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IEEE TRANSACTIONS ON SMART GRID, VOL. 4, NO. 1, MARCH 2013 353

Advanced Power Distribution System Configurationfor Smart Grid

Jae-Chul Kim, Member, IEEE, Sung-Min Cho, Member, IEEE, and Hee-Sang Shin

AbstractPower distribution systems should meet demandssuch as high reliability, efficiency, and penetration of renewableenergy generators (REGs) in a smart grid. In general, power dis-

tribution systems are radial in nature. One-way power flow is theadvantage of a radial system. However, the introduction of REGs

causes bidirectional power flow. Furthermore, there are limitsto improvements in reliability and efficiency in a radial system.Therefore, the upgrading of primary feeders from a radial to aloop configuration has been considered in the Korea Smart Distri-

bution Project. An advanced power distribution system (APDS),

in which primary feeders operate in a loop configuration, has beenexplored in this paper. First, the design scheme of a conventionalpower distribution system configuration that adopts distribution

automation is introduced. Subsequently, an upgrading scheme ofloop configuration using normally opened tie switches and a tie

switch selection algorithm for loss minimization are described.

Finally, the advantages of the upgraded configuration are reportedthrough case studies. It is observed that the APDS configurationcan integrate more REGs from the viewpoint of voltage regulation.An advanced distribution system allowing greater use of REGswill be a major contribution to smart grid implementation.

Index TermsPower distribution system configuration, renew-able energy generation, smart distribution, voltage regulation.

I. INTRODUCTION

C ONCERNS about global climate change have increasedthe penetration of REGs, which are connected to powerdistribution systems. The Korean government announced a na-

tional objective to increase the share of REGs, which was 2.24%

in 2006, to 11% by 2030. In general, power distribution sys-

tems are radial in nature. In addition, given their relatively late

development, REGs were not considered in the design phase

of current power distribution systems. For these reasons, con-

necting distributed generation units (i.e., REGs) to the distri-

bution system may cause various interconnection problems in-

cluding harmonic concerns, system overvoltage, fault coordina-

tion, increased fault currents, and islanding concerns. To miti-

gate these problems, the IEEE Std. 1547 for distributed gener-ator (DG) interconnection was published in 2003 [1]. In Korea,

Manuscript received November 26, 2012; accepted December 09, 2012. Dateof publication February 06, 2013; date of current version February 27, 2013.This work was supported by the Power Generation & Electricity Delivery ofthe Korea Institute of Energy Technology Evaluation and Planning (KETEP)grant funded by the Korea government Ministry of Knowledge Economy (No.2009T100200067). Paper no. TSG-00821-2012.

S.-M. Cho (corresponding author) is with the Department of Electrical Engi-neering, Soongsil University, Seoul 156-743, Korea (e-mail: dannyone@ssu.ac.kr).

J.-C. Kim and H.-S. Shin are with the Department of Electrical Engi-neering, Soongsil University, Seoul 156-743, Korea (e-mail: jckim@ssu.ac.kr;shs8828@ssu.ac.kr).

Digital Object Identifier 10.1109/TSG.2012.2233771

TABLE ICAPACITYLIMITS OFDG [2]

KEPCO (Korea Electric Power Cooperation) also published a

DG interconnection manual in 2005. In this manual, the capacity

of a DG is limited by the voltage level and line configuration.

Table I shows capacity limits of a DG [2].

The main goal of these DG installment limitations is to keep

the distribution system voltage within a permissible range. In

Korean power distribution systems, the distribution voltage is

controlled by an on-load tap changer (OLTC) through the line

drop compensation (LDC) method. In some cases, the high pen-

etration of REGs may cause voltage regulation failures [3].

A fault occurrence in a feeder or lateral that is connectedto an REG leads to its interruption. For stable reclosing oper-

ation and maintenance crew safety, the REGs must detect is-

landing operationand be disconnected within 0.5 s according to

the KEPCO DG interconnecting manual. Furthermore, REGs

should wait 5 min to re-connect after the distribution system

becomes stable [1], [2]. Therefore, the reliability of the power

distribution system is important for REGs as well as for cus-

tomers.

For several decades, the configuration of power distribu-

tion systems has been typically designed in a radial form for

easy control [4], [5]. However, some power utilities such as

Taipower, Florida Power Company, Hong Kong Electric Com-

pany, and Singapore Power have adopted normally closed loop

configurations to serve their customers with high reliability

[6][9]. Loop power distribution systems have the advantages

of reliability and voltage regulation. To accommodate these

strengths, an APDS including loop configuration is being

developed in the Korea Smart Distribution System Project.

