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1. Title
POWER QUALITY IMPROVEMENT AND FAULT RIDE THROUGH OF
GRID CONNECTED WIND ENERGY CONVERSION SYSTEMS.

2. Specific Area
Power Systems
Name of the students
Bharadwaj S [18]
Bharat Singh [20]
Karthick S [38]
Mohammed Riyaz S [53]
Supervisor name
Mr. Venkatesan C (Assistant Professor)

3. Objectives
To improve the power quality by compensating reactive power with Active
Power Filters.
To analyze Fault Ride Through of Grid connected wind energy conversion
systems.

4. Description
 Power quality depends upon voltage, frequency, and waveform.
 Good power quality can be defined as a steady supply voltage that stays within
the prescribed range, steady a.c. frequency close to the rated value, and
smooth voltage curve waveform. As far as power system is concerned, there
are many problems related to power quality.
 Faults in line can cause voltage sags, swells and harmonics which lower the
quality of power that may eventually affect the consumers. So, to improve
power quality of a distribution system, devices such as Active Power
Filters(APF) are used.
 Non-linear loads produce harmonics and Shunt APF is used to mitigate such
problems and make source current sinusoidal and distortion free.
 Series APF is used to mitigate problems caused due to voltage distortion and
unbalance present in source voltage and make load voltage perfectly balanced
and regulated. The proposed work is simulated and verified using MATLAB
Simulink.

5.  This power quality mitigation is applied to the Wind Energy Conversion
Systems.
 Fault Ride Through is the capability of electric generators to stay
connected to the grid in times of voltage dip. The analysis of the dynamic
behavior of the machine during grid disturbances is done under a voltage
sag condition. The Wind Turbine Model is also designed to drive the DFIG
machine in such a way WECS (Wind Energy Conversion Systems) has been
implemented.
 The Fault Ride Through capability of DFIG is analyzed by using a Crowbar
Protection Scheme to enhance the power quality of the overall system and
the control of DFIG machine is met through Vector Control Strategy and
hence two of the prime grid code requirements have been met.
02/13/19

6. What is a voltage dip?
Voltage sags also known as voltage dips are a common cause of power
disturbances. They are a temporary RMS voltage drops to low 10 - 90% of the
nominal voltage caused by abrupt increases in loads, short circuits or faults.
Instantaneous sags can typically last for 0.5 to 30 cycles, momentary sags can be
up to 3 seconds and temporary sags can last for about a minute. Sags or dips can
happen internally by major loads or some wiring problems.
Many types of electronic equipment are sensitive to voltage sags but not limited to
VFDs, motors and PLCs. A constant voltage transformer and an Un-interrupted
Power Supply can help to regulate power coming in. It is to be noted that this not
to be confused with the under voltage condition where the duration of an event
usually lasts longer and approaches steady state.
02/13/19

7. Literature Survey
S.NO Theory/
Method
Title and
Author
Description Pros Cons
1. INSTANT
ANEOUS
REACTIVE
POWER
THEORY
Instantaneous
Reactive
Power
Theory: A
Reference
in the
Nonlinear
Loads
Compensation
,Reyes S.
Herrera and
Patricio
Salmeron.
To calculate the real and
reactive power requirements
of the load instantaneously.
This method is mostly
applied to estimate the
reference current of the
shunt filter. The p-q theory
consists of an algebraic
transformation (Clarke’s
transformation) of the three-
phase voltages and currents
in the a-b-c coordinates to
the (alpha-beta-zero)
coordinates, followed by the
calculation of the p-q theory
instantaneous power
components.
This theory
is very
efficient
and flexible
in designing
controllers
for power
conditioner
s based on
power
electronic
devices.
It
interprets
the load
imbalance
as a
loading
that causes
only a
active and
reactive
currents.

