17-07-2012, 11:27 AM
Power Electronics in Wind Turbine Systems
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Abstract
The global electrical energy
consumption is still rising and there is a steady
demand to increase the power capacity. The
production, distribution and the use of the energy
should be as technological efficient as possible and
incentives to save energy at the end-user should be set
up. The deregulation of energy has lowered the
investment in larger power plants, which means the
need for new electrical power sources may be very
high in the near future. Two major technologies will
play important roles to solve the future problems.
One is to change the electrical power production
sources from the conventional, fossil (and short term)
based energy sources to renewable energy resources.
The other is to use high efficient power electronics in
power systems, power production and end-user
application. This paper discuss the most emerging
renewable energy source, wind energy, which by
means of power electronics is changing from being a
minor energy source to be acting as an important
power source in the energy system. By that wind
power is also getting an added value in the power
system operation.
I. INTRODUCTION
In classical power systems, large power generation
plants located at adequate geographical places produce
most of the power, which is then transferred towards
large consumption centers over long distance
transmission lines. The system control centers monitor
and control the power system continuously to ensure the
quality of the power, namely the frequency and the
voltage. However, now the overall power system is
changing, a large number of dispersed generation (DG)
units, including both renewable and non-renewable
sources such as wind turbines, wave generators,
photovoltaic (PV) generators, small hydro, fuel cells and
gas/steam powered Combined Heat and Power (CHP)
stations, are being developed [1]-[2] and installed. A
wide-spread use of renewable energy sources in
distribution networks and a high penetration level will be
seen in the near future many places. E.g. Denmark has a
high penetration (> 20%) of wind energy in major areas
of the country and today 18% of the whole electrical
energy consumption is covered by wind energy. The
main advantages of using renewable energy sources are
the elimination of harmful emissions and the
inexhaustible resources of the primary energy. However,
the main disadvantage, apart from the higher costs, e.g.
photovoltaic, is the uncontrollability. The availability of
renewable energy sources has strong daily and seasonal
patterns and the power demand by the consumers could
have a very different characteristic. Therefore, it is
difficult to operate a power system installed with only
renewable generation units due to the characteristic
differences and the high uncertainty in the availability of
the renewable energy sources.
The wind turbine technology is one of the most
emerging renewable technologies. It started in the
1980’es with a few tens of kW production power to
today with Multi-MW range wind turbines that are being
installed. This also means that wind power production in
the beginning did not have any impact on the power
system control but now due to their size they have to
play an active part in the grid. The technology used in
wind turbines was in the beginning based on a
squirrel-cage induction generator connected directly to
the grid. By that power pulsations in the wind are almost
directly transferred to the electrical grid. Furthermore
there is no control of the active and reactive power,
which typically are important control parameters to
regulate the frequency and the voltage. As the power
range of the turbines increases those control parameters
become more important and it is necessary to introduce
power electronics [3] as an interface between the wind
turbine and the grid. The power electronics is changing
the basic characteristic of the wind turbine from being an
energy source to be an active power source. The
electrical technology used in wind turbine is not new. It
has been discussed for several years [6]-[46] but now the
price pr. produced kWh is so low, that solutions with
power electronics are very attractive.
This paper will first discuss the basic development
in power electronics and power electronic conversion.
Then different wind turbine configurations will be
explained both aerodynamically and electrically. Also
different control methods will be explained for a turbine.
Wind turbines are now more often installed in remote
areas with good wind conditions (off-shore, on-shore)
and different possible configurations are shown and
compared. Finally, a general technology status of the
wind power is presented demonstrating a still more
efficient and attractive power source.
II. MODERN POWER ELECTRONICS AND SYSTEMS
Power electronics has changed rapidly during the
last thirty years and the number of applications has been
increasing, mainly due to the developments of the
semiconductor devices and the microprocessor
technology. For both cases higher performance is
steadily given for the same area of silicon, and at the
same time they are continuously reducing the price. Fig.
