Loop Restoration Impact

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Published in IET Generation, Transmission & Distribution

Received on 31st December 2008

Revised on 14th April 2009

doi: 10.1049/iet-gtd.2008.0658

ISSN 1751-8687

Impacts of automatic control systems of loop restoration scheme on the distributionsystem reliability

S. Kazemi 1,3

M. Fotuhi-Firuzabad 1

M. Sanaye-Pasand 2

M. Lehtonen3

1Center of Excellence in Power System Control and Management, Department of Electrical Engineering,

Sharif University of Technology, Azadi Avenue, P.O. Box 11365-9363, Tehran, Iran2

Department of Electrical and Computer Engineering, University of Tehran, Tehran, Iran3

Department of Electrical Engineering, Helsinki University of Technology, Espoo, Finland

E-mail: [email protected]

Abstract: Loop restoration scheme (LRS) is a special feeder automation (FA) scheme, which is used by utilities to

improve distribution system reliability. The LRS is controlled and managed by its automatic control system (ACS).

The impacts on distribution system reliability indices of implementing LRS mainly depend on the type of its ACS.

Two common types of ACS of LRS are presented and used in this study. Successful operation of ACS is dependenton the protection and automatic control functions of switching devices of LRS. Different failure modes of these

switching devices can therefore affect the procedure of ACS in fault detecting, isolating and service restoration.

The impacts of failure of protection and automatic control functions of switching devices and fuse of lateral

distributors on reliability indices are illustrated. The worth of implementing LRS and its ACS type is

represented by the reduction in expected customer interruption cost. A distribution test system is utilised to

examine the impacts of two common types of ACS of LRS on the distribution system reliability. Selecting the

type of ACS of LRS by utilities relies on the desired level of load-point and system reliability improvement.

This study aims to quantitatively assess the impacts of two common types of ACS of LRS on the distribution

system reliability.

1 Introduction

As distribution utilities shift from non-profit public utilitiesto profit-driven business enterprises, the question of maintaining and improving service reliability while keeping electricity rates lower and protect shareholders’ interestsbecomes more difficult to answer [1]. Analysis of customer failure statistics in most utilities indicates that distributionsystems make the greatest individual contribution to theunavailability of supply to customers [2]. In an attempt tofacilitate rapid response to outages as well as efficient day-to-day operation of distribution systems, some utilities

worldwide have begun the prototype or large-scale projectsfor implementing distribution automation (DA) into thedesign of their distribution networks [3–5].

Loop restoration scheme (LRS) is a special DA method inthe feeder automation (FA) level, which is used by utilities toimprove distribution system reliability. The LRS is controlledand managed by its automatic control system (ACS). The

ACS is tuned by a set of algorithms to provide automaticcontrol operations of switching devices of LRS to remove thefaulted section and restore the unfaulted sections of thefeeder. There are two common types of ACS of LRS. Thetask of ACS is to detect the fault, isolate the faulted sectionand restore the service to the disconnected healthy sections.

The first type of ACS performs its task using the localinformation. The second one has communication link that

can gather and use remote data for that purpose [6–8].Successful operation of ACS is dependent on the protectionand automatic control functions of switching devices of LRS.

IET Gener. Transm. Distrib., 2009, Vol. 3, Iss. 10, pp. 891 – 902 891

doi: 10.1049/iet-gtd.2008.0658 & The Institution of Engineering and Technology 2009

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Therefore different failure modes of these switching devicescanaffect the procedure of ACS in performing its task.

Different regulatory environments put different weightingson reliability indices, such as system average interruptionfrequency index (SAIFI), system average interruption duration

index (SAIDI), momentary average interruption frequency index (MAIFI) and momentary average interruption event frequency index (MAIFIE). On the other hand, the impactsof implementing LRS on reliability indices of a givendistribution system mainly depend on the type of its ACS.

Therefore selecting the type of ACS of LRS by utilities relieson the level of improvement required at load points andsystem-oriented reliability indices and also budget constraints.

An approach is proposed in this paper to quantitatively assessthe impacts of the two common types of ACS of LRS ondistribution system reliability. The proposed technique is

based on the event tree method and the concepts of conditional probability approach. The general concepts behindthe proposed technique have been previously published in a few research works including those developed by the authors.However, the reliability assessment technique proposed in thispaper includes considerable advantages in comparison withthe previously published works. First, the evaluation procedureof this new work is generalised and can be used for reliability assessment of distribution systems in presence of any kinds of automated fault management schemes. This, however, cannot be performed using the existing approaches and tools. Second,it introduces some new equations for deducing the load pointsand overall system reliability indices associated with thesustained and momentary interruptions. Third, the new setsof generalised equations for deducing reliability worth indicesof an automated distribution system have been developed.

