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Submitted to the Department of Electrical Engineering in Partial
Fulfillment of the Requirement for the Degree
Of
0DVWHU RI 6FLHQFH
In
(OHFWULFDO (QJLQHHULQJ
By
Eng..Mahmoud Abd-El Wahab Mossa Mohamed
B.Sc. in Electrical Engineering (Power and machines)
- Faculty of Engineering
Minia University (2008)
EL-Minia
2013
$&.12:/('*0(176
In the name of Allah, the most Gracious and most Merciful
$OO GHHSHVW WKDQNV DUH GXH WR ALLAH WKH PHUFLIXO DQG WKH
FRPSDVVLRQDWH IRU WKH XQFRXQWDEOH JLIWV JLYHQ WR PH
I would like to express my great thanks to
Prof. Dr. Ahmed Abd-Al twab hassan
, Professor of Electrical Machines, El
Minia University for his discussions and encouragement. I would like to
express my deepest thanks to him for his kind supervision, generous advice,
clarifying suggestions and support during each step of this work.
I also would like to express my great thanks to
Prof. Dr. Yehia Sayed Mohamed, Professor of Electrical Machines, EL
Minia University for his discussions, advices and encouragement. I would
like to express my deepest thanks to him for his kind supervision, generous
advice.
I also would like to express my great thanks to
Prof. Dr. Mohamed Mahmoud Hamada, Professor of electrical power
systems, EL Minia University for his discussions and encouragement. I
would like to express my deepest thanks to him for his kind supervision,
generous advice.
I would like to thank all members and friends in the Electrical
Engineering Department, EL Minia University, for their valuable
cooperation that was highly needed during the conduction of this study.
I must not forget to express my deepest thanks to my family especially
my lovely mother whose prayers, cooperation at all stages of this work and
against all odds, have been simply overwhelming.
Mahmoud,
2013
Abstract
ii
$%675$&7
Wind electrical power systems are recently getting lot of attention, because
they are cost competitive, environmental clean and safe renewable power
source, as compared with fossil fuel and nuclear power generation. A special
type of induction generator, called a doubly fed induction generator (DFIG),
is used extensively for high-power wind applications. They are used more
and more in wind turbine applications due to ease controllability, high
energy efficiency and improved power quality.
This thesis aims to develop a method of a field orientation scheme for
control both the active and reactive powers of a DFIG driven by a wind
turbine. The proposed control system consists of a wind turbine that drives a
DFIG connected to the utility grid through AC-DC-AC link. The main
control objective is to regulate the dc link voltage for operation at maximum
available wind power. This is achieved by controlling the
and
axes
components of voltages and currents for both rotor side and line side
converters using PI controllers. The complete dynamic model of the
proposed system is described in detail. Computer simulations have been
carried out in order to validate the effectiveness of the proposed system
during the variation of wind speed. The results prove that , better overall
performances are achieved, quick recover from wind speed disturbances in
addition to good tracking ability.
Generally, any abnormalities associated with grid asymmetrical faults are
going to affect the system performance considerably. During grid faults,
unbalanced currents cause negative effects like overheating problems and
mechanical stress due to high torque pulsations that can damage the rotor
Abstract
iii
shaft, gearbox or blade assembly. Therefore, the dynamic model of the
DFIG, driven by a wind turbine during grid faults has been analyzed and
developed using the method of symmetrical components. The dynamic
performance of the DFIG during unbalanced grid conditions is analyzed and
described in detail using digital simulations.
A novel fault ride-through (FRT) capability is proposed (i.e. the ability of
the power system to remain connected to the grid during faults) with suitable
control strategy in this thesis. In this scheme, the input mechanical energy of
the wind turbine during grid faults is stored and utilized at the moment of
fault clearance, instead of being dissipated in the resistors of the crowbar
circuit as in the existing FRT schemes. Consequently, torque balance
between the electrical and mechanical quantities is achieved and hence the
rotor speed deviation and electromagnetic torque fluctuations are reduced.
This results in a reduction of reactive power requirement and rapid
reestablishment of terminal voltage on fault clearance.
Extensive simulation study has been carried out employing
MATLAB/SIMULINK software to validate the effectiveness of the
proposed system during grid faults. The results demonstrate that the
potential capabilities of the proposed scheme in enhancing the performance
of DFIG based wind farms to fault ride-through are excellent.
