International Electrical Engineering Journal (IEEJ)
Vol. 2 (2011) No. 3, pp. 550-554
ISSN 2078-2365
Interline Power Flow Controller: Review
Paper
Akhilesh A. Nimje , Chinmoy Kumar Panigrahi , Ajaya Kumar Mohanty

Abstract – The Interline Power Flow Controller (IPFC)
proposed is a recent concept for the compensation and effective
power flow management of multi – line transmission systems. In
its general form, the IPFC employs a number of inverters with a
common DC link, each to provide series compensation for a
selected line of the transmission system [1],[2] & [3]. This paper
investigates the use of IPFC, which are dc/ac converters linked
by common DC terminals, in a DG-power system from an
economy perspective [4]. Because of the common link, any
inverter within the IPFC is able to transfer real power to any
other and thereby facilitate real power transfer among the lines
of the transmission system. Since each inverter is able to provide
reactive compensation, the IPFC is able to carry out an overall
real and reactive power compensation of the total transmission
system. This capability makes it possible to equalize both real
and reactive power flow between the lines, transfer power from
overloaded to under loaded lines, compensate against reactive
voltage drops and corresponding reactive line power and to
increase the effectiveness of the compensating system against
dynamic disturbances.
Keywords: FACTS, Compensation, power flow, IPFC, voltage
stability.
I. INTRODUCTION
The evolution of power industry in recent years has imposed
many challenges due to the radical changes in the energy
market as power demand is more than the availability [4]. Due
to heavy demand of power, distribution networks are always
in stress which results in reduced voltage across the load and
it affect on the performance. It is necessary to improve the
performance of power system to received quality power at the
consumer end. Reactive power compensation is the main
measure to keep power network running with high voltage
stability, high power quality and minimum system loss.
thyristor operation techniques. FACTS controllers are broadly
classified as series and shunt, both used to modify the natural
electrical characteristics of ac power system. Series compensation
modifies the transmission or distribution system parameters, while
shunt compensation changes the equivalent impedance of the
load. In both the cases the reactive power that flows through
the system can be effectively controlled by FACTS, which
improves the overall performance of ac power system. The
introduction of the Flexible AC Transmission systems has
been a considerable effort in the recent years on the
development of the power electronic based power flow
controllers. These controllers use thyristor switched
capacitors or reactors to provide reactive shunt and series
compensation. Active Power Filters, Universal Power
Line Conditioners, mainly Unified Power Flow
Controllers and Unified Power Quality Conditioners are
in stage of hard researches and increasingly applied.
Their possible functions are enlarging and include
power flow control, current and voltage harmonic
compensation, voltage imbalance, reactive power,
negative sequence current compensation. To one of the
most powerful arrangements we can add so called
UPFC (Unified Power Flow Controllers). Those systems
are the classical series-parallel filters (or special matrix
converter), which can control active and reactive powers
transmitted through the line. The major purpose of the
parallel filter is to keep voltage on the source element on
constant value. The series filter has to inject controllable
(with angle and magnitude) voltage and in this way
control power flow. One of the disadvantages of this
solution is need to equip every transmission line with
independent UPFC system.
Flexible AC transmission system (FACTS) devices are found to be
very effective controller to enhance the system performance.
FACTS Technology invented in 1986 by N. G. Hingorani from the
Electric Power Research Institute (EPRI) USA and it is based on
Corresponding Author is Akhilesh A. Nimje, School of Electrical
Engineering,KIIT
University, Bhubaneswar
751024, India Email:
buntynimje@yahoo.co.in
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Akhilesh A. Nimje et al. Interline Power Flow Controller: Review Paper
CB
Series
Transformer
Shunt
Transformer
the effectiveness of the overall compensating system for
dynamic disturbances. In other words, the IPFC can
potentially provide a highly effective scheme for power
transmission management at a multi-line substation.
(i)
Intermediate
Transformer
Intermediate
Transformer
SW1
VSC1
VSC2
VSC1
VSC2
SW2
Fig. 2. A Two Converter IPFC
FIG. 1. A UPFC SCHEMATIC
The closing of switches 1 and 2 enable the two
