Flicker mitigation by individual pitch control of variable speed wind turbines with DFIG

A MINI PROJECT REPORT
ON
“FLICKER MITIGATION BY INDIVIDUAL PITCH CONTROL
OF VARIABLE SPEED WIND TURBINES WITH DFIG”
Submitted in partial fulfilment of the requirements for the award of degree of
BACHELOR OF TECHNOLOGY
in
ELECTRICAL AND ELECTRONICS ENGINEERING
By
CHANDUPATLA SRIKANTH (13B71A0211)
Under the guidance of
Ms. N. VIDYA RANI
(Assistant professor)
DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING
SINDHURA COLLEGE OF ENGINEERING & TECHNOLOGY
RAMAGUNDAM, GODHAVARIKHANI, DIST: PEDDAPALLI-505209
Affiliated to JNTU, Kukatpally, Hyderabad-500085
Approved by AICTE, New Delhi and recognized by Govt of T.S.
2013-2017

(Approved by AICTE New Delhi, affiliated to JNTUH, Hyderabad)
DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING
CERTIFICATE
This is to certify that the mini Project report entitled “FLICKER
MITIGATION BY INDIVIDUAL PITCH CONTROL OF
VARIABLE SPEED WIND TURBINES WITH DFIG” is submitted in
the particular fulfilment for the award of the degree of BACHELOR OF
TECHNOLOGY in ELECTRICAL AND ELECTRONICS ENGINEERING by
CHANDUPATLA SRIKANTH (13B71A0211) Bonafide student of
During the academic year 2016-2017
Ms. N.VIDYA RANI Mr. U.KRISHNA PRASAD
INTERNAL GUIDE H.O.D
Mr. R.NARAYAN DAS EXTERNAL EXAMINER
PRINCIPAL

ACKNOWLEDGEMENT
I express my sincere gratitude to my internal guide Ms. N.VIDYA RANI
Assistant Professor and Mr. MADANLAL CHOWDARY technical coordinator for
their constant support and Valuable suggestions during the work.
I express my sincere gratitude to Mr. U.KRISHNA PRASAD, Head of
Department, Electrical and Electronics Engineering, for providing me with
adequate facilities, ways and means by which I was able to complete this seminar.
I express my sincere thanks to our principal Mr. R.NARAYAN DAS for his
Encouragement and support throughout the endeavor.
I express my immense pleasure and thanks to all the teachers and staff of the
Department of Electrical and Electronics Engineering, SCET for their cooperation and
support.
I thank all others and especially our classmates and our family members who
in one way or another helped me in the successful completion of this work.

DECLARATION
I am the student of B-tech in electrical and electronics engineering, Sindhura
College of engineering and technology, Ramagundam hereby declare that the mini
project report entitled “FLICKER MITIGATION BY INDIVIDUAL PITCH
CONTROL OF VARIABLE SPEED WIND TURBINE WITH DFIG” is the
original work carried out by me to the best of my knowledge and belief. I hereby
declare this mini project bears to resemblance to any other project submitted at
Sindhura College of engineering and technology, Ramagundam or any other college
affiliated by JNTUH for the award of degree
CH.SRIKANTH
(13B71A0211)

CONTENTS
CHAPTER NAME PAGE NO
ABSTRACT i
LIST OF FIGURES ii
CHAPTER-1
INTRODUCTION 1
CHAPTER-2
DOUBLE FED INDUCTION GENERATOR (DFIG)
2.1 Principle OF a DFIG Connected To A Wind Turbine 3
2.2 Wound-Rotor DFIG
2.2.1 Construction 5
2.2.2 Electronic control 6
2.2.3 Efficiency 6
2.2.4 Power density 7
2.2.5 Cost 7
2.3 Doubly-Fed Induction Generator Models
2.3.1 DFIG model expressed in the ABC 8
reference frame
2.3.2 DFIG Model 8
CHAPTER-3
WIND TURBINE CONFIGURATION
3.1 Fast 10
3.2 Variable Speed Wind Turbine
3.2.1 Background 11
3.3 operating Strategies for Variable speed Wind turbines
3.3.1 Stall regulated 14

3.3.2 Below rated power 15
3.3.3 Rated power and above 15
3.3.4 Pitch regulated 16
3.3.5 Above rated power 17
3.4 Gearboxes 17
3.5 Generators 17
3.6 Grid connections 17
3.7 Power converters
3.7.1 Pulse width modulation 18
3.8 Mechanical drive train 21
CHAPTER-4
WIND TURBINE CONTROL & FLICKER EMISSION ANALYSIS
4.1 Control of back-to-back converter 22
4.2 The reason of flicker 23
4.3 Pitch control 24
4.4 Flicker Emission in Normal Operation 24
CHAPTER-5
INDIVIDUAL PITCH CONTROL FOR FLICKER MITIGATION
5.1 Individual Pitch Control 27
5.2 IPC for Flicker Mitigation 28
5.2.1 Design of BPF 29
5.2.2 Signal Processing 30
5.2.3 Individual Pitch Controller design 32
CHAPTER-6
MATLAB/SIMULINK CIRCUITS & RESULTS 33
CONCLUSION 37
REFERENCES 38

i
FLICKER MITIGATION BY INDIVIDUAL PITCH CONTROL
OF VARIABLE SPEED WIND TURBINES WITH DFIG
ABSTRACT
Due to the wind speed variation, wind shear and tower shadow effects, grid
connected wind turbines are the sources of power fluctuations which may produce
flicker during continuous operation. This paper presents a model of an MW-level
variable speed wind turbine with a doubly fed induction generator to investigate the
flicker emission and mitigation issues. An individual pitch control (IPC) strategy is
proposed to reduce the flicker emission at different wind speed conditions. The IPC
scheme is proposed and the individual pitch controller is designed according to the
generator active power and the azimuth angle of the wind turbine. The simulations are
performed on the NREL (National Renewable Energy Laboratory) 1.5-MW upwind
reference wind turbine model. Simulation results show that damping the generator
active power by IPC is an effective means for flicker mitigation of variable speed
wind turbines during continuous operation.

