International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, April 2013)
Control Strategy on Doubly Fed Induction Generator Using
PSIM
Niraj Danidhariya1, Kaumil B Shah2
1
P.G Student, M.E 4th Semester, Sankalchand Patel College of Engineering, Visnagar.
Abstract- This paper deals with doubly fed induction
generator control strategy. This paper includes the results
performed in PSIM-9 software such as Performance
comparison of output voltage under different loading
conditions and output power.
III. BACKGROUND
The mechanical power that can be extracted from a wind
turbine is given by
Pw = 0.5 ϱACpV3
Where ϱ is the air density, A is the area swept by the
turbine blades, Cp is the performance coefficient of the
turbine and v is the wind velocity. A typical performance
coefficient curve is shown in Figure where β is the blade
pitch angle.
Keywords— Doubly Fed Induction Generator (DFIG),
variable speed wind turbine
I. INTRODUCTION
The conventional energy sources are limited and have
less 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
economically viable In India. Many applications of wind
power can be found in a wide power range from a few
kilowatts to several megawatts in small scale off-grid
standalone systems or large scale grid-connected wind
farms. This type of dispersed power generation causes
problems in the electrical connected system. So this
requires accurate modeling, control and selection of
appropriate wind energy conversion system.
Figure 1 Typical performance coefficient vs. tip speed ratio curve
showing the effect of varying the blade pitch angle.
The performance coefficient is dependent on the tip
speed ratio λ is given by
II. DOUBLY FED INDUCTION GENERATOR
Variable speed operation is essential for large wind
turbines in order to optimize the energy capture under
variable speed conditions. Variable speed wind turbine
requires a power electronic interface converter to permit
connection with the grid. The power electronics can be
either partially-rated or fully-rated. A popular interference
method for large wind turbines that is based on a partially
rated interference is the Doubly Fed Induction Generator
(DFIG) System. In the DFIG system, the power electronic
interface controls the rotor currents in order to control the
electrical torque and thus the rotational speed. Because the
power electronics only process the rotor power, which is
typically less than 25% of the overall output power[1], the
DFIG offers the advantages of speed control for a reduction
in cost and power losses. The paper presents DFIG system
simulation in PSIM.
λ
Where t is the rotational speed of the turbine and R is
the turbine radius. It can be seen that λ should be held
constant to harness maximum power from the wind. The
turbine rotational speed must therefore increase as the wind
speed increases. When the wind turbine reaches its
maximum rotational speed however, blade pitch angle
control can be employed to shed the excess wind power.
Increasing the blade pitch angle decreases the optimum Cp
and λ value as shown in Figure 1.
IV. SIMULATION ON DOUBLY FED INDUCTION GENERATOR
A. Simulation on wind turbine
General simulation of wind turbine using PSIM can be
done by using math function block.
214
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, April 2013)
In this simulation the quantities related to turbine is used
in that math function block. Before simulation of DFIG
system, simulation of wind turbine is necessary. Although
wind turbine generators can be interfaced directly with the
power system, the use of a power electronic interface is
preferred since it permits variable speed operation and thus
offers increased power extraction from the wind. Figure for
turbine simulation is shown in figure 2 given below:
Figure 5 PSIM Simulations on Doubly Fed Induction Generator
Here output voltage of DFIG system is measured and
rotor circuit has to give 3-phase external source. As
mentioned that DFIG is capable of supplying power both
the side. In this simulation we came to know that the
voltage supplied from grid side to DFIG through converter
is 5% of the total output. It means that it takes 5% from the
grid and supplies other 95% to the grid and frequency is
also the same like it[2]. Output voltage from DFIG system
is shown in figure 6 given below:
Figure 3 wind turbine simulation
Torque output from wind turbine is constant and it is
shown in figure 4 given below:
Va
400
200
0
-200
-400
Vb
600
400
200
0
-200
-400
Figure 4 output torque of wind turbine
-600
Vc
B. General simulation of DFIG
In doubly fed induction generator both stator and rotor
are able to supply the power, but the direction of active
power flow through the rotor circuit is dependent on the
wind speed and accordingly the generator speed.
