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Various Protection Schemes for Closed Loop Feederswith Synchronous Machine Based DistributedGeneration and Their Impacts on Reliability

Ferry A. Viawan, Student Member, IEEE, Daniel Karlsson, Senior Member, IEEE.

Abstract This paper presents various protection schemes for

distribution feeders in a closed loop operation. Firstly, radial and

closed loop operation of conventional distribution feeders, what

kind of protection schemes can be implemented for the closed

loop operation and how those schemes will affect the reliability

are briefly investigated. The protection schemes for closed loop

conventional distribution feeders are then adopted for

distribution feeders with distributed generation (DG) by taking

into account the stability of the DG. Cost benefit of each scheme

to the reliability of the system is investigated. It is shown that

upgrading distribution feeders from radial to closed loop

operation without properly upgrading the protection system will

decrease the reliability of the system. On the other hand, when

the protection scheme is properly upgraded, the closed loop

operation will increase the reliability of the system, where the

level of upgrade needed will depend on the expected reliability

gain.

Index TermsDistribution feeders, distributed generation,

protection schemes, voltage dips, stability, reliability.

I. INTRODUCTIONEDIUM voltage (MV) feeders are mostly operated in

radial, though they are normally provided with alternate

route of power supply to ensure backup connections to

minimize the impact of permanent faults and to prevent load

unserved during a scheduled outage. Therefore, in

conventional MV feeders, protection systems are designed for

radial operation and for power flows in one direction only,

which allows the use of protection system without directional

discrimination [1]-[2].

Radial operation has been adopted in the operation of

conventional MV feeders for several decades and offers

simplicity for the feeder operation, especially in the protectionof the feeders. However, in line with increasing interest in the

improvement of the quality of electricity distribution, closed

loop feeder operation is becoming a subject of increasing

interest [3]. In comparison with radial feeders, closed loop

feeders are known to have advantages of decreasing power

Ferry A. Viawan and Daniel Karlsson are with the Division of ElectricPower Engineering, Department of Electric Power Engineering, ChalmersUniversity of Technology, Gothenburg, Sweden. Daniel Karlsson is also withGothia Power, Gothenburg, Sweden.

Corresponding e-mail for the paper: ferry.viawan@ chalmers.se

losses, improving feeder voltage profile and increasing

flexibility to cope with the load growth [3]-[4]. On distribution

feeders with distributed generation, closed loop operation also

increases the maximum allowed DG capacity in the feeders

[4]. Further, closed loop operation will also decrease the risk

of unintentional islanding operation of the DG. When the

upgrading of the feeders from radial to normally closed loops

is followed by the proper upgrade of the feeder protection

scheme, closed loop operation will significantly increase the

reliability of the feeders [5]. On the other hand, besidesincreasing the complexity of the protection scheme of the

feeders, closed loop operation gives disadvantages of

increasing the short circuit currents and increasing the voltage

dips frequency and the severity of the dips [3]-[5].

This paper firstly gives brief investigations on radial and

closed loop operation of conventional distribution feeders,

what kind of protection schemes can be implemented for the

closed loop operation and how those schemes will affect the

reliability. The protection schemes for closed loop

conventional distribution feeders are then adopted for

distribution feeders with DG by taking into account the

stability of the DG. Cost benefit of each scheme to the

reliability of the feeders is presented.

II. PROTECTION AND RELIABILITY OF RADIAL AND CLOSEDLOOP CONVENTIONAL DISTRIBUTION FEEDERS

For a brief overview of protection and reliability of radial

and closed loop distribution feeders, see the one-line diagram

in Fig. 1. The two feeders are connected through a normally

open switch (tie switch X). The tie switch is used for example

as an alternate supply to serve load under recloser R11, when

there is maintenance on feeder-1 upstream of recloserR11.

2

Feeder-2

1

Feeder-1

R11

A C

1

F

X

B

R12

R21

R22D E

2

Feeder-2

1

Feeder-1

R11

A C

1

FF

X

BB

R12

R21

R22DD EE

Fig. 1. One-line diagram of a simple distribution system for radial and closed

loop feeder operation studies.


