Asabk1

STANDARD

DOCUMENT CLASSIFICATION CONTROLLED DISCLOSURE
REFERENCE REV
ESKASABK1 1
TITLE: DETERMINATION OF CONDUCTOR DATE: May 2005
CURRENT RATINGS IN ESKOM PAGE 1 OF 12
REVISION DATE:
May 2008
TESCOD APPROVED
COMPILED BY FUNCTIONAL RESP APPROVED BY AUTHORISED BY

Signed Signed Signed Signed
.............................. ................................. ................................. .................................
M Rapapa R Stephen A Bekker MN Bailey
Corporate Consultant for TESCOD DTM for MD (R&S)
This document has been seen and accepted by:

R Stephen Study Committee
A Bekker for TESCOD
MN Bailey DTM

Contents
Page
Introduction......................................................................................................................................... 2
1 Scope ............................................................................................................................................. 2
2 Normative References.................................................................................................................... 2
3 Definition......................................................................................................................................... 3
4 Requirements ................................................................................................................................. 3
4.1 General........................................................................................................................................ 3
4.2 Probabilistic methods .................................................................................................................. 4
4.3 Application of the absolute method in Eskom ............................................................................. 5
4.4 Ampacity tables ........................................................................................................................... 8
4.5 Application of ampacity tables................................................................................................... 10
5 Revisions ...................................................................................................................................... 11
Annex
A Impact assessment ...................................................................................................................... 12

DOCUMENT CLASSIFIACTION: CONTROLLED DISCLOSURE
DETERMINATION OF CONDUCTOR REFERENCE REV
CURRENT RATINGS IN ESKOM ESKASABK1 1
PAGE 2 OF 12

Introduction
The power transfer on transmission lines affects the sag of the conductor and hence the height of the
conductor above the ground. This in turn affects the safety of the line. The determination of the
allowable power transfer is thus not only a function of the properties of the conductor but also of the
safety to the public. It is thus essential that the designers are aware of the factors that affect the safety
of a transmission line as well as the types of accidents or factors that are pertinent to the utility.

The present situation in Eskom is that the conductor thermal rating or ampacity is determined by a
deterministic method using conservative ambient conditions. These conservative ambient conditions
of 40 ºC ambient temperature, 1120W/m2 solar radiation and 0,44m/s wind speed were used, together
with equations derived in the 1940s by Hutchins and Tuck and described in a book by Butterworth, to
determine the conductor current rating.

Ratings were calculated for normal and emergency conditions at 75 °C and 90 °C. The lines were
then templated at 50 °C according to an internal Eskom directive, EED 15/6/1-1 1970. This means that
if the conductor temperature reached 50 °C, the height of the conductor above the ground would be at
the height prescribed by law. It follows that if the line was operated at the rated normal current and the
severe ambient conditions were present, the conductor temperature would be near 75 ºC, which
would result in the line being under clearance, in terms of legislation. The directive stated that the
probability of this occurring was so low that it was acceptable to template at 50 °C and determine the
current rating for 75 °C and 90 °C. This probability was not quantified.

This practice served Eskom well for almost thirty years and there were no known incidents of a
contact occurring due to the thermal limit being exceeded. However in today’s economic environment
it is necessary to use assets more effectively and, on power lines, costs can be deferred or saved by
finding ways to operate the lines closer to the safe design limits.

One way to do this is to provide the means to calculate the line ratings at different templating
temperatures which was not possible using the previous directive.

This document provides this means. It also quantifies the probability of an unsafe condition arising
associated with the rating and keeps this constant for conductors of a similar type.

It is important to note that the probabilities applied are based on the present practices so that if the
line is utilized at a higher temperature, the probability of an unsafe condition arising is no more than
the probability designed for at present. The lines are therefore just as safe as in the past albeit they
are operating at a higher temperature with a higher rating.

1 Scope
The document covers the different means of determining ampacity and gives the reasons for the
methods chosen. It then presents the ratings for different templating temperatures and conductor
types in the form of simple tables. The application of the table by planners, designers and operators is
also discussed.

The use of local conditions to determine the likely increase in ampacity by using real time monitoring
on certain lines is not covered in this document.

