0. ECE 333 Renewable Energy Systems
Lecture 9: Wind Power Systems
Prof. Tom Overbye
Dept. of Electrical and Computer Engineering
University of Illinois at Urbana-Champaign

1. Announcements Read Chapter 7
HW 4 is 7.1, 7.2, 7.4, 7.5; it will be covered by an in-class quiz on Thursday Feb 20

2. In the News: On Feb 12 AWEA Released Report on Wind Reliablity
Report addressed issue of how much wind energy could be integrated into the US grid
Finding is wind could provide more than 40% of our total electric energy
In 2013 Iowa and South Dakota got 25% of their electricity from wind, and for ERCOT it as 10.6%
Key to integrating large amounts of wind is that the wind plant outputs are not correlated across large areas
Changes in the wind tend to cancel out
Report: awea.files.cms-plus.com/AWEA%20Reliability%20White%20Paper%20-% pdf

3. North American Power Grid Load/Generation Contour
Image contours the load (green) and generation (red)

4. Maximum Rotor Efficiency
Rotor efficiency CP vs. wind speed ratio λ. Recall λ is the ratio between the downstream wind velocity and the upstream velocity

5. Tip-Speed Ratio (TSR)
Efficiency is a function of how fast the rotor turns
Tip-Speed Ratio (TSR) is the speed of the outer tip of the blade divided by wind speed
D = rotor diameter (m)
v = upwind undisturbed wind speed (m/s)
rpm = rotor speed, (revolutions/min)
One meter per second = 2.24 miles per hour

6. Tip-Speed Ratio (TSR) TSR for various rotor types
If blade turns too slow then wind passes through without hitting blade; too fast results in turbulence
Rotors with fewer blades reach their maximum efficiency at higher tip-speed ratios
Figure 7.18
A higher TSR is needed when there are fewer blades

7. Example
40-m wind turbine, three-blades, 600 kW, wind speed is 14 m/s, air density is kg/m3
a. Find the rpm of the rotor if it operates at a TSR of 4.0
b. Find the tip speed of the rotor
c. What gear ratio is needed to match the rotor speed to the generator speed if the generator must turn at 1800 rpm?
d. What is the efficiency of the wind turbine under these conditions?

8. Example a. Find the rpm of the rotor if it operates at a TSR of 4.0
Rewriting (7.30),
We can also express this as seconds per revolution:

9. Example
b. Tip speed From (7.30): c. Gear Ratio

10. Example
d. Efficiency of the complete wind turbine (blades, gear box, generator) under these conditions
From (7.7):
Overall efficiency:

11. Converting Wind into Electric Energy
Design challenge is to convert rotating mechanical energy into electrical energy
This is, of course, commonly done in most power plants. But the added challenges with wind turbines are 1) the shaft is often rotating a variable speed [because of changes in the wind speed], and 2) the rate of rotation is relatively slow (dozens of rpm)
Early wind turbines used a near fixed speed design, which allowed use of simple and well proven induction generators, but gave up aerodynamic efficiency. Modern turbines tend to use a variable speed design to keep tip-to-speed ratio near optimal

12. Electric Machines
Electric machines can usually function as either a motor or as a generator
Three main types of electric machines
DC machines: Advantage is they can directly operate at variable speed. For grid application the disadvantage is they produce a dc output. Used for small wind turbines.
AC synchronous machines
Operate at fixed speed. Used extensively for traditional power generation. The fixed speed had been a disadvantage for wind.
AC induction machines
Very rugged and allow some speed variation but usually not a lot for efficient operation.

13. Types of Wind Turbines by Machine
From an electric point of view there are four main types of large-scale wind turbines (IEEE naming convention)
Type 1: Induction generator with fixed rotor resistance
Type 2: Induction generators with variable rotor resistance
Type 3: Doubly-fed induction generators
Type 4: Full converter generators which main use either a synchronous generator or an induction generator
Most new wind turbines are either Type 3 or Type 4
In Europe these are sometimes called Types A, B, C, D respectively.

