Overcurrent Protection Issues for Radial Distribution Systems with Distributed Generators

1 Overcurrent Protection Issues for Radial Distribution Systems with Distributed Generators Karen L. Butler-Purry, Senior Member, IEEE, Hamed B. Funmilayo Abstract The integration of distributed generators (DGs) into distribution systems is being proposed as a solution to meet increasing load demands and to utilize more renewable energy. Existing overprotection schemes must be modified to address the new system characteristics of radial distribution systems with DGs. Minimum off-the-shelf overcurrent protection devices and distributed generators were added to the 123 Nodes Radial Test feeder. A commercial power systems analysis tool was used to determine the coordination issues that arise in the distribution system due to the integration of DGs. The results of these studies are presented and discussed. Index Terms-- Power distribution protection, Overcurrent protection, Power distribution lines, Software tools. W I. INTRODUCTION ITH the proposed migration to the inclusion of distributed generation in distribution systems, it is critical that the protection schemes be adapted to address these new and very different system characteristics. The impact of the integration of DGs in radial distribution systems on the coordination of existing overcurrent protection schemes were studied using the IEEE 123 Nodes Radial Test Feeder [1]. Overcurrent protective devices and distributed generators were added to the test feeder. Coordination studies were performed on the feeder with and without distributed generators. This presentation will discuss the findings of these studies. II. OVERCURRENT PROTECTION FOR 123 NODES RADIAL TEST FEEDER The IEEE 123 Node radial test feeder consists of 1-phase, 2- phase and 3-phase lines. The system operates on a 3.86 MVA base with a 115/4.16 kv substation transformer rating and a 4.16/4.16kV transformer rating downstream of the feeder. The circuit is characterized by 85 spot loads modeled as constant power, impedance, and current loads. The spot loads are combined as 1-phase, 2-phase, and 3-phase loads. The total load is 3.524 KW. There are four voltage regulators This work was supported in part by the U.S. National Science Foundation under Grant ECS-02-18309. Dr. Karen. L. Butler-Purry and H. B. Funmilayo, are with the Electrical and Computer Engineering Department, Texas A&M University, TX. 77843 USA. (Emails: klbulter@ece.tamu.edu, hfunmilayo@gmail.com) 978-1-4244-4241-6/09/$25.00 2009 IEEE at specific locations on the feeder. A total of eleven switches, six of which are normally closed labeled as sw # and the remaining five are normally open, provide the feeder with different configuration options. Fig.1 illustrates the test feeder with the added OCPDs. The fuse saving OCP scheme of the radial feeder in Fig. 1 includes a recloser on the main and 29 fuses on the existing laterals. The defined protection zone for each fuse corresponds to the lateral on which the fuse is placed. The number following the letter(s) indicates the lateral number with the exception of the underground line fuse (F-UG) and capacitor bank fuses (FC18, FC21, and FC22). Three laterals have reclosers (RL1, RL2, and RL4) instead of fuses because none of the fuses in the protective device database in DIgSILENT would satisfy the minimum and maximum fault requirements for these laterals. These reclosers were set to operate on a delayed curve only [2] and were coordinated with the feeder recloser. Table I provides the list of protection devices added to the IEEE 123 nodes radial test feeder and their settings. The first and second columns give the names of the nodes which demarcate the locations of the protective devices. The third column provides the names of the protective devices, while the fourth column shows the devices manufacturer s names. The remaining columns show the settings for each of the protective devices. Fuse F25 and F29 are three single phase fuses on the 3-phase laterals of the feeder while F11 consist of two single phase fuses for the 2- phase (phases A and B) lateral. The OCPDs on a radial distribution feeder normally operate on steady state fault current since the feeder is inherently passive. The generation and transmission systems are farther away from the feeder such that any transients from the generation system would have decayed over a period of time before the fault reaches the distribution feeder. However, when DG is added to the feeder lateral, generation is closer to the load and large transients may occur during a fault on the feeder. Depending on the DG penetration level (size) on the laterals, the fuses may be subject to unnecessary damage especially during a fault. Three OCP problems that arise include fuse fatigue, nuisance fuse blowing, and fuse misoperation[3], [4]. Fig. 2 illustrates how these OCP problems develop after adding DG to a lateral of a feeder.