And finally, the effects of short-circuit failures of automaticswitching devices, operational failure of protection and/or control functions of automatic switching devices andoperational failure of fuse of lateral distributors have also beenconsidered in the proposed technique. In order to demonstratethe proposed technique, comparative studies and sensitivity analyses areconducted using a distribution reliability test system.

2 LRS and its ACSPower distribution engineers have developed different restoration schemes to automate the process of fault detecting, isolating and service restoration (FDISR). Theseschemes exploit the intelligence found in power distribution system equipment [9–11].

The LRS typically utilises a predetermined number of reclosers installed in series between two distribution feeders.It is applied to two feeders by installing a normally open tierecloser at a tie point between two feeders, as shown inFig. 1. Also, a normally closed sectionalising recloser is

installed on each feeder. The substation circuit breaker that operates to protect the distribution feeder has also reclosing capability. This scheme provides the isolation of the faulted

section within the distribution feeder and restores service toas much as possible of customers unaffected by the faulted

section within a relatively short period of time. This schemeis controlled and managed by a special ACS. In this paper,two common types of ACS with and without communication link are considered in the analysis.

2.1 ACS without communication link

The operating functions of this type of ACS are distributedamong switching devices and there is no communication link between these switching devices. The LRS may therefore beactivated when the switching devices act in accordance withthe pre-defined roles of the stored instructions and extracted

local information. Consider the schematic diagram of Fig. 1. The LRS which is equipped with ACS without communication link is designed to work as follows [6–8]:

† When a permanent fault occurs on section 1, substationcircuit breaker CB1 first opens and recloses and after re-ignition of the fault opens again and then locks out. The

ACS at the normally closed sectionalising recloser R 1senses loss of the source side voltage, and the ACS at thenormally open tie recloser R 3 senses voltage loss on its R 1side. Timers of both reclosers begin to operate. The controlsystem is designed in a way that the time delay at R 1expires first, and then R 1 opens and locks out. The time

delay at R 3 expires next, and R 3 closes and restores serviceto the unfaulted feeder on sections between R 1 and R 3.

After locating the fault by maintenance team, sectionalising switch SW 1 can be opened and R 1 is closed manually andrestores service to the unfaulted feeder on section 2.

† When a permanent fault occurs on section 2, a proceduresimilar to the case when a permanent fault occurs on section 1

will follow. Except, after fault locating, SW 1 can be openedand CB1 is closed manually and restores service to theunfaulted feeder on section 1.

†

When a permanent fault occurs on section 3, R 1 first opens and recloses and after re-ignition of the fault opensagain and then locks out. Tie recloser R 3 senses voltage

Figure 1 Typical configuration of loop restoration scheme

892 IET Gener. Transm. Distrib., 2009, Vol. 3, Iss. 10, pp. 891–902

& The Institution of Engineering and Technology 2009 doi: 10.1049/iet-gtd.2008.0658

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loss on its R 1 side. After expiration of its time delay, it closesand senses fault current, then trips and locks out. Theunfaulted portion of the feeder, between CB1 and R 1,remains in service. After fault locating, sectionalising switchSW 2 can be opened and R 3 is closed manually and restoresservice to the unfaulted feeder on section 4.

† When a permanent fault occurs on section 4, R 1 and R 3open and lock out after their fault protection sequences.

The unfaulted portion of the feeder, between CB1 and R 1,remains in service. After fault locating, SW 2 can be openedand R 1 is closed manually and restores service to theunfaulted feeder on section 3.

† When a temporary fault occurs on section 1 or 2, CB1

performs its normal reclosing sequence and remains closedonce the fault is cleared. During these sequences, allcustomers along the feeder are momentarily interrupted.

† When a temporary fault occurs on section 3 or 4, R 1performs its normal reclosing sequence and remains closedonce the fault is cleared. During these sequences, only customers between R 1 and R 3 are momentarily interrupted.

2.2 ACS with communication link

Although the above-discussed ACS without communicationlink is easy to implement, the following disadvantages exist.First, when a permanent fault occurs between CB1 and R 1or between CB2 and R 2, the LRS reconfiguration occursafter CB1 or CB2 performs its complete reclosing sequence

(four reclosing shot in most cases). This operation severely impacts the whole feeder. Second, automatic restoration tothe normal system configuration is difficult to achieve.

