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Advancements in the Harvesting
and Utilization of Wind Energy
K.V. Vidyanandan, Member, IEEE
Abstract—Growing pollution concerns, depleting natural
resources and increasing fuel costs are driving the nations
world over to reorganize their energy mix, with more thrust to
develop diverse sources of non-polluting, renewable and local
energy. Wind, as an energy source, has incredible potential to
meet all the above requirements. Wind energy along with
equally promising technologies such as solar energy and fuel
cells are going to be the possible solutions for our quest for an
environmental friendly energy future. However, wind energy is
a technology of variable output, with very little controllability
and despatchability, which needs to be integrated into a highly
dynamic electric system. This paper is an attempt to present an
overview of major wind energy conversion methods, some of
the challenges that required to be addressed for integrating
this advanced technology with the existing power system.
Index Terms - droop, frequency control, inertia, wind turbine.
I. INTRODUCTION
W
IND energy has a number of advantages compared to
traditional energy sources. Unlike fossil fuels which
are not uniformly distributed and whose combustion that
contributes to pollution and global warning (a major issue
plaguing the planet), wind is a more distributed, clean,
inexhaustible, and zero fuel cost energy source. Besides, it
does not require water either as an energy carrier or as a
coolant. Wind energy got the boost in 1973 when the oil
crisis increased the price of oil based electricity. Wind
energy conversion (WEC) technology has evolved
considerably during the last two decades in terms of
increasing power capacity and rotor diameter, as shown in
Fig. 1. Large capacity wind turbines, typically 4-5 MW and
above, is usually of offshore type, whereas small and
medium capacity wind machines are of onshore or offshore.
Fig. 1. Evolution of horizontal axis wind turbine sizes.
Increased reliance on wind energy will have many
positive impacts: a) on environment (due to reduced
pollution by way of reduced fossil fuel consumption), b)
energy security and stability in electricity price (being a
K. V. Vidyanandan is a Senior Faculty Member with the Power
Management Institute, NTPC Ltd., NOIDA, Uttar Pradesh, India (e- mail:
[email protected]).
local energy source, prices are not subject to fuel volatility),
c) local economics (creates new income source and jobs for
local population), and d) water savings (wind systems uses
almost no water in the energy conversion process).
A typical 1 MW wind turbine in an average location will
annually displace nearly 2,000 tonnes of CO2. Assuming a
life span of 20 years, a typical wind turbine on average takes
only two to three months to recover all the energy spent to
build and operate it. Compared to many other power
generation methods, WEC systems need higher initial
capital costs (generation capacity and transmission system),
but has reduced O&M costs as the fuel cost is zero.
However the technical challenges associated with this new
technology is much more complex than the matured
technologies.
The advances in various fields of engineering such as
turbine blade material, blade aerodynamics, electric
machines, power electronics, and control system have made
wind energy the fastest growing and one of the most
promising and establishing energy technologies of the
present time. Due to better conversion efficiency, reliability,
reducing capital costs, and exponential yearly growth rate,
wind energy is becoming increasingly competitive with
other power generation options.
Wind results due to the uneven heating of the earth’s
land, water and atmosphere, which cause air to move over
the surface. The energy available in wind is the kinetic
energy of large masses of moving air. Wind turbines (WTs)
extract energy from the wind by using aerodynamically
shaped blades, which produce a lift force along the length of
the blade. This force produces the torque on the turbine
shaft, which in turn drives the electric generator. Wind
turbines are generally grouped together into a wind farm,
and generate bulk electric power. Electricity from these
wind farms is fed into the utility power grid and distributed
to the customers, just like any conventional power plant.
The aim of this article is to provide an overview of major
wind energy conversion methods, generator configurations,
control methods and issues associated in the integration of
wind energy with the electric network. The rest of the paper
is organized as follows: an overview of various wind energy
technologies are discussed in Section II, integration of wind
energy with power system is presented in Section III, and
the conclusions are presented in Section IV.
II. OVERVIEW OF WIND ENERGY CONVERSION SYSTEMS
According to the orientation of the axis of rotation with
respect to the wind direction, wind turbines are classified
into two categories: a) Vertical-axis wind turbines (VAWT)
and b) Horizontal-axis wind turbines (HAWT). In VAWTs,
the blades circle around a vertical axis and its rotor has the
shape of an egg beater. This type of WTGs was used in the
past because of certain structural advantages. However,
most modern wind turbines use horizontal-axis design.
