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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 11, NOVEMBER 2014 5813

Line Loss Minimization in Isolated Substations and

Multiple Loop Distribution Systems Using the UPFCMahmoud A. Sayed , Member, IEEE , and Takaharu Takeshita , Member, IEEE

Abstract—This paper presents the line loss minimum conditionin isolated substations and same substation multiple loop distribu-tion systems by using the unified power flow controller (UPFC).In each case, the mathematical model is presented and the lineloss minimum conditions are obtained based on the line param-eters of the distribution feeders. Since multiple loop distributionsystem is fed from same substation, the line loss minimization canbe achieved by compensating the summation of the line reactancevoltage drop. In an isolated substation loop distribution system,the line loss minimization can be achieved by compensating thesummation of the line reactance voltage drop in addition to thevoltage difference of the substations. Realization of both cases canbe achieved if the loop current is eliminated from the loop system.

The series compensation technique applied by the UPFC is used toeliminate the loop current from the loop distribution system andhence minimize the total line loss. Theeffectiveness of theproposedcontrol schemes of the UPFC have been verified experimentally.

Index Terms—FACTS, line loss minimization, loop distributionsystem, series compensation, unified power flow controller(UPFC),voltage regulation.

I. INTRODUCTION

GREENHOUSE gas emissions are considered one of the

most important problems that affect our environment and

causes climate change. Electrical Energy supplies are consid-

ered as oneof themain contributors in globalwarming since they

emit carbon dioxide (CO2 ), which is the most abundant green-

house gas. Therefore, concerns over global warming problems

have led to change the operation and design standards for dis-

tribution networks. The most important of these is the energy

saving. Line loss minimization in distribution systems is the

most efficient solution in order to achieve energy saving. In ad-

dition, line loss minimization enhances the voltage profile along

the distribution feeders [1], [2].

Distribution networks are mainly classified as either radial

or loop. The radial system is commonly used because it has

simple and inexpensive protection schemes. Also, its voltage

profile can be controlled easily. However, radial systems are of-

ten quite weak because of the long distances involved and thehigh resistance-to-inductance ratio of the lines that are used,

Manuscript received July 13, 2013; revised October 23, 2013 and December3, 2013; accepted January 8, 2014. Date of publication January 29, 2014; dateof current version July 8, 2014. Recommended for publication by AssociateEditor P. Barbosa.

M. A. Sayed is with the Department of Electrical Engineering, Fac-ulty of Engineering, South Valley University, 83523 Qena, Egypt (e-mail:

[email protected]).T. Takeshita is with the Department of Electrical and Computer Engi-

neering, Nagoya Institute of Technology, 466-8555 Nagoya, Japan (e-mail:[email protected]).

Digital Object Identifier 10.1109/TPEL.2014.2301833

Fig. 1. Configuration of the distribution system.

and the continuous increasing of the power demand. Hence, as

the demand power increases on these networks, power quality

issues such as high power loss often become a significant prob-

lem. Ideally, the power loss in electrical systems should not be

more than 6%. However, in developed countries, the power loss

is more than 10%, whereas in developing countries, it is more

than 20% [3]. Therefore, distribution system engineers haveproposed a loop system to achieve the merits of line loss reduc-

tion and to enhance the load voltage profile along the feeders.

Although, the loop distribution system requires complex pro-

tection schemes [4], [5], it is also more reliable service that is

offered for critical loads such as hospitals. As a result, much

of recent research works have considered reconfiguring the ra-

dial distribution system to loop one in order to achieve these

objectives [6]–[9].

Reconfiguration of radial distribution systems to the closed-

loop lines, using the existing infrastructure, is done in the

medium voltage by connecting the ends of adjacent radial feed-

ers. Based on the feeder substation, two different types of the

loop system may result. These types are the isolated substationsloop system and multiple loop system [8], [10], as shown in

Fig. 1. The isolated substations loop system results from re-

configuring two radial feeders fed from different substations

to perform one loop, whereas the multiple loop system results

from reconfiguring more than two radial feeders fed from same

substation to perform at least two adjacent loops. In the isolated

substations loop system, a loop current may results from the

voltage difference between substation voltages in addition to

the asymmetrical line parameters of the feeders. In a multiple

loop system, a loop current may results from the asymmetrical

line parameters of the feeders, only, since its feeders are fed

from the same substation.