Third, if the fault occurs between R 1 and R 3 or betweenR 2 and R 3, the loss-of-voltage closure command by the

ACS without communication link logic could cause thefault to be momentarily placed on the other feeder. Theseconcerns are prevented with the ability to exchange data between switching devices via communication link.Reconsider the schematic diagram of Fig. 1. The LRS,

which is equipped with ACS with communication link, isdesigned to work as follows [8]:

† When a permanent or temporary fault occurs on section 1or 2, CB1 and R 1 instantly open. R 1 is activated through a command sent by CB1. After recognising the status of other switching devices, R 3 closes and restores service tothe unfaulted feeder on sections between R 1 and R 3. After that, CB1 recloses in a normal fashion. If successful (in thecase of a temporary fault), the loop is automatically restoredto its normal situation. Otherwise (in the case of a permanent fault), CB1 opens and locks out. After determining the fault location, SW 1 can be opened and R 1or CB1 is closed manually and restores service to the

unfaulted feeder on the section between SW 1 and R 1 or CB1 and SW 1, respectively. Comparing with the similar case using the ACS without communication link, there is

no need to wait until CB1 goes through all reclosing sequences even for temporary faults. In this way, reclosing impact on the whole feeder is minimised.

† When a temporary fault occurs on section 3 or 4, R 1performs its normal reclosing sequence and remains closed

once the fault is cleared. For permanent faults on thesesections, R 3 is blocked from closing into the fault. Contrary to the ACS without communication link, this type of ACSprevents the fault to be momentarily placed on the other feeder.

3 Evaluation procedure

3.1 General concepts

A modular approach is used to evaluate the impacts of theabove-discussed ACSs of LRS on distribution systemreliability. Successful operation of ACS depends on theprotection and automatic control functions of switching devices of LRS. Therefore different failure modes of theseswitching devices should be considered in the procedure of

ACS in fault detection, isolation and service restoration. Inthe proposed approach, the ACS is divided into modules that can be analysed independently. The reliability data associated

with each module can be either derived by a separatereliability analysis or obtained from a data collection scheme.For illustration purposes, the ACS for the LRS of Fig. 1 canbe divided into five modules, corresponding to each switching device. These modules have no shared components and areconsidered to be independent. Each module contains

switchgear, power supply units, timers, relays, sensors,processing units and communication systems (if available).

Based on the type of implemented ACS of LRS, theprocedure of ACS in performing its task, when a permanent or temporary fault occurs on a component, isidentified. This procedure involves the sequential operationlogic of a set of protection and automatic control functionsof switching devices of LRS. Therefore the consequence of the availability and unavailability of each protection andautomatic control functions of these switching devices onthe ACS operating procedure is analysed using the event

tree method [12–14]. Using this approach, various possibleclasses of load points and their associated probabilities areidentified. Then, based on the concepts of conditionalprobability approach, the interruption frequency andrestoration time of the load points are calculated using these outcomes. System-oriented reliability indices are thendetermined by aggregating the load-point indices.

3.2 Classification of interruptions and load points

Three classes of interruptions, designated as momentary

interruption, momentary interruption event and sustainedinterruption are considered in this paper [15]. Thedefinitions of these interruptions are found in [15].



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When a fault occurs, interruption frequency andrestoration time of different load points of a feeder are not the same because of the operating procedure of theimplemented ACS of LRS and availability of theprotection and automatic control functions of switching devices of LRS. Therefore during a permanent or a

temporary faulty condition on a component, load points of a distribution system equipped with LRS can be dividedinto different classes as tabulated in Table 1.

3.3 Steps of evaluation procedure

A comprehensive study is performed to develop appropriateformulas, models and evaluation procedure for determining the impacts of various ACSs of LRS on the distributionsystem reliability. They can be summarised in the following steps:

1. Based on the above-described general concepts, anappropriate model is deduced for each switching device of LRS from operational failure point of view. This modelshould be able to show the probabilities associated withsuccess or failure operation of protection and controlfunctions of switching devices. In addition, if failureoperation of fuse of lateral distributors needs to beconsidered, an appropriate model should be deduced for this component too. It should be noted that probability of operational failure is the conditional probability that a device will not operate if it is required to operate. The

developed models shown in Fig. 2 can have any degree of complexity, depending on available data and study targets.

