AbstractâDirectional inverse time overcurrent relays are used in protection systems of ... This paper proposes the addition of fault current direction constraint to ...
2014 5th IEEE PES Innovative Smart Grid Technologies Europe (ISGT Europe), October 12-15, Istanbul 1
Protection Coordination of Directional Overcurrent Relays Considering Fault Current Direction Hebatallah Mohamed Sharaf1, Student Member, H. H. Zeineldin1,2, Senior Member IEEE, Doaa Khalil Ibrahim1, Senior Member IEEE, Essam El Din Abo El Zahab1 1 2
Department of Electrical Power Engineering, Faculty of Engineering, Cairo University, Egypt
Department of Electrical Engineering and Computer Science, Masdar Institute, Abu Dhabi, UAE
Abstract—Directional inverse time overcurrent relays are used in protection systems of meshed networks to operate for fault currents in its forward zone of operation. They are used for primary and backup protection and their operation needs to be coordinated to assure selectivity. Different optimization techniques have been used to select their settings to achieve protection coordination, however the optimal protection coordination problem formulation doesn’t consider the fault current direction to assure correct operation especially for backup relays. This paper proposes the addition of fault current direction constraint to the formulation of the optimal directional inverse overcurrent relays coordination. The proposed formulation is tested on the distribution portion of the IEEE 30 bus system considering the effect of distributed generation addition. Results show that the proposed formulation gives more accurate optimal relay settings and thus highlighting the importance of inclusion of the proposed direction constraint in the protection coordination problem.
of DG addition is changing the short circuit levels in the system which may lead to relays' protection coordination failure [2].
Index Terms-Distributed Generation, Directional Overcurrent Relay, Meshed Distribution Systems, Protection Coordination, Relay Settings.
Concerning short circuit current direction detection, there are many approaches for detecting the fault current direction [8], and the fundamental concept is based on the angle between the relay fault current and its phase voltage (IscA& VA for example) or for more stable analysis the phase fault current and a polarizing quantity that is not affected by the fault (IscA& VBC). Fig. 1a shows the angle of the short circuit current when it is in the forward operation of a relay while Fig.1b shows the angle of the short current seen by the relay when it is in its reverse direction.
I.
INTRODUCTION
With the steps towards the “Smart Grid”, there is an increasing trend for adding distributed generation units in networks at the distribution voltage level. One of the basic impacts of this addition is transforming the commonly radial distribution network into meshed and looped structure with bidirectional power flow leading to the essential dependence on directional inverse time overcurrent relays (DOCRs) in distribution systems protection [1], [2]. Protection coordination plays a vital role in the selective operation of DOCRs. The DOCRs coordination is accomplished by adjusting the two main settings: time dial setting (TDS) and pick up current (Ip). Different optimization methods, including conventional and heuristic techniques, have been applied to determine the optimal TDS and Ip settings of the relays that guarantee coordination and minimum total relay operating times[3]-[5]. Another impact
Generally while performing an optimal coordination study, the backup scheme is designed assuming that the direction of the current will be “always” seen by the backup relays in its forward zone of operation. The short circuit current direction constraint is not taken into consideration while trying to perform protection coordination as in [6]-[7].This can result in a backup DOCR relay (based on the assumed/ expected short circuit current direction) that will actually not operate because the actual direction of the short circuit current is in the reverse operation zone for this DOCR. Adding a direction detection constraint in the optimal coordination formulation assures that every DOCR in the backup scheme will detect the short circuit current in its forward zone or otherwise it will be excluded.
Reverse Direction
Isc
Forward Direction
V
T sc (a)
Isc
Reverse Direction
Forward Direction
T sc
V
(b)
Fig. 1 Angle between short circuit current and polarizing voltage (a) in forward direction (b) in reverse direction
978-1-4799-7720-8/14/$31.00 ©2014 IEEE
2
II.
PROPOSED COORDINATION PROBLEM FORMULATION CONSIDERING FAULT CURRENT DETECTION
The protection coordination problem can be optimized such that the optimization objective is to minimize the coordination times of all the relays while maintaining the conditions of protection coordination. The objective function is taken to be the sum, T, of the coordination times of all the relays which needs to be minimized as follows: N
M
¦¦ (t
T
pij
i 1 j 1
¦ t bij )
(1)
III.
