Overhead Distribution Conductor Ratings
Rosie Roessel
Energy Systems Engineering
Introduction
• Conductor ratings: the maximum amount of electrical current that a conductor can carry safely and reliably
• Conductor properties such as the diameter, material, emissivity, absorptivity, and electrical current have significant impacts on the ratings
• When the conductors are heated, they expand and lengthen, tension decreases, and sag occurs
• Ambient weather significantly impacts sag
Problem Statement
Conductor Ratings
• Are ratings in use too conservative? Can they be updated with more current weather data?
• Major assumptions: conductor temperature (Tc), ambient
temperature (Ta), wind speed (Vw)
• qc = convective heat loss, qr = radiated heat loss, qs = solar
heat gain, R(Tc) = AC resistance of the conductor at
temperature Tc
Types of Conductors
• 477 KCMIL AAC 37-strand, 336 KCMIL AAC 19-strand
WEATHER
DATA
Summer
Average
Summer Winter
Maximum Average
Temperature
(°F)
73.5
104
Wind (mph)
4.9
43
32
7.2
Winter
Weather Data
Minimum
• 4 airports within PPL EU’s service territory:
KABE (Allentown), KAVP (Wilkes-Barre),
-8
KIPT (Williamsport), KMDT (Harrisburg)
• Winters (December- February) 2009-2012,
40
Summers (June – August) 2010-2013
SAG10 Software
• Input factors affecting sag: span length, tension,
ambient temperature and initial sag at construction,
creep
• Creep: long-term elongation of the conductors from
prolonged use
• Sag must meet NESC clearance specification of 27”
between the neutral conductor and primary
conductor
• Conductor temperatures were varied to determine
sag limiting temperature
336
Normal
CONDUCTOR
RATINGS
Rating (amps) 463
Emergency Winter
Minimum
(-8/43)
446
513
Wind Speed Analysis
• Probability of wind speeds for winter and
summer at 4 airports over 4 years
• Monte Carlo simulation for comparison
• Computed conductor ratings with
temperature assumptions from 1979
study that inform ratings
Average Winter Wind Speed
14
Percent Frequency
12
10
8
Actual Data
Monte Carlo
6
4
2
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Wind Speed (mph)
Peak Load Data
• Peak loads match up with summer maxima
and winter minima
336 SAG LIMITING Summer Summer Winter
Winter
TEMPERATURES Average Maximum Average Minimum
Temperature (°F)
Winter
Thermal Summer Summer
Average
Maximum Average
(32/94)
(104/184) (73.5/184)
594
640
721
861
Wind Speed
(mph)
0
1
1.34
2
3
4
5
6
7
8
Probability (%)
.4%
3.7%
6.0%
8.6%
9.2%
10.8%
11.7%
9.1%
7.8%
227*
94
43
Conductor Ratings
• Computed with new weather data and sag limiting
conductor temperatures
• Comparison made to normal, emergency, and thermal
ratings in use
• The maximum conductor operating temperature for the
336 AAC is 184 °F, so the sag limiting temperature is
reduced to this value
• Note “(73.5/184)” implies Ta = 73.5 °F, Tc = 184 °F
Cumulative
Probability (%)
.4%
4.1%
10.1%
18.7%
27.9%
38.7%
50.4%
59.5%
67.3%
186*
Current
(amps)
491
593
642
715
798
863
926
980
1028
1072
Conclusion
• Separate ratings for winter and
summer seasons should be used
• Conductor ratings can be improved
using probable wind speeds in the
calculation
Future Work
• Determine impact of ice loads on sag and conductor ratings
• Calculate dynamic ratings based on temperature, wind speed, and ice load considered simultaneously
• Analyze different creep temperatures’ impact on sag limiting temperature
Acknowledgements:
Thank you to my mentor Mike Dicks and subject matter
expert Bill Keller, both from PPL Electric Utilities, Inc. ,
as well as Prof. Martha Dodge for their guidance.
March 12, 2014