establish ratings which will produce the
limiting amount of annealing. Fortunately, convergence is rapid, since ratings
too low give negligible annealing, and those
too high exceed the limit of annealing in
short order. In a graphical method, these
effects are easily noted.
Transmission Conductor Ratings
G. M. BEERS
MEMBER IEEE
S. R. GILLIGAN
SENIOR MEMBER IEEE
H. W. LIS
MEMBER IEEE
J. M. SCHAMBERGER
SENIOR MEMBER IEEE
Summary: The purpose of this paper is to
describe a method of calculating transmission conductor ratings. These ratings
are limited by the amount of annealing which
can be tolerated during the life of the conductor. The method includes an approach
to weather-data analysis, use of a digital
computer for bulk calculations, and a
graphical method of calculating annealing.
The resulting ratings can be used with confidence, since they reflect both local weather
experience and operating practices.
SEVERAL operating electric com-
panies in Connecticut and Massachusetts* jointly plan new generation and
transmission facilities in order to coordinate both area and local requirements.
The need for a common basis of establishing transmission conductor ratings was
recognized and a joint study group was
formed to determine both the basis and
the ratings.
General
The basic method itself is general. It
may be used in any area where routine
weather observations have been made and
where tr4nsmission operating practices
are known. It will remain for any user
of this method to determine his own
limiting physical parameters and life of
the plant. Several previous papers have
suggested the use of local weather observations or the use of a computer in making
calculations.' New data are available
on conductor annealing and resistance
characteristics.2 The method to be described makes use of these techniques
and data to establish unique ratings for
local conditions.
Conductors must be rated in order to
limit the total loss of strength due to
annealing to a prescribed amount during
the stated life of the conductor. In
addition, ratings must be based on the
maximum conductor temperature to be
tolerated. Depending upon local weather
conditions or operating practices, several
ratings for a single conductor may be
desired, such as summer versus winter, or
normal versus emergency, ratings. These
ratings can be established only if the
operation under the various conditions
can be predicted. The method consists of
the following steps:
1. Reduction of local weather observations
to a table of total hours of occurrence of
each condition of wind and ambient temperature which may be expected to produce
any appreciable annealing during the life
operation of the conductor.
2. Assignment of the load characteristics
to be assumed, giving the load duration in
one form or another, together with any
assumptions or requirements for emergency
operation.
3. Assignment of the maximum loss of
conductor strength due to annealing which
can be permitted over life of the conductor.
4. Preparation of a table of conductor
temperatures as a function of current, wind
velocity, and ambient temperature.
5. Calculation of the total annealing by a
step-by-step graphical method. Some trialand-error work is required here, in order to
The Connecticut Light & Power Company,
The Hartford Electric Light Company, The
United Illuminating Company, and the Western
Massachusetts Electric Company.
OCTOBER 1963
Factors Affecting Ampere Rating
CONDUCTOR CHARACTERISTICS
There are several characteristics associated with transmission line conductors
which may establish the maximum rating.
These are, principally: loss of strength due
to annealing, conductor sag increase due
to operating temperatures, and the ability
of line hardware, splices, and connectors
to meet the capability of the conductor
itself. Most important of these characteristics is the permanent loss of tensile
strength due to annealing. This effect
is cumulative and depends upon conductor temperature and time.
300
o
250
ACTUAL
IN
z
0
Paper 63-86, recommended by the AIEE Transmission and Distribution Committee and approved
by the AIEE Technical Operations Department for
presentation at the IEEE Winter General Meeting,
New York, N. Y., January 27-February 1, 1963.
Manuscript submitted October 18, 1962; made
available for printing November 30, 1962.
G. M. BEERS iS with the United Illuminating
Company, New Haven, Conn.; S. R. GILLIGAN is
with the Hartford Electric Light Company,
Hartford, Conn.; H. W. Lis is with the Western
Massachusetts Electric Company, West Springfield, Mass.; and J. M. SCHAMBERGER is with The
Connecticut Light & Power Company, Berlin,
Conn.
*
The method has one particular advantage: The resulting ratings are tailored
to local conditions and requirements. As
such, they can be used with confidence.
The use of arbitrary or uncertain average
temperatures or wind velocities is avoided,
as are unrealistic "continuous" ratings.
Despite this advantage, the method can
produce some disconcerting results: In
the light of actual local weather experience, even a modest ampere rating may,
at times, result in conductor temperatures (and sags) far exceeding those
which formed the basis of line design.
Conversely, at other times, the line may
be capable of currents which exceed the
capabilities of series devices or terminal
equipment.
5
200
ASSUME
DISTRI
m
o
Fig. 1. Wind distribution at 70 F
(68 F-72 F)
b0
150
z
100
WIND
VELOCITY
-
KNOTS
Beers, Gilligan, Lis, Schamberger-Conductor Ratings
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767
Fig. 2. Wind hours
at 70 F (68 F-72 F)
.0
I
OBSERVATIONS
AT EACH INTERVAL
(HOURS IN 5 YRS.)
U)
0
'F-
50
O0
0
0.375
1.0
.5
2.o
0
WIND
VELOCITY
Based upon the line design and its economic life it should be possible to state
the loss of tensile strength that can be
permitted over the life of the conductor.
This method of calculating conductor
ratings is based entirely on the annealing
limitation. For an existing line, the
resulting sag and the demands on various
line devices and hardware must be
checked. For a new line, a co-ordinated
design can be based upon the ratings
established by loss of tensile strength.
AMBIENT CONDITIONS
In order to evaluate annealing, it is
necessary to know the conductor temperature and the time period at such temperatures. The principal source of heat
tending to raise the conductor temperature is the I2R loss; cooling is primarily
by convection in the surrounding air.
The degree of cooling is based on air
temperature and wind velocity. The
most important of these aspects is the
wind. For example, if we can count on a
minimum 10-fps (feet per second) wind
velocity, ambient temperature will have
little effect on the cooling, and the rating
will be high. On the other hand, if there
is no wind, cooling is entirely by radiation and natural convection. Under this
condition, ambient temperature does play
a part, but the ratings will be low due
to the lack of wind. After examining
the equations for conductor heating and
cooling found in the House and Tuttle
paper,3 it becomes apparent that essentially all annealing will take place well
below 10-fps wind velocity and that it is
necessary to know the conditions of wind
and temperature which are within this
range.
768
LOAD CHARACTERISTICS
The third factor affecting conductor
ratings is the loading of the line. It is
important to know whether a line will be
operated at a continuous load, which is
rare, or whether peak loading will occur
for only a short time each day. Similarly,
the requirements for emergency load conditions must be known or assumed.
From such requirements can be determined whether a single rating may be
established, or if several ratings will be
desirable. For example, a winter peak
load may justify the use of a high winter
rating, if a reduced summer capability is
acceptable; or a single year-round value
may more nearly match the load requirements.
Weather Data
In order to determine the conductor
temperatures at times when annealing is
appreciable, it is necessary to know what
the expected frequency of low-velocity
winds combined with temperatures may
be for the local area. These data are
available in the "Local Climatalogical
Data (Supplement)" which is prepared
monthly from local observations by the
Weather Bureau of the U.S. Department
of Commerce. From such data at two
locations in Connecticut, about 2,100
simultaneous hourly observations of wind
velocity and temperature representing a
5-year period were transcribed to punched
cards for sorting and tabulation. Only
the hours between 7 a.m. and 10 p.m. were
included, as this is the time period in
which the lines can be expected to be
heavily loaded. Readings were grouped
in intervals of 5 F (degrees Fahrenheit)
and one knot (nautical mile per hour) for
all observed temperatures and for velocities up to 6 knots (about 10 fps). A
typical group of velocities for the 70 F
interval (68-72 F) is shown plotted in Fig.