8. S. No Theory/
Method
Title and
Author
Description Pros Cons
2. SYNCHRONO
US
REFERENCE
FRAME
THEORY
(SRFT)
A Comparison
of Control
Algorithms
For
DSTATCOM,
Bhim Singh
and Jitendra
Solanki
SRF based control
strategy used in SAF,
which mainly
compensates the voltage
related power quality
problems. This theory is
aimed to compute
the three phase
reference voltage at the
load terminal. It is used
in series active filter for
generating reference
voltage signal. The
supply voltages Va,Vb,Vc
are transformed into d-
q-0 coordinates and
followed by the inverse
tranformation of it
generates the PWM
signal for the switching
devices.
Exhibits fast
response and
is able to
maintain the
voltage level,
solves the
voltage
related power
quality
problems
such as,
voltage
sag, voltage
swell and
voltage
harmonics.
It is not
simple as
it works
well to
estimate
referenc
e
currents
rather
than to
determin
e
referenc
e
voltages.

9. 02/13/19
S. No Method/
Theory
Title and
Author
Description Pros Cons
3. INSTANTA
NEOUS
REACTIVE
POWER
THEORY
Simulation of
Instantaneous
Power Theory
for
Active Power
Filter,
Vasundhara
Mahajan,
Pramod
Agarwal, Hari
Om Gupta.
This method is mostly
applied to estimate the
reference current of the
shunt filter. The p-q theory
consists of an algebraic
transformation (Clarke’s
transformation) of the
three-phase voltages and
currents in the a-b-c
coordinates to the (alpha-
beta-zero) coordinates,
followed by the calculation
of the p-q theory
instantaneous power
components. Basically it
calculates the real and
reactive power
requirements
This theory
is very
efficient
and flexible
in designing
controllers
for power
conditioner
s based on
power
electronic
devices.
It interprets
the load
imbalance
as a loading
that causes
only a active
and reactive
currents.

10. 02/13/19
S. No Method of
compensati
on
Title and
Author
Description Pros Cons
4. Active
Power
Filters
(Shunt and
Series)
Impact of
Unified
Power-
Quality
Conditioner
Allocation on
Line Loading,
Losses, and
Voltage
Stability of
Radial
Distribution
Systems,
Sanjib
Ganguly.
It is used to compensate
voltage distortions and
voltage unbalances in a
power system so that
the voltage at load side
is completely balance
and sinusoidal &
perfectly regulated and
also it is used to
compensate for load
current harmonics so
that the current at the
source side is perfectly
sinusoidal and free from
distortions and
harmonics. Here, two
voltage source inverters
are connected back to
back through a DC link
capacitor.
Effective
response and
good
accuracy.
Complicatio
ns in
choosing
the
capacitor
ratings.

11. 02/13/19
S. No Method Title and
Author
Description Pros Cons
5. Crowbar
protectio
n design.
Fault Ride
through and
Power Quality
Improvement of
Doubly-Fed
Induction
Generator
based Wind
Turbine System
during Grid
Fault
with Novel
Active Crowbar
Protection
Design,
Snehaprava
Swain and
Pravat Kumar
Ray.
The proposed protection
scheme is designed with
a capacitor in series with
the resistor unlike
conventional Crowbar
(CB) having only
resistors. It plays an
important role in
maintaining the
connection
of DFIG with the grid
during fault to provide
continuous power
supply to the loads.
It prevent
the
converters
from getting
damage by
the large
value of
rotor
currents and
DC-bus
voltage
caused by
the voltage
dips.
It is not
highly
effective.

12. 02/13/19
S. No Method Title and
Author
Description Pros
6. Rotor
side
Vector
Control
of DFIG.
POWER
ELECTRONICS
FOR
RENEWABLE
ENERGY
SYSTEMS,
TRANSPORTATI
ON AND
INDUSTRIAL
APPLICATIONS,
Textbook,
Haitham Abu-
Rub, Mariusz
Malinowski and
Kamal Al-
Haddad.
It describes the control
of the DFIM. The control
is described in different
steps: first, by studying
the current control
loops. After that, some
interesting steady-state
analyses are developed
before finally describing
the control at an
unbalanced grid voltage.
The vector control of the
DFIM is performed in a
synchronously rotating
dq frame.
It is probably
the most
extended
and
established
one.

