M. Koteswara Rao, T.Ganeshkumar and PappuPawan
Mitigation of Voltage Sag and Voltage Swell by
Using D-STATCOM and PWM Switched
Autotransformer
M. Koteswara Rao, T.Ganeshkumar and PappuPawan Puthra
Abstract— This paper proposes a novel distribution-level
voltage control scheme that can compensate voltage Sag and
Swellconditionsin three-phase power systems. Faults occurring
in power distribution systems or facilities in plants generally
cause the voltage sag or swell. Sensitivity to voltage sags and
swells varies within different applications. For sensitive loads,
even the slightest voltage sag for short duration can cause
serious problems. Normally, a voltage interruption triggersa
protection device, which causes shut down the entire load.. In
order to mitigate power interruptions, this paper proposes a
voltage sag support based on a pulse width modulated
autotransformer and D-STATCOM. The proposed devices
quickly recognize the voltage sag and voltage swell conditions
and correct the voltage by either boosting the input voltage
during voltage sag events or reducing the voltage during swell
events. Simulation analysis of these devices is performed in
PSCAD/EMTDC and performance analysis of the system is
presented for various levels of sag and swell. Simulation results
are presented for various conditions of sag and swell
disturbances in the supply voltage to show the compensation
effectiveness.
Index terms—D-STATCOM, Pulse Width Modulation (PWM)
I.
INTRODUTION
With an increase in the use of sensitive loads, the power
quality issues have become an increasing concern. Poor
distribution power quality results in power disruption for the
user and huge economic losses due to the interruption of
production processes. According to an Electric Power
Research Institute (EPRI) report, the economic losses due to
poor power quality are $400 billion dollars a year in the
U.S. alone [1]. Many power quality surveys have been done,
which show that voltage sags have been identified as the
most serious power quality problem facing industrial
customers today.
Voltage sag is a momentary decrease of the voltage RMS
value with the duration of half a cycle up to many cycles.
Voltage sags are given a great deal of attention because of
the wide usage of voltage-sensitive loads such as adjustable
speed drives (ASD), process control equipment, and
computers.
Sag can cause serious problem to sensitive loads that use
voltage-sensitive components such as adjustable speed
drives, process control equipment, and computers [2], [3].
Power systems supply power for a wide variety of
different user applications, and sensitivity to voltage sags
and swells varies widely for different applications. Some
applications such
as
automated manufacturing
processes are more sensitive to voltage sags and swells
than other applications. For sensitive loads, even voltage
sag of short duration can cause serious problems in the
manufacturing process. Normally,
a
voltage
interruption triggers a protection device, which causes
the entire branch of the system to shut down.
Various voltage sag mitigation schemes are based on inverter
systems consisting of energy storage and switches. The DSTATCOM has emerged as a promising device to provide
not only for voltage sag mitigation but a host of other
power quality solutions such as voltage stabilization, flicker
suppression, power factor correction and harmonic control
[4].The D-STATCOM has additional capability to sustain
reactive current at low voltage and can be developed as a
voltage and frequency support by replacing capacitors with
batteries as energy storage. The D-STATCOM, which
consists of a thyristor-based voltage source inverter [5], can
provide fast capacitive and inductive compensation and is
able to control its output current independently of the AC
system voltage. This feature of the compensator makes it
highly effective in improving the transient stability.
In an effort to achieve the advantages of a fast response
time, but at a significantly lower cost, the PWM switched
autotransformer is proposed here [6]. The proposed system
has only one PWM switch per phase with no energy storage,
which is a very low cost solution for voltage sag mitigation.
Any power electronic switch for a high voltage application
is expensive, and the peripheral circuits such as gate drivers
and power supplies are even more expensive than the device
itself. The overall cost of power electronics-based equipment
is nearly linearly dependent on the overall number
of
switches in the circuit topology. Hence, this paper suggests a
scheme that uses only one PWM switch with no energy
storage. Here the control circuit based on RMS voltage is
used to identify the sag and swell disturbances. Simulation
of the compensator is performed using PSCAD/EMTDC and
performance results are presented.
