Power Flow Control through Transmission Line
with UPFC to Mitigate Contingency
Amit Shiwalkar & N. D. Ghawghawe
G.C.O.E. Amravati
E-mail : amitashiwalkar@gmail.com, g_nit@rediffmail.com
the transmission line can be controlled in a
predetermined manner. Flexible AC Transmission
System (FACTS) uses advanced power electronics to
control the parameters in the power system in order to
fully utilize the existing transmission facilities. Modern
power systems are becoming increasingly stressed
because of growing demand and variety of factors. In
recent years, energy, environment, right-of-way, and
cost problems have delayed the construction of new
transmission lines, while the demand for electric power
has continued to grow. This situation has necessitated a
review of the traditional power system concepts and
practices to achieve greater operating flexibility and
better utilization of existing power systems.
Representative of the FACTS devices are SVC (Static
Var Compensators), TCSC (Thyristor Controlled Series
Capacitor), STATCOM, the Unified Power Flow
Controller (UPFC), a phase shifters, etc. These devices
may be connected in shunt/series or a combination of
shunt and series. Variety of advantages like
improvement in steady state power flow, transient
stability, voltage stability etc. can be derived.
Performance of an existing transmission system can be
improved. The UPFC is the most versatile and complex
of the FACTS devices, combining the features of
STATCOM and the SSSC. The UPFC can provide
simultaneous control of all basic power system
parameters viz., line voltage, impedance and phase
angle [1].
Abstract – This paper is based on practical contingency
condition of a transmission line, over which a study using
UPFC is done without sacrificing the available generation
capacity. The Unified Power Flow Controller (UPFC) is
the most versatile and complex power electronic
equipment that has emerged for the control and
optimization of power flow in electrical power
transmission system. Installing the UPFC makes it possible
to control an amount of real power flow through the line.
This paper presents control of power flow through
overloaded/under loaded line under study. Simulation
were carried out using MATLAB software to overcome the
contingency condition by using UPFC .
Keywords – Contingency, FACTS (UPFC), Power flow,
WPCL (Wardha Power Co Ltd).
I.
INTRODUCTION
Today’s power system is highly complex and
requires careful design of new devices taking into
consideration the already existing equipments. Now a
day, numbers of private generating units are getting
commissioned due to power generation policy and open
access to transfer power. But, due to variety of
environmental and regulatory concern, the expansion of
electric power transmission facilities is restricted. Power
Transmission and Generation utility would be benefited
if they could increase line power capability while being
able to delay the construction of new transmission lines.
So as to meet, additional power transfer capability,
beyond the present limit, will be needed to meet the
requirement and to provide adequate margin. Power
transfer capability is constrained by facility overloads
and reactive power deficiencies. For optimum utilization
of available transmission facilities, FACTS (UPFC) can
be used to mitigate the problems [1][2].
II. UNIFIED POWER FLOW CONTROLLER
A. Basic Concepts
Submit your The UPFC consists of two series and
shunt converters with AC transmission systems. It is a
combination of STATCOM and SSSC. These parts are
coupled together with common DC link (capacitor) to
allow the real power flow between the series and shunt
converter output terminals bi-directionally, and to
provide real and reactive line compensation without an
external energy source. UPFC is able to control the
It is well known that the power flow through
transmission line is a function of line impedance,
magnitude and phase angle of bus voltage. If these
parameters can be controlled, the power flow through
ISSN (Print): 2278-8948, Volume-1, Issue-2, 2012
64
International Journal of Advanced Electrical and Electronics Engineering (IJAEEE)
series voltage injection by the means of unconstrained
angle. The transmission line with UPFC, which is
extended in AC transmission network, is shown in the
Figure 1 [7][10].
The series inverter is controlled to inject a
symmetrical three phase voltage of controllable
magnitude and phase angle in series with the line to
control active and reactive power flows on the
transmission line. So, this inverter will exchange active
and reactive power with the line. The reactive power is
electronically provided by the series inverter, and the
active power is transmitted to the dc terminals. The
shunt inverter is operated in such a way as to demand
this dc terminal power (positive or negative) from the
line keeping the voltage across the storage capacitor
Vdc constant. So, the net real power absorbed from the
line by the UPFC is equal only to the losses of the
inverters and their transformers. The remaining capacity
of the shunt inverter can be used to exchange reactive
power with the line so to provide a voltage regulation at
the connection point.
