Wind Power Generation
2. introduction to Wind Power Generator
Yuan-Kang Wu
National Chung-Cheng University
Wu Yuan-Kang
Few Laughs for Today
• What's the difference between stress, tension and
panic?
– Stress is when wife is pregnant,
– Tension is when girlfriend is pregnant,
– and Panic is when both are pregnant.
Stress: 壓力，緊張
Tension: 拉緊, 繃緊
Panic: 恐慌, 驚慌
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Few Laughs for Today
• A young boy asks his Dad, "What is the difference
between confident and confidential?”
• Dad says, "You are my son, I'm confident about
that.
• Your friend over there, is also my son, that's
confidential."
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Main components of a wind turbine system
• The main electrical power components that may be included
in a wind turbine system are illustrated in the Fig, including
turbine rotor, gearbox, generator, power electronics and a
transformer for grid connection.
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Rotational speed and terminal voltage of a wind turbine
• For large multi megawatt-scale wind turbines, the rotational
speed will usually be 10 to 15 rpm (i.e. rotation period 4 to 6
seconds) . The traditional way to convert such low-speed,
high-torque mechanical power to electrical power is with a
gear-box to increase the shaft speed to a 4 or 6 pole-pair
standard generator rotating at about 1,500 rpm or 1,000 rpm.
However, the gear box may not be necessary for multi-pole
generators.
• Each turbine has its own adjacent transformer to raise
voltage from the generator terminal voltage (usually under
1,000 V, for example, 690V) to distribution system site
voltage.
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Squirrel Cage Induction Generators
• Induction generators are much less used than synchronous
generators for large scale conventional power generation.
This is because the inductive magnetisation results in
relatively larger energy losses in the rotor. The reactive
power necessary to energize the magnetic circuits of
induction machines must be supplied from the network.
• For simple induction machines, the terminal voltage and
reactive power cannot be directly controlled. Simple
induction generators in wind turbines may suffer from
voltage instability which is a significant concern if used with
large-scale wind farms.
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Wound-rotor Induction Machine
• A wound-rotor induction machine has a rotor with copper
wire windings, which can be connected to an external
resistor or to ac systems via power electronic systems.
Such a system provides a partial variable speed operation
with a small power electronic converter, therefore increased
energy capture and reduced mechanical load to the system.
This type of system is an economical way to obtain variable
speed and supply reactive power and to increase energy
yield at low wind-speeds. Also reduced torque loads in the
drive train make the whole mechanical construction simple
and reliability increased.
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Synchronous Generator
• For power generation applications, electromagnets excited
by an externally applied direct current are used on the rotor.
The rotor operates at exactly the same 'fixed' speed as the
stator magnetic field but it leads the stator field by an
• In practice, synchronous generators powered by wind
turbines cannot be directly connected to an ac grid.
However, if an indirect coupling is used (e.g. rectifier/inverter
solid state electronics 'converter'), synchronous generators
can be used beneficially, since the turbine rotor can then
have variable-speed and the blade rotation can be matched
more efficiently to the variable wind-speed. Hence,
synchronous generators can be used, if coupled to the grid
through full-rate power-electronic converters.
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Synchronous Generators
• Synchronous generators can be classified as electrical
excited synchronous generators (EESGs) and permanent
magnet generators(PMGs).
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Direct-drive generators for wind turbines
• The power output of a rotating electrical machine is
propositional to the machine length, rotor area and
rotor speed. Thus if a smaller rotational speed is
adopted, it is necessary either to lengthen the
generator or to increase the diameter to provide the
same level of power. It is cheaper to increase the
diameter. Thus, direct-drive generators for wind
turbines tend to have rather large
diameters.This also allows many pole-pairs to be
fitted on the large diameter.
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Design on air gap between induction generator and
synchronous generator
• Induction generators require a rather small air gap to ensure
an adequate air-gap magnetic flux density as the excitation
is provided from the stator. In contrast, synchronous
generators have excitation systems on the rotor and so can
operate with larger air-gaps. It is difficult to manufacture
large diameter electrical machines with small air gaps for
mechanical and thermal reasons. Hence direct-drive wind
turbines use synchronous generators (either with
permanent magnet excitation or with an electromagnet).
