Download ENHANCED CONTROL OF A DFIG-BASED WIND

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ENHANCED CONTROL OF A DFIG-BASED WIND-POWER GENERATION SYSTEM WITH
SERIES GRID-SIDE CONVERTER UNDER UNBALANCED GRID VOLTAGE CONDITIONS
Abstract: This paper presents an enhanced control method
for a doubly fed induction generator (DFIG)-based windpower generation system with series grid-side converter
(SGSC) under unbalanced grid voltage conditions. The
behaviors of the DFIG system with SGSC during network
unbalance are described. By injecting a series control
voltage generated from the SGSC to balance the stator
voltage, the adverse effects of voltage unbalance upon the
DFIG, such as stator and rotor current unbalances,
electromagnetic torque, and power pulsations, can be
removed, and then the conventional vector control strategy
for the rotor-side converter remains in full force under
unbalanced conditions. Meanwhile, three control targets for
the parallel grid-side converter (PGSC) are identified,
including eliminating the oscillations in the total active
power or reactive power, or eliminating negative-sequence
current injected to the grid. Furthermore, a precise current
reference generation strategy for the PGSC has been
proposed for the PGSC to further improve the operation
performance of the whole system. Finally, the proposed
coordinated control strategy for the DFIG system with
SGSC has been validated by the simulation results of a 2MW-DFIG-based wind turbine with SGSC and
experimental results on a laboratory-scale experimental rig
under small steady-state grid voltage unbalance.
Index Terms: Doubly Fed Induction Generator (DFIG),
Series Grid-Side Converter (SGSC), Parallel Grid-Side
Converter (PGSC)
I. INTRODUCTION
In recent years with growing concerns over carbon
emission and uncertainties in fossil fuel supplies, there is an
increasing interest in clean and renewable electrical energy
generation. Among various renewable energy sources, wind
power is currently the fastest growing form of electric
generation. Although wind power currently only provides
about 3% of European electricity and 2% of the U.S.'
electrical energy demands, it is reasonable to expect a high
penetration of wind power into the existing power system in
the near future, e.g., by 2030.With the rapid increase in
penetration of wind power in the power system, it becomes
necessary to require wind farms to behave as much as
possible as conventional power plants to support the
network active power, voltage and frequency.
Modern
variable-speed
wind
power
systems,
predominantly based on the Doubly-Fed Induction
Generator (DFIG) technology[1], are equipped with backto-back, AC/DC/AC power electronic converters whose
intermediate DC voltage and excellent controllability
renders them technically attractive to incorporate energy
storage devices such as a flywheels, super capacitors,
batteries, etc., it is shown that a DFIG-based windpower/storage system can deliver a pre-specified amount of
power to the grid, despite wind power fluctuations.
In my work a wind farm equipped with doubly-fed
induction generator (DFIG) wind turbines, where each
WTG’s is equipped with a super capacitor energy storage
system (ESS) to maintain constant active power control to
the grid. Two different control schemes are developed one
for Rotor side control (RSC) using stator flux reference
frame and the other for Grid side control (GSC) using
current reference frame are developed to provide the firing
pulses to the converters. A wind farm supervisory control
(WFSC) is developed to generate the active power reference
to the RSC and GSC. WTG controllers then regulate each
DFIG wind turbine to generate the desired amount of active
power, where the deviations between the available wind
energy input and desired active power output are
compensated by the ESS.
Simulation is done in Matlab for the grid connected wind
farm; the wind farm consists of 15 DFIG wind turbines each
with 4MW capacity at different and constant wind speeds. A
constant active power Pref is taken at 38MW which can be
specified by the grid operator and the wind farm has to
supply this constant active power. Simulation results are
studied for the active power supplied by wind farm with and
without Energy Storage System (ESS). Here we can observe
that with ESS the active power supplied by wind farm is
almost constant.
II. WIND ENERGY
2.1 Introduction
Wind is abundant almost in any part of the world. Wind
is caused due to the heating of earth’s surface by sun and
due to the earth’s rotation. The conventional ways of
generating electricity using non-renewable resources such as
coal, natural gas, oil and so on, have great impacts on the
environment as it contributes vast quantities of carbon
dioxide to the earth’s atmosphere which in turn will cause
the temperature of the earth’s surface to increase, known as
the greenhouse effect. Hence, with the advances in science
and technology, ways of generating electricity using
renewable energy resources such as the wind, solar, biomass
are developed. Nowadays, the cost of wind power that is
connected to the grid is as cheap as the cost of generating
electricity using coal and oil. Thus, the increasing popularity
of green electricity means the demand of electricity
produced by using non-renewable energy is also increased
accordingly.