13. 02/13/19
S. No Method Title and
Author
Description Pros
7. Grid side
Vector
Control
of DFIG.
DOUBLY FED
INDUCTION
MACHINE
MODELING AND
CONTROL
FOR WIND
ENERGY
GENERATION,
Textbook,
Gonzalo Abad
Jesu´s Lo´pez
Miguel A. Rodrı
´guez
Luis Marroyo
Grzegorz
Iwanski.
The grid side converter is
in charge of controlling
part of the power flow
of the DFIM. The power
generated by the wind
turbine is partially
delivered though the
rotor of the DFIM as
advanced in the previous
chapter. This power flow
that goes through the
rotor flows also through
the DC link and finally is
transmitted by the grid
side converter to the
grid.
It is probably
the most
extended
and
established
one.

14. 02/13/19
S. No Method Title and
Author
Description Pros
8. Design of
PI
regulator
s for
current
loops of
DFIG.
POWER
ELECTRONICS
AND ELECTRIC
DRIVES FOR
TRACTION
APPLICATIONS,
Textbook,
Gonzalo Abad.
This helps in choosing
equal proportional-
integral (PI) regulators
for the current loops.
The equivalent closed-
loop systems of
both current loops are
equal to a second-order
system with two poles
and a zero that can be
placed by
classic control theory
choosing the
appropriate gains of the
PI regulators.
It is more
accurate

15. Block Diagram of Grid connected DFIG system
02/13/19
Wind
turbine
Doubly Fed
Induction
Machine
Grid
Grid side voltage
source converter
(VSC)
Rotor side
voltage source
converter (VSC)
Rotor side
control
Grid side
control
Stator
Rotor
DC
link
Grid side
filter
Wind
Active
Power
Filters

16. Block diagram of Active Power Filters
DC
Storage
Shunt Filter (VSI)
Series Filter (VSI)
Filter (HPF) Filter (LPF)
Non-linear load
Coupling
Transformer
3 Phase
Supply

17. Electromagnetic force induced during a voltage dip
 When the total voltage dip occurs in case if the machine is met with a short
circuit right across its terminals, the stator has no voltage and hence the
machine will be demagnetized. There is no stator flux and no EMF is induced in
the rotor windings. In the steady state, the flux is proportional to the stator
voltage and therefore if the dip is long enough, the machine will demagnetize
completely. However, the flux cannot be discontinuous as it is a state variable.
On the contrary, the flux evolves from its initial value pre-fault to zero (evolves
from the flux before the dip arises), resulting in a transient EMF induced in the
rotor terminals.
02/13/19

18.  One important characteristic of the flux during the dip is that it doesn’t rotate as
it is fixed with the stator. In other words, the flux which was rotating at the grid
frequency before the dip, freeze during the dip. Its amplitude decays
exponentially from its initial value to zero with the time constant of the stator
(instigates till how long its field strength can last). In multi-megawatt machines,
this time constant ranges between 0.8 and 1.5 s, much longer than the average
duration of a voltage dip.
 As it has been stated previously, during a total voltage dip there is flux inside the
machine even if there is no voltage in the grid. This transitory flux induces an
EMF in the rotor windings in the same way as a steady state flux does during
normal operation. However, both situations are quite different, the steady state
flux rotates synchronously at a speed very similar to the rotor speed, whereas
the transitory flux during a dip is static and does not rotate. Thus regarding the
rotor windings, the steady state flux rotates very slowly at the slip frequency. On
the contrary, the transitory flux is seen by the rotor as rotating much faster at
rotor speed. Consequently, the EMF induced by the transitory flux will be much
higher than the EMF induced by the steady state during normal operation. It is to
be noted that maximum voltage is induced at the beginning of the dip due to the
sudden transients (sudden voltage change when the dip arises) as it decays
exponentially. The amplitude of the EMF induced at the beginning of a total dip
can be 3 to 5 times higher than during normal operation.
02/13/19

19. Simulation circuit – Grid connected DFIG model
02/13/19

20. Rotor Side Control of DFIG
02/13/19

21. Grid Side Control of DFIG

22. Choosing PI gains
Transfer function of current control loops of DFIG,
By comparing the above equation with the Second-order Differential equation,
…we get and gains for the PI Regulators as follows,
02/13/19
r
i
r
p
r
r
i
d
d
L
K
L
s
K
R
s
L
K
r
q
I
r
q
I
σ
σ
σ
+
+
+
=
)
(
)
(
)
(
2
*
2
2
2
2 n
n
n
s
s ω
ςω
ω
+
+
σ
ω
ςσ
ω
r
n
i
r
r
n
p
L
K
R
L
K
2
)
2
(
=
−
=
p
K i
K