(2.1)
(2.2)
The real and reactive power flow of the
transmission line at the receiving end with UPFC is
given by following equations (2.1) and (2.2). Real
power of the system with UPFC:
(2.3)
From equation (2.1) and (2.3)
(2.4)
(2.5)
Reactive power of the system with UPFC
(2.6)
From equation (2.2) and (2.6)
(2.7)
Fig. 1
(2.8)
The voltage represented by a phasor V is
injected by SSSC in series with the transmission line.
The magnitude of ΔV is limited from 0 to 0.5 pu and its
angle Ø is constrained from 0° to 360° [2]. This voltage
can be added vectorally to the sending end voltage
(Vs δ), and the transmission line voltage can be seen as
Vs + V, as shown by the Figure 1. Thus, the sending
end voltage (Vef ) is effectively on the line voltage.
According to this situation, UPFC affects both the
magnitude and the phase angle of the line voltage on the
transmission line. Hence, it is reasonable to expect its
ability to control the transmittable real and reactive
power demand by varying the magnitude and phase
angle of the line at any given transmission phase angle
between the sending and receiving end voltages.
The transmitted real (P0) and reactive power (Q0) of
the transmission above without UPFC (from sending to
receiving end bus) are given by the expression
To provide the required real and reactive power, the
magnitude and phase angle of the series-injected voltage
from equation (2.5) and (2.8) are given by following
equations:
(2.9)
Phase angle of injected voltage,
(2.10)
The equations (2.9) and (2.10) give information
about V and the phase angle of series-injected voltage
to get the demanded real and reactive power by the
system within the specified range defined by the
maximum limit of injected V. The phasor diagram
based on the mathematical derivation above, which
gives the magnitude and phase angle of injected voltage
ISSN (Print): 2278-8948, Volume-1, Issue-2, 2012
65
International Journal of Advanced Electrical and Electronics Engineering (IJAEEE)
to meet the demand of real and reactive power of the
system, is obtained, as shown in Figure 2 [2].
Fig. 2
The transmittable of the real and reactive power
will be specified by the maximum limit of injected ΔV
.The wide range of controlling the transmitted power
independent from the transmission angle (δ ) indicates
not only superior capability of UPFC in power flow
applications, but also promises powerful capacity for
transient stability improvement and power oscillation
damping. The controllable of the real power (P) and the
reactive power (Q) at the receiving end between any
transmission angle (δ) can be seen in the following
mathematical equations. The powerful capabilities of
UPFC are discussed above in terms of conventional
transmission control concepts. It can be integrated into a
generalized power flow controller that is able to
maintain prescribed controllable real power and reactive
power Q in the line.
Fig. 4
The equations mentioned above verify that this
control region is a circle with a center defined by the
coordinates defined by PO(δ) and QO(δ), and the radius
is given by following equation:
(2.11)
The circular control regions are defined by the
equation (2.11). These regions are shown in the Figure
4.3 and 4.4. The center of these circuses are Po(δ) and
Qo(δ) at angles δ= 0° , δ = 30°, δ = 60° and δ = 90°
respectively. The focus of the centers are indicated by
the + sign as δ varies between 0° and 90°.
B. Controlling the Real and Reactive Power Flow
When the transmission line angle (δ) and amplitude
are zero, the real power Po(δ) and reactive power
Qo(δ)flows are zero. According to this situation, the
system is at standstill at the origin of the P and Q
coordinates. It is illustrated as a case in the Figure 3.
The circle around the origin of the plane is the locus of
the corresponding Q and P values (at receiving end). It
is obtained as the voltage phasor V which is rotated
around itself by a full revolution (0O<<380O) with its
maximum value Vmax .The boundary of the circle in the
(P, Q) plane shows all P and Q values obtainable with
UPFC in accordance with given rating. The UPFC with
the specific voltage rating of 0.5 pu is able to obtain 0.5
pu power flowing into either direction without imposing
any additional reactive power demanded on either the
sending or the receiving end bus, as seen from Figure 3
and 4.