In Korea, KEPCO has already adopted a distribution automa-

tion system (DAS) with many normally opened tie switches to

improve reliability [10]. Therefore, without feeders or lateral

extensions, the power distribution system can simply be up-

graded to an APDS through tie switch closure. However, to op-

erate in loop configuration, the protection system should be up-

graded as well. All reclosers and circuit breakers (CBs) in the

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354 IEEE TRANSACTIONS ON SMART GRID, VOL. 4, NO. 1, MARCH 2013

Fig. 1. Conventional power distribution configuration in KEPCO.

loop path should detect bi-directional fault currents. Thus, ap-

propriate tie switches are selected to upgrade the radial configu-

ration to a loop configuration with the minimum upgrading cost.

The purposeof this paper is to introduce the advantages of the

APDS, which is being studied in the Korea Smart Distribution

Project, and to propose an algorithm for appropriate tie switch

selection. In section two, the conventional power distribution

system in Korea is described. The basic scheme of an APDS and

the algorithm for tie switch selection are introduced in section

three. In section four, the advantages of the APDS are examined

through a case study.

II. CONVENTIONALPOWERDISTRIBUTIONCONFIGURATION

A. Design Scheme Which Considers Reliability

The DAS is very useful for reliability enhancement. To min-

imize interruption time, KEPCO has adopted the DAS for de-termination of fault location, fault isolation, and service restora-

tion. A conventional power distribution configuration adopting

the DAS is shown in Fig. 1.

In the figure, CB, RA, and GA are substation circuit breakers,

automatic reclosers, and remote-controlled switches, respec-

tively. Distribution feeders are normally divided into three

sections. Each section has one or more normally opened tie

switches for service restoration. For example, if a fault occurs

in section two as illustrated in Fig. 2, automatic equipment de-

vices which experience a fault current generate a fault indicator

(FI) signal within 30 s. At the same time, RA1 is opened to clear

the fault. Subsequently, by considering the FI message, thefault location is determined. Next, GA3 and GA4 are opened to

isolate the fault. Finally, RA1 and GA10 or GA6 are closed for

service restoration. As these processes are completed within 5

min, the DAS is very useful in improving reliability. However,

because the REG interconnected in section three experiences

momentary interruptions, it should be disconnected within 0.5

s by an anti-islanding detection function. Furthermore, the

REGs should wait 5 min to re-connect after service restoration

is complete [2], [10] .

In the conventional power distribution system, a service

restoration scheme using normally opened tie switches is useful

for minimizing interruption time. However, REG operation is

sensitive to momentary interruptions due to anti-islanding de-

tection. Therefore, the service restoration process in an APDS

Fig. 2. Diagram of a conventional LDC voltage regulation method.

Fig. 3. Diagram of a conventional LDC voltage regulation method.

should reduce momentary interruptions to assure continuous

operation of REGs.

B. Voltage Regulation Scheme

Power utilities are required to keep customers voltage pro-

files on feeders close to therated value under all load conditions.

In urban areas, the power transformer located in the substation

with an under-load tap changer (ULTC) is the main voltage reg-

ulation equipment. Pole-mounted voltage regulators (PVRs) are

additionally installed in rural feeders. A ULTC controlled by the

LDC method is used to keep the voltage constant at a fictitious

regulation point (FRP) regardless of the magnitude or power

factor of the load. Sending currents and voltages , as

shown in Fig. 3, are used to calculate the FRP voltage according

to (1). The power distribution system operator should set up pa-

rameters such as dead band, time delay, reference voltage, resis-tance , and reactance for automatic voltage regulation

[3][5].

(1)

where

Fictitious regulation point (FRP) voltage.

Sending voltage.

Sending current.

Resistance of a feeder from ULTC to FRP.

Reactance of a feeder from ULTC to FRP.

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KIMet al.: ADVANCED POWER DISTRIBUTION SYSTEM CONFIGURATION FOR SMART GRID 355

In Korean power distribution systems, a power transformer

bank has 6 to 8 feeders. The voltage of the feeders depends on

the ULTC at the power transformer. If the load imbalance be-

tween feeders exceeds an acceptable range, voltage regulation

using only ULTC may fail. Furthermore, if many REGs are con-

nected to the feeders, LDC detects sending currents that are less

than the actual values. The output of REGs depends on weather

conditions such as wind, solar radiation, and temperature, and is

therefore uncontrollable. For this reason, it becomes more diffi-

cult for the distribution system operator to predict the load bal-

ance in advance as the penetration of REGs increases. There-

fore, high penetration of REGs may cause voltage regulation

failures [3].

III. ADVANCEDPOWERDISTRIBUTIONCONFIGURATION

A. Basic Scheme

The configuration of a conventional power distribution

system is radial because of its simplicity. However, as men-tioned above, the upgrading of primary feeders is needed

because there are numerous problems for a radial structure to

accommodate many REGs. In Korea, primary feeders have

at least three normally opened tie switches. By closing the

opened tie switches, the radial distribution configuration can

be upgraded to a loop structure without installing additional

electric power lines. Fig. 3 shows the upgrading scheme of an

ADPS, which includes a loop feeder structure as an example.

If tie switch GA4 is closed, feeder1 and feeder2 form a loop

structure. In this case, other tie switches should be opened to

avoid a mesh structure. Although additional electric power

lines do not need to be installed, the protection devices shouldbe upgraded to operate the power distribution system in a

loop structure. Therefore, the optimum tie switches should be

selected to maximize the profits of ADPS upgrading for the

loop structure.

B. Loop Configuration Selection Algorithm for Loss

Minimization

Load imbalance between feeders increases loss in the power

distribution system. In a radial structure, network reconfigura-

tion is used for load imbalance alleviation, loss minimization

and others [11]. However, in a loop structure, a loop path con-

necting a heavily loaded feeder and a lightly loaded feeder canalleviate the load imbalance to minimize loss.

The voltage drop in a heavily loaded feeder is larger than that

in lightly loaded feeders. Therefore, by considering the voltage

across the opened tie switch, we can infer which side of the

tie switch is the heavily loaded feeder. If the voltage across an

opened tie switch is high, loop operation using the tie switch

is more effective for loss minimization. Therefore, we present

a loop path selection algorithm using the voltage across open

tie switches. The loop path selection algorithm considers a 24 h

load profile because the output is for representing stable rather

than temporary states. A flowchart of the loop path selection

algorithm for loss minimization is shown in Fig. 4.

The loop path selection algorithm is as follows.

Step 1) Generate tie switch cases for loop configuration.

Fig. 4. Flowchart of loop path selection algorithm for loss minimization.

Step 2) Carry out powerflow analysis for 24 h.

Step 3) Calculate accumulated switch voltage (ASV) for all

tie switches according to (2).

(2)

where

Tie switch number.

Hours.

Open voltage of switch n at hour h.

Step 4) If all ASV calculations for tie switches are com-

pleted, calculate the total ASV according to (3) for

each case.

(3)

Step 5) Select the case with the maximum total ASV for loss

minimization.

IV. CASESTUDY

In this section, we compared the conventional radial structure

and the APDS loop structure to explore the advantages of the

APDS from the perspective of loss reduction and voltage profile.

The conventional Korean power distribution system adopting

the DAS, which is shown in Fig. 5, is used as the test distribu-

tion system model [12]. In the test model, there are four feeders

and six normally opened tie switches. There are three available

cases for upgrading the primary feeder from a radial to a loop

configuration. All test cases are summarized in Table II. Case 1

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356 IEEE TRANSACTIONS ON SMART GRID, VOL. 4, NO. 1, MARCH 2013

Fig. 5. Conventional Korea power distribution system adopting DAS.

TABLE IITEST CASES FORCASESTUDY

Fig. 6. Load profile of each feeder in the test distribution system model.

involves a radial structure and cases 2, 3, and 4 involve a loop

structure.

Each feeder supports electric power of various loads for res-

idential, commercial, and industrial customers. Most load pro-

files depend on the type of customer. Fig. 6 shows feeder load

profiles in the test model. Feeder 3 shows a relatively light load

throughout the day. Alternatively, feeders 1 and 2 show a heavy

load. The load profiles are derived from actual load patterns in

Korea.

Fig. 7. Losses during 24 h in the test distribution system model.

A. Loss Reduction Study

Powerflow analysis was performed for loss analysis using the

cases summarized in Table II. Fig. 7 shows the losses accumu-

lated during 24 h for each case. Upgrading to case 3, in which

the loss is reduced from 6.74 to 6.42 MWh, is the best solution

for loss minimization. However, to compare loss reduction be-

tween cases, powerflow analysis should be conducted for eachcase. Yet if there are more tie switches, the processing time will

increase. Therefore, to compare loss reduction more efficiently,

we applied the proposed loop path selection algorithm to test the

power distribution system model. In the radial case, the ASVs

for each tie switch calculated by (2) are shown in Fig. 8.

The total ASVs for each case are summarized in Table III.

The total ASV is highest in case 3. Therefore, this case is the

best loop-upgrading solution for loss minimization. This shows

that we can select a loop-upgrading path without conducting

powerflow analysis for each case.

B. Voltage Control Study

Voltage regulation in the test power distribution system

model mainly depends on ULTC control by the LDC method.

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Fig. 8. ASV for each tie switch.

TABLE IIICALCULATEDTOTAL ASV

Fig. 9. Maximum and minimum voltage profiles for case 1 and case 3.

Maximum and minimum voltage profiles for cases 1 and 3 arecompared in Fig. 9. The difference between the maximum and

minimum voltages for case 3 is narrower than that for case 1

(radial). Therefore,voltage regulation is easier in case 3.

The voltage regulation of power distribution systems in

which many REGs are interconnected may fail. Therefore, we

compared the robustness of voltage regulation between cases

1 and 3. The REGs were interconnected at line sections F25,

F36, and F414, respectively. Subsequently, we changed the

generation capacity from 1 to 10 MW. The maximum and

minimum voltages in the test power distribution system are

shown in Fig. 10. In case 1, undervoltage occurred because

the LDC method failed as a result of the 24 MW generated bythe REGs. In contrast, in case 3, the power distribution system

could accommodate the 30 MW generated by the REGs within

a permissible voltage range.

V. CONCLUSION

In this paper, we analyzed the advantages of an APDS loop

structure from the perspective of loss reduction and voltage reg-

ulation. In addition, we presented a loop path selection algo-

rithm for loss minimization. The results of case studies using

the test power distribution system are summarized as follows:

1) Appropriate upgrading of primary feeders from a radial to

a loop configuration reduces loss in the power distribution

system

Fig. 10. Maximum and minimum voltages with REGs of case 1 and case 3.

2) The robustness of voltage regulation in the loop structure

allows the APDS to accommodate more REGs than the

radial structure.

3) The proposed loop path selection algorithm gives the best

solution for loss minimization using only a power flow

analysis of the radial system.

Coordination of protection using communication technology

may permit the upgrading of primary feeders from radial to loop

configurations. We expect the APDS presented here to support

a more suitable environment for high penetration of REGs in

smart grids.

APPENDIX

The line impedance and load profiles of the test power dis-

tribution system used in the case study are listed in Tables IV

and V, respectively. All per units are based on 22.9 kV and 100

MVA. All power factors are 0.9.

TABLE IVLINE IMPEDANCE

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TABLE VAVERAGE LOAD AT END OFLINE SECTION

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[8] W. T. Huang, T. H. Chen, G. C. Pu, Y. F. Hsu, and T. Y. Guo, As-sessment of upgrading existing primary feeders from radial to nor-mally closed loop arrangement, inProc. 2002 IEEE Power Eng. Soc.Transm. Distrib. Conf., pp. 21232128.

[9] T. H. Chen, W. T. Huang, J. C. Gu, G. C. Pu, Y. F. Hsu, and T. Y. Guo,Feasibility study of upgrading primary feeders from radial and open-loop to normally closed-loop arrangement, IEEE Trans. Power Syst.,vol. 19, no. 3, pp. 13081316, 2004.

[10] N.-G. James, Robert Wilson, Control and Automation of ElectricalPower Distribution Systems. Boca Raton, FL, USA: CRC, 2006.

[11] S. M. Cho, H. S. Shin, J. H. Park, and J. C. Kim, Distribution systemreconfiguration consideringcustomer and DG reliability cost,J. Elect.

Eng. Technol., vol. 7, no. 4, Jul. 2012.[12] H. T. Lee, A study on the reliability analysis of loop power distribu-

tion systems with microgrid structure, Ph.D. dissertation, Dept Electr.Eng., Soongsil Univ., Seoul, Korea, 2009.

Jae-Chul Kim (M84) received the B.S. degree from Soongsil University,Korea, in 1979, and M.S. and Ph.D. degrees from Seoul National University,Korea, in 1983 and 1987, respectively.

He hasbeena professor of ElectricalEngineering at Soongsil University since1988. His research interests include power system reliability, smart distributionsystems, and smart grids.

Sung-Min Cho(S08M13) received the B.S., M.S. and Ph.D. degrees in elec-trical engineering from Soongsil University, Korea, in 2003, 2008 and 2012, re-spectively.

Currently, he is a Postdoctoral Researcher with Soongsil University. His re-searchinterests include powersystem reliability, smart distribution systems, anddistributed generation interconnection.

Hee-Sang Shin received the B.S. and M.S. degrees in electrical engineeringfrom Soongsil University, Korea, in 2007 and 2009, respectively. Currently, heis working toward the Ph.D. degree at Soongsil University Graduate School.

His research interests include smart distribution systems, and electric railways.