International Journal of Emerging Trends in Electrical and Electronics (IJETEE – ISSN:
Vol. 7, Issue. 1,
M. Koteswara Rao, T.Ganeshkumar and PappuPawan
II.DISTRIBUTIONSTATIC COMPENSATOR
(DSTATCOM)
In its most basic function, the DSTATCOM configuration
consist of a two level voltage source converter (VSC), a dc
energy storage device, a coupling transformer connected in
shunt with the ac system, and associated control circuit [7,
8] as shown in Fig 1. More sophisticated configurations use
multipulse and/or multilevel configurations as discussed in
[9]. The VSC converts the dc voltage across the storage
device into a set of three phase ac output voltages. These
voltages are in phase and coupled with the ac system through
the reactance of the coupling transformer. Suitable
adjustment of the phase and magnitude of the DSTATCOM
output voltages allows effective control of active and reactive
power exchanges between the DSTATCOM and the ac
system.
Fig. 1.Schematic diagram of the DSTATCOM as a custom power
controller
The VSC connected in shunt with the ac system provides a
multifunctional topology which can be used for up to three
quite distinct purposes [10]:
i. Voltage
regulation
and
compensation
of
reactive power;
ii. Correction of power factor;
iii. Elimination of current harmonics.
The design approach of the control system determines the
priorities and functions developed in each case. In this case,
DSTATCOM is used to regulate voltage at the point of
connection. The control is based on sinusoidal PWM and
only requires the measurement of the rms voltage at the load
point.
angle between the inverter voltage and the line voltage is
dynamically adjusted so that the DSTATCOM generates or
absorbs the desired VAR at the point of connection.
Fig. 2.Building blocks of DSTATCOM
The phase of the output voltage of the thyristor based
converter, Vi, is controlled in the same way as the
distribution system voltage, Vs. Figure 3 shows the three
basic operation modes of the DSTATCOM output current, I,
which varies depending upon Vi. For instance, if Vi is equal
to Vs, the reactive power is zero and the DSTATCOM does
not generate or absorb reactive power. When Vi is greater
than Vs, the DSTATCOM ‘sees’ an inductive reactance
connected at its terminal. Hence, the system sees the
DSTATCOM as a capacitive reactance. The current, I, flows
through the transformer reactance from the DSTATCOM to
the ac system, and the device generates capacitive reactive
power. Furthermore, if Vs is greater than Vi, the system
‘sees’ and inductive reactance connected at its terminal and
the DSTATCOM ‘sees’ the system as a capacitive reactance,
then the current flows from the ac system to the
DSTATCOM, resulting in the device absorbing inductive
reactive power.
A. Basic configuration and function of D-statcom
The DSTATCOM is a three phase and shunt connected
power electronics based device. It is connected near the load
at the distribution systems. The major components of the
DSTATCOM are shown in Fig 2. It consists of a dc
capacitor, three phase inverter module such as IGBT or
thyristor, ac filter, coupling transformer and a control
strategy. The basic electronic block of the DSTATCOM is
the voltage sourced converter that converts an input dc
voltage into three phase output voltage at fundamental
frequency.
Referring to Fig 2, the controller of the DSTATCOM is
used to operate the inverter in such a way that the phase
Fig. 3.Operation modes of a DSTATCOM
International Journal of Emerging Trends in Electrical and Electronics (IJETEE – ISSN:
Vol. 7, Issue. 1,
M. Koteswara Rao, T.Ganeshkumar and PappuPawan
III. APWM SWITCHED AUTOTRANSFORMER
The proposed device for mitigating voltage sag and swell in
the system consists of a PWM switched power electronic
device connected to an autotransformer in series with the
load. Fig. 4 shows the single phase circuit configuration of
the mitigating device and the control circuit logic used in the
system. It consists of a single PWM insulated gate bipolar
transistor (IGBT) switch in a bridge configuration, a
thyristorbypass switch, an autotransformer, and voltage
controller.