The use of a synchronous generator, in turn, leads to the
requirement for power electronic conversion system to decouple the generator from the network.
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Permanent magnet synchronous machine
• Permanent magnet synchronous machines are
also used for wind turbine systems, especially
for multipole machines. In this case, slip rings for
field excitation are eliminated, together with the
power loss of an excitation system; however, the
machine terminal voltage cannot be easily
controlled with permanent magnet excitation.
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Classification of Wind Power Systems
• 1. With gearbox / without gearbox
• 2. Rotational speed (fixed speed / variable speed)
• 3. Power conversion (Type A,B,C,D)
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Sketch of a nacelle with gearbox, in this case of a constant speed NEG Micon
wind turbine
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Sketch of a gearless nacelle, in this case of an Enercon E-66
directdrive wind turbine
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Type A
SCIG = squirrel cage induction generator
• This configuration denotes the fixed-speed wind turbine with
an asynchronous squirrel cage induction generator (SCIG)
directly connected to the grid via a transformer.
• Since the SCIG always draws reactive power from the grid,
this configuration uses a capacitor bank for reactive power
compensation.
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Type A: Advantage and Disadvantage
• Easy and relatively cheap for mass production.
• The speed is not controllable and variable only over a very narrow
range, in which only speeds higher than the synchronous speed are
possible for generator operation.
• The fixed speed concept means that wind speed fluctuations are directly
translated into electromechanical torque variations, this causes high
mechanical and fatigue stresses on the system and may result in swing
oscillations between turbine and generator shaft.
• The turbine speed cannot be adjusted with the wind speed to optimize
the aerodynamic efficiency.
• In most cases, capacitors are connected in parallel to the generator to
compensate for the reactive power consumption.
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Type A
SCIG
(FSWT)
Squirrel-Cage
Induction
Generator
Advantages
Disadvantages
• Easier to design, construct
and control
• Robust operation
• Low cost
• Low energy yield
• No active/reactive power
controllability
• High mechanical stress
• High losses on gear
( A three-stage gearbox in the
drive train is necessary for this
wind turbine concept)
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Type B
WRIG = wound rotor induction generator
•
•
•
The limited variable speed wind turbine with variable generator rotor
resistance, known as OptiSlip.(2) It uses a wound rotor induction generator
(WRIG).
The generator is directly connected to the grid. A capacitor bank performs the
reactive power compensation.
The unique feature of this concept is that it has a variable additional rotor
resistance, which can be changed by an optically controlled converter mounted
on the rotor shaft. The rotor resistance can be changed and thus controls the
slip. This way, the power output in the system is controlled. The range of the
dynamic speed control depends on the size of the variable rotor resistance.
Typically, the speed range is 0–10%above synchronous speed.
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Type B
• Variable-speed operation can be achieved by controlling the energy
extracted from the WRIG rotor. With the increase in variable speed
range, a higher slip means a high power extracted by the rotor, and
the lower generator efficiency, so that the rating of the resistor must
also be higher.
• Therefore the dynamic speed control range depends on the size of
the variable rotor resistance.
• A typical limited variable speed range is less than 10% above the
synchronous speed. Additionally, the slip rings may be avoided, for
example, the wind turbine manufacturer Vestas built the power
converter and resistor on the rotor, the control signals are
transmitted to the rotating electronics by an optical coupling.
• Furthermore, reactive power compensation and a soft-starter are
also required for this concept.
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Type C
Doubly fed induction generator (DFIG)
30%
• For a typical DFIG, the power converters are connected to
the rotor and, for a restricted speed range, are rated at a
fraction of the machine nominal power, i.e., typically 30% of
this value.
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Doubly fed induction machine based wind turbine
• The stator is connected directly to the grid and the rotor is fed by a
bidirectional converter that is also connected to the grid
• Using vector control techniques, the bidirectional converter
assures energy generation at nominal grid frequency and nominal
grid voltage independently of the rotor speed.
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Two sides of converter
• The rotor side converter (RSC) : control the generator torque
and the reactive power exchanged between the stator and the
grid.
• The grid side converter (GSC): The active power is indirectly
controlled by means of the DC bus controller and the reactive
power.