23. Design of PI Regulators
02/13/19

24. Design equations of Wind Turbine model
02/13/19
Air density Torque coefficient
Blade pitch angle Gearbox ratio
- Power coefficients corresponding to various wind speeds
−
ρ
−
β
6
1 C
C −
−
t
C
−
N

25. Wind Turbine modeling
02/13/19

26. Maximum Power Point Tracking Curve
02/13/19
Tip speed ratio
Turbine speed
Radius of the turbine blades
Power coefficient Wind speed
v
t
V
RΩ
=
λ
−
λ
−
Ωt
−
R
−
p
C −
Vv

27. Maximum Power Point Tracking – Indirect
Speed Control
02/13/19
3
3
max
5
5
.
0
N
C
R
K
opt
p
opt
λ
ρπ
=
1
.
8
,
48
.
0
max =
= opt
p
C λ

28. Simulation results - Switching pulses for the
converters
02/13/19

29. Analysis of DFIG during a voltage dip
02/13/19

30. 02/13/19

31. Implementation of Active Power Filters
02/13/19

32. Estimation of reference voltages




















+
−
+
−
−
−
−
=










0
2
/
1
)
3
/
2
sin(
)
3
/
2
cos(
2
/
1
)
3
/
2
sin(
)
3
/
2
cos(
2
/
1
sin
cos
3
2
'
'
'
V
V
V
t
t
t
t
t
t
V
V
V
q
d
c
b
a
π
ω
π
ω
π
ω
π
ω
ω
ω




















+
−
−
−
−
+
−
=










c
b
a
q
d
V
V
V
wt
t
t
wt
t
t
V
V
V
2
/
1
2
/
1
2
/
1
)
3
/
2
sin(
)
3
/
2
sin(
sin
)
3
/
2
cos(
)
3
/
2
cos(
cos
3
2
0
π
π
ω
ω
π
π
ω
ω

33. Control of Series filter – Synchronous
Reference Frame Theory
02/13/19

34. Estimation of reference currents




















−
−
−
=










c
b
a
V
V
V
V
V
V
2
/
3
2
/
3
0
2
/
1
2
/
1
1
2
/
1
2
/
1
2
/
1
3
2
0
β
α




















−
−
−
=










c
b
a
i
i
i
i
i
i
2
/
3
2
/
3
0
2
/
1
2
/
1
1
2
/
1
2
/
1
2
/
1
3
2
0
β
α




















=










β
α
α
β
β
α
i
i
i
V
V
V
V
V
p
p
p 0
0
0
0
0
0
0
−
0
p
−
p
−
q
Instantaneous zero-sequence power
Instantaneous real power
Instantaneous imaginary power

35. 









−
−
−












−
+
+
=












q
p
p
V
V
V
V
V
V
V
V
V
V
V
V
V
i
i
i
~
0
0
0
0
)
2
2
(
)
2
2
(
1
*
*
*
0
0
0
0
0
0
α
β
β
α
β
α
β
α
β
α
)
(
)
2
2
(
)
~
(
)
2
2
(
* q
V
V
V
p
V
V
V
i −
+
+
−
+
−
=
β
α
β
β
α
α
α
)
(
)
2
2
(
)
~
(
)
2
2
(
* q
V
V
V
p
V
V
V
i −
+
−
−
+
=
β
α
α
β
α
β
β
0
0
0
*
V
p
i −
=

36. Control of Shunt filter – Instantaneous Reactive
Power Theory
02/13/19

37. Capacitor rating
- dc-link current
- switching pulse period of
- maximum allowed ripple 610 volts
- dc-link voltage
- capacitance value
02/13/19
F
C
u
T
i
C
DC
DC
SW
DC
DC
µ
2
.
4277
2
)
(
≥
∆
≥
DC
i
SW
T
DC
u
∆
DC
u
DC
C
%
10
400
=
∆
=
DC
SW
u
s
T µ
DC
u
=
DC
u