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5814 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 11, NOVEMBER 2014

Recent research in distribution networks has been focused

on the minimizing power loss and enhance the load voltage

profile along the feeders by using power electronics technolo-

gies [11]–[18]. The idea given in [11]–[15] has been achieved

using STATCOM, SSSC, and active filters-based shunt and se-

ries power converters to compensate the reactive power in dis-

tribution systems, and hence, control the power flow. In [16]

and [17], power balancing between feeders using a back-to-

back converter has been proposed to reduce the loop power loss

and improve the load voltage profile. In [18], a loop power con-

troller has been used to reconfigure two radial systems to loop

in order to reduce the power loss and mitigate power flow over-

loading. In [19]–[21], the authors have proposed the line loss

minimum conditions in loop distribution systems, and experi-

mentally achieved these conditions by using the unified power

flow controller (UPFC). In [20], the authors proposed load volt-

age regulation to be equal in the magnitude to the source volt-

age under line loss minimum condition. However, the proposed

technique cannot guarantee the other load voltages to be within

the permissible voltage range. In [21], the authors proposed anew technique for controlling all load voltages to be within the

permissible voltage range under line loss minimum condition

by controlling both the load voltage magnitude and phase an-

gle to follow reference values. The reference load phase angle

was calculated from the line loss minimum condition, whereas

the reference voltage magnitude was calculated based on the

voltage magnitude of each load in the loop system. Line loss

minimization can be achieved if the loop current is eliminated

from the loop system. This condition can be achieved if, at least,

the line parameters of all loop lines have the same ratio between

the resistance and inductance. This can be realized in the same

substation loop distribution system since it has only one substa-tion for the whole loop, whereas in an isolated substations loop

system this condition cannot be realized due to the voltage dif-

ference between them, which may result from small difference

in the magnitude or phase angle. The difference between the

substation voltages results in an excessive loop current that will

flow in the loop lines even if the loop line parameter ratios are

same.

In this paper, the mathematical model of the isolated substa-

tions and the same substation multiple loop distribution system

is presented. Also, the line loss minimum condition is inves-

tigated by using the UPFC in both loop systems. The UPFC

shunt converter is used to regulate the dc-link voltage, whereas

the series converter is used to control the power flow in order toachieve line loss minimum condition. Since the series converter

performs the main function of the UPFC, its proposed control

schemes in both cases are presented. The effectiveness of the

proposed control schemes are investigated experimentally using

a laboratory prototype of 200 V, 6 kVA.

II. MODEL OF THE ISOLATED SUBSTATIONS LOOP

DISTRIBUTION SYSTEM WITH THE UPFC

The UPFC is used in a loop distribution system as a series

compensator to control the power flow in the loop lines in order

to achieve line loss minimum condition. Fig. 2 shows a simple

Fig. 2. Model of the isolated substations loop distribution system.

model of the isolated substations loop distribution system with

the UPFC equivalent circuit. Since the UPFC shunt converter is

used to regulate the dc-link voltage, its current is very small andcan be neglected. Therefore, the UPFC is simplified as a series

voltage source that represents the series converter voltage.

The loop system is represented by three distribution lines

(Line 1, Line 2, and Line 3). Each line has its resistance (Ri) andinductance (Li ), where i is the line number (i = 1, 2, 3). Theline impedance is represented by Ż i = Ri + jωLi . Since theloop system is isolated substations, it is supplied from different

substations represented by two voltage sources ( V̇ 1 and V̇ 2 ). Thesystem feeds two loads represented by constant current sources

( İ L1 and İ L2 ). The line current of each line in the loop systemflows in the same direction. The UPFC series injected voltage

is represented by the voltage source ( V̇ c).