Table of contents
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List of tables
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Parameters and data specifications of the DFIG system 39
Sequence and mode of operation of the FRT scheme 113
List of figures
i
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Figure 2-1: Induction machine (SCIG) based wind turbine...12
Figure 2-2: Doubly Fed Wound Rotor Induction Generator wind
based system...13
Figure 2-3:
General structure of a field oriented control in
a synchronous reference frame for an induction machine...16
Figure 2-4: Structure of a direct field oriented control of a wind
driven DFIG...18
Figure 2-5: Structure of indirect field oriented control of a wind
driven DFIG...19
Figure 2-6: Crowbar circuits. a] Antiparallel thyristor crowbar
b] Diode bridge crowbar... 22
Figure 2-7: Series antiparallel thyristors for LVRT...25
),*85(6 2) +$37(5
Figure 3-1: Doubly-fed induction Generator driven by a wind
turbine system... 29
Figure 3-2: Wind turbine control system...
...
30
Figure 3-3: Power flow through dc-link element
...
32
Figure 3-4: Proposed control scheme of the DFIG driven by a wind
turbine based on field orientation... 38
Figure 3-5: Performance of the proposed DFIG driven by a wind
turbine system with wind speed step change... 41
List of figures
x
Figure 3-5-a: Wind speed variation...40
Figure 3-5-b: Rotor speed variation...40
Figure 3-5-c: Generated active power...40
Figure 3-5-d: Generated reactive power...41
Figure 3-5-e: DC link voltage...41
Figure 3-6: Dynamic response of the proposed system with
sinusoidal variation of wind speed... 44
Figure 3-6-a: Wind speed variation... 43
Figure 3-6-b: Rotor speed variation... 43
Figure 3-6-c: Generated active power... 43
Figure 3-6-d: Generated reactive power... 44
Figure 3-6-e: DC link voltage... 44
Figure 3-7: Performance of the proposed DFIG driven by a wind
turbine system with linear bi-directional variation
of wind speed...46
Figure 3-7-a: Wind speed variation...45
Figure 3-7-b: Rotor speed variation...45
Figure 3-7-c: Generated active power... 45
Figure 3-7-d: Generated reactive power...46
Figure 3-7-e: DC link voltage...46
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Figure 4-1: Equivalent circuit of a DFIG in the synchronous
List of figures
x
reference frame rotating at a speed of
s
...49
Figure 4-2: Relationships between the (-) reference frame and the
52
(
and (
reference frames...
Figure 4-3: DFIG driven by a wind turbine based on field
orientation control during grid fault conditions...56
Figure 4-4: performance of the proposed system under a single
phase to ground fault during a constant wind speed...61
Figure 4-4-a: Rotor speed variation...57
Figure 4-4-b: Generated active power...57
Figure 4-4-c: Generated reactive power...58
Figure 4-4-d: DC link voltage...58
Figure 4-4-e: Mechanical torque...58
Figure 4-4-f: Electromagnetic torque...59
Figure 4-4-g: Voltage of phase A... 59
Figure 4-4-h: Voltage of phase B... 59
Figure 4-4-i: Voltage of phase C... 60
Figure 4-4-j: Current of phase A... 60
Figure 4-4-k: Current of phase B... 60
Figure 4-4-l: Current of phase C... 61
Figure 4-4-m: Phase A rotor current... 61
Figure 4-5: performance of the proposed system under a double
phase to ground fault during a constant wind speed...67
Figure 4-5-a: Rotor speed variation...63
Figure 4-5-b: Generated active power...63
List of figures
xi
Figure 4-5-c: Generated reactive power...64
Figure 4-5-d: DC link voltage...64
Figure 4-5-e: Mechanical torque...64
Figure 4-5-f: Electromagnetic torque...65
Figure 4-5-g: Voltage of phase A...65
Figure 4-5-h: Voltage of phase B...65
Figure 4-5-i: Voltage of phase C...66
Figure 4-5-j: Current of phase A...66
Figure 4-5-k: Current of phase B...66
Figure 4-5-l: Current of phase C...67
Figure 4-5-m: Phase A rotor current. ...67
Figure 4-6: performance of the proposed system under a three
phase to ground fault during a constant wind speed...72
Figure 4-6-a: Rotor speed variation...69
Figure 4-6-b: Generated active power...69
Figure 4-6-c: Generated reactive power...70
Figure 4-6-d: DC link voltage...70
Figure 4-6-e: Mechanical torque...70
Figure 4-6-f: Electromagnetic torque...71