converters to exchange real power flow between the two
converters. The reactive power can be either absorbed
or supplied by the series connected converter. The
provision of a controllable power source on the DC side
of the series connected converter, results in the control
of both real and reactive power flow in the line
(measured at the receiving end). The shunt connected
converter not only provides the necessary power
required, but also the reactive current injected at the
converter bus Thus, a UPFC has three degree of
freedom unlike other FACTS controllers which have
only one degree of freedom (controlled variable). This
solution is not attractive from economical point of view.
The Interline Power Flow Controller (IPFC) concept
proposed in this paper addresses the problem of compensating
a number of transmission lines at a given substation.
Conventionally, series capacitive compensation (fixed,
thyristor-controlled or SSSC based) is employed to increase
the transmittable real power over a given line and also to
balance the loading of a normally encountered multi-line
transmission system. However, independent of their
implementation, series reactive compensators are unable to
control the reactive power flow in, and thus the proper load
balancing of, the lines. This problem becomes particularly
evident in those cases where the ratio of reactive to resistive
line impedance (Xm) is relatively low. Series reactive
compensation reduces only the effective reactive impedance
X and, thus, significantly decreases the effective X/R ratio
and thereby increases the reactive power flow and losses in
the line. The IPFC scheme proposed provides, together with
independently controllable reactive series compensation of
each individual line, a capability to directly transfer real
power between the compensated lines. This capability makes
it possible to: equalize both real and reactive power flow
between the lines; transfer power demand from overloaded to
under loaded lines; compensate against resistive line voltage
drops and the corresponding reactive power demand; increase
A pure series reactive (controllable) compensation in the
form of TCSC or SSSC can be used to control or regulate the
active power flow in the line, the control of reactive power is
not feasible unless active (real) voltage in phase with the line
current is not injected. The application of a TCSC (or SSSC
with impedance emulation) results in the reduction of net
series reactance of the line. However, X/R ratio is reduced
significantly and thereby increases the reactive power flow
(injected at the receiving end) and losses in the line. The
interline power flow controller (IPFC) provides, in addition to
the facility for independently controllable reactive (series)
compensation of each individual line, a capability to directly
transfer or exchange real power between the compensated
lines. This is achieved by coupling the series connected VSC
in individual lines on the DC side, by connecting all the DC
capacitors of individual converters in parallel. Since all the
series converters are located inside the substation in close
proximity, this is feasible.
II. BASIC PRINCIPLE OF IPFC
Conv1
+
Control
_
Conv2
….
Conv3
DC Bus
_
Optical Links
Fig. 3 IPFC Comprising n Converters
551 | P a g e
International Electrical Engineering Journal (IEEJ)
Vol. 2 (2011) No. 3, pp. 550-554
ISSN 2078-2365
An IPFC with two converters compensating two lines is
similar to UPFC in which the magnitude and phase angle of
the injected voltage in the prime system (or line) can be
controlled by exchanging real power with the support system
(which is also a series converter in the second line). The basic
difference with a UPFC is that the support system in the later
case is the shunt converter instead of a series converter. The
series converter associated with the prime system of one IPFC
is termed as the master converter while the series converter
associated with the support system is termed as slave
converter. The master converter controls both active and
reactive voltage (within limits) while the slave converter
controls the DC voltage across the capacitor and the reactive
voltage magnitude.
Similar equations also apply to the support line ( 2) except
that Vp2 is not independent. It is related to Vp1 by the equation.
For the system shown in figure 4, the received power
and the injected reactive power at the receiving end of the
prime line ( 1) can be expressed as:
The concept of combining two or more converters
can be extended to provide flexibility and additional degrees
of freedom. A generalized UPFC refers to three or more
converters out of which one is shunt connected while the
remaining converters are series connected as shown in figure
5.
1
Vp1
Vr1
+

