Download Lecture #9

ECE 333
Renewable Energy Systems
Lecture 9: Wind Power Systems
Prof. Tom Overbye
Dept. of Electrical and Computer Engineering
University of Illinois at Urbana-Champaign
[email protected]
Announcements
•
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Read Chapter 7
HW 4 is 7.1, 7.2, 7.4, 7.5; it will be covered by an inclass quiz on Thursday Feb 20
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In the News: On Feb 12 AWEA
Released Report on Wind Reliablity
• Report addressed issue of how much wind energy
•
could be integrated into the US grid
Finding is wind could provide more than 40% of our
total electric energy
–
•
In 2013 Iowa and South Dakota got 25% of their electricity
from wind, and for ERCOT it as 10.6%
Key to integrating large amounts of
wind is that the wind plant outputs
are not correlated across large areas
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Changes in the wind tend to cancel out
Report: awea.files.cms-plus.com/AWEA%20Reliability%20White%20Paper%20-%202-12-15.pdf
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North American Power Grid
Load/Generation Contour
Image contours the load (green) and generation (red)
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Maximum Rotor Efficiency
Rotor efficiency
CP vs. wind
speed ratio λ.
Recall λ is the
ratio between the
downstream
wind velocity
and the upstream
velocity
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Tip-Speed Ratio (TSR)
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Efficiency is a function of how fast the rotor turns
Tip-Speed Ratio (TSR) is the speed of the outer tip
of the blade divided by wind speed
Rotor tip speed rpm   D
Tip-Speed-Ratio (TSR) 
=
(7.30)
Wind speed
60v
•
•
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D = rotor diameter (m)
v = upwind undisturbed wind speed (m/s)
rpm = rotor speed, (revolutions/min)
One meter per second = 2.24 miles per hour
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Tip-Speed Ratio (TSR)
•
•
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TSR for various rotor
types
If blade turns too slow
then wind passes
through without hitting
blade; too fast
results in turbulence
Rotors with fewer
blades reach their
maximum efficiency at
higher tip-speed ratios
Figure 7.18
A higher TSR is needed
when there are fewer blades
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Example
•
40-m wind turbine, three-blades, 600 kW, wind speed
is 14 m/s, air density is 1.225 kg/m3
a. Find the rpm of the rotor if it operates at a TSR of 4.0
b. Find the tip speed of the rotor
c. What gear ratio is needed to match the rotor speed to
the generator speed if the generator must turn at 1800
rpm?
d. What is the efficiency of the wind turbine under these
conditions?
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Example
a. Find the rpm of the rotor if it operates at a TSR of
4.0
Rewriting (7.30),
Tip-Speed-Ratio (TSR)  60v
rpm 
D
4.0  60sec/min 14m/s
rpm 
= 26.7 rev/min
  40m/rev
We can also express this as seconds per revolution:
26.7 rev/min
rpm 
= 0.445 rev/sec or 2.24 sec/rev
60 sec/min
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Example
b. Tip speed
rpm   D
From (7.30): Rotor tip speed=
60 sec/min
Rotor tip speed = (rev/sec)   D
Rotor tip speed = 0.445 rev/sec   40 m/rev = 55.92 m/s
c. Gear Ratio
Generator rpm 1800
Gear Ratio =
=
= 67.4
Rotor rpm
26.7
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Example
d. Efficiency of the complete wind turbine (blades,
gear box, generator) under these conditions
From (7.7):
1
1

