Journal of Engineering, Computers & Applied Sciences (JEC&AS) ISSN No: 2319-5606 Volume 2, No.8, August 2013 _________________________________________________________________________________ www.borjournals.com Blue Ocean Research Journals 45 Modeling and Simulation of Doubly Fed Induction Generator Coupled With Wind Turbine-An Overview Ankit Gupta, M.Tech Student, Alternate Hydro Energy Centre, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand (India) S.N. Singh, Senior Scientific Officer, Alternate Hydro Energy Centre, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand (India) Dheeraj K. Khatod, Assistant Professor, Alternate Hydro Energy Centre, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand (India) ABSTRACT This paper gives an overview of Modeling and simulation of Doubly Fed Induction generator (DFIG) coupled with wind turbine. Latest researches and developments which have been published in imminent journals through rigorous review are overviewed in this paper. Because of the advantages of the DFIG over other generators it is being used for most of the wind applications. Various researches have been done in modelling and simulation field of DFIG coupled with wind turbine. This paper summarises the researches in the area of study of DFIG, steady state and transient analysis, its modelling, simulation, reactive power control strategies and performance analysis of DFIG coupled with wind turbine. The response of DFIG wind turbine system to grid disturbances, which is simulated and verified experimentally, is overviewed here. The behaviour of DFIG wind turbine system for different faults is also overviewed in this paper. Keywords: Doubly-fed induction generator, wind turbine, wind energy, wind energy conversion system, voltage sag. Introduction The wind energy industry is booming due to its capability of producing ecologically sustainable energy. China has the most installed wind energy capacity, followed by the United States, Germany, Spain and India. Wind energy is one of the fastest growing industries at present and it will continue to grow worldwide, as many countries have plans for future development. According to the Centre for Wind Energy Technology, Government of India- Growing concern for the environmental degradation has led to the world's interest in renewable energy resources. Wind is commercially and operationally the most viable renewable energy resource and accordingly, emerging as one of the largest source in terms of the renewable energy sector. The Indian wind energy sector has an installed capacity of 18.551 GW (up to 31.02.2013) [22]. In terms of wind power installed capacity, India is ranked 5 th in the World [23]. Today India is a major player in the global wind energy market. The potential is far from exhausted. Indian Wind Energy Association has estimated that with the current level of technology, the „on-shore‟ potential for utilization of wind energy for electricity generation is of the order of 102 GW [24]. Wind turbines 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. 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 these large wind turbines. Doubly-fed induction generators are commonly used by the wind turbine industry for larger wind turbines [21].
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Journal of Engineering, Computers & Applied Sciences (JEC&AS) ISSN No: 2319-5606 Volume 2, No.8, August 2013 _________________________________________________________________________________
www.borjournals.com Blue Ocean Research Journals 45
Modeling and Simulation of Doubly Fed Induction
Generator Coupled With Wind Turbine-An Overview
Ankit Gupta, M.Tech Student, Alternate Hydro Energy Centre, Indian Institute of Technology Roorkee, Roorkee,
Uttarakhand (India)
S.N. Singh, Senior Scientific Officer, Alternate Hydro Energy Centre, Indian Institute of Technology Roorkee,
Roorkee, Uttarakhand (India)
Dheeraj K. Khatod, Assistant Professor, Alternate Hydro Energy Centre, Indian Institute of Technology Roorkee,
Roorkee, Uttarakhand (India)
ABSTRACT This paper gives an overview of Modeling and simulation of Doubly Fed Induction generator (DFIG) coupled with
wind turbine. Latest researches and developments which have been published in imminent journals through
rigorous review are overviewed in this paper. Because of the advantages of the DFIG over other generators it is
being used for most of the wind applications. Various researches have been done in modelling and simulation field
of DFIG coupled with wind turbine. This paper summarises the researches in the area of study of DFIG, steady
state and transient analysis, its modelling, simulation, reactive power control strategies and performance analysis
of DFIG coupled with wind turbine. The response of DFIG wind turbine system to grid disturbances, which is
simulated and verified experimentally, is overviewed here. The behaviour of DFIG wind turbine system for different
faults is also overviewed in this paper.
Keywords: Doubly-fed induction generator, wind turbine, wind energy, wind energy conversion system, voltage
sag.
