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© CREDIT Dundalk IT
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
© CREDIT Dundalk IT
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
© CREDIT Dundalk IT
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!!
© CREDIT Dundalk IT
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
© CREDIT Dundalk IT
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
© CREDIT Dundalk IT
Coefficient of Performance
• Coefficient of performance varies with Tip Speed Ratio
• Characterized by Cp vs Tip Speed Ratio curve
© CREDIT Dundalk IT
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
© CREDIT Dundalk IT
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
© CREDIT Dundalk IT
11
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
© CREDIT Dundalk IT
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
© CREDIT Dundalk IT
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
© CREDIT Dundalk IT
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
© CREDIT Dundalk IT
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
© CREDIT Dundalk IT
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
© CREDIT Dundalk IT
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
© CREDIT Dundalk IT
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
© CREDIT Dundalk IT
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
© CREDIT Dundalk IT
24
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% )
© CREDIT Dundalk IT
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
© CREDIT Dundalk IT
Health and Safety Aspects
• Rotor Overspeed!
• Vibrations (e.g. tower)
• Electrical Safety- Protection and Signage
© CREDIT Dundalk IT
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
© CREDIT Dundalk IT
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
© CREDIT Dundalk IT
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)
© CREDIT Dundalk IT
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