Power Electronics in Wind Turbine Systems

I.INTRODUCTION
The wind turbine technology is one of the most emerging renewable technologies. It started in
the 1980es 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 chapter 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 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.

Fig. 1. Power electronic system with the grid, load/source, power converter 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.

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

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.

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. 3.
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.

Fig. 3. Converting wind power to electrical power in a wind turbine [17 ].

The possible technical solutions are many and Fig. 4. 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 Wind turbines capture the
power from the wind by means of aerodynamically designed blades and convert it
different wind turbine configurations will be presented and compared.

Fig. 4. Road-map for wind energy conversion. PE: Power Electronics. DF: Doubly-fed [15], [22].

IV. FIXED SPEED WIND TURBINES

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. 4) and the last is the full-scale power electronic
interfaced wind turbine systems (large PE converter in Fig. 4). Fig. 5. shows different topologies
for the first category of wind turbines where the wind turbine speed is fixed.
I

Fig. 5. 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. 5. 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. 5) 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. 6. where the power is limited aerodynamically. Fig. 6. 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.

Fig. 7. Power characteristics of fixed speed wind turbines.

a) Stall control b) Active stall control c) Pitch control

V. VARIABLE SPEED WIND TURBINES

The next category is wind turbines with partially rated power converters and by that improved
control performance can be obtained. Fig 7. shows two such solutions.

Fig. 7. Wind turbine topologies with partially rated power electronics

and limited speed range.
a) Rotor-resistance converter (System IV)
b) Doubly-fed induction generator (System V)

Fig. 7 shows a wind turbine system where the generator is an induction generator with a
wounded rotor. An extra resistance is added in the rotor, which can be controlled by power
electronics. This is a dynamic slip controller and it gives typically a speed range of 2 - 5 %.
The power converter for the rotor resistance control is for low voltage but high currents. At the
same time an extra control freedom is obtained at higher wind speeds in order to keep the
output power fixed. This solution still needs a softstarter and a reactive power compensator,
which is in continuous operation.

A second solution of using a medium scale power converter with a wounded rotor induction
generator is shown in Fig. 7b. Slip-rings are making the electrical connection to the rotor. A
power converter controls the rotor currents.
If the generator is running super-synchronously electrical power is delivered through both
the rotor and the stator. If the generator is running sub-synchronously electrical power is only
delivered into the rotor from the grid. A speed variation of 30 % around synchronous speed can
be obtained by the use of a power converter of 30 % of nominal power. Furthermore, it is
possible to control both active (Pref) and reactive power (Qref), which gives a better grid
performance, and the power electronics is enabling the wind turbine to act as a more dynamic
power source to the grid.
The last solution needs neither a soft-starter nor a reactive power compensator. The solution
is naturally a little bit more expensive compared to the classical solutions shown before in Fig. 7
and Fig. 8a. However, it is possible to save money on the safety margin of gear, reactive power
compensation units as well it is possible to capture more energy from the wind.
The third category is wind turbines with a full-scale power converter between the generator
and grid, which are the ultimate solutions technically. It gives extra losses in the power
conversion but it may be gained by the added technical performance. Fig. 8 shows four possible,
but not exhaustive, solutions with full-scale power converters.
The solutions shown in Fig. 8a and Fig. 8b are characterized by having a gear. A
synchronous generator solution shown in Fig. 8b needs a small power converter for field
excitation. Multi-pole systems with the synchronous generator without a gear are shown in Fig.
8c and Fig. 8d. The last solution is using permanent magnets, which are still becoming cheaper
and thereby more attractive. All four solutions have the same controllable characteristics since
the generator is decoupled from the grid by a dc-link.

Fig. 8. Wind turbine systems with full-scale power converters.

a) Induction generator with gear (System VI)
b) Synchronous generator with gear (System VII)
c) Multi-pole synchronous generator (System VIII)
d) Multi-pole permanent magnet synchronous generator (System IX)

The power converter to the grid enables the system to control active and reactive power very
fast. However, the negative side is a more complex system with more sensitive electronic parts.
Comparing the different wind turbine systems in respect to performance shows a contradiction
between cost and the performance to the grid. Table I shows a technical comparison of the
presented wind turbine systems, where issues on grid control, cost, maintenance, internal turbine
performance are given. By introducing power electronics many of the wind turbine systems get a
performance like a power plant. In respect to control performance they are faster but of course
the produced real power depends on the available wind. The reactive power can in some
solutions be delivered without having any wind producing active power.
Fig. 9 is also indicating other important issues for wind turbines in order to act as a real
power source for the grid. They are able to be active when a fault appears at the grid and where it
is necessary to build the grid voltage up again; having the possibility to lower the power
production even though more power is available in the wind and thereby act as a rolling capacity
for the power system. Finally, some systems are able to work in island operation in the case of a
grid collapse. The market share in 2001 (Globally and in Germany) between the dominant
system topologies is shown Table II.

Table I. TECHNICAL COMPARISON OF THE PRESENTED WIND TURBIN

SYSTEM.

TABLE II. WIND TURBINE TOPOLOGIES MARKET IN 2002.

(Source: [4])

As it can be seen the most sold technology in 2001 is the doubly-fed induction generator
system which occupies about 50% of the whole market. More than 75% of all sold wind turbines
in 2001 are controlled by power electronics. That is even more in 2003.