1. An apparent inconsistency is noted
in the small number of observations at 1
and 2 knots. From discussion with
Weather Bureau personnel, it developed
that the indicating anemometers are not
accurate below about 4 knots and, in fact,
will not start to turn much below 2 or 3
knots. For these reasons, and because
the low-velocity winds are such an important factor, it was decided to "smooth"
the readings between zero and 4 knots,
assuming that the probability of a true
zero velocity approached zero. This was
done with the curve shown in Fig. 1. The
curve was adjusted so that the total number of hourly observations was correct;
i.e., the area under the curve between
zero and 6.5 knots was made just equal
to the total observations.
Preliminary calculations of conductor
temperatures (and annealing) indicated
that for the local ambient temperature
range, only those wind velocities of 3 fps
and below would involve the conductor
in appreciable annealing.
The curve of Fig. 1 is shown replotted
in Fig. 2 on a larger scale, and block
intervals centered at C.375, 1.0, 1.5, 2.0,
2.5, and 3.0 fps are shown. The areas of
each interval were measured and represent the equivalent number of observations (hours) in each block. Similar
constructions were made for each 5 F
block of temperatures, and from the results, a wind-velocity-amnbient-temperature chart was prepared. The 5-year
totals were prorated to 30 years and a
correction factor of 4 included. This
f actor is a wrultiplier, and is used to
Table 1. Assumed Hours of Weather in
30 Years for a Typical Protected Area
Wind Velocity-Fps
Ambient
Temperature,
2.0
2.5
Degrees F 0.375 1.0 1.5
90.
85 .
80.
75 .
70 .
65 .
60 .
55 .
50 .
45 .
40 .
35 .
30 .
25 .
20.
Beers, Gilligan, Lis, Schamberger-Conductor Ratings
15 .
10.
5.
3.0
3... 7... 13... 20... 28... 38
4... 18 ... 36... 58... 84.. 115
5.. .23 ... 47... 77.. 113.. 155
31.. .77.. .140.. .223.. 312.. 395
38.. 92 ... 168.. 271.. 363.. 500
38.. .92 ... 168.. 271.. 363.. .500
24.. 68 ... 130.. 208.. 294.. .376
24.. 68 .. 130.. .208.. 294.. 376
22.. 58 .. 105... 163.. 235.. 309
23.. 62 .. 118.. .191.. 276.. .353
31.. .77 .. 140.. 223.. .312.. .395
35.. 86. .160.. 260.. 371.. .471
35.. 86 .. 160.. 260.. 371.. 471
22.. 55 ... 96.. 146.. .206.. 267
9.. 28... 55... 87... 127.. .168
5.. .18... 36... 57... 83.. 110
4... 15 ... 30... 47... 65... 83
4.. .15 ... 30... 46... 62... 78
OCTOBER 1963
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Fig.13. Typical anneaing of aluminum
(from reference 2,
permission of Alcoa)
HEATING PERIOD IN HOURS
acknowledge the fact that the observations were made in elevated and exposed
locations, while most of the lines run
through hilly, wooded terrain which may
be quite sheltered from the wind. The
factor of 4 was determined by comparing
simultaneous observation from both sheltered and exposed locations. A similar
relationship was noted in- the Schurig and
Frick article in 1930. However, there is
still a distinct need for more accurate data
on the variability of wind, particularly
with respect to terrain, elevation, and
locale.
The summary of hours of wind velocity
and temperature for 30 years is shown
in Table I.
Conductor Temperature Calculations
The second step in determining annealing is to prepare a table of conductor temperatures as a function of current and the
ambient conditions derived previously.
The basic equations used were those determined by House and Tuttle in their 1958
paper, and include the effects of I2R and
solar heat sources, and convection and
OCTOBF-R 1963
radiation heat losses. The equations for
convection heat loss cover two situations.
The first is for forced convection cooling
where the wind velocity is 2 fps or greater,
and the second covers natural vertical
convection (wind velocity of zero).
Since the significant area of conductor annealing was known to exist between 0 and
3 fps of wind, it was necessary to modify
the two equations to accept the velocity
values of 0.375, 1.0, and 1.5 fps. This
was done by calculating the vertical
natural convection velocities which could
be combined with horizontal wind to resuilt in the. correct cooling effects at the
zero (natural convection) and2-fps points.
Since a table of conductor temperatures
at discrete current steps was required, the
solution of the equations for conductor
temperature involves "cut and try"
methods. This led to the use of a
high-speed digital computer to produce,
by iterative methods, the required tables
of conductor temperature. Tables were
prepared for 9 different ACSR (steel-reinforced aluminum cable) conductors, with
about 800 temperature values calculated
for each conductor. Included in the com-
puter program is automatic temperature
correction of conductor resistance, which
has an important effect on the ratings.
Information regarding this program [for
IBM (International Business Machines
Corporation) 70701 and its availability
can be obtained from the authors. A portion of the table for 795-MCM (thousand
circular mils) ACSR "Tern" is reproduced as Tab-le V. It will be noted
that the first velocity column is headed
"0.0" fps. Although zero wind was not
used in calculating total annealing, it provides an indication of the maximum attainable conductor temperatures. While
the probability of these temperatures ever
occurring is assumed to be zero, they
represent a design limit in considering
sags and clearances.
Calculation of Rating
The final step in determining the ratings
is to calculate the actual conductor loss of
tensilestrength due to annealing, using the
tables previously described in combination with an assumed loading schedule.
The annealing characteristics of aluminum
(or the aluminum portion of ACSR),
Beers, Gilligan, Lis, Schamberger-Conductor Ratings
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769
Table II. Reduced Hours of Observed
Weather in 30 Years for a Typical Protected
Area (5%o of Total Hours of Table I)
Ambient
Temperature,
Degrees F
90.
85.
80.
75.
70.
65.
60.
55 .
50 .
45
40.
35.
30.
25 .
20 .
15.
10.
5.
Wind Velocity-Fps
0.375 1.0
1.5
2.0
2.5
3.0
0... 1... 1 .... 1.... 1.... 2
0...
1...
2...
.
1... 2 .... 3.... 4.... 6
1... 2 .... 4.... 6.... 8
4... 7 ... 11... 16... 20
2... 5... 8 .... 14.... 18... 25
2... 5... 8 .... 14... 18... 25
...
l.3 .6 ... 10... 15... 19
1... 3... 6.... 10... 15... 19
1... 3... 5.... 8.... 12.... 15
1.... 3... 6... .10... 14... 18
2... .4 .7 ... 11... 16... 20
2... .4 .8 ... 13... .19... 24
2... 4... 8 ... 13... 19... 24
1... 3... 5 .... 7... 10... 13
1.... 1... 3 .... 4.... 6.... 8
0... .1... 2 .... 3.... 4.... 5
0. 1...2 .... 2.... 3.... 4
...
0... ..... 1.... 2.... 3.... 4
Table Ill. 795-MCM (45/7) ACSR Conductor Accumulated Hours at Annealing
Temperatures for 1,060 Amperes, Normal
Operation
Conductor
Temperature,
Degrees C
130
125
120
115
Expected Hours
in 30 Years
..
..
..
..
1tO ..
105 ..
100 ...21.
95 ..
90 ..
85 ..
80 ..
75 ..
70 ..
65* ..
1
4
4
4
11
13
42
60
59
117
99
76
41
552
* Hours below 65 C cause only negligible
annealing.
for example, are shown in Fig. 3, taken
from reference 2. On this family of
curves, it is possible to plot the cumulative
effects of annealing for all conductor temperatures 65 C (degrees centigrade) and
above, and so to determine the loss of
tensile strength.