14. Wind Generator Types

15. Rotating Magnetic Field
Imagine coils in the stator of this 3-phase generator
Positive current iA flows from A to A’
Magnetic fields from positive currents are shown by the bold arrows
Magnetic flux is proportional to current, with direction given by the right-hand rule (from Ampere's circuit law)

16. Rotating Magnetic Field
Three-phase currents are flowing in the stator
At ωt = 0, iA is at the maximum positive value and iB=iC are both negative
Resultant magnetic flux points vertically down

17. Rotating Magnetic Field Demo

18. Magnetic Poles
Synchronous speed depends on the electrical frequency and the number of poles, with
Image source

19. Synchronous Machines
Spin at a rotational speed determined by the number of poles and by the frequency (3600 rpm at 60Hz, 2 pole)
The magnetic field is created on their rotors
Create the magnetic field by running DC through windings around the core
A permanent magnet can also be used
A gear box if often needed between the blades and the generator
Some newer machines are designed without a gear box
Slip rings are needed to get a dc current on the rotor

20. Asynchronous Induction Machines
Do not turn at a fixed speed
Acts as a motor during start up; can act as a generator when spun faster then synchronous speed
Do not require exciter, brushes, and slip rings
Less expensive, require less maintenance
The magnetic field is created on the stator not the rotor
Current is induced in the rotor (Faraday's law: v= dl/dt)
Lorenz force on wire with current in magnetic field:

21. Squirrel Cage Rotor
The rotor of many induction generators has copper or aluminum bars shorted together at the ends, looks like a cage
Can be thought of as a pair of magnets spinning around a cage
Rotor current iR flows easily through the thick conductor bars

22. Squirrel Cage Rotor
Instead of thinking of a rotating stator field, you can think of a stationary stator field and the rotor moving counterclockwise
The conductor experiences a clockwise force
Figure 6.16

23. The Inductance Machine as a Motor
The rotating magnetic field in the stator causes the rotor to spin in the same direction
As rotor approaches synchronous speed of the rotating magnetic field, the relative motion becomes less and less
If the rotor could move at synchronous speed, there would be no relative motion, no current, and no force to keep the rotor going
Thus, an induction machine as a motor always spins somewhat slower than synchronous speed

24. Slip The difference in speed between the stator and the rotor
s = rotor slip – positive for a motor, negative for a generator
NS = no-load synchronous speed (rpm)
f = frequency (Hz)
p = number of poles
NR = rotor speed (rpm)

25. The Induction Machine as a Motor
As load on motor increases, rotor slows down
When rotor slows down, slip increases
“Breakdown torque” increasing slip no longer satisfies the load and rotor stops
Braking- rotor is forced to operate in the opposite direction to the stator field
Torque- slip curve for an induction motor

26. 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
Single-phase, self-excited, induction generator

27. The Induction Machine as a Generator
Slip is negative because the rotor spins faster than synchronous speed
Slip is normally less than 1% for grid-connected generator
Typical rotor speed

28. Speed Control Necessary to be able to shed wind in high-speed winds
Rotor efficiency changes for different Tip-Speed Ratios (TSR), and TSR is a function of windspeed
To maintain a constant TSR, blade speed should change as wind speed changes
A challenge is to design machines that can accommodate variable rotor speed and fixed generator speed

29. Blade Efficiency vs. Windspeed
At lower windspeeds, the best efficiency is achieved at a lower rotational speed

30. Power Delivered vs. Windspeed
Impact of rotational speed adjustment on delivered power, assuming gear and generator efficiency is 70%

31. Pole-Changing Induction Generators
Being able to change the number of poles allows you to change operating speeds
A 2 pole, 60 Hz, 3600 rpm generator can switch to 4 poles and 1800 rpm
Can do this by switching external connections to the stator and no change is needed in the rotor
Common approach for 2-3 speed appliance motors like those in washing machines and exhaust fans
Increasingly this approach is being replaced by machine drives that convert ac at grid frequency to ac at a varying frequency (covered in ECE 464)

32. Variable-Slip Induction Generators
Purposely add variable resistance to the rotor
External adjustable resistors - this can mean using a wound rotor with slip rings and brushes which requires more maintenance
Mount resistors and control electronics on the rotor and use an optical fiber link to send the rotor a signal for how much resistance to provide

33. Effect of Rotor Resistance on Induction Machine Power-Speed Curves
Left plot shows the torque-power curve from slip of -1 to 1 with external resistance = 0.05; right plot is with external resistance set to 0.99 pu.

34. Variable Slip Example: Vestas V80 1.8 MW
The Vestas V MW turbine is an example in which an induction generator is operated with variable rotor resistance (opti-slip).
Adjusting the rotor resistance changes the torque-speed curve
Operates between 9 and 19 rpm
Source: Vestas V80 brochure