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IJRREST
INTERNATIONAL JOURNAL OF RESEARCH REVIEW IN ENGINEERING SCIENCE & TECHNOLOGY
(ISSN 2278–6643)
VOLUME-4, ISSUE-3, November - 2015
IJRREST, ijrrest.org 67 | P a g e
STUDY OF DYNAMIC PERFORMANCE OF GRID CONNECTED DOUBLY FED
INDUCTION GENERATOR 1Rupak Sharma,
2Dr. Amit Manocha
1Department of Electrical Engineering, GIMT, Kanipla, Haryana, India
2Professor, Department of Electrical Engineering, GIMT, Kanipla, Haryana, India
Abstract- The work is carried out with the objective to study the operational aspects of grid connected DFIG and simulate the dynamic
performance of grid connected DFIG. The various aspects of aerodynamic conversion using wind turbine, dynamic model, converter
model has been studied. The dynamic model and control scheme of grid connected DFIG has been realized under simulink environment.
The stator flux oriented (SFO) and stator voltage oriented (SVO) control scheme are implemented on the rotor and grid side converter
respectively. The DFIG is operated in speed control mode. The modeling and dynamic performance of the wind driven DFIG system is
studied for change in wind speed and change in reference speed. The doubly-fed induction generator (DFIG) is widely used in variable
speed wind energy conversion systems (WECS). This paper presents a review on study of dynamic performance of grid connected
doubly fed induction generator. The operation of DFIG based on both slip ring and brushless arrangement has been discussed. The grid
integration of DFIG and its influence on system stability, system reliability, power quality and power transmission is also reviewed.
Keywords—
Doubly-fed induction generator, DFIG, wind energy conversion system, WECS, MPPT, grid operation.
1. INTRODUCTION
Wind is a renewable form of energy, as most of the
renewable sources of energy come from the sun „directly‟ or
„indirectly‟ wind comes under „indirect solar energy. About
1 to 2 per cent of the energy coming from the sun is
converted into wind energy. Wind energy is energy
collected from motion caused by heavy winds. Air in motion
arises from a pressure gradient. On a global basis one
primary forcing function causing surface winds from the
poles toward the equator is convective circulation. Solar
radiation heat the air near the equator, and this low density
heated air is buoyed up. At the surface is displaced by more
dense higher pressure air flowing from the poles. In the
upper atmosphere near the equator the air thus tends to flow
back towards the poles and away from the equator. So it is
clear that wind is basically caused by the solar energy
irradiating the earth.
Now this wind energy is collected in turbines with
propellers that spin when the wind blows and turn the
motion of the propeller into energy that can be used in the
electrical grid. Wind energy is a clean, renewable
energy source that is abundant in windy areas. Large wind
farms are often located outside of cities, supplying power
for electrical grids within the city [1].Wind turbines come in
a variety of sizes that can be used to supply power to
individual buildings or feed electricity into a grid system.
They must be located above nearby buildings and trees to
work effectively for a home or building. Considerations to
be taken when installing a wind turbine include location,
average wind speed, the height of the surrounding buildings
and trees, and the building‟s connection to the electrical
grid.
The present work should be carried out to study the
dynamics of grid connected doubly fed induction generator
by doing mathematical modelling of components and
Further, the performance of doubly fed induction generator
is observed by changing various system parameters like base
wind speed, line length and grid voltage. The complete
study should be done in matlab & simulink environment.
The stator is directly connected to the AC mains, while the
wound rotor is fed from the Power Electronics Converter via
slip rings to allow DIFG to operate at a variety of speeds in
response to changing wind speed. Indeed, the basic concept
is to interpose a frequency converter between the variable
frequency induction generator and fixed frequency grid.
The DC capacitor linking stator- and rotor-side converters
allows the storage of power from induction generator for
further generation. To achieve full control of grid current,
the DC-link voltage must be boosted to a level higher than
the amplitude of grid line-to-line voltage. The slip power
can flow in both directions, i.e. to the rotor from the supply
and from supply to the rotor and hence the speed of the
machine can be controlled from either rotor- or stator-side
converter in both super and sub-synchronous speed ranges.
As a result, the machine can be controlled as a generator or a
motor in both super and sub-synchronous operating modes
realizing four operating modes. Below the synchronous
speed in the motoring mode and above the synchronous
speed in the generating mode, rotor-side converter operates
as a rectifier and stator-side converter as an inverter, where
slip power is returned to the stator. Below the synchronous
speed in the generating mode and above the synchronous
speed in the motoring mode, rotor-side converter operates as
an inverter and stator side converter as a rectifier, where slip
power is supplied to the rotor. At the synchronous speed,
IJRREST
INTERNATIONAL JOURNAL OF RESEARCH REVIEW IN ENGINEERING SCIENCE & TECHNOLOGY
(ISSN 2278–6643)
VOLUME-4, ISSUE-3, November - 2015
IJRREST, ijrrest.org 68 | P a g e
slip power is taken from supply to excite the rotor windings
and in this case machine behaves as a synchronous machine.
