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TABLE OF CONTENTS
Page
ACKNOWLEDGMENTS... i
ABSTRACT... ii
TABLE OF CONTENTS... iv
LIST OF TABLES... vii
LIST OF FIGURES ... viii
LIST OF SYMBOLS... xiv
CHAPTER (1) INTRODUCTION 1
1-1 General... 1
1-2 Research Objectives... 3
1-3 Research Outlines... 3
CHAPTER (2) LITERATURE REVIEW 6
2-1 Introduction... 6
2-2 Synchronous Generators Driven by a wind
Turbine... 7
2-2-1 Wound Field Synchronous Generator
(WFSG)
Driven by a wind turbine... 7
2-2-2 Permanent-Magnet Synchronous Generator
(PMSG) Driven by a wind turbine... 8
2-3 Induction Generators Driven by a variable
speed wind turbine ... 8
2-3-1
Squirrel Cage Induction Generator (SCIG)
Driven by a wind turbine... 8
2-3-2 Doubly Fed Induction Generator (DFIG) Driven
by a wind turbine... 10
2-4 Field oriented Control of an Induction machine 11
iv
2-4-1 Direct field oriented control of a wind driven
DFIG... 13
2-4-2 Indirect field oriented control of a wind driven
DFIG... 14
2-5 Enhancement techniques of DFIG performance
during grid faults ... 15
2-5-1 Traditional techniques for protection of wind
turbines during grid faults... 16
2-5-2 Crowbar protection technique... 16
2-5-2-1 Series antiparallel thyristors LVRT technique 18
CHAPTER (3) Field Orientation Control of a Wind Driven
DFIG Connected to the Grid...
... 21
3-1 Introduction... 21
3-2 System description... 21
3-3 Dynamic modeling of the DFIG... 22
3-3-1 Turbine model... 22
3-3-2 Induction machine model... 24
3-4 DC Link model ... 24
3-5 Complete system model... 25
3-6 Field oriented control of a DFIG... 25
3-7 Complete system configuration... 27
3-8 Simulation results and discussions... 31
CHAPTER (4) Dynamic Performance of a Wind Driven Doubly
Fed Induction Generator During Grid Faults... 38
4-1 Introduction... 38
4-2 Dynamic Model of a DFIG System... 39
4-3 Mathematical Model of DFIG System Under
Unbalanced Grid Voltage... 40
4-4 System Description... 44
4-5 Simulation results and discussions... 45
v
CHAPTER (5) Enhancement of Fault Ride through Capability
of a Wind Driven Doubly Fed Induction
Generator Connected to the Grid... 62
5-1 Introduction... 62
5-2 System under study and proposed FRT scheme.. 63
5-3 Control strategy of the proposed FRT scheme... 64
5-4 Choice of size of storage inductor... 65
5-5 Simulation results and discussions ... 67
CHAPTER (6) Conclusions and Recommendations... 85
6-1 Conclusions... 85
6-2 Recommendations for future work... 86
REFERENCES ... 88
Appendix A ... 93
Appendix B ... 94
Appendix C ... 95
vi
LIST OF TABLES
TABLES Page
3-1 Parameters and data specifications of the DFIG system 31
C.2 Sequence and mode of operation of the FRT scheme 96
vii
LIST OF FIGURES
FIGURES OF CHAPTER (2) Page
Figure 2-1: Induction machine (SCIG) based wind turbine... 9
Figure 2-2: Doubly Fed Wound Rotor Induction Generator wind
based system... 10
Figure 2-3: General structure of a field oriented control in
a synchronous reference frame for an induction machine... 12
Figure 2-4: Structure of a direct field oriented control of a wind
driven DFIG... 14
Figure 2-5: Structure of indirect field oriented control of a wind
driven DFIG... 15
Figure 2-6: Crowbar circuits. a] Antiparallel thyristor crowbar
b] Diode bridge crowbar... 17
Figure 2-7: Series antiparallel thyristors for LVRT... 19
FIGURES OF CHAPTER (3)
Figure 3-1: Doubly-fed induction Generator driven by a wind
turbine system... 22
Figure 3-2: Wind turbine control system... 23
Figure 3-3: Power flow through dc-link element... 25
Figure 3-4: Proposed control scheme of the DFIG driven by a wind
turbine based on field orientation... 30
Figure 3-5: Performance of the proposed DFIG driven by a wind