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Operating the feeder in a closed loop without upgrading the

protection system will deteriorate the reliability of the system.

For example, a fault on location C will interrupt loads under

both reclosersR12andR22if the tie switch X is closed.

The reliability of that radial system can be improved when

the normally open tie switch X is coordinated with recloser R12

andR22, where the switch will automatically close when either

recloserR12or R22senses no power at a certain period. By this

scheme, when there is a permanent fault at location B forinstance, after a certain period, the recloser R12will open and

then switch X will close, to prevent loads under recloser R12(between R12 and X) from experiencing a long interruption.

However, those loads still experience a short interruption due

to the fault.

When it is demanded that loads under recloser R12 should

not experience either long or short interruptions due to the

fault at location B, the feeders can be operated in a normally

closed loop with appropriate upgrading of the protection

scheme, with the expense that the severity of the voltage dips

will increase. Some distribution utilities in the world have

adopted normally closed loop operation to serve their

customers, such as Taiwan Power Company, Florida Power

Company, Hong Kong Electric Company and Singapore

Power [5].

Depending on the expected reliability gained, various

protection schemes can be implemented for distribution

systems operated in closed loops, such as:

1) The least expensive upgrade is by replacing the tie switchX with an overcurrent relay plus breaker (tie breaker). The

overcurrent relay operating the breaker is set to operate

instantaneously to break up the loop for any fault occurring in

feeder-1 and feeder-2. Other protective devices will then

operate based on normal protection coordination for radial

feeder operation. This method will not increase the reliabilityof the feeders, because the tie breaker will always open

following a fault, and thereby, a fault at location A, for

instance, will always cause interruption to all loads under

feeder-1. Closed loop operation with this protection scheme

will here be called closed loop scheme-1.

2) When reliability improvement is expected, all recloserscan be replaced with directional relays (and breakers).

Reclosers can not be used as the reclosers do not have

directional features [6]. Both instantaneous and time delayed

operations are used. With this scheme, the number of loads

interrupted due to a fault can be limited only for loads between

two breakers. The fault will be cleared immediately when thefault is within the reach of the instantaneous operation. The

problem may occur when, for example, the fault occurs at A

and beyond the reach of instantaneous operation of relay-1.

With the time coordination between two subsequent relays at

least 0.25 second, to maintain the proper coordination [2], the

fault clearing time by time delayed overcurrent relay-1 for the

fault at location A can be 2 seconds or more (as there are six

relays in series that have to have shorter time delays than

relay-1). A long lasting fault close to the substation may

endanger the distribution system. This scheme will here be

called closed loop scheme-2.

3) In order to keep the fault cleared with a short time delay,the directional overcurrent protection in closed loop scheme-2

can be replaced by distance protection. If the relays used in

scheme-2 are digital multifunction relays where the distance

protection is already available in the relay, scheme-2 can be

upgraded to this scheme without additional investment cost.

However, when the relay in scheme-2 is an electromechanical

or a single function relay, upgrading from directional

overcurrent to distance needs investment costs and installation

new distance relays. This scheme will here be called closed

loop scheme-3.

4) The problem with distance relays in scheme-3 may occurfor faults close to the next breaker, where the relay has to

operate with time delay, and on other hand, there are many

dip-sensitive-loads connected to the feeder. Though the time

delay of the distance relay is in order of a couple of hundreds

millisecond, the closed loop operation however increases the

severity of the dip, which may cause the sensitive equipment

to trip, when the relay operates with a time delay. In this case,

when the distribution system is expected to have a very high

reliability, i.e., the sensitive equipment tripping due to voltage

dips should be avoided, the distance protection in scheme-3

can be operated with a permissive overreaching transfer trip

(POTT) pilot scheme. With this scheme, the relays are set to

overreach in such a way any fault within the protected area

will always be cleared without time delayed relay operation.

This offers a high level of distribution system reliability, with

the expense that it needs a huge investment where

communication facilities to all relays are needed. This scheme

will here be called as closed loop scheme-4.

