-
Maximum power point tracking and power
smoothing in wind Energy conversion system using
fuzzy logic pitch controller
A Thesis submitted
in partial fulfillment of the requirement for the award of the
degree
of MASTER OF TECHNOLOGY
in POWER ELECTRONICS & ASIC DESIGN
by
Pankaj Shukla (Reg. No. 2008PE19)
Under the Guidance of
Dr. S.R.Mohanty Asst. Professor, EED
DEPARTMENT OF ELECTRICAL ENGINEERING
MOTILAL NEHRU NATIONAL INSTITUTE OF TECHNOLOGY
(DEEMED UNIVERSITY) ALLAHABAD-211004
JUNE 2010 MOTILAL NEHRU NATIONAL INSTITUTE OF TECHNOLOGY
-
ALLAHABAD
CERTIFICATE This is to certify that the thesis entitled, Maximum
power point tracking and power
smoothing in wind Energy conversion system using fuzzy logic
pitch controller
submitted by Mr. Pankaj Shukla in partial fulfillment of the
requirement of the award of
the degree of Master of Technology in Electrical Engineering
with specialization in
Power Electronics & ASIC Design to the Motilal Nehru
National Institute of Technology,
Allahabad (Deemed University) during the academic year 2009-10.
The results
embodied in this thesis have not been submitted for the award of
any other degree. We
approve his submission for the above mentioned degree.
Date: June 2010 (Dr. S.R.Mohanty) Place: Allahabad (U.P.)
Assistant Professor, EED
-
CANDIDATESS DECLARATION
I, Pankaj Shukla hereby submit the thesis, as approved by the
thesis supervisors Assistant Professor, Dr.S.R.Mohanty, Assistant
Professor, Electrical Engineering Department, MNNIT, Allahabad. I
hereby declare that the work presented in this thesis is an
authentic work carried out by me during July 2009-June- 2010. I
have read and understand the Institutes rule relating to the
thesis, inventions, innovations and other work and agree to be
bound by them. I also declare that, to the best of my knowledge and
belief, this work has not been submitted earlier for the award of
any other degree or thesis.
June, 2010 (Pankaj shukla) Allahabad Reg.No.2008PE19
-
Dedicated
To
My parents
-
ACKNOWLEDGEMENTS
I would like to express my sincere thanks and deepest to my
honorable Thesis Supervisors Dr.S.R.Mohanthy, Assistant Professor,
Department of Electrical Engineering, Motilal Nehru National
Institute of Technology, Allahabad, for their invaluable guidance,
motivation, support, advice and supervision during the entire
period of this thesis. Their meticulous guidance, constructive and
valuable suggestions, timely discussions and clarifications of my
doubts increased my cognitive awareness and helped me for making a
deeper analysis of the subject under study.
I also express my sincere thanks to my Head of the Department,
Prof. Dinesh Chandra, for his invaluable support and encouragement
throughout the thesis.
I also express my heartfelt gratitude to the Department of
Electrical Engineering MNNIT Allahabad for giving us this
opportunity, which has enriched our knowledge and experience
immensely.
Lastly, I wish to express thanks to my parents, family members
and friends for their patient encouragement and cooperation, which
has gone along way in making this report a success.
(Pankaj Shukla) Reg. No. 2008PE19
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ABSTRACT
In recent years, there has been a growing interest in wind
energy as it is a potential source for electricity generation with
minimal environmental impact. With the advancement of aerodynamic
designs, wind turbines, which can capture hundreds of kilowatts of
power, are readily available. When such wind energy conversion
systems (WECS) are integrated to the grid, they produce a
substantial amount of power, which can supplement the base power
generated by thermal, nuclear, or hydropower plants. The purpose of
this work is to develop a maximum power tracking control strategy
for variable speed wind turbine systems. In this thesis, four
different methods of tracking the peak power in a wind energy
conversion system (WECS) is discussed. The algorithms search for
the peak power by varying the speed in the desired direction. The
generator is operated in the speed control mode with the speed
reference being dynamically modified in accordance with the
magnitude change of active power. The peak power points in the P
curve correspond to dP/d =0. This fact is made use of in the
optimum point search algorithm. The generator considered is a wound
rotor induction machine whose stator is connected directly to the
grid and the rotor is fed through back-to-back
pulse-width-modulation (PWM) converters. Pitch angle control is the
most common means for adjusting the power output of the wind
turbine when wind speed is above rated speed and various
controlling variables may be chosen, such as wind speed, generator
speed and generator power. As conventional pitch control usually
use PI controller, the mathematical model of the system should be
well known. A fuzzy logic pitch angle controller is developed in
this thesis, in which it does not need well known about the system.
The design of the fuzzy logic controller and the comparisons with
conventional pitch angle control strategies with various
controlling variables are carried out. The simulation results show
that the fuzzy logic controller can achieve better control
performances than other three methods of maximum power point
control strategies. The output power of WECS is also effectively
smoothened using the proposed method.
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CHAPTER-1 INTRODUCTION
1.1 INTRODUCTION
Wind energy is one of the most available and exploitable forms
of renewable energy.
Wind blows from a region of higher atmospheric pressure to one
of the lower
atmospheric pressure. The difference in pressure is caused
by:
(A) The fact that earths surface is not uniformly heated by the
sun and
(B) The earths rotation.
The global electrical energy is rising and there is a steady
rise of the demand on power
generation, transmission, distribution and utilization. The
maximum extractable energy
from the 0-100m layer of air has been estimated to be the order
of 1012
KWh/annum,
which is of the same order as hydroelectric potential.
Wind Energy, energy contained in the force of the winds blowing
across the earths
surface.When harnessed, wind energy can be converted into
mechanical energy for
performing work such as pumping water, grinding grain, and
milling lumber. By
connecting a spinning rotor (an assembly of blades attached to a
hub) to an electric
generator, modern wind turbines convert wind energy, which turns
the rotor, into
electrical energy [1].
Since earliest recorded history, wind power has been used to
move ships, grind grain and
pump water. This is the evidence that wind energy was used to
propel boats along the
Nile River as early 5000 B.C. within several centuries before
Christ; simple windmills
were used in china to pump water.
In the United States, millions of windmills were erected as the
American West was
developed during the late 19th century. Most of them were used
to pump water for farms
and ranches. By 1900, small electric wind systems were developed
to generate current,
but most of these units fail into disuse as inexpensive grid
power was extended to rural
areas during the 1930s. By 1910, wind turbine generators were
producing electricity in
many European countries.
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Wind turbines are available in a variety of size, and therefore
power ratings. The largest
machine, such as the one built in Hawaii, has propellers that
span the more than the
length of a football field and stands 20 building stories high,
and produces enough
electricity to power 1400 homes. A small home-sized wind machine
has rotors between 8
and 25 feet in diameter and stands upwards of 30 feet and can
supply the power needs of
an all-electric home or small business.
All electric-generating wind turbines, no matter what size, are
comprised of a few basic
components: the (the part that actually rotates in the wind),
the electrical generator, a
speed control system, and a tower. Some wind machine have fail-
safe shutdown system
so that if part of the machine fails, the shutdown system turn
the blades out of the wind or
puts brakes.
In Fig.1.1, the data showing the present situation of installed
units in different countries
of the world. It shows that the maximum units is installed in
U.S.A (31.62%), then in
China (23.83%) and then in India (6.57%) [44].
Fig.1.1.Installed units of wind power in different countries in
percentage.
1.1.1- Benefits of wind power
A wind energy system can provide a cushion against electric
power price increases. If
you live in a remote location, a small wind energy system could
help you avoid the high
USA, 31.62
CHINA, 23.83
INDIA, 6.57
GERMANY, 6.3SPAIN, 0.9
ITALY, 0.82
FRANCE, 3.59
PORTUGAL, 3.29REST OF THE
WORLD, 14.88
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costs of having utility power lines extended to your site.
Although wind energy system
involves a significant initial investment, they can be
competitive with conventional
energy sources when you account for a lifetime of reduced or
altogether avoided utility
costs. The length of the payback period the time before the
savings resulting from your
system equal the cost of the system itself- depends on the
system you choose the wind
resource on your site, electricity costs in your area, and how
you use your wind system.
Wind energy is the world's fastest-growing energy source and
will power industry,
businesses and homes with clean, renewable electricity for many
years to come. In India,
wind power plants have been installed in Gujarat, Orissa,
Maharashtra and Tamil Nadu,
where wind blows at speed of 30 km/h during summer. On the
whole, the wind power
potential of India has been estimated to be around 20,000 MW
[2].