1 shows a typical power electronic system consisting of a
power converter, a load/source and a control unit.
Power converter
Reference (local/centralized)
Control
Power flow
Load /
generator
Appliance
Industry
Communication
Wind
Photo-voltaic
Fuel cell
Other sources
2-3 2-3
Fig. 1. Power electronic system with the grid, load/source, power
onverter and control.
The power converter is the interface between the
load/generator and the grid. The power may flow in both
directions, of course, dependent on topology and
applications. Three important issues are of concern using
such a system. The first one is reliability; the second is
efficiency and the third one is cost. For the moment the
cost of power semiconductor devices is decreasing 2-5 %
every year for the same output performance and the price
pr. kW for a power electronic system is also decreasing.
A high competitive power electronic system is adjustable
speed drives (ASD) and the trend of weight, size,
number of components and functions in a standard
Danfoss Drives A/S frequency converter can be seen in
Fig. 2. It clearly shows that power electronic conversion
is shrinking in volume and weight. It also shows that
more integration is an important key to be competitive as
well as more functions become available in such a
product.
The key driver of this development is that the power
electronic device technology is still undergoing
important progress. Fig. 3 shows different key power
devices and the areas where the development is still
going on.
The only power device which is not under
development any more is the silicon-based power bipolar
transistor because MOS-gated devices are preferable in
the sense of easy control. The breakdown voltage and/or
current carrying capability of the components are also
continuously increasing. Also important research is
going on to change the material from silicon to silicon
carbide. This may dramatically increase the power
density of power converters but silicon carbide based
transistors on a commercial basis with a competitive
price will still take some years to appear on the market.
0
50
100
150
200
1968
1983
1988
1993
1998
Year
Relative unit
Components
Functions
(a)
0
20
40
60
80
100
120
1968
1988
1998
Year
%
Size (volume)
Weight
(b)
Fig. 2. Development of a 4 kW standard industrially adjustable speed
drive during the last 25 years [5].
a) Relative number of components and functions
b) Relative size and weight
Silicon carbide FETs
MOSFETs
Insulated-gate
bipolar transistors
MOS-gated thyristors
Silicon
Bipolar transistors
1950 ?60 ?70 ?80 ?90 2000 2006 2010
Year
Trench
Coolmos
IGCT
Diode
IGTO
Fig. 3. Development of power semiconductor devices in the past and
in the future [34]
Power conversion &
power control
Wind power
Power converter
(optional)
Power conversion &
power control
Power transmission Power conversion Power transmission
Supply grid
Consumer
Rotor Gearbox (optional) Generator
Electrical Power
C o n su m er/ lo a d
Fig. 4. Converting wind power to electrical power in a wind turbine [17 ].
M ec h a n ica l E ne rgy S ou rce
Varia ble S peed
D irec t G e arbo x
M ultipo la r S ynch ronous
& N ov el M achines
Con ve n tio n a l
Syn ch ron ous M ac hines
Ind uc tion M ac hines
W o u n d R otor
( fie ld c o n tro l)
La rge P E
c onve rter
P erm a ne n t
M a g n e t
L a rg e P E
c onv e rter
C age
R otor M /C
Large PE
conve rte r
W o u n d R o tor o r
Brus h less D F
W ound
E lec trica l E n e rgy S o urce
F ixe d F requency o r D C
R otor
S ta tor
M achine
typ e
T ransm is sion
W ound W ound W ound
G rid
c onnect ion
Outp u t
Sm all P E
conv erter
Inp u t
P ower
conve rsion
He a t los s
dum p load
Wind Energy
Mechanical Energy Source
Fixed/Variable Speed
Fig. 5. Road-map for wind energy conversion. PE: Power Electronics. DF: Doubly-fed [15], [22].
III. WIND ENERGY CONVERSION
Wind turbines capture the power from the wind by
means of aerodynamically designed blades and convert it
to rotating mechanical power. The number of blades is
normally three. As the blade tip-speed typically should
be lower than half the speed of sound the rotational
speed will decrease as the radius of the blade increases.