III. PROTECTION OF CLOSED LOOP DISTRIBUTION FEEDERSWITH THE PRESENCE OF DISTRIBUTED GENERATION

The presence of DG in radial feeders leads to unnecessary

DG disconnection for faults occurring upstream the DG

connection point. For example, assume that a DG is connected

between recloser R12and tie switch X in Fig. 1. This DG will

always be disconnected for every fault occurring in feeder-1.

This may not be a problem when the DG is small. But when

the DG is considerably large, this unnecessary disconnection

should not be acceptable. This can be mitigated by, for

example, operating feeder-1 in a closed loop with feeder-2,

and when the fault occurs between the substation and R11, bothrelay-1 and R11 will open and the DG will deliver the power

through feeder-2. Similarly, when the fault occurs betweenR11

andR12, both R11and R12 will open and the DG power can be

delivered through feeder-2. Hence, operating the feeders in a

closed loop, when DG is present in the feeders, gives

additional reliability advantage to the ones explained in

Section II.

In addition, the presence of synchronous machine based DG

will give voltage support to the feeder during a fault that will

decrease the severity of the voltage dips [7]. Hence, the


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presence of synchronous machine based DG can be expected

to decrease the voltage dip magnitude or even to mitigate the

negative impact of closed loop operation on causing loads to

trip due to voltage dips.

Operating feeders containing DG in a closed loop will

decrease the electrical distance between the DG (on one

feeder) and the fault (on the pair feeder). This decrease may

lead to the decrease of the critical fault clearing time (the

maximum fault clearing time that will not cause DG to lose itsstability). For example, assume that a DG is installed at point F

in Fig. 1. The three-phase fault at point C may cause the DG to

lose its stability in a few hundreds milliseconds when the tie

switch X is closed, meanwhile when the tie switch is opened,

the DG may still be stable for the same fault lasting for a few

seconds. Hence, the stability of DG has to be taken into

account in the reliability evaluation of operating feeders with

DG in a closed loop. The closed loop operation of feeders

containing DG may demand an increase of the protection

speed in order not to cause instability to the DG.

IV. CASE STUDYThe protection schemes in closed loop feeders with and

without the presence of synchronous machine based DG was

tested in an 11 kV distribution system, fed from a 132 kV grid,

as shown in Fig. 2. The detailed specification of the system is

presented in Appendix I. Loads shown in the figure are lumped

loads, which is a simplification of feeder branches with loads

connected on them.

X

2

1

2 3 4 5

6 7 8 9 10

11

DG-1

3x3.5 MVA

DG-2

2x3.5 MVA

DG-31x3.5 MVA

Feeder-1

Feeder-2

R11

R21

R22

X

2

1

2 3 4 5

6 7 8 9 10

11

DG-1

3x3.5 MVA

DG-2

2x3.5 MVA

DG-31x3.5 MVA

Feeder-1

Feeder-2

X

2

1

2 3 4 5

6 7 8 9 10

11

DG-1

3x3.5 MVA

DG-2

2x3.5 MVA

DG-31x3.5 MVA

X

2

1

X

2

11

2 3 4 5

6 7 8 9 10

11

DG-1

3x3.5 MVA

DG-2

2x3.5 MVA

DG-31x3.5 MVA

Feeder-1

Feeder-2

R11

R21

R22

Fig. 2. One-line diagram of the system under study.

Assume that the branches have their own protection in such

a way that only the protection of the main feeders, as shown in

the figure, is of interest. Feeders are protected with overcurrent

relays (and their corresponding circuit breakers) in the

substation and reclosers (recloser is basically an overcurrentrelay and a breaker in one unit) at some buses along the

feeders.

Both conditions, without and with DG connected, are

investigated. DG is assumed to be synchronous machine based

DG. In the radial operation, the two feeders are connected

through a normally open switch X at bus-11. The switch X is

replaced with breaker X in the closed loop operation. The

reclosers may be replaced with breakers (plus relays) in the

closed loop operation. Hence, the term of breakers and

reclosers are considered to refer to the same thing in the closed

loop operation.

