B.Vaikundaselvan et al., International Journal of Advanced Engineering Technology
E-ISSN 0976-3945
Research Paper
REACTIVE POWER CONTROL OF DOUBLY FED INDUCTION
GENERATOR USING DIRECT POWER CONTROL
1
B.Vaikundaselvan, 2 M.Kannan
Address for Correspondence
1, 2
Professor, Department of EEE, Kathir College of Engineering, Coimbatore, Tamil Nadu, India
ABSTRACT
Active and reactive power control of DFIG using back to back converter, operation is reviewed and the mathematical model
for real and reactive powers are derived from its equivalent circuit. The control circuit for Grid Side Converter (GSC) and
Rotor Side Converter (RSC) have been prepared and implemented by using DPC scheme. Direct power control technique
based on grid voltage orientation has been proposed. The DPC gets the same control performance as vector control, but has
better robustness and simple structure. This DPC approaches the Instantaneous Reactive Power (IRP) p-q theory, which is
based on the Clarke transform of voltages and currents in three-phase systems into α and β orthogonal coordinates. In this
proposed control, DPC is used to control both active and reactive power under steady state condition. The simulation of a
1hp DFIG based GCWECS with PI controller is carried out using MATLAB/Simulink. The steady state behavior of system
was analyzed for synchronous (157 rad/sec), sub synchronous (135 rad/sec) and super synchronous speed (179 rad/sec). The
reactive power supplied by DFIG for three modes of operations (synchronous, sub synchronous and super synchronous
speeds) are zero, and has been validated by the simulation results. The hardware results also confirmed the reactive power as
zero for all the three conditions.
KEYWORDS— Renewable energy, DFIG, Back to back converter, Direct power control.
I. INTRODUCTION
Owing to rapid decrease in fossil fuels and increase
in global warming, our attention is diverted towards
the importance of locally available natural resources.
The natural resources will provide an alternative
energy source with less cost and also be helpful in
maintaining a pure and healthy atmosphere. Increased
use of renewable energy sources such as wind
energy, bio-gas, solar, and hydro potential has
become essential to adopt a low - cost generating
system, which is feasible for operating in remote
areas. Out of all renewable energy sources, wind
energy seems to be prominent and quite promising
for electric power generation. Wind Energy
Conversion (WEC) has been found to be economic
compared to the cost of fossil fuels which are rising
at a much faster rate.
Therefore, the study of Wind Energy Conversion
System (WECS) has regained importance, as they are
particularly suitable for wind power plants. Quite
recently the developments in wind turbine technology
have been taking place consistently. India possesses a
long coast line of about 7500 km and interestingly it
is estimated that it has a wind energy capacity of
48000 MW at 60 Meters height from the ground.
However, the installed capacity is only about 14000
MW (Indian Wind Energy Association 2011). The
wind turbine installation commenced between the
late fifties and early sixties.
Since then, the numbers of wind turbine installations
have increased in different parts of the country,
mostly due to government encouragement. There is
still a large potential, which is still untapped. The
reasons for lack of entrepreneurs’ interest may be due
to lower conversion efficiencies, higher cost of
renewable energy, alternative energy availability and
removal of government subsidies. The lower
conversion efficiencies enhance the wind energy
extraction cost, making uneconomical. The price for
energy generation with wind turbine is several times
higher than energy obtained from conventional
sources. Hence, research studies are undertaken to
determine ways and means for enhancing the
conversion efficiency.
Wind energy is conversion of kinetic energy (i.e.
energy of motion of the wind) into mechanical
energy that can be utilized to generate electricity. The
wind blows against the blades and they rotate about
Int J Adv Engg Tech/Vol. VII/Issue I/Jan.-March,2016/841-850
the axis. The rotational motion is converted to energy
by wind turbines because wind turbines produce
rotational motion. Wind-energy is readily converted
into electrical energy by converting the turbine into
an electrical generator.
Wind energy generation has attracted much interest
during the last few years. Large wind farms have
been planned and installed in various locations
around the world. Many of these wind farms are
based on the Doubly Fed Induction Generator (DFIG)
technology with converter ratings around 30 percent
of the generator ratings. DFIG have more advantages
over synchronous and induction generators when
used in wind farms, such as robustness, reliability,
low price, variable speed operation, active and
reactive power control, relatively high efficiency, and
lower converter cost.