The simplest loading schedule to consider is a single year-round continuous
load. In this situation, the conductor will
experience all the hours of simultaneous
wind and temperature obtained from the
analysis of weather data. Total annealing is calculated for an assumed value of
current by combining Tables I and V to
determine the hours of operation at each 5
C interval of conductor temperature.
This in turn is plotted on annealing curves
of Fig. 3. Total annealing is calculated
for several values of current until, by
approximation or interpolation, a value
770
of current is found which will just produce the maximum allowable loss of
strength. This is the single continuous
rating for year-round use.
The first modification might be a
requirement for a summer rating and a
winter rating. In this case, the weather
data should be set up as two tables, one
covering the summer period and the other
the winter. It would now have to be
determined which period should be
favored in establishing the ratings. This
is to say, whether more of the allowable
annealing should take place in the warm
weather or in cold weather.
A second modification might be to have
dual ratings independent of the season of
the year-a normal rating and an emergency or contingent rating which could
be tolerated during abnormal system conditions. In this case, it would be necessary to assume (by estimation or past
experience) how often and for how long
these abnormal conditions might exist.
In the sample calculation of the Appendix,
for example, it was assumed -that contingency conditions would exist for no
more than 600 hours in the 30-year conductor life. If it is further assumed that
these hours occur at random, it is possible
to build a new table of expected hours of
emergency operation when unfavorable
ambient conditions might exist. Under
such assumptions, contingency ratings
appreciably higher than normal can be
achieved. These ratings have the added
quality that they can be permitted to
persist whenever they occur, without
load curtailment or apprehension about
conductor damage. In the study whose
method this paper describes, the contingency rating was felt to be as significant as normal loading and approximately
half of the total loss of tensile strength
were allocated to the contingency period.
In fact, the system operation under normal conditions is determined by the contingency rating to the extent that the
study assumed that normal maximum
loading would occur for not more than 5%
of the hours in the life of the conductor.
To assume that normal loading is cQntinuous (100% load factor) is costly in
terms of amperes and would result in
ratings well below any of the currently
published figures.
Conclusions
The method of determining transmission-line conductor ratings discussed
here is a general method by which any
conductor can be given a rating suitable to
the area and conditions. Despite the
considerable data required and the step-
by-step calculation, this rating method
will permit taking maximum advantage
of local conditions and practices.
It may be of interest to see some of the
ratings established by the actual study,
and these are shown in Table VI. Readers are cautioned that these values are
valid only under the conditions and assumptions outlined in the Appendix.
An interesting and unexpected situation developed when it was noted that
different sizes of similar type conductors
developed the same maximum conductor
temperature when loaded for a given percent loss of tensile strength due to annealing. Thus, it was possible for other sizes
to be rated by a single direct calculation
using the maximum conductor temperature to determine the ampere value.
The range of conductor temperatures,
from 140 C to 180 C, suggests conductor
sags well above those anticipated a few
years ago. Existing lines must be
checked before the new ratings are applied.
Appendix
To illustrate the application of the
method outlined in the text, a sample calculation of the ratings for the 795-MCM
(45/7) ACSR conductor will be made. This
Table IV. 795-MCM 45/7-Strand ACSR
Conductor Accumulated Hours at Annealing
Temperatures for 1,340 Amperes, Contingent
Operation
Conductor
Temperature,
Degrees C
Hours in 30
Years
Probable
Actual Hours
3
190............
185 ............
9
180 ...........
69.
*1
62 ..1.
0
175 ...........
170
.
1 ........... *1
165.154 ........... *1
.
160
1....27.....127. 1.0
155
.
-265
.
2.0O
(1. 0+ *1)
150 ............
327 ............ 1.2
145 ............
393 ............ 1.4
140 ............
684 ............ 2.5
135 ............
654 ............ 2.4
130 ............ 1.574. ........... 5.7
125 ............ 1,156 ............. 4.2
120 ............ 1,512 ............ 5.5
.869 .
6.88
115...
110 ............ 1,290 ............ 4.7
105
......
. .....2,100 ............ 7.6
12,319 .49
Probability that the combination of wind and ambient temperature will cause conductor temperatures above 100 C with a current of 1,340 amperes:
12,319 = 0.075
164,000
Probable number of hours of operation at these
temperatures:
0.075 X 600
=
45 hours
* Four extra hours were inserted at temperatures
above 150 C to provide a safety factor of 2 in the
high-temnperature range.
Beers, Gilligan, Lis, Schamberger.-*Conductor Ratings
OCTOBER 1 963
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calculation will use the data and assumptions
used in the original study.
Normal and Contingent Rating
Table V. Conductor Temperatures of 795-MCM (45/7) ACSR in Degrees C
Current,
Amperes
For our systems, we chose to develop yearround normal and contingent ratings for
each conductor size. The normal rating is
defined as the rating for continuous operation. The contingent rating is defined as
the rating which will be permitted following
an unscheduled outage on the system.
In developing the normal rating, it is
assumed that no line can reasonably be
expected to operate at, or near, its normal
rating for more than 5% of the hours
in its 30-year life. Either economic loading,
or the possible loss of another facility,
would dictate a lower loading. In addition,
the load factor of the system would indicate
very few hours per year that a line would be
loaded at or near its miaximum normal
rating. For these reasons, the hours of
wind for various ambient temperatures,
shown in Table I, are reduced to 5% of the
total hours, anld are shown in Table II. It
is believed that this assumption is conservative for most lines in our system.
In developing the contingent rating, it is
assumed that the number of hours of emergency operation would not exceed 600 in the
life of the conductor. It is further assumed
that these 600 hours occur in a random
fashion.
Annealing
Annealing for the aluminum portion of the
various sizes of ACSR was permitted to fall
between 12% and 15%. The permissible
annealing was varied, depending upon the
percentage of the conductor strength accounted for by the steel core. In general, an
over-all reduction in conductor strength of
7'0-8<%o was permitted.
Information for the air density, absolute
air viscosity, and the coefficient of thermal
conductivity are found in the tables of reference 3. A computer printout using the
equations of reference 3 at various currents
for the 795-MCM (45/7) ACSR conductor
is shown in Table V.
Calculation of the Normal Current
Rating
In the calculation of the annealing associated with the normal and contingency
ratings, the step-by-step graphical method
is used. To determine the normal rating,
it was first assumed that 1,060 amperes
would produce a satisfactory amounlt of
OCTOBER 1 963
0.375
1.5
1.0
2.0
2.5
3.0
41.1 ..... 34.8
44.3 ..... 38.0
47.4 ..... 41.1
50.6 ..... 44.3
53.8 ..... 47.4
787 .....
56.9 . 50.6
81.6.....
60.1. 53.8
84.5.....
87.4 .....
63.3. 56.9
90.4.....
66.4.... 60.0
69.5 ..... 63.2
93.3 .....
1,060........ 445 .
72.7 ..... 66.3
50 .
116.7... 112.8..... 96.2 .....
75.8 . 69.4
55 .
119.4....'115.5 . 99.1 .....
'''' 72.6
79.0 .....
60 .
122.1... 118.2 ...101.9 .....
82.1 ..... 75.7
65 .......... 124.8..... 120.9.....104.8.....
78.8
85.2......
70 .
127.5... 123.6 ... 107.7.....
88.3 ..... 82.0
75 .......... 130.2..... 126.2 ..... 110.6 .....
85.1
91.4
.
.
80
132.8 ... 128.9 .....1134...
