This presentation contains, i. Basics of Control Systems, ii. Wind Turbine Controls iii. Basics about Wind Farm and Control iv. Wind Turbine Gearbox v. Wind Turbine Generator vi. Grids

1. Lecture 5
Wind Energy Systems
ITE – 1883
Lecture Delivered By
Umair N. Mughal

2. What do these two have in common?
• Tornado • Boeing 777
• Highly nonlinear, complicated dynamics!
• Both are capable of transporting goods and people over long distances
BUT
• One is controlled, and the other is not.
• Control is “the hidden technology that you meet every day”
• It heavily relies on the notion of “feedback”
January 11, 2005 2

3. Basic Concepts
• System
– A collection of components which are coordinated together to perform a function.
• Dynamic System
– A system with a memory.
– For example, the input value at time t will influence the output at future instant.
• A system interact with their environment through a controlled boundary.

4. Basic Concepts
• The interaction is defined in terms of variables.
i. System input
ii. System output
iii. Environmental disturbances

5. System Variables
• The system’s boundary depends upon the defined objective function of the system.
• The system’s function is expressed in terms of measured output variables.
• The system’s operation is manipulated through control input variables.
• The system’s operation is also affected in an uncontrolled manner through
disturbance input variables.

6. Block Diagram
• Component or process to be controlled can be represented by a block diagram.
• The input-output relationship represents the cause and effect of the process.
Input Process Output
• Control systems can be classified into two categories:
i. Open-loop control system
ii. Closed-loop feedback control system

7. Control System
• Control is the process of causing a system variable to conform to some desired
value.
• Manual control Automatic control (involving machines only).
• A control system is an interconnection of components forming a system
configuration that will provide a desired system response.
Control
System
Output
Signal
Input
Signal
Energy
Source

8. Manual Vs Automatic Control
• Control is a process of causing a system variable such as temperature or position to
conform to some desired value or trajectory, called reference value or trajectory.
• For example, driving a car implies controlling the vehicle to follow the desired path to
arrive safely at a planned destination.
i. If you are driving the car yourself, you are performing manual control of the car.
ii. If you use design a machine, or use a computer to do it, then you have built an automatic control
system.

9. Control System Classification
• An open-loop control system utilizes an actuating device to control the process directly without
using feedback.
Actuating
Device
Process Output
Desired Output
Response
• A closed-loop feedback control system uses a measurement of the output and feedback of the
output signal to compare it with the desired output or reference.
Desired
Output
Response
Comparison Controller Process Output
Measurement
Single Input Single Output (SISO) System

10. Control System Components
i. System, plant or process
– To be controlled
ii. Actuators
– Converts the control signal to a power signal
iii. Sensors
– Provides measurement of the system output
iv. Reference input
– Represents the desired output

11. General Control System
+
+ Controller Actuator + Process
Sensor
Set-point
or
Reference
input
Actual
Output
Error
Controlled
Signal
Disturbance
Manipulated
Variable
Feedback Signal
+
-
+
1. Measure system response, including effects of disturbances, using (noisy) sensors
2. Compare actual system response to desired system response at each time
“Error” signal(s) = (Desired response)-(Actual response)
1. Use “error” signals to drive compensator (controller) so as to generate real-time control corrections so as to keep
“errors” small for all time.

12. Industrial Control Example - Level 0 and 1
Logical
Signal
Controller Actuators
Inputs
Sensors Output
Variables
Input
Parameters
(Level 2)
Process
Error
Feedback Signal

13. Managerial Control Model
13
If
Inadequate
If Adequate
Adjust Standards
Adjust Performance
Feedback
Establish
Strategic
Goals.
1. Establish
standards of
performance.
2. Measure
actual
performance.
3. Compare
performance to
standards.
4. Take
corrective
action.
4. Do nothing or
provide
reinforcement.

14. Blood Sugar Control

15. 15
Negative Feedback:.
Stimulus triggers response to counteract further change in the same direction.
Negative-feedback mechanisms prevent small changes from becoming too large.

