Journal of Engineering and Economic Development, 2(2), 1-11, July 2015 1
Fast and Practical Protection Coordination Strategy for Radial Distribution Systems with
Distributed Generations
Hadi Zayandehroodi
Department of Electrical Engineering, Kerman Branch, Islamic Azad University, Kerman, Iran.
Email: h.zayandehroodi@yahoo.com
Abstract
This paper presents a new protection method of coordinating the open/close states of the
overcurrent relays in a power distribution system with DG units. A faulty line is initially identified,
and all downstream and upstream paths leading to the DG units are determined using the
backtracking algorithm. The main protection algorithm then determines the open/close states of
relays, thereby isolating the faulty line. A backup protection algorithm is considered in completing
the protection coordination scheme in case the primary protection scheme fails. The accuracy of
the proposed protection coordination method has been tested on a 22-bus test system. Results
illustrate that the proposed protection strategy can accurately coordinate the relays in a power
distribution system with DG units.
Keywords: Coordination, Distributed Generated, Fault Location, Overcurrent Relay, Protection,
Radial Distribution Network,
Introduction
Conventional distribution systems are designed based on the assumption that power flow is
unidirectional, from the generating stations to the transmission network and finally to the
distribution network (Zayandehroodi, et al. 2013). When DG units are connected in a system, the
distribution system is no longer radial, and the assumption of a unidirectional power flow no longer
holds. Consequently, the system has to be analyzed as a mesh network (Ghosh, et al. 2010). Thus,
Journal of Engineering and Economic Development, 2(2), 1-11, July 2015 2
DG units may cause the loss of coordination among network protection devices (Brahma and
Girgis 2004, Zayandehroodi, et al. 2010).
Given that distribution systems typically have radial configurations with only one source,
distribution systems have simple protection schemes, which are usually implemented using fuses,
reclosers, and overcurrent (OC) relays (Quezada, et al. 2006). In OC relay coordination, a fault is
sensed as soon as it occurs by both the primary and backup protection. However, finding a proper
OC relay setting to meet this requirement is difficult using traditional distribution network methods
with DG units (Zayandehroodi, et al. 2011). Previous literature has introduced several new
approaches to solve the OC relay coordination problem. Checking the protection coordination of
a system after connecting each DG unit to a distribution network is suggested in (Hadjsaid, et al.
1999). However, this approach is applicable only in systems with low DG penetration. In (Girgis
and Brahma 2001), it was proven that protective devices in radial distribution systems can
maintain coordination using the available margin in the curves of the protective devices.
A study was performed to solve the fuse coordination problem, in which the suggested solution
was to disconnect all DG units whenever a fault occurs (Brahma and Girgis 2002). The
disadvantage of this solution is that it leads to the disconnection of all DG units even when transient
faults occur. The recloser–fuse coordination with DG units using microprocessor-based reclosers
was also addressed. An adaptive protection scheme based on network clustering into several zones
was presented to solve the disconnection problem of all downstream DGs (Brahma and Girgis
2004). A new protection coordination strategy was developed to maximize generation in a
distribution network and to avoid unacceptably high fault clearing times (Jager, et al. 2004). In
this scheme, the OC principle and the time-dependent characteristics of the current were used. An
adaptive OC pickup scheme that updates the OC relay minimum pickup current based on the fault
Journal of Engineering and Economic Development, 2(2), 1-11, July 2015 3
analysis of the distribution system was presented in (Zayandehroodi, et al. 2015) . However, this
study focuses only on studying inverter-interfaced DG units. Another protection coordination
scheme for radial distribution networks with DG units employs directional relays at the beginning
of the feeders embedding the DG units (Zamani, et al. 2010).
An expert system was previously applied for protective relay coordination in a radial distribution
network with small DG units (Tuitemwong and Premrudeepreechacharn 2011). The expert system
employs a knowledge base and an inference process to improve the coordination settings of relays,
thereby accommodating DG penetration. Furthermore, some studies (Wan, et al. 2008) proposed
a multi-agent-based protection scheme for distribution systems with DG units (Hui, et al. 2010).
The coordination strategy is embedded in every relay agent. In the coordination strategy, relay
settings and time are not the only parameters determining the relay coordination. Efforts are
currently underway to improve the performance of multi-agent systems in coping with protection
coordination within more complex systems.
Backtracking Algorithm
The backtracking algorithm is implemented by multi-stage confirmed step by step search method.
At every stage, there are multi-selection branches necessary to pick out one branch from the
available set of alternative branches. If no solution can be found, the algorithm must backtrack to
the searched node and select another node. If no solution can be found after testing the entire set
of branches at a particular node, the algorithm must backtrack to the quondam node which is called
as the backtracking node which is satisfied by the backtracking condition (Wang, et al. 2002). In
this paper, the backtracking algorithm is applied to determine the location of DG units when a fault
occurs in a distribution network with DG units. Finding the feasible solution for a problem can be
formulated by finding a solution path in an implicitly directed state-space tree from an initial bus
to a goal bus. When the last generated bus has a DG unit, the searching algorithm is completed in
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that path. Figure 1 illustrates the backtracking search algorithm in a sample distribution network
with two DG units.
Figure 1: Backtracking search in a distribution network with several DG units
Proposed Protection Coordination Strategy
The proposed protection coordination strategy can be divided into two sub-strategies. In the first
strategy, the main protection algorithm supervises the entire network to clear the fault at the time
of occurrence. The second strategy, which is called backup protection algorithm, operates as a
complement in standby mode to clear a fault when the main protection is misoperated.