.....95.7.....
102.6
..... 88.2
94.5
85 .
135.5 ... 131.6 ... 116.3 ... 105.6..... 98.7.
..... 91.3
i90 . 138.1 ... 134.2 ... 119.1... 108.5 ..... 101.7. 97.6 .....
0..
.
5.
10 .
15 .
20 .
25 .
30 .
35 .
40 .
1 ,130........ 45 .
50 .
55 .
60 .
65 .
70 .
75 .
80 .
85 .
90 .
1,270........
In calculating the conductor temperatures
using the equation found itn the House and
Tuttle paper,3 the following were used:
west
0.0
0.
5.
10 .
15 .
20 .
25 .
30 .
35 .
40 .
Calculation of Conductor Temperature
1. Coefficient of emissivity = 0.5
2. Coefficient of solar absorption 0.5
3. Altitude of sun = 65 degrees
4. Azimuth of sun= 148.5 degrees for 41.5
degrees north latitude
5. Azimuth of line = 270 degrees, or east to
Wind Velocity-Fps
Ambient
Temperature,
Degrees F
49.7.
52.9.
56.1.
59.2.
66.8...... 62.4.
69.8 ..... 65.6.
72.8 ..... 68.7.
75.8 ..... 71.2.
78.8.71.5.
81.8.74.5.
84.8..... 77.6.
87.8 ..... 80.6.
90.8 ..... 83.6.
93.9 ..... 86.7.....
96.7..... 89.7.
99.7..... 92.7 ....
66.8 ..... 54.6.....
69.8 ..... 57.6 .....
72.7 .....60.7 .
75.7 ..... 63.7.....
101.3..... 98.2 ..... 78.4 ..... 64.7 ..... 59.6 ..... 49.5 ..... 42.2
104.2 ....
107.0 ....
109.8 ....
112.6 ....
115.4 ....
118.1 ....
120.9 ....
123.6 ....
126.3 ....
129.0 ....
131.7 ....
134.4 ....
137.1 ....
139.7 ....
142.4 ....
145.0 ....
147.6 ....
150.3 ....
100.9 ..... 81.3 ..... 67.8 ..... 62.8 ..... 52.8 .....
103.7 ..... 84.3 ..... 70.9..... 66.0 ..... 56.0 .....
106.4..... 87.3..... 73.9.
'''' 69.2 ..... 59.2 .....
109.1..... 90.2 ..... 77.0 ..... 72.5..... 62.5.....
111.8 ..... 93.2.... 80.1 ..... 75.6 ..... 65.7 .....
74.8 ..... 68.9.....
114.6 ..... 96.1 ..... 83.1.
''''
117.3 ..... 99.0 ..... 86.1 ..... 77.9. 72.1.....
120.0 ..... 102.0,.... 89.1 ..... 81.0..... 75.3 .....
122.7 ..... 104.9 ..... 92.2 ..... 84.1 ..... 78.5.....
125.4 ..... 107.8..... 95.2 ..... 87.2 ..... 81.7.....
128.0 ..... 110.7 ..... 98.2 ..... 90.2 ..... 84.9 .....
130.7 ..... 113.6.....101.2 ..... 93.3 ..... 88.0.....
133.4 ..... 116.4 ..... 104.2 ..... 96.4..... 91.2.....
136.0 ..... 119.3.... 107.1 ..... 99.4..... 94.4 .....
138.7.....122.2 ..... 110.1.102.4 ..... 97.5.....
141.4.....125.1 ..... 113.1.105.5 ..... 100.7.....
144.0 ..... 127.9 ..... 116.1. 108.5 ..... 103.9 .....
146.7 ..... 130.7 ..... 119.0 ..... 111.5 ..... 107.0 .....
45.5
48.7
51.9
55.1
58.3
61.5
64.7
67.9
71.1
74.3
77.5
80.7
83.9
87.0
90.2
93.4
96.5
99.7
69.5 ..... 59.8
72.9..... 63.1
76.3..... 66.5
79,.6..... 69.8
82.9...... 73.2
86.3 ..... 76.5
89.6..... 79.8
92.9 ..... 83.2
96.2..... 86.5
131.1 ... 116.0 ..... 106.4 ..... 99.5..... 89.8
134.0 ... 119.0 ..... 109.5 ..... 102.8 ..... 93.1
136.9 ... 122.0 ..... 112.7 ..... 106.1 ..... 96.4
139.8... 125.0 ...... 115.8 ..... 109.4..... 99.7
142.6 ... 128.1 ..... 118.9 ..... 112.7.....103.0
145.5 ... 131.0 ..... 122.0 ..... 115.9 ..... 106.2
148.3 ... 134.0 ..... 125.0 ..... 119.2 ..... 109.5
151.2 ... 137.0 ..... 128.1.....122.4 ..... 112.8
154.0 ... 140.0 ..... 131.2 ..... 124.8 ..... 116.0
156.8 ...143.0 ..... 134.2 ..... 124.2 ..... 119.3
0... 129.2 .... 126.1 ..... 104.7 ..... 88.2 ..... 82.9 .....
132.0 .... 128.9 ..... 107.7 ..... 91.3 ..... 86.2 .....
134.7 .... 131.6.....110.6 ..... 94.5 ..... 85.2 .....
137.5 .... 134.3 ..... 113.6 ..... 97.6 ..... 87.4 .....
140.2 .... 137.0 ..... 116.5 ..... 100.6...... 90.6 .....
142.9 ..... 139.6 .... 119.5..... 103.7 ..... 93.8.....
145.6 ... 142.3... 122.4 ... 106.8 ..... 97.0 .....
148.3 ... 145.0 ... 125.3 ... 109.9 ..... 100.1.....
151.0 ... 147.7 ... 128.2 ... 112.9 ..... 103.3 .....
5.
10 .
15 .
20 .
25 .........
30 .
35 .
40 .
45 .
50 .
55 .
60 .
65 .
70 .
75 .
80 .
1,340 ....
88.8 ..... 85.5 .....
91.7 ..... 88.2.....
94.5 ..... 91.0 .....
97.3 ..... 93.7 .....
100.1 ..... 96.5 .....
102.9 . 99.2 .....
105.7 ... 102.0 .....
108.5 ... 104.7 .....
111.2 ... 107.4.....
114.0 ... 110.1 .....
153.6 ...
156.2 ...
158.9 ...
161.5 ...
164.1 ...
166.6 ...
169.2
171.8 ...
174.3...
150.3 ...
153.0 ...
155.6 ...
158.2...
160.9 ...
163.5...
166.1 ...
168.7 ...
171.3 ...
173.9 ...
85 .
90 .
176.9 ...
0.
5.
10 .
15 .
20 .
25 .
30 .
35 .
40 .
45 .
50 .
55 .
60 .
65 .
70 .
75 .
80 .
85 .
101.7 ..... 90.9..... 81.3..... 70.1
104.8 ..... 93.8 ..... 84.7..... 73.5
149.9 ... 146.8 ...