DPC technique based on grid voltage orientation has
been proposed in this chapter. The DPC gets the
same control performances as vector control, but has
better robustness and simple structure. This DPC
approaches the Instantaneous Reactive Power (IRP)
p-q theory, which is based on the Clarke transform of
voltages and currents in three-phase systems into α
and β orthogonal coordinates. In this proposed
control, DPC is used to control both active power and
reactive power under steady state condition.
Simulation
is
accomplished
using
MATLAB/Simulink software. The simulations are
verified with laboratory hardware setup.
II. OPERATION OF DFIG
Fig. 1 show the basic configuration of DFIG. Based
on the operation such as sub synchronous,
synchronous and super synchronous speed of the
DFIG, the power flow is from rotor to grid or grid to
rotor. The total power supplied to grid is sum of the
power from stator and rotor power.
Fig. 1Basic configuration of DFIG wind turbine
DFIG supplied the power to grid from its stator When
Ps > 0. Rotor received the power from grid when ωr<
B.Vaikundaselvan et al., International Journal of Advanced Engineering Technology
ωs, it means that Pr < 0. Rotor also supplied power to
grid when Pr > ωs, it means that Pr > 0. Here ωr, ωs, Pr,
Ps are rotor speed, stator speed, power from rotor and
power from stator respectively. Hence by operating
the DFIG in sub synchronous, synchronous and super
synchronous modes and the outputs in terms of rotor
powers are Pr < 0, Pr = 0 and Pr > 0 respectively.
A three-phase wound-rotor induction machine can be
setup as a doubly-fed induction motor. In this case,
the machine operates like asynchronous motor whose
synchronous speed can be varied by adjusting the
frequency of the ac currents fed into the rotor
windings. The same wound-rotor induction machine
setup can also serve as a doubly-fed induction
generator. In this case, mechanical power at the
machine shaft is converted into electrical power
supplied to the ac power network via both the stator
and rotor windings.
Furthermore, the machine operates like a
synchronous generator whose synchronous speed
(speed at which the generator shaft must rotate to
generate power at the AC power network frequency)
can be varied by adjusting the frequency of the AC
currents fed into the rotor windings. In a conventional
three-phase synchronous generator, when an external
source of mechanical power (prime mover) makes the
rotor of the generator rotate, the static magnetic field
created by the dc current fed into the generator rotor
winding rotates at the same speed as the rotor. As a
result, a continually changing magnetic flux passes
through the stator windings as the rotor magnetic
field rotates, inducing an alternating voltage across
the stator windings. Mechanical power applied to the
generator shaft by the prime mover is thus converted
to electrical power that is available at the stator
windings. In conventional (singly-fed) induction
generators, the relationship between the frequency of
the ac voltages induced across the stator windings of
the generator and the rotor speed is expressed using
the Equation (1).
ω
f =
(1)
f - Frequency of the ac voltages induced across the
stator windings
− Speed of rotor in rps
Ps – Number of poles in the DFIG per phase
From the above Equation (1), it is very clear that,
when the speed of the generator rotor is equal to the
generator synchronous speed, the frequency of the
AC voltages induced across the stator windings of the
generator is equal to the frequency of the ac power
network.
The same operating principles apply in a doubly-fed
induction generator as in a conventional (singly-fed)
induction generator. The only difference is that the
magnetic field created in the rotor is not static (not
static means, it is created using three-phase AC
current instead of dc current), but rather rotates at a
speed proportional to the frequency of the ac currents
fed into the generator rotor windings. This means that
the rotating magnetic field passing through the
generator stator windings not only rotates due to the
rotation of the generator rotor, but also due to the
rotational effect produced by the ac currents fed into
the generator rotor windings. Therefore, in a DFIG
both the rotation speed of the rotor and the frequency
of the AC currents fed into the rotor windings
determine the speed of the rotating magnetic field
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E-ISSN 0976-3945
passing through the stator windings, and thus, the
frequency of the alternating voltage is induced across
the stator windings.
Taking into account the principles of operation of
doubly-fed induction generators, it can thus be
determined that, when the magnetic field at the rotor
rotates in the same direction as the generator rotor,
the rotor speed and the speed of the rotor magnetic
field add up. The frequency of the voltages induced
across the stator windings of the generator can thus
be calculated using the Equation (2).