Main Protection Algorithm
After identifying the faulty line number using the radial basis function neural network (RBFNN)
method, as illustrated in (Zayandehroodi, et al. 2011), the main protection algorithm functions by
determining all downstream paths leading to the DG units using the backtracking algorithm. When
a DG is present in the downstream path, a signal is sent to the open relay at the faulty line, N, as
well as all the relays attached to the immediate downstream bus leading to the DG units. Otherwise,
the control signal is only sent to the relay at the faulty line. Figure 2 shows a flowchart of the
implementation procedure of the proposed main protection algorithm.
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Specify a faulty line, N
Find all downstream paths
leading to DG units using the
Backtracking Aalgorithm
Open the relay at
line N
No
Is the number of
DG paths > 0
Yes
Open all the relays attached to immediate
downstream bus leading to DG units and
relay at line N
End of Main protection
Figure 2: Proposed main protection algorithm
Backup Protection Algorithm
In case the main protection system misoperated, the backup protection system is activated from its
standby mode to protect the network. For this purpose, the location of the misoperated relay should
be determined, which may be placed at either the immediate upstream or the downstream bus of
the faulty line, N. If the relay is located at the immediate upstream bus of the faulty line, the main
protection algorithm is implemented for the upstream line of the faulty line. Otherwise, the main
protection algorithm is implemented for the downstream line where the misoperation has occurred.
This process continues until the absolute isolation of the fault is achieved. Figure 3 shows the
flowchart of the proposed backup protection algorithm.
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Receive the main
protection relays status
Whether all
relays operate during
main protection
Yes
End of backup
protection
No
Whether
the misoperated
relays connected to
immediate Upstream
bus of faulty
line, N
No
Yes
Consider the downstream line
where misoperation occurred
Consider the immediate line
connected upstream to the faulty line
Find all downstream paths
leading to DG units using the
Backtracking Algorithm
Find all downstream and upstream paths
leading to DG units and main source
using the Backtracking Algorithm
Is the number
of DG paths in
downstream > 0
No
End of backup
protection
No
Is the number
of DG paths in
upstream > 0
Yes
Yes
Open all the relays connected to
the immediate downstream bus
leading to DG units
Open the relay at immediate line
connected upstream to the faulty
line
Figure 3: Proposed backup protection algorithm
Results and Discussion
Figure 4 shows the 22-bus, 20 kV distribution network with two DG units used to verify the
performance and accuracy of the proposed protection coordination strategy. The system data can
be find in (Zayandehroodi, et al. 2010).
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Figure 4: Single line diagram of a 22-bus test system
In this study, DIgSILENT Power Factory 14.0.524 software is used to simulate the abovementioned test system; various types of faults are created in each line. The two-stage RBFNN
method is applied and implemented using MATLAB software to estimate the fault distance from
each source and from the faulty line number (Zayandehroodi, et al. 2012, Zayandehroodi, et al.
2010). The proposed main and backup protection schemes shown in Figures 2 and 3 are coded in
MATLAB to determine the operation sequence of the main and backup relays, thus completing
the protection scheme. Various fault types, including single phase to ground fault, phase to phase
fault, two phases to ground fault, and three-phase fault, have been selected randomly for testing to
confirm the accuracy and effectiveness of the proposed protection scheme at the time of fault
occurrence. At first by creating a fault in line 8, the proposed method identifying two upstream
paths and one downstream path using the backtracking algorithm. At this time, the opening signals
are sent to relays R8 and R9 via the main protection algorithm to isolate the faulty line 8, as shown
in Figure 5. Figure 6 illustrates that when the main protection operates accurately, the backup
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protection does not operate and stays in the closed mode. The rest of the network will receive
power from the main source S and DG units DG1 and DG2.
Figure 5: Status of the circuit breakers of R8 and R9 as main protection with a fault at line 8
Figure 6: Status of R1, R2, and R10 with a fault at line 8 and the main protection operates
In the case of relay misoperation in the main protection system, the backup protection is activated
through the activation of the backup relays from standby mode, thus protecting the network. Figure
7 also illustrates the generated on/off command for the main protection relays and misoperation of
relay R8. Figure 8 shows the on/off command for the backup protection relays to cover the
misoperation of relay R8.
Figure 7: Status of R8 and R9 as main protection with a fault at line 8 when R8 does not operate
as the first backup protection
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Figure 8: Status of R1, R2, and R10 with a fault at line 8 when R8 does not operate as the first
backup protection.
In case of the misoperation of relay R9 in the main protection system, the backup protection
system is activated from its standby mode to protect the network. Relay R10 operates with a 247 ms
delay as a backup for relay R9. The operation performances of the related relays are shown in
Figures 9 and 10. As shown in the figures, the calculated implementation times of the proposed
protection coordination method in the main and backup protection for various fault types are
around 111 and 247 ms, respectively, which are quite fast and meet the Institute of Electrical and
Electronics Engineers Standard 242-2001 requirements (IEEE 2001).
Figure 9: Status of R8 and R9 as main protection with a fault at line 8 when R9 does not operate
as the first backup protection
Figure 10: Status of R1, R2, and R10 with fault in Line 8 when R9 does not operate as the first
backup protection.
Journal of Engineering and Economic Development, 2(2), 1-11, July 2015 10
Conclusion
A new protection coordination strategy for a distribution network with DG units has been
presented. In the proposed approach, all downstream paths leading to DG units are determined
using the backtracking algorithm after determining the location of the faulty line. The faulty line
number is then sent to the main protection system to determine the open/close states of the relays
for isolating the faulty line. A backup protection system is also considered to complete the
protection scheme in case the main protection system misoperates.
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