2.2 Power Contained in Wind
2
The power contained in wind is given by kinetic energy
of the flowing air mass per unit time. That is,
Pwind =
π
8
3
dD2 Vwind
(1)
Where Pwind is the power contained in wind, d is the air
density, A is the rotor area and Vwind is the wind velocity.
There are several factors that contribute to the
efficiency of the wind turbine in extracting the power from
the wind. Firstly, the wind speed is one of the important
factors in determining how much power can be extracted
from the wind. This is because the power produced from the
wind turbine is a function of the cubed of the wind speed
thus, the wind speed if doubled; the power produced will be
increased by eight times the original power. Then, location
of the wind farm plays an important role in order for the
wind turbine to extract the most available power form the
wind.
force in the direction of rotation. The blades were made of
sails or wooden slats.
2.3.2 Multi-blade water-pumping windmills
Modern water-pumping windmills have a large number
of blades generally wooden or metallic slats driving a
reciprocating pump. As the mill has to be placed directly
over the well, the criterion for site selection concerns water
availability and not windiness. Therefore, the mill must be
able to operate at slow winds. The large number of blades
gives high torque, required for driving a centrifugal pump,
even at low winds. Hence sometimes these are called fanmills.
2.3.3 High-speed propeller type windmills
The horizontal-axis wind turbines that are used today for
electrical power generation do not operate on thrust force.
They depend mainly on aerodynamic forces that develop
when wind flows around a blade of aerofoil design. The
wind stream at the top of the foil has to traverse a longer
path than that at the bottom, leading to a difference in
velocities. This gives rise to a difference in pressure given
by Bernoulli’s principle, from which a loft force results.
There is of course another force that tries to push the
aerofoil back in the direction of the wind. This is called as
drag force. The aggregate force on the aerofoil is then
determined by the resultant of these two forces.
Fig.1. Power-Wind speed Curve
2.3.4 The Savonius rotor
The second important factor of the wind turbine is the
rotor blade. The rotor blades and length of the wind turbine
is one of the important aspects of the wind turbine since the
power produced from the wind is also proportional to the
swept area of the rotor blades i.e. the square of the diameter
of the swept area. Hence, by doubling the diameter of the
swept area, the power produced will be fourfold increased.
It is required for the rotor blades to be strong and light and
durable. As the blade length increases, these qualities of the
rotor blades become more elusive. But with the recent
advances in fiber glass and carbon-fibre technology, the
production of lightweight and strong rotor blades between
20 to 30 meters long is possible. Wind turbines with the size
of these rotor blades are capable to produce up to 1
megawatt of power.
The savonius rotor is an extremely simple vertical-axis
device that works entirely because of the thrust force of the
wind. The basic equipment is a drum cut into two halves
vertically. The two parts are attached to the two opposite’s
sides of a vertical shaft. As the wind blowing into the
structure meets with two dissimilar surfaces, one convex
and other concave the two forces exerted on the two
surfaces are different, which gives the rotor a torque. The
savoinus rotor is inexpensive and simple, and the material
required for it is generally available in any rural area,
enabling onsite construction of such windmills. However,
its utility is limited to pumping water because of its
relatively low efficiency.
2.3.5 The Darrieus rotor
2.3 Types of Wind Turbines
Depending on their axis alignment Wind turbines can be
categorized as:
2.3.1 Dutch-type grain-grinding windmills
These grain-grinding windmills that were widely used
in Europe since the middle ages were Dutch. These
windmills operated on thrust exerted by wind. The blades,
generally four, were inclined at an angle to the plane of
rotation. The wind, being deflected by the blades, exerted a
In 1931, a vertical-axis device for wind energy
conversion was invented by G.J. Darrieus. It has two or
more flexible blades are attached to a vertical shaft. The
blades bow outward, taking approximately the shape of a
parabola, and are of symmetrical aerofoil section. At first
the forces on the blades at the two sides of the shaft should
be the same, producing no torque i.e., torque is zero when
the rotor is stationary. Is develops a positive toque only
when it is already rotating. This means that such a rotor has
no starting torque and has to be started using some external
means. The Darrieus rotor has high efficiency and high
speed and is perfectly suited for electrical power generation.