ii
LIST OF FIGURES
S.NO FIGURE NAME PAGE NO
Fig 1.1 Overall scheme of the DFIG-based wind turbine system………………….2
Fig 2.1 double fed induction genenrator..................................................................3
Fig 2.2 principle of DFIG connected to the wind turbine……………………….....4
Fig 2.3 Cross sectional view of a wound rotor induction machine………………..8
Fig 2.4 D −q equivalent circuit of DFIG at synchronously rotating
reference frame…………………………………………………………...9
Fig 3.1 Cp-λ curve for a typical wind turbine……………………………………...11
Fig 3.2 Plot for the relationship between torque and rotor speed of WT…………13
Fig 3.3 The depiction of the apparent wind speed, as seen by a blade…………...13
Fig 3.4 torque rotor speed diagram for a stall regulated wind turbine……………14
Fig 3.5 torque rotor speed diagram for a below rated power wind turbine……….15
Fig 3.6 torque rotor speed diagram for a above rated power wind turbine……….16
Fig 3.7 Two mass model of drive train……………………………………………21
Fig. 4.1 PI controller with anti wind up…………………………………………...22
Fig 4.2 Simplified diagram of a grid connected wind turbine…………………….23
Fig 4.3 Spectral density of the generator output power…………………………...25
Fig 4.4 Flicker severity Pst between the cases with 3p, higher harmonics
and wind speed variation (square), and the case with only wind
speed variation (circle)……………………………………………………..25
Fig 5.1 Proposed individual pitch control scheme………………………………..29
Fig 5.2 Bode diagram of the BPF (high wind speed)……………………………30
Fig 6.1 Matlab/simulink circuit for high wind with IPC scheme…………………33
Fig 6.2 Matlab/simulink circuit for high wind without IPC scheme……………...33
Fig 6.3 Long-term view of the generator active power without and with IPC,
and pitch angle (high wind speed)………………………………………....34

iii
Fig 6.4 Matlab/simulink circuit for low wind with IPC scheme………………….35
Fig 6.5 Matlab/simulink circuit for low wind without IPC scheme………………35
Fig 6.2 Long-term view of the generator active power without and with IPC,
and pitch angle (low wind speed)………………………………………….36

iv
LIST OF TABLES
S.NO TABLE NAME PAGE NO
Table 1 Control principle of individual pitch control………………….31

FLICKER MITIGATION BY INDIVIDUAL PITCH CONTROL OF VARIABLE
SPEED WIND TURBINES WITH DFIG
SIND, EEE Page 1
CHAPTER-1
INTRODUCTION
During the last few decades, with the growing concerns about energy shortage
and environmental pollution, great efforts have been taken around the world to
implement renewable energy projects, especially wind power projects. With the
increase of wind power penetration into the grid, the power quality becomes an
important issue. One important aspect of power quality is flicker since it could
become a limiting factor for integrating wind turbines into weak grids, and even into
relatively strong grids if the wind power penetration levels are high. Flicker is defined
as “an impression of unsteadiness of visual sensation induced by a light stimulus,
whose luminance or spectral distribution fluctuates with time”. Flicker is induced by
voltage fluctuations, which are caused by load flow changes in the grid. Grid-
connected variable speed wind turbines are fluctuating power sources during
continuous operation. The power fluctuations caused by wind speed variation, wind
shear, tower shadow, yaw errors, etc., lead to the voltage fluctuations in the network,
which may produce flicker. Apart from the wind power source conditions, the power
system characteristics also have impact on flicker emission of grid-connected wind
turbines, such as short-circuit capacity and grid impedance angle. The flicker
emission with different types of wind turbines is quite different. Though variable-
speed wind turbines have better performance with regard to the flicker emission than
fixed-speed wind turbines, with the large increase of wind power penetration level,
the flicker study on variable speed wind turbines becomes necessary and imperative.
A number of solutions have been presented to mitigate the flicker emission of
grid-connected wind turbines. The most commonly adopted technique is the reactive
power compensation. However, the flicker mitigation technique shows its limits in
some distribution networks where the grid impedance angle is low. When the wind
speed is high and the grid impedance angle is 10◦, the reactive power needed for
flicker mitigation is 3.26 per unit. It is difficult for a grid-side converter (GSC) to
generate this amount of reactive power, especially for the doubly fed induction
generator (DFIG) system, of which the converter capacity is only around 0.3 per unit.
The STATCOM which receives much attention is also adopted to reduce flicker

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emission. However, it is unlikely to be financially viable for distributed generation
applications. Active power control by varying the dc-link voltage of the back-to-back
converter is presented to attenuate the flicker emission. However, a big dc-link
capacitor is required, and the lifetime of the capacitor will be shortened to store of the
fluctuation power in the dc link. An open-loop pitch control is used to investigate the
flicker emission in high wind speeds, however, the pitch actuation system (PAS) is
not taken into account. Because the pitch rate and the time delay of the PAS make
great contributions to the results of the flicker emission of variable-speed wind
turbines, it is necessary to take these factors into consideration. In recent years, IPC
which is a promising way for loads reduction has been proposed, from which it is
notable that the IPC for structural load reduction has little impact on the electrical
power. However in this paper, an IPC scheme is proposed for flicker mitigation of
grid-connected wind turbines. The power oscillations are attenuated by individual
pitch angle adjustment according to the generator active power feedback and the wind
turbine azimuth angle in such a way that the voltage fluctuations are smoothed
prominently, leading to the flicker mitigation. The influence of the flicker emission on
the structural load is also investigated. The FAST (Fatigue, Aerodynamics, Structures,
and Turbulence) code which is capable of simulating three-bladed wind turbines is
used in the simulation.
Fig 1 Overall scheme of the DFIG-based wind turbine system

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CHAPTER-2
DOUBLE FED INDUCTION GENERATOR (DFIG)
DFIG is an abbreviation for Double Fed Induction Generator, a generating
principle widely used in wind turbines. It is based on an induction generator with a
multiphase wound rotor and a multiphase slip ring assembly with brushes for access
to the rotor windings. It is possible to avoid the multiphase slip ring assembly (see
brushless doubly-fed electric machines), but there are problems with efficiency, cost
and size. A better alternative is a brushless wound-rotor doubly-fed electric machine.
Fig 2.1 double fed induction genenrator
2.1 PRINCIPLE OF A DFIG CONNECTED TO A WIND TURBINE
The principle of the DFIG is that rotor windings are connected to the grid via
slip rings and back-to-back voltage source converter that controls both the rotor and
the grid currents. Thus rotor frequency can freely differ from the grid frequency (50
or 60 Hz). By using the converter to control the rotor currents, it is possible to adjust
the active and reactive power fed to the grid from the stator independently of the
generator's turning speed. The control principle used is either the two-axis current
vector control or direct torque control (DTC). DTC has turned out to have better
stability than current vector control especially when high reactive currents are
required from the generator.

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Fig 2.2 principle of DFIG connected to the wind turbine
The doubly-fed generator rotors are typically wound with 2 to 3 times the
number of turns of the stator. This means that the rotor voltages will be higher and
currents respectively lower. Thus in the typical ± 30 % operational speed range
around the synchronous speed, the rated current of the converter is accordingly lower
which leads to a lower cost of the converter. The drawback is that controlled
operation outside the operational speed range is impossible because of the higher than
rated rotor voltage. Further, the voltage transients due to the grid disturbances (three-
and two-phase voltage dips, especially) will also be magnified. In order to prevent
high rotor voltages - and high currents resulting from these voltages - from destroying
the IGBTs and diodes of the converter, a protection circuit (called crowbar) is used.
The crowbar will short-circuit the rotor windings through a small resistance
when excessive currents or voltages are detected. In order to be able to continue the
operation as quickly as possible an active crowbar has to be used. The active crowbar
can remove the rotor short in a controlled way and thus the rotor side converter can be
started only after 20-60 ms from the start of the grid disturbance. Thus it is possible to
generate reactive current to the grid during the rest of the voltage dip and in this way
help the grid to recover from the fault.

