CHAPTER ONE

INTRODUCTION

1.1 General Concept

With increased penetration of wind generation into power systems over the years,
there are requirements for power sources to contribute to network support rather
than being disconnected from the network then abnormal grid voltage is detected.
For wind farms connected with long transmission lines to the ac grid, voltage
unbalance may arise due to a number of reasons, such as asymmetric line
impedances and loads. The unbalanced voltage can have a significant effect on the
performance and stability of connected equipment (e.g. induction machines). Most
early wind farms use wind turbines based on the fixed-speed induction generator
(FSIG) whereas many recent wind farms use doubly-fed induction generator (DFIG)
based wind turbines [1].
In order to meet power needs, taking into account economical and environmental
factors, wind energy conversion is gradually gaining interest as a suitable source of
renewable energy. The electromagnetic conversion is usually achieved by induction
machines or synchronous and permanent magnet generators. Squirrel cage induction
generators are widely used because of their lower cost, reliability, construction and
simplicity of maintenance. But when it is directly connected to a power network,
which imposes the frequency, the speed must be set to a constant value by a
mechanical device on the wind turbine. Then, for a high value of wind speed, the
totality of the theoretical power can be extracted. To overcome this problem, a
converter, which must be dimensioned for the totality of the power exchanged, can
be placed between the stator and the network. In order to enable variable speed
operations with a lower rated power converter, double-fed induction generator
(DFIG) can be used. The stator is directly connected to the grid and the rotor is fed
to magnetize the machine [2].

1
For a wind farm to be connected to the network it must comply with standards
specified in the relevant grid code. One area of particular concern is fault ride-
through. Studies have shown that FSIG based wind farms have difficulties in
meeting the requirements for fault ride-through without dynamic reactive power
compensation [3].
The conventional energy sources are limited and have pollution to the environment.
So more attention and interest have been paid to the utilization of renewable energy
sources such as wind energy, fuel cell and solar energy etc. Wind energy is the
fastest growing and most promising renewable energy source among them due to
economical viable. Doubly-fed induction machines are receiving increasing
attention for wind energy conversion system during such situation. Because the
main advantages of such machines is that, if the rotor current is governed applying
field orientation control-carried out using commercial double sided PWM inverter,
decoupled control of stator side active and reactive power results and the power
processed by the power convertor is only a small fraction of the total system power
[4].
Some methods of conventional power production shall be replaced in the future by
improving the efficiency of electricity use, conversion to renewable forms of energy
and other environmentally acceptable electricity production technologies. One of the
solutions in this case is the wind power use [5].
There are two basic types of wind turbines used nowadays
 Fixed-speed wind turbines
 Variable-speed wind turbines
Fixed-speed wind turbines are mainly equipped with squirrel-cage induction
generators, which are also known as self-excited generators. Self-excited generators
work within a limited wind speed range, which is one of their main drawbacks in
comparison with variable-speed wind turbines. They can be manufactured as one- or
two-speed versions and they are suitable for low power ranges up to 2 MW. The
majority of modern megawatt wind turbines are variable-speed wind turbines

2
equipped with a Doubly Fed Induction Generator (DFIG) that is coupled to the
power grid via a main transformer and supplied to the rotor from a frequency
convertor. The main advantage of DFIG wind turbines is their ability to supply
power at a constant voltage and frequency while the rotor speed varies. The DFIG
concept of a variable-speed wind turbine also makes possible the controlling of the
active and reactive power, which is a significant advantage as regards grid
integration [6].
The doubly fed induction generator (DFIG) can supply power at constant voltage
and constant frequency while the rotor speed varies. This makes it suitable for
variable speed wind energy application. Additionally, when a bidirectional AC-AC
converter is used in the rotor circuit, the speed range can be extended above
synchronous speed and power can be generated both from the stator and the rotor.
An advantage of this type of DFIG drive is that the rotor converter need only be
rated for a fraction of the total output power, the fraction depending on the allowable
sub- and super-synchronous speed range [7].
Figure (1.1) shows the general concept of the doubly-fed induction generator. The
mechanical power generated by the wind turbine is transformed into electrical power
by an induction generator and is fed into the main grid through the stator and the
rotor windings. The rotor winding is connected to the main grid by self commutated
AC/DC converters allowing controlling the slip ring voltage of the induction
machine in magnitude and phase angle. In contrast to a conventional, singly-fed
induction generator, the electrical power of a doubly-fed induction machine is
independent from the speed. Therefore, it is possible to realize a variable speed wind
generator allowing adjusting the mechanical speed to the wind speed and hence
operating the turbine at the aerodynamically optimal point for a certain wind speed
range [8].

3
 Figure 1.1 Doubly-fed induction generator system

1.2 Motivations
To improve wind generation system efficiency and power quality, adjustable speed
wind generators are used to track maximum wind power output and to control the
system more flexible, therefore doubly-fed induction generators are developed and
applied widely in the large wind generation system. While most conventional wind
farms were based on fixed speed induction generators (FSIG), more recent
instilments tend to use the doubly-fed induction generators (DFIG). DFIGs offer
several advantages when compared with FSIGs. These advantages, including speed
control, reduced flicker, and four-quadrant active and reactive power capabilities,
are primarily achieved via control of the rotor side converter (RSC), which is
typically rated at 30% of the generator rating for a given rotor speed operating range
of 0.75-1.25 pu [3].
Doubly fed induction generator (DFIGs) are widely used in modern wind turbines
due to their full power control capability, variable speed operation, low converter
cost, and reduced power loss compared to other solutions such as fixed speed

4
induction generators or fully rated converter system [9]. Doubly fed induction
generators can also contribute to reduced network unbalance due to its control
flexibility.
1.3 The Research Objectives
 To understanding the doubly-fed induction generators.
 Study the operating characteristics of the DFIG.
 Develop and discuss a dynamic model of the DFIG used in wind turbine.
 To develop a control strategy to control real and reactive power from a
generator to ensure maximum output power and voltage stability.
 Simulation analysis is performed to investigate a variety of DFIG
characteristics.
1.4 Problem Statement
Analysis of the Doubly-Fed Induction Generator (DFIG) for Wind Turbines
application both during steady-state operation and transient operation .In order to
analyze the DFIG during transient operation both the control and modeling of the
system is of importance. Hence, the control and the modeling are also important
parts of the thesis. The main contribution of this thesis is dynamic and steady-state
analysis of the doubly-fed induction generators.
1.5 Methodology
The steady state characteristics of a Wind energy conversion system using doubly
fed induction generator (DFIG) are analyzed using MATLAB. The dynamic steady-
state simulation model of the DFIG is developed using MATLAB. Simulation
analysis is performed to investigate a variety of DFIG characteristics, including
torque-speed, real and reactive-power over speed characteristics. Based on the
analysis, the DFIG operating characteristics are studied.
1.6 Thesis layout
This dissertation consists of five chapters. The literature review of doubly fed
induction generators modeling, control and DFIG in wind turbine are introduced in

5
chapter two. The mathematical models of doubly fed induction generators and
control are presented in chapter three. Chapter four presents the simulation results
and discussion. Finally, chapter five draws the conclusions, recommendations for
future research and author’s contribution.