107.9..... 97.0..... 88.2..... 76.9
152.6 ... 149.5 ... 128.3... 111.1 ..... 100.2 ..... 91.6..... 80.3
155.3 ... 152.1... 131.3 ... 114.2 ... 103.4 ..... 95.0. 83.7
158.0 ... 154.8 ... 134.2 ... 117.3 ... 106.6 ... 98.4 . 87.1
160.6 ... 157.4 ... 137.1 ... 120.3... 109.8... 101.8 ..... 90.5
163.2 ... 160.1... 140.0 ... 123.4 ... 113.0 ... 105.1..... 93.9
165.9 ... 162.7 ... 142.9 ... 126.5... 116.1 ... 108.5 ..... 97.3
168.5 ... 165.3 ... 145.8 ... 129.5... 119.3 ... 111.9.....100.7
171.0 ... 168.0 ... 148.6... 132.5 ... 122.4 ... 115.2 ..... 104.0
173.6 ... 170.6... 151.5 ... 135.6 ... 125.6 ... 118.5 ..... 107.4
176.2 ... 173.2 ... 154.3... 138.6 ...128.7 ... 121.8 ..... 110.7
178.7 ... 175.8 ... 157.2 ... 141.6 ... 131.8 ... 125.2 ..... 114.1
181.3... 178.4... 160.0 ... 144.6 ... 135.0... 128.4 ..... 117.4
183.8 ... 180.9 ... 162.8 ... 147.6 ... 138.1... 130.3.....120.7
186.3... 183.5 ... 165.6 ... 150.6 ... 141.1... 129.6 ..... 124.1
188.8 . 186.1 ... 168.4 ... 153.5 ... 144.2... 132.8 ... 127.4
191.2 . 188.7 ... 171.3 ... 156.5 ... 147.3 ... 135.9 ... 130.7
t90
.
144.5 ...
147.2 ...
141.4 ...
144.1...
annealing. The amount of annealing associated with this rating was found as follows:
Entering Table V at 1,060 aniperes to obtain
conductor temperatures at this loading, and
using the hours of Table II, the first significant point of annealing is one hour at an
ambient temperature of 80 F and 0.375-fps
119.5 ...
122.4 ...
125.4 ...
The corresponding temis 128.9 C, rounded to
130 C. At a 75 F and 70 F ambient temperature with a 0.375-fps wind velocity, the
conductor temperatures are 126.2 C
(rounded to 125 C) and 123.6 C (rounded to
125 C). This 125 C conductor tempera-
wind
(Table II).
peratnre (Table V)
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771
Table VI. Transmission Conductor Ratings
Rating in Amperes
Wire Size, ACSR
2/0 (6/1)
4/0 (6/1)
336.4 MCM
336.4 MCM
397.5 MCM
556.5 MCM
795 MCM
1, 113 MCM
1,272 MCM
Normal
Contingency
270 ...... 320
400 ...... 470
(18/1) ....... 580 ...... 685
(26/7) ....... 610 ...... 725
(18/1) ....... 660 ...... 770
(24/7) ....... 860 ...... 1,020
(45/7).......1,075 ...... 1,285
(45/7) ....... 1,365 ...... 1,640
.......
.......
(45/7)....1...1,475 ...... 1,770
Note: Normal ratings result in a conductor temperature of about 140 C with a 90 F ambient air
temperature and no wind.
Contingent ratings result in a conductor temperature
of about 180 C with a 90 F ambient air temperature
and no wind.
ture can be expected for 4 hours every 30
years. Table III is made up in this manner
to tabulate the accumulated hours at
annealing temperatures. The accunmulated
hours at the various temnperatures are used
with the annealing curves of Fig. 3 to determine the conductor annealing. Assuniing
the initial tensile strength of the aluminum
at 27,100 psi (pounds per square inch) and
projecting 41 hours along the 65 C annealing
curve, reveals that the tensile strength of
the aluminum is reduced to 26,985 psi.
Projecting 76 hours along the 70 C annealing
curve from the reduced (26,985-psi) tensile strength, shows that the alunminum
strength is now reduced to 26,850 psi. By
projecting the remaining values on the
annealing curves in the foregoing' manner,
the total annealing for the normnal conductor
rating will result in a 5.7% loss of tensile
strength of the aluminum portion of the
conductor.
Calculation of the Contingent Rating
To determine the contingent rating, it
was first assumed that'1,340 amperes would
produce a satisfactory amount of annealing.
The amount of annealing associated with
this rating was found as follows: Entering
Table V at 1,340 amperes to obtain conductor temperatures at this loading and using
the hours of Table I, it is seen that there
are 3 hours in 30 years in which the ambient
temperature is 90 F and the wind velocity is
0.375 fps. This results in a conductor temperature of 188.7 C (rounded to 190 C).
The first two columns of Table IV are made
up in this manner. The total hours of
column 2 are then divided by the total
number of hours between 7 a.m. and 10 p.m.
in 30 years to give the probability that any
one contingent hour would fall at a time
when the ambient temperature and wind
velocity would produce any appreciable
annealing. This probability is then multiplied by 600 to give the probable hours of
actual operation under contingent loading.
The total probable hours of operation at the
contingent ratings were then spread on the
basis of probability of their occurrence and
are shown in the third column of Table IV.
The probable hours, together with the appropriate temperatures, are used to compute
the annealing during contingent loading.
The annealing is computed in the same step-
772
by-step method as for the normal rating.
From Fig. 3, this resulted in an additional
10.2% loss of tensile strength.
Final Conductor Ratings
From the curves of Fig. 3, it is seen that
the total annealing was 15.9%, which was
beyond the allowable limits of 12% to 15%.
Adjustments for the normal and contingent
ratings therefore had to be made.
Because an infinite number of ratings
could be developed for the normal and contingent ratings and still be within the limits
set for allowable annealing, it was decided
to set temperature limits for the conductor
at normal and contingent loading. As noted
in the conclusion, ratings obtained by annealing limits for different sizes of similar
conductors generally resulted in about the
same maximum temperature. Normal ratings generally resulted in a conductor
temperature of about 140 C with a 90 F
ambient air temperature and no wind, and
the contingency ratings generally resulted
in a conductor temperature of about 180 C
with a 90 F ambient air temperature and no
wind. Interpolation of the computer data
of Table V at these temperatures shows
that the normal rating would be 1,075
amperes and the contingent rating 1,285
amperes. Following the same procedure as
outlined previously, annealing was again
determined for these new ratings and was
found to be approximately 12.7% which is
within the limits set for allowable annealing.
Conductor ratings as calculated for nine
ACSR conductors are shown in Table VI.
References
1. AMPERE LOAD LIMITS FOR COPPER IN OVERHEAD LINEs, A. H. Kidder, C. B. Woodward,
AIEE Transactions, vol. 62, Mar. 1943, pp. 148-52.
2. ALCOA ALUMINUM OVERHEAD CONDUCTOR
ENGINEERING DATA. Aluminum Company of
America, Pittsburgh, Pa., sect. 5, 1960; sect. 7, 1959.
3. CURRENT-CARRYING CAPACITY oF ACSR,
H. E. House, P. D. Tuttle. AIEE Transactions,
pt. III (Power Apparatus and Systems), vol. 77, 1958
(Feb. 1959 section), pp. 1169-77.
4. HEATING AND CURRENT-CARRYING CAPACITY
OF BARE CONDUCTORS FOR OUTDOOR SERVICE,
0. R. Schurig, C. W. Frick. General Electric
Review, Schenectady, N. Y., 1930, pp. 141-57.
Discussion
E. Jaboolian (Gibbs & Hill, Inc., New
York, N. Y.): The authors have presented
a comprehensive method for calculating
conductor ratings to be used for any transmission line where adequate weather conditions can be determined.
Their method, in general, equates accumulated annealing with conductor currents
under existing long-time ambient temperature and wind conditions. It is a very
timely paper, since many operating companies now feel that published ratings are
too conservative, and consequently, allow
higher currents in both new and existing
lines. However, these increases have often
been based on judgment and experience.
The method, as outlined by the authors,
permits a rigorous answer.