Now below the synchronous speed, the active power
flows from the grid to the rotor side and the rotor side
converter (RSC) acts as voltage source inverter while grid
side converter (GSC) acts as a rectifier above the
synchronous speed[7], RSC acts as the rectifier and GSC
acts as the inverter.
So doubly fed induction generator system is superior
then conventional system. Simulation of DFIG is shown
below:
600
400
200
0
-200
-400
-600
0.14
0.16
0.18
0.2
0.22
0.24
0.26
Time (s)
Va
Vb
Vc
400
200
0
-200
-400
0.16
0.18
0.2
0.22
Time (s)
Figure 6 simulation view of output voltage(y axis div. 1
section=200volts)
215
0.24
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, April 2013)
C. Rotor Side Converter circuit of DFIG
Variable speed operation is essential for large wind
turbines in order to optimize the energy capture under
variable wind speed conditions[14]. Variable speed wind
turbines require a power electronic interface converter to
permit connection with the grid. The power electronics can
be either partially-rated or fully-rated.
Although wind turbine generators can be interfaced
directly with the power system, the use of a power
electronic interface is preferred since it permits variable
speed operation and thus offers increased power extraction
from the wind. Variable speed operation also allows wind
gusts to be absorbed in the mechanical inertia of the
turbine, reducing torque pulsations and fluctuations in the
output power. The partially-rated interface processes a
portion of the power output of the turbine and offers
control flexibility over most of the operating range of the
wind turbine[8][9]. A partially-rated interface is typically
used with a DFIG where the power electronic interface
only processes the rotor power. The main advantages of
this scheme are a reduction in the size and cost of the
interface converter and a consequent reduction in overall
converter losses.
In the DFIG system, the power electronic interface
controls the rotor currents in order to control the electrical
torque and thus the rotational speed. Because the power
electronics only process the rotor power, which is typically
less than 25% of the overall output power[10], the DFIG
offers the advantages of speed control for a reduction in
cost and power losses. Rotor side converter circuit using
PSIM is shown in figure 7 given below:
Although wind turbine generators can be interfaced
directly with the power system, the use of a power
electronic interface is preferred since it permits variable
speed operation and thus offers increased power extraction
from the wind[13]. Variable speed operation also allows
wind gusts to be absorbed in the mechanical inertia of the
turbine, reducing torque pulsations and fluctuations in the
output power. The partially-rated interface processes a
portion of the power output of the turbine and offers
control flexibility over most of the operating range of the
wind turbine. A partially-rated interface is typically used
with a DFIG where the power electronic interface only
processes the rotor power. The main advantages of this
scheme are a reduction in the size and cost of the interface
converter and a consequent reduction in overall converter
losses. The purpose of the rotor converter is to control the
generator speed to achieve maximum power from the wind
over a range of wind velocities. The rotor converter control
scheme is based on a multi tiered structure that comprises a
speed, power and current control loop. Speed control is
implemented by controlling the real power reference to the
power control loop[6]. The current controller tracks the
power reference by controlling the rotor currents. A
partially-rated interface is typically used with a DFIG
where the power electronic interface only processes the
rotor power. Rotor side converter circuit is rotor side
converter which is used for control of rotor.
Simulation result for rotor side converter circuit is
shown in figure 8 given below:
Figure 8 Rotor Side Converter circuit output results
(y axis div. 1 section=500 Volts)
V. RELATIONSHIP BETWEEN TIP SPEED RATIO AND POWER
COFFICIENT
The power coefficient (Cp) indicates how efficiently the
conversion of wind power to rotational mechanical power
is performed by the wind turbine.
Figure 7 Rotor side Converter circuit of DFIG
216
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, April 2013)
The Betz limit is the maximum theoretic value reached
by the power coefficient which is 0.59 for three blades
horizontal axis wind turbine used to model the dynamics of
the Cp. The values of C1–C9 presented in Table were
suggested to represent the aerodynamics of modern wind
turbines[3][4],
VI. CLOSED LOOP SIMULATION OF DFIG
Cp ( , ) = C1 (
Optimized values of Cp curve equations is given below
C1
0.73
C2
151
C3
0.58
C4
0.002
C5
2.14
C6
13.2
C7
18.4
C8
0.02
C9
0.003
At lower wind speed, the pitch angle is set to a null
value, because, the maximum power co-efficient is
obtained for this angle. Pitch angle control operates only
when the value for wind speed is greater than the nominal
wind speed. The Cp curves were calculated for different tip
speed ratio (λ) and different blade pitch angle (β˚). For
better visualization, they are shown in Figure 9 given
below:
Figure 9 MATLAB/Simulink Relationships between power &
Wind speed at different blade pitch angle β.
