A brief overview of the different types of wind energy conversion schemes used at present.

1. Electrical Machines for Wind Power Generation
Hassan Tirmizi
Department of Electrical Engineering
Polytechnical Institute of Coimbra
Coimbra, Portugal
Email: hassantirmizi100@gmail.com
Abstract—Wind energy is an important source of electricity
production since it is a free energy source and exists widely in
nature. In recent years the emphasis on producing power from
renewables has increased tremendously to cut the over reliance
on fossil fuels and move towards a grid that constitutes a balance
between conventional and alternate energy production.
Through modern technology efficiency of wind turbines has
markedly improved but still power production through wind
turbines requires dealing with the subtle changes of wind speed,
atmospheric pressure and other natural variations. Therefore
to deal with these problems we need electrical machines with
intelligent and efficient control algorithms.
This report starts of with a brief introduction to the principles
of wind power production and then moving on towards electrical
wind power generation schemes which are currently in use
worldwide.
I. INTRODUCTION
The growth and prominence of wind energy development for
power generation in the last decade has been unprecedented,
and is currently the most used of any renewable energy
technology [1]. Wind generation of electrical energy is one
of the most promising future energy resources due to its
sustainable nature and very low net carbon impact.
A typical modern windmill looks as shown in Fig1. The
Fig. 1. Modern wind turbine [2]
wind-mill consists of three blades equidistant from each other
mounted on a horizontal axis and installed on a tower. A
turbine connected to a generator is fixed about the horizontal
axis.
Like the weather which is highly prone to change, the wind
also depends on the vagaries of nature. It varies from place to
place thus there are major challenges with regards to obtaining
a reliable and constant supply.
Wind has the advantage that it exists widely in nature and
as long as there will be life on earth, human beings will be
able to harvest wind energy to power their lives. However at
the same time it is a diffused energy source that cannot be
contained or stored for use elsewhere or at another time thus
presenting major challenges.
A. Classification of Wind turbines
1) Horizontal axis wind turbines: Horizontal axis wind
turbines are the most common wind turbine design. The blades
of a wind turbine rotate about an axis and in the case of
a horizontal wind turbine the plane about which the blades
are rotating is parallel to the flow of wind. The plane also
happens to be parallel to the ground. These machines are
usually designed to operate in an upwind mode [2], with the
blades upwind of the tower. In this case, a tail vane is usually
used to keep the blades facing into the wind. loss of power
from the interference is significantly reduced as compared to
a downwind turbine [4].
The towers are relatively tall so that the blades intercept the
wind at a point where the wind is flowing at a very fast speed
and is free from turbulence. In some places, the power output
of the wind turbine could increase up to 30% every ten meters
in altitude because the wind speed is increased by 20%.
Also because the blades are always perpendicular to the wind
they receive more power [2].
2) Vertical axis wind turbines: The vertical axis turbines
do not have to be pointed towards the wind to be effective.
Thus to the engineer it gives more leverage with regards to
the placement of the turbine as the changing wind directions
do not affect the overall operation
These types of turbines are mostly useful during minimal wind
speed.There curved propellers are designed so as to ensure
proper operation even with a small amount of wind
These towers are normally smaller in height and as a conse-
quence the wind speed is slower and more turbulent resulting
in a lower efficiency[5].
II. TYPES OF WIND ENERGY CONVERSION SYSTEMS
Over the past few years new wind energy conversion
techniques have been developed to cater to the multi-pronged

2. Fig. 2. types of wind turbines [2]
market needs of reduced cost, enhanced efficiency and in-
creased reliability.Some of the popular types of wind energy
conversion systems(WECS) are the following [3].
1) constant speed WECS connected to the grid directly .
2) Variable speed WECS with a doubly-fed Induction gen-
erator(DFIG).
3) Direct drive WECS with a Synchronous generator.
4) Direct drive WECS with a permanent magnet Syn-
chronous generator (PMSG).
The variable speed WECS with a DFIG and the direct-drive
WECS with a PMSG are the most popular schemes in the
market as of now which is evident from the Table3. In the
Fig. 3. typical products manufactured by the companies [3]
section to follow the basic design, method and topology of
these wind energy conversion systems are discussed succinctly
with the goals of making a comparative study.