2 Normative References
The following documents contain provisions that, through reference in the text, constitute
requirements of this standard. At the time of publication, the edition indicated was valid. All controlled
documents are subject to revision, and parties to agreements based on this standard are encouraged
to investigate the possibility of applying the most recent edition of the documents listed below.
Information on currently valid national and international standards and specifications can be obtained
from the Information Centre and Eskom Documentation Centre at Megawatt Park.

The following documents contain provisions that, through reference in the text, constitute
requirements of this standard. At the time of publication, the editions indicated were valid. All

PAGE 3 OF 12

standards and specifications are subject to revision, and parties to agreements based on this standard
are encouraged to investigate the possibility of applying the most recent editions of the documents
listed below. Information on currently valid national and international standards and specifications can
be obtained from the Information Centre and Technology Standardization Department at Megawatt
Park.

EED 15/6/1-1:1970, Thermal limits of transmission line and busbar conductors

ERA Publications OT/4:1953, Electrical characteristics of overhead lines (S. Butterworth)

Swan, J. November 1995. Determination of conductor ampacity - A probabilistic approach. A
dissertation submitted to the School of Electrical Engineering at Vaal Triangle Technikon South Africa,
in fulfilment of the requirements for the Magister Technologiae Degree.

Working Group 12 Cigre:1992, The thermal behaviour of overhead conductors − Sections 1 and 2
Mathematical model for evaluation of conductor temperature in the steady state and the application
thereof (Electra number 144 October 1992 pages 107 to 125).

Working Group 12 Cigre:1996, Probabilistic determination of conductor current rating (Electra
Number 164 February 1996 pages 103 to 119).

3 Definitions and Abbreviations

3.1 Definitions

3.1.1 ampacity: The ampacity of a conductor is that current that will meet the design, security and
safety criteria of a particular line on which the conductor is used.

4 Requirements

4.1 General
The formulas used in the determination of the ampacity tables were obtained from the Cigre Working
Group 12 document, “The thermal behaviour of overhead conductors Sections 1 and 2 Mathematical
model for evaluation of conductor temperature in the steady state and the application thereof”. These
formulas take into account the wind direction, conductor absorbtivity, more accurate forced convection
formulas as well as a mixed convection rather than the simplistic forced and natural convection
formulas found in EED 1970.

There are two methods of calculating conductor ampacity tables: the deterministic approach and the
probabilistic approach.

The deterministic approach assumes certain bad cooling conditions (low wind speed, high ambient
temperature, etc.) and calculates the current that would result in the design temperature of the line
being reached. The line templating or design temperature, is that temperature, at which the height of
the conductor above the ground is the minimum permissible. The deterministic approach has been
used by utilities for a number of years. It is a quick and simple method. Bad cooling conditions are
assumed and the current that will result in the line design temperature being achieved is calculated.
The drawback is that the method does not address the safety or the relationship between safety and
the power transfer capability.

Eskom is at present designing and operating its lines and power systems based on, inter alia, the
allowable current (or ampacity) that can flow down the line. This current is usually calculated using a
deterministic approach with assumed bad cooling conditions. It is assumed that by limiting the current
the safety criteria will be met and the line will not contravene any regulations.

It is known however, that conditions may result at some stage in the conductor exceeding the line
design temperature causing the line to be under clearance. What is needed therefore is the
quantification of the safety aspect of the design.

PAGE 4 OF 12

The probabilistic approach uses the actual weather data and conditions prevailing on the line or in the
area to determine the likelihood or probability of a certain condition occurring. Such a condition could
be, for example, the conductor temperature rising above the design temperature. These methods
have been developed to include a measure of safety of the line. This can be used as a means of
comparison of practices between utilities in all countries.

There may be a problem in obtaining accurate low wind speed data. Very low wind speeds (< 1,0 m/s)
are not recorded accurately by cup anemometers generally used by national weather services. Data
received from these services may, therefore, be of limited use.

4.2 Probabilistic methods
There are three main methods available at present.

The first is the method whereby the probability of an accident occurring can be quantified. The
benefits of this method are that an absolute measure of safety is achieved. The drawback is that the
nature of the parameters (later described in 4.2.1) is extremely difficult to determine. In addition the
correlation between the parameters, for example, the weather parameters need to be determined.