For multi-MW wind turbines the rotational speed will be
10-15 rpm. The most weight efficient way to convert the
low-speed, high-torque power to electrical power is
using a gear-box and a standard fixed speed generator as
illustrated in Fig. 4.
The gear-box is optional as multi-pole generator
systems are possible solutions. Between the grid and the
generator a power converter can be inserted.
The possible technical solutions are many and Fig.
5 shows a technological roadmap starting with wind
energy/power and converting the mechanical power into
electrical power. It involves solutions with and without
gearbox as well as solutions with or without power
electronic conversion. The electrical output can either be
ac or dc. In the last case a power converter will be used
as interface to the grid. In the following sections, some
different wind turbine configurations will be presented
and compared.
IV. FIXED SPEED WIND TURBINES
The development in wind turbine systems has been
steady for the last 25 years and four to five generations
of wind turbines exist. It is now proven technology. The
conversion of wind power to mechanical power is as
mentioned before done aerodynamically. It is important
to be able to control and limit the converted mechanical
power at higher wind speed, as the power in the wind is a
cube of the wind speed. The power limitation may be
done either by stall control (the blade position is fixed
but stall of the wind appears along the blade at higher
wind speed), active stall (the blade angle is adjusted in
order to create stall along the blades) or pitch control (the
blades are turned out of the wind at higher wind speed).
The wind turbines technology can basically be divided
into three categories: the first category is systems
without power electronics, the second category is wind
turbines with partially rated power electronics (small PE
converter in Fig. 5) and the last is the full-scale power
electronic interfaced wind turbine systems (large PE
converter in Fig. 5). Fig. 6 shows different topologies for
the first category of wind turbines where the wind
turbine speed is fixed.
Gearbox
Induction
generator
(a)
Grid
Reactive
compensator
I
Pitch
Gearbox
Induction
generator
(b)
Grid
Reactive
compensator
II
Stall
Gear
Induction
generator
Active
Stall
Grid
Reactive
compensator
III
©
Fig. 6. Wind turbine systems without power converter but with
aerodynamic power control.
a) Pitch controlled (System I)
b) Stall controlled (System II)
c) Active stall controlled (System III)
The wind turbine systems in Fig. 6 are using
induction generators, which almost independent of
torque variation operate at a fixed speed (variation of
1-2%). The power is limited aerodynamically either by
stall, active stall or by pitch control. All three systems are
using a soft-starter (not shown in Fig. 6) in order to
reduce the inrush current and thereby limit flicker
problems on the grid. They also need a reactive power
compensator to reduce (almost eliminate) the reactive
power demand from the turbine generators to the grid. It
is usually done by continuously switching capacitor
banks following the production variation (5-25 steps).
Those solutions are attractive due to cost and reliability
but they are not able very fast (within a few ms) to
control the active power. Furthermore wind-gusts may
cause torque pulsations in the drive-drain and load the
gear-box significantly. The basic power characteristics of
the three different fixed speed concepts are shown in Fig.
7 where the power is limited aerodynamically.
Power [PU]
Vindhastighed
[m/s]
P
0.25
0.50
0.75
1
5 10
15
20
25
30
Stall control
Wind speed [m/s]
(a)
5 10
15
20
25
30
0.25
0.50
0.75
1
Power [PU] Active Stall control
30
Wind speed [m/s]
(b)
0.25
0.50
0.75
1
5 10
15
20
25
30
Wind speed [m/s]
©
Power [PU] Pitch
Fig. 7. Power characteristics of fixed speed wind turbines.
a) Stall control b) Active stall control c) Pitch control
Fig. 7 shows that by rotating the blades either by
pitch or active stall control it is possible precise to limit
the power while the measured power for the stall
controlled turbine shows a small overshoot. This depends
a lot on the final aerodynamic design.