This control voltage is then compared with the triangular
voltage Vtri to generate the PWM pulses VG which are
applied to the IGBT to regulate the output voltage. Hence
the IGBT switch operates only during voltage sag or swells
condition and regulates the output voltage according to the
PWM duty-cycle. To suppress the over voltage when the
switches are turned off, RC snubber circuits are connected
across the IGBT and thyristor.
B. Voltage sag compensation
The ac converter topology is employed for realizing the
voltage sag compensator. This paper considers the voltage
mitigation scheme that use only one shunt type PWM switch
[11] for output voltage control as shown in Fig. 5. The
autotransformer shown in Fig.5 is used in the proposed
system to boost the input voltage instead of a two winding
transformer. Switch IGBT is on the primary side of the
autotransformer.
Fig. 4. Block diagram of the voltage sag/swell mitigation scheme.
A. Principle of operation
An IGBT is used as power electronic device to inject
the error voltage into the line so as to maintain the load
voltage constant. Four power diodes (D1 to D4) connected to
IGBT switch (SW) controls the direction of power flow and
connected in ac voltage controller configuration. This
combination with a suitable control circuit maintains constant
rms load voltage. In this scheme sinusoidal PWM
Pulse technique is used. RMS value of the load voltage VL
Is calculated and compared with the reference rms voltage
Vref. Under normal condition when there is no voltage
disturbance the power flow is through the anti-parallel
thyristors used as the ac bypass switch. Output filters
containing a main capacitor filter and a notch filter are used
at the output side to filter out the switching noise and reduce
harmonics. During this normal condition, VL = Vref and the
error voltage Verr is zero. The gate pulses are blocked to
IGBT. A sag or swell occurs in the system due to sudden
increase or decrease in the load, or due to faults. The supply
voltage VS and hence VL decreases. When the sensing
circuit detects an error voltage Verr greater than ±10% of the
normal voltage the voltage controller acts immediately to
switch off the thyristors. Voltage Verr applied to the pi
controller gives the phase angle δ. The control voltage
given in (1) is constructed at power frequency f= 50 Hz..
Vcontrol= ma * sin (wt+δ)
(1)
where ma is the modulation index.
The phase angle δ is dependent on the percentage of
disturbance and hence controls the magnitude of Vcontrol.
Fig. 5. Voltage sag/swell mitigating device.
The voltage and current distribution in the autotransformer
is shown in Fig. 6. It does not provide electrical isolation
between primary side and secondary side but has advantages
of high efficiency with small volume. The compensator
considered is a shunt type as the control voltage developed
is injected in shunt. The relationships of the autotransformer
voltage and current are expressed in (2),
VL
VH
 a
IH
IL
, a 
N1
N1 N 2
(2)
where a is the turns ratio,
VL= Primary voltage
VH = Secondary voltage = Load voltage
IL,IH= Primary and secondary currents, respectively
IS= Source current
A transformer with N1:N2 = 1:1 ratio is used as an
autotransformer to boost the voltage on the load side when
sag is detected. With this the device can mitigate up to 50%
voltage sag. As the turns ratio equals 1:2 in autotransformer
mode, the magnitude of the load current IL (high voltage
side) is the same as that of the primary current IL (low
voltage side). From (2), it is clear that VL = 2VP and IS = 2IL.
The switch is located in the autotransformer’s primary side
and the magnitude of the switch current equals the load
current. The voltage across the switch in the off-state is
equal to the magnitude of the input voltage. When sag is
detected by the voltage controller, IGBT switched ON and
is regulated by the PWM pulses. The primary voltage VP
is such that the
International Journal of Emerging Trends in Electrical and Electronics (IJETEE – ISSN:
Vol. 7, Issue. 1,
M. Koteswara Rao, T.Ganeshkumar and PappuPawan
load voltage on the secondary of autotransformer is the
desired rms voltage.