Variable speed operation is essential for large wind
turbines in order to optimize the energy capture under
variable wind speed conditions. Variable speed wind
turbines require a power electronic interface converter to
permit connection with the grid. The power electronics can
be either partially-rated or fully-rated. A popular interface
method for large wind turbines that is based on a partially
rated interface is the doubly-fed induction generator
system. In the DFIG system, the power electronic interface
controls the rotor currents in order to control the electrical
torque and thus the rotational speed. Because the power
electronics only process the rotor power, which is typically
less than 25% of the overall output power, the DFIG offers
the advantages of speed control for a reduction in cost and
power losses[11][12]. The purpose of grid side converter is
to maintain capacitor voltage constant.
The purpose of the grid converter is to control the DC
link capacitor voltage constant to achieve maximum power
from the wind over a range of wind velocities. The grid
converter control scheme is based on a multi tiered
structure that comprises a speed, power and current control
loop.
Simulation is shown in figure given below:
Figure 10 Grid side Converter Circuit
The purpose of the grid converter is to control the DC
link capacitor voltage constant to achieve maximum power
from the wind over a range of wind velocities. The grid
converter control scheme is based on a multi tiered
structure that comprises a speed, power and current control
loop. Result for this circuit is shown in figure given below:
As the velocity increase, tip speed ratio decrease and
power co-efficient must increase but here as velocity starts
form zero tip speed ratio starts from zero tip speed ratio
starts from its max so to obtain the range of graph from tip
speed vs. Cp at different β
217
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Figure 11 DC Link capacitor constant voltage
DFIG characteristics are affected by its injected rotor
voltage. By varying the amplitude and phase angle of the
rotor injected voltage, the DFIG torque speed
characteristics are shifted from the over-synchronous to
sub-synchronous speed range to generate electricity and
also increases the DFIG pushover torque, thereby
improving the stability of operation. 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[5]. The
increase of Vq results in the expansion of the DFIG torque
and real power characteristics for its generating mode, but
at the same time increase the inductive reactive power
demand from the grid. Whereas, the increase of voltage can
not only expand DFIG torque and real power
characteristics for its generating mode but also reduces the
DFIG inductive power demand and may even change it to
capacitive. For both motoring and generating modes, the
DFIG sends additional real power through its rotor to the
grid. Unlike the stator power, the characteristics of rotor
power are mainly influenced by the rotor injected voltage.
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 subsynchronous speed range and the pushover power of the
DFIG rises. DFIG with closed loop is shown in figure
given below:
Figure 12 Doubly Fed Induction Generator Closed loop
Simulation results for doubly fed induction generator are
shown in figure given below:
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REFERENCE
[1]
[2]
[3]
[4]
Figure 13 simulation results of DFIG wind turbine
[5]
So, from above simulation we are able to know function
of DFIG system and also we know the power flow of DFIG
system.
The doubly fed machine is a transformer at standstill.
The transformer-like characteristics are also present when it
is rotating, manifesting itself especially during transients in
the grid. Due to the voltage and current behavior described
above the rotor will either require, or generate, active
power depending on the speed and torque. If the machine is
producing torque and operating as a motor, the rotor will
generate power if the speed is below synchronous speed.
Doubly fed machines are used in applications that require
varying speed of the machine's shaft for a fix power system
frequency. So doubly fed induction generator system is
superior then conventional wind turbine system and has
more advantage. So it is very useful in future.
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VII. CONCLUSION
[13]
After all this simulation doubly fed induction generator
can be controlled by rotor side converter as well as grid
side converter and to maintain output power to the grid we
must have control on that two converter. So that control of
doubly fed induction generator through vector control, P-Q
calculation and also can apply other controlling methods to
doubly fed induction generator to achieve good power
output.
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