Small wind energy systems can be used in connection with
grid-connected systems, or in
stand-alone application that are not connected to the utility
grid. A grid-connected wind
turbine can reduce consumption of utility-supplied electricity
for lighting, appliances, and
electric heat. If the turbine cannot deliver the amount of
energy you need, the utility
makes up the difference. When the wind system produces more
electricity than the
household requires, the excess can be returned to the grid. With
the interconnection
available today, switching takes place automatically.
Stand-alone wind energy systems
can be appropriate for homes, farms, or even entire communities
that are far from the
nearest utility lines.
Either type of the system can be practical if the following
condition exist
Some few requirements of wind generation system [4]
1. Wind generation is dependent on the quality and quantity of
the wind hitting the
blades. The better the wind you have the more power you will
generate.
2. The power available in wind increases by the cube of the wind
speed if wind
speed doubles, power output increases by eight.
3. Turbulent wind (from obstruction, geographical features,
etc.) will reduce the
power output as the turbine swings back and forth hunting for
the wind.
4. These are the few requirements of site for wind generation
system:
5. The higher a turbine, the more power is generated, the better
quality the wind.
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6. A wind turbine should be at least 40 ft above any object
within a 400 ft radius.
Note there is often exception to this rule depending on your
site.
7. It is often more economical to install a higher tower than
purchasing a larger
turbine.
8. Space: generally locations with an acre or more will be
suitable. A guyed tower
requires the height of the tower as a radius at a minimum for
location of anchor
points. Space is also required for ground assembly and erection
of the tower.
Lattice towers require less surface area, but are more complex
and expensive to
install.
Wind energy has been the subject of much recent research and
development. In order
to overcome the problems associated with fixed speed wind
turbine system and to
maximize the Wind energy capture, many new wind farms will
employ variable speed
wind turbine. DFIG is one of the components of Variable speed
wind turbine system.
DFIG offers several advantages when compared with fixed speed
generators including
speed control. These merits are primarily achieved via control
of the rotor side converter.
Many works have been proposed for studying the behavior of DFIG
based wind turbine
system connected to the grid. Most existing models widely use
vector control Double Fed
Induction Generator. The stator is directly connected to the
grid and the rotor is fed to
magnetize the machine [3].
Large wind farms have been installed or planned around the world
and the power ratings
of the wind turbines are increasing. Wind energy generation
equipment is most often
installed in remote, rural areas. These remote areas usually
have weak grids, often with
voltage unbalances and under voltage conditions. [5].
Many places also do not have the potential for generating hydel
power. Nuclear power
generation was once treated with great optimism, but with the
knowledge of the
environmental hazard with the possible leakage from nuclear
power plants, most
countries have decided not to install them anymore [42]. It is,
however, only since the
1980s that the technology has become sufficiently mature to
produce electricity
efficiently and reliably from the wind. Over the last two
decades, a variety of wind
energy systems have been developed. Power extracted from wind
energy contributes a
-
significant proportion of consumers electrical power demands. In
recent years, many
power converter techniques have been developed for integrating
with electrical grid [2].
The first wind turbines were probably simple vertical-axis, such
as those used in
Persia as early as about 200 B.C for grinding grain. The use of
these vertical-axis mills
subsequently spread throughout the Islamic world Later,
horizontal-axis windmills,
consisting of up to ten booms, rigged with jibs sails, were
developed. In middle ages,
by the eleventh century A.D, windmills were in extensive use in
the Middle East and
were introduced to Europe in the thirteen century by returning
crusaders. By the fourteen
century the Dutch had taken the lead in improving the design of
windmills, and used
them extensively thereafter for draining the marshes and lakes
of the Rhine River delta.
Between 1608 and 1612, Beemster Polder, wetland area which was
about 10 feet below
sea level, was drained by 26 windmills of unto 50 hours power
(hp) each, operating in
two stages .The first oil mill was built in Holland in 1582 and
paper mill 1586. By the
middle of the nineteenth century, some 9000 windmills were being
used in the
Netherlands [1, 3].
By 1960, fewer than 1000 were still in working condition due to
introduction of
steam engine. The Dutch introducing many improvements in the
design of windmills and
particular, the rotors, large industrial mills could deliver up
to 90 hp in high winds.
Industrialization, first in Europe and later in America, led to
a gradual decline in the use
of windmills. The steam engine replaced European water-pumping
windmills. In the
1930s, the Rural Electrification Administration's programs
brought inexpensive electric
power to most rural areas in the United States [1, 3].
Since the end of the 19th
century the wind power used to generate electricity. In
1888, Charles F. Brush built the first automatically operating
wind turbine of 12 KW with
a rotor diameter of 17 meter and 144 rotor blades made of cedar
wood for electricity
generation. The Danish Poul la Cour (1846-1908), another pioneer
of electricity
generating wind turbines, discovered fast rotating wind turbines
with few rotor blades in
1897 in Askov (Denmark). It was more efficient for electricity
production than the slow
moving ones.
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Modern wind turbine technology has been accomplished with the
help of many areas,
such as material science, computer science, aerodynamics,
analytical methods, testing,
and power electronics. Without the help of these areas the rapid
development of new
technologies would not be possible. A relatively new area for
wind turbines is power
electronics based variable speed drives. Power electronic
systems allow
synchronization between the wind turbine system and the utility
grid and operate the
wind turbine at variable speeds, increasing the energy
production of the system. In
addition, power electronics provide a means to transfer energy
to and from storage
units, which can allow the storage of excess energy generation
for later use[2].
It is important to find an alternative form of energy before the
worlds fossil fuels
are depleted. It is predicted that oil and gas reserves will be
depleted by 2032. Due to the
combustion of fossil fuels, carbon dioxide is released into the
atmosphere causing the
atmosphere to trap solar radiation that then leads to global
warming or the green house
effect.
1.2 LITERATURE REVIEW
A lot of research work has been carried out in the area of wind
power
technologies in power systems which led to the development of
different methodologies
and approaches. Both grids connected and stand-alone operation
is feasible. A lot of
research work has been reported in the area of wind energy
conversion systems,
Since wind availability is sporadic and unpredictable. A brief
literature review of these
methodologies and approaches is present below.
J.G. Slootweg et al. [41] presented dynamic model (d-q frame) of
wind turbine
concept namely a doubly fed (wound rotor) induction generator
with a voltage source
converter feeding the rotor. Thus wind turbine concept is
equipped with rotor speed, pitch
angle and terminal voltage controllers. The wind turbine
response is simulated in this
paper.
In [1], the authors focused on future concepts to increase the
penetration of wind
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power in power system, where Offers broad coverage ranging from
basic network
interconnection issues to industry deregulation and future
concepts for wind turbines and
power system. Discusses wind turbine technology, industry
standards and regulations
along with power quality issue. [1] presents models for
simulating wind turbines in power
system.
The [2, 4] are added with almost all existing machines, but they
introduced new control
concepts on different motors and its drives. Introduced
matlab/simulink model for The
doubly fed (wound rotor) induction generator control through a
rotor connected
bidirectional a.c., d.c., a.c. PWM converter that is used for
pump storage hydro and wind
energy conversion today.All the renewable sources, non renewable
sources and other
energy sources are discussed in [3].
F. Mei and B. Pal [5] investigated the modal analysis of a grid
connected doubly fed
induction generator (DFIG). The change in modal properties for
different system
parameters, operating points, and grid strengths are computed
and observed. The results
offer a better understanding of the DFIG intrinsic dynamics,
which can also be useful for
control design and model justification. L. Szabo et al. [1]
presented simulation tool for
induction generators. In this paper, a mathematical model of
doubly-fed induction
generator was built to control active and reactive power in wind
power plants. In this
model parks transformation is used where three phased stator and
rotor symmetrical
windings are transformed in orthogonal axis systems to improve
power quality, high
energy efficiency and controllability.
The wind farm power collection system, grounding of wind farms
against power system
faults and transient over voltages and Wind turbine lightning
protection systems are
discussed in [5]. The Embedded wind generation, Electrical
distribution networks and the
impact of dispersed generation, the per-unit system, power flows
and voltages in simple
radial distribution networks, connection of embedded wind
generation, power system
studies, Power (voltage) quality. Voltage flicker, harmonics
from variable speed wind
turbines, measurement and assessment of power quality of grid
connected wind turbines
also be mention[5].[6]This book devoted to wind power and solar
photovoltaic
technologies, their engineering fundamentals, conversion
characteristics, operational
-
considerations to maximize output, and emerging trends also
includes new and
specialized technologies and explore the large-scale energy
storage technologies, overall
electrical system performance[6].
