Topic 6 Quiz, February 2017 Impedance and Fault Current Calculations For Radial Systems TLC ONLY!!!!! DUE DATE FOR TLC- February 14, 2017

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1 Topic 6 Quiz, February 2017 Impedance and Fault Current Calculations For Radial Systems TLC ONLY!!!!! DUE DATE FOR TLC- February 14, 2017 NAME: LOCATION: 1. The primitive self-inductance per foot of length of a straight wire of circular cross section, as used for surge arrester ground leads or pole ground leads, is independent of the total length of the wire. T/F 2. If the diameter of the wire for a surge arrester ground lead is doubled, the primitive self-inductance per foot of length will be halved. T/F 3. In a single-phase two-wire circuit, the phase or hot wire and the neutral wire may be different size conductors (reduced size neutral). The magnitude of the longitudinal voltage drop from the near end to the far end on the hot wire always equals the magnitude of the longitudinal voltage drop from the far end to the near end of the neutral wire. T/F 4. In a single-phase circuit made with ROMEX cable having # 14 solid copper wire for the phase conductor (black wire) and # 14 solid copper wire for the neutral conductor (white wire), the circuit resistance is much greater than the circuit reactance. T/F 5. In an overhead single-phase 240-volt circuit (service) to a residence, where the two phase wires are made from 1/0 aluminum with their centers separated by 8 inches, the circuit resistance is more than 20 times the circuit reactance. T/F 6. Carson s equations for the self and mutual impedances of conductors with earth return have many terms, some of which contain the height of the conductor(s) above the surface of the earth. If the height terms are neglected when calculating the self-impedance of a conductor with earth return, and the mutual impedance between two conductors with common earth return, for the range of parameters associated with overhead distribution circuits, extremely large errors result. T/F 7. In a single-phase multi-grounded neutral line consisting of one phase conductor, and one neutral conductor which is grounded (connected to earth) at different points along the length of the circuit, 100 percent of the current in the phase conductor returns in the neutral, with no current returning in the earth. T/F 8. For a three-phase overhead distribution circuit, increasing the geometric mean spacing between the three phase conductors (ie: putting the conductors farther apart) increases the positive-sequence inductive reactance. T/F 9. If a multi-grounded neutral conductor is added to an otherwise three-phase threewire overhead distribution line, addition of the multi-grounded neutral conductor practically has no effect on the positive-sequence reactance (X 11 ) of the line. T/F 10. For practical purposes, the positive-sequence resistance in Ω per mile of a 1

2 three-phase overhead distribution line, with the same size conductor in each phase, equals the phase conductor resistance in Ω per mile. T/F 11. Adding a multi-grounded neutral conductor to an otherwise three-phase threewire overhead distribution line will reduce the zero-sequence reactance of the line. T/F 12. Adding a multi-grounded neutral conductor to an otherwise three-phase threewire overhead distribution line always will reduce the zero-sequence resistance of the line, regardless of the size of the neutral conductor. T/F 13. The positive-sequence resistance of a circuit made with three single-conductor lead-sheath cables, or three multi-wire concentric neutral cables, is always equal to or greater than the resistance of the phase conductor. T/F 14. The positive-sequence reactance of a circuit made with three single-conductor lead-sheath cables, or three multi-wire concentric neutral cables, is always less than the positive-sequence reactance the circuit would exhibit if the sheaths or neutrals were not present (ie: the sheaths on each cable have an infinite resistance such that there are no circulating currents in the sheaths when the phase currents are perfectly balanced). T/F 15. For a three-phase circuit made with three single-conductor multi-wire concentric neutral cables, the spacing between the centers of the cables has minimal effect on the zero-sequence resistance and the zero-sequence reactance of the circuit. T/F 16. For a three-phase circuit made with single-conductor multi-wire concentric neutral cables or single-conductor lead-sheath cables, increasing the spacing between cable centers from 2 inches to 15 inches will have no effect on the circulating currents induced into the neutrals or lead sheaths when the phase currents are perfectly balanced (only positive-sequence currents in the phase conductors). T/F 17. The positive-sequence reactance of an overhead distribution circuit, where the line uses cross arm construction with an 8 foot arm, can be taken as 0.6 Ohms per mile of line when specific information is not available on the size of the phase conductor. T/F 18. When a fault occurs in a distribution circuit, the total current consists of a dc component which decays exponentially with time, and the ac (steady-state) component of current which flows until interrupted by a breaker or fuse. Only the ac (steady-state) component of the fault current needs to be considered in selecting the momentary rating and interrupting rating of equipment in distribution circuits, and when determining the physical forces on buses. T/F 19. When the distribution substation transformer (Main transformers in area substations) high-voltage windings are connected in delta and the medium voltage windings are connected in wye, the neutral point (X0 bushing) of the medium- 2

3 voltage windings can be solidly grounded ( X0 connected to the station ground mat without any impedance inserted in the connection), or else grounded through an impedance. When solidly grounded, the current for the bolted single phaseto-ground fault at the medium-voltage terminals of the substation transformer can be higher than the current for a bolted three-phase fault at the transformer terminals. T/F 20. Installing a neutral reactor between the X0 bushing of the substation transformer medium-voltage winding and the ground mat in the area substation will cause a reduction of the current for the single phase-to-ground fault. T/F 21. Installing a neutral reactor between the X0 bushing of the substation transformer medium-voltage winding and the ground mat results in a reduction of the current for both the three-phase ungrounded fault and the ungrounded phase-to-phase fault (faults between phase conductors that do not involve ground). T/F 22. When rotating machines (generators and large motors) are remote from the point of fault in the distribution system, the current for the ungrounded phase-to-phase fault is 86.6 % of the current for the three-phase fault at the same location. T/F 23. In both a 13 kv system, and a 27 kv system, the three-phase fault current on the substation bus is 20,000 amperes. Both substations have overhead lines exiting with the same size conductor and spacing between phase conductors. That is, the impedances in Ω/mile for the 13 kv line are identical to those of the 27 kv line. The figure below shows the fault-current profile curves (plot of available threephase fault current versus distance of the fault from the substation) for the 13 kv & 27 kv systems. Curve A applies to the 27 kv system, & Curve B applies to the 13 kv system. T/F THREE-PHASE FAULT CURRENT IN KA SUBSTATION FAULT CURRENT = 20 KA, X/R = 20.0 OVERHEAD LINE Z 1 = j Ω/MILE CURVE A CURVE B C:\CON ED DIST COURSE 2011\TOPIC 6\Topic 6 Quiz Prob 24.XLS-2014 REV DISTANCE FROM SUBSTATION TO FAULT POINT IN MILES 3

4 24. In overhead open wire distribution feeders, the current for an arcing fault between any two phases typically will not exceed 10% of the current for a bolted fault between the same two phases at the same location. T/F 25. A down conductor fault in overhead distribution systems, a fault where a phase conductor breaks and falls on the surface of the earth without contacting any other phase or neutral conductor, does not create a hazard as it always draws sufficient current to operate relays, reclosers, and fuses in less than 10 cycles. T/F 4

5 Name: Location QUIZ 1 ANSWER SHEET IMPEDANCE AND FAULT CURRENT CALCULATIONS FOR RADIAL SYSTEMS 1. T F 13. T F 2. T F 14. T F 3. T F 15. T F 4. T F 16. T F 5. T F 17. T F 6. T F 18. T F 7. T F 19. T F 8. T F 20. T F 9. T F 21. T F 10. T F 22. T F 11. T F 23. T F 12. T F 24. T F 25. T F 5