2 33 27 150 32 29 25 0 30 25 1 51 28 50 31 49 25 47 48 46 26 45 23 44 43 24 42 41 21 40 22 38 19 35 36 13 5 20 18 37 14 57 11 58 59 9 2 10 52 53 152 7 8 13 94 149 1 96 34 12 17 64 65 63 66 39 35 0 11 1 11 0 112 113 114 151 30 0 10 9 10 7 62 60 10 8 10 6 104 451 10 3 45 0 10 5 102 10 0 10 1 99 98 71 19 7 70 97 69 16 0 67 68 75 74 73 72 54 61 610 77 78 55 56 76 80 76 92 90 88 81 85 79 84 3 4 5 6 15 16 95 19 5 93 91 89 87 86 82 83 Fig. 1. IEEE 123 Node Test feeder with overcurrent protective devices (Modified from [1]) TABLE I OVERCURRENT PROTECTION DEVICES SETTINGS ON THE IEEE 123 NODE TEST FEEDER From To Protective Nomenclature Manufacturer's I rated I asymetric Node Node Device Type Name (A) (KA) 1 149 R main SEL351R* SEL 800 12.00 1 2 RL1 SEL351R* SEL 60 12.00 1 6 RL2 SEL351R* SEL 60 12.00 8 11 F3 DOMF3-150 Dominion 150 9.85 8 12 RL4 SEL351R* SEL 60 12.00 13 17 F5 R400TL-80E Westinghouse 80E 9.78 18 20 F6 DOMF3-100 Dominion 100 10.00 21 22 F7 DOMF3-100 Dominion 100 10.00 23 24 F8 DOMF3-100 Dominion 100 10.00 26 32 F9 DOMF3-100 Dominion 100 10.00 27 33 F10 NEMAT-50T Cooper 50T 8.17 35 39 F11 DOMF3-150 Dominion 150 9.85 40 41 F12 NEMAT-50T Cooper 50T 8.17 42 43 F13 NEMAT-50T Cooper 50T 8.17 44 46 F14 NEMAT-50T Cooper 50T 8.17 57 59 F15 DOMF3-100 Dominion 100 10.00 67 71 F16 DOMF3-100 Dominion 100 10.00 72 75 F17 DOMF3-100 Dominion 100 10.00 76 85 F18 DOMF3-100 Dominion 100 10.00 87 88 F20 NEMAT-50T Cooper 50T 8.17 89 90 F21 NEMAT-50T Cooper 50T 8.17 91 92 F22 NEMAT-50T Cooper 50T 8.17 93 94 F23 NEMAT-50T Cooper 50T 8.17 95 96 F24 NEMAT-50T Cooper 50T 8.17 97 450 F25 DOMF3-100 Dominion 100 10.00 101 104 F26 DOMF3-100 Dominion 100 10.00 105 107 F27 DOMF3-100 Dominion 100 10.00 108 114 F28 DOMF3-100 Dominion 100 10.00 47 48 F29 R400TL-65E Westinghouse 65E 9.78 For the given fault at the location shown in Fig. 2, two sources (the substation/grid and the DG) now supply the fault current. Therefore, the currents through the OCPDs now become functions of the substation ( I SUB ) and the infeed current from the DG ( I DG ). The recloser current ( I REC ) is a function of the substation, the FUSE1 current ( I FUSE1 ) is a function of the DG, and the FUSE2 current ( I FUSE2 ) is a function of the substation and infeed current from the DG. The OCP problems may arise as a result of the redistributed fault current and other factors such as DG penetration level. Fig. 2 A Portion of a Typical Radial Feeder with a DG, Recloser, and Fuses Fuse fatigue or damage arises when the fuse link begins to melt before the recloser s fast operation [5]. The occurrence of fuse fatigue may reduce the lifetime of the fuse but will not cause the fuse to blow or result in a permanent outage. As the DG penetration level increases the fault current through FUSE2 may increase as well. The lateral may therefore suffer a permanent outage during a fault which may be potentially temporary. The nuisance fuse blowing issue occurs when the infeed current from the DG causes the fuse to operate on the MC

3 curve prior to the recloser s fast operation during a fault. Hence if the fault were temporary, this would be an incorrect operation and cause an unwarranted permanent outage. In the radial feeder, the protection zone for a fuse is the lateral and a fuse would operate to isolate all faults on the lateral protected by the fuse. However when the DG is added to the feeder lateral, the feeder is no longer radial and the fuse may operate for faults outside the lateral (on the main or on other laterals). This condition is known as fuse misoperation. The