ω
f =
+ f
(2)
f - Frequency of rotor current fed into the rotor of
DFIG
Conversely, when the magnetic field at the rotor
rotates in the direction opposite to that of the
generator rotor, the rotor speed and the speed of the
rotor magnetic field are subtracted from each other.
The frequency of the voltages induced across the
stator windings of the generator can thus be
calculated using the Equation (3).
ω
f =
− f
(3)
III. DFIG MODELLING
The DFIG has two sets of three-phase windings that
display self and mutual inductances. Mutual
inductances change as the machine turns and the
angle between stator and rotor circuits varies with
time, which ultimately leads to a time-varying
mathematical model of the machine. This angle
dependency in DFIG’s model and the associated
complexities can be surmounted by making a
transformation from three-phase magnitudes to twoaxis magnitudes (Clark’s transformation) and
transforming the magnitudes into direct and
quadrature components referred to a synchronously
rotating reference frame (Park’s transformation). The
DFIG model in the synchronous reference frame can
be expressed as Equations (4) to (12), where the
model corresponds to the angular speed of the
rotating reference frame.
v = R i + sψ + ω ψ
(4)
v = R i + sψ − ω ψ
(5)
v ′ = R′ i′ + sψ′ + (ω − ω )ψ′
(6)
v ′ = R′ i′ + sψ′ − (ω − ω )ψ′
ψ =L i +L ′
ψ′ = L′ i′ + L i
(7)
(8)
(9)
(10)
ψ′ = L′ i′ + L i
(11)
ψ
=L i
+L
′
T = P (ψ i − ψ i )
(12)
Where,L = L + L ; L′ = L′ + L
; L = 3 L ⁄2
A. Mathematical Representation of DFIG
The mathematical model is similar to the squirrel
cage induction machine; the only difference is that
the rotor voltage is non-zero in DFIG. In order to
simulate the Wind Energy Conversion System a
proven method is needed to represent the
characteristics of a DFIG. The rest of this section is
devoted to the mathematical equations that describe a
DFIG. The voltage equations to represent a 3-phase
induction machine can be expressed as follows,
[V
] = [r ][i
]+ρ ψ
(13)
[
] = [ ][
]+ρ ψ
(14)
=
+
(15)
(16)
Q = V i −V i
B.Vaikundaselvan et al., International Journal of Advanced Engineering Technology
Where ν, r, i, and ψ respectively refer to the voltage,
resistance, current, and flux linkage of the phase
windings. The subscripts a, b, and c refer to their
phase component. The subscripts r and s refer to the
stator and rotor windings. The term ρ represents the
derivative (d/dt). The flux linkages shown in Fig. 2
for a linear magnetically coupled circuit are
expressed as Equation (17) to (20).
Fig. 2 Equivalent circuit of induction machine in d-q
reference frame
v⃗ = R ı ⃗ +
ψ⃗
+ jω ψ⃗
(17)
ψ⃗
v⃗ = R ı⃗ +
+ j(ω − ω )ψ⃗
(18)
ψ⃗ = L ı⃗ + Lmı⃗
(19)
ψ⃗ = L ı⃗ + L ı⃗
(20)
Where and are rotor and stator voltages
and
are rotor and stator fluxes
R , R , L , and L , are rotor and stator resistance &
inductance.
= Mutual inductance
ω and ω are Rotor & Synchronous angular speed
In rotating
reference frame the machine model is
v
v
v
=R i
=R i
=R i
+
ψ
− ω ψd +
ψ
−ω ψ
+ (ω − ω )ψ
v = R i + (ω − ω )ψ
ψ =L i +L i
ψ =L i +L i
ψ =L i +L i
ψ =L i +L i
Torque produced is
(21)
(22)
+
ψ
+
ψ
(23)
(24)
(25)
(26)
(27)
(28)
T = p(ψd iq − ψq id ) = pL (id iq − iq id )
(29)
The mechanical part of the model is
ω
= (T − T )
(30)
The active and reactive powers inputs from the
network can be calculated as
P = (v i + v i
(31)
Q = (v i − v i )
(32)
P = V i +V i
(33)
Q = V i −V i
(34)
IV. CONTROL METHODS
Different control methods are popular in the industry,
such as scalar method (V/f), vector control, direct and
indirect field oriented control, rotor and stator flux
control, adaptive flux observer, stator flux orientation
and field weakening control. All the said methods are
complex, for reducing the complexity to choose the
direct power control scheme to control the machine.
