ISSN 2278-3083

International Journal of Science and Applied Information Technology (IJSAIT), Vol. 3 , No.4, Pages : 22 - 28 (2014)
Special Issue of ICECT 2014 - Held during September 01, 2014 in The Golkonda Hotel, Hyderabad, India

Mitigation of Voltage Sag and Swell in Transmission Line

using DPFC with PI and Fuzzy Logic Control
1

Sai Lakshmi K1, Mrs. G. Ujwala2,Dr. K. Ramesh Reddy3

Department of EEE, GNITS, Hyderabad, Telangana, India. sailakshmi.kurnuthala@gmail.com
2
Department of EEE, GNITS, Hyderabad, Telangana, India. gjujwala@gmail.com
3
Dean & HOD, Department of EEE, GNITS, Shaikpet, Hyderabad, Telangana, India.

Abstract The Power Quality problems during the last two

decades has been the major concern of the power companies.
The operation of power systems has become complex due to
growing consumption and increased number of non-linear
loads because of which compensation of multiple power
quality issues has become an compulsion. A new component
within the flexible AC-transmission system (FACTS) family,
called Distributed Power-flow controller (DPFC) is presented
in this paper. DPFC is derived from the unified power-flow
controller (UPFC). DPFC can be considered as a UPFC with
an eliminated common dc link. The active power exchange
between the shunt and series converters, which is through the
common dc link in the UPFC, is now through the transmission
lines at the third-harmonic frequency. The DPFC employs the
distributed FACTS (D-FACTS) concept, which is to use
multiple small-size single-phase converters instead of the one
large-size three-phase series converter in the UPFC. Power
quality issues are studied and DPFC is used to mitigate the
voltage deviation and improve power quality. In this paper, the
capability of DPFC is observed for the transmission line based
on PI and fuzzy logic controllers (FLC). On comparing the
two controllers performance, we can say that Fuzzy Logic
Controller based DPFC gives better compensation than PI
Controller based DPFC. Simulink models are developed with
and without the controllers. The three phase fault is created
near the load. Simulation results show the effectiveness
between the two controllers.

Depending on how devices are connected in systems, PFCDs

can be divided into shunt devices, series devices, and
combined devices(both in shunt and series with the system).
Based on the implemented technology, PFCDs can be
categorized into mechanical-based devices and power
electronics(PE)-based devices. Mechanical PFCDs consist of
fixed or mechanical interchangeable passive components, such
as inductors or capacitors, together with transformers. PE
PFCDs also contain passive components, but include
additional PE switches to achieve smaller steps and faster
adjustments. There is another term- Flexible AC Transmission
System(FACTS) - that overlaps with the PE PFCDs.
According to the IEEE, FACTS is defined as an alternating
current transmission system incorporating power electronic
based and other static controllers to enhance controllability
and increase power transfer capability. Normally, the High
Voltage DC transmission(HVDC) and PE devices that are
applied at the distribution network, such as a Dynamic
Voltage Restorer (DVR), are also considered as FACTS
controllers. Most of the FACTS controllers can be used for
power flow control. However, the HVDC and the DVR are out
of the scope of the PFCD. PE combined PFCDs (also referred
to as combined FACTS) have the best control capability
among all PFCDs. They inherit the advantages of PE PFCDs
and combined PFCDs, which is the fast adjustment of multiple
system parameters. The FACTS devices, such as unified
power flow controller(UPFC) is currently the most used
PFCD's and synchronous static compensator (STAT-COM),
are used to alleviate the disturbance and improve the power
system quality and reliability.
The same as the UPFC, DPFC introduced as a new FACTS
device, is used to mitigate voltage and current waveform
deviation and improve power quality in a matter of seconds.
The DPFC structure is derived from the UPFC structure with
one shunt converter and several small independent series
converters, as shown in Fig. 1. The DPFC has same capability
as UPFC to balance the line parameters, i.e., line impedance,
transmission angle, and bus voltage magnitude.

KeywordsPower Quality,D-FACTS,DPFC,Voltage Sag,

Swell, PI Controller,Fuzzy Logic Controller

INTRODUCTION
This paper discusses the concept of a distributed approach for
realizing FACTS devices in particular series FACTS devices.
The future power system will be a meshed network and the
power flow within this network, both the direction and
quantity, will be controlled. To keep the system stable during
faults or weather variations, the response time of the power
flow control should be within several cycles to minutes.
Without proper controls, the power cannot flow as required,
because it follows the path determined by the parameters of
generation, consumption and transmission .To fulfill the
power flow requirements for the future network, power flow
controlling devices are needed. The device that attempts to
vary system parameters to control the power flow can be
described as a Power Flow Controlling Device(PFCD).

Fig 1: Flow Chart from UPFC to DPFC

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ISSN 2278-3083
International Journal of Science and Applied Information Technology (IJSAIT), Vol. 3 , No.4, Pages : 22 - 28 (2014)
Special Issue of ICECT 2014 - Held during September 01, 2014 in The Golkonda Hotel, Hyderabad, India
DPFC TOPOLOGY

through the transmission line. This harmonic current controls

the dc voltage of series capacitors. Fig. 3 illustrates how the
active power is exchanged between the shunt and series
converters in the DPFC using third harmonic components.

