International Electrical Engineering Journal (IEEJ)

Vol. 2 (2011) No. 3, pp. 550-554

ISSN 2078-2365

550

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. 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

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.

Interline Power Flow Controller: Review

Paper

Akhilesh A. Nimje , Chinmoy Kumar Panigrahi , Ajaya Kumar Mohanty

Akhilesh A. Nimje et al. Interline Power Flow Controller: Review Paper

551 | P a g e

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

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)

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

Intermediate Transformer

Intermediate Transformer

VSC1 VSC2

SW1

CB

Shunt Transformer

Series Transformer

SW2

FIG. 1. A UPFC SCHEMATIC

Fig. 2. A Two Converter IPFC

VSC1 VSC2

+ _ DC Bus

Conv1 Conv2 Conv3 ….

Optical Links

Fig. 3 IPFC Comprising n Converters

Control

_

International Electrical Engineering Journal (IEEJ)

Vol. 2 (2011) No. 3, pp. 550-554

ISSN 2078-2365

552

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.

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:

)1(2

cos2

sin 12

1

11

1

1

1

101

X

VV

X

VVPP rp

)2(2

sin2

cos 12

1

11

1

1

1

101

X

VV

X

VVQQ rp

where 1 = 1 - 2,

2sin2

sin1

1

V

Vp

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:

)3(cos1,sin

1

1

2

10

1

1

2

10

X

VQ

X

VP

Similar equations also apply to the support line ( 2) except

that Vp2 is not independent. It is related to Vp1 by the equation.

Vp1 I1 + Vp2 I2 = 0. (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 = - R2I2

2 (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:

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.

III. MODELLING OF MULTI – CONVERTER FACTS

DEVICES

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.

1. Power flow studies

2. Dynamic stability

V1

Vp1

Vp2

Vr1

Vr2

+ +

+ +

j X1

j X2

V2

V3

1 = 1 - 2 2 = 1 - 3

1 2

3

I1

I2

1

111

2

II

2

222

2

II

Fig.4. Representation of IPFC

Fig. 5. A Three Converter GUPFC

VSC1 VSC2 Series Series Shunt

VSC

Akhilesh A. Nimje et al. Interline Power Flow Controller: Review Paper

553 | P a g e

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.

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.

The phasor injV

is expressed differently for the shunt

and series converters. For the shunt converter,

.|| 1

shinj VV For the series converter,

.||

seinj VV

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.

V11

Vinj

+ V22

I

Lt Rt

Fig. 6. Model of a SVC

International Electrical Engineering Journal (IEEJ)

Vol. 2 (2011) No. 3, pp. 550-554

ISSN 2078-2365

554

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] Narain G. Hingorani, Laszlo Gyugyi, “Understanding FACTS

Concepts and Technology of Flexible AC Transmission Systems”, IEEE Press, Standard Publishers Distributors, Delhi.

[2] K. R Padiyar, “FACTS Controllers in Power Transmission and Distribution”, New Age International Publishers (formerly Wiley Eastern Limited), New Delhi.

[3] 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.

[4] 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.

[5] R. Strzelecki, G. Benysek, “Interline Power Flow Controller – New Concept in Multiline Transmission Systems”.