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Energy
Procedia
Energy Procedia
Energy00Procedia
(2011) 000–000
12 (2011) 263 – 270
www.elsevier.com/locate/procedia

ICSGCE 2011: 27–30 September 2011, Chengdu, China

Coordination Return of Protective Devices in Distribution

Systems in Presence of Distributed Generation
B. Abdi*, M. Abroshan, M. H. Aslinezhad, A. Alimardani
Damavand Branch, Islamic Azad University, Damavand, Iran

Abstract

With the presence of distributed generation (DG) units in distribution systems, its function is generally going to be
changed. These systems deeply effects on distribution systems protection. A method to decrease DG effects on
distribution systems protection is re-coordination of protective devices. To prevent high cost, the system
configuration should be change to decrease the effect of DG sources when a fault occurs. One of the suggested
methods to do that is the use of fault current limiter (FCL). A sample ring network at DIgSILENT is simulated in this
paper. An overview to protective devices and their protective coordination will be presented. Then an optimal
protective coordination would be created by utilization of FCL in presence of DG.
© 2011 Published by Elsevier Ltd. Open access under CC BY-NC-ND license.
Selection and/or peer-review under responsibility of University of Electronic
Science and Technology of China (UESTC).
Keywords: DG, FCL, Protective coordination, Distribution system

1. Introduction

Distributed generation (DG) presence in power systems is one of attractive phenomena in power
industry. With the presence of DG units in distribution systems, its function would generally be changed
and it would variously be affected by these units. One of the most important effects of these units is on
distribution systems protection. Since the number of DG units not only can be varied but also has a great
deal of wide spread, Distribution systems protective devices manner is completely changed with the
presence of distributed generation.[1-4]
DG units are electrical energy souses which are connected to distribution systems and in comparison
with the large scale power stations, have the lower generation capacity and also have a lower starting cost.
There are some cases in which the use of DG should be paid attention such as: economical problems in
power stations developing, high efficiency of these sources, decreasing of environmental pollution,

* Corresponding author. Tel.: +98 21 77522304.

E-mail address: babakabdi@ieee.org.

1876-6102 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of University of Electronic Science and Technology of
China (UESTC). Open access under CC BY-NC-ND license.
doi:10.1016/j.egypro.2011.10.036
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increasing of power quality, decreasing of loss in distribution systems, improvement of voltage profile
and so one. The most important distributed sources are fuel cells, micro turbines, batteries, wind and
hydropower station, earth heating systems [1-3].
the fault current would increase in system when the distribution generation sources is installed so it is
necessary to set some of the protective system points again after installation DG sources[1-3].
The problems of DG sources on distribution systems protection generally are: feeders misconception
trip, misconception trip of generation units, increasing or decreasing of short circuit surface, unwanted
islanding, prevention of automatic reclosing and unsynchronized reclosing. [4]
One of the ways to decrease DG effects on distribution systems protection which needed high cost is
re-coordination of protective devices and replacement of low power breakers and fuses instead of high
power breakers and fuses.
To prevent this high cost, it should be made a change in system to decrease the effect of DG sources
when a fault occurs. Of course it shouldn’t make a change on system operation in normal conditions. One
of the suggested methods to do that utilizes the fault current limiter (FCL). FCL makes no change on
system operation in normal conditions and limit DG current at fault accordance conditions.
In this paper, sample ring network at DIgSILENT is simulated. By the reviewing protective devices
and their protective coordination and using algorithm an optimal protective coordination would be created
by the use of FCL in presence of DG.

2. Protective Devices Characteristics

The protective devices such as: fuse, re-closer and breakers are used in the distribution systems.
Breakers and re-closers are used in main feeders and fuses are used in lateral ones. In normal condition
breakers and re-closers are equipped with reverse time over current relays. General characteristic of these
relays is:
A
= t ( I ) TD( P + B) (1)
M −1
where A, B, p are constants for particular curve characteristics. And t is operating time of device
M: ratio of I I pickup ( I pickup is the relay current set point)
TD: time dial setting.
The characteristic of fuses is similar to reverse time over current characteristic. General equation of
fuses follows this relationship:
=log ( t ) a log ( I ) + b (2)
where and are the associated operating time and current, and the coefficients and are calculated from
curve fitting.