turbine system with wind speed step change... 33
Figure 3-5-a: Wind speed variation... 32
Figure 3-5-b: Rotor speed variation... 32
Figure 3-5-c: Generated active power... 32
viii
Figure 3-5-d: Generated reactive power... 33
Figure 3-5-e: DC link voltage... 33
Figure 3-6: Dynamic response of the proposed system with
sinusoidal variation of wind speed... 35
Figure 3-6-a: Wind speed variation... 34
Figure 3-6-b: Rotor speed variation... 34
Figure 3-6-c: Generated active power... 34
Figure 3-6-d: Generated reactive power... 35
Figure 3-6-e: DC link voltage... 35
Figure 3-7: Performance of the proposed DFIG driven by a wind
turbine system with linear bi-directional variation
of wind speed... 37
Figure 3-7-a: Wind speed variation... 36
Figure 3-7-b: Rotor speed variation... 36
Figure 3-7-c: Generated active power... 36
Figure 3-7-d: Generated reactive power... 37
Figure 3-7-e: DC link voltage... 37
FIGURES OF CHAPTER (4)
Figure 4-1: Equivalent circuit of a DFIG in the synchronous
reference frame rotating at a speed of
s
... 39
Figure 4-2: Relationships between the (-) reference frame and the
41
(
d - q)
+
and (
d - q)
-
reference frames...
Figure 4-3: DFIG driven by a wind turbine based on field
orientation control during grid fault conditions... 44
Figure 4-4: performance of the proposed system under a single
ix
phase to ground fault during a constant wind speed... 49
Figure 4-4-a: Rotor speed variation... 45
Figure 4-4-b: Generated active power... 45
Figure 4-4-c: Generated reactive power... 46
Figure 4-4-d: DC link voltage... 46
Figure 4-4-e: Mechanical torque... 46
Figure 4-4-f: Electromagnetic torque... 47
Figure 4-4-g: Voltage of phase A... 47
Figure 4-4-h: Voltage of phase B... 47
Figure 4-4-i: Voltage of phase C... 48
Figure 4-4-j: Current of phase A... 48
Figure 4-4-k: Current of phase B... 48
Figure 4-4-l: Current of phase C... 49
Figure 4-4-m: Phase A rotor current... 49
Figure 4-5: performance of the proposed system under a double
phase to ground fault during a constant wind speed... 55
Figure 4-5-a: Rotor speed variation... 51
Figure 4-5-b: Generated active power... 51
Figure 4-5-c: Generated reactive power... 52
Figure 4-5-d: DC link voltage... 52
Figure 4-5-e: Mechanical torque... 52
Figure 4-5-f: Electromagnetic torque... 53
Figure 4-5-g: Voltage of phase A... 53
Figure 4-5-h: Voltage of phase B... 53
Figure 4-5-i: Voltage of phase C... 54
Figure 4-5-j: Current of phase A... 54
Figure 4-5-k: Current of phase B... 54
x
Figure 4-5-l: Current of phase C... 55
Figure 4-5-m: Phase A rotor current. ... 55
Figure 4-6: performance of the proposed system under a three
phase to ground fault during a constant wind speed... 60
Figure 4-6-a: Rotor speed variation... 57
Figure 4-6-b: Generated active power... 57
Figure 4-6-c: Generated reactive power... 58
Figure 4-6-d: DC link voltage... 58
Figure 4-6-e: Mechanical torque... 58
Figure 4-6-f: Electromagnetic torque... 59
Figure 4-6-g: Phase voltages... 59
Figure 4-6-h: Current of phase A... 59
Figure 4-6-i: Current of phase B... 60
Figure 4-6-j: Current of phase C... 60
Figure 4-6-k: Phase A rotor current... 60
FIGURES OF CHAPTER (5)
Figure 5-1: Proposed fault ride-through (FRT) scheme and field
oriented control for DFIG system... 63
Figure 5-2: Performance of the proposed DFIG system with
crowbar resistance and with FRT scheme during a
single phase to ground fault... 72
Figure 5-2-a: Rotor speed variation... 68
Figure 5-2-b: Generated active power... 68
Figure 5-2-c: Generated reactive power... 69
Figure 5-2-d: DC link voltage... 69
Figure 5-2-e: Mechanical input torque... 69
Figure 5-2-f: Electromagnetic torque... 70
xi
Figure 5-2-g: Phase voltages... 71
