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Direct Torque Control for Doubly Fed Induction
Machine-Based Wind Turbines
Under Voltage Dips and Without Crowbar
Protection
Abstract:-
This paper proposes a rotor flux amplitude reference generation
strategy for doubly fed induction machine based wind turbines.
It is specially designed to address perturbations, such as voltage dips,
keeping controlled the torque of the wind turbine, and considerably
reducing the stator and rotor over currents during faults. In addition,
a direct torque control strategy that provides fast dynamic response
accompanies the overall control of the wind turbine. Despite the
fact that the proposed control does not totally eliminate the necessity
of the typical crowbar protection for this kind of turbines, it eliminates
the activation of this protection during low depth voltage dips.
Aim:-
The main aim of the project is to analyze the performance of the double
fed induction generator(DFIG) which is an integral part of the wind
energy generation system under unbalanced grid fault condition. . And
we have to control the speed of the induction generator to produce
constant current even in voltage dips and un balanced load conditions
by using a new method Direct torque control method with out
using crowbar protections.
INTRODUCTION
Here we discuss about the analysis on the control of doubly fed
induction machine (DFIM) based high-power wind turbines when they
operate under presence of voltage dips.
Most of the wind turbine manufacturers build this kind of wind turbines
with a back-to-back converter sized to approximately 30% of the
nominal power.
This reduced converter design provokes that when the machine is
affected by voltage dips, it needs a special crowbar protection in order
to avoid damages in the wind turbine and meet the grid-code
requirements.
The main objective of the control strategy proposed in this project is to
eliminate the necessity of the crowbar protection when low-depth
voltage dips occurs.
Hence, by using direct torque control (DTC), with a proper rotor flux
generation strategy, during the fault it will be possible to maintain the
machine connected to the grid, generating power from the wind,
reducing over currents, and eliminating the torque oscillations that
normally produce such voltage dips
Conventional Methods:-
Sensor methods
Power electronic converters (VSI)
Sensor less methods (Adaptive observer method)
Crowbar protection
Disadvantages:-
• Increases the size of the machine
• Noise pollution
• Drift effects are increased
– By using A.O techniques
• Require more memory space,
• Have to check for number of estimated values,
• Difficult to tune with number of values,
• More complex of calculating part.
Proposed method:-
• Direct torque control method is one of the best control strategies which
allow a torque control in steady state and transient operation of
induction motor.
• The main aim of direct torque control strategies is to effectively control
the torque and flux of induction motor.
• Direct torque control method made the motor more accurate and fast
torque control, high dynamic speed response and simple to control.
Advantages:-
– No need to use sensors.
– Efficient output.
– No chance of drift effects and power losses.
– No need to use a crowbar protection.
– Reduce the stator and rotor over currents during faults.
Wind Generation
• When wind strikes the stationary blade of the wind turbine forcely then it starts rotating.
This blades are connected to hub it is connected to low speed shaft.
This shaft starts rotate with the speed of wind and give
dynamic energy to Gear box.
This gear box connected to Generator with high speed level
shaft to give more dynamic energy to the Generator
This Generator converts this Gearbox M.E into E.E.
This is the general process to generate electricity through
Wind energy.
Why we use DFIG?
• Generally in wind generation we use DFIG.
• The reason is the speed of the rotor is based upon the wind speed.
• Wind speed is varies at every time. Due to this variation of speed
the rotor shaft will be damaged.
• To overcome this problem in wind generation we use a specially
designed machine “Double fed induction generator”
• It can withstand under presence of voltage dips,
• Ability to control rotor currents.
• Allows for reactive power control and variable speed operation.
Methods of Speed Control of Induction motors
(1) Stator voltage Control
(2) Stator Frequency Control
(3) Stator Current Control
(4) V/F Control
(5) Slip power recovery Control ( Wound Rotor Induction Machine)
15
• General control techniques to control the speed of the Induction
Motor are
– Stator side
– Rotor side
• But it is difficult to control through Rotor side and Stator side.
The next step is to control the I.M is by using Sensors.
Disadvantages of sensors:-
If we use sensors to control IM,
I. We have to put voltage and current sensors at both stator and rotor
sides.
II. So It increases the size of the machine and
III. It increases the cost of the machine.
IV. If we use sensors in get some noisy, and sound polluted.
V. Drift effects are increased.
To over come this problems the next step to control the I.M we
go for sensor less techniques.
By using this AO Techniques we get accuracy output, but the
main disadvantages are
a) Require more memory space,
b) Have to check for number of estimated values,
c) Difficult to tune with number of values,
d) More complex of calculating part.
To overcome this problems we use Crow-Bar protection.
Crowbar protection.• It is used to mitigate the high voltages and high current
ratings.
• A crowbar thyristor is connected across the input dc
terminals.