It is interesting to note that, under the
observed conditions of 5 years of Connecticut
weather, actual load characteristics of four
companies, and a permissible loss of 15%
in aluminum tensile strength, the norinal
ratings of Table VI average within a quarter
of 1 % Alcoa ratings published in Section
6 for rises 40 C to 100 C in a 2-fps wind.
Although the averages are equal, for the
smaller conductors Table VI ratings are a
little lower than Alcoa and for the larger
conductors a little higher.
The Table VI contingency ratings average 19% higher than the normal with a
range of only 117 to 120%. The increase
is constant enough to use 119% and omit
calculation of contingency annealing, at
least for Connecticut weather and the 15%
tensile loss limit.
The paper states that, "based upon the
line design and its economic life, it should be
possible to state the loss of tensile strength
that can be permitted over the life of the
conductor." Evidently, the authors derived the annealing limits for the conductors
studied to be economically 15% for the
aluminum portion or 7 to 8% when the tensile strength of the steel core was included.
This raises the question: Why not pick
a conductor with more percentage steel
so that the aluminum could be permiitted to
anneal far enough to contribute much less,
or even none, of the tension required in the
total conductor because of sag considerations?
The loss of tensile strength then in the
aluminum would not be a limitation. The
new limit would be the ability of all types
of compression hardware, bolted clamps,
splices, etc., to preserve their tightness on
completely annealed aluminum. With the
hardware, rather than the aluminum
strength, limiting the conductor temperatures, much higher ratings could be tolerated,
at least for the contingency basis. Theoretically, the 15% loss in tensile strength
limitation could become 100%.
It should be realized that the ratings of
Table VI are based on 15% annealing in 30
years. An economic life of more than 30
years, other conditions remaining the same,
would lower the ratings. However, the
method described can be applied to any
annealing and life condition.
If the assumed conditions result in ratings
much higher than nominal, economics should
be studied so that the 12R loss in dollars
is not more than the annual investment
charges would be for a larger wire with less
losses.
As a final comment, it is hoped that the
NESC (National Electric Safety Code) will
define more rigorously their required vertical
clearances. The Code now defines clearances for bare wire sags at 60 F corrected for
voltages and maximum sag increase. Presumably, these requirements are stringent
enough to allow for greater sags under
higher wire temperatures. However, it
would be more satisfactory to specify the
minimum clearance under any condition
of wire temperature so that the various
utilities would not have to use varying judgments for hot-wire clearance.
Peter Ralston (Hydro-Electric Power Commission of Ontario, Toronto, Ont., Canada):
The problem of annealing aluminum and
ACSR conductorsunder high loads isone that
Beers, Gilligan, Lis, Schamberger-Conductor Ratings
OC-TOBER 1963
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Fig. 4. Ampacity of 795MCM (Drake) ACSR operating at 160 F (71 C) with
varying wind speeds
%TIME AMPACITY IS LESS THAN
ORDI NATE
Fig. 5. Ampacity of 795
MCM (Drake) ACSR on
per cent of time base operating at 160 F (71 C). (Computed from Malton, Ont.,
weather data, 1947-1961
included)
A-Winter season, Novem.
ber-March included
B-Summer season, Apri 1October included
has been with us for some time, particularly with older transmission lines which
are expected to carry loads greater than
they were designed for. The authors
have made a significant contribution in this
direction by attempting to establish the
amount of annealing on a statistical basis.
In principle, the approach appears to be
sound, although details raise questions.
The authors assume that the maximum
normal loading occurs for not more than 5%
of the hours in the life of the conductor, anid
calculate the amount of annealing under
normal loading on that basis. Do they
consider that somewhat lesser loads, say
80% to 95% of normal maximum, have no
annealing effect? The authors also assume
that with the loads of the order contemplated no annealing can take place if the
effective wind is greater than 3 fps.
To explain what is intended by these two
comments, Fig. 4 of this discussion shows the
current-carrying capacity of a 795-MCM
26 X 7 Drake conductor at 160 F, with
varying wind speeds and ambient temperature. This corresponds to a conductor temperature of 71 C, which is just above what
is treated in this paper as the start of the
annealing range. Since these curves have
been calculated by the use of slightly different values of solar. absorption and emissivity,
with wind speeds in miles per hour
rather than fps, an exact comparison cannot
be made with the data given in the paper.
Nevertheless, they are close enough to
illustrate the point.
It can be seen that at 1,075 amperes (the
normal rating assigned to this conductor by
the authors), when the ambient temperature is high, effective wind speeds up to
5 mph (or 7 fps) must be taken into consideration when calculating the number of
hours in the annealing range. Similarly,
loads as low as 80% of the normal rating
(860 amperes) can bring the conductor into
the annealing range. Thus, the shape of
the load curve should be considered. For
instance, Table VII is an approximate tabulation taken from a typical daily load curve
of one of Ontario Hydro's stations.
It can be seen that the load can be more
than 80% of the daily peak for more than
29% of the time. This is not an extreme
case, the daily load factor being higher
on a system basis. This illustrates the importance of considering the load-duration
curves of the system being studied.
One also wonders whether the contingent
ratings assigned by the authors might be
Fig. 5 shows the
unduly restrictive.
ampacity of Drake conductor at 71 C against
per cent of time, based on an analysis of the
Malton, Ont., weather records for the period
1947-1961, inclusive. It can be seen that
even with such a low conductor temperature most of the time the ampacity is well
above the contingent rating of 1,285 amperes
during the winter season. Although it
might not be economic to operate lines normally with higher loads, at times it may be
necessary. At such times nomograms such
as suggested by Waghorne and Ogorodnikov
might prove valuable.'
It would be interesting to see the results
of a study along lines similar to the method
suggested in this paper, whereby the prob-
,OCTOBERR 1963
ability of high loads occurring coinicident
with adverse weather conditions is taken
into account. Would the authors care to
comment on the feasibility of such a study?
Perhaps some utilities are already unknowingly operating their lines in a partially
annealed state. This hlso raises the question
of how far we should go in this direction,
perhaps even as far as fully annealed aluminum. Precisely what are the problems involved, and how can these problems best be
solved? It is perhaps time that we had some
of the answers, so that the problem of
whether to anneal or not to anneal can be
decided strictly on the basis of economics
rather than by limitations in technology.
Another very significant point is mentioned by the authors. The loads which
they suggest for various conductors can,
under some conditions, raise the conductor
temperature to 140 C for normal ratings,
and up to 180 C for contingency ratings.
Would the authors care to comment on
which is the governing consideration, and on
what basis they would design lines with regard to sags and clearances? That is, although the contingencies mlight be of such
short duration that the amount of annealing
can be neglected, the high conductor temperature will, of course, cause greater sag.
This discusser would like to see permissible
clearances reduced during "contingencies,"
and taking this one step further, let any
adverse weather conditions worse than 90 F
and 2-fps wind also be considered a contingency, even with nortmal loads.
REFERENCE
1. CURRENT CARRYING CAPACITY OF ACSR
CONDUCTORS, J. H. Waghorne, V. E. Ogorodnikov.
AIEE Transactions, vol. 70, pt. II, 1951, pp. 115962.
D. H. Sandell (Aluminum Company of
America, Pittsburgh, Pa.): The authors
have shown effectively that fundamental
laboratory test data and basic engineering
analyses can be applied to specific problems
relating to normal and emergency conductor
current ratings. They have correctly concluded that each electric system must be
analyzed individually in order to determine
safe current ratings.
For the general case, we believe it is
somewhat risky to plan even contingent current ratings that will result in conductor
temperatures above 300 F (150 C). The
authors correctly analyze the behavior of
the conductor mechanical properties at
higher temperatures, but sometimes weak
links in the circuit, such as possibly splices or
clamps, can fail at these high contingent
Table VIl. Duration of Loads as Percentage of
Daily Peak
Duration,
Percentage
of Peak
100
96
93
86
80
Total.