SETUP AND SIMULATION
The proposed optimized protection coordination while considering fault current direction detection is tested on the distribution portion of the IEEE 30 bus test case shown in Fig.2. This system which represents a portion of the American Electric Power System and its parameters are available in [9]. The system has three distribution substations 132/33kV connected to 15 feeders which are protected by 28 directional overcurrent relays. Fault nodes (F15-F30) are added on the feeders. 14
13 R28
where t p and
F29
t b are the operating time of the primary and
A
TDS j (
I sci B ) 1 I pj
12
R26 R25
R24
10 R22
F27
11
R23
F26
(2)
R11 3 1
R8
R16
F25
F17 F15
R18 4 R12
R6 R7
F19
F20
R13
F22 F21
R21
6 R19
F18
F16
R5
5 R17
R10 R9
Before adding the relay operating time to the objective function in (1), its direction is checked using the following constraint:
T min d T sc d T max
R27
F28
backup protection relays respectively. N represents the total number of relays and M denotes the total number of fault locations. The inverse time current relay characteristic can be represented as follows:
tj
F30
R14
9 R20 7 R15
F24
F23 R2
R1
R3
R4 8
2
(3)
where T min , and T max are the angle limits which represent the forward operation zone of the relay taken to be -135° and 45°, respectively based on [8].If this equation is fulfilled, the relay time is added to the objective function and the primary/backup pair scheme is not modified otherwise the relay operating time is excluded and the relay primary/backup pair is modified taking into account the fault current direction. Constraints (4) to (6) have to be fulfilled while solving the protection coordination problem. CTI is the coordination time interval which indicates the minimum time between the primary and the backup relay and it usually takes values between 0.2-0.5s and it is chosen to be 0.3s in this analysis. The minimum and maximum pickup current will depend on the system’s rated load currents and system’s short circuit levels. TDS depends on the permissible relay time dial setting range and could take a value between 0.1 and 3.
tbij t pij t CTI
(4)
I pj min d I pj d I pj max
(5)
TDS min d TDS j d TDS max
(6)
Fig.2. Distribution portion of the IEEE 30 bus system
The tested system is constructed and the protection coordination problem is formulated as a nonlinear programming problem using Matlab developed m-files and the optimization is solved using the built in function fmincon(find minimum of constrained nonlinear multivariable function) in the Matlab Optimization Toolbox which relies on the gradient-based method that is designed to work on problems where the objective and constraint functions are both continuous and have continuous first derivatives [10]. The simulation study includes performing the short circuit analysis for the test system when the fault nodes (F15-F30) are in different locations on the feeders with an emphasis on how the direction of the currents seen by the backup relays changes with the different fault locations. The effect of adding synchronous based DG units on the short circuit current direction is also tested. IV.
RESULTS AND ANAYSIS
A. Investigating the effect of fault location on fault current direction Fault location is an important factor that affects short circuit current levels and direction.Case studies including far, midpoint and near ends faults from indicated buses are tested
3
where far end indicates that the fault node is at a distance of 99% of the line length from the indicated bus and near end fault is at a distance of 1% the of line length from the indicated bus.Applying equation (3), the short circuit current angle between -135° and 45°will indicate that the fault current is in the forward zone of operation of the DOCR. The results of the short circuit analysis with direction detection for the test system shows that there are some backup relays in which the fault current will pass in a direction opposite to the expected one considered in the backup scheme. This will cause misoperation of these relays. Table I summarizes these cases for faults at