ISSN 2278-3083
International Journal of Science and Applied Information Technology (IJSAIT), Vol.5 , No.1, Pages : 05-09 (2016)
Special Issue of ICECT 2016 - Held on February 27, 2016 in Hyderabad Marriot Hotel & Convention Centre, Hyderabad
http://warse.org/IJSAIT/static/pdf/Issue/icect2016sp02.pdf
Transient Voltage stability Enhancement of a grid
connected wind energy system using SVC and
STATCOM - A Comparison
K.SREE LATHA
Dr.M.VIJAYA KUMAR
Assoc.Prof., Dept of EEE
GNITC, HYDERABAD, INDIA
Professor, Dept of EEE
JNTU,ANANTAPUR, INDIA
latha.dharani@gmail.com
mvk2004@rediffmail.com
Abstract : Wind energy is one of the fastest growing
renewable energy resources due to global warming. The
high electrical power can be generated from an
aggregation of multiple wind turbines as a wind farm or
wind park. To interconnect it to the utility grid, control
system must ensure high power quality and stability.
Hence a better analysis of the wind farm is required as it
contains induction generators which exhibit a different
behaviors. This paper mainly deals with the study of
transient stability of a grid connected DFIG wind farm
when it has been integrated to a weak grid. A
comparative analysis when it is supported with FACTS
devices like SVC and STATCOM with fuzzy controller
is studied. A single case of three phase fault is simulated
and the results thus prove that the system can regain after
disturbance with less recovery time when FACTS are
implemented
Keywords: Wind farms, Grid, SVC, STATCOM,
recovery time, Fuzzy.
ratings multiplied by the total hours in a year since
the wind speed is variable. The capacity factor of a
wind farm is the ratio of the actual productivity in a
year to the theoretical maximum[5]. Thus the
capacity factor of the wind farm is affected by
several parameters such as the variability of the
wind at the site and the generator size.
Electricity generated from wind power can be
highly variable, instantaneous electrical generation
and consumption must remain in balance to
maintain grid stability otherwise this variability can
present substantial challenges while incorporating
large amount of power generated from wind farms
to grid. This work investigates the possible
methods to enhance the stability of the system
when connected to grid using FACTS devices[3].
Unlike classical sources of energy, wind farm
supply real power variations into the upstream grid
and these variations cause problems with voltage
stability and transient stability.
There are many different types of generators used
for wind power applications today. This paper
mainly focuses on DFIG due to advantages such as
high energy efficiency and controllability. This is
basically a wound rotor induction generator with a
voltage source converter connected to the slip
rings of the rotor. The stator winding is coupled
directly to the grid and the rotor winding is
connected to power converter.
INTRODUCTION
Complex power system networks have evolved due
to continuously increase in demand and economic
growth. With increase in industrialization growth
and population in the world, more energy is
required to satisfy the required needs of the
consumers and improve the standards of the human
welfare. In order to make availability of the energy
economical and environment free, the need for the
renewable energy sources has increased.
Based on the Global Wind Energy Council Report
(GWEC), India was the fifth largest globally in
2014 by adding 2,315MW of new wind energy to
reach a total of 22.5GW[4]. Among renewables,
wind power accounted for almost 2/3rd of the
installed capacity. The Indian government
presently having 6.9 % share expects it to grow to
at least 15% in the next five years. Wind energy
shares in the total power of the country was 3% for
the calendar year 2014. The recent announcements
by the Indian Ministry of new Renewable Energy
(MNRE) indicate that the India plans to achieve
60,000MW in total wind power installations by
2022 with emerging states include Andhra Pradesh
and Madhya Pradesh.