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Except for the rotor, all other systems are the same in both
designs, with some changes in their placement [1]-[2].
Typical arrangement of vertical axis and horizontal axis
wind turbine generators is shown in Fig. 2.
In order to limit the size and thereby cost of magnetics,
electric generators are generally of high speed type, and
therefore, turbine speed of 20-30 rpm will not be compatible
for direct connection with generators. A gearbox is thus
required to interface the high speed generator shaft with the
low speed turbine shaft. The gearbox ratio depends on the
number of magnetic poles and the type of generator.
Fig. 3. a) Aerodynamic characteristics of wind turbine and
b) Rotational speed range of high capacity wind turbine.
a. Vertical axis WT
b. Horizontal axis WT
Fig. 2. Arrangement of vertical axis & horizontal axis WTs
Due to the variations in the wind speed, the frequency of
electric power generated will not be at the nominal
frequency of 50/60 Hz. Hence, a frequency converter and a
step-up transformer are used for interfacing the WTG with
the power network. Fig. 4 shows various stages involved in
the conversion of energy from the wind resource to
electricity [3].
The mechanical power (Pm) extracted from the wind by a
wind turbine is calculated as
1
(1)
ρ AU w3 CP (λ , β )
2
where, ρ = Air density, A = Rotor sweep area, Uw= Wind
speed, = Blade pitch angle, Cp = Performance coefficient,
and = Tip speed ratio.
An analysis of the above equation reveals the following:
in order to obtain a higher wind power, (a) it requires a
higher wind speed, (b) a longer blade length for realizing a
larger sweep area and (c) a higher air density. Because the
wind power output is proportional to the cubic power of the
mean wind speed, a small variation in wind speed can result
in a large change in wind power.
Energy in the wind depends on the density of the air as
kinetic energy of a moving body is proportional to its mass
(mass=volume*density). Air density is a function of altitude, temperature, pressure and humidity. Deviations in these
parameters can vary the air density up to ±10%. Increase in
ambient temperature reduces air density; at higher altitudes
the reduced atmospheric pressure also reduces air density.
The outcome of these effects will be a reduced power output
from the wind turbine for the same wind speed.
Rotor sweep area is a function of the turbine blade radius.
A modern offshore wind turbine with 150 m diameter will
have a rotor sweep area of nearly 18,000 m2. Since the
sweep area is proportional to the square of the rotor radius,
doubling the rotor diameter will increase the power output
by four times, for the same wind speed.
The tip speed ratio, which is the ratio between blade tip
speed and wind speed, is limited to 80 - 100 m/s so that as
turbines get bigger, their rotational speed reduces such that
large turbines rotate slowly at around 20-30 rpm. Typical
wind turbine aerodynamic power output as a function of
rotational speed for different wind speeds is shown in Fig.
3a and the turbine rotor speed is shown in Fig. 3b.
Pm =
Fig. 4. Stages in the wind to electricity conversion process.
In certain latest designs, direct drive generators without
gearbox is also available. By using large diameter
generators with multiple poles, like the ones used in hydro
generators, a gearless drive arrangement is possible. This
has reliability advantages since gearbox is the weakest link
in the wind energy conversion chain. Yet, this type is
presently not very popular because of very high cost since
high performance permanent magnet material is virtually
indigenous to China.
Wind turbines are usually designed to start rotating at a
cut-in wind speed of around 3-5 m/s. WTGs produce the
rated power output at wind speeds around 13-14 m/s. In
order to avoid damage to turbine, generator and converter,
WTGs are taken out of service during high wind speeds of
more than 25 m/s. This wind speed is known as the cut-out
wind speed. The operating regions of a WTG as a function
of wind speed are shown in Fig. 5.
Fig. 5. Operating range of a WT as function of wind speed.
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All energy in the wind cannot be harvested by the turbine
blades due to a phenomenon known as rotor wake. The
wake or shadow effect occurs because the air immediately
behind the turbine blades moves very slowly due to lack of
energy. After some distance behind the blades, as the less
energetic wind stream mixes with the surrounding air, it
regains some speed and moves faster. However, the almost
dead air behind the turbine will prevent the free flow of
fresh air and thus limits the energy captured by the blades.
Theoretical limit of aerodynamic efficiency (Cp) is 59.26%,
known as the Betz limit, which is comparable with the
Carnot efficiency in thermodynamics. Modern WTs have Cp
between 35-45%.