3. Protective Coordination

The selection of over current protective devices with paying attention to their time-current setting in
the distribution line length to remove of faults in lines and other equipment concerning the order of
previous function is called protective devices coordination.
The regulated device to operate at first is called main protection which is probable to fault and it
operates quickly. The other device operates as the backup protection and it operates when main protective
device doesn’t operate.
In order not to enter in both main and backup relays operation in fault occurrence, there should be a
time interval between main and backup relays operation time. In a protective system, relay operation time
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should make the nearest breaker trip when a fault occurs. In order to prevent of serious damage on system,
it also shouldn’t be so longer that whether inaction of main protection occurs or backup relay trips. The
time interval depends on following factors:
1) Required time to fault current trip by breaker.
2) Time of backup flay over shoot.
3) Error (fault) of equipments as relays and current transformers (CT) and short circuit currents
calculation.
4) Safe margin to be sure backup relay inaction.
To obtain coordination time interval (CTI) of two relays, mention times should be added. Both in past and
at the present time, in many cases relays are coordinated by choosing a constant time. In the past this time
was 0.5 second, but nowadays because of being breakers faster and decreasing of time related to relay
overshoot it is 0.4 second. However it should be less than 0.3 second in best possible conditions. The
characteristics of relays (R1&R2&R3) which are main and backup relays are noticed in Fig.1respectively.
CTI is time interval between main and backup relays.

Fig. 1.main and back up relays coordination time interval Fig. 2. Modified network of IEEE 30 buses system.

4. Objective Function

One of the most important issues in power systems protection is protective relays coordination.
Because of the importance of the issue and quick protection basis, it is a long time that it is brought forth
minimizing of operation time of protective design and several formulations are presented as well.
Objective function which is used in optimum coordination of relays is:
Min F = min ∑ Wi Tik (3)
That
Wi : vector with positive coefficient for relay "i".
Tik : vector of indicator of main (i) and backup(k) over current relays operation times.
One of the above constraints function is coordination constrain:
Tnk − Tik ≥ ΔT (4)
Tnk is first backup relay ( Rn ) operation time for relay Ri for a supposed fault in K protective Zone. ΔT
is coordination time interval and its amount is between 0.2 to 0.5 second. In this paper, selected ΔT
equal to 0.3 second.
Borders constraints on relay setting and relays operation times are:
TDSi min ≤ TDSi ≤ TDSi max (5)
Ipi min ≤ Ipi ≤ Ipi max (6)
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in which TDSi and Ipi in relay Ri are respectively time during setting and pickup current. In this paper,
normal reversed over current relays are used with following characteristic:
0.14 × TDSi
Tik = (7)
[( I ik / Ipi )0.02 − 1]
in which Iik is short circuit current of relay Ri . Above equation is normal reversed characteristic of
"SPCS2D26" relay made in A.B.B Company.

5. Simulation Results

5.1. Introducing of sample system

Nowadays, distribution systems are constantly complicated. As 30 bus IEEE tested network has
appropriate number and complicated bus, it is used in this paper. Although, distribution level is the main
end of this paper only the distribution level (33kV) portions are studied.
Modified network of 30 buses system, has 22 buses and 43 relays that shown in Fig.2. Connection to
132kV substations are designed by infinite bus with 200 MVA short circuit power. It can be found the
data lines and loads and also the data of main and backup relays system in this paper appendix.
All of needed process using MATLAB and DIgSILENT softwares is simulated and they are executed
on illustrated distribution network. To study, it is used DIgSILENT software for sample network
simulation and short circuit and load flow calculations, and it is also used MATLAB software for
algorithm execution and math equations.

5.2. Optimum coordination results

In order to execution of optimum coordination program in MATLAB, it is used genetic algorithm.

Since suiTable tools in MATLAB, exist for genetic algorithm execution, it is not needed to direct
programming of genetic algorithm and it is just needed constraints and optimization function definition.
Obtained time based on genetic optimization is 35/03 seconds. Important parameters of genetic
optimization are shown in Table 1.
Paying attention to system complication, this time is seemed reasonable. Obtained minimum of CTI
amount is presented in Table2. This time is approximately 0.27 second.

