Page 1 of 8
European Journal of Business &
Social Sciences
Available at https://ejbss.org/
ISSN: 2235-767X
Volume 07 Issue 04
April 2019
Available online:https://ejbss.org/ P a g e | 1461
PERFORMANCE ENHANCEMENT OF GRID CONNECTED
DOUBLY FED INDUCTION GENERATOR FOR SOURCE AND
GRID SIDE ISSUES
MEENAKSHI SUNDARAM .K.S
PG Student
PRIST Deemed to be University, Thanjavur
Abstract—This paper describes the controller design for a DFIG
based wind energy generation system using the static output
feedback technique through the LMI Toolbox. The features of
the DFIG, its converters and their controllers are discussed. The
lower order nominal representation of DFIG is obtained using
numerical differentiation of the SIMULINK model which is
subsequently used for PID controller design. The obtained results
are compared with existing methods for performance
enhancement of the DFIG and wind energy conversion systems.
Keywords-DFIG; Wind turbine; MATLAB SIMULINK models;
PID controller; linear matrix inequalities; SOF technique
Abbreviations: DFIG: Doubly Fed Induction Generator, PI:
Proportional Integral, SOF: Static Output Feedback, LMI: Linear
Matrix Inequality, FACT: Flexible AC Transmission, PFC: Power
Factor Control, VSC: Voltage Source Converter, WEC: Wind Energy
Conversion, PCC: Point of Common Coupling
I. INTRODUCTION
The investigation of progressivelyalternate energy sources,
demand for electric energy is increasing very fast. Among the
obtainable alternate energy sources, wind energy, solar energy
and fuel cells have drawn considerable attention. Additional, all
of these alternate energy sources are also of renewable nature.
Among the mentioned alternate energy sources, wind power
generation systems have been the most cost competitive
alternative. Since the direction and speed of winds may vary
from location to location as well as from time to time, the
variable speed wind turbine technology offers inherent
advantages over the fixed speed one [1]. The doubly fed
induction generator (DFIG) is used in cycle with the wind
turbine to produce electric energy. The DFIG through the use
of the two back to back converters, rotor side and grid side
converters, is able to deal with a wide variation of wind speeds
by injecting a compensating variable frequency current
component in the rotor circuit. This facilitates both super and
sub synchronous operations of DFIG. It is well known that the
induction machine is widely used in industrial application due
to its low cost, simplicity of construction and low maintenance
cost. Such type of machines can be used for electric generation
where the speed of the prime mover is constant i.e. just above
the synchronous speed. However, it is a fact that the wind
speed varies drastically depending upon the environmental
conditions and time of operation. Thus, there is a large margin
of speed variation. Such large margins of speed variation
makes wound rotor induction machines suitable for generation
of wind energy [2]. In addition to its large speed variation, the
wound rotor induction machine offers additional advantage of
bidirectional transfer of the rotor power which depends on the
rotor speed and field speed [3]. The DFIG is essentially a
wound rotor induction machine capable of operating in super
synchronous as well as sub synchronous mode. The advantages
of DFIG over the fixed speed induction generators are
improved power quality, reduced mechanical stress and
fluctuation and advanced energy capture [4].
The operations of DFIG connected to the grid are facilitated
with the help of rotor side and grid side converter. It is the
responsibility of the inverter connected to the rotor side to
provide the necessary complementary frequency to maintain
the stator frequency at a constant level, despite fluctuations in
the mechanical power. The control of DFIG poses a twofold
problem to compensate the speed fluctuations as well as
reactive power. The stability and performance of the overall
setup is to be maintained in the face of model uncertainties,
external noise, variation of the internal machine parameters and
speed. The problem of control becomes more involved in case
of unbalanced grid connected operations. It is well known that
unbalanced operations lead to the flow of negative sequence
currents which in turn may lead to localized heating as well as
pulsations in the electric torque. In critical applications, DFIGs
must be disconnected from the grid when the voltage
imbalance is more than 6% [5]. It was reported that even within
this margin, torque pulsations can be reduced by injecting a
compensation current in the DFIG rotor, but with their
Page 2 of 8
European Journal of Business &
Social Sciences
Available at https://ejbss.org/
ISSN: 2235-767X
Volume 07 Issue 04
April 2019
Available online:https://ejbss.org/ P a g e | 1462
(a)
(b)
technique the torque pulsations could not be completely
eliminated. The design of control strategy for the DFIG
requires its simplified models which can be integrated with
flexible AC transmission system (FACTs) based grid models.