Hours
Percentage
of Day
1...1.............
4.17
1............... 4.17
2.............. 8.34
...
...
1..............
4.17
2.............. 8.34
.
.
..
..
7............. 29.19
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773
Table Vil. Comparison of Conductor Clearance to Ground for Elevated Temperature Operation
(2)
(1)
Maximum
ACSR Conductor
Size and Stranding
Dfsign
Tension,
Pounds
(3)
Span
Length,
Feet
230-K? Lines:
954 MCM 54/7 ....... 12,000 .......
138-Kv Lines:
477 MCM 30/7 ........ 9,000 .......
795 MCM 45/7 ........ 9,000
69-Kv Lines:
(5)
5th Edition 6th Edition
NESC
NESC
(6)
Sag (Feet)
60 F
(7)
(8)
(9)
Increase Over Sag at
60 Degrees (Feet)
120 F
200 F
300 F
(10)
Required
Clearance
at Elevated
Temperature*
(11)
(12)
Clearance of Line
Designed Under
5th Edition NESC
When Operating at
200 Ft
300 Ft
600 ...... 30.5 ...... 25.73 ........ 10.0 ....... 3.0 ..... 5.5 ...... 7.5 ....... 22.73 ....... 25.0 ...... 23.0
800 ...
30.5 ...... 25.73 ...
30.5 ...... 25.73 ...
18.0 ..
28.0 ..
3.3 .
4.0.
6.3
7.9
......
......
9.0.
10.4 ..
600 .
800 ..
8.2 .
26.07 ... 23.38 ...
26.07 . 23.38 .
16.0 ....
26.0 ...
26.07 . 23.38 .
26.3 ...... 23.8 ........ 14.8 .
2.9 .
3.0 .
3.0 .
3.2 .
4.8 ......
5.0 ......
5.8 ......
6.7 ......
6.9 .
7.9 ...
8.4 ...
9.1 .
1,000 ...
1,000 ..
3,000 .
4/0-6/1 ..........
336.4 MCM 18/1 ...... 4,000 .
*
(4)
Required Ground
Clearance (Feet) at
60 F
660 .
22.43 ...
21.73 ...
24.2 ...... 21.5
22.6 ...... 20.10
20.48 ....-.21.27 ....
21.07 .
20.38 .
20.38 .
20.27 .
20.6 ........ 19.6 .....
19.17
18.17
17.67
17.2
250 . 22.2 ...... 21.5 ........ 2.5 ..
1.8 . 2.4 ...... 3.3 .
19.7 ........ 19.8 ..... 18.9
350 ...... 22.2 ...... 21.5 ........ 5.8........ 2.0...... 2.7 ...... 4.0 ........ 19.5 ........ 19.5 ...... 18.2
2.2 . 4.65 ...... 7.1 .
350 . 22.4 ...... 21.6 ........ 4.7 .
19.4 ........ 17.75.;.....15.3
Column (5) less column (7).
t Column (4) less column (8).
2 Column (4) less column (9).
temperatures.
Any temperature
limita-
tion selected is, of course, arbitrary. We
have selected 300 F for purposes of standardization when making sag and tension computations, and this standard could also be used
for evaluating any device that would be
used on the conductor. We have also
selected 200 F (93 C) for maximum normal temperature and use this for making
sag-tension computations. Aluminum will
be significantly annealed above 200 F, as
indicated in reference 2 of the authors'
paper.
C. Robert North (Philadelphia Electric
Company, Philadelphia, Pa.): The Philadelphia Electric Company has employed
probability theory in rating of aerial conductors for approximately 20 years. The
basic principles and data used in these
ratings were published in 1943 in reference 1
of the paper. Our experience with the
ratings based on these principles has been
very good and investment in oversized conductors has been avoided. The Philadelphia Electric Company has initiated a review
of weather conditions similar to that outlined in the paper. The methods described
should be most helpful in setting up the data
handling for digital-computer anialysis.
It is unfortunate that accurate wind
velocity data could not be obtained at the
critical conditions of less than 4 knots.
The authors made a logical assumption in adjusting the curve of Fig. 1 of the paper.
However, confirmation of this critical
region would be very valuable since it is so
important to the conclusions.
Whenever preparing standard ratings for
conductors on a system, or area-wide, basis
it is necessary to make a number of very
broad assumptions, such as loss of. aluminum strength to be tolerated due to annealing, hours of the day during which critical
weather conditions could occur, factors for
variations in wind velocities over the area
served, percentage of total hours that the
conductors will operate at the normal rating,
and the probable number of hours of contingency or emergency operation. Each
designer must choose the values which will
most nearly describe the actual service conditions. His choices will greatly affect the
answers provided by the computers in the
774
form of printed out ampacity tables. The
Philadelphia Electric Company has used
aerial conductor ratings generally higher
than any other utility that has come to our
attention.
The calculation methods used, apart from
the assumptions mentioned, yield results
comparable to our ratings, indicating agreement on method.
Loss of strength in the aluminum strands
is important in determining pernlissible
ratings, but there are other factors to be
considered. Recent tests conducted by the
Philadelphia Electrid Company on 4/0
Awg (American wire gage) 6/1 Penguin
ACSR conductors indicated that the aluminum strands actually go into compression
and "birdcage" from the steel strand.
For this conductor birdcaging occtrs at a
temperature of approximately 100 C when
installed with standard sags. At this
temperature the steel strand is supporting
100% of the weight of the conductor. After
4 consecutive hours of operation at 140 C,
the aluminum strands tightened around the
steel core when cooled to ambient. After 4
consecutive hours of operation at 170 C,
the aluminum strands failed to tighten
around the steel core wire. If this should
occur in service, the core would be exposed
to possible corrosion and the increased
diameter would permit a larger build-up of
sleet or ice exceeding the design limits at a
time when the conductor is actually in a
weakened condition. The harmful effects
of residual birdcaging of the aluminum
strands must be considered when there is a
possibility of ACSR conductors operating at
teniperatures above 140 C. Actually, the
temperature at which this effect becomes
harmful will vary with the conductor strand
make-up and the installation conditions.
R. H. Sarikas and Z. J. Andracki (Illinois
Power Company, Decatur, Ill.): The
authors note in their paper that when
operating at elevated temperatures there is
an increase in sag and this results in reduced
clearances over ground, as well as other
utility lines. We would be interested in
knowing the policy used in determining the'
amount of clearance reduction permitted
without taking remedial action.
The NESC specifies the required clearance
at 60 F, and then makes provision by the use
of the so-called "maximum sag increase" for
increasing the required clearance by an
amount very nearly equal to the difference
between 60 F final sag and 120 F final sag or
sag under heavy load conditions, for all
spans of the length ordinarily associated with
transmission lines. While not stated in the
language of the code, the intent appears to
be one of basing clearance requirements at
60 F on the assumption that the line will not
operate at temperatures much in excess of
120 F. As a consequence, it seems appropriate to provide additional clearance if
conductor temperatures are to exceed 120 F
by a significant amount. An appropriate increase, in the opinion of the. discussers, is
one that will provide an absolute minimum
clearance equal to that required under the
6th edition at 120 F. Table VIII of
this discussion shows pertinent data for
operation at 200 F and 300 F, respectively,
for ACSR conductor sizes in common use
on 230-, 138-, and 69-kv lines.