The second method uses the existing weather data to determine the temperature of the line
conductors for a given current flow. The amount of time that the temperature exceeds the line design
temperature can be determined for each current level. The utility can then decide on the current level
to use based on the percentage of excursion or "exceedence". The advantage of this method is that it
is relatively easy to determine the percentages and decide on a level by which to operate. The
disadvantage is that there is no way of determining the difference in safety (to the public) between, for
example, the 5 % and 6 % excursion levels.

An adaptation of this method is to simulate the weather data and the current flow to determine the
cumulative distribution of the conductor temperature as a function of current. This curve could be used
to determine the current and excursion level.

The third approach is to simulate the safety of a transmission line by incorporating all the factors that
affect the safety of a line. From this method a measure of safety can be developed whereby the
practices in different countries can be compared on an objective basis. The advantage of this method
is that all factors are considered. The variation of the occurrence of objects under the line e.g. a traffic
pattern can be related to the safety of a line. Designers can use a wider range of methods to increase
the thermal rating of the line not generally used before. An example of this is the reduction of surge
magnitudes or the number of surges per year can be used to increase the current carrying capacity of
a line.

By using the measure of safety, system planners and line designers are in a position to determine the
consequences of decisions in an objective way, rather than a subjective way.

Similarly System Operators, by using the measure of safety together with data from a real time
monitoring system, could operate transmission lines at higher than rated currents during emergencies.

Utilities worldwide would be in a position to determine the safety of their lines in relation to other
utilities.

This standard deals exclusively with the absolute probability method, as this method is the one
preferred for the generation of ampacity tables.

4.2.1 Determination of the absolute probability of an unsafe condition arising

Research to date has primarily been confined to attempts at determining the probability of an unsafe
condition arising. This is determined by ascertaining the probability of each factor occurring and
multiplying the probabilities.

PAGE 5 OF 12

This is represented as:

P(acc) = P(CT) × P(I) × P(obj) × P(surge) (Stephen 1991)
where

P(acc) is the probability of the accident arising.
P(CT) is the probability of a certain temperature being reached by the conductor and is
calculated from existing weather conditions, conductor types and an assumed current.
P(I) is the probability of the assumed current being reached and is determined from the
actual current being measured on a system.
P(obj) is the probability of the electrical clearance being decreased by an object or person.
P(surge) is the probability of a voltage surge occurring in the line and may be determined from
fault records kept by the power utility as well as simulations on switching surge
overvoltages on the system. If the surge occurs simultaneously with the object being
under the line the likelihood of a flashover is increased.
Each of the above is considered to be determined independently.

P(CT) is determined by the Monte Carlo simulation technique sampling from distributions of ambient
temperature, wind speed, wind direction and solar radiation to calculate the probability of a certain
temperature being reached given a current transfer. The ambient temperature, solar radiation, wind
speed and wind direction are sampled independently to form a set of parameters from which the
temperature of the conductor is determined.

The problem with this method is that it assumes there is no correlation between weather parameters
or the current, object and surge occurrences. This may not be correct in all cases. The correlation
between the individual weather parameters, as well as the weather parameters and the surge
occurrences and objects being under the line, must be ascertained.

This problem can be partly overcome by using sets of weather parameters, measured at the same
time. These sets will be used to determine the P(CT). Since each set used is determined from actual
recordings of ambient temperature, solar radiation, wind speed and wind direction taken at the same
time, the correlation between the parameters is taken care of.

4.3 Application of the absolute method in Eskom
In system planning and design, overhead line transmission capacity is a parameter of major
importance. It is therefore necessary to have exhaustive information regarding the factors affecting
this capacity in order to be able to design a transmission system under the best possible technical and
economic conditions.

The power transfer capability of transmission lines is limited by economic, physical and statutory
constraints. Conductor current and temperatures generally determine the amount of power that can be
transmitted over a given circuit. The maximum temperature at which a conductor can safely operate is
determined by:

a) permissible sag, that is governed by statutory requirements;
b) annealing and long term creep, and;
c) the reliability of joints and fittings.
In addition, limits imposed by temperature, line transfer limit or losses may limit the load capability of
specific transmission lines.

Because of the economic pressures to increase the current carrying capacity of both existing and
planned overhead lines, there is a growing interest in using probabilistic methods which take into
account the variability of the stochastic nature of the meteorological parameters.

