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2015 IEEE Rural Electric Power Conference
Asheville, North Carolina
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
Why Interest now in Conductor Motion?

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.
Catenary Parameters and Basic Equation
Figure 1: Catenary Parameters
(Equation 1b)

D = the sag of a level span.

S = the span length.

H = the conductor tension.

w = the unit weight of the conductor.
Magnetic Forces Between Conductors
(Equation 2)

F = the force in pounds per foot of conductor.

d = the spacing between the conductors in feet.

I = the symmetrical short-circuit current.
Wind Forces on Conductors
(Equation 3)

d = the conductor diameter, in.

VW = the wind speed, mph.

FH = the horizontal wind force, lbs/ft.
Conductor Displacement Due to Horizontal Forces
Figure 2: Conductor Swing
(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.
Calculation of Motion
(Equation 2)
(Equation 4a)
(Equation 4b)

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.
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).
Importance of Conductor Tension
Typical 250' Span Conductor Final Sag - (IN.)
Conductor
Design
60°F
Initial #
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
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 Ambient

Values are combined effect of using 40°C (104°F) ambient and various wind angles.
Typical RUS Structures
Figure 4A: RUS C1

Phase-to-Phase faults more critical.
Figure 4B: RUS DC-C1
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
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)
Time for Maximum Reverse Swing
3000 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
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.