3
2
PW   Av = 1.225    40 143  2112 kW
2
2
4

Overall efficiency:
600 kW

 28.4%
2112 kW
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Converting Wind into Electric Energy
•
Design challenge is to convert rotating mechanical
energy into electrical energy
–
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This is, of course, commonly done in most power plants.
But the added challenges with wind turbines are 1) the shaft
is often rotating a variable speed [because of changes in the
wind speed], and 2) the rate of rotation is relatively slow
(dozens of rpm)
Early wind turbines used a near fixed speed design,
which allowed use of simple and well proven
induction generators, but gave up aerodynamic
efficiency. Modern turbines tend to use a variable
speed design to keep tip-to-speed ratio near optimal
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Electric Machines
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Electric machines can usually function as either a
motor or as a generator
Three main types of electric machines
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DC machines: Advantage is they can directly operate at
variable speed. For grid application the disadvantage is they
produce a dc output. Used for small wind turbines.
AC synchronous machines

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Operate at fixed speed. Used extensively for traditional power
generation. The fixed speed had been a disadvantage for wind.
AC induction machines

Very rugged and allow some speed variation but usually not a lot for
efficient operation.
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Types of Wind Turbines by Machine
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From an electric point of view there are four main
types of large-scale wind turbines (IEEE naming
convention)
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Type 1: Induction generator with fixed rotor resistance
Type 2: Induction generators with variable rotor resistance
Type 3: Doubly-fed induction generators
Type 4: Full converter generators which main use either a
synchronous generator or an induction generator
Most new wind turbines are either Type 3 or Type 4
In Europe these are sometimes called Types A, B, C,
D respectively.
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Wind Generator Types
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Rotating Magnetic Field
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Imagine coils in the stator of this 3-phase generator
Positive current iA flows from A to A’
Magnetic fields from positive currents are shown by the
bold arrows
Magnetic flux is proportional to current, with direction
given by the right-hand rule (from Ampere's circuit law)
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Rotating Magnetic Field
•
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Three-phase currents are flowing in the stator
At ωt = 0, iA is at the maximum positive value and
iB=iC are both negative
Resultant magnetic flux points
vertically down
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Rotating Magnetic Field Demo
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Magnetic Poles
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Synchronous speed depends on the electrical
frequency and the number of poles, with
2 fe
fm 
where f e is electrical frequency
P
P is the number of poles, f m is mechanical frequency
Image source :cnx.org/contents/cbb3bd3b-430a-487b-9c53-b17d79e3367c@1/Chapter_5:_Synchronous_Machine
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Synchronous Machines
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Spin at a rotational speed determined by the number of
poles and by the frequency (3600 rpm at 60Hz, 2 pole)
The magnetic field is created on their rotors
Create the magnetic field by running DC through
windings around the core
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A gear box if often needed between the blades and the
generator
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A permanent magnet can also be used
Some newer machines are designed without a gear box
Slip rings are needed to get a dc current on the rotor
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Asynchronous Induction Machines
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Do not turn at a fixed speed
Acts as a motor during start up; can act as a generator
when spun faster then synchronous speed
Do not require exciter, brushes, and slip rings
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Less expensive, require less maintenance
The magnetic field is created on the stator not the rotor
Current is induced in the rotor
(Faraday's law: v= dl/dt)
Lorenz force on wire with current in magnetic field:
F  Il  B
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Squirrel Cage Rotor
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The rotor of many induction generators has copper
or aluminum bars shorted together at the ends, looks
like a cage
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Can be thought of as a
pair of magnets
spinning around a cage
Rotor current iR flows
easily through the thick
conductor bars
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Squirrel Cage Rotor
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Instead of thinking of a rotating stator field, you can
think of a stationary stator field and the rotor
moving counterclockwise
The conductor experiences a clockwise force
Figure 6.16
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The Inductance Machine as a Motor
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The rotating magnetic field in the stator causes the
rotor to spin in the same direction
As rotor approaches synchronous speed of the
rotating magnetic field, the relative motion becomes
less and less
If the rotor could move at synchronous speed, there
would be no relative motion, no current, and no
force to keep the rotor going
Thus, an induction machine as a motor always spins
somewhat slower than synchronous speed
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Slip
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The difference in speed between the stator and the
rotor
NS  NR
NR