The annual energy production of a wind farm is
not equal to the sum of the generator name plate
DFIG MODEL & FAULT RIDE THROUGH
SOLUTIONS
a) Doubly-Fed Induction Generator (DFIG)
The major aspects that determine the grid behavior
of wind turbines are that majority of these are
consisting of Induction Generators. Unlike the
conventional generators these require capacitor
banks for VAR support otherwise from the grid
reactive power will be drawn which affects the
voltage profile at PCC. However, if we use wind
turbines of variable type which use wound rotor or
permanent magnet synchronous generator, reactive
compensation will not be a issue but issues like
harmonics generated , maintenance of voltage
5
ISSN 2278-3083
International Journal of Science and Applied Information Technology (IJSAIT), Vol.5 , No.1, Pages : 05-09 (2016)
Special Issue of ICECT 2016 - Held on February 27, 2016 in Hyderabad Marriot Hotel & Convention Centre, Hyderabad
http://warse.org/IJSAIT/static/pdf/Issue/icect2016sp02.pdf
profile during disturbance/faults need to be
considered. In this paper Doubly-fed Induction
Generator (DFIG) system is used in which the
stator is directly connected to the grid where as the
rotor connected to the three- phase converter.
Variable speed wind turbines are connected to the
grid using power electronic technology and
maximize effective turbine speed control.
The discrete simulink model constructed presented
here is based on the phasor model available in the
MATLAB/ Sim power systems library[6] but with
a frequency of 50 HZ. Fig 1 shows the doubly - fed
induction generator and wind turbine model.
Generally the absolute value of slip is much lesser
than 1 and consequently Pr is only a fraction of Ps.
Since Tm is positive for power generation and since
ωs is positive and constant for a constant frequency
grid voltage, the sign of Pr is a function of the slip
sign. Pr is positive for negative slip and is negative
for positive slip. For super - synchronous speed
operation Pr is transmitted to DC bus capacitor and
tends to rise the DC voltage. For sub synchronous
operations , Pr is taken out of DC bus capacitor and
tends to decrease the DC voltage. Cgrid is used to
generate or absorb the power Pgc in order to keep
the DC voltage constant. In steady- state for a
lossless AC/DC/AC converter pgc is equal to Pr and
the speed of the wind turbine is determined by the
power Pr absorbed or generated by Crotor. The
phase sequence of teh AC voltage generated by
Crotor is positive for sub-synchronous speed and
negative for super - synchronous speed. The
frequency of this voltage is equal to the product of
the grid frequency and the absolute value of the
slip. Crotor and Cgrid have the capability of
generating or absorbing reactive power and could
be used to control the reactive power or the voltage
at the grid terminals.
Fig.1 DFIG & Wind turbine model
The AC/DC/AC converter is divided in to two
components grid- side converter Cgrid and rotor-side
converter Crotor. Both the converters are Voltagesourced converters that use the IGBTs to convert
AC voltage from DC. The coupling inductor L is
used to connect C grid to the grid where as a
capacitor is connected on to the DC side to act as
DC voltage source. The 3Φ rotor winding is
connected to the Crotor by sliprings and brushes but
the stator is directly connected to the grid. The
power taken from the wind turbine is fed to the grid
by the stator and the rotor windings by converting
it into electrical power by the induction generator.
The control system that is present generates the
command signals of voltage to the rotor and the
grid in order to control the power of the wind
turbine ,DC bus voltage and the voltage at the grid
terminals[6].
The mechanical power and the stator electric power
are compared as follows
Pm = ωr Tm ...........(1)
Ps = ωs Tem ...........(2)
For a loss less generator the mechanical equation is
J
= Tm - Tem .
In steady - state at fixed speed for a loss less
generator
Tm = Tem and Pm = Ps +Pr .
It follows that :
Pr = Pm - Ps = Tmωr - Temωs = -Tm
ωs =
-sTmωs= -sPs.
Where s is defined as the slip of the generator.
Fig.2 Power Flow Study
b) Fault ride through solutions:
Generally wind farm research and development
studies illustrate that wind farms are suffering more
due to grid faults which are symmetric and non
symmetric in nature. Grid code requirement only
determines symmetrical faults where as non
symmetrical faults are also more severe due to the
presence of DFIG[2].
During the occurrence of Voltage dip the stator
flux will not be able to follow the sudden variations
in its voltage[1] and becomes nearly stationary.
