Class-13 These slides majorly focus on wind turbine components and wind turbine characteristics. Later based on this basic idea, we will discuss about the various control strategies for wind generators.

1. Design Considerations for
Wind Turbine Systems
Prof. (Dr.) Pravat Kumar Rout
Department of EEE
ITER
Siksha ‘O’ Anusandhan (Deemed to be University),
Bhubaneswar, Odisha, India
1

2. Introduction
 There is an air turbine of large blades
attached on the top of a supporting
tower of sufficient height.
 When wind strikes on the turbine
blades, the turbine rotates due to the
design and alignment of rotor blades.
The shaft of the turbine is coupled with
an electrical generator.
 The output of the generator is
collected through electric power
cables.
2

3. How does a wind turbine work?
 1: Wind (moving air that contains kinetic energy)blows toward the turbine's rotor blades.
 2: The rotors spin around, capturing some of the kinetic energy from the wind, and turning
the central drive shaft that supports them. Although the outer edges of the rotor blades
move very fast, the central axle (drive shaft) they're connected to turns quite slowly.
 3: In most large modern turbines, the rotor blades can swivel on the hub at the front so
they meet the wind at the best angle (or "pitch") for harvesting energy. This is called the
pitch control mechanism. On big turbines, small electric motors or hydraulic rams swivel
the blades back and forth under precise electronic control. On smaller turbines, the pitch
control is often completely mechanical. However, many turbines have fixed rotors and no
pitch control at all.
3

4. Continue…
 4: Inside the nacelle (the main body of the turbine sitting on top of the tower and behind
the blades), the gearbox converts the low-speed rotation of the drive shaft (perhaps, 16
revolutions per minute, rpm) into high-speed (perhaps, 1600 rpm) rotation fast enough to
drive the generator efficiently.
 5: The generator, immediately behind the gearbox, takes kinetic energy from the spinning
drive shaft and turns it into electrical energy. Running at maximum capacity, a typical
2MW turbine generator will produce 2 million watts of power at about 700 volts.
 6: Anemometers (automatic speed measuring devices) and wind vanes on the back of
the nacelle provide measurements of the wind speed and direction.
 7: Using these measurements, the entire top part of the turbine (the rotors and nacelle)
can be rotated by a yaw motor, mounted between the nacelle and the tower, so it faces
directly into the oncoming wind and captures the maximum amount of energy. If it's too
windy or turbulent, brakes are applied to stop the rotors from turning (for safety reasons).
The brakes are also applied during routine maintenance.
4

5. Continue…
 The electric current produced by the generator flows through a cable running down
through the inside of the turbine tower.
 A step-up transformer converts the electricity to about 50 times higher voltage so it
can be transmitted efficiently to the power grid (or to nearby buildings or
communities). If the electricity is flowing to the grid, it's converted to an even higher
voltage (130,000 volts or more) by a substation nearby, which services many turbines.
5

6. 6

7. Pros
 Very low carbon dioxide emissions (effectively zero once constructed).
 No air or water pollution.
 No environmental impacts from mining or drilling.
 No fuel to pay for—ever!
 Completely sustainable—unlike fossil fuels, wind will never run out.
 Turbines work almost anywhere in the world where it's reliably windy, unlike fossil-
fuel deposits that are concentrated only in certain regions.
 Unlike fossil-fueled power, wind energy operating costs are predictable years in
advance.
 Freedom from energy prices and political volatility of oil and gas supplies from
other countries.
 Wind energy prices will become increasingly competitive as fossil fuel prices rise
and wind technology matures.
 New jobs in construction, operation, and manufacture of turbines.
7