Introduction The wind energy industry is booming due to its
capability of producing ecologically sustainable
energy. China has the most installed wind energy
capacity, followed by the United States, Germany,
Spain and India. Wind energy is one of the fastest
growing industries at present and it will continue to
grow worldwide, as many countries have plans for
future development.
According to the Centre for Wind Energy
Technology, Government of India- Growing concern
for the environmental degradation has led to the
world's interest in renewable energy resources. Wind
is commercially and operationally the most viable
renewable energy resource and accordingly,
emerging as one of the largest source in terms of the
renewable energy sector.
The Indian wind energy sector has an installed
capacity of 18.551 GW (up to 31.02.2013) [22]. In
terms of wind power installed capacity, India is
ranked 5th in the World [23]. Today India is a major
player in the global wind energy market. The
potential is far from exhausted. Indian Wind Energy
Association has estimated that with the current level
of technology, the „on-shore‟ potential for utilization
of wind energy for electricity generation is of the
order of 102 GW [24].
Wind turbines 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. 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 these large wind turbines. Doubly-fed
induction generators are commonly used by the wind
turbine industry for larger wind turbines [21].
Journal of Engineering, Computers & Applied Sciences (JEC&AS) ISSN No: 2319-5606 Volume 2, No.8, August 2013 _________________________________________________________________________________
www.borjournals.com Blue Ocean Research Journals 46
Litrature Review Some of the important literature related to DFIG
modeling, simulation and analysis is presented in this
section. Wu et al. [1] presented a detailed model of
WT with DFIG and its associated controllers based
on which the small signal stability model is derived
which shows that the DFIG control can significantly
improve the stability of WT system. They performed
the dynamic simulations to illustrate the control
performance. The oscillation after the disturbances
was damped out very quickly. The peak DC link
voltage was reduced noticeably, which is very
beneficial to the operation of the fed back converters.
Fig.1, 2 and 3 shows the terminal voltage, active
power output and DC-link voltage results achieved
by them. Petersson et al. [7] simulated and verified
the DFIG WT system to grid disturbances. A full-
order model with a reduced order model was used for
simulatioon. Response were verified to the
symmetrical and unsymmetrical voltage sags. 80%
voltage sag was handled very well. They measured
the power quality impact by the DFIG WT system
and found that the flicker emission is very low, the
reactive power is close to zero and the current THD
is always less than 5%.
Fig.1. Terminal Voltage [1]
Fig. 2. Active Power output [1]
Fig. 3. DC- Link Voltage [1]
Lima et al. [8] studied a simplified model for DFIG
based WT system. They presented a new dynamic
model for wind turbines, based on DFIG, able of
representing accurately its behaviour during both the
steady state and the transient of the grid voltage. The
accuracy of the performance of the model was tested
under different conditions, by means of
PSCAD/EMTDC simulations. They concluded that
their model can be useful when simulating large scale
wind power applications. Babu and Mohanty [9]
presented the modeling and simulation of wind
turbine driven DFIG which feeds power to the utility
grid. DFIG model used by them was based on the
vectorized dynamic approach and was applicable for
all types of induction generator configurations for
steady state and transient analysis. The power flow
control was obtained by connecting two back to back
PWM converters between rotor and utility grid.
Ostadi et al. [10] studied the DFIG based wind power
system connected to a series-compensated
transmission line. They developed a nonlinear
mathematical model that takes into account dynamics
of DFIG flux observer, phase-locked loop (PLL),
controllers of the power-electronic converter, and
wind turbine. Choudhury et al. [2] simulated DFIG
system with a back-to-back converter at the rotor end
using PI controllers. They analysed the system
performance under steady state and for a sudden
change in grid voltage as the fig.4 shows the
simulation results under voltage sag. The generated
stator voltages and currents, active power supplied to
grid, VAR requirement for the DFIG are observed
and it is concluded that with the implemented vector
control strategy, the DFIG system under simulation
study is suitable under sudden change in grid voltage.