Such models can also be used for first hand analysis of the
overall system. Ekanayake et al. in [6] presented a comparative
study of the simplified models, wherein the authors have
compared the fifth and third order model of DFIG followed by
the study under faulted conditions. Z. Wang and Y. Sun in [7]
presented the magnitude and frequency control of grid
connected DFIG based on synchronized model for wind power
generation. In this paper the numerical differentiation based
incremental model around the nominal operating point of DFIG
is used. The usefulness of such models can be verified from the
results presented. It is worth to mention here that the typical
ratings of wind turbines are between 800kW to 3 MW, whereas
wind farms range from 1 MW to 200MW [8].
II. AN OVERVIEW OF WIND TURBINES
The wind turbine is a machine that converts kinetic energy
from the wind into mechanical energy. If the mechanical
energy is used to produce electrical energy, the device may be
called a wind generator or wind changer. If the mechanical
energy is used to drive machinery, for example to grinding
grain or pumping water, the device is called a windmill or wind
pump. Today's wind turbines are manufactured in a range of
vertical and horizontal axis types. The nominal turbines are
used for applications such as battery charging or auxiliary
power on seafaring boats; whereas large grid connected arrays
of turbines are becoming an increasingly large source of
commercial electric power. The basic principle of wind turbine
is as follows, the aerodynamic power is converted into
mechanical power and then electricalpower.
A. Basic Perception of DFIG Based Wind Turbine:
The mechanical power which is produced by the wind
turbine is proportional to the cube of the wind speed i.e. Pm
v
3
. Where Pm is the mechanical power of the wind and v is
the velocity of the wind speed. The maximum power which is
obtained by the rotationalspeed of the wind turbine defers from
different wind speeds. Hence the operation of variable speed is
necessary to maximize the energy. The variable speed turbines
are fed to the grid via a power factor control (PFC) which
decouples the rotational speed of the wind turbine fromthe grid
frequency, to enable variable speed operation. There are two
basic concepts that exist for variable speed turbines. The first
concepts are electric generator with a converter connected with
in the stator windings and the grid network which is shown in
Figure 1(a). For the rated power of the wind turbine the
converter is to be designed. The generator is a synchronous
machine which is mostly a permanent magnet. For the direct
drive concepts a wind turbine with a DFIG has a converter
connected to the rotor windings of the wound rotor induction
machine, which is shown in Figure 1(b). This type of generator
can be defined for a fraction (~30%) of the rated power. But
the system ensures competent power conversion appropriate to
variable rotor speed, which adjusts automaticallyin accordance
with prevailing wind speeds [9]. The speed changing is
possible by the directionally dependent transfer of slip power,
which changes as follows. In the sub synchronous operating
mode, the stator of the DFIG supplies power to the grid. In the
super synchronous operating mode, both stator output power
and the rotor slip power are fed into the grid. A variable speed
wind turbine with full size converter and doubly fed induction
generator is shown in Figure 1.
However, the converter has to be intended for the rated
power of the turbine. This problem can be taken care of by
using the DFIG, which has a converter connected to the rotor
winding of the wound rotor induction machine (Figure 1(b)).
Rated power has been reduced to (25% - 35%) in the case of
DFIG. The main components of the wind turbine are given as
follows.
Fig. 1. Variable speed wind turbines (a) with full-size converter
(b) with doubly-fed induction generator
1) Drive Train along with Aerodynamics:
The drive train has turbine, gear box, shafts and other
mechanical components of wind turbine; a multi mass (in
general two mass) model to be used for dynamic studies of
wind turbines through DFIG [10]. A simplified aerodynamic
model is sufficient, when the speed and pitch angle changes on
the aerodynamic power during the grid faults. For stability
investigation, the drive train system have to be approximated
by the at least a two mass spring as well as damper model
while the system response to heavy disturbance [11]. There is a
flexible shaft during the turbine and generator masses are
associated.