8. CONS…
 High up-front cost (just as for large nuclear or fossil-fueled plants).
 Economic subsidies needed to make wind energy viable (though other power forms
are subsidised too, either economically or because they don't pay the economic
and social cost of the pollution they make).
 Extra cost and complexity of balancing variable wind power with other forms of
power.
 Extra cost of upgrading the power grid and transmission lines, though the whole
system often benefits.
 Variable output—though that problem is reduced by operating wind farms in
different areas and (in the case of Europe) using interconnectors between
neighboring countries.
 Large overall land take—though at least 95 percent of wind farm land can still be
used for farming, and offshore turbines can be built at sea.
 Can't supply 100 percent of a country's power all year round, the way fossil fuels,
nuclear, hydroelectric, and biomass power can.
 Loss of jobs for people working in mining and drilling.
8

9. 9

10. Some options wind turbine topologies
 Rotor axis orientation: horizontal or vertical;
 Rotor position: upwind or downwind of tower;
 Rotor speed: fixed or variable;
 Hub: rigid, teetering, gimbaled or hinged blades;
 Rigidity: still or flexible;
 Number of blades: one, two, three or even more;
 Power control: stall, pitch, yaw or aerodynamic surfaces;
 Yaw control: active or free.
10

11. Types of Wind Turbines
Modern wind turbines fall into two basic
groups:
 HORIZONTAL-AXIS TURBINES
Horizontal-axis wind turbines are what many
people picture when thinking of wind
turbines. Most commonly, they have three
blades and operate "upwind," with the
turbine pivoting at the top of the tower so
the blades face into the wind.
 VERTICAL-AXIS TURBINES
Vertical-axis wind turbines come in several
varieties, including the eggbeater-style
Darrieus model, named after its French
inventor. These turbines are omnidirectional,
meaning they don’t need to be adjusted to
point into the wind to operate.
11

12. Sizes of Wind Turbines
 UTILITY-SCALE WIND TURBINES
Utility-scale wind turbines range in size from 100
kilowatts to as large as several megawatts.
Larger wind turbines are more cost effective
and are grouped together into wind plants,
which provide bulk power to the electrical
grid.
 OFFSHORE WIND TURBINES
Offshore wind turbines tend to be massive, and
taller than the Statue of Liberty. They do not
have the same transportation challenges of
land-based wind installations, as the large
components can be transported on ships
instead of on roads. These turbines are able to
capture powerful ocean winds and generate
vast amounts of energy.
12

13. Continue…
 SINGLE SMALL TURBINES
Single small turbines—below 100 kilowatts—are
typically used for residential, agricultural, and small
commercial and industrial applications. Small turbines
can be used in hybrid energy systems with other
distributed energy resources, such as micro-grids
powered by diesel generators, batteries, and photo-
voltaics. These systems are called hybrid wind systems
and are typically used in remote, off-grid locations(
where a connection to the utility grid is not available)
and are becoming more common in grid-connected
applications for resiliency.
 DISTRIBUTED WIND
When wind turbines of any size are installed on the
"customer" side of the electric meter, or are installed at
or near the place where the energy they produce will
be used, they're called "distributed wind.
13

14. 1: Rotor
 Rotor is the rotating part of the
wind turbine.
 It transfers the energy in the wind
to the shaft.
 The rotor hub holds the wind
turbine blades while connected
to the gearbox via the low speed
shaft.
14

15. 2: Pitch
 Pitch is the mechanism of
adjusting the angle of attack of
the rotor blades.
 Blades are turned in their
longitudinal axis to change the
angle of attack according to the
wind directions.
15

16. 3: Load-speed Shaft
 Shaft is divided into two parts:
low and high speed.
 The low-speed shaft transfers
mechanical energy from rotor
to gear box, while the high
speed shaft transfers
mechanical energy from
gearbox to generator.
16

17. 4: Gearbox
 Gearbox is a mechanical
component that is used to
increase or decrease the
rotational speed.
 In wind turbines, the gearbox is
used to control the rotational
speed of the generator.
17

