Download ISSN (Online): 2347-2820, Volume -1, Issue-2, 2013

International Journal of Electrical, Electronics and Computer Systems, (IJEECS)
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EFFECTS OF UPFC FOR POWER FLOW CONTROL AND
DAMPING OF OSCILLATIONS IN A POWER SYSTEM
1
1,2,3,4,5
Rudresh S J, 2Kumudeesh K C, 3Neetha H.M, 4Chaitra. C, 5Vishwas.S
Assistant Professors, Department of Electrical and Electronics, PES Institute of Technology and Management,
Shimoga, Karnataka, India
Email : [email protected]
number blackouts in different parts of the world. The
reasons behind the above fault sequences may be due to
the systematical errors in planning and operation, weak
interconnection of the power system, lack of
maintenance or due to overload of the network. In the
late 1980’s the Electric Power Research Institute (EPRI)
introduced a concept of technology to improve the
power flow, improve the system stability and reliability
with the existing power systems. This technology of
power electronic devices is termed as Flexible
Alternating Current Transmission Systems (FACTS)
technology. The two main objectives of FACTS are to
increase the transmission capacity and control power
flow over designated transmission routes. FACTS
controllers are capable of controlling the network
condition in a very fast manner and this feature of
FACTS can be exploited to improve the voltage
stability, and steady state and transient stabilities of a
complex power system.The improvements in the field of
power electronics have had major impact on the
development of facts controllers. These controllers are
based on voltage source converters and include devices
such as Static Var Compensators (SVC), Static
Synchronous Compensators (STATCOM), Thyristor
Controlled Series Compensators (TCSC), the Static
Synchronous Series Compensators (SSSC), and the
Unified Power Flow Controller (UPFC).
Abstract - Electrical power system is a large
interconnected network that requires a careful design to
maintain the system with continuous power flow operation
without any limitations. The technology of power system
utilities around the world has rapidly evolved with
considerable changes in the technology along with
improvements in power system structures and operation.
The ongoing expansions and growth in the technology,
demand a more optimal and profitable operation of a
power system with respect to generation, transmission and
distribution systems. The solution is the use of FACTS
devices especially the use of UPFC. The objective of this
paper is to study the effect of UPFC with its various modes
of operation such as power flow control, voltage injection.
Later, damping of oscillations in a power system is also
studied. In this paper the MATLAB/Simulink software is
used to model the power system by installing UPFC in
transmission link to study its use as power flow controller
and damping the power system oscillations. The
simulations results were presented and found satisfactory.
I. INTRODUCTION
The technology of power system utilities around the
world has rapidly evolved with considerable changes in
the technology along with improvements in power
system structures and operation. The ongoing
expansions and growth in the technology, demand a
more optimal and profitable operation of a power system
with respect to generation, transmission and distribution
systems. In the present scenario, most of the power
systems in the developing countries with large
interconnected networks share the generation reserves to
increase the reliability of the power system. However,
the increasing complexities of large interconnected
networks had fluctuations in reliability of power supply,
which resulted in system instability, difficult to control
the power flow and security problems that resulted large
II. UNIFIED POWER FLOW CONTROLLER
(UPFC)
UPFC is capable of both supplying to and absorbing
from the power system through the excitation converter
and transformer a controllable amount of reactive power
and inserting a voltage of controllable magnitude and
phase angle in series with the transmission system
through the converter and transformer. The UPFC can
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ISSN (Online): 2347-2820, Volume -1, Issue-2, 2013
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International Journal of Electrical, Electronics and Computer Systems, (IJEECS)
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provide simultaneous and/or independent control of all
basic power system parameters, which are transmission
voltage, impedance and phase angle. Such "new"
FACTS device combines together the features of two
"old" FACTS devices: the Static Synchronous
Compensator (STATCOM) and the Static Synchronous
Series Compensator (SSSC).
The arrangement shown in the figure 3.1 functions as an
ideal ac-to-ac power converter in which the real power
can freely flow in either direction between the ac
terminals of the two converters, and each converter can
independently generate (or absorb) reactive power at its
own ac output terminal.
Converter 2 provides the main function of the UPFC by