6
 CHAPTER TWO
LITERATURE REVIEW

2.1 Introduction
Renewable energy including solar, wind, tidal, small hydro geothermal, refused
derived fuel cell energies is sustainable, reusable and environment-ally friendly and
clean. With the increasing shortage in fossil fuels, and pollution problems renewable
energy has become an important energy source. Among the other renewable energy
sources wind energy has proven to be one of the most economical one. Earlier
constant speed wind energy conversion systems (WECS) were proposed to generate
constant frequency voltages from the variable wind. However, Variable speed wind
energy conversion system operations can be considered advantageous, because
additional energy can be collected as the wind speed increases. Variable speed
WECS must use a power electronic converter. They are classified as full power
handling WECS and partial power handling WECS. In full power handling WECS,
the power converter is in series with the induction or synchronous generator, in
order to transform the variable amplitude/frequency voltages into constant
amplitude/frequency voltages and the converter must handle the full power. In a
partial power handling WECS, the converter processes only a portion of the total
generated power (eg slip power) which poses an advantage in terms of the reduced
cost converter of the system and increased efficiency of the system [11].
Due to the energy crisis, alternative renewable resources have assumed increased
importance leading to relevant technological efforts. Wind energy has been
identified as an affordable promising resource for such exploitation. The wind
energy conversion system safely and efficiently turns wind into electrical energy. It
is predicated that nearly 10% of the world energy needs could be met by the wind
energy by the year 2020 [12].

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In the 1990s, wind power turbines were characterized by a fixed-speed operation.
Basically, they consisted of the coupling of a wind turbine, a gearbox and an
induction machine directly connected to the grid. Additionally, they used a soft
starter to energize the machine and a bank of capacitor to compensate the machine
power reactive absorption. Although being simple, reliable and robust, the fixed-
speed wind turbines were inefficient and power fluctuations were transmitted to the
network due to wind fluctuations. In the mid- 1990s, variable-speed wind power
turbines gave an impulse to the wind power industry. A better turbine control
reduces power fluctuations. In addition, optimal power extraction from wind was
possible by operating the turbine at optimal speed. Among the different
configurations of variable-speed wind power turbine, the doubly-fed induction
generator, at present, is the most used in the development of wind farm projects.
This configuration consists of the coupling of a turbine, a gearbox and an induction
machine doubly connected to the grid directly connected from the stator circuits and
indirectly connected from the rotor circuits by using converters [13].
2.2 Wind Turbine System
Wind turbines system (WTS) can either operate at fixed speed or variable speed. For
a fixed speed wind turbine the generator is directly connected to the electrical grid.
For a variable speed wind turbine the generator is controlled by power electronic
equipment. There are several reasons for using variable-speed operation of wind
turbines; among those are possibilities to reduce stresses of the mechanical structure,
acoustic noise reduction and the possibility to control active and reactive power
[5],[10].
Most of the major wind turbine manufactures are developing new larger wind
turbines in the 3-to-5 MW range. These large wind turbines are all based on variable
speed operation with pitch control using a direct driven synchronous generator
(without gearbox) or a doubly-fed induction generator (DFIG). Fixed-speed
induction generators with stall control are regarded as unfeasible for this large wind

8
turbine. Today, doubly-fed induction generators are commonly used by the wind
turbine industry for larger wind turbines [5],[10].
The power electronic equipment makes it possible to control the rotor speed. In this
way the power fluctuation caused by wind variations can be more or less absorbed
by changing the rotor speed and thus power variation originating from the wind
conversion and the drive train can be reduced. Hence the power quality impact
caused by the wind turbine can be improved compared to a fixed-speed turbine.
The rotational speed of a wind turbine is fairly low and must therefore be adjusted to
the electrical frequency. This can be done in two ways: with a gearbox or with the
number of pole pairs of the generator. The number of pole pairs sets the mechanical
speed of the generator with respect to the electrical frequency and the gearbox
adjusts the rotor speed of the turbine to the mechanical speed of the generator [12].
Variable-speed turbines have many advantages in comparison with constant-speed
turbines. Variable-speed improves the dynamic behavior of the turbine, increases the
power production and reduces noise at low wind speed. In addition, by using a
voltage source converter for the variable-speed system, grid currents can be
controlled to be sinusoidal without low-frequency harmonics and the reactive power
can be chosen freely [12].
2.3 Doubly-Fed Induction Generator System For Wind Turbines
For variable-speed systems with limited variable-speed range, e.g.±30% of
synchronous speed, the DFIG can be an interesting solution. As mentioned earlier
the reasons for this are that power electronic convertor only has to handle a fraction
(20-30%) of total power. This means that the losses in power electronic converter
can be reduced compared to a system where the converter has to handle the total
power. In addition, the cost of the converter becomes lower. The stator circuit of the
DFIG is connected to the grid while the rotor circuit is connected to a convertor via
slip ring, see Figure 2.1[10].

9
 Figure 2.1 Principle of the doubly-fed induction generator

A more detailed picture of the DFIG system with a back-to-back converter can be
seen in Figure 2.2 [5],[10]. The back-to-back convertors consist of two convertors,
i.e. machine side and grid side convertor that are connected back-to-back. Between
the two convertors a dc-link capacitor is placed, as energy storage, in order to keep
the voltage variation (or ripple) in the dc-link voltage small.

Figure 2.2 DFIG systems with a back-to-back convertor

With the machine side convertor it is possible to control the torque or the speed of
the DFIG and also the power factor at the stator terminals, while the main objective
for the grid side convertor is to keep the dc-link voltage constant [5],[10].