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A doubly fed induction machine is a wound-rotor doubly-fed electric machine
and has several advantages over a conventional induction machine in wind power
applications. First, as the rotor circuit is controlled by a power electronics converter,
the induction generator is able to both import and export reactive power. This has
important consequences for power system stability and allows the machine to support
the grid during severe voltage disturbances (low voltage ride through, LVRT).
Second, the control of the rotor voltages and currents enables the induction
machine to remain synchronized with the grid while the wind turbine speed varies. A
variable speed wind turbine utilizes the available wind resource more efficiently than
a fixed speed wind turbine, especially during light wind conditions. Third, the cost of
the converter is low when compared with other variable speed solutions because only
a fraction of the mechanical power, typically 25-30 %, is fed to the grid through the
converter, the rest being fed to grid directly from the stator. The efficiency of the
DFIG is very good for the same reason.
2.2 WOUND-ROTOR DOUBLY-FED ELECTRIC GENERATOR
2.2.1 CONSTRUCTION
Two multiphase winding sets with similar pole-pairs are placed on the rotor
and stator bodies, respectively. The wound-rotor doubly-fed electric machine is the
only electric machine with two independent active winding sets, the rotor and stator
winding sets, occupying the same core volume as other electric machines. Since the
rotor winding set actively participates in the energy conversion process with the stator
winding set, utilization of the magnetic core real estate is optimized. The doubly fed
generator operation at unity stator power factor requires higher flux in the air-gap of
the machine than when the machine is used as wound rotor induction machine. It is
quite common that wound rotor machines not designed to doubly fed operation
saturate heavily if doubly fed operation at rated stator voltage is attempted. Thus a
special design for doubly fed operation is necessary. A multiphase slip ring assembly
(i.e., sliding electrical contacts) is traditionally used to transfer power to the rotating
(moving) winding set and to allow independent control of the rotor winding set. The
slip ring assembly requires maintenance and compromises system reliability, cost and

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efficiency. Attempts to avoid the slip ring assembly are constantly being researched
with limited success (see Brushless doubly-fed induction electric machines).
2.2.2 ELECTRONIC CONTROL
The electronic controller, a frequency converter, conditions bi-directional (i.e.,
four quadrant), speed synchronized, and multiphase electrical power to at least one of
the winding sets (generally, the rotor winding set). Using four quadrant control, which
must be continuously stable throughout the speed range, a wound-rotor doubly-fed
electric machine with two poles (i.e., one pole-pair) has a constant torque speed range
of 7200 rpm when operating at 60 Hz. However, in high power applications two or
three pole-pair machines with respectively lower maximum speeds are common. The
electronic controller is smaller, less expensive, more efficient, and more compact than
electronic controllers of singly-fed electric machine because in the simplest
configuration, only the power of the rotating (or moving) active winding set is
controlled, which is less than half the total power output of the electric machine. Due
to the lack of damper windings used in synchronous machines, the doubly fed electric
machines are susceptible to instability without stabilizing control. Like any
synchronous machine, losing synchronism will result in alternating torque pulsation
and other related consequences. Doubly-fed electric machines require electronic
control for practical operation and should be considered an electric machine system or
more appropriately, an adjustable-speed drive.
2.2.3 EFFICIENCY
Neglecting the slip ring assembly, the theoretical electrical loss of the wound-
rotor doubly-fed machine in super synchronous operation is comparable to the most
efficient electric machine systems available (i.e., the synchronous electric machine
with permanent magnet assembly) with similar operating metrics because the total
current is split between the rotor and stator winding sets while the electrical loss of
the winding set is proportional to the square product of the current flowing through
the winding set. Further considering the electronic controller conditions less than 50%
of the power of the machine, the wound-rotor doubly-fed electric motor or generator
(without brushes and with stable control at any speed) theoretically shows nearly half

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the electrical loss (i.e., winding set loss) of other electric motor or generator systems
of similar rating.
2.2.4 POWER DENSITY
Neglecting the slip ring assembly and considering similar air-gap flux density,
the physical size of the magnetic core of the wound-rotor doubly-fed electric machine
is smaller than other electric machines because the two active winding sets are
individually placed on the rotor and stator bodies, respectively, with virtually no real-
estate penalty. In all other electric machines, the rotor assembly is passive real estate
that does not actively contribute to power production. The potential of higher speed
for a given frequency of excitation, alone, is an indication of higher power density
potential. The constant-torque speed range is up to 7200 rpm at 60 Hz with 2 poles
compared to 3600 rpm at 60 Hz with 2 poles for other electric machines. In theory,
the core volume is nearly half the physical size (i.e., winding set loss) of other electric
motor or generator systems of similar rating.
2.2.5 COST
Neglecting the slip ring assembly, the theoretical system cost is nearly 50%
less than other machines of similar rating because the power rating of the electronic
controller, which is the significant cost of any electric machine system, is 50% (or
less) than other electric motor or generator systems of similar rating.
2.3 DOUBLY-FED INDUCTION GENERATOR (DFIG) MODELS
For the purposes of better understanding and designing vector control schemes in a
wind turbine-generator system, it is necessary to know the dynamic model of the
machine subjected to control. A model of the electrical machine which is adequate for
designing the control system must preferably incorporate all the important dynamic
effects occurring during steady state and transient operations. It should be valid for
any arbitrary time variations of the voltages and currents generated by the
converter which supplies the machine. In this section, such a model which is valid for
any instantaneous variations of the voltages and currents, and can adequately describe
the performance of the machine under both steady state and transient operations, will

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be developed in both the ABC reference frame and several different dqo reference
frames.
2.3.1 DFIG MODEL EXPRESSED IN THE ABC REFERENCE FRAME
For simplicity, a wound rotor induction machine is considered with symmetrical
two poles and three-phase windings. The cross-sectional view of the machine
under consideration, where the effects of slotting have been neglected.
Fig 2.3 Cross sectional view of a wound rotor induction machine
2.3.2 DFIG MODEL
The model of the DFIG is based on dq equivalent model. All electrical variables are
referred to the stator. uds, uqs, udr, uqr, ids, iqs, idr, iqr and ψds, ψqs, ψdr, ψqr are the
voltages, currents, and ﬂux linkages of the stator and rotor in d- and q-axes, rs and rr
are the resistances of the stator and rotor windings, Ls, Lr, Lm are the stator, rotor,

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Fig 2.4 D −q equivalent circuit of DFIG at synchronously rotating reference frame
and mutual inductances, L1s,L1r are the stator and rotor leakage inductances, w1 is
the speed of the reference frame, ws is the slip angular electrical speed. The RSC of
DFIG is controlled in a synchronously rotating d-q reference frame with the d-axis
aligned along the stator ﬂux position. The electrical torque Te, active power Ps, and
reactive power Qs of DFIG can be expressed as
where p is the number of pole pairs, ψs is the stator ﬂux, us is the magnitude of the
stator phase voltage. From (4) and (5), due to the constant stator voltage, the active
power and reactive power can be controlled via iqr and idr.

