WIND ENERGY – PRINCIPLES AND APPLICATIONS
Sasi K K
Professor, Department of Electrical and Electronics Engineering
Amrita Vishwa Vidyapeetham, Coimbatore 641105
Introduction
Wind electricity generation first appearing in Denmark in 1891 and undergoing a very slow
development till 1974 emerged as a serious candidate amongst the renewables after the first oil shock. The
American wind power programme receiving a boost from President Carter's new energy policy led to the
setting up of California windfarms with thousands of rotating wind turbine-generators producing grid
quality electricity.
The serious accident of 1986 at the nuclear power station at Chernobyl in
Ukraine set the trend in the decline of nuclear power in almost every country through a general showing
down of expansions or cancellation of new nuclear power plants. The environmental awareness also
heightened and green house effect or global warming due to increased concentration of carbon-di-oxide in
atmosphere attracted so much of international attention that an International Panel on Climatic Change
(IPCC) was constituted under the auspices of the United Nations for a scientific study of the phenomenon
and advice on the remedial measures. A policy of shift from traditional polluting technologies to clean
technologies was adopted by the international governments and solar energy was advocated by the energy
experts as the desired change. Wind energy in general and wind electricity in particular received finally the
due attention for a sustained growth and the status of an acknowledged source for inclusion in national
energy planning. The cost of wind electricity has also declined expectedly due to advancement of
technology. Amongst the renewable sources for electricity generation wind is entitled to a distinct
preference as indicated in Table-1 below.
Table-1: Capital Costs and the Typical Cost of Generated Electricity from the Renewable Options
Sl.No.
Source
1
2
3
4
5
Capital Cost
(Crores of
Rs/MW)
5.00-6.00
4.00-5.00
4.00
3.5
1.94
Estimated Cost of
Generation Per
Unit (Rs/kWh)
1.50-2.50
2.00-3.00
2.50-3.50
2.50-3.00
2.50-3.50
Small Hydro-Power
Wind Power
Bio-mass Power
Bagasse Cogeneration
Bio-mass
Gasifier
6
Solar Photovoltaic
26.5
15.00-20.00
7
Energy from Waste
2.50-10.0
2.50-7.50
Source: Ministry of Non-Conventional Energy Sources, Planning Commission (2005)
History of Wind Electricity
The Danes are the pioneers in world wind electricity generation. Professor Poul La Cour, the
‘Danish Edison’, began experimenting with it in his Polytechnic of Askov and finally erected one as early
as 1891. Professor La Cour's generator was a stand alone DC dynamo because AC was not very popular
then. By 1910 several hundred units ranging in capacity from 5 to 25 kW were in operation in Denmark.
While the Industrial Revolution replaced water pumping and grinding windmills by steam engines in the
nineteenth century the advent of fossil fuel based power generation censored the growth of wind electricity
early in the Twentieth century. But the long wartime blockade forced the Danes rely on wind for both
mechanical and electric power. Electricity from the crude wind machines of an estimated total installed
capacity of 3 MW was a valuable source of power to an impoverished rural Danish population during the
First World War. When the situation changed after the war the little wind power stations of Denmark
eventually could not compete in upkeep or convenience with larger and more efficient steam turbogenerators. Yet, the Danes returned to wind for electricity during the Second World War too when there
was acute oil shortage.
The demand for electricity in the rural areas of United States in the 1920s was largely satisfied by
small wind electric generators (WEGs), But WEGs in America became anachronisms with creation of
Rural Electrification Administration (REA) in 1935. In 1924, within seven years of the Russian revolution,
USSR produced a 40 kW WEG which used a synchronous machine. In another seven years they built and
tested the world's first grid-connected WEG at Balaclava which reportedly worked well for 10 years. Being
partially inspired by this Russian adventure and also being reportedly annoyed by his electricity bill Palmer
C. Putnam of United States started working on a larger wind electricity generator in 1934. The well known
Smith-Putnam machine of 1.25 MW capacity generated power over a period of two years until a bearing
failed and a blade fell off in 1943 and no replacement or repair was available due to war time priorities. In
the Fifties a Royal Commission headed by Professor E. W. Golding was instituted in the U.K. to explore
wind electricity generation in the country. In Denmark and France also such initiatives were taken.