The DFIG control level performs the control of the
rotor side and the grid-side back-to-back converters.
A vector control approach is adopted for the rotor
controller, while two cross coupled controllers adjust
Int J Adv Engg Tech/Vol. VII/Issue I/Jan.-March,2016/841-850
E-ISSN 0976-3945
the speed and power of the system. The goals of such
controllers are to track the optimum operation point,
limit the power in the case of high wind speeds, and
control the reactive power exchanged between the
wind turbine generator and the grid. The control of
the grid-side converter keeps a constant dc-link
voltage while injecting the active power to the grid.
Internal current loops in both converters are typically
using proportional integral (PI) controllers.
DTC and DPC schemes have been presented as
alternative methods which directly control machine
flux and torque via the selection of suitable voltage
vectors. It has been shown that DPC is a more
efficient approach compared to modified DTC.
However, the DPC method also depends on the
estimation of machine parameters and it requires a
protection mechanism to avoid over current during a
fault in the system.
In induction wind generators, unbalanced three phase
stator voltages cause a number of problems,
including overheating and stress on the mechanical
components from torque pulsations. Therefore,
beyond a certain amount of imbalance (6%),
induction wind generators are switched out of the
network. In DFIG control of rotor currents allows for
adjustable speed operation and reactive power
control. In addition, it is possible to control the rotor
currents to correct the problems caused by
unbalanced stator voltages. This paper presents a
novel controller design for a doubly-fed induction
generator that provides adjustable speed and reactive
power control while greatly reducing torque
pulsations.
B. Rotor Side converter
The RSC supplies the voltage to the rotor windings
of the DFIG. The purpose of the rotor-side converter
is to control the rotor currents such that the rotor flux
position is optimally oriented with respect to the
stator flux in order that the desired torque is
developed. The rotor-side converter uses a torque
controller to regulate the wind turbine output power
and the voltage (or reactive power) measured at the
machine stator terminals.
The power is controlled in order to follow a predefined turbine power-speed characteristic to track
the maximum power point. The actual electrical
output power from the generator terminals, added to
the total power losses (mechanical and electrical) is
compared with the reference power obtained from the
wind turbine characteristic. Usually, a ProportionalIntegral (PI) regulator is used at the outer control
loop to reduce the power error (or rotor speed error)
to zero. The output of this regulator is the reference
rotor current irqref that must be injected in the rotor
winding by rotor-side converter. This q-axis
component controls the electromagnetic torque Te.
The actual irq component of rotor current is compared
with irqref and the error is reduced to zero by a current
PI regulator at the inner control loop. Fig. 3 show the
rotor side converter control scheme.
The output of this current controller is the voltage vrq
generated by the rotor-side converter. With another
similarly regulated ird and vrd component the required
3-phase voltages applied to the rotor winding are
obtained. The generic power control loop is
illustrated in the next section.
(6.30)
B.Vaikundaselvan et al., International Journal of Advanced Engineering Technology
E-ISSN 0976-3945
Fig. 3 Rotor side converter control scheme
The grid side converter is used to partly control the
flow of real and reactive power from the turbine
system to the grid. The grid-side converter feeds the
grid through a set of interfacing inductors. The gridside converter (a voltage source inverter) can
generate a balanced set of three-phase voltages at the
supply frequency and that the voltage (E) can have a
controllable magnitude and phase. Load angle control
is used to illustrate the basics of real and reactive
power control, though in practice, a more
sophisticated control is used which provides superior
transient response. Essentially, load angle control
uses the angle, δ, between the voltage generated by
the grid-side converter, E, and the grid voltage, V,
The steady-state equations relate to the real and
reactive power flow from the grid-side converter to
the grid are shown in Equation (35) and (36).
δ
(35)
P=
and
Q = − cos δl
P=
δ
and
Q=
−
(36)
Showing that P can be controlled using load angle δl,
and Q can be controlled using the magnitude of E.
The combination of control and power electronics
enables the grid-side converter to produce the
necessary voltage magnitude, E, and load angle δ, in
order to meet a required Ps and Qs demand set by the
main system controller. The controller should be able
to synchronize the grid frequency and phase, in order
to connect and supply power.