10
2.4 Types of Doubly-Fed Induction Machines
In this section a short presentation of other kinds of doubly-fed induction machines
is made: a cascaded doubly-fed induction machines, a single-frame cascaded
doubly-fed induction machines, a brushless doubly-fed induction machines and
doubly-fed reluctance machine.
2.4.1 Cascaded Doubly-Fed Induction Machine
The cascaded doubly-fed induction machine consists of two doubly-fed induction
machines with wound rotors that are connected mechanically through the rotor and
electrically through the rotor circuits. The stator circuit one of the machines is
directly connected to the grid whiles the other machines stator is connected via a
convertor to the grid. Since the rotor voltages of both machines are equal, it is
possible to control the induction machine that is directly connected to the grid with
the other induction machine.
2.4.2 Single-Frame Cascaded Doubly-Fed Induction Machine
The single-frame cascaded doubly-fed induction machine is a cascaded doubly-fed
induction machine, but with the two induction machines in one common frame.
Although this machine is mechanically more robust than cascaded doubly-fed
induction machine, it suffers from comparative low efficiency.
2.4.3 Brushless Doubly-Fed Induction Machine
This is an induction machine with two stator windings in the same slot. That is, one
winding for the power and one winding for the control. To avoid a direct
transformer coupling between the two-stator windings, they cannot have the same
number of pole pairs. Furthermore, to avoid unbalanced magnetic pull on the rotor
the difference between the pole pairs must be great than one. The number of poles in
the rotor must equal the sum of the number of poles in the two stator winding.
2.4.4 Doubly-Fed Reluctance Machine
The stator of the doubly-fed reluctance machine is identical to the brushless doubly-
fed induction machine, while the rotor is based on the principle of reluctance [10].

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2.5 Doubly-Fed Induction Machines In Wind Turbine
Wind energy is popular renewable energy source. Doubly-fed induction generator
(DFIG) wind turbines are used widely by all the wind generator manufactures. In
Figure 2.3 a basic layout of DFIG wind turbine system is shown. The rotor circuit is
connected through slip rings to the back to back convertors arrangement controlled
by PWM strategies.

Figure 2.3 DFIG wind turbine system

2.6 Doubly-Fed Induction Generator Control

Doubly fed induction generator system have two major control areas: electrical
control of the convertors and mechanical control of the blade pitch angle. Control of
the DFIG is more complicated than the control of a standard induction machines. In
order to control the DFIG the rotor current is controlled by a power electronic
convertor. One common way of controlling the rotor current is by means of field-
oriented (vector) control. One common way is to control the rotor current with
stator-flux orientation, or with air-gap flux orientation. If the stator resistance can be
considered small, stator-flux orientation gives in principle orientation also with the
stator voltage (grid-flux orientation) [5],[10].

12
Traditionally, the DFIG is controlled using vector control (VC), which decouples the
rotor currents into active power (or torque) and reactive power (or flux) components
and adjusts them separately in a reference frame fixed to either the stator flux or
voltage. Current controllers are then utilized to regulate the rotor currents. The main
drawback for VC is its linear nature which does not consider the discrete operation
of power electronics converters. Thus, in order to maintain system stability over the
whole operation range and adequate dynamic response under both normal and
abnormal conditions, the current controller and its control parameters must be
carefully turned [9],[14].
The rotor side converter is used to provide speed/stator output active power control,
along with terminal voltage and reactive power control, by decoupling the rotor
current into real and imaginary parts, using stator flux oriented vector control. The
real current component reference is taken from an optimal power tracking current,
which allows the generator to produce maximum power for a given wind speed [15].
2.7 Summary
A wind energy conversion system (WECS) differs from a conventional power
system. The power output of a conventional power plant can be controlled whereas
the power output of a (WECS) depends on the wind. The doubly-fed induction
generator used as a wind turbine generator has recently received a great attention
from the industrial and scientific communities. The reason is twofold: first, this
machine easily produces a fixed frequency voltage from the stator winding when the
rotor is driven at variable speed; second: the excitation power electronic convertor
feeding the rotor windings needs to be rated at a fraction of the nominal power of
the generator. Consequently, this machine is often the natural choice for electricity
generation from renewable energy sources.

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 CHAPTER THREE
MATHEMATICAL MODEL OF DFIG AND
CONTROL
3.1 Introduction
The Induction Machine can be represented by the transformer per phase equivalent
circuit model where Rr and Xr represent rotor resistance and reactance referred to the
stator side. The primary internal stator induced voltage Esi is coupled to the
secondary rotor induced voltage Er by an ideal transformer with an effective turn
ratio aeff. But the equivalent circuit of Figure 3.1 differs from the transformer
equivalent circuit primarily in the effects of varying rotor frequency on the rotor
voltage Er [11].

Figure 3.1 Conventional induction machine equivalent circuit

In the case of doubly-fed induction machines, however there is a voltage injected in

to the rotor windings so that the normal induction machine equivalent circuit of
Figure 3.1 needs to be modified by adding a rotor injected voltage as shown in
Figure 3.2 [11].

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 Figure 3.2 Equivalent circuit of DFIG
From the equivalent circuit, for a doubly-fed induction machine the real and reactive
power of stator Psw, PsQ and rotor Prw, PrQ can be derived as follows:
PSW = 3V1 I1 cos Φv1 − Φi1 (3.1)
PSQ = 3V1 I1 sin Φv1 − Φi1 (3.2)
Prw = 3V2 I2 cos Φv2 − Φi2 (3.3)
PrQ = 3V2 I2 sin Φv2 − Φi2 (3.4)
As mentioned earlier the DFIG system consist of a DFIG and back-to-back voltage
source converter with a dc link. The back-to-back converters consist of a grid-side
converter (GSC) and machine side converter (MSC) [10].
3.2 DFIG Model
Due to its simplicity for deriving control laws for the DFIG, the T representation of
the induction generator model will be used. Note, that from a dynamic point of view,
the rotor and the stator leakage inductance have the same effect. Therefore, it is
possible to use a different representation of the park model in which the leakage
inductance is placed in the rotor circuit, the so-called T representation of the
induction machine. The name is due to the formation of a “T” of the inductances see
Figure 3.3 [10].