However, the success of the controlled nuclear fission experiment in 1942 and subsequent emergence of
nuclear electricity generation in the late fifties shadowed almost all efforts in this field. Since then and upto
the early Seventies only isolated attempts were made in wind electricity research. Yet, two of the most
significant contributions to the WEG technology had been during this period : namely, the Gedser wind
generator of Denmark and the Hutter's machine of Germany.
Johannes Juul of Denmark developed an IG (induction generator) based WEG in the year 1957
which was a 200 kW machine installed at Gedser which operated until 1968. The Gedser machine is
believed to be the forerunner of all later Danish WEGs. Ulrich Hutter of Germany built a 100 kW machine
in 1957 which worked for 11 years and contributed much to the design of WEGs of the future.
Table-2 shows the wind electric capacity of various countries. The world wind power capacity
doubled in 5 years from 1992 whereas the growth between 1990 and 1995 was 150%. The European wind
industry had a five fold growth in five years from 1990. The world's total installed wind power capacity
crossed 10,000 MW in 1998 with a European share of 6553 MW. The annual capacity additions of grid
connected WEGs in the country are shown in Fig-1.
Table-2 : World Leaders in Wind Power Installation
Country
Germany
Spain
USA
India
Denmark
Capacity in MW
18427
10027
9149
4430
3128
The System Operation
The WEG comprises a wind turbine (mostly horizontal axis type), a gear for speed matching and
an electric generator (mostly induction type) connected to the grid. The WEG generates power above a cutin wind speed Vc and up to a cut-off wind speed Vf as specified by the machine. It is so because a wind
speed below Vc is incapable of rotating the induction generator above its synchronous speed, and also at
such wind speeds the machine losses are greater than the realisable power in the wind. Where V is any
wind speed available at the site and Vr is the rated wind speed of the WEG, for Vc<V< Vr the WEG output
power P is less than its rated value Pr. For wind speeds above Vr the turbine power coefficient Cp is so
controlled either by stall regulation or by pitch control that P remains almost constant. But P is proportional
to V3 and hence operating the unit at very high wind speeds will likely affect the mechanical stability of the
entire system. As such Vf is chosen as the feasible safety limit above which the system is shut down. The PV characteristics of a WEG which is a plot of P against V is known as the WEG power curve. It has a near
linear shape between Vc and Vr with P increasing and then P remains constant from Vr to Vf.
Power and Energy
Power in the wind is (½  AV3) watts, where  is the air density in kg/m3, A is the swept area of
the wind turbine rotor in m3 and V is the wind speed in m/s. Therefore the output power of a WEG
operating at a wind speed V is
P = ½  AV3 (V) watts
----------(1)
where (V) is the overall conversion efficiency of the machine corresponding to V. (V) is a function of V
and therefore not a constant. Then the rated power output of the WEG at a given site is
Pr = ½  AVr3 (Vr) watts
----------(2)
where Vr is the rated wind speed of the WEG and (Vr) is the corresponding value of overall efficiency.
The energy produced per annum by the machine can be expressed as
E = Pr  CF  8760 watt-hours
----------(3)
because the capacity factor(CF) of a WEG has been defined as the ratio of actual energy output to its rated
value on an annual basis. CF can be estimated by use of wind regime parameters described below.