VI. CONTROL OF WIND TURBINES.

Controlling a wind turbine involves both fast and slow control. Overall the power has to be
controlled by means of the aerodynamic system and has to react based on a set-point given by
dispatched center or locally with the goal to maximize the production based on the available
wind power.

Fig. 9. Control of wind turbine with doubly-fed induction generator system [35 ].

Fig. 10. Basic control of active and reactive power in a wind turbine [17].
a) Doubly-fed induction generator system (System V)
b) Multi-pole synchronous PM-generator system (System IX)

The power control system should also be able to limit the power. An example of an overall
control scheme of a wind turbine with a doubly-fed generator system is shown in Fig. 9.
Below maximum power production the wind turbine will typically vary the speed proportional
with the wind speed and keep the pitch angle fixed. At very low wind the speed of the turbine
will be fixed at the maximum allowable slip in order not to have over voltage. A pitch angle
controller will limit the power
when the turbine reaches nominal power. The generated electrical power is done by controlling
the doubly-fed generator through the rotor-side converter. The control of the grid-side converter
is simply just keeping the dc-link voltage fixed. Internal current loops in both converters are used
which typically are linear PI-controllers, as it is illustrated in Fig. 10a. The power converters to
the grid-side and the rotor-side are voltage source inverters.
Another solution for the electrical power control is to use the multi-pole synchronous generator.
A passive rectifier and a boost converter are used in order to boost the voltage at low speed. The
system is industrially used today. It is possible to control the active power from the generator.
The topology is shown in Fig. 11b. A grid inverter is interfacing the dc-link to the grid. Here it is
also possible to control the reactive power to the grid. Common for both systems are they are
able to control reactive to control the reactive power to the grid. Common for both systems are
they are able to control reactive and active power very fast and thereby the turbine can take part
in the power system control.

VII. ON-SHORE WIND FARM TOPOLOGIES.

In many countries energy planning is going on with a high penetration of wind energy, which
will be covered by large onshore wind farms. These wind farms may in the future present a
significant power contribution to the national grid, and therefore, play an important role on the
power quality and the control of power systems.
Consequently, very high technical demands are expected to be met by these generation units,
such as to perform frequency and voltage control, regulation of active and reactive power, quick
responses under power system transient and dynamic situations, for example, to reduce the
power from the nominal power to 20 % power within 2 seconds. The power electronic
technology is again an important part in both the system configurations and the control of the
offshore wind farms in order to fulfill the future demands.

One on-shore wind farm equipped with power electronic converters can perform both real and
reactive power control and also operate the wind turbines in variable speed to maximize the
energy captured as well as reduce the mechanical stress and noise. This solution is shown in
Fig.12a and it is in operation in Denmark as a 160 MW on-shore wind power station.
For long distance transmission of power from on-shore wind farm, HVDC may be an interesting
option. In an HVDC transmission, the low or medium AC voltage at the wind farm is converted
into a high dc voltage on the transmission side and the dc power is transferred to the onshore
system where the dc voltage is converted back into ac voltage as shown in Fig. 12c. For certain
power level, an HVDC transmission system, based on voltage source converter technology, may
be used in such a system instead of the conventional thyristor based HVDC technology.
The topology may even be able to vary the speed on the wind turbines in the complete wind
farm.
Another possible dc transmission system configuration is shown in Fig. 12d, where each wind
turbine has its own power electronic converter, so it is possible to operate each wind turbine at an
individual optimal speed.

Fig. 11. Wind farm solutions.

a) Doubly-fed induction generator system with ac-grid (System A).
b) Induction generator with ac-grid (System B).
c) Speed controlled induction generator with common dc-bus and
control of active and reactive power (System C).
d) Speed controlled induction generator with common ac-grid and
dc transmission (System D).

As it can be seen the wind farms have interesting features in order to act as a power source to the
grid. Some have better abilities than others. Bottom-line will always be a total cost scenario
including production, investment, maintenance and reliability. This may be different depending
on the planned site.

VIII. WIND POWER TRENDS.

The installed power in wind energy has grown rapidly in many years. Today more than 45000
MW are installed globally with recently an annual market of 8000 MW. This is illustrated in Fig.
12.

Fig. 12. Annually installed and accumulated wind power globally.

The expectations for the future are also very positive as many countries have progressive
plans. Table IV gives an estimate for the installed wind power in 2010 based on official
statements from different European countries.
It can be seen that many countries will increase their wind power capacity in large scales. In
Denmark the installed capacity is expected to approach saturation as the problems of a too high
capacity compared to the load level are appearing. However, energy cost rise can
change this.
The power scaling has been an important tool to reduce the price pr. kWh. Fig. 14 shows the
average size of the installed wind turbines in Denmark as well as their produced energy pr. m2
swept area pr. year. It can be seen that the technology is improving and it is possible to produce
more than 900 kWh/m2/year. This depends of course on location and from experience off-shore
wind-farms are able to produce much more energy
The influence on the power scaling can also be seen at the prices pr. kWh for different windturbine sizes in two different landscape classes and it is shown in Fig. 15. The key to reduce
price is to increase the power and today prototype turbines of 4-5 MW are seen around the world
being tested. Finally, the development of wind turbines is illustrated in Fig. 16. It is expected 10
MW wind turbines will be present in 2010.

Fig. 13. Average size of wind turbines and produced energy pr.
m2 swept area pr. year in Denmark.

Fig. 14. Price pr. produced kWh at different landscape classes.

Fig. 15. Development of wind turbines during the last 25 years.