small scale wind technologies part 2 - creeslough.com slides 2.pdf · inverter power vs input...

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Small Scale Wind TechnologiesPart 2

Centre for Renewable Energy at Dundalk IT

CREDIT

© CREDIT Dundalk IT

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Part 2

Small and large scale wind turbine

technologies


Overview of small scale grid connected system

Variable (wild) AC

DC

Grid quality AC

Rectifier/Controller

Grid TieInverter

Grid

FusePanel

Meter

Wind Turbinewith PMG

Note: Isolator/protection switches not shown

Dump Load


Airfoil Shape

Just like the wings of an airplane, wind turbine blades use the airfoil shape to create lift and maximize efficiency.

The Bernoulli Effect


Basic Aerodynamics

• The Lift Force is perpendicular to the direction of motion. We want to make this force

BIG.

• The Drag Force is parallel to the direction of motion. We want to make this force small.

α = low

α = medium<10 degrees

α = HighStall!!


Lift/Drag Forces on rotating blade section

V

LΩRW

D

Head Wind

Direction ofrotation

V = Wind speedW = Apparent wind speedΩ= rotational speed ΩR = speed of blade section at a distance R from hubL = Lift

D =Drag


Tip-Speed Ratio

• Tip-speed ratio is the ratio of the speed of the rotating blade tip to the speed of the free stream wind

• There is an optimum angle of attack which creates the highest lift to drag ratio

• Because angle of attack is dependant on wind speed, there is an optimum tip-speed ratio

ΩRU

TSR =Where,

Ω = rotational speed in radians/sec

R = Rotor Radius

U = Wind “Free Stream” Velocity

ΩR

R


Coefficient of Performance

• Coefficient of performance varies with Tip Speed Ratio

• Characterized by Cp vs Tip Speed Ratio curve


Betz Limit

• All wind power cannot be

captured by rotor or air

would be completely still

behind rotor and not allow

more wind to pass

through

• Theoretical limit of rotor

efficiency is 59%

(impossible to achieve)

• Most modern wind

turbines are in the 35 –

45% range


Rotor Solidity

Solidity is the ratio of total rotor planform area to total swept area

Low solidity (0.10) = high speed, low torque

High solidity (>0.80) = low speed, high torque

A

R

a

Solidity = 3a/A


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Power from the rotor

Wind Turbine Mechanical Output Power at rotor shaft:

P = 0.5 x p x A x Cp x U3

Cp = Coefficient of performance (varies with wind

speed!!)

= Ratio of wind turbine mechanical output power of

the wind turbine to power in the wind that intercepts the

rotor


Generator Overview

• Synchronous Generators

• Asynchronous Generators

Synchronous means the rotor magnetic field rotates

exactly the same rate as the grid frequency

The rotor magnetic field in an asynchronous generator

does not rotate exactly at the same rate as the grid

frequency


Generator Overview- Permanent Magnet

•The permanent magnet generator (PMG)

-Most popular in small scale wind systems at present

-There is no control over the magnetic field strength of the

magnets

-Voltage amplitude and frequency varies with RPM (i.e. wild AC)

=> cannot be connected directly to grid


Generator Overview- Permanent Magnet


Inverters

• Is a device that converts DC power to AC power

• Two Types– Non grid tie: modified sine wave, square wave

and pure sine wave

– Grid tie – Only grid tie inverters should be used to connect to grid


Non Grid Tie Inverters

Modified Sine Wave Inverters –They only suit certain types of load where power quality is not a major issue (e.g. light bulbs)

Pure Sine Wave Inverters

Note: These type must never be connected to grid


Micro Generation Interface Protection Requirements (ESB Networks -EN50438)


Grid Tie Inverters


Grid Tie Inverters

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

P/Pnom (%)

Eff

(%

)

SB50 SB25

Efficiency


Grid Tie Inverters

Inverter Power vs Input Voltage Curve e.g. Windy Boy 1100LV

The input voltage vs Power can be programmed

Some inverters have maximum power point tracking MPP capability


ESB Grid connection of micro-generation

systems (www.esb.ie)

•Micro-generation – Small wind, Solar PV, Micro CHP, Micro-

hydro

•Single phase 230V connection 25 A => 6kW (rated power)

•Three Phase (230/400V) 16A => 11kW (rated power)

•Applications are made on NC6 notification form

•http://www.esb.ie/esbnetworks/en/generator-connections/micro_gen_connections.jsp

•No grid connection application fee for NC6

•Need to install and interval meter to measure imports ands exports

http://www.esb.ie/esbnetworks/en/generator-connections/micro_gen_connections.jsp




Overview of small scale grid connected system

Variable (wild) AC

DC

Grid quality AC

Rectifier/Controller

Grid TieInverter

Grid

FusePanel

Meter

Wind Turbinewith PMG

Note: Isolation/protection not shown

Dump Load

0.4

0.3

0.2

0.1

0.0

Cp

12 10 8 6 4 2 0

Tip Speed Ratio

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

P/Pnom (%)

Eff

(%

)

SB50 SB25


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Power from the Wind Turbine

Wind Turbine Electrical Output Power:

P = 0.5 x p x A x Cp x U3 x Nb x Ng

Ng = generator efficiency (50% for car

alternator, 80% or possibly more for a

permanent magnet generator or grid-connected

induction generator)

Nb = gearbox/bearings efficiency (a good

quality gearbox could be as high as 95% )


Overview of small scale standalone system

Note: Isolator/protection not shown

Basic idea- other variants exist

Rectifier/Controller Inverter

Battery BankGenerator (optional)

Master Control(System Manager)

AC loads


Health and Safety Aspects

• Rotor Overspeed!

• Vibrations (e.g. tower)

• Electrical Safety- Protection and Signage


Health and Safety Aspects

• Rotor over speed is a very significant issue – If a turbine rotor is allowed to run out of control into over speed it can self destruct and have costly (even fatal!) consequences

• Commons methods of over speed prevention– Furling tail

– Electric braking dump loads

– Bade pitching mechanisms

• Each of these methods has shortcomings


Furling tail

Furling tail is hinged off centre to rotor shaft. In high winds

above rated power the tail folds and yaws turbine out of wind

It is designed so that that a spring or gravity returns the

turbine back to the normal upwind position at normal operating

wind speeds


Electrical braking dump loads

•As the rotor RPM increases beyond the normal

operating range the voltage increases to a level

where the controller switches on dump resistor loads

rated above rated power. These loads’ slow down

the rotor preventing overspeed.

•Shorting the output of a permanent magnet

generator will stop the rotor altogether (can be done

when parking/ servicing the turbine)


Blade pitching mechanisms

•Changing pitch of blade to reduce the lift and

preventing rotor over speed

•Controlled active blade pitching mechanism

commonly used on large scale wind turbines

•Hinged spring loaded mechanisms used on

some scale wind systems


Blade pitching mechanisms

small scale wind technologies part 2 - creeslough.com slides 2.pdf · inverter power vs input...

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