Figure 3.3 T representation of the Induction Generator

15
This model is described by the following space-vector equations in stator:
𝑑Ѱ𝑠𝑠
𝑉𝑠𝑠 = 𝑅𝑠 𝐼𝑠𝑠 + (3.5)
𝑑𝑡
𝑑Ѱ𝑠𝑅
𝑉𝑅𝑠 = 𝑅𝑅 𝐼𝑅𝑠 + − 𝑗𝜔Ѱ𝑅𝑠 (3.6)
𝑑𝑡

Where superscript s indicates stator coordinates. The model can also be described in
synchronous coordinates as:
𝑑Ѱ𝑆
𝑉𝑆 = 𝑅𝑆 𝐼𝑆 + + 𝑗𝜔1 Ѱ𝑆 (3.7)
𝑑𝑡
𝑑Ѱ𝑅
𝑉𝑅 = 𝑅𝑅 𝐼𝑅 + + 𝑗𝜔2 Ѱ𝑅 (3.8)
𝑑𝑡

Where the following notation is used:

Vs ⇒ Stator voltage Ψs ⇒ Stator flux
VR ⇒ Rotor voltage ΨR ⇒ Rotor flux
Is ⇒ Stator current Rs ⇒ Stator resistance
IR ⇒ Rotor current RR ⇒ Rotor resistance
ω1 ⇒ Synchronous frequency ω2 ⇒ Slip frequency
The stator flux, rotor flux and electromechanical torque are given by:
𝛹𝑠 = 𝐿𝑀 𝑖𝑠 + 𝑖𝑅 (3.9)
𝛹𝑅 = 𝐿𝑀 + 𝐿𝜎 𝑖𝑅 + 𝐿𝑀 𝑖𝑠 = 𝛹𝑠 + 𝐿𝜎 𝑖𝑅 (3.10)
𝑇𝑒 = 3𝑛𝑝𝐼𝑚 𝛹𝑠 𝑖𝑅∗ (3.11)

Where LM is the magnetizing inductance, Lσ is the leakage inductance and np is the

number of pole pairs. Finally, the mechanical dynamics of the induction machine are
described by:
𝐽 𝑑𝜔 𝑟
= 𝑇𝑒 − 𝑇𝑠 (3.12)
𝑛𝑝 𝑑𝑡

Where J is the inertia and Ts is the shaft torque [10].

16
3.2.1 Grid-Filter Model
In the Figure 3.4 the equivalent circuit of the grid filter in stator coordinates can be
seen. The filter consist of an inductance Lf and its resistance Rf. Applying
Kirchhoff’s voltage law to the circuit in the figure the following model in
synchronous coordinates can be found:
𝑑𝑖 𝑓
𝐸𝑔 = − 𝑅𝑓 + 𝑗𝜔1 𝐿𝑓 𝑖𝑓 − 𝐿𝑓 + 𝑉𝑓 (3.13)
𝑑𝑡

Where 𝐸𝑔 is the grid voltage, if is the grid-filter current, and Vf is the grid-filter
voltage supplied from the grid-side converter [10].

Figure 3.4 Grid-filter model in stator coordinates

3.2.2 DC-Link Model

The energy, Wdc stored in the dc-link capacitor, Cdc is given by
1 2
𝑊𝑑𝑐 = 𝐶𝑑𝑐 𝑉𝑑𝑐 (3.14)
2

Where Vdc is the dc-link voltage, In Figure 3.5 an equivalent circuit of the dc-link
model, where the definition of the power flow through the grid-side converter (GSC)
and the machine-side converter (MSC), can be seen. Moreover if the losses in the
actual converter can be considered small and thereby be neglected, the energy in the
dc-link capacitor is dependent on the power delivered to the grid filter, Pf and the
power delivered to the rotor circuit of the DFIG Pr [10].

17
 Figure 3.5 DC-Link Model

𝑑𝑊 𝑑𝑐 1 𝑑 2
= 𝐶𝑑𝑐 𝑉𝑑𝑐 = −𝑃𝑓 − 𝑃𝑟 (3.15)
𝑑𝑡 2 𝑑𝑡

This means that the dc-link voltage will vary as

𝑑𝑉 𝑑𝑐
𝐶𝑑𝑐 𝑉𝑑𝑐 = −𝑃𝑓 − 𝑃𝑟 (3.16)
𝑑𝑡

This means that Pf = Pr for a constant dc-link voltage.

The total model of the DFIG system presented in Figure 3.6 and can be summarized
in synchronous coordinate, as
𝑑𝛹 𝑠
= 𝐸𝑔 − 𝑅𝑠 𝑖𝑠 − 𝑗𝜔1 𝛹𝑠 (3.17)
𝑑𝑡
𝑑𝛹 𝑅
= 𝑉𝑅 − 𝑅𝑅 𝑖𝑅 − 𝑗𝜔2 𝛹𝑅 (3.18)
𝑑𝑡
𝑑𝑖 𝑓
𝐿𝑓 = 𝑉𝑓 − 𝑅𝑓 + 𝑗𝜔1 𝐿𝑓 𝑖𝑓 − 𝐸𝑔 (3.19)
𝑑𝑡
𝑑𝑉 𝑑𝑐
𝐶𝑑𝑐 𝑉𝑑𝑐 = −𝑃𝑓 − 𝑃𝑟 (3.20)
𝑑𝑡
𝐽 𝑑𝜔 𝑟
= 𝑇𝑒 − 𝑇𝑠 (3.21)
𝑛𝑝 𝑑𝑡

Where:
𝛹𝑠 = 𝐿𝑀 𝑖𝑠 + 𝑖𝑅 (3.22)
𝛹𝑅 = 𝐿𝜎 𝑖𝑅 + 𝐿𝑀 𝑖𝑠 + 𝑖𝑅 (3.23)
𝑇𝑒 = 3𝑛𝑝𝐼𝑚 𝛹𝑠 𝑖𝑅∗ (3.24)
𝑃𝑟 = 3𝑅𝑒 𝑉𝑅 𝑖𝑅∗ (3.25)
𝑃𝑓 = 3𝑅𝑒 𝑉𝑓 𝑖𝑓∗ (3.26)