During this time rotor keeps spinning and develops
high slip causing over voltages and over currents in
the rotor circuit, which destructs the power
converters and the rotor circuit. Thus in order to
protect the system a proper protection scheme is
required which can be achieved by using
 Arrangement of crow bar in the rotor
circuit
 Using Active crow bar protection
 Series Anti-parallel thyristors
 Implementation of FACTS
6
ISSN 2278-3083
International Journal of Science and Applied Information Technology (IJSAIT), Vol.5 , No.1, Pages : 05-09 (2016)
Special Issue of ICECT 2016 - Held on February 27, 2016 in Hyderabad Marriot Hotel & Convention Centre, Hyderabad
http://warse.org/IJSAIT/static/pdf/Issue/icect2016sp02.pdf
General solution for the voltage dips is to
connect a crow bar to the rotor of the circuit which
is to limit the high currents by providing bypass
resistors [5]. This may be antiparallel thyristor
converter or diode bridge crowbar. In both the
cases the crow bar is activated when the rotor
current exceeds a particular value and will remain
in the circuit till stator is disconnected. But the new
grid code requires the wind farm to be remained
during the voltage dip.
The next advancement is to remove the
crowbar from the circuit as early as possible which
can be done by using Active crow bar control
method that implements fully controllable switches
called IGBT’s[7]. In this method the stator of the
machine is not disconnected from the grid and the
crowbar is removed as soon as possible to retrieve
the machine. Sometimes during the switching
operations there is a possibility of production of
surge current transients for which its operation
must be retrieved.
New method [8] is been proposed which is
going for the implementation of IGBTs in the
converter connected to the rotors that are designed
for higher ratings, and antiparallel thyristors are
placed. In this method during normal operations the
thyristors are kept on and because of which
conduction losses will be very high . This could be
avoided by bypassing the thyristors with
commutators or going for the implementation of
high rating thyristors with control methods.
All the above advantages can be reduced by
going for the implementation of FACTS devices
like SSSC, SVC, STATCOM etc., can be
implemented. This papers deals with the
implementation of SVC and STATCOM.
Implementation of SVC will help in reducing the
reacting power generated during the voltage dip
thus enhancement in voltage can be achieved. The
other crucial solution is going for the
implementation of a SSSC also called as Dynamic
voltage restorer (DVR) which can isolate the wind
turbine during the voltage dip [9].The drawback of
this method is because of the incorporated power
converters. The next possibility is going for the
implementation of voltage source inverter as
STATCOM [10]. More control techniques can be
incorporated to enhance the operation of the
controllers without bypassing the wind farm from
the grid.
turbines each of 1.5MW. The generator used is a
wound rotor type induction generator (WTDFIG)
with AC-DC-AC converter which is divided into
two components CRotor rotor-side converter and
CGrid Grid side converter. These are voltage sourced
converters (VSCs) that use forced- commutated
IGBTs to synthesize an AC voltage from DC
voltage source. The wind farm transmits a power of
120KV grid via a transmission line of 30KM line.
Fig.3 System under study
SIMULATED MODEL & RESULTS
Data
of
various
components
used
in
Matlab/Simulink are given in appendix Fig 4. Here
the doubly fed induction machine acts as a
induction generator which is fluctuating in nature
due to series of excursions in the system and due to
variation in wind speeds. In order to study the
transient behavior of the system a symmetric 3Φ
fault is applied at time t= 5 sec and variation is
studied until t= 5.2 secs. This causes variation in
the voltage dip which is increased after clearance of
the fault using FACTS devices . For improvement
in voltage FACTS controllers like SVC and
STATCOM are added and the results are
compared. The simulation model runs under
discrete mode. Simulation results obtained with the
two models are shown in figure.
Case (a) At first the system is incorporated with
SVC(TSC-TCR) at time t=5 sec the variation of
voltage v/s time and P & Q at bus B4 are shown in
fig4(a) and 4(b) shows the results incorporated
with fuzzy controller. Fig 4 (C ) shows the voltages
at all the four buses .The results thus obtained
prove that by going for the implementation of the
fuzzy controller to SVC the transient stability of
the system can be improved better when compared
with SVC.