When Considering the equations (2.9) and (2.10),
the real and reactive power change from the
uncompensated values PO (δ) and QO(δ)as a function of
the magnitude and the phase angle of the injected
voltage ΔV As the angle  is an unrestricted variable
0O<<380O, the attainable boundary of the control
region for P(δ,) and Q(δ,) is obtained from a complete
rotation of the phasor with its maximum ΔVmax.
The skills of UPFC demonstrated in the Figures 3
and 4 shows the capability of controlling the real and
reactive power flow independently at any transmission
angle provide a powerful tool for AC transmission
control.
Fig. 3
ISSN (Print): 2278-8948, Volume-1, Issue-2, 2012
66
International Journal of Advanced Electrical and Electronics Engineering (IJAEEE)
III. SYSTEM UNDER STUDY
A. Transmission Line under Study
A 540MW WPCL generation plant and 480MW
Koradi Power plant are interconnected by two separate
220kV line. One line directly connected to Ambazari
substation, Nagpur and other through 220kV Wardha
substation, a load center, through 160 km transmission
line. The load center is modeled by a 150MW load at
220kV Wardha S/stn. The load is fed by the WPCL
plant and remained generation fed to grid at Ambazari
substation is shown in Figure 5.
Fig. 6a
Similarly, the load flow using Gauss-Seidel and
N-R methods were calculated for the system parameters
give in Table-I and its results for the real power flow
through the system under study is compared with
simulation results.
Fig. 5
B. Modelled Load Flow without UPFC
The above said system with parameters given in
Table I, is modeled in MATLAB and contingency
condition is studied. In which it is found that the line
WPCL-Wardha (L2) is getting overloaded. Whereas, the
WPCL-Ambazari (L1) line remain underloaded. With
the generation of 4x135MW at WPCL and no CTPS
Generation, the power through L1 and L2 were 190MW
and 257MW, as in Figure 6.
IV. SIMULATION STUDY
In order to improve power flow, the transmission
line is compensated by an UPFC. The UPFC is
connected to line facing contingency condition i.e. lineL2 and adjacent to CTPS-WPCL station, as shown in
Figure 7. UPFC Phasor Model has been used for this
simulation. The existing system is naturally unbalanced
without UPFC, for all Gen-Sets at WPCL as per
simulation results shown in Figure 6.
Fig. 7
At this location the UPFC is used to reduce the
power flow in the line L2 to desired limit so as to push
the power in underloaded line L1. The simulation result
of the system configuration, in MATLAB Simulink is as
shown in Figure 6. At 5sec the UPFC is brought into
circuit, with required power flow reference, The line L2
is controlled to carry 235MW power through it and
remaining 212MW power carried over by L1 [3][6][10].
Fig. 6
Comparison with the realistic load flow data from
field is done, where 02 generating sets evacuating the
power through the system and the modeled system in
MATLAB. With these 2 gensets practical power flow
through line L1 and L2 were 76MW and 146MW
respectively which is equal to simulated result as shown
in Figure 6a. With 03 gensets the simulation result of
power flow is 133MW and 202MW through L1 and L2
respectively.
V. EFFECT OF UPFC ON LINE PERFORMANCE
The transmission line under study with parameters
as given in Table I is considered. UPFC is placed in
series with line L2 as shown in Figure 7. Bypass option
blocks the UPFC for the first 5 second. Voltage, active
ISSN (Print): 2278-8948, Volume-1, Issue-2, 2012
67
International Journal of Advanced Electrical and Electronics Engineering (IJAEEE)
power, reactive power and current variations in the
transmission line with UPFC and without UPFC are
studied and compared. When the transmission line is
without UPFC, the sending-end and receiving-end p.u.
voltages, real power and reactive power are shown in
first 5 seconds of Figure 8. When UPFC is placed across
the same transmission line, after 5 seconds, the voltages
at buses were improved, desired power flow through the
line L2 by which the power through line L1 gone up.
The Mw loss of line L2 without UPFC was 8.1MW,
whereas with UPFC it reduced to 6.5 MW (instead of
6.75MW which the loss for reduced power flow) [6][9].