IV.
SIMULATIONANALYSIS AND RESULTS
Simulation analysis is performed on 230/11kv three phase
systems in PSCAD version 4.2 to study the performance of
D-STATCOM and PWM Switched autotransformer. The
system data as follows
TABLE I
System Parameters Used For Simulation
3-Phase100 MVA, 230KV, 50 Hz ,
Transformer
Fig. 6. Voltage and current relations in an autotransformer.
C. Ripple filter design
The output voltage VP is given by the IGBT is the pulse
containing fundamental component of 50 Hz and harmonics
at switching frequency. Hence there is a necessity to design
a suitable ripple filter at the output of the IGBT to obtain the
load voltage THD within the limits. A combination of notch
filters to remove the harmonics and a low pass filter for the
fundamental component as shown in Fig. 1 is used.
Capacitor Cr1 in combination with source inductance and
leakage inductance form the low pass filter. The notch filter
is designed with a center frequency of PWM switching
frequency by using a series LC filter. A resistor may be
added to limit the current. The impedance of the filter is
given by (3)


= +
−



where R, Lr and Cr2 are the notch filter resistance,
inductance and capacitances respectively.
The resonant frequency of the notch filter is tuned to the
PWM switching frequency. The capacitor is designed by
considering its kVA to be 25% of the system kVA.
Capacitor value (Ctotal) thus obtained is divided into
Cr1and Cr2 equally. The notch filter designed for switching
frequency resonance condition is capacitive in nature for
frequencies less than its resonance frequency. Hence at
fundamental frequency it is capacitive of value Cr2 and is in
parallel with Cr1 resulting to Ctotal.










230KV / 11KV / 11KV,100MVA,50Hz

R = 12.1 Ω, L =
R= 0.05Ω,
Capacitor
PI controller gain
Switching frequency
Duty cycle
(A).Voltage sag/swell mitigation by using D- STATCOM:
Fig. 7 shows the test system implemented in PSCAD to
carry out simulations for the D-STATCOM. The test system
comprises a 230 kV transmission system, represented by a
Thévenin equivalent, feeding into the primary side of a 3winding transformer. A varying load is connected to the 11
kV, secondary side of the transformer. A two-level DSTATCOM is connected to the 11 kV tertiary winding to
provide instantaneous voltage support at the load point. A
750F capacitor on the dc side provides the D-STATCOM
energy storage capabilities.
The set of switches shown in Fig. 7 were used to assist
different loading scenarios being simulated with ease. To show
the effectiveness of this controller in providing continuous
voltage regulation, simulations were carried out with and with
no D-STATCOM connected to the system.
A set of simulations was carried out for the test system shown
in Fig. 7. The simulations relate to three main operating
conditions.
1) In the simulation the load is increased by closing Switch
BRK3. In this case, the voltage drops by almost 27% with
respect to the reference value
2) The switch BRK3 is opened and remains so throughout
the rest of the simulation. The load voltage is very close to
the reference value, i.e., 1 pu.
3) Three phase faults also applied in the study system to
study the performance of the device.
4) In the simulation Switch BRK1 is closed, connecting a
capacitor bank to the high voltage side of the network. The
rms voltage increases 27% with respect to the reference
voltage
International Journal of Emerging Trends in Electrical and Electronics (IJETEE – ISSN:
Vol. 7, Issue. 1,
M. Koteswara Rao, T.Ganeshkumar and PappuPawan
Fig. 7. Test system implemented in PSCAD for D- STATCOM simulation.
International Journal of Emerging Trends in Electrical and Electronics (IJETEE – ISSN:
Vol. 7, Issue. 1,
M. Koteswara Rao, T.Ganeshkumar and PappuPawan
Fig. 8. Simulation results for load voltage during voltage sag without
D-STATCOM
Under normal conditions D-STATCOM continuous
monitors the load voltage and generates the error voltage.