3
The cost of construction is low because the generator and
the gear assembly can e located at ground level, drastically
reducing the cost of the tower. However, then it is unable to
take advantage of the high wind speeds available at higher
altitudes.
2.4 Components of Wind Turbine
The main important components of a wind turbine are
shown in Fig.2 below
mounted on the top of the nacelle sensing the relative wind
direction, and the wind turbine controller. In some designs,
the nacelle is yawed to attain reduction in power during high
winds. In extremity, the turbine can be stopped with nacelle
turned such that the rotor axis is at right angles to the wind
direction. One of the more difficult parts of a wind turbine
designs is the yaw system, though it is apparently simple.
Especially in turbulent wind conditions, the prediction of
yaw loads is uncertain.
2.4.3 Gearbox
Many wind turbines have a gearbox that increases the
rotational speed of the shaft. A low-speed shaft feeds into
the gearbox and a high-speed shaft feeds from the gearbox
into the generator. Some turbines use direct drive generators
that are capable of producing electricity at a lower rotational
speed. These turbines do not require a gearbox.
2.4.5 Generators
Fig.2. Components of wind turbine
2.4.1 Blades
Most wind turbines have three blades, though there are
some with two blades. Blades are generally 30 to 50 meters
(100 to165 feet) long, with the most common sizes around
40 meters (130 feet). Longer blades are being designed and
tested. Blade weights vary, depending on the design and
materials—a 40 meter LM Glass fibre blade for a 1.5 MW
turbine weighs 5,780 kg (6.4 tons) and one for a 2.0 MW
turbine weighs 6,290 kg (6.9 tons).
2.4.2 Controller
There is a controller in the nacelle and one at the base of
the turbine. The controller monitors the condition of the
turbine and controls the turbine movement. Different types
of controllers used are:
a) Pitch Angle Control: This system changes the pitch
angle of the blades according to the variation of wind speed.
On a pitch controlled machine, as the wind speed exceeds its
rated speed, the blades are gradually turned about the
longitudinal axis and out of the wind to increase the pitch
angle which reduces the aerodynamic efficiency of the rotor,
and the rotor output power decreases. During the operation
below the rated speed the control system endeavors to pitch
the blade at an angle that maximizes the rotor efficiency.
b) Yaw Control: It turns the nacelle according to the
actuator engaging on a gear ring at the top of the tower.
Yaw control is the arrangement in which the entire rotor is
rotated horizontally or yawed out of the wind. During
normal operation of the system, the wind direction should
be perpendicular to the swept area of the rotor. The yaw
drive is controlled by a slow closed- loop control system.
The yaw drive is operated by a wind vane, which is usually
Wind turbines typically have a single AC generator that
converts the mechanical energy from the wind turbines
rotation into electrical energy. Clipper Wind power uses a
different design that features four DC generators.
2.4.6 Nacelles
The nacelle houses the main components of the wind
turbine, such as the controller, gearbox, generator and
shafts.
2.4.7 Rotor
The rotor includes both the blades and the hub (the
component to which the blades are attached).
2.4.8 Towers
Towers are usually tubular steel towers 60 to 80 meters
(about 195 to 260 feet) high that consist of three sections of
varying heights. (There are some towers with heights around
100 meters (330 feet)).
2.5 Classification of Wind Turbines based on speed
Based on speeds the wind turbines can be classified as:
2.5.1 Fixed-Speed Wind Turbine
For the fixed-speed wind turbine the induction generator is
directly connected to the electrical grid according to Fig. 3.
4
Fig.3. Fixed speed wind turbine with a an Induction
Generator
The rotor speed of the fixed-speed wind turbine is in
principle determined by a gearbox and the pole-pair number
of the generator. The fixed-speed wind turbine system has
often two fixed speeds. This is accomplished by using two
generators with different ratings and pole pairs, or it can be
a generator with two windings having different ratings and
pole pairs. This leads to increased aerodynamic capture as
well as reduced magnetizing losses at low wind speeds. This
system (one or two-speed) was the “conventional” concept
used by many Danish manufacturers in the 1980s and
1990s.