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The typical characteristics of these converters
• Vector control or direct torque control (DTC) for generator and grid
converter control;
• Two-level, three-phase converter with IGTBs, at switching frequency
of 2.5–5 kHz
• LCL filter for the GSC, and dv/dt filter for the RSC
• Nominal power: 500 to 2500 kVAs
• Nominal voltage 690V +10% to -15%.
• Nominal DC bus voltage 1000V
• Very low total harmonic distortion (THD<3%)
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Type C
• It is the limited variable speed wind turbine with a wound
rotor induction generator (WRIG) and partial scale
frequency converter (rated at approximately 30% of
nominal generator power) on the rotor circuit
• The partial scale frequency converter performs the
reactive power compensation and the smoother grid
connection.
• It has a wider range of dynamic speed control compared
with the OptiSlip, depending on the size of the frequency
converter.
• Typically, the variable speed range is +/- 30% around
the synchronous speed. The smaller frequency converter
makes this concept attractive from an economical point
of view. Its main drawbacks are the use of slip rings and
protection in the case of grid faults.
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Type C
• The speed range is limited, and slip rings are
required in order to connect the machine-side
converter to the rotor.
• For WECSs based on DFIGs, gearboxes are
still required because a multi-pole low-speed
DFIG is not technically feasible.
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Type C
Advantages
DFIG
(VSWTPSPC)
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•High energy yield
•High active/reactive
power controllability
•Lower cost on
PEC( power electronic
conversion)
•Lower losses by PEC
•Less mechanical stress
•Compact size
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Disadvantages
• Existence of
brush/slip ring
• High losses on
gear
•Others
Disadvantage of DFIG wind turbines
• A multi-stage gearbox is still necessary in the drive train because the
speed range for DFIG is far from a common turbine speed of 10–25 rpm.
• The slip ring is used to transfer the rotor power by means of a partialscale converter, which requires a regular maintenance, and maybe
result in machine failures and electrical losses.
• Under grid fault conditions, on the one hand, large stator currents result
in large rotor currents, so that the power electronic converter needs to
be protected from destroy; on the other hand, large stator peak currents
may cause high torque loads on the drive train of wind turbines.
• According to grid connection requirements for wind turbines, in case of
grid disturbances, a ride-through capability of DFIG is also required,
so that the corresponding control strategies may be complicated.
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Type D
• This configuration corresponds to the full variable speed
wind turbine, with the generator connected to the grid
through a full-scale frequency converter. The frequency
converter performs the reactive power compensation and
the smoother grid connection.
• The generator can be excited electrically WRSG or
WRIG or by a PMSG.
• Some full variable-speed wind turbine systems have no
gearbox. In these cases, a direct driven multi-pole
generator with a large diameter is used,
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Full Converter Geared Solutions
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Full Converter Direct Drive Solutions
• Two types:
– Multi pole permanent magnet generator (MPMG)
– Multi pole wound rotor synchronous generator (WRSG)
• Disadvantage:
– The size of the bidirectional converter, which must be of the same
power level as the alternator.
– The harmonic distortion generated by the converter must be
eliminated by a nominal power filter system.
• Advantage:
– the elimination of the mechanical converter (gearbox coupling)
because the machine can operate at low speed.
– multipole machine requires an elevated number of poles, with the
size of the machine being bigger than the generators with the
gearbox coupling.
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Electrically excited synchronous generator (EESG)
• The amplitude and frequency of the voltage can be fully
controlled by the power electronic at the generator side, so
that the generator speed is fully controllable over a wide
range, even to very low speeds.
• The EESG has the opportunities of controlling the flux for a
minimized loss in different power ranges, because the
excitation current can be controlled by means of the power
converter in the rotor side.
• It is the mostly used direct-drive generator type in the
current market. The typical manufacturer is Enercon,
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Type D
Advantages
WRSG
(VSWTFSPC)
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•High energy yield
•Higher active/reactive
power controllability
•Absence of brush/slipring
•Low mechanical stress
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Disadvantages
•Higher cost on
PEC
•Higher losses on
PEC
•Large size
Disadvantage of EESG
• In order to arrange space for excitation windings and
pole shoes, the pole pitch has to be large enough for the
large diameter-specific design, so a larger number of
parts and windings probably make it a heavy weight and
expensive solution.