At any instance, the power exported by the GSC is
determined by the state of the DC- link voltage. The
grid-side converter controller monitors the DC- link
voltage. If the DC- link voltage rises, the grid-side
converter can export more real power by increasing
the load angle in order that the DC- link voltage
moves back towards it nominal value. If more power
is being exported by the GSC than is currently being
generated by the RSC, the DC link voltage will fall
below its nominal value.
The grid-side controller will then reduce the exported
real power to allow the DC link voltage to recover to
its nominal value. In this sense, the DC link voltage
indicates power flow balance between the generated
energy and the exported energy in the rotor side. If
the input and output power to the dc link capacitor do
not match, then the DC link voltage will change. The
quality of the energy supplied to the network must
Int J Adv Engg Tech/Vol. VII/Issue I/Jan.-March,2016/841-850
meet basic requirements and it will be set by the grid
code in force at the PCC.
The grid code specifies many performance indicators
of the quality of the energy supplied by the grid-side
converter, along with other important issues such as
fault level. The relevant grid code(s) in operation
must be determined prior to tendering for work on the
turbine power electronics and control. The grid code
has important implications on the control system of
the turbine. If the generator-side controller continues
to generate power, the DC link capacitance will be
over charged. Therefore, a grid fault will require the
generator to stop generating energy, which then
means that there is no longer a restraining torque to
control the blade speed.
In a wind turbine, a loss of supply will cause an over
speed condition, as the blade system will accelerate
due to the aerodynamic torque produced by the
blades. Shorting resistors, or a crowbar circuit, is
often switched across the rotor circuit of the
generator in order that the energy generated by the
blade system can be absorbed and the high speed
condition controlled to a safe and manageable level.
In addition, there are often aerodynamic (pitch
control) and mechanical braking mechanisms
included in wind turbines as an additional over-speed
safety measure.
C. Grid Side Converter
Different control strategies are used to perform the
control of the grid side converter. They all are
focused on the same topics, the control of the DClink voltage, active and reactive power delivered to
the grid, grid synchronization and to ensure high
quality of the injected power .They can be classified
depending on the reference frame used in the control
structure. In this project the focus is on the
synchronous and stationary reference frame control
strategies. In both cases, the control strategy contains
two cascaded loops. Fig. 4 shows the grid side
converter control scheme.
The inner loops control the grid currents and the
outer loops control the DC-link voltage and the
reactive power. The current loops are responsible for
the power quality, thus harmonic compensation can
be added to the action of the current controllers to
improve it. The outer loops regulate the power flow
of the system by controlling the active and reactive
power delivered to the grid. The total current
B.Vaikundaselvan et al., International Journal of Advanced Engineering Technology
delivered into the grid is the sum of the currents from
stator side and grid side. Since the negative-sequence
current of the DFIG stator is eliminated with the
effective control of GSC.
Fig. 4 Grid side converter control scheme
D. Direc Power Control
The DPC inherits most of its theoretical background
from the DTC. DPC also exploits the geometrical
relationship between stator and rotor fluxes and the
fact that the rotor flux can be fully controlled through
the RSC.
This distinction change in the control approach has a
significant impact on the robustness and simplicity of
the DPC strategy, whose main features include:
1) Independence from machine parameters
2) Reduced number of electrical magnitudes to be
measured and
E-ISSN 0976-3945
3) No need for reference frame transformations
V. SIMULATION RESULTS
Simulations of the proposed control strategy for a
DFIG based wind power generation system were
carried out, using MATLAB/Simulink. Discrete
models were used with a simulation time step of 1.5s.
The DFIG is rated at 1.3 kW. The nominal converter
DC- link voltage was set at 1200 V. The grid side
converter has to maintain a constant DC-link voltage,
and it is controlled by a method similar to the DC
voltage controller in a PWM voltage source rectifier.
During simulations, a sampling frequency of 10 kHz
was used for the proposed control strategy. A highfrequency ac filter is connected to the stator side to
absorb the switching harmonics generated by the two
converters.
The filter is a single-tuned one with inductance and
resistance connected in parallel instead of series,
which results in a wide band filter having impedance
at high frequencies limited by the resistance. In a
practical system, voltages and currents are sampled at
the beginning of each sampling period. The required
rotor control voltage for the sampling period is then
calculated and passed to the SVM (Support Vector
Machine) module. Impossible to avoid, there is a
time delay between the instant sampling and SVM
modulator’s receiving the required rotor control
voltage and updating its register values.