A.Tapia et al. [33] described the modeling of the machine
considers operating conditions
below and above synchronous speed, which are actually achieved
by means of a double-
sided PWM converter joining the machine rotor to the grid. In
order to decouple the
active and reactive powers generated by the machine,
stator-flux-oriented vector control
is applied. The wind generator mathematical model developed in
this paper is used to
show how such a control strategy offers the possibility of
controlling the power factor of
the energy to be generated.
In [6], a new control scheme implemented for the variable speed
grid connected
wind energy generation system, that helps a induction generator
driven by an emulated
wind turbine with two back to back voltages fed PWM inverters to
interface the generator
and grid. The machine currents are controlled using an indirect
vector control technique
[6]. The generator torque is controlled to drive the machine to
the speed for maximum
wind turbine aerodynamic efficiency [6]. In order to implement
the separated positive
and negative sequence controllers of DFIG, two methods to
separate positive and
negative sequence in real time are compared [7]. The features of
each generator
converter configuration are considered in the context of wind
turbine systems [8].
H. Karimi-Davijani et al. [34] presented fuzzy logic control of
Doubly Fed Induction
Generator (DFIG) wind turbine in a sample power system.. Fuzzy
logic controller is
applied to rotor side converter for active power control and
voltage regulation of wind
turbine. Wei Qiao et al. [35] presented an approach to use the
particle swarm
optimization algorithm to design the optimal PI controllers for
the rotor-side converter of
the DFIG. A new time-domain fitness function is defined to
measure the performance of
the controllers. Simulation results show that the proposed
design approach is efficient to
find the optimal parameters of the PI controllers and therefore
improves the transient
performance of the WTGS over a wide range of operating
conditions.
Rohin M. Hilloowala, and Adel M. [35] presented a rule-based
fuzzy logic controller to control
the output power of a pulse width modulated (PWM) inverter used
in a standalone wind energy
conversion scheme (SAWECS).
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C. A. M. Amendola, and D. P. Gonzaga [36] presented the energy
capture control is
made applying a fuzzy-logic controller directly on the turbine
pitch-angle and the speed
control is made by a field-oriented fuzzy-logic controller, that
acts on DFIG
electromotive torque so that to follow the reference value
generated by an optimum
angular speed estimator. Yongchang and Z. Zhengming [38]
presented a conventional
PI controller, sliding mode controller (SMC) and fuzzy logic
controller (FLC) for rotor
field oriented controlled (RFOC) induction motor drives are
studied comparatively. PI is
simple but sensitive to parameter variations. SMC provides
strong robustness to
parameters variations, disturbance rejection and system order
reduction. FLC does not
need exact system mathematical model and can handle intricate
nonlinearity, but its
implementation is more complicated than that of PI and SMC.
Comparative study of PI,
SMC and FLC are carried out from four aspects: dynamic
performance and steady-state
accuracy, parameter robustness, and complexity of
implementation.
In the [10, 11] developed a 30kW electrical power conversion
system for a
variable speed wind turbine system.. As the voltage and
frequency of generator output
vary along the wind speed change, a dc-dc boosting chopper is
utilized to maintain
constant dc link voltage. The input dc current was regulated to
follow the optimized
current reference for maximum power point operation of turbine
system. Line side PWM
inverter supply currents into the utility line by regulating the
dc link voltage. The active
power was controlled by q-axis current whereas the reactive
power can be controlled by
d-axis current. The phase angle of utility voltage was detected
using software PLL
(Phased Locked Loop) in d-q synchronous reference frame[9, 10]
.Proposed scheme
gives a low cost and high quality power conversion solution for
variable speed WECS.
A switch-by-switch representation of the PWM converters with a
carrier-based
Sinusoidal PWM modulation for both rotor- and stator-side
converters has been
proposed. Stator-Flux Oriented vector control approach is
deployed for both stator- and
rotor-side converters to provide independent control of active
and reactive power and
keep the DC-link voltage constant [12]. In order to set
synchronous vector controllers,
decoupled design based on Internal Model Control approach is
applied, where dynamics
of the PWM converters is taken into account [12, 14].
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After controlling method for the power of variable speed DFIG a
method of
tracking the peak power proposed which is independent of turbine
parameters and air
density is proposed. The algorithm searches for peak power
points by varying the speed
in desired direction. The generated is operated in speed control
mode with the speed
reference being dynamically modified in accordance with the
magnitude and direction of
change of active power [14, 15, 16]. But this method is rotor
speed dependent, then a
method proposed that doesnt depend on wind generator speed and
rotor speed ratings
nor the dc/dc power converter rating [17, 19]. The two methods
utilize the turbine
characteristics (torque, power and power coefficient curves) to
determine the
operating point that results in maximum power capture [20, 22].
The only difference
between the two methods presented is that one requires an
anemometer so that the wind
speed is physically measured while and the second method
calculates the wind speed
using electrical parameters [ 22].
These methods are advantageous for fast optimum point
determination and
easy implementation since all the physical characteristics of
the turbine are programmed
directly and optimum operation point is determined by simply
examining the
characteristics. A disadvantage of these strategies however, is
that they are customized
for a particular turbine.. Another drawback of this algorithm is
that it cannot take into
account the atmospheric changes in air density, since for all
its calculations, it assumes a
certain value. But for overall efficiency improvement and to
reduce the cost PWM
converters were used with reduced switch count power converters
[23].
In [41] The complete system is modeled and simulated in the
MATLAB Simulink
environment in such a way that it can be suited for modeling of
all types of induction
generator configurations. The model makes use of rotor reference
frame using dynamic
vector approach.
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1.3 OBJECTIVE AND OGANIZATION OF THESIS
Objective
The objective is to develop a model and control methodology for
a Doubly Fed
Induction Generator and maximum power point tracking for this
model that can be
achieved.
Thesis Outline
Chapter -2 deals with the types of wind energy conversion system
and configuration.
This chapter also deals with wind energy back ground and wind
turbine characteristics.
Chapter-3 deals with induction machine with basic dynamic d-q
model, axes
transformation and also describe dc drive analogy and vector
control of induction
machine in brief.
Chapter-4 deals with modeling of wind turbine, pitch angle
control, rotor side
controller, grid side controller of DFIG and also deals with
detail modeling of wind
turbine coupled with DFIG.
Chapter-5 deals with DFIG under Maximum Power Point Tracking
(MPPT) and power
smoothing using fuzzy pitch controller.
Chapter-6 deals with simulation model and parameter
initializations.
Chapter-7 deals with simulation results and discussion between
different results.
Chapter-8 deals with conclusion and future work related to
DFIG.
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CHAPTER-2 TYPES AND CONFIGURATIONS OF WIND ENERGY CONVERSION
SYSTEMS
In this chapter various types and configurations of wind energy
conversion
systems are discussed i.e. the fixed speed wind energy
conversion systems and
variable-speed wind energy conversion systems. Also wind turbine
characteristic
which are specific to each turbine and depends on the
aerodynamic design of the
turbine and the site location of wind power plant are discussed
.But in this thesis only
variable speed wind turbines will be considered [26].
2.1 General
A special type of induction generator, called a doubly fed
induction generator (DFIG), is
used extensively for high-power wind applications. DFIGs ability
to control rotor
currents allows for reactive power control and variable speed
operation, so it can operate
at maximum efficiency over a wide range of wind speeds. The
Doubly-Fed Induction
Generator (DFIG) is widely used for variable-speed generation,
and it is one of the most
important generators for Wind Energy Conversion Systems (WECS).
Both grid
connected and stand-alone operation is feasible. For variable
speed operation, the
standard power electronics interface consists of a rotor and
stator side PWM inverters
that are connected back-to-back. These inverters are rated, for
restricted speed range
operation, to a fraction of the machine rated power. Applying
vector control techniques
yields current control with high dynamic response. In
grid-connected applications, the
DFIG may be installed in remote, rural areas where weak grids
with unbalanced voltages
are not uncommon. As reported in induction machines are
particularly sensitive to
unbalanced operation since localized heating can occur in the
stator and the lifetime of
-
the machine can be severely affected. Furthermore,
negative-sequence currents in the
machine produce pulsations in the electrical torque, increasing
the acoustic noise and
reducing the life span of the gearbox, blade assembly and other
components of a typical
WECS. To protect the machine, in some applications, DFIGs are
disconnected from the
grid when the phase-to-phase voltage unbalance is above 6%.
