Journal of Reliable Power - SEL

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Alternatively, we could use additional comparators to make blinders parallel to the transmission line characteristic, to limit the impedance-plane coverage, and exclude load from the tripping characteristic. Or, we could build quadrilateral characteristics, which boxout load. All traditional solutions have the same common approach: shape the operating characteristic of the relay to avoid load. The traditional solutions have two major disadvantages: 1. Reducing the size of the relay characteristic desensitizes the relay to faults with resistance. Avoiding a small area of load encroachment often requires sacrificing much larger areas of fault coverage. 2. From a user's point of view, the more complex shapes become hard to define, and the relays are harder to set. A new approach does not modify the relay characteristic shape directly. Instead, it defines the load regions in the impedance plane, and blocks operation of distance elements if the impedance is in either of the load regions. Figure 13 shows the new approach applied to a four-zone mho relay. The mho characteristics are conventional, and are not modified to exclude load. Figure 13: Improved Load-Encroachment Method There are two load regions shown (load-in and load-out). The relay calculates the complex positive-sequence impedance and tests it against the boundaries of these load regions. If the impedance is inside either region, then the relay concludes the impedance represents load, and blocks the mho elements. If the impedance is outside both load regions, the mho elements are permitted to operate. The advantages of the new approach include: 1. Greater coverage for faults is possible because only the overlap between the load characteristic and the relay characteristic is blocked. 2. The load-encroachment characteristics are easy to set, because they can be directly related to the maximum load conditions. Once we have maximum load-in and load-out conditions, and the range of power factors of the load, the load-impedance areas can be found. No customization of relay characteristics is required to avoid load. 3. Separate characteristics for load-in and load-out are easy to define. 4. Since the characteristics for faults and loads are independent, there is less chance of setting errors. The logic applies to three-phase faults only. Loadencroachment blocking is not required or desired for unbalanced faults (e.g., AB, BC, CA, AG, BG, CG, ABG, BCG, CAG faults). VIII. FAULT-TYPE SELECTION CONSIDERATIONS For security, distance relay schemes must consider the behavior of the distance elements in all six fault loops (AG, BG, CG, AB, BC, and CA) under very broad and general system, load, and fault conditions. There are two major concerns: 1. Ground distance elements can overreach for line-lineground (LLG) faults. 2. Phase distance elements can operate for close-in lineground (LG) faults. The first concern is generally considered a problem in all applications. The second concern is a problem in single-poletrip schemes, and a targeting nuisance. How can we reliably prevent unwanted relay elements from interfering with the performance of the overall scheme If we know the fault is an AG fault, then we can block the AB and CA elements in order to avoid a three-pole trip for a LG fault. If we know the fault is a BCG fault, then we can block the BG and CG elements, avoiding possible overreach by the BG and CG elements. (The BG element tends to overreach for a BCG fault with resistance to ground. The CG element tends to overreach for a BCG fault with resistance between the phases.) A. Selection Using the Angle Between I 0 and I 2 The angle between the negative-sequence current and the zero-sequence current is a frequently used and very useful indicator. Figure 14 shows the sequence networks for AG and BCG faults. The symmetrical component currents are referenced to phase A. That is, I 0 means I A0 , and I 2 means I A2 . For these two faults, the angle between I 2 and I 0 is zero degrees. Figure 15 shows phasor diagrams for AG, BG, and CG faults. It also shows the phase relationships between I 0 and I 2 for the three faults. 12 | Journal of Reliable Power