Weibull Parameters
It has been widely accepted that the two parameter Weibull distribution has a good match with the
wind regime in most of the places. The Weibull probability density function of a wind speed V is given by
f(V) = (k/c) (V/c)k-1 exp[-(V/c)k] ----------(4)
= F(V)  8760
----------(5)
where k is the dimensionless Weibull shape parameter, c is the Weibull scale parameter having the same
dimension as that of V and F(V) is the duration for which V prevailed at the site during the year under
consideration. V and c are normally expressed in m/s and the unit of F(V) is hours. k and c are estimated
from the annual average wind speed at the site, Vm and the corresponding annual average value of its
standard deviation,  as,
k = ( / Vm)-1.086
----------(6)
c = Vm /  (1 + [1/k])
----------(7)
and,
Computation of CF
The annual energy contribution by the wind speed V is
E = PF(V)
----------(8)
So E can be computed as the sum of individual contributions by all the wind speeds prevalent at the site
during the year. Therefore,
Vr
Vf
E =  (A/2)  (V) V3 F(V) dv + Pr  F(V) dv ----------(9)
Vc
Vr
Then an expression for CF can be obtained by dividing Eqn.(9) by (Pr  8760). That is,
Vr
Vf
CF = { 1/( (Vr) Vr3 ) }  (V) V3 f(V) dv +  f(V) dv
Vc
----------(10)
Vr
Then
 f(V) dv =  exp[ (V/c)k]
----------(11)
(V) V3 f(V) = f '(V).
----------(12)
and
By use of numerical integration with a tabular interval, h
Vr
Vr - h
 f '(V) dv = h  { f '(V) + f '(Vr)  f '(Vc) }
Vc
V = Vc
2
----------(13)
where f '(Vr) and f '(Vc) are the values of f '(V) at V = Vr and V = Vc respectively. The value of f '(Vc) will
be either zero or negligibly small and hence it can be neglected. Then,
Vr
Vr - h
 f '(V) dv = h  { f '(V) + f '(Vr) }
Vc
V = Vc
2
----------(14)
Eqn.(10) with Eqns.(11),(12) and (14) substituted in it can be used along with Eqn.(3) for the
prediction of annual energy production and corresponding capacity factor of a specific WEG at a given
site. The output power rating specified for the WEG should be corrected for the site's air density value
before using it in Eqn.(3).
That is,
Pr = Pr '  / '
----------(15)
where Pr ' and ' are the rated power and the air density specified for the power curve of the WEG. (V) V characteristics can be deduced from the WEG power curve by dividing values of P ' by (½ ' A V3)
where P ' is the output power corresponding to V in the power curve. (V) can then be expressed as a
polynomial of V by a curve fitting method so that a computer programme can be used for the computations.
It may be noted that the expressions assume a 100% grid-machine availability.
Description of WEG Systems
A WEG unit consists of a wind turbine (WT) which is coupled to the electric generator through a
gear. The WT can be of stall-regulated type or pitch-controlled type the former being normally used for
constant speed operation and the latter for variable speed operation. A synchronous WEG has its generator
synchronised to the grid while in operation whereas an asynchronous WEG has an asynchronous interface
(a DC link converter-inverter) between generator and the grid. The latter has a technical advantage over the
former that variations in grid parameters like the voltage and the frequency will not affect the generator
operation and vice versa. With an IG based WEG, asynchronous link helps in not disturbing the grid power
factor too.
The commercial systems available today are the following:
(i) a synchronous WEG comprising a stall -regulated WT, gear and squirrel cage IG, named as a WEGIG,
(ii) a synchronous dual speed WEG comprising a stall -regulated WT, gear and a doubly wound squirrel
cage IG, named as a WEG-DWIG,
(iii) an asynchronous WEG with a stall-regulated WT, gear and a slip ring IG having its stator synchronised
to the grid and the rotor connected to the grid through an asynchronous link, called as a WEG-DOIG,
and
(iv) an asynchronous WEG employing a multipole PMG directly coupled to a pitch-controlled WT and
connected to the grid through a DC link inverter, called as a WEG-PMG.
The wind power converted into a mechanical rotatory motion by the WT in a WEG-IG is
transfered through the gear to the generator shaft for onward conversion into electric power which is then
directly fed into the electric power grid. The WEG-IG scheme has been illustrated in Fig-2 below.
Fig-2: WEG-IG Configuration
1
2
3
4
1 - Wind Turbine : Stall regulated
2 - Speed reduction gear
3 - Electric Generator : 3 phase squirrel cage induction generator
4 - Power grid
The WT is a stall-regulated one although pitch-controlled ones are also in use in commercial
designs.