This is mainly due to the fact that the power electronic
converter only has to handle a fraction (20–30%) of the total
power. Therefore, the losses in the power electronic
converter can be reduced, compared to a system where the
converter has to handle the total power. In addition, the cost
of the converter becomes lower. There exists a variant of the
DFIG method that uses controllable external rotor
resistances (compare to slip power recovery). Some of the
drawbacks of this method are that energy is unnecessary
dissipated in the external rotor resistances and that it is not
possible to control the reactive power.
III.DOUBLY-FED INDUCTION GENERATOR&
ENERGY STORAGE SYSTEM
2.5.2 Variable-Speed Wind Turbine
3.1 Introduction
The system presented in Fig.4 consists of a wind turbine
equipped with a converter connected to the stator of the
generator.
Induction machines are often described as the
‘workhorse of industry’. They are cheap to manufacture,
rugged and reliable and find their way in most possible
applications. Variable speed drives require inexpensive
power electronics and computer hardware, and allowed
induction machines to become more versatile. In particular,
vector or field oriented control allows induction motors to
replace DC motors in many applications
The stator of an induction machine is a typical three phase
one. The rotor can be one of two major types. Either
Fig.4. Variable speed driven (gear less) wind turbine
with a Synchronous Generator
The generator could either be a cage-bar induction
generator or a synchronous generator. The gearbox is
designed so that maximum rotor speed corresponds to rated
speed of the generator. Synchronous generators or
permanent-magnet synchronous generators can be designed
with multiple poles which imply that there is no need for a
gearbox. Since this “full-power” converter/generator system
is commonly used for other applications, one advantage
with this system is its well-developed and robust control.
2.5.3 Variable-Speed Wind Turbine with Doubly-Fed
Induction Generator
This system, see Fig.5, consists of a wind turbine with
doubly-fed induction generator. This means that the stator is
directly connected to the grid while the rotor winding is
connected via slip rings to a converter. This system has
recently become very popular as generators for variablespeed wind turbines.
Fig.5. Variable speed wind turbine with a Doubly Fed
Induction Generator (DFIG)

It is wound in a fashion similar to that of the stator
with the terminals led to slip rings on the shaft.
 It is made with shorted Wound rotor slip rings and
connections bars.
3.2 Operation of Induction Machine
As these rotor windings or bars rotate within the
magnetic field created by the stator magnetizing currents,
voltages are induced in them. If the rotor is in standstill,
then the induced voltages would be very similar to those
induced in the stator windings. In the case of squirrel cage
rotor, the voltage induced in the bars will be slightly out of
phase with the voltage in the next one, since the flux
linkages will change in it after a short delay. If the rotor is
moving at synchronous speed, together with the field, no
voltage will be induced in the bars or the windings.
Generally when the synchronous speed is as = 2πfs, and
the rotor speed ω0, the frequency of the induced voltages
will be fro, where 2πfr =as -ω0. Maxwell’s equation becomes
here:
έ = v x Big
(1)
Where v is the relative velocity of the rotor with respect to
the field:
v = as -ω0
(2)
Since a voltage is induced in the bars, and these are short
circuited, currents will flow in them. The current density J
(θ) will be:
J (θ) = (1/ρ) έ
J (θ) = (1/ρ).as -ω0Bg (θ)
J (θ) = (1/ρ). (ωs -ω0)BG sin (θ)
(3)
5
This induced voltage magnitude is dependent upon the
speed difference between the rotating stator field and the
rotor. The speed difference is maximum during starting
when the motor draws large current. As the motor starts to
rotate, the speed difference is reduced, which results in
reduction on the frequency of the induced voltage in the
rotor and reduced magnitude of rotor current and induced
voltage. The force generated in the rotating field induces
current in the bar and the current and field interaction
generates the driving force.
Force = B rotating L Ir
(4)
Where L is the length of the rotor and
Ir is the current induced in the rotor.
This force drives the motor and if the rotor speed is equal
to the angular speed of the stator field, the induced voltage,
current and torque become zero. Therefore the motor speed
must be less than the synchronous speed. Thus for motor
operation requires speed difference between the stator
generated rotating field and the actual rotor speed.
The speed difference is called slip (s) and defined as:
s = (ns - nr) / ns
Where ns = 2 f / p
At starting the speed is zero, hence s = 1, and at
synchronous speed, ωs = ω0, hence s = 0. Above
synchronous speed s < 0, and when the rotor rotates in a
direction opposite of the magnetic field 1< s.
The frequency of the rotor current is fr = s f and the slip in
normal operation is between 1 and 5 %. The torque slip
characteristic of an Induction Machine is shown in Fig.6.
