Voltage Sag and Harmonics Mitigation using Distributed Power Flow Controller

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 04 Issue: 08 | Aug -2017 www.irjet.net p-ISSN: 2395-0072
© 2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 1504
Voltage Sag and Harmonics Mitigation Using Distributed Power Flow
Controller
Vikash Kumar Goutam1, Dr. Deependra Singh 2
1PG Scholar, Department of Electrical Engineering, KNIT Sultanpur, India
2Professor, Department of Electrical Engineering, KNIT Sultanpur, India
---------------------------------------------------------------------***---------------------------------------------------------------------
Abstract - According to growth of electricity demand and
the increased number of non-linear loads in power grids,
providing a high quality electrical power should be
considered. In this work, a new power flow controlling
device called Distributed Power Flow Controller (DPFC) is
presented that offers the same control capability as the
UPFC, at a reduced cost and with an increased reliability.
The DPFC eliminates the common DC link within the UPFC,
to enable the independent operation of the shunt and the
series converter. The D-FACTS concept is employed to the
series converter to increase the reliability. Multiple low-
rating single-phase converters replace the high-rating
three-phase series converter, which greatly reduces the cost
and increases the reliability. The active power that is
exchanged through the common DC link in the UPFC is now
transferred through the transmission line at the 3rd
harmonic frequency. Depending upon the growth of
electricity demand the problem of harmonics and voltage
sag is occurred. DPFC is used to mitigate the voltage
deviation and improve power quality. Application of DPFC
in power quality enhancement is simulated in
MATLAB/Simulink environment. In this work three different
types of fault analysis (L-G fault, L-L fault and L-L-L-G fault)
is done to validate the DPFC ability to improve the power
quality.
Keywords- Power flow control, harmonics, voltage sag
mitigation, Distributed power flow controller.
1. INTRODUCTION
Flexible ac Transmission Systems (FACTS) devices are
used to control power flow in the transmission grid to
relieve congestion and limit loop flows [1]. All electrical
devices are prone to failure or malfunction when exposed
to one or more power quality problems [2]. A power
quality problem can be defined as any problem is
manifested on voltage, current, or frequency deviation
that results in power failure [3]. To control power flow, it
is necessary to be able to maintain or change line
impedances, bus voltage magnitudes, or phase angle
differences. To control the power flow UPFC is the most
powerful FACTS device, which can simultaneously control
all the parameters of the system. The cost of the UPFC is
high because UPFC handle the voltages and currents with
high rating.
Fig -1: Simplified representation of UPFC
The UPFC is the combination of a static synchronous
compensator (STATCOM) and a static synchronous series
compensator (SSSC), which are coupled via a common dc
link [4]. Due to the common dc-link interconnection, a
failure that happens at one converter will influence the
whole system, which reduces the reliability of the system.
Due to lower reliability and higher cost the UPFC has not
been commercially used, even though, it has the most
advanced control capabilities.
In this paper, the UPFC is further developed into a new
FACTS device that is Distributed Power Flow Controller
(DPFC), by applying the two approaches i.e., eliminating
the common DC link and distributing the series converter.
Within the DPFC, the common dc link between the shunt
and series converters is eliminated, which provides
flexibility for independent placement of series and shunt
converter [5].
Fig -2: Flowchart from UPFC to DPFC

Fig -3: Configuration of DPFC from UPFC
Distributed power flow controller having two major
advantages over Unified power flow controller: 1) The
cost of DPFC is low because of the low component rating of
the series converter and low voltage isolation, and 2) The
reliability of the DPFC is high due to elimination of d.c. link
between series and shunt converters.
In this work, the DPFC is used to mitigate the voltage sag
and harmonics in three different cases. Line to ground,
Line to line and 3-phase to ground faults are created to
establish voltage sag and harmonics in the system and
DPFC is used to mitigate both the problem.
2. DPFC CONSTRUCTION
The DPFC is derived from the UPFC by considering two
approaches as follows. First, eliminating commonly
connected dc link of UPFC [6] and the second is
distributing series converter with D-FACTS concept to
series converter, which consists of multiple units that are
connected in series with the transmission lines as shown
in figure.2.
The unique control capability of the UPFC is due to the
back-to-back connection between the shunt and series
converters, which allows the active power to freely
exchange. In DPFC the active power exchange between
converters with an eliminated DC link is provided through
the transmission line present between the AC ports of the
shunt and the series converters.
3. RELATED WORK
Distributed Power Flow Controller developed by
eliminating the common DC link and distributing the
series converter, which is based upon Distributed static
series compensator concept. Both the concepts are
explained below-
3.1 Elimination of D.C. link
Within the DPFC, the transmission line is used as a
connection between shunt converter output and AC port
of series converters, instead of using DC-link for power
exchange between converters [7]. The active power
exchange in DPFC is based on non-sinusoidal voltage and
current, which can presented as the sum of sinusoidal
components at different frequencies. It is the main result
of Fourier analysis.
Fig - 4: Active power exchange between DPFC converters
The power supply generates the active power and the
shunt converter absorbs it in fundamental frequency of
current. Meanwhile, the third harmonic component is
trapped in Y-Δ transformer [8]. The third harmonic
current is injected into the neutral of Δ-Y transformer
through the output terminal of the shunt converter. This
harmonic current controls the dc voltage of series
capacitors. According to the amount of required active
power at the fundamental frequency, the DPFC series
converters generate a voltage at the harmonic frequency,
thereby absorbing the active power from harmonic
components. At the receiving end transformer a high pass
filter is connected, which is used to blocks the
fundamental frequency components and allows the
harmonic components to pass. High pass filter provides a
return path for the harmonic component.
3.2 Distributed static series compensator
The Distributed Static Series Compensator (DSSC) is a
distributed SSSC, which keeps the functionality of the SSSC
with a much lower cost and higher reliability. The concept
of DSSC uses a large number of units with low power
ratings instead of one controller with a large power rating.
The configuration of a DSSC unit is shown in Fig.5. This
figure shows a single unit of series converter. It is a single-
phase converter which is attached to transmission lines by
a single-turn transformer. The transmission line is acts as
secondary winding for single-turn transformer, inserting
controllable impedance into the line directly. Costly high