Fig-3: WEG-DWIG Configuration
1
2
4
3
1 - Wind Turbine : Stall regulated
2 - Speed reduction gear
3 - Electric Generator : 3 phase squirrel cage induction generator with two stator
windings (eg. 4 pole and 6 pole)
4 - Power grid
In the WEG-DWIG scheme the generator has two stator windings, for example, one for 4 poles
and the other for 6 poles. The 6-pole winding is connected to the grid when the wind speed is low and the
4-pole winding connected otherwise. It has a change- over switch activated by the wind speed sensor.
Power in the wind at low wind speeds being low this arrangement helps the WT operate at a low speed so
that the mechanical torque developed is relatively high. Both WEG-IG and WEG-DWIG use identical
WTs. Their generators differ in the size of the stator as the DWIG has to accommodate two windings. The
slip of the induction generator being very small the rotor speed variation of WEG-IG and WEG-DWIG
(for a particular connection) will be negligible. Both the systems draw the required reactive power from the
grid.
Fig-4: WEG-DOIG Configuration
4
1
2
3
5
1 - Wind Turbine : Stall regulated
2 - Speed reduction gear
3 - Electric Generator : 3 phase wound rotor induction generator
4 - Asynchronous grid interface : AC-DC-AC converter
5 - Power grid
The generator of the WEG-DOIG has a wound rotor which is connected to the grid
through a DC link converter whereas the stator is directly connected to the same grid. Both the stator and
the rotor will feed power to the grid. Power output of the rotor being the slip power depends on the amount
of slip at which it operates. The generator works at a full load slip as high as 50%. The reactive power
drawn by the stator from the grid can be compensated by adjusting the phase angle of the voltage applied to
the rotor through the DC link. Therefore the machine does not cause a VAR drain from the grid. The WT
used in a WEG-DOIG is of stall-regulated type. But it will opearte at a variable speed as the slip of DOIG
changes considerably. A variable speed operation improves the overall power conversion efficiency of a
WT and the DOIG achieves it without providing pitch-control which is otherwise necessary.
Fig-5: WEG-PMG Configuration
1
2
3
4
1 - Wind Turbine : Pitch regulated
2 - Electric Generator : 3 phase multipole permanent magnet AC generator (directly
coupled to the turbine)
3 - Asynchronous grid interface : AC-DC-AC converter
4 - Power grid
Unlike the three systems described above a WEG-PMG employs a synchronous generator. To
avoid complexities in the field excitation system it uses permanent magnet poles. Use of permanent magnet
facilitates reduction of pole size and consequently reduction of the synchronous speed of the generator by
increasing the number of poles. As a result the WEG-PMG does not need a gear. The grid interface in the
case of WEG-PMG is an asynchronous link. With an asynchronous grid interface the problem of armature
voltage regulation demands a variable speed operation of the prime mover. As such a pitch-controlled WT
is essential for this system because unlike an induction machine the PMG does not have a command on
speed on its own. Though it is costlier than a stall-regulated WT owing to sophisticated control mechanism,
the overall power conversion efficiency of the pitch-controlled WT is superior to that of a constant speed
stall regulated one. The power loss in the grid interface of a WEG-PMG is expectedly higher than that of a
WEG-DOIG of equal capacity because the former delivers the entire load current through the asynchronous
link unlike the latter.
Issues in the Developmental Phases
The major technical issues in the development of wind electricity so far have been enumerated and
reviewed below:
1. The Choice of Turbine
La Cour's WEG was essentially a combination of a windmill and an electric generator. The
concept of a generator-friendly wind turbine was evolved much later. The Vertical Axis Wind Turbine
VAWT was invented in 1920s by the French engineer G.J.M.Darrieus. Anton Flettner of Germany and
S.N.Savonius of Finland also developed VAWTs in the same decade. Sandia National Laboratories of
USA further developed the Darrieus technology in the late Seventies. But VAWT could not compete
with the Horizontal Axis Wind Turbines (HAWT) in the race of commercialisation.
However, VAWT has the advantages of installing the generator on ground and operating
omnidirectional. But it can not take advantage of the higher wind velocity and lower turbulence at higher
elevations. High cut-in wind speed and low cut-off wind speed as well as low starting torque had been
the discredits of the earlier Darrieus VAWT designs. Efforts are being made to increase the operating
wind speed range.