A. Before Installing the UPFC

The line currents İ i (i = 1, 2, 3), that flow in each line of the loop system, before installing the UPFC, can be formulated

using Superposition theorem as follows:

İ 1 = ( Ż 2 + Ż 3 ) İ L1 + Ż 2 İ L2 + ( V̇ 1 − V̇ 2 )

Ż loop

İ 2 = − Ż 1 İ L1 − ( Ż 1 + Ż 3 ) İ L2 + ( V̇ 1 − V̇ 2 )

Ż loop

İ 3 = − Ż 1 İ L1 + Ż 2 İ L2 + ( V̇ 1 − V̇ 2 )Ż loop

(1)

where

Ż loop =3

i=1

Ż i , Rloop =3

i=1

Ri , Lloop =3

i=1

Li . (2)

According to the line loss minimization theory in the loop

distribution system [19]–[21], the line currents can be divided

into two currents. The first one is the current flow in each line

during the line loss minimum condition [ İ mi , (i = 1, 2, 3)], andthe second one is the loop current İ loop that circulates in the

loop lines, as shown in Fig. 2. Therefore, the line current can be


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SAYED AND TAKESHITA: LINE LOSS MINIMIZATION IN ISOLATED SUBSTATIONS AND MULTIPLE LOOP DISTRIBUTION SYSTEMS 5815

Fig. 3. Equivalent circuit of the loop system under line loss minimization.

formulated as follows:

İ i = İ mi + İ loop . (3)

The line currents that flow in each line of the loop system

during line loss minimization, [ İ mi , (i = 1, 2, 3)], can be for-mulated as follows:

İ m1 = {(R2 + R3 ) İ L1 + R2 İ L2}/Rloop

İ m2 = {−R1 İ L1 − (R1 + R3 ) İ L2}/Rloop

İ m3 = {−R1 İ L1 + R2 İ L2}/Rloop

. (4)

Considering the voltage difference in substations, the loop

current İ loop that circulates in the loop system lines can beformulated as follows:

İ loop = − 1

Rloop

3i= 1

jωLi İ i + ( V̇ 2 − V̇ 1 )

. (5)

The total power loss in the isolated substation loop systemlines can be formulated as follows:

P l =3

i=1

Riİ i2

=

3i=1

Riİ mi2 + 2

3i=1

(Ri İ mi)

· İ loop

+ Rloopİ loop 2 . (6)

According to (6), the line loss minimum condition can be

achieved in the loop system if the loop current İ Loop is elimi-

nated from the system. In this case, the line currents that flowin the loop lines after eliminating the loop current are [ İ mi ,(i = 1, 2, 3)]. Fig. 3 shows the equivalent circuit of the isolatedloop distribution system in case of line loss minimization. There-

fore, the resultant total line loss can be formulated as follows:

P lmin =3

i= 1

Riİ mi2 . (7)

The line loss minimum conditions can be obtained by equat-

ing the loop current, shown in (5) with zero, as follows:

3

i=1

jωLi İ i + ( V̇ 2 − V̇ 1 ) = 0. (8)

The loop current can be eliminated from the loop system if

the summation of the reactance voltage drop in addition to the

voltage difference between the substation voltages equals zero.

The summation of the reactance voltage drop equals zero if the

resistance-to-inductance ratio in each line of the loop system is

the same. Therefore, the loop current can be eliminated from

the loop system if the following conditions have been realized,

simultaneously:

R1/L1 = R2/L2 = R3/L3

V̇ 2 − V̇ 1 = 0

. (9)

B. After Installing the UPFC for Line Loss Minimization

The UPFC is installed in an isolated substations loop sys-

tem in order to eliminate the loop current from the system by

inserting a controlled series voltage that can compensate the

difference between substation voltages in addition to the sum-

mation of the reactance voltage drop in loop lines. In order to

compensate the summation of the reactance voltage drop, two

control schemes are proposed based on (8) and (9). These con-trol schemes are line inductance compensation and line voltage

compensation. Both schemes have a common part that is used

to compensate the difference between the substation voltages.

In the line voltage compensation scheme, the series converter

voltage of the UPFC can be formulated as follows:

V̇ c =3

i=1

jωLi İ i + ( V̇ 2 − V̇ 1 ). (10)

As a function of time, the UPFC series converter voltage can be


vc =

3i= 1

Lidi

idt + (v

2 − v1 ). (11)

In the line inductance compensation scheme, the UPFC is

used to insert a virtual inductance, Lc , which realize same ra-tio between the resistance and inductance in each line of the

loop system. The inserted inductance Lc can realize the sameresistance-to-inductance ratio in the loop system as follows:

R1/(L1 + Lc) = R2/L2 = R3/L3 . (12)

Therefore, the inserted inductance Lc can be calculated asfollows:

Lc = (R1/R2 )L2 − L1 . (13)

Based on loop line parameters, the value of the inserted con-

trolled inductance can be positive or negative [19]. The UPFC

series converter voltage, for line loss minimization, can be for-

mulated as follows:

V̇ c = − jωLc İ 1 + ( V̇ 2 − V̇ 1 ). (14)

As a function of time, the UPFC series converter voltage can be


vc = −Lcdi1dt

+ (v2 − v1 ). (15)

It is clear that the line inductance compensation scheme can

be used in the loop distribution system if there is only one line


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Fig. 4. Model of the multiple loop distribution system.

that has a different resistance-to-inductance ratio. Therefore,

only this line current in addition to the substation voltages are

required to obtain the reference voltage of the UPFC series

converter. On the other hand, the line voltage compensation

scheme can be used in the loop system if the resistance-to-

inductance ratio is not the same in, at least, two lines of the loop.

Also, in both schemes, the line loss minimum condition does

not need the load information. Both schemes depend on loop

line currents in addition to line impedance parameters to achieve

line loss minimization. Therefore, the system performance will

not be affected by load type.

III. MODEL OF THE SAME SUBSTATION MULTIPLE LOOP

DISTRIBUTION SYSTEM WITH THE UPFC

Fig. 4 shows the model of the same substation multiple loop

distribution system. It consists of three radial feeders fed from

the same substation. The loads fed from the radial lines are con-

nected in parallel in order to obtain two adjacent loop systems.

For mathematical model simplification, the loads are assumed

to be adjacent. Therefore, the impedance parameters of the line

connecting them are neglected. Line 1 has a step voltage regula-

tor (SVR), a common device used to compensate load voltages

along distribution feeders. It is assumed that the UPFC is in-

stalled in series with line 1 to achieve line loss minimization by

controlling the loop current in the two adjacent loop systems.

The line currents in loop lines should flow in the same direction.

Therefore, it is assumed that the currents İ 1 and İ 2 flow in the

counter clockwise direction, whereas the currents İ 2 and İ 3 flowin the clockwise direction.

A. Before Installing the UPFC

Considering that the UPFC output voltage is zero, the three

load currents ( İ L1 , İ L2 , and İ L3 ) can be lumped together ina single load current ( İ L ). According to the location of theSVR in line 1, the line parameter will be determined referred

to the secondary side. Fig. 5 shows the equivalent circuit of the

multiple loop system, shown in Fig. 4. The line currents in each

feeder of the loop distribution system model, shown in Fig. 5,

Fig. 5. Equivalent circuit of the multiple loop system.

can be formulated as follows:

İ 1 = 1

Ż 1 +Ż 2 Ż 3

Ż 2 + Ż 3

Ż 2 Ż 3

Ż 2 + Ż 3İ L + (n−1) V̇ s

İ 2 = −1Ż 2 +

Ż 1 Ż 3

Ż 1 + Ż 3

˙Z 1

˙Z 3

Ż 1 + Ż 3İ L−

˙Z 3

Ż 1 + Ż 3(n−1) V̇ s

İ 3 = 1

Ż 3 +Ż 1 Ż 2

Ż 1 + Ż 2

Ż 1 Ż 2

Ż 1 + Ż 2İ L−

Ż 2

Ż 1 + Ż 2(n−1) V̇ s

(16)

where

Ż 1 = R1 + jωL1

R1 = n2R1a + R1b

L1 = n2L1a + L1b

(17)

İ L = İ L1 + İ L2 + İ L3 . (18)

Since the distribution system is connected to perform two

adjacent loop systems, the line currents in each feeder can be

divided into the loop circulating currents ( İ loop1 and İ loop2 ) in

addition to the currents at line loss minimum condition ([ İ mi ,(i = 1, 2, 3)]). Therefore, the line current can be formulated asfollows:

İ 1 = İ m1 + İ loop1İ 2 = İ m2 + İ loop1 + İ loop2İ 3 = İ m3 + İ loop2

. (19)

The loop circulating current in each loop system can be for-

mulated as follows:

İ loop1 =

−V̇ m1 + V̇ m2 −(n−1) V̇ s

+

Ż 2

Ż 2 + Ż 3( V̇ m2 + V̇ m3 )

Ż 1 +Ż 2 Ż 3

Ż 2 + Ż 3

İ loop2 =

Ż 2

Ż 1 + Ż 2

V̇ m1 + V̇ m2 −(n−1) V̇ s

−( V̇ m2 + V̇ m3 )

Ż 3 +Ż 1 Ż 2

Ż 1 + Ż 2

(20)

V̇ mi = Ż i İ mi(i = 1, 2, 3). (21)


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Fig. 6. Equivalent model at line loss minimum condition.