2) Pitch Angle Control System:
The “Pitch Control” is a technique so as to mechanically
adjust the blade pitch angle to change the curve of the power
coefficient of the turbine [12]. PI control is used to realize the
pitch angle, in servo mechanism model with time control Tservo
accounts for the practical response in the pitch angle control
system. For the duration of the grid faults how quick the
aerodynamic power can be reduced in order to stop more
speeding is decided by the rate of change limit.
B. Modeling of Wind Turbine:
In this section wind turbine model is discussed for optimal
operations of the wind turbine at different wind speeds [13]. It
must be operated at its maximum power coefficient
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European Journal of Business &
Social Sciences
Available at https://ejbss.org/
ISSN: 2235-767X
Volume 07 Issue 04
April 2019
Available online:https://ejbss.org/ P a g e | 1463
6
(CPoptimum=0.3-0.5) i.e. at a constant tip speed ratio, intended for
operation approximately its maximum power coefficient. J
dr
dt (Tm Tem )
The aerodynamic power generated by wind turbine is given as
follows.
At steady state, the mechanical torque balances the
electromagnetic torque substitute on the machine and hence the
relations are as follows.
P
aero
=0.5 A C (, )v
3
c
c 5
(1) Tm Tem and Pm (Ps Pr ) it follows that:
Cp ( , ) =c
1
(
2
c 3 . c 4
).e
i c .
(2) Pr (Pm Ps ) = Tmr Tems sPS
1
1
i
0 .35 , and the coefficients where s (s r ) / s is defined as the slip of the
i 0 .08 .
3 1
generator in per unit and J is inertia constant.
c 1 =0.5176, c 2 =116, c 3 =0.4, c 4 =5, c 5 =21, c 6 =0.0068
And
T .R
v
A=Swept area of the blades (=πR2
),
Tip ratio speed,
T Rotational speed of the rotor, = Pitch angle, R= Radius of the area covered through the blades.
The speed of a wind turbine determines the conversion efficiency from wind energy to mechanical energy, for a given wind
velocity, blades geometry, andturbine
III. AN OVERVIEW OF THE DFIG OPERATING PRINCIPLE
The overview and operating principle of DFIG discussed in this section is also mentioned in [14]. The basic diagram of DFIG
with converters is shown in Figure 2. The AC/DC/AC converter is divided into two components: The rotor side converter (Crotor)
as well as the grid side converter (Cgrid). Crotor and Cgrid are voltage sourced converters with a common DC link via a capacitor. The
grid converter uses a coupling inductor L to connect to the grid. The three phase rotor winding is connected to the Crotor through slip
rings as well asbrushes, while the three phase stator winding is directly connected to the grid.
The power captured by the wind turbines is converted into electrical power by the induction generator and it istransmitted to the
grid through both the stator as well as rotor windings. The control system generates the control signals to control the active,
reactive power as well as currents, the injected frequency compensation to the rotor windings and lastly the DC voltage control of
the common coupling link capacitor. Rotor side converter operates as an inverter and stator side converter operates as rectifier.
When rotor moves below the synchronous speed in generating slip power is supplied to the rotor.
The mechanical power as well as the stator electric power output is given as below VOLTAGE SOURCE CONVERTER
CONTROLLER
A voltage source converter (VSC) is used in DFIG to control the rotor and grid side converter as well as DC link voltage
also. The general schematic diagram of a DFIG and Wind Turbine along with VSC Converters with a common DC link is
shown in Figure 3. The advantage of VSC based converter is that it uses very small power converter on the rotor side. The rotor
side converter supplies the compensated current for frequency compensation. Wide variation in the wind speed is taken care of
by gear train with adjustable gear ratio. VSC control circuitry provides the necessary controlsignals for both rotor side as well as
grid side converters. Further, the control strategy maintains a constant DC voltage of the common DC link.
Fig. 2. Basic diagram of Doubly Fed Induction generator with converter
p