• It is necessary to excite the rotor winding with DC, using
slip rings and brushes, or brushless exciter, employing a
rotating rectifier, and the field losses are inevitable.
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Advantages of PM synchronous generator
• Higher efficiency and energy yield
• No additional power supply for the magnet field
excitation.
• Improvement in the thermal characteristics of the
PM machine due to the absence of the field
losses
• Higher reliability due to the absence of
mechanical components such as slip rings
• Lighter and therefore higher power to weight ratio.
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Disadvantages of PM synchronous generator
• High cost of PM material,
• Difficulties to handle in manufacture,
• Demagnetization of PM at high temperature.
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Review on Asynchronous Generators
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Future trend for direct-drive topologies
• Although the current DFIG technology is well developed and
it is a very cost effective solution, it has some drawbacks.
The use of a gearbox increases the weight of a nacelle, with
more power loss and increased costs, especially in the
offshore applications when the power rating goes up. The
losses in the gearbox represent 65% of the total power loss
in the generation system. Thus, a direct drive topology is
preferred, although most researchers do not commonly
consider induction generators for direct drive applications.
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COMMERCIAL WIND TURBINE WITH SYNCHRONOUS
MACHINE(LARGE THAN 1MW)
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TOP 10 WIND TURBINE MANUFACTURERS OF 2009, CURRENTLY
USED GENERATOR CONCEPTS AND POWER RANGES
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COMPARISON OF FOUR WIND TURBINE GENERATOR CONCEPTS,
+: STRENGTH, - WEAKNESS.
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The comparison of the six different WTG concepts
good
good
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Comparison among wind turbines
• Characteristics of three of the most−popular wind
generators, in the 3-MW power range, are
presented and compared
Direct-drive
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Comparison among wind turbines
• The most-efficient generator is the direct-drive
permanent-magnet SG (PMSG) with power losses of
about 65% of that of a typical DFIG-based WECS
• However, in terms of costs, weight, and size, the DFIG has
advantages over the direct-drive generators.
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Comparison among wind turbines
• Multi-pole PMSGs with full-power back-to-back converters
appears to be the configuration to be adopted by most of
the large wind-turbine manufactures in the near future,
gradually replacing the doubly fed generator as the main
generator in the wind-energy market.
• An additional advantage of direct-drive generators is the
noise reduction achieved when the gearbox is eliminated
from the WECS. For offshore applications, the increased
reliability and elimination of possible oil spills from the
gearbox is another advantage.
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Comparison among wind turbines
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Medium-speed + Single-stage gearbox (ratio:6~10)
• The Multibrid (now Areva) company developed a WECS
composed of a medium-speed PMSG and a single-stage
gearbox with a gear ratio of about 6–10.
• This allows reducing the weight and the size of the
generators combined with the advantages of using a
gearbox technology, which is lighter, more reliable, and
cheaper than that of the standard three-stage gearbox with
a typical ratio of 80–100 times.
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SUMMARY OF THE COMMERCIALLY AVAILABLE WECS
IN THE 1.5- TO 3-MW RANGE
Number of models
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for “number of models
SUMMARY OF THE COMMERCIALLY AVAILABLE WECS
IN THE 1.5- TO 3-MW RANGE
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SUMMARY OF THE COMMERCIALLY AVAILABLE WECS
IN THE 1.5- TO 3-MW RANGE
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Type of WECS
• According to the information in the Table, in the
power range of 1.5–3 MW,
• 60 models of WECSs implemented using DFIGs,
• 66 models implemented with PMSGs,
• 18 models implemented with IGs (cage or
wound-rotor machines)
• 19 models of WECSs implemented using
wound-rotor SGs with external excitation.
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World market share of yearly installed wind
power during 1995 – 2004
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LARGE WIND TURBINES FOR OFFSHORE APPLICATIONS
• The 10-MW Britannia manufactured by Clipper Wind Power
will be used in “Round Three” in U.K., 2011).
• The direct-drive WECSs of 6 MW already available in the
market.
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