Line loss minimization in the loop distribution system can

be achieved if the loop current is eliminated from the loop

system. This can be realized by compensating the inductance

voltage drop in each feeder constituting the loop system by

using a series compensator. Therefore, the equivalent circuit of

the loop distribution system, shown in Fig. 5, in case of line

loss minimization can be represented by a feeder resistanceonly as shown in Fig. 6. The line currents of each feeder in the

system shown in Fig. 6, at line loss minimum condition, can be


İ m1 = 1

R1 + R2R3R2 +R3

R2R3R2 +R3

İ L

İ m2 = −1

R2 + R1R3R1 +R3

R1R3R1 +R3

İ L

İ m3 = 1

R3 + R1R2R1 +R2

R1R2R1 +R2

İ L

. (22)

Based on (19), (20), and (22), the loop current in each loop

system ( İ loop1 and İ loop2 ) can be formulated as follows:

İ loop1 =−V̇ 1 + V̇ 2 −(n−1) V̇ s

+

R2R2 +R3

( V̇ 2 + V̇ 3 )

R1 + R2R3R2 +R3

İ loop2 =

R2R1 +R2

V̇ 1 + V̇ 2 −(n−1) V̇ s

−( V̇ 2 + V̇ 3 )

R3 + R1R2R1 +R2

(23)

˙V i = jωLi

˙I i (i = 1, 2, 3). (24)

In this case, the total power loss can be formulated as a function

of the line currents and the loop currents as follows:

P l =3

i= 1

Riİ i2

=

3i= 1

Riİ mi2 + 2(R1 İ m1 + R2 İ m2 ) · İ loop1

+ 2(R2 İ m2 + R3 İ m3 ) · İ loop2 + R1

İ loop1

2

+ R2İ loop1 + İ loop2

2+ R3

İ loop2

2. (25)

According to (25), the line loss minimum condition can be

achieved if both loop currents ( İ loop1 and İ loop2 ) are eliminatedfrom the system. In this case, the resultant total power loss can

be formulated as follows:

P lmin =3

i= 1

Riİ mi2

. (26)

It is clear from (23) that eliminating both loop currents from

the loop system, in order to realize line loss minimum condition,

can be achieved if the following two conditions are realized,

simultaneously:

V̇ 1 + V̇ 2 −(n−1) V̇ s = 0

V̇ 2 + V̇ 3 = 0

. (27)

B. After Installing the UPFC for Line Loss Minimization

The main objective of installing the UPFC in the same sub-

station multiple loop distribution system is to realize the line

loss minimum condition by controlling the loop power flow,using the series compensation scheme, provided by the inserted

controlled series converter voltage. Fig. 7(a) shows the multiple

loop distribution system with the UPFC in case of line loss min-

imization. The voltages (V 1 , V 2 , and V 3 ) represent the inductivevoltage drop in each line (Line 1, Line 2, and Line 3). Using

Thevenin’s theorem, the system can be approximated as shown

in Fig 7 (b) and (c) in order to obtain the reference voltage of the

UPFC series converter. The parallel feeders (Line 2 and Line 3)

are approximated by a resistor Ż 0 in series with a voltage sourceV̇ 0 , which can be formulated as follows:

Ż 0 =

R2R3R2 +R3

V̇ 0 = R3R2 +R3

V̇ 2 − R2R2 +R3

V̇ 3

. (28)

According to the approximated loop system shown in Fig. 7,

the reference voltage of the UPFC series converter can be for-

mulated as follows:

V̇ c = V̇ 1 + V̇ 0 −(n−1) V̇ s . (29)

Therefore, inserting a controlled series voltage by the UPFC

series converter as given in (29) results in loop currents İ loop1

and İ loop2 to be as follows:

İ loop1 = 0

İ loop2 = −V̇ 2 + V̇

3

R2 + R3

(30)

where

V̇ i = jωLi İ i

İ 1 = İ m1

İ 2 = İ m2 + İ loop2

İ 3 = İ m3 + İ loop2

. (31)


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Fig. 7. Multiple loop system with the UPFC and its equivalent circuit. (a)

Model of the multiple loop distribution system with the UPFC. (b) Thevenin’sequivalent circuit of line 2 and line 3. (c) Equivalent circuit of the multiple loopsystem with the UPFC.