 1
NS
NS
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s = rotor slip – positive for a motor, negative for a
generator
NS = no-load synchronous speed (rpm)
f = frequency (Hz)
120 f
NS 
p = number of poles
p
NR = rotor speed (rpm)
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The Induction Machine as a Motor
Torque- slip curve for an induction motor
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As load on motor increases, rotor slows down
When rotor slows down, slip increases
“Breakdown torque” increasing slip no longer
satisfies the load and rotor stops
Braking- rotor is forced to operate in the opposite
direction to the stator field
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The Induction Machine as a
Generator
• The stator requires excitation current
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–
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from the grid if it is grid-connected or
by incorporating external capacitors
Single-phase, self-excited, induction generator
Wind speed forces generator shaft to exceed
synchronous speed
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The Induction Machine as a
Generator
• Slip is negative because the rotor spins faster than
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synchronous speed
Slip is normally less than 1% for grid-connected
generator
Typical rotor speed
N R  (1  s) N S  [1  (0.01)]  3600  3636 rpm
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Speed Control
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Necessary to be able to shed wind in high-speed
winds
Rotor efficiency changes for different Tip-Speed
Ratios (TSR), and TSR is a function of windspeed
To maintain a constant TSR, blade speed should
change as wind speed changes
A challenge is to design machines that can
accommodate variable rotor speed and fixed
generator speed
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Blade Efficiency vs. Windspeed
At lower windspeeds, the best efficiency is achieved
at a lower rotational speed
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Power Delivered vs. Windspeed
Impact of rotational speed adjustment on delivered
power, assuming gear and generator efficiency is 70%
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Pole-Changing Induction
Generators
• Being able to change the number of poles allows
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•
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you to change operating speeds
A 2 pole, 60 Hz, 3600 rpm generator can switch to 4
poles and 1800 rpm
Can do this by switching external connections to the
stator and no change is needed in the rotor
Common approach for 2-3 speed appliance motors
like those in washing machines and exhaust fans
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Increasingly this approach is being replaced by machine
drives that convert ac at grid frequency to ac at a varying
frequency (covered in ECE 464)
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Variable-Slip Induction Generators
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Purposely add variable resistance to the rotor
External adjustable resistors - this can mean using a
wound rotor with slip rings and brushes which
requires more maintenance
Mount resistors and control electronics on the rotor
and use an optical fiber link to send the rotor a
signal for how much resistance to provide
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Effect of Rotor Resistance on Induction
Machine Power-Speed Curves
Real Pow er
Real Pow er
0.9
1.6
0.8
1.4
0.7
1.2
0.6
1
0.5
0.8
0.4
0.6
0.3
0.2
Real Power
Real Power
0.4
0.2
0
0.1
0
-0.1
-0.2
-0.2
-0.4
-0.3
-0.6
-0.4
-0.8
-0.5
-1
-0.6
-0.7
-1.2
-0.8
-1.4
-0.9
-1.6
-0.95
-0.9
-0.85
-0.8
-0.75
-0.7
-0.65
-0.6
-0.55
-0.5
-0.45
-0.4
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
-0.0500.050.10.150.20.250.30.350.40.450.50.550.60.650.70.750.80.850.90.951
Slip
-0.95
-0.9
-0.85
-0.8
-0.75
-0.7
-0.65
-0.6
-0.55
-0.5
-0.45
-0.4
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
-0.0500.050.10.150.20.250.30.350.40.450.50.550.60.650.70.750.80.850.90.951
Slip
Real Pow er
Real Pow er
Left plot shows the torque-power curve from slip of -1 to
1 with external resistance = 0.05; right plot is with
external resistance set to 0.99 pu.
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Variable Slip Example: Vestas V80
1.8 MW
• The Vestas V80 1.8 MW turbine is an
•
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example in which an induction
generator is operated
with variable rotor resistance
(opti-slip).
Adjusting the rotor resistance
changes the torque-speed curve
Operates between 9 and 19 rpm
Source: Vestas V80 brochure
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