By introducing the two approaches ( elimination of the

common DC link and distribution of the series converter) into
the UPFC, the DPFC is achieved. Similar as the UPFC, the
DPFC consists of shunt and series connected converters. The
shunt converter is similar as a STATCOM, while the series
converter employs the DSSC concept, which is to use multiple
single-phase converters instead of one three-phase converter.
Each converter within the DPFC is independent and has its
own DC capacitor to provide the required DC voltage. The
configuration of the DPFC is shown in figure 2.

Fig 3: Active Power exchange between DPFC Converters in

the transmission line at the third harmonic frequency
Fig 2: The DPFC Structure

Advantages and Limitation of the DPFC

DPFC Operating principle

The DPFC in comparison with UPFC has some advantages,
as follows:

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. The method of power exchange in DPFC
is based on power theory of non-sinusoidal components. Nonsinusoidal voltage and current can be presented as the sum of
sinusoidal components at different frequencies. It is the main
result of Fourier analysis. The product of voltage and current
components provides the active power. Since the integral of
some terms with different frequencies are zero, so the active
power equation is as follow:

1) High control capability.

The DPFC can control all parameters of transmission network:
line impedance, transmission angle and bus voltage
magnitude.
2) High reliability.
The series converters redundancy increases the DPFC
reliability during converters operation [10]. It means, if one of
series converters fails, the others can continue to work.
3) Low cost.
The single-phase converters rating, in comparison with threephase converters is very low. Furthermore, the series
converters, in this configuration, no need to any voltage
isolation to connect in line. We can use the single turn
transformers for series converters hanging.

where Vi and Ii are the voltage and current at the ith harmonic
frequency, respectively, and i is the angle between the
voltage and current at the same frequency. Equation. 1
expresses the active powers at different frequencies are
independent from each others. Thus, the converter can absorb
the active power in one frequency and generates output power
in another frequency.

However, there is a limitation using the DPFC:

Extra currents: Because the exchange of power between the
converters takes place through the same transmission line as
the main power, extra currents at the 3rd harmonic frequency
are introduced. These currents reduce the capacity of the
transmission line and result in extra losses within the line and
the two Y- transformers. However, because this extra current
is at the 3rd harmonic frequency, the increase in the RMS
value of the line current is not large and through the design
process can be limited to less than 5% of the nominal current.

Active Power exchange between DPFC Converters

Assume the DPFC is located in transmission line of a two-bus
system; therefore, 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- transformer. Output terminal of the shunt converter
injects the third harmonic current into the neutral of -Y
transformer. Consequently, the harmonic current flows

DPFC CONTROL AND MODELLING

The DPFC has three control strategies: central controller,
series control, and shunt control, as shown in Fig. 4

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ISSN 2278-3083
International Journal of Science and Applied Information Technology (IJSAIT), Vol. 3 , No.4, Pages : 22 - 28 (2014)
Special Issue of ICECT 2014 - Held during September 01, 2014 in The Golkonda Hotel, Hyderabad, India
Central Control
This controller manages all the series and shunt controllers
and sends reference signals to both of them.

Fig 6: Control Scheme of Series Converter

Fig 4: DPFC Control Structure
PI Controller based 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 control
scheme of series converter is shown in figure 5 and the
simulated diagram of the series controller is shown in figure 6.

Fig 7: Control scheme of the shunt converter (a)for the

fundamental frequency components; (b) for the 3rd harmonic
frequency components
FUZZY LOGIC BASED DISTRIBUTED POWER FLOW
CONTROLLER
Fig 5: Simplified
Matlab/Simulink

Diagram

of

Series

Converter

Design of Fuzzy Logic Controller

in

Fuzzy logic (FL) controller is one of the most successful

operations of fuzzy set theory; its major features are the use of
linguistic variables rather than numerical variables. This
control technique relies on human capability to understand the
systems behavior and is based on quality control rules. Fuzzy
Logic provides a simple way to arrive at a definite conclusion
based upon vague, ambiguous, imprecise, noisy, or missing
input information.

PI Controller based Shunt Control

The shunt converter includes a three-phase converter which is
back-to-back connected to a single-phase converter. The threephase converter absorbs active power from grid at
fundamental frequency and controls the dc voltage of
capacitor between this converter and single-phase one. The
shunt control structure block diagram is shown in Fig. 6

A fuzzy logic controller is based on set of rules called as fuzzy

rules among the linguistic variables. These rules are expressed
in the form of conditional statements. Our basic structure of
the fuzzy logic coordination controller to damp out the
oscillations in the power system consists of 3 important parts,
viz. fuzzification, knowledge base-decision making logic
(inference system)
and the defuzzification, which are

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ISSN 2278-3083
International Journal of Science and Applied Information Technology (IJSAIT), Vol. 3 , No.4, Pages : 22 - 28 (2014)
Special Issue of ICECT 2014 - Held during September 01, 2014 in The Golkonda Hotel, Hyderabad, India
explained in brief as follows. Fig.8 shows the Fuzzy Editor
with inputs and output.