Since most existing lines have been built
with the clearances specified in the 5th
edition of the NESC, use of the reduced
clearances permitted under the 6th edition
may permit operation at the elevated temperatures and still meet the criterion suggested by the discussers. Table VIII of this
discussion shows that an existing 230-kv line
using 954-MCM 54/7 ACSR conductor constructed under the 5th edition of the NESC
can be operated at 200 F without modification but not at 300 F. A 138-kv line could
operate at 200 F with slight modification,
depending upon span length and conductor
type, but could not operate at 300 F. Most
69-kv lines would require modification to
operate at 200 F. Operation at elevated
tenperatures will probably discourage the
use of all aluminum conductors in view of
the large sag increase and possible annealing
associated with such conductors.
If the authors design initially for increased
clearances, has the relative economics of
increased conductor size versus increased
structure height been compared? Also, if
additional clearances are provided, what
efforts have been made to insure that the
future construction of other utilities will not
then begin to encroach uponl these additional
clearances?
Beers, Gilligan, Lis, Schamberger-Conductor Ratings
OCTOBER 10863
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G. M. Beers, S. R. Gilligan, H. W. Lis, and
J. M. Schamberger: There has been some
indication on the authors' systems that
failures of splices can and do occur under
normal daily loadings more often than under
extreme conditions. This is probably because of the very infrequent occurrence of the
extreme conductor temperatures and the
fact that a deteriorating splice matures to
failure in the long normal periods. Mr.
Sandell's arbitrary limit of 300 F is practically coincident with the limitation established for ACSR (356 F) in that there are
only 7 hours in 30 years when conductor
temperatures in the 300 F-356 F range are
expected. There did not seem to be any
practical load limit that would guarantee
no failures of joints.
As Mr. Jaboolian notes, similar types of
conductors attained fairly uniform per-cent
normal and contingency ratings over other
base figures. The authors have used this
feature to obtain ratings for intermediate
conductors without the full sequence of calculation. The limiting loss of tensile
strength was broadly set at between 5% and
10% of the initial value without economic
analysis. It was felt that this would be a
practical amount of annealing which could
be absorbed by both existing plant and future
designs without special provisions. One deterrent from greater annealing is the fact
that each increment of loss of strength
produces an even smaller marginal increase
in rating. This is because the heating
increases with 12, and annealing accelerates
with increased temperature,, at least until
most of the cold worked strength is gone.
Mr. Jaboolian, Mr. Ralston, Mr. Sarikas,
and Mr. Andracki all raise the question
about clearances at high temperatures and
the interpretation of the NESC. The
authors' companies presently follow the
policy outlined by Mr. Sarikas and Mr.
Andracki; namely, that the calculated sag
increase (all, not 75%) between 120 F and
the maximum expected operating temperature (356 F for ACSR) is added to the 60 F
sag determined from NESC 6th editioil.
This results in no less clearance than the
Code provides at 120 F operation. On the
practical side, we felt that the 120 F 6th
edition Code clearances, particularly at
wire crossings, were the smallest values we
could accept under present operating and
construction practices. Our experience with
designs based on these higher temperatures
indicates a penalty of about 5 feet of additional structure height at 115 kv.
Mr. North comments on the possibility of
permanent birdcaging of a single layer
ACSR at or about 170 C (338 F). This
would seem to add further emphasis to Mr.
Sandell's suggested 300 F limit and indicates that only very limited operation
should be permitted at these temperatures.
We feel that there is very little probability
of obtaining anything like 4 hours of continuous operation at 170 C.
Mr. Ralston is quite correct in saying
that some additional conductor annealing
will take place (a) with wind velocities above
3.0 fps and (b) at loading less than maximum
normal rating. The amount of this additional annealing is insignificant, however,
when the annealing curves are considered.
For example, in a rigorous calculation,
there will be about 415 hours of additional
operation at normal loading (1,075 amperes)
when winds are between 3.0 and 5.5 fps,
which will produce conductor temperatures
of 65 C to 85 C. These hours, when
plotted on the annealing curves, along with
the hours of higher conductor temperature,
reduce the tensile strength of the aluminum
from 25,570 psi to 25,560 psi--a truly insignificant amount. It is characteristic of
this annealing property that a very few
hours at high annealing temperatures will
overshadow many hours at the low temperatures.
This same characteristic applies to annealing accumulations at loads lower than
normal maximum. For example, if we assume another 5% (of 30 years) block of
hours at 900-ampere operation (84% of
Radio Interference from
Lines
High-Voltage
Part 11-Distri6ution and Correlation
R. J. MATHER
SENIOR MEMBER IEEE
Summary: Randomly varying quantities,
contingent upon other independent variables,
are not amenable to easy analysis. Continuously recorded radio noise data collected
from operating lines over a 3-year period
have been subjected to statistical techniques.
Some anticipated relationships were established; others failed to be verified. The
results presented are based upon records
obtained at two high-voltage levels and with
three different single-conductor diameters.
OCTOBER 1 963
B. M. BAILEY
MEMBER IEEE
SCIENTIFIC research normally involves one of two general approaches:
deductive or inductive. One either may
postulate a general relationship and test
the experimental results for validity, or
avoid basic premises and derive conclusions from the experimental results regardless of their apparent logic. Many
times, certain relationships have been
maximum), it turns out that there are only
305 additional hours when the conductor
will be at 65 C or above, and most of these
are at 65, 70, and 75 C. The additional annealing is so small that it cannot be measured on the graphical plot. All of these are
fortutnate circumstances and permit considerable simplification of the assumptions.
Mr. Ralston's comparison of the yearround contingency rating of 1,285 amperes
(in effect determined by summer load requirements in southern New England) with
the winter capabilities of the same conductor in Ontario, seems to emphasize the need
to analyze weather, loads, and operating
practices in establishing the basis of line
ratings. If variable rating charts are used
(and Mr. Ralston does not explain how he
uses his), it would seeni to require continuous analysis of load and weather. It does
not seem practical to rate a line at 1,400
amperes one day, and 1,000 amperes the
next day, unless loads are somehow controlled or interruptible.
In the method we have outlined, it was
assumed that weather and loads were independent random variables having the distribution characteristics outlined. As Mr.
Ralston suggests, this is probably not the
case. We know, for example, that system
loads tend to increase with extremes of
temperature. Seemingly, many systems
might have available enough data to permit
more refined analysis of these relationships.
On the other hand, as we have discussed
previously, great refinement of data or
method is not likely to produce more than
a small increase in rating; a rapidlv diminishing return.
Several discussers have touched on the
economic aspects of line loadings. For
any new line, the conductor should be
selected on the basis of a sound application
of engineering economics. The authors'
companies presently design all new construction with adequate clearances to permit the use of contingent conductor
ratings.
accepted as axiomatic and only the data
which seemed to confirm this relationship
were accepted and published.
Both the deductive and inductive approach have value and have influenced
radio noise investigations, but the authors
feel that the intuitive attitude has predominated in much of the published literature. Adams' series of papersl-3 estabPaper 63-92, recommended by the AIEE Transmission and Distribution Committee and approved
by the AIEE Technical Operations Department for
presentation at the IEEE Winter General Meeting,
New York, N. Y., January 27-February 1, 1963.
Manuscript submitted April 17, 1962; made
available for printing November 20, 1962.
R. J. MATHER is with the Federal Power Commission, Washington, D. C.; and B. M. BAILEY
is with the Bonneville Power Administration,
Portland, Oreg.
The authors wish to express their appreciation to
M. G. Poland, through whose efforts these data
were obtained, and to many other members of the
Bonneville Power Administration, who assisted
in designing and maintaining the complex instrumentation necessary for collecting the information.
Oather, Bailey-Radio Interference from High- Voltage Lines-I7
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