Table1: Important parameters of genetic optimization

Parameter Amount
Cross-Over coefficient 0.9
The number of population 20
The number of generation 50

Table 2: Obtained minimum of CTI

Relay Numerical Relays

Relay
Current Operating Time
Unit CTI (sec)
(Amp) (sec)
R1 7093 0.9116 ---------
R24,1 608 1.1808 0.2691
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5.3. DG effects on protective coordination

DG influence can be considered by following ways:

• DG different locations consideration:
DG is placed on different buses to study the effects of DG replacement in different places in sample
network. the recorded results as released coordination amount based on low CTI and mis-coordination are
illustrated in various states of DG in Table 3.
Most of the times, putting DG in a bus often makes coordination of margin decrease that it is shown as
low CTI in Table 3. However sometimes DG effects so much that coordination between pairs of main and
backup relays are completely missed. It is illustrated as mis-cordination in
Table 3.
• DG different capacities consideration:
In order to effective study of DG on relays coordination, a DG unit is placed on the network and DG
capacity amount gradually is increased. The result of DG capacity increasing effects is shown on CTI of
distribution system in Fig.3. As it can be seen through increasing DG capacity, relays CTI margin is less
than its ideal, i.e. 0.3 second.The negative effects of increased DG capacity on decreasing of CTI margin
and also coordination margin in C(34) index which is related to main relay 21and backup relay 19,are
shown in Table 4 which is maximum deviation of CTI according to DG capacity amounts.
Table 3: type of CTI deviation (DG=20MVA)

DG @ Number of DG @ Number ofMis-

Bus low CTI Bus Coordination
10 9 10 0
12 12 12 0
15 13 15 0
16 12 16 0
17 8 17 2
18 16 18 0
19 14 19 0
21 9 21 5
24 8 24 3
27 6 27 0
30 4 30 0

Table 4: maximum deviation of CTI according to DG capacity

DG Size (MW) Max (CTI-0.3)

0 0.0309
5 0.0715
10 0.1124
15 0.1421

Fig. 3.CTI of distribution system relays with DG capacity increasing Fig. 4.CTI of distribution system with different amount of
FCL
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5.4. Coordination return by FCL application

I) FCL:
FCL is a device that it is set in network series form and it limited fault current in considered level and
make a few losses in normal conditions of operations [5-6].
In using FCL some cases as losses in normal conditions, reliability and economical problems should be
considered. Some of the advantages of FCL are fault current limiting and system stability improvement
and voltage say decrease.
It is necessary to study the location of installation of FCL, because it should be selected so that because
of economical problems, low number of FCL is used. After various studying on these fields, researchers
have found out that FCL should be installed near to DG so that the effect of FCL on DG is maximized.
II) Coordination return:
The share of DG in fault current can be reduced by use of FCL. In this paper, we are going to
compensate the effects of DG on CTI by changing the amount of FCL. For the first step, a FCL is located
on the bus 12 which is connected to DG unit. Impedance of FCL is increased 0-80 ohms. The results are
shown at Fig.4. As it can be noticed with increasing of FCL capacity, CTI amount gets closer to the ideal
amount. The considered subject is obviously distinguished by the comparison with maximum deviation of
CTI corresponding with different amount of FCL. It is shown in Table 5 in index C(34).

Table 5 maximum deviation of CTI corresponding with different amount of FCL

FCL Size (Ohm) Max (c)

0 0.1422
20 0.0814
40 0.0591
60 0.0499
80 0.0457

According to the Table, the positive effect of FCL for decreasing CTI and returning of coordination is
distinguished. Considering the effects of DG on maximum deviation of coordination margin, it could
make a decision to used or not to used FCL
application for a DG (in a specific bus). If 0.07 is selected as minimum CTI, it would be distinguished
that there is no need to FCL for DG units having 0-5 MW in bus 12. It is necessary to apply an
appropriate FCL if the DG capacity would be more than mentioned amount in future develops of
distribution system. So considering economical and technical points, the amount of FCL is determined for
maximum DG capacity in bus 12 (15MW).
According to simulation results and paying attention to Table 5, it can set CTI to 0.05 by 60 ohms
FCL, that it is technically suiTable.
Through repetitive optimum, the best obtained result for coordination margin would be 0.02.