38. Gating signals for the switching devices
For shunt filter For series filter
02/13/19

39. Power quality mitigation results
Voltage waveform before compensating with Active Power Filters
Voltage waveform after compensating with Active Power Filters
02/13/19

40. Conclusion
Thus, a study has been carried out to improve the fault ride through capability
and the power quality of the power system by compensating reactive power
with Active Power Filters. The power quality mitigation is applied to the Wind
Energy Conversion Systems thus performing both fault ride through. This is all
about wind energy generation system and the way it must be operated to be
stay connected to the utility grid. The proposed system instigates by matching
up with the reactive power as soon as when the voltage level imbalances occur
in the lines. Hence, this shows that the time delay of the fault clearing time after
the disconnecting the grid and the machine has been much reduced. This
eliminates the use of Crowbar Protection Circuit to bypass the high rotor
currents in the event of any fault. It improves the overall power quality of the
system and is fast in action as possible. The MATLAB/ Simulink based simulation
results show the analysis of DFIG under voltage sag conditions and how it
compensates for the above said conditions.
02/13/19

41. References
1. Haitham Abu-Rub, Mariusz Malinowski and Kamal Al-Haddad, “POWER
ELECTRONICS FOR RENEWABLE ENERGY SYSTEMS, TRANSPORTATION AND
INDUSTRIAL APPLICATIONS”, Textbook, 2014, A co-publication of IEEE Press and
John Wiley & Sons Ltd.
2. Gonzalo Abad, Jesu´s Lo´pez, Miguel A. Rodrı´guez, Luis Marroyo and Grzegorz
Iwanski, “DOUBLY FED INDUCTION MACHINE MODELING AND CONTROL FOR
WIND ENERGY GENERATION”, Textbook, 2011, Published by John Wiley & Sons,
Inc., Hoboken, New Jersey.
3. GONZALO ABAD, “Power Electronics and Electric Drives for Traction
Applications”, Textbook, 2017, John Wiley & Sons, Ltd.
4. K.R Padiyar, “FACTS Controllers in Power Transmission and Distribution”,
Textbook, 2014, Department of Electrical Engineering, IIS Bangalore, New Age
International Publishers.
5. R. Mohan Mathur, Rajiv K. Varma, “Thyristor-Based FACTS Controllers for
Electrical Transmission Systems”, Textbook, 2012, Wiley India Publishers.
6. TING LEI , “DOUBLY-FED INDUCTION GENERATOR WIND TURBINE MODELLING,
CONTROL AND RELIABILITY”, A thesis submitted to The University of
Manchester for the degree of Doctor of Philosophy in the Faculty of Engineering
and Physical Sciences, 2014.

42. 7. G. Aruna Jyothi, DR. P. V. R. L. Narasimham, “Implementation of Instantaneous
Reactive Power Theory for Current Harmonic Reduction and Reactive Power
Compensation in Three Phase Four Wire Power System” (2015), IOSR Journal of
Electrical and Electronics Engineering (IOSR-JEEE).
8. Krupa M. Patel, Prof. Kaushik K Patel, “SYNCHRONOUS REFERENCE FRAME
THEORY FOR REDUCTION OF VOLTAGE SWELL”,
May 2015, International Journal For Technological Research In Engineering.
9. Rajiv Kumar Sinku, “Study of Unified Power Quality Conditioner for Power
Quality Improvement”, May 2015, National Institute of Technology, Rourkela.
10. R.V.L. Narayana Divakar, P.Kishore, C.H.Ravi Kumar, V.Madhu Kishore,
V.Pradeep Kumar, “Power Quality Improvement of Non-Linear Load by Using
Instantaneous P-Q Theory”, March 2015, International Journal of Electrical and
Electronics Research.
11. George G. Karady, “Effects of Voltage Sags on Loads in a Distribution System”,
October 2005, Final Report, Power Systems Engineering Research.
12. Sahasrabuddhe, Saurabh, "Islanding technique in power systems to avoid
cascading failure“, 2014, Masters Theses. 7310.
http://scholarsmine.mst.edu/masters_theses/7310.
02/13/19

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