A schematic of power flow in DFIG wind energy conversion
system is shown in fig. 1 below.
Fig. 1 Power flow in DFIG WECS
2. MATHEMATICAL MODELING OF INDUCTION
GENERATOR
The mechanical power and the stator electric power output
are computed as
Pm = Tem* ωm
(1)
*s em sP T
(2)
For a loss less generator the mechanical equation is:
rm em
dJ T T
dx
(3)
In steady-state at fixed speed for a loss less generator
m emT T and m s rP P P
And it follows that:
r m s m r em s sP P P T T sP
Where
r s
s
S
„S‟ is defined as the slip of the generator. Generally the
absolute value of slip is much lower than 1 and,
consequently, Pr is only a fraction of Ps. Since Tm is positive
for power generation and since ωs is positive and constant
for a constant frequency grid voltage, the sign of Pr is a
function of the slip sign. Pr is positive for negative slip
(speed greater than synchronous speed) and it is negative for
positive slip (speed lower than synchronous speed).
For super synchronous speed operation, Pr is transmitted to
DC bus capacitor and tends to rise the DC voltage. For sub-
synchronous speed operation, Pr is taken out of DC bus
capacitor and tends to decrease the DC voltage. Cgrid is used
to generate or absorb the power Pgc in order to keep the DC
voltage constant. In steady-state for a lossless AC/DC/AC
converter Pgc is equal to Pr and the speed of the wind turbine
is determined by the power Pr absorbed or generated by
Crotor.
The phase-sequence of the AC voltage generated by Crotor 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. Crotor and Cgrid have the capability for generating or
absorbing reactive power and could be used to control the
reactive power or the voltage at the grid terminals.
2.1 Dynamic model of Induction Machine
There are two commonly used dynamic models for the
induction generator. One is based on space vector theory
and the other is the d-q axis model derived from the space
vector model. The space vector model features compact
mathematical expressions and a single equivalent circuit but
requires complex (real and imaginary part) variables,
whereas the d-q frame model is composed of two equivalent
circuits, one for each axis.
These models are closely related to each other and are
equally valid for the analysis of transient and steady-state
performance of the induction generator. But here only dq-
frame model of induction machine is discussed.
2.1.1 D-Q model of induction machine
The Asynchronous Machine block operates in either
generator or motor mode. The mode of operation is dictated
by the sign of the mechanical torque:
When Tm is positive, the machine acts as a motor.
When Tm is negative, the machine acts as a
generator.
The electrical part of the machine is represented by a fourth-
order state-space model and the mechanical part by a
second-order system. All electrical variables and parameters
are referred to the stator. This is indicated by the prime signs
in the machine equations given below. All stator and rotor
quantities are in the arbitrary two-axis reference frame (d-q
frame) as shown in Fig. 2(a) and Fig. 2(b).
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Fig.2a Equivalent Representation of D-Q model of
induction machine.(q-axis) [34]
Fig.2b Equivalent Representation of D-Q model of
induction machine.(d-axis) [34]
(a) Voltage equations:
Stator Voltage Equations:
qs s qs qs dsV R i p (4)
ds s ds ds s qsV R i p
(5)
Rotor Voltage Equations:
qr r qr qr r drV R i p (6)
dr r dr dr r qrV R i p
(7)
(b) Flux Linkage Equations:
Stator Flux Equations:
qs ls m qs m qrL L i L i (8)
ds ls m ds m drL L i L i (9)
Rotor Flux Equations:
qr lr m qr m qsL L i L i (10)
dr ls m dr m dsL L i L i (11)
(c) Power and Torque Equations:
All the equations above are induction motor equations.
When the induction motor operates as a generator, current
direction will be opposite. Assuming negligible power
losses in stator and rotor resistances, the active and reactive
power outputs from stator and rotor side are given as:
3/ 2 s ds ds qs sP V i V i (12)
3 / 2 s qs ds ds qsQ V i V i (13)
3 / 2 r dr dr qr rP V i V i (14)
3 / 2 s qr dr dr qrQ V i V i (15)
The total active and reactive power generated by DFIG is:
Total s rP P P (16)
Total s rQ Q Q (17)
If PTotal and QTotal is positive, DFIG is supplying power to
the power grid, else it is drawing power from the grid. Total
P Total Q. In the induction machine, the electromagnetic
torque is developed by the interaction of air-gap flux and the
rotor magneto-motive force (MMF).