voltage isolation is not required because converters are
hanging on the line.
Fig - 5: D-FACTS unit configuration
Most of the voltage injected by a DSSC unit is in
quadrature with the line current, to emulate inductive or
capacitive impedance. A small part of the voltage is in
phase with the line current and serves to self-power of
DSSC unit. The DSSC is remotely controlled via wireless
communication or a PLC (Power Line Communication).
Rating of the device is very low (in the range of 10-20
KVA).
To control the transmission line, number of DSSC devices
need to be attached to the line as per the requirement. At
the highest level, DSSC can maximize the utilization of
asset, reduction in congestion of system, available transfer
capacity (ATC) of the system, enhance system reliability
and capacity under contingencies, enhance system
stability and can do so with lower capital and operating
cost than most conventional single-point ‘lumped’
solutions, such as FACTS devices.
As shown in Fig. 5, Single Turn Transformer (STT) with a
NC (normally closed) switch which makes the device in
bypass condition until the inverter is activated. The dc
control power supply transformer is excited by the
current that flows in the STT secondary winding. As the
switch is opened, DC bus of the inverter charged up and
operation of inverter is initiated.
4. CONTROL STRATEGY OF DPFC
The DPFC system consists of two types of converters, and
each type of converter requires a different control scheme.
The DPFC has three control strategies: central controller,
series control and shunt control. The shunt and series
control are local controllers and are responsible for
maintaining their own converters parameters.
Fig - 6: DPFC control structure
4.1 Central control
The central controller manages all the series and shunt
controllers and sends reference signals to both of them.
According to the system requirement it gives
corresponding reactive current signal for the shunt
converter and voltage reference signals for the series
converters. Central controller gives all the reference
signals at the fundamental frequency.
4.2 Series control
Each single-phase converter has its own series control
through the line. This controller inputs are series capacitor
voltages, line current and series voltage reference in dq-
frame. Any series controller has one low-pass and one 3rd-
pass filter to create fundamental and third harmonic
current respectively. Two single-phase phase lock loop
(PLL) are used to take frequency and phase information
from network. The simulated diagram of series controller
is shown in Fig.7.
Fig - 7: Series control structure

4.3 Shunt control
The shunt converter is controlled to inject a constant 3rd
harmonic current into the transmission line, which is
intended to supply active power for the series converters.
The shunt converter extracts some active power from the
grid at the fundamental frequency to maintain its dc
voltage.
The dc voltage of the shunt converter is controlled by the d
component of the current at the fundamental frequency,
and the q component is utilized for reactive power
compensation. The series converters generate a voltage
with controllable phase angle at fundamental frequency,
and use the voltage at the 3rd frequency to absorb active
power to maintain its dc voltages at a constant value. The
power flow control function is realized by an outer control
loop, the power flow control block. This block gets its
reference signals from the system operator, and the
control signals for DPFC series converters are sent
remotely via wireless or PLC communication method.
Fig - 8: Shunt control configuration: (a) for fundamental
frequency (b) for third-harmonic frequency
5. SIMULATION
The simulation of the DPFC has been done in
Matlab/simulink environment, which is shown in Fig. 9.
The DPFC model is simulated with a three phase source,
which is connected to a non-linear load. The three phase
source is connected to the load through parallel
transmission lines(line 1 and line 2) with the same length.
The series converter of DPFC is distributed through the
line 2 and the shunt converter of DPFC is connected to the
line 2 and ground through a Y-Δ three-phase transformer.
For examine the performance of DPFC, a fault is created
near the load. The result analysis have been done in three
different cases. In case 1 single line to ground fault is
considered, in case 2 line to line fault is considered and in
case 3 three phase to ground fault is considered. The time
duration of fault is 0.1 seconds (200-300 millisecond). The
simulation parameters is listed in table 4.
Fig - 9: Simulation model of DPFC
6. SIMULATION RESULTS AND DISCUSSION
Case-A: Single line to ground fault
Fig - 10: Load voltage waveform without DPFC
Fig - 11: Load voltage THD without DPFC