Journal of Engineering, Computers & Applied Sciences (JEC&AS) ISSN No: 2319-5606 Volume 2, No.8, August 2013 _________________________________________________________________________________
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Fig. 4. Simulation result under voltage sag [2]
Masaud and Sen [11] proposed a detailed DFIG
model in Matlab/Simulink and simulated it by vector
control strategy based in stator flux oriented frames
with satisfactorily results. They presented a new
vector control strategy based on the rotor flux
oriented reference frame and compared with the
stator flux oriented vector control. Zhang and Wang
[3] analysed characteristics of every part of control
system of wind turbine by entirely dynamic
mathematical model with good precision for variable-
speed variable-pitch wind turbine. Then they took
step and turbulent wind speed signal and simulated
for typical control strategy in Matlab/Simulink
environment as shown in the fig. 5 and 6. Fig. 5
shows the simulation result for step wind step signal
it a, b and c parts shows the torque change, rotation
speed change and pitch angle change curves of
generator while a, b and c parts of fig. 6 shows the
dynamic torque, dynamic rotation and dynamic
power change curves of generator.
(a) Torque change curve of generator
(b) Rotation speed change curve of generator
(c) Pitch angle change curve
Fig. 5. Simulation result of step wind speed signal [3]
Journal of Engineering, Computers & Applied Sciences (JEC&AS) ISSN No: 2319-5606 Volume 2, No.8, August 2013 _________________________________________________________________________________
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(a) Dynamic torque change curve of generator
(b) Dynamic rotation speed curve of generator
(c) Dynamic power change curve of generator
Fig. 6. Simulation result of turbulent wind speed
signal [3]
Zhang et al. [12] analysed the steady state
characteristics of DFIG in detail and proved from the
analytic expression of DFIG electromagnetic power
that it has a very extensive stability region and
intrinsic stability. It‟s all operating points are stable.
They did the stability assessment of stator-flux-
oriented system and stator-voltage-oriented system
comparably by using small signal stability analysis
scheme by linearizing the DFIG model. Alkandari et
al. [13] did the steady state analysis of DFIG, they
assumed that the machine is excited on the rotor side
by a slip-frequency current injected from an exciter
mounted on the same shaft of the machine. The
resulting magnetic field rotates at the synchronous
speed. Effects of the excitation voltage magnitude
and angle on both the active and reactive power when
the machine runs at constant speed are investigated
and shown that controlling the excitation voltage
magnitude and phase angle controls the mode of the
operation of the machine. Sediki et al. [4] established
the steady state characteristics of a DFIM under unity
power factor operation. Based on the forth
synchronized mathematical model, analytic
determination of the control laws is presented and
illustrated by various figures to understand the effect
of the applied rotor voltage on the speed and the
active power. They included the stator resistance
while other previous works neglected that. The
analytical expressions lead to a very interesting and
easy open loop control of the DFIM without any
sensors. Because of more simplicity it is more
reliable. Fig.7 shows their simulation results for
reference speed and DFIM speed, fig.8 shows the
simulation results of stator active power and reactive
power and fig. 9 shows the simulation results of
stator current components (d and q).
Fig. 7. Simulated reference speed and DFIM speed
[4]
Fig. 8. Simulated stator active and reactive power [4]
Journal of Engineering, Computers & Applied Sciences (JEC&AS) ISSN No: 2319-5606 Volume 2, No.8, August 2013 _________________________________________________________________________________
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Fig.9. Simulated stator current [4]
Lei et al. [14] first reviewed the electrical equations
of the induction machine then by eliminating the flux
linkage variables in these equations, a DFIG model
which is compatible with transient analysis programs
was obtained. Then they simulated the independent
control of torque and reactive power for wind
turbines based on the assumption that the frequency
converter is ideal and simulated as a controllable
voltage source. To test the performance of the
proposed model, wind turbine responses both to a
step increase in wind speed and to a voltage dip
caused by an electrical fault were simulated using
PSS/E and compared with detailed models developed
by others.
This model is computationally efficient and suitable
for large scale power system analysis. However, due
to the assumption adopted, the model cannot be used
to study the internal dynamics of the power
converter. Wei Qiao [15] developed two different
models in PSCAD/EMTDC to represent a wind
turbine equipped with a DFIG. One is a detailed
switching-level (SL) model, in which the variable
frequency converter (VFC) is fully represented by
individual IGBT switches and a dc-link capacitor.