18
Note that in (3.17) that the stator voltage, Vs, has been changed to the grid voltage,
𝐸𝑔 [10].

Figure 3.6 Equivalent circuit of the DFIG system

3.3 Control of DFIG

The control system consists of two controllers. One controller is a speed controller
which is used to control the torque of generator to make the wind turbine absorb the
maximum power from the wind when wind speed is under rated speed. The feed-
forward compensator and loop shaping are used for this controller design. The
design of the speed controller is easy and has better robustness than PI controller.
The other is a pitch controller that it used to regulate the pitch angle of the blade.
Considering the grate mechanical inertia of the blade, we set five given pitch-angles
in this controller. This can simplify the design of pitch controller and is easy to
implement in practice [16].
The phase sequence of the AC voltage generated by the rotor side converter is
positive for sub synchronous speed and negative for super synchronous speed. The
frequency of this voltage is equal to the product of the grid frequency and the
absolute value of the slip. The rotor side and the grid side converter have the
capability of generating or absorbing reactive power and could be used to control the
reactive power or the voltage at the grid terminals. The rotor side converter is used
19
to control the wind turbine output power and the voltage (reactive power) measured
at the grid terminals. The grid side converter is used to regulate the voltage of the
DC bus capacitor [11].
Different strategies were proposed in the literature to solve the DFIG control
problem. A vector controlled doubly-fed induction generator is an attractive solution
for high performance restricted speed-range electric drives and energy generation
application [17][18].
In order to control the rotor current of a DFIG by means of vector control, the
reference frame has to be aligned with a flux linkage. One common way is to control
the rotor currents with stator-flux orientation, or with air-gap-flux orientation. If the
stator resistance is considered to be small, stator-flux orientation gives orientation
also with the stator voltage [10].
3.4 Control of Machine-Side Converter
The main task of the machine-side converter is, of course, to control the machine.
This is done by having an inner fast field-oriented current control loop that controls
the rotor current. The field orientation could, for example, either be aligned with the
stator flux of the DFIG or the grid flux. For both reference frames the q component
of the rotor current largely determines the produced torque while the d component
can be used to control for instance, the reactive power at the stator terminals [10].
3.4.1 Current Control
As mentioned earlier, it is common to control the rotor current with either stator-flux
orientation or grid-flux orientation. In order to derive the rotor-current control law, it
is advantageous is and ΨR from (3.7) and (3.8) which yields
𝑑𝛹 𝑠 𝑅𝑠
𝑉𝑠 = −𝑅𝑠 𝑖𝑅 + + + 𝑗𝜔1 𝛹𝑠 (3.27)
𝑑𝑡 𝐿𝑀
𝑑𝑖 𝑅 𝑑𝛹 𝑠
𝑉𝑅 = 𝑅𝑅 + 𝑗𝜔2 𝐿𝜎 𝑖𝑅 + 𝐿𝜎 + + 𝑗𝜔2 𝛹𝑠
𝑑𝑡 𝑑𝑡
𝑑𝑖 𝑅
= 𝑅𝑅 + 𝑅𝑠 + 𝑗𝜔2 𝐿𝜎 𝑖𝑅 + 𝐿𝜎 +𝐸 (3.28)
𝑑𝑡
𝑅𝑠
𝐸 = 𝑉𝑠 − + 𝑗𝜔𝑟 𝛹𝑠 (3.29)
𝐿𝑀

20
Where E is the back EMF, the rotor current dynamics formed by the inner loop in
Figure 3.7 bellow

Figure 3.7 Block diagram of the current control system

3.4.2 Torque Control

The electromechanical torque can be found from (3.11) as
𝑇𝑒 = 3𝑛𝑝𝐼𝑚 𝛹𝑠 𝑖𝑅∗ ≈ −3𝑛𝑝𝛹𝑠 𝑖𝑅𝑞 (3.30)
For a stator-flux-oriented system the above approximation is actually equality. Since
the stator flux, Ψs is almost fixed to the stator voltage; the torque can be controlled
by the q component of the rotor current, iRq. Since it is difficult to measure the
torque, it is most often controlled in an open-loop manner. Therefore the q
𝑟𝑒𝑓
component reference current, 𝑖𝑅𝑞 , can be determined from the reference torque
𝑟𝑒𝑓
𝑇𝑒 as [4].
𝑟𝑒𝑓
𝑟𝑒𝑓 𝑇𝑒
𝑖𝑅𝑞 = − (3.31)
3𝑛𝑝 𝛹𝑠

Fig.(3-8) shows a block diagram of the open-loop torque control scheme.

Figure 3.8 Block diagram of the open-loop torque control

21
3.4.3 Speed Control
Since the current dynamics, that is, with the band width αc should be set much faster
than the speed dynamics, the speed can be controlled in cascade with the current.
The mechanical dynamics are described by
𝐽 𝑑𝜔 𝑟
= 𝑇𝑒 − 𝑇𝑠 (3.32)
𝑛𝑝 𝑑𝑡

Where Te is the electromechanical torque and Ts is the shaft torque. The

electromechanical torque can be expressed, under the assumption that the current
dynamics are much faster than the speed dynamics, as
𝑟𝑒𝑓
𝑇𝑒 = 𝑇𝑒 (3.33)
Where the reference torque is set to
𝑟𝑒𝑓 𝑟𝑒𝑓
𝑇𝑒 = 𝑇′𝑒 − 𝐵𝑎 𝜔𝑟 (3.34)
The Figure 3.9 shows a block diagram of the speed control system [10].

Figure 3.9 Speed control loop

3.5 Control of Grid-Side Converter

The main objective of the grid-side converter is to control the dc-link voltage. The
control of the grid-side converter consists of a fast inner current control loop, which
controls the current through the grid filter, and an outer slower control loop that
controls the dc-link voltage. The reference frame of the inner current control loop
will be aligned with the grid flux. This means that the q component of the grid filter
current will control the active power delivered from the converter and the d

22
component of the filter current will, accordingly, control the reactive power. This
implies that the outer dc-link voltage control loop has to act on the q component of
the grid-filter current [10].
3.5.1 Current Control of Grid Filter
In (3.19) the dynamic of the grid filter are described
𝑑𝑖 𝑓
𝐿𝑓 = 𝑉𝑓 − 𝑅𝑓 + 𝑗𝜔1 𝐿𝑓 𝑖𝑓 − 𝐸𝑔 (3.35)
𝑑𝑡

In order to introduce “active damping” and decouple the d and the q components of
the grid-filter current, the applied grid-filter voltage, Vf is chosen as
𝑉𝑓 = 𝑉′𝑓 − 𝑅𝑎𝑓 − 𝑗𝜔1 𝐿𝑓 𝑖𝑓 (3.36)
This means that the inner closed-loop transfer function, assuming ideal parameters,
becomes
𝑖𝑓 𝑝 1
𝐺 𝑝 = = (3.37)
𝑉′ 𝑓 𝑝 𝐿𝑓𝑝 +𝑅𝑓 +𝑅𝑎𝑓

3.5.2 DC-Link Voltage Control

One way of simplifying the control of the dc-link voltage is by utilizing feedback
linearization, i.e. the nonlinear dynamics of the dc-link are transformed into an
equivalent linear system where linear control techniques can be applied. A block
diagram of the dc-link voltage controller is depicted in Figure 3-10 [10].

Figure 3.10 Dc-link voltage control loop

23
3.6 Summary
Mathematical modeling of converter system is realized by using various types of
models, which can be broadly divided into two groups: mathematical functional
models and mathematical physical models. Owing to the fact that DFIG controls
have a significant influence on the system dynamics, vector control is applied for
both stator and rotor side converters to increase the degree of controllability, where
fixed-frequency internal model controller approach is adopted to design the
controllers precisely.