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CHAPTER-3
WIND TURBINE CONFIGURATION
The overall scheme of a DFIG-based wind turbine system is shown in Fig. 1, which
consists of a wind turbine, gearbox, DFIG, a back-to-back converter which is
composed of a rotor side converter RSC and GSC, and a dc-link capacitor as energy
storage placed between the two converters. In this paper, FAST is used to simulate the
mechanical parts of wind turbine and the drive train. The pitch and converter
controllers, DFIG, and power system are modeled by Simulink blocks.
3.1 FAST
The open source code FAST is developed at the National Renewable Energy
Laboratory (NREL) and accessible and free to the public. FAST can be used to model
both two and three bladed, horizontal-axis wind turbines. It uses Blade Element
Momentum theory to calculate blade aerodynamic forces and uses an assumed
approach to formulate the motion equations of the wind turbine. For three-bladed
wind turbines, 24 degree of freedoms (DOFs) are used to describe the turbine
dynamics. Their models include rigid parts and ﬂexible parts. The rigid parts include
earth, base plate, nacelle, generator, and hub. The ﬂexible parts include blades, shaft,
andtower.FASTrunssignificantlyfastbecauseoftheuseofthemodalapproachwithfewer
DOFs to describe the most important parts of turbine dynamics.
3.2 VARIABLE SPEED WIND TURBINE
Original models of wind turbines were fixed speed turbines; that is, the rotor speed
was a constant for all wind speeds. The tip-speed ratio for a wind turbine is given by
the following formula:
where is the rotor speed (in radians per second), R is the length of a blade, and is
the wind speed. That is to say, for a fixed-speed wind turbine, the value of the tip-
speed ratio is only changed by wind speed variations. In reference to a - graph,

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which illustrates the relationship between Tip-speed ratio and efficiency, it is evident
that only one value of yields the highest efficiency. That is, the fixed speed wind
turbine is not operating at peak efficiency across a range of wind speeds. This was a
motivator for the development of variable speed wind turbines.
3.2.1 BACKGROUND
All wind turbines that generated electricity were variable speed before 1939.[1]
All
grid-connected wind turbines, from the first one in 1939 until the development of
variable-speed grid-connected wind turbines in the 1970s, were fixed-speed wind
turbines. As of 2003, nearly all grid-connected wind turbines operate at exactly
constant speed (synchronous generators) or within a few percent of constant speed
(induction generators).[1]
Cp-λ curves Below is an illustration of the Cp-λ curve for a
typical wind turbine.
Fig 3.1 Cp-λ curve for a typical wind turbine
Maximum efficiency occurs at one tip-speed ratio only. Since tip-speed ratio is given
by the aforementioned expression, variable speed wind turbines can operate at
maximum efficiency over all wind speeds (ideally).
Torque Rotor-speed diagrams
For a wind turbine, the power harvested is given by the following formula:

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where is the power, is the density of the air, is the length of the blade, is the
velocity of the wind, and is the power co-efficient for the wind turbine. The power
co-efficient is a representation of how much of the available power in the wind is
captured by the wind turbine.
The torque, , on the blades is given by the ratio of the power extracted to the rotor
speed, :
The rotor speed can be related to the wind speed, , through the tip-speed ratio, :
Thus we can get the following expressions for torque and power:
and
From the above equation, we can construct a torque- rotor speed diagram for a wind
turbine. This consists of multiple curves: a constant power curve which plots the
relationship between torque and rotor speed for constant power (green curve);
constant wind speed curves, which plot the relationship between torque and rotor
speed for constant wind speeds (dashed grey curves); and constant efficiency curves,
which plot the relationship between torque and rotor speed for constant efficiencies,
. This diagram is presented below:

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Fig 3.2 Plot for the relationship between torque and rotor speed of WT
Notes
Green curve: Plot of power = rated power
Grey curve: Wind speed, , is held constant
Blue curve: Constant
Blade forces
For further details, see Blade Element Momentum Theory
Consider the following figure:
Fig 3.3 The depiction of the apparent wind speed, as seen by a blade

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This is the depiction of the apparent wind speed, as seen by a blade (left of
figure). The apparent wind speed is influenced by both the free-stream velocity of the
air, and the rotor speed. From this figure, we can see that both the angle and the
apparent wind speed are functions of the rotor speed, . By extension, the lift and
drag forces will also be functions of. This means that the axial and tangential forces
that act on the blade vary with rotor speed. The force in the axial direction is given by
the following formula:
3.3 OPERATING STRATEGIES FOR VARIABLE SPEED WIND
TURBINES
3.3.1 STALL REGULATED
As discussed earlier, a wind turbine would ideally operate at its maximum efficiency
for below rated power. Once rated power has been hit, the power is limited. This is for
two reasons: ratings on the drive train equipment, such as the generator; and second to
reduce the loads on the blades. An operating strategy for a wind turbine can thus be
divided into a sub-rated-power component, and a rated-power component.
Fig 3.4 torque rotor speed diagram for a stall regulated wind turbine

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3.3.2 BELOW RATED POWER
Below rated power, the wind turbine will ideally operate in such a way that
. On a Torque-rotor speeds diagram, this looks as follows
where the black line represents the initial section of the operating strategy for a
variable speed stall-regulated wind turbine. Ideally, we would want to stay on the
maximum efficiency curve until rated power is hit. However, as the rotor speed
increases, the noise levels increase. To counter this, the rotor speed is not allowed to
increase above a certain value. This is illustrated in the figure below:
Fig 3.5 torque rotor speed diagram for a below rated power wind turbine
3.3.3 RATED POWER AND ABOVE
Once the wind speed has reached a certain level, called rated wind speed, the turbine
should not be able to produce any greater levels of power for higher wind speeds. A
stall-regulated variable speed wind turbine has no pitching mechanism. However, the
rotor speed is variable. The rotor speed can either be increased or decreased by an
appropriately designed controller. In reference to the figure illustrated in the blade
forces section, it is evident that the angle between the apparent wind speed and the
plane of rotation is dependent upon the rotor speed. This angle is termed the angle of
attack. The lift and drag co-efficients for an airfoil are related to the angle of attack.
Specifically, for high angles of attack, an airfoil stalls. That is, the drag substantially