occurrence of the three described issues will affect the reliability of the feeder during temporary or permanent faults. III. DISTRIBUTED GENERATION MODEL DG is defined as a subset of Distributed Resources (DR) that may be employed in smaller capacities as micro (~1W < 5kW), small (5kW < 5 MW), medium (5 MW < 50 MW), and large (50 MW < 300 MW). DG technologies include photovoltaics, wind turbines, fuel cells, small and micro-sized turbine modules, sterling-engine based generators, and internal combustion engine generators [6]. Many of these technologies use renewable energy resources. DG can serve as a viable option to provide additional substation and feeder capacity in anticipation of future load growth. Some immediate benefits of operating DG in parallel with the feeder include reduction of feeder power losses and Transmission & Distribution (T&D) costs. In the studies reported in this presentation, DGs were added at two locations on the 123 Nodes Radial Test feeder as shown in fig. 1. Each DG operates in parallel with the feeder through an isolated transformer connected in a delta-delta configuration. The kva rating of the transformer matched the DG kva rating. The synchronous generator represents the most prevalent type of DG and therefore was used in the studies as the DG model. The synchronous, transient, sub-transient and zero sequence reactance were taken from [3],[8]. The generator was chosen to be power factor controlled (PQ) with an initial power factor of 0.854 [9]. Table II shows the parameters for the four sizes of DGs used in the studies. The first column gives the name and unit of the generator parameters. The remaining columns show the generator parameter for each of the four generators. The subtransient quadrature reactance (x q ) and the stator resistance (R s ) were not available in [10] for the 3.85MVA DG. Therefore the two parameters were duplicated from the 2.5MVA DG. TABLE II SYNCHRONOUS GENERATOR PARAMETERS FOR DG UNITS Generator Generator Size (MVA) Parameter 0.406 1.075 2.5 3.85 V n (Volts rms ) 460 460 460 460 Freq (hz) 60 60 60 60 T d ' (s) 0.080 0.185 0.330 3.300 T d '' (s) 0.019 0.025 0.030 0.015 T q '' (s) 0.019 0.025 0.030 0.050 x d (pu) 2.900 2.890 2.400 2.320 x d ' (pu) 0.170 0.250 0.200 0.260 x d ''(pu) 0.120 0.170 0.150 0.160 x q (pu) 2.440 1.720 1.770 1.180 x q '' (pu) 0.340 0.290 0.260 0.26* x l (pu) 0.070 0.080 0.050 0.150 H (s) 0.194 0.322 0.347 1.010 R s (pu) 0.003 0.003 0.003 0.003* IV. STUDIES AND RESULTS Coordination studies were performed using DIgSILENT Powerfactory software [11] to identify the three overcurrent protection issues caused by the inclusion of DGs. Two cases are presented which describe the nuisance fuse blowing and fuse fatigue issues as conveyed in the 123 Nodes Radial Test Feeder. A. Case 1 In one of the studies, a temporary L-G fault was placed downstream of a single phase fuse (F10) on a 1-phase lateral (phase A type). The fault was initiated at 0.01 seconds with a 10 cycle duration. During the temporary fault duration, it is expected that the feeder recloser will trip to clear the fault and prevent F10 from melting before the recloser s fast operation began. Fig. 3 provides the operation of F10 and the feeder recloser during the fault period. Here the status of the fuse remained at 1 and changed to 0 when melting began. As shown, after the fault was initiated, R opened at 0.053 seconds and closed at 0.220 seconds. However, F10 became fatigued at 0.053 seconds, around the same time the feeder recloser began its fast operation.