2. The operational procedure of implemented ACS of LRS for FDISR, when a permanent or a temporary short-circuit fault occurs on each component of distribution network (including

switching devices of LRS), is identified. These procedureshave been described in Sections 2.1 and 2.2 for both ACS

without and with communication link. Then, using thededuced models for switching devices and fuse of lateraldistributors, the event trees are developed. The sequences of events together with the associated outcomes in the ACSoperating procedure are then identified. Eventually, basedon the outcomes of the developed event trees, the classes of each load point (corresponding to those shown in Table 1)and their associated probabilities are determined. The reader is invited to refer [12–14] for more detailed explanations onhow event trees are developed and interpreted.

3. Based on the results obtained from Step 2, thecontribution to the sustained, momentary and momentary interruption event frequency of L j because of a permanent and a temporary fault on C i are determined using theconcepts of expectations

lSC i L j

¼ (lPC i jB PC i L j

)Â P (B PC i L j ) þ (lP

C i jC PC i L j

)P (C PC i L j )

þ (l TC i j J TC i L j

) Â P ( J TC i L j )þ (l T

C i jK TC i L j

)Â P (K TC i L j )

(1)

Table 1 Classification of load points when a fault occurs on a component

Class of load point Fault type Interruption class Interruption frequency Interruption duration

A permanent not interrupted 0 0

B SI RPF RT

C SI RPF MST

D MI 1 Â RPF LRT

E MI NROFB Â RPF DFCT

F MI NRO Â RPF LRT

G temporary not interrupted 0 0

H MI 1 Â RTF LRT

I MI NROTFC Â RTF FFCT

J SI RTF FRT

K SI RTF MST

FRT: fuse replacement time; MI: momentary interruption; SI: sustained interruption; MST: manual

switching time; NRO: number of reclosing operations to lock out; NRO FB: number of reclosing

operations to blow the fuse of faulted lateral distributors; NROTFC: number of reclosing operations to

clear temporary faults; RPF: rate of permanent fault of the failed component; RTF: rate of temporaryfault of the failed component; RT: repair time of the failed component; LRT: loop restoration time;

DFCT: delayed fault clearing time; FFCT: fast fault clearing time



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lMC i L j

¼ ((lPC i Â 1)jD

PC i L j

) Â P (D PC i L j

)

þ ((lPC i ÂNROFB)j E PC i L j

)Â P ( E PC i L j )

þ ((lPC i ÂNRO)jF PC i L j

) Â P (F PC i L j )

þ ((l TC i Â 1)jH

TC i L j

)Â P (H TC i L j

)

þ ((l TC i ÂNRO TFC)j I

TC i L j

) Â P ( I TC i L j

) (2)

l

MEC i L j ¼ ((l

PC i Â 1)jD

PC i L j ) Â P (D

PC i L j ) þ ((l

PC i Â 1)j E

PC i L j )

Â P ( E PC i L j )þ ((lP

C i Â 1)jF PC i L j

)Â P (F PC i L j )

þ ((l TC i Â 1)jH

TC i L j

)Â P (H TC i L j

)

þ ((l TC i Â 1)j I

TC i L j

)Â P ( I TC i L j

) (3)

where lSC i L j

is the contribution to the sustained interruptionfrequency of L j because of permanent and temporary fault onC i , l

MC i L j

is the contribution to the momentary interruptionfrequency of L j because of permanent and temporary faults

on C i , l

ME

C i L j is the contribution to the momentary interruption event frequency of L j because of permanent and temporary faults on C i , l

PC i

is the rate of permanent

fault on C i , l TC i

is the rate of temporary fault on C i , C i isthe component number i and L j is the load-point number j .

In these equations, B PC i L j and P (B PC i L j

) are, respectively, theevent and its associated probability, so that load point L j

because of a permanent fault on C i is categorised as ClassB. Similar notations are used for referring to other classesof load points. Superscripts P and T are, respectively, usedfor representing permanent and temporary faults on C i .Other parameters have been defined in Table 1.

4. The contribution to the sustained annual outage time of L j

of a permanent and a temporary fault on C i is determinedbased on the concepts of conditional probability theory andusing the results obtained in Step 2

U S

C i L j ¼ ((r C i

Â lPC i

)jB PC i L j

) Â P (B PC i L j

)

þ ((MST Â lPC i

)jC PC i L j )Â P (C PC i L j

)

þ ((FRT Â l TC i

)j J TC i L j )Â P ( J TC i L j

)

þ ((MST Â l TC

i

)jK TC i L

j

)Â P (K TC i L

j

) (4)

where U S

C i L j is the contribution to the sustained annual

Figure 2 Deduced models for operational failures

a Switching devices of LRS

b Fuse of lateral distributors



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outage time of L j because of permanent and temporary fault on C i and r C i

is the repair time of C i .