Fig. 5 Grid side converter phase volt and current for a sub synchronous speed (135 rad/s)
Fig. 6 Grid side converter phase volt and current for synchronous speed (157 rad/s)
Rotor voltage calculation of the proposed DPC
strategy is relatively simple, and the time delay
should be adroit small. In spite of that, the calculated
output rotor voltage is delayed by 4 ms (one sampling
period) to closely represent a practical DPC control
system. During the simulation, the grid side converter
is enabled first, so that the converter DC-link voltage
is regulated. The generator is then excited by rotorInt J Adv Engg Tech/Vol. VII/Issue I/Jan.-March,2016/841-850
side converter with the rotor rotating at a fixed speed
till the stator voltage matches with the network
voltage, such that the DFIG system is switched into
grid-connected operation. The Figures 5 to 7 shows
the grid side converter phase volt and current for a
sub
synchronous,
synchronous
and
super
synchronous speed respectively.
B.Vaikundaselvan et al., International Journal of Advanced Engineering Technology
E-ISSN 0976-3945
Figure 7 Grid side converter phase volt and current for super synchronous speed (179 rad/s)
Fig. 8 Rotor side converter phase voltage and current for sub synchronous speed (135 rad/sec)
The Fig. 8 to 10 shows the rotor side converter phase
volt and current for a sub synchronous, synchronous and
super synchronous speed respectively. The Fig. 11 to 13
shows the stator phase volt and current for a sub
synchronous, synchronous and super synchronous speed
respectively.
Fig. 14 shows the Load voltage, load Current with
different rotor angle changes and Fig. 15 shows the
constant reactive power for different power
generation.
Fig. 9 Rotor side converter phase voltage and current for synchronous speed (157 rad/sec)
Fig. 10 Rotor side converter phase voltage and current for super synchronous speed (179 rad/sec)
Int J Adv Engg Tech/Vol. VII/Issue I/Jan.-March,2016/841-850
B.Vaikundaselvan et al., International Journal of Advanced Engineering Technology
E-ISSN 0976-3945
Fig. 11 Stator phase voltage and current for sub synchronous speed (135 rad/sec)
Fig. 12 Stator phase volt and current for synchronous speed (157 rad/sec)
Fig. 13 Stator phase volt and current for super synchronous speed (179 rad/sec)
Fig. 14 Load voltage, load current with rotor angle changes
E. Description of the Experimental Work-Bench
As it can be seen in the picture shown in Fig. 16, the
bench is composed of two controlled electrical drives
one is the wind turbine emulator (DC motor) and the
Int J Adv Engg Tech/Vol. VII/Issue I/Jan.-March,2016/841-850
other one is the electric generator (DFIG), coupled on
the same shaft. The wind turbine emulator (WTE)
consists of a DC motor driven by a Siemens AC/ DC
converter that can be easily configurable.
B.Vaikundaselvan et al., International Journal of Advanced Engineering Technology
Fig. 15 Constant reactive power
Fig. 16 DFIG and DC motor coupled on the same shaft
Fig. 17 Experimental test rig of DFIG
Fig. 17 shows a proposed grid connected DFIG with
all supporting components like GSC, RSC,
measuring units (Power quality analyser) and FPGAcontroller. The specific components used in the
presented proposed grid connected DFIG is:

A 1.3 kW, 220 V, 1500 rpm, DC shunt
machine. It is mechanically coupled to the
generator to transmit the desired torque.

An AC/DC Siemens converter for DC drives.
It supplies the suitable voltage or current to
the DC machine in order to achieve the
specified torque reference.

A 1 hp, 1410 rpm, 4 poles, 415 V wound rotor
asynchronous machine an AC/DC, 1200 V, 25
A three-phase controlled IGBT converter.
When working below the synchronous speed,
it rectifies the voltage or currents from the
grid to feed the DC bus from which the
generator converter will feed the wind
generator rotor. Above that speed, it will
control the DC bus voltage delivering power
to the grid. At all times, the control system of
this converter regulates the reactive power
exchanged with the grid to which the
generator’s rotor is connected. A DC/AC 1200
V, 25 A, three-phase controlled IGBT’s
converter feeds the DFIG’s rotor with AC
obtaining the energy from the DC bus. The
control system calculates how the rotor phases
must be fed so that the generator’s stator
delivers the maximum electric power
associated to different wind conditions to the
grid. The control system also regulates the
reactive power exchanged with the grid
through the generator’s stator by means of this
converter.