Since the two loops are adjacent, the loop currents İ loop1 and

İ loop2 affect each other. The inserted controlled series voltage

has theability to eliminate the loop current İ loop1 , which reduces

the loop current İ loop2 . However, the loop current İ loop2 can be

eliminated, simultaneously with the loop current İ loop1 , if itsloop lines have the same resistance-to-inductance ratio. The ref-

erence voltage of the UPFC series converter can be formulated,

as a function of time and line parameters, as follows:

vc = L1di1dt

+ R3R2 +R3

L2di2dt

− R2R2 +R3

L3di3dt

− (n−1)vs . (32)

Based on the line loss minimum conditions in both isolated sub-

stations and same substation multiple loop distribution system,

it is clear that the controlled series voltage depends mainly on

the line currents or line parameters. The change in line param-

eters due to the temperature is very small and it will be in all

line parameters, simultaneously. Therefore, it will not affect the

system performance. However, detecting all line currents of the

Fig. 8. Configuration of the UPFC.

loop system is difficult. Line currents in the loop system can be

estimated as given in [22]. Moreover, the protection schemes of

the loop system add another challenge in the practical system.

Since the short-circuit current in loop distribution systems is

high, recent research works focus on protective devices for fault

isolation and fault location detecting methods [23], [24].

IV. EXPERIMENTAL RESULTS

Two laboratory prototype systems have been carried out in

order to demonstrate the effectiveness of the line loss minimiza-

tion in isolated substations and same substation multiple loop

distribution systems. In both systems, the UPFC is installed to

achieve line loss minimum condition. Experimental waveforms

of the line currents in all studied cases have been captured.

Bar charts have been presented to compare theoretical and ex-

perimental results in order to demonstrate the accuracy of the

experimental system and the proposed control schemes. All the

line currents, voltages, and total power loss are measured byusing Digital Power Meters (Yokogawa WT1600) that are con-

nected simultaneously in the sending and receiving ends of each

line in addition to the input and output of the UPFC converter

in the experimental system. The power loss in each line is cal-

culated from the difference between the sending and receiving

powers.

A. UPFC Circuit Configuration

One of the most promising FACTS devices is the UPFC,

which has been introduced by Gyugiy in 1991 [25]. Fig. 8

shows the configuration of the UPFC used in the experimen-

tal system. It consists of combined series and shunt convertersconnected back to back to each other through a common dc-

link capacitor. The series converter, which acts as a controllable

voltage source vc , is used to inject a controlled voltage in series

with the distribution line and thereby to force the power flow to

a desired value. In general, the series converter exchanges the

active and reactive power with the line while performing this

duty. The reactive power is electronically provided by the series

converter itself, whereas the active power is transmitted to the

dc terminals. The main function of the shunt converter, which

acts as a controllable current source ic , is to regulate the dc-link

voltage by adjusting the amount of the active power drawn from

the distribution line to meet the real power needed by the series


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Fig. 9. Experimental configuration of the isolated substations loop system.

converter. In addition, the shunt converter has the capability of

controlling the reactive power [26]–[29].

The UPFC series and shunt converters, shown in Fig. 8,

have been built as a three-phase pulse width modulation con-

verter with IGBT SKM100GB124D as the power device. The

Expert-III DSP system-based TMS320VC33 is selected as the

controller for both converters. The shunt converter is connected

in parallel with the distribution line via a three-phase trans-

former. The series converter, multilevel converter [30], [31],

consists of three single-phase H-bridge converters. The ac ter-

minals of each H-bridge converter are connected in series with

the distribution line through a single-phase transformer. The

switching and sampling frequency for series and shunt convert-

ers are 2.45 and 4.9 kHz, respectively.