Figure 4-6-g: Phase voltages...71
Figure 4-6-h: Current of phase A...71
Figure 4-6-i: Current of phase B...72
Figure 4-6-j: Current of phase C...72
Figure 4-6-k: Phase A rotor current...72
List of figures
xii
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Figure 5-1: Proposed fault ride-through (FRT) scheme and field
oriented control for DFIG system...77
Figure 5-2: Performance of the proposed DFIG system with
crowbar resistance and with FRT scheme during a
single phase to ground fault...87
Figure 5-2-a: Rotor speed variation...83
Figure 5-2-b: Generated active power...83
Figure 5-2-c: Generated reactive power...84
Figure 5-2-d: DC link voltage...84
Figure 5-2-e: Mechanical input torque...84
Figure 5-2-f: Electromagnetic torque...85
Figure 5-2-g: Phase voltages...86
Figure 5-2-h: Stator currents...87
Figure 5-2-i: Phase A rotor current...87
Figure 5-3: Performance of the proposed DFIG system with
crowbar resistance and with FRT scheme during a
double phase to ground fault... 93
Figure 5-3-a: Rotor speed variation... 89
Figure 5-3-b: Generated active power... 89
Figure 5-3-c: Generated reactive power...90
Figure 5-3-d: DC link voltage...90
Figure 5-3-e: Mechanical input torque...90
Figure 5-3-f: Electromagnetic torque...91
List of figures
xi
Figure 5-3-g: Phase voltages...91
Figure 5-3-h: Stator currents...92
Figure 5-3-i: Phase A rotor current...93
Figure 5-4: Performance of the proposed DFIG system with
crowbar resistance and with FRT scheme during a
three phase to ground fault...98
Figure 5-4-a: Rotor speed variation...95
Figure 5-4-b: Generated active power...95
Figure 5-4-c: Generated reactive power...96
Figure 5-4-d: DC link voltage...96
Figure 5-4-e: Mechanical input torque...96
Figure 5-4-f: Electromagnetic torque...97
Figure 5-4-g: Stator currents...98
Figure 5-4-h: Phase A rotor current...98
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Figure App.A: Simulink program for normal operation of the
proposed DFIG system during wind speed
variation...110
Figure App. B: Simulink program of the proposed DFIG system
during unbalanced network conditions...111
Figure App. C.1: Simulink program of the proposed DFIG system
during grid faults with the application of FRT
scheme...112
List of figures
x v
Figure App. C.2: Mode and Sequence of operation of the FRT
scheme...113
Figure App. C.3: Simulink program of the proposed DFIG system
during grid faults with the application of Crowbar
resistance...114
List of symbols
xv
LIST OF SYMBOLS
SYMBOLS
, V
qs
e
d
e
-axis and q
e
-axis stator voltages, (V).
,
d
e
-axis and q
e
-axis stator currents, (A)
.
,
d
e
-axis and q
e
-axis rotor voltages, (V).
,
d
e
-axis and q
e
-axis rotor currents, (A).
,
d
e
-axis and q
e
-axis magnetizing currents, (A).
V , V
d
s
-axis and q
s
-axis stator voltages, (V).
,
d
s
-axis and q
s
-axis stator currents
, (A).
,
d
s
-axis and q
s
-axis rotor currents, (A).
R
s
Stator winding resistance, (
).
R
r
Rotor winding resistance, (
).
L
m
Magnetizing inductance, (H).
L
s
Stator self inductance, (H).
L
r
Rotor self inductance, (H).
L
ls
Stator leakage inductance, (H).
L
lr
Rotor leakage inductance, (H).
r
Electrical rotor angular speed in (rad./sec).
V
W
Wind speed, (m./sec).
p
d/dt, the differential operator.
T
m
Mechanical torque on the shaft, (N.m).
T
e
Electromagnetic torque, (N.m).
B
Friction damping coefficient,( N.m./rad./sec).
J
m
Machine moment of inertia, (Kg.m
2
).
P
s
, Q
s
Stator active and reactive powers, (W).
P
m
Turbine mechanical power, (W).
P
Number of pole pairs.
e
Electrical stator flux angle, degree.
r
Electrical rotor angular position, degree.
slip
Electrical slip flux angle, degree.
Blade pitch angle, degree.
µ
Ratio of the rotor blade tip speed and wind speed (rad)
Specific density of the air, (Kg.m
3
).
A
Swept area of the blades, (m
2
).
( , µ)
Turbine power coefficient.
D
r
Rotor diameter in meters.
rdc
I
Rectified Rotor current, (A).
cw
R
Crowbar resistance, (
).
f
t
Fault duration, (sec).
List of symbols
xvi
L
Storage inductance, (H).
The leakage factor.
Subscripts
+, -
Positive and negative sequence.
r, s
Rotor/stator reference.
*
Denote the reference value.
^
Denote the estimated value.
d-q
Direct and quadrature axis.