Controller design parameters for the operation of induction
generators in unbalanced
grids have been reported in, where it is proposed to inject
compensating current in the
DFIG rotor to eliminate or reduce torque pulsations. The main
disadvantage of this
method is that the stator current unbalance is not eliminated.
Therefore, even when the
torque pulsations are reduced, the induction machine power
output is rerated, because the
machine current limit is reached by only one of the stator
phase. Compensation of
unbalanced voltages and currents in power systems are addressed
in where a STATCOM
is used to compensate voltage unbalances.
However, the application of the control method to DFIGs is not
discussed. No formal
methodology for the design of the control systems is presented
and only simulation
results are discussed in. In this thesis, a controller design is
specified, which compensates
the stator current unbalance in grid-connected and stand-alone
DFIG operation.
The strategy uses two revolving axes theory (rotating
synchronously at to obtain the d
q components of the negative and positive-sequence currents in
the stator and grid/load.
The unbalance is compensated by the rotor side converter. The
positive-sequence current
is conventionally controlled to regulate the dc link voltage,
whereas negative-sequence
current is regulated to reduce or eliminate the grid voltage
unbalance.
2.2 Type of Wind turbines
Wind turbine converts mechanical energy into generator torque
and the generator
converts this torque into electricity and feeds it into the grid
as other generation processes
does. The only difference from other generation processes is
that the mechanical energy
is from wind. There are currently three main types of wind
turbines available as shown in
Fig.2.1.[20]
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Gear Box Grid IG
(a)Fixed speed wind turbine with an induction
generator
Gear Box Grid IG
(b)Variable-speed wind turbine with a doubly-fed induction
generator
RSC GSC
Gear Box Grid PM
(c)Variable-speed wind turbine with a permanent magnet
synchronous
generator
RSC GSC
Blades
converters
Fig. 2.1 General structures of three different types of wind
turbines
-
Fig.2.1 shows the structures of three different types of wind
turbines in Fig.2.1 (a), (b)
and (c) shows as:
(a) Fixed speed wind turbine with an asynchronous squirrel cage
induction generator (IG)
directly connected to the grid via a transformer.
(b) Variable speed wind turbine with a doubly fed induction
generator (DFIG) and blade
pitch control.
(c) Variable speed wind turbine using a permanent magnet
synchronous generator that is
connected to the grid through a full-scale frequency converter.
This is called direct
drive (DD) wind turbine.
However, indirect grid connected wind turbines still need many
improvements to
compete with other conventional electricity generation
technologies. Firstly, as Fig. 2.1
shows, the indirect grid connected wind turbines will need a
rectifier and two inverters,
one to control the stator current, and another to generate the
output current, but it may
change as the cost of power electronics decreases. Secondly,
there are energy losses
associated with AC/DC/AC conversion process, and harmonic
distortions of the
alternating current may be introduced in the electrical grid by
power electronics devices,
thus reducing power quality.
To improve the performance of wind turbines, different
technologies are being applied to
them. Now two types of indirect grid connected wind turbines
dominate the market. The
DD type of wind turbines is mainly built by Enercon (Germany).
This type of wind
turbines is combined with synchronous permanent magnet generator
and AC/DC/AC
converter with a rating of 100% of the rated wind turbine power.
Since it does not need
the gear box, the weight at the hub height can be lowered a lot,
and the operation and
maintenance of the gear box are not needed. But because the
capacity of the converter has
to match the maximum output power of the generator, its cost is
highest among all types
of wind turbines. Also the generator is bigger than other types
of wind turbines. In the
long term, the operation and maintenance costs of the gear box
can be saved.
-
The other type of indirect grid connected wind turbine is a
variable speed wind turbine
with DFIG, which dominates the market with their total share to
be around 84.5%-86%.
The wind turbine with DFIG is combined with gear box, induction
generator, and
AC/DC/AC converter with a rating of only 20%30% of the rated
wind turbine power..
The cost of DFIG system is lower than the direct drive system
because its power
converter is approximately one-third the size of the direct
drive system. But the control
system of a DFIG is more complex than that of a DD.
2. 3 TYPES OF WIND ENERGY CONVERTION SYSTEMS
Wind electric conversion systems can be broadly classified
as;
Constant speed constant frequency (CSCF);
Variable speed constant frequency (VSCF);
Variable speed variable frequency (VSVF);
2.3.1 Constant speed constant frequency (CSCF)
In the CSCF scheme, the rotor is held constant by continuously
adjusting the
blade pitch and/or generator characteristics. For synchronous
generators, the requirement
of constant speed is very rigid and only minor fluctuations of
about 1% for short duration
could be allowed [5].As the wind fluctuates, a control mechanism
becomes necessary to
vary the pitch of the rotor so that the power derived from the
wind system is held fairly
constant. Such a control system is necessary since wind power
varies with the cube of
wind velocity. During gusty periods, the machine is subjected to
rapid changes in the
input power. The control mechanism must be sensitive enough to
damp out these
transient so that the machine output does not become unstable.
Such a mechanism is
expensive and adds complexity to the system [21].
2.3.2 Variable speed constant frequency (VSCF)
The variable speed operation of wind electric system yields
higher output for both
low and high wind speeds. This results in higher annual energy
per rated installed
-
capacity. Both horizontal and vertical axis wind turbines
exhibit this gain under variable
speed operation [17]. In this scheme, the need for a costly
blade control mechanism is
avoided. Generation schemes involving speed rotors are more
complicated than constant
speed systems. Variable frequency power must be converted to
constant frequency
power, and this can be done by using power electronics [21].
2.3.3 Variable speed variable frequency (VSVF)
General, resistive heating loads are less frequency sensitive.
Synchronous
generators can be affected at variable speed, corresponding to
the changing drive speed.
For this purpose, self-excited induction generator can be
conveniently used. This scheme
is gaining importance for standalone wind power applications [5,
18, 21].
2.4 WIND GENERATORS
According to the turbine position the wind generators are
divided into two axes
that are horizontal axis and vertical axis generators.
2.4.1 Horizontal axis wind generators
Horizontal axis wind generators have the main rotor shaft and
electrical generator
at the top of a tower, and must be pointed into the wind. Small
generators are pointed by
a simple wind vane or tail. Large generators often use a wind
sensor coupled with a
servomotor. Most large wind generators use a gearbox, which
turns the slow rotation of
the blades into a quicker rotation that is more suitable for
generating electricity [4, 5].
2.4.2 Vertical axis wind generators
Vertical axis wind generators have the main rotor shaft running
vertically. The
advantages of this configuration are that the generator and/or
gearbox can be placed at the
bottom, near the ground, so the tower doesn't need to support
the additional weight, and
that the generator doesn't need to be pointed into the wind.
They generally also operate at
-
lower wind speeds. However, they are not as efficient at
extracting energy from the wind
[4, 5].
2.5 CHOICE OF GENERATORS
Basically, a wind turbine can be equipped with any type of 3
phase generator.
Today, the demand for grid-compatible electric current can be
met by connecting
frequency converters, even if generator supplies AC of variable
frequency or DC. Several
general types of generators may be used in WT [4, 5, 21].
1. Permanent magnet generators,
2. Caged rotor induction generators,
3. Synchronous generators,
4. Doubly fed induction generators.
2.5.1 Permanent magnet synchronous generators
Permanent magnet excitation is generally favored in newer
smaller scale turbine
designs, since it allows for higher efficiency and smaller wind
turbine blade diameter.
While recent research has considered larger scale designs, the
economics of large
volumes of permanent magnet material has limited their practical
application. The
primary advantage of permanent magnet synchronous generators
(PMSG) is that they do
not require any external excitation current. A major cost
benefit in using the PMSG is the
fact that a diode bridge rectifier may be used at the generator
terminals since no external
excitation current is needed. Flexibility in design allows for
smaller and lighter designs
and higher output level may be achieved without the need to
increase generator size.
Lower maintenance cost and operating costs, bearings last
longer, there is no significant
losses generated in the rotor and the Generator speed can be
regulated without the need
for gears or gearbox .Very high torque can be achieved at low
speeds and Eliminates the
need for separate excitation or cooling systems[5].
But some disadvantages are there in PMSG that Higher initial
cost due to high
price of magnets used and Permanent magnet costs restricts
production of such generators
-
for large scale grid connected turbine designs. High
temperatures and sever overloading
and short circuit conditions can demagnetize permanent magnets.
Use of diode rectifier in
initial stage of power conversion reduces the controllability of
overall system [5, 17].