A.The Impacts of Closed Loop Operation on Voltage DipsClosed loop operation decreases the voltage dip magnitude

(which in this paper refers to the remaining voltage during the

dips). For an example, voltage dips at bus-2 and bus-5 in

feeder-1 due to a three-phase fault at bus-6 and bus-10 in

feeder-2 are shown in Fig. 3 - Fig. 4. The figures indicate that: The closed loop operation increases the severity of the dips

(decreases the dips magnitude).

Severity of the dips (sensed by the load) due to closed loopoperation increases with the closer the location of the load

to the closed loop point.

The presence of DGs increases dip magnitudes. Dependingon the location of the DG, the fault and the load; the

increase can either minimize or mitigate the impact of the

closed loop operation on increasing the severity of dips.

Fig. 3. Voltage dips at bus-2 and bus-5 in feeder-1 due to a three-phase fault

at bus-6 in feeder-2.

Fig. 4. Voltage dips at bus-2 and bus-5 in feeder-1 due to a three-phase fault

at bus-10 in feeder-2.

In order to ensure that the upgrading of the feeders from

radial to closed loop operation will not decrease the reliability

of the feeder (many loads trip due to the dips), the voltage dips

should be investigated. This dip magnitude should then be

compared with the voltage dip immunity of the loads, in order


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to find the relay maximum operation time before a dips causes

loads to trip. However, this is beyond the scope of this paper.

Nevertheless, it can be concluded that closed loop scheme-1

will most probably decrease the reliability of the feeders, as it

does not decrease the number of loads/DGs interrupted during

the fault, and it increases the severity of the dips. Further, the

maximum dip duration that load can withstand is normally

inversely proportional to the square of the voltage deficit

(below the minimum specified voltage) [8]. Therefore,protection scheme-4 will give the highest prevention on load

trip due to dips, followed by protection scheme-3.

B.The Impact of Closed Loop Operation on LoadInterruption

When the effect of voltage dips on the trip of sensitive loads

is neglected or when there is no dip sensitive load connected to

the feeder, the impact of closed loop operation on increasing

the feeder reliability (decreasing the load interruption) is

obvious. The load interruption due to faults on various buses is

then as shown in Table I. As load interruption is affected by

the part of the distribution system that is isolated during a

fault, closed loop scheme-1 and radial operation yield the same

load interruption. Further, load interruption is not affected by

the speed of protection system, hence closed loop operation

with protection scheme-2, 3 and 4 yield the same load

interruptions.

TABLE ILOAD INTERRUPTION DUE TO FAULTS OF THE SYSTEM UNDER STUDY

WITH RADIAL AND CLOSED LOOP OPERATIONS

Number of loads interruptedFaulted Bus Radial and Closed Loop

Scheme-1Closed Loop

Scheme-2, 3 and 4

2 4 23 4 24 2 25 2 26 6 27 6 28 4 29 4 2

10 2 211 2 2

Total 36 20

C.The Impacts of Closed Loop Operation on DG ReliabilityAs previously explained, closed loop operation will affect

the stability of the DG with respect to the fault in the feeders.

For example, Fig. 5 shows the rotor angle of DG-3 due to 0.3

second three-phase fault at bus-5. The figure indicates that theclosed loop operation decreases the critical fault clearing time,

i.e., for a three-phase fault at bus-5 lasting for 0.3 second, DG-

3 is stable when the feeders are operated in radial and not

stable when the feeders are operated in a closed loop.

The fault clearing time 0.3 second for closed loop operation

in Fig. 5 means that both R11 and X are opened after 0.3

second. In practice, those two breakers may not open

simultaneously, which will further give different effect on the

stability. For example, the rotor angle of DG-3 when the three-

phase fault at bus-5 is cleared by firstly breaking up the loop

(by opening breaker X) and then followed by opening R11 at

0.3 second is shown in Fig. 6. The figure indicates that DG-3

is stable with this protection scheme, although it can be seen

that this closed loop operation decreases the stability margin of

the DG (compare Fig. 6 with the left side of Fig. 5). It can also

be concluded that the stability margin decreases with further

delay of the opening of breakerX.