During high wind speeds, it is necessary to limit the
aerodynamic power developed by the turbine blades to
avoid rotor over speeding and over loading of electric
machine and the power electronics converters. There are
three methods used for controlling the wind turbine power
output during high wind speeds, these are: stall control,
pitch control and active stall control.
Stall control is used in small capacity WTGs, in which
the turbine blades are designed in such a way that flow
separation and thereby turbulence is created on the sides of
the blade when the wind speed exceeds some specific value.
The main drawback of this method is that at low wind
speeds, the energy conversion efficiency will be low and at
high wind speeds, increased vibrations.
Pitch control uses separate actuators by which the turbine
blades are pitched slightly out of the wind to reduce the
aerodynamic efficiency and the power generated. The blades
will be pitched back again once the wind speed drops. Pitch
control is commonly used in variable speed WTGs.
Active stall controlled WTGs also use pitchable blades.
At low wind speeds, active stall turbines will operate like
pitch controlled turbines. At high wind speeds, they will
pitch the blades in the opposite direction than the normal
pitching direction and force the blades into stall. This allows
maintaining the constant rated power at all high wind speeds
till the cut-off speed. The main disadvantage of the pitch
control and the active stall control is the extra complexity
and added cost.
Based on the speed of rotation, WEC systems can be can
be broadly classified into three categories: a) fixed speed, b)
limited variable speed, and c) variable speed WTGs.
In fixed speed WTGs the electric generator is usually of
cage rotor type induction generators, which are directly
connected to the grid through a transformer as shown in Fig.
6. Since the rotational speed of cage induction machine
(CIG) is limited to a narrow range governed by the
generator slip (2%), these types of generators are called
fixed-speed wind turbines.
Fig. 6. Fixed speed induction generator based wind energy
conversion system.
The main advantages of CIG are: they are robust, cheap
and need low maintenance. However, due to fixed speed
operation, any fluctuation in the wind speed will lead to
torque fluctuation, which in turn will be reflected as power
fluctuation. Besides, the constant speed operation also
results in reduced energy conversion efficiency. Since
reactive power needed by the CIG is drawn from its stator
terminal, these machines cannot support grid voltage
control. To meet the reactive power requirements, usually,
capacitors are connected across the stator terminals and this
may lead to overvoltage in islanding condition.
For harvesting optimum energy from the wind, variable
speed WTGs is the preferred choice. As reported, the energy
capture in variable speed WTs can be 8-15% more than
fixed speed WTs in some specific sites with specific
technology. Variable speed WTs use power electronic
controllers in the generator for regulating the rotor speed.
Besides better conversion efficiency, variable speed WTGs
have many other advantages such as improved power
quality, reduced stresses on the mechanical components,
acoustic noise reduction and possibility to control active and
reactive power independently.
There are many configurations of variable speed WTGs
discussed in the literature, ranging from limited speed
control of 10% to full speed limit.
The limited variable speed (LVS) WEC system is the
simplest and cheapest among all the variable speed WTG
technologies. LVS-WEC system uses a wound rotor
induction generator; with its stator winding is directly
connected to the grid whereas the rotor winding is
connected to a variable resistance through a power
electronic converter. Speed control in the range of 1-1.1 pu
is achieved by controlling the rotor resistance. The range of
speed control in this method is limited to a low value as the
increased speed deviation from the synchronous speed
results in increased slip power extracted from the rotor,
which is dissipated in the external resistance, causing a
reduction in efficiency. The configuration of limited
variable speed WTG is shown in Fig. 7
Fig. 7. Limited variable speed wind energy conversion
system.
For wider speed range and thereby achieving optimum
energy conversion efficiency, the electric machines typically
used in variable speed WTs are the Doubly-Fed Induction
Generator (DFIG) and the multi-pole Permanent Magnet
Synchronous Generator (PMSG). The term doubly-fed
means both stator and rotor windings actively participate in
the electromechanical energy conversion process.
Due to variable speed operation, the power output of
variable speed WTGs will not be at the nominal frequency
of 50/60 Hz. Therefore, they require power electronic
converters for interfacing the electric machine with the
power system. DFIGs use partial scale power electronic
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converters in their rotor circuit, while PMSGs use full scale
power electronic converters in their stator circuit for grid
connection.