The main function of the UPFC series converter is to realize

line loss minimization in the isolated substations and multiple

loop distribution systems by injecting a controlled series voltage

based on the line lossminimum condition in each case. The main

function of the shunt converter is to regulate the dc-link voltage.

Therefore, in the following discussion, the shunt converter is

disregarded because its current, ic , is very small.

B. Isolated Substations Loop Distribution System

Fig. 9 shows the 6-kVA, 200-V laboratory prototype of thedistribution system and the UPFC converters. The distribution

system consists of two radial feeders fed from different sources

that have same magnitude but a difference of 10◦ in the phase

angle. The two radial feeders can be transformed to the loop

system by connecting the loads in parallel. The parameters of

the whole system are listed in Table I. The proposed line in-

ductance compensation scheme and line voltage compensation

scheme have been used, respectively, to control the UPFC se-

ries converter in order to realize line loss minimization. In each

control scheme, all line currents and loop current in addition

to the source voltage difference are measured before and after

installing UPFC.

TABLE IPARAMETERS OF THE ISOLATED SUBSTATIONS LOOP SYSTEM

Fig. 10. Experimental waveforms using theline voltage compensation schemein the isolated substations loop system.

1) Controlling the UPFC Using the Line Voltage Compen-

sation Scheme: In order to control the UPFC series converter

using the line voltage compensation scheme, all line currentsand source voltages are detected to calculate the reference volt-

age as given in (11). In this scheme, the UPFC is considered as

a voltage source to compensate the line inductance voltage drop

in addition to the difference in source voltages. Fig. 10 shows

the experimental waveforms of source voltages difference ∆v,reference voltage of the UPFC series converter (vc), line 1 cur-

rent (i1 ), line 2 current (i2 ), and loop current (iloop ) of the loop

system before and after installing the UPFC. It is clear that after

installing the UPFC, the loop current is eliminated from the loop

system, and hence, the line loss minimum condition is realized.

The total power loss in the UPFC converters is 83.58 W.

2) Controlling the UPFC Using the Line Inductance Com-

pensation Scheme: In order to control the UPFC series con-verter using the line inductance compensation scheme, only the

UPFC line current and the source voltages are detected to cal-

culate the reference voltage as given in (15). In this scheme, the

UPFC is considered as an inductance (Lc ) in series with a volt-age source. The inductance (Lc ) compensates the loop systemline parameters in order to achieve equal ratio of the resistance

to inductance for all loop lines, whereas the additional voltage

source compensates the difference in source voltages. Accord-

ing to (13), the inserted inductance Lc = 0.13mH. Fig. 11 showsthe experimental waveforms of the source voltages difference

∆v, reference voltage of the UPFC series converter (vc ), line

1 current (i1 ), line 2 current (i2 ), and loop current (iloop ) of


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Fig. 11. Experimental waveforms using the line inductance compensationscheme in the isolated substations loop system.

Fig. 12. Comparison between theoretical and experimental results for radial,isolated substations loop system without the UPFC, and isolated substations

loop system with the UPFC.

the loop system before and after installing the UPFC. Exper-

imental results show that after installing the UPFC, the loop

current is eliminated from the loop system, and hence, the lineloss minimum condition is realized. The total power loss in the

UPFC converters is 80.95 W. Also, it is clear that the experimen-

tal results obtained by both schemes coincide with each other

since they provide the same function for the same system with

different control techniques.

The bar chart shown in Fig. 12 presents a comparison be-

tween theoretical and experimental results of the experimental

system that works as radial, isolated substations loop before in-

stalling the UPFC, and isolated substations loop after installing

the UPFC controlled by the line voltage and line inductance

compensation schemes. In both control schemes, after installing

the UPFC, the total line loss is reduced by 37.3% compared with

Fig. 13. Experimental configuration of the same substation multiple loop

system.

TABLE I I

PARAMETERS OF THE SAME SUBSTATION MULTIPLE LOOP SYSTEM

loop before installing the UPFC, and reduced by 16.5% com-

pared with radial.

C. Same Substation Multiple Loop Distribution System

Fig. 13 shows the 6-kVA, 200-V laboratory prototype of the

same substation distribution system that consists of three radial

feeders, which can be reconfigured to loop by connecting the

loads in parallel. SVR is taken into consideration by installing

in series with line 1. The parameters of the whole system are

listed in Table II. The UPFC is installed in the loop system as

shown in Fig. 13 to achieve the line loss minimum condition

by eliminating the loop currents from the two mesh circuits. In

order to achieve that, the UPFC has been controlled to insert a

reference series voltage as given in (32).