The voltage sag can be created by using either load
switching or by using three phase fault. The load voltage
corresponding to sag is shown in Fig 8.The d-statcom can
mitigate the sag as shown in Fig 9.
Fig.9.Simulation results for load voltage during voltage sag with DSTATCOM
Fig.10.Simulation results for load voltage during voltage swell Without
D-STATCOM.
Voltage swell created by using a capacitor bank switching
during a period of 0.3s to 0.6s.under this condition voltage
swell is experienced. By using D-statcom it can be
eliminated. The corresponding wave forms are shown in
Figs 10 and Fig 11.
Fig.11.Simulation results for load voltage during voltage swell with DSTATCOM
(B).VOLTAGE SAG/SWELL MITIGATION BY USING
A PWM SWITCHED AUTOTRANSFORMER
Under normal condition, the power flow is through the
antiparallel SCRs and the gate pulses are inhibited to IGBT.
International Journal of Emerging Trends in Electrical and Electronics (IJETEE – ISSN:
Vol. 7, Issue. 1,
M. Koteswara Rao, T.Ganeshkumar and PappuPawan
The load voltage and current are same as supply voltage and
current. When a disturbance occurs, an error voltage which
is the difference between the reference rms voltage and the
load rms voltage is generated. The PI controller thus gives
the angle δ. Control voltage at fundamental frequency (50
Hz) is generated and compared with the carrier frequency
triangular wave of carrier frequency 1.5 kHz. The PWM
pulses now drive the IGBT switch. The simulation modeling
.
t
of PWM switched autotransformer used as mitigating device
along with its control circuit is shown in Fig. 12. The
autotransformer rating in each phase is 6.35/6.35 kV (as line
voltage is 11 kV) with 1:1 turns ratio. The effective voltage
available at the primary of autotransformer is such that the
load voltage is maintained at desired rms value (6.35 kV or 1
pu).The simulation results of load voltages are shown in fig
13-fig 16 during voltage sag and voltage swell disturbances
PWM Switched autotransformer control circuit
Fig. 12.Test implemented in PSCAD for simulation of PWM Switched Autotransformer
International Journal of Emerging Trends in Electrical and Electronics (IJETEE – ISSN:
Vol. 7, Issue. 1,
M. Koteswara Rao, T.Ganeshkumar and PappuPawan
Fig.13.simulation result of load voltage during voltage sag without
compensator
Fig. 14. Simulation result of load voltage during voltage sag
with compensator
Fig 15. Simulation result of load voltage during voltage swell without
compensator
Fig. 16.Simulation result of load voltage during voltage swell with
compensator
International Journal of Emerging Trends in Electrical and Electronics (IJETEE – ISSN:
Vol. 7, Issue. 1,
M. Koteswara Rao, T.Ganeshkumar and PappuPawan
[4]
TABLE II
THDSOf Load Voltage For Voltage Sag Of D-Statcom
Type of disturbance
Vs (rms)
LLLG Fault(sag)
(VDC=30kv)
Inductive load
switching(sag)
(VDC=20kv)
Capacitive load
switching(swell)
VLoad (rms)
THD (%)
0.9645
3.445
0.9651
5.011
0.9766
6.291
0.975
2.456
1.012
3.978
TABLE III
THDSOf Load Voltage For Voltage Sag Of Pwm Switched
Type of disturbance
VLoad (rms)
THD (%)
0.9732
LLLG fault(sag)
0.9673
0.9622
Inductive load
switching(sag)
Capacitive load
switching(swell)
3.711
0.9823
Autotransformer
Tables II and table III summarizes the simulation results for
both these devices for various sag conditions and swells
Gareth A. Taylor, “Power quality hardware solutions for
distribution systems: Custom power”, IEE North Eastern
Centre Power Section Symposium, Durham, UK, 1995, pp.