Figure 5-2-h: Stator currents... 72
Figure 5-2-i: Phase A rotor current... 72
Figure 5-3: Performance of the proposed DFIG system with
crowbar resistance and with FRT scheme during a
double phase to ground fault... 78
Figure 5-3-a: Rotor speed variation... 74
Figure 5-3-b: Generated active power... 74
Figure 5-3-c: Generated reactive power... 75
Figure 5-3-d: DC link voltage... 75
Figure 5-3-e: Mechanical input torque... 75
Figure 5-3-f: Electromagnetic torque... 76
Figure 5-3-g: Phase voltages... 76
Figure 5-3-h: Stator currents... 77
Figure 5-3-i: Phase A rotor current... 78
Figure 5-4: Performance of the proposed DFIG system with
crowbar resistance and with FRT scheme during a
three phase to ground fault... 83
Figure 5-4-a: Rotor speed variation... 80
Figure 5-4-b: Generated active power... 80
Figure 5-4-c: Generated reactive power... 81
Figure 5-4-d: DC link voltage... 81
Figure 5-4-e: Mechanical input torque... 81
Figure 5-4-f: Electromagnetic torque... 82
Figure 5-4-g: Stator currents... 83
Figure 5-4-h: Phase A rotor current... 83
xii
FIGURES OF APPENDICES
Figure App.A: Simulink program for normal operation of the
proposed DFIG system during wind speed
variation... 93
Figure App. B: Simulink program of the proposed DFIG system
during unbalanced network conditions... 94
Figure App. C.1: Simulink program of the proposed DFIG system
during grid faults with the application of FRT
scheme... 95
Figure App. C.2: Mode and Sequence of operation of the FRT
scheme... 96
Figure App. C.3: Simulink program of the proposed DFIG system
during grid faults with the application of Crowbar
resistance... 97
xiii
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.
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
).
(, )
Turbine power coefficient.
D
r
Rotor diameter in meters.
rdc
I
Rectified Rotor current, (A).
cw
R
Crowbar resistance, ().
f
t
Fault duration, (sec).
L
Storage inductance, (H).
The leakage factor.
Subscripts
+, -
Positive and negative sequence.
r, s
Rotor/stator reference.
xiv
*
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.
xv
Chapter 1
INTRODUCTION
1.1.
General
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 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
1
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 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 research, 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.
2
1.2.
Research Objectives:
In view of the foregoing brief discussion, the objectives of the research 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.
1.3.
Research outlines:
The present research is organized in six chapters.
Chapter 1 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 research.
Chapter 2 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.
3
Chapter 3 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.
Chapter 4 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.
Chapter 5 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 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.
Chapter 6 is entitled ''Conclusion and recommendations for future work''. It summarizes
the main conclusions drawn from this research along with recommendations for future
work.
In addition of these chapters, a quite useful list of references pertinent to the topics
treated in the research is given. For related details, the research is ended with three
appendices summarized as follows:
4
- Appendix A, which gives the simulink model for normal operation of the proposed DFIG
system during different wind speed variations.
- Appendix B, which presents the simulink model of the proposed DFIG system during
unbalanced grid conditions.