Nomenclature
RSC
Rotor side converter.
GSC
Grid side converter.
IGBT
Insulated gate bipolar transistor.
FRT
Fault ride through.
PI
Proportional integral controller.
C.T
Co-ordinate transformation.
SPWM
Sinusoidal pulse width modulation.
LVRT
Low voltage ride through.
ZVRT
Zero voltage ride through.
VSI
Voltage source inverter.
FOC
Field orientation control.
Chapter 1: Introduction
1
KDSWHU
,1752'87,21
*HQHUDO
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 employ variable
speed wind turbine. (DFIG) Double Fed Induction Generator 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.
The reason for the world wide interest in developing wind generation plants is the
rapidly increasing demand for electrical energy and the depletion of the reserves of
fossil fuels, namely, oil and coal. Many places also do not have the potential for
generating hydro electrical power. The growing awareness of these problems led to
heightened research efforts for developing alternatives of energy sources. The most
desirable source would be one that non-pollutant, available in abundance,
renewable and can be harnessed at an acceptable cost in both large-scale and small
scale systems. The most promising source satisfying these entire requirements is
wind. Since earliest recorded history, wind power has been used to move ships,
grind grains and pump water. Wind energy was used to propel boats along the Nile
Chapter 1: Introduction
2
River as early 5000 B.C. within several centuries before Christ; simple windmills
were used in china to pump water [1].
All electric-generating wind turbines, no matter what size, are comprised of a few
basic components: the part that actually rotates in the wind, the electrical
generator, a speed control system, and a tower. Some wind machines 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 [2].
Just like solar electric system, wind
powered system can be used in two ways: off-grid or on-grid is when your home or
business is entirely disconnected from electric utility company and we generate
absolutely all of the electricity we need. Usually these systems cost about 30%
more than an on-grid (or grid-tie system).
DFIG is used extensively for high-power wind applications. DFIG has the ability
to control rotor currents that allow reactive power control and variable speed
operation. Both grid connected and stand-alone operation is feasible. For variable
speed operation, the standard power electronics interface consists of a rotor and
grid side pulse width modulator (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 field oriented 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,
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, which can result in acoustic noise due to torque
Chapter 1: Introduction
3
pulsations at low levels and at high levels can damage the rotor shaft, gearbox, or
blade assembly. Also an induction generator connected to an unbalanced grid will
draw unbalanced current. These unbalanced current tend to magnify the grid
voltage unbalance and cause over current problems as well.
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
[2]. 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. In this thesis, a novel FRT scheme is proposed. In this scheme, the
input mechanical energy of the wind turbine during grid fault is stored and utilized
at the moment of fault clearance, instead of being dissipated in the resistors of the
crowbar circuit as in the existing FRT schemes.
Chapter 1: Introduction
4
7KHVLV 2EMHFWLYHV
In view of the foregoing brief discussion, the objectives of the thesis are
summarized as follows:
1.
Modeling of a variable speed wind energy conversion system
(WECS) including a doubly fed induction generator as an electrical
power generation unit.
2. Controlling and improving the performance of a doubly fed induction
generator driven by a wind turbine system during wind speed
variations based on field orientation control principle.
3. Investigating the effect of the grid faults on the dynamic performance
of variable speed wind-driven doubly fed induction generator
connected to the grid.
4. Enhancing the capability of a wind driven doubly fed induction
generator to fault-ride through during grid faults.
7KHVLV RXWOLQHV
The present thesis is organized in six chapters.
KDSWHU is entitled ''Introduction''. It gives an overview about the
importance of the wind energy conversion system (WECS). Also, it presents
the motivations and objectives of the thesis and the contents of this thesis.
Chapter 1: Introduction
5
KDSWHU is entitled ''Literature review''. It contains a brief review of types
of wind generation systems and the types of generators used in each system.
Literature review of different control methods of a wind driven doubly fed
induction generator have been presented. The available literature covering
the methods used for enhancing the performance of the doubly fed induction
generator during grid fault intervals and a detailed comparison between
these methods.
KDSWHU is entitled ''Field orientation control of a wind driven doubly fed
induction generator connected to the grid''. It presents a dynamic model of
the proposed wind generation system, and developing an excellent control
technique for controlling both the active and reactive power of the doubly
fed induction generator based on field orientation control technique. Also the
performance of the wind generation system has been tested for different
wind speed profiles variations to emphasize the validity of the proposed
control method.
KDSWHU is entitled ''Dynamic performance of a wind driven doubly fed
induction generator during grid fault''. It presents a dynamic mathematical
model of the wind driven doubly fed induction generator during grid faults.