As well known, with the increase of power flow the
reactive power also increases. With UPFC this reactive
power is maintained to desired level so as to maintain
voltage profile and losses due to reactive current
through line.
Fig. 9
Also, with UPFC in transmission line, results in
improvement of transient stability of the system, which
is an added advantage along with the power flow control
[5], improved Plant Utilization Factor, better Voltage
Profile.
Table-I
Line to line voltage
Transmission rating
Length of the transmission
line
Resistance of the line
Inductive reactance of the
line
Capacitive reactance of the
line
220 kV
260 MVA
160 Km
0.069947 Ω/Km
0.001355 Ω/Km
VIII. REFERENCES
Fig. 8
[1]
N.G. Hingorani and L. Gyugyi, “Understanding FACTS:
concepts and technology of flexible ac transmission systems”,
IEEE Press, NY, 1999.
[2]
Y.H. Song and A.T. Johns, “Flexible ac transmission systems
(FACTS)”, The Institute of Electrical Engineers, London,
1999.
[3]
M. Noroozian, L. Angquist, M. Ghandhari, and G. Andersson,
“Use of UPFC for optimal power flow control”, IEEE Trans.
on Power Delivery, Vol. 12, No. 4, pp. 1629-1634, 1997.
[4]
L. Gyugyi, “Dynamic compensation of ac transmission line by
solid-state synchronous voltage sources”, IEEE Trans. Power
Delivery, Vol. 9, pp. 904-911, Apr.1994.
[5]
R. MihaliEet.al, “Improvement of Transient Stability Using
Unified Power Flow Controller”, IEEE Transactions on Power
Delivery. Vol.11,No1. January 1996.
[6]
A. Edris, A.S. Mehraban, L. Gyugyi, S. Arabi, T. Reitman,
"Controlling the Flow of Real and Reactive Power ", IEEE
Computer Applications in Power, pp. 20-25, January 1998.
[7]
L.Gyugyi, C.D. Schauder, S.L. Williams, T.R. Reitman, D. R.
Torgerson, A. Edris, "The Unified Power Flow Controller: A
New Approach to Power Transmission Control,” IEEE Trans.
VI. UPFC PARAMETER
The real power flow through line L2 is regulated to
235MW from 257MW due to that the power flow in line
L1 gone up from 190MW to 212MW, and the reactive
power to 14.04MVAr. for this power flow adjustment it
is found that series insertion voltage of 0.085 p u -70
Degree needs to be injected, as shown in Figure 9.
VII. CONCLUSION
In this study, the MATLAB environment using
phasor model of UPFC connected to a three phase-three
wire transmission system. This paper presents control
and performance of UPFC intended for installation on a
transmission line. Simulation results show the
effectiveness of UPFC in controlling real and reactive
power through the line.
68
ISSN (Print): 2278-8948, Volume-1, Issue-2, 2012
International Journal of Advanced Electrical and Electronics Engineering (IJAEEE)
on Power Delivery, Volume 10, No. 2, pp. 1085-1097, April
1995
[8]
[9]
Ying Xiao; Song, Y.H.; Chen-Ching Liu; Sun, Y.Z.;
“Available transfer capability enhancement using FACTS
devices”, Power Systems, IEEE Transactions on Volume: 18,
Issue: 1, 2003 , Page(s): 305 – 312.
Farhangfar, A.; Sajjadi, S.J.; Afsharnia, S.; “Power flow
control and loss minimization with Unified Power Flow
Controller (UPFC)”, IEEE Conference on Electrical and
Computer Engineering, 2004, Page(s): 385 - 388 Vol.1.
[10]
Das, G.T.; Kalyani, S.T.; “Control and
performance of UPFC connected to a transmission line”,
Power Engineering Conference, 2007. IPEC 2007, Page(s):
302 –307.
[11] Amit Shiwalkar; N. D. Ghawghawe; “Power Flow Control
Through Transmission Line with UPFC”, The Institution of
Engineers (India), Nagpur, Oct-2012.

ISSN (Print): 2278-8948, Volume-1, Issue-2, 2012
69