Hence the stator voltage equation is given as
V1 = E1+ I1 (R1+ j X1)
(5)
This E1 induced voltage generates a voltage E2 in the
rotor through the magnetic coupling. If the rotor is at stand
still, the induced voltage E2 is proportional to E1 times the
turn ratio. T = Nstat / Nrot = N1 /N2 and is given as
E2 = E1 (N2 /N1) = E1 / T
If the rotor is rotating, the voltage induced in the rotor is
multiplied by the slip s, because the induced voltage is
proportional to the speed difference between the stator field
and rotor.
E2 = s E 1 / T
The rotor induced voltage is equal to the sum of the
voltage drop across the rotor resistance (I2 R2), and the
leakage inductance (I2 X2). The voltage drop across the
secondary leakage inductance L2 is given by
I2 j wr L2 = I2 j (2 π fr) L2 = I2 j (2 π f) s L2 = I2 j s (w L2) = I2
j s X2
Therefore the rotor voltage equation is can be written as
E2 = I2 (R2 + j s X2)
(6)
But we know that
E1 = E2 T / s
I2 = I1 (N1/ N2) = I1 T
Substituting the value of E2 for eq 6 we get
E1= T I2 (R2 + j s X2) /s = I1 T2 (R2 /s + j X2)
= I1 [(R2 T2 /s) + j (T2 X2)]
E1= I1 (R*2 /s) + j X*2)
(7)
Where R*2 = R2 T2 and X*2 = T2 X2 are rotor resistance and
reactance referred to the stator.
Fig.6. Torque-slip Characteristics of Induction Machine
3.3 Development of equivalent circuit of Induction
Machine
The induction motor consists of a two magnetically
connected systems: Stator and rotor. This is similar to a
transformer that also has two magnetically connected
systems primary and secondary windings. The stator is
supplied by a balanced three-phase voltage that drives a
three-phase current through the winding. This current
induces a voltage in the rotor. The applied voltage (V1)
across phase A is equal to the sum of the induced voltage
(E1), the voltage drop across the stator resistance (I 1 R1) and
the voltage drop across the stator leakage reactance (I 1 j X1).
By substituting eq.7 in eq.5 we obtain the following
equation for the induction machine
V1 = I1 (R2* / s + j X2*) + I1 (R1+ j X1)
= I1 [(R1 + R2* / s) + j (X1+ X2*)]
Therefore the final equation can be written as
V1 = I1 [(R1 + R2* / s) + j (X1+ X2*)]
(8)
This equation suggests that the induction motor
equivalent circuit contains two resistances and reactance’s
connected in series and the magnetizing current can be
represented by a resistance Rc and a reactance Xm
connected in parallel. Here the resistance represents the
hysteresis and eddy current losses and the reactance
represents the magnetizing current that generates the air-gap
magnetizing flux. Therefore the equivalent circuit of an
Induction Machine is shown if Fig.7.
6
A typical application, as mentioned earlier, for DFIG is
wind turbines, since they operate in a limited speed range of
approximately ±30%. Other applications, besides wind
turbines, for the DFIG systems are, for example, flywheel
energy storage system, stand-alone diesel systems, pumped
storage power plants, or rotating converters feeding a
railway grid from a constant frequency public grid
Fig.7. Equivalent Circuit of Induction Machine
3.4 Doubly-Fed Induction Generator Systems
For variable-speed systems with limited variable-speed
range, e.g. ±30% of synchronous speed, the DFIG can be an
interesting solution. As mentioned earlier the reason for this
is that power electronic converter only has to handle a
fraction (20–30%) of the total power. This means that the
losses in the power electronic converter can be reduced
compared to a system where the converter has to handle the
total power. In addition, the cost of the converter becomes
lower. The stator circuit of the DFIG is connected to the
grid while the rotor circuit is connected to a converter via
slip rings. DFIG system with a back-to-back converter can
be seen in Fig.8.
3.5 Development of Equivalent Circuit of the DoublyFed Induction Generator
The equivalent circuit of the doubly-fed induction
generator, with inclusion of the magnetizing losses, can be
seen in Fig.10. This equivalent circuit is valid for one
equivalent Y phase and for steady-state calculations. In the
case that the DFIG is Δ-connected the machine can still be
represented by this equivalent Y representation. In this
section the jω-method is adopted for calculations.