• A current sensing resistor detects the value of converter
current. If it exceeds preset value, gate circuit provides the
signal to crowbar SCR and turns it on in a few microseconds.
• If we use Crow-Bar protection if any fault occurs
– we have to replace the fuse and Thyristor.
– The circuit became complex to design
– The size and cost of the equipment increased.
– Externally we have to add another device to reduce voltage dips.
In order to overcome these problems and to reduce the faults with in the transmission here we use a new method
Direct Torque Control Method.
Principles of Vector Control
The basic conceptual implementation of vector control is illustrated in the below block diagram:
Note: The inverter is omitted from this diagram.
The motor phase currents, ia, ib and ic are converted to idss and
iqss in the stationary reference frame. These are then
converted to the synchronously rotating reference frame d-q
currents, ids and iqs.
In the controller two inverse transforms are performed:
1) From the synchronous d-q to the
stationary d-q reference frame;
2) From d*-q* to a*, b*, c*.
There are two approaches to vector control:
1) Direct field oriented current control
- here the rotation angle of the iqse vector with respect to the
stator flux qr’s is being directly determined (e.g. by measuring
air gap flux)
2) Indirect field oriented current control
- here the rotor angle is being measured indirectly, such as by
measuring slip speed.
Direct Vector Control
In direct vector control the field angle is calculated by using
terminal voltages and current or Hall sensors or flux sense
windings.
A block diagram of a direct vector control method using a PWM
voltage-fed inverter is shown on the next slide.
Direct Vector Control (cont’d)
The principal vector control parameters, ids* and iqs
*,
which are dc values in the synchronously rotating reference
frame, are converted to the stationary reference frame (using
the vector rotation (VR) block) by using the unit vector cose and
sine. These stationary reference frame control parameters idss*
and iqss* are then changed to the phase current command
signals, ia*, ib
*, and ic* which are fed to the PWM inverter.
A flux control loop is used to precisely control the flux. Torque
control is achieved through the current iqs* which is generated
from the speed control loop (which includes a bipolar limiter
that is not shown). The torque can be negative which will
result in a negative phase orientation for iqs in the phasor
diagram.
How do we maintain idsand iqs orthogonality? This is explained in
the next slide.
Why FOC ?
• IM is superior to DC machine with respect to size, weight,
inertia, cost, speed
• DC motor is superior to IM with respect to ease of control
– High performance with simple control due de-coupling
component of torque and flux
• FOC transforms the dynamics of IM to become similar to the
DC motor’s – decoupling the torque and flux components
Basic Principles DC machine
Current in
Current out
a
f
By keeping flux constant, torque can be controlled by controlling armature current
Te = k If Ia
Basic Principles of IM
a
b
b’c’
c
Stator current produce stator flux
s r
Interaction between stator and rotor fluxes produces torque
Space angle between stator and rotor fluxes varies with load, and speed
Stator flux induces rotor current produces rotor flux
FOC of IM drive
Torque equation :
sse i2
p
2
3T
srr
me i
L
L
2
p
2
3T
In d-q axis :
)ii(LL
2p
23
T sdrqsqrdr
me
FOC of IM drive
In d-q axis :
)ii(LL
2p
23
T sdrqsqrdr
me
Choose a frame such that:
rrdr
0r
rq
FOC of IM drive
Choose a frame such that:
rrdr
0r
rq
FOC of IM drive
)ii(LL
2p
23
T sdrqsqrdr
me
Choose a frame such that:
rrdr
0r
rq
qs
ds
si
r
sqi
rq
sdird
As seen by stator reference frame:
FOC of IM drive
rsqr
r
me i
LL
2p
23
T
si
)ii(LL
2p
23
T sdrqsqrdr
me
Choose a frame such that:
rrdr
0r
rq
qs
ds
r dr
qr
rsdi
rsqi
Rotating reference frame:
FOC of IM driveTo implement rotor flux FOC need to know rotor flux position:
(i) Indirect FOC
grrg
grg
sr
rmgr
r
r )(jdt
di
LRL
LR
0
rsliprr
sqr
sdr
rmr
r
r )(jdt
djii
LRL
LR
0
Synchronous speed obtain by adding slip speed and rotor speed
Rotor voltage equation:
grrg
grg
rr )(jdt
diR0
gsm
grr
gr iLiL
Rotor flux equation:
FOC of IM drive - indirect
dtd
iLRL
LR
0 rrsd
r
rmr
r
r
d component
rslipr
sqr
rm )(iLRL
0
q component
rsliprr
sqr
sdr
rmr
r
r )(jdt
djii
LRL
LR
0
FOC of IM drive - indirect
m
r
r
*e
sq LL
p3T4
*i r
dtd
iLRL
LR
0 rrsd
r
rmr
r
r
d component
rslipr
sqr
rm )(iLRL
0
q component
rsliprr
sqr
sdr
rmr