SIND, EEE Page 16
increases. The lift and drag forces influence the power production of a wind turbine.
This can be seen from an analysis of the forces acting on a blade as air interacts with
the blade (see the following link). Thus, forcing the airfoil to stall can result in power
limiting. So it can be established that if the angle of attack needs to be increased to
limit the power production of the wind turbine, the rotor speed must be reduced.
Again, this can be seen from the figure in the blade forces section. It can also be seen
from considering the torque-rotor speed diagram. In reference to the above torque-
rotor speed diagram, by reducing the rotor speed at high wind speeds, the turbine
enters the stall region, thus bringing some limiting to the power output.
Fig 3.6 torque rotor speed diagram for a above rated power wind turbine
3.3.4 PITCH REGULATED
Pitch regulation allows the wind turbine to actively change the angle of attack of the
air on the blades. This is preferred over a stall-regulated wind turbine as it enables far
greater control of the power output.
Below rated power
Identical to the stall-regulated variable-speed wind turbine, the initial operating
strategy is to operate on the curve. However, due to constraints such as noise
levels, this is not possible for the full range of sub-rated wind speeds. Below the rated
wind speed, the following operating strategy is employed:

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3.3.5 ABOVE RATED POWER
Above the rated wind speed, the pitching mechanism is employed. This allows a good
level of control over the angle of attack, thus control over the torque. The previous
torque rotor-speed diagrams are all plots when the pitch angle, , is zero. A three
dimensional plot can be produced which includes variations in pitch angle.
Ultimately, in the 2D plot, above rated wind speed, the turbine will operate at the
point marked 'x' on the diagram below.
3.4 GEARBOXES
A variable speed may or may not have a gearbox, depending on the manufacturer's
desires. Wind turbines without gearboxes are called direct-drive wind turbines. An
advantage of a gearbox is that generators are typically designed to have the rotor
rotating at a high speed within the stator. Direct drive wind turbines do not exhibit this
feature. A disadvantage of a gearbox is reliability and failure rates An example of a
wind turbine without a gearbox is the Enercon E82.
3.5 GENERATORS
For variable speed wind turbines, one of two types of generators can be used:
a DFIG (Doubly-fed induction generator) or an FRC (fully rated converter). A DFIG
generator draws reactive power from the transmission system; this can increase the
vulnerability of a transmission system in the event of a failure. A DFIG configuration
will require the generator to be a wound rotor; squirrel cage rotors cannot be used for
such a configuration. A fully rated converter can either be an induction generator or a
permanent magnet generator. Unlike the DFIG, the FRC can employ a squirrel cage
rotor in the generator; an example of this is the Siemens SWT 3.6-107, which is
termed the industry workhorse. An example of a permanent magnet generator is the
Siemens SWT-2.3-113. A disadvantage of a permanent magnet generator is the cost
of materials that need to be included.
3.6 GRID CONNECTIONS
Consider a variable speed wind turbine with a permanent magnet synchronous
generator. The generator produces AC electricity. The frequency of the AC voltage

SIND, EEE Page 18
generated by the wind turbine is a function of the speed of the rotor within the
generator:
where is the rotor speed, is the number of poles in the generator, and is the
frequency of the output Voltage. That is, as the wind speed varies, the rotor speed
varies, and so the frequency of the Voltage varies. This form of electricity cannot be
directly connected to a transmission system. Instead, it must be corrected such that its
frequency is constant. For this, power converters are employed, which results in the
de-coupling of the wind turbine from the transmission system. As more wind turbines
are included in a national power system, the inertia is decreased. This means that the
frequency of the transmission system is more strongly affected by the loss of a single
generating unit.
3.7 POWER CONVERTERS
As already mentioned, the voltage generated by a variable speed wind turbine is non-
grid compliant. In order to supply the transmission network with power from these
turbines, the signal must be passed through a power converter, which ensures that the
frequency of the voltage of the electricity being generated by the wind turbine is the
frequency of the transmission system when it is transferred onto the transmission
system. Power converters first convert the signal to DC, and then convert the DC
signal to an AC signal. Techniques used include pulse width modulation
3.7.1 PULSE WIDTH MODULATION
PWM is the most effective means to achieve constant voltage battery charging by
switching the solar system controller’s power devices. When in PWM regulation, the
current from the solar array tapers according to the battery’s condition and recharging
needs Consider a waveform such as this: it is a voltage switching between 0v and 12v.
It is fairly obvious that, since the voltage is at 12v for exactly as long as it is at 0v,
then a 'suitable device' connected to its output will see the average voltage and think it
is being fed 6v - exactly half of 12v. So by varying the width of the positive pulse -
we can vary the 'average' voltage.

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Similarly, if the switches keep the voltage at 12 for 3 times as long as at 0v,
the average will be 3/4 of 12v - or 9v, as shown below
and if the output pulse of 12v lasts only 25% of the overall time, then the average is
By varying - or 'modulating' - the time that the output is at 12v (i.e. the width
of the positive pulse) we can alter the average voltage. So we are doing 'pulse width
modulation'. I said earlier that the output had to feed 'a suitable device'. A radio would
not work from this: the radio would see 12v then 0v, and would probably not work
properly. However a device such as a motor will respond to the average, so PWM is a
natural for motor control.
So, how do we generate a PWM waveform? It's actually very easy, there are
circuits available in the TEC site. First you generate a triangle waveform as shown in
the diagram below. You compare this with a d c voltage, which you adjust to control
the ratio of on to off time that you require. When the triangle is above the 'demand'
voltage, the output goes high. When the triangle is below the demand voltage, the

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When the demand speed it in the middle (A) you get a 50:50 output, as in
black. Half the time the output is high and half the time it is low. Fortunately, there is
an IC (Integrated circuit) called a comparator: these come usually 4 sections in a
single package. One can be used as the oscillator to produce the triangular waveform
and another to do the comparing, so a complete oscillator and modulator can be done
with half an IC and maybe 7 other bits.
The triangle waveform, which has approximately equal rise and fall slopes, is
one of the commonest used, but you can use a saw tooth (where the voltage falls
quickly and rinses slowly). You could use other waveforms and the exact linearity
(how good the rise and fall are) is not too important.
Traditional solenoid driver electronics rely on linear control, which is the
application of a constant voltage across a resistance to produce an output current that
is directly proportional to the voltage. Feedback can be used to achieve an output that
matches exactly the control signal. However, this scheme dissipates a lot of power as
heat, and it is therefore very inefficient.
A more efficient technique employs pulse width modulation (PWM) to
produce the constant current through the coil. A PWM signal is not constant. Rather,
the signal is on for part of its period, and off for the rest. The duty cycle, D, refers to
the percentage of the period for which the signal is on. The duty cycle can be
anywhere
from 0, the signal is always off, to 1, where the signal is constantly on. A 50% D
results in a perfect square wave. (Figure 1)

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A solenoid is a length of wire wound in a coil. Because of this configuration,
the solenoid has, in addition to its resistance, R, a certain inductance, L. When a
voltage, V, is applied across an inductive element, the current, I, produced in that
element does not jump up to its constant value, but gradually rises to its maximum
over a period of time called the rise time (Figure 2). Conversely, I does not disappear
instantaneously, even if V is removed abruptly, but decreases back to zero in the same
amount of time as the rise time.
3.8 MECHANICAL DRIVETRAIN
In order to take into account the effects of the generator and drivetrain on the wind
turbine, two-mass model shown below
Fig 3.7 Two mass model of drive train
Which is suitable for transient stability analysis issued The drive train modeling is
implemented in FAST, and all values are referred to the wind turbine side. The
equations for modeling the drive train are given by
where Jw and Jg are the moment of inertia of wind turbine and generator, respectively,
Tw, Te are the wind turbine torque and generator electromagnetic torque, respectively,
θw, θg are the mechanical angle of wind turbine and generator, K is the drive train
torsional spring, D is the drive train torsional damper.