Fig.10.Equivalent circuit of DFIG
Fig.8. DFIG system with a Back to Back Converter
The back-to-back converter consists of two converters,
i.e., machine-side converter and grid-side converter that are
connected “back-to-back.” Between the two converters a dclink capacitor is placed, as energy storage, in order to keep
the voltage variations (or ripple) in the dc-link voltage
small. With the machine-side converter it is possible to
control the torque or the speed of the DFIG and also the
power factor at the stator terminals, while the main
objective for the grid-side converter is to keep the dc-link
voltage constant. The speed–torque characteristics of the
DFIG system can be seen in Fig.9, as also seen in the figure,
the DFIG can operate both in motor and generator operation
with a rotor-speed range of ±Δωmaxr around the synchronous
speed, ω1.
Fig.9. Speed-Torque Characteristics of DFIG
Note that if the rotor voltage, Vr, in Fig.10 is short
circuited then the equivalent circuit for the DFIG becomes
the ordinary equivalent circuit for a cage-bar induction
machine. Applying Kirchhoff’s voltage law to the circuit in
Fig.10 yields
Vs = RsIs + jω1LsλIs + jω1Lm(Is + Ir + IRm)
(9)
Vr/s=Rr/sIr + jω1LrλIr + jω1Lm(Is + Ir + IRm)
(10)
0 = RmIRm + jω1Lm(Is + Ir + IRm)
(11)
Where Vs stator voltage; Rs stator resistance
Vr rotor voltage; Rr rotor resistance
Is stator current; Rm magnetizing resistance
Ir rotor current; Lsλ stator leakage inductance
IRm magnetizing resistance current; Lrλ rotor
leakage inductance
ω1 stator frequency
Lm magnetizing inductance and
s is slip.
The slip, s, equals s =ω1 – ωr/ω1=ω2/ω1
where ωr is the rotor speed and ω2 is the slip frequency.
Moreover, if the air-gap fluxes, stator flux and rotor flux are
defined as
Ψm = Lm(Is + Ir + IRm)
(12)
Ψs= LsλIs + Lm(Is + Ir + IRm) = LsλIs +Ψm
(13)
Ψr= LrλIr + Lm(Is + Ir + IRm) = LrλIr +Ψm
(14)
The equations describing the equivalent circuit, i.e., (9)–
(11), can be rewritten as
Vs = RsIs+ jω1Ψs
Vrs=RrsIr + jω1Ψr
0 = RmIRm+ jω1Ψm
The resistive losses of the induction generator are
Ploss = 3Rs|Is|2 + Rr|Ir|2 + Rm|IRm|2
7
and it is possible to express the electro-mechanical torque,
Te, as
Te = 3npImΨmI∗r= 3npImΨrI∗r
(15)
Where np is the number of pole pairs.
3.6 Power Flow in DFIG
In order to investigate the power flow of the DFIG
system the apparent power that is fed to the DFIG via the
stator and rotor circuit has to be determined. The stator
apparent power Ss and rotor apparent power Sr can be found
as
Ss = 3VsI∗s = 3Rs |Is|2 + j3ω1Lsλ |Is|2 + j3ω1ΨmI∗s
(16)
Sr = 3VrI∗r = 3Rr |Ir|2 + j3ω1sLrλ |Ir|2 + j3ω1sΨmI∗r
3.7 Modeling of the
Induction Generator
Wind-Turbine
Doubly-Fed
The wind turbine and the doubly-fed induction generator
are shown in Fig.12. The AC/DC/AC converter is divided
into two components, the rotor-side converter (Crotor) and the
grid-side converter (Cgrid). Crotor and Cgrid are Voltage-Source
Converters that use forced-commutated power electronic
devices (IGBTs) to synthesize an AC voltage from a DC
voltage source. A capacitor connected on the DC side acts
as the DC voltage source. A coupling inductor L is used to
connect Cgrid to the grid. The three-phase rotor winding is
connected to Crotor by slip rings and brushes and the threephase stator winding is directly connected to the grid.
(17)
The above equations 16-17 can be rewritten using
equations12-14 as
Ss = 3Rs |Is|2 + j3ω1Lsλ |Is|2 + j3ω1 |Ψm|2Lm+ 3Rm |IRm|2 −
j3ω1ΨmI∗r
Sr = 3Rr |Ir|2 + j3ω1sLrλ |Ir|2 + j3ω1sΨmI∗r .