16. Human Closed Loop System
brain
spinal cord
muscles
joints
movement
muscle sensors
(length and force)
see
skin
touch
sensory feedback
reflex!
another layer of
sensory feedback

17. Human Closed Loop System (Box Diagram)
17
Brain Spinal cord Muscles Joints Movement
Controller Actuator
+ Joints
(muscles)
+
_
Desired
Movement
Central Nervous System
Central Nervous System

18. Cruise Control Block Diagram
18
Magnet and Coil Sensor
set point, SP
(desired speed)
Gas Pedal
Control
Element
disturbances
measured “click rate” (hills, wind, etc.)
PV signal
controller error
e(t) = (SP – PV)
manipulated
gas flow rate
to car engine car speed
controller output, CO
signal to gas pedal
+-
Car Speed
Process
Cruise
Controller
Copyright © 2007 by Control Station, Inc.
All Rights Reserved

19. MIMO Control System
DISTURBANCES
COMMANDS CONTROLS OUTPUTS
DYNAMIC SYSTEM
(PLANT)
CONTROLLER
(COMPENSATOR)

20. Response Characteristics
Transient response: Gradual change of output from initial to the desired condition
Steady-state response: Approximation to the desired response
Overshoot
Settling time
Controlled
variable
Steady state error
Time
Reference
%
Transient State Steady State

21. Performance Specs
Stability
Stable Unstable

22. Wind Turbine Design is an Interdisciplinary Problem
Aerodynamics
Structures,
Structural Dynamics,
Vibrations, Stability,
Fatigue Life
Control systems for
RPM, Pitch, Yaw
Transmission,
gears, tower,
power systems,
etc.
Cost
Noise,
aesthetics

23. Control System Design Process
1. Establish control goals
2. Identify the variables to control
3. Write the specifications for the variables
4. Establish the system configuration and identify the actuator
5. Obtain a model of the process, the actuator and the sensor
6. Describe a controller and select key parameters to be adjusted
7. Optimize the parameters and analyze the performance
If the performance meet the specifications, then finalize design
If the performance does not
meet specifications, then
iterate the configuration
and actuator

24. WIND ENERGY CONVERSION SYSTEMS
•Power is transferred from the wind to the rotor then passed through the gearbox, generator, and power electronics
until it finally reaches the gird.
•Each stage of the power transfer has a certain efficiency. Therefore, each power transfer stage presents an
opportunity to reduce the cost of energy from a wind turbine.

26. KidWind Project | www.kidwind.org

29. Control Objectives
• Speed control
– Maintain rated rotor speed in above rated winds
• Load control
– Oscillations occur in the Low Speed Shaft (LSS)
– Reduce loads in LSS

30. Power Capture
Power
[kW]
Torque Control
Pitch Control
Wind Speed
v [m/s]
Pwind
 v3
Ideal turbine
(max. 60% efficient)
Prated
vcut-in v vcut-out rated
Source: Dr. Karl Stol, UoA
Region 1
Region 2
Region 3
Region 1:
 Turbine is stopped
Region 2:
 Maintain constant tip speed ratio to
produce maximum power below
rated wind speed.
Region 3:
 Maintain rated rotor speed and
power.
Maintain Rotor Speed
 Keep the best tip speed ratios in
Region 2
 Not exceed rated velocity in Region 3
 Have smooth power output
Reducing DYNAMIC loading on
the turbine.
 Blade flap
 Tower fore-aft vibration
 Drive train torsional vibration

31. Modern Control Objectives for the Wind Turbine
31

32. Individual Blade Pitching
With modern control (MIMO) we can control the load on each
blade individually
This now allows mitigation of ASSYMETRIC loading:
Wind shear
Tower shadow
Inertial loads
Turbulence across the swept area

33. Common Power Control Methods
• Pitch control
- blade pitch and blade angle of attack is decreased with wind speed greater than rated speed.
- Wind speed and power output and power out put are continuous monitored by sensors
- Need sophisticated control mechanism
• Stall control
- blades are designed in such a that with increase in wind speed, the angle of attack increases.
- Pressure variation at the tp and bottom surface changes causing flow separation and vortex shedding
- kills lift forces and leads to blades stalling
- Need very sophisticated blade aerodynamic design
• Active stall-Controlled power regulation
- The blades are pitched to to attain its best performance.
- As the wind speed exceeds the rated velocity, the blades are turned in the opposite direction to increase the angle
of attack and forces the blade to stall region.
• Yaw Control
- The rotor is partly pushed away from the wind direction at higher wind speeds.
- The rotor spin axis is pushed to an angle to the incoming wind direction

34. Control of power
Reducing the power at high windspeed
At high wind the power is reduced by pitching the blades. This can be done in two ways.
• Reducing the lift and
over speeding called
Pitch variable speed
• Reducing the lift by
generating stall
Flow on upper and lower
surface equal  no lift

