation
A smoother grid connection is achieved by using a soft-starter
The main drawbacks of this concept are that it does not support any speed control , it requires a stiff grid and its mechanical construction must be able to tolerate high mechanical stress

22. Wind Power Systems – Speed Control for Maximum Power
Type B: limited variable speed
This configuration corresponds to the limited variable speed wind turbine with variable generator rotor resistance
It uses a wound rotor induction generator (WRIG) and the generator is directly connected to the grid
A capacitor bank performs the reactive power compensation and a smoother grid connection is achieved by using a soft-starter
OptiSlip® is such a design with a variable additional rotor resistance, which can be changed by an optically controlled converter mounted on the rotor shaft and the optical coupling eliminates the need for costly slip rings and brushes

23. Wind Power Systems – Speed Control for Maximum Power
Type C: variable speed with partial scale frequency converter
This configuration , known as the doubly fed induction generator (DFIG) , corresponds to the limited variable speed wind turbine with a wound rotor induction generator (WRIG) and partial scale frequency converter (rated at approximately 30% of its nominal generator power) on the rotor circuit
The frequency converter performs the reactive power compensation and the smoother of grid connection
It has a wider range of dynamic speed control compared with the Type B

24. Wind Power Systems – Speed Control for Maximum Power
Rotor is fed from a three-phase variable frequency source, thus allowing variable speed operation
reduction of mechanical stress, higher overall efficiency, reduced acoustical noise
The variable frequency supply to rotor is attained through the use of two voltage-source converters linked via a capacitor
Cbc
DFIG
Grid
Grid side
converter
DC Link
Rotor side
converter
Reactor
Chopper

25. Wind Power Systems – Speed Control for Maximum Power
The converters handle ac quantities:
rotor-side converter carries slip frequency current
grid-side converter carries grid frequency current
The converters are controlled using vector-control techniques:
based on the concept of a rotating reference frame and projecting currents on such a reference
such projections referred to as d- and q-axis components
With a suitable choice of reference frame, AC quantities appear as DC quantities in the steady state

26. Wind Power Systems – Speed Control for Maximum Power
In flux-based rotating frames:
changes in the d-axis component of current will lead to reactive power changes
changes in the q-axis component will vary active power
This allows independent control of active and reactive power of the stator
Implemented through rotor-side converter control
An important aspect of the DFIG concept
Since rotor flux tracks the stator flux, air gap torque provides no damping of shaft oscillations
additional modulating signal has to be added

27. Wind Power Systems – Speed Control for Maximum Power
Type D: variable speed with full scale frequency converter
This configuration corresponds to the full variable speed wind turbine, with the generator connected to the grid through a full-scale frequency converter
The frequency converter performs the reactive power compensation and the smoother of grid connection
The generator can be excited electrically [wound rotor synchronous generator (WRSG) or WRIG] or by permanent magnet [permanent magnet synchronous generator (PMSG)]
Some full variable-speed wind turbine systems have no gearbox and a direct driven multipole generator with a large diameter is used