Fig 8: FIS Fuzzy Editor with 2 inputs and 1 output

Memembership Functions used in Fuzzification process are
shown in Fig. 9

Fig 10: Simulink Model of a Shunt Controller with Fuzzy

Logic Control
SIMULATION SET UP OF DPFC
System model under study is shown in Fig. 11 Simulation
studies are carried out to analyze the performance of DPFC for
voltage sag and swell conditions in a transmission system with
a voltage of 230kV and 60Hz. A three phase fault with fault
resistance of 50 near the load is said to be introduced into
the system. Due to this voltage sag is created with a value 0.5
per unit. The voltage sag and swell are said to be compensated
in the transmission line using DPFC with PI controller and
Fuzzy Logic controller

Fig 9(a) : The membership function for input in this model

Fig 9(b): The membership function for output in this model

In this controller 49 rules have been used.

A simulink block of the shunt converter control for

fundamental components using a Fuzzy Logic Controller in
DPFC is shown in the Fig. 10.

Fig 11: Simulation Model of DPFC

EXAMINING SIMULATION RESULTS
Simulation results for three types of connections are taken
considering sag/swell condition using two types of controllers
1. DPFC with PI controller.
2. DPFC with Fuzzy Logic controller
Results showing the load voltage harmonic analysis using fast
fourier transform (FFT) of power GUI window by simulink
are as follows.

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ISSN 2278-3083
International Journal of Science and Applied Information Technology (IJSAIT), Vol. 3 , No.4, Pages : 22 - 28 (2014)
Special Issue of ICECT 2014 - Held during September 01, 2014 in The Golkonda Hotel, Hyderabad, India

Simulation results without DPFC

Simulation Results with DPFC using PI

Controller

Fig 12: Voltage Sag without DPFC

Fig 15: Voltage sag Compensation with DPFC using PI

Controller

Fig 13: Current Swell without DPFC

Fig 16: Current Swell Compensation with DPFC using PI
Controller

Fig 14: THD without DPFC

Fig 17: THD with DPFC using PI Controller

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ISSN 2278-3083
International Journal of Science and Applied Information Technology (IJSAIT), Vol. 3 , No.4, Pages : 22 - 28 (2014)
Special Issue of ICECT 2014 - Held during September 01, 2014 in The Golkonda Hotel, Hyderabad, India
APPENDIX
Table1 shows Simulation System Parameters

Simulation Results with DPFC using Fuzzy

Logic Controller

Parameters

Values

Three Phase Source

Rated Voltage

230 kV

Rated Power/Frequency

100 MW/60 Hz

X/R

Short Circuit Capacity

11000 MW

Transmission Line
Fig 18: Voltage Sag Compensation with DPFC using Fuzzy
Logic Controller

Resistance

0.012 pu/km

Inductance/Capacitive Reactance

0.12/0.12 pu/km

Length of Transmission Line

100 km

Shunt Converter 3-Phase

Nominal Power

60 MVAR

DC link Capacitor

600 F

Coupling Transformer (Shunt)

Nominal Power

100 MVA

Rated Voltage

230/15 kV

Series Converters
Fig l9: Current Swell Compensation with DPFC using Fuzzy
Logic Controller

Rated Voltage

6 kV

Nominal Power

6 MVAR

Three-phase fault
Type

ABC-G

Ground Resistance

0.01

CONCLUSION
In this study mitigation of power quality issues like voltage
sag and swell are simulated in Matlab/Simulink environment
employing a new FACTS device called Distributed Power
Flow Controller(DPFC). The DPFC is emerged from the
UPFC and inherits the control capability of the UPFC, which
is the simultaneous adjustment of the line impedance, the
transmission angle, and the bus voltage magnitude. The
common dc link between the shunt and series converters,
which is used for exchanging active power in the UPFC, is
eliminated. This power is now transmitted through the
transmission line at the third harmonic frequency. The series
converter of the DPFC employs the D FACTS concept, which
uses multiple small single phase converters instead of one
large size converter. The reliability of the DPFC is greatly
increased because of the redundancy of the series converters.
The total cost of the DPFC is also much lower than the

Fig 20: THD with DPFC using Fuzzy Logic Controller

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ISSN 2278-3083
International Journal of Science and Applied Information Technology (IJSAIT), Vol. 3 , No.4, Pages : 22 - 28 (2014)
Special Issue of ICECT 2014 - Held during September 01, 2014 in The Golkonda Hotel, Hyderabad, India
UPFC, because no high voltage isolation is required at the
series converter part and the rating of the components of is
low. It is proved that the shunt and series converters in the
DPFC can exchange active power at the third harmonic
frequency, and the series converters are able to inject
controllable active and reactive power at the fundamental
frequency .Also the performance of DPFC is simulated using
two mechanisms i.e., with PI and Fuzzy Logic controllers.The
results prove that the DPFC with Fuzzy controller gives better
voltage compensation than DPFC with PI controller.
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