SYSTEM UNDER SIMULATION
The wind farm equipped with SVC and
STATCOM
is
simulated
using
MATLAB/SIMULINK tool box. Fig.3 shows the
system considered for study. It comprises of a
windfarm of 3MW , which consists of 2 wind
7
ISSN 2278-3083
International Journal of Science and Applied Information Technology (IJSAIT), Vol.5 , No.1, Pages : 05-09 (2016)
Special Issue of ICECT 2016 - Held on February 27, 2016 in Hyderabad Marriot Hotel & Convention Centre, Hyderabad
http://warse.org/IJSAIT/static/pdf/Issue/icect2016sp02.pdf
shown in fig 5 (a) and 5(b) using STATCOM with
FUZZY controller.
fig.4(a) Voltage and P, Q at bus 4 using SVC
without fuzzy
Fig 5(a) Voltage and P, Q at bus 4 using
STATCOM with out fuzzy.
fig.4(b) Voltage and P, Q at bus 4 using SVC with
fuzzy.
Fig 5(b) Voltage and P, Q at bus 4 using
STATCOM with fuzzy.
4(c )Three Phase Voltages at all the four buses with
Fuzzy SVC
Case (b) The study system is now incorporated
with STATCOM to improve the transient stability
of the system at time t=5 sec with the same
3MVAR rating as that of SVC and the results are
5(C) Voltages at all the buses with Fuzzy based
Statcom
8
ISSN 2278-3083
International Journal of Science and Applied Information Technology (IJSAIT), Vol.5 , No.1, Pages : 05-09 (2016)
Special Issue of ICECT 2016 - Held on February 27, 2016 in Hyderabad Marriot Hotel & Convention Centre, Hyderabad
http://warse.org/IJSAIT/static/pdf/Issue/icect2016sp02.pdf
[6] The Math Works "Sim Power Systems For Use
with Simulink ",User's Guide .
[7]J.Matevosyan, T.ackermann , S.Bolik , L.soder
“Comparision of International Regulations for
connection of Wind Turbines to the network”
Nordic wind power Conference , 1-2 March 2004.
Table 2 shows results used with different FACTS
controllers
S.No
Name
of
controller
1.
SVC
2.
SVC with Fuzzy
Controller
STATCOM
0.015
STATCOM with
Fuzzy Controller
0.03
3.
4.
the
Recover
y time
period(
Sec)
0.025
0.05
Power
system
stability
[8]T.Thiringer , A.Peterson , T.petru ,”Grid
disturbance response of wind turbines equipped
with induction generator and doubly-fed induction
generator”, Power engineering society annual
meeting, Toronto, Canada july 2003.
Less
Effective
Better
Control
Good
Control
More
Effective
[9]ELTRA, “specifications for connecting wind
farms to the transmission network”, 2000.
[10]K.Sree
latha,
Dr.M.Vijaya
Kumar,
”Enhancement of voltage stability using
STATCOM in a grid connected wind generation
system”,PESTSE 2014.
CONCLUSIONS
This paper mainly emphasizes on the voltage
stability enhancement of a grid connected wind
energy systems with DFIG by using various
FACTS devices with better controllers. The voltage
stability of the system is compared with different
devices. The performance of the STATCOM with
Fuzzy controller proves to provide better voltage
stability during the fault when compared to other
controllers. The essential features of FACTS
controllers and their potential to improve system
stability is the prime concern for effectiveness in
restoring the system stability after grid disturbance.
REFERENCES
[1] Ahmed G. Abo-Khalil " Impacts of Wind Farms
on Power System Stability " in " Wind Farm" book,
ISBN 980-953-307-562-9.
[2]Ahmed G. Abo-Khalil " Synchronization of
DFIG Output Voltage to Utility Grid in Wind
Power System"" Elsevier Journal of Renewable
Energy, Vol. 44, Sept. 2012, PP. 193-198
[3] N.G. Hingorani, L. Gyugyi, Understanding
FACTS: Concepts and Technology of Flexible AC
Transmission Systems, New York, Wiley-IEEE
Press, 1999
[4].Global Wind Energy council GWEC,"Global
Wind Report Annual Market Update 2014".
[5]J.Niiranen”Voltage Dip ride through of a doubly
fed system” Worldwide Energy Conference, 2004.
9