24
 CHAPTER FOUR
RESULTS AND DISCUSSIONS

4.1 Introduction
Large Wind turbines are often equipped with doubly-fed induction generators.
There are several advantages by using adjustable speed generators. Modern wind
turbines use complex technologies including power electronic converters and
sophisticates control systems. Electromagnetic transients need to be simulated and
analyzed in order to study the impact of these generators on the power systems.
Methods and tools for simulation of wind turbines in large power systems are
therefore needed [19].
In this dissertation a detailed model from Matlab & Simulink SimPower systems
library is used. The detailed model includes detailed representation of power
electronic IGBT, the model must be discredited at a relatively small time step (5
microsecond). This model is well suited for observing harmonics and control
systems dynamic performance over relatively short periods of times (typically
hundreds of milliseconds to one second) [20].
4.2 Circuit Description
A 9 MW wind farm consisting of six 1.5 MW wind turbines connected to a 25 KV
distribution system exports power to a 120 KV grid through a 30 Km, 25 KV
feeder, as shown in Figure 4.1 below

Transformer 1 Transformer 2
Transmission

Line
Synchronous DFIG
Generator

Figure 4.1 detailed model of DFIG

25
Wind turbines using a doubly-fed induction generator (DFIG) consist of a wound
rotor induction generator and an AC/DC/AC IGBT-based PWM converter. The
stator winding is connected directly to the 60 Hz grid while the rotor is fed at
variable frequency through the AC/DC/AC converter. The DFIG technology
allows extracting maximum energy from the wind for low wind speed by
optimizing the turbine speed, while minimizing mechanical stresses on the turbine
during gusts of wind. In this model the wind speed is maintained constant at
15m/s. The control system uses a torque controller in order to maintain the speed
at 1.2 pu. The reactive power produced by the wind turbine is regulated at 0 Mvar.
[20].
4.3 Modeling of wind turbine doubly-fed induction generator
The wind turbine and the doubly-fed induction generator are shown in Figure 4.2.
The AC/DC/AC converter is divided into two components: the rotor side converter
C rotor and the grid-side converter C grid. C rotor and C grid are Voltage-Sourced
Converters that use forced-commutated power electronic devices (IGBTs) to
synthesize an AC voltage from a DC voltage source. A capacitor connected on the
DC side acts as the DC voltage source. A coupling inductor L is used to connect C
grid to the grid. The three-phase rotor winding is connected to C rotor by slip rings
and brushes and the three-phase stator winding is directly connected to the grid
[19], [21].
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 C rotor and C grid respectively in order to control the
power of the wind turbine, the DC bus voltage and the voltage at the grid
terminals. The phase-sequence of the AC voltage generated by C rotor is positive for
sub-synchronous speed and negative for super-synchronous speed. The frequency
of this voltage is equal to the product of the grid frequency and the absolute value
of the slip. C rotor and C grid have the capability for generating or absorbing reactive
26
power and could be used to control the reactive power or the voltage at the grid
terminals [19],[21].

Figure 4.2 the wind turbine and doubly-fed induction generator.

4.3.1 Rotor Side Converter Control system

The rotor side converter is used to control the wind turbine output power and
voltage or the output power and reactive power measured at the grid terminals. The
power is controlled in order to follow a pre-defined power-speed characteristic
named tracking characteristic. This characteristic is illustrated by the ABCD curve
in Figure 4.3 superimposed to the mechanical power characteristics of the turbine
obtained at different wind speed. The actual speed of the turbine ωr 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
defined by four points: A, B, C and D. From zero speed to speed of point A the
reference power is zero. Between point A and point B the tracking characteristic is
a straight line. Between point B and point C the tracking characteristic is the locus

27
of the maximum power of the turbine (maxima of the turbine power vs turbine
speed curves). The tracking characteristic is a straight line from point C and point
D. The power at point D is one per unit (1 p.u.). Beyond point D the reference
power is a constant equal to one per unit (1 p.u.).

Figure 4.3 Turbine characteristics and tracking characteristic

The generic power control loop illustrated in Figure 4.4. For the rotor-side
controller the d-axis of the rotating reference frame used for d-q transformation
is aligned with air-gap flux. 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.

28
 Figure 4.4 rotor-side controller

4.3.2 Grid Side Converter Control system

The grid-side converter is used to regulate the voltage of the DC bus capacitor.
For the grid-side controller the d-axis of the rotating reference frame used for d-q
transformation is aligned with the positive-sequence of grid voltage. This
controller consists of:
 Measurement system measuring the d and q components of AC current to
be controlled as well as the DC voltage Vdc.
 An outer regulation loop consisting of a DC voltage regulator. The output
of the DC voltage regulator is the reference current Idgc-ref for the
current regulator (Idgc = current in phase with grid voltage which controls
active power flow).
 An inner current regulation loop consisting of a current regulator. The
current regulatory controls the magnitude and phase of the voltage

29
 generated by converter C grid (Vgc) from the Idgc-ref produced by the DC
voltage regulator and specified Iq-ref reference. The current regulator is
assisted by feed forward terms which predict the C grid output voltage [19],
[21].

Figure 4.5 Grid-side controllers.

4.4 Simulink Diagram

This is simulink diagram for a doubly-fed induction generator connected to the
grid side. The system is connected to a 120 KV, 3-phase source which is
connected to a 9 MW wind farm (6 of 1.5 MW each).
The wind-turbine model is a detailed model. The sample time used to discretize
the model (Ts=50 microseconds) is specified in the initialization function of the
model properties. For a wind speed of 15 m/s, the turbine output power is 1.0 p.u
of its rated power, the pitch angle is 8.7 degrees and the generator speed is 1.2 p.u.

30
 Figure 4.6 Network model schematic.
4.4.1 Generator Data
In the table below implements a model of a variable speed pitch controlled wind
turbine using a doubly-fed induction generator (DFIG).
Table 4.1 Generator data.
Number of wind turbines 6
Nominal power 1.5e6 VA
Line to line voltage 575 Vrms
Frequency 60 Hz
Stator resistance 0.023 p.u
Stator inductance 0.18 p.u
Rotor resistance 0.016 p.u
Rotor inductance 0.16 p.u
Magnetizing inductance 2.9 p.u
Inertia constant 0.685 s
Fraction factor 0.01 p.u
Pairs of poles 3

31
 4.4.2 Control Parameters
This is table for control parameters showing different modes of operation in
which we can select the voltage regulation mode and Var regulation mode. Here
we input the required values of voltage regulator gains (both proportional and
integral), power regulator gains, current regulator gains and their respective rate
of change.
Table 4.2 Control parameters.
Gains of PI-controller Kp Ki
DC bus voltage regulator 8 400
Grid-side converter current regulator 0.83 5
Rotor-side converter current regulator 0.6 8
Speed regulator 3 0.6
Reactive power(var) and voltage(volt) reulator 0.05 20
Pitch compensation 3 30
Pitch controller 150 -
Frequency of the grid-side PWM carrier 2700 Hz
Frequency of the rotor-side PWM carrier 1620 Hz
Maximum pitch angle 27 (deg)
Maximum rate of change of pitch angle 10 (deg/s)

4.5 Simulation Results and Discussion

In this model we are observe the steady-state operation of the doubly-fed induction
generator and its dynamic response to voltage sag resulting from a remote fault on
the 120-KV system.