midpoint, far and near ends of the feeders. Incase of faults at the middle of the lines, relay R21 will misoperate to act as backup protection for faults at nodes F22, F23 and F24. Another misoperation was recorded incase of fault at node F25.Incase of near/far end faults, the same misoperations are also recoreded with two additional misoperation of R9as a backup for fault at F15for a far end fault from bus 2 and R16 as a backup for fault at F26 when it is far end fault from bus 3. These results show that the direction of the fault currents is affected by the fault location. B. Investigating the effect of DG addition on fault current direction The effect of DG addition on fault current direction is investigated in this section. Short circuit analysis with direction detection is performed in presence of synchronous based DG units with ratings range from 1 up to 20 MVA at different system buses. Both, the effect of adding one DG unit at one bus and 2 DGsat2 different system buses are tested in case of midpoint, far and near fault locations. With respect to the base case results (Table I), some DG locations and ratings cause changes in short circuit current direction as shown in Table II. As an example, for a fault at F22at midpoint of the line, adding DGs 5 MVA at bus 5 and 18 MVA at bus 9 changes the direction of the fault current to be in the forward operation zone of relays R21 (originally it was in the reverse zone) so R21 can be included in the backup scheme for this case study. On the contrary, adding DGs of 20 MVA at buses 1&9 reversed the direction of the short circuit current seen by R4due to a fault at F26 when it is far from bus 10 (the current was in the forward zone in the base case and the relay was operating properly as a backup relay for this fault) so R21 should be excluded from the backup scheme for this case study. C. Protection coordination with proposed formulation versus conventional one The previous results showed the importance of adding a direction constraint in the protection coordination formulation to avoid adding the operating times of relays which actually don’t operate and should be excluded from the primary/backup scheme. The proposed directional overcurrent relays coordination formulation with direction checking is applied on the system base case and the cases with DGs considering midpoint faults. The results are
compared to performing the optimal coordination using the conventional formulation. For the base case, Table III shows the optimal relay settings (TDS & Ip) using the conventional formulation without the direction detection constraint. The total relay operating time using conventional formulation is 63.27 sec. Table IV shows the relay operating time of the system’s relays. Although the relays R20 & R21 will not operate in the backup protection scheme of faults from F22 to F25based on the short circuit analysis results in Table I due to reverse current, the operating times are calculated and added to the objective function (these relays are highlighted in Table IV). This means that the conventional formulation gives inaccurate results concerning the optimal settings, optimal relay operating times and the total relay operating time. TABLE I RELAY MISOPERATION CASES DUE TO REVERSE DIRECTION IN IEEE 30-BUS SYSTEM - D IFFERENT FAULT L OCATIONS Fault Node
MisoperatedBackup Relays Midpoint Faults
F22 F23 F24 F25
R21 R21 R21 R20 Near/Far End Faults
F22 near end/bus 6 F23 near end/bus 7 F24 near end/bus 9 F15 far end/bus 2 F22 far end/bus 6 F23 far end/bus 7 F24 far end/bus 9 F26 far end/bus 3
R21 R21 R21 R9 R21 R21 R21 R16
TABLE II CHANGES IN RELAY MISOPERATION CASES FOR D IFFERENT FAULT LOCATIONS IN IEEE 30-BUS SYSTEM DUE TO DG ADDITION Fault Node
DG Size/Location
F22
5 MVA @ bus 5 and 18 MVA @ bus 9
Misoperated Relays
without DG Midpoint Fault
with DG
R21 misoperates
R21 operates
Near/Far End Faults
F26 far end/bus 3 F22 far end/bus 8 F26 far end/bus 10 F26 far end/bus 10
15 MVA @ bus 4, or 5 or 6 or 7 or 9 20 MVA @ bus 1 and bus 9 20 MVA @ bus 1 and bus 10
R16 misoperates
R16 operates
R21 misoperates
R21 operates