- Appendix C.1, which introduces the simulink model of the proposed DFIG system
during grid faults with the application of FRT scheme.
- Appendix C.2, contains a table that illustrates the mode and sequence of operation of the
FRT scheme.
- Appendix C.3, that shows the simulink model of the proposed DFIG system during grid
faults with the application of crowbar resistance.
5
Chapter 2
Literature Review
2.1. Introduction
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:
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].
6
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:
2.2.
Synchronous Generators Driven by a Wind Turbine
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.
2.2.1.
Wound Field Synchronous Generator (WFSG) Driven by a Wind Turbine
The stator winding is connected to network through a four-quadrant power converter comprised
of two back-to-back sinusoidal PWM. The machine side converter regulates the electromagnetic
torque, while the grid side converter regulates the real and reactive power delivered by the WPS
to the utility. The Wound Field Synchronous Generator has some advantages that are:
x The efficiency of this machine is usually high, because it employs the whole stator
current for the electromagnetic torque production [3].
x The main benefit of the employment of wound field synchronous generator with salient
pole is that it allows the direct control of the power factor of the machine, consequently
the stator current may be minimized at any operation circumstances.
The existence of a winding circuit in the rotor may be a drawback as compared with permanent
magnet synchronous generator. In addition, to regulate the active and reactive power generated,
the converter must be sized typically 1.2 times of the WPS rated power [4].
7
2.2.2.
Permanent-Magnet Synchronous Generator (PMSG) Driven by a Wind Turbine.
Many configuration schemes using a permanent magnet synchronous generator for power
generation had been adopted. In one of them a permanent magnet synchronous generator was
connected to a three-phase rectifier followed by boost converter. In this case, the boost converter
controls the electromagnet torque. One drawback of this configuration is the use of diode
rectifier that increases the current amplitude and distortion of the PMSG [5]. As a result this
configuration has been considered for small size wind power system (WPS) (smaller than 50
kW).
In another scheme using PMSG, the PWM rectifier is placed between the generator and the DC
link, while another PWM inverter is connected to the network. The advantage of this system
regarding the use of field orientation control (FOC) is that it allows the generator to operate near
its optimal working point in order to minimize the losses in the generator and power electronic
circuit. However, the performance is dependent on the good knowledge of the generator
parameter that varies with temperature and frequency. The main drawbacks, in the use of PMSG,
are the cost of permanent magnet that increase the price of machine, demagnetization of the
permanent magnet material and it is not possible to control the power factor of the machine [6].
2.3.
Induction Generators Driven by a Variable Speed Wind Turbine
The AC generator type that has most often been used in wind turbines is the induction generator.
There are two kinds of induction generator used in wind turbines that are: squirrel cage and
wound rotor.
2.3.1.
Squirrel Cage Induction Generator (SCIG) Driven by a Wind Turbine
Three-phase squirrel cage induction generators are usually implemented in standalone power
systems that employ renewable energy resources, like hydro-power and wind energy. This is due
to the advantages of these generators over conventional synchronous generators. The main
advantages are: reduced unit cost, absence of a separate d.c. source for excitation, ruggedness,
brushless rotor construction and ease of maintenance. A three-phase induction machine can be
operated as a self excited induction generator if its rotor is externally driven at a suitable speed
8
and a three-phase capacitor bank of a sufficient value is connected across its stator terminals. The
stator winding in this generation system is connected to the grid through a four-quadrant power
converter comprised of two PWM VSI connects back-to-back trough a DC link, this can be
shown in figure 2.1.
Wind
speed
Machine
side
Converter
Grid side
Converter
DC Link
C
T
m
IG
V
dc
1 : n
Grid
Step up
Transformer
Figure 2.1: Squirrel Cage Induction machine (SCIG) driven by a wind turbine.