r
r )(jdt
djii
LRL
LR
0
m
*r
sd L*i r
r
sqr
*r
rmslip i
LRL
)(
FOC of IM drive - indirect
T*
*2/3
1/s
irsq*
irsd*
isq*
isd*
ia*
ib*
ic*
CCVSI
slip r
+ +
Rotating frame Stationary frame
m
*r
sd L*i r
m
r
r
*e
sq LL
p3T4
*i r
rsq
r*r
rmslip i
LRL
)(
ej
FOC of IM drive
grr
rs
r
rmr
r
r jdt
di
L
RL
L
R0
srr
me i
LL
2p
23
T
(ii) Direct FOC
Rotor flux can be estimated by:
Rotor flux estimated from motor’s terminal variables
Express in stationary frame
FOC of IM drive
grr
rs
r
rmr
r
r jdt
di
LRL
LR
0
(ii) Direct FOC
)j(jdt
)j(di)jii(
LRL
)j(LR
0 rqrdrrqrd
sqsdr
rmrqrd
r
r
dti
LRL
LR
rqrsdr
rmrd
r
rrd
dti
LRL
LR
rdrsqr
rmrq
r
rrq
d q
rd
rq
2rq
2rdr
FOC of IM drive - direct
grr
rs
r
rmr
r
r jdt
di
L
RL
L
R0
srr
me i
LL
2p
23
T
T*
r*2/3
isq*
isd*
ia*
ib*
ic*
CCVSI
TC
FC
irsq*
irsd*ej
Te
r
Rotating frame Stationary frame
Working:-
Stator is directly connected Grid
Rotor is connected supply through Back-Back converters
We control the speed of the I.M by changing the firing angles at
different level by taking the reference of Direct torque.
DTC that controls the machine’s torque (Tem ) and the rotor flux
amplitude (|ψr |) with high dynamic capacity, and a second block
that generates the rotor flux amplitude reference, in order to
handle with the voltage dips.
What is MATLAB?
MATLAB (Matrix Laboratory)– MATLAB is developed by The Math Works, Inc.– MATLAB is a high-level technical computing
language and interactive environment for algorithm development, data visualization, data analysis, and numeric computation.
– MATLAB can be install on Unix, Windows
• About MATLAB/Simulink:
– Matlab is a high-performance language for technical
computing.
– It integrates computation, visualization, and programming in an
easy-to-use environment where problems and solutions are
expressed in familiar mathematical notation.
History of MATLAB:
Fortran subroutines for solving linear (LINPACK) and eigen value (EISPACK)
problems
Developed primarily by Cleve Moler in the 1970’s
Later, when teaching courses in mathematics, Moler wanted his
students to be able to use LINPACK and EISPACK without requiring
knowledge of Fortran
MATLAB developed as an interactive system to access LINPACK and
EISPACK.
MATLAB gained popularity primarily through word of mouth because it
was not officially distributed.
In the 1980’s, MATLAB was rewritten in C with more functionality (such as
plotting routines)
The Mathworks, Inc. was created in 1984
The Mathworks is now responsible for development, sale, and
support for MATLAB
The Mathworks is located in Natick,
The Mathworks is an employer that hires co-ops through our co-op
program
Strengths of MATLAB• MATLAB is relatively easy to learn.
• MATLAB code is optimized to be relatively quick when
performing matrix operations.
• MATLAB may behave like a calculator or as a
programming language.
• MATLAB is interpreted, errors are easier to fix.
• Although primarily procedural, MATLAB does have
some object-oriented elements.
Other Features
• 2-D and 3-D graphics functions for visualizing data
• Tools for building custom graphical user interfaces
• Functions for integrating MATLAB based
algorithms with external applications and
languages, such as C, C++, Fortran, Java, COM, and
Microsoft Excel
Weaknesses of MATLAB
• MATLAB is NOT a general purpose programming
language.
• MATLAB is an interpreted language (making it for
the most part slower than a compiled language
such as C++).
• MATLAB is designed for scientific computation and
is not suitable for some things (such as parsing
text).
Components of MATLAB Interface
• Workspace• Current Directory• Command History• Command Window
SIMULINK
It is a commercial tool for modeling, simulating and
analyzing multidomain dynamic systems.
Its primary interface is a graphical block diagramming
tool and a customizable set of block libraries.
Simulink is widely used in control theory and digital
signal processing for multidomain simulation
and Model-based design.
• Generally there are three ways to open Simulink
1.By using start in Matlab
2.By typing Simulink in Command prompt.
3.By clicking Simulink icon in toolbar.
Cont……
Simulink is widely used in control theory and digital signal processing for multidomain simulation and Model-based design.
Simulation diagram
Simulation results
Simulation results
• Conclusion:-
we control the speed of the induction generator to produce
constant current even in voltage dips and un balanced load
conditions with out using any crowbar protections and by using
a new method Direct torque control method.