PAGE 6 OF 12

The probability of a certain load current, that will result in the template temperature being met, is equal
to the product of the individual probabilities of the weather conditions and conductor surface
temperature (Swan1995).

P(Tc) = P(I) × P(Ta) × P(GSR) × P(WS) × P(WD) ==> P(I) = P(Tc) /(P(Ta) × P(GSR) × P(WS) × P(WD))
where

P(Tc) is the probability of a conductor temperature;
P(I) is the probability of a current;
P(Ta) is the probability of an ambient temperature;
P(GSR) is the probability of global solar radiation;
P(WS) is the probability of wind speed; and
P(WD) is the probability of wind direction.
The weather model was constructed from historical hourly weather data between the hours of 11h00
and 15h00 (inclusive) for a period of 30 years. The reason for this choice is that the highest conductor
temperature will occur during that time of the day under full load conditions, considered to be the worst
case condition. The data has been tested statistically for correlation in order to ensure that no special
simulation techniques are required. The highest correlation was found to be in the order of 3 %. No
special simulation techniques are required for such low correlation.

A range of conductor current values are generated from the above that will result in the template
temperature being met. In order to identify the optimal current from the range, it is necessary to
identify conditions that may lead to a possible dangerous condition. Typical high-risk factors are:

a) with high traffic density road crossings and
b) the possibility of a flashover from the conductor to an object underneath the conductor.
The main factors that may cause a flashover are:

a) a vehicle, at least 4,65 m high, underneath the conductor;
b) full load current;
c) weather conditions that together with full load current will result in the conductor surface
temperature being equal to the template temperature;
d) maximum system voltage; and
e) an impulse, switching or as result of lightning, that will transiently raise the system voltage to at
least 2 per unit (p.u).
The above are assumed to be occurring independently. Therefore the probability of an unsafe
condition or accident is equal to the product of the individual probabilities (Swan 1995).

P(acc) = ((P(Ta) × P(GSR) × P(WS) × P(WD))/P(Tc)) × P(OBJ) × P(S.I) × P(U.max) × P(2,5p.u)
where

P(acc) is the probability of an unsafe condition occurring, calculated for the Eskom design practice
prior to 1987 i.e. 75 °C conductor thermal rating and 50 °C template temperature;
P(OBJ) is the probability of an object under the line, based on the Ben Schoeman Highway traffic
patterns i.e. 800 vehicles per hour of which 40 % are trucks with a maximum height of
4,2 m;
P(S.I) is the probability of switching impulse occurring, calculated based on transmission
performance database;
P(U.max) is the probability of maximum system voltage, assumed to be 1; and
P(2.5p.u) is the probability of the surge magnitude being 2 p.u based on a simulation representative
line in the system.

PAGE 7 OF 12

For the purpose of generating the table the probability of an unsafe condition occurring was calculated
for the Eskom design philosophy prior to 1987. With this philosophy the conductor was thermally rated
for a 75 °C electrical rating, but the line template temperature was at 50 °C. The probability of an
unsafe condition was then kept constant and the ratings at different temperatures were then
calculated. The table of ampacity values will therefore not increase the Eskom operational risk. For
example, the probability for Wolf conductor at 370 A at a templating temperature of 50 °C for normal
operation was then calculated. This probability was kept constant in order to calculate the rating of
Wolf conductor at different templating temperatures. This method was in turn used for other
conductors. The probability used for Wolf conductor was used for all double layer ACSR conductors. A
similar method was used for other conductor configurations. The same approach was used for the
emergency ratings in that the probability was calculated for the present emergency conditions and
kept constant for the different templating temperatures.

The use of hourly weather data between 11h00 and 15h00 will not affect the calculated values in the
table. A more comprehensive database will however affect the calculated value of the probability of an
unsafe condition {P(acc)}. If this method is to be used for international benchmarking exercises, the use
of a comprehensive weather database will be required.