At synchronous speed, the rotor cannot see the moving
magnetic field; as a result, there is no question of induced
EMF as well as the rotor current, so the torque becomes
zero. But at any speed other than synchronous speed, the
machine will experience torque. That is true in case of
motor, where as in case of wind turbine generator;
electromechanical torque is provided by means of prime
mover which is wind turbine in DFIG-based WECS. The
rotor speed dynamics of the DFIG is given as:
( )2
r m e r
d PT T F
dt J
(18)
where P is the number of poles of the machine, is friction
coefficient, J is inertia of the rotor, Tm is the mechanical
torque generated by wind turbine, and Te is the
electromagnetic torque generated by the machine which can
be written in terms of flux linkages and currents as follows:
3 / 2 e qs ds ds qsT i i (19)
Where, negative (positive) values of Te means DFIG works
as a generator (motor).
3. SIMULATION ANALYSIS OF GRID CONNECTED DFIG
The performance is analyzed for the wind speed of 9 m/s
and 12 m/s at a constant rotor speed of ωref = 1.05pu.
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Fig. 3 Model of Grid Connected DFIG
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4. E 4. EFF ECT OF WIND SPEED ON DFIG PERFORMANCE
4. EFFECT OF WIND SPEED ON DFIG PERFORMANCE
The performance for ωref = 1.05 pu for different wind
speeds as 9 m/s and 12 m/s are shown in following
result analysis.
4.1 Effect of Wind Speed on Turbine torque at 9 m/s
and 12 m/s is shown below in fig 4 and 5 respectively
Fig. 4 Turbine torque at 9 m/s
Fig. 5 Turbine torque at 12 m/s
As shown in Fig.4 & 5 with the increase in wind speeds
the torque generated by the wind turbine increases. The
initial transients of about 0.1 sec duration are also
resulted.
4.2 Effect of Wind Speed on electromagnetic torque at
9 m/s and 12 m/s shown in fig.6 & fig.7 respectively.
Fig. 6 Electromagnetic torque at 9 m/s
Fig. 7 Electromagnetic torque at 12 m/s
As shown in Fig.6 & 7 after the initial transients of 0.15
sec, the steady state value is violated, electromagnetic
torque increases with increase in wind speed as shown
in the above figures. the steady state value of 0.27 and
0.6 N/m is resulted for wind speed of 9 m/s and 12 m/s
respectively.
4.3 Effect of Wind Speed on the active power
generated by DFIG
As shown in Fig. 8 & 9 the active power generated by
the DFIG also increases with the increase in wind
speed.
Fig 8 Active power at 9 m/s
Fig. 9 Active power at 12 m/s
In line with the observation of Fig.8 and 9, initially due
to operation in the motoring mode and settles to
negative value due to operation in generating mode.
Increase in active power with increase in speed takes
place up to a certain speed of wind, after that the
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
-0.05
0
Time
Tu
rb
ine
To
rq
ue
Tm
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
Time
Tu
rb
ine
To
rq
ue
Tm
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5-5
0
5
10
15
20
Time
Ele
ctr
om
ag
ne
tic T
orq
ue
(T
em
)
Tem
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
-5
0
5
10
15
20
Time
Ele
ctr
om
ag
ne
tic to
rqu
e(T
em
)
Tem
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
Time
Active
Po
we
r
Active Power
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5-1
-0.5
0
0.5
1
Time
Active
Po
we
r
Active Powerdata1
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generated power will saturate and will not increase
further. In case the wind speed exceeds the safe limit,
the turbine will enter into parking mode and no
generation of power takes place.
4.4 Effect of Wind Speed on the reactive power
The nature of reactive power for different speeds is
depicted in Fig.10 & 11. The reactive power in the
system increases in magnitude and frequency as the
wind speed increases. As observed from Fig.10,
although the frequency and magnitude of reactive
power is changing, the pattern is symmetric to „zero‟
value. Thus average will be „zero‟.
Fig. 10 Reactive power steady state at 9 m/s
Fig. 11 Reactive Power steady state at 12 m/s
4.5 Rotor Speed, stator voltage and dc link voltage
Fig. 12 Rotor speed
Fig. 13 Stator voltage
Fig. 14 DC link voltage
The rotor speed, stator voltage and DC-link voltage is
illustrated in Fig.12, 13 and 14 respectively. It is
observed that, these parameters remain unchanged due
to change in wind speed. This is because the system is
used in speed control mode.