32
 13

12

11

10

Power(Mw)
9

4
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time(sec)

Figure 4.7 Power curve at wind speed 10m/s

Figure 4.7 shows the result of output power when the speed is 10m/s. Initially
the power is steady-state and constant in 9 Mw with simple oscillation up to
t=0.031 s and at this time the power starting increasing smoothly. As a result of
acceleration of the rotor, it is found that the electro motive force decreases and
then reduces the torque and thus power value. The power decreases smoothly at
t=0.0353s and it is again start increasing at t=0.1301s, due to rotor speed
reduction. Also it is found that the maximum power 12.91 Mw at t=0.152s and
the minimum power 4.869 Mw at t=0.0526s and the power at t=0.2s is 9.319Mw.
13

12

11

10
Power(Mw)

4
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time(sec)

Figure 4.8 Power curve at wind speed 15m/s.

Figure 4.8 shows the result of output power when the speed is 15m/s. Initially
the power is steady-state and constant in 9 Mw with simple oscillation up to
t=0.031 s and in this time the power starting increasing smoothly. As a result of
acceleration of the rotor, it is found that the electro motive force decreases and

33
then reduces the torque and thus power value. The power decreases smoothly at
t=0.0353s and it is again start increasing at t=0.1301s, due to rotor speed
reduction. Also it is found that the maximum power 12.96 Mw at t=0.152s and
the minimum power 4.87 Mw at t=0.0526s and the power at t=0.2s is 9.528Mw.
14

13

12

11

10
Power(Mw)

4
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time(sec)

Figure 4.9 Power curve at wind speed 20m/s.

Figure 4.9 shows the result of output power when the speed is 20m/s. Initially
the power is steady-state and constant in 9 Mw with simple oscillation up to
t=0.031 s and in this time the power starting increasing smoothly. As a result of
acceleration of the rotor, it is found that the electro motive force decreases and
then reduces the torque and thus power value. The power decreases smoothly at
t=0.0353s and it is again start increasing at t=0.1301s, due to rotor speed
reduction. Also it is found that the maximum power 13.02 Mw at t=0.152s and
the minimum power 4.877 Mw at t=0.0526s and the power at t=0.2s is 9.805Mw.
1.218

1.216

1.214

1.212

1.21
Speed(pu)

1.208

1.206

1.204

1.202

1.2

1.198
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time(sec)

Figure 4.10 Rotor speed curve at wind speed 10m/s

34
Figure 4.10 shows the result of the rotor speed, when the wind speed is 10m/s.
At t=0s the rotor speed is 1.2 pu, power is 9Mw. Due to acceleration of wind the
rotor speed increasing smoothly at t=0.0357s and continuous increasing until it
reach its maximum, 1.215pu at t=0.134s, after that the rotor speed starting
decreases due to operation of control system. At t=0.2s the rotor speed is
1.198pu.
1.22

1.215

1.21
Speed(pu)

1.205

1.2

1.195
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time(sec)

Figure 4.11 Rotor speed curve at wind speed 15m/s

Figure 4.11 shows the result of the rotor speed, when the wind speed is 15m/s.
At t=0s the rotor speed is 1.2 pu, power is 9Mw. Due to acceleration of wind the
rotor speed increasing smoothly at t=0.0357s and continuous increasing until it
reach its maximum, 1.217pu at t=0.134s, after that the rotor speed starting
decreases due to operation of control system. At t=0.2s the rotor speed is
1.204pu.
1.23

1.225

1.22

1.215
Speed(pu)

1.21

1.205

1.2

1.195
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time(sec)

Figure 4.12 Rotor speed curve at wind speed 20m/s

35
Figure 4.12 shows the result of the rotor speed, when the wind speed is 20m/s.
At t=0s the rotor speed is 1.2 pu, power is 9Mw. Due to acceleration of wind the
rotor speed increasing smoothly at t=0.0357s and continuous increasing until it
reach its maximum, 1.22pu at t=0.134s, after that the rotor speed starting
decreases due to operation of control system. At t=0.2s the rotor speed is
1.212pu.
5

3
Reactive power(Mvar)

-1

-2
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time(sec)

Figure 4.13 Reactive power curve at wind speed 10m/s

Figure 4.13 shows the result of reactive power when the wind speed is 10m/s.
Initially the reactive power is constant at 0 Mvar. Due to the active power
decreases the reactive power increasing smoothly at t=0.0308s and continuous
increasing until it reach its maximum, 4.186Mvar at t=0.042s. Then the reactive
power starting decreases because increasing of active power. At t=0.2s the
reactive power is 1.565Mvar.
5

3
Reactive power(Mvar)

-1

-2
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time(sec)

Figure 4.14 Reactive power curve at wind speed 15m/s

36
Figure 4.14 shows the result of reactive power when the wind speed is 15m/s.
Initially the reactive power is constant at 0 Mvar. Due to the active power
decreases the reactive power increasing smoothly at t=0.0308s and continuous
increasing until it reach its maximum, 4.179Mvar at t=0.0424s. Then the reactive
power starting decreases because increasing of active power. At t=0.2s the
reactive power is 1.553Mvar.
5

3
Reactive power (Mvar)

-1

-2
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time(sec)

Figure 4.15 Reactive power curve at wind speed 20m/s

Figure 4.15 shows the result of reactive power when the wind speed is 20m/s.
Initially the reactive power is constant at 0 Mvar. Due to the active power
decreases the reactive power increasing smoothly at t=0.0308s and continuous
increasing until it reach its maximum, 4.184Mvar at t=0.0424s. Then the reactive
power starting decreases because increasing of active power. At t=0.2s the
reactive power is 1.533Mvar.
1.5

0.5
Vabc-B575

-0.5

-1

-1.5
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time(sec)

Figure 4.16 Vabc-B575 curves at wind speed 10m/s

37
Figure 4.16 shows the positive-sequence voltage curves at busbar 575. Initially
the doubly-fed induction generator wind farm produces 9 MW. The
corresponding turbine speed is 1.2 p.u of generator synchronous speed. And the
positive-sequence voltage is 1.0 pu. At t=0.03s the positive-sequence voltage
suddenly drops to 0.5 pu until t=0.13s and at this time the voltage returned to 1.0
pu.