In addition, transmission lines in service may be further optimized using hourly weather data and the
actual load profile of the line. The uprating of lines with this method would not result in the same
increase in power transfer capacity that would be possible with real time monitoring. It is however less
costly, it requires a once-off resource, and may potentially increase the transfer capacity up to 25 %.
The potential increase of 25 % is in most cases sufficient to delay capital expenditure. In some cases
capital expenditure may even be deferred indefinitely. The potential increase in power is dependent on
a number of factors i.e. the terrain, the original design criteria, survey tolerances, equivalent spans
etc. The successful uprating of a line can only be achieved once the impact of all these factors has
been accessed in terms of the safety and reliability of the line in question.

When the new ampacity values are used for the planning and design of new lines, it is of vital
importance that the template temperature and conductor thermal rating are the same. If the template
temperature and conductor thermal rating are different the probability of an unsafe condition {P(acc)}
will not be the same as the calculated values in this table. The operational risk to Eskom and the
safety to the public will therefore be adversely affected. Please note that no lines should be templated
above 80oC for reasons listed in 4.5.3.

PAGE 8 OF 12

4.4 Ampacity tables
Double TT Normal Emergency Triple layer TT Normal Emergency Single TT Normal Emergency AAAC TT Normal Emergency
layer Deg C Amps Amps Amps Amps layer Amps Amps Amps Amps

TIGER 50 330 425 TERN 50 611 814 SQUIRREL 50 106 135 ACACIA 50 114 142
WOLF 50 378 501 BUNTING 50 807 1077 FOX 50 148 192 35 50 163 204
WOLF 60 473 602 BUNTING 60 1029 1321 FOX 60 184 228 35 60 200 243
WOLF 70 548 683 BUNTING 70 1198 1506 FOX 70 210 258 35 70 229 275
WOLF 80 610 751 BUNTING 80 1347 1663 FOX 80 233 283 35 80 254 303
LYNX 50 417 557 ZEBRA 50 642 859 MINK 50 209 272 PINE 50 227 288
CHICADEE 50 433 576 DINOSAUR 50 852 1151 HARE 50 292 380 OAK 50 312 402

PAGE 9 OF 12

Double layer TT Normal Emergency Triplelayer TT Normal Emergency Single layer TT Normal Emergency AAAC TT Normal Emergency
Amps Amps Amps Amps Amps Amps Amps Amps
PANTHER 50 459 612 BERSFORT 50 875 1183 MAGPIE 50 33 40 ASH 50 410 539
PELICAN 50 487 654 YEW 50 792 1057
PELICAN 60 611 785 YEW 60 978 1268
PELICAN 70 709 892 YEW 70 1131 1440
PELICAN 80 789 980 Quadlayer TT Normal Emergency COPPER YEW 80 1264 1584
BEAR 50 529 715 Amps Amps SYCAMORE 50 583 772
BEAR 60 663 860 800-IEC-72/7 50 930 1339 C7/0.104 50 197 246 SYCAMORE 60 717 921
KINGBIRD 50 590 793 800-IEC-72/7 80 1592 2066 C7/0.104 80 306 364 ELM 50 455 599
KINGBIRD 60 738 955 C7/0.152 50 316 402 ELM 60 563 716
KINGBIRD 70 855 1088 C7/0.152 60 388 480 ELM 70 648 810
KINGBIRD 80 956 1196 C7/0.152 70 446 544 ELM 80 719 889
GOAT 50 607 822 C7/0.152 80 496 597
GOAT 60 763 990
GOAT 70 884 1130
GOAT 80 988 1240
The template temperature (TT) and the conductor thermal rating shall be the same if the above table values are being used.

PAGE 10 OF 12

4.5 Application of ampacity tables
The tables have been generated for all the conductor types, with different templating temperatures.
The planners now have a far wider choice of options than previously. If, for example, a planner
requires a power transfer of 600A the planner can choose a Wolf conductor at 80 °C, a Lynx
conductor at 70 °C, a Panther conductor at 60 °C, or a Goat conductor at 50 °C. This will depend on
what the line is to be used for. This is covered in 4.5.1 and 4.5.2. The same applies with lines for
upgrading: if a 50 °C Wolf line exhibits a load of 600A under emergency conditions, it is possible that
investigations can be done to see if the templating temperatures cannot be increased by 10 °C.

4.5.1 Short line with high emergency load requirement. Peaky load profile, no voltage
or stability problems (no line length dependent transfer limit)

For this condition it is preferable to use a quad or hex bundle with small conductors. This increases
the cooling efficiency, improves corona performance and surge impedance loading (SIL). It also allows
the optimum aluminium area to be used. However, the templating temperature is fixed to a maximum
of 80 °C with regard to line service considerations listed in 4.5.3 below.