If the angle is near +120°, then the fault is CG or ABG. • Enable CG and AB elements only. • One approach is to assign 120° sectors to AG, BG and CG faults. For example, angles between ±60° belong to AG and BCG faults. B. Fault Resistance Can Affect This Scheme Figure 16 shows the effects of introducing fault resistance between the point where phases B and C are shorted together and ground. Rf appears in the zero-sequence net-work, and the angle of the I 0 leads the angle of I 2 , because the network zerosequence impedance angle is less than that of the negativesequence impedance angle. Figure 14: Resistance) I 0 and I 2 Relationship for AG and BCG Faults (Without Fault Figure 15: I 0 and I 2 Relationship for AG, BG, and CG Faults Given that the fault is a single-line-ground (SLG) fault, the angle between I 0 and I 2 in the fault is a very reliable indicator of the fault type: centered around zero degrees for AG, –120° for BG (I 2 lags I 0 by 120°), and +120° for CG (I 2 leads I 0 by 120°). Fortunately, these angles do not change much over a broad range of system conditions. Thinking about Figures 14 and 15 at the same time, we conclude: If the angle is near zero, then the fault is AG or BCG. • If it is AG, then AB or CA could also pick up and three-pole trip for a single-line-ground fault. • If it is BCG, then the BG and CG ground distance elements may overreach. • THEREFORE: Enable AG, BC elements only. If the angle is near –120°, then the fault is BG or CAG. Enable BG and CA elements only. Figure 16: Effects of Increasing Rf for a BCG Fault When Rf is big enough, there may be confusion as to whether the fault is AG/BCG or BG/CAG, because the angle could be less than –60°, as shown in the bottom of the figure. Distance Relay Element Design | 13
Page 1 and 2: $12.00 Journal of Reliable Power Vo
Page 3 and 4: Journal of Reliable Power Volume 1
Page 5 and 6: Capable of running protection calcu
Page 7 and 8: Both P and Q are two-input phase an
Page 9 and 10: TABLE 1: MHO ELEMENT POLARIZING CHO
Page 11 and 12: For an Aφ-ground fault on the syst
Page 13: The most onerous 3φ fault is one w
Page 17 and 18: Impedance-Based Fault Location Expe
Page 19 and 20: A. Simple Reactance Method From Fig
Page 21 and 22: Figure 9 shows a screen capture of
Page 23 and 24: The relay then performs “torque
Page 25 and 26: IX. LAB TEST SETUP Several line fau
Page 27 and 28: APPENDIX The following are the rela
Page 29 and 30: The following shows the analog math
Page 31 and 32: Digital Communications for Power Sy
Page 33 and 34: increasing security by a factor of
Page 35 and 36: The receiver amplifier is the major
Page 37 and 38: Noise from faults or other sources
Page 39 and 40: 80 60 40 20 R F 100 3 3 S 2 5 6 4 1
Page 41 and 42: 15. A POTT scheme implemented over
Page 43 and 44: Transmission Line Protection System
Page 45 and 46: One-Cycle Directional Element Figur
Page 47 and 48: element misoperation. Shunt reactor
Page 49 and 50: • SW2 is in Position 1 when there
Page 51 and 52: Complete reclosing relay supervisio
Page 53 and 54: IX. BIOGRAPHIES Armando Guzmán, P.
Page 55 and 56: 2 II. COMMON ZONE TIMING In June 20
Page 57 and 58: 4 ment 67N2 after a 1-cycle carrier
Page 59 and 60: 6 VI. ZONE 1 OVERREACH DUE TO CVT I
Page 61 and 62: 8 1_VA 1_VB 1_VC 2_VA 2_VB 2_VC 3_V
Page 63 and 64: 10 The phasors during the fault are
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12 The prefault data from the backu
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1 Adaptive Phase and Ground Quadril
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3 C. Short Line Applications A shor
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5 II. QUADRILATERAL DISTANCE ELEMEN
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7 For the purpose of implementing t
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9 The three-phase fault detection e
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11 Zone 1 distance elements should
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13 Fig. 24 shows the apparent resis
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15 B. Adaptive Resistance Element F
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17 Fig. 34 and Fig. 35 illustrate t
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Line Protection Bibliography Issue
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Hou, Daqing, and Jeff Roberts. “C
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©2010 Schweitzer Engineering Labor
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