It is interesting to note that almost all WEG models available in the market today have the wind
speed rating in the range of 12-15 m/s. While it is true that such values of wind speed rating are suitable
for the rated power capacities (200 to 750 kW) for which these are designed, it has to be recognised that
there is a need of different capacities for different wind regimes.
2. The Choice of Generator
The generator options tried in the Seventies were:(i) conventional synchronous generator (SG), (ii)
squirrel cage induction generator (IG), (iii) double output slip ring induction generator(DOIG) and (iv)
AC commutator generator (ACCG). All except the IG needed slip rings and brushes on tower top and the
AC commutator machines burdened with commutator-brush maintenance virtually lost its candidature for
use in WEG. DOIG was tried in the German programme of Growian I at Hamburg in 1982. It was the
largest WEG ever made in the history having a rotor diameter of 100 m. and a rated capacity of 3 MW.
With its giant size and ignominious weight it turned out to be an aeronautical failure. Till its dismantling
in 1987 it operated only for 420 hours thus denying a chance to study thoroughly the DOIG
performance. SG and IG were used in many projects which proved that the latter had an edge over the
former owing to (i) lower cost, (ii) absence of synchronising units, (iii) reliability, and (iv) simpler
construction. The Eighties witnessed the successful commercialisation of IG based WEG. Both pitchcontrolled and stall-regulated WEGs were produced. Stall-regulated single speed systems (the operating
speeds of different models lie between 28 and 40 r.p.m.) became more popular thanks to lower cost and
less maintenance. Double winding IG of pole-changing type was also brought to the market which
operated at either of the two rotor speeds as selected based on the wind speed at hub height. ACCE,
because of their relatively high maintenance of the commutator brush gear, did not find favour from
system designers for higher ratings. At lower ratings these offer advantages of free running speeds.
IG having established as the right choice for a commercial WEG the next major hurdle was
eradication of VAR drain it caused. WEG manufacturers paid attention to this problem and efforts started
in the first half of Nineties. Methods adopted for the purpose included (i) use of asynchronous grid
interface comprising power electronic devices (Kenetech model), (ii) use of SG asynchronously
interfaced to the grid (Enercon model), and (iii) use of DOIG having asynchronous interface only in the
rotor circuit (Windtec model). All these aimed at, apart from mitigating the reactive power demand,
variable speed operation of WEG thereby increasing the overall energy conversion efficiency.
3. The VAR Drain
VAR drain to windfarms has been a cause of concern to Indian power grid on two accounts : (i)
inadequate capacitor compensation of WEGs, and (ii) existing VAR load of induction motors in the
agricultural sector. Poor maintenance of VAR compensating capacitors of the installed WEGs at
Muppandal resulted in very large inflow of reactive current through the Tuticorin-Muppandal
transmission line that forced TNEB to enforce a penalty on VAR consumption by induction generators in
1995. VAR drain has been effectively blocked in the new generation machines but at an extra high cost
of the DC link interface.
4. The Windfarm Operation and Control
Effects of windfarm behaviour under storm induced conditions have been a matter of concern in
the power system research sector in the West. Output power quality of WEGs using asynchronous
interface has also been questioned. WEG performance affected by poor grid power quality remains as a
major irritant in windfarm development and operation in India.
5. The Capacity Credit
That a grid-connected windfarm does not have a capacity credit notwithstanding its energy credit
of any amount is a generic phenomenon concerned with wind electric generation.
6. The Power System Reliability
Power system analysts claim that a low level grid penetration of wind electricity improves the
overall system reliability which then decreases with increasing wind penetration level.
7. The Grid Penetration Limit
The grid penetration limit of wind electricity has been an issue of debate globally. Researchers in
different countries have tackled the issue differently. It is but certain that utility based energy storage
will be a requirement at higher levels of grid penetration of wind electricity.
Meeting the correct solution of a problem at the right time brings forth the healthy development of
a technology. Quoting the oft-quoted lines of the Time magazine may be relevant here : "If wind power
does not fulfill its promises as a major energy source by the end of the century, it will not be a failure of
technology. It will be a failure of vision on the part of society to make the necessary commitment".
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