2.5.2 Induction generators
The use of induction generators (IG) is advantageous since they
are relatively
inexpensive, robust and they require low maintenance. The nature
of IG is unlike that of
PMSG, Lower capital cost for construction of the generator and
Known as rugged
machines that have a very simple design. Higher availability
especially for large scale
grid connected designs and Excellent damping of torque pulsation
caused by sudden wind
gusts, relatively low contribution to system fault levels
[5].
Disadvantages of this generator is Increased converter cost
since converter must
be rated at the full system power then Results in increased
losses through converter due
to large converter size needed for IG Generator requires
reactive power and therefore
increases cost of initial ACDC conversion stage of converter and
May experience a
large in-rush current when first connected to the grid .it also
increased control complexity
due to increased number of switches in converter [5, 17].
2.5.3 Synchronous generators
The major advantages of synchronous generator is that its
reactive power
characteristics can be controlled, and therefore such machine
can be used to supply
reactive power to systems that require reactive power. The
application of synchronous
generators (SG) in wind power generation has also been
researched. A brief description
of one possible converter-control scheme is given for a small
wind energy conversion
system. The use of a diode rectifier along with a DC/DC boosts
stage and inverter as a
power electronic interface for grid connection. It possesses
Minimum mechanical wear
due to slow machine rotation. Due to direct drive applicable
further reducing cost since
gearbox not needed. it allow for reactive power control as they
are self excited machines
-
that do not require reactive power injection and Readily
accepted by electrically isolated
systems for grid connection. It Allow for independent control of
both real and reactive
power [5].
Disadvantages are typically having higher maintenance costs
again in comparison
to that of an IG and magnet used which is necessary for
synchronization is expensive. But
magnet tends to become demagnetized while working in the
powerful magnetic fields
inside the generator. It requires synchronizing relay in order
to properly synchronize with
the grid [5].
2.5.4 Doubly fed induction generators
As the PMSG has received much attention in wind energy
conversion, the doubly
fed induction generator has received just as much consideration,
if not more. If a wound
rotor induction machine is used, it is possible to control the
generator by accessing the
rotor circuits. A significant advantage in using doubly fed
induction generators (DFIG) is
the ability to output more than its rated power without becoming
overheated. It is able to
transfer maximum power over a wide speed range in both sub- and
super-synchronous
modes. The DFIG along with induction generators are excellent
for high power
applications in the MW range. More importantly, converter power
rating is reduced since
it .is connected to the rotor, while the majority of the power
flows through the stator [5].
-
Fig. 2.2: Typical wind generators.
2.6 MODELING OF DFIG SYSTEM
2.6.1 Blade modeling (wind modeling)
An aerodynamic model of the wind turbines is a common part of
the dynamic models of
the electricity-producing wind turbines. The captured
aerodynamic power is given by:
=1
2
2 , (2.1)
where is the captured power from wind, is the air density, v is
the wind speed, A
is the swept area of the blade, ( ) is the power coefficient, is
the ratio between
blade tip speed and wind speed at hub height, is the pitch
angle. (, ) can be
obtained from wind turbine manufacturers.
2.6.2 Drive train modeling
The mechanical construction of the wind turbines is simply
modeled as a lumped-mass
system with the lumped combined inertia constant of the turbine
rotor and the generator
rotor. The shaft dynamic equation is [15]:
-
2
= ( ) (2.2)
2
= ( ) (2.3)
= ( ) (2.4)
where JT and JG are the inertia constant of the turbine rotor
and the generator rotor,
respectively, Ks and Ds are the shaft stiffness and damping
constant respectively, TG is
the electrical twist angle of the shaft, o is the base value of
angular speed, T and G are
the angular speeds of shaft at the ends of turbine and
generator, respectively, TT and TE
are the mechanical and electrical torque, respectively.
2.6.3Generator modeling
As mentioned earlier, there are three types of generators used
in wind turbines: one is
induction generator, the second one is doubly fed induction
generator, and the other is
permanent magnetic synchronous generator.
1) Induction generator (IG)
The equivalent circuit of the induction generator is shown in
Fig.2.3, and the electric and
magnetic equations of the model are described by equations
(2.5)-(2.10) [20].
-
Fig.2.3 Equivalent circuit of the induction generator
Stator Voltage is given by:
= +dds
dt (2.5)
= + +dqs
dt (2.6)
Rotor Voltage is given by:
= +ddr
dt (2.7)
= + +dqr
dt (2.8)
Flux Linkage is given by:
=
=
=
= (2.9)
+
-
jws +rdq
Vsdq
Rs Rr Ls Lr
Lm V+rdq
J(s-r)+
rdq
-
Electronicmagnetic Toque is:
= (2.10)
where vs, is and s are stator voltage, current and flux
respectively; vr, ir and r are rotor
voltage, current and flux respectively; s is the angular
velocity of the chosen frame of
reference; d and q represent d and q axis, respectively. Lm is
the mutual inductance; Lsl
and Lrl are the stator and rotor leakage inductances,
respectively.
2) Doubly fed induction generator (DFIG)
Doubly fed induction generator is a modified version of IG where
two rotor windings
receive electrical excitation from external sources. As a
result, the rotor equations are
modified as presented in section D below. Rests of the equations
are same as IG.
3) Permanent magnet synchronous generator
The generator for a direct drive wind turbine is different from
the other types. It is a
permanent magnet synchronous generator, and using Parks
transformation, can be
expressed by the following equations [20] and [18].
= +dds
dt
= +dqs
dt (2.11)
= + +
= + (2.12)
where r is the mechanical angular velocity of the rotor at any
instant, d and q represent d
and q axis respectively, m is the flux produced by the permanent
magnets.
Electronicmagnetic Toque is given by:
-
= (3
2 )(
2 )( ) (2.13)
Where p is the number of poles.
2.6.4 Converter modeling and control
With the assumption that the converters are lossless, the
equations of converters are as
follows:
1) DFIG converter
The power at the rotor side (also called slip power) is given
by:
= +
= (2.14)
And the power at the stator side is given by:
= +
= (2.15)
So the total output power is:
= + = + + +
= + = + (2.16)
2) Frequency converter
For a direct drive system, all the power produced by the
generator goes from the stator
and pass through the converter.
= = +
= =
2.7 WIND ENERGY BACKGROUND
-
The amount of power captured from a wind turbine is specific to
each turbine and
is governed by [1].
(2.17)
Where:
Pt = the turbine power(W),
= the air density (kg/m),
A= the swept turbine area (m^3),
CP = the coefficient of performance
vw = is the wind speed(m/s).
The coefficient of performance of a wind turbine is influenced
by the tip-speed to
wind speed ratio or TSR given by
(2.18)
Where w is the turbine rotational speed and r is the turbine
radius. A typical
relationship, as shown in Fig. 2.4, indicates that there is one
specific TSR at which the
turbine is most efficient. In order to achieve maximum power,
the TSR should be kept at
the optimal operating point for all wind speeds. The turbine
power output can be plotted
versus the turbine rotational speed for different wind speeds,
an example of which is
shown in Fig. 2.4. The curves indicate that the maximum power
point increases and
decreases as wind speed rises and falls [4, 22, 23],
3
2
1wpt vACP
,wv
wrTSR
-
0.4
0.3
0.2
0.1
0.0
Cp
12 10 8 6 4 2 0
Tip Speed Ratio
Fig. 2.4: Typical coefficient of power curve
P1 max
P2 max
P (W)
(rad/s)
V2 > V1
Fig. 2.5: Turbine output power characteristic
CHAPTER -3
PRNCIPLE OF DOUBLY FED INDUCTION GENERATOR
-
3.1 INTRODUCTION
Variable speed ac drives have been used in the past to perform
relatively
undemanding roles in application which preclude the use of dc
motors, either because of
the working environment limits. Because of the high cost
efficient, fast switching
frequency static inverter. The lower cost of ac motors has also
been a decisive economic
factor in multi motor systems. However as a result of the
progress in the field of
power electronics, the continuing trend is towards cheaper and
more effective power
converters, and a single motor ac drives complete favorably on a
purely economic basis
with a dc drives. Among the various ac drive systems, those
which contain the cage
induction motor have a particular cost advantage. The cage motor
is simple and rugged
and is one of the cheapest machines available at all power
ratings. Owing to their
excellent control capabilities, the variable speed drives
incorporating ac motors and
employing modern static converters and torque control can well
complete with high
performance four quadrant dc drives [27].