4.6 Effect of Wind Speed on 3- phase stator current
Transient State
The effect of different speeds on 3-phase stator currents
is shown in Fig 15 to 18 for both initial transient and
after attaining steady state. The steady state value of
stator current is increased from 0.6 to 0.9. It is observed
that with the increase in speed the current at the stator
terminal increases. On observing the effect of different
speeds on stator current it is concluded that with the
increase in speed the current at the stator terminal
increases. This is in agreement with the increase in
electromagnetic torque and active power as observed in
pervious results.
Fig. 15 Isabc at 9 m/s
2 2.5 3 3.5 4 4.5 5-0.01
-0.008
-0.006
-0.004
-0.002
0
0.002
0.004
0.006
0.008
0.01
Time
Re
active
Po
we
r
reactive Power
2 2.5 3 3.5 4 4.5 5-0.01
-0.008
-0.006
-0.004
-0.002
0
0.002
0.004
0.006
0.008
0.01
Time
Re
active
Po
we
r
reactive power
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.5
1
1.5
Time
Ro
tor S
pe
ed
Rotor speed
0 0.05 0.1 0.15 0.2 0.25
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Time
Vsa
bc
Vsa
Vsb
Vsc
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 51198.4
1198.6
1198.8
1199
1199.2
1199.4
1199.6
1199.8
1200
1200.2
Time
DC
Lin
k V
olta
ge
DC link v/g
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-20
-15
-10
-5
0
5
10
15
20
Time
Isb
c
Isa
Isb
Isc
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Fig. 16 Isabc at 12m/s Steady state
Fig. 17 Isabc at 9 m/s
Fig. 18 Isabc at 12m/s
4.7 Effect of Wind Speed on 3- phase rotor current
Transient state
Fig. 19 Irabc at 9 m/s
Fig. 20 Irabc at 12 m/s
4.8 Steady state
Fig 21 Irabc at 9 m/s
Fig. 22 Irabc at 12 m/s
The effect of wind speed on 3-phase rotor currents is
shown in Fig.19 to 22. It is evident that the magnitude
and frequency of rotor current increases with increase in
wind speed. It is similar to the pattern of reactive power
shown in previous results. The rotor current at steady
state vary from 0.2 to 0.5 at 9 and 12 m/s wind speed,
respectively.
4.9 Influence of Wind Speed on rotor and stator
current in terms of d-q components
The influence of wind speed on the d-q components of
rotor current and stator current is shown in results. The
magnitude of rotor current (Idr,Iqr) and stator currents
(Ids,Iqs) are summarized in Table 1 and Table 2
respectively. No significant change in transients is
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-20
-15
-10
-5
0
5
10
15
20
25
Time
Isa
bc
Isa
Isb
Isc
4.75 4.8 4.85 4.9 4.95 5-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Time
Isa
bc
Isa
Isb
Isc
4.75 4.8 4.85 4.9 4.95 5-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Time
Isa
bc
Isa
Isb
Isc
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
-15
-10
-5
0
5
10
15
20
Time
Irabc
Ira
Irb
Irc
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-20
-15
-10
-5
0
5
10
15
20
Time
Ira
bc
Ira
Irb
Irc
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observed.
Table 1- Summary of rotor current in d-q terms
Wind speed
(m/s)
Magnitude
of Iqr (pu)
Magnitude of Idr
(pu)
9 0.21 0.003
12 0.54 0.006
Table 2- Summary of stator current in d-q terms
Wind speed
(m/s)
Magnitude of Ids
(pu)
Magnitude of
Iqs (pu)
9 -0.78 -0.11
12 -0.78 -0.22
It is observed, from Table 1, that „Idr‟ is around to zero
which control the reactive power and maintain it around
„zero‟ while the „Iqr‟control the active power. On steady
state, the Ids assume the same value for all two wind
speeds. Since on grid side „Ids‟ control the DC link
voltage and maintain it so its magnitude remains the
same, while „Iqs‟ control the reactive power of the grid,
so its value vary around zero, to maintain the reactive
power.
5. CONCLUSION
During this work the operational aspects of grid
connected DFIG has been studied and the dynamic
performance of grid connected DFIG has been
simulated. The dynamic model and control scheme has
been realized under simulink environment. The stator
flux oriented and stator voltage oriented control
schemes are implemented on the rotor and grid side
converter respectively while the DFIG is operated in
speed control mode. The performance for change in
wind speed is analyzed. The following conclusions are
drawn-
For the given reference speed, the performance
characteristics is affected by wind speed. The higher
wind speed results in higher amount of torque and
power.
The rotor speed under steady state follows the
reference speed irrespective to the varying wind
speed.
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VOLUME-4, ISSUE-3, November - 2015
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