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CHAPTER-4
WIND TURBINE CONTROL AND FLICKER EMISSION
ANALYSIS
For a DFIG-based variable speed wind turbine, the control objective is
different according to different wind speed. In low wind speed, the control goal is to
keep the tip speed ratio optimum, so that the maximum power can be captured from
the wind. In high wind speed, since the available power is beyond the wind turbine
capacity, which could overload the system, the control objective is to keep the
extracted power constant at its rated value.
4.1 CONTROL OF BACK-TO-BACK CONVERTER
Vector control techniques are the most commonly used methods for a back-to-
back converter in a wind turbine system. Two vector control schemes are illustrated,
respectively, for the RSC and GSC, as shown, where vs, and is are the stator
Fig. 4.1 PI controller with anti wind up
Voltage and current, ir is the rotor current, vg is the grid voltage, ig is the GSC
currents, wg is the generator speed, E is the dc-link voltage, Ps ref, and Qs ref are the
reference values of the stator active and reactive power, Qr ref is the reference value
of the reactive power ﬂow between the grid and the GSC, Eref is the reference value of
the dc-link voltage, C is the dc-link capacitor. The vector control objective for RSC is
to implement maximum power tracking from the wind by controlling the electrical
torque of DFIG. The reference value of the generator speed ωref is obtained via a
lookup table to enable the optimal tip speed ratio. The objective of GSC is to keep the

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dc-link voltage constant, while keeping sinusoidal grid currents. It may also be
responsible for controlling the reactive power ﬂow between the grid and the grid-side
converter by adjusting Qg ref. usually, the values of reactive power of RSC and GSC
are set to zero to ensure unity power factor operation and reduce the current of RSC
and GSC.
4.2 THE REASON OF FLICKER
Flicker is induced by voltage fluctuations, which are caused by load flow
changes in the grid. Grid-connected wind turbines may have considerable fluctuations
in output power, which depend on the wind power generation technology applied. The
flicker emission produced by grid-connected wind turbines during continuous
operation is mainly caused by fluctuations in the output power due to wind speed
variations, the wind gradient and the tower shadow effect. As a consequence of the
combination of wind speed variations, the wind gradient and the tower shadow effect,
an output power drop will appear three times per revolution for a three blade wind
turbine. This frequency is normally referred to as p 3. For fixed speed wind turbines
with induction generators, power pulsations up to 20% of the average power at the
frequency of p 3 can be generated [6]. Frequency analyses of the output power from
grid connected wind turbines show [7, 8], in addition to the dominating periodic
component p 3 , the p 6 , p 9 , p 12 and p 18 components are visible too. The
simplified diagram of a grid-connected wind turbine is shown
Fig 4.2 Simplified diagram of a grid connected wind turbine
The generator represents the wind turbine which is connected to the grid
through a line, PCC represents the Point of Common Coupling, E& is the voltage at
the PCC [V], V& is the voltage of the external grid [V], R and X are the line
resistance and reactance [Ω], P and Qare the active [W] and reactive [Var] power flow
produced by the wind turbine respectively.

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The voltage change across the connection linemay be approximately calculated with
the following formula [9]:
Assuming the grid voltage is constant, any fluctuations in the active or reactive power
produced by the wind turbine results in voltage fluctuations and flicker at the PCC.
4.3 PITCH CONTROL
Normally, pitch control is used to limit the aerodynamic power captured from
the wind. In low wind speeds, the wind turbine should simply try to produce as much
power as possible, so there is no need to pitch the blades. For wind speeds above the
rated value, the pitch control scheme is responsible for limiting the output power. The
PI controller used for adjusting the pitch angles works well in normal operation,
however, the performance of the pitch control system will degrade when a rapid
change in wind speed from low to high wind speed is applied to the turbine rotor. It
takes a long time for a positive power error contribution to cancel the effects of the
negative pitch angle contribution that has been built up from integration of these
negative power errors. The integrator anti windup scheme is implemented as shown in
Fig. 4, in which the anti windup term with gain Kaw is fed back to the integrator only.
This prevents the integrated power error from accumulating when the rotor is
operating in low wind speeds. The value for Kaw may be turbine dependent. When
the pitch angle is not saturated, this anti windup feedback term is zero [14].
4.4 FLICKER EMISSION IN NORMAL OPERATION
As discussed in Section, flicker emission of a grid-connected wind turbine
system is induced by voltage fluctuations which are caused by load flow changes in
the network, so it is necessary to analyze the electrical power to the grid. Therefore, a
simulation is conducted when the mean wind speed is 13 m/s based on the model as
shown in Fig. 1. The parameters of the wind turbine system are given in the
Appendix. In this case, the turbine speed is around 0.345 Hz, which corresponds to

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Fig 4.3 Spectral density of the generator output power
Fig 4.4 Flicker severity Pst between the cases with 3p, higher harmonics and wind speed
variation (square), and the case with only wind speed variation (circle).

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The 3p frequency of 1.035 Hz, which is in conformation with the
spectrumshowninFig.5.Itisclearlyseenthatinadditiontothe 3p frequency, 6p, 9p, and
higher frequencies are also included in the generator output power. These components
will induce voltage fluctuations and flicker emission in the power grid. Further, the
flicker emission of a variable-speed wind turbine with DFIG is studied. The level of
flicker is quantified by the short-term flicker severity Pst, which is normally measured
over a 10-min period. According to IEC standard IEC 61000-4-15, a flicker meter
model is adopted to calculate the short-term flicker severity Pst, the variation of
flicker severity Pst with different mean wind speed between the cases with 3p, higher
harmonics and wind speed variation and with only wind speed variation, respectively.
In the first case, in low wind speeds, with the increase of mean wind speed the Pst
increases accordingly, because higher mean wind speed with the same turbulence
intensity means larger power oscillation and larger wind shear and tower shadow
effects, leading to higher flicker severity.
For high wind speeds, where the wind turbine reaches rated power, the flicker
level decreases due to the introduction of PI blade pitch control which could reduce
the power oscillation in low frequency prominently, but it cannot effectively mitigate
the power oscillations with 3p, 6p, 9p, and higher frequencies. As the power
oscillation is bigger for higher wind speeds when the wind speed is above the rated
wind speed, the flicker level continues to rise with the increase of mean wind speed.
In the case with only the wind speed variation, in low wind speeds the flicker
emission has the similar situation, only the Pst is relatively smaller. In high wind
speed, the Pst is much smaller, since the power oscillation contains little 3p and
higher harmonics. From this figure, it can be concluded that the 3p and higher
harmonics make a great contribution to the flicker emission of variable speed wind
turbines with DFIG during continuous operation, especially in high wind speeds as
shown. It is recommended that the flicker contribution from the wind farm at the point
of common coupling shall be limited so that a flicker emission of Pst below0.35 is
considered acceptable. The maximum Pst is above 0.35 in this investigation where the
turbulence intensity is 10%. As proved in [6], Pst will increase with the increase of
the turbulence intensity; therefore, it is necessary to reduce the flicker emission. For
this reason, a new control scheme for flicker mitigation by individual pitch control is
proposed in next section.