Fig - 12: Load voltage waveform with DPFC
Fig - 13: Load voltage THD with DPFC
Case-B: Line to line fault
Case-C: Three phase to ground fault

7. COMPARATIVE ANALYSIS OF RESULT
When any type of fault is occurred in the transmission line
then parameters of the line gets affected. During fault
condition the problem of voltage sag, current and increase
in harmonics occurred. Without using any compensation
these problems will remains in the system. After
implementation of DPFC in the system the above problems
are mitigated. The comparative analysis is done on the
following three cases.
Table -1: Comparative analysis for single line to ground
fault
Without DPFC With DPFC
Phases (THD) Voltage
sag
(THD) Voltage
sag
Phase A
3.18 %
0.50 P.U.
0.25 %
Nominal
Phase B 0.75 P.U. Nominal
Phase C Nominal Nominal
Table -2: Comparative analysis for line to line fault
Phases (THD) Voltage
sag
(THD) Voltage
sag
0.60 P.U. Nominal
Phase A
6.28
%
0.69 %
Phase B
0.35 P.U. Nominal
Phase C
Nominal Nominal
Table -3: Comparative analysis for three phase to ground
fault
Total
harmonics
distortion
(THD)
Voltage sag Total
harmonics
distortion
(THD)
Voltage sag
6.28 % 0.4 P.U. 0.68 % Nominal
8. CONCLUSIONS
In this study, the application of DPFC as a new FACTS
device, in the voltage sag mitigation of a system composed
of a three-phase source connected to a non-linear load
through the parallel transmission lines is simulated in
Matlab/Simulink environment. The voltage dip is analyzed
by implementing a fault close to the system load. To detect
the voltage sags and determine the three single phase
reference voltages of DPFC, the SRF method is used as a
detection and determination method. The obtained
simulation results show the effectiveness of DPFC in
power quality enhancement, especially in sag mitigation.
In this work the performance analysis of the distributed
power flow controller is done in three different cases as
below-
Case-A: Single line to ground fault
 In this work, single phase to ground fault is
created with the help of MATLAB. When DPFC is
not connected with the transmission line then the
value of voltage sag is 0.5 per unit in phase A,
which is completely mitigated when DPFC is
connected with the transmission line.
 Total harmonics distortion (THD) is also reduced
from 3.18% to 0.25% in line.
 The voltage sag in this case is 50% of the nominal
voltage.
Case-B: Line to line fault
 In this work, line to line fault is created with the
help of MATLAB. When DPFC is not connected
with the transmission line then the value of
voltage sag is 0.6 per unit in phase A and 0.35 per
unit in phase B, which is completely mitigated
when DPFC is connected with the transmission
line.

 In this case total harmonics distortion (THD) is
also reduced from 6.28% to 0.69% in line.
Case-C: Three phase to ground fault
 In this work, three phase to ground fault is
created with the help of MATLAB. When DPFC is
not connected with the transmission line then the
value of voltage sag is 0.4 per unit in all the
phases, Which is completely mitigated when DPFC
is connected with the transmission line.
 In this case total harmonics distortion (THD) is
also reduced from 6.28% to 0.68% in line.
Table - 4: System Parameters
Parameters Rating
Source voltage 230 KV
Frequency 60 Hz
X/R 3
Transformer rating 100 MVA
Magnetization resistance 500 (P.U)
Magnetization inductance 500 (P.U)
Resistance per unit length of
line
0.01273
Inductance per unit length of
line
0.9337 mH
Capacitance per unit length of
line
12.74 nF
Length of line 100 Km
Snubber resistance 105 ohm
D.C link capacitance 25 x 10-3
Ground resistance 0.001 ohm
REFERENCES
[1] Deepak Divan and Harjeet Johal, “Distributed FACTS-
A new concept for realizing power flow control,” IEEE
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2253-2260, 2007.
[2] Kevin Kumar G. Raythaththa and Bhargav Y. Vyas,
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line
using Distributedstatic Series Compensator (DSSC),”IE
EE International Conference on Energy Efficient
Technologies for Sustainability (ICEETS), p.p. 459-
463, 2016.
[3] A.N.V.V. Rajasekhar and Naveen Babu, “Harmonics
reduction and power quality improvement by using
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BIOGRAPHIES
Vikash Kumar Goutam received
his B. Tech. degree in Electrical
Engineering from MPEC Kanpur in
2006. Presently he is pursuing his
M.Tech in Electrical Engineering
(Power system) from KNIT,
Sultanpur India.
Dr. Deependra Singh is currently
working as a Professor in Electrical
Engineering department in KNIT,
Sultanpur, India. His area of
interest is Measurement &
Instrumentation, Power Systems,
and Distributed Generation.

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