The induction motors were evolved from being a constant speed
motors to a
variable speed. In addition, the most famous method for
controlling induction motor is by
varying the stator voltage or frequency. To use this method, the
ratio of the motor voltage
and frequency should be approximately constant. With the
invention of Field Orientated
Control, the complex induction motor can be modeled as a DC
motor by performing
simple transformations. In a similar manner to a dc machine, in
induction motor the
armature winding is also on the rotor, while the field is
generated by currents in the stator
winding. However the rotor current is not directly derived from
an external source but
results from the emf induced in the winding as a result of the
relative motion of the rotor
conductors with respect to the stator field. In other words, the
stator current is the source
of both the magnetic field and armature current. In the most
commonly used, squirrel
cage motor, only the stator current can directly be controlled,
since the rotor winding is
not accessible. Optimal torque production condition are not
inherent due to the absence of
a fixed physical disposition between the stator and rotor
fields, and the torque equation is
non linear. In effect, independent and efficient control of the
field and torque is not as
-
simple and straightforward as in the dc motor [27, 28].
The concept of the steady state torque control of an induction
motor is
extended to transient states of operation in the high
performance, vector control ac drive system
based on the field operation principle defines condition for
decoupling the field control
from the torque control. A field oriented induction motor
emulates a separately exited dc motor
in two aspects [27].
I - Both the magnetic field and torque developed in the motor
can be controlled
independently.
II - Optimal condition for the torque production, resulting in
the maximum torque per unit
ampere, occurs in the motor both in steady state and in
transient condition of operation.
3.2 DC MOTOR ANALOGY
Fig-3.1 DC motor analogy
Where torque (T) Ia.If
And where Ia represents the torque component and If the
field.
The orthogonal or perpendicular relationship between flux and
mmf axes is
independent of the speed of rotation and so the electromagnetic
torque of the dc motor is
proportional to the product of the field flux and armature
current. Assuming negligible
magnetic saturation, field flux is proportional to field current
and is unaffected by armature
current because of the orthogonal orientation of the stator and
rotor field. Thus in a
separately excited dc motor with constant value of field flux,
torque is directly proportional
to armature current [27, 28].
Ia
If
Ia If
-
Fig. 3.2: Separately excited
The principle behind the field oriented control or the vector
control is that the
machine flux and torque are controlled independently, in a
similar fashion to a separately
excited DC machine. Instantaneous stator currents are
transformed to a reference frame
rotating at synchronous speed aligned with the rotor stator or
air gap flux vectors, to
produce a d-axis component current and a q-axis component
current. (SRRF).In this work,
SRRF is aligned with rotor mmf space vector, the stator current
space vector is split into
two decoupled components, one controls the flux and the other
controls the torque
respectively [27, 28].
3.3 INDUCTION MOTOR ANALOGY
An induction motor is said to be in vector control mode, if the
decoupled
components of the stator current space vector and he reference
decoupled components
defined by the vector controller in the SRRF match each other
respectively. Alternatively,
instead of matching the two phase currents (reference and
actual) in the SRRF, the close
match can also be made in the three phase currents (reference
and actual) in the stationary
reference frame. Hence in spite of induction machines non linear
and highly interacting
multivariable control structure [28].its control has becomes
easy with the help of FOC.
Therefore FOC technique operates the induction motor like a
separately excitedly DC
motor.
The transformation from the stationary reference frame to the
rotating reference
frame is done and controlled by with reference to specific flux
vector (stator flux linkage,
-
rotor flux linkage) or magnetizing flux linkage). In general,
there exits three possibilities
for such selection and hence, three vector controls. They are
stator flux oriented control,
rotor flux oriented control and magnetizing flux oriented
control. As the torque producing
component in this type of control is controlled only after
transformation is done and is not
the main input reference, such control is known as indirect
torque control. The most
challenging and ultimately, the limiting feature of field
orientation is the method whereby
the flux angle is measured or estimated. Depending on the method
of measurement, the
vector control is sub divided into two sub categories: direct
vector and indirect vector
control. In direct vector control, the flux measurement is done
by using flux sensing coils
or the hall devices [27, 28].
FOC uses a d-q coordinates having the d-axis aligned with rotor
flux vector that
rotates at the stator frequency. The particular solution allows
the flux and torque to be
separately controlled by the stator current d-q components. The
rotor flux is a flux of the d-
axis component stator current ids .The developed torque is
controlled by the q axis
component of the stator current iqs. The decoupling between
torque and flux is achieved
only if the rotor flux position is accurately known. This can be
done using direct flux
sensors or by using a flux estimator [28].
3.3.1 Vector control techniques of induction motor
The synchronously rotating reference frame (SRRF) can be aligned
with the stator
flux or rotor flux or magnetizing flux (field flux) space
vectors respectively. Accordingly,
vector control is also known as stator flux oriented control or
rotor flux oriented control or
magnetizing flux oriented control. Generally in induction
motors, the rotor flux oriented
control is preferred. This is due to the fact that by aligning
the SRRF with the rotor flux,
the vector control structure becomes simpler and dynamic
response of the drive is observed
to be better than any other alignment of the SRRF.
The vector control can be classified into (i) Direct vector
control and (ii) indirect vector
control [28].
-
Fig. 3.3: Vector controlled induction motor
In vector control the dynamic performance of the induction motor
improves to a
great extent. The squirrel cage induction motor behaves similar
to a separately excited dc
motor with control of field and torque being independent of each
other. Therefore the drive
exhibits quick starting response, fat reversal response and
quick change over from one
operating point to another. With proper choice of speed
controller, the drive can be further
improved in terms of performance indices such as starting time,
reversal time, and dip in
speed on load application, overshoot in speed on load removal,
steady state speed error on
load etc [27, 28].
3.3.2 DYNAMIC DQ MODEL
R.H. Park in 1920's proposed a model for synchronous machine
with respect to
stationary reference frame. H.C. Stanley in 1930's proposed a
model for induction
machine with respect to stationary reference frame. Later G.
Bryons proposed a
transformation of both stator and rotor variables to a
synchronously rotating reference
frame that moves with the rotating magnetic field. Lastly Krause
and Thomas proposed a
model for induction machine with respect to stationary reference
frame.
Transformation: - the stator winding axes as-bs-cs with voltage
with respect
to stationary reference frame, the voltages are referred as
[29].
csbsas vvv &,
qsds vv &
-
Fig. 3.4: Stationary frame a-b-c to dq.
Fig. 3.5: Stationary frame to synchronous rotating frame
3.3.2.1 Synchronously rotating reference frame-Dynamic model
(Kron's equation)
The dynamic model of DFIG is derived from the two-phase
synchronous
reference frame in which the q-axis is 90 ahead of the d-axis
with respect to the direction
of rotation. The electrical model of DFIG in the synchronous
reference frame, here the
quantities on the rotor side have been referred to the stator
side. The model is composed
of two groups, i.e. the first one is the voltage equations and
the other is the flux ones. The
general model for wound rotor induction machine is similar to
any fixed-speed induction
generator [12].
The DFIG system consists of stator, rotor and turbine. So the
model design according
these, the followings parameters are used to modeling the
DFIG:
-
3.3.2.2 Voltage equations
Stator Voltage Equations:
(1)
(2)
Fig. 3.6: d-axes transform
Rotor Voltage Equations:
(3)
(4)
Fig. 3.7: q-axes transform
3.3.2.3 Power Equations:
(5)
(6)
3.3.2.4 Torque Equation:
qssdsqsqs iRPV
dssqsqsds iRPV
qrrdrqrqr iRrPV )(
drrqrdrdr iRwrwPV )(
)(2/3 qsqsdsdss iViVP
)(2/3 qsdsdsqss iViVQ
-
(7)
3.3.2.5 Flux Linkage Equations:
Stator Flux Equations:
(8)
(9)
Rotor Flux Equations:
(10)
Then, the d-axis of reference frame to be along the stator flux
linkage (stator flux oriented
control) will be
(11)
And hence from stator flux equation:
(12)
Substituting for in torque equation will result in:
(13)
For to remain unchanged at zero, must be zero. Substituting for
using
stator voltage equation we get
(14)
Neglecting stator resistance will lead to ; substituting this,
the power equation
simplified as
)(22
3dsqsqsdse ii
p
qrmqsmlsqs iLiLL )(
drmdsmlsds iLiLL )(
dsmdrmlrdr
qsmqrmlrqr
iLiLL
iLiLL
)(
)(
0eqs
e
qr
mls
me
qs iLL
Li
e
qsi
e
qr
e
ds
mls
m
e iLL
Lp
22
3
e
dse
dspe
dsp
e
dss
e
ds irV
0edsV
-
(15)
Therefore, the above equations show that active and reactive
powers of the stator can be
controlled independently.