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CHAPTER-5
INDIVIDUAL PITCH CONTROL FOR FLICKER
MITIGATION
5.1 INDIVIDUAL PITCH CONTROL
How can designers build wind turbines with longer lifetimes? Recent
economic and technical developments such as the pressure to reduce the overall cost
of electricity generated by wind turbines, the necessity to reduce O&M costs as well
as increased emphasis on reliability and predictability of power production make it
urgent to find a technical solution to that question.
Load reduction is a key element of the solution. In addition, load reduction
gains an increasing importance due to the trend towards larger wind turbines.
Individual pitch control (IPC) plays a key role in compensating loads. So what is IPC?
Any pitch control system allows control of the turbine speed and consequently the
power output. It also acts as a brake, stopping the rotor by turning the blades.
Moreover, pitch control, especially an IPC system, has a role in reducing fatigue
loads on the turbine structures. Recently developed wind turbines are variable speed
turbines capable of adapting to various wind conditions. This adaption is realized via
new generator concepts on the one hand, and a pitch control system on the other hand.
Pitch control means the turning of rotor blades between 0° and 90°.
When wind speeds are below rated power, typically below 12 m/s, the rotor
blades are turned fully towards the wind which means that the pitch is positioned at
0°. At increasing wind speeds the pitch of the blades is controlled in order to limit the
power output of the turbine to its nominal value. When wind speeds reach a
predefined threshold, typically 28 m/s, the turbine stops power production by turning
the blades to a 90° position. Collective pitch control adjusts the pitch of all rotor
blades to the same angle at the same time.
In contrast, IPC dynamically and individually adjusts the pitch of each rotor blade.
Based on current individual loads this pitch adjustment is carried out in real-time. The
main benefit of IPC is the reduction of fatigue loads on the rotor blades, the hub, and

SIND, EEE Page 28
mainframe and tower structures. In order to compensate these loads, especially
symmetric loads caused by inhomogeneous wind fields, the pitch of each rotor blade
has to be adjusted independently from the other blades. A reduction of fatigue loads
has two considerable advantages: It allows lighter designs and translates into longer
lifetimes of wind turbines. What is meant by lighter designs? In cases where
components are designed according to fatigue loads, a reduction of these loads allows
savings in cost and material notably for the rotor blades and the tower structure,
which are the most expensive elements of a wind turbine.
Moreover, lighter rotor blades enable a more efficient turbine, especially in low wind
conditions. Finally the load reduction through IPC gives designers the option to
develop low wind turbines from existing designs, which means a reduction of time to
market.
5.2 IPC FOR FLICKER MITIGATION
This section concentrates on flicker mitigation of variable speed wind turbines
with DFIG during continuous operation using IPC. The flicker emission produced by
grid connected wind turbines during continuous operation is mainly caused by
fluctuations in the generator active power. As illustrated in Fig. 6, the flicker emission
will be mitigated effectively if the 3p and higher harmonics of the generator power
can be reduced. When the wind speed is above the rated wind speed, the pitch angle
should be tuned by a traditional collective pitch control (CPC) to keep the output
power at its rated value in order not to overload the system, and normally the 3p effect
is not taken into consideration. For attenuating the generator power oscillation caused
by the 3p effect, each of the three pitch angles can be added by a small pitch angle
increment, which is dependent on the generator active power and wind turbine
azimuth angle. When the wind speed is below the rated wind speed, usually the
control objective of the wind turbine is to implement maximum power track in g by
generator electrical torque control. Pitch control is not used in this area. However if
the pitch angles can be adjusted around a small average value, the 3p effect can also
be reduced. For this purpose, the output of the CPC should leave a small amount of
residual for pitch movement. This means a small part of wind energy will be lost.
Based on this concept, a novel IPC strategy is proposed. The control scheme is shown
below. The control scheme consists of two control loops: CPC loop and IPC loop.

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Fig 5.1 Proposed individual pitch control scheme.
The CPC loop is responsible for limiting the output power. In this loop, Pg ref is the
reference generator power which can be calculated according to different wind speed,
Pg is the generator active power, β is the collective pitch angle, of which the
minimum value βmin can be obtained by simulations under different wind speed such
that the mitigation of generator power ﬂuctuation should compromise the wind power
loss. In the individual pitch control loop, the Band Pass Filter (BPF) is to let the
frequency of 3p generator active power Pg3p through and block all other frequencies.
Pg3p is fed to the signal processing (SP) block, since the power signal has to be
transferred to the pitch signal βs which subsequently is passed to the individual pitch
controller to output a pitch increment for a speciﬁc blade. The three pitch angles
β1,2,3 which are, respectively, the sum of collective pitch angles, and three pitch
angle increments are sent to the PAS to adjust the three pitch angles to implement the
mitigation of the generator active power oscillation.
5.2.1 DESIGN OF BPF
The transfer function of the BPF can be expressed as follows
where ωc is the center frequency, K is the gain, and Q is the quality factor. ωc which
corresponds to the 3p frequency can be calculated by the measurement of the
generator speed ωg. ωc = 3ωg/N, where N is the gear ratio. The gain of the BPF at the

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center frequency is designed as 1 in order to let all the 3p frequencies pass the ﬁlter
(F(s)=KQ/ωc = 1). Q which is responsible for the bandwidth of the BPF should be
adjusted to let only the 3p component pass. In this case, Q is designed as Q = ωc. the
Bode diagram of the BPF when the wind speed is above the rated value. In this case,
the 3p frequency is 6.44 rad/s, and the bandwidth of the BPF which is around is 0.16
Hz (1 rad/s) is shown with the dotted lines.
5.2.2 SIGNAL PROCESSING
The SP block has to produce a pitch signal to offset the power oscillation, in such
away that the generator power will oscillate in a much smaller range.
Fig 5.2 Bode diagram of the BPF (high wind speed)
Due to the time delay caused by the PAS and the power transfer from wind turbine
rotor to the power grid, etc., the phase of the generator active power lags the phase of
the pitch signal. In order to produce the correct phase angle shift of the SP block, it is
very important to get the phase deviation of the component with 3p frequency of β

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and Pg3p. For this reason, the system is operated in high wind speed without the IPC
loop. In this case, the collective pitch angle β contains the component with 3p
frequency. The phase angle shift can be obtained by the component of β with 3p
frequency and Pg3p. The SP block can be implemented with a first-order lag element,
which delays the phase angle at 3p frequency. The SP block can be represented as
follows:
The angular contribution of (7) is
Hence, the time constant Tsp can be calculated with the required angular contribution
δ at ω3p, shown as follows:
Azimuth angle (α) βs
0<α<2π/3 βΔ1
4π/3>α>2π/3 βΔ2
2π>α>4π/3 βΔ3
Table 1 control principle of individual pitch control
where ω3p is the center frequency of the BPF. The gain Ksp can be tuned by testing,
as it has no contribution to the phase shift of the SP block. Increasing Ksp can
accelerate the flicker mitigation; however, a big value of Ksp might increase the
flicker emission of the wind turbine.