injecting a voltage Vpq with controllable magnitude
Vpq and phase angle in series with the line via an
insertion transformer. This injected voltage acts
essentially as a synchronous ac voltage source. The
transmission line current flows through this voltage
source resulting in reactive and real power exchange
between it and the ac system. The reactive power
exchanged at the ac terminal is generated internally by
the converter. The real power exchanged at the ac
terminal is converted into dc power which appears at the
dc link as a positive or negative real power demand.
In practice, these two devices are two Voltage Source
Inverters (VSI’s) connected respectively in shunt with
the transmission line through a shunt transformer and in
series with the transmission line through a series
transformer, connected to each other by a common dc
link including a storage capacitor. The shunt inverter is
used for voltage regulation at the point of connection
injecting an opportune reactive power flow into the line
and to balance the real power flow exchanged between
the series inverter and the transmission line. The series
inverter can be used to control the real and reactive line
power flow inserting an opportune voltage with
controllable magnitude and phase in series with the
transmission line. Thereby, the UPFC can fulfill
functions of reactive shunt compensation, active and
reactive series compensation and phase shifting.
The basic function of converter 1 is to supply or absorb
the real power demanded by converter 2 at the common
dc link to support the real power exchange resulting
from the series voltage injection. This dc link power
demand of converter 2 is converted back to ac by
converter 1 and coupled to the transmission line bus via
a shunt connected transformer. In addition to the real
power need of converter 2, converter 1 can also generate
or absorb controllable reactive power, if it is desired,
and thereby provide independent shunt reactive
compensation for the line.
Besides, the UPFC allows a secondary but important
function such as stability control to suppress power
system oscillations improving the transient stability of
power system. As the need for flexible and fast power
flow controllers, such as the UPFC, is expected to grow
in the future due to the changes in the electricity
markets, there is a corresponding need for reliable and
realistic models of these controllers to investigate the
impact of them on the performance of the power system.
III. CASE STUDY AND SIMULATION
RESULTS
2.1 Basic operating principles of UPFC
3.1 Case study 1-Power Flow Control with the UPFC
The main function of UPFC is to control the flow of real
and reactive power by injection of a voltage in series
with transmission line. Both the magnitude and the
phase angle of the voltage can be varied independently.
Real and reactive power flow control can allow for
power flow in prescribed routes; loading of transmission
lines closer to their thermal limits and can be utilized for
improving transient and small signal stability of the
power system. The UPFC consists of two voltage
sourced converters, connected back-to-back and are
operated from a common dc link provided by a storage
capacitor as shown in the figure 1
To illustrate the study and effects of UPFC the Test
system is considered and its Single Line diagram is
shown in figure 2.
Figure 2. Single Line Diagram of the Multimachine
system
Explanation of Single Line Diagram
In a 500 kV /230 kV transmission system, which is
connected in a loop configuration as shown in the figure
4 consists essentially of five buses (B1to B5)
interconnected through three transmission lines (L1, L2,
.
Figure 1. Implementation of the UPFC by two back-toback voltage sourced converter
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International Journal of Electrical, Electronics and Computer Systems, (IJEECS)
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L3) and two 500 kV/230 kV transformer banks Tr1 and
Tr2. Two power plants located on the 230kV system
generate a total of 1500 MW which is transmitted to a
500 kV, 15000 MVA equivalent connected at bus B5
and to a 200 MW load connected at bus B3. The plant
model includes a speed regulator, an excitation system
as well as a power system stabilizer (PSS).
The series converter of UPFC can inject a maximum of
10% of nominal line-to-ground voltage (28.87 KV) in
series with line L2.
In normal operation, most of the 1200 MW generation
capacity of power plant #2 is exported to the 500 kV
equivalents through two 400 MVA transformers
connected between buses B4 and B5. For this illustration
we consider a case where only two transformers out of
three are available (Tr2= 2*400 MVA = 800 MVA). The
simulation shows that most of the power generated by
plant #2 is transmitted through the 800 MVA
transformer bank (901 MW out of 1000 MW) and that
92.58 MW is circulating in the loop. Transformer Tr2 is
therefore overloaded by 101 MVA. This power
congestion can be relieved by placing the UPFC in the
transmission line as shown in figure 3.
Figure 4. MATLAB- SIMULINK Model of single line
diagram with UPFC
SIMULATION RESULTS:

Power Flow Control with the UPFC
Figure 3. Single Line Diagram of the Test system with
UPFC
The above test system is modeled in MATLABSIMULINK environment as shown in figure 4.
Parameters of the UPFC: the series converter is rated
100 MVA with a maximum voltage injection of 0.1 pu.
The shunt converter is also rated 100 MVA. The DC
link nominal voltage (Vdc) is 40KV and DC link total
equivalent capacitance(C) is 750µF.
Figure 5. P Pref, Q Qref, Voltage Mag (p.u), Voltage
phase (deg) of the UPFC
The UPFC located at the right end of line L2 as shown
in figure 4.4 is used to control the active and reactive
powers at the 500 KV bus B3, as well as the voltage at
bus B_UPFC. The UPFC consists of two 100 MVA,
IGBT-based, converters (one shunt converter and one
series converter interconnected through a DC bus).The
important keys to note in the block diagram are:
 Use of Bypass breaker – Used to connect or
disconnect UPFC Block from Power System
Figure 6. Voltage, Real Power and Reactive power at
Buses
 The reference power inputs [Pref, Qref] – Reference
for power flow control
In figure 5 it shows how P and Q measured at bus B3
follow the reference values of converter. The Bypass
breaker is opened at t=5sec the natural power is diverted
from the breaker to the UPFC series branch without
noticeable transient. At t=10 sec, the power increases to
684MW at a rate of 1 pu/sec. This increase of 100 MW
 The reference voltage Vdqref – Reference for
voltage injection
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ISSN (Online): 2347-2820, Volume -1, Issue-2, 2013
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International Journal of Electrical, Electronics and Computer Systems, (IJEECS)
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at bus B3 is achieved by injecting a series voltage of
0.091 pu with an angle of 95 degrees. This results in an
approximate 100 MW decrease in the active power
flowing through Tr2 which now carries an acceptable
load i.e. active power decrease from 901 MW to 799
MW at bus 4.
Case C-
Multimachine system with PSS and without
UPFC
Case D-
Multimachine system with PSS and UPFC.
In this multimachine system the damping of oscillations
is tested for all the above cases with respect to Rotor
Speed (wm) and Load angle (Delta) and simulation
results are presented.
The figure 6 shows the variations of voltages, active
powers and reactive powers at buses B1 to B5 reflect the
above.
Case A- Multimachine system without PSS as well
as UPFC
4. Case study 2- Performance of UPFC on damping
of power system oscillations
In the figure 8 and 9 it is observed that oscillations in the
Rotor Speed (wm) and Load angle (Delta) of Generators
G1 and G2 are sustained in the system.
Low frequency electromechanical oscillations are
inevitable characteristics of power systems and they
greatly affect the transmission line transfer capability
and power system stability. PSS and UPFC devices can
help the damping of power system oscillations.
SIMULATION RESULTS:
In this case the same test system shown in the figure 3 is
used to study the effect of UPFC on damping the power
system oscillations under fault condition. A three phase
fault is considered near to the bus B1 in the test system
and is modeled in the simulink.
Figure 8. Rotor speed variations of G1 and G2 without
PSS as well as UPFC
Figure 9. Load angle (Delta) variations of G1 and G2
without PSS as well as UPFC
Figure 7. MATLAB-SIMULINK Model of the single
line diagram with Fault
Case B- Multimachine system without PSS but
having UPFC
In this case the study of effect of UPFC on damping the
power system oscillations under fault condition is
studied with and without PSS.
Simulation results are shown below, it is observed that
Rotor Speed (wm) and Load angle (Delta) oscillations
are damped by the UPFC is shown in figure 10 and 11.
The two machines are equipped with a Hydraulic
Turbine and Governor (HTG), Excitation system and
Power System Stabilizer (PSS). These blocks are located
in the two 'Turbine and Regulator' subsystems.
In this case a 3-phase fault is applied near to the bus B1
using Fault breaker and select the parameters "Switching
of phase A, B and C" in the Fault breaker to simulate a
three-phase fault.
Figure 10. Rotor speed (Delta) variations of G1 and G2
without PSS but having UPFC
The magnitude of the fault and the time of application of
the fault are kept constant.
The transition time is set as 1s to 1.15s. This means that
the fault will be applied at 1 s and is cleared at 1.15s.
The various cases considered under the fault condition
are:
Case A - Multimachine system without PSS as well as
UPFC
Case B-
Figure 11. Load angle (Delta) variations of G1 and G2
without PSS but having UPFC
Multimachine system without PSS but having
UPFC
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International Journal of Electrical, Electronics and Computer Systems, (IJEECS)
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Therefore to control the power from one end to another
end, the concept of power flow control and the concept
of voltage injection using UPFC are applied. Modeling
the system in MATLAB-SIMULINK and studying the
simulation have given an indication that UPFC are very
useful to control power flow in a transmission line using
its various modes of operation
Case C-Multimachine system with PSS and without
UPFC
Simulation result for this case is shown in figure 12 and
13 with respect to Rotor Speed (wm) and Load angle
(Delta).
The Low frequency electromechanical oscillations are
inevitable characteristics of power systems and they
greatly affect the transmission line transfer capability
and power system stability. Hence the Damping of
oscillations using UPFC is analyzed in the system under
fault condition. In brief the Following conclusions are
made
Figure 12. Rotor speed (wm) variations of G1 and G2
with PSS and without UPFC