In case of DFIG, the rotor power output at slip frequency
is converted to line frequency power using the back-to-back
converter. When the generator speed is below synchronous
speed, power is fed into the rotor circuit from the grid and if
the rotor speed is above synchronous speed, rotor power is
fed in to the system. By regulating the rotor power flow and
its direction, the speed of DFIG-WTGs can be controlled in
the range of 0.67 pu-1.33 pu.
Because of the full scale converters, speed control range
of PMSG is much higher than partial scale converter based
DFIGs. Since only a small fraction of the generated power
flows through the DFIG rotor circuit, the converter required
in these types is only 20-30% of the full capacity.
Performance of the full-scale converter based PMSGs is far
better compared to partial-scale converter based DFIGWTGs over the entire speed range. However, full scale
converter based systems are more costly and have higher
losses in the power electronics, since all the generated
power has to pass through the power converters. The
arrangement of the DFIG based WEC system is shown in
Fig. 8 and the PMSG based WEC system is shown in Fig. 9.
output, if the machine were running at its rated (maximum)
power during the entire 8760 hours of the year. The annual
capacity factor of a wind farm typically ranges from 20-40%
[4]. This does not mean that a wind farm will only generate
power for 20-40% of the year. Wind turbines customarily
generate useful power for 70-85% of the year, but not
necessarily at their full capacity. A term generally used to
represent the percentage of load demand met by the wind
energy in a certain grid, on an annual basis, is the wind
energy penetration. The wind (energy) penetration in
percentage is defined as:
Total wind electricity produced
Wind energy penetration =
Gross annual electricity demand
The percentage share of wind energy in the power system
is continuously increasing and many countries are targeting
for 20% energy share from wind systems by the year 2030.
Though technically this could be feasible, a few European
countries like Denmark and Spain have already crossed 10%
wind penetration, the issues that need to be addressed for
realizing this ambitious task are enormous.
In the classical energy conversion methods by using
synchronous generators, driven by steam, hydro or gas
turbines, both real and reactive power can be independently
regulated as all these parameters are controllable. However,
such is not the case with the WTGs. The major challenges of
wind as an energy resource are largely related to its variable
nature of speed and direction, as both these can change by
the season, month, day, hour and minute. A typical wind
pattern over a small time period is shown in Fig. 11.
Fig. 8. DFIG based wind energy conversion system.
Fig. 11. Random nature of the wind resource.
Fig. 9. PMSG based wind energy conversion system.
Of the two most popular WTG technologies (i.e. DFIG
and PMSG), DFIG based wind turbines are more commonly
used. Fig. 10 shows the conversion efficiencies of various
intermediate stages of a typical 1.2 MW DFIG based
variable speed wind energy conversion system at nominal
operating condition [3].
Fig. 10. Conversion efficiency of various stages of a
variable speed WEC system at nominal operating condition.
III. INTEGRATION OF WIND ENERGY WITH POWER SYSTEM
The annual energy output from a wind farm is usually
represented as capacity factor, which denotes the actual
annual energy output divided by the theoretical maximum
As per the latest grid codes of many countries, wind
farms must participate in the system frequency and voltage
control functions, exactly like the conventional generating
units. In order to participate in frequency control, wind
farms must modulate the active power, including primary
response and secondary response. In order to participate in
voltage control, wind farms must generate or consume
reactive power [5]-[7].
Most of the present-day WTGs get disconnected from the
network when disturbances like large voltage sags develop
in the grid. Once the WTG has been disconnected, it takes
some time before the unit is reconnected to the grid.
Disconnection of wind turbines during grid disturbances
deteriorates the grid stability further and eventually the
cascading effects may result in partial system collapse.
New grid codes also stipulate that wind turbines to ride
through voltage sags. This means that WTGs should support
the power system to overcome the ill effects of disturbances
in the same way the conventional synchronous machines do.
With modern power electronics based controls, it is possible
for the WTGs to deliver fault ride through capability,
voltage control, and reactive power support to the grid. Few
methods such as anti-parallel thyristors in the stator circuit,
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active crowbar in rotor circuit etc. are being proposed in the
literature to enable WTGs to ride through low voltage. Fault
ride through capability of a typical wind turbine generator is
shown in Fig. 12.
Fig. 13. Reduction in the power system inertia with
increasing penetration of wind energy.
Fig. 12. Typical fault ride through capability of a WTG.