A. Constant speed WECS with multiple stage gearbox
.The constant speed WECS consists of an Induction Gen-
erator connected to the wind turbine through a multiple-stage
gearbox .The design is popular and convenient because of it’s
simplicity and low cost. The schematic for the constant speed
WECS is shown in Fig4.
Fig. 4. Scheme of a constant speed WECS[3]
The major disadvantage of this topology is that for the
Induction generator reactive power still has to be supplied for
establishing the magnetic field in the airgap. Hence a capacitor
bank is needed for reactive power compensation.
In this system the fluctuations in wind speed are directly
converted to mechanical torque because the gearbox merely
scales the speed of rotation of the generator shaft[5]. Thus the
ensuing oscillations cause a great stress on the whole system.
B. Variable speed WECS with a Doubly fed Induction gener-
ator
The scheme of the variable speed WECS with a multiple-
stage gearbox, a DFIG and a partial-scale power converter is
shown in Fig5. The rotor of the DFIG is connected to the grid
through a back-to-back converter while the stator is directly
connected to the grid. The DFIG can deliver wind energy to
the grid in a wide speed range i.e from a speed greater than
that of the revolving magnetic field due to the stator currents
to a speed that is less than the synchronous speed.
Fig. 5. Scheme of a variable speed WECS with DFIG[3]
By controlling the active power output from the rotor
side converter the generator speed can be regulated so as
to optimize the power extracted from the wind.However the
drawback with this scheme is that the slip rings and brushes
require frequent maintenance and may lead to machine fail-
ures.
C. Induction Generator with a power converter
The multiple-stage geared Induction generator is connected
to the grid through a full-scale back-to-back converter.
The generator speed can be varied by controlling the first
thyristor-bridge rectifier on the generator side while the DC
bus voltage and the reactive power transmitted to the grid can
be controlled through the grid-side converter.

3. The reactive power has a more dominant effect on the terminal
voltage as compared to the active power. Therefore the reactive
power supplied to the grid is varied so as to achieve a constant
terminal voltage. The scheme for this type of WECS is shown
in Fig6.
Fig. 6. Scheme of a variable speed Induction generator with full scale
converter[3]
D. Synchronous Generator with a full scale power converter
The variable speed WECS with a multiple-stage geared
synchronous generator and a full-scale power converter is
shown in Fig7.
The usage of the multiple-stage gearbox can reduce the volume
and weight of the Synchronous generator while comparing
with the direct-drive system since the machine can be designed
with less number of poles.
f = ns ∗ p (1)
.where ns is the speed in rev/s and p is the number of poles.
The gearbox increases the speed of rotation of the generator
shaft so as a result the number of poles required are less and
the overall system is less bulky. This will in turn decrease the
difficulty of designing and manufacturing the generator.
Fig. 7. Scheme of a PMSG with full scale converter[3]
Both electrically excited synchronous generator (EESG) and
permanent magnet synchronous generator (PMSG) can be
used, however in EESG the rotor windings are supplied with
direct current to setup the field flux. Despite the excitation
being DC there is still hysteresis losses associated with this
technique and cooling mechanisms are required for a viable
operation.
Thus efficiency of PMSG is better than that of EESG. However
the cost of the PMSG is higher than all of the aforementioned
schemes i.e (EESG, SCIG and DFIG) due to the expensive
permanent magnet materials.
E. Electrically excited Synchronous Generator with a full
scale power converter
The Electrically excited Synchronous Generator (EESG) is
built with a rotor carrying the field system supplied with a DC
excitation.
Mostly this scheme is employed with a direct drive operation
without using the gearbox[3]. As a result generator must
be designed with high number of poles to make the gear-
less system possible .Consequently, the volume and weight
of these low speed generators are much greater than those
of the multiple-stage geared generators. The wind energy
conversion system with an EESG and full scale converter is
shown in Fig8. This scheme has the drawback that slip rings
Fig. 8. Scheme of a EESG with full scale converter[3]
and carbon brushes are required for the field winding which
produces power loss as well as requires extra maintenance.
Thus with EESG the robustness and ruggedness of the system
is compromised.