In terms of rotor current component
(16)
Where =
CHAPTER-4 CONTROLLER FOR DOUBLY FED INDUCTION GENERATOR
4.1 DFIG WITH BACK TO BACK CONVERTER
A double fed induction generator is a standard, wound rotor
induction machine
)(2
3
)(2
3
e
ds
e
qs
e
s
e
qs
e
qs
e
s
iVQ
iV
)(
)(
s
s
e
drme
m
e
s
e
qr
s
me
m
e
s
L
iLVQ
iL
LV
sL mls LL
-
with its stator windings is directly connected to grid and its
rotor windings is
connected to the grid through an AC/DC/AC converter. AC/DC
converter connected to
rotor winding is called rotor side converter and another DC/AC
is grid side converter.
Doubly fed induction generator (DFIG) ability to control rotor
currents allows for
reactive power control and variable speed operation, so it can
operate at maximum
efficiency over a wide range of wind speeds [43].
Fig. 4.1: Wind Energy System.
In modern DFIG designs, the frequency converter is built by
self-commutated
PWM converters, a machine-side converter, with an intermediate
DC voltage link.
Variable speed operation is obtained by injecting a variable
voltage into the rotor at slip
frequency. By controlling the converters, the DFIG
characteristics can be adjusted so as
to achieve maximum of effective power conversion or capturing
capability for a wind
turbine and to control its power generation with less
fluctuation. The DFIG is a WRIG
with the stator windings connected directly to the three phases,
constant-frequency grid
and the rotor windings connected to a back-to-back voltage
source converter. Thus, the
term doubly-fed comes from the fact that the stator voltage is
applied from the grid and
the rotor voltage is impressed by the power converter [5,
41].
Vector control of a doubly fed induction generator drive for
variable
speed wind power generation is described. The control scheme
uses stator flux-oriented
GEAR BOX
3~
DOUBLY FED INDUCTION GENERATOR
AC DC
DC AC
TRANSFORMER
GRIDDDD
-
control for the rotor side converter bridge control and grid
voltage vector control for the
grid side converter bridge. The purpose of the grid side
converter is to maintain the dc
link voltage constant. It has control over the active and
reactive power transfer between
the rotor and the grid, while the rotor side converter is
responsible for control of the flux,
and thus, the stator active and reactive powers. A complete
simulation model is
developed for the control of the active and reactive powers of
the doubly fed generator
under variable speed operation [6, 9, 43].
Fig. 4.2: DFIG with converter control signal.
The wind turbine and the doubly-fed induction generator (DFIG)
is shown in the
Fig. 4.2. The AC/DC/AC converter is divided into two components:
the rotor-side
converter (Crotor) and the grid-side converter (Cgrid).A
capacitor connected on the DC side
acts as the DC voltage source. A coupling inductor L is used to
connect Cgrid to the grid.
The three-phase rotor winding is connected to Crotor by slip
rings and brushes and the
three-phase stator winding is directly connected to the grid.
The power captured by the
wind turbine is converted into electrical power by the induction
generator and it is
transmitted to the grid by the stator and the rotor windings.
The control system generates
the pitch angle command and the voltage command signals Vr and
Vgc for Crotor and Cgrid
respectively in order to control the power of the wind turbine,
the DC bus voltage and the
reactive power or the voltage at the grid terminals.[6, 9,
43].
4.2 POWER-SPEED CHARACTERISTIC
control
Sign Vc,Vg
Pitch angle
-
From previous discussion it is clear that the controller, i.e
Crotor and Cgrid have the
capability of generating or absorbing reactive power and control
the reactive power or the
voltage at the grid terminals. The power is controlled is follow
the power-speed
characteristic (Fig. 4.3).
Fig. 4.3: Power-speed characteristic.
The above ABCD curve shows the power characteristics. The actual
speed of the
turbine r is measured and the corresponding mechanical power of
the tracking
characteristic is used as the reference power for the power
control loop. The tracking
characteristic is obtained over four points. From zero speed to
speed of point A the
reference power is zero. Between point A and point B the
characteristic is a straight line,
the speed of point B must be greater than the speed of point A.
Between point B and
point C the tracking characteristic is the locus of the maximum
power of the turbine. The
tracking characteristic is a straight line from point C and
point D. The power at point D is
1 pu and the speed of the point D must be greater than the speed
of point C. Beyond point
D the reference power is a constant equal to 1 pu [43].
-
4.3 WIND-TURBINE MODEL.
Wind turbines convert aerodynamic power into electrical energy.
In a wind
turbine two conversion processes take place. The aerodynamic
power is first converted
into mechanical power. Next, that mechanical power is converted
into electrical power.
Wind energy conversion systems are systems that generate
electrical power from
mechanical power derived from the wind. The major components of
a typical wind
energy conversion system include a wind turbine, a generator and
control systems as
shown in Fig. 4.2.
Cp is the power coefficient which, in turn, is a function of tip
speed ratio and
blade angle .i.e. Cp = Cp (, ) and = (*r)/ v; One common way to
control the
active power of a wind turbine is by regulating the value of the
rotor turbine. In the
model, the value of the turbine rotor is approximated using a
non-linier function [7,
15].
(4.1)
Where the tip is speed ratio and is the pitch angle. The value
is given
according to the following relation.
(4.2)
The maximum value of can be found using a graphical method, this
tip speed value is
assigned as the optimum tip speed. Based on this value, the
optimum turbine speed curve
at any given wind speed can be obtained. This curve is then used
as a reference in the
active power control. The variation of Cp as a function of
assuming constant pitch
angle = const [43].
The out put power from turbine: (4.3)
Torque is
pc
pc
reCi
P
5.12
)54.116
(22.0),(
i
1
035.
08.
112
i
pc
3
2
1wpt vACP
rtm PT /
-
Fig. 4.4: simulation model of turbine.
4.4 PITCH ANGLE CONTROL
The pitch angle control is a common control method to regulate
the aerodynamic
power from the turbine. Pitch angle controller controls the wind
flow around the wind
turbine blade, thereby controlling the toque exerted on the
turbine shaft. If the wind speed
is less than the rated wind speed of the wind turbine, the pitch
angle is kept constant at its
optimum value. It should be noted that the pitch angle can
change at a finite rate, which
may be quite low due to the size of the rotor blades. Small
change in pitch angle can have
a dramatic effect on the power output. The maximum rate of
change of the pitch angle is
in the order of 3 to 10 degrees/second. In this controller a
slight over-speeding of the
rotor above its nominal value can be allowed without causing
problems for the wind
turbine structure. The relationship between the pitch angle and
the wind speed is shown
Tm (pu)1
wind_speed 3^
u(1) 3^
pu->pu
-K-
pu->pu
-K-
lambda_nom
-K-
cp(lambda,beta)
lambda
betacp
Scope1
Scope
Product
Product
-K-
Avoid divisionby zero
Avoid divisionby zero
1/wind_base
-K-
1/cp_nom
-K-
Wind speed(m/s)
3
Pitch angle (deg)2
Generator speed (pu)1
Pwind_puPm_pu
lambda
cp_pu
cp_pulambda_pu
wind_speed_pu
-
in Figure 4.6.[11,43].
Fig. 4.5: Pitch Angle control
Fig. 4.6: Relationship between Pitch Angle and Wind Speed
The pitch angle controller employs a PI (proportional integral)
controller as
shown below.
In Fig.4..5. When the wind turbine output power Pmeasured is
lower than the rated power
Pref of the wind turbine, the error signal is negative and pitch
angle is kept at its optimum
value. When the wind turbine output power Pmeasured exceeds the
rated power Pref, the
error signal is positive and the pitch angle changes to a new
value, at a finite rate, thereby
reducing the effective area of the blade resulting in the
reduced power output. The PI
controller inputs are in per-unit.
4.5 ROTOR SIDE CONVERTER
The rotor-side converter is used to control the wind turbine
output power and the voltage
or reactive power measured at the grid terminals.
-
Fig. 4.7: Rotor side and Grid _side converter control
circuit.