35. Control of Power Pitching
Low wind High wind Stop
Pitch variable
speed and optislip
Passive stall
Active stall

36. Control of power
Iso-power map wind speed and pitch angle
― Pitch control
2500 kW
2000 kW
1500 kW
1000 kW
500 kW
0 kW
25
20
15
10
5
-10 0 +10 +20 +30
Pitch angle (deg)
-20
72 m rotor 2MW turbine
― Stall control
This diagram show how much the turbine is able to
produce depending on the pitch angles and wind speed.
Other blades and dimensions will have different diagrams
but the principles will be the same. The red and green
lines show the pitch angle for a pitch regulated and a stall
regulated turbine. Up to rated power the pitch angle is
identical for the two different systems. From this diagram
you can see it is possible to limit the maximum power to 2
MW on a stall turbine just by setting the pitch angle to -4°
without adjusting. To reach the same by pitching it is
needed to pitch from 0 to +25° this means the demands
to the pitch system is high in order to avoid power peaks
when the wind is increasing. If the wind is increasing from
17 to 19 m/s and the pitch is not adjusted the power is
increased from 2 to 3 MW which means 50 % overload.

37. The Betz Power

38. Wind Turbine
Control

39. Wind Turbine
Control
(more detail)

41. Control systems
Fixed speed
Parking
brake
Rotor
bearing
Bypass
contactor
Soft start
equipment
WTG
control
Asynchronous generator
Step-up
transformer
6 ... 33 kV, f = 50 Hz/
6 ... 34,5 kV, f = 60 Hz
HV
switchgear
ABB drawing
Gearbox
Passive Stall
Generator
switchgear
AC
f = constant
n = costant

42. Control systems
Fixed speed
Parking
brake
Rotor
bearing
Bypass
contactor
Soft start
equipment
WTG
control
Asynchronous generator
Step-up
transformer
HV
switchgear
ABB drawing
Active Stall, Pitch Control
Gearbox
Generator
switchgear
AC
f = constant
n = costant
Pitch
drive
6 ... 33 kV, f = 50 Hz/
6 ... 34,5 kV, f = 60 Hz

43. Control systems
Semi-variable speed
ABB drawing
Parking
brake
Variable slip, pitch control
Rotor
bearing
Bypass
contactor
Soft start
equipment
WTG
control
Asynchronous
generator
Step-up
transformer
HV
switchgear
Gearbox
Generator
switchgear
AC
f = constant
n = semi-variable
Pitch
drive
RCC
unit
RCC
control
HEAT
6 ... 33 kV, f = 50 Hz/
6 ... 34,5 kV, f = 60 Hz

44. Control system
Variable speed
ABB drawing
Variable speed control DFIG (doubly fed induction generator)
Parking
brake
Rotor
bearing
WTG
control
Doubly-fed
asynchronous
generator Step-up
transformer
HV
switchgear
Gearbox
Generator
switchgear
AC
f = constant
n = variable
Pitch
drive
Generator
side
converter
Grid
side
converter
Converter
control
6 ... 33 kV, f = 50 Hz/
6 ... 34,5 kV, f = 60 Hz

45. Control system
Variable speed
ABB drawing
Variable speed control with full scale converter
Parking
brake
Rotor
bearing
WTG
control
Step-up
transformer
HV
switchgear
Gearbox
Generator
switchgear
AC
f = variable
n = variable
Pitch
drive
Converter
control
6 ... 33 kV, f = 50 Hz/
6 ... 34,5 kV, f = 60 Hz
Asynchronous or
synchrounous generator
Converter

46. Gearbox
A gear box is commonly used for stepping up the speed of the generator, although designs may also use direct
drive of an annular generator. Some models operate at constant speed, but more energy can be collected by
variable-speed turbines which use a solid-state power converter to interface to the transmission system. All
turbines are equipped with protective features to avoid damage at high wind speeds, by feathering the blades
into the wind which ceases their rotation, supplemented by brakes.
Gear Transmission
- Speed of a typical rotor may be 30 rpm to 50 rpm.
- Generator speed may be around 1000 rpm to 1500 rpm.
- Need gear trains in the transmission line to manipulate the speed
- May need multiple stages to achieve the speed ratio.
- Connects the low speed shaft of the rotor to the high sped shaft of the generator.
Additional Requirements,
Heat dissipation, power losses and cooling, Compact design, Weight, Bearing system
Lower
speed shaft
Low Speed Gear
Higher
Speed
shaft
High Speed
Gear

47. Gearing designs
Spur
(external
contact)
47
Spur
(internal
contact)
Worm Helical Planetary
“parallel shaft”
Parallel (spur) gears can achieve gear ratios of 1:5.
Planetary gears can achieve gear ratios of 1:12.
Wind turbines almost always require 2-3 stages.