Fig. 14 shows the experimental waveforms of the UPFC series

converter voltage vc , line 1 current on both side of the SVR (i1aand i1b ), line 2 current i2 , line 3 current i3 , and the two loop

currents (iloop1 and iloop2 ) before and after installing the UPFC.

Before installing the UPFC, the difference in the resistance-to-

inductance ratio in the line parameters of loop 1 causes the loop

current to pass in this loop (I loop1 = 2.05 A). Although the lineparameters of loop 2 have the same resistance-to-inductance

ratio, there is a loop current (I loop2 = 1.04 A), which resultsfrom the effect of loop 1 current. Since the injected power of

the UPFC is small, the total power loss in the UPFC converter is

20 W. It is clear that after installing the UPFC, both loop currents

have been diminished from the distribution system, and hence,

the line loss minimum condition has been realized.


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Fig. 14. Experimentalwaveforms of thesamesubstation multiple loop system.

Fig. 15. Comparison between theoretical and experimental results for radial,same substation multiple loop system without the UPFC, and same substationmultiple loop system with the UPFC.

Fig. 15 shows a comparison between theoretical and exper-

imental results of both loop currents and total line loss for the

system that works as radial, same substation multiple loop with-

out the UPFC, and same substation multiple loop after installing

the UPFC. It is clear that after installing the UPFC, the total line

loss is reduced by 12.5% compared with loop before installing

the UPFC, and reduced by 24.6% compared with radial.

V. CONCLUSION

This paper has presented the line loss minimum condition

and the power flow control schemes of the UPFC to realize line

loss minimization in the isolated substations and the same sub-

station multiple loop distribution systems, along with a detailed

mathematical analysis of both systems. The line loss minimum

condition has been realized in the loop distribution system bycompensating the line reactance voltage drop in addition to the

difference of substation voltages in the case of isolated sub-

stations loop system. Two control schemes of the UPFC series

converter have been proposed to achieve line loss minimum

condition in loop systems. These control schemes are the line

inductance compensation and line voltage compensation. The

effectiveness of the proposed control schemes has been verified

experimentally using a laboratory prototype 6 kVA, 200 V. A

comparison between experimental and theoretical results has

been presented to evaluate the accuracy of the results and the

validity of line loss minimum condition theory. Theoretical and

experimental results agree well. Experimental results prove that

the UPFC has a great capability to achieve line loss minimiza-

tion in isolated substations and the same substation multiple

loop distribution systems.

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Mahmoud A. Sayed (M’09) was born in Qena Pre-fecture, Egypt, in 1974. He received the B.Sc. andM.Sc. degrees in electrical engineering from MiniaUniversity, El-Minia, Egypt, in 1997 and 2001, re-spectively, and the Ph.D. degree from the NagoyaInstitute of Technology, Nagoya, Japan, in 2010.

Since 1999, he has been with the Department of Electrical Engineering, Faculty of Energy Engineer-ing, Aswan University, Aswan, Egypt, first as an Ad-

ministrator and since 2001 as a Research Assistant.Currently, he is an Assistant Professor in the Depart-

ment of ElectricalEngineering,Faculty of Engineering, SouthValley University,Qena, Egypt. His research interests include series and shunt compensation of

electrical distribution systems for voltage regulation and loss reduction usingseries and shunt PWM converters in addition to renewable energy applicationsand machine drives.

Dr. Sayed is a Member of the IEEE Power Electronics Society.

Takaharu Takeshita (M’92) was born in Aichi,

Japan, on August 23, 1959. He received the B.S.

and M.S. degrees in electrical engineering from theNagoya Institute of Technology, Nagoya Japan, in1982 and 1984, respectively, and the Ph.D. degreefrom Nagoya University, Nagoya, Japan, in 1990.

Since 1991, he has been with the Nagoya Instituteof Technology, where he is currently a Full Professorandis engaged in research on power convertersystemand motor control.

Dr. Takeshita is a Member of the Society of In-strument and Control Engineers.

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