Power flow control is achieved and congestion is
less.

Improved Voltage Profile

Faster Steady State achievement.

Damping of oscillations in a power system.
Figure 13. Load angle (Delta) variations of G1 and G2
with PSS and without UPFC
REFERENCES
Case D-Multimachine system with PSS and UPFC.
Simulation result for this case is shown in figure 14 and
15 with respect to Rotor Speed (wm) and Load angle
(Delta).From the result it is observed that the oscillations
are well damped in the system with UPFC.
[1]
N.G. Hingorani and L. Gyugyi, “Understanding
FACTS concepts and technology of flexible ac
transmission systems”, IEEE Press, NY, 1999.
[2]
John J. Paserba, Fellow, IEEE “How FACTS
Controllers Benefit AC Transmission Systems “,
pp.447-454
[3]
L.Gyugui,“A Unified Power Flow Control
Concept of Flexible AC Transmission
Systems” international conference on AC and DC
power Transmission, 17-20 sept.1991.
[4]
L. Gyugyi, C.D. Schauder, S.I. Williams, T.R.
Reitman, D.R. Torgerson, and A. Edris,
1995,“The Unified Power Flow Controller: A
new approach to power Transmission control”,
IEEE Trans. on Power Delivery, 10(2), pp. 10851097
[5]
Vibhor Gupta “Study and Effects of UPFC and
its Control System for Power Flow Control and
Voltage Injection in a Power System”,
International Journal of Engineering Science and
Technology Vol. 2(7), 2010, 2558-2566.
[6]
Mr. R. H. Adware, Prof. P. P. Jagtap and Dr.
J.B. Helonde,
“power
system oscillations
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Third International Conference on Emerging
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Figure 14. Rotor speed (wm) variations of G1 and G2
with PSS and UPFC
Figure 15. Load angle (Delta) variations of G1 and G2
with PSS and UPFC
CONCLUSION
In power system transmission, it is desirable to maintain
the voltage magnitude, phase angle and line impedance
to improve power system operation and stability.

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