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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
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.
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.
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
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.
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.
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
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).
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].
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.
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.
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]
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
Recommended