V1
+

Vp2
j X1
+


1 = 1 - 2


I 1  I 1  1  1 
2

(4)
The above equation shows that Vp2 is negative if Vp1 is
positive. With the resistance emulation, we have
Vp1 = -R1I1, Vp2 = - R2I2.
(5)
Substitute equation (5) in equation (4), we get the constraint
involving R1 and R2 as R1I12 = - R2I22
(6)
The constraint equation (4) and (6) can limit the utility of
IPFC. In such a case, an additional shunt converter (forming a
GUPFC) will be useful as shown in figure 5 below:
V2
I1
Vr2
+
2
Vp1 I1 + Vp2 I2 = 0.
j X2
V3
I2
2 = 1 - 3
3


I 2  I 2  2   2 
 2

Fig.4. Representation of IPFC
P1  P10 
VV p1
X1
Series
VSC1
Shunt
VSC

 VV


sin 1  1   r1 cos 2  1  (1)
2
 X1
 2

Series
VSC2
Fig. 5. A Three Converter GUPFC

 VV


Q1  Q10 
cos 1  1   r1 sin 2  1  (2)
X1
2
 X1
 2

V p1
III. MODELLING OF MULTI – CONVERTER FACTS
where 1 = 1 - 2, sin 1 

DEVICES
2V sin
2
VV p1
P10 and Q10 are the real power and reactive power in the line 1
(at the receiving end ) when both Vp1 and Vr1 are zero. These
are expressed as:
V 2 sin  1
V2
1  cos  1 
P10 
, Q10 
X1
X1
The studies of multi converter FACTS devices are carried out
from the objectives of planning and operational analysis. The
broad spectrum of the required studies is listed below with
increasing order of complexity.
(3) 1. Power flow studies
2.
Dynamic stability
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Akhilesh A. Nimje et al. Interline Power Flow Controller: Review Paper
3.
Transient analysis neglecting harmonics
4.
Detailed transient analysis considering switching action
in the converters.

The power flow studies involve the computation of
solution of non-linear algebraic equations that relate the
specifications to the system state variables. The constraints
are usually handled by modifying the specifications. For
example, limits on the reactive current/ power are handled by
changing the voltage (magnitude) specification.
The dynamic stability refers to the stability of a power
system influenced by various controllers (AVR, PSS and
network controllers including HVDC and FACTS). There are
different mechanisms of system instability.
Both power flow and dynamic stability analysis are based
on the single - phase models of the network. Since dynamic
stability analysis involves phenomena of frequency below 5
Hz, the network variables (voltage and currents) are
represented by phasors that vary slowly.
However it is essential to test the controller performance
using detailed three phase models to validate the simplified
analysis. For example, the design of AC voltage regulator for
shunt converter requires the study electromagnetic
interactions that result from the network transients. In general,
this is true for all fast acting controllers. The detailed transient
simulation considers three phase nonlinear models of all
relevant components.
For the analysis and simulation of SSR, network
transients (below third harmonic) need to be modeled by
approximate models. For example, a transmission line can be
modeled by a single  equivalent model. There is no need to
consider the switching action in the converters and the
resulting harmonics. The FACTS controllers can be modeled
using dynamic phasors or d – q variables referred to a
synchronously rotating reference frame.
Vinj
Lt
V11
+
Rt
V22
I
Fig. 6. Model of a SVC
It would be desirable to employ a common model for all
types of studies. For multi-converter circuits, a converter can
be modeled by a variable voltage source in series with
inductive impedance as shown in figure 6. Here the voltage
source is related to the voltage across the DC capacitor based
on the converter topology and control action. For three phase
models, the voltage source is defined instantaneously and
contains harmonics. Neglecting harmonics, we can represent
the voltage by d – q components (dynamic phasors) that are
determined by exact controller models.
V inj  | Vsh | 1   . For the series converter,
V inj  | Vse |    .