4.5.2 Long line, level load profile voltage and stability problems

In most cases with these problems use the smallest number of conductors in a bundle that is possible.
The more conductors that are used in a bundle the higher the resultant mechanical loading on the
towers and foundations. The hardware and damping are also more expensive. Corona limits (at high
voltages such as 400 kV) are likely to be the limiting criteria. The large aluminium area will ensure the
lowest life cycle cost. Compactness should be an option to improve the SIL. If possible, series
compensation or FACTS (Flexible AC Transmission Systems) devices such as thyristor switched
capacitors shall be used. The templating temperature can be reduced due to the low current density. A
probabilistic approach using the ampacity tables could result in the optimum templating temperature
being established.

It is stressed that the use of the tables is a first pass at the final conductor selection. The best tower,
conductor combination will depend on the terrain, cost of hardware, towers and construction. The best
thermal solution of bundled conductor with small conductors may not prove the best mechanical
solution due to high wind load and hardware costs. The main benefit of using the tables is that it opens
far more options to the planner.

4.5.3 Thermal constraints when applying ampacity tables

In the light of thermal constraints considered for a line in service, the templating temperature has been
reduced to 80 °C on all conductor sizes and types as listed on the ampacity tables.

The main reasons are:

a) The accelerated rate of annealing of the aluminium at temperatures exceeding 80C with a
consequential loss of mechanical strength. The implication is more sag than anticipated and under
clearance of lines.
b) High temperature operation increases the risk of joint failures that will impact negatively on the
reliability of the circuit/system.
c) The I^2R losses become significant hence the system losses become extensively high to offset
the initial potential cost savings.
d) With greased conductors the protective grease melts and oozes out of the conductor leaving the
conductor steel without corrosion protection.
e) Bird caging may occur transferring the entire mechanical load onto the steel portion of the
conductor. In addition, during bird caging, the aluminium goes into compression and therefore
adding an additional mechanical load onto the steel portion of the conductor. Sag will be more

PAGE 11 OF 12

than anticipated and Eskom will be in contravention of statutory requirements due to sag and
exceeding design safety factors.
f) Voltage regulation requirements may not be achieved due to high voltage drop on the
circuit/system (95-105%).
g) Smaller conductors at high current may not satisfy system stability criteria and requirements.

5 Equipment/Software

DATE REV. NO. NOTES

Jun ‘00 0 Document issued.

Oct ’04 0(A) Templating Temperature reduced from 100oC to 80oC

April ‘05 1 Document approved.

PAGE 12 OF 12

Annex A
(informative)

Impact Assessment

Impact assessment
Document title: Determination of conductor current ratings in
Eskom
Document no: ESKASABK1 Revision no: 1
Activity Detail
1. What training is required to implement this document? N/A
(e.g. Awareness training, practical / on job, module.)
2. Who will require training? (State designations.) N/A
3. What prerequisites are needed for students? N/A
4. What equipment will be required for training? (Computers etc.) N/A
5. What special tools will be required for training? N/A
6. What special requirements are needed for the trainer? N/A
7. Time period for training to be completed? N/A
8. What special tools / equipment will be needed to be purchased by N/A
the Region to effectively implement?
9. Are there stock numbers available for the new equipment? N/A
9. Does the document affect the budget? N/A
10. Time period for implementation of requirements after training is N/A
completed?
11. Does the Buyers Guide or Buyers List need updating? N/A
12. What Buyer’s Guides have been created? N/A
13. Was Training & Development consulted w.r.t training requirements? N/A
14. Were the critical points in the document determined? Yes
15. Is any training material available on the subject in this document? N/A
16. Was the document SCSPVABE0 adhered to? Yes

Total implementation
period
Total training cost
Total cost of tools /
equipment
Total cost involved
Comments:

Assessment Compiled by: Recommended by (Functional Responsibility):
Name: M Rapapa Name: J Swan
Designation: Engineer Designation: Transmission Consultant
Dept: Distribution Technology Dept: Transmission
Date: March 2005 Date: March 2005

Asabk1

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