Now the stator and rotor power can be determined as
Ps = Re [Ss] = 3Rs |Is|2 + 3Rm |IRm|2 + 3ω1Im [ΨmI∗r ] ≈ 3ω1Im
[ΨmI∗r ]
Pr = Re [Sr] = 3Rr |Ir|2 − 3ω1sIm [ΨmI∗r] ≈ −3ω1sIm [ΨmI∗r]
From the above equations the mechanical power
produced by the DFIG can be determined as the sum of the
stator and rotor power as
Pmech = 3ω1Im [ΨmI∗r ] − 3ω1sIm [ΨmI∗r ]
= 3ωrIm [ΨmI∗r]
Then, by dividing P mech with mechanical rotor speed, ωm
= ωr/np, the produced electromechanical torque, as given in
equation 15, can be found. Moreover, this means that
Ps ≈ Pmech/ (1 − s) and
Pr ≈ −sPmech/ (1 − s).
In Fig.11 the power flow of a “lossless” DFIG system
can be seen. In the figure it can be seen how the mechanical
power divides between the stator and rotor circuits and that
it is dependent on the slip. Moreover, the rotor power is
approximately minus the stator power times the slip: P r ≈
−sPs. Therefore, as mentioned earlier, the rotor converter
can be rated as a fraction of the rated power of the DFIG if
the maximum slip is low.
Fig.12. DFIG connected to Wind Turbine
The power captured by the wind turbine is converted into
electrical power by the induction generator and it is
transmitted to the grid by the stator and the rotor windings.
The control system generates the pitch angle command and
the voltage command signals Vr and Vgc for Crotor and Cgrid
respectively in order to control the power of the wind
turbine, the DC bus voltage and the voltage at the grid
terminals. An average model of the AC/DC/AC converter is
used for real-time simulation The DC bus is simulated by a
controlled current source feeding the DC capacitor. The
current source is computed on the basis of instantaneous
power conservation principle: the power that flows inside
the two AC-sides of the converter is equal to the power
absorbed by the DC capacitor.
3.8 DFIG with Energy Storage System (ESS)
The ESS consists of a super capacitor bank and a twoquadrant DC/DC converter connected to the dc link of the
DFIG as shown in Fig.13. The ESS serves as either a source
or a sink of active power, and therefore, contributes to
control the generated active power of the WTG.
Fig.13. DFIG of Wind Turbine connected with Energy
Storage System (ESS)
Fig.11. Power flow of a lossless DFIG
8
The dc/dc converter contains two IGBT switches S1 and
S2. Their duty ratios are controlled to regulate the active
power Pg that the GSC exchanges with the grid. In this
configuration, the dc/dc converter can operate in two
different modes, i.e., buck or boost mode, depending on the
status of the two IGBT switches. If S1 is closed and S2 is
open, the dc/dc converter operates in the buck mode; if S1 is
open and S2 is closed, the dc/dc converter operates in the
boost mode.
The duty ratio D1 of S1 can be approximately expressed
as and the duty ratio D2 of S2 is D2 = 1 – D1.
D1 
VSC
Vdc
Also the nominal dc-voltage ratio Scan/Vdc,n is 0.5,
where VSC,n and Vdc,n are the nominal voltages of the
super capacitor bank and the DFIG dc link, respectively.
Therefore, the nominal duty ratio D1,nof S1 is 0.5.The duty
ratio D1 of the dc/dc converter is controlled depending on
the relationship between the active powers (Pr)of the RSC
and (Pg)of the GSC. If Pr is greater than Pg, D1 is
controlled greater than 0.5. Consequently, the super
capacitor bank serves as a sink to absorb active power,
which results in the increase of its voltage VSC. On the
contrary, if Pg is greater than Pr then D1 is controlled less
than 0.5. Consequently, the super capacitor bank serves as a
source to supply active power, which results in the decrease
of its voltage VSC. Therefore, by controlling the duty ratio
of the dc/dc converter, the ESS serves as either a source or a
sink of active power to control the generated active power
of the WTG.
connected to it which each has a rating of 4 MW and 575v.
The wind turbines WTG1, WTG6, WTG9 are provided with
different wind speeds within the range of 10-14 m/s. And all
the remaining wind turbines are provided with constant
wind speed of 15 m/s.