The mathematical model is based on symmetrical components analyzing
method, and it is used for studying and explaining the transient behavior of
the DFIG during different types of unbalanced conditions.
KDSWHU is entitled ''Enhancement of fault ride through capability of a
wind driven DFIG connected to the grid''. It introduces a novel scheme used
for improving the performance and enhancing the fault ride through
capability of the wind driven doubly fed induction generator scheme. In this
Chapter 1: Introduction
6
scheme, the input mechanical energy of the wind turbine during grid fault is
stored and utilized at the moment of fault clearance, instead of being
dissipated in the resistors of the crowbar circuit as in the existing FRT
schemes. Furthermore, the stored electromagnetic energy in the inductor is
transferred into the dc link capacitor on fault clearance and hence the grid
side converter is relieved from charging the dc link capacitor.
KDSWHU is entitled ''Conclusion and recommendations for future work''. It
summarizes the main conclusions drawn from this thesis along with
recommendations for future work.
In addition of these chapters, a quite useful list of references pertinent to the
topics treated in the thesis is given. For related details, the thesis is ended
with three appendices summarized as follows:
-
$SSHQGL[ $, which gives the simulink model for normal operation of the
proposed DFIG system during different wind speed variations.
-
$SSHQGL[ %, which presents the simulink model of the proposed DFIG
system during unbalanced grid conditions.
-
$SSHQGL[ which introduces the simulink model of the proposed DFIG
system during grid faults with the application of FRT scheme.
-
$SSHQGL[ contains a table that illustrates the mode and sequence of
operation of the FRT scheme.
Chapter 1: Introduction
7
-
$SSHQGL[ that shows the simulink model of the proposed DFIG system
during grid faults with the application of crowbar resistance.
Chapter 2: Literature review
8
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Electrical power is the most widely used source of energy for our homes, work
places and industries. Population and industrial growth have led to significant
increases in power consumption over the past three decades. Natural resources like
coal, petroleum and gas which drive our power plants, industries and vehicles for
many decades are becoming depleted at a very fast rate. This serious issue has
motivated nations across the world to think about alternative forms of energy
which utilize inexhaustible natural resources.
Wind plants have benefited from
steady advances in technology made over past 15 years. Much of the advancement
has been made in the components dealing with grid integration, the electrical
machine, power converters, and control capability. The days of the simple
induction machine with soft start are long gone. We are now able to control the
real and reactive power of the machine, limit power output and control voltage and
speed [1]. There is a lot of research going on around the world in this area and
technology is being developed that offers great deal of capability. It requires an
understanding of power systems, machines and applications of power electronic
converters and control schemes put together on a common platform.
Unlike a
conventional power plant that uses synchronous generators, a wind turbine can
operate as fixed-speed or variable-speed. In a fixed-speed wind turbine, the stator
of the generator is directly connected to the grid. However, in a variable-speed
wind turbine, the machine is controlled and connected to the power grid through a
power electronic converter. There are various reasons for using a variable-speed
wind turbine:
Chapter 2: Literature review
9
i.
Variable-speed wind turbines offer a higher energy yield in comparison to
constant speed turbines.
ii.
The reduction of mechanical loads and simple pitch control can be achieved
by variable speed operation.
iii.
Variable-speed wind turbines offer acoustic noise reduction and extensive
controllability of both active and reactive power.
iv.
Variable-speed wind turbines show less fluctuation in the output power [1]
and [2].
The use of renewable energy sources for electric power generation is gaining
importance in order to reduce global warming and environmental pollution, this is
in addition to meeting the escalating power demand of the consumers. Among
various renewable energy technologies, grid integration of wind energy electric
conversion system is being installed in huge numbers due to their clean and
economical energy conversion. Recent advancements in wind turbine technology
and power electronic systems are also more instrumental for the brisk option of
grid integration of wind energy conversion system [3]. Generally, wind power
generation uses either fixed speed or variable speed turbines, the main
configurations of generators and converters used for grid connected variable speed
wind power system (WPS) are presented in the following sections:
6\QFKURQRXV *HQHUDWRUV 'ULYHQ E\ D :LQG 7XUELQH
A synchronous generator usually consist of a stator holding a set of three-phase
windings, which supplies the external load, and a rotor that provides a source of
magnetic field. The rotor may be supplied either from permanent magnetic or from
a direct current flowing in a wound field.
Excerpt out of 131 pages
- Quote paper
- Mahmoud Mossa (Author), 2013, Control of a Wind Driven Doubly Fed Induction Generator During Grid Faults, Munich, GRIN Verlag, https://www.grin.com/document/209083
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