48. Gearing designs
Tradeoffs between size,
mass, and relative cost.
Source: E. Hau, “Wind turbines: fundamentals, technologies, application,
economics, 2nd edition, Springer 2006.
48

49. Type of generator
Synchronous
Asynchronous
Common in fossil fuel
powerplants, but rare in wind
turbines. Rotation speed is
synchronized with the grid
frequency
If the rotor were to rotate at the same frequency as the
electric field in the stator, no electricity would be produced
When the rotor of the generator rotates faster than the
stator, a strong current is induced in the rotor
The harder one cranks on the rotor, the more power that is
transferred as electromagnetic force to the stator,
converted to electricity, and fed to the grid
The difference in the rotation speed between no power
and peak power is about 1%, but this slip reduces stress on
the rotor and smoothes out power variations

50. Type of generator
Fixed speed
asynchronous generator
50 Hz
6-poled stator Rotational speed
60 x frequency
number of pole pairs
rpm =
1000rpm
+ kW
(generator)
- kW
(motor)

51. Type of generator
Variable speed asynchronous generators
50 Hz
AC
DC AC
DC
Stator field = 1000 rpm
Rotor mechanically = 1100 rpm

52. Electric Generators
generator
full power
Plant
Feeders
ac
to
dc
dc
to
ac
generator
partial power
Plant
Feeders
ac
to
dc
dc
to
ac
generator
Slip power
as heat loss
Plant
Feeders
PF control
capacitor s
ac
to
dc
generator
Plant
Feeders
PF control
capacitor s
Type 1 (Asynchronous)
Conventional Induction
Generator (fixed speed)
Type 2
Wound-rotor Induction
Generator w/variable rotor
resistance
Type 3 (Synchronous)
Doubly-Fed Induction
Generator (variable speed)
Type 4 (Asynchronous)
Full-converter interface
52

53. Type 3 Doubly Fed Induction Generator (Synchronous)
generator
partial power
Plant
Feeders
ac
to
dc
dc
to
ac
•Most common technology today
• Provides variable speed via rotor freq control
• Converter rating only 1/3 of full power rating
• Eliminates wind gust-induced power spikes
• More efficient over wide wind speed
• Provides voltage control
53

54. Conventional Induction
Generator (fixed speed)
• Direct connected.
• Simplest.
• Requires switch to prevent motoring.
• Draws reactive power with no reactive control.

55. Wind Generator Topologies
• Doubly-fed.
• The doubly-fed topology is the most common for high power.
• Rotor control allows for speed control of around 25% of synchronous.
• Rotor converter rating is only around 25% of total generator rating.
• Reactive power control.

56. Wind Generator Topologies
• Full-rated converter connected.
• Lower cost generator than DFIG. Lower maintenance.
• Converter must be full-rated.
• Full-rated converter allows for complete speed and reactive power control.
• Could also be used with a synchronous generator.

57. Generator Design Considerations
Other Factors:
• Weight
• Starting overcurrent
• Dynamic response behavior
• Speed range

58. Connection to the grid

59. Connection to grid
Direct
Grid frequency AC
PCC
Grid frequency AC

60. Connection to grid
Indirect
Variable frequency AC
Rectifier Inverter PCC
(e.g. from synchronous generator)
DC
Irregular switched AC Grid frequency AC

61. Control System Architecture of Wind Farm
Wind
turbine
Sensors
Positions, speeds,
accelerations, stresses,
strains, temperature,
electrical & fluid
characteristics, etc.
Supervisor
Choice of operating condition:
• Start up
• Power production
• Emergency shut-down …
Observers
Wind, tower & blades
Active control
system
Control strategy
Wind farm
supervisor
Communication and reporting
Actuators
Actuator control
system

62. Supervisory Control System
Main tasks:
• Operational managing and monitoring
• Diagnostics, safety
• Communication, reporting and data logging
• …
Operational states:
• Idling
• Start Up
• Normal power production
• Normal shut down
• Emergency shut down
Main input data:
• Wind speed
• Rotor speed
• Blade pitch
• Electrical power
• Temperatures in critical area
• Accelerations
• …
but also
• Stresses, strains (blades, tower)
• Position, speed (yaw, blade, actuators,
teetering angle, rotor tilt, …)
• Fluid properties and levels
• Electrical systems (voltages, grid
characteristics, …)
• Icing conditions, humidity, lighting, …