The other is a simplified fundamental-frequency (FF)
model, in which the VFC is represented by two
current-controlled voltage sources which take into
account the dc-link dynamics. He simulated the 3.6
MW DFIG wind turbine using both the models and
concluded that both the models provides same level
of accuracy and FF model should be used in order to
speed up the simulation process. He also concluded
that the two mass model (for shaft system models)
should be used for the study of power system
transient dynamics. Babypriya and Anita [16]
simulated operating characteristics of DFIG using
Matlab. The simulated stator real power
characteristics of the DFIG show that with increase in
the rotor injected voltage, the DFIG real power
characteristics shifts more in to the sub-synchronous
speed range and the pushover power of the DFIG
rises. For both motoring and generating modes, the
DFIG sends additional real power through its rotor to
the grid. The characteristics of rotor power are
mainly influenced by the rotor injected voltage. Rotor
power is normally smaller than the stator power and
the difference between the two depends on the values
of Vd and Vq and slip. It can also be seen that the
DFIG rotor power is capacitive when the DFIG
operates in the generating mode under a sub
synchronous speed and is inductive otherwise.
Tremblay et al. [17] presented the comparison of
three most widespread and well performing control
strategies (vector control, direct torque control and
direct power control) for controlling DFIG in wind
energy conversion system. They implemented it in an
experimental setup based on a digital signal
processor. They concluded that VC strategy imposes
lower instrumentation constraints and has the lowest
THD, the direct method are up to four times faster
than VC in transitory response. They also concluded
that the DTC strategy was outperformed by the other
two strategies. It is acknowledged that (more
complex) variations of DTC, DPC, and VC could
yield different results. Vicatos and Teqopoulos [18]
investigated the overall performance of the DFIG
under synchronous operation. Stator and rotor
currents, active and reactive power as well as
mechanical power and electromagnetic torque are
expressed as a function of slip, the rotor excitation
voltage, the angle and the DFIG parameters.
Variable-speed constant-frequency operation can be
performed by supplying the rotor with a voltage
phasor having frequency equal to the difference
between the actual speed and the synchronous speed.
Active power can be controlled by angle and reactive
power can be controlled by varying the magnitude of
the rotor excitation voltage. Then operation of DFIG
connected to isolated passive load is analysed. They
also concluded that the principle used by them may
also be applied to hydroelectric generators. Vieira et
al. [5] developed a steady state model of DFIG-based
wind turbines and demonstrated its application for
load flow analysis. They assessed the steady state
behaviour of the DFIG, under varying system
conditions. They proposed the method based on
Newton-Raphson algorithm. The results obtained are
directly usable as initial values in a fifth order
dynamic model of the DFIG. Fig. 10 shows the graph
of terminal voltage with respect to time and fig 11
Journal of Engineering, Computers & Applied Sciences (JEC&AS) ISSN No: 2319-5606 Volume 2, No.8, August 2013 _________________________________________________________________________________
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shows the variation of stator active power with
respect to time.
Fig. 10. DFIG Terminal voltage [5]
Fig. 11. DFIG Stator Active Power [5]
Zhang et al. [6] discussed the steady-state operation
characteristics and power relations in detail on the
basis of the equivalent circuit of DFIG. Then they
simulated the DFIG‟s steady state characteristics.
Their paper has laid the foundation for making in
depth study of the DFIG. Fig. 12 shows the
characteristics curve of the rotor voltage and speed
obtained by them and fig. 13 shows the
characteristics curve of active power with respect to
speed obtained in their paper.
Fig. 12. The curve of the rotor voltage and speed [6]
Fig. 13. The curve of the active power and the speed
[6]
Li et al. [19] compared real and reactive power
control for a DFIG based wind turbine using stator-
voltage and stator-flux oriented frames and presented
both DFIG steady-state and transient models in d-q
reference frame. They used steady-state model to
obtain the general relationship between rotor d/q
current and stator real/reactive power references
using stator-flux and stator-voltage oriented frames.