18. 5: Generator
 Generator is the component
that converts the mechanical
energy from the rotor to the
electrical energy.
 The most electrical generators
used in wind turbines are
induction generators (IGs),
doubly fed induction generators
(DFIGs), and permanent magnet
synchronous generators (PMSGs)
18

19. 6: Controller
 Controller is the brain of the
wind turbine.
 It monitors constantly the
condition of the wind
turbine and controls the
pitch and yaw systems to
extract optimum power from
the wind.
19

20. 7: Anemometer
 Anemometer is a type of
sensor that is used to measure
the wind speed.
 The wind speed information
may be necessary for
maximum power tracking
and protection in emergency
cases.
20

21. 8: Wind Vane
 Wind Vane is a type of sensor
that is used to measure the wind
direction.
 The wind direction information is
important for the yaw control
system to operate.
21

22. 9: Nacelle
 Nacelle is the enclosure of the
wind turbine generator,
gearbox, and internal
equipment.
 It protects the turbine’s internal
components from the
surrounding environment.
22

23. 10: High-speed Shaft
 Shaft is divided into two parts:
low and high speed.
 The low-speed shaft transfers
mechanical energy from rotor to
gear box, while the high speed
shaft transfers mechanical
energy from gearbox to
generator.
23

24. 11: Tower
 Tower is the physical structure that
holds the wind turbine
 It supports the rotor, nacelle,
blades, and other wind turbine
equipment.
 Typical commercial wind towers
are usually 50-120 meters long and
they are constructed from
concrete or reinforced steel.
24

25. 12: Blades
 Blades are physical structures, which
are aerodynamically optimized to
help capture the maximum power
from the wind in normal operation
with a wind speed in the range of
about 3-15 m/s.
 Each blade is usually 20 meters or
more in length, depending on the
power level.
25

26. 13: Yaw Motor and Yaw Drive
 Yaw is the horizontal moving part of the
turbine.
 It turns clockwise and anticlockwise to
face the wind.
 The Yaw has two main parts.: The Yaw
motor and the Yaw Drive.
 The Yaw drive keeps the rotor facing the
wind when the wind direction varies.
 The Yaw motor is used to move the Yaw.
26

27. 14: Brake
 Brake is a mechanical part
connected to the high speed
shaft in order to reduce the
rotational speed or stop the
wind turbine overstepping or
during emergency conditions.
27

28. Wind Turbine Characteristics
 Many of the factors need to be focussed to regulate the control criteria
of a wind energy generator.
 The characteristics of wind energy generator helps in designing the
adequate control strategy for the following:
✓ Active and reactive power control
✓ Torque oscillations
✓ Controllability
✓ Power factor control
✓ Speed regulation
✓ Speed response against surges
✓ Connectivity
✓ Synchronisation for grid connection
✓ Speed regulation
✓ Ride through capability
28
pC

29. Power Coefficient of Wind Turbine
 Power Coefficient (Cp) is a measure of wind turbine efficiency.
 Cp is the ratio of actual electric power produced by a wind turbine divided by the total wind
power flowing into the turbine blades at specific wind speed.
 The power coefficient represents the combined efficiency of the various wind power system
components which include the turbine blades, the shaft bearings and gear train, the
generator and power electronics.
 If you know the Cp at a given wind speed for a specific turbine you can use it to estimate the
electrical power output.
 The Cp of a particular wind turbine varies with operating conditions such as wind speed,
turbine blade angle, turbine rotation speed, and other parameters.
 It is a measure of a particular wind turbine's overall system efficiency.
29

30. 30 Continue…

31. 31
Mechanical Power versus Wind Speed Curve
➢The power characteristics of a wind
turbine are defined by the power curve,
which relates the mechanical power of
the turbine to the wind speed.
➢The power curve is a wind turbine's
certificate of performance that is
guaranteed by the manufacturer.