R4 operates
R4 misoperates
R21 operates R4 operates
R21 misoperates R4 misoperates
TABLE III OPTIMAL RELAY TDS AND Ip SETTINGS FOR IEEE 30-BUS SYSTEM USING THE CONVENTIONAL COORDINATION FORMULATION Relay 1 2 3 4 5 6 7 8 9
TDS(s) 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
Ip(p.u) 0.855 0.6811 1.0708 0.1395 0.0768 0.6758 0.7312 0.0196 0.3229
Relay 15 16 17 18 19 20 21 22 23
TDS(s) 0.3513 0.2773 0.1 0.1 0.1 0.2888 0.5966 0.2243 0.4176
Ip(p.u) 0.0684 0.0605 0.4648 0.4558 0.2412 0.0789 0.0166 0.0961 0.0627
4
10 11 12 13 14
0.1 0.1 0.1 0.1 0.1
0.5952 0.1975 0.2967 0.538 0.48
24 25 26 27 28
0.1 0.2836 0.1 0.1 0.1
0.1661 0.1502 0.2174 0.0622 0.04
TABLE IV OPTIMAL PRIMARY AND BACKUP RELAY OPERATING TIMES USING THE CONVENTIONAL PROTECTION COORDINATION FORMULATION Fault Location F15 F16
F17
F18
F19
F20
F21
F22
F23
F24
F25
F26
F27
F28 F29 F30
Operating times of relays in sec. (p=primary, b=backup) p b1 b2 b3 b4 R5 R9 R12 0.1862 0.8136 0.9223 R8 R6 R16 R22 0.1359 0.9274 0.8168 0.8457 R6 R12 0.4931 0.7931 R22 R9 R16 0.7411 0.4411 0.7411 R22 R6 R10 0.8380 0.8380 0.5380 R18 R16 0.9214 0.6214 R9 R7 0.8544 0.5544 R14 R12 0.7451 0.4451 R17 R10 0.5484 0.8484 R18 R2 0.5816 0.8816 R13 R7 0.6839 0.9839 R14 R1 0.4753 0.7753 R21 R20 R15 R2 R23 1.8241 1.3743 0.6136 1.5297 1.5697 R17 R19 0.7283 0.4283 R1 R19 R20 R21 R23 0.4942 1.0210 1.0408 1.1631 1.0793 R15 R13 0.8053 1.1053 R21 R19 R15 R23 R3 0.9011 0.9011 0.9843 0.6011 0.9011 R23 R4 R20 0.9843 0.9843 0.6843 R4 R15 R19 R20 0.3073 0.9495 0.6073 1.1985 R21 R3 R25 0.8317 1.1317 1.2304 R23 R11 0.9304 1.2304 R11 R6 R16 0.3166 1.1198 0.9365 R22 R4 R21 R25 0.5627 1.0324 1.2515 1.4612 R21 R11 R24 R4 1.1825 0.8724 0.3191 0.8111 R25 0.8182 R26 R24 0.3416 0.6416 R27 0.1963 R28 0.2179
R24 0.5460 R26 0.5179
-
-
-
Applying the proposed formulation with the short circuit current direction constraint, the operating times of the relays that will not operate in the backup scheme are excluded. The new total relay operating time is 48.7036secwithaccurate optimal settings and relay operating time for the different faults, as shown in Table V and VI. R20 and R21 are excluded from the backup scheme of faults F22-F25. As an example, fault at F22based on the conventional formulation will have primary protection relay R2 with operating time 0.6136 sec and backup relays R15, R20, R21 and R23 with operating times 1.3743, 1.8241, 1.5297 and 1.5697 sec respectively. These results are inaccurate because R21should be excluded and this is achieved when using the direction detection constraint giving the accurate results in Table VI where R2 will operate in 0.62 sec and the backup relays are R15, R20 and R23 only with operating times 1.27, 1.88 and 1.29 sec respectively. The new formulation results in less backup relay operating time for some other faults such as R9 and R12 for fault F15which decreased from 0.8136 and 0.9223 sec to 0.72 and 0.7 sec respectively. TABLE V OPTIMAL RELAY TDS AND Ip SETTINGS FOR IEEE 30-BUS SYSTEM USING PROPOSED PROTECTION COORDINATION FORMULATION Relay 1 2 3 4 5 6 7 8 9 10 11 12 13 14
TDS(s) 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
Ip(p.u) 0.81 0.65 0.56 0.11 0.08 0.41 0.66 0.02 0.24 0.06 0.17 0.21 0.46 0.45
Relay 15 16 17 18 19 20 21 22 23 24 25 26 27 28
TDS(s) 0.24 0.24 0.1 0.1 0.1 0.17 0.1 0.13 0.31 0.1 0.22 0.1 0.1 0.1
Ip(p.u) 0.07 0.06 0.35 0.44 0.15 0.1 0.17 0.2 0.06 0.16 0.18 0.22 0.06 0.04
TABLE VI OPTIMAL PRIMARY AND BACKUP RELAY OPERATING TIMESUSING THE PROPOSED PROTECTION C OORDINATION FORMULATION Fault Location F15 F16
F17
F18
F19
F20
Operating times of relays in sec. (p=primary, b=backup) p b1 b2 b3 b4 R5 R9 R12 0.1862 0.72 0.7 R8 R6 R16 R22 0.1359 0.37 0.78 0.88 R12 R6 0.37 0.67 R22 R9 R16 0.72 0.42 0.72 R10 R6 R22 0.19 0.52 0.9 R16 R18 0.6214 0.9214 R7 R9 0.53 0.83 R12 R14 0.39 0.69 R17 R10 0.51 0.22 R18 R2 0.59 0.89
5
F21
F22
F23
F24
F25
F26
F27
F28 F29 F30
R13 0.6839 R14 0.46 R2 0.62 R19 0.36 R1 0.4942 R15 0.62 R3 0.41 R20 0.47 R4 0.29 R21 0.29 R23 0.73 R11 0.3166 R22 0.47 R24 0.3191 R25 0.68 R26 0.3425
R7 0.9839 R1 0.76 R15 1.27 R17 0.66 R19 0.8 R13 0.92 R15 0.71 R4 0.77 R15 0.75 R3 0.59 R11 1.03 R6 0.61 R4 0.77 R4 0.62 R24 0.6425