The actual electrical output power, measured at the grid
terminals of the wind turbine, is
added to the total power losses (mechanical and electrical) and
is compared with the
reference power obtained from the tracking characteristic. A
Proportional-Integral (PI)
regulator is used to reduce the power error to zero. The output
of this regulator is the
reference rotor current Iqr_ref that must be injected in the
rotor by converter Crotor. This is
the current component that produces the electromagnetic torque
Tem. The actual Iqr is
compared to Iqr_ref and the error is reduced to zero by a
current regulator (PI). The
output of this current controller is the voltage Vqr generated
by Crotor. The current
regulator output is Vqr [8, 13].Reactive and Active Power at
grid terminals is controlled
by the reactive and active current flowing in the converter
Crotor
The output of the voltage regulator or the var regulator is the
reference d-axis
current Idr_ref that must be injected in the rotor by converter
Crotor. The same current
regulator as for the power control is used to regulate the
actual Idr. The output of this
regulator is the d-axis voltage Vdr generated by Crotor. The
current regulator output is
Vdr. Vdr and Vqr are respectively the d-axis and q-axis of the
voltage Vr [8, 30].
-
Fig. 4.8: Rotor side converter control.
4.5.1 MATLAB SIMULATION MODEL
VAR
MEASUREMENT
TRACKING CHA
POWER
MEASUREMENT
Var
CURRENT
POWER
REGULATOR
CURRENT
REGULATOR
V
I
V
I
Wr
P
Pref
Q
Qref
I Id
Iq
Id ref
Iq ref
+
-
+
-
+
-
-
+
-
Fig. 4.9: Simulation Model of Rotor-Side Controller.
4.6 GRID SIDE CONVERTER
The converter Cgrid is used to regulate the voltage of the DC
bus capacitor. In this
thesis , this model Cgrid converter to generate or absorb
reactive power. In this control
system ,measuring the d and q components of AC currents to be
controlled as well as the
DC voltage Vdc. The output of the DC voltage regulator is the
reference current Idgc_ref
for the current regulator. The current regulator controls the
magnitude and phase of the
voltage generated by converter Cgrid (Vgc) from the Idgc_ref
produced by the DC voltage
regulator and specified Iq_ref reference. The current regulator
give the Cgrid output
voltage [6, 43].
The magnitude of the reference grid converter current Igc_ref is
equal to
.The maximum value of this current is limited to a value
defined by the converter maximum power at nominal voltage. When
Idgc_ref and Iq_ref
rotor side control voltage
Vabc_r
1
turbine power charecteristics
wr
idqs
Vdqs
Freq
Iqr *
reactive power
Q_ref
QIdr *
dq 2 abc
Vdq*
Vdc
Angle
Uctrl_r
abc_dq
Theta
Iabc _s
Idq _s
abc to dqr
In1
angle _rotor
Iabc _r
Idq _r
r_angle
abc to dq
Theta
Vabc
Vdq
Vq calculation
f(u)
Vd calculation
f(u)
1/z1/2
F
50
PI
Discrete
3-phase PLL
Vabc (pu )
Freq
wt
Sin _Cos
Demux
Demux
Demux
Demux
Vdc
8
angle _rotor
7
Q
6
Iabc_r1
5
Iabc_s
4
wr
3
Q_ref
2
Vabc
1
Idr*
w-wr
Idr
Iqr
vd'
vq'Iqr*
22 __ refIqrefIdgc
-
are such that the magnitude is higher than this maximum value
the Iq_ref component is
reduced in order to bring back the magnitude to its maximum
value [9].
Fig. 4.10: Grid side converter control.
4.6.1 MATLAB SIMULATION MODEL
DC VOLTAGE
REGULATOR
CURRENT
MEASUREMENTCURRENT
REGULATORIg Idg
Iqg
Idg ref
Iqg ref
Vq
+
+
-
-
+
-
Vd
Vdc ref
-
Fig. 4.11: Simulation Model of Grid-Side controller and
Power.
dq2abc converter voltage
P3
Q
2
Vabc_g
1
curent to volatge tf
Idq
Idq_ref
vdq ref
active and reactive power
Vabc
Iabc
T_F
Q
P
abc to dq 1
Iabc
Theta1
Idq
abc to dq
Theta
Vabc
Vdq
Vdcref
Vdc_nom
1/z
Theta
Vdq*_g
Vdc
control _g
-K-
Discrete
3-phase PLL
Vabc (pu)
Freq
wt
Sin_Cos
Demux
Demux
DC to Idq ref
Vdc_ref
Vdc
Iq_ref
Idq_ref
Iabc
5
Iq_ref
4Vdc
3
Iabc_g
2
Vabc
1vd'
vq'
Icr
10
Ibr
9
Iar
8
Ics
7
Ibs6
Ias
5
Q1
4
P1
3
Te
2
Nr
1
dq to abc 1
Iq
Id
We-Wr
Ia
Ib
Ic
dq to abc
Iq
Id
We
Ia
Ib
Ic
abc 2 dq
Va
Vb
Vc
Vq
Vd
We
Subsystem
Vds
Ids
Vqs
Iqs
P1
Q1
IM model
Vqs
Vds
We
Vqr
Vdr
TL
Iqs
Ids
Iqr
Idr
Wr
Te
Gain
-K-
Constant 1
0
Constant
0
Add
Vcs
4
Vbs
3
Vas
2
TL
1
-
Fig. 4.12: Induction machine model
CHAPTER-5 MAXIMUM POWER POINT TRACKING AND POWER SMOOTHING
5.1 INTRODUCTION
In this thesis different way to track the maximum power were
implemented. All
these tracking characteristic process are previously
implemented, but here these processes
are compared and new one is implemented in different way. The
variable speed control is
Te
6
Wr
5
Idr
4
Iqr
3
Ids
2
Iqs
1
Subsystem 2
Iqs
Ids
Iqr
Idr
TL
Te
Wr
Subsystem 1
Fqs
Fds
Fqr
Fdr
Iqs
Ids
Iqr
Idr
Subsystem
Vqs
Vds
Vqr
Vdr
Iqs
Ids
Iqr
Idr
We
Wr
Fqs
Fds
Fqr
Fdr
TL
6
Vdr
5Vqr
4
We
3
Vds
2Vqs
1
-
done based on the optimal power curve that shows the relation
between the maximum
output of the system (output) and the generator speed (input),
namely maximum power
point tracking (MPPT). The wind speed control or the generator
speed control is adopted
for MPPT.
At a given wind velocity, the mechanical power available from a
wind
turbine is a function of its shaft speed. To maximize the power
captured from the wind,
the shaft speed has to be controlled. For a given shaft speed
turbine power increases with
increase in wind velocity v. Also peak power points of turbine
power occurs at
different turbine speed for different wind velocity and shaft
speed corresponding to
maximum power increases with increase in wind speed. To trap
maximum power from
the wind some control algorithm should be incorporate such that
rotational speed of
the wind turbine adapts the to the wind speed v automatically
leading to maximum
power point operation. This is known as maximum power point
operation of wind
turbine, and the process of keeping track of peak Power points
with change in wind speed
is Maximum Power Point Tracking MPPT [17, 22].
The conventional method is to generate a control law to produce
the target
generator torque Te, which provides wind turbine with sufficient
acceleration or
deceleration torque to attain particular angular velocity
leading to maximum power point
operation. Irrespective of the generator used for a variable
speed wind energy
conversion system the output energy depends on the method of
tracking the peak
power points on the turbine characteristics due to fluctuating
wind. The generator is
operated in speed control mode with the speed reference being
dynamically modified in
accordance with the magnitude and direction of change of active
power. If we operate
at a peak power point a small increase or decrease in turbine
speed would result in
no change in output power because necessary condition for the
speed to be a
maximum power point is dP/dw =0 [14].
5.2 First Method using Power Point Tracking Characteristics
-
The ABCD curve shows the power characteristics (Fig. 5.1). The
actual speed of
the turbine r is measured and the corresponding mechanical power
of the tracking
characteristic is used as the reference power for the power
control loop. The tracking
characteristic is obtained over four points. From zero speed to
speed of point A the
reference power is zero. Between point A and point B the
characteristic is a straight line,
the speed of point B must be greater than the speed of point A.
Between point B and
point C the tracking characteristic is the locus of the maximum
power of the turbine. The
tracking characteristic is a straight line from point C and
point D. The power at point D is
1 pu and the speed of the point D must be greater than the speed
of point C. Beyond point
D the reference power is a constant equal to 1 pu [43].
Fig. 5.1: Power Point Tracking Characteristics.
-
5.2.1 MATLAB MODEL
Fig. 5.2: Simulation of Power Point Tracking
Characteristics.
5.3 Second Method using MPPT curve implemented as look-up
table
From the above discussion it can be conclude that for the
maximum power
characteristic divided in different region, then using slop
equation manipulates the value
of power which is used as reference power for the simulation.
Here same characteristics
is used as look-up table ,where the power only measured only
some few wind velocity
like at A,B,C & D. at other poin