A FPGA - Spartan 6 based control system
composed development board has been used
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E-ISSN 0976-3945
as controller. This hardware allows for the
simultaneous controlling of the IGBT firing
signals in the DC/AC generator side converter
and those of the AC/DC grid side converter.
The interface included with this system allows
for the real-time tracking of the change of any
variable in the entire generation system. A
Hall Effect sensors box for adapting voltage
and current measurements to the FPGA input
channels.
F. System Operation
The described system is designed to be operated in
the following manner
1.
The DC machine coupled with DFIG act as
wind turbine emulator and is initiated using a
specific wind reference. The wind turbine
emulator control system (Siemens control)
reads the turbine speed from the generator
control system (FPGA).
2.
When the generator control system detects the
minimum speed at which the DFIG must be
started, both the grid converter and the
generator converter are started in order to
prepare the system for connection to the grid.
3.
Once the DFIG stator generates three voltages
equal to those of the grid to which it will be
connected, the grid connection switch is
closed.
4.
When the generator stator is connected to the
grid, the generator control system can operates
the specified active and reactive power (zero).
5.
At the same time, the grid converter control
system maintains the DC bus voltage
equivalent to the reference value.
6.
From this point, the system works
automatically and the only parameter that can
be modified is the DC machine current or
torque reference.
7.
At any turbine speed, the DFIG will deliver
the maximum active power established by a
reference table, including the higher speed
values at which the pitch angle must be
changed. The generator control system
operates to maintain zero reactive power
interchanged with the grid.
8.
According to the grid converter control
system, it is important to remember that when
the wind turbine operates below the
synchronous speed, this converter (GSC) acts
as a rectifier, allowing the energy to flow from
the grid to the rotor. But when the turbine
speed increases above the synchronous value,
it works as an inverter, delivering power from
the rotor to the grid. Moreover, this converter
also collaborates in the global reactive power
control.
G. Hardware Results
To demonstrate the operating possibilities of this
system, we present the results of a specific test
consisting of the production of an increment of the
wind speed while operating the system on manual
reference mode. The test results are presented in Fig.
18 to 20. The intention of this test is that the wind
increases carries an increment of the machine rotating
speed which increases from 1400 r.p.m., under
synchronous speed 1500 r.p.m, to a supersynchronous speed 1280 r.p.m.
B.Vaikundaselvan et al., International Journal of Advanced Engineering Technology
E-ISSN 0976-3945
Fig. 18 Stator voltage, stator current and rotor current for rotor frequency as 27.830 Hz
Fig.19 Stator voltage, stator current and rotor current for rotor frequency as 16.588 Hz
Fig. 18 to 20 show the evolution of the voltage
amplitude, the frequency, and the three phase
currents injected into the grid, as well as the value of
the active (P) and reactive (Q) power interchanged
with the grid through the stator for different rotor
frequencies, whose values follow at all times the
constant references established for this test. Fig. 21
shows that, the indication of zero reactive power for
the rotor frequency as 30.442 Hz.
Fig. 20 Stator voltage, stator current and rotor current for rotor frequency as 15.693 Hz
Int J Adv Engg Tech/Vol. VII/Issue I/Jan.-March,2016/841-850
B.Vaikundaselvan et al., International Journal of Advanced Engineering Technology
E-ISSN 0976-3945
Fig. 21 Indication of zero reactive power for the rotor frequency as 30.442 Hz
VI. CONCLUSION
The operation of DFIG was reviewed and the
mathematical model for real and reactive powers are
derived from its equivalent circuit. The control circuit
for GSC and RSC have been prepared by using direct
power control scheme and implemented. The
simulation of a 1.3 kW DFIG based GCWECS with
PI controller is carried out using MATLAB/simulink.
The steady state behavior of system was analyzed for
synchronous (157 rad/sec), sub synchronous (135
rad/sec) and super synchronous speed (179 rad/sec).
The reactive power supplied by DFIG for three
modes of operations (synchronous, sub synchronous
and super synchronous speeds) is zero, and has been
justified by the simulation results. The hardware
results also confirmed the reactive power as zero for
all the three conditions.
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