1.5

0.5
Vabc-B575

-0.5

-1

-1.5
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time(sec)

Figure 4.17 Vabc-B575 curves at wind speed 15m/s

1.5

0.5
Vabc-B575

-0.5

-1

-1.5
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time(sec)

Figure 4.18 Vabc-B575 curves at wind speed 20m/s

Figure 4.17 and 4.18 are shows the positive-sequence voltage curves at busbar
575 at wind speed 15m/s and 20m/s respectively. There are no changes in curves.

38
 2

1.5

0.5

Iabc-B575(pu) 0

-0.5

-1

-1.5

-2
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time(sec)

Figure 4.19 Iabc-B575 curves at wind speed 10m/s

Figure 4.19 shows the currents curves in busbar 575 at wind speed 10m/s.
Initially the values of currents are 0.8887pu. At t=0.03 the values of current are
increasing due to voltage sag up to t=0.13 the values of current are starting
decreases.

1.5

0.5
Iabc-B575(pu)

-0.5

-1

-1.5

-2
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time(sec)

Figure 4.20 Iabc-B575 curves at wind speed 15m/s

39
 2

1.5

0.5

Iabc-B575(pu) 0

-0.5

-1

-1.5

-2
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time(sec)

Figure 4.21 Iabc-B575 curves at wind speed 20m/s

Figure 4.20 and 4.21 are shows currents curves in busbar 575 at wind speed
15m/s and 20m/s respectively. There are no changes in curves.

1200

1190

1180

1170
Vdc (V)

1160

1150

1140

1130

1120
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time(sec)

Figure 4.22 DC voltage curve at wind speed 10m/s

Figure 4.24 shows the DC voltage curve at wind speed 10m/s. The DC voltage is
regulated at 1150V and reactive power is kept at 0 Mvar. At t=0.03 second the
positive-sequence voltage suddenly drops to 0.5 p.u causing an oscillation on the
DC bus voltage and on the DFIG output power. During the voltage sag the

40
control system tries to regulate DC voltage and reactive power at their set points
(1150V, 0 Mvar).

1200

1190

1180

1170
DC voltage(V)

1160

1150

1140

1130

1120
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time(sec)

Figure 4.23 DC voltage curve at wind speed 15m/s

1200

1190

1180

1170
DC voltage (V)

1160

1150

1140

1130

1120
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time(sec)

Figure 4.24 DC voltage curve at wind speed 20m/s

Figure 4.23 and 4.24 are shows the DC voltage curve at wind speed 15m/s and
20m/s respectively. There are no changes in curves.

41
 1.5

0.5

Vabc-B25(pu)
0

-0.5

-1

-1.5
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time(sec)

Figure 4.25 Vabc-B25 curves at wind speed 10m/s

Figure 4.25 shows the positive-sequence voltage curves at busbar 25. Initially the
positive-sequence voltage is 1.0 pu. At t=0.03s the positive-sequence voltage
suddenly drops to 0.5 pu until t=0.13s and at this time the voltage returned to 1.0
pu.

1.5

0.5
Vabc-B25(pu)

-0.5

-1

-1.5
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time(sec)

Figure 4.26 Vabc-B25 curves at wind speed 15m/s

42
 1.5

0.5

Vabc-B25(pu) 0

-0.5

-1

-1.5
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time(sec)

Figure 4.27 Vabc-B25 curves at wind speed 20m/s

Figure 4.26 and 4.27 are shows the positive-sequence voltage curves at busbar 25
at wind speed 15m/s and 20m/s respectively. There are no changes in curves.

1.5

0.5
Iabc-B25(pu)

-0.5

-1

-1.5

-2
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time(sec)

Figure 4.28 Iabc-B25 curves at wind speed 10m/s

Figure 4.30 shows the currents curves in busbar 25 at wind speed 10m/s. Initially
the values of currents are 0.8881pu. At t=0.03 the values of current are
increasing due to voltage sag up to t=0.13 the values of current are starting
decreases.

43
 2

1.5

0.5

Iabc-B25(pu)
0

-0.5

-1

-1.5

-2
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time(sec)

Figure 4.29 Iabc-B25 curves at wind speed 15m/s

1.5

0.5
Iabc-B25(pu)

-0.5

-1

-1.5

-2
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time(sec)

Figure 4.30 Iabc-B25 curves at wind speed 20m/s

Figure 4.20 and 4.30 are shows currents curves in bus-bar 25 at wind speed
15m/s and 20m/s respectively. There are no changes in curves.
4.6 Summary
From the results we are found that if the wind speeds increasing the active power
increasing also the maximum rotor speed increasing. The reactive power increasing
and then decreases due to the changes in active power. The positive-sequence
voltage suddenly drops to 0.5pu then it returned to 1.0pu after 0.1s. The values of
currents are changes due to the changes in the voltage. The drops in positive-
sequence voltage causing an oscillation on the DC bus-bar voltage and on the DFIG
output power.

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 CHAPTER FIVE
CONCLUSIONS AND RECOMONDATIONS

5.1 Conclusions
The main objective of this dissertation is to give an overview of research and
development in the field of modeling and simulation of DFIG coupled with wind
turbine. Wind energy conversion system, DFIG equivalent circuit, modeling of
different parts and control of DFIG is discussed. So the reader should be familiar
with the DFIG Wind Turbine systems. The basic operation of DFIG and it is
controls using AC/DC/AC converter have discussed. For best efficiency the DFIG
system is used which is connected to grid side and has better control. The rotor side
converter (RSC) usually provides active and reactive power control of the machine
while the grid side converter (GSC) keeps the voltage of the DC-link constant.
The model is a discrete-time version of the wind turbine doubly-fed induction
generator (detailed type) of Matlab-Sim Power system and three values of wind
speed were taken. The DFIG is able to provide a considerable contribution to grid
voltage support during short circuit periods. Considering the result it can be said that
doubly-fed induction generator proved to be more reliable and stable system when
connected to grid-side with the proper converter control systems.
The system model of a wind farm 9MW consisting of six 1.5MW wind turbines has
been presented. The model has simulated using Matlab/Simulink software. The
results of the model illustrated that the maximum extract possible output power from
variable wind can be achieved by controlling the back to back convertors systems.

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5.2 Recommendations
 In this thesis the response of the doubly-fed induction generator to grid
disturbances have been investigated. As always there are many more
interesting aspect that can be considered, such as; unsymmetrical voltage
dips, voltage harmonics, phase shifts, and frequency dips in the grid voltage.
 Also future work can involve using multi-level STATCOM to reduce the
harmonics of the system.
 The transient behaviors of the DFIG-based wind turbine system under
disturbances of grid failures should be studied.

46
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