5. The contribution to the expected interruption cost of L j by a permanent and a temporary fault on C i is determined basedon the concepts of conditional probability theory and using

the results obtained in Step 2

ECOSTC i L j ¼ La L j

ÂlPC i Â {(CICL j

(r C i )jB PC i L j

)ÂP (B PC i L j )

þ (CICL j (MST)jC PC i L j

)ÂP (C PC i L j )

þ (CICL j (LRT)jD

PC i L j

)ÂP (D PC i L j

)

þ (CICL j (DFCT)j E PC i L j

)ÂP ( E PC i L j )

þ (CICL j (LRT)jF

PC i L j

)ÂP (F PC i L j

:)}þLa L j

Â l TC i Â {(CICL j (LRT)jH TC i L j )ÂP (H TC i L j )

þ (CICL j (FFCT)j I TC i L j

)ÂP ( I TC i L j )

þ (CICL j (FRT)j J

TC i L j

)ÂP ( J TC i L j

)

þ (CICL j (MST)jK

TC i L j

)ÂP (K TC i L j

)} (5)

where ECOSTC i L j is the contribution to expected interruption

cost of L j because of permanent and temporary fault on C i ,La L j

is the average load connected to L j and CICL j (t ) is

the per unit interruption cost of L j because of aninterruption with duration of t in (EUR /kW).

6. The load-point and system-oriented reliability indices aredetermined by analysing the contributions associated withdifferent failure events. For L j , the reliability indices arededuced by aggregating the calculated l

SC i L j

, lMC i L j

, lMEC i L j

,U

SC i L j

and ECOSTC i L j . The load-point reliability indices

are calculated as follows

lSL j ¼XNC

i ¼1

lSC i L j

(6)

lML j ¼XNC

i ¼1

lMC i L j

(7)

lMEL j

¼XNC

i ¼1

lMEC i L j

(8)

U SL j ¼XNC

i ¼1

U SC i L j (9)

r SL j ¼

U SL j

lSL j

(10)

EENSSL j ¼ U SL j

Â La L j (11)

ECOSTL j ¼XNC

i ¼1

ECOSTC i L j (12)

IEAR L j ¼

ECOSTL j

EENSL j

(13)

where lSL j

is the average sustained interruption frequency of

L j , lML j

is the average momentary interruption frequency

of L j , lMEL j

is the average momentary interruption event

frequency of L j , U S

L j is the average sustained annual outage

time of L j , r SL j is the average sustained outage time of

L j , EENSSL j

is the average expected energy not supplied

of L j , ECOSTL j is the average expected interruption cost

of L j , IEAR L j is the average interrupted energy assessment

rate of L j and NC is the number of components.

The following system-oriented reliability indices can be

calculated using the load-point indices

SAIFI ¼

PNLP j ¼1 (lS

L j Â N L j

)PNLP

j ¼1 N L j

(14)

MAIFI ¼

PNLP j ¼1 (lM

L j Â N L j

)PNLP

j ¼1 N L j

(15)

MAIFIE ¼

PNLP j ¼1 (lME

L j Â N L j

)PNLP

j ¼1 N L j

(16)

SAIDI ¼

PNLP j ¼1 (U

SL j Â N L j

)PNLP

j ¼1 N L j

(17)

ASAI ¼

PNLP j ¼1 (8760 Â N L j

)ÀPNLP

j ¼1 (U S

L j Â N L j

)PNLP

j ¼1 ( N L j Â 8760)

(18)

EENS ¼XNLP

j ¼1

(U S

L j Â La L j

) ¼XNLP

j ¼1

EENSSL j

(19)

ECOST ¼XNLP

j ¼1

ECOSTL j (20)

IEAR ¼ECOST

EENS(21)

where N L j is the number of customers of L j and NLP is the

number of load points.