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5.2.3 INDIVIDUAL PITCH CONTROLLER DESIGN
The individual pitch controller will output the three pitch angle increments
βΔ1,Δ2,,Δ3 for each blade based on the pitch signal βs and the azimuth angle θ. In
this paper, the wind turbine is simulated by FAST, in which blade 3 is ahead of blade
2, which is ahead of blade 1, so that the order of blades passing through a given
azimuth is 3– 2–1-repeat. The individual pitch controller will output a pitch increment
signal which will be added to the collective pitch angle for a specificblade, dependent
on the blade azimuth angle. The principle of the individual pitch controller is
described in Table. For example, if the azimuth angle belongs to the area of (0, 2π/3),
then βΔ2 equals βs, and both βΔ1 and βΔ3 equal 0. The three pitch increments will
be, respectively, added with the collective pitch angle to give three total pitch angle
demands. The three pitch angle signals will be sent to the PAS. The PAS can be
represented using a first-order transfer function:
where Tpas which is a turbine dependent time constant of the PAS. In this case
Tpas = 0.1. The control scheme shown in Fig. 7 is used for mitigation of the 3p
component of the generator active power, leading to the reduction of the flicker
emission which is caused by the 3p effect. Similar method can also be used to reduce
the 6p component of the generator active power. However, this 6p component
mitigation needs a much faster pitch actuation rate, which is not taken into account in
this paper

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CHAPTER-6
MATLAB/SIMULINK CIRCUITS & RESULTS
The ﬂicker mitigation using IPC is tested in many wind speed conditions. The
variable speed wind turbine with DFIG and back-to-back converter are simulated with
the proposed IPC method. The parameters of NREL 1.5-MW wind turbine with DFIG
are shown in the Appendix. Figs. 9 and 10 illustrate the short-term view and long-
term view of the generator active power as well as the three pitch angles when the
mean wind speed is above the rated wind speed. From these ﬁgures, it is shown that
the generator active power to the grid is smoothed prominently. It is noted that when a
power
Fig 6.1 Matlab/simulink circuit for high wind with IPC scheme
Fig 6.2 Matlab/simulink circuit for high wind without IPC scheme

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Fig 6.3 Long-term view of the generator active power without and with IPC, and pitch angle
(high wind speed)
drop occurs which is caused by wind shear, tower shadow, and wind speed variation,
etc., one of the blades will accordingly reduce its pitch angle, thus the generator active
power will not drop so dramatically, in such a way that the power oscillation is
limited in a much smaller range. A spectral density analysis of the generator active
power into the grid has been conducted with IPC. Compared with the spectral density
of generator active power without IPC, the 3p oscillation frequency component which
is significant in flicker emission of variable speed wind turbines during continuous
operation is damped evidently with IPC. As a consequence, the flicker level may be
reduced by using IPC. The wind turbine system employing IPC is also carried out
when the mean wind speed is below the rated wind speed. As a small pitch angle
movement will contribute to high power variation, in this case, the minimum pitch
angle βmin in the CPC loop is set to 2◦ (0.0349 rad), leaving a small amount of residual
for IPC to mitigate the power oscillation. The performance of the generator active
power in Fig. 12 demonstrates that the IPC also works well in low wind speeds at the
cost of some power loss due to the pitch movement. The variation of short-term

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flicker severity Pst with different mean wind speed between the case without IPC and
the case with IPC. It can be concluded that damping the active power oscillation by
using IPC is an effective means for flicker mitigation of variable speed wind turbines
during continuous operation at different wind speed. Since many IPC algorithms can
mitigate the wind turbine loads, the proposed new IPC which can mitigate the flicker
emission might have some impact on the wind turbine load. Therefore, the spectra of
the blade root bending moment of blade 1 without and with IPC are plotted,
respectively, which obviously shows that the load on the blade consists of 1p, 2p, 3p,
and higher harmonics, and it demonstrates that the proposed IPC has little impact on
the blade root bending moment. Due to the relationship between the rotor tilt and yaw
moments and the blade root bending moments, it can also be inferred that the
proposed IPC has little impact on the tilt and yaw loads.
Fig 6.4 Matlab/simulink circuit for low wind with IPC scheme
Fig 6.5 Matlab/simulink circuit for low wind without IPC scheme

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Fig 6.6 Long-term view of the generator active power without and with IPC, and pitch angle
(low wind speed)
The mechanical torque of the wind turbine by using the proposed IPC, showing that
compared with previous flicker emission methods, the 3p component of the
mechanical torque is much reduced by using the presented IPC algorithm. As a
consequence, the fatigue load of the wind turbine rotor is relatively smaller in
comparison with the previous flicker mitigation methods, leading to the lifetime
increase of the drive train. There are also drawbacks of the proposed IPC method,
such also a small amount of wind energy in low wind speed and high demand of the
PAS. There is an alternative flicker mitigation method, which is the turbine rotor
speed control taking advantage of the large rotor inertia. In this way, the wind power
fluctuations can be stored in the wind turbine rotor, leading to the flicker mitigation.
However, this paper is focused on the IPC method. The IPC method for flicker
mitigation proposed in this paper may be equally applicable to other types of variable
speed wind turbines, such as a permanent magnet synchronous generator or a doubly
salient permanent magnet generator, etc.

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CONCLUSION
This Report describes a method of flicker mitigation by IPC of variable-speed
wind turbines with MW-level DFIG. The modeling of the wind turbine system is
carried out using FAST and Simulink. On the basis of the presented model, flicker
emission is analyzed and investigated in different mean wind speeds. To reduce the
flicker emission, a novel control scheme by IPC is proposed. The generator active
power oscillation which leads to flicker emission is damped prominently by the IPC in
both high and low wind speeds. It can be concluded from the simulation results that
damping the generator active power oscillation by IPC is an effective means for
flicker mitigation of variable speed wind turbines during continuous operation.

SIND, EEE Page 38
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