Here the simulation is done in order to maintain the real
power supplied by the wind farm is to be maintained
constant. The constant real power is given as P ref to the wind
turbines under different conditions like wind turbines
operating without any energy storage system, operating with
energy storage system with two layer conventional
controllers. The amount of real power that has to be
maintained constant i.e., Pref is specified by the grid operator
and in this case the power is to be maintained at 36MW.
Also we will observe the variations of wind speed and
variations in voltages in the energy storage system for
WTG1, WTG6, WTG11 and the response of the system for
step changes in input power demand given by the grid
operator.
In Fig.14 the reference signal Pg* is generated by the
high-layer WFSC.
Fig.15. Doubly-Fed Induction Generator (DFIG) of wind
turbine
Fig.14. Configuration and control of ESS for DFIG
Wind Turbine
IV.CASE STUDY AND RESULTS
4.1 Case Study
Simulation studies are carried out to verify the
effectiveness of the proposed control schemes under various
operating conditions. Some typical results are shown and
discussed in this section. At one end grid is taken as 120kv
and it is stepped down to 25kv and it is further stepped
down to 575v and is given to different loads. At the other a
wind farm is connected which has 15 wind turbines
Fig.16. Control structure for Wind Turbine
9
power, or balancing the total current generated from the
overall system.
5.2 Future Scope
By the proposed system and schemes the control of
Reactive power in the wind farm, with few modifications in
the control scheme can also be obtained so that in case of
system sags or swells and during faults the wind farm
should be able to supply or consume the reactive power and
maintain the system voltage. So, as futures work the reactive
power control can also be done.
Fig.17. Grid side Control
VI. REFERENCES
Vd
1
Demux
L_RL
Vdqs
[1] E. Muljadi, T. Batan, D. Yildirim, and C. P. Butterfield,
“Understanding
[w_pu]
[iqr]
[idr]
R_RL
Id_ref
4
[idr]
Demux
Idq_ref
Id
v d'
PI
2
Demux
1
Demux
Vdq*
v q'
[2] The unbalanced-voltage problem in wind turbine
generation,” in Proc.
Idq
Iq
[3] IEEE Ind. Appl. Conf., 1999, vol. 2, pp. 1359–1365.
Iq_ref
[4] T. Brekken and N. Mohan, “A novel doubly-fed
induction wind generator
[iqr]
[iqr]
R_RL
[idr]
L_RL
Vq
[w_pu]
Fig.18. Grid side control Current regulator & Rotor side
control
3
Freq
[w_pu]
[5] Control scheme for reactive power control and torque
pulsation compensation.
Fnom
Fnom
V. CONCLUSION AND FUTURE SCOPE
[6] Under unbalanced grid voltage conditions,” in Proc.
IEEE Power
5.1 Conclusion
[7] Expo. Spec. Conf., Jun. 2003, vol. 2, pp. 760–766.
Enhanced control of a grid-connected DFIG-based wind
turbine with SGSC under unbalanced grid voltage
conditions has been investigated in this paper. The precise
current reference generation strategy for the PGSC has been
proposed, and a coordinated control strategy for the SGSC,
PGSC, and RSC is discussed. Three selective control targets
for PGSC including eliminating the oscillations in the total
active power or reactive power, or generating total balanced
current from the whole system have been obtained,
respectively, by coordinating control of the PGSC and
SGSC, while the RSC is controlled with the conventional
VC strategy to achieve the goals of zero oscillations in
DFIG’s active and reactive power and balanced stator and
rotor currents in the three-phase windings under unbalanced
voltage conditions. Furthermore, the function of SGSC need
not be changed during both the normal grid condition and
the unbalanced voltage condition for the three control
strategies, and the required dynamic regulation performance
can be provided with the proposed dual PI controllers for
the SGSC and PGSC. The proposed coordinated control
strategies have been validated using both simulation and
laboratory-scale experimental tests. The results show that
the enhanced operation and control of the DFIG-based
wind-power generation system with SGSC under
unbalanced conditions can be significantly improved by
eliminating the oscillations in the total active or reactive
[8] R. Piwko, N.Miller, J. Sanchez-Gasca, X. Yuan, R. Dai,
and J. Lyons, “Integrating
[9] Large wind farms into weak power grids with long
transmission
[10] Lines,” in Proc. IEEE Int. Power Electron. Motion
Control Conf., 2006,
[11] vol. 3, pp. 1122–1128.
[12] J. Kearney, M. F. Conlon, and E. Coyle, “The
integrated control of the