F. Permanent magnet synchronous Generator with a full scale
power converter
The direct-dive WECS with permanent magnet synchronous
generator (PMSG) is the most promising system currently in
use as shown in Fig9 where the permanent magnets also known
as hard magnetic materials setup the flux in the airgap without
the need of external excitation. This is possible because of the
high coercivity values of the hard magnetic materials which
is a measure of the capability of the magnet to setup flux in
the airgap
In this way the efficiency and reliability of the system im-
proves considerably. Weight and size of the PMSG are also
smaller than those of the EESG because with the discovery of
ever new rare earth permanent materials with extraordinarily
high values of retentivity and coercivity, even a smaller portion
of the magnet will suffice for a reliable operation.
An additional advantage of this system is the noise reduction
due to the absence of the gearbox and independent excitation
system. For the increasing power levels and decreasing turbine
Fig. 9. Scheme of a direct drive WECS with PMSG and a full scale converter
speeds, the direct-drive generators are becoming larger, heavier
and more expensive. There is a trade-off between the generator
size and reliability and thus for a optimal solution sometimes
the WECS with a single-stage gearbox, a medium-speed
PMSG and a full scale power converter is sometimes used.
Hence this system is also called Multibrid system[3].

4. III. CONCLUSIONS
Now we will look at the merits and demerits of the afore-
mentioned techniques and compare them on various criterias
to see which ones fare better than others.
A. Comparison on the basis of volume and weight
The doubly fed Induction generator with multiple gearboxes
(DFIGMG) generator is the smallest and lightest one in the
variable speed systems[3],making it the most popular scheme
currently.
The constant speed squirrel cage induction generator is a
bit smaller and lighter than doubly fed Induction Generator
however both these schemes are not used much lately because
it doesn’t make the best use of wind energy.
The EESG Direct drive generator without the use of a multi-
stage gearbox is the biggest and heaviest solution[3]. Due to
the properties of the permanent magnet materials it is possible
to make the PMSG-DD system more compact than the EESG-
DD system.
However by using PMSG with one gearbox, the volume,
weight and size of the generator can be further decreased and
thus the PMSG-1G system is preferred over the PMSG-DD
system. Therefore the PMSG-1G system is attractive for high
power wind turbines.
B. Comparison of Cost and efficiency
By comparing the structures of the six systems, one can
infer that the cost of the constant speed squirrel cage Induction
generator has to be the least because of the straightforward
design of cage rotors. With the rotor windings made up of
aluminium bars inserted in the rotor slots, this simple design
almost requires no maintenance at all and the operation is also
rugged.
No need of an extra converter however without it the energy
yield of the constant speed system is much lower than that of
the variable speed system.
The EESGDD is the most expensive system, while the PMS-
GDD is the second most expensive system because of the full-
scale converters required for power control and large multi-
pole generators. On the other hand the efficiency of the PMSG-
DD is the highest owing to the extraordinary properties of hard
magnetic materials and absence of the gearbox which produces
noise.
The efficiency of the EESG-DD is lower than that of PMSG-
DD because of the losses in the field windings as discussed
earlier.
C. Comparison on the basis of system reliability
The multiple-stage gearbox is vulnerable to damage in
high speed systems such as the constant speed squirrel cage
induction generators and doubly fed induction generators.
Thus even though they allow a better energy yield overall
the system fares poorly on the reliability standards since any
problem in it could cause the whole system to malfunction.
In the sameway brushes and slip rings used in the DFIG and
EESG necessitate constant care and maintenance which in-
creases the maintenance cost and reduce the system reliability.
In the light of the above mentioned two factors the PMSG
scheme with direct drive or even with a single stage gearbox
would rank higher on the reliability standards since both
the problems of multiple-stage gearbox and slip rings is
eliminated.
REFERENCES
[1] L. Mastny (Ed.) Renewable 2015: Global Status Report. Renewable
Energy Policy Network for the 21st Century; REN21 Secretariat, Paris,
France. Available at: www.ren21.net
[2] https://sites.google.com/a/temple.edu/urbanwind/services/turbine-options-
and-specifications
[3] Cheng, Ming, and Ying Zhu. ”The state of the art of wind energy
conversion systems and technologies: A review.” Energy Conversion and
Management 88 (2014): 332-347.
[4] http://www.power-talk.net/upwind-turbine.html
[5] ISEC Power Plants Notes on Wind Energy