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Overhead Conductor Motion During Short Circuits
Edward S. Thomas, PE & Richard A. BarberUtility Electrical Consultants, PC
“Serving Utilities Since 1995”
Raleigh, North Carolina
2015 IEEE Rural Electric Power Conference
Asheville, North Carolina
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Factors Affecting Conductor Motion
Conductor Unit Weight (lbs. per ft.)
Fault Current in Conductor (amperes)
Duration of Fault Current (seconds)
Conductor Spacing
Conductor Tension & Sag
Mechanical Damping
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Larger substations that increase available fault currents.
Closer conductor spacing in order to minimize aesthetic impact of overhead lines.
Increased customer sensitivity to momentary interruptions and voltage dips.
Increased conductor sag due to the use of larger conductors while maintaining
distribution tension limits.
Why Interest now in Conductor Motion?
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D = the sag of a level span.
S = the span length.
H = the conductor tension.
w = the unit weight of the conductor.
Figure 1: Catenary Parameters(Equation 1b)
Catenary Parameters and Basic Equation
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F = the force in pounds per foot of conductor.
d = the spacing between the conductors in feet.
I = the symmetrical short-circuit current.
(Equation 2)
Magnetic Forces Between Conductors
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(Equation 3)
d = the conductor diameter, in.
VW = the wind speed, mph.
FH = the horizontal wind force, lbs/ft.
Wind Forces on Conductors
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(Equation 4a)
WC = the conductor weight per unit length, lbs/ft.
(Equation 4b)
XH = the horizontal deflection at midpoint of span, ft.
D = the midpoint sag of conductor at specified wind
and conductor temperature, ft.
Figure 2: Conductor Swing
Conductor Displacement Due to Horizontal Forces
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Calculation of Motion
(Equation 4a) (Equation 4b)(Equation 2)
Apply Equations 4a & 4b to determine displacement using initial force based
on conductor ‘at rest’ position with Equation 2.
Calculate displacement for 0.01 seconds.
Apply Equation 2 for new horizontal separation and reiterate with
Equations 4a & 4b.
Continue iterations until limit is reached. Limit is when gravity vector
equals vertical vector component of horizontal force acting on displaced
conductor or horizontal position is reached. Also limit iterations to fault
current duration.
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NESC Requirements for Horizontal Spacing
NESC requirements are based principally on clearances to minimize
contact during wind events.
NESC requirements are basically the same as in NBS Handbook 81 (1961).
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Importance of Conductor Tension
Typical 250' Span Conductor Final Sag - (IN.)
Conductor Design 60°FInitial # 60°F 90°F 167°F 75% - 60°F
Initial # 60°F 90°F 167°F
1/0 ACSR 1243 910 20.5 29.5 45.6 682 27.1 37.3 52.1
4/0 ACSR 2000 1394 26 36 52 1046 32.8 42.8 58.1
336 ACSR 2000 1172 38 49.1 72.2 879 46.1 56.3 77.8
556 ACSR 2000 1149 54.7 64 83.8 862 69.4 77.2 94
556 ACSR 3000 1812 39.8 50.6 73.4 1360 43.7 54 76.1
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Why Worry about 167°F (75°C)?
Conductor Ampacity
Wind Angle Book* 90°** 45°** 0°**
1/0 ACSR 243 198 182 121
4/0 ACSR 366 294 271 180
336 ACSR 519 419 386 262
556 ACSR 711 571 526 369
All values at 75°C (167°F) conductor temperature with 2 FPS
wind.
* 25°C Ambient
** 40°C AmbientValues are combined effect of using 40°C (104°F) ambient and various wind
angles.
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Typical RUS Structures
Figure 4A: RUS C1 Figure 4B: RUS DC-C1
Phase-to-Phase faults more critical.
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Conductor Size Required for 10,000 AMP Fault Soft Drawn CU - Start Temp 40ºC
Example of Structure for Phase-to-Ground Fault
Figure 5: Conductor Conflict for a C9 STRUCTURE
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Conductor Size Required for 14,500 AMP Fault Soft Drawn CU - Start Temp 40ºC
Conductor Temperature Effect
Conductor 3 kA - 58~ 10 kA - 10~
1/0 ACSR @ 90°F 27" 37" (Horiz)
1/0 ACSR @ 75% & 167°F 45" 47" (Horiz)
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Time for Maximum Reverse Swing3000 A for 58~
Conductor Return Time (sec.) Displacement (inches)
1/0 ACSR 1.45 - 1.82 27 - 45
4/0 ACSR 1.58 - 1.90 22 - 33
336 ACSR 1.79 - 2.14 25 - 41
556 ACSR 1.99 - 2.32 21 - 28
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Conclusions
Consider conductor temperature under load currents when
determining maximum sags.
Consider maximum operating sags and available short circuit
currents when evaluating allowable span lengths, design tensions
and conductor spacing.
Include measurements of actual conductor sag/tension during
inspections of conductor installations.
When investigating the occurrence of apparent miscoordination,
consider the possibility of conductor clash on the source side of
suspected fault locations.