6. Conclusion

DG is placed on different buses to study the effects of DG replacement in different places in a sample
network. Most of the times, putting DG in a bus makes coordination of margin decrease (low CTI) and
sometimes DG effects so much that coordination between pairs of main and backup relays are completely
missed (mis-coordination). In order to effective study of DG on relays coordination, a DG unit is placed
on the network and it can be seen through increasing DG capacity, relays CTI margin is less than its ideal,
i.e. 0.3 second. In this paper, we compensated the effects of DG on CTI by changing the amount of FCL.
It can be noticed with increasing of FCL capacity, CTI amount gets closer to the ideal amount.
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7. Appendix

A. Lines information B. Main/backup relays of network

From Bus To Bus R(pu) X (pu) B (pu) V line Primary Secondary Primary Secondary
1 2 0.0192 0.0575 0.0528 132 Relay Relay Relay Relay
1 3 0.0452 0.1652 0.0408 132 1 19 23 2
2 4 0.057 0.1737 0.0368 132 1 22 23 19
3 4 0.0132 0.0379 0.0084 132 1 24 23 22
2 5 0.0472 0.1983 0.0418 132 2 4 24 25
2 6 0.0581 0.1763 0.0374 132 3 1 24 28
4 6 0.0119 0.0414 0.009 132 4 6 25 21
5 7 0.046 0.116 0.0204 132 5 3 26 23
6 7 0.0267 0.082 0.017 132 6 8 26 28
6 8 0.012 0.042 0.009 132 6 12 27 23
6 9 0 0.208 0 132 7 5 27 25
6 10 0 0.556 0 132 7 12 28 30
9 11 0 0.208 0 132 8 10 28 41
9 10 0 0.11 0 132 9 7 29 27
4 12 0 0.256 0 132 10 11 29 41
12 13 0 0.14 0 33
10 14 30 33
12 14 0.1231 0.2559 0 33
10 42 30 31
12 15 0.0662 0.1304 0 33
11 5 31 29
12 16 0.0945 0.1987 0 33
11 8 31 33
14 15 0.221 0.1997 0 33
12 9 32 29
16 17 0.0524 0.1923 0 33
12 14 32 31
15 18 0.1073 0.2185 0 33
18 19 0.0639 0.1292 0 33 12 43 33 37
19 20 0.034 0.068 0 33 13 9 33 35
10 20 0.0936 0.209 0 33 13 11 34 37
10 17 0.0324 0.0845 0 33 13 42 34 32
10 21 0.0348 0.0749 0 33 14 16 35 38
10 22 0.0727 0.1499 0 33 15 13 36 35
21 22 0.0116 0.0236 0 33 16 18 36 32
15 23 0.1 0.202 0 33 17 15 37 39
22 24 0.115 0.179 0 33 18 20 38 36
23 24 0.132 0.27 0 33 19 17 39 34
24 25 0.1885 0.3292 0 33 20 2 40 30
25 26 0.2544 0.38 0 33 20 22 40 27
25 27 0.1093 0.2087 0 33 20 24 41 43
28 27 0 0.396 0 33 21 2 42 40
27 29 0.2198 0.4153 0 33 21 19 43 9
27 30 0.3202 0.6027 0 33 21 24 43 14
29 30 0.2399 0.4533 0 33 22 26 43 11
8 28 0.0636 0.2 0.0428 132
6 28 0.0169 0.0599 0.013 132

C. Load information
Bus Base kV Load (MW) Load(MVAr)
10 33 5.8 2
12 33 11.2 7.5
14 33 6.2 1.6
15 33 8.2 2.5
16 33 3.5 1.8
17 33 9 5.8
18 33 3.2 0.9
19 33 9.5 3.4
20 33 2.2 0.7
21 33 17.5 11.2
22 33 0 0
23 33 3.2 1.6
24 33 8.7 6.7
25 33 0 0
26 33 3.5 2.3
27 33 0 0
29 33 2.4 0.9
30 33 10.6 1.9
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