63. Supervisory Control System
Idling
Power
production
Start up
V > V cut-in RPM > Wcut-in
Emergency shut
down
Normal shut down
V > V cut-off
V < V cut-in
• Failures
• Overspeed & high rotor accel.
• Vibrations
Representative operational state monitoring logic:

64. Capacity Factor
• This is the mean (average) power output of the turbine divided by the peak (or rated) power output
• The mean power output is computed as the power output in the centre of each wind speed interval, times the
probability of that interval, summed over all intervals and divided by the total probability (which is 1.0)

65. Variable Speed Generators
• Becoming more common
• Rotation rate of rotor varies with wind speed from 8 rpm to 16 rpm
• Results in less stress on the structure and more uniform variation in
power output
• Requires more complex electronics and gearbox to always produce
electricity at the fixed grid frequency

66. Wind Energy in Cold Regions
• Wind is a widely accepted source of clean energy.
• Arctic regions has good resources of wind energy.
• Atmospheric ice accretion on wind turbine blades in arctic is a major hindrance
in proper use of wind energy.
• Icing is mainly caused by the impingement of super cooled water droplets.
• Average annual power production losses of wind turbines due to atmospheric
ice accretion are estimated to be about 20%.

67. Effects of Atmospheric Icing
• Main effects of atmospheric ice accretion on wind turbine are:
– Disrupted blade aerodynamics.
– Increased fatigue due to mass imbalance.
– Human’s harm due to ice shedding.
– Instrumental measurement errors.
– Loss of power production.
– Complete stop of power production.
• Active anti icing and de-icing systems are installed on wind turbines
to minimize these effects.
– This complicates the design and increase the cost.

68. Plot shows the calculated power curve for a pitch controlled wind turbine with different ice accretions; [1].

69. Research Need
• Keeping in view the potential of wind resources in cold regions, it is important:
– To understand the physics of atmospheric ice accretion on large wind turbine blades.
– To analyze the effect of various operating and blade geometric parameters on rate and
shape of ice growth.
– To estimate the resulting performance losses.
– To purpose actions to minimize performance losses.

70. Atmospheric Icing: Physics
• Atmospheric icing on wind turbines mainly occurs due to collision of super
cooled water droplets with the exposed surface.
• Mainly three types of atmospheric icing occurs:
– Light rime ice
– Hard rime ice
– Glaze ice
• The type of ice formed on a surface, heavily depends upon the surface
thermal balance.

71. 71
Type of atmospheric ice
71
In-Cloud Icing
Rime
Hard Rime Soft Rime
Glaze
Precipitation Icing
Wet Snow
Freezing
Rain
Frost

72. freezing adiabatic kinetic sub,evap Conduction Convective Radiation Q Q Q  Q Q Q Q
Heat flux balance for iced surface; [2].
Q = Heat Flux

73. Wind Turbines : Atmospheric Icing
• Atmospheric ice accretion on wind turbines mainly
depends upon following variables:
1. Point of operation (Location, Altitude etc).
2. Geometry of wind turbine blade.
3. Relative wind velocity.
4. Atmospheric temperature.
5. Droplet diameter.
6. Liquid water content.
• Field measurements and numerical modeling
techniques were used to understand the atmospheric
ice growth on large wind turbines

74. Ice Monitoring: Field Measurements
• Field measurements were taken at Nygårdsfjell wind park (2.3 MW, pitch
controlled Siemens wind turbines), mainly focusing on ice monitoring and
its resultant effects on power production.
• Field measurements were taken using HoloOptics icing sensors,
anemometers, web cameras, data acquisition system and an onsite
weather station.
• Power losses were calculated for wind speed > 5 m/s, and temp > +2 C
(summer) and < 0.5 C (winter).
• Due to instrumental problems , the average production losses at
Nygårdsfjell was recorded 0.5 % due to ice.

75. Anemometer, icing sensor, Web Camera installed on Siemens SWT-2.3 MW wind turbine at Nygårdsfjell.

76. Average Production Losses: Comparison
• Data from three different wind park sites (Nygårdsfjell , Sveg &
Aapua) in Sweden and Norway was compared to estimate the
average annual performance losses due to ice accretion.
Site
Wind Turbine Power loss
(summer)
Power loss
(winter)
Nygårdsfjell Siemens SWT-
2.3 MW
0.3 % 0.5 %
Sveg Vestas V90 -
2MW
- 5%
Aapua Vestas V82 –
1.5 MW
6.6 % 28 %