Wind energy is often located in remote rural areas and
the rural grids are generally weak and prone to voltage sags,
faults, and unbalances. These disturbances will cause many
problems for induction generators and connected loads. The
integration of wind energy systems into the existing power
grid depends on a number of factors, both technical and
regulatory. The regulatory aspects include the percentage
wind energy share, location of common point of coupling,
WTG technology, generation mix of electricity in the
system, and the size and characteristics of the network in
which it is connected. The technical aspects include the
effect on the system inertia and the droop [8]-[11].
Conventional synchronous generators possess two key
qualities which are extremely essential in controlling the
grid frequency. These are: a) system inertia and b) speed
droop.
Inertia is the property of a body to oppose any change in
its motion. Inertia of the power system is proportional to the
amount of rotating masses in the system [12]-[14]. It
determines the rate of change of frequency following a load
event. The larger the system inertia, the less is the frequency
rate of change following a power imbalance.
The initial turbine governor action immediately
following a load event, known as primary control,
establishes the active power balance between generation and
demand by using proportional control action. This control
action, known as droop control, arrests the frequency
deviations due to the change in load [15]-[16].The droop
parameter in the governor control loop allows multiple units
to share common loads.
For optimizing the energy conversion efficiency, most of
the wind farms use variable speed WTG technologies. In
order to prevent the reproduction of wind speed variation as
frequency variation in the grid, variable speed WTGs use
back-to-back ac-dc-ac power electronic converters for the
grid connection. The intermediate dc bus in the converter
creates an electrical decoupling between the machine and
the grid. Because of this ac-dc-ac decoupling, the wind
turbines, though they are very heavy, appear lighter to the
system. Thus the increasing presence of WEC systems in the
power system will reduce the effects of conventional
synchronous generators that supply the major portion of the
active power needed in the grid. The entire system will then
behave as a lighter system. A lighter system will experience
larger changes in the frequency even for small mismatches
in the supply and demand. Graphical representation of the
reduction in system inertia with increasing penetration of
wind energy in the power grid is shown in Fig. 13.
The increasing penetration of wind energy in the power
grid increases the equivalent droop parameter of the system
[17]. For instance, with a 20% wind penetration, the
conventional generating capacity capable of providing
primary frequency response reduces to 100/120=83.33%.
The effective stiffness of the system decreases as the speed
droop increases to R/0.8333=1.2R, where R is the initial
value of permanent droop. The variation in the system droop
due to the penetration of wind energy in the conventional
system is shown in Fig. 14. The increase in droop value
translates into a weaker system, less responsive to load
changes and consequently, more frequency excursions result
after every system event.
Fig. 14. Variations in system droop due to wind penetration.
a) No wind energy, equivalent system droop = 100%
b) 20% wind penetration, conventional system has
reduced to 100/120 = 83.33%
c) Equivalent system droop is 1/0.8333 = 120% of
original
Since the droop parameter is incorporated in the primary
speed control loop of a prime-mover governor, increase in
the droop value will reduce the overall frequency control
capability of the generating unit.
IV. CONCLUSION
For harvesting optimum energy from the wind resource,
modern wind energy conversion systems generally use
variable speed wind turbine generators, connected to the
electric network through power electronic converters. The
use of power electronic interface results in an electrical
decoupling between the generator and the grid, due to which
certain very important capabilities such as inertia and droop
will be missing. Reduced effects of these parameters will
reduce the frequency regulation capability of the wind
generating units. When connected to the network, WTGs
must also share reactive power requirements of the system.
This is necessary to maintain voltage stability in the grid.
The present day research in the wind energy systems is
mainly focusing on these aspects so that by the end of the
current decade, the reliance on conventional thermal units
for maintaining system frequency and voltage will reduce
drastically. The anticipated target of 20% wind energy by
year 2030 could be realized without much difficulty.
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K.V. Vidyanandan (M’10) received his B. Tech from the National
Institute of Technology (NIT), Calicut, D.C.A. from Raipur and M.Tech
from the Indian Institute of Technology (IIT), Delhi, India. He is a Faculty
Member in the Power Management Institute (PMI), NTPC Ltd., NOIDA,
Uttar Pradesh, India. Prior to his joining at PMI, he has worked at Farakka,
Korba, and Singrauli super thermal stations of NTPC. He is currently
pursuing Ph.D. degree in Electrical Engineering at the IIT, Delhi, India. His
research interests are wind energy conversion systems, energy storage,
microgrid and load frequency control.
E- mail: [email protected]