Fig. 5. DG-3 rotor angle for a 0.3 second three-phase fault at bus 5.

Left: radial operation, right: closed loop operation.

Fig. 6. DG-3 rotor angle for a 0.3 second three-phase fault at bus 5, where

the feeders are operated in a closed loop and breakerXis opened

at 0.1 second (left plot) and 0.25 second (right plot).

The DGs that will be disconnected following the fault

isolation and the critical clearing time for three-phase faults at

different buses are presented in Table II. For the closed loop

operation, the table is obtained by opening the first breaker at 5cycles (0.1 second), which is obtained by assuming 1 cycle for

the relay instantaneous operation, 3 cycles for breaker

operation and 1 cycle for other delays. The maximum critical

clearing time of interest is limited to 3 second, assuming that

time delay of the breaker opening will, however, never reach 3

second.

Similar with the ones illustrated in Fig. 5 and Fig. 6, the

table shows that the critical clearing time becomes shorter with

the upgrade from the radial to the closed loop operation


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REFERENCES

[1] P.M. Anderson, Power System Protection, IEEE Press, 1999.[2] J.L. Blackburn, Protective Relaying Principles and Applications, Marcel

Dekker, 1988.[3] A. Nikander, S. Repo and P. Jrventausta, Utilizing the Ring Operation

Mode of Medium Voltage Distribution Feeders, in Proc of 17 thInternational Conference on Electricity Distribution, 2003.

[4] G. Celli, F. Pilo and G. Pisano, Meshed Distribution Networks toIncrease the Maximum Allowable Distributed Generation Capacity, in

Proc. of 2005 CIGRE Symposium.[5] 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 Transactions onPower Systems, Vol. 19, No. 3, 2004.

[6] S.M. Brahma and A.A. Girgis, Development of Adaptive ProtectiveScheme for Distribution Systems With High Penetration of DistributedGeneration, IEEE Transaction on Power Delivery, vol. 19, no.1,January 2004.

[7] M.H.J. Bollen and M. Hger, Impact of Increasing Penetration ofDistributed Generation on the Number of Voltage Dips Experienced byEnd-Customers, in Proc. of. 2005 International Conference onElectricity Distribution CIRED.

[8] J.C. Gomez and M.M. Morcos, Volatge Sag and Recovery Time inRepetitive Events, IEEE Transactions on Power Delivery, vol. 17, no.4, October 2002.

BIOGRAPHIES

Ferry A. Viawan received his B.Sc. and M.Sc. degrees from BandungInstitute of Technology, Indonesia in 1996, and Chalmers University ofTechnology, Sweden in 2003, respectively. He worked as a Power SystemEngineer at PT Caltex Pacific Indonesia (now Chevron) from 1996 to 2003,where he worked on operation, planning and protection of a transmission anddistribution system. Since 2004, he is a PhD Student at the Division of ElectricPower Engineering, Department of Energy and Environment, ChalmersUniversity of Technology, Gothenburg, Sweden. Since February 2007, he isalso with ABB, Corporate Research, Vsters, Sweden.

Daniel Karlsson received his Ph.D in Electrical Engineering fromChalmers University in Sweden 1992. Between 1985 and April 1999 heworked as an analysis engineer at the Power System Analysis Group withinthe Operation Department of the Sydkraft utility. From 1994 until he leftSydkraft in 1999 he was appointed Power System Expert and promoted ChiefEngineer. His work has been in the protection and power system analysis areaand the research has been on voltage stability and collapse phenomena withemphasis on the influence of loads, on-load tap-changers and generatorreactive power limitations. His work has comprised theoretical investigationsat academic level, as well as extensive field measurements in power systems.Most recently Dr. Karlsson hold a position as Application Senior Specialist atABB Automation Technology Products and now he is with Gothia Power.Through the years he has been active in several Cigr and IEEE workinggroups. Dr. Karlsson is a member of Cigr and a senior member of IEEE. Hehas also supervised a number of diploma-workers and Ph. D students atSwedish universities.

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