11/1-11/9.
[5] HendriMasid, Norman Moriun, Senan Mahmud, Azah
Mohamed and Sallehuddin Yusuf, “Design of a
prototype D-Statcom for Voltage Sag Mitigation,” in Proc.
2004 National Power and Energy. Conf.,Kuallampur, Malaysia,
Nov. 2004, pp. 61-66.
[6] J. R. Rostron and D.-M. Lee, “Voltage Sag and Over
Voltage Compensation Device With Pulse Width Modulating
Switch Connected in Series With Autotransformer,” U.S. Patent
6 750 563, Jun. 2004.
[7] A. Hernandez, K. E. Chong, G. Gallegos, and E. Acha “The
implementation of a solid state voltage source in
PSCAD/EMTDC,” IEEE Power Eng. Rev., pp. 61-62, Dec
1998.
[8] L. Xu, Anaya-Lara, V. G. Agelidis, and E. Acha “Development
of custom power devices for power quality enhancement,” in
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[9] Y. Chen and B. T. Ooi, “STATCOM based on multimodules of
multilevel converters under multiple regulation feedback
control,” IEEE Trans. Power Electron., vol. 14, pp.959-965,
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[10] Dong-Myung Lee, Thomas G. Habetler, Ronald G. Harley,
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BIBILIOGRAPHY
V. CONCLUSION
A New Voltage sag mitigation topology called
PWM switched autotransformer is modeled and simulated
with RMS voltage as a reference. This topology requires
only one PWM switch per phase as compared to
DSTATCOM requires two switches per phase. The PWM
switched autotransformer does not require energy storage
device for mitigation of voltage sag as compared to
DSTATCOM requires energy storage elements. The voltage
mitigation capability of D-STATCOM depends on energy
storage device.
Hence it is shows that PWM switched
autotransformer is more economical than D-statacom.
The PWM switched autotransformer and D-STATCOM for
mitigation of voltage sag/swell could identify the
disturbance and capable of mitigating the disturbance by
maintaining the load voltage at desired magnitude and THD
within limits.
VI. REFERENCES
[1] Electric Power Research Institute (EPRI), “Power quality in
commercial buildings,” Tech. Rep. BR-105018.
[2] M. F. Mc Granaghan, D. R. Muller and M. J. Samotyj,
“Voltage sags in industrial systems,” IEEE Trans. Ind. Appl.,
vol. 29, no. 2, pp. 397–403,Mar./Apr. 1993.
[3] M. H. J. Bollen, Understanding Power Quality Problems:
Voltage Sags and Interruptions. New York: IEEE Press, 2000.
M. Koteswara Rao was born in july 14th1988.He
graduated in 2011 from Vignan’s Lara Institute Of
Technology And Science,Guntur, India in
Electrical Engineering. He is currently pursing
M.TECH. in GVP college of engineering,
Visakhapatnam. His area of interests are
Renewable Energy and Power Quality.
T.Ganeshkumar was born in April 24th 1987.He
graduated in 2008 from Sri Sai Aditya institute of
science and technology, Rajahmundry, India in
Electrical Engineering. He is currently pursing
M.Tech from GVP college of Engineering,
Visakhapatnam. His area of interest is
Power Quality.
P. PawanPuthra was born in 15thNov 1983. He
graduated in JNTU Hyderabad from St.Theresa
Institute of Engg.& Tech., Garividi India in the year
2006. He obtained his post graduation from Vellore
Institute of Technology (VIT) with Specialization
Power Electronics & Drives in year 2008.
Presently he is working as an Asst. professor in
GVP College of Engineering. His main area of research is Power Electronics
& Drives, FACTS, and HVDC Transmission Systems
International Journal of Emerging Trends in Electrical and Electronics (IJETEE – ISSN:
Vol. 7, Issue. 1,