4 Fig. 3. Fuse fatigue on F10 of 123 Nodes Radial Test Feeder B. Case 2 In another scenario in which nuisance fuse blowing was investigated, a L-G fault downstream of F14 caused the fuse to blow prior to the feeder recloser completing its first operation. This situation occurred when the maximum DG size was placed on the feeder as well. The illustration in Fig. 4 shows the operation of F14 and the feeder recloser, while Fig. 5 shows the fault current through F14 during the nuisance blowing situation. As the fault is initiated at 0.01seconds, R opened at 0.045seconds and reclosed at 0.211 seconds. However, F14 blew at 0.171 seconds. The vertical axis in Fig. 5 gives the phase fault current through the fuse, while the horizontal axis corresponds to the fault current duration in seconds. Fig. 5 Fault Current through the Fuse F14 TABLE III SUMMARY OF OCP ISSUES ON THE IEEE 123 NODE RADIAL TEST FEEDER OCP Issues on Total the Radial Feeder # Fuse Fatigue 34 Nuisance Fuse Blowing 13 Fuse Misoperation 6 V. CONCLUSION The IEEE 123 Nodes Radial Test Feeder was modeled and provided with conventional protective devices and a fuse-saving overcurrent protection scheme. Load flow and short circuit analysis studies were conducted on these feeders using DIgSILENT Powerfactory software to determine the settings and coordination of the protective devices. Four sizes of the salient pole synchronous generator models were then customized in DIgSILENT and added to the Radial Test Feeders. Coordination studies were performed to identify overcurrent protection issues caused by the inclusion of DGs. Two of the issues are presented. The analysis results are being used to determine how overcurrent protection must be modified to properly protect radial distribution systems with DGs. Fig. 4 Nuisance fuse blowing on F14 of 123 Nodes Radial Test Feeder Table III summarizes results of case studies conducted during maximum fault studies on the IEEE 123 nodes radial test feeder for the DGs placed at the two locations shown in Fig.1. The first column of the table provides the three OCP issues, while the second column gives the number of issues found on the feeder with the existing OCP scheme. VI. REFERENCES [1] "Radial Test Feeders - IEEE Distribution System Analysis Subcommittee [Online]. Available: http://www.ewh.ieee.org/soc/pes/dsacom/testfeeders.html.. [2] Electrical Distribution-System Protection: Cooper Power systems, 2005. [3] R.C. Dugan and D.T. Rizy, "Electric distribution protection problems associated with the interconnection of small, dispersed generation devices," IEEE Trans. Power Apparatus and Systems, vol. PAS103, pp. 1121-1127, 1984. [4] A. Girgis and S. Brahma, "Effect of distributed generation on protective device coordination in distribution system," in Proc. Power Engineering Large Engineering Systems Halifax Canada, 2001, pp. 115-119. [5] Electrical Distribution-System Protection: Cooper Power systems, 2005. [6] M. Davis, D. Costyk, and A. Narang, Distributed and Electric Power System Aggregation model and Field Configuration Equivalency Validation Testing, 2003 [Online]. Available: http://www.nrel.gov/docs/fy03osti/33909.pdf.

5 [7] "SimPower Systems MATLAB Manual 2007," [Online] Available: www.mathworks.com. [8] "System Impact Assessment Report Windsor Energy Center," 2007. [Online] Available: www.ieso.ca/imoweb/pubs/caa/caa_siareport_2007-281.pdf. [9] W. D. Stevenson, Jr., Elements of Power System Analysis, Fourth ed. New York: McGraw-Hill Book Company, 1982. [10] "System Impact Assessment Report Windsor Energy Center," 2007. [Online] Available: www.ieso.ca/imoweb/pubs/caa/caa_siareport_2007-281.pdf. [11] DIgSILENT (2007). DIgSILENT PowerFactory Manual [Online]. Available: www.digsilent.com. Hamed B. Funmilayo (S 04) received the B.Sc degree in electrical engineering from Kansas State University, Manhattan, KS, in 2005 and Master s degree in electrical engineering from Texas A&M University in 2008. He was a graduate research assistant at Texas A&M University s Power System Automation Lab from 2006-2008. He is currently employed by Centerpoint Energy. His research includes distributed generation with emphasis on overcurrent protection. Mr. Funmilayo is a member of Eta Kappa Nu and the Power Engineering Society. VII. BIOGRAPHIES Karen L. Butler-Purry (SM 01) received her B.S. (summa cum laude) in Electrical Engineering in 1985 from Southern University in Baton Rouge, LA. She was awarded her M.S. degree in 1987 from the University of Texas at Austin. She was awarded her Ph.D. in Electrical Engineering in 1994 from Howard University, Washington, D.C. She joined Texas A&M University in 1994, where she currently serves as Associate Head and Professor in the Department of Electrical and Computer Engineering. Her research interests are in the areas of distribution automation and intelligent systems for power quality, equipment deterioration and fault diagnosis. Dr. Butler-Purry is a member of the Power Engineering Society, the American Society for Engineering Education and the Louisiana Engineering Society. She is a registered professional engineer in the states of Louisiana, Texas and Mississippi. She received the National Science Foundation Faculty Career Award (1995) and the Office of Naval Research Young Investigator Award (1999).