Lecture 5

Wind Energy Systems ITE – 1883

Lecture Delivered ByUmair N. Mughal

2

What do these two have in common?

• Tornado • Boeing 777

January 11, 2005

• 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”

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.

Basic Concepts

• The interaction is defined in terms of variables.i. System inputii. System outputiii. Environmental disturbances

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.

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.

• Control systems can be classified into two categories:

i. Open-loop control system

ii. Closed-loop feedback control system

Process Output

Input

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 Signa

l

Energy

Source

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.

Control System Classification• An open-loop control system utilizes an actuating device to control the process directly without

using feedback.

• 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.

Actuating Device Process Output

Desired Output

Response

Desired Output

Response

Measurement

Output

Controller

ProcessComparison

Single Input Single Output (SISO) System

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

General Control System

Sensor

Actuator ProcessController ++

Set-point or

Reference input

Actual Outpu

t

ErrorControlled Signal

Disturbance

Manipulated

Variable

Feedback Signal

+

-

++

1. Measure system response, including effects of disturbances, using (noisy) sensors2. Compare actual system response to desired system response at each time “Error” signal(s) = (Desired response)-(Actual response)3. Use “error” signals to drive compensator (controller) so as to generate real-time control corrections so as to keep

“errors” small for all time.

Logical Signal

Industrial Control Example - Level 0 and 1

Controller Actuators

Sensors OutputVariables

InputParameters(Level 2)

Process

Inputs

Error

Feedback Signal

13

Managerial Control Model

If

InadequateIf Adequate

Adjust StandardsAdjust 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.

Blood Sugar Control

15

Negative Feedback:.

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

Human Closed Loop System brain

spinal cord

muscles

joints

movementsee

skintouch

muscle sensors(length and force)

sensory feedback

reflex!another layer ofsensory feedback

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Human Closed Loop System (Box Diagram)

Brain Spinal cord Muscles Joints Movement

Controller Actuator(muscles)

Joints++_

DesiredMovement

Central Nervous System

Central Nervous System

Cruise Control Block Diagram

18

Magnet and Coil Sensor

set point, SP

(desired speed)

Gas Pedal ControlElement

disturbances

(hills, wind, etc.)measured “click rate”PV signal

controller errore(t) = (SP – PV)

manipulated gas flow rateto car engine car speed

controller output, COsignal to gas pedal

+-+-Car Speed

ProcessCruise

Controller

Copyright © 2007 by Control Station, Inc. All Rights Reserved

MIMO Control System

DISTURBANCES

DYNAMIC SYSTEM (PLANT)

CONTROLLER (COMPENSATOR)

COMMANDS CONTROLS OUTPUTS

Transient response: Gradual change of output from initial to the desired conditionSteady-state response: Approximation to the desired response

Settling time

Overshoot

Controlledvariable

Time

Reference

%

Steady StateTransient State

Steady state error

Response Characteristics

Performance SpecsStability

UnstableStable

Wind Turbine Design is an Interdisciplinary Problem

Aerodynamics

Structures,Structural Dynamics,Vibrations, Stability,

Fatigue Life

Control systems forRPM, Pitch, Yaw

Transmission, gears, tower,

power systems, etc.

Cost

Noise,aesthetics

Control System Design Process1. 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

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.

KidWind Project | www.kidwind.org

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

Power Capture

Pow

er

[kW

]Wind Speed v [m/s]

Pwind

v3Ideal turbine

(max. 60% efficient)

Prated

vcut-invcut-outvrated

Source: Dr. Karl Stol, UoA

Torque Control

Pitch Control

Reg

ion

1

Reg

ion

2

Reg

ion

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

Modern Control Objectives for the Wind Turbine

31

Individual Blade Pitching

With modern control (MIMO) we can control the load on each blade individually

This now allows mitigation of ASSYMETRIC loading:Wind shearTower shadowInertial loadsTurbulence across the swept area

32

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

Control of powerReducing the power at high windspeed

• 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

Wind attack point

Wind attack point

At high wind the power is reduced by pitching the blades. This can be done in two ways.

Control of Power PitchingLow wind High wind Stop

Pitch variable speed and optislip

Active stall

Passive stall

Control of powerIso-power map wind speed and pitch angle

― Pitch control

0 kW

500 kW

1000 kW

1500 kW

2000 kW

2500 kW

Win

d s

pe

ed m

/s

5

25

15

10

20

-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.

The Betz Power

Wind Turbine Control

Wind Turbine Control

(more detail)

Control systems Fixed speed

Get

rieb

e 1:

50

Parkingbrake

Rotorbearing

Bypasscontactor

Soft startequipment

WTGcontrol

Asynchronous generator

6 ... 33 kV, f = 50 Hz/6 ... 34,5 kV, f = 60 Hz

Step-uptransformer

HVswitchgear

ABB drawing

Passive Stall

Gearbox

Generatorswitchgear

ACf = constantn = costant

Control systems Fixed speed

Get

rieb

e 1:

50

Parkingbrake

Rotorbearing

Bypasscontactor

Soft startequipment

WTGcontrol

Asynchronous generator

Step-uptransformer

HVswitchgear

ABB drawing

Active Stall, Pitch Control

Gearbox

Generatorswitchgear

ACf = constantn = costant

Pitchdrive

6 ... 33 kV, f = 50 Hz/6 ... 34,5 kV, f = 60 Hz

Control systems Semi-variable speed

ABB drawing

Variable slip, pitch control

Get

rieb

e 1:

50

Parkingbrake

Rotorbearing

Bypasscontactor

Soft startequipment

WTGcontrol

Asynchronous generator

Step-uptransformer

HVswitchgear

Gearbox

Generatorswitchgear

ACf = constantn = semi-variable

Pitchdrive

RCCunit

RCCcontrol

HEAT

6 ... 33 kV, f = 50 Hz/6 ... 34,5 kV, f = 60 Hz

Control systemVariable speed

ABB drawing

Variable speed control DFIG (doubly fed induction generator)

Get

rieb

e 1:

50

Parkingbrake

Rotorbearing

WTGcontrol

Doubly-fedasynchronous

generator Step-uptransformer

HVswitchgear

Gearbox

Generatorswitchgear

ACf = constantn = variable

Pitchdrive

Generatorside

converter

Gridside

converter

Converter control

6 ... 33 kV, f = 50 Hz/6 ... 34,5 kV, f = 60 Hz

Control system Variable speed

ABB drawing

Variable speed control with full scale converter

Get

rieb

e 1:

50

Parkingbrake

Rotorbearing

WTGcontrol

Step-uptransformer

HVswitchgear

Gearbox

Generatorswitchgear

ACf = variablen = variable

Pitchdrive

Converter control

6 ... 33 kV, f = 50 Hz/6 ... 34,5 kV, f = 60 Hz

Asynchronous or synchrounous generator

Converter

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

Higher Speed shaft

Low Speed Gear

High Speed Gear

Gearing designs

47

Spur (external contact)

Spur (internal contact)

Helical PlanetaryWorm

“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.

Gearing designs

48

Source: E. Hau, “Wind turbines: fundamentals, technologies, application, economics, 2nd edition, Springer 2006.

Tradeoffs between size, mass, and relative cost.

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 producedWhen the rotor of the generator rotates faster than the stator, a strong current is induced in the rotorThe 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 gridThe 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

Type of generatorFixed speed

asynchronous generator

50 Hz

60 x frequencynumber of pole pairsrpm =

Rotational speed6-poled stator

rpm1000

+ kW (generator)

- kW(motor)

Type of generator Variable speed asynchronous generators

50 Hz

AC

DC AC

DC

Stator field = 1000 rpm

Rotor mechanically = 1100 rpm

Electric Generators

generator

full power

PlantFeeders

actodc

dctoac

generator

partia l power

PlantFeeders

actodc

dctoac

generator

Slip poweras heat loss

PlantFeeders

PF controlcapacitor s

actodc

generator

PlantFeeders

PF controlcapacitor s

Type 1 (Asynchronous)Conventional Induction Generator (fixed speed)

Type 2Wound-rotor Induction

Generator w/variable rotor resistance

Type 3 (Synchronous)Doubly-Fed Induction

Generator (variable speed)

Type 4 (Asynchronous)Full-converter interface

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Type 3 Doubly Fed Induction Generator (Synchronous)

generator

partia l power

PlantFeeders

actodc

dctoac

•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

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Conventional Induction Generator (fixed speed)

• Direct connected.• Simplest.• Requires switch to prevent motoring.• Draws reactive power with no reactive control.

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.

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.

Generator Design Considerations

Other Factors:• Weight• Starting overcurrent• Dynamic response behavior• Speed range

Connection to the grid

Connection to grid Direct

Grid frequency AC

PCC

Grid frequency AC

Connection to gridIndirect

Variable frequency AC

(e.g. from synchronous generator)DC

Irregular switched AC Grid frequency AC

Rectifier Inverter PCC

Control System Architecture of Wind Farm

Wind turbine

SensorsPositions, speeds, accelerations, stresses, strains, temperature, electrical & fluid characteristics, etc.

SensorsPositions, speeds, accelerations, stresses, strains, temperature, electrical & fluid characteristics, etc.

SupervisorChoice of operating condition:• Start up• Power production• Emergency shut-down …

SupervisorChoice of operating condition:• Start up• Power production• Emergency shut-down …

Active control systemControl strategy

Active control systemControl strategy

ObserversWind, tower & bladesObserversWind, tower & blades

Wind farm supervisorWind farm supervisor

Communication and reporting Communication and reporting

ActuatorsActuator control system

ActuatorsActuator control system

Supervisory Control SystemSupervisory 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, …

Supervisory Control SystemSupervisory Control System

IdlingIdling Power production

Power productionStart upStart up

Normal shut downNormal shut down

Emergency shut down

Emergency shut down

V > V cut-inV > V cut-in RPM > Wcut-in

RPM > Wcut-in

V > V cut-offV > V cut-off

V < V cut-inV < V cut-in

• Failures• Overspeed & high rotor accel.

• Vibrations

• Failures• Overspeed & high rotor accel.

• Vibrations

Representative operational state monitoring logic:

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)

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

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%.

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.

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

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.

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

Type of atmospheric ice

71

In-Cloud Icing

Rime

Hard Rime Soft Rime

Glaze

Precipitation Icing

Wet Snow Freezing Rain

Frost

Heat flux balance for iced surface; [2].

,freezing adiabatic kinetic sub evap Conduction Convective RadiationQ Q Q Q Q Q Q

Q = Heat Flux

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

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.

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

• 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.

Average Production Losses: Comparison

SiteWind 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 %