32. Continue…
 A typical power curve is characterized by
three wind speeds: cut-in wind speed, rated
wind speed, and cut-out wind speed.
 The cut-in wind speed, as the name
suggests, is the wind speed at which the
turbine starts to operate and deliver power.
The blade should be able to capture
enough power to compensate for the
turbine power losses.
 The rated wind speed is the speed at which
the system produces nominal power, which
is also the rated output power of the
generator.
 The cut-out wind speed is the highest wind
speed at which the turbine is allowed to
operate before it is shut down. For wind
speeds above the cut-out speed, the
turbine must be stopped, preventing
damage from excessive wind.
32

33. Continue…
 The wind turbine starts to capture power at the cut in wind speed.
 The power captured by the blades is a cubic function of wind speed until
the wind speed reaches its rated value.
 To deliver captured power to the grid at different wind speeds, the wind
generator should be properly controlled with variable speed operation.
 As the wind speed increases beyond the rated speed, aerodynamic power
control of blades is required to keep the power at the rated value.
 This task is performed by three main techniques: passive stall, active stall,
and pitch control.
33

34. 34
Continue…

35. Wind Turbine Controls
 You can use different control methods
to either optimize or limit power output.
 You can control a turbine by controlling
the generator speed, blade angle
adjustment, and rotation of the entire
wind turbine.
 Blade angle adjustment and turbine
rotation are also known as pitch and
yaw control, respectively.
 A visual representation of pitch and yaw
adjustment is shown in Figures.
35
Pitch
Adjustment
Yaw
Adjustment

36. Aerodynamic Power Control: Passive Stall, Active
Stall, and Pitch Control
 The aerodynamics of wind turbines are very similar to that of airplanes.
 The blade rotates in the wind because the air flowing along the surface that is not
facing the wind moves faster than that on the surface against the wind.
 This creates a lift force to pull the blade to rotate.
 The angle of attack of the blade plays a critical role in determining the amount of force
and torque generated by the turbine. Therefore, it is an effective means to control the
amount of captured power.
 There are three aerodynamic methods to control the capture of power for large wind
turbines: passive stall, active stall, and pitch control.
36

37. Passive-Stall Control
 In passive-stall-controlled wind turbines, the blade is fixed onto
the rotor hub at an optimal (rated) angle of attack.
 When the wind speed is below or at the rated value, the
turbine blades with the rated angle of attack can capture
the maximum possible power from the wind.
 With the wind speed exceeding the rated value, the strong
wind can cause turbulence on the surface of the blade not
facing the wind. As a result, the lifting force will be reduced
and eventually disappear with the increase of the wind
speed, slowing down the turbine rotational speed.
 This phenomenon is called stall. The stall phenomenon is
undesirable for airplanes, but it provides an effective means
to limit the power capture to prevent turbine damage.
37

38. Continue…
38
 The blade profile is aerodynamically designed to ensure that stall
occurs only when the wind speed exceeds the rated value.
 To ensure that the blade stall occurs gradually rather than abruptly,
the blades for large wind turbines are usually twisted along the
longitudinal axis by a couple of degrees.
 The passive-stall-controlled wind turbines do not need complex pitch
mechanisms, but the blades require a complex aerodynamic
design.
 The passive stall may not be able to keep the captured power PM at
a constant value.
 It may exceed the rated power at some wind speeds, which is not a
desirable feature.

39. Active Stall Control
 In active-stall turbines, the stall phenomenon can be
induced not only by higher wind speeds, but also by
increasing the angle of attack of the blade.
 Thus, active-stall wind turbines have adjustable blades
with a pitch control mechanism.
 When the wind speed exceeds the rated value, the
blades are controlled to turn more into the wind, leading
to the reduction of captured power.
 The captured power can, therefore, be maintained at
the rated value by adjusting the blade angle of attack.
39

40. Continue…
When the blade is turned
completely into the wind, the
blade loses all interaction with
the wind and causes the rotor
to stop. This operating
condition can be used above
the cut- out wind speed to
stop the turbine and protect it
from damage.
40