For transient or dynamic stability analysis, the converter
model shown above can be represented conveniently by
Norton equivalent that simplifies the network solution using
the admittance matrix. For power flow analysis, a shunt
converter in isolation can be modeled as synchronous
condenser with the specification of bus voltage (magnitude).
The two control variables |Vsh| and  are calculated from the
specified voltage magnitude and the constraint equation that
relates the power drawn to the losses in the converter. For the
series converter, the specification in the line power flow (P)
and the constraint is the power supplied by the series
converter which may be assumed as zero. For the coupled
converters such as UPFC, the four control variables, |Vsh|,
|Vse|,  and  can be computed from the three specified
variables, (say V1, P2, Q2) and the constraint that relates the
power balance in the DC circuit.
IV. APPLICATION CONSIDERATIONS
The concept and basic operating principles of the IPFC
are explained in this paper. In practical applications the IPFC
would, in general, have to manage the power flow control of a
complex, multi-line system in which the length, voltage, and
capacity of the individual lines could widely differ. One of the
attractive features of the IPFC is that, although it may pose
engineering challenges particularly in the area of control, it is
inherently flexible to accommodate complex systems and diverse
operating requirements. A few relevant points to consider are briefly
mentioned below.
(1) The IPFC is particularly advantageous when controlled
series compensation or other series power flow control
(e.g., phase shifting) is contemplated. This is because the
IPFC simply combines the otherwise independent series
compensators (SSSCs), without any significant hardware
addition, and provides some of those with greatly
enhanced functional capability.
(2) The operating areas of the individual inverters of the
IPFC can differ significantly, depending on the voltage
and power ratings of the individual lines and on the
amount of compensation desired. It is evident that a high
power line may supply the necessary real power for a low
capacity line to optimize its power transmission, without
significantly affecting its own transmission.
(3) The IPFC is an ideal solution to balance both the real and
reactive power flow in a multi-line system.
(4) The prime inverters of the IPFC can be controlled to
provide totally different operating functions, e.g.,
independent P and Q control, phase shifting
(transmission angle regulation), transmission impedance
control, etc. These functions can be selected according to
prevailing system operating requirements.

The phasor V inj is expressed differently for the shunt
and series converters. For the shunt converter,
553 | P a g e
International Electrical Engineering Journal (IEEJ)
Vol. 2 (2011) No. 3, pp. 550-554
ISSN 2078-2365
V. CONCLUSION
IPFC like other FACTS Controller contribute to the
optimal system operation by reducing the power loss and
improving the voltage profile. The IPFC is a kind of
combined compensators, which combines at least two SSSCs
via a common DC voltage link. This DC voltage link provides
the device with an active power transfer path among the
converters, which enables the IPFC to compensate multiple
transmission lines at a given substation. This is a very
attractive feature of this FACTS device.
REFERENCES
[1]
[2]
[3]
[4]
[5]
Narain G. Hingorani, Laszlo Gyugyi, “Understanding FACTS
Concepts and Technology of Flexible AC Transmission Systems”,
IEEE Press, Standard Publishers Distributors, Delhi.
K. R Padiyar, “FACTS Controllers in Power Transmission and
Distribution”, New Age International Publishers (formerly Wiley
Eastern Limited), New Delhi.
Laszlo Gyugyi, Kalyan K. Sen, Colin D. Schauder, “ The Interline
Power Flow Controller Concept : A New Approach to the Power Flow
Management, ” IEEE Trans. on Power Delivery, Vol.14, no. 3, pp 1115
– 1123, July 1999.
Kishor Porate, K. L. Thakre, G. L. Bodhe, “ Voltage Stability
Enhancement of Low Voltage Radial Distribution Network Using
Static VAR Compensator: A Case Study”, WSEAS Transactions on
Power Systems, Issue 1, vol. 4, pp 32 – 41, January 2009.
R. Strzelecki, G. Benysek, “Interline Power Flow Controller –
New Concept in Multiline Transmission Systems”.
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