R20 1.88 R20 0.8 R19 0.71 R23 0.77 R19 0.92 R25 1.22 R16 0.97 R21 0.77 R11 0.83 -
R21 R21 R21 R20 R25 1.22 R21 0.62 -
R23 1.29 R23 0.85 R23 0.77 -
R27 0.1968 R28 0.2182
R24 0.5475 R26 0.5182
-
-
-
The proposed formulation is also applied to achieve optimal coordination in one of the cases where DG addition causes a change in the fault current direction: adding 5 MVA atbus 5 and 18 MVA atbus 9 which caused R21 to operate as backup for fault atF22. The short circuit current direction constraint in the proposed formulation detected that the current seen by R21will be changed to be in its forward zone as a result of DG addition. New relays’ optimal settings and operating times are calculated, R21 is included in the backup scheme with operating time equals 1.8917 sec as shown in Table VII. TABLE VII OPTIMAL PRIMARY AND BACKUP RELAY OPERATING TIMES CONSIDERING THE PROPOSED PROTECTION COORDINATION FORMULATION FOR T EST SYSTEM WITH DG Fault Location F22
F23
F24
Operating times of relays in sec. (p=primary, b=backup) p b1 b2 b3 b4 R2 R15 R20 R21 R23 0.61 1.6348 1.7585 1.6446 1.8917 R19 R17 0.3862 0.6862 R23 R21 R1 R19 R20 0.8976 0.4816 0.8961 0.7816 R15 R13 0.6645 0.9645 R3 R15 R19 R21 R23 0.462 0.762 0.762 0.7802
F25
R20 0.4802 R4 0.2727 R21 0.5255 R23 0.7214
R4 1.9283 R15 0.8207 R3 0.8255 R11 1.0214
V.
R23 0.7802 R19 1.0078 R25 1.0214 -
R20 -
-
CONCLUSION
This paper highlights the importance of checking short circuit current direction when performing directional overcurrent relays coordination especially with the increasing complexity in power system and changes that accompany the addition of DGs. A modified formulation is proposed for performing the optimized directional overcurrent relays coordination that checks the direction of the short circuit current in the backup relays before performing the coordination. The proposed formulation is tested on the distribution system portion of the benchmark IEEE-30 bus system. Simulation results show the need of short circuit current direction checking in the protection coordination formulation. The proposed formulation shows effectiveness over the conventional coordination formulation that does not take into account direction of short circuit current in all the relays. REFERENCES [1]
Mozina, C.J., “ Impact of Smart Grids and Green Power Generation on Distribution Systems,” IEEE Transaction on Industry Applications, vol. 49, no. 3, pp. 1079-1090, May 2013. [2] T.K. Abdel-Galil, A. E.B. Abu-Elanien, E. F. El-Saadany, A. Girgis, Yasser A.-R. I. Mohamed, M. M. A. Salama, H. H. M. Zeineldin, Protection Coordination Planning With Distributed Generation, CETC Sept. 2007. [3] H.H.Zeineldin, “Optimal Coordination of Microprocessor based Directional Overcurrent Relays,” in Proc. Canadian Conference on Electrical and Computer Engineering, CCECE 2008. [4] Hebatallah Mohamed Sharaf, H.H.Zeineldin, Doaa Khalil Ibrahim and Essam El-Din .Abou El-Zahab, “Directional Inverse Time Overcurrent Relay for Meshed Distribution Systems with Distributed Generation with Additional Continuous Relay Settings,” in Proc. 12th IET Development in Power System Protection, DPSP 2014. [5] D.Pirla, R.Maheshwari and H.O.Gupta, “A New Nonlinear Directional Overcurrent Relay Coordination Technique, and Banes and Boons of Near End Faults based Approach,” IEEE Transactions on Power Delivery, vol.23, No.3, pp. 1176-1182, July 2006. [6] Mansour Ojaghi, Zeinab Sudi and Jawad Faiz, “Implementation of full adaptive technique to optimal coordination of overcurrent relays,” IEEE Transactions on Power Delivery, vol. 28, no.1, pp. 235-243, January 2013. [7] Turaj Amraee, “Coordination of Directional Overcurrent Relays using Seeker Algorithm,” IEEE Transactions on Power Delivery, vol.27, no.3, pp.1415-1422, July 2012. [8] John Horak and Walt Babic, “Directional Overcurrent Relaying (67) Concepts”, in Rural Electric Power Conference, IEEE, 2006, pp. 1-8. [9] Univ. Washington, Power Systems Test Case Archive, Seattle, WA. March 2006[Online]. Available: http://www.ee.washington.edu [10] www.Mathworks.com