4 Study results

The application of the proposed technique to a multi-load-point distribution system is illustrated using the distributionsystem shown in Fig. 3. The test system is the distributionsystem connected to bus 2 of the Roy Billinton test system[16]. The required reliability data are given in [16]. The data



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related to the interruption costs for various types of customersare shown in Table 2. These data are based on the results of the Finnish reliability worth study [17]. A computer programhas been developed to perform the necessary computations.In the study results presented in this paper, any failures in the

equipment of 11 kV feeders are considered, but failures in the33 kV system and the 33/11 kV substation are ignored.

Temporary short-circuit failure rates of equipments areassumed to be two times of their permanent short-circuit failure rates. It is also assumed that a spare transformer isavailable for the low-voltage transformer in order to reduce

the effect of transformer failure. It is assumed that substationcircuit breakers have reclosing capability. Also, whenemploying the LRS for the test system, the intermediatedsection switches on each feeder and loop switches are replacedby normally closed and normally open tie reclosers,respectively. Any two adjacent feeders that are interconnectedto each other through a normally open tie recloser areconsidered as a LRS with its own ACS.

4.1 Comparative studies

In order to quantitatively assess the impacts of various ACSs

of LRS on the distribution system reliability, threecomparative case studies are conducted. An overall brief description of these case studies is as follows:

† Case 1 – The LRS is not implemented.

† Case 2 – The LRS is implemented and equipped with ACS without communication link.

† Case 3 – The LRS is implemented and equipped with ACS with communication link.

The basic data tabulated in Table 3 are used in the abovethree case studies. In order to compare the results associated with these case studies, it is assumed that both protectionand automatic control functions of switching devices of LRSand also fuse of lateral distributors are fully reliable. Their impacts will be considered in the next subsection.

Fig. 4 shows some of the load-point reliability indices of the test system for the comparative studies. Figs. 4a and b

show that the sustained interruption frequency and thesustained annual outage time of all load points decrease

when LRS is implemented in the test system. The

Figure 3 Distribution reliability test system [16]

Table 2 Interruption costs in EUR/kW for various types of

customers [17]

Interruption duration Customer type

Residential Public Commercial Industry

1 s 0.23 1.9 1.8 1.9

2 min 0.84 2.6 3.0 2.5

1 h 5.8 13.6 27.6 17.0

4 h – 52.1 67.8 –

8 h – 70.6 117.2 104.412 h 43.8 91.3 163.0 132.7



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improvement in these indices is the same for both Cases 2and 3. However, in the case of reliability indices associated with momentary interruptions, two groups of load pointsare distinguishable from Figs. 4c and d . The first group(FG) consists of those load points located betweensubstation circuit breakers and midpoint reclosers, that is,load points 1–4, 8, 10–12 and 16–19. The second group(SG) consists of those load points located betweenmidpoint reclosers and normally open tie reclosers, that is,load points 5–7, 9, 13–15 and 20–22. Figs. 4c and d show that the momentary interruption frequency and themomentary interruption event frequency of the FG of loadpoints decrease when LRS is implemented in the test

system (Cases 2 and 3) compared to the similar results inthe absence of LRS (Case 1). This improvement is thesame for both Cases 2 and 3. However, it can be seen fromFig. 4c that the momentary interruption frequency of theSG of load points decreases and increases for Cases 3 and2, respectively. Fig. 4d also shows that the momentary interruption event frequency of the SG of load pointsincreases by implementing LRS.

Actually, installation of the midpoint recloser has causedthe FG of load points to be almost unaffected frompermanent and temporary faults on the downstream

sections. However, for the SG of load points, when a permanent fault occurs upstream of the midpoint recloser,the downstream load points are restored after some reclosing shots of upstream recloser (2, 3 or 4 shots depending on thetype and location of fault) in Case 2 and one reclosing shot for Case 3. These operational procedures will improve thereliability indices associated with sustained interruptions for the SG of load points compared to the similar results in theabsence of LRS. But, the SG of load points, in addition tomomentary interruptions arising from the temporary faultson upstream and downstream sections, will experience someadditional momentary interruptions which are due to the

permanent faults on upstream sections. Therefore as shownin Fig. 4d , the SG of load points will suffer moremomentary interruption events in Cases 2 and 3 compared

to the similar results in the absence of LRS (Case 1).However, for faults on the upstream sections, as mentionedabove, less reclosing operation is required to complete therestoration sequence compared to that of Case 2. Themomentary interruption frequency index of the SG of loadpoints is less for Case 3 compared to the same index in

Cases 1 and 2. This can be seen in Fig. 4c .