The transient model, together with the analysis based
on DFIG d-q steady state equivalent circuit, was used
to develop and design DFIG controller. They
concluded that it is easier to estimate a stator-voltage
space vector position than a stator-flux space vector
position and controller design using both stator-
voltage and stator-flux oriented frames has equivalent
performance. Islam et al. [20] explored the steady
state characteristics of a DFIG in wind power
generation system using Matlab. Stator and rotor real
and reactive power as well as electromagnetic torque
Journal of Engineering, Computers & Applied Sciences (JEC&AS) ISSN No: 2319-5606 Volume 2, No.8, August 2013 _________________________________________________________________________________
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are analysed as function of slip, the rotor injected
voltage and the angle α. From the simulation results it
is clear that the characteristics of DFIG are affected
by its injected rotor voltage.
Wind Energy Conversion System A wind turbine catches the wind through its rotor
blades and transfers it to the rotor hub. The rotor hub
is attached to a low speed shaft through a gear box.
The high speed shaft drives an electric generator
which converts the mechanical energy to electric
energy and delivers it to the grid. As the wind speed
varies, the power captured, converted and transmitted
to the grid also varies[16]. The output power of the
turbine is given by the following equation.
Pm = Cp (λ, β) wind (1)
Where, Pm is Mechanical output power of the turbine
(W), Cp is performance coefficient of the turbine, ρ
is the air density (kg/m3), A is the turbine swept area
(m2), Vwind is wind speed (m/s), λ is tip speed ratio of
the rotor blade tip speed to wind speed and β is the
blade pitch angle (deg).
A conceptual diagram of wind turbine – DFIG based
system connected with the electric grid is shown in
Fig. 14. The stator of the wound rotor induction
machine is connected to the three-phase grid and the
rotor side is fed via the back-to-back IGBT voltage-
source inverters with a common DC bus. The grid
side converter controls the power flow between the
DC bus and the AC side and allows the system to be
operated in sub synchronous and super synchronous
speed. The general control strategy of a DFIG can be
divided into three different control levels as given
below:
Fig 14: Conceptual Diagram of Wind Turbine – DFIG based system [21]
Control level I regulates the power flow between the
grid and the electrical generator. The rotor side
converter is controlled in such a way that it provides
independent control of the electromechanical torque
of the generator and the stator reactive power.
Journal of Engineering, Computers & Applied Sciences (JEC&AS) ISSN No: 2319-5606 Volume 2, No.8, August 2013 _________________________________________________________________________________
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Control level II is responsible for controlling wind
energy conversion into mechanical energy, that is, the
amount of energy extracted from the wind by the
wind turbine rotor. This control level calculates the
references for control level I.
Control level III is dedicated to the wind turbine–
grid integration. This control level provides the
voltage (Vgrid) & frequency (fgrid) control and
responds to active and reactive power references
from the grid operator [21].
DFIG Model The DFIG model using d-q synchronous reference
frame is being presented here with the equations,
Modeling can be of different type depending upon
the type of study but basically modeling consists of
the mathematical equations of DFIG which can be
achieved from the equivalent circuit of the DFIG.
a. Equivalent circuit of DFIG: The equivalent
circuit of the DFIG, with inclusion of the
magnetizing losses, can be seen in Fig. 15. This
equivalent circuit is valid for one equivalent Y
phase and for steady-state with the jω-method for
calculations.
Fig. 15: Equivalent circuit of the DFIG. [21]
Applying Kirchhoff‟s voltage law to the circuit in
Fig. 1.4 yields
Vs= RsIs+ jω1LsλIs + jω1Lm(Is + Ir+ IRm)……(2)
Vr/s=(Rr/s)Ir+jw1LrλIr+j1Lm(Is+Ir+IRm) ……(3)
0 = RmIRm+ jω1Lm(Is+ Ir+ IRm) ……… (4)
Where, Vs is stator voltage, Rs is stator resistance, Vr
is rotor voltage, Rr is rotor resistance, Is is stator
current, Rm is magnetizing resistance, Ir is rotor
current, Lsλ is stator leakage inductance, IRm is
magnetizing resistance current, Lrλ is rotor leakage
inductance, ω1 is stator frequency, Lm is
magnetizing inductance, s is slip.
The slip,
s = (w1 – wr) / w1 = w2 / w1 …………. (5)
Where, ωr is the rotor speed and ω2 is the slip
frequency. Moreover, if the air-gap flux, stator flux