The control system of the machine side converter regulates the electromagnetic torque and
supplies the reactive power to maintain the machine magnetized [7]. The grid side converter
regulates the real and reactive power delivered from the system to the utility and regulates the
DC link, but the uses of squirrel cage induction generator have some drawbacks as following:
x Complex system control whose performance is dependent on the good knowledge of the
generator parameter that varies with magnetic saturation, temperature and frequency [7].
x The stator side converter must be oversized 30-50% with respect to rated power, in order
to supply the magnetizing requirement of the machine [7].
9
2.3.2.
Doubly Fed Induction Generator (DFIG) Driven by a Wind Turbine
Wind
speed
Rotor side
Converter
Grid side
Converter
DC Link
T
m
IG
V
dc
1 : n
Step up
Transformer
Gear
Box
Step up
Transformer
Universal
Grid
A
B
C
A
B
C
1 : n
Figure 2.2: Doubly fed wound rotor induction generator driven by a wind turbine
The wind power system shown in Figure 2.2 consists of a doubly fed induction generator
(DFIG), where the stator winding is directly connected to the network and the rotor winding is
connected to the network through a four quadrant power converter comprised of two back-to-
back sinusoidal PWM. The thyristor converter can be used but they have limited performance.
Usually, the controller of the rotor side converter regulates the electromagnetic torque and
supplies part of the reactive power to maintain the magnetization of the machine. On the other
hand, the controller of the grid side converter regulates the DC link [8]. Compared to
synchronous generator, this DFIG offers the following advantages:
x Reduced inverter cost, because inverter rating typically 25% of the total system power.
This is because the converters only need to control the slip power of the rotor.
x Reduced cost of the inverter filter, because filters rated for 0.25 p.u. total system power,
and inverter harmonics represent a smaller fraction of total system harmonics.
x Robustness and stable response of this machine facing against external disturbance [8].
10
2.4.
Field Oriented Control of an Induction Machine
The control system of a variable speed wind turbine with DFIG has goals to control the reactive
power interchanged between the generator and the grid and the active power drawn from the
wind turbine in order to track the wind turbine optimum operation point or to limit the power in
the case of high wind speeds. Each wind turbine system contains subsystems (aero dynamical,
mechanical, electrical) with different ranges of time constants, i.e. the electrical dynamics are
typically much faster than the mechanical. This difference in time constants becomes even bigger
in the case of a variable speed wind turbine, due to the presence of the power electronics. Such
more complicated electrical system requires a more sophisticated control system too. The DFIG
control system contains two decoupled control channels: one for the rotor side converter and one
for the grid side converter. As the pulse-width modulation factor PWM is the control variable of
the converter, each of these control channels generates a pulse-width modulation factor PWM,
for the respective converter. This control variable is a complex number and therefore can control
simultaneously two variables, such as the magnitude and phase angle of the rotor induced
voltage. For example, for a predefined DC voltage and a control variable (pulse width
modulation factor PWM), the line-to-line AC-voltage is determined. On the other hand, the wind
turbine control is a control with slow dynamic responses. The wind turbine control contains two
cross-coupled controllers: a torque controller and a power limitation controller. It supervises both
the pitch angle actuator system of the wind turbine and the reactive power set point of the DFIG
control level. It thus provides both a reference pitch angle directly to the pitch actuator and a
converter reference reactive power signal for the measurement grid point.
Nowadays many variable speed wind turbines (WT) are based on DFIGs, which are connected to
the grid through back to back converters. The major advantage of these facilities lies in the fact
that the power rate of the inverters is around the 25-30% of the nominal generator power. This
feature permits to regulate the electrical power production within this range, something that has
been proven to be a good tradeoff between optimal operation and costs. The most used power
control systems for DFIG-WTs are normally based on voltage oriented control (VOC)
algorithms. The most extended version of such systems takes advantage of the field oriented
control (FOC) principle. Regarding this method, an accurate synchronization with the stator flux
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- , 2014, Modeling, Analysis and Enhancement of the performance of a Wind Driven DFIG During steady state and transient conditions, Hamburg, Bedey Media GmbH, https://www.anchor-publishing.com/document/287514
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