41. Pitch Control
 Similar to the active-stall control, pitch-controlled wind
turbines have adjustable blades on the rotor hub.
 When the wind speed exceeds the rated value, the pitch
controller will reduce the angle of attack, turning the blades
(pitching) gradually out of the wind.
 The pressure difference in front and on the back of the blade
is reduced, leading to a reduction in the lifting force on the
blade.
 When the wind is below or at the rated speed, the blade
angle of attack is kept at its rated (optimal) value.
 With higher than the rated wind, the angle of attack of the
blade is reduced, causing a reduction in lift force.
41

42. Continue…
 When the blade is fully pitched,
the blade angle of attack is
aligned with the wind, and no lift
force will be produced.
 The turbine will stop rotating and
then be locked by the
mechanical brake for protection.
42

43. Wind Turbine Characteristics
Let the wind approaches the turbine with constant velocity, with homogeneous
properties (such as temperature and density), and without turbulence.
Under this condition the fraction of the power extracted from the wind by a real wind
turbine can be defined by the symbol Cp, , that is , the coefficient of performance or
power coefficient. The actual mechanical power output Pm from the wind turbine is
expressed in the following equation:
43
),(
2
1
)
2
1
( 323
 pwwpm CvRAvCP ==
Where R is the blade radius of the wind turbine (m)
is the wind speed (m/s)
is the air density in

3
/mkg
wv
wv


44. Continue…
 The coefficient of performance varies with the wind speed, the
rotational speed of the turbine, and turbine blade parameters, that
is, blade pitch angle and angle of attack. Therefore, the power
coefficients, , is mainly a function of tip-speed ratio , and blade
pitch angle [deg.]. The tip speed ratio is defined as:
44
Where
is the mechanical angular velocity of the turbine rotor (rad/s)
is the wind speed (m/s)
W
R
V
Rw
=
wvRw


pC

45. Continue…
 The angular velocity, , is determined from the rotational speed, (r/min) as
follows:
45
The wind turbine characteristics can be found in the manufacture datasheet.
In order to calculate for the given values of and , the following numerical
approximations can be used, as indicated in the following equations:
1
03.0
02.0
1
1
3
+
−
+
=

i
i
eC
i
p




4.18
14.2
]2.13002.058.0
151
[73.0),(
−
−−−=
pC  
Rw n
60
2 n
wR

=

46. Continue…46
Power = Torque x angular speed

47. Control Strategies
 Recall that controlling the pitch of the blade
and speed of the generator are the most
effective methods to adjust output power.
 The following control strategies use pitch and
generator speed control to manage turbine
functionality throughout the power curve:
fixed-speed fixed-pitch (FSFP), fixed-speed
variable-pitch (FSVP), variable-speed fixed-
pitch (VSFP), and variable-speed variable-
pitch (FSVP).
47

48. Betz limit
➢ The Betz limit is the theoretical maximum efficiency for a wind
turbine, conjectured by German physicist Albert Betz in 1919.Betz
concluded that this value is 59.3%, meaning that at most only
59.3% of the kinetic energy from wind can be used to spin the
turbine and generate electricity.
➢ In reality, turbines cannot reach the Betz limit, and common
efficiencies are in the 35-45% range.
48

49. References
 Rekioua, Djamila. "Wind Power Electric Systems." Green Energy and
Technology. Springer, 2014.
 Anaya-Lara, O., Jenkins, N., Ekanayake, J. B., Cartwright, P., & Hughes, M.
(2011). Wind energy generation: modelling and control. John Wiley & Sons.
49

50. Questions
 Explain about pitch control of wind generators.
 Explain about Yaw control of wind generators.
 What is power coefficient and mention its significance to wind turbine
operation?
 Why the pitch and yaw adjustments are necessary for wind generators?
 Explain about the passive and active stall control.
 What is Betz constant and mention its significance to wind turbine
operation?
50