Table 4 shows the reliability indices of total (T), FG andSG of load points of the test system for the three casestudies. It can be seen from the results that the indices arealmost improved when LRS is implemented (Cases 2 and3) in the test system compared to those obtained in theabsence of LRS (Case 1). Improvements in reliability indices associated with sustained interruption are the samefor both Cases 2 and 3. However, indices associated withmomentary interruptions are different for the FG and theSG of load points. In addition, when employing LRS, the

ECOST index for Case 3 is reduced more than that of Case 2. The ECOST index integrates the costs associated

with both momentary and sustained interruptions. For thisreason, the reliability performance of different ACSs of LRS is more understandable from this index.

4.2 Sensitivity analyses

The results presented in the previous section were based onthis assumption that both protection and automatic controlfunctions of switching devices of LRS and also fuse of lateral distributors are fully reliable. The impacts on system

reliability indices of operational failure in protection andautomatic control functions of switching devices and fuseof lateral distributors are investigated in this section. Allthe sensitivity analyses are conducted for the three casestudies described in the previous subsection. In the analysespresented here, the basic data given in Table 3 remainunchanged.

Table 5 shows the effects of operational failure of the fuseof lateral distributors on the system reliability indices of thetest system. The fuses are assumed to successfully operate

with a probability of 0.9 if required. Also, Table 6 showsthe situation where there are no fuses in the lateral

distributors of the test system. It can be seen from theresults presented in Tables 4– 6 that operational failure of fuses or operating of lateral distributors without fuse hasnegative impact on reliability indices associated with thesustained interruption. However, these indices are improved

when LRS is implemented in the test system compared tothose obtained in the absence of LRS. Although reliability indices associated with sustained interruption improvedconsiderably for both Cases 2 and 3, the reliability indicesassociated with momentary interruption and also theECOST index in Tables 4 –6 show that Case 3 couldprovide better performance than Case 2.

Tables 7 and 8, respectively, show the effects of operationalfailures of protection function and automatic control function

Table 3 Basic data used in comparative studies and

sensitivity analyses

MST, h 1

FRT, h 0.5

LRT (Case 2), s 120

LRT (Case 3), s 10

NRO 4

NROFB 3

NROTFC 2

FFCT, s 5

DFCT, s 15



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Figure 4 Load-point reliability indices for comparative studies

a Sustained interruption frequency

b Sustained annual outage timec Momentary interruption frequencyd Momentary interruption event frequency



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Table 4 System reliability indices for comparative studies

Case study SAIFI, int/cust.yr SAIDI, h/cust.yr MAIFI, int/cust.yr

FG SG T FG SG T FG SG T

Case 1 0.28 0.28 0.28 0.82 0.80 0.82 3.53 3.50 3.53

Case 2 0.18 0.18 0.18 0.72 0.70 0.72 1.80 3.91 1.85

Case 3 0.18 0.18 0.18 0.72 0.70 0.72 1.80 2.50 1.82

Case study MAIFIE, int.eve/cust.yr EENS, kwh/yr ECOST, EUR/yr


Case 1 1.59 1.57 1.59 5053 4420 9473 34 845 68 840 1 03 686

Case 2 0.82 1.68 0.84 4484 3882 8366 26 810 60 180 86 990

Case 3 0.82 1.68 0.84 4484 3882 8366 26 810 59 832 86 641

Table 5 System reliability indices when successful operational probability of fuses is equal to 0.9



Case 1 0.32 0.31 0.32 0.85 0.84 0.85 3.42 3.39 3.42

Case 2 0.20 0.19 0.20 0.74 0.71 0.74 1.75 3.89 1.80

Case 3 0.20 0.19 0.20 0.74 0.71 0.74 1.75 2.46 1.77



Case 1 1.55 1.54 1.55 5256 4588 9844 36 482 71 734 1 08 216

Case 2 0.80 1.66 0.82 4575 3941 8516 27 533 61 295 88 828

Case 3 0.80 1.66 0.82 4575 3941 8516 27 533 60 862 88 394

Table 6 System reliability indices when there are no fuses in the lateral distributors of the test system



Case 1 0.63 0.63 0.63 1.17 1.15 1.17 2.47 2.45 2.47

Case 2 0.35 0.31 0.34 0.88 0.83 0.88 1.32 3.74 1.38

Case 3 0.35 0.31 0.34 0.88 0.83 0.88 1.32 2.12 1.34



Case 1 1.23 1.22 1.23 7077 6106 13 183 51 216 97 773 1 48 988

Case 2 0.66 1.55 0.68 5389 4480 9869 34 038 71 329 1 05 368Case 3 0.66 1.55 0.68 5389 4480 9869 34 038 70 133 1 04 171



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of switching devices of LRS on the system reliability indices. The successful operational probability of these functions isassumed to be 90%. By comparing the results of Tables 7and 8 with those shown of Table 4, it can be found that

the operational failure of protection and/or automaticcontrol function of switching devices has negative impact on system reliability indices. Although the indices areimproved when LRS is implemented in the test systemcompared to those obtained in the absence of LRS, it canbe found that Case 2 could provide better performancethan Case 3. Close examination of the results presented inthese tables indicate that the ability of Case 3 in improving the reliability indices of the SG of load points decreases

when protection and/or automatic control functions of switching devices of LRS have a chance to fail. This is dueto the strict dependability of operating procedure of ACS

with communication link, which needs all LRS switching devices work properly at the same time. In contrast,operating procedure of ACS without communication link is

not fully dependent on successful operation of all switching devices and therefore can have chances to restore thedownstream load points for faults on upstream section insituation like operational failure of some protection and/or

automatic control functions of switching devices.

5 Conclusion

This paper proposes an approach for assessing the impacts of two common types of ACS of LRS on the distributionsystem reliability indices. In order to demonstrate theproposed technique, comparative studies and sensitivity analyses were conducted using a typically distributionreliability test system.

Comparative studies were directed to illustrate how two

common types of ACS could affect load-point and systemreliability indices. Successful operation of these ACSs isdependent on the protection and automatic control

Table 7 System reliability indices when successful operational probability of protection function of switching devices is equal

to 0.9



Case 1 0.34 0.34 0.34 0.92 0.87 0.92 3.31 3.29 3.31

Case 2 0.24 0.24 0.24 0.78 0.75 0.78 1.74 3.63 1.78

Case 3 0.24 0.25 0.24 0.78 0.77 0.78 1.74 2.54 1.75

Case study MAIFIE, int.eve/cust.yr EENS, kwhr/yr ECOST, EUR/yr


Case 1 1.57 1.56 1.57 5702 4808 10 510 40 293 76 110 1 16 402

Case 2 0.84 1.61 0.85 4859 4201 9059 30 477 65 688 96 165

Case 3 0.84 1.59 0.85 4850 4244 9094 30 256 65 922 96 178

Table 8 System reliability indices when successful operational probability of automatic control function of switching devices is

equal to 0.9



Case 1 0.44 0.44 0.44 1.02 0.97 1.02 3.18 3.15 3.18

Case 2 0.26 0.23 0.26 0.81 0.75 0.81 1.63 3.58 1.67

Case 3 0.27 0.36 0.27 0.81 0.89 0.81 1.62 2.42 1.64

Case study MAIFIE, int.eve/cust.yr EENS, kwhr/yr ECOST, EUR/yr


Case 1 1.43 1.42 1.43 6253 5256 11 509 43 805 82 859 1 26 664

Case 2 0.74 1.63 0.76 4979 4126 9105 30 771 64 559 95 330

Case 3 0.74 1.49 0.76 5014 4814 9828 31 051 75 720 1 06 770



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functions of switching devices of LRS. Therefore sensitivity analyses were performed to illustrate the impacts onreliability indices of operational failure of the protectionand automatic control functions of switching devices of LRS and also fuse of lateral distributors.

The results presented indicate the benefits of employing the LRS for automating distribution feeders. They alsoshow the different effects of the two studied ACSs of LRSon the reliability indices associated with sustained andmomentary interruptions. However, when comparing thesetwo ACSs for selection, one should not just consider thereduction in ECOST and MAIFI as decision making criteria. A suitable business case should be developed for such analysis, which considers the combination of hard andsoft benefits – those that can be economically quantifiedand those that are intangible but influence the perceptionof a utility’s performance. In this business case, in addition

to performance of these ACSs from distribution systemreliability point of view, other parameters such as different

weightings that regulatory agencies puts on the reliability indices, purchase, installation, operation and maintenancecosts of LRS equipment, the number of reclosing operations of ACS procedure for fault detecting, isolating and service restoration (which severely impacts the wholefeeder components and load points), automatic restorationcapability to the normal system configuration (whichdecreases the operational cost), and also avoiding the faultsoccurs downstream of mid-recloser to